Antibody-free photoelectrochemical biosensor for DNA carboxylation detection based on SnS₂@Ti₃C₂ heterojunction

As an important epigenetic modification, 5-carboxycytosine (5caC) played an important role in gene regulation, cell differentiation and growth. 5caC existed in many cells and tissues, but it was highly similar to the structure of other cytosine derivatives and had less content in the genome. Therefore, it was urgent to develop a sensitive and highly selective trace biosensor to detect 5caC. A novel photoelectrochemical biosensor was fabricated for 5-carboxy-2'-deoxycytidine-5'-triphosphate (5cadCTP) detection, where SnS₂@Ti₃C₂ nanocomposite was employed as photoactive material, polyethyleneimine was used as 5cadCTP recognition and capture reagent, and Ru(NH₃)₆³⁺ was used as photosensitizer for signal amplification. Due the good conductivity of Ti₃C₂ MXene and the matched energy band between Ti₃C₂ MXene and SnS₂, SnS₂@Ti₃C₂ nanocomposite presented strong photoactivity, which was beneficial to the high detection sensitivity. For specific recognition of 5cadCTP, the covalent interaction of -NH₂ in 5cadCTP with -COOH on the substrate electrode was used, which was beneficial to the high detection selectivity. A broad linear relationship between photocurrent and 5cadCTP concentration was observed ranging from 1 pM to 0.2 μM. The low detection limit of 260 fM was achieved. The developed method has high detection specificity and can even distinguish 5caC with its derivatives. In addition, the applicability was evaluated by detecting the content change of 5caC in the genomic DNA of rice seedlings after cultured with environmental pollutants. This work provides a novel platform for 5cadCTP detection, and it can also be applied to detect other cytosine derivatives with suitable recognition strategies.