Development and micro-gap flow evaluation of electro-rheological nano-suspensions

Electro-rheological (ER) nano-suspensions based on titanium dioxide nano-particles with particle diameter on the order of 100 nm were prepared. Micro-gap flow behavior and microstructure were evaluated for the suspensions with a particle volume fraction of 8.8 vol%. For the nano-suspension based on a silicone oil, the ER effect and flow behavior were found to be stable during the sharing time on the order of 10²?s. The ER response measured at a given strength of the electric field for the nano-suspension based on a chemically modified silicone oil was effectively induced by the electric field, especially with low strengths, and it was larger than that of the nano-suspension based on the silicone oil but slightly lower than that based on a hydrocarbon oil. Furthermore, the relaxation of the ER response after removal of the electric field was somewhat slow for the nano-suspension based on the hydrocarbon oil, whereas it was fairly well for the nano-suspension based on the modified silicone oil.     

Integrated microfluidic device with an electroplated palladium decoupler for more sensitive amperometric detection of the 8-hydroxy-deoxyguanosine (8-OH-dG) DNA adduct

8-Hydroxy-deoxyguanosine (8-OH-dG) DNA adduct is one of the most frequently used biomarkers reporting on the oxidative stress that leads to DNA damage. More sensitive and reliable microfluidic devices are needed for the detection of these biomarkers of interest. We have developed a capillary electrophoresis (CE)-based microfluidic device with an electroplated palladium decoupler that provides significantly improved detection limit, separation efficiency, and resolving power. The poly(dimethylsiloxane) (PDMS)/glass hybrid device has fully integrated gold microelectrodes covered in situ with palladium nanoparticles using an electroplating technique. The performance and coverage of the electrodes electroplated with palladium particles were evaluated electrochemically and via scanning electron microscope (SEM) imaging, respectively. The performance of the device was tested and evaluated with different buffer systems, pH values, and electric field strengths. The results showed that this device has significantly improved resolving power, even at separation electric field strengths as high as 600 V cm⁻¹. The detection limit for the 8-OH-dG adduct is about 20 attomoles; the concentration limit is on the order of 100 nM (S/N = 3). A linear response is reported for both 8-OH-dG and dG in the range from 100 nM to 150 μM (≈100 pA μM⁻¹) with separation efficiencies of approximately 120,000–170,000 plates m⁻¹.     

Electrorheology and creep-recovery behavior of conducting polythiophene/poly(oxymethylene)-blend suspensions

Electrorheological properties and creep-recovery behavior of polythiophene/polyoxymethylene-blend having PT（50%）/POM（50%） composition were investigated.Particle size,conductivity and dielectric values were measured to be 24.77μm,3.85×10^-5 S·m^-1 and 26.75,respectively.Sedimentation ratio was measured to be 64%at the end of 16 days.The effects of dispersed particle volume fraction,external electric field strength,shear rate,frequency and temperature on ER properties and storage modulus of PT/POM-blend/silicone oil（SO） suspensions were examined.Enhancement were observed in the electric field viscosities of the suspensions and thus they were classified as a smart material.Shear thinning non-Newtonian viscoelastic behavior was determined for PT/POM-blend/SO system.Further,time-dependent deformation was examined by creep-recovery tests and recoverable viscoelastic deformation established.     

Characterization of a capacitance-coupled contactless conductivity detection system with sidewall electrodes on a low-voltage-driven electrophoresis microchip

A new type of capacitance-coupled contactless conductivity detection (C⁴D) system with sidewall electrodes was proposed for integration on a silicon-on-isolator–poly(dimethylsiloxane) (SOI-PDMS) hybrid low-voltage-driven electrophoresis microchip. By a microelectromechanical system process, the sidewall electrodes were fabricated precisely at either side of the separation channel. The area of the capacitor electrodes was the maximum value to improve the detection sensitivity with an enhanced capacitance effect. According to the simulation results, the structural parameters of the sidewall electrodes were determined as 550-μm length, 15-μm width, 80-μm separation distance, and 1-μm isolator thickness. The integrated microdevice with the SOI-PDMS hybrid electrophoresis microchip was very compact and the size was only 15 cm × 15 cm × 10 cm (width × length × height), which permitted miniaturization and portability. The detector performance was evaluated by K⁺ testing. The detection limit of the conductivity detector was determined to be 10^-9 and 10^-6 M for K⁺ in the static and electric-driven modes, respectively. Finally, the C⁴D was applied to low-voltage-driven electrophoresis on a microchip to carry out real-time measurement of the separation of amino acids. The separations of 10^-4 M lysine and phenylalanine in the low-voltage-driven electrophoresis mode were performed with an electric field of 300 V/cm and were completed in less than 15 min with a resolution of 1.3. The separation efficiency was found to be 1.3 × 10³ and 2.8 × 10³ plates for lysine and phenylalanine, respectively, with a migration time reproducibility of 2.7 and 3.2%. The conductivity detection limit of amino acids achieved was 10^-6 M. The proposed method for the construction of a novel C⁴D integrated on an SOI-PDMS hybrid low-voltage-driven electrophoresis microchip showed the most extensive integration and miniaturization of a microdevice, which is a further crucial step toward the realization of the “lab-on-a-chip” concept.     

3D nanogap interdigitated electrode array biosensors

Three-dimensional interdigitated electrodes (IDEs) have been investigated as sensing elements for biosensors. Electric field and current density were simulated in the vicinity of these electrodes as a function of the electrode width, gap, and height to determine the optimum geometry. Both the height and the gap between the electrodes were found to have significant effect on the magnitude and distribution of the electric field and current density near the electrode surface, while the width of the electrodes was found to have a smaller effect on field strength and current density. IDEs were fabricated based on these simulations and their performance tested by detecting C-reactive protein (CRP), a stress-related protein and an important biomarker for inflammation, cardiovascular disease risk indicator, and postsurgical recuperation. CRP-specific antibodies were immobilized on the electrode surface and the formation of an immunocomplex (IC) with CRP was monitored. Electrochemical impedance spectroscopy (EIS) was employed as the detection technique. EIS data at various concentrations (1 pg/mL to 10 μg/mL) of CRP spiked in buffer or diluted human serum was collected and fitted into an equivalent electrical circuit model. Change in resistance was found to be the parameter most sensitive to change in CRP concentration. The sensor response was linear from 0.1 ng/mL to 1 μg/mL in both buffer and 5% human serum samples. The CRP samples were validated using a commercially available ELISA for CRP detection. Hence, the viability of IDEs and EIS for the detection of serum biomarkers was established without using labeled or probe molecules.     

Growth and characterisation of diffusion junctions between paired gold electrodes: diffusion effects in generator–collector mode

A diffusion junction between two paired gold electrodes is created in a bipotentiostatic electro-deposition process. Gold metal is deposited simultaneously on two adjacent disc electrodes (100 μm diameter, approximately 125 μm separation) until short-circuit conditions trigger the end point of the electro-deposition. Symmetric gold junctions with typically 5 μm average inter-electrode gap size, 140 μm gap length, and approximately 18 μm junction depth are obtained. These paired gold electrodes are employed in generator–collector mode to give well-defined steady-state feedback currents even for extremely low concentrations of analyte (sub-μM) and without any contributions from capacitive charging. Four redox systems are investigated spanning a wide range of diffusion coefficients: (1) the one-electron oxidation of iodide to iodine, (2) the two-electron oxidation of hydroquinone to benzoquinone, (3) the two-electron reduction of alizarin red S, and (4) the one-electron oxidation of the redox protein cytochrome c. Consistent results for these redox systems suggest that (1) the junction zone between the two electrodes is dominating the behaviour of the electrode in particular for the slower diffusing systems and (2) the paired gold electrode junction can be calibrated and employed for electroanalysis at very low concentrations and for a wider range of analytically relevant redox systems. Dedicated to Professor Keith B. Oldham, on the occasion of his 80th birthday     

Fast capillary electrophoresis-time-of-flight mass spectrometry using capillaries with inner diameters ranging from 75 to 5 μm

Fast electrophoretic separations in fused silica capillaries (CE) coupled to time-of-flight mass spectrometry (TOF-MS) are presented. CE separations of the model analytes (epinephrine, norepinephrine, dopamine, histidine, and isoproterenol) under conditions of high electric field strengths of up to 1.25 kV cm⁻¹ are completed in 20 s. Coupling of CE with TOF-MS is accomplished using a coaxial sheath liquid electrospray ionization interface. The influence of parameters inherent to the interface and their effects, including suction pressure and dilution, are discussed. In addition to standard capillaries of 75 and 50 μm inner diameter (ID), separations in capillaries with IDs of 25, 15, and 5 μm have been successfully applied to this setup. The analytical performance is compared over this range of capillary dimensions, and both advantages and disadvantages are discussed.     

Dynamic non-equilibrium SPME combined with GC, PICI, and ion trap MS for determination of organophosphate esters in air

Methodology for time-weighted average (TWA) air measurements of semivolatile organophosphate triesters, widely used flame-retardants and plasticizers, and common indoor pollutants is presented. Dynamic non-equilibrium solid-phase microextraction (SPME) for air sampling, in combination with GC/PICI and ion trap tandem MS, yields a fast, almost solvent-free method with low detection limits. Methanol was used as reagent gas for PICI, yielding stable protonated molecules and few fragments. A field sampler, in which a pumped airflow over three polydimethylsiloxane (PDMS) 100-μm fibers in series was applied, was constructed, evaluated, and used for the measurements. The method LODs were in the range 2–26 ng m⁻³ for a sampling period of 2 h. The uptake on the SPME fibers was shown to be about five times faster for triphenyl phosphate compared to the other investigated organophosphate esters, most likely due to more lipophilic properties of the aromatic compound. The boundary layer for triphenyl phosphate when using a 100-μm PDMS sorbent was determined to 0.08 mm at a linear air velocity of 34 cm s⁻¹. Five different organophosphate triesters were detected in air from a laboratory and a lecture hall, at concentrations ranging from 7 ng m⁻³ up to 2.8 μg m⁻³.     

First principle electric field response of single-walled boron nitride nanotube: a case study of zigzag (4,0) model

Structural and electrical responses of the (4,0) zigzag model of single-walled boron nitride (BN) nanotube (NT) (with edges terminated by H atoms) have been investigated under the external electric fields (parallel and transverse) with strengths 0−2.0 × 10⁻² a.u. using DFT-B3LYP/6-31G* method. Calculated electric dipole moment shows a significant change in the presence of the parallel and perpendicular external electric fields which result in much stronger interactions at higher electric field strengths. Natural bond orbital (NBO) atomic charges analysis shows that the separation of the center of the positive and the center of the negative electric charges of (4,0) zigzag BNNT increase with increase the applied parallel and transverse electric field strengths. The applied fields change the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energies and decrease the HOMO–LUMO gap (HLG) values. The calculated electronic spatial extent (ESE) showed small changes of <0.63% and <1.53% over the entire range of the applied parallel and perpendicular electric field strengths, respectively. Results of this study indicate that the properties of BNNTs can be controlled by applying the proper external electric field. Electronic supplementary material  The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Fast counter-electroosmotic capillary electrophoresis–time-of-flight mass spectrometry of hyaluronan oligosaccharides

Fast capillary electrophoresis–mass spectrometry measurements under counter-electroosmotic analyte migration conditions are presented. Efficient separations of a homologous series of six hyaluronan oligosaccharides (comprising 1–6 hyalobiuronic acid moieties) could be completed in 65 s. Separations were achieved in short-length fused silica capillaries under high electric field strengths of up to 1.25 kV·cm⁻¹. Capillary inner diameters ranging from 5 to 50 μm were investigated, resulting in an optimal value of 15 μm. The influence of capillary dimensions and buffer composition on separation efficiency and sensitivity are discussed. Optimal separations were achieved using a 28 cm × 15 μm capillary, a separation high voltage of 35 kV, a background electrolyte of 25 mM ammonium acetate adjusted to pH 8.5, and negative ionization mode. The optimized method was successfully applied to a bovine testicular hyaluronidase digest of hyaluronan. Only minimal sample pretreatment for protein-containing samples is required. The simple manual injection procedure and fast separations allow for a sample throughput of 35 samples per hour.     

Mapping alternating current electroosmotic flow at the dielectrophoresis crossover frequency of a colloidal probe

AC electroosmotic (ACEO) flow above the gap between coplanar electrodes is mapped by the measurement of Stokes forces on an optically trapped polystyrene colloidal particle. E²‐dependent forces on the probe particle are selected by amplitude modulation (AM) of the ACEO electric field (E) and lock‐in detection at twice the AM frequency. E²‐dependent DEP of the probe is eliminated by driving the ACEO at the probe's DEP crossover frequency. The location‐independent DEP crossover frequency is determined, in a separate experiment, as the limiting frequency of zero horizontal force as the probe is moved toward the midpoint between the electrodes. The ACEO velocity field, uncoupled from probe DEP effects, was mapped in the region 1–9 μm above a 28 μm gap between the electrodes. By use of variously sized probes, each at its DEP crossover frequency, the frequency dependence of the ACEO flow was determined at a point 3 μm above the electrode gap and 4 μm from an electrode tip. At this location the ACEO flow was maximal at ～117 kHz for a low salt solution. This optical trapping method, by eliminating DEP forces on the probe, provides unambiguous mapping of the ACEO velocity field.     

Diamond paste-based electrodes for the determination of sildenafil citrate (Viagra)

Three types of monocrystalline diamond: natural diamond 1 μm, synthetic diamond 50 μm (synthetic-1), and synthetic diamond 1 μm (synthetic-2) were used for the design of diamond paste electrodes for the determination of sildenafil citrate (Viagra) using square wave voltammetry. The linear concentration ranges recorded for sildenafil citrate when natural diamond, synthetic-1, and synthetic-2 based electrodes were used were between 10⁻¹² and 10⁻⁸, 10⁻¹² and 10⁻⁹, and 10⁻¹¹ and 10⁻⁹ mol/L, respectively. Low detection limits which lie between 0.1 and 1 pmol/L proves the sensitivity of the electrodes. It was found that sildenafil citrate yielded a peak at about +0.175 ± 0.025 V (versus Ag/AgCl) for all the electrodes. Sildenafil citrate was determined with high reliability from its pharmaceutical formulation.     

Localized Visualization of Chemical Cross-Talk in Microsensor Arrays by Using Scanning Electrochemical Microscopy

 An investigation of an array of four Pt microband electrodes 25 μm wide and spaced 25 μm apart was performed with the scanning electrochemical microscope (SECM). Where possible the SECM measurements were confirmed with conventional electrochemical measurements. It is shown how the sensiti- vity of the SECM recycling current to the activity of the underlying surface can be used to probe the homogeneity of enzyme-modified microelectrodes. The diffusion of H₂O₂ between these micro enzyme- electrodes and unmodified electrodes was investigated and it was demonstrated how the SECM can be a powerful tool in the elucidation of the properties of these electrodes. Received June 8, 1998. Revision November 12, 1998.     

Reduction of metol on mechanically renewed metal electrodes

The electroreduction of metol on mechanically renewed metallic electrodes is studied by direct voltammetry with linear potential sweep. Reduction peaks of metol are found in a neutral supporting electrolyte (0.02–0.5 M Na₂SO₄) on nickel, silver, and mercury electrodes before the potential of hydrogen liberation from the supporting electrolyte. The shape and parameters of the cathodic peak depend on an electrode material, and also on the composition and pH of the supporting solution. A probable mechanism of the electroreduction of metol is proposed. The regeneration of nickel and silver electrodes by in situ mechanical cutting of a 0.5-μm surface layer provides good reproducibility of the value of peak current; it is proportional to the concentration of metol in the range 2 × 10⁻³–1.8 × 10⁻² M.     

Electrochemical impedance of microelectrodes

The influence of the radius (10 <a < 1500 μm) of a Pt disk electrode on the impedance frequency spectrum of the [Fe(CN)₆]^3-/4- reversible system is studied in the frequency range 5 × 10^-3 to 10³ Hz. The impedance is calculated by applying the Fourier transform to potential and current pulses of various lengths obtained when imposing a step potential. For electrodes witha < 100 μm, the spectrum in the complex plane has the form characteristic of microelectrodes, while for electrodes of higher radii, it corresponds to a finite Warburg impedance. The main impedance parameters, such as the charge transfer resistance, the diffusion resistance, and the frequency in the maximum of the imaginary constituent are determined. The latter decreases with an increase ina by the lawf* ∼ 1/a² at lowa and is independent ofa on electrodes witha > 100 μm, which agrees with the impedance theory for microelectrodes     

Fabrication of TiO2/Ti nanotube electrode and the photoelectrochemical behaviors in NaCl solutions

Titanium oxide nanotube electrodes were successfully prepared by anodic oxidation on pure Ti sheets in 0.5 wt.% NH₄F + 1 wt.% (NH₄)₂SO₄ + 90 wt.% glycerol mixed solutions. Nanotubes with diameter 40–60 nm and length 7.4 μm were observed by field emission scanning electron microscope. The electrochemical and photoelectrochemical characteristics of TiO₂ nanotube electrode were investigated using linear polarization and electrochemical impedance spectroscopy techniques. The open-circuit potential dropped markedly under irradiation and with the increase of Cl⁻ concentrations. A saturated photocurrent of approximately 1.3 mA cm⁻² was observed under 10-W low-mercury lamp irradiation in 0.1 M NaCl solution, which was much higher than film electrode. Meanwhile, the highest photocurrent in NaCl solution implied that the photogenerated holes preferred to combine with Cl⁻. Thus, a significant synergetic effect on active chlorine production was observed in photoelectrocatalytic processes. Furthermore, the generation efficiency for active chlorine was about two times that using TiO₂/Ti film electrode by sol–gel method. Finally, the effects of initial pH and Cl⁻ concentration were also discussed.     

Gamma-ray absorption using rubber—lead mixtures as radiation protection shields

In the present study five samples of special rubber–lead were fabricated each of them consists of lead and rubber with different weight ratios. The fabrication was carried out through the process of mixing under compression pressure. Gamma-ray transmission method was employed to determine the linear attenuation coefficient for narrow collimated mono-energetic beams of gamma-rays emitted from ²⁴¹Am 0.059, ¹⁵²Eu 0.13 and ¹³⁷Cs 0.662 MeV. The linear attenuation coefficient of standard rubber–lead shield was also experimentally determined. The percentage of lead in standard rubber–lead shield was determined through the calibration curve or by a simple computer program written in MATLAB. All prepared samples are characterized as flexible and gives a good homogeneity. samples no. 4 & 5 offers the best performance as a radiation protection shields. The results showed an inverse proportionality between the linear attenuation coefficient μ_{l and E}, and μ_l has a direct proportionality with mixing ratios (sample density). The results showed an inverse proportional between the half value layers and the average linear attenuation coefficients of the various samples.     

Determination of triazine herbicides: development of an electroanalytical method utilizing a solid amalgam electrode that minimizes toxic waste residues, and a comparative study between voltammetric and chromatographic techniques

The use of a copper solid amalgam electrode (CuSAE) for the analytical determination of triazine herbicides (atrazine and ametryne) instead of the conventional hanging mercury drop electrode (HMDE) is reported. The results obtained using electroanalytical methods utilizing each of these electrodes were also compared with those provided by the HPLC technique. The results indicated that the CuSAE electrode can be used to detect the herbicides studied, since the detection limits reached using the electrode (3.06 μg L⁻¹ and 3.78 μg L⁻¹ for atrazine and ametryne, respectively) are lower than the maximum values permitted by CONAMA (Brazilian National Council for the Environment) for wastewaters (50 μg L⁻¹) and by the US EPA (Environmental Protection Agency of the United States) in natural water samples (10.00 μg L⁻¹). An electroanalytical methodology employing CuSAE and square wave voltammetry (SWV) was successfully applied to the determination of atrazine and ametryne in natural water samples, yielding good recoveries (70.30%–79.40%). This indicates that the CuSAE provides a convenient substitute for the HMDE, particularly since the CuSAE minimizes the toxic waste residues produced by the use of mercury in HDME-based analyses.     

Peak compression in semi-preparative supercritical fluid chromatography

Summary Peak compression of a dihydropyridine drug, clevidipine, is obtained in both analytical and semi-preparative scale supercritical fluid chromatography, resulting in extremely high apparent efficiencies. The observed effect, when utilising a carbon dioxide/2-propanol mobile phase with a bare silica stationary phase, is achieved when the retention of the clevidipine peak is controlled to coalesce with a system peak, generated as a result of having water in the sample. Apparent efficiencies of 350,000 and 170,000 plates meter⁻¹ were obtained when 0.25 and 0.5 mg, respectively, are directly injected to a 200×4.6 mm ID 5 μm Hypersil silica packed column. The effect was extended to a semi-preparative system where apparent efficiencies in the region of 2,000,000 plates meter⁻¹ were observed when 0.3 mg of a clevidipine sample containing 80% water was injected to a 250×10 mm I.D. column containing 5-μm Hypersil silica particles.     

Gas chromatography on 10 um silica particles

Summary Porous silica microparticles designed for modern liquid chromatography have proven effective in gas chromatography. Columns of 35–50 cm gave plate heights as low as 3.3 particle diameters and speeds of 2400 theoretical plates per second or 500 effective theoretical plates per second. Inlet pressures up to 70 atmospheres were required using hydrogen as carrier gas. The particles as received were too retentive for fast chromatography and gave asymmetric peaks. A coating of fluorosilicone oil overcame both problems. Other coatings were less effective. Bonded phases proved less satisfactory on both counts and also gave substantially less efficient columns and greater flow resistance. Column efficiency and flow resistance were sharply dependent on physical properties of the particles. The most efficient packing was clearly spherical particles of 5–10 μm diameter with narrow size distribution, pore diamters about 50 nm, BET surface areas of 25–50 m²/g and surfaces modified with trifluoropropyl silicone. A six-component hydrocarbon sample was separated in 33 s with a resolution of 4 for the most difficult pair and in 2.6 s with a minimal resolution. Performance was limited by end effects and by available pressure so that much better performance can be expected from longer columns and higher pressures.