Imaging of Flow Patterns with Fluorescent Molecular Rotors |
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Authors: | Adnan Mustafic Hsuan-Ming Huang Emmanuel A Theodorakis and Mark A Haidekker |
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Institution: | (1) Faculty of Engineering, Driftmier Engineering Center, University of Georgia, Athens, GA 30602, USA;(2) Department of Biomedical Engineering, Case Western Reserve University, Cleveland, OH 44106, USA;(3) Department of Chemistry and Biochemistry, University of California, San Diego, 9500 Gilman Drive, San Diego, CA 92093, USA; |
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Abstract: | Molecular rotors are a group of fluorescent molecules that form twisted intramolecular charge transfer states (TICT) upon
photoexcitation. Some classes of molecular rotors, among them those that are built on the benzylidene malononitrile motif,
return to the ground state either by nonradiative intramolecular rotation or by fluorescence emission. In low-viscosity solvents,
intramolecular rotation dominates, and the fluorescence quantum yield is low. Higher solvent viscosities reduce the intramolecular
rotation rate, thus increasing the quantum yield. We recently described a different mechanism whereby the fluorescence quantum
yield of the molecular rotor also depends on the shear stress of the solvent. In this study, we examined a possible application
for shear-sensitive molecular rotors for imaging flow patterns in fluidic chambers. Flow chambers with different geometries
were constructed from polycarbonate or acrylic. Solutions of molecular rotors in ethylene glycol were injected into the chamber
under controlled flow rates. LED-induced fluorescence (LIF) images of the flow chambers were taken with a digital camera,
and the intensity difference between flow and no-flow images was visualized and compared to computed fluid dynamics (CFD)
simulations. Intensity differences were detectable with average flow rates as low as 0.1 mm/s, and an exponential association
between flow rate and intensity increase was found. Furthermore, a good qualitative match to computed fluid dynamics simulations
was seen. On the other hand, prolonged exposure to light reduced the emission intensity. With its high sensitivity and high
spatial and temporal resolution, imaging of flow patterns with molecular rotors may become a useful tool in microfluidics,
flow measurement, and control. |
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