Label-free measurement of reaction rate constants in solution using fluidic dielectrophoresis

Abstract

We use the fluidic dielectrophoresis (fDEP) motion of a polarizable liquid-liquid electrical interface to quantify reaction rate kinetics of binding interactions in solution. Using a T-shaped microfluidic channel we co-flow two reactant streams side-by-side. Their resulting interface enables controlled diffusive mixing and product formation. We electrokinetically engineer the interface such it is electrical in nature so that one liquid stream possesses a higher electrical conductivity and the other a higher permittivity. When the interface is exposed to a perpendicular alternating current (AC) electric field the electrical mismatch drives it to polarize and displace across the microchannel width in a direction and magnitude that is a function of the electrical properties of the interface and of the field frequency applied. At a low frequency below the inverse interfacial charge relaxation timescale, the high conductive stream displaces into the high permittivity stream. At frequencies above this inverse timescale the direction reverses. At an intermediate “crossover” frequency (COF), the polarization of the interface tends to zero and the displacement ceases. Because interfacial reactions influence the electrical properties, binding dynamics can be quantified using the COF. We measure the COF down the axial length of an interface subjected to binding reactions between CaGreen and Ca2+, and between avidin and biotin. The experimental measurements are in good agreement with an electrokinetically coupled two-dimensional simulation of the reaction-diffusion equations. From this work, we show that the motion of an electrical interface in an electrical field can be utilized to measure kinetic reaction rates in solution phase.