Ultrasensitive miRNA-21 biosensing: a metal-free photoinduced ROMP strategy via 3'-end oxidation

MicroRNA-21 (miRNA-21) is frequently overexpressed in various solid tumors, making its reliable quantification important for cancer-related biomarker analysis. Herein, we report an electrochemical sensing strategy that integrates RNA 3'-end oxidation with metal-free photo-induced ring-opening metathesis polymerization (photo-ROMP) for miRNA-21 detection. After hybridization with a thiolated DNA probe immobilized on a gold electrode, the 3'-terminal cis-diol group of miRNA-21 was selectively oxidized by sodium periodate to generate reactive aldehyde groups. The vinyl ether initiator 3-aminopropanol vinyl ether (APVE) was subsequently anchored to the oxidized miRNA-21 through aldehyde-amine condensation followed by NaCNBH₃-mediated reductive amination. Under blue-light irradiation, photoexcited 2,4,6-tris(4-methoxyphenyl)pyrylium tetrafluoroborate (MeO-TPT) activated the interfacial APVE and initiated photo-ROMP of the cyclopentadienyl ferrocene (CFc), producing electroactive polymer chains on the electrode surface and amplifying the square-wave voltammetric response. Under optimized conditions, the biosensor exhibited a linear response toward miRNA-21 from 100 aM to 100 pM, with a calculated limit of detection of 28.48 aM. The sensor also showed good sequence selectivity, reproducibility, and short-term stability. Matrix-matched calibration in 10% diluted human serum yielded recoveries of 106.2-114.5% with relative standard deviations of 2.37-4.99%. These results demonstrate that target-end-triggered photo-ROMP provides an effective enzyme-free polymer amplification strategy for electrochemical miRNA detection in buffer and diluted serum matrices.