Dual-Signal Amplification Strategy for Ultrasensitive Photoelectrochemical Immunosensing of α-Fetoprotein

An ultrasensitive photoelectrochemical immunoassay of cancer biomarker alpha-fetoprotein (AFP) is proposed that uses titanium dioxide (TiO2) coupled with AFP-CdTe-GOx bioconjugate, which featured AFP antigen and glucose oxidase (GOx) labels linked to CdTe quantum dots (QDs) for signal amplification. The synthesized CdTe QDs yielded a homogeneous and narrow size distribution, which allowed the binding of AFP and GOx on CdTe QDs. Greatly enhanced sensitivity for AFP came from a dual signal amplification strategy. First, an effective matching of energy levels between the conduction bands of CdTe QDs and TiO2 allowed for fast electron injection from excited CdTe QDs to TiO2 upon irradiation, which reduced the recombination process of electron-hole pairs and prompted photoelectrochemical performance. Second, GOx enzyme could catalyze glucose to produce H2O2, which acted as a sacrificial electron donor to scavenge the photogenerated holes in the valence band of CdTe QDs, further causing an enhanced photocurrent. Thus, on the basis of the dual signal amplification strategy, the competitive immunosensor based on the specific binding of anti-AFP antibodies to AFP and AFP-CdTe-GOx bioconjugates was achieved. This proposed biosensor for AFP possessed largely increased linear detection range from 0.5 pg/mL to 10 mu g/mL with a detection limit of 0.13 pg/mL. The proposed amplification strategy shows high sensitivity, stability, and reproducibility and can become a promising platform for other protein detection.