Development and Characterization of pH Sensors for Oral Cavity
Objectives: Our goal is to develop electrochemical pH sensors to aid in detecting inflammation and/or disease of oral tissues. Miniaturized pH-sensing (iridium oxide, IrOx) and reference (silver/silver chloride, Ag/AgCl) electrodes were combined on a single, planar platform. Response of sensors with sputter-deposited and electrodeposited IrOx to pH and temperature changes were compared.
Methods: Two gold electrodes and electrical connections were fabricated on silicon/silicon oxide or polyimide (the platform) using microfabrication methods. Ag (3-5µm) was electrodeposited on one electrode in commercial plating solution and then oxidized in hydrochloric acid to make Ag/AgCl. The other electrode was modified by depositing IrOx by reactive sputtering (750-1000nm) of iridium and oxygen or electrodeposition (200-600nm) in iridium(IV) solution. Sensors were tested by measuring voltage between IrOx- and Ag/AgCl-modified electrodes in standard buffers with varying pH, chloride concentration ([Cl-]), and temperature with a high-impedance (>100TΩ) electrometer. Physical characterization included optical microscopy, scanning electron microscopy (SEM), and profilometry.
Results: At room temperature (21-22°C), sputtered IrOx sensors exhibited a linear response (-47 to -55mV/pH) for pH 2-10 whereas electrodeposited IrOx was linear (-56 to -71mV/pH) for pH 4-8. Voltage stabilized within 10-120s at each pH value. Ag/AgCl responded linearly to [Cl-] (58mV/-log([Cl-]) as well as varied by <2mV over pH 4-8. Repetitions of well-behaving sensors at one pH were <±5mV (≈0.1pH). pH sensitivity of electrodeposited IrOx was higher (-76mV/pH) at 37°C than at 21°C.
Conclusions: Planar pH sensors with pH-sensitive IrOx and stable Ag/AgCl reference electrodes were fabricated successfully using microfabrication techniques to make the platform with electrodes and contacts and electrodepostion to modify the electrodes. pH sensitivity is linear for range of expected oral pH for sputtered IrOx, but time response needs to be decreased for clinical applications. Because pH response is sensitive to temperature, measuring temperature and pH simultaneously will be necessary in oral environments.