In-situ engineered MXene-TiO₂/ BiVO₄ hybrid as an efficient photoelectrochemical platform for sensitive detection of...

Interfacial charge-carrier recombination is a bottle-neck issue restricting photoelectrochemical biosensors advancement in the wearable clinical electronics. In this study, we propose a simple approach to construct a highly efficient photoactive heterojunction capable of functioning as an active substrate in PEC biosensing of CD44 proteins. Taking the advantage of high photocatalytic activity of BiVO₄, and biocompatible yet conductive 2D-Ti₃C₂T_x nanosheets, a workable heterojunction was constructed between in-situ formed TiO₂ from the partially oxidized Ti₃C₂T_x and lysine functionalized BiVO₄ (TiO₂/MX-BiVO₄). The interfacial arrangement was ideal for promoting fast charge transfer from photo-excited BiVO₄ and TiO₂ to Ti₃C₂T_x, constructing an energy level-cascade that permits minimal charge-carrier recombination besides robust photocatalytic redox activity. The PEC biosensor relies on the ligand-protein interaction, where hyaluronic acid was directly immobilized over TiO₂/MX-BiVO₄ based on the interactions between carboxyl of lysine and amino moieties of hyaluronic acid. The PEC biosensor response depends on the inhibition in the measured photo-oxidation current of mediator species, i.e., ascorbic acid after the addition of CD44 proteins. The superior photo-activity, and robust heterojunction arrangement, produced a sensitive signal capable of recognizing CD44 in the wide concentration window of 2.2 × 10^-4 ng mL^-1 to 3.2 ng mL^-1 with a low-detection limit of 1.4 × 10^-2 pg mL^-1. The strong interaction between lysine functionalized BiVO₄ and hyaluronic acid enabled biosensor to exhibit robust antifouling characteristics towards similar proteins such as PSA and NSE. The quantification of CD44 protein from real-blood serum samples further confirmed the biosensor's reliability for clinical application.