(297d) Coupling Dielectrophoresis At Nano-Constrictions to Concentration Polarization Effects for Enhanced Protein Pre-Concentration

The challenge of sensing rare numbers of biomarkers against a background of high concentration of other matrix proteins within physiologically relevant media requires methodologies for selective pre-concentration of the biomarker in proximity of the sensor. Antibody affinity based chemical depletion methods are unable to achieve the necessary degree of pre-concentration, since the biomarkers are present at billion-fold lower levels than the background proteins in blood [1]. Hence, there is great interest in applying electrokinetic methods, especially within nanofluidic devices, where million-fold levels of pre-concentration have been reported using concentration polarization effects to cause ion exclusion-enrichment due to electrical double layer overlap at the micro-to-nanofluidic interface [2]. However, these methods require several tens of minutes to achieve this pre-concentration and they cannot effectively separate target biomarker proteins against a background of similarly charged and sized proteins in samples of physiological fluids. Dielectrophoresis enables highly selective trapping of bio-particles based on the characteristic frequency response of the dielectric permittivity of the bio-particle versus that of the medium, and can be applied in principle towards selective pre-concentration based on differing dielectric frequency response. However, its application to trapping nanoscale bio-particles, such as ss-DNA and proteins, requires methods to enhance the local field to offset the steep fall in dielectrophoretic trapping forces with particle size [3]. Furthermore, within physiological media of high conductivity, dielectrophoretic trapping is in competition with electrothermal flow due to Joule heating [4]. Herein, we explore the coupling of a DC field offset to negative dielectrophoretic trapping of streptavidin protein biomolecules at nano-constrictions by AC fields (200 V_pp/cm, 1 MHz) to enhance the degree of pre-concentration through an electrokinetic force balance [5]. At sub-100 nm constrictions, the addition of a critical DC field offset to the AC field causes the emergence of concentration polarization effects in addition to dielectrophoresis to enable an exponentially enhanced extent of the protein depletion zone across the device, thereby resulting in steep and ultra-fast protein pre-concentration. Using a potential energy diagram across the device to explain protein trapping in terms of energy wells due to energy barriers caused by the constriction and potential profile tilting caused by the DC field, we develop a design parameter to characterize the effectiveness of constrictions for protein trapping based on depth and sharpness of the energy well. We envision that these protein pre-concentration methodologies may be applied towards biomarker discovery, protein crystallization, and rare target sensing for early disease diagnostics.

[1] Polaskova, V.; Kapur, A.; Khan, A.; Molloy, M. P.; Baker, M. S. Electrophoresis 2010, 471-482, 31

[2] Wang, Y. C.; L., S. A.; Han, J. Million-fold Preconcentration of Proteins and Peptides by Nanofluidic Filter. Analytical Chemistry 2005, 4293.

[3] Swami, N.; Chou, C.-F.; Ramamurthy, V.; Chaurey, V. Enhancing DNA hybridization kinetics through constriction-based. Lab on a Chip 2009, 9, 3212–3220.

[4] Chaurey, V.; Polanco, C. F.; Chou, C.-F.; Swami, N. S. Floating electrode enhanced constriction dielectrophoresis for biomolecular trapping in physiological media of high conductivity. Biomicrofluidics 2012, 6, 012806.

[5] Liao, K.T.; Tsegaye, M.; Chou, C.F.; Swami, N. “Nano-constriction device for rapid protein pre-concentration in physiological media through a balance of electrokinetic forces”, Electrophoresis (2012) (Accepted)