Continuous-flow performance of carbon electrodes modified with immobilized Fe(II)/Fe(III) centers Amperometric response to N(2)O, NO and NO(2)

The amperometric performance of two types of chemically modified carbon electrodes developed for the determination of oxides of nitrogen in continuous-flow systems is presented. The modification consists in immobilization of reversible Fe(II)/Fe(III) centers. The first type of electrode is a simple modification made by direct mixing of carbon paste with tris-4,7-diphenyl-1,10-phenanthroline-iron(II) perchlorate; the other is a glassy-carbon surface modified by oxidative electropolymerization of tris-[5-amino-1,10-phenanthroline]iron(II) perchlorate. Detection is accomplished by transporting an injected sample plug to the sensing surface with the aid of gravitational flow of an aqueous solution of supporting electrolyte. The polymer-coated electrochemical detector compares favourably with the chemically modified carbon paste. It offers excellent resistance to poisoning and a competitive limit of detection [about 2 ppb (2 parts in 10(9)) v/v], at + 1.0 V vs. Ag/AgCl, and good selectivity for NO(2) when used in a thin-layer cell. Incorporation of the cell in a continuous-flow system allows injection of about 120 samples per hour. The typical concentration range amenable to determination is 2-25 ppb v/v but depends on the thickness of the polymeric film. Nitrogen monoxide can also be detected but only in undiluted, pure form. Dinitrogen oxide gives no amperometric signal at any of the modified surfaces.     

Electrocatalytic properties of guanine, adenine, guanosine-5'-monophosphate, and ssDNA by Fe(II) bis(2,2':6',2'-terpyridine), Fe(II) tris(1,10-phenanthroline), and poly-Fe(II) tris(5-amino-1,10-phenanthroline)

The electrocatalytic oxidations of guanine, adenine, guanosine-5'-monophosphate(GMP) and ssDNA were performed in the presence of Fe(II) bis(2,2':6',2'-terpyridine) and Fe(II) tris(1,10-phenanthroline) complexes as homogeneous catalysts by cyclic voltammetric methods. The Fe(II/III) redox couple of these compounds is responsible for their catalytic properties. The electrocatalytic oxidation current of above substrates were developed from the anodic peak currents of Fe(II) bis(2,2':6',2'-terpyridine) and Fe(II) tris(1,10-phenanthroline) complexes at about +0.93 V and 0.97 V, respectively. The electrocatalytic oxidative properties of guanine by Fe(II) bis(2,2':6',2'-terpyridine) complex was measured by amperometry method using the rotating disk electrodes. Electropolymerization of Fe(II) tris(5-amino-1,10-phenanthroline) complex produced thin polymer films on gold and glassy carbon electrodes. The electrochemical quartz crystal microbalance (EQCM) and cyclic voltammetry were used to study the in situ growth of the polymer. The poly(FeII(5-NH(2)-1,10-phen)(3)) exhibited a good electrocatalytic oxidation towards guanine and also for the mixture of guanine and adenine too.     

Electrochemical behavior of 5-amino-1,10-phenanthroline and oxidative electropolymerization of tris[5-amino-1,10-phenanthroline]iron(II)

The electrochemical behavior of 5-amino-1,10-phenanthroline and tris[5-amino-1,10-phenanthroline]-iron(II) at carbon paste, glassy carbon, and platinum electrodes is reported. The iron complex undergoes electrochemically induced oxidative polymerization from acetonitrile solutions and the resulting polymers are very stable. Charge transport through the polymer films occurs with a charge transfer diffusion coefficient, D_ct, equal to 3.1 × 10⁻⁸ cm² s⁻¹ corresponding to an electron self-exchange rate of 5.2×10⁷M⁻¹ s⁻¹. The activation energy and the entropy change for the charge transfer diffusion process are (approximate values) 32.0 ± 0.12 kJ mol⁻¹ and −24.7 ± 0.4 J K⁻¹ mol⁻¹, respectively.     

Chemical species produced in the reaction between ethanedial (glyoxal) and 5-amino-1,10-phenanthroline and their iron(II) complexes: electrochemical studies and analytical applications

The cyclic voltammetric behavior of carbon paste electrodes modified by direct admixing with the products of the reactions between ethanedial (glyoxal) and 5-amino-1,10-phenanthroline at 100°C and that of their iron(II) complexes is reported. The ligand(s) produced in absence of iron(II) are able to complex iron(II) and copper(II) ions reversibly, but other ions such as nickel(II), cobalt(II), cadmium(II) and manganese(II), if complexed, show no electrochemical activity. Admixing with the products of the reaction in the presence of excess of iron(II) ion, because of high insolubility and fast electron exchange, produces surfaces useful for amperometric detection in continuous-flow systems. The voltammetric and amperometric behavior in the presence of HSO^?₃ ions is reported in order to illustrate this application.     

Anion selectivity studies on liquid membrane electrodes

Selectivity coefficients of liquid-membrane electrodes for common inorganic anions were measured in electrodes containing tris(l,10-phenanthroline)iron(II), tris(4,7-diphenyl-1,10-phenanthroline)iron(II) or tetraheptylammonium ion in nitrobenzene, and tris(4,7-diphenyl-1,10-phenanthroline)iron(II) ion in nitrobenzene, chloroform or n-amyl alcohol as the liquid membrane. With the exception of the amyl alcohol electrode, selectivity coefficients were relatively independent of membrane composition and followed a common sequence of decreasing selectivity: PF₆^?> ClO₄^?>SCN^?~I^?~BF₄^?>NO₃^?>Br^?>Cl^?. This sequence parallels the order of increasing anion hydration energy, suggesting that aqueous phase solvation energies play a predominant role in determining electrode selectivity for these ions. Time-dependent behavior of liquid-membrane electrodes on transfer between solutions containing different ions also is described. Instantaneous e.m.f. readings were used to determine selectivity coefficients.     

A Bifunctional Electrocatalyst Containing Tris(2,2′‐bipyridine) Ruthenium(II) and 12‐Molybdophosphate Bulk‐Modified Carbon Paste Electrode

The electroactive composite containing tris(2,2′‐bipyridine) ruthenium(II) and 12‐molybdophosphate (RuPMo₁₂) was synthesized and first used as a bifunctional electrocatalyst to fabricate a chemically bulk‐modified carbon paste electrode (RuPMo₁₂‐CPE) by direct mixing. The electrochemical behavior of the RuPMo₁₂‐CPE was studied by cyclic voltammetry. The RuPMo₁₂‐CPE presents good electrocatalytic activity not only toward the reduction of hydrogen peroxide and bromate, which is attributed to the function of molybdophosphate, but also toward the oxidation of arsenite, which is primarily attributed to the function of tris(2,2′‐bipyridine) ruthenium(II). The remarkable advantage of the RuPMo₁₂‐CPE is its good stability owing to the insolubility of RuPMo₁₂ and reproducibility of surface renewal.     

Spectrophotometric determination of aluminum and iron in calcium

A simple, rapid, and accurate method is described whereby micro amounts of aluminum and iron can be determined in calcium metal. The iron is determined spectrophotometrically by measuring the absorbency of the reddish-orange color of the tris(1,10-phenanthroline)iron(II)ion, [(C₁₂H₈N₂)₃Fe]⁺⁺. This complex effectively removes the iron interference, and the aluminum is determined spectrophotometrically in the solution remaining from the iron determination by extracting it with a chloroform solution of 8-quinolinol and measuring the absorbency of the yellow color of the tris(8-quinolinolo)aluminum(III) in the chloroform solution.     

Electrochemical Determination of Uric Acid,Dopamine and Tryptophan at Zinc Hexacyanoferrate Clay Modified Electrode

A Cameroonian smectite clay has been transformed into Zn²⁺ homoionic form and then used to prepare film modified glassy carbon electrodes and carbon paste electrodes. These electrodes containing Zn²⁺ were exploited to prepare a mixed valence zinc hexacyanoferrate (ZnHCF). Cyclic voltammetry has been employed to monitor the in situ growth of ZnHCF on clay modified electrodes. Although interesting electrocatalytic properties toward UA were observed with these modified electrodes, the modified carbon paste electrodes were the most suitable for dopamine, uric acid and tryptophan detection and exhibited for these analytes extended linear range, high sensitivities, selectivity and low limit.     

Thin-layer chromatography of tris(1,10-phenanthroline)iron(II) and tris(2,2′-bipyridine)iron(II) on Sephadex G gels

Summary Gel chromatographic behaviour of tris(1, 10-phenanthroline)iron(II), tris(2,2′-bipyridine)iron(II) and tris(glycinato)cobalt(III) on Sephadex G-10 or G-25 was investigated by TLC with 0.001–1.0M NaCl as the eluent. The zone shapes and R_M values of tris(1,10-phenanthroline)iron(II) and tris(2,2′-bipyridine)iron(II) were appreciably dependent on the sample and eluent concentration, while the neutral complex, tris(glycinato) cobalt(III), exhibited the round zones with constant R_M values. The order of R_M values was found to be tris(glycinato)cobalt(III<tris(2,2∔pyridine)iron(II)<tris-(1,10-phenanthroline)iron(II) in all systems studied, although the reverse trend was expected when assuming the chromatographic behaviour of solute compounds to be controlled by the “sieving effect”. The comparison of the behaviour on Sephadex G gels with that on CM-cellulose revealed that the predominant mechanism involved is not the sieving effect, but ion-exchange and/or hydrophobic interaction.     

A new method for the spectrophotometric determination of pertechnetate with tris(1,10-phenanthroline)iron(II) ion

A new spectrophotometric determination of technetium has been developed by means of the solvent extraction of tris(1,10-phenanthroline)iron(II) ([Fe(phen)₃ ²⁺]) with pertechnetate into nitrobenzene. The concentration of technetium can be determined by measuring the characteristic absorbance at 516 nm (=11,700M^–1·cm^–1) in the organic phase. An important feature of the proposed method is that the concentration of pertechnetate can be determined without complicated processes such as the reduction of pertechnetate and the subsequent formation of a colored chelate.     

Oxygen reduction at the surface of glassy carbon electrodes modified with anthraquinone derivatives and dyes

The preparation, electrochemical and catalytic behaviour of glassy carbon electrodes modified by anthra-9,10-quinone, its amino derivatives and dyes were investigated. The stability of the modified electrodes was studied by cyclic voltammetry in acidic and neutral media. The electrocatalytic ability of the modified electrodes for the reduction of dioxygen to hydrogen peroxide was examined by cyclic voltammetry, chronoamperometry and chronocoulometry techniques. The influence of pH on the electrochemical and catalytic behaviour was studied and pH 5.0–8.0 was chosen as the optimum working pH by comparing the shift in oxygen reduction potential. The anthraquinone-adsorbed glassy carbon electrodes possess excellent electrocatalytic abilities for dioxygen reduction with overpotential ranging from 280 to 560 mV lower than that at a plain glassy carbon electrode. Hydrodynamic voltammetric studies were performed to determine the heterogeneous rate constants for the reduction of O₂ at the surface of the modified electrodes, mass specific activity of the anthraquinones used and the apparent diffusion coefficient of O₂ in buffered aqueous O₂-saturated solutions. Studies showed the involvement of two electrons in dioxygen reduction.     

Solvent effects on the redox properties of tris-(1,10-phenanthroline)iron(II/III) complexes

The redox properties of the system Fe(tmphen)₃(II/III) (tmphen=3,4,7,8-tetramethyl-1,10-phenanthroline) have been studied in the solvents nitromethane, acetonitrile, propanediol-1,2-carbonate, dimethylformamide, dimethylacetamide, dimethylsulfoxide and of the systems Fe(phen)₃(II/III) (phen=1,10-phenanthroline) and Fe(niphen)₃(II/III) (niphen=5-nitro-1,10-phenanthroline) in the solvents nitromethane, acetonitrile, propanediol-1,2-carbonate and acetone. The redox potentials of Fe(tmphen)₃(II/III) are nearly independent of the solvent suggesting that the system might be used as a reference redox couple similar to the systems ferrocene/ferricinium or bisbiphenylchromium(0/I). In contrast the redox potentials of Fe(niphen)₃(II/III) show a significant decrease with increasing donor number of the solvent which can be explained by nucleophilic attack of solvent molecules at the iron. It is shown that such a mechanism is consistent with the known solvent and salt effects on the kinetics of dissociation of ferroin and ferriin type complexes.     

Electroanalytical Study of Prussian Blue Modified Glassy Carbon Paste Electrodes

Two types of glassy carbon (GC) powder (i.e., Sigradur K and Sigradur G) have been mixed with mineral oil to obtain glassy carbon paste electrodes (GCPE's). The electrochemical behavior of such electrodes at different percentages of glassy carbon has been evaluated with respect to the electrochemistry of ferricyanide as revealed with cyclic voltammetry and the best paste composition was chosen. GC was then modified with Prussian Blue (PB), mixed at different percentages with unmodified GC and with a fixed amount of mineral oil in order to obtain PB modified glassy carbon paste electrodes (PB‐GCPE's). PB‐GCPE's with different percentages of GC modified with PB (PB‐GC) were compared and the dependence on the amount of PB on their performances was evaluated by studying the parameters of cyclic voltammetry (i.e., current peak, ΔE_p, anodic and cathodic current ratio, charge density) and the amperometric response to H₂O₂. Data interpretation based on the GC surface area is presented. GCPE's with a selected amount of PB‐GC were then tested as H₂O₂ probes and all the analytical parameters together with the dependence on pH were evaluated. Some preliminary experiments with these electrodes assembled as glucose, lysine and lactate biosensors are also reported.     

A saccharinate–iron(II) complex with a free saccharin molecule present, [Fe(phen)3]sac2·sacH·6H2O

The crystal structure of the title compound, tris(1,10-phenanthroline-N,N′)­iron(II) bis­[1,2-benziso­thia­zol-3(2H)-onate 1,1-dioxide] 1,2-benziso­thia­zol-3(2H)-one 1,1-dioxide hexahydrate, [Fe(C₁₂H₈N₂)₃](C₇H₄NO₃S)₂·C₇H₅NO₃S·6H₂O, at 120 K consists of slightly distorted octahedral [Fe(phen)₃]²⁺ cations (phen is 1,10-phenanthroline), two saccharinate anions and a free saccharin mol­ecule, as well as six waters of crystallization. The compound has been confirmed as diamagnetic low-spin iron(II) by magnetic measurements. There is extensive hydrogen bonding leading to a three-dimensional network.     

Hemoglobin Modified Carbon Paste Electrode: Direct Electrochemistry and Electrocatalysis

Abstract

A new hemoglobin (Hb) modified carbon paste (CP) electrode was fabricated by simply mixing the hemoglobin with carbon powder and paraffin homogeneously. To prevent the leakage of Hb from the electrode surface, a Nafion film was further applied on the surface of Hb-carbon composite paste electrode. Direct electrochemistry of hemoglobin in the paste electrode was easily achieved, and a pair of well-defined quasi-reversible redox peak of heme Fe(III)/Fe(II) couple appeared with the formal potential (E^0′) as ?0.335 V (vs. Saturated calomel electrode; CE) in pH 7.0 phosphate buffer solution (PBS). The fabricated Hb modified electrode showed good electrocatalytic ability to the reduction of trichloroacetic acid (TCA) and H₂O₂.     

The spectrophotometric determination of anions by solvent extraction with metal chelate cations. part XX : Trichloroacetic acid with tris(1,10-phenanthroline)iron(ii) chelate

Trichloroacetic acid can be extracted from an aqueous solution by nitrobenzene with tris(1,10-phenanthroline)iron(II) chelate, and can be determined spectrophotometrically by measuring the extract at 516 nm. The extracted species is probably [Fe(phen)₃].(CCl₃COO)₂. Beer's law is obeyed over the concentration range 1.0·10^-5–1.0·10^-4M trichloroacetic acid in aqueous solution. Large amounts of phosphate and sulfate and moderate amounts of chloride, acetic acid, and monochloroacetic acid do not interfere, equal amounts of dichloroacetic acid give a slight positive error     

Kinetics of reactions of the tris(2,2′-bi(4H-5,6-dihydrothiazine))iron(II) cation

Summary We report rate constants for reactions of tris(2,2-bi(4H-5,6-dihydrothiazine)iron(II), Fe(btz) ₃ ²⁺ , in acid solution, with hydroxide, with peroxodisulphate, with silver(I), with mercury(II), and with cadmium(II), in aqueous solution, and for the reaction of Fe(btz)_{3 3} ²⁺ with 1,10-phenanthroline and with 2,2-bipyridyl in aqueous methanol. Mechanisms and relative reactivities are discussed.     

Lead ion sensor with electrodes modified by imidazole-functionalized polyaniline

Glassy carbon electrodes (GCE) and carbon paste electrodes (CPE) were modified with imidazole functionalized polyaniline with the aim to develop a sensor for lead (II) in both acidic and basic aqueous solution. The electrodes were characterized by cyclic voltammetry and differential pulse adsorptive stripping voltammetry. The limit of detections obtained with glassy carbon electrode and carbon paste electrode are 20?ng?mL^-1 and 2?ng?mL^-1 of lead ion, respectively. An interference study was carried out with Cd(II), As(III), Hg(II) and Co(II) ions. Cd(II) ions interfere significantly (peak overlap) and As(III) has a depressing effect on the lead signal. The influence of pH was investigated indicating that bare and modified GCE and CPE show optimum response at pH?4.0 ± 0.05.

Myoglobin/sol-gel film modified electrode: direct electrochemistry and electrochemical catalysis

Direct electrochemical and electrocatalytic behavior of myoglobin (Mb) immobilized on carbon paste electrode (CPE) by a silica sol-gel film derived from tetraethyl orthosilicate was investigated for the first time. Mb/sol-gel film modified electrodes show a pair of well-defined and nearly reversible cyclic voltammetric peaks for the Mb Fe(III)/Fe(II) redox couple at about -0.298 V (vs Ag/AgCl) in a pH 7.0 phosphate buffer solution. The formal potential of the Mb heme Fe(III)/Fe(II) couple shifted linearly with pH with a slope of 52.4 mV/pH, denoting that an electron transfer accompanies single-proton transportation. An FTIR and UV-vis spectroscopy study confirms that the secondary structure of Mb immobilized on an electrode by a sol-gel film still maintains the original arrangement. The immobilized Mb displays the features of a peroxidase and acts in an electrocatalytic manner in the reduction of oxygen, trichloroacetic acid (TCA), and nitrite. In comparison to other electrodes, the chemically modified electrodes used in this study for direct electrochemistry and electrocatalysis of Mb are easy to fabricate and fairly inexpensive. Consequently, the Mb/sol-gel film modified electrode provides a convenient way to perform electrochemical research on this kind of protein. It also has potential use in the fabrication of bioreactors and third-generation biosensors.     

Initial state and transition state contributions to solvent effects on rate constants for racemisation and dissociation of the [Fe(phen)3]2+ cation in DMSO-water mixtures

Summary Rate constants for racemisation and dissociation of the tris(1,10-phenanthroline)iron(II) cation, and solubilities of its picrate, are reported for dimethyl sulphoxide-water mixtures (0 to 40% by vol DMSO) at 298.2 K. The observed solvent effects on reactivity are analysed in terms of initial state and transition state contributions, and the pattern established compared with patterns for methanol- and acetone-water mixtures. In all cases the hydrophobic periphery of the ligand is dominant. Rate constants are also reported for racemisation and dissociation of the analogous complex [Fe(sb)₃]²⁺, where sb = the Schiff base from 2-benzoylpyridine andp-toluidine, in aqueous methanol.