Assembly of Bimetallic Gold−Silver Nanoparticles via Selective Interparticle Dicarboxylate−Silver Linkages

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This article describes findings of an investigation of the thin-film assembly of binary metal nanoparticles via interparticle linkages of dicarboxylic acid mediators at selective metal sites. Decanethiol- (DT-) capped gold−silver (Au−Ag) alloy nanoparticles of ∼3-nm core sizes were chosen as a model system because of the unique carboxylate−Ag+ binding chemistry and the well-defined thiolate-capping capability at Au. The dicarboxylate-mediated thin-film assembly of Au−Ag nanoparticles was characterized using UV−vis, TEM, XPS, FTIR, and NMR techniques. The results from controlling the oxidative or reductive conditions reveal that the oxidation of Ag to Ag+ on the alloy nanocrystal surface played an important role in the selective ligand exchange and interparticle linkage by dicarboxylic acids (e.g., hexadecanedioic acid, HDA). The formation of the thin-film assembly is dependent on both the alloy composition of the Au−Ag nanoparticles and the chain length of the dicarboxylic acids. The degree for an HDA−DT/Au−Ag exchanging and linking system is determined to yield a value of 16% of DTs per Au−Ag particle being displaced by HDAs. In comparison to similar thin-film assemblies of Au−Ag and Au nanoparticles by other mediators, the results support the conclusion that the HDA-mediated assembly of Au−Ag nanoparticles occurs via −CO2-−Ag+ linkages at silver sites forming a mediator (HDA)−template (DT) nanostructure. The nanostructured thin-film assemblies exhibit interesting optical and electronic properties, which have important implications for the exploration of binary nanoparticle-structured materials for sensing and catalytic applications where fine-tuning of either the binary nanocrystal cores or the bifunctional interparticle structures is desired.