• Sangeeta Saini

      Articles written in Journal of Chemical Sciences

    • Fluorescence resonance energy transfer (FRET) in chemistry and biology: Non-Förster distance dependence of the FRET rate

      Sangeeta Saini Harjinder Singh Biman Bagchi

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      Fluorescence resonance energy transfer (FRET) is a popular tool to study equilibrium and dynamical properties of polymers and biopolymers in condensed phases and is now widely used in conjunction with single molecule spectroscopy. In the data analysis, one usually employs the Förster expression which predicts (l/R6) distance dependence of the energy transfer rate. However, critical analysis shows that this expression can be of rather limited validity in many cases. We demonstrate this by explicitly considering a donor-acceptor system, polyfluorene (PF6)-tetraphenylporphyrin (TPP), where the size of both donor and acceptor is comparable to the distance separating them. In such cases, one may expect much weaker distance (as l/R2 or even weaker) dependence. We have also considered the case of energy transfer from a dye to a nanoparticle. Here we find l/R4 distance dependence at large separations, completely different from Förster. We also discuss recent application of FRET to study polymer conformational dynamics.

    • An alternative decomposition reaction pathway for CO oxidation at Au(5-x-y)AgxCuy (2 ≤ x + y ≤ 4) nanoclusters


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      Different reaction pathways of CO oxidation at the surface of trimetallic nanoclusters, Au(5-x-y)AgxCuy of varying compositions with 2 ≤ x + y ≤ 4 (x,y >0) are investigated. The starting point of all pathways is the surface reaction between adsorbed CO and O2 via the Langmuir-Hinshelwood mechanism. The conventional reaction pathway leads to the formation of intermediate Au(5-x-y)AgxCuy.O* followed by areaction with CO to release CO2. However, on these trimetallic clusters, the formation of the nanocluster-carbonate adduct is greatly facilitated, which is known to poison the catalytic surface because of its greater stability. Here, we propose a possible Eley-Rideal (ER) pathway leading to carbonate decomposition and catalytic surface regeneration. This ER-pathway involves interaction between nanocluster-carbonate adductand CO in the gaseous state, forming Au(5-x-y)AgxCuy.O2COCO*, which decomposes to release CO2. The study suggests the best trimetallic clusters for CO oxidation are the ones where both reaction sites are Agsites.

      Different reaction pathways of CO oxidation at the surface of trimetallic nanoclusters, Au(5-x-y)AgxCuy, are investigated. These include the conventional LH-LH pathway and a newer alternative LH-ER pathway. The proposed LH-ER pathway involves an interaction between nanocluster-carbonate adduct and CO(g), forming Au(5-x-y)AgxCuy.O2COCO*, which decomposes to release CO2.

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