Chandra Nath Roy

Synthesis of Excitation Independent Highly Luminescent Graphene Quantum Dots through Perchloric Acid Oxidation

We demonstrate a facile liquid phase exfoliation method by only using perchloric acid to synthesize graphene quantum dots (GQDs) having excitation independent strong emission with a quantum yield of about 14%. The proposed simplified synthesis strategy can help in overcoming the limitations of existing aqueous routes which produce GQDs with excitation dependent emission and of low quantum efficiency. Photoluminescence (PL) properties of GQDs have been studied in detail to understand the origin of emission. As-synthesized GQDs show excitation independent photoluminesce (PL) which suggests that the synthesized materials do not have any significant defects. Spectral analysis suggests that the PL emission of the well-defined GQDs originates mainly from the peripheral functional groups conjugated with carbon backbone planes. We also demonstrate a relatively longer PL lifetime (average lifetime of about 10 ns) of the synthesized GQDs determined by time correlated single photon counting (TCSPC) measurement and this high lifetime suggests that the synthesized GQDs may be suitable for biomacromolecular probing. In addition, as-synthesized GQDs interestingly show delayed fluorescence and steady state anisotropy, which make the material an appropriate candidate for application in sensing and bioimaging of cells and organisms.

Modulation of catalytic functionality of alkaline phosphatase induced by semiconductor quantum dots: evidence of substrate-mediated protection

Enzymes provide the critical means by which almost all biological reactions are catalyzed in a controlled manner. Methods to harness and exploit their properties are of strong current interest to the growing field of biotechnology. In view of many unique physical and optical characteristics that are advantageous for studying enzyme activity at quantum dot (QD)–bioconjugate interfaces, we endeavored to explore the possible influence on conformation and catalytic functionality of alkaline phosphatase (ALP), a clinical marker enzyme, by conducting in vitro enzymatic activity assay and using molecular spectroscopic techniques such as UV-visible absorption, fluorescence, circular dichroism (CD) and Fourier transform infrared (FTIR) spectroscopy. The resulting experimental data were then analyzed in the context of the classical Michaelis–Menten model. The results show almost 60% and 30% decrease in enzyme activity on interaction with 10 nM Cys-CdTe or Cys-CdS QDs, respectively, suggesting inhibition dependence on particle nature. CD spectra measurements indicated that QDs induced a decrease of α-helical content and an increase of β-sheet structure in ALP, resulting in the loosening and unfolding of the protein skeleton. Interestingly, when QDs and substrate were added simultaneously to enzyme molecules, catalytic inhibition was significantly reduced indicating protection of enzyme structure by substrate molecules. Further, we demonstrate that protein-encapsulated QDs do not affect the catalytic functionality of enzyme molecules. The present study provides valuable information which may have significant ramifications in various biomedical applications, such as biosensing, drug delivery, cellular imaging, etc.

Determination of the energetics of formation of semiconductor/dendrimer nanohybrid materials: implications on the size and size distribution of nanocrystals

The synthesis of inorganic–organic hybrid nanomaterials has attracted considerable interest in recent years because of their multifaceted applications, such as in optoelectronics, cellular imaging, drug delivery, etc. The maneuvering of controlling parameters is key to the successful fabrication of good quality materials. However, the fundamental aspects pertaining to the thermodynamics of growth of such nano hybrid materials has so far not been unraveled. Here, we have investigated the energetics behind the formation of semiconductor–dendrimer nanohybrid materials using isothermal calorimetry. It is apparent from the observed energy release profile that the heat change for the formation of nanoclusters in phase II saturates faster with an increase in starting materials or monomer concentrations. We have also shown variation of the thermodynamic parameters with changes in the synthesis conditions, such as temperature, dendrimer generation and dendrimer core or surface groups. Based on a bi-phasic thermogram and the dependence of thermodynamic parameters on the dendrimer core and surface functionalities, it is suggested that nanoparticles are formed inside dendrimer molecules in the initial time period and on the outer surface at a longer time scale. Furthermore, it is observed that the formation of quantum dot–dendrimer hybrid materials is an exothermic, spontaneous and enthalpy driven process. Also, a lower temperature thermodynamically favors formation in the core of dendrimer molecules leading to smaller particles with a narrower distribution. The observed results suggest that higher values of formation constant and enthalpy are likely to make dendrimers of higher generation better templates for the synthesis of nanoparticles. The dependence of the ratio of concentrations of reacting metal ions (Cd or Zn) to sulfide ions shows a differential size pattern for CdS and ZnS nanoparticles, which has been interpreted in terms of binding constants determined calorimetrically. It is shown that enthalpy–entropy compensation takes place in the synthesis process affording favorable free energy. Such investigation can provide useful guidelines for the synthesis of better quality semiconductor–dendrimer hybrid nanomaterials.

A comparative evaluation of the activity modulation of flavo and non-flavo enzymes induced by graphene oxide

Graphene, and its water soluble derivative graphene oxide, has shown great promise in various biomedical applications, such as cancer therapeutics, drug delivery, etc. and in industrial applications such as enzyme immobilization, etc. Thus, modulation of the activities of different classes of enzymes by graphene materials is an important aspect in the formulation of different biological applications. We have demonstrated here how flavin adenine dinucleotide (FAD) moieties protect the binding site from conformational change in the presence of an inhibitor, graphene oxide, and also explore differences in the mode of interactions between flavo and non-flavo enzymes. It was shown that there was a much greater loss of activity with the non-flavo enzyme, l-lactate dehydrogenase (LDH), of ∼74% compared to that with the flavo-enzyme, glucose oxidase (GOX), of ∼45%, in the presence of equal concentrations of GO. Furthermore, GO acts as an enzyme inhibitor and the mode of inhibition is uncompetitive for GOX and competitive for LDH. Circular dichromism measurements showed a 21% decrease in the α helix of GOX and a 31% decrease in the α helix of LDH in the presence of a given concentration of GO (0.5 mg mL-1). There was a slight change in the average emission lifetime of tryptophan in GOX in the presence of GO from 3.2 to 2.6 ns. In contrast, there was no change in the average emission lifetime of tryptophan in LDH in the presence of GO. The extents of fluorescence quenching for GOX and LDH were 39% and 70% upon addition of a certain amount of GO. The present study provides insight into the development of sensors through the immobilization of enzymes and the possible formulation of a multifunctional protein and graphene composite system for various biomedical applications such as bio-sensing, gene and drug delivery, etc.

Aqueous Synthesis of Protein Encapsulated ZnSe Quantum Dots and Physical Significance of Semiconductor Induced Cu(II) Ion Sensing

In view of their promising bio-applicability, we have synthesized water-soluble bovine serum albumin (BSA)-encapsulated ZnSe quantum dots (QDs) with visible emission with longer average luminescence lifetimes of approximately 125 ns at ambient conditions. BSA-ZnSe QDs are shown to be efficient selective copper ion probes in the presence of physiologically important metal ions through luminescence quenching with a high Stern-Volmer constant (3.3×105  m-1 ). The mechanism of sensing has been explained in terms of electron transfer processes and the apparent rate of electron transfer (Ket ) from ZnSe QDs to Cu2+ has been calculated to be 2.8×108  s-1 . It is demonstrated that the negative conduction band potential plays a major role in the feasibility of the electron transfer process, which is reflected in the higher efficacy of ZnSe QDs in sensing copper(II) ions over other group II-VI quantum dots, namely, CdSe, ZnS, or CdS. The results observed with cysteine-capped QDs are almost identical to those with BSA-encapsulated QDs and this presumably negates the possible reason of CuII ion induced quenching ascribed to its binding with surface groups or replacement of metal sites as proposed by several groups previously.

SERS Enhancement on the Basis of Temperature-Dependent Chemisorption: Microcalorimetric Evidence.

The mechanism of surface-enhanced Raman spectroscopy (SERS) is not very clear in view of the magnitude of the contribution of electromagnetic factor as well as the chemical mechanism. This report presents the extent of adsorption at different temperatures in terms of signal enhancements in SERS employing silver nanoparticles (AgNPs) of various shapes as substrate and dye molecules, crystal violet or Rhodamine 6G, as model Raman probes. Initially, the SERS signal increases with increasing temperature until a maximum intensity is reached, before it gradually decreases with increasing temperature. This trend is independent of the shape of the Raman substrates and probes. However, the temperature at which maximum intensity is obtained may depend upon the nature of the Raman probe. The energetics involved in the chemisorption process between dye molecules and AgNPs were determined through isothermal titration calorimetry and their implications for the observed SERS signals were assessed. The maximum heat change occurred at the temperature at which the maximum signal enhancement in SERS was obtained and the enhanced interaction at optimum temperature was confirmed by absorption spectroscopy.

Interactions of graphene oxide with luminescent biofunctionalized semiconductor nanoparticles: simultaneous monitoring in a protein–semiconductor coupled system

We have demonstrated the physicochemical aspects of the interactions of free graphene oxide (GO) with bovine serum albumin (BSA) encapsulated ZnSe NPs as a representative protein–semiconductor coupled system. The well-resolved emissions of tryptophan and ZnSe NPs in the chosen biofunctional nanomaterial enables to follow interactions of GO with protein and semiconductor components simultaneously. The long average emission lifetime of semiconductor nanoparticles in BSA–ZnSe NPs changed significantly on interactions with GO from 131.5 to 108.6 ns, while there was little change from 5.24–5.08 ns for protein component. Influence of solvent polarity on steady-state emissions provide evidence of non-electrostatic interactions of BSA and charge transfer from ZnSe NPs towards GO sheet. Circular dichroism spectral measurements suggest change in protein secondary structure and iodide quenching studies provide a quantitative estimate of decrease in accessibility of tryptophan residues (fa) towards polar environment (fa changes from 42% to 17%) on interactions of GO with BSA–ZnSe NPs. These results are consistent with the observed changes in UV-vis absorption and zeta potential, which also indicate hydrophobic association of GO with BSA–ZnSe NPs. Further, electron transfer process is evident from Raman peak shift and the observed changes in ID/IG ratio, which indicate strong interactive nature of BSA–ZnSe NPs towards GO. We also justified the thermodynamic feasibility of electron transfer process and calculated the rate of electron transfer from semiconductor component in BSA–ZnSe NPs to the GO surface to be 2.06 × 109 s−1. Thus, the present study provides useful information for future fabrication of multifunctional single platform combining the graphene, semiconductor and protein molecules.