Ionel Catalin Popescu

Reagentless biosensors based on self-deposited redox polyelectrolyte-oxidoreductases architectures

Reagentless fructose and alcohol biosensors have been produced with a versatile enzyme immobilisation technique which mimics natural interactions and flexibility of living systems. The electrode architecture is built up on electrostatic interactions by the sequential adsorption of redox polyelectrolytes and redox enzymes giving rise to the efficient transformation of substrate fluxes into electrocatalytic currents. All investigated multilayer structures were self-deposited on 3-mercapto-1-propanesulfonic acid monolayers self-assembled on gold electrodes. Fructose dehydrogenase, horseradish peroxidase (HRP) and the couple HRP-alcohol oxidase were electrochemically connected with a cationic poly[(vinylpyridine)Os(bpy)2Cl] redox polymer (RP) interface in a layer-by-layer self-deposited architecture. The dependence of the distance on the electrochemical response of this interface was also studied showing a clear decrease in the Faradaic current when the distance to the electrode surface was increased. The sensitivities obtained for each biosensor were 19.3, 58.1 and 10.6 mA M(-1) cm(-1) for fructose, H2O2 and methanol, respectively. The sensitivity values can be easily controlled by a rational deposition and manipulation of the charge in the catalytic layers. The electrostatic assembly of the electrochemical interface and the catalytic layers resulted in integrated biochemical systems in which mass transfer diffusion and heterogeneous catalytic and electron transfer steps are efficiently coupled and can be easily manipulated.

Influence of graphite powder, additives and enzyme immobilization procedures on a mediatorless HRP-modified carbon paste electrode for amperometric flow-injection detection of H2O2

The behaviour of horseradish peroxidase modified carbon paste electrodes (HRP-CPEs) for mediatorless amperometric flow-injection (FI) detection of hydrogen peroxide is described. The influence of various commercially available graphite powders and additives was studied, using adsorption and covalent binding (at different pHs) as enzyme immobilization techniques. The HRP-CPE was used as the working electrode in an FI system. An optimal applied potential for electrocatalytic reduction of H2O2 was found at −50 mV vs. Ag/AgCl (0.1 M KCl). The highest sensitivity (≈ 78 μA cm−2 mM−1) obtained in the FI mode was observed for electrodes made of graphite powder having the highest surface area, with HRP immobilized by adsorption (at pH 8) in the presence of lactitol as enzyme stabilizer. The lowest detection limit (≈ 0.1 μM) was noted for HRP-CPE incorporating the graphite powder having the lowest background current.

Biosensors for phenol derivatives using biochemical signal amplification

Two different approaches, both exploiting two enzymes cooperative functioning, to enhance the sensitivity of tyrosinase (PPO) based biosensor for amperometric detection of phenols have been compared. For this purpose, one monoenzyme electrode (PPO) and two bienzyme electrodes (PPO and d-glucose dehydrogenase, GDH; PPO and horseradish peroxidase, HRP) were constructed using agar-agar gel as enzyme immobilization matrix. The biosensors responses for l-tyrosine detection were recorded at -50 mV versus saturated calomel electrode (SCE). The highest sensitivity (74 mA M(-1)) was observed for the PPO-GDH couple, while that recorded for PPO-HRP couple system was only 32 times higher than that measured for monoenzyme electrode (0.01 mA M(-1)). The ability of the PPO-, PPO-GDH-, PPO-HRP-based biosensors to assay phenols was demonstrated by quantitative determination of phenol, 1,2-dihydroxybenzene, 1,3-dihydroxybenzene, 1,4-dihydroxybenzene, 2-amino-3 (4-hydroxyphenyl) propanoic acid, 2-hydroxytoluene, 3-hydroxytoluene, 4-hydroxytoluene, 4-clorophenol, 3-clorophenol, 2-clorophenol, 4-hydroxybenzoic acid.

Kinetic and morphological investigation of copper electrodeposition from sulfate electrolytes in the presence of an additive based on ethoxyacetic alcohol and triethyl-benzyl-ammonium chloride

Abstract A comparative study of the influence of a new additive (IT-85), based on ethoxyacetic alcohol and triethyl-benzyl-ammonium chloride, and thiourea on copper electrodeposition from acidic sulfate solutions has been performed in order to obtain information about the kinetics of the cathodic process. In spite of their different chemical nature, both additives were found to be efficient as leveling agents, leading to fine-grained cathodic deposits. The study was based on scanning electron microscopy and X-ray dispersive analysis of the Cu deposits, coupled with an electrochemical investigation using cyclic voltammetry (CV) and steady-state polarization measurements at a rotating disc electrode. CV results and kinetic parameters obtained from Tafel plots led to the conclusion that both additives have a pronounced inhibiting effect on Cu2+ discharge, the strongest inhibition being observed for thiourea. An induction period related to a slow nucleation, increasing with additive concentration, was clearly put on evidence on the polarization curves, for both additives.

Modified electrodes with new phenothiazine derivatives for electrocatyltic oxidation of NADH

Abstract Phenothiazine derivatives containing two linearly condensed phenothiazine moieties, 16H,18H-dibenzo[c,l]-7,9-dithia-16,18-diazapentacene (I), dibenzo[c,l]-16,18-diacetyl-7,9-dithia-16,18-diazapentacene (II) dibenzo[c,l]-16,18-dibenzoyl-7,9-dithia-16,18-diazapentacene (III), and 16H,18H-dibenzo[c,l]-7,9-dithia-16,18-diazapentacene-7,7,9,9-bis-dioxide (IV), strongly adsorb on spectrographic graphite resulting in modified electrodes with electrocatalytic activity for NADH oxidation. From cyclic voltammetry measurements, performed in aqueous buffer solutions at different potential scan rates and pH values, the rate constants of the heterogeneous electron transfer and the transfer coefficients were estimated. The linear dependence between the peak current and the potential scan rate, corroborated with the slope of the formal standard potential versus pH linear regression, pointed out to a quasi-reversible, surface confined redox process involving 1e−/1H+. The electrocatalytical efficiency, evaluated from cyclic voltammetry, and the second order electrocatalytical rate constant (k1), calculated from rotating disk electrode experiments, revealed the same sequence of the activity decrease: III>II>IV>I. Compound III-modified electrodes were characterized by the highest k1 (1.8×103 M−1 s−1, at pH 7.0) as well as by the highest stability, expressed by the lowest rate of the surface coverage depleting under continuous potential cycling.

Indophenol and o-quinone derivaties immobilized on zirconium phosphate for NADH electro-oxidation

Abstract The electrocatalytic properties for NADH oxidation of five organic dyes, two indophenol derivatives (2,6-dichlorophenolindophenol and indophenol) and three o-quinone derivatives (phenanthrenequinone, pyrroloquinoline-quinone, and 1,2-naphthoquinone) were compared when adsorbed on zirconium phosphate entrapped in carbon paste. The electrochemical behavior of the immobilized dyes was investigated with cyclic voltammetry, performed in different aqueous buffers, at different potential scan rates and pH values. The electrocatalytic efficiency for NADH oxidation was evaluated from cyclic voltammetry, and the second order electrocatalytic rate constant was calculated from rotating disk electrode experiments, at various concentrations of NADH and pH values. These studies indicate that the mechanism of such electro-oxidation proceeds via the formation of an intermediate complex. A positive effect with the addition of Ca2+ cations to the solution was observed and the reaction rate for NADH oxidation increased. The highest second order rate constant was obtained for 2,6-dichlorophenolindophenol and found to be 4.6 × 106 M−1 s−1.

Sensitive Detection of Organophosphorus Pesticides Using a Needle Type Amperometric Acetylcholinesterase-based Bioelectrode. Thiocholine Electrochemistry and Immobilised Enzyme Inhibition

An acetylcholinesterase (AChE) based amperometric bioelectrode for a selective detection of low concentrations of organophosphorus pesticides has been developed. The amperometric needle type bioelectrode consists of a bare cavity in a PTFE isolated Pt-Ir wire, where the AChE was entrapped into a photopolymerised polymer of polyvinyl alcohol bearing styrylpyridinium groups (PVA-SbQ). Cyclic voltammetry, performed at Pt and AChE/Pt disk electrodes, confirmed the irreversible, monoelectronic thiocholine oxidation process and showed that a working potential of +0.410 V vs. Ag/AgCl, KCl sat was suitable for a selective and sensitive amperometric detection of thiocholine. The acetylthiocholine detection under enzyme kinetic control was found in the range of 0.01-0.3 U cm −2 of immobilised AChE. The detection limit, calculated for an inhibition ratio of 10%, was found to reach 5 μM for dipterex and 0.4 μM for paraoxon. A kinetic analysis of the AChE-pesticide interaction process using Hanes-Woolf or Lineweaver-Burk linearisations and secondary plots allowed identification of the immobilised enzyme inhibition process as a mixed one (non/uncompetitive) for both dipterex and paraoxon. The deviation from classical Michaelis Menten kinetics induced from the studied pesticides was evaluated using Hill plots.

Kinetics of copper electrodeposition in the presence of triethyl-benzyl ammonium chloride

Quasi-steady state hydrodynamic voltammetry at a rotating-disc electrode and electrochemical impedance spectroscopy were used to investigate the influence of triethyl-benzyl-ammonium (TEBA) chloride on the kinetics of copper electrodeposition from sulphate acidic electrolytes. SEM and X-ray diffraction analysis were used to examine the morphology and the structure of copper deposits. The kinetic parameters (i0, αc, k0), obtained by both Tafel and Koutecky–Levich interpretations lead to the conclusion that TEBA acts as an inhibitor of copper electrodeposition process, as a consequence of its adsorption on the electrode surface. The influence of TEBA on the kinetics of copper electrodeposition was explained in terms of a reaction model confirmed by the simulated impedance spectra. TEBA acts only as a blocking agent competing for adsorption active sites of the cathodic surface with cuprous ions without changing the reaction pathway corresponding to the absence of the additive.

Kinetic analysis of horseradish peroxidase “wiring” in redox polyelectrolyte–peroxidase multilayer assemblies

Abstract A hydrogen peroxide sensitive interface based on a self-deposited redox polyelectrolyte–peroxidase assembly was used as model system for investigation of the electron transfer between chemically modified horseradish peroxidase and the redox active centers of poly[(vinylpyridine)Os(bpy) 2 Cl] redox polymer. The assembly was built up by sequential adsorption of the redox polymer and the peroxidase on gold electrodes modified with a self-assembled monolayer of 3-mercapto-1-propane sulfonic acid. Making use of a kinetic model, formulated for steady-state conditions, and of the rotating disc electrode technique it was possible to estimate: (i) the second order rate constant of reaction between the modified peroxidase and hydrogen peroxide ( 2.74×10 5 M −1 s −1 ) and (ii) the apparent turnover number of the oxidized enzyme regeneration by the osmium redox center ( 2.14 s −1 ), i.e., two parameters that reflect the efficiency of the electrical communication within the investigated interface.

Electropolymerized Architecture Entrapping a Trilacunary Keggin‐Type Polyoxometalate for Assembling a Glucose Biosensor

A new design to obtain an amperometric glucose biosensor based on glucose oxidase (GOx) immobilization on an electropolymerized matrix [poly (1,8 DAN)] entrapping a trilacunary Keggin type polyoxometalate (Fe4POM) deposited on a graphite electrode was proposed. Some experimental evidences suggest that the modified conducting polymeric matrix consists in a multilayer architecture, formed from positively charged conductive poly (1,8 DAN) alternating with the negatively charged (Fe4POM). At an applied potential of 0.04 V (vs. Ag/AgCl, KClsat), the batch amperometric response to glucose of the G/Fe4POM-poly(1,8 DAN)/GOx modified electrode is characterized by a sensitivity of 19.8 2.4 AM 1 cm 2 , a linear range between 2.5 to 20 mM glucose, a response time less than 40 s and a detection limit of 1.2 mM glucose (for S/N 3).