Voltammetric behaviour of ascorbic acid at a graphite—epoxy composite electrode chemically modified with cobalt phthalocyanine and its amperometric determination in multivitamin preparations

New photosensitive nanometric graphite oxide composites as antimicrobial material with prolonged action

A new conjugate material based on partially reduced graphite oxide (rGO), silver nanoparticles (Ag), and bis(lysinato)zirconium(IV) phthalocyanine complex (ZrPc) was obtained. Its optical properties (absorption and photoluminescence) after dispersion in solvents were examined. The antimicrobial properties were tested to determine the effect of the composite on the following bacterial strains: Staphylococcus aureus, Pseudomonas aeruginosa, and Escherichia coli, which are responsible for many infections and are one of the pathogens the most difficult to treatment. The results obtained for rGO-ZrPc-Ag composite were compared with the properties of GO, GO-ZrPc, and rGO-Ag structures. The influence of the near-infrared irradiation on the antimicrobial activity of ZrPc- and Ag-doped materials against bacteria was observed for very low concentration (0.32mg/mL) of GO-ZrPc to stop the growth of P. aeruginosa in comparison to the nonirradiated sample (41mg/mL). The usefulness of this material in therapy, such as wound infection treatment or endodontic treatment, as antibacterial agent with sustained action was discussed.

Oxides in silver–graphene nanocomposites: electrochemical signatures and electrocatalytic implications

Electrochemical investigations establish that the electroreduction of oxide form of silver present in Ag-reduced graphene oxide nanoassembly enhances its efficiency towards electro-oxidation of ascorbic acid (AA).

Cobalt phthalocyanine modified electrodes utilised in electroanalysis: nano-structured modified electrodes vs. bulk modified screen-printed electrodes

Cobalt phthalocyanine (CoPC) compounds have been reported to provide electrocatalytic performances towards a substantial number of analytes. In these configurations, electrodes are typically constructed via drop casting the CoPC onto a supporting electrode substrate, while in other cases the CoPC complex is incorporated within the ink of a screen-printed sensor, providing a one-shot economical and disposable electrode configuration. In this paper we critically compare CoPC modified electrodes prepared by drop casting CoPC nanoparticles (nano-CoPC) onto a range of carbon based electrode substrates with that of CoPC bulk modified screen-printed electrodes in the sensing of the model analytes l-ascorbic acid, oxygen and hydrazine. It is found that no “electrocatalysis” is observed towards l-ascorbic acid using either of these CoPC modified electrode configurations and that the bare underlying carbon electrode is the origin of the obtained voltammetric signal, which gives rise to useful electroanalytical signatures, providing new insights into literature reports where “electrocatalysis” has been reported with no clear control experiments undertaken. On the other hand true electrocatalysis is observed towards hydrazine, where no such voltammetric features are witnessed on the bare underlying electrode substrate.

Synthesis of Nitrogen Doped Carbon and Its Enhanced Electrochemical Activity towards Ascorbic Acid Electrooxidation

Nitrogen doped carbon, synthesized by a novel way of carbonizing polyaniline in an inert atmosphere at a constant temperature of800°C, exhibits several unique features. The carbon: nitrogen ratio is found to increase with the treatment duration up to 120 minutes and a mass reduction of 60 wt% is observed with an interesting observation of the retention of the bulk polymer morphology, surprisingly, even after the carbonization process. The electrochemical activity evaluated with potassium hexacyanoferrate and hexamine ruthenium redox systems at a regular time interval helps to tune the catalytic activity. This type of nitrogen doped carbon prepared from polyaniline base exhibits excellent electrocatalytic activity as illustrated by the oxidation of ascorbic acid in neutral medium.

Surface electrochemistry and electrocatalytic activity of ion pairs formed from oppositely-charged phthalocyanine and porphyrin species

Self-assembled ion pairs are deposited onto a graphite electrode surface by dipping the electrode sequentially into phthalocyanine cations and anions, or a phthalocyanine cation and porphyrin anion. The electrochemical properties of these surface-bound ion pairs are compared with the properties of the individual surface-bound components. The order of deposition is shown to be crucial, implying that these species lie flat on the surface. A detailed study of the electrocatalytic oxidation of oxalate anion is used to explore their reactivity as a function of deposition order.

Electro-oxidation of ascorbic acid catalyzed on cobalt hydroxide-modified glassy carbon electrode

The electrochemical behavior of ascorbic acid on a cobalt hydroxide modified glassy carbon (CHM-GC) electrode in alkaline solution was investigated. The process of the involved oxidation and its kinetics were established using the cyclic voltammetry, chronoamperometry techniques, as well as by steady state polarization measurements. The results revealed that cobalt hydroxide promotes the rate of oxidation by increasing the peak current; hence ascorbic acid is oxidized at lower potentials, which is thermodynamically more favorable. The cyclic voltammograms and chronoamperometry indicate a catalytic EC mechanism is operative with the electrogeneration of Co(IV) as the electrochemical process. Also, the process is diffusion-controlled and the current- time responses follow Cottrellian behavior. This result was confirmed by steady state measurements. The rate constants of the catalytic oxidation of ascorbic acid and the electron-transfer coefficient are reported.

Determination of l-dopa using electropolymerized 3,3′,3″,3‴-tetraaminophthalocyanatonickel(II) film on glassy carbon electrode

Electropolymerized film of 3,3',3'',3'''-tetraaminophthalocyanatonickel(II) (p-Ni(II)TAPc) on glassy carbon (GC) electrode was used for the selective and stable determination of 3,4-dihydroxy-L-phenylalanine (L-dopa) in acetate buffer (pH 4.0) solution. Bare GC electrode fails to determine the concentration of L-dopa accurately in acetate buffer solution due to the cyclization reaction of dopaquinone to cyclodopa in solution. On the other hand, p-Ni(II)TAPc electrode successfully determines the concentration of L-dopa accurately because the cyclization reaction was prevented at this electrode. It was found that the electrochemical reaction of L-dopa at the modified electrode is faster than that at the bare GC electrode. This was confirmed from the higher heterogeneous electron transfer rate constant (k(0)) of L-dopa at p-Ni(II)TAPc electrode (3.35 x 10(-2) cms(-1)) when compared to that at the bare GC electrode (5.18 x 10(-3) cms(-1)). Further, it was found that p-Ni(II)TAPc electrode separates the signals of ascorbic acid (AA) and L-dopa in a mixture with a peak separation of 220 mV. Lowest detection limit of 100 nM was achieved at the modified electrode using amperometric method. Common physiological interferents like uric acid, glucose and urea does not show any interference within the potential window of L-dopa oxidation. The present electrode system was also successfully applied to estimate the concentration of L-dopa in the commercially available tablets.

Simultaneous detection of ascorbate and uric acid using a selectively catalytic surface

The direct and selective detection of ascorbate at conventional carbon or metal electrodes is difficult due to its large overpotential and fouling by oxidation products. Electrode modification by electrochemical reduction of diazonium salts of different aryl derivatives is useful for catalytic, analytical and biotechnological applications. A monolayer of o-aminophenol (o-AP) was grafted on a glassy carbon electrode (GCE) via the electrochemical reduction of its in situ prepared diazonium salts in aqueous solution. The o-aminophenol confined surface was characterized by cyclic voltammetry. The grafted film demonstrated an excellent electrocatalytic activity towards the oxidation of ascorbate in phosphate buffer of pH 7.0 shifting the overpotential from +462 to +263 mV versus Ag/AgCl. Cyclic voltammetry and d.c. amperometric measurements were carried out for the quantitative determination of ascorbate and uric acid. The catalytic oxidation peak current was linearly dependent on the ascorbate concentration and a linear calibration curve was obtained using d.c. amperometry in the range of 2-20 microM of ascorbate with a correlation coefficient 0.9998, and limit of detection 0.3 microM. The effect of H2O2 on the electrocatalytic oxidation of ascorbate at o-aminophenol modified GC electrode has been studied, the half-life time and rate constant was estimated as 270 s, and 2.57x10(-3) s(-1), respectively. The catalytically selective electrode was applied to the simultaneous detection of ascorbate and uric acid, and used for their determination in real urine samples. This o-AP/GCE showed high stability with time, and was used as a simple and precise amperometric sensor for the selective determination of ascorbate.

Detecting thiols in a microchip device using micromolded carbon ink electrodes modified with cobalt phthalocyanine

This paper describes the fabrication and evaluation of a chemically modified carbon ink microelectrode to detect thiols of biological interest. The detection of thiols, such as homocysteine and cysteine, is necessary to monitor various disease states. The biological implications of these thiols generate the need for miniaturized detection systems that enable portable monitoring as well as quantitative results. In this work, we utilize a microchip device that incorporates a micromolded carbon ink electrode modified with cobalt phthalocyanine to detect thiols. Cobalt phthalocyanine (CoPC) is an electrocatalyst that lowers the potential needed for the oxidation of thiols. The CoPC/carbon ink composition was optimized for the micromolding method and the resulting microelectrode was characterized with microchip-based flow injection analysis. It was found that CoPC lowers the overpotential for thiols but, as compared to direct amperometric detection, a pulsed detection scheme was needed to constantly regenerate the electrocatalyst surface, leading to improved peak reproducibility and limits of detection. Using the pulsed method, cysteine exhibited a linear response between 10-250 microM (r(2) = 0.9991) with a limit of detection (S/N = 3) of 7.5 microM, while homocysteine exhibited a linear response between 10-500 microM (r(2) = 0.9967) with a limit of detection of 6.9 microM. Finally, to demonstrate the ability to measure thiols in a biological sample using a microchip device, the CoPC-modified microelectrode was utilized for the detection of cysteine in the presence of rabbit erythrocytes.

Chemically modified carbon paste electrode for the potentiometric flow injection analysis of piribedil in pharmaceutical preparation and urine

A new carbon paste electrode selective for piribedil (PD) was prepared and fully characterized in terms of composition, usable pH range, response time and thermal stability. The electrode active recognition is by liquid ion-exchange mechanism via the use of piribedil phosphomolybdate as ion-exchanger dissolved in tricresyl phosphate as a more suitable solvent mediator for the paste. The modified electrode showed a Nernstian slope of 58.4+/-0.6 mV over the concentration range of 7.5 x 10(-7) to 1 x 10(-3)M with an average recovery of 98.3-101.0% and R.S.D. of 0.45-1.31%. The electrode exhibits good selectivity for PD with respect to a large number of inorganic cations, organic cations, sugars and amino acids. The developed electrode was successfully used for the potentiometric determination of PD in its aqueous solutions, pharmaceutical preparation, and urine in batch and flow injection analysis (FIA).

Chemically Modified Carbon Paste Electrode for the Potentiometric Determination of Dicylomine Hydrochloride in Batch and in FIA Conditions

A carbon-paste electrode for dicyclomine hydrochloride (DcCl) was prepared and fully characterized in terms of composition, life span, usable pH range and temperature. The electrode was applied to the potentiometric determination of dicylominium ion in its pure state and in pharmaceutical preparations in batch and flow injection conditions and in biological fluids using standard additions method. The electrode is based on a mixture of two ion-exchangers, namely, dicyclominium-phosphomolybdate and dicylominium-tetraphenylborate, dissolved in dioctyl phthalate as pasting liquid. The modified electrode showed a near-Nernstian slope of 58 +/- 2 mV over the concentration range of 1.2 x 10(-5)-1.6 x 10(-2) M with an average recovery of 97-102% and a RSD of 0.090-1.00. The electrode exhibits good selectivity for DcCl with respect to a large number of inorganic cations, sugars, amino acids and organic substances of biological fluids. Potentiometric titrations of DcCl with several titrants have been monitored using this modified carbon paste electrode as an end-point indicator electrode.

Electrocatalytic Characteristics of a 1-[4-(Ferrocenyl ethynyl)phenyl]-1-ethanone Modified Carbon-Paste Electrode in the Oxidation of Ascorbic Acid

A carbon-paste electrode spiked with 1-[4-(ferrocenyl ethynyl)phenyl]-1-ethanone was constructed by the incorporation of 1-[4-(ferrocenyl ethynyl)phenyl]-1-ethanone in a graphite powder silicon oil matrix. It shown by cyclic voltammetry and double potential-step chronoamperometry, which this ferrocene derivative modified a carbon-paste electrode, can catalyze the ascorbic acid oxidation in an aqueous buffered solution. It has been found that under the optimum conditions (pH 7.00), the oxidation of ascorbic acid at the surface of this carbon paste modified electrode occurs at a potential of about 260 mV less positive than that of an unmodified carbon-paste electrode. The catalytic oxidation peak current was linearly dependent on the ascorbic acid concentration, and a linear calibration curve was obtained in the range of 6 x 10(-5) M-7 x 10(-3) M of ascorbic acid with a correlation coefficient of 0.9997. The detection limit (2sigma) was determined to be 6.3 x 10(-5) M. This method was also used for the determination of ascorbic acid in some pharmaceutical samples, such as effervescent tablets, ampoules and multivitamin syrup, by using a standard addition method. The reliability of the method was established by a parallel determination against the official method.