Comparing two cortisol aptamers for label-free fluorescent and colorimetric biosensors

Abstract

Cortisol is a stress hormone and abnormal secretion of cortisol is related to many chronical diseases and neurological disorders. To detect cortisol, two DNA aptamers have been reported so far: 15-1 and CSS.1. In this work, two truncated aptamers were compared: 40-mer 15-1a and 42-mer CSS.1 using isothermal titration calorimetry, where only CSS.1 showed binding with a K_d of 245 nM. We then explored the label-free fluorescence and colorimetric detection methods of cortisol. Using SYBR Green I to stain the aptamers, CSS.1 showed 53% saturated fluorescence enhancement, and this assay allowed us to conclude that its binding was independent of Na⁺ and only weakly dependent on Mg²⁺. Moreover, two mutants and three truncated aptamers of CSS.1 were evaluated. In addition, cortisol had no effect on the colloidal stability of gold nanoparticles (AuNPs) and did not affect DNA adsorption by AuNPs, allowing a colorimetric assay of cortisol binding. While a cortisol-dependent color change was observed, this assay was interfered by other molecules that can cause aggregation of AuNPs such as dopamine. Since none of these assays showed cortisol binding with the truncated 15-1a aptamer, we concluded that 15-1a cannot bind to cortisol, and a more careful truncation study on the original aptamer is needed.