Magnetic control: Switchable ultrahigh magnetic gradients at Fe₃O₄ nanoparticles to enhance solution-phase mass transport

Enhancing mass transport to electrodes is desired in almost all types of electrochemical sensing, electrocatalysis, and energy storage or conversion. Here, a method of doing so by means of the magnetic gradient force generated at magnetic-nanoparticle-modified electrodes is presented. It is shown using Fe₃O₄-nanoparticle-modified electrodes that the ultrahigh magnetic gradients (> 10⁸ T·m^–1) established at the magnetized Fe₃O₄ nanoparticles speed up the transport of reactants and products at the electrode surface. Using the Fe(Ⅲ)/Fe(Ⅱ)-hexacyanoferrate redox couple, it is demonstrated that this mass transport enhancement can conveniently and repeatedly be switched on and off by applying and removing an external magnetic field, owing to the superparamagnetic properties of magnetite nanoparticles. Thus, it is shown for the first time that magnetic nanoparticles can be used to control mass transport in electrochemical systems. Importantly, this approach does not require any means of mechanical agitation and is therefore particularly interesting for application in micro- and nanofluidic systems and devices.