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Polymer Science and Engineering: The Shifting Research Frontiers
thereafter, of moieties that respond in some detectable way to the presence of the chemical to be analyzed. For example, polymers that are modified to bind dyes that respond to blood chemistry (oxygen, carbon dioxide, acidity) or to immobilize enzymes that produce reactions with substances of biological interest, such as glucose, are used to construct biosensors for in vivo application.
Another polymer property used in such sensors is permeability. The polymer allows diffusive transport of the chemical to the immobilized functionality to enable interaction and subsequent detection of the reaction products. When increased transport kinetics are required, the polymer may be fabricated in a porous state or may be engineered to swell or expand in the medium in which the sensor is immersed, such as water. In other cases, polymers may be engineered as a controlled-release material, supplying reagents to the surrounding medium for local detection.
The polymer properties described here are being used in the development of fiber-optic chemical sensors. These sensors employ dye molecules incorporated into transparent polymers that form either part of the fiber structure or part of an active element, termed the "optrode," located at the terminus of the fiber. The sensors may incorporate either absorbing or fluorescent dyes for detection of specific chemical species. Light injected into the fiber, at a location remote from the chemical environment being probed, interacts with the dye and is absorbed or produces fluorescence. When a chemical species permeates the polymer and alters the absorption or fluorescence of the dye, the light output of the fiber returning from the optrode is altered in a quantitatively detectable manner.
Chemically modified electrode sensors rely on the measurement of electrical potentials produced by selective electrochemical reactions involving the chemical species to be determined. The development of thin polymer coatings to chemically modify the electrodes is an important topic of research in this field. The polymers are chemically and physically modified to concentrate electroactive sites at the electrode surfaces, to provide large ion and electron mobility, and to ensure a stable environment for the desired electrochemical reactions. Especially promising areas of investigation include the development of such sensors for determination of specific ions and products of biochemical reactions with enzymes or antibodies immobilized in the polymer film.
A related sensor type in the chemical and biomedical fields is the microsensor based on integrated solid-state electronic devices, for example, CHEMFETS. These sensors incorporate chemically sensitive polymer films placed in contact with the gate of a field effect transistor on a transducing silicon chip. The electrical current output of the device is modulated by the chemical environment at its surface. The polymer films are tailored in their chemical and physical properties to optimize specific solubility interactions and/or chemical activity with the substance to be sensed, thereby controlling the sensitivity and selectivity of the sensor. Polymers used for this purpose must often be deposited and patterned using the standard photolithographic techniques of the semiconductor