Closed bipolar electrode-enabled dual-cell electrochromic detectors for chemical sensing

Abstract

Bipolar electrodes (BPE) are electrically floating metallic elements placed in electrified fluids that enable the coupling of anodic and cathodic redox reactions at the opposite ends by electron transfer through the electrode. One particularly compelling application allows electron transfer reactions at one end of a closed BPE to be read out optically by inducing a redox-initiated change in the optical response function of a reporter system at the other end. Here, a BPE-enabled method for electrochemical sensing based on the electrochromic response of a methyl viologen (MV) reporter is developed, characterized, and rendered in a field-deployable format. BPE-enabled devices based on two thin-layer-cells of ITO and Pt were fabricated to couple an analytical reaction in one cell with an MV reporter reaction, producing a color change in the complementary cell. Using Fe(CN)₆^3/4− as a model analyte, the electrochemically induced color change of MV was determined initially by measuring its absorbance via a CCD camera coupled to a microscope. Then, smartphone-based detection and RGB analysis were employed to further simplify the sensing scheme. Both methods produced a linear relationship between the analyte concentration, the quantity of MV generated, and the colorimetric response, yielding a limit of detection of 1.0 μM. Similar responses were observed in the detection of dopamine and acetaminophen. Further evolution of the device replaced the potentiostat with batteries to control potential, demonstrating the simplicity and portability of the device. Finally, the physical separation of the reporter and analytical cells renders the device competent to detect analytes in different (e.g. non-aqueous) phases, as demonstrated by using the electrochromic behavior of aqueous MV to detect ferrocene in acetonitrile in the analytical cell.