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The ready market availability of porous membranes with cylindrical pores of 15 - 200 nm and a thickness of 6 - 10 'im facilitates the development of three dimensional analytical unit operation devices on an atta Liter 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 nano capillary 1. 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 ('i TAS), 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 (dimethyl siloxane) (PDMS), a common polymer with non-polar side groups. PDMS is durable, highly flexible and elastic, oxygen permeable and very hydrophobic 2. 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 nanopore's 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 system 3. 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 Debt length (^e- 1) of the ionic interactions in solution (1 nm < ^e- 1 < 50 nm) when the ionic strength is in the millimmolar range 1. The small physical character of the nano pore allows for a change in ionic strength of the solution to be sufficient to alter the interaction between the solution and the nano pore. By merely changing the concentration, the nature of the flow induced by electrical potential can be switched between electrophoresis and electro osmosis 1. The direction of the flow can be controlled by the size of the nano pore.
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 system 1. 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 potential 3. As a result of this, the charge density of the nanopore's overpower that of the micro channels when the size of the nanopore's falls below 100 nm, and the bias is reversed as shown in Figure 1. Finally, the size of the nanopore's 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 pore 3. The analytes used to observe these results were fluorescein disodium salt and various fluorescein isothiocyanate-labeled dextran's that ranged in molecular weight from 4 kDa to 2 MDa 1. These analytes, both ionic and non-polar, can be clearly observed with single spot laser-induced fluorescence and fluorescent microscopic imaging 1. This enabled not only the accurate magnitude of effluence through the membrane, but also the speed of response to changing conditions and the direction of the bias. The 488 nm light of the Ar+ laser affords more precision in sensing than is needed for the scale of the plots such as are shown in Figure 1.
The method of analysis by counting the photons released by a controlled fluorescent analyte is reliable and persuasive. Dr. Bohn is optimistic about this development in nano / microfluidic's , and sees future study of the unique and powerful properties of the nanoscale membrane as a pursuit of single molecule / single pore measurements, molecular recognition much more specific than that of molecular weight, and sequential operations within a multidimensional device. These possibilities require a degree of precision in fabrication that has not been achieved so far.
The micro channels are made of a highly hydrophobic polymer, and require an opening in order to interface with another via a nano porous membrane. These holes must then be lined up perfectly in order to minimize the leakage that has not yet been eliminated 1. This limits the number of devices that can be etched per unit area. What distance between these hybrid devices is required to eliminate the effects of this leakage? What percentage of failure rate will these devices have? Will the ratio of possible devices per cm 2 to failure per cm 2 be maximized to the point of feasibility?
Figures and References Figure 1. Fluorescence intensity (solid line) and applied bias "AV (dashed line) as a function of time in the receiving channel showing transport of 0. 17 'iM fluorescein in in 5 mM pH 8 Phosphate Buffer across PCTE membranes with the following pore diameters: (a) 15 nm, (b) 30 nm, (c) 100 nm, (d) 200 nm 1. 1. Kuo, T-C. ; Cannon Jr. , D. M. ; Shannon, M. A. ; Bohn, P. W. ; Sweedler J.
V. Sens. Actual. A 2003, 102 / 3, 223 - 233. 2. Allcock, H.
R. ; Lampe, F. W. ; Mark, J. E. ; Contemporary Polymer Chemistry 3 rd ed. ; Pearson Education International: Upper Saddle River, NJ, 2003. 3. Kuo, T-C. ; Cannon Jr. , D. M. ; Chen, Y. ; Tulock, J. J. ; Shannon, M.
A. ; Sweedler J. V. ; Bohn, P. W. Analyst. Chem. 2003, 75, 18
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