Temperature control is key to microfluidic-based Lab-on-a-Chip devices for a variety of applications such as polymerase chain reaction for DNA amplification and isoelectric focusing for protein separation where pH gradients are thermally generated. The most widely used temperature measurement method involves the mixing of the buffer solution with a fluorescent dye, which has a temperature-dependent fluorescent intensity. The temperature distribution in the liquid can be obtained by monitoring the fluorescent intensity distribution in the channel. However, this method can not be easily applied to polymer-made microfluidic chips because of dye absorption and penetration into polymer chips, electrophoresis of dye which causes artificial temperature gradients, and inevitable photobleaching of fluorescent dye. Therefore, a novel method is developed and presented here for temperature measurement by utilizing photobleaching of fluorescent dye. This method includes two novel contributions: i) a specially developed model for converting temperature-dependent photobleaching speed distribution to temperature distribution, and ii) an introduction of a thin polydimethylsiloxane (PDMS) layer with saturated Rhodamine B for solving the above-mentioned dye diffusion and electrophoresis problems. In this new method, a thin PDMS layer saturated with Rhodamine B is bonded with another PDMS layer with microchannels instead of mixing the dye with the buffer solution. Therefore, the problems associated with dye diffusion into PDMS chips and electrophoresis when an electrical field is applied to channels are avoided. The developed theory is validated by comparing the experimentally measured temperature distribution with numerical predicted results. The theory and its validation will be presented and discussed.

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