dispersion of carbon nanotubes

Journal Pre-proof Modification strategies for improving the solubility/dispersion of carbon nanotubes Syed Tayyab Raza Naqvi, Tahir Rasheed, Dilshad Hussain, Muhammad Najam ul Haq, Saadat Majeed, Sameera shafi, Nisar Ahmed, Rahat Nawaz PII:

S0167-7322(19)34304-1

DOI:

https://doi.org/10.1016/j.molliq.2019.111919

Reference:

MOLLIQ 111919

To appear in:

Journal of Molecular Liquids

Received Date: 31 July 2019 Revised Date:

5 October 2019

Accepted Date: 12 October 2019

Please cite this article as: S.T.R. Naqvi, T. Rasheed, D. Hussain, M. Najam ul Haq, S. Majeed, S. shafi, N. Ahmed, R. Nawaz, Modification strategies for improving the solubility/dispersion of carbon nanotubes, Journal of Molecular Liquids (2019), doi: https://doi.org/10.1016/j.molliq.2019.111919. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

Graphical Abstract

Modification Strategies for Improving the Solubility/Dispersion of Carbon Nanotubes Syed Tayyab Raza Naqvi,a Tahir Rasheedb ⃰, Dilshad Hussaina, Muhammad Najam ul Haqa, Saadat Majeeda ⃰, Sameera shafib, Nisar Ahmedc ,Rahat Nawaza a

Division of Analytical Chemistry, Institute of Chemical Sciences, Bahauddin Zakariya

University, Multan 60800, Pakistan. bSchool of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China; cSchool of Chemistry, University of Bristol, Bristol, BS8 1TS, United Kingdom. Corresponding author

*

E-mail:[email protected]

(T.Rasheed): [email protected] (S. Majeed) Abstract Carbon nanotubes (CNTs) are an important part of diverse carbon family. CNTs, either single walled or multi walled are used in different applications due to their versatile attributes like the unique structure which is crucial for its specific properties. Its high surface area, easier surface modifications and hollow tubular structure impart versatility in terms of properties and applications. Applications of CNTs are often hindered by its limited dispersibility and solubility in different solvents. Variety of modification methods have been employed to improve the solubility and dispersibility of CNTs. These modifications impart special characteristics to nanotube structure, apart from improved dispersibility. This review presents a comprehensive overview of the methods in improving the dispersibility and solubility of CNTs, maximizing their application in diverse fields. Furthermore, conditions and applicability of each modification method is discussed and its effect on dispersion/solubility of CNTs is also evaluated. Keywords: Carbon nanotube; enhanced solubility; oxidation; dispersion; functionalization; nanoparticle 1.

Introduction

Among all carbon-based nanomaterials, carbon nanotubes (CNTs) have tempted researchers by

their discovery due to cylindrical shape and larger length to diameter ratio (132,000,000:1) which is remarkably larger than for any other nanomaterial [1]. The name “nanotube” is assigned due to long hollow structures which are graphene sheets rolled in a tubular shape. The characteristic behavior of nanotubes is determined by rolling angle and radius. On the other hands, independent nanotube is of a metallic or semiconductor nature. The CNTs inherently oriented themselves into "ropes" held together with week van-der-Waal forces of attraction, and can be classified as single-walled carbon nanotubes(SWCNTs), double wall carbon nanotubes (DWCNTs) and multi-walled carbon nanotubes(MWCNTs) [2]. CNTs have been studied for omnidirectional applications including science, clinical and engineering [3]. Due to their properties, they are chemically and mechanically stable offering high electrical and thermal conductance with larger surface area accompanied by increased strength and functionalization capability. Additionally, their possible uses in sensing and energy storage devices, chemiluminescent antitumor imaging, environmental pollutants removal and drug delivery are some potential fields [4]. CNTs have been synthesized by different methodologies like arc discharge (AD), laser ablation (LA) and chemical vapor deposition (CVD) as well as synthesized by some low standard strategies including pyrolysis and hydrothermal [5-8] AD and LA were developed earlier and executed at high temperature ( >1700 ˚C). Although, they have been replaced with CVD due to its low operating temperature ( <800 ˚C) [9]. In this contribution, recent work regarding the modification and functionalization methods enhancing the solubility/dispersion of CNTs in variety of solvents (aqueous or organic) has been encompassed. Despite controversy about terms solubility or dispersion, this work includes CNTs modified with conducting polymers, metallic nanoparticles, organic functional moieties, nanostructured oxides and sidewall modifications helping in de-bundling of nanotubes [10,11]. The

modification of CNTs with variety of functional groups makes them specialized for enzyme immobilization for electrochemical analysis and sensing [12]. The critical review of existing modification methods for CNTs can help in understanding new methods, which can be employed/developed for enhancing the solubility and dispersion of CNTs. 2.

Modifications Assisting Solubilization of CNTs

Carbon nanotubes are generated in entangled form because of Vander-Waals forces and cannot be isolated from each other, which limit their utility as materials [13].CNTs therefore be debundled and dispersed in solvents for the desired applications. Rapid and uniform dispersion and solubilization methods for CNTs have been developed for decades as they can be integrated in biochemical and electrochemical platforms [14]. The simplest and the easiest method often used in the laboratories is ultra-sonication. The drawback of sonication lies in the formation of aggregates on stopping sonication [15]. Number of methods has been developed for improving solubility and dispersion of CNTs [16]. There has been advances in functionalization methodologies for improving the dispersion of CNTs in aqueous and non-aqueous solvents. Factors like structural variations, size dispersal, surface-to-volume ratio, surface chemistry and purity are responsible for the CNTs reactivity, hydrophobicity, aggregation and toxicity. Surface modification of CNTs attaches specific molecules, which impart chemical specificity for specific applications. The modification in CNTs reduces toxicity of nanotubes in biological systems. The strategies include chemical modification of surfaces by sidewall/end functionalization, wet mechano-chemical method, surfactants, polymer wrapping, biomolecules, transition metal complexes immobilization on CNT sidewalls by covalent binding, protonation by super acids and reduction by alkali metals, electro-grafting of aryldiazonium salts,18electro-polymerization of functionalized pyrroles and pyrenes and other supramolecular bindings onto CNTs [17, 18].

The detailed investigation of functionalization has revealed the chirality of CNTs and modifying groups, π-πstacking, weak attractive forces (London dispersion forces, Van der Waals forces, charge-transfer interactions) and substitution of hydrophilic molecules along with diameter of nanotubes contributing in enhancing the solubility and uniform dispersion of CNTs [19]. 3.

Chemical Modifications

The inadequate solubility of CNTs in common aqueous and non-aqueous medium retards their applicability. Murry et al. presented first report on chemical modification of carbon materials [20]. The chemical alteration of carbon material is inexpensive and overcomes the shortcomings associated with their utility [21]. The modification generally shortens length of nanotubes however functionalized CNTs withhold intrinsic properties. The de-bundling of nanotubes is dependent on the process and its parameters like centrifugation speed, power, temperature of sonication, and CNT to modifying moiety ratio [22]. The modified CNTs are characterized by atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), optical absorption spectroscopy (OAS), attenuated total reflectance (ATR) and Raman spectroscopy. 3.1

Oxidation of Carbon Nanotubes

Raw CNTs are often purified by their oxidation. Number of oxidation methods have been employed to etch, purify and exfoliate the surfaces of CNTs (Fig. 1a). The oxidation is generally performed by refluxing the CNTs in highly acidic environment like H2SO4, HNO3, HClO4, KMnO4, H2O2,oxygen and chlorine [23]. The blocked active sites of CNTs open up in strong oxidation conditions producing carboxylic, hydroxyl, enone, formyl (-CHO), epoxide, and sometimes quinoidal and ester groups at the blockedsites [24]. Furthermore, some defect sites are created on outer surface of nanotubes walls [25]. The functional groups on CNTs are controlled by mild conditions as per the requirements of application. In polymer nanocomposites formation,

hydroxyl groups act as binding sites for the silanization process [26]. Sometimes carboxyl moieties on CNTs are acylated with thionyl chloride (SOCl2), and subsequently reacted with desired functional groups [27]. The oxidation of CNTs enhances the solubility by increasing number of acidic groups on nanotubes. The acid treatment however affects structural properties of CNTs and causes defect-based chemistry over surfaces (Fig. 1b). The heavily derivatized water-soluble carboxylated CNTs have pores on outer surface to make them permeable for fluids [28]. Peroxidase was immobilized on MWCNTs to develop nano drug carrier, sensing and biocatalyst. Peroxidase was immobilized on MWCNTs via covalent linkage following oxidation of MWCNTs with help of oxidizing agents [29]. Carbon based nanomaterials have been used as lubricant additives to increase their tri-biological properties. However, stability and dispersibility of CNTs in the lubricants caused obstacles in their utilization [30]. However, oxidation of CNTs have been done by chemically treating them with sulphuric acid, nitric acid and refluxed with stearic acid to increase their dispersibility in lubricant. Synthesis of PVA films coated oxidized CNTs@PEI was accomplished by solvent casting technique to make CNTs highly stable dispersible in aqueous solvent. Subsequently, PVA coated ox-CNTs@PEI was studied to check antimicrobial activity [31]. 3.2

Esterification and Amidation of Oxidized CNTs

The amidation and esterification are used to fabricate the conjugates of water soluble organic molecules including polymers, nucleic acids and proteins with oxidized CNTs [32]. The functional groups are linked to CNTs through carboxylic groups which provide active sites for functionalization in multidimensional applications. Modification process fabricates reactive CNTs with improved dispersion [33]. Amidation of CNTs was performed by modifying with acetonitrile and maleimide followed by the comparison was made between acetonitrile and maleimide. This study depictes that amidation through acetonitrile was easy and cheap while the

ustilization of maleimide require number of preparatory steps [34]. Flux and anti-fouling capability of ultrafiltration membrane, made up of MWCNTs, was enhanced when it was modified by triethylenetetramine (TETA) via generating –NH2 on side wall of MWCNTs. Modified membrane was characterized by AFM, showed better flux, antifouling, hydrophilicity, roughness and no detrimental effect as compared to membrane comprised of pristine CNTs. Amidation of carboxylic acid on side wall of CNTs was done to conjugate insulin on CNTs (figure 2). Insulin conjugation was confirmed by UV, SEM and TEM as well as amount of insulin conjugated was determined by performing bicinchoninic acid assay [35]. Metal catalyst assisted generation of amine groups on sidewalls of CNTs has also been studied. Recently, CuI has been used as catalyst in direct amidation on CNTs while Cu2+ and Ni2+ salts presented less desirable results as compared to Cu1+ salts. Copper catalyzed amidation offers polyamine groups on surface which in turn showed excellent aqueous dispersibility, developed electronic semiconductor behavior and high capability to entrap CO2 [36]. DFT study on aminotriethylene glycol (AMTG) modified SWCNTs has been accomplished to investigate the electronic and solubility behavior. AMTG-SWCNTs presented increased solubility and binding energy while decrease in salvation free energy as number of AMTG group increased on side walls of SWCNTs [37]. N,N-dihexylamide (DHA) modified CNTs were prepared to remove heavy metals (especially lanthanide) from low activity waste water release as effluent via nuclear fuel recycling. This was an effort to study sorption behavior of trivalent lanthanides by DHA-CNTs. Sorption behavior was studied by all isotherm models including Langmuir, Freundlich, Temkin and Dubinin-Radushkevich isotherms [38]. Amide functionalized CNTs have been playing their role in separation science. Polysaccharides and polyethylene glycol have been separated with the assistance of diethanolamine modified CNTs as centrifugal filler units [39].

3.3

Functionalization of CNTs Sidewalls by Cycloaddition Mechanism

Oxidation reaction generates defect sites on CNTs while cycloaddition reactions generate chemical linkageson sidewalls and blocked ends [40]. This modification occurs during improper creation of carboxylic groups. The cycloaddition puckers CNTs surface by releasing the strain produced by functional groups. Ghini et al .reported nitrone 1,3-cycloaddition to the Sp2 network to achieve water soluble CNTs (Figure 3) [41]. Azomethine-ylides attach pyrrolidine ring on the walls of CNTs following 1,3-cycloaddition,resulting in de-bundling of nanotubes. The organic grafting by1,3-cycloadditions onto CNT network increases solubility in aqueous and organic solvents with stability over several weeks. These reactions however require longer times of several days [42]. 3.4 CNTs Sidewalls Functionalization by Radical Additions The other covalent modification methods include the reduction and functionalization of CNTs by radical addition method, Bingel reaction, Grignard reagent and reduction by alkali metals including organo-lithium reagents and per fluoropolyether radicals [43,44]. The aryl groups of aryl diazonium salts covalently attach onto the skeleton of carbon nanomaterials by radical addition method [45]. The solubility behavior of functionalized CNTs has dependence on aniline substituted derivatization and degree of functionalization. The aryl-modified carbon nanomaterials with polar functional groups (carboxylic acid and oligoethylene branches) are solubilized in polar medium such asTHF (tetrahydrofuran), methanol and CHCl3 (chloroform) [46].This solubility is due to positively charged ammonium molecules and ethyleneglycosidic polar chains of phenylring [47]. Covalent modifications are tedious while non-covalent functionalizations are comparatively simpler [48]. The non-linear optical behavior of these modified nanomaterials and ultrafast response can be employed in devices of optoelectronics and

photonics [49]. A non-covalent π-π interaction between naphthalen-1-ylmethylphosphonic acid (NYPA) and multiwall carbon nanotubes (MWCNTs) is used to anchor phosphonate groups onto MWCNTs. The oxidized MWCNTs are sonicated for 2 h in ice bath with NYPA aqueous solution at pH 9. NYPA-grafted-MWCNTs hybrid is water soluble with the average size of 215 nm and stability up to three months [50]. CNTs with high and stable dispersibility were synthesized by combination of mussel stimulated chemistry and free radical polymerization. PDA modified CNTs were prepared by mussel chemistry which was further coated by PDMC (amino-terminated-polymer) with the assistance of free radical addition polymerization (figure 4). SEM, FT-IR, XPS and TGA validated CNTs/PDA/PMDC functionalization. Results revealed that CNTs/PDA/PDMC showed high dispersibility in aqueous as well as organic solvents [51]. 4.

Modifications with Polymers

CNTs are ideal materials for strengthening the fibers due to their distinctive mechanical properties. The nanotube polymer composites show applications in aeronautics, optoelectronics, biomedical science, tissue engineering, neuro-science, genetics, and biosensor technology [52]. Covalent addition of CNTs to polymers is an interface that follows “grafting to” and “grafting from” approaches [53]. The covalent modifications of CNTs alter the structural characteristics [54]. Several synthetic procedures have been developed for the homogeneous introduction of CNTs in polymer backbone network, for example, polymerization of monomers in the presence of nanotubes, atom transfer radical polymerization, ring-opening polymerization, surfactant assisted polymer wrappings, reversible addition-fragmentation chain transfer, and by click chemistry protocols [55]. The hydrophobic portions of molecules wrap the nanotubes through non-covalent interactions like Vander Waals and π-π, while the hydrophilic ends of modified material direct themselves toward the polar ends of solvent and enhance dispersion. The non-

covalent modification as compared to covalent preserves CNTs for their electronic properties [56]. These modifications increase the stability and de-bundling of nanotubes to many folds in range of solvents. Polyaniline, polyethyleneimine, polypyrrole and their derivatives are used to functionalize CNTs [57]. Polymer modification is done on both oxidized and non-oxidized CNTs. The synthesis of chemically modified MWCNTs with pyrrole monomers at room temperature without using any oxidant causes lesser impurity of ions than in oxidative polymerization [58]. Xia et al. and Soriano et al. prepare polymer-encapsulated MWCNTs via ultra-sonication [59]. Functionalization of MWCNTs with methylmethacrylate and nbutylacrylate has been ultrasonically done through emulsion polymerization. CNTs modified via emulsion polymerization offer higher surface to volume ratio, after three days of Soxhlet extraction with acetone, and studied by transmission electron microscopy (TEM) [60]. The dispersion of CNTs has been studied in combination with macromolecules. The cyclic cucurbit [n]-urilmolecular containers selectively solubilize MWCNTs structures [61]. The selective binding can be controlled by high concentration of molecular containers or by regulating the concentration of ions in solution. The C shaped large molecular wrap around nanotubes enhances solubility to many folds with structural preservation of CNTs [62]. The induced redox radical polymerization (RRP) method fabricates water-soluble nanotubes in the presence of metal ions [63]. Modified CNTs are dispersed in aqueous solution of ceric ions and acrylamide monomer and stirred for 4 hours at room temperature. Ceric ions induce redox process for the polymerization of acrylamide monomer. The RRP with CNTs bearing functionalities not only modifies the surfaces but also provides study of their side wall [64]. The hydrophilic and biocompatible MWCNTs-grafted-poly(vinyl alcohol) and graphene oxidelinked-PVA have been synthesized by non-covalent π–π interactions [31]. These MWCNTs are

loaded with poorly water-soluble drug, camptothecin (CPT), and used as anticancer in chemotherapy of breast and skin cancer. The PVA modified MWCNTs loaded with CPT drug is more cytotoxic than CPT alone. The described strategy provides route for loading water insoluble, aromatic drugs on modified CNTs and graphene based nano structures and are appropriate water soluble nano-carriers [65]. The MWCNTs have been modified in deionized water through the esterification process of poly ethylene oxide (PEO) and p-formyl benzoic acid followed by imination with polyaniline (PANI) [66]. The conjugated block copolymer (BCP) consisting of PEO−PANI is applied to non-covalently integrated MWNTs via π-π interactions. BCP modified MWCNTs show solubility for several weeks in common solvents including water [67]. The non-covalently modified nanotubes with hydrophilic BCP are nanomaterials for nanoelectronics, drug delivery, electrochemical sensors, thermal and chemiluminescence sensors (Figure 5) [67, 68]. Less dispersibility of CNTs in polar and non-polar solvent was major hurdle in executing as nanocarriers for drug delivery. MWCNTs covalently combined with polyethyleneimine PI, Fluorescein isothiocynate FI and Hyaluronic acid HA were synthesized to deliver the DOXs (anticancer drug) at targeted site [69]. Type-2 photo-stimulated PS-2 strategy played an active role in synthesizing water soluble MWCNTs via noncovalent interactions. By PS-2 strategy, 2-(diethylamino) ethyl methacrylate and MMA was deployed to develop hyper branched co-polymer that was used to modify the MWCNTs (figure 6) and made them water soluble [70]. Similarly, block copolymer containing bilayer hydrophilicity disperses the oxidized MWCNTs

in

aqueous

and

non-aqueous

solvents

by

non-covalent

interactions.

Poly(ethyleneoxide)-b-poly [2,(N,N-dimethylamino) ethylmethacrylate] is a biocompatible and water-soluble copolymer [71]. It is attached to the oxidized MWCNTs through multiple sites, showing zwitterionic interactions. This is the non-covalent interaction created between the amino

groups (block copolymer) and the carboxyl groups of oxidized CNTs. A stable dispersion of nanotubes through non-covalent interaction between water-soluble carboxy-methyl chitosan derivatives with pyrene groups (CMCSPy) wrapped on MWCNTs is prepared [72]. The stabilizing and templating CMCSPy disperses nanotubes more in hydrophilic system than hydrophobic system and in the gels. These modified MWCNTs are in-situ incorporated to biofriendly sol-gel silica polymer for electrochemical study of hydrogen peroxide. The poly(organophosphazene) polymers are highly stable macromolecules [73]. Excellent dispersion of the nanocomposites are achieved in aqueous as well as non-aqueous solvents. The solubility and thermal stability of MWCNTs/poly(organophosphazene) hybrid systems can be controlled by variation in the side groups of polymer [74]. Linear structured polyphosphazenes can be used as dispersant for SWCNTs [75]. They dispersed SWCNTs in aqueous medium of PZS (sulfonated poly(organophosphazene)) by supramolecular conjugation. The polymer wraps nanotubes of about 10 nm with total thickness of 4 nm. Poly(organophosphazene) with CNTs and the functionalized CNTs/PZS hybrid molecules offer better solubility and thermal characteristics [76]. Non-covalent modifications use organic molecules ranging from lower to higher molecular weights. Supramolecular chemistry produces CNTs with high dispersion in solvents of varying nature. The functional groups on sidewalls of these polymers maintain dispersions. Thymine-decorated CNTs dispersion can be stabilized by nanotube/molecule conjugation utilizing any polymer chain decorated with DAT soluble in a solvent. Steric hindrances among polymer chains secure dispersions stability [77]. Polyethyleneimine combines with SWCNTs providing water solubility and stability, as well as playing role as an adsorbent [78]. Li et al. reports covalent interactions between SWCNTs and thermosensitive polymer to make CNT-polymeric micelle hybrid which provides stability, biocompatibility and water

solubility [79]. Reversible addition fragmentation chain transfer (RAFT) polymerization synthesizes azide group containing PDMA-b-P[poly N,(N-dimethylacrylamide)-b-poly], and NIPAM-co-NAS

(N-isopropylacrylamide-co-N-acryl-

oxysuccinimide)

copolymer.

This

covalent azide derivatization of PDMA-b-P - (NIPAM-co-NAS) is achieved through nitrene addition reaction. The thermosensitive copolymer-integrated SWCNTs hybrid micelle is covalently placed onto the sidewalls of nanotubes. During the micelle synthesis, free copolymers are also produced as result of side reaction (figure 7) [79]. In most of the studies, polyelectrolyte has been used to functionalize CNTs. The polymers possessing anionic, cationic or zwitterionic functional groups have been incorporated into CNTs networkthrough non-covalent attachments [80]. These polymeric moieties grafted to CNTs enhance dispersion in aqueous media by anionic/cation-pi interactions. The supramolecular adduct of water soluble allylamine polymer (PAL) with CNTs is also reported [49]. The amino groups of PAL link with CNTs and envelop around the tubes. The approach provides distance between the functional groups on CNTs by alterations in the polymer backbone structure and sidewall chains, which in turn resists the bundling of nanotubes. Ionic 1,3-glucans wrap around SWCNTs and DWCNTs (double wall nanotubes) to form water-soluble hybrid complexes with ionic groups on the wall [81]. The method produces water soluble CNTs based hollow capsules from natural-1,3-glucan polysaccharides and CNTs complexes. The electrostatic forces of attractions between curdlans, high molecular weight polymer of glucose, fabricated /SWCNTs hybrid complexes from positively charged ammonium-modified and negatively charged sulfonate-modified hybrid complexes [82] fabricate the SWCNTs. The apparently transparent black solution indicates the solubilization of DWCNTs in aqueous solvent. The complexes are obtained with 0.5-1.2µm length and 3.1-3.6nm height. The hollow capsule’s spherical shape and the structure are stable in

aqueous solvents even after hydrogen fluoride etching [83]. An ultra-thin cross-linked polymer film method has been used to modify the CNTs with non-ionic surfactants. Self-polymerization of n-dodecyl glycerylitaconate (DGI) monomer in the presence of linker N, N’-methylene bis (acrylamide) casts thin film on individual nanotubes [84]. DGI attaches to nanotubes through hydrophobic interactions, while hydrophilic head groups of polymer are oriented towards the water phase. The thickness of thin film over nanotubes varies from 8to 26 nm upon increasing the polymer concentration. The approach provides stable and monodisperse suspension of CNTs in water for several days without altering the chemical and physical properties. Water soluble CNTs are obtained when citric acid monomer is polycondensed with sorbitol [85]. Modified MWCNTs suspension is stable over three months. Citric acid polymerizes with D-sorbitol by an anhydride intermediate mechanism on MWCNTs with the mole ratios of greater than 4:1.The electrospinning generates isolated fine fibers of PANI/PEO/MWCNTs which are engineered by addition of sulphate moieties to emeraldine salt derived from PANI [86]. These fibers have strong water solubility and are electro-conductive. The morphology and fiber diameter from 181 to 217 nm exhibit constant thickness across the fibers [87]. The modification approaches, using molecules with hydrophilic and hydrophobic nature, range from single micro-molecule to complex macromolecules. Adsorption or wrapping of amphiphilic molecules, such as surfactants, onto CNTs is an efficient way to make CNTs dispersion. This modification has been done at room temperature in common solvents by sonication of nanotubes with surfactant. Surfactant assists CNTs dispersal in variety of solvents to form polymer-CNTs composites [88]. The non-specific molecules adsorb on CNTs while retaining the hybridization of CNTs network. The CNTs dispersion relies on the type and nature of surfactants. Many types of surfactants are in use to functionalize CNTs. The common surfactants used are negatively

charged such as SDS(sodium dodecyl sulphate), SDBS (sodium dodecylbenzene sulphonate), positively chargedsuch as alkyl-substituted imidazolium derivatives, benzalkonium chloride, CTAB(cetyltrimethyl ammonium bromide), and non-ionic surfactants such Triton-X. The hydrophilic and hydrophobic portions of surfactants interact with polar solvents and CNTs, respectively and stabilize nanotubes in solvents. Sulphonic acid sodium salts are the important agents that assist in the solubility of polymers-modified CNTs in aqueous solutions [89]. A water-soluble derivative of copper phthalocyanines and sodium salt of sulphonic acid blended with CNTs has been used in the development of hole-withdrawing electrode and inorganic bilayer donor solar cell [90]. The sulphonic acid moieties increase the ionization potential and phthalocyanines macrocylce tend to be soluble in aqueous media by exhibiting the negative mesomeric effect. The modified CNTs are stable for more than 6 months. Ionic liquid based surfactant de-bundles CNTs in the gels and solvents. The ionic liquids instead of the organic solvents exhibit the properties of inflammability, stability at room temperature, and are nonvolatilein nature. These liquids have been used to fabricate nanotubes [91]. 1-hexadecyl-3vinylimidazolium bromide exploits properties of both ionic liquid and surfactant when mixed with CNTs and ultra-sonicated for several hours in aqueous conditions. The obtained suspension is stable over a period of 2-3 months [92]. Micelle is another common modification method to encapsulate CNTs with surfactants. Number of CNT−micelle hybrids has been fabricated through covalent intermolecular forces by cross-linked polymeric micelles on CNTs. The use of surfactants in excessive amount and behavior of micelle complex structures generally limits their applications [93]. Dendrimers are class of highly branched polymers. They provide extended π-π conjugated electrons delocalization to CNTs upon modification. The commonly used dendrimer, poly-

amidoamine (PAMAM) possesses distinctive surface with plentiful amine groups [94]. PAMAM tethered CNTs composite has been synthesized by a multi step Michael addition of methylacrylate followed by the addition of amide group, amidation, with 1,2-ethylenediamine [95]. The solubility of PMMA-MWCNTs in water is attributed to polar interactions between amine groups and the solvent. The water-soluble polypyridyl (ppy) complexes having general formula [Rux (bpy) yL] 2+ (L = dppz, dppn, tpphz) with elongated planar π systems solubilize SWCNTs via π-π interactions. The obtained dispersion is stable for several weeks and the method provides advantages regarding facile synthesis, and unfolding in the absence of any further chemical functionalization of SWCNTs [96]. Pyrene derivatives are prepared with variety of functional groups to solubilize CNTs. Pyrene is a polyaromatic molecule having affinity towards CNTs through π-πinteractions. The complex formations with pyrene molecules cause partial unfolding of CNTs bundles and bring them into separate tube fibers in solution. These molecular bindings containπ-π interactions instead of covalent bonding, resulting in disturbance of nanotube complex system [97]. The solubility of CNTs is difficult to regulate which can restrain their applications to some extent. Attempts have been made in this regards by controlling experimental conditions. The solubility of hydrophobic sites on conjugating complexes depends on the solution pH. Consequently, solubility of nanotubes can be controlled with ease by reversible properties [98]. Polymer functionalized CNTs were deeply investigated in electronics and optoelectronics. SWCNTs functionalized with degradable co-polymer s-tetrazine were synthesized to develop thin film transistors and sensors showing improved sensitivity [99]. MWCNTs were also integrated with poly-amide-imide to synthesize a composite so that to regulate conductive and mechanical and electrical behavior of CNTs. Structural investigation has been made on PAA-PAI block copolymer functionalized CNTs to observe its effect on solubility

in aqueous media. Investigation revealed that presence of hydrophobic interactions offered improved solubility in water [100]. 5. Surface Modifications of Carbon Nanotubes with Polysaccharides Multi-walled carbon nanotubes with enhanced ability to disperse in usual solvents have been synthesized by chemically integrating the surfaces with monosaccharides and polysaccharides. CNTs integrated by polysaccharide chains via non-covalent forces of attraction are soluble and dispersive in water containing environments, and have diverse chemical, environmental, energy storage and biomedical applications [101,102]. The wrapping mechanism is not known, but generally the ends of CNTs play role in wrapping mechanism. MWCNTs were functionalized with glucose and fructose to enhance their dispersibility in aqueous solvents. Comparison was made between PAI-MWCNTs and glucose or fructose modified MWCNTs [103]. An amylose chain of polysaccharide and SWCNTs are combined to generate hybrid complex, which has selfassembling tendency via atomistic molecular dynamics simulation, along with its structural characteristics in aqueous environment. To explore edge effects, two relative shapes are allocated i.e., parallel and orthogonal if the middle and tail ends of the SWCNTs have been selected as two starting wrapping sites. The results show that amyl group, starting from tail end of SWCNTs, can envelop spontaneously around the tubular surface, done by both Vander Waals forces of attraction and hydrophobic interactions. As a result, compact and helical conformation is formed which is maintained by wreathed hydrogen-bond network. The stabilized helical conformation involves gradual submergence of hydrophobic ends of polysaccharide chain, which are developed by hydrophobic forces. On the contrary, if wrapping proceeds inefficiently, selfassembling into a helical shape is not observed due to strong Vander Waals forces of attractions suppressing the hydrophobic interaction of the amyl group of polysaccharide chain with the

tubular surface [104]. Carboxyl functionalized SWCNTs were further modified with chitosan, a polysaccharide, to improve water dispersibility and biocompatibility. Polysaccharide functionalized CNTs have been widely involved in sensing application. To make stable dispersion of electrode materials, CNTs have been functionalized with polysaccharides. Recently, polyaniline-CNTs-starch nanocomposite has been synthesized to develop biocompatible and dispersible electrode material for electrochemical sensing of H2O2 and glucose [105]. Algal modified CNTs show good solubility being deposited on dioctyldimethylammonium bromide single layer film to increase urease incorporation capacity. Alginate and chitosan modified SWCNTs were prepared as nanocarrier for anticancer drug such as curcumin to targeted site. SWCNTs were modified with alginate and chitosan to attain high drug stability, bio distribution and dispersion. CNTs have potential to act as good supporting material for drug in DDS. Lentinan ( a polysaccharide extracted from mashrooms act as immunostimulatory drug) modified CNTs were prepared as antigen delivery system in cancer treatment. Lentinan also improved the solubility of CNTs [106]. 6.

Surface Modifications of Carbon Nanotubes with Biomolecules

The biomolecules are used to make CNTs dispersive and biocompatible. The biomolecules enhance hydrophilicity and biocompatibility of CNTs. CNTs are coupled with DNA, proteins, amino acids, polypeptides, hemoglobin, enzymes, cytochrome c, glucose dehydrogenase and glucose oxidase, and horseradish peroxidase [12, 107]. The coupling of biomolecules with CNTs has diverse biomedical applications like drug delivery,therapeutics and diagnostic techniques owing to their translocation/transport into cytoplasm and cell nucleus through cell membrane. Solubility and toxicity of CNTs has been addressed by the non-covalent interactions between

biomolecules and CNTs. They have developed molecular conformation of cell surfaces, and are water-soluble, biologically improved surfaces. In a recent study, functionalization of SWCNTs have been done with tri-acyl-glycerol acylhydrolases (TGAD) enzyme to make SWCNTs soluble in variety of solvents to increase their biocompatibility and interaction with other molecules [108]. TGAD functionalized SWCNTs exhibit homogenous distribution with high solubility for more than 30 days. The high cost, sensitivity and selectivity of these biomolecules render their use in several applications. The strategy follows the physical mixing of host and guest specie in the solvent to prepare composite. Many synthetic biocompatible, polymers and dyes, like polyvinyl pyrrolidine, polyethylene glycol, chitosan, amylase, and Congo red have been used to modify CNTs [109]. Chitosan and its polymer derivatives are biocompatible dispersants to make unfolding and dispersion of CNTs in neutral aqueous media. The chitosan-modified surfaces provide ade-bundling method to immobilize the enzymes like horseradish peroxide and offer platform for bio-nano medical field applications [110]. Similarly, the composite of amylosemodified chitosan derivatives with SWCNTs has been prepared to increase the dispersion stability and biocompatibility of SWCNTs in aqueous solvents. These homogeneous distributions of SWCNTs in amylose grafted cellulose matrix improve functionalized nanotubes’ mechanical and electro-catalytic activity for enzymes like hydrogen peroxide. The homogeneous black aqueous suspension of complex is stable for two weeks without any precipitation [111]. Congo red functionalized MWCNTs have high solubility in water as well as excellent conductivity which help to form homogeneous and stable tangled nanostructures and generate higher surface area resulting in number of binding sites [112]. The Neutral-red dye functionalized MWCNTs deposited on electrode structure improve the sensitivity of electrochemical sensor for acetone [113]. Similarly, water-soluble hyperbranched polyamine

functionalized multi-walled carbon nanotubes efficiently remove organic dyes [114]. CNTs have also been functionalized to offer electrochemically active sensing sites for analyte and show strong binding interactions between electrode and metallic nanoparticles. This scheme is suitable for the conversion of oxidized solid SWCNTs into aqueous SWCNTs dispersions in the presence of 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid and human bovine serum albumin (BSA) (Figure 8). This protocol has been used to produce dispersion of SWCNTs consisting of short length individual fibers and can be employed to remove heavy metal contaminants [115]. Direct electron transfer has synthesized a highly water-soluble CNT-hemoglobin (HGB) composite. The as-synthesized CNTs modified HGB composite play role of a highly sensitive electrochemical sensor for the detection of hydrogen peroxide to a detection limit as low as 0.1 nM [116]. The modified CNTs are platform for sensing of glucose in aqueous solution. Solubility of CNTs can be enhanced by immobilization of enzymes on their surfaces. Recently, [NiFeSe]-hydrogenase decorated CNTs was synthesized via rotational interaction between enzyme and hydrophobic CNTs. The enzyme functionalized CNTs were employed as catalyst in H2-fuel cells (figure 9) to develop a renewable energy source [117]. Phospholipids for example poly-ethyleneglycol (PEG) adsorb onto sidewalls of SWCNTs. The functionalized CNTs are used for photo-thermal and imaging fluorescence; owing to well-dispersed behavior in deionized water. The modification is based on π-stacking between pyrenyl (Py) group and side wall ofSWCNTs [118]. The hydrophobic part of protein causes non-specific bindings with sidewalls of SWCNTs. When protein adsorptive nanotubes make nanotubes-protein composite then nanotubes become more water soluble due to the hydrophilic nature of the surfaces [119]. The water soluble SWCNTs-protein conjugate has also been obtained by sonicating oxidized SWCNTs with other kinds of proteins (i.e. BSA, HRP, or Cytochrome C) in deionized water for

sometimes followed by centrifugation. The obtained suspension is stable over two weeks. These water-soluble conjugates suffer from non-specific bindings as they arrange in stacks by hydrophobic sidewalls of SWCNTs interaction with each other as well as with hydrophobic end of the charged proteins. The increase in size of globular protein structures indicates a massing of numerous proteins together over CNTs surface [120]. CNTs were also modified with chitosanNH2 as drug carrier in drug delivery system (DDS). As modified CNTs were executed as carrier for the delivery of BSA, act as antibiotic, at targeted site [121]. Aptamer functionalized MWCNTs were prepared to sense thrombin potentiomerically. Moreover, nucleic acid-aptamer functionalized MWCNTs were also prepared as electrochemical sensor to sense thrombin [122]. Glyco-conjugated functionalized CNTs provide biocompatible surfaces stable over long period. Glyco-conjugates including glycolipids, glycol-dendrimers, and glycol-polymers have similar extended structures with CNTs through π-π interactions, and degree of solubility in water and DMF depend on the functionalization process [77]. The wrapping of CNTs with DNA has been demonstrated in aqueous media. Nucleic acid moieties like DNA and RNA disperse through their hydrophobic and hydrophilic ends consisting of bases and phosphate/ribose ends, respectively [123]. The alternative approaches to disperse CNTs use thiolated DNA. Wrapping of CNTs with DNA is achieved by sonication through π-π interactions between hydrophobic parts of thiolated DNA. The hydrophilic ends of DNA provide solubility and de-bundling of CNTs in aqueous solution. This modification does not need chemical change to CNTsand is helpful in introducing desired chemical functionalities into DNA strand [124]. Biomolecule modified CNTs were also prepared as renewable energy source. Enzyme based biofluid cells (EBFC) were developed to generate energy from glucose and oxygen present in living organisms. Suherman et al devised EBFC based on chitosan modified MWCNTs (figure 10) for the adsorption and desorption of

laccase. Laccase adsorption and desorption was studied by CV and spectrophotometer. Chitosan increased the sorption capacity upto c.a 92.02% being good protector and stabilizer for laccase on 3D bio-electrode [125].

7.

Other Modification Methods

The chemical modification methods are cost effective and simple to carry out. There are other methods including plasma modification, microwave assisted modification, chemical vapor deposition,and electron beam-induced surface modification [126]. These are developed to fabricate the CNTs with better efficiency, but these methods are expensive. Work is also carried out on methods like nano-engineering of CNTs with arc plasma and the modification of MWCNTs with styrene (St) monomer using plasma polymerization [127]. The compositional and structural investigations show better dispersion of plasma-treated MWCNTs than the untreated ones in non-polar solvent. The chemically functionalized MWCNTs containing carboxylic groups have been further modified with creatinine and aromatic aldehydes through microwave irradiation [128]. Titanium functionalized nanotubes and regular bead-shaped structures of iron-containing CNTs prepared by CVD process provide efficient modification methods for better dispersion, and may be utilized as functional nanomaterials in coming days. CNTs can be made dispersible or soluble via defect free modification method as CNTs first functionalized with PDA and then by PEA (polyetheramine). CNTs/PDA/PEA has synthesized and utilized in development of commercial polymer with high tensile strength [129]. Solubility of CNTs can be enhanced by ultra-sonication. Currently, Price et al modified CNTs by exposing CNTs with 20 kHz ultrasonic waves in the presence of dilute HNO3 and H2SO4. Ultrasonic treatment of CNTs enhances their dispersibility in ethanol or chitosan. Ultrasonically chitosan-

CNTs offered improved tensile strength [130]. 8.

Conclusion

Modification and functionalization of CNTs improve solubility, sensitivity and selectivity of CNTs. The modification process can be a practical strategy to prepare highly dispersed and stable suspension of nanotubes, which paves the way for wide potential applications. The modified nanotubes are highly soluble in polar solvents such as water, ethanol and dimethylformamide, depending on degree of functionalization and nature of the functional groups attached. Numbers of modification methods are being investigated utilizing covalent and noncovalent attachments. Care is taken during modifications to preserve CNTs characteristics however, changes in shape, size and properties can occur. Sometimes the processes are timeconsuming, and to control modification parameters is also cumbersome. Therefore, simple, easy, green, cost-effective and less destructive modification methodologies are still of concern. Conflict of interest Authors describe no conflict of interest in this work. Acknowledgements We are thankful to office of research innovation and commercialization Bahuddin Zakariya University Multan, Pakistan for financial support and cooperation. Marie Skłodowska-Curie Actions COFUND Fellowshipto Dr. Nisar Ahmed is gratefully acknowledged. References [1]. (a) Sanchez-Valencia, J. R.; Dienel, T.; Gröning, O.; Shorubalko, I.; Mueller, A.; Jansen, M.; Amsharov, K.; Ruffieux, P.; Fasel, R., Controlled synthesis of single-chirality carbon nanotubes. Nature 2014, 512 (7512), 61; (b) Feng, C.; Khulbe, K.; Matsuura, T.; Tabe, S.; Ismail, A., Preparation and characterization of electro-spun nanofiber membranes and their possible applications in water treatment. Separation and Purification Technology 2013, 102, 118-135. [2]. Tasis, D.; Tagmatarchis, N.; Bianco, A.; Prato, M., Chemistry of carbon nanotubes. Chemical reviews 2006, 106 (3), 1105-1136. [3]. Loh, K. P.; Ho, D.; Chiu, G. N. C.; Leong, D. T.; Pastorin, G.; Chow, E. K. H., Clinical Applications of Carbon Nanomaterials in Diagnostics and Therapy. Advanced Materials 2018,

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Figure1: (a). Methods for the oxidation of CNTs (b) Schematic illustration of outer wall selective oxidation of DWNTs by H2SO4/HNO3. H2SO4 intercalation opens the diffusion pathways

for

HNO3 to access nanotubes embedded in a rope and react with the exposed outer walls selectively. For clarity, the inner tubes are omitted. Reprinted with the permission from Ref. [25b].Copyright 2010, American Chemical Society. Figure 2. Reaction pathway for the formation of insulin-conjugated sWcNT. Reprinted from reference [35], an open access article distributed under the Creative Commons Attribution License. Figure 3: Schematic presentations of Nitrone 1,3-dipolar cycloaddition onto MWNTs Reprinted with the permission from Ref. [41].Copyright 2010, Royal Society of Chemistry. Figure 4. Schematic representation for the synthesis of CNT-PDA-PDMC nanocomposites via combination of mussel inspired chemistry and chain transfer free radical living polymerization. Step 1: PDMC-NH2 was synthesized via chain transfer free radical polymerization under rather mild reaction conditions using cysteamine hydrochloride as the chain transfer agent and DMC as the monomer. Step 2: the PDMC was further conjugated onto surface of PDA functionalized CNTs (CNT-PDA) in Tris buffer solution for 6 h at room temperature (pH value is about 8.5).. Reprinted from reference [51], with permission from Elsevier. Copyright (2015) Elsevier B.V. Figure 5.Schematic illustration of the formation of hierarchical MWNT@BCP Thorn. Reprinted with the permission from Ref. [68].Copyright 2013, American Chemical Society. Figure 6. Modification of MWCNT by noncovalent interactions using pyrene functional hyperbranched copolymers and quaternization of amine groups on nanotube surface by HCl. Reprinted from reference [70], with permission from Elsevier. Copyright (2018) Elsevier B.V. Figure 7: Covalent functionalization of SWCNTs with PDMA-b-P(NIPAM-co-NAS) by nitrene chemistry. Reprinted with the permission from Ref. [79].Copyright 2012, American Chemical

Society. Figure 8. Preparation of SWNT Dispersions. Reprinted with the permission from Ref. [115].Copyright 2011, American Chemical Society. Figure 9. Schematic representation of the H2/air enzymatic fuel cell. Reprinted with the permission from Ref. [117].Copyright 2018, American Chemical Society. Figure 10. The Bio-cathode Modification Strategy used to Prevent Laccase Desorption. Reprinted from Ref. [125] under a Creative Commons Attribution-Share Alike 4.0 International License.

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Highlights
Enhancement of solubility of carbon nanotubes is reviewed.
Dispersibilty and solubility related challenges are discussed.
Conditions and applicability of each modification method is discussed.
Effects of these modifications on dispersion/solubility of CNTs is also evaluated.

Conflict of interest Authors describe no conflict of interest in this work.