Microelectrochemical Transistors Based on Electrostatic Binding of Electroactive Metal Complexes in Protonated Poly(4-vinylpyridine): Devices That Respond to Two Chemical Stimuli

This article reports the fabrication and characterization of a microelectrochemical transistor derived from coating and connecting two closely spaced (<1.7 µm) microelectrodes (~80 µm long × ~2.4 µm wide × 0.1 µm thick) with poly-(4-vinyipyridine), (4-VPy)_n. The transistor turns on only when two chemical criteria are met: the pH of the medium in contact with the (4-VPy)_n must be sufficiently low to protonate the polymer to give (4-VPyH⁺)_n and an anionic redox couple must be present to be electrostatically bound into the polymer, e.g., Fe(CN)₆^3-/4-. By use of conventional redox species as the mechanism for transporting charge from one microelectrode (source) to the other microelectrode (drain), the device shows a narrow region of gate voltage, V_G, where the source-drain current, I_D, Is nonzero: I_D(peak) occurs at V_G≈ E^º,(Fe(CN)₆^3-/4-) for a small difference (<50 mV) In potential between source and drain, V_o. The temperature dependence of Io shows an Arrhenius activation energy of ~50 kJ/mol, consistent with previous results for the (4-VPyH⁺¹/₃Fe(CN)₆^3-)_n polymer. The transistor can be turned alternately on and off in a reproducible manner by alternate exposure of the device to pH 3 Fe(CN)₆^3-/4- and pH 3 Co-(CN)₆^3-, respectively, at V_G = +0.2 V vs. SCE, consistent with dynamic exchange In and out of the (4-VPyH⁺)_n by electroactive (Fe(CN)₆^3-/4-) and nonelectroactive (Co(CN)₆^3-) anions. At pHs above the pK_a of the (4-VPy)_n exposure of the device to Fe(CN)₆^3-/4- does not turn on the device.