Core-Size Dependent Fluorescent Gold Nanoclusters and Ultrasensitive Detection of Pb2+ Ion

Gold nanoclusters (Au NCs) are new class of fluorescent nanomaterials with widespread applications in energy, water and healthcare. Here, we report a green synthesis of Au NCs with tunable emission wavelength from 590 to 510 nm in aqueous medium by core etching and ligand exchange method. Investigation reveals that the number of Au atoms present in the core of nanoclusters controls the emission wavelength. The quantum yield (QY) of nanoclusters increases from 0.57 to 3.15% with changing core from Au12 to Au6. Time resolved spectroscopic study reveals that the emission with higher lifetime (>100 ns) originates from ligand to metal charge transfer (LMCT; S to gold core of NCs). It is demonstrated that the highly green emitting NCs (Au-510) are more sensitive than orange emitting NCs (Au-590) toward Pb2+. The detection limit of Pb2+ is found to be 10 nM which is much lower than allowed concentration of Pb2+ in drinking water. Thus, Au NCs based optical sensor is promising for the selective detection of Pb2+ in drinking water.