Direct electrochemistry of nanoparticulate Fe₂O₃ in aqueous solution and adsorbed onto tin-doped indium oxide

Nanoparticulate iron oxides occur naturally, for example, in soil, water, and in the cytoplasm of living cells. The redox properties and detection of these nanoparticles are therefore of considerable importance. Understanding and mimicking nanoparticle-based redox reactions may lead to new types of water-based electrochemical processes. In this study, the electrochemical detection of 4-5 nm diameter Fe₂O₃ nanoparticles dissolved in aqueous buffer solutions is investigated as a model system. Voltammetric experiments with nanoparticulate Fe₂O₃ are reported based on two complementary approaches: (i) Fe₂O₃ nanoparticles adsorbed onto tin-doped indium oxide (ITO) electrodes are shown to give well-defined voltammetric reduction responses and (ii) hydrodynamic voltammety in the presence of fast (24 kHz ultrasound-enhanced) mass transport is shown to allow the direct detection of Fe₂O₃ nanoparticles in solution. Both the adhesion and the electrochemical reactivity of Fe₂O₃ nanoparticles at ITO electrode surfaces are strongly affected by the solution composition and the pH.