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AbstractAbnormal levels of ascorbic acid in the human body can induce various diseases. We reported a nickel-doped hydroxyapatite (Ni-HAp)-based biosensor for ascorbic acid and confirmed it through various spectroscopic techniques. Ni-HAp demonstrated peroxidase-like activity that was detected through the chromogenic substrate. The analyte produces a color change from blue-green to colorless. The optimized sensor has a correlation coefficient of 0.997, a limit of detection of 0.11 µM, a limit of quantification value of 0.36 µM, and a linear range from 2 to 47 µM for ascorbic acid. The synthesized sensor was used to determine ascorbic acid in spiked physiological solutions. [ABSTRACT FROM AUTHOR]

Ascorbic acid (AA) determination is of high importance in the diagnosis and treatment of diseases. Herein, we report a new ratiometric electrochemical sensor for ascorbic acid using a glassy carbon electrode (GCE) modified with a copper nanoparticle@resin nanosphere nanocomposite (CuNPs@RNS). The ratiometric strategy was established by immobilizing an internal reference (thionine) on the modified electrode using cyclic voltammetry (CV). The performance of the modified electrodes as well as the newly established ratiometric strategy was explored. The sensing platform had good electrocatalytic ability, reproducibility, and stability. In addition, the ratiometric strategy significantly improved the performance of electrochemical sensing with a wide linear range (0.0837 to 15.5 μM) and a low detection limit of 0.0279 μM under the optimal conditions. The original ratiometric electrochemical sensor was successfully applied to monitor ascorbic acid in tablets and urine. These results showed that the sensor provides a new strategy and broad prospects for biomolecular sensing with reliability and high sensitivity. [ABSTRACT FROM AUTHOR]

A biosensor based on the optical signal of gold nanorods (GNRs) and an aptamer as the recognition element is reported to perform the rapid and selective spectrophotometric determination of profenofos. The GNR synthesized by the seed growth method have unique structures which binds with the aptamer through electrostatic adsorption and is well dispersed in a salt-containing solution. Upon the addition of profenofos, specific recognition caused aggregation of the GNRs. The optical signal therefore decreased and the response linearly related to the concentration of profenofos. The results demonstrate that ascorbic acid as the reducing agent plays an important role in the morphology of the GNRs. GNRs with the length-diameter ratio (L/D) of 12 stably combined with the aptamer. For the optimum conditions, spectrophotometric determination of profenofos was achieved using the biosensor with a linear relationship between the absorbance of GNR at 508 nm and the concentration of profenofos from 40 to 460 µg/L. The limit of detection of the method was 31.93 µg/L. In addition, this approach shows good selectivity for the analysis of fruits and vegetable. A valuable new method is reported for the determination of profenofos. [ABSTRACT FROM AUTHOR]

A new electrochemical biosensor was designed for the determination of uric acid by modifying the surface of the glassy carbon electrode with electrodeposited tartrazine and copper nanoparticles (Cu NPs/TAT) nanocomposite. Electrochemical investigations were administered using cyclic voltammetry and differential pulse voltammetry. The modified Cu NPs/TAT nanocomposite electrode was characterized by field emission scanning electron microscopy, mapping, X-ray diffraction, energy-dispersive X-ray spectroscopy, diffuse reflectance spectroscopy, and electrochemical impedance spectroscopy. The results showed an increase in surface conductivity. The modified electrode had linear ranges from 0.03 to 97 µM with a sensitivity of 0.258 µA µM−1 and from 115 to 1100 µM with a sensitivity of 0.0368 µA µM−1. The modified electrode showed a detection limit as low as 2.9 nM at a signal-to-noise ratio of three. In the interference study, this modified electrode had the ability to separate dopamine, uric acid, and ascorbic acid peaks. In addition, the sensor displayed high selectivity and sensitivity, good repeatability, and good stability, and was employed for uric acid determination in human serum samples with satisfactory results. [ABSTRACT FROM AUTHOR]

The determination of antioxidant capacity, total phenolics, and ascorbic acid using accurate, efficient, low cost, and rapid methods has great analytical importance. The antioxidant capacity, total phenolics, and ascorbic acid content of apricots, arugula, banana, cranberries, spinach, and strawberries were investigated with an electrochemical approach and examined for compatibility with conventional methods. The antioxidant activity was determined by ABTS and DPPH assays. The total phenolic content was determined by the Folin Ciocalteu method and ascorbic acid was quantified by high-performance liquid chromatography (HPLC). In order to perform sensitive and simultaneous electrochemical measurements, the surface of a glassy carbon electrode (GCE) was modified by electropolymerization with p-toluene sulfonic acid (TSA). The TSA/GCE modified sensor was used for the first time for the determination of antioxidant capacity and total phenolic content. The surface of the modified sensor was characterized by cyclic voltammetry and scanning electron microscopy. The TSA/GCE was linearly correlated with the differential pulse voltammetry (DPV) for Trolox and gallic acid in 0.1 M NaNO3 and ascorbic acid in pH 7.4 phosphate buffer. Electrochemical methods offer a promising alternative for the determination of antioxidant capacity and total phenolic content due to their simplicity, rapid response, low cost, sensitivity, and reproducibility. [ABSTRACT FROM AUTHOR]

A practical and sensitive electrochemical sensor for hemoglobin (Hb) detection was developed. The main electrochemical reaction was the reduction of Fe (III) in the Hb structure. For the construction of the sensor, Hb was physically immobilized onto a carbon felt electrode (CFE) without using any linker or applying any pre-functioning or electrochemical procedures. After examining the electrochemical mechanism, the experimental conditions, such as pH, were optimized and the analytical characteristics were investigated. A linear range of 0.05 µM to 3.00 µM with a detection limit of 4.4 nM was obtained for Hb. The relative standard deviation value for 3 replicate determinations of 0.05 µM Hb was calculated as1.35%. The developed sensor was successfully applied for Hb detection via standard addition method in synthetic physiological serum sample. Meanwhile, the influences of interfering reagents, such as cysteine, ascorbic acid, bovine serum albumin, and glucose, on Hb detection were also examined. [ABSTRACT FROM AUTHOR]

Alkaline phosphatase (ALP) is a significant biomarker of diseases, such as osteoblastic bone cancer, hepatitis, and myeloma, and hence its determination is essential and important. MnO2/Mn2+ conversion-based magnetic relaxation sensing was developed to directly determine ALP in blood. In the presence of ALP, 2-phospho-L-ascorbic acid trisodium salt was catalyzed to produce ascorbic acid (AA), which reduced MnO2 to Mn2+, causing transverse relaxation time (T2) decay. A linear relationship of ALP from 5 mU·L−1 to 500 mU·L−1 with T2 signal and a low limit of detection (LOD) of 3.32 mU·L−1 were obtained using the optimized conditions of 200 µM of ascorbic acid-phosphate (AAP), 10 µg·mL−1 of MnO2, 37 °C, pH 7.4, and a 30 min reaction time. When directly measuring ALP in serum with the proposed method, the recoveries were from 99.2% to 101.3%, in accordance with values obtained by a commercial assay. In conclusion, a sensitive one-step method for ALP in complex samples was established which has potential for clinical applications. [ABSTRACT FROM AUTHOR]

A surface base polymeric film was fabricated onto a glassy carbon electrode (GCE) via reversible voltammetric scanning in acidic electrolyte containing a (2,4,6-triaminopyrmidine) monomer (TP). The assembled poly(2,4,6-triaminopyrmidine) (PTP) surface material was further modified by electrochemical attachment of tris(2,2'-bipyridine) ruthenium(II) described as Ru(II)-complex. The structure and electrochemical properties of the resulting (Ru-PTP-GCE) surface network were characterized using X-ray photoelectron spectroscopy (XPS), electrochemical impedance spectroscopy (EIS), cyclic voltammetry (CV) and differential pulse voltammetry (DPV). The apparent estimated value of electron transfer rate constant (kapp) of the prepared sensor was three times greater than the virgin PTP. The cooperative effects of the (PTP) and Ru(II)-complex composite increased the efficiency and the stability of the proposed sensor for the selective determination of dopamine (DA) in the presence of ascorbic acid (AA) and uric acid (UA). The detection limit (DL3σ) of DA in this environment was 0.021 μM. The fabricated sensor was successfully applied for DA detection in a blood serum sample with satisfactory recovery. [ABSTRACT FROM AUTHOR]

This work presents a simple, low cost, and portable spectrophotometer based on a digital microscope as a detector for the determination of the antioxidant activity and total phenolic content (TPC). The antioxidant activity was expressed as ascorbic acid equivalents (AAE) using the 2,2-diphenyl-1-picrylhydrazyl radical (DPPH) assay. The TPC value was measured using the Folin-Ciocalteu method. The color of the products was captured using a digital microscope powered through the USB output of a laptop. A digital microscope, held in a 3D printing device was used to take photos of the reaction product that was illuminated with the stable light source of the digital microscope. The pictures were analyzed for quantitation. For the DPPH assay, the developed method provided a working range from 0.6 to 6 mg L−1 of ascorbic acid with a limit of detection (LOD) of 0.2 mg L−1 and a relative standard deviation (RSD) of 2.2%. The TPC method provided a working range from 4.6 to 100 mg L−1 for gallic acid, with a LOD of 1.4 mg L−1 and a RSD of 1.3%. Results of the proposed system agree well with a conventional spectrophotometric method and have been successfully applied to measure the antioxidant activity and TPC of local Thai tea samples. [ABSTRACT FROM AUTHOR]

Neurotransmitters, the material bases for nerve signaling, play an important role in the physiological activities in the body. Accurate analysis of neurotransmitter concentration is of great significance for disease prevention and control. They are secreted in low concentrations; therefore highly sensitive detection methods are needed. In this paper, 3 spherical gold nanoparticles were prepared with diameters of 5 nm, 20 nm, and 50 nm. The gold nanoparticles were deposited on the surface of carbon fiber microelectrodes by a one-step potentiostatic process to obtain nanogold-modified carbon fiber microelectrodes (AuNPs/CFME). These electrodes were electrochemically characterized before and after modification. The electrochemical properties of dopamine, norepinephrine, ascorbic acid, and potassium ferricyanide were also studied on the microelectrodes before and after modification. The results demonstrated that AuNPs/CFME had an electrocatalytic influence upon catecholamines with positive charges with shielding effects against negatively charged substances. The smaller spherical gold nanoparticles had better catalytic shielding properties. Under the optimum conditions, the differential pulse voltammetry (DPV) peak current and the concentration of dopamine with high concentration of ascorbic acid displayed a linear range of 0.1 μM to 10 μM with a detection limit of 0.013 μM based on a signal-to-noise ratio of 3. This work provides a simple and easy approach to selectively determine positively charged substances and shielding negatively charged compounds. The prepared gold nanoparticles were employed for the selective determination of dopamine in the presence of high concentrations of ascorbic acid in the mouse serum samples with excellent analytical performance. This work shows high selectivity for dopamine, which can be applied to its selective determination in the presence of high concentrations of ascorbic acid in biological systems. [ABSTRACT FROM AUTHOR]

A new automated system based on a multi-pumping flow system (MPFS) for the spectrophotometric determination of total iron in various wine samples is presented. This analytical protocol is based on the complex formation between Fe (II) and 2-(5-bromo-2-pyridylazo)-5-(diethylamino)-phenol (Br-PADAP). The addition of ascorbic acid allows the determination of total iron by reducing the Fe (III) present in the samples to Fe (II). Several physical and chemical parameters, including the mixing coil length, the sample volume, the chromogenic reagent concentration, the pH, and the ascorbic acid concentration have been characterized in order to optimize the analytical conditions. The limits of detection and quantification were 34 µg L−1 and 114 µg L−1 respectively. The developed procedure requires minimal sample preparation. [ABSTRACT FROM AUTHOR]

Here we present a novel strategy for the quantification of ascorbic acid (AA) (vitamin C). The developed method is based on the quenching effect produced by AA on the time-resolved luminescence signal of europium ions using excitation and emission wavelengths of 388 nm and 617 nm. The method presents a linear dynamic range from 0.2 to 2 µg mL−1 with a detection limit of 0.06 µg mL−1 and a relative standard deviation lower than 6%. The method was applied to the determination of AA in pharmaceutical preparations, human serum and human urine. The obtained results in pharmaceuticals were compared with a reference method; the recovery yields were close to 100% in all cases. The accuracy of the method was statistically assessed, demonstrating the suitability of this alternative method for the determination of AA in pharmaceuticals and biological samples, requiring no sample treatment except for a dilution step. [ABSTRACT FROM AUTHOR]

Thin composite layers of poly(3,4-ethylenedioxythiophene) and poly(4-lithium styrene sulfonic acid) were formed on a platinum surface. The modified electrode platinum/polyethylenedioxythiophene:poly(4-lithium styrene sulfonic acid) was characterized by cyclic voltammetry, chronocoulometry, and electrochemical impedance spectroscopy. The results showed that the electrode may be used as a working electrode for electroanalytical methods. This electrode was used for the determination of ascorbic acid. The effect of composite layer thickness on the functionality of the electrode was characterized for ascorbic acid determination. Another investigated aspect was the process of ascorbic acid oxidation in solutions of various pH values. The results showed that the modified electrode was suitable for the determination of ascorbic acid in various electrolytes from pH 2 to 7.6 in the presence and absence of buffer solutions. The linear dynamic range of ascorbic acid concentrations corresponded to its concentrations in physiological fluids. [ABSTRACT FROM PUBLISHER]

A novel strategy for preparing polyethylene open tubular reactors coated with MnO2(s) was pioneered for flow analysis in order to minimize the inherent drawbacks of packed solid reagents. As an application, the determination of ascorbic acid in pharmaceuticals was selected. The method involved reduction of Mn(IV) by ascorbic acid, release of the formed Mn2+ion, reaction with formaldoxime, and spectrophotometric monitoring. The influence of flow pattern was investigated by using two similar flow systems relying on constant or pulsed flow delivered by peristaltic or solenoid pumps. With pulsed flow, system versatility and ruggedness were improved, and the analytical sensitivity (0.01143 L mg−1ascorbic acid), detection limit (0.6 mg L−1ascorbic acid), and sampling rate (60 h−1) evaluated compared to constant flow were improved by 2.1%, 40.0%, and 6.5%, respectively. At the same time, the reagent consumption (2.8 mg per determination) and analytical precision (relative standard deviation of approximately 2.5%) were maintained. One can conclude that the mixing conditions did not limit the analyte/reagent interaction. The results were in agreement with the British Pharmacopoeia method. [ABSTRACT FROM AUTHOR]

A facile and green synthesis was employed to synthesize water-soluble carbon dots that were shown to be uniform monodisperse spheres with an average diameter of approximately 4 nanometers. Because of their strong adsorption, the carbon dots were assembled on a glassy carbon electrode without a bridging agent. The modified electrode was employed for the sensitive determination of dopamine. In 0.1 mole per liter phosphate buffer at pH 6.0, the peak currents of dopamine increased at the modified electrode. A calibration curve for dopamine was obtained from 1.5 × 10−7to 1.5 × 10−4 mole per liter with a limit of detection of 2.6 × 10−8 mole per liter. The sensor was employed to determine dopamine in human plasma without interferences from uric acid and ascorbic acid. The carbon dot modified electrode also exhibited high stability and excellent precision. The results demonstrate the facile fabrication of a carbon dot-based sensor and a sensitive method for the determination of dopamine. [ABSTRACT FROM PUBLISHER]

The simultaneous determination of ascorbic acid and uric acid has a major importance in biomedical and pharmaceutical science. A glassy carbon electrode modified with β-cyclodextrin entrapped in polyethyleneimine was developed for the determination of ascorbic and uric acids in binary mixtures using square wave voltammetry. In order to characterize the electrode modification, Raman spectra were obtained. The modified electrode was successfully applied for the determination of ascorbic and uric acid in urine with good recoveries. The limit of detection for ascorbic acid was 0.22 µM. [ABSTRACT FROM PUBLISHER]

A hydride generation laser induced fluorescence (HG LIF) approach has been investigated for trace level measurements of bismuth. The technique uses a tunable dye laser operating at 306.7 nm as the excitation source and bismuth fluorescence is measured at 472 nm. The optimized HCl and NaBH4concentrations for bismuth measurements were 1.2 M and 2.0%, respectively. The current technique has a limit of detection of 0.03 ppb and 0.01 ppb for blank measurements performed with the laser tuned on and off the bismuth excitation wavelength, respectively. Measurements of bismuth in different sample matrices have demonstrated the effectiveness of thiourea and ascorbic acid as masking agents for measuring samples containing interfering ions. Measurements of bismuth have been performed in reference materials, a bismuth-containing medication, and tea leaves. The results demonstrate that the HG LIF approach has feasibility for measuring bismuth in various samples at environmentally relevant concentrations. [ABSTRACT FROM AUTHOR]

Magnetic chitosan microspheres (MCMS) and thionine were incorporated in a modified electrode for the simultaneous sensitive determination of dopamine (DA) and uric acid (UA). Due to the unique properties of the MCMS and the electron mediation of thionine, this modified electrode showed excellent electrocatalytic oxidation toward dopamine and uric acid with a large separation of peak potentials and a significant enhancement of peak currents. However, the electrochemical behavior of ascorbic acid may be depressed at the modified electrode. Differential pulse voltammetry was used for the simultaneous sensitive determination of dopamine and uric acid in the presence of excess ascorbic acid at this modified electrode. The current responses showed excellent linear relationships in the range of 2–30 µM and 9–100 µM for dopamine and uric acid, respectively. The detection limits were estimated to be 0.5 µM and 2.3 µM for dopamine and uric acid, respectively. In addition, this modified electrode showed excellent repeatability, good stability, and satisfactory reliability, thus indicating potential for the practical applications. [ABSTRACT FROM AUTHOR]

A simple electrochemical sensor for epinephrine (EP) has been developed by modification of a carbon film electrode (CFE) with multiwalled carbon nanotubes (MWCNTs) in a chitosan matrix. Cyclic voltammetry (CV) was employed for the evaluation of the electrochemical oxidation of EP at modified electrodes MWCNT/CFE in different pH electrolytes. Under the optimum conditions (pH 7.0), the MWCNT/CFE electrode showed significant electrocatalytic oxidation of EP: a decrease of about 200 mV in the overpotential and an 11-fold increase in the peak current, compared with the unmodified CFE. Detection of EP was carried out by CV, fixed potential amperometry, and differential pulse voltammetry (DPV); the most sensitive response with the lowest detection limit of 0.9 µM obtained by DPV with preconcentration. At MWCNT/CFE, a separation of 175 mV between EP and ascorbic acid peaks was found. The sensor exhibited excellent stability over a period of 6 months and was successfully applied to the analysis of injectable adrenaline solutions. [ABSTRACT FROM AUTHOR]

In pH 7.2 Tris-HCl buffer solution, the substrate strand DNA (SDNA) was hybridized to the enzyme strand DNA (EDNA) forming a double strand DNA (dsDNA). The SDNA in dsDNA could be cleaved by lead(II) to release a cleavaged single-stranded (ssDNA) that prevented the gold nanoparticles (AuNPs) from forming a stable AuNPs-ssDNA conjugate. The unconjugated AuNPs were aggregated to form AuNP aggregation (AuNPsA) that appeared as a resonance Rayleigh scattering (RS) peak at 532 nm. When the lead(II) concentration increased, the AuNPs-ssDNA increased, the AuNPsA decreased, the color changed from blue to red, and the RS intensity at 532 nm decreased. The decreased RS intensity ΔI 532 nm was linear to the lead(II) concentration in the range of 0.67–60 nmol/L, with a detection limit of 0.3 nmol/L. The AuNPs-ssDNA exhibited a strong catalytic effect on the reaction between chloroauric acid and vitamin C (VC) that can be detected by an RS method at 620 nm. When the lead(II) concentration increased, the intensity at 620 nm increased, and the increased intensity ΔI 620 nm was linear to the lead(II) concentration in the range of 1.33–120 pmol/L, with a detection limit of 0.5 pmol/L. The proposed method was applied to detect lead(II) in water samples, with satisfactory results. [ABSTRACT FROM AUTHOR]

Eight disposable stochastic dot sensors based on porphyrins and modified diamond or carbon pastes were employed for the assay of ascorbic acid in pharmaceutical, beverages, and biological samples. The advantage of using such sensors for the assay of ascorbic acid is the possibility of qualitative and quantitative assay of ascorbic acid in one run of the experiment. The covered linear concentration range for these sensors was between 10-14 and 10-5 mol/L with high sensitivities. The proposed dot sensors were used reliably (RSD < 1%) for the assay of ascorbic acid from different samples for more than 6 months, with a recovery higher than 92.00%. [ABSTRACT FROM AUTHOR]

Two-electron reduced heteropolyblue is formed very fast at pH 3.75-4.75 in the reaction between Dawson-type molybdophosphate HPA [image omitted] and ascorbic acid (AsA) (ε814 = 1.15 × 104 L · mol-1 cm-1). Simple, fast, and direct SIA method for AsA determination was developed. Under found optimal conditions, linear calibration curve was obtained over the range 3 × 10-6-3.5 × 10-4 mol L-1 AsA, and detection limit (3 s) was 1 × 10-6 mol L-1. The proposed SIA method has high sample throughput of 60 h-1 and small reagent consumption. The procedure was applied to the analysis of pharmaceuticals and juices. [ABSTRACT FROM AUTHOR]

Development and use of highly ordered, vertically aligned TiO2 nanotube arrays modified with gold nanoparticles for the selective detection of ascorbic acid (AA) in the presence of uric acid and glucose are reported here. Gold nanoparticles were electrodeposited on the Nanotube arrays by CV. The sensor was characterized using SEM, EDS, CV, and EIS. It showed very good performance with a sensitivity of 46.8 μA mM-1 cm-2, response time below 2 seconds and linearity in the range of 1 μM to 5 mM with a detection limit of 0.1 μM and was tested for the AA concentration in pharmaceutical preparations. [ABSTRACT FROM AUTHOR]

A carbon paste electrode (CPE), modified with novel hydroquinone/TiO2 nanoparticles, was designed and used for simultaneous determination of ascorbic acid (AA), uric acid (UA) and folic acid (FA). The magnitude of the peak current for modified TiO2-nanoparticle CPE (MTNCPE) increased sharply in the presence of ascorbic acid and was proportional to its concentration. A dynamic range of 1.0-1400.0 μM, with the detection limit of 6.4 × 10-7 M for AA, was obtained using the DPV technique (pH = 7.0). The prepared electrode was successfully applied for the determination of AA, UA, and FA in real samples. [ABSTRACT FROM AUTHOR]

A modified carbon paste electrode that incorporates the anionic surfactant sodium dodecyl sulfate (SDS) in the paste (SDS-CP) is reported. It is shown that the SDS-CP electrode discriminates effectively between the cationic form of dopamine and the anionic electroactive species existing in biological fluids at the physiological pH. Therefore, a differential pulse voltammetry method has been developed. It has the detection limit comprised in the submicromolar range and the capability to remove the interference of the ascorbic acid and to diminish the interference of the uric acid. The method has been tested with good results on real samples of deproteinized serum. [ABSTRACT FROM AUTHOR]

A three-dimensional L-cysteine (L-cys) monolayer assembled on gold nanoparticles (GNP) providing simultaneous detection of uric acid (UA) and ascorbic acid (AA) was studied in this work. The cyclic voltammetry demonstrated that, at a bare glassy carbon electrode (GCE) or planar gold electrode, the mixture of UA and AA showed one overlapped oxidation peak; whereas when the electrode was modified with GNP, the oxidation peaks for UA and AA were separated. While a GNP modified electrode was further modified with L-cys monolayer (L-cys/GNP/GCE), namely, three-dimensional L-cys monolayer, a better separation for UA and AA response was obtained. Interestingly, the L-cys monolayer-modified planar gold electrode presented a block effect on the oxidation of AA, which was facilitated by the three-dimensional L-cys monolayer attributed to its distinct structure. The pH of solution presented a noticeable effect on the separation of UA and AA at GNP modified electrodes with or without L-cys monolayer. Wide concentration ranges from 2 × 10-6-1 × 10-3 M to UA and 2 × 10-6-8 × 10-4 M to AA could be obtained at L-cys/GNP/GCE. [ABSTRACT FROM AUTHOR]

A carbon paste electrode (CPE) modified with a ferrous pentacyanonitrosylferrate (FePCNF) was prepared via two consecutive procedures. The modified electrode showed a well-defined redox couple in 0.5 M KNO3 solution. The results showed that the peak potential and peak current varied with different metal cations, while anions such as [image omitted], Cl-, [image omitted], and [image omitted] at the same concentration had no effect. An extensive study showed that the response of the modified electrode was affected by pH within a range of 1 ∼ 12. The FePCNF/CPE shows excellent electrocatalytic activity toward the oxidation of ascorbic acid in 0.5 M KNO3 solution. The plot anodic peak current Ip vs. concentration of ascorbic acid was linear and has been given by the following equation: Ip = - 19.7 - 0.38C, R = 0.998 for N = 13 in the concentration range 10 ∼ 120 µM. The detection limit was 4 µM. The electrode exhibited a high stability and reproducibility. [ABSTRACT FROM AUTHOR]

Fourier transform-infrared (FT-IR) spectrometry was used for the rapid, direct measurement of ascorbic acid (vitamin C) and biotin (vitamin H) in different pharmaceutical products. Conventional KBr spectra were compared for the best determination of active substances in drug preparations. The Beer-Lambert law and chemometric approaches were applied in data processing. [ABSTRACT FROM AUTHOR]

Iron nanoparticles (INPs) were dispersed in Nafion solution to obtain a homogeneous INP-Nafion dispersion, and then a drop of this dispersion was cast on the surface of a carbon paste electrode (CPE) to fabricate an INP-Nafion-modified electrode. The electrochemical behavior of dopamine (DA) at this modified electrode was studied by cyclic voltammetry in a pH 7.0 Britton-Robinson (B-R) buffer solution. The result showed that the modified CPE exhibited an obvious electrocatalytical response toward DA, with the anodic and cathodic peak potentials shifted negatively and positively respectively, and great enhance of the peak currents at the scan rate of 100 mV s-1. The effects of carbon paste constitution, amount of the dispersion, pH, and scan rate were investigated. Under the optimum experimental conditions, the peak currents determined by differential pulse voltammetry showed an excellent linear relationship with DA concentration in the range from 10 to 110 µM with the detection limit of 3.3 µM. In addition, ascorbic acid and some other possible interferents did not interfere with the voltammetric sensing of DA, and this method also had good stability and reproducibility. [ABSTRACT FROM AUTHOR]

Electrochemical investigation of the interaction of Ascorbic acid (AA) with bovine serum albumin (BSA) on a glassy carbon electrode is reported for the first time. In a 0.1 mol/l, pH 7.3 NaCl solution, AA had a well-defined voltammetric oxidative peak at +0.2086 V (versus Ag/AgCl) on a GC electrode. After the addition of BSA into the AA solution, the oxidative peak current decreased significantly without a shift of the peak potential, and no new peak appeared. The experimental results showed that a new nonelectrochemical supramolecular complex was formed after the interaction of BSA with AA, which resulted in a decrease of the diffusion coefficient, and then a decrease of the oxidative peak current. The interaction conditions and the electrochemical detection conditions were carefully investigated. The stoichiometry of this supramolecular complex was calculated based on voltammetric data with a binding number of two and a binding constant of 1.38×107. [ABSTRACT FROM AUTHOR]

A novel chemiluminescence (CL) reaction between hydroxyl radical and ascorbic acid is described in this paper. Hydroxyl radical generated on line by the reaction between Fe3+ solution and H2O2 solution in HCl medium could oxidize rhodamine 6G to produce weak chemiluminescence. It was found that ascorbic acid could enhance the chemiluminescence and the excited rhodamine 6G was the emitter of the chemiluminescence reaction. The possible mechanism of the CL system was also discussed. Ascorbic acid can be determined in the range of 2.0×10-6-8.0×10-4 mg/ml with a detection limit of 1×10-6 mg/ml (3σ). A complete analysis could be done in 1 minute with the relative standard deviation of 3.1% for 5.0×10-5 mg/ml (n=11). In order to study the chemiluminescence reaction further, the application to the determination of ascorbic acid in food using the chemiluminescence reaction combined with flow injection is investigated. [ABSTRACT FROM AUTHOR]

Cd2+ ion was used as an electrochemical indicator to detect VB1 or Vc using square ware voltammetry (SWV) at a mercury film-coated glassy carbon (GC) electrode. At pH=10 NH3-NH4Cl buffer, a new cathodic peak was found at -0.360 V (vs.SCE) by addition of thiamine, and the peak current of SWV was linear with the concentration of thiamine in the range of 1×10-6 to 4×10-3 M. On the other hand, the SWV peak current of Cd2+ at -0.856 V linearly decreased with addition of ascorbic acid in the range of 6×10-6∼10-3 M. The effects of interference, such as citric acid, DL-malic acid, and calcium panlothenate, on thiamine or ascorbic acid determination were investigated. This method was successfully applied to the determination of thiamine or in pharmaceutical preparation. [ABSTRACT FROM AUTHOR]

This paper describes a kinetic spectrophotometric method for the determination of L-ascorbic acid (AA) and thiols (RSH). Absorbance of Fe(II)-phen complex formed during the reaction of AA or RSH with Fe(III)-phen was continuously measured at 510 nm by double-beam spectrophotometer with flow cell. For determination some thiols, the catalytic effect of Cu2+ ions was used. AA and RSH can be determined in concentration ranges from 4.0×10-6 to 4.0×10-5 M and from 8.0×10-6 to 8.0×10-5 M, respectively. The applicability of the proposed method was demonstrated by determination of chosen compounds in pharmaceutical dosage forms. [ABSTRACT FROM AUTHOR]

The biological catalysts adsorbed on the carbon surface gradually lost their activities. In order to prolong stabilization of the enzyme activity, polymaleimidostyrene (PMS) was used to preserve enzyme conformation. Sulfhydryl and amino groups of enzyme or biological tissue can be covalently bound to PMS. By using the carbon material coated by PMS, the stability of L‐ascorbic acid sensor response using purified enzyme was significantly improved, and its initial response was not decreased at all for 150 days, although the purified enzymes adsorbed to the carbon without PMS lost their activity 22.5% after 10 days. [ABSTRACT FROM AUTHOR]

Methylene blue (MB) was incorporated into titanium phosphate (TiP) after pretreatment of TiP with the gas, n-butyl amine. The dye is strongly retained and not easily leached from the layered host matrix. The adsorbed MB on TiP was used to prepare modified carbon paste electrodes (MCPE), which were studied voltammetrically and in amperometric flow injection (FI) mode for the electrocatalytic oxidation of ascorbic acid (AA). The electrochemical behavior of the immobilized dye was investigated with cyclic voltammetry, at a pH 7.0 phosphate buffer containing 0.5 M KCl, at different potential scan rates. The MB immobilized on the support underwent a quasi-reversible electrochemical redox reaction. A homemade flow-through electrochemical cell with a suitable transparent window for irradiation of the electrode surface was constructed and used for amperometric FI studies. The photoamperometric-FI conditions were optimized for sensitivity and reproducibility at a flow rate of 1.5 mL/min, a transmission tubing length of 25 cm, a sample injection volume of 100 µL, and a constant applied potential of +100 mV vs. SCE. The calibration curve for AA was linear over the concentration range from 1.0×10 -6 to 2.5×10 -5 mol l -1 for both amperometric and photoamperometric studies. But the slope of the photoelectrocatalytic FIA procedure was improved about 52% compared with those obtained without irradiation. The results obtained for AA determination in some pharmaceutical products are in good agreement with those obtained using the procedure involving the reaction between triiodide and AA. [ABSTRACT FROM AUTHOR]

A new method for the determination of trace vanadium(V) by solid substrate room temperature phosphorimetry(SS-RTP) has been established. It is based on the facts that in the presence of sulfuric acid and sodium dodecyl benzene sulfonate(DBS), vanadium activated by vitamin C(Vc) can catalyze potassium bromate to oxidize safranine O(R), the oxidation products of safranine O can emit intense and stable solid substrate room temperature phosphorescence(SS-RTP) on nitrocellulose membrane. The catalytic reaction is a first-order reaction, with a reaction rate constant(k) of 7.3 × 10−4 sec−1, and the apparent activation energy(E) is 46.09 kJ/mol. The phosphorescence intensity(ΔIp) will be increased 10 times if Vc is added. The linear range of this method is 1.6–16.0 fg/spot within whichΔIp obeyed Beer's Law, the detection limit(LD) is 0.38 fg/spot(0.4 µL sample solution per spot), and the regression equation for working curve isΔIp = 52.17 + 3.181mV(V)(fg/spot), n = 6, correlation coefficient r is 0.9993. This sensitive method has been applied to the determination of trace vanadium in tea and human hair samples successfully. The mechanism of the catalytic reaction was also discussed. [ABSTRACT FROM AUTHOR]

A highly selective and sensitive electrochemical sensor modified with carboxymethylated β-cyclodextrin (CM-β-CDP) polymer film was fabricated for the determination of dopamine (DA) in the presence of ascorbic acid (AA). The electrochemical behavior of DA and AA at the chemically modified electrode was investigated by cyclic voltammetry (CV) and differential pulse voltammetry. The response mechanism of CM-β-CDP film for DA was based on the combination of electrostatic and inclusion interaction of CM-β-CDP for DA, which was distinguished from the response mechanism of the charged polymer film modified electrode that only relied upon the electrostatic interaction between charged polymer film and DA (or AA) to realize the separation of overlapping peak potential of DA and AA. The CM-β-CDP film showed preferable analytical performance characteristics in catalytic oxidation of DA compared with the β-CDP polymer film. Under optimized conditions, it was feasible for the electrode modified with the CM-β-CDP film to selectively determine DA in the presence of a large excess of AA. A linear calibration plot was obtained over the range 0.8–60 µM with a sensitivity of 65 nA/µM and a detection limit (signal/noise (S/N) = 3) of 0.2 µM in the presence of 1.0 mM AA. This electrochemical sensor showed excellent sensitivity, repeatability, stability, and recovery for the determination of DA. The interference of AA with the determination of DA could be efficiently eliminated. [ABSTRACT FROM AUTHOR]

Novel membrane sensors for triiodide consisting of triiodide-fluphenazine (FP) and triiodide-trifluoperazine (TFP) ion pair complexes dispersed in PVC matrix plasticized with 2-nitrophenyl octal ether with unique selectivity toward I3− ions are described. The influence of the membrane composition, pH of the test solutions, and foreign ions on the electrode performance were investigated. The optimized membranes demonstrate Nernstian response for triiodide ions over a wide linear range from 1.0 × 10−2 to 5.0 × 10−6 M at 25°C. The electrodes could be used over a wide pH range 2–9, and have the advantages of easy to prepare, good selectivity, fast response time, long lifetime (over 3 months), and small interferences from hydrogen ion. They were successfully used as indicator electrode in the potentiometric titration of triiodide ions and ascorbic acid. [ABSTRACT FROM AUTHOR]

A porphyrin 5-(4-hydroxyethoxyphenyl)-10,15,20-tris-phenylporphyrin (HEPP) was synthesized. The voltammetric behavior of gold electrodes modified with neutral free-base and positively charged cobaltous center porphyrin films through exposed hydroxyl and pre-assembled carboxyl terminal via coupled reaction were investigated using Fe(CN)63−/4−, ascorbic acid (AA), and dopamine (DA) as probe species. The voltammetric sensing of AA at the physiological level in the presence of DA can be achieved on functionalized self-assembled monolayer (SAM) of short chain cobaltous porphyrin. The selectivity of response can be improved by introducing structural defects and positively charged center (Co2+) in monolayer modified electrode. The results showed that the current of AA anodic peak and its concentration had a linear relationship in a wide range of concentration from 4 × 10−7 to 2 × 10−2 mol L−1. The detection limit obtained by differential pulse voltammetry was 6.9 × 10−8 mol L−1. [ABSTRACT FROM AUTHOR]

A kinetic spectrophotometric method was developed for simultaneous determination of ascorbic acid (AA) and L-cysteine (Cys), based on detecting the colored final product. These compounds react differentially with Fe(III) in neutral medium, and the product of reduction, Fe(II), will form a colored compound with chromogenic reagent 1,10-phenanthroline, which shows a maximum absorbance at 510 nm. The absorbance was measured at 510 nm every 2 sec from 10 to 190 sec, and the data obtained were processed by chemometric approach, partial least squares (PLS). The experimental calibration set was composed with 25 sample solutions using orthogonal calibration design for two component mixtures. The concentration ranges for AA and Cys were 2.00–11.82 and 1.50–14.99 ppm, respectively. The effect of pH and temperature was studied. The proposed method was successfully applied to the simultaneous determination of both analytes in pharmaceutical formulation. [ABSTRACT FROM AUTHOR]

A new spectrophotometric method for the on‐site determination of arsenic in contaminated water samples has been developed. The method is simple, rapid, and selective. It is based on reduction of As(V) to As(III) with ascorbic acid, complexation of As(III) with I− ions in strong HCl medium and extraction of the yellow colored iodo‐complex with N‐octylacetamide into chloroform solution. The optimum acidity range is 4.5–6.0 M HCl/2.5–3.5 M H2SO4. The apparent value (i.e., at two fold enrichment factor) of molar absorptivity in the term of arsenic is (5.00) × 104 L mol−1 cm−1 at λmax, 420 nm. The detection limit (i.e., absorbance causing more absorbance than 3s) of the method is 10 µg L−1 As. The method is free from interference of the ions commonly found to be associated with arsenic in environmental samples. The method has been tested for the field determination of arsenic in contaminated waters (i.e., surface, ground, rice field) of central India. #Accepted for poster presentation in IMA conference to held in Thessaloniki, Greece, 23–27 Sept., 2003. [ABSTRACT FROM AUTHOR]

A chemiluminescence (CL) flow-through sensor for ascorbic acid, based on permanganate electrostatically immobilized on an anion exchange column is described. Ascorbic acid can be measured by the CL reaction directly with the immobilized reagent. The sensors response was linear in the range 0.05–10 µg/mL with a detection limit of 5 ng/mL. The analysis could be performed within 30 s including sampling and washing, giving a throughout of about 120 samples per hour with RSD of 1.6% for 2 µg/mL ascorbic acid. The sensor was stable for over 500 determinations and has been successfully applied to the determination of ascorbic acid in tablets and vegetables. [ABSTRACT FROM AUTHOR]

A rapid voltammetric method was developed for the determination of L-ascorbate using methylene blue as a mediator with a glassy carbon electrode. The magnitude of the peak current that occurs near -0.04 V vs. Ag/AgCl is proportional to L-ascorbate concentration in the range of 1.2 × 10[SUP-6) to 1.12 × 10[SUP-3] M. The limit of detection is 1 × 10[SUP-6] M. Eight replicate analyses of 5.0 × 10[SUP-5] M and 4.5 ×10[SUP4] L-ascorbate yielded RSD values of 1.9 and 1.6%, respectively. The method was applied to the determination of ascorbic acid in fruit juices and vitamin C tablets. [ABSTRACT FROM AUTHOR]

The nano-gold modified glassy carbon electrode, which can catalytically oxidize and accumulate epinephrine was obtained by electrodeposition. The effect of pH value and concentration of phosphate buffer solution (PBS) on the electrochemical behavior of epinephrine was studied. Two separating oxidation peaks of epinephrine and ascorbic acid can be observed at the nano-gold modified glassy carbon electrode in cyclic voltammogram. So it can be applied for determination of epinephrine in the presence of ascorbic acid. The peak current of epinephrine is in linear relationship with its concentration in the range of 1.0 x 10[sup -4]-1.0 x 10[sup -6]M. Furthermore, the FT-IR spectrometry was employed to study the oxidation of epinephrine. [ABSTRACT FROM AUTHOR]

The insertion of Co(II) into a thiol-derivatized free-base porphyrin monolayers pre-assembled on gold has been conducted by refluxing the Co(II) solution in which the monolayer-coated gold electrode was immersed. The monolayers have been used for the electrocatalytic oxidation of ascorbic acid (AA). A large decrease (ca. 243 mV) in the overpotential for the oxidation of AA has been observed at the self-assembled monolayer modified Au electrode. Catalytic currents increased linearly with AA concentration in the range of 8.2 x 10[sup -8]-3.9 x 10[sup -4] mol/L with the correlation coefficient of 0.9948. The detection limit was 4.6 x 10[sup -9] mol/L. The modified electrode exhibited good sensitivity and stability. The proposed method was applied to the determination of AA in real samples with satisfactory results. [ABSTRACT FROM AUTHOR]

Copper (II) tetrasulphophthalocyanine, CuPc[sup 4-], was encapsulated in a n-propylpyridinium silsesquioxane polymer, SiPy[sup +]Cl[sup -], immobilized on the SiO[sub 2]/Al[sub 2]O[sub 3] surface. The material obtained incorporated in a carbon paste electrode, presented the property of electrocatalysing the oxidation of ascorbic acid at 0.18 vs SCE. It was observed that the anodic peak potential at different solution pH remained practically constant between pH 5 and 8 and shifted to more positive potentials below pH 5. The electrode showed a very reproducible response and was chemically stable under various oxidation-reduction cycles. By amperometric measurements, a linear response was obtained in the range of 1 × 10[sup -4]-1.4 × 10[sup -] mol1[sup -1] of ascorbic acid. The modified electrode was used to determine ascorbic acid (vitamin C) in tablets. The results were validated by comparison with the results obtained by using the standard 2,6-dichlorophenolindophenol titration method. [ABSTRACT FROM AUTHOR]

Ascorbic acid present in pharmaceuticals and fruit juice was rapidly determined by the spectrophotometric method in association with flow injection analysis. The method is based on the inhibitory effect of ascorbic acid on the oxidation of pyrogallol red by iodate in sulfuric acid media. The calibration graph is linear in the range of 2.0 × 10[sup -6] M up to 1.0 × 10[sup -4] M. The throughput is 20 × 5 samples per hour and the detection limit is 0.5 µM ascorbic acid. The relative standard deviation for ten replicate determinations of 8.00 µM and 50.0 µM ascorbic acid is 3.4 and 0.60%, respectively. The influence of various species was studied for this determination. [ABSTRACT FROM AUTHOR]

An electrochemically pretreated glassy carbon electrode (PGCE) was achieved by anodic oxidation at 1.8 V and following potential cycling in the potential range from -0.8 to + 1.0 V. The resulting PGCE gave well-defined redox peaks with E[SUBpa]=-0.073 V, E[SUBpc] = 0.044V. The PGCE showed good activity in improving the electrochemical responses of some water-soluble Vitamins, such as Vitamin (VB[SUB2]), Vitamin (VB[SUB6]) and Vitamin (VC). VB[SUB2], VB[SUB6] could be adsorbed at PGCE and VC proceeded with an electrocatalytic process. The electrochemical behavior of VB[SUB6] at PGCE depended on the potential scanning method and the pretreatment process of the electrode. The PGCE was rather stable and can be used to simultaneously determine these vitamins in multivitamin tablets. [ABSTRACT FROM AUTHOR]

This paper reports the preparation of the glutamic acid chemically modified graphite electrode and its electrocatalytic function toward dopamine and ascorbic acid. The overlapped oxidation peaks of dopamine and ascorbic acid can be separated up to about 200 mV, which is large enough to determine both of them simultaneously in a mixture. The catalytic peak currents obtained from the cyclic voltammograms are linearly related to both dopamine and ascorbic acid concentrations in the ranges of 6.0 × 10[SUP6]-8.0 × 10[SUP-4]mol/L with correlation coefficient of 0.9976 for dopamine and 4.0 × 10[SUP-5]-0.1 mol/L with correlation coefficient of 0.9982 for ascorbic acid. The detection limits (3σ) for dopamine and ascorbic acid are 1.8 × 10[SUP-6]mol/L and 1.3 × 10[SUP-5]mol/L, respectively. The modified electrodes showed high catalyticability and good stability. The proposed method has been successfully applied to the simultaneous determination of dopamine and ascorbic acid in samples. [ABSTRACT FROM AUTHOR]

A simple, sensitive and selected fluorimetry for determination of ascorbic acid (AA) is described. The method is based on the reaction of ascorbic acid in alkaline medium with 2,3-diaminonaphthalene without oxidant. The addition of βcyclodextrin(β-CD) greatly enhance the fluorescence of the system. Under optimum conditions, a linear relationship was obtained between the fluorescence intensity and the concentration of ascorbic acid in the range of 0.05-600 μg/ml, the detection limit is 8ng/ml. In AA-DAN-β-CD system, the inclusion complex is formed. Its inclusion constant is 1.1 x 10[SUP6] L[SUP2]moL[SUP2]:. [ABSTRACT FROM AUTHOR]