Tissue-based sensors

Of all types of biosensors, metabolism sensors based on the molecular analyte recognition and conversion have been most intensively studied. According to the degree of integration of the biocomponents they can be classified into monoenzyme sensors, biosensors using coupled enzyme reactions, organelle, microbial, and tissue-based sensors. The sequence of the following sections corresponds to this classification. 

Slices and other parts of tissues of animal or plant origin are the most complex biosystems so far applied in biosensors. Tissues containing large amounts of the enzymes of interest have been deliberately used. An overview of tissue-based sensors is given in Table 18. 

Biomimetic/single cell/ tissue-based sensors, instrumented cells (nanocanary), e.g., B-cell sensors... 

He X. Rechnitz G.A., Plant tissue-based pyruvate fiberoptic sensor, Anal. Chim. Acta 1995 316(1) 57-63. 

Vegetable tissue based electrochemical sensors can be divided into two groups according to their principle of operation potentiometric and amperometric. Such devices are usually prepared in a manner similar to that of conventional enzyme electrodes, with the detection of an electroactive species that is consumed or produced by the enzyme present in the vegetable tissue. 

The potentiometric biosensor is a combination of an ion-selective electrode (ISE) base sensor with a vegetable tissue (the source of enzyme), which provides a highly selective and sensitive method for the determination of a given substrate. Advantages of such potentiometric biosensors are simplicity of instrumentation (only a pH meter is needed),... 

Tables 17.1, 17.2 and 17.4 give a list of some vegetable tissue based and crude extract electrodes that have been prepared for analysis of several substrates in environmental, food and pharmaceutical samples. These tables also present the sample matrix, the tissue or crude extract used, the sensor, the range of determinable concentration, the LD, the response time and the stability and/or lifetime of the biosensor.

The stability of enzyme electrodes is difficult to define because an enzyme can lose some of its activity. Deterioration of immobilized enzyme in the potentiometric electrodes can be seen by three changes in the response characteristics (a) with age the upper limit will decrease (e.g., from 10-2 to 10 3 moll-1), (b) the slope of the analytical (calibration) curve of potential vs. log [analyte] decrease from 59.2 mV per decade (Nernstian response) to lower value, and (c) the response time of the biosensor will become longer as the enzyme ages [59]. The overall lifetime of the biosensor depends on the frequency with which the biosensor is used and the stability depends on the type of entrapment used, the concentration of enzyme in the tissue or crude extract, the optimum conditions of enzyme, the leaching out of loosely bound cofactor from the active site, a cofactor that is needed for the enzymatic activity and the stability of the base sensor. 

Flow-Based Systems Needle-type sensors with a fluid flowing over the sensor tip seem to resist biofouling and extend sensor lifetime.31 There are numerous methods that have been investigated for flow-based sensors, such as microperfusion systems,75 microdialysis,76 77 and ultrafiltration.78 Reduced fouling was found with an open microflow system where slow flow of protein-free fluid over the sensor surface at the implant site is effected.73 Different from the other flow-based sensors, the open microflow is controlled by the subcutaneous tissue hydrostatic pressure and does not require a pump. 

The measured signal from an in vivo fluorescence-based sensor depends on both the quantum yield of the fluorophore, which is the ratio of emitted photons to absorbed photons, and the absorption of the excited and emitted light by the surrounding tissue.12-15 While SWNT have a much lower quantum yield than many visible fluorophores, the most important factor for depth of implantation actually turns out to be the absorption coefficient of the surrounding medium.16,17 A one-dimensional absorption-fluorescence model can be used to compare the suitability of fluorophores for in vivo applications ... 

Enzyme sensors can measure analytes that are the substrates of enzymatic reactions. Thermometric sensors can measure the heat produced by the enzyme reaction [31], while optical or electrochemical transducers measure a product produced or cofactor consumed in the reaction. For example, several urea sensors are based on the hydrolysis of urea by urease producing ammonia, which can be detected by an ammonium ion-selective ISE or ISFET [48] or a conductometric device [49]. Amperometric enzyme sensors are based on the measurement of an electroactive product or cofactor [50] an example is the glucose oxidase-based sensor for glucose, the most commercially successful biosensor. Enzymes are incorporated in amperometric sensors in functionalised monolayers [51], entrapped in polymers [52], carbon pastes [53] or zeolites [54]. Other catalytic biological systems such as micro-organisms, abzymes, organelles and tissue slices have also been combined with electrochemical transducers. 

The development of analytically useful enzyme electrodes is limited by the availability of purified and stable enzyme preparations. In an effort to extend the range of measurable species using ISE devices further, Rechnitz and co-workers (Rl) recently introduced bacterial- and tissue-based bio-selective electrode systems. These sensors are prepared in much the same manner as the enzyme probes except that whole intact cells are utilized as the immobilized reagents. There are several potential advantages to this novel approach, including (1) no need to extract and purify the enzymes involved, i. e., low cost (2) enzymes which are unstable when extracted from the cell may be used in situ to maximize and preserve their activity (3) if desired enzyme reactions require cofactors, these co ctors need not be added to the assay mixture because they are already present in the intact cell and (4) analytical reactions involving multistep enzyme sequences already present in the cells may be used to detect given analytes. 

Most EIA use HRP as the marker enzyme. The activity of HRP can be measured photometrically as well as electrochemically. Using the catalase activity of liver tissue, Mascini and Palleschi (1983b) developed a tissue-based electrode for the measurement of hydrogen peroxide and combined the sensor with commercial test kits for digoxin and insulin. The HRP-labeled hormones of the test kit compete with antigen in the sample in a test tube. The bound HRP activity is inversely proportional to the concentration of insulin and digoxine, respectively. 

Carbon-fiber-based sensors are too fragile to be used as deep-tissue measurements. However, these sensors can be protected by insertion into an intravenous catheter that has been implanted in tissue or blood vessels." - - To make the catheter-protected sensor, a bundle of carbon fibers is mounted inside a truncated needle (Figure ll). " After curing, the shaft of the truncated needle is coated with nonconducting epoxy. Then conductive porphyrinic film... 

In similar fashion to DNA and protein microarrays the ability to create arrays of cells and even tissues at high-density offers the potential for the development of cell-based sensors... 

See also-. Enzymes Enzyme-Based Electrodes. Flow Injection Analysis Detection Techniques. Liquid Chromatography Principles. Process Analysis Sensors. Sensors Amperometric Oxygen Sensors Tissue-Based. Titrimetry Overview. Water Analysis Sewage Biochemical Oxygen Demand Chemical Oxygen Demand. 

See also Sensors Overview Amperometric Oxygen Sensors Calorimetric/Enthalpimetric Chemically Modified Electrodes Microorganism-Based Photometric Tissue-Based. 

Another group of hybrid sensors uses several bioactive components that are hybridized at membrane electrodes. For example, sensors have been proposed in which a microorganism plus an auxiliary enxyme are coimmobilized [118] the function of the enzyme is to facilitate the overall biocatalytic scheme by conversion of the initial substrate, an intermediate, or the product of the microbial metabolism. Auxiliary enzymes have also been used in conjunction with both plant and animal-tissue based membrane electrodes [119]. 

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