Simultaneous measurements of electrophoretic and dielectrophoretic forces using optical tweezers

Herein, charged microbeads handled with optical tweezers are used as a sensitive probe for simultaneous measurements of electrophoretic and dielectrophoretic forces. We first determine the electric charge carried by a single bead by keeping it in a predictable uniform electric field produced by two parallel planar electrodes, then, we examine same bead's response in proximity to a tip electrode. In this case, besides electric forces, the bead simultaneously experiences non-negligible dielectrophoretic forces produced by the strong electric field gradient. The stochastic and deterministic motions of the trapped bead are theoretically and experimentally analysed in terms of the autocorrelation function. By fitting the experimental data, we are able to extract simultaneously the spatial distribution of electrophoretic and dielectrophoretic forces around the tip. Our approach can be used for determining actual, total force components in the presence of high-curvature electrodes or metal scanning probe tips.Comment: 9 pages, 3 figure

Numerical modeling of dielectrophoretic effect for manipulation of bio-particles

This thesis was submitted for the degree of Doctor of Philosophy and awarded by Brunel University.This text describes different aspects of the design of a Doctor-on-a-Chip device. Doctor-on-a-Chip is a DNA analysis system integrated on a single chip, which should provide all of the advantages that stem from the system integration, such as small sample volume, fast and accurate analysis, and low cost. The text describes all of the steps of the on-chip sample analysis, including DNA extraction from the sample, purification, PCR amplification, novel dielectrophoretic sorting of the DNA molecules, and finally detection. The overview is given of the technologies which are available to make the integration on a single chip possible. The microfluidic technologies that are used to manipulate the sample and other chemical reagents are already known and in this text they are analyzed in terms of their feasibility in the on-chip system integration. These microfluidic technologies include, but are not limited to, microvalves, micromixers, micropumps, and chambers for PCR amplification. The novelty in the DNA analysis brought by Doctor-on-a-Chip is the way in which the different DNA molecules in the sample (for example, human and virus DNA) are sorted into different populations. This is done by means of dielectrophoresis – the force experienced by dielectric particles (such as DNA molecules) when subject to a non-uniform electric field. Different DNA molecules within a sample experience different dielectrophoretic forces within the same electric field, which makes their separation, and therefore detection, possible. In this text, the emphasis is put on numerical modelling of the dielectrophoretic effect on biological particles. The importance of numerical modelling lies in the fact that with the accurate model it is easier to design systems of microelectrodes for dielectrophoretic separation, and tune their sub-micrometre features to achieve the maximum separation efficacy. The numerical model described in this text is also experimentally verified with the novel microelectrodes design for dielectrophoretic separation, which is successfully used to separate the mixture of different particles in the micron and sub-micron range

PROBLEMS IN THE STUDY AND USE OF AC DIELECTROPHORESIS AND THEIR CONSEQUENCES: A STUDY BASED ON COMSOL MULTIPHYSICS MODELING

Dielectrophoresis (or DEP) is an important phenomenon which is induced when a dielectric particle is placed in a non-uniform electric field. The force generated by DEP has been exploited for various micro and nano fluidics applications like positioning, sorting and separation of particles involved in medical diagnostics, drug discovery, cell therapeutics, biosensors, microfluidics, nanoassembly, particle filtration etc. The integration of DEP systems into the microfluidics enables inexpensive, fast, highly sensitive, highly selective, label-free detection and also the analysis of target bioparticles. This work aims to provide a complete compilation of the factors affecting the DEP force. It elucidates the underlying mechanisms using COMSOL Multiphysics and sheds new insight into the mechanisms for the separation and sorting of different types of particles. This research identifies the problems in the literature and uses COMSOL to analyze the impact of these problems on the end results. It examines four factors that affect the DEP force: physical conditions, electrode setup, properties of the particles and suspension medium. Moreover, it analyzes the influence of the Clausius-Mossotti factor (CM factor) and its cross-over upon the magnitude and direction of the DEP force. From the analysis, it becomes clear that particle size not only affects the magnitude of the DEP force but also the conductivity of the particle. This factor, which is largely ignored, could lead to a shift in the crossover frequency. Shell model plays an important role in determining the dielectric properties of particles that are not homogenous. In such a situation assuming uniform dielectric properties may lead to inconclusive results. The presence of an electric double layer surrounding a particle affects the conductivity of the particle. Also, assuming DEP force to be the only force acting on a particle suspended in a non-uniform electric field leads to errors in the end results. This research provides knowledge on the basic characteristics of the DEP force and its mechanism. It provides a better understanding by examining numerous works carried out in the past and brings out the problems and their consequences

On-chip high-speed sorting of micron-sized particles for high-throughput analysis

A new design of particle sorting chip is presented. The device employs a dielectrophoretic gate that deflects particles into one of two microfluidic channels at high speed. The device operates by focussing particles into the central streamline of the main flow channel using dielectrophoretic focussing. At the sorting junction (T- or Y-junction) two sets of electrodes produce a small dielectrophoretic force that pushes the particle into one or other of the outlet channels, where they are carried under the pressure-driven fluid flow to the outlet. For a 40mm wide and high channel, it is shown that 6micron diameter particles can be deflected at a rate of 300particles/s. The principle of a fully automated sorting device is demonstrated by separating fluorescent from non-fluorescent latex beads

Coaxial Atomic Force Microscope Tweezers

We demonstrate coaxial atomic force microscope (AFM) tweezers that can trap and place small objects using dielectrophoresis (DEP). An attractive force is generated at the tip of a coaxial AFM probe by applying a radio frequency voltage between the center conductor and a grounded shield; the origin of the force is found to be DEP by measuring the pull-off force vs. applied voltage. We show that the coaxial AFM tweezers (CAT) can perform three dimensional assembly by picking up a specified silica microsphere, imaging with the microsphere at the end of the tip, and placing it at a target destination.Comment: 9 pages, 3 figures, in review at Applied Physics Letter

Dielectrophoresis: An Approach to Increase Sensitivity, Reduce Response Time and to Suppress Nonspecific Binding in Biosensors?

The performance of receptor-based biosensors is often limited by either diffusion of the analyte causing unreasonable long assay times or a lack of specificity limiting the sensitivity due to the noise of nonspecific binding. Alternating current (AC) electrokinetics and its effect on biosensing is an increasing field of research dedicated to address this issue and can improve mass transfer of the analyte by electrothermal effects, electroosmosis, or dielectrophoresis (DEP). Accordingly, several works have shown improved sensitivity and lowered assay times by order of magnitude thanks to the improved mass transfer with these techniques. To realize high sensitivity in real samples with realistic sample matrix avoiding nonspecific binding is critical and the improved mass transfer should ideally be specific to the target analyte. In this paper we cover recent approaches to combine biosensors with DEP, which is the AC kinetic approach with the highest selectivity. We conclude that while associated with many challenges, for several applications the approach could be beneficial, especially if more work is dedicated to minimizing nonspecific bindings, for which DEP offers interesting perspectives

Dielectrophoretic (DEP) Tweezers: New Tool for Molecular Force Spectroscopy

Many recent advances in DNA sequencing technology have taken advantage of single-molecule techniques using fluorescently-labeled oligonucleotides as the principle mode of detection. In spite of the successes of fluorescent-based sequencers, avoidance of labeled nucleotides could substantially reduce the costs of sequencing. This dissertation will describe the development of an alternative sequencing method, in which unlabeled DNA can be manipulated directly on a massively parallel scale using single molecule force spectroscopy. We demonstrated that a combination of a wide-field optical detection technique (evanescent field excitation) with dielectrophoretic (DEP) tweezers could determine the amount of the double-stranded character of DNA. This thesis discusses all aspects of the implementation of DEP tweezers, including the principle of operation, making of polymer force probes, numerical modeling of various designs, fabrication of electrode and disposable chip, force calibration, and the assembly of the device. The feasibility of this technique was demonstrated by conducting force spectroscopy on single DNA molecules using DEP tweezers. The development of such a single molecule force spectroscopy technique shows great potential for genome sequencing and other analytical applications that employ direct manipulation of biomolecules

Higher Order Electrokinetic Effects for Applied Biological Analytics

abstract: Microfluidic systems have gained popularity in the last two decades for their potential applications in manipulating micro- and nano- particulates of interest. Several different microfluidics devices have been built capable of rapidly probing, sorting, and trapping analytes of interest. Microfluidics can be combined with separation science to address challenges of obtaining a concentrated and pure distinct analyte from mixtures of increasingly similar entities. Many of these techniques have been developed to assess biological analytes of interest; one of which is dielectrophoresis (DEP), a force which acts on polarizable analytes in the presence of a non-uniform electric fields. This method can achieve high resolution separations with the unique attribute of concentrating, rather than diluting, analytes upon separation. Studies utilizing DEP have manipulated a wide range of analytes including various cell types, proteins, DNA, and viruses. These analytes range from approximately 50 nm to 1 µm in size. Many of the currently-utilized techniques for assessing these analytes are time intensive, cost prohibitive, and require specialized equipment and technical skills. The work presented in this dissertation focuses on developing and utilizing insulator-based dielectrophoresis (iDEP) to probe a wide range of analytes; where the intrinsic properties of an analyte will determine its behavior in a microchannel. This is based on the analyte’s interactions with the electrokinetic and dielectrophoretic forces present. Novel applications of this technique to probe the biophysical difference(s) between serovars of the foodborne pathogen, Listeria monocytogenes, and surface modified Escherichia coli, are investigated. Both of these applications demonstrate the capabilities of iDEP to achieve high resolution separations and probe slight changes in the biophysical properties of an analyte of interest. To improve upon existing iDEP strategies a novel insulator design which streamlines analytes in an iDEP device while still achieving the desirable forces for separation is developed, fabricated, and tested. Finally, pioneering work to develop an iDEP device capable of manipulating larger analytes, which range in size 10-250 µm, is presented.Dissertation/ThesisDoctoral Dissertation Chemistry 201

Improving the Design and Application of Insulator-Based Dielectrophoretic Devices for the Assessment of Complex Mixtures

Dielectrophoresis (DEP) is an electrokinetic (EK) transport mechanism that exploits polarization effects when particles are exposed to a non-uniform electric field. This dissertation focused on the development of high-performance insulator-based DEP (iDEP) devices. A detailed analysis of the spatial forces that contribute to particle movement in an iDEP device is provided. In particular, this analysis shows how particle size and shape affects the regions where particles are likely to be retained due to dielectrophoretic trapping. The performance of these trapping regions was optimized using a systematic approach that integrates the geometrical parameters of the array of insulating structures. Devices that decrease the required electrical potential by ~80% where found. The optimization strategy enabled the detection of structures that promote and discourage particle trapping. By combining the best and worst structures in a single asymmetric structure, a novel iDEP device was designed. This device selectively enriches the larger particles in a sample and drives the smaller particles away from the enrichment region. A quick enrichment and elution of large cells was achieved. This is important when dealing with samples containing eukaryotic cells, which can be harmed by the electrical treatment. Yeast cells were successfully separated from polystyrene particles in under 40 seconds using this device and a high cell viability of 85% was achieved. Finally, an enhancement of traditional iDEP devices is proposed, where some insulating posts are replaced by conducting structures. That is, insulating and conductive posts are intimately combined within the same array. The performance of this hybrid device is presented to show the advantage of using insulating structures with microelectrodes in the same array to dominate particle movement