The dispersion, solubilization and stabilization in “solution” of single-walled carbon nanotubes

Abstract

A series of contemporary techniques, used for SWCNTs solubilization, from physical (classic ultrasound (stable and inertial cavitation types of ultrasonic treatment are emphasized), radiation treatment or UV and visible light influence) to chemical and biological, applying inorganic (nanodiamonds, iodine, metallic sodium in liquid ammonia, peroxides, and mineral acids) and organic (acids, salts, polymers, dyes, natural products and biomolecules) compounds, have been reviewed. van der Waals interactions, π–π stacking interactions between aromatic rings in organic compounds and nanotubes, and hydrophobic interactions are major factors that are responsible for CNTs dispersion. Despite the similarity of dispersion methods for all types of nanotubes, several differences are observed for SWCNTs, DWCNTs and MWCNTs. Among other aspects for SWCNTs dispersion, a variety of functionalization routes are discussed, including double-covalent functionalization of nanotubes, as well as thermodynamic and kinetic approaches towards the stability of the formed CNTs dispersions and the prevention of SWCNTs reaggregation.

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Introduction

Carbon nanotubes (CNTs), objects or classic investigations in nanotechnology and nanochemistry,¹ form bundle-type structures possessing complex morphologies, which contain a high number of van der Waals interactions. These forces cause the very poor solubility of both single-walled CNTs (SWCNTs) and multi-walled CNTs (MWCNTs) in water and organic solvents. During last two decades, hard efforts have been dedicated to develop approaches for their separation and dispersion in a variety of aqueous and non-aqueous media. It has been well established that the π–π stacking, hydrophobic and van der Waals interactions are the principal factors obstaculizing their “dissolution”. Recently, several chapters and reviews^2–8 have been published, which are devoted to particular aspects of CNTs dispersion and stability in liquid media. Analyzing the reports, it was observed⁹ that there is frequently an important misunderstanding between the terms “solubilization” and “dispersion”. In many studies, these terms are used interchangeably, which causes confusion. The fundamental question discussing CNTs in the liquid phase is: are they dissolved or dispersed? It has been suggested¹⁰ to apply the term “dispersion” rather than “solution”. In addition, a 2005 NASA-NIST workshop made a clear distinction between the “macrodispersion” of CNT bundles and “nanodispersion” of individual CNTs.¹¹

Multi-walled carbon nanotubes (MWCNTs) and single-walled carbon nanotubes (SWCNTs) are frequently objects of comparison of their certain properties,^12–16 in particular their dispersion in polymers^17,18 or chlorosulfonic acid.^19,20 In the case of SWCNTs, in general the methods for their dispersion are similar to those used for MWCNTs and based, for instance, on physical (density gradient ultracentrifugation,²¹ gel electrophoresis,²² dielectrophoresis,^23,24 plasma, chromatography and irradiation techniques) and chemical (ozonolysis, diazonium salts, functionalization with bromine, porphyrins, pyrene, DNA, peptides and other biomolecules (needed for surface engineering of CNTs for targeting purposes²⁵), various amines, polymers, and use of dendrimers²⁶) methods. Many these methods have problems with their effectiveness, dissolution sensitivity, and scalability. The quantitative evaluation of SWCNT solubilization is generally carried out by UV-visible absorbance of SWCNT suspensions,^27,28 as well as applying Design-Expert® software (Version 7.0.0, Stat-Ease, Inc. Minneapolis, USA) for additional calculations.²⁹

In this study, we emphasize the main dispersion methods for SWCNTs only, paying attention to the recent achievements reported between 2010 and 2016.

Ultrasound and other physical methods

The use of ultrasound (weak in the form of ultrasonic cleaner or a powerful source with a sound) continues to be a classic method for SWCNTs dispersion and is frequently used to accompany many functionalization manipulations with carbon nanotubes.† Under ultrasound action, the cavitation process produces a strong shear force, leading to the exfoliation of SWCNTs bundles, bubble formation and collapse,³⁰ providing homogeneity of nanosuspension.³¹ To enhance the dispersion efficiency of CNTs, a double ultrasonic source can also be applied (Fig. 1).³² Despite being a fundamental dispersion technique, several novelties in ultrasonic applications have been recently observed. Thus, the role of sonication energy on SWCNTs and MWCNTs (with distinct lengths) dispersions in the presence of sodium dodecylbenzene sulfonate (SDBS, see below) was evaluated.³³ It was revealed, in particular, that the concentration of dispersed carbon nanotubes in the SDBS solution depends on the sonication energy, but not the output power of the sonicator alone or the sonication time. Higher quantities of CNTs are dissolved upon the addition of more SDBS; smaller or shorter nanotubes lead to less nanotube dispersion. It was also established that the optimal energy depends on the CNT diameter and it was independent of the amount of CNTs or SDBS, surface functional groups and CNT length. In another report,³⁴ the authors distinguished between stable cavitation, leading to chemical modification of the CNTs surface and inertial cavitation, favoring CNT length reduction and exfoliation. The second type of cavitation (which is dependant on the surfactant concentration) was found to be responsible for effective CNTs dispersion.

Fig. 1 A typical double sonication system used in dispersing SWCNTs. The system consists of an ultrasonic bath and an ultrasonic probe on the right. Reproduced with permission of the Scientific Research: An Academic Publisher.

Modification of the SWCNTs surface in dispersions upon distinct sonication parameters was monitored using several techniques, in particular, by Raman spectroscopy.³⁵ It was shown that standard conditions in water (1–6 h sonication, power 3–80 W) do not result in covalent modifications. In the case upon the addition of single-stranded DNA (ssDNA) to this dispersion, it can be non-covalently absorbed onto the SWCNTs surface without covalent linkage. Less than 10% of defect sites can appear upon higher sonication power or longer sonication durations. Studying the length and diameter of the SWCNTs under the sonication conditions (in the presence of 1 w/v% sodium deoxycholate) by AFM,³⁶ the authors considered the sonication power to be most important for SWCNTs dispersion. A shortening of the SWCNTs length was observed with a long sonication time. Moreover, ultracentrifugation was shown to be useful in eliminating large-diameter bundles from the SWCNTs dispersion to obtain a homogeneous system. The monitoring of SWCNTs surface was also carried out using SEM and UV-vis-NIR spectroscopy,³⁷ which showed that the maximum concentration of dispersed SWCNTs corresponds to the maximum UV-vis-NIR absorbance of the solution. To disperse higher SWCNTs concentrations, a longer time is required for sonication. The optimal conditions are as follows: 1.5 wt% of surfactant and 0.4 mg mL⁻¹ of SWCNTs dispersed. In a related report,³⁸ these data are slightly different: 9–10 mg mL⁻¹ for SDBS and 8–9 mg mL⁻¹ for sodium deoxycholate lead to 0.25 mg mL⁻¹ of SWCNTs dispersion (0.7–2.5 nm of SWCNTs diameter). The SEM analysis also showed the presence of surfactant particles on the SWCNTs surface. In the case of organic solvents, 4 h of treatment is sufficient to obtain the best dispersions in 1,2-DCE or DMF.

Ultrasound-assisted studies used to enhance CNTs dispersibility without the application of surfactants have been made and can sometimes lead to useful applications of carbon nanotubes. Thus, an efficient cooling cell for probe-type ultrasonication was described.³⁹ Comparing it with a standard cylindrical cell, the SWCNTs concentration in water after ultracentrifugation was found to be almost double in a rosette cooling cell. The efficiency of this cell was attributed to the cooling of the SWCNTs dispersion and higher efficiency in circulation, as well as an enhancement of the cavitation process. A systematic calorimetry-based method was developed to standardize the SWCNTs dispersion technique using a ultra-dismembrator.⁴⁰ This protocol was applied to prepare aqueous SWCNTs suspensions sonicating SWCNTs in a range of input energy (18–100 kJ). It was shown that the suspended mass of SWCNTs increased up to 18 kJ of energy input with no further enhancement upon continued energy input. The considerable changes that were observed were limited to the morphological properties, i.e., debundled, shorter length, and sharp edged SWCNTs and fractal cluster formation (lower D_f) with increased input energy. In conclusion, ultrasonic treatment is often used for the dispersion of carbon nanotubes (and other nanomaterials) together with the chemical methods described below and the resulting CNTs nanocomposites can be applied, in particular, for cement reinforcement⁴¹ or for the creation of pharmaceuticals.⁴²

Radiation methods

Radiation methods⁴³ have also been applied for CNTs dispersion. Thus, a technique for very efficient SWCNTs functionalization with DNA wrapping included exposure of the SWCNTs to γ-irradiation (50 kGy), which lowered by one order of magnitude the amount of single stranded deoxyribonucleic acid (ssDNA) required for SWCNT modification.⁴⁴ γ-Irradiation in 3 distinct media considerably improved the SWCNT dispersion procedure and irradiation in ammonia medium was found to be the most efficient. These ssDNA-functionalized γ-irradiated SWCNTs were stabilized by electrostatic forces and could be used for biomedical applications. In addition, the application of a ball-milling method (up to 12 h; 9 h optimal time for shortening and dispersion of SWCNTs) for SWCNTs dispersion and further formation of their nanocomposites with ZnO is known.⁴⁵

UV and visible light influence

Photochemical reactions involving colloidal dispersions of carboxylated SWNT-COOH in sunlight have been examined.⁴⁶ The production of reactive oxygen species (ROS) during irradiation occurs and is evidence for potential further phototransformation and may be significant in assessing their overall environmental impact. In aerated samples exposed to sunlight or to lamps that emit light only within the solar spectrum, the probe compounds, furfuryl alcohol (FFA), tetrazolium salts (NBT²⁺ and XTT), and p-chlorobenzoic acid (pCBA), were used to indicate the production of ¹O₂, O₂˙⁻, and ˙OH, respectively. All three ROS were produced in the presence of SWNT-COOH and molecular oxygen (³O₂). ¹O₂ production was confirmed by observing the enhanced FFA decay in deuterium oxide, attenuated decay of FFA in the presence of azide ion and the lack of decay of FFA in deoxygenated solutions. Moreover, SWCNTs were functionalized by the covalent attachment of 2-propanol-2-yl radicals, generated by the photolysis of 2-hydroxy-2-methyl-1-phenyl-1-propanone under UV light, to their surface in THF solution.⁴⁷ A loss of Van Hove singularities and a decrease in the intensity ratio of the G band and D band (I_G/I_D) were observed. The solubility in common organic solvents was improved and the original electronic structure of the SWCNTs was retained without severe modification that damages the nanotubes. Possible applications of SWCNTs, modified by photoinitiators, could have applications for the creation of new materials.

The use of inorganic compounds

Several simple inorganic compounds have been used for SWCNTs dispersion. Thus, nanodiamond particles were applied⁴⁸ to disperse CNTs, leading to their stable colloidal suspensions. Both MWCNTs and SWCNTs could be suspended in deionized water using either high pressure and high temperature NDs or detonation NDs. Negatively charged NDs were found to suspend CNTs in deionized water better, when compared to positively charged particles, which is possibly caused by electrostatic interactions. Because individual NDs have biomedical applications,⁴⁹ their soluble composites with SWCNTs could also have drug delivery, bioimaging and tissue engineering applications. The combination of nitric acid application with further physical treatments has also been developed. For example, individual SWCNTs were prepared in an aqueous solution on a large scale via 3 processing steps: refluxing in concentrated HNO₃, low speed and high-speed centrifugation.⁵⁰ The bulk production (10 g of starting SWCNTs) resulted in a concentration of 0.2 mg mL⁻¹ individual SWCNTs stably dispersed in deionized water without any external protection. It was revealed that the aqueous dispersion contained approximately 80% individual SWCNTs (concentration 0.05 mg mL⁻¹ at pH 5, tube lengths 500 nm to 1 mcm, and absolute zeta potential ∼72 mV). The authors assumed that this high zeta potential resulting from an electrical double layer produces the repulsion to overcome the van der Waals attraction and so maintained the SWCNTs dispersed.

Few examples of applications of inorganic salts for CNTs dispersion have been reported. The effects of inorganic monovalent salts (NaI and NaCl) on the dispersion stability of CNTs were studied, performing all-atom MD simulations using non-polarizable interaction models to compute the potential of mean force between two (10,10) SWCNTs in the presence of NaCl/NaI and used to compare the potential of mean force between SWCNTs in pure water.⁵¹ The addition of salts enhances the stability of the contact state between two SWCNTs on the order of 4 kcal mol⁻¹. Iodide anion directly stabilizes the contact state to a much greater extent than chloride anion. The enhanced stability arises from the locally repulsive forces imposed on nanotubes by the surface-segregated iodide anion. Within the time scale of these simulations, both NaCl and NaI solutions stabilize the contact state by equivalent amounts.

Iodine-doping

Iodine-doping into SWCNTs can be effectively carried out electrochemically⁵² and controlled by changing the polarity. Iodine molecules were found to be mainly inserted into the hollow core of SWCNTs and the resulting iodine-doped SWCNTs can be homogeneously dispersed in an aqueous medium at low temperature (ca. <15 °C). However, in our opinion, the most intriguing example in this section is an unusual application of the Na–NH_3(liq.) system, well-known in classic inorganic chemistry courses. Thus, a scalable method for SWNT separation and dispersion, using a reductive treatment in sodium metal–ammonia solutions, was discussed (Fig. 2).⁵³ The ability to isolate individual nanotubes was confirmed by AFM. The soluble fraction contained predominately large SWCNTs; they can indeed be unbundled to obtain individual tubes in solution by reductive charging in ammonia. Following the removal of the liquid ammonia, a dry powder of sodium “nanotubide” was formed (where “nanotubide” was proposed by authors as a term for a pure nanotube anion). These nanotubide salts can be dissolved in dry amide solvents to form solutions of individual charged SWCNTs that can be readily handled. It was emphasized that no stirring and no ultrasound were used at any step. This spontaneous dissolution was presumably driven by the solvation of the cations, leading to repulsion between the solvated nanotubide anions and the formation of an electrostatically stabilized colloid (or polyelectrolyte molecule).

Fig. 2 Liquid ammonia reduction of ARC SWCNTs. The process scheme for the reduction, solvation, and subsequent dissolution in sodium–ammonia, illustrated by atomistic models (Na ions in pink) and images of the relevant phases (M : C 1 : 20). Reproduced with permission of the American Chemical Society.

Ionic salts of organic acids

Ionic salts of organic acids are common surfactants for both MWCNTs and SWCNTs; for example, cationic⁵⁴ or anionic surfactants known long ago.⁵⁵ Among the classic ionic surfactants, sodium dodecylbenzene sulfonate 1 (SDBS) and sodium dodecyl sulfate 2 (SDS) are very popular and mostly frequently used⁵⁶ due to their outstanding properties for CNTs dispersion, first of all dispersion stability in distinct temperature conditions and solvents. Thus, as a representative example, the stability of dispersions of SWCNTs stabilized by SDBS in binary polar solvents “water + antifreeze” (glycerol, polyethyleneglycol) with eutectic compositions was studied.^57,58 The absorption spectra of the suspensions demonstrated no change during 1 year of storage with temperature spanning from −40 to +40 °C. Aqueous dispersions of the nanotubes exhibited considerable enhancement of optical limiting parameters alongside an increase of the bundled material populace. Among other important studies, the aggregation kinetics for SWCNTs and MWCNTs carbon nanotubes dispersed using SDBS were investigated⁵⁹ using time-resolved dynamic light scattering (DLS), in the presence of several electrolytes, and humic acid (HA). The CNTs could be effectively suspended in an aqueous solution using the SDBS and the increased electrolyte concentrations induced aggregation. Increases in the solution pH from 3 to 10 led to a significant decrease in CNT aggregation, indicating the presence of functional groups on the CNTs' surface.

Studies of SWCNTs suspensions in SDS aqueous solutions and saturated fatty acids (C_n) indicated that an enhanced individualization of SWCNTs occurs in the dispersions for C_n's with an alkyl chain longer than SDS.⁶⁰ This elevated SWCNTs solubilization was interpreted in terms of the increased binding energy to the nanotube wall and the reduced electrostatic repulsion within the surfactant aggregates. In addition, it was quantified how changing the counter-ion (Cs⁺ instead of Na⁺) affected the morphology of dodecyl sulfate surfactants adsorbed on CNTs.⁶¹ Using atomistic molecular dynamics, aqueous cesium dodecyl sulfate (CsDS) adsorbed on (6,6), (12,12), and (20,20) SWCNTs at r.t. were simulated. It was suggested that CsDS should be more effective than SDS at stabilizing aqueous carbon nanotubes dispersions. More importantly, these results were obtained only for the (6,6) nanotubes simulated. In addition, a series of comparative studies for SDBS and SDS between each other, as well as with frequently used non-sulfonate surfactants, have been carried out. Thus, the SDBS contributed to a better dispersibility and electrical conductivity of SWCNTs than SDS 2, sodium cholate 3 (SC), and cetyltrimethyl ammonium bromide 4 (CTAB).⁶² In general, the use of these surfactants is a classic way to disperse CNTs and the resulting dispersions can be widely applied, in particular for nanodevice fabrication, biochemistry and biomedical engineering.

Other organic and coordination compounds

Among the relatively simple organic compounds, we note microwave-assisted chemical functionalization of SWCNTs with undecyl groups decomposed from lauroyl peroxide 5.⁶³ This rapid efficient procedure reduced the reaction time to 10 min and obtained the products with a higher functionalized degree than that obtained using the conventional refluxing method. A longer treatment time leads to partial defunctionalization and higher microwave power (higher than 900 W) can reduce the functionalized degree by removing some initially attached functional groups. The resulting SWNTs have enhanced dispersivity in organic solvents when compared to the pristine nanotubes. Moreover, the one-step dissolution of SWCNTs was demonstrated through the use of dimethylacetamide (DMA) as a stable solvent for both sodium naphthalide and reduced SWCNTs, enabling the synthesis of concentrated solutions of nanotubide.⁶⁴ In addition, the use of DMF (after strong acid treatment of SWCNTs) was reported for SWCNTs dispersion.⁶⁵

In addition to salts of organic acids, described in the previous section, other organic non-biological substances for SWCNTs dispersion are mostly aromatic and polyaromatic compounds, frequently containing heterocycles, for instance porphyrin (their applications are known, for example, chiral Zn–porphyrins can serve as significant host molecules for separating SWNTs, providing optically active ones with limited (n,m) structures⁶⁶ or, for the case of porphyrin–CNTs composites, for delivering immunotherapeutics and drugs⁶⁷ or as optoelectronic and photovoltaic devices⁶⁸) or thiophene. Thus, as an example of an S-heterocycle, the hydrogenation of 2-nitrothiophene gave 2-aminothiophene 6 that was used⁶⁹ for the amidation of SWCNTs functionalized with carboxylic acid groups (SWCNT–COOH). In these modified carbon nanotubes, the thiophenes were covalently attached to the SWCNTs via amide linkages. The modified SWCNTs showed enhanced solubility and thus better dispersion in common organic solvents and were used as the dopant in polymer-fullerene photovoltaic cells. The relative polymers, poly(3-alkylthiophene)s (7–9), were also scalably used to sort SWCNTs at a dispersing temperature of ∼50 °C in toluene.⁷⁰ On the basis of their geometrical models, it was suggested that the selective dispersion is possible due to the formation of a supramolecular structure, constructed through the interdigitation of side chains wrapping around the SWCNTs. The wrapping process induces the formation of a “polymer shell”, in which the shell diameter matches well with the selected SWCNTs. Other interesting examples of bifunctional sulfur-containing aromatic molecules are as follows: large aromatic surfaces of electron-donor and -acceptor molecules {highly soluble tetrathiafulvalene (TTF) derivatives 10 and 11 as the electron donors direct them onto the surface of the graphene/SWCNTs through π–π stacking interactions, whereas the alkyl chains as well as the glycol chains on the aromatic molecules promote the solubility, thereby giving stable dispersions of SWNT/graphene composites in non-polar solvents.⁷¹

The dispersion of SWCNTs in the presence of water soluble polypyridyl complexes with the general formula [Ru_x(bpy)_yL]²⁺ (L = dppz, dppn, tpphz) have been reported.⁷² These ligands have extended planar π-systems, which aid in the solubilization of SWCNTs via π–π interactions. A preparation method for individually dispersed SWCNTs in both liquid and dried solid states was developed using triphenylene 12 derivatives.⁷³ On its basis, a highly concentrated solution of SWCNTs (0.7–0.8%) was prepared and the SWCNTs were dispersed well after the addition of methanol (up to 70%) for a long time. These properties (redispersion ability and dispersion to organic solvent) were not observed in typical dispersing surfactants such as SFBS and sodium cholate. A triptycene-13 (Trp) and amphiphilic naphthalene-based surfactant based surfactant was designed with the ability to solubilize SWCNTs and C₆₀ in water through non-covalent interactions,⁷⁴ as well as sugar-functionalized triptycenes.⁷⁵ The prepared compounds were designed with either two ionic or non-ionic tails to ensure a large number of supramolecular interactions with the solvent, thereby promoting strong solubilization. The surfactants produced stable suspensions in which the SWCNTs were dispersed and the surfactant/SWNT complexes formed are stable for more than one year. In related research,⁷⁶ supramolecular surface modification of SWCNTs using amphiphilic molecules containing a bent triptycene moiety and a hydrophilic oligo(ethylene glycol) chain 14–15 was described (see more information on polyethylene glycol below). The surface modification was realized through the binding of the triptycene moiety onto the sidewalls of the SWCNTs through π–π stacking interactions and the oligo(ethylene glycol) chains extend into the water and act as dispersing agents, thus yielding an aqueous SWCNT dispersion. This dispersion was found to be stable for more than six months and contained a high concentration of SWCNTs. Based on shape-fitting of SWCNTs and the triptycene moiety, the stacking of triptycene moieties on the SWCNT sidewalls showed a nice selectivity for SWCNTs with a diameter of 1.0 nm. The stacking of Trp-TEG molecules onto the SWCNT sidewalls is illustrated in Fig. 3. Among other polyaromatics, p-terphenyl (16) is also known as a surfactant for SWCNTs dispersion in toluene.⁷⁷

Fig. 3 An illustration of the non-covalent surface modification of SWCNTs using amphiphilic Trp-TEG molecules. Reproduced with permission of the Elsevier Science.

An organogelator, N,N′-bis(octadecyl)-l-(1-pyrenebutyric acid)-glutamic diamide (LPG), was designed and its interaction with pristine SWCNTs in the gel state investigated.⁷⁸ It was found that LPG can form organogels with various types of organic solvents and SWCNTs can be well dispersed into the LPG gel. The gelation process and the properties of the resulting nanocomposites were found to be closely related to the pyrene group in the gelator. The textures of the nanocomposites were altered from a layered structure to intertwined fibers upon the incorporation of SWCNTs, which also supported the effective mixing of SWCNTs into the LPG organogels. In addition to this LPG gel, hybrid organogels, wherein SWCNTs were incorporated⁷⁹ into organogel fibers, were prepared. The SWCNTs were covalently functionalized with organic branches {1,8-bis[3,4,5-tris(decyloxy)benzoylamino]octane and N-(8-aminooctyl)-3,4,5-tris(decyloxy)benzamide} (Fig. 4) that had a similar structure to the organogelator. The functionalized SWCNTs in the hybrid organogel formed in decane were mainly located inside or on the surface of the organogel fibers, while the f-SWCNTs in the hybrid organogel formed in DMF were distributed evenly over the sample. When an organogelator had a different chemical structure to that of an organic functional group on the SWNT surface, the SWCNTs existed as large aggregates, or long bundles, which were not incorporated inside of the organogel fibers. The dispersion properties of the f-SWCNTs in the organogels were greatly dependent on their relative interactions with the solvent and the organogelator, and can be exploited to optimize the properties of hybrid gels bearing CNTs.

Fig. 4 The covalent functionalization of a SWCNT with N-(8-aminooctyl)-3,4,5-tris(decyloxy)benzamide. Reproduced with permission of the Elsevier Science.

Representative examples of organic agents for SWCNTs dispersion are a series of dyes, with possible applications for the removal of dye molecules. Thus, SWCNTs were dispersed in water in the presence of various dyes and fluorophores (17–25). The authors established that their ability to disperse SWCNTs was not correlated with the stability of the formed hybrids, supposing that the on-rate of dye/fluorophore binding to SWCNTs may dominate the process efficiency.⁸⁰ These results could have potential applications in the delivery of poor cell-penetrating fluorophore molecules. In the case of another CNTs-dye thermodynamically stable colloidal system,⁸¹ by adding sodium chloride electrolyte, the SWCNTs flocculated and settled out due to the destabilization of colloidal systems initiated by the increase in ionic strength. The dye molecules can be removed by heat treatment at 300 °C for 5 h following washing with water.

The fluorophores/dyes used to disperse SWCNTs: FITC, fluorescein isothiocyanate; FSS, fluorescein sodium salt; FLUO, fluorescein; RB, rhodamine B isothiocyanate; TB, trypan blue; OII, orange II sodium salt; PR, phenol red sodium salt; MG, malachite green; TAS, thionin acetate salt.

The slow diffusion of Tween 80 surfactant molecules in SWCNTs aqueous dispersion was directly observed using the pulsed field gradient nuclear magnetic resonance method.⁸² The slow diffusion of Tween 80 molecules was attributed to the strongly adsorbed molecules on the SWCNTs in the aqueous dispersion. The amount of bound Tween 80 molecules was estimated to be approximately 12% of the total amount of Tween 80 molecules, contributing to the stability of the SWCNT aqueous dispersion. This SWCNT/Tween 80 aqueous dispersion was found to be very stable for at least 3 weeks. The observed zeta potentials of this SWCNT dispersion were between −10 and 0 mV, indicating that the stability of the SWCNTs in the Tween 80 solution was maintained by the steric interactions between the small amount of adsorbed Tween 80 molecules on the SWCNTs, whereas the effect of electrostatic interactions between adsorbed Tween 80 was minimal.

Polymer-assisted SWCNTs dispersion

Polyethylene glycol

Polyethylene glycol 26 (PEG), mentioned above, and its derivatives are also very common in CNTs functionalization processes. Thus, the CNTs–polyethylene glycol graft copolymer was synthesized⁸³ via the covalent functionalization of electric arc-produced SWCNTs with monofunctional, tetrahydrofurfuryl-terminated polyethylene glycol PEG–THFF (molecular weight ∼ 200), to obtain a material composed of 80 wt% SWCNTs. The sequential processing of the resulting material by ultrasonication and high-shear mixing provided a means to disperse the SWNT–PEG–THFF macromolecules on two different length scales and leads to highly viscous solutions; at a concentration of 10 mg mL⁻¹ the kinematic viscosity (v) of an aqueous SWNT–PEG–THFF dispersion reached a value of v > 1000 cSt (for water v ∼ 1 cSt). Analysis of this procedure by means of viscosity measurements and AFM showed that ultrasonication is effective in disrupting the SWNT bundles, whereas the high shear mixing disperses the individual SWCNTs. To improve the biocompatibility of SWNHox (carbon nanohorns, new materials that are similar to SWCNTs but have more comparative advantages than SWCNTs), carboxyl polyethylene glycol distearoyl phosphatidylethanolamine (DSPE–PEG–COOH) was chosen⁸⁴ to modify them. Different concentrations of DSPE–PEG–COOH were used in water and phosphate buffer solution (PBS) followed by determining the zeta potentials and monitoring the coagulation times. It was shown that 0.25 mg mL⁻¹ was the optimal concentration of DSPE–PEG–COOH to achieve the best dispersion in PBS. Without DSPE–PEG–COOH, H₂O₂ oxidation assisted by Xe lamp in 1 h was found to be the most effective method because it generates a large amount of oxygenated groups on the surface of the SWNHs, which are of great help for their dispersion. In addition, starting from experimental evidence of the goodness of poly(ethylene glycol-bl-propylene sulfide), PEG-PPS, to disperse SWCNTs, atomistic molecular dynamics simulations were performed to study SWCNTs/polymer systems in the presence of water molecules in solution.⁸⁵ It was revealed that the hydrophobic nature of PPS systematically ensures a higher SWCNT surface coverage, higher interstitial water depletion and a much lower degree of water ordering when compared to the PEG homopolymer.

A light-switchable type of “smart” SWCNTs was developed via the reversible host–guest interaction between azobenzene-terminal polyethylene oxide (AzoPEO) and a pyrene-labeled host attached on the sidewalls of nanotubes via π–π stacking.⁸⁶ These SWCNTs hybrids not only were found to be well dispersed in pure water, but also exhibited switchable dispersion/aggregation states upon the alternate irradiation of UV and visible light. Such a reversible host–guest interaction system may open up the possibility to control the dispersion state of SWCNTs by other common polymers. In the case of polyacrylamide (PAA, 27) derivatives, the SWCNTs were homogeneously dispersed in the aqueous solution of poly(N-isopropylacrylamide-co-acrylic acid) {P(NIPAm-co-AA), 28} with the assistance of sonification.⁸⁷ The mixture was sonicated at 25 °C for 3 h to form the P(NIPAmco-AA)–SWCNTs or PNIPAm–SWCNTs complex and then centrifuged at 2000 rpm for 30 min. In addition, the SWCNTs were endowed with pH- and thermo-sensitivity at the same time. The SWCNTs switched reversibly between the aggregated and the well-exfoliated states using pH or temperature as a stimulus (Fig. 5). Moreover, factors, including the solvent composition, while preparing P(NIPAm-co-AA), concentration and composition of P(NIPAm-co-AA), showed evident influence on the dispersing stability of the SWCNTs. To control the level of carbon nanotube exfoliation in water, pH-responsive polymers (i.e., weak polyelectrolytes, PAA, poly(methacrylic acid) (PMAA, 29), poly(allylamine) (PAAm, 30)), and branched polyethyleneimine (BPEI, 31) were used⁸⁸ as stabilizers in water. This non-covalent functionalization of SWCNTs resulted in suspensions whose dispersion state can be altered by simply changing the pH (Fig. 6 and 7), similarly to poly(N-isopropylacrylamide) (PNIPAAm, 32) and poly-L-lysine (PLL, 33).⁸⁹ The SWCNTs stabilized with these polymers showed a pH tailorable exfoliation and bundling in water. The composite films prepared by drying these aqueous suspensions suggest that nanotube microstructure in the liquid state was largely preserved in the solid composites, with more bundled/networked structures showing higher electrical conductivity.

Fig. 5 Schematic showing the reversible changes of SWCNTs between the aggregated and well-exfoliated states with pH and temperature of the P(NIPAm-co-AA) aqueous solution. At high pH or low temperature, the polymer was in the extended state and the SWCNTs were homogeneously dispersed in the aqueous solution. At low pH or high temperature, the polymer was in the coiled state and the SWCNTs aggregated or precipitated from the aqueous solution. Reproduced with permission of the Elsevier Science.

Fig. 6 The effect of pH on the chain conformations of PAA, PMAA, PAAm and BPEI. PAA and PMAA have neutral charge at low pH and become negatively charged at high pH. PAAm and BPEI are neutral at high pH and attain a positive charge at low pH. Reproduced with permission of the Springer.

Fig. 7 Images of the aqueous nanotube suspensions after centrifugation at different pH. All suspensions contained 0.11 wt% of SWCNTs in 1 wt% of the aqueous polymer solution. Reproduced with permission of the Springer.

Two types of polyfluorenes 34 bearing two lateral pyrene terminated alkyl chains and two alkyl chains per repeating unit were synthesized via a Suzuki polycondensation and used to disperse SWCNTs in organic solvents.⁹⁰ A stable polymer–SWCNT complex can be formed via the multivalent stacking interactions of the lateral pyrene functional groups and the polyfluorene backbone with the outer surface of the carbon nanotubes; moreover, the lateral alkyl chains can impart good solubility to the complex. Polyfluorenes bearing lateral pyrene functional groups and octyl chains exhibited much higher CNTs solubility in common organic solvents than the corresponding polyfluorenes bearing only octyl chains. The selective dispersion of SWCNT species (n,m) with conjugated polymers such as poly(9,9-dioctylfluorene) (PFO, 35) and its analogue poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) in organic solvents depends not only on the type of solvent, but also on the molecular weight of the polymer.⁹¹ The solution viscosity was found to be one of the factors influencing the apparent selectivity by changing the reaggregation rate of the SWCNTs. Poly(4-vinylpyridine) (P4VP 36) is a widely studied polymer for applications in catalytic, humidity sensitive and antimicrobial materials due to its pyridine group exhibiting coordinative reactivity with transition metals. The non-covalent functionalization of SWCNTs with P4VP in CO₂-expanded liquids has been reported,⁹² showing that P4VP stabilized SWCNTs revealed good dispersion in both organic solvent and aqueous solution (pH = 2). The ability to manipulate the dispersion state of the CNTs in water with P4VP will likely benefit many biological applications such as drug delivery and optical sensors. Applications also include the fabrication of high on/off ratio solution-processed thin film transistors on the basis of dithiafulvalene/thiophene copolymers.⁹³ Other reports on polymer-assisted SWCNTs dispersions are also known,^94–96 devoted to selective dispersion of SWCNTs according to either diameter, chiral angle⁹⁷ or influence of SWCNTs dispersion parameters on mechanical and morphological characteristics for SWCNT/polyacrylonitrile (PAN)/polyvinylpyrrolidone (PVP) composite nanofibers.⁹⁸

Use of natural products

Biopolymer dispersant gellan gum (a water-soluble polysaccharide produced by Sphingomonas elodea, a bacterium) was used to achieve aqueous dispersion of highly concentrated SWCNTs, which can be used to form the SWCNT liquid crystal phase.⁹⁹ To achieve the alignment of SWCNTs, purification of SWCNTs was found to be very important and it was achieved by a facile and non-destructive physical method that can prepare large volumes of SWCNTs in high yield for experimental use. Composite membranes of aligned SWCNTs could be obtained via the simple evaporation of the SWCNT liquid crystals. Co-dispersion of native cellulose and SWCNTs in water was demonstrated,¹⁰⁰ showing that the pH of the water should be between 6 and 10 for better dispersion and being useful for biological applications. The co-solubility is likely caused through disruption of the intramolecular hydrogen bonds in cellulose by hydroxyl groups present on nanotubes surface and the creation of intermolecular hydrogen bonds between cellulose and the carbon nanotubes. In addition, the polyphenol curcumin (37) was loaded onto PEG-functionalized SWCNTs through π–π stacking (loading capacity of 235–327 mg g⁻¹ (curcumin/f-SWCNTs)) and the resulting composite was found to form good dispersion stability in water.¹⁰¹ Its excellent biocompatibility as a drug carrier was confirmed.

MWCNTs and SWCNTs were surface modified¹⁰² with humic acids HA 38 {the major organic constituents of soil (humus), peat, coal, many upland streams, dystrophic lakes, and ocean water} from different sources and with surfactants of different ionic types. Both humic acid and surfactant could effectively disperse MWCNTs, but not SWCNTs, into stable suspensions under the studied conditions. The inhibitory effect of peat humic acid was relatively stronger than that of soil humic acid, but the two surfactants had a similar inhibitory effect on atrazine adsorption by the two CNT types. Increases in the surfactant concentration resulted in rapid decreases in the adsorption of atrazine by the CNTs when the surfactant concentration was less than 0.5 of the critical micelle concentration. Moreover, green tea (tea extracts as sources of polyphenols are widely used in “greener chemistry” approaches for obtaining nanoparticles¹⁰³) was reported^104,105 to be a good dispersant of SWCNTs in aqueous media and organic solvents. Dimethyl sulfoxide (DMSO) was found to be a good solvent of green tea extract for dispersing SWCNTs. A combination of green tea (dispersant)/DMSO (solvent)/polyvinyl alcohol (PVA) (nanotube wrap) was obtained that resulted in the dispersion of SWCNTs almost into individual nanotubes or to very thin nanotube bundles. Dispersions of SWCNTs in various surfactant solutions were also evaluated using natural products, including catechins 39, phenolic acids, and flavonoids (a class of plant secondary metabolites).¹⁰⁶

Sugars and related compounds

Carbon nanotube composites are well-known to have numerous medical applications, in particular those functionalized with sugars,¹⁰⁷ so a variety of sugars are actively used for SWCNTs modification. Thus, a covalent microwave-assisted functionalization of pristine SWCNTs directly with three sugar azides (Fig. 8), 2,3,4,6-tetra-O-acetyl-β-D-glucopyranosyl, 2,3,4,6-tetra-O-acetyl-β-D-galactopyranosyl or 2,3,4,6-tetra-O-acetyl-β-D-mannopyranosyl azide¹⁰⁸ was carried out for SWCNTs prepared using the HiPCO method (high-pressure carbon monoxide process). Deacetylation of the functionalized tubes by sodium methoxide yielded nitrogen-linked, sugar-functionalized CNTs that formed stable dispersions in water. The water solubility was found to be highest for galactopyranosyl and lowest for the gluco and mannopyranosyl derivatives varying between 0.6 and 1.3 mg mL⁻¹. Based on the water solubility, it was calculated that ca. 16–35% of functionalized SWCNTs dispersed in water depending on the reactant sugar azide from which galactopyranosyl azide was most reactive. These functionalized SWCNTs were not soluble in ethanol, methanol or ethyl acetate and thereby showed similar solubility properties as the corresponding untreated sugars. Moreover, a single, yet multifunctional, hyaluronic acid (40, HA, an anionic, non-sulfated glycosaminoglycan)-based biosurfactant was used to simultaneously disperse nanocarbons and target SWCNTs to CD44 receptor positive tumor cells with prompt uptake.¹⁰⁹ In vivo photoacoustic, fluorescence, and positron emission tomography imaging of the coated SWCNTs displayed high tumor targeting capability, while providing long-term, fluorescence molecular imaging of targeted enzyme events.

Fig. 8 Functionalization of HiPCO SWCNTs by sugar azides under microwave conditions yielding nitrogen-linked functionalized CNTs. Deacetylation of the product with sodium methoxide produces water-dispersible nitrogen-linked b-D-pyranosyl-functionalized carbon nanotubes. Reproduced with permission of Elsevier Science.

A dispersion of SWCNTs in an aqueous medium was prepared¹¹⁰ by functionalizing the SWCNT with D-glucosamine 41. The grafting resulted in a good dispersion of the SWCNTs in water of an amount less than 1 mg mL⁻¹. β-1,3-Glucan polysaccharides have potential for producing gene carriers and bio-nanomaterials. Carboxylic curdlan (CurCOOH) bearing the β-1,3-polyglucuronic acid structure was prepared¹¹¹ from one of them, β-1,3-glucan polysaccharide curdlan (Cur), by one-step oxidation using a 4-acetamido-TEMPO/NaClO/NaClO₂ system as the oxidant. Its further complexation with SWCNTs resulted in a water-soluble 1D architecture, which formed dispersion in an aqueous solution, stable for several months, and much more stable than SWCNTs complexes of the similar negatively-charged polyacrylic acid (PAA) and polymethacrylic acid (PMAA). It was shown that in the complex, SWCNTs are effectively wrapped by a small amount of CurCOOH, enabling them to avoid electrostatic repulsion.

The debundling and selective dispersion of semiconducting SWCNTs was demonstrated¹¹² using a neutral pH water-soluble chitosan (42) derivative, N-acetylated chitosan 43 (NACHI), which was synthesized via the controlled N-acetylation of chitosan using acetic anhydride (reactions in Fig. 9). The SWNT-NACHI supernatant solution demonstrated semiconductor-enriched properties owing to the preferential adsorption of N-groups of the NACHI on semiconducting nanotubes with a fairly weak charge transfer. Another chitosan derivative, neutral pH water-soluble chitosan–hydroxyphenyl acetamide, prepared by functionalizing the amino groups of chitosan with 4-hydroxyphenyl acetic acid, was also found to be an efficient biocompatible dispersant to effectively debundle and individually dispersed SWCNTs in a neutral aqueous solution.¹¹³

Fig. 9 The controlled N-acetylation of chitosan using acetic anhydride.

Biomolecules

A variety of biomolecules have been applied for SWCNTs functionalization and dispersion, especially DNA (see below); these investigations are obviously directed mainly towards biomedical purposes. Among these compounds, to achieve stable high-concentration SWCNTs suspensions in various alcohols, cholic acid 44 (one of the most important human bile acids) was applied as a useful additive.¹¹⁴ SWCNTs solubility in alcohols was found to depend on its concentration. The best solvent for all alkanol–cholic acid mixtures was found to be EtOH–surfactant mixture (0.018 mol kg⁻¹), decreasing in the order ethanol–isopropanol–t-butanol–butanol–propanol. The natural lung surfactant Survanta® was used to disperse SWCNTs in a biological medium¹¹⁵ without causing a cytotoxic or fibrogenic effect. The forming composite stimulated proliferation of lung epithelial cells at low doses (0.04–0.12 μg mL⁻¹ or 0.02–0.06 μg cm⁻² exposed surface area) but had a suppressive effect at high doses. These effects were not observed for non-dispersed SWCNTs.

A composite material was developed on the basis of SWCNTs and artificially designed peptides,¹¹⁶ which were designed to form a β-sheet structure that would be suitable for wrapping SWCNTs. The composite SWCNT–peptide showed good dispersibility in aqueous media and was considerably stable even in the absence of an excess amount of peptide in the media. The authors suggested the potential of the SWCNT–peptide composite as a molecular platform on which a desirable structure and/or function can be constructed for biomedical and industrial applications. Comparing¹¹⁷ the dispersion capacity of peptides, DNA, low-molecular-weight surfactants, and a water-soluble polymer, the peptide aptamer, A2 (IFRLSWGTYFS, Fig. 10), exhibited the highest dispersion capability below the critical micelle concentration at a concentration of 0.02 w/v%. The aromatic groups of this peptide aptamer and SWCNT walls provide binding capacity due to the π–π interactions between them. In addition, cholesterol-based dipeptide carboxylates 45–49 were applied for the pH responsive reversible dispersion and precipitation of SWCNTs (Fig. 11) in water specifically at tumorogenic environmental pH (6.0–6.5), showing an excellent pH responsive drug release in this range.¹¹⁸ These nanocomposites possess a higher ability to fight cancer cells in comparison with native drug and their action is selective (normal cells are less affected). In addition, a study of the dispersion properties of SWCNTs, DWCNTs and MWCNTs, functionalized with a row of surfactants (Fig. 12) by non-covalent attachment, revealed¹¹⁹ that phospholipids with PL–PEG–NH₂ for SWCNTs and LysoPC for DWCNTs and MWCNTs are the best conditions. The yields are less for DWCNTs in comparison with the two other nanotube types.

Fig. 10 The representative conformation of A2 on a SWCNT. (a) Side view and (b) front view. Reproduced with permission of American Chemical Society.

Fig. 11 Schematic of pH-responsive SWCNT precipitation and drug release. Reproduced with permission of John Wiley & Sons.

Fig. 12 The surfactant structures: (a) benzalkonium chloride, (b) pyrenemethylamine (PMA), (c) polyethylenimine (PEI), (d) 1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-[amino(polyethylene glycosl)2000] (PL-PEG-NH₂), (e) 1-stearoyl-2-hydroxy-sn-glycero-3-phosphocholine (Lyso PC), (f) 1,2-dipalmitoyl-sn-glycero-3-phosphoethanolamine (DPPE), (g) poly(Lys : Phe, 1 : 1) and (h) poly(Lys : Tyr, 1 : 9).

Structures of the synthesized amphiphiles used for dispersing SWCNTs and the corresponding percentage dispersion using the amphiphiles.

Several important (due to useful applications, for instance electrochemical analysis and sensing¹²⁰) reports are dedicated to dispersions of DNA–SWCNTs hybrids, in particular for the determination of polymerase chain reaction (PCR) efficiency.¹²¹ Thus, SWCNTs, effectively dispersed by both pristine and denatured DNA were used to prepare transparent conductive films on PET substrates.¹²² If these films are treated with acid, the DNA molecules can be easily eliminated (Fig. 13). In these hybrids, electrons are transferred from DNA to the SWCNTs and the interaction between the counterparts is strong. Moreover, the DNA–SWCNTs hybrids were also investigated from a reverse point of view: the aggregation of dispersed SWCNTs from aqueous dispersions. It was shown¹²³ that the kinetics of SWCNTs aggregation in aqueous media strongly depended on the overall surface charge. SWCNTs having a greater number of surface charges showed faster aggregation. At the beginning, the microfilaments appear, they then grow forming larger aggregates, which are difficult to redisperse into water. The electrostatic interactions dominate instead of van der Waals interactions, the interactions among dispersed SWCNTs in aqueous media.

Fig. 13 Schematic of the degradation and removal of DNA from the SWCNTs; blue chains, DNA backbone; red rings, DNA bases. Reproduced with permission of Royal Society of Chemistry.

Special investigations on SWCNTs dispersion

Several investigations, both theoretical and experimental, have been carried out to understand the global dispersion processes of SWCNTs in aqueous and organic media, and their particular aspects. Thus, to predict the dispersibility of SWCNTs in a variety of organic solvents (50–78), a quantitative structure–activity/property relationship (QSAR/QSPR) approach was used.¹²⁴ It was established that (1) heavier solvents (and small in size) are most probably the better solvents for SWCNTs and (2) a higher polarizability of the solvent molecule increases the dispersibility. The role of ultracentrifugation in the dispersion processes was studied for arc discharge produced SWCNTs and then functionalized by several routes, dispersed in different aqueous media containing dispersants (Table 1) and purified by ultracentrifugation.¹²⁵ Among the other important results, the purity of all the dispersions considerably improved; however, the centrifugation yield, the degree of purity, and the spectrum profile were influenced by the surface functional groups, the SWCNT type, and the dispersion medium. After SWCNTs dispersion, the ultracentrifugation process always leads to substantially purified supernatant dispersions. Moreover, a good dispersion, orientation and concomitant-polarised photoluminescence of SWCNTs in a nematic chromonic liquid crystal is also known,¹²⁶ causing an alignment of the individual SWCNTs (aligned parallel to the liquid crystal director) over stable macroscopically large domains.

Table 1 The dispersants used for modeling the dispersibility of SWCNTs. Reproduced with permission of the Elsevier Science

Abbreviation	Name	Structure	Category	C [wt/vol%]
SDBS, 1	Sodium dodecylbenzenesulfonate		Anionic surfactant
CTAB, 4	Hexadecyltrimethylammonium bromide		Cationic surfactant	0.5
Plu	Pluronic® F-68		Block copolymer	1
GA	Gum Arabic from acacia tree		Natural gum (branched polysaccharide)	0.5

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SWCNTs functionalization and consequently, their dispersion ability depend on the presence of impurities on the nanotube wall surfaces, in particular other carbon allotropes. It was established that, applying standard nitric acid treatment, the functionalization can be increased by the initial removal of the amorphous carbon on the SWCNTs surface.¹²⁷ The amorphous carbon is the most reactive carbon form on the SWCNTs surface, its presence leads to the formation of oxidation debris and undesirable prevention of SWCNTs surface against functionalization. In addition, an intriguing double-covalent functionalization of SWCNTs (the stepwise functionalization of the tube surface with two different organic moieties), involving both solubilizing ionic liquids and electroactive moieties, was reported.¹²⁸ Oxidized SWCNTs were first amidated with ionic liquid precursors and further treated with n-butyl bromide to afford SWCNTs functionalized with 1-butylimidazolium bromide. This allowed the modified SWCNTs to achieve a relative solubility in water. Electron-acceptor units control the electronic properties of the SWCNTs and the imidazolium units modulate the solubility.

The stability of the CNTs dispersions formed is also important in terms of how to slow down or prevent CNTs re-aggregation.¹²⁹ Both a thermodynamic approach (choosing a compatible solvent or surfactant) and kinetic approach (using a highly viscous solution/melt) were discussed. The following aspects were taken into account: (1) the strong van der Waals binding energies of the CNT aggregates, (2) the ratio between the gravitational and Brownian forces, (3) the apparently enhanced dispersion through “sonication cutting”, (4) the limiting tube length achievable by ultrasonication, (5) the use of appropriate surfactants, for example, octadecylamine, (6) non-covalent dispersion techniques, using appropriate molecules of surfactant or solvent having favorable interaction with the curved graphene wall of the CNT surface, and (7) the zeta-potential of the surfactant/CNT complex, among others. The mechanisms of binding and stabilization are shown in Table 2.

Table 2 A summary of dispersion mechanisms for the various surfactants. The surfactants are divided into six generic groups, labeled A–F. Those in E and F are for dispersion in organic solvents, whereas the rest are for dispersion in an aqueous solution. Reproduced with permission of the MDPI

Stabilization	Binding
	Hydrophobic	Stacking/amine e-pair donation	Stacking
Electrostatic repulsion	A	B	C
	Sodium dodecyl sulfate & related salts phospholipid	Single-stranded DNA; water-soluble proteins	Pyrene acid derived salts
Steric hindrance	D	E	F
	Triton X/Pluronic range; Tween/polysorbate range	Polyvinylpyrrolidone (aqueous, NMP)	Conjugated block-copolymers pyrene siloxane (non-polar organic solvent) conjugated polymer (polar organic solvent)

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Dispersion of different types of CNTs

In general, but not always, all methods described above are for all types of carbon nanotubes, SWCNTs, DWCNTs, and MWCNTs. The grade of dispersion could considerably differ depending on tube lengths, diameters, bundle sizes and applied methods. Sometimes, the nanotube type is important; thus, as it was shown above (see section “Use of natural products”), MWCNTs can be dispersed using humic acid, but not SWCNTs. In a representative and very important report¹³⁰ in this area, 11 different dispersion techniques (Nanomizer, high-pressure jet mill, probe sonicator, ball milling, bead milling, paint shaker, ball collision milling, cone milling, rotor milling, high-shear batch disperser, and thin-film spin mixing) were used as a comparative performance for the dispersion of long SWCNTs, short MWCNTs, and short SWCNTs to elucidate the most appropriate dispersion methods for the different types of CNTs. The authors found an unique effect (“when long SWCNTs were dispersed using a turbulent flow method, the resulting composites showed the highest performance when compared to (1) composites with other types of CNTs and (2) composites prepared using other dispersion methods”) and proposed a fundamental mechanism to explain it. We can affirm that this is the first fundamental systematic study investigating the comparative performance of the different dispersion methods.

Conclusions and further outlook

Carbon nanotubes continue to be one of the most interesting topics in nanoscience and nanotechnology and therefore the development of their functionalization and dispersion methods is and will be an important research area, at least for next 5 years. A series of contemporary techniques (Fig. 14) are being used for SWCNTs (as well as MWCNTs) solubilization, from physical (classic ultrasound or radiation treatment) to chemical and biological, applying inorganic (nanodiamonds, iodine, metallic sodium in liquid ammonia, peroxides, and mineral acids) and organic (acids, salts, polymers, dyes, antibiotics,¹³¹ natural products, and biomolecules) compounds, as well as micelles¹³² on their basis. Carbon nanotube dispersion in nematic liquid crystals is also known.¹³³ In some cases, successive steps can be applied, for instance the use of low- and high-weight surfactants, mineral acid treatment for (1) the elimination of amorphous carbon or (2) the creation of –OH and –COOH groups and their further interaction with organic molecules. It has been suggested that van der Waals interactions, π–π stacking interactions between aromatic rings in organic compounds and CNTs, and hydrophobic interactions are the major factors responsible for CNTs dispersion. For appropriate solvent selection, it was established that (1) heavier solvents (and small in size) were most probably the better solvents for SWCNTs and (2) a higher polarizability of the solvent molecule increases the dispersibility.

Fig. 14 Selected methods for SWCNTs dispersion.

To choose surfactants that are able to stabilize CNTs in water, it is necessary to employ dispersing agents that (a) strongly adsorb onto the nanotube surface, (b) present hydrophilic groups, better if rigid, that extend towards the aqueous phase, (c) are not very mobile on the nanotube surface, and (d) show aggregates with a structural dependence on the nanotube diameter and chirality. When comparing SWCNTs with MWCNTs, sometimes unpredictable results could be obtained, for example, in the case of the action of humic acid to effectively disperse MWCNTs, but not SWCNTs, into stable suspensions under the studied conditions. Studies of SWCNTs dispersions could have useful applications, for example SWCNTs networks can be combined with dissolvable electrodes, dielectrics, and substrates to create transistors that offer transient behavior due to disintegration and dissolution in water.¹³⁴

The ability to manipulate the dispersion state of CNTs leads to a variety of useful applications for the functionalized SWCNTs dispersions in both water and organic solvents, including the fabrication of devices, creation of membranes and optical sensors, drug delivery and other medical uses, and the possible regulation of environmental spread and organic pollutants (for instance dyes) adsorbed onto carbon nanotubes. Taking into account the facts that the carbon nanotube area is currently an enormous field of research and the applications and CNTs consumption is being increased annually, the demand for their solubilized/dispersed forms in liquid and solid phases will be obviously dramatically enhanced in the next decade.

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Footnote

† In the case of MWCNTs, an ultrasound treatment of nanotubes in the presence of highly water-soluble theraphthal (cobalt phthalocyanine salt with 7 –COONa and 1 –COOH groups) led to the total destruction of nanotubes and the formation of nanoonions, among other products instead of nanotube solubilization (RSC Advances, 2015, 5, 57764–57770).

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