Square wave voltammetric determination of nitrofurantoin in pharmaceutical formulations on highly boron-doped diamond electrodes at different boron-doping contents

The influence of the boron-doping levels in boron-doped diamond film electrodes on the electrochemical response of nitrofurantoin (NFT) and the development of an electroanalytical procedure for NFT determination were investigated. The investigations were carried out using the techniques of cyclic voltammetry and square wave voltammetry on diamond film electrodes with different boron-doping levels (i.e., 5000, 10,000 and 20,000 mg L⁻¹). The level of boron-doping in the diamond film electrodes influenced the electrochemical reduction of NFT. The appropriate cyclic voltammetric response of NFT was obtained with Britton-Robinson buffer at pH 4 and for diamond films doped with 10,000 and 20,000 mg L⁻¹ of boron. These two films were selected for the development of the electroanalytical procedure. The use of square wave voltammetry with the optimized parameters demonstrated a good linear relationship between the peak current and the NFT concentration for a wide range of concentration. The lower limit of detection for the electrodes doped with 10,000 and 20,000 mg L⁻¹ of boron were 2.69 × 10⁻⁸ mol L⁻¹ (6.40 μg L⁻¹) and 8.15 × 10⁻⁹ mol L⁻¹ (1.94 μg L⁻¹), respectively, while the lower limits of quantification were 8.96 × 10⁻⁸ mol L⁻¹ (21.33 μg L⁻¹) and 2.72 × 10⁻⁸ mol L⁻¹ (6.47 μg L⁻¹), respectively. The applicability of the proposed procedure was tested using a commercial pharmaceutical formulation of NFT, and the results were compared with the procedure recommended by the British Pharmacopeia. The proposed procedure was sensitive, accurate and precise for analysis of NFT and did not require complex preparations or renovations of the electrode surface. This presents the advantage of eliminating mercury waste and minimizing the adsorptive problems related to the use of other electrodic solid surfaces.     

Simultaneous voltammetric determination of paracetamol and caffeine in pharmaceutical formulations using a boron-doped diamond electrode

A simple and highly selective electrochemical method was developed for the single or simultaneous determination of paracetamol (N-acetyl-p-aminophenol, acetaminophen) and caffeine (3,7-dihydro-1,3,7-trimethyl-1H-purine-2,6-dione) in aqueous media (acetate buffer, pH 4.5) on a boron-doped diamond (BDD) electrode using square wave voltammetry (SWV) or differential pulse voltammetry (DPV). Using DPV with the cathodically pre-treated BDD electrode, a separation of about 550 mV between the peak oxidation potentials of paracetamol and caffeine present in binary mixtures was obtained. The calibration curves for the simultaneous determination of paracetamol and caffeine showed an excellent linear response, ranging from 5.0 × 10⁻⁷ mol L⁻¹ to 8.3 × 10⁻⁵ mol L⁻¹ for both compounds. The detection limits for the simultaneous determination of paracetamol and caffeine were 4.9 × 10⁻⁷ mol L⁻¹ and 3.5 × 10⁻⁸ mol L⁻¹, respectively. The proposed method was successfully applied in the simultaneous determination of paracetamol and caffeine in several pharmaceutical formulations (tablets), with results similar to those obtained using a high-performance liquid chromatography method (at 95% confidence level).     

Carbon and diamond paste microelectrodes based on Mn(III) porphyrins for the determination of dopamine

Three Mn(III) porphyrins were used for the design of carbon paste and diamond paste based microelectrodes, which were employed for the determination of dopamine in pharmaceutical and biological samples using differential pulse voltammetry (DPV). The limits of detection lie between 1.6 × 10⁻¹³ and 2.0 × 10⁻⁶ mol L⁻¹ while the sensitivities were between 230 pA μmol L⁻¹ and 3.24 μA mol L⁻¹. Dopamine was recovered reliable from pharmaceutical and biological samples in percentages higher than 91.00% and 92.00%, respectively. The surface of the microelectrodes can easily be renewed by simple polishing, obtaining a fresh surface ready for use in a new assay.     

Electrochemical behavior of dopamine at a 3,3′-dithiodipropionic acid self-assembled monolayers

Monolayers of 3,3′-dithiodipropionic acid (DTDPA) were prepared on a polycrystalline gold electrode through a self-assembly procedure to produce a gold 3,3′-dithiodipropionic acid self-assembled monolayer (AuDTDPA) modified electrode. The characterization of the AuDTDPA electrode was investigated by cyclic voltammetry and ac impedance using the [Fe(CN)₆]^3−/4− redox couple. The electrochemical behavior of DA on the modified electrode AuDTDPA was studied by cyclic and square-wave voltammetries, using phosphate buffer as supporting electrolyte. The oxidation peak current for DA increases linearly with concentration in the range of 0.35 × 10⁻⁵ to 3.4 × 10⁻⁵ mol L⁻¹. The performance of the AuDTDPA modified electrode was evaluated for the electroanalytical determination of dopamine (DA) in a pharmaceutical formulation. The AuDTDPA modified electrode showed a stable behavior and the presence of surface-COOH groups avoided the passivation of the electrode surface during the dopamine oxidation.     

Preliminary evaluation of monolithic column high-performance liquid chromatography with tris(2,2′-bipyridyl)ruthenium(II) chemiluminescence detection for the determination of quetiapine in human body fluids

High-performance liquid chromatography (HPLC) with tris(2,2′-bipyridyl)ruthenium(II) chemiluminescence detection methodology is reported for the determination of the atypical antipsychotic drug quetiapine and the observation of its major active and inactive metabolites in human urine and serum. The method uses a monolithic chromatographic column allowing high flow rates of 3 mL min⁻¹ enabling rapid quantification. Flow injection analysis (FIA) with tris(2,2′-bipyridyl)ruthenium(II) chemiluminescence detection and HPLC time of flight mass spectrometry (TOF-MS) were used for the determination of quetiapine in a pharmaceutical preparation to establish its suitability as a calibration standard. The limit of detection achieved with FIA was 2 × 10⁻¹¹ mol L⁻¹ in simple aqueous solution. The limits of detection achieved with HPLC were 7 × 10⁻⁸ and 2 × 10⁻¹⁰ mol L⁻¹ in urine and serum, respectively. The calibration range for FIA was between 5 × 10⁻⁹ and 1 × 10⁻⁶ mol L⁻¹. The calibration ranges for HPLC were between 1 × 10⁻⁷-1 × 10⁻⁴ and 1 × 10⁻⁸-1 × 10⁻⁴ mol L⁻¹ in urine and serum, respectively. The quetiapine concentrations in patient samples were found to be 3 × 10⁻⁶ mol L⁻¹ in urine and 7 × 10⁻⁷ mol L⁻¹ in serum. Without the need for preconcentration, the HPLC detection limits compared favourably with those in previously published methodologies. The metabolites were identified using HPLC-TOF-MS.     

Amorphous carbon nitride as an alternative electrode material in electroanalysis: Simultaneous determination of dopamine and ascorbic acid

Boron-doped diamond (BDD) films are excellent electrode materials, whose electrochemical activity for some analytes can be tuned by controlling their surface termination, most commonly either to predominantly hydrogen or oxygen. This tuning can be accomplished by e.g. suitable cathodic or anodic electrochemical pretreatments. Recently, it has been shown that amorphous carbon nitride (a-CN_x) films may present electrochemical characteristics similar to those of BDD, including the influence of surface termination on their electrochemical activity toward some analytes. In this work, we report for the first time a complete electroanalytical method using an a-CN_x electrode. Thus, an a-CN_x film deposited on a stainless steel foil by DC magnetron sputtering is proposed as an alternative electrode for the simultaneous determination of dopamine (DA) and ascorbic acid (AA) in synthetic biological samples by square-wave voltammetry. The obtained results are compared with those attained using a BDD electrode. For both electrodes, a same anodic pretreatment in 0.1 mol L⁻¹ KOH was necessary to attain an adequate and equivalent separation of the DA and AA oxidation potential peaks of about 330 mV. The detection limits obtained for the simultaneous determination of these analytes using the a-CN_x electrode were 0.0656 μmol L⁻¹ for DA and 1.05 μmol L⁻¹ for AA, whereas with the BDD electrode these values were 0.283 μmol L⁻¹ and 0.968 μmol L⁻¹, respectively. Furthermore, the results obtained in the analysis of the analytes in synthetic biological samples were satisfactory, attesting the potential application of the a-CN_x electrode in electroanalysis.     

An optical redox chemical sensor for determination of iodide

A novel optical sensor based on a redox reaction for the determination of iodide has been developed. The optode membrane is constructed by immobilization of methyltrioctylammonium chloride on triacetylcellulose polymer. The exchange of chloride as counter ion with iodate in the membrane changes the color to yellow, when it is placed in acidic solution of iodide. The sensor can readily be regenerated by 0.1 mol L⁻¹ NaOH in less than 15 s. The optode has a linear range of 3.94 × 10⁻⁶ to 5.51 × 10⁻⁵ mol L⁻¹ of iodide ions with a limit of detection 7.44 × 10⁻⁷ mol L⁻¹. The relative standard deviation for eight replicate measurements of 3.94 × 10⁻⁶ and 1.57 × 10⁻⁵ mol L⁻¹ of iodide was 2.83 and 1.38%, respectively. The sensor was successfully applied to the determination of iodide in tablet, powdered milk and urine samples.     

Rapid simultaneous analysis of 1-hydroxypyrene, 2-hydroxyfluorene, 9-hydroxyphenanthrene, 1- and 2-naphthol in urine by first derivative synchronous fluorescence spectrometry using Tween-20 as a sensitizer

A novel method of first derivative synchronous fluorescence was developed for the rapid simultaneous analysis of trace 1-hydroxypyrene (1-OHP), 1-naphthol (1-NAP), 2-naphthol (2-NAP), 9-hydroxyphenanthrene (9-OHPe) and 2-hydroxyfluorene (2-OHFlu) in human urine. Only one single scan was needed for quantitative determination of five compounds simultaneously when Δλ = 10 nm was chosen. In the optimal experimental conditions, there was a linear relationship between the fluorescence intensity and the concentration of 1-OHP, 1-NAP, 2-NAP, 9-OHPe and 2-OHFlu in the range of 1.75 × 10⁻⁹ to 4.50 × 10⁻⁶ mol L⁻¹, 3.64 × 10⁻⁸ to 2.20 × 10⁻⁴ mol L⁻¹, 8.18 × 10⁻⁹ to 1.20 × 10⁻⁴ mol L⁻¹, 3.26 × 10⁻⁹ to 8.50 × 10⁻⁵ mol L⁻¹ and 4.88 × 10⁻⁹ to 5.50 × 10⁻⁶ mol L⁻¹, respectively. The limits of detection (LOD) were found to be 5.25 × 10⁻¹⁰ mol L⁻¹ for 1-OHP, 1.10 × 10⁻⁸ mol L⁻¹ for 1-NAP, 2.46 × 10⁻⁹ mol L⁻¹ for 2-NAP, 9.77 × 10⁻¹⁰ mol L⁻¹ for 9-OHPe and 1.46 × 10⁻⁹ mol L⁻¹ for 2-OHFlu. The proposed method is reliable, selective and sensitive, and has been used successfully in the determination of traces of 1-OHP, 1-NAP, 2-NAP, 9-OHPe and 2-OHFlu in human urine samples, whose results were in good agreement with those gained by the HPLC method.     

Colloidal-gold cysteamine-modified carbon paste electrodes as suitable electrode materials for the electrochemical determination of sulphur-containing compounds Application to the determination of methionine

The suitability of colloidal-gold cysteamine-modified carbon paste electrodes (nAu-Cyst-CPE) for the electrochemical determination of sulphur-containing compounds is illustrated in this work by determining the amino acid methionine in real samples, as well as a methionine-based peptide. Voltammograms from methionine solutions at nAu-Cyst-CPE exhibited improved electroanalytical characteristics when compared with colloidal-gold cysteamine-modified Au disk electrodes (nAu-Cyst-AuE). Differential pulse voltammetry in phosphate buffer of pH 7.0 was used for the determination of methionine, with a range of linearity of (1.0-100) × 10⁻⁶ mol l⁻¹ and a detection limit of 5.9 × 10⁻⁷ mol l⁻¹. This detection limit is remarkably lower than those reported previously using other modified electrodes or amperometric detection. Methionine peptides also exhibited anodic peaks suitable to detect this kind of molecules. Methionine was determined with good results in a pharmaceutical product containing several vitamins, amino acids and other compounds, and in spiked meat peptone, a complex sample containing enzimatically digested protein.     

Determination of epinephrine based on its enhancement for electrochemiluminescence of lucigenin

Epinephrine was found to be able to strongly enhance the electrochemiluminescence (ECL) of lucigenin system by using the anodic potential sweep. Based on which, a novel ECL method for the determination of epinephrine was developed. Under the optimum condition, the enhanced ECL intensity was linear with the epinephrine concentration in the range of 4.0 × 10⁻⁸ to 2.0 × 10⁻⁷ mol L⁻¹. The detection limit (defined as S/N = 3) was 2.4 × 10⁻⁸ mol L⁻¹, and the relative standard deviation was 2.7% for 1.0 × 10⁻⁷ mol L⁻¹ epinephrine (n = 11). The method was successfully applied to the determination of epinephrine in pharmaceutical samples with satisfactory results. In addition, the possible mechanism for the lucigenin ECL system in the presence of epinephrine has also been discussed.     

Polarographic determination of sunscreen agents in cosmetic products in micellar media

This paper introduces a simple, fast and reliable electroanalytical method for differential-pulse polarography based on electrochemical reduction at a dropping mercury electrode. The method was validated for the determination of 2-ethylhexyl-4-methoxycinnamate (EHMC) alone and in association with 4-methylbenzylidene camphor (MBC) or 2-hydroxy-4-methoxybenzophenone (BENZ-3) in samples of commercial cosmetic preparations. The supporting electrolyte that provided the best-defined and most intense peak current for EHMC determination was Britton-Robinson buffer (pH 4.0) in the presence of a cationic surfactant. Under optimized conditions, EHMC exhibited one single peak of reduction at −1.49 V versus Ag/AgCl. A limit of detection of 3.76 × 10⁻⁸ mol L⁻¹ and a limit of quantitation of 1.25 × 10⁻⁷ mol L⁻¹ were found for the pure EHMC standard. A good average recovery rate was reached for all the samples analyzed.     

Manganese detection in marine sediments: anodic vs. cathodic stripping voltammetry

Three different electroanalytical techniques for the detection of manganese in marine sediments are evaluated. The anodic stripping voltammetry of manganese at an in situ bismuth-film-modified boron-doped diamond electrode and cathodic stripping voltammetry at a carbon paste electrode are shown to lack the required sensitivity and reproducibility whereas cathodic stripping voltammetry at a bare boron-doped diamond electrode is shown to be reliable and selective with a limit of detection, from applying a 60 s accumulation period of 7.4 × 10⁻⁷ M and a sensitivity of 0.24 A M⁻¹. The method was used to evaluate the manganese content of marine sediments taken from Šibenik, Croatia.     

Simultaneous determination of N-acetyl-p-aminophenol and p-aminophenol with poly(3,4-ethylenedioxythiophene) modified glassy carbon electrode

A sensitive and selective method was developed for the determination of N-acetyl-p-aminophenol (APAP) and p-aminophenol (PAP) using poly(3,4-ethylenedioxythiophene) (PEDOT)-modified glassy carbon electrode (GCE). Cyclic voltammetry and differential pulse voltammetry were used to investigate the electrochemical reaction of APAP and PAP at the modified electrode. Both APAP and PAP showed quasireversible redox reactions with formal potentials of 367 mV and 101 mV (vs. Ag/AgCl), respectively, in phosphate buffer solution of pH 7.0. The significant peak potential difference (266 mV) between APAP and PAP enabled the simultaneous determination both species based on differential pulse voltammetry. The voltammetric responses gave linear ranges of 1.0 × 10⁻⁶-1.0 × 10⁻⁴ mol L⁻¹ and 4.0 × 10⁻⁶-3.2 × 10⁻⁴ mol L⁻¹, with detection limits of 4.0 × 10⁻⁷ mol L⁻¹ and 1.2 × 10⁻⁶ mol L⁻¹ for APAP and PAP, respectively. The method was successfully applied for the determination of APAP and PAP in pharmaceutical formulations and biological samples.     

On-site fuel electroanalysis: Determination of lead,copper and mercury in fuel bioethanol by anodic stripping voltammetry using screen-printed gold electrodes

The potential application of commercial screen-printed gold electrodes (SPGEs) for the trace determination of lead, copper, and mercury in fuel bioethanol is demonstrated. Samples were simply diluted in 0.067 mol L⁻¹ HCl solution prior to square-wave anodic stripping voltammetry (SWASV) measurements recorded with a portable potentiostat. The proposed method presented a low detection limit (<2 μg L⁻¹) for a 240 s deposition time, linear range between 5 and 300 μg L⁻¹, and adequate recovery values (96–104%) for spiked samples. This analytical method shows great promise for on-site trace metal determination in fuel bioethanol once there is no requirement for sample treatment or electrode modification.     

High selective SiO2-Al2O3 mixed-oxide modified carbon paste electrode for anodic stripping voltammetric determination of Pb(II)

The main purpose of this study is to develop an inexpensive, simple, selective and especially highly selective modified mixed-oxide carbon paste electrode (CPE) for voltammetric determination of Pb(II). For the preliminary screening purpose, the catalyst was prepared by modification of SiO₂-Al₂O₃ mixed-oxide and characterized by TG, CHN elemental analysis and FTIR spectroscopy. Using cyclic voltammetry the electroanalytical characteristics of the catalyst have been determined, and consequently the modified mixed-oxide carbon paste electrode was constructed and applied for determination of Pb(II). The electroanalytical procedure for determination of the Pb(II) comprises two steps: the chemical accumulation of the analyte under open-circuit conditions followed by the electrochemical detection of the preconcentrated species using differential pulse anodic stripping voltammetry. During the preconcentration step, Pb(II) was accumulated on the surface of the modifier by the formation of a complex with the nitrogen atoms of the pyridyl groups in the modifier. The peak currents increases linearly with Pb(II) concentration over the range of 2.0 × 10⁻⁹ to 5.2 × 10⁻⁵ mol L⁻¹ (r² = 0.9995).The detection limit (three times signal-to-noise) was found to be 1.07 × 10⁻⁹ mol L⁻¹ Pb(II). The chemical and instrumental parameters have been optimized and the effect of the interferences has been determined. The Proposed method was used for determination of lead ion in the real samples.     

Square wave adsorptive stripping voltametric determination of the mixture of nalidixic acid and its main metabolite (7-hydroxymethylnalidixic acid) by multivariate methods and artificial neural network

Nalidixic acid (NA) and its main metabolite, 7-hydroximethylnalidixic acid (OHNA), are quinolones antibacterial used as agents used for the treatment of urinary tract infection. For both compounds an adsorption process on a hanging mercury electrode (HMDE). On this basis, a square wave adsorptive stripping voltammetry (SWadSV) method has been developed for the individual and simultaneous determination of NA and OHNA. The variables that affect to accumulation process, such as concentration of perchloric acid, accumulation potential and accumulation time have been optimised by using an experimental design (concretely a Box-Behnken design with three levels) together with the response surface methodology (RSM). Calibration curves were linear in the range (0-1.38) × 10⁻⁷ mol L⁻¹ for NA and (0-3.23) × 10⁻⁸ mol L⁻¹ for OHNA, in the optimized conditions, with detection limits of 9.48 × 10⁻⁹ mol L⁻¹ and 8.06 × 10⁻¹⁰ mol L⁻¹ for NA and OHNA, respectively. The method was applied to urine samples containing only one of the analytes with satisfactory recoveries. As the voltammetric signals of these compounds show a high overlapping, different chemometric methods, such as classical least squares (CLS), partial least squares (PLS), principal component regression (PCR) and artificial neural network (ANN) have been used for the resolution of the mixture. The analysis of these compounds in urine samples were carried out using the different chemometric tools and the best recoveries were obtained by using ANN. No pre-treatment of the sample was necessary.     

Simultaneous determination of butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) in food samples using a carbon composite electrode modified with Cu3(PO4)2 immobilized in polyester resin

A simple electrochemical method was developed for the single and simultaneous determination of butylated hydroxyanisole (BHA) and butylated hydroxytoluene (BHT) in food samples using square-wave voltammetry (SWV). A carbon composite electrode modified (MCCE) with copper (II) phosphate immobilized in a polyester resin was proposed. The modified electrode allowed the detection of BHA and BHT at potentials lower than those observed at unmodified electrodes. A separation of about 430 mV between the peak oxidation potentials of BHA and BHT in binary mixtures was obtained. The calibration curves for the simultaneous determination of BHA and BHT demonstrated an excellent linear response in the range from 3.4 × 10⁻⁷ to 4.1 × 10⁻⁵ mol L⁻¹ for both compounds. The detection limits for the simultaneous determination of BHA and BHT were 7.2 × 10⁻⁸ and 9.3 × 10⁻⁸ mol L⁻¹, respectively. In addition, the stability and repeatability of the electrode were determined. The proposed method was successfully applied in the simultaneous determination of BHA and BHT in several food samples, and the results obtained were found to be similar to those obtained using the high performance liquid chromatography method with agreement at 95% confidence level.     

Flow injection determination of thyroxine in pharmaceutical preparations using tris(2,2′-bipyridyl)ruthenium(III)-NADH chemiluminescence detection

A flow injection (FI) method is reported for the determination of thyroxine based on its enhancement of chemiluminescence (CL) from the Ru(bpy)₃³⁺-NADH system. The calibration graph was linear over the range 2.0-10 × 10⁻⁸ mol L⁻¹ (r² = 0.9989) with relative standard deviations (R.S.D.) in the range 2.0-4.5% (n = 4). The limit of detection (3σ blank) was 1.0 × 10⁻⁹ mol L⁻¹ with sample throughput of 120 h⁻¹. The effect of some organic compounds, anions and cations were studied for l-thyroxine determination. The method was applied to pharmaceutical preparations and the results obtained were in reasonable agreement with the amount labeled. The method was statistically compared with the results obtained by RIA; no significant disagreement at 95% confidence limit was observed. A calibration graph of NADH over the range 1.3 × 10⁻⁸-1.3 × 10⁻⁶ mol L⁻¹ was also established (r² = 0.9992) with R.S.D. in the range1.0-3.5% (n = 4). The limit of detection (3σ) was 1.0 × 10⁻¹⁰ mol L⁻¹ NADH.     

Determination of thiodicarb using a biosensor based on alfalfa sprout peroxidase immobilized in self-assembled monolayers

A biosensor based on alfalfa sprout (Medicago sativa) homogenate as a source of peroxidase is proposed for the determination of thiodicarb by square-wave voltammetry. This enzyme was immobilized in self-assembled monolayers of l-cysteine on a gold electrode. Several parameters were investigated to evaluate the optimum conditions for operation of the biosensor. The analytical curve was linear for thiodicarb concentrations of 2.27 × 10⁻⁶ to 4.40 × 10⁻⁵ mol L⁻¹ with a detection limit of 5.75 × 10⁻⁷ mol L⁻¹. The lifetime of the Au-alfalfa sprout-SAMs was 20 days (at least 220 determinations). The average recovery of thiodicarb from samples of vegetable extracts ranged from 99.02 to 101.04%. The results obtained for thiodicarb in vegetable extracts using the proposed method are in close agreement with those using a high performance liquid chromatography procedure at the 95% confidence level.     

Ultrasensitive cysteine sensing using citrate-capped CdS quantum dots

The importance of cysteine (Cys) in biological systems has stimulated a great deal of efforts in the development of analytical methods for the determination of this amino acid. In this work, a novel fluorescent probe for Cys based on citrate (Cit)-capped CdS quantum dots (QDs) is reported. The Cit-capped CdS QDs fluorescent probe offers good sensitivity and selectivity for detecting Cys. A good linear relationship was obtained from 1.0 × 10⁻⁸ mol L⁻¹ to 5.0 × 10⁻⁵ mol L⁻¹ for Cys. The detection limit was calculated as 5.4 × 10⁻⁹ mol L⁻¹. The proposed method was applied to detect Cys in human urine samples, which showed satisfactory results. This assay is based on both the lability of Cit and the strong affinity of thiols to the surface of CdS QDs. The addition of Cys improved the passivation of the surface traps of CdS QDs and enhanced the fluorescence intensity.