291392 Nano-Bioparticle Separations Based On Frequency Response for Double-Layer Polarization

Tuesday, October 30, 2012
Hall B (Convention Center )
Yi-Hsuan Su, Mikiyas Tsegaye, Walter Varhue and Nathan Swami, Electrical & Computer Engineering, University of Virginia, Charlottesville, VA

Diagnosis of diseases and detection of their pathogenesis requires the quantification of a spectrum of closely related biomarkers. However, these closely related biomarkers are present in extremely small quantities (~ng-pg/mL) and need to be detected in complex biological fluids, such as blood serum, which contain several other proteins at million to billion-fold higher concentration levels (mg/mL). To address this need for separations and selective enrichment of closely-related biomarkers over other proteins in the bio-fluid media, we herein explore frequency-selective methods for polarization of the electrical double-layer around the bio-particles. Dielectrophoresis (DEP) enables highly selective translation of polarized bio-particles based on the characteristic frequency response of the dielectric permittivity of the bio-particle versus that of the medium, and it has been extensively applied towards sorting of somewhat similar sized biological cells with differing dielectric frequency response. However, its application to nanoscale bio-particles, such as ss-DNA, proteins, and nanostructures requires micro- or nano-device geometries to enhance the local field to offset the steep fall in dielectrophoretic trapping forces with particle size. Herein, through utilizing a microfluidic electrode-less DEP device with 1000-fold lateral constrictions to enhance the localized field, we demonstrate that the relaxation time constant for polarization of the electrical double layer around nanoscale bio-particles is highly dependent on its size and charge density. The double layer around nanoscale bio-particles is completely formed at ~kHz frequencies and the ensuing screening of the external electrical field by the double layer causes negative DEP behavior. On the other hand, at 0.1-1 MHz frequencies where the double layer is not fully formed, the bio-particles exhibit positive DEP due to interfacial polarization. We demonstrate that this crossover from negative to positive DEP is highly sensitive to small differences in size and charge density of bio-particles. Bio-particles of smaller size and/or higher charge density exhibit substantially faster relaxation frequencies, thereby extending field screening and negative DEP to higher frequency ranges. This enables more effective bio-particle separations, based on the magnitude as well as direction of the DEP force. We envision the application of this methodology towards separations for biomarker discovery.

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