This report demonstrates the fi rst pH-dependent synthesis of pepsinmediated gold nanoclusters (AuNCs) with blue-, green-, and red-fl uorescent emission from Au 5 (Au 8 ), Au 13 , and Au 25 , respectively. Pepsin is a gastric aspartic proteinase (molecular weight, 34 550 g/mol) that plays an integral role in the digestive process of vertebrates. It was found that the pH of the reaction solution was critical in determining the size of Au NCs (i.e., the number of gold atoms of AuNCs). Interestingly, enzyme function of pepsin contributes to the formation of these AuNCs. The photo-stability of the Au 25 (or Au 13 ) NCs is much higher than that of Au 5 NCs (i.e., Au 25 ∼ Au 13 > > Au 5 ). The pepsin-mediated Au 25 NCs were also found to be useful as fl uorescent sensors for the detection of Pb 2 + ions by enhanced fl uorescence and the detection of Hg 2 + ions by fl uorescence quenching. Although the detailed formation mechanisms of these AuNCs require further analysis, the synthetic route using proteinase demonstrated here is promising for preparing new types of fl uorescent metal nanoclusters for application in catalysis, optics, biological labeling, and sensing.

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.

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