High-Entropy Oxides as Nanozymes for Non-Immunological Electrochemical Monitoring of Cortisol

High entropy oxides (HEOs) are a class of materials that have garnered significant attention in recent years due to their potential use as catalysts in energy-related applications[1]. These materials are derived from high entropy compounds, which consist of equimolar or near equimolar ratios of multiple primary elements and have configurational entropy in the formation of a single-phase structure. The cocktail effect of HEOs gives rise to various catalytically active sites due to the synergistic interaction of the different elements, and lattice distortion during the formation of a single-phase solid solution results in the generation of abundant oxygen vacancies, which enhances the adsorption activity of HEOs and improves their catalytic performance[2]. HEOs can also be cost-efficient by utilizing low-cost transition metal elements as their active centers. Despite their widespread use as catalysts, the application of HEOs in electrocatalysis for molecule monitoring and early diagnosis remains largely unexplored. Cortisol, a glucocorticoid hormone that regulates many aspects of metabolism, stress, and immune response, is closely related to various diseases and disorders[3]. Current methods for noninvasive monitoring of cortisol relies upon immunological assays and typically limited to labor-intensive and time-consuming laboratory techniques. As a consequence, detecting cortisol in a label-free manner that is low cost and offers significant sensor stability and high sensitivity is desirable for many emerging applications, particularly for on-body monitoring. Here, we present work that examines the catalytic activities of HEOs as nanozymes for the electrocatalysis of cortisol. The electrochemical cortisol sensor is based on a novel one-pot microwave-assisted reaction synthesized (Fe, Co, Mn, Ni, Cu) Ox. The catalytic mechanism of electrochemical sensors depends on the formation of various valence metal redox species pairs, where the higher valence state oxidizes cortisol species present on the electrode surface, accompanied by a reduced lower valence state. The one-pot microwave-assisted synthesized HEOs exhibited ultra-high catalytic activity towards cortisol species as measured by cyclic voltammetry and chronoamperometry. Experimental results show a superior sensing performance including a detection limit as low as 0.5 µM (s/N=3), a good linear detection range between 0.01 ~ 4.01 µM, and a sensitivity of 46.5 µA µM-1 cm-2. References: [1] Y. Pan, J.-X. Liu, T.-Z. Tu, W. Wang, and G.-J. Zhang, “High-entropy oxides for catalysis: a diamond in the rough,” Chem. Eng. J. , p. 138659, 2022. [2] D. Feng et al. , “Holey lamellar high-entropy oxide as an ultra-high-activity heterogeneous catalyst for solvent-free aerobic oxidation of benzyl alcohol,” Angew. Chem. , vol. 132, no. 44, pp. 19671–19677, 2020. [3] B. Wang et al. , “Wearable aptamer-field-effect transistor sensing system for noninvasive cortisol monitoring,” Sci. Adv. , vol. 8, no. 1, p. eabk0967, 2022.