Branched Polyethylenimine-Modified Upconversion Nanohybrid-Mediated Photoelectrochemical Immunoassay with Synergistic Effect of Dual-Purpose Copper Ions.

This work developed a near-infrared (near-IR) light-activated non-enzymatic signal-off photoelectrochemical (PEC) immunoassay for the ultrasensitive detection of α-fetoprotein (AFP) on the basis of branched polyethylenimine (BPEI)-modified upconversion nanoparticle (UCNP)@CdTe quantum dot (QD) nanostructures by coupling with the synergistic effect of dual-purpose copper ions. Emission light originated from NaYF4:Yb,Er UCNP was directly utilized through the electrostatic bonding of CdTe QDs to excite the separation of electron-hole pairs, resulting in the generation of obvious photocurrent under a 980 nm laser. By using polyclonal antibody-labeled cupric oxide nanoparticle as the secondary antibody, the nanolabel was introduced into the monoclonal anti-AFP antibody-modified microplates in the presence of target AFP. After treatment with acid, the as-released copper ion decreased the photocurrent through the synergistic effect with two issues: one was initially to form coordination with BPEI on the surface of UCNP, and then the near-IR excitation light and upconversion luminescence were attenuated due to the internal filter effect; another was to snatch the electrons flowing from the valence band of CdTe QD as the exciton trapping sites. Under optimal conditions, the dual-purpose Cu2+-activated signal-off PEC immunosensing platform exhibited a dynamic linear range from 10 pg mL-1 to 50 ng mL-1, accompanying the decreasing photocurrent with the increment of AFP concentration at an experimental detection limit of 1.2 pg mL-1. Importantly, good accuracy was achieved by this method in comparison with the results with human AFP ELISA kit for analysis of human serum samples. This dual-purpose Cu2+-activated PEC immunoassay brings a promising, enzyme-free and innovative thinking for the detection of low-abundance biomarkers.