In situ growth of carbon dots on TiO₂ nanotube arrays for PEC enzyme biosensors with visible light response

Carbon dots (CDs) were grown in situ on secondary anodized TiO₂ nanotube arrays (TiO₂ NTAs) via a hydrothermal method. The combination of CDs and TiO₂ NTAs enhanced the photoelectrochemical performance. Morphology, structure, and elemental composition of the CDs were characterized. No simple physical adsorption was found between the CDs and TiO₂, but chemical bonds were formed. UV-vis absorption and fluorescence spectroscopy showed that the CDs could enhance the absorption of TiO₂ in the visible and near-infrared regions. Owing to their up-conversion fluorescence properties, the CDs could convert low-energy photon absorption into high-energy photons, which may be used to excite TiO₂ to produce a stronger photoelectric response. Moreover, the CDs could effectively transport electrons and accept holes, thus contributing to the effective separation of electrons and holes during photoexcitation. Finally, the PEC biosensor was prepared by immobilizing glucose oxidase (GOx) on the surface of the composite. The PEC biosensor exhibited a broad range of 0.1-18 mM with a detection limit of 0.027 mM under visible irradiation because the composite material reflected strong light absorption for visible light, good conductivity, and good biocompatibility.