Spontaneous formation of gold nanoparticles triggered by hydrophobic interfaces.

Gold nanoparticles (AuNPs) underpin advances across numerous applications, yet most syntheses still rely on added chemical reductants and organic additives, constraining sustainability. Here, we report an aqueous, reductant-free route to AuNPs that leverages hydrophobic interfaces under mild conditions. When NaOH is added to aqueous HAuCl4 to reach basic conditions (pH 10-13), where [Au(OH)4]- predominates, AuNPs form spontaneously upon contact with hydrophobic fluoropolymer surfaces (e.g., PFA) without added reductants or surfactants. In contrast, almost no AuNP formation is observed on hydrophilic glass under otherwise identical conditions, indicating that interfacial rather than bulk properties govern nucleation and growth. Systematic pH tuning revealed that AuNP yield reaches a pronounced maximum at pH ≈ 12 and becomes negligible at very high alkalinity (pH 14), while particle size is tunable by varying HAuCl4 and NaOH concentrations. These results, together with the suppression of AuNP formation at high ionic strength, indicate that interfacial ion distributions, rather than bulk pH alone, play a decisive role in the reaction. A consistent interpretation is that hydrophobic interfaces promote preferential adsorption of OH-, giving rise to an electric double layer (EDL) with an ion distribution distinct from the bulk. Within this nanoscale-confined environment, ultrasmall Au(III) hydroxide-like species may form and, owing to strong size effects, undergo low-temperature transformation to yield AuNPs. These results establish interfacial EDL confinement as a basis for sustainable, reductant-free nanomaterial synthesis and suggest extension of this principle to other aqueous metal systems.