Reliable biomarker monitoring at microneedle aptamer biosensors using a dual-frequency ratiometric approach: Overcoming signal drifts.

Abstract

Electrochemical aptamer-based (E-AB) microneedle (MN) sensors offer a promising route for continuous monitoring of endogenous biomarkers. However, long-term operation is often limited by signal attenuation arising from surface biofouling, aptamer detachment, and redox label degradation. Here, we investigated the origin of signal drift by monitoring changes in the peak current (I_P) of the redox label during prolonged square-wave voltammetry (SWV) operation in PBS, serum, and artificial ISF. To address this drift, we introduced a dual-frequency ratiometric approach based on the ratio between the I_P measured at frequencies showing maximum (F_max) and minimum (F_min) responses. Prolonged SWV measurements in PBS, where biofouling is negligible, showed a gradual decrease in I_P, and cyclic voltammetry measurements obtained before and after SWV operation confirmed a decrease in the MB faradaic current. These results suggest that SWV-induced aptamer detachment and/or MB degradation are major source of intrinsic signal drift. In serum and artificial ISF, this drift was further exacerbated by biofouling, resulting in an approximately 30% decrease in I_P measured at F_max and F_min (IF_max and IF_min) over 48 h. In contrast, the ratiometric signal (I_R = IF_max/IF_min), remained stable because similar signal drift in IF_max and IF_min was effectively compensated. In addition, the ratiometric approach reduced sensor-to-sensor variation, enabling reliable continuous cortisol monitoring with quantified concentrations that closely matched the applied levels, whereas IF_max-based quantification resulted in large deviations. These findings establish dual-frequency ratiometric E-AB MN sensors as a reliable platform for stable and accurate continuous biomarker monitoring in complex biological fluids.