Electrocatalytic oxidation of l-cysteine with a stable copper-cobalt hexacyanoferrate electrochemically modified carbon paste electrode

A stable composition of hybrid copper-cobalt hexacyanoferrate (Cu-CoHCF) film was electrodeposited on a carbon paste electrode (CPE). There are a few reports for using this hybrid as a mediator, but all of them require almost 12 h conditioning time before usage. Contrary to previous reports this electrode does not require any conditioning and can be used immediately after film formation. The electrocatalytic activity of this film was investigated and showed a good electrocatalytic effect for oxidation of l-cysteine (Cys) in phosphate buffer solution (PBS) in pH range of 1-7. A linear range of 6 μM to 1 mM of Cys and an experimental detection limit of 5 μM of Cys were obtained using cyclic voltammetry method. The diffusion coefficient of Cys and catalytic rate constant for electrocatalytic reaction were also calculated. The major problem reported in electro oxidation of Cys is poisoning of electrode surface with reaction product, but in this study oxidation of Cys had no significant fouling effect on the modified electrode surface for the concentrations below 0.5 mM of Cys.     

Electrocatalytic reduction of hydrogen peroxide at nanostructured copper modified electrode

The electrocatalytic reduction of hydrogen peroxide (H₂O₂) has been studied at nanostructured copper (Cu_nano) modified glassy carbon (GC/Cu_nano) electrode in phosphate buffer (pH 7.2). The electrical properties of GC/Cu_nano modified electrodes were studied by electrochemical impedance spectroscopy (EIS). Surface and electrochemical characterization were carried out by using atomic force microscopy (AFM) and cyclic voltammetry. A well-defined H₂O₂ reduction signal, which is due to mediation of a surface active site redox transition exhibits at the GC/Cu_nano electrode. The Cu_nano is acting as a bridge without the aid of any other electron mediator, which enables the direct electron transfer between the modified electrode and the substrate. The results are compared with bulk copper macroelectrode and emphasized the efficiency of the Cu_nano modified electrode. Systematic investigations were made to optimize the experimental parameter, such as applied potential (E_app) for copper electrodeposition. The calibration curve obtained from chronoamperometric studies was found to be linear in the range 0.5 to 8.0 μM H₂O₂ with a detection limit of ca.10 nM (S/N = 3) at the GC/Cu_nano electrode. The modified electrode is stable for 1 week in phosphate buffer after repetitive measurements.     

Electrocatalytic oxidation of some anti-inflammatory drugs on a nickel hydroxide-modified nickel electrode

The electrocatalytic oxidation of several anti-inflammatory drugs (mefenamic acid, diclofenac and indomethacin) was investigated on a nickel hydroxide-modified nickel (NHMN) electrode in alkaline solution. This oxidation process and its kinetics were studied using cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy techniques. Voltammetric studies indicated that in the presence of drugs, the anodic peak current of low-valence nickel species increases, followed by a decrease in the corresponding cathodic current. This pattern indicates that drugs were oxidized on the redox mediator immobilized on the electrode surface via an electrocatalytic mechanism. A mechanism based on the electrochemical generation of Ni(III) active sites and their subsequent consumption by drugs was also investigated. The corresponding rate law under the control of charge transfer was developed and kinetic parameters were derived. In this context, the charge-transfer resistance accessible both theoretically and through impedancemetry was used as a criterion. The rate constants of the catalytic oxidation of drugs and the electron-transfer coefficients are reported. A sensitive, simple and time-saving amperometric procedure was developed for the analysis of these drugs in bulk form and for the direct assay of tablets, using the NHMN electrode.     

Electrocatalytic hydrazine oxidation on quinizarine modified glassy carbon electrode

The electrocatalytic oxidation of hydrazine has been studied on glassy carbon modified by electrodeposition of quinizarine, using cyclic voltammetry and chronoamperometry techniques. It has been shown that the oxidation of hydrazine to nitrogen occurs at a potential where oxidation is not observed at the bare glassy carbon electrode. The apparent charge transfer rate constant and transfer coefficient for electron transfer between the electrode surface and immobilized quinizarine were calculated as 4.44 s⁻¹ and 0.66, respectively. The heterogenous rate constant for oxidation of hydrazine at the quinizarine modified electrode surface was also determined and found to be about 4.83 × 10³ M⁻¹ s⁻¹. The diffusion coefficient of hydrazine was also estimated as 1.1 × 10⁻⁶ cm² s⁻¹ for the experimental conditions, using chronoamperometry.     

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.     

Electrocatalytic oxidation of deoxyguanosine on a glassy carbon electrode modified with a ruthenium oxide hexacyanoferrate film

The electrocatalytic oxidation of deoxyguanosine on a ruthenium hexacyanoferrate (RuOHCF) glassy carbon (GC) modified electrode was investigated in acid medium by using rotating disc electrode (RDE) voltammetry. Chronoamperometric experiments allowed information on the charge transport rate through the RuOHCF film and at a very short time window a diffusion-like behavior was observed with a D_ct value of 2.7 × 10⁻¹¹ cm² s⁻¹ for a film with Γ = 4.47 × 10⁻⁹ mol cm⁻². The influence of systematic variation of rotation rate, film thickness and the electrode potential indicates that the rate of cross-chemical reaction between Ru(IV) centers immobilized into the film and deoxyguanosine controls the overall electrodic process and the value of the rate constant was found to be 3.2 × 10⁶ mol⁻¹ L¹ s⁻¹. The relatively high rate constant of the cross-reaction, the facile penetration of the substrate through the film and the fast transport of electrons suggest that the electrocatalytic process occurs throughout the film layer.     

Towards the conception of an amperometric sensor of l-tyrosine based on Hemin/PAMAM/MWCNT modified glassy carbon electrode

A novel amperometric sensor was fabricated based on the immobilization of hemin onto the poly (amidoamine)/multi-walled carbon nanotube (PAMAM/MWCNT) nanocomposite film modified glassy carbon electrode (GCE). Electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and ultraviolet visible (UV-vis) adsorption spectroscopy were used to investigate the possible state and electrochemical activity of the immobilized hemin. In the Hemin/PAMAM/MWCNT nanocomposite film, MWCNT layer possessed excellent inherent conductivity to enhance the electron transfer rate, while the layer of PAMAM greatly enlarged the surface average concentration of hemin (Γ) on the modified electrode. Therefore, the nanocomposite film showed enhanced electrocatalytical activity towards the oxidation of l-tyrosine. The kinetic parameters of the modified electrode were investigated. In pH 7.0 phosphate buffer solution (PBS), the sensor exhibits a wide linear range from 0.1 μM to 28.8 μM l-tyrosine with a detection limit of 0.01 μM and a high sensitivity of 0.31 μA μM⁻¹ cm⁻². In addition, the response time of the l-tyrosine sensor is less than 5 s. The excellent performance of the sensor is largely attributed to the electro-generated high reactive oxoiron (IV) porphyrin (O = Fe^IV-P) which effectively catalyzed the oxidation of l-tyrosine. A mechanism was herein proposed for the catalytic oxidation of l-tyrosine by oxoiron (IV) porphyrin complexes.     

Detection of NADH via electrocatalytic oxidation at single-walled carbon nanotubes modified with Variamine blue

Screen-printed electrodes (SPEs) modified with Variamine blue (VB), covalently attached to the oxidized single-walled carbon nanotubes (SWCNTs-COOH), were developed and used as chemical sensors for the detection of the reduced nicotinamide adenine dinucleotide (NADH). The Variamine blue redox mediator was covalently linked to the SWCNTs-COOH by the N,N′-dicyclohexylcarbodiimide (DCC) and N-hydroxysuccinimide (NHS) chemistry. Infrared Fourier transform (FT-IR) spectroscopy revealed the presence of the amide bands situated at 1623 cm⁻¹ (I band), 1577 cm⁻¹ (II band) and 1437 cm⁻¹ (III band) demonstrating the covalent linkage of Variamine blue to SWCNTs-COOH. The heterogeneous electron transfer rate, k_obs., was 13,850 M⁻¹ s⁻¹, and the k_s and α were 0.8 s⁻¹ and 0.56, respectively. The pH dependence was also investigated. SPEs modified with Variamine blue by using the DCC/NHS conjugation method, showed a variation of −36 mV per pH unit.A successful application was the development of a lactate biosensor obtained by the immobilization of the l-lactate dehydrogenase on the NADH sensor.     

Electrocatalytic reduction of nitrite on tetraruthenated metalloporphyrins/Nafion glassy carbon modified electrode

This paper describes the electrochemical reduction of nitrite ion in neutral aqueous solution mediated by tetraruthenated metalloporphyrins (Co(II), Ni(II) and Zn(II)) electrostatically assembled onto a Nafion film previously adsorbed on glassy carbon or ITO electrodes. Scanning electron microscope (SEM-EDX) and transmission electron microscopy (TEM) results have shown that on ITO electrodes the macrocycles forms multiple layers with a disordered stacking orientation over the Nafion film occupying hydrophobic and hydrophilic sites in the polyelectrolyte. Atomic force microscopy (AFM) results demonstrated that the Nafion film is 35 nm thick and tetraruthenated metalloporphyrins layers 190 nm thick presenting a thin but compacted morphology. Scanning electrochemical microscopy (SECM) images shows that the Co(II) tetraruthenated porphyrins/Nf/GC modified electrode is more electrochemically active than their Ni and Zn analogues.These modified electrodes are able to reduce nitrite at −660 mV showing enhanced reduction current and a decrease in the required overpotential compared to bare glassy carbon electrode. Controlled potential electrolysis experiments verify the production of ammonia, hydrazine and hydroxylamine at potentials where reduction of solvent is plausible demonstrating some selectivity toward the nitrite ion. Rotating disc electrode voltammetry shows that the factor that governs the kinetics of nitrite reduction is the charge propagation in the film.     

Electrocatalysis of oxidation of 2-mercaptoethanol, l-cysteine and reduced glutathione by adsorbed and electrodeposited cobalt tetra phenoxypyrrole and tetra ethoxythiophene substituted phthalocyanines

Catalytic activity of cobalt tetra ethoxythiophene and cobalt tetra phenoxypyrrole phthalocyanine complexes towards oxidation of 2-mercaptoethanol, l-cysteine and reduced glutathione is reported. It was found that the activity of the complexes depends on the substitution of the phthalocyanine ring, pH, film thickness and method of electrode modification. The high electrocatalytic activity obtained with adsorbed complexes in alkaline medium clearly demonstrates the necessity of modifying bare carbon electrodes to endow them with the desired behaviour.     

Modification of glassy carbon electrodes by 4-chloromethylphenyl units and d-glucosaminic acid

The present work is dealing with the attachment of d-glucosaminic acid (D-GA) on glassy carbon electrode by two different methods. Firstly, the electrode was modified by chloromethylphenyl groups by reduction of 4-chloromethylphenyldiazonium cations followed by the nucleophilic substitution of the chlorine by the amine functionality of D-GA and secondly by the direct immobilization of the amine terminated molecule. The generality of the nucleophilic substitution reaction and the direct immobilization of an amine were also demonstrated with reactants bearing an electroactive ferrocene moiety; 4-nitrophenylferrocene (NFc) and 4-ferrocenylaniline (FcA). The surfaces modified with FcA and NFc were investigated by cyclic voltammetry, and the D-GA modified electrodes were characterized by X-ray photoelectron spectroscopy. A preliminary evaluation of the efficiency of these surface modifiers to prevent protein adsorption was realized by scanning electron microscopy.     

Electroenzymatic strategies for deracemization, stereoinversion and asymmetric synthesis of amino acids

A combination of a selective enzymatic oxidation with an unselective electrochemical reduction step was applied for deracemization, stereoinversion and asymmetric synthesis of l-leucine (starting from racemic leucine, d-leucine or 4-methyl-2-oxovaleric acid) in a batch reactor. d-Amino acid oxidase (d-AAO) from Trigonopsis variabilis was used as enzyme. Reaction conditions for the electrochemical and enzymatic reactions were investigated separately and finally combined to an electroenzymatic synthesis, yielding 3.5 mmol L⁻¹ d⁻¹ of l-leucine (ee 91%).     

Use of a 12-molybdovanadate(V) modified ionic liquid carbon paste electrode as a bifunctional electrochemical sensor

Vanadium (V) was used as the central heteroatom in the synthesis of the Keggin-type polyoxometalate {[CH₃(CH₂)₇]₄N}₃[VMo₁₂O₄₀] (1). This compound was characterized by elemental analysis and single-crystal X-ray diffraction. Compound 1 was used as a bulk-modifier to fabricate the chemically modified ionic liquid 1-n-butyl-3-methylimidazolium hexafluorophosphate ([BMIM][PF₆]) carbon paste electrode (1-ILCPE) by direct mixing. The bulk-modified 1-ILCPE showed well-defined cyclic voltammograms with four reversible redox couples in H₂SO₄ + Na₂SO₄. The first wave could be ascribed to the reduction of the central V and the last three waves corresponded to the redox processes of the peripheral Mo atoms. The 1-ILCPE presents good electro-catalytic activity not only toward the oxidation of l-cysteine, which is attributed to the function of the central V, but also toward the reduction of hydrogen peroxide, which is attributed to the function of the Mo atoms. Compared with the conventional carbon paste electrode (CPE), the excellent experimental results suggest that the newly developed paste electrode holds great promise in the design of electrochemical devices, such as sensors and biosensors. The promising results are ascribed to the use of an ionic liquid, which forms an excellent charge-transfer bridge and wide electrochemical windows in the bulk of carbon paste electrode.     

Ring-opening polymerization of l-lactide catalyzed by a biocompatible calcium complex

A novel calcium complex, [(DAIP)₂Ca]₂ (where DAIP-H = 2-[(2-dimethylamino-ethylimino)methyl]phenol), is prepared in a one flask reaction by condensation of Ca(OMe)₂ with DAIP-H in toluene/THF. Experimental results show that in the presence of various alcohols, [(DAIP)₂Ca]₂ efficiently initiates the ring-opening polymerization of l-lactide in a controlled fashion, yielding polymers with expectative molecular weight and low polydispersity indexes. Furthermore, kinetic studies show a first-order dependency on both [LA] and [BnOH].     

Improved preparative electrochemical oxidation of d-glucose to d-glucaric acid

The 2,2,6,6-tetramethyl-1-piperidinyloxy free radical (TEMPO) mediated electrochemical oxidation of d-glucose to d-glucaric acid on a synthetically useful scale is reported. Using TEMPO and a graphite felt anode combined with a stainless steel cathode, d-glucose was oxidized under different conditions (pH, temperature, co-oxidant), and the reaction outcomes were analyzed. Optimized conditions for such oxidation are provided along with few new interesting results unique to this reaction, such as the appearance of a novel triacid.     

Glass transition behavior and dynamic fragility in polylactides containing mobile and rigid amorphous fractions

The segmental dynamics of polylactide chains covering the T_g − 30 °C to T_g + 30 °C range was studied in absence and presence of a crystalline phase by dynamic mechanical analysis (DMA) using the framework provided by the WLF theory and the Angell's dynamic fragility concept. An appropriate selection of stereoisomers combined with a thermal conditioning strategy to promote crystallization (above T_g) or relaxation of chains (below T_g) was revealed as an efficient method to tune the ratio of the rigid and mobile amorphous phases in polylactides. A single bulklike mobile amorphous phase was taken for poly(d,l-lactide) (PDLLA). In turn three phases, comprising a mobile amorphous fraction (MAF, X_MA), a rigid amorphous fraction (RAF, X_RA) and a crystalline fraction (X_c) were determined in poly(l-lactide) (PLLA) by modulated differential scanning calorimetry (MDSC) according to a three-phase model. The analysis of results confirms that crystallinity and RAF not only elevate the T_g and the breadth of the glass transition region but also yields an increase in dynamic fragility parameter (m) which entails the existence of a smaller length-scale of cooperativity of polylactide chains in confined environments. Consequently it is proposed that crystallinity is acting in polymeric systems as a topological constraint that, preventing longer range dynamics, provides a faster segmental dynamics by the temperature dependence of relaxation times according to the strong-fragile scheme.     

Broadband dielectric spectroscopic characterization of the hydrolytic degradation of carboxylic acid-terminated poly(d,l-lactide) materials

Broadband dielectric spectroscopy was used to examine carboxylic acid-terminated poly(d,l-lactide) samples that were hydrolytically degraded in 7.4 pH phosphate buffer solutions at 37 °C. The dielectric spectral signatures of degraded samples were considerably more distinct than those of undegraded samples and a T_g-related relaxation associated with long range chain segmental mobility was seen. For both degraded and undegraded samples, a relaxation peak just beneath a DSC-based T_g was observed, which shifts to higher frequency with increasing temperature. Thus, this feature is assigned as the glass transition as viewed from the dielectric relaxation perspective. Linear segments on log-log plots of loss permittivity vs. frequency, in the low frequency regime, are attributed to d.c. conductivity. An upward shift in relaxation peak maximum, f_max, observed especially after 145 d of immersion in buffer, implies a decrease in the time scale of long range segmental motions with increased degradation time.Permittivity data for degraded and undegraded materials were fitted to the Havriliak-Negami equation with subtraction of the d.c. conductivity contribution to uncover pure relaxation peaks. Parameters extracted from these fits were used to construct Vogel-Fulcher-Tammann-Hesse (VFTH) curves and distribution of relaxation time, G(τ), curves for all samples. It was seen that the relaxation times for the α-transition in both degraded and undegraded samples showed VFTH temperature behavior. G(τ) curves showed a general broadening and shift to lower τ with degradation, which can be explained in terms of a broadening of molecular weight within degraded samples and faster chain motions.     

Electrocatalytic oxidation of thioglycolic acid at carbon paste electrode modified with cobalt phthalocyanine: application as a potentiometric sensor

The voltammetric behavior of thioglycolic acid (TGA) was studied at a carbon paste electrode modified with cobalt phthalocyanine (CoPc). The CoPc-modified electrode shows high electrocatalytic activity toward oxidation of TGA, lowering substantially the overpotential of anodic reaction. Results of the cyclic voltammetry show that TGA undergoes a two-step oxidation (each step with one electron) resulting the dimer of thiol. Enhancement of the rate of electron transfer results in a near-Nernstian behavior of modified electrode to the concentration of TGA and makes it as a suitable potentiometric sensor for the detection of this compound. This electrode shows a near-Nernstian response in a wide linear range of the concentration TGA (4 orders of magnitude). The modified electrode was used successfully for the determination of TGA and its salts in hair-treatment products and also in culture media. The modified electrode exhibited a fast response time (<10 s), very good stability, and had an extended lifetime.     

Ring-opening polymerization of six-membered cyclic esters catalyzed by tetrahydroborate complexes of rare earth metals

Catalytic behavior of tetrahydroborate complexes of rare earth metals, Ln(BH₄)₃(THF)_x (1: Ln = La, x = 3; 2: Ln = Pr, x = 2; 3: Ln = Nd, x = 3; 4: Ln = Sm, x = 3; 5: Ln = Y, x = 2.5; 6: Ln = Yb, x = 3), for ring-opening polymerization (ROP) of six-membered cyclic esters, δ-valerolactone (VL) and d,l-lactide (d,l-LA), was studied. The controlled polymerization of VL with 1-6 proceeded in THF at 60 °C. The catalytic activities of these complexes for the ROP of VL were observed to be in order of the ionic radii of the metals: 1(La) ≥ 2(Pr) ≥ 3(Nd) > 4(Sm) > 5(Y) > 6(Yb). The obtained polymers were demonstrated to be hydroxy-telechelic by ¹H NMR and MALDI-TOF MS spectroscopy. The controlled ROP of d,l-LA also proceeded by these complexes. The activities of these complexes for the d,l-LA ROP were also in order of the ionic radii of the metals.     

Functionalized micelles from new ABC polyglycidol-poly(ethylene oxide)-poly(d,l-lactide) terpolymers

New ABC type terpolymers of poly(ethoxyethyl glycidyl ether)/poly(ethylene oxide)/poly(d,l-lactide) were obtained by multi-mode anionic polymerization. After successive deprotection of the ethoxyethyl groups from the first block, highly hydroxyl functionalized copolymers of polyglycidol/poly(ethylene oxide)/poly(d,l-lactide) were obtained. These copolymers form elongated ellipsoidal micelles by direct dissolution in water. The micelles consist of a poly(d,l-lactide) core and stabilizing shell of polyglycidol/poly(ethylene oxide). The hydroxyl groups of polyglycidol blocks situated at the micelle surface provide high functionality, which could be engaged in further chemical modification resulting in a potential drug targeting agents. The micellization process of the copolymers in aqueous media was studied by hydrophobic dye solubilization, static and dynamic light scattering, and transmission electron microscopy.