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© 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim wileyonlinelibrary.com (1 of 29) 1602423

Green Processing of Carbon Nanomaterials

Masuki Kawamoto, Pan He, and Yoshihiro Ito*

DOI: 10.1002/adma.201602423

1. Introduction

Carbon nanomaterials (CNMs) have received a great deal of attention over the past three decades because of their unique characteristics.[1] Kroto et al. prepared the sphere-like nano-carbon fullerene, a so-called buckyball, in 1985.[2] In 1991, Iijima discovered the cylindrical structure of a carbon nanotube (CNT) using electron microscopy,[3] and Novoselov et al. iso-lated and characterized a single atomic plane of graphene from graphite using a Scotch tape technique in 2004.[4] Because these three carbon allotropes are composed of an extended hexagonal lattice of sp2-bonded carbon atoms, they possess exceptional

Carbon nanomaterials (CNMs) from fullerenes, carbon nanotubes, and graphene are promising carbon allotropes for various applications such as energy-conversion devices and biosensors. Because pristine CNMs show substantial van der Waals interactions and a hydrophobic nature, precipita-tion is observed immediately in most organic solvents and water. This inevi-table aggregation leads to poor processability and diminishes the intrinsic properties of the CNMs. Highly toxic and hazardous chemicals are used for chemical and physical modification of CNMs, even though efficient dispersed solutions are obtained. The development of an environmentally friendly dispersion method for both safe and practical processing is a great challenge. Recent green processing approaches for the manipulation of CNMs using chemical and physical modification are highlighted. A summary of the current research progress on: i) energy-efficient and less-toxic chemical modification of CNMs using covalent-bonding functionality and ii) non-covalent-bonding methodologies through physical modification using green solvents and dispersants, and chemical-free mechanical stimuli is provided. Based on these experimental studies, recent advances and challenges for the potential application of green-processable energy-conversion and biological devices are provided. Finally, a conclusion section is provided summarizing the insights from the present studies as well as some future perspectives.

Dr. M. Kawamoto, Dr. P. He, Dr. Y. ItoEmergent Bioengineering Materials Research TeamRIKEN Center for Emergent Matter Science (CEMS)2-1 Hirosawa, Wako, Saitama 351-0198, JapanE-mail: [email protected]. M. Kawamoto, Dr. Y. ItoNano Medical Engineering LaboratoryRIKEN, 2-1 Hirosawa, Wako, Saitama 351-0198, JapanDr. M. KawamotoPhotocatalysis International Research CenterTokyo University of Science2641 Yamazaki, Noda, Chiba 278–8510, Japan

electrical, chemical, and mechanical prop-erties that lead to many potential appli-cations such as energy conversion and storage devices, catalysts, carbon fibers, and biosensors.[5–7] Currently, graphene has been widely explored in research and development (R&D) in the United States, Europe, China, South Korea, and Japan. Particularly, China has focused not only on R&D projects but also on technology transfer from basic research to commer-cial products. Local patents from China for graphene technologies account for 38% of all the graphene patents in the world.[8] Investment committed to gra-phene research in the world is still lim-ited (US$12 million in 2013); however, the marketing research firm Lux Research predicted the global market for graphene would be worth US$349 million by 2025.[9]

Graphite is a simple layered two-dimen-sional (2D) structure, which is composed of an sp2-hybridized honeycomb lattice with alkene bond length of 0.142 nm (Figure 1). Each graphitic layer has non-covalent binding through van der Waals

interaction with an interlayer spacing of 0.335 nm.[10] When breaking of the weak interactions between the sheets is induced chemically or physically, a single layer of graphene is obtained. The resultant graphene exhibits specific properties: exceptional tensile strength of 130 GPa,[11] remarkable electron mobility of 15 000 cm2 V−1 s−1 at room temperature,[12] and a huge theoret-ical surface area of 2630 m2 g−1,[13] even though the thickness of graphene is at least six orders of magnitude thinner than that of a sheet of paper.[14]

If 2D graphene is wrapped or rolled, the carbon allotropes of zero-dimensional fullerenes or one-dimensional (1D) CNTs are observed (Figure 1). Although fullerenes are the first iso-lated CNMs, the use of fullerenes is inconspicuous compared with that of CNTs and graphene. However, fullerenes are a good candidate as electron acceptors with low reorientation energies for organic photovoltaic cells (OPVs). After the report of photoinduced electron transfer from electron-donating conducting polymers to fullerenes,[15] bulk heterojunctions, bicontinuous and interpenetrating donor and acceptor net-works, have been developed.[16] A soluble fullerene derivative of [6]-phenyl-C61-butyric acid methyl ester (PC61BM) is one of the most effective electron acceptors, and is a practical choice for solution-processed OPV applications.[17]

Single-walled carbon nanotubes (SWCNTs) are composed of a single layer of graphene, and multi-walled carbon nanotubes (MWCNTs) consist of several layers of graphene. The electronic

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properties of CNTs depend on the curvatures determined by the chiral vector, which identifies the circumference with respect to the graphene lattice. SWCNTs exhibit metallic or semicon-ducting properties depending on their chiral vectors, whereas MWCNTs show metallic properties with current carrying capacity of 109 A cm−1. Furthermore, CNTs possess specific mechanical properties (tensile strength: 100 GPa) with flex-ibility (elastic modulus: 1 TPa), even though they are 105 times thinner than human hair.[18] Carbon nanohorns (CNHs) are dahlia-like aggregated nanocarbons consisting of horn-shaped graphene with a tubular length of 40–50 nm and a diameter of 2–3 nm.[19] In comparison with SWCNTs, CNHs have practical advantages because of their facile synthesis without the use of a toxic metal catalyst at room temperature, and potential large-scale production (1 kg day−1) at high purity (≈95%).[20] Because the sidewall of the nanohorns can be opened by thermal oxi-dation, the resultant CNHs exhibit nanoscale internal space, which leads to a large surface area and pore volume up to 1300 m2 g−1 and 0.9 mL g−1, respectively.[21]

CNMs have made important contributions to practical appli-cations and development of science. Unfortunately, common to all pristine carbon allotropes is that they are difficult to dis-perse as a result of fatal aggregation. Because the materials show substantial van der Waals interactions and a hydrophobic nature, precipitation is observed immediately in most organic solvents and water. This inevitable aggregation leads to poor processability and diminishes the intrinsic properties of CNMs. To overcome these drawbacks, improvement of the dispersion properties has been made by chemical and physical modifica-tion. Chemical modification is based on the covalent linkage of functional groups to the surface of hexagonal sp2-bonded carbon atoms or in defect sites on the sidewalls of CNMs.[22,23] Chemical functionalization is expected to be one of the most promising strategies for increasing the dispersion ability. For example, Hummer’s method is an effective and conventional

Masuki Kawamoto received his M.Sc. and Ph.D. in polymer chemistry from Tokyo Institute of Technology under the supervision of Professor Tomiki Ikeda in 2000 and 2003, respectively. He was a postdoctoral scholar with Professor Shaw H. Chen and Professor Ching W. Tang at University of Rochester, NY, USA and then

joined RIKEN in 2004. He is currently a Senior Research Scientist of Emergent Bioengineering Materials Research Team at RIKEN Center for Emergent Matter Science (CEMS), and Nano Medical Engineering Laboratory at RIKEN. His current research focuses on development of green-processable functionalized polymers for energy-conversion and sensing devices.

Pan He obtained her Ph.D. in Polymer Chemistry and Physics from the Graduate University of Chinese Academy of Sciences, Changchun Institute of Applied Chemistry under the supervision of Professor Xuesi Chen in 2012. Her research activities include functionalized biodegrad-able polymers for drug and

gene delivery. She is currently a postdoctoral researcher in Emergent Bioengineering Materials Research Team at RIKEN CEMS. Her current research interest is devel-opment of thermocleavable conjugated polymers and aqueous-processed carbon nanomaterials for thermoelec-tric devices.

Yoshihiro Ito is Team Leader of Emergent Bioengineering Materials Research Team at RIKEN CEMS (from 2013), and Chief Scientist and Director of the Nano Medical Engineering Laboratory at RIKEN (from 2004). He received his doctorate in engineering from Kyoto University in 1987. Since then he has held posts

including Assistant and Associate Professor at Kyoto University, Professor at the University of Tokushima, and Project Leader at the Kanagawa Academy of Science and Technology. His research focuses on biomaterial science, combinatorial bioengineering for the creation of functional-ized polymers, and soft nanotechnology.

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Figure 1. Structures of CNMs. Graphene is a two-dimensional (2D) building material for CNMs of all other dimensionalities. It can be wrapped up into zero-dimensional fullerenes, rolled into one-dimensional (1D) nanotubes or stacked into three-dimensional (3D) graphite. Reproduced with permission.[12] Copyright 2007, Nature Publishing Group.

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oxidation process that can be used to yield water-dispersed gra-phene oxide (GO) through graphite, sodium nitrate, and sul-furic acid in an aqueous solution in the presence of potassium permanganate.[24] However, the resultant GO exhibits chemical functionalization of the hydrophilic groups with structural defects owing to the change in the carbon hybridization from sp2 to sp3. Furthermore, toxic gasses such as nitrogen dioxide and nitrogen peroxide are observed under the harsh reaction conditions.

Conversely, physical modification seems to be a “soft” dis-persion approach[25] because the dispersed solution is achieved using non-covalent interactions between nanocarbons and dis-persants such as organic solvents, surfactants, polymers, and biomaterials. When the dispersants are attached to the surface of the CNMs through hydrophobic or delocalized π stacking interactions, minimal perturbation of the electronic structures can be induced after dispersion. Ultrasonication is an effective external stimulus to obtain homogeneous dispersions in com-bination with dispersants, however, substantial damage such as formation of structural defects and change in size and mor-phology are observed from longer sonication times and high sonication intensities. Furthermore, the intrinsic properties of the dispersants at the surface of the CNMs directly affect their electrical conductivity, chemical stability, and biocompatibility. These drawbacks are a disadvantage for potential applications. Therefore, the development of safe and practical chemical and physical modification is highly desirable.

Solution-processed carbon-based device fabrication is attrac-tive owing to the low-cost, large-area deposition technologies such as roll-to-roll printing.[26,27] Despite the distinctive advan-tages of solution-processed methods, strong acids and bases, hazardous oxidants and reductants, and chlorinated organic solvents such as o-dichlorobenzene (o-DCB) are highly toxic, even though efficient dispersed solutions are obtained using chemical and physical modification. These poisonous chemi-cals are harmful to human health and to the environment. The Occupational Safety and Health Administration (OSHA), an agency of the United States Department of Labor, has estab-lished permissible exposure limits (PELs) for chemical con-taminants.[28] PELs denote the maximum permitted 8 h time-weighted average concentration of an airborne contaminant for an employee. PEL values of conventional organic solvents for dispersion of CNMs such as 1-methyl-2-pyrrolidinone (NMP), N,N-dimethylformamide (DMF), and o-DCB are 1, 10, and 25 ppm, respectively. These chemicals, which can be absorbed into the bloodstream through the skin, are not suitable for bio-logical applications.

The development of environmentally friendly dispersion methods for both safe and practical processing is a great challenge. Among all the chemicals, water is the most abun-dant and biologically safe material on earth. Researchers have investigated the dispersion properties of aqueous-processed CNMs, which offers a potential approach for preventing the use of harmful chemicals.[29] Furthermore, an attractive way to develop mechanical exfoliation of CNMs without any solvents would also be a promising approach. If energy-efficient dis-persion procedures can be established using little or no toxic chemicals, these methodologies would be classified as green processing approaches.

This review aims to describe recent green processing approaches for the manipulation of CNMs using chemical and physical modification (Figure 2). Section 2 focuses on chemical modification of CNMs by covalent bonding. Sections 3 and 4 discuss the physical modification of CNMs using non-covalent-bonding approaches. Section 3 describes solvent-assisted dis-persion methods and solvent-free procedures for CNMs. Sec-tion 4 is devoted to the description of non-covalent-bonding methodologies to disperse CNMs using various dispersants for diverse potential applications in nanotechnology. In Section 5 we highlight recent advances and challenges for energy conver-sion and biotechnological applications and discuss the future potential. Because of limitations in space and the number of references, the discussion is limited to only the most popular examples in recent years.

2. Chemical Modification

2.1. Modification of the sp2 Carbon on the Side Wall

2.1.1. Radical Additions

Because CNMs consist of a hexagonal lattice of sp2-bonded carbon atoms, they give rise to a wide range of chemical reac-tions based on the addition of organic species to alkene bonds (Figure 3). The most effective organic species that have been incorporated into the π bonds of carbon allotropes are organic free radicals. Aryl diazonium salts are commonly used organic intermediates in radical additions (Figure 3a). Bahr et al.

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Figure 2. Graphic outline of this Progress Report. Section 2 discusses the chemical modification of CNMs by covalent bonding. Section 3 focuses on physical mixing dispersion methods for CNMs including solvent-assisted and solvent-free procedures. Section 4 describes non-covalent-bonding methodologies to disperse CNMs using various dispersants. Section 5 is devoted to description of recent advances and challenges for energy conversion and biotechnological applications.

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reported the chemical and electrochemical synthesis of highly functionalized SWCNTs using aryl diazonium salts. The ther-mally induced reaction of SWCNTs with diazonium com-pounds was an effective and mild functionalization method.[30] The resultant functionalized SWCNTs were generated by the action of isoamyl nitrite on aniline derivatives at 60 °C for 12 h. The derivatization of SWCNTs has successfully been induced by electrochemical reduction of aryl diazonium salts.[31] The degree of functionalization by 4-tert-butylbenzene was esti-mated as high as one out of every 20 carbons in the nanotubes bearing a functionalized moiety. Unfortunately, the modified SWCNTs did not show good aqueous dispersion even with the addition of a surfactant.

Dyke et al. demonstrated that aryl diazonium salts can react efficiently with sodium dodecyl sulfate (SDS)-coated SWCNTs in water to form aryl functionalized SWCNTs.[32] The aqueous-processable SWCNTs exhibited improved solubility in DMF (0.8 mg mL−1), chloroform (0.6 mg mL−1), o-DCB (0.7 mg mL−1), and tetrahydrofuran (THF) (0.6 mg mL−1), respectively. They also demonstrated solvent-free chemical modification of SWCNTs, leading the way for large-scale functionalization of the materials.[33] After isoamyl nitrite was added to mixtures of SWCNTs and 4-substituted anilines, and the reactants were stirred vigorously at 60 °C. The degree of functionalization after the reaction was checked by the decrease in weight as a result of the decomposition of the functional segments using thermo-gravimetric analysis (TGA), which indicated that as low as 20% of weight loss occurred at 750 °C.

Chen et al. examined a solvent-free chemical process to obtain functionalized MWCNTs using in situ-generated diazonium salts in the presence of ammonium persul-fate and 2,2′-azoisobutyronitrile.[34] The resultant MWCNTs obtained from a greener chemical process exhibited solubility in water, chloroform, and THF. Although the diazotization reaction is a useful strategy for functionalization of CNTs,

nitrogen-containing moieties such as biocompatible segments are undesirable owing to the formation of N-nitroso species.

Hudson et al. discovered a novel approach for function-alization of SWCNTs by preventing side reactions using tri-azene compounds as stable precursors to diazonium salts in aqueous media (Figure 4a).[35] The triazene moiety was added to an aqueous suspension of SWCNTs and SDS, and then a film formed on the water. After adjusting the solution to pH 2 using 6 M HCl, the triazene was converted to the water-soluble diazonium salt and the film disappeared. The suspension was adjusted to pH 10 with 6 m NaOH to complete the reaction. The authors successfully demonstrated that a biocompatible compound of biotin amide with the triazene moiety could be attached to SWCNTs without any side reaction. This method has been exploited to make diazonium precursors for applica-tions in biosensors, drug delivery, and medical diagnostics.

Stephenson et al. developed a method for the bulk prepara-tion of water-soluble SWCNTs that contain multifunctional, acid-sensitive addends.[36] The multifunctional SWCNTs could undergo repetitive functionalization using diazonium salts. Water- and phosphate buffer saline (PBS)-soluble carboxy-lated ultrashort SWCNTs were prepared by functionalization using mixtures of oleum nitric acid at various temperatures; the average length obtained was between 98 nm and 32 nm at temperatures ranging from 30 °C to 70 °C, respectively. Amide-functionalized SWCNTs with poly(ethylene glycol) (PEG) chains were obtained at the defect site of the carboxylic acid using N,N-dicyclohexylcarbodiimide in dry DMF. After repeating the aryl diazonium salt functionalization of the resultant SWCNTs, the authors successfully introduced the biotin moiety, which shows chemical sensitivity to acid. These results indicate that repetitive functionalization is applicable for synthesis of water-soluble SWCNTs containing biologically important molecules, which are not accessible using the oleum–nitric acid method.

Doyle et al. examined environmentally friendly functionali-zation of SWCNTs in molten urea.[37] Stirring of SWCNTs in a mixture of molten urea, 3,4,5-trisubstituted aniline derivatives, and sodium nitrite, evolution of nitrogen gas occurred within 15 min, resulting from the generation of aryl diazonium salts and the incorporation of functional units at the surface of the SWCNTs. The authors suggested that the facile method brings about a rapid and eco-friendly synthetic protocol to produce covalently functionalized SWCNTs.

Dispersion of graphene sheets in organic solvents was achieved using aryl diazonium chemistry. Surfactant-wrapped graphene sheets obtained from the reduction of GO with hydra-zine were prepared using aryl diazonium salts.[38] GO was dis-persed in aqueous sodium dodecylbenzenesulfonate (SDBS) with homogenization and sonication. After adjustment of the solution to pH 10 using a sodium hydroxide aqueous solu-tion, the resulting GO was reduced with hydrazine hydrate at 80 °C for 24 h. Finally, the obtained chemically converted gra-phene with surfactant wrapping was reacted with related aryl diazonium salts for 1 h at room temperature. Up to 1 mg mL−1 of the functionalized graphene could be dispersed in DMF, N,N-dimethylacetamide, and NMP. Although there has been significant research activity in the field of functionalized gra-phene, most of the work has focused on GO, which requires initial oxidative damage of the sp2-bonded carbon atoms.

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Figure 3. Chemical functionalization of CNMs. a) aryl diazonium radical addition; b) 1,3-dipolar cycloaddition; c) nitrene cycloaddition; d) aryne cycloaddition; e) oxidation reaction; f) Hummer’s method for graphene oxide; and g) amidation reaction. The red circles indicate functional groups.

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Englert et al. reported direct covalent bulk functionalization of graphene by reduction of graphite with a sodium/potassium alloy (Figure 4b).[39] Exfoliation of the negative charge of the graphite was obtained by charge repulsion with the help of the electron-positive alloy. After addition of 4-tert-butylphenyldiazo-nium tetrafluoroborate, the aryl radicals led to covalent addition to the conjugated π-electron of the graphene.

Jin et al. demonstrated that graphene could be reacted with 4-propargyloxybenzenediazonium tetrafluoroborate and sub-sequently reacted with an azide containing an oligo(ethylene) oxide chain and carboxylic acid using click chemistry in aqueous media.[40] Water-dispersed graphene was formed by the addition of sodium chlorate without oxidative treatment. Diazonium-functionalized graphene was obtained after the diazonium salt was added to the water-dispersed solution. Finally, azide–alkyne cycloaddition reaction occurred in the presence of copper sul-fate, tris(3-hydroxypropyltriazolylmethyl)amine, and sodium ascorbate in the aqueous solution. The resultant graphene was dispersed in water at a concentration of 14.2 μg mL−1.

2.1.2. Cycloaddition Reactions

The 1,3-dipolar cycloaddition on the sp2 carbon of CNMs is one of the most common functionalization methods; however, it still attracts the interest of a great number of scientists (Figure 3b). Bianco et al. was one of the first to investigate amino acid func-tionalization of water-soluble CNTs through 1,3-dipolar cycloaddition.[41] An amino acid with a N-tert-butoxycarbonyl (t-Boc) pro-tecting group and paraformaldehyde was added to a suspension of CNTs in DMF at 130 °C for 96 h to yield the functionalized CNTs. Deprotection of the t-Boc at the ter-minal group occurred after the addition of HCl gas. After activation using N-hydroxy-benzotriazole and diisopropylcarbodiimide, N-α-(9-fluorenylmethoxycarbonyl)glycine was added to a suspension of the CNTs in dichloromethane to yield water-soluble CNTs with N-protected amino acids in an elegant way. Synthesis of 111In-labeled water-soluble SWCNTs containing diethylenetriamine-pentaacetic acid as a chelating molecule for a radiotracer was also evaluated using the amino-functionalization method.[42] Intra-venous administration followed by radio-activity tracing using gamma scintigraphy indicated that functionalized SWCNTs were not retained in reticuloendothelial system organs. Furthermore, these materials were cleared with a half-life of 3 h, from systemic blood circulation through the renal excretion route.

Brunetti et al. found an efficient synthetic strategy to produce functionalized SWCNTs using microwave irradiation.[43] Raw SWCNTs were functionalized by 1,3-dipolar cycloaddi-tion using different aldehydes and sarcosine

in solvent-free conditions under microwave irradiation. The 1,3-dipolar cycloaddition could be achieved after microwave irradiation for 1 h instead of the 5 days necessary under conven-tional thermal conditions. Interestingly, the use of diazonium salts with microwave irradiation led to further functionaliza-tion of the arene radical addition in water. This clean and facile synthetic method is useful for developing novel multifunctional SWCNTs to avoid the use of long reaction times, toxic solvents, and extreme conditions.

Georgakilas et al. investigated the covalent functionalization of SWCNTs with phenol groups through 1,3-dipolar cycloaddi-tion.[44] Functionalization of MWCNTs could also be achieved in a similar manner, and the resultant MWCNTs were dispersed in ethanol (2 mg mL−1), DMF (0.4 mg mL−1), and ethanol/water mixture (0.1 mg mL−1) with a moderate volume ratio. Furthermore, uniform and transparent polymer–MWCNT composite films were obtained by simple mixing of a dispersed MWCNTs solution and a polymer solution of polyacrylonitrile or poly(ethylene vinyl acetate). Laponite–MWCNT composite

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Figure 4. a) Chemical functionalization of SWCNTs by triazenes. Reproduced with permis-sion.[35] Copyright 2006, American Chemical Society. b) Representation of the intercalation and exfoliation of graphite with subsequent functionalization of intermediately generated reduced graphene yielding 4-tert-butylphenyl functionalized graphene (double bonds in the basal planes have been omitted for clarity). 1,2-DME, 1,2-dimethoxyethane. Reproduced with permission.[39] Copyright 2011, Nature Publishing Group. c) One-step preparation of functionalized MWCNTs. Reproduced with permission.[54] Copyright 2009, American Chemical Society.

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gels were also prepared by adhesion of Laponite sheets at the surface of functionalized MWCNTs with ion-exchange prop-erties. The composite gels could potentially immobilize var-ious cationic species such as dyes, drugs, biomolecules, and polyelectrolytes.

Georgakilas et al. demonstrated for the first time the applica-tion of 1,3-dipolar cycloaddition of azomethine ylides to obtain virtually defect-free graphenes.[45] The dispersed solution of the functionalized graphene sheet in ethanol was obtained without solid residues. According to atomic force microscopy (AFM) images, the thickness of the dispersed graphene was 1.5 nm, indicating that the value consists of a monolayer of graphene (thickness: 0.6–0.9 nm) with functional groups across the sheet. Zhang et al. demonstrated a one-pot functionalization of a gra-phene sheet with porphyrin derivatives using a similar reac-tion.[46] Interestingly, porphyrin-functionalized graphene exhib-ited significant fluorescence quenching; fluorescence quantum yields were reduced from 4% for a porphyrin segment to 0.3% for the functionalized graphene. From the results of the luminescence lifetimes of materials, either energy or electron transfer between graphene and the covalently bonded porphy-rins occurred after photoexcitation.

The 1,3-dipolar cycloaddition of azomethine ylides has his-torically been an effective methodology for the functionalization of fullerenes.[47] Georgakilas et al. reported the formation of a supramolecular assembly of fullerenes in water.[48] After func-tionalization of fullerene attached to hydrophilic ammonium groups through 1,3-dipolar cycloaddition, the self-assembled spherical and tubular objects were obtained after sonication in water. The supramolecular assembly originates as a result of a subtle balance between the hydrophobic and hydrophilic prop-erties of fullerene and the ammonium side chain, and direc-tional interactions between the molecules. Recently, Guryanov et al. evaluated microwave-assisted functionalization of fuller-enes and SWCNTs using an ionic liquid.[49] The cycloaddition of azomethine ylides to fullerene under microwave irradia-tion was achieved within 10 min with conversion up to 98%. Furthermore, functionalization of SWCNTs in the presence of bucky gels, SWCNT–ionic liquid composites, was achieved under microwave irradiation for 1 h.

Alvaro et al. reported the microwave-assisted 1,3-cyclo-addition of a nitrile oxide on the sidewall of SWCNTs.[50] The reaction mixtures of 4-pyridyl nitrile oxide and pentyl ester-modified SWCNTs in the presence of triethylamine were irra-diated for 45 min to afford 4-pyridyl isoxazolino-SWCNTs. The resulting SWCNTs formed a supramolecular complex with zinc porphyrin through axial coordination. Steady-state fluorescence and transient absorption spectroscopy revealed that energy transfer quenching of the zinc porphyrin singlet excited state by 4-pyridyl isoxazolino–SWCNTs. However, no evidence was observed for the occurrence of electron transfer.

The cycloaddition of nitrile imines to CNMs to prepare 2-pyrazolino-functionalized CNMs is a versatile procedure for chemical functionalization. Intermediate hydrazones are avail-able from aldehydes in a single step, and the corresponding cycloadducts are obtained under microwave irradiation.[51] Pal et al. examined charge recombination behavior of poly(3-hexylthiophene) (P3HT)-3′-(3,5-bis-trifluoromethyl-phenyl)-1′-(4-nitrophenyl) pyrazolino[70]fullerene ([70]BTPF) in a bulk

heterojunction film for OPVs.[52] Hole–electron charge recombin-ation in the P3HT-[70]BTPF film was faster than that in P3HT- PC61BM. The authors suggested that the first-order charge recombination occurred from a state in which the electrons and holes were separated by different distances in the various materials. Ondera et al. prepared 2-pyrazolino-functionalized SWCNTs for rapid detection and killing of bacteria under micro-wave irradiation.[53] The functional SWCNTs containing the thiol units in the sidewall could undergo surface adsorption of gold nanopopcorn through gold–sulfur bonds, yielding hybrid nano-materials with good aqueous stability. The resultant nanomate-rials exhibited effective photothermal agents (killing efficiency: ≈97%) with rapid detection (≈15 min) toward Escherichia coli under laser irradiation of 670 nm at 1.5–2.5 W cm−2.

Nitrene is also a frequently used intermediate in [2+1] cycloaddition reactions of CNMs. Triazolines are formed by the cycloaddition reactions between organic azides and sp2 carbons at the graphitic surface, and then a concomitant nitrogen loss occurs upon thermolysis or photolysis (Figure 3c). Gao et al. evaluated a facile, green, and efficient one-step methodology for the synthesis of functionalized MWCNTs (Figure 4c).[54] The desired products were obtained after raw MWCNTs were simply reacted with azides in NMP at 160 °C for 18 h. The func-tionalized MWCNTs containing hydroxyl, amino, and carboxyl units on the end groups exhibited good dispersion properties in water and polar organic solvents. Furthermore, chemical reac-tions on the functionalized MWCNTs were demonstrated using ring-opening metathesis polymerization and atom transfer radical polymerization to induce polymer-coated MWCNTs. He et al. investigated the covalent functionalization of graphene nanosheets with nitrene using a similar approach.[55] The func-tionalized graphenes could undergo surface-initiated polymeri-zation, amidation, and reduction of metal ions, yielding flexible and conducting graphene films. The electrical conductivities of the resultant graphene films were in the range of 10−1 to 103 S m−1, which was less than that of polycrystalline graphene (≈105 S m−1), but comparable to that of reduced GO (r-GO, ≈102 to 104 S m−1) and noncovalent functionalized graphene (≈102 S m−1).

Liu et al. found direct functionalization of a pristine gra-phene nanosheet using perfluorophenyl azides through nitrene cycloaddition.[56] After a one-pot functionalization with azides containing an alkyl chain, ethylene oxide, or perfluoroalkyl groups, the resultant graphenes were dispersed into water or o-DCB. They also demonstrated photochemical functionali-zation of graphene using nitrene chemistry upon irradiation with a medium-pressure mercury lamp for 1 h in o-DCB under ambient conditions.

Arynes can also be used for efficient chemical modification of CNMs based on cycloaddition reactions (Figure 3d). Zhong et al. reported the preparation of functionalized graphenes through aryne cycloaddition under mild reaction conditions of stirring at 45 °C for 12 h in acetonitrile.[57] Fluoride-induced 1,2-elimi-nation of o-trimethylsilylphenyl triflate gives rise to benzynes, which are highly reactive species, to induce the cycloaddition with sp2 carbons. After incorporation of the phenyl units on the pristine graphene sheets, the dispersion properties dramati-cally changed in DMF, chloroform, toluene, ethanol and even in water. Criado et al. investigated efficient aryne cycloaddition

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to SWCNTs under microwave irradiation.[58] They compared the aryne cycloaddition reaction using microwave irradiation and a classical bench-top reaction. The reaction time was clearly reduced from 12 h for the classical reaction to 15 min for the microwave reaction. Furthermore, a high degree of functionali-zation of the SWCNTs was observed with microwave treatment, ranging from 6.4% to 15.1%, depending on the aryne units.

2.2. Modification of Defect Sites

2.2.1. Oxidation Reactions

Defect sites in the hexagonal lattice on the sidewall of CNMs are observed under oxidizing conditions (Figure 3e). The pres-ence of defects leads to an enhancement of chemical reactivity. Functionalization of the defects generates sp3 carbons, which disrupt the π conjugation along the sp2 carbons, resulting in a decrease in electrical conductivity. Oxidation reactions using strong acids do not seem to be a green processing approach. However, these procedures are one of the most efficient disper-sion methods for CNMs in organic solvents and water that are currently available.

Wang et al. demonstrated that carboxylated and acid-sul-fonated SWCNTs were rapidly obtained by microwave irradia-tion of raw SWCNTs in a 1:1 mixture of 70% nitric acid and 97% sulfuric acid in water for 3 min.[59] The resultant SWCNTs exhibited high dispersion ability in ethanol and water at concen-trations of 2.5 mg mL−1 and 10 mg mL−1, respectively. Current–voltage characteristics revealed that the electrical conductivity of a microwave-assisted SWCNT membrane was reduced by 33% compared with a pristine SWCNT membrane.

Tchoul et al. investigated the mild nitric acid oxidation of SWCNTs. Sonication in 8 M nitric acid for 1 h at 40 °C led to an increase in solubility in DMF, methanol, and water owing to functionalization.[60] Chemical treatments using oxidizing reagents gave rise to smaller bundles and individual tubes. The dispersion ability increased with increasing number of func-tional groups in the SWCNTs, indicating that attractive forces between the solvents and SWCNTs was a crucial factor for good dispersion properties. Aviles et al. examined a similar approach to functionalize MWCNTs using a relatively low concentration of nitric acid, sulfonic acid, and hydrogen peroxide aqueous solutions with sonication.[61] A low power sonochemical treat-ment employing 3 m nitric acid for 2 h followed by identical treatment with hydrogen peroxide was found to be the most effective oxidation conditions with minimal damage to the side-wall of MWCNTs.

Naeimi et al. performed a one pot functionalization of MWCNTs by oxidation with ozone in the presence of hydrogen peroxide under mild conditions.[62] Ozone acts as a good oxi-dizing agent and is attached to sp2 carbons to generate oxidative ozonide. Unstable ozonide is converted to functional groups such as hydroxyl, carboxyl, and carbonyl units with the help of the cleavage reagent hydrogen peroxide. A mixture of MWCNTs and hydrogen peroxide are mixed in a reactor, and then gaseous ozone is passed through the samples to generate functionalized MWCNTs. Owing to the formation of hydrogen bonding inter-action between carboxyl and hydroxyl groups in the oxidized

MWCNTs and solvents, stable dispersion solutions were obtained in water, ethanol, DMF, and THF even after 30 days.

GO, oxidized graphene, contains reactive oxygen functional groups, which are good candidates for use in a great variety of applications through chemical modifications.[63] Because GO can be dispersed well in water and organic solvents and is reduced to r-GO, potential applications have been demonstrated in many areas such as transparent electrodes, sensors, and energy-conversion devices.[23]

Basically, GO is obtained from graphite using strong acids under hazardous conditions. Hummers’ method is the fastest and most efficient protocol for obtaining GO prepared by reacting graphite with a mixture of sodium nitrate and concen-trated sulfuric acid in the presence of potassium permanganate (Figure 3f).[24] However, the method has drawbacks: exposure to toxic gasses such as nitrogen dioxide and nitrogen peroxide during the oxidation process and difficulty in removing residual sodium ions and nitrates from the water containing GO after the oxidation reaction. Although Hummers’ method is a harsh oxidative treatment, it is a facile functionalization procedure with a relatively high ratio of carbon to oxygen even in large-scale production of GO.

Marcano et al. investigated an improved Hummers’ method for preparation of GO.[64] GO was prepared by mixing graphite with a 9:1 mixture of concentrated sulfuric acid and phosphoric acid in the presence of potassium permanganate at 50 °C. TGA and solid-state 13C NMR revealed that the overall oxida-tion of GO using the improved Hummers’ method was higher than that of using the original method. More importantly, the new method did not generate the toxic and explosive gases of nitrogen dioxide and nitrogen peroxide during the process. The electrical conductivity of r-GO prepared using the modified Hummers’ method after annealing at 900 °C was estimated to be 400 ± 220 S m−1. These values were comparable to those of r-GO using the original Hummers’ method (375 ± 215 S m−1). Chen et al. also examined an improved Hummers’ method without using sodium nitrate.[65] Graphite powder was reacted with concentrated sulfuric acid and potassium permanganate at 40 °C for 30 min without toxic gases being generated. The func-tionalized GO obtained had almost the same properties as the GO prepared by the conventional Hummers’ method.

Tang et al. developed a bottom-up synthesis of GO nanosheets using a green and facile fabrication method (Figure 5a).[66] In comparison with a top-down approach such as Hummers’ method, the GO nanosheets were prepared by a hydrothermal method using self-assembled glucose sheets in aqueous media. The thickness of the GO nanosheets could be tuned from 1 nm to 1500 nm by using various concentrations of glucose. After thermal treatment, the semiconducting properties of the annealed GO nanosheets were comparable to those of r-GO prepared by the top-down method. p-Type carrier mobility was estimated to be 10−5 cm2 V−1 s−1 using a field-effect transistor (FET). Furthermore, visible-light-driven GO-based photodetec-tors in the wavelength range of 400–650 nm were fabricated onto a silicon oxide substrate using the film transfer method, as shown in Figure 5a. The authors mentioned that this method is environmentally friendly, facile, and low-cost, as well as large-scale for mass production without the use of strong acids and harsh conditions.

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Reduction of GO is the most efficient and useful reaction to recover electrical conductivity. The resultant r-GO includes residual oxygen-based functional groups such as hydroxyl, carboxyl, and epoxy units. Owing to these functional groups, good dispersion of r-GO is obtained without aggregation even in water. Chemical modification of GO is crucial for function-alization of graphene for solution-processed devices. Basically, r-GO is prepared by toxic reductants such as hydrazine mono-hydrate,[67] dimethyl hydrazine,[68] and sodium borohydride.[69] Because r-GO prepared under hazardous conditions still con-tains many defects on the surface, mild and environmentally friendly reduction methods needed to be developed.

Fernández-Merino et al. found that vitamin C (ascorbic acid) is a safe reducing reagent for the preparation of stable suspen-sions of r-GO in water and organic solvents.[70] r-GO was easily obtained by stirring with vitamin C at 95 °C for 15 min in water. The electrical conductivity of r-GO prepared using vitamin C (7700 S m−1) was comparable with that of the r-GO prepared using the conventional reductant hydrazine (9960 S m−1),

indicating that the electrical conjugation of the obtained r-GO was restored after chemical modification. Zhu et al. investi-gated a green and facile approach for the preparation of r-GO using sugars (Figure 5b).[71] Sugars such as d-glucose, d-fruc-tose, and sucrose were added into an aqueous solution of GO with an ammonia solution, which led to synergistic augmenta-tion of the reaction rate. After stirring at 95 °C for 60 min, the resultant r-GO was obtained with good dispersion properties at a concentration of 0.1 mg mL−1 in water and was stable for more than 1 month.

Wang et al. successfully demonstrated that a one-pot reduc-tion and functionalization of GO could be achieved using tea polyphenols.[72] After extraction of polyphenols from green tea powder in water, GO was added to the aqueous solution with stirring at 90 °C to yield r-GO. The X-ray diffraction pattern showed good evidence for the formation of r-GO: d-spacing of r-GO (2.1 nm) was larger than that of GO (0.91 nm), sug-gesting adsorption of polyphenols at the surface of the r-GO sheets. Because of the adsorption of insulating polyphenols,

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Figure 5. a) Schematic illustration of the preparation of a graphene oxide nanosheet (GON). i) Glucose is dissolved in water to obtain a colorless transparent source solution. ii) Polymerization occurs under hydrothermal conditions as glucose molecules dehydrate intermolecularly to form a GON, which floats onto the surface of the solution owing to its hydrophobicity. iii) The as-grown GON can be transferred onto any substrate by dipping and lifting. iv) The transferred GON is rinsed by dipping into water to remove the residue. v) The as-grown GON is annealed at a certain temperature to dehydrate and graphitize for tuning its electrical, optical, and structural properties. vi) The annealed GON with desired thickness and electrical proper-ties. Reproduced with permission.[66] Copyright 2012, The Royal Society of Chemistry. b) Schematic illustration of preparation of r-GO based on glucose reduction. Reproduced with permission.[71] Copyright 2010, American Chemical Society.

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the electrical conductivity of r-GO prepared using polyphenols (53 S m−1) is smaller than that of r-GO prepared using vitamin C (7700 S m−1);[70] however, this non-toxic method has been applied for green fabrication of conducting nanosheets.

2.2.2. Esterification–Amidation Reactions

As we mentioned above, the oxidation of CNMs is a widely accepted methodology for covalent functionalization in the pres-ence of various oxidizing reagents such as nitric acid, sulfonic acid, and potassium permanganate. Carboxylation is used for preparation of precursors for further functionalization through esterification and amidation reactions (Figure 3g). Haddon et al. reported a conventional and facile method for amide function-alization of SWCNTs.[73] The carboxylic groups at the defect sites of the oxidized SWCNTs were converted to acyl chloride using thionyl chloride and reacted with octadecylamine to form amide linkages. Because of the introduction of long aliphatic chains, the dispersion ability clearly changed in organic solvents such as chloroform, toluene, and 1,2-dichlorobenzene. In a similar manner, aqueous-dispersed functionalized SWCNTs can be achieved by simple and effective amidation between the carboxyl groups and hydrophilic polymers or biomolecules for useful biosensing applications.

Yim et al. found that DNAzyme connected with MWCNTs exhibited classical enzyme activities over 400 catalytic turnovers (Figure 6a).[74] Streptavidin (STV) was reacted with carboxylated MWCNTs through 1-(3-dimethylaminopropyl)-3-ethyl-carbodiimide

hydrochloride (EDC) and N-hydroxysuccinimide (NHS) cou-pling to yield amide linkages. Biotinylated DNAzyme, which consists of a 38-mer containing the activated site of 17E against Pb2+, was attached to STV–MWCNT to obtain aqueous-dispersed DNAzyme-MWCNT conjugates using biotin–STV specific affinity. 6-Carboxylfluorescein hydrate (FAM)-bound substrate DNA, which contains a cleavage site of a single embedded ribonucleotide, could be cleaved to give two single-stranded DNA fragments in the presence of lead acetate. The authors pointed out that 15 × 10−6 m of the DNA substrate was cleaved using 0.1 mg mL−1 of the DNAzyme–MWCNT conju-gates including 0.036 × 10−6 m of DNAzyme. After this reaction, a similar turnover rate was obtained after further addition of the substrate, suggesting that a lack of product inhibition was observed under the conditions.

Tiraferri et al. reported antimicrobial properties of a SWCNT–polyamide composite membrane prepared in aqueous media.[75] The surface reaction of the membrane occurred through EDC and NHS coupling to yield amine-reactive esters in 2-(N-morpholino)ethanesulfonic acid (MES) buffer. The acti-vated ester was reacted with ethylenediamine to form an amide linkage in 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES) buffer. Finally, carboxylated SWCNTs after ozone treatment were attached to terminal amines to obtain the amide-conjugated SWCNT–polyamide composites in the pres-ence of EDC and NHS under sonication. Enhancement of the bacterial cytotoxicity of E coli. cells using the resultant mem-brane was observed to achieve up to 60% of inactivation within 1 h after contact. The antimicrobial activity of the SWCNTs was

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Figure 6. a) i) Sequence of the 17E DNAzyme and schematic showing the procedure for immobilizing it onto MWCNTs. ii) Sequence of FAM-bound substrate DNA, which contains a single embedded ribonucleotide, along with a cartoon depicting its hybridization with DNAzyme. Reproduced with permission.[74] Copyright 2005, American Chemical Society. b) Synthesis of alkyl amine (NH2), PEG, and PEI-functionalized MWCNTs through the esterification or amidation reaction for the detection of pulmonary toxicity. Reproduced with permission.[76] Copyright 2013, American Chemical Society. c) Synthesis of carboxylated GO (CG) via the Johnson–Claisen rearrangement. Reproduced with permission.[77] Copyright 2013, WILEY-VCH.

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induced by direct contact with the bacterial cells and a combi-nation of membrane perturbation and oxidative stress.

Li et al. investigated the detection of pulmonary toxicity using covalently functionalized MWCNTs showing different surface charge (Figure 6b).[76] PEG, polyetherimide (PEI), and alkyl amine groups were incorporated into the carboxyl MWCNTs through an esterification or amidation reaction, and an aqueous dispersion of the resultant MWCNTs was obtained. Cell screening in BEAS-2B cells of an immortal-ized human bronchial epithelial cell line and THP-1 cells of a macrophage-like cell line revealed that the introduction of ani-onic carboxyl- and PEG-modified MWCNTs led to a decrease in the production of pro-fibrogenic cytokines and growth factors. In contrast, cationic PEI-functionalized MWCNTs exhibited clear biological effects of pulmonary fibrosis, and the amine-conjugated MWCNTs showed similar fibrogenic properties compared with bare MWCNTs. The change in the pulmonary fibrogenic behavior was ascribed to different cellular uptake of the modified MWCNTs and NLRP3 inflammasome activation, which depends on the propensity toward lysosomal damage and cathepsin B release in macrophages.

Recently, Sydlik and Swager developed novel method-ologies for the esterification and amidation of GO using a Johnson−Claisen rearrangement.[77] The unsaturated ester occurs in the presence of allylic alcohol and orthoesters such as triethyl orthoacetate (TEOA) with a catalytic amount of acid (Figure 6c).[78] The authors proposed that the functional ester groups were formed by the rearrangement between hydroxyl groups onto GO and TEOA as a solvent and reagent. Func-tionalized GO-containing ethyl esters were obtained after the reaction mixture of GO, TEOA, and catalytic para-toluene sul-fonic acid was refluxed for 36 h. The reduced GO containing the ester groups exhibited a good electrical conductivity of 39 S m−1 with a low sheet resistance of 1.6 kΩ sq−1. More importantly, a simple and efficient multistep modification from the functionalized GO was demonstrated. Hydrolysis led to carboxylated GO that could be dispersed in water, with a large zeta potential of −75 mV at pH = 9. Furthermore, car-boxylated GO was converted to acyl chloride GO using oxalyl chloride with catalytic DMF, and then propargyl amide GO was obtained from amidation of the acyl chloride and prop-argylamine. The resultant propargyl amide GO was subjected to a “click” reaction of the terminal alkyne, which reacted with an azide derivative through a copper-catalyzed cycloaddition. Sulfonate- or PEG-incorporated GO were successfully syn-thesized using related azide derivatives to yield an aqueous dispersion.

Song et al. investigated catalysis-free synthesis of nano-porous GO networks using a Johnson−Claisen rearrange-ment.[79] After the formation of ethyl ester-functionalized GO through the rearrangement of a hydroxyl group on GO and TEOA, GO networks with arylimidazole linkages were obtained by the condensation reaction of the ester groups and tetraami-noaryl moieties. The resultant porous materials exhibited high surface areas up to 732 m2 g−1 with carbon dioxide (CO2) uptake capability of 3.75 mmol g−1 and with CO2/N2 selectivity of 130 under 1 bar at 273 K. This synthetic procedure can con-tribute to the development of functionalized CNMs with a facile chemical modification.

3. Dispersion of CNMs in Solvents with a Low Environmental Burden

Because of their substantial van der Waal attractions and spe-cific hydrophobic interactions, CNMs tend to aggregate par-ticularly well in aqueous media.[29] To maximize the advantage of CNMs, various dispersants have been developed to exfoliate or stabilize the CNMs in organic solvents such as chloroform, toluene, and chlorobenzene, and in polar liquids such as NMP, acetonitrile, and acetone.[6] Here we summarize dispersion methodologies for CNMs using green solvents such as ethanol and water, as well as a chemical-free mechanical process.

3.1. Dispersion of CNMs in Pure Water

CNMs as pure carbon allotropes are not at all soluble in water. It is a great challenge to prepare stable aqueous dispersion of CNMs without the assistance of dispersants. As far as we know, there has been no report of dispersion of CNTs in water without any stabilizer or without chemical modification. To directly disperse the chemically converted graphene sheets in pure water, Wallace et al.[80] added ammonia in the reduction reaction step to maximize the charge density on the resulting graphene. The obtained graphene sheets were produced in large scale and could readily form stable aqueous colloids through electrostatic stabilization. The electric conductivity of graphene paper obtained by vacuum filtration of this dispersion was found to be ≈7200 S m−1 at room temperature. Although the chemical reduction method produced GO in large quanti-ties, the quality of the generated defect-laden GO was lower than that of graphene produced by physical methods.[81]

Recently, Kim et al. reported a simple method for exfoliation of 2D materials including graphene without any chemical treat-ment.[82] By merely controlling the temperature of the sonica-tion bath and storage, graphene was dispersed in pure water without the help of any chemicals or surfactants (Figure 7a). A thin-layer graphene sample was found to exhibit the highest stability among the several 2D materials, with 90% of the gra-phene present in the suspension even after 1 month at 60 °C (Figure 7b). In addition, by filtering the solution through an Anodisc filter, the obtained 2.5 μm-thick graphene film exhib-ited a high electric conductivity of 440 S m−1. They also per-formed inkjet printing on a hard or flexible substrate using the dispersed solutions (Figure 7c). A mesh pattern was fabricated on the flexible substrate shown in Figure 7d. The exfoliation mechanism in pure water is still not clear. However, X-ray photo electron spectroscopic (XPS) spectra of graphene after high-temperature sonication revealed that the resultant mate-rial was functionalized with carboxyl and hydroxyl groups (Figure 7e), which may contribute to the dispersion stability in water. This work is a milestone in green processing of CNMs using pure water. What needs to be improved is the low con-centration of the exfoliated graphene in the dispersion.

Fullerenes have been dispersed in pure water as stable col-loids using functionalization,[83] solvent exchange,[84] or even after prolonged mixing of [60]fullerene (C60) and water.[85] Deguchi et al. developed a simple method for the disper-sion of fullerenes by injecting a saturated THF solution of

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fullerene into water, followed by THF removal by purging gaseous nitrogen. The obtained C60, [70]fullerene (C70) clus-ters are 60 nm in diameter, negatively charged, and display excellent colloidal stability for 9 months in water (Figure 7f).[86] Why are the fullerene particles so stable in water? This question has lingered for a long time in the mind of researchers and so far several different mechanisms have been proposed to explain it. One possibility is the stabiliza-tion of fullerene–water colloidal systems both by the charge transfer from oxygen atoms of water to C60 and by the for-

mation of ordered, hydrogen-bonded and sphere-like hydrated shells around the fullerene.[87] Another group provided spec-troscopic evidence for the surface hydroxy-lation of the initially hydrophobic C60 in water after sonication and suggested the formation of alcohols for dissolution of fullerene in water.[88] Recently, Ritter et al. reported that the surface hydroxylation of C60 was the most likely mechanism of stabi-lization in water, which was independent of the dispersion methods for fullerenes in the aqueous solution (Figure 7g).[89] Because limitations remain in these explanations, the mechanism of stability of the fullerene dispersion in water deserves further study.

3.2. Dispersion of CNMs in other Non-Toxic Solvents

The C60 nanoparticles obtained by hand-grinding could be dispersed in a variety of organic solvents,[90] in which C60 is only spar-ingly soluble.[91,92] For example, the disper-sion concentrations of C60 nanoparticles were up to 98 μg mL−1 in ethanol, 274 μg mL−1 in silicone oil, 330 μg mL−1 in 1-propanol, and 341 μg mL−1 in acetone. These values using the hand-grinding method were one order of magnitude higher than those using conven-tional methods. The same group discovered stable colloidal dispersions of C60 and C70 in combination with a good solvent (toluene, 1-methylnaphthalene, and NMP) and a poor polar solvent (acetonitrile, ethanol, or acetone).[93] Their dispersion solutions con-taining microcrystals of fullerenes with low polydispersity may be utilized for practical applications.

Supercritical water has been discovered as a new medium for opening and thin-ning of MWCNTs in the presence and in the absence of oxygen.[94] Instead of strong acids, the presence of oxygen in supercritical water (≈2 mmol) could improve the thinning of MWCNTs with the collapsed outer gra-phene layers toward the inner layers. Khlo-bystov et al. discovered that SWCNTs could

be efficiently filled with fullerenes in supercritical CO2 at tem-peratures as low as 30–50 °C.[95] The obtained fullerene arrays in nanotubes might be employed for catalysis and quantum computing. Recently, exfoliation of graphite into monolayered and few-layered graphene sheets was achieved in supercritical DMF[96] and in supercritical ethanol,[97] and also in the emul-sion microenvironment of supercritical CO2.[98] These tech-niques using supercritical liquids lead to simple, economic, feasible, and green approaches for functionalization and exfo-liation of CNMs.

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Figure 7. Dispersion of CNMs in pure water. a) Photograph of pure water (left) and graphene solutions dispersed in deionized water after 1 month (right). b) Graphene was sonicated at the high (60 °C) and low (30 °C) temperature and stored at high (60 °C) and low (20 °C) tem-peratures. Two types of triangles almost overlap because of fast precipitation. c) Lines printed with pure-water inks of graphene on Si substrates. The scale bar is 300 μm. d) A mesh pattern printed on poly(ethylene terephthalate) substrate with a mixed PEO–water graphene ink. The scale bar is 100 μm. e) XPS spectra of dispersed graphene in water. Reproduced with permis-sion.[82] Copyright 2015, Macmillan Publishers Limited. f) TEM images of C60 clusters (left) and C70 clusters (right) in water. Reproduced with permission.[86] Copyright 2001, American Chemical Society. g) Molecular structure of hydroxylated fullerene in water. Reproduced with permission.[89] Copyright 2014, American Chemical Society.

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Besides supercritical liquids, other solvents with a low envi-ronmental burden have been employed to exfoliate and dis-perse graphite. Using p-phenylene diamine as a reducing rea-gent, Chen et al. obtained graphene monolayers dispersed in ethanol that were so stable that the nanosheets were not sep-arated out even by intensive centrifugation at 12 000 rpm.[99] The positively charged graphene films prepared by electro-phoretic deposition on indium tin oxide exhibited high con-ductivity up to 15 000 S m−1, which was six orders of mag-nitude higher than that of the GO film. Nonomura et al. demonstrated a highly stable dispersion of graphite in water/acetone mixtures, with negative charges on the sur-face of the graphite particles.[100] Acetone is proposed to disintegrate the graphite aggregation during sonication. Treatment of GO with a variety of inexpensive and relatively non-toxic alcohols (methanol, ethanol, isopropyl alcohol, and benzyl alcohol), could afford highly reduced GO, with high conductivities of up to 4600 S m−1 and good specific capacitances.[101]

3.3. Exfoliation of CNMs in Ionic Liquids

Srinivasan et al. investigated the facile formation of graphene–ionic-liquid hybrid gels through π–π and π–cation interac-tions.[102] Dispersed GO in the imidazolium-based ionic liquids underwent thermal reduction to r-GO/ionic liquid gels at 150 °C for 1 h. The dried gels were prepared by diluting the composite gels in DMF and then by drying under vacuum. Scanning elec-tron microscopic (SEM) images revealed that the porous net-works were observed in the r-GO sheets. Ionic liquids have emerged as promising solvents for the exfoliation of graphite because the surface energies of ionic liquids are close to that of graphene.[103] 1-Butyl-3-methylimidazolium bis(trifluoro-methane-sulfonyl)imide led to a stable dispersion of graphene nanosheets at a concentration of 0.95 mg mL−1 after sonication for 1 h.[104] Fukushima et al. reported that ionic liquids based on imidazolium salts were highly effective for the exfoliation of bundled SWCNTs to generate carbon-based soft gels of “bucky gels” (Figure 8a).[105] Imidazolium ions were proposed to

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Figure 8. a) Photographs of a bucky gel prepared by grinding a suspension of SWCNTs in an ionic liquid b) Chemical structures of ionic liquids for dispersion of SWCNTs. Reproduced with permission.[105,106] Copyright 2011, The Royal Society of Chemistry. c) Schematic illustration of the experi-mental procedure for microwave-assisted liquid-phase exfoliation of graphite in an ionic liquid. d) Chemical structures of ionic liquids for dispersion of graphene. Reproduced with permission.[107] Copyright 2015, Macmillan Publishers Limited.

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adhere strongly to the π-conjugated graphitic surface of SWCNT, owing to cation–π inter-actions (Figure 8b). This method allowed for the fabrication of highly electroconductive polymer/nanotube composite materials.[106] Matsumoto et al. demonstrated that micro-wave irradiation of graphite suspended in molecularly engineered oligomeric ionic liq-uids exhibited ultrahigh-efficiency exfoliation (93% yield) of single-layer graphene (thick-nesses: <1 nm) with a high selectivity (95%) in 30 min[107] (Figure 8c,d).

3.4. Mechanical (Chemical-Free) Exfoliation of CNMs

The excessive sonication applied to disperse CNMs in a solution may change the intrinsic size and structures of CNMs. If CNMs can be exfoliated directly in the solid state without any toxic solvents, it would be an environmentally friendly method. Deguchi et al. reported the preparation of fullerene nanoparticles using hand-grinding as the chemical-free processing method.[108] Nano-particles with a diameter of 20 nm were readily obtained by grinding the bulk C60 solid in an agate mortar (Figure 9a). X-ray diffraction patterns revealed that the crys-talline structure of the fullerene remained unchanged. The obtained C60 nanoparticles could disperse in water at a high concentra-tion of 5 mg mL−1 with the help of the sur-factant SDS. The C60 nanoparticles, which were completely free from residual organic solvents, showed negligible toxicity to E coli. at 5 μg mL−1. Furthermore, these C60 nanoparticles were found to form stable dispersions in various non-toxic organic solvents (Figure 9b).[90] Interestingly, rubbing bulk solids of C60 between fingertips could also generate nanoparticles with a diameter of 256.8 + 1.1 nm (Figure 9c), without deliberate engineering.[109] In addition to their solvent-free nature, these top-down approaches showed higher concentrations of the dis-persed fullerene nanoparticles.

The discovery of a graphene sheet originated from the mechanical splitting of graphite into individual atomic planes, which can also be considered as smart chemical-free processing of CNMs.[110] Although this method provides high quality crys-tals, the low processability is not suitable for practical purposes. Another technique for exfoliation of CNMs in the solid state is ball milling. SWCNTs were successfully cut in relatively homo-geneous sizes using a planetary mill process.[111] The same group reported that few-layer graphene were achieved from ball milling of graphite with melamine.[112] The resultant exfoli-ated graphene was dispersed in DMF or in water. Jeon et al. prepared edge-selective carboxylated graphite (ECG) by simple ball milling of pristine graphite in the presence of dry ice. The resultant ECG can disperse well in various solvents by self-

exfoliating into a single layer and a few layers graphene.[113] The ball-milling technique can be scaled-up easily but usually only produces micrometer-sized particles.

Paton et al. reported the synthesis of large quantities of defect-free graphene nanosheets by shear exfoliation.[114] When the local shear rate exceeded 104 s−1, exfoliation of few-layer graphene was obtained in NMP or an aqueous solution of sodium cholate as a dispersant. The resultant graphene nanosheets exhibited unoxi-dized and free-of-basal-plane defects. Scale-up shear exfoliation of graphene could be achieved up to 100 L with production rates exceeding 100 g h−1. Inspired by this finding, shear exfoliation of graphene was also achieved using kitchen blenders.[115,116] In terms of turbulent flows in the kitchen blender, extraordinary graphite–graphite collisions could be induced, resulting from an increase in Reynolds stress during the shear-mixing process. Pattammattel et al. examined efficient shear exfoliation of gra-phene using edible proteins and the kitchen blender in water.[117] Exfoliation efficiencies exceeding 4 mg mL−1 h−1 were obtained by bovine serum albumin. The authors also revealed that the number of negative charges on the proteins was crucial for increasing the exfoliation rate and storage stability of graphene in aqueous media.

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Figure 9. a) A top-down approach for the formation of fullerene nanoparticles by simple hand-grinding of a fullerene solid in an agate mortar. Reproduced with permission.[108] Copyright 2006, John Wiley & Sons, Inc. b) Photographs of dispersed fullerene nanoparticles in various solvents: i) methanol, ii) ethanol, iii) 2-propanol, iv) 1-octanol, v) acetone, and vi) silicone oil. Reproduced with permission.[90] Copyright 2006, Chemical Society of Japan. c) i) Preparation of fullerene nanoparticles formed by rubbing of fullerene solids between glass slides. ii) A scanning electron microscope (SEM) image of the fullerene nanoparticles. The scale bar is 200 nm, iii) Aqueous dispersed solution of the fullerene nanoparticles containing 1 wt% of SDS. iv) Size distribution of the fullerene nanoparticles. Reproduced with permis-sion.[109] Copyright 2013, Macmillan Publishers Limited.

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3.5. Electrochemical Exfoliation of Graphite

Electrochemical exfoliation of graphite is expected to emerge as a green processing method.[118] This method uses inexpensive equipment involving a graphite electrode as an anode or cathode, a counter electrode, an electrolyte, and an external power source. When an electrochemical reaction occurs between the graphite electrode and the electrolyte in an aqueous or nonaqueous solution, structural expansion at the electrode is observed owing to intercala-tion of the electrolyte ions. To decrease the carbon-based π interac-tions between the graphitic layers, exfoliated graphene is obtained.

Su et al. demonstrated electrochemical exfoliation of highly ordered pyrolytic graphite in an aqueous solution of sulfuric acid and potassium hydroxide with an applied voltage of 2.5 V for 1 min.[119] The thickness of the resultant graphene was lower than 3 nm and the lateral size of these graphene sheets ranged from 1 to 40 μm. A dispersed graphene solution in DMF was achieved at a concentration of 0.085 mg mL−1. Parvez et al. found that exfoliation of the graphite electrode with inorganic sulfate salts led to the mass production of high-quality graphene sheets in a similar manner.[120] The graphene sheets obtained at a high production rate of 16.3 g h−1 exhibited remarkable properties including a high yield (>85%), large lateral size (up to 44 μm), low oxidation degree (a carbon/oxygen ratio of 17.2), and hole mobility of 310 cm2 V−1 s−1. Further-more, graphene films with a low sheet resistance (11 Ω sq−1) could be fabricated by painting a dispersed graphene solution in DMF (10 mg mL−1) on A4 size paper using a brush.

Recently, Rao et al. investigated glycine-bisulfate ionic-com-plex-assisted electrochemical exfoliation of a graphite electrode for soft processing of graphene nanosheets.[121] In the presence of glycine, the ionic complex formed surface molecule nuclei through the polymerization of intercalated monomeric bisulfate and sulfate ions. Intercalation of the active species resulted in the exfoliation of the graphite electrode to yield two to five layers of the graphene nanosheets. Because exfoliation of the graphite electrode occurs at a low applied voltage of 1 V for 5 min, this technique is useful for the development of a mild, environmen-tally friendly, and cost-effective exfoliation methodology.

4. Dispersants for Exfoliation of CNMs

Large-scale processing of CNMs is a key challenge for practical applications. CNMs can be dispersed in water by the assistance of small molecules of surfactants and macromolecules of water-soluble polymers, DNA, and peptide aptamers. Their ability to suspend CNMs as an individualized dispersion state in water opens the door for material and biomedical researchers and engineers.[122] Stabilizers for the aqueous dispersions of CNMs are also important for green processing, which may result in novel composite nanomaterials.

4.1. Chemicals as Dispersants

4.1.1. Surfactants

Usually, the surfactants possess hydrophobic parts of aliphatic groups and hydrophilic parts of ions or ethylene oxide groups.

SDS,[123] cholesterol derivatives,[124] cetrimonium bromide (CTAB), and non-ionic Triton X-100 are typical surfactants for the preparation of dispersed CNMs in water (Figure 10a). Mechanical force during sonication overcomes the van der Waals interactions between CNMs, leading to their exfolia-tion. Meanwhile, surfactants are adsorbed onto the surface of the exfoliated CNMs, which can provide steric or electrostatic repulsion between individual CNMs. Compared with the hydrophobic interactions between CNMs and conventional surfactants, aromatic surfactants exhibit exfoliation of CNMs through carbon-based π–π interactions (Figure 10b).[125]

Bonard et al. found that an aqueous dispersion of CNTs was obtained using 1 wt% solution of SDS after sonication.[126] Smalley’s group investigated the dispersion ability of a series of anionic, cationic, and non-ionic surfactants for individual SWCNTs.[122] They found that SDBS-dispersed CNTs showed the most well-resolved spectral features from all of the ionic sur-factants. High concentrations of SWCNTs could be suspended by high molecular weight non-ionic surfactants because of their enhanced steric stabilization with longer hydrophilic groups. Islam et al. reported that 63 ± 5% of CNT bundles could be exfoliated into single tubes in water by the non-specific phys-ical adsorption of SDBS even at SWCNT concentrations of 20 mg mL−1.[127] Hirsch et al. discovered a perylene bisimide derivative that was more effective than SDBS for dispersing SWCNTs, even though the concentration of the perylene was as low as 0.01 wt%.[128] The same group reviewed a variety of perylene-based surfactants for exfoliation and dispersion of CNMs.[129]

Aqueous dispersion and purification of single-walled carbon nanohorns (SWCNHs) were achieved using a surfactant. Five milligrams of as-grown CNHs can be dispersed in deuterium oxide (25 mL) with 0.5 wt% SDBS using ultrasonication. After ultracentrifugation at 220 000g for 1 h, smaller size SWCNHs (70–80 nm in diameter) were successfully separated from the aqueous dispersion.[130] Zhang et al. investigated the isolation of individual SWCNHs from oxidized CNH aggregates in an aqueous solution of sodium cholate using sucrose density gra-dient centrifugation.[131] The individual SWCNHs possessed several types of aggregated structures. The diameters and lengths of the resultant SWCNHs were in the range of 2–10 nm and 10–70 nm, respectively.

Coleman et al. prepared dispersed solutions of graphene in water stabilized by twelve different types of ionic and non-ionic surfactants.[124] The size of the graphene flake was similar in each case, typically 750 nm long and four layers thick on average. However, the concentrations of the dispersed solutions varied from 11 μg mL−1 for SDS to 26 μg mL−1 for sodium cho-late. For the stabilization mechanism of ionic surfactants, they suggested that the magnitude of the zeta potential ζ, which stabilizes the surfactant-coated flakes against aggregation, controlled the concentration of the dispersed graphene. For non-ionic surfactants, the stabilization mechanism might be based on steric effects. However, additional stabilization factors were found for non-ionic surfactants, i.e., the presence of acid groups and ether linkages interacting with water and the pres-ence of negative ζ values owing to adsorbed impurities.[124,132]

The stability of dispersed CNMs is also influenced by the characteristics of the absorbed surfactants such as quantity,

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charge, and morphology. Graphene sheets were dispersed in different concentrations of SDS,[133] which includes four stages of adsorption onto functionalized graphene sheets (Figure 10c): i) adsorption of isolated surfactant monomers with their alkyl chains oriented parallel to the carbon surface, ii) subsequent formation of a surfactant monolayer, iii) formation of hemi-cylindrical micelles prior to micelle formation in the bulk solu-tion under the critical surface aggregation concentration, and (iv) formation of micelles in the bulk solution above the critical micelle concentration (CMC).[134]

Torres et al. examined the dispersion behavior of fullerene using nine different surfactants and evaluated its antioxidant activity. They found that non-ionic surfactants such as Triton X-100 and poly(oxyethylene) lauryl ether had better solubiliza-tion ability for fullerene. Furthermore, their micellar solutions of fullerene exhibited high radical scavenging activity.[135]

A highly water-soluble perylene-based surfactant was designed to exfoliate graphite by π–π stacking interactions and the hydrophobic effect in water.[136] Furthermore, an amphi-philic compound consisting of an aromatic ring and an ethylene

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Figure 10. a) Chemical structures of typical anionic, cationic, and non-ionic surfactants. b) Aromatic surfactants for dispersion of CNMs. c) Schematic representation of SDS adsorption onto functionalized graphene. Reproduced with permission.[134] Copyright 2013, American Chemical Society.

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oxide dendron could selectively exfoliate graphite powder into single or double layers of graphene sheets in an aqueous solu-tion.[137] In another case, graphene was successfully exfoliated using 7,7,8,8-tetracyanoquinodimethane anion as a stabilizer in water, DMF, and dimethyl sulfoxide (DMSO).[138]

Recently, the 2,9-dimethyldiazaperopyrenium di-cation (MP2+)[81] was synthesized to directly exfoliate powdered graphite to graphene sheets through the exploitation of π–π interactions (Figure 11a) in mild conditions at 23 °C in water. Furthermore, electrostatic repulsion between positively charged regions in MP2+ decreased the aggregation of graphene layers and helped in sustaining a stable dispersion of the graphene sheets in water. The strong fluorescence associated with the chloride salt of MP2+ (MP∙2Cl) is almost entirely quenched in the presence of graphene (Figure 11b). A variety of designed perylene-based surfactants for exfoliation of SWCNTs in water were reviewed.[129] Srinivasan et al. prepared supramolecular gels based on GO and tetracationic cyclophanes under mild conditions.[139] Exfoliation and gelation of GO were observed

by ultrasonication, resulting from intercalation of the cationic macrocycles between GO sheets in DMF. Rheology studies revealed that the elastic properties of the GO/cyclophane gels were maintained with a strain amplitude of 10%.

4.1.2. Polymers as Dispersants

Dispersion of CNMs using biocompatible polymers and conju-gated polymers in aqueous solution has been explored. Aqueous suspensions of MWCNTs were prepared through non-covalent functionalization with a biocompatible carboxymethylcellulose or gum arabic (GA) after sonication.[140] The natural cationic polymer, chitosan, was found to selectively wrap and disperse “smaller-diameter’’ (0.91 and 0.82 nm) SWCNTs in the aqueous supernatant while ‘‘larger-diameter’’ SWCNTs were observed in the precipitate.[141] An ionic conjugated polymer of poly(phenylene ethynylene) (PPE) exhibited efficient exfoliation of SWCNTs in water.[142] The obtained SWCNTs were highly dispersed (≈80%),

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Figure 11. a) Schematic representation of exfoliation of graphite using MP2+ di-cation. b) Photographs of only graphite (i,iv), only MP∙2Cl (ii,v) and MP∙2Cl–graphene (iii,vi) in water under ambient light (i–iii) and under UV light (iv–vi). Reproduced with permission.[81] Copyright 2013, John Wiley & Sons, Inc. c) Dispersed SWCNTs in aqueous media under ultrasonication using amphiphilic PPE-based conjugated polymers. The self-assembled superstructure of the conjugated polymer–SWCNT composite was evaluated by AFM and TEM images. Reproduced with permission.[142] Copyright 2009, American Chemical Society. d) Schematic illustration of the interaction between tetronic block copolymers and graphene. Reproduced with permission.[150] Copyright 2011, American Chemical Society.

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as shown by AFM and transmission electron microscopy (TEM) (Figure 11c). They revealed that the origin of the aqueous dispersion of SWCNTs was the wrapping of the nanotube surface by a self-assembled helical superstruc-ture of PPE.

Size-tunable stable aqueous fullerene nan-oparticles (60–150 nm) were prepared with different concentrations of biocompatible pluronic acid (PA) triblock copolymer[143] by a sonochemical technique, which presented an enhanced stability in saline solutions owing to PA-induced steric hindrance. C60 can be dispersed in water by supramolecular encapsulation using gamma-cyclodextrin (γ-CD), which is an eight-membered sugar macrocycle.[144] C60 is sandwiched between two hydrophilic γ-CD moieties, leading to the aqueous dispersion of the γ-CD–C60 complex. Ikeda et al. explored the stability properties of the γ-CD-based complexes using C60 derivatives in water.[145] Using experimental and theoretical investigations, the authors determined that the stabilities of the complexes in the aqueous solutions were affected not only by steric hindrance but also by the polarities of the C60 derivatives. Nubusawa et al. investigated a pyridyl-substi-tuted γ-CD forming a sandwiched complex with C60 at a concentration of more than 70 mm.[146] The value of the resultant com-plex was 90 times higher than that of the nonsubstituted γ-CD–C60 complex. Recently, an aqueous dispersion of C60 was achieved by complexation of C60 between hyperbranched polyglycerol (HPG)-linked β-CDs.[147] The size of the monodispersed complex with a 1:2 ratio of C60 and β-CD-g-HPG was esti-mated to be 60 nm using dynamic light scattering. The biomedical applications using fullerene deriva-tives is discussed in Section 5.3.

The first polymer-coated graphitic nanoplatelets in stable aqueous dispersions were prepared through exfoliation via in situ reduction of graphite oxide in the presence of the amphiphilic polymer poly(sodium 4-styrenesulfonate).[148] Pyrene-terminated poly(N-isopropylacrylamide) was attached onto the basal plane of graphene sheets through π–π stacking. The resulting composites exhibited ther-moresponsive behavior of reversible transparent–turbid response in an aqueous solution at 24 °C.[149] Pristine gra-phene was dispersed by biocompatible poly(ethylene oxide)–poly(propylene oxide) (PEO–PPO) block copolymers at con-centrations exceeding 0.07 mg mL−1 in water. The thickness of the dispersed graphene sheets ranged from one to 10 layers of graphene.[150] The hydrophobic PPO segments are expected to interact strongly with the graphene faces leaving the hydrophilic PEO chains free to interface with other nearby PEO chains and the surrounding aqueous environ-ment (Figure 11d).

4.2. Biomacromolecules

Self-assembly of biomolecules is considered a valuable pathway for green processing of CNMs, owing to the high specificity, diverse chemical and biological functionalities, and environmen-tally benign processing. Single-stranded DNA (ssDNA) was found to effectively separate bundled SWCNTs even into an individual-ized dispersion state in water. Molecular modeling suggested that ssDNA could bind to SWCNTs through π–π stacking, resulting in helical wrapping at the surface (Figure 12a,b).[151,152] Kato et al. found that the addition of ssDNA to a surfactant-dispersed SWCNT solution led to clear absorption shifts in the near infrared region with an isosbestic point owing to the thermodynamic exchange from surfactant to ssDNA (Figure 12c,d).[153] Water-soluble mono-layers and bilayers of graphene sheets could be fabricated by soni-cation of graphite flakes in the presence of pyrene-labeled ssDNAs. The immobilized DNA on the graphene surface was further hybrid-ized with gold-nanoparticles-labeled complementary DNA to gen-erate graphene composites with nanomaterials.[154] This composite could be used for nanosensors and nanoelectronic applications.

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Figure 12. a) Schematic illustration of the DNA–SWCNT nanocomposite. b) The composite structure is viewed along the tube axis. Orange: thymine, green: adenine, yellow ribbons: backbones. Reproduced with permission.[152] Copyright 2009, Macmillan Publishers Limited. c) A schematic illustration of the exchange reaction between the surfactant (SC)–SWCNTs and ssDNA–SWCNTs composite. d) Absorption spectra of SWCNTs in the mixed solution of SC containing ssDNA of 0 (black), 0.0625 × 10−6 (red), 0.156 × 10−6 (orange), 0.313 × 10−6 (yellow), 0.469 × 10−6 (green), 0.938 × 10−6 (blue), and 15.6 × 10−6 m (purple) at 25 °C. Reproduced with permission.[125] Copyright 2015, National Institute for Materials Science. e) Aqueous dispersion of SWCNTs using peptide aptamer of A2 (IFRLSWGTYFS) below the CMC. f) The representative conformation of A2 on a SWCNT. (i) Side view (ii) front view. Reproduced with permission.[158] Copyright 2015, American Chemical Society.

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Peptides are regarded as promising non-covalent dispersants for SWCNTs in water, because the peptide backbone forms a folded helical conformation stabilized by hydrogen bonds. Jagota et al. demonstrated the affinity of peptides for CNTs by their direct binding to surface-immobilized phage. This pep-tide could act as a dispersant by hydrophobic interactions with CNT.[155] An interesting approach to the computational design of peptides, such as Hex (AEGESALEYGQQALEKGQLA-LQAGRQALKA), was to use self-organizing dispersants for SWCNTs.[156] Recently, our group successfully demonstrated that a ribosome display from a diverse random library was applied for selecting peptide aptamers with a higher binding affinity to SWCNTs.[157] The screened peptide aptamer named as A2 (IFRLSWGTYFS), could efficiently exfoliate and dis-perse SWCNTs in water owing to π–π interaction between aromatic groups in the peptide and the sidewalls of SWCNTs (Figure 12e,f). Among the dispersants, including low-mole-cular-weight surfactants, peptides, DNA, and a water-soluble polymer, A2 exhibited the highest dispersion capability below the CMC at a concentration of 0.02 w/v%.[158] In addition to CNTs, hybrid assembly of peptides and graphene into core/shell nanowires[159] was also demonstrated.

4.3. Carbon Allotropes as Dispersants for CNMs

It is a rather exciting discovery that carbon allotropes act as superior dispersants for other CNMs, especially in water. GO was found to disperse pristine CNTs into water to form stable suspensions through supramolecular interactions.[160] The GO–CNT nanocomposite could further promote the electrochemical conversion of GO to graphene, which exhibited high perfor-mance electrodes for supercapacitors. This discovery offers a very simple, cost-effective and environmentally friendly strategy for solution-processed all-carbon-nanocomposites.

Recently, the same group extended the application of GO as a stabilizer and systematically investigated the dispersion behavior of SWCNTs, fullerenes, and graphene in aqueous media (Figure 13a).[161] GO could be considered as a polyelectrolyte with surfactant-like characteristics that exfoliates CNMs through strong π–π interaction between the hydrophobic central plane of GO and the other conjugated sp2 network structures. Dispersed SWCNTs (Figure 13b) and C60 nanoparticles (Figure 13c) in water were obtained by exfoliating with GO sheets.

5. Applications of CNMs

During the last three decades, CNMs have attracted significant attention not only from the scientific community[6] but also for practical applications.[18] Our focus is devoted to the green processing of CNMs for energy conversion and biomedical applications.

5.1. CNMs for Energy-Conversion Devices

The energy conversion (e.g., solar cells and fuel cells) and storage (e.g., supercapacitors and batteries) device efficiencies depend strongly on the electrical properties of the materials.

In comparison with conventional energy materials, CNMs composed of sp2-bonded graphitic carbon exhibit unusual size-/surface-dependent properties that are useful for the enhance-ment of energy-conversion efficiencies.[162]

5.1.1. Green-Processed Photovoltaics

Solar energy is the most abundant, clean, and renewable energy source on earth. Scientists and engineers have made tre-mendous efforts to the convert energy of sunlight directly into electricity by the photovoltaic effect. Although inorganic solar cells show a high power conversion efficiency (PCE) up to 38% in the lab scale,[163] they are still too expensive to meet our daily energy needs. Fullerene-based OPVs have played an important role in cost-effective solar cells, while CNTs and graphene have been developed as transparent conductors.[164,165]

After the first report of photoinduced carrier transport from poly[2-methoxy-5-(2′-ethyl-hexyloxy)-p-phenylene vinylene] (MEH-PPV) to C60 in 1992, fullerenes have been recognized as efficient electron-accepting materials for OPVs.[166] Owing to the poor solubility of these materials, solution-processed OPVs are prepared using toxic organic solvents such as chloro-form, toluene, and dichlorobenzene. The use of green solvents instead of the toxic organic solvents is expected to be useful not only for human health but also for the environment.[167]

Toward green solvent-processed OPVs, both an alcohol-soluble polymer (PCDTBT-N) and C70 derivative (PC71BM-N) (Figure 14a), which were functionalized with pendant tertiary amino groups, were developed by Duan et al.[168] The cur-rent density–voltage characteristic of the as-cast PCDTBT-N:PC71BM-N solar cell from the n-butyl alcohol solution showed a weak photovoltaic response owing to the hole trap nature of the amine units. This contribution sheds light on the

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Figure 13. a) Dispersion behavior of CNMs using GO. b) TEM image of the GO–SWCNT composite. c) TEM image of the GO–fullerene composite. Reproduced with permission.[161] Copyright 2013, American Chemical Society.

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Figure 14. a) Chemical structures of alcohol-soluble PCDTBT-N and PC71BM-N for OPVs. Reproduced with permission.[168] Copyright 2013, The Royal Society of Chemistry. b) Chemicals structures of a water-soluble polythiophene showing thermo-cleavage behavior and a water-soluble PC61BM derivative. c) Schematic illustrations of conventional processing steps and all-water-processing steps for OPVs. Reproduced with permission.[167] Copyright 2011, John Wiley & Sons, Inc. d) i) Chemical structures of PffBT4T–C9C13 for green-processed OPVs using non-toxic hydrocarbon solvents of 1,2,4-trimethylbenzene (TMB) and 1-phenylnaphthalene (PN). ii) Current density–voltage characteristics. iii) External quantum efficiency spectra of the PffBT4T–C9C13:PC71BM films. Reproduced with permission.[169] Copyright 2015, Macmillan Publishers Limited.

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essentials of developing green-processed photovoltaic materials using an alcohol solution, and also provides new insights into the underlying working mechanisms.

Krebs et al. designed and synthesized a water-soluble PC61BM derivative and thermocleavable polythiophene for OPVs (Figure 14b).[167] The fullerene containing an ester sub-stituent of a triethyleneglycol group was soluble in polar sol-vents such as DMF, DMSO, and THF. The active layer was pre-pared by spin-coating from mixtures of water, isopropyl alcohol, and THF. An insoluble film was obtained after the thermal-cleavage of polythiophene in the side chain at 310 °C for 10 s. Finally, a metal electrode was formed by screen-printing using aqueous metal ink. The device characteristics of a representa-tive solar cell showed an open-circuit voltage of 0.49 V, short-circuit current of 4.46 mA cm−2, fill factor of 0.32, and PCE of 0.70%. Despite the low value of PCE, these results should be considered an essential step on the way to an environmen-tally friendly large-scale production of OPVs using aqueous-processing techniques (Figure 14c). Very recently, high-per-formance OPVs were achieved using a non-toxic hydrocarbon solvent.[169] A PCE of 11.7% was obtained from the resultant OPVs using 1,2,4-trimethylbenzene and 1-phenylnaphthalene (Figure 14d). This is a promising result for development of green-processing OPVs.

Ramuz et al. fabricated the first solution-processable, all-carbon solar cells to replace conventional indium–tin oxide (ITO) and metal electrodes including precious metals.[170] The anode and cathode were composed of r-GO and n-doped SWCNTs, respectively. A PCE of 5.7 × 10−3% was obtained from the active layer of poly(3-dodecylthiophene)-sorted SWCNTs and C60 under air mass 1.5 illumination.

Costa et al. fabricated SWCNH-doped efficient dye-sensitized solar cells (DSSCs) that showed an enhancement of the device characteristics.[171] A maximum energy conversion efficiency of 7.98% was achieved when 0.5 wt% of SWCNHs was incorpo-rated into the titanium oxide electrode. The authors suggested that an increase in the surface area of the electrode, quantity of dye adsorption, charge transport across the electrode interface, and charge recombination were influenced by the presence of SWCNHs in the DSSCs.

5.1.2. CNTs for Thermoelectric Devices

Thermoelectric devices are one of the most promising energy conversion devices from the viewpoint of global warming because they could generate electricity directly from heat. The thermoelectric performance of materials can be evaluated by the dimensionless figure of merit (ZT = (α2σ/κ)T), where α is the Seebeck coefficient, σ is electric conductivity, κ is the thermal conductivity, and T is the temperature.[172] A high power factor (α2σ) and a low thermal conductivity are expected to show good energy conversion properties.

CNTs have emerged as promising thermoelectric mate-rials for flexible film devices because they have a high σ, are lightweight, and have a high specific tensile strength. Yu et al. demon strated that the thermoelectric proper-ties could be enhanced by electric junctions between the CNTs using water-soluble poly(3,4-ethylenedioxythiophene)

poly(styrenesulfonate) (PEDOT:PSS).[173] To compare carrier-transporting properties, electrically insulating GA was also used for the preparation of the polymer–CNT composite. Figure 15a,b show the dispersion behavior of CNTs using GA and PEDOT:PSS in water. Formation of a segregated network during drying of the water-based polymer emulsions is illus-trated in Figure 15c,d. Initially, the nanotubes and polymer particles were uniformly dispersed in water (left). During evap-oration of water, the polymer particles pushed the nanotubes into interstitial spaces to form a segregated network (right). The PEDOT:PSS–CNT composite film showed high electrical conductivities up to 40 000 S m−1 with ZT ≈ 0.02, and a power factor of 25 μW m–1 K–2.

Yao et al. developed polyaniline–SWCNT nanocomposite films with a high power factor of 176 μW m–1 K–2 at room temperature.[174] Müller et al. reported that P3HT–SWCNT composite films doped with ferric chloride exhibited a power factor of 95 μW m–1 K–2 at room temperature.[175] A P3HT–SWCNT composite film that was obtained by wire-bar-coating showed a high power factor of 105 μW m–1 K–2 at room tem-perature without chemical doping.[176] Hewitt et al. fabricated poly(vinylidene fluoride)–MWCNT multilayered composite films for felt fabric thermoelectric devices (Figure 15e).[172] The power output in the device increased with increasing number of layers in the composite film (Figure 15f,g). A maximum power of 137 nW with a load resistance of 1270 Ω was obtained from the 72-layered composite film at ΔT = 50 K (Figure 15h). The fabric modules should be realistic about a lightweight, flex-ible, and portable thermoelectric device.

Aqueous-processed CNTs with non-conjugated polymers such as poly(vinyl acetate) (PVA)[177] and Nafion[178] were also explored for thermoelectric devices. However, the composite materials showed insufficient electrical contact between the dispersed CNTs owing to the intrinsic properties of insulating polymers. Efficient CNT-based thermoelectric devices by green processing need to be developed in the future.

5.1.3. Green-Processed Graphene-Based Energy-Storage Devices

Exfoliated graphene sheets have been used in a wide variety of applications owing to their extraordinary physical properties.[134] For example, the introduction of conducting graphenes in metal oxide electrodes led to an increase in the power density of lithium-ion batteries.[179] After addition of graphene into liquid fuels, lower ignition temperatures and enhanced combustion rates were proposed by molecular dynamics simulations.[180] Graphene has been most employed in supercapacitor devices. Ultracapacitors with extremely high specific capacitance of 120 F g−1 were prepared using dispersed graphene in water.[181]

Kim et al. developed electroconductive nanowires composed of multilayered graphene. A 1,1,1,3,3,3-hexafluoro-2-propanol (HFP) solution of peptides was poured into an aqueous r-GO dispersion to obtain a nanocomposite with a self-assembled core–shell structure (Figure 16a–c).[159] As shown in Figure 16d, the resultant core–shell nanowires exhibited good conductivity through their continuous graphene shell. Conversely, bare peptide nanowires without a graphene shell showed insu-lating behavior. After calcination of the peptide cores at 400 °C

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for 20 min, thin hollow shells consisting of 10 layers of r-GO were obtained (Figure 16e,f). The hollow-graphene-shell-net-work showed redox activity with a high specific capacitance of 157 F g−1 (Figure 16g).

Yang et al. prepared multilayered graphene films showing bioinspired prevention of restacking for the use in supercapaci-tors.[182] The resultant graphene layers exhibited an open pore structure, allowing an electrolyte solution to permeate indi-vidual sheets. A maximum power density of 414 kW kg−1 at a discharge current of 108 A g−1 could be achieved. Furthermore, the graphene-based supercapacitors showed excellent cycla-bility over 10 000 cycles even under a high operation current of 100 A g−1. r-GO was obtained by chemical reduction of GO using zinc powder in an ammonia solution under ultrasonication for 10 min at room temperature.[183] The specific surface area of the as-prepared graphene nanosheets was induced up to 262 m2 g−1, and a maximum specific capacitance of 116 F g−1 was achieved in an aqueous electrolyte solution of potassium hydroxide.

5.2. CNMs-Based Composite Materials

Multifunctional nanofillers composed of single-layer graphene were demonstrated to significantly improve the mechanical properties, lower the gas permeability, and improve the elec-trical conductivity of three distinct elastomers: natural rubber, styrene–butadiene rubber, and polydimethylsiloxane.[184] A study of aqueous-processed polyvinyl alcohol (PVA)–GO or PVA–graphene nanocomposites[185] revealed that GO exhib-ited better dispersion and exfoliation abilities, whereas the introduction of graphene gave rise to enhancement of the mechanical properties, electrical conductivity, and thermal stability.

Gong et al. developed nacre-inspired nanocomposites con-sisting of r-GO, double-walled carbon nanotubes (DWNTs), and 10,12-pentacosadiyn-1-ol (PCDO) through synergistic tough-ening.[186] The tensile strength and toughness of the r-GO–DWNT–PCDO composites reached 374.1 ± 22.8 MPa and

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Figure 15. Formation of 3D CNT networks along the surface of spherical emulsion particles. a,b) Dispersion behavior of CNTs using GA and PEDOT:PSS. c,d) Schematic illustrations of segregated networks before (left) and after (right) drying of water-based polymer emulsions. Reproduced with permission.[173] Copyright 2010, American Chemical Society. e) Photographs of the poly(vinylidene fluoride) (PVDF)–MWCNT multilayered ther-moelectric device. f) Device configuration for thermoelectric behavior. g) The thermoelectric voltage as a function of the number of PVDF–MWCNT layers. h) Thermoelectric power as a function of resistance of the 72-layered composite film. Reproduced with permission[172] Copyright 2012, American Chemical Society.

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9.2 ± 0.8 MJ m−3, which was 2.6 and 3.3 times higher than that of the pure r-GO film, respectively. They also demon-strated another artificial nacre nanocomposite by replacing the DWNTs with inexpensive nanofibrillar cellulose (NFC).[187] r-GO–NFC–PCDO nanocomposites exhibited an ultimate stress of 314.6 ± 11.7 MPa and a toughness of 9.8 ± 1.0 MJ m−3, respectively. Furthermore, electrical conduc-tivity as high as 162.6 S cm−1 was achieved from the bioin-spired nanocomposites.

MWCNTs were first used as conducting fillers in plastics because they take advantage of the high aspect ratio to form a percolation network at low concentrations.[18] Polymer–MWCNT composites showed good electrical conductivities as high as 10 000 S m−1 at 10 wt% loading of MWCNTs.[188] Furthermore, MWCNTs could act as a better flame-retardant additive for plastics compared with traditional nanoclay-based

flame retardants.[189] These CNT additives are more commer-cially attractive as a replacement for halogenated flame retard-ants, which have restricted use because of environmental regu-lations. Another environmentally friendly application of the CNM-based composite is water purification. Nanoporous fil-ters composed of CNT networks have been commercialized by Seldon Technologies for purification of contaminated drinking water.[18]

Vilela et al. prepared GO-based microrobots for removal and recovery of toxic heavy metals from water.[190] The composite structures consisted of multilayers of GO, nickel, and plat-inum. The outer layer of GO exhibited adsorption of lead on the surface, and the inner layer of platinum acted as an engine decomposing hydrogen peroxide fuel. The middle layer of nickel showed control of the microrobots driven by the external magnetic field. A decrease in the lead concentration from

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Figure 16. a) Schematic illustration of the self-assembly of peptide–graphene nanowires by mixing of a HFP solution of peptides and an aqueous dispersion of r-GO. b,c) Low-magnification (b) and high-magnification (c) SEM images of core–shell nanowires. d,e) TEM (d) and high-resolution TEM (e) images of the hollow graphene–shell structure after calcination. f) Current–voltage characteristics of a peptide–graphene composite nanowire and a bare peptide nanowire. g) Cyclic voltammograms of graphene-based supercapacitors in a 0.1 m H2SO4 aqueous solution. Reproduced with permis-sion.[159] Copyright 2010, John Wiley & Sons, Inc.

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1000 ppb to 50 ppb could be achieved within 60 min using the mobile microrobots.

5.3. CNMs for Biomedical Applications

The unique intrinsic fluorescence of CNMs has been widely employed for in vitro cell imaging and tissue staining.[5] CNM-based biosensors can exhibit large changes in electric imped-ance and optical properties in response to the surrounding environment, which is typically adjusted by adsorption–release of biomolecules on the surface of CNM.[18] Furthermore, the favorable sizes of CNMs have been attractive for drug loading and delivery to specific targets.[5]

There have been many reports of CNT-based electrochem-ical, FET, and optical biosensors for detection of diverse bio-logical structures such as DNA,[191] viruses, antigens,[192] disease markers, and whole cells.[193] A novel potentiometric biosensor based on aptamer-modified SWCNTs was devel-oped for specific real-time detection of a single pathogen at the single-bacterium level.[192] For microscale or nanoscale CNT-based FET, the change in the Schottky-barrier and/or charge transfer is the dominant mechanism responsible for biosensing analysis.[194] Near infrared (NIR) luminescence of semiconducting SWCNTs is particularly interesting for optical biosensing applications, as NIR emission is not absorbed by biological tissue. Furthermore, it can be used for monitoring biological samples or organisms.[191] Robinson et al. fabricated a r-GO-based electrical gas sensor, which was able to detect toxic gases at levels of parts per billion.[195] Graphene shows reproducible sensing responses because of intrinsic electro-chemical properties owing to less or no catalytic residues after its preparation.[193]

Recently, aptamer-modified r-GO-based catalytic micro-motors were developed for the detection of ricin.[196] The micro-motors were composed of r-GO and platinum modified with a ricin B aptamer tagged with FAM. The continuous movement of the motor could induce the enhanced specific binding of the ricin B toxin to the aptamer–dyeconjugate, leading to real-time fluorescent detection. The authors suggested that the aptamer micromotor concept could be expanded for detecting multiple biothreats and could hold considerable promise for a variety of biodefense applications.

The unique physicochemical properties of CNHs, including their large surface area, suggest that they could have potential applications in biosensors. Ojeda et al. developed simple and relatively low-cost immune sensors for determination of fibrin-ogen in human plasma and urine.[197] These devices were fabri-cated by the immobilization of fibrinogen onto activated CNHs deposited on screen-printed carbon electrodes. The implemen-tation of an indirect competitive assay was conducted using antifibrinogen labeled with horseradish peroxidase and hyd-roquinone as the redox mediator. A CNH-based protease bio-sensing platform was constructed to selectively detect thrombin down to the picomolar level.[198] After noncovalent bonding interactions of a fluorescein-labeled peptide were induced, exceptionally high fluorescence quenching of CNHs occurred. These results suggested that CNHs can be used as scaffold nanomaterials to improve biosensing properties.

CNMs for biological imaging has been summarized in another review.[5] Here we only introduce recent works using CNMs for drug delivery and imaging applications. The size of CNMs offers additional benefits in morbid regions such as tumors. The regions show increased retention of nanoma-terials owing to enhanced permeability and retention effects. During retention, CNMs can selectively destruct the cancerous bodies through photothermal therapy[199] by utilizing their high absorbance of NIR light or by activating photosensitizing agents.[200] Dai et al. found a PEG-functionalized SWCNT–dox-orubicin (DOX) complex exhibited extremely high drug loading efficiency of up to 60 wt% on CNTs compared with 8–10 wt% on conventional liposomes.[201] DOX-loaded SWCNTs have also demonstrated significantly enhanced in vivo therapeutic effi-cacy with greatly mitigated toxicity to treated mice in compar-ison with free DOX and liposome-solubilized DOX.[202]

By the physical incorporation and release of bioactive units either on their sidewalls or in their internal empty space, opened SWCNHs can be used as effective drug carriers. For example, Murakami et al. explored in vitro binding of the anti-inflammatory glucocorticoid dexamethasone (DEX) onto oxi-dized SWCNHs in ethanol/H2O mixtures.[203] The obtained DEX–SWCNH composites exhibited continuous release of biologically active DEX in mammalian cells without significant side effects.

Very recently, dihydroartemisinin (DHA) and transferrin (Tf) functionalized-GO (DHA–GO–Tf) was developed for pH-triggered chemotherapy (Figure 17a).[204] DHA is a novel anticancer drug utilizing its reactive oxygen species yielding mechanism for interacting with ferrous ion (Fe(II)). Tf has a dual function as a pilot for the nanoparticle to target tumor cell with overexpressed Tf receptors and as a Fe(II) carrier. The green fluorescence of dichlorodihydrofluorescein diacetate was observed in cytoplasm after Murine mammary tumor line (EMT6) cells were co-incubated with DHA–GO–Tf, resulting from an intracellular reactive oxygen species (ROS) genera-tion (Figure 17b). In comparison with DHA alone for tumor therapy, the DHA–GO–Tf composite resulted in a significantly enhanced specificity with minimal side effects and longer sur-vival rate in mice (Figure 17c).

In addition to the small molecules of anticancer drugs, biological therapeutics can be successfully attached to CNTs for translocation across the cell membrane. Proteins such as the epidermal growth factor receptor inhibitor were routinely attached to CNTs for the targeted delivery of drug payloads to enhance the efficacy of chemotherapy.[205] Carbon dots[206] and nanodiamonds[207] are newer members of the CNM family. They have been functionalized with positively charged poly-mers to bind and deliver DNA and small interfering RNA,[208] which showed promise as novel gene therapy agents. The intrinsic fluorescence of these two CNMs makes them particu-larly useful as multifunctional gene delivery vectors that allow for real-time tracking of their locations in live cells through microscopic fluorescence imaging.[5]

Water-soluble fullerenes and their composites have been emer-gent biomedical materials for DNA photo-cleavage agents,[209] anti-HIV protease inhibitors,[210] X-ray contrast agents,[211] and antibacterial agents.[212] Photocatalytic production of ROS was the most publicized mechanism for the cytotoxic and antibacterial

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properties of water suspension of fullerene (nC60). However, when nC60 was prepared using THF as a solvent, it can act as an oxidant and exert ROS-independent oxidative stress.[213] Fullerenes can also be used for photo dynamic therapy, because photoexcited C60 gener-ates ROS, resulting in efficient destruction of the vicinal tissue. To overcome the poor solubility of fullerenes in water, 6-amino-γ-cyclodextrin was successfully applied as a pH responsive carrier for C60 delivery to cancer cells. By adjusting the electrostatic repulsion between ammonium groups, C60 was rap-idly released from its inclusion complex and was released on slightly acidic surfaces of the cancer cell (Figure 17d). These results indicate that fullerenes are a good candidate for photo-dynamic therapy.[214]

To elucidate the effects of CNMs on cell functions is important for their biomedical applications. Mechanical property reflects organization of cell cytoskeleton, in particular the actin cytoskeletal. Molak et al. found that treatment with SWCNTs significantly increased the Young’s modulus of chondro-cytes[215] and other types of cells.[216] Mao et al. prepared collagen-functionalized SWCNTs (Col–SWCNTs), which were well dispersed in aqueous solutions for months.[217] After cellular uptake, Col–SWCNTs distributed in the perinuclear region showed no negative effects and were retained in the cells for more than 1 week. As imaging probes for labeling of human mesenchymal stem cells (hMSCs), Col–SWCNTs exhibited efficient cellular internalization without affecting their pro-liferation and differentiation.[218] For further biomedical application of CNTs, the same group fabricated porous collagen sponges containing SWCNTs for 3D culturing of bovine articular chondrocytes.[219,220] The incorporated SWCNTs in the porous sponges promoted cell proliferation and production of sulfated glycosaminoglycans. Even though there has been great progress made in the research of CNMs for biotechnical applica-tions, further in vivo medical use of CNMs requires more consideration, such as the immune response to CNMs and the defi-nition of the exposure dose. The retention time of CNMs within the body should also be critically controlled to prevent undesirable accumulation.[18]

Figure 17. a) Preparation of a DHA–GO–Tf composite used for pH-triggered chemo-therapy b) Fluorescent images of the DHA–GO–Tf composite in EMT6 cells. The scale bar is 10 μm. c) In vivo antitumor effect by intravenous injection of DHA–GO–Tf. Tumor growth curves of mice with different treatments (left). Survival curves of mice bearing EMT6 tumor with different treatments (right). Reproduced with permission.[204] Copyright 2015, Elsevier. d) pH-Responsive carrier of C60 with 6-amino-γ-cyclodextrin (ACD) inclusion complex

for photodynamic therapy: i) Direct insertion to the cell membranes and ii) intra cellular uptake of C60 aggregation surrounded by protonated ACDs. Reproduced with permission.[214] Copyright 2012, The Royal Society of Chemistry.

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6. Conclusions

Representative examples of green processing of CNMs using covalent- and non-covalent-bonding methods have been dis-cussed. Chemical functionalization based on covalent bonding might be beneficial for the reliable use of nanocarbon-based devices. Many efforts have been made to investigate effective dispersion using fewer amounts of harmful chemicals or less toxic reagents with mild reaction conditions. The resultant CNMs have improved as far as dispersion behavior not only in organic solvents but also in water. However, critical issues such as a decrease in conductivity owing to deformation of the hexag-onal sp2-bonded carbon atoms and change in biocompatibility owing to introduction of the solubilizing groups are frequently encountered. Thus, the major challenges for the near future are development of effective and precise control of chemical functionalization at specific reactive sites while maintaining the conjugated structure, which is responsible for the outstanding properties of CNMs.

Physical exfoliation using green solvents is a simple and clean processing method under mild conditions, because of no additional chemicals for stabilization of the dispersed solu-tions. However, because the dispersion properties are affected by the solubility of the CNMs, sometimes pretreatment of the materials is needed for stable dispersion properties, for example, oxidation of graphite to GO. The use of ball milling and ultrasonication techniques without any solvents is also a promising green and facile methodology that can be used to avoid harsh chemical treatments. Unfortunately, changes in the size and structures of the resulting dispersed materials are observed, depending on the processing time and the applied power. These drawbacks will be improved by means of a facile technique combining both mechanical forces from equipment and the physical properties of the solvents such as polarity, density, and boiling and melting points.

The use of dispersants is the most effective approach for physical exfoliation and dispersion of CNMs. Surfactants and biomolecules as efficient water-soluble dispersants are used to assist aqueous dispersion by wrapping the surface of the CNMs. Despite forming stable dispersed solutions, a decrease in electrical conductivity occurs significantly owing to the dis-ruption of the conducting pathway through the carbon-based π−π interactions. The development of aqueous-processable con-ducting and semiconducting dispersants will provide practical opportunities to create hybrid materials such as water-based conducting inks for electronic applications.

CNMs promise to open new opportunities for the develop-ment of next-generation applications from flexible electronics (energy conversion and storage, memory, and sensors) to lightweight composites (electromagnetic shielding packages, water filters, and heat dissipation sheets) for commercial prod-ucts.[18,221] However, real breakthroughs are still to come, even though innovative materials have been reported in laboratory-scale research. The present cost is a major roadblock to com-mercialization. For example, the graphene films that are used as transparent electrodes for touchscreens are prepared on hot copper foil using chemical vapor deposition. Unfortunately, the total cost for the graphene film including heating the foil, maintaining clean rooms, and separating the graphene film

from the foil is two times higher than that for the typical ITO transparent electrode.[9]

In contrast, solution processing such as drop-casting, spin-coating, ink-jet printing, and roll-to-roll manufacturing are promising methodologies.[222,223] These techniques contribute to low-cost and large-area thin-film device fabrication. However, scalable approaches to obtain stable dispersions of CNMs are still limited owing to lower efficiency. Recently, significant pro-gress has been reported for large-scale aqueous dispersions of CNMs by using simple gel chromatography for single-chirality separation of SWCNTs,[224] and shear exfoliation of defect-free few-layer graphene.[114] Undoubtedly, greener approaches are beneficial and facilitate processing for the development of a sustainable society. We believe that green and cost-effective processing of CNMs will be particularly useful not only in environmentally friendly fabrication methodologies but also in industrial applications in fields ranging from electronics to biotechnology.

Received: May 7, 2016Revised: July 11, 2016

Published online: November 17, 2016

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