Mechanically stabilized UiO-66-NH₂-MB screen printed carbon electrode for high-performance electrochemical ratiometric quantification of miR-21-5p

The ratiometric sensing strategy, which uses dual-signal output, drastically compensates for the background noise and interference from the detection environment, compared to the sensing methods that rely on a single-signal output. However, the stability of the reference signal has become the primary challenge in constructing a ratiometric detection sensor. Therefore, in order to achieve stable ratiometric signal sensing, methylene blue (MB) was encapsulated in the UiO-66-NH₂ framework and printed as a reference signal onto a screen-printed carbon electrode (SPCE), facilitating the precise detection of miR-21-5p. Subsequently, based on the ultra-sensitive detection mechanism of catalytic hairpin assembly (CHA), the combination of miR-21-5p with H₁ sequence on the Au-deposited SPCE triggered the loop-open of H1. After that, ferrocene-labeled H₂ (H₂-Fc) and H₃-Fc sequences were sequentially added to form a stable "T-shaped" structure, and miR-21-5p was released into the next cycle. Thus, the detection of miR-21-5p was quantified by the current ratio of Fc to MB, obtaining an ultra-low detection limit of 2.7 fM. This ratiometric sensing strategy based on SPCE offers a promising pathway for highly sensitive sensing platforms.