Highly sensitive and selective electrochemical cortisol sensor using bifunctional protein interlayer-modified graphene electrodes

Highly accurate and prompt monitoring of cortisol is imperative for the prevention of stress-related diseases. In this study, we present a novel electrochemical biosensor for cortisol detection harnessing an electrochemical sensing platform consisting of a reduced graphene oxide (rGO) electrode, a bifunctional protein interlayer, and an antibody probe that specifically recognizes cortisol molecules. A thermally denatured bovine serum albumin (d-BSA) protein layer was directly adsorbed as an interlayer onto the rGO electrode surface via pi-stacking interactions, which provides the bifunctionality of creating covalent anchoring sites for anti-cortisol antibody probes and preventing the nonspecific binding of undefined substances. Electrochemical impedance spectroscopic (EIS) measurements show that the constructed sensing platform (antibody/d-BSA/rGO) exhibits picomolar-range sensitivity cortisol detection with a wide linear dynamic range spanning 10 pM-100 nM. In addition, our sensor exhibits high selectivity toward cortisol with negligible cross-binding reactivity to the cortisol analogs, aldosterone and progesterone. Moreover, almost complete spike recovery was confirmed with cortisol present in human saliva samples containing several interfering compounds. The demonstrated strategy for constructing the protein/rGO composite sensing platform is simple but versatile. Therefore, it is expected to facilitate efficient interfacing of various biomolecular probes with graphene-based and/or other electrode materials with hydrophobic surfaces. (C) 2016 Elsevier B.V. All rights reserved.