Neurotransmitter dopamine applied in electrochemical determination of aluminum in drinking waters and biological samples.

It was demonstrated that the decrease of the differential pulse voltammetric (DPV) anodic peak current of dopamine (3,4-dihydroxyphenylethylamine, DA) was linear with the increase of aluminum (Al) concentration. Under optimum experimental conditions (pH 4.6, 1.2 x 10(-3) M DA, and 0.04 M NaAc-HAc buffer solution), the linear range is 4.0 x 10(-7)-8.0 x 10(-5) M, the detection limit is 1.4 x 10(-7) M, and the relative standard deviation for 4 x 10(-5) M Al(III) is 3.5% (n=8). Many foreign species, especially some low-molecule-weight biological molecules, were chosen for interference testing. The proposed method was applied to the determination of Al in biological samples such as synthetic renal dialysate, Ringer's solution, human blood, cerebrospinal fluid of a patient, and urine of a diabetic patient. The corresponding recoveries were generally between 95 and 105%. The basic principle of the method was determined by examining Al complexed with DA. This results in the blockage of the electroactive sites on DA, followed eventually by the reduction of the electrochemical response of DA. This result was verified by examining the behavior of DA, both in the presence and absence of Al, using electrochemical, UV-Vis, Raman, and (13)C NMR spectroscopic methods.     

Self-assembled films of hemoglobin/laponite/chitosan: application for the direct electrochemistry and catalysis to hydrogen peroxide

A highly stable biological film was formed on the functional glassy carbon electrode (GCE) via step-by-step self-assembly of chitosan (CHT), laponite, and hemoglobin (Hb). Cyclic voltammetry (CV) of the Hb/laponite/CHT/GCE showed a pair of stable and quasi-reversible peaks for the Hb-Fe(III)/Fe(II) redox couple at about -0.035 V versus a saturated calomel electrode in pH 6.0 phosphate buffer at a scan rate of 0.1 V s(-1). The electrochemical reaction of Hb entrapped on the laponite/CHT self-assembled film exhibited a surface-controlled electrode process. The formal potential of the Hb-heme-Fe(III)/Fe(II) couple varied linearly with the increase of pH over the range of 3.0-8.0 with a slope of -63 mV pH(-1), which implied that an electron transfer was accompanied by single-proton transfer in the electrochemical reaction. The position of the Soret absorption band of this self-assembled Hb/laponite/CHT film suggested that the entrapped Hb kept its secondary structure similar to its native state. The self-assembled film showed excellent long-term stability, the CV peak potentials kept in the same positions, and the cathodic peak currents retained 90% of their values after 60 days. The film was used as a biological catalyst to catalyze the reduction of hydrogen peroxide. The electrocatalytic response showed a linear dependence on the H2O2 concentration ranging widely from 6.2 x 10(-6) to 2.55 x 10(-3) M with a detection limit of 6.2 x 10(-6) M at 3 sigma.     

An electrocatalytic transducer for l-cysteine detection based on cobalt hexacyanoferrate nanoparticles with a core–shell structure

The electrocatalytic oxidation of l-cysteine (CySH) was studied on cobalt hexacyanoferrate nanoparticles with a core–shell structure (iron(III) oxide core–cobalt hexacyanoferrate shell) using cyclic voltammetry and chronoamperometry. Voltammetric studies represented two quasi-reversible redox transitions for the nanoparticles in phosphate buffer solution (pH 7.4). In the presence of CySH, the anodic peak current of the Fe(II)/Fe(III) transition was increased, followed by a decrease in the corresponding cathodic peak current, whereas the peak currents related to the Co(II)/Co(III) transition almost remained unchanged. The results indicated that the nanoparticles oxidized CySH via a surface mediation electrocatalytic mechanism. The catalytic rate constant, the electron transfer coefficient, and the diffusion coefficient involved in the electrooxidation process of CySH are reported here. Ultrasensitive and time-saving determination procedures were developed for the analysis of the CySH, and the corresponding analytical parameters are reported. According to the proposed methods, CySH was determined with detection limits of 40 and 20 nm in batch and flow systems, respectively. The proposed amperometric method was also applied to the analysis of CySH in human urine and serum blood samples.     

The effect of antioxidants on electrocatalytic activity of cytochrome P450 3A4

The electrochemical analysis of cytochrome P450 3A4 catalytic activity has shown that vitamins C, A and E influence reduction of cytochrome P450 3A4. These data suggest a possibility of cross effects and interference of vitamins-antioxidants with drugs metabolised by cytochrome P450 3A4, during complex therapy of patients. These vitamins demonstrate antioxidant properties that lead to the increase of the cathodic current corresponding to heme reduction of this functionally significant hemoprotein. Ascorbic acid (0.028–0.56 mM) stimulated the cathodic peak (an electrochemical signal) of cytochrome P450 3A4. In the presence of diclofenac (Voltaren), a typical substrate of cytochrome P450 3A4, the increase in the catalytic current suggesting electrocatalysis and stimulating action of ascorbic acid was observed. In the presence of vitamins A and E the dose-dependent increase in the catalytic current of cytochrome P450 3A4 was observed in the range of vitamin concentrations from 10 to 100 μM. The maximal increase of 229 ± 20 and 162 ± 10% was observed at 100 μM vitamin A and vitamin E, respectively. In contrast to vitamin A, vitamin E in the presence of the cytochrome P450 inhibitor itraconazole did not increase the catalytic current. The latter implies existence of some substrate properties in vitamin E. The electrochemical approach for the analysis of catalytic activity of cytochrome P450 3A4 and studies of the effect of biologically active compounds on electrocatalysis is the sensitive and effective sensor approach, allowing to use low concentration of protein on an electrode (up to 10–15 mol/electrode), to carry out the analysis without involvement of protein redox partners, and to reveal drug-drug or drug-vitamins interaction in pre-clinical experiments.     

Applying cathodically polarised substrata to the restoration of a high value coral

Larval settlement of the high value red coral, Corallium rubrum, was studied on three different CaCO(3) substrata, viz. lithogenic (marble), electro-accreted calcium carbonate in the presence and in the absence of cathodic polarisation. The last two substrata consisted of stainless steel plates galvanically coupled with Zn anodes. The electrochemical characterization of the settlement device was studied in order to investigate correlations between cathodic parameters (polarisation potential, current density, calcareous deposit composition) and larval settlement. The results obtained in the natural habitat (at 35 m depth) showed that settlement was five times lower on the electro-accreted aragonite in the presence of low cathodic current densities (i≤1 μA cm(-2)) compared to both marble tiles and electro-accreted aragonite in the absence of polarisation. These last two substrata showed similar settlement values. The implications of these findings on restoration strategies for C. rubrum are discussed.     

A theoretical study of the principles regulating the specificity for Al(III) against Mg(II) in protein cavities

Several toxic effects arise from Al's presence in living systems, one of them being the alteration of the natural role of enzymes and non-enzyme proteins. Al(III) is capable of entering protein active sites that in normal conditions should be occupied by other metals. Even if Mg(II) is an ubiquitous metal in biological systems, the interference of aluminium in magnesium metabolism is not clear yet. In this work, a systematic study of the affinity of Al(III) for different protein binding sites is presented, with special attention on structural parameters, the role of the charge and the presence of different ligands in the protein cavity. The specificity of the binding site for Al(III) against Mg(II) has been studied, and also the thermodynamical propensity of a Mg(II)/Al(III) exchange. Quantum mechanical methods that proved to be reliable in previous works have been used, namely, the density functional theory (DFT) and polarizable continuum model (PCM).     

Nano-composition of riboflavin-nafion functional film and its application in biosensing

A novel nafion-riboflavin membrane was constructed and characterized by the scanning electron microscopy (SEM), transmission electron microscopy (TEM), UV-visible spectroscopy and cyclic voltammetric techniques. The estimated average diameter of the designed nanoparticles was about 60 nm. The functional membrane showed a quasi-reversible electrochemical behaviour with a formal potential of -562 +/- 5 mV (vs Ag/AgCl) on the gold electrode. Some electrochemical parameters were estimated, indicating that the system has good and stable electron transfer properties. Moreover, horseradish peroxidase (HRP) was immobilized on the riboflavin-nafion functional membrane. The electrochemical behaviour of HRP was quasi-reversible with a formal potential of 80 +/- 5 mV (vs Ag/AgCl). The HRP in the film exhibited good catalytic activity towards the reduction of H2O2. It shows a linear dependence of its cathodic peak current on the concentration of H2O2, ranging from 10 to 300 (micro)M.     

The effects of Al(III) speciation on the activity of trypsin

The effects of the different forms of Al(III) on the catalytic activity of the serine protease trypsin were studied. Enzyme activity was measured by BAEE assay in the presence of AlCl(3), Al(III):lactic acid 1:3, Al(III):maltol 1:3 or Al(III):nitrilotriacetic acid (NTA) 1:1 at a nominal Al(III) concentration of 0.01 M, and the ligand alone at pH 7.4 at 25 degrees C. Maltol and NTA caused approximately 30% inhibition, while that for the corresponding Al(III) complex was less than half of this. Al(III) in the form of the chloride or in three equivalents of lactic acid did not influence the activity of the enzyme, probably because most of the Al(III) was precipitated as Al(OH)(3). No direct interaction could be detected between the enzyme and the Al(III) complexes, either by ultrafiltration or by CD spectroscopy. These results strongly suggest that there is no direct involvement of Al(III) in the enzymatic reactions of trypsin.     

Rapid diagnosis of multidrug resistance in cancer by electrochemical sensor based on carbon nanotubes-drug supramolecular nanocomposites

The multidrug resistance (MDR) in cancer is a major chemotherapy obstacle, rendering many currently available chemotherapeutic drugs ineffective. The aim of this study was to explore the new strategy to early diagnose the MDR by electrochemical sensor based on carbon nanotubes-drug supramolecular interaction. The carbon nanotubes modified glassy carbon electrodes (CNTs/GCE) were directly immersed into the cells suspension of the sensitive leukemia cells K562 and/or its MDR cells K562/A02 to detect the response of the electrochemical probe of daunorubicin (DNR) residues after incubated with cells for 1h. The fresh evidence from the electrochemical studies based on CNTs/GCE demonstrated that the homogeneous, label-free strategy could directly measure the function of cell membrane transporters in MDR cancer cells, identify the cell phenotype (sensitive or MDR). When the different ratios of the sensitive leukemia cells K562 and its MDR ones K562/A02 were applied as a model of MDR levels to simulate the MDR occurrence in cancer, the cathodic peak current showed good linear response to the fraction of MDR with a correlation coefficient of 0.995. Therefore, the MDR fraction can be easily predicted based on the calibration curve of the cathodic peak current versus the fraction of MDR. These results indicated that the sensing strategy could provide a powerful tool for assessment of MDR in cancer. The new electrochemical sensor based on carbon nanotubes-drug supramolecular nanocomposites could represent promising approach in the rapid diagnosis of MDR in cancer.     

Preparation of a manganese titanate nanosensor: Application in electrochemical studies of captopril in the presence of para-aminobenzoic acid

This study reports the synthesis and characterization of a novel nanostructure-based electrode for electrochemical studies and determination of captopril (CP). At first manganese titanate nanoceramics were synthesized by the sol–gel method. The structural evaluations of the pure nanopowders were investigated by different techniques such as X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM). Then it was used to prepare a new nanostructured manganese titanate carbon paste electrode (MnTiO₃/CPE). The characterization of the modified sensor was carried out by comprehensive techniques such as electrochemical impedance spectroscopy (EIS), SEM, and voltammetry. Subsequently, the modified electrode was used for CP catalytic oxidation in the presence of para-aminobenzoic acid (PABA) as a mediator. The results showed that PABA has high catalytic activity for CP oxidation. The electrochemical behavior of CP was studied by cyclic voltammetry (CV), linear sweep voltammetry (LSV), chronoamperometry (CHA), and differential pulse voltammetry (DPV) techniques. Under the optimized conditions, the catalytic oxidation peak current of CP showed two linear dynamic concentration ranges of 1.0 × 10⁻⁸ to 1.0 × 10⁻⁷ and 1.0 × 10⁻⁷ to 1.0 × 10⁻⁶, with a detection limit of 1.6 nM (signal/noise = 3), using the DPV technique. Finally, the proposed method was successfully applied for determination of CP in pharmaceutical and biological samples.     

Novel electrochemical catalysis as signal amplified strategy for label-free detection of neuron-specific enolase

A label-free electrochemical immunoassay for neuron-specific enolase (NSE), a kind of lung cancer marker, was developed in this work via novel electrochemical catalysis for signal amplification. The new amplified strategy was based on the electrochemical catalysis of nickel hexacyanoferrates nanoparticles (NiHCFNPs) in the presence of dopamine (DA). NiHCFNPs, which were assembled on the porous gold nanocrystals (AuNCs) modified glassy carbon electrode (GCE), could exhibit a distinct pair of redox peaks corresponding to anodic and cathodic reactions of hexacyanoferrate (II/III). Subsequently, gold nanoparticles functionalized graphene nanosheets (Au-Gra) were coated on the surface of NiHCFNPs/AuNCs film. Then an enhanced amount of neuron-specific enolase antibody (anti-NSE) could be loaded to obtain a sensitive immunosensor of anti-NSE/Au-Gra/NiHCFNPs/AuNCs/GCE due to the strong adsorption capacity and large specific surface area of Au-Gra. More importantly, the oxidation peak current can be enormously enhanced towards the electrocatalytic oxidation of DA based on NiHCFNPs, resulting in the further improvement of the immunosensor sensitivity. Under optimal conditions, the electrochemical immunosensor exhibited a linear range of 0.001-100 ng/mL with a detection limit of 0.3 pg/mL (S/N=3). Thus, the proposed immunosensor provides a rapid, simple, and sensitive immunoassay protocol for NSE detection, which may hold a promise for clinical diagnosis.     

Electrocatalytic oxidation and determination of deferasirox and deferiprone on a nickel oxyhydroxide-modified electrode

The electrocatalytic oxidation of two orally administered iron chelator drugs (deferiprone, CP20, and deferasirox, ICL670) was investigated on a nickel oxyhydroxide-modified nickel electrode in alkaline solution. The oxidation process involved and its kinetics were investigated using cyclic voltammetry, chronoamperometry, and electrochemical impedance spectroscopy techniques, as well as steady-state polarization measurements. Voltammetric studies indicated that in the presence of the drugs under study, the anodic peak current of low-valence nickel species increased, followed by a decrease in the corresponding cathodic current. This result indicates that the drugs were oxidized via oxyhydroxide species immobilized on the electrode surface via an EC' mechanism. A mechanism based on the electrochemical generation of Ni(III) active sites and their subsequent consumption by the drugs in question 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 the drugs and the electron-transfer coefficients are reported. A sensitive, simple, and time-saving amperometric procedure was developed for the analysis of deferasirox and deferiprone, with detection limits of 28 and 19 microM, respectively. The electrode was used for the direct assay of deferasirox and deferiprone tablets.     

Mechanism of control of cytochrome oxidase activity by the electrochemical-potential gradient.

Cytochrome c oxidation by bovine cytochrome oxidase embedded into liposomal vesicles with high respiratory control ratio (RCR = 6-10) has been studied by rapid-mixing experiments in the presence and absence of different ionophores. Kinetic analysis of the reaction indicates a linkage between the intrinsic activity of the enzyme, the efficiency of coupling and the electrochemical potential across the membrane. A simple model, based on two allosteric states with different catalytic properties in rapid equilibrium, is presented and successfully applied in the simulation of the observed time-course.     

The electrochemical preparation of FAD/ZnO with hemoglobin film-modified electrodes and their electroanalytical properties

Flavin adenine dinucleotide (FAD)-modified zinc oxide self-assembly films were prepared using repeated cyclic voltammetry. The electrochemical reaction of the hemoglobin with the FAD/ZnO self-assembly film-modified electrodes and their electrocatalytic properties were investigated. This paper describes the successful loading of the electrochemically active molecules of hemoglobin and FAD along with ZnO by electrochemical method. In addition to the cyclic voltammetry, an electrochemical quartz crystal microbalance was used to study the growth mechanism and the properties of the films. The FAD/zinc oxide films exhibited a single redox couple, which corresponded to the FAD redox couple. The electrocatalytic properties of the O2, H2O2, trichloroacetic acid and SO(3)2- were studied by the FAD/zinc oxide films in the absence or in the presence of hemoglobin. The electrocatalytic reduction current had been developed from the cathodic peak of the FAD/zinc oxide redox couple. The electrocatalytic process involved an interaction of hemoglobin and FAD/GC film-modified electrode to increase the electrocatalytic reduction current. The electrocatalytic reduction of O2 using the FAD/zinc oxide films was investigated by cyclic voltammetry and rotating ring-disk electrode methods.     

Amperometric sensors based on Ni/Al and Co/Al layered double hydroxides modified electrode and their application for hydrogen peroxide detection

Two different hydrogen peroxide sensors were constructed with Ni/Al and Co/Al layered double hydroxides (LDHs) modified glassy carbon electrodes (GCE). Ni (Co)/Al-LDHs were synthesized by electrochemical method and were characterized by scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). The advantages and shortcoming of the two hydrogen peroxide sensors were described in detail. Compared to Co/Al-LDHs modified electrode, sensors fabricated by Ni/Al-LDHs showed quicker heterogeneous electron transfer rate constants (k(s)), lower detection and better reproducibility. But Co/Al-LDHs modified electrode held the advantages of wider linear range and higher sensitivity. Further more, the different catalytic redox mechanisms of hydrogen peroxide on the Ni/Al/GCE and Co/Al/GCE were firstly comparatively explored.     

Unusual electrochemical oxidation of cholesterol

It has been found that cholesterol undergoes direct electrochemical oxidation on platinum electrode in dichloromethane. Voltammetric measurements show that the process is controlled by the rate of electron transfer and the height of the oxidation peak is linear vs. concentration of cholesterol. Preparative electrolysis with separated cathodic and anodic compartments afforded dicholesteryl ether in a relatively high material yield. Depending on electrolysis conditions (composition of supporting electrolyte and electrolytic cell construction) various by-products with a 3beta-chloro, 3beta-acetoxy, or 3beta-acetylamino group were obtained.     

Biomimetic sensor for certain catecholamines employing copper(II) complex and silver nanoparticle modified glassy carbon paste electrode

A dimeric Cu(II) complex [Cu(μ(2)-hep)(hep-H)](2)·2ClO(4) (1) containing bidentate (hep-H=2-(2-hydroxyethyl)pyridine) ligand was synthesized and characterized by single crystal X-ray diffraction studies. Each Cu-ion in 1 is in a distorted square pyramidal geometry. Further 1 along with silver nanoparticles (SNPs) have been used as modifier in the construction of a biomimetic sensor (1-SNP-GCPE) for determining certain catecholamines viz., dopamine (DA), levodopa (l-Dopa), epinephrine (EP) and norepinephrine (NE) using cyclic voltammetry, chronocoulometry, electrochemical impedance spectroscopy and adsorptive stripping square wave voltammetry (AdSSWV). Finally, the catalytic properties of the sensor were characterized by chronoamperometry. Employing AdSSWV, the calibration curves showed linear response ranging between 10(-6) and 10(-9)M for all the four analytes with detection limits (S/N=3) of 8.52×10(-10)M, 2.41×10(-9)M, 3.96×10(-10)M and 3.54×10(-10)M for DA, l-Dopa, EP and NE respectively. The lifetime of the biomimetic sensor was 3 months at room temperature. The prepared modified electrode shows several advantages such as simple preparation method, high sensitivity, high stability, ease of preparation and regeneration of the electrode surface by simple polishing along with excellent reproducibility. The method has been applied for the selective and precise analysis of DA, l-Dopa, EP and NE in pharmaceutical formulations, urine and blood serum samples.     

The kinetics and mechanisms of the reactions of aluminium(III) with gallic acid, gallic acid methyl ester and adrenaline

The kinetics and mechanisms of the reactions of gallic acid, gallic acid methyl ester and adrenaline with aluminium(III) have been investigated in aqueous solution at 25 degrees C and an ionic strength of 0.5 M. A mechanism has been proposed which accounts satisfactorily for the kinetic data. This is consistent with a mechanism in which complex formation takes place almost exclusively by reaction of [Al(H2O)5OH]2+ with the ligands. [Al(H2O)5OH]2+ reacts with gallic acid, gallic acid methyl ester and adrenaline with rate constants of 1145, 1330 and 316 M(-1) s(-1) respectively. These data together with the equilibrium data enable the rate constants for reaction of [Al(H2O)6]3+ with both gallic acid and gallic acid methyl ester to be calculated. In view of the dissociative nature of water exchange on [Al(H2O)6]3+ and [Al(H2O)5(OH)]2+ the complex formation rate constants are discussed in terms of the Eigen-Wilkins-Tamm mechanism. The overall mechanisms have been validated using global analysis. The results are compared with previously published data on the complex formation reactions of aluminium(III). In addition, the rate constants and mechanisms for replacement of maltol by gallic acid methyl ester and diethylenetriaminepentaacetic acid (dtpa) have been investigated.     

Electrochemical approach for acute myocardial infarction diagnosis based on direct antibodies-free analysis of human blood plasma

A novel direct antibodies-free electrochemical approach for acute myocardial infarction (AMI) diagnosis has been developed. For this purpose, a combination of the electrochemical assay of plasma samples with chemometrics was proposed. Screen printed carbon electrodes modified with didodecyldimethylammonium bromide were used for plasma charactrerization by cyclic (CV) and square wave voltammetry and square wave (SWV) voltammetry. It was shown that the cathodic peak in voltammograms at about -250 mV vs. Ag/AgCl can be associated with AMI. In parallel tests, cardiac myoglobin and troponin I, the AMI biomarkers, were determined in each sample by RAMP immunoassay. The applicability of the electrochemical testing for AMI diagnostics was confirmed by statistical methods: generalized linear model (GLM), linear discriminant analysis (LDA) and quadratic discriminant analysis (QDA), artificial neural net (multi-layer perception, MLP), and support vector machine (SVM), all of which were created to obtain the "True-False" distribution prediction where "True" and "False" are, respectively, positive and negative decision about an illness event.     

Highly Sensitive Assay for Dopamine-β-Hydroxylase Activity in Human Cerebrospinal Fluid by High Performance Liquid Chromatography-Electrochemical Detection: Properties of the Enzyme

Abstract: This paper describes a new, sensitive assay for dopamine-β-hydroxylase (DBH) activity in human cerebrospinal fluid (CSF), serum and brain tissues by high performance liquid chromatography (HPLC) with electrochemical detection (ED). Dopamine (DA) was used as a substrate and was incubated under optimal conditions. Norepinephrine (NE) formed enzymatically from DA was isolated by a double-column procedure, the first column of Dowex-50-H⁺ and the second column of aluminum oxide. NE was adsorbed on the second aluminum oxide column and then eluted with 0.5 M-hydrochloric acid and assayed by HPLC-ED. Epinephrine (EN) was added to each incubation mixture as an internal standard, and this assay was therefore highly reproducible. The peak height in HPLC was linear from 500 fmol to 100 pmol of NE and EN. The lower limit of detection for NE formed enzymatically was about 30 pmol, which indicated that the sensitivity of this procedure was comparable to that of radioassay procedures. We applied the method to measurement of the activity of and examination of some of the characteristics of DBH in human CSF. DBH activity in CSF of Parkinsonian patients was lower than that of control patients. The properties of DBH in human CSF were similar to those in serum and adrenal medulla.