Stochastic collisions of aqueous nanodroplets (AnDs) on a microelectrode were observed in situ by electrochemistry. Reduction of Cu2+ ions enclosed in the reacting AnDs resulted in surfactant-free synthesis of copper nanoparticles on the electrode surface. The particle size distribution was reasonably controllable by the modulation of electrode voltage. The versatility of the synthetic method was established by its application in synthesizing nanoparticles of silver and cobalt oxyhydroxide.