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The collection of surface‐enhanced Raman scattering (SERS) by silver nanocolloids was used to identify the critical micelle concentration (CMC) of three cationic surfactants in 0.5‐M NaCl solutions: hexadecyltrimethylammonium bromide, dodecyltrimethylammonium bromide, and benzalkonium chloride. Despite structural similarity between the three test surfactans, differences of the shape and size of the respective micelles affected the corresponding SERS signals. The CMC values determined by this method agreed with the respective values measured by conventional methods, including fluorescence probing and electrical conductivity measurements. The potential for the application of SERS technique for low CMC determination without surface modification of silver nanoparticles by a Raman‐active molecule was also demonstrated. [ABSTRACT FROM AUTHOR]

• SERS revealed reorientation and packing density of BG with respect to the AgNP surface as a function of its concentration. • The presence of the cationic surfactant CTAB improved the SERS enhancement of BG. • Micelles create the optimum distances for the generation of "hotspots". • The presence of CTAB at CMC created the optimum distances between AgNPs for appropriate SERS active "hotspots". • SERRS has been applied to detect BG below its maximum allowable discharge concentration from ClearBal in marine water. 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. [ABSTRACT FROM AUTHOR]