Investigation of water diffusion in poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) by generalized two-dimensional correlation ATR-FTIR spectroscopy

Water diffusion process in biodegradable poly(3-hydroxybutyrate-co-3-hydroxyhexanoate) (PHBHHx, HHx = 12 mol%) was investigated by generalized 2D correlation time-resolved ATR-FTIR spectroscopy based on the analysis of v(OH) stretching and δ(OH) bending bands of water as well as v(CO) and v(C-O-C) stretching bands of PHBHHx. Three states of water were figured out during water diffusion process. They are bulk water, bound water and free water. The water diffusion mechanism was suggested as: water molecules firstly diffuse into the micro-voids in bulk water form or are dispersed on the surface in free water form, and then penetrate into the polymer matrix in hydrogen bound water with the hydrophilic groups of PHBHHx. Moreover, water molecules diffuse into the loose amorphous phase and then into compact crystalline phase. Water diffusion coefficient in PHBHHx was thus evaluated as 7.8 ± 0.7 × 10⁻⁸ cm² s⁻¹ for the PHBHHx with crystallinity of 16.2 ± 0.3% at 293 K.     

Cyclic voltammetry and scanning electrochemical microscopy studies of methylene blue immobilized on the self-assembled monolayer of n-dodecanethiol

Electron transfer (ET) kinetics through n-dodecanethiol (C₁₂SH) self-assembled monolayer on gold electrode was studied using cyclic voltammetry (CV), scanning electrochemical microscopy (SECM) and electrochemical impedance spectroscopy (EIS). An SECM model for compensating pinhole contribution, was used to measure the ET kinetics of solution-phase probes of ferrocyanide/ferricyanide (Fe(CN)₆^4−/3−) and ferrocenemethanol/ferrociniummethanol (FMC^0/+) through the C₁₂SH monolayer yielding standard tunneling rate constant () of (4 ± 1) × 10⁻¹¹ and (3 ± 1) × 10⁻¹⁰ cm s⁻¹ for Fe(CN)₆^4−/3− and FMC^0/+ respectively. Decay tunneling constants (β) of 0.97 and 0.96 Å⁻¹ for saturated alkane thiol chains were obtained using Fe(CN)₆⁴⁻ and FMC respectively. Also, it was found that methylene blue (MB) molecules are effectively immobilized on the C₁₂SH monolayer and can mediate the ET between the solution-phase probes and underlying gold substrate. SECM-mediated model was used to simultaneously measure the bimolecular ET between the solution-phase probes and the monolayer-immobilized MB molecules, as well as tunneling ET between the monolayer-immobilized MB molecules and the underlying gold electrode, allowing the measurement of k_BI = (5 ± 1) × 10⁶ and (4 ± 2) × 10⁷ cm³ mol⁻¹ s⁻¹ for the bimolecular ET and and (7 ± 3) × 10⁻² s⁻¹ for the standard tunneling rate constant of ET using Fe(CN)₆^4−/3− and FMC^0/+ probes respectively.     

Electrochemistry of thorium in LiCl-KCl eutectic melts

This work presents a study of the electrochemical properties of Th chloride ions dissolved in a molten LiCl-KCl eutectic, in a temperature range of 693-823 K. Transient electrochemical techniques such as cyclic voltammetry, chronopotentiommetry and chronoamperometry have been used in order to investigate the reduction mechanism on a tungsten electrode and the diffusion coefficient of dissolved Th ions. All techniques showed that only one valence state was stable in the melt. The reduction into Th metal was found to occur according to a one-step mechanism, through a nucleation-controlled process which requires an overpotential of several 100 mV. At 723 K, the diffusion coefficient is D_Th(723 K) = 3.15 ± 0.15 × 10⁻⁵ cm² s⁻¹. EMF measurements indicated that, at 723 K, the standard apparent potential is (723 K) = −2.582 V versus Cl₂/Cl⁻, and the activity coefficient γThCl₄ (723 K) = 4.6 × 10⁻⁴ on the mole fraction scale (based on a pure liquid reference state).     

Analysis of the chemical diffusion coefficient of lithium ions in Li3V2(PO4)3 cathode material

The chemical diffusion coefficients of lithium ions (DLi⁺) in Li₃V₂(PO₄)₃ between 3.0 and 4.8 V are systematically determined by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS) and galvanostatic intermittent titration technique (GITT). The DLi⁺ values are found to be dependent on the voltage state of charge and discharge. Based on the results from all the three techniques, the true diffusion coefficients measured in single-phase region are in the range of 10⁻⁹ to 10⁻¹⁰ cm² s⁻¹. Its apparent diffusion coefficients measured in two-phase regions by CV and GITT range from 10⁻¹⁰ to 10⁻¹¹ cm² s⁻¹ and 10⁻⁸ to 10⁻¹³ cm² s⁻¹, respectively, depending on the potentials. By the GITT, the DLi⁺ varies non-linearly in a “W” shape with the charge-discharge voltage, which is ascribed to the strong interactions of Li⁺ with surrounding ions. Finally, the chemical diffusion coefficients of lithium ions measured by CV, EIS and GITT are compared to each other.     

Investigation on diffusion behavior of Li in LiFePO4 by capacity intermittent titration technique (CITT)

Capacity intermittent titration technique (CITT) was used to investigate the chemical diffusion coefficient () of lithium-ion in LiFePO₄ cathode material. The values of at the galvano-charge current of 0.2 and 0.4 mA were respectively found to range from 8.8 × 10⁻¹⁶ to 8.9 × 10⁻¹⁴ cm² s⁻¹ and from 1.2 × 10⁻¹⁶ to 8.5 × 10⁻¹⁴ cm² s⁻¹ in the voltage range from 3.2 to 4 V (vs. Li⁺/Li). The transfer coefficients of cathode (0.32-0.42) and anodic (0.26-0.3), and the standard rate constant (1.58 × 10⁻⁹ to 1.30 × 10⁻⁸ cm s⁻¹) were measured from the Tafel plots of LiFePO₄ in the equilibrium potential range from 3.06 to 3.45 V. From these kinetic parameters, the finite kinetics at interface was taken into account to revise the above values of . The revised values of at the galvano-charge current of 0.2 and 0.4 mA were respectively found to range from 2.44 × 10⁻¹⁵ to 2.21 × 10⁻¹³ cm² s⁻¹ and from 5.81 × 10⁻¹⁶ to 3.22 × 10⁻¹³ cm² s⁻¹ in the voltage range from 3.2 to 4 V. Results show that the approximation of infinite charge-transfer kinetics leads to a spurious value of which is lower than the revised value, and the spurious extent depends on the galvano-charge current of CITT experiment.     

Molecular characteristics of poly(1-trimethylsilyl-1-propyne) in dilute solutions

Samples and fractions of a membrane-forming polymer, poly(1-trimethylsilyl-1-propyne) (PTMSP), were studied by methods of molecular hydrodynamics (velocity sedimentation, translational isothermal diffusion and viscometry) in cyclohexane in the molecular mass range 60<M×10⁻³ g mol⁻¹<430. The following molecular-mass dependencies of the hydrodynamic characteristics (intrinsic viscosity [η] (cm³ g⁻¹), sedimentation coefficient s₀(s) and translational diffusion coefficient D₀ (cm² s⁻¹)) were established: [η]=0.198 M^0.50±0.06; s₀=8.66×10⁻¹⁶M^0.50±0.04; D₀=9.30×10⁻⁵M^{−0.50±0.04}. On the basis of the hydrodynamics data the equilibrium rigidity and hydrodynamic diameter of PTMSP chains were evaluated. The equilibrium properties of the different disubstituted polyacetylenes molecules are compared on the base of the normalised scaling plots.     

Characterization of manganese tetraarylthiosubstituted phthalocyanines self assembled monolayers

Manganese tetraarylthiosubstituted phthalocyanines (complexes 1-5) have been deposited on Au electrode surfaces through the self assembled monolayer (SAM) technique. SAM characteristics reported in this work are: ion barrier factor (∼1); interfacial capacitance (303-539 μF cm⁻²) and surface coverage (1.06 × 10⁻¹⁰-2.80 × 10⁻¹⁰ mol cm⁻²). Atomic force microscopy was employed in characterizing a SAM. SAMs of complexes 1-5 were employed to detect l-cysteine (with limit of detection ranging from 2.83 × 10⁻⁷ to 3.14 × 10⁻⁷ M at potentials of 0.68-0.75 V vs. Ag|AgCl) and nitrite (limit of detection ranging from 1.78 × 10⁻⁷ to 3.02 × 10⁻⁷ M at potentials of 0.69-0.76 V vs. Ag|AgCl).     

Preparation of Co-Sn alloys by electroreduction of Co(II) and Sn(II) in molten LiCl-KCl

Co-Sn alloys were prepared by an electrochemical route in molten LiCl-KCl between 400 and 550 °C. The Sn(IV)/Sn(II), Sn(II)/Sn(0) and Co(II)/Co(0) redox couples were studied by cyclic voltammetry and/or chronopotentiometry over the temperature range. The diffusion coefficient values of Co(II) ions were measured. For example, it was found that the D_Co(II) values deduced from chronopotentiometry range from D_Co(II) = 1.65 × 10⁻⁵ cm² s⁻¹ at 400 °C to 4.95 × 10⁻⁵ cm² s⁻¹ at 550 °C. The standard potential of the Co(II)/Co(0) redox couple in molten LiCl-KCl was measured at 400 °C: vs Cl₂/Cl⁻. Finally, Co-Sn alloys were prepared in potentiostatic mode. The influence of the temperature of molten LiCl-KCl, the applied potential and the deposition time on the morphology and the composition of the Co-Sn alloys were also investigated. For T > 450 °C, the following tendency has been observed: the more negative the potential, the higher the Sn content in the deposited alloy. Thus, depending on the operating conditions, pure CoSn or CoSn₂ can be prepared.     

Ionophores in polymeric membranes for selective ion recognition; impedance studies

Synthetic calix[4]arene-crown ionophores for selective Na⁺ (ionophore L1) and Cs⁺-ions (ionophore L2) recognition find application in ion-selective membrane electrodes (ISE) for analytical purpose. Selectivity coefficients for the electrodes with compounds L1 and L2 are  = −2.6 and  = −2.4, respectively. Electrodes of two different construction: all-solid-state (ASS) (with conducting polymer layer on glassy carbon or platinum as ion-to-electron transducer) and conventional ion-selective electrode (ISE) (with liquid electrolyte and Ag/AgCl) are presented and their properties and lifetime are being compared. Resistance of PVC membrane with ionophores L1 and L2 were within the range 0.15-1.4 MΩ depending on the type of the outer electrolyte and its concentration. Conductivity of the membranes was in the range 0.7 × 10⁻⁸ to 6 × 10⁻⁸ Ω⁻¹ cm⁻¹. Warburg coefficients σ were within 0.16 × 10⁴ to 12.7 × 10⁴ Ω s^−1/2, dielectric constant values ? were in a range 28-60 depending mainly on the type of plasticizer.     

A study of water diffusion, in both radial and axial directions, into biodegradable monolithic depots using ion beam analysis

Water diffusion into cylindrical biodegradable monolithic depots fabricated from extruded mixtures of poly(dl-lactide) and a peptidic drug, goserelin, containing 20, 30 and 40% drug by weight has been studied using an ion beam analysis technique. A series of depots were immersed in a phosphate buffered saline/heavy water solution at 37 °C for times ranging from 1 h to 7 days. One-dimensional radial profiles showing the diffusion of water into the depots were produced at points along the length of the sample and, for some short immersion times, axial profiles were obtained for the cylinder ends. The changes in weight, radius, drug release and water uptake of the depots with time were also studied. Using the water uptake measurements the one-dimensional radial profiles were normalised. From appropriate one-dimensional profiles at the shorter times Fickian diffusion coefficients were obtained for initial water diffusion. The average radial diffusion coefficients were (1.07±0.22)×10⁻⁸ cm² s⁻¹ for the 20% drug-loaded depots, (1.54±0.27)×10⁻⁸ cm² s⁻¹ for the 30% drug-loaded depots and (2.00±0.83)×10⁻⁸ cm² s⁻¹ for the 40% drug-loaded depots—in the ratio of the drug loadings i.e. 2:3:4, implying the water associated with drug during its uptake into the monoliths. The axial diffusion coefficients were found to be of the same order of magnitude as the radial diffusion coefficients, in accord with this hypothesis. At longer times there is a subsequent non-Fickian increase in the water concentration profile. In the case of depots loaded with 40% by weight of goserelin, the substantial fraction of the hydrophilic drug released at times greater than one day is accompanied by a decrease in radius and a decrease in water concentration near the depot surface.     

Reduction process of uranium(IV) and uranium(III) in molten fluorides

This study focused on the electroreduction process of uranium cations in molten fluorides. It involved cyclic voltammetry, chronopotentiometry with and without current reversal, and square wave voltammetry.The results indicate a two-step reduction process for uranium(IV). The first step U(IV)/U(III) exchanging one electron corresponds to a soluble/soluble system and is limited by U(IV) diffusion with D_U(IV) = 1.25 ± 0.35 × 10⁻⁵ cm² s⁻¹ in LiF-NaF at 720 °C.In order to perform a thorough study of the second step U(III)/U(0) in the reduction process, the melt was chemically reduced in U(III) with U metal as reducing agent. Alternatively to the use of LiF-NaF where U metal is unstable at 720 °C, the chemical reduction of U(IV) in U(III) was performed in a LiF-CaF₂-UF₄ solution containing U metal at 810 °C. It has been confirmed that the reduction of U(III) proceeds in one step exchanging three electrons and by a diffusion controlled process with D_U(III) = 2.2 ± 0.7 × 10⁻⁵ cm² s⁻¹ in LiF-CaF₂ at 810 °C.     

Electrochemical behaviour of americium ions in LiCl-KCl eutectic melt

This work presents a study on the electrochemical properties of AmCl₃ in a molten LiCl-KCl eutectic, at a temperature range of 733-833 K. Transient electrochemical techniques, such as cyclic voltammetry and chronopotentiometry, on inert metallic tungsten working electrode have been used to investigate the reduction mechanism of Am³⁺ ions. The results show that Am³⁺ is reduced to Am metal by a two-step mechanism corresponding to the Am³⁺/Am²⁺ and Am²⁺/Am⁰ transitions. Formal standard potentials of Am³⁺/Am²⁺ ( versus Cl₂/Cl⁻ at 733 K) and Am²⁺/Am⁰ ( versus Cl₂/Cl⁻ at 733 K) redox couples as well as diffusion coefficients of Am³⁺ and Am²⁺ (2.4 × 10⁻⁵ and 1.15 × 10⁻⁵ cm² s⁻¹ at 733 K, respectively) have been calculated at three different temperatures. In the studied range of temperature, the DAm3+/DAm2+ ratio was found to be around 2. In addition, thermodynamic properties have been calculated for Am³⁺ () and Am²⁺ () and compared to thermodynamic reference data in order to estimate activity coefficients (Am³⁺ = 4.7 × 10⁻³ and Am²⁺ = 2.7 × 10⁻² at 733 K) in the molten LiCl-KCl eutectic.     

Adsorption dynamics and interfacial properties of thiol-based cobalt terpyridine monolayers

Spontaneously adsorbed monolayers of [Co(ttp-CH₂-SH)₂](PF₆)₂ have been formed on platinum microelectrodes by exposure to micromolar solutions of the complex in 0.1 M TBABF₄ in acetonitrile, ttp-CH₂-SH is 4′-(p-(thiolmethyl)-phenyl)-2,2′:6′,2″-terpyridine. Resonance Raman spectroscopy on roughened polycrystalline platinum macro electrodes show that the molecule undergoes adsorption through the sulphur atom onto the platinum surface. The monolayers show reversible and well defined cyclic voltammetry when switched between Co²⁺ and Co³⁺ forms, with a peak to peak splitting of 0.040 ± 0.005 V up to 200 V s⁻¹ and an FWHM of 0.138 ± 0.010 V. Adsorption is irreversible leading to the maximum surface coverage, 6.3 ± 0.3 × 10⁻¹¹ mol cm⁻² for 2.5 ≤ [Co(ttp-CH₂-SH)₂] ≤ 10 μM. The rate of monolayer formation appears to be controlled not by mass transport or interfacial binding but by surface diffusion of the complex. The surface diffusion coefficient is 5.5 ± 1.1 × 10⁻⁷ cm² s⁻¹ indicating that prior to formation of an equilibrated monolayer, the adsorbates have significant mobility on the surface. The electron transfer process across the monolayer-electrode interface has been probed by high speed chronoamperometry and the standard heterogeneous electron transfer rate constant, k°, is approximately 3.06 ± 0.03 × 10⁴ s⁻¹. The reorganization energy is at least 18.5 kJ mol⁻¹.     

Electrochemical impedance spectroscopy of electrodes modified with thin films of MgMnCO3 layered double hydroxides

The electrochemical impedance spectra of MgMnCO₃ LDH films oxidized at different dc potentials were recorded. The results were fitted to a Randles type cell by replacing the Warburg impedance with a mass transfer resistance in parallel with a constant phase element. The films charge transfer resistances decreased dramatically at the onset of manganese oxidation. In thin films, R_ct decreased from 10⁴ Ω for an un-oxidized film to a minimum of 40 Ω in a film oxidized at 0.32 V, before increasing back to 10⁴ Ω in a film oxidized at 0.5 V. Iodometry measurements show these changes correspond to increases in the manganese average valence in the films from 3.09+ prior to oxidation, to 3.80+ at 0.32 V and 3.95+ at 0.5 V. In thicker films, however, a much higher dc potential, 1.0 V, was required to return R_ct to 10⁴ Ω. There was also less change in the manganese average valence in the thicker films. Oxidation at 1.0 V only increased the manganese valence to 3.33+. For the partially oxidized films, the Nyquist plots consisted of depressed semicircles at high frequency, followed by linear regions at lower frequency where the impedance was controlled by mass transport. The effective diffusion coefficient estimated from the low frequency impedance was 1 × 10⁻⁹ cm² s⁻¹, consistent with proton diffusion in solid electrodes. The impedance spectrum of a partially oxidized film reduced at −0.2 V was similar to that of the un-oxidized film.     

Electrocatalytic oxidation of deferiprone and its determination on a carbon nanotube-modified glassy carbon electrode

The electrochemical behavior of the anti-thalassemia and anti-HIV replication drug, deferiprone, was investigated on a carbon nanotube-modified glassy carbon (GC-CNT) electrode in phosphate buffer solution, pH 7.40 (PBS). During oxidation of deferiprone, two irreversible anodic peaks, with and , appeared, using GC-CNT. Cyclic voltammetric study indicated that the oxidation process is irreversible and diffusion controlled. The number of exchanged electrons in the electro-oxidation process was obtained, and the data indicated that deferiprone is oxidized via two two-electron steps. The results revealed that carbon nanotube (CNT) promotes the rate of oxidation by increasing the peak current, so that deferiprone is oxidized at lower potentials, which thermodynamically is more favorable. This result was confirmed by impedance measurements. The diffusion coefficient, electron-transfer coefficient and heterogeneous electron-transfer rate constant of deferiprone were found to be 1.49 × 10⁻⁶ cm² s⁻¹, 0.44, and 3.83 × 10⁻³ cm s⁻¹, respectively. A sensitive, simple and time-saving differential-pulse voltammetric procedure was developed for the analysis of deferiprone. Using the proposed method, deferiprone can be determined with a detection limit of 5.25 × 10⁻⁷ M. The applicability of the method to direct assays of spiked human serum and urine fluids is described.     

Copolymerization of aniline and 4,4′-diaminodiphenyl sulphone and characterization of formed nano size copolymer

Aniline was copolymerized chemically in presence of five different concentrations of 4,4′-diaminodiphenyl sulphone using potassium persulphate. The copolymer exhibited good solubility in DMF and DMSO. Copolymers were characterized by UV-VIS, FTIR, XRD and SEM studies. The formation of polymer through N-H group was understood from the single N-H stretching vibrational frequency at 3378 cm⁻¹ and bands at 1630 and 1494 cm⁻¹ for quinonoid and benzenoid structures, respectively. The stretching vibration of sulphone SO at 1115 cm⁻¹ clearly indicated the presence of DDS in the copolymer. The X-ray diffraction studies revealed the formation of nano sized crystalline copolymer. When more DDS was incorporated in the copolymer the crystalline nature changed from less to more. The grain size of the copolymer calculated from Scherrer's formula was 83 nm. The nano size copolymer formation was also confirmed through surface morphology (100 nm) studies. The electrical property of the copolymer was studied by four-probe conductivity meter. The synthesized polymers have conductivity of 7.21 × 10⁻³ to 2.07 × 10⁻³ S cm⁻¹. The voltammetric and spectroelectrochemical results were also presented.     

Li-ion diffusion kinetics in LiMn2O4 thin films prepared by pulsed laser deposition

LiMn₂O₄ thin films were deposited on Au substrates by pulsed laser deposition (PLD). The Li-ion chemical diffusion coefficients of the films, , were measured by cyclic voltammetry (CV), galvanostatic intermittent titration technique (GITT), potentiostatic intermittent titration technique (PITT), and electrochemical impedance spectroscopy (EIS). It was found that the values by CV and PITT were in the order of 10⁻¹³ cm² s⁻¹, and those by EIS and GITT were in the range of 10⁻¹³ to 10⁻¹¹ and 10⁻¹⁴ to 10⁻¹¹ cm² s⁻¹, respectively. These data were compared with the previously reported values.     

Electroreduction of peroxycitric acid coexisting with hydrogen peroxide in aqueous solution

The electrochemical reduction of peroxycitric acid (PCA) coexisting with citric acid and hydrogen peroxide (H₂O₂) in the equilibrium mixture was extensively studied at a gold electrode in acetate buffer solutions containing 0.1 M Na₂SO₄ (pH 2.0-6.0) using cyclic and hydrodynamic voltammetric, and hydrodynamic chronocoulometric measurements. The reduction of PCA was characterized to be an irreversible, diffusion-controlled process, and the cyclic voltammetric reduction peak potential () was found to be more positive by ca. 1.0 V than that of the coexisting H₂O₂, e.g., the values obtained at 0.1 V s⁻¹ for PCA and H₂O₂ were 0.35 and −0.35 V, respectively, vs. Ag|AgCl|KCl (sat.) at pH 3.3. The of PCA was found to depend on pH, i.e., at pH > 4.5, the plot of vs. pH gave the slope (−64 mV decade⁻¹) which is close to the theoretical value (−59 mV decade⁻¹) for an electrode process involving the equal number of electron and proton in the rate-determining step, while at pH < 4.5, the was almost independent of pH. The relevant electrochemical parameters, Tafel slope, number of electrons, formal potential (E⁰′), cathodic transfer coefficient and standard heterogeneous rate constant (k⁰′) for the reduction of PCA and the diffusion coefficient of PCA were determined to be ca. 100 mV decade⁻¹, 2, 1.53 V (at pH 2.6), 0.29, 1.2 × 10⁻¹² cm s⁻¹ and 0.29 × 10⁻⁵ cm² s⁻¹, respectively, and except for E⁰′, the obtained values were almost independent of the solution pH. The overall mechanism of the reduction of PCA was discussed.     

Interaction between promethazine hydrochloride and DNA and its application in electrochemical detection of DNA hybridization

The interactions of promethazine hydrochloride (PZH) with thiolated single-stranded DNA (HS-ssDNA) and double-stranded DNA (HS-dsDNA) self-assembled on gold electrodes have been studied electrochemically. The binding of PZH with ssDNA shows a mechanism containing an electrostatic interaction, while the mode of PZH interaction with dsDNA contains both electrostatic and intercalative bindings. The redox system belongs to the category of diffusion control approved by cyclic voltammetry (CV). The diffusion coefficients of PZH at the bare, HS-dsDNA and HS-ssDNA modified gold electrodes decrease regularly as 1.34 × 10⁻³ cm² s⁻¹, 1.04 × 10⁻³ cm² s⁻¹, 7.47 × 10⁻⁴ cm² s⁻¹, respectively. The electron transfer standard rate constant k_s of PZH at bare gold, HS-ssDNA and HS-dsDNA modified electrodes are 0.419 s⁻¹, 0.131 s⁻¹, and 0.154 s⁻¹, respectively. The presence of adsorbed dsDNA results in a great increase in the peak currents of PZH in comparison with those obtained at a bare or ssDNA adsorbed gold electrode. The difference between interactions of PZH with HS-ssDNA and HS-dsDNA has been used for hybridization recognition of 14-mer DNA oligonucleotide. The peak current (i_pa) of PZH is linearly proportional to the logarithmic concentration of complementary target DNA in the range from 2.0 × 10⁻⁹ mol L⁻¹ to 5.0 × 10⁻⁷ mol L⁻¹ with the detection limit of 3.8 × 10⁻¹⁰ mol L⁻¹.     

Voltammetric detection of the interactions between RNO2 and electron acceptors in aqueous medium at highly boron doped diamond electrode (HBDDE)

This paper describes the electrochemical behavior of the nitrofurazone (NFZ), in predominantly aqueous medium, in the absence and presence of glutathione (reduced form) (GSH), l-cysteine (Cys) and O₂ using a highly boron doped diamond electrode (HBDDE). In presence of [Thiol] ≥ 3.7 × 10⁻² mol L⁻¹ NFZ is directly reduced to RNO-Thiol adducts in an electrochemical process involving two electrons and two protons. On the other side, O₂ acts as a RNO₂^•− scavenger and the velocity constant for the reaction, kO₂, is 60 L mol⁻¹ s⁻¹. The process is catalytic and can be used to the analytical determination of NFZ in the range of 9.9 × 10⁻⁷ ≤ [NFZ] ≤ 1.1 × 10⁻⁵ mol L⁻¹ at pH 8.0, with sensitivity of 2.2 × 10⁶ μA mol⁻¹ cm⁻² and detection limit of 3.4 × 10⁻⁷ mol L⁻¹. The analytical parameters were similar to those obtained at pH 4.0 using the direct reduction of NFZ to the respective amine derivative in a process involving six electrons and six protons. The characterization of NFZ global reduction process in aqueous medium and at relative low scan rate, 100 mV s⁻¹, was only possible due the intrinsic superficial characteristics of the HBDDE, which stabilize the RNO₂⁻ free radical, allowing to work in a large potential window, without losing the RNO₂⁻ oxidation signal.