Extremely high stability of glutathionate-protected Au25 clusters against core etching.

It is well known that so-called magic-numbered clusters can be preferentially populated by dissociative excitation of larger precursors, because the energy required for removal of a single atom from a magic-numbered cluster is higher than from a neighbor. Thus, if the Au atoms can be removed sequentially from preformed thiolated-protected gold (Au:SR) clusters, one can anticipate a population growth of certain stable Aun:SR clusters. Chemical etching by free thiols is one feasible method for core size reduction of the Au:SR clusters. The etching rate of Aun:SR clusters must be determined as a function of core size, in order to provide a synthesis for welldefined Aun(SR)m clusters in large quantity, as well as to provide information regarding the stability of Aun(SR)m. In the present paper, we studied etching reactions of Aun(SG)m clusters with (n,m) = (10,10), (15,13), (18,14), (22,16), (25,18), (29,20), (33,22), (39,24) by free glutathione (GSH). It was found that Au25:SG clusters show higher stability against etching than the others and as a result two different reaction modes are operative depending on the core size. The Aun(SG)m (n < 25) clusters are completely oxidized to Au(I):SG complexes while Aun(SG)m (n ≥ 25) clusters are etched into Au25: SG by free GSH molecules. On the basis of this observation, a model is proposed to explain our recent finding that Au25 (SG)18 clusters are selectively formed during the reaction of triphenylphosphine-stabilized Au11 clusters and an excess amount of GSH.

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