Despite the promise of electrochemical biosensors in amplified nucleic acid diagnostics, existing high-sensitivity platforms often rely on a multilayer surface assembly and cascade amplification confined to the electrode interface. These stepwise strategies suffer from inefficient enzyme activity, poor mass transport, and inconsistent probe orientation, which compromise the amplification efficiency, reproducibility, and practical applicability. To address these limitations, we report a programmable dual-phase electrochemical biosensing system that decouples amplification from signal transduction. In the homogeneous phase, a palindromic allosteric hairpin probe undergoes target-triggered polymerization and bidirectional strand extension, generating double-stranded DNA (dsDNA) amplicons. These dsDNAs activate CRISPR/Cas12a complexes, which, in turn, cleave thiolated reporter DNA immobilized on a gold electrode. The exposed 3'-hydroxyl termini then initiate terminal deoxynucleotidyl transferase (TdT)-mediated polyguanine (poly-G) extension in the presence of dGTP. Methylene blue, selectively binding to G-rich sequences, produces a strong voltammetric signal proportional to the original miRNA-21 concentration. By integration of homogeneous amplification with localized electrochemical signal generation, this dual-phase design circumvents the drawbacks of interface-bound cascades while leveraging their sensitivity advantages. The system achieves a detection limit of 25 attomolar for miRNA-21, excellent sequence specificity, and reliable performance in human blood samples. This approach provides a robust and generalizable platform for nucleic acid diagnostics with high sensitivity, modularity, and operational simplicity.
