Electron-Transfer Engineered Biointerface With Detonation Nanodiamonds for Next-Generation Cortisol Electrochemical Immunoassay.

Detonation nanodiamonds (DND) emerge as versatile nano building blocks for next generation biosensing, owing to their exceptional physicochemical properties and biocompatibility. In this study, a robust label-free electrochemical immunosensor for cortisol detection is based on screen printed carbon electrodes (SPE) modified with pristine DND. By utilizing EDC/NHS, efficient covalent crosslinking and stable antibody immobilization is achieved, as shown by comprehensive morphological and spectroscopic characterization (AFM, SEM, Raman, FTIR). Electrochemical analyses demonstrate that the DND interlayer functions as efficient multifunctional nanoscale mediator, enhancing interfacial electron transfer kinetics and providing stable biointerface. The resulting immunosensor exhibits high sensitivity with relevant linear range of 0.1 to 100 nM  and detection limit of 10 pM, alongside high selectivity against structurally related hormones and robust long-term storage stability. Validation in artificial saliva matrices demonstrates reliable performance and successful recovery. Benchmarking confirms that the DND-engineered SPE can offer a scalable, low-cost, reliable, competitive platform for non-invasive cortisol monitoring in point-of-care applications.