Electrokinetic translocation of a deformable nanoparticle controlled by field effect in nanopores

Nanopores have become a popular single-molecule manipulation and detection technology. In this paper, we have constructed a continuum model of the nanopore; the arbitrary Lagrangian-Eulerian (ALE) method is used to describe the motion of particles and fluid. The mathematical model couples the stress-strain equation for the dynamics of a deformable particle, the Poisson equation for the electric field, the Navier-Stokes equations for the flow field, and the Nernst-Planck equations for ionic transport. Based on the model, the mechanism of field-effect regulation of particles passing through a nanopore is investigated. The results show that the transport of particles which is controlled by the field effect depends on the electroosmotic flow (EOF) generated by the gate electrode in the nanopore and the electrostatic interaction between the nanopore and particles. That also explains the asymmetry of particle transport velocity in the nanopore with a gate electrode. When the gate potential is negative, or the gate electrode length is small, the maximum deformation of the particles is increased. The field-effect regulation in the nanopore provides an active and compatible method for nanopore detection, and provides a convenient method for the active control of the particle deformation in the nanopore.     

Numerical modeling of surface reaction kinetics in electrokinetically actuated microfluidic devices

We outline a comprehensive numerical procedure for modeling of species transport and surface reaction kinetics in electrokinetically actuated microfluidic devices of rectangular cross section. Our results confirm the findings of previous simplified approaches that a concentration wave is created for sufficiently long microreactors. An analytical solution, developed for the wave propagation speed, shows that, when normalizing with the fluid mean velocity, it becomes a function of three parameters comprising the channel aspect ratio, the relative adsorption capacity, and the kinetic equilibrium constant. Our studies also reveal that the reactor geometry idealized as a slit, instead of a rectangular shape, gives rise to the underestimation of the saturation time. The extent of this underestimation increases by increasing the Damkohler number or decreasing the dimensionless Debye–Hückel parameter. Moreover, increasing the values of the Damkohler number, the dimensionless Debye–Hückel parameter, the relative adsorption capacity, and the velocity scale ratio results in lower saturation times.     

Numerical analysis of the thermal effect on electroosmotic flow and electrokinetic mass transport in microchannels

Joule heating is present in electrokinetically driven flow and mass transport in microfluidic systems. Nowadays, there is a trend of replacing costly glass-based microfluidic systems by the disposable, cheap polymer-based microfluidic systems. Due to poor thermal conductivity of polymer materials, the thermal management of the polymer-based microfluidic systems may become a problem. In this study, numerical analysis is presented for transient temperature development due to Joule heating and its effect on the electroosmotic flow (EOF) and mass species transport in microchannels. The proposed model includes the coupling Poisson-Boltzmann (P-B) equation, the modified Navier-Stokes (N-S) equations, the conjugate energy equation, and the mass species transport equation. The results show that the time development for both the electroosmotic flow field and the Joule heating induced temperature field are less than 1 s. The Joule heating induced temperature field is strongly dependent on channel size, electrolyte concentration, and applied electric field strength. The simulations reveal that the presence of the Joule heating can result in significantly different characteristics of the electroosmotic flow and electrokinetic mass transport in microchannels.     

Continuous and precise particle separation by electroosmotic flow control in microfluidic devices

A new scheme has been described for continuous particle separation using EOF in microfluidic devices. We have previously reported a method for particle separation, called "pinched flow fractionation (PFF)", in which size-dependent and continuous particle separation can be achieved by introducing pressure-driven flows with and without particles into a pinched microchannel. In this study, EOF was employed to transport fluid flows inside a microchannel. By controlling the applied voltage to electrodes inserted in each inlet/outlet port, the flow rates from both inlets, and flow rates distributed to each outlet could be accurately tuned, thus enabling more effective separation compared to the pressure-driven scheme. In the experiment, the particle behaviors were compared between EOF and pressure-driven flow schemes. In addition, micrometer- and submicrometer-sized particles were accurately separated and individually collected using a microchannel with multiple outlet branch channels, demonstrating the high efficiency of the presented scheme.     

离子交换-原子捕集-火焰原子吸收光谱法测定水中的超痕量镍

在文中所述条件下,镍的原子捕集灵敏度较常规法高320倍,即在离子交换法浓缩仅40倍的情况下,可使镍的测定灵敏度提高12800倍,从而成功实现了ppt级超痕量镍的测定。     

氢化物发生-冷阱捕获-色谱分离-原子吸收法测定天然水中砷的形态

研究建立了氢化物发生－冷阱捕获－色谱分离－原子吸收方法测定天然水中四种主要砷形态。测试系统自行组装，色谱住填料采用ChromosorbGAW－DMCS（粒径0．3～0．45mm），其上涂布3％OV－101。方法的检出限以砷计分别为：As（V）0．51ng，As（Ⅲ）0．43ng，MMA0．38ng，DMA067ng；12ng砷标准偏差As（Ⅴ）4．21％，As（Ⅲ）3．56％，MMA3．23％，DMA5．46％。在0～50ng砷量范围标准曲线线性关系良好。该法适用性广，已用于湖水、河水、海水和地下水等不同水样神形态测定，加标回收率93．5％～104．9％。给出了上述水样四种砷形态的分析结果。     

Trapping of fatty acid allyl radicals generated in lipoxygenase reactions in biological fluids by nitroxyl radical

Nitroxyl radicals can trap fatty acid allyl radicals on ferric‐lipoxygenases at lower oxygen content, which are an intermediate in the lipoxygenase reaction. In the present study, we examined whether nitroxyl radical‐trapping of fatty acid allyl radicals on the enzyme proceeds in biological fluids with abundant antioxidants. The fatty acid allyl radical–nitroxyl radical adducts were quantified by HPLC with electrochemical detection (HPLC‐ECD); the adducts in eluate degraded into nitroxyl radical by passing through heating coil at 100°C, and then nitroxyl radical was detected by electrochemical detector. Soybean 15‐lipoxygenase and nitroxyl radical (3‐carbamoyl‐2,2,5,5‐tetramethyl‐3‐pyrroline‐N‐oxyl, CmΔP) were mixed with rat serum prepared from fresh venous blood, and the solution was stood at 37°C for 1 h. One volume of the solution was mixed with 5 vols of cold acetonitrile. After centrifugation, the supernatant was subjected to HPLC‐ECD. Arachidonate allyl radical–CmΔP adducts as well as linoleate allyl radical–CmΔP adducts were detected in the solution, and the content of these adducts remarkably increased in the presence of phospholipase A₂. It is proved for the first time that nitroxyl radical traps fatty acid allyl radicals generated in the lipoxygenase reaction in biological fluid without competition from endogenous antioxidants. Copyright © 2010 John Wiley & Sons, Ltd.     

Chemodosimetric Mechanistic Insight through Trapping Intermediate for Selective Detection of Picric Acid in Aqueous Medium by a Dinuclear CdII Complex

A fluorescent dinuclear cadmium(II) based discrete metal complex of composition [Cd^II₂L(μ-Cl)Cl₂]( 1 ) is used {HL=2,6-bis[2-(methylamino)ethyliminomethyl]-4-Ethylphenol} for the specific recognition of 2,4,6-trinitrophenol (picric acid; PA) via fluorescence quenching phenomenon among various nitroaromatic compounds through a chemodosimetric approach. It has been established that 1 is a chemodosimeter in pure water. We have successfully been able to isolate three compounds: chemodosimeter 1 ; an intermediate complex 2 of composition [Cd^II(LH₂)Cl₂](Picrate) and final association complex 3 of composition [NH₃(CH₂CH₂)NH₂CH₃](Picrate)₂. Compounds have been characterised by CHN elemental analyses, single crystal X-ray crystallography, PXRD, NMR and FTIR. Selective interaction of 1 with PA was evaluated by fluorescence, UV-Vis and life time measurements. Fluorescence quenching of 1 occurs definitely due to the formation of compound 3 via intermediate 2 involving partial decomplexation, hydrolysis and proton transfer phenomena in solution during the course of sensing. The quenching constant (K_sv), association constant (K_a) and detection limit (LOD) of the complex 1 for picric acid are ∼1.55×10⁵ M⁻¹, ∼1.8×10¹⁰ M⁻² and ∼0.47 μM (0.108 ppm), respectively. Mechanism of sensing is proposed and the very rare case of isolation and characterization of intermediate in picric acid sensing is discussed.     

导数-原子捕集-火焰原子吸收光谱法测定中草药中的微量铜

研究了在原有基础上对原子捕集装置的改进，并与导数原子吸收光谱法结合，使铜的测定灵敏度有较大提高。在10mV/min灵敏度档下，捕集时间为2min时，方法的检出限和特征浓度分别为0.52和0.85μg/L，分别较常规火焰原子吸收法改善1和2个数量级。利用该法成功测定了甘草、柴胡等10味中药中的微量铜，平均回收率为94.2%－104％。     

吹扫捕集采样-气相色谱法测定海水淡化过程产生的浓盐水中氯乙酸

提出了吹扫捕集-气相色谱法测定海水淡化过程产生的浓盐水中氯乙酸含量的方法。样品(25.00mL)在60℃下经硫酸-甲醇(1+9)溶液衍生化60min。取衍生化后的溶液20mL,用吹扫捕集进样,并用HP-5石英毛细管色谱柱分离,电子捕获检测器检测。二氯乙酸和三氯乙酸的质量浓度在120.0μg·L-1以内与其峰面积均呈线性关系,检出限(3S/N)分别为0.15,0.84μg·L-1。以浓盐水样品为基体进行加标回收试验,所得回收率在87.2%~112%之间,测定值的相对标准偏差(n=6)在1.1%~2.7%之间。     

Hydrodynamic Instabilities Driven by Acid-base Neutralization Reaction in Immiscible System

The hydrodynamic instabilities driven by an acid-base neutralization reaction, in contact along a plane interface, placed in a Hele-Shaw cell under the gravitational field are reported.The system consists of the heavier aqueous tetramethyle-ammonium hydroxide below the lighter layer of organic phase with propionic acid as reacting specie. The effect of chemical composition on hydrodynamic instabilities during interfacial mass transfer accompanied by a neutralization reaction is investigated. Depending on the initial concentration of the reacting species, Marangoni convection in the form of roll cells or trains of waves is observed. Mach-Zehnder interferometer is used to measure the change in base concentration at the time of instability formation. The results show that the instabilities resulted from the convection flow are more efficient to the mechanism of mass transfer and can drastically alter pattern formation in the system.     

Improved Conductivity Induced by Photodesorption in SnO2 Thin Films Grown by a Sol-Gel Dip Coating Technique

Thin films of undoped and Sb-doped SnO2 have been prepared by a sol-gel dip-coating technique. For the high doping level (2–3 mol% Sb) n-type degenerate conduction is expected, however, measurements of resistance as a function of temperature show that doped samples exhibit strong electron trapping, with capture levels at 39 and 81 meV. Heating in a vacuum and irradiation with UV monochromatic light (305 nm) improves the electrical characteristics, decreasing the carrier capture at low temperature. This suggests an oxygen related level, which can be eliminated by a photodesorption process. Absorption spectral dependence indicates an indirect bandgap transition with Eg 3.5 eV. Current-voltage characteristics indicate a thermionic emission mechanism through interfacial states.     

On‐line sample preconcentration by polarity switching in floating electrode‐integrated microchannel

In this study, we found that the polarity switching was effective to enrich and separate fluorescent analytes which have weakly‐dissociated groups in a floating platinum electrode (width, 50 µm; thickness, 2.5 µm)‐integrated straight‐channel in microchip electrophoresis (MCE). In the straight channel filled with an Alexa Flour 488 (AF488) solution, a sharp peak was observed after the polarity inversion with a 530‐fold enhancement of the sensitivity relative to the conventional MCE analysis. By using a fluorescent pH indicator, we verified that a sharp high‐pH zone was generated nearby the floating electrode and moved toward the anode with maintaining the high pH, which induced the sample enrichment like a dynamic pH junction mechanism. In the floating electrode‐embedded channel, the mixture of AF488‐labeled proteins was also well concentrated and separated within 100 s.     

Determination of Germanium and Tin and Inorganic Tin Species by Hydride Generation in Situ Trapping Flame Atomic Absorption Spectrometry

The analytical performances of a coupled hydride generation, integrated atom trap (HG-IAT) atomizer flame atomic absorption spectrometry (FAAS) system were evaluated for determination of Ge and Sn and inorganic tin species in environmental samples. Germanium and tin hydrides were atomized in air-acetylene flame-heated IAT. A new design of HG-IAT-FAAS hyphenated technique that would exceed the operational capabilities of existing arrangements (a water-cooled single silica tube, double-slotted quartz tube) permitting construction of an “integrated trap” was investigated. For the estimation of Sn(II) and Sn(IV) concentrations in samples, the difference between the analytical sensitivities of the absorbance singnals obtained for tin hydride without and with previous treatment of samples with L-cysteine could be used. The concentration of Sn(IV) was calculated by the difference between total Sn(tot) and Sn(II). An improvement in limit of detection was achieved compared with that obtained using any of the aforementioned atom trapping techniques separately. The concentration limits of detection were 25 and 8 ng mL^?1 for Ge and Sn, respectively. For a 2 min in situ preconcentration time, sensitivity enhancement, compared to FAAS, were 10 and 14 folds for Ge and Sn, respectively, using hydride generation atom trapping technique. The sensitivity can be further improved by increasing the collection time. The relative standard deviations (RSDs) are of the order of 5–10% for this hyphenated technique. The designs studied include slotted tube, single silica tube, and integrated atom trap water-cooled atom traps. The accuracy of this method was tested by analyses of BCS-CRM No.346 (Alloy), GBW 07302 (Stream Sediment), PACS-1 (Marine Sediment), NBS SRM 1633a (Coal Fly Ash), and NIST SRM 1643e (Trace Element in Water) certified reference materials. Agreement between analytical results and certified values for the test elements Ge and Sn (in the range of 1.7–91.0 µg g^?1) were in agreement with recommended values. The measured Ge and Sn contents in five referenced materials were in satisfactory agreement with the certified values. The hyphenated technique was applied for germanium and tin determination in coal fly ash, soil, sediment, garlic, sewage, and river water.     

方波伏安法评估聚苯胺膜修饰电极的电荷传输速度

研究导电聚合物的电荷传输机制与速度对于其在应用方面的研究极为重要［‘］．例如在电池的研究中，充放电的速度决定于导电聚合物的电荷传输速度；在电致变色器件的研究中，响应时间也是决定于导电聚合物在氧化还原态间的变化速度，即电荷传输速度．显然较低的电荷传输速度不利于器件的实用化，因此人们必须找到一种能够迅速准确评价电荷传输速度的方法，由于导电聚合物是一个特殊的体系，用一般的电化学方法对其电荷传输速度进行测定时常常遇到这样或那样的问题［‘j．一般说来，导电聚合物的氧化还原过程中均伴随着抗衡离子的运动．体系…     

An Unusual Phase Transition Driven by Vibrational Entropy Changes in a Hybrid Organic–Inorganic Perovskite

The driving forces for the phase transitions of ABX₃ hybrid organic–inorganic perovskites have been limited to the octahedral tilting, order–disorder, and displacement. Now, a complex structural phase transition has been explored in a HOIP, [CH₃NH₃][Mn(N₃)₃], based on structural characterizations and ab initio lattice dynamics calculations. This unusual first‐order phase transition between two ordered phases at about 265 K is primarily driven by changes in the collective atomic vibrations of the whole lattice, along with concurrent molecular displacements and an unusual octahedral tilting. A significant entropy difference (4.35 J K^?1 mol^?1) is observed between the low‐ and high‐temperature structures induced by such atomic vibrations, which plays a main role in driving the transition. This finding offers an alternative pathway for designing new ferroic phase transitions and related physical properties in HOIPs and other hybrid crystals.     

Determination of Total Selenium Content in Cereals and Bakery Products by Flow Injection Hydride Generation Graphite Furnace Atomic Absorption Spectrometry Applying in-situ Trapping on Iridium-Treated Graphite Platforms

A flow injection hydride generation graphite furnace atomic absorption spectrometric (FI-HG-GFAAS) method was applied to the determination of Se in Se-doped and undoped cereals and bakery products. For the purpose of doping, the soils used for the cultivation of the cereals were dosed with Se-doped foliar fertilizers. The samples were dissolved in a mixture of HNO₃ and H₂O₂ solutions using microwave-assisted digestion. The decomposition of H₂Se generated from the sample solutions and the trapping of elemental Se were performed at a temperature of 300°C on an Ir-pretreated integrated graphite platform of a transversally heated graphite atomizer (THGA). For release of the trapped Se within a fairly short atomization time (5s), an atomization temperature of 2200°C was observed to be optimal. The overall efficiency of hydride generation, transport and trapping was 86%.The upper limit of the linear dynamic range of calibration was 10µgL^–1, which corresponds to 0.5µgg^–1 for solid samples. Recovery of the samples spiked with Se^VI solutions was found to be 93±6% on average. The relative standard deviation of the determinations was less than 8%. The limit of detection was found to be 0.06µgL^–1, corresponding to 3ngg^–1 for solid samples. The accuracy of the method was verified with the use of IAEA-155 (whey powder) certified reference material. End-capped THGA tubes resulted in an extension of the linear calibration range compared to that of standard THGAs.The Se content in bakery products made of undoped cereals ranged from 7.7 to 68ngg^–1 (wet weight) in 18 samples, whereas the Se content of the corresponding cereals was found to be below 100ngg^–1 (wet weight). The Se level of cereals grown on soils treated with Se-doped fertilizers ranged from 128 to 1046ngg^–1 (wet weight), and it depended linearly on the Se concentration of the corresponding foliar fertilizer.     

Determination of Ascorbic Acid in the Presence of Uric Acid and Folic Acid by a Nanostructured Electrochemical Sensor Based on a TiO2 Nanoparticle Carbon Paste Electrode

A carbon paste electrode (CPE), modified with novel hydroquinone/TiO₂ nanoparticles, was designed and used for simultaneous determination of ascorbic acid (AA), uric acid (UA) and folic acid (FA). The magnitude of the peak current for modified TiO₂-nanoparticle CPE (MTNCPE) increased sharply in the presence of ascorbic acid and was proportional to its concentration. A dynamic range of 1.0–1400.0 μM, with the detection limit of 6.4 × 10^?7 M for AA, was obtained using the DPV technique (pH = 7.0). The prepared electrode was successfully applied for the determination of AA, UA, and FA in real samples.     

Improvement of the Detection Limit in Capillary Gas Chromatographic Trace Analysis of C2–C4 Hydrocarbons by Post Column Cryogenic Trapping

A method for the improvement of signal-to-noise ratios and hence detection limits in capillary gas chromatographic trace analysis of C₂–C₄ hydrocarbons was developed. It is based on the post column cryogenic trapping of the separated hydrocarbons using liquid nitrogen as coolant and its fast reinjection into the detector by rapid heating of the capillary tubing used for trapping. The improved signal-to-noise ratios were applied to decrease the sample volume and/or to lower the detection limit 10 to 27 times. The concentrations of these hydrocarbons in an urban air sample determined without and with post column cryogenic trapping were in good agreement but the precision was better when applying the cryogenic trapping mode.     

Nonlinear Optical Switching Driven by the Lone Pair Electron Effect in a Hybrid Compound: (1S, 4S)-(+)-2-Aza-5-oxabicyclo[2.2.1] heptane)2[SbCl5]

In order to take advantage of hybrid materials in their structural diversity and richness of their photoelectric properties, the research in polar materials has been extended to metal-organic compounds in recent years. The main synthesis strategy is to use the molecular dynamics. Here, we describe that the second harmonic generation (SHG) switching effect can be effectively achieved by combination of coordination distortion of metal ions with s² electrons and molecular dynamics. We synthesized a noncentrosymmetric complex, (1S,4S)-(+)-2-aza-5-oxabicyclo[2.2.1]heptane)₂[SbCl₅], and found that it undergoes an isomorphic phase transition at 382 K. Accompanying the phase transition, the SHG experiences a switching process, and its strength changes from 0.2 times of that of KDP to only noise level. Systematic characterization reveals that the significant change of coordination distortion of the Sb³⁺ and the change of thermal motion of organic cations lead to the SHG switching effect. This finding shows that the combination of lone pair electron effect of metal ion and molecular dynamics characteristics would be a feasible strategy in development of polar hybrid materials.