Showing papers in "Electroanalysis in 1999"

Polymer Membrane Ion-Selective Electrodes–What are the Limits?

Abstract: This article reviews recent advances in the field of potentiometric solvent polymeric membrane electrodes. These sensors have found widespread applications in a variety of fields, especially in the area of clinical diagnostics. Emphasis is given on the discussion of the theoretical and practical limits of ionophore-based ion-selective electrodes, with a special focus on electrode sensitivities, characterization of selectivities and dramatic improvements in detection limits. Advances in ionophore design and in the underlying model assumptions are also discussed. It is shown that a multitude of exciting new research possibilities have recently emerged in this well—established field.

Electropolymerized Azines: A New Group of Electroactive Polymers

Abstract: The electropolymerization of different azines from aqueous solutions was investigated. The structure of monomers was systematically varied changing both the nature of the second heteroatom and the substituents in the aromatic rings. Considering the electropolymerization process and the properties of the resulting polymers one can denote polyazines as a new group of electroactive polymers. The electrochemical and spectroelectrochemical investigation of polyazines was done. A hypothesis on azine polymer structure is presented.

A Feasibility Study of a Capacitive Biosensor for Direct Detection of DNA Hybridization

Abstract: This preliminary study was performed to prove the feasibility of a direct capacitive DNA biosensor for detection of nucleic acids. Two different methods for immobilization of the oligonucleotide probes were used. The first type of sensor was composed of a gold rod with a self-assembled monolayer of a 26-base long oligonucleotide probe, modified with an SH-group at the 5 0 -end. Coverage studies showed that only around 20% of the surface was covered, probably due to the bulky nature of the probes. Hybridization studies performed in a flowthrough cell showed selectivity towards a DNA sample containing single stranded fragments of cytomegalo virus (CMV) possessing a complementary sequence. As few as 25 molecules could be detected at sample concentrations of 0.2 attomolar with an injection volume of 250mL. Controls with fragments of double-stranded CMV and single-stranded hepatitis B virus and tyrosinase mRNA gave all lower responses. The other type of sensor was modified by covalent immobilization of a phosphorylated 8-base long oligonucleotide probe to a self-assembled monolayer of cysteamine. This biosensor also showed selectivity against single stranded fragments of CMV and also in this case as few as 25 molecules could be detected.

Analytical Applications of Silica-Modified Electrodes -A Comprehensive Review

Abstract: This article aims to review the electroanalytical applications of chemically modified electrodes involving silica and silica-containing matrices. After a brief introduction to silica chemistry, the implication of pure and grafted silicas in electrochemistry, as well as that of the silica-based materials originating from the sol-gel technology, will be discussed. The interest of silica-modified electrodes (SiO2-MEs) with respect to electroanalysis will be pointed out by means of several examples. In the last five years, successful applications of SiO2-MEs have been achieved. Among them, silica grafted with inorganic films or organic moieties was successfully applied to the selective preconcentration or the electrocatalytic detection of various analytes. Also, the ability of the sol-gel chemistry to encapsulate enzymes within a silica matrix without preventing its activity was exploited in the development of new amperometric biosensors. These promising fields are thought to lead to significant advances in the near future, because of the explosive growth in both the synthesis of novel mesoporous organically-modified silica-based materials and the huge development of sol-gel chemistry in tailoring and construction of new modified electrodes.

Determination of Uranyl Ions Using Poly(vinyl chloride) Based 4‐tert‐Butylcalix[6]arene Membrane Sensor

Abstract: Membranes of 4-tert-butylcalix[6]arene I as an ion active material in poly(vinyl chloride) (PVC) based matrix have been tried for uranyl-selective sensors. The effect of anion excluder, sodium tetraphenylborate (NaTPB) and plasticizers, tris(2-ethylhexyl)phosphate (TEP), tributyl phosphate (TBP), dibutyl phthalate (DBP), 1-chloronaphthalene (CN) and diphenyl ether (DPE) on the performance of the membrane electrodes has also been studied. It was observed that the membrane having the composition I: PVC:NaTPB:TBP in the ratio 7:52:6:35 gave the best results with a wide working concentration range of3.9× 10–5–1.0–× 10–1 M, Nernstian slope of 29.1 mV per decade of activity, fast response time of 10 s and moderate selectivity over a number of mono-, bi- and trivalent cations. The sensor works well in the pH range 2.2–3.2 and can be successfully employed for the estimation ofUO22+in partially nonaqueous medium having upto 20% (v/v) nonaqueous content. The sensor was used for the estimation of uranium as uranyl ions in simulated mixtures where the results are comparable to those obtained from radiochemical methods.

Electropolymerized Azines: Part II. In a Search of the Best Electrocatalyst of NADH Oxidation

Abstract: A comparative investigation of catalytic activity of different polyazines in NADH electrooxidation is reported. The structure of azine monomers taken for electropolymerization was varied systematically changing both the second heteroatom and the substituents of aromatic rings. It was found that the monomer structure affects catalytic activity of the resulting polymer in the following way: (i) additional substitution of benzene ring by alkyl group reduced the catalytic activity, (ii) polymerized phenoxazine Brilliant Cresyl Blue was a better electrocatalyst than the corresponding phenothiazine (o-Toluidine Blue), (iii) ring substitution with only tertiary nitrogen atoms as ligands provides higher catalytic activity, (iv) higher redox potential of the polymer also provides higher catalytic activity.

Determination of Surface pKa of SAM Using an Electrochemical Titration Method

Abstract: A self-assembled monolayer (SAM) of 3-mercaptopropionic acid (HSCH2CH2COOH) was formed on a gold electrode. The effect of the charge of the end group on the electrochemical response of Fe(CN)(6)(3-) at the SAM modified electrode was studied by using cyclic voltammetry. At high pH, when the -COOH groups are dissociated, Fe(CN)(6)(3-) current is suppressed; as the solution pH is lowered, Fe(CN)(6)(3-) current increases. The electrochemical titration curve was obtained by correlating the currents to the different electrolyte pH values, from which the surface pK(a) was obtained to be 5.2+/-0.1. Furthermore, a calculation equation was presented to simulate the electrochemical titration. As comparison, the surface pK(a) was also measured by contact angle titration as 5.6+/-0.1. The surface pK(a) values determined by the two methods in our work are consistent and accurate.

Incorporation of Erythrocytes into Polypyrrole to Form the Basis of a Biosensor to Screen for Rhesus (D) Blood Groups and Rhesus (D) Antibodies

Abstract: Antibodies to Rhesus (Rh) antigens are important indicators in screening for haemolytic disease of the new-born (HDN) and autoimmune haemolytic anaemia (AIHA). Identification of the Rh antibodies formed by immune stimulation is also essential in order to maximize the in vivo survival time of transfused erythrocytes. Currently this is performed by agglutination based assays that are time consuming. A prototype of an immuno-biosensor for detecting antibodies recognizing the Rhesus blood group antigen, Rh (D), was constructed. Human erythrocytes were incorporated into a conducting polypyrrole, polyelectrolyte matrix. The process was followed by using oximetry and light microscopy to demonstrate the integrity of the erythrocytes in the polymerization solution and in the polymer matrix; cyclic voltammetry and resistometry for electrochemical characterization of the polymer and then agglutination, ELISA techniques and cyclic resistometry for analysis of the immuno response from antigen/antibody binding. Antigen/antibody binding could be detected qualitatively by using resistometry while cycling the polymer between +0.35 V and –0.7 V (vs. Ag/AgCl). A characteristic cyclic change in resistance (a resistogram) was recorded. After addition of Anti-Rh (D) antibody (250 µg/mL), the change in resistance during the resistogram decreased by 1.1 Ω (p<0.0008) in polymers containing Rh (D) positive erythrocytes, whereas polymers without erythrocytes showed no significant change.

Potentiometric Anion Selective Sensors

Abstract: In comparison with selective receptors (and sensors) for cationic species, work on the selective complexation and detection of anions is of more recent date. There are three important components for a sensor, a transducer element, a membrane material that separates the transducer element and the aqueous solution, and the receptor molecule that introduces the selectivity. This review deals with potentiometric transduction elements that convert membrane potentials into a signal. The structure and properties of membrane materials is discussed. The nature of the anion receptor ultimately determines the selectivity. Both coordination chemistry and hydrogen bonding have been used to design anion receptor molecules. The integration of all three elements by covalent linkage of all elements in durable sensorsystem concludes the review.

A Comparative Study of the Electrocatalytic Activities of Some Metal Hexacyanoferrates for the Oxidation of Hydrazine

Abstract: The electrocatalytic properties of electrodes modified with metal [Cr(III), Mn(II), Fe(II), Co(II), Ni(II), Cu(II), Zn(II), Ag(I), Cd(II) and In(III)] hexacyanoferrates(II) were studied by cyclic voltammetry using hydrazine as substrate. The electrodes were prepared by mechanically transferring microparticles of the hexacyanoferrates onto the surface of a paraffin impregnated graphite electrode. The catalytic activities of these compounds were compared based on the ratio of the total current for the catalytic oxidation to the current for the modified electrode. This ratio has been calculated at the peak potentials of the cyclic voltammograms and also at some fixed anodic potentials. As expected for an electrocatalytic reaction, it was observed that the current ratio is higher at slow scan rates and lower at high scan rates. The catalytic activities of hexacyanoferrates are rather complex and they do not follow any uniform pattern. The hexacyanoferrates of manganese, zinc and indium turned out to possess the highest catalytic activity.

Application of Electronic Tongue for Quantitative Analysis of Mineral Water and Wine

Abstract: An electronic tongue comprising 29 potentiometric chemical sensors and pattern recognition tools for the data processing has been applied for the analysis of Italian produced mineral waters and dry red wines. The electronic tongue appeared to be capable to distinguish between different sorts of mineral waters, between contaminated by organic matter waters and pure ones and between Barbera wine samples of the same denomination but from different vineyards. Simultaneously with qualitative recognition, quantitative determination of some components in the water and the wine has been performed. The electronic tongue can be highly valuable in food quality control.

Mercury(II) Ion‐Selective Electrode Based on Dibenzo‐diazathia‐18‐crown‐6‐dione

Abstract: A PVC-based membrane of dibenzodiazathia-18-crown-6-dione reveals a Nernstian potentiometric response for Hg2+over a wide concentration range (8.0×10−6 to 1.0×10−2mol dm−3). It has a response time of about 10s and can be used for at least 3 months without any divergence. The proposed electrode revealed very good selectivities for Hg2+ over a wide variety of alkali, alkaline earth, transition and heavy metal ions and could be used in a pH range of 0.5–2.5. It was successfully used as an indicator electrode in potentiometric titration of mercury ions.

DNA Electrochemical Biosensor for the Detection of Short DNA Sequences Related to the Hepatitis B Virus

Abstract: Nucleic acid hybridization forms the basis for the diagnosis of genetic and infectious diseases. Electrochemical biosensors, coupling the inherent specificity of DNA recognition reactions with the high sensitivity of physical transducers, thus hold great promise for sequence-specific detection. An electrochemical biosensor for the voltammetric detection of DNA sequences related to the hepatitis B virus (HBV) is described. Synthetic single-stranded oligonucleotides (“probe”) have been immobilized onto carbon paste electrodes with the adsorption at a controlled potential. The probes were hybridized with different concentrations of complementary (‘target’) sequences. The formed hybrids on the electrode surface were evaluated by differential pulse voltammetry using cobalt phenanthroline, [Co(phen)33+] as the indicator of hybridization reaction.

A Thin‐Layer Electrochemical Flow Cell Coupled On‐Line with Electrospray‐Mass Spectrometry for the Study of Biological Redox Reactions

Abstract: A three electrode thin-layer, flow-by electrode cell was coupled on-line with electrospray-mass spectrometry (i.e., EC/ES-MS) for the study of biological redox reactions. The cell made use of a commercially available 6.0 mm diameter, offset glassy carbon disk working electrode, a Ag/AgCl reference electrode, and a working/counter electrode spacing gasket (16 μm), along with a home-built PEEK counter electrode block with a Pt foil (7.0 mm wide) counter electrode (cell volume ca. 1.1 μL). Off-line hydrodynamic voltammetry experiments and electrolytic conversion efficiency measurements using chronocoulometry were used to characterize the performance of the cell. On-line EC/ES-MS conversion efficiency was found consistent with the off-line results and the EC/ES-MS response time was measured as 5.1 s at a flow rate of 30 μL/min and 2.4 s at 62 μL/min. The use of this hybrid system for the study of the products of biologically relevant redox reactions was demonstrated using the oxidation of dopamine in aqueous CH3OH (pH 4.0) as a test case. Tandem mass spectrometry experiments provided evidence to conclude that the structure of the two major dopamine oxidation products observed in the ES mass spectra were 5,6-dihydroxyindoline and 5,6-hydroxyindole. Reaction of oxidized dopamine with the surrogate biogenic nucleophile benzene thiol resulted in the formation of mono-, di-, and tri-benzene thiol addition products. These spectra also indicated that the reaction of oxidized dopamine with benzene thiol was significantly faster than the intermolecular cyclization reaction of this intermediate, which normally produces 5,6-dihydroxyindoline.

Glucose Nanosensor Based on Prussian-Blue Modified Carbon-Fiber Cone Nanoelectrode and an Integrated Reference Electrode

Abstract: An enzyme nanosensor, based on a carbon fiber cone nanoelectrode modified by codeposition of Prussian blue (PB) and glucose oxidase (GOD), has been constructed. The nanosensor displays a low-potential electrocatalytic detection of the enzymatically liberated hydrogen peroxide, along with good reproducibility and high selectivity. The dependence of the nanosensor response upon parameters of the sensor preparation (e.g., amount of GOD, PB deposition time, Nafion coverage), and upon measurements conditions (e.g., pH, applied potential) was evaluated. An operating potential of –0.1 V (vs. Ag/AgCl) yielded the highest selectivity towards glucose, with no interference from ascorbic acid. The lifetime of the nanosensor is more than two weeks. Such miniaturization and analytical performance offer great promise for measurements of glucose in extremely small volumes.

Remote Biosensor for In-Situ Monitoring of Organophosphate Nerve Agents

Abstract: A remote electrochemical biosensor for field monitoring of organophosphate nerve agents is described. The new sensor relies on the coupling of the effective biocatalytic action of organophosphorus hydrolase (OPH) with a submersible amperometric probe design. This combination results in a fast, sensitive, selective, and stable response at large sample-instrument distances. Such attractive performance is illustrated for direct measurements of micromolar levels of paraoxon and methyl parathion in untreated river water samples. Unlike multi-step inhibition biosensors, the remote OPH probe offers single-step direct measurements, and is thus highly suitable for the continuous monitoring task. Variables relevant to field operations are discussed, along with prospects for remote monitoring and early detection of nerve agents.

Biosensors Based on Thin Lipid Films and Liposomes

Abstract: This article reports the theory and analytical applications of thin lipid films. Recent advances of electrochemical devices based on lipid membranes have lead to reports of construction of biosensors for environmental and food applications, and may provide opportunities for commercial fabrication. The methods of formation of lipid membranes on various supports including metals (silver, gold, stainless steel), agar, conducting polymers and ultrafiltration membranes have provided stabilization of lipid films with a diversity of analytical applications in real samples. Methods of immobilization and incorporation of various functional macromolecules are summarized. Several examples of the application of various methods for study of physical properties of supported bilayer lipid membranes are described. Applications of lipid-based biosensors in analytical chemistry for determination of compounds are demonstrated, including a diversity of chemical compounds such as environmental pollutants (ammonia and carbon dioxide, cyanide ions, etc.) and food toxins (aflatoxin M1and direct detection of toxin in real samples such as milk and milk preparations). Methods for application of liposomes as a sensing system are also summarized.

Cytochrome C Based Superoxide Sensor for In Vivo Application

Abstract: A superoxide sensor based on immobilized cytochrome c has been applied for short time implantation. It has a “stabilization time” of several minutes and the working stability during implantation allows the investigation of the role of superoxide during ischaemia and reperfusion. Its current output is in the pA range.

Electrocatalytic Reduction of Dioxygen at the Surface of Glassy Carbon Electrodes Modified by Some Anthraquinone Substituted Podands

Abstract: The preparation and electrochemical characterization of glassy carbon electrodes modified by some recently synthesized anthraquinone substituted podands were investigated. These compounds were found to be strongly and irreversibly adsorbed on the preanodized glassy carbon electrodes. The electrochemical behavior and stability of the modified electrodes were studied by cyclic voltammetry in acidic aqueous solutions. The influence of pH on the electrochemical behavior of the electrodes was studied and a pH of 4.5 was chosen as the optimum working pH. The ability of the modified electrodes as potential electrocatalysts for the reduction of dioxygen to hydrogen peroxide was tested by cyclic voltammetry, chronomperometry and rotating disk electrode voltammetry. They showed excellent electrocatalytic ability for the reduction of O2at pH 4.5 with overpotentials ranging from 380 to 470 mV lower than the plain glassy carbon electrode. The heterogeneous rate constants for the reduction of O2 at the surface of these modified electrodes were determined by the use of Koutecky-Levich plots. In addition, the apparent diffusion coefficients of O2 in buffered aqueous O2-saturated solution was estimated.

Electrocatalytic Oxidation of Hydrazine at Glassy Carbon Electrode Modified with Electrodeposited Film Derived from Caffeic Acid

Abstract: A caffeic acid modified glassy carbon electrode was constructed for the electrocatalytic detection of hydrazine. This modified electrode enabled hydrazine to be catalytically oxidized at a greatly reduced overpotential and in a wide operational pH range (pH 5.0–9.0). The overall number of electrons involved in the catalytic oxidation of hydrazine and the number of electrons involved in the rate determining step are 4 and 1, respectively. The rate constant for the catalytic reaction was evaluated by chronoamperometry, cyclic voltammetry and RDE voltammetry and was found to be around 10–3 cm s–1. Experiments were performed to characterize the electrode as an amperometric sensor for the determination of hydrazine and linear calibration plots were obtained over the range of 0.01 to 2.0 mM for linear sweep voltammetry and chronoamperometry. Amperometry in stirred solution exhibits a linear dynamic range of 2.5 μM to 1.0 mM (correlation coefficient 0.9993) and a detection limit of 0.4 M. The resulting modified electrode retains its initial response for at least 2 weeks if stored in phosphate buffer (pH 4). The precision of amperometry was found to be 1.8 % for replicate determinations of a 1.0 mM solution of hydrazine (n=12).

Molecularly Imprinted Polymer-Coated Quartz Crystal Microbalance for Detection of Biological Hormone

Abstract: A piezoelectric sensor with a molecularly imprinted synthetic polymer receptor was developed. A plant hormone, indole-3-acetic acid (IAA) was used as the model target molecule, and an IAA-imprinted polymer was coated onto a 9 MHz AT-cut quartz crystal microbalance. The sensor showed a selective response and gave a linear relationship between frequency shift and amount of IAA in the range from 10 to 200 nmol.

Electrochemical Study of Flavins, Phenazines, Phenoxazines and Phenothiazines Immobilized on Zirconium Phosphate

Abstract: Adsorption of a number of flavins, phenazines, phenoxazines, and phenothiazines on zirconium phosphate (ZP) was carried out in aqueous solution. The adsorbed organic dyes on ZP were used to prepare modified carbon paste electrodes. The electrochemical properties of the immobilized dyes were investigated and also their possible use to electrocatalytically oxidize NADH. The formal potential (E0′) of most of the adsorbed flavins, phenoxazines, and phenothiazines shifted markedly towards more positive potentials compared with their values in solution. The pH of the contacting solution did not affect their E0′-values between pH 1 and 9. The phenazines did neither present good electrochemical response nor electrocatalytic activity for NADH oxidation and their E0′-values remained pH dependent. In contrast, adsorbed flavins, phenoxazines, and phenothiazines presented good electron transfer rates between the electrode and the adsorbed molecule dye at pH 7.0.

Cytochrome c/Clay‐Modified Electrode

Abstract: The direct electrochemical behavior of a small metalloprotein cytochrome c incorporated in a montmorillonite-modified membrane is described. The interaction of cytochrome c with the clay colloidal particles was investigated using electrochemical and spectrophotometric methods. The incorporated cytochrome c in the clay-modified membrane displayed a diffusion-controlled electrode process and still maintained its biological activity.

A New Glucose Biosensor Based on Sandwich Configuration of Organically Modified Sol-Gel Glass

Abstract: A new glucose biosensor was developed based on the sandwich configuration of organically modified sol-gel glasses. The new sol-gel glass was developed using 3-aminopropyltrimethoxy silane and 2-(3,4-epoxycyclohexyl)-ethyltrimethoxy silane. Two types of sol-gel glasses were used to develop glucose biosensors that differ in absence (A) and the presence of graphite powder [particle size 1–2 μ] (B). An additional additive (polyethylene glycol, Mol. wt. 6000) was also incorporated in both types of the upper sol-gel glass layer. The new sol-gel matrix with immobilized glucose oxidase was analyzed by scanning electron microscopy (SEM).The sandwich configuration was developed using a bilayer of sol-gel glasses having a layer of glucose oxidase in between the bilayer. This electrode with special configuration was used to form a layer of sol-gel glass of ca. 0.2 mm thickness. The performance of sol-gel glasses (A & B) was analyzed based on cyclic voltammetry using ferrocene monocarboxylic acid. The results show a diffusion limited condition of ferrocene across the sol-gel matrix. The characterization of sol-gel glass based biosensor was recorded based on the cyclic voltammograms in absence and presence of glucose. The results show an increase in anodic current which is also characteristic of hydrogen peroxide oxidation in both cases (A & B). The responses of the sol-gel glasses based biosensors were analyzed based on chronoamperometric measurements. An amplified signal on the addition of the same concentrations of glucose was recorded with the B-type sol-gel glass electrode which was attributed to its relatively high porosity and better conductivity of the graphite loaded sol-gel glass. These observations were in accordance with the results on the diffusion of ferrocene and the magnitude of anodic current resulting from hydrogen peroxide oxidation. The calibration plots for glucose analysis using both type of sensors are reported. Data on the mediated electrochemical oxidation of glucose oxidase using soluble ferrocene were also reported based on cyclic voltammograms and amperometric measurement.

Voltammetric Detection of the DNA Interaction with Copper Complex Compounds and Damage to DNA

Abstract: Binding reactions of copper(II) complexes of 1,10-phenanthroline (phen) and tetraaza macrocyclic ligand TAAB with calf thymus DNA have been investigated voltammetrically at bare and DNA modified glassy carbon electrodes. The values of binding constant, binding site size, ratio of binding constants for the reduced and oxidized copper complex forms as well as information on the electrostatic and intercalative binding modes were obtained by solution and surface-based methods. The Cu(phen)22+complex mediates the dsDNA cleavage to a higher degree than Cu(TAAB)2+as indicated by the redox marker Co(phen)33+and the anodic signal of the DNA base. A procedure for the damage to DNA detection using dsDNA/GCE biosensor is proposed.

Steady‐State Model Calculations Predicting the Influence of Key Parameters on the Lower Detection Limit and Ruggedness of Solvent Polymeric Membrane Ion‐Selective Electrodes

Abstract: Using a previously described steady-state model, the influence of various parameters on the response function of ion-selective electrodes is investigated. The parameters studied include activities of interfering ions of the sample and inner solution, selectivity coefficients, lipophilicity of the analyte, concentrations of ionophore and lipophilic ionic sites in the membrane, the thickness of the Nernstian diffusion layer and the membrane as well as the diffusion coefficients in both phases. The results obtained provide indications as to how the lower detection limit of potentiometric sensors and their ruggedness can be improved.

Sono-cathodic stripping voltammetry of lead at a polished boron-doped diamond electrode: Application to the determination of lead in river sediment

Abstract: Ultrasonically-assisted cathodic stripping voltammetry at a boron-doped diamond electrode, has been developed for the detection of lead. At concentrations above 3 µM, linear sweep voltammetry was used to give the analytical signal from a cathodic strip of electrodeposited PbO2; linearity was observed from 3–100 µM, with 3 µM being the lower detection limit. Square-wave voltammetry was then employed for the cathodic stripping step, to lower the detection limits of the technique while retaining linearity to the order of 10–8 M. The procedure involves ultrasonic electrode cleaning, cathodic preconditioning and sono-anodic deposition of PbO2. This novel analytical tool is mercury-free, oxygen insensitive and highly specific towards lead, yet still offers scope for further elemental diversity, particularly for the detection of copper and iron. The square-wave sono-cathodic stripping voltammetry technique was combined with an ultrasonically assisted acid digestion protocol to successfully determine the lead content of a contaminated sample of river sediment, offering significant time saving over the currently used analytical procedure.

Thiocyanate-Selective Membrane Electrode Based on (Octabromotetraphenylporphyrinato)manganese(III) Chloride

Abstract: The response characteristics of a new solvent-polymeric membrane electrode with unique selectivity and detection limit toward SCN– ion are reported. The electrode is prepared by incorporating (octabromotetraphenylporphyrinato)manganese(III) chloride into a plasticized PVC-membrane. The resulting sensor exhibits anti-Hofmeister selectivity pattern with high specificity for thiocyanate over other anions, including lipophilic inorganic anions. The electrode has a linear dynamic range between 1.0 and 4.8×10–7 M, with a Nernstian slope of 58.3±0.8 mV/decade and a detection limit of 3.2×10–7 M. The electrode can be used for at least 3 months without any considerable divergence in potential. It was used as an indicator electrode in potentiometric titration of SCN– ion and in direct determination of thiocyanate in urine samples.

A Pyruvate Oxidase Electrode Based on an Electrochemically Deposited Redox Polymer

Abstract: A redox polymer-modified, multilayer biosensor for the determination of pyruvate and phosphate in oxygen-free samples has been developed. The new, highly conductive redox polymer was produced by potentiostatic copolymerization (+1.4 V vs. Ag/AgCl) of Os(bipy)2pyCl-modified pyrrole monomer (6×10–3 mol L–1) and thiophene (1×10–3 mol L–1) on top of a platinized glassy-carbon electrode. The redox polymer-coated platinum black layer with increased active surface area permitted the adsorption of pyruvate oxidase as the biological recognition element and efficient electron transfer from enzyme-bound FAD-groups to the electrode. Pyruvate was detected at anodic potentials (350–500 mV) in oxygen-free solution in the presence of phosphate as the cosubstrate with a linear range from 0.02×10–3 to 0.3×10–3 mol L–1. A sensitivity as high as 0.2 A cm–2 mol–1 L was obtained. Phosphate was measured similarly between 0.02×10–3 and 0.5×10–3 mol L–1 in the presence of pyruvate as co-substrate. The sensitivity of the sensor dropped to about 12 % after 10 days. Since interference by ascorbate, due to the high formal potential of the used Os(bipy)2pyCl-group, could be a problem in real samples, coverage of the adsorbed enzyme by a polycationic size exclusion layer of polypyrrole was investigated. Compared to former enzyme electrodes utilizing pyruvate oxidase, the new approach offered an unprecedentedly high sensitivity, O2- and thiamindiphosphate-independent operation and presented a large step towards electrochemical pyruvate determination in vivo.