Gas-assisted focused ion beam fabrication of gold nanoelectrode arrays in electron-beam evaporated alumina films for microfluidic electrochemical sensors

This work reports the fabrication details used to prepare gold nanoelectrode arrays as working electrodes in a microfluidic electrochemical sensor system. We emphasize on the design criteria and the methods used to construct the nanoelectrode arrays for the purpose of enhancing mass transport of the redox species to the electrodes. The fabrication techniques include electron-beam evaporation of Ti and Au to create the electrode platform on a glass substrate, followed by electron-beam evaporation of thick alumina films as a passivation layer. I2-assisted focused ion beam (FIB) milling is employed to produce high-aspect-ratio pores in the alumina films. The final nanopores with diameters around 60-120 nm are achieved by ion beam sculpting after the initial milling process. The nanopores are filled with Au via electrodeposition to obtain a nanoelectrode structure. The current responses of Ru(NH3)63+/Ru(NH3)62+ redox species measured at three-electrode cells in polydimethylsiloxane (PDMS) microfluidic chips are investigated by cyclic voltammetry (CV). The current density obtained at the nanoelectrode arrays is increased by two orders of magnitude compared to that obtained from the microelectrode system. In addition, due to the effect of enhanced mass transport, the voltammetric response curve acquired from the nanoelectrode array exhibits large deviations from the classical diffusion-limited current transport theory.