A Simple, sensitive, and selective electrochemical aptasensor for cortisol based on rGO-AuNPs

Cortisol is a steroid hormone naturally produced by the adrenal glands. It participates in and controls several processes in the body and is considered an important physiological biomarker. Due to its very low concentrations in body fluids, its detection requires high sensitivity and specificity. Here, we present a simple electrochemical biosensor based on reduced graphene oxide (rGO) modified with gold nanoparticles (AuNPs) for the immobilization of the cortisol-specific aptamer (Ap) (MCH-Ap-AurGO/GCE). Important analytical parameters for identifying the target analyte were optimized, such as conditions and amount of immobilized Ap, the influence of the concentration and nature of the supporting electrolyte, pH of the medium, and incubation time. The optimized conditions for the aptasensor were: concentration of Ap 1.0 x 10(-6) mol L-1, support electrolyte Tris/HCl 50 mmol L-1, MgCl2 10 mmol L-1, and NaCl 10 mmol L-1, at pH 5.0 and incubation time of 15 min. A linear response range was obtained from 1 x 10(-18) up to 1 x 10(-11) mol L-1 of cortisol with a detection limit (LOD) of 1.0 x 10(-18) mol L-1. A curve adjusted for operational purposes in the saliva sample was fitted for the concentration range between 0.5 x 10(-14) and 1 x 10(-11) mol L-1, with a linear regression equation Delta Rtc/Rtc(1) = 2.70 + 0.17 x log([Cortisol]). The aptasensor demonstrated a great potential for detecting cortisol in a simple, fast, and highly sensitive way, opening its path for application in real samples, which present levels below the concentration in which cortisol is commonly found in body fluids.