Fluorine-free anodized TiO2 nanotube arrays for photoelectrochemical H2O2 sensing: Role of anatase-rutile phase transformation

Hydrogen peroxide (H₂O₂) is a ubiquitous reactive oxygen species (ROS) involved in diverse physiological and pathological processes. Elevated H₂O₂ levels are often present in both tumor and stromal cells during different stages of cancer progression. Therefore, developing stable and efficient methods for H₂O₂ detection is of great importance. In this study, one-dimensional (1D) ordered TiO₂ nanotubes were successfully synthesized via electrochemical anodization in a fluoride-free electrolyte and subsequently annealed at various temperatures and demonstrated promising potential as photoelectrochemical (PEC) catalysts for hydrogen peroxide (H₂O₂) sensing in human serum samples. The morphology and physicochemical properties of the 1D TiO₂ nanotubes were systematically characterized using field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), synchrotron-based X-ray absorption spectroscopy (XAS), wide-angle X-ray scattering (WAXS), and X-ray excited optical luminescence (XEOL). These results demonstrate that thermal annealing enables precise control over phase composition (anatase, rutile, or mixed), as well as the distribution of Ti³⁺/Ti⁴⁺ species and oxygen vacancies. The resulting optimized nanotubes, with tailored surface and structural features, show enhanced PEC activity toward H₂O₂ sensing. The developed TiO₂ nanotube arrays displayed a linear detection range from 0 to 0.75 mM, a sensitivity of 14.98 μA mM^–1 cm^–2, and a low limit of detection of 131 μM, along with acceptable selectivity. Additionally, the sensor performed effectively in human serum samples, achieving a recovery of 101.33 % and a relative standard deviation (RSD) of 1.23 %. This study highlights the potential of anodized 1D TiO₂ nanotubes formed in fluoride-free electrolytes as an environmentally friendly approach, demonstrating significant (photo)electrocatalytic activity for H₂O₂ sensing and supporting their promise for practical biomedical applications.