Stationary Chemical Gradients for Concentration Gradient-Based
Separation and Focusing in Nanofluidic Channels
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Abstract
Previous
work has demonstrated the simultaneous concentration and
separation of proteins via a stable ion concentration gradient established
within a nanochannel (Inglis Angew. Chem., Int. Ed. 2001, 50, 7546−7550). To gain a better understanding of how this
novel technique works, we here examine experimentally and numerically
how the underlying electric potential controlled ion concentration
gradients can be formed and controlled. Four nanochannel geometries
are considered. Measured fluorescence profiles, a direct indicator
of ion concentrations within the Tris–fluorescein buffer solution,
closely match depth-averaged fluorescence profiles calculated from
the simulations. The simulations include multiple reacting species
within the fluid bulk and surface wall charge regulation whereby the
deprotonation of silica-bound silanol groups is governed by the local
pH. The three-dimensional system is simulated in two dimensions by
averaging the governing equations across the (varying) nanochannel
width, allowing accurate numerical results to be generated for the
computationally challenging high aspect ratio nanochannel geometries.
An electrokinetic circuit analysis is incorporated to directly relate
the potential drop across the (simulated) nanochannel to that applied
across the experimental chip device (which includes serially connected
microchannels). The merit of the thick double layer, potential-controlled
concentration gradient as a particle focusing and separation tool
is discussed, linking this work to the previously presented protein
trapping experiments. We explain why stable traps are formed when
the flow is in the opposite direction to the concentration gradient,
allowing particle separation near the low concentration end of the
nanochannel. We predict that tapered, rather than straight nanochannels
are better at separating particles of different electrophoretic mobilities