Dielectrophoretic separation of nano-particles

Abstract

Abstract. We show for the first time that it is possible to separate a population of nanoparticles into two subpopulations solely on the basis of their dielectric properties. Using nanofabricated electrode arrays it has been shown that a solution of 93 nm diameter latex beads with a distribution of surface charge can be separated by the application of non-uniform AC electric fields (dielectrophoresis). The mixture separated into two populations, one experiencing positive dielectrophoresis and the other negative dielectrophoresis.The movement of polarizable particles in non-uniform fields, termed dielectrophoresis by Pohl [1], has been exploited as a rapid non-invasive tool for the diagnosis and separation of mammalian cells and micro-organisms [2]. It has found practical applications in medicine for separating cancer cells from blood [3], enriching CD34+ cells in bone marrow samples [4] and for detecting the viability of pathogenic organisms in water [5].Separation of particles is based on the fact that when placed in a non-uniform AC electric field, polarized particles experience a variable translational force, depending on the applied field frequency. For particles whose polarizability is greater than the medium the net movement is to regions of highest field strength, whereas particles whose polarizability is less than the medium move to the region of lowest field gradient. Because the particle's polarization is frequency dependent, the net force is also frequency dependent. Therefore, by judicious choice of suspending medium conductivity and applied frequency, even particles with very similar dielectric properties can be efficiently separated.The sum total of the forces on a given particle is: F total = F deterministic + F random . The deterministic force is the sum of the sedimentation, hydrodynamic and dielectrophoretic forces. The random force is due to Brownian motion and for particles with a diameter of the order of 100 nm this force is considerable. The mean free path of the movement is inversely dependent on mass, implying that decreases in the particle diameter require significant increases in the applied electrostatic energy (or field strength). The stable trapping of viruses and beads has been demonstrated previously [6][7][8][9]. Here we show that nanometre sized latex spheres can be both stably trapped in nanofabricated electrode arrays and, more importantly, that a heterogeneous mixture can be separated into two populations using dielectrophoresis. Experiments were conducted on charged latex spheres of 93 nm diameter, using planar electrodes manufactured by electron-beam lithography. The applied electric field strength was 2.5 × 10 5 V m −1 and the suspending medium conductivity was 0.018 S m −1 . Large electric field strengths cause localized heating of electrode structures which in turn gives rise to discontinuities in conductivity, permittivity and viscosity of the medium. The result is the inception

A detailed study of p–n junction solar cells by means of collection efficiency

“…Eq. (7) is manifestly in the self-adjoint (Sturm-Liouville) form and the reciprocity theorem follows directly [5].…”

“…[5,7,8]. For simplicity of the analysis, the authors consider the charge carrier transport to be onedimensional along a linear coordinate x, and the injected minority carrier densities to be small compared to the majority carrier densities, i.e., low injection assumption.…”

“…The parameter Z c (x) is sometimes called the spatial collection efficiency, for example by Sinkkonen et al [4], or the collection probability, for example by Donolato [2], Green [5] and Brendel and Rau [6].…”

“…In the present work, this parameter is considered in a different way based on the use of the diffusion equation for minority carriers derived by del Alamo and Swanson [7] and a reciprocity relationship between the charge collection and the dark carrier distribution [5,[8][9][10][11][12]. The results obtained from the calculation are applied to study the performance of a solar cell.…”

The role of electrohydrodynamic forces in the dielectrophoretic manipulation and separation of particles

“…Such systems are capable of providing high electric fields with small applied voltages. It has been shown that a wide range of particles such as cells, latex spheres, and microorganisms can be selectively manipulated using dieleetrophoretic techniques [1][2][3] and more recently, sub-micrometre particles sueh as latex spheres, maeromoleeules, DNA and viruses have been manipulated and charaeterised [4][5][6][7][8][9][10]. For sub-mierometre particles it was believed Brownian motion would prevent deterministic movement by DEP, but this premise has been disproved.…”