Caged-Fluorescence Microscopy for Electrokinetics
Motivation
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.
Project Description
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.
(a)
(b)
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.
References
Herr, A.H., J.I. Molho, J.G. Santiago, M.G. Mungal, T.W. Kenny, and M.G. Garguilo, "Electroosmotic Capillary Flow with Non-Uniform Z-Potential", Analytical Chemistry, Vol. 72, No. 5, pp. 1053-1057, 2001.
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.