Gold nanoparticles (AuNPs) show unique optical properties and catalytic activities, and their synthesis from gold ions has been widely studied. One of the additive-reagent-free and noncontact production procedures is the reduction of gold ions in solution by femtosecond laser pulses; however, the aggregation of AuNPs is unavoidable in homogeneous solution. Here, we report the synthesis of single-nanometer-sized AuNPs in a mixture of aqueous HAuCl4 solution and n-hexane (the mixture) and in aqueous HAuCl4 solution (the aqueous solution) by femtosecond laser irradiation in the absence of any additive reagents. Transmission electron microscopy revealed that circlelike colonies consisting of well-separated AuNPs were obtained from the mixture, while highly stacked and agglomerated AuNPs were obtained from the aqueous solution. The mean size of AuNPs in the mixture was nearly independent of the laser irradiation time, whereas that obtained in aqueous solution was gradually shifted to smaller size by laser irradiation period. We propose that the adsorption of primary AuNPs on the surface of hexane microdroplets and the fragmentation of large AuNPs in water by successive laser pulses retain single-nanometer-sized AuNPs in the mixture. The use of liquid-liquid interface on hexane microdroplets in aqueous solution provides a simple and useful environment to synthesize small AuNPs without the aid of surfactants or capping agents.
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