E. Martínez-Fàbregas

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.

Flow injection system for on-line potentiometric monitoring of ammonia in freshwater streams

A flow injection system based on potentiometric detection and designed for on-line monitoring of ammonia in freshwater streams is described. A specially constructed all-solid state tubular flow-through ammonium electrode was used in conjunction with a gas-diffusion chamber to enhance its selectivity. Construction of the electrode involved the in situ formation and direct casting of the sensing nonactin-poly(vinyl chloride) membrane on to a mouldable conducting filled polymer (epoxy resin loaded with graphite), which acted as an internal solid contact, without an inner solution. The electrode response under dynamic conditions both with and without the gas-diffusion chamber was evaluated. After optimisation of the flow system parameters, over 30 samples with a wide range of ammonia concentrations (2 × 10–5–10–2M) could be processed per hour with a detection limit of ca. 10–6M. The performance of the monitoring system under on-line conditions in a water treatment plant is also described.

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.

Biosensors based on conducting filled polymer all-solid-state pvc matrix membrane electrodes

Abstract Transducers consisting of PVC matrix membranes applied to conducting filled polymers and featuring no inner reference solution are proposed as viable alternatives to the construction of potentiometric biosensors. The construction procedure basically involves the application of biocatalyst layers (e.g. immobilized enzymes or whole bacterium cells) on mobilecarrier PVC membranes formed in-situ and deposited directly on a conductive plastic support which functions as an internal solid contact. The chief advantage of this construction procedure is the ease with which sensors in different shapes and sizes can be prepared. Thus this technique allows the obtainment of flow-through sensors for use in flow-injection analysis, of which those with sandwich configuration are particularly suitable for the construction of flow-through electrochemical biosensors. The results obtained with various potentiometric urea sensors in different configurations and based on all-solid-state ammonium-selective electrodes constructed by the proposed technique are reported.

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.

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.

Amperometric glucose biosensor based on an electrocatalytically bulk-modified epoxy-graphite biocomposite

Abstract An inexpensive, robust, polishable and easily machinable amperometric glucose transducer was constructed. The biocomposite materials used were graphite, palladium—gold, non-conducting epoxy resin and glucose oxidase. The enzyme retains its bioactivity in the rigid epoxy-graphite matrix. The electrocatalytic oxidation of hydrogen peroxide is enhanced by gold—palladium and it is used as the analytical signal (at 900 mV vs. Ag/AgCl in a pH 7.00 buffered solution with 0.1 M phosphate and 0.1 M KCl). This biosensor exhibits a linear response to glucose in the 0.01–2 mM range and produces steady-state signals within a few seconds (6 s for 95% signal). Simple polishing procedures can be used to generate a fresh bioactive transducer surface.

Covalent binding of urease on ammonium-selective potentiometric membranes☆

As part of the development of disposable urea bioselective probes, the covalent binding of urease on ammonium-selective potentiometric membranes has been assessed. Nonactin/bis(1-butylpentyl)adipate/poly(vinylchloride) (PVC) membranes, directly applied to an internal solid contact (conductive epoxy-graphite composite), has been used as a support for covalent immobilization of urease. Two types of all-solid-state construction process have been assayed: thin layers of cellulose acetate (CA) were coated on the PVC ammonium-selective membranes (type 1) and blends of PVC and CA at various ratios were used as ammonium-selective membrane matrices (type 2). Urease was covalently attached to CA via aldehyde groups. These groups were created on the polysaccharide with sodium periodate to which the enzyme was immobilized through a spacer (hexamethylenediamine). The viability of both types of probe for the determination of ammonium ions was assessed after each step of the activation process. Results indicated that type 2 potentiometric probes are altered after the treatment with sodium periodate. Good results were obtained with type 1 probes. Their dynamic concentration range of response to urea was from 2 x 10(-5) to 0.01 M with a sensibility of 50 mV/decade.