Zinc-vacancy defects in ZnO nanorods induced visible-light activity of photoelectrochemical glucose sensing: experimental and DFT+U analysis

Non-enzymatic photoelectrochemical (PEC) glucose sensing using ZnO nanorods (ZnO NRs) is typically constrained to UV excitation, as stoichiometric ZnO possesses a wide band gap. In this work, this UV-only limitation is addressed by deliberately exploiting intrinsic defect chemistry: zinc-vacancy (VZn) defects generated during low-temperature electrodeposition are engineered as the central design knob to activate visible-light PEC glucose sensing—without intentional doping, noble-metal decoration, or multi-component heterostructures. Vertically aligned ZnO NRs grown on fluorine-doped tin oxide (FTO) retain the wurtzite structure but exhibit a reduced optical band gap of ∼2.32 eV (UV–Vis/Tauc), consistent with defect-related electronic states. Under 460 nm illumination in phosphate buffer (pH 7.4), the ZnO NRs/FTO electrode delivers a stable, concentration-dependent photocurrent for glucose oxidation with a linear range of 0.5–10 mM and a detection limit of 12.5 μM. A synergistic combination of spatially resolved EDX compositional analysis and DFT + U electronic-structure cal-culations demonstrates that VZn induces localized band-tail/sub-bandgap states near the band edges, lowering the effective excitation energy and facilitating carrier generation and interfacial charge transfer under visible light. These results establish intrinsic zinc-vacancy engineering as a scalable, metal-free strategy to push ZnO-based non-enzymatic PEC glucose sensing from UV-only operation toward solar-compatible, visible-light activation.