Gold-nanostars for FT-SERS measurements: a comparison between CTAB-protected and unprotected particles

Gold-nanostars were tested as SERS substrates for FT-Raman spectroscopy using a thiolic derivative of naphtalimide as ligand molecule. In particular, two different synthesis procedures were used providing surfactant-stabilized (namely CTAB-stabilized) [1] and surfactant-free [2] nanostars, both having the main plasmonic band at 960-970 nm that is suitable for excitation at 1064 nm for FT-Raman. The enhancement effect was measured on the colloidal solutions bearing a 1.6?10-6 M concentration of the analyte. Also the effect of the surfactant concentration was investigated for CTAB-stabilized particles and the hindering of the interaction between the metallic surface and the reporter molecule was pointed out to prevent the SERS detection. On the contrary, surfactant-free nanostars tended to agglomerate giving a reduced efficiency. For a quantitative comparison of the enhancing activity of the two batches of nanostars, the limit of detection (LOD) and the effective enhancement effect EFeff, which was obtained from the experimental EF through a shielding-correction factor, were measured and compared. On the whole, both protected- and unprotected-gold nanostars demonstrated to behave as good substrates for FT-SERS. In fact effective EFs values were in the range 5?104 - 5?105, comparable with the best results reported in literature for similar structures probed in solution but measured with shorter wavelengths. In particular, the most favourable conditions were reached on surfactant-nanostars once the CTAB excess was eliminated through centrifugation, which provided a nanomolar limit of detection of the reporter molecule.