Surface enhanced Raman scattering of brilliant green: Packing density and stabilizing effect of the cationic surfactant CTAB on the “hotspot” spacing

Surface-enhanced Raman scattering (SERS) has been applied utilizing colloidal suspensions of silver nanoparticles (AgNPs), for the detection of brilliant green (BG) at very low concentrations. Proper stimulated surface-enhanced resonance Raman scattering (SERRS) spectra collected from BG aqueous solutions at different low concentration ranges revealed a variation of the relative intensity ratio of certain BG bands attributed to a respective alteration of the molecular orientation of the analyte with respect to the AgNPs. This behavior is closely related to a different packing of the BG molecules on the nanoparticle surface when the concentration increases and the available space decreases. Additionally, the addition of suitable amounts of cetyl thrimethyl ammonium bromide (CTAB) to the BG aqueous solutions enhanced the detection limit of the analyte. This enlightens the role of the cationic surfactant in tuning the interparticle distances of AgNPs in the colloidal suspension; this presumably allows an optimization of the SERS enhancement factor. The concentration of CTAB that maximizes the SERS of BG is the same with the critical micelle concentration (CMC) of the surfactant. Most probably, CTAB at the CMC acts as a spacer keeping the AgNPs at a certain distance suitable for the generation of SE(R)RS hotspots.