Electrochemical Signal Enhancement via Redox Cycling Involving Iron Oxide Magnetic Particles (Adaptable, Reversible Redox Reservoirs) and Its Application in Sensitive Cu2+ Detection

Electrochemical signals may be affected by the presence of iron oxide magnetic particles (MPs) on an electrode owing to their distinct magnetic and redox properties. Recognizing their significance in electrochemical detection, we investigated the changes in electrochemical signals in the presence of MPs and their underlying causes. In the presence of MPs, the cyclic voltammograms of (quasi)reversible redox species (e.g., Fe(CN)64– and Ru(NH3)63+) exhibit different current-enhancing behaviors, depending on their formal potentials. Several redox species, such as Fe(CN)63– and Os(2,2′-bipyridyl)2Cl2, display non-zero initial currents at non-oxidizing or -reducing applied potentials in the presence of MPs. These findings are primarily attributed to the rapid redox reaction between the redox species and MP rather than the enhancement of mass transfer via magnetoconvection. The reaction between a redox species and an MP leads to a positive or negative shift in the equilibrium potential of the MP, which depends on the formal potential of the redox species. This enables MPs to act as adaptable, reversible redox reservoirs, facilitating current enhancement via redox cycling involving the MPs at specific potentials or during anodic and cathodic scanning. We applied signal enhancement via redox cycling to electrochemical Cu2+ detection. Cu2+ is rapidly reduced to Cu+ by the MPs during incubation, and Cu+ is then measured by using redox cycling. The calculated detection limit is approximately 15 nM, which is ∼100-fold lower than that observed without using MPs.