M. Emilia Ghica

Electrochemical sensors and biosensors based on redox polymer/carbon nanotube modified electrodes: a review.

The aim of this review is to present the contributions to the development of electrochemical sensors and biosensors based on polyphenazine or polytriphenylmethane redox polymers together with carbon nanotubes (CNT) during recent years. Phenazine polymers have been widely used in analytical applications due to their inherent charge transport properties and electrocatalytic effects. At the same time, since the first report on a CNT-based sensor, their application in the electroanalytical chemistry field has demonstrated that the unique structure and properties of CNT are ideal for the design of electrochemical (bio)sensors. We describe here that the specific combination of phenazine/triphenylmethane polymers with CNT leads to an improved performance of the resulting sensing devices, because of their complementary electrical, electrochemical and mechanical properties, and also due to synergistic effects. The preparation of polymer/CNT modified electrodes will be presented together with their electrochemical and surface characterization, with emphasis on the contribution of each component on the overall properties of the modified electrodes. Their importance in analytical chemistry is demonstrated by the numerous applications based on polymer/CNT-driven electrocatalytic effects, and their analytical performance as (bio) sensors is discussed.

Enzyme immobilisation on electroactive nanostructured membranes (ENM): Optimised architectures for biosensing

Electroactive nanostructured membranes have been produced by the layer-by-layer (LbL) technique, and used to make electrochemical enzyme biosensors for glucose by modification with cobalt hexacyanoferrate redox mediator and immobilisation of glucose oxidase enzyme. Indium tin oxide (ITO) glass electrodes were modified with up to three bilayers of polyamidoamine (PAMAM) dendrimers containing gold nanoparticles and poly(vinylsulfonate) (PVS). The gold nanoparticles were covered with cobalt hexacyanoferrate that functioned as a redox mediator, allowing the modified electrode to be used to detect H(2)O(2), the product of the oxidase enzymatic reaction, at 0.0 V vs. SCE. Enzyme was then immobilised by cross-linking with glutaraldehyde. Several parameters for optimisation of the glucose biosensor were investigated, including the number of deposited bilayers, the enzyme immobilisation protocol and the concentrations of immobilised enzyme and of the protein that was crosslinked with PAMAM. The latter was used to provide glucose oxidase with a friendly environment, in order to preserve its bioactivity. The optimised biosensor, with three bilayers, has high sensitivity and operational stability, with a detection limit of 6.1 microM and an apparent Michaelis-Menten constant of 0.20mM. It showed good selectivity against interferents and is suitable for glucose measurements in natural samples.

A novel amperometric sensor for ascorbic acid based on poly(Nile blue A) and functionalised multi-walled carbon nanotube modified electrodes

A new type of modified electrode sensor for ascorbic acid has been prepared by deposition of multi-walled carbon nanotubes (MWCNT) and poly(Nile blue A) on the surface of glassy carbon electrodes. Nile blue A was electropolymerised either beneath (directly on glassy carbon) or onto the MWCNT layer by potential cycling in phosphate buffer solution at pH 6.0. Characterisation of the modified electrodes was carried out by cyclic voltammetry and electrochemical impedance spectroscopy. Quantitative determination of ascorbate was achieved by cyclic voltammetry and fixed potential amperometry in phosphate buffer solution at pH 5.3. The modified electrodes exhibited good sensitivity, wide linear range, a detection limit of 1.6 μM and good stability, showing that they can be used as sensors for ascorbic acid. There is no interference from compounds commonly found in clinical and pharmaceutical samples and the determination of ascorbic acid in commercial tablet samples was successfully performed.

Poly(neutral red) based hydrogen peroxide biosensor for chromium determination by inhibition measurements

Amperometric hydrogen peroxide enzyme inhibition biosensors based on horseradish peroxidase (HRP) immobilised on electropolymerised neutral red (NR) or directly on the surface of carbon film electrodes (CFE) have been successfully applied to the determination of toxic Cr(III) and Cr(VI). Parameters influencing the performance of the biosensor including the enzyme immobilisation method, the amount of hydrogen peroxide, applied potential and electrolyte pH were optimised. The inhibition of horseradish peroxidase by the chromium species was studied under the optimised conditions. Results from the quantitative analysis of chromium ions are discussed in terms of detection limit, linear range and sensitivity. The HRP kinetic interactions reveal mixed binding of Cr(III) with I50=3.8μM and inhibition binding constant Ki=11.3μM at HRP/PNR/CFE biosensors and uncompetitive binding of Cr(VI) with I50=3.9μM and Ki=0.78μM at HRP/CFE biosensors in the presence of H2O2 substrate. Interferences from other heavy metal ions were studied and the inhibition show very good selectivity towards Cr(III) and Cr(VI).

Poly(thionine)-carbon nanotube modified carbon film electrodes and application to the simultaneous determination of acetaminophen and dipyrone

Multi-walled carbon nanotube (MWCNT)-poly(thionine) (PTH) nanostructures were formed on carbon film electrodes (CFE), the polymer film being formed beneath or on top of MWCNT, to give two different sensor architectures, PTH/MWCNT/CFE or MWCNT/PTH/CFE. Characterization of the novel nanostructures was carried out by cyclic voltammetry, electrochemical impedance spectroscopy and scanning electron microscopy. The modified electrodes were then used for sensing acetaminophen (ACOP) and dipyrone by differential pulse voltammetry and fixed-potential amperometry independently and simultaneously. Analytical parameters were determined and the results compared with similarly modified electrodes reported in the literature. An interference study as well as recovery measurements in pharmaceutical samples was successfully performed.

Poly(brilliant green) and poly(thionine) modified carbon nanotube coated carbon film electrodes for glucose and uric acid biosensors

Poly(brilliant green) (PBG) and poly(thionine) (PTH) films have been formed on carbon film electrodes (CFEs) modified with carbon nanotubes (CNT) by electropolymerisation using potential cycling. Voltammetric and electrochemical impedance characterisation were performed. Glucose oxidase and uricase, as model enzymes, were immobilised on top of PBG/CNT/CFE and PTH/CNT/CFE for glucose and uric acid (UA) biosensing. Amperometric determination of glucose and UA was carried out in phosphate buffer pH 7.0 at -0.20 and +0.30 V vs. SCE, respectively, and the results were compared with other similarly modified electrodes existing in the literature. An interference study and recovery measurements in natural samples were successfully performed, indicating these architectures to be good and promising biosensor platforms.

Comparison of Cobalt Hexacyanoferrate and Poly(Neutral Red) Modified Carbon Film Electrodes for the Amperometric Detection of Heavy Metals Based on Glucose Oxidase Enzyme Inhibition

Electrochemical biosensors have been developed for the determination of cadmium, cobalt, and copper cations through their inhibitory effect on glucose oxidase activity, using cobalt hexacyanoferrate or poly(neutral red) as redox mediators. Cobalt hexacyanoferrate and poly(neutral red) were used to modify carbon film electrodes by electrodeposition and glucose oxidase was then immobilized on the electrode by crosslinking with glutaraldehyde. A comparison between the two redox mediated biosensors for the determination of the metal ions was performed under the same experimental conditions. In fixed-potential amperometry at −0.35 V vs. the saturated calomel electrode, the detection limits for the metal cations studied were in the low micromolar range, lower at glucose oxidase/cobalt hexacyanoferrate/carbon film electrode, and the lowest achieved with glucose oxidase inhibition measurements. The linear dynamic ranges were wider at the glucose oxidase/poly(neutral red)/carbon film electrode for all three metal ions. The 10% and 50% inhibition values (I10 and I50) for the metal cations were calculated and the degree of inhibition at the biosensors was compared. Copper ions inhibited glucose oxidase to the highest degree for both biosensors. The application for the analysis of tap water was demonstrated.