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Electrokinetic Transport of Charged Analytes through Nanofluidic Channels

Debashis Dutta, University of Wyoming, Department of Chemistry, Dept 3838, 1000 E. University Avenue, Laramie, WY 82071

Nanofluidics is an area of immense interest to the scientific community due to its potential to analyze chemical/biochemical species at very small volumes. However, as the dimensions of the fluidic ducts are reduced to the sub-micrometer/nanometer scale (nanofluidics), several surface effects, which may be ignored in microfluidic devices, become significant. For example, the concentration profile of charged analytes across the lateral dimension of a nanochannel can be significantly different from that in a micrometer sized conduit due to the stronger interaction of the solute species with the Debye layer around the channel walls. In the case of a glass device, the electric field induced by the static negative charges on the channel surface tends to concentrate positively charged analytes near the walls while repelling negatively charged species towards the channel center. When such a concentration distribution is combined with non-uniform fluid flow e.g., electroosmotic flow under Debye layer overlap conditions, the transport rate of solute samples in the system is significantly altered from that observed in a microchannel. In general, positively charged species are slowed down and negatively charged analytes are speeded-up relative to the average fluid velocity when flown through glass nanochannels under strong Debye layer overlap conditions [1].

In this work, we present a mathematical description of this transport process based on the Gouy-Chapman picture for the Debye layer around the channel walls. The electric potential induced by the charges at the channel surface has been determined here by solving the non-linear Poisson equation. The diffusion-convection equation has been then simplified using the Method of Moments technique to derive expressions for the average velocity and the Taylor-Aris dispersivity of the analyte bands as they travel through a nanofabricated duct. Our study shows that the effect of the lateral electric field on the solutal motion is critically determined by the ratio of an analyte's electrophoretic to diffusive transport rate across the lateral dimension of the nanochannel. The model further predicts that these effects can be exploited to enhance electrophoretic separations over that realized in a micrometer-sized conduit by tuning the extent of the Debye layer overlap in the system. This occurs as the axial transport rates in these devices exhibits an exponential dependence on the electrophoretic mobility of the analyte molecule rather than a linear dependence as observed in the limit of thin Debye layers. Such sharper variations in the transit time with the solute's electrophoretic mobility can allow the realization of difficult separations in shorter channels permitting further miniaturization of chip based analysis systems.

Reference:

[1] Pennathur, S; Santiago, J.G. Anal. Chem. 2005, 77(21), 6782-6789