Valence-Engineered Regulation of Built-In Electric Field in Hollow CuS@CeO2 Heterojunction for Photoelectrochemical Immunoassay

Abstract Work function (Φ) strongly affects interfacial charge distribution and built-in electric field (BIEF) formation, fundamentally determining the photoelectric conversion efficiency of photoelectrochemical (PEC) sensors. Exploring the inherent correlation between Φ and BIEF is therefore essential to guide the rational design of PEC sensors. In this work, a PEC sensing platform was fabricated based on hollow CuS@CeO2 heterojunctions, where BIEF intensity was precisely regulated by cerium valence-state engineering. A robust BIEF was formed in high-valence CuS@CeO2(IV), which facilitates efficient photogenerated carrier separation and yields a high initial PEC signal. Following the sandwich-type immunoassay recognition of carcinoembryonic antigen (CEA), hydrogen peroxide (H2O2) generated by enzymatic reactions was used to react with high-valence CuS@CeO2(IV). After CuS@CeO2(IV) underwent a reduction reaction with H2O2, the Ce atoms in the heterojunction were reduced to form low-valence CuS@CeO2(III). The reduction in Ce valence states was accompanied by a decrease in work function difference (ΔΦ) values, which weakened the BIEF intensity. The weakened BIEF is unfavorable for the photogenerated carrier separation, thereby inducing an obvious decline in the PEC signal. Relying on this distinct signal variation, sensitive CEA detection is successfully realized by the PEC platform, which exhibits a linear detection range of 0.02−50 ng/mL and a low detection limit of 0.53 pg/mL. This study validates the efficacy of valence engineering in BIEF regulation, offering a promising strategy for the development of advanced PEC sensors for clinical diagnostic applications.