Size-Dependent Electrochemical and Morphological Properties of Magnetite Nanoparticles Adsorbed on Electrodes

We investigated the influence of particle size on the electrochemical behavior of Fe₃O₄ magnetite nanoparticles (MNPs) electrostatically adsorbed onto graphite electrodes modified with a preadsorbed poly-(ethylenimine) polycation layer. Three hydrodynamic sizes (50, 100, and 200 nm) were selected to assess size-dependent differences in electrochemical response using cyclic voltammetry under well-controlled adsorption and measurement conditions. The 50 nm MNPs exhibited the highest electroactive response and peroxidase-like electrocatalytic currents, which are consistent with greater surface area-to-volume ratios. Qualitative image analysis from atomic force microscopy and scanning electron microscopy revealed closer particle spacing and more extended surface contact for the smaller MNPs, in contrast to isolated aggregates formed by larger particles. These surface-level differences were reflected in the electrochemical signals, where the 50 nm particles yielded higher electroactive surface coverage. The study demonstrates how particle size and interfacial organization influence electrochemical readouts, underscoring the utility of correlating microscopy with electrochemical data to evaluate nanoparticle-based sensing interfaces.