We report on the effect of convection on electrochemically active collisions between individual Pt nanoparticles (PtNPs) and Hg and Au electrodes. Compared to standard electrochemical cells utilizing Hg and Au ultramicroelectrodes (UMEs) used in previous studies of electrocatalytic amplification, microelectrochemical devices offer two major advantages. First, the PtNP limit of detection (0.084 pM) is ∼8 times lower than the lowest concentration measured using UMEs. Second, convection enhances the mass transfer of PtNPs to the electrode surface, which enhances the collision frequency from ∼0.02 pM-1 s-1 on UMEs to ∼0.07 pM-1 s-1 in microelectrochemical devices. We also show that the size of PtNPs can be measured in flowing systems using data from collision experiments and then validate this finding using multiphysics simulations.