Towards Reliable and Quantitative Surface-Enhanced Raman Scattering (SERS): From Key Parameters to Good Analytical Practice

Abstract

Experimental results obtained in different laboratories world-wide by researchers using surface-enhanced Raman scattering (SERS) can differ significantly. We, an international team of scientists with long-standing expertise in SERS, address this issue from our perspective by presenting considerations on reliable and quantitative SERS. The central idea of this joint effort is to highlight key parameters and pitfalls that are often encountered in the literature. To that end, we provide here a series of recommendations on: a) the characterization of solid and colloidal SERS substrates by correlative electron and optical microscopy and spectroscopy, b) on the determination of the SERS enhancement factor (EF), including suitable Raman reporter/probe molecules, and finally on c) good analytical practice. We hope that both newcomers and specialists will benefit from these recommendations to increase the inter-laboratory comparability of experimental SERS results and further establish SERS as an analytical tool.

Keywords: Raman spectroscopy; SERS; enhancement factor; quantitative analysis.

Figure 1

Key parameters in SERS experiments. Energy level diagrams for the plasmon resonance as well as the for molecular electronic resonances of the adsorbed and free molecule. In surface‐enhanced resonance Raman scattering (SERRS) λ_exc.1 matches both the plasmon and the molecular (S_o→S₁) electronic resonance.

Figure 2

Major categories of SERS substrates for Raman signal enhancement.

Figure 3

Calculated enhancement factor (EF) distribution around the hot spot in a dimer of silver spheres (radius 30 nm, gap 2 nm). The excitation wavelength is taken at the dipolar localized surface plasmon resonance, which provides the maximum SERS EF. The logarithmic false color maps show graphically the surface SERS EF distribution in the |E|⁴ approximation. The plot on the left shows the SERS EF in the plane of incidence as a function of arc length L along the surface together with the maximum SERS EF (F _max), the surface‐averaged SERS EF (⟨F⟩) and the relative area a₈₀ from which 80 % of the total SERS signal originates. Figure adapted from Ref. 21.