Crystal facet engineering-modulated electron transfer between nano-FeCuO and BiOCl nanosheets for photoelectrochemical immunoassay of alpha-fetoprotein in hepatocellular carcinoma

Targeted modulation of the charge carrier pathways can significantly enhance the sensitivity of the photoelectrochemical (PEC) immunoassays. In this study, we designed an atomic-level anisotropic electric field triggered through target molecules into BiOCl nanosheets semiconductors to enable the sensitive screening of hepatocellular carcinoma-related cancer biomarker (alpha-fetoprotein: AFP) in biological fluids. Uniformly exposed {110} facets on BiOCl nanosheets were achieved by coupling with a thermochemical treatment method. In the presence of target AFP, the analyte was captured to form a sandwich-type immunocomplex between capture antibody (cAb) and nano-FeCuO-labeled detection antibody (dAb). Under acid treatment, the abundant Fe and Cu ions released from the nanolabels could form the coordination complexes on the BiOCl surface to in-situ construct an anisotropic internal electric field and enable directional control of charge carrier pathways. Density-functional theory (DFT) calculations, complemented by XPS and solid-state UV–vis absorption spectroscopy, confirmed that Fe and Cu introduced intermediate energy levels within the intrinsic BiOCl semiconductor. These elements simultaneously establish anisotropic electric fields, synergistically enhancing the sensor's detection sensitivity. Under optimized conditions, the developed PEC immunosensing platform, utilizing an interface-independent anisotropic electric field strategy, achieved an ultra-wide linear range (0.005 – 100 ng mL−1) and an ultra-low detection limit (0.47 pg mL−1) for target AFP. This work provides a new electric field modulation approach for designing high-performance PEC immunosensing systems.