With an objective to understand the differences in the behavior of monocationic and dicationic ionic liquids (ILs) in their interaction with protein, we have investigated the binding interaction of lysozyme enzyme with two monocation ionic liquids (MILs), [C 3 MIm][Br], [C 6 MIm][Br], and one dicationic ionic liquid (DIL), [C 6 (MIm) 2 ][Br] 2 , by exploiting various experimental methods. These ILs are purposefully chosen so that the effect of both hydrophobicity and structural arrangements of the cationic moiety of ionic liquids (ILs), if any, on the interaction event is understood. Both average ensemble and single molecule pathways have been adopted to obtain a comprehensive picture. For ensemble averaged measurements, the interaction events have been investigated by steady-state and time-resolved fluorescence spectroscopy, whereas for single molecule measurements, fluorescence correlation spectroscopy (FCS) has been utilized. Additionally, the behavior of protein in the absence and presence of ILs has also been investigated through circular dichroism (CD) measurements. The investigations have revealed that MILs and DIL interact differently with the protein. In particular, as compared to MILs, the influence of DIL toward protein is observed to be significantly less in terms of change in the structure and dynamics of protein. The outcome of the present work has demonstrated that imidazolium-based DIL can be a better choice over MILs for retaining native structure of protein in aqueous medium.

Highly efficient energy transfer from a water soluble quantum dot to organic J-aggregates in an inorganic–organic nanohybrid associate.

With an aim to understand the mechanism of interaction between quantum dots (QDs) and various metal ions, fluorescence response of less-toxic and water-soluble glutathione-capped Zn−Ag− In−S (GSH@ZAIS) QDs in the presence of different metal ions has been investigated at both ensemble and single-molecule level. Fourier transform infrared (FT-IR) spectroscopy has also been performed to obtain a molecular level understanding of the interaction event. The steady-state data reveal no significant change in QD emission for alkali and alkaline earth metal ions, while there is a decrease in fluorescence intensity for transition metal (TM) and some heavy transition metal (HTM) ions. Interestingly, a significant fluorescent enhancement (FE) (19−96%) of QDs is found for Cd 2+ ions. Time-resolved fluorescence studies reveal that all the three decay components of QDs decrease in the presence of first-row TM ions. However, in the case of Cd 2+ , the shorter component is found to increase while the longer one decreases. The analysis of data reveals that photoinduced electron transfer is responsible for fluorescence quenching of QDs in the presence of first-row TM ions and destruction/removal of trap/ defect states in the case of Cd 2+ causes the FE. In FT-IR experiments, a prominent peak at 670 cm −1 , corresponding to Cd−S stretching vibrations, indicates strong ground-state interactions between the −SH of GSH and Cd 2+ ions. Moreover, a decrease in the diffusion coefficient of QDs in the presence of Cd 2+ ions during fluorescence correlation spectroscopy (FCS) studies further substantiates the removal of GSH by Cd 2+ from the surface of QDs. The optical output of this study demonstrates that ZAIS can be used for fluorescence signaling of various metal ions and in particular selective detection of Cd 2+ . More importantly, these results also suggest that Cd 2+ can effectively be used for enhancing the fluorescence quantum yield of thiol-capped QDs such as GSH@ZAIS.

With an aim to understand the intermolecular/particle interaction and the optical properties of the inorganic-organic hybrid nanostructured materials, Förster resonance energy transfer (FRET) between negatively charged CdS quantum dots (donor) and positively charged Oxazine 170 perchlorate (acceptor) has been investigated by employing steady-state and time-resolved fluorescence spectroscopy. Investigations revealed that size-dependent changes in the FRET efficiency of different QD-dye FRET pairs occurred mainly due to the electrostatic effects. Interestingly, the present study also reveals that at a higher concentration of dye molecules, aggregation occurs on the QD surface and the quenching of dye fluorescence occurs due to homo-FRET process. The homo-FRET process in this case has been established by exploiting steady-state fluorescence anisotropy measurements. The feasibility of aggregate formation and the homo-FRET interaction between the dye molecules has also been demonstrated through quantum mechanical calculations.