Nano gold-doped molecularly imprinted electrochemical sensor for rapid and ultrasensitive cortisol detection

Rapid and sensitive detection of steroid hormone cortisol can benefit the diagnosis of diseases related to adrenal gland disorders and chronic stress. We report a molecularly imprinted polymer (MIP)-based electrochemical sensor that utilized nano gold-doped poly o-phenylenediamine (poly-o-PD) film to selectively determine trace level cortisol with enhanced sensitivity. The sensor detected cortisol levels by measuring the current change of the redox-active probes in response to the binding of target cortisol to the imprinted sites in the polymer. The gold-doped MIP (Au@MIP) sensor was prepared using a facile one-step in situ gold reduction and electropolymerization method to distribute high-density gold nanoparticles in the vicinity of the binding cavities. The in situ gold reduction promote the polymerization reaction, enlarging the effective surface area of the sensor. The nano gold doping also facilitated charge transfer when exposed to redox reagents. It enabled efficient blocking of the charge transfer upon the occupation of the cavities by cortisol, resulting in enhanced detection response and sensitivity. The Au@MIP sensor exhibited a high affinity toward cortisol binding with a dissociation constant K_d of ∼0.47 nM, a linear detection range from 1 pM to 500 nM with a detection limit of ∼200 fM, and satisfied specificity over other steroid hormones with highly similar structures. The sensor was successfully demonstrated to determine cortisol levels in spiked saliva in normal and elevated ranges. The facile antibody-free cortisol detection method was proved to be highly sensitive and selective, suitable for point-of-care testing applications.