Bifunctional S-doping-mediated interfacial gradient electric field for in-situ amplified photoelectrochemical immunoassay.

Ultrasensitive chemical reactions at the photoanode interface provide new ideas for the development of novel photoelectrochemical (PEC) immunoassays. Herein, we reported an in situ-promoted all-inorganic semiconductor reaction realizing an ultrasensitive PEC analysis of carcinoembryonic antigen (CEA). Uniform In2O3 nanocubes were synthesized through one-step in situ growth, and composite In2OxS3-x was obtained by one-step post-modification sulfurization, achieving ultra-high light-to-dark current switching ratios (169 times). S doping, on the one hand, lowered the band gap of In2O3 and established a gradient electric field to enhance charge separation, resulting in a substantial enhancement of the photocurrent; on the other hand, it reacted with Cu2+ released from the detection probes during the detection process to further amplify the photocurrent signal. The presence of a built-in gradient electric field of In2OxS3-x was determined by in situ X-ray photoelectron spectroscopy (XPS) and density functional theory (DFT). In the presence of CEA, CuO modified on the detection probe formed Cu2+ by exogenous acidification and therefore caused a sudden crossing of the photocurrent by forming a robust Cu-S bond with the vulcanized photoanode. Under optimized conditions, the developed PEC immunosensing system based on photoanodic interfacial reaction exhibited an ultra-wide operating range (0.05-100 ng mL-1), and an ultra-low limit of detection (13.5 pg mL-1). In conclusion, this work provides a promising in situ ultrasensitive monitoring strategy for efficient PEC bio-immunosensor, expanding the range of potential applications in early cancer analysis and bedside diagnostics.