Chemically Etched Silver Decorated Titanium Carbonitride MXene (Ag/Ti₃CN) for Enhanced Electrochemical Detection of Cortisol in Biological Fluid

Stress and unhealthy lifestyles have an undue effect on people’s physical and mental health. As a key hormone responsible for maintaining the normal functioning of human systems, cortisol plays a vital role in regulating physiological activities and monitoring psychological stress. Various nanomaterials, mainly two-dimensional (2D) nanomaterials, hold great promise in developing rapid electrochemical sensors due to their high surface area, excellent conductivity, and tunable properties. MXenes, a 2D transition metal carbide/nitride or carbonitrides, exhibit transformative characteristics related to their physical, chemical, and environmental properties, making them attractive candidates for sensor applications. The design of metal-doped MXene-based electrochemical sensors holds the potential to revolutionize biomarker monitoring. A novel material based on the chemical etching of Al from the (Ti₃AlCN) MAX phase without oxidation resulted in silver-decorated (Ag/Ti₃CN) MXene, which showed enhanced electrochemical properties. The fabrication of Ag/Ti₃CN was characterized using FTIR, XRD, SEM, AFM, and XPS techniques. The resulting MXene-based composite is used as a multilayer electrochemical sensor for highly sensitive cortisol detection to achieve a sub pg/mL detection limit. Fabrication of the disposable and miniturized cortisol sensor is realized on the in-house produced porous laser engraved graphene electrode (PLEG) on thin polyimide (PI) films. The laser engraving employs a CO₂ laser to directly pattern porous and multilayered graphene onto a thin PI surface. The Raman spectrum of the PLEG electrode exhibited three distinguished peaks (D, G, and 2D peaks), confirming the formation of graphene. The fixed ratio of Ag/Ti₃CN dispersed into the water was sonicated (15 min), drop-casted onto an electrochemically cleaned PLEG electrode, and dried. Later, the electrode surface (Ag/Ti₃CN/PLEG) was washed to remove the unbound fraction. To quantify cortisol, a differential pulse voltammetric signal was deployed to capture the oxidation current of metabolized cortisol due to the nanozyme activity of the Ag/Ti₃CN composite using a wireless readout. This was attributed to silver-catalyzed cortisol metabolism and Ag/Ti₃CN MXene’s excellent conductivity and large electroactive surface area, facilitating charge transfer at the electrode surface and enhancing electrocatalytic activity for cortisol detection. Under optimal experimental conditions, the proposed MXene-based sensor electrodes could detect cortisol in a broad concentration range from 0.10 pg/mL to 100 pg/mL with a good correlation of (R² = 0.9891, n=3). The higher sensitivity and notable LOD of 0.012 pg/mL cortisol, with good repeatability and reproducibility. The real-time application of the proposed MXene sensor electrodes was confirmed by testing in buffer and spiked artificial saliva samples with excellent recoveries from 97.8 to 102 % (n=3), demonstrating the practicality and reliability of the platform.<br/><b>Keywords:</b> MXene, 2D material, Graphene, Electrochemical Sensor, Cortisol<br/><b>Acknowledgements: </b>This work was partially funded by the Excellence Initiative: Research University (IDUB) program, Warsaw University of Technology (POST DOC II), and the National Science Centre within the OPUS 18 (UMO 2019/35/B/ST5/02538) project.