Waveguide confined Raman spectroscopy for microfluidic interrogation.

We report the first implementation of the fiber based microfluidic Raman spectroscopic detection scheme, which can be scaled down to micrometre dimensions, allowing it to be combined with other microfluidic functional devices. This novel Raman spectroscopic detection scheme, which we termed as Waveguide Confined Raman Spectroscopy (WCRS), is achieved through embedding fibers on-chip in a geometry that confines the Raman excitation and collection region which ensures maximum Raman signal collection. This results in a microfluidic chip with completely alignment-free Raman spectroscopic detection scheme, which does not give any background from the substrate of the chip. These features allow a WCRS based microfluidic chip to be fabricated in polydimethylsiloxane (PDMS) which is a relatively cheap material but has inherent Raman signatures in fingerprint region. The effects of length, collection angle, and fiber core size on the collection efficiency and fluorescence background of WCRS were investigated. The ability of the device to predict the concentration was studied using urea as a model analyte. A major advantage of WCRS is its scalability that allows it to be combined with many existing microfluidic functional devices. The applicability of WCRS is demonstrated through two microfluidic applications: reaction monitoring in a microreactor and detection of analyte in a microdroplet based microfluidic system. The WCRS approach may lead to wider use of Raman spectroscopy based detection in microfluidics, and the development of portable, alignment-free microfluidic devices.