On-line laser Raman spectroscopic probing of droplets engineered in microfluidic devices

Sub-nanoliter droplets engineered in microfluidic devices constitute ideal microreactors to
investigate the kinetics of chemical reactions on the millisecond time scale. Up to date, fluorescence
detection has been extensively used in chemistry and biology to probe reactants and resultant products
within such nanodroplets. However, although fluorescence is a very sensitive technique, it lacks
intrinsic specificity as frequently fluorescent labels need to be attached to the species of interest. This
weakness can be overcome by using vibrational spectroscopy analysis. As an illustrative example, we
use confocal Raman microspectroscopy in order to probe the concentration profiles of two
interdiffusing solutes within nanoliter droplets transported through a straight microchannel. We
establish the feasibility of the experimental method and discuss various aspects related to the spacetime
resolution and the quantitativeness of the Raman measurements. Finally, we demonstrate that the
droplet internal molecular mixing is strongly affected by the droplet internal flow.

Publié dans Lab on a Chip