Bimalendu Adhikari

Self-assembling tripeptide based hydrogels and their use in removal of dyes from waste-water

A new class of hydrogelators based on synthetic self-assembling N-terminally Boc-protected tripeptides has been developed. A series of five tripeptides have been synthesized to study their self-assembling behavior in aqueous medium. Three of them form thermoreversible translucent gels at basic pH (pH 11.5–13.5). These hydrogels were characterized by FT-IR spectroscopy, circular dichroism (CD), small angle X-ray diffraction analysis (SAXRD), field-emission scanning electron microscopic (FE-SEM), transmission electron microscopic (TEM) and atomic force microscopic (AFM) studies. These hydrogels can be potentially utilized for the treatment of waste-water and the organic dyes (Rhodamine B, Reactive Blue 4 and Direct Red 80) that are widely used in textile industries can be efficiently removed. Moreover, peptide gelators can be recovered very easily just by changing the pH of the medium.

Short-Peptide-Based Hydrogel: A Template for the In Situ Synthesis of Fluorescent Silver Nanoclusters by Using Sunlight

N-terminally Fmoc-protected dipeptide, Fmoc-Val-Asp-OH, forms a transparent, stable hydrogel with a minimum gelation concentration of 0.2% w/v. The gelation property of the hydrogel was investigated by using methods such as transmission electron microscopy, field-emission scanning electron microscopy, atomic force microscopy and Fourier transform infrared spectroscopy. The silver-ion-encapsulating hydrogel can efficiently and spontaneously produce fluorescent silver nanoclusters under sunlight at physiological pH (7.46) by using a green chemistry approach. Interestingly, in the absence of any conventional reducing agent but in the presence of sunlight, silver ions were reduced by the carboxylate group of a gelator peptide that contains an aspartic acid residue. These clusters were investigated by using UV/Vis spectroscopy, photoluminescence spectroscopy, high-resolution transmission electron microscopy (HR-TEM), atomic force microscopy (AFM) and X-ray diffraction (XRD) studies. Mass spectrometric analysis shows the presence of a few atoms in nanoclusters containing only Ag(2). The reported fluorescent Ag nanoclusters show excellent optical properties, including a very narrow emission profile and large Stokes shift (>100 nm). The reported fluorescent Ag nanoclusters within hydrogel are very stable even after 6 months storage in the dark at 4 °C. The as-prepared hydrogel-nanocluster conjugate could have applications in antibacterial preparations, bioimaging and other purposes.

Pyrene-containing peptide-based fluorescent organogels: inclusion of graphene into the organogel.

The N-terminally pyrene-conjugated oligopeptide, Py-Phe-Phe-Ala-OMe, (Py=pyrene 1-butyryl acyl) forms transparent, stable, supramolecular fluorescent organogels in various organic solvents. One of these organogels was thoroughly studied using various techniques including transmission electron microscopy (TEM), field-emission scanning electron microscopy (FESEM), atomic force microscopy (AFM), Fourier-transform infrared (FTIR) spectroscopy, photoluminescence (PL) spectroscopy, and rheology. Unfunctionalized and non-oxidized graphene was successfully incorporated into this fluorescent organogel in o-dichlorobenzene (ODCB) to form a stable hybrid organogel. Graphene is well dispersed into the gel medium by using non-covalent π-π stacking interactions with the pyrene-conjugated gelator peptide. In the presence of graphene, the minimum gelation concentration (mgc) of the hybrid organogel was lowered significantly. This suggests that there is a favorable interaction between the graphene and the gelator peptide within the hybrid organogel system. This hybrid organogel was characterized using TEM, AFM, FTIR, PL, and rheological studies. The TEM study of graphene-containing hybrid organogel revealed the presence of both graphene sheets and entangled gel nanofibers. The AFM study indicated the presence of 3 to 4 layers in exfoliated graphene in ODCB and the presence of both graphene nanosheets and the network of gel nanofibers in the hybrid gel system. The rheological investigation suggested that the flow of the hybrid organogel had become more resistant towards the applied angular frequency upon the incorporation of graphene into the organogel. The hybrid gel is about seven times more rigid than that of the native gel.

Short peptide based hydrogels: incorporation of graphene into the hydrogel

Stable supramolecular hydrogels were obtained from Fmoc (N-fluorenyl-9-methoxycarbonyl) protected synthetic dipeptides, Fmoc-Xaa-Asp-OH (Xaa = Tyr, Phe). These hydrogels were characterized by various methods including transmission electron microscopy (TEM), field-emission scanning electron microscopy (FE-SEM), atomic force microscopy (AFM), fluorescence spectroscopy and rheology. Different microscopic studies showed the presence of an entangled uniform nanofibrillar network structure in the gel state. These gelator molecules containing aromatic moieties in the side chain (Tyr/Phe) and in the N-terminus (fluorenyl group) can be useful in interacting with graphene sheets using π–π stacking interactions. One of these peptide based hydrogels (Fmoc-Tyr-Asp-OH) was utilized for the successful incorporation of reduced graphene oxide (RGO) into the hydrogel to make a well-dispersed RGO containing stable hybrid hydrogel. This study demonstrates that RGO is stabilized within the peptide based hydrogel system without the help of any external stabilizing agent. The RGO containing hybrid hydrogel was characterized using transmission electron microscopy, selected area electron diffraction, atomic force microscopy, Raman spectroscopy, and rheology. Morphological studies reveal the presence of a nano-hybrid system containing graphene (RGO) sheets and gel nanofibrils. The morphology of the peptide hydrogel does not change significantly even after the incorporation of RGO as it is evident from TEM and AFM studies. Rheological studies suggest the formation of a more rigid and ‘solid-like’ hybrid hydrogel after the incorporation of RGO into the native hydrogel.

A Gel‐Based Trihybrid System Containing Nanofibers, Nanosheets, and Nanoparticles: Modulation of the Rheological Property and Catalysis

Wonderful gels: A trihybrid gel was prepared by incorporating graphene oxide and in situ synthesized gold nanoparticles (AuNPs) into an amino-acid-based native gel matrix (see pict). The morphology of this system indicates the presence of three distinctly different nanostructures: nanofibers, nanosheets, and nanoparticles. The catalytic efficiency of this trihybrid system is enhanced relative to that of AuNPs in a dihybrid system.

Multicomponent hydrogels from enantiomeric amino acid derivatives: helical nanofibers, handedness and self-sorting

In this study, chiral helical nanofibers have been obtained from suitable, co-assembling, two oppositely charged amino acid based two component hydrogels. An equimolar mixture of an N-terminally protected amino acid Fmoc-(L/D)Glu (Fmoc: N-fluorenyl-9-methoxycarbonyl, Glu: glutamic acid) and (L/D)Lys (Lys: lysine) can co-assemble to form hydrogels. These hydrogels have been characterised using circular dichroism (CD), atomic force microscopy (AFM), transmission electron microscopy (TEM), X-ray powder diffraction, fluorescence spectroscopic and rheological studies. CD and AFM studies have been extensively used to examine the chiral/achiral nature of fibers obtained from different hydrogel systems. The equimolar mixture of two L-isomers, {Fmoc-(L)Glu + (L)Lys} in the assembled state, leads to the exclusive formation of left-handed helical nanofibers, whereas an equimolar mixture of two D-isomers, {Fmoc-(D)Glu + (D)Lys}, gives rise to right-handed helical nanofibers. The CD study of the gel obtained from the {Fmoc-(L)Glu + (L)Lys} system is exactly the mirror image of the CD signal obtained from the gel of the {Fmoc-(D)Glu + (D)Lys} system. These results suggest that the molecular chirality is being translated into the supramolecular helicity and the handedness of these fibers depends on the corresponding molecular chirality in the mixture of the two component system. Reversing the handedness of helical fibers is possible by using enantiomeric building blocks. Co-assembly of racemic and equimolar mixtures of all four components, i.e., [{Fmoc-(L)Glu + (L)Lys} + {Fmoc-(D)Glu + (D)Lys}] can also form hydrogels. Interestingly, in this racemic mixture self-sorting has been observed with the presence of almost equal amount of left- and right-handed helical nanofibers. The equimolar mixture of Fmoc-(L)Glu and L-ornithine/L-arginine also produces hydrogel with left-handed helical fibers. Moreover, the straight fiber has been observed from the two component hydrogel {Fmoc-(L)Glu + (L)Lys} system in the presence of Ca2+/Mg2+ ions. This indicates the straight nanofibers are obtained under suitable conditions and acid–base interaction is responsible for making the helical fibers at the nanoscale.

Self-assembly of guanosine and deoxy-guanosine into hydrogels: monovalent cation guided modulation of gelation, morphology and self-healing properties

In this study, we report the effect of monovalent cations including Na+, K+, Rb+, Ag+, Au+, Tl+, Hg+ and NH4 + on the stimulation of purine nucleosides guanosine (G) and deoxy-guanosine (dG) to self-assemble into hydrogels. The gelation properties in terms of gel crystallization, lifetime stability, thermo-reversibility, minimum gelation concentration, gel melting temperature, thixotropic property and others were thoroughly investigated and compared not only between two nucleosides but also among different metal ions. A few metal ions were found to induce G/dG to form gels with much improved lifetime stability. The results revealed that dG is a much better gelator than G for introduction of a thixotropic property. Interestingly, morphological, fluorescence and rheological (thixotropic) properties of the gels were found to modulate significantly by changing the metal ions. In the presence of K+ ions, G produces a self supporting tight gel but it has poor lifetime stability. In contrast, the presence of K+ ions stimulates dG to form a very loose gel but with excellent lifetime stability and thixotropic property. In this context, we have successfully engineered a self-supporting stable co-gel using a simple co-gelation method by mixing an equimolar amount of K+ induced G and dG gels and this co-gel shows long lifetime stability, self-healing and injectable properties which may allow for a broad range of biological applications. Furthermore, an Ag+ induced G gel was exploited for the light triggered in situ fabrication of uniform AgNPs within a gel to make a nano-bio hybrid material.

Sensitive electrochemical detection of Salmonella with chitosan–gold nanoparticles composite film

An ultrasensitive electrochemical immunosensor for detection of Salmonella has been developed based on using high density gold nanoparticles (GNPs) well dispersed in chitosan hydrogel and modified glassy carbon electrode. The composite film has been oxidized in NaCl solution and used as a platform for the immobilization of capture antibody (Ab1) for biorecognition. After incubation in Salmonella suspension and horseradish peroxidase (HRP) conjugated secondary antibody (Ab2) solution, a sandwich electrochemical immunosensor has been constructed. The electrochemical signal was obtained and improved by comparing the composite film with chitosan film. The result has shown that the constructed sensor provides a wide linear range from 10 to 10(5) CFU/mL with a low detection limit of 5 CFU/mL (at the ratio of signal to noise, S/N=3:1). Furthermore, the proposed immunosensor has demonstrated good selectivity and reproducibility, which indicates its potential in the clinical diagnosis of Salmonella contaminations.

Amino Acid Chirality and Ferrocene Conformation Guided Self-Assembly and Gelation of Ferrocene–Peptide Conjugates

The self-assembly and gelation behavior of a series of mono- and disubstituted ferrocene (Fc)-peptide conjugates as a function of ferrocene conformation and amino acid chirality are described. The results reveal that ferrocene-peptide conjugates self-assemble into organogels by controlling the conformation of the central ferrocene core, through inter- versus intramolecular hydrogen bonding in the attached peptide chain(s). The chirality controlled assembling studies showed that two monosubstituted Fc conjugates FcCO-LFLFLA-OMe and FcCO-LFLFDA-OMe form gels with nanofibrillar network structures, whereas the other two diastereomers FcCO-DFLFLA-OMe and FcCO-LFDFLA-OMe exclusively produced straight nanorods and non-interconnected small fibers, respectively. This suggests the potential tuning of gelation behavior and nanoscale morphology by altering the chirality of constituted amino acids. The current study confirms the profound effect of diastereomerism and no influence of enantiomers on gelation. Correspondingly, the diastereomeric and enantiomeric Fc[CO-FFA-OMe]2 were constructed for the study of chirality-organized structures.

Development of photocatalysts for selective and efficient organic transformations

One of the main goals of organic chemists is to find easy, environmentally friendly, and cost effective methods for the synthesis of industrially important compounds. Photocatalysts have brought revolution in this regard as they make use of unlimited source of energy (the solar light) to carry out the synthesis of organic compounds having otherwise complex synthetic procedures. However, selectivity of the products has been a major issue since the beginning of photocatalysis. The present article encompasses state of the art accomplishments in harvesting light energy for selective organic transformations using photocatalysts. Several approaches for the development of photocatalysts for selective organic conversions have been critically discussed with the objective of developing efficient, selective, environmental friendly and high yield photocatalytic methodologies.