On‐chip pressure generation using a gel membrane fabricated outside of the microfluidic network

On‐chip generation of pressure gradients via electrokinetic means can offer several advantages to microfluidic assay design and operation in a variety of applications. In this article, we describe a simple approach to realizing this capability by employing a polyacrylamide‐based gel structure fabricated within a fluid reservoir located at the terminating end of a microchannel. Application of an electric field across this membrane has been shown to block a majority of the electroosmotic flow generated within the open duct yielding a high pressure at the channel–membrane junction. Experiments show the realization of higher pressure‐driven velocities in an electric field‐free separation channel integrated to the micropump with this design compared to other similar micropumps described in the literature. In addition, the noted velocity was found to be less sensitive to the extent of Debye layer overlap in the channel network, and therefore more impressive when working with background electrolytes having higher ionic strengths. With the current system, pressure‐driven velocities up to 3.6 mm/s were realized in a 300‐nm‐deep separation channel applying a maximum voltage of 3 kV at a channel terminal. To demonstrate the separative performance of our device, a nanofluidic pressure‐driven ion‐chromatographic analysis was subsequently implemented that relied on the slower migration of cationic analytes relative to the neutral and anionic ones in the separation channel likely due to their strong electrostatic interaction with the channel surface charges. A mixture of amino acids was thus separated with resolutions greater than those reported by our group for a similar analysis previously.     

Protein separation using free‐flow electrophoresis microchip etched in a single step

A one‐step etching method was developed to fabricate glass free‐flow electrophoresis microchips with a rectangle separation microchamber (42 mm‐long, 23 mm‐wide and 28 μm‐deep), in which two glass bridges (0.5 mm‐wide) were made simultaneously to prevent bubbles formed by electrolysis near the Pt electrode from entering the separation chamber. By microchip free‐flow zone electrophoresis, with 200 V voltage applied, the baseline separation of three FITC labeled proteins, ribonuclease B, myoglobin and β‐lactoglobulin, was achieved, with resolution over 1.78. Furthermore, with 2.5 mM Na₂SO₄ added into the electrode buffer to form higher electrical field strength across separation microchamber than electrode compartments, similar resolution of samples was achieved with the applied voltage decreased to 75 V, which could obviously decrease Joule heat during continuous separation. All these results demonstrate that the free‐flow electrophoresis microchip fabricated by one‐step etching method is suitable for the continuous separation of proteins, which might become an effective pre‐fractionation method for proteome study.     

Experimental investigation of the flow induced by artificial cilia

The fluid transport produced by rectangular shaped, magnetically actuated artificial cilia of 70 μm length and 20 μm width was determined by means of phase-locked Micro Particle Image Velocimetry (μPIV) measurements in a closed microfluidic chamber. The phase-averaged flow produced by the artificial cilia reached up to 130 μm s(-1) with an actuation cycle frequency of 10 Hz. Analysis of the measured flow data indicate that the present system is capable of achieving volume flow rates of V[combining dot above](cilia) = 14 ± 4 μl min(-1) in a micro channel of 0.5 × 5 mm(2) cross-sectional area when no back pressure is built up. This corresponds to an effective pressure gradient of 6 ± 1 Pa m(-1), which equals a pressure difference of 0.6 ± 0.1 mPa over a distance of 100 μm between two rows of cilia. These results were derived analytically from the measured velocity profile by treating the cilia as a thin boundary layer. While the cilia produce phase-averaged velocities of the order of O(10(2)μm s(-1)), time-resolved measurements showed that the flow field reverses two times during one actuation cycle inducing instantaneous velocities of up to approximately 2 mm s(-1). This shows that the flow field is dominated by fluid oscillations and flow rates are expected to increase if the beating motion of the cilia is further improved.     

Impact of Taylor‐Aris diffusivity on analyte and system zone dispersion in CZE assessed by computer simulation and experimental validation

Application of pressure‐driven laminar flow has an impact on zone and boundary dispersion in open tubular CE. The GENTRANS dynamic simulator for electrophoresis was extended with Taylor‐Aris diffusivity which accounts for dispersion due to the parabolic flow profile associated with pressure‐driven flow. Effective diffusivity of analyte and system zones as functions of the capillary diameter and the amount of flow in comparison to molecular diffusion alone were studied for configurations with concomitant action of imposed hydrodynamic flow and electroosmosis. For selected examples under realistic experimental conditions, simulation data are compared with those monitored experimentally using modular CE setups featuring both capacitively coupled contactless conductivity and UV absorbance detection along a 50 μm id fused‐silica capillary of 90 cm total length. The data presented indicate that inclusion of flow profile based Taylor‐Aris diffusivity provides realistic simulation data for analyte and system peaks, particularly those monitored in CE with conductivity detection.     

Boron‐Doped Diamond Dual‐Plate Deep‐Microtrench Device for Generator‐Collector Sulfide Sensing

A BDD‐BDD dual‐plate microtrench electrode with 6 μm inter‐electrode spacing is investigated using generator‐collector electrochemistry and shown to give microtrench depth‐dependent sulfide detection down to the μM levels. The effect of the microtrench depth is compared for a “shallow” 44 μm and a “deep” 180 μm microtrench and linked to the reduction of oxygen to hydrogen peroxide which interferes with sulfide redox cycling. With a deeper microtrench and a fixed collector potential at ?1.4 V vs. SCE, two distinct redox cycling potential domains are observed at 0.0 V vs. SCE (2‐electron) and at 1.1 V vs. SCE (6‐electron).     

Chiralcel OD-RH直接拆分五味子乙素对映体

采用反相手性色谱柱Chiralcel OD-RH(纤维素3,5二甲基苯基氨基甲酸酯涂敷在5 μm硅胶上)建立了五味子乙素对映体的反相高效液相色谱拆分方法。考察了流动相组成、柱温和流速对五味子乙素手性对映体拆分的影响。以甲醇-水(90∶10)为流动相,流速0.5 mL/min,柱温20 ℃,检测波长254 nm,在Chiralcel OD-RH手性柱上成功拆分了五味子乙素对映体,其中R-构型先出峰。用lnk对1/T作图得到的Van-t Hoff曲线具有良好线性,相关系数（r）均大于0.99,计算了对映体与固定相相互作用的焓变以及焓变差值和熵变差值等热力学参数。结果显示,五味子乙素对映体的拆分过程为焓控过程,即氢键、π-π及偶极-偶极等作用方式对对映体的拆分起重要作用。     

Monolithic Silica Columns for HPLC,Micro‐HPLC,and CEC

Two types of monolithic silica columns derivatized to form an ODS phase, one prepared in a fused silica capillary (SR‐FS) and the other prepared in a mold and clad with an engineering plastic (poly‐ether‐ether‐ketone) (SR‐PEEK), were evaluated. The column efficiency and pressure drop were compared with those of a column packed with 5‐μm ODS‐silica particles and of an ODS‐silica monolith prepared in a mold and wrapped with PTFE tubing (SR‐PTFE). SR‐FS gave a lower pressure drop than a column packed with 5‐μm particles by a factor of 20, and a plate height of 20 μm at a linear velocity below 1 mm/s. SR‐PEEK showed higher flow‐resistance than the other monolithic silica columns, but they still showed a minimum plate height of 8–10 μm and a lower pressure drop than popular commercial columns packed with 5‐μm particles. The evaluation of SR‐FS columns in a CEC mode showed much higher efficiency than in a pressure‐driven mode.     

Sodium silicate based sol–gel structures for generating pressure-driven flow in microfluidic channels

In this article, we report the design of a microchip based hydraulic pump that employs a sodium silicate derived sol–gel structure for generating pressure-driven flow within a microfluidic network. The reported sol–gel structure was fabricated in a chosen location of our device by selectively retaining sodium silicate solution within a sub-micrometer deep segment via capillary forces, and then providing the precursor material appropriate thermal treatment. It was shown that while the molecular weight cut-off for these membranes is at least an order of magnitude smaller than their photo-polymerized counterparts, their electrical conductance is significant. Moreover, unlike their polymeric counterparts these structures were found to be capable of blocking electroosmotic flow, thereby generating a pressure-gradient around their interface with an open microchannel upon application of an electric field across the microchannel–membrane junction. In this work, a fraction of the resulting hydrodynamic flow was successfully guided to an electric field-free analysis channel to implement a pressure-driven assay. Our experiments show that the pressure-driven velocity produced in the analysis channel of our device varied linearly with the voltage applied across the sol–gel membrane and was nearly independent of the cross-sectional dimensions of the membrane and the microfluidic channels. With our current design pressure-driven velocities up to 1.7 mm/s were generated for an applied voltage of 2 kV, which easily covers the range of flow speeds that can minimize the plate height in most microfluidic separations. Finally, the functionality of our device was demonstrated by implementing a reverse phase chromatographic separation in the analysis channel of our device using the pressure-driven flow generated on-chip.     

Separation of natural product using columns packed with Fused‐Core particles

Three HPLC columns packed with 3 μm, sub‐2 μm, and 2.7 μm Fused‐Core (superficially porous) particles were compared in separation performance using two natural product mixtures containing 15 structurally related components. The Ascentis Express^TM C18 column packed with Fused‐Core particles showed an 18% increase in column efficiency (theoretical plates), a 76% increase in plate number per meter, a 65% enhancement in separation speed and a 19% increase in back pressure compared to the Atlantis T3^TM C18 column packed with 3 μm particles. Column lot‐to‐lot variability for critical pairs in the natural product mixture was observed with both columns, with the Atlantis T3 column exhibiting a higher degree of variability. The Ascentis Express column was also compared with the Acquity^TM BEH column packed with sub‐2 μm particles. Although the peak efficiencies obtained by the Ascentis Express column were only about 74% of those obtained by the Acquity BEH column, the 50% lower back pressure and comparable separation speed allowed high‐efficiency and high‐speed separation to be performed using conventional HPLC instrumentation.     

Dynamic evaluation of polypropylene capillary‐channeled fibers as a stationary phase in high‐performance liquid chromatography

Polypropylene (PP) capillary‐channeled polymer (C‐CP) fiber stationary phases are investigated for applications in HPLC. Specifically, the roles that fiber size and shape, linear velocity, interstitial fraction, and column inner diameter play in separation efficiency were evaluated using a uracil and butylparaben mixture eluted under isocratic conditions. Four fiber types, having nominal diameters ranging from 30 to 65 μm, were used in 250 mm × 2.1 mm columns. Optimum flow characteristics, as judged by plate height and resolution, were observed for 40 μm diameter PP C‐CP fibers packed at an interstitial fraction of ～0.63, over a broad range of linear velocities (～2 to 37 mm/s). The influence of column inner diameter was studied on 1.5, 2.1, and 4.6 mm columns packed at the optimal interstitial fraction. The best performing column in terms of plate height and resolution was the 2.1 mm inner diameter. C‐CP columns were also evaluated for the separation of a protein mixture composed of ribonuclease A, cytochrome c, and transferrin. Results obtained with the biomacromolecules mixture validate the optimal structural and operative conditions determined with the small solutes, laying the groundwork towards biomacromolecule applications, focusing more on the chemical aspects of separations.     

超临界流体色谱法分析非对映异构体d4T-5’-N-磷酰化苯丙氨酸甲酯

采用超临界CO2流体色谱技术,分析d4T-5’-N-磷酰化苯丙氨酸甲酯手性磷的非对映异构体。色谱柱为Hpersil ODS2(250 mm×4.6 mm,5μm),流动相为夹带改性剂甲醇、乙醇和异丙醇的超临界CO2流体。以容量因子、选择性和分离度为指标,考察改性剂、背压和柱温对分离的影响。在甲醇、乙醇和异丙醇3种改性剂中,甲醇为最好的改性剂,其中在7%甲醇改性剂下,该化合物的分离度可达到3.35。在7%甲醇改性剂条件下,考察了压力(10～20 MPa)和温度(303.15～318.15 K)的影响。在优化的分离条件(改性剂为7%甲醇,流速为2 mL/min,柱温为308.15 K,背压为15 M Pa)下,d4T-5’-N-磷酰化苯丙氨酸甲酯的两种非对映异构体完全达到基线分离,分离时间约15 min。     

DC-biased AC-electrokinetics: a conductivity gradient driven fluid flow

This paper studies the principles of fluid flow manipulation based on DC-biased AC-electrokinetics. This method makes use of planar parallel electrodes in a microfluidic channel in contact with an electrolyte solution, with a DC biased AC electrical signal applied to the electrode pair. Due to the application of DC bias, incipient Faradaic electrolytic reactions take place resulting in an increase of the ionic content of the bulk solution. The ionic content was found to be dissimilar at the cathodic and anodic sides of the channel and a conductivity difference of approximately 10% was measured for 2 V(DC). Fluid flow is generated by the action of the DC biased AC electric signal acting on the transverse conductivity gradient generated across the microchannel. The induced flow in the form of vortex was characterized experimentally and the results substantiated theoretically. The velocity of the induced flow vortex under the employed experimental conditions was ~600 to 700 μm s(-1) which is faster than those obtained in conventional AC-electroosmosis and AC-electrothermal types of flows.     

Anion separations with pressure-assisted capillary electrophoresis using a sequential injection analysis manifold and contactless conductivity detection

It is demonstrated that a hydrodynamic flow superimposed on the mobility of analyte anions can be used for the optimization of analysis time in capillary zone electrophoresis. It was also possible to use the approach for counter‐balancing the electroosmotic flow and this works as well as the use of surface modifiers. To avoid any band‐broadening due to the bulk flow narrow capillaries of 10 μm internal diameter were employed. This was enabled by the use of capacitively coupled contactless conductivity detection, which does not suffer from the downscaling, and detection down to between 1 and 20 μM for a range of inorganic and small organic anions was found feasible. Precisely controlled hydrodynamic flow was generated with a sequential injection manifold based on a syringe pump. Sample injection was carried out with a new design relying on a simple piece of capillary tubing to achieve the appropriate back‐pressure for the required split‐injection procedure.     

Scanning Electrochemical Microscopy as an In Vitro Technique for Measuring Convective Flow Rates Across Dentine and the Efficacy of Surface Blocking Treatments

Scanning electrochemical microscopy (SECM) is shown to be a powerful technique for both the measurement of local solution velocities through human dentine slices, in vitro, and for assessing quantitatively the effect of surface treatments on the flow process. SECM employs a small ultramicroelectrode (micron dimensions) as an imaging probe to provide information on the topography and transport characteristics of dentine, with high spatial resolution. In these studies the dentine sample is a membrane in a two compartment cell, which contains solutions of identical composition, including a redox active mediator (Fe(CN) . In the absence of an applied pressure, the transport‐limited current response at the probe electrode is due to diffusion of Fe(CN) to the UME, which depends on the probe to sample separation. Under an applied hydrostatic pressure, hydrodynamic flow across the sample enhances mass transport to the UME. With this methodology it was possible to accurately measure effective fluid velocities, by recording tip currents with and without pressure, and assess the efficacy of potential flow retarding agents for the treatment of dentinal hypersensitivity. For native dentine, the solution velocity was found to vary dramatically with location on the sample. The application of a glycerol monooleate ‐ base paste treatment to the surface of dentine was found to lower local flow velocities significantly. This electroanalytical methodology is simple to implement and is generally applicable to assessing the efficacy and mode of action of a wide variety of potential fluid flow retarding agents.     

Michrochip chromatography using an open‐tubular microchannel and a ternary water–ACN–ethyl acetate mixture carrier solution

A capillary chromatography system has been developed using a ternary mixed‐solvents solution, i.e. water–hydrophilic/hydrophobic organic solvent mixture as a carrier solution. Here, we tried to carry out the chromatographic system on a microchip incorporating the open‐tubular microchannels. A model analyte solution of isoluminol isothiocyanate (ILITC) and ILITC‐labeled biomolecule was injected to the double T‐junction part on the microchip. The analyte solution was delivered in the separation microchannel (40 μm deep, 100 μm wide, and 22 cm long) with the ternary water–ACN–ethyl acetate mixture carrier solution (3:8:4 volume ratio, the organic solvent rich or 15:3:2 volume ratio, the water‐rich). The analyte, free‐ILITC and labeled BSA mixture, was separated through the microchannel, where the carrier solvents were radially distributed in the separation channel generating inner and outer phases. The outer phase acts as a pseudo‐stationary phase under laminar flow conditions in the system. The ILITC and the labeled BSA were eluted and detected with chemiluminescence reaction.     

Enhancing separation in short‐capillary electrophoresis via pressure‐driven backflow

We present a novel easy‐to‐operate and efficient method to improve the separation efficiency in short‐capillary electrophoresis by introducing steady backflow to counterbalance electro‐osmotic flow without the use of any external pressure. The backflow was easily generated by tapering the capillary end, which was achieved by heating a straight capillary and stretching it with a constant force. We investigated the net fluidic transport rate under different tip lengths and separation voltages. Good run‐to‐run repeatability and capillary‐to‐capillary reproducibility of the present method were obtained with RSD less than 1.5%, indicating the stability of the fluid transport rate in the tapered capillary, which ensures the quantification and repeatability of capillary zone electrophoresis (CZE) analysis. Enhanced separation of the tapered short capillary electrophoresis was demonstrated by CZE analyzing amino acids and positional isomers. Baseline separations were achieved in less than 60 s using a tapered capillary with the effective length of 5 cm, while no separation was achieved using a normal capillary without a tapered tip. The present study provides a promising method to use pressure‐driven backflow to enhance separation efficiency in short‐capillary electrophoresis, which would be of potential value in a wide application for fast analysis of complex samples.     

Dielectrophoretic manipulation and separation of microparticles using curved microelectrodes

This paper presents the development and experimental analysis of a dielectrophoresis (DEP) system, which is used for the manipulation and separation of microparticles in liquid flow. The system is composed of arrays of microelectrodes integrated to a microchannel. Novel curved microelectrodes are symmetrically placed with respect to the centre of the microchannel with a minimum gap of 40 μm. Computational fluid dynamics method is utilised to characterise the DEP field and predict the dynamics of particles. The performance of the system is assessed with microspheres of 1, 5 and 12 μm diameters. When a high‐frequency potential is applied to microelectrodes a spatially varying electric field is induced in the microchannel, which creates the DEP force. Negative‐DEP behaviour is observed with particles being repelled from the microelectrodes. The particles of different dimensions experience different DEP forces and thus settle to separate equilibrium zones across the microchannel. Experiments demonstrate the capability of the system as a field flow fraction tool for sorting microparticles according to their dimensions and dielectric properties.     

A simple chip free‐flow electrophoresis for monosaccharide sensing via supermolecule interaction of boronic acid functionalized quencher and fluorescent dye

Here, a simple micro free‐flow electrophoresis (μFFE) was developed for fluorescence sensing of monosaccharide via supermolecule interaction of synthesized boronic acid functionalized benzyl viologen (ο‐BBV) and fluorescent dye. The μFFE contained two open electrode cavities and an ion‐exchange membrane was sandwiched between two polymethylmethacrylate plates. The experiments demonstrated the following merits of developed μFFE: (i) up to 90.5% of voltage efficiency due to high conductivity of ion‐exchange membrane; (ii) a strong ability against influence of bubble produced in two electrodes due to open design of electrode cavities; and (iii) reusable and washable separation chamber (45 mm × 17 mm × 100 μm, 77 μL) avoiding the discard of μFFE due to blockage of solute precipitation in chamber. Remarkably, the μFFE was first designed for the sensing of monosaccharide via the supermolecule interaction of synthesized ο‐BBV, fluorescent dye, and monosaccharide. Under the optimized conditions, the minimum concentration of monosaccharide that could be detected was 1 × 10^?11 M. Finally, the developed device was used for the detection of 0.3 mM glucose spiked in human urine. All of the results demonstrated the feasibility of monosaccharide detection via the μFFE.     

Fluorescence Imaging In Vivo at Wavelengths beyond 1500 nm

Compared to imaging in the visible and near‐infrared regions below 900 nm, imaging in the second near‐infrared window (NIR‐II, 1000–1700 nm) is a promising method for deep‐tissue high‐resolution optical imaging in vivo mainly owing to the reduced scattering of photons traversing through biological tissues. Herein, semiconducting single‐walled carbon nanotubes with large diameters were used for in vivo fluorescence imaging in the long‐wavelength NIR region (1500–1700 nm, NIR‐IIb). With this imaging agent, 3–4 μm wide capillary blood vessels at a depth of about 3 mm could be resolved. Meanwhile, the blood‐flow speeds in multiple individual vessels could be mapped simultaneously. Furthermore, NIR‐IIb tumor imaging of a live mouse was explored. NIR‐IIb imaging can be generalized to a wide range of fluorophores emitting at up to 1700 nm for high‐performance in vivo optical imaging.     

Rapid determination of parabens in seafood sauces by high‐performance liquid chromatography: A practical comparison of core–shell particles and sub‐2 μm fully porous particles

In this work, the chromatographic performance of superficially porous particles (Halo core–shell C₁₈ column, 50 mm × 2.1 mm, 2.7 μm) was compared with that of sub‐2 μm fully porous particles (Acquity BEH C₁₈, 50 mm × 2.1 mm, 1.7 μm). Four parabens, methylparaben, ethylparaben, propylparaben, and butylparaben, were used as representative compounds for calculating the plate heights in a wide flow rate range and analyzed on the basis of the Van Deemter and Knox equations. Theoretical Poppe plots were constructed for each column to compare their kinetic performance. Both phases gave similar minimum plate heights when using nonreduced coordinates. Meanwhile, the flat C‐term of the core–shell column provided the possibilities for applying high flow rates without significant loss in efficiency. The low backpressure of core–shell particles allowed this kind of column, especially compatible with conventional high‐performance liquid chromatography systems. Based on these factors, a simple high‐performance liquid chromatography method was established and validated for the determination of parabens in various seafood sauces using the Halo core–shell C₁₈ column for separation.