Three-layer substrates for surface-enhanced Raman scattering: Preparation and preliminary evaluation

In surface-enhanced Raman scattering (SERS), the magnitude of enhancement is highly dependent on the nanometer-scale morphological and optical properties of the substrate employed. This paper reports the development of an improved SERS-active substrate that is based on a combination of nanoparticle assembly and evaporated thin films. Highly enhancing, three-layer SERS-active substrates were constructed by evaporation of an Ag film over an Ag-clad colloidal Au submonolayer; the distinguishing feature of the three-layer substrate is the formation of evaporated Ag islands between the surface-bound colloidal particles. UV-visible absorption spectroscopy, atomic force microscopy and field emission scanning electron microscopy were used to characterize the substrate morphology. The thickness of the evaporated layer was varied in order to determine the optimal substrate architecture. The estimated enhancement factor for the optimized substrate architecture was 7 × 10⁴. Furthermore, better than 15% reproducibility in the Raman spectra for adsorbed analytes was observed from spot to spot on a given substrate, and also from substrate to substrate. Finally, a sub-picogram detection limit was demonstrated with the environmental pollutant 1,3,5-triazine.