Towards electrochemical diagnostics of biochemical free radical species in aqueous microliter volumes

Reactive oxygen species (ROS) are free radicals often implicated in disease states. In probing related enzyme processes, reactant consumption must be minimized. Levitated drops show potential as microreactors where radicals are present as reactants or products. Solid/liquid interfaces are absent or minimized, avoiding adsorption and interfacial reaction of conventional microfluidics. Probing small volumes electrochemically requires an integrated sensor with reference, counter-, and one or more working electrodes. Microfabrication allows production of planar gold sensor microelectrodes. A design that placed eight copies of the sensor on a single wafer maximized throughput and use of available substrate area. Titanium was found to be superior to chromium in corrosion resistance while providing adequate adhesion between gold and the polyimide-passivated silicon substrate. A lithographic process using image reversal with lift-off proved superior to etching in terms of line width range, allowing resolution of narrow electrodes (