Silver Nanoparticles-Loaded Cross-Linking Branched Polyethylenimine as a Spongy Flexible Substrate for Surface-Enhanced Raman Spectroscopy

Surface-enhanced Raman spectroscopy (SERS) can significantly enhance Raman scattering signals of samples located at or very close to the Ag nanoparticles. The flexible SERS substrates may broaden the application of SERS technology because of high SERS efficiency, conveniently collecting or in situ detecting liquid samples. The glutaraldehyde cross-linked branched polyethylenimine loading silver nanoparticles (bPEI/AgNPs) composite with dual three-dimensional (3D) network structures was in situ synthesized under vortex at room temperature and lyophilization. The first molecular-level 3D network with a topological cross-linking structure provided uniformity and stability of Ag nanoparticles in the flexible bPEI/AgNPs composite, while the second micro-3D network with a coarsely porous structure further endowed the flexibility of the bPEI/AgNPs composite, and rapid and effective absorbing the liquid sample, in addition to bringing about the SERS effect. UV–vis spectroscopy, X-ray diffraction analysis, and energy dispersive spectroscopy confirmed the formation of Ag nanoparticles in the bPEI/AgNPs composite. The microimage of the spongy bPEI/AgNPs composite with 3D porous microstructure and the shapes of Ag nanoparticles were analyzed using scanning electron microscopy and transmission electron microscopy. Mechanical property analysis showed good flexibility of the bPEI/AgNPs composite. The bPEI/AgNPs composite exhibited the strong SERS effect for Rhodamine 6G (R6G), thiram, and bovine serum albumin (BSA), of which the detection limits were 1.0 × 10^–6, 1.0 × 10^–5, and 5.0 × 10^–6 mol/L, respectively. The SERS enhancement factor of R6G was further determined to be 2.0 × 10⁵. The 3D rough and porous microstructure of the bPEI/AgNPs composite absorbing R6G was observed in the 3D micro-Raman image. The Raman bands of the amino acid residues and the second structural domains of BSA molecules approaching Ag nanoparticles were significantly enhanced by the bPEI/AgNPs composite. The bPEI/AgNPs composite is thus promising for use as a spongy flexible SERS substrate for Raman active compound analysis through convenient and fast sampling.