Photoelectrochemical biosensor for 5caC-DNA and Exo III detection based on Schottky junction integrated with type-I heterojunction of BiOCl/Bi2S3/ CS-MXene and hybridization chain reaction

The 5-carboxylcytosine (5caC), an active DNA demethylation intermediate, is present in many cells and tissues and plays a key role in the regulation of gene expression. 5caC is highly similar in structure to 5-methylcytosine, 5-hydroxymethylcytosine and 5-formylcytosine, but its abundance is extremely low. To explore the biologic function of 5caC, the sensitive detection technique is required. To achieve this goal, a new photoelectrochemical (PEC) biosensor was developed for 5caC-DNA detection using Schottky junction integrated with type-I heterojunction of BiOCl/Bi2S3/CS-MXene tetany composite as photoactive material and 5caC-hairpin DNA hybridization triggered hybridization chain reaction (HCR) as signal amplification mode. Based on the double heterojunctions, the photoactivity of BiOCl/Bi2S3/CS-MXene improved greatly, leading to a high photocurrent response, and achieving high detection sensitivity. To perform HCR amplification, an ingenious hairpin DNA containing 5caC was designed to inhibit the cleavage activity of exonuclease III (Exo III). However, after hybridization with S1DNA, the unfolded hairpin DNA triggered the excision of double-stranded DNA to release the 5caC-DNA fragment, which further induced the HCR amplification and capture electron donor of methylene blue (MB) onto the electrode surface, causing an increased photocurrent response, and achieving the highly sensitive detection of 5cac-DNA and Exo III. The proposed PEC biosensor showed a detection range of 1 pM - 1 nM and 1-100 U/mL with detection limits of 0.16 pM and 0.34 U/mL (S/N = 3) for 5caC DNA and Exo III, respectively. The developed method can also be applied to screen Exo III inhibitor and evaluate the ecotoxic effect of pollutant.