Synthesis of thiolated Ag/Au bimetallic nanoclusters exhibiting an anti-galvanic reduction mechanism and composition-dependent fluorescence

We report a direct one-pot approach, employing 11-mercaptoundecanoic acid (11-MUA) as a reducing and capping agent, for the preparation of fluorescent Ag/Au bimetallic nanoclusters (Ag/AuNCs) from HAuCl4 and AgNO3 in basic aqueous solution at room temperature. Unlike the monocomponent 11-MUA-AgNCs with negligible fluorescence, the as-prepared 11-MUA-Ag/AuNCs exhibit similar average diameters of similar to 1.8 nm but much stronger and tunable fluorescence by varying the concentration ratios of AgNO3 to HAuCl4 in the synthesis. The quantum yield of the 11-MUA-Ag/AuNCs can reach 6.81% at a Ag-to-Au molar ratio of 1 : 2, nearly 3-fold higher than the monocomponent 11-MUA-AuNCs (2.38%) of similar sizes. More importantly, the occurrence of the reaction between Ag+ ions and Au-0 of 11-MUA-AuNCs (which is opposite to the classic galvanic theory) and the coexistence of Ag-0 and Au+ in the final NC products, are revealed by thorough analysis of XPS data. The anti-galvanic reduction, that is, metal ions are replaced by less reactive metals, has been further demonstrated through the reaction between the as-obtained purified monocomponent 11-MUA-AuNCs and Ag+ ions. Direct electrochemical insights into the anti-galvanic reduction mechanism were provided by cyclic voltammetry characterizations of the NCs, and the universality of the anti-galvanic reduction was tested by preparing thiolated Pt/Au bimetallic NCs using platinum instead of silver in our synthetic system