Page 106 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

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Encyclopedia of Physical Science and Technology EN005F-954 June 15, 2001 20:48

Fiber-Optic Chemical Sensors 819

FIGURE 15 Schematic diagram of three different immobilization techniques. (a) Absorption/electrostatic, (b) entrap-
ment, (c) covalent immobilization.

a. Adsorption/electrostatic immobilization. Ad- In order to prevent leaching, strong ion exchangers are
sorption immobilization methods involve adsorbing employed.
the sensing material onto a solid surface or polymer The absorption immobilization method is very easy and
matrix. Sensing materials can be adsorbed onto the highly reproducible. Sensing material loading can be con-
end of optical fibers either directly or with the aid of a trolled easily by changing the immobilization time. Also,
thin polymer film. This immobilization method is the the immobilized reagents are easily accessible to analyte
simplest of all the methods (described below); however, because they are situated at sites on the polymer surface.
the adsorbed sensing material tends gradually to leach out
from the solid support, decreasing sensing performance b. Entrapment immobilization. In the entrapment
and/or lifetime. The surface may first be modified with method, the sensing material is physically trapped within
complementary functional groups to enhance retention of a porous matrix. Many different indicators, such as flu-
the material. For example, a hydrophobic surface can be orophores or biological materials, may be immobilized
prepared to immobilize a hydrophobic species. in this fashion. When this immobilization method is de-
Electrostatic immobilization employs similar comple- signed properly, trapped reagents either do not leach out or
mentarity. The fiber surface can be coated with a thin leach out very slowly from the matrix. The rate of leach-
polyelectrolyte membrane, which interacts electrostati- ing depends on the indicator size, molecular weight, and
callywithoppositelychargedsensingmaterials.Forexam- porosity of the matrix. Organic polymer matrices have
ple, a negative surface, such as sulfonated polystyrene, can been widely used for entrapping sensing materials in the
be prepared to efficiently immobilize a positively charged void volume of polymers. Sensing materials are first dis-
dye. Similarly, cationic polymers can bind various anions. solved in a common solvent with an appropriate polymer
Ionicindicatorsorbiologicalmolecules(i.e.,eithercations or polymer precursor and then the optical fiber is either
or anions) can be immobilized by electrostatic interaction. spin-coated or dip-coated with the viscous solution. Upon