Probing traces of hydrogen peroxide by use of a biosensor based on mediator-free DNA and horseradish peroxidase immobilized on silver nanoparticles

A new electrochemical biosensor for determination of hydrogen peroxide (H₂O₂) has been developed by immobilizing horseradish peroxidase (HRP) on silver colloids (nanosilver) and use of a DNA-functionalized interface. In the presence of the DNA and the nanosilver the immobilized HRP gives a pair of well-defined redox peaks with an electron-transfer rate constant of 3.27 ± 0.91 s⁻¹ in pH 7.0 PBS. The presence of DNA also provides a biocompatible microenvironment for enzyme molecules, greatly amplifies the amount of HRP molecules immobilized on the electrode surface, and improves the sensitivity of the biosensor. Under optimum conditions the biosensor has electrocatalytic activity in the reduction of hydrogen peroxide with linear dependence on H₂O₂ concentration in the range 1.5 × 10⁻⁶ to 2.0 × 10⁻³ mol L⁻¹; the detection limit is 5.0 × 10⁻⁷ mol L⁻¹ at a signal-to-noise ratio of 3. The value of HRP in the composite membrane was found to be 1.62 mmol L⁻¹. These results suggest that the properties of the complex film, with its bioelectrochemical catalytic activity, could make it useful for development of bioelectronic devices and for investigation of protein electrochemistry at functional interfaces.     

Bioelectrochemical response of a choline biosensor fabricated by using polyaniline

On the basis of the isoelectric point of an enzyme and the doping principle of conducting polymers, choline oxidase was doped in a polyaniline film to form a biosensor. The amperometric detection of choline is based on the oxidation of the H₂O₂ enzymatically produced on the choline biosensor. The response current of the biosensor as a function of temperature was determined from 3 to 40°C. An apparent activation energy of 22.8 kJ·mol⁻¹ was obtained. The biosensor had a wide linear response range from 5 × 10⁻⁷ to 1 × 10⁻⁴ M choline with a correlation coefficient of 0.9999 and a detection limit of 0.2 μM, and had a high sensitivity of 61.9 mA·M⁻¹·cm⁻² at 0.50 V and at pH 8.0. The apparent Michaelis constant and the optimum pH for the immobilized enzyme are 1.4 mM choline and 8.4, respectively, which are very close to those of choline oxidase in solution. The effect of selected organic compounds on the response of the choline biosensor was studied.     

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.     

Immobilization of Hemoglobin on the Gold Colloid Modified Pretreated Glassy Carbon Electrode for Preparing a Novel Hydrogen Peroxide Biosensor

A novel hydrogen peroxide (H₂O₂) biosensor was developed by immobilizing hemoglobin on the gold colloid modified electrochemical pretreated glassy carbon electrode (PGCE) via the bridging of an ethylenediamine monolayer. This biosensor was characterized by UV-vis reflection spectroscopy (UV-vis), electrochemical impendence spectroscopy (EIS) and cyclic voltammetry (CV). The immobilized Hb exhibited excellent electrocatalytic activity for hydrogen peroxide. The Michaelis–Menten constant (K _m) was 3.6 mM. The currents were proportional to the H₂O₂ concentration from 2.6 × 10⁻⁷ to 7.0 × 10⁻³ M, and the detection limit was as low as 1.0 × 10⁻⁷ M (S/N = 3).     

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.     

An amperometric hydrogen peroxide biosensor based on immobilization of horseradish peroxidase on an electrode modified with magnetic dextran microspheres

A new kind of magnetic dextran microsphere (MDMS) with uniform shape and narrow diameter distribution has been prepared from magnetic iron nanoparticles and dextran. Horseradish peroxidase (HRP) was successfully immobilized on the surface of an MDMS-modified glassy-carbon electrode (GCE), and the immobilized HRP displayed excellent electrocatalytic activity in the reduction of H₂O₂ in the presence of the mediator hydroquinone (HQ). The effects of experimental variables such as the concentration of HQ, solution pH, and the working potential were investigated for optimum analytical performance. This biosensor had a fast response to H₂O₂ of less than 10 s and an excellent linear relationship was obtained in the concentration range 0.20 μmol L⁻¹–0.68 mmol L⁻¹, with a detection limit of 0.078 μmol L⁻¹ (S/N = 3) under the optimum conditions. The response showed Michaelis–Menten behavior at larger H₂O₂ concentrations, and the apparent Michaelis–Menten constant was estimated to be 1.38 mmol L⁻¹. Moreover, the selectivity, stability, and reproducibility of the biosensor were evaluated, with satisfactory results. Figure Amperometric response of the biosensor to successive additions of H₂O₂ and the plot of amperometric response vs. H₂O₂ concentration     

Highly sensitive choline biosensor based on carbon nanotube-modified Pt electrode combined with sol-gel immobilization

A novel amperometric choline biosensor has been fabricated with choline oxidase (ChOx) immobilized by the sol-gel method on the surface of multi-walled carbon nanotubes (MWCNT) modified platinum electrode to improve the sensitivity and the anti-interferential property of the sensor. By analyzing the electrocatalytic activity of the modified electrode by MWCNT, it was found that MWCNT could not only improve the current response to H₂O₂ but also decrease the electrocatalytic potential. The effects of experimental variables such as the buffer solutions, pH and the amount of loading enzyme were investigated for the optimum analytical performance. This sensor shows sensitive determination of choline with a linear range from 5.0 × 10⁻⁶ to 1.0 × 10⁻⁴ mol/L when the operating pH and potential are 7.2 and 0.15 V, respectively. The detection limit of choline was 5.0 × 10⁻⁷ mol/L. Selectivity for choline was 9.48 μA·(mmol/L)⁻¹. The biosensor exhibits excellent anti-interferential property and good stability, retaining 85% of its original current value even after a month. It has been applied to the determination of choline in human serum. Translated from Chinese Journal of Analytical Chemistry, 2006, 34(7): 910–914 (in Chinese)     

Experimental Determination of the Isotherm at 15°C of the System Mg2+/Cl?, SO42–H2O

 The diagram of the ternary system Mg²⁺/Cl⁻, SO₄ ²⁻–H₂O was established at 15°C by means of analytical and conductimetric measurements. Three compounds were found in this diagram, which are MgSO₄·6H₂O, MgSO₄·7H₂O, and MgCl₂·6H₂O. The solubility field of MgSO₄·7H₂O is important whereas those of MgSO₄·6H₂O and MgCl₂·6H₂O are small. The compositions (mass-%) of the two invariant points determined by the two methods are: MgSO₄:MgCl₂=2.73:33.80 and MgSO₄: MgCl₂=3.38:28.91. Both the measured and the calculated isotherm at 15°C have been used for modelling of the diagram Mg²⁺/Cl⁻, SO₄ ²⁻–H₂O between 0 and 35°C. The polythermal invariant point was approximately located between 15 and 10°C.     

New electrochemiluminescent biosensors combining polyluminol and an enzymatic matrix

Performant reagentless electrochemiluminescent (ECL) (bio)sensors have been developed using polymeric luminol as the luminophore. The polyluminol film is obtained by cyclic voltammetry (CV) on a screen-printed electrode either in a commonly used H₂SO₄ medium or under more original near-neutral buffered conditions. ECL responses obtained after performing polymerization either at acidic pH or at pH 6 have been compared. It appears that polyluminol formed in near-neutral medium gives the best responses for hydrogen peroxide detection. Polymerization at pH 6 by cyclic voltammetry gives a linear range extending from 8 × 10⁻⁸ to 1.3 × 10⁻⁴ M H₂O₂ concentrations. Based on this performant sensor for hydrogen peroxide detection, an enzymatic biosensor has been developed by associating the polyluminol film with an H₂O₂-producing oxidase. Here, choline oxidase (ChOD) has been chosen as a model enzyme. To develop the biosensor, luminol has been polymerized at pH 6 by CV, and then an enzyme-entrapping matrix has been formed on the above modified working electrode. Different biological (chitosan, agarose, and alginate) and chemical (silica gels, photopolymers, or reticulated matrices) gels have been tested. Best performances have been obtained by associating a ChOD-immobilizing photopolymer with the polyluminol film. In this case, choline can be detected with a linear range extending from 8 × 10⁻⁸ to 1.3 × 10⁻⁴ M. This paper is based on the results presented in a poster that received a Poster Award on the occasion of XIII International Symposium on Luminescence Spectrometry in Bologna, Italy, on September, 7th-11th, 2008.     

Synthesis, characterization and studies on the third-order optical non-linearities of novel charge-transfer heteropoly complexes (C8H12N2)5H7PMo12O40 and (C8H12N2)3H3PMo12O40·5H2O

Summary Two novel charge-transfer (CT) heteropoly complexes, (C₈H₁₂N₂)₅H₇PMo₁₂O₄₀ (1) and (C₈H₁₂N₂)₃H₃-PMo₁₂O₄₀·5H₂O (2), prepared by reacting p-Me₂NC₆H₄NH₂ with the four-electron heteropoly blue H₇PMo₁₂O₄₀·12H₂O and heteropoly acid H₃PMo₁₂O₄₀· xH₂O, respectively, were characterized by elemental analysis, and u.v., i.r., XPS and e.s.r. spectroscopies. A sizable electron-transfer interaction occurs within the product molecules and the heteropoly anions retain their Keggin structure. Their third-order optical non-linearity coefficients were measured using the Z-scan technique at a concentration of 4.68 × 10⁻⁶ mol dm⁻³ for (1) and 2.79 × 10⁻⁶ mol dm⁻³ for (2), with I ₀ = 2.38 × 10¹³ w m⁻² and λ = 532nm. The |χ⁽³⁾| for (1) is 2.61 × 10⁻¹⁰ esu and |χ⁽³⁾| for (2) is 1.05 × 10⁻¹⁰ esu.     

Design of an amperometric xanthine biosensor based on a graphite transducer patterned with noble metal microparticles

A mesoporous graphite material micro-structured with palladium-platinum deposits (mixed in the ratio of 70:30% Pd:Pt) has been used as a cost-effective electrode material for designing an amperometric biosensor for xanthine. The here reported biosensor shows significantly improved operational parameters as compared to previously published results. At a constant applied potential of −0.05 V (vs. Ag/AgCl) it is distinguished with enhanced selectivity of the determination: at the working potential the current from the electrochemical transformation of various electrochemically active substances usually attending biological fluids (incl. uric acid, L-ascorbic acid, glutathione and paracetamol) has been eliminated. The effect of both the temperature and buffer composition on the analytical performance of the sensor has been investigated. Under optimal operational conditions (25°C, −0.05 V vs. Ag/AgCl, phosphate buffer, pH 8.4), the following have been defined for the biosensor: sensitivity 0.39 μA μM⁻¹, strict linearity of the response up to xanthine concentration 70 μM, detection limit of 1.5 μM (S/N=3) and a response time of at most 60 s.     

A supramolecular complex based on poly-Keggin-anion chains: synthesis, structure characterization, and conductivity

Abstract  

A proton-conductive supramolecular complex, {[Cu(H₂O)₈][H(H₂O)₃](HINO)₄(PMo₁₂O₄₀)} _n , was constructed by a self-assembly of H⁺(H₂O)₃ clusters, [Cu(H₂O)₈]²⁺ clusters, [PMo₁₂O₄₀]³⁻ anions, and isonicotinic acid N-oxide (HINO). Single-crystal X-ray diffraction analysis at 293 K revealed that the complex presented the three-dimensional (3D) supramolecular framework built from non-covalent interactions. Interestingly, [PMo₁₂O₄₀]³⁻ anions self-assembled into poly-Keggin-anion chains in the supramolecular framework. Thermogravimetric analysis shows no weight loss in the temperature range of 20–100 °C, indicating that all water molecules in the unit structure are not easily lost below 100 °C. Surprisingly, the proton conductivity of the complex in the temperature range of 85–100 °C under 98% RH condition reached good proton conductivity of 10⁻³ S cm⁻¹. A possible mechanism of the proton conduction was proposed according to the experimental results.     

Formations of Active Species and By-Products in Water by Pulsed High-Voltage Discharge

Formations of active species and by-products are different from bubbling different gases in a pulsed high-voltage discharge reactor. The identification of all the products and the formation rate determination of active species are quite important as the process is applied to wastewater disposal. Serials of measurements were conducted to do the identifications and determinations in this paper. Amounts of · OH all increased but that of H₂O₂ all decreased by bubbling gas. The · OH formation rate was 3.49 × 10⁻⁷, 3.56 × 10⁻⁷, 3.21 × 10⁻⁷ and 1.94 × 10⁻⁷ mol l⁻¹ s⁻¹ with bubbling nitrogen, argon, air and oxygen respectively, but it was 1.61 × 10⁻⁷ mol s⁻¹ l⁻¹ without bubbling. Without any bubbling, the H₂O₂ formation rate was up to 6.53 × 10⁻⁶ mol l⁻¹ s⁻¹, while it was 9.97 × 10⁻⁷, 1.663 × 10⁻⁷, 1.73 × 10⁻⁶ and 3.14 × 10⁻⁶ mol l⁻¹ s⁻¹ with bubbling nitrogen, argon, air and oxygen, respectively. NO₂⁻ and NO₃⁻ was detected in discharged water with bubbling nitrogenous gas. Their formation made the pH decreased.     

Heat capacity,enthalpy and entropy of calcium niobates

Heat capacity and enthalpy increments of calcium niobates CaNb₂O₆ and Ca₂Nb₂O₇ were measured by the relaxation time method (2–300 K), DSC (260–360 K) and drop calorimetry (669–1421 K). Temperature dependencies of the molar heat capacity in the form C _pm=200.4+0.03432T−3.450·10⁶/T ² J K⁻¹ mol⁻¹ for CaNb₂O₆ and C _pm=257.2+0.03621T−4.435·10⁶/T ² J K⁻¹ mol⁻¹ for Ca₂Nb₂O₇ were derived by the least-squares method from the experimental data. The molar entropies at 298.15 K, S _m⁰(CaNb₂O₆, 298.15 K)=167.3±0.9 J K⁻¹ mol⁻¹ and S _m⁰(Ca₂Nb₂O₇, 298.15 K)=212.4±1.2 J K⁻¹ mol⁻¹, were evaluated from the low temperature heat capacity measurements. Standard enthalpies of formation at 298.15 K were derived using published values of Gibbs energy of formation and presented heat capacity and entropy data: Δ_f H ⁰(CaNb₂O₆, 298.15 K)= −2664.52 kJ mol^t-1 and Δ_f H ⁰(Ca₂Nb₂O₇, 298.15 K)= −3346.91 kJ mol⁻¹.     

Experimental Determination of the Isotherm at 15°C of the System Mg<Superscript>2+</Superscript>/Cl<Superscript>−</Superscript>, SO<Subscript>4</Subscript><Superscript>2</Superscript>–H<Subscript>2</Subscript>O

Summary.  The diagram of the ternary system Mg²⁺/Cl⁻, SO₄ ²⁻–H₂O was established at 15°C by means of analytical and conductimetric measurements. Three compounds were found in this diagram, which are MgSO₄·6H₂O, MgSO₄·7H₂O, and MgCl₂·6H₂O. The solubility field of MgSO₄·7H₂O is important whereas those of MgSO₄·6H₂O and MgCl₂·6H₂O are small. The compositions (mass-%) of the two invariant points determined by the two methods are: MgSO₄:MgCl₂=2.73:33.80 and MgSO₄: MgCl₂=3.38:28.91. Both the measured and the calculated isotherm at 15°C have been used for modelling of the diagram Mg²⁺/Cl⁻, SO₄ ²⁻–H₂O between 0 and 35°C. The polythermal invariant point was approximately located between 15 and 10°C.  Corresponding author. E-mail: ariguib@planet.tn Received October 16, 2002; accepted (revised) December 3, 2002 Published online April 24, 2003 RID="a" ID="a" Dedicated to Prof. Dr. Heinz Gamsj?ger on the occasion of his 70^th birthday     

Voltammetric study of the interaction of the ofloxacin–copper complex with DNA, and its analytical application

The electrochemical behavior of the ofloxacin–copper complex, Cu(II)L₂, at a mercury electrode, and the interaction of DNA with the complex have been investigated. The experiments indicate that the electrode reaction of Cu(II)L₂ is an irreversible surface electrochemical reaction and that the reactant is of adsorbed character. In the presence of DNA, the formation of the electrochemically non-active complexes Cu(II)L₂-DNA, results in the decrease of the peak current of Cu(II)L₂. Based on the electrochemical behavior of the Cu(II)L₂ with DNA, binding by electrostatic interaction is suggested and a new method for determining nucleic acid is proposed. Under the optimum conditions, the decrease of the peak current is in proportional to the concentration of nucleic acids in the range from 3 × 10⁻⁸ to 3 × 10⁻⁶ g · mL⁻¹ for calf thymus DNA, from 1.6 × 10⁻⁸ to 9.0 × 10⁻⁷ g · mL⁻¹ for fish sperm DNA, and from 3.3 × 10⁻⁸ to 5.5 × 10⁻⁷ g · mL⁻¹ for yeast RNA. The detection limits are 3.3 × 10⁻⁹, 6.7 × 10⁻⁹ and 8.0 × 10⁻⁹ g · mL⁻¹, respectively. The method exhibits good recovery and high sensitivity in synthetic samples and in real samples.     

Kinetics of the Formation of the Blue Complex CrO(O2)2 Formed by Dichromate and H2O2 in Acid Solutions. A Stopped-Flow Investigation Using Rapid-Scan UV-VIS Detection

 The kinetics of the CrO(O₂)₂ formation by H₂O₂ and Cr₂O₇ ²⁻ in aqueous acidic media was measured at 293 ± 2 K in a pH range between 2.5 and 3.3. Using the stopped-flow method with rapid scan UV-VIS detection, the rate law of the formation of CrO(O₂)₂ was determined. For the media HClO₄, HNO₃ and CH₃COOH, the reaction order in the Cr₂O₇ ²⁻ concentration was found to be 0.5. For [H₂O₂] as well as for [H⁺], the reaction was first order in all acids used. In HCl and H₂SO₄ media the reaction was first order in Cr₂O₇ ²⁻. At T = 293 ± 2 K the rate constant for the formation of Cr(O)(O₂)₂ was found to be (7.3 ± 1.9) · 10² M^−3/2 s⁻¹ in HClO₄.     

Kinetics of the Formation of the Blue Complex CrO(O2)2 Formed by Dichromate and H2O2 in Acid Solutions. A Stopped-Flow Investigation Using Rapid-Scan UV-VIS Detection

Summary.  The kinetics of the CrO(O₂)₂ formation by H₂O₂ and Cr₂O₇ ²⁻ in aqueous acidic media was measured at 293 ± 2 K in a pH range between 2.5 and 3.3. Using the stopped-flow method with rapid scan UV-VIS detection, the rate law of the formation of CrO(O₂)₂ was determined. For the media HClO₄, HNO₃ and CH₃COOH, the reaction order in the Cr₂O₇ ²⁻ concentration was found to be 0.5. For [H₂O₂] as well as for [H⁺], the reaction was first order in all acids used. In HCl and H₂SO₄ media the reaction was first order in Cr₂O₇ ²⁻. At T = 293 ± 2 K the rate constant for the formation of Cr(O)(O₂)₂ was found to be (7.3 ± 1.9) · 10² M^−3/2 s⁻¹ in HClO₄. Corresponding author. E-mail: grampp@ptc.tu-graz.ac.at Received January 30, 2002; accepted (revised) June 5, 2002     

Electrogeneration of polyluminol and chemiluminescence for new disposable reagentless optical sensors

A performant reagentless electrochemiluminescent (ECL) detection system for H₂O₂ is presented, based on an electropolymerized polyluminol film prepared under near-neutral conditions. Such an original polyluminol electrodeposition is reported for the first time and on a screen-printed electrode (SPE) surface. Electropolymerized luminol acts as an active luminophore of the electrochemiluminescent reaction, as the monomer does. Polymerization conditions have been optimized in order to obtain the best ECL responses to H₂O₂. By performing electrodeposition in a potentiostatic mode, at 425 mV vs. Ag|AgCl, in 0.1 mol L⁻¹ phosphate/0.1 mol L⁻¹ KCl pH 6 and 1 mmol L⁻¹ luminol, with a total charge of 0.5 mC, the linear range for H₂O₂ detection extends from 7.9 × 10⁻⁸ mol L⁻¹ to 1.3 × 10⁻³ mol L⁻¹. Such performant disposable reagentless easy-to-use miniaturized systems based on SPEs should be applicable to the electrochemiluminescent detection of many oxidase-substrate compounds. Figure An original polyluminol electrodeposition process on a screen-printed electrode surface is reported for the first time. The polymeric structure is demonstrated to behave as an electrochemiluminescent luminophore, allowing disposable reagentless easy-to-use optical sensors for hydrogen peroxide detection to be designed.