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Abstract
Dielectrophoresis (DEP) has increasingly been used for the assessment of the electrical properties of molecular scale objects including proteins, DNA, nanotubes and nanowires. However, whilst techniques have been developed for the electrical characterisation of frequency-dependent DEP response, biomolecular study is usually limited to observation using fluorescent markers, limiting its applicability as a characterisation tool. In this paper we present a label-free, impedance-based method of characterisation applied to the determination of the electrical properties of colloidal protein molecules, specifically Bovine Serum Albumin (BSA). By monitoring the impedance between electrodes as proteins collect, it is shown to be possible to observe multi-dispersion behaviour. A DEP dispersion exhibited at 400 kHz is attributable to the orientational dispersion of the molecule, whilst a second, higher-frequency dispersion is attributed to a Maxwell-Wagner type dispersion; changes in behaviour with medium conductivity suggest that this is strongly influenced by the electrical double layer surrounding the molecule.
Highlights
For more than fifty years, dielectrophoresis (DEP) has been used for the manipulation of a variety of particles, both organic and inorganic, and for myriad applications including separation, fabrication and analysis.[1,2] First described by Herbert Pohl in 1951, DEP is the motion of a particle due to its polarization in the presence of non-uniform electric field.[3]
For many years it was believed that DEP force, which scales according to the volume of the manipulated particle, was too small at molecular levels to overcome Brownian motion; the first demonstration of DEP manipulation of biomolecules was exhibited by Masao Washizu and co-workers in 1994.4 Since this time, a number of demonstrations of biomolecular DEP have been performed, many of which are reviewed elsewhere.[5]
Two dispersions could be observed in the Bovine Serum Albumin (BSA) spectrum, and in different proportions at different medium conductivities
Summary
For more than fifty years, dielectrophoresis (DEP) has been used for the manipulation of a variety of particles, both organic and inorganic, and for myriad applications including separation, fabrication and analysis.[1,2] First described by Herbert Pohl in 1951, DEP is the motion of a particle due to its polarization in the presence of non-uniform electric field.[3] The magnitude and direction of the DEP force is regulated by the relative polarizability of the particle, which depends on the dielectric properties of both the particle and the suspending medium, as well as the frequency of the energising field. Kotsuki et al recently published organic semiconductor single crystals formed a field effect transistor using DEP,[8] and lately in 2016, there were inorganic DEP
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