Surface-Enhanced Raman Scattering Spectral Imaging for the Attomolar Range Detection of Crystal Violet in Contaminated Water

A series of nanocomposites based on polyamide (NL16, PA) filter membranes containing metal nanoparticles (NPs) have been prepared by filtration under reduced pressure of the metal colloids. The ensuing materials were then investigated as substrates for surface-enhanced Raman scattering (SERS) imaging studies envisaging the spectroscopic detection of vestigial organic pollutants dissolved in contaminated water. The organic dye crystal violet (CV) was used here as a model pollutant because it is a hazardous compound present in certain effluent waters. Moreover this compound is well-known for its strong SERS activity, which is clearly advantageous in the context of material development for SERS. Indeed, several preparative strategies were employed to prepare PA-based composites, and the impact on SERS detection was investigated. These include the use of chemical and morphological distinct plasmonic NPs (Ag, Au), a variable metal load and changing the order of addition of the analytical specimens. These studies demonstrate that the parameters employed in the fabrication of the SERS substrates have a strong impact on the Raman signal enhancement. The use of Raman imaging during the fabrication process allows establishing improvements that translate to better performances of the substrates in the analyte detection. The results have been interpreted by considering an integrated set of operational parameters that include the affinity of CV molecules to the substrate, amount and dispersion of NPs in the PA membranes, and the detection method.  Noteworthy the use of SERS analysis assisted with Raman imaging allowed achieving a detection limit for CV as low as 100 aM in ultrapure water and 10 fM in real samples.