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

Abstract

Synthesis of luminescent metal cluster for selective sensing of speci c 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 Fe 3+ and Hg 2+ ions, AuAg-SG cluster showed turn-off response for Cu 2+ , Fe 3+ and Hg 2+ , but luminescent enhancement (turn-on) response for Cd 2+ ions. The highest detection limit obtained for Cu 2+ ion by AuAg-SG cluster is 20 nM while for Cd 2+ ion it is 75 nM. From TCSPC and DLS measurements we postulated that except Cd 2+ , all other metal ions cause aggregation of clusters through ligation with SG ligands while Cd 2+ 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 con rms static quenching/enhancement process through formation of stable ground state adducts.