Tailoring the Photoelectrochemical Activity of Hexametaphosphate-Capped CdS Quantum Dots by Ca2+-Triggered Surface Charge Regulation: A New Signaling Strategy for Sensitive Immunoassay.

The development of efficient signaling strategies is highly important for photoelectrochemical (PEC) immunoassay. We report here a new and efficient strategy for sensitive PEC immunoassay by tailoring the electrostatic interaction between the photoactive material and the electron donor. The photoelectric conversion of hexametaphosphate (HMP)-capped CdS quantum dots (QDs) in Na2SO3 solution is significantly boosted after Ca2+ incubation. The negative surface charges on CdS@HMP QDs decrease because of the complexation reaction between HMP and Ca2+, and the electrostatic repulsion between CdS@HMP QDs and electron donor (SO32-) becomes weak accordingly, leading to an improved electron-hole separation efficiency. Inspired by the PEC response of CdS@HMP QDs to Ca2+, a novel "signal-on" PEC immunoassay platform is established by employing CaCO3 nanoparticles as labels. By regulating the surface charge of CdS@HMP QDs with in situ-generated Ca2+ from CaCO3 labels, sensitive detection of the carcinoembryonic antigen (CEA) is achieved. The linear detection range is 0.005-50 ng mL-1 and the detection limit is 1 pg mL-1 for CEA detection. Our work not only provides a facile route to tailor the photoelectric conversion but also lays the foundation for sensitive PEC immunoassay by simply regulating the surface charge of photoactive materials.