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 ∼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 Au0 of 11-MUA–AuNCs (which is opposite to the classic galvanic theory) and the coexistence of Ag0 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.