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Cite This: Ind. Eng. Chem. Res. 2018, 57, 12624−12645
Solubilization and Dispersion of Carbon Allotropes in Water and Non-aqueous Solvents Oxana V. Kharissova, Ceś ar Max́ imo Oliva Gonzaĺ ez, and Boris I. Kharisov*
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Universidad Autónoma de Nuevo León, Ave. Universidad, 66455 San Nicolás de los Garza, NL, Mexico ABSTRACT: Available methods for solubilization and dispersion of several carbon allotropes (with the 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 the “solubility” of carbon allotropes include chemical functionalization of the carbon surface (covalent and non-covalent functionalization, oxidation via ozonation, insertion of electron-withdrawing atoms (fluorination), polymer grafting, swelling and biomolecule treatments, use of molecular “wedges” for intercalation, thermal reduction, etc.). The combination of functionalization with the use of surfactants, ultrasound, laser ablation, milling, microwave expansion, hydrothermal methods, and other treatments leads to better results. The possibility of dispersion and the size of the formed particles depend on a series of factors, in particular the nature of the solvent, its viscosity and hydrogen-bond donation ability, dispersion interactions, π−π stacking between the carbon surface and reagents, and the nature of the surfactants, among others.
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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 of the available methods leading to dispersion of carbon allotropes in water and organic solvents and present the state of the art for this very important problem of nanoscience and nanotechnology. CNTs are examined here in a very condensed form, being fully generalized in a recent book6 and reviews cited therein. In brief, single-wall CNTs (SWCNTs) and multiwall CNTs (MWCNTs) can be solubilized by a host of physical, chemical, and biological methods, yielding short- or long-term dispersions in water or organic solvents.
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, and medical applications (in particular for drug delivery purposes), and many others, we note that the main difficulty in integrating 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 environments and thus facilitates the development of novel biotechnology, biomedicine, and bioengineering. For example, nanodiamonds (NDs) have a series of distinct applications in various areas, in particular medicine, electrochemistry, and the creation of novel materials. Biomedical applications of NDs are well-developed and related to the recently established fact that carbon NDs are much more biocompatible than most other carbon nanomaterials, including carbon blacks, fullerenes, and carbon nanotubes (CNTs).1 Their tiny size, large surface area, and ease of functionalization with biomolecules make 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 Many of these applications, as mentioned above, require increased “solubility” (dispersibility) of carbon allotropes in © 2018 American Chemical Society
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SOLUBILIZATION OF FULLERENES
The solubility of fullerenes (generally C60) has been intensively studied in the last decade of the 20th century and at the beginning of this millennium, in the era of the boom of fullerene research,7 although more recent investigations8−15 (in particular, using biomolecules16) and reviews17 are also available. Their solubilization studies are really needed because of 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 Received: Revised: Accepted: Published: 12624
June 11, 2018 July 31, 2018 September 6, 2018 September 6, 2018 DOI: 10.1021/acs.iecr.8b02593 Ind. Eng. Chem. Res. 2018, 57, 12624−12645
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Industrial & Engineering Chemistry Research
Figure 1. Space-filling model of the most stable complex between γ-CD and C60 with 2:1 stoichiometry, calculated in vacuo. Reprinted from ref 36. Copyright 2010 American Chemical Society.
and several other solvents (bromobenzene, o-xylene) is due to the formation of solid solvates.27 The temperature of maximum solubility in these solvents coincides with the incongruent melting point of the solvate with a mole ratio of C60 to solvent of about 1:1.8. In addition, it was established that the oxidation 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 similar to that of C60, being mostly soluble in carbon disulfide and o-dichlorobenzene (DCB), less soluble in toluene and benzene, and only slightly soluble (but more soluble than C60) in dichloromethane. The 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 at fairly dilute solution concentrations ranging from 1.39 to 2.11 mg/mL at 23−26 °C. Among recent reports, the solubilities of light fullerenes (C60 and C70) in n-nonane was investigated in the pressure range of 0.1−100 MPa and the temperature range of 298.3−353.3 K.30 Under isothermal conditions, the solubility, expressed as the 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 nonmonotonic (in contrast to the C70−nnonane system), being related to desolvation of the C60−nC9H20 solvate. Solubility in Water. Several reports are dedicated to the interactions of C60 with cyclodextrins (CDs, or their derivatives), which, being water-soluble, solubilize the fullerene in water through bond formation. Thus, γ-CD was found to catalyze the reaction of C60 with water during reflux; watersoluble 1:1 and 2:1 complexes of some fullerene derivatives are formed.31 A chemical transformation of C60 during the solubilization or in solution in CD is caused mainly by oxygen and/or water and catalyzed by light or γ-CD.32 Also, various hydrophilic γ-CD 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 (Figure 1). These complexes show a much lower aggregation tendency than the corresponding ones of native γCD, which, on the other hand, form nanoparticles with C60.
Before the discussion of specific features of fullerene solubility, we note that the solubility of fullerenes as well as the dispersion of other carbon nanostructures can be described by Hansen solubility parameters (HSPs) as a way of predicting whether one material will dissolve in another and form a solution.19 Thus, the solubility of C60 was correlated with HSPs 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 forces between molecules), and δH (the energy from hydrogen bonds between molecules) for this fullerene were determined to be 19.7, 2.9, and 2.7 MPa1/2, respectively. These parameters were used to identify 55 additional predicted good solvents for C60. Solubilization of C60. The 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 have been established, for instance that the solubility of C60 is related to the solvent polarizability and the solvent polarity.23 Among fundamental studies, a multiparameter linear model for C60 solubility in different solvents using solvent empirical parameters was proposed,24 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 solubilities of C60 in various solvents. In addition, the translational diffusion constant, D, of C60 was determined in the even nalkanes 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. Organic Solvents. Solubilization of fullerenes in common organic solvents is of current interest because of 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 The unusual temperature dependence of the solubility of C60 fullerene in toluene 12625
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followed by partial hydroxylation of the nanoparticle surface, which 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 the concentrations of its fullerene derivatives were found to be ∼0.30 mM and the average particle sizes were ∼90 nm. On the whole, we note that natural surfactants indeed contribute to better fullerene solubilization in water. Thus, mixing of pristine C60 in water with the natural surfactant proteins latherin and ranaspumin-2 (Rsn-2) at low concentrations yields stable aqueous dispersions in which the concentrations are compatible with clusters approximating 1:1 protein:C60 stoichiometry.42 Representative data on the solubilities of C60 in selected solvents are shown in Table 1.
Water solutions of the tris(malonic acid) derivative of C60, {C60[C(COOH)2]3}, were investigated by pycnometry, isothermal saturation, dynamic light scattering, and pH potentiometry,34 which provided information on the temperature dependence of the solubility in water, the dependence of the concentration on the hydrogen ion concentration, the specific conductivity, the molar conductivity, and the dissociation constant. In particular, it was established that the temperature dependence of the solubility is nonlinear (the maximum solubility was found to occur at 333 K) and is related to several thermal effects, corresponding to decarbonylation with dehydration (release of CO and H2O) from the three different malonate groups. Also, spherical fullerene− glycine derivatives are completely soluble in water, yielding clear brown solutions,35 and, compared with the fullerene complex, they exhibit mortality and apoptosis of the cells that increase with increasing 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 DCB, dioxane, etc., having solubility behavior similar to that of glycine itself and totally unlike that of C60. This fact demonstrates the strong effect of glycine attachment. Optimization of the aqueous solubility can be carried out by the 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, with the latter case resulting in 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 were produced by optimization of a solvent exchange method using toluene, tetrahydrofuran (THF), 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 C60 solubility to one of low C60 solubility, thereby gradually inducing particle aggregation. It was suggested that with the addition of THF to a solution of C60 in toluene, the more polar solvent induces nucleation of small aggregates, resulting in a transition from a solution to a suspension. After particle seeding is started, 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 of THF and acetone. In addition, we note an intriguing theoretical study on oxidationinduced water solubilization and chemical functionalization of the fullerenes C60, Gd@C60, and
[email protected] Their H2O2 oxidations can readily proceed under alkaline conditions, resulting in the formation of the oxygen-containing groups of the fullerenols (hydroxyl, carbonyl, hemiacetal, and deprotonated vicinal diol) and, as a consequence, an increase in their solubility. The large-scale production of stable aqueous C60 dispersions was offered by mixing of a solution of the crystalline fullerene in N-methylpyrrolidone (NMP) with water followed by exhaustive dialysis against water.40 Addition of amino acids or sugars at low concentration before dialysis increased the stability of the dispersion. It was speculated that the process underlying C60 solubilization involves the formation of complexes of C60 molecules or their clusters with NMP
Table 1. Solubilities (S) of C60 in Selected Solvents43 S (g/L)
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 carnation essential oils lemon essential oils orange essential oils Inorganic Solvents water tin(IV) chloride (SnCl4) hexachlorodisilane ((SiCl3)2) Alcohols methanol (CH3OH) ethanol (C2H5OH) butan-1-ol (C4H9OH) Aromatic Hydrocarbons benzene (C6H6) toluene (C7H8) chlorobenzene (C6H5Cl)
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 1.3 × 10−11 0.09 0.38 0.000035 0.0014 0.0345 1.7 2.9 7
Solubilization of C70, Higher Fullerenes, and Their Mixtures. Several data are also available for higher fullerenes. As in case of C60, adduct formation can contribute to an increase in the C70 fullerene solubility. Thus, a C70 derivative, indene−C70 bisadduct, was prepared in high yield (58%) by a one-pot reaction of indene and C70 and was highly soluble in common organic solvents.44 Among other investigations, the 12626
DOI: 10.1021/acs.iecr.8b02593 Ind. Eng. Chem. Res. 2018, 57, 12624−12645
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contrast, the similarly prepared colloidal suspensions of fluorinated CNO (F-CNO) (obtained by direct fluorination of CNOs at 350, 410, and 480 °C; the reaction is shown in Figure 2) in ethanol were light-green-colored and remained
Table 2. Solubilities of C70 in Organic Solvents45 solvent
S (μg/mL)
solvent
S (μg/mL)
pentane hexane heptane octane decane dodecane cyclohexane acetone
2 13 47 42 53 98 80 1.9
isopropanol CCl4 p-xylene toluene benzene CS2 dichloromethane dichlorobenzene
2.1 121 3985 1406 1300 9875 80 36210
Figure 2. Representation of the fluorination of CNOs. Reproduced from ref 49. Copyright 2007 American Chemical Society.
stable for a long time (more than 1 year). On the whole, improved solubility was observed in organic solvents, such as alcohols and N,N-dimethylformamide (DMF), compared with pristine CNOs as a result of the functionalization through direct fluorination, similar to NDs and CNTs. As a result, direct fluorination can be used as the first step in the organic functionalization of CNOs 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 a 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 room temperature. 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 soluble as similarly functionalized SWCNTs in all three polar solvents tested. The fluorinated CNOs are insoluble because of their hydrophobic nature, whereas CNOs functionalized with the highly hydrophilic sucrose form dark solutions. However, the addition of a small amount of HCl followed by sonication caused complete precipitation of water-solubilized CNOs due to cleavage of the sucrose groups. Figure 3 shows a comparison of the solubilities of CNOs with other nanocarbons after sonication followed by standing for 1 week. Because of 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
exception of CS2) is correlated with solvent properties such as the 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 The Table 3. Solubilities of C70 in Ionic Liquids46 IL
S (μg/L)
IL
S (μg/L)
[BMIM][BF4] [BDMIM][Tf2N] [MOIM][BF4] [MOIM][PF6] [MOIM][Tf2N] [DMIM][BF4]
0 20 40 40 40 60
[MTOA][Cl] [MOTOA][Tf2N] [THTDP]]Cl] [THTDP][Tf2N] methylcyclohexane yoluene
50 80 40 60 380 1406
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) as well as the polythermal solubility of fullerene C70 over the temperature range 20−80 °C in these acids (Table 4).47 Finally, the polythermal solubilities of individual light fullerenes (C60 and C70) and an 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, grapeseed oil, pignolia oil, and walnut oil) and animal fats (pork fat, chicken fat, beef fat, margarine, lamb fat, and desi) were also investigated.48
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SOLUBILIZATION OF NANOONIONS Fluorination, Ozonation, and Acid Treatments. Pristine carbon nanoonions (multishell fullerenes), formed black-colored colloidal suspensions in ethanol.49 The majority of the CNO material precipitated from the suspension within 10 days, leaving a gray-colored supernatant solution. In
Table 4. Solubilities of C70 in Monocarboxylic Acids at Different Temperatures47 S (mg/L) solvent
20 °C
30 °C
40 °C
50 °C
60 °C
70 °C
80 °C
butyric valeric caproic enanthic caprylic pelargonic
13.3 13.8 125 330 110 56.2
16.1 19.7 131 357 135 56.9
18.5 21.7 140 380 152 58.1
22.1 25.9 146 381 153 57.1
37.1 30.9 150 401 154 84
36.1 33.4 150 447 154 101
47.9 38.9 154 439 166 117
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Figure 3. Photograph of water dispersions of pristine and sucrose (Suc)-functionalized nanocarbons (left to right): SWCNTs, Suc-SWCNTs, NDs, Suc-NDs, CNOs, and Suc-CNOs. Reproduced with permission from 50. Copyright 2010 Springer.
Figure 4. (a) SEM, (b) TEM, and (c) HRTEM images of water-soluble CNOs showing the surface defects and d spacing of CNOs. (d) Diameter distribution histogram of water-soluble CNOs. Reproduced with permission from ref 52. Copyright 2011 Wiley.
carboxylation occurred upon oxidation because of the presence of an appreciable amount of surface defects. Oral ingestion of up to 4 ppm soluble CNOs allowed optical fluorescence microscopy imaging of all of the stages of the fruit fly life cycle without showing any toxic effects. However, it was noted that although the nitric acid treatment is effective for the carboxylation of CNOs, the case of CNTs is different: the density of such carboxylate groups is very low, and therefore, thus-functionalized CNTs are not readily soluble in water. Organic Functionalization. Functionalization of CNOs with polyaniline (PANI) can be easily achieved by in situ polymerization by using phenyleneamine-terminated CNO structures (the reaction is shown in Figure 5).53 The modified CNOs were found to be soluble in polar solvents such as water, methanol, and THF. The hydrophilicity and wettability of the CNOs was increased here in aqueous electrolytes, facilitating aniline polymerization on the surface, without damage or structural changes on the carbon surfaces. The CNOs were
THF. Solution ozonolysis of CNOs provides both increased hydrophilicity and conductivity for improved performance of aqueous-type electrical double layer capacitors. Oxidation of CNOs by ozonation is more effective than oxidation by concentrated acid solutions; it is considered as a “green” process without the 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. These CNOs (Figure 4), which are readily soluble in water because of the presence of extensive carboxylate groups on the surface, were synthesized from wood waste by pyrolysis of wood wool at 600 °C in a muffle furnace under a flow of a 95:5 nitrogen/oxygen gas mixture for 2 h and a series of further treatments, including a step with concentrated HNO3. The CNOs colorfully imaged all of the development phases of D. melanogaster from egg to adulthood. Such dense 12628
DOI: 10.1021/acs.iecr.8b02593 Ind. Eng. Chem. Res. 2018, 57, 12624−12645
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Figure 5. Synthesis of CNO/PANI composites. Reproduced with permission from ref 53. Copyright 2012 Wiley.
Figure 6. Representation of the synthesis of CNOs modified with β-CD and polymer with ferrocene: (a) reaction of the CNOs with H2SO4/ HNO3; (b) amidation reaction with β-CD-NH2, EDC, NHS, and DMAP; (c) addition of polymer with ferrocene in water. Reproduced from ref 54. Copyright 2014 American Chemical Society.
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also functionalized54 with carboxylic groups by chemical oxidation and then reacted with β-CD. A biocompatible dextran polymer with grafted ferrocene groups (Fc-Dex) was employed for the supramolecular self-assembly on the β-CD− CNO surfaces. The ferrocene groups in Fc-Dex interact with the β-CD on the surface of the CNOs; the β-CDs act as hosts and the polymer ferrocene groups as guests. The reaction for the functionalization is shown in Figure 6. The functionalized CNOs showed increased solubility in aqueous solutions, revealing that the functionalized CNOs have a homogeneous size. However, when an excess of sodium adamantanecarboxylate was added, displacement of ferrocene molecules from the β-CD cavity caused the precipitation of CNOs. Other important information on the solubilization of nanoonions can be seen in recent reports.55,56
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 monograph, so here we present this material in a very shortened version. The history of solubilization of CNTs is not long. One of the first fundamental reviews in the field of CNT dissolution57 and, more recently, a book58 and several chapters59−63 and reviews64−74 have been published that are devoted to particular aspects of the dispersion and stability of CNTs in liquid media. In particular, some reviews have been dedicated to strategic approaches toward the solubilization of CNTs using chemical and physical modifications,75−77 environmental, toxicological, and pharmacological studies related to the use of CNTs,78,79 the main methods for the modification of CNTs with polymers,80,81 applications of 12629
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Figure 7. Colloidal dispersions obtained after liquid-phase exfoliation of graphite using the indicated perfluorinated aromatic solvents. Reproduced with permission from ref 115. Copyright 2009 Wiley.
functionalized CNTs,75,82, in particular as biosensors,83 and discussions of the possibility of the existence of SWCNTs in organic solvents in the form of clusters.84 In addition, certain attention has been paid to CNT nanofluids.85−89 A series of contemporary techniques are being used for solubilization of CNTs, from physical methods (classic ultrasound, plasma treatment, UV light, dielectrophoresis,90,91 gel electrophoresis,92,93 density gradient ultracentrifugation,94,95 irradiation techniques, and chromatography,96 among others) to chemical and biological ones, applying inorganic (other carbon allotropes, iodine, bromine, metallic sodium in liquid ammonia, CO2, peroxides, ozone, metal salts, and mineral acids) and organic (acids, salts, polymers, dyes, natural products, in particular diazonium salts,97 functionalization with porphyrins, amines, pyrene, polymers, sugars, etc.) compounds and biomolecules (DNA, peptides, amino acids, etc.) as well as micelles based on them and some metal complexes. CNTs can be functionalized by oxidation (peroxyacids; metal oxidants such as osmium tetraoxide, potassium permanganate, and chromates; ozone, oxygen, and superoxides) or reduction (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, the use of low- and highweight surfactants, mineral acid treatment to create −OH and −COOH groups, and their further interaction with organic molecules. Dispersion of CNTs in nematic liquid crystals is also known. Many of the chemical agents mentioned above are amphiphiles, which are 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 attention98,99 as good dispersants for CNTs) were designed to achieve programmed pH-dependent dispersions of CNTs.100 All of these methods suffer from problems with scalability, effectiveness, and often dissolution sensitivity. Several selected methods and strategies101 based on organic reactions and leading to functionalization of CNTs102 emphasize their universality for solubilization of CNTs. At the same time, not all methods are sometimes desirable for use, since some of them can, for example, heavily damage SWCNTs and MWCNTs. Scalability is a separate problem that needs a profound investigation, taking into account (1) the possibility of scaling up a laboratory-developed process, (2) the availability and cost of equipment and chemicals, (3) possible economic effects, and (4) environmental aspects based on the toxicities of CNTs and the chemicals used. Among solvents, water is most preferable because of the much larger number of existing and potential applications of CNTs, in particular for medical and biological purposes. NMP, N-dodecylpyrrolidi-
none (N12P), acetone, THF, DMF, N,N-dimethylacetamide (DMA), and cyclohexylpyrrolidinone (CHP) have been considered as suitable solvents for dispersion of CNTs. In contrast, much precipitation can be obviously observed for systems of CNTs in water, ethanol, and toluene. Nonfunctionalized CNTs can be solubilized in suitably chosen organic solvents, and furthermore, their solubility can be understood in terms of the Hansen solubility parameters,103 similarly as for fullerenes (see above).
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SOLUBILIZATION OF GRAPHITE There are several reports on the solubilization of graphite and oxidized graphite (graphite oxide, GrO),104−111 resulting mainly in graphene. 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, such as polyynes. Indeed, as has been observed, most reports on graphite solubilization are dedicated to attempts to optimize graphene fabrication. Both polar and nonpolar solvents have been used in these studies. Thus, GrO can be exfoliated directly in several polar solvents, such as water, ethylene glycol (EG), DMF, NMP, and THF, to form dispersions of GrO at a concentration of around 0.5 mg mL−1.112 Ultrasound-Assisted Solubilization. As in the case of CNTs and other carbon allotropes, 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.113 Dispersing graphite in surfactant/water solutions using ultrasound leads to large-scale exfoliation, resulting in large quantities of multilayer graphene (fewer than five layers) and smaller quantities of monolayer graphene.114 The formed dispersions are stable for up to 6 weeks and can be transformed to reasonably conductive and semitransparent films by vacuum filtration. The graphene basal plane was found to have only low levels of defects or oxides. It was proposed that the dispersions are stabilized by electrostatic repulsion between surfactant (sodium dodecylbenzenesulfonate (SDBS))-coated graphene flakes, similar to CNT dispersions in several surfactants. Treatment of graphite powder with a series of certain aromatic solvents (electron-deficient perfluorinated aromatic compounds and the aromatic heterocycle pyridine) under sonication leads to a homologous set of clear, stable colloidal dispersions containing solubilized graphenes (Figure 7), which can be transformed into gold−graphene hybrids using wet-chemistry routes.115 The mechanism probably includes charge transfer through π−π stacking from the electron-rich carbon layers to the electron-deficient 12630
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of polymerization was explained by the difference in the competition of polymerization and hydrogenation in these processes. Also, water-soluble carbon nanocrystals (CNCs) possessing electrochemiluminescence (ECL) activity were formed in aqueous solution by applying a scanning potential to a graphite rod.119 Being initially immobilized in a graphite rod, the CNCs are electrochemically released into the supporting electrolyte. In more detail, the proposed CNC formation mechanism is as follows: CNCs are initially immobilized in porous graphite, and then the CNCs exposed to phosphate buffer solution (PBS) are electrochemically oxidized to be water-soluble particles and released into the water phase. The features of the formed CNCs are easy labeling, good stability, excellent water solubility, environmental friendliness, and low cytotoxicity. This type of CNCs could have applications in the creation of biosensors (e.g., replacing Ru(bpy)32+ as a fine ECL reagent in the detection of DNAs and proteins). Chemical Solubilization and Combined Methods. Other approaches for graphite solubilization involve chemical treatments, frequently in combination with ultrasound (see above), classic and microwave heating, among others. Thus, thermal reduction of GrO dispersed in solvent (H2O, DMF, dimethyl sulfoxide (DMSO), or EG) at 100 and 150 °C was studied, revealing the solvent-assisted decomposition of carboxylic and carbonyl groups under these conditions.120 At 150 °C, DMF accelerates the GrO reduction rate significantly, while DMSO has a smaller acceleration effect and EG reduces the reduction rate compared with dry conditions. It was concluded that the dipole−dipole interactions promote the functional group migration by polarizing the carbon−oxygen bond and thus accelerate the reduction rate of GrO. Chemical treatment of the acidic functional groups in GrO with octadecylamine (ODA) renders graphite soluble in common organic solvents.121 GrO was refluxed in SOCl2 in the presence of DMF at 70 °C for 24 h using a CaCl2 guard tube. Further interaction with ODA yielded octadecylamidographite, which has a solubility of 0.5 mg/mL in THF and is also soluble in CCl4 and 1,2-dichloroethane. Also, sulfuric acid intercalation, microwave expansion, and ultrasonic dispersion were combined to make graphite suspensions.122 The resulting natural graphite was exfoliated into graphite flakes (Figure 10) with diameters of several micrometers and thicknesses of several tens of nanometers (with a distance of 0.335 nm between planes; the hexagonal lattice contains several layers), which were then mixed with EG and poly(α-olefin) oil (PAO), yielding a stable suspension without additional surfactants. The formed conductive fluids may find applications in many energy systems, such as thermal convection and thermal storage. Combination of the Hummers process for graphite oxidation, an amine-coupling process to make oleylamine (OA)-functionalized graphite oxide (OA-GrO), and a
aromatic molecules; the presence of strong electron-withdrawing fluorine atoms increases this effect. Liquid-phase ultrasound-assisted exfoliation of graphite in organic solvents with the addition of naphthalene produced graphene sheets containing few-layer graphene flakes without strong defects.116 Naphthalene serves as a “molecular wedge” to intercalate into the edge of graphite, which plays a key role during sonication and significantly improves the production yield of graphene. The graphene concentration in the NMP dispersion was found to be 0.15 mg/mL without addition of a surfactant/polymer stabilizer. In a related report,117 liquid exfoliation of graphite using organic solvents (DMF, isopropanol) led to multilayer graphene and graphite nanoflakes following a log-normal distribution, with a higher fraction of large-sized flakes (Figure 8) compared with a
Figure 8. SEM image of the graphene flakes deposited from a suspension sonicated for 240 min in DMF and not centrifuged. Scale bar = 20 μm. Reproduced with permission from ref 117. Copyright 2014 Elsevier.
conventional normal distribution. A similar distribution was also observed for the nanoflake thickness. Their formation mechanism is based on the appearance and collapse of cavitation bubbles. The sonication and centrifugation times have a notable effect on the mean flake size. Longer processes resulted in a reduced fraction of larger flakes. Changing key process parameters and selection of the solvent can lead to the desired size range of flakes and formation of high-quality nanoflakes without defects. Laser Ablation and Electrochemical Solubilization. Laser ablation of graphite particles suspended in benzene, toluene, or hexane solution using a Nd laser118 led to the formation of linear-chain polyynes (CnH2, n = 8−16, even numbers; Figure 9). The degree of polymerization of polyynes depends on the nature of the solvent. The change in the degree
Figure 9. Mechanism of formation of polyynes by laser ablation of graphite suspensions. Reproduced with permission from ref 118. Copyright 2002 Elsevier Science. 12631
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in nonpolar organic solvents.123 The thus-obtained reduced OA-G remained stable in moderately polar and nonpolar organic solvents (THF and DCB) for a long time. TOP functions as both an aggregation-prevention surfactant and reducing agent in the reduction of OA-GO. Finally, the effect of carboxymethyl cellulose swelling on the stability of natural graphite particles in an aqueous medium was studied,124 revealing that carboxymethyl cellulose functions as a thickening agent, preventing graphite particles from sedimentation. The stability of the graphite particles was found to be dependent on the electrostatic surface potential of the particles.
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SOLUBILIZATION OF GRAPHENE Among thousands of reports in the graphene area, one of the hottest topics in nanotechnology, a certain number of investigations have been dedicated to stabilization of graphene (G), GO, and rGO in solutions125−130 in both aqueous media and organic solvents (e.g., NMP, ethanol, DMSO, acetonitrile, acetone, THF, diethyl ether, toluene, and DCB131) because of multiple current and potential applications of its solubilized and nonsolubilized forms. Graphene is generally acknowledged to be a hydrophobic material, and it is not known to be soluble in any solvent. On the whole, recent studies of the dispersion of graphene in water and some other solvents using surfactants, polymers, and other dispersants showed that nearly completely exfoliated graphene may be obtained at concentrations from 0.001 to 5% by weight in water.132 Among dispersants, imidazolium-based polymeric surfactants and nanolatexes have demonstrated the highest degrees of graphene dispersion, up to 4.9 wt %. Generally, the obtained concentrations are too low for large-volume applications. Graphene, possessing sp2 surfaces, has favorable binding affinity to compounds possessing delocalized π systems, such as polymers or polyaromatic rings. The dispersion of G and GO in various solvents has been reviewed.133−137 Generally, discussions of graphene solubilization include methods of formation of solvent dispersions without stabilizers, surfactant-stabilized graphene exfoliation, GO- and rGO-based dispersions, and the use of polymer solutions as stabilizers. In addition, alternative techniques are known, such as electrochemical exfoliation and mechanochemical milling. Dispersion approaches include (1) exfoliation of graphene in a particular solvent in the absence of any surface modifying agent, surfactant, or polymer; (2) production of chemical functionality on the graphene surface, which promotes solvation and exfoliation; (3) use of surfactants, polymers, and other agents (dispersion aids) that physically
Figure 10. Micrographs of graphite flakes. (a) SEM image of graphite flakes obtained by H2SO4 intercalation, microwave expansion, and ultrasonic exfoliation of natural graphite. Scale bar = 2 μm. (b) Typical TEM image of a graphite flake. Scale bar = 500 nm. (c) HRTEM image of a graphite flake. Scale bar = 10 nm. Reproduced from ref 122. Copyright 2011 American Chemical Society.
reduction process using trioctylphosphine (TOP) led to graphene functionalized with OA (OA-G), which is soluble
Table 5. Dipole Moments, Surface Tensions, and Hildebrand Parameters of Various Solvents and GO and rGO Solubilities for All Solvents Studied138 solvent
GO solubility (μg/mL)
rGO solubility (μg/mL)
solvent
GO solubility (μg/mL)
rGO solubility (μg/mL)
DI water acetone methanol ethanol 2-propanol EG THF DMF NMP
6.6 0.8 0.16 0.25 1.82 5.5 2.15 1.96 8.7
4.74 0.9 0.52 0.91 1.2 4.9 1.44 1.73 9.4
n-hexane dichloromethane chloroform toluene chlorobenzene DCB 1-chloronaphthalene acetylacetone diethyl ether
0.1 0.21 1.3 1.57 1.62 1.91 1.8 1.5 0.72
0.61 1.16 4.6 4.14 3.4 8.94 8.1 1.02 0.4
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Figure 11. Digital picture of GO and rGO dispersions after 2 weeks, showing the long-term stability of different solutions. Reproduced with permission from ref 138. Copyright 2014 Elsevier Science.
as the best solvent for massive graphene production. This is also clear on the basis of another report on a nondispersion strategy for large-scale production of ultrahigh-concentration graphene slurries (5 wt %, 50 mg mL−1) in water with high production efficiencies (82−170 g/h).140 No destructive chemical oxidation processes are applied, allowing the safe storage and transportation of graphene. The as-exfoliated graphene slurry can be directly used for fabrication of conductive graphene aerogels and 3D printing. Nature of the Surfactant. Aqueous graphene dispersions were obtained by exfoliation of pristine graphite in the presence of a wide range of surfactants (Table 6).141 It has
adsorb onto graphene surfaces and stabilize them in a given solvent against flocculation. Characterization of graphene dispersions includes centrifugation and sedimentation as well as scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic force microscopy (AFM), and UV and Raman spectroscopy. The following aspects are important in consideration of G and GO solubilization in various solvents. Nature of the Solvent. The dispersion behavior of GO and rGO in a wide range of organic solvents was investigated (Table 5), taking into consideration the solvent polarity.138 rGO concentrations up to 9 μg/mL in chlorinated solvents were achieved, demonstrating an efficient solubilization strategy (Figure 11). On the basis of the Hansen and Hildebrand parameters of GO and rGO, it was revealed that the reduction process has a strong effect on the solubility and stability. Solutions of GO in NMP, EG, and water presented significant long-term stability, with the solubility reaching ∼8.7 μg/mL for NMP, while the dispersion behavior of GO changed after its reduction, presenting better interaction with solvents like DCB (∼9 μg/mL) and 1-chloronaphthalene (∼8.1 μg/mL). Another approach was investigated that involved treatment of graphite powder with a series of other solvents (NMP, DMA, γ-butyrolactone, 1,3-dimethyl-2-imidazolidinone, and benzyl benzoate) under sonication, leading to a homologous set of moderately dark-gray stable colloidal dispersions containing solubilized graphenes.115 To choose solvents, their first set comprised certain electron-deficient perfluorinated aromatic compounds, whereas the second set was the aromatic heterocycle pyridine. The key parameter for suitable solvents was established that the solvent−graphene interactions must be at least comparable to those existing between the stacked graphenes in graphite. The mechanism of solubilization most likely involves charge transfer through π−π stacking from the electron-rich carbon layers to the electrondeficient aromatic molecules, the latter containing strong electron-withdrawing fluorine atoms. Taking into account that all brilliant ideas are generally simple, we emphasize that a recent green technique139 for large-scale production of aqueous-compatible graphene nanoplatelets is very promising, where only water is used as the solvent under sonication, hydrothermal treatment and mechanical stirring of graphite. Without using any additives, the prepared graphene can be highly dispersed in water (concentration of 0.55 g/L) for several months at room temperature. Therefore, from our point of view, water remains
Table 6. List of Surfactants Used for Graphene Solubilization and Their Acronyms141 surfactant name
acronym
Nonionic Pluronic P-123 Tween 80 Brij 700 gum arabic from acacia tree Triton X-100 Tween 85 Brij 30 polyvinylpyrrolidone
P-123
PVP Ionic
poly(sodium 4-styrenesulfonate) 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate sodium deoxycholate sodium dodecylbenzenesulfonate 1-pyrenebutyric acid sodium dodecyl sulfate sodium taurodeoxycholate hydrate hexadecyltrimethylammonium bromide
PSS CHAPS DOC SDBS PBA SDS TDOC HTAB
been shown that nonionic surfactants can exfoliate and disperse pristine graphene from graphite in water at significant concentrations. High graphene concentrations, up to about 1 mg/mL, were obtained with their use. The resulting dispersions are constituted of single- and few-layer graphene sheets whose basal planes are essentially free of structural defects, even of the smallest size (point defects). Covalent and Non-covalent Functionalization of the Graphene Surface. Functionalized graphene layers that are soluble in halogenated solvents such as DCB and dichloro12633
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Industrial & Engineering Chemistry Research methane, were readily formed by alkylation of graphite fluoride with n-butyl- or n-hexyllithium reagents (Figure 12).142 The
Figure 12. Preparation of functionalized graphene layers starting from graphite fluoride. Reproduced with permission from ref 142. Copyright 2007 Elsevier Science.
powders were found to disperse readily and were stable in solution for months. The hexyl products were found to be more soluble than the butyl products (up to 0.54 g/L), having solubility comparable to that of edge-functionalized ODA graphene nanoplatelets (0.5 g/L). Upon annealing, the intrinsic properties of the graphene layers can be partially restored. Dealkylation occurs when the bulk samples are annealed, further extending the π system and thus restoring the electronic properties of the nanocrystalline graphene. Solubilization of graphene was reached using SDBS starting from graphite under ultrasonic treatment in water aided by a highly water-soluble perylene-based bolaamphiphile detergent (Scheme 1).143 While ultrasonication serves to overcome the
Figure 13. (a) Scheme for the preparation of graphene−coronene composites. (b) Aqueous solutions of CS, graphite oxide (EG)−CS and hydrogen atmosphere (HG)−CS). (c) Corresponding images under UV−vis irradiation. (d) Schematic illustration of the exfoliation of few-layer graphene with CS. Reproduced with permission from ref 144. Copyright 2010 Wiley.
charge transfer and π−π-stacking interactions. The negative charge of CS prevents both inter- and intra-π−π stacking of graphenes, leading to stabilization of the functionalized graphene sheets (Figure 13d). Highly soluble tetrathiafulvalene (TTF) derivatives as the electron donors and a coronene bisimide (CBI) derivative as the electron acceptor (Figure 14) were used.145 In an amphiphilic TTF derivative (Amph-TTF), two dodecyl chains on one end were replaced by tetra(ethylene glycol) chains. The long alkyl chains as well as the glycol chains on the aromatic molecules promote the solubility of the graphene composites in organic solvents. Both TTF and CBI derivatives have large planar aromatic surfaces that can strongly direct them onto the surface of the graphene sheets through π−π-stacking interactions, greatly enhancing the graphene solubility. Among the various organic solvents, chloroform was found to be more effective than DMF and toluene in solubilizing graphene, and stable dispersions could be made with both TTF and CBI derivatives. Graphene could also be effectively dispersed in nonpolar solvents like toluene and polar solvents like DMF, but the dispersions were stable for only up to 2−3 h. Polymer Grafting of Graphene. Water-soluble polymergrafted graphene or negatively charged graphene are known to be effective for graphene solubilization in water.146 Meanwhile, in non-covalent chemistry, various π-rich water-soluble polyelectrolytes such as poly(4-styrenesulfonate) (PSS) or sulfonated polyethynylphenylene (PPE-SO3−) produced stable graphene dispersions via π−π interactions with graphene and subsequent charge repulsion between polyelectrolyte/graphene conjugates. Examples of polymers used for graphene grafting are as follows. An electrocatalytic biosensing platform was designed by the functionalization of rGO sheets with a conducting polypyrrole graft copolymer, poly(styrenesulfonic acid-g-pyrrole) (PSSA-g-PPY), via non-covalent π−π interactions.147 The resulting nanocomposite could disperse well in
Scheme 1. Perylene-Based Bolaamphiphile Detergent
strong van der Waals interactions between the graphene planes in the graphite mother crystals, the detergent guarantees stabilization of the exfoliated sheets of few- and single-layer graphene in water driven by π−π-stacking interactions and the hydrophobic effect, comparable to other aromatic systems. Final dispersions contained monolayer graphene as well as residual stacks containing more than one layer. This method did not cause the formation of any defects, leaving the sp2 hybridization of the entire carbon sheets intact. Another example of non-covalent functionalization is the use of an anionic coronene derivative, the tetrapotassium salt of coronene tetracarboxylic acid (CS; Figure 13a), which was chosen144 for the non-covalent functionalization of graphene to make stable aqueous solutions of single- and few-layer graphenes by exploiting non-covalent interactions with a coronene carboxylate acceptor molecule. CS is highly soluble in water, showing blue emission (Figure 13b,c) and has a very large planar aromatic surface allowing strong interactions with the graphene sheet surface through synergistic non-covalent 12634
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Figure 14. Molecular structures of electron-donor TTF derivatives and electron-acceptor coronene bisimide (CBI) derivatives. Reproduced with permission from ref 145. Copyright 2010 Elsevier Science.
water for at least 2 months with a solubility of 3.0 mg/mL, showed uniform sheet nanostructure, and exhibited high conductivity. The hybrid nanocomposite possessed excellent electrocatalytic activity toward electrooxidation of both H2O2 and uric acid. On the basis of the electrocatalysis activity, a biosensor was constructed for fast detection of hypoxanthine by an enzymatic cycle. Also, complete exfoliation of graphene aggregates in water at concentrations of up to 5% by weight was reached using recently developed triblock copolymers (Scheme 2) and copolymeric nanolatexes based on a reactive Scheme 2. Triblock Copolymers
Figure 15. Grafting-on functionalization of GF with N3-PCL. Reproduced with permission from ref 149. Copyright 2015 Wiley.
leading to relatively high concentration suspensions of large exfoliated graphene sheets. Graphite exfoliation was first accomplished in CHCl3 (a common good solvent for the different arms of the star copolymer dispersing agent), followed by phase transfer to the immiscible acidified water phase. At the CHCl3−H2O (pH 2) interface, protonation of the P2VP arms of the star copolymer, physisorbed onto the graphene surface by π−π-stacking interactions of the polystyrene (PS) arms, takes place. Graphene can be transferred in a variety of different media by using the same copolymer as a dispersing agent in relatively high concentrations and good solubilization yields in either nonselective low-boiling-point organic solvents, aqueous media, or ionic liquids. The overall exfoliation yield, including concentration, solubilization yield, monolayer percentage, and large graphene size, was found to be among the highest observed to date using polymeric stabilizers. Among other intriguing recent results, environmentally friendly aqueous dispersion of polyurethane nanocomposites based on p-tert-butylcalix[4]arene (BC4A)and sodium p-sulfonatocalix[4]arene (SC4A)-modified GO nanosheets (SGO and CGO) as green corrosion-protective coatings was recently reported.151
ionic liquid acrylate surfactant.148 The delocalized π systems of the imidazolium groups allow backbone groups to physically adsorb on graphenic sp2 surfaces. These aqueous graphene dispersions are suitable as conducting inks. The grafting-on reaction between azido-terminated poly(εcaprolactone) (N3-PCL) and ultrasonication-assisted exfoliated graphene flakes (GF) was carried out to obtain PCLgrafted GF (PCL-g-GF), which showed good dispersibility in a wide variety of organic solvents (dichloromethane, THF, benzene, and NMP), although the dispersibility was bad in methanol and hexane.149 The PCL macromolecules were covalently introduced on the surface of GF without disrupting the structure of GF (Figure 15). At least 75 wt % grafting of PCL macromolecules onto GF was enough to provide good dispersibility of GF. In addition, a two-step isolation process to obtain nearly defect-free mono- and few-layer graphenes in various media was achieved by liquid-phase pre-exfoliation of pristine graphite in the presence of an ionizable poly(2vinylpyridine) (P2VP) heteroarm star copolymer in an organic solvent and subsequent graphene shuttle between immiscible media (organic solvent/water and water/ionic liquid),150 12635
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Industrial & Engineering Chemistry Research Functionalization with Biomolecules. In a graphene/ heparin conjugate, a non-covalent interaction between chemically reduced graphene and heparin (Scheme 3) takes place.146 Scheme 3. Structure of Heparin
Figure 16. Yellow-green fluorescent image of water-soluble FMCNDs in aqueous solution upon exposure to UV irradiation. Reproduced from ref 154. Copyright 2008 American Chemical Society.
It is known that biomolecules such as heparin (which is a blood thinner) non-covalently interacting on the graphene plate preserve their bioactivity after conjugation with graphene. It was revealed that unfractioned heparin (UFH) with higher molecular weight was effective for graphene solubilization, while low-molecular-weight heparin (LMWH) was not. Such graphene/biomolecule conjugates could give multiple bioactivities such as biocompatibility, targeting, delivery, and cell viability to the graphene-based biomaterials or devices. The very practical graphene dispersion concentrations provide great promise for many low-cost and high-volume processes for producing double-layer supercapacitors, solar cells,152 highly electrically conducting inks and thin layers, highly thermally conducting coatings, etc.
current ways of deaggregation of nanodiamonds, using watersoluble nontoxic crystalline compounds such as sodium chloride or sucrose. Upon completion of the milling, the medium can be easily removed from the product by simple water rinsing. Highly transparent colloidal solutions of detonation CNDs in organic solvents such as THF, methyl ethyl ketone (MEK), and acetone were obtained by oxidation, and then dispersion of CNDs into solvents was achieved by bead milling with the addition of the surfactant oleylamine.156 It was confirmed that a readily apparent number of Lewis acid sites composed of mainly carboxylic acid and cyclic acid anhydrides were derived on the surface of thermally oxidized nanodiamonds (T-CNDs) and were chemically active and favored the formation of charge transfer complexes with amino-containing surfactants such as OA and octadecylamine (ODA). The T-CND showed good dispersion stability in organic solvents for at least 3 months. The CNDs functionalized with ODA, which have potential applications in biomedical imaging, showed good dispersion in dichloromethane, toluene, and chloroform without the use of sonication, surfactants, or other special means of dispersion.157 The blue-fluorescent ODA-CNDs added another color to the set of red and green fluorescent CNDs. Good solubility is known to be extremely important for many applications of CNDs. In particular, ODA-CNDs can be used in lubricants or motor oil additives as well as in other applications where stable ND suspensions in hydrophobic systems (fuels, polymers, or oils) are required. Fluorination as the First or Final Step. The presence of fluorine atoms in CND functionalities was observed to provide certain dispersibility. Similarly to the case of carbon nanoonions, solubilization of CNDs in water and other polar solvents (DMF, ethanol) was achieved through their covalent functionalization by fluorination and subsequent derivatization with sucrose (Figure 17).50 The solubility of CNDs is increased by the abundance of hydroxyl groups available for hydrogen bonding. In a difference with the fluorinated nanocarbons, which because of their hydrophobic nature do not disperse and remain on top of water, the sucrosefunctionalized samples form light-yellow solutions. It is important to note that further addition of small amounts of HCl caused complete precipitation of the water-solubilized CNDs due to cleavage of sucrose groups under acidic conditions. On the whole, pH sensitivity is normal for suspensions of nanocarbons. A similar efficient two-step
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SOLUBILIZATION OF NANODIAMONDS By the use of physical methods, with or without combinations with the use of additional surfactants or other chemicals, it is possible to obtain solubilized carbon nanodiamonds both in water and in organic solvents and apply them for a series of purposes. Thus, microwave (MW)-assisted chemical functionalization of detonation CNDs led to the appearance of −COOH groups and the formation of stable dispersions in aqueous as well as organic solvents, with higher solubilities in water, THF, and DMSO.153 The diamond core is known not to be a microwave absorber, so CNDs required relatively long reaction times. The resulting carboxylation of CNDs led to altered colloidal behavior in terms of reduced in agglomeration in solvents, which was accompanied by increased solubility. Magnetic CNDs were prepared via a microwave-arcing process using pristine CNDs and ferrocene as precursors.154 The resulting magnetic CNDs were found to be composed of iron nanoparticles chemically bound to CNDs and encapsulated by graphene layers on the surface of CNDs. The composite was surface-grafted with poly(acrylic acid)s and fluorescein omethacrylate (FM) to create a fluorescence property. These fluorescent magnetic CNDs are water-soluble with a solubility of ∼2.1 g/L (Figure 16), collectible with an external magnet, and readily ingested by HeLa cells, likely via non-receptormediated endocytosis, to enter and remain in the cytoplasm regime without entering the nucleus. In addition to the MW treatment described above, mechanical methods have been used. It is known that by dry-medium-assisted milling with subsequent pH adjustment, stable aqueous CND colloidal solutions with particles
