Silk fibroin-decorated with tunable Au/Ag nanodendrites: A plastic near-infrared SERS substrate with periodic microstructures for ultra-sensitive monitoring of lactic acid in human sweat

The flexible and plastic near-infrared surface-enhanced Raman spectroscopy (NIR-SERS) substrate, with efficient tunable surface microstructures for probe molecular detection on irregular targets, has great potential for ultra sensitive monitoring in real-world scenarios. Herein, we propose an interesting 785 nm laser-excited plastic SERS substrate based on the silk fibroin fibers (SFF) decorated with multiple-branched Au/Ag nanodendrites (NDs). Compared with the original smooth flexible substrate, the further constructed plastic SFF-Au/Ag NDs with concave pyramid surface microstructures provides a relatively higher NIR-SERS activity at bottom of the inward concave region. Moreover, the enhanced NIR-SERS activity is also highly related to the content of Au/Ag NDs in the hybrid flexible substrate. The maximal NIR-SERS signal intensity can be realized at the content of 0.15 wt% Au/Ag NDs in the SFF-Au/Ag NDs, which is about 8-fold enhancement compared to that obtained at the content of 0.01 wt%. Then, the molecular detection limits of crystal violet (CV) and 4-mercaptobenzoic acid (4-MBA) are calculated about 10(-15) and 10(-13) M, respectively, reaching the ultra-sensitive requirements of NIR-SERS trace detection. Moreover, the substrate can detect lactic acid (LA) molecules with a concentration as low as 10(-9) M, and can effectively reflect slight LA fluctuation in human sweat, providing a basis for the realization of ultra sensitive NIR-SERS health monitoring.