The strategy proposed herein employs the scanning electrochemical microscope in a generation collection like mode to depict the mechanism implied in the electrosynthesis of metal oxides nanoparticles, NPs. It offers a simultaneous generation of the NPs precursors and a single NP-level electroanalysis by nanoimpact coulometry. The former process is operated under controlled fluxes within the wide field of precursor diffusion in the inter-MEs gap thus forming a tunable reaction layer allowing the growth of NPs size gradient within this gap. The latter process exploits the much slower diffusion of NPs, spatially frozen, in the near field of a collecting microelectrode. It then allows to dynamically and in situ monitor the modes of growth of NPs without perturbing their synthesis. As a proof of concept, the synthesis of Ag 2 O nanocubes, NCs, is described, using an Ag microelectrode to generate Ag + ions while a facing Au microelectrode both electrogenerates HOand collect the as-synthetized Ag 2 O NCs. The dynamic analysis of the NCs reductive electrochemical impacts, allows getting insights in their growth and stability. Particularly it suggests a two steps growth mechanism starting from a quasi-instantaneous nucleation of a >260 nm nucleus followed by a mass-transfer driven crystallization over the nuclei.