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Nanotechnology and Applications: An All-Language Literature Survey Including Books and Patents

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Larry J Kricka, Paolo Fortina, Nanotechnology and Applications: An All-Language Literature Survey Including Books and Patents, Clinical Chemistry , Volume 48, Issue 4, 1 April 2002, Pages 662–665, https://doi.org/10.1093/clinchem/48.4.662

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This all-language literature survey categorizes and lists books, book chapters, reviews, editorials, papers, abstracts, and patents on the topic of nanotechnology that have been published up to the middle of 2001. It can serve as a convenient entry point into the nanotechnology literature for those wishing to gain an insight into the scope and diversity of this important and rapidly expanding branch of science. The database has been compiled from searches of OVID Medline, INSPEC, BIOSIS, PubMed, various patent databases, and the personal databases of members the IFCC Working Group on Nanotechnology. The listing of references for each of the 19 categories can be found at Clinical Chemistry Online ( www.clinchem.org/content/vol48/issue4/ ).

The science of nanotechnology “is concerned with materials and systems whose structures and components exhibit novel and significantly improved physical, chemical and biological properties, phenomena and processes because of their small nanoscale size. Structural features in the range of ∼10 −9 to 10 −7 m (1 to 100 nanometers) determine important changes as compared to the behavior of isolated molecules (1 nanometer) or of bulk materials” ( 1 ). More generally, nanotechnology can be defined as any technique able to work at a submicron scale. It includes molecular nanotechnological processes in which devices are constructed atom by atom or molecule by molecule using “assemblers” that facilitate the precise handling and positioning of atomic or molecular building blocks (the so-called, “bottom-up” approach to fabrication).

The seminal lecture by Richard Feynman in 1959, entitled “There’s Plenty of Room at the Bottom”, is widely acknowledged as a key event in the development of the field of nanotechnology ( 2 ). The field of nanotechnology was further enabled by the many advances in microscopic techniques that permit atomic resolution imaging of surfaces and manipulation of atoms and molecules ( 3 ). The current range of scanning probe techniques can be traced back to the work of Gerd Binnig and Heinrich Rohrer on the scanning tunneling microscope at IBM (Zurich) in 1981 for which they were awarded the 1986 Nobel prize in physics ( 4 ). Additional impetus to this nascent field was the publication of the book, Engines of Creation , by K. Eric Drexler in 1986, which explored and advocated the possible applications of nanotechnology ( 5 ). Forty years after Feynman’s initial theoretical propositions, the field of nanotechnology has progressed to real examples of devices and objects ( 6 ). The current range of nanofabricated objects includes nanorods (e.g., nanometer-diameter whiskers of silicon carbide); nanotubes (hollow nanometer-wide tubes of carbon atoms bonded in a graphite-like structure, also called microtubules or fibrils); nanopores; nanoparticles; nanocrystals; nanowires; and quantum dots [droplets of electric charge produced in a nanometer-sized piece (“dot”) of semiconductor]. It is anticipated that materials and devices produced using nanotechnology will have a major impact on many aspects of our daily life, e.g., carbon nanotubes are stronger than steel and hold great promise for applications where light weight and strength are a premium ( 7 ). Already, nanotubes and nanowires are being used to build electronic display panels ( 8 ), and more generally, nanoelectronics is being pursued as a means to produce smaller and faster electronic devices ( 9 )( 10 )( 11 )( 12 ). Nanoparticles are finding applications as drug carriers in therapy ( 13 )( 14 )( 15 )( 16 ) and as labels in analytical methods ( 17 )( 18 )( 19 ). Finally, nanotubes are being exploited as probes of molecular structure at the single-molecule level, in the so-called scanometric approach to analysis ( 20 )( 21 ). The influence of nanotechnology on routine analytical methods is difficult to predict, but the recent advances in scanometric methods coupled with the rapid progress in nanoparticle labels suggest an important future for nanotechnology in the clinical laboratory.

We divided the nanotechnology literature into 19 categories and listed documents in each category in chronological order and in alphabetical order of first author within each year. Table 1 provides a list of key words for each category to provide a more detailed view of the scope of each of these sections. In the interest of simplicity, citations have been assigned to just one category. A more detailed and comprehensive listing of references for particular topics can be obtained by searching the online database (including title, keywords, and abstracts), using the appropriate keyword or keyword combinations. We have provided the total database and the database for the 19 categories for the convenience of the user [available through Clinical Chemistry Online ( www.clinchem.org/content/vol48/issue4/ )]. Please note that in many cases we have relied on the accuracy of the abstraction service for the citation details. The Internet is also a rich source of information on nanotechnology (e.g., see http://www.foresight.org/ and http://itri.loyola.edu/nanobase/ ), and a selection of nanotechnology-related Internet sites is listed at Clinical Chemistry Online ( www.clinchem.org/content/vol48/issue4/ ).

Major categories and keywords in nanotechnology.

This compilation is based in part on a survey undertaken by the IFCC Working Group on Nanotechnology, chaired by Dr Larry J. Kricka. Members of the Working Group are listed in the data supplement that accompanies this article at Clinical Chemistry Online ( http://www.clinchem.org/content/vol48/issue4/ ).

Nanotechnology. Prepared written statement and supplemental material of R. E. Smalley, Rice University, June 22, 1999. http://www.house.gov/science/smalley_062299.htm (Accessed February 2002)..

Feynman RP. There’s plenty of room at the bottom (Presented at the Annual Meeting of the American Physical Society, December 29, 1959, California Institute of Technology, Pasadena, CA). http://www.zyvex.com/nanotech/feynman.html (Accessed February 2002)..

Lucent Technologies. Bell Labs topical gallery: scanning probe microscopy. http://www.bell-labs.com/new/gallery/spm.html (Accessed February 2002)..

Gerd Binnig. The Nobel Prize Internet archive. http://www.almaz.com/nobel/physics/1986b.html (Accessed February 2002)..

Drexler KE. Engines of creation. The coming era of nanotechnology. 1986 : 298pp Anchor Press/Doubleday Garden City, NY. .

. Nanotech—the science of small gets down to business. Special nanotechnology issue. Sci Am 2001 ; 285 : 32 -91.

Yu M-F, Files BS, Arepalli S, Ruoff RS. Tensile loading of ropes of single wall carbon nanotubes and their mechanical properties. Phys Rev Lett 2000 ; 84 : 5552 -5555.

Collins PG, Arnold MS, Avouris P. Engineering carbon nanotubes and nanotube circuits using electrical breakdown. Science 2001 ; 292 : 706 -709.

Duan X, Huang Y, Cui Y, Wang J, Lieber CM. Indium phosphide nanowires as building blocks for nanoscale electronic and optoelectronic devices. Nature 2001 ; 409 : 66 -69.

Cui Y, Lieber CM. Functional nanoscale electronic devices assembled using silicon nanowire building blocks. Science 2001 ; 291 : 851 -853.

Kim P, Lieber CM. Nanotube nanotweezers. Science 1999 ; 286 : 2148 -2150.

Cobden DH. Molecular electronics. Nanowires begin to shine. Nature 2001 ; 409 : 32 -33.

Mehnert W, Mader K. Solid lipid nanoparticles: production, characterization and applications. Adv Drug Deliv Rev 2001 ; 47 : 165 -196.

Schwarz C, Mehnert W. Solid lipid nanoparticles (SLN) for controlled drug delivery. II. Drug incorporation and physicochemical characterization. J Microencapsul 1999 ; 16 : 205 -213.

Moghimi SM, Hunter AC, Murray JC. Long-circulating and target-specific nanoparticles: theory to practice. Pharmacol Rev 2001 ; 53 : 283 -318.

Lambert G, Fattal E, Couvreur P. Nanoparticulate systems for the delivery of antisense oligonucleotides. Adv Drug Deliv Rev 2001 ; 47 : 99 -112.

Reichert J, Csaki A, Kohler JM, Fritzsche W. Chip-based optical detection of DNA hybridization by means of nanobead labeling. Anal Chem 2000 ; 72 : 6025 -6029.

Taton TA, Mirkin CA, Letsinger RL. Scanometric DNA array detection with nanoparticle probes. Science 2000 ; 289 : 1757 -1760.

Chemla YR, Grossman HL, Poon Y, McDermott R, Stevens R, Alper MD, et al. Ultrasensitive magnetic biosensor for homogeneous immunoassay. Proc Natl Acad Sci U S A 2000 ; 97 : 14268 -14272.

Woolley AT, Guillemette C, Li Cheung C, Housman DE, Lieber CM. Direct haplotyping of kilobase-size DNA using carbon nanotube probes. Nat Biotechnol 2000 ; 18 : 760 -763.

Cheung CL, Hafner JH, Lieber CM. Carbon nanotube atomic force microscopy tips: direct growth by chemical vapor deposition and application to high-resolution imaging. Proc Natl Acad Sci U S A 2001 ; 97 : 3809 -3813.

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Royal Society of Chemistry

Nanomaterials: a review of synthesis methods, properties, recent progress, and challenges

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First published on 24th February 2021

Nanomaterials have emerged as an amazing class of materials that consists of a broad spectrum of examples with at least one dimension in the range of 1 to 100 nm. Exceptionally high surface areas can be achieved through the rational design of nanomaterials. Nanomaterials can be produced with outstanding magnetic, electrical, optical, mechanical, and catalytic properties that are substantially different from their bulk counterparts. The nanomaterial properties can be tuned as desired via precisely controlling the size, shape, synthesis conditions, and appropriate functionalization. This review discusses a brief history of nanomaterials and their use throughout history to trigger advances in nanotechnology development. In particular, we describe and define various terms relating to nanomaterials. Various nanomaterial synthesis methods, including top-down and bottom-up approaches, are discussed. The unique features of nanomaterials are highlighted throughout the review. This review describes advances in nanomaterials, specifically fullerenes, carbon nanotubes, graphene, carbon quantum dots, nanodiamonds, carbon nanohorns, nanoporous materials, core–shell nanoparticles, silicene, antimonene, MXenes, 2D MOF nanosheets, boron nitride nanosheets, layered double hydroxides, and metal-based nanomaterials. Finally, we conclude by discussing challenges and future perspectives relating to nanomaterials.

1. Introduction

The term nanometer was first used in 1914 by Richard Adolf Zsigmondy. 5 The American physicist and Nobel Prize laureate Richard Feynman introduced the specific concept of nanotechnology in 1959 in his speech during the American Physical Society's annual meeting. This is considered to be the first academic talk about nanotechnology. 5 He presented a lecture that was entitled “There's Plenty of Room at the Bottom”. During this meeting, the following concept was presented: “why can’t we write the entire 24 volumes of the Encyclopedia Britannica on the head of a pin?” The vision was to develop smaller machines, down to the molecular level. 6,7 In this talk, Feynman explained that the laws of nature do not limit our ability to work at the atomic and molecular levels, but rather it is a lack of appropriate equipment and techniques that limit this. 8 Through this, the concept of modern technology was seeded. Due to this, he is often considered the father of modern nanotechnology. Norio Taniguchi might be the first person who used the term nanotechnology, in 1974. Norio Taniguchi stated: “nano-technology mainly consists of the processing of, separation, consolidation, and deformation of materials by one atom or one molecule.” 5,9 Before the 1980s, nanotechnology remained only an area for discussion, but the concept of nanotechnology was seeded in the minds of researchers with the potential for future development.

The invention of various spectroscopic techniques sped up research and innovations in the field of nanotechnology. IBM researchers developed scanning tunneling microscopy (STM) in 1982, and with STM it became feasible to attain images of single atoms on “flat” ( i.e. , not a tip) surfaces. 10 Atomic force microscopy (AFM) was invented in 1986, and it has become the most crucial scanning probe microscope technique. 11 The motivation to develop hard discs with high storage density stimulated the measurement of electrostatic and magnetic forces. This led to the development of Kelvin-probe-, electrostatic-, and magnetic-force microscopy. 12 Currently, nanotechnology is rapidly evolving and becoming part of almost every field related to materials chemistry. The field of nanotechnology is evolving every day, and now powerful characterization and synthesis tools are available for producing nanomaterials with better-controlled dimensions.

Nanotechnology is an excellent example of an emerging technology, offering engineered nanomaterials with the great potential for producing products with substantially improved performances. 13 Currently, nanomaterials find commercial roles in scratch-free paints, surface coatings, electronics, cosmetics, environmental remediation, sports equipment, sensors, and energy-storage devices. 14 This review attempts to provide information in a single platform about the basic concepts, advances, and trends relating to nanomaterials via covering the related information and discussing synthesis methods, properties, and possible opportunities relating to the broad and fascinating area of nanomaterials ( Scheme 1 ). It is not easy to cover all the literature related to nanomaterials, but important papers from history and the current literature are discussed in this review. This review provides fundamental insight for researchers, quickly capturing the advances in and properties of various classes of nanomaterials in one place.

2. Descriptions of terms associated with nanomaterials

3. approaches for the synthesis of nanomaterials, 3.1. top-down approaches.

The conditions under which arc discharge takes place play a significant role in achieving new forms of nanomaterials. The conditions under which different carbon-based nanomaterials are formed via the arc discharge method are explained in Fig. 6 . Various carbon-based nanomaterials are collected from different positions during the arc discharge method, as their growth mechanisms differ. 44 MWCNTs, high-purity polyhedral graphite particles, pyrolytic graphite, and nano-graphite particles can be collected from either anode or cathode deposits or deposits at both electrodes. 46–48 Apart from the electrodes, carbon-based nanomaterials can also be collected from the inner chamber. Different morphologies of single-wall carbon nanohorns (SWCNHs) can be obtained under different atmospheres. For example, ‘dahlia-like’ SWCNHs are produced under an ambient atmosphere, whereas ‘bud-like’ SWCNHs are generated under CO and CO 2 atmospheres. 49 The arc discharge method can be used to efficiently achieve graphene nanostructures. The conditions present during the synthesis of graphene can affect its properties. Graphene sheets prepared via a hydrogen arc discharge exfoliation method are found to be superior in terms of electrical conductivity and have good thermal stability compared to those obtained via argon arc discharge. 50

3.2. Bottom-up approaches

The hard template method is also called nano-casting. Well-designed solid materials are used as templates, and the solid template pores are filled with precursor molecules to achieve nanostructures for required applications ( Fig. 10 ). 78 The selection of the hard template is critical for developing well-ordered mesoporous materials. It is desirable that such hard templates should maintain a mesoporous structure during the precursor conversion process, and they should be easily removable without disrupting the produced nanostructure. A range of materials has been used as hard templates, not limited to carbon black, silica, carbon nanotubes, particles, colloidal crystals, and wood shells. 85 Three main steps are involved in the synthetic pathway for obtaining nanostructures via templating methods. In the first step, the appropriate original template is developed or selected. Then, a targeted precursor is filled into the template mesopores to convert them into an inorganic solid. In the final step, the original template is removed to achieve the mesoporous replica. 86 Via using mesoporous templates, unique nanostructured materials such as nanowires, nanorods, 3D nanostructured materials, nanostructured metal oxides, and many other nanoparticles can be produced. 87 From this brief discussion, it can be seen that a wide range of unique structured nanomaterials can be produced using soft and hard template methods.

4. Unique nanomaterial features

The electronic properties of semiconductors in the 1–10 nm range are controlled by quantum mechanical considerations. Thus, nanospheres with diameters in the range of 1–10 nm are known as quantum dots. The optical properties of nanomaterials such as quantum dots strongly depend upon their shape and size. 96 A photogenerated electron–hole pair has an exciton diameter on the scale of 1–10 nm. Thus, the absorption and emission of light by semiconductors could be controlled via tuning the nanoparticle size in this range. However, in the case of metals, the mean free path of electrons is ∼10–100 nm and, due to this, electronic and optical effects are expected to be observed in the range of ∼10–100 nm. The colors of aqueous solutions of metal nanoparticles can be changed via changing the aspect ratio. Aqueous solutions of Ag NPs show different colors at different aspect ratios. A red shift in the absorption band appears with an increase in the aspect ratio ( Fig. 12 ). 21

Among a range of unique properties, the following key properties can be obtained upon tuning the sizes and morphologies of nanomaterials.

4.1. Surface area

4.2. magnetism, 4.3. quantum effects, 4.4. high thermal and electrical conductivity, 4.5. excellent mechanical properties, 4.6. excellent support for catalysts, 4.7. antimicrobial activity.

Overall, these features have made nanoscale materials valuable for a wide range of applications, substantially boosting the performances of various devices and materials in a number of fields. Details of various nanomaterials, their properties, and applications in various fields will be discussed below.

5. Nanomaterials, characteristics, and applications

5.1. special carbon-based nanomaterials.

In the carbon-based nanomaterial family, fullerenes were the first symmetric material, and they provided new perspectives in the nanomaterials field. This led to the discovery of other carbon-based nanostructured materials, such as carbon nanotubes and graphene. 110 Fullerenes are present in nature and interstellar space. 111 Interestingly, fullerenes were the molecule of the year in 1991 and attracted the most research projects compared to other scientific subjects during that period. 112 Fullerenes possess several unique features that make them attractive for applications in different fields. Fullerenes display solubility to some extent in a range of solvents, and these characteristics make them unique compared to the other allotropes of carbon. 108

The chemical modification of fullerenes is an exciting subject, improving their effectiveness for applications. There are two main ways to modify fullerenes: 113 fullerene inner-space modification, and fullerene outer-surface modification.

Endohedral and exohedral doping examples are shown in Fig. 13 . 114 Fullerenes are hollow cages, and the interior acts as a robust nano-container for host target species when forming endohedral fullerene. 115 Endohedral fullerenes do not always follow the isolated pentagon rule (IPR). 116 To date, fullerene nanocages have received substantial consideration in the materials chemistry field due to their useful potential applications. Neutral and charged single atoms in free space are highly reactive and unstable. In the confined environment of fullerenes, these reactive species can be stabilized; for example, the LaC 60 + ion does not react with the NH 3 , O 2 , H 2 , or NO. Thus, reactive metals can be protected from the surrounding environment by trapping them inside fullerene cages. 117 Another emerging carbon nanomaterial is endohedral fullerene containing lithium (Li@C 60 ). 118 Lithium metal is very reactive, and extreme controlled environmental conditions are required to preserve or use it. In other words, secure structures are required for lithium storage. Li-Based endohedral fullerene shows unique solid-state properties. The encapsulation of lithium atoms in fullerene helps to protect lithium atoms from external agents. Li-Based endohedral fullerenes have the potential to open the door to nanoscale lithium batteries. 119 For the development of endohedral metallofullerenes, larger fullerenes are generally required, as they possess large cages to accommodate lanthanide and transition metal atoms more smoothly. 118 Fullerene nanocages are useful for the storage of gases. Fullerene is under consideration for hydrogen storage. 120,121

Exohedral fullerenes carry more potential for applications as outer surfaces can be more easily modified or functionalized. The exohedral doping of metals into fullerenes strongly affects the electronic properties via shifting electrons from the metal to the fullerene nanocage. 122 The practical application of fullerenes can be achieved with tailor-made fullerene derivatives via chemical functionalization. As fullerene chemistry has matured, a wide range of functionalized fullerenes has been realized through simple synthetic routes. 123 The combination of hydrogen-bonding motifs and fullerenes has allowed the modulation of 1D, 2D, and 3D fullerene-based architectures. 124 The excellent electron affinities of fullerenes have shown great potential for eliminating reactive oxygen species. The presence of excess reactive oxygen species can cause biological dysfunction or other health issues. The surfaces of fullerenes have been functionalized via mussel-inspired chemistry and Michael addition reactions for the fabrication of C 60 –PDA–GSH. The developed C 60 –PDA–GSH nanoparticles demonstrated excellent potential for scavenging reactive oxygen species. 125

Amphiphiles have great importance in industrial processes and daily life applications. Amphiphilic molecules consist of hydrophilic and hydrophobic parts, and they perform functions in water via forming two- and three-dimensional assemblies. Recently, conical fullerene amphiphiles 126 have emerged as a new class of amphiphiles, in which a nonpolar apex is supplied by fullerenes and a hydrophilic part is achieved through functionalization. The selective functionalization of the fullerene on one side helps to achieve a supramolecule due to unique interfacial behavior. The unique supramolecular structure formed via the spontaneous assembly of one-sided selectively functionalized fullerenes through strong hydrophobic interactions between the fullerene apexes and polar functionalized portions is soluble in water. Conical fullerene amphiphiles are mechanically robust. Via upholding the structural integrity, conical fullerene amphiphiles can be readily aggregated with nanomaterials and biomolecules to form multicomponent agglomerates with controllable structural features. 127 Fullerenes, after suitable surface modification, have excellent potential for use in drug delivery, but there have only been limited explorations of their drug delivery applications. 128,129 Fullerene-based nano-vesicles have been developed for the delayed release of drugs. 130 Water-soluble proteins have great potential in the field of nanomedicine. The water-soluble cationic fullerene, tetra(piperazino)[60] fullerene epoxide (TPFE), has been used for the targeted delivery of DNA and siRNA specifically to the lungs. 131 For diseases in lungs or any other organ, efficient treatment requires the targeted delivery of active agents to a targeted place in the organ. The accumulation of micrometer-sized carriers in the lung makes lung-selective delivery difficult, as this may induce embolization and inflammation in the lungs. Size-controlled blood vessel carrier vehicles have been developed using tetra(piperazino)fullerene epoxide (TPFE). TPFE and siRNA agglutinate in the bloodstream with plasma proteins and, as a result, micrometer-sized particles are formed. These particles clog the lung capillaries and release siRNA into lungs cells; after siRNA delivery, they are immediately cleared from the lungs ( Fig. 14 ). 132

The supramolecular organization of fullerene (C 60 ) is a unique approach for producing shape-controlled moieties on the nano-, micro-, and macro-scale. Nano-, micro-, and macro-scale supramolecular assemblies can be controlled via manipulating the preparation conditions to achieve unique optoelectronic properties. 133 The development of well-ordered and organized 1D, 2D, and 3D fullerene assemblies is essential for achieving advanced optical and organic-based electronic devices. 134 Fullerene-based nanostructured materials with new dimensions are being developed from zero-dimensional fullerene and tuned to achieve the desired characteristics. 1D C 60 fullerene nanowires have received substantial attention over other crystalline forms due to their excellent features of potential quantum confinement effects, low dimensionality, and large surface areas. 135

Carbon nanomaterials are also used as supports for catalysts, and the main reasons to use them are their high surface areas and electrical conductivities. Carbon supports strongly influence the properties of metal nanoparticles. In fuel cells, the carbon support strongly affects the stability, electronic conductivity, mass transport properties, and electroactive surface area of the supported catalyst. 136 In fuel cells, the degradation of some catalysts, such as platinum-based examples, and carbon is correlated and reinforced as a result of both being present. Carbon support oxidation is catalyzed by platinum and the oxidation of carbon accelerates platinum-catalyst release. Overall, this results in a loss of catalytically active surface area. 137 Fullerenes are considered suitable support materials due to their excellent electrochemical activities and stability during electrochemical reactions. 138 Due to their high stability and good conductivity, fullerenes can replace conventional carbon as catalyst support materials. Fullerenes are also used for the development of efficient solar cells. 139

Apart from the applications mentioned above, fullerenes have a broader spectrum of applications where they can be used to improve outcomes considerably. Fullerenes have the potential to be used in the development of superconductors. 140 The strong covalent bonds in fullerenes make them useful nanomaterials for improving the mechanical properties of composites. 141 The combination of fullerenes with polymers can result in good flame-retardant and thermal properties. 142 Fullerenes and their derivatives are used for the development of advanced lubricants. They are used as modifiers for greases and individual solid lubricants. 138 Fullerenes have tremendous medicinal importance due to their anticancer, antioxidant, anti-bacterial, and anti-viral activities. 104

Fullerenes are vital members of the carbon-based nanomaterial family and they certainly possess exceptional properties. This discussion further emphasizes their importance for advanced applications. However, the discovery of other carbon-based nanomaterials has put fullerenes in the shade, and the pace of their exploration has been reduced. As fullerenes are highly symmetrical molecules with unique properties, they can act as performance boosters, but more attention is needed from researchers for their practical expansion. 110

Single-walled carbon nanotubes consist of a seamless one-atom-thick graphitic layer, in which carbon atoms are connected through strong covalent bonds. 146 Double-walled carbon nanotubes consist of two single-walled carbon nanotubes. One carbon nanotube is nested in another nanotube to construct a double-walled carbon nanotube. 147 In multi-walled carbon nanotubes, multiple sheets of single-layer carbon atom are rolled up. In other words, many single-walled carbon nanotubes are nested within each other. From different types of nanotubes, it can be concluded that the nanotubes may consist of one, tens, or hundreds of concentric carbon shells, and these shells are separated from each other with a distance of ∼0.34 nm. 148 Carbon nanotubes can be synthesized via chemical vapor deposition, 149 laser ablation, 150 arc-discharge, 143 and gas-phase catalytic growth. 151

Single-walled carbon nanotubes display a diameter of 0.4 to 2 nm. The inner wall distance between double-walled carbon nanotubes was found to be in the range of 0.33 to 0.42 nm. MWCNT diameters are usually in the range of 2–100 nm, and the inner wall distance is about 0.34 nm. 147,152 However, it is essential to note that the diameters and lengths of carbon nanotubes are not well defined, and they depend on the synthesis route and many other factors. The electrical conductivities of SWCNTs and MWCNTs are about 10 2 –10 6 S cm −1 and 10 3 –10 5 S cm −1 , respectively. SWCNTs and MWCNTs also display excellent thermal conductivities of ∼6000 W m −1 K −1 and ∼2000 W m −1 K −1 , respectively. CNTs remain stable in air at temperatures higher than 600 °C. 153 These properties indicate that CNTs have obvious advantages over graphite.

Single-walled carbon nanotubes can display metallic or semiconducting behavior. Whether carbon nanotubes show metallic or semiconducting behavior depends on the diameter and helicity of the graphitic rings. 154 The rolling of graphene sheets leads to three different types of CNTs: chiral, armchair, and zigzag ( Fig. 15 ). 155

Carbon nanotubes demonstrate some amazing characteristics that make them valuable nanomaterials for possible practical applications. Theoretical and experimental studies of carbon nanotubes have revealed their extraordinary tensile properties. J. R. Xiao et al. used an analytical molecular structural mechanics model to predict SWCNT tensile strengths of 94.5 (zigzag nanotubes) and 126.2 (armchair nanotubes) GPa. 156 In another study, the Young's modulus and average tensile strength of millimeter-long multi-walled carbon nanotubes were analyzed and found to be 34.65 GPa and 0.85 GPa, respectively. 157 Carbon nanotubes possess a high aspect ratio. Due to their high tensile strength, carbon nanotubes are used to enhance the mechanical properties of composites.

Carbon nanotubes have become an important industrial material and hundreds of tonnes are produced for applications. 158 Their high tensile strength and high aspect ratio have made carbon nanotubes an ideal reinforcing agent. 159 Carbon nanotubes are lightweight in nature and are used to produce lightweight and biodegradable nanocomposite foams. 160 The structural parameters of carbon nanotubes define whether they will be semiconducting or metallic in nature. This property of carbon nanotubes is considered to be effective for their use as a central element in the design of electronic devices such as rectifying diodes, 161 single-electron transistors, 162 and field-effect transistors. 163 The chemical stability, nano-size, high electrical conductivity, and amazing structural perfection of carbon nanotubes make them suitable for electron field emitter applications. 164 The unique set of mechanical and electrochemical properties make CNTs a valuable smart candidate for use in lithium-ion batteries. 165 CNTs have the full potential to be used as a binderless free-standing electrode for active lithium-ion storage. CNT-based anodes can have reversible lithium-ion capacities exceeding 1000 mA h g −1 , and this is a substantial improvement compared with conventional graphite anodes. In short, the following factors play a role in controlling and optimizing the performances of CNT-based composites: 166 (i) the volume fraction of carbon nanotubes; (ii) the CNT orientation; (iii) the CNT matrix adhesion; (iv) the CNT aspect ratio; and (iv) the composite homogeneity.

For some applications, a proper stable aqueous dispersion of CNTs at a high concentration is pivotal to allow the system to perform its function efficiently and effectively. 167 One of the major issues associated with carbon nanotubes is their poor dispersion in aqueous media due to their hydrophobic nature. Clusters of CNTs are formed due to van der Waals attraction, π–π stacking, and hydrophobicity. The CNT clusters, due to their strong interactions, hinder solubility or dispersion in water or even organic-solvent-based systems. 168 This challenging dispersion associated with CNTs has limited their use for promising applications, such as in biomedical devices, drug delivery, cell biology, and drug delivery. 167 Carbon nanotube applications and inherent characteristics can be further tuned via suitable functionalization. The functionalization of carbon nanotubes helps scientists to manipulate the properties of carbon nanotubes and, without functionalization, some properties are not attainable. 169 The functionalization of nanotubes can be divided into two main categories: covalent functionalization and non-covalent functionalization.

The heating of CNTs under strongly acidic and oxidative conditions results in the formation of oxygen-containing functionalities. These functional groups, such as carboxylic acid, react further with other functional groups, such as amines or alcohols, to produce amide or ester linkages on the carbon nanotubes. 172 One of the main issues preventing the utilization of CNTs for biomedical applications is their toxicity. The cytotoxicity of pristine carbon nanotubes can be reduced via introducing carbonyl, –COOH, and –OH functional groups. Apart from functionalization through oxidized CNTs, the direct functionalization of CNTs is also possible. However, direct functionalization requires more reactive species to directly react with the CNTs, such as free radicals. Addition reactions to CNTs can cause a transformation from sp 2 hybridization to sp 3 hybridization at the point of addition. At the point where functionalization has taken place, the local bond geometry is changed from trigonal planar to tetrahedral geometry. Some addition reactions to the sidewalls of CNTs are shown in Fig. 16 . 155

It is important to discuss how the covalent functionalization of carbon nanotubes comes at the price of the degradation of the carbon sp 2 network. This substantially affects the electronic, thermal, and optoelectronic properties of the carbon nanotubes. 169 Efforts are being made to introduce a new method of covalent functionalization that can keep the π network of CNTs intact. Antonio Setaro et al. introduced a new [2+1] cycloaddition reaction for the non-destructive, covalent, gram-scale functionalization of single-walled carbon nanotubes. The reaction rebuilds the extended π-network, and the carbon nanotubes retained their outstanding quantum optoelectronic properties ( Fig. 17 ). 173

Polymers are frequently combined with CNTs to enhance their dispersion capabilities. Polymers interact with CNTs through CH–π and π–π interactions. 174 Hexanes and cycloalkanes are poor CNT solvents but the good solubility or dispersion of CNTs in these solvents is required for surface coating applications. Poly(dimethylsiloxane) (PDMS) macromer-grafted polymers have been prepared using PDMS macromers and pyrene-containing monomers that strongly adsorb on CNTs, thus improved the solubility of CNTs in chloroform and hexane. 176 The use of head–tail surfactants is another attractive way to achieve a fine dispersion of CNTs in an aqueous medium. In head–tail surfactants, the tail is hydrophobic and interacts with the CNT sidewalls, and the hydrophilic head groups interact with the aqueous environment to provide a fine dispersion. 177

For electrical applications, non-covalently functionalized CNTs are more preferred because the electrical properties of the CNTs are not compromised. CNTs have been non-covalently functionalized with a variety of biomolecules for the fabrication of electrochemical biosensors. 175 Non-covalently functionalized SWCNTs are used for energy applications. Single-walled carbon nanotubes (SWCNTs) have been non-covalently functionalized with 3d transition metal( II ) phthalocyanines, lowering the potential of the oxygen evolution reaction by approximately 120 mV compared with unmodified SWCNTs. 178 The toxicity of pristine CNTs toward living organisms can be lessened via using surfactant-functionalized CNTs. 170 However, in some cases, during polymer non-covalent functionalization, the polymer may wrap CNT bundles and make it difficult to separate the CNTs from each other. Polymers can develop into insulating wrapping that affects the CNT conductivity.

In the literature, several graphene-related materials have been reported, such as graphene oxide and reduced graphene oxide. 187 Among graphenoids, graphene oxide is a more reported and explored graphene-related material as a precursor for chemically modified graphene. The synthetic route to graphene oxide is straightforward, and it is synthesized from inexpensive graphite powder that is readily available. 188 Graphene oxide has many oxygen-containing functional groups, such as epoxy, hydroxyl, carboxyl, and carbonyl groups. The basal plane of graphene oxide is generally decorated with epoxide and hydroxyl groups, whereas the edges presumably contain carboxyl- and carbonyl-based functional groups. 189 The presence of active functional groups in graphene oxide allows its further functionalization with different polymers, small organic compounds, or other nanomaterials to realize several applications. 190

Graphene oxide, due to its oxygen functionality, is insulating in nature and displays poor electrochemical performance. The presence of oxygen functionalities in graphene oxide breaks the conjugated structure and localizes the π-electron network, resulting in poor carrier mobility and carrier concentration. 196 Its electrochemical performance is improved substantially after removing the oxygen-containing functional groups. 197 These functional groups can be removed or reduced via thermal, electrochemical, and chemical means. The product obtained after removing or reducing oxygen moieties is called reduced graphene oxide. The properties of reduced graphene oxide depend upon the effective removal of oxygen moieties from graphene oxide. The process used to remove oxygen-containing functionalities from graphene oxide will determine the extent to which reduced the properties of graphene oxide resemble pristine graphene. 198

Reduced graphene oxide is extensively used to improve the performances of various electrochemical devices. 199 It is essential to mention that even after reducing graphene oxide, some residual sp 3 carbon bonded to oxygen still exists, which somehow disturbs the movement of charge through the delocalized electronic cloud of the sp 2 carbon network. 200 Apart from this, the electrochemical activity of reduced graphene oxide is substantially high enough to manufacture electrochemical devices with improved performances. Recently, the demand for super-performance electrochemical devices has increased to overcome modern challenges relating to electronics and energy-storage devices. 201 Graphene-based materials are considered to be excellent electrode materials, and they can be proved to be revolutionary for use in energy-storage devices such as supercapacitors (SC) and batteries. Graphene-based electrodes improve the performances of existing batteries (lithium-ion batteries) and they are considered useful for developing next-generation batteries such as sodium-ion batteries, lithium–O 2 batteries, and lithium–sulfur batteries ( Fig. 18 ). Being flat in nature, each carbon atom of graphene is available, and ions can easily access the surface due to low diffusion resistance, which provides high electrochemical activity. 202

Graphene and its derivatives are extensively used for the development of electrochemical sensors. 203 The surfaces of bare electrodes are usually not able to sense analytes at trace levels and they cannot differentiate between analytes that have close electrooxidation properties due to their poor surface kinetics. The addition of graphene layers to the surfaces of electrodes can substantially improve the electrocatalytic activity and surface sensitivity towards analytes. 204 Graphene has definite advantages over other materials that are used as electrode materials for sensor applications. Graphene has a substantially high surface-to-volume ratio and atomic thickness, making it extremely sensitive to any changes in its local environment. This is an essential factor in developing advanced sensing tools, as all the carbon atoms are available to interact with target species.

Consequently, graphene exhibits higher sensitivity than its counterparts such as CNTs and silicon nanowires. 205 Graphene has two main advantages over CNTs for the development of electrochemical sensors. First, graphene is mostly produced from graphite, which is a cost-effective route, and second, graphene does not contain metallic impurities like CNTs can. Graphene offers many other advantages when developing sensors and biosensors, such as biocompatibility and π–π stacking interactions with biomolecules. 206 Graphene-based materials are ideal for the construction of nanostructured sensors and biosensors.

The mechanical properties of graphene are used to fabricate highly desired stretchable and flexible sensors. 207 Graphene can be utilized to develop transparent electrodes with excellent optical transmittance. It displays good piezoresistive sensitivity. Researchers are making efforts to replace conventional brittle indium tin oxide (ITO) electrodes with flexible graphene electrodes in optoelectronic devices such as liquid-crystal displays and organic light-emitting diodes. 208 For human–machine interfaces, transparent and flexible tactile sensors with high sensitivity have become essential. Graphene film (GF) and PET have been applied to develop transparent tactile sensors that exhibit outstanding cycling stability, fast response times, and excellent sensitivity ( Fig. 19 ). 209 Similarly, graphene is applied for the fabrication of pressure sensors. 210 Overall, graphene is an excellent material for developing transparent and flexible devices. 211,212

The use of graphene-based materials is an effective way to deal with a broad spectrum of pollutants. 213 There are many ways to deal with environmental pollution; among these, adsorption is an effective and cost-effective method. 214,215 Graphene-based adsorbents are found to be useful in the removal of organic, 216 inorganic, and gaseous contaminants. Graphene-based materials have some obvious advantages over CNT-based adsorbents. For example, graphene sheets offer two basal planes for contaminant adsorption, enhancing their effectiveness as an adsorbent. 192 GO contains several oxygen functional groups that impart hydrophilic features. Due to appropriate hydrophilicity, GO-based adsorbents can efficiently operate in water to remove contaminants. Moreover, graphene-oxide-based materials can be functionalized further through reactive moieties with various organic molecules to enhance their adsorption capacities. 217

In short, extensive research must continue in order to develop graphene-based materials with high performance and bring them to the market. Massive focus on graphene research is also justified due to the extraordinary features described in extensive theoretical and experimental research works.

Nanodiamonds possess a core–shell-like structure and display rich surface chemistry, and numerous functional groups are present on their surface. Several functional groups, such as amide, aldehyde, ketone, carboxylic acid, alkene, hydroperoxide, nitroso, carbonate ester, and alcohol groups, are present on nanodiamond surfaces, assisting in their further functionalization for desired applications ( Fig. 20 ). 226

Furthermore, nanodiamond surfaces can be homogenized with a single type of functional group according to the application requirements. 227 The use of nanodiamond particles as a reinforcing material in polymer composites has attracted great attention for improving the performance of polymer composite materials. The superior mechanical properties and rich surface chemistry of nanodiamonds have made them a superior material for tuning and reinforcing polymer composites. Nanodiamonds might operate via changing the interphase properties and forming a robust covalent interface with the matrix. 228 Nanodiamond (ND)-reinforced polymer composites have shown superior thermal stabilities, mechanical properties, and thermal conductivities. Nanodiamonds have shown great potential for energy storage applications. 229 Nanodiamonds and their composites are also used in sensor fabrication, environmental remediation, and wastewater treatment. 230,231 Their stable fluorescence and long fluorescence lifetimes have made nanodiamonds useful for imaging and cancer treatment. For biomedical applications, the rational engineering of nanodiamond particle surfaces has played a crucial role in the carrying of bioactive substances, target ligands, and nucleic acids, resisting their aggregation. 232,233 Nanodiamonds have a great future in nanotechnology due to their amazing surface chemistry and unique characteristics.

Carbon quantum dots can be synthesized through several chemical routes. 241–245 Some methodologies for synthesizing carbon dots are described in Fig. 21 . 246–248 Carbon itself is a black material and displays low solubility in water. In contrast, carbon quantum dots are attractive due to their excellent solubility in water. They contain a plethora of oxygen-containing functional groups on their surface, such as carboxylic acids. These functional moieties allow for further functionalization with biological, inorganic, polymeric, and organic species.

Carbon quantum dots are also called carbon nano-lights due to their strong luminescence. 248 In particular, carbon quantum dots offer enhanced chemiluminescence, 249,250 fluorescent emission, 251 two-photon luminescence under near-infrared pulsed-laser excitation, 252 and tunable excitation-dependent fluorescence. 253 The luminescence characteristics of carbon quantum dots have been used to develop highly sensitive and selective sensors. In most cases, a simple principle is involved in sensing with luminescent carbon quantum dots: their photoluminescence intensity changes upon the addition of an analyte. 254 Based on this principle, several efficient sensors have been developed using carbon quantum dots. 255–257 They can be used as sensitive and selective tools for sensing explosives such as TNT. Recognition molecules on the surfaces of carbon quantum dots can help to sense targeted analytes. Amino-group-functionalized carbon quantum dot fluorescence is quenched in the presence of TNT through a photo-induced electron-transfer effect between TNT and primary amino groups. This quenching phenomenon can help to sense the target analyte ( Fig. 22 ). 258 Chiral carbon quantum dots (cCQDs) can exhibit an enantioselective response. The PL responses of cCQDs were evaluated toward 17 amino acids and it was found that the PL intensity of the cCQDs was only substantially enhanced in the presence of L -Lys ( Fig. 22 ). 254

Carbon quantum dots have received significant interest in the fields of biological imaging and nanomedicine ( Fig. 23 ). 239 Direct images of RNA and DNA are essential for understanding cell anatomy. Due to the limitations of current imaging probes, tracking the dynamics of these biological macromolecules is not an easy job. Recently, membrane-penetrating carbon quantum dots have been developed for the imaging of nucleic acids in live organisms. 259 It is important to note that most of the carbon quantum dots utilized to attain cell imaging under UV excitation emit blue radiation. Some biological tissue also emits blue light, specifically that involving carbohydrates, and this interferes with cell imaging carried out with blue-emitting CQDs. This seriously hinders their potential in the field of biomedical imaging. Due to this reason, researchers are focusing on tuning CQDs in a way that their emission peak is red-shifted to avoid interference. 260 Carbon quantum dots with yellow and green fluorescence have been reported for bioimaging purposes. 261,262 The suitable doping of carbon quantum dots can red-shift the emission to enhance the bioimaging effectiveness. 263 Doped carbon quantum dots are capable of biological imaging and display advanced capabilities for scavenging reactive oxygen species. 264

Carbon quantum dots demonstrate photo-induced electron transfer properties 265 that make them valuable for photocatalytic, light-energy conversion, and other related applications. 266 Carbon quantum dots enhance the activities of other photocatalysts to which they are attached. Carbon quantum dots, along with photocatalysts, provide better charge separation and suppress the regeneration of photogenerated electron–hole pairs. Moreover, the proper implantation of carbon quantum dots into photocatalysts can broaden the photo-absorption region. Implanted carbon quantum dots form micro-regional heterostructures that facilitate photo-electron transport. 267 The implantation of carbon quantum dots into g-C 3 N 4 can substantially enhance charge transfer and separation efficiencies, prevent photoexcited carrier recombination, narrow the bandgap, and red shift the absorption edge. 268 The intrinsic catalytic activity of polymeric carbon nitride is improved as a result of the nano-frame heterojunctions formed with the help of CQDs. 269

Carbon quantum dots offer many advantages over conventional semiconductor-based QDs and, thus, they have attracted considerable researcher attention. 244 Due to their remarkable features, they have shown importance in recent years in the fields of light-emitting diodes, nanomedicine, solar cells, sensors, catalysis, and bioimaging. 236

The production of carbon nanohorns has some obvious advantages over carbon nanotubes, such as the ability for toxic-metal-catalyst-free synthesis and large-scale production at room temperature. Carbon nanotube synthesis involves metal particles, and harsh conditions, such as the use of strong acids, are required to remove metallic catalysts. This process introduces many defects into CNT structures and may cause a loss of carbon material. 270 Carbon nanohorns possess a wide diameter compared to CNTs. CNHs possess good absorption capabilities and their interiors are also available after partial oxidation, which provides direct access to their internal parts. Heat treatment under acidic or oxidative conditions facilitates the facile introduction of holes into carbon nanohorns. Holes in graphene sheets of single-walled carbon nanohorns can be produced with O 2 gas at high temperatures. A large quantity of material can be stored inside CNH tubes. 274 The surface area of CNHs is substantially enhanced upon opening the horns to make their interiors accessible. 275 Carbon nanohorns have great potential for energy storage, 275 electrochemiluminescence, 276 adsorption, 277 catalyst support, 278 electrochemical sensing, 279 and drug delivery system 273 uses. CNHs as catalyst supports can provide a homogeneous dispersion of Pt nanoparticles ( Fig. 25 ). The current density of Pt supported on single-walled carbon nanohorns is double compared to a fuel cell made from Pt supported on carbon black. 280 Thus, carbon nanohorns provide a better uniform dispersion that facilitates a high surface area and better catalyst performance.

5.2. Nanoporous materials

In nanoporous materials, the size distributions, volumes, and shapes of the pores directly affect the performances of porous materials for particular applications. It has become a hot area of research to develop materials with precisely controlled pores and arrangements. Recent research has focused more on the precise control of the shapes, sizes, and volumes of pores to produce nanoporous materials with high performance. Several state-of-the-art reviews are present in the literature that focus explicitly on the synthesis, properties, advances, and applications of nanoporous materials. 85,287–289 Based on the materials used, nanoporous materials can be divided into three main groups: inorganic nanoporous materials; carbonaceous nanoporous materials; and organic polymeric nanoporous materials.

Inorganic nanoporous materials include porous silicas, clays, porous metal oxides, and zeolites. The generation of pores in the material can introduce striking features into the material that are absent in non-porous materials. Nanoporous materials offer rich surface compositions with versatile characteristics. Nanoporous materials exhibit high surface-to-volume ratios. Their outstanding features and nanoporous framework structures have made these materials valuable in the fields of environmental remediation, adsorption, catalysis, sensing, energy conversion, purification, and medicine. 284,290

Porous silica is a crucial member of the inorganic nanoporous family. Over the decades, it has generated significant research interest for use in fuel cells, chemical engineering, ceramics, and biomedicine. It is essential to note that specific morphologies and pore size diameters are required for each application, and these can be achieved via tuning during the synthesis process. Nanoporous silica offers two functional surfaces: one is the cylindrical pore surfaces, and the second is the exterior surfaces of the nanoporous silica particles. The surfaces of nanoporous silica can be easily functionalized for the desired applications. The nanoporous silica surface is heavily covered with many silanol groups that act as reactive sites for functionalization ( Fig. 26 ). 291,292 For biomedical applications, mesoporous silica has emerged as a new generation of inorganic platform materials compared to other integrated nanostructured materials. Several factors make it a unique material for biomedical applications: 293,294 (a) its ordered porous structure; (b) its tunable particle size; (c) its large pore volume and surface area; (d) its biocompatibility; (e) its biodegradation, biodistribution, and excretion properties; and (f) its two functional surfaces. For instance, ordered MCM-48 nanoporous silica was used for the delivery of the poorly soluble drug indomethacin. It has been found that surface modification can control drug release. 295 Mesoporous silica-based materials have emerged as excellent materials for use in sustained drug delivery systems (SDDSs), immediate drug delivery systems (IDDSs), targeted drug delivery systems (TDDSs), and stimuli-responsive controlled drug delivery systems (CDDSs). The drug release rate from mesoporous silica can also be controlled via introducing appropriate polymers or functional groups, such as CN, SH, NH 2 , and Cl. Researchers are currently focusing on developing MSN-based (MSN = mesoporous silica nanoparticle) multifunctional drug delivery systems that can release antitumor drugs on demand in a targeted fashion via minimizing the premature release of the drug ( Fig. 27 ). 296

Hierarchically nanoporous zeolites are a vital member of the nanoporous material family. They are crystalline aluminosilicate minerals whose structures comprise uniform, regular arrays of nanopores with molecular dimensions. The microporous structures of zeolites contain pores that are usually below 1 nm in diameter. In zeolites, the micropores are uniform in shape and size, and these pores can effectively discriminate between molecules based on shape and size. 297 Currently, based on crystallography, more than 200 zeolites have been classified. 298 Zeolites have been proved to be useful materials in the field of host–guest chemistry. In solid catalysis, about 40% of the entire solid catalyst field is taken up by zeolites in chemical industry. The excellent catalysis success of zeolites is based on their framework stability, shape-selective porosity, solid acidity, and ion-exchange capacity. Oxygen tetrahedrally coordinates with the Al atoms in most zeolite crystalline silicate frameworks, resulting in charge mismatch between the oxide framework and Al. Extra-framework Na + ions compensate for this charge mismatch. The Na + ions are exchangeable for other cations such as H + and K + . 298 The zeolite crystalline networks are remarkable in that they provide high mechanical and hydrothermal stabilities. The most crucial task facing the zeolite community is to find new structures with desired functions and apply them more effectively for different applications.

Apart from these inorganic porous materials, several other metal- and metal-oxide-based nanoporous materials have been introduced that are more prominent for use in electrode material, catalyst, photodegradation, energy storage, and energy conversion applications. 299–302 Nanoporous metal-based materials are famous due to the nanosized crystalline walls, interconnected porous networks, and numerous surface metal sites that provide them with unique physical/chemical properties compared with their bulk counterparts and other nanostructured materials. 303 For example, nanoporous WO 3 films were developed via tuning the anodization conditions for photoelectrochemical water oxidation. It has been observed that the morphology of the film strongly affected the photoelectrochemical performance. 304 Nanoporous alumina is also a unique material in the inorganic nanoporous family due to several aspects. Nanoporous alumina can be prepared in a controlled fashion with any size and shape in polyprotic aqueous media via the anodic oxidation of the aluminum surface. The parallel arrangement of pores on alumina can easily be controlled from 5 nm to 300 nm, and alumina is stable in the range of 1000 °C. The anodizing time plays a significant role in controlling the pore length. Nanoporous alumina membranes offer various unique properties, such as pores of variable widths/lengths, temperature stability, and optical transparency. Nanoporous alumina pores can be filled with magnetically and optically active elements to produce the desired applications at the nanoscale level. Photoluminescent alumina membranes can be produced via introducing cadmium sulfide, gallium nitride, and siloxenes inside nanoporous alumina using appropriate precursors. 305 Porous alumina also acts as an efficient support and template for the designing of other nanomaterials. Palladium nanowires, 306 high aspect ratio cobalt nanowires, 307 and highly aligned Cu nanowires 308 were developed using porous alumina as a template. Ni–Pd as a catalyst was supported on porous alumina for hydrogenation and oxidation reactions. 309 Nanoporous anodic alumina is also considered to be an efficient material for the development of biosensors due to the ease of fabrication, tunable properties, optical/electrochemical properties, and excellent stability in aqueous environments. 310

Nanoporous carbon-based materials are a hot topic in the field of materials chemistry. Nanoporous carbon materials have become ubiquitous choices in the environmental, energy, catalysis, and sensing fields due to their unique morphologies, large pore volumes, controlled porous structures, mechanical, thermal, and chemical stabilities, and high specific surface areas ( Fig. 28A ). 311 Nanoporous materials are found to be useful in the treatment of water. The separation of spilled oil and organic pollutants from water has emerged as a significant challenge. 312–314 The design of materials that can allow the efficient separation of organic, dye, and metal contaminants from water has become a leading environmental research area. 315,316 Nanoporous carbon can be derived from different natural and synthetic sources. 317–319 Nanoporous carbon foam can be derived from natural sources, such as flour, pectin, and agar, via table-salt-assisted pyrolysis. The agar-derived nanoporous carbon foam showed high absorption capacities, a maximum of 202 times its own weight, for oil and organic solvents. Air filtration paper developed from carbon nanoporous materials and non-woven fabrics has shown a filtration efficiency of greater than 99% ( Fig. 28B ). 320 Nanoporous carbon can also be produced from other porous frameworks, such as metal–organic frameworks. MOF- and COF-based materials are promising precursors for nanoporous carbon-based materials. The direct carbonization of amino-functionalized aluminum terephthalate metal–organic frameworks has produced nitrogen-doped nanoporous carbon that shows an adequate removal capacity of 98.5% for methyl orange under the optimum conditions. 321 Fe 3 O 4 /nanoporous carbon was also produced with Fe salts as a magnetic precursor and MOF-5 as a carbon precursor for removing the organic dye methylene blue (MB) from aqueous solutions. 322 The mesoporous carbon removal efficiency could be further enhanced via modifying or functionalizing the surface with various materials. Unmodified mesoporous carbon has shown a mercury removal efficiency of 54.5%. This efficiency can be substantially improved to 81.6% and 94% upon modification with the anionic surfactant sodium dodecyl sulfate and cationic surfactant cetyltrimethyl ammonium bromide (CTAB), respectively. 323

Ordered nanoporous carbon, CNTs, and fullerenes are extensively applied for energy and environmental applications. The complicated synthesis routes required for fullerenes and CNTs have slowed down the full exploitation of their potential for highly demanding applications. In comparison, the synthesis of highly ordered nanoporous carbon is facile, and the properties of ordered nanoporous carbon are also appealing for energy and environmental applications. 311 CO 2 is a greenhouse gas, and its sustainable conversion into value-added products has become the subject of extensive research. A nitrogen-doped nanoporous-carbon/carbon-nanotube composite membrane is a high-performance gas-diffusion electrode applied for the electrocatalytic conversion of CO 2 into formate. A faradaic efficiency of 81% was found for the production of formate. 324 Nanoporous carbon materials modified with the non-precious elements P, S, N, and B have emerged as efficient electrode materials for use in the oxygen evolution reaction (OER), hydrogen evolution reaction (HER), oxygen reduction reaction (ORR), batteries, and fuel cells. 311,325–327

Nanoporous polymers, including nanoporous coordination polymers and crystalline nanoporous polymers, have emerged as impressive nanoporous materials. 328 Nanoporous polymers have many applications, and these materials are extensively being evaluated for gas separation and gas storage. The great interest in these applications arises from the presence of pores providing an exceptionally high Brunauer–Emmett–Teller (BET) surface area. Recently, new classes of metal organic framework and covalent organic framework porous materials have been reported that have shown exceptionally high and unprecedented surface areas. For instance, in 2010, a MOF was reported with a surface area of 6143 m 2 g −1 ; 329 in 2012, a MOF was reported with a surface area greater than 7000 m 2 g −1 ; 330 and in 2018, a MOF (DUT-60) was reported with a record surface area of 7836 m 2 g −1 . 331 Mesoporous DUT-60 has also shown a high free volume of 90.3% with a density of 0.187 g cm −3 . 331

Due to their exceptionally high surface areas and porous networks, these MOFs and COFs are ideal for gas storage. Air separation and post-combustion CO 2 capture have become integral parts of mainstream industries related to the energy sector in order to avoid substantial economic penalties. Due to the inefficiencies of available technology and the critical importance of this area, earnest efforts are being made to design gas-selective porous materials for the selective adsorption of desired gases. Nanoporous MOF- and COF-based materials can significantly capture CO 2 and help reach zero or minimum CO 2 emission levels. For instance, nanoporous fluorinated metal–organic frameworks have shown the selective adsorption of CO 2 over H 2 and CH 4 . 332 Hasmukh A. Patel et al. developed N 2 -phobic nanoporous covalent organic polymers for the selective adsorption of CO 2 over N 2 . The azo groups in the framework rejected N 2 , leading to CO 2 selectivity. 333 Nanoporous polymers that are superhydrophobic in nature can also be used for volatile organic compounds and organic contaminants. 334 Nanoporous polymers, due to the presence of a porous network, have been considered as highly suitable materials for catalyst supports. Furthermore, organocatalytic functional groups can be introduced pre-synthetically and post-synthetically into solid catalysts. 335

Nanoporous polymeric materials are amazingly heading towards being extremely lightweight with exceptionally high surface areas. These high surface areas and the fine-tuning of the nanopores has made these nanoporous materials, specifically MOFs and zeolites, ideal support materials for encapsulating ultrasmall metal nanoparticles inside void spaces to produce nanocatalysts with exceptionally high efficiencies. 336 In the coming years, more exponential growth of nanoporous materials is expected in the energy, targeted drug delivery, catalysis, and water treatment fields.

5.3. Ultrathin two-dimensional nanomaterials beyond graphene

However, from a material synthesis standpoint, a graphite-like layered form of Si does not exist in nature and there is no conventional exfoliation process that can generate 2D silicene, although single-walled 351 and multi-walled 352 silicon nanotubes and even monolayers of silicon have been synthesized via exfoliation methods. 353 Forming honeycomb Si nanostructures on substrates like Ag(001) and Ag(110) via molecular beam deposition, so-called “epitaxial growth”, was then proposed as a method for the architectural design of silicene sheets. 354–356 The successful synthesis of a silicene monolayer was first achieved on Ag(111) and ZrB 2 (0001) substrates in 2012; 357,358 later, various demonstrations were made using Ir(111), ZrB 2 (001), ZrC(111), and MoS 2 surfaces as the silicene growth substrates. 359–361 Despite various extensive studies to date involving the “epitaxial growth” of silicene on different substrates and investigations of the electronic properties, 357,362–364 the limited nanometer size, difficulties relating to substrate removal, and air stability issues have substantially impeded the practical applications of silicene. Bearing in mind all these known difficulties, Akinwande and co-workers recently reported a growth–transfer–fabrication process for novel silicene-based field-effect transistor development that involved silicene-encapsulated delamination with native electrodes. 365 An etch-back approach was used to define source/drain contacts in Ag film. Without causing any damage to the silicene, a novel potassium-iodide-based iodine-containing solution was used to etch Ag, avoiding rapid oxidation, unlike other commonly used Ag etchants. The results demonstrated that this was the first proof-of-concept study confirming the Dirac-like ambipolar charge transport predictions made about silicene devices. Comparative studies with a graphene system, the low residual carrier density, and the high gate modulation suggested the opening of a small bandgap in the experimental devices, proving that silicene can be considered a viable 2D nanomaterial beyond graphene.

Nonetheless, the synthesis of silicene on a large-scale is greatly limited, as “epitaxial growth” is the only promising method for obtaining high-quality silicene, and this presents an enduring challenge in relation to silicene research and development. Xu and co-workers recently introduced liquid oxidation and the exfoliation of CaSi 2 as a means for the first scalable preparation of high-quality silicene nanosheets. 366 This new synthetic strategy successfully induced the exfoliation of stacked silicene layers via the mild oxidation of the (Si 2 n ) 2 n layers in CaSi 2 into neutral Si 2 n layers without damage to the pristine silicene structure ( Fig. 29 ). The selective oxidation of pristine CaSi 2 into free-standing silicene sheets without any damage to the original Si framework was carried out via exfoliation in the presence of I 2 in acetonitrile solvent. Furthermore, the obtained silicene sheets yielded ultrathin monolayers or layers with few-layer thickness and exhibited excellent crystallinity. This 2D silicene nanosheet material was extensively explored as a novel anode, which was unlike previously developed silicon-based anodes for lithium-ion batteries. It displayed a theoretical capacity of 721 mA h g −1 at 0.1 A g −1 and superior cycling stability of 1800 cycles. Overall, during the last decade, silicene has been widely accepted as an ideal 2D material with many fascinating properties, suggesting great promise for a future beyond graphene.

Like other 2D materials, MXenes exhibit crystal geometry with a hexagonal close-packed structure based on the equivalent MAX-phase precursor, and the close-packed structure is formed from M atoms with X atoms occupying octahedral sites. 371 According to the formula, there are three representative structures of MXenes: M 2 XT x , M 3 X2T x , and M 4 X3T x . In these combinations, X atoms are formed with n layers, whereas M atoms have n + 1 layers ( Fig. 30 ). 372 Apart from graphene, MXenes are considered the most dynamic developing material, and they have incredible innovation potential amongst typical 2D nanomaterials because of their remarkable properties, such as hydrophilicity, conductivity, considerable adsorption abilities, and catalytic activity. These vital properties of MXenes suggest their use for various potential applications, including in the photocatalysis, electrocatalysis, 373,374 energy, 375 membrane-based separation, 376,377 and biological therapy 378 fields. In this section, we focus on describing new developments relating to MXenes that are utilized for electrocatalytic and energy storage applications, competing as alternatives to graphene materials.

Interestingly, due to the presence of abundant terminal groups, mainly –O, –OH, and –F, and their modifying nature, MXenes can exhibit outstanding hydrophilic properties and high conductivity and charge carrier mobility, making them a very attractive material for various electrocatalytic applications, such as the hydrogen evolution reaction, oxygen evolution reaction, oxygen reduction reaction, nitrogen reduction reaction, and CO 2 reduction reaction. To further increase their electrocatalytic activities, recent works involving MXenes have included incorporation with CNTs, 379 g-C 3 N 4 , 380 FeNi-LDH, 381 NiFeCo-LDH, 382 and metal–organic frameworks. 383

Cho and co-workers designed and developed MXene–TiO 2 2D nanosheets via the surface oxidation of MXene with defect-free control. These MXene–TiO 2 2D nanosheets were successfully implemented in nano-floating-gate transistor memory (NFGTM) providing a floating gate ( i.e. , multilayer MXene) and tunneling dielectric ( i.e. , the TiO 2 layer). A process of oxidation in water further represented a cost-effective and environmentally benign method, as depicted in Fig. 31 . The MXene NFGTM with an optimal oxidation process displayed exceptional nonvolatile memory features, having a great memory window, high programming/erasing current ratio, long term retention, and high durability. 384

There have been some exciting reports on 2D materials from the pnictogen family, particularly phosphorene. Recently, more attention has also been given to the remaining group 15 elements, 390 with the novel 2D materials arsenene, antimonene, and bismuthene being obtained from the key elements arsenic, antimony, and bismuth, respectively. It is reported that 2D monolayers of group 15 elements, including phosphorene allotropes, have five distinct honeycomb (α, β, γ, δ, and ε) and four distinct non-honeycomb (ζ, η, θ, and ι) structures, as depicted in Fig. 32 . Dissimilar crystal orientations were found for single-layered As, Sb, and Bi. Zeng and co-workers also reported comprehensive density functional theory (DFT) computations that proved the energetic stability and broad-range application of these materials in 2D semiconductors. 391 Previously, following theoretical predictions, Wu and co-workers successfully demonstrated that α-phosphorene showed lowest energy configurations in both honeycomb and non-honeycomb nanosheets. 392 In contrast, Zeng and co-workers proved that the buckled forms of 2D sheets of As, Sb, and Bi allotropes are the most stable structures, particularly their β phases. 391

Among monolayer group 15 family materials, 2D sheets of arsenic (As) and antimony (Sb) have gained considerable attention from researchers. 393,394 Studies have shown that As and Sb exhibit better stability than black phosphorus; they are highly stable at room temperature and less reactive to air, likely inhibiting the oxidization process. 395–398 Nevertheless, it has been demonstrated that the oxidation process is perhaps favorable for fine-tuning the electronic properties; increases in the indirect band gaps ranging from 0 to a maximum of 2.49 eV are found in free-standing arsenene and antimonene semiconductors. 399–403 Simultaneously, arsenene and antimonene can also be transformed into semiconductors with direct band gaps. These two 2D nanosheets can be used to design mechanical sensors, moving beyond common electronic and optoelectronic applications. These two extraordinary 2D nanosheets have been studied for their structural–property relationships via first-principles methods. 403–405

Continuing the characterization and structural property studies of arsenene carried out by Kamal 404 et al. and Zhang 403 et al. , Anurag Srivastava and co-workers analyzed applications of arsenene to explore the possibility of improving sensor devices that can be utilized to detect ammonia (NH 3 ) and nitrogen dioxide (NO 2 ) molecules. 406,407 They investigated the affinities of NH 3 and NO 2 molecules for pristine arsenene sheets, examining the binding energies, bonding distances, density distributions, and current–voltage features. The results showed that arsenene 2D sheets are highly durable, with significant electronic charge transfer. They also considered germanium-doped arsenene and characterized the 2D lattice based on molecular affinity relationships with respect to the dopant.

However, the incorporation of any dopants into 2D nanomaterials not only results in experimental difficulty but it also lowers the stability of 2D materials. 408 Recently, Dameng Liu and co-workers reported the electronic structures, focusing on band structures, band offsets, and intrinsic defect properties, of few-layer arsenic and antimony. 409 The spontaneous oxide passivation layer that is formed naturally on pristine antimonene provides excellent stability. 410 Very recently, Stefan Wolff and co-workers conducted DFT calculations on various single or few-layer antimony oxide structures to describe the stoichiometry and bonding type. Interestingly, the samples exhibited various structural stabilities and electronic properties with a wide range of direct and indirect band gaps. Showing band gaps between 2.0 and 4.9 eV, these 2D layers of antimonene exhibited the potential to be used as insulators or semiconductors. 411 The same group also analyzed Raman spectra and discussed identifying the predicted antimonene oxide structures experimentally. The enduring task of exploring the utility of antimonene has boosted recent research interest in 2D nanomaterials due to the broad range of potential applications, such as their use in electrochemical sensors, 412,413 stable organic solar cells, 414 and supercapacitors 415 to name a few.

The 2D MOF nanosheets are also evaluated for the development of high-performance power-storage devices. For example, Li et al. 427 recently reported two novel Mn-2D MOFs and Ni-2D MOFs as anode materials for rechargeable lithium batteries. The Mn-based ultrathin metal–organic-framework nanosheets, due to thinner nanosheets, a higher specific surface area, and smaller metal ion radius, had structural advantages over Ni-based ultrathin metal–organic-framework nanosheets. Due to these features, the Mn-based ultrathin metal–organic-framework nanosheets displayed a high reversible capacity of 1187 mA h g −1 at 100 mA g −1 for 100 cycles and a rate capability of 701 mA h g −1 even at 2 A g −1 .

The expensive metal oxides utilized in the catalytic process can be replaced in due course by 2D-MOF-based nanosheets with exposed metal sites that impart an adjustable pore structure, ultrathin thickness, a high surface-to-volume atom ratio, and high design flexibility. As a result, 2D-MOFs have extensively been explored for various electrocatalytic applications, including the hydrogen evolution reaction (HER), oxygen evolution reaction (OER), oxygen reduction reaction (ORR), and carbon dioxide reduction reaction (CO 2 RR). For example, Marinescu et al. 428 combined cobalt dithiolene species with benzenehexathiol (BHT) and yielded 2D-MOFs capable of acting as electrocatalysts for the HER in water ( Fig. 34 ). In the presence of 2D-MOF sheets, a high current density of 41 mA cm −2 , at −0.8 V vs. SHE and a pH value of 1.3, is observed. Similarly, Feng et al. 429 also developed single-layer Ni-based 2D-MOF sheets that are highly effective for electrocatalytic hydrogen evolution. Later, Patra et al. 430 reported similar 2D sheets from covalent organic frameworks (2D-COFs) as metal-free catalysts for HER applications. 2D-MOFs are also being explored as active catalysts for the OER process. For example, Xu et al. 431 reported the preparation of 2D Co-MOF sheets using polyvinylpyrrolidone as a surfactant under mild solvothermal conditions. These novel 2D Co-MOFs displayed ultrathin nanosheets with many surface-based metal active sites, improving the overall OER performance.

Interestingly, experimental electrochemical measurement data showed that Co-MOF sheets offer a low overpotential ( i.e. , 263 mV at 10 mA cm −2 ). Similarly, Wang et al. 432 also reported that double-metal 2D-sheets (2D NiFe MOFs) consisting of a very ultrathin structure with a thickness of ∼10 nm further offer a low overpotential of 260 mV at 10 mA cm −2 . In other reports, Zhang et al. 433 successfully performed the OER process with ultrathin 2D-MOF sheets prepared via electrochemical and chemical exfoliation strategies.

Recent work on the catalytic activity of 2D-MOFs has also been reported in relation to the ORR and CO 2 RR because of their layered crystal structures and high-volume modifiable porous structures. For example, Dincă et al. 434 demonstrated that ultrathin layered conductive sheets of the 2D-MOF Ni 3 (HITP) 2 (HITP = 2,3,6,7,10,11-hexaiminotriphenylene) could actively be utilized as a catalyst in an alkaline medium for the ORR process. These 2D-MOF sheets show high stability while retaining 88% of the initial current density over 8 h at 0.77 V vs. RHE. In another report, through fabricating Co x Zn 2− x (bim) 4 2D-sheets as precursors, Zhao et al. 435 successfully synthesized cobalt nanodots (Co-NDs) with bimetallic Co x Zn 2− x (bim) 4 nanosheets encapsulating few-layer graphene (Co@FLG). For the CO 2 RR, a cobalt–porphyrin-containing 2D-MOF was achieved for the selective electrochemical reduction of CO 2 to CO with enhanced stability by Peidong Yang and co-workers. 436 The results further proved that these thin-film catalysts have the highest selectivity for CO ( i.e. , 76%) at −0.7 V vs. RHE with the little-to-no substantial decrease in activity over 7 h at −0.7 V vs. RHE, and 16 mL of CO was produced. Besides, like many other porous materials, 2D-MOFs were also shown to be a supporting platform for catalytic nanoparticles because of their high specific surface areas and favorable porosity distributions. To this end, an example can be noted from Wang et al. 437 reporting that fine porous MOF-5 nanosheets can be utilized to immobilize Pd nanoparticles.

5.4. Metal-based nanostructured materials

As discussed, catalysis is one of the main uses of metal-based nanostructured materials. A continuous increase in the demand for energy, the rapid depletion of conventional energy reservoirs, and rising concerns over the emission of CO 2 have increased the challenges and urgency in the energy field. 460 Metal-based nanostructured materials are extensively being explored to produce alternative clean and renewable energy sources. A range of metal-based nanomaterials has been evaluated and is under consideration for developing robust electrodes that can be effectively applied to water splitting, batteries, and solar cells.

High energy demands have led to more pressure to improve the performances of existing highly demanded lithium-ion batteries. Researchers have focused on improving their lifetimes, sizes, and safety. 462 Nanostructured metal-oxide-based materials are promising electrode materials for use in high-performance charge-storage devices. A metal-based nanostructured electrode is evaluated as both the anode and cathode to overcome the challenges of conventional electrodes. 463 In a conventional LIB, LiCoO 2 was used as the cathode material. Controlled morphology plays a crucial role in determining the performance of a material. Powder composed of spherical particles of LiNi 0.8 Co 0.2 O 2 showed a higher tap density compared to irregular particles and the material substantially improved the power density of secondary lithium batteries. 464 Hierarchical nanostructures of metal-based oxides (such as 3D hierarchical ZnCo 2 O 4 nanostructures) have emerged as a new trend for the development of high-capacity electrodes for lithium-ion batteries. 465 Since their commercialization by Sony in the early 1990s, LIBs have achieved tremendous success in bringing portable electronic devices to the market. However, their sustainable development on the grid-scale is hampered due to limited Li resources in nature, and this is causing a continuous increase in cost. 466 Sodium-ion batteries are in the spotlight to replace powerful lithium-ion batteries due to the widespread availability of sodium and its lower cost compared with lithium. 467 It is essential to note that, in terms of energy densities for SIBs, it is difficult to bypass LIBs because of the low standard electrochemical potential and higher weight of Na. SIBs could be proved to be ideal for those applications where cost is a critical factor compared to energy density. 466

SIBs also operate similarly to LIBs, based on an intercalation mechanism. SIBs also consist of cathode and anode electrodes separated through an electrolyte. During the charging process, sodium ions are extracted from the cathode and inserted into the anode via the electrolyte. In the discharging process, the electrons leave the anode through an external circuit to reach the cathode, providing electricity to the load, whereas Na + moves to the cathode during this process. The radius of Na + (1.02 Å) is greater than that of Li + (0.76 Å), making it challenging to intercalate into electrode materials. 468 Thus, appropriate electrode materials are required in which fast Na-ion insertion and extraction is possible. However, SIBs are suffering from a lack of appropriate electrode materials. It is important to develop electrode materials that have enough interstitial space within their crystallographic structures and better electrochemical performance. Among the various proposed electrode materials, Na x MO 2 layered transition-metal oxides (M = V, Fe, Cu, Co, Ni, Cr, Mn, and their combinations) are considered to be promising electrode materials for SIBs. Layered metal oxides are considered to be promising electrode materials due to their facile scalable synthesis, simple structures, appropriate operating potentials, and high capacities. 469,470 Large volume expansion and poor kinetics during the charge–discharge process can severely affect the cyclability and performance of SIBs. One of the effective strategies to deal with the mechanical stress triggered by large volume changes is the design of hollow or porous structures. In response, three-dimensional network-based Sb 2 O 3 @Sb composite anode materials can help to relieve the volume-change-related stress through their uniform porous networks and provide better transportation channels for Na + . 471

The large volume expansion of electrodes can also be buffered via designing 2D metal-oxide materials with large interlayer spacing. The ultrathin nanosheets provide high reversible capacity with enhanced cycling stability and contribute to providing reaction sites for electrons/ions, decreasing the diffusion distance, providing effective diffusion channels, and facilitating fast charge/discharge for sodium and lithium. 2D SnO nanosheet anodes were evaluated for SIBs. The capacity and cyclic stability improved, as the number of atomic SnO layers is decreased in the sheets. 472 Sb is a promising anode material, but during the sodiation/desodiation processes, huge volume expansion of 390% is observed, which hinders its practical use. Nanostructured Sb in the form of nanorod arrays with large interval spacing displays the great capacity to accommodate volume changes during cycling. 473 A comparison of various nanostructured metal-based electrodes for various charge storage purposes is shown in Table 3 . Overall, well-structured metal or metal-based oxide nanomaterials have the capacity to resolve current issues relating to charge storage devices.

Recently, an immense focus of research has been to produce H 2 fuel via water-splitting to replace conventional fossil fuels. This will help to eliminate emissions from the use of carbonaceous species. 484 Electrochemical method are considered simple water splitting approaches, as these methods only require an applied voltage and water as inputs to produce hydrogen fuel. 485 The coupling of solar irradiation to electrochemical water splitting has enhanced the performance and reduced the process cost. Due to these reasons, this has become a hot area of research. 486 During water electrolysis, H 2 is produced through the hydrogen evolution reaction at the cathode and O 2 is produced through the oxygen evolution reaction at the anode. However, water splitting is not so straightforward, and it requires an efficient catalyst that can facilitate the splitting of water. Metal- and metal-oxide-based catalysts are extensively being explored for water splitting. For the HER reaction, Pt-based catalysts are found to be suitable, whereas for OER reactions, Ir-/Ru-based compounds are found to be benchmark catalysts. Scarcity and high cost have limited the widespread use of these metals. The barrier of noble-metal cost can be mitigated through developing noble-metal nanostructured surfaces that produce more active sites or via depositing monolayers of noble metals on low-cost materials. The alloying of noble metals with other metals has enhanced site-specific activity. 484 At present, more focus is being placed on developing noble-metal-free catalysts for water splitting. 485 Usually, an efficient electrocatalyst is characterized by: 487 a low overpotential; high stability; low production costs; and high electrocatalytic activity.

The nano-structuring of catalysts is an effective tool to boost their surface areas. The electrolysis of water occurs at the surface of a catalyst, and nanostructured catalysts provide more active sites and the better diffusion of ions and electrolytes. 484 Non-noble metals that are under observation for the development of HER electrocatalysts include nickel (Ni), tungsten (W), iron (Fe), molybdenum (Mo), cobalt (Co), and copper (Cu). 487 For instance, a noble metal-free catalyst, carbon-decorated Co 3 O 4 nanoarrays on carbon paper, required a small overpotential of 370 mV to reach a current density of 10 mA cm −2 . It can maintain a current density of 100 mA cm −2 for 413.8 h and 86.8 h under alkaline and acidic conditions, respectively. 488

Metal-based semiconductor materials play a crucial role in a range of applications. For photoelectrochemical water splitting, the semiconductor material plays a central role in the solar-to-hydrogen conversion efficiency. Some critical features are prerequisites when it comes to selecting the right semiconductor material for the photoelectrochemical splitting of water: 489 an extraordinary capacity to absorb visible light; an appropriate bandgap; suitable valence and conduction band positions; commercial feasibility; and chemical stability.

For an ideal semiconductor for water splitting, the valence band and conduction band edge positions must straddle the oxidation and the reduction potentials of water. Metal oxides have received significant attention among semiconductors due to their wide band gap distributions, remarkable photo-electrochemical stabilities, and favorable band edge positions. 490 Semiconductor-based photoelectrodes become excited upon light irradiation, and electrons from the valence band move to the unoccupied conduction band. Some of the generated electrons at the cathode surface reduce protons to hydrogen gas, whereas holes at the photoanode produce oxygen gas via water splitting. 490 As a result, various nanostructured metal oxides can be used as photoelectrode materials, such as WO 3 , 491 Cu 2 O, 492 TiO 2 , 493 ZnO, 494 SnO 2 , 495 BiVO 4 , 496 and α-Fe 2 O 3 , 490 for the efficient splitting of water. As discussed, the nano-structuring of semiconductors can significantly impact the electrode photoelectrochemical performance during water splitting.

Metal-based nanomaterials have been used for the development of sensitive sensors. These metal-based sensors can replace the complex and expensive instruments that are conventionally used for the sensing of analytes. Metal-oxide-based sensors have the interesting characteristics of low detection limits, low cost, high sensitivity, and facile operation. 497 Mostly, semiconducting metal-oxide-based sensors are used for the sensing of toxic, flammable, and exhaust gases. Semiconductor metal oxides with a size in the range of 1–100 nm have been significantly investigated as gas sensors due to their size-dependent properties. The geometry and size of a nanomaterial can considerably affect the hole and electron movement in semiconductors. 498 The surface-to-volume ratio and surface area are substantially enhanced at the nanoscale level, and this is amazingly beneficial for sensing. Chemiresistive semiconducting metal oxides are potential candidates for gas sensing due to the following features: 499 rapid response times; fast recovery times; low cost; simple electronic interfaces; user-friendliness and low maintenance; and abilities to sense a wide range of gases.

Electrode materials decorated with metal- or metal-oxide-based nanostructured materials have shown better responses and selectivity for determining various analytes over conventional electrode materials. The nano-sized metal structures act as an electrocatalyst and electronic wires to provide rapid electron transfer between the transducers and analyte molecules. 500 The electrochemical redox reaction of H 2 O 2 can be improved via the thermally controlled anchoring of Pt NPs on the electrode surface. 501

Currently, researchers are not just concentrating on the development of randomly shaped nanomaterials; instead, they are very focused on and interested in the rational design of materials with controlled nano-architectures for boosting their performances for specific applications. As a result, extensive research has been carried out to develop metal-based materials with controlled dimensions to achieve better catalytic responses. Particle morphology is a crucial factor in the performance of nanomaterials for specific applications. Laifa Shen et al. rationally designed an electrode architecture via growing mesoporous NiCo 2 O 4 nanowire arrays on carbon textiles, which boosted the electrode performance ( Fig. 37 ). 474

The same materials with different morphologies can produce different outcomes. For instance, MnO 2 nanoflowers have provided high initial sodium-ion storage capacity compared with MnO 2 nanorods. 481 Radha Narayanan and Mostafa A. El-Sayed have analyzed various nanoscale morphologies of Pt, such as tetrahedral, cubic, and near-spherical nanoparticles. The highest rate constant is observed with tetrahedral nanoparticles and the lowest rate constant was observed with cubic nanoparticles, whereas spherical nanoparticles exhibited an intermediate rate constant during catalysis. 502 Xiaowei Xie et al. found that Co 3 O 4 nanorods show high activity compared to conventional Co 3 O 4 nanoparticles for the low-temperature oxidation of CO. 503 The catalytic activity of metal-based nanomaterials is strongly affected by their shape. 504 Shape-defined mesoporous materials (TiO 2 ) have shown superior photoanode activities ( Fig. 38 ). 505 As a result, in the literature, several nanostructured morphologies of metal-based materials, such as nanotubes, 506,507 nanorods, 508,509 nanoflowers, 510 nanosheets, 511 nanowires, 512 nanocubes, 513 nanospheres, 514,515 nanocages, 516 and nanoboxes, 517 have been reported for a range of applications.

Hollow nanostructures have surfaced as an amazing class of nanostructured material, and they have received significant attention from researchers. Hollow nanostructures have the unique features of: 518,519 low density; abundant inner void spaces; large surface areas; and the ability to act as nanoscale containers with high loading capacity, nanoreactors, and nanocarriers.

Various metal-based hollow nanostructures, such as hollow SnO 2 , 520 hollow palladium nanocrystals, 521 Co–Mn mixed oxide double-shell hollow spheres, 521 hollow Cu 2 O nanocages, 522 three-dimensional hollow SnO 2 @TiO 2 spheres, 523 hollow ZnO/Co 3 O 4 nano-heterostructure, 524 triple-shell hollow α-Fe 2 O 3 , 525 and hierarchical hollow Mn-doped Ni(OH) 2 nanostructures, 526 have been developed for various applications. The presence of nanoscale hollow interiors and functional shells imparts them with great potential for gas sensing, catalysis, biomedicine, energy storage, and conversion. 519

From this discussion, it can be concluded that metal-based nanostructured materials have great potential compared to their bulk counterparts. The conversion of materials to the nanoscale is not enough to achieve high performance with better selectivity. Now, research is switching from conventional nanomaterials to more advanced and smartly designed nanomaterials. In modern research, nanomaterials are being designed with better-controlled morphologies and regulated features.

5.5. Core–shell nanoparticles

A spherical nanoparticle core–shell nanostructure is a practical way to introduce multiple functionalities on the nanoscopic length scale. 528 The properties arising from the core or shell can be different, and these properties can be tuned via controlling the ratio of the constituent materials. The shape, size, and composition play a critical role in tuning the core–shell nanoparticle properties. 529 The shell material can help to improve the chemical and thermal stabilities of the core material. The core–shell design has become effective where an inexpensive material cannot be used directly due to its instability or easily oxidizable nature. The core can consist of an easily oxidizable inexpensive metal, whereas the shell might consist of noble metals, oxides, polymers, or silica. 530 For instance, magnetic nanoparticles when prepared can be sensitive toward air, acids, and bases. Magnetic nanoparticles can be protected via coating with organic or inorganic shells. 528

Core–shell metal nanoparticles are an emerging nanostructured material with great potential in the fields of energy and catalysis. 531 The first report of core–shell nanoparticles (2007) for supercapacitor applications consisted of a polyaniline/multi-walled-carbon-nanotube composite (PANI/MWNTs). 532 Metal-based core–shell structured nanoparticles have shown enhanced catalytic performance due to their shape-controlled properties. 533 Ming-Yu Kuo et al. developed Au@Cu 2 O core–shell particles with controllable shell thicknesses that acted as a dual-functional catalyst. The shell thickness of Cu 2 O increased with an increasing concentration of Cu 2+ precursor. The thicknesses of the shells of Au@Cu 2 O-1.5 (12.2 ± 1.7 nm), Au@Cu 2 O-2 (13.2 ± 1.8 nm), Au@Cu 2 O-3 (18.2 ± 2.2 nm), and Au@Cu 2 O-4 (20.8 ± 2.5 nm) due to various concentrations are shown in Fig. 40 . 534 A NiO@SiO 2 core–shell catalyst provided a higher yield of acrylic acid from acetylene hydroxycarbonylation. 535 Core–shell architecture can be used to prevent active metal nanoparticles from oxidation during operation. For instance, a plasmonic photocatalyst was developed that consisted of silver nanoparticles embedded in titanium dioxide. The direct contact of Ag with TiO 2 could lead to its oxidization; this is prevented via developing core–shell architecture in which Ag is used as the core and SiO 2 is used as a shell to protect it. 536 Another excellent option is to replace an expensive core with a non-noble metal to reduce the core–shell cost while using a thin layer of a noble metal that consumes a small amount of metal as the shell. This will ensure the prolonged stability of the catalyst during operation. 533 Overall, core–shell morphologies provide better catalytic activity due to the synergistic effect of the metallic core–shell components. 152

Among the several classes of nanomaterials, core–shell nanoparticles are found to be more promising for different biomedical applications. For instance, magnetic nanoparticles are considered to be useful for biomedical applications due to the following reasons: (a) aggregation is prevented due to superparamagnetism; (b) delivery and separation can be controlled using an external magnetic field; (c) they can be appropriately dispersed; and (d) there is the possibility of functionalization. A range of magnetic nanoparticles is available, such as NiO, Ni, Co, and Mn 3 O 4 . The most famous example is iron oxide, but uncoated iron oxides are unstable under physiological conditions. This may result in controlled drug delivery failure due to improper ligand surface binding and the promotion of the formation of harmful free radicals. Therefore, the formation of shells around magnetic nanoparticles has tremendous significance for biomedical applications. 537 One of the approaches is to use gold shells on magnetic nanoparticles. Au NPs are also called surface plasmons and they substantially enhanced the absorption of light in the visible and near-infrared regions. Thus, coating magnetic nanoparticles with a Au shell can result in a core–shell nanostructure that displays both optical and magnetic functionality in combination. 529

Numerous biocompatible core–shell nanoparticles are being developed for photothermal therapy, as core–shell materials are found to be useful for photothermal therapy. Hui Wang et al. have developed bifunctional core–shell nanoparticles for dual-modal imaging-guided photothermal therapy. The core–shell nanoparticles consist of a magnetic ∼9.1 nm core of Fe 3 O 4 covered by an approximately 3.4 nm fluorescent carbon shell. The Fe 3 O 4 core leads to superparamagnetic behavior, whereas the carbon shell provides near-infrared (NIR) fluorescence properties. The bifunctional nanoparticles have shown dual-modal imaging capacity both in vivo and in vitro . The iron oxide–carbon core–shell nanoparticles absorbed and converted near-infrared light to heat, facilitating photothermal therapy. 538 Au-Based core–shell structures are also being prepared for photothermal therapy. Bulk gold is biocompatible, but Au NPs can accumulate in the spleen and liver, causing severe toxicity. Koo Chul Kwon et al. have developed Au-NP-based core–shell structures that did not result in any gross or histological lesions in the major organs of mice, which revealed that this is a potent and safe agent for photothermal cancer therapy. The core–shell nanoparticles consisted of proteinticle/gold (PGCS-NP) and were developed via proteinticle surface engineering. PGCS-NP was injected intravenously into mice with tumors, and the injected core–shell nanoparticles successfully reached the EGFR-expressing tumor cells. The tumor size was significantly reduced upon exposure to near-infrared laser irradiation ( Fig. 41 ). No accumulation of Au NPs was observed in the mice organs, which indicated that PGCS-NP disassembled into many tiny gold dots, which were easily excreted by the kidneys and liver without causing any toxicity. 539 In another example, multifunctional Au@graphene oxide nanocolloid core@shell nanoparticles were developed, in which the core and shell consisted of gold and a graphene oxide nanocolloid, respectively. The developed core–shell structure showed multifunctional properties, allowing Raman bioimaging and photothermal/photodynamic therapy with low toxicity. 540 Apart from this, numerous other core–shell nanoparticles, such as polydopamine–mesoporous silica core–shell nanoparticles, 541 AuPd@PVP core–shell nanoparticles, 542 Au@Cu 2− x S core–shell nanoparticles, 543 bismuth sulfide@mesoporous silica core–shell nanoparticles, 544 and Ag@S-nitrosothiol core–shell nanoparticles, have been used for photothermal therapy. 545

Due to their unique features and the combination of properties from the shell and core, these core–shell nanoparticles have received considerable interest in many fields, ranging from materials chemistry to the biomedical field. For electrochemical reactions, the core–shell structure conductivity can be enhanced via conducting polymers, carbon materials, and metals. Core–shell nanoparticles as electrode materials showed better performance compared to single components. Most of the core materials are prepared via hydrothermal methods, and shells can be prepared via hydrothermal or electrodeposition methods. 546 Even though significant progress has been made relating to the synthesis methods of core–shell materials, a major challenge is the high-quality production of core–shell materials in more effective ways for required applications, specifically biomedical applications.

6. Challenges and future perspectives

(a) The presence of defects in nanomaterials can affect their performance and their inherent characteristics can be compromised. For instance, carbon nanotubes are one of the strongest materials that are known. However, impurities, discontinuous tube lengths, defects, and random orientations can substantially impair the tensile strength of carbon nanotubes. 547

(b) The synthesis of nanomaterials through cost-effective routes is another major challenge. High-quality nanomaterials are generally produced using sophisticated instrumentation and harsh conditions, limiting their large-scale production. This issue is more critical for the synthesis of 2D nanomaterials. Most of the methods that have been adopted for large-scale production are low cost, and these methods generally produce materials with defects that are of poor quality. The controlled synthesis of nanomaterials is still a challenging job. For example, a crucial challenge associated with carbon nanotube synthesis is to achieve chiral selectivity, conductivity, and precisely controlled diameters. 548,549 Obtaining structurally pure nanomaterials is the only way to achieve the theoretically calculated characteristics described in the literature. More focused efforts are required to develop new synthesis methods that overcome the challenges associated with conventional methods.

(c) The agglomeration of particles at the nanoscale level is an inherent issue that substantially damages performance in relevant fields. Most nanomaterials start to agglomerate when they encounter each other. The process of agglomeration may be due to physical entanglement, electrostatic interactions, or high surface energy. 550 CNTs undergo van der Waals interactions and form bundles, making it difficult to align or properly disperse them in polymer matrices. 159 Similarly, graphene agglomeration is triggered by the basal planes of graphene sheets due to π–π interactions and van der Waals forces. Due to severe agglomeration, the high surface areas and other unique graphene features are compromised. These challenges hinder the practical application of high-throughput electrode materials or composite materials for various applications. 551

(d) The efficiency of nanomaterials can be tuned via developing 3D architectures. 3D architectures have been tried with several nanomaterials, such as graphene, to improve their inherent features. 3D architectures of 2D graphene have provided high specific surface areas and fast mass and electron transport kinetics. This has become possible due to the combination of the exceptional intrinsic properties of graphene and 3D porous structures. 194,552 The combination of graphene and CNT assemblies into 3-D architectures has emerged as the most investigated nanotechnology research area. Porous architectures of other nanomaterials can be developed to enhance their catalysis performance through providing nanomaterial interior availability.

(e) 2D ultrathin materials are an outstanding class of nanomaterial with promising theoretical properties; however, very little experimental evaluation of these materials has been done, apart from the case of graphene. The synthesis and stability of 2D ultrathin materials are some of the major challenges associated with them. In the future, more focus is anticipated to be placed on their synthesis and practical utilization.

(f) Nanomaterial utilization in industry is being increased, and there is also demand for nanoscale material production at higher rates. Moreover, nanotechnology research has vast horizons; the exploration of new nanomaterials with fascinating features will continue and, in the future, more areas will be discovered. One of the significant concerns relating to nanomaterials that cannot be overlooked is their toxicity, which is still poorly understood, and this is a serious concern relating to their environmental, domestic, and industrial use. The extent to which nanoparticle-based materials can contribute to cellular toxicity is unclear. 553 There is a need for the scientific community to put efforts into reducing the knowledge gap between the rapid development of nanomaterials and their possible in vivo toxicity. A proper and systematic understanding of the interaction of nanomaterials with cells, tissues, and proteins is critical for the safe design and commercialization of nanotechnology. 14

The future of advanced technology is linked with advancements in the field of nanotechnology. The dream of clean energy production is becoming possible with the advancement of nanomaterial-based engineering strategies. These materials have shown promising results, leading to new generations of hydrogen fuel cells and solar cells, acting as efficient catalysts for water splitting, and showing excellent capacity for hydrogen storage. Nanomaterials have a great future in the field of nanomedicine. Nanocarriers can be used for the delivery of therapeutic molecules.

7. Conclusions

Conflicts of interest, acknowledgements.

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literature survey on nanotechnology

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Nanotechnology-enabled biomedical engineering: Current trends, future scopes, and perspectives

Applications of nanotechnology in biomedical engineering are vast and span several interdisciplinary areas of nanomedicine, diagnostics, and nanotheranostics. Herein, we provide a brief perspective on nanotechnology as an enabling tool for the design of new functional materials and devices for medical applications. Semiconductor nanocrystals, also known as quantum dots, are commonly used in optical imaging to diagnose diseases such as cancer. Varieties of metal and metal oxide nanoparticles, and two-dimensional carbon-based nanostructures, are prospective therapeutics and may also be used in protective antiviral/antibacterial applications. Similarly, a number of nanomaterials have shown the potential to overcome the drawbacks of conventional antiviral drugs. However, assessing the adverse effects and toxicities of nanoparticles in medicine and therapeutics is becoming more critical. This article discusses the latest developments of nanomaterials in diagnosis, nanotheranostics, and nanomedicines, with particular emphasis on the importance of nanomaterials in fighting against coronavirus disease. Further, we considered the safety and toxicity of nanomaterials in the context of biomedical applications. Finally, we provided our perspective on the future of nanotechnology in emerging biomedical engineering fields.

Graphical abstract

1 Introduction

Nanotechnology is an enabling tool for designing new materials and devices, which opens a new era of biomedical engineering. Biomedical engineering is an expeditious field that bridges the gap between technology, biology, and medicine. The potential use of nanotechnology in biomedical engineering includes the early detection and treatment of diseases. Cognizance in physical/chemical biology, fabrication principles, and the development of predictive methods to control them, are likely to lead the major advances in nanomedicine and nanodevices.

Paul Ehrlich, a medical doctor who expressed great interest in bacteriology and immunology, proposed the concept of targeted drug delivery that was mediated by nanoparticles in 1954 [ 1 ]. He called the delivery system “Zauberkugeln,” which translates to “Magic bullets” in English. This concept gained momentum in 1950–1960 and the controlled release of drugs has since then attracted significant attention. The historical development of nanoparticles (NPs) for biomedical applications over the years is depicted in Figure 1 . Professor Peter Paul Speiser and his group first investigated polyacrylic beads for oral administration and developed NPs for drug delivery purposes and vaccines in 1973 [ 1 , 2 ]. Several vaccinations, such as those for tetanus and diphtheria, require multiple injections to build up sufficient antibodies for protection. It was anticipated that the sustained release mechanism of NPs would provide constant immune stimulation, and hence, one injection would be sufficient to achieve the necessary antibody response. Within a short time, the Johns Hopkins Medical Institutions invented albumin NPs and, in subsequent years, Kramer used a similar method to produce magnetic NPs. One of the most important applications of NPs, which is for the detection of tumor cells and cancer therapy, was developed between 1979 and 1986 [ 3 , 4 ]. Later, it was demonstrated that NPs could deliver anti-infective drugs [ 5 ], DNA fragments, and genes [ 6 ]. The covalent attachment of antibodies on human serum albumin NPs enabled drug transport across the blood–brain barrier [ 7 ]. NPs have been introduced in pharmaceutical and medical applications 67 years ago and have great potential for future biomedical applications.

Figure 1 
               Historical development of nanoparticles for biomedical applications.

Historical development of nanoparticles for biomedical applications.

Nanotechnology has revolutionized medical diagnosis, immunization, treatment, and even healthcare products. The coupling of biological agents with different types of NPs can be achieved either via chemical conjugation, physical encapsulation, or adsorption [ 8 ]. The suitable dosage of these nanosubstances depends on the application purpose. They can be used to deliver various chemicals (drugs, chemotherapeutic agents, or imaging substances) and biological substances (antigens, antibodies, RNA, or DNA) through endocytosis, and transmit light and heat to target cells in therapies [ 8 ]. The emerging field of nanotechnology for biomedical application is vast and can be classified into four categories, as shown in Figure 2 . Diagnostic methods involve different imaging techniques. Drug delivery, tissue engineering, prosthetic orthopedic implants, and gene and stem cell therapies fall under the category of therapeutic applications. Other emerging fields include antiviral applications of NPs in conjunction with treatments used in combating drug-resistant bacteria, as well as immunization and vaccine production. Therefore, because of the growing importance and the need to draw the attention of current researchers towards the application of nanotechnology in biomedical fields, we hereby provide a brief account of the latest developments in the use of nanomaterials in diagnosis, nanotheranostics, and nanomedicines. The importance of nanotechnology in fighting against the coronavirus disease, safety, and toxicity of nanomaterials is also discussed. Finally, we propose possible future directions in the context of the current challenges.

Figure 2 
               Application of nanotechnology for biomedical purposes [9,26,51]. Reproduced with permission from ref. [9], © American Chemical Society. Reproduced with permission from ref. [51], reproduced with permission from ref. [9], © 2011 Elsevier Science Ltd., and reproduced with permission from ref. [51]; © 2011 Elsevier Science Ltd.

Application of nanotechnology for biomedical purposes [ 9 , 26 , 51 ]. Reproduced with permission from ref. [ 9 ], © American Chemical Society. Reproduced with permission from ref. [ 51 ], reproduced with permission from ref. [ 9 ], © 2011 Elsevier Science Ltd., and reproduced with permission from ref. [ 51 ]; © 2011 Elsevier Science Ltd.

2 Nanotechnology in diagnostics

NPs used in imaging applications, such as optical imaging and magnetic resonance imaging (MRI), are well established and widely used for diagnosis. Semiconductor nanocrystals, known as quantum dots (QDs), are also commonly used in optical imaging [ 10 ]. QDs are 100 times brighter than organic dye molecules. The number of NPs in the cell cytoplasm is critical to illuminate the cells within deep tissue. Although QDs are useful tagging materials, they have several disadvantages. There is a risk of an increase in toxicity due to the increased number of NPs required to illuminate the cell. Another drawback is the blinking behavior, which hinders the tracking of QD-targeted biomolecules [ 10 ]. Hence, the development of fluorescent NPs for in vivo imaging is still an open challenge. Compared to QDs, silicon nanocrystals are more appealing because they are nontoxic to the cells and do not require a thick surface coating to protect the nanocrystal core from the environment [ 10 ]. Table 1 shows the comparative advantages of silicon nanocrystals over QDs and organic dyes. Silicon nanocrystals are small and have high photostability and moderate quantum yield (a measure of the efficiency of photon emission, as defined by the ratio of the number of photons emitted to the number of photons absorbed).

Comparative properties of silicon nanocrystals with QDs and organic dyes [ 10 ]. Reproduced with permission from ref. [ 10 ]; this work is licensed under the Creative Commons Attribution 4.0

In contrast, superparamagnetic iron oxide (Fe 2 O 3 ) and manganese oxide (MnO) NPs are suitable for MRI. Further, research has shown that metal NPs, fullerenes, dendrimers, and polymer-coated copper sulfide nanocrystals are suitable for diagnostic imaging [ 9 , 10 , 11 , 12 ]. An example of the use of Au@MnS@ZnS core/shell/shell NPs with poly(ethylene glycol) functionalization, in the diagnosis of a tumor, is shown in Figure 1 [ 9 ]. The figure shows efficient accumulation and retention of NPs in the tumors of mice after intravenous injection. Moreover, exposure to X-rays can significantly inhibit the growth of tumors, which shows the possibility of using such NPs in therapeutic applications. In recent years, multipurpose nanosized sensors have been designed to detect different pathological parameters, antigens, and toxic substances.

Chen et al. [ 13 ] have reported a method that utilizes a small-molecule peptide, the asparagine-glycine-arginine (NGR), as a capture probe for the selective enrichment and isolation of circulating tumor cells (CTCs). The multiscale TiO 2 nanofibers are obtained by electrospinning and calcination. Bovine serum albumin (BSA) is decorated onto TiO 2 nanofiber surfaces to inhibit nontarget cell adhesion. At the same time, NGR peptides are conjugated onto the TiO 2 –BSA surface through the glutaraldehyde (GA) to specifically capture the target cells.

3 Nanotechnology in therapeutics

3.1 treatment of hyperthermia.

Metallic NPs, being excellent conductors of heat, efficiently transmit heat generated within them to adjacent tissues. When administered intravenously, they can accumulate preferentially in tumors. Magnetic NPs, gold (Au) NPs, and carbon nanotubes (CNTs) have been used in therapeutic applications. Typical examples of magnetic NPs are Fe 2 O 3 NPs, superparamagnetic Fe 2 O 3 NPs, and doped Fe 2 O 3 NPs. Magnetic NPs are activated in an alternating magnetic field to generate heat, which damages the tumor cells without harming the healthy tissues. Superparamagnetic Fe 2 O 3 NPs are efficient heat generators that can be localized within the center of the tumor and have low toxicity [ 14 ]. The presence of dopants in Fe 2 O 3 NPs increases their specific absorption rate and enhances the heat generation efficiency in an alternating magnetic field [ 14 ]. Recently, electrospun fibers containing NPs have received attention for postsurgical treatment. Superparamagnetic Fe 2 O 3 , graphene oxide, and doxorubicin-incorporated nanofibers are proven to eradicate breast cancer’s regional recurrence and enhance tissue regeneration [ 15 , 16 ]. In contrast to magnetic NPs, Au NPs are activated photothermally. When Au NPs are illuminated by light, the light energy is converted into heat energy. CNTs absorb incident energy over a broad frequency spectrum, including visible light, near-infrared light, and even radiofrequency waves, and the absorption is considerably higher than that of natural chromophores such as melanin, hemoglobin, and water. Electrospun nanofibers composed of NPs (such as copper sulfide nanoflowers) can be used for postsurgical skin cancer treatment and skin tissue regeneration. While copper sulfide plays an important role in photothermal performance, the nanofibers enhance the adhesion, proliferation, and migration of cells [ 15 ].

3.2 Drug delivery

Various lipids are also used as antiviral drug carriers. Lipids are biodegradable, biocompatible, inert, nontoxic, nonimmunogenic, easily available, and cheaper. Moreover, their unique characteristics include a smaller size, larger surface area, high drug-loading capacity, improved interface interactions, controlled release, and enhanced overall performance of the drug they deliver [ 17 ].

A drug called antisense oligonucleotide (ASO) can inhibit the mRNA processing and translation that leads to many diseases. The disadvantages of ASO-based therapies include poor biological stability, short half-life in circulation, and limited cellular uptake of free ASO. Wartlick et al. [ 7 ] showed that NPs consisting of human serum albumin (HSA), cross-linked with glutaraldehyde and the drug ASO, enhances the cellular uptake of ASO. Confocal laser scanning microscopic images presented in Figure 3 confirm the presence of the NPs containing drugs in different types of cells (MDA-MB-468, MCF-7, and BT-474 are breast cancer cells; A549 is lung cancer cells). At an incubation temperature of 37°C (similar to our body temperature), NPs were spotted after 24 h, and rapid uptake of the NPs was observed in MDA-MB-468 and MCF-7 cells.

Figure 3 
                  Confocal laser scanning microscopic images showing the presence of the nanoparticles containing the drug in different types of cells: (1) MDA-MB-468, (2) MCF-7, and (3) BT-474 are breast cancer cells, and (4) A549 is lung cancer cells. Reproduced with permission from ref. [7].

Confocal laser scanning microscopic images showing the presence of the nanoparticles containing the drug in different types of cells: (1) MDA-MB-468, (2) MCF-7, and (3) BT-474 are breast cancer cells, and (4) A549 is lung cancer cells. Reproduced with permission from ref. [ 7 ].

Drug delivery systems fabricated with functional electrospun fibers show promising results for local cancer therapy due to their porous structure, relatively large surface area, high drug loading capacity, and stimuli-responsive drug release [ 15 ]. For instance, pH-responsive electrospun fibers can be fabricated by direct blending or core–shell structures comprised of pH-responsive inorganic components or polymer chains and anticancer drugs. Similarly, thermo-responsive, magnetic-responsive, light-responsive electrospun fibers contain NPs of respective properties and the target drug to treat a specific disease.

3.3 Stem cell applications

NPs are beneficial for regenerative medicine, stem cell growth, and differentiation. Pluripotent stem cells generally need to be cultured before they are transferred to the human body. Due to their large surface area and biocompatibility at low concentrations, two-dimensional nanostructures are proven to be felicitous for the fabrication of scaffolds suitable for the culture of pluripotent stem cells. Asil et al. [ 18 ] used graphene oxide (GO) and nanofiber scaffolds to culture neural stem cells, as shown in Figure 4 . Moreover, GO allows the spontaneous differentiation of the stem cells. Superparamagnetic Fe 2 O 3 NPs promote the proliferation of human multipotent stem cells found in the bone marrow [ 19 ]. Metal oxide NPs such as Fe 2 O 3 , zinc oxide (ZnO), and Au NPs are used to track the stem cells [ 20 , 21 ].

Figure 4 
                  Schematic diagram of the scaffold structure fabricated from graphene oxide and nanofibers for the differentiation of neural stem cells [18]. Reproduced with permission from ref. [18].

Schematic diagram of the scaffold structure fabricated from graphene oxide and nanofibers for the differentiation of neural stem cells [ 18 ]. Reproduced with permission from ref. [ 18 ].

To treat patients with Batten disease, neural stem cells are administered to the brain by multiple subcortical or intraventricular injections. Batten disease, caused by a genetic defect, can cause loss of vision, and a progressive decline in motor and cognitive responses. A number of studies have shown that the number of neural stem cells injected can be tolerated with immunosuppressant therapy. The use of immunosuppressant drugs is common during organ and tissue transplants because they stop the immune system from overreacting and damaging transplanted organs and tissues. A similar procedure can be used for brain tumor diagnosis and therapy, provided that neural stem cells containing NPs are administered [ 22 ]. The characteristics of stem cells can be altered by using loaded stem cells. Chung et al. [ 23 ] recently showed that the migration of stem cells toward tumor cells improves when stem cells are loaded with Fe 2 O 3 NPs. Figure 5 shows a schematic diagram of the proposed chronological order of events that take place during the intraoperative transplantation of stem cells carrying drug-loaded NPs into the human brain after tumor resection [ 22 ]. Nevertheless, the efficacy of this therapy is yet to be proven.

Figure 5 
                  Intraoperative transplantation of stem cells carrying drug-loaded NPs into the human brain after tumor resection [22]. Reproduced with permission from ref. [22].

Intraoperative transplantation of stem cells carrying drug-loaded NPs into the human brain after tumor resection [ 22 ]. Reproduced with permission from ref. [ 22 ].

3.4 Gene therapy

Gene therapy introduces normal exogenous genes into the target cells to compensate for the defective and abnormal disease. Usually, viral vectors are used as transporters to deliver the desired gene to the cell. However, there are many limitations, including infection-related cell damage [ 24 ]. Nanoparticles bound to DNA have shown promising results when used for delivering genes into stem cell carriers [ 25 , 26 ].

A recent study showed that DNA-containing surface-modified Au NPs provide synergistic photothermal/gene therapy during photoacoustic imaging [ 27 ]. Figure 6 shows a schematic illustration of synergistic photothermal/gene therapy for breast cancer. The procedural sequence used to study the efficacy of this NP is presented in Figure 7 [ 27 ]. Tumor-bearing mice were divided into four groups: (i) control, (ii) gene therapy group (J-ACP/p53, where J-ACP represents the surface-modified Au NPs and p53 is DNA), (iii) photoacoustic imaging group (J-ACP + NIR; NIR stands for near-infrared radiation), and (iv) complementary photothermal/gene therapy J-ACP/p53 + NIR (refer to Figure 7(a) ). It is evident from Figure 7(b) that the volume, average weight, and size of the tumors in the control group grew rapidly over time. For monotherapy groups (Groups ii and iii), the growth of tumors was inhibited by the photothermal killing effect, the high transfection efficiency of J-ACP, and the excellent antitumor function of p53. However, the monotherapy was not sufficient because the average volume of tumors was still ≈3.5 times as large as the tumors before the treatment. The tumor growth of the complementary photothermal/gene therapy group (Group iv) was completely inhibited and the tumors were significantly smaller than the monotherapy groups after treatment. Therefore, such synergistic therapies seem promising for use in the treatment of cancer.

Figure 6 
                  Schematic illustration of synergistic photothermal/gene therapy for breast cancer. J-ACP: surface-modified gold NP and J-ACP/p53: DNA containing surface-modified gold NP [27]. Reproduced with permission from ref. [27].

Schematic illustration of synergistic photothermal/gene therapy for breast cancer. J-ACP: surface-modified gold NP and J-ACP/p53: DNA containing surface-modified gold NP [ 27 ]. Reproduced with permission from ref. [ 27 ].

Figure 7 
                  (a) Schematic illustration of the treatment process of photoacoustic-imaging guided therapy and (b) time-dependent growth curves of tumors, average weights, and representative photographs of excised tumors of mice [26]. Reproduced with permission from ref. [26].

(a) Schematic illustration of the treatment process of photoacoustic-imaging guided therapy and (b) time-dependent growth curves of tumors, average weights, and representative photographs of excised tumors of mice [ 26 ]. Reproduced with permission from ref. [ 26 ].

Gene transfer is a new mode of cancer treatment that introduces new genes into a cancerous cell or the surrounding tissue to cause cell death or slow cancer’s growth, or correct genetic errors to reverse the malignant state. Typical therapeutic approaches include angiogenic gene therapy, suicide gene therapy, immunotherapy, small interfering RNAs (siRNA) therapy, pro-apoptotic gene therapy, oncolytic virotherapy, and gene directed-enzyme prodrug therapy [ 28 ]. Angiogenic gene therapy (angiogenesis) promotes the formation of new capillary blood vessels from existent microvessels. Suicide gene therapy uses a drug to kill cancer cells. siRNA therapy refers to the transient silencing of a gene of interest. Pro-apoptotic gene therapy prevents apoptosis, a process of programmed cell death. Oncolytic virotherapy uses viruses to infect and destroy cancer cells. Gene for a nonendogenous enzyme is directed to target tissues in gene-directed enzyme prodrug therapy, which activates subsequently administered prodrug. Development of safe and effective vectors for gene delivery and understanding the activity of nucleases will facilitate future genome editing as new treatment approaches for cancer.

4 Nanotechnology in antiviral/antibacterial protective applications

Infectious diseases caused by viruses pose a significant risk to human health worldwide. Aerosol particles play a critical role in the spreading of airborne viruses. Different sizes of aerosol particles infect the host’s respiratory tract through different mechanisms, as shown in Figure 8 [ 29 , 30 ]. Aerosol particles are released while breathing, speaking, sneezing, or coughing. Smaller particles (<0.2 µm) infect the lungs via diffusion and particles that are <2.0 µm in diameter can infect the respiratory tract as well as the lungs. Pathogen inactivation and protection depend on filtration efficiency, face seal, and infrastructure to reduce cross-infection and minimize environmental contamination.

Figure 8 
               Respiratory protection and airborne transmission intertwine system [29]. Reproduced with permission from ref. [29].

Respiratory protection and airborne transmission intertwine system [ 29 ]. Reproduced with permission from ref. [ 29 ].

One common protective tool is a facemask. Medical protective masks are typically made of a functional wet-resistant spun-bonded nonwoven layer, a melt-blown nonwoven layer, and a skin-friendly spun-bonded nonwoven layer, as depicted in Figure 9(a) [ 30 ]. Facemasks allow gas/steam perspiration but prevent the penetration of aerosols, blood, or fluids. Facemasks are mandatory to fend off the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2) infection and the development of innovative self-sterilized and re-useable facemasks has been stipulated. Sunlight-mediated sterilization of facemasks appears to be a promising avenue for recyclability [ 31 , 32 , 33 ]. Meanwhile, photothermal materials such as carbon dots, graphene, and silver (Ag) NP-based materials are being designed for facemasks. Figure 9(b) shows a schematic diagram of nanoporous membranes comprising carbon dots and a polymer [ 33 ]. The resulting composite film exhibits a hydrophobic surface, which prevents moisture accumulation. A compact nanopore network allows both breathability and effective filtration of particles above 100 nm in diameter.

Figure 9 
               (a) Schematic diagram of facemasks [29]; (b) schematic structure of self-sterilization facemasks containing carbon dot/polymer composite [33]; and (c) temperatures of the polymer-containing membrane and the membrane comprising carbon dot/polymer composite at different irradiation times [32]. Reproduced with permission from ref. [32].

(a) Schematic diagram of facemasks [ 29 ]; (b) schematic structure of self-sterilization facemasks containing carbon dot/polymer composite [ 33 ]; and (c) temperatures of the polymer-containing membrane and the membrane comprising carbon dot/polymer composite at different irradiation times [ 32 ]. Reproduced with permission from ref. [ 32 ].

Moreover, self-sterilization occurs upon simulated solar irradiation as the embedded carbon dots absorb visible light and concurrently increase the temperature through heat dissipation. Figure 9(c) shows the temperatures of the polymer-containing membrane and the membrane composed of carbon dot/polymer composite at different irradiation times. The authors tested the bacterial proliferation of E. coli to simulate the SARS-CoV-2 (due to safety precautions) and found that E. coli does not survive at elevated temperatures (<60°C) [ 33 ]. Hence, such a composite can be considered for facemasks.

The viral infection process involves attachment, penetration, uncoating, replication, assembly, and release, as illustrated in Figure 10(a) . Viruses enter host cells through specific receptors on the host cell membrane using attachment proteins in the viral capsid [ 34 ]. Self-disinfecting devices, therefore, can prevent infections to a certain extent. Metal oxide NPs ( e.g ., copper oxide, nickel oxide, and titanium dioxide), QDs, GOs, carbon dots, and Ag NPs can reduce the virus’ viability on surfaces when associated with polymers and textiles [ 34 , 35 ]. Metal oxide NPs produce reactive oxygen species when exposed to illumination [ 30 , 35 ]. Metal-based NPs inhibit viral activities in three stages, as shown in Figure 10(b) . First, NP attachment to the virus prevents the penetration of the virus into the cell. It then produces reactive oxygen species (ROS), ions, and radicals to destroy the structure and function of viral proteins and nucleic acids. Finally, it is simulated that the nucleus increases the immune response of the host cell and inhibits the budding and spreading of the virus. GO can inactivate viruses due to its ability to destroy the viral envelope and capsid (protein shell of a virus) [ 30 ]. For instance, Au NPs could significantly reduce viral infections (a reduction of 92% after a 6 h interaction) by preventing the penetration of the virus into the cells.

Figure 10 
               (a) Mechanism of entry of the virus into the host cells [34] and (b) schematic diagram of an antiviral mechanism of metal-based NPs [30]. Reproduced with permission from ref. [30] and [34], respectively.

(a) Mechanism of entry of the virus into the host cells [ 34 ] and (b) schematic diagram of an antiviral mechanism of metal-based NPs [ 30 ]. Reproduced with permission from ref. [ 30 ] and [ 34 ], respectively.

Carbon-based NPs, such as graphene or GO, bind with a virus, as shown in Figure 11 [ 36 ]. As a result, the virus loses its structural integrity with the destruction of the spike structures and the leakage of RNA. The extracted RNA can then be used to detect the virus. Graphene decorated with copper( i ) oxide, silicon carbide, manganese dioxide, and molybdenum sulfide exhibits antibacterial properties in a similar manner [ 37 , 38 ].

Figure 11 
               Schematic diagram of the antiviral mechanism of carbon‑based NPs, such as graphene or graphene oxide [36]. Reproduced with permission from ref. [36].

Schematic diagram of the antiviral mechanism of carbon‑based NPs, such as graphene or graphene oxide [ 36 ]. Reproduced with permission from ref. [ 36 ].

Recent advances in borophene technology have demonstrated the potential for combined photothermal chemotherapy to treat cancer [ 39 ]. As graphene is a two-dimensional planar monolayer of carbon, borophene is a two-dimensional planar monolayer of boron. Because of its unique physical, chemical, optical, and electronic properties, it can be used as a biosensor in a variety of bioimage-guided therapies. It can also be used in boron neutron capture therapy, provided its biocompatibility, cytotoxicity, and stability are investigated.

5 Importance of nanotechnology in nanomedicine and nanotheranostics

There is a continuous effort to discover new antiviral drugs and therapies to improve the quality of life of patients suffering from viral infections. However, long-term treatment with antiviral drugs has toxic side effects. Other challenges with conventional treatments include the development of drug resistance, the inability to deal with critical diseases, patient noncompliance, as well as the issue of bioavailability and having a short half-life ( t 1/2 ) [ 40 ]. The smaller size and high surface-to-volume ratio of NPs result in characteristics such as increased bioavailability, targeted drug delivery, and increased therapeutic efficacy. As a result, nanotechnology can revolutionize nanomedicine and nanotheranostics.

It is noteworthy to mention that according to StatNano, up until February 2020, 9,217 patents were registered as having the coronavirus disease, and 5.2% of these are being treated based on the application of nanotechnology in diagnostics, therapeutics, vaccines, and filters [ 40 ]. Nanoparticulates composed of naturally occurring inflammatory compounds and fat as a core, and vitamin E as an envelope, were developed to reduce hyperinflammation [ 41 ]. Arcturus Therapeutic in San Diego as well as Duke-NUS Medical School and a research-intensive medical school in Singapore collaborated to develop an mRNA-based vaccine against the coronavirus disease [ 40 ]. Lipid-based NPs encapsulate the mRNA to trigger rapid and prolonged antigen expression within host cells, resulting in protective immunity against infectious pathogens. Scientists at the University of Waterloo, Canada, proposed the delivery of therapeutic DNA to target tissues through the nasal spray to produce antigenic proteins for the SARS-CoV-2 (virus-like particles), which are harmless but produce an immunogenic response against the virus [ 40 ]. Respilon Group incorporated copper oxide into nanofibers to produce a mask that can trap and destroy the virus [ 40 ]. The Advanced Institute of Science and Technology (KAIST) in Korea, developed nanofiber-based nanofilters that maintain their filtering efficiency even after 20 washes with ethanol. An MIT spin out start-up company has developed strips, based on AU NPs, which could give a color reaction within 20 min of the start of the test. The strip is coated with antibodies that bind to the specific viral protein of the SARS-CoV-2 and the second antibody is attached to AU NPs. The patient’s sample is placed on the strip, and if it has a viral antigen that binds to both these antibodies, a colored spot appears on the strip.

6 Safety of NPs

The direct association of NPs with an organism’s cell surface causing damage to the cell membrane.

Dissolution of the material by releasing toxic ions that affect the organism by impairing through direct interaction with a cell’s DNA or important enzyme functions.

The generation of ROS and subsequent oxidative stress in an organism, which can also damage an organism’s genetic material or important enzymes [ 42 ].

NPs can generate oxidative stress and inflammation in various tissues, which damage the biological molecules ( e.g ., proteins, lipids, and DNA) of the cell [ 40 ]. The most important organs in our study are the liver, lungs, spleen, kidneys, and heart. Hence, NPs may cause hepatotoxicity, nephrotoxicity, cardiotoxicity, immunotoxicity, and genotoxicity [ 40 ]. Additionally, Au NPs can undergo cyanidation and oxidation in the body and generate toxic products. These products are heavily absorbed in the kidneys and may cause nephrotoxicity [ 43 ]. Sun et al. [ 44 , 45 ] studied the shape-dependent toxicity of Au NPs and found that rod-shaped Au NPs are more toxic than cube-shaped Au NPs. The spherical Au NPs exhibited the best biocompatibility [ 44 , 45 ]. Similar to the biodistribution of other nanomedicines, traces of Au NPs can be found in the blood, brain, lungs, heart, kidneys, liver, and spleen [ 46 , 47 ]. The unprotected QDs showed the release of toxic cadmium upon exposure to ultraviolet radiation [ 45 ]. Yoisungnern et al. [ 48 ] reported that Ag NPs are a potential cytotoxic agent for sperm cells and exert adverse effects, possibly via the induction of oxidative stress. Copper oxide NPs have shown the highest level of DNA damage and cytotoxicity in vitro [ 49 ]. In general, carbon-based nanostructures are biocompatible but metal-based impurities and the agglomeration state are considered to be the major factors responsible for the toxicity of CNTs [ 46 , 47 ]. Furthermore, long-term retention of long CNTs leads to severe inflammation and progressive fibrosis in mice [ 50 ]. Figure 12 shows the histological examination of chest wall samples from mice injected with NTtang1 (CNTs measuring 15–20 μm in diameter and 1–5 μm in length) and NTlong2 (CNTs measuring 20–100 μm in diameter and 56 μm in length) at day 1 and day 7 following injection [ 50 ]. It can be seen from the figure that the aggregates of inflammatory cells are present in both NTtang1 and NTlong2 samples at day 1. However, they were present only in NTlong2 samples at day 7. The authors have concluded that short nanofibers either resolve no inflammation or modestly resolve inflammation, with no parietal pleural pathological features. Furthermore, surface-modified CNTs create more toxic effects than pristine CNTs [ 51 ]. The toxicity of NPs depends on the dose and duration of exposure. Understanding the mechanism of toxicity will guide the redesign strategies based on the major modes of toxicity.

Figure 12 
               Histological examination of chest wall samples from mice injected with NTtang1 and NTlong2 at day 1 and day 7 following injection. The diameters of NTtang1 and NTlong2 are approximately 15–20 and 20–100 μm, respectively; while the length of NTtang1 and NTlong2 are approximately 1–5 and 56 μm, respectively. The arrowhead indicates long CNT aggregates in ×100 magnification of NTlong2 (7-day sample). Scale bar = 20 μm [50]. Reproduced with permission from ref. [50].

Histological examination of chest wall samples from mice injected with NTtang1 and NTlong2 at day 1 and day 7 following injection. The diameters of NTtang1 and NTlong2 are approximately 15–20 and 20–100 μm, respectively; while the length of NTtang1 and NTlong2 are approximately 1–5 and 56 μm, respectively. The arrowhead indicates long CNT aggregates in ×100 magnification of NTlong2 (7-day sample). Scale bar = 20 μm [ 50 ]. Reproduced with permission from ref. [ 50 ].

7 Future scopes

In many ways, nanotechnology can shape the future of biomedical engineering and medical treatments. For instance, the FDA-approved “pill cam” technology can be expanded to monitor when medication is taken, based on the response of the body, and may also assist with aspects such as the adjustment of the prescribed drug dosage [ 52 ]. Globally, researchers are working on controlled drug release to target cancer cells as an alternative to chemotherapy [ 52 ]. The global nanomedicine market revenue trends illustrated in Figure 13(a) indicate that the market is growing [ 53 ]. The US nanomedicine market size for by-products (including diagnostics, therapeutics, and vaccines) is also showing a growing trend, as depicted in Figure 13(b) [ 54 ]. In order to produce the vaccine, antigens can either be conjugated to the surface of the nanoparticles or encapsulated in the core of the particles [ 55 ]. The potential pipeline of nanotechnology-based products and associated nanotechnology-based devices is anticipated to drive the market with potential avenues for growth. The presence of approximately 40% of the products in phase II of clinical development is anticipated to result in a number of key commercialization opportunities over the coming decade.

Figure 13 
               (a) The global nanomedicine market revenue trends [53] and (b) US nanomedicine market size for by-products [54]. Reproduced with permission from ref. [53].

(a) The global nanomedicine market revenue trends [ 53 ] and (b) US nanomedicine market size for by-products [ 54 ]. Reproduced with permission from ref. [ 53 ].

Nanoscale sensors embedded directly into the implant or surrounding area can reduce the risk of post-surgery inflammation and infection through early detection [ 52 ]. Based on the advancements made in understanding type 2 diabetes, the possibility of devising a smart therapy for the reversal of type 2 diabetes in the near future can save many lives [ 45 ]. The correlation and reproducibility between the toxicity testing methods, and adequate knowledge of in vivo and in vitro findings, are necessary to implement nanotechnology in biomedical applications. Biopharmaceutical and toxicity regulation agencies are moving forward rapidly to new metrics to keep pace with the changing paradigms introduced by nanomedicines. Through the realization of the challenges of the recent coronavirus disease in the health care system, nanotechnology can ameliorate the breathability, antiviral capabilities, and entrapping potential of personal protective equipment.

8 Conclusion

In this article, we have presented the recent developments in a variety of nanotechnology-based solutions for emerging biomedical engineering fields. A number of applications, namely pharmaceutical engineering ( e.g ., drug engineering and targeted drug delivery), tissue engineering ( e.g ., creating artificial organs), genetic engineering ( e.g ., gene slicing and modification/manipulation), medical devices, imaging, and antiviral use, which have been reported in the recent literature, were discussed. However, the effectiveness of any such application is determined through clinical evaluation, its compliance with performance standards, or demonstrations of substantial equivalence with an already marketed device. Currently, a number of nanomedicines are undergoing phase II of clinical trials for practical applications. Moreover, the current challenges associated with the use of nanomaterials in biomedical fields were discussed, and suitable directions for future research were proposed. Despite the fact that recent research showed superior performance of stimuli-responsive electrospun fibers for local cancer therapy, several issues must be resolved before clinical trials. A deeper understanding is required of the inflammatory and immune response, consequence of reaction at the interface of tissue/blood and the fibers, concentration of drug and the consequence of burst release, and so on. In summary, the authors believe that the various research contributions summarized in this article will help to accelerate the application of nanotechnology in different challenging biomedical engineering fields.

Acknowledgments

The authors gratefully acknowledge the assistance from Prof. S. S. Ray.

Funding information: JB would like to the Council for Scientific and Industrial Research (C1VPP99) for financial support.

Author contributions: All the authors have accepted responsibility for the entire content of this manuscript and approved its submission.

Conflict of interest: The authors state no conflict of interest.

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  • Beilstein J Nanotechnol

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Nanotechnology in the real world: Redeveloping the nanomaterial consumer products inventory

Marina e vance.

1 Institute for Critical Technology and Applied Science, Virginia Tech, 410 Kelly Hall (0194), 235 Stanger St., Blacksburg, VA 24061, United States

Todd Kuiken

2 Woodrow Wilson International Center for Scholars, One Woodrow Wilson Plaza - 1300 Pennsylvania Ave., NW, Washington, DC 20004, United States

Eric P Vejerano

3 Department of Civil & Environmental Engineering, Virginia Tech, 418 Durham Hall (0246), Blacksburg, VA 24061, United States

Sean P McGinnis

4 Department of Materials Science and Engineering, Virginia Tech, Holden Hall (0237), Blacksburg, VA 24061, United States

Michael F Hochella, Jr.

5 Department of Geosciences, Virginia Tech, 4044 Derring Hall (0420), Blacksburg, VA 24061, United States

David Rejeski

Matthew s hull, associated data.

To document the marketing and distribution of nano-enabled products into the commercial marketplace, the Woodrow Wilson International Center for Scholars and the Project on Emerging Nanotechnologies created the Nanotechnology Consumer Products Inventory (CPI) in 2005. The objective of this present work is to redevelop the CPI by leading a research effort to increase the usefulness and reliability of this inventory. We created eight new descriptors for consumer products, including information pertaining to the nanomaterials contained in each product. The project was motivated by the recognition that a diverse group of stakeholders from academia, industry, and state/federal government had become highly dependent on the inventory as an important resource and bellweather of the pervasiveness of nanotechnology in society. We interviewed 68 nanotechnology experts to assess key information needs. Their answers guided inventory modifications by providing a clear conceptual framework best suited for user expectations. The revised inventory was released in October 2013. It currently lists 1814 consumer products from 622 companies in 32 countries. The Health and Fitness category contains the most products (762, or 42% of the total). Silver is the most frequently used nanomaterial (435 products, or 24%); however, 49% of the products (889) included in the CPI do not provide the composition of the nanomaterial used in them. About 29% of the CPI (528 products) contain nanomaterials suspended in a variety of liquid media and dermal contact is the most likely exposure scenario from their use. The majority (1288 products, or 71%) of the products do not present enough supporting information to corroborate the claim that nanomaterials are used. The modified CPI has enabled crowdsourcing capabilities, which allow users to suggest edits to any entry and permits researchers to upload new findings ranging from human and environmental exposure data to complete life cycle assessments. There are inherent limitations to this type of database, but these modifications to the inventory addressed the majority of criticisms raised in published literature and in surveys of nanotechnology stakeholders and experts. The development of standardized methods and metrics for nanomaterial characterization and labelling in consumer products can lead to greater understanding between the key stakeholders in nanotechnology, especially consumers, researchers, regulators, and industry.

Introduction

Advancements in the fields of nanoscience and nanotechnology have resulted in myriad possibilities for consumer product applications, many of which have already migrated from laboratory benches into store shelves and e-commerce websites. Nanomaterials have been increasingly incorporated into consumer products, although research is still ongoing on their potential effects to the environment and human health. This research will continue long into the future.

To document the penetration of nanotechnology in the consumer marketplace, the Woodrow Wilson International Center for Scholars and the Project on Emerging Nanotechnology created the Nanotechnology Consumer Product Inventory (CPI) in 2005, listing 54 products [ 1 ]. This first-of-its-kind inventory has become one of the most frequently cited resources showcasing the widespread applications of nanotechnology in consumer products. In 2010, the CPI listed 1012 products from 409 companies in 24 countries. Even though it did not go through substantial updates in the period between 2010 and 2013, it continued being heavily cited in government reports [ 2 ] and the scientific literature – the website http://www.nanotechproject.org has been cited over 2,580 times in articles according to Google Scholar – and became a popular indicator of the prevalence of nanotechnology in everyday life and the need to further study its potential social, economical, and environmental impacts [ 3 – 6 ]. The CPI has also been criticized due to its lack of science-based data to support manufacturer claims. Other longstanding suggestions for improvement included: more frequent updates, indications when products were no longer available for purchase by consumers, and the inclusion of more product categories to improve the searchability of the CPI database [ 7 ].

Since the creation of the CPI, other nanotechnology-related inventories have been developed around the world. In 2006, a German company launched a freely accessible internet database of nanotechnology products [ 8 ]. The website associated with this database was not accessible at the time of this writing and its last available record is from May 2014, when 586 products were listed. In 2007, Japan’s National Institute of Advanced Industrial Science and Technology created an inventory of “nanotechnology-claimed consumer products” available in Japan [ 2 ]. This inventory is freely accessible online and it acknowledges the CPI in its website. At the time of this writing, the inventory listed 541 product lines and 1241 products; its last update occurred in 2010 [ 9 ]. In 2009, two European consumer organizations, the European Consumers Organization (BEUC) and the European Consumer Voice in Standardization (ANEC), joined efforts to develop an inventory of “consumer products with nano-claims” available to consumers in Europe [ 10 ]. A new inventory was generated annually from 2009 to 2012, but the 2011 and 2012 versions focused exclusively on products containing silver nanoparticles (nanosilver); the latest version in 2012 listed 141 nanosilver products. This inventory does not provide a searchable online database, but it can be downloaded for free as an Excel spreadsheet. In 2012, the Danish Consumer Council and Ecological Council and the Technical University of Denmark’s Department of Environmental Engineering launched “The Nanodatabase”, an inventory of products available for purchase that are claimed to contain nanomaterials and are available in the European consumer market [ 11 ]. This inventory has been continually updated and it currently lists 1423 products.

These worldwide efforts to understand the transition of nanotechnology from the laboratory bench to the commercial marketplace substantiate the need for applying the concept of nanoinformatics to a nanotechnology-enabled consumer products database, which is to determine the most relevant and useful information needed by a variety of stakeholders and to develop tools for its most effective use [ 12 ]. Databases such as the CPI offer information useful and relevant to a variety of stakeholders who are interested in a) understanding which consumer products incorporate nanotechnology and b) developing strategies, tools, and policies that may be needed to ensure safe and responsible use of those products.

Nanomaterials are regulated without specific provisions in the U.S. as hazardous chemical substances and pesticides, under the EPA’s Toxic Substances Control Act (TSCA) [ 13 ] and the Federal Insecticide, Fungicide, and Rodenticide Act (FIFRA) [ 14 ]. When used as food additives, drugs, or cosmetics, nanomaterials are regulated under the Federal Food, Drug, and Cosmetic Act (FFDCA).

In the European Union, nanomaterials are regulated under the Concerning the Registration, Evaluation, Authorization and Restriction of Chemicals (REACH) and the Classification, Labeling, and Packaging (CLP) regulations when those are classified by the Commission as hazardous chemical substances [ 15 ]. The Biocidal Products Regulation (BPR) has special provisions for biocidal materials that consist of nanoparticles, aggregates, or agglomerates in which at least 50% of primary particles have at least one dimension between 1 and 100 nm, with no provisions for “novel properties” stemming from their small size [ 16 ]. Cosmetics that contain nanomaterials are also regulated by the European Commission, and although the use of nanoscale titanium dioxide is permitted, zinc oxide is not [ 17 ]. The German Federal Environment Agency performed an Impact Assessment of a European Register of Products Containing Nanomaterials and determined that when compared to the implementation of a variety of national registries, an unified European registry would bring many advantages, including a lower cost for industries and, ultimately, a registry would benefit consumers, companies, and governments [ 18 ].

The objective of this work was to modify the CPI to improve its functionality, reliability, and utility to the diverse group of stakeholders who have come to depend on it as a critical resource for current information on nano-enabled consumer products. Specific objectives were (1) to update the CPI data to gain an insight into the penetration of nanotechnology in the consumer products market over the past decade; (2) to determine and implement improvements to the CPI based on the scientific literature and a survey of nanotechnology experts and CPI users; and to (3) develop a sustainable model to facilitate future CPI maintenance using crowdsourcing tools.

Below, we present a brief history of this inventory over a decade of existence. We also describe the specific changes made in the inventory during this project (referred here as CPI 2.0). Finally, we present an overview of the current data present in the CPI after the completion of this project.

Results and Discussion

Cpi growth over time.

Table 1 lists the growth of the CPI since 2005. In 2011, before this current project, the CPI described 1314 products. Since then, 489 products that are no longer available or marketed as containing nanotechnology have been archived and 500 products have been added. The new total of 1814 products as of March 2015 represents a thirty-fold increase over the 54 products originally listed in 2005 – which is not a complete representation of the growth of this market, as our methodology has also evolved over time. Based on our review, the CPI is the largest online inventory of nanotechnology consumer products available. Products come from 622 companies located in 32 countries ( Supporting Information File 1 , Table S1).

Number of products in the CPI over time.

a The CPI now has crowdsourcing capabilities, so these numbers are a snapshot in time and will not represent the CPI at the time of reading.

The products listed on the CPI 2.0 satisfy three criteria: (1) they can be readily purchased by consumers; (2) they are claimed to contain nanomaterials by the manufacturer or another source; and (3) their claim to contain nanomaterials appears reasonable to CPI curatorial staff.

Although the steady growth of the inventory indicates that the popularity of products claimed to incorporate nanotechnology is continually increasing, not all products have persisted in the consumer market. In the past seven years, 34% of the entries in the inventory have been archived because the product is not currently available in the market or their claim to contain nanotechnology can no longer be verified. One example of a claim that can no longer be verified is a product that is still available for purchase on a manufacturer’s website but no longer references, explicitly, the incorporation of nanotechnology into that product. Even after archiving, a product can return to the main inventory listing if a third party makes the claim that the product indeed contains nanomaterials or if the manufacturer restates their nanomaterial claim.

In the CPI, entries are grouped under eight generally accepted consumer goods categories that are loosely based on publicly available consumer product classification systems ( Figure 1 ) [ 19 ]. The Health and Fitness category includes the largest listing of products in the CPI, comprising 42% of listed products (excluding archived products). Within the Health and Fitness category, Personal Care products (e.g., toothbrushes, lotions, and hairstyling tools and products) comprise the largest subcategory (39% of products). Starting in 2012, a large continual effort has been put into periodically checking products for their current availability and current claim to contain nanotechnology. This effort resulted in archiving 316 products in the Health and Fitness category – mainly in the Personal Care and Clothing subcategories – with 86 and 78 products archived between 2012 and 2014, respectively.

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Object name is Beilstein_J_Nanotechnol-06-1769-g002.jpg

Number of available products over time (since 2007) in each major category and in the Health and Fitness subcategories.

New nanomaterial descriptors

Eight new product descriptors were introduced to facilitate the use of this database by a variety of stakeholders (namely industry and the scientific and regulatory communities):

  • main nanomaterial composition or type,
  • nanomaterial shape and size,
  • nanomaterial coating or stabilizing agent,
  • nanomaterial location within the product,
  • nanomaterial function in the product,
  • potential exposure pathways,
  • “how much we know”,
  • “researchers say”.

The experimental section of this paper describes all new product descriptors. The results of the five new quantitative descriptors are presented and discussed below. Since the “nanomaterial shape and size”, “coating and stabilizing agent”, and the “researchers say” categories are text-entry data fields, thus qualitative information at this point, we have not included their analysis in this paper.

Nanomaterial composition

Of the 1814 products listed in the CPI, 47% (846 products) advertise the composition of at least one nanomaterial component and 62 of those products list more than one nanomaterial component (e.g., a product comprised of both silver and titanium dioxide nanomaterials). There are 39 different types of nanomaterial components listed in the inventory (listed in Supporting Information File 1 , Table S2), which have been grouped into five major categories in Figure 2 and Figure 4 , to improve their legibility: metal, carbonaceous, silicon, not advertised, and other. Nominally, metals and metal oxides comprise the largest nanomaterial composition group advertised in the inventory, listed in 37% of products.

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Object name is Beilstein_J_Nanotechnol-06-1769-g003.jpg

(a) Claimed composition of nanomaterials listed in the CPI, grouped into five major categories: not advertised, metal (including metals and metal oxides), carbonaceous nanomaterials (carbon black, carbon nanotubes, fullerenes, graphene), silicon-based nanomaterials (silicon and silica), and other (organics, polymers, ceramics, etc.). (b) Claimed elemental composition of nanomaterials listed in the metals category: silver, titanium, zinc, gold, and other metals (magnesium, aluminum oxide, copper, platinum, iron and iron oxides, etc.). (c) Claimed carbonaceous nanomaterials (CNT = carbon nanotubes).

An external file that holds a picture, illustration, etc.
Object name is Beilstein_J_Nanotechnol-06-1769-g005.jpg

Major nanomaterial composition pairs in consumer products. Carbonaceous nanomaterials (carbon black, carbon nanotubes, fullerene, and graphene) were combined into the same category (carbon). Grey boxes in the diagonal represent the total times each nanomaterial composition has been listed with other compositions in the same product.

Titanium dioxide (TiO 2 ), silicon dioxide, and zinc oxide are the most produced nanomaterials worldwide (on a mass basis) and the global annual production of silver nanoparticles represents only 2% of that of TiO 2 [ 20 – 21 ]. However, silver nanoparticles are the most popular advertised nanomaterial in the CPI, present in 438 products (24%). The CPI reports the numbers of different consumer products and product lines available in the market, so there is no implication on mass, volume, or concentration of nanomaterials incorporated into products or the production volume of each product.

Of carbonaceous nanomaterials (89 products), the majority of products listed contains carbon nanoparticles (sometimes described as carbon black, 39 products) and single- or multi-walled carbon nanotubes (CNT, 38 products). Unfortunately, 891 (49%) of the products included in the CPI do not present the composition or a detailed description of the nanomaterial used ( Figure 2 ).

The percentages of nanomaterial compositions in the CPI 2.0 are somewhat in agreement with those of the Danish Nanodatabase. The Nanodatabase also lists a high fraction of products with unknown nanomaterial composition (944 products or 66%) and, among known compositions, silver is also the most frequently advertised nanomaterial component, with 207 products or 14.5% [ 11 ]. Silver nanoparticles are popular consumer product additives due to their well-documented antimicrobial properties [ 22 ].

Figure 3 shows how the availability of these major nanomaterial composition groups changed over time. Since the start of the CPI 2.0 project (2012), products with unknown (not advertised) nanomaterial compositions have decreased by 12%, which is partially due to these products being archived and of their composition being identified and added to the inventory. Products advertising to contain metal and metal oxide nanomaterials, silicon-based nanomaterials (mostly SiO 2 nanoparticles), and a variety of other nanomaterial components (organics, ceramics, polymers, clays, nanocellulose, liposomes, nano micelles, carnauba wax, etc.) have been growing in popularity. During the same period, carbonaceous nanomaterials have remained stable at around 50 products available in the market.

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Object name is Beilstein_J_Nanotechnol-06-1769-g004.jpg

Major nanomaterial composition groups over time. Carbon = carbonaceous nanomaterials (carbon black, carbon nanotubes, fullerenes, graphene). Other = organics, ceramics, polymers, clays, nanocellulose, liposomes, nano micelles, carnauba wax, etc. Note the difference in scale between the top and bottom panels in this plot.

Of the 846 products listed in the CPI for which we were able to determine a nanomaterial composition, 61 products (7%) advertise to contain more than one main nanomaterial component. Figure 4 presents 11 nanomaterial components that were most frequently listed with others in the same product.

Silver and titanium dioxide are the nanomaterial components most likely to be combined with other nanomaterials in consumer products, with 35 and 30 product combinations, respectively. Silver and titanium dioxide were paired with each other in 10 products (cosmetics and electronics); titanium dioxide and zinc oxide were paired in 10 products (sunscreens, cosmetics, and paints). The European Commission’s Cosmetics Regulation has permitted the use of nanoscale titanium dioxide in sunscreens, but not zinc oxide [ 17 ].

Calcium and magnesium were listed together in dietary supplements. Nano-ceramics and silver are used in combination in water filtration products, cosmetics, and a humidifier. These results demonstrate the use of nanohybrids [ 23 ] in consumer products and indicate that the use of nanotechnology-based consumer products in the home may, in some cases, lead to multiple exposures from a combination of nanomaterial compositions. These results suggest the need to examine nanomaterial toxicity effects that could be synergistic, additive, or even antagonistic.

Nanomaterial location

About 29% of consumer products in the CPI (528 products) contain nanomaterials suspended in a variety of fluids (e.g., water, skin lotion, oil, car lubricant). The second largest group in this category – with 307 products – comprises solid products with surface-bound nanoparticles (e.g., hair curling and flat irons, textiles). Figure 5 shows the location of nanomaterials for which a composition has been identified [ 24 ].

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Locations of nanomaterials in consumer products for which a nanomaterial composition has been identified.

The majority (64%) of carbonaceous nanomaterials are embedded in solid products, whereas products of all other compositions are more commonly suspended in liquid. Of the few bulk nanomaterials that are available for purchase by consumers, the largest group (42%) consists of metal and metal oxide nanomaterials. Metals and metal oxides were also the largest composition for surface-bound particles and those suspended in liquid products. The majority (67%) of products with nanostructured surfaces consist of nanomaterials of undetermined composition. An example of such product is a liquid or spray products that forms a nanofilm upon application over a surface. Of nanostructured bulk materials, the majority (57%) are silicon-based nanomaterials (e.g., computer processor parts). It is interesting to note that we expect nano-electronics to exist now in massive numbers of consumer products, such as mobile devices, where field effect transistors, the heart of chip technology, have components (sources, gates, collectors, channels) that are now in the nanoscale [ 25 ] and would fit into the nanostructured bulk category. However, because most of these products do not advertise their use of nanomaterials, we believe that they are grossly underrepresented in the CPI.

Nanomaterial function

Of the 1814 inventory entries, 1244 were grouped according to the expected benefits of adding such nanomaterials to the product ( Figure 6 ). A significant portion of products in the CPI (31% of products analyzed) utilize nanomaterials – mostly silver nanoparticles, but also titanium dioxide and others – to confer antimicrobial protection. Nanomaterials such as titanium dioxide and silicon dioxide are used to provide protective coatings (15%) and for environmental treatment (to protect products against environmental damage or to treat air and water in the home, 15%). Cosmetic products (12%) are advertised to contain a variety of nanomaterials such as silver nanoparticles, titanium dioxide, nano-organics, gold, and others. A wide variety of nanomaterial compositions (silver, nano-organics, calcium, gold, silicon dioxide, magnesium, ceramics, etc.) were also advertised to be used for health applications, such as dietary supplements (11%).

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Object name is Beilstein_J_Nanotechnol-06-1769-g007.jpg

Expected benefits of incorporating nanomaterial additives into consumer products.

Potential exposure pathways

Since critical information such as nanomaterial size and concentration are not known for most products listed on the CPI, the actual health risks of these products remain largely unknown. Nevertheless, the CPI may be useful for inferring potential exposure pathways from the expected normal use of listed products. To investigate this utility, we analyzed a subset of 770 products from the CPI to determine their most likely route(s) of exposure ( Figure 7 ).

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Object name is Beilstein_J_Nanotechnol-06-1769-g008.jpg

Potential exposure pathways from the expected normal use of consumer products, grouped by major nanomaterial composition categories.

We identified the skin as the primary route of exposure for nanomaterials from the use of consumer products (58% of products evaluated). This is because many entries in the CPI consist of (1) solid products that contain nanomaterials on their surfaces and are meant to be touched or (2) liquid products containing nanomaterial suspensions which are meant to be applied on the skin or hair. Of the products evaluated, 25% present nanomaterials that can possibly be inhaled during normal use (e.g., sprays and hair driers) and 16% contain nanomaterials that may be ingested (e.g., supplements and throat sprays). Hansen et al. developed a framework for exposure assessment in consumer products. In this framework, products that contain nanomaterials suspended in liquid and products that may emit airborne nanoparticles during use are expected to cause exposure [ 26 ].

Since metals and metal oxides are the most common nanomaterial composition in the CPI, they are also the most likely materials to which consumers will be exposed during the normal use of product via dermal, ingestion, and inhalation routes. Products containing nanomaterials of unknown composition are most likely to lead to exposure via the dermal route.

Berube et al. [ 7 ] offered a critique of the original CPI in 2010, which focused primarily on the lack of data pertinent to the dosages of nanomaterials to which consumers might be exposed through CPI-listed products. This is a valid criticism given that information used to populate the CPI is based primarily on marketing claims made by manufacturers. However, the most recent modifications of the CPI offer a potential remedy for data gaps through the contributions of third-party research teams. These modifications are especially timely as there is a growing number of published studies assessing consumer exposure to nanomaterials released during the use of nanotechnology-enhanced consumer products [ 27 ], such as cosmetic powders [ 28 ], sprays [ 29 – 30 ], general household products [ 31 ], and products for children [ 32 – 33 ]. One challenge is that there are no standardized methods for assessing consumer risks from using nanotechnology-enabled consumer products or a set of agreed-upon metrics for characterizing nanomaterials to determine environmentally relevant concentrations [ 34 ]. The development of such standards is seen as a top strategy for safe and sustainable nanotechnology development in the next decade [ 35 ]. The Consumer Product Safety Commission recently requested $7 million to establish the Center for Consumer Product Applications and Safety Implications of Nanotechnology to help develop methods to identify nanomaterials in consumer products and to understand human exposure to those materials [ 36 ].

How much we know

Through the “How much we know” descriptor, inventory entries are rated according to the reliability of the manufacturer’s claim that products contain nanomaterials. We evaluated 1259 products present in the inventory for the “How much we know” descriptor and the majority (71%) of products are not accompanied by information sufficient to support claims that nanomaterials are indeed used in the products, such as a manufacturer datasheet containing technical information about nanomaterial components (e.g., median size, size distribution, morphology, concentration). Only nine products have been classified in Category 1, “Extensively verified claim” due to the availability of scientific papers or patents describing the nanomaterials used in these products ( Figure 8 ). The experimental section, below, presents a full description of these categories.

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Distribution of products into the “How much we know” categories.

Hansen [ 37 ] performed interviews with 26 nanotechnology stakeholders who agreed on an incremental approach to nanomaterial regulation in consumer products, including classification and labeling. The European Commission’s Classification, Labeling, and Packaging (CLP) regulation covers nanomaterials that are classified by the Commission as hazardous chemical substances [ 15 ]. Becker [ 38 ] reported that there are diverging opinions in the nanotechnology industry with regards to labeling, ranging from ‘‘If it’s a nano-scale material, people should know, hands down” to not supporting labeling because “it wouldn’t accurately inform consumers of anything and would be bad for business because it would scare consumers.”

Appropriate nanomaterial labeling containing sufficient technical information (i.e., at a minimum, nanomaterial composition, concentration, and median size) would better inform consumers and highly benefit researchers interested in understanding consumers’ exposure and nanomaterial fate and transport in the environment.

Crowdsourcing

Since October 29, 2013, when the modified inventory (CPI 2.0) was released, 557 new user accounts have been requested. Of these, only approximately 10 users who were not directly or indirectly involved in the research team performing the CPI upgrade and maintenance suggested updates or edits to CPI entries. These edits have all been suggested by users from industry and academia.

Future work is needed to better educate users on their role as curators of CPI 2.0 and the importance of the data they contribute. Providing the supporting technical data required to verify the nature and quantity of nanomaterial components in CPI-listed products is a massive undertaking, and no single laboratory can accomplish it on its own or within a short amount of time. A long-term solution is to promote the importance of crowd-sourcing data collection and implementing standard data collection and reporting best practices that can help reliably populate the CPI with much needed supporting data. The new crowd-sourcing capability can also be used to provide high school-, undergraduate- and graduate-level educators with meaningful assignments that can help teach students about the prevalence of nanotechnology in everyday products and will contribute to the continued growth of this resource.

Nanotechnology expert survey

The survey was submitted to 147 people who have published research papers or reports in the applications of nanotechnology in consumer products and its potential impacts, participated in recent conferences in the field, or were notably involved in the field of nanotechnology and the consumer products industry. The survey had a 46% response rate (68 respondents), which is in the expected range for this type of survey [ 39 ]. The majority of respondents (59%) had six to ten years of experience working with nanotechnology and 38% of respondents had more than ten years of experience. Half (51%) of respondents work in academic institutions and 25% work in governmental agencies. Most respondents (88%) have previously used the CPI in their work, and all respondents believe they will or may use it again in the future.

Results convey a general belief or hope that the CPI will become more useful after the modifications reported in this publication. When asked the following open-ended questions: “How did you use the CPI in your work?” and “To what end do you think you might use the CPI in the future?”, answers could be easily grouped into three main categories: (1) for raising awareness, teaching, or for urging the need for regulation, (2) to justify the need for research in research proposals or papers, and (3) to use the inventory data for research ( Figure 9 ).

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Nanotechnology survey answers on how respondents have used the CPI in the past and how they might use it in the future.

Half the respondents (51%) have used the CPI in the past to gather data for research (e.g., searching for consumer products of a certain nanomaterial composition to understand their potential applications or consumer exposure) while 74% believe they will use the CPI for that purpose in the future. The majority (79%) of survey respondents believed the modified CPI would present more products than its previous version, which indicates their belief in the growing prevalence of nanotechnology in consumer products.

Survey respondents suggested a number of new categories of information for the CPI 2.0, including nanomaterial type or composition, location of nanomaterial within the product, nanomaterial size, relevant scientific publications that describe the products in the inventory, a summary of known toxicity of the advertised nanomaterial, supply chain information, volume produced, and life cycle assessment information.

Most of these suggestions were included in the CPI 2.0 as the new categories described in this work. Others, such as known nanomaterials toxicity were not pursued since toxicity can vary greatly depending on particle size, coating, and exposure route (e.g., inhalation versus ingestion).

Piccinno et al. and Keller et al. provide global estimates for production and major applications of nanomaterials [ 20 – 21 ]. We recommend that future work associated with this inventory or others include information on the production volumes for each product, since this information is presently unavailable.

Additional results from this survey are available in Supporting Information File 1 .

The modified version of the Wilson Center’s nanotechnology consumer products inventory (CPI 2.0) was released in October 2013. We improved the searchability and utility of the inventory by including new descriptors for both the consumer products and the nanomaterial components of those products (e.g., size, concentration, and potential exposure routes). The updated CPI 2.0 now links listed products to published scientific information, where available, and includes a metric to assess the reliability of the data associated with each entry. Finally, the CPI 2.0 has enabled crowdsourcing capabilities, which allow registered users to upload new findings such as basic product composition information, human and environmental exposure data, and complete life cycle assessments. There are inherent limitations to this type of database, but recent improvements address the majority of issues raised in published literature and in a survey of nanotechnology experts.

Improvements to the CPI were motivated, in part, by the recognition that it represents and will continue to represent an important information resource for a broad range of stakeholders, especially consumers and the academic and regulatory communities. The CPI is a useful interactive database for educating consumers and legislators on the real-world applications of nanotechnology. Michaelson stated that the CPI transformed “the face of nanotechnology away from innovations in the realm of science fiction to the iconic images of everyday consumer products” [ 2 ]. The academic community can continue to make use of this inventory to help prioritize, for example, which types of products or nanomaterial components to evaluate in human exposure or toxicity studies, life cycle assessments, and nanomaterial release studies.

The CPI is useful for policy makers interested in regulating nanotechnology in consumer products by understanding their increasing numbers in the market, the main nanomaterial components that are chosen by manufacturers, and the likelihood for exposure. Beaudrie et al. [ 40 ] urge that there should be regulatory reforms to improve oversight of nanomaterials throughout their life cycle.

Finally, the current lack of global standardized methods and metrics for nanomaterial characterization and labeling in consumer products is an issue that, if addressed, can lead to greater understanding between the key stakeholders in nanotechnology, especially researchers, regulators, and industry. Further, as we recognize the growing importance of tools like the CPI for the needs of diverse stakeholder groups, steps should be taken to help ensure that those tools are fully developed and refined to meet those needs.

Experimental

To determine potentially useful improvements for the CPI, we developed a web-based survey to gather the informed opinions of nanotechnology experts – mostly in US-based academic institutions, governmental agencies, and research centers. Their answers guided the CPI modifications and provided an idea of the expectations related to the inventory. The survey questions are presented in the Supporting Information File 1 .

New descriptors

To improve the utility and searchability of this database, seven product descriptors were created. Entries in the inventory were revised to go beyond a categorization of the consumer products and instead, to include more information on the nanomaterials themselves. We searched for this information mainly on the internet – on manufacturer’s websites, retailer’s websites, news sites and blogs, patents – and, when available, product labels.

The main composition of the nanomaterials used. This information, when available, was added to the database in the form of a check-box list, in which more than one nanomaterial composition can be selected for each consumer product.

Nanomaterial shape and size

Because there are many different ways in which manufacturers can measure and describe the shape and size of nanomaterials in consumer products (i.e., units of nanometers or micrometers, thickness of nanofilms, diameter or length of fibers or tubes, diameter or radius of nanoparticles, maximum, median, average, or minimum size), this descriptor was added as a text entry field in the database, which allows for any form of data entry but makes data analysis cumbersome.

We created another text entry field in the CPI to include any available information on the coatings or stabilizing agent used along the nanomaterials in each product.

To assist CPI users in understanding the potential for nanomaterial release and exposure scenarios from the use of these consumer products, we created a qualitative descriptor for the location of nanomaterials within each product. We adapted the categorization framework for nanomaterials from Hansen et al. [ 24 ] to determine the following nanomaterial locations within products:

  • Bulk: Nanomaterials sold in powder form or in liquid suspensions
  • Nanostructured bulk: Products or parts that contain nanostructured features in bulk (e.g., nanoscale computer processors)
  • Nanostructured surface: Products or parts that contain nanostructured features on their surface (e.g., nanofilm-coated products)
  • Surface-bound particles: Nanoparticles added to the surface of a solid product or part (e.g., a computer keyboard coated with silver nanoparticles for antimicrobial protection)
  • Suspended in liquid: Nanomaterials suspended in a liquid product (e.g., disinfecting sprays, liquid supplements)
  • Suspended in solid: Nanomaterials suspended in a solid matrix, usually plastic or metal (e.g., composites of carbon nanotubes in a plastic matrix to confer strength).

We created a metric to describe the reason why nanotechnology was added to each consumer product or the function it performs within each product. We investigated a subset of 1244 products in the CPI for each product’s intended use, the manufacturer claims, and, most importantly, the type or composition of nanomaterials used to infer potential nanomaterial functions (e.g., antimicrobial protection, hardness and strength, pigment).

Using methodology similar to that applied for the “nanomaterial functions” category, we investigated the CPI entries for possible exposure scenarios resulting from the expected normal use of each consumer product. Entries were only populated when a potential exposure risk was identified.

In an effort to verify the data associated with each product listed on the CPI, we created a metric called “How much we know”. Products were divided into five categories based on the information available to substantiate manufacturer claims that a particular product contains nanomaterial components ( Table 2 ). Category 4, “Unsupported claim”, is the default category for products added to the CPI based soley on a manufacturer’s marketing claims. A product can rise in ranking according to the amount of information that is available to corroborate the manufacturer’s claim that the product contains nanomaterials. If the manufacturer provides supporting information (e.g., a datasheet containing electron micrographs showing the nanomaterials or a particle size distribution), the product is placed in Category 3, “Manufacturer-supported claim”. If a third-party further supports the information provided by the manufacturer, such as through a publication or technical report, then the product can be placed into Category 2, “Verified claim”. If a product is backed by multiple science-based sources (e.g., a peer-reviewed scientific paper or patent documentation), it is then placed in Category 1, “Extensively verified claim”. Category 5, “Not advertised by the manufacturer”, is a special class for products that have been shown to contain nanomaterials but the manufacturer does not advertise this fact anywhere in product labeling or other informational materials. Category 5 has been added in recognition of the fact that not all nano-enabled products are marketed by manufacturers as such.

“How much we know” categorization, based on the information available to substantiate manufacturer claims that a particular product contains nanomaterial components.

Researchers say

In order to add available scientific information to the inventory, we created a text-entry database field named “Researchers say”, which makes it possible to include an extract from a research paper (such as the abstract), author citation, and a link to the paper.

We added a new crowdsourcing capability to the CPI website so that consumers, manufacturers, and the greater scientific community can contribute new information on nanomaterial composition of CPI products to the inventory. New contributors must request an account by completing a form with their contact information, and they must provide a reason why they would like to gain access to this crowdsourcing tool. Accounts are manually reviewed. Access is granted to all requesters who complete the form and have a legitimate purpose for contributing information. Once an account is created, users may sign in and suggest edits to any product (including the archiving of products no longer available or no longer advertising to contain nanomaterials) or suggest new products to the inventory. As a quality control measure, suggestions and new product forms contributed by registered users must be approved by a CPI curator before updates or revisions are posted to the inventory.

Supporting Information

A compilation of company and product numbers listed by country of origin. A list of all nanomaterial components included in the inventory. Nanotechnology expert survey questions. Additional nanotechnology expert survey results.

Acknowledgments

Funding for this work was provided by the Institute for Critical Technology and Applied Science (ICTAS) at Virginia Tech and the Virginia Tech Center for Sustainable Nanotechnology (VTSuN). We acknowledge the important help of J. Rousso, E. Bruning, S. Guldin, J. Wang, D. Yang, X. Zhou, L. Marr, the VTSuN graduate students in updating inventory entries, and the Laboratory for Interdisciplinary Statistical Analysis at Virginia Tech. We also acknowledge the Center for the Environmental Implications of Nanotechnology, funded under NSF Cooperative Agreement EF-0830093, for helping to inform our understanding of the broad world of manufactured nanomaterials. Co-author M. Hull acknowledges helpful discussions with A. Maynard of the Arizona State University Risk Innovation Lab that provided important motivation for this work.

This article is part of the Thematic Series "Nanoinformatics for environmental health and biomedicine".

Animal and non-animal experiments in nanotechnology - the results of a critical literature survey

Affiliation.

  • 1 Scientific Consultancy - Animal Welfare, Neubiberg/Munich, Germany.
  • PMID: 19565168
  • DOI: 10.14573/altex.2009.2.109

A literature survey funded by the Foundation Animalfree Research was performed to obtain an overview on animal experiments in nanotechnology. Scientific articles from Germany, France, the United Kingdom, Italy, the Netherlands and Switzerland published between 2004 and 2007 were collected. A total of 164 articles was retrieved covering in vivo nanotechnological research. The majority of animal experiments were conducted in "nanomedicine", i.e. nanotechnology in the health care area, to study targeted drug, vaccine or gene delivery. Further areas of research relate to nanotechnology-based imaging technologies, the toxicity of nanomaterials, tissue engineering for regenerative treatments, and magnetic tumour thermotherapy. Many experiments were classified as moderately and even severely distressful to the animals. Due to the significance of the scientific topics pursued, the possible scientific benefit of the research depicted in the articles is also assigned to be moderate to high. Nevertheless, it has to be asked whether such animal experiments are truly the only means to answer the scientific questions addressed in nanotechnology. An overview on non-animal test methods used in nanotechnological research revealed a broad spectrum of methodologies applied in a broad spectrum of scientific areas, including those for which animal experiments are being performed. Explicit incentives to avoid animal experiments in nanotechnology currently can only be found in the area of nanotoxicology, but not in the area of nanomedicine. From the point of view of animal welfare, not least because of the new technologies that arise due to nanotechnology, it is time for a paradigm change both in fundamental and applied biomedical research to found research strategies on non-animal test methods.

Publication types

  • Research Support, Non-U.S. Gov't
  • Animal Testing Alternatives / methods*
  • Animal Welfare
  • Cost-Benefit Analysis
  • Drug Delivery Systems
  • Nanotechnology / economics*
  • Nanotechnology / methods*

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