Ion Permeation through Silica Coating of Silver Nanoparticles Functionalized with 2-Mercaptoethanesulfonate Anions: SiO2-Encapsulated SERS Probes for Metal Cations

摘要

In this work, the first use of thick-silica-protected Ag nanocomposites (Ag-MES@SiO2) in surface enhanced Raman scattering (SERS) detection of metal cations is presented. Several factors which are important for the successful synthesis of silica-encapsulated citrate-free Ag nanoparticles functionalized with sodium 2-mercaptoethanesulfonate (MES) are elucidated. It is shown that negatively charged MES anions adsorbed on Ag surface effectively protect Ag NPs from both aggregation and dissolution in the presence of ammonia. Simultaneously, MES acts as a primer in the modified Steber reaction. Owing to the MES functionalization of the silver core, Ag-MES@SiO2 nanoparticles exhibit intense and stable SERS spectra that are sensitive to metal cations. Based on cation-sensitive SERS spectra of silica-encapsulated MES tags it is proved that 30 nm thick silica shell is permeable for metal cations in short spans of time. Differences between several metal cations in transport through the SiO2 layer are observed, with smaller cations reaching the Ag core more efficiently. The Ag-MES@SiO2 SERS nanosensor exhibits better sensitivity than unprotected Ag-MES sensor.