Inhomogeneous AC Electric Field Research Articles

AC electrokinetic immobilization of organic dye molecules

The application of inhomogeneous AC electric fields for molecular immobilization is a very fast and simple method that does not require any adaptions to the molecule’s functional groups or charges. Here, the method is applied to a completely new category of molecules: small organic fluorescence dyes, whose dimensions amount to only 1 nm or even less. The presented setup and the electric field parameters used allow immobilization of dye molecules on the whole electrode surface as opposed to pure dielectrophoretic applications, where molecules are attracted only to regions of high electric field gradients, i.e., to the electrode tips and edges. In addition to dielectrophoresis and AC electrokinetic flow, molecular scale interactions and electrophoresis at short time scales are discussed as further mechanisms leading to migration and immobilization of the molecules.Graphical

Simple, Fast And Highly Sensitive Detection Of Gram-Negative Bacteria By A Novel Electrical Biosensor.

This work presents a rapid, low-cost, highly sensitive and specific capacitive sensor for detection of Gram negative bacteria in a field setting. Recognition of Gramnegative bacteria is based on specific detection of lipopolysaccharides (LPS) by LPS-specific aptamer probe immobilized on electrode sensors. An inhomogeneous AC electric field is applied on sensor electrodes and induces positive dielectrophoresis that attracts LPS particles to the sensor electrodes for rapid detection. The same AC signal is also used to detect the binding reactions occurred on the sensor surface. The AC signal was optimized, and the binding between LPS and the specific aptamer was demonstrated. The detection limit reaches as low as 4.9 fg/mL for free LPS molecules and 53 #/mL of bacteria within a 30s' response time, meeting the needs of onsite bacteria detection.

A PCR-free point-of-care capacitive immunoassay for influenza A virus

This article describes a highly sensitive and specific capacitive immunosensor for rapid, low cost and simple-to-use detection of virus particles from clinical swab samples. An inhomogeneous AC electric field is applied on sensor electrodes. This induces positive dielectrophoresis that attracts virus particles to the sensor electrodes. As a result, rapid and sensitive detection of influenza A virus is accomplished without the need for nucleotide isolation and amplification. The same AC signal is used to detect the binding of virus particle to the sensor surface immobilized with the antibody probe. The assay is highly suitable for point-of-care use. When testing clinical swab samples, the response of samples at various dilutions is analyzed, and an optimal dilution is found and used for subsequent blind tests of clinical swab samples. Analytical experiments on standard influenza virus sample demonstrate a limit of detection of 0.25 pg⋅mL−1. Other figures of merit include (a) an assay time of 30 seconds; (b) a diagnostic sensitivity of 90%; and (c) a specificity of 70%. Blind tests are conducted for a panel of twenty nasal swab samples, and the results are in good agreement with those by using the commercial reverse transcription polymerase chain reaction.

Combination of Au Dielectrophoresis Chip and Optical Tweezers for Cell Culture

Dielectrophoresis (DEP) force will arise when an inhomogeneous AC electric field with sinusoidal wave is applied to microelectrodes. By using DEP, we could distinguish between viable and non-viable cells by their movement through a non-uniform electric field. In this paper, we propose a yeast cell separation system, which utilizes an Au DEP chip and an optical tweezers. The Au DEP chip is planar quadrupole microelectrodes, which were fabricated by Au thin-film and a box cutter. This fabrication method is low cost and simpler than previous existing methods. The tip of the optical tweezers was fabricated by dynamic chemical etching in a mixture of hydrogen fluoride and toluene. The optical tweezers has the feature of high manipulation performance. That does not require objective lens for focusing light because the tip of optical tweezers has conical shape. By using both the Au DEP chip and optical tweezers, we could obtain three-dimensional manipulation of specific cells after viability separation.

Colloidal crystal grain boundary formation and motion

The ability to assemble nano- and micro- sized colloidal components into highly ordered configurations is often cited as the basis for developing advanced materials. However, the dynamics of stochastic grain boundary formation and motion have not been quantified, which limits the ability to control and anneal polycrystallinity in colloidal based materials. Here we use optical microscopy, Brownian Dynamic simulations, and a new dynamic analysis to study grain boundary motion in quasi-2D colloidal bicrystals formed within inhomogeneous AC electric fields. We introduce “low-dimensional” models using reaction coordinates for condensation and global order that capture first passage times between critical configurations at each applied voltage. The resulting models reveal that equal sized domains at a maximum misorientation angle show relaxation dominated by friction limited grain boundary diffusion; and in contrast, asymmetrically sized domains with less misorientation display much faster grain boundary migration due to significant thermodynamic driving forces. By quantifying such dynamics vs. compression (voltage), kinetic bottlenecks associated with slow grain boundary relaxation are understood, which can be used to guide the temporal assembly of defect-free single domain colloidal crystals.

A Simplified Fabrication Method of Dielectrophoresis Chip Using Au Thin-Film and Box Cutter

We propose fabrication method of a planar quadrupole microelectrode for dielectrophoresis (DEP), which is fabricated by Au thin-film, ion coater and box cutter. This method is more cost effective and simpler than previous existing methods. We conducted two experiments for confirming usefulness of the Au DEP chip. Those are separation of yeast cells and trap force of DEP. To separate yeast cells, we used viable and non-viable cells. DEP force arises when an inhomogeneous AC electric field and sine wave frequency were applied to microelectrode. The Au DEP chip is able to distinguish between viable and non-viable cells and separate them by frequency dependence and the flow with Syringe pump. The chip can obtain viable cells which were trapped without contact to the microelectrode. The viable cells can use for fusion or cell culture. Furthermore, we carried out another experiment to investigate the trap force. The trap force of Negative-DEP becomes gradually weak when the frequency increases with 0.1, 0.5 and 1MHz. We were able to confirm relation between trap force and frequency by the Au DEP chip. The trap force has frequency dependence. Through the two experiments, we have established usefulness of the Au DEP chip.

A Simplified Fabrication Method of Dielectrophoresis Chip Using Au Thin-Film and Box Cutter

We propose fabrication method of a planar quadrupole microelectrode for dielectrophoresis (DEP), which is fabricated by Au thin-film, ion coater and box cutter. This method is more cost effective and simpler than previous existing methods. We conducted two experiments for confirming usefulness of the Au DEP chip. Those are separation of yeast cells and trap force of DEP. To separate yeast cells, we used viable and non-viable cells. DEP force arises when an inhomogeneous AC electric field and sine wave frequency were applied to microelectrode. The Au DEP chip is able to distinguish between viable and non-viable cells and separate them by frequency dependence and the flow with Syringe pump. The chip can obtain viable cells which were trapped without contact to the microelectrode. The viable cells can use for fusion or cell culture. Furthermore, we carried out another experiment to investigate the trap force. The trap force of Negative-DEP becomes gradually weak when the frequency increases with 0.1, 0.5 and 1MHz. We were able to confirm relation between trap force and frequency by the Au DEP chip. The trap force has frequency dependence. Through the two experiments, we have established usefulness of the Au DEP chip.

KT-Scale Colloidal Interactions in High-Frequency Inhomogeneous AC Electric Fields. II. Concentrated Ensembles

We report nonintrusive optical microscopy measurements of ensembles of polystyrene colloids in inhomogeneous AC electric fields as a function of field frequency and particle size. By using an inverse Monte Carlo (MC) simulation analysis of time-averaged particle microstructures, we sensitively measure induced dipole-dipole interactions on the kT energy scale. Measurements are reported for frequencies when the particle polarizability is greater and less than the medium, as well as the crossover between these conditions when dipole-dipole interactions vanish. By using measured single dipole-field interactions and associated parameters from Part I as input in the inverse analysis, the dipole-dipole interactions in this work are accurately modeled with no adjustable parameters for conditions away from the crossover frequency (i.e., |f(CM)| > 0). As dipolar interactions vanish at the crossover, a single frequency-dependent parameter is introduced to account for microstructures that appear to result from weak AC electro-osmotic flow induced interactions. By connecting quantitative measures of equilibrium microstructures and kT-scale dipole-field and dipole-dipole interactions, our findings provide a basis to understand colloidal assembly in inhomogeneous AC electric fields.

KT-Scale Colloidal Interactions in High Frequency Inhomogeneous AC Electric Fields. I. Single Particles

We report nonintrusive optical microscopy measurements of single micrometer-sized silica and polystyrene colloids in inhomogeneous AC electric fields as a function of field amplitude and frequency. By using a Boltzmann inversion of the time-averaged sampling of single particles within inhomogeneous electric fields, we sensitively measure induced dipole-field interactions on the kT energy scale and fN force scale. Measurements are reported for frequencies when the particle polarizability is greater and less than the medium, as well as the crossover between these conditions when dipole-field interactions vanish. For all cases, the measured interactions are well-described by theoretical potentials by fitting a nondimensional induced dipole-field magnitude. While silica dipole-field magnitudes are well-described by existing electrokinetic models, the polystyrene results suggest an anomalously high surface conductance. Sensitive measurements of dipole-field interactions in this work provide a basis to understand dipole-dipole interactions in particle ensembles in the same measurement geometry in part II.

Simulation studies on electrothermal fluid flow induced in a dielectrophoretic microelectrode system

The inhomogeneous ac electric fields used for particle manipulation in dielectrophoresis(DEP)-based microdevices not only produce forces on the particle, but also generate volume forces in the liquid by producing gradients in conductivity and permittivity due to local heating. The forces on the liquid give rise to fluid motion, which is referred to as electrothermal flow. This paper presents a numerical study on the electrothermally induced fluid flow on the dielectrophoretic microelectrode array. The fluid movement is numerically solved by coupling electrical, thermal and mechanical equations. A number of parameters including frequency, electrode structure, conductivity of the fluid and external heating that influence the fluid flow patterns are investigated. Particle behavior under the effects of electrothermal flow is studied. The viscous drag force on the particles arising from the electrothermal fluid flow becomes apparent as the particle size is reduced to the sub-micrometer scale. In certain circumstances, the drag force may be of the same order as or much greater than the DEP force. Under the effect of the electrothermal fluid flow, small particles may exhibit movements differing from that in the common DEP environment. These results provide significant suggestions for the manipulation of nanoparticles using ac electric fields under the normal DEP conditions.

Dielectrophoretic ratchets.

We have experimentally applied some concepts of "force-free" motion to micron size particles (latex beads). The coupling of dissipation and local spatial asymmetry of the potential experienced by the beads can put them into motion. The potentials used in these experiments are of dielectrophoretic nature. To that end, electrodes of particular shapes were used in order to submit the considered suspensions to inhomogeneous ac electric fields. Two regimes were explored: i-the Brownian ratchet case in which a Brownian particle is successively trapped in a factory roof-like potential and left free to diffuse. ii-the shifted ratchets case in which two potentials exhibiting similar characteristics are applied successively, one of them being shifted by a fraction of their common period relatively to the other. In both cases, a good agreement with the theoretical predictions was observed. In particular, particles of different sizes were characterized by different macroscopic velocities leading to the prospect of promising separation techniques. (c) 1998 American Institute of Physics.

Dielectrophoretic separation of cancer cells from blood

Recent measurements have demonstrated that the dielectric properties of cells depend on their type and physiological status. For example, MDA-231 human breast cancer cells were found to have a mean plasma membrane specific capacitance of 26 mF/m(2), more than double the value (11 mF/m(2)) observed for resting T-lymphocytes. When an inhomogeneous ac electric field is applied to a particle, a dielectrophoretic (DEP) force arises that depends on the particle dielectric properties. Therefore, cells having different dielectric characteristics will experience differential DEP forces when subjected to such a field. In this article, we demonstrate the use of differential DEP forces for the separation of several different cancerous cell types from blood in a dielectric affinity column. These separations were accomplished using thin, flat chambers having microelectrode arrays on the bottom wall. DEP forces generated by the application of ac fields to the electrodes were used to influence the rate of elution of cells from the chamber by hydrodynamic forces within a parabolic fluid flow profile. Electrorotation measurements were first made on the various cell types found within cell mixtures to be separated, and theoretical modeling was used to derive the cell dielectric parameters. Optimum separation conditions were then predicted from the frequency and suspension conductivity dependencies of cell DEP responses defined by these parameters. Cell separations were then undertaken for various ratios of cancerous to normal cells at different concentrations. Eluted cells were characterized in terms of separation efficiency, cell viability, and separation speed. For example, 100% efficiency was achieved for purging MDA-231 cells from blood at the tumor to normal cell ratio 1:1 x 10(5) or 1:3 x 10(5), cell viability was not compromised, and separation rates were at least 10(3) cells/s. Theoretical and experimental criteria for the design and operation of such separators are presented.