Page 107 - Academic Press Encyclopedia of Physical Science and Technology 3rd BioTechnology

P1: GNB Final Pages
Encyclopedia of Physical Science and Technology EN005F-954 June 15, 2001 20:48

820 Fiber-Optic Chemical Sensors

curing or cross-linking, the polymer forms and entraps reaction with an isothiocyanate-containing dye. Sensing
the material within its pores. Alternatively, complemen- materials are also covalently immobilized to a fiber sur-
tary solubility between polymer and sensing material can face by using similar chemistries to those employed with
be used to effect the entrapment. For example, lipophilic thin polymer or sol–gel glass films.
indicators will dissolve in lipophilic polymers and are not An alternative method for dye immobilization is based
easily leached. Another way to entrap an indicator is to on photopolymerization. Dye-doped monomers or dyes
initiate polymerization of a monomer solution contain- containing polymerizable groups can be copolymerized
ing the indicator. When the polymer is formed, it entraps with a monomer on a silanized fiber tip. In these proce-
the indicator. Such polymers can be either thermally or dures, it is important not to modify the sensing material in
photochemically initiated and attached to the fiber surface such a way as to disturb its ability to bind to the analyte
by dip-coating procedures. Silanization of the fiber sur- or transduce the signal. Covalent immobilization methods
face with polymerizable groups, such as acrylates or vinyl are usually complicated and time-consuming compared
residues, enhances adhesion of the polymer to the surface. with the other two immobilization techniques, but are very
Optically transparent sol–gel glasses are also used for reliable since the dye or indicator is not likely to leach out.
sensing material entrapment. Sol–gel glasses are produced Covalent binding may result in a reduced response of the
by hydrolysis and polycondensation of organometallic sensing material due to bond formation and reduced de-
compounds, such as tetraethyl orthosilicate, Si(OCH 3 ) 4 . grees of freedom upon immobilization.
A sensing material is added to the reaction mixture at
some time during the formation of the sol or gel. This
3. Biological Sensing Materials (Fiber-Optic
viscous sol–gel solution is coated onto an optical fiber to Based Biosensors)
form a sensing element. Sol–gel glasses prepared by this
method contain interconnected pores formed by a three- Fiber-optic biosensors are a subtype of fiber-optic chemi-
dimensional SiO 2 network. As a result, the sensing ma- cal sensors that rely on sensing materials of biological ori-
terial is trapped but small analytes can readily diffuse in gin. In fiber-optic biosensors, biological recognition com-
and out of the pores. One advantage of sol–gel glass im- ponents such as enzymes, antibodies, DNA, or cells are
mobilization is its compatibility with many inorganic and attached to the optical fiber sensing layer in order to alter
organic reagents allowing many types of sensing materials the specificity of the sensor.
to be entrapped. Also, sol–gel glasses are chemically, pho- In nature, interaction between biological molecules,
tochemically, and mechanically stable and solvent resis- suchasreceptor–ligand,antibody–antigen,ortwocomple-
tant, and can therefore be useful in harsh conditions. Dis- mentary DNA strands, are highly specific. Some of these
advantages of sol–gel glass immobilization are the slow recognition molecules can be purified and used in fiber-
response times in aqueous media and the fragility of the optic biosensors. Moreover, by using genetic engineering,
thin sol–gel glass films compared with polymer films. recombinant recognition molecules can be produced and
used. Fiber-optic biosensors can be miniaturized and used
c. Covalent immobilization. Robust sensing mate- in portable analytical devices for clinical, environmental,
rials are formed by covalent binding to the substrate. Both and industrial applications. Clinical fiber-optic biosensors
the substrate and the sensing materials must contain re- have been developed for detecting cancer cells, pathogenic
active groups for covalent attachment to occur. There are bacteria, viruses, toxic proteins, hormones, and drugs.
many methods for covalently immobilizing a sensing ma- Environmentally important analytes such as pesticides,
terial to a fiber surface. One simple way is to covalently heavymetals,andcarcinogeniccompoundscanalsobede-
modify the fiber surface by silanizing with trialkoxysi- tected using fiber-optic biosensors. Industrial applications

lyl compounds of the type (RO) 3 SiR , with R being ethyl of fiber-optic biosensors include on-line process monitor-
or methyl and R being aminopropyl, 3-chloropropyl, 3- ing of bacteria or mammalian cell-based bioprocesses.

glycidyloxy, vinyl, or a long-chain amine. The functional Biological sensing molecules enable the detection of
group on the fiber surface then reacts with the sensing an expanded number of analytes. Such molecules tend to
materials. In some cases, the sensing material must first be sensitive to pH or temperature changes and have poor
be activated for reaction with the substrate. For example, stability, resulting in short lifetimes. Another important
dyes possessing COOH groups can be converted to N- limitation is the high cost of purified biological sensing
hydroxysuccinimidoyl esters that can react with an amine- compounds.Biologicalsensingcompoundscanbedivided
modified surface. Amine-modified surfaces can also be into two categories based on their bioactivity: biocatalysts
derivatized with amine-containing dyes by using bifunc- (enzymes and cells) and bioaffinity molecules (antibodies,
tional cross-linkers such as glutaraldehyde or by direct receptors, and nucleic acids).