Self-Powered and Sensitive Assay of Caspase-3 Activity Based on an Ultralarge Photocathodic Current Amplified by Polydopamine Modification and a CdIn₂S₄/CdS Heterojunction Photoanode

Photocathodic analysis has an excellent anti-interference property but suffers from a weak response signal, restricting its sensitivity and applicability in trace-level assays. Here, a 55-fold amplification of the photocathodic response signal was achieved. On the one hand, polydopamine (PDA) modification of CuO to form a new Z-scheme CuO@PDA photocathode was used as a preliminary amplifier to amplify the cathodic photocurrent by 2.7 times. On the other hand, the developed CdIn₂S₄/CdS photoanode with an ultralarge anodic photocurrent was used as an external signal amplifier to further enhance the cathodic photocurrent by 20.6 times. Consequently, a self-powered dual-photoelectrode photoelectrochemical (PEC) sensing platform with an ultralarge cathodic photocurrent was developed for a sensitive activity assay of caspase-3, the key mediator of cell apoptosis. Ac-GDGDEVDCC-NH₂ peptide (HS-Peptide) was designed and modified onto the CuO@PDA/ITO photocathode through the Michael addition reaction between the carbonyl group of PDA and the thiol group of HS-Peptide, and then, the electrode was further blocked by 6-mercaptohexanol. Due to the steric hindrance caused by the HS-Peptide, the PEC signal was significantly reduced. When caspase-3 was present, the DEVD sequence in the HS-Peptide was specifically recognized and cleaved, resulting in the obvious recovery of the PEC response, thus achieving the sensitive activity assay of caspase-3 (linear response range, 10 fg mL^-1 to 1 μg mL^-1; detection limit, 1.1 fg mL^-1). Besides, the biosensing platform showed satisfactory recovery in the cell lysate. In view of the ultralarge photocathodic current, the developed self-powered dual-photoelectrode PEC sensing platform should have great promising applications in bioanalysis and early disease diagnosis.