Electrochemical behavior and determination of epinephrine at a mercaptoacetic acid self-assembled gold electrode

The electrochemical behavior of epinephrine (EP) at a mercaptoacetic acid (MAA) self-assembled monolayer modified gold electrode was studied. The MAA/Au electrode is demonstrated to promote the electrochemical response of epinephrine by cyclic voltammetry. The possible reaction mechanism is also discussed. The diffusion coefficient D of EP is 6.85 × 10⁻⁶ cm² s⁻¹. In 0.1 mol L⁻¹ phosphate buffer (pH 7.20), a sensitive oxidation peak was observed at 0.177 V, and the peak current is proportional to the concentration of EP in the range of 1.0 × 10⁻⁵–2.0 × 10⁻⁴ mol L⁻¹ and 1.0 × 10⁻⁷–1.0 × 10⁻⁶ mol L⁻¹. The detection limit is 5 × 10⁻⁸ mol L⁻¹. The modified electrode is highly stable and can be applied to the determination of EP in practical injection samples. The method is simple, quick, sensitive and accurate.     

An “off–on” fluorescence probe for chromium(III) ion determination in aqueous solution

An “off–on” rhodamine-based fluorescence probe for the selective signaling of Cr(III) has been designed by exploiting the guest-induced structure transform mechanism. This system shows a sharp Cr(III)-selective fluorescence enhancement response in 100% aqueous system under physiological pH value and possesses high selectivity against the background of environmentally and biologically relevant metal ions including Cr(VI), Al(III), Fe(III), Cd(II), Co(II), Cu(II), Ni(II), Zn(II), Mg(II), Ba(II), Pb(II), Na(I), and K(I). Under optimum conditions, the fluorescence intensity enhancement of this system is linearly proportional to Cr(III) concentration from 5.0 × 10⁻⁸ to 7.0 × 10⁻⁶ mol L⁻¹ with a detection limit of 1.6 × 10⁻⁸ mol L⁻¹.     

Ultra-trace level determination of nitrite in human saliva by spectrofluorimetry using 1,3,5,7-tetramethyl-8-(3,4-diaminophenyl)-difluoroboradiaza-s-indacene

A rapid, highly sensitive and selective fluorogenic method for the determination of traces of nitrite is described. It is based on the reaction of weakly fluorescent 1,3,5,7-tetramethyl-8-(3,4-diaminophenyl)-difluoroboradiaza-s-indacence (DAMBO) and nitrite in acidic aqueous solution to give 1,3,5,7-tetramethyl-8-(5-benzotriazolyl)-difluoroboradiaza-s-indacene (DAMBO-T), which is highly fluorescent. The optimum reaction conditions and other analytical parameters are investigated to enhance the sensitivity of the method. The fluorescence enhancement at 507 nm is linearly related to the concentration of nitrite in the range of 6.0 × 10⁻⁹–5.0 × 10⁻⁷ mol L⁻¹ with a correlation coefficient of R = 0.9995 (n = 10) and a detection limit of 1.0 × 10⁻¹⁰ mol L⁻¹. The R.S.D. is 1.12% (n = 10). The method is applied to the determination of nitrite in human saliva samples with the recoveries of 96. 24–105.30%. Correspondence: Ke-Jing Huang, Department of Chemistry, Wuhan University, Wuhan 430072, P.R. China     

Electrochemical behavior of uric acid at a penicillamine self-assembled gold electrode

The fabrication and electrochemical characteristics of a penicillamine (PCA) self-assembled monolayer modified gold electrode were investigated. The electrode can enhance the electrochemical response of uric acid (UA), and the electrochemical reaction of UA on the PCA electrode has been studied by cyclic voltammetry and differential pulse voltammetry. Some electrochemical parameters, such as diffusion coefficient, standard rate constant, electron transfer coefficient and proton transfer number have been determined for the electrochemical behavior on the PCA self-assembled monolayer electrode. The electrode reaction of UA is an irreversible process, which is controlled by the diffusion of UA with two electrons and two protons transfer at the PCA/Au electrode. In phosphate buffer (pH 5.0), the peak current is proportional to the concentration of UA in the range of 6.0 × 10⁻⁵–7.0 × 10⁻⁴ mol L⁻¹ and 2.0 × 10⁻⁵–7.0 × 10⁻⁴ mol L⁻¹ for the cyclic voltammetry and differential pulse voltammetry methods with the detection limits of 5.0 × 10⁻⁶ and 3.0 × 10⁻⁶ mol L⁻¹, respectively. The method can be applied to determine UA concentration in real samples.     

Tripodal chelating ligand-based sensor for selective determination of Zn(II) in biological and environmental samples

Potassium hydrotris(N-tert-butyl-2-thioimidazolyl)borate [KTt^t-Bu] and potassium hydrotris(3-tert-butyl-5-isopropyl-l-pyrazolyl)borate [KTp^t-Bu,i-Pr] have been synthesized and evaluated as ionophores for preparation of a poly(vinyl chloride) (PVC) membrane sensor for Zn(II) ions. The effect of different plasticizers, viz. benzyl acetate (BA), dioctyl phthalate (DOP), dibutyl phthalate (DBP), tributyl phosphate (TBP), and o-nitrophenyl octyl ether (o-NPOE), and the anion excluders sodium tetraphenylborate (NaTPB), potassium tetrakis(p-chlorophenyl)borate (KTpClPB), and oleic acid (OA) were studied to improve the performance of the membrane sensor. The best performance was obtained from a sensor with a of [KTt^t-Bu] membrane of composition (mg): [KTt^t-Bu] (15), PVC (150), DBP (275), and NaTPB (4). This sensor had a Nernstian response (slope, 29.4 ± 0.2 mV decade of activity) for Zn²⁺ ions over a wide concentration range (1.4 × 10⁻⁷ to 1.0 × 10⁻¹ mol L⁻¹) with a limit of detection of 9.5 × 10⁻⁸ mol L⁻¹. It had a relatively fast response time (12 s) and could be used for 3 months without substantial change of the potential. The membrane sensor had very good selectivity for Zn²⁺ ions over a wide variety of other cations and could be used in a working pH range of 3.5–7.8. The sensor was also found to work satisfactorily in partially non-aqueous media and could be successfully used for estimation of zinc at trace levels in biological and environmental samples. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

Simultaneous Determination of Dopamine and Ascorbic Acid at a Triazole Self-Assembled Monolayer-Modified Gold Electrode

A 3-amino-5-mercapto-1,2,4-triazole (TA) self-assembled monolayer-modified gold electrode (TA SAM/Au) is characterized by X-ray photoelectron spectroscopy, A.C. impedance, cyclic voltammetry, chronoamperometry and chronocoulometry. The TA SAM/Au exhibited good promotion of the electrochemical oxidation of dopamine. Some electrochemical parameters of dopamine such as electron transfer number, exchange current density, standard heterogeneous rate constant, diffusion coefficient, etc., were measured by different electrochemical methods. The peak currents of dopamine were linearly dependent on its concentration in the range of 1.5 × 10⁻⁶–1.0 × 10⁻⁴ mol L⁻¹, with a detection limit of 5.0 × 10⁻⁷ mol L⁻¹. The oxidative peak potentials of dopamine and ascorbic acid were well separated at about 190 ± 10 mV in pH 2.0 BR buffers at TA SAM/Au, the oxidation peak current increases approximately linearly with increasing concentration of both dopamine and ascorbic acid in the concentration range of 9.98 × 10⁻⁶–4.54 × 10⁻⁴ mol L⁻¹. It can be used for simultaneous determination of dopamine and ascorbic acid.     

Chemiluminescence Method for the Determination of Glutathione in Human Serum Using the Ru(phen)3 2+ – KMnO4 System

A simple, rapid and specific chemiluminescence (CL) method for the determination of glutathione has been developed. The method is based on the enhanced CL of the reaction between Ru(phen)₃ ²⁺ (phen = 1,10-phenanthroline) and KMnO₄ by glutathione in HCl medium. Under the optimum conditions, the response is linearly proportional to the concentration of glutathione between 1.5 × 10⁻⁷ and 1.0 × 10⁻⁵ mol L⁻¹. The dection limit for glutathione (5.8 × 10⁻⁸ mol L⁻¹) is about 10 and 200 times better than those of the spectrophotometric method using Ellman regent and the Lucigenin – CL method, respectively. The final procedure allows the determination of glutathione in human serum with recoveries of 92%–108%. A satisfactory agreement was obtained with a mean relative difference of 2.5% compared to the HPLC method.     

Voltammetric Analysis of the Novel Atypical Antipsychotic Drug Quetiapine in Human Serum and Urine

The electrooxidative behaviour and determination of quetiapine (QTP), a dibenzothiazepine derivative and antipsychotic agent, on a glassy carbon disc electrode was investigated using cyclic (CV), linear sweep (LSV), differential pulse (DPV) and Osteryoung square wave voltammetry (OSWV). Fully validated DP and SW voltammetric procedures are described for the determination of QTP. QTP in pH 3.5 acetate buffer solution presents a well-defined anodic response, studied by the proposed methods. This main response was due to the irreversible, diffusion-controlled, one-electron and one-proton oxidation of the aliphatic nitrogen of the piperazine ring. Under optimal conditions, a detection limit of 4.0 × 10⁻⁸ mol L⁻¹ for DPV and 1.33 × 10⁻⁷ mol L⁻¹ for OSWV, and a linear calibration graph in the range from 4.0 × 10⁻⁶ to 2.0 × 10⁻⁴ mol L⁻¹ were obtained for both methods. The procedure was successfully applied to the determination of the drug in tablets, human serum and human urine with good recoveries. The detection limits were 6.20 × 10⁻⁷ mol L⁻¹ and 5.92 × 10⁻⁷ mol L⁻¹ in human serum and 1.44 × 10⁻⁷ mol L⁻¹ and 1.31 × 10⁻⁶ mol L⁻¹ in human urine, for the DPV and OSWV method, respectively.     

Simple sensor for simultaneous determination of dihydroxybenzene isomers

A simple sensor based on bare carbon ionic liquid electrode was fabricated for simultaneous determination of dihydroxybenzene isomers in 0.1 mol L⁻¹ phosphate buffer solution (pH 6.0). The oxidation peak potential of hydroquinone was about 0.136 V, catechol was about 0.240 V, and resorcinol 0.632 V by differential pulse voltammetric measurements, which indicated that the dihydroxybenzene isomers could be separated absolutely. The sensor showed wide linear behaviors in the range of 5.0 × 10⁻⁷–2.0 × 10⁻⁴ mol L⁻¹ for hydroquinone and catechol, 3.5 × 10⁻⁶–1.535 × 10⁻⁴ mol L⁻¹ for resorcinol, respectively. And the detection limits of the three dihydroxybenzene isomers were 5.0 × 10⁻⁸, 2.0 × 10⁻⁷, 5.0 × 10⁻⁷ mol L⁻¹, respectively (S/N = 3). The proposed method could be applied to the determination of dihydroxybenzene isomers in artificial wastewater and the recovery was from 93.9% to 104.6%.     

A new amperometric H<Subscript>2</Subscript>O<Subscript>2</Subscript> biosensor based on nanocomposite films of chitosan–MWNTs,hemoglobin, and silver nanoparticles

A new H₂O₂ biosensor was fabricated on the basis of nanocomposite films of hemoglobin (Hb), silver nanoparticles (AgNPs), and multiwalled carbon nanotubes (MWNTs)–chitosan (Chit) dispersed solution immobilized on glassy carbon electrode (GCE). The immobilized Hb displayed a pair of well-defined and reversible redox peaks with a formal potential (E ^θ′) of −22.5 mV in 0.1 M pH 7.0 phosphate buffer solution. The apparent heterogeneous electron transfer rate constants (k _s) in the Chit–MWNTs film was evaluated as 2.58 s⁻¹ according to Laviron’s equation. The surface concentration (Γ*) of the electroactive Hb in the Chit–MWNTs film was estimated to be (2.48 ± 0.25) × 10⁻⁹ mol cm⁻². Meanwhile, the Chit–MWNTs/Hb/AgNPs/GCE demonstrated excellently electrocatalytical ability to H₂O₂. Its apparent Michaelis–Menten constant (K _M^app) for H₂O₂ was 0.0032 mM, showing a good affinity. Under optimal conditions, the biosensors could be used for the determination of H₂O₂ ranging from 6.25 × 10⁻⁶ to 9.30 × 10⁻⁵ mol L⁻¹ with a detection limit of 3.47 × 10⁻⁷ mol L⁻¹ (S/N = 3). Furthermore, the biosensor possessed rapid response to H₂O₂ and good stability, selectivity, and reproducibility.     

Electrochemical Behavior of Uric Acid and Epinephrine at a Meso-2,3-Dimercaptosuccinic Acid Self-Assembled Gold Electrode

A self-assembled electrode with a meso-2,3-dimercaptosuccinic acid (DMSA) monolayer has been characterized by electrochemical quartz crystal microbalance and complex impedance analysis, surface enhanced Raman spectroscopy and cyclic voltammetry. The self-assembled electrode was used for the simultaneous electrochemical detection of epinephrine (EP) and uric acid (UA) in phosphate buffer of pH 7.7. The simultaneous oxidation of EP and UA was performed by cyclic voltammetry (CV) and differential pulse voltammetry (DPV), and the signals for each method were well separated with a potential difference of over 330 mV and without interference by each other. The detection limit of EP is 5.4 × 10⁻⁸ mol L⁻¹ by CV and 5.3 × 10⁻⁸ mol L⁻¹ by DPV and that of UA is 8.4 × 10⁻⁸ mol L⁻¹ by CV and 4.2 × 10⁻⁸ mol L⁻¹ by DPV. The DMSA self-assembled electrode can be applied to the simultaneous determination of EP and UA.     

Sensitive voltammetric sensor of dihydromyricetin based on Nafion/SWNT-modified glassy carbon electrode

A novel voltammetric sensor, based on single-walled carbon nanotubes (SWNT) dispersed in Nafion and modified glassy carbon electrode (GCE), was fabricated and used to determine the trace amounts of dihydromyricetin (DMY). The electrochemical behavior of DMY at this sensor was investigated in 0.1 mol L⁻¹ sulfuric acid solutions + 0.1 mol L⁻¹ NaCl by cyclic voltammetry and squarewave voltammetry. Compared with bare GCE, the electrode presented an excellent response of DMY through an adsorption-controlled quasi-reversible process. Under the optimum conditions, the response peak currents were linear relationship with the DMY concentrations in the range of 1.0 × 10⁻⁷–1.0 × 10⁻⁵ mol L⁻¹ with a detection limit of 9 × 10⁻⁸ mol L⁻¹. Based on this voltammetric sensor, a simple and sensitive electroanalytical method for DMY was proposed and applied to quantitative determination of DMY in Ampelopsis grossedentata samples. In addition, the oxidation mechanism was proposed and discussed, which could be a reference for the pharmacological action of DMY in clinical study.     

Solid Phase Spectrophotometry for the Determination of Cobalt in Pharmaceutical Preparations

 A new sensitive method exploiting solid-phase spectrophotometry is proposed for the determination of cobalt in pharmaceutical preparations. The chromogenic reagent 1-(2-thiazolylazo)-2-naphthol (TAN) was immobilized on C18 bonded silica loaded into a home-made cell with 1.5 mm of optical path for cobalt determination. Cobalt(II) reacts with TAN on C18 material, at pH 6.0–7.5, to give a coloured complex which has maximum absorption at 572 nm. In this way, the sample was passed through the cell and Co(II) ions were quantitatively retained on the solid-phase. After the direct measurement of light-absorption in the solid phase, only the cobalt was eluted with 0.1 mol L⁻¹ hydrochloric acid. The cell was washed with water and then another sample solution could be passed through the cell. The procedure allowed the determination of cobalt in the range of 10–160 μg L⁻¹ with coefficient of variation of 4.7% (n=10) and apparent molar absorptivity of 2.62 × 10⁶ L mol⁻¹ cm⁻¹ using sample volume of 3-mL. Received May 15, 2000. Revision August 28, 2000.     

Complex Formation Equilibria of Amine-Bridged Dinuclearpalladium(II) Complexes with DNA Constituents

The complex formation equilibria involving trans-diamminepalladium(II) chloride (Pd^II), 1,6-hexanediamine (HDA), and DNA constituents were investigated. The formation constant of all possible mononuclear and binuclear complexes were determined at 25 °C and 0.1 mol⋅L⁻¹ NaNO₃. The speciation diagrams of the binuclear complex of Pd^II–HDA–DNA reveal that these complexes predominate in the physiological pH range and the reaction of the binuclear complex Pd^II–HDA–Pd^II with DNA constituents is quite feasible.     

Ultrasensitive Assay of Rhein in Medicine Based on its Enhanced Luminol-K3Fe(CN)6 Chemiluminescence Reaction Using the Flow Injection Technique

Determination of the effective components in traditional Chinese medicine is one of the key steps for its identification. In this paper a novel and sensitive chemiluminescence (CL) method for the determination of rhein coupled with flow-injection analysis (FIA) is developed. It is based on the strong sensitizing effect on the weak CL reaction between luminol and ferricyanide in alkaline solution. Under optimal experimental conditions, the relative CL intensity is proportional to the concentration of rhein in the range of 7.0 × 10⁻¹²–7.0 × 10⁻¹⁰ mol L⁻¹ and 1.0 × 10⁻⁹–4.0 × 10⁻⁵ mol L⁻¹, the detection limit is 1.478 × 10⁻¹³ mol L⁻¹, and the relative standard deviation (RSD) for 9 parallel measurements of 1.408 × 10⁻⁷ mol L⁻¹ rhein is 3.4%. The method was successfully applied to the determination of rhein in pharmaceutical preparations. The possible mechanism of CL is also briefly discussed.     

Covalent Modification of Glassy Carbon Electrode with L-Cysteine for the Determination of Acetaminophen

A novel L-cysteine film modified electrode has been fabricated by means of an electrochemical oxidation procedure, and it was successfully applied to the electrochemical determination of acetaminophen. This method utilizes the electrooxidation of amines to their analogous cation radicals to form a chemically stable covalent linkage between the nitrogen atom of the amine and edge plane sites at the glassy carbon electrode surface. The electrochemical behaviour of acetaminophen at the film electrode was investigated in 0.1 mol L⁻¹ phosphate buffer (pH 6.20). It was found that the redox peak current of acetaminophen was enhanced greatly on the film electrode. Linearity between the oxidation peak current and the acetaminophen concentration was obtained in the range of 1.0 × 10⁻⁴–2.0 × 10⁻⁷ mol L⁻¹ with a detection limit of 5.0 × 10⁻⁸ mol L⁻¹. For seven parallel detections of 1.0 × 10⁻⁵ mol L⁻¹ acetaminophen, the relative standard deviation (RSD) was 1.46%, suggesting that the film electrode has excellent reproducibility. Application to the determination of acetaminophen in drug tablets and human urine demonstrated that the film electrode has good stability and high sensitivity.     

A novel method for the determination of Pb2+ based on the quenching of the fluorescence of CdTe quantum dots

A novel method for the determination of Pb²⁺ has been developed based on quenching of the fluorescence of thiol-capped CdTe quantum dots (QDs) by Pb²⁺ in aqueous solutions. Under optimum conditions, the relative fluorescence intensity was linearly proportional to the concentration of Pb²⁺ between 2.0 × 10⁻⁶ and 1.0 × 10⁻⁴ mol L⁻¹ with a detection limit of 2.7 × 10⁻⁷ mol L⁻¹. The relative standard deviation (RSD) was 4.6% for a 4.0 × 10⁻⁵ mol L⁻¹ Pb²⁺ solution (N = 5). As an application, the proposed method was successfully applied to the analysis of Pb²⁺ in food samples, and the results were satisfactory, i.e. consistent with those of flame atomic absorption spectrometry (FAAS). Correspondence: Heyou Han, College of Science, Institute of Chemical Biology, State Key Laboratory of Agricultural Microbiology, Huazhong Agricultural University, Wuhan 430070, P.R. China     

The electrochemical polymerization of <Emphasis Type="Italic">o</Emphasis>-phenylenediamine on <Emphasis Type="SmallCaps">l</Emphasis>-tyrosine functionalized glassy carbon electrode and its application

The polymerization of o-phenylenediamine (OPD) on l-tyrosine (Tyr) functionalized glassy carbon electrode (GCE) and its electro-catalytic oxidation towards ascorbic acid (AA) had been studied in this report. l-Tyrosine was first covalently grafted on GCE surface via electrochemical oxidation, which was followed by the electrochemical polymerization of OPD on the l-tyrosine functionalized GCE. Then, the poly(o-phenylenediamine)/l-tyrosine composite film modified GCE (POPD-Tyr/GCE) was obtained. X-ray photo-electron spectroscopy (XPS), field emission scanning electron microscope (SEM), and electrochemical techniques have been used to characterize the grafting of l-tyrosine and the polymerization and morphology of OPD film on GCE surface. Due to the doping of the carboxylic functionalities in l-tyrosine molecules, the POPD film showed good redox activity in neutral medium, and thus, the POPD-Tyr/GCE exhibited excellent electrocatalytic response to AA in 0.1 mol l⁻¹ phosphate buffer solution (PBS, pH 6.8). The anode peak potential of AA shifted from 0.58 V at GCE to 0.35 V at POPD-Tyr/GCE with a greatly enhanced current response. A linear calibration graph was obtained over the AA concentration range of 2.5 × 10⁻⁴–1.5 × 10^–3 mol l⁻¹ with a correlation coefficient of 0.9998. The detection limit (3δ) for AA was 9.2 × 10⁻⁵ mol l⁻¹. The modified electrode showed good stability and reproducibility and had been used for the determination of AA content in vitamin C tablet with satisfactory results.     

A hydrogen peroxide biosensor based on direct electrochemistry of hemoglobin in palladium nanoparticles/graphene-chitosan nanocomposite film

Thermally two-dimensional lattice graphene (GR) and biocompatibility chitosan (CS) act as a suitable support for the deposition of palladium nanoparticles (PdNPs). A novel hydrogen peroxide (H₂O₂) biosensor based on immobilization of hemoglobin (Hb) in thin film of CS containing GR and PdNPs was developed. The surface morphologies of a set of representative membranes were characterized by means of scanning electron microscopy and showed that the PdNPs are of a sphere shape and an average diameter of 50 nm. Under the optimal conditions, the immobilized Hb showed fast and excellent electrocatalytic activity to H₂O₂ with a small Michaelis–Menten constant of 16 μmol L⁻¹, a linear range from 2.0 × 10⁻⁶ to 1.1 × 10⁻³ mol L⁻¹, and a detection limit of 6.6 × 10⁻⁷ mol L⁻¹. The biosensor also exhibited other advantages, good reproducibility, and long-term stability, and PdNPs/GR–CS nanocomposites film would be a promising material in the preparation of third generation biosensor.     

Chromium(III) Hydroxide Solubility in the Aqueous K+-H+-OH?-CO2-HCO 3? -CO 32? -H2O System: A?Thermodynamic Model

Chromium(III)-carbonate reactions are expected to be important in managing high-level radioactive wastes. Extensive studies on the solubility of amorphous Cr(III) hydroxide solid in a wide range of pH (3–13) at two different fixed partial pressures of CO₂(g) (0.003 or 0.03 atm.), and as functions of K₂CO₃ concentrations (0.01 to 5.8 mol⋅kg⁻¹) in the presence of 0.01 mol⋅dm⁻³ KOH and KHCO₃ concentrations (0.001 to 0.826 mol⋅kg⁻¹) at room temperature (22±2 °C) were carried out to obtain reliable thermodynamic data for important Cr(III)-carbonate reactions. A combination of techniques (XRD, XANES, EXAFS, UV-Vis-NIR spectroscopy, thermodynamic analyses of solubility data, and quantum mechanical calculations) was used to characterize the solid and aqueous species. The Pitzer ion-interaction approach was used to interpret the solubility data. Only two aqueous species [Cr(OH)(CO₃)₂²⁻ and Cr(OH)₄CO₃³⁻] are required to explain Cr(III)-carbonate reactions in a wide range of pH, CO₂(g) partial pressures, and bicarbonate and carbonate concentrations. Calculations based on density functional theory support the existence of these species. The log ₁₀ K° values of reactions involving these species [{Cr(OH)₃(am) + 2CO₂(g)⇌Cr(OH)(CO₃)₂²⁻+2H⁺} and {Cr(OH)₃(am) + OH⁻+CO₃²⁻ ⇌Cr(OH)₄CO₃³⁻}] were found to be −(19.07±0.41) and −(4.19±0.19), respectively. No other data on any Cr(III)-carbonato complexes are available for comparisons.