Internal Intensity Standards for Quantitative SERS

Another important issue of quantitative Raman detection is an internal intensity standard introduced into the experiment. The Raman signal of standard can be used to normalize intensity in Raman spectrum of studied analyte. This can eliminate problems connected with experimental factors, such as laser power fluctuations, focusing, drift in the optical alignment and positioning of the sample (Bell and Sirimuthu 2008 Bell and Stewart 2011). 

Recently, Zhou and co-workers introduced a new strategy to quantitatively measure the SERS signals of analytes based on Au-core/Ag-shell NPs with embedded molecule (4-ATP) serving as the intensity standard (Zhou et al. 2015). In this approach, a molecular layer of 4-ATP is sandwiched between the core and shell of two metals. They chose Au nanospheres to obtain a highly uniform core and silver as the shell to achieve the highest possible enhancement. Since the 4-ATP was embedded inside the shell and the analyte was adsorbed on the shell surface, they did not compete for the surface sites. Thus, the 4-ATP signal was not influenced by the detection environment. Successful detection of two analytes, toluidine blue O in aqueous solution (detection limit of 0.1 pM) and melamine in milk (see Sect. 4.3.2) was demonstrated. Shen and co-workers proved this concept using 

In the case of microfluidic devices, the implementation of the SERS-active internal standard can compensate for the influence of the properties of the colloidal NPs on the SERS signal of analyte. Marz and co-workers showed that the 

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