NIR-induced label-free subcutaneous PEC biosensor UC@Cu₂O@TiO₂ for detection of <scp>l</scp>-cysteine

Conventional photoelectrochemical (PEC) biosensors based on ZnO and TiO₂ face challenges in applications <i>in vivo</i> due to the poor penetration of excitation light through the skin. To address this limitation, we designed a novel near-infrared (NIR)-driven FTO@UC@Cu₂O@TiO₂ heterojunction biosensor for subcutaneous L-cysteine detection. NIR light penetrates the skin and excites the upconversion material (called UC for short, NaYF₄:Er, Yb), which emits visible light at 540 nm. This emission activates the Cu₂O semiconductor to generate photocurrent. The Cu₂O@TiO₂ heterojunction facilitates charge carrier separation and suppresses electron-hole recombination. The biosensor demonstrates stable, sensitive, and reproducible detection of L-cysteine over a range of 10 μM to 10 pM, with a detection limit of 5.2 pM. Notably, it also enables subcutaneous detection covering a range from 10 μM to 100 pM, with a detection limit of 74 pM. These results provide a promising strategy for real-time <i>in situ</i> monitoring of L-cysteine in clinical applications.