Confocal Raman thermometer for microfluidic devices

Abstract

A confocal Raman microscopy technique has been designed and demonstrated that measures the temperature rise and profile in microfluidic devices. The system is based in the deformation of the water Raman peak assigned to the O-H stretching at 3400 cm−1 that occurs with temperature keeping an isosbectic point at 3425 cm−1 . Hence two photon counting detectors that sample the Raman emission at each side of the isosbectic point are used to monitor the water temperature. Using a confocal detection scheme the spatial resolution of a confocal microscope can be obtained to map the temperature profile within small microfluidic structure in a noninvasive manner. The differential signal between the two channels normalized by the added signals has a linear dependence with temperature that yields a sensitivity of 0.8 K using a 1s integration time and a count rate per channel of 1.5 · 105 . The pump laser used had a 405 nm wavelength and 20 mW average power. The confocal collection was performed by a single mode optical fiber and the explored volume was of about 40 μm3 . The temperature rise in electrotrofluidic devices was studied showing that the temperature increase depended on the power used to move the sample along the channel (electroosmotic flow) and the particular design and structure of the device that determines the heat dissipation mechanism. The scheme proved useful to evaluate and prevent detrimental temperature effects with the advantage that no specific temperature sensitive particle needs to be added to the fluid, and has the additional virtue of allowing spatial scans in 3D.