Electrochromic, Closed-Bipolar Electrodes Employing Aptamer-Based Recognition for Direct Colorimetric Sensing Visualization

In this paper, we adapt the electrochemical, aptamer-based (E-AB) sensor platform to develop colorimetric aptamer-based sensors using a closed-bipolar electrode (C-BPE) system. The C-BPE E-AB sensors provide quantitative detection of target molecules based on the rate of color change of an electrochromic Prussian blue (PB) thin-film indicator electrode. The C-BPE cathode, or sensing electrode, is modified with a redox-labeled aptamer that binds to a specific target. More specifically, we employed sequences specific for adenosine triphosphate (ATP) and tobramycin as test-bed targets because these sequences are well vetted. The C-BPE anode, or indicator electrode, was coated with an electrochromic thin film comprising Prussian white (PW) that, when reduced to PB, is accompanied by a corresponding color change used for analytical detection. The rate of color change from PW to PB is facilitated by a potassium ferricyanide-catalyzed oxidation of leucomethylene blue (LB) to methylene blue (MB), the redox label conjugated to the aptamer on the sensing electrode. We demonstrate that the rate of color change is quantitatively related to the concentration of target analyte, which provides a means for naked eye determination. When combined with smartphone-based colorimetric detection, these C-BPE E-AB sensors present a user-friendly alternative to traditional E-AB sensors that rely on voltammetric analysis and a potentiostat, opening up the possibility of point-of-use applications.