(252e) Frequency-Selective Polarization of the Electrical Double-Layer Around Nano-Colloids

The direct observation of dielectrophoretic behavior of nanoparticles (sub-100 nm) due to Maxwell-Wagner interfacial polarization is limited by the dissipative action of electrothermal flow due to Joule heating, especially within media of moderate to high conductivity [1]. However, for nano-colloids of sizes comparable to their double layer thickness, the induced electrical dipoles are stronger than that predicted by Maxwell-Wagner theory due to significant ion migration in the normal direction to the applied field as a result of the substantial extent of the double-layer [2]. Prior work has demonstrated that the resulting ion adsorption onto the Stern region of the double-layer enhances its polarization and leads to a wider frequency spread for positive dielectrophoresis than observed for larger particles with thinner double-layers [3]. Herein, we study the influence of this ion migration normal to the field on the polarization behavior of the diffuse region of the double-layer around silica nano-colloids. Through utilizing successively smaller nano-colloids, we demonstrate that the net dipole can be reversed for particles below a critical size to cause negative dielectrophoresis, presumably due to the polarization of the diffuse region of the double-layer. Furthermore, we demonstrate that the frequency window for negative dielectrophoresis behavior can be enhanced through polarization of the nano-colloid within media of successively higher ionic strengths, to enhance ion migration. Since the conformation of biomolecules strongly determines the surface conductance and polarization behavior of the diffuse region of the double-layer [4], we envision that this phenomenon can be applied towards the frequency-selective electrokinetic separation of biomolecules, such as DNA aptamers, based on their molecular conformation state.

[1] V. Chaurey, A. Rohani, Y.-H. Su, W. Varhue, K.T. Liao, C. F. Chou, N. S. Swami, “Scaling down constriction-based dielectrophoresis for trapping nanoscale biomolecules in high conductivity media”,  Electrophoresis (2013), 34, 1097-1104.

[2] S. Basuray and H. -C. Chang, "Induced dipoles and dielectrophoresis of nanocolloids in electrolytes," Phys. Rev. E, vol. 75, p. 060501 (R), 2007.

[3] I. Ermolina and H. Morgan, "The electrokinetic properties of latex particles: comparison of electrophoresis and dielectrophoresis," J of Colloid and Interfacial Science, vol. 285, pp. 419-428, 2005.

[4] M. Hughes and H. Morgan, "Dielectrophoretic characterization and separation of antibody-coated submicrometer latex spheres," Anal. Chem., vol. 71, no. 16, pp. 3441-3445, 1999.