Disposable amperometric sensor for neurotransmitters based on screen-printed electrodes modified with a thin iridium oxide film.

Potential cycling in the range from -0.2 to +1.2 V is used for the electrodeposition of hydrous iridium oxide films onto a screen-printed electrode from a saturated solution of alkaline iridium(III) solution. The iridium oxide redox couple shows a stable and obvious reversible redox, with the formal potential being pH dependent in the range 1-14. The properties, stability and electrochemical properties of iridium oxide films were investigated by cyclic voltammetry. A modified electrode showed excellent catalytic activity toward the oxidation of neurotransmitters (catecholamines) over a wide pH range (2-8). The electrocatalytic behavior is further exploited as a sensitive detection scheme for adrenaline and dopamine by hydrodynamic amperometry. Under the optimized conditions, the calibration curves are linear in the concentration range 0.1-70 and 0.1-15 microM for dopamine and adrenaline determination, respectively. The detection limit and sensitivity are 30 nM and 30 nA/microM for adrenaline and 15 nM and 80 nA/microM for dopamine. Finally, the analytical performance of the modified electrode was demonstrated for the elimination of interference by uric acid in catecholamines determination when present in a 1000-fold concentration excess.     

High performance electrochemical sensor based on modified screen-printed electrodes with cost-effective dispersion of nanostructured carbon black

A novel sensor based on a screen-printed electrode (SPE) modified with a stable dispersion of commercially available carbon black (CB) N220 was developed. This probe showed significantly enhanced electrochemical activity relative to a bare SPE when tested with ferricyanide, epinephrine, norepinephrine, benzoquinone and NADH. When challenged in amperometric batch mode with NADH, the response was stable and revealed a linear dependence up to 2·10^?4 mol L^?1 with a detection limit of 3·10^?7 mol L^?1. The analytical performance, coupled with the low cost of the CB nanomaterial, suggests that this sensor holds promise for electrochemical applications.     

Multiwalled carbon nanotube modified screen-printed electrodes for the detection of p-aminophenol: optimisation and application in alkaline phosphatase-based assays

Carboxylated multiwalled carbon nanotubes (MWCNT-COOH) were used to modify the working electrode surface of different screen-printed electrodes. The effect of this modification on the electrodic characteristics (double layer capacitance, electroactive area and heterogeneous rate constants for the electron transfer) was evaluated and optimized for the cyclic voltammetric determination of p-aminophenol. The enzymatic hydrolysis of p-aminophenylphosphate was employed for the quantification of alkaline phosphatase, one of the most important label enzymes in immunoassays. Finally, ELISA assays were carried out to quantify pneumolysin using this enzymatic system. Results obtained indicated that low superficial densities of MWCNT-COOH (0.03-0.06 μg mm⁻²) yielded the same electrodic improvements but with better analytical properties.     

Electrochemical non-enzymatic sensing of oxalic acid based on PdPt-modified electrodes: application to the analysis of vegetable samples

Monatshefte für Chemie - Chemical Monthly - The electrocatalytic properties of carbonaceous electrodes, electrochemically modified with microquantities of PdPt (low-platinum-content alloy),...     

Detection of carbamic and organophosphorous pesticides in water samples using a cholinesterase biosensor based on Prussian Blue-modified screen-printed electrode

In the present paper, a comparative study using Co-phthalocyanine and Prussian Blue-modified screen-printed electrodes, has been performed. Both the electrodes have demonstrated an easiness of preparation together with high sensitivity towards thicoholine (LOD = 5 × 10⁻⁷ and 5 × 10⁻⁶ M for Co-phthalocyanine and Prussian Blue, respectively) with high potentialities for pesticide measurement. Prussian Blue-modified screen-printed electrodes were then selected for successive enzyme immobilization due to their higher operative stability demonstrated in previous works. AChE and BChE enzymes were used and inhibition effect of different pesticides was studied with both the enzymes. AChE-based biosensors have demonstrated a higher sensitivity towards aldicarb (50% inhibition with 50 ppb) and carbaryl (50% inhibition with 85 ppb) while BChE biosensors have shown a higher affinity towards paraoxon (50% inhibition with 4 ppb) and chlorpyrifos-methyl oxon (50% inhibition with 1 ppb). Real samples were also tested in order to evaluate the matrix effect and recovery values comprised between 79 and 123% were obtained.     

Highly sensitive detection of silybin based on adsorptive stripping analysis at single-sided heated screen-printed carbon electrodes modified with multi-walled carbon nanotubes with direct current heating

A new disposable multi-walled carbon nanotubes modified single-sided heated screen-printed carbon electrode (MWNT/ss-HSPCE) was fabricated. The electrochemical behavior of silybin was investigated by cyclic voltammetry and the probable electrode reaction mechanism was proposed. A simple and cheap direct current heating supplier was used to heating the electrode for adsorptive accumulation of silybin. The square wave voltammetric stripping peak current of silybin at MWNT/ss-HSPCE with an elevated electrode temperature of 50 °C only during accumulation step was dramatically improved compared with that at bare single-sided heated screen-printed carbon electrode (ss-HSPCE) without heating. This enhancement was mainly contributed to the combination of the advantages of multi-walled carbon nanotubes and electrically heated electrodes. Under optimum conditions, two detection linear ranges of silybin were from 1.0 × 10⁻⁹ to 1.0 × 10⁻⁷ M and 3.0 × 10⁻⁷ to 1.0 × 10⁻⁶ M. A detection limit of 5.0 × 10⁻¹⁰ M could be obtained (S/N = 3), which was more than two magnitudes lower than that at bare ss-HSPCE without heating. To the best of our knowledge, this was also at least two magnitudes lower than any others for electrochemical detection of silybin in the literature. Finally, the method was successfully applied to the determination of silybin in pharmaceutical tablets.     

Characterization of the gold-catalyzed deposition of silver on graphite screen-printed electrodes and their application to the development of impedimetric immunosensors

This paper describes the characterization of the gold-catalyzed deposition of silver on graphite screen-printed electrodes (SPEs) using electrochemical impedance spectroscopy (EIS) and the application of this approach to the development of impedimetric immunosensors. After applying −0.1 V for 45 s, the amount of electrodeposited silver quantitatively changes the magnitude of two elements of the electrical equivalent circuit: the interface capacitance, Ci, and the charge-transfer resistance, R_CT. Better correlations have been found when considering the R_CT since this parameter is almost exclusively dependent on the amount of deposited silver under these experimental conditions. This approach has been successfully applied to the development of an impedimetric immunosensor for aflatoxin M₁. The R_CT magnitude shows good correlation with the amount of gold immobilized on the electrode surface after a competitive assay and thus, with the toxin concentration. This approach has been found sensitive in a wide range of concentrations, from 15 to 1000 free-AFM₁ ppt with a limit of detection of 12 ppt.     

Electrocatalytic reduction of hydrogen peroxide on modified graphite electrodes: application to the development of glucose biosensors

The electrocatalytic activities of a series of compact graphites modified with microquantities of platinum metals (Pd or Pt+Pd) towards the electrochemical reduction of hydrogen peroxide were characterised. Operational parameters such as the optimal working potential, the influence of temperature and the resulting electrode characteristics were examined. The benefits of using graphite modified with Pt+Pd (mixture ratio 30%:70%) as the basic transducer in a glucose biosensor with improved sensitivity were demonstrated. It was proven that, under the working conditions chosen, the selected electrode (whether bare or covered with an enzyme layer) did not respond to any glutathione, uric acid or ascorbic acid (which all normally occur in biological fluids) present.     

NADH screen-printed electrodes modified with zirconium phosphate,Meldola blue,and Reinecke salt. Application to the detection of glycerol by FIA

Bulk screen-printed electrodes (bSPEs) modified with zirconium phosphate (ZrP) and Meldola blue (MB) and by electrochemical deposition of a Reineckate film (bMBZrPRs-SPEs) have been constructed and used as NADH sensors. Cyclic voltammetric investigation of these bulk electrochemically modified screen-printed electrodes revealed stable catalytic activity in oxidation of the reduced form of the coenzyme nicotinamide adenine dinucleotide (NADH). Flow-injection analysis (FIA) coupled with amperometric detection confirmed the improved stability of the bMBZrPRs-SPEs (10⁻⁴ mol L⁻¹ NADH, %RSD = 4.2, n = 90, pH 7.0). Other conditions, for example applied working potential (+50 mV relative to Ag|AgCl), flow rate (0.30 mL min⁻¹) and pH-dependence (range 4.0–10.0) were evaluated and optimized. A glycerol biosensor, prepared by immobilizing glycerol dehydrogenase (GDH) on the working electrode area of a bMBZrPRs-SPE, was also assembled. The biosensor was most stable at pH 8.5 (%RSD = 5.6, n = 70, 0.25 mmol L⁻¹ glycerol). The detection and quantification limits were 2.8 × 10⁻⁶ and 9.4 × 10⁻⁶ mol L⁻¹, respectively, and the linear working range was between 1.0 × 10⁻⁵ and 1.0 × 10⁻⁴ mol L⁻¹. To assess the effect of interferences, and recovery by the probe we analyzed samples taken during fermentation of chemically defined grape juice medium and compared the results with those obtained by HPLC.     

Electrochemical characterization of a superoxide biosensor based on the co-immobilization of cytochrome c and XOD on SAM-modified gold electrodes and application to garlic samples

This paper describes the characterization and optimization of an amperometric cytochrome c (cyt c)-based sensor for the determination of the antioxidant capacity of pure substances and natural samples. The cyt c and the xanthine oxidase (XOD) enzyme were co-immobilized on the electrode using the combination of several long-chain thiols. The self-assembled monolayer (SAM) was optimized in terms of composition and ratio between thiols. The immobilization protocol for both cyt c and XOD and the SAM formation time were evaluated through electrochemical methods, such as cyclic voltammetry (CV), square wave voltammetry (SWV), chronoamperometry (CA) and impedance spectroscopy (IS). Finally, the biosensor was applied to the determination of the antioxidant capacity of pure alliin and two compounds extracted from garlic bulbs.     

Immobilization of glucose oxidase on carbon paper electrodes modified with conducting polymer and its application to a glucose fuel cell

A carbon paper electrode was modified with the conducting copolymer of 3-methylthiopene and thiophene-3-acetic acid prepared electrochemically on the electrode, and an enzyme electrode was fabricated by covalent immobilization of glucose oxidase on the modified electrode. The modification with the conducting copolymer increased the surface area of the electrode and the amount of the immobilized enzyme. As a result, the enzyme electrode showed a high catalytic activity. Moreover, it was found that the increased surface area led to a high rate of electron transfer reaction between the electrode and p-benzoquinone employed as an electron mediator. The enzyme electrode fabricated with the modified carbon paper gave a larger glucose oxidation current than that fabricated with the bare one. In addition, the glucose oxidation current was found to increase with increasing content of the conducting copolymer in the modified carbon paper. Corresponding to the large glucose oxidation current, high performance was confirmed for the glucose fuel cell constructed with the enzyme electrode based on the modified carbon paper.     

Critical considerations on the practical application of orion ion-selective electrodes to sea and other natural water samples

Summary The paper deals with some practically important aspects of the application of Orion ion-selective electrodes in water analysis. The effects of the salinity of sea water, sample temperature and response time on the normal electrode function are considered. It is experimentally shown that carefull checking of these factors must precede the practical use of fluoride, ammonia, chloride, nitrate and lead electrodes in sea, lake, river and drinking water samples. This is of particular importance for automatic or continuous systems, where samples with similar concentrations are analysed.

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Kritische Betrachtungen zur praktischen Anwendung von Orion Ionen-selektiven Elektroden in Meer- und anderen Naturwasserproben

Zusammenfassung Einige praktisch wichtige Aspekte bei der Anwendung von Orion-Elektroden in der Wasseranalyse werden mitgeteilt. Die Einflüsse des Salzgehalts von Meerwasser, der Temperatur der Probe sowie der Einstellzeit auf die normale Funktion der Elektrode wurden untersucht. Es wurde experimentell festgestellt, daß eine sorgfältige Prüfung dieser Faktoren vor der praktische Anwendung von Fluorid-,Ammoniak-, Chlorid-, Nitrat- und Bleielektroden in Meer-, See-, Fluß- und Trinkwasser vorgenommen werden muß. Das gilt besonders für automatische oder kontinuierliche Systeme, bei denen die Konzentrationen ähnlich sind.

     

Nonenzymatic glucose sensor based on glassy carbon electrode modified with a nanocomposite composed of nickel hydroxide and graphene

We report on a nonenzymatic glucose sensor based on a glassy carbon electrode that was electrochemically modified with a nanocomposite prepared from nickel hydroxide and graphene. Scanning electron microscopy revealed that the nickel hydroxide in the nanocomposite was present in the form of a nanostructure of three-dimensional spheres that were assembled by many densely arranged nanosheets. The electrocatalytic activity of the electrode toward the oxidation of glucose was investigated by chronoamperometry. The current response was linearly related to the glucose concentration in the range from 1 to 10?μM, with a sensitivity of 494?μA?mM^–1?cm^–2 and a correlation coefficient of 0.9990, and a second range (from 10 to 1000?μM with a sensitivity of 328?μA?mM^–1?cm^–2 and a correlation coefficient of 0.9990). The detection limit was 0.6?μM at a signal-to-noise ratio of 3, and the response time was as short as 2?s.

Immobilization of acetylcholinesterase on screen-printed electrodes: comparative study between three immobilization methods and applications to the detection of organophosphorus insecticides

The analytical performance of three acetylcholinesterase (AChE) screen-printed biosensors designed for the detection of pesticides are evaluated. Bioencapsulation of the enzyme in a sol-gel composite and immobilization by metal-chelate affinity were compared with the entrapment of the enzyme in a photopolymerisable polymer. A very low amount of enzyme ranging between 0.8 and 1.2 mIU was immobilized on the electrode surface in each approach. The sensors exhibited a storage stability of over 6 months when the enzyme was encapsulated in a polymer film. Pesticide concentrations in the range of 10⁻⁸ to 10⁻⁹ M paraoxon, dichlorvos and chlorpyrifos ethyl oxon could be detected according to each configuration by following an incubation time of 20 min.     

Choline biosensors based on a bi-electrocatalytic property of MnO2 nanoparticles modified electrodes to H2O2

An interesting mode of reactivity of MnO₂ nanoparticles modified electrode in the presence of H₂O₂ is reported. The MnO₂ nanoparticles modified electrodes show a bi-direction electrocatalytic ability toward the reduction/oxidation of H₂O₂. Based on this property, a choline biosensor was fabricated via a direct and facile electrochemical deposition of a biocomposite that was made of chitosan hydrogel, choline oxidase (ChOx) and MnO₂ nanoparticles onto a glassy carbon (GC) electrode. The biocomposite is homogeneous and easily prepared and provides a shelter for the enzyme to retain its bioactivity. The results of square wave voltammetry showed that the electrocatalytic reduction currents increased linearly with the increase of choline chloride concentration in the range of 1.0 × 10⁻⁵ –2.1 × 10⁻³ M and no obvious interference from ascorbic acid and uric acid was observed. Good reproducibility and stability were obtained. A possible reaction mechanism was proposed.     

Mechanical and barrier properties of ternary nanocomposite films based on polycarbonate/amorphous polyamide blends modified with a nanoclay

Ternary polycarbonate (PC)/amorphous polyamide–nanoclay (naPA) nanocomposite (PC/naPA) films were obtained by melt mixing and drawing, and the effects of the naPA content and the draw ratio (DR) on the structure, morphology and mechanical and barrier properties were studied. Despite the presence of nanoclay, the films exhibited a negligible roughness and the excellent optical properties of PC and amorphous polyamide (aPA). The dispersed naPA phase was pure and small, indicating compatibility. The naPA did not hinder the drawing ability of PC. At low DRs the dispersed phase was elongated and oriented along the machine direction (extrusion flow direction), but at high DRs, it fibrillated due to the higher non‐isothermal elongational flow induced by drawing. The laminar structure of the nanoclay allowed the films to be reinforced both in the machine and the transverse directions. The oxygen permeability of PC was reduced by 42% in the nanocomposite with 25% of naPA, and dropped further with the DR, which is attributed to the increased tortuosity of the oxygen path induced by fibrillation. Copyright © 2015 John Wiley & Sons, Ltd.     

Fabrication of ferrocene functionalised ionic liquid/carbon nanotube nanocomposite modified carbon-ceramic electrode: application to the determination of hydrazine

A novel ionic liquid, 1-(ferrocenyl butyl)-3-methylimidazolium tetrafluoroborate (Fc-IL), was synthesised. The nanocomposite of Fc-IL and multi-walled carbon nanotubes (MWCNTs) was constructed and used for surface modification of carbon-ceramic electrode. The modified electrode was applied to the determination of hydrazine. Operational parameters such as pH of the solution, ionic liquid volume and amount of carbon nanotubes, which affect the analytical performance of the modified electrode, were optimised. The linear range of the modified electrode toward hydrazine concentration was 0.96–106.10 μg L^–1 with a detection limit of 0.64 μg L^–1 (S/N = 3). The modified electrode displayed high repeatability, reproducibility, long-term life time and low response time (<3 s). The applicability of this method was further tested by analysing the hydrazine content in boiler-feed water samples containing different concentrations of hydrazine and the results were in good agreement with the spectrophotometry method.     

Detection of Hg2+ with NBD-based colorimetric chemosensor: Practical application to water samples and test strips

A new nitrobenzoxadiazole (NBD)-based chemosensor N′-(7-nitrobenzo[c][1,2,5]oxadiazol-4-yl)thiophene-2-carbohydrazide ( NTCH) was synthesized for detecting Hg²⁺. NTCH could sense Hg²⁺ through a color change method from pale yellow to pink. Binding ratio of NTCH and Hg²⁺ was determined to be 1:1 with the analysis of electrospray ionization mass spectrometry and Job plot. Detection limit turned out to be 0.69 μM. In addition, NTCH could be successfully utilized for detecting Hg²⁺ in real water samples and visible color test strips. The probing mechanism of NTCH to Hg²⁺ was explained with ¹H nuclear magnetic resonance (NMR) titration, Job plot, electrospray ionization mass spectrometry (ESI-MS), and theoretical calculations.     

An analytical model of gene evolution with six mutation parameters: an application to archaeal circular codes

We develop here an analytical evolutionary model based on a trinucleotide mutation matrix 64 x 64 with six substitution parameters associated with the transitions and transversions in the three trinucleotide sites. It generalizes the previous models based on the nucleotide mutation matrices 4 x 4 and the trinucleotide mutation matrix 64 x 64 with three parameters. It determines at some time t the exact occurrence probabilities of trinucleotides mutating randomly according to six substitution parameters. An application of this model allows an evolutionary study of the common circular code COM and the 15 archaeal circular codes X which have been recently identified in several archaeal genomes. The main property of a circular code is the retrieval of the reading frames in genes, both locally, i.e. anywhere in genes and in particular without a start codon, and automatically with a window of a few nucleotides. In genes, the circular code is superimposed on the traditional genetic one. Very unexpectedly, the evolutionary model demonstrates that the archaeal circular codes can derive from the common circular code subjected to random substitutions with particular values for six substitutions parameters. It has a strong correlation with the statistical observations of three archaeal codes in actual genes. Furthermore, the properties of these substitution rates allow proposal of an evolutionary classification of the 15 archaeal codes into three main classes according to this model. In almost all the cases, they agree with the actual degeneracy of the genetic code with substitutions more frequent in the third trinucleotide site and with transitions more frequent that transversions in any trinucleotide site.