Preparation and characterization of gold nanoparticle-loaded silica-gel films for localized surface plasmon resonance sensing

Abstract

Gold nanoparticles (AuNPs) were investigated as a sensing material because their absorption wavelength due to localized surface plasmon resonance (LSPR) shifts depending on the surrounding environment. For accurate sensing, the immobilization of dispersed AuNPs on a substrate is desirable. However, attaching AuNPs to smooth surfaces is difficult because the adhesion forces between AuNPs and most substrates are very weak. In this study, thick and porous silica-gel films were prepared via a dip-coating method using polyvinylpyrrolidone-containing silica sols. The AuNPs were precipitated in the silica-gel films by immersion in HAuCl4 solution followed by heating. The effects of the silica-gel film calcination temperature and the silica-sol aging conditions on sample structures, LSPR peak wavelengths, and LSPR sensing abilities against the change in refractive index were investigated. When the calcination temperature decreased, the intensity of LSPR absorption increased significantly because the silica gel calcined at low temperatures had a sparse structure and a significant quantity of AuNPs in the silica gel pores. The aging conditions of the silica sol also affected the structures of the AuNP-loaded silica gel films and their LSPR sensing abilities. The sample prepared from a silica sol aged at 25 °C contained much larger AuNPs and exhibited a higher sensing ability than the sample prepared from a silica sol aged at 40 °C. The high sensing ability of the sample prepared from the silica sol aged at 25 °C was considered to result from both the large size of the AuNPs and the low silica gel density around them.