Simultaneous Determination of Dopamine and Uric Acid by Electrocatalytic Oxidation on a Carbon Paste Electrode Using Pyrogallol Red as a Mediator

A sensitive and selective electrochemical method for the simultaneous determination of dopamine (DA) and uric acid (UA) was developed using a pyrogallol red modified carbon paste electrode. Under the optimized conditions, the peak current was linearly dependent on 1.0–700.0 μmol L^?1 DA and 50.0–1000.0 μmol L^?1 UA. The detection limits for DA and UA were 0.78 μmol L^?1 and 35 μmol L^?1, respectively. Finally, this method was also examined for the determination of DA and uric acid in real samples such as drugs and urine.     

Sensitive Analysis of Malondialdehyde in Human Urine by Derivatization with Pentafluorophenylhydrazine then Headspace GC–MS

A sensitive method based on derivatization with pentafluorophenylhydrazine then headspace gas chromatography–mass spectrometry has been used for analysis of malondialdehyde in human urine. Preparation of urine sample by one-step derivatization/evaporation was performed by reaction of malondialdehyde with pentafluorophenylhydrazine in a headspace vial for 10 min; the derivatives were then injected in GC–MS analysis. The reaction was performed at pH 3, and total analysis time was 35 min. The method detection limit was 0.04 μg L^?1. For MDA concentrations of 2.0 and 10.0 μg L^?1 the relative standard deviation was less then 5%. The concentration of MDA in urine was measured to be 0.199 ± 0.252 μmol g^?1 creatinine (0.022 ± 0.028 μmol mmol^?1 creatinine).     

A thin poly(acridine orange) film containing reduced graphene oxide for voltammetric simultaneous sensing of ascorbic acid and uric acid

We have fabricated, in a single step, carbon ceramic electrodes modified with a poly(acridine orange) film containing reduced graphene oxide. They display electrocatalytic activity to ascorbic acid (AA) and uric acid (UA) at pH 4.5. The anodic peak potentials of AA and UA are separated by 276 mV so that they can be well resolved in cyclic voltammetry. UA and AA were simultaneously determined in a mixture at working potentials of 170 and 400 mV, respectively. Under optimized conditions, the calibration curves for AA and UA cover the 0.8–5,000 μM and 0.6–900 μM concentration range, respectively, while detection limits are 0.3 μM and 0.2 μM. The electrode was applied to determine AA and UA in urine samples.

CE Determination of Creatinine and Uric Acid in Saliva and Urine During Exercise

A simple and reliable method based on capillary electrophoresis with electrochemical detection (CE–ED) was applied to study the effect of aerobic exercises on creatinine and uric acid concertration in saliva and urine. The pH value, the running buffer concentration, the SDS concentration, separation voltage, injection time and the potential applied to the working electrode were investigated to find the optimum conditions. The detection limits (S/N = 3) for creatinine and uric acid were 3.6 μmol L^?1 and 0.86 μmol L^?1, respectively. This method was successfully used in the rapid analysis of creatinine and uric acid in saliva samples. After aerobic exercises, creatinine concentration decreased, and uric acid concentration increased in saliva. In urine, the concentrations of creatinine and uric acid both increased after exercise.     

Flow injection amperometric determination of hydrogen peroxide in household commercial products with a ruthenium oxide hexacyanoferrate modified electrode

Hydrogen peroxide was determined in oral antiseptic and bleach samples using a flow-injection system with amperometric detection. A glassy carbon electrode modified by electrochemical deposition of ruthenium oxide hexacyanoferrate was used as working electrode and a homemade Ag/AgCl (saturated KCl) electrode and a platinum wire were used as reference and counter electrodes, respectively. The electrocatalytic reduction process allowed the determination of hydrogen peroxide at 0.0 V. A linear relationship between the cathodic peak current and concentration of hydrogen peroxide was obtained in the range 10–5000 μmol L^?1 with detection and quantification limits of 1.7 (S/N?=?3) and 5.9 (S/N?=?10) μmol L^?1, respectively. The repeatability of the method was evaluated using a 500 μmol L^?1 hydrogen peroxide solution, the value obtained being 1.6% (n?=?14). A sampling rate of 112 samples h^?1 was achieved at optimised conditions. The method was employed for the quantification of hydrogen peroxide in two commercial samples and the results were in agreement with those obtained by using a recommended procedure.     

Highly selective determination of ascorbic acid,epinephrine, and uric acid by differential pulse voltammetry using poly(Adizol Black B)-modified glassy carbon electrode

A polymerized film of Adizol Black B (ABB) on the surface of glassy carbon (GC) electrode was prepared for the simultaneous determination of ascorbic acid (AA), epinephrine (EP), and uric acid (UA). This new electrode presented an excellent electrocatalytic activity towards the oxidation of AA, EP, and UA by differential pulse voltammetry method. The oxidation peaks of the three compounds were well defined and had the enhanced peak currents. The separation of the oxidation peak potentials for AA–EP and EP–UA were about 180 and 130 mV, respectively. The calibration curves obtained for AA, EP, and UA were in the ranges of 2.0–1,970.0, 0.1–64.0, and 0.1–1,700.0 μmol L^–1, respectively. The detection limits (S/N?=?3) were 0.01, 0.007, and 0.02 μmol L^–1 for AA, EP, and UA, respectively. The diffusion coefficient and the catalytic rate constant for the oxidation reaction of EP at poly(ABB) film-coated GC electrode were calculated as 1.54(±0.10)?×?10^?4 cm² s^?1 and 4.5?×?10³ mol^?1 L s^?1, respectively. The present method was applied to the determination of EP in pharmaceutical, AA in commercially available vitamin C tablet, and UA in urine samples.     

Electrochemical investigation of the interaction between lysozyme-shelled microbubbles and vitamin C

We report loading of vitamin C (ascorbic acid) on to lysozyme-shelled microbubbles. The interaction between lysozyme-shelled microbubbles and vitamin C was studied by use of cyclic and differential pulse voltammetry, zeta potential measurements, and scanning electron microscopy. The effect of microbubbles on electrochemical measurement of ascorbic acid was evaluated. The linear range for ascorbic acid obtained for differential pulse measurement in the presence of 1 mg mL^?1 microbubbles was 1–50 μmol L^?1 (y?=?0.067x?+?0.130, r ²?=?0.995), with a detection limit of 0.5 μmol L^?1. The experimental conditions, i.e., pH and ionic strength, were optimized to improve the interaction between ascorbic acid and lysozyme-shelled microbubbles. The results were satisfactory when the interaction was performed for 1 h in aqueous solution at pH 6. The amount of vitamin C loaded on the microbubbles (90 % of the analyte added, RSD _inter-expt. = 3 %, n?=?6) and the stability of microbubbles–ascorbic acid complex (until 72 h at 25 °C) were also evaluated by use of differential pulse voltammetry and zeta potential measurements.

A Flow Injection Chemiluminescence Method for the Determination of Protein Using Copper(II)-Alizarin Red S Complex as an Efficient Chemiluminescent Probe

A sensitive flow injection chemiluminescence (CL) method is proposed for the determination of bovine serum albumin (BSA) using Copper(II)-Alizarin Red S (ARS) complex as an efficient chemiluminescent probe. The detection is based on the binding of the copper(II)-ARS complex to proteins and the catalytic activity of copper(II)-ARS in the luminol-H₂O₂ CL system. Under the selected conditions, the CL intensity is linear with the concentration of BSA in the range of 5.0 × 10^?11 to 1.0 × 10^?9 mol · L^?1. The detection limit was 2.0 × 10^?11 mol · L^?1. The method is successfully applied to the determination of protein in urine.     

Analysis of Glycocholic Acid in Human Plasma and Urine from Hepatocellular Carcinoma Patients

Glycocholic acid (GCA) has been identified as endogenous biomarker for hepatocellular carcinoma (HCC). To dissolve protein and liberate GCA from protein, ionic liquids (ILs) that contain chaotropic ions were used for pretreatment of liquid biological samples. Coupling with solid-phase extraction (SPE) and reversed-phase high-performance liquid chromatography (RP-HPLC), the novel sample pretreatment method was applied for quantitative determination of GCA in urine and plasma samples. Compared with the traditional organic solvents pretreatment of biological samples, the proposed method is “greener” and simpler, due to no use of volatile organic solvent, and avoiding centrifugation. Under the optimal conditions, when the GCA-free urine and plasma samples were spiked with GCA at 0.05–1.0 and 0.2–10 μmol L^?1, the recoveries of GCA ranged between 95.8–101.6 and 96.9–100.4%, respectively. These procedures only required 1 mL of urine and 3 mL of 3 mM ILs aqueous solution and 100 μL of plasma and 4 mL of 2 mM ILs aqueous solution, respectively. The proposed method has been successfully validated on a small sample size of 14 HCC patients and 14 healthy volunteers. For HCC patients, the mean concentration of GCA was 24.79 ± 6.86 and 31.98 ± 11.12 μmol L^?1 in urine and plasma samples, and was about 3 times and 24 times as much as in healthy people, respectively. The proposed method opens up a new possibility in early diagnosis of HCC.     

The corrected structure of depressoside,an antioxidative iridoid glucoside extracted from the flowers of Gentiana urnula Harry Sm.

Three known iridoid glucosides (gentiournoside A, gentiournoside E and depressoside) were isolated from the flowers of Gentiana urnula Harry Sm. through activity-guided fractionations with a 1,1-diphenyl-2-picrylhydrazyl (DPPH) assay. All three compounds exhibited excellent DPPH radical scavenging activities (IC₅₀: 10–20 μmol L^?1) comparable to that of ascorbic acid and Trolox. However, examination of the NMR data revealed that the reported chemical structure of depressoside, previously isolated from the leaves of G. depressa, needed correcting due to incorrect elucidation around C-7 of the iridane skeleton, and was corrected to 6-β-(2,3-dihydroxyphenyl)-d-glucosyl 7-O-(2,3-dihydroxybenzoyl)-loganate. Depressoside exhibited a much higher scavenging activity against superoxide radicals (IC₅₀: 45.5 μmol L^?1) than the other two extracted compounds (IC₅₀: more than 900 μmol L^?1) due to the crucial presence of a pyrogallyl unit.     

Modified Screen-Printed Electrode for Potentiometric Determination of Copper(II) in Water Samples

Modified screen printed (SPE) and carbon paste electrodes (CPE) with phenanthroline–tetraphenyl borate ionophore [Phen:TPB] were fabricated for the determination of copper(II). The modified electrodes have linear responses over a wide concentration range (1 × 10^?6–1 × 10^?2 mol·L^?1) of copper(II) ion at 25 °C with divalent cationic slopes of 29.85 ± 0.58 and 29.45 ± 0.81 mV·decade^?1 and exhibit a detection limit of 1 × 10^?6 mol·L^?1 for SPE and CPE. The selectivity coefficient was measured using the match potential method in acetate buffer of pH = 4.2. The modified SPE and CPE sensors show high selectivity and sensitivity for determination of copper(II) and also show stable and reproducible response over a period of five and three months for SPE and CPE sensors, respectively. This method can be used for determination of copper(II) in water, soil, plant and fish tissue samples and the results obtained agreed with those obtained with atomic absorption spectrometer (AAS).     

Covalent grafting tyrosinase and its application in phenolic compounds detection

A stepwise strategy is reported for the design of a meditor-free amperometric tyrosinase biosensor. It is based on the azide-alkyne click reaction and carbodiimide coupling. Firstly, azide-terminated alkane thiols monolayers were self-assembled on the Au electrode surface. Then, nitrophenyl groups were covalent attached to the self-assembled monolayers (SAMs) via the click reaction of copper(I)-catalyzed 1,3-dipolar cycloadditions of azide-alkyne. Finally, the nitrophenyl group terminated SAMs were converted to aminophenyl-terminated interface by electrochemical reduction, and tyrosinase was covalent immobilized onto the Au electrode via carbodiimide reaction. Based on the stepwise strategy, a meditor-free amperometric tyrosinase biosensor was farbricated, and it showed good electrocatalytic reduction ability toward phenol, pyrocatechol and m-Cresol. Their linear ranges were over the range of 0.2 to 15.0 μmol·L^?1, 0.2 to 73.0 μmol·L^?1, and 0.2 to 33.0 μmol·L^?1, respectively.     

Comparative Analysis of the Distribution of Copper in Green and Brown Brazilian Propolis Extracts

We proposed here a novel analytical procedure for copper speciation in green and brown propolis extracts using SEC—HPLC—GFAAS with 0.5% m v^?1 SDS in 2.5 m mol L^?1 Tris–HCl (pH 7.4) as the mobile phase buffer solution. Both basic (0.05 mol L^?1 NaOH) and acid (0.05 mol L^?1 HCl) conditions were evaluated for sample extraction. Depending on the extraction procedure, differences in copper distribution were identified. Copper was mainly associated with high-molecular-weight (HMW) fractions in green propolis extract when extracted with basic solution, whereas with acid extraction solution, only low-molecular-weight (LMW) fractions were obtained in both samples. Furthermore, combined analysis of results obtained using SEC-UV and GF AAS confirmed the association of copper with LMW and HMW species.     

Determination of Methadone in Biological Samples Using Liquid Phase Microextraction with Back Extraction Combined with LC

Liquid phase microextraction with back extraction (LPME-BE) combined with liquid chromatography-ultra violet (LC-UV) was applied for the extraction and determination of methadone in biological fluids. At the optimized conditions, an enrichment factor of 386 and detection limit (LOD) of 0.2 μg L^?1 were obtained. The calibration curve was linear (r ² = 0.989) in the concentration range of 0.6–1,000 μg L^?1. Within-day relative standard deviation RSD (S/N = 3) and between-day RSD were 2.7 and 5.9%, respectively. The feasibility of the proposed method was evaluated by extraction and determination of methadone in plasma and urine samples and satisfactory results were obtained.     

A Direct Chemiluminescence Method for the Determination of Prulifloxacin Using Tris-(4,7-Diphenyl-1,10-Phenanthrolinedisulfonic Acid) Ruthenium(II)–Cerium(IV) System

Abstract

A simple, rapid, injection chemiluminescence method is described for the determination of prulifloxacin, a commonly used antibiotic. A strong chemiluminescence signal was detected when a mixture of the analyte and tris-(4,7-diphenyl-1,10-phenanthrolinedisulfonic acid)ruthenium(II) was injected into cerium(IV) sulfate. The chemiluminescence signal is proportional to the concentration of prulifloxacin in the range 4.0 × 10^?8–9.0 × 10^?6 mol L^?1. The detection limit is 1.0 × 10^?8 mol L^?1, and the relative standard deviation is 2.2% (n = 11) for the determination of 8.0 × 10^?7 mol L^?1 prulifloxacin. The proposed method was successfully applied to the determination of prulifloxacin in pharmaceutical preparations in capsules, spiked serum, and urine samples.     

Detection of silver(I) ion based on mixed surfactant-adsorbed CdS quantum dots

Mixed cationic and anionic surfactants were adsorbed on cadmium sulfide quantum dots (CdS QDs) capped with mercaptoacetic acid. The CdS QDs can be extracted into acetonitrile with 98 % efficiency in a single step. Phase separation only occurs at a molar ratio of 1:1.5 between cationic and anionic surfactants. The surfactant-adsorbed QDs in acetonitrile solution display stronger and more stable photoluminescence than in water solution. The method was applied for determination of silver(I) ion based on its luminescence enhancement of the QDs. Under the optimum conditions, the relative fluorescence intensity is linearly proportional to the concentration of silver(I) ion in the range between 50 pmol L^?1and 4 μmol L^?1, with a 20 pmol L^?1 detection limit. The relative standard deviation was 1.93 % for 9 replicate measurements of a 0.2 μmol L^?1 solution of Ag(I).

Used a new aza-thia-macrocycle as a suitable carrier in potentiometric sensor of copper (II)

A new modified carbon paste electrode for determination of Cu²⁺ made in our laboratory that used a new synthesized macrocycle 7,16-diaza-1-thia-4,10,13,19-tetraoxa-6,17-dioxo-2,3;20,21-dinaphtho-cyclouneicosane as modifier. This sensor exhibits a good affinity toward copper (II) ions over a wide variety of other metal ions. The electrode exhibits a Nernstian slope of 30 (±0.5) mV per decade for copper (II) ions over a wide concentration range (1.0 × 10^?8–1.0 × 10^?2 mol L^?1), with a limit of detection of 7.0 × 10^?9 mol L^?1 (~0.45 ppb). It has a response time of 30 s and can be used for at least 3 months without any considerable divergence in responses. The potentiometric response of the electrode is independent of the pH of test solution in the pH range 3.5–7.5. Finally, it was successfully used as an indicator electrode for determination of copper (II) in real samples such as Karoun river and tap water.     

Rapid and simple electrochemical detection of morphine on graphene–palladium-hybrid-modified glassy carbon electrode

A hybrid of reduced graphene oxide–palladium (RGO–Pd) nano- to submicron-scale particles was simultaneously chemically prepared using microwave irradiation. The electrochemical investigation of the resulting hybrid was achieved using cyclic voltammetry and differential pulse voltammetry. RGO–Pd had a higher current response than unmodified RGO toward the oxidation of morphine. Several factors that can affect the electrochemical response were studied, including accumulation time and potential, Pd loading, scan rate, and pH of electrolyte. At the optimum conditions, the concentration of morphine was determined using differential pulse voltammetry in a linear range from 0.34 to 12 μmol L^?1 and from 14 to 100 μmol L^?1, with detection limits of 12.95 nmol L^?1 for the first range. The electrode had high sensitivity toward morphine oxidation in the presence of dopamine (DA) and of the interference compounds ascorbic acid (AA) and uric acid (UA). Electrochemical determination of morphine in a spiked urine sample was performed, and a low detection limit was obtained. Validation conditions including reproducibility, sensitivity, and recovery were evaluated successfully in the determination of morphine in diluted human urine.

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Determination of nitrite in environmental waters by flow injection catalytic spectrophotometry

ABSTRACT

The flow injection catalytic spectrophotometry is proposed for the determination of nitrite based on the catalytic effect on the redox reaction between methylene blue and potassium bromate in acidic medium. The reaction is monitored spectrophotometrically by measuring the decrease in the absorbance of methylene blue at the maximum absorption wavelength of 664 nm. The method is characterised by low solvent consumption and easy automatic continuous analysis. It has higher sensitivity and lower detection limit. Experimental analysis conditions of the flow injection-catalytic photometry are optimised, and the best analysis conditions are: the concentration of the potassium bromate is 0.068 mol L^?1; the concentration of the phosphoric acid in oxidation liquid is 0.045 mol L^?1; the concentration of the methylene blue in colour-substrate solution is 2.4 mg L^?1, the volume of sample ring is 200 μL; the reaction coil is around 7 m in length; the inject time is 50 s and analysis time is 70 s. Under the optimal conditions, the linear range is from 10 to 500 μg L^?1 and the detection limit is 1 μg L^?1. The nitrate standard solution is continuously determined with a mass concentration of 300 μg L^?1. The RSD is determined to be 1.41% (n = 10). The nitrite in water samples, which were from the Half Acre pond, the Ink River and the Small West lake in a campus, was determined respectively by this method. A satisfactory standard addition recovery of 96.7%–103.9% was obtained.     

Chromatographic Properties of Ethanol/Water Mobile Phases on Silica Based Monolithic C18

A simple and reliable method based on capillary electrophoresis with electrochemical detection (CE–ED) was applied to study the effect of aerobic exercises on creatinine and uric acid concertration in saliva and urine. The pH value, the running buffer concentration, the SDS concentration, separation voltage, injection time and the potential applied to the working electrode were investigated to find the optimum conditions. The detection limits (S/N = 3) for creatinine and uric acid were 3.6 μmol L⁻¹ and 0.86 μmol L⁻¹, respectively. This method was successfully used in the rapid analysis of creatinine and uric acid in saliva samples. After aerobic exercises, creatinine concentration decreased, and uric acid concentration increased in saliva. In urine, the concentrations of creatinine and uric acid both increased after exercise.