Chemical reaction imaging within microfluidic devices using confocal Raman spectroscopy: The case of water and deuterium oxide as a model system

Microfluidic devices face presently a tremendous interest, especially for the development of labs-on-a-chip systems. One of the primary challenges for such applications is the ability to perform local chemical detection and analysis from various species. In this paper, we investigate the use of confocal Raman spectroscopy from both qualitative and quantitative sides, to obtain spatially resolved concentration maps of chemically reactive fluids flowing in different channels networks. As a model chemical reaction, we used the isotopic exchange reaction between D2O and H2O, which is diffusion-controlled and whose equilibrium states exhibit distinct Raman signatures depending on the composition. Two types of chip technologies were studied, which are typical of those used for chemical kinetics investigations. In the first one, reagent mixing occurs by molecular interdiffusion of the two streams (H2O and D2O) flowing side by side in the same channel; in the second one, reagents are hosted in droplets moving in winding channels that enhance the mixing. In the first series of experiments, we were able to extract Raman images of H2O, D2O, and HOD concentrations in the main channel together with an estimate of an interdiffusion coefficient, and in the second one, we evidenced the influence of channel wiggles on mixing efficiency.