S. Shankara Narayanan

Validation and Divergence of the Activation Energy Barrier Crossing Transition at the AOT/Lecithin Reverse Micellar Interface

In this report, the validity and divergence of the activation energy barrier crossing model for the bound to free type water transition at the interface of the AOT/lecithin mixed reverse micelle (RM) has been investigated for the first time in a wide range of temperatures by time-resolved solvation of fluorophores. Here, picosecond-resolved solvation dynamics of two fluorescent probes, ANS (1-anilino-8-naphthalenesulfonic acid, ammonium salt) and Coumarin 500 (C-500), in the mixed RM have been carefully examined at 293, 313, 328, and 343 K. Using the dynamic light scattering (DLS) technique, the size of the mixed RMs at different temperatures was found to have an insignificant change. The solvation process at the reverse micellar interface has been found to be the activation energy barrier crossing type, in which interface-bound type water molecules get converted into free type water molecules. The activation energies, Ea, calculated for ANS and C-500 are 7.4 and 3.9 kcal mol(-1), respectively, which are in good agreement with that obtained by molecular dynamics simulation studies. However, deviation from the regular Arrhenius type behavior was observed for ANS around 343 K, which has been attributed to the spatial heterogeneity of the probe environments. Time-resolved fluorescence anisotropy decay of the probes has indicated the existence of the dyes in a range of locations in RM. With the increase in temperature, the overall anisotropy decay becomes faster revealing the lability of the microenvironment at elevated temperatures.

Time-resolved photoluminescence decay characteristics of bovine serum albumin-conjugated semiconductor nanocrystallites

We report processing and luminescence decay characteristics of Cd1− x Zn x S composite nanocrystals (NCs) conjugated with bovine serum albumin (BSA) proteins. Time-resolved study on unconjugate NCs (with dimensions less than the bulk exciton Bohr radius) suggests that in the radiative emission, the fast (τ 1) and the slow (τ 2) carrier components are equally competitive for a given stoichiometry. Conversely, bioconjugate NCs advocate that the decay component due to the free exciton recombination is ∼9 times faster than the component due to the surface recombination emission. The observation of two distinct decay parameters is due to the fact that the NCs have experienced photostability by way of binding and protecting NC surface with biomolecules (BSA) as binding agents. The occurrence of two decay constants would help in extracting information with regard to the nature of surface recombination, free-exciton relaxation along with the strength of emission. Furthermore, with the increase in % Zn, slow carrier component gets slower owing to the incorporation of extra surface traps due to Zn/Cd incompatibility while making perfect lattice sites in the NCs. As a result, surface emission intensity gets improved compared to the radiative intensity due to core-state direct transitions. Understanding photoluminescence decay of bioconjugated NCs, on a comparative basis, would find scope for biomolecular labelling, sensing and electrophysiology applications.