Subhasish Roy

Amino acid based smart hydrogel: formation, characterization and fluorescence properties of silver nanoclusters within the hydrogel matrix

N-Terminally Fmoc protected amino acid, Fmoc-Phe-OH (Fmoc-L-Phenylalanine-OH), forms an efficient, stable and transparent hydrogel with a minimum gelation concentration of 0.1% w/v. This hydrogel has been nicely utilized to prepare and stabilize fluorescent few-atom silver nanoclusters. Interestingly, in absence of any toxic reducing agents in a water medium, silver ions are complexed with the carboxylate group of the Fmoc-Phe-OH gelator, and they are reduced spontaneously in the presence of diffused sunlight at physiological pH (7.46) and room temperature to form silver nanoclusters. The three dimensional structure provided by the hydrogel helps to stabilize newly formed silver nanoclusters within the hydrogel matrix. These clusters have been examined using UV-Vis, photoluminescence spectroscopy, high resolution transmission electron microscopy (HR-TEM), X-ray powder diffraction (XRPD) and matrix-assisted laser-desorption ionization (MALDI) mass spectrometric analysis. MALDI mass spectrometric analysis shows the presence of just a few atoms within the silver cluster as Ag4. These silver nanoclusters exhibit interesting fluorescent properties including large Stokes shift (more than 110 nm), narrow emission band width (36 ± 1 nm) and a quantum yield of 3.76%. These silver nanoclusters are stable for up to 4 months of storage at 4 °C in the dark. The morphology of the hydrogel changes after the encapsulation of silver ions within the gel and this altered morphology is retained after the formation of silver nanoclusters within the gel. Interestingly, the rheological properties of the hydrogel alone are different from that of the silver nanocluster-containing hydrogel.

Assembly of an injectable noncytotoxic peptide-based hydrogelator for sustained release of drugs.

A new synthetic tripeptide-based hydrogel has been discovered at physiological pH and temperature. This hydrogel has been thoroughly characterized using different techniques including field emission scanning electron microscopic (FE-SEM) and high-resolution transmission electron microscopic (HR-TEM) imaging, small- and wide-angle X-ray diffraction analyses, FT-IR, circular dichroism, and rheometric analyses. Moreover, this gel exhibits thixotropy and injectability. This hydrogel has been used for entrapment and sustained release of an antibiotic vancomycin and vitamin B12 at physiological pH and temperature for about 2 days. Interestingly, MTT assay of these gelator molecules shows almost 100% cell viability of this peptide gelator, indicating its noncytotoxicity.

Fluorescent Au@Ag Core–Shell Nanoparticles with Controlled Shell Thickness and HgII Sensing

Au-Ag core-shell nanoparticles have been synthesized using synthetic fluorescent dipeptide β-Ala-Trp (β-Ala is β-alanine; Trp is l-tryptophan) in water at pH 6.94 and at room temperature. The synthesis of the Au-Ag core-shell nanomaterial does not involve any external reducing and stabilizing agents, and the constituents of dipeptide β-alanine and l-tryptophan are naturally occurring. Therefore, the synthesis procedure is ecofriendly. Moreover, the shell thickness has also been controlled, and the optical property of the core-shell nanomaterial varies with the shell thickness. The core-shell nanomaterial exhibits a fascinating fluorescence property. This fluorescent Au@Ag core-shell nanoparticle can detect toxic Hg(II) ions ultrasensitively (with a lower limit of detection of 9 nM) even in presence of Zn(II), Cd(II), and other bivalent metal ions (Ca(II), Mg(II), Ni(II), Mn(II), Ba(II), Sr(II), Pb(II), and Fe(II)). Au-Ag core-shell nanomaterials can also be reused for sensing Hg(II) ions.

An Amino‐Acid‐Based Self‐Healing Hydrogel: Modulation of the Self‐Healing Properties by Incorporating Carbon‐Based Nanomaterials

An amino-acid-based (11-(4-(pyrene-1-yl)butanamido)undecanoic acid) self-repairing hydrogel is reported. The native hydrogel, as well as hybrid hydrogels, have been thoroughly characterized by using various microscopic techniques, including transmission electron microscopy (TEM), atomic force microscopy (AFM), Raman spectroscopy, fluorescence spectroscopy, FTIR spectroscopy, X-ray diffraction, and by using rheological experiments. The native hydrogel exhibited interesting fluorescence properties, as well as a self-healing property. Interestingly, the self-healing, thixotropy, and stiffness of the native hydrogel can be successfully modulated by incorporating carbon-based nanomaterials, including graphene, pristine single-walled carbon nanotubes (Pr-SWCNTs), and both graphene and Pr-SWCNTs, within the native gel system. The self-recovery time of the gel was shortened by the inclusion of reduced graphene oxide (RGO), Pr-SWCNTs, or both RGO and Pr-SWCNTs. Moreover, hybrid gels that contained RGO and/or Pr-SWCNTs exhibited interesting semiconducting behavior.

Tuning of Silver Cluster Emission from Blue to Red Using a Bio-Active Peptide in Water

Blue, green, and red emitting silver quantum clusters have been prepared through green chemical approach by using a bio-active peptide glutathione (reduced) in a 50 mM phosphate buffer at pH 7.46. This study describes fluorescence emission tuning of the silver clusters by making different sized Ag clusters using slightly different reaction conditions keeping the same stabilizing ligand, reducing agent, solvent system, and silver salt precursor. The preparation procedure of these silver quantum clusters is new and highly reproducible. Each of these clusters shows very interesting fluorescence properties with large stokes shifts, and the quantum yields of blue, green, and red clusters are 2.08%, 0.125%, and 1.39%, respectively. These silver quantum clusters have been characterized by using different techniques including fluorescence spectroscopy, UV-vis spectroscopy, field-emission gun transmission electron microscopic (FEG-TEM) imaging and MALDI-TOF MS analyses. MALDI-TOF MS analyses show that the size of these blue, green and red emitting silver clusters are Ag5 (NC1, nanoclusters 1), Ag8 (NC2, nanoclusters 2) and Ag13 (NC3, nanoclusters 3), respectively, by using 2,5-dihydroxybenzoic acid as a matrix. These clusters are stable in broad ranges of pH. The NC3 (red emitting) has been successfully utilized for selective and sensitive detection of toxic Hg(II) ions in water by using even naked eyes, fluorometric, and calorimetric studies. The lower limit of detection of Hg(II) ions in water has been estimated to be 126 and 245 nM from fluorometric and UV-vis analyses, respectively. Enthalpy change (ΔH) during this Hg(II) sensing process is 2508 KJ mol(-1).

Self-Assembling Dipeptide-Based Nontoxic Vesicles as Carriers for Drugs and Other Biologically Important Molecules

Self-assembling short peptides can offer an opportunity to make useful nano-/microstructures that find potential application in drug delivery. We report here the formation of multivesicular structures from self-assembling water-soluble synthetic amphiphilic dipeptides containing a glutamic acid residue at the C-terminus. These vesicular structures are stable over a wide range of pH (pH 2-12). However, they are sensitive towards calcium ions. This causes the rupturing of these vesicles. Interestingly, these vesicles can not only encapsulate an anticancer drug and a fluorescent dye, but also can release them in the presence of calcium ions. Moreover, these multivesicular structures have the potential to carry biologically important molecules like cyclic adenosine monophosphate (cAMP) within the cells keeping their biological functions intact. A MTT cell-survival assay suggests the almost nontoxic nature of these vesicles. Thus, these peptide vesicles can be used as biocompatible delivery vehicles for carrying drugs and other bioactive molecules.

A new hydrogel from an amino acid-based perylene bisimide and its semiconducting, photo-switching behaviour

A new, stable, semiconducting, photo-responsive, amino acid (L-tyrosine) based perylene bisimide derivative (PBI-Y) was found to form pH-sensitive hydrogels. This gelator molecule (PBI-Y) has undergone hydrogelation at 50 mM phosphate buffer at pH 5.00–9.00. Different minimum gelation concentration values (MGC) were recorded in different pHs at the same phosphate buffer strength (50 mM). The hydrogel was also characterized using different techniques, including X-ray diffraction analysis, rheology and FT-IR study. The fluorescence and cyclic voltammetric responses of the PBI-Y hydrogelator can be tuned by changing the pH. The PBI-Y hydrogelator has shown self-assembled nanofibrillar network structures, as it has been evident from the field emission scanning electron microscopic (FE-SEM), as well as transmission electron microscopic analyses (TEM). This PBI-Y hydrogel has also shown interesting semiconducting behaviour. This PBI-Y hydrogelator-based photoconductor was found to exhibit excellent photo-switching behaviour with a high photo-response value. This holds a future promise for the creation of PBI-conjugated functional soft-materials-based photodetectors and photovoltaics.

Functionalized single walled carbon nanotube containing amino acid based hydrogel: a hybrid nanomaterial

Fmoc-protected amino acid based (Fmoc-Phe-OH) hydrogel has been developed in our laboratory previously (S. Roy and A. Banerjee, Soft Matter, 2011, 7, 5300–5308). This hydrogel has been used to incorporate and disperse functionalized single-walled carbon nanotube (f-SWCNT) within the gel phase to make a hybrid hydrogel at physiological pH and temperature and this is a convenient procedure to create a f-SWCNT based hybrid nanomaterial. The hybrid hydrogel has been characterized using different microscopic studies including transmission electron microscopic imaging (TEM) and atomic force microscopic imaging (AFM) and rheological studies. The AFM study demonstrates a nice 1D alignment of f-SWCNTs on the surface of the gel nanofibers. Functionalized SWCNT containing hybrid hydrogels are more thermally stable than the native gel. Rheological experiments show that the hybrid hydrogel is a more elastic material than the native hydrogel obtained from the Fmoc-Phe-OH. Moreover, G′ (storage modulus) of the native hydrogel has been increased about 16 times upon the inclusion of f-SWCNT into the gel matrix. Conductivity of the f-SWCNT containing hydrogel has been found to be 3.12 S cm−1 and conductivity of the f-SWCNT has dropped after incorporation of f-SWCNT into the hydrogel system.

A Peptide-Based Mechano-sensitive, Proteolytically Stable Hydrogel with Remarkable Antibacterial Properties

A long-chain amino acid containing dipeptide has been found to form a hydrogel in phosphate buffer whose pH ranges from 6.0 to 8.8. The hydrogel formed at pH 7.46 has been characterized by small-angle X-ray scattering (SAXS), wide-angle powder X-ray diffraction (PXRD), Fourier transform infrared (FT-IR) spectroscopy, field-emission scanning electron microscopy (FE-SEM), high-resolution transmission electron microscopy (HR-TEM) imaging and rheological analyses. The microscopic imaging studies suggest the formation of a nanofibrillar three-dimensional (3D) network for the hydrogel. As observed visually and confirmed rheologically, the hydrogel at pH 7.46 exhibits thixotropy. This thixotropic property can be exploited to inject the peptide. Furthermore, the hydrogel exhibits remarkable antibacterial activity against Gram-negative bacteria Escherichia coli and Pseudomonas aeruginosa, which are responsible for many common diseases. The hydrogel has practical applicability due to its biocompatibility with human red blood cells and human fibroblast cells. Interestingly, this hydrogel shows high resistance toward proteolytic enzymes, making it a new potential antimicrobial agent for future applications. It has also been observed that a small change in molecular structure of the gelator peptide not only turns the gelator into a nongelator molecule under similar conditions, but it also has a significant negative impact on its bactericidal character.

A bolaamphiphilic amino acid appended photo-switching supramolecular gel and tuning of photo-switching behaviour

Self-assembled bolaamphiphilic perylene bisimide (PBI) containing an amino acid appended fluorescent semiconducting soft material (hydrogel) has been discovered at physiological pH. This new organic material based on self-assembled perylene bisimide appended amino acid-based bolaamphiphile (PBI-C11-Y) has been well characterized using various techniques including UV-Vis, fluorescence, X-ray diffraction, FT-IR, transmission electron microscopic (TEM) and atomic force microscopic (AFM) studies. Interestingly, the UV-Vis absorption properties of the soft-material are dependent on the pH of the medium. This PBI-conjugated amino acid appended gelator molecule contains a centrally located perylene bisimide moiety as well as an aromatic amino acid l-tyrosine at the side chains, which are extremely useful for interacting with the delocalized large π-surface of GO (graphene oxide) or RGO (reduced graphene oxide) to form a GO/RGO containing hybrid hydrogel. Graphene oxide and reduced graphene oxide have been successfully incorporated into the nanofibrillar network structure of the PBI-C11-Y based gel to make nanohybrid systems. The I-V profile of the semiconducting photo-responsive soft-material of the PBI-C11-Y has been successfully tuned upon the incorporation of GO and RGO within the gel-based soft material. This PBI-C11-Y xerogel based structure shows photo-switching behaviour upon exposure to white light. The ON/OFF ratio of the PBI-C11-Y can be modulated upon the inclusion of GO and RGO within the hydrogel matrix. Furthermore, the OFF state stability of the PBI-C11-Y xerogel material has been increased upon the inclusion of RGO. Regulation of the photo-switching behaviour of the PBI-C11-Y based xerogel holds promise for making PBI-containing amino acid appended biomaterials with interesting properties in future.