Moderately clustered gold nanoparticles preserved in in-situ polymerized hydrogels towards facile SERS sensing

Plasmonic nanoparticles such as gold nanoparticles exhibit Surface Enhanced Raman Scattering (SERS), enabling highly sensitive Raman spectroscopy for molecular sensing. SERS effect can be strongly enhanced by making plasmonic nanoparticles anisotropic or by assembling them. However, synthesis of such nanoarchitecture requires complicated and time-consuming processes. In this work, we employed a facile immobilization method for spherical plasmonic nanoparticles by incorporating them into the three-dimensional network structure of hydrogels. Nanoparticle aggregation that was induced by an increase in ionic strength was kinetically arrested by in-situ polymerization of hydrogels, thereby preserving small nanoparticle clusters in a gel film. The fabricated composite films were directly applicable as SERS substrates. The composite films containing gold nanoparticle clusters with moderate sizes (around 90 nm in hydrodynamic diameter) demonstrated superior SERS performance. The analytically estimated enhancement factor for the substrates with moderately clustered nanoparticles was approximately 70-fold higher than those with non-clustered nanoparticles. The proposed method for producing moderately clustered nanoparticles enables facile preparation of SERS substrates with superior performance, and moreover, is applicable to various types of nanoparticles to strengthen their properties.