The need to make better and more reproducible SERS measurement with metal nanoparticles is apparent and NPL is tackling the issues behind this.

The use of metal nanoparticles in SERS is still widespread, because they are cheap and easy to synthesise. Another advantage of their use is that they can be dispersed easily, even inside macrostructures, such as cells and tissue samples. The need to make better and more reproducible SERS measurement with metal nanoparticles is apparent and NPL is tackling issues behind this. Current research in this area at NPL not only tries to understand more about the mechanism behind SERS using metal nanoparticles, but also aims to improve protocols for SERS analysis, in particular the aggregation step prior to spectral acquisition.  We have used a variety of chemometric techniques to help us better understand the effect of a number of complex variables on the performance of these substrates.

The most commonly used type of SERS nanoparticle substrate is made from gold or silver particles in the 10 – 100 nm size range. A SERS analysis involves the mixing of such nanoparticles along with the analyte of interest and a suitable aggregating agent, such as potassium chloride (KCl). The final choice and concentration of aggregating agent is very much dependent on the nature of the analyte of interest, and this has previously been an area of active research for NPL. In the case of Rhodamine 6G analyte, we have determined that a concentration of approximately 20 mM KCl is needed for optimum SERS detection, as shown in the Figure below.

Average Raman intensity at 1650 cm^-1 of an aqueous solution of 0.25 μM rhodamine-6G and citrate-reduced silver colloids, against variations in added KCl concentration (●).  For comparison, the second axis plots the change in the wavelength of maximum absorption in the UV-Vis spectra (∆).