Salvador Alegret

Configurations used in the design of screen-printed enzymatic biosensors. A review

Abstract Different thick-film biosensor configurations designed during the last decade are revised. These planar configurations are classified in three groups: (a) multiple-layer deposition (biological deposition by hand or electrochemically), (b) screen-printing of composite inks or pastes using two or more steps (biological deposition by screen-printing), (c) one-step deposition layer or biocomposite strategy. Different enzyme immobilisation procedures corresponding to these configurations are also revised. These procedures consist mainly of enzyme adsorption onto transducer surface, biological immobilisation by cross-linking in a glutaraldehyde layer, and physical, electro- or UV-polymerisation entrapment. Immobilisation into the bulk of different carbon-based inks or pastes is also presented.

Electrochemical genosensor design: immobilisation of oligonucleotides onto transducer surfaces and detection methods

The present report reviews immobilisation techniques of purified oligonucleotides on electrochemical transducers and their corresponding detection techniques. Most of the literature reviewed was published in the 1990s. The immobilisation techniques of a DNA probe to the surface of an electrochemical transducer made from carbon, gold, platinum or polypyrrole, ranged from simple adsorption to covalent bonding. Recent efforts to couple the recognition layer containing the immobilised nucleic acid recognition layer with the electrochemical signal transducer are discussed. Special attention is given to hybridisation biosensing based on electroactive indicators.

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.

Flow-through tubular PVC matrix membrane electrode without inner reference solution for flow injection analysis

A simply constructed tubular PVC matrix membrane electrode without inner reference solution suitable for flow injection analysis is described. The nitrate-selective electrode developed allows simple replacement of an exhausted membrane, or change of function with a new sensor membrane. The compact design ensures mechanical stability and the tubular configuration minimizes distortion of the hydrodynamic flow in the detector. Nitrate can be determined in the range 10−1–10−5 M, under different flow conditions, with a sampling frequency of about 120 h−1.

A unified physicochemical description of the protonation and metal ion complexation equilibria of natural organic acids (humic and fulvic acids). 2. Influence of polyelectrolyte properties and functional group heterogeneity on the protonation equilibria of fulvic acid.

Potentiometric studies of the neutralization of several fulvic acid sources with standard base in aqueous and nonaqueous media have been conducted. Analysis of the results with a recently developed unified physicochemical model has shown that the protonation behavior of these fulvic acid sources is a reflection of their polyelectrolyte nature and their heterogeneity. It has been possible to ascribe the polyelectrolyte properties observed to a rather inflexible fulvic acid molecule whose variably charged surface is impermeable to simple electrolyte. 42 references, 24 figures, 4 tables.

Application of a potentiometric electronic tongue as a classification tool in food analysis

This paper reports on the application of a potentiometric sensor array to the food analysis field, in order to distinguish simple tastes and to classify food samples. This array is formed by a set of non-specific all-solid-state potentiometric sensors and has been used in combination with principal component analysis (PCA) for the classification of food samples in batch and in flow injection mode. First attempt was to classify synthetic samples prepared with controlled variability. Once this ability is proven, satisfactory classification results are presented for commercial waters, orange-based drinks and tea samples. An interesting correlation is achieved between the natural juice content and its first calculated component, which allows for a very simple tool for screening purposes.

New materials for electrochemical sensing. IV. Molecular imprinted polymers

Molecular imprinted polymers (MIPs) have become an important tool in the preparation of artificial and robust recognition materials capable of mimicking natural systems. When compared with natural molecular recognition products, such as antibodies, MIPs bring several advantages, such as low-cost, predictable specificity, durability and mass production. An overview of the use of MIPs for the design of electrochemical sensors based on different signal-transduction schemes is presented here. The revised transduction schemes include capacitive, conductometric, voltammetric and potentiometric sensors. The coupling of MIPs with these transducers is still at an early stage.

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.

Chemical sensors, biosensors and thick-film technology

Abstract Trends in thick-film technology as applied in chemical sensor and biosensor fabrication are briefly reviewed and discussed. Current work aims at the production of sensor devices with a wide application range and commercial viability. Planar technologies are being employed for developing solid-state sensors having low cost, small size and high reproducibility. These technologies are compatible with silicon-based procedures that permit integration of the sensor and the associated electronic circuitry in the same chip. The use of planar technologies to produce sensors based on thick films can be very convenient since it calls for an inexpensive infrastructure, and is feasible for small to medium production runs. Once developed, a thickfilm fabrication process can easily be combined with other planar technologies (chemical vapor deposition, plasma etching, etc.). The resulting devices can be produced in high volume at lower cost, with greater reproducibility and with further miniaturization.