WS₂/Bi/BiOBr Nanostructures for Photoelectrochemical Sensing of 5-Formyluracil-2′-deoxyuridine-5′-triphosphate through Hemin/G-Quadruplex Double Signal Amplification

5-Formyluracil (5fU) is considered an important epigenetic modification agent involved in gene regulation, cell differentiation, and development. 5fU exists in many cells and tissues but is highly similar to the structure of 5-formylcytosine (5fC) and is less abundant in genome. Therefore, the development of a rapid, sensitive, and highly selective method to detect 5fU is desired. In this study, a photoelectrochemical biosensor based on WS2/Bi/BiOBr nanocomposites, black TiO2 (b-TiO2), and hemin/G-quadruplex was described, which was found to be highly sensitive and selective for 5-formyluracil-2′-deoxyuridine-5′-triphosphate (5fdUTP) detection. WS2/Bi/BiOBr was first employed as a photosensitive material, and 3,4-diaminobenzoic acid (DABA) was adopted as a 5fdUTP recognition and capture reagent, where DABA was captured by polyethylene polyamine through covalent reaction to form a stable amid bond. Then, a benzimidazole bond was formed between DABA and −CHO of 5fdUTP, and 5fdUTP was selectively captured on the electrode. Using b-TiO2 as a cross-linker, a guanine-rich ssDNA sequence was further immobilized on the electrode surface by covalent reaction with the 5fdUTP phosphate group. With the assistance of K+ and hemin, the hemin/G-quadruplex structure was formed on the electrode. The hemin molecules in the structure of hemin/G-quadruplex obviously enhanced the PEC response of the biosensor for detecting 5fdUTP, with a linear range of 0.005–500 nM and a detection limit of 1.53 pM (S/N = 3). In addition, this method exhibits optimistic selectivity. The precision and accuracy of this method were evaluated by studying the content of 5fdUTP in genome DNA of crop tissues with standard addition recovery experiments.