Using electrophoretic exclusion to manipulate small molecules and particles on a microdevice

Abstract

Electrophoretic exclusion, a novel separations technique that differentiates species in bulk solution using the opposing forces of electrophoretic velocity and hydrodynamic flow, has been adapted to a microscale device. Proof-of-principle experiments indicate the device was able to exclude small particles (1 μm polystyrene microspheres) and fluorescent dye molecules (rhodamine 123) from the entrance of a channel. Additionally, differentiation of the rhodamine 123 and polystyrene spheres was demonstrated. The current studies focus on the direct observation of the electrophoretic exclusion behavior on a microchip.

Development of the resolution theory for electrophoretic exclusion

“…Aside from traditional CE studies, there are several examples in the literature where species have been differentiated at an appropriate interface that allows direct comparison to these predictions, including GEMBE and previous electrophoretic exclusion studies . To compare these studies, the variance of the concentration gradient at the entrance must be determined; however, it is difficult to quantitatively assess these data.…”

“…These previous studies were indirect and the dynamic strategies used are not especially helpful in clarifying exactly what resolution is possible with this overall strategy. Fortunately, direct observation of the local interface on a planar microfluidic chip is available and indicates very sharp concentration gradients and the flow and electric field forces of the interface have been experimentally quantified . The concentration gradients are shown to be less than 100 μ m wide for small molecules (fluorescent dye).…”

“…The success of electrophoretic exclusion has been demonstrated experimentally using both mesoscale and microscale devices. The technique has proven to be applicable to a variety of analytes with various properties and sizes, including small molecules, polystyrene microspheres, and proteins.…”

Simulation and visualization of velocity fields in simple electrokinetic devices

“…While this thought experiment itself is unimportant, it certainly foretells of more recent work exploiting a stagnation region near a capillary or channel entrance which exceeds diffusion/dispersion. 5,10,11,27,28 Within the chosen system, the velocities vary dramatically and are a good test of the simulation and visualization. Four points are noted on the 2D velocity map (Figure 4), where points B and D are noted to contrast the predominance of either horizontal or vertical components while A and C differ greatly in magnitude.…”

“…In the example shown ( Figure 5), the device area covered by the streamlines and the final profile during a steady flow (parabolic flow profile with higher velocities near the center) was well behaved. In the design of an electrophoresis experimental setup, and specifically for the ones where capture of some particular species is envisioned, 5,10,11,27,28 it is important to know within-the-fluid connection paths between points within the device. While spatial velocity components might exist for the complete region-of-interest (ROI) (and apparent from the velocity vector fields) this does not inherently indicate the possibility of movement (transfer) of particles from one point to another within the ROI.…”