Potassium-regulated Immobilization of Cortisol Aptamer for Field-effect Transistor Biosensor to Detect Changes in Charge Distribution with Aptamer Transformation

Salivary cortisol concentration and its circadian variation were detected by optimizing the ionic concentration of a solution during the immobilization of aptamers on a field-effect transistor biosensor. This was achieved by modifying our previously developed technique of detecting uncharged cortisol using the transformation of negatively charged aptamers by controlling the aptamer spacing. This spacing control was achieved by synthesizing pseudo-cortisol-binding aptamers with a guanine-quadruplex during the immobilization of the aptamers in a highly concentrated solution of K+. To detect uncharged cortisol, FETs were prepared by immobilizing a cortisol aptamer that contracts to form a guanine-quadruplex (G4). While simple immobilization reduces sensitivity due to steric hindrance during detection (top), high concentrations of K+ optimized the immobilization conditions of the pseudo-cortisol-bound aptamer upon G4 formation (bottom), allowing measurement of saliva samples.