Caged-Fluorescence Microscopy for Electrokinetics
The design of complex microfluidic bioanalytical systems that perform biological analyses on a chip often requires a detailed understanding of their fluid flow fields. An important class of systems are those that use electrokinetic effects to handle sub-picoliter sample volumes, achieve high-speed separations, and perform a number of assays in series or parallel. Caged-fluorescence microscopy is a technique that can provide in-situ visualizations of scalar fields in electrokinetic systems.
Stanford Microfluidics Lab has designed and built an advanced caged-fluorescence microscopy system to image flow fields of microfabricated electrokinetic systems. This system takes advantage of a custom-built Nd:YAG laser (New Wave Research), an Olympus epifluorescent microscope, a 1030 x 1300 pixel cooled interline-transfer Princeton Instruments CCD camera, and the blue (488 nm) line of a Lexel argon ion laser to produce images of the (scalar) concentration fields of electrokinetic systems with sub-millisecond time resolution.
Figure 1. These images show a fundamental difference in the dynamics of sample dispersion between electroosmotically-driven and pressure-driven flows. This visualization was performed using a molecular tagging technique (caged fluorescence visualization described later on in the chapter) and shows the reduced sample dispersion for (a) electroosmotic flow (in a capillary with a rectangular cross section 200 micron wide and 9 micron deep) as compared to (b) pressure-driven flow (rectangular cross-section 250 micron wide and 70 micron deep).
Figure 2. This is a sequence of images showing an uncaged band entering a sudden expansion in a microfluidic system. Flow is from left to right into the larger channel which has a width of 1000 micron and is 9 micron deep. This electrokinetic flow field is approximately two dimensional, with an applied electric field of 200 V/cm (within the small channel). The channel outline has been highlighted for clarity.
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Molho, J.I., A.E. Herr, B.P. Mosier, J.G. Santiago, T.W. Kenny, R.A. Brennen, G.B. Gordon, B. Mohammadi, “Optimization of Turn Geometries for On-Chip Electrophoresis,” Analytical Chemistry, Vol. 73, No. 6, pp. 1350-1360, 2001.