Biosensors electrochemical

Electrochemical transducers work based on either an amperometric, potentio-metric, or conductometric principle. Further, chemically sensitive semiconductors are under development. Commercially available today are sensors for carbohydrates, such as glucose, sucrose, lactose, maltose, galactose, the artificial sweetener NutraSweet, for urea, creatinine, uric acid, lactate, ascorbate, aspirin, alcohol, amino acids and aspartate. The determinations are mainly based on the detection of simple co-substrates and products such as 02, H202, NH3, or C02 [142]. 

Amperometric transducers measure the current (flux of electrons) caused by oxidation or reduction of the species of interest when a voltage is applied between the working and the reference electrode. Often, oxygen serves as electron acceptor but interferences have encouraged development of new methods to avoid this, e.g. controlled oxygen supply and the application of mediators, such as ferrocene, e.g. [169]. Other determinations focus on the 

Potentiometric transducers measure the potential between the sensing element and a reference element. Thus, in contrast to amperometric transducers, practically no mass transport occurs the response depends on the development of the thermodynamic equilibrium. pH changes often correlate with the measured substance because many enzymatic reactions consume or produce protons. 

Conductometric transducers consist of two pairs of identical electrodes, one of which contains an immobilized enzyme. As the enzyme-catalyzed reaction causes concentration changes in the electrolyte the conductivity alters and can be detected. 

Elechochemical biosensors combme tire analytical power of elechochemical techniques with the specificity of biological recognition processes. The aun is to 

Undoubtedly, two of the biggest growth areas for electrochemical sensing devices and instrumentation lie in biochemical and biomedical applications. Laboratory developments have involved a remarkably wide range of species but we will restrict this section to those sensors which are available commercially and which have become acceptable to biochemists and biomedical practitioners. 

While there is considerable overlap, the application areas for biosensors include  

Laboratory measurements of pH, p02, glucose, penicillin and certain enzymes in a range of media including tissue extracts and body fluids such as blood, saliva and urine. 

Laboratory immunoassays in which the sensitivity of electrochemical detection is combined with the specificity of an antibody-antigen interaction. 

Tn-vivo measurements, where the sensor is inserted into the body or perhaps into a pumped recycle loop incorporating a body fluid. 

Following the actual definition (Chap. l,Sect. 1.2),biosensors are characterized by a receptor function which is implemented by biologically active substances that are able to recognize selectively other substances by a biological mechanism. This biological recognition is based on the lock-and-key principle in the majority of cases. It means that molecules are identified by their size. 

The selectivity of biosensors is astonishing. Bioactive substances sometimes may identifj reliably one specific substance among a matrix of millions of others. 

Biosensors either work as biocatalytic or as bioaffinity sensors. In biocat-alytic sensors, mostly enzymes are immobihzed at an electrode surface to act as selective catalysts. Enzymes catalyse slow reactions. The reaction rate, under appropriate conditions, is a quantitative measure for the substrate concentration. If the sample is one of the reactants, i.e. if the sample itself is the substrate, then chemical sensors can be created on this basis. The reaction rate can be measured in terms of an electrolysis current if amperometric biosensors are appHed. An alternative approach is to indicate the reaction product of the catalysed reaction selectively, as is done with potentiometric biosensors, hi bioaffinity sensors, as the second group, commonly very stable complexes with sample molecules are formed and bound strongly to the sensor surface. The extent of this complex formation is a quantitative measure of the sample concentration. It can be measured indirectly, since many properties of the electrode are changed by complex formation. In bioaffinity sensors, mostly the antibody-antigen reaction is utilized. 

Nucleic acid sensors can be classified as another group of biosensors. 

Bioactive substances designed to act as receptors must be immobilized at an electrode surface. There are some differences depending on the question whether biocatalytic or bioaffinity sensors must be realized. Potentiometric and amperometric sensors also are characterized by different requirements. In principle, with potentiometric sensors a higher impedance can be tolerated, whereas amperometric sensors require a good electric conductivity. Nevertheless, immobilization techniques are similar for both groups. 

The interaction between the enzyme and substrate produces an electrochemical signal that can be detected by an electrochemical detector. These are based on mediated or unmediated electrochemistry. 

Potentiometry is a rarely used detection method employed in biosensors, with enzymes immobilised in an electrodeposited polymer layer, although certain advantages over 

Biosensors based on conducting polymers have found potential applications in healthcare, veterinary medicine, environmental monitoring, immunosensing, etc. 

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The following sources provide additional information on electrochemical biosensors. 

In Vivo Biosensing. In vivo biosensing involves the use of a sensitive probe to make chemical and physical measurements in living, functioning systems (60—62). Thus it is no longer necessary to decapitate an animal in order to study its brain. Rather, an electrochemical biosensor is employed to monitor interceUular or intraceUular events. These probes must be small, fast, sensitive, selective, stable, mgged, and have a linear response. 

Entrapment of biochemically reactive molecules into conductive polymer substrates is being used to develop electrochemical biosensors (212). This has proven especially useful for the incorporation of enzymes that retain their specific chemical reactivity. Electropolymerization of pyrrole in an aqueous solution containing glucose oxidase (GO) leads to a polypyrrole in which the GO enzyme is co-deposited with the polymer. These polymer-entrapped GO electrodes have been used as glucose sensors. A direct relationship is seen between the electrode response and the glucose concentration in the solution which was analyzed with a typical measurement taking between 20 to 40 s. 

ENVIRONMENTAL AND FOOD APPLICATIONS WITH DNA ELECTROCHEMICAL BIOSENSORS... 

Similarly to the above-mentioned entrapment of proteins by biomimetic routes, the sol-gel procedure is a useful method for the encapsulation of enzymes and other biological material due to the mild conditions required for the preparation of the silica networks [54,55]. The confinement of the enzyme in the pores of the silica matrix preserves its catalytic activity, since it prevents irreversible structural deformations in the biomolecule. The silica matrix may exert a protective effect against enzyme denaturation even under harsh conditions, as recently reported by Frenkel-Mullerad and Avnir [56] for physically trapped phosphatase enzymes within silica matrices (Figure 1.3). A wide number of organoalkoxy- and alkoxy-silanes have been employed for this purpose, as extensively reviewed by Gill and Ballesteros [57], and the resulting materials have been applied in the construction of optical and electrochemical biosensor devices. Optimization of the sol-gel process is required to prevent denaturation of encapsulated enzymes. Alcohol released during the... 

Thevenot D.R., Toth K., Durst R.A., Wilson G.S., Electrochemical biosensors Recommended definitions and classification, Pure Appl. Chem. 1999 71 2333-2348. 

MWNTs favored the detection of insecticide from 1.5 to 80 nM with a detection limit of InM at an inhibition of 10% (Fig. 2.7). Bucur et al. [58] employed two kinds of AChE, wild type Drosophila melanogaster and a mutant E69W, for the pesticide detection using flow injection analysis. Mutant AChE showed lower detection limit (1 X 10-7 M) than the wild type (1 X 10 6 M) for omethoate. An amperometric FIA biosensor was reported by immobilizing OPH on aminopropyl control pore glass beads [27], The amperometric response of the biosensor was linear up to 120 and 140 pM for paraoxon and methyl-parathion, respectively, with a detection limit of 20 nM (for both the pesticides). Neufeld et al. [59] reported a sensitive, rapid, small, and inexpensive amperometric microflow injection electrochemical biosensor for the identification and quantification of dimethyl 2,2 -dichlorovinyl phosphate (DDVP) on the spot. The electrochemical cell was made up of a screen-printed electrode covered with an enzymatic membrane and combined with a flow cell and computer-controlled potentiostat. Potassium hexacyanoferrate (III) was used as mediator to generate very sharp, rapid, and reproducible electric signals. Other reports on pesticide biosensors could be found in review [17],... 

M. Bernabai, C. Cremisini, M. Mascini, and G. Palleschi, Determination of organophosphorus and car-bamic pesticides with a choline and acetylcholine electrochemical biosensor. Anal. Lett. 24, 1317-1331 (1991). 

T. Neufeld, I. Eshkenazi, E. Cohen, and J. Rishpon, A micro flow injection electrochemical biosensor for organophosphorus pesticides. Biosens. Bioelectron. 15, 323-329 (2000). 

J. Frew and H.A. Hill, Electrochemical biosensors. Anal. Chem. 59, 933A-939A (1987). 

P. Hilditch and M. Green, Disposable electrochemical biosensors. Analyst 116, 1217-1220 (1991). 

S. Sasso, R. Pierce, R. Walla, and A. Yacynych, Electropolymerized 1,2-diaminobenzene as a means to prevent interferences and fouling and to stabilize immobilized enzyme in electrochemical biosensors. Anal. Chem. 62, 1111-1117 (1990). 

J. Min and A.J. Baeumner, Characterization and optimization of interdigitated ultramicroelectrode arrays as electrochemical biosensor transducers. Electroanalysis 16, 724—729 (2004). 

M. Ozsoz, A. Erdem, P. Kara, K. Kerman, and D. Ozkan, Electrochemical biosensor for the detection of interaction between arsenic trioxide and DNA based on guanine signal. Electroanal. 15, 613-619... 

Accurate, rapid, cheap, and selective analysis is required nowadays for clinical and industrial laboratories. Electrochemical biosensors seem to accomplish this function. 

Electrochemical biosensors based on detection of hydrogen peroxide at platinized electrodes were found to be more versatile allowing a decrease in detection limit down to 1 i,mol L 1 [109]. However, all biological liquids contain a variety of electrochemically easily oxidizable reductants, e.g. ascorbate, urate, bilirubin, catecholamines, etc., which are oxidized at similar potentials and dramatically affect biosensor selectivity producing parasitic anodic current [110]. 

CNTs offer an exciting possibility for developing ultrasensitive electrochemical biosensors because of their unique electrical properties and biocompatible nanostructures. Luong et al. have fabricated a glucose biosensor based on the immobilization of GOx on CNTs solubilized in 3-aminopropyltriethoxysilane (APTES). The as-prepared CNT-based biosensor using a carbon fiber has achieved a picoamperometric response current with the response time of less than 5 s and a detection limit of 5-10 pM [109], When Nation is used to solubilize CNTs and combine with platinum nanoparticles, it displays strong interactions with Pt nanoparticles to form a network that connects Pt nanoparticles to the electrode surface. The Pt-CNT nanohybrid-based glucose biosensor... 

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