Three-dimensional in situ temperature measurement in microsystems using Brownian motion of nanoparticles

We report an in situ method for three-dimensionally resolved temperature measurement in microsystems. The temperature of the surrounding fluid is correlated from Brownian diffusion of suspended nanoparticles. We use video-microscopy in combination with image analysis software to selectively track nanoparticles in the focal plane. This method is superior with regards to reproducibility and reduced systematic errors since measuring Brownian diffusivity does not rely on fluorescence intensity or lifetime of fluorophores. The efficacy of the method is demonstrated by measuring spatial temperature profiles in various microfluidic devices that generate temperature gradients and by comparing these results with numerical simulations. We show that the method is accurate and can be used to extract spatial temperature variations in three dimensions. Compared to conventional methods that require expensive multiphoton optical sectioning setups, this technique is simple and inexpensive. In addition, we demonstrate the capability of this method as an in situ tool for simultaneously observing live cells under the microscope and monitoring the local temperature of the cell medium without biochemical interference, which is crucial for quantitative studies of cells in microfluidic devices.