Potentiometric Determination of Acids and Bases using Silicone Rubber Based Graphite Electrode

Abstract

A silicone rubber based graphite electrode has been used as an indicator electrode for potentiometric acid-base titrations. The electrode potential changes an average of 30 mV per pH-unit. If the electrodes are pretreated with a solution of an oxidant before use, the pH-sensitivity is increased. The electrode can be used in both aqueous and non-aqueous solutions. A measuring cell, composed of a silicone rubber based graphite electrode of small surface area and a chloride-selective reference electrode, can be used for acid-base titrations in the micro range.     

Electrochemical RNase A Detection Using an Electrode with Immobilized Ferrocenyl Deoxyribooligonucleotide Containing Cytidine Residue

A ferrocenyl deoxyribooligonucleotide (FcODN(rC)) with contiguous cytosine bases and a single ribonucleotide, cytidine, was immobilized on a gold electrode, and this electrode was used to detect RNase A. RNase A activity in a solution was assessed using cyclic voltammetry, and it was found that the current response of the sensor electrode decreased with increasing enzyme concentration. An extremely low detection limit of 1.0×10^?11 g mL^?1 RNase A was observed, with 15–90 % changes in the current signal. RNase activity can be an indicator of a number of diseases; therefore, this probe has great potential for applications in medical diagnostics.     

Electrocatalytic Reduction and Flow Injection Analysis of Organic Peroxides at Polymeric Tetra-Amino Iron Phthalocyanine Modified Electrode

Abstract

It has been demonstrated that tetra-amino iron phthalocyanine can be polymerized onto a glassy carbon electrode by electrooxidation of the monomer in solution. Organic peroxides can be catalytically reduced at such polymeric film modified electrode. While on a bare glassy carbon electrode, none of the peroxides can be reduced within the potential range studied. When used in flow injection analysis, the modified electrode permits the detection of these compounds by reduction mechanism at higher potentials without the interference from oxygen. The response is stable and has a wide dynamic range.     

A Micro Planar Ampehometric Glucose Sensor Using an Isfet as a Reference Electrode

Abstract

A very small glucose sensor has been realized, which consists of a gold working electrode with a glucose oxidase immobilized membrane on it, and a gold counter electrode, all made on a sapphire substrate. By using the pH sensitive ISFET as a reference electrode, the potential for a solution, whose pH is constant, can be measured and irreversible metal electrodes, such as gold or platinum, can be used as working electrode and counter electrode. The sensor is very suitable for miniaturizing and mass production, because the Integrated Circuit (IC) fabrication process can be applied. The glucose oxidase immobilized membrane was also deposited by a lift off method, one of the IC processes. A glucose concentration, from 1 to 100 mg/dl, was measured with good linear current output.     

Cyclic Voltammetry and Quartz Crystal Microbalance Electrochemical Studies on Thiosalicylic Acid and Dithiodibenzoic Acid

ABSTRACT

Cyclic voltammetry (CV) and electrochemical quartz crystal microbalance (EQCM) were used to study the behavior of thiosalicylic acid (KTS) and dithiodibenzois acid (DTDB) at the controlled-growth mercury drop electrode (CGMDE), at the gold electrode, and at the carbon electrode. The CV method was used to study peak current intensities and peak potentials in relation to pH of the solution, to scan rate and to the concentration of the analyte. Optimum measurement parameters were established and stock solutions developed. The electrode activity was found to be primarily due to the oxidation of the sulhydryl group. The currents observed are diffusion controlled. Electrochemical studies on complexes of KTS with Cu(II) were undertaken at varying constituent proportions and the cathodic peak was found to rise by ca. 70% whereby the sensitivity of the determination was considerably increased. The EQCM method allowed to establish that, as KTS is oxidized, the electrode slightly gains in weight (ca. 80 ng). As DTDB is reduced, the electrode slightly looses in weight (ca 40 ng). Studies on the equimolecular KTS-Cu(II) complex showed the electrode to gain about 125 ng more in weight than in the solutions containing KTS alone, a fact evidencing for the formation of the complex and for its deposition on the electrode surface. A procedure was developed to determine KTS and DTDB in the substance by the CV method. Statistical evaluation of data showed the results to be characterized by good accuracy and precision (RSD 0.80% and 0.61%, respectively).     

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.     

Carbon paste electrode modified by cobalt ions dispersed into poly (N-methylaniline) preparing in the presence of SDS: application in electrocatalytic oxidation of hydrogen peroxide

Conducting and stable poly (N-methylaniline) film was prepared by using the repeated potential cycling technique in aqueous solution containing N-methylaniline, sulfuric acid, and sodium dodecyl sulfate (SDS) at the surface of carbon paste electrode (CPE). The transition metal ions of Co(ІІ) were incorporated to the polymer by immersion of the modified electrode in 0.1 M cobalt chloride solution for 10 min. The electrochemical characterization of this modified electrode exhibits stable redox behavior of Co(ІІ)→Co(ІІІ) and formation of insoluble oxide/hydroxide cobalt species on the CPE surface. The modified electrode showed well-defined and stable redox couples in alkaline aqueous solution. The modified electrode showed excellent electrocatalytic activity for oxidation of hydrogen peroxide. The response of modified electrode toward the H₂O₂ oxidation was examined using cyclic voltammetry, differential pulse voltammetry, square wave voltammetry, and chronoamperometry. This modified electrode has many advantages such as simple preparation procedure, good reproducibility, and high catalytic activity toward the hydrogen peroxide oxidation. Such characteristics were explored for the specific determination of hydrogen peroxide in cosmetics product sample, giving results in excellent agreement with those obtained by standard method.     

Correlation of the electrochemical reversibility and resolution of the anode and cathode peaks of the electrode processes of hydroquinone and metol on graphite and platinum

Cyclic and direct voltammetry with linear potential sweep was used to study the potential difference of the anode and cathode peaks for hydroquinone and metol in the cyclic voltammograms (CVA) of their individual solutions on the main factors affecting the reversibility of the electrochemical process on electrodes of a graphite-epoxy composite (GEC), namely, the state of the surface of the indicator electrode, the pH of the buffer solution, and the mode of polarization. The surface state of the GEC electrode was affected by its passivation in air for various periods of time; the reversibility of the electrode process was judged by the difference between the anode and cathode potentials on the CVAs of hydroquinone and metol. A correlation was found between the degree of reversibility and the difference of peak potentials for similar electrode processes of hydroquinone and metol on the GEC electrode and other solid electrodes made of graphite materials and platinum.     

New Highly Reliable Smde Based Computer Controlled Polarggraphic System For Wet Chemical Analysis

Abstract

A highly reliable static mercury drop electrode is described for polarographic analysis. the electrode was employed as working electrode in a new computer controlled automatic analytical system developed in this laboratory. the system can be well used for the serial analysis of small volumed solution samples or for monitor In addition to this, the system was employed in monitoring the drug content of dissolution media in the course of drug dissolution studies. This will be discussed in lurther paper¹¹     

Prototypical Nonelectrochemical Method for Surface Regeneration of an Integrated Electrode in a PDMS Microfluidic Chip

Abstract

A prototypical method for surface regeneration of an integrated electrode in a microfluidic chip is demonstrated. A platinum wire as working electrode was mounted in a polydimethylsiloxane chip vertical to the chip through the channel. The regeneration of the electrode was easily achieved by drawing the platinum wire out for 5 mm, because the area exposed to the channel or the stream would be altered. With continuous motion, the wire electrode can maintain a fresh surface just like a dropping mercury electrode. The current–time curve and open circuit potential (OCP) of dopamine solution show the performance of this prototypical system.     

Preparation of PbS Nanoparticles by Phase-Transfer Method and Application to Pb2+-Selective Electrode Based on PVC Membrane

Abstract

A novel approach to prepare homogeneous PbS nanoparticles by phase-transfer method was developed. The preparatory conditions were studied in detail, and the nanoparticles were characterized by transmission electron microscopy (TEM) and UV-vis spectroscopy. Then a novel lead ion-selective electrode of polyvinyl chloride (PVC) membrane based on these lead sulfide nanoparticles was prepared, and the optimum ratio of components in the membrane was determined. The results indicated that the sensor exhibited a wide concentration range of 1.0 × 10^?5 to 1.0 × 10^?2 mol·L^?1. The response time of the electrode was about 10 s, and the optimal pH in which the electrode could be used was from 3.0 to 7.0. Selectivity coefficients indicated that the electrode was selective to the primary ion over the interfering ion. The electrode can be used for at least 3 months without any divergence in potential. It was successfully applied to directly determine lead ions in solution and used as an indicator electrode in potentiometric titration of lead ions with EDTA.     

Attachment of Nanoparticles to Pyrolytic Graphite Electrode and Its Application for the Direct Electrochemistry and Electrocatalytic Behavior of Catalase

Abstract

A highly hydrophilic, nontoxic, and conductive effect of colloidal gold nanoparticles (GNP) and multi-walled carbon nanotubes (MWCNT) on pyrolytic graphite electrode has been demonstrated. The direct electron transfer of catalase (CAT) was achieved based on the immobilization of MWCNT/CAT-GNP on a pyrolytic graphite electrode by a Nafion film. The immobilized catalase displayed a pair of well-defined and nearly reversible redox peaks in 0.1 M phosphate buffer solution (PBS) (pH 6.98). The dependence of E°′on solution pH indicated that the direct electron transfer reaction of catalase was a single-electron-transfer coupled with single-proton-transfer reaction process. The immobilized catalase maintained its biological activity, showing a surface controlled electrode process with an apparent heterogeneous electron transfer rate constant (k _s) of 1.387±0.1 s^?1 and charge-transfer coefficient (α) of 0.49, and displayed electrocatalytic activity in the electrocatalytic reduction of hydrogen peroxide. Therefore, the resulting modified electrode can be used as a biosensor for detecting hydrogen peroxide.     

Glucose Oxidase Electrode Based on Graphite and Methylthio Tetrathiafulvalene as Mediator

Abstract

A glucose sensor based on glucose oxidase and a new mediator - 4,5-dimethyl-4′-methylthio-Δ 2,2′-bi-1,3-dithiole (MTTTF) is described. The background for sensor action is the effective MTTTF cation interaction (apparent bimolecular constant (2.0+/-0.5)?10⁶ M^?1 s^?1 at 25°C and pH 7.0) with reduced glucose oxidase and the high electrochemical rate of mediator transformation.

A glucose sensor was prepared by adsorbing mediator (MTTTF) and glucose oxidase on graphite rods. The sensor responds to glucose at electrode potentials higher than 50 mV vs SCE, but the maximal activity is obtained at a potential of 250 mV. In air saturated solution the electrode shows a non-linear calibration curve with a half-saturation concentration 10.4 mM and Hill coefficient 2.08 at 250 mV. Sensor response changes little at pH 6.5–8.0. The energy of activation of the sensor response calculated from the Arrhenius equation was 64.5 kJ/mol, and the temperature coefficient at 25°C was 9.2%.     

Lignosulfonate‐Modified Electrode for Electrocatalytic Reduction of Acidic Nitrite

Sodium lignosulfonate (LS) undergoes oxidative electropolymerization on a glassy carbon (GC) electrode from sulfuric acid solution to form a chemically modified electrode exhibiting anionic character and redox activity. Cyclic voltammetry reveals the existence of two redox systems at E°′ values of +0.29 and +0.53 V, respectively. Peak currents are proportional to the scan rate as expected for surface confined systems. The GC|poly‐LS electrode shows electrocatalytic activity toward the reduction of acidic nitrite. When operating in a constant potential amperometric mode (at 0.0 V, vs. Ag/AgCl), a linear relationship between nitrite concentration and reduction current is observed over the range of 1 to 250 μM. The detection limit reaches 0.3 μM (S/N=3). The electrode may be practically applied for nitrite determination in human saliva.     

A Novel Adsorptive Square Wave Voltammetric Method for Pico Molar Monitoring of Lorazepam at Gold Ultra Microelectrode in a Flow Injection System by Application of Fast Fourier Transform Analysis

Abstract

This paper describes development of a new analysis system for determination of lorazepam by a novel square wave voltammetry method to perform a very sensitive method. The method used for determination of lorazepam involves measuring the changes in admittance voltammogram of a gold ultramicroelectrode (in 0.05 M H₃PO₄ solution) caused by adsorption of the lorazepam on the electrode surface. Variation of admittance in the detection process is created by inhibition of oxidation reaction of the electrode surface, by adsorbed lorazepam. Furthermore, signal-to-noise ratio is significantly increased by application of discrete fast Fourier transform (FFT) method, background subtraction, and two-dimensional integration of the electrode response over a selected potential range and time window. Also in this work, some parameters such as SW frequency, eluent pH, and accumulation time were optimized. Calibration plots are given for solutions containing 10^?6–10^?11 M of lorazepam. The detection limit is calculated to be 6.0 × 10^?12 M (～ 2 pg/ml). The relative standard deviation at concentration 3.0 × 10^?8 M is 6.1% for 5 reported measurements.     

Voltammetric Characteristics of Adenine and Adenosine at a Copper Electrode and Their Application to End-Column Amperometric Detection for Capillary Zone Electrophoresis

Abstract

The voltammetric characteristics of adenine and adenosine at a copper electrode were investigated in different supporting electrolytes. It was found that citric acid had some activating effect at the copper electrode and the oxidation reaction of adenine and adenosine at the copper electrode could be observed in the solution, if PO₄ ^3? and citric acid exist simultaneously. The oxidation reaction of adenine and adenosine could be applied to capillary zone electrophoresis with end column amperometric detection for their determination. The method was applied to the determination of adenine in yeast-RNA.     

A Solid Potassium Ion Selective Electrode

Abstract

A solid membrane potassium selective electrode has been developed. The following parameters of the potassium ion electrode were investigated; Nernstian response to potassium ion activity, response time to step changes in potassium ion activity, the effect of pH on potassium response, and the electrode response to other cations.     

Flow Injection Analysis of Lactate and Lactate Dehydrogenase Using an Enzyme Membrane in Conjunction with a Modified Electrode

Abstract

A modified electrode for H₂O₂ oxidation, consisting of Pd/Au sputtered on carbon was covered with a lactate oxidase membrane and used in a FIA manifold for selective determination of lactate. The linear range was 0.01-3 mM lactate and up to 200 samples per hour were measured with a relative standard deviation of 1%. Interferences from ascorbic acid and NADH were small because of the low potential of the modified electrode. The lactate oxidase membrane electrode was also used for measurement of lactate dehydrogenase activity using direct injection of the sample into a carrier stream containing pyruvate and NAD⁺.     

Electrochemical Determination of Cholinesterase Activity and Indication of Its Inhibitors Using A Thick-Film Metallized Platinum Electrode

Abstract

A thick-film metallized platinum electrode operated in the voltammetric mode is used to determine the cholinesterase activity in human serum and erythocytes. Inhibitors of the enzyme can also be quanitified by the electrode with cholinesterase either immobilized directly on the electrode surface or dissolved in the test medium.     

Original Potentiometric Ytterbium(III) PVC-Membrane Sensor Based on N1,N2-Bis-[1-(2-hydroxy-1,2-diphenyl)ethylidene]ethanedihydrazide

Abstract

In this work, we describe the construction, performance, and applications of an original ytterbium(III) sensor based on N¹,N²-bis-[1-(2-hydroxy-1,2-diphenyl)ethylidene]ethanedihydrazide (BHDEH), which acts as a suitable carrier. Because it has a low detection limit of 4.2 × 10^?7 M, the sensor response for the Yb(III) ion is Nernstian over a wide concentration range: four decades of concentration (1.0 × 10^?6 to 1.0 × 10^?2 M). The response time of the electrode is less than 10 s, it can be used in the pH range of 3.2–8.3, and its duration is at least 2 months without any considerable potential divergence. The sensor revealed very good selectivity for Yb(III) in the presence of several metal ions. To investigate the sensor analytical applicability, it was tested as an indicator electrode in the potentiometric titration of Yb(III) solution with standard EDTA solution. The proposed electrode was also used to determine fluoride ions in mouthwash.