The Application of Fluorescence Lifetime Imaging Microscopy to Quantitatively Map Mixing and Temperature in Microfluidic Systems

The technique of Fluorescence Lifetime Imaging Microscopy (FLIM) has been employed to quantitatively and spatially map the fluid composition and temperature within microfluidic systems. A molecular probe with a solvent-sensitive fluorescence lifetime has been exploited to investigate and map the diffusional mixing of fluid streams under laminar flow conditions within a microfluidic device. Using FLIM, the fluid composition is mapped with high quantification and spatial resolution to assess the extent of mixing. This technique was extended to quantitatively evaluate the mixing efficiency of a range of commercial microfluidic mixers employing various mixing strategies, including the use of obstacles fabricated within the channels. A fluorescently labelled polymer has been investigated as a new probe for mapping temperature within microfluidic devices using FLIM. Time Correlated Single Photon Counting (TCSPC) measurements showed that the average fluorescence lifetime displayed by an aqueous solution of the polymer depended strongly on temperature, increasing from 3 ns to 13.5 ns between 23 and 38 oC. This effect was exploited using FLIM to provide high spatial resolution temperature mapping with sub-degree temperature resolution within microfluidic devices. A temperature-sensitive, water-soluble derivative of the rhodamine B fluorophore, effective over a wide dynamic temperature range (25 to 91 oC) has been used to map the temperature distribution during the mixing of fluid streams of different temperatures within a microchannel. In addition, this probe was employed to quantify the fluid temperature in a prototype microfluidic system for DNA amplification. FLIM has been demonstrated to provide a superior approach to the imaging within microfluidic systems over other commonly used techniques, such as fluorescence intensity and colourimetric imaging