Solubilization and Dispersion of Carbon Allotropes in Water and Non

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Solubilization and Dispersion of Carbon Allotropes in Water and Non-Aqueous Solvents. Oxana Vasilievna Kharissova, Cesar Maximo Oliva González, and Boris Ildusovich Kharisov Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b02593 • Publication Date (Web): 06 Sep 2018 Downloaded from http://pubs.acs.org on September 7, 2018

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Industrial & Engineering Chemistry Research

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Solubilization and Dispersion of Carbon Allotropes in Water and Non-Aqueous Solvents. Oxana V. Kharissova, César Máximo Oliva González, Boris I. Kharisov*. Universidad Autónoma de Nuevo León, Ave. Universidad, San Nicolás de los Garza, N.L., Mexico, 66455. E-mail [email protected] Abstract Available methods for solubilization and dispersion of several carbon allotropes (with exception of dispersions in polymers) in organic solvents and water are reviewed. Main attention is paid to graphite, graphene, fullerenes, nanoonions, nanodiamonds, carbon nanotubes, and carbon nanodots. The techniques to increase “solubility” of carbon allotropes include chemical functionalization of carbon surface (covalent and non-covalent functionalization, oxidation via ozonation, insertion of electron-withdrawing atoms (fluorination), polymer grafting, swelling and biomolecule treatments, use molecular “wedges” for intercalation, thermal reduction, among others). Combination of functionalization with use of surfactants, ultrasonic, laser ablation, milling, microwave expansion, hydrothermal metods and other treatments leads to better results. The possibility of dispersion and size of formed particles depends on a series of factors, in particular solvent nature, its viscosity and hydrogen bond donation ability, dispersion interactions, ππ-stacking between carbon surface and reagents, nature of surfactants, among others. Keywords: carbon allotropes; nanocarbons, dispersion; solubilization; nanodiamonds; nanoonions; graphene; graphite; carbon nanotubes; functionalization; Hansen Solubility Parameters.

Introduction Currently, the area of carbon allotropes, in particular nanocarbons, is one of the most developing fields in chemistry and nanotechnology, where carbon nanotubes and graphene are leaders in the number of publications. Taking into account their existing and potential technical, biological, medical applications (in particular for drug delivery purposes), and many others, we note that the main difficulty to integrate such materials into devices and biological systems derives from their lack of solubility in organic and physiological solutions. Functionalization of carbon allotropes with the assistance of biological molecules remarkably improves their solubility in aqueous or organic environment and, thus, facilitates the development of novel biotechnology, biomedicine, and bioengineering. For example, the nanodiamonds (NDs) have got a series of distinct applications in various areas, in particular medicine, electrochemistry and creation of novel materials. Biomedical applications of NDs are well-developed and related with the recently established fact that carbon NDs are much more biocompatible than most other carbon nanomaterials, including carbon blacks, fullerenes and carbon nanotubes.1 Their tiny size, large surface area and ease functionalization with biomolecules, makes NDs attractive for various biomedical applications both in vitro and in vivo, for instance for single particle imaging in cells, drug delivery, protein separation and biosensing.2 3 Similarly, water-soluble carbon nanoonions (CNOs) are used for biological imaging4 and as promising theranostic agents.5

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Many of these applications, as it was mentioned above, require an increased “solubility” (dispersibility) of carbon allotropes in solvents, first of all in water, especially for biological applications. This could be reached by their functionalization and other routes. In this review, we discuss all available methods leading to dispersion of carbon allotropes in water and organic solvents and present a state of the art for this very important problem of nanoscience and nanotechnology. The carbon nanotubes are examined here in a very condensed form, being fully generalized in a recent book6 and cited reviews therein. In brief, SWCNTs and MWCNTs can be solubilized by a host of physical, chemical, and biological methods, yielding short- or long-term dispersions in water or organic solvents. Solubilization of fullerenes Solubility of fullerenes (generally C60) has been intensively studied in the last decade of XX century and at the beginning of this millennium, in the era of boom of fullerene research,7 although more recent investigations8 9 10 11 12 13 14 15 (in particular, using biomolecules16) and reviews17 are also available. Their solubilization studies are really needed due to a lot of fullerene applications. One example is that aqueous suspensions of C60 and C70, nC60 and nC70, respectively, exhibit many similar physicochemical properties, yet nC70 appears to be significantly more photoactive than nC60.18 Before the discussion of specific features of fullerene solubility, we note that the solubility of fullerenes as well as dispersion of other carbon nanostructures can be described by Hansen solubility parameters (HSP, as a way of predicting if one material will dissolve in another and form a solution).19 Thus, the solubility of C60 was correlated with HSP in a database of 89 organic solvents.20 The parameters (indicative of an essentially nonpolar solid) δD (the energy from dispersion forces between molecules), δP (the energy from dipolar intermolecular force between molecules), and δH (the energy from hydrogen bonds between molecules) for this fullerene were determined as 19.7, 2.9, and 2.7, respectively, with units MPa1/2. These parameters were used to identify 55 additional predicted good solvents for C60. C60 solubilization. C60 molecule was found to be quite soluble in numerous organic solvents;21 its molecular aqueous solubility recently was reported at 7.96 ng/L,22 so molecular C60 does exist in water. Several peculiarities of fullerene solubility depending on solvent properties and fullerene-solvent interactions have been observed and corresponding relationships were established, for instance that the solubility of C60 is related with solvent polarizability and the solvent polarity.23 Among fundamental studies, a multiparameter linear model was proposed24 for C60 solubility in different solvents using solvent empirical parameters, covering more than 81 and 87% of the variance in the training and test sets, respectively. It was shown that hydrogen bond donation ability, basicity scale and dispersion interactions were some of the effective parameters for correlating the solubility of C60 in various solvents. In addition, the translational diffusion constant, D, of C60 was determined in the even n-alkanes n-C6H14 to n-C16H34 using microcapillary techniques and Taylor-Aris dispersion theory.25 It was established that the solute size decreased as the solvent viscosity increased, suggesting that the smoothness of C60’s surface diminishes the degree of interaction between the solute and solvent.


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Organic solvents. Solubilization of fullerene in common organic solvents is of an interest up to date due to promising applications. Thus, fluoroalkylpyrrolidine-substituted fullerene derivatives bearing a C4F9-phenyl group were found to be soluble in common organic solvents, which could be applied in the fabrication of photovoltaic cells.26 Unusual solubility versus temperature dependence for C60 fullerene in toluene and several other solvents (bromobenzene, o-xylene) is due to the formation of solid solvates.27 Temperature maximum of solubility in these solvents coincides with the incongruent melting point of the solvate with the mole ratio of C60 to solvent about 1:1.8. In addition, it was established that the oxidation products of C60 with m-chloroperoxybenzoic acid led to a fullerene diepoxide with both oxygen atoms positioned over 6:6 ring junctions on a common six-membered face of the carbon cage.28 This compound was found to have solubility in organic solvents that is similar to that of C60, being mostly soluble in carbon disulfide and odichlorobenzene, less soluble in toluene and benzene, and only slightly soluble (but more soluble than C60) in dichloromethane. Solution behavior of C60 in benzene was investigated by laser light scattering using an incident wavelength of 790 nm.29 C60 was found to aggregate slowly even in fairly dilute solution concentrations ranging from 1.39 to 2.11 mg/ml at 23-26°C. Among recent reports, solubility of light fullerenes (C60 and C70) in n-nonane was investigated in the ranges of pressure (0.1-100 MPa) and temperature (298.3-353.3 K).30 Under isothermal conditions, the solubility, expressed as weight fraction of the fullerene in the solution, increases monotonously with increasing pressure. At ambient pressure, it was found that the temperature dependence of the solubility of C60 in n-nonane is non-monotonic (in contrast with the C70–n-nonane system), being related with desolvatation of the C60-n-C9H20 solvate. Solubility in water. Several reports are dedicated to the interactions in the pair C60cyclodextrin (CD, or its derivatives), which, being water-soluble, solubilize the fullerene in water due to bond formation. Thus, γ-CD was found to catalyze the reaction of C60 with water during reflux; the water soluble 1:1 and 2:1 complexes some fullerene derivatives are formed.31 A chemical transformation of C60 during the solubilization or in solution in CD caused mainly by oxygen and/or water and catalyzed by light or γ-cyclodextrin.32 Also, various hydrophilic γ-cyclodextrin thioethers, containing neutral or ionic side arms were found to form molecular disperse solutions of C60 in water reaching concentrations of 15 mg/L,33 forming complexes (Fig. 1). These complexes show a much lower aggregation tendency than the corresponding ones of native γ-CD, which, on the other hand, form nanoparticles with C60. C60-tris-malonic derivative {C60[C(COOH)2]3} water solutions were investigated by the pycnometer, isothermal saturation, dynamic light scattering, and pH-potentiometric methods,34 providing information on the temperature dependence of solubility in water, concentration dependence on the hydrogen ion concentration, specific conductivity, molar conductivity, and dissociation constant. In particular, it was established that the temperature dependence of solubility is non-linear (the maximum of solubility was found to be at 333 K) and is related with several thermal effects, corresponding to decarboxylation with dehydrogenation (C=O and H2O releasing) from the three different malonate groups. Also, spherical fullerene–glycine derivatives are completely soluble in water, yielding a clear brown solution,35 and, being compared with the fullerene complex, they exhibit mortality



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and apoptosis of the cells which increased with the increase of fullerene–glycine derivative concentration. The solubility of derivatives in water is ~15 mg/mL (calculated as C60 residue), which could be explained by the intrinsic solubility of glycine. In addition to H2O, the fullerene–glycine derivatives are extremely soluble in a variety of organic solvents such as dichlorobenzene, dioxane, etc., having the solubility behavior similar to glycine itself and totally unlike C60. This fact which demonstrates the strong effect of glycine attachment.

Fig. 1. Space-filling model of the most stable complex between γ-CD and C60 with 1:2 stoichiometry, calculated in vacuo. Reprinted with permission from.36 Copyright 2010, American Chemical Society. Optimization of aqueous solubility can be carried out by solvent exchange method. Thus, Stable aqueous suspensions of colloidal C60 free of toxic organic solvents were prepared by two methods: ethanol to water solvent exchange and extended mixing in water, resulting in the last case the formation of larger and less negatively charged nC60 colloids than nC60 prepared by ethanol to water solvent exchange.37 In addition, 75 nm diameter particles of fullerene C60 aggregates (nC60) as aqueous suspensions can be produced under optimization of a solvent exchange method using toluene, tetrahydrofuran, acetone, and water as solvents.38 The standard solvent addition order of toluene, THF, acetone, and finally water, was chosen to gradually shift the solvent system from one of high-solubility of C60 to one of low solubility of C60, thereby gradually inducing particle aggregation. It was suggested that, with the addition of THF to a solution of C60 in toluene, more polar solvent induces nucleation of small aggregates, resulting in a transition from solution to a suspension. After starting particle seeding, acetone facilitates the transport into the aqueous phase, allowing additional seeding of the colloid and particle growth. The resulting particle sizes can be tuned by maintaining a 1:10 volume ratio between THF and acetone. In addition, we note an intriguing theoretical study on oxidation-induced water-solubilization and chemical functionalization of fullerenes C60, Gd@C60 and [email protected] Their H2O2 oxidations can readily proceed under alkaline conditions, resulting formation of the oxygen-containing groups of the fullerenols (hydroxyl, carbonyl, hemiacetal and deprotonated vic-diol) and, as a consequence, an increase of their solubility. A large-scale production of well-stable aqueous C60 dispersions of fullerene was offered by mixing a solution of crystalline fullerene in N-methylpyrrolidone (NMP) with water


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followed by exhaustive dialysis against water.40 Addition of amino acids or sugars at low concentration before dialysis increases the stability of the dispersion. It could be speculated that the process underlying C60 solubilization can involve a formation of the complexes of C60 molecules or their clusters with NMP followed by partial hydroxylation of the nanoparticle surface that is capable of stabilizing the C60 aggregates. This method was noted as promising for the preparation of aqueous solutions of endofullerenes. Discussing sugars, we emphasize, for instance, β-(1,3-1,6)-D-glucan,41 where concentrations of its fullerene derivatives were found to be ≈0.30 mM and the average particle sizes were ≈90 nm. In whole, we note that natural surfactants indeed contribute to a better fullerene solubilization in water: thus, mixing of pristine C60 in water with natural surfactant proteins latherin and ranaspumin-2 (Rsn-2) at low concentrations yields stable aqueous dispersions, where concentrations are compatible with clusters approximating 1:1 protein:C60 stoichiometry.42 Several representative data on C60 solubility in selected solvents are shown in Table 1. Table 1. C60 solubility in selected solvents. Partially reproduced with permission from.43 Copyright 2010, American Chemical Society. Solubility (S/g•L-1)

Solvent Alkanes Hexane (C6H14) Heptane (C8H16) Octane (C8H18)

0.052 0.027 0.302 Cyclic Alkanes

Cyclopentane (C5H10) Cyclohexane (C6H12 1-methyl-1-cyclohexane (C6H9CH3) Haloalkanes Dichloromethane (CH2Cl2) Cyclohexyl iodide (C6H11I) Tribromomethane (CHBr3) Natural oils Sunflower oil (Venus) Olive oil Linseed oil Animal Fats Pork fat Beef fat Margarine Essential oils Essential oils carnation Essential oils lemon Essential oils orange


0.036 0.054 1.03 0.26 8.06 5.64 6.91 0.909 0.365 0.052 0.041 0.019 1.551 18.8 21.3


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Inorganic solvents Water Tin (IV) chloride (SnCl4)

1.3E-11 0.09

Hexachlorodisilane ((SiCl3)2)

0.38

Alcohols Methanol (C2H5OH) Ethanol (C2H5OH) Butan-1-ol (C4H9OH) Aromatic Hydrocarbons Benzene (C6H6) Toluene (C7H8) Chlorobenzene (C6H5Cl)

0.000035 0.0014 0.0345 1.7 2.9 7

Solubilization of C70, higher fullerenes and their mixtures. Several data are available also for higher fullerenes. As in case of C60, the adduct formation can contribute to an increase of C70 fullerene solubility. Thus, a C70 derivative, indene-C70 bis-adduct, highly soluble in common organic solvents, was prepared with high yield (58 %) by a one-pot reaction of indene and C70.44 Among other investigations, the solubility of C70 in various solvents was measured at 303 K,45 revealing that the solubilities vary from 2 µg/mL in pentane to 36 mg/mL in o-dichlorobenzene (Table 2). The solubility (with exception of CS2) is correlated with solvent properties such as Hildebrand solubility parameter and polarizability. Pristine fullerene C70 can be also solubilized in imidazolium, ammonium and phosphonium based ionic liquids (ILs) bearing long alkyl chains (C8 or higher; see Table 3).46 Isothermal (20°C) solubility of fullerene C70 was also studied in solvents of the homologous series of monocarboxylic acids Cn−1H2n−1COOH (n = 1–9) and polythermal solubility over the temperature range 20–80°C of fullerene C70 in solvents of these acids (Table 4).47 At last, polythermal solubility of individual light fullerenes (C60, C70) and industrial fullerene mixture (60% C60, 39% C70, 1% C76–90) in natural oils (unrefined and refined sunflower‐ seed oil, corn oil, olive oil, linseed‐oil, apricot‐kernel oil, grape‐seed oil, pignolia oil, walnut oil) and animal fats (pork fat, chicken fat, beef fat, margarine, lamb fat, desi) was also investigated.48 Table 2. Solubility of C70 in organic solvents. Reproduced with permission from.45 Copyright 1994, Taylor & Francis. Solvent µg/mL Solvent µg/mL Pentane 2 Isopropanol 2.1 Hexane 13 CCl4 121 Heptane 47 p-Xylene 3985 Octane 42 Toluene 1406 Decane 53 Benzene 1300 Dodecane 98 CS2 9875


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Cyclohexane

80

Dichloromethane

80

Acetone

1.9

Dichlorobenzene

36210

Table 3. Solubility of C70 in ionic liquids. Reproducing with permission from.46 Copyright 2010, Elsevier Science. Solubility Solubility Ionic liquid Ionic liquid (µg/L) (µg/L) [BMIM][BF4]

0

[MTOA][Cl]

50

[BDMIM][Tf2N]

20

[MOTOA][Tf2N]

80

[MOIM][BF4]

40

[THTDP]]Cl]

40

[MOIM][PF6]

40

[THTDP][Tf2N]

60

[MOIM][Tf2N]

40

Methylcyclohexane

380

[DMIM][BF4]

60

Toluene

1406

Table 4. Solubility of C70 in monocarboxylic acids at different temperatures. Reproduced with permission from.47 Copyright 2008, Springer. Solubility (mg/L) 20°C 30°C 40°C 50°C 60°C 70°C 80°C Solvent Butyric 13.3 16.1 18.5 22.1 37.1 36.1 47.9 Valeric 13.8 19.7 21.7 25.9 30.9 33.4 38.9 Caproic 125 131 140 146 150 150 154 Enanthic 330 357 380 381 401 447 439 Caprylic 110 135 152 153 154 154 166 Pelargonic 56.2 56.9 58.1 57.1 84 101 117 Solubilization of nanoonions Fluorination, ozonation and acid treatments. Multi-shell fullerenes, the pristine carbon nanoonions (CNO) form a black-colored colloidal suspension in ethanol.49 The majority of the CNO material precipitated from the suspension within 10 days, leaving a gray-colored supernatant solution. In contrast, the similarly prepared colloidal suspensions of fluorinated F-CNO materials (obtained by direct fluorination of CNOs at 350, 410, and 480°C; the reaction is shown in the Fig. 2) in ethanol were light green-colored and remained stable for



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a long time, even after 1 year. In whole, an improved solubility was observed in organic solvents, such as alcohols and DMF, as compared to the pristine CNO, due to functionalization through direct fluorination, similar to nanodiamond and carbon nanotubes. As a result, direct fluorination can be used as the first step in the organic functionalization of CNO for nanocomposites, paints, coatings, and biomedical applications. In a related report,50 water solubilization of CNOs was carried out through their covalent functionalization by fluorination and subsequent derivatization with sucrose. The formation of covalent bond between sucrose and the surface of the fluorinated NOs was reached by a one-step fluorine substitution reaction with sucrose-derived lithium monosucrate under sonication in DMF at r.t. The functionalized CNOs were found to be soluble in water, DMF, ethanol, and other polar solvents. The solubilities of CNOs in water, EtOH and DMF are 200, 220, and 400 mg/L, respectively. It was found that the functionalized CNOs (as well as nanodiamonds, also studied in this report) are about twice as much soluble as similarly functionalized SWCNTs in all three polar solvents tested. The fluorinated CNOs are insoluble due to their hydrophobic nature, meanwhile CNOs functionalized with the highly hydrophilic sucrose form dark solution. However, the addition of small amount of HCl followed by sonication caused complete precipitation of water-solubilized CNOs due to cleavage of sucrose groups. Fig. 3 shows a comparison of solubilities of CNOs with other nanocarbons after sonication followed by one week standing. Due to the abundance of hydroxyl groups available for hydrogen bonding and further chemical modification, these soluble CNOs are expected to be biocompatible. The room temperature oxidation of CNOs with ozone was performed51 under mild conditions and oxidized products with high concentrations of oxygen-containing functional groups were obtained. Thus ozonized CNOs were found to be highly soluble in polar solvents such as water, methanol, and THF. The solution ozonolysis of CNOs provides both increased hydrophilicity and conductivity for improved performance of aqueous type EDLC. The oxidation of CNOs by ozonation is more effective than by concentrated acid solutions; it is considered as a “green” process without use of hazardous chemicals, representing an effective and easy to implement method to modify the surface of carbon nanostructures. In addition, water-soluble CNOs were introduced52 as a nontoxic, fluorescent reagent enabling Drosophila melanogaster (fruit flies) to be imaged alive. It was demonstrated that these CNOs (Fig. 4; readily soluble in water due to the presence of extensive carboxylate groups on the surface), synthesized from wood waste (by pyrolysis of wood wool at 600°C in a muffle furnace under a flow of nitrogen:oxygen (95:5) gas mixture for 2 h and a series of further treatments including the step with concentrated HNO3), colorfully image all the development phases of Drosophila melanogaster from its egg to adulthood. Such dense carboxylation occurred upon oxidation due to the presence of an appreciable amount of surface defects. Oral ingestion of up to 4 ppm of soluble CNOs allowed the optical fluorescence microscopy imaging of all the stages of the fruit fly life cycle without showing any toxic effects. It was, however, noted that the nitric acid treatment is effective for the carboxylation of CNOs, but in the case of the CNTs is different: the density of such carboxylate groups is very low and therefore thus functionalized CNTs are not readily soluble in water.


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Fig. 2. Representation of the fluorination of CNOs. Reproduced with permission from.49 Copyright 2007, American Chemical Society.

Fig. 3. Photograph of water dispersions of pristine and sucrose functionalized nanocarbons: SWCNTs (1), Suc-SWCNTs (2), ND (3), Suc-NDs (4), CNOs (5), and Suc-CNOs (6). Reproduced with permission from.50 Copyright, 2010, Springer.

Fig. 4. Microscope images of the water-soluble CNOs under a) SEM; b) TEM; c) HRTEM. These show the surface defects and d-spacing for CNOs; d) A diameter distribution histogram of water-soluble CNOs. Reproduced with permission from.52 Copyright 2011, Wiley.



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Organic functionalization. The functionalization of CNOs with polyaniline (PANI) can be easily achieved by in situ polymerization by using phenyleneamine-terminated CNO structures (reaction is shown in the Fig. 5).53 The modified CNOs were found to be soluble in polar solvents such as water, methanol, and THF. The hydrophilicity and the wettability of the CNOs was increased here in aqueous electrolytes, facilitating aniline polymerization on the surface, without damages or structural changes in the carbon surfaces. The CNOs were also functionalized54 with carboxylic groups by chemical oxidation and then reacted with β-cyclodextrins (βCD). A biocompatible dextran polymer with grafted ferrocene groups (Fc-Dex) was employed for the supramolecular self-assembly on the βCD-CNO surfaces. The groups of ferrocene in the Fc-Dex interacts with the βCD in the surface of the CNOs; the βCDs act as hosts and the polymer ferrocene groups as guest. The reaction for the functionalization is illustrated in the Fig. 6. The functionalized CNOs showed an increased solubility in aqueous solutions, revealing that the functionalized CNOs have a homogeneous size. However, when excess of sodium adamantine carboxylate is added, a displacement of ferrocene molecules from the βCD cavity caused the precipitation of CNOs. Other important information on the solubilization of nanoonions can be seen in recent reports.55 56

Fig. 5. Synthesis of CNO/PANI composites. Reproduced with permission from.53 Copyright 2012, Wiley.


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Fig. 6. Representation of the synthesis of CNOs modified with βCD and polymer with ferrocene: (a) reaction of the CNOs with H2SO4:HNO3 (b) amidation reaction with βCDNH2, EDC, NHS, and DMAP; (c) addition polymer with ferrocene in water. Reproduced with permission from.54 Copyright 2015, American Chemical Society. Solubilization of carbon nanotubes As we noted above, complete information on the solubilization and dispersion of SWCNTs and MWCNTs can be found in a recent Springer´s monograph, so here we present this material in a very shortened version. The history of CNTs solubilization is not long. One of the first fundamental reviews57 in the field of CNTs dissolution and, more recently, a book,58 several chapters59 60 61 62 63 and reviews64 65 66 67 68 69 70 71 72 73 74 75 76 have been published, which are devoted to particular aspects of CNTs dispersion and stability in liquid media. In particular, some reviews were dedicated to the strategic approaches toward the solubilization of CNTs using chemical and physical modifications,77 78 79 environmental, toxicological and pharmacological studies related with use of CNTs,80 81 the main methods for the modification of CNTs with polymers,82 83 applications of functionalized CNTs,84 85 in particular as biosensors,86 and discussions on the possibility of the existence of SWCNTs in organic solvents in the form of clusters.87 In addition, certain attention is paid to CNTs nanofluids.88 89 90 91 92 A series of contemporary techniques are being used for CNTs solubilization, from physical (classic ultrasound, plasma treatment, UV-light, dielectrophoresis,93 94 gel electrophoresis,95 96 density gradient ultracentrifugation,97 98 irradiation techniques, and chromatography,99 among others) to chemical and biological, applying inorganic (other carbon allotropes, iodine, bromine, metallic sodium in liquid ammonia, CO2, peroxides, ozone, metal salts and



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mineral acids) and organic (acids, salts, polymers, dyes, natural products, in particular diazonium salts,100 functionalization with porphyrins, amines, pyrene, polymers, sugars, etc.) compounds, and biomolecules (DNA, peptides, amino acids, etc.), as well as well as micelles on their basis and some metal complexes. CNTs can be functionalized by oxidation (peroxyacids, metal oxidants, such as osmium tetraoxide, potassium permanganate, chromates; ozone, oxygen, superoxides) or reduction reactions (interactions with thiols, carbenes, dienes, etc.). Frequently, physical action (more frequently ultrasound, more rarely hydrothermal technique) is combined with chemical/biological treatment. In some cases, successive steps can be applied, for instance use of low- and high-weight surfactants, mineral acid treatment for creation of –OH and –COOH groups and their further interaction with organic molecules. Carbon nanotube dispersion in nematic liquid crystals is also known. Many of these chemical agents above are amphiphiles, known to be suitable molecules to disperse CNTs in water by shielding their highly hydrophobic surfaces. Tailored anionic, nonionic, and cationic photopolymerizable amphiphiles (objects of permanent attention101 102 as good dispersants for CNTs) were designed to achieve programmed pH-dependent dispersions of CNTs.103 All of these methods suffer from problems with scalability, effectiveness, and often dissolution sensitivity. Several selected methods and strategies,104 based on organic reactions and leading to CNTs functionalization,105 emphasize their universality for CNTs solubilization. At the same time, not all methods are sometimes desirable for use, since some of them, for example, can heavily damage SWCNTs and MWCNTs. Scalability is a separate problem, which needs a profound investigation, taking into account the 1) possibility to scale up a laboratorydeveloped process, 2) availability and cost of equipment and chemicals, 3) possible economical effect, and 4) environmental aspects based on toxicity of CNTs and chemicals used. Among solvents, water is most preferable due to much more existing and potential CNTs applications, in particular more for medical and biological purposes. N-methyl-2pyrrolidinone (NMP), N-dodecyl-pyrrolidinone (N12P), acetone, tetrahydrofuran (THF), N,N-dimethylformamide (DMF), N,N-dimethylacetamide (DMA), cyclohexylpyrrolidinone (CHP) have been considered as suitable solvents for CNTs dispersion, meanwhile, in contrast, much precipitation can be obviously observed for systems of the CNTs in water, ethanol and toluene. Non-functionalized CNTs can be solubilized in suitably chosen organic solvents and, furthermore, their solubility could be understood in terms of the Hansen Solubility Parameters,106 similarly as for fullerenes (see above).

Solubilization of graphite There are several reports on the solubilization of graphite and oxidized graphite (graphite oxide, GrO)107 108 109 110 111 112 113 114 resulting mainly graphene (G). In the dispersed state, the resulting carbon mainly appears in these processes in the form of graphite flakes with distinct sizes or, more frequently, graphene. Other species could also be formed, for instance polyynes. Indeed, as it has been observed, most reports on graphite solubilization are dedicated to the attempts of optimization of graphene fabrication. Both polar and non-


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polar solvents have been used in these studies. Thus, the GrO can be exfoliated directly in several polar solvents, such as water, ethylene glycol, N,N-dimethylformamide (DMF), Nmethylpyrrolidone (NMP), and tetrahydrofuran (THF), to form dispersion solutions of GrO at a concentration of around 0.5 mg mL-1.115 Ultrasound-assisted solubilization. As well as for the case of carbon nanotubes and other carbon allotropes, the ultrasonic treatment has been widely applied as a principal or supportive technique for graphite disaggregation both in water and organic solvents. Thus, graphene layers were synthesized using ultrasonic dispersion of graphite flakes followed by ultracentrifugation in sodium cholate and polyoxyethylene nonylphenyl ether aqueous solutions.116 Dispersing graphite in surfactant−water solutions applying ultrasound leads to a large-scale exfoliation resulting large quantities of multilayer graphene (

Solubilization and Dispersion of Carbon Allotropes in Water and Non

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