Spring Expanding-like\nPolarity Reversal in Photoelectrochemical\nBiosensing

Although\npolarity-reversal photoelectrochemical (PEC)\nanalysis\ncan effectively eliminate false-positive and negative signals caused\nby interferents, achieving high sensitivity and accuracy is still\na challenge. Hence, a spring expanding-like polarity reversal strategy\nwith bipolar signal synergistic amplification is first proposed to\nhelp build a high-performance PEC analysis system. In this study, l-cysteine (l-cys) is discovered to not only act as\na polarity regulator to elaborately reverse photocurrent via its covalent\nbond to Cu and Bi but also provide a relatively stable electron donor\nto effectively consume the photogenerated holes compared with commonly\nused H<sub>2</sub>O<sub>2</sub> and ascorbic acid. More importantly,\nthe amino and electron-rich functional acridine groups in the dye\nacriflavine endow an electrochemical activity to accelerate electron\ntransfer between the electrode and solution, thus enabling bipolar\nsynergistic signal amplification for acquiring an extremely enlarged\nphotocurrent variation that is of great significance to overcome the\nrigorous signal prereversal depression and reversal amplification\nin traditional polarity-reversal systems. Accordingly, the PEC biosensor\nwith the proposed spring expanding-like polarity reversal strategy\nexhibits excellent sensitivity and accuracy, reflecting ultralow detection\nlimits of 0.04 fM toward lead ions (Pb<sup>2+</sup>) and good anti-interference\nability in the detection of natural water samples. This work provides\nan avenue for exploring a new polarity reversal strategy for accomplishing\nhigh-performance PEC bioanalysis, expected to be widely applied in\nenvironmental monitoring, clinical diagnosis, and food supervision.