Micro pH Sensors Based on Iridium Oxide Nanotubes

Iridium oxide (IrO_x) nanotubes have been grown on microelectrodes utilizing a patterned nanoporous aluminum oxide (AAO) template to form a micro pH sensor. A layer of 900-nm thick aluminum sputtered on a silicon wafer was locally anodized at various voltages to investigate the morphology of the patterned AAO templates. The electrode position of IrO_x inside the supportive AAO template was carried out to yield IrO_x nanotubes with a diameter ranging from 80 to 110 nm, corresponding to the AAO pore sizes. The fabrication processes produced free-standing IrO_x nanotubes on top of the 50 × 100 μm² micro-scale electrodes. Material characterization along with fabrication parameters was investigated. Potentiometric responses of the miniature sensors to hydrogen ions in terms of sensitivity, repeatability, response time, and reliability were performed. Super-Nernstian response was achieved. pH measurements for higher resolution targeting biological applications and sensor temperature dependence were also conducted.