Synthesis, characterization, and mechanistic study involved in the self-doping of TiO₂ nanotubes simultaneously to the impedimetric detection of methylene blue

This paper reports that the self-doped TiO₂ nanotubes (SD–TNT) electrode obtained by cathodic polarization promotes a significant decrease in the band gap value and an increase in electrode conductivity, due to the formation of Ti³⁺ sites and oxygen vacancies. A mechanism for the electrochemical self-doping of TNT electrode and its potential applicability as an impedimetric sensing platform for the target molecule methylene blue (MB) dye was proposed. Cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) studies made it possible to determine that the charge transfer process in the self-doping process is associated with the stability of H⁺ species. A 2² full factorial design (FFD) combined with response surface methodology (RSM) was employed to find the optimal operational conditions of the impedimetric sensor, which were equal to −1.4 V at pH 7.0. In this optimized condition, a linear relationship (R² = 0.999) for the MB concentration range of 0.10 to 1.75 mg L⁻¹ as a function of the charge transfer resistance (R_ct) was found. Finally, it was possible to correlate the self-doping process of TNT with the impedimetric behavior, indicating that this system can be used for other cationic compounds of interest.