Fluoro-Substituted Covalent Organic Framework Particles Anchored on TiO₂ Nanotube Arrays for Photoelectrochemical Determination of Dopamine

In this work, the photoelectrochemical (PEC) properties of TiO2 nanotube arrays (NTAs) were enhanced by forming heterostructures with non-substituted covalent organic frameworks (H-COFs) and fluoro-substituted covalent organic frameworks (F-COFs) through a facile in situ hydrothermal synthesis method. The composites H-COF/TiO2 NTA and F-COF/TiO2 NTA were fully characterized using scanning electron microscopy, energy-dispersive spectrometry, transmission electron microscopy, X-ray diffraction, attenuated total reflection–Fourier transform infrared, 13C CP/MAS NMR, and X-ray photoelectron spectroscopy. It was observed via UV–vis diffuse reflection spectroscopy that both H-COF/TiO2 NTA and F-COF/TiO2 NTA exhibited enhanced visible light absorption compared to bare TiO2. The modification of TiO2 with fluoro-substituted COF was found to be the most effective in enhancing the photoelectric response of TiO2 by producing photoelectrons with the longest lifetime based on the calculation of transient time constant (τ). The potential application of F-COF/TiO2 NTA as a PEC sensor for dopamine (DA) detection was investigated, and the resultant sensor exhibited an extended linear range for DA quantification (0.1–300 μM) with a detection limit of 0.032 μM (S/N = 3). The formation of a type II heterojunction was proposed to be responsible for the “signal on” mode for DA quantification using F-COF/TiO2 NTA.