Ag triangle nanoplates assembled on PVC/SEBS membrane as flexible SERS substrates for skin cortisol sensing

Surface-enhanced Raman scattering (SERS) based on rigid substrates has been widely used in biomedical detection due to its high sensitivity and specificity. However, the tedious operation steps for preparing SERS rigid substrates limited their applications in real-world detection. Compared with general rigid substrate, the flexible substrate has the advantages of simple preparation and easy portability, which are suitable for rapid, wearable and personalized detection in the field of point-of-care test. Herein, the flexible SERS substrates employing copolymer were fabricated and used for detection of skin cortisol, a biomarker for evaluating psychological stress in sweat. Silver triangle nanoplates with sharp corner were used as enhanced particles, and transferred to polyvinyl chloride/styrene-ethylene-butene-styrene copolymer (PVC/SEBS) film through three-phase interface self-assembly. By adjusting the size of silver nanoparticles, the ratio of PVC to SEBS in the polymer film, and the number of transfers of self-assembled silver films, the enhancement effect of the flexible SERS substrate was maximized. In addition, functionalization of the flexible SERS substrate with cortisol antibodies is used to achieve specific detection of cortisol on the skin surface. Under the optimal conditions, the Raman peak intensities at 1268 and 1500 cm^-1 of the cortisol had a good linear relationship with the logarithm of its concentration in the range of 10^-7 to 10^-3 M, and the detection limits were 5.47 × 10^-8 M and 5.51 × 10^-8 M, respectively. The flexible silver triangle nanoplates SERS substrate constructed by self-assembly in the three-phase interface has the characteristics of good specificity and high sensitivity, which has potential for transdermal cortisol wearable detection, providing a feasible method for the rapid evaluating psychological stress level.