Rasa Pauliukaite

Comparative study of different cross-linking agents for the immobilization of functionalized carbon nanotubes within a chitosan film supported on a graphite-epoxy composite electrode.

The effectiveness of immobilization of functionalized carbon nanotubes into chitosan using different cross-linking agents has been evaluated. The cross-linkers used were glyoxal (GO), glutaraldehyde (GA), epichlorohydrin (ECH), and 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide together with N-hydroxysuccinimide (EDC-NHS), and the nanotubes were retained on graphite epoxy resin composite electrodes. The nanotube modified electrodes have been characterized by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS). Using CV and EIS in the presence of potassium hexacyanoferrate(III), the electroactive area of all types of electrodes was determined and the redox process analyzed, leading to the conclusion that ECH and EDC-NHS are better for immobilization of functionalized carbon nanotubes inside the chitosan matrix. The modified electrodes were successfully applied to the determination of hydrogen peroxide by fixed potential amperometry at -0.1 V vs SCE, the highest response being exhibited when using ECH.

Phenazines and Polyphenazines in Electrochemical Sensors and Biosensors

Phenazine dyes and their polymers are reviewed with respect to their application in electrochemical sensors and biosensors, for which they are finding increasing application as redox mediators due to their unique redox and also chromatic properties, being used for the development of (bio)sensors for direct or indirect analyte determination. Electrocatalytic effects decrease the overpotential of a number of important analytes and help increase sensitivity and decrease the detection limit, in some cases, allowing the simultaneous determination of several analytes; their properties as redox mediators can be improved when combined with carbon nanotubes. Future perspectives are indicated.

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.

AFM nanometer surface morphological study of in situ electropolymerized neutral red redox mediator oxysilane sol-gel encapsulated glucose oxidase electrochemical biosensors

Four different silica sol-gel films: methyltrimethoxysilane (MTMOS), tetraethoxysilane (TEOS), 3-aminopropyltriethoxysilane (APTOS) and 3-glycidoxypropyl-trimethoxysilane (GOPMOS) assembled onto highly oriented pyrolytic graphite (HOPG) were characterized using atomic force microscopy (AFM), due to their use in the development of glucose biosensors. The chemical structure of the oxysilane precursor and the composition of the sol-gel mixture both influenced the roughness, the size and the distribution of pores in the sol-gel films, which is relevant for enzyme encapsulation. The GOPMOS sol-gel film fulfils all the morphological characteristics required for good encapsulation of the enzyme, due to a smooth topography with very dense and uniform distribution of only small, 50 nm diameter, pores at the surface. APTOS and MTMOS sol-gel films developed small pores together with large ones of 300-400 nm that allow the leakage of enzymes, while the TEOS film formed a rough and incomplete network on the electrode, less suitable for enzyme immobilisation. GOPMOS sol-gel film with encapsulated glucose oxidase and poly(neutral red) redox mediator, prepared by in situ electropolymerization, were also morphologically characterized by AFM. The AFM results explain the variation of the stability in time, sensitivity and limit of detection obtained with different oxysilane sol-gel encapsulated glucose oxidase biosensors with redox mediator.

ELECTROCHEMICAL CHARACTERIZATION OF AND STRIPPING VOLTAMMETRY AT SCREEN PRINTED ELECTRODES MODIFIED WITH DIFFERENT BRANDS OF MULTIWALL CARBON NANOTUBES AND BISMUTH FILMS

A screen-printed electrode sensor has been fabricated by modifying the carbon ink surface with different brands of multiwall carbon nanotubes (MWCNTs) and bismuth film (BiF) for the determination of traces of lead, cadmium and zinc ions by square wave anodic stripping voltammetry. The MWCNTs, from three different sources, were functionalized and dispersed in Nafion (MWCNT-Nafion) solution and placed on screen printed electrodes (MWCNT-Nafion/SPE); bismuth films were then prepared by ex-situ plating of bismuth onto the MWCNT-Nafion/SPE electrodes. The electrochemical characteristics of BiF/MWCNT-Nafion/SPE/ were examined by electrochemical impedance spectroscopy and showed differences; the charge transfer resistance tends to decrease with negative applied potentials. After optimizing the experimental conditions, the square-wave peak current signal is linear in the nmol L−1 range. The lowest limit of detection found for the separate determination of lead, cadmium and zinc were 0.7 nmol L−1, 1.5 nmol L−1, and 11.1 nmol L−1, respectively, with a 120 s deposition time.

Graphite-Epoxy Electrodes Modified with Functionalised Carbon Nanotubes and Chitosan for the Rapid Electrochemical Determination of Dipyrone

A rapid electrochemical procedure for the determination of dipyrone was successfully developed at a carbon nanotube modified graphite-epoxy resin composite (GrEC) electrode. The composite electrode was used as support on which multi-walled carbon nanotubes (MWCNT) were immobilised by 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide together with N-hydroxysuccinimide (EDC-NHS) in a chitosan (Chit) matrix. The electrochemical behaviour of dipyrone at this electrode in different buffer electrolytes with pH values between 5.0 and 8.0 was explored using cyclic voltammetry, differential pulse voltammetry, and electrochemical impedance spectroscopy, and comparison with a conventional glassy carbon electrode was made. Dipyrone was best determined by differential pulse voltammetry with a low limit of detection of 1.4 microM. Application to commercial samples was demonstrated.

Polarographic Determination of Formaldehyde According to the Anodic Oxidation Wave in Alkaline Solutions

The polarographic waves of CH2O oxidation in 2–3 M NaOH solutions containing 0.04–0.10 M (+)-tartrate are diffusion controlled: the dependences of limiting current (ilim) on the square root of the Hg column height are linear and the temperature coefficient of the limiting current is relative small, i.e., ∼2% grad–1. The anodic waves of CH2O oxidation could be used for analytical purposes. ilimis proportional to CH2O concentration in the range 2×10–4–4×10–3 M. The precision of CH2O determination is typical for DC-polarography. A RSD smaller than 3 % has been obtained for 20–200 μmol; CH2O. The method proposed is suitable for the direct determination of formaldehyde in electroless copper plating solutions containing (+)-tartrate or glycerol as Cu(II) ions ligands.

Spectrometric and electrochemical investigation of vanadium(V) and vanadium(IV) tartrate complexes in solution

Spectroscopic and electrochemical studies were carried out to characterize the vanadium(IV) and (V) complexes that form in solution and their interconversions. 51V NMR spectroscopy coupled with 1H NMR, 13C NMR and EPR spectroscopy were used to characterize the vanadium(V)-tartrate complexes that form in the vanadium-tartrate system. The major complex that forms over most of the pH range is a 1:1 complex. In addition a minor 1:2 complex forms. The 1:2 complex formed from enantiomerically pure tartaric acid was less stable than complex formed from racemic tartaric acid. These complexes are different than the 2:2 complex that is the major contributor in the vanadium(IV)-tartrate system. The polarographic and cyclic voltammetic investigations of the electrochemical behavior of V(V) in the presence of tartrate demonstrated a complex formation. The diffusion coefficient values of free vanadium ions and vanadium-tartrate complexes were determined. The minimum average values for the diffusion coefficient for the vanadium(IV)-tartrate 2:2 complexes were determined to range from 3 ´ 10-6 cm2 s-1 to 17 ´ 10-6 cm2 s-1 depending on pH.

Influence of Ionic Strength and Cation Nature on the Deprotonation of Methanediol in Alkaline Solution

The pKa values of methanediol deprotonation were found to decrease with increase in solution ionic strength as estimated by a 13C NMR titration method.

Interaction of (+)-Tartrate with Methanediol in AlkalineSolutions

The equilibrium of acetal-type compound formation from (+)-tartrate and methanediol in alkaline solutions has been studied and characterized quantitatively.