Ag/Poly(N-isopropylacrylamide)-laponite Hydrogel Surface-Enhanced Raman Membrane Substrate for Rapid Separation, Concentration and Detection of Hydrophilic Compounds in Complex Sample All-in-One

In this work, Ag nanoparticles (AgNPs) were embedded into a poly(N-isopropylacrylamide)-laponite (Ag/PNIP-LAP) hydrogel membrane for highly sensitive surface-enhancement Raman scattering (SERS) detection. In situ polymerization was initiated by UV light to encapsulate AgNPs in a PNIP-LAP hydrogel to prepare a highly active SERS membrane with a three-dimensional structure. Due to the surface plasmon resonance and high swelling/shrinkage ratio of the Ag/PNIP-LAP hydrogel SERS membrane, its network structure has a “sieving” effect, which makes it easier for hydrophilic small-molecule targets to enter the sterically confined hydrogel, and the AgNPs are close to each other to form a Raman “hot spot” through the shrinkage of the hydrogel, at the same time, the analyte is enriched in the confined space and close to the AgNPs to form a stronger SERS signal. The characterization of the SERS activity of the Ag/PNIP-LAP hydrogel showed that the prepared three-dimensional membrane had high detection sensitivity for urotropine, 2,5-dimethylpyrazine, pyrazinamide, and pyrazine; the detection limits (S/N = 3) were 17.4, 31.0, 53.1, and 1.11 μg/L, and the analytical time was 35 min. Due to the hydrophilicity of the Ag/PNIP-LAP hydrogel membrane, the small molecules can easily enter the SERS membrane, and the hydrophobic macromolecules are blocked outside the SERS membrane. The SERS method has good selectivity, stability, and reproducibility. The SERS method was applied to the detection of urotropine in dried bean curd sticks, 2,5-dimethylpyrazine in nuts and potato chips, and pyrazinamide in human plasma with recoveries of 81.8–116.8% and the relative standard deviation within 4.9–9.9%. The results matched well with that found by the corresponding chromatographic methods. The proposed method has the advantages of simple sample pretreatment, speediness, high sensitivity, and good selectivity to hydrophilic compounds and has potential application in the rapid on-site detection.