Hydrogel Microneedle‐Integrated Electrochemical Sensor With Negative‐Potential Readout Toward Wash‐Free and Anti‐Interference Monitoring of miRNAs in Tumor Interstitial Fluid

Electrochemical quantification of microRNAs (miRNAs) in tumor interstitial fluid (TIF) may provide complementary molecular information for tumor microenvironment assessment, but analytical accuracy is often limited by extraction-induced biomarker loss/degradation and background interference from endogenous electroactive metabolites. Herein, a hydrogel microneedle-integrated electrochemical biosensor is developed as a complementary ex vivo molecular sensing approach for wash-free and anti-interference monitoring of miRNAs via negative-potential readout. The sensing platform employs a four-way junction (FWJ) nucleic acid nanoreservoir to encapsulate the electroactive indicator doxorubicin (DOX), which is released upon target miRNA-triggered strand displacement. The characteristic oxidation potential of DOX (-0.65 V) enables detection in a negative potential window, thereby reducing co-oxidation interference by endogenous electroactive species. The porous hydrogel microneedle architecture facilitates rapid TIF sampling and directional diffusion of released DOX toward the electrode interface. The biosensor achieved a detection limit of 1.25 pM for miRNA-21 with an interassay variability of <1.9%. Preliminary analysis of clinical TIF samples from breast cancer and healthy donors further supported the feasibility of direct miRNA detection, yielding an AUC of 0.92. This work offers a novel strategy by integrating ex vivo in situ TIF sampling with negative-potential readout to achieve wash-free and anti-interference detection of biomarkers in complex physiological matrices.