Anion-exchange reactions: facile and general access to sensitive photoelectrochemical platforms for biomarker...

Affordable methodologies for detecting tumor markers at low concentrations are very important for early diagnosis. The two key requirements for designing ultrasensitive photoelectrochemical (PEC) immunosensors are highly active PEC platforms and amplified recognition events on the transducer surfaces. Herein we report the development of an effective and general strategy for the sensitive immunoassay of biomarkers using a novel PEC platform. In particular, in contrast to traditional photoanode materials prepared by introducing well-stabilized quantum dots onto an electrode, in this work CdSe nanocrystal (NC)-based PEC films were first formed directly on an ITO surface by an anion-exchange reaction and followed by coverage with organic stabilizers. The compacted CdSe film produces ultrahigh photocurrent signals under visible-light irradiation (λ = 470 nm). Using rabbit immunoglobulin G (RIgG) as a model biomarker, we showed that through biotin-avidin bridges, biotin-functionalized peroxidase (B-HRP) could be further assembled and could catalyze the conversion of its substrate, 4-chloro-1-naphthol (4-CN), to realize precipitation of nonconductive benzo-4-chlorohexadienone on the CdSe surface, thus resulting in blocking of the electron donors and absorption of the incident light. As the resulting photocurrent decrease was directly related to the target concentration, a sensitive PEC immunoassay could be constructed. The target RIgG was detected over a concentration range from 1.0 fg mL^-1 to 10 μg mL^-1 with an ultralow detection limit of 0.5 fg mL^-1. This study presents a promising and general strategy for the development of highly sensitive PEC biosensors, which can be extended to the detection of other enzymes and biomolecules.