Electrochemical synthesis and characterization of a novel thiazole-based copolymer and its application in biosensor

A novel copolymer based on 2-aminothiazole (AT) and 2-amino-4-thiazoleacetic acid (ATA) is electrochemically synthesized and then characterized using UV–visible absorption spectroscopy (UV–vis), Fourier transform infrared spectroscopy (FT-IR), scanning electron microscopy (SEM) and electrochemical techniques. The results confirmed that the obtained polymer was a copolymer rather than a blend or a composite of the respective homopolymers and could improve the electrochemical response of both positively and negatively charged analytes. Therefore, the copolythiazole film was applied to determine ascorbic acid (AA), dopamine (DA) and uric acid (UA). It is found that the peak separating degrees and peak currents of AA, DA and UA at copolythiazole modified electrode were much better than those at bare GCE. Under the optimum conditions, the linear calibration curves were obtained in the range of 10–2000 μM for AA, 1–150 μM for DA, 1–180 μM for UA. The detection limits of AA, DA and UA were 2, 0.04 and 0.4 μM, respectively (S/N = 3). The practical application of the modified electrode was demonstrated by the determination of UA in urine sample.     

Selective electrochemical determination of dopamine in the presence of ascorbic acid using sodium dodecyl sulfate micelles as masking agent

From differential pulse voltammetry, DPV, it was determined the electrochemical behaviour of dopamine, DA, and ascorbic acid, AA, using a carbon paste electrode, CPE, with the surfactant sodium dodecyl sulfate, SDS, present at concentrations around the critical micellar concentration, CMC. It is shown that without SDS in solution, the oxidation peak potential of AA, E_AA, and DA, E_DA, completely overlap, however when [SDS] > CMC separation is possible between them (E_AA ? E_DA = 238 mV). Moreover, at variance with other proposed systems, the DA potential peak becomes less positive than AA potential peak. Therefore, separation of signals allows selective determination of DA even when AA is present in the same solution. Optimization of the experimental conditions permitted to obtain a calibration plot for the DA in the presence of the AA, having a 0.01–0.2 mM linear range, a detection limit of 5 μM and a 0.04812 μA μM^?1 sensitivity. This new approach was used effectively to determine dopamine in real pharmaceuticals obtaining satisfactory results.     

Simultaneous determination of ascorbic acid, dopamine and uric acid by use of a MWCNT modified carbon-ceramic electrode and differential pulse voltammetry

Multi walled carbon nanotube modified carbon-ceramic electrode (MWCNT/CCE) was employed for the simultaneous determination of ascorbic acid (AA), dopamine (DA) and uric acid (UA). The MWCNT/CCE displayed very good electrochemical catalytic activities with respect to CCE. The oxidation over-potentials of AA, DA and UA decreased dramatically, and their oxidation peak currents increased significantly at MWCNT/CCE compared to those obtained at the bare CCE. Differential pulse voltammetry was used for the simultaneous determination of AA, DA and UA in their ternary mixture. The peak separation between AA and DA, and DA and UA was large up to 205 and 160 mV, respectively. The calibration curves for AA, DA and UA were obtained in the range of 15.00-800.00, 0.50-100.00, and 0.55-90.00 μM, respectively. The detection limits (S/N = 3) were 7.71, 0.31, and 0.42 μM for AA, DA and UA, respectively.The present method was applied to the determination of AA, DA and UA in human serum and some commercial pharmaceutical samples, using standard adding method and the results were quite promising.     

A nickel hexacyanoferrate and poly(1-naphthol) hybrid film modified electrode used in the selective electroanalysis of dopamine

A mixed-valent nickel hexacyanoferrate and poly(1-naphthol) hybrid (NiHCF–PNH) film was prepared on a gold (Au) electrode by a galvanostatic method, which led to stable and homogeneous hybrid film. The film was characterized using scanning electronic microscopy, cyclic voltammetry, and electrochemical impedance spectroscopy. This electrode showed excellent catalytic properties toward dopamine (DA) detection, using cyclic voltammetry and differential pulse voltammetry methods. The electrocatalytic oxidations of DA at different electrodes, such as a bare Au electrode or a poly(1-naphthol)/Au-, or NiHCF–PNH/Au-modified electrode, were investigated in a phosphate buffer solution (pH 7). Interestingly, the NiHCF–PNH-modified electrode facilitated the oxidation of DA, but it did not responded to other electroactive biomolecules, such as ascorbic acid and uric acid_. The DA electrochemical sensor exhibited a linear response from 0.1 to 4.3 μM (R² = 0.9984) and from 4.3 to 9.6 μM (R² = 0.9969), with a detection limit of 2.1 × 10^?8 M, and a short response time (3 s) for DA determination. In addition, the NiHCF–PNH-modified electrode exhibited distinct advantages by its simple preparation, specificity, and stability.     

Controllable synthesis of nitrogen-doped graphene and its effect on the simultaneous electrochemical determination of ascorbic acid,dopamine, and uric acid

A simple, low-cost method for fabricating nitrogen-doped graphene (NG) is demonstrated by combining the ultrafast thermal exfoliation and covalent transformation from the melamine (MA)–graphene oxide (GO) mixture. NGs prepared at 300, 600, and 900 °C were systematically characterized by X-ray photoelectron spectroscopy (XPS), in which pyridinic-N, pyrrolic-N and graphitic-N are the main nitrogen-doped structures in various ratios. These NGs possess large specific surface area and porous microstructures, confirmed by the N₂ adsorption–desorption isotherms. The NG-modified screen-printed carbon electrodes (SPCEs) were fabricated to detect ascorbic acid (AA), dopamine (DA) and uric acid (UA) simultaneously by cyclic voltammetry (CV) and linear sweep voltammetry (LSV). Due to the large specific surface area, mesoporous structures and nitrogen-doped sites, these NGs show highly electrochemical sensitivity for AA, DA and UA. Notably, the pyrrolic-N structure makes the negative shift in the oxidation peak potential of these biomolecules, showing the better catalytic activity than pyridinic-N and graphitic-N structures. The large surface area of NGs provides more nitrogen-doped sites to oxidize bio-compounds and enhances the corresponding currents. The good sensitivity of NG-modified SPCEs makes them become effective sensors for determining AA, DA and UA simultaneously. The discrimination to peak potential and current among these NGs can be observed.     

Electrochemical characterization of poly(eriochrome black T) modified glassy carbon electrode and its application to simultaneous determination of dopamine, ascorbic acid and uric acid

A polymerized film of eriochrome black T (EBT) was prepared on the surface of a glassy carbon (GC) electrode in alkaline solution by cyclic voltammetry (CV). The redox response of the poly(EBT) film at the GC electrode appeared in a couple of redox peak in 0.1 M hydrochloride and the pH dependent peak potential was −55.1 mV/pH which was close to the Nernst behavior. The poly(EBT) film-coated GC electrode exhibited excellent electrocatalytic activity towards the oxidations of dopamine (DA), ascorbic acid (AA) and uric acid (UA) in 0.05 mM phosphate buffer solution (pH 4.0) and lowered the overpotential for oxidation of DA. The polymer film modified GC electrode conspicuously enhanced the redox currents of DA, AA and UA, and could sensitively and separately determine DA at its low concentration (0.1 μM) in the presence of 4000 and 700 times higher concentrations of AA and UA, respectively. The separations of anodic peak potentials of DA-AA and UA-DA reached 210 mV and 170 mV, respectively, by cyclic voltammetry. Using differential pulse voltammetry, the calibration curves for DA, AA and UA were obtained over the range of 0.1-200 μM, 0.15-1 mM and 10-130 μM, respectively. With good selectivity and sensitivity, the present method provides a simple method for selective detection of DA, AA and UA in biological samples.     

The effect of surface treatment on oxidation of oxalic acid at nanocrystalline diamond films

The surface investigation of undoped and boron doped nanocrystalline diamond (NCD/BDND) films associated to their electrochemical behavior of oxalic acid after four pre-treatments was studied. The films were produced using Hot Filament CVD technique on Si substrate with a gas mixture of CH₄/H₂/Ar. Surface pre-treatments were carried out to analyze the surface chemical changes induced by hydrogen and oxygen plasma and as well as cathodic and anodic treatments performed in 0.1 mol L^? 1 HClO₄. The films wetting analyzed by contact angle presented a strong dependence of their surface before and after each treatment was also confirmed by the electrochemical response from cyclic voltammograms. Independent of the surface pre-treatments, all the electrodes exhibited response for oxalic acid oxidation, but the electrode submitted to hydrogen plasma presented the lowest starting oxidation potential and the highest current density. Nonetheless, the BDND electrode presented higher oxidation current than that for NCD electrodes, after all pre-treatments studied. The use of square wave voltammetry with BDND electrode treated by hydrogen plasma for the analytical determination of oxalic acid is described. The detection limits of 0.75 μmol was obtained from the linear relationship between the peak currents of voltammograms as a function of the oxalic acid concentrations.     

Electro-oxidation and determination of gemcitabine hydrochloride,an anticancer drug at gold electrode

Electrochemical behavior of an anticancer drug, gemcitabine hydrochloride (GMB) was studied in 10.4 pH with 0.2 M phosphate buffer solution as supporting electrolyte at 25 ± 0.1 °C at gold electrode (GE) using different voltammetric techniques. The electrochemical process was observed to be diffusion controlled, irreversible and involving one-electron oxidation. Under optimal conditions, the peak current was proportional to GMB concentration in the range of 0.1–15 μM with a detection limit of 0.06 μM by differential pulse voltammetry. The method was developed for the determination of GMB in pharmaceutical formulations and urine as a real sample.     

Simultaneous detection of ascorbic acid, uric acid and homovanillic acid at copper modified electrode

The copper was deposited on glassy carbon (GC) and indium tin oxide (ITO) electrodes by electrochemical method. The copper structures on electrode were characterized by atomic force microscope, X-ray diffractometeric pattern and differential pulse voltammetric studies. Optimal conditions for uniform growth of copper structures on the electrode were established. Voltammetric sensor was fabricated using the copper deposited GC electrode for the simultaneous detection and determination of uric acid (UA) and homovanillic acid (HVA) in the presence of excess concentrations of ascorbic acid (AA). The voltammetric signals due to AA and UA oxidation were well separated with a potential difference of 400 mV and AA did not interfere with the measurement of UA and HVA at the GC/Cu electrode. Linear calibration curves were obtained in the concentration range 1-40 μM for AA and 20-50 μM for UA at physiological pH and a detection limit of 10 nM of UA in the presence of 10-fold excess concentrations of AA was achieved. The simultaneous detection of submicromolar concentrations of AA, UA and HVA was achieved at the GC/Cu electrode. The practical utility of the present GC/Cu modified electrode was demonstrated by measuring the AA content in Vitamin C tablet, UA content in human urine and blood serum samples with satisfactory results.     

Boron-doped diamond electrochemical sensor for sensitive determination of nicotine in tobacco products and anti-smoking pharmaceuticals

A sensitive, selective and reliable electrochemical method for the determination of nicotine using differential pulse voltammetry on a bare boron-doped diamond electrode has been developed. Nicotine yielded a single oxidation peak at a highly positive potential of + 1.45 V (vs. Ag/AgCl/3 M KCl) in Britton–Robinson buffer solution at pH 8. The influence of supporting electrolyte, pH and scan rate on the current response of NIC was investigated. At optimized experimental conditions, a linear relation between peak current and concentration of nicotine was found for the range from 0.5 to 200 μM (0.08–32.9 mg L^− 1) with a detection limit of 0.3 μM (0.05 mg L^− 1) and a good repeatability (relative standard deviation of 2.1% at 25 μM concentration level, n = 10) was achieved without any electrode surface modification. The practical usefulness of the developed procedure was successfully demonstrated with the determination of nicotine in tobacco products and anti-smoking pharmaceuticals with results similar to those obtained by a high-performance liquid chromatography and to the contents declared by the manufacturer, respectively. Prior to analysis, the sample pretreatment includes only sonication and/or simple liquid–liquid extraction. The proposed sensor represents an effective electrochemical tool and a promising alternative for quality control analysis of products in tobacco and pharmaceutical industry.     

Simultaneous determination of dopamine and uric acid on nafion/sodium dodecylbenzenesulfonate composite film modified glassy carbon electrode

A novel electrochemical sensor has been constructed by using a glassy carbon electrode (GCE) coated with nafion/sodium dodecylbenzenesulfonate (SDBS). Differential pulse voltammetry (DPV) was used to study the electrochemical behaviors of dopamine (DA) and uric acid (UA). An optimum of 5 mM SDBS together with 0.05 wt% of nafion was used to improve the resolution and the determination sensitivity successfully. In 0.1 M phosphate buffer solution (pH 6.5), the modified electrode exhibited high electrocatalytical activity toward the oxidation of DA and UA with obvious reduction of overpotential. Compared with bare GCE, the modified electrode resolved the voltammetric response of DA and UA into two well-defined voltammetric peaks by DPV, which can be used for simultaneous determination of these species in mixture. The peak currents obtained from DPV were linearly related to the concentrations of DA and UA in the ranges of 4.0 × 10⁻⁷–8.0 × 10⁻⁵ M and 4.0 × 10⁻⁶–8.0 × 10⁻⁴ M, respectively. The detection limit of DA and UA (signal-to-noise ration was 3) were 5.0 × 10⁻⁸ and 4.0 × 10⁻⁷ M, respectively.     

PEDOT/Palladium composite material: synthesis, characterization and application to simultaneous determination of dopamine and uric acid

Palladium (Pd) incorporated poly (3,4-ethylenedioxythiophene) (PEDOT) films were synthesized through an electrochemical route and characterized using scanning electron microscopy (SEM) and atomic force microscopy (AFM). The electrochemical study showed catalytic oxidation of dopamine (DA) with optimum loading of Pd. DA and uric acid (UA) were detected using differential pulse voltammetry (DPV). In the presence of ascorbic acid (AA), DA-AA showed peak potential separation of 0.19 V while 0.32 V between UA-AA on Pd-incorporated PEDOT. These peak separations are large enough for sensing DA and UA in the presence of AA. DA and UA exhibited linear calibration plots and the minimum detection limits are 0.5 and 7 μM respectively. On Pd-PEDOT, the reversibility of DA oxidation was found to increase compared to bare glassy carbon electrode (GCE) and PEDOT modified GCE. Fouling effects were also found to be minimal making Pd-PEDOT composite suitable for electroanalysis.     

Electrochemical synthesis of a novel poly(2,5-dimethoxy aniline) nanorod for ultrasensitive glucose biosensor application

We report for the first time a rapid electrochemical synthesis of one-dimensional poly(2,5-dimethoxyaniline) nanorods (PDMA-NR) in the presence of surfactant. FE-SEM and TEM images confirm the PDMA-NR formation and the average diameter of single rod sizes in the range of ∼200–300 nm. An enzymatic glucose biosensor was fabricated through immobilizing glucose oxidase (GOx) into PDMA-NR matrix. The amperometric current response of PDMA-NR/GOx to glucose is linear in the concentration range between 1 and 10 μM with a detection limit of 0.5 μM (S/N = 3). The PDMA-NR/GOx electrode possesses high sensitivity (5.03 μA/μM), selectivity, stability, and reproducibility toward glucose.     

Development of an amperometric sensor for simultaneous determination of uric acid and ascorbic acid using 2-[bis(2-aminoethyl)amino]ethanol, 4,4′-bipyridine bridged dicopper(II) complex

Binuclear copper complex (2-[bis(2-aminoethyl)amino]ethanol, 4,4′-bipyridine bridged dicopper(II) complex) was grafted onto the surface of a glassy carbon electrode (GCE) using the cyclic voltammetric method in a phosphate buffer solution (PBS). The modified electrode resulted in efficient electrocatalytic activity for anodic oxidation of uric acid (UA) and ascorbic acid (AA) via a substantial decrease in anodic over-potentials for both compounds. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) using this modified electrode result in two well-resolved anodic waves for the oxidation of UA and AA in mixed solution, making possible the simultaneous determination of both compounds. Linear analytical curves were obtained in the ranges 5.0–300.0 μM and 5.0–160.0 μM for UA and AA concentrations through DPV methods, respectively. The detection limits were 2.0 μM of UA and AA. This electrode was used for UA and AA determinations in urine samples with satisfactory results.     

Graphene oxide nanoribbon and polyhedral oligomeric silsesquioxane assembled composite frameworks for pre-concentrating and electrochemical sensing of 1-hydroxypyrene

1-Hydroxypyrene (1-OHP) is a ubiquitous metabolite of polycyclic aromatic hydrocarbons (PAHs) and has been widely used as a biomarker for evaluation of human exposure to PAHs. In this paper, an electrochemical sensor consisted of graphene oxide nanoribbon (GON)/octa(3-aminopropyl)octasilae quioxane octahydrochloride (OA-POSS) framework films (GON/OA-POSS) modified electrode for pre-concentrating and sensing of 1-OHP was described. The GON/OA-POSS composite films were prepared by hierarchically layer-by-layer assembly via electrostatic interactions. GON, which has higher electrical conductivity than that of graphene oxide, offers residual π-conjugated systems for adsorption of 1-OHP via π–π stacking interaction. Meanwhile, OA-POSS nanocages make the GON layers frameworked to enhance contacting between GON and 1-OHP. When 1-OHP was pre-concentrated on GON/OA-POSS frameworks modified electrode, the response signal and adsorption stability was enhanced significantly. Differential pulse voltammetry was used for direct determination of 1-OHP. The electrochemical sensor calibration curve had a range of 0.1–12.55 μM (R = 0.995) with a detection limit of 0.04 μM. The results showed that the GON/OA-POSS framework films modified electrode established a new way for simple and sensitive analysis of 1-OHP.     

Simultaneous voltammetric determination of ascorbic acid, dopamine and uric acid by methylene blue adsorbed on a phosphorylated zirconia-silica composite electrode

This paper describes the preparation, characterization and application of a composite electrode based on methylene blue adsorption to phosphorylated zirconia-silica mixed oxide particles prepared by a sol-gel process. This electrode electrocatalytically oxidizes ascorbic acid (AA), dopamine (DA) and uric acid (UA), allowing their simultaneous voltammetric detection. Well-defined and -separated oxidation peaks were observed by differential pulse voltammetry in a 0.35 mol l⁻¹ Tris-HCl buffer solution (pH 7.4) containing 0.5 mol l⁻¹ KCl. The anodic peak currents observed at −74, 94 and 181 mV increased with increasing concentrations of AA, DA and UA, respectively. Linear calibration plots were obtained over the range of 100-1600 μmol l⁻¹ for ascorbic acid, 6-100 μmol l⁻¹ for dopamine and 22-350 μmol l⁻¹ for uric acid with detection limits of 8.3 ± 0.1, 1.7 ± 0.1 and 3.7 ± 0.2 μmol l⁻¹, respectively. DA and UA concentrations could also be determined under conditions of excess AA (1 mmol l⁻¹).     

Electroanalysis of ascorbic acid (vitamin C) using nano-ZnO/poly(luminol) hybrid film modified electrode

Electrochemical analysis of ascorbic acid (AsA) in physiological condition using a new hybrid film modified electrode is described. Electrochemical polymerization of luminol in 0.1 M H₂SO₄ solution was carried out using ZnO nanoparticles (ZnO-NPs) coated glassy carbon electrode (GCE) as working electrode. This hybrid film coated electrode noted as poly(luminol)/ZnO-NPs hybrid film modified GCE (PLu/ZnO-NPs/GCE). The atomic force microscope (AFM) and scanning electron microscope (SEM) studies were demonstrated that PLu/ZnO-NPs hybrid film covered the electrode surface and the ZnO-NPs particle sizes were <100 nm. The visible blue colored organic–inorganic (PLu/ZnO-NPs) hybrid films were observed on the electrode surface. Electrochemical studies proved that PLu/ZnO-NPs hybrid film modified electrode is electroactive in the pH range from 1 to 11 and the poly(luminol) (PLu) redox peak was pH dependent with a slope of ?53 mV/pH. The PLu/ZnO-NPs modified electrodes electroactivity also investigated by catalyzing the oxidation of AsA, demonstrating its great potential applications in electroanalysis of AsA. The resulting, AsA electrochemical sensor exhibited a wide linear response range (from 1 × 10^?6 to 3.6 × 10^?4 M, r² = 0.9989), lower detection limit (1 × 10^?6 M) and fast response time (3 s) for AsA determination. Our results show that PLu/ZnO-NPs hybrid film provides a novel and efficient platform for the oxidation of AsA and realizing efficient electrocatalysis and that the materials have potential applications in the fabrication of electrochemical sensors. Analysis of commercial vitamin C samples using PLu/ZnO-NPs hybrid film modified electrode was demonstrated and the obtained results are good agreement with the labeled amount.     

Voltammetric studies of a novel bicopper complex modified glassy carbon electrode for the simultaneous determination of dopamine and ascorbic acid

A novel modified glassy carbon electrode (GCE) with a binuclear copper complex was fabricated using a cyclic voltammetric method in phosphate buffer solution. This modified electrode shows very efficient electrocatalytic activity for anodic oxidation of both dopamine (DA) and ascorbic acid (AA) via substantial decrease in anodic overpotentials for both compounds. Cyclic voltammetry (CV) and differential pulse voltammetry (DPV) using this modified electrode show two well-resolved anodic waves for the oxidation of DA and AA in mixed solution, which makes it possible for simultaneous determination of both compounds. Linear analytical curves were obtained in the ranges 2.0–120.0 μM and 5.0–160.0 μM for DA and AA concentrations by using DPV methods, respectively. The detection limits were 1.4 × 10⁻⁶ M of DA and 2.8 × 10⁻⁶ M of AA. This electrode was used for AA and DA determinations in medicine and foodstuff samples with satisfactory results.     

Selective electrochemical behavior of highly conductive boron-doped diamond electrodes for fluvastatin sodium oxidation

Boron-doped diamond electrodes have received much attention for electrochemical determination due to their attractive electrochemical properties over other electrodes. The electrooxidation of fluvastatin sodium at boron-doped diamond electrode was investigated using cyclic, differential pulse and square wave voltammetry. The possible mechanism of oxidation was discussed with model compounds that have indole oxidation. The dependence of the peak current and potentials on pH, concentration, scan rate, and the nature of the buffer were investigated. The oxidation of fluvastatin was irreversible and exhibited a diffusion-controlled fashion. The slope of the log i_p–log v linear plot was 0.44 indicating the diffusion control for pH 10.00 Britton–Robinson buffer solution. The linear response was obtained in the ranges of 1 × 10^− 6 M–6 × 10^− 4 M in pH 10.00 BR buffer solution. The detection limit of the standard solution is estimated to be 1.37 × 10^− 7 M for DPV, 1.44 × 10^− 7 M for SWV. The repeatability of the methods was found as 0.66 and 0.15 RSD % for peak currents for differential pulse and square wave voltammetry, respectively. The practical analytical value of the method is demonstrated by quantitative determination of fluvastatin in pharmaceutical formulation and human serum, without the need for separation or complex sample preparation, since there was no interference from the excipients and endogenous substances. Selectivity, reproducibility and accuracy of the developed methods were demonstrated by recovery studies.     

Anodic stripping voltammetry coupled with design of experiments for simultaneous determination of Zn+2, Cu+2, Pb+2, and Cd+2 in gasoline

In this paper a rapid method of anodic stripping voltammetry (ASV) coupled with experimental design was developed for simultaneous determination of Zn²⁺, Cd²⁺, Pb²⁺, and Cu²⁺ in gasoline samples. A microwave assisted gasoline digestion procedure for sample preparation was also established. The determination limit was found to be 0.24 μg L^?1 for Zn²⁺; 8.58 × 10^?4 μg L^?1 for Cd²⁺; 0.13 μg L^?1 for Pb²⁺, and 0.87 μg L^?1 for Cu²⁺. It was found that the interferences of concomitant metal ions were negligible. Gasoline samples collected in the city of São Luis (MA), Brazil were analyzed using this developed method, and the results demonstrated that this method was reliable and accurate. It is expected that this method can be used as a routine technique for metal ion determination in gasoline.