ZnO Nanoparticle-Based Electrochemical Immunosensor for One-Step Quantification of Cortisol in Saliva

The determination of cortisol is the most important aspect to monitor the stress level of human beings. Currently, cortisol levels are monitored through blood samples. However, multiple sampling and frequent monitoring are highly difficult. Few notable studies reveal the application of gas chromatography-mass spectroscopy (GC-MS) and liquid chromatography-mass spectroscopy (LC-MS) techniques for saliva cortisol quantification. Since quantification through GC-MS and LC-MS are not cost-effective and involves a tedious process of sample preparation, the development of appropriate quantification techniques for cortisol levels in saliva is much needed. In this work, we have reported the development of an electrochemical sensor modified with zinc nanoparticles on a glassy carbon electrode (GCE) to determine the cortisol level in saliva. The modification process involved electrodeposition of the ZnO nanoparticles on the surface of the GCE to form GCE (ZnO/GCE) at -1.0 V by the chronopotentiometric technique, followed by treatment with the 3-aminopropyltriethoxysilane (APTES) cross-linker in order to improve cortisol antibody (C-Mab) immobilization and stability (Zn-APT-AC). Employing different analytical tools, including a particle size analyzer, Fourier transform infrared spectroscopy (FT-IR), and scanning electron microscopy (SEM), structural analyses of ZnO nanoparticles and antibody-loaded ZnO on a GCE were verified. To evaluate the electrochemical immunosensor behavior of the developed modified electrodes, the differential pulse voltammetry (DPV) electrochemical method was used. The immunosensor exhibited a steady, selective, and sensitive response at cortisol concentrations between 10^-7 and 10² nM, in artificial saliva, at room temperature. These results indicate that the ZnO NP-modified GCE sensor could be a very promising candidate for saliva-based cortisol quantification.