We report on trypsin-stabilized fluorescent gold nanoclusters (Au NCs) for the sensitive and selective detection of Hg 2+ ions. The Au NCs have an average size of 1 nm and show a red emission at 645 nm. The photostable properties of the trypsin-stabilized Au NCs were examined, and their photochemical stability was found to be similar to that of CdSe quantum dots. The fluorescence was particularly quenched by Hg 2+ , and therefore the Au NCs can be used as fluorescent sensors for sensitive and selective Hg 2+ detection to a detection limit of 50 ± 10 nM and the quantitative detection of Hg 2+ in wide and low concentration range of 50 -600 nM.