DNA-templated synthesis of PtAu bimetallic nanoparticle/graphene nanocomposites and their application in glucose biosensor

In this paper, single-stranded DNA (ss-DNA) is demonstrated to functionalize graphene (GR) and to further guide the growth of PtAu bimetallic nanoparticles (PtAuNPs) on GR with high densities and dispersion. The obtained nanocomposites (PtAuNPs/ss-DNA/GR) were characterized by transmission electron microscopy (TEM), energy-dispersive X-ray spectrometer (EDS), and electrochemical techniques. Then, an enzyme nanoassembly was prepared by self-assembling glucose oxidase (GOD) on PtAuNP/ss-DNA/GR nanocomposites (GOD/PtAuNPs/ss-DNA/GR). The nanocomposites provided a suitable microenvironment for GOD to retain its biological activity. The direct and reversible electron transfer process between the active site of GOD and the modified electrode was realized without any extra electron mediator. Thus, the prepared GOD/PtAuNP/ss-DNA/GR electrode was proposed as a biosensor for the quantification of glucose. The effects of pH, applied potential, and temperature on the performance of the biosensor were discussed in detail and were optimized. Under optimal conditions, the biosensor showed a linearity with glucose concentration in the range of 1.0 to 1,800 μM with a detection limit of 0.3 μM (S/N = 3). The results demonstrate that the developed approach provides a promising strategy to improve the sensitivity and enzyme activity of electrochemical biosensors.     

Neodymium(III) cathodic processes in molten fluorides

The electrochemical behaviour of the Nd(III)/Nd(0) system has been investigated in several molten media and more particularly in LiF-CaF₂. A preliminary study based both on thermodynamic and experimental data showed that it is not possible to observe the Nd(III)/Nd(0) system in LiF-KF and LiF-NaF melts; because the K⁺ and Na⁺ cation reduction waves hide the Nd³⁺ reduction wave. Then, the Nd(III)/Nd(0) system has been investigated at 810 °C using solutions of NdF₃ in fluoride solvents without K⁺ and Na⁺ ions, such as LiF-CaF₂, by cyclic voltammetry, chronopotentiometry and square wave voltammetry. Experimental results show that neodymium trifluoride is reduced in Nd(0) in a one-step process exchanging three electrons (Nd(III) + 3e⁻ → Nd(0)). The electrode process is shown to be diffusion controlled. Nd(III) diffusion coefficient is in the range of 1.1-1.3 × 10⁻⁵ cm² s⁻¹ at 810 °C.     

Preparation and characterisation of dipotassium trizinc hexacyanoferrate (II)

This paper summarises our findings on the properties and cation exchange behaviour of a new silica-based dipotassium trizinc hexacyanoferrate (II) (DTH) prepared under varying conditions of precipitation which possesses good ion-exchange properties. The product is characterised by i.r. spectra, thermal decomposition, X-ray diffractogram and its ion-exchange properties have been studied under different conditions. The separation of Rb⁺ and Cs⁺ is described.     

Layer by layer assembly of glucose oxidase and thiourea onto glassy carbon electrode: Fabrication of glucose biosensor

For the first time a novel, simple and facile approach is described to construct highly stable glucose oxidase (GOx) multilayer onto glassy carbon (GC) electrode using thiourea (TU) as a covalent attachment cross-linker. The layer by layer (LBL) attachment process was confirmed by cyclic voltammetry, electrochemical impedance spectroscopy and Fourier transform infrared reflection spectroscopy (FT-IR-RS) techniques. Immobilized GOx shows excellent electrocatalytic activity toward glucose oxidation using ferrocenemethanol as artificial electron transfer mediator and biosensor response was directly correlated to the number of bilayers. The surface coverage of active GOx per bilayer, heterogeneous electron transfer rate constant (k_s) and Michaelis–Menten constant (K_M), of immobilized GOx were 1.50 × 10⁻¹² mol cm⁻², 9.2 ± 0.5 s⁻¹ and 3.42(±0.2) mM, respectively. The biosensor constructed with four-bilayers of TU/GOx showed good stability, high reproducibility, long life-time, fast amperometric response (5 s) with the high sensitivity of 5.73 μA mM⁻¹ cm⁻² and low detection limit of 6 μM at concentration range up to 5.5 mM.     

Mechanistic studies of ruthenium(III)-catalyzed oxidation of DL-methionine by hexacyanoferrate(III) in an alkaline medium

The kinetics and mechanism of ruthenium(III) catalyzed oxidation of dl-methionine by alkaline hexacyanoferrate(III) (HCF(III)) in an alkaline medium were studied spectrophotometrically at 30±0.1°C. The reaction was first-order-dependent each on [HCF(III)] and [ruthenium(III)] and fractional-order-dependent on [alkali]. The rate of the reaction was found to be decreased with the increase in [methionine]. The main product of oxidation was methionine sulfone nitrile (3-(methylsulfonyl)propanenitrile) and it was identified and confirmed by FT-IR and mass spectral studies. Further, no effect of added reaction product was observed. A plausible mechanism was proposed involving complexation between methionine and ruthenium(III) species, [Ru(H₂O)₅OH]²⁺. Thermodynamic parameters for the reaction, E _a and Δ S ^#, were computed using linear least squares method and are found to be 65.83±1.03 kJ/mol and?249.58±3.35 J/K mol, respectively.

     

Studies on an araldite-based membrane of copper hexacyanoferrate (III) as a caesium ion-sensitive electrode

Solid membranes of copper hexacyanoferrate (III) in araldite are evaluated as a caesium ion-sensitive electrode. The electrode can be used for caesium determination in the concentration range of 10⁻¹ to 10-⁴M . The potentials generated across the membrane are reproducible and steady potentials are attained in about 1 to 2min. The same electrode can be used over a period of 6 months without significant change in potential. The electrode can be used in the pH ranges 2.5–6.0 at 10⁻² M Cs⁺ and 3.0–6.0 at 10⁻³ M CS⁺, and in presence of a number of interfering ions. Potentiometric titration of caesium nitrate with 12-moIybdophosphoric acid was also carried out using the membrane as an end point indicator.     

Catalytic reduction of hydrogen peroxide at metal hexacyanoferrate composite electrodes and applications in enzymatic analysis

The electrocatalytic activity of various metal hexacyanoferrates (Mhcfs) (i) immobilized on graphite electrodes, and (ii) as components of a composite electrode was investigated with respect to the reduction of hydrogen peroxide. The flow-through working electrode was a thin layer consisting of a composite of Mhcf, graphite, and polymethylmetacrylate (PMMA) as a binder, sandwiched between two Plexiglas plates. Among the pure Mhcfs immobilized on a graphite electrode, iron(III) hexacyanoferrate (Prussian blue) exhibits the highest electrocatalytic effect, whereas in the composite electrodes chromium(III) hexacyanoferrate (Crhcf) shows the highest activity and best performance and reproducibility for the electrochemical reduction of H₂O₂. The Crhcf electrode provides a linear dependence on H₂O₂ concentration in the range 2.5 × 10⁻⁶ mol L⁻¹ (LOD) to 1 × 10⁻⁴ mol L⁻¹ (phosphate buffer, pH 7). The sensor was applied for the detection of H₂O₂ enzymatically produced by glucose oxidase. The optimal conditions for the peroxide injection were 2 min after the beginning of the reaction and 25 °C with a detection limit of 7.0 × 10⁻⁶ mol L⁻¹ for glucose.     

Study of the biosensor based on platinum nanoparticles supported on carbon nanotubes and sugar–lectin biospecific interactions for the determination of glucose

Highly sensitive electrochemical platform based on Pt nanoparticles supported on carbon nanotubes (Pt_nano-CNTs) and sugar–lectin biospecific interactions is developed for the direct electrochemistry of glucose oxidase (GOD). Firstly, Pt_nano-CNTs nanocomposites were prepared in the presence of carbon nanotubes (CNTs), and then the mixture was cast on a glassy carbon electrode (GCE) using chitosan as a binder. Thereafter, concanavalin A (Con A) was adsorbed onto the precursor film by the electrostatic force between positively charged chitosan and the negatively charged Con A. Finally, the multilayers of Con A/GOD films were prepared based on biospecific affinity of Con A and GOD via layer-by-layer (LBL) self-assembly technique. The electrochemical behavior of the sensor was studied using cyclic voltammetry and chronoamperometry. The electrochemical parameters of GOD in the film were calculated with the results of the electron transfer coefficient (α) and the apparent heterogeneous electron transfer rate constant (k_s) as 0.5 and 5.093 s⁻¹, respectively. Experimental results show that the biosensor responded linearly to glucose in the range from 1.2 × 10⁻⁶ to 2.0 × 10⁻³ M, with a detection limit of 4.0 × 10⁻⁷ M under optimized conditions.     

Novel amperometric glucose biosensor based on an ether-linked cobalt(II) phthalocyanine-cobalt(II) tetraphenylporphyrin pentamer as a redox mediator

The development of cobalt(II) phthalocyanine-cobalt(II) tetra(5-phenoxy-10,15,20-triphenylporphyrin), (CoPc-(CoTPP)₄) pentamer as a novel redox mediator for amperometric enzyme electrode sensitive to glucose is described. A glassy carbon electrode (GCE) was first modified with the pentamer, then followed by the immobilization onto the GCE-CoPc-(CoTPP)₄ with glucose oxidase (GOx) through cross-linking with glutaraldehyde in the presence of bovine serum albumin (BSA) and Nafion^® cation-exchange polymer. The proposed biosensor displayed good amperometric respose charateristics to glucose in pH 7.0 PBS solution; such as low overpotentials (+400 mV versus Ag|AgCl), very fast amperometric response time (∼5 s), linear concentration range extended up to 11 mM, with 10 μM detection limit. The biosensor exhibited electrochemical Michaelis-Menten kinetics and showed an average apparent Michaelis-Menten constant (K^′M) of 14.91 ± 0.46 mM over a storage period of 2 weeks.     

A hybrid photocatalytic and enzymatic process using glucose oxidase immobilized on TiO2/polyurethane for removal of a dye

A rectangular recycling photo-bioreactor using glucose oxidase (GOx) immobilized on TiO₂/polyurethane (PU) was developed as a novel coupling of photodegradation and enzymatic process. This method was tested for removal of Acid Orange 7 (AO7), as a model pollutant. High efficiency of decolorization (>99%) was achieved after 22 min using the GOx/TiO₂/PU photo-biocatalyst. Roles of various processes including photodegradation (TiO₂/PU), enzymatic process (GOx/PU) and a coupling of photocatalytic–enzymatic (GOx/TiO₂/PU) process were investigated in the presence and absence of UV light. All the experiments were performed in a circulation photoreactor equipped with a 6 W UV lamp with rate of 5 mL/min.     

Dual oxygen and Ir oxide regeneration of glucose oxidase in nanostructured thin film glucose sensors

A nanoparticulate iridium oxide (IrOx) thin film has been developed as a redox-active matrix material for an advanced generation glucose biosensor, in which IrOx serves as the non-physiological mediator, replacing oxygen in the enzymatic re-oxidation of glucose oxidase (GOx). Ethanolic solutions of Nafion and an Ir sol were mixed with an aqueous GOx solution and then deposited on a Au support. The Ir nanoparticles were then oxidized electrochemically to IrOx and the resulting films (IrOx-GOx-Nafion) were tested for their glucose response in both oxygen- and argon-saturated solutions, with the oxygen content in both solutions monitored by a Pt electrode. The sensors that are regenerated largely by O₂ are characterized by a Michaelis-Menten K^′m value of ∼30 mM or more and i_max values of at least 20 μA cm⁻². Under fully deareated conditions, the sensors lose only ∼50% of their response to glucose, clearly indicating that a dual oxygen-regeneration and IrOx mediation mechanism is operative for the biosensor under these conditions. Under optimized conditions, involving a controlled GOx:Ir ratio, only the Ir oxide sites in the film serve to mediate GOx regeneration, giving K^′m (10-15 mM) and i_max values that are independent of the O₂ content of the solution.     

Amperometric glucose biosensor based on immobilization of glucose oxidase in electropolymerized o-aminophenol film at Prussian blue-modified platinum electrode

An amperometric glucose biosensor is developed, based on immobilization of glucose oxidase (GO_X) in an electrochemically polymerized, non-conducting poly(o-aminophenol) (POAP) film at Prussian blue (PB)-modified platinum (Pt) microelectrode. Effects of polymerization cycle number for POAP and PB, applied potential used in the determination, pH value of the detection solution and electroactive compounds on the amperometric response of the sensor were investigated and discussed. The electroactive property and rough surface of PB film result in the improvement of the detection limit and the increase of the maximum response current and sensitivity. The biosensor based on Pt/PB/POAP/GO_X electrode has two times lower detection limit, five times larger maximum current and nine times higher sensitivity than those of the biosensor based on Pt/POAP/GO_X electrode. Additionally, the biosensor shows fast response time, large response current, and good anti-interferent ability for l-ascorbic acid, uric acid and acetaminophen. Excellent reproducibility and stability of biosensor are also observed.     

Electrochemical reduction of Gd(III) and Nd(III) on reactive cathode material in molten fluoride media

The electrochemical reduction of two lanthanides (neodymium Nd and gadolinium Gd) was investigated in the 800–950 °C temperature range on nickel and copper electrodes. These materials react with lanthanides (Ln) to form intermetallic compounds. The formation mechanism of the alloys was determined by coupling electrochemical techniques and Scanning Electron Microscopy (SEM) after electrolyses runs; this also allowed the identification of the binary compounds formed. In addition, from the electrochemical results, we calculated the Gibbs energies of Nd/Ni, Gd/Ni, Gd/Cu and Nd/Cu.     

Analysis of impedance measurements performed on the platinum hexacyanoferrate (II) and (III) system in phosphate media

Impedance measurements of Fe(CN)³⁻₆ and Fe(CN)⁴⁻₆ solutions were taken in a potentiostatic mode in phosphate media at pH 7.2 in contact with a polished platinum electrode. Complementary data required to demonstrate the autocoherence of the results were deduced from thin layer spectroelectrochemistry measurements and from experiments carried out on a rotating disc electrode. The standard heterogeneous transfer rate constant was 0.1 cm s⁻¹ and decreased, in the presence of oxygen, when the contact time between the electrode and the solution increased. In addition, we confirm the accuracy of the impedance method for the characterization of an electrode surface before an experiment.     

Mechanistic investigation of the oxidation of Cefuroxime by hexacyanoferrate(III) in alkaline conditions

The reactions of Cefuroxime (CFA) by hexacyanoferrate (III) (HCF(III)) in alkaline medium at a constant ionic strength has been studied spectrophotometrically. It is a first order reaction, but fractional order in both CFA and alkali. Decrease in dielectric constant of the medium decreases the rate of reaction. The effect of added products and ionic strength has also been investigated. A mechanism involving free radicals is proposed. In a composite equilibrium step, CFA binds to HCF(III) to form a complex that subsequently decomposes to the products. The main two products were separated and identified by column chromatography, TLC and FT-IR. There is good agreement between the observed and calculated rate constants under different experimental conditions. The reaction was studied at different temperatures and activation parameters were computed with respect to the slow step of the proposed mechanism.     

A new metal chelate sorbent for glucose oxidase: Cu(II)- and Co(II)-chelated poly(N-vinylimidazole) gels

Poly(N-vinylimidazole) (PVIm) gels were prepared by irradiating a binary mixture of N-vinylimidazole (VIm)–water in a ⁶⁰Co-γ source having 4.5 kGy/h dose rate. In the glucose oxidase (GOx) adsorption studies, affinity gels with a swelling ratio of 1100% for PVIm and 40 and 55% for Cu(II)- and Co(II)-chelated PVIm gels, respectively, at pH 6.5 in phosphate buffer were used. FTIR spectra were taken for PVIm and Cu(II)- and Co(II)-chelated PVIm, and glucose oxidase adsorption on these gels, to characterize the nature of the interactions in each species. The results show that PVIm–glucose oxidase interaction is mainly electrostatic and metal ion–chelated PVIm gel–glucose oxidase interaction is of coordinate covalent nature. Cu(II) and Co(II) ions were chelated within the gels via amine groups on the imidazole ring of the gel. Different amounts of Cu(II) and Co(II) ions [maximum 3.64 mmol/g dry gel for Cu(II) and 1.72 mmol/g dry gel for Co(II)] were loaded on the gels by changing the initial concentration of Cu(II) and Co(II) ions at pH 7.0. GOx adsorption on these gels from aqueous solutions containing different amounts of GOx at different pH was investigated in batch reactors. GOx adsorption capacity was further increased when Cu(II) and Co(II) ions were attached [up to 0.53 g GOx/g dry Co(II)-chelated PVIm gels]. More than 90% of the adsorbed GOx was desorbed in 5 h in desorption medium containing 1.0M KSCN at pH 7.0 for plain gel and 0.05M EDTA at pH 4.9 for metal-chelated gel. Nonspecific glucose oxidase adsorption on/in the metal ion–chelated PVIm gel was investigated using 0.02M of phosphate buffer solution. The nonspecific GOx adsorption was determined to be about 18% for PVIm and 8% for the metal ion–chelated PVIm gels. The ionic strength effect was investigated both on PVIm and on the metal ion–chelated PVIm gels for the glucose oxidase adsorption. It was found that ionic strength was more effective on the PVIm gel because of the electrostatic interaction between protonated gel and the deprotonated glucose oxidase side chain. © 2001 John Wiley & Sons, Inc. J Appl Polym Sci 82: 446–453, 2001     

Spectroscopic Study of Neodymium(III) in Sodium Tellurite Glass

The local environment of Nd-O in a sodium tellurite glass was elucidated as a function of Nd₂O₃ concentration, from 0.1 to 2.5 mol%, using optical spectroscopy. The Judd-Ofelt (J-O) theory was used to determine the oscillator strength parameters (J-O parameters) of the glasses. According to the J-O parameters, a significant change in the local environment of Nd-O was suggested for the glass containing 1 mol% Nd₂O₃; the asymmetry of the Nd-O crystal field was shown to be at a maximum and the bond covalency at a minimum. The results were further supported by a significant shift of the structural hypersensitive band of the glass with 1 mol% Nd₂O₃, as compared with those of the other glasses.     

Bioelectrochemistry and enzymatic activity of glucose oxidase immobilized onto the bamboo-shaped CNx nanotubes

The novel bamboo-shaped CN_x nanotubes, synthesized by nitrogen atoms doping into carbon nanotubes, were used for the immobilization of a relatively large enzyme glucose oxidase (GOx) and its bioelectrochemical studies. The morphologies and adsorptions of GOx immobilization onto CN_x nanotubes were clearly observed by transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM). Electrochemical impedance spectroscopy (EIS) was also used to feature the GOx adsorbed onto the surface of CN_x nanotubes. The immobilized GOx incorporated into CN_x nanotubes films exhibited a well-defined nearly reversible cyclic voltammetric peaks for the electroactive centers of GOx and a fast heterogeneous electron transfer rate with the rate constant (K_s) of 1.96 s⁻¹. The immobilized GOx onto the CN_x nanotubes exhibited its bioelectrocatalytic activity for the oxidation of glucose. The obtained results suggest that with a large amount of defective/active sites on the tube surfaces, a special bamboo structure and a suitable C-N microenvironment introduced by nitrogen doping, CN_x nanotubes could not only facilitate the direct electron transfer between the enzyme and electrode, but also retain the high enzyme loading and the enzymatic bioactivity.     

In situ growth of copper nanoparticles on multiwalled carbon nanotubes and their application as non-enzymatic glucose sensor materials

Cu-nanoparticles were coated on the sidewall of multiwalled carbon nanotubes (MWCNTs) by a facile and effective in situ approach via the template of a polyelectrolyte (polyethylenimine or poly(sodium 4-styrene sulfonate)) noncovalently functionalized on MWCNTs. Extensive characterizations of the fabricated nanocomposites have been studied using X-ray diffraction, transmission electron microscopy, selected area electron diffraction, X-ray photoelectron spectroscopy, thermal gravimetric analysis and inductively coupled plasma. The results demonstrate that Cu-nanoparticles were well distributed on the surface of MWCNTs. The nanocomposites can be easily modified on the glassy carbon electrodes due to the presence of polyelectrolyte. The electrocatalytic activity of the modified electrodes towards glucose oxidation was investigated by cyclic voltammetry and chronoamperometry. The nanocomposites showed good non-enzymatic electrocatalytic responses to glucose in alkaline media, and can be used for the development of enzyme-free glucose sensors.