Construction of a Near-Infrared-Light-Triggered Dual-Signal Biosensing Platform for Microcystin-RR Detection Based on the Z-Type Heterojunction of Bi₂S₃/In(OH)₃

Traditional single-signal detection modes are susceptible to false positives and false negatives. In contrast, dual-signal detection modes enable the cross-validation of results, thereby enhancing detection accuracy. This study integrates near-infrared (NIR) light-triggered photoelectrochemical (PEC) detection with square wave voltammetry (SWV) to achieve simultaneous dual-signal output for microcystin-RR (MC-RR) sensing. The PEC-SWV platform was designed using upconversion fluorescent nanoparticles as photoconversion centers to convert NIR light to visible light. Moreover, a Z-type heterojunction of Bi₂S₃/In(OH)₃ was first prepared and adopted as the visible-light-responsive PEC active material, with ascorbic acid (AA) as the electron donor and ferrocene as the SWV signal-output molecule. In the presence of MC-RR, single-stranded DNA modified with ferrocene on magnetic beads was released and subsequently captured by a stationary probe on the paper-based electrode. This gives rise to a steric hindrance effect that interferes with the electron transport of AA on the electrode surface, reducing the photocurrent generated by Bi₂S₃/In(OH)₃. Conversely, the SWV signal is enhanced because of ferrocene acting as a typical electrical signaling molecule. The fabricated PEC-SWV dual-signal biosensing platform exhibits excellent linear detection ranges (PEC, 10 to 1 × 10⁷ fg/mL; SWV, 1 × 10² to 1 × 10⁷ fg/mL) and low detection limits (PEC, 1.9 fg/mL; SWV, 31.6 fg/mL) (S/N = 3) for MC-RR detection. The ultralow detection limits and wide linear ranges render it ideal for trace-level MC-RR detection in environmental samples, such as drinking water and freshwater reservoirs, with potential applications in environmental monitoring and public health.