Deflagration to detonation transition in fast flames and tracking with chemical explosive mode analysis

In the context of vapour cloud explosion, the flame acceleration process can lead to conditions promoting deflagration to detonation transition (DDT), potentially leading to increased damages in accidental scenarios. This study focuses on this phenomenon by performing simulations of detonation reinitiation for fast flames in the Chapman–Jouguet deflagration regime. It is obtained experimentally by the attenuation of an incident detonation by an array of obstacles. A primary objective of the paper is to demonstrate the ability of the numerical model to reproduce the major experimental trends, namely the variation of the reinitiation propensity for different initial pressures and blockage ratios (BRs). Chemical explosive mode analysis (CEMA) is also adapted to the context of this study, in order to identify locally the propagation regime and to provide insights on the reinitiation mechanism. An a priori validation of the CEMA methodology is first performed on relevant canonical one-dimensional configurations. Subsequently, ensembles of five realizations are computed at different initial pressures and BRs and compared to experimental data. They are shown to reproduce the major observed trends in terms of detonation reinitiation length with respect to the operating conditions, with significant variability from one realization to another. In addition, the reinitiation mechanism is also found to be consistent with experimental observations and a previous numerical study of the same configuration. The CEMA methodology adapted to this context is able to identify locally the different propagation regimes, and to track the highly reactive zones that coherently couple with transverse pressure perturbations, leading to the formation of a strongly reacting kernel which eventually triggers the detonation reinitiation.     

Molecularly Imprinted Polymers Combined with Electrochemical Sensors for Food Contaminants Analysis

Detection of relevant contaminants using screening approaches is a key issue to ensure food safety and respect for the regulatory limits established. Electrochemical sensors present several advantages such as rapidity; ease of use; possibility of on-site analysis and low cost. The lack of selectivity for electrochemical sensors working in complex samples as food may be overcome by coupling them with molecularly imprinted polymers (MIPs). MIPs are synthetic materials that mimic biological receptors and are produced by the polymerization of functional monomers in presence of a target analyte. This paper critically reviews and discusses the recent progress in MIP-based electrochemical sensors for food safety. A brief introduction on MIPs and electrochemical sensors is given; followed by a discussion of the recent achievements for various MIPs-based electrochemical sensors for food contaminants analysis. Both electropolymerization and chemical synthesis of MIP-based electrochemical sensing are discussed as well as the relevant applications of MIPs used in sample preparation and then coupled to electrochemical analysis. Future perspectives and challenges have been eventually given.     

A novel carbon/chitosan paste electrode for electrochemical detection of normetanephrine in the urine

Normetanephrine is a marker for pheochromocytoma, a rare catecholamine-secreting and neuroendocrine tumor, that arises from sympathetic and parasympathetic paraganglia. In this work, a novel carbon/chitosan electrode paste was used for sensitive voltammetric determination of normetanephrine and dopamine in the presence of ascorbic acid and uric acid. The modified electrode has shown an increase in the effective area of up to 68%, well-separated oxidation peaks, and an excellent electrocatalytic activity. The electrochemical response characteristics were investigated by cyclic and differential pulse voltammetry. Interestingly, high sensitivity and selectivity in the linear range of normetanephrine, dopamine, ascorbic acid, and uric acid concentrations were observed. The present method was applied in the urine sample and satisfactory results were obtained showing that this electrode is very suitable in pharmaceutical and clinical preparations.     

Cyclic voltammetric responses of horseradish peroxidase multilayers on electrodes

The catalytic responses obtained with step-by-step neutravidin-biotin deposition of successive monolayers of HRP are analyzed by means of cyclic voltammetry. The theoretical tools that have been developed allowed full characterization of the multilayered HRP coatings by means of a combination between closed-form analysis of limiting behaviors and finite difference numerical computations. An analysis of the experiments in which the number of monolayers was extended to 16 allowed an approximate determination of the average thickness of each monolayer, pointing to a compact arrangement of neutravidin and biotinylated HRP. The piling up of so many monolayers on the electrode allowed an improvement of the catalytic current by a factor of ca. 10, leading to very good sensitivities in term of cosubstrate detection.     

Cyclic voltammetric studies of the antipsychotic chlorpromazine using an alkylthiol/phospholipid-modified gold electrode

Self-assembled monolayers of alkanethiols on gold have been reported to be highly stable for voltammetry experiments in aqueous electrolyte. In this work a gold electrode has been modified by first depositing one layer of an alkylthiol (S-C₁₈) and then coating by phospholipid multilayers. Voltammetric oxidation of the antipsychotic chlorpromazine at this two-step modified electrode was followed by means of cyclic voltammetry measurements. The results give important information concerning the behaviour of the pharmacological agent at the lipid-water interface. Measurements made using the pre-concentration method allow good sensitivity improvement after 5 min accumulation time. The ability of chlorpromazine to penetrate inside the phospholipid multilayers has also been investigated under different conditions such as the nature of the phospholipid and the pH of the medium. The accumulation process seems to be closely related to the charge carried by the phospholipid and by the molecule, while the incorporation process seems to be independent of the charge carried by the phospholipid and dependent of the degree of fluidity of their hydrocarbon chains. We found through this work that the acid-base equilibrium of chlorpromazine together with its amphiphilic properties (as compared with the results of similar studies on phenothiazine) could be responsible for governing the principal aspects of the drug's behaviour toward biological membranes. Received: 6 January 1997 / Accepted: 27 February 1997     

A Carbon Paste Electrode Modified by Bentonite and l‐Cysteine for Simultaneous Determination of Ascorbic and Uric Acids: Application in Biological Fluids

A novel modification of a paste carbon electrode by Bentonite (Bent) and l‐Cysteine (l‐Cyst) was carried out for uric acid (UA) and ascorbic acid (AA) detection and quantification. Morphological and compositional characterization of the electrode surface were carried out using electrochemical impedance spectroscopy (EIS), scanning electron microscopy (SEM) and energy dispersive X‐ray spectroscopic analysis (EDS). Cyclic voltammetry (CV) and square wave voltammetry (SWV) techniques were used to analyze UA and AA. The obtained sensor shows a good stability, sensibility, selectivity, and regeneration ability. Accordingly, the limit of detection (LOD) is found to be 0.031 μm and 9.6 μm for UA and AA, respectively. A good linearity in the range of 0.1 to 100 μm for UA and 10 to 1000 μm for AA was obtained. The peak‐to‐peak separation of UA‐AA (ΔE _UA‐AA) was determined to be 330 mV. In addition, the sensor is applied successfully to monitor UA and AA in serum samples.     

Quantitative analysis of catalysis and inhibition at horseradish peroxidase monolayers immobilized on an electrode surface

Out of several tries, biotinylation of the electrode surface by means of a sacrificial biotinylated immunoglobulin, followed by the anchoring of an avidin-enzyme conjugate appears as the best procedure for depositing a horseradish peroxidase (HRP) monolayer onto an electrode surface, allowing a high-yield immobilization of the enzyme within a stable and highly catalytic coating. Cyclic voltammetry is an efficient means for analyzing the catalytic reduction of H(2)O(2) at such HRP monolayer electrodes in the presence of [Os(III)(bpy)(2)pyCl](2+) (with bpy = bipyridine and py = pyridine) as a one-electron reversible cosubstrate. The odd shapes of current-potential responses, unusual bell-shaped variation of the peak or plateau current with the substrate concentration, hysteresis and trace crossing phenomena, and dependence or lack of dependence with the scan rate, can all be explained and quantitatively analyzed in the framework of the same catalysis/inhibition mechanism as previously demonstrated for homogeneous systems, taking substrate and cosubstrate mass transport of into account. According to H(2)O(2) concentration, limiting-behavior analyses based on the dominant factors or complete numerical simulation were used in the treatment of experimental data. The kinetic characteristics derived from these quantitative treatments implemented by the determination of the amount of enzyme deposited by the newly developed droplet depletion method allowed a comparison with homogeneous characteristics to be drawn. It shows that HRP remains nearly fully active once anchored on the electrode surface through the avidin-biotin linkage. On the basis of this full mechanistic and kinetic characterization, the analytical performances in H(2)O(2) detection and amperometric immunosensor applications are finally discussed.     

Copper‐Catalyzed Arylation of Nitrogen Heterocycles from Anilines under Ligand‐Free Conditions

The arylation of pyrazole and derivatives can be achieved by coupling arenediazonium species (formed in situ from anilines) by using a catalytic system that employs low‐toxicity and inexpensive copper metal under very mild and ligand‐free conditions (T=20 °C). From other nitrogen heterocycles, the presence of an additive (NBu₄I) significantly improves the efficiency of the catalytic system. These results represent the first examples of C?N bond formation from arenediazonium species.     

Time scale analysis of the homogeneous flame inhibition by alkali metals

A time scale analysis of the homogeneous flame inhibition problem is carried out to identify the main parameters controlling the gas phase chemical interaction of the alkali metal inhibitors with the flame chemistry. First, kinetic sub-models for the interaction of alkali metals with the flame are analyzed to show that a simplified 2-step inhibition cycle can capture the essential features of this interaction. Second, it is shown that this cycle is auto-catalytic, which explains the high efficiency of alkali metals in inhibiting flames even at low concentrations. Third, the time scales associated to this inhibition cycle are linked to the free flame termination time scale via a non-dimensional parameter characterizing the efficiency of an inhibitor at promoting radical scavenging. It is shown that this parameter accounts for the main trends observed in the literature and can also be used to provide estimates for the chemical flame suppression limit.