Sensitive stripping voltammetry of heavy metals by using a composite sensor based on a built-in bismuth precursor

A new graphite-epoxy composite electrode (GECE) containing Bi(NO(3))(3) as a built-in bismuth precursor for simultaneous and individual anodic stripping analysis of heavy trace metals like lead and cadmium is reported. The developed Bi(NO(3))(3)-GECE is compatible with bismuth film electrodes reported previously including the composite electrodes (Bi-GECE) recently reported by our group. Bi(NO(3))(3)-GECE displays the ability for the detection of both individual and simultaneous determination of heavy trace metals and exhibits well defined, reproducible and sharp stripping signals. The sensitive response is combined with the minimal toxicity of Bi(NO(3))(3). This novel sensor would be an appropriate alternative tool to sensors using bismuth in solution during their utilization in environmental quality monitoring as well as other applications.     

Micro-enzyme field effect transistor sensor using direct covalent bonding of urease

Enzyme field effect transistors (ENFETs), which have been previously proposed for the detection of urea, consist of a pH ion-sensitive FET with urease enzyme immobilized in a polymeric membrane. A new means of preparing the enzymatic sensing layer is proposed in which urease is directly covalently bonded onto the silica insulator. The sensitivity, lifetime and response time of the ENFETs obtained are fairly good.     

Amperometric determination of pesticides using a biosensor based on a polishable graphie-epoxy biocomposite

The determination of organophosphorus and carbamate pesticides was carried out using an amperometric transducer based on a robust, polishable and easily mechinable biocomposite. The biocomposite material contains graphite powder, a non-conducting epoxy resin and acetylcholinesterase. The enzyme retains its bioactivity in the rigid epoxy-graphite matric. Measurements were carried out with acetylhiocholine as a substrate. Thiocholine produced by enzymatic hydrolysis was oxidized electrochemically at 70 mV (vs. Ag/AgCl in pH 7.0 buffered solution with 0.1 M phosphate and 0.1 m KCl). The decrease rate of substrate steady-state current after the addition of pesticide was used for evaluation. The method of construction allows for the repetitive use of the electrode. Simple polishing procedures are used to regenerate the bioactive transducer surface.     

Sequential injection system with higher dimensional electrochemical sensor signals Part 1. Voltammetric e-tongue for the determination of oxidizable compounds

A sequential injection analysis (SIA) system was developed with the aim of obtaining an automatic and versatile way to prepare standards needed in the study of systems with higher dimensional sensor signals. To illustrate this, different analytical techniques were used in determinations of several analytes. Automated potentiometric calibrations of different potentiometric sensors, with and without interference, were carried out. Useful determinations of selectivity coefficients with two degrees of freedom were obtained. Simultaneous voltammetric determinations have also been done. Firstly, simultaneous determinations of lead and cadmium, using epoxy-graphite composite as the working electrode, have enabled a separate calibration for each metal to be obtained. Next, a voltammetric electronic tongue was designed and applied to the determination of oxidizable species. The use of artificial neural networks has solved the overlapped signal of ascorbic acid, 4-aminophenol and 4-acetamidophenol (paracetamol). A set of 63 data points was prepared automatically and has facilitated the training of an electronic tongue for these three analytes. Accurate predictions of test solutions, in the range of 12–410 μM for ascorbic acid, 17–530 μM for 4-aminophenol and 10–420 μM for paracetamol, have been achieved with RMSEs lower than 0.10 μM.     

Application of Graphite-Epoxy Composite Electrodes in Differential Pulse Anodic Stripping Voltammetry of Heavy Metals

An analytical study on the use of graphite-epoxy composite (GEC) electrodes for differential pulse anodic stripping voltammetry (DPASV) of heavy metals is presented. This study is accompanied by microscopic observations of the electrode surface before and after the stripping step in comparison to glassy carbon electrode. GEC electrodes show much better accumulation properties and consequently acceptable behaviour which makes them suitable as working electrodes in the DPASV of heavy metals. Lead determination in real water samples in a batch system as well as some preliminary results in a flow-through system are presented. The detection limits in batch measurements were 100ppb for Cd, 10pb for lead and 50ppb for copper. The detection limit for lead in the flow-through system was similar to that in the batch. The results obtained show that these low cost and easy to prepare materials can be of interest in future research concerning stripping techniques of heavy metals and other analytes.     

Sequential injection system with higher dimensional electrochemical sensor signals Part 2. Potentiometric e-tongue for the determination of alkaline ions

An intelligent, automatic system based on an array of non-specific-response chemical sensors was developed. As a great amount of information is required for its correct modelling, we propose a system generating it itself. The sequential injection analysis (SIA) technique was chosen as it enables the processes of training, calibration, validation and operation to be automated simply. Detection was carried out using an array of potentiometric sensors based on PVC membranes of different selectivity. The diluted standard solutions needed for system learning and response modelling are automatically prepared from more concentrated standards. The electrodes used were characterised with respect to one and two analytes, by means of high-dimensionality calibrations, and the response surface of each was represented; this characterisation enabled an interference study of great practical utility. The combined response was modelled by means of artificial neural networks (ANNs), and thus it was possible to obtain an automated electronic tongue based on SIA. In order to identify the ANN which provided the best model of the electrode responses, some of the network's parameters were optimised and its usefulness in determining NH₄⁺, K⁺ and Na⁺ ions in synthetic samples was then tested. Finally, it was used to determine these ions in commercial fertilisers, the obtained results being compared with reference methods.     

Electronic structure of IPR and non-IPR endohedral metallofullerenes: Connecting orbital and topological rules

The electronic structure of endohedral metallofullerenes is rationalized by connecting the apparently independent orbital and topological rules that explain the stability of this family of fullerenes. The separation of the 12 pentagons of the fullerene, which is maximized in order to minimize the Coulomb repulsion, is found to be correlated with the orbital energies of the cage that accepts the electron transfer from the internal cluster. An explanation for the absence of non-IPR cages in large-size EMFs is also provided.     

Carbon composite electrodes: surface and electrochemical properties

Electrodes based on particulate carbon-epoxy or silicone composites have been formed and characterised using electrochemical methods, scanning electron microscopy and scanning electrochemical microscopy. These composites are rigid, exhibit high electrical conductivity and are stable in organic solvents for prolonged periods. The bulk resistance of the Araldite-M and Araldite-CW2215 based electrodes is low, 130+/-12 and 185+/-15 ohms, respectively. In contrast, the bulk resistance of the silicone based electrodes is 1480+/-112 ohms. The uncompensated resistance of electrochemical cells where the composites act as working electrodes is significantly larger than that expected on the basis of solution resistance alone, i.e., up to 7.5 kohms in the case of the silicone composites. These results are interpreted in terms of the presence of pores within the composite material. The response times of the composite electrodes to changes in the applied potential is between 3.1 and 7.2 ms which, although almost an order of magnitude longer than a comparable glassy carbon electrode, is sufficiently rapid to give useful voltammetric data for scan rates of several V s(-1). Close to ideal reversible cyclic voltammetry is observed for ferrocene under semi-infinite diffusion control for scan rates between 0.01 and 0.1 V s(-1) at the Araldite composites. In contrast, the large resistance associated with the silicone based materials causes quasi-reversible responses to be observed over this range of scan rate. Scan rate dependent cyclic voltammetry and time resolved chronoamperometry responses observed for ferrocene in solution are consistent with those expected for a random array of microelectrodes. Scanning electron microscopy and scanning electrochemical microscopy has been used to image the shape, size and electrochemical activity of the electroactive zones. In the case of Araldite-M, the quality of the electrode surface has been probed by comparing the rate of heterogeneous electron transfer at a composite microelectrode with that found for a carbon fibre electrode. The standard heterogeneous electron transfer rate constant, k degrees , is 6.0+/-0.1 x 10(-3) cm s(-1) for the composite compared to 1.5+/-0.1 x 10(-1) cm s(-1) for the carbon fibre electrode. While the smaller rate constant found for the composite suggests a less pristine surface, k degrees is sufficiently large to support reversible, electron transfer under typical electroanalytical conditions. These fundamental measurements will underpin the development of enzyme based biosensors for use in organic solvents.     

Determination of Pb and Cu by Anodic Stripping Voltammetry Using Glassy Carbon Electrodes Modified with Mercury or Mercury-Nafion Films

 Two different approaches for the modification of glassy carbon electrodes using a mercury film and mercury-nafion are compared. The mixture of mercury(II) chloride solution with a nafion solution diluted in ethanol is used to coat the polished glassy carbon surface. The modified working electrodes are compared when measuring Pb and Cu in real seawater samples. An optimisation of the parameters during the formation of the films was done to obtain well-defined stripping peaks. The type of inert supporting electrolyte and the pH play an important role on the sensitivity of the measurement. Results for Pb and Cu determinations show the advantages of Hg-nafion modification as an alternative method. These advantages include a shorter modification time, the avoidance of Hg solutions during the formation of the Hg film and an improved sensitivity for Pb determination. Received March 16, 1999. Revision April 24, 2000.     

Bingel-Hirsch reactions on non-IPR Gd3N@C2n (2n = 82 and 84)

The Bingel-Hirsch reactions on non-isolated pentagon rule (non-IPR) Gd(3)N@C(2n) (2n = 82, 84) are studied. Computational results show that the two metallofullerenes display similar reactivity according to their related topologies. Long C-C bonds with large pyramidalization angles lead to the most stable adducts, the [5,6] bonds in the adjacent pentagon pair being especially favored. The lesser regioselectivity observed for Gd(3)N@C(82) is probably due to the activation of some C-C bonds by means of the metal cluster.