Structural Basis and Conformational Switching of Capture-SELEX Aptamers for Designing Equol Sensors

Expanding healthy life expectancy requires technologies for continuous and noninvasive health monitoring. Sensors detecting health-related biomarkers in urine can be used for such applications. Aptamers are nucleic acids that interact with biomolecules through the formation of unique tertiary structures, some of which undergo target-induced conformational switching. The structural and dynamic properties of aptamers can be utilized for constructing sensors. In this study, DNA aptamers targeting equol, a gut microbiota-derived metabolite with various health benefits, were obtained by Capture-SELEX. Highly enriched 5 aptamers, Eq1 to Eq5, exhibited equilibrium association constants more than 10⁶ M^-1 toward equol. The aptamers were applied to the construction of fluorometric and colorimetric sensors. The fluorometric sensor employed fluorescence indicator displacement based on the static tertiary structure of the aptamer, while the colorimetric sensor utilized aggregation of gold nanoparticles (AuNP) based on dynamic conformational switching of the aptamer dissociating from a capture strand immobilized on AuNP. Both assays quantified equol in urine samples with LOD values of 2.73 μM and 846 nM, and linear detection ranges of 0-5 μM and 0-6 μM, for fluorometric and colorimetric assays, respectively. The sensitivities were sufficient to evaluate favorable equol production in urine. The promising proof-of-concept sensor demonstrated in this study offers a versatile platform for aptamer-based health-monitoring devices.