Soft-Nanocomposites of Nanoparticles and Nanocarbons with

Review pubs.acs.org/CR

Soft-Nanocomposites of Nanoparticles and Nanocarbons with Supramolecular and Polymer Gels and Their Applications Santanu Bhattacharya*,†,‡ and Suman K. Samanta‡ †

Department of Organic Chemistry, Indian Institute of Science, Bangalore 560 012, India Director’s Research Unit, Indian Association for the Cultivation of Science, Kolkata 700032, India

‡

ABSTRACT: Gel-nanocomposites are rapidly emerging functional advanced materials having widespread applications in materials and biological sciences. Herein, we review syntheses, properties, and applications of various gel-nanocomposites assembled from different metal-based nanoparticles or nanocarbons [fullerene, carbon nanotubes (CNTs), and graphenes] with tailor-made supramolecular (small molecular) or polymeric physical organogels and hydrogels. Dynamic supramolecular self-assembly of gelators prove to be excellent hosts for the incorporation of these dimensionally different nanomaterials. Thus, gel-nanocomposites doped with preformed/in situ synthesized nanoparticles show magnetic or near-infrared-responsive, catalytic or antibacterial properties. Fullerene-based gelnanocomposites show applications in organic solar cells. Gel-nanocomposites based on CNTs and graphenes and their functionalized (covalent/noncovalent) analogues find interesting properties including electrical conductivity, viscoelasticity, thermal robustness, magnetic, phase-selective, redox and near-infrared radiation sensitive properties. We present appropriate rationale to explain most of these phenomena at the molecular level, which provide useful perspectives for future designs and new spin-offs. Finally, a possible outlook is projected for the design and syntheses of next generation multifunctional gelnanocomposites, which could be achieved by increasing the complexity of the system upon adding selective nanomaterials with desired properties in a multicomponent mixture following a de novo design in order to take advantage of their individual properties.

CONTENTS 1. Introduction 1.1. Supramolecular and Polymer Gels as Hosts 1.2. Different Nanomaterials as Guests 1.3. Host−Guest Interactions: Formation of Soft Gel-Nanocomposites 1.4. Functionalization of Nanomaterials: Covalent vs Noncovalent 1.5. Focus of the Review: Scope and Limitations 2. Gel-Nanocomposites with Metal Nanoparticles 2.1. Gel-Nanocomposites from Supramolecular and Polymer Gels Incorporated with MetalBased Nanoparticles 2.1.1. Gold-Nanoparticle Incorporated GelNanocomposites 2.1.2. Silver-Nanoparticle Incorporated GelNanocomposites 2.1.3. CdS/CdSe Incorporated Gel-Nanocomposites 2.1.4. Incorporation of Other Nanoparticles 2.2. Supramolecular and Polymer Gel Assisted Synthesis of Nanoparticles and Gel-Nanocomposites 2.2.1. Silica Nanoparticles 2.2.2. Gold Nanoparticles 2.2.3. Silver Nanoparticles 2.2.4. Other Nanoparticles

© 2016 American Chemical Society

2.3. Applications of Gel-Nanoparticle Composites 3. Gel-Nanocomposites with Fullerene 3.1. Applications of Fullerene-Based Gel-Nanocomposites 4. Gel-Nanocomposites with Carbon Nanotubes 4.1. Incorporation of CNTs in Supramolecular Organogels and Hydrogels 4.2. Incorporation of CNTs in Polymer Organogels and Hydrogels 4.3. Applications of Carbon Nanotube Based Gel-Nanocomposites 5. Gel-Nanocomposites with Graphenes 5.1. Preparation, Functionalization, and Gelation of Graphenes 5.2. Supramolecular Gelator Based Gel-Nanocomposites with Graphenes 5.3. Polymer-Based Gel-Nanocomposites with Graphenes Thermal and pH-Regulated GO-Polymer Hydrogels Mechanical Properties of GO-Polymer Hydrogels Electrical Properties of GO-Polymer Hydrogels Stimuli Responsive Hydrogels

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Received: April 6, 2016 Published: August 26, 2016 11967

DOI: 10.1021/acs.chemrev.6b00221 Chem. Rev. 2016, 116, 11967−12028

Chemical Reviews Multifunctional GO-Hydrogels 5.4. Gelation of Graphene Oxide Effect of pH Effect of Dimension Effect of Cations Effect of Surfactants Effect of Ionic liquids Effect of Cross-Linkers (Small molecules/ Polymers) Gelation-Assisted Isolation of Graphene from Graphene-GO Mixture 5.5. Applications of Graphenes-Based Gel-Nanocomposites 6. Applications of Gel-Nanocomposites 7. Conclusions and Outlook Author Information Corresponding Author Notes Biographies Acknowledgments References

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Figure 1. Schematic representation showing fate of supersaturated hot solutions under cooling toward formation of self-assembled physical gels. Inset: an example of “sol”-to-“gel” transformation for oligo(pphenylenevinylene)-based gelators (for example ref 49).

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also be tuned. In this way, generation of different nanostructures in turn creates microdomains with solvent pockets, and hence during gelation, immobilization of significant amount of solvent takes place. Such pockets inside the intertwined fibrillar assemblies can also retain nanomaterials such as nanoparticles, nanotubes, and even nanosheets. In the case of the polymeric gelators, physical cross-linking originating from either noncovalent supramolecular interactions among polymer chains or entanglements within dynamic macromolecular species leads to polymer gel formation.69−72 Due to the physical cross-linking nature, polymer-based physical gels are generally reversible (by thermo-, mechano-, or other stimuli) in nature. Polymer-based physical gels in organic solvents emerge from polystyrene,73−75 poly(methyl methacrylate),76,77 poly(p-phenylenevinylene),78 poly(p-phenylene ethynylene),79 etc. In contrast, polymer-based hydrogels are generally derived from poly(N-isopropylacrylamide),80 poly(ethylene glycol),81 poly(vinyl alcohol),82,83 poly(acrylic acid),84 poly(vinylpyrrolidone),85 poly(vinyl imidazole),86 etc. Recent reviews document widespread applications of such polymer gels in the fields of stimuli responsive “smart” polymer hydrogels,87 3D-printing,88 energy storage applications89 and high-performance electrochemical devices,90 electrochemical biosensing,91 3D cell culture,92 tissue engineering,93 and drug delivery.94 Thus, in the case of polymer gels, physical cross-linking creates three-dimensional (3D)-network structures with nanoscopic spaces or domains, which could host different nanomaterials. However, incorporation of nanomaterials could also alter the gelation and associated 3D-fiber network structures due to intermolecular interactions with either polymer or small molecular (supramolecular) gelators in organogel or hydrogel medium. While gelation of small molecules or polymers is possible in both aqueous and organic medium, the resulting hydrogel or organogel has certain advantages over the other in definite fields, and these are discussed in further sections. Reversibility is one such criterion for which physical gels were generally preferred for the incorporation of nanomaterials; as in this case gelation could be regained after the addition of certain nanomaterials and mixing by heating, sonicating, or any other means. Suitably tailored nanoscopic entities have the potential to interact at the molecular level with the gels derived from either small molecules or polymers. Therefore, the gel matrix can act as a “host” to accommodate different nanomaterials as interactive “guest” systems to the formation of gel nanocomposites.

1. INTRODUCTION 1.1. Supramolecular and Polymer Gels as Hosts

Supramolecular association of small molecules often leads to physical gel formation in a chosen solvent medium.1,2 The supramolecular gels comprise flexible, nanofibrillar networks held together by noncovalent interactions among the gelator molecules. Such soft materials are useful in numerous applications in widespread area of material science and energy,3−8 biology and medicine,9−12 as well as chemical ecology.13 Several gelators made of mainly aliphatic backbones are known. These include among others, cholesterol derivatives,14,15 surfactants,16,17 carbohydrate-based molecules,18−20 amides,21,22 ureas,23,24 long-chain hydrocarbons,25 and amino acid derivatives.26−28 However, gelators based on aromatic backbone often provide extra handle (π−π stacking, chromophore) to probe the aggregation-induced photophysical properties.29 These are derived mainly from pyrene,30−34 tetrathiafulvalene,35,36 fluorene,37−39 porphyrin,40−42 perylene, 43,44 conjugated oligothiophenes, 45,46 and oligo(pphenylenevinylene)s.47−55 Gelation takes place through the interplay of various noncovalent interactions among the gelator molecules involving hydrogen bonding, π−π stacking, van der Waals, dipole−dipole, and electrostatic forces.56 Such interactions lead to the formation of various interesting nanoscopic structures such as fibrils, rods, tubes, spheres, coils, and sheets.57,58 Thus, during the thermally induced gelation process, cooling a supersaturated, homogeneous, hot solution of the gelator molecules leads to a metastable state depending on the optimal (above-mentioned) intermolecular forces, and if the resulting mass does not flow upon inverting the container, it ensures the gel formation (Figure 1). However, in the due course, there could be two other possibilities: (i) formation of a highly ordered aggregates leading to crystallization and (ii) a random aggregation resulting precipitation.59−61 Apart from thermally induced gels, several other protocols are known for the gel formation. These include sonication,62 light irradiation,63 heat-set gels,64 incorporation of ions/additives,65−67 chemical reactions among gel precursors,68 etc. Thus, molecular arrangements in the gel phase are unique in nature which could

1.2. Different Nanomaterials as Guests

Various nanomaterials including metal nanoparticles and nanocarbons [e.g., C60 fullerene, carbon nanotubes (CNTs) 11968


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1.3. Host−Guest Interactions: Formation of Soft Gel-Nanocomposites

and graphenes] may be incorporated in small molecular- and polymer-based physical organogels and hydrogels. Among other nanomaterials, the above-mentioned ones are the ones which have been studied extensively due to their interesting intrinsic properties and high potential for futuristic applications.95,96 Nanoparticles are mainly clusters of elementary metal atoms or compounds (e.g., metal oxides and metal sulfides) commonly stabilized with some capping agents. They could be of different size and shape in the nanometer dimension, and depending on their size, their optical properties change significantly.97,98 Different metal-based nanoparticles showed widespread optoelectronic and biological applications. Among carbon-based nanomaterials, fullerene (C60, unless otherwise stated) is a “ball-shaped”, zero-dimensional (0D) aromatic molecule composed of 20 hexagons and 12 pentagons.99 Like most of the functionalized nanoparticles, it is soluble in solvents such as toluene. Fullerene is extensively applied in organic solar cells as electron acceptors. CNTs are next generation 1D nanocarbons that emerge as rolled up graphene sheets in tubular shape. CNTs are of many kinds e.g., depending on the number of layers they could be either single-walled nanotubes (SWNTs) or multiwalled nanotubes (MWNTs), and based on the rolling axis they could be chiral or achiral. The spatial arrangement of the hexagonal sp2 carbon atoms classifies SWNTs as metallic or semiconducting.100 Graphenes represent 2D nanocarbons composed of hexagonal honeycomb lattice like a peeled single layer of graphite. They could also be either single-layer or multilayer. CNTs and graphenes possess an extended π-aromatic surface. They show high electrical conductivity and good mechanical properties. As the physical chemical properties of these nanomaterials stem mainly from their size, shape, and interparticle separations, the properties of such nanomaterials are quite different from those of the bulk materials. These are also distinctly different from those of simple molecules. Figure 2 depicts relative dimension (based on shape) of the nanomaterials that are presented in this review. Depending on

Several nanoparticles and nanocarbons have been incorporated successfully into the physical gel matrices where the host and guest interact specifically via noncovalent forces. Accordingly, the composite gels thus formed often show strikingly superior properties compared to the native gel or the mere physical mixtures of a gelator and an additive. Incorporation of the nanomaterials in gel that interact with host gelators may induce improved physical properties, e.g., increase in viscoelasticity, gelmelting temperature, variation in thermal mesophases behavior, alteration in optical birefringence etc. Thus, in order to reap the exceptional properties of the nanomaterials, rapidly growing fields involving de novo design of gel based nanocomposite materials have emerged. The nature of host−guest interaction in such assemblies depends primarily on the molecular design of gelators. The nature of intermolecular interactions among the gelator molecules and with the incoming guest molecules dictates the extent of mixing. The gelators and the nanomaterials may interact in many different ways that include π−π stacking between aromatic sites, van der Waals through the aliphatic long chains, hydrogen bonding, dipolar, and electrostatic interactions through the polar functional groups. Gel-nanocomposite formation may take place involving either one or more of these interactions. Gold, silver, and other metal nanoparticles when capped with suitable organic ligands produce stable dispersions in common lipophilic solvents. Such surface cappings enable them to exert van der Waals and other noncovalent interactions with the host gel matrix. However, unfunctionalized fullerene, CNT, and graphene possess π-aromatic surfaces. Therefore, π−π stacking interaction with these materials would predominantly facilitate the composite formation. Thus, the choice of gelators certainly assumes a significant role to become a superior host component. Aromatic gelators are composed of π-aromatic moiety (fused/conjugated aromatics including well-known chromophores, benzene-to-larger aromatics) connected with aliphatic, hydrogen bonding, electrostatic, or dipolar interacting groups. Thus, π−π interactions from the aromatic moiety along with other crucial noncovalent intermolecular interactions lead to self-assembled gel formation. Figure 3 depicts schematic representation of an aromatic based small molecular gelator composed of an oligo-p-phenylenevinylene backbone as the aromatic unit. It forms self-assembled fiber networks often through the aromatic units imparting π−π stacking interaction, end-functional groups inducing hydrogen bonding, and the aliphatic long chains responsible for van der Waals interactions leading to the formation of physical gels.48,49 Incorporation of nanomaterials such as pristine CNTs or graphene would allow favorable interactions mainly through π−π stacking between the gelator and the π-surface of CNTs or graphene leading to the formation of gel-nanocomposites. Thus, the nature of interaction of these gelators with the guest molecules could either favor or disfavor stable composite formation. However, mismatch in these interactions does not promote a proper composite formation. For example, an aliphatic-based gelator due to the lack of π-stacking moiety seldom accommodates nanomaterials endowed with aromatic residues due to the lack of significant interactions.30 However, polymer-based gelators have the advantage of wrapping around and encapsulating nanomaterials of even higher dimension. Therefore, covalent functionalization of such nanomaterials with aromatic or

Figure 2. Schematic representation of dimensionally different nanoparticle and nanocarbons used in the present review.

the size and shape of the nanomaterials, their properties differ significantly. Thus, while nanoparticle-based composites were realized predominantly for optoelectronic or catalytic applications, CNT- and graphene-based composites were used for electrical conductivity or mechanical properties. Therefore, while higher dimensional nanomaterials (CNTs and graphene) can act as reinforcing backbone for the gelator network, zerodimensional (0D) nanoparticles and fullerene mainly act as nanofillers in the composite materials. Several of these aspects are described in the following sections. 11969


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Figure 3. Schematic representation of self-assembled gelation and intermolecular interactions with nanocarbons (for example, refs 102 and 103).

framework, which leads to distortion and damage in their electronic and physical properties.104−110 General strategies for the covalent functionalization of CNTs and graphene (described later in detail) include oxidation with acids, amidation/esterification of −COOH groups obtained by oxidation, cycloadditions, addition of free radicals to sp2 carbon atoms, etc. Thus, the resulting functionalized materials are easily dispersible in common solvents. Incorporation of these substances into the gel matrix can form hybrid gel-nanocomposites due to the additional interaction through the added functionality with the gelator molecules. The added functionality promotes even nonaromatic gelators for the gel-nanocomposite synthesis. Therefore, covalent functionalization, even though it introduces defects and thus affects the performance of the materials in certain areas such as electronic applications, it eventually helps to form intimate hybrid materials by a range of different gelators, which shows improved mechanical properties as an example. To increase the dispersibility of CNTs and graphenes and eventual synthesis of gel-nanocomposites, noncovalent functionalization strategies are therefore more desirable. Pristine CNTs and graphenes are dispersed mainly in aromatic-based gelators as they exert π−π stacking interactions among them as described in Figure 3. Polymer chain wrapping or lesser effective van der Waals interactions through alkyl chains can also lead to the dispersion and gel-nanocomposite formation. The extent of synergistic noncovalent interactions between the nanomaterials and the host gelators dictate the degree of functionalization and effective synthesis of gel-nanocomposites. In this context, physical gels are considered to be among the best hosts because of the reversibility and easy modulation of the metastable organization of the gelator molecules upon incorporation of different nanomaterials. Thus, excellent mechanical, thermal, electrical, and various other properties of the nanomaterials could be harnessed through the formation of gel-nanocomposites. Therefore, this strategy is highly restricted to the choice of the gelators; however, the main advantage of noncovalent strategy is that the integrity of the nanomaterials remains intact, and hence, the resulting composites could reap all the functionality that the pristine nanomaterial could offer.

aliphatic moieties could be an effective strategy to overcome this problem toward the formation of gel-nanocomposites efficiently. For example, aliphatic long chain functionalized CNTs or graphenes can interact with aliphatic-based gelators through the van der Waals interactions between aliphatic chains, and thus gel-nanocomposites can be synthesized.101 Also, unfunctionalized CNTs and graphenes tend to coaggregate and phase-separate because of which functionalization (either covalent or noncovalent) of CNTs and graphenes is generally undertaken. 1.4. Functionalization of Nanomaterials: Covalent vs Noncovalent

Pristine CNTs and graphenes have limited solubility in common organic and other solvents. Thus, their applications are often restricted because of their poor solubility in spite of having interesting properties. Therefore, functionalization of nanocarbons is mainly carried out to increase their dispersibility in a particular solvent. There are mainly two strategies for functionalization: (i) covalent and (ii) noncovalent. Fullerene is soluble on its own in solvents such as toluene. Thus, a solution of unfunctionalized fullerene could be incorporated in the gel matrices to form gel-nanocomposites through π−π stacking interaction between fullerene and gelator molecules. However, sometimes phase separation of fullerene occurs from the composite and thus covalent functionalization is undertaken in order to increase the interaction with the host matrix. Covalent functionalization of fullerene is mostly achieved through cycloadditions or nucleophilic addition reactions, and the resulting gel-nanomaterials are shown to have intimate interaction between the gelator and fullerene. Thus, functionalized fullerene, phenyl-C61-butyric acid methyl ester (PCBM), is one of the important components in organic solar cells with different donor−acceptor polymers. Functionalization helps to increase its solubility, reduce phase separation of fullerene molecules from the mixture, as well as optimize the bandgap. Nanoparticles are mainly covalently functionalized, and the effect of their incorporation in nanocomposites is 2-fold: (i) suitable capping agents (ligands and gelators) mainly stabilize them and (ii) structurally matching or shape complementary capping agents attached to the nanoparticle surface communicate with the host gelator due to favorable interactions between them through molecular or shape recognition pathways. In both cases, ligands are added prior to the formation of nanoparticles and the ligands are attached with the nanoparticles mostly through thiol, carboxylate, or amine groups. In the case of CNTs and graphene, covalent functionalization generally introduces “defects” by destroying their skeletal

1.5. Focus of the Review: Scope and Limitations

In the previous sections, selection criteria for suitable gel matrices and nanomaterials are discussed in the context of their interactions to the formation of gel-nanocomposites. We brought up crucial factors to generate such soft-nanocomposites and the possibility of synergy in properties from individual components in the composites. Generally, “soft”nanocomposites are those in which the physical state of the 11970


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level interactions in order to analyze and draw correlation between gelator structure and their functionality as well as to address crucial factors in order to prepare better strategies for the future. Among nanomaterials, we review only metal and metal-based nanoparticles, which have been employed to prepare the gel-nanocomposites. We exclude organic nanoparticles as those are beyond the scope of the present review. We also exclude other nanomaterials such as boron nitride nanotubes, nanohorns, nanospheres, doped nanocarbons, metal nanotubes, etc. In this review, citing specific literature, we analyze various aspects of the spatiotemporal modes of interaction that pervade between the gelators and the nanomaterials, including πstacking, van der Waals, electrostatic, H-bonding, etc. The ability of the host organic compounds (small molecules or polymers) to disperse different nanomaterials in a given solvent is discussed. The effect of external stimuli (such as mechanical stress, thermal fluctuation, photoirradiation, etc.) to the gels and nanocomposites is also presented. Depending on several factors, they often lead to the generation of task-specific, functional composites with interesting new properties, including electrical conductivity, viscoelasticity, thermal robustness, magnetic properties, phase-selective properties, redox behavior, and near-infrared (NIR) sensitive properties. When these composites are derived from biomaterials such as DNA, protein, lipid membranes, etc., they lead to the formation of nanobiocomposites, which show various biological applications including reduced cytotoxicity, increased cell viability, antibacterial activity, drug delivery, as well as cancer therapeutics. We present appropriate rationale to explain most of these phenomena at the molecular level, which provide useful perspectives for future designs and new spin-offs. Finally, a summary of the review and a possible outlook is projected toward the design and syntheses of new generation nanocomposites.

material can be modulated by external stimuli such as thermal or mechanical fluctuations. Reversibility of physical gels and corresponding gel-nanocomposites therefore includes them in the category of soft nanocomposites. Several reviews on nanomaterials and their composites with small molecules or polymers were reported containing nanoparticles as well as different nanocarbons,111 including fullerene,112 CNTs,113−116 and graphenes.117−119 However, recently a review based on gel-nanocomposites has appeared describing in situ synthesis of various nanoparticles such as silica, TiO2, CdS, Au/Ag, and other metal nanoparticles using gel networks as templates.120 Another report describes in situ synthesis as well as incorporation of preformed nanoparticles in the gel network.121 Reviews on nanoparticle-hydrogel composites based on small molecules and polymers have also appeared recently.122−124 Another report on polymer-based gel-nanoparticle composites describe different properties of such composites.123 Nanocarbon-based gel-nanocomposites have also received enormous attention in recent years due to their potential applications in widespread area of materials, energy, and biology. Recently, reviews describing three-dimensional graphene-based macrostructures assembled mainly from graphene oxide via gel formation have appeared.125−128 Therefore, in this review, we discuss the effects of incorporation of various carbon-based nanomaterials and different types of metal-based nanoparticles into various gel networks, a field in which there is a strong surge in current interest. Accordingly, others and we firmly believe that the resulting nanocomposites should be considered as integrated systems, which could lead to novel materials with unprecedented properties. In this review, we describe synthesis, properties, and applications of gel-nanocomposites assembled from various metal-based nanoparticles and nanocarbons such as fullerene, CNTs, and graphene and their functionalized analogues with small molecular or polymeric physical organogels and hydrogels. We put an emphasis on the role of small molecules or polymers and the effect of functionalization of the nanomaterials for the evolution of new composite materials through noncovalent interactions. The basic aspect of the review is shown in a flow chart in Scheme 1. Due to the sheer volume of the topic, we exclude examples of nanocomposites formed by small molecules or polymers with such nanomaterials that are not related to the physical gels. Therefore, in this review, we exclude chemical gels, aerogels, and also nongelling aggregates. Discussion of only physical gels enabled us to discuss various noncovalent forces and molecular

2. GEL-NANOCOMPOSITES WITH METAL NANOPARTICLES Various metal nanoparticles, including gold, silver, platinum, and palladium, as well as metal-based nanoparticles such as CdS, ZnO, TiO2, Fe2O3, etc. were developed in recent decades due to their interesting optical, electronic, magnetic, and catalytic properties129−134 and widespread applications.135−138 Metal nanoparticles are generally prepared by reducing their corresponding salts. During the reduction process, various capping agents (mostly thiol, carboxylate, or amine-containing small molecules) are generally added. These are required for the stabilization of the resulting nanoparticles particularly in solution. Nanoparticles are often characterized from their intrinsic surface plasmon resonance (SPR) bands.139 Depending on a few crucial factors such as concentration of the precursor, equivalent of reducing agent, reduction time, temperature, etc., size of the nanoparticles vary, which in turn modulates their optical properties.140,141 For example, CdSe nanoparticles are well-known for their size-dependent optical properties with the emission color spanning over the entire visible region.142 Particularly, quantum dot based nanoparticles (core−shell) show potential applications in solar cells.143 Gold and silver nanoparticles find potential applications in optoelectronic, 144 surface-enhanced Raman scattering (SERS),145,146 catalysis,147,148 and biological149,150 fields. Platinum- and palladium-based nanoparticles were mainly applied for catalysis.151 Magnetic nanoparticles were mostly

Scheme 1. Showing the Assembly of Soft-Nanocomposites from Various Components of Gelators and Additives

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derived from iron oxides.152 These nanoparticles were incorporated inside gel network to prepare gel-nanocomposites where the individual properties of the nanoparticles remain intact and were executed in the hybrid materials through synergy in properties from both the components. 2.1. Gel-Nanocomposites from Supramolecular and Polymer Gels Incorporated with Metal-Based Nanoparticles

Design and syntheses of several gel-nanocomposites were accomplished in the past few years. Some of them were documented in some recent reviews by focusing mainly on the small molecule based supramolecular gel systems.120,121 Different types of nanoparticles were either incorporated into preformed gel matrices or synthesized in situ inside the gel media in order to modulate the properties of the native gels. Therefore, in this section, we discuss the two main possibilities categorically in terms of different nanoparticles and different gelators based on small molecules or polymers in organogel or hydrogel media. We first discuss about the incorporation of various nanoparticles (mainly gold, silver, CdS/CdSe, and others) into preformed gels, and then we describe in situ syntheses of nanoparticles in the gel media for the preparation of gel-nanoparticle composites. Strategies to incorporate nanoparticles include (i) appropriate capping agent functionalized gold nanoparticles for achieving intermolecular interactions with the host gelator, (ii) structurally matching capping agent to that of the gelators for promoting complementary interactions, or (iii) ligand exchange with incorporated nanoparticles and the gelator. 2.1.1. Gold-Nanoparticle Incorporated Gel-Nanocomposites. Many research groups successfully accomplished syntheses of gel-nanocomposites from gold nanoparticles. Simple incorporation of gold nanoparticles into the gel matrix was reported by us as well as others. Functionalized gold nanoparticles with different capping ligands (derived from nalkanethiols, cholesterol based thiol, p-thiocresol, and phytanyl thiol) have been incorporated into an organogel system formed by N-lauroyl L-alanine (1) in toluene (Figure 4).153,154 Gelator 1 forms a stable, transparent organogel in toluene as a consequence of van der Waals interaction among the aliphatic chains and hydrogen-bonding interactions via the COOH and amide linkages.27,155 Properties of the resulting nanocomposites depend on the information inscribed on the nanoparticle surfaces. Incorporation of a minute quantity of nanoparticles alters the microstructures of the gel aggregates dramatically and imposes directional alignment of the nanoparticles along the fibers. The directionality of the nanoparticles depends upon the extent of interdigitation of the capped ligands on the nanoparticles and the fibers of the gelators through van der Waals interactions. Thus, directional ordering was observed in each case with longer hydrophobic moieties but not with pthiocresol-capped nanoparticles due to the lack of adequate “matching” interactions. The composite gel exhibited increased viscoelasticity and thermal phase transition temperature because of the reinforcement of the individual fibers by the gold nanoparticles. Other amino acid based small molecular gelators have been used subsequently for the incorporation of nanoparticles to produce hybrid materials. Thus, Coates et al. showed that gels derived from 2a and 2b (Chart 1) dispersed gold (and gold−palladium) nanoparticles through direct interaction with the gel fibers leading to the formation of aligned nanoparticles in gels.156 The interactions between the

Figure 4. L-Alanine-based gelator 1 and different capping ligands on the gold nanoparticles. Negative-stained TEM images showing the long-range assembly of gold nanoparticles on gel fibers for (a) AuPhy and a random arrangement for (b) AuPhMe. Reproduced with permission from ref 154. Copyright 2009 Wiley-VCH.

nanoparticles and the biologically derived cysteine building blocks (2a) are further mediated by the S−Au interactions with the nanoparticle surface coated n-octadecyl thiols. This afforded highly dynamic nanohybrid materials which evolve in response to an external stimulus (e.g., heating) as a consequence of the noncovalent interactions operational among them. These reports took advantage of the obvious affinity of gold nanoparticles with thiol-containing groups leading to easy functionalization of gold nanoparticles. Thus, Sangeetha et al. incorporated alkane-thiol-capped gold nanoparticles in a selfstanding organogel derived from a 2,3-didecyloxyanthracene (3a) gelator.157 Homogeneous dispersion of the nanoparticles inside the gel fibers took place due to the interaction of alkanethiol capping agents with alkyl groups of the gelator. Thus, these gel-nanocomposites showed improved mechanical and thermal stability. Intimate interactions between the gel fibers and the nanoparticles could be increased significantly by appropriate modification of the surface of the nanoparticles. Kimura et al. reported incorporation of octanethiol-stabilized gold nanoparticles in a self-standing organogel of 4.158 The gelnanocomposite was formed in due course by the replacement of the octanethiol groups with the thiol groups of the gelator via site-exchange reactions. Thus, in this way, a gold nanoparticle organized fiber network based gel-nanocomposite was synthesized. This type of nanoparticle organization was also shown in two-component gels. Yamamoto et al. prepared fluorescent gold nanoparticle composites from two-component organogels derived from a mixture (1:1) of pseudoenantiomeric ethynylhelicene oligomers (M)-4SS4 (5a) and (P)-3SS3 (5b), both of which contain disulfide linkage.159 Formation of the 11972


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Chart 1. Molecular Structures of Supramolecular Gelators for the Incorporation of Gold-Nanoparticles

Chart 2. Molecular Structures of Supramolecular Gelators and Structurally Alike Capping Ligands on Gold-Nanoparticles for Intimate Interactions through Molecular Recognition

benzylcysteine (6a)- and homophenylalanine (6b)-based organogelators for the synthesis of gel-nanocomposites with gold nanoparticles.160 The gelators 6a−6b self-assembled in a three-dimensional loofah-like nanoscale network to form selfstanding organogels. Incorporation of gold nanoparticles, however, led to the formation of a composite gel only with 6a having the sulfur containing cysteine moiety due to the S− Au interactions. Thus, the composite gel showed 100-times greater mechanical stability and 43 °C higher gel melting temperature than the native gel.

nanocomposite was promoted by the high reactivity of the disulfide moiety and thus the hybrid yielded a novel emission at 600−800 nm from the gold nanoparticles while the gelator emission appears at 450 nm. Notably, the emission at 600−800 nm was observed in the self-assembled hybrid gel or in the solid state but not in the sol state. The authors suggest that the appearance of this emission was due to the energy transfer from the (M)-4SS4/(P)-3SS3 to the self-assembled directionally arranged gold nanoparticles in the nanocomposite. He et al. used polyhedral oligomeric silsesquioxane core-appended 11973


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Chart 3. General Backbone Structure of the Polymers Mostly Used for Gel-Nanocomposite Synthesis

oligo-(p-phenylenevinylene)-based organogelator 8a.53 Due to having similarity in structural motifs, the gelator and the capping agent interacted with each other intimately and led to the formation of a hybrid gel with nanotape morphology. Intercalation of gold nanoparticle containing 8b into the gelfibers of 8a showed pairs of parallel rows of metal particles aligned to both sides of the tapelike fibers. Thus, the hybrid system communicates electronically where the excitation energy was transferred all the way to the gold core through ́ the π-conjugated tapes. In a similar report, Puigmarti-Luis et al. doped amide-containing tetrathiafulvalene-capped gold nanoparticle (9b) into an organogel of structurally alike tetrathiofulvalene-based gelator (9a).35 Thus, incorporation of minute quantity (1%) of the nanoparticles exerts favorable interaction with the gelator molecule leading to the formation of supramolecular fibers which showed metallic (ohmic) conductivity. However, this type of favorable interaction was not observed with the structurally unlike nanoparticle (9c). Steroid-based systems were also employed for this type of complementary interaction. Maitra and co-workers synthesized a series of facially amphiphilic thiols (10a−10c) as capping agents to stabilize gold nanoparticles.162 A steroid-derived super hydrogelator (10) was obtained to act as an excellent medium for stabilizing the structurally “analogous” nanoparticles, giving rise to nanocomposite materials. The supramolecular structures created inside the gel could stabilize the

Directional arrangements of the nanoparticles in the gel media originate due to intimate molecular interactions and surface complementary recognitions. Thus, Tsunashima et al. achieved a one-dimensional array of nanoparticles by the Langmuir−Blodgett method in assistance with the fibrillar network formed by certain self-assembling molecules.161 The Langmuir−Blodgett film of 7a (Chart 2) formed highly crystalline nanofibrillar structures in addition to a uniaxial molecular assembly and uniform domain on the mica surface. The fibrous structure of 7a possessed hydrophilic surface due to the presence of the hydroxyl groups. Thus, doping with molecules containing ammonium group such as 7b into 7a could obtain a fibrous structure with a positively charged surface. Compounds 7a and 7b exhibited similar gelation ability in organic solvents owing to a similar type of self-assembly patterns, while hydrogelation was observed only in the case of 7b. As the gelator, 7b was capable of forming a uniform Langmuir−Blodgett film, and incorporation of the gold nanoparticles in the mixture of 7a and 7b (80:20) led to the production of a one-dimensional array of the nanoparticles. Therefore, the fibrous structure acted as a template for the gold nanoparticles for their directional arrangements. To acquire these types of directional arrangements, molecular recognition based complementary interactions were also developed. Thus, van Herrikhuyzen et al. incorporated π-conjugated oligo-(pphenylenevinylene) (8b)-capped gold nanoparticles in another 11974


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steroid-capped gold nanoparticles due to the association of the steroid units present both on the nanoparticle and on the gelator. Thus, a regular directional arrangement of the nanoparticles along the axis of the fiber is because of the physical arrest of the nanoparticles during the fiber formation. Apart from small molecular gelators, polymers were also used to prepare gel-nanoparticle composites. Chart 3 listed the general backbone chemical structure of the polymers that were mostly used for the synthesis of gel-nanocomposites. Tanaka et al. assembled three-dimensional nanostructured protein-based hydrogel composed of “loosely” interconnected protein− polymer hybrid nanoparticles using different proteins (streptavidin, protein A, and Mouse IgG) and a polymer poly[(Nacryloylmorpholine)-co-(N-acryloxysuccinimide)].163 The protein hydrogel has been accomplished by spotting a protein/ polymer mixture on a substrate. The nanocomposites were prepared by the addition of gold nanoparticles containing protein molecules, which could easily diffuse into the hydrogel matrix through pores and spaces. Therefore, the protein chip obtained in this way showed improved sensitivity in detecting protein−protein interactions compared with that by direct protein immobilization methods. Gold nanoparticles were doped in polymer gels to modify the redox properties in the resulting nanocomposite. Sheeney-Haj-Ichia et al. used poly(Nisopropylacrylamide) (PNIPAM) hydrogels for the incorporation of gold nanoparticles, and the resulting gel-nanocomposites were probed for electronic properties.164 Thermal phase transitions of the polymer reversibly controlled the electronic properties of the polymer film associated with an electrode. Gold nanoparticles were incorporated into the polymer by a thermal “breathing-in” process involving cyclic swelling/shrinking thermal transitions of the polymer. The resulting gel-nanocomposite showed lower resistance of the shrunken polymer interface. A decreased electron transfer resistance for the redox transformations at the nanocomposite functionalized electrode was also observed. Gold nanoparticle incorporated gel-nanocomposites were used for drug/dye release properties. Mitamura et al. reported composite hydrogels from gold nanorods incorporated into alginate gel in the presence of calcium ions.165 The gel-nanocomposite showed release of gold nanorods from the gel phase in saline as well as consequent collapse of the gel. This type of characteristics of alginate gel in saline medium could be useful for applications in living body as drug delivery system by controlling the decomposition of the hybrid gel. Salgueiro et al. reported incorporation of spherical and rod-shaped gold nanoparticles within thermosensitive κ-carrageenan hydrogels.166 Release of a model drug (methylene blue) from the gel-nanocomposite was shown to depend upon the type of gold nanoparticle used. 2.1.2. Silver-Nanoparticle Incorporated Gel-Nanocomposites. In a similar strategy to that of gold nanoparticle based composites, silver nanoparticles were also incorporated into preformed gels. Effect of incorporation of silver nanoparticles toward the alteration of morphology of the resulting composites is particularly important. Khan et al. reported biscarbamate derivative 11 (Chart 4) which formed selfsupporting thermoreversible organogel in benzonitrile medium exhibiting thin sheetlike structures that wrap into hollow fibers.167 Incorporation of silver nanoparticles filled up the hollow fibers. These fibers contain uniform distribution of the nanoparticles without showing any aggregation on the outer surfaces of the fibers. Thus, a partial melting of one such

Chart 4. Molecular Structures of Supramolecular Gelators for the Incorporation of Silver-Nanoparticles

nanofibers showed lack of nanoparticle aggregation on the surface while a coaxial array of the nanoparticles was noted. Small molecular hydrogels were also involved in the preparation of gel-nanocomposites. Nanda et al. used Nterminally Boc (tert-butyloxycarbonyl) protected synthetic tripeptide (Boc-Phe-Phe-Ala−OH)-based hydrogel (12) for the incorporation of silver nanoparticles that were stabilized with cysteine or cysteine-containing dipeptides.168 Fabrication of silver nanoparticles along the hydrogel nanofibers were observed in the resulting gel-nanocomposite due to the interaction among the amino acid moieties present in the nanoparticles and the gelator. Mechanical stability of the gelnanocomposite was shown to depend on the factors such as the nature of the stabilizing ligands, size, and amount of the silver nanoparticles used for the preparation of gel-nanocomposites. Miljanić et al. reported the incorporation of silver nanoparticles inside gel-networks derived from amino acid based gelator (13) to induce surface-enhanced Raman scattering (SERS) phenomenon.169,170 Enhanced Raman signals of the gelator molecules associated with the silver nanoparticles allowed study of the molecular self-assembly in such gel-nanocomposites. Raman intensity of the amide and carboxylic acid vibrational bands of the gelator in the composites indicated that the gelator molecules are associated closely to the silver nanoparticles through the benzene units. Organization of the gel structure has been supported by the intermolecular hydrogen bonding between the oxalyl amide and the −COOH groups in the gelator. Such SERS phenomenon was also reported by Saha et al. for polysaccharide alginate gel supported monometallic and bimetallic Au and Ag nanoclusters.171 The effectiveness of the gel-nanocomposites for SERS detection was probed using 2aminothiophenol and 1,10-phenanthroline, and in this case, Aucomposites showed better response than corresponding Agcomposites. 2.1.3. CdS/CdSe Incorporated Gel-Nanocomposites. Cd-based nanoparticles and quantum dots are well-known for their interesting optoelectronic properties. In an early report, John and co-workers reported incorporation of CdS nanoparticles into an organogel formed by sodium bis (2-ethylhexyl) sulfosuccinate (14) in the presence of p-chlorophenol in isooctane medium (Figure 5).172,173 The self-standing organogel was formed from a reverse micellar solution of 14 with the addition of phenol as a dopant. Nanoparticles can be synthesized in water-in-oil microemulsions, where the water droplets act as microreactors to control their dimensions. Therefore, in the water-in-oil microemulsion-containing nanoparticles when converted to an organogel, the particles took an 11975


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Wadhavane et al. reported incorporation of different types of CdSe nanoparticles such as CdSe core and CdSe/ZnS core− shell capped with trioctylphosphine oxide and n-octadecylamine ligands in a preformed organogel obtained from a pseudo peptidic macrocyclic gelator (16).176 Synergistic effect between the CdSe quantum dots and the gelator was observed, which led to a decrease in the minimum gelator concentration (MGC) for the formation of stable and thermoreversible hybrid gels. Notably, the intrinsic optical properties of the CdSe nanoparticles remained intact in the hybrid gel. However, interestingly the luminescence intensity and lifetime of the nanoparticles showed significant increment in the hybrid gels. Yan et al. incorporated multicolored CdSeS nanocrystal-based quantum dots (green, yellow, orange, and red) inside a diphenylalanine dipeptide gel (L-Phe-L-Phe, 17).177 Being one of the smallest dipeptide gelators, diphenylalanine forms gel in organic solvents through hydrogen bond of peptide main chains and the π−π interactions between the aromatic residues of the peptide. The gel could encapsulate lipophilic quantum dots by improving their stability and protecting them from oxidation. Photoluminescent properties of the incorporated quantum dots remained intact in the composite gel. 2.1.4. Incorporation of Other Nanoparticles. Various other nanoparticles were incorporated inside the preformed gel matrices. Das et al. reported incorporation of capped zinc oxide nanoparticles inside the photochromic organogel formed by 2,3-didecyloxyanthracene molecules (3a, Chart 1).178 The ZnO capping agent 3b was chosen in such a way to achieve an optimal gelator-nanoparticle interaction. 2,3-Didecyloxyanthracene-based capping ligand 3b showed improved dispersibility of the nanoparticles into the organogel matrix due to the complementary interaction between the ZnO nanoparticles and the structurally similar 2,3-didecyloxyanthracene molecules. Thus, reinforcement of the fibrillar network of the host gel could be achieved with the incorporation of a minute amount of ZnO nanoparticles. In addition to these, the photoactive gelator 3a also allowed construction of a light-harvesting matrix leading to photoinduced processes at extremely low loading of ZnO-nanoparticles. Kotal et al. reported nanoparticle induced gel formation. Ultrasound-induced in situ formation of coordination organogels have been described with various isobutyric acids (18, Chart 6) (including 2-methylisobutyric

Figure 5. Gelling molecules and the schematic showing nanoparticles incorporated gel formation mechanism. Reproduced from ref 172. Copyright 2002 American Chemical Society.

intrinsic part of the gel, as opposed to a seeded system where the particles may be randomly located throughout the system. The gel thus acquired properties conferred by the nanoparticles. Thus, incorporation of CdS quantum dots brought in luminescent properties to the gels. Furthermore, incorporation of paramagnetic ferrites in the gel phase induced magnetic properties in the resulting gel-nanocomposites. Palui et al. reported incorporation of luminescent CdS nanoparticles into pH-responsive self-assembling hydrogels derived from short synthetic peptides 15a−15c (Chart 5).174 Chart 5. Molecular Structures of Supramolecular Gelators for the Incorporation of Cd-Based Nanoparticles

Chart 6. Molecular Structures of Supramolecular Gelators for the Incorporation of ZnO and Lanthanide-Based Nanoparticles

Interaction of CdS nanoparticles with the gelator molecules in the gel matrix led to the formation of definite array on the nanofibrillar structures. Thus, the CdS nanoparticles showed tuning of the optoelectronic properties in the nanocomposite without changing the size of the nanoparticles. Korala et al. reported fabrication of CdSe (ZnS) semiconductor quantum dots in micron thickness xerogel films with high transparency and luminescence by using sol−gel methods.175 The xerogel films were prepared by dipping glass substrates horizontally into a sol of 11-mercaptoundecanoic acid-capped CdSe (ZnS) followed by gelling and drying under ambient conditions. The xerogel films thus obtained showed conductivity on the order of 10−3 S cm−1 possibly due to the formation of an electrically connected network of CdSe (ZnS) within the xerogel film. 11976


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acid and 2-bromoisobutyric acid) and ZnO nanoparticles.179 Ultrasound irradiation triggers quick dissolution of the ZnO nanoparticles by isobutyric acids. This induced an in situ formation of the zinc isobutyrate complexes, which undergoes fast sono-crystallization into gel fibers. Formation of welldefined networks of fibers of long aspect ratio due to lamellar organization was observed, and the average diameter of the fiber ranged from 30−65 nm, depending on the nature of the isobutyric acids used. Wu et al. reported incorporation of upconversion rare earth nanophosphors (NaYF4) containing lanthanide dopants (Yb, Er, and Tm) in the peptide-based gelator 19a−19b.180 These dispersed nanoparticles acted as physical cross-links between the supramolecular fibers and thus improved stability of the gel by reinforcing the gel structure. Thus, these hybrid gels appeared as multicolor gels and the nanoparticles retained their nanophosphor properties in the gel state. Electrically conductive or magnetic nanoparticles in this context are important in order to prepare gel-nanocomposites. Taboada et al. reported nanocomposites composed of the conductive and magnetic oleate-stabilized iron oxide nanoparticles and a tetrathiofulvalene (TTF)-based organogelator (9a, Chart 2).181 The n-octadecyl chains present on the nanoparticle surface increased its solubility in n-hexane allowing progressively increasing proportions of the nanoparticles loading into the gel matrix (Figure 6). However, a uniform

Polymer gels were used as a host material to incorporate metal nanoparticles. Bhargavi et al. incorporated magnetic FePt

Figure 7. TEM images of polyacrylamide (PAM) nanowires and goldnanoparticle-containing PAM nanowires dispersed from an aqueous solution. Bottom part showing high-magnification SEM image of a cross-section of an anodic aluminum oxide (AAO) film containing Aunanoparticle/PAM after calcination treatments. Reproduced from ref 185. Copyright 2003, American Chemical Society.

nanoparticles into a polymer-based nematic liquid crystal system to achieve a hybrid material executing both magnetic and liquid-crystalline properties.182 Gelation is achieved by the incorporation of 12-hydroxystearic acid which is a well-known gelator of organic solvents.183 As the gels undergo thermal sol− gel transitions, a switch in the properties of the hybrid gels is obtained. These include the composites that exhibit anisotropic sol-to-gel transformations instead of the isotropic sol state, and the two anisotropic states have a nematic liquid crystal behavior. The orientational correlations of the nematic liquid crystals are slightly strengthened by the presence of the nanoparticles and the gelator, and as a result, the mechanical rigidity of the gels increases by 2 orders of magnitude. While the FePt nanoparticles exhibit superparamagnetism, the composites show diamagnetic behavior. Kimizuka et al. described 2D organization of water-soluble nanoparticles by a hydrogel surface as a substrate, which provided the gel-assisted transfer of nanoparticles onto the solid substrates.184 Colloidal dispersions of silica nanoparticles (diameter 100 nm) were spread onto an agarose hydrogel surface. The density of the nanoparticles on the gel surface could be controlled by changing the concentration of the spreading solution. These spread particles could be transferred onto various solid surfaces due to the inherent nature of the agarose nanostructures which act as “molecular glues”. The stability of the nanoparticles, transferred in this way, was enhanced by the electrostatic attraction in the case of polyethylenimine-coated mica. This technique could find applications for the use of gel surfaces to achieve molecular organization of nonamphiphilic, hydrophilic materials, and their immobilization on the solid substrates. In another report, Guo et al. reported preparation and characterization of highly dispersed metal nanoparticles in porous anodic aluminum oxide films using a hydrogel-nanowire-assisted technique.185 First, cross-linked polyacrylamide hydrogel nanowires were organized within the pores of aluminum oxide template by electropolymerization of acrylamide. Then, metal nanoparticles (Au or Pt) were incorporated into these nanowires using a “breathing” mechanism where the shrunken polymer nanowires were allowed to swell in an aqueous solution containing metal nanoparticles followed by reshrinking in acetone. The loading amount and distribution of nano-

Figure 6. (a) Formation of the gel-nanocomposites with increasing percentage of the Fe2O3 nanoparticles in hexane. TEM images showing (b) the native gel network and (c) 1% nanoparticle loaded aligned network. (d) I−V conductivity measurements of the nanocomposites. Reproduced with permission from ref 181. Copyright 2011 Royal Society of Chemistry.

distribution of the iron oxide nanoparticles through the fibrous organic materials takes place when the percentage of the inorganic component is below 10% in weight, whereas above this loading %, domains of the pure nanoparticles are observed. Electrical conductivity of these composites decreased with an increasing amount of nanoparticle loading due to the interruption of conducting pathways by the inorganic domains. However, the fibers formed by the gelator are directionally aligned at low loadings of the nanoparticles, indicating a structural role played by the nanoparticles. These observations could be useful for the fabrication of electronic devices and related applications. 11977


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hydrogel which led to the successful synthesis of gelnanocomposites.188 In situ synthesis of gold nanoparticles and corresponding gel-nanocomposites were successfully synthesized from dimeric bile-acid derivatives.189 Examples describing successful syntheses of nanoparticles in the gel medium leading to gel-nanocomposites are discussed below. 2.2.1. Silica Nanoparticles. Shinkai and co-workers documented the first report on the successful use of organogels as templates for the synthesis of porous silica with nanotubular structures.191 A cholesterol-based organogelator 20 (Chart 7) and tetraethoxysilane were used as the precursor. The polymerization of tetraethoxysilane templated by the gelator in the gel medium provided the route toward the synthesis of the silica nanotubes. Recently, Wei et al. reported the description of a hybrid hydrogel based on an anionic surfactant, sodium laurate with silica nanoparticles with the aid of potassium chloride.192 It was mentioned that the hydrogel was formed due to the coassembly of silica nanoparticles and sodium laurate building blocks through hydrogen bond and hydrophobic effect leading to intertwined 1D fibers. 2.2.2. Gold Nanoparticles. Synthesis of gold nanoparticles in the gel medium could be achieved either by adding external reducing agent or reduction by the gelator itself. However, in both the instances often the gelator acts as the stabilizing agent for the in situ generated nanoparticles. Vemula et al. reported urea-based aryl derivatives (21) that formed hydro/organogel which were used for the in situ preparation of gold nanoparticles.193 In this case, the gelator molecules acted as both reducing and capping agents for the as-synthesized gold nanoparticles. The native gel retained its gelation properties even after the nanoparticle synthesis, and thus, it led to the gelnanocomposite formation. In another report from the same authors, ascorbic acid has been used as an in situ reducing agent to produce gold nanoparticles inside a gel network.18 In situ synthesis and stabilization of gold nanoparticles have been demonstrated in various self-assembled systems and liquid crystals using amphiphilic ascorbic acid derivatives (22). The hybrid materials generated in this way were stable for several months. Lu et al. reported in situ synthesis of gold nanoparticles in the organogel medium of bis-triterpenoid derivative 23.194 The organogel medium acted as a template which could sustain and stabilize the as-prepared gold nanostructures. Several amino acid derived gelators were used for the in situ synthesis of gold nanoparticles. Gelator 24 has been synthesized in order to control the synthesis and modification of the architectures of nanoscale assemblies constructed from gold.195 HAuCl4 was phase transferred into the toluene gel of 24, which slowly diffused to produce an even distribution of nanoparticles. This was followed by UV-irradiation, which produced localization of nanoparticles inside the gel network, which also acted as a stabilizer. Therefore, this method provides a route to the syntheses of hybrid organic−inorganic materials consisting of both organic fibers and inorganic nanoparticles because of simple irradiative methodology. Furthermore, modification of the peripheral groups on the gelator molecule allows tuning of specific interactions between the organic and inorganic parts in the hybrid materials, in order to yield systems with an additional degree of nanostructuring. Delbecq et al. reported gel-mediated syntheses of gold nanoparticles in the hydrogel/organogels derived from N-stearoyl amino acids (25a−25c) using different methods of reduction of HAuCl4 dispersed in the gel matrix such as LiEt3BH and photo-

particles in the nanowires were controlled by varying the number of breathing cycles. After calcinations, the nanoparticle/hydrogel composites were transformed into highly dispersed metal nanoparticles supported on aluminum oxide (Figure 7). Therefore, this strategy could be applicable in loading and dispersing various nanoparticles into the channels of porous materials as well as into the polymer nanowires. 2.2. Supramolecular and Polymer Gel Assisted Synthesis of Nanoparticles and Gel-Nanocomposites

Gels were also used as media for the syntheses of inorganic and metal nanoparticles.186 As mentioned before, nanoparticles are prepared by reducing their corresponding salts in the gel media. Therefore, a general method of preparation of such nanocomposites starts by preparing first the small molecule or polymer-based gels. Then incorporation of the desired nanoparticle precursor (such as Ag+ or Au+ ions in case of Ag or Au nanoparticles respectively) followed by the reduction in the gel-phase produced the well-dispersed homogeneous nanoparticles adhering the gel fibrillar networks. An aqueous medium is thus better suited for the preparation of nanoparticles due to the ready solubility of the nanoparticle precursors. In order to stabilize such in situ generated nanoparticles, various capping agents (mostly thiol, carboxylate, or amine-containing small molecules) are either added externally or, in many cases, the gelator itself acts as the stabilizing agent. In this way, nanoparticles could be formed in different size and shape as depicted in Figure 8 depending on

Figure 8. Hydrogel networks serves as templates to synthesize different sizes of nanostructures of silver nanoparticles. Reproduced with permission from ref 190. Copyright 2010, Elsevier.

few crucial factors such as concentration of the precursor, equivalent of reducing agent, reduction time, temperature, etc. Various systems were tried for the synthesis of such nanoparticle in the gel medium. For example, riboflavin− melamine equimolar supramolecular assembly was used for in situ silver nanoparticle formation which induced tuning of morphology and photoluminescence property of the native gel.187 However, in this case, the gelation was abolished after the formation of the nanoparticles due to the breakdown of the crucial intermolecular forces responsible for the gel formation. Also, during the metal sulfide nanoparticle synthesis from metal cholate hydrogels (with metal ions- Cu2+, Co2+, Zn2+, Cd2+, Hg2+, and Ag+), the gelation was abolished in each case after the addition of Na2S.188 However, several other instances are known where successful synthesis of gel-nanoparticle composites has been demonstrated. For example, syntheses of metal sulfide nanoparticles (CuS, CoS, ZnS, CdS, and HgS) as well as gold and silver nanoparticles were achieved in calcium cholate 11978


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Chart 7. Molecular Structures of Supramolecular Gelators for the Synthesis of Silica and Gold-Nanoparticles

irradiation process.197 The side-chain amino groups participate in stabilizing the in situ formed gold nanoparticles in the gelnetwork. Thus, a uniform dispersion of gold nanoparticles was achieved either from the toluene gels of 25a−25c after reduction with LiEt3BH or from the hydrogel of 25c after photoirradiation. The size and distribution of the gold nanoparticles inside the gel-network depends upon the gel elasticity and the phase transition temperature of the organogels. The photoirradiation process does not involve any additional reducing agent and thus avoids any contamination of the gels. Das and co-workers reported in situ synthesis of gold nanoparticles of varying shapes by the reduction of HAuCl4 within tryptophan-based (26a−26d) small molecular hydrogel matrix at room temperature in the absence of any foreign ion or external reducing/capping agent (Figure 9).196 It has been proposed by the authors that the chloroaurate ions are entrapped in the gel fibers followed by in situ reduction by the tryptophan moiety of the host peptide amphiphiles, which led to the generation of gold nanoparticles stabilized by the gelators. Depending on the structural variation of the gelators, they led to different 3D-supramolecular fibrillar morphology, which in turn modulated the shapes of the as-synthesized gold nanoparticles. Therefore, such tryptophan-based peptide amphiphiles hydrogelators acted as structure-directing templates for the in situ shape-controlled preparation and stabilization of gold nanoparticles. In a similar report by the same authors, gold nanoparticles of different shapes were synthesized in the L-phenylalanine-based amphiphilic gelators (e.g., compound 27a−b showed octahedron shaped gold nanoparticles).198 However, the shapes of the nanoparticles depend on the ratio of the precursor HAuCl4 and the gelator. In another report, these authors also reported the synthesis of gold nanoparticle-gel nanocomposites from amino acid based amphiphilic gelators 28 (carboxylate salts, Figure 10).199 In this case, the gelator itself acted as a reducing agent for in situ synthesis of gold nanoparticles in hydrogel networks at room temperature. Upon decreasing the pH, the carboxylate salts of the gelator were converted to the −COOH and these water insoluble carboxylic acids spontaneously transferred from the aqueous phase to the nonpolar organic media (toluene) along

with the synthesized gold nanoparticles to form the gold nanoparticle-organogel composite. The supramolecular gel network stabilized the gold nanoparticles and increased the viscoelasticity of the gels. Liu et al. used gelators based on assemblies of alkylamine (such as n-hexadecylamine) and polyol (for example ethylene glycol or glycerol) as microreactors for gold nanoparticles synthesis.200 Upon formation of gold nanoparticles in the gel assembly, the fibrillar morphology was found to be either broken (in case of ethylene glycol) or underwent a phase transition (in case of glycerol) due to the disruption of the molecular packing of the native gels in the presence of gold nanoparticles. Gold nanoparticles were also synthesized inside the polymer-based hydrogels. Thus, Wang et al. reported thermoresponsive PNIPAM-containing thiol groups as the hydrogel medium for the preparation of gold nanoparticles with controlled structure.80 These Au-PNIPAM nanocomposites showed significant changes in the swelling degree and thermal phase transition because of the interaction between the gold nanoparticles and PNIPAM side-chains. 2.2.3. Silver Nanoparticles. Several amino acid based gelators were used for in situ synthesis of silver nanoparticles. Mantion et al. reported L-valine-based oligopeptides 29 and 30 (Chart 8) for the synthesis of Ag nanoparticles in the gel medium.201 These two peptide gelators and their mixtures form stable and efficient organogels in n-butanol, which gave highly ordered peptide fibers with a predominant β-sheet structure. The fibers were mineralized with Ag nanoparticles using DMF as a reducing agent. In the mixture, the fraction of the sulfurcontaining peptide (30) controls the shape and size of the resulting nanoparticles. At higher concentration of 30, small spherical nanoparticles were formed, and at lower concentrations, bigger particles were formed. The interactions between the peptide and the Ag nanoparticles occur through the complexation of the silver ions to the sulfur atom of the thioether moiety, which plays a crucial role in controlling the particle formation. In another report, Banerjee and co-workers used a short N-fluorenyl-9-methoxycarbonyl (Fmoc)-protected dipeptide (Fmoc-Val-Asp-OH, 31) to synthesize and stabilize silver nanoparticles in hydrogel medium.37 In this case, the nanoparticles were formed without any external reducing agent. 11979


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Figure 10. Molecular structure and photograph of (A) in situ synthesized gold nanoparticle-hydrogel of 28, (B) transfer of gold nanoparticle from hydrogel to toluene layer, and (C) formation of gold nanoparticle-organogel nanocomposite after phase transfer. Reproduced with permission from ref 199. Copyright 2010 Royal Society of Chemistry.

hydrogelator (33) under mild conditions without using high temperature, alkaline medium, or external reducing agent.202 Li et al. prepared chiral Ag nanoparticles and chiral silver nanoparticulate films using a silver(I) ion-coordinated organogel as template by in situ reduction.203 First, the Ag-coordinated compound (34) was taken in ethanol to make a self-supporting organogel. This was followed by reduction with hydroquinone, which produced the desired nanocomposite. The absorption band of the Ag nanoparticles at 408 nm emerged as bisignated signals in the nanocomposite which ensured the chiral nature of the nanoparticles. Syntheses of gel−Ag−nanoparticle composite materials were accomplished by us from a two-component hydrogel comprising stearic acid and 3,3′-iminobis(propyl-amine) (35a−35b).204,205 The nanocomposite was obtained by reducing AgNO3 with NaBH4 in the melted hydrosol of the two-component system, which upon cooling afforded the nanocomposites in which the Ag colloids were stabilized in situ by the gel fibers. These as-prepared gel-nanocomposites were found to be thermoreversible in nature, and on repeated heating and cooling the gelation behavior remained intact. The Ag nanoparticles were found to be directionally aligned along the gel fibers in an extended assembly. Zhang et al. reported narrowly dispersed Ag nanoparticles in the hydrogel matrix from cholic acid conjugated polyethylene glycol based gelator (36).206 This polymeric gelator was used to form a hydrogel, and it served the dual purpose both as the stabilizer preventing the silver nanoparticles from further growth and aggregation and as a reducing agent in natural light at ambient temperature. Thus, well-dispersed Ag nanoparticles were obtained in the hydrogel system through a single step procedure. Few gelators were reported in which multiple nanoparticles could be synthesized. For example, Banerjee and co-workers demonstrated in situ preparation of Ag and Au nanoparticles inside the gel networks using tyrosine-containing oligopeptidebased organogelators (37a−37c).207 The tyrosine residue(s) of the gelators acted as the reducing agent for Au3+/Ag+ into colloidal Au0/Ag0 nanoparticles, respectively. In this way, the “nascent” metal nanoparticles were trapped and stabilized

Figure 9. Molecular structures of supramolecular gelators for the synthesis of gold-nanoparticles of varying shapes and TEM images of directly synthesized gold-nanoparticles (a, b in gel 26a; d, e in gel 26b; g, h in gel 26c; and j, k in gel 26d) at MGC and the corresponding SAED pattern (c in gel 26a; f in gel 26b; i in gel 26c; and l in gel 26d). Reproduced from ref 196. Copyright 2008 American Chemical Society.

The silver ion encapsulated hydrogel formed fluorescent silver nanoclusters efficiently and spontaneously under sunlight at physiological pH (7.46). In presence of sunlight, the silver ions were reduced by the carboxylate group of the aspartic acid residues present in the peptide-based gelator. The silver nanoclusters prepared in this way were very stable (more than 6 months) and showed a narrow emission profile and large Stokes shift (>100 nm). In a similar report by the same authors, Fmoc-protected L-phenylalanine (32) was used to synthesize and stabilize fluorescent few-atom silver nanoclusters in the presence of diffused sunlight at physiological pH (7.46) and at room temperature without using any other external reducing agents.38 Das and co-workers reported in situ synthesis of silver nanoparticles within L-tryptophan containing amphiphilic 11980


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Chart 8. Molecular Structures of Supramolecular Gelators for the Synthesis of Silver-Nanoparticles

Chart 9. Molecular Structures of Supramolecular Gelators for the Synthesis of CdS- and Pd-Nanoparticles

modified 1,3:2,4-dibenzylidenesorbitol (39) to extract gold/ silver salts from model waste where the uptake of heavy metals were preferred over other common metals.209 Spontaneous in situ reduction of the salts in the gel nanofibers led to the generation of metal nanoparticles and hence the corresponding composite gel. Thus, the resulting gel-nanocomposite showed electrochemical activity such as higher conductance compared to the same gels doped with carbon nanotubes mainly due to high accumulation of the nanoparticles in the gel matrix. These composite materials could find applications in electrode surface modification or enhancing electrocatalysis. Yadav et al.

within the gel network. Such in situ synthesized nanoparticles show directional alignment along the nanostructured gel fibers of the peptides. Maitra and co-workers used lithocholyl− dipeptide conjugate hydrogels (such as 38) for the chemical and photoinduced in situ synthesis of silver and gold nanoparticles without addition of an external stabilizing agent.208 In addition, the color, size, and shape of the silver nanoparticles synthesized by photoreduction were shown to depend on the amino acid residue of the side chain present in bile acid-peptide conjugates. In another recent report, Smith and co-workers have shown hydrogels of acyl hydrazide 11981


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ature sensitive N-isopropylacrylamide hydrogels cross-linked with ethylene glycol dimethacrylate, tetraethylene glycol dimethacrylate, and poly(ethylene glycol) 400 dimethacrylate using UV free-radical polymerization protocol.217 Iron oxide nanoparticles were incorporated into the hydrogel matrix to prepare the magnetic nanocomposites. These systems hold potential for applications in the microscale and nanoscale devices and in various biomedical applications. Daniel-da-Silva et al.218,219 and Jones et al.220 reported synthesis of magnetic nanoparticle (magnetite) within natural polysaccharides carrageenan-based hydrogels. These gel-nanocomposites were further explored for drug delivery and bioapplications. Gelators 44 and 45 were introduced for the templatedirected TiO2 nanoparticle synthesis (Figure 11).221 These

synthesized silver and ZnO nanoparticles in the organogel derived from thiazole-based amide derivatives 40.210 The gelator molecules assisted the nanoparticle formation by encapsulating and stabilizing the as-synthesized nanoparticles in the gel phase. The size of the nanoparticle varied depending on the aliphatic chain length of the gelator as the pore size of the entangaled fiber network depending on the chain length. Recently, Trivedi et al. prepared agarose- and chitosan-based gels in an ionic liquid and synthesized 1-butyl-3-methylimidazolium chloride and silver oxide (Ag2O) nanoparticles in situ to form nanocomposites in a single step where the biopolymers acted as both reducing and stabilizing agents.211 The nanocomposite ionogels showed significantly high mechanical strength possibly due to the additional bridging effect of the nanoparticles between the polymer chains leading to a network structure apart from the usual electrostatic and hydrogenbonding interactions. 2.2.4. Other Nanoparticles. Several other nanoparticles such as CdS, Sn, Pd, TiO2, lanthanides, and magnetic nanoparticles were also synthesized in situ within the gel medium. Stupp and co-workers synthesized CdS nanoparticles in the form of nanoscale helical morphology using selfassembled organogel (41, Chart 9) as a template.212 Selfassembled helical nanoribbons of 41 were generated first in ethyl methacrylate as a solvent. Addition of Cd2+ ions accumulated near the hydrophilic segment in the nanoribbon followed by reduction with H2S led to the in situ formation of CdS nanoparticles assuming the helical shape. In another report, Xue et al. used cholesterol-based organogelator 42 for the syntheses of CdS nanoparticles.213 The fibrillar network morphology in the organogel medium acted as a template for the growth of the nanoparticle. Depending on time, the gel fibers were either decorated with CdS nanoparticles or fully encapsulated to form pearl-necklace porous CdS nanofibers by continuous growth of the nanoparticles on the surface of the primary CdS nanoparticles distributed on the gel fibers. Huang et al. developed thermoreversible organogels based on selfassembly of oleic acid and hexadecylamine in ethylene glycol with the addition of NaOH.214 This gelator acted as a template and carbon source for the syntheses of Sn nanoparticles encapsulated with carbon. Bhowmik et al. reported room temperature in situ synthesis of lanthanide trifluoride nanoparticles in a lanthanide (III) cholate hydrogel where the gelator itself acted as the stabilizing agent for the finely sized (3−5 nm) and homogeneously dispersed nanoparticles.215 Das and co-workers reported peptide bola amphiphiles containing tryptophan and tyrosine moiety (43) which underwent selfassembly to give hydrogel upon brief sonication.216 Pd nanoparticles were synthesized in the gel matrix from PdCl2 without any external reducing agent. The as-prepared Pd nanoparticles were decorated on the gel nanofibers, which also provide extra stability to the Pd nanoparticles either due to the interaction with the charges on COO− in the peptide bola amphiphile or through the interaction of the CO and NH of amide bonds, which do not participate in hydrogen bonding. Interestingly, the peptide-nanofiber-supported Pd nanoparticles showed effective catalytic activity toward the deprotection of different types of N-terminus (Boc-, Cbz-, Fmoc-, and Nmoc) amino acids and peptides. Various nanocomposites based on magnetic field responsive hydrogel networks containing magnetic nanoparticles and temperature responsive hydrogels were developed. The nanocomposite hydrogels were synthesized based on the temper-

Figure 11. Molecular structures of the gelators and a schematic showing template directed nanoparticle superstructure formation. (a) Formation of nanotube with gelator 44 and (b) formation of nanoparticle with gelator 45. Reproduced with permission from ref 221. Copyright 2006 Royal Society of Chemistry.

organogels controlled the nanostructure formation depending on whether the gelators were neutral or charged. The gelators can act as templates as they form different superstructures such as nanofibers, nanoribbons, nanorods, and nanoparticles depending on their molecular structures. A sol−gel polymerization was performed in the presence of the gelators, [Ti(OiPr)4], and propylamine catalyst followed by calcinations at higher temperature to produce the corresponding nanoparticle superstructures. In this process, propylamine acted as catalyst for the sol−gel polymerization through partial reaction with the −COOH groups of 44. This induced a peripheral positive charge around the self-assembled gel fibers, and thus, the TiO2 precursor was attracted electrostatically toward the gel fibers. Thus, nanotubes were generated from the gelator 44. However, nanoparticles were created from the gelator 45 as the nanofibers of 45 acquired negative charges, and the polymerization around the nanofibers was inhibited due to the electrostatic repulsion between nanofibers and the sol−gel precursors. However, a uniform size distribution of the 11982


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Chart 10. Molecular Structures of Supramolecular Gelators for the Synthesis/Incorporation of Different Nanoparticles for Various Applications of Resulting Gel-Nanocomposites

Apart from catalysis, gel-nanoparticle composites are promising in the field of sensor devices. Zhao et al. dispersed gold nanoparticles into a hydrogel matrix of PNIPAM to accomplish nanocomposites through the copolymerization of functional gold nanoparticles with the polymer.225,226 The nanocomposites showed thermo-switchable electrical properties. Electrical conductivity of the nanocomposites showed a shift by 2 orders of magnitude under the temperature stimuli (Figure 12). In addition, the change in the electrical properties

nanoparticles was obtained, which indicated that the sol−gel polymerization took place in the nanospaces in the 3D network formed by the self-assembled nanofibers of 45. 2.3. Applications of Gel-Nanoparticle Composites

Several gel-nanoparticle composites were shown to have catalytic activity in different reactions. Platinum nanoparticlebased gel-nanocomposites have been explored for hydrogenation reactions. Maity et al. reported sonication-induced self-assembly of peptide-based bola-amphiphile 46 (Chart 10), which led to hydrogelation through a synergic effect of intermolecular hydrogen bonding and π-stacking interactions.222 This tyrosine rich gelator self-assembled into nanofibrillar structures, and it was used as a template for in situ synthesis of platinum nanoparticles. To test the catalytic activity of the Pt nanoparticle-hydrogel composite, it was added to the p-nitroaniline solution in the phosphate buffer and methanol followed by addition of sodium borohydride. The hydrogenation reaction was successfully completed within 1 h, and pnitroaniline was converted into p-phenylenediamine. Gold nanoparticle based composites were used for catalytic reduction of p-nitrophenol to p-aminophenol. Kumar et al. used urea/ amide-based gelators such as 47a−47b for the synthesis of gold nanoparticles by in situ reduction of HAuCl4 by the gelator molecules itself in the absence of any external reducing agent.223 Interestingly, these gel-gold nanoparticle composites showed spontaneous reduction of p-nitrophenol to p-aminophenol also without addition of any external reducing agent. The gold nanoparticles prepared in the gel in nitrobenzene were fluorescent in nature, and it was used for sensing of Hg(II). Palladium nanoparticle-based gel-nanocomposites were used for aromatic C−C coupling (Suzuki) reactions. Maity et al. synthesized palladium nanoparticles in a calcium-cholate (48) hydrogel by in situ reduction of potassium tetrachloropalladate(II) with sodium cyanoborohydride.224 The resulting palladium nanoparticles in the gel-nanocomposites showed good catalytic activity for Suzuki reaction under aqueous aerobic condition. Thus, reactions between phenyl boronic acid and derivatives of bromo/iodobenzene in the presence of the dried gel-nanoparticle composite led to good yields of the corresponding biphenyl derivatives. The catalytic activity of the composite-based nanoparticles retained up to 4cycles of reuse, even in storage for several months.

Figure 12. Electrical conductivity as a function of heating−cooling cycles between 10 °C (●) and 40 °C (■). Reproduced with permission from ref 225. Copyright 2005 Wiley-VCH.

is reversible during heating and cooling cycles. Therefore, the thermo-switchable electrical properties could lead to the design of smart materials that may be potentially useful as sensors. Hydrogel-based nanocomposites are suitable for the incorporation of bioactive drugs or biomacromolecules. Therefore, stimuli responsive release/degradation of the host gel matrix could specifically deliver the drug on-demand. Recently, Yan et al. incorporated nanoparticles into certain photodegradable hydrogels in order to achieve NIR light triggered release of biomacromolecules.227 The hydrogel is composed of cross-linked hybrid polyacrylamide-poly(ethylene glycol) structure containing photoresponsive o-nitrobenzyl groups incorporated with core−shell NaYF4:TmYb nanoparticles and different 11983


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Figure 13. (a) Schematic depiction of the NIR-light-induced degradation of hydrogel using the UV light generated by the encapsulated nanoparticles (polymer chains, black line; photo cleavable cross-links, red triangles; nanoparticles, green spheres; and trapped biomacromolecules, yellow rods). (b) Chemical structure of the polymer-containing photo cleavable o-nitrobenzyl moieties in the cross-linker and the NIR-light-triggered photoreaction of the hydrogel. Reproduced from ref 227. Copyright 2012 American Chemical Society.

viscoelastic and thixotropic self-recovery properties. This hybrid gel also exhibited antibacterial activity against both Grampositive and Gram-negative bacteria and showed high biocompatibility toward mammalian CHO cells. Murali Mohan et al. developed polymer-based hydrogels for a simple and facile synthetic strategy to control the size and shape of the Ag nanoparticles.190 Variations in the cross-link density of the polymer network composed of poly(acrylamide) with poly(ethylene glycol) as the cross-linking agent could control the size of the nanoparticles and this regulates the shape of the nanostructures such as nanorods, nanocubes, etc. within the hydrogel networks (Figure 8). In this process, the free-space in the hydrogel networks acted as a template for the nucleation of the nanoparticles and provided long-term stability. These hybrid nanocomposites released nanoparticles progressively over time, which could be utilized as antibacterial materials, as Ag nanoparticles are among the most widely used antibacterial agents that offer a number of advantages. In this way, a higher degree of biocompatibility and long-term antibacterial activity could be both achieved with the hydrogel-based Ag nanó particle composites. Garcia-Astrain et al. reported bionanocomposite hydrogels based on furan-appended gelatin and chondroitin sulfate cocross-linked with benzotriazole maleimide functionalized silver nanoparticles.230 In the composite, maleimide-coated silver nanoparticles were covalently crosslinked with furan-containing gelatin through Diels−Alder cycloaddition, and also amide coupling between remaining free ε-amino groups of furfuryl gelatin and carboxylic groups of chondroitin sulfate eventually led to stable hydrogel formation.

biomacromolecules such as proteins (Figure 13). NIR light irradiation of the composite hydrogel was absorbed by the nanoparticles which in turn produced UV light that was required for cleaving the o-nitrobenzyl groups. This resulted in the conversion of the gel-to-sol and consequently the release of the biomacromolecules. This type of release device has also been derived from magneto-responsive hydrogel. Xu and co-workers reported a magneto-rheological hydrogel assembly by doping a small amount of magnetic nanoparticle coated with a ligand (50a, Figure 14) which is structurally similar to the host hydrogelator (50b).228 Therefore, incorporation of a minute amount of 50acoated Fe3O4 nanoparticles could effectively interact with the gelator molecules through a molecular recognition pathway leading to the gel-nanocomposite formation. Exposing the gelnanocomposite to a small bar-magnet could efficiently transform the gel into a sol by breaking the assembly as the magnetic nanoparticles were attracted toward the magnet (Figure 14). Therefore, this design strategy could allow for drug delivery applications by releasing specific drugs encapsulated in the hydrogel in the presence of magnet as stimuli. Detailed biological studies were performed with the hydrogel-based nanocomposites incorporated with appropriate bioactive nanoparticle, such as antibacterial activity of silver nanoparticles. Das and co-workers reported naphthalene appended L-lysine-based amphiphilic hydrogelator (49, Chart 10) for the synthesis of Ag nanoparticle within the hydrogel by in situ photoreduction of AgNO3 under sunlight.229 The hydrogel was syringe-injectable due to its appropriate 11984


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type sandwich complex formation (ligand:fullerene = 2:1). Therefore, an addition of fullerene increased the stability of the gels and the gel melting temperatures significantly. In another instance, porphyrin-based compounds 52a and 52b were also developed. Compound 52a self-organized in aromatic hydrocarbon solvents with J-type aggregation mode, whereas compound 52b adopted H-type aggregation. Inclusion of fullerene into these organogels increased the stability of the gels and formed a 2:1 complex between the gelator and fullerene arranged in a one-dimensional array. Thus, the sheetlike organizations formed by the gelators alone were converted into one-dimensional multicapsular structures in the presence of fullerene.237,238 An ex-tetrathiafulvalene-based organogelator with L-glutamide-derived lipid unit, 53 (Figure 16), has been synthesized. Incorporation of fullerene into it enhances the gelation ability of 53 in organic solvents such as toluene, dimethyl sulfoxide, and ethanol.36 This has resulted due to the intermolecular interaction between the ex-tetrathiafulvalene unit and fullerene through π−π stacking as well as solvophobic and concave− convex shape complementary interactions. This increases the gel melting temperature significantly. Gelation-induced CD signals of the gelator alone and the gel-fullerene composites exhibit opposite sense in the CD signals, indicating that the composite assembles into a different chiral structure compared to the chiral assembly structure of the gelator alone. Bisureabased gelator 54 (Chart 11) was developed for supramolecular gel-assisted self-assembly of fullerene in organic media.239 While the gelator 54 alone self-assembled into helical nanobelts, incorporation of fullerene transformed the morphology into nanorods. As a mechanistic interpretation, the molecularly dispersed fullerenes in the gel state started crystallization in orderly domains based on specific hydrophobic, hydrophilic, and other weak interactions (strong hydrogen bonding and π−π interactions) exerted by the gelator molecules and thus it led to the formation of nanorods wrapped by the gelators. Apart from inclusion of unfunctionalized fullerene directly into the gel network, fullerene has been also connected covalently with the gelator molecules. Fullerene coupled amphiphile 55 (Chart 11) transformed into an organogel upon keeping a methanolic solution for a few days.240 Transformation of a less-ordered into a more-ordered structure took place due to the change in the molecular orientation which led to the alteration in morphology from globular to fibrous aggregates. In another report, fullerene-coupled cholesterol 56 was shown to form chirally ordered fullerene assemblies in the organogel state.241 Due to the cooperative and cohesive association among cholesterol−cholesterol and fullerene−fullerene systems, a columnar 1D-packing of the cholesterol moieties was observed where the fullerene moieties were chirally oriented outside the helical column. Thus, ordered fullerene assemblies were created in the physical organogel medium. A fullerene-linked 3,4,5-tris (dodecyloxy)benzamide derivative 57 was synthesized that formed nanowire structures through self-assembly using Langmuir−Blodgett (LB) method.242 The surface morphology of the LB film of 57 changed from a homogeneous monolayer to a bilayer structure via a fibrous monolayer structure depending on the holding time at the air/water interface before deposition. It is proposed that the morpohological change appears because of the close packing of the fullerene moiety through π-stacking interaction among the fullerene moieties and intermolecular H-bonding within the

Figure 14. Molecular structures and the magnet-induced gel-to-sol transition showing (a) solution of 50b (0.35 wt %), (b) gelnanocomposites of 50b+50a−Fe3O4, (c) change in shape of the hydrogel after application of the magnet, and (d) gel-to-sol transformation and the precipitate of 50a−Fe3O4. Reproduced with permission from ref 228. Copyright 2007 Elsevier.

The gel-nanocomposite showed significantly higher (78%) controlled release of a model drug antibiotic chloramphenicol. The nanocomposite hydrogel showed adequate level of cytotoxicity, and the cell viability was higher than 70% for the first 24 h and close to 100% after 48 h of incubation. Therefore, this bionanocomposite hydrogel shows potential for biomedical applications.

3. GEL-NANOCOMPOSITES WITH FULLERENE Carbon-based nanomaterials such as fullerene, CNTs, graphene, and their functionalized analogues were incorporated in the fibrillar gel networks. Zero-dimensional fullerene-based host−guest complexes are extensively investigated mainly with small molecules,231,232 dendrimers,233,234 and supramolecular polymers235 based on π-extended tetrathiafulvalene moiety; however, these are not assembled in the gel phase. Fullerene has been doped into the self-supporting physical gels to introduce the electro-optical properties of fullerene into the resulting composite.236 Several small molecular gelators were chosen to prepare these nanocomposites, and in some cases, these are also connected with fullerene covalently. Fullerenecomposites were developed predominantly in the organogel medium due to its intrinsic hydrophobicity. These are discussed below. A gel-fullerene composite was achieved first by Shinkai et al. from a porphyrin-based low molecular mass organogelator 51 (Figure 15). Zn(II)-Porphyrin-based gelator was first prepared, and the gelation was observed in the aromatic hydrocarbons.41,42 The gel matrix was then stabilized by incorporating fullerene through its reinforced interaction via host−guest11985


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Figure 15. Gelator molecules and the one-dimensional aggregates by 51 (a) in the absence and (b) in the presence of fullerene in the gel phase. Reproduced from ref 42. Copyright 2001 American Chemical Society.

Figure 16. Gelator molecule 53 and the possible molecular arrangement of 53 in the gel phases in absence and presence of fullerene. Reproduced from ref 36. Copyright 2010 American Chemical Society.

monolayer fibrous structures. Fullerene coupled with an Lglutamide-derivative (58a and 58b) also showed greater extent

of dispersibility in organic solvents, and they formed physical gel in various organic solvents.243 The presence of L-glutamide 11986


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Chart 11. Molecular Structures of Supramolecular Gelators for the Formation of Fullerene-Based Gel-Nanocomposites

comprising mixtures of donors and acceptors showed remarkably high photocurrent generation. Efficient photoluminescence quenching of the neat xerogel of 60a by 60b is due to a photoinduced electron transfer process. Photocurrent measurements of the hybrid xerogel films showed stable photocurrents appearing immediately after the irradiation and dropping instantly when the illumination was switched off. Further, this process was found to be reversible in nature. Complexes between zinc and copper bisporphyrin (61a) and fullerene derivatives (61b) were developed to accomplish gelation and harness liquid-crystalline properties.246 The porphyrin tweezers complexed with the metal ions and fullerene or its derivatives form 1:1 complexes. The inherent gelation ability of the metal-porphyrin tweezers increases upon incorporation of fullerene derivatives. The fullerene derivative (61b) did not form liquid-crystalline phase on its own. However, its 1:1 mixture with the metal-porphyrin tweezer showed existence of smectic liquid-crystalline phases. Apart from small molecules, fullerene has also been linked with poly(ethylene glycol) (PEG) through biradicals formed by thermal decomposition of (azo-PEG)n.247 At the feed ratio between PEG/fullerene less than 3, while PEG-grafted-fullerene was formed, the gelation did not take place. However, gelation occurred with the feed ratio greater than 4. Incorporation of fullerene increased the thermal stability of fullerene-PEG gel in

moiety imparted an intermolecular H-bonding and van der Waals interactions, which favored the gel formation. Gelation ability of 58a increased upon addition of an ex-tetrathiafulvalene derivative (58b), which is a strong electron donor and thus could specifically bind fullerene. Self-assembly of 58a in dimethyl sulfoxide led to spherical particles, and the surfaces of these spherical particles were hierarchically structured with nanofibers. In addition, the thin-films of these spherical particles exerted water-repellent superhydrophobic properties with a water contact angle of 148.2° In another instance, the porphyrin−fullerene complex system has been developed using the fullerene-based gelator 59a and porphyrin-based gelator 59b.244 This complex showed photoinduced electron transfer from the porphyrin to the fullerene assisted by the coaggregation with ordered structures through the glutamide moiety. The efficiency of electron transfer increases either by increasing the amount of fullerene analogue or by decreasing the temperature of the mixture. Another electron donor− acceptor assembly was obtained using a π-conjugated organogelator 60a acting as an electron donor and the fullerene analogue 60b which acted as the electron acceptor.245 The gelator 60a self-assembled into 1D nanofibers through hydrogen bonding and π−π stacking interactions. The donor−acceptor assembly also generated a 1D array in the thin film. The resulting interdigitated and ordered assemblies 11987


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Chart 12. Molecular Structures of Supramolecular Gelators Forming Fullerene-Based Gel-Nanocomposites for Electronic Device Applications

swelling decreased and with increasing shear strain the amount of PVP/fullerene solution release could be increased (up to 95−98% of gel volume at about 20000 strain units). Also, the rate of PVP/fullerene release was faster (by about 30%) with a higher concentration of fullerene inside the gel.

comparison with that of (azo-PEG)n. The fullerene-PEG composite gel could swell in solvents, such as water, methanol, and THF but not in hexanes or related solvents, and the degree of swelling was recorded to be ∼5.1−7.8%. In another report, the poly(vinylpyrrolidone)/fullerene complex (PVP/fullerene) was incorporated inside a microgel formed by sodium polyacrylate.248 The microgel was swollen in aqueous solutions containing PVP/fullerene to absorb the complexes, which were then released under mechanical shear stress. It was shown that with the increase in PVP/fullerene concentration, the degree of

3.1. Applications of Fullerene-Based Gel-Nanocomposites

Fullerene and its derivatives have considerable applications in the design and engineering of organic solar cells as electron accepting materials.249 Composites of mainly donor−acceptor11988


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4. GEL-NANOCOMPOSITES WITH CARBON NANOTUBES One-dimensional CNTs including SWNTs and MWNTs have been incorporated in certain gels in organic and aqueous media. The number of reports for such gel-CNT composites is significantly greater compared to that of fullerene, perhaps indicating an increasing interest in developing CNT-based nanocomposites. CNTs have attracted a lot of attention due to their high aspect ratio, extraordinary mechanical properties, and enhanced electrical conductivity which make them useful in many aspects ranging from optoelectronics to even in biomedical applications.253,254 CNT-based composites are exploited majorly for achieving enhanced conductivity,255 electrostatic charging,256 optical emitting devices,257 flexible and stretchable electronics,258 moldable all-carbon integrated circuits,259 or in the preparation of lightweight, high strength composites,260 etc. Conjugated polymers were also employed to wrap and disperse CNTs to explore their possible applications in the field effect transistors and solar cells.100 One of the goals of studying interactions of different small molecules and polymers with SWNTs is to separate different nanotube species in terms of their electrical conductivity (metallic and semiconducting) and also chirality.100,261−265 Small molecular and polymer gelators were employed to improve dispersion of CNTs in the medium through molecular level interactions and the resulting gel-nanocomposites manifested mainly optoelectronic, mechanical, thermal, biological, and NIR-sensitive properties. These are discussed below by citing appropriate references.

based small molecules or polymers blended with fullerene or its derivatives (mostly PCBM) were used as the active materials in the bulk heterojunction solar cells. Such composites were also derived from physical gels in order to explore the ordered chromophore assembly. Potential applications of fullerenebased gel-nanocomposites include photocurrent generation and also organic solar cell device fabrication. These are discussed below. Fullerene-based carboxylic acid 62 (Chart 12) has been incorporated by Xue et al. into an organogel derived from a donor−acceptor-based gelator 64 in order to use the composite for photocurrent generation.250 Due to the intermolecular hydrogen bonding between the gelator and the fullerene derivative, a two-component gel was formed and thus 1D channels of the electron donor and acceptor were achieved. Large and reversible photocurrent was generated upon whitelight irradiation when the gel film was used as an active layer in a photovoltaic cell. These authors have also reported incorporation of the same fullerene-based carboxylic acid 62 inside another organogel formed by a donor−acceptor-based gelator 65.251 A two-component hybrid gel was again formed through hydrogen bonding between the gelator and fullerene derivative which led to the formation of self-assembled 1Dnanofibers representing close association of p- and n-type channels. Stable and reversible photocurrent was also generated in the nanofibrous xerogel film upon switching between darkto-visible light. Therefore, these two reports indicate photoinduced electron transfer from the gelator to fullerene derivative in such self-assembled gels. In these two cases, the fullerene derivative acts as a hydrogen-bonding donor and the gelator as the corresponding hydrogen-bonding acceptor. Another report depicts the same principle; however, in this case, the gelator (66) acts as a hydrogen-bonding donor and the fullerene derivative (63) as a hydrogen-bonding acceptor. In this case, also the two-component gel showed photoinduced electron transfer from the gelator to the fullerene derivative and thus a large photocurrent could be generated.252 Organogel-based fullerene nanocomposites have been used to fabricate bulk heterojunction organic solar cells. Stupp and co-workers reported hairpin-shaped sexithiophene molecule 67 for the generation of self-assembled, grooved nanowires as the donor component, and the same acted as a host to fill in the grooves with the fullerene derivative PCBM through noncovalent interactions.46 The dried gel retained the selfassembled structure, and at the interfacial heterojunction between the assembled gelator and PCBM, some PCBM molecules can occupy the grooves of the gelator. This interfacial organization also remained unaffected during the annealing process. Thus, better complexation of PCBM with the grooved shape of the gelator nanowires prevented excessive phase separation and promoted higher quality interfaces for exciton splitting if compared with the composites of PCBM with a linear sexithiophene moiety 68. This was reflected in the power conversion efficiencies of the solar cell devices as well. While the mean power conversion efficiencies of the devices made of 67 was 0.42%, the same with 68 was 0.36%. Therefore, the self-assembled architecture obtained through the gelnanocomposite formation provided better efficiency in terms of device performance. Therefore, such examples depict the possibility and potential of the gel-nanocomposites for future applications in these fields.

4.1. Incorporation of CNTs in Supramolecular Organogels and Hydrogels

Several aliphatic and aromatic small molecular gelators were shown to interact with pristine and covalently functionalized CNTs both in organic and aqueous media depending on the molecular features of the gelators. Formation of gel-nanocomposite occurs via their noncovalent interaction at the molecular level. In this section, we first describe the molecular structures of aliphatic gelators and their ability to form gelnanocomposites and then discuss the advantages of having fused/conjugated aromatic-based gelators. In the case of aliphatic gelators, covalently functionalized CNTs were mostly used in order to ensure their optimal association with the gelators through hydrogen bonding or van der Waals forces. Structurally simple L-alanine-based small molecular organogelator 1 (Figure 4) has been tested by us for the dispersion of both pristine and functionalized (with n-dodecyl and nhexadecyl carboxamide) SWNTs.101 It has been shown that the type of SWNTs controls the gelation process and influences the properties of the resulting nanocomposites. Dispersibility of the functionalized SWNTs in the gel matrix was greater compared to the pristine SWNTs because the gelator lacks to induce π−π stacking interactions (Figure 17). This indicates that van der Waals interactions between the aliphatic hydrocarbon chains of the gelator and those of the functionalized SWNTs are responsible for the stable dispersion. The gel-nanocomposites showed increased viscoelasticity and the increase was greater in the case of functionalized SWNTs possessing longer hydrocarbon chains. This suggests an effective interdigitation of the pendant aliphatic hydrocarbon chains with the fatty acid amide chains of L-alanine in the gel assembly. Due to this interaction, the morphology and organization of the gel-nanocomposites alter significantly 11989


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amino acid or dipeptide backbone (71a−71c) induced dispersal of a significantly high amount of pristine SWNTs (2−3.5% w/ v) without sacrificing the gelation properties.268 The π− electronic interactions between the SWNT surface, aromatic moiety of the amino acids, and imidazolium moiety propel efficient CNT exfoliation, which enabled these gelators to disperse such high quantity of CNTs in water. The soft nanocomposites thus obtained for example from 71b by incorporating 1.5% w/v SWNTs in 2% w/v of the gelator exhibited about 85-times higher mechanical viscoelasticity (G′ value ≈ 15700 Pa) compared to the native hydrogel. Banerjee and co-workers reported N-fluorenyl-9-methoxycarbonyl (Fmoc) protected amino acid L-phenylalanine (Fmoc-PheOH, 72) as the host hydrogelator for the incorporation of carboxylic acid functionalized SWNTs.269 Gel-nanocomposite was formed efficiently due to the strong π−π stacking interactions of the gelator with the nanotubes. Thus, in the nanocomposite, SWNTs were aligned directionally in a 1D fashion in line with the gel nanofibers. The nanocomposites also showed increased thermal stability and improved mechanical stability (16 times more viscoelastic) than the native hydrogel. We have recently reported effective dispersion of SWNTs in a two-component hydrogel system composed of the viologen derivative (73a) and L-alanine-based amphiphile (73b).270 The resulting gel-nanocomposite under electrochemical analysis showed that incorporation of SWNTs into the hydrogels converted the otherwise electrochemically irreversible hydrogel to a quasi-reversible system. Therefore, these reports indicate that the presence of an aromatic moiety in the gelator would effectively enhance the gel-nanocomposite formation. The effect of aromatic π−π stacking interactions for the dispersion of pristine SWNTs could be best discriminated by involving an aliphatic- and aromatic-based gelator having similar molecular structure. Thus, Tian et al. employed an identical aliphatic tail connected gelator 74a and a pyrene appended gelator 74b to clearly show the difference in interactions with pristine SWNTs.30 It was shown that the incorporation of SWNTs led to a homogeneous dispersion in the case of 74b (denoted as LPG), even after several heating and cooling cycles, but it precipitated in the case of 74a (LBG) only after one heating−cooling cycle (Figure 18). Therefore, pyrene moiety in the 74b acts as an anchor to SWNTs through π−π stacking interactions. This induced decreases in the critical gelation concentration (CGC) of the LPG, decreased sol-to-gel transition temperature, and increased viscoelasticity. However, in case of 74a which is devoid of pyrene moiety, no significant change was seen in the CGC, thermal, and mechanical properties upon addition of the same amount of SWNTs. Therefore, attachment of aromatic moieties with the gelator molecule appears to be an effective strategy for the synthesis of improved gel-nanocomposites. Pyrene is well-known for inducing strong π−π stacking interactions with CNTs. Nagarajan et al. investigated pyreneappended sugar-based gelator 75 (Chart 14) for the interaction with SWNTs and modulation of the gelation properties.32 It was observed that interaction of the pyrene moiety with SWNTs led to noncovalent functionalization of SWNTs by the gelator, and the mixture did not induce gelation. This indicates that strong π−π stacking interactions between the gelator and SWNTs in turn weakened the intergelator interactions (πstacking interaction through the pyrene moieties) which did not allow the gel formation. This report indicates that for the

Figure 17. (i) Suspension of (A) pristine SWNTs and (B) C16− SWNTs in toluene. (C) Organogel of 1 in toluene and composite gel with (D) pristine SWNTs, (E) C16−SWNTs in toluene. (ii) Plots of yield stress as a function of wt % of different SWNTs incorporated in the organogels. Reproduced with permission from ref 101. Copyright 2008 Royal Society of Chemistry.

upon incorporation of SWNTs. Further, an irradiation using near IR laser (1064 nm) for a short duration (1 min) at room temperature induced a gel-to-sol phase transition selectively for the nanocomposites. Moniruzzaman et al. used another simple organogelator 12-hydroxystearic acid (69, Chart 13) to incorporate pristine and carboxylated MWNT (MWNTCOOH).266 It has been demonstrated that the carboxylated MWNTs improved the mechanical strength of the gel compared to the pristine MWNTs. This is due to the better dispersibility of the carboxylated MWNT in the gel matrix through hydrogen bonding between the gelator and the COOH groups of the MWNTs. However, pristine MWNT was more effective in improving the electrical conductivity of the nanocomposites. Das and co-workers showed amino acid based amphiphilic gelators 70a−70b containing tryptophan as the aromatic residue for the interaction with carboxylic acid functionalized SWNTs (SWNT-COOH).267 Inclusion of a minute amount of the carbon nanotubes can improve the gelation efficiency significantly. Gelation efficiency increased by ∼17 times for 70a and ∼12 times for 70b on addition of 0.1% w/v SWNT-COOH in toluene. It was suggested that strong π−π stacking interactions between the SWNT-COOH and the aromatic tryptophan moiety was responsible for such increase in gelation efficiency. The π−π stacking contribution from the tryptophan moiety of the gelator was verified by the replacement of this residue with L-phenylalanine and L-valine, which did not show this type of improved gelation behavior. These authors have also reported hydrogelators based on imidazolium-appended 11990


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Chart 13. Molecular Structures of Supramolecular Gelators for the Incorporation of CNTs

(76a−76c) to act as ambidextrous gelators (i.e., it would induce efficient gelation in both aqueous and organic solvents).33 Gelation in water at acidic pH (pH 2.0−5.0) took place with the MGC of 0.5−1.1% w/v while 0.7−5.0% w/v in organic solvents. Gel-SWNT nanohybrid was prepared upon incorporation of 0.025−0.6% w/v of SWNTs in either media without compromising the gelation efficiency. The reinforcement of the hybrid gels by SWNTs was reflected in increased gel melting temperature and increased viscoelasticity. Eventually, π-conjugated small molecular aromatic gelators were tested for the synthesis of gel-nanocomposites. For example, Malicka et al. reported gallic acid/squaraine dye-based gelator (77) showing SWNT-induced stable gel formation through heterogeneous nucleation and growth processes.271 Addition of a minute amount of SWNT into the gelator could influence the sonication-induced gelation significantly. As a result, the hybrid gel showed higher gel melting temperature and morphology transition from nanoring to nanotape. Ajayaghosh and co-workers incorporated CNTs into the πorganogel assembled from oligo (p-phenylenevinylene) (OPV)-based gelators (80).47 Due to the π−π stacking interactions between the gelator and the CNTs, the association of OPV molecules is promoted toward the formation of hybrid π-conjugated gels (Figure 19). The gel network was reinforced due to the encapsulation of CNTs, which increased the stability of the gels, gel-to-sol transition temperature, and viscoelasticity of the hybrid gel. The inherent birefringent texture of the gelator (80) altered significantly on addition of CNTs. In the hybrid gel, CNT acts as a physical cross-linking agent without losing its long aspect ratio and electronic properties.

Figure 18. Molecular structures of the gelators 74a−74b. Photographs of the organogels after 1 cycle of heating−cooling show phase separation of SWNTs from LBG gels but a homogeneous dispersion in case of LPG-SWNT gels. Reproduced with permission from ref 30. Copyright 2010 Royal Society of Chemistry.

synthesis of gel-nanocomposites, while it is important to have an optimal interaction between the gelator and SWNTs (and in general with nanomaterial additives), retention of intergelator interactions is crucial for the maintenance of gelation. Thus, in a recent report, Mandal et al. showed pyrene-containing amino acid (L-alanine, L-phenylalanine, and L-tryptophan) derivatives 11991


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Chart 14. Molecular Structures of Supramolecular Gelators Having Fused/Extended Aromatics for the Formation of CNTBased Gel-Nanocomposites

Interaction of pristine and n-hexadecanoyl amide-functionalized SWNTs were also investigated by us with the organogels derived from OPV-based gelator 81.102 Addition of a small amount of either version of SWNTs on the viscous aggregates of 81 just below its CGC converted the mixture into a rigid gel (Figure 20). This indicates a strong interaction between the gelator and the SWNTs through π−π stacking and van der Waals interactions. This interaction induced quenching of the UV−vis and fluorescence spectral intensity, increased the solto-gel transition temperature, and increased the viscoelasticity progressively with gradual addition of the SWNTs. Inclusion of CNTs also altered the intrinsic birefringent textures of the gelator 81. These composites manifested electrical semiconducting properties, and their conductivity increased in the composites compared to that of the native gel. The nanocomposites containing functionalized SWNTs showed greater conductivity compared to that of pristine SWNTs since the integrity of the electronic framework of the SWNTs is lost partly during functionalization. In addition, the conductivity

increases progressively with an increasing amount of the SWNTs. To further explore π-conjugated aromatic-based gelators, functionally dense electrically conducting or redox active groups were incorporated in the gelator. Canevet et al. doped SWNTs in an organogelator containing two distinct πfunctional units (78, Chart 14).31 One of them is made of a tetrathiafulvalene derivative, which often shows electrically conducting behavior, while the other unit is based on pyrene to impart π-stacking with SWNTs. This judicious choice of gelators leads to nanocomposites with SWNTs, which impose a structuring of the supramolecular fibers. The nanocomposites act as the conducting supramolecular wires after doping with iodine vapor, and the magnitude of current increases with the samples containing SWNTs. It has been shown that the increase in the conductivity is due to the π-stacking interactions of the gelator with the SWNTs, and the SWNTs alone may not be directly responsible for the increase in the conductivity. In another report, Brunetti et al. prepared electron donor− 11992


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Figure 19. Schematic showing CNT-induced organogel formation for the gelator 80. Reproduced with permission from ref 47. Copyright 2008 Wiley-VCH.

where the molecular structure of the host gelator and the functionalizing ligand of the nanomaterials were identical. A similar strategy has also been used for the preparation of gelnanocomposites based on SWNTs. Oh et al. developed a hybrid organogel system based on gelator 82a and SWNTs (82b) functionalized with an organic moiety having a similar molecular structure to that of the gelator (Chart 15).273 The microscopic morphology of the hybrid organogel depended upon the relative interactions between the SWNTs, organogelator, and the solvent molecules. The electron microscopy examination of the hybrid organogels prepared in n-decane showed that the SWNTs were placed inside or on the surface of the organogel fibers. However, in the dimethylformamide gel, the SWNTs were distributed evenly throughout the sample. The gel melting temperature remained unaltered in the hybrid gels obtained from dimethylformamide as compared to the native gel, but it decreased with increasing SWNTs concentration in the case of the decane gel. In another report, Lascialfari et al. also covalently connected nanocarbons with molecular gelators in order to produce homogeneously dispersed soft-nanocomposites.274 This allowed high loading of nanomaterials (up to 20 wt %) to achieve gel-nanocomposite materials. While the precursors 83a−83c formed organogel, the adducts 83d and 83e did not form self-standing gel on their own. The fullerene adduct 83d was obtained after coupling fullerene with 83a in the presence of N-methylglycine (sarcosine) in refluxing toluene, while the adduct 83e was obtained from 83b through an aryldiazonium salt intermediate. Composite gels were prepared by mixing 83d+83c (10−30 wt % of 83d) and 83e+83c (10−40 wt % of 83e). All nanocomposite gels were found to be thermoreversible in nature. Interaction between 83d and 83c was evident from the increased molar ellipticity in the composites compared to that of the individual components. Incorporation of fullerene or MWNT-functionalized compounds (83d and 83e) into the preformed gel of 83c did not affect the regular self-assembly of 83c. Therefore, the mutual interaction between the gelator and the functionalized nanocarbons leads to the formation of

Figure 20. Molecular structure of the gelator 81. (i) Photograph of the viscous solution of 81 in toluene below its MGC (left) and Pr-SWNT induced gel (right). (ii) I−V measurements of 81, 81 + 0.99 wt % PrSWNT, and 81 + 0.99 wt % C16−SWNT (left y axis) and 81 + 4.76 wt % Pr-SWNT (right y axis). Reproduced with permission from ref 102. Copyright 2010 Royal Society of Chemistry.

acceptor nanohybrids using SWNTs and a photo- and redoxactive water-soluble ex-tetrathiafulvalene-based dendrimer 79 to achieve supramolecular organization from the nano- to the macro-scale and probing their photophysical properties.272 Photoexcitation of the nanocomposite made of 79+SWNT showed SWNT-centered excited states, which transformed into long-lived charge-separated states in water. Stability of these charge-separated species increased with an increase in the longrange order in the self-assembly. Thus, the nanocomposite obtained via noncovalent functionalization of SWNTs afforded unique p-/n-type nanohybrids. Molecular recognition strategies were described previously for both nanoparticles and fullerene-based gel-nanocomposites, 11993


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Chart 15. Molecular Structures of Supramolecular Gelators and Structurally Similar Functionalizing Moieties for GelNanocomposite Preparation through Molecular Recognition

In another report, Yu et al. described electroluminescence behavior of transparent SWNT-polymer [poly(tert-butyl acrylate)] composite electrodes prepared through photopolymerization of the liquid mixture of SWNT and the polymer.277 Interaction of SWNTs with the polymer in the composite through interpenetrating networks induced a low sheet resistance, high transparency, high compliance, and low surface roughness of the film. The composite electrode films could be stretched reversibly by up to 45% strain. Hybrid hydrogel-containing MWNTs was prepared using gelatin as a host, and its swelling behavior was examined.278 Pure gelatin gel reached swelling equilibrium at a faster rate than the hybrid gel; however, the swelling ratio of the hybrid gel was higher at the starting than that of the native gelatin gel, indicating that MWNTs inhibited the swelling of the gel matrix. In another report, CNTs were incorporated into PVA hydrogels (CNT-PVA) by the freezing/thawing method.279 The CNT-PVA hybrid hydrogels exhibited a dramatic improvement in the mechanical and swelling properties compared to the PVA hydrogel alone. Huang et al. reported wrapping of MWNTs with poly(vinylpyrrolidone) to form composite hydrogels in the presence of PVA.85 Addition of poly(vinylpyrrolidone) brings about uniform distribution of the nanotubes in the matrix. The mechanical properties such as tensile strength, modulus, toughness, tear strength, and friction coefficient of the composite hydrogels were improved significantly with the addition of less than 2 wt % of poly(vinylpyrrolidone) treated MWNTs. The frictional coefficient of the composite hydrogel was found to decrease due to

supramolecular nanostructures, which is useful in many aspects such as generation of superhydrophobic surfaces, conductive plastic devices, etc. 4.2. Incorporation of CNTs in Polymer Organogels and Hydrogels

Gel-nanocomposites based on pristine and functionalized CNTs were achieved with polymeric gelators. In some cases, the nanotube species were directly introduced into the gel medium. However, in many cases gelation was achieved only after mixing different components of nanotubes and precursors for polymer gel formation. For example, Chen et al. obtained stable gel-nanocomposite by noncovalent functionalization of SWNTs with ferrocene-grafted poly(p-phenyleneethynylene).275 The gel forms a robust 3D nanotube network through physical cross-linking the functionalized nanotubes. Strong π−π stacking interactions between the poly(p-phenyleneethynylene) and the nanotube surface led to the extensive cross-linking. Thus, the composite gels obtained in this way were so robust that it could not be redispersed in solvents even on sonication. The shift in the photoluminescence of SWNTs at NIR emission in a biocompatible poly(vinyl alcohol) (PVA) hydrogel has been recorded because of hydrogel swelling.276 These photoluminescence shifts arise depending on the change of the hydrogel cross-linking density and the state of hydration. Due to these changes, the Raman spectrum of the G-band shows upward shifts, indicating a deformation of the SWNT lattice. It has been concluded that the PL shifts result from the changes in the local dielectric environment around the SWNTs. 11994


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the lubrication effect of poly(vinylpyrrolidone), and it was independent of the MWNT concentration. Vaysse et al. reported effect of SWNTs to generate porosity in the organogels formed by poly(methyl methacrylate) (PMMA) using nonsolvents as porogens such as water, methanol, propanol, and cyclohexane was also investigated.280 It has been shown that the incorporation of SWNTs alone does not generate porous morphology unless a nonsolvent is added. Addition of SWNTs during gelation by the porogens imparts multiple factors, such as it increases the elongation ratio of the gel, forms a network around the pores, and thus it acts as a reinforcing filler for the gel network. Covalently functionalized nanotubes, SWNT-COOH could be dispersed efficiently in an aqueous solution of hyaluronic acid which was then converted into a hybrid hydrogel by crosslinking with divinyl sulfone.281 While both the native and hybrid gels showed a shear thinning behavior, the SWNT composite gel showed improved viscoelasticity. The improvement in the viscoelasticity is due to the participation of the hydroxyl functionality on the SWNT surface on the crosslinking of hyaluronic acid by divinyl sulfone. However, high water uptake capacity of the native gel and the hybrid gel remained almost unchanged, even after 2 wt % of SWNT incorporation. Therefore, this kind of hybrid gels could find potential application in tissue engineering due to its unique properties like water uptake capacity and increased viscoelasticity. In another report, porous hybrid hydrogels were prepared from polyacrylamide and MWNT or hydroxyl functionalized MWNT (MWNT−OH).282 Incorporation of nanotubes showed an improved swelling behavior, high compression strength, and low retention rate of the elastic deformation. This indicates that the nanotube chains might be wrapped or inserted in the polyacrylamide networks through noncovalent interactions such as intermolecular hydrogen bonding. Pyridine-functionalized SWNTs have been synthesized by the reaction of pyridine diazonium salt with SWNTs to use them as cross-linkers and hydrogen bond to poly(acrylic acid) to form SWNT hydrogels.84 Due to electrostatic interactions between the pyridine moieties with gold nanoparticles, the location and distribution of the functional groups on the SWNT surface could be determined. The pyridinefunctionalized SWNTs were dispersed easily in aqueous solution of the polymer, and the dispersion at pH 5.8 led to hydrogel formation, while pristine SWNT did not form a gel. This indicated possible interaction of pyridine-functionalized SWNTs with the polymer through hydrogen bonding which was absent in the case of pristine SWNTs. The molecular recognition principle has been applied to the formation of polymer based gel-CNT nanocomposites. You et al. reported functionalization of MWNTs with hyperbranched poly(amidoamine) moieties through multistep Michael addition reaction to facilitate the aggregation among them.283 Formation of composite organogel took place in the presence of linear poly(amidoamine) in dimethylformamide by the external ultrasound-stimulation, which induced the assembly via intermolecular hydrogen bonding (Figure 21). MWNTs were homogeneously dispersed in such gel, and the sol−gel switching was accomplished by heating and ultrasonication. Sonication breaks down the van der Waals interactions between the individual MWNTs resulting in debundling and dispersal in organic media. These gels are responsive to external stimuli such as heating, vigorous agitation, addition of water, and acids and salts (NaBr) which could convert the gel to a sol. In a

Figure 21. (A) Schematic showing the mode of self-assembly leading to the switchable sol−gel, (B) images of the MWNT induced sol−gel phenomena, (C) the structure of linear poly(amidoamine). Reproduced with permission from ref 283. Copyright 2009 Royal Society of Chemistry.

similar report, Li et al. demonstrated dynamic covalently crosslinked polymer gel from surface functionalized SWNTs with diol-containing polymer and a phenylboronic acid-containing polymer to create a hybrid system with reversible sol−gel transition.284 Thus, this hybrid system emerged as reversible chemical polymeric gels depending on the pH of the medium through a reversible formation and breakage of the phenylboronate ester linker. The hybrid gel showed as high as 1285% (from 7.58 to 105 kPa) increase in the storage modulus (G′) with only 0.02 wt % of functionalized SWNT content compared with the native polymer gel, indicating the reinforcement of the nanotubes. However, a further increase of the G′ value to 176 kPa was observed upon increasing the SWNT content to 0.06 wt %. It has also been shown that the polymer needs to be bound with the surface of SWNTs to show such increase in viscoelasticity. Furthermore, interesting selfhealing property has been observed for the hybrid gel, which occurred autonomously under mild conditions without the need of any additives. Ogoshi et al. reported multicomponent chemically responsive supramolecular hybrid hydrogels based on β-cyclodextrin (β-CD) coupled with pyrene and SWNTs in the presence of a 11995


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water-soluble polymer poly(acrylic acid) carrying 2 mol % of ndodecyl groups (PAA2).285 Pyrene-derivatized β-CD (Py-βCD) were anchored on the surface of the SWNTs through π−π stacking interactions. The water solubility of β-CD renders the SWNT-composites dispersible in water. The vacant cavities of β-CD can associate with the guest molecules, and through these host−guest interactions with a polymer (PAA2), hybrid hydrogels are derived (Figure 22). Therefore, either addition

Figure 23. Viscosity (η) change for the hydrogels after repeated irradiations with UV (365 nm) and visible light (430 nm). The red line denotes CD-CUR/SWNT/pAC12Azo and the blue line represents CD-CUR/SWNT/pAC12Azo with tetraethylene glycol. Green and pink lines are the CD-CUR/SWNT/pAC12Azo with C12CA2Na2 and with α-CD, respectively. Reproduced with permission from ref 287. Copyright 2011 Wiley-VCH. Figure 22. Schematic showing Py-β-CD/SWNT hydrogels with PAA2. Reproduced from ref 285. Copyright 2007 American Chemical Society.

nanotube network as a function of the overlap distance at a constant pressure. The conductivity of the composites depended crucially on the connectivity of the nanotube networks. It has been shown that the contact resistance is inversely proportional to the conductivity of the nanocomposites. Hong et al. reported polymeric ionic liquidSWNT hydrogel synthesized by noncovalent functionalization of oxidized SWNT surfaces with imidazolium-based poly(ionic liquids) monomer using an in situ radical polymerization protocol.289 The composite formed hydrogel via π-stacking and electrostatic attractions between SWNT surface and the ionic liquids. The composite hydrogel was converted into organogels through substitution of hydrophilic anions (Br−) with hydrophobic anions [TFSI−, bis (trifluoromethanesulfonyl) imide] by anion exchange. Owing to this, it was easily dispersed in a variety of organic solvents such as acetone, propylene carbonate, dimethylformamide, tetrahydrofuran, N-methylpyrrolidone, nitromethane, and methyl ethyl ketone. Electrical properties of the nanocomposites show that SWNT induces a decrease in the surface resistivity with increasing SWNT content due to the formation of conducting gel-networks. Nanocomposites derived from the covalent attachment of the hydrogel forming polymers to the SWNTs were also demonstrated. Covalently functionalized SWNTs with poly(ethylene glycol) methacrylate formed optically transparent hybrid hydrogel by a cross-linking reaction with poly(ethylene glycol) diacrylate prepolymer using dual photo-UV and thermal initiations.291 Another optically transparent (Figure 24) and electrically conducting composite of MWNTs and poly Nhydroxymethyl acrylamide was achieved by a simple in situ polymerization technique.290 The MWNT suspension was introduced as filler before the polymerization process ensuring the homogeneity of the composite. These composite films show electrical conductivity as high as 103 S/m depending upon the MWNT concentration along with the optical transparency greater than 95%. CNT-based hydrogels were investigated using systems primarily based on SWNTs and water-soluble polymers.292 The mixing of the two often leads to hydrogel formation where SWNTs assist the formation of the assembly. The composite gels exhibit increased thermal and mechanical properties. In addition to that, these systems owing to their solubility in water, show certain properties such as redox behavior and

of competitive guest molecules such as sodium adamantane carboxylate or competitive host molecules such as α-CD could selectively induce a gel-to-sol transition. SWNTs were incorporated into the supramolecular hydrogel derived from Pluronic copolymer and α-cyclodextrin (αCD).286 The hydrogel plays a dual role in dispersing SWNTs by the polymer and the formation of inclusion complexes with α-CD. The inherent characteristics such as shear-thinning and reversible temperature-responsive properties of the native hydrogel were retained even after the addition of SWNTs. It has been shown that the addition of SWNTs accelerates the gelation process, while the viscoelasticity of the hybrid gel decreases with the increase in the amount of SWNTs. The sol containing well-dispersed SWNTs formed a self-standing hydrogel in ∼25 min, while the native sol system took ∼60 min for the gelation. In another report, Tamesue et al. described the wrapping of α-cyclodextrin-modified polysaccharide Curdlan (CD-CUR) on the SWNT surface to disperse them in water.287 Supramolecular hydrogel was formed by the addition of poly(acrylic acid)-modified azobenzenes (pAC12Azo) into the aqueous solution of the CD-CUR/SWNT composites due to an inclusion complex formation between cyclodextrin (host) and azobenzene (guest). Therefore, addition of a competitive host such as α-CD or a guest [e.g., sodium 1,12-decanedicarboxylate (C12CA2Na2) to a preformed hydrogel of CD-CUR/SWNT/pAC12Azo] led to the gel-to-sol transformation. However, addition of tetraethylene glycol as a competitive guest molecule showed an insignificant effect on the gelation stability. Due to the presence of the azo-moiety, the hydrogels showed photoresponsiveness to cause gel−sol phase transition (Figure 23). Viscosity of the hydrogel decreased upon irradiation of UV light (365 nm) due to gel melting, and it reversibly converted again to gel upon irradiation with visible light (430 nm). Electrical properties of CNTs were also explored in the polymer-based gel-nanocomposites. Songmee et al. prepared nanocomposites of SWNTs and MWNTs with gellan gum, xanthan gum, and triton X-100 specifically to estimate their electrical conductivity properties.288 The conductivity was measured by evaluating the resistance of two overlapping 11996


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toward dispersibility in aqueous media in which they form selfassembled viscous hydrogels. Dense monolayer films of these oxidized SWNTs were prepared through the binding to aminecoated surfaces. These thin films show ohmic current−voltage behavior, while their electrical resistivity was nearly 3 orders of magnitude greater than that of the starting SWNTs. Sankar et al. prepared a pH-sensitive polyampholyte nanogel by copolymerizing vinylimidazole (VIM) with acrylic acid (AA) using functionalized SWNTs and cyanuric chloride via an intermolecular quaternization reaction where SWNTs act as reinforcing material.293 The nanogel showed improved physical properties such as increased viscoelasticity and thermal stability compared to the native gel without containing SWNTs. The nanogel also showed biocompatibility and cell viability, and thus it was used to deliver water-soluble drugs such as promethazine hydrochloride. 4.3. Applications of Carbon Nanotube Based Gel-Nanocomposites

Figure 24. Optical photographs of composite thin films with increasing MWCNT contents (from a−f) 0, 0.02, 0.04, 0.05, 0.06, and 0.08 mg/mL, respectively. Reproduced with permission from ref 290. Copyright 2012 Elsevier.

Gel-nanocomposites prepared using CNTs as dopants find applications in many areas such as oil absorption and recovery, NIR radiation induced photothermal and programmable devices, NIR-driven drug release, improved catalytic activity as well as antimicrobial activity. In this aspect, polymer-based gel-nanocomposites appear to be more useful compared to small molecular gelator-based composites. These are discussed below.

incorporation of DNA or drugs, which results in an evolution of new bionanocomposites. Chemical oxidation of SWNTs are generally undertaken to increase the extent of exfoliation from its bundles. These oxidized SWNTs showed greater propensity

Figure 25. (A) NIR triggered gel-to-sol phase transitions of SWNT−gel composites at 20 °C derived from (A) gelator 1: (a) 0.13 wt % pristine SWNT in 15 mg mL−1 toluene gel, (b) 0.13 wt % C16−SWNT in 15 mg mL−1 toluene gel, (c) 1.315 wt % C16−SWNT in 15 mg mL−1 toluene gel, (d) 15 mg mL−1 toluene gel and (B) gelator 81: (a) 81−Pr-SWNT, (b) 81−C16−SWNT, and (c) native gel at 25 °C ([81] in toluene = 10 mg mL−1 and [Pr-SWNT] = [C16−SWNT] = 0.5 wt%). Reproduced with permission from ref 101. Copyright 2008 Royal Society of Chemistry. Reproduced with permission from ref 102. Copyright 2010 Royal Society of Chemistry. 11997


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progressively higher temperatures, while addition of MWNTs to the hydrogels decreased the extent of swelling because of the hydrophobic effects. Therefore, this type of “smart” stimuli responsive gel-nanocomposites shows potential applications in release devices, drug delivery, or therapeutics. Convenient patterning method was developed by Miyako et al. using a composite hydrogel containing sodium dodecyl sulfate (SDS), SWNT, and agarose gel involving a template of microfabricated polydimethylsiloxane toward the development of NIR laser-driven high-performance photothermal switching nanomaterials.300 This enabled a photothermal phenomenon for the thermal control of the micropatterned nanotube−gel composites. The composites also exhibit fast control of temperature of a solution in microspaces when illuminated with the NIR laser. A honeycomb-type micropatterned composite has been prepared with a polydimethylsiloxane template which created a thin and smooth surface of the nanotube−gel composite where the SWNTs are buried alongside the surface of the film. Real-time photothermal behavior of the nanocomposite was observed when the laser is set to “ON’ or ‘OFF” modes (Figure 26). The photothermal behavior mainly occurs due to the high thermal conductivity of nanotubes and the low heat capacity of the microstructures.

Oil absorption property of polymer-based gel-MWNT composites were explored thanks to its swelling behavior. Pourjavadi et al. synthesized MWNT-based organogels by cross-linking polymerization of n-dodecyl methacrylate with vinyl-group-modified MWNTs or pristine MWNTs to use as an oil-adsorbent for its recovery.294,295 The hydrophobic polymer network created in the gel-nanocomposite is responsible for the oil adsorption. Thus, adsorption of oil was ensured by the swelling behavior, which showed greater swelling in the case of CNT-based gelator compared to a polymeric gelator without containing any CNTs. For example, vinyl-modified MWNTs showed adsorption up to 42.6 g toluene and 36.0 g crude oil per gram of the oil absorbent. Nishihama et al. reported photoswing extraction of lanthanides [e.g., Eu(III), La(III), and Lu(III)] through sufficient absorption and release by thermosensitive copolymers phosphoxy ethyl methacrylate and PNIPAM incorporated with SWNTs.296 Swelling and shrinking phase transition of the composite gel takes place due to the heat generation by the SWNTs upon photoirradiation. Thus, the extraction ability of different lanthanides depend upon changes in the volume and hydrophobicity of the composite gel upon photoirradiation. NIR-induced exothermicity of CNTs was shown in gelnanocomposites composed of either polymer or small molecule-based organo- or hydrogels. Nakashima and coworkers reported reversible phase transition (volume change) of SWNT-incorporated PNIPAM hydrogel by ON/OFF switching of NIR laser light for more than 1200 cycles.297 Therefore, in such gel-nanocomposite, SWNTs acted as a molecular heater via photothermal conversion and thus increased the local temperature of the gel. These authors have also reported gel-nanocomposites of SWNTs and PNIPAM hydrogels for drug absorption and release studies.298 In the composite, SWNTs acted as a molecular container, and thus it could effectively bind small hydrophilic molecules such as an antineoplastic agent, doxorubicin hydrochloride in basic aqueous media. However, the adsorbed doxorubicin molecules were released back to the bulk aqueous solution upon lowering the pH. Release of doxorubicin from the gel-nanocomposite was also shown by NIR-light irradiation due to the photothermal conversion effect of SWNTs. We have shown NIRinduced exothermicity of SWNTs leading to gel-to-sol transition of composite organogels derived from both aliphatic (1)- and aromatic (81)-based gelators (Figure 25).101,102 Due to the characteristic Van Hove singularities of SWNTs, it absorbs NIR light (700−1100 nm) and shows exothermicity due to their high thermal conductivity. Thus, NIR laser irradiation (1064 nm) induces gel-to-sol transition for the composites derived from pristine or alkylamide-functionalized SWNTs. However, due to better dispersibility (and hence greater mixing) of functionalized nanotubes in the gel matrix, it took less time for the gel melting to occur compared to that of the pristine nanotubes while the native organogel under identical conditions did not cause any of such gel-to-sol conversion. Satarkar et al. reported radio frequency induced exothermicity of nanocomposites derived from MWNTs and N-isopropylacrylamide and acrylamide cross-linked hydrogels.299 Temperature of the nanocomposite increased as a function of time and MWNTs loading (0−5 wt %). For example, the surface temperature of the nanocomposite increased from 25 to 46 °C in 4 min of 13.56 MHz radio frequency exposure. In addition, increase in the amount of acrylamide in the mixture shifted the swelling transition to

Figure 26. Photothermal behavior of the nanotube−gel composite. (a) Fast temperature control of the nanocomposite. The white arrow indicates the direction and position of the laser beam. The two white squares indicate the locations where the temperatures were analyzed. The white circle shows the location where the composite was partially destroyed by laser irradiation. Scale bars, 350 mm; laser power, 2W; wavelength, 1064 nm. (b) Temperature curves of the photo-induced nanocomposite under continuous NIR laser irradiation. Reproduced with permission from ref 300. Copyright 2009 Wiley-VCH.

Zhang et al. reported composites made of SWNT and PNIPAM to fabricate reversible, thermally and optically responsive actuators that led to the design of complex and programmable self-folding materials such as cubes and flowers.301 The composites containing a minute quantity of SWNTs (0.75 mg/mL) exhibited up to a 5-fold enhancement in the thermal response time compared to that of the pure PNIPAM hydrogel. Absorption of NIR radiation by SWNTs demonstrated manifestation of an ultrafast NIR-responsive composite hydrogels. Programmable devices were made using these composite hydrogel hinged in a complex assembly which showed cube folding and unfolding based on thermal stimulation (Figure 27). Thus, when this complex arrangement was placed inside warm water (48 °C), it led to a cube-shaped folding due to the strain induced by the shrinkage of the hydrogel patches. Reversible unfolding of the cube took place at 11998


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Figure 27. Programmable folding of the cube at 48 °C and unfolding of the same cube at 20 °C based on SWNT-PNIPAM composite gels. Reproduced from ref 301. Copyright 2011 American Chemical Society.

a temperature of the water at 20 °C. The cube folding process was significantly fast and took ∼35 s for the composite gels, which otherwise took ∼150 s for the PNIPAM hydrogel alone to complete the same function. Tan et al. developed composite hydrogels made of SWNTs and a bile salt sodium deoxycholate for the preparation of nanowires and nanopatterns by a direct printing using the hydrogel as a “solid” ink.302 This hydrogel showed excellent viscoelastic properties, and it could be extended about 50-fold along the direction of the stretching force. The viscoelasticity of the composites increased a noticeable extent (up to 106 Pa) upon increasing the amount of SWNT from 1% to 3% in the resulting composites. This indicates that these hydrogels could be used in applications as stretchable and flexible electronic devices. Solvent exchange treatment of these hydrogel wires with ethanol showed a shift from nonlinear to linear current− voltage graph indicating an ohmic response. Carbon nanotube based gels and nanocomposites have been used widely in biology particularly for acting controlled drug release, drug delivery, and tissue engineering. Cheng et al. reported DNA/SWNT hybrid hydrogel for selective sol−gel transitions based on the pH of the solution. 303 The viscoelasticity of the hydrogel could be controlled by varying the concentration of the components. Importantly, various inorganic substances could be incorporated in DNA-based hydrogels in this way. Koga et al. reported NIR-responsive gelnanocomposites made of agarose and CNTs and cell patterning technique by NIR irradiation.304 A tissue culture plate coated with the gel-nanocomposite upon irradiation with NIR laser showed heat generation by CNTs which resulted in local solation of the composite gel and exposed the selective celladhesive regions. Therefore, this method is useful for generating stepwise/complex cell patterning. Kar et al. reported the effect of pristine SWNT on the hydrogels derived from amphiphilic dipeptide carboxylate-based gelators (84a−84b, Chart 16) and enzyme activity of the resulting gel-nanocomposite.305 Due to the higher polarizability of sulfur atoms, 84a−84b formed composite gels by efficient dispersion of SWNTs. The composite hydrogel showed ∼10-fold higher mechanical strength than the native gel, indicating a reinforcement of SWNTs in the gel matrix. Immobilization of an enzyme cytochrome c inside the gel-network improved its

Chart 16. Molecular Structures of Supramolecular Gelators for the Preparation of Gel-Nanocomposites with SWNTs for Improved Enzyme Activity

activity (70-fold for 84a and 58-fold for 84b) compared to that in water alone. Interestingly, further improvements in the enzyme activity were obtained when the SWNT-composite gel was used (120-fold for SWNT+84a and 95-fold for SWNT +84b), probably due to the presence of amphiphobic SWNTs in amphiphilic networks and greater area of contact between cytochrome c and the substrate. Stimuli responsive controlled drug release from gel-SWNT composites was reported in multiple occasions. Kam et al. demonstrated NIR radiation-induced drug delivery and cancer therapy by CNT-DNA composite nanomaterials.306 Fujigaya et al. prepared polymer-based SWNT hydrogels as a molecular reservoir for controlled drug release upon NIR laser irradiation.298 SWNT-embedded hydrogels were prepared from three different polymers, poly-N-isopropylacrylamide, polyacrylamide, and poly-N,N-dimethylacrylamide. Because of debundling of SWNTs in the gel matrix, it generated a wide surface area, which adsorbed small hydrophilic molecules such as antineoplastic agent, doxorubicin hydrochloride. The doxorubicin molecules adhered with the SWNTs and were released into the solution by lowering the pH of the medium. Release of doxorubicin also took place due to a rapid volume of phase transition of the gel upon photothermal conversion of SWNTs. As the NIR light can penetrate the human body without causing any harm, the gel-SWNT composite may be potentially useful for the design of an NIR-driven controllable releasing drug reservoir. Sankar et al. prepared a composite hydrogel using an amphiphilic copolymer poly(vinyl imidazole) and carboxylic acid functionalized SWNTs in order to study drug loading and its control release behavior.86 The gelation 11999


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takes place through various noncovalent interactions such as Hbonding, π-stacking, and ionic forces to maximize the attractive forces between polymers and nanotubes (Figure 28). The

Figure 29. (i) Adhesions of the hydrogel of (A) EPE/CD and (B) EPE/CD/CNT hybrids on human skin. (ii) Phase-contrast images of L-929 cells with (A) EPE/CD hydrogel and (B) hybrid EPE/CD/ CNT hydrogel for 24 h. Reproduced with permission from ref 309. Copyright 2010 Wiley Periodicals, Inc.

(10 mg/mL) for 24 h showed good cellular growth indicating that these gels are practically noncytotoxic in nature. CNT-based hydrogels were explored extensively for biological applications, particularly for drug delivery applications.310 Several interesting work from Khademhosseini and co-workers,311 Merino et al.,312 Kostarelos,313 Mezzenga et al.,314 Wu et al.,315 and many more are reported. However, a detailed review on CNT-based gel-nanocomposites for extensive biological applications is beyond the scope of this review.

Figure 28. Schematic representation of the composite nanogel assisted by various noncovalent interactions. Inset showing the polymer with MWNTs and nanogel formed by the polymer, MWNTs, and crosslinking agent. Reproduced with permission from ref 86. Copyright 2012 Royal Society of Chemistry.

composite gels showed excellent biocompatibility and swelling behavior in water as well as in organic solvents. This enabled the hydrogel toward loading different model drugs such as an antidepressant, amitriptyline, and a micronutrient, riboflavin (vitamin B2). The drug-release was pH-dependent and the release rate was faster in acidic medium than in the neutral one. CNT-based hydrogels were shown to be noncytotoxic in nature. Shin et al. prepared reinforced hybrid hydrogels of CNT and gelatin methacrylate which provided biocompatible, cellresponsive platforms for creating cell-laden three-dimensional constructs.307 The porous structure of the gel matrix also helped spread the cell growth. The encapsulated cells present inside these hybrid hydrogels showed high levels of cellular viability, elongation, and proliferation. Kawaguchi et al. reported gel-nanocomposite with acid treated CNTs incorporated into alginate hydrogels.308 The composite gels showed higher mechanical strength compared to alginate gel and showed saline absorption capability and noncytotoxicity. Hui et al. prepared CNT-based supramolecular hydrogels through the formation of the inclusion complex with α-CD onto poly(ethylene oxide) (PEO) segments of a triblock copolymer PEO-block-poly(propylene oxide)-block-PEO(EPE) in order to use it for biomedical applications.309 It has been shown that gelation of EPE/α-CD accelerated in the presence of CNT. Good adhesive property of both hybrid and native hydrogels on the scarfskin of humans make these promising for the wound dressing applications (Figure 29). The composite hydrogel containing CNT showed a better antimicrobial activity against E. coli and Staphylococcus aureus compared to the native hydrogel. Furthermore, the composite gels showed cell viability above 60% even after 24 h of incubation at concentration of 10 mg/mL against L-929 cell growth. Phase contrast images of L929 cell treated with EPE/CD and EPE/CD/CNT hydrogels

5. GEL-NANOCOMPOSITES WITH GRAPHENES Pristine graphene is a two-dimensional planar carbon allotrope316 with outstanding material properties and has tremendous potential for numerous applications. The family of graphene includes single- and multilayered graphene, graphene oxide (GO), and reduced graphene oxide (RGO). Graphenes exhibit highly conducting, 317,318 optoelectronic,319,320 interesting mechanical properties,321 high Young’s modulus,322 impressive thermal conductivity,323 and extremely high charge-carrier mobilities at room temperature324 which enable them to be useful in diverse areas. Graphene is also a zero-band gap semiconductor showing long-range ballistic transport and holds great promise for potential applications in fabricating electronic devices, such as field-effect transistors,325 ultrasensitive sensors,326 lithium-ion batteries,327 fuel cells328 and transparent conductive electrodes used in touch screens and displays.329 Several of the above-mentioned properties supported on three-dimensional (3D) matrices derived from graphene-based assemblies are, however, highly limited to date. Therefore, composites of graphenes with small molecule or polymer gels through interaction at the molecular level could enable effective harnessing of such properties. 5.1. Preparation, Functionalization, and Gelation of Graphenes

Preparation of different graphene analogues (single- and multilayered graphene, GO, and RGO) are described briefly. “Few-layer” graphenes were prepared from the exfoliation of graphite where the individual layers were peeled off from the surface of graphite.330 Single-layer graphenes are obtained through simple mechanical exfoliation of graphite using a Scotch tape.316 Thermal exfoliation process leads to the preparation of graphene through the formation of GO 12000


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Scheme 2. Schematic Molecular Structure of Pristine Graphene, GO, and RGOa

a

For example, ref 351.

Chart 17. Molecular Structures of Supramolecular Gelators for the Preparation of Graphenes-Based Gel-Nanocomposites

intermediate as described in Hummers method.331 Thus, GO is formed through the oxidation of graphite which upon reduction at high temperature produce “few-layers” graphenes.332 Other ways to synthesize graphene includes epitaxial growth on an insulator to form multilayer graphene, ball-milling of graphite with small aromatic molecules such as melamine,333,334 chemical vapor deposition on metal surfaces, and longitudinal “unzipping” of CNTs to form single-layer graphene.117,335 Pristine graphene appears as wrinkled flakes with no functional groups attached to it. Therefore, it is highly hydrophobic in nature and thus insoluble in aqueous medium or in any other

common solvents, whereas, GO contains polar functional groups such as −OH, −COOH, and −CO− and thus is soluble in water and forms a stable brownish aqueous suspension. RGO is prepared through mild reduction of GO using N2H4, H2O2, NaBH4, or amino acids which removes most of the carbonyl groups although the hydroxyl groups still remain intact, which makes RGO hydrophilic in nature.336 However, the extent of functional groups attached to RGO depends mainly on the reduction conditions. The basic backbone structure and the attached functional group in pristine graphene, GO, and RGO is depicted in Scheme 2. 12001


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Functionalization of graphene increases its “solubility” or dispersibility in a particular solvent.335 For instance, covalent functionalization of graphene with long aliphatic hydrocarbon chains increases their dispersibility in various lipophilic organic solvents.103,337 Other chemical modification such as oxidation (to produce GO) introduces functionality such as carboxylic acid, epoxide, and hydroxide at the expense of their structural integrity. This in turn makes GO soluble in water, and in special situations, hydrogels are obtained via supramolecular interactions. Unfortunately, however, covalent functionalizations often destroy the native “graphitic” networks which affect their intrinsic properties. For example, electrical conductivity of graphene analogues vary in the order: pristine graphene > RGO > GO. The ballistic electrical transport in pristine graphene turned into nearly electrically insulating in the case of GO due to disruption of the “graphitic” networks by oxygen-containing functionalities. However, the conductivity was regained in RGO (upon reduction of GO), while the value was in some case 3times less than pristine graphene.338 This is due to the lattice vacancies created in RGO, which cannot be cured during the reduction process. The structure of RGO is therefore best described as the sheets of nanometer-sized graphitic domains separated by defect clusters as depicted in Scheme 2. Noncovalent strategies are thus generally more desirable to increase their dispersibility.339,340 Interaction of the π-surface of graphene takes place mainly through π−π stacking with different small molecules containing aromatic moieties such as pyrene,341−345 methylene green,346 perylene tetracarboxylic dianhydride, 347 and polymeric substances that include peptides348 and other polymers.349 In this regard, supramolecular gels can act as hosts for the incorporation of graphenes through intermolecular noncovalent interactions (described below). In this way, the integrity of the graphene frameworks remains intact. Therefore, pristine graphene could form gel-nanocomposites with aromatic gelators exclusively through π−π stacking interactions. However, GO or RGO could further assist gel formation additionally through hydrogen-bonding interactions. Therefore, composite gel with either pristine graphene or functionalized graphene (GO, RGO, alkyl amide chain containing graphene) possesses certain advantages and disadvantages.350 Hence, in the fields where backbone structural integrity is crucial (such as electronic transport applications), pristine graphene is to be considered while GO or RGO is to be chosen for applications such as mechanically rigid composites. Intrinsic solubility of GO is one such criterion which leads to the formation of homogeneous composites quite easily, and therefore, composites based on GO (and RGO) were developed predominantly than pristine graphene, as described below.

its various physical properties. Devoid of any functional groups, pristine graphene is hydrophobic in nature and hence the gelnanocomposites from it were derived mainly in organic media. For example, Adhikari et al. reported pyrene-conjugated oligopeptide 85 (Chart 17) for the incorporation of nonfunctionalized and unoxidized graphene.353 Gelator 85 forms transparent, stable, supramolecular fluorescent organogels in various aliphatic and aromatic hydrocarbons, including odichlorobenzene in which it forms a stable hybrid gelnanocomposite. Homogeneous dispersion of pristine graphene in the gel medium was obtained through noncovalent π−π stacking interactions with the pyrene moiety. Minimum gelator concentration of the gel decreased, and the viscoelasticity increased (∼7 times) in the presence of graphene, suggesting a favorable interaction between the graphene and the gelator in the hybrid organogels. Therefore, molecular structure of gelators is a crucial factor which decides the interactions with graphenes to be either favorable or unfavorable. Thus, two structurally different gelators based on all-trans tri-p-phenylenevinylene bis-aldoxime (81) and n-lauroyl-L-alanine (1) were employed by us for the gel-nanocomposite synthesis with exfoliated graphene (EG) and dodecyl- and hexadecyl-chain functionalized EG (EG12 and EG16, respectively).103 The nature of interactions between the gelators and graphene analogues were also compared with that of SWNTs and fullerene. These carbon nanomaterials were incorporated in the preformed organogels of the individual gelators in toluene. Gelation of individual compounds takes place through hydrogen-bonding and van der Waals interactions for both 81 and 1 in addition to π−π stacking interaction specifically for 81 (Figure 30). Graphenes, SWNTs, and fullerene possess extended π-conjugated aromatic surface, and hence, they interact efficiently with 81 through π−π stacking interactions to the formation of densely wrapped carbon nanomaterial encapsulated fibrous networks. Thus, the interaction was pronounced in case of the aromatic gelator 81 compared to the aliphatic gelator 1. Exfoliation of graphene sheets took place due to the π−π stacking-mediated association of graphene sheets with gelator 81. The gel-nanocomposites showed significantly increased electrical conductivity compared to that of the organogel alone, even if a trace quantity of carbon nanomaterial was doped in the gel matrix. Rheological studies of the composites confirmed the formation of rigid and viscoelastic solid-like assemblies due to the reinforced aggregation of the gelators on carbon nanomaterials. Importantly, a synergistic behavior was observed in case of the composite gel of 81 containing a mixture of EG and SWNT when compared with mixtures of other carbon nanomaterials in all combinations with EG (Figure 30b). Thus, these gelnanocomposites provide significant understanding of the nature of molecular level interactions between dimensionally different carbon nanomaterials with structurally different gelators and allow achievement of tunable properties in the supramolecular nanocomposites. This type of comparative study involving both graphene and SWNTs for the preparation of gel-nanocomposites has also been performed by Zhang et al. by involving a bi-1,3,4-oxadiazole-based organogelator 86.354 While incorporation of SWNTs into the organogels of 86 induced aggregation of the gelator molecules, incorporation of GO particularly induced formation of J-aggregation, which led to a lower CGC and different morphologies compared to the native organogel.

5.2. Supramolecular Gelator Based Gel-Nanocomposites with Graphenes

Hydrogels or organogels derived from aromatic small molecular gelators were used as hosts for the incorporation of graphene analogues in the development of gel-nanocomposite materials. Self-standing thermoreversible gels are formed based on various noncovalent intermolecular forces such as π-stacking, hydrogen bonding, van der Waals, and electrostatic interactions.352 Dispersion of graphene into the gel matrix could occur via π−π stacking interactions between the gelators and graphene analogues leading to exfoliation of the graphene layers through supramolecular complex formation. A minute quantity of graphene could significantly reinforce the gel matrix and tune 12002


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compared to the native gel. These authors also reported stabilization of GO within a dual stimuli-responsive organogel from 1-octadecyl-ureido-naphthalene (89) which brought about interesting changes in mechanical strength, fluorescent emissions, as well as thermal stability.356 Recently, Banerjee and co-workers employed pyrene-based gelator 90 for the incorporation of carbon nanomaterials such as RGO and pristine SWNTs.357 The self-healing, thixotropy, and stiffness property of the native hydrogel could be tuned in the presence of carbon nanomaterials; for example, the selfrecovery time of the gel was shortened by the inclusion of either RGO or pristine SWNTs or both. These authors also used a pyrene-appended tryptophan-based gelator (91) to develop functional trihybrid hydrogel system incorporated with GO and Au nanoparticles that showed catalytic activity toward the reduction of aromatic nitro-compounds.358 The pyrene moiety took advantage of the aromatic surface of GO to induce π−π stacking interactions, while the tryptophan moiety mediated in situ reduction of Au+ to form gold nanoparticles giving rise to the trihybrid system. The stiffness of the composite gels increased in a stepwise fashion upon progressive increase of mixing of the native gel with the nanomaterials. They also reported amino acid appended perylene bisimide based hydrogelator (92) to prepare gel-nanocomposites with GO and RGO, and the resulting composite showed photoswitching behavior.44 The ohmic nature of the I−V curve for the native gel could be tuned in the presence of GO and RGO and thus the nanocomposites showed a greater extent of current in the dark. Interestingly, the native gel as well as the nanocomposites showed photoswitching behavior upon exposure to white light. The device performed reversibly when the white-light was turned “on” and “off”, which also showed a steady and reproducible photoresponse. Cheng et al. reported gel-nanocomposites in both hydrogel and organogel from GO and pyrene-appended sugar-based gelator 93.359 Gelation took place because of the strong π−π stacking interactions between pyrene and graphene surface as well as hydrogen-bonding interactions between the hydrophilic groups. The hydrogelbased composite showed dye-absorption ability when tested with basic fuchsine as a model dye. Das and co-workers reported pyrene-appended fluorescent, ambidextrous gelator 94 for the gel-nanocomposite preparation with GO and RGO.360 Presence of complementary π-stacking interactions between the π-electronic surface of graphene sheet and the pyrene moieties of the gelator induced the formation of composite gels with improved viscoelastic properties. Wu et al. reported gelnanocomposites of ethynyl-pyrene-modified platinum−acetylide organogelator 95 and RGO by the strong interaction between aromatic ethynyl-pyrene moiety of the gelator and the large aromatic surface of graphene through charge transfer interaction.361 Rajamalli et al. reported linear sugar (glucose)based aryl ether dendron 96 for the incorporation of GO to prepare gel-nanocomposites, which showed improved mechanical strength compared to the native gel.362 Therefore, the above examples describe preferable interaction of graphene analogues with aromatic gelators leading to the formation of gel-nanocomposites which showed interesting optical, mechanical, and electrical properties.

Figure 30. (a) Bar diagram showing the storage modulus (G′) of gels of 81 with carbon nanomaterials and mixtures of carbon nanomaterials obtained under oscillatory frequency sweep experiment (G′ values are at 10 rad/s, 0.83 wt % carbon nanomaterials were added individually, [81] = 10 mg/mL); inset showing gel of 81 in toluene and its composite with EG. (b) Electrical conductivity showing I−V characteristics of 81+EG (left Y axis), 81, 81+EG12, and 81+EG16 (right Y axis). [81] = 10 mg/mL, [EGs] = 0.99 wt % with respect to 81 (“1” in the figure represents gelator 81). Reproduced with permission from ref 103. Copyright 2012 Wiley-VCH.

Extensive preparation of gel-nanocomposites based on GO and RGO were achieved mostly in hydrogel medium. Hydrogels derived from Fmoc (N-fluorenyl-9-methoxycarbonyl) protected dipeptides 87a and 87b (Chart 17) have been used by Adhikari et al. to incorporate RGO.355 The aromatic moieties present in the amino acid side chain (Tyr/Phe) and in the N-terminus (fluorenyl group) presumably induced effective π-stacking interactions with exfoliated graphene sheets leading to a homogeneous composite preparation. Improved mechanical rigidity due to the formation of “solid-like” hybrid hydrogel was obtained after incorporation of a small amount of RGO into the native hydrogel. In another report, Xing et al. reported Fmoc linked amino acids (88) to be a gelator in a mixture of “good” solvents such as dimethyl sulfoxide, tetrahydrofuran, acetone, etc. and “bad” solvents like water.39 The intrinsic aggregation induced emission of the gelator was quenched about 2500 times on addition of GO, and thus, efficient crosslinking between gel-nanofibers and GO sheets was obtained in a homogeneously dispersed nanocomposite. This also indicates the presence of strong π−π stacking interactions between the gelators and GO sheets which was further reflected from the rheological behavior; for example, 2 wt % of Fmoc−Glu gel incorporated with 0.2 wt % GO showed a 10-fold increase in G′

5.3. Polymer-Based Gel-Nanocomposites with Graphenes

Gelation of graphene analogues was observed in several instances with the assistance of different polymeric systems. In each case, the polymer-graphene composites emerge as new 12003


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Figure 31. Photographs of (i) a PVDF-HFP gel and (ii) a PVDF-HFP/graphene gel. SEM images of (a) PVDF-HFP and (b) PVDF-HFP/graphene dried gels. The inset optical images show a water drop sitting on these porous dried gels. The contact angles of the PVDF-HFP and PVDF-HFP/ graphene hybrid composites are 132.7 ± 2.7 and 151.6 ± 1.4°, respectively. (c and d) Magnified surface morphology of PVDF-HFP/graphene microspheres. Reproduced from ref 363. Copyright 2011 American Chemical Society.

Figure 32. (i) Photograph of (A) reduced graphite oxide (1.0 mg/mL) and (B) copolymer-coated graphene in aqueous solution. (ii) Hybrid supramolecular hydrogel (on left) compared with the native polymer hydrogel. (iii) Proposed structure of the copolymer coated graphene (a) and supramolecular well-dispersed graphene sheet containing hybrid hydrogel (b). Reproduced from ref 367. Copyright 2009 American Chemical Society.

nanomaterials with novel properties. Gelation was achieved in both organic solvents and mostly in aqueous media. While gelnanocomposites were obtained from each of the graphene analogues mentioned, GO based composites were largely conceived probably because of its easy preparation and high aqueous solubility. Apart from several polymers, different small molecules and ionic species also induce gelation of graphenes, particularly GO. The effect of external stimuli on the gelation was shown to be multifaceted. These will be discussed in the following sections. We first describe pristine and reduced

graphene oxide based composites followed by GO-based gelnanocomposites. Zhang et al. used pristine few-layer graphene to prepare superhydrophobic poly(vinylidene fluoride-hexafluoro-propylene) (PVDF-HFP)/graphene composite microspheres with uniform size distribution through gelation in organic solvent.363 Hybrid gel was obtained through the absorption of water vapor by the suspension of PVDF-HFP/graphene (0.25 wt % with respect to PVDF-HFP) in DMF (Figure 31). The hybrid gel showed microspheres with a diameter of 8−10 μm after solvent 12004


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adhesive to form the composite. Thus, the gel-nanocomposites showed improved mechanical performances. Shen et al. used starlike block copolymer PEO−PPO−PEO for its supramolecular hydrogelation with α-cyclodextrin and gel-nanocomposite formation with graphene as well as GO.369 Inclusion complex formation by α-CDs and PEO blocks led to supramolecular polymer networks via physical cross-links, and the resulting hydrogel was used to incorporate graphene and GO. Different properties like phase behavior, morphology, and mechanical strength of the native hydrogel was shown to be modulated to a greater extent by GO compared to graphene. The GO-based gel-nanocomposite also showed absorption of water-soluble dyes. Kuwahara et al. reported gel-nanocomposite of electrically conducting RGO and organically dispersible polyaniline incorporated in small molecule based two-component organogel 97 (Chart 17).370 The native gel retained its thixotropic behavior even after incorporation of the nanomaterials. Electrical conductivity of the as prepared gel (2.5 × 10−5 S cm−1) improved significantly upon drying (3.2 S cm−1). Interestingly, the composite gel could be filled into a ballpoint pen and the lines drawn from this pen also showed electrical conductivity. This may have future applications in electrical soft materials. Several instances of nanocomposite hydrogels were reported involving GO. Due to the presence of hydrophilic functionality (−COOH, −OH, −CO−), GO can interact with hydrophilic groups in the polymer. This led to physical cross-linking among the polymer domains, and thus, 3D networks were formed which often appears as hydrogels. Several aspects of such GOpolymer hydrogels were described in the following sections, including thermal, mechanical, electrical and stimuli responsive (e.g., pH) properties. Thermal and pH-Regulated GO-Polymer Hydrogels. Sun et al. developed GO interpenetrating PNIPAM hydrogel networks through an efficient cross-linking reaction.371 Resulting hydrogels showed improved mechanical strength, and they exhibited dual thermal and pH response with good reversibility. Incorporation of GO sheets into PNIPAM microgel networks showed a uniform dispersion, and it manifested an increased sol-to-gel transition temperature. GO/PNIPAM hydrogel and PNIPAM microgel network are shown to be pH sensitive. As the pH increases, the sol−gel transition temperature increases. With increasing pH, ionization of the carboxylic acid groups increases, which reduces the number of their hydrogen bonds resulting in a much more swollen structure or a higher transition temperature. Mechanical Properties of GO-Polymer Hydrogels. Zhang et al. reported the role of graphene oxide nanosheets as the nanofillers for the enhancement of mechanical properties in composite materials.372 The GO/polyacrylamide hydrogels were prepared through an in situ radical polymerization of acrylamide in the presence of GO nanosheets. In the composite, polyacrylamide macromolecules were grafted onto the GO nanosheets by a radical chain transfer reaction during polymerization. Due to this, the compatibility between the polymer and GO improved and, consequently, the composite hydrogels displayed superior mechanical properties in both dry and wet states. The compressive strength of the soft composite hydrogels increased by 6-fold in comparison to that of the pure polymer. Zhang et al. also prepared GO/PVA composite hydrogels by a freeze/thaw method.373 Compared to the pure PVA hydrogels, the composite hydrogels showed an increase in

exchange and freeze-drying. In contrast, the gel of pure PVDFHFP showed a cellular morphology by the liquid−liquid demixing process. Thus, addition of graphene sheets induced the formation of spherical microstructure in the PVDF-HFP/ graphene gels. The aggregation of PVDF-HFP chains around the entangled graphene sheets suggests a strong interaction in between them. The hybrid gel exhibits superhydrophobicity due to the presence of microspheres with nanoscaled surface roughness. Therefore, this hybrid gel could find potential application as a catalyst-supporting material with water repellence, or it can be dispersed in polymer solutions as an additive to produce superhydrophobic polymer coatings. Das et al. reported polymerization of acrylamide monomer in polyacrylamide-stabilized pristine graphene dispersion for the production of graphene-polyacrylamide hydrogels in an in situ polymerization process where pristine graphene acts as a physical cross-linker.364 The polymer chains interact with the graphene surface through physisorption process, and thus, the graphene sheets were dispersed homogeneously in the hybrid hydrogel. These hybrid hydrogels show improved mechanical stability and toughness and exhibit self-healing and swelling properties. However, this type of improvement in the mechanical stability was not observed in the case of gelnanocomposites prepared by incorporating graphene nanoplatelets in a triblock copolymer gel consisting of poly(methyl methacrylate)−poly(n-butyl acrylate)−poly(methyl methacrylate), as reported by Zabet et al.365 In this case, graphene nanoplatelets influence the self-assembly of the polymer as shown by the decrease in gelation temperature, while they did not show any elastic contribution to the gel-nanocomposites. In another instance, Alzari et al. prepared stimuli-responsive PNIPAM hydrogels containing partially exfoliated graphite by frontal polymerization.366 An increase in the amount of graphene in the composite in turn increases the maximum temperature reached by the front (Tmax). In addition, a sharp increment in the Tmax is observed at a concentration of graphite greater than ∼0.07 wt %. The graphite-containing samples exhibit a lower critical solution temperature (LCST) than that of the neat PNIPAM hydrogel (28 °C). However, LCST increases with an increasing amount of the graphite fillers. Zu et al. reported a facile strategy to disperse graphene by using commercially available triblock copolymer (poly(ethylene oxide)-block-poly(propylene oxide)-block-poly(ethylene oxide), PEO-b-PPO-b-PEO).367 The polymer coating on graphene produced a stable aqueous dispersion (Figure 32). This could be due to the presence of hydrophobic PPO segments, which were bound to the apolar surfaces of graphene via hydrophobic effect, whereas the hydrophilic PEO chains were extended into water. Thus, due to having a dual role for this pluronic copolymer, it could form supramolecular hydrogel with α-CDs through the penetration of PEO chains into the cyclodextrin cavities. Therefore, this hybrid hydrogel retained the basic rheological characteristics of supramolecular hydrogel such as the shear-thinning property, while the viscoelasticity of the hybrid hydrogels decreased significantly as compared to those of the native hydrogel. However, the supramolecular hybrid hydrogel could be transformed into a flowing sol by increasing temperature. In a similar strategy, Li et al. reported covalent functionalization of GO with poly(γ-benzyl-L-glutamate) by using a combination of ring-opening polymerization and amidation reactions.368 The resulting functionalized GO formed organogel in the presence of the unbound poly(γbenzyl-L-glutamate) where the GO platelets acted as 2D 12005


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nature due to its hydrophilic edges and hydrophobic basal plane.381 This makes GO a promising 2D-building block for the generation of various graphene-based supramolecular architectures.382 Gelation of GO could take place through the supramolecular interaction of GO with small molecules, surfactants, metal ions, ionic liquids, various cross-linkers, and even the effect of pH (Figure 33).383 Several of these factors leading to GO-based gels are described below.

tensile strength, breaking elongation, and compressive strength. Thus, as much as 132% increase in tensile strength and 36% improvement in compressive strength were achieved at 0.8 wt % of GO doped into the PVA. Shen et al. incorporated GO into poly(acrylic acid) in order to improve the mechanical properties of the native polymer hydrogel.374 The otherwise weak and brittle polymer hydrogel of poly(acrylic acid) turned tough and exhibited good strength upon incorporation of GO. Electrical Properties of GO-Polymer Hydrogels. Wu et al. reported grafting of chemically modified graphene with 2aminoanthraquinone followed by mild reduction.375 The resulting conjugate formed self-assembled macroporous hydrogel, which was used for supercapacitor electrodes. The composite electrode showed a high specific capacitance of 258 F/g at a discharge current density of 0.3 A/g, which is much greater than pure graphene hydrogels (193 F/g). The composite hydrogel-based electrode exhibited improved rate capability of charge/discharge performance with excellent cycle stability. The redox capacitance ability of the composite gel occurred primarily due to the covalently bound 2-aminoanthraquinone moieties. The charge transfer and electrolyte diffusion took place due to the highly conductive reduced graphene hydrogel scaffold, which provided a large specific surface area to form electrical double layers. Stimuli Responsive Hydrogels. Lo et al. described a lightresponsive PNIPAM hydrogel nanocomposite formation with glycidyl methacrylate functionalized GO.376 A large volumetric change of this nanocomposite hydrogel was observed upon infrared light illumination, which ensured a highly efficient photothermal conversion. The composite hydrogel also showed a large swelling ratio by water uptake. The transition temperature of the gel is decreased by ∼10 °C due to the addition of GO. It has been demonstrated that the application of such hydrogel nanocomposite in a microvalve in a microfluidic channel can control the fluidic flow within the microchannel through remote IR light actuation. In another report, Wang et al. reported supramolecular hydrogelation of ethylenediamine appended RGO in the presence of β-CD and poly(acrylic acid) carrying 2 mol % of the n-dodecyl group.377 This graphene-based chemically responsive supramolecular hydrogel undergoes gel-to-sol transformation upon addition of a competitive guest sodium adamantane carboxylate or competitive host α-cyclodextrin (α-CD). Multifunctional GO-Hydrogels. Xu et al. prepared multifunctional self-assembled graphene hydrogel by one step hydrothermal reduction of GO dispersion in water.378 The gel contained ∼2.6% of graphene sheets and 97.4% water, and thus, it is highly hydrated in nature. The hydrogel shows high electrical conductivity (5 × 10−3 S/cm), impressive thermal stability in the temperature range of 25−100 °C, and its storage modulus (450−490 kPa) is about 1−3 orders of magnitude higher than those of conventional self-assembled hydrogels. The gel acted as a 3D supercapacitor electrode, exhibiting high specific capacitance (175 F/g) in an aqueous electrolyte.

Figure 33. Possible ways of hydrogel formation by GO and the morphology as revealed under SEM image. Reproduced from ref 383. Copyright 2011, American Chemical Society.

Effect of pH. Depending on the pH of the medium GO can form hydrogel on its own. As stated, due to the presence of hydrophilic groups (such as −OH, −COOH, and −CO−) GO shows amphiphilic character and as a result it forms a stable colloidal dispersion in aqueous medium. However, aggregation of the GO sheets in aqueous medium is inhibited by the electrostatic repulsion between the ionized carboxyl groups.383 Therefore, acidification of the GO dispersion weakens the electrostatic repulsion and leads to the formation of an aggregated state. The zeta potential (ζ) increases with decreasing pH of the GO dispersion, indicating the protonation of the COO− groups (Figure 34). In strong acidic medium,

Figure 34. Zero-shear viscosities and zeta potentials of a GO (5 mg/ mL) aqueous solution at different pHs. Insets show the snapshots of a GO solution at pH = 4.6 and a hydrogel at pH = 0.6. Reproduced from ref 383. Copyright 2011 American Chemical Society.

GO sheets undergo aggregation and phase separation because of insufficient mutual repulsion. However, at higher concentration of GO (>4 mg/mL), hydrogel formation was observed upon acidification, as confirmed by the tube inversion method. The zero-shear viscosity also increases with decreasing pH of the solution. Therefore, the gelation ensured the formation of 3D GO networks.384 Effect of Dimension. Hydrogel formation was shown to depend on the lateral dimension of GO sheets.383 A stable hydrogel on acidification was achieved for GO sheets having the lateral dimensions of several micrometers. Gelation did not

5.4. Gelation of Graphene Oxide

Apart from small molecule and polymer-based gelators for the gelation of graphene analogues, several other small molecular/ ionic nongelator additives also induce gelation of mainly GO in aqueous medium. Thus, in recent years, gelation of GO has attracted much attention toward gel formation. GO is particularly important due to its unique structure and its physical and chemical properties.379,380 GO is amphiphilic in 12006


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take place irrespective of pH if the lateral dimensions of most of the GO sheets are smaller than 1 μm even if the concentration of GO is as high as 9 mg/mL. In this case, the acidified solutions of small GO sheets turn cloudy in several minutes and subsequently precipitate. Due to the amphiphilic nature of GO, it self-assembles in a dilute solution leading to a weak dynamic network depending on the balance between the electrostatic repulsion and binding interactions (hydrogen bonding, πstacking, hydrophobic effect, etc.). Thus, gelation occurs through a dominating reinforcement of the GO network by enhancing the bonding forces and weakening the repulsion force. Both gelation and precipitation of GO sheets may be induced by the optimization of these two opposing forces, which primarily depend on the stacking states of the GO sheets. In the hydrogel the GO sheets are randomly orientated, whereas they adopt a parallel arrangement in their precipitates. The parallel stacking mode is energetically more favorable because of the larger area of contact between the GO sheets. The mobility of large GO sheets in solution is strongly limited and thus precipitation is a kinetically slower process. Thus, a very slow precipitation process occurs after 2 weeks of the formation of hydrogels. However, if the sizes of GO sheets are reduced (smaller than 1 μm), they adopt parallel orientations in solution more easily and favor precipitation. Effect of Cations. A variety of metal ions were shown to induce gelation of GO. While, monovalent ions (e.g., K+, Li+, and Ag+) cannot induce gelation of GO, divalent and trivalent ions (e.g., Ca2+, Mg2+, Cu2+, Pb2+, Cr3+, and Fe3+) promote the formation of GO hydrogels.383 The gelation takes place through the coordination of metal ions with hydroxyl and carboxylic acid groups on the GO sheets, which is the primary driving force for the assembly of the GO sheets. Greater coordination stability of the multivalent transition-metal ions than the alkali and alkaline-earth metal ions induces stronger cross-linking abilities for the former ions leading to gel formation. Jiang et al. reported gelation of RGO sheets connected with divalent cations (Ca2+, Ni2+, or Co2+) and water molecules.385 In the gels, the RGO sheets, water molecules, and divalent ions play the role of skeleton, filler, and linker, respectively, through the formation of chemical bonds and hydrogen bonds. RGO sheets were obtained through partial reduction of GO (obtained by Hummers method) with H2O2. The RGO sheets in the gels mainly exist as single or double layers by the isolation of the interlamellar water molecules as shown in the proposed model (Figure 35). A large amount of the water content in the gel (∼99 wt %) facilitates the introduction of a polymeric strengthening agent such as poly(vinyl alcohol) which could lead to the generation of microporous structures of the freeze-dried gels. Cong et al. reported one-step fabrication of macroscopic multifunctional graphene-based hydrogels through reduction of GO by ferrous ions (α-FeOOH) and also in situ deposition of nanoparticles (Fe3O4) on graphene sheets at different pH under mild conditions.386 These gel-nanocomposites were capable of removing pollutants and thus applicable as adsorbents for water purification. Effect of Surfactants. Guardia et al. reported nonionic surfactants such as Brij 700- and P-123-mediated exfoliation and dispersion of pristine graphene in water at significant concentrations (1 mg/mL).387 The increase in the amount of graphene suspension in water with sonication time was also observed for surfactants including Brij 700, hexadecyltrimethylammonium bromide (HTAB), poly(ethylene glycol)-poly-

Figure 35. Schematic illustration of RGO gelation with divalent metal ions. M2+ represents the divalent ion (Ca2+, Ni2+, or Co2+). Reproduced from ref 385. Copyright 2010 American Chemical Society.

(propylene glycol)-poly(ethylene glycol) (P-123), poly(styrenesulfonate) (PSS), sodium dodecylbenzenesulfonate (SDBS), taurodeoxycholic acid sodium salt(TDOC), and polyoxyethylene sorbitan monooleate (Tween 80). These dispersions are stable for several weeks and are constituted by single- and few-layer graphene sheets with “defect”-free basal planes. Vacuum filtration of these dispersions through polycarbonate membranes leads to potentially useful macroscopic materials, such as paper-like films, which has significant electrical conductivity. Supramolecular hydrogels were prepared from the P-123 stabilized graphene dispersions through the formation of inclusion complexes between α-cyclodextrin and the poly(ethylene oxide) (PEO) chains. However, in the films, the electrical contacts between the neighboring graphene sheets are hindered by the presence of surfactant molecules and this restricts the rise in the electrical conductivity. Jiao et al. reported self-assembled organogel formation from composites of GO and various cationic amphiphiles such as cetyltrimethylammonium bromide, cetylpyridinium bromide, and 1,1′dihexadecyl-4,4′-bipyridinium bromide as well as gemini amphiphiles.388,389 Self-assembly of the GO-based composite was shown to depend on headgroups of the amphiphiles and the ammonium headgroup was more favorable for gelation over the pyridinium headgroup. Effect of Ionic liquids. Homogeneous nanocomposite gel has been obtained from a mixture of graphene and an ionic liquid 1-butyl-3-methylimidazolium hexafluorophosphate.390 A gel was prepared from 16 mg of graphene mixed with 1.6 mL of the ionic liquid in an agate mortar upon grinding for 30 min. A uniform nanocomposite was formed through cation−π and/or π−π interactions between graphene and ionic liquid (Figure 36). The nanocomposite gel shows porous structures, and thus, it can act as a stoichiometric electron-acceptor for the electrondonating guest species such as nitric oxide. Due to the presence of a large electroactive surface area of the nanocomposite gel electrode, it shows a fast response, high sensitivity, and low detection limit for nitric oxide. The high performance of the gel might be due to its porous structure, high specific surface area, and superior conductivity. These results suggest that the graphene/ionic liquid nanocomposites are promising materials for sensitive nitric oxide detection and can also be an advanced electrode material in various other electrochemical devices. 12007


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Figure 36. Schematic representation of the fabrication of the 3D graphene/ionic liquid nanocomposite as well as its photograph. Reproduced with permission from ref 390. Copyright 2011 Wiley-VCH.

Chart 18. Small Molecular and Polymeric Cross-Linkers for GO Gelation

Effect of Cross-Linkers (Small molecules/Polymers). Addition of a cross-linker (Chart 18) can increase the binding force among the GO sheets leading to the increased gelation abilities.383 Thus, addition of a small amount of poly(vinyl alcohol) (PVA) could induce gelation of GO due to the additional hydrogen-bonding interactions between the GO sheets and PVA chains. Hydrogels are also obtained by mixing small amounts of poly(ethylene oxide) (PEO), hydroxypropylcellulose (HPC), and poly(vinylpyrrolidone) (PVP) into the GO dispersions. Among these three polymers, PEO and PVP are hydrogen-bond acceptors, whereas HPC is both a hydrogen bond donor and an acceptor. Formation of GO/polymer composite hydrogels is controlled by several factors. These include the concentration of GO, pH, the concentration and the molar mass of the polymer. With increasing molar masses of the polymer cross-linker, the viscoelasticity of the hydrogels increases. A longer polymer chain may induce more hydrogen bonds with the GO sheets and has a larger probability of interacting with two or more GO sheets. Thus, HPC is an effective cross-linker for the formation of a GO hydrogel, whereas monosaccharide glucose fails to become a cross-linker because of its smaller molecular size. Electrostatic interactions also play significant role on the supramolecular self-assembly of the graphene sheets. Polydimethyl diallylammonium chloride (PDDA) acts as an efficient cross-linker, which is devoid of any hydrogen bond forming groups.383 Only electrostatic interactions between the quater-

nary ammonium groups of PDDA and the carboxylic acid groups of the GO sheets are available to induce gelation. In the case of polyethylenimine, hydrogen bonding and electrostatic interaction between the GO sheets and polyethylenimine chains leads to hydrogel formation. GO hydrogels also form in the presence of a small amount of quaternary ammonium salts, such as cetyltrimethylammonium bromide (CTAB) and tetramethylammonium chloride (TMAC) because of the long-range electrostatic force. Furthermore, melamine, which is a strong hydrogen-bonding acceptor and multiaminated small molecule in water, can also exhibit good cross-linking ability, leading to the formation of GO hydrogels. Recently, vitamin C was also used for the reduction and stabilization of GO to form RGO hydrogels.391 The electrical properties depend on the appropriate interlayer distance and grain size of GO as the vitamin C molecules could enter in the internal spaces between the layers for the reduction process. Huang et al. reported GO hydrogels using glucono-δ-lactone as hydrogel promoter in the presence of either multivalent metal ions (e.g., La3+, Co2+, and Ni2+) or protonized polyamines [e.g., polyethylenimine, melamine, and poly(amidoamine)].392 These hydrogels showed stimuli responsive selective reversible gel−sol transformation on addition of EDTA. Gelation-Assisted Isolation of Graphene from Graphene-GO Mixture. A simple and rapid coagulation technique has been developed to separate graphenes from GO and other impurities through the formation of GO-based 12008


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hydrogels.393 GO has been prepared through Hummers method and reduced with NaBH4 followed by N2H4 treatment. This yields a partially reduced GO containing some remnants of oxidized graphene moieties including small nitrogen impurities. This was suspended in deionized water and sonicated for 10 min. To this dispersion, a poly(vinyl alcohol) (PVA) solution in water was added and the mixture was heated. When the temperature of the solution reached 45−50 °C, graphene floated on the surface of the solution, and GO settled down rapidly (Figure 37). The floating graphene layer appeared Figure 38. (a) Photograph of the oriented graphene hydrogel films peeled off from the filter membrane; (b) SEM image of the crosssection of a freeze-dried graphene hydrogel film. Scale bar: 1 mm. Reproduced with permission from ref 394. Copyright 2011 WileyVCH.

repulsions. The directionally oriented conductive gel film possess exceptional chemical and structural stability, high electrical conductivity (0.58 S/cm), and mechanical flexibility which make it more suited for the use in flexible electronics and biomedical devices compared to the 3D rigid hydrogels. Hou et al. incorporated magnetic nanoparticles into the hybrid gel network through one-step solvothermal method for the synthesis of high strength graphene/Fe3O4 composites.395 In the composite, the spherical Fe3O4 particles were distributed randomly on the surfaces and edges of the graphene nanosheets. PNIPAM/graphene/Fe3O4 gels (P/G) were prepared by solution polymerization using the as-prepared graphene/Fe3O4 hybrid as a cross-linker. In addition, to compare the properties of the nanocomposite, PNIPAM/ hectorite (P/H) composite hydrogel was prepared using hectorite as a cross-linker. The gel without any cross-linkers was broken into pieces under an applied external pressure (Figure 39). However, the P/H gel (b) and P/G gel (c) showed high strength and excellent resilience on the response of the external pressure, indicating an effective wrapping of the polymer chains around the graphene/Fe3O4 cross linker. Furthermore, the dried P/G gel showed a good response (attraction) to the external magnet. Therefore, owing to these properties, the graphene/Fe3O4-functionalized hydrogels are highly promising for the light-driven and magnetically controlled switching in microreactors. Biomolecules were also employed for the preparation of graphene-based gel-nanocomposites. Huang et al. reported stable composite hydrogels of GO sheets with hemoglobin.396 Strong electrostatic attraction between the positively charged hemoglobin and negatively charged GO sheets is responsible for the formation of composite hydrogel at pH 1.87, which is much lower than the isoelectric point of hemoglobin (pH 6). The hydrogel showed a higher activity and stability than free hemoglobin or GO for catalyzing oxidation of pyrogallol by H2O2 in organic solvents. Thus, the hydrogel provided an aqueous microenvironment that protected the enzyme from deactivation, and it acted as a transit station to attract the substrate and exclude the product. Liu et al. reported hemoglobin and graphene-based hydrogels containing chitosan-dimethylformamide (CS-DMF) for an amperometric nitrite (NO2−) sensor.397 The surface morphologies of the modified electrode (hemoglobin-CS-DMF/graphene film) exhibited a three-dimensional porous structure. The electrochemical behavior of the composite indicated that bioactivity of hemoglobin remained intact on the surface of the graphene-

Figure 37. PVA-mediated separation of graphene. GO reinforced PVA hydrogels were settled at the bottom of the container. Reproduced with permission from ref 393. Copyright 2010 Elsevier.

as fluffy flakes, which were collected on a substrate and dried. The precipitate was a gelatinous sticky mass of PVAfunctionalized GO, which formed graphene-reinforced PVA hydrogels. Due to the presence of hydrophilic functionalities in the GO layers, the water molecules can readily intercalate into the interlayer galleries. The hydroxyl groups in PVA are hydrogen-bonding acceptors, which increased its interaction with the water molecules making PVA soluble in water. Therefore, the hydroxyl groups of PVA form strong hydrogen bonds with the hydroxyl and carboxylic acid groups present on GO, thereby forming hydrogen-bonded cross-links between GO and PVA molecules leading them to the hydrogel formation. 5.5. Applications of Graphenes-Based Gel-Nanocomposites

Graphene-based hydrogels were shown to have multiple applications. Due to the unique properties of graphene and graphene analogues, the gel-nanocomposites find potential applications in the area of electronics, dye-absorption and selfhealing, high strength and magneto-responsive materials, sensing devices and in biology. Yang et al. reported selfgelation of chemically converted graphene (CCG, obtained by the reduction of mildly oxidized graphene oxide) during filtration at the solution−filter membrane interface which showed mechanically flexible, high electrical conductivity.394 CCG sheets are deposited in a sheet-by-sheet array on the filter membrane (Figure 38). A layer of water molecules could be placed in between the CCG sheets due to the presence of hydrophilic oxygen-containing groups on the CCG surface. Thus, the hydrated CCG sheets in the film are linked by a partial π−π stacking interaction and a strong hydration force by the layer of water present in between prevent them from being fully collapsed. It has been observed that the thickness of such gel film increases with increasing pH of the solution. This is because with increasing pH the ionization of the carboxylic groups increases which induces greater intersheet electrostatic 12009


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additional stability to the gel. The GO/DNA gel showed effective dye adsorption. A 0.2 mL of the gel in a 0.6 mL of an aqueous solution of safranine O (0.1 mg/mL) showed ∼80% dye absorption after 12 h at room temperature. The dye absorption reached nearly 100% after 24 h as confirmed from the digital images and absorption spectra (Figure 40, panels a

Figure 39. (i) Digital images of the PNIPAM gels using (a) no crosslinker, (b) hectorite as a cross-linker and (c) graphene/Fe3O4 hybrids as a cross-linker. (ii) Photographic image showing response of the PNIPAAm/graphene/Fe3O4 gel to a magnet. Reproduced with permission from ref 395. Copyright 2011 Elsevier.

Figure 40. (a) Photographs and (b) absorption spectra of an aqueous solution of safranine O after adsorption by GO/DNA gel at different times; 0.6 mL of dye solution (0.1 mg/mL) was added onto 0.2 mL of the gel. (c) Self-healing process: the as-prepared free-standing gel was cut with a razor into three small blocks and the blocks could adhere to each other by pushing the freshly formed surfaces to contact together followed by heating at 90 °C in air for 3 min. Reproduced from ref 398. Copyright 2010 American Chemical Society.

modified electrode as it exhibited a surface-controlled electrochemical behavior and showed fast heterogeneous electron transfer rate. Thus, graphene facilitates the electron transfer between hemoglobin molecules due to the excellent electrical conductivity of graphene. As a result, the composite showed a good sensitivity and stability toward the amperometric determination of nitrite. Xu et al. reported a facile 3D self-assembly of GO and DNA to form composite hydrogels having high mechanical strength, excellent environmental stability, high dye-adsorption capacity, and self-healing function.398 The hydrogel has been prepared by mixing equal volumes of aqueous dispersion of GO (6 mg/ mL) and double-stranded DNA (10 mg/mL) followed by heating the mixture at 90 °C for 5 min. Upon heating, the double-stranded DNA unwound to single-stranded DNA, which bridged adjacent GO sheets via strong noncovalent interactions to produce stable hydrogels. Thus, the in situ generated single-stranded DNA acted as “glue” for the binding of GO sheets to form 3D networks. The GO/DNA hydrogel was stable in a variety of harsh conditions such as strong acidic (pH 2), basic (pH 13), or salty (1 M NaCl) aqueous media, or even in organic solvents such as tetrahydrofuran (THF). The impressive environmental stability of the gels could be due to the strong binding of DNA chains to GO sheets via multiple noncovalent interactions, including π−π stacking and hydrophobic forces between the DNA bases and the graphitic domains of GO. Electrostatic and hydrogen-bonding interactions between the amine groups in DNA bases and the oxygen-containing functional groups of GO also imparted

and b). Total dye-loading capacity of the gel for safranine O was estimated to be ∼960 mg/g of GO. Therefore, high dyeloading ability of the gel is due to the strong electrostatic interactions between the positively charged safranine O and negatively charged GO and DNA as well as the porous structures of the gels. This GO/DNA gel also possesses a selfhealing property. When a long free-standing gel was cut into pieces and the blocks were pushed together to make the surfaces touch each other followed by heating at 90 °C, it showed that the blocks adhered to each other (Figure 40c). The resulting “self-healed” gel was strong enough to bridge two posts horizontally. The excess single-stranded DNA present in the gel, which is unbound, or weakly bound to GO could transform into double-stranded DNA during the cooling process. As the cut gel surfaces were pushed together and heated, the free double-stranded DNA would unwind again to produce in situ single-stranded DNA which linked the adjacent GO sheets at the interfaces to “repair” the damage. Therefore, these multifunctional aspects of this GO/DNA hydrogels impart promises for a variety of biological and environmental applications such as tissue engineering, drug delivery, and for removing organic pollutants. Song et al. employed another biomolecule hyaluronic acid to prepare nanocomposite hydrogel with GO through horseradish peroxidase catalyzed in situ cross-linking process.399 Compared to pure hyaluronic acid hydrogel, mechanical stability of the composite gel was 12010


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Figure 41. (A) 0.05 mmol L−1 MB aqueous solution (a) without and (b) with freeze-dried PVA (0.002 g). (B) MB aqueous solution (a) without and (b−e) with freeze-dried GO (0.002 g) at (b) 0, (c) 60, (d) 120, and (e) 240 min, respectively. (C) MB aqueous solution (a) without and (b−e) with freeze-dried m-PVA−GO xerogel (0.002 g) at (b) 0, (c) 15, (d) 30, and (e) 60 min, respectively. (D) UV−vis spectra of MB aqueous solution with m-PVA−GO xerogel at different times. Reproduced with permission from ref 401. Copyright 2015 Royal Society of Chemistry.

due to the GO sheets while the chitosan molecules hold the gelation process of the GO sheets. Several gel-nanocomposites derived from graphene analogues were used in electronic applications such as supercapacitors, dye-sensitized solar cells, and photovoltaic devices. Shi and coworkers reported self-assembled graphene (RGO) organogel with 3D macrostructure prepared by in situ solvothermal reduction of GO in propylene carbonate for application in electrochemical capacitors.403 The supercapacitor showed a high specific capacitance of 140 F g−1 and a high maximum energy density of 43.5 Wh kg−1. These authors also reported composite organogels of RGO incorporated with activated carbon in propylene carbonate which showed specific capacitance of 116.5 ± 2.2 F g−1 at a current density of 1 A g−1.404 Park et al. reported transparent and flexible composite films for electrothermal switchable haze.405 The composite film was composed of paraffin-adsorbed polydimethylsiloxane laminated on graphene and supported by polyethylene terephthalate substrate. On applying bias, graphene present in the film showed Joule-heating which led to microstructural transformation of paraffin-adsorbed polydimethylsiloxane layer and this in turn causes the modulation of light scattering. Thus, when the bias on the polymer layer was optically transparent due to the heating by graphene and upon switching off the bias, the film returned to the haze and the process was reproducible for a number of cycles. Gun et al. prepared low concentration (0.4 wt %) GO organogels in acetonitrile-containing iodide/triiodide for the quasi-solid electrolyte for dye-sensitized solar cells.406 These solar cells showed energy conversion efficiency of 7.5% for GO gel electrolyte-based devices compared to 6.9% for correspond-

significantly improved and thus it prolonged the release of rhodamine B as a model drug. The composite gel also showed pH-dependent release of rhodamine B. Cytotoxicity results of incorporation of mouse embryonic fibroblasts (BALB 3T3 cells) in the composite hydrogel showed cytocompatibility of both the enzymatic cross-linking process and the hyaluronic acid/GO composite hydrogels (cell viability 90.6 ± 4.25%). Several other groups have also reported potential dye removal application of GO-based gel-nanocomposites. Li et al. reported functionalization of poly(vinyl alcohol) (PVA) hydrogels with GO for dye removal applications.400 Incorporation of GO increased the macromolecular chain mobility of the polymer in composite hydrogels. This in turn increased the swelling ratio and dye removal ability of methylene blue in water. Xue et al. reported composite hydrogels from PVA and GO for dye-absorption studies.401 Three different molecular weight PVA was used for the gel-nanocomposite formation. Among them the higher molecular weight PVA was shown to form effective composite formation as confirmed from their low critical gelation concentration. The medium molecular weight PVA-based nanocomposite was chosen for the dye-absorption studies of methylene blue. In the study, PVA alone was shown to be incapable of absorbing methylene blue (Figure 41). PVAGO composite showed absorption of methylene blue by encapsulation at a significantly higher rate compared to GO alone which was also evident by UV−vis spectra. Zhao et al. reported GO-chitosan composite hydrogels for dye removal from wastewater which showed efficient removal of three tested dye molecules, Congo red, methylene blue, and Rhodamine B.402 In such gel-nanocomposites, the dye adsorption occurs 12011


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Figure 42. Photograph showing various nanoparticles dispersed in the gel. From left-to-right: gel with no nanoparticles, gold, gold−palladium, magnetite, 25 nm silica, 80 nm silica, and TiO2. Gel composition in all cases: 0.40 M AOT, 12% w/v gelatin. Demonstration of physical stability of a magnetic gel composite with incorporated Fe3O4 particles and response toward external magnets. Reproduced with permission from ref 412. Copyright 2010 Royal Society of Chemistry.

components and their extent of synergy in the mixture. Therefore, performance of these composite materials depends on many crucial factors, such as homogeneity of the mixture, extent of intermolecular interactions between the components, extent of phase separations, organization of the nanomaterials inside the host matrix (such as specific organizations of the electron donor components and electron-accepting fullerene leading to p- and n- conduction channels for electron transport), integrity of the nanomaterials (for example, covalent functionalization-induced damage of CNTs and graphenes), etc. We mentioned in the introduction section that the fate of a supersaturated solution could lead to the formation of crystal, precipitate or physical gel depending on the intermolecular interactions, and degree of order in the molecular arrangements. Since in crystals there is a high degree of order in the molecular arrangements and therefore incorporation of nanomaterials would breakdown the intermolecular interactions as well as associated modes of crystal packing, a composite crystal is generally hard to achieve. However, in a precipitate, there is a very limited degree of order in the molecular arrangements, and hence, incorporation of nanomaterials may lead to amorphous aggregates which may lack in synergy in properties and corresponding applications. Therefore, physical gel is such a metastable state where the intermolecular interactions are optimum, and hence, when the composites were formed from physical gels the added nanomaterials can adjust themselves inside the gel network. Thus, it could preferably fulfill the above-mentioned criteria of homogeneity, optimum intermolecular interactions, less phase separation, proper organization, and arrangements of the nanomaterials as well as retention in the integrity of the materials by noncovalent functionalization strategy. Such molecular arrangements in the gel network could be correlated with the arrangements present in the natural photosynthetic assembly. In the photosynthetic pathway, the light-absorbing magnesium porphyrin complex was placed in the biological soft tissues in such a way that it favors the sunlight induced electron transfer efficiently to the reaction center leading to photosynthesis.411 In this way, communication between the nanomaterial and the host gelator leads to synergy in properties. Furthermore, the preparation of gel-nanocomposite is quite convenient and often straightforward. The synthetic strategy could be as simple as addition of nanomaterials into a preformed gel followed by mixing (through heating/sonicating, etc.) and resting the mixture to set the gel. However, many other strategies are possible as already discussed. Evolution of nanoparticle-based gel-nanocomposites syntheses progressed from adding externally synthesized nanoparticles

ing liquid ones. Neo et al. prepared gel electrolytes by dispersing iodine, 1-methyl-3-propylimidazolium iodide, guanidine thiocynate, and 4-tert butylpyridine into organogels of GO and 3-methoxypropionitrile to fabricate high-performance quasi-solid state dye-sensitized solar cells.407 The gel electrolyte showed a photovoltaic efficiency of 6.70% under AM 1.5 G illumination (100 mW cm−2). Lim et al. used gel-nanocomposites from poly(3,4-ethylene dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) incorporated with graphene quantum dots (diameter of about 8−10 nm) as hole extraction layer in organic photovoltaic device applications.408 Efficiency of the bulk heterojunction solar cells (using P3HT/PCBM as active material) was 4.74% as compared to 3.77% for pristine PEDOT:PSS. Tung et al. reported composite hydrogels of GO and PEDOT:PSS which showed improvement in the conductivity of PEDOT.409 The gel-nanocomposite acted as a metal-free solder for creating electrical connections, and thus high-efficiency polymer tandem solar cells were successfully fabricated. Interesting properties of graphenes offer potential advantages for specific applications in biological sciences, especially in the areas of bioelectronics, biosensors, and drug delivery. In a recent review,410 some of these applications have been discussed briefly. These include biofunctionalization of graphene with DNA, proteins and other biomolecules, graphene-based FRET (fluorescence resonance energy transfer) biosensors for the detection of DNA and various ions, small molecules and proteins, graphene-based biotechnology for living cell studies, and even in drug delivery and cell imaging. Due to the excellent solubility of GO in aqueous media, it exhibits widespread applications in biological systems. For this purpose, several hosts were selected for the delivery of the GO inside cellular systems. Also, GO has been used as a carrier for the delivery of bioactive drugs or even DNA.

6. APPLICATIONS OF GEL-NANOCOMPOSITES We have separately described the applications of gel-nanocomposites assembled from each of the nanomaterials (nanoparticles, fullerene, CNTs, and graphenes) at the end of each section for the discussion of a particular nanomaterial-based composite. We summarize here the crucial applications demonstrated by each nanomaterial-based composite toward the generation of multifunctional future advanced materials. Here we give rationale for using physical gel medium to harness interesting properties of different nanomaterials. Even though several composite materials have been prepared in solution or solid phase by involving small molecules or polymers, their applicability depended on the properties of the individual 12012


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based composites were developed for antibacterial activity. Drug delivery vehicles were composed of hydrogels in the presence of stimuli-responsive nanoparticles such as NIR responsive NaYF4:TmYb or magnetoresponsive Fe3O4. These applications of gel-nanocomposites in specific fields are summarized in Table 1. Fullerene-based gel-nanocomposites were mostly achieved by adding unfunctionalized fullerene to the gel network for a host−guest-mediated inclusion complex formation through π−π stacking interactions. However, such simple strategy often suffers with a limitation of phase separation. Therefore, one of the strategies for functionalization of fullerene is covalent attachment of small molecular entity that is complementary to the molecular structure of the host gelator in order to form gel-nanocomposites through molecular recognition. In the next generation strategy, fullerene has been functionalized in such a way that with the added moieties they can interact with the host gelator additionally through hydrogen bonding and van der Waals forces along with π−π stacking interactions. In this way, proximity among the gelator and fullerene remain maintained as these are particularly important for photoinduced electron transfer in organic solar cells, as already described.46,250−252 Further improvements in the strategy are in functionalizing fullerene with chromophoric groups to tune the electronic band gap for improved solar cell performance. CNTs are extensively studied nanocarbons. CNT-based gelnanocomposites find applications in oil absorption and recovery, NIR laser-induced photothermal performances, drug release, antimicrobial activity, etc. as summarized in Table 1. CNT-based gel-nanocomposite synthesis strategies are quite similar to that of fullerene. However, in the case of CNTs, small molecules as well as the large extent of polymers were employed for the nanocomposite formation and also their applications. Apart from incorporation of CNTs alone into the gel matrix, different intermixing of nanomaterials was also tried in past few years. Therefore, ternary or quaternary complexes were prepared with a host gelator mixed with more than one of the nanomaterials CNTs, nanoparticles, fullerene, or graphenes. Mazzier et al. reported terminally protected hydrophobic dipeptide 99 for the synthesis of 99/C60/MWNT ternary composite organogels.415 This ternary coassembled material afforded catalytic reduction of water-soluble azo-compounds in the presence of NaBH4 and UV-light and also reduction of benzoic acid to benzyl alcohol in the presence of NaBH4. Graphene-based composites are a widely pursued topic in recent years, and many more gel-based nanocomposites are yet to emerge. Graphene-based gel-nanocomposites find applications in highly electrically conductive films, mechanically high strength materials, catalytic oxidation/reduction, sensing, as well as dye-absorption studies. In recent years, multicomponent nanomaterials based gel-nanocomposites have emerged significantly using graphene analogues. In these composites, graphene or GO along with various intermixing of nanoparticles, fullerene, and CNTs were achieved which are supported by small molecular or polymeric gelators for selfassembled gel formation. For example, Banerjee and co-workers reported in situ synthesis of gold nanoparticles inside hydrogels assembled from GO and tryptophan to form nanohybrid materials.416 Tryptophan moiety acted here as the reducing agent without any other external reducing and stabilizing agents. Sui et al. reported vitamin C induced reduction of GO to form RGO hydrogels.417 Consequently, hybrid hydrogels of

into preformed gels to in situ syntheses of nanoparticles in the gel media. Further progress in the preparation of the second category describes the use of reducing agent for nanoparticle synthesis to environmentally friendly photo/bioreduced nanoparticle synthesis. Advancement in the field of next generation nanoparticles is based on improved functionality such as magneto-responsive or NIR-responsive as well as core−shellbased nanoparticles. Progress in terms of gelator is emerging toward universality in hosting multiple nanoparticles. Shen et al. reported sodium salts of bile acid-based hydrogels for the in situ fabrication of silver and gold nanoparticles through environmentally friendly method of photoreduction, without adding any external reducing or stabilizing agents.413 Trickett et al. dispersed various inorganic nanoparticles inside organogels developed from oil/water microemulsions from AOT [sodium bis(2-ethylhexyl) sulfosuccinate] with added gelatin.412 The nanoparticles (Au, Au−Pd, Fe3O4, silica, and TiO2) are stable inside the gels for long periods without leaching when the gel was pelleted and stirred in organic solvents (Figure 42). The properties of the individual gels and nanoparticles are retained in the inorganic−organic hybrid materials. Thus, Fe3O4containing gel-nanocomposite emerged as a “meltable” thermo-reversible magnetic organogel responsive to external magnets. As described previously, this type of magnetic gelnanocomposites were reported in several instances.172,181,182,228 In another report, Peveler et al. reported sugar-based small molecular organogelator 98 (Chart 19) for the synthesis of a Chart 19. Molecular Structures of Supramolecular Gelators for the Preparation of Multi-Component GelNanocomposites

range of functional “smart” gel-nanocomposites by incorporating/in situ synthesizing variety of nanomaterials, including CdSe/ZnS quantum dots, Au, Ni, Fe3O4, CoO nanoparticles, and MWNTs.414 Gel-nanocomposites were achieved successfully with each of these nanomaterials and showed specific applications, respectively. The gel-nanocomposites impregnated with CdSe/ZnS quantum dots showed fluorescent gels and also fluorescence quenching in the presence of nitrobenzene. Magnetic gel was obtained from superparamagnetic Fe3O4 nanoparticles. Ni nanoparticle incorporated gel showed catalytic activity for H2 generation from aqueous NH3BH3. Thus, nanoparticle-based gel-nanocomposites are emerging as a multifunctional soft material for widespread utility. Gel-nanocomposites based on metal-based nanoparticles show potential applications mainly in the field of catalysis, drug delivery, and antibacterial activities. Thus, while Pt- and Pd-based gel-nanocomposites showed catalytic activity, Ag12013


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Table 1. Applications of Gel-Nanocomposites for Different Choice of Nanomaterials and Gelators refs nanoparticles as dopants Maity et al.222 Kumar et al.223 Maity et al.224 Zhao et al.225,226 Yan et al.227 Yang et al.228 Mandal et al.229 Murali Mohan et al.190 Peveler et al.414 Fullerene as dopant Xue et al.250−252 Tevis et al.46 CNTs as dopants Pourjavadi et al.294,295 Bhattacharya et al.101,102 Satarkar et al.299 Miyako et al.300 Zhang et al.301

gel-nanocomposite system

potential applications

criteria of merit

small molecular hydrogel + Pt nanoparticles small molecular hydrogel + Pt nanoparticles small molecular hydrogel + Pd nanoparticles polymer hydrogel + Au nanoparticles polymer hydrogel + NaYF4: TmYb nanoparticles small molecular hydrogel + Fe3O4 nanoparticle xmall molecular hydrogel + Ag nanoparticles polymer hydrogel + Ag nanoparticles small molecular organogel + Ni nanoparticles

catalytic hydrogenation reactions in water catalytic reduction of p-nitrophenol in water aromatic C−C coupling (Suzuki) reactions thermo-switchable electrical properties NIR induced delivery of biomacromolecules

hydrogenation catalyst catalytic reducing agent synthetic method external trigger materials delivery

magneto-responsive hydrogel for delivery antibacterial activity antibacterial activity catalytic H2 generation from aqueous NH3BH3

stimuli sensitive system biocidal agent biocidal agent hydrogenation catalyst

small molecular organogel + fullerene small molecular organogel + fullerene

photocurrent generation in photovoltaic cell bulk heterojunction organic solar cells

energy application energy application

polymer organogel + MWNTs small molecular organogel + SWNTs

oil absorption and recovery NIR-induced exothermicity

oil spill recovery stimuli sensitive system

polymer hydrogel + MWNTs polymer hydrogel + SWNTs polymer hydrogel + SWNTs

radio-frequency-induced exothermicity NIR laser-driven high-performance photothermal NIR responsive actuators for programmable selffolding materials nanopatterning by direct printing improved enzyme activity NIR-induced controlled drug release drug loading and pH-dependent controlled drug release antimicrobial activity

stimuli sensitive system stimuli sensitive system stimuli sensitive system

electronics design

Tan et al.302 Kar et al.305 Fujigaya et al.298 Sankar et al.86

small molecular hydrogel + SWNTs small molecular hydrogel + SWNTs polymer hydrogel + SWNTs polymer hydrogel + SWNTs

Hui et al.309 graphenes as dopants Yang et al.394

polymer hydrogel + CNTs

Hou et al.395 Huang et al.396 Liu et al.397 Xu et al.398 Xue et al.401 Zhao et al.402 Shi and coworkers403,404 Park et al.405 Gun et al.,406 Neo et al.407 Lim et al.408 ternary gelnanocomposites Mazzier et al.415

polymer hydrogel + graphene + Fe3O4 GO + hemoglobin GO + hemoglobin + chitosan GO + DNA polymer hydrogel + GO polymer hydrogel + GO polymer organogel + RGO

highly electrically conductive films for flexible electronics high strength and excellent resilient material Catalytic oxidation amperometric nitrite sensor Dye-adsorption and self-healing dye-adsorption dye removal supercapacitor

polymer organogel + graphene GO organogel

electrothermal optical switching dye sensitized solar cells

electronic devices energy applications

polymer organogel + graphene quantum dots

organic bulk heterojunction solar cells

energy applications

small molecular organogel + C60 + MWNTs

Banerjee and coworkers358 Banerjee and coworkers418 Jiao et al.420

small molecular hydrogel + GO + Au nanoparticles RGO + polyamines + Au nanoparticles

catalytic reduction of azo-compounds and benzoic acid catalytic reduction of aromatic nitro-compounds

catalytic reagents for organic synthesis catalytic reagents for organic synthesis catalytic reagents for organic synthesis environmental cleanup

Banerjee and coworkers422

small molecular hydrogel + GO/RGO + Ag nanoparticles

chemically converted graphene hydrogel

RGO + Chitosan + Ag nanoparticles

catalytic reduction of aromatic nitro to amino group photocatalytic dye degration for wastewater treatments catalytic reduction of nitroarenes

nanolithography bioengineering drug delivery drug delivery biocidal gel

advanced materials Oxidation catalyst sensor environmental cleanup environmental cleanup environmental cleanup electronic devices

catalytic reagents for organic synthesis

particles (Au, Ag, and Pt) by in situ reduction with vitamin C.418 The Au nanoparticle containing RGO-based hybrid hydrogel showed efficient catalytic reduction of aromatic nitro to amino group. Jiao et al. reported gel-nanocomposites by in situ coreduction of GO and silver acetate using environmentally friendly reducing agent vitamin C within the hydrogel matrix of GO-polyethylenimine to form RGO-silver nanoparticle com-

GO/MWNT or GO/Pt nanoparticle was achieved upon reduction or coreduction of MWNTs added and stabilized with GO sheets or H2PtCl6 incorporated with GO sheets, respectively. The residual vitamin C was shown to have a slow release from the composite, and hence, it could be used for its controlled delivery. Banerjee and co-workers prepared composite hydrogels of polyamines, RGO, and metal nano12014


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posite hydrogels.419 These gel-nanocomposites showed efficient catalytic activity for organic dye removal and wastewater treatment. In another report, they prepared composite hydrogels of RGO, chitosan, and silver nanoparticles for photocatalytic dye degration studies for wastewater treatments.420 The photocatalytic activity of the gel-nanocomposite was acquired by the silver nanoparticles, while the gelation process was facilitated by the chitosan molecules. Thus, the gelnanocomposite showed good photocatalytic performance for the removal of dyes such as Rhodamine B and methylene blue in single or mixed solutions. Lim et al. reported fabrication of nanocomposite hydrogels from polyacrylamide (PAAm) along with RGO, Ag nanoparticles, and polyethylenedioxythiophenepolystyrenesulfonate mixture (PEDOT/PSS) by photopolymerization method.421 The gel-nanocomposite showed improved mechanical, thermal, and electrical properties [e.g., compressive strength increased 1.57 times upon incorporation of 0.3 wt % of nanomaterials (RGO-Ag-PEDOT/PSS) and the electrical conductivity of the composite hydrogel appeared at 3.91 × 10−5 S cm−1]. In another report, Banerjee and co-workers also reported sunlight induced in situ preparation of silver nanoparticles inside hydrogel network composed of GO or RGO in the presence of small molecular gelator 100 which constitutes trinanohybrid system.422 With dependence on sunlight exposure time, the initial trihybrid system (nanofibers, nanosheets, and silver nanoclusters) transformed into a final trihybrid system (nanospheres, nanosheets, and silver nanoparticles) through an intermediate tetra-nanohybrid system (nanofibers, nanosheets, nanospheres, and silver nanoparticles). These trihybrid systems showed catalytic reduction of nitroarenes to aminoarenes. We have also reported a tetrananohybrid system composed of fullerene, SWNT, and graphene incorporated in a small molecular organogelator 81.103 Synergistic interactions among the nanomaterials were addressed in terms of mechanical properties upon variation of the number of components in the mixture. Subsequently, another trihybrid thixotropic metallo(organo)gel was also reported by us, which was assembled from silver nanoparticle, single-walled carbon nanohorns, and an oligo(p-phenylenevinylene) derivative.423 Banerjee and co-workers reported synthesis of different metal nanoparticles (Au, Ag, and Pd) in vitamin B2/B12 doped GO hydrogels, which emerged as a ternary nanocomposite.424 The nanoparticles containing GO gels were then reduced with another vitamin (vitamin C) to produce RGO-based hydrogels laced with metal nanoparticles. Furthermore, the Pd nanoparticle-based hybrid hydrogel showed efficient catalytic activity for C−C bond-forming Suzuki−Miyaura coupling reaction with good yields and high recyclability. Chen et al. reported such ternary nanocomposite hydrogels assembled from GO/nanosulfur/polypyrrole for supercapacitor applications which showed high specific capacitance, excellent cycling life, and excellent conductivity.425 Fan et al. prepared hydrogels of Ag nanoparticles and GO by cross-linking acrylic acid and N,N′methylene bis(acrylamide). Thus, antibacterial activity of Ag nanoparticles and water-retaining ability of GO enabled these composite hydrogels for accelerated wound-healing ability.426

supramolecular chemistry of small molecular and polymer gelators as well as syntheses and physical chemical characterization of novel nanomaterials have been pursued. Integration of these diverse areas into a single discipline represents the most recent and extremely important development in the field of multifunctional nanocomposites. Dynamic supramolecular metastable self-assembly of gelators appears to be an excellent host for incorporation of different nanomaterials. While metal-based nanoparticles were studied extensively for gel-nanocomposite syntheses, their practical applications were realized only recently. Significant improvements in the design strategy for nanoparticle synthesis as well as composite preparation may be correlated to their intended applications. Compared to doping preformed nanoparticles, in situ syntheses of nanoparticles inside the self-assembled network of a gel is a step forward, especially when it is performed under environmentally friendly conditions (e.g., photo/bioreduction). Several types of multifunctional gel-nanocomposites were realized at present and will also be pursued in future from nanoparticles with improved functionality such as magneto-responsive, NIRresponsive, catalytic, or antibacterial properties assembled by appropriate gelators that can host these specially tailored nanoparticles. Although fullerene is a couple of decades old, composites of it with small molecular gelators were explored only recently for the organic solar cell applications, thanks to the high electronaccepting property of fullerene. Fullerene derivatives (such as PCBM) are, however, an active component in the donor− acceptor polymer-based solar cells. Recently, donor−acceptorbased small molecular solar cells in the presence of different fullerenes (C 70 ) showed exceptionally high solar cell efficiency.427,428 Therefore, applications of fullerene-based gelnanocomposites in this direction continue to be a hot topic in the future as the gel-based materials can also reinforce the selfassembly strategies. In addition, covalent functionalization of fullerene with different chromophoric molecules could prove advantageous for the modulation of electronic bandgap and charge-transfer properties for improved solar cell performance. Higher-dimensional nanocarbons (CNTs and graphenes)based gel-nanocomposites find interesting applications in diverse unrelated fields, including electronics, material science, mechanical engineering, and biology. Functionalization of CNTs and graphene remains an issue to disperse them easily in commonly employed solvent medium in order to retain their integrity and to reap benefits from their interesting properties. Two different strategies for their functionalization, covalent and noncovalent, have been described for nanocomposite formation along with their advantages and disadvantages. Complementary molecular recognition is one such strategy, which exclusively applied in each of the nanomaterial-based gel-nanocomposites either with small molecule or polymer where the functionalizing moiety is structurally identical with that of the host gelator. Complexity of the gel-nanocomposite system could be increased multifold to generate ternary or quaternary systems composed of different nanomaterial species in order to take advantage of each of their individual properties gathered in the composites. Thus, nanocomposites with desired properties could be tailored by selectively adding different nanomaterials (e.g., nanocomposites of polymer hydrogels incorporated with GO and Ag nanoparticles showed both antibacterial property and high water content which led to this composite suitable for wound-healing ability).426 This strategy enables preparation of different nanomaterials with interesting properties that could be

7. CONCLUSIONS AND OUTLOOK In summary, the composites of gel-nanoparticles and gelnanocarbons are rapidly emerging as advanced materials that exhibit properties of both soft matter like gel and nanomaterials. Thus, a vast number of investigations involving the 12015


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Scheme 3. Representation Showing Diverse Ways By Which Various Nanocomposites are Formed and Stabilized

For instance, since the flow behavior of such soft composite materials can be tuned by the choice of nanodopants depending on their shape, functionality, and interactibility with gelators at molecular level, one can imagine achieving control over potential applications even in civil engineering. One can imagine how vulcanization has dramatically improved the commercial utility of natural polymers like rubber. Similar benefits may be envisioned with the advent of intelligent application involving new chemistry at the interface of synthetic soft materials and advanced nanomaterials. Some of these have been highlighted in this review in specific sections. In spite of these early developments, many aspects of the blending of nanoparticles or nanocarbons into gels are more often than not, understood at the molecular level. The improvement and application of these composites will depend on how effectively we can handle most of these challenges.

constantly explored for many hitherto unknown applications. Gel-nanocomposites laced with boron nitride nanotubes showed improved viscoelasticity and thermal conductivity.429,430 Interestingly, in boron nitride nanotubes, the boron centers are electron deficient in nature and thus Lewis acid− base adduct formation with different amines or phosphines may lead to convenient noncovalent functionalization. In this way, the integrity of the nanotubes remains unaffected upon functionalization. More recently, carbon nanohorns were employed for gel-nanocomposites syntheses.423,431 Understanding the modes of self-assembly of dimensionally different nanomaterials (nanoparticles, fullerene, CNTs, and graphenes) in the resulting gel-nanocomposites could enable exploration of such multifunctional gel-nanocomposites (Scheme 3). We must accept the fact that most of the gelation-induced nanocomposite syntheses are in their early days of preparation and characterization. A lot of fine-tuning will be required before their eventual applications in any critical fields such as drug delivery, tissue engineering, etc. Different synthetic methods are being evolved continuously for the preparation of various bionanocomposites, and the process still strives to achieve an optimum route for the generation of task-specific composites endowed with well-defined and improved properties. The main purpose of these hybrid nanocomposites are that they often provide materials with properties that are superior to the individual components or their noninteractive physical mixtures alone. Therefore, the future development of these nanocomposites may be best realized with the advent of de novo designs and through clever use of newer and novel functionalization strategies of the corresponding nanomaterials with the organic compounds or biological macromolecules. Thus, an understanding of the host−guest interactions among the host gelators with different guest nanomaterials and their intelligent utilization in achieving a particular application becomes the key in the evolution of the hybrid properties.

AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. Biographies Santanu Bhattacharya obtained his Ph.D. (Prof. Robert A. Moss) from Rutgers University, New Brunswick, NJ. Then he had a three year postdoctoral stint at Massachusetts Institute of Technology under the tutelage of Nobel Laureate Prof. H. Gobind Khorana. He is currently the Director of Indian Association for the Cultivation of Science, Kolkata. Concurrently, he is also a Professor of the Department of Organic Chemistry at the Indian Institute of Science and an honorary Professor of the JNCASR, Bangalore. He has authored over 230 refereed research articles in international journals. His research interests are at the interfaces of biology and materials science. Some topics where he is actively involved include supramolecular chemistry, 12016


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molecular sensors, soft materials, self-assemblies, chemical biology, and nanoscience. He is a recipient of the TWAS prize in Chemistry and an elected fellow of the TWAS, Trieste, INSA, New Delhi and the IASc, Bangalore. Suman K. Samanta received his M.Sc. in Chemistry from the University of Calcutta in 2006. He received a Ph.D. under the guidance of Prof. Santanu Bhattacharya at the Indian Institute of Science. Following a brief postdoctoral stay in the same lab, he joined Prof. Ullrich Scherf, Germany, as an Alexander von Humboldt fellow. His research interests include synthesis of functional organic molecules, nanomaterials, self-assembly, physical gels, photophysical properties, novel surfactants, conjugated polymers, microporous polymer networks, and theoretical studies.

ACKNOWLEDGMENTS We thank the facilities provided by the Indian Institute of Science, Bangalore, and the Indian Association for the Cultivation of Science, Kolkata, India. We also take this opportunity to acknowledge the contributions of many of our co-workers who have contributed to the development of some of the areas described in this review, whose names appear in the references. S.B. thanks DST for the J C Bose National Fellowship. S.K.S. acknowledges receipt of Research Associateship at IACS. REFERENCES (1) Terech, P.; Weiss, R. G. Low Molecular Mass Gelators of Organic Liquids and the Properties of Their Gels. Chem. Rev. 1997, 97, 3133− 3160. (2) Du, X.; Zhou, J.; Shi, J.; Xu, B. Supramolecular Hydrogelators and Hydrogels: From Soft Matter to Molecular Biomaterials. Chem. Rev. 2015, 115, 13165−13307. (3) Ajayaghosh, A.; Praveen, V. K.; Vijayakumar, C. Organogels as Scaffolds for Excitation Energy Transfer and Light Harvesting. Chem. Soc. Rev. 2008, 37, 109−122. (4) Carretti, E.; Bonini, M.; Dei, L.; Berrie, B. H.; Angelova, L. V.; Baglioni, P.; Weiss, R. G. New Frontiers in Materials Science for Art Conservation: Responsive Gels and Beyond. Acc. Chem. Res. 2010, 43, 751−760. (5) Escuder, B.; Rodriguez-Llansola, F.; Miravet, J. F. Supramolecular Gels as Active Media for Organic Reactions and Catalysis. New J. Chem. 2010, 34, 1044−1054. (6) Amabilino, D. B.; Puigmarti-Luis, J. Gels as a Soft Matter Route to Conducting Nanostructured Organic and Composite Materials. Soft Matter 2010, 6, 1605−1612. (7) Diaz Diaz, D.; Kuhbeck, D.; Koopmans, R. J. Stimuli-Responsive Gels as Reaction Vessels and Reusable Catalysts. Chem. Soc. Rev. 2011, 40, 427−448. (8) John, G.; Jadhav, S. R.; Menon, V. M.; John, V. T. Flexible Optics: Recent Developments in Molecular Gels. Angew. Chem., Int. Ed. 2012, 51, 1760−1762. (9) Xu, B. Gels as Functional Nanomaterials for Biology and Medicine. Langmuir 2009, 25, 8375−8377. (10) Vintiloiu, A.; Leroux, J.-C. Organogels and their Use in Drug Delivery  A Review. J. Controlled Release 2008, 125, 179−192. (11) Tomatsu, I.; Peng, K.; Kros, A. Photoresponsive Hydrogels for Biomedical Applications. Adv. Drug Delivery Rev. 2011, 63, 1257− 1266. (12) Truong, W. T.; Su, Y.; Meijer, J. T.; Thordarson, P.; Braet, F. Self-Assembled Gels for Biomedical Applications. Chem. - Asian J. 2011, 6, 30−42. (13) Bhagat, D.; Samanta, S. K.; Bhattacharya, S. Efficient Management of Fruit Pests by Pheromone Nanogels. Sci. Rep. 2013, 3, 1294. (14) Geiger, C.; Stanescu, M.; Chen, L.; Whitten, D. G. Organogels Resulting from Competing Self-Assembly Units in the Gelator: 12017


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