A reliable l-lactate electrode with a new membrane for enzyme immobilization for amperometric assay of lactate

Lactate oxidase from Pediococcus species can be rapidly and simply immobilized on a commercially available pre-activated membrane fixed to an amperometric probe detecting hydrogen peroxide, to provide a very sensitive practical l-lactate sensor. At 25°C, in a phosphate buffer pH 7.1, the detection limit is 1.25 × 10^?7 M, calibration is linear between 2.5 × 10^?7 and 2.5 × 10^?4 M, the response time is <2 min, and the probe can be used for hundreds of assays over several weeks. With a microprocessor-based instrument including the same type of electrode, sample injections can be made at 90-s intervals, the response being displayed after only 30 s. High selectivity is achieved because the differential measurement system continuously subtracts currents at the chosen potential arising from the presence of electroactive species. Samples (20 μl) from sera and dairy products were successfully tested without pretreatment; a relative standard deviation between 1 and 3% was routinely obtained. Correlation of the data with data obtained by the conventional spectrophotometric method was excellent.     

Bioluminescence-based fibre-optic sensor with entrapped co-reactant: an approach for designing a self-contained biosensor

The possibility of designing a self-contained fibre-optic biosensor, i.e., that can be used without renewing reagents in the probe, was investigated. A probe specific for NADH involves immobilized bioluminescent enzymes which require the presence of two co-reactants [a flavin substrate (FMN) and an aldehyde] to catalyse light emission. The FMN was non-covalently immobilized in a synthetic film and was internally released in the vicinity of the bound enzymes, at the sensing tip of the bioprobe. Release of FMN was achieved from the two different matrices tested: a collagen film and a poly(vinyl alcohol) (PVA) network. Continuous-flow assays of NADH could be performed over a linear dynamic range from 10 pmol to 1 nmol. A PVA matrix appears to be a promising support for designing a self-contained biosensor, as 30–35 reliable measurements (R.S.D.=5%) could be achieved without a decrease in the sensor signal, compared with only 10–15 assays with a collagen film.     

Direct biosensor coating with a photopolymerised microlayer through a controllable step-procedure

Biosensors are based on the intimate association of a transducer and of a sensing layer. The latter can be a preformed membrane further connected to the transducer, or a thin film directly deposited on its surface. As the stability is a key parameter to be considered, a polymer with high potentialities for this purpose was chosen to form a direct surface coating: a poly(vinyl alcohol) bearing styrylpyridinium groups (PVA-SbQ). Choline oxidase was entrapped in this photo-cross-linkable gel for making an enzyme electrode through controllable steps. The influence of the ratio PVA-SbQ/enzyme was studied and the stability of the resulting modified electrodes was determined. After deposition of minute volumes (10–45 l) including no more than one unit of choline oxidase and 0.3 mg of polymer, an efficient choline sensor was obtained. It exhibited a fast response time lower than 30 s, a low detection limit of 2.5 nM choline, a wide linear range extending up toca. 10^–4 M and good stability, both operational and on storage. This method appears promising for making miniaturized biosensors.     

Enthalpie de formation de la whitlockite Ca18Mg2H2(PO4)14

Pure and well crystallised whitlockite Ca₁₈Mg₂H₂(PO₄)₁₄ has been synthesized by precipitation from the magnesium and calcium nitrates and the diammonic phosphate. The product of the reaction has been characterized by X-ray diffraction, IR spectroscopy and chemical analysis. Using differential conduction calorimeters the enthalpies of solution of the whitlockite and of a mixture of the solid reactants - the tricalcium, the trimagnesium and the dicalcium phosphates - have been measured at 25°C for various concentrations of solid in a 46 wt% nitric acid solution. A combination of the enthalpies of solution with the enthalpies of formation of the reactants allows us to determine the standard enthalpy of formation of the whitlockite. The value deduced, -27,93·10³kJ mol^-1, is compared to the standard enthalpies of formation of the trimagnesium and the tricalcium phosphates. This revised version was published online in August 2006 with corrections to the Cover Date.     

Small angle x-ray scattering of a supercritical electrolyte solution: the effect of density fluctuations on the hydration of ions

Synchrotron small angle x-ray scattering measurements on water and zinc bromide ZnBr2 aqueous solutions were carried out from ambient to supercritical conditions. For both systems several isobars (between 285 and 600 bars) were followed beyond the critical isochore. The data were analyzed through an Ornstein-Zernike formalism in terms of correlation length and null angle structure factor. The results for pure water are in agreement with previously published values. Solutions of different electrolyte concentrations were studied. In each case, the values of the correlation length and null angle structure factor are larger than those of pure water. This effect is more pronounced for higher concentrations and/or for pressure closer to the critical point of pure water. This is in agreement with the shift of the critical point determined in the literature for NaCl solutions. Comparing these results to previous x-ray absorption measurements carried out on identical samples we propose the following two step sequence for ionic hydration up to supercritical conditions: (1) from ambient to about 300 degrees C, an increase of ion pairing and formation of multi-ionic complexes which can be correlated to the decrease of the dielectric constant; (2) an enhancement of the local solvation shell of ions due to the onset of the thermal density fluctuations at high temperature, leading to a screening effect between ions and inhibiting the ion pairing processes.     

Multi-function fibre-optic sensor for the bioluminescent flow determination of ATP or NADH

A multi-function biosensor for the determination of either ATP or NADH using a single bioluminescence-based fibre-optic probe is described. This was made possible by co-immobilizing the firefly luciferase from Photinus pyralis for ATP analysis with the bacterial/oxidoreductase system from Vibrio harveyi for NADH analysis, on the same preactivated polyamide membrane. Compatible analytical conditions with regard to the activity and stability of each bioluminescent system were selected, enabling them to attain their highest performances. It was possible to perform continuous-flow measurements of ATP and NADH over a wide (log-log) linear calibration range with a relative standard deviation of 4.0–4.5% and detection limits of 0.25 pmol ATP and 5 pmol NADH.     

Bioluminescent determination of reduced nicotinamide adenine dinucleotide with immobilized bacterial luciferase and flavin mononucleotide oxidoreductase on collagen film

Bacterial luciferase flavin mononucleotide oxidoreductase were co-immobilized on collagen strips. Reduced nicotinamide adenine dinucletodie was determined in the range 1 X 10^?9?2 X 10^?5 M, with a precision of 5%. The immobilized system retained 70% of its initial activity after two weeks.     

Sensitive biosensor for choline and acetylcholine involving fast immobilization of a bienzyme system on a disposable membrane

A biosensor was prepared for the determination of choline or acetylcholine by co-immobilizing choline oxidase and cholinesterase on a chemically preactivated membrane ready for use. This rapid procedure allows the coupling to be performed in a few minutes. The determination is based on the electrochemical detection of enzymatically generated hydrogen peroxide. This sensor has a detection limit of 5 × 10^?8 M. The response was obtained in 2 min and was linear up to 2 × 10^?5 M.