Electrochemical sensors electrode

Immobilized Enzymes. The immobilized enzyme electrode is the most common immobilized biopolymer sensor, consisting of a thin layer of enzyme immobilized on the surface of an electrochemical sensor as shown in Figure 6. The enzyme catalyzes a reaction that converts the target substrate into a product that is detected electrochemicaHy. The advantages of immobilized enzyme electrodes include minimal pretreatment of the sample matrix, small sample volume, and the recovery of the enzyme for repeated use (49). Several reviews and books have been pubHshed on immobilized enzyme electrodes (50—52). 

Moreover, disposable electrochemical sensors for the detection of a specific sequence of DNA were realised by immobilising synthetic single-stranded oligonucleotides onto a graphite or a gold screen-printed electrode. Tire probes became hybridised with different concentrations of complementary sequences present in the sample. 

Microfabrication technology has made a considerable impact on the miniaturization of electrochemical sensors and systems. Such technology allows replacement of traditional bulky electrodes and beaker-type cells with mass-producible, easy-to-use sensor strips. These strips can be considered as disposable electrochemical cells onto which the sample droplet is placed. The development of microfabricated electrochemical systems has the potential to revolutionize the field of electroanaly-tical chemistry. 

Particularly attractive for numerous bioanalytical applications are colloidal metal (e.g., gold) and semiconductor quantum dot nanoparticles. The conductivity and catalytic properties of such systems have been employed for developing electrochemical gas sensors, electrochemical sensors based on molecular- or polymer-functionalized nanoparticle sensing interfaces, and for the construction of different biosensors including enzyme-based electrodes, immunosensors, and DNA sensors. Advances in the application of molecular and biomolecular functionalized metal, semiconductor, and magnetic particles for electroanalytical and bio-electroanalytical applications have been reviewed by Katz et al. [142]. 

Indicator electrodes are used both for analytical purposes (in determining the concentrations of different substances from values of the open-circuit potential or from characteristic features of the polarization curves) and for the detection and quantitative characterization of various phenomena and processes (as electrochemical sensors or signal transducers). One variety of indicator electrode are the reference electrodes, which have stable and reproducible values of potential and thus can be used to measure the potentials of other electrodes. 

The ISFET is an electrochemical sensor based on a modification of the metal oxide semiconductor field effect transistor (MOSFET). The metal gate of the MOSFET is replaced by a reference electrode and the gate insulator is exposed to the analyte solution or is coated with an ion-selective membrane as illustrated in Fig. 

The high specificity required for the analysis of physiological fluids often necessitates the incorporation of permselective membranes between the sample and the sensor. A typical configuration is presented in Fig. 7, where the membrane system comprises three distinct layers. The outer membrane. A, which encounters the sample solution is indicated by the dashed lines. It most commonly serves to eliminate high molecular weight interferences, such as other enzymes and proteins. The substrate, S, and other small molecules are allowed to enter the enzyme layer, B, which typically consist of a gelatinous material or a porous solid support. The immobilized enzyme catalyzes the conversion of substrate, S, to product, P. The substrate, product or a cofactor may be the species detected electrochemically. In many cases the electrochemical sensor may be prone to interferences and a permselective membrane, C, is required. The response time and sensitivity of the enzyme electrode will depend on the rate of permeation through layers A, B and C the kinetics of enzymatic conversion as well as the charac-... 

If the electrochemical sensor does not require a permselective membrane, immobilization of the enzyme onto the surface of the electrode is possible. Glassy carbon graphite reticulated vitreous carbon and carbon paste electrodes... 

A rational manipulation of the electrode surface functionality by immobilizing selected types of molecules is an essential key for the development of electrochemical sensors and devices. One of the recent studies in this area has focused on monolayer formation on gold surfaces from organosulfur precursors [11]. Stable attachment of organic molecules to gold... 

Special electrochemical sensors that operate on the principle of the voltammetric cell have been developed. The area of chemically modified solid electrodes (CMSEs) is a rapidly growing field, giving rise to the development of new electroanalytical methods with increased selectivity and sensitivity for the determination of a wide variety of analytes [490]. CMSEs are typically used to preconcentrate the electroactive target analyte(s) from the solution. The use of polymer coatings showing electrocatalytic activity to modify electrode surfaces constitutes an interesting approach to fabricate sensing surfaces useful for analytical purposes [491]. 

There is an increasing interest in the development of electrochemical sensors and microsensors for detecting and monitoring NO or N02, due to their importance in clinical and environmental analysis. It has been suggested that transition metal electrocatalysts active for NO or N02 coordination and reduction could be exploited for the development of metal-complex film electrodes for N02 and NO sensing. However, most of the sensory devices reported so... 

Chitosan-clay bio-nanocomposites are very stable materials without significant desorption of the biopolymer when they are treated with aqueous salt solutions for long periods of time. In this way, they act as active phases of electrochemical sensors for detection of ions (Figure 1.8). The particular nanostructuration of the biopolymer in the interlayer region drives the selective uptake of monovalent versus polyvalent anions, which has been applied in electrode arrays of electronic tongues [132]. 

S. Centi, S. Laschi, M. Franek, and M. Mascini, A disposable immunomagnetic electrochemical sensor based on functionalized magnetic beads and carbon-based screen-printed electrodes (SPCEs) for the detection of polychlorinated biphenyls (PCBs). Anal. Chim. Acta 538, 205—212 (2005). 

T. Garcia, E. Casero, E. Lorenzo, and F. Pariente, Electrochemical sensor for sulfite determination based on iron hexacyanoferrate film modified electrodes. Sens. Actuators B chem. 106, 803 (2005). 

The electrodes modified with vertically aligned CNTs have recently received much interest for the purpose of designing CNT-based electrodes and electrochemical sensors. 

Many methods including photometric, fluorimetric, chromatographic, and electrochemical methods have been used to detect the antioxidants so far. Recently, electrochemical methods have intensively been used for antioxidant detection. Among the electrochemical methods, the detection of antioxidant based on the direct redox transformation of cyt c has been studied over the decade. Since cyt c can act as an oxidant of superoxide, the superoxide level in solution can be detected as an oxidation current at the sensor electrode due to electron transfer from the radical via cyt c to the electrode. 

Electrochemical sensors with a liquid electrolyte are widely used for the detection of corrosive or toxic gases in the workplace. Portable monitors are used in short time measurements of exhaust gases as well. These sensors work amperometri-cally - an external voltage supply is connected with the electrode on both sides of the measuring cell. 

The concept of thin films of a molecularly imprinted sol-gel polymer with specific binding sites for a target analyte is general and can be applied also to electrochemical sensors. For example, a sensor to detect parathion in aqueous solutions is based on films cast on glass substrates and on glassy carbon electrodes (Figure 6.14).12... 

Electrochemical sensors have several disadvantages with respect to optical sensors (i) they are based on electrodes and require a reference electrode (ii) the liquid-liquid junction is easily perturbed by external factors (iii) they are sensitive to electrical interferences (iv) miniaturization is not easy and their cost is relatively high. However, optical sensors also have some disadvantages (i) ambient light can interfere (ii) the range over which the concentration of an analyte can be accurately measured is often limited (iii) they have generally limited long-term stability. 

Huang, H., et ah, Fabrication of new magnetic nanoparticles (Fe304) grafted multiwall carbon nanotubes and heterocyclic compound modified electrode for electrochemical sensor. Electroanalysis, 2010. 22(4) p. 433-438. 

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