A coated-metal enzyme electrode for urea determinations

A potentiometric enzyme electrode is reported in which an enzyme immobilized in polyvinyl chloride is used to coat an antimony metal electrode to detect changes in pH when the electrode is immersed in a solution of the enzyme substrat. As an example, urea is determined in solution by using immobilized urease on an antimony electrode, giving a linear concentration range of 5.0 × 10^-4–1.0 × 10^-2 M urea with a slope of 44 mV per decade change in urea concentration. The response slope is stable for about 1 week, with response times in the range 1–2 min, but with absolute potential changes occurring from day to day.     

Determination of trace amounts of silver with a chemically modified carbon paste electrode

A method for the determination of trace amounts of silver with a chemically modified carbon paste electrode is described. The modified electrode is prepared by simply mixing a chelating resin (a polythioether backbone and dioxymonosulphur polyethylene polyimines in the side-chain polymer) with graphite powder and Nujol oil. By immersing the electrode in a silver sample solution (pH = 6.5–7.5), silver can be adsorbed on the electrode surface and then determined by voltammetry in a separate blank solution. The response depends on the concentration of silver and the preconcentration time. For a preconcentration time of 5 min, the detection limit is about 3 × 10^?10 M and the linear range is from 5 × 10^?10 to 1 × 10^?7 M with a relative standard deviation of 4%. Many common metal ions have no or little effect on the determination of silver. The recommended procedure was applied to the determination of trace amounts of silver in waste water.     

Application of semidifferential electroanalysis to anodic stripping voltammetry

A new voltammetric technique, Semidifferential electroanalysis, in which the semiderivative, e, of the current, i, is measured as a function of electrode potential, has been applied for detection in anodic stripping voltammetry. The semiderivative of the current is defined by [fx131-1.tif] Cd²⁺, Pb²⁺, and Tl⁺ in 0.1 M KNO₃ at different pH values were tested as samples on a hanging mercury drop working electrode. Symmetrical sharp peaks were observed for the re-dissolution processes of metal amalgams formed during pre-electrolysis at -1.0 V vs. SCE. The peak potentials of e vs. E curves for the above three amalgams agreed well with the literature values for d.c. polarographic half-wave potentials. The peak heights were proportional to the pre-electrolysis time up to about 5 min, to the potential scan rate in the range 60–160 mV s^-1, and to the concentrations of Cd²⁺, Pb²⁺, and Tl⁺ in the original solution in the range 10^-6–10^-9 M. The relative standard deviation for the determination of Cd²⁺ was about ±4% at the 2 × 10^-5 M level.     

Electrolytic determination of nanomolar concentrations of silver in solution with a piezoelectric quartz crystal

Silver in solution is determined in situ by the frequency change of a piezoelectric quartz crystal on electrodeposition on the electrode of the crystal. The electrolyte solution flows through a cell containing the platinum-plated electrode (cathode) of the quartz crystal, a coiled platinum-wire anode and a silver—silver(I) chloride reference electrode, and is electrolyzed at —0.2 V vs. $\text{Ag}\text{AgCl}$. The frequency change is proportional to the silver concentration in the range 10^-5–5 × 10^-7 M after electrodeposition for 5 min, and in the range 10^-8–10^-9 M by recycling 20 ml of the solution over the electrodes for 3 h.     

Voltammetric Determination of Epinephrine with a 3‐Mercaptopropionic Acid Self‐Assembled Monolayer Modified Gold Electrode

Epinephrine (EP) could exhibit an anodic peak at a bare gold electrode, but it was very insensitive. However, when the bare gold electrode was modified with 3‐mercaptopropionic acid (3MPA) self‐assembled monolayer (3MPA SAM), the peaks of EP became more reversible and sensitive due to the accumulation and mediate efficiency of 3MPA SAM. Conditions such as solution pH, concentration of supporting electrolyte and accumulation time were optimized. Under the selected conditions (i.e., 0.02 M pH 6.8 sodium phosphate buffer, accumulation time: 2 min under open‐ circuit.), the height of the anodic peak at about 0.18 V (vs. SCE) was linear to EP concentration in the range of 2×10^?7 ?1×10^?6 M and 1×10^?6?5×10^?4 M with correlation coefficient of 0.995 and 0.999, respectively. When the 3MPA/Au was further modified with cysteamine, the interference of H₂O₂ and BrO₃^? was eliminated. But the resulting electrode still suffered from the interference of ascorbic acid. This method was used to determine the content of EP in adrenaline hydrochloride injections, and the recovery was in the range of 97.0% to 105.1%.     

Determination of ethanol by air-stream separation with flow injection and electrochemical detection at a nickel oxide electrode

Ethanol is separated by an air stream from a 1-ml sample with collection in 1 ml of 0.2 mol l^-1 sodium hydroxide. Measurement is by voltammetry at a tubular, catalytic nickel oxide electrode with 30-μl sample injected into a continuous flow stream. Relative standard deviation for repetitive measurements is 1.8% for synthetic samples. Serum samples extracted for 100 s and then measured yielded linear results for concentrations of 2 × 10^-4–5 × 10^-2 mol l^-1 ethanol (0.001–0.23%), with relative standard deviation of 2.0%. The time per determination was about 2 min.     

Construction of a guanine enzyme electrode and determination of guanase in human blood serum with an ammonia gas sensor

The guanine electrode is based on guanase used with an ammonia gas-sensing membrane electrode; immobilization of the enzyme is optimized. Guanine in the range 10^-4–10^-2 M gives a linear potential vs. log(concentration) plot with a response time of 4–1.5 min over the range specified. Guanase (0.12–12 I.U. I^-1) is determined in serum by adding guanine to the sample, and measuring the ammonia evolved with the gas-sensing electrode. Results compare favourably with the xanthine oxidase method.     

Tungstate-Selective Electrode

A chemical sensor for tungstate ions has been developed and implemented in two versions on the basis of the poorly soluble nickel(II) hexatungstonickelate(II) heteropoly compound. The electrode function is linear within the range 10^–5–10^–1 M tungsten for the film electrode and within the range 10^–4–10^–1 M tungsten for the coated wire electrode at pH 6–9. The slope of the electrode function is 28–29 mV. The time it takes to attain an equilibrium electrode potential is 1–5 min, depending on the ion concentration to be determined. The selectivity coefficients are found for the tungstate-selective electrode in the presence of chloride, sulfate, nitrate, and perchlorate.     

Copper(II)-selective electrode based on dithioacetal

A PVC membrane electrode for copper ion based on 1,3-dithiane,2-(4-methoxy phenyl) as ionophore and o-nitrophenyl octyl ether as a plasticizer is demonstrated. The electrode exhibits a Nernstian slope of 29.5±1 mV per decade in a linear range of 3.0×10⁻⁶ to 5.0×10⁻² M for Cu²⁺ ion. The detection limit of this electrode is 1.0×10⁻⁶ mol/l. This sensor has a very short response time of about 5 s and could be used in a pH range of 4.0-7.0. High selectivity was obtained over a wide variety of metal ions. The proposed electrode was successfully applied as an indicator electrode for the potentiometric titration of copper ion with EDTA and for the direct determination of copper in river water.     

Voltammetric Determination of Uric Acid with a Glassy Carbon Electrode Coated by Paste of Multiwalled Carbon Nanotubes and Ionic Liquid

The voltammetric behavior of uric acid (UA) has been studied at a multiwalled carbon nanotube‐ionic liquid (i.e., 1‐butyl‐3‐methylimidazolium hexafluorophosphate, BMIMPF₆) paste coated glassy carbon electrode (MWNTs‐BMIMPF₆/GC). It is found that UA can effectively accumulate at this electrode and cause a sensitive anodic peak at about 0.49 V (vs. SCE) in pH 4.0 phosphate buffer solutions. Experimental parameters influencing the response of the electrode, such as solution pH and accumulation time, are optimized for uric acid determination. Under the optimum conditions, the anodic peak current is linear to UA concentration in the range of 1.0×10^?8 M to 1.0×10^?6 M and 2.0×10^?6 M to 2.0×10^?5 M. The detection limit is 5.0×10^?9 M for 180 s accumulation on open circuit. The electrode can be regenerated by successively cycling in a blank solution for about 3 min and exhibits good reproducibility. A 1.0×10^?6 M UA solution is measured for eight times using the same electrode regenerated after every determination, and the relative standard deviation (RSD) of the peak current is 3.2%. As for different electrodes fabricated by the same way the RSD (i.e., the electrode to electrode deviation) is 4.2%(n=9). This method has been applied to the determination of UA in human urine samples, and the recoveries are 99%–100.6%. In addition, comparison is made between MWNTs‐BMIMPF₆/GC and MWNTs/GC. Results show that the MWNTs‐BMIMPF₆/GC exhibits higher sensitivity, selectivity and ratio of peak current to background current.     

Determination of Thiophosphamide in Aqueous Solutions by Cathodic Stripping Voltammetry at a Silver Electrode

It was demonstrated that thiophosphamide can be determined in aqueous solutions by cathodic stripping voltammetry at a silver electrode. The determination conditions were found. The detection limit for thiophosphamide in the Britton buffer solution (pH 11.2) was 5 × 10^–4M (relative standard deviation 5%), and the upper boundary of the analytical range was 3 × 10^–3M (relative standard deviation 1.5%) for a time of electroaccumulation of 5 min.     

Construction of a N-Acetyl-L-methionine Electrode and Determination of Acylase with an Ammonia Gas Sensor

Abstract

The N-acetyl-L-methionine electrode is based on a coupled enzymatic system consisting of acylase and L-amino acid oxidase with an ammonia gas sensor; conditions of imobilization are optimized. N-acetyl-L-methionine in the range 4×10^?5–2×10^?3M gives a linear potential vs. log(concentration) plot with a response time of 2–5 min over the range specified. This electrode combined with an L-methionine electrode, based only on L-amino acid oxidase and an ammonia gas sensor, can be used for the determination of both substrates in mixtures, thus extending the feasibility of the method. Acylase (0.1–2.00) is determined in aqueous solutions by adding N-acetyl-L-methionine to the sample, and measuring the ammonia evolved with the gas-sensing electrode.     

Induced bacterial electrode for the potentiometric measurement of tyrosine

A bacterial tyrosine-selective potentiometric electrode is proposed in which the desired biocatalytic activity is biochemically induced during growth of the bacterial cells. As the result of this induction, a normally ineffective biocatalyst, Aeromonas phenologenes ATCC 29063 can be coupled with an ammonia gas-sensing electrode in order to produce a useful tyrosine-selective electrode. The sensor shows excellent response characteristics, having a slope of 50–58 mV/decade, a range of logarithmic response from 8.3 × 10^-5 M to 1.0 × 10^-3 M tyrosine, a lower limit of detection of 3.3 × 10^-5 M tyrosine, response times of 4–6 min, and a useful lifetime in excess of one week. Specific enzyme inhibitors are employed to enhance the selectivity of the electrode while maintaining high biocatalytic activity with respect to tyrosine.     

Amperometric Biosensor for the Determination of Phenols Using a Crude Extract of Sweet Potato (Ipomoea Batatas (L.) Lam.)

Abstract

An amperometric biosensor for the determination of phenols is proposed using a crude extract of sweet potato (Ipomoea batatas (L.) Lam.) as an enzymatic source of polyphenol oxidase (PPO; tyrosinase; catechol oxidase; EC 1.14.18.1). The biosensor is constructed by the immobilization of sweet potato crude extract with glutaraldehyde and bovine serum albumin onto an oxygen membrane. This biosensor provides a linear response for catechol, pyrogallol, phenol and p-cresol in the concentration ranges of 2.0×10^?5-4.3×10^?4mol L^?1, 2.0×10^?5-4.3×10^?4 mol L^?1, 2.0×10^?5-4.5×10^?4 mol L^?1 and 2.0×10^?5-4.5×10^?4mol L^?1, respectively. The response time was about 3–5 min for the useful response range, and the lifetime of this electrode was excellent for fifteen days (over 220 determinations for each enzymatic membrane). Application of this biosensor for the determination of phenols in industrial wastewaters is presented.     

Thin-layer differential pulse voltammetric determination of chlorpromazine in biological fluids

The combination of a thin-layer electrochemical cell with differential pulse voltammetry can be used to determine chlorpromazine in plasma and urine. The thin-layer cell (23 μl capacity) has a wax-impregnated graphite electrode. Direct determination of chlorpromazine in urine gave a linear calibration curve for the range 4.8 × 10^-3–2.4 × 10^-4 M with 97% recovery. No interference from glutethimide, dextropropoxyphene, meprobamate, diazepam, and methaqualone-HCl was detected. Direct measurement of chlorpromazine in plasma gave a linear calibration curve for the range 2.4 × 10^-5–4.8 × 10^-4 M with 89% recovery. The procedure for plasma and urine requires only 2 min per determination. Detection levels are below that required for monitoring therapeutic levels of chlorpromazine in urine.     

In vivo measurements with a potassium ion-selective microelectrode based on a new bis(crown ether)

Potassium ion-selective microelectrodes with a tip diameter of 10 μm are described. The ionophore used is 2,2′-bis[3,4-(15-crown-5)-2-nitrophenylcarbamoxymethyl] tetradecane in a conventional mixture with sodium tetraphenylborate, an ether and PVC. The electrode provides Nernstian response over the range 10^?5?10^?1 M potassium activity. Its selectivity is shown to be similar to that of a valinomycin-based microelectrode. After the electrode has aged for 24 h, the 95% response time at the 10^?3 M potassium ion level is 2.0 ± 0.5 s. The applicability of the bis(crown ether) electrode for measurements in vivo is proved by monitoring the changes in potassium activity in different areas of the brain of anaesthetized rats after administration of veratrine.     

Adsorptive Stripping Voltammetric Determination of Phenol at an Electrochemically Pretreated Carbon-Paste Electrode with Solid Paraffin as a Binder

An adsorptive stripping voltammetric method for determination of phenol at an electrochemically pretreated carbon-paste electrode has been developed. Solid paraffin was used as the binder of the carbon-paste electrode. The carbon-paste electrode was pretreated in the solution of 0.001 mol L⁻¹NaOH by holding it at +1.8 V (versus an Ag/AgCl electrode) for 5 min. On the pretreated electrode, the adsorption of phenol was greatly enhanced. Phenol was accumulated in NH₃–NH₄Cl (pH 9.25) medium at the potential of +0.1 V (versus Ag/AgCl electrode) for a certain time and then determined by second order differential anodic stripping voltammetry. An oxidative peak was observed at about +0.66 V. The relationship between second order peak current and phenol concentration was linear in the range of 2.5 × 10⁻⁷–5.0 × 10⁻⁶mol L⁻¹phenol, and the detection limit was 5.0 × 10⁻⁸mol L⁻¹. The method has been applied to the determination of phenol in tap water and waste water. The relative standard deviation (six determinations) was less than 3.5%.     

Determination of lead (II) ion by highly selective and sensitive lead (II) membrane electrode based on 2-(((E)-2-((E)-1-(2-hydroxyphenyl) methyliden)hydrazono)metyl)phenol

A PVC (poly vinyl chloride) membrane electrode for lead ion based on 2-(((E)-2-((E)-1-(2-hydroxyphenyl)methyliden)hydrazono)metyl)phenol (HMHMP) as a membrane carrier was prepared. This electrode exhibited linear response with Nernstian slope of 29.2?±?0.2?mV per decade within the concentration range of 2.0?×?10^?7–1.0?×?10^?1?M lead ion. The limit of detection, as determined from the intersection of the extrapolated linear segments of the calibration plot, was 8.0?×?10^?8 M. The electrode exhibited high selectivity for Pb (II). The response time of the electrode was about 5–10?s for different concentrations. The electrode is suitable for use in aqueous solutions in a pH range of 5.0–7.5. It was used as an indicator electrode in a titration of Pb (II) with chromate at constant pH. This electrode was used for the determination of lead in ore samples, and the results were in agreement with those obtained with an atomic absorption spectroscopy (AAS) method. Also lead selective electrode was used for monitoring of lead in spiked samples of the Zayanderud River and waste water by the potentiometry technique.     

Potentiometric stripping analysis for mercury

In potentiometric stripping analysis for mercury, elemental mercury is deposited on a glassy carbon electrode surface by means of potentiostatic reduction. It is then oxidized by potassium permanganate added to the sample prior to analysis and the “redox titration curve” thus obtained is recorded on a high-input impedance recorder. Deaeration of the sample is unnecessary. The analytical range is 5 × 10^-9–10^-3 M mercury(II), the times needed for potentiostatic accumulation ranging from 64 min at 10^-8 M to 1 min at concentrations above 10^-6 M. The chemistry of the stripping process is discussed and an automatic instrument for potentiometric stripping analysis is described.     

A PVC-based 1,8-diaminonaphthalen electrode for selective determination of vanadyl ion

A PVC membrane vanadyl (VO²⁺) ion-selective electrode was constructed using 1,8-diaminonaphthalen (DAN) as a neutral carrier. The electrode shows good Nernstian response for VO²⁺ ions over a wide concentration range (1.0×10⁻¹-1.0×10⁻⁵ M). The optimum composition of the membrane was 55 wt.% poly(vinylchloride), 35 wt.% 2-nitrophenyl octyl ether (NPOE), 5 wt.% ionophore, and 5 wt.% potassium tetrakis(p-chlorophenyl)borate (KTpClPB). It has relatively fast response time and can be used at least for 5 weeks without any considerable divergence in potentials. The proposed electrode revealed relatively good selectivity for VO²⁺ over wide variety of other metal ions. The electrode was used for the potentiometric titration of VO²⁺ ions with EDTA.