Electrochemical properties of an array of closely spaced (1.2 μm) Au or Pt microelectrodes (∼2 μm wide × ∼50 μm long × 0.1 μm high) coated by a 0.15-μm-thick layer of polycrystalline WO3 are reported. The WO3 is deposited on the electrodes by radio frequency (rf) sputtering of a WO3 target. The cyclic voltammetry of these microelectrodes indicates that WO3 connects individual microelectrodes, since the voltammogram of a pair of microelectrodes driven together is indistinguishable from that of an individual microelectrode. WO3 becomes a good conductor upon electrochemical reduction in aqueous solutions. The change in resistance of WO3 connecting two microelectrodes as a function of electrochemical potential spans 4 orders of magnitude, from ∼106 to ∼102 ohm. A pair of WO3-connected microelectrodes functions as a microelectrochemical transistor that is sensitive to pH. The cyclic voltammetry is pH-dependent and consistent with pH-dependent transistor characteristics, which indicate that the device is turned on at more positive electrochemical potentials in acidic media. In basic solutions, more negative potentials are needed to turn on WO3-based transistors. The maximum slope of the drain current, ID, vs. gate voltage, VG, plot at fixed drain voltage, VD, gives a transconductance of 12 mS/mm of gate width. Potential step and potential sweep measurements indicate that the WO3-based transistor can be reversibly turned off and on in seconds; furthermore, the gate current, IG, and ID can be measured simultaneously, allowing demonstration of power gain for a sinusoidal variation of VG at fixed VD. Operating at a frequency of 1 Hz, the power amplification by the WO3-based transistor is 200, at pH 1. The power amplification decreases at both higher pH and higher frequency. The properties of the WO3-based microelectrochemical transistor allow its use as a real-time pH sensor: a reproducible change in ID, at fixed VG and VD, is obtained rapidly as the pH of a stream flowed continuously past the electrode is repetitively changed from pH 3.9 to pH 7.2.
All Science Journal Classification (ASJC) codes
- Physical and Theoretical Chemistry