Electrokinetic Velocity Characterization of Microparticles in Glass Microchannels

Insulator-based dielectrophoresis (iDEP) is an efficient technique with great potential for miniaturization. It has been applied successfully for the manipulation and concentration of a wide array of particles, including bioparticles such as macromolecules and microorganisms. When iDEP is applied employing DC electric fields, other electrokinetic transport mechanisms are present: electrophoresis and electroosmotic flow. In order to achieve dielectrophoretic trapping of bioparticles, dielectrophoresis has to overcome electrokinetics (electroosmosis and electrophoresis). Therefore, to improve and optimize iDEP-based separations, it is necessary to characterize these electrokinetic mechanisms under the operating conditions employed for dielectrophoretic separations. The main objective of this work was to identify the operating conditions that will benefit dielectrophoretic trapping and concentration of particles when electrokinetics is present. This study presents the estimation of the electrokinetic mobility of microparticles suspended inside a microchannel. Micro Particle Image Velocimetry (μPIV) was employed to measure the velocity of 1-μm-diameter inert polystyrene particles suspended in a 3-cm-long, 10-μm-deep, 1-mm-wide, straight glass microchannel. A parametric study was carried out by varying the properties of the suspending medium (conductivity and pH) as well as the magnitude of the applied DC electric field. The results obtained using μPIV allowed to identify the conditions under which the electrokinetic force (i.e. particle velocity) is lowest, i.e., optimal conditions for dielectrophoretic trapping. It was shown that high conductivity and low pH values for the suspending medium produce lower electrokinetic mobilities, i.e., low electrokinetic force, thus benefiting dielectrophoretic trapping. These findings were proved by carrying out dielectrophoretic trapping of microparticles employing a glass microchannel that contained cylindrical insulating structures. The results obtained in this study will provide with guidelines for the optimization of iDEP-based separations. Introduction Dielectrophoresis (DEP) is an electrokinetic transport mechanism in which a force is exerted on a particle when it is subjected to a non-uniform electric field. DEP has been used successfully for the manipulation and concentration of a wide array for bioparticles, from macromolecules to parasites (Ozuna-Chacon et al., 2008). Majority of these studies have used electrode-based DEP, where nonuniform electric field is generated employing an array of electrodes and AC fields (Markx et al., 1994; Washizu, 1995; Rousselet et al.,1998). However, there are some drawbacks with this approach: high cost of electrode construction, complex fabrication processes and decrease of functionality due to fouling effects. The use of insulators rather than electrode arrays to produce non-uniform electric field has some advantages: they retain their functionality despite surface fouling; in addition insulators can be made from a variety of materials, including plastics, which facilitates the fabrication of equipment to handle higher flow rates. Additionally, a DC field can be used for solution and particle flow through the device by electroosmotic flow, which eliminates the need of a micro-pump (Lapizco-Encinas et al., 2004a). There is growing number of studies that employ insulator-based DEP (iDEP). In iDEP the nonuniform electric field is obtained by employing arrays of insulating structures and only two electrodes. The electric field is applied along an array of micro-insulating structures creating zones of higher and lower electric field intensity throughout the array, creating dielectrophoretic traps of several biological particles like DNA (Chou et al. 2002), protein (Lapizco-Encinas et al., 2008), yeast cells (Zhou et al., 2002; Suehiro et al. ,2003) and microorganisms (LapizcoEncinas et al.,2004a; Lapizco-Encinas et al., 2004b;). When iDEP is applied through DC voltage other transport mechanisms must be considered: electrophoresis and electroosmotic flow. Electrokinesis is the superposition of electrophoresis and electroosmosis. In order to achieve dielectrophoretic trapping and concentration of particles, DEP must overcome electrokinesis and pressure-driven flow, if present. Suspending medium properties have a strong influence on the magnitude of electrokinetic force (Kirby and Hasselbrink, 2004). In this work an examination of the optimal conditions of the suspending medium (pH and conductivity) for the dielectrophoretic trapping is made through by measuring the electrokinetic mobility of microparticles suspended inside a microchannel. Microparticle image velocimetry (μPIV) was used to determinate the velocity of 1-μm-diameter polystyrene carboxylated microspheres suspended inside in a 3-cm-long, 1-mm-wide and 10-μm-deep microchannel made from glass. From this measurement was possible to determine the operating conditions at which electrokinetic force is the lowest, thus, benefiting dielectrophoretic trapping. These results were confirmed by carrying out dielectrophoretic trapping of the same microparticles employing a glass microchannel containing an array of cylindrical insulating posts. Theory Electrokinetic phenomena are related to the formation of an electrical double layer (EDL) as result of the interaction of an aqueous solution with the static charges in dielectric surfaces. There is a conventionally introduced slipping plane that separates mobile fluid from fluid that remains attached to the surface. Electric potential at this plane is called electrokinetic potential or ζ zeta potential. Usually this potential is used to estimate the degree of charge of the EDL and it is related to the thickness of the Debye Length, Equation 1 shows the relation between the zeta potential and the electroosmotic mobility (Kuhn and Hoffstetter-Kuhn, 1993).