Gold nanostars as SERS-active substrates for FT-Raman spectroscopy

Gold nanostars having extinction maxima in the near infrared spectral region were synthesized according to surfactant-free or CTAB-based procedures. The SERS activity of these colloidal suspensions by 1064 nm excitation was tested with a thiolic derivative of naphtalimide as a ligand molecule. The shielding effect of the colloid on the observed Raman response was taken into account as well as the influence of nanoparticle concentration and the addition of chitosan, as stabilizer. A nanomolar limit of detection and a Raman enhancement factor of more than 105 were found. In particular, an important finding is that CTAB-free nanostars, even in the more biocompatible chitosan-stabilized form, still behaved as good SERS substrates in the near infrared where the occurrence of the “optical tissue window” points to their use in biological systems with minimum fluorescence background.