魔芋多糖固定化肌红蛋白的直接电化学与电催化

用魔芋多糖(KGM)和N,N-二甲基甲酰胺(DMF)的加合物,将肌红蛋白(Mb)固定在玻碳电极(GCE)上,制备了稳定的Mb-KGM-DMF/GCE修饰电极,并研究了Mb在修饰电极上的直接电化学行为和电催化性能。该电极在pH=7.0的磷酸盐缓冲溶液(PBS)中,-0.38 V(E0′)处有一对氧化还原峰,峰电位差ΔEp=70 mV,该峰正是Mb中血红素辅基FeⅢ/FeⅡ电对的氧化还原特征峰。在0.2~9.0 V/s扫速的范围内,氧化还原峰峰电流大小和扫描速率成正比,呈现出表面控制行为。在pH为5.0~12.0的范围内,式电位和pH值呈线性关系,表明电子传递过程伴随着质子转移。同时,Mb-KGM-DMF/GCE修饰电极表现出良好的电催化性能,对氧、H2O2有显著的催化作用。在4.70~75.0μmol/L的范围内,其催化峰电流大小与H2O2的浓度有良好的线性关系,其线性回归方程i=0.127 0.093C,r=0.9989,表观米氏常数为80.8μmol/L。     

<Emphasis Type="Italic">S,S,S</Emphasis>-Tris(2-ethylhexyl) phosphorotrithioate as an effective solvent mediator for a mexiletine-sensitive membrane electrode

S,S,S-Tris(2-ethylhexyl) phosphorotrithioate proved to be an effective solvent mediator for constructing a mexiletine-sensitive membrane electrode in combination with an ion-exchanger, sodium tetrakis[3,5-bis(2-methoxyhexafluoro-2-propyl)phenyl]borate. Among a series of phosphorus compounds containing phosphoryl (P=O) groups, this solvent mediator showed the highest sensitivity to mexiletine in phosphate-buffered physiological saline containing 0.15 mol L⁻¹ NaCl and 0.01 mol L⁻¹ NaH₂PO₄/Na₂HPO₄ (pH 7.4), giving a detection limit of 2 × 10⁻⁶ mol L⁻¹ with a slope of 58.8 mV decade⁻¹. This is the best reported detection limit of any mexiletine-sensitive electrode developed to date. Owing to its high selectivity toward inorganic cations, the electrode was used to determine the level of mexiletine in saliva, the monitoring of which is quite effective for controlling the dose of this drug noninvasively. The mexiletine concentrations determined with the mexiletine-sensitive electrode compared favorably with those determined by high-performance liquid chromatography.     

Spectrophotometric determination of vanadium in steel and alloys with 2-(5-chloro-2-pyridylazo)-5-dimethylaminophenol

Using spectrophotometric methods, the protopysis constant of the 5.ClDMPAP reagent (pK_a1 = −0.19; pK_a2 = 1.97; pK_a3 = 11.98) and the stability constant of its vanadic complex (6.0 ± 0.11) × 10¹⁴ were determined. A high-sensitivity spectrophotometric method was developed to determine V(V) using 0.1–1.2 ppm and pH = 3.8. ε₅₈₆ = 55,300 ± 400 liters · mol⁻¹ · cm⁻¹. A study on the most important interferences and the way to eliminate them was carried out. The method can be applied to the determination of the element in steels and ferrovanadiums.     

Physical immobilization of laccase on an electrode by means of poly(ethyleneimine) microcapsules

Microcapsules of poly(ethyleneimine) were used to immobilize laccase on the surface of an electrode and its mediated electron transfer was studied with the redox mediator p-phenylenediamine (PPD). The microcapsules consisted of a cross-linked PEI wall generated from an emulsion of an aqueous phase containing the enzyme. The reaction of encapsulated laccase with PPD was studied by spectrophotometry and oxygen consumption. We found that the encapsulation resulted in a small shift for the optimum pH and a lower K_m value when compared to free laccase. These differences are attributed to the charged micro-environment offered by the microcapsules. The microcapsules were then deposited on a glassy carbon electrode and chronoamperometry was used to evaluate the mediated electron transfer between the enzyme and the electrode. No significant differences in term of optimum pH and K_m occurred upon capsules deposition on the electrode. The response time of the electrode for PPD oxidation was higher than those found in the literature, which suggests that the PEI capsule wall offers some resistance to mediator permeation, an hypothesis that was verified by RDE measurements. The charged nature of the PEI membrane appeared to affect several parameters of the laccase-mediator reaction and the effect of pH and mediator charge on this reaction are reported. The immobilization platform under study can be applied to different enzyme-mediator systems than the laccase-PPD used here and is relevant to the development of bioelectrocatalytic systems.     

Accurate pKa Calculation of the Conjugate Acids of Alkanolamines,Alkaloids and Nucleotide Bases by Quantum Chemical Methods

The pK_a of the conjugate acids of alkanolamines, neurotransmitters, alkaloid drugs and nucleotide bases are calculated with density functional methods (B3LYP, M08‐HX and M11‐L) and ab initio methods (SCS‐MP2, G3). Implicit solvent effects are included with a conductor‐like polarizable continuum model (CPCM) and universal solvation models (SMD, SM8). G3, SCS‐MP2 and M11‐L methods coupled with SMD and SM8 solvation models perform well for alkanolamines with mean unsigned errors below 0.20 pK_a units, in all cases. Extending this method to the pK_a calculation of 35 nitrogen‐containing compounds spanning 12 pK_a units showed an excellent correlation between experimental and computational pK_a values of these 35 amines with the computationally low‐cost SM8/M11‐L density functional approach.     

A Tin Oxide Transparent Electrode Provides the Means for Rapid Time‐resolved pH Measurements: Application to Photoinduced Proton Transfer of Bacteriorhodopsin and Proteorhodopsin†

An electrochemical cell was previously reported in which bacteriorhodopsin (BR, purple membrane) was adsorbed on the surface of a transparent SnO₂ electrode, and illumination resulted in potential or current changes (Koyama et al., Science 265:762–765, 1994; Robertson and Lukashev, Biophys. J. 68:1507–1517, 1995; Koyama et al., Photochem. Photobiol. 68:400–406, 1998). In this paper, we concluded that pH changes caused by proton transfer by the deposited BR or proteorhodopsin (PR) films lead to the flash‐induced potential change in the SnO₂ electrode. Thus, the signals originate from BR and PR acting as light‐driven proton pumps. This conclusion was drawn from the following observations. (1) The relation between the potential of a bare electrode and pH is linear for a wide pH range. (2) The flash‐induced potential changes decrease with an increase in the buffer concentration. (3) The action spectrum of PR agrees well with the absorption spectrum. (4) The present electrode can monitor the pH change in the time range from 10 ms to several hundred milliseconds, as deduced by comparing the SnO₂ signal with the signals of pH‐sensitive dyes. Using this electrode system, flash‐induced proton transfer by BR was measured for a wide pH range from 2 to 10. From these data, we reconfirmed various pK_a values reported previously, indicating that the present method can give the correct pK_a values. This is the first report to estimate these pK_a values directly from the proton transfer. We then applied this method to flash‐induced proton transfer of PR. We observed proton uptake followed by release for the pH range from 4 to 9.5, and in other pH ranges, proton release followed by uptake was observed.     

Acid—base equilibria in the mixed solvent 80% dimethyl sulfoxide—20% water : Part I. definition of ph scale and determination of pk values of aliphatic monocarboxylic acids

The acid—base equilibria in the mixed solvent, 80% dimethyl sulfoxide—water, have been investigated by potentiometric measurements with a glass—silver—silver chloride electrode couple. The response of the glass electrode is quick and reproducible over the whole pH scale. The autoprotolysis constant (itK_SH) of the mixed solvent has been determined; the value obtained, pK_SH = 18.40, agrees well with reported data. The pK_a values of the following aliphatic monocarboxylic acids are reported: formic, acetic, propionic, n-butyric, iso-butyric, n-valeric, iso-valeric, capric, pentadecanoic, stearic, monochloroacetic. The mixed solvent offers better titration conditions than water for the determination of these acids.     

Homooxacalix[3]arene Derivatives as Ionophores for Serotonin‐Selective Membrane Electrodes

Homooxacalix[3]arene derivatives are effective ionophores for constructing serotonin‐selective membrane electrodes. An electrode based on one of the derivatives, tris(methoxyphenylpropyloxy)hexahomooxacalix[3]arene‐triethyl ether, with potassium tetrakis(p‐chlorophenyl)borate (20 mol% relative to the ionophore) as an ionic additive and bis(2‐ethylhexyl) sebacate as a solvent mediator in a poly(vinyl chloride) membrane matrix, displayed much better selectivity for serotonin than for various organic ammonium ions and inorganic cations. The electrode exhibited a near‐Nernstian response to serotonin in the concentration range of 2×10^?4 to 1×10^?2 M with a slope of 56.4 mV per concentration decade in physiological saline containing 150 mM NaCl and 10 mM Na₂HPO₄/NaH₂PO₄ (pH 7.4). The limit of the detection was 8×10^?5 M. The selectivity pattern of this electrode was quite different from that of an electrode using calix[6]arene‐hexaacetic acid hexaethyl ester, a well‐known ionophore for primary organic ammonium ions, which did not induce an enhanced response to serotonin. The developed electrode was used for the active loading of serotonin in liposomes induced by transmembrane pH gradients.     

3-(4-Tolylazo)phenylboronic acid as the active component of polyhydroxy compounds-selective electrodes

Ionophoric, extraction, acidic and hydrophobic properties of 3-(4-tolylazo)phenylboronic acid (TAPBA) were studied. Determined K_d value equals to 36±2, pK_a equals to 8.6±0.5. TAPBA extracts dobutamine from water into chloroform and transports it across a bulk chloroform membrane. The recovery is 83% (pH=7.5), the transport rate – (6.5±0.5)×10⁻⁷ mol/h. ¹H and ¹³C NMR data confirm the formation of an 1:1 complex between arylboronic acid and catecholamine. TAPBA was used as electrode-active component of plasticized membrane electrodes with cationic and anionic responses to catecholamines and phenolic acids, respectively. For the diethyl sebacate-plasticized membrane, a slope of electrode function to dobutamine is 56±2 mV/decade; the detection limit is 1.3×10⁻⁵ mol/l; the linear range – 5×10⁻⁵–1×10⁻² mol/l; the working pH-range – 4.8–7.6; the response time – 5–10 s. ISE gives incomplete cationic function to less lipophilic catecholamines. The membrane with cationic additive shows an anionic response to caffeic acid in wide pH range.     

Poly(aquachlorobis(1,10–phenanthroline)copper(II)iodidemonohydrate)/GCE for simultaneous determination of caffeine and theophylline in human serum,tea, and tablet samples

This paper presents metal complex based polymer film modified electrode for simultaneous determination of caffeine, and theophylline. Potentiodynamic fabrication of poly(aquachlorobis(1,10– phenanthroline)copper(II)iodidemonohydrate) modified glassy carbon electrode (poly(ACP₂CuIH)/GCE) was verified using cyclic voltammetric and electrochemical impedance spectroscopic techniques. In contrast to the unmodified glassy carbon electrode, the poly(ACP₂CuIH)/GCE in equi-molar mixture of theophylline and caffeine revealed sufficiently separated oxidative peaks with much enhanced peak current showing electrocatalytic property of the polymer film towards the oxidation of theophylline and caffeine. Under optimized solution pH and square wave voltammetric parameters, oxidative peak current response of poly(ACP₂CuIH)/GCE showed linear dependence on the concentration of caffeine and theophylline in the concentration range 1.0–200.0 µM with limit of detection 8.92 × 10^-3 µM for theophylline, and 1.02 × 10^{-2 µ}M for caffeine. Spike recovery in the range 97.0-102.4% for theophylline, and 95.4-100.0% for caffeine, interference recovery in the range 96.0–101.0% for theophylline, and 95.7–104.3% for caffeine, agreement of the detected amounts of theophylline and caffeine in tablet samples with the nominal values, and stability of the modified electrode all validated the developed method for simultaneous determination of theophylline and caffeine in wide range of real samples. The method was applied for simultaneous determination of both theophylline and caffeine in three tea brands (Black lion, Addis, and Wush wush), pharmaceutical tablet brands (Panadol extra, and Theodrine), and human blood serum samples making the method an excellent candidate.     

Biphenol as an Electron Transfer Mediator for Glucose Oxidase

Abstract

Biphenol (4,4′-dihydroxy-biphenyl) was found to be an electron transfer mediator for glucose oxidase (GOD) of Aspergillus niger. At a glassy carbon electrode, a 1.44×10^-4 M solution of biphenol in phosphate-buffered saline (PBS) at pH 7.4 gives an quasi-reversible, one-electron, pH-sensitive couple at 255mV (relative to the standard calomel electrode). The apparent second-order rate constant for electron transfer from reduced GOD to oxidized biphenol was determined to be 3×10⁵ M^-1 s^-1. When biphenol and GOD are cophysiadsorbed on a graphite electrode immersed in PBS and held at 400mV, a glucose-dependent current response is noted. In addition to the predominant quasi-reversible biphenol redox couple, repetitive cyclic voltammetry at a graphite electrode gave rise to a polybiphenol polymer; this is most marked at a pH above the biphenol pK_a of about 9.5. At pH 7.4, the polymerization is less significant. Polybiphenol formed either side of the pK_a remains electrochemically active (E°_app. = 245mV) but no longer mediates GOD.     

A Novel Potentiometric Sensor for Thiocyanate Based on an Amide‐Linked Manganese Diporphyrin Xanthene

The synthesis of a new compound, amide‐linked manganese diporphyrin xanthene (Mn₂Cl₂ADPX), and its application for preparation of thiocyanate selective electrodes was described. The electrode was prepared with a PVC membrane combining Mn₂Cl₂ADPX as an electro active material, 2‐nitrophenyl octyl ether (o‐NPOE) as a plasticizer in the percentage ratio of 3 : 65 : 32 (Mn₂Cl₂ADPX: o‐NPOE: PVC, w : w : w). The electrode exhibited linear response within the concentration range of 2.4×10^?6 to 1.0×10^?1 M SCN^?, with a working pH range from 3.0 to 8.0 and a fast response time of less than 60 s. Several electroactive materials and solvent mediators have been compared and the experimental conditions were optimized. The Mn₂Cl₂ADPX based electrode shows obviously better response characteristics than that of monoporphyrin manganese in terms of working concentration range and slope. Selectivity coefficients for SCN^? relative to a number of interfering ions were investigated. The electrode exhibits anti‐Hofmeister selectivity toward SCN^? with respect to common coexisting anions. The electrode was applied to the determination of SCN^? in body urine with satisfactory results.     

A novel H(2)O(2) amperometric biosensor based on gold nanoparticles/self-doped polyaniline nanofibers

A new kind of gold nanoparticles/self-doped polyaniline nanofibers (Au/SPAN) with grooves has been prepared for the immobilization of horseradish peroxidase (HRP) on the surface of glassy carbon electrode (GCE). The ratio of gold in the composite nanofibers was up to 64%, which could promote the conductivity and biocompatibility of SPAN and increase the immobilized amount of HRP molecules greatly. The electrode exhibits enhanced electrocatalytic activity in the reduction of H₂O₂ in the presence of the mediator hydroquinone (HQ). The effects of concentration of HQ, solution pH and the working potential on the current response of the modified electrode toward H₂O₂ were optimized to obtain the maximal sensitivity. The proposed biosensor exhibited a good linear response in the range from 10 to 2000 μM with a detection limit of 1.6 μM (S/N = 3) under the optimum conditions. The response showed Michaelis–Menten behavior at larger H₂O₂ concentrations, and the apparent Michaelis–Menten constant K_m was estimated to be 2.21 mM. The detection of H₂O₂ concentration in real sample showed acceptable accuracy with the traditional potassium permanganate titration.     

Glucose biosensor based on glucose oxidase immobilized in sol–gel chitosan/silica hybrid composite film on Prussian blue modified glass carbon electrode

An improved amperometric glucose biosensor based on glucose oxidase immobilized in sol–gel chitosan/silica hybrid composite film, which was prepared from chitosan (CS) and methyltrimethoxysilane (MTOS), on the surface of Prussian blue (PB)-modified glass carbon electrode was developed. The film was characterized by FT-IR. Effects of some experimental variables such as ratio of CS to silica, buffer pH, temperature, and applied potential on the current response of the biosensor were investigated. The biosensor fabricated under optimal conditions had a linear response to glucose over the range 5.0×10^–5 to 2.6×10^–2 M with a correlation coefficient of 0.9948 and a detection limit of 8.0×10^–6 M based on S/N =3. The biosensor had a fast response time of less than 10 s, a high sensitivity of 420 nA mM^–1, a long-term stability of over 60 days, and a good selectivity. The apparent Michaelis–Menten constant K_m was found to be 3.2×10^–3 M. The activation energy for enzymatic reaction was calculated to be 21.9 kJ mol^–1. This method has been used to determine the glucose concentration in real human blood samples.     

A New PVC-Membrane Electrode Based on a Macrocyclic Ionophore for Selective Determination of Ni (II) Ions

A poly(vinyl chloride)-based membrane of 3,4:12,13-dibenzo-1,6,10,15-tetraazacyclooctadecane with sodium tetraphenyl borate (NaTPB) as an anion excluder and dibutylphthalate (DBP), dibutylbutylphosphonate (DBBP), tris(2-ethylhexyl) phosphate (TEHP) and tributyl phosphate (TBP) as plasticizing solvent mediator was prepared and investigated as a Ni(II)-selective electrode. The best performance was observed with the membrane having the ligand-PVC-TEHP-NaTPB composition 1:4:1:1, which worked well over a wide concentration range (2.82 ×10⁻⁶−1.00 × 10⁻¹ M) with a Nernstian slope of 30.5 mV per decade of activity between pH 2.5–7.5. This electrode showed a fast response time of 18 s and was used over a period of 4 months with good reproducibility (S=0.4 mV). The selectivity coefficient over mono-, di- and trivalent cations indicate excellent selectivity for Ni (II) ions over a large number of cations. Anions such as Cl⁻ and NO₃⁻ do not interfere and the electrode also works satisfactorily in a partially non-aqueous medium. It has been used successfully for determination of Ni (II) in real samples.     

Amperometric Biosensor for Hydrogen Peroxide,Using Supramolecularly Immobilized Horseradish Peroxidase on the β‐Cyclodextrin‐Coated Gold Electrode

Horseradish peroxidase, previously modified with 1‐adamantane moieties, was supramolecularly immobilized on gold electrodes coated with perthiolated β‐cyclodextrin. The functionalized electrode was employed for the construction of an amperometric biosensor device for hydrogen peroxide using 1 mM hydroquinone as electrochemical mediator. The biosensor exhibited a fast amperometric response (6 s) and a good linear response toward H₂O₂ concentration between 12 μM and 450 μM. The biosensor showed a sensitivity of 1.02 mA/M cm², and a very low detection limit of 5 μM. The electrode retained 97% of its initial electrocatalytic activity after 30 days of storage at 4 ⁰C in 50 mM sodium phosphate buffer, pH 7.0.     

Spectrophotometric study of acid-base equilibria of 4-(2-benzothiazolylazo)resorcinol and 4-(2-benzothiazolylazo)salicylic acid in water-organic solvent media

The visible absorption spectra of 4-(2-benzothiazolylazo)resorcinol (BTAR) and 4-(2-benzothiazolylazo)salicylic acid (BTAS) have been recorded in water-organic solvent mixtures in the pH range 0.5–12.0. The organic solvents used are methanol, ethanol,n-propanol, iso-propanol, acetone, dioxane and dimethyl formamide. The spectral changes have been explained in terms of shifts in equilibria among different molecular and ionic species of BTAR and BTAS existing in solution. The pK _a values corresponding to the different ionization steps have been determined at 25°C and I=0.1M (KNO₃) by graphic analysis of the absorbance-pH curves. The results obtained are discussed in terms of the molecular structure of the reagents and the nature of the organic co-solvent used.     

Hydrogen bond interactions at the TiO2 surface: Their contribution to the pH dependent photo-catalytic degradation of p-nitrophenol

We have obtained pK_a values of p-nitrophenol–TiO₂ by measuring the adsorption equilibrium constants of p-nitrophenol (PNP) on the TiO₂ surface at different pH values. These values have been obtained from Langmuir isotherms and from a plot of 1/rate vs. 1/[PNP]_o obtained during TiO₂ catalyzed solar light photo-degradation of PNP. Two limit equilibrium constants are readily obtained depending on the solution pH: at pH 5 at which the TiO₂ surface is mainly positively charged and at pH 8 when it is negatively charged. With these and other adsorption equilibrium constants and the PNP pK_a value in solution, thermodynamic cycles are established in order to obtain the PNP pK_a when it is adsorbed on positively charged, neutral and negatively charged TiO₂ surfaces. From these pK_a values useful information on the PNP–TiO₂ interaction is readily obtained. For instance, the PNP nitro group interacts with the TiO₂ surface via a hydrogen bond, arising from the complex of water molecules with the Ti⁴⁺ ions on its surface. The weaker the hydrogen bond donor, the stronger the oxygen nitro group basicity. Therefore, pK_a changes on the phenolic hydroxyl group result from these interactions. Linear free energy correlations, maximum PNP adsorption capacity values (Q_L) and FTIR ATR, spectrum support this proposal. A k_obs vs. pH degradation profile of p-nitrophenol is also provided.     

Dual Fluorescence and Photoprototropic Characteristics of 2-Aminodiphenylsulphone-β-Cyclodextrin Inclusion Complex

The absorption and fluorescence spectral characteristics of 2-aminodiphenylsulphone (2ADPS) have been investigated in the presence of β-cyclodextrin (β-CDx) in water. Dual emission is observed upon the complexation of 2ADPS in β-CDx. The stoichiometry of the host:guest inclusion complex is found to be 2:1. Steady state and time-resolved fluorescence spectral analysis support the formation of 2:1 complex between β-CDx and 2ADPS. The large enhancement in fluorescence intensity of twisted intramolecular charge transfer (TICT) band in aqueous β-CDx solution is due to the decrease in non-radiative processes. The ground and the excited state pK_a values for the monocation-neutral equilibrium of 2ADPS in β-CDx are found to be different from the pK_a values in aqueous solution. In the presence of β-CDx, 2ADPS is found to be less basic in the ground and the excited states.     

Formation of a novel electroactive film by electropolymerization of 5-amino-1-naphthol

The electrooxidation of 5-amino-1-naphthol in acetonitrile solutions led to the formation of a purplish gray, air-stable, polymeric film (poly(5-amino-1-naphthol), poly(5,1-ANT)), which presents a nonfibrillar morphology, on basal-plane pyrolytic graphite and In-Sn oxide conducting glass (ITO) electrodes. The film showed a reversible, well-defined oxidation-reduction response in both aqueous solutions of pH 1–13 and protic non-aqueous solutions, and was semiconducting. The formal redox potential (E^o') was 0.075 V vs. a sodium chloride saturated calomel electrode in 0.2 M NaClO₄ aqueous solution (pH 1.0). The pH dependence of (E^o') suggested that the electrode processes at pH 1-5 and pH 8–13 are 1e⁻ − 1H⁺and 1e^t- − 2 H⁺ reactions, respectively. The poly(5,1-ANT) film-coated ITO electrode displayed an electrochromic character: the oxidized form is purplish gray and the reduced one is pale brownish yellow. In addition, based on the electrochemical and IR absorption spectroscopic characterization of the film, some possible candidates for the structures of electroactive moieties of poly(5,1-ANT) are proposed.