Towards a quantitative SERS approach--online monitoring of analytes in a microfluidic system with isotope-edited internal standards.

In this contribution a new approach for quantitative measurements using surface-enhanced Raman spectroscopy (SERS) is presented. Combining the application of isotope-edited internal standard with the advantages of the liquid-liquid segmented-flow-based approach for flow-through SERS detection seems to be a promising means for quantitative SERS analysis. For the investigations discussed here a newly designed flow cell, tested for ideal mixing efficiency on the basis of grayscale-value measurements, is implemented. Measurements with the heteroaromatics nicotine and pyridine using their respective deuterated isotopomers as internal standards show that the integration of an isotopically labeled internal standard in the used liquid-liquid two-phase segmented flow leads to reproducible and comparable SERS spectra independent from the used colloid. With the implementation of an internal standard into the microfluidic device the influence of the properties of the colloid on the SERS activity can be compensated. Thus, the problem of a poor batch-to-batch reproducibility of the needed nanoparticle solutions is solved. To the best of our knowledge these are the first measurements combining the above mentioned concepts in order to correct for differences in the enhancement behaviour of the respective colloid.