High-Resolution Charge-Based Electrokinetic Separation of Almost Identical Microparticles

Abstract

Well-established techniques, e.g., chromatography and capillary electrophoresis, are available for separating nanosized particles, such as proteins. However, similar techniques for separating micron-sized particles are still needed. Insulator-based electrokinetic (iEK) systems can achieve efficient microparticle separations by combining linear and nonlinear EK phenomena. Of particular interest are charge-based separations, which could be employed for separating similar microorganisms, such as bacterial cells of the same size, same genus, or same strain. Several groups have reported charge-based separations of microparticles where a zeta potential difference of at least 40 mV between the microparticles was required. The present work pushes the limit of the discriminatory capabilities of iEK systems by reporting the charged-based separation of two microparticles of the same size (5.1 μm), same shape, same substrate material, and with a small difference in particle zeta potentials of only 3.6 mV, which is less than 10% of the difference in previous studies. By building an accurate COMSOL Multiphysics model, which correctly accounts for dielectrophoresis and electrophoresis of the second kind, it was possible to identify the conditions to achieve this challenging separation. Furthermore, the COMSOL model allowed predicting particle retention times (t R,p) which were compared with experimental values (t R,e). The separations results had excellent reproducibility in terms of t R,e with variations of only 9% and 11% between repetitions. These findings demonstrate that, by following a robust protocol that involves modeling and experimental work, it is possible to discriminate between highly similar particles, with much smaller differences in electrical charge than previously reported.

Separation of Cells and Microparticles in Insulator-Based Electrokinetic Systems

“…14,38 The accuracy of this model has enabled the design of challenging separations, our group recently reported the separation of two types of almost identical microparticles. 38 The results of separation 1, the mixture of the 2 μm red and 5.1 μm green particles, are shown in Figure 1B,C. The image in Figure 1B shows the particles as they migrate across the asymmetric insulating post array, forming "zones," with the 2 μm red particles moving ahead of the green 5.1 μm particles at ΔV = 1500 V between reservoirs B and D. Figure 1C presents the electropherogram of this separation, which was built from the fluorescence signal from the particles as they eluted the post array.…”

“…After analyzing the COMSOL predictions, the voltages listed in Table for separation 1 were selected, as according to the model, these voltages should produce well-resolved peaks ( R s > 1.5). It is important to mention that the COMSOL model employed here considers the effects of nonlinear EP, a phenomenon that had been ignored in the modeling of many iEK systems stimulated with DC and low-frequency AC potentials. , The accuracy of this model has enabled the design of challenging separations, our group recently reported the separation of two types of almost identical microparticles . The results of separation 1, the mixture of the 2 μm red and 5.1 μm green particles, are shown in Figure B,C.…”

“…24,34−37 A recent study illustrated that carefully selected operating conditions and accurate mathematical modeling allow for the successful separation of almost identical microparticles with a slight difference in electrical charge of only 3.6 mV. 38 Several studies by this group have targeted the separation and trapping of mixtures of cells. In 2010, they reported the simultaneous separation and concentration of a mixture of yeast and E. coli cells in an iEK system in less than 2 min.…”

“…The same method was used to discriminate between the strains of pathogenic Streptococcus mitis and Pseudomonas aeruginosa . The Lapizco-Encinas research group has published several studies on the EK separations of microparticles and cells in an array of iEK systems. ,− A recent study illustrated that carefully selected operating conditions and accurate mathematical modeling allow for the successful separation of almost identical microparticles with a slight difference in electrical charge of only 3.6 mV . Several studies by this group have targeted the separation and trapping of mixtures of cells.…”

Dielectrophoresis: An Approach to Increase Sensitivity, Reduce Response Time and to Suppress Nonspecific Binding in Biosensors?

“…It has been widely accepted that the DEP force is responsible for trapping the particles regardless of if an DC voltage or an AC voltage is applied. Recent work suggests, however, that electrophoresis and electroosmosis are the most important forces present when working in DC mode, and it is better referred to as DC insulator-based electrokinetic [ 40 , 41 , 42 , 43 ]. Readers should have this in mind when works on DC iDEP are cited in this review.…”