Electroactivity, stability and application in an enzyme switch at pH 7 of poly(aniline)–poly(styrenesulfonate) composite films

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Abstract
By incorporating high molecular weight poly(styrenesulfonate) as the counter-ion in poly(aniline) films good electroactivity and stability can be achieved for the resulting molecular composite in buffered aqueous solution at pH 7. For short times (e.g. 30 min) in pH 7 solution the redox process is reversible and reproducible. After 80 consecutive electrochemical cycles between –0.4 V and +0.4 V vs. SCE, the composite film retains around 85% of its original redox activity as determined by cyclic voltammetry at 20 mV s–1. On prolonged contact with solution at pH 7 (overnight), a significant decrease in electroactivity of the composite is observed. The electroactivity of the composite at pH 7 is affected by the concentration of poly(styrenesulfonate) used in the growth solution, initially rising with increasing poly(styrenesulfonate) concentration and then reaching a plateau. In situ steady-state resistance measurements on the composite films show that the potential corresponding to the transition from the insulating pernigraniline to the conducting emeraldine state shifts from +280 mV at pH 5.0 to around +90 mV vs. SCE at pH 7. The composite film, deposited across the gap of a dual microband electrode, was successfully employed in a microelectrochemical enzyme switch responsive to glucose operating at pH 7. In the enzyme switch studies good switching rates and reusability of the composite films were obtained indicating that films of this type could form the basis of useful microelectrochemical enzyme switches operating in neutral solution.