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Anal Chem. 2017 Feb 7;89(3):1716-1723. doi: 10.1021/acs.analchem.6b03943. Epub 2017 Jan 9.

Quantitative Chemical Imaging of Nonplanar Microfluidics.

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Beckman Institute for Advanced Science and Technology, ‡Department of Bioengineering, §Department of Mechanical Science and Engineering, and ∥Departments of Electrical & Computer Engineering, Chemical and Biomolecular Engineering and Chemistry, University of Illinois , Urbana, Illinois 61801, United States.


Confocal and multiphoton optical imaging techniques have been powerful tools for evaluating the performance of and monitoring experiments within microfluidic devices, but this application suffers from two pitfalls. The first is that obtaining the necessary imaging contrast often requires the introduction of an optical label which can potentially change the behavior of the system. The emerging analytical technique stimulated Raman scattering (SRS) microscopy promises a solution, as it can rapidly measure 3D concentration maps based on vibrational spectra, label-free; however, when using any optical imaging technique, including SRS, there is an additional problem of optical aberration due to refractive index mismatch between the fluid and the device walls. New approaches such as 3D printing are extending the range of materials from which microfluidic devices can be fabricated; thus, the problem of aberration can be obviated simply by selecting a chip material that matches the refractive index of the desired fluid. To demonstrate complete chemical imaging of a geometrically complex device, we first use sacrificial molding of a freeform 3D printed template to create a round-channel, 3D helical micromixer in a low-refractive-index polymer. We then use SRS to image the mixing of aqueous glucose and salt solutions throughout the entire helix volume. This fabrication approach enables truly nonperturbative 3D chemical imaging with low aberration, and the concentration profiles measured within the device agree closely with numerical simulations.

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