Mechanistic investigation of nanoparticle motion in pulsed voltage miniaturized electrical field flow fractionation device by in situ fluorescence imaging

In our previous study, we reported a miniaturized electrical field flow fractionation device (micro-EFFF) that used a pulsed voltage (PV) to increase the effective electric field and, hence, improved the separation performance. In this work, we developed two micro-EFFFs with planar or segmented electrode design and investigated the particle movement in the flow channels under a PV. Numerical simulation was used to understand the electric field distribution in the micro-EFFFs. When the calculations for the micro-EFFF with a segmented electrode (segmented micro-EFFF) and the micro-EFFF with planar electrodes (planar micro-EFFF) are compared, a stronger electric field at the top electrode segments is found in the segmented micro-EFFF, with the strongest field at the edges of the electrode segments. Nanoparticle motion in both devices was in situ visualized by using a fluorescence microscope equipped with a CCD-camera. Results reveal that electrophoresis governs the nanoparticle movement in the planar micro-EFFF and dielectrophoresis dominates the movement in the segmented micro-EFFF. Two models are postulated to explain the experimental observations of the nanoparticle movement. The mechanistic understanding of controlling nanoparticle motion in a miniaturized environment will help the design and application of micro-EFFF for the separation of charged biomolecules (proteins and DNAs).     

Electric circuit model for electrical field flow fractionation

In electrical field flow fractionation (EFFF or ElFFF), an electric potential is applied across a narrow gap filled with a weak electrolyte fluid. Charge buildup at the two poles (electrodes) and the formation of an electric double layer shields the channel, making the effective field in the bulk fluid very weak. Recent computational research suggests that pulsed field protocols, however, should improve retention and may enhance separation in EFFF through systematic disruptions of the double layer resulting in a stronger effective field in the bulk fluid. Improved retention has already been demonstrated experimentally. Accurate modeling and subsequent device optimization and design, however, depends, in part, on formulating a suitable model for the capacitative response of the channel and double layer at the electrode surfaces. Early models do not correctly describe experimentally observed current-time response and are not physically meaningful even when accurate mathematical fits of the data are realized. A new model and conceptual framework based on electrical resistance and capacitance variations of the double layer is suggested here. Physical interpretations of the electrical response have been developed and compared to published experimental data sets.     

Geometric scaling effects in electrical field flow fractionation. 2. Experimental results

Geometric scaling of microelectrical field flow fractionation (micro-EFFF) systems is investigated experimentally and compared to theory and to macroscale EFFF systems. Experimental results are presented to demonstrate that the miniaturized system operates according to the scaling theory associated with the system. Demonstrated improvements in the channels include increased retention and resolution and decreased peak broadening, electrical time constants, relaxation time, power consumption, and sample size. Additionally, scaling effects related to the compression of separation zones in the miniaturized EFFF systems are discussed.     

Nanoparticle characterization by cyclical electrical field-flow fractionation

In this work, the analytical potential of cyclical electrical field flow fractionation (CyElFFF) for nanomaterial and colloidal particle characterization has been experimentally demonstrated. Different operating parameters were investigated in order to evaluate their effect on the mechanisms of retention and fractionation power of CyElFFF. The voltage and frequency of the oscillating electrical field appeared to be the most influential parameters controlling the separation mode. Mobile phase flow rate was also found to be a key parameter controlling the fractionation efficiency. This work allowed the definition of operating conditions such that a reliable CyElFFF analysis could be performed on different nanoparticles on the basis of the direct comparison of their theoretical and experimental behavior. The results show that this technique in optimized conditions is a powerful tool for electrophoretic mobility based separation and characterization of various nanoparticles.     

Microchip capillary electrophoresis with an integrated indium tin oxide electrode-based electrochemiluminescence detector

This paper describes an indium tin oxide (ITO) electrode-based Ru(bpy)3(2+) electrochemiluminecence (ECL) detector for a microchip capillary electrophoresis (CE). The microchip CE-ECL system described in this article consists of a poly(dimethylsiloxane) (PDMS) layer containing separation and injection channels and an electrode plate with an ITO electrode fabricated by a photolithographic method. The PDMS layer was reversibly bound to the ITO electrode plate, which greatly simplified the alignment of the separation channel with the working electrode and enhanced the photon-capturing efficiency. In our study, the high separation electric field had no significant influence on the ECL detector, and decouplers for isolating the separation electric field were not needed in the microchip CE-ECL system. The ITO electrodes employed in the experiments displayed good durability and stability in the analytical procedures. Proline was selected to perform the microchip device with a limit of detection of 1.2 microM (S/N = 3) and a linear range from 5 to 600 microM.     

A microfabricated device for subcellular organelle sorting

We report a microfabricated field flow fractionation device for continuous separation of subcellular organelles by isoelectric focusing. The microdevice provides fast separation in very small samples while avoiding large voltages and heating effects typically associated with conventional electrophoresis-based devices. The basis of the separation is the presence of membrane proteins that give rise to the effective isoelectric points of the organelles. Simulations of isoelectric focusing of mitochondria in microchannels are used to assess design parameters, such as dimensions and time scales. In addition, a model of Joule heating effects in the microdevice during operation indicates that there is no significant heating, even without active cooling. The device is fabricated using a combination of photolithography, thin-film metal deposition/patterning, and electroplating techniques. We demonstrate that in the microfluidic devices, mitochondria from cultured cells migrate under the influence of an electric field into a focused band in less than 6 min, consistent with model predictions. We also illustrate separation of mitochondria from whole cells and nuclei as well as the separation of two mitochondrial subpopulations. When automated and operated in parallel, these microdevices should facilitate high-throughput analysis in studies requiring separation of organelles.     

Using channel depth to isolate and control flow in a micro free-flow electrophoresis device

A multiple-depth micro free-flow electrophoresis chip (mu-FFE) has been fabricated with a 20-microm-deep separation channel and 78-microm-deep electrode channels. Due to the difference in channel heights, the linear velocity of buffer in the electrode channels is approximately 15 times that of the buffer in the separation channel. Previous mu-FFE devices have been limited by electrolysis product formation at the electrodes. These electrolysis products, manifested as bubbles, decreased the electric field and disrupted the buffer flow profile, limiting performance and preventing continuous operation. Using channel depth to control buffer flow over the electrodes and in the separation channel effectively removes electrolysis products, allowing continuous operation. The linear velocities in the channels were confirmed using particle velocimetry and compared well with values predicted using lubrication theory. A separation potential of 645 V could be applied before significant Joule heating was observed. This corresponded to an electric field of 586 V/cm in the separation channel, a 4-fold increase over our previous design. A separation of fluorescent standards was demonstrated using the new mu-FFE device. Resolution increased by a factor of 1.3 over our previous design, even when operated under similar conditions, suggesting that effective removal of electrolysis products is more important than originally thought.     

Programmed field decay thermal field flow fractionation of polymers: a calibration method

The universal calibration procedure typical of thermal field flow fractionation (ThFFF) under constant thermal field operation was extended to thermal field programming (TFP) operation. The method requires knowledge of the following: (a) the programming function, which only depends on the thermal field decay function, (b) the physicochemical properties of the solvent, and (c) the calibration plot under varying channel cold wall temperatures (T(c)). Two field flow fractionation field programming conditions, with either a constant or a variable in time carrier flow velocity, are exploited. The method is based on determination, for each retention time position, of the average lambda retention value typical of TFP ThFFF. This parameter is then used to obtain the calibration plot (i.e., the molecular weight of the species as a function of the retention time position) by using the programming function and the calibration plot under varying T(c) values. The procedure approximation errors are also derived as a function of the programming type and solute-solvent system. To properly test the procedure, the calibration plot for the system constituted by polystyrene (PS) in cis-trans Decalin was determined, under varying conditions T(c) and thermal gradients, by using a set of monodisperse PS standards of different molecular weights (M). The procedure was first validated by simulation under two typical cases of TFP ThFFF operation. The approximation errors were found acceptable (in the worse cases, the accuracy in M prediction was 3%) and are in agreement with the theory. The procedure was then experimentally validated under varying programming decay function conditions. The reproducibility and accuracy of the M determination are both better than 2%.     

Flow-through sampling for electrophoresis-based microchips and their applications for protein analysis

This work presents a model behind the operation of a flow-through sampling chip and its application for immunoseparation, as well as its integration with a wash/elution bed for protein purification, concentration, and detection. This device used hydrodynamic pressure to drive the sample flow, and a gating voltage was applied to the electrophoretic channel on the microchip to control the sample loading for the separation and to inhibit sample leakage. The deduced model indicates that the critical gating voltage (VC) that is defined as the minimum gating voltage applied to the microchip for sampling is a function of the pump flow rate, the configuration of the microchannel on the chip, and the electroosmosis of the buffer solution. It was found that the theoretical V(C) values calculated from the measured electroosmotic mobilities and flow split ratios were comparable to those experimentally obtained from two microchips with different sampling channel sizes. This had an error percentage ranging from 1 to 20%. Because the hydrodynamic flow is insensitive to electrophoretic mobility, this electrophoresis-based microchip device was free of injection bias due to different ionic strength and electrophoretic mobility in the sample. Additionally, the usefulness of this device was demonstrated for the study of affinity interactions. Mixtures of Cy5-labeled bovine serum albumin (Cy5-BSA) and anti-BSA in various proportions were introduced into the microchip via a syringe pump, and the immunocomplex was electrophoretically separated from the free Cy5-BSA on the microchip. Based on the relative intensity of the free and complex BSA, the binding constant of BSA and anti-BSA was estimated as 3.3 x 10(7) M(-1). Furthermore, a C18 microcartridge (20 microL) was connected to the hydrodynamic inlet of the microchip. Using this device, the wash/elution step can be integrated on-line with the electrophoretic separation and detection on the microchip. Results show that the calibration curve of Cy5-BSA obtained from this integrated device has an R2 value greater than 0.99 and a minimum of quantitation at approximately 10 ng. This direct sampling method is another means of subfractionation, resulting in a relatively greater concentration factor than the average concentration of the whole fraction. Moreover, the electrical field-free bed ensures that the protein interaction will not be affected by the electric field during the wash/elution step.     

Entropic recoil separation of long DNA molecules

A novel technique that can rapidly separate long-strand polymers according to length is presented. The separation mechanism is mediated by a confinement-induced entropic force at the abrupt interface between regions of vastly different configuration entropy. To demonstrate this technique, DNA molecules were partially inserted into a dense array of nanopillars (an entropically unfavorable region) using a pulsed electric field and allowed to relax to their natural state by removal of the field. Molecules of dissimilar lengths (T2 and T7 coliphage DNA) were inserted into this region in such a way that shorter molecules were fully inserted in this region, while longer molecules remained partially across the interface. The longer T2 molecules were observed to recoil entirely out of the pillar array, leaving the shorter T7 molecules inserted, and effecting separation of the two species in a single step. To show how this method can be used for separation of unknown samples, the inserting electric field was pulsed for progressively longer times, allowing passage of progressively longer molecules and producing the equivalent of a conventional electropherogram. The effects limiting resolution in this device are discussed, and the expected separating power of a multistage device is reported. The extracted resolution and running separation time compare favorably with current conventional separation techniques.     

Insights into the power law relationships that describe mass deposition rates during electrospinning

This work explores how in electrospinning, mass deposition rate and electric current relate to applied voltage and electrode separation, factors giving a range of applied electric fields. Mass deposition rate was measured by quantifying the rate of dry fibre deposited over time. Electric current was measured using a current feedback from the high voltage supply. The deposition of fibre was observed to occur at a constant rate for deposition times of up to 30 min. Both the mass deposition rate and electric current were found to vary with the applied voltage according to a power law. The relationship between the electric current and mass deposition rate was found to be linear for all combinations of applied voltage and electrode separation. This means that for all combinations of applied voltage and electrode separation, hence for all applied electric field conditions, there is a constant charge density of 96.1 C/kg for poly(vinyl alcohol).     

Large-Area Arrays of Polymer-Tethered Gold Nanorods with Controllable Orientation and Their Application in Nano-Floating-Gate Memory Devices

In this work, it is reported that large-area (centimeter-scale) arrays of non-close-packed polystyrene-tethered gold nanorod (AuNR@PS) can be prepared through a liquid–liquid interfacial assembly method. Most importantly, the orientation of AuNRs in the arrays can be controlled by changing the intensity and direction of electric field applied in the solvent annealing process. The interparticle distance of AuNR can be tuned by varying the length of polymer ligands. Moreover, the AuNR@PS with short PS ligand are favorited to form orientated arrays with the assistance of electric field, while long PS ligands make the orientation of AuNRs difficult. The orientated AuNR@PS arrays are employed as the nano-floating gate of field-effect transistor memory device. Tunable charge trapping and retention characteristics in the device can be realized by electrical pulse with visible light illumination. The memory device with orientated AuNR@PS array required less illumination time (1 s) at the same onset voltage in programming operation, compared to the control device with disordered AuNR@PS array (illumination time: 3 s). Moreover, the orientated AuNR@PS array-based memory device can maintain the stored data for more than 9000 s, and exhibits stable endurance characteristic without significant degradation in 50 programming/reading/erasing/reading cycles.     

Discrete element numerical simulation of fly ash triboelectrostatic separation in a nonlinear electric field

Fly ash is solid waste produced by thermal power generation, and its carbon content is a key factor affecting its recycling. Due to the large difference in fly ash quality and insufficient tribocharging, the parallel plate electric field with constant electric field strength cannot meet the practical needs of efficient decarbonization of fly ash particles with wide charge range or small charge to mass ratio (CMR). Therefore, a nonlinear electric field structure is proposed. The separation process of fly ash particles in the nonlinear electric field is explored through the establishment of geometric model and the application of CFD-DEM coupled calculation method, and the main influencing factors of fly ash electrostatic dry separation are studied. The results show that the nonlinear electric field structure is feasible to achieve high efficiency decarbonization of fly ash. With the increase of air flow velocity, the loss on ignition of positive electrode first increases and then decreases. The loss on ignition (LOI) of positive electrode products is directly proportional to the voltage and the CMR of the input, but inversely proportional to the feed quantity. Air flow velocity of 20 m/s, voltage of 30 kV, charge-mass ratio of 1.1–1.2 nC/g and feed quantity of 5000/s are suitable conditions for efficient decarbonization of fly ash. Compared with parallel plates, hyperbolic nonlinear electric field has higher decarbonization efficiency and lower energy consumption in experiment.     

Use of a mixed-mode packing and voltage tuning for peptide mixture separation in pressurized capillary electrochromatography with an ion trap storage/reflectron time-of-flight mass spectrometer detector.

A mixed-mode (reversed-phase/anion-exchange) stationary phase has been used as the capillary column packing for investigation of the separation of peptide mixtures in pressurized capillary electrochromatography (pCEC). This stationary phase contains both octadecylsilanes and dialkylamines. The amine groups of the stationary phase determine the charge density on the surface of the packing and can produce a strong and constant electroosmotic flow (EOF) at low pH. A comparison was made in terms of the capability of separating tryptic digests between the mixed-mode phase and C18 reversed phase. In addition, the constant EOF enabled the tuning of the retention and the selectivity of the separation by adjusting the mobile phase pH from 2 to 5. Furthermore, the magnitude and the polarity of the electric voltage were demonstrated to greatly influence the elution profiles of the peptides in pCEC. An ion trap storage/reflectron time-of-flight mass spectrometer was used as an on-line detector in these experiments due to its ability to provide rapid and accurate mass detection of the sample components eluting from the separation column.     

ITO平面微电极上P(VDF-TrFE)薄膜的制备及性能表征

在ITO平面微电极上设计了P(VDF-TrFE)纳米级薄膜,以构建仅电场作用的模式,并避免其他因素的干扰.通过旋涂法制备了不同厚度的P(VDF-TrFE)薄膜,当厚度达到470 nm时,薄膜具有足够的致密度和绝缘性,能隔绝电极上的微电流和可能产生的电化学产物.由于电极上覆有薄膜,在生物电刺激的应用中,还可消除电极材料不...     

抗强干扰快速测量真空规管及仪器的研究

托卡马克装置中存在强磁场、强电场,放电过程中真空度变化迅速。为满足这种特殊环境的真空测量要求,开展了抗强干扰快速测量真空规及仪器的研究。快规使用平板电极结构,在普通热阴极电离规管的基础上增加了调制极;仪器电路除完成稳发射、为加速极供电、离子流放大等基本功能外,它可以提供更大的灯丝电流,为调制极提供周期性脉冲电压,离子流和电子流放大器、调制/解调电路和锁相环电路相互配合,确保快速准确测量。仪器的主控制单元以微处理器为核心,可以设定系统运行的相关参数,离子流、发射电流、灯丝电流可供主控单元采集,以配合完成自动调整和相应控制。     

Detection of surface antigens on living cells through incorporation of immunorecognition into the distinct positioning of cells with positive and negative dielectrophoresis

Rapid determination of surface antigens on cells is possible by immobilization of cells accumulated by positive dielectrophoresis (p-DEP) via effective surface immunoreactions and removal of unbound cells by negative DEP (n-DEP). The DEP device for cell manipulation comprises a microfluidic channel with an upper indium tin oxide (ITO) electrode and a lower ITO microband array electrode (band electrode) modified with an antibody. Cells with the surface antigen introduced into the channel immediately accumulated on the surface of the band electrode during p-DEP generated by the application of ac voltage between the ITO electrode and the band electrode to immobilize by the specific antibody. The removal of accumulated cells to the gap region during n-DEP was used for rapid estimation of the residual cells with a specific surface antigen. We demonstrate here that human promyelocytic leukemia cells with the surface antigen CD33 can be captured on a band electrode modified with anti-CD33 antibody. The time required for the determination of the surface antigen using this compelled accumulation of cells by p-DEP and the separation of unbound cells by n-DEP is decreased to 60 s compared to that required by a cell binding assay using microtiter plates (30 min). Furthermore, the present method for a novel cell binding assay does not require pretreatment such as target labeling or washing of unbound cells and thereby enhancing throughput in the clinic and in cytobiology studies.     

Characteristics of organic light emitting diodes with vanadium oxide solution-treated indium tin oxide

The effect of indium tin oxide (ITO) in organic light emitting diodes (OLEDs) treated by vanadium penoxide (V2O5) saturation solution was studied. The device with ITO treated by ultraviolet-ozone (UV-ozone) was fabricated in the same run for comparison. It was found that the V2O5 solution-treated devices have much higher current efficiency compared to the device with bare ITO and UV-ozone-treated ITO as the anode. The turn-on voltages were reduced by around 2.5 V, and the luminances were about 1.69 times greater than that of the conventional device at 16 V driving voltage. Series resistances were determined by the slope of the current-voltage curve and the ac impedance spectroscopy technique.     

Enhanced Electrostatics for Low-Voltage Operations in Nanocrystal Based Nanotube/Nanowire Memories

The metal nanocrystal (NC)/carbon nanotube (CNT) based nonvolatile memory has been proposed recently in comparison to the microfabricated Si channel and Si NCs in ultranarrow channel structure. The electrostatics of metal NC-CNT devices during memory operations differ significantly from the metal NC memory with planar silicon channel. In this paper, we present the theoretical analysis on the three-dimensional (3-D) electrostatics of the NC-CNT device during memory operations, to illustrate the experimentally observed large number of charge storage at low gate bias (5 V) despite a 100-nm-thick bottom-gate control dielectric. NCs are electrostatically more strongly coupled to the two-dimensional (2-D) gate electrode than to the one-dimensional (1-D) channel, even when the NCs are in much closer proximity to the 1-D channel, for efficient tunneling and low-voltage program operation. Under the retention condition, the NC-CNT devices have lower electric field across tunneling oxide than that in the case of a 2-D channel. This increasing electric field difference with respect to program versus retention operations indicates larger ratio between program and retention times. Together with the large number of electrons stored per NC, this enhanced electrostatics can be utilized either to reduce the operating voltage or to reduce statistical fluctuation of the information storage     

PLD grown ZnO-K3[Fe(CN)6] composite thin film for biosensing application

Zinc oxide-potassium ferricyanide (ZnO-KFCN) composite film was prepared on ITO coated corning glass using pulsed laser deposition (PLD). The composite film has proved to be a suitable platform for enzyme immobilization. The composite matrix exhibits the advantages of ZnO along with enhanced redox property due to the presence of a mediator in the matrix. Glucose oxidase (GOx) has been chosen as the model enzyme for studying the application of the present matrix to biosensing. The sensing response of the bio-electrode, GOx/ZnO-KFCN/ITO/glass, towards glucose was studied by electrochemical and photometric techniques. The bio-electrode exhibits good linearity and low value of Michaelis-Menten constant. Due to efficient biosensing in a mediator-less system the present bio-electrode should lead to a hand held integrated lab-on-chip device.