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.     

Kinetics of catalyst-free thermal and photo-oxidation of cumene

Kinetics of thermal and photo-oxidation of cumene in the absence of catalyst was studied using high-pressure differential scanning calorimetry and low-pressure photocalorimetry. Kinetics of oxidation was followed by cumene hydroperoxide (CHP), acetophenone, and phenol formation. The amount of CHP formed was deduced from the total heat of reaction of thermal degradation of CHP at 453 K and using a new gas chromatographic method. CHP solution in cumene oxidized at 453 K and 680 psi of oxygen reproducibly with the heat of reaction linearly dependent on peroxide concentration in cumene. It was confirmed that cumene thermal oxidation was slow at <453 K, but at ≥453 K could occur explosively. Autocatalysis by CHP during thermo-oxidation was confirmed. Apparent activation energy of the photo-oxidation of cumene was found to be E _a = 22.3 kJ mol^?1. The value corresponds to radical chain process of the cumene autoxidation. Under assumption of pseudo-first order reaction, the rate constant of CHP formation was found to change from k _CHP ≈ 0.76 s^?1 during the first 4 h of photo-oxidation to k _CHP ≈ 0.2 s^?1 at the later stages at 2.0 W cm^?2 of UV exposure dose. It was established that the initial presence of the CHP in cumene does not change the photo-oxidation kinetics, but shifts the kinetic curve to earlier time. Finite difference method was employed to numerically model kinetics of cumene oxidation. The result indicated higher than expected thermal and photo-stability of both, cumene and CHP.     

Über die anodische oxidation von hydrazinderivaten an platin im sauren elektrolyten I. Phenylhydrazin

The investigation of the anodic oxidation of phenylhydrazine at smooth platinum in acid solutions by the triangular voltage scan method, by controlled potential electrolysis and by potential step measurements yields the following results:(1) The overall-reaction yields 4 electrons per PH-molecule.(2) Within the PH-oxidation an inhibiting intermediate is developed, whose further reaction is limited, and which can be reduced by hydrogen.(3) The kinetics of the oxidation reaction are determined by a 1-electron step. The anodic reaction orders of phenylhydrazine and H⁺-ions are z_PH = 0.4 and z_H⁺ = ?0.8, respectively.These results are interpreted by means of a reaction mechanism. The kinetics of the oxidation are explained with the aid of the Temkin adsorption isotherm.     

Synthesis,characterization, and catalytic properties of a cobalt(II) complex supported by an amine-bis(phenolate) ligand

A cobalt(II) complex [L′CoPy] 1 was prepared by the reaction of dimethylaminoethylamino-N,N-bis(2,4-dibromo)phenol (H₂L′) with CoCl₂. Electrochemical studies indicate that this complex is among the most efficient homogeneous catalysts for water reduction, with a turnover frequency of 917.7 mol of hydrogen per mole of catalyst per hour at an overpotential of 636.7 mV (pH 7.0). Additionally, under photoirradiation with blue light (λ _max = 469 nm), complex 1 in combination with [Ru(bpy)₃]Cl₂ and ascorbic acid (pH 4.0 in aqueous solution) also produces hydrogen with a turnover number of 4.9 × 10⁵ mol of H₂ per mol of catalyst.     

Synthesis and cure kinetics of diphenyl(diphenylethynyl)silane monomer

Diphenyl(diphenylethynyl)silane ((ph–C≡C)₂–Si–ph₂) (DPDPES) was synthesized by the Grignard reaction. The corresponding isothermal and non-isothermal cure kinetics of DPDPES were analyzed by using differential scanning calorimetry (DSC), and the molecular structure was characterized by H-NMR. The results showed that all the cure curves were typically sigmoid shape and cure reactions could be described by an autocatalytic kinetic model by isothermal DSC. The kinetic data, for example, activation energy (E) and frequency factor (A), were 119.22 kJ/mol and 4.67 × 10⁷ (s^?1), respectively. The non-isothermal DSC analyses showed that E and A were 162.12 kJ/mol and 1.32 × 10⁹ (s^?1), respectively, and the reaction order was 0.94. Based on the research work of this paper, it can be said that the cure reaction of DPDPES monomer was of autocatalytic and diffusion-controlled characteristics, and the effect of the diffusion was more evident at low temperature. The cure reaction of DPDPES was a first-order kinetic reaction.     

Study on thermal decomposition and oxidation kinetics of cation exchange resins using non-isothermal TG analysis

Kinetics of two successive thermal decomposition reaction steps of cationic ion exchange resins and oxidation of the first thermal decomposition residue were investigated using a non-isothermal thermogravimetric analysis. Reaction mechanisms and kinetic parameters for three different reaction steps, which were identified from a FTIR gas analysis, were established from an analysis of TG analysis data using an isoconversional method and a master-plot method. Primary thermal dissociation of SO₃H⁺ from divinylbenzene copolymer was well described by an Avrami–Erofeev type reaction (n = 2, g(α) = [?ln(1 ? α)]^1/2]), and its activation energy was determined to be 46.8 ± 2.8 kJ mol^?1. Thermal decomposition of remaining polymeric materials at temperatures above 400 °C was described by one-dimensional diffusion (g(α) = α ²), and its activation energy was determined to be 49.1 ± 3.1 kJ mol^?1. The oxidation of remaining polymeric materials after thermal dissociation of SO₃H⁺ was described by a phase boundary reaction (contracting volume, g(α) = 1?(1 ? α)^1/3). The activation energy and the order of oxygen power dependency were determined to be 101.3 ± 13.4 and 1.05 ± 0.17 kJ mol^?1, respectively.     

Iron Incorporated Mesoporous Molecular Sieves Synthesized by a Microwave-Hydrothermal Process and Their Application in Catalytic Oxidation

Fe-FSM-16 and Fe-containing mesoporous materials (Fe-JLU-15) prepared by using semifluorinated surfactant as a template, have been synthesized by microwave-hydrothermal (M-H) process and characterized by several spectroscopic techniques. The catalytic activity of these materials was tested for the phenol hydroxylation and wet phenol oxidation with H₂O₂ under mild reaction conditions. The effect of pH, H₂O₂/PhOH molar ratio and stability of the catalyst on the oxidation process was also investigated. Phenol oxidation and H₂O₂ decomposition show that the Fe-JLU-15 is more active than Fe-FSM-16 and more stable in aqueous solution. The total amount of dissolved iron is less than 5 wt% of the iron initially contained in the catalyst. In phenol hydroxylation, these two solids can effectively catalyze the phenol hydroxylation. Catechol and hydroquinone were observed as the major products, with a difference in the product distribution for these solids. The Fe-JLU-15 has a high selectivity for catechol (63.5 % phenol conversion, CAT/HQ = 2.7) while the Fe-FSM-16 shows a high selectivity for hydroquinone (56.8 % phenol conversion, CAT/HQ < 1) under the same reaction conditions.     

Determination of catalytic oxidation products of phenol by RP-HPLC

A reversed-phase high-performance liquid chromatography (RP-HPLC) with ultraviolet detection was established for the determination of phenol, catechol, hydroquinone, and p-benzoquinone in the reaction solution of catalytic oxidation of phenol using hydrogen peroxide as the oxidant and copper-doped FeSBA-15 zeolite as the catalyst. Separation was accomplished on a reversed-phase C₁₈ column, and the elution condition was optimized by changing the composition of the mobile phase. A good resolution of all of the relative components in the reaction solution was achieved when the mobile phase was methanol–water–1% acetic acid aqueous solution = 10:50:40 (v/v/v). The concentrations of phenol, catechol, hydroquinone, and p-benzoquinone were determined in 11 different reaction solutions by the external standard method. The proposed HPLC method was simple, accurate, reliable, and suitable for tracing the amount of target products during the catalytic oxidation reaction of phenol. The results can provide data support for evaluating the properties of catalysts, and, thus, guide the selection of catalysts for the industrial production of dihydric phenol.     

Poly(2-amino-5-(4-pyridinyl)-1, 3, 4-thiadiazole) film modified electrode for the simultaneous determinations of dopamine, uric acid and nitrite

Poly(2-amino-5-(4-pyridinyl)-1,3,4-thiadiazole) (PAPT) modified glassy carbon electrode (GCE) was fabricated and used for the simultaneous determinations of dopamine (DA), uric acid (UA) and nitrite (NO₂ ^?) in 0.1 mol?L^?1 phosphate buffer solution (PBS, pH 5.0) by using cyclic voltammetry and differential pulse voltammetry (DPV) techniques. The results showed that the PAPT modified GCE (PAPT/GCE) not only exhibited electrocatalytic activities towards the oxidation of DA, UA and NO₂ ^? but also could resolve the overlapped voltammetric signals of DA, UA and NO₂ ^? at bare GCE into three strong and well-defined oxidation peaks with enhanced current responses. The peak potential separations are 130 mV for DA–UA and 380 mV for UA–NO₂ ^? using DPV, which are large enough for the simultaneous determinations of DA, UA and NO₂ ^?. Under the optimal conditions, the anodic peak currents were correspondent linearly to the concentrations of DA, UA and NO₂ ^? in the ranges of 0.95–380 μmol?L^?1, 2.0–1,000 μmol?L^?1 and 2.0–1,200 μmol?L^?1 for DA, UA and NO₂ ^?, respectively. The correlation coefficients were 0.9989, 0.9970 and 0.9968, and the detection limits were 0.2, 0.35 and 0.6 μmol?L^?1 for DA, UA and NO₂ ^?, respectively. In 0.1 mol?L^?1 PBS pH 5.0, the PAPT film exhibited good electrochemical activity, showing a surface-controlled electrode process with the apparent heterogeneous electron transfer rate constant (k _s) of 25.9 s^?1 and the charge–transfer coefficient (α) of 0.49, and thus displayed the features of an electrocatalyst. Due to its high sensitivity, good selectivity and stability, the modified electrode had been successfully applied to the determination of analytes in serum and urine samples.     

The surface oxidation behavior of Ni–45.16%Ti shape memory alloys at different temperatures

The isothermal oxidation behavior of Ni–45.16%Ti (composition in atomic percent) alloy was investigated by thermogravimetric analysis, and differential scanning calorimeter (DSC) methods. It was found that Ni-rich NiTi alloy exhibits a different oxidation behavior at temperatures above 400 °C in oxygen atmosphere. The alloy was exposed to oxygen atmosphere isothermally, i.e., between 400 and 800 °C, for 1 h. A gravimetric method was used to determine the oxidation kinetics and it was seen that the oxidation constant increases significantly with isothermal temperature. The activation energy of oxidation reaction for NiTi alloy was determined to be 65.47 kJ mol^?1. According to DSC measurements, the transformation temperature of alloy (M _s, M _f, A _s and A _f) was increased and also R phase disappeared above 500 °C. The formal oxides were determined by means of SEM–EDX measurements and obtained oxides are TiO and TiO₂ oxides.     

99mTc-labeling of a dithiocarbamate-DWAY fragment using [99mTcN]2+ core for the preparation of potential 5HT1A receptor imaging agents

1-(2-methoxy phenyl) piperazine fragment of WAY100635 or its phenolic analogue, derived from DWAY is used to design the desired structure of 5HT_1A receptor imaging agents. In this study a DWAY analogue was labeled with ^99mTc-nitrido ([^99mTcN]²⁺) core via dithiocarbamate. 2-(piperazin-1-yl) phenol dithiocarbamate was synthesized by the reaction of 2-(piperazin-1-yl) phenol with an equivalent amount of carbon disulfide in KOH solution then radiolabeled with ^99mTc-nitrido core. The final complex was characterized by HPLC and its radiochemical purity was more than 90 %. In vitro stability studies have shown the complex was stable at least 4 h after labeling at room temperature. The n-octanol/water partition coefficient experiment demonstrated log p = 1.34 for ^99mTcN–OHPP–DTC. Biodistribution results showed that radio tracer had moderate brain uptake (0.39 ± 0.03 %ID/g at 15 min and 0.29 ± 0.02 %ID/g at 120 min) and good retention, suggesting that this complex may lead to a further development of a radiotracer with specific binding to 5-HT_1A receptor.     

Partial Purification and Characterization of a Novel Extracellular Tyrosinase from Auricularia auricula

Extracellular tyrosinase from Auricularia auricula RF201 was purified in a three-step procedure involving ammonium sulfate precipitation, Sephadex G-100, and DEAE-Sepharose column chromatography. The partially purified enzyme showed a single protein band of 12.6 kDa on SDS-PAGE. The optimum pH for tyrosinase activity was 7, and the enzyme was stable between pH 6 and 9. Tyrosinase has optimal activity at 40 °C and retained most of its activity between 4 and 50 °C. A. auricula tyrosinase could oxidize l-tyrosine, l-DOPA, catechol, and caffeic acid and displayed dark brown or peach color. However, the enzyme was unable to catalyze l-phenylalanine and ferulic acid. In comparison with other substrates, l-tyrosine displayed the highest affinity (K _m of 0.11 mM) and the maximal reaction velocity (V _max of 102.58 μmol/min). Tyrosinase activity was reduced in the presence of numerous tested compounds. Particularly SDS, it significantly inhibited enzyme activity. CuSO₄ and NaCl showed an activation effect on enzyme activity, with the maximum activation found in the presence of CuSO₄.     

Supramolecular immobilization of glucose oxidase on gold coated with cyclodextrin-modified cysteamine core PAMAM G-4 dendron/Pt nanoparticles for mediatorless biosensor design

Cysteamine core polyamidoamine G-4 dendron branched with β-cyclodextrins was chemisorbed on the surface of Au electrodes and further coated with Pt nanoparticles. Adamantane-modified glucose oxidase was subsequently immobilized on the nanostructured electrode surface by supramolecular association. This enzyme electrode was used to construct a reagentless amperometric biosensor for glucose, making use of the electrochemical oxidation of H₂O₂ generated in the enzyme reaction. The amperometric response of the biosensor was rapid (6 s) and a linear function of glucose concentration between 5 and 705 μmol?L^?1. The biosensor had a low detection limit of 2.0 μmol?L^?1, sensitivity of 197 mA?mol^?1?L?cm^?2, and retained 94 % of its initial response after storage for nine days at 4 °C.     

Synthesis, structure and catalytic activity of an oxo-bridged dinuclear oxovanadium complex of an isonicotinohydrazide ligand

A mononuclear dioxo vanadium(V) complex of a hydrazone ONO donor ligand, [V^VO₂(L¹)] (1), was synthesized by the reaction of V₂O₅ and terephthalic acid with H₂L¹ in 1:1:1 mol ratio, while an oxo-bridged bis(vanadium(IV)oxo) complex, [μ ₂–O–{V^IVO(L²)}₂] (2), was synthesized by the treatment of isonicotinic acid hydrazide, salicylaldehyde and CoSO₄·7H₂O with bis(acetylacetonato)oxovanadium(IV) (H₂L¹ = isonicotinic acid(2-hydroxy-benzylidene)-hydrazide, H₂L² = isonicotinic acid (1-methyl-3-oxo-butylidene)-hydrazide). The complexes were characterized by elemental analyses and spectroscopic methods. The crystal structure of complex 2 was determined by X-ray analysis. The complexes were tested as catalysts for the oxidation of cycloalkenes and benzyl alcohol using H₂O₂ as terminal oxidant. Excellent selectivity was achieved in the oxidation of cyclohexene.     

Photocatalytic water purification: Destruction of resorcinol by irradiated titania

Ethanedial, butanoic acid anhydride and trihydroxybenzenes have been determined as intermediates of resorcinol photocatalytic oxidation. The photoreaction rate shows a 1st order dependence on resorcinol concentration. The calculated reaction rate constant is 2.1×10⁻⁸ mol dm⁻³ s⁻¹, which is similar to the rate constant of phenol photocatalytic oxidation on TiO₂ reported beore.     

Sensitive phenol determination based on co-modifying tyrosinase and palygorskite on glassy carbon electrode

A sensitive tyrosinase biosensor, based on co-modifying tyrosinase and palygorskite on glassy carbon electrode, was developed for phenol analysis. Palygorskite, a kind of natural one-dimensional clay with good biocompatibility, high specific surface area and porous morphology, works as a perfect matrix of enzyme. Tyrosinase retains its inherent bioactivity when immobilized in palygorskite, which leads to a high sensitivity of 1.897 A mol^?1 L. The sensor response achieves 95% of steady-state-current in no more than 3 s, and the linear range of the bioelectrode spans the concentration of phenol from 5?×?10^?8 to 1?×?10^?4 mol L^?1 with a correlation coefficient of 0.9992. The results show no apparent decrease in the response over 2 weeks, and about 80% of the response was retained after 2 months when the electrode was stored at 4–5 °C.     

Kinetics and Mechanism of Cr(VI) Reduction in a Water Cathode Induced by Atmospheric Pressure DC Discharge in Air

The process of reduction of Cr⁶⁺ ions (solution of potassium dichromate, K₂Cr₂O₇) in a water cathode was studied during a DC discharge in air. The concentration range of Cr⁶⁺ was (5.7–19) ×10^?5 mol/l and discharge current range was 20–80 mA. Cr⁶⁺ ions were shown to be reversibly reduced under a discharge action. The equilibrium degree of reduction increased with increasing initial concentration of the solution at fixed discharge current. At fixed initial concentration the reduction degree increased with increasing discharge current. The reduction degrees so obtained were 0.34–0.84. A kinetic scheme of the processes taking place in a solution was proposed. The calculated data obtained as a result of application of this scheme described well the experimental results on Cr⁶⁺ kinetics. The main processes of Cr⁶⁺ reduction and Cr³⁺ oxidation were revealed. HO ₂ ^· radicals and hydrogen peroxide were shown to be responsible for Cr⁶⁺ reduction whereas ^·OH radicals and O₂ molecules provide the reverse process of Cr³⁺ oxidation to Cr⁶⁺. The mechanism of action of phenol additives improving the process efficiency is discussed. The efficiency of phenol action as a radical scavenger was shown to be determined with its mass-transfer to the reaction area rather than chemical reaction rate.     

Green and efficient extraction strategy to lithium isotope separation with double ionic liquids as the medium and ionic associated agent

The paper reported a green and efficient extraction strategy to lithium isotope separation. A 4-methyl-10-hydroxybenzoquinoline (ROH), hydrophobic ionic liquid—1,3-di(isooctyl)imidazolium hexafluorophosphate ([D(i-C₈)IM][PF₆]), and hydrophilic ionic liquid—1-butyl-3-methylimidazolium chloride (ILCl) were used as the chelating agent, extraction medium and ionic associated agent. Lithium ion (Li⁺) first reacted with ROH in strong alkali solution to produce a lithium complex anion. It then associated with IL⁺ to form the Li(RO)₂IL complex, which was rapidly extracted into the organic phase. Factors for effect on the lithium isotope separation were examined. To obtain high extraction efficiency, a saturated ROH in the [D(i-C₈)IM][PF₆] (0.3 mol l^?1), mixed aqueous solution containing 0.3 mol l^?1 lithium chloride, 1.6 mol l^?1 sodium hydroxide and 0.8 mol l^?1 ILCl and 3:1 were selected as the organic phase, aqueous phase and phase ratio (o/a). Under optimized conditions, the single-stage extraction efficiency was found to be 52 %. The saturated lithium concentration in the organic phase was up to 0.15 mol l^?1. The free energy change (ΔG), enthalpy change (ΔH) and entropy change (ΔS) of the extraction process were ?0.097 J mol^?1, ?14.70 J mol K^?1 and ?48.17 J mol^?1 K^?1, indicating a exothermic process. The partition coefficients of lithium will enhance with decrease of the temperature. Thus, a 25 °C of operating temperature was employed for total lithium isotope separation process. Lithium in Li(RO)₂IL was stripped by the sodium chloride of 5 mol l^?1 with a phase ratio (o/a) of 4. The lithium isotope exchange reaction in the interface between organic phase and aqueous phase reached the equilibrium within 1 min. The single-stage isotope separation factor of ⁷Li–⁶Li was up to 1.023 ± 0.002, indicating that ⁷Li was concentrated in organic phase and ⁶Li was concentrated in aqueous phase. All chemical reagents used can be well recycled. The extraction strategy offers green nature, low product cost, high efficiency and good application prospect to lithium isotope separation.     

Oxygen Dependence in a Dual‐Phase Electrochemical Biosensing System

Oxygen dependence of a tyrosinase‐based electrochemical biosensor for determination of phenol in aqueous and organic media was systematically investigated. The result demonstrated that the enzymatic coupling reaction rate of tyrosinase (deoxy form) and O₂ to regenerate tyrosinase (oxy form) is a kinetically fast reaction, and the significant change of O₂ concentration in aqueous solution did not affect the coupling reaction. The further increase of O₂ concentration did not increase the overall oxidation reaction rate of the substrate at low substrate concentration (e.g.,<10 μM phenol) when O₂ concentration was greater than 8.9 ppm. The oxygen dependence was observed in the case of high substrate concentration due to insufficient amount of O₂ available for the regeneration of tyrosinase. In other words, the upper linear range is oxygen dependent for tyrosinase biosensors. The phenol biosensors employing microelectrodes had wider upper linear ranges than macroelectrodes in both aqueous and organic phase, which can be explained by the oxygen dependence.     

One-step construction of an electrode modified with electrodeposited Au/SiO2 nanoparticles, and its application to the determination of NADH and ethanol

A new electrode was developed by one-step potentiostatic electrodeposition (at ?2.0 V for 20 s) of Au/SiO₂ nanoparticles on a glassy carbon electrode. The resulting electrode (nano-Au/SiO₂/GCE) was characterized by scanning electronic microscopy, X-ray photoelectron spectroscopy and electrochemical techniques. The electrochemical behavior of dihydronicotinamide adenine dinucleotide (NADH) at the nano-Au/SiO₂/GCE were thoroughly investigated. Compared to the unmodified electrode, the overpotential decreased by about 300 mV, and the current response significantly increased. These changes indicated that the modified electrode showed excellent catalytic activity in the oxidation of NADH. A linear relationship was obtained in the NADH concentration range from 1.0?×?10^?6 to 1.0?×?10^?4 mol?L^?1. In addition, amperometric sensing of ethanol at the nano-Au/SiO₂/GCE in combination with alcohol dehydrogenase and nicotinamide adenine dinucleotide was successfully demonstrated. A wide linear response was also found for ethanol in the range from 5.0?×?10^?5 to 1.0?×?10^?3 mol?L^?1 and 1.0?×?10^?3 to 1.0?×?10^?2 mol?L^?1, respectively. The method was successfully applied to determine ethanol in beer and biological samples.