Poly(3-methylthiophene)-coated electrodes: Optical and electrical properties as a function of redox potential and amplification of electrical and chemical signals using poly(3-methylthiophene)-based microelectrochemical transistors

Optical and electrical properties of anodically grown poly(3-methylthiophene) are reported as a function of redox potential in CH₃CN/0.1 M [n-Bu₄N]ClO₄. Poly(3-methylthiophene) can be grown by the oxidation of 3-methylthiophene and deposited onto Au or Pt electrode surfaces. The poly(3-methylthiophene) can be used to "connect" two or more closely spaced (1.2 μm) microelectrodes (2.4 μm wide x 50 μm long x 0.1 μm thick) of a microelectrode array fabricated by conventional microfabrication techniques. A pair of poly(3-methylthiophene)-connected microelectrodes can function as a transistor where one of the electrodes is regarded as a "source" and the other as a "drain" with the source being referenced to the solution as a gate. The poly(3-methylthiophene) is the analogue of the channel of a solid-state field-effect transistor, since its conductivity changes by >10⁸ depending on the gate potential. Large optical (300-800 nm) and electrical changes for the poly(3-methylthiophene) occur between ∼+0.3 and ∼+0.8 V vs. SCE. The reduced material has an absorption maximum at 490 nm and the oxidized material has an absorption maximum at 750 nm. The optical density changes parallel the resistance changes that occur as the potential of the polymer changes between ∼+0.3 and ∼+0.8 V vs. SCE. The "charging" of a 1.5-μm-thick film of poly(3-methylthiophene) involves ∼10⁴ times more charge per unit of projected area than a smooth Pt electrode, consistent with a large effective internal surface area for the conducting polymer. The resistance of poly(3-methylthiophene) falls by greater than eight orders of magnitude upon oxidation, and the drain current, I_D, vs. gate voltage, V_G, at a fixed drain potential, V_D, of a poly(3-methylthiophene)-based transistor gives a transconductance, ∂I_D/∂V_G, of 120 mS/mm of channel width, competitive with high-quality Si MOSFET devices. Power amplification by a factor of about 1000 is possible at frequencies of 10 Hz. In aqueous 0.1 M NaClO₄/0.05 M NaH₂PO₄ (pH 5.3) the poly(3-methylthiophene)-based transistor is durable and the resistance can be varied by >10⁵ by variation in gate potential. A poly(3-methylthiophene)-based electrochemical transistor placed in the effluent stream of a high-pressure liquid chromatograph reproducibly turns on or off in response to a chemical oxidant (IrCl₆^2-) or reductant (Fe(CN)₆^4-), respectively, introduced into the system via an automatic injection of 40 μL of 0.01 M redox reagent. The microelectrochemical transistor responds to <10^-9 mol of oxidant injected. Since only ∼8 × 10^-5% of the injected material passes by the transistor, the amount of oxidant to which the device responds is <10^-15 mol.