Tanmoy Kar

Gel-nanocomposites: materials with promising applications

The race to develop newer materials with superior properties/applications in diversified fields is gathering momentum in modern day science. In this context, an exciting avenue of research deals with the development of hybrid materials resulting from the combination of gels with nanoparticles of different origins. These varying kinds of nanoparticles (inorganic nanoparticles, Au/Ag based nanoparticles and carbonaceous nanostructures like carbon nanotube and graphene) are being used in conjunction with diverse self-assemblies to develop gel-nanocomposites with the scope of generating advanced applications. The present review will track the noteworthy progress of gel-nanocomposites and also will highlight the recent advances in their synthesis, improved properties/features and applications for developing mechanically robust materials to antimicrobial hydrogels.

Organogelation and Hydrogelation of Low-Molecular-Weight Amphiphilic Dipeptides: pH Responsiveness in Phase-Selective Gelation and Dye Removal†

The search for efficient low-molecular-weight gelators (LMWGs) with possible structure-activity correlation is on the rise. The present work reports a novel set of amphiphilic dipeptide-based carboxylic acids capable of efficiently gelating organic solvents. More interestingly, their sodium salts showed enhanced efficiency in organogelation with the additional ability to gelate water. Electrostatic interactions present in the aggregation of the sodium carboxylates of amphiphilic dipeptides seem to be important because some of the nongelator carboxylic acids turned out to be excellent gelators upon salt formation. The combinations and sequence of the amino acids in the dipeptide moiety were systematically altered to understand the collective importance of the nonpolar aliphatic/aromatic substitution in amino acids in the self-assembling behavior of amphiphiles. Almost a 20-fold enhancement in the gelation ability was observed on reversing the sequence of the amino acid residues, and in some cases, nongelators were transformed to efficient gelators. Spectroscopic and microscopic studies of these thermoreversible organo/hydrogels revealed that balanced participation of the noncovalent interactions including hydrogen bonding and van der Waals interactions are crucial for organo/hydrogelation. These dipeptides selectively gelate organic solvents from their mixtures with water, and the xerogels prepared from these organogels showed time-dependent adsorption of dyes such as crystal violet. The most remarkable feature of these gelators is the pH responsiveness, which was aptly utilized for the pH-dependent phase-selective gelation of either solvent in a biphasic mixture of oil and water. The dissimilar gelation ability of the acid and its salt originating from the pH responsiveness of the amphiphilic dipeptide was employed in the instant removal of large amounts of dyes for wastewater treatment.

Counterion-induced modulation in the antimicrobial activity and biocompatibility of amphiphilic hydrogelators: influence of in-situ-synthesized Ag-nanoparticle on the bactericidal property.

The necessity for the development of new antimicrobial agents due to the ever increasing threat from microbes is causing a rapid surge in research. In the present work, we have shown the efficient antimicrobial activity of a series of amino acid-based hydrogelating amphiphiles through alteration in their counterion. The subtle variation in the counterion from chloride to various organic carboxylates had a significant impact on the antimicrobial properties with notable improvement in biocompatibility toward mammalian cells. Incorporation of a hydrophobic moiety in the counterion augmented the antibacterial property of the amphiphilic hydrogelator as minimum inhibitory concentration (MIC) against the Gram-positive bacterial strain, Bacillus subtilis decreased up to 5-fold (with respect to the chloride) in the case of n-hexanoate. These counterion-varied amphiphilic hydrogelators were also found to be effective against fungal strains (Candida albicans and Saccharomyces cerevisiae) where they exhibited MICs in the range of 1.0-12.5 μg/mL. To widen the spectrum of antibacterial activity, particularly against Gram-negative bacteria, silver nanoparticles (AgNPs) were synthesized in situ within the supramolecular assemblies of the carboxylate hydrogelators. These AgNP-amphiphile soft-nanocomposites showed bactericidal property against both Gram-positive and Gram-negative bacteria. Encouragingly, these carboxylate hydrogelators showed superior biocompatibility toward mammalian cells, HepG2 and NIH3T3, as compared to the chloride analogue at a concentration range of 10-200 μg/mL. Importantly, the AgNP composites also showed sufficient viability to mammalian cells. Because of the intrinsic hydrogelation ability of these counterion-varied amphiphiles, the resulting soft materials and the nanocomposites could find applications in biomedicine and tissue engineering.

pH-Triggered conversion of soft nanocomposites: in situ synthesized AuNP-hydrogel to AuNP-organogel

Amino acid based amphiphilic gelators (carboxylate salts) were employed for the in situ synthesis of gold nanoparticles (GNPs) in hydrogel networks at room temperature without using any external reducing or capping agents for the development of AuNP-hydrogel soft composite. Synthesized AuNP-hydrogel composites were then successfully converted to AuNP-organogel composites simply by lowering the pH of the aqueous medium, as the hydrogelating amphiphilic carboxylates were transformed to corresponding carboxylic acids that are efficient organogelators. These water insoluble carboxylic acids spontaneously moved from the aqueous phase to the nonpolar organic media (toluene) along with the synthesized GNPs to form the AuNP-organogel composite. The phase transfer of the GNPs from a hydrogel network to an organogel network was investigated by UV-Vis spectroscopy, field emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD) studies. Supramolecular networks of both the gels played a crucial role in stabilization of the GNPs. Fluorescence spectroscopy was used to investigate the mechanistic detail of the in situ GNP synthesis. The characterizations indicated the formation of spherical and uniform sized GNPs and even phase transfer of the nanoparticles did not result in change of shape or size of the nanoparticles. Rational designing of the gelator/nongelator molecules helped us to recognize the key structural components required for the efficient synthesis and stabilization of the GNPs in both the phases. Rheological study suggested that the hydrogel-GNP composites possess improved viscoelastic property than the native hydrogel.

Pyrene-Based Fluorescent Ambidextrous Gelators: Scaffolds for Mechanically Robust SWNT-Gel Nanocomposites

With the rapid progress in the development of supramolecular soft materials, examples of low-molecular-weight gelators (LMWGs) with the ability to immobilise both water and organic solvents by the same structural scaffold are very limited. In this paper, we report the development of pyrene-containing peptide-based ambidextrous gelators (AGs) with the ability to efficiently gelate both organic and aqueous solvents. The organo- and hydrogelation efficiencies of these gelators are in the range 0.7-1.1% w/v in various organic solvents and 0.5-5% w/v in water at certain acidic pH values (pH 2.0-4.0). Moreover, for the first time, AGs have been utilised to prepare single-walled carbon-nanotube (SWNT)-included soft nanocomposites in both hydro- and organogel matrices. The influence of different non-covalent interactions such as hydrogen bonding, hydrophobic, π-π and van der Waals interactions in self-assembled gelation has been studied in detail by circular dichroism, FTIR, variable-temperature NMR, 2D NOESY and luminescence spectroscopy. Interestingly, the presence of the pyrene moiety in the structure rendered these AGs intrinsically fluorescent, which was quenched upon successful integration of the SWNTs within the gel. The prepared hydro- and organogels along with their SWNT-integrated nanocomposites are thermoreversible in nature. The supramolecular morphologies of the dried gels and SWNT-gel nanocomposites have been studied by transmission electron microscopy, fluorescence microscopy and polarising optical microscopy, which confirmed the presence of three-dimensional self-assembled fibrillar networks (SAFINs) as well as the integrated SWNTs. Importantly, rheological studies revealed that the inclusion of SWNTs within the ambidextrous gels improved the mechanical rigidity of the resulting soft nanocomposites up to 3.8-fold relative to the native gels.

Structure and properties of cholesterol-based hydrogelators with varying hydrophilic terminals: biocompatibility and development of antibacterial soft nanocomposites.

The present work demonstrates a rational designing and synthesis of cholesterol-based amino acid containing hydrogelators with the aim to improve the biocompatibility of these amphiphilic molecules. A thorough structure-property correlation of these hydrogelators was carried out by varying the hydrophilic terminal from a neutral amine to a quaternized ammonium chloride. The amphiphiles having a cationic polar head as the hydrophilic domain and cholesterol as the hydrophobic unit showed better water gelation efficiency (minimum gelation concentration (MGC) ∼0.9-3.1%, w/v) than the analogous free amines. Presumably, the additional ionic interactions for the quaternized amphiphiles might have played the crucial role in gelation as counterions also got involved in hydrogen bonding with solvent molecules. Hence, the attainment of desired hydrophilic-lipophilic balance (HLB) of hydrophobic cholesterol in combination with the appropriate hydrophilic terminal led to the development of efficient hydrogels. Microscopic investigations revealed the formation of various supramolecular morphologies of hydrogels due to the variation in the molecular structure of the amphiphile. Spectroscopic investigations showed the involvement of hydrogen-bonding, hydrophobic, and π-π interactions in the self-assembled gelation. Importantly, biocompatibility of all the cholesterol-based hydrogelators tested against human hepatic cancer derived HepG2 cells showed increased cell viability than the previously reported alkyl-chain-based amphiphilic hydrogelators. In order to incorporate broad spectrum antibacterial properties, silver nanoparticles (AgNPs) were synthesized in situ within the hydrogels using sunlight. The amphiphile-AgNP soft nanocomposite exhibited notable bactericidal property against both gram-positive and gram-negative bacteria.

Organogel-hydrogel transformation by simple removal or inclusion of N-Boc-protection.

Development of organo- and hydrogelators is on the rise because of their extensive applications, from advanced materials to biomedicine. However, designing both types of gelator from a common structural scaffold is challenging, and becomes more significant if transformation between them can be achieved by a simple method. The present work reports the design and synthesis of both organo- and hydrogelators from amino acid/peptide-based amphiphilic precursors with a naphthyl group at the N terminus and a primary amine-containing hydrophilic ethyleneoxy unit at the C terminus. In alkaline medium, tert-butyloxycarbonyl (Boc) protection at the primary amine of the amphiphiles resulted in efficient organogelators (minimum-gelation concentration (MGC)=0.075-1.5% w/v). Interestingly, removal of the Boc protection from the ethyleneoxy unit, under acidic conditions, yielded amphiphiles capable of gelating water (MGC=0.9-3.0% w/v). Simple protection and deprotection chemistry was used to achieve transformation between the organogel and hydrogel by alteration of the pH. Combinations of different aliphatic and aromatic amino acids were investigated to discover their cumulative effect on the gelation properties. Field-emission scanning electron microscopy (FESEM) and transmission electron microscopy (TEM) were employed to investigate the supramolecular morphology of the thermoreversible gels. Spectroscopic investigations (FTIR, photoluminescence, XRD) revealed that noncovalent interactions, such as hydrogen bonding, π-π stacking, and van der Waals interactions play a decisive role in self-assembled gelation.

Organogelation through self-assembly of low-molecular-mass amphiphilic peptide

Development of supramolecular soft materials using naturally occurring precursors is of great interest. The present work reports the development of an amphiphilic peptide based low molecular weight (LMW) organogelator comprising of L-phenylalanine, L-lysine and tert-butyloxycarbonyl (Boc) protected 6-amino caproic acid. The peptide molecule is capable of gelating a wide range of aromatic and aliphatic organic solvents like benzene, toluene, ethyl acetate, tetrahydrofuran, chloroform and many others. The prepared organogels were thermoreversible in nature. The morphology of this organogelator was characterised by transmission electron microscopy (TEM). Morphological studies of the xerogels have revealed the formation of three dimensional (3D) self-assembled fibrillar networks (SAFINs). Involvement of different non-covalent interactions like hydrogen bonding, hydrophobic, π–π and van der Waals interactions in supramolecular gelation is studied by circular dichroism, X-ray diffraction, FTIR, photoluminescence spectroscopy, temperature dependent NMR and 2D NOESY experiments. Importantly, the mechanical strength of the organogelator in various solvents is found to be very high and the storage modulus value varies from 5000 to 45 000 Pa. Interestingly, this organogelator exhibits phase selective gelation by solidifying both aromatic and aliphatic organic solvents from their mixtures with water. Furthermore, the corresponding xerogel has an excellent ability of removing toxic dyes from water within a short period of time.

pH-responsive reversible dispersion of biocompatible SWNT/graphene–amphiphile hybrids

The present work reports synthesis of cholesterol based peptide carboxylates as efficient dispersing agents for single walled carbon nanotubes (SWNTs) as well as graphene in water. Variation of the amino acids within the peptide moiety exhibited interesting changes in their SWNTs dispersion efficacy. The dipeptide carboxylate comprising of two alanine residues showed 80% SWNTs dispersion which is ∼2 fold higher than that obtained by using the common surfactant, SDBS. The dipeptide amphiphiles also efficiently dispersed the 2D-allotrope of carbon, graphene, in water. As to our objective, the terminal carboxylate moiety in these cholesterol based carboxylates exhibited pH-sensitivity towards the reversible solubilization and precipitation of the nanohybrids. Acidification of the nanohybrids with HCl converted the carboxylates to the water insoluble carboxylic acids leading to the precipitation of carbon nanomaterials. Most importantly, addition of an equivalent amount of NaOH resulted in the restoration of stable aqueous dispersion of SWNT/graphene. This reversible precipitation–dispersion cycle was performed time and again. The conversion of the carboxylate salt to the corresponding acid and vice versa is the main reason for such reversible switching between precipitation and dissolution under acidic and basic pH. Indeed the presence of the amino acid/peptide moiety in the structure of cholesterol carboxylates was found to be indispensable for efficient dispersion of carbon nanomaterials. Significant stability of these SWNT dispersions was observed in the presence of high salt and protein concentration. Moreover, the nanohybrids were highly biocompatible with mammalian cells, which increases their future prospects in biomedicine.

Pyrene-Based Fluorescent Supramolecular Hydrogel: Scaffold for Energy Transfer

The self-assembled gelation of an amino-acid-based low molecular weight gelator having a pyrene moiety at the N terminus and a bis-ethyleneoxy unit linked with succinic acid at the C terminus is reported. This amphiphile is capable of gelating binary mixtures (1/3 v/v) of CH3CN/water, DMSO/water, and DMF/water, and the minimum gelation concentration (MGC) varied from 0.2 to 0.3% w/v. The sodium salt of the amphiphile efficiently gelates water with an MGC of 1.5% w/v. The participation of different noncovalent interactions in supramolecular gelation by formation of fibrillar networks was investigated by spectroscopic and microscopic methods. High mechanical strength of the supramolecular gels is indicated by storage moduli on the order of 10(3) Pa. The hydrogel was utilized for energy transfer, whereby inclusion of only 0.00075% w/v of acridine orange resulted in about 50% quenching of the fluorescence intensity of the gel through fluorescence resonance energy transfer.