Luminescent Quantum Clusters of Gold in Bulk by Albumin-Induced Core Etching of Nanoparticles: Metal Ion Sensing, Metal-Enhanced Luminescence, and Biolabeling

The synthesis of a luminescent quantum cluster (QC) of gold with a quantum yield of similar to 4% is reported. It was synthesized in gram quantities by the core etching of mercaptosuccinic acid protected gold nanoparticles by bovine serum albumin (BSA), abbreviated as Au-QC@BSA. The cluster was characterized and a core of Au-38 was assigned tentatively from mass spectrometric analysis. Luminescence of the QC is exploited as a "turn-off" sensor for Cu2+ ions and a "turn-on" sensor for glutathione detection. Metal-enhanced luminescence (MEL) of this QC in the presence of silver nanoparticles is demonstrated and a ninefold maximum enhancement is seen. This is the first report of the observation of MEL from QCs. Folic acid conjugated Au-QC@BSA was found to be internalized to a significant extent by oral carcinoma KB cells through folic acid mediated endocytosis. The inherent luminescence of the internalized Au-QC@BSA was used in cell imaging.