Synthesis of silica-coated gold nanorod as Raman tags by modulating cetyltrimethylammonium bromide concentration

The applications of colloidal gold nanorods (AuNRs) as substrate for surface-enhanced Raman scattering (SERS) inevitably have to deal with a bilayer of cetyltrimethylammonium bromide (CTAB) around the AuNR surface. Here, we report an improved fabrication of silica-coated gold nanorods as SERS tags by mainly controlling the amount of CTAB to facilitate the synthesis. CTAB molecules on AuNRs were partially removed by centrifugation twice to allow the adsorption of Raman marker molecules and were re-added to promote the growth of silica shells. The zeta potential was used to monitor the charge state of AuNRs during the synthesis. A model of the morphology changes of CTAB assemblies is proposed based on the measurement of zeta potential and effects of pH value and CTAB/TEOS molar ratio. SERS tags with single AuNR enclosed in a round and 40 nm-thick silica shell were produced within 24 h. The SERS signal of a thus-prepared tag was compared with that of a nanoaggregate tag by a model probe–target system made of a biotin-functionalized polystyrene (PS) microsphere. Results showed that the SERS signal from the streptavidin-functionalized AuNR tag on a single PS microsphere is comparable to that from the nanoaggregate-tag counterpart.