pH-sensitive WO₃-based microelectrochemical transistors

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, I_D, vs. gate voltage, V_G, plot at fixed drain voltage, V_D, 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, I_G, and I_D can be measured simultaneously, allowing demonstration of power gain for a sinusoidal variation of V_G at fixed V_D. 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 I_D, at fixed V_G and V_D, 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.