Photoelectrochemical detection of microRNAs based on target-triggered self-assembly of energy band position-matched CdS QDs and C3N4 nanosheets.

An ultrasensitive photochemical biosensor based on the target miRNA-triggered catalytic hairpin assembly (CHA) reaction between Au nanoparticles (AuNPs)/C3N4 nanosheets and CdS quantum dots (QDs) was developed for the determination of miRNAs. Firstly, AuNPs/C3N4 nanosheets were immobilized onto a working glassy carbon electrode. Then, the hairpin probe 1 (H1) was loaded through Au-S bonding. Afterward, the unbound sites were blocked with 6-mercaptohexanol to avoid nonspecific adsorption. In the presence of the target miRNA, the CHA reaction between the H1 and hairpin probe 2-CdS QDs (H2-CdS QDs) could be triggered. As a result, the AuNPs/C3N4 nanosheet and CdS QDs were linked by the double helix structure H1-H2. Unlike the other CHA reactions, H2 used in this work is longer than H1 so that the AuNPs/C3N4 nanosheets could touch the CdS QDs. Given the matched energy band positions between the C3N4 nanosheet and CdS QDs, a strong photocurrent could be obtained after the CHA reaction was triggered by the target miRNA. In addition, p-type C3N4 nanosheets and n-type CdS QDs presented reduction photocurrents and oxidation photocurrents, respectively. Therefore, the photocurrents were vectors in this design that can eliminate the interference of nonspecific adsorption and avoid the generation of false-positive signals. Under the optimal conditions, the limit of detection was 92 aM. The constructed photoelectrochemical biosensor showed good reproducibility and selectivity in the analysis of serum samples, which indicates its great prospects in disease diagnostics and bioanalysis.