Nabil El Murr

Viability of Saccharomyces cerevisiae cells exposed to low-amperage electrolysis as assessed by staining procedure and ATP content.

Assessment of yeast viability by plate counts, ATP determination and FUN-1 viability staining was performed to study sublethal injury of Saccharomyces cerevisiae cells submitted to low-amperage electrolysis. Lethal effects of electrolysis were confirmed by all methods, demonstrated by the decrease in viable counts observed during electrolysis. FUN-1 viability staining and ATP determination appeared to demonstrate higher survivors than plate counts. To study possible recovery of certain yeast cells damaged by electrolysis thus rendering them nonculturable, yeast suspensions were stored in phosphate buffer at 4 and 20 degrees C. Increase in viable counts and ATP content of treated yeast cells was observed during storage at 20 degrees C, whereas viable counts of treated and control yeast cells were shown to decrease during storage at 4 degrees C. The increase in the number of viable cells appeared to be the result of repair of damaged cells rather than regrowth of few cells remaining culturable. The lethal efficacy of electrolysis might be overestimated by plate counts. Further experiments must be done to evaluate the lethal efficacy of electrolysis on microorganisms in real conditions encountered in food products.

Electrochemical Behavior of Cu(II) on Carbon Paste Electrode Modified by Humic Acid, Cyclic Voltammetry Study

Polarography at dropping mercury electrode and infrared spectroscopy were used to confirm the reaction of copper(II) ions with humic acid in solution. Polarography allowed observing the reaction to follow the concentration of the Cu(II) in solution while infrared permitted one to follow the humic acid complexation. Cyclic voltammetry experiments, at stationary carbon paste electrode, showed original properties of humic acid which adsorbs at the surface of carbon paste electrode that becomes appropriate for the preconcentration and identification of Cu(II) species. Although very promising the use of adsorption showed the disadvantage of not being reproducible therefore we developed a new preparation method for the carbon paste electrodes modified with humic acid that allow highly repeatable measurements. Humic acid was dissolved in base solution before mixing it with carbon graphite powder and freeze-drying to lead to a fine powder that allows reproducible batches of electrodes. The modification of carbon paste electrode with two different humic acids revealed that the electrochemical behavior of Cu(II) solutions depends on the nature of the humic material. The effects on the electrochemical behavior of Cu(II) of the solutions pH and the ratio of humic acid in the bulk of the electrode were also examined. Such carbon paste electrodes modified with humic acid extracted from a peat area in the Mogi GuaAu River at Sao Paulo State in Brazil showed high reproducibility and particular stability in solution (pH range of 3.0 to 5.5) entitling them to be very good candidates for analytical purposes.

Development of renewable surface biosensors to meet industrial needs for measurement of glucose in fruit juices.

Most biosensors reported to date have been prepared, studied and used under laboratory conditions. The feasibility of a very great number of biosensors seems to be demonstrated and their characteristics, very often, established as corresponding to the demands of the modern analysis. The operational stability of the biosensors, according to authors, is almost always acceptable. The long term storage, with analytical quality conservation that is necessary to commercialise products, has rarely been studied. The stability of biosensors has to remain not only during the fabrication step or their subsequent utilisation, but also throughout the whole commercial shelf-life of the sensor, from producer to end user, through wholesaler and/or retailer. We developed the manufacturing processes, on a large scale, of renewable surface electrodes modified with enzymes such as oxidoreductases. The process consisting of several steps is described and the analytical behaviours of resulting biosensors is studied and correlated with the effects of different constraints applied during the fabrication process.

Use of Ferricinium Exchanged Zeolite for Mediator Stabilization and Analytical Performances Enhancement of Oxidase-Based Carbon Paste Biosensors

Abstract The carbon paste design allows bulk modification of the entire electrode material with the bio-component and the incorporation of a variety of different additives such as enzyme stabilizers, mediators, activators, or other compounds that can increase the stability, amplitude, and selectivity of the response signal. The incorporation of heterogeneous particles such zeolites in the modified carbon paste electrodes might engender an improvement of their analytical performances. We show here that the incorporation of zeolite, already exchanged with the cationic form of the redox mediator (YFcH), within GOx carbon paste bioelectrodes improves the analytical performances of the mediated glucose biosensor. Not only the inclusion of the redox mediator into the zeolite particles permits to extend the linear range and to enhance significantly the operational and storage stabilities, but unexpectedly it permits to speed up the response time. †To Pierre Coulet: Thank you for showing us that honesty is essential for stable and ever-lasting friendships.

Sensitive and reproducible quantification of Cu2+ by stripping with a carbon paste electrode modified with humic acid

Abstract The preparation of a humic acid modified carbon paste electrode (HA-MCPE) as well as the behavior of its surface as complexing agent toward Cu2+ cations are described. Electrochemical studies of the reduction of the complexed cations and of the anodic stripping oxidation of the resulting copper are outlined. The anodic stripping current was correlated to the Cu2+ concentrations. A well-defined method for the preparation of reproducible electrodes is described. The effects on the current response obtained by cyclic voltammetry of the humic acid ratio, the pH, the accumulation time, and the speed scan rate were studied. Calibration graphs were linear over the range 3 × 10−8 − 10−5 mol L−1 Cu2+ and the relative standard deviation (R.S.D.) was 1.2% (n = 5) for [Cu2+] = 1.6 × 10−5 mol L−1. 5 min accumulation time for [Cu2+]>10−7 mol L−1 and 10 min for [Cu2+] < 10−7 mol L−1 were sufficient to permit sensitive and reproducible measurements. The electrode was successfully used to measure Cu2+ in real samples and the results were compared to those obtained by the standard method with differential pulse anodic stripping voltammetry.

“No calibration” type sensor in routine amperometric bio-sensing: An example of a disposable hydrogen peroxide biosensor

The predicted potential market size for the use of biosensors in the biomedical sector has correlated closely to the actual global market sales of biosensor devices. Unlike glucose measurement in blood, biosensors for environmental or food analysis are niche markets in which the demand varies and the sale volumes are low. Until now the use of biosensors in these markets has been limited. In contrast, spectrophotometric methods, and particularly enzyme-coupled assays, packaged in simple to use kit form, have proved successful in such fragmented markets and have become, in many cases, the recommended standard method for industrial analysis. The popularity of such kits is the result of disposable consumables, reliable detection methods and most importantly, no requirement for tedious calibration steps (if the Beer-Lambert law is applicable). Disadvantages include sample preparation, multiple and sequential addition of several reagents during the analysis process. We believe that within the environmental and agri-food industries, amperometric biosensor technology could be a valid competitor to the standard spectrophotometric methods; provided that cheap and easy to use methods are proposed. According to industrial users within these fragmented markets, the biggest obstacle for the take up of biosensor technology as a standard method is the requirement for complex pre-calibration. It seems that, to meet the demand of these markets, an essential improvement would be the removal of calibration steps before the use of the biosensor. The electrochemical signal of a large number of amperometric biosensors is dependent upon enzymatic or chemical kinetics that are reliant on operating conditions (enzyme concentrations, temperature, pH) and so necessarily requires a calibration step prior to use. Despite their excellent analytical performances this makes them unattractive for industries that want to use these analytical tools in remote locations using unskilled operators. With spectroscopic methods these calibration steps are unnecessary because, for a given wavelength and a fixed light path, the measured absorbance is directly proportional to the concentration of sample material. This is true for a concentration range for which the Beer-