Bilinear temperature gradient focusing in a hybrid PDMS/glass microfluidic chip integrated with planar heaters for generating temperature gradients

Temperature gradient focusing (TGF) is a counterflow gradient focusing technique, which utilizes a temperature gradient across a microchannel or capillary to separate analytes. With an appropriate buffer, the temperature gradient creates a gradient in both the electric field and electrophoretic velocity. Combined with a bulk counter flow, ionic species concentrate at a unique point where the total velocity sums to zero and separate from each other. Scanning TGF uses varying bulk flow so that a large number of analytes that have large differences in electrophoretic mobility can be sequentially focused and passed by a single detection point. Up to now, scanning TGF examples have been performed using a linear temperature gradient which has limitations in improving peak capacity and resolution at the same time. In this work, we develop a bilinear temperature gradient along the separation channel that improves both peak capacity and separation resolution simultaneously. The temperature profile along the channel consists of a very sharp gradient used to preconcentrate the sample followed by a shallow gradient that increases separation resolution. A specialized design is developed for the heaters to achieve the bilinear profile using both analytical and numerical modeling. The heaters are integrated onto a hybrid PDMS/glass chip fabricated using conventional sputtering and soft-lithography techniques. Separation performance is characterized by separating several different dyes and amino acids that have close electrophoretic mobilities. Experiments show a dramatic improvement in peak capacity and resolution in comparison to the standard linear temperature gradient.     

Simultaneous concentration and separation of enantiomers with chiral temperature gradient focusing

A new technique is demonstrated for the simultaneous concentration and high-resolution separation of chiral compounds. With temperature gradient focusing, a combination of a temperature gradient, an applied electric field, and a buffer with a temperature-dependent ionic strength is used to cause analytes to move to equilibrium, zero-velocity points along a microchannel or capillary. Different analytes are thus separated spatially and concentrated in a manner that resembles isoelectric focusing but that is applicable to a greater variety of analytes including small chiral drug molecules. Chiral separations are accomplished by the addition of a chiral selector, which causes the different enantiomers of an analyte to focus at different positions along a microchannel or capillary. This new technique is demonstrated to provide high performance in a number of areas desirable for chiral separations including rapid separation optimization and method development, facile reversal of peak order (desirable for analysis of trace enantiomeric impurities), and high resolving power (comparable to capillary electrophoresis) in combination with greater than 1000-fold concentration enhancement enabling improved detection limits. In addition, chiral temperature gradient focusing allows for real-time monitoring of the interaction of chiral analyte molecules with chiral selectors that could potentially be applied to the study of other molecular interactions. Finally, unlike CE, which requires long channels or capillaries for high-resolution separations, separations of equivalent resolution can be performed with TGF in very short microchannels (mm); thus, TGF is inherently much more suited to miniaturization and integration into lab-on-a-chip-devices.     

Temperature gradient focusing with field-amplified continuous sample injection for dual-stage analyte enrichment and separation

We describe the serial combination of temperature gradient focusing (TGF) and field-amplified continuous sample injection (FACSI) for improved analyte enrichment and electrophoretic separation. TGF is a counterflow equilibrium gradient method for the simultaneous concentration and separation of analytes. When TGF is implemented with a low conductivity sample buffer and a (relatively) high conductivity separation buffer, a form of sample enrichment similar to field-amplified sample stacking (FASS) or field-amplified sample injection (FASI) is achieved in addition to the normal TGF sample enrichment. FACSI-TGF differs from FASI in two important respects: continuous sample injection, versus a discrete injection, is utilized; because of the counterflow employed for TGF, the stacking interface exists in a pseudo-stationary region outside of the separation column. Notably, analyte concentration enrichment factors greater than the ratio of separation and sample conductivities (gamma) were achieved in this method. For gamma=6.1, the concentration factor for one model analyte (Oregon Green 488) was found to be 36-fold higher with FACSI-TGF as compared to TGF without FACSI. A separation of five fluorescently labeled amino acids is also demonstrated with the technique, yielding an average enrichment of greater than 1000-fold.     

聚焦型热声转换装置的特性

设计了一种新型的聚焦型热声转换装置,主要由加热层、绝缘层和储热层三层结构构成。当加热层输入交变电信号时,由于焦耳热效应及各层材料的热力学特性,其表面附近区域内气体压力产生交变的振荡,加热层的凹球表面会使产生的声波在某一区域聚焦。通过对装置进行数值模拟与实验研究,得出声波聚焦区域及聚焦点声压强度随声波频率的变化情况。装置可作为一种新型的声学聚焦换能装置,工作频率涉及可听及超声频域,无共振,无运动部件。这一研究具有一定的应用价值。     

Reexamination of Dependence of Plate Number on SDS Concentration in Micellar Electrokinetic Chromatography

Chromatograms of hydrophilic, hydrophobic, and intermediate-polarity analytes were developed in 50-μm capillaries by micellar electrokinetic chromatography at field strengths less than 31 kV/m. The analytes were solubilized by phosphate/borate buffers containing 15, 50, and 100 mM sodium dodecyl sulfate (SDS). The plate numbers N of the analytes, as well as those of the electroosmotic flow and micellar markers, were compared to predictions of N estimated by a simple model based on longitudinal diffusion and plug size. Good to fair agreement between theory and experiment was obtained for the hydrophilic and intermediate-polarity analytes in all buffers over the entire field strength range. Good agreement between theory and experiment was obtained for the hydrophobic analyte and micellar marker in all buffers at low field strengths; however, these compounds were subject to dispersion at higher field strengths by what appears to be Joule heating. The magnitudes of other, closely related Joule heating losses are quantified here using temperature profile measurements by Morris and co-workers and Taylor dispersion calculations. In contrast to the commonly reported increase of N with media concentration, the Ns of the hydrophilic and intermediate-polarity analytes were found to be essentially independent of SDS concentration over the investigated SDS range, and the Ns of the hydrophobic species were found to be independent of SDS concentration until (what appears to be) Joule heating became significant. These results were compared to those of Sepaniak and Cole. A critique of some previous studies of N vs SDS concentration is presented, in which quantitative explanations for some dispersions are offered as alternatives to surfactant concentration effects.     

Scanning temperature gradient focusing

Temperature gradient focusing (TGF) is a recently developed technique for the simultaneous concentration and electrophoretic separation of ionic analytes in microfluidic channels. One drawback to TGF as it has previously been described is the limited peak capacity; only a small number of analyte peaks (approximately 2-3) can be simultaneously focused and separated. In this paper, we report on a variation of the TGF method whereby the bulk flow rate is varied over time so that a large number of analytes can be sequentially focused, moved past a fixed detection point, and flushed to waste. In addition to improved peak capacity, the detection limits of the scanning TGF method can be adjusted on-the-fly, as needed for different samples. Finally, scanning TGF provides a technique by which high-resolution, high-peak-capacity electrophoretic separations can be performed in simple, straight, and short microfluidic channels.     

Poly(ethylene glycol)-functionalized devices for electric field gradient focusing

Electric field gradient focusing (EFGF) is an equilibrium gradient focusing technique that depends on an electric field gradient and a hydrodynamic counterflow to focus, concentrate, and separate charged analytes. In this work, EFGF devices were fabricated from poly(ethylene glycol) (PEG)-functionalized acrylic plastic. The separation channel was formed in an ionically conductive and protein-resistant PEG-functionalized hydrogel, which was cast in a changing cross-sectional cavity in the plastic device. A linear electric field gradient was obtained by applying a voltage lengthwise across the shaped hydrogel. Standard proteins were used as analytes to demonstrate the performance of these EFGF devices. With an increase in counterflow rate or decrease in applied voltage, analyte bands broadened, but resolution increased in agreement with theory. To reduce analyte band dispersion and improve focusing performance, a protein-compatible PEG-functionalized monolith was incorporated in the EFGF channel. Compared with focusing in an open channel, protein bands in the monolith-filled EFGF channel were significantly narrower.     

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.     

Electrokinetically based approach for single-nucleotide polymorphism discrimination using a microfluidic device

In this work, we describe and implement an electrokinetic approach for single-nucleotide polymorphism (SNP) discrimination using a PDMS/glass-based microfluidic chip. The technique takes advantage of precise control of the coupled thermal (Joule heating), shear (electroosmosis), and electrical (electrophoresis) energies present at an array of probes afforded by the application of external electrical potentials. Temperature controllers and embedded thermal devices are not required. The chips can be easily and inexpensively fabricated using standard microarray printing methods combined with soft-lithography patterned PDMS fluidics, making these systems easily adaptable to applications using higher density arrays. Extensive numerical simulations of the coupled flow and thermal properties and microscale thermometry experiments are described and used to characterize the in-channel conditions. It was found that optimal conditions for SNP detection occur at a lower temperature on-chip than for typical microarray experiments, thereby revealing the importance of the electrical and shear forces to the overall process. To demonstrate the clinical utility of the technique, the detection of single-base pair mutations in the survival motor neuron gene, associated with the childhood disease spinal muscular atrophy, is conducted.     

Time-weighted average sampling with solid-phase microextraction device: implications for enhanced personal exposure monitoring to airborne pollutants

The solid-phase microextraction (SPME) device is used as a time-weighted average (TWA) sampler for gas-phase analytes by retracting the coated fiber a known distance into its needle housing during the sampling period. Unlike in conventional spot sampling with SPME, the TWA sampling approach does not allow the analytes to reach equilibrium with the fiber coating, but rather they diffuse through the opening in the needle to the location of the sorbent. The amount of analytes accumulated over time gives the measurement of the average concentration to which the device was exposed to. Depending on the sorbent used as the sink, TWA sampling for various analytes is possible with times ranging from 15 min to at least 16 h. Both the poly(dimethylsiloxane) (PDMS) and poly(dimethylsiloxane)/divinylbenzene (PDMS/DVB) fiber coating phases were tested, with the latter employing on-fiber derivatization for reactive carbonyl compounds, e.g., formaldehyde. Described herein are the theoretical and practical considerations for using the SPME device as a TWA sampler.     

Simultaneous concentration and separation of coumarins using a molecular micelle in micellar affinity gradient focusing

We report the use of a molecular micelle for the simultaneous separation and concentration of neutral and hydrophobic analytes using micellar affinity gradient focusing (MAGF). The technique, MAGF, combines the favorable features of micellar electrokinetic chromatography and temperature gradient focusing. The focusing of neutral coumarin analytes was accomplished by the use the molecular micelle, poly(sodium undecenyl sulfate) (poly-SUS). Concentration enhancements of 10-25-fold/min were achieved for focusing of the coumarin dyes. The effect of varying the temperature gradient on the resolution of two of the coumarin dyes was also investigated, demonstrating that improved resolution could be achieved by reducing the steepness of the temperature gradient. In addition, with scanning-mode MAGF (in which the peaks are sequentially scanned past a fixed detection point by varying the buffer counterflow velocity), the use of poly-SUS was shown to produce repeatable and quantitative analyte peaks, making quantitative separations possible with the MAGF technique. Finally, it was shown that peak areas could be increased in scanning MAGF by reducing the scan rate so that the sensitivity of the method can be adjusted as needed.     

Potential of New Superconductivity Produced by Electrostatic Field and Diffusion Current in Semiconductor

This paper proposes a unique device that uses a constant-current source to demonstrate a new superconductivity concept. This device is considerably different from conventional superconductors, and is based on the idea that the voltage that produces Joule heating can be in proportion to the voltage derived from the line integral of an internal electric field employing a condenser when the current is supplied to a doped semiconductor by a current source. In this case, the charge-carrier concentration is spatially nonuniform. The concentration gradient of the current source leads to diffusion of the charge carriers, and the motion of these carriers contributes to the current density. An electric field is not needed to move the charge carriers, because they move by diffusion and not by drift. Because the voltage associated with Joule heating is proportional to the voltage of the internal electric field, the total voltage in the semiconductor is zero; however, the current carried from the current source prevents the total current from being zero. We show that this property of the device results in a superconducting state arising from the diffusion-current state. In our theoretical analysis, we demonstrate that two electrons in the device form a pair and that Bose?CEinstein condensation of all pairs is produced. From this result, we derive the existence of a superconducting current without voltage. Furthermore, we have developed an experimental setup and confirmed zero electric resistance and energy emission from the semiconductor. Therefore, we conclude that a new type of superconductivity had been achieved. This phenomenon was observed multiple times and thus can be reproduced.     

Calculation of the temperature gradient in the melt zone during the electric-transport liquid phase epitaxy of silicon carbide based solid solutions

The temperature distribution in a growth cell and the temperature gradient in the melt zone during the electric-field liquid phase epitaxy of silicon carbide based solid solutions (ytterbium-gallium, ytterbium-aluminum) were calculated with an allowance for the growth cell geometry. The analysis was based on a solution of the stationary thermal conductivity equations in all five regions of the standard growth cell. The solution was obtained taking into account the following factors: (i) Joule’s heating; (ii) Peltier’s heating (cooling) at the electrode-source (substrate)-melt zone interfaces; (iii) contact heat liberated at the electrode-source (substrate) interface; (iv) dissolution heat; and (v) crystallization heat. Expressions for the temperature gradient ?T in the melt zone as a function of the current density and the dimensions of regions in the growth cell are obtained.     

Localized temperature and chemical reaction control in nanoscale space by nanowire array

We introduce a novel method for chemical reaction control with nanoscale spatial resolution based on localized heating by using a well-aligned nanowire array. Numerical and experimental analysis shows that each individual nanowire could be selectively and rapidly Joule heated for local and ultrafast temperature modulation in nanoscale space (e.g., maximum temperature gradient 2.2 K/nm at the nanowire edge; heating/cooling time < 2 μs). By taking advantage of this capability, several nanoscale chemical reactions such as polymer decomposition/cross-linking and direct and localized hydrothermal synthesis of metal oxide nanowires were demonstrated.     

Electric field gradient focusing of proteins based on shaped ionically conductive acrylic polymer

Electric field gradient focusing (EFGF) is a separation technique that uses an electric field gradient and an opposing hydrodynamic flow to separate and concentrate charged analytes. This work describes miniaturized EFGF devices that are used for protein analysis. These devices employ a unique ionically conductive polymer that enables the required electric field gradient to be established. This polymer has good protein compatibility and allows the transport of small buffer ions while retaining large analytes such as proteins. With the use of an EFGF device, green fluorescent protein was concentrated 10 000-fold and the separation of a protein mixture was demonstrated. The development of these ionically conductive polymer-based devices represents a step toward making EFGF a useful analytical tool for proteomics investigations.     

Helical sorbent for fast sorption and desorption in solid-phase microextraction-gas chromatographic analysis

A new technique for solid-phase microextraction (SPME) of analytes using a helical solid sorbent followed by thermal desorption into a gas chromatographic injector is reported. The main factors that affect the mass transport of analytes in sorption and thermal desorption process using a poly(dimethylsiloxane) (PDMS) helical sorbent are described. The sorption and thermal desorption were achieved in a few seconds, being very close by the theoretical prediction. Both processes were very fast by the reduction of the thickness of boundary layer between sorbent and gaseous sample as a result of a turbulent rotational flow of the headspace air on the surface of sorbent, which is generated by the helical configuration of the sorbent. The thermal desorption was also reduced by improving heat transfer into a thin boundary layer and by increasing the temperature of the heat transporter (carrier gas). The sorption and desorption with PDMS helical sorbent were compared with those of the PDMS silica rod. The extraction time was as much as 15 times faster with the PDMS helical sorbent than with the PDMS silica rod. The desorption with the PDMS helical sorbent was very fast, giving narrow peaks without tailing and a high efficiency of separation in comparison with PDMS silica rod.     

Experimental Determination of the Kapitza Conductance Across Thin Film Bolometer Interfaces

Thin film bolometers are widely used in low temperature experiments. They are always connected in an electrical measuring circuit which involves a current, so an unavoidable Joule heating effect is generated. As a result the detector temperature can be appreciably higher than that of the cell, especially in the low temperature range. It is therefore important to be able to determine the bolometer net heat loss in order to evaluate the temperature gradient which could take place. We propose a simple analysis that provides an experimental determination of the values of the heat conductance across the interfaces of a thin film bolometer deposited onto a substrate. In this way, both temperatures of the bolometer and of the part of the helium film which covers it, as well as the respective heat flows through the film and the substrate are easily obtained. This method could prove useful in the future for a better understanding of surface exchanges and surface characterization.     

Joule Heating Activation of Template-Mediated Carbon Fibers with Significantly Enhanced Capacitive Performance

Carbon fibers (CFs) have been innovated in the application of fiber-shaped and wearable energy storage devices recently. However, time- and energy-consuming activation process under harsh conditions is generally required to modify the surface structure of the CF. Herein, a fast and effective Joule heating activation method is proposed with the nickel–cobalt layered double hydroxide-derived nanometal particles as the template. The resultant Joule heating-activated CF (JACF) demonstrates excellent capacitive performance with an electrode capacitance of 268 F g⁻¹. After assembling into a solid-state fiber-shaped supercapacitor (FSC), the device shows both enhanced specific capacitance and energy/power densities, as well as the long cycling stability of over 5000 cycles. The Joule heating method reported in this work is time and energy saving for CF activation compared with the traditional chemical etching or high-temperature annealing processes, and it is promising for the large-scale production of high-performance fiber-shaped and wearable energy devices.     

Effect of voltage on the conduction of a high-density polyethylene/carbon black composite at the NTC region

The time dependences of current and Joule heating under various voltages are studied to high-density polyethylene (HDPE) filled with carbon black (CB) of 0.082 in volume at an ambient temperature of 158 °C located at the region of negative temperature coefficient (NTC) of resistivity. The composite shows either a transition from the intrinsic to the steady conductions or an irreversible electrical breakdown depending on the applied voltage. The intrinsic conduction in the absence of Joule heating is explained combining two electron tunneling processes while the electric field induced NTC effect is related to the production of additional conducting pathways resulted from cold-emission.     

Counterflow rejection of adsorbing proteins for characterization of biomolecular interactions by temperature gradient focusing

A new technique is described for the analysis of small molecules in samples containing serum proteins and for the measurement of the binding of small molecules to serum proteins. The new technique is based on temperature gradient focusing (TGF) and takes advantage of the counterflow used with TGF to exclude serum proteins from the analysis channel while small molecules are focused for detection. The technique is demonstrated for the measurement of the binding constant between a small molecule and serum albumin using both a direct measurement of the free fraction of the small molecule as well as using a competitive binding assay.