1. Plasmonically-Powered Chemical Systems
Organization of plasmonic nanostructures: KGT group has realized the significance of assembling plasmonic nanostructures for the development of various plasmonically- powered systems, which gained international attention. The experimental verification of interplasmon coupling in Au nanorods by adopting electrostatic/supramolecular/covalent approaches was first demonstrated by his group. For e.g., KGT group has effectively utilized this approach for the selective detection of micromolar concentrations of cysteine/glutathione in the presence of various other α-amino acids. His research contributions in the area of gold nanorods have gained international attention among chemists and biologists. Experimental verification of enhanced potential at the edges of anisotropic nanomaterials such as Au nanorods was demonstrated by KGT group. The potential applications of chromophore functionalized metal nanoparticles in the design of sensors for metal cations, light-induced controlled release systems for aminoacids such as DOPA and nanophoshors were also demonstrated. The light-regulated changes in the topographic properties of spiropyran-capped Au nanoparticles (i.e., interconversion between the zwitterionic and neutral forms) were exploited for the assembly and release of amino acids such as L-DOPA.
Plasmonically-active assemblies for SERS: Following the assembling strategy, KGT group has developed methodologies for enhancing the intensity of electric field (hot spots) at the junctions of plasmonically-active assemblies by varying the distance between the plasmonic nanoparticles. KGT group has developed SERS substrates (i) by coating Ag@SiO2 nanoparticles on the inner walls of the glass capillaries resulting in very high enhancement factor and (ii) for sieving and sensing applications. The fundamental knowledge acquired on plasmon hybridization and hot-spot generation was further translated to a real device by undertaking a project on the “Design of a Surface-Enhanced Spectroscopy based Device for the Rapid Detection of Organophosphate Pesticides and Pyrethroid Insecticides in Fruits and Vegetables,” addressing a severe problem faced by many developing and underdeveloped countries. Jointly with experts from other branches of science, a customized tabletop Raman spectroscopic device was built, which is interfaced with a plasmonic platform and data processing software for the rapid detection of pesticide/insecticide residues from vegetable/fruit extracts. The device is now identified by the funding agency as one of the 14 top-performing products having translational potential (https://youtu.be/vl7xySLKOnQ).
Plasmon-exciton coupling: Herein, we investigate the interactions between plasmons on metal particles/nanorods and excitons on dyes or their assemblies as ensemble and in single particle level by combining dark-field microscopy and high resolution scanning electron microscopy. The role of various optical parameters dictating the plasmon-exciton interactions and formation of hybrid states were investigated in collaboration with theoretical groups and established guidelines for the design of plexcitonic systems, by choosing plasmonic and excitonic systems with high oscillator strength and narrow spectral width.
2. Photochemistry and photophysics of Semiconductor quantum dots
Crystal Structure Dependent Luminescence of QDs: Optimizing the shell thickness of core-shell quantum dots (QDs), which can suppress the undesired electron-transfer process and provide maximum emission yield is crucial for the design of luminescence materials. We have developed methodologies for obtaining quantitative information on the optimum shell thickness ZnS overcoated on CdSe for various luminescence applications. It is reported that the cubic zinc blende CdSe (Zb-CdSe) exhibited excellent photostability and high luminescence quantum yield compared to hexagonal wurtzite CdSe. However, the reason for the crystal-dependent luminescence properties of CdSe was not understood. Our studies have revealed a fundamentally important finding that the surface of Zb-CdSe is rich with Cd2+ ions, which leads to the formation of Type I core-shell nanostructures responsible for the enhanced luminescence.
Indium Phosphide QDs for Photoinduced Energy Transfer: We have demonstrated that InP is a versatile and environmentally friendly material for energy transfer applications, meeting various photophysical requirements. The structural changes in peptide monomer, on addition of heparin are further monitored at nanomolar concentrations through the inhibition of energy transfer from InP quantum dots to rhodamine dye.
Deep Trap State in InP and Shallow Trap States in CdSe: It is theoretically proposed that the degree of covalency in QDs influences the depth of surface trap states, which in turn affects the charge recombination process in hybrid systems. We have experimentally investigated these aspects by taking relatively ionic cadmium selenide (CdSe) QDs with shallow trap states and covalent indium phosphide (InP) QDs having deep trap states as examples, using a viologen derivative as an electron acceptor. Time-resolved spectroscopic studies showed the presence of deep hole trap states in InP QDs which retard the charge recombination to hundreds of milliseconds and to even seconds. In contrast, charge recombination in CdSe with shallow trap states occurs in picosecond time scale.
3. Chiroptical properties of nanostructures
Design of chiral nanostructures