Anomalous Dielectrophoresis of Nanoparticles: A Rapid and Sensitive Characterization by Single-Particle Laser Spectroscopy

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Abstract

Although much experimentation has focused on the dielectrophoretic behavior of submicrometer colloids, few studies have provided advanced understanding of the ac electric polarization mechanism for nanoparticles, largely due to the paucity of sensitive and rapid experimental techniques. By using ultrafast fluorescence correlation spectroscopy (FCS) at a single-particle level, we have examined the dielectrophersis (DEP) behavior of nanoparticles of varied radii, a = 10−50 nm, in aqueous media under varied ac electric fields in situ, where direct and real-time microscopic methods become inapplicable for nanoparticles of a < 50 nm. The characteristic dipole relaxation frequency (the DEP crossover frequency, ωc) is determined from the ac-frequency-dependent nanocolloidal concentration profile by FCS. In stark contrast to a single ωc predicted from classical DEP theory, we have observed two distinct ωc's with a strong dependence on particle size and medium conductivity. The anomalous lower ωc inversely scales with the screening length and particle size, suggesting the dynamic double-layer charging effect on the interfacial polarization of nanocolloids whose size is comparable to the double-layer thickness.

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