Improved Quantitative SERS Enabled by Surface Plasmon Enhanced Elastic Light Scattering

Abstract

The application of surface-enhanced Raman spectroscopy (SERS) for everyday quantitative analysis is hindered by the point-to-point variability of SERS substrates that arises due to the heterogeneous distribution of localized electromagnetic fields across a suite of plasmonic nanostructures. Herein, we adopt surface-enhanced elastic scattering as a SERS internal standard. Both elastic and inelastic (i.e., Raman) scattering are simultaneously enhanced by a given “hot spot”, and thus, the surface-enhanced elastic scattering signal provides a localized intrinsic internal standard that scales across all of the plasmon-enhanced electromagnetic fields within a substrate. Elastically scattered light originates from the amplified spontaneous emission (ASE) of the commercial laser, leading to the formation of a low-wavenumber pseudo band that arises from the interaction of the ASE and the edge filter. A theoretical model was developed to illustrate the underlying mechanism supporting this normalization approach. The normalized Raman signals are independent of the incident laser intensity and the density of “hot spots” for numerous SERS substrates. Following “hot-spot” (HS) normalization, the coefficient of variation for the tested SERS substrates decreases from 10 to 60% to 2%–7%. This approach significantly improves SERS quantitation of four chloroanilines and enables collection of highly reproducible analyte adsorption results under both static and dynamic imaging conditions. Overall, this approach provides a simple means to improve SERS reproducibility without the need to use additional chemicals as internal standards.

Application of SERS on the chemical speciation of individual Aitken mode particles after condensational growth

“…To verify if the peaks are actually enhanced or not by the presence of additional AS, we normalized the peak intensity at 965 cm À1 by that at 40 cm À1 (Figure 5a). Wei et al (2018) reported that the intensity of the surface plasmon enhanced elastic scattering signal (at 40 cm À1 in this case) scales linearly with the integrated "hot spot" signals within a laser excitation volume. Thus, intensity of other peaks can be quantified by using the peak intensity at 40 cm À1 as a normalizing factor (Wei et al 2019).…”

“…Figure 4 shows that all SERS spectra (from LG and AS), as well as NaCl spectra, already had peaks at 40 cm À1 and 965 cm À1 . The low frequency Raman scattering at 40 cm À1 has spontaneously emerged due to the interaction of elastically scattered light and the edge filter (Wei et al 2018). The latter peak at 965 cm À1 is more problematic in this case, since it interferes with the characteristic peak of enhanced sulfate (SO 4 2-) at $970 cm À1 , which is known to be red-shifted in SERS experiments (Gen and Chan 2017;Fu et al 2017;Tirella et al 2018).…”

“…Despite the absence of any interfering background peak at 2950 cm À1 , we also used the peak intensity at 40 cm À1 as a normalization factor to minimize potential point-to-point SERS signal variability. The signal intensity on every spectrum may change depending on the difference on the "hot spot" distribution within a laser excitation volume (Wei et al 2018). We therefore needed to implement such normalization for comparison.…”

Experimental methods in chemical engineering: Raman spectroscopy

“…More recently, a correction has been suggested for SERS based on normalizing against the elastic scattering. [ 70 ]…”

SERS background imaging – A versatile tool towards more reliable SERS analytics

“…Spectral acquisition delivers average spectra of the diffraction-limited areas. The uniformity of the signal enhancement on a SERS substrate is most often checked by pointwise [22][23][24] or linewise 25 Raman mapping or by randomly selecting different measurement spots on the substrate 15,26,27 . However, the long spectral acquisition times hinder the imaging of large areas in high resolution in short time, for example to study the distribution of hot spots.…”