Hofmeister effect-regulated poly(vinyl alcohol) @ cellulose nanofiber hydrogel-based SERS enhancement and sensing...

Owing to high water absorption and molecular capture capabilities, hydrogels are ideal substrates for integrating with metal nanoparticles and widely used in surface-enhanced Raman scattering (SERS) sensing. Herein, a strategy is proposed to versatilely alter the SERS properties of hydrogels by tuning the aggregation states of polymer chains through ion-specific effects derived from the Hofmeister series. A polyvinyl alcohol@cellulose nanofiber/silver nanoparticle (PVA@CNF/Ag NPs) hybrid hydrogel was fabricated, incorporating both sulfate and nitrate anions into this hydrogel with enhancement of SERS signals. The kosmotropic Na₂SO₄ drives chain bundling in the PVA@CNF hydrogel matrix, thereby indirectly modulating the interparticle distance of Ag NPs therein. This facilitates the formation of electromagnetic "hot spots", resulting in a marked improvement in SERS performance. Furthermore, the SERS performance of the hydrogels can be readily tuned across a broad range via adjusting anion type, concentration, and treatment duration. The optimal hydrogel exhibits a linear detection range of 0.001-100 ppm for methamphetamine (MAMP), with a coefficient of determination (R²) of 0.9836. When applied to urine analysis, it shows 85-110 % recoveries and a relative standard deviation (RSD) ~ 10 %. We further developed an indirect quantitative method for glucose, which features a lower limit of detection (10^-7 M).