Multimode optoelectrochemical detection of cysteine based on an electrochromic Prussian blue electrode

In the present work, multiple detection modes of an electrochromic (EC) sensor were studied and compared. We utilized a Prussian blue (PB) thin film modified F-doped tin oxide (FTO) electrode to detect l-cysteine (Cys), a naturally occurring amino acid bearing a thiol group. The sensing was triggered by a cyclic voltammetric (CV) scan and functioned upon the catalytic oxidation of Cys at the surface of a PB/FTO electrode that had been oxidized to the Berlin green (BG) state. With the EC property of PB and the use of transparent FTO electrode, the Cys concentration, denoted as [Cys], could be detected both electrochemically and optically. As a result, the following four types of calibration curves were obtained simultaneously from the same in situ optoelectrochemical measurement: amperometry (current versus [Cys]), coulometry (charge versus [Cys]), potentiometry (potential versus [Cys]), and absorptometry (optical density change versus [Cys]). This proof-of-concept study suggests that the EC detection can provide more informative results than pure electrochemical or optical sensing approach does.