(Almost) Stationary Isotachophoretic Concentration Boundary in a Nanofluidic Channel Using Charge Inversion

The present work is an experimental study of a new means to induce a quasi-stationary boundary for concentration or separation in a nanochannel induced by charge inversion. Instead of using pressure-driven counter-flow to keep the front stationary, we exploit charge inversion by a highly charged electrolyte, Ru(bpy)3Cl2, that changes the sign of the zeta potential in part of the channel from negative to positive. Having a non-charge inverting electrolyte (MgCl2) in the other part of the channel and applying an electric field can create a standing front at the interface between them without added dispersion due to an externally applied pressure-driven counterflow. The resulting slow moving front position can be easily imaged optically since Ru(bpy)3Cl2 is fluorescent. A simple analytical model for the velocity field and front axial position that reproduces the experimental location of the front shows that the location can be tuned by changing the concentration of the electrolytes (and thus local zeta potential). Both of these give the charge inversion-mediated boundary significant advantages over current methods of concentration and separation and the method is, therefore, of particular importance to chemical and biochemical analysis systems such as chromatography and separations and for enhancing the stacking performance of field amplified sample injection and isotachophoresis. By choosing a non-charge inverting electrolyte other than MgCl2, either this electrolyte or the Ru(bpy)3Cl2 solution can be made to be the leading or trailing electrolyte.