Syamantak Khan

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.

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.

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.

Single-molecule analysis of fluorescent carbon dots towards localization-based super-resolution microscopy

The advancement of high-resolution bioimaging has always been dependent on the discovery of bright and easily available fluorescent probes. Fluorescent carbon nanodots, an interesting class of relatively new nanomaterials, have emerged as a versatile alternative due to their superior optical properties, non-toxicity, cell penetrability and easy routes to synthesis. Although a plethora of reports is available on bioimaging using carbon dots, single-molecule-based super-resolution imaging is rare in the literature. In this study, we have systematically characterized the single-molecule fluorescence of three carbon dots and compared them with a standard fluorescent probe. Each of these carbon dots showed a long-lived dark state in the presence of an electron acceptor. The electron transfer mechanism was investigated in single-molecule as well as in ensemble experiments. The average on-off rate between the fluorescent bright and dark states, which is one of the important parameters for single-molecule localization-based super-resolution microscopy, was measured by changing the laser power. We report that the photon budget and on-off rate of these carbon dots were good enough to achieve single-molecule localization with a precision of ~35 nm.

Optimizing the underlying parameters for protein-nanoparticle interaction: advancement in theoretical simulation

Abstract The interaction of nanosized materials with living organisms is the central concern in the key applications of nanotechnology. In particular, the protein adsorption to nanomaterial surface has been a major focus of study in the past decade. Unfortunately, the underlying principles and molecular mechanisms are still not well understood, and there have been various approaches to address the issue. Bottom-up approaches like computational simulations at the atomistic level have already proved their potential. Several force fields and models have been developed to simulate realistic dynamics to mimic the interaction of solid surfaces and peptides, even in some cases, the whole protein. However, there are a few major limitations and bottlenecks of these studies, which remain mostly ignored and unexplored. Here, we review the studies that have been the major contributors to our present understanding of the nanoparticle (NP)-protein interaction. As the complexity of this phenomenon arises from different stages, the study of protein-NP interactions from multiple directions is necessary. In the perspective of bioapplications, we discuss the major challenges of this field and future scopes of research that can be designed rationally, sometimes coupled with numerous available experimental techniques to understand NP-protein docking in a more realistic manner.

Charge-Driven Fluorescence Blinking in Carbon Nanodots

This study focuses on the mechanism of fluorescence blinking of single carbon nanodots, which is one of their key but less understood properties. The results of our single-particle fluorescence study show that the mechanism of carbon nanodots blinking has remarkable similarities with that of semiconductor quantum dots. In particular, the temporal behavior of carbon nanodot blinking follows a power law both at room and at cryogenic temperatures. Our experimental data suggest that static quenching via Dexter-type electron transfer between surface groups of a nanoparticle plays a major role in the transition of carbon nanodots to off or gray states, whereas the transition back to on states is governed by an electron tunneling from the particles core. These findings advance our understanding of the complex mechanism of carbon nanodots emission, which is one of the key steps for their application in fluorescence imaging.

Role of melting temperature in intermixing of miscible metal/metal bilayers induced by swift heavy ions

In the present work, we have investigated the role of melting temperature in miscible metallic systems, where one component is sensitive and the other component is insensitive to electronic energy deposition. The metal/metal bilayer system (W/Ti) was prepared by thermal evaporation and the samples were irradiated by 120 MeV Au ions at different fluences ranging from 3×1013 to 1×1014 ions/cm2. Rutherford backscattering spectrometry measurements of the pristine and the irradiated samples show that there is no interface mixing due to swift heavy ion (SHI) irradiation. The role of melting temperature in intermixing miscible metal/metal bilayers induced by SHIs has been successfully explained using equations based on calorimetric principle and thermal spike calculations.