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Microfluidic Systems

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Microfluidic Systems

The ready market availability of porous membranes with cylindrical pores of 15-200 nm and a thickness of 6-10 мm facilitates the development of three dimensional analytical unit operation devices on an attaLiter scale. By employing these membranes as gates at the interface of two crossed microfluidic channels, the rate and direction of the fluid exchange can be controlled with electrical potential, polarity, solution ionic strength or diameter of the nanocapillary1. The microfluidic channels, fabricated by soft lithography, have been used for a decade. Dr. Paul W. Bohn, Centennial Professor of Chemical Sciences at the University of Illinois at Urbana-Champaign, sees the advance to multilayered liquid chromatography as a key step in the development of micro total analysis systems (мTAS), which would involve such new applications as injection, collection, mixing, switching and detection. Recently he has been studying the analyte responses to various constraints applied to the system and its deviations in behavior from that of a similar system on the macro scale.

Microfluidic channels are a convenient and durable means of fluid transport made of poly(dimethylsiloxane) (PDMS), a common polymer with non-polar side groups. PDMS is durable, highly flexible and elastic, oxygen permeable and very hydrophobic2. It also has negative surface charge density at pH 81. The method of soft lithography allows for rapid deposition of complex crossed two dimensional fluid pathways on a silicon wafer.

The membrane containing these nanopores is a 6 - 10 micron thick polycarbonate nuclear track-etched membrane (PCTE) that has been coated with poly(vinylpyrrolidone) (PVP) to make it hydrophilic. This coating results in a pH of 8 in the system3. The pores in the membrane are cylindrical and of diameters in the range of 15 - 200 nm. The size of these pores are of the same order of magnitude of the Debye length (к-1) of the ionic interactions in solution (1 nm < к-1 < 50 nm) when the ionic strength is in the millimmolar range1.

The small physical character of the nanopore allows for a change in ionic strength of the solution to be sufficient to alter the interaction between the solution and the nanopore. By merely changing the concentration, the nature of the flow induced by electrical potential can be switched between electrophoresis and electro osmosis1.

The direction of the flow can be controlled by the size of the nanopore. At large pore sizes, the negative surface charge density on the microfluidic channel caused by the slightly basic pH of the system

causes it to dictate a forward direction of the electrokinetic flow when voltage is applied to the system1. However, increasing surface-to-volume ratios increase the fraction of charge that is trapped on the walls of the tube, increasing the magnitude of the potential3. As a result of this, the charge density of the nanopores overpower that of the micro channels when the size of the nanopores falls below 100 nm, and the bias is reversed as shown in Figure 1.

Finally, the size of the nanopores dictate a maximum size of molecule that is allowable through the membrane. For any pore size, the maximum molecular size can be determined experimentally. These limits range from ~10 kDa for a 15 nm pore to 2 MDa for a 200 nm pore3.

The analytes used to observe these results were fluoroscein disodium salt and various fluoroscein isothiocyanate-labeled dextrans that ranged in molecular weight from 4 kDa to 2MDa1. These analytes, both ionic and non-polar, can be clearly observed with single spot laser-induced



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