Najih Naser

Amperometric biosensing of organic peroxides with peroxidase-modified electrodes

Abstract An amperometric enzyme electrode for organic peroxides, based on the incorporation of horseradish peroxidase (HRP) into a carbon paste matrix, is reported. The electrode responds rapidly to low (micromolar) concentrations of such peroxides, in accordance to the following sensitivity trend: butanone peroxide > tert-butyl peroxibenzoate > cumene hydroperoxides > triphenylmethyl hydroperoxide > tert-butyl hydroperoxide > tert-amyl perbenzoate > tertbutyl peroxiacetate. The effect of various operational parameters is explored for optimum analytical performance. The dynamic properties of this electrode are exploited for detection in flow injection systems. Work in mixed nonaqueous-aqueous media allows the quantitation of highly hydrophobic peroxides and extension of the linear range. Similar organic peroxide detection is reported for the incorporation of HRP within a rigid graphite-epoxy matrix, and for a tissue (horseradish-root) modified electrode. Applicability of the HRP electrode for assays of drinking water is illustrated and prospects for its utility in environmental work are discussed. Analogous measurements using another (fungal) peroxidase are reported.

Tissue bioelectrode for organic-phase enzymatic assays

Abstract The utility of tissue bioelectrodes to serve as effective organic-phase biosensors is demonstrated. The organic-phase operation enables a simple immobilization procedure, based on spreading a thin layer of the tissue onto a rough graphite disk. Mushroom, banana or horseradish root layers (rich with tyrosinase or horseradish peroxidase) were thus immobilized and employed for quantifying the corresponding phenolic and peroxide species in chloroform media. The thin tissue layer and its immediate proximity to the graphite surface results in a very fast and sensitive amperometric response (8–20 s for steady state, and a detection limit 3 × 10 −5 M). The influence of numerous experimental variables is explored, and future prospects are discussed.

Plant tissue-based amperometric electrode for eliminating ascorbic acid interferences

Abstract An application of tissue-based electrodes aimed at eliminating interferences from co-existing electroactive constituents is described. The concept is illustrated using a zucchini-containing carbon paste electrode. The presence of the enzyme ascorbic acid oxidase (AAO) in the zucchini tissue effectively eliminates contributions from ascorbic acid, thus allowing selective measurement of dopamine or norepinephrine. In comparison with analogous enzyme-based AAO electrodes, the tissue “eliminator” electrode offers high biocatalytic stability and activity and extremely low cost. The effects of various experimental variables are studied using pulse voltammetry, chronoamperometry and flow-injection amperometry. The electrode has a useful lifetime of 4 weeks. Simultaneous elimination of uric acid interferences is obtained via the co-immobilization of uricase. Oxygen background currents are eliminated in the presence of ascorbic acid.

Modified carbon-wax composite electrodes

Abstract Chemically-modified carbon-wax composite electrodes were fabricated by incorporating appropriate quantities of model modifiers into melted wax/graphite-powder mixtures prior to the solidification step. Such melting and solidification processes result in homogeneous dispersion of the modifier and mechanical rigidity of the surface. It also maintains the attractive features of analogous carbon paste electrodes, including low background current, flexible modifier loading, surface renewal, speed and low cost. The compatibility of different modifiers with carbon-wax composites is illustrated in connection with preconcentrating/voltammetric measurements of nickel and copper at DMG- and cation-exchanger resin containing surfaces, as well as electrocatalytic monitoring of hydrazines, oxalic acid or butanone peroxide at cobaltphthalocyanine and metalized carbon-based composites.

Electroanalysis at modified carbon-paste electrodes containing natural ionic polysaccharides

The strong affinity of natural ionic polysaccharides for certain metal ions is exploited in the design of a new class of voltammetric sensing devices. In particular, carbon-paste electrodes containing pectic and alginic acids are used for the nonelectrolytic collection and subsequent voltammetric determination of copper and lead, respectively. Cyclic and differential pulse voltammetry are used to quantify the accumulated ions. The response is characterized with respect to modifier loading, preconcentration period, metal concentration, reproducibility, possible interferences and other variables. Titrimetric experiments illustrate the potential of polysaccharide electrodes for speciation work. Preliminary data are also given for analogous measurements of copper at heparin-modified electrodes. Detection limits are 1 mug/ml and the relative standard deviation is 4.8%.

Use of tyrosinase for enzymatic elimination of acetaminophen interference in amperometric sensing

Abstract An effective enzymatic strategy for eliminating the common acetaminophen interference in amperometric sensing is described. The method relies on the in situ biocatalytic removal of acetaminophen by the surface-bound tyrosinase. Mushroom-containing carbon paste electrodes (rich with tyrosinase) are particularly useful for eliminating contributions from acetaminophen. Various factors affecting the efficiency of the tyrosinase-based elimination capability are explored. Selective biosensing of glucose is accomplished by the coimmobilization of glucose oxidase.

Enzyme Nanoband Electrodes

Abstract Enzyme nanoelectrodes have been constructed by immobilizing glucose oxidase, alcohol oxidase or tyrosinase onto ultrathin carbon films (of 35–50 nm thickness). The enzyme immobilization is accomplished via entrapment within electropolymerized poly(o-phenylenediamine) coatings. Cyclic voltammetry and controlled-potential amperometry are used to characterize the performance of the new nanoscopic biosensors under different preparation and operation conditions. The resulting electrodes offer convenient and rapid measurements of millimolar substrate concentrations, and (to the best of our knowledge) are the smallest enzyme probes reported to date.

Organic-phase biosensing of secondary alcohols with a Ta. brockii alcohol dehydrogenase electrode

An organic-phase enzyme electrode for monitoring secondary alcohols is described. The new biosensor relies on the biocatalytic activity of Ta. brockii alcohol dehydrogenase in non-aqueous media. The Eastman-AQ 55D polymeric coating, known for its stability in organic solvents, is used to entrap the enzyme and its NADP+ cofactor onto the glassy carbon surface. Various experimental variables, influencing the response of the reagentless sensor, are explored for optimum performance. The solvent-induced changes in the enzymatic activity result in an extended linear range. The work expands the scope of organic-phase biosensors towards additional substrates of analytical significance.

Reticulated vitreous carbon-plant tissue composite bioelectrodes

Abstract The construction and dynamic response characteristics of a tissue-porous electrode combination, composed of a plant tissue which is packed into reticulated vitreous carbon (RVC), are described. The open-cell structure of RVC thus serves as a template for the biocomponent. The immediate proximity of the tissue to the carbon surface results in a very rapid response. High sensitivity accrues from the high tissue “loading” and large electrode area. Such advantages are illustrated with mushroom-, potato- and horseradish root-packed RVC matrices. Bioaccumulation at alga-“loaded” RVC composites is also demonstrated. The effects of various operational variables on the response were evaluated. The tissue-RVC composite electrodes exhibit sigmoidal cyclic voltammograms, characteristic of partially blocked surfaces.

Horseradish-Root Tissue Tubular Reactor for Flow Injection Detection of Hydrogen Peroxide

Abstract The use of a horseradish-root tissue as a on-line reactor in connection with flow injection measurements of hydrogen peroxide is described. Such tissue reactor is very suitable for flow operation, as it couples the advantages of very high enzyme loading and good durability, with extreme simplicity and economy. The high conversion efficiency is evaluated under various experimental conditions (reactor dimensions, flow rate, etc.). Response characteristics, such as linearity, reproducibility or response time, are reported. The incorporation of glucose oxidase in the carrier solution allows convenient, low potential, monitoring of glucose (through the liberation and conversion of hydrogen peroxide). Analogous measurements of dopamine, based on an open-tubular apple reactor, are reported. Tissue biocatalytic reactors thus offer a very attractive alternative to enzyme reactors used for flow analysis.