Development of a creatinine ELISA and an amperometric antibody-based creatine sensor with a detection limit in the nanomolar range

Creatinine-specific antibodies have been generated and used for highly sensitive and specific immunochemical creatinine determinations. Creatinine was derivatized at N3 and coupled to KLH carrier protein. On the basis of this immunogen, monoclonal antibodies were developed by hybridoma technology. Antibodies from various clones have been characterized with BIAcore 2000 with respect to the dissociation constant and specificity. Antibodies of clone B90-AH5 exhibited the lowest dissociation constant (0.74 microM) and the highest specificity for creatinine and were chosen for the development of a competitive ELISA and an amperometric creatinine sensor. The creatinine sensor was constructed by fixing a creatinine-modified membrane on the top of a platinum working electrode which was then incorporated into a stirred electrochemical measuring cell. For creatinine determination the creatinine-containing sample was incubated with B90-AH5 and anti-IgG(mouse)-glucose oxidase conjugate and applied to the measuring cell. After a washing step glucose was added and the produced hydrogen peroxide was registered at Eappl = +600 mV vs Ag/AgCl. The measuring range was 0.01-10 microg/mL. The highest sensitivity for creatinine was achieved at 330 ng/mL (3 microM) and the lower detection limit at 4.5 ng/mL (40 nM). This is far below the relevant clinical range, which is 5-17 microg/mL (44-150 microM) and allows a reliable determination of very low creatinine concentrations in serum, where standard methods cannot be applied. After each measurement the sensor was regenerated with 10 mM HCl without any loss in binding activity.     

Effect of lipophilic ion-exchanger leaching on the detection limit of carrier-based ion-selective electrodes

The equilibrium partitioning of lipophilic ion-exchanger salts from ion-selective polymeric membrane electrodes (ISEs) and its possible effect on the lower detection limit of these sensors is described. Predictions are made on the basis of various parameters, including the knowledge of tetraphenylborate potassium salt partitioning constants, the selectivity of ionophore-free ion-exchanger membranes, and ionophore stability constants in the membrane. Ion-exchanger lipophilicities are significantly increased if the membrane contains an ionophore that strongly binds the primary ion. Predicted detection limits are on the order of 10(-5)-10(-8) M for ionophore-free membranes, and may reach levels as low as 10(-18) M with adequate ionophores in the membrane. Experiments are performed for well-described lead-selective membranes containing different tetraphenylborate derivatives, and detection limits appear to be independent of the ion-exchanger used. However, they are much higher if a more hydrophilic carborane cation-exchanger is incorporated in the membrane. The first finding confirms recent theory, which states that transmembrane ion fluxes, given by a small level of ion-exchange at the sample side by interfering ions, normally dictate the detection limit of these sensing systems. Predicted detection limits on the basis of ion-exchanger leaching alone are here listed for a number of analytically relevant cases. For potassium-selective electrodes containing BME-44 and tetraphenylborate as ion-exchanger, the experimental detection limits are in agreement with predicted values. These results suggest that the detection limit of many current ISEs for ultratrace level analysis are, in optimal cases, dictated by transmembrane ion fluxes; however, because improved chemical solutions are being developed to reduce such effects, simple ion-exchanger partitioning may indeed become an important mechanism that can give higher detection limits than practically desired, and should not be ruled out.     

Rotating electrode potentiometry: lowering the detection limits of nonequilibrium polyion-sensitive membrane electrodes

A rotating electrode configuration is evaluated as a means to lower the detection limits of newly devised polyion-sensitive membrane electrodes (PSEs). Planar potentiometric polycation and polyanion PSEs are prepared by incorporating tridodecylmethylammonium chloride and calcium dinonylnaphthalenesulfonate, respectively, into plasticized PVC or polyurethane membranes and mounting disks of such films on an electrode body housed in a conventional rotating disk electrode apparatus. Rotation of the PSEs at 5000 rpm results in an enhancement in the detection limits toward heparin (polyanion) and protamine (polycation) of at least 1 order of magnitude (to 0.01 unit/mL for heparin; 0.02 microg/mL for protamine) over that observed when the EMF responses of the same electrodes are assessed using a stir-bar to achieve convective mass transport. A linear relationship between omega(-1/2), where omega is the rotating angular frequency, and C1/2, the polyion concentration corresponding to half the total maximum deltaEMF response toward the polyion species, is observed. It is further shown that the rotating polycation sensor can be used as an end-point detector to greatly enhance (relative to nonrotated indicator electrode) the analytical resolution and precision for measurement of low concentrations of heparin when such samples are titrated with protamine. The theoretical basis for lowering the detection limits by rotating PSEs is discussed based on the unique nonequilibrium response mechanism of such sensors.     

Electrochemical sample matrix elimination for trace-level potentiometric detection with polymeric membrane ion-selective electrodes

Potentiometric sensors are today sufficiently well understood and optimized to reach ultratrace level (subnanomolar) detection limits for numerous ions. In many cases of practical relevance, however, a high electrolyte background hampers the attainable detection limits. A particularly difficult sample matrix for potentiometric detection is seawater, where the high saline concentration forms a major interfering background and reduces the activity of most trace metals by complexation. This paper describes for the first time a hyphenated system for the online electrochemically modulated preconcentration and matrix elimination of trace metals, combined with a downstream potentiometric detection with solid contact polymeric membrane ion-selective microelectrodes. Following the preconcentration at the bismuth-coated electrode, the deposited metals are oxidized and released to a medium favorable to potentiometric detection, in this case calcium nitrate. Matrix interferences arising from the saline sample medium are thus circumvented. This concept is successfully evaluated with cadmium as a model trace element and offers potentiometric detection down to low parts per billion levels in samples containing 0.5 M NaCl background electrolyte.     

Improving the blood compatibility of ion-selective electrodes by employing poly(MPC-co-BMA), a copolymer containing phosphorylcholine,as a membrane coating

The hydrogelpoly(2-methacryloyloxyethylphosphorylcholine-co-butyl methacrylate), or poly(MPC-co-BMA), was used as a coating for polyurethane- and poly(vinyl chloride)-based membranes to develop ion-selective electrodes (ISEs) with enhanced blood compatibility. Adverse interactions of poly(MPC-co-BMA) with blood were diminished due to the phosphorylcholine functionalities of the hydrogel, which mimic the phospholipid polar groups present on the surface of many cell membranes. As demonstrated by immunostaining, hydrogel-coated PVC membranes soaked in platelet-rich plasma showed less adhesion and activation of platelets than uncoated PVC membranes, indicating an improvement in biocompatibility owing to the hydrogel. Furthermore, little differences in the potentiometric response characteristics, e.g., slope, detection limit, and selectivity, of ISEs employing uncoated and coated membranes were observed.     

Stripping analysis of nanomolar perchlorate in drinking water with a voltammetric ion-selective electrode based on thin-layer liquid membrane

A highly sensitive analytical method is required for the assessment of nanomolar perchlorate contamination in drinking water as an emerging environmental problem. We developed the novel approach based on a voltammetric ion-selective electrode to enable the electrochemical detection of "redox-inactive" perchlorate at a nanomolar level without its electrolysis. The perchlorate-selective electrode is based on the submicrometer-thick plasticized poly(vinyl chloride) membrane spin-coated on the poly(3-octylthiophene)-modified gold electrode. The liquid membrane serves as the first thin-layer cell for ion-transfer stripping voltammetry to give low detection limits of 0.2-0.5 nM perchlorate in deionized water, commercial bottled water, and tap water under a rotating electrode configuration. The detection limits are not only much lower than the action limit (approximately 246 nM) set by the U.S. Environmental Protection Agency but also are comparable to the detection limits of the most sensitive analytical methods for detecting perchlorate, that is, ion chromatography coupled with a suppressed conductivity detector (0.55 nM) or electrospray ionization mass spectrometry (0.20-0.25 nM). The mass transfer of perchlorate in the thin-layer liquid membrane and aqueous sample as well as its transfer at the interface between the two phases were studied experimentally and theoretically to achieve the low detection limits. The advantages of ion-transfer stripping voltammetry with a thin-layer liquid membrane against traditional ion-selective potentiometry are demonstrated in terms of a detection limit, a response time, and selectivity.     

Air-cooled cold trap channel integrated in a microfluidic device for monitoring airborne BTEX with an improved detection limit

We integrated an air-cooled cold trap (CT) channel in a microfluidic device for monitoring airborne benzene, toluene, ethylbenzene, and xylene (BTEX) gases and demonstrated its effect on improving the detection limit of the microfluidic device. The device consists of concentration and detection cells formed of 3 x 1 cm Pyrex plates. We first introduced a sample gas into the concentration cell, and the gas was adsorbed onto an adsorbent in the channel. We then raised the temperature using a thin-film heater and introduced the desorbed gas into the detection cell. To prevent dilution of the gas before detection, we propose an improvement to the concentration cell structure that involves the integration of the CT channel. We examined the CT effect by comparing three types of concentration cell with different channel structures. We found that we could detect a gas concentration about 2 orders of magnitude lower than in our previous work by optimizing the channel structure and integrating a CT channel. As an example of BTEX detection,we obtained a 0.05 ppm detection limit for toluene gas with a sampling time of 30 min.     

Impedimetric and potentiometric investigation of a sulfate anion-selective electrode: experiment and simulation

A new anion-selective polyvinyl chloride (PVC) membrane electrode based on {6,6'-diethoxy-2,2'-[2,2-dimethylpropane-1,3-diylbis(nitrilomethylidyne)]diphenolato}nickel(II)monohydrate as a carrier for the sulfate anion is reported. In this work, a new strategy for optimizing membrane components by electrochemical impedance spectroscopy (EIS) is presented. The performance of this electrode was investigated using potentiometric and EIS techniques. The potentiometric results indicated that the prepared electrode had a Nernstian slope of -28.9 ± 0.1 mV in a linear concentrations range of 1.0 × 10(-6) to 3.0 × 10(-1) M, a detection limit of 6.3 × 10(-7) M, an applied pH range of 4.0-9.0, and a response time of less than 15 s; while using the EIS technique, the linear concentrations range was 1.0 × 10(-9) to 1.0 × 10(-1) M and the pH range increased to 4.0-10.0. Finally, the impedance spectra were simulated using the Maple 13 software. A comparison of the experimental data and information obtained from the simulation confirmed the accuracy of the impedance measurement of this electrode.     

Improving the compatibility of contact conductivity detection with microchip electrophoresis using a bubble cell

A new approach for improving the compatibility between contact conductivity detection and microchip electrophoresis was developed. Contact conductivity has traditionally been limited by the interaction of the separation voltage with the detection electrodes because the applied field creates a voltage difference between the electrodes, leading to unwanted electrochemical reactions. To minimize the voltage drop between the conductivity electrodes and therefore improve compatibility, a novel bubble cell detection zone was designed. The bubble cell permitted higher separation field strengths (600 V/cm) and reduced background noise by minimizing unwanted electrochemical reactions. The impact of the bubble cell on separation efficiency was measured by imaging fluorescein during electrophoresis. A bubble cell four times as wide as the separation channel led to a decrease of only 3% in separation efficiency at the point of detection. Increasing the bubble cell width caused larger decreases in separation efficiency, and a 4-fold expansion provided the best compromise between loss of separation efficiency and maintaining higher field strengths. A commercial chromatography conductivity detector (Dionex CD20) was used to evaluate the performance of contact conductivity detection with the bubble cell. Mass detection limits (S/N = 3) were as low as 89 +/- 9 amol, providing concentration detection limits as low as 71 +/- 7 nM with gated injection. The linear range was measured to be greater than 2 orders of magnitude, from 1.3 to 600 microM for sulfamate. The bubble cell improves the compatibility and applicability of contact conductivity detection in microchip electrophoresis, and similar designs may have broader application in electrochemical detection as the expanded detection zone provides increased electrode surface area and reduced analyte velocity in addition to the reduction of separation field effects.     

Improving speed behaviour: the potential of in-car speed assistance and speed limit credibility

Speeding is still a common practice on many roads and it contributes to a significant number of crashes. Two new approaches to solve speeding issues are focused on: intelligent speed assistance systems (ISA) and speed limit credibility. Research has indicated that ISA is promising with respect to improving speed behaviour but has not been widely implemented yet. Another promising approach to reduce speeding involves adjusting the environment to improve the speed limit credibility. The aim here is to investigate the potential of both approaches and particularly the potential of the combination of these measures. A driving simulator study was conducted to investigate the individual and combined effects of the use of an ISA system and the speed limit credibility on drivers? average speed and the amount of time spent speeding. The results indicated that both the informative ISA system used here and the speed limit credibility significantly improved speed behaviour. Drivers not using ISA appeared to be more susceptible to the speed limit credibility than those using ISA. It is concluded that both the measures can be effective to improve speed behaviour. The results obtained suggest that the properties of this particular informing and warning type of ISA could have resulted in the speed limit credibility neither affecting the amount of time speeding nor the average speed.     

Rate-limiting steps of tyrosinase-modified electrodes for the detection of catechol

The response currents obtained for tyrosinase-modified Teflon/graphite, carbon paste, and solid graphite electrodes in the presence of catechol are analyzed primarily using rotating disk electrode experiments. The rate-limiting steps, such as the electrochemical reduction of o-quinones and the enzymatic reduction of oxygen as well as the enzymatic oxidation of catechol, are theoretically considered and experimentally demonstrated for the different electrode configurations.     

Resonances of an unbounded piezoelectric plate with circular electrodes

We consider an unbounded piezoelectric plate with two circular electrodes. The polarization direction of the material of the plate is perpendicular to the faces of the plate. One electrode is at the upper surface and the other one at the lower surface, symmetrically with respect to the middle plane. The resonances of this system are investigated. Three types of resonances are introduced. The dependence of the occurring of the resonances on the material constants is discussed.     

气相色谱仪火焰离子化检测器检出限的不确定度分析

该文根据新实施的JJG700——2016检定规程的新要求和新变化,结合不确定度测量的分析,全面讨论和研究气相色谱仪火焰离子化检测器检出限的测量不确定来源和影响程度,并对其进行全面的评定分析,从而为气相色谱仪离子化检测器的检定和使用者提供参考。     

Lipophilic polymer membrane optical sensor with a synthetic receptor for saccharide detection

An optical sensing approach for the detection of saccharides based on reversible boronic acid-diol complexation in a lipophilic polymer membrane is presented. The complexation of saccharides with phenyl boronic acids that are immobilized in a hydrophobic polymer matrix produces a stable boronate anion and liberates a mobile hydrogen ion. The change in fluorescence intensity of the sensing films resulting from the increase in hydrogen ion concentration is thus directly related to the ambient saccharide concentration. By monitoring the pH change in a bulk optical film, we were able to detect D-glucose, D-fructose, D-galactose, and D-sorbitol with a concentration range from 0.1 to 100 mM at physiological pH 7.4.     

Modification of indium tin oxide electrodes with repeat polynucleotides: electrochemical detection of trinucleotide repeat expansion.

Genomic expansion of the triplet repeat sequences 5'-(CTG)n and 5'-(CGG)n leads to myotonic dystrophy and fragile X syndrome, respectively. Methods for determining the number of repeats in unprocessed nucleic acids would be useful in diagnosing diseases based on triplet repeat expansion. Electrochemical reactions based on the oxidation of guanine were expected to give larger signals per strand for expansion of repeats containing guanine. A novel PCR reaction was used to generate fragments containing 150, 230, 400, and 830 repeats of (CTG)n, which codes for myotonic dystrophy, and 130 and 600 repeats of (CGG)n, which codes for fragile X syndrome. These PCR fragments were immobilized to indium tin oxide electrodes, and oxidation of guanine in the fragments was realized using electrocatalysis by Ru(bpy)3(2+) (bpy = 2,2'-bipyridine). The catalytic currents due to oxidation of the immobilized guanines by Ru(bpy)3(3+) increased with the number of repeats and were a linear function of the repeat number when normalized to the number of strands immobilized. These results suggest a sensing strategy for repeat length based on the combination of the electrocatalytic strategy for determining the repeat length combined with existing methods for determining the number of strands.     

Anodic current density of an electric arc with carbon electrodes

We present the results obtained in measuring the current density of anode spots in a carbon arc in air and we show its relationship to the magnitude of the arc current, analogous to the cathode spot.     

Performance characteristics of an electric-arc gas heater with segmental electrodes

Relations are established for the arc length as a function of the electric field intensity in an arc column and for the heat loss in the base spot as a function of the electric current and of the gas flow rate.Translated from Inzhenerno-Fizicheskii Zhurnal, Vol. 25, No. 1, pp. 73–76, July, 1973.     

Experimental study of an impulse electric discharge with liquid electrodes

Experimental results are presented on transition of a streamer shape multichannel discharge into a multichannel spark discharge. We present current–voltage characteristics of a multichannel streamer discharge at atmospheric and reduced pressures as well as temperature distribution between a plate-like copper electrode and technical water.