Francisco Céspedes

New materials for electrochemical sensing I. Rigid conducting composites

Abstract The development of composites based on conductive phases dispersed in polymeric matrices has led to important advances in analytical electrochemistry, particularly insensor devices. These new materials combine the electrical properties of graphite with the ease of processing of plastics (epoxy, methacrylate, Teflon, etc.) and show attractive electrochemical, physical, mechanical and economical features compared to the classic conductors (gold, platinum, graphite, etc.). The properties of these composites are described, along with their application to the construction of conductometric, potentiometric and amperometric sensors. The chemical modification of the composites by blending fillers that improve the analytical characteristics of the resulting sensors is discussed, particularly for the case of amperometric devices.

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

Abstract 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.

Determination of organophosphorus and carbamate pesticides using a biosensor based on a polishable, 7,7,8,8-tetracyanoquino-dimethane-modified, graphite—epoxy biocomposite

Abstract An amperometric biosensor for the determination of organophosphorus and carbamate pesticides is described. The biosensor is based on robust, polishable and easily machinable biocomposites containing graphite powder, a non-conducting epoxy resin, the electronic mediator 7,7,8,8-tetracyanoquinodimethane (TCNQ) and the enzyme acetylcholinesterase (AChE) or butyrylcholinesterase (BChE) immobilized on aminated silica particles. Determinations were done with acetylthiocholine (ATCh) or butyrylthiocholine (BTCh) as substrate. Thiocholine produced by enzymatic hydrolysis is oxidized electrocatalytically. The biosensor operates at a potential of 300 mV vs. Ag/AgCl in a pH buffered solution with 0.1 M phosphate and 0.1 M KCl. The decrease rate of the steady-state current after the addition of pesticides was used for the determination. The materials employed in the construction of the reported biosensors allow for a prolonged use of the device if a slight polish of the surface is performed from time to time.

Determination of total polyphenol index in wines employing a voltammetric electronic tongue

This work reports the application of a voltammetric electronic tongue system (ET) made from an array of modified graphite-epoxy composites plus a gold microelectrode in the qualitative and quantitative analysis of polyphenols found in wine. Wine samples were analyzed using cyclic voltammetry without any sample pretreatment. The obtained responses were preprocessed employing discrete wavelet transform (DWT) in order to compress and extract significant features from the voltammetric signals, and the obtained approximation coefficients fed a multivariate calibration method (artificial neural network-ANN-or partial least squares-PLS-) which accomplished the quantification of total polyphenol content. External test subset samples results were compared with the ones obtained with the Folin-Ciocalteu (FC) method and UV absorbance polyphenol index (I(280)) as reference values, with highly significant correlation coefficients of 0.979 and 0.963 in the range from 50 to 2400 mg L(-1) gallic acid equivalents, respectively. In a separate experiment, qualitative discrimination of different polyphenols found in wine was also assessed by principal component analysis (PCA).

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 12-410muM for ascorbic acid, 17-530muM for 4-aminophenol and 10-420muM for paracetamol, have been achieved with RMSEs lower than 0.10muM.

Hydrogen peroxide amperometric biosensor based on a peroxidase-graphite-epoxy biocomposite

Abstract An amperometric biosensor sensitive to hydrogen peroxide has been built, using a HRP-graphite-epoxy biocomposite. The proximity of the redox centers of the enzyme and the conductive sites at the surface of the electrode permits a direct regeneration of the enzyme. The response characteristics of this biosensor have been improved by adding powdered platinum to the matrix of the biocomposite. When Pt is absent, the working potential is −300 mV vs. Ag AgCl and the response is linear between 0.03 mM and 7 mM. When 3% wt platinum is added to the biocomposite, a stable response is produced at −50 mV vs. Ag AgCl and the linearity range lies between 0.09 mM and 9 mM. The response time is approximately 2s for both the cases.

Carbon-polymer biocomposites for amperometric sensing☆

Abstract New electrochemical sensing biocomposite materials are reported. These materials are based on polymer technology and are prepared mixing graphite powder, a non-conducting polymer resin and a lyophilized enzyme. The resulting biosensing material is inexpensive, robust, polishable and easy to machine. A survey of potentially suitable polymeric matrices was carried out. Epoxy, silicone, methacrylate and polyester polymers have been used to prepare rigid conducting composite materials of the graphite-polymer type. For each material, an optimal graphite content was determined. Amperometric transducers built with these materials were characterized electrochemically using cyclic voltammetry and linear-sweep voltammetry. Their linear response to hydrogen peroxide was evaluated. The applicability of these conducting polymer-graphite composites has been extended to the construction of conventional glucose biosensors. In these devices the conducting composite is bulk-modified with the addition of glucose oxidase. The amperometric detection of hydrogen peroxide serves as the analytical signal. Following the same construction method, it is also possible to obtain other biosensing systems. New biocomposites have been prepared, using a different enzyme in each case, i.e. acetylcholinesterase and peroxidase.

Comparison of methods for the processing of voltammetric electronic tongues data

AbstractWe are making a numerical comparison of various preprocessing strategies for dealing with data from voltammetric electronic tongues in order to reduce the high dimensionality of the response matrices. Different modelling tools are presented and briefly described. We then compare combinations of four preprocessing strategies (principal component analysis, fast Fourier transform, discrete wavelet transform, voltammogram-windowed slicing integral) with four modelling alternatives (principal component regression, partial least squares regression, multi-way partial least squares regression, artificial neural networks) by employing data from a voltammetric bioelectronic tongue, an array formed by enzyme-modified biosensors and applied to the discrimination and quantification of phenolic compounds. FigureWe are making a numerical comparison of various preprocessing strategies for dealing with data from voltammetric electronic tongues in order to reduce the high dimensionality of the response matrices

BioElectronic Tongue for the quantification of total polyphenol content in wine

This work reports the application of a BioElectronic Tongue (BioET) in the estimation of polyphenol content in wine. The approach used an array of enzyme biosensors capable of giving a wide and complete response of the analyzed species, plus a chemometric processing tool able to interpret the chemical signals and extract meaningful data from the complex readings. In our case, the proposed BioET was formed by an array of four voltammetric enzymatic biosensors based on epoxy-graphite composites, one blank electrode and the other three bulk-modified with tyrosinase and laccase on one side, and copper nanoparticles on the other; these modifiers were used in order to incorporate differentiated or catalytic response to different polyphenols present in wine and aimed to the determination of its total polyphenol content value. The obtained voltammetric responses were pre-processed employing the Fast Fourier Transform (FFT); this was used to compress the relevant information whereas the obtained coefficients fed an Artificial Neural Network (ANN) model that accomplished the quantification of total polyphenol content. For comparison purposes, obtained polyphenol content was compared against the one assessed by two different reference methods: Folin-Ciocalteu and UV polyphenol index (I(280)); good prediction ability was attained with correlation coefficients higher than 0.949 when comparing against reference methods. Qualitative discrimination of individual polyphenols found in wine was also assessed by means of Principal Component Analysis which allowed the discrimination of the individual polyphenols under study.

Amperometric enzymatic glucose electrode based on an epoxy-graphite composite

Abstract An inexpensive, robust, polishable and easy to mechanize amperometric transducer showing a long lifetime, based on a composite material made of graphite and non-conducting epoxy resin (Epo-Tek H77), was constructed. This composite was characterised electrochemically using cyclic voltammetry and linear-sweep voltammetry. The applicability of this amperometric transducer was demonstrated in the construction of a glucose biosensor based on the amperometric detection of hydrogen peroxide produced by the catalytic action of glucose oxidase covalently immobilized on a nylon mesh. The sensor shows a linear response range for glucose in the range 10 −5 −10 −2 M when a potential of 1150 mV is applied with respect to an Ag/AgCl electrode in a pH 7.00 buffered solution with 0.1 M phosphate and 0.1 M KCl. The resulting biosensor was compared with a commercial glucose analyser.