Real‐Time Nitrophenol Detection Using Single‐Walled Carbon Nanotube Based Devices

Single‐walled carbon nanotube (SWNT) based devices have been developed for the real‐time detection of nitrophenols in aqueous solution. SWNTs are assembled to electrodes using AC dielectrophoresis technique. The SWNT devices exhibit not only high sensitivity to nitrophenol compounds, but also good reusability. Charge transfer between nitro group and SWNTs, and the metal‐nanotube interface modification are hypothesized to be the possible origins of conductance change. These results indicated that the SWNT devices can be utilized as a simple, low cost, sensitive, and reusable platform for real‐time detection of nitrophenol compounds.     

Dielectrophoretic characterization and isolation of jellyfish stinging capsules

Jellyfish stinging capsules known as nematocysts are explosive, natural‐injection systems with high potential as a natural drug‐delivery system. These organelles consist of a capsule containing a highly folded thin needle‐like tubule and a matrix highly concentrated with charged constituents that enable the tubule to fire and penetrate a target. For the purpose of using these nematocysts as drug delivery system it is first required to purify subpopulations from heterogeneous population of capsules and to investigate each subpopulation's distinct function and characteristics. Here, the nematocysts’ dielectric properties were experimentally investigated using dielectrophoretic and electrorotational spectra with best fits derived from theoretical models. The dielectric characterization adds to our understanding of the nematocysts’ structure and function and is necessary for the dielectrophoretic isolation and manipulation of populations. As expected, the effect of monovalent and divalent exchange cations resulted in higher inner conductivity for the NaCl treated capsules; this result stands in agreement with their relative higher osmotic pressure. In addition, an efficient dielectrophoretic isolation of different nematocyst subpopulations was demonstrated, paving the way to an understanding of nematocysts’ functional diversity and the development of an efficient drug delivery platform.     

Interdigitated electrode design and optimization for dielectrophoresis cell separation actuators

Among all particle separation approaches, dielectrophoresis actuators which use electric properties difference between particles, have turned into strong separating tools. This way, the particles in the fluid within non-uniform electric field experience the dielectrophoresis force. The amount and direction of this force depend on the fluid and particle polarization, particle size and electric field gradient. In this paper after presenting governing equations concerning the dielectrophoresis phenomenon, a microfluidic actuator introduced in which an interdigitated electrode pattern is applied in. Voltage, pitch, and width to pitch ratio of electrode as well as channel height are of the most important geometrical parameters of this actuator whose individual effect on particles separation was investigated using finite element analysis (FEM). The simulation results showed that if the actuator is intended to work in the efficient conditions, channel height and electrodes pitch should be near to each other, height needs to be as minimum as possible while voltage as maximum as possible in order to reach to the least time duration and the highest quantity for particles separation. Then, using theoretical equations and simulation results, a flowchart is introduced to design and optimize dielectrophoresis separation actuators. Finally, experimental results for k562 cell separation, as a biological particle, from Polystyrene, as a standard particle, is presented. In the fabricated actuator recovery and purity efficiency are 93% and about 100% respectively.     

碳纳米管介电电泳精确及可控组装技术

碳纳米管(CNTs)具有优良的电学、热学、光学、力学性能和大的长径比,使得碳纳米管在能源存储、生物医药学、催化剂载体、水气过滤、复合材料等领域存在极大的应用价值。碳纳米管在金属电极之间的精确可控组装是实现其诸多应用的前提,介电电泳法是目前最常用且最具前景的组装方法之一。文中介绍了介电电泳法组装碳纳米管的原理,分析了介电电泳组装碳纳米管的影响因素,分别从碳纳米管的精确定位组装和数量可控组装两个方面进行了综述。     

Analysis of microorganisms with nonlinear electrokinetic microsystems

Nonlinear electrokinetics (EK), specifically electrophoresis of the second kind, dielectrophoresis (DEP) and electrorotation (EROT), have gained significant interest recently for their flexibility and labeless discriminant manner of operation. The current applications of these technologies are a clear advancement from what they were when first discovered, but also still show strong signs of future growth. The present review article presents a discussion of the current uses of microscale nonlinear EK technologies as analytical, sensing, and purification tools for microorganisms. The discussion is focused on some of the latest discoveries with various nonlinear EK microfluidic techniques, such as DEP particle trapping and EROT for particle assessments, for the analysis of microorganisms ranging from viruses to parasites. Along the way, special focus was given to key research articles from within the past two years to provide the most up-to-date knowledge on the current state-of-the-art within the field of microscale EK, and from there, an outlook on where the future of the field is headed is also included.     

Dielectrophoresis force spectroscopy for colloidal clusters

Optical trapping-based force spectroscopy was used to measure the frequency-dependent DEP forces and DEP crossover frequencies of colloidal polymethyl methacrylate spheres and clusters. A single sphere or cluster, held by an optical tweezer, was positioned near the center of a pair of gold-film electrodes where alternating current elecroosmosis flow was negligible. Use of amplitude modulation and phase-sensitive lock-in detection for accurate measurement of the DEP force yielded new insight into dielectric relaxation mechanisms near the crossover frequencies. On one hand, the size dependence of the DEP force near the crossover frequencies indicates that the dominant polarization mechanism is a volume effect. On the other hand, the power-law dependence of the crossover frequency on the particle radius with an exponent of -2 indicates the dielectric relaxation is more likely because of ionic diffusion across the particle surface, suggesting the dominant polarization mechanism may be a surface polarization effect. Better theories are needed to explain the experiment. Nevertheless, the strong size dependence of the crossover frequencies suggests the use of DEP for size sorting of micron-sized particles.     

Analytical solutions and validation of electric field and dielectrophoretic force in a bio-microfluidic channel

In a microbiological device, cell or particle manipulation and characterization require the use of electric field on different electrodes in several configurations and shapes. To efficiently design microelectrodes within a microfluidic channel for dielectrophoresis focusing, manipulation and characterization of cells, the designer will seek the exact distribution of the electric potential, electric field and hence dielectrophoresis force exerted on the cell within the microdevice. In this paper we describe the approach attaining the analytical solution of the dielectrophoretic force expression within a microchannel with parallel facing same size electrodes present on the two faces of channel substrates, with opposite voltages on the pair electrodes. Simple Fourier series mathematical expressions are derived for electric potential, electric field and dielectric force between two distant finite‐size electrodes. Excellent agreement is found by comparing the analytical results calculated using MATLAB? with numerical ones obtained by Comsol. This analytical result can help the designer to perform simple design parametric analysis. Bio‐microdevices are also designed and fabricated to illustrate the theoretical solution results with the experimental data. Experiments with red blood cells show the dielectrophoretic force contour plots of the analytical data matched to the experimental results.     

The effect of solvent on the morphology of ZnO nanostructure assembly by dielectrophoresis and its device applications

Different zinc oxide nanostructured morphologies were grown on photolithographically patterned silicon/silicon dioxide substrates by dielectrophoresis technique using different solvents, such as water and ethanol, obtaining rod-like and net-like nanostructures, respectively. The formation of continuous nanostructures was confirmed by scanning electron microscopic, atomic force microscopic images, and electrical characterizations. The rod-like zinc oxide nanostructures were observed in the 10 μm gap between the fingers in the pattern, whereas net-like nanostructures were formed independently of microgap. A qualitative study about the mechanism for the assembly of zinc oxide continuous nanostructures was presented. Devices were electrically characterized, at room temperature, in controlled environment to measure the conductance behavior in ultraviolet and humidity environment. Devices based on zinc oxide nanostructures grown in ethanol medium show better responses under both ultraviolet and humidity, because of the net-like structure with high surface-to-volume ratio.     

Interaction of nanoparticles at the DEP microelectrode interface under high conductance conditions

The separation of nanoparticles from micron size particles in high conductance buffers was achieved using an AC dielectrophoretic (DEP) microarray device with hydrogel over-coated microelectrodes. While nanoparticles could be selectively concentrated into high field regions directly over the platinum microelectrodes, micro-bubbling and electrode darkening was also observed. For similar experiments using un-coated microelectrodes, SEM analysis showed severe erosion of the platinum microelectrodes and fusion of nanoparticles due to the aggressive electrochemistry.     

Characterization of electrokinetic mobility of microparticles in order to improve dielectrophoretic concentration

Insulator-based dielectrophoresis (iDEP), an efficient technique with great potential for miniaturization, has been successfully applied for the manipulation of a wide variety of bioparticles. When iDEP is applied employing direct current (DC) electric fields, other electrokinetic transport mechanisms are present: electrophoresis and electroosmotic flow. In order to concentrate particles, iDEP has to overcome electrokinetics. This study presents the characterization of electrokinetic flow under the operating conditions employed with iDEP; in order to identify the optimal conditions for particle concentration employing DC-iDEP, microparticle image velocimetry (μPIV) was employed to measure the velocity of 1-μm-diameter inert polystyrene particles suspended inside a microchannel made from glass. Experiments were carried out by varying the properties of the suspending medium (conductivity from 25 to 100 μS/cm and pH from 6 to 9) and the strength of the applied electric field (50–300 V/cm); the velocities values obtained ranged from 100 to 700 μm/s. These showed that higher conductivity and lower pH values for the suspending medium produced the lowest electrokinetic flow, improving iDEP concentration of particles, which decreases voltage requirements. These ideal conditions for iDEP trapping (pH = 6 and σ _m = 100 μS/cm) were tested experimentally and with the aid of mathematical modeling. The μPIV measurements allowed obtaining values for the electrokinetic mobilities of the particles and the zeta potential of the glass surface; these values were used with a mathematical model built with COMSOL Multiphysics software in order to predict the dielectrophoretic and electrokinetic forces exerted on the particles; the modeling results confirmed the μPIV findings. Experiments with iDEP were carried out employing the same microparticles and a glass microchannel that contained an array of cylindrical insulating structures. By applying DC electric fields across the insulating structures array, it was seen that the dielectrophoretic trapping was improved when the electrokinetic force was the lowest; as predicted by μPIV measurements and the mathematical model. The results of this study provide guidelines for the selection of optimal operating conditions for improving insulator-based dielectrophoretic separations and have the potential to be extended to bioparticle applications. Figure Comparison of experimental measurements and mathematical modeling of electrokinetic and dielectrophoretic effects on microparticles