Synthesis of a low-molecular-weight fluorescent ambidextrous gelator: development of graphene- and graphene-oxide-included gel nanocomposites

Low-molecular-weight fluorescent supramolecular ambidextrous gelators have drawn enormous attention owing to their ever-increasing number of potential applications. In this study, we synthesized a low-molecular-weight fluorescent ambidextrous gelator consisting of hydrophobic pyrenebutyric acid moieties at the N termini of a central l-lysine, and an ethyleneoxy unit coupled with 1-(3-aminopropyl)imidazole via a succinic acid linker as a hydrophilic moiety at the C terminus. This compound showed efficient hydrogelation and organogelation, having minimum gelation concentrations of 0.5 % (w/v) in a DMSO/water mixture (1:4 v/v) and 0.2 % (w/v) in nitrobenzene. The presence of the pyrene moieties in the structure rendered the ambidextrous gelator fluorescent. Significant amounts of both graphene oxide and reduced graphene oxide were included within these gels to form a stable soft nanohybrid. Hydrogen bonding, π–π stacking and van der Waals interactions are the key factors for self-assembly leading to gelation. The complementary interactions between the π-electronic surface of 2D graphene sheet and the aromatic pyrene moieties of the gelator play key roles in the inclusion of the nanomaterials into the gels. The inclusion of carbon nanomaterials within the gels resulted in a notable improvement in the stiffness of the soft nanocomposites compared to native gels. The native gels and soft nanocomposites have shear thinning properties and are thixotropic in nature

Pristine carbon-nanotube-included supramolecular hydrogels with tunable viscoelastic properties

Research investigations involving pristine carbon nanotubes (CNTs) and their applications in diversified fields have been gathering enormous impetus in recent times. One such emerging domain deals with the hybridization of CNTs within hydrogels to form soft nanocomposites with superior properties. However, till now, reports on the inclusion of pristine CNTs within low-molecular-weight hydrogels are very scarce due to their intrinsic feature of remaining in the bundled state and strong repulsive behavior to the aqueous milieu. Herein, the synthesis of a series of amino acid/dipeptide-based amphiphilic hydrogelators having a quaternary ammonium/imidazolium moiety at the polar head and a C16 hydrocarbon chain as the hydrophobic segment is reported. The synthesized amphiphiles exhibited excellent hydrogelation (minimum gelation concentration (MGC) ≈0.7–5 % w/v) as well as single-walled carbon nanotube (SWNT) dispersion ability in aqueous medium. Interestingly, the dispersed SWNTs were incorporated into the supramolecular hydrogel formed by amphiphiles with an imidazolium moiety at the polar end through complementary cation–π and π–π interactions. More importantly, the newly synthesized hydrogelators were able to accommodate a significantly high amount of pristine SWNTs (2–3.5 % w/v) at their MGCs without affecting the gelating properties. This is the first time that such a huge amount of SWNTs has been successfully incorporated within hydrogels. The efficient inclusion of SWNTs to develop soft nanocomposites was thoroughly investigated by spectroscopic and microscopic methods. Remarkably, the developed nanocomposites showed manifold enhancement (≈85-fold) in their mechanical strength compared with native hydrogel without SWNTs. The viscoelastic properties of these nanocomposites were readily tuned by varying the amount of incorporated CNTs

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

Influence of pristine SWNTs in supramolecular hydrogelation: scaffold for superior peroxidase activity of cytochrome c

The influence of pristine SWNTs in supramolecular hydrogelation of amphiphilic dipeptide carboxylates is delineated. The developed SWNT-nanohybrids with notable mechanical strength act as a scaffold for superior peroxidase activity of cytochrome c in organic media

In situ synthesised silver nanoparticle-infused L-lysine-based injectable hydrogel: development of a biocompatible, antibacterial, soft nanocomposite

The synthesis of L-lysine-based amphiphilic hydrogelator consisting of a naphthalene moiety at the N termini and an ethyleneoxy unit with free primary amine at the C terminus is reported. The amphiphile showed good gelation ability with minimum gelation concentration 0.6 % w/v in binary mixtures of dimethyl sulfoxide/phosphate buffer (1:4 v/v, pH 7.4). The hydrogel was characterised by spectroscopic and microscopic studies to delineate the role of non-covalent interactions in self-assembly gelation. Subsequently, Ag nanoparticles were synthesised within the hydrogel by in situ photo-reduction of AgNO3 under sunlight, in which the gelators act as reducing/stabilising agents. The nanocomposites were characterised by transmission electron microscopy, UV/Vis and X-ray diffraction spectroscopy and thermogravimetric analysis. Rheology of the soft nanocomposite showed significant mechanical strength and thixotropic self-recovery properties, which made the composite suitable for use as a syringe-injectable hydrogel. These soft nanocomposites exhibited excellent antibacterial activity against both Gram-positive and Gram-negative bacteria. They showed low haemolytic activity and high biocompatibility to mammalian (Chinese hamster ovarian) cells. In addition, agar–gelatin film infused with these nanocomposites allowed normal growth of mammalian cells on its surface while being lethal towards bacteria

The striking influence of SWNT–COOH on self-assembled gelation

A miniscule amount of f-SWNTs remarkably improved (∼17-fold) the gelation efficiency of amphiphilic molecules by triggering the formation of interconnecting self-assembled fibrillar networks (SAFIN) in supramolecular gelation

Fabrication of SWCNT-Ag Nanoparticle Hybrid Included Self-Assemblies for Antibacterial Applications

<div><p>The present article reports the development of soft nanohybrids comprising of single walled carbon nanotube (SWCNT) included silver nanoparticles (AgNPs) having superior antibacterial property. In this regard aqueous dispersing agent of carbon nanotube (CNT) containing a silver ion reducing unit was synthesised by the inclusion of tryptophan and tyrosine within the backbone of the amphiphile. The dispersions were characterized spectroscopically and microscopically using TEM, AFM and Raman spectroscopy. The nanotube-nanoparticle conjugates were prepared by the <i>in situ</i> photoreduction of AgNO₃. The phenolate residue and the indole moieties of tyrosine and tryptophan, respectively reduces the sliver ion as well as acts as stabilizing agents for the synthesized AgNPs. The nanohybrids were characterized using TEM and AFM. The antibacterial activity of the nanohybrids was studied against Gram-positive (<i>Bacillus subtilis</i> and <i>Micrococcus luteus</i>) and Gram-negative bacteria (<i>Escherichia coli</i> and <i>Klebsiella aerogenes</i>). The SWCNT dispersions showed moderate killing ability (40–60%) against Gram-positive bacteria however no antibacterial activity was observed against the Gram negative ones. Interestingly, the developed SWCNT-amphiphile-AgNP nanohybrids exhibited significant killing ability (∼90%) against all bacteria. Importantly, the cell viability of these newly developed self-assemblies was checked towards chinese hamster ovarian cells and high cell viability was observed after 24 h of incubation. This specific killing of bacterial cells may have been achieved due to the presence of higher –SH containing proteins in the cell walls of the bacteria. The developed nanohybrids were subsequently infused into tissue engineering scaffold agar-gelatin films and the films similarly showed bactericidal activity towards both kinds of bacterial strains while allowing normal growth of eukaryotic cells on the surface of the films.</p></div

Structure of the dispersing agents.

<p>PEG =  -(CH₂CH₂O)₁₂CH₃.</p

Raman Spectra of dispersed SWCNT-1 and SWCNT-2 using 514.5 nm excitation.

<p>Raman Spectra of dispersed SWCNT-1 and SWCNT-2 using 514.5 nm excitation.</p