A novel, label-free, fluorescent, turn-on sensor for biological thiol detection that uses highly fluorescent gold nanoclusters (AuNCs), prepared by a bovine serum albumin (BSA)-templated green synthetic route, has been developed. The assay relies on blocking Hg2+-induced quenching of the fluorescence of AuNCs, caused by metallophilic Hg2+-Au+ interactions, through selective coordination of biological thiols with Hg2+ ions. Biological thiols entrap added Hg2+ ions via a robust Hg-S interaction. This phenomenon prevents Hg2+-induced quenching and results in fluorescence from AuNCs. By employing this turn-on sensor, biological thiols, such as cysteine (Cys), glutathione (GSH) and homocysteine (Hcy), are successfully detected at concentrations as low as 8.3 nM for Cys, 9.4 nM for GSH, and 14.9 nM for Hcy. The diagnostic capability and potential in practical applications of this method have been demonstrated by detecting biological thiols in human blood serum. Crown Copyright (C) 2013 Published by Elsevier B.V. All rights reserved.