A Direct Z-Scheme AgBr/CuBi2O4 Photocathode for Ultrasensitive Detection of Ciprofloxacin and Ofloxacin by Controlling the Release of Luminol in Self-Powered Microfluidic Photoelectrochemical Aptasensors

An innovative self-powered microfluidic photoelectrochemical (PEC) aptasensor was developed that uses photoactive AgBr/CuBi₂O₄ (ACO) composites as the photocathode matrix for ultrasensitive detection of ciprofloxacin (CIP) and ofloxacin (OFL). The formation of direct Z-scheme heterojunctions in ACO composites greatly aided electron/hole pair separation. Meanwhile, ZnIn₂S₄-decorated CdS nanorod arrays (CZIS) as the photoanode were used instead of a platinum counter electrode to provide electrons. The "signal-off" CIP detection was accomplished through the steric hindrance effect in the photoanode due to the combination of aptamer_(CIP) and CIP. To increase the cathodic photocurrent intensity for OFL determination, controlled release of luminol was first used. Luminol molecules were successfully embedded in the porous structure of silicon dioxide nanospheres (PSiO₂) by the electrostatic adsorption between PSiO₂ and aptamer_(OFL). The luminol released by specific recognition between OFL and aptamer_(OFL) could not only react with •O₂^- but also produce chemiluminescence emission, resulting in the "signal-on" state. Because of the signal "on-off-on", the proposed aptasensor exhibited wide linear ranges for CIP (0.001-100 ng/mL) and OFL (0.0005-100 ng/mL) detection. Furthermore, the low detection limits of CIP (0.06 pg/mL) and OFL (0.022 pg/mL) could achieve the ultrasensitive analysis.