Synthesis of Orange-Red Emissive Au-SG and AuAg-SG Nanoclusters and Their Turn-OFF vs. Turn-ON Metal Ion Sensing.

Synthesis of luminescent metal cluster for selective sensing of specific analyte with detail mechanistic understanding is very important for real world applications as well as for developing new emissive materials. In the present work, we have synthesized L-glutathione stabilized gold (Au-SG) and gold-silver bimetallic (AuAg-SG) clusters under identical experimental conditions with orange red emissive characteristics for both. Detail photo physical analysis reveals that both clusters are phosphorescent in nature with moderate quantum yield of 7% and 19% for Au-SG and AuAg-SG respectively and their excited state lifetime values are in the range of 1-2 µs. While Au-SG cluster showed luminescence quenching response (turn-off) in presence of Fe3+ and Hg2+ ions, AuAg-SG cluster showed turn-off response for Cu2+, Fe3+ and Hg2+, but luminescent enhancement (turn-on) response for Cd2+ ions. The highest detection limit obtained for Cu2+ ion by AuAg-SG cluster is 20 nM while for Cd2+ ion it is 75 nM. From Time Correlated Single Photo Counting (TCSPC) and Dynamic Light Scattering (DLS) measurements we postulated that except Cd2+, all other metal ions cause aggregation of clusters through ligation with SG ligands while Cd2+ ion does not induce any cluster aggregation but binds to cluster surface atoms. The near constant life time values of both clusters during gradual addition of respective metal ions confirms static quenching/enhancement process through formation of stable ground state adducts.

4-Iodophenylboronic Acid Stabilized Gold Cluster as a New Fluorescent Chemosensor for Saccharides Based on Excimer Emission Quenching.

4-iodophenylboronic acid (IPBA) ligated luminescent gold cluster was synthesized by mixing an aqueous solution of IBPA and polyvinylpyrrolidone stabilized gold cluster (Au:PVP) in water at room temperature through chemisorption of iodine on gold nano surface. Transmission Electron microscopy (TEM) and matrix assisted laser desorption ionization (MALDI) analysis revealed that the size of these Au-clusters (1.4±0.2 nm) remain unchanged without any noticeable aggregation during synthesis. Owing to the formation of excimer between aryl moieties grafted over Au surface, the cluster exhibit strong emission peak at 335 nm. This luminescent gold cluster is used for sensing different saccharides in water at physiological pH through quenching of excimer emission peak. This strong excimer emission is significantly quenched in presence of saccharides through interaction with boronic acid moieties. The selectivity for different saccharides follows the order: fructose > galactose > maltose > glucose ~ ribose > sorbitol with hight affinity for fructose (KSV = 1.54 × 104 M-1) with Limit of Detection (LOD) of 100 µM.