Chayan Kanti Nandi

Time-Resolved Emission Reveals Ensemble of Emissive States as the Origin of Multicolor Fluorescence in Carbon Dots

The origin of photoluminescence in carbon dots has baffled scientists since its discovery. We show that the photoluminescence spectra of carbon dots are inhomogeneously broadened due to the slower relaxation of the solvent molecules around it. This gives rise to excitation-dependent fluorescence that violates the Kasha-Vavilov rule. The time-resolved experiment shows significant energy redistribution, relaxation among the emitting states, and spectral migration of fluorescence spectra in the nanosecond time scale. The excitation-dependent multicolor emission in time-integrated spectra is typically governed by the relative population of these emitting states.

Carbon dots for naked eye colorimetric ultrasensitive arsenic and glutathione detection

A novel one-step method for the synthesis of bright, multicolor fluorescent sulphur doped carbon dots (CNDs) has been developed by using simple microwave assisted pyrolysis of citric acid and sodium thiosulphate. The synthesized CNDs showed dual mode naked eye colorimetric ultrasensitive sensing capability both for arsenic [As (III)] and glutathione (GSH) with high selectivity. Using fluorometric assay, the detection limit (DL) for As (III) was found to be as low as 32pM. The selectivity data show that the newly developed CNDs is very specific for As (III) even with interference by high concentrations of other metal ions. The CNDs were also able to detect GSH very selectively over other biothiols like cysteine (Cys) and homo-cysteine (H-cys) with a DL of 43nM, even in blood plasma. The fast kinetic data suggests that the present CNDs assay could be used onsite As (III) detection. The CNDs, further, showed its potential application in high resolution bioimaging of bacterial nucleoid segregation.

Kinetics of protein adsorption on gold nanoparticle with variable protein structure and nanoparticle size.

The spontaneous protein adsorption on nanomaterial surfaces and the formation of a protein corona around nanoparticles are poorly understood physical phenomena, with high biological relevance. The complexity arises mainly due to the poor knowledge of the structural orientation of the adsorbed proteins onto the nanoparticle surface and difficulties in correlating the protein nanoparticle interaction to the protein corona in real time scale. Here, we provide quantitative insights into the kinetics, number, and binding orientation of a few common blood proteins when they interact with citrate and cetyltriethylammoniumbromide stabilized spherical gold nanoparticles with variable sizes. The kinetics of the protein adsorption was studied experimentally by monitoring the change in hydrodynamic diameter and zeta potential of the nanoparticle-protein complex. To understand the competitive binding of human serum albumin and hemoglobin, time dependent fluorescence quenching was studied using dual fluorophore tags. We have performed molecular docking of three different proteins--human serum albumin, bovine serum albumin, and hemoglobin--on different nanoparticle surfaces to elucidate the possible structural orientation of the adsorbed protein. Our data show that the growth kinetics of a protein corona is exclusively dependent on both protein structure and surface chemistry of the nanoparticles. The study quantitatively suggests that a general physical law of protein adsorption is unlikely to exist as the interaction is unique and specific for a given pair.

Reversible Photoswitching of Carbon Dots

We present a method of reversible photoswitching in carbon nanodots with red emission. A mechanism of electron transfer is proposed. The cationic dark state, formed by the exposure of red light, is revived back to the bright state with the very short exposure of blue light. Additionally, the natural on-off state of carbon dot fluorescence was tuned using an electron acceptor molecule. Our observation can make the carbon dots as an excellent candidate for the super-resolution imaging of nanoscale biomolecules within the cell.

Direct Visualization of Lead Corona and Its Nanomolar Colorimetric Detection Using Anisotropic Gold Nanoparticles

The study presents dithiothreitol (DTT) functionalized anisotropic gold nanoparticles (GNP) based colorimetric sensor for detection of toxic lead ions in water. Our results demonstrate the selectivity and sensitivity of the developed sensor over various heavy metal ions with detection limit of ∼9 nM. The mechanism of sensing is explained on the basis of unique corona formation around the DTT functionalized anisotropic GNP.

Lysine and dithiothreitol promoted ultrasensitive optical and colorimetric detection of mercury using anisotropic gold nanoparticles

Using lysine (Lys) and dithiothreitol (DTT) as aggregation promoters, an ultrasensitive and highly selective colorimetric assay for detection of mercury (Hg2+) ions has been developed with anisotropic gold nanoparticles (GNP). The detection limit of the assay is determined to be 27 pM and 58 pM for Hg2+ in deionized water and tap water, respectively. The mechanism of sensing is explained on the basis of possible mercury-gold amalgam formation and subsequent aggregation of the GNP upon addition of Lys and DTT. The kinetics data indicate that the sensor can be effectively used for on-site and real-time Hg2+ detection.

Orientational switching of protein conformation as a function of nanoparticle curvature and their geometrical fitting

Among the various surface properties, nanoparticle curvature has a direct effect on the inner root of protein nanoparticle interaction. However, the orientation of adsorbed proteins onto the nanoparticle surface and its binding mechanism still remains elusive because of the lack of in-depth knowledge at the molecular level. Here, we demonstrate detail molecular insights of the orientational switching of several serum proteins as a function of nanoparticle curvature using theoretical simulation along with some experimental results. With the variation of binding stability, four distinctly different classes of orientation were observed for human serum albumin, whereas only two unique classes of conformations were observed for ubiquitin, insulin, and haemoglobin. As a general observation, our data suggested that orientations were exclusively dependent on the specific protein structure and the geometrical fitting onto the nanoparticle surface.

One pot synthesis of doxorubicin loaded gold nanoparticles for sustained drug release

Here, we report a facile, versatile and simple one-pot synthesis of doxorubicin (Dox) loaded gold nanoparticles (Dox–GNP conjugate), where Dox can act both as a reducing as well as a capping agent. Interestingly, when the conjugate was placed into the transporter protein environment, it avoided the undesirable multilayer protein corona formation, which is very common for nanomaterials. The in vitro drug release kinetic studies and the cytotoxicity assay and cellular update efficiency advocates that the system is capable of sustained release of the drug even in the presence of a complex biological environment.

Effect of surface chemistry and morphology of gold nanoparticle on the structure and activity of common blood proteins

Keeping the native structure and activity of proteins, while adsorbed onto a nanoparticle surface, is one of the pre-requisites for the real biological applications of nanoparticles. However, this phenomenon is poorly understood because of the lack of in-depth knowledge on the structural orientation of the adsorbed protein, complex surface chemistry and morphology of the nanoparticle. In this study, we present quantitative information on the structure and the activity of a few major blood proteins when adsorbed onto different morphological and surface functionalized gold nanoparticles (GNPs). A profound effect of both the particle anisotropy and the surface ligands on the secondary structural change and consequently the activity of the proteins were observed. Furthermore, a prominent effect on the cell viability assay was also observed, when the MTT assay was performed using MDA-MB 231 cell lines.

Gold nanoparticle chitosan composite hydrogel beads show efficient removal of methyl parathion from waste water

Removal of pesticides from waste water is extremely important for the avoidance of serious health risks. We report an environmentally benign synthetic approach to prepare sorbent–hydrogel composite beads, composed of gold nanoparticles (GNP) embedded in a cross-linked biocompatible chitosan polymer matrix, for removal of the pesticide methyl parathion (MP). The equilibrium sorption and kinetic data, at different initial MP concentrations, are analysed using various sorption isotherm and kinetic models. It is observed that the sorption process can be best described by the Langmuir isotherm model and that pseudo second-order kinetics are followed throughout the studied concentration range. The monolayer capacity of the beads with and without the GNP composite are determined to be 58 μmol g−1 and 11 μmol g−1, respectively, suggesting the potential usefulness of the former for the removal of MP. The results show that the synthesized sorbent has good potential for pesticide removal.