Ageliki B. Florou

Electrocatalytic Oxidation of NADH in Flow Analysis by Graphite Electrode Modified with 2,6-Dichlorophenolindophenol Salts

The preparation of a 2,6-dichlorophenolindophenol (DCPI) modified graphite electrode is described. DCPI was successfully immobilized by physical adsorption onto a plain graphite electrode (DCPI-CME) and onto graphite electrodes pretreated with lanthanum nitrate (DCPI-La-CME) or thorium nitrate (DCPI-Th-CME). The electrochemical behavior of DCPI-CME was extensively studied using cyclic voltammetry. The electrochemical redox reaction of DCPI was found to be fairly reversible at low coverage with Eo′ = +55 mV (vs. Ag/AgCl/3M KCl) at pH 6.5. A pKa value of 5.8 ± 0.1 for immobilized form of DCPI is determined from the intersection of the lines in the plot Eo′ vs. pH. The current Ip has a linear relationship with the scan rate up to 1200 mV s−1, which is indicative for very fast electron transfer kinetics. The calculated value of the standard rate constant is ko = 18 ± 4 s−1. No decrease of either the anodic or the cathodic current of the cyclic voltammogram was observed after 500 runs of successive sweeps. The influence of the morphology of the electrode surface on the electrochemical behavior of the DCPI-CME was studied and a mathematical model is proposed, which partly describes the dependence of the geometrical area of the electrode surface on the grid of the emery paper. The modified electrodes were mounted in a flow-injection manifold, poised at +60 mV (vs. Ag/AgCl/3M KCl) and a catalytic current due to the oxidation of NADH was observed reducing thus the oxidation overpotential of NADH for about 400 mV. Interference from various reductive species present in real samples was investigated. The repeatability was 1.2 % RSD (n = 10 for 0.1 mM NADH). The sensor showed good operational and storage stability.

Electrocatalysis of sulphide with a cellulose acetate film bearing 2,6-dichlorophenolindophenol. Application to sewage using a fully automated flow injection manifold

The application of an electrochemical sensor, based on a glassy carbon electrode, modified with a cellulose acetate polymeric film bearing 2,6-dichlorophenolindophenol (CA/DCPI-CME), for flow injection analysis of sulphide, is described. The overall reaction was found to obey a catalytic regeneration mechanism (EC mechanism) and the electrochemical rate constant k(f) for the electrocatalytic oxidation of sulphide was evaluated. The modified electrodes were mounted in a flow-injection manifold, poised at +0.08 mV versus Ag/AgCl/3 M KCl at pH 7.25 and utilised for the determination of sulphide in urban waste samples. The proposed method correlates well with a colorimetric method. Parameters such as working pH, sample size, flow rate and temperature were studied. Interferants of various compounds normally present in real samples were also tested. Calibration graphs were linear over the range 0.02-1 mM sodium sulphide for CA/DCPI-CMEs hydrolysed in KOH for 6 min. The throughput was 25 samples per h and the R.S.D. was 1.2% (n=7) for 0.1 mM sodium sulphide. Recoveries for spiked urban waste samples ranged from 99 to 120%. The sensor remained active for more than 2 weeks under specified conditions.

The Importance of Surface Coverage in the Electrochemical Study of Chemically Modified Electrodes

The importance of the surface coverage in the behavior of chemically modified electrodes and its effect on different electrochemical parameters such as the peak potential separation ΔEp, the potential of full width at half peak height Efwhm and the apparent electrochemical rate constant k°, is discussed. The influence of the morphology of the surface of the electrode on the value of surface coverage is also demonstrated.

Lab-on-a-membrane foldable devices for duplex drop-volume electrochemical biosensing using quantum dot tags

This work describes a new type of integrated lab-on-a-membrane foldable device suitable for on-site duplex electrochemical biosensing using drop-size sample volumes. The devices are fabricated entirely by screen-printing on a nylon membrane and feature two assay zones which are located symmetrically on either side of a three-electrode voltammetric cell with a bismuth citrate-loaded graphite working electrode. After the completion of two spatially separated drop-volume competitive immunoassays on the assay zones using biotinylated antibodies labeled with streptavidin-conjugated Pb- and Cd-based quantum dots (QDs), respectively, the QD labels are dissolved releasing Pb(II) and Cd(II) in the assay zones. Then, the two assay zones are folded over, and they are brought in contact with the voltammetric cell for simultaneous anodic stripping voltammetric (ASV) determination of Pb(II) and Cd(II) at the bismuth nanostructured layer formed on the working electrode by reduction of the bismuth citrate during the preconcentration step. The fabrication of the devices is discussed in detail, and their operational characteristics are exhaustively studied. In order to demonstrate their applicability to the analysis in complex matrices, duplex ASV-QDs-based determination of bovine casein and bovine immunoglobulin G is carried out in milk samples yielding limits of detection of 0.04 μg mL(-1) and 0.02 μg mL(-1), respectively. The potential of the devices to detect milk adulteration is further demonstrated. These new membrane devices enable duplex biosensing with distinct advantages over existing approaches in terms of cost, fabrication, and operational simplicity and rapidity, portability, sample size, disposability, sensitivity, and suitability for field analysis.

Electrochemical Behavior and Analytical Utility of a Controlled Porosity Cellulose Acetate Film Bearing 2,6-Dichlorophenolindophenol

Glassy carbon electrodes coated with a cellulose acetate film incorporating 2,6-dichlorophenolindophenol (CA/DCPI) were developed. At this multifunctional coating DCPI serves as a mediator for the electron transfer kinetics and in conjunction with the cellulose acetate’s size exclusion properties it results in a chemical sensor with great selectivity and stability. Access to the surface can be manipulated via controlled hydrolysis of the film in KOH or ZnCl2 solutions. Different permeabilities are obtainable by hydrolyzing the film over different time periods. Diffusion coefficients Dapp of these films for analytes of different molecular sizes were determined with double step chronocoulometry. The electrochemical characteristics of the immobilized DCPI were explored using cyclic voltammetry. The formal potentials of the immobilized DCPI coating hydrolyzed in KOH, ZnCl2 and with ZnCl2 in the casting solution were found to be 70, 75, and 79 mV (vs. Ag/AgCl/ 3M KCl), respectively, at pH 6.5. The dissociation constants of the DCPI redox couple were found to be 5.2+ 0.1 (pKr) and 7.4+ 0.05 (pKo). The electrochemical rate constant k o of the of DCPI redox couple within the film was also evaluated. The behavior of the sensor towards different reducing compounds was investigated. The sensors showed good operational and storage stability.

Sensitive Determination of Iron Using Disposable Nafion-Coated Screen-Printed Graphite Electrodes

ABSTRACT Iron was determined in drinking water using low-cost, disposable, and highly reproducible modified graphite screen-printed electrodes. Nafion is a chemically inert polymer and endows remarkable sensitivity for Fe(III) through interaction with the sulfonic groups on the polymer surface. The concentration of Nafion on the modified electrodes was optimized and the best results were obtained using 2 μL of 0.2% Nafion in ethanol. The preconcentration of analytes was performed in an electrodeless mode with stirring in 0.01 mol L−l HCl. Under selected conditions and for a preconcentration time of 60 s, square-wave voltammograms exhibited a cathodic peak, the height of which was linearly dependent on the concentration of iron from 0.05 to 5.00 µmol L−l. The limit of detection was 15 nmol L−l Fe(III). The developed electrodes were successfully used to determine iron in tap water. Accuracy of the method was evaluated with recovery measurements in spiked samples. The values were 108 ± 3%. Possible interferences from the coexisting ions were also investigated. The results show that the sensors are sensitive, selective, rapid, reliable, and suitable for the determination of Fe(III) in water.