SERS-based pump-free microfluidic chip sensor for highly sensitive competitive immunoassay of cortisol in human sweat†

Cortisol, known as the “stress hormone”, is secreted by the adrenal cortex. Measuring cortisol levels in body fluids is essential for evaluating stress levels, adrenal function, hormone imbalance, and psychological well-being. Early diagnosis and management of related conditions depend on this measurement. A rapid detection method that combines immunoassay and surface-enhanced Raman scattering (SERS) technology has become widely used in bioanalysis, offering benefits such as fast detection, high throughput, integrated microsystems, and high specificity. This study introduces a pump-free microfluidic chip integrating a solid-state SERS substrate to detect trace amounts of cortisol in bodily fluids through immunoassay. The method relies on a competitive reaction between cortisol and SERS tags with cortisol antigens immobilized on gold nanostructured substrates in a microfluidic environment. Two detection channels are used to provide controls and enhance measurement efficiency and accuracy. Solid-state gold nanostructured substrates offer a larger surface area for antibody capture and act as SERS-active substrates, which significantly enhance the Raman signal and improve the microsystem's sensitivity and applicability. Driven by a capillary pump, the sample can be loaded within 60 seconds, with the entire detection process taking less than 10 min, significantly reducing the detection time. Results indicate that the detection limit for cortisol is 10 pg mL⁻¹, meeting clinical biomarker thresholds. The integrated SERS microfluidic chip shows great promise as an analytical tool for the rapid and sensitive diagnosis of cortisol in bodily fluids.