Label-free photoelectrochemical immunosensor for carcinoembryonic antigen detection based on g-C3N4 nanosheets hybridized with Zn0.1Cd0.9S nanocrystals

A label-free photoelectrochemical (PEC) immunosensor for carcinoembryonic antigen (CEA) detection was developed using two-dimensional (2D) ultrathin graphitic carbon nitride (g-C3N4) nanosheets hybridized with Zn0.1Cd0.9S nanocrystals as the photoactive matrix. The g-C3N4 ultrathin nanosheets, which are 2D conjugated polymers, were synthesized through a thermal polymerization-exfoliation method that can generate a high specific surface area. Next, the Zn0.1Cd0.9S nanocrystals was successfully grown on the g-C3N4 ultrathin nanosheets (Zn0.1Cd0.9S/g-C3N4), forming a well-matched band structure where the presence of Zn0.1Cd0.9S/g-C3N4 accelerated the separation and transfer of photogenerated electron-hole pairs. The as-synthesized Zn0.1Cd0.9S/g-C3N4 composites showed excellent photoelectrochemical activity, considerably higher than that of pure Zn0.1Cd0.9S and g-C3N4. It was worth noting that the heterojunction with strong interfacial interaction formed between Zn0.1Cd0.9S and g-C3N4 was adopted in the construction of an immunosensor for the first time. CEA antibody (anti-CEA) was immobilized on the surface of a modified electrode using a chemical bonding effect, with mercaptoacetic acid (MPA) and EDC/NHS being used as bridging media. The highly specific introduction of CEA resulted in increased steric hindrance via the antigen-antibody specific immunoreaction; Thus, the concentrations of CEA could be detected by observing the decrease in photocurrent. Under optimal conditions, the linear range for the immunosensor was from 0.005 ng mL−1 to 20 ng mL−1. The detection limit was 1.4 pg mL−1.