Reversible Chemical Tuning of Charge Carriers for Enhanced Photoelectrochemical Conversion and Probing of Living Cells

A facile, solution method for reversible tuning of oxygen vacancies inside TiO₂ nanowires, in which the reducing treatment of TiO₂ by NaBH₄ leads to 2.4‐fold increase of photocurrent density, compared to pristine TiO₂ nanowires, is reported. Subsequent oxidizing treatment using KMnO₄ or annealing in air can reset the photocurrent density to the original values. The incident photo‐to‐current conversion efficiency measurement exhibits that the reduced TiO₂ nanowires present both enhanced photoactivity in both UV and visible regions. Density functional theory calculations reveal that the oxygen vacancies in the reduced TiO₂ cause defect states in the band structure and result in enhanced carrier density and conductivity. In addition, the enhanced solar energy‐driven photoelectrochemical conversion allows real‐time, sensitive chemical probing of living cells that are directly grown on the TiO₂ nanowire photoanodes. As proofs‐of‐concept, after functionalized with horseradish peroxidase (HRP) on the surface, the reduced TiO₂ NWs demonstrate sensitive, real‐time monitoring of the H₂O₂ levels in several distinctive living cell lines, with the lowest detectable H₂O₂ concentration of 7.7 nm . This reversible tuning of oxygen vacancies suggests a facile means for transition metal oxides, with enhanced photoconversion activity and electrochemical sensitivity.