Biosensors on Enzymes, Tissues, and Cells

The chemical and topographical characteristics of surfaces have profoimd effects on cellular, tissue, and host responses to synthetic materials [11, 31]. Consequently, surface modifications of chemistry and roughness have been introduced to improve performance in virtually all materials used in biotechnological [e.g., tissue culture and enzyme-linked immimosorbent assay (ELISA) plates, gene and protein array chips, bioseparation and bioprocess matrices] and biomedical (e.g., vascular grafts, orthopedic and dental implants, biosensors, catheters) appUcations. This review focuses on interfaces controlling cell-biomaterial adhesive interactions via manipulations of material surface chemistry to modulate protein adsorption and activity. 

Chemical sensors can be of gas, liquid, and solid particulate sensors based on the phases of the analyte. Depending on the operating principle of transducer in a chemical sensor, it can be used as optical, electrochemical, thermometric, and gravimetric sensor. Chemical sensors also include a special branch referred to as biosensors for the recognition of biochemicals and bio-reactions. The use of biological elements such as organisms, enzymes, antibodies, tissues, and cells as receptors differentiates biosensors from conventional chemical sensors. 

Tissue electrodes [2, 3, 4, 5, 45,57], In these biosensors, a thin layer of tissue is attached to the internal sensor. The enzymic reactions taking place in the tissue liberate products sensed by the internal sensor. In the glutamine electrode [5, 45], a thick layer (about 0.05 mm) of porcine liver is used and in the adenosine-5 -monophosphate electrode [4], a layer of rabbit muscle tissue. In both cases, the ammonia gas probe is the indicator electrode. Various types of enzyme, bacterial and tissue electrodes were compared [2]. In an adenosine electrode a mixture of cells obtained from the outer (mucosal) side of a mouse small intestine was used [3j. The stability of all these electrodes increases in the presence of sodium azide in the solution that prevents bacterial decomposition of the tissue. In an electrode specific for the antidiuretic hormone [57], toad bladder is placed over the membrane of a sodium-sensitive glass electrode. In the presence of the antidiuretic hormone, sodium ions are transported through the bladder and the sodium electrode response depends on the hormone concentration. 

Besides being used as adsorbent for gas molecules, both SWCNTs and MWC-NTs can be cast as a random network or a porous thin film on metal electrodes [57-59] or used as a three-dimensional scaffold [41,42] for biosensors. CNTs serve both as large immobilization matrices and as mediators to improve the electron transfer between the active enzyme site and the electrochemical transducer. Various enzymes, such as glucose oxidase and flavin adenine dinucleotide (FAD) can adsorb onto the CNT surface spontaneously and maintain their substrate-specific enzyme activity over prolonged times [57]. Recently, cells have been grown on CNT scaffolds which provide a three-dimensional permeable environment, simulating the natural extracellular matrix in a tissue [60-62]. 

The immobilization of enzymes, cell cultures, tissues, and other biologically active substances on various electrode substrates have been explored extensively for the development of amperometric biosensors for various substances. This approach has been useful in improving the selectivity of amperometric measurements and hence for developing new amperometric... 

Use of microorganisms and plant and animal tissues as a biological component of biosensor are also described in the literature (9, 10). The principle is based on the use of the natural bio-reactive systems. They have several advantages over the isolated enzymes and receptors. Isolation of enzymes and receptors are often required to increase the response time. Enzymes and receptors retained in the cells are more stable and have longer lifetimes. Cell-based biosensors are also economical as no purification step is required. 

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