Page 35 - Academic Press Encyclopedia of Physical Science and Technology 3rd Analytical Chemistry

P1: GJB Revised Pages
Encyclopedia of Physical Science and Technology En001f25 May 7, 2001 13:58

574 Analytical Chemistry

industrial chemical processes, long-term remote environ- selective membrane. The variation of potential of one sur-
mentalmonitoring,automationofclinicalanalyses,invivo face of this membrane can control any electronic current
drug monitoring, and feedback control of artificial organs. through the transistor known as the drain current. A simi-
All chemical sensors employ a basic union between a lar arrangement employing an enzyme–substrate receptor
selective receiving site for ionic or molecular binding and system at the gate also provides for ionic charge control
a transducer that is capable of translating a chemical bind- of transistor electronic current conduction, resulting in a
ing into a useful analytical signal. Transducers can be di- chemically sensitive field effect transistor (CHEMFET).
vided into four categories: semiconductors, conventional Multiple enzyme systems, where one enzyme produces
ion-selective electrodes, optical devices, and piezoelectric an electrochemically inactive product that is consumed as
devices. Other than the ion-selective electrodes described a substrate by another enzyme to form an active prod-
previously, the semiconductor-based gas sensors are the uct, have been successfully used to extend enzyme se-
best known chemical sensors. These devices usually op- lectivity. The selectivity of immunochemical systems has
erate at elevated temperatures, allowing certain gases to been employed by implementation of enzyme-linked as-
interact chemically with the semiconductor surface to alter says. Direct coupling of redox relay centers of enzymes
its electrical characteristics. Such devices may be selective to conductive electrodes has been achieved by a technique
to certain gases such as CO 2 and O 2 or to certain classes known as molecular wiring and avoids the indirect anal-
of molecules such as hydrocarbons and may take the form ysis of products of enzyme–substrate reactions. This fast
of chemiresistors or thin-film metal oxides. and sensitive technique measures current flow and is com-
Better selectivity can be attained by the use of special- mercially available.
ized chemical receptors. The ion-exchange chemistry of
ion-selective membranes is one manner in which selec- 2. Optical Devices
tivity is enhanced for simple inorganic ions and gases.
Nature provides the materials for the complicated task of In analogy to electrodes, chemically selective optical de-
selectively complexing biochemicals through molecular vices have been termed optrodes. These systems employ
recognition processes. The selectivity of these receptors is absorption and luminescence strategies and make use of
based on the tertiary structure of proteins and polypeptides waveguide fiber-optic and laser technology for miniatur-
and on nucleic acid complementary pairing. Enzyme– ization. Light in the ultraviolet or visible region of the
substrate, antibody–antigen, lectin–saccharide, and hor- electromagnetic spectrum can be passed through an opti-
mone receptor–hormone selective reactions have all been cal fiber to a remote reaction vessel. Detection may de-
employed in conjunction with “building block” construc- pend on observation of light passing through the reaction
tion and a variety of transducers to produce sensitive and zone, or being transmitted toward a detector by another
selective devices. Such selective chemistry has also found fiber, or by the evanescent wave phenomenon encoun-
wide application in techniques such as immunoassay. The tered in techniques such as attenuated total reflection or
development of catalytic antibodies (synzymes) and ge- total internal reflection fluorescence spectroscopy. In con-
netic engineering to express proteins of DNA of inter- trast to electrochemical devices, these systems offer ad-
est for chemical selectivity and the introduction of artifi- vantages such as insensitivity to electrical interference,
cial receptor sites and selective surfaces have substantially elimination of reference electrodes, possibility for change
broadened the commercial potential for long-term devel- of reagent phase, distributed multisensor operation with
opment of a wide range of chemical sensors. the use of one optical detector (spectrophotometer), mul-
tiwavelength analysis, time-resolved analysis, and a com-
plementary range of analytes that may not be electrochem-
1. Electrochemical Devices
ically active. One example is a pH sensor, based on optical
Thesedevicesarebasedonthemeasurementofeitherelec- absorption changes of an indicator dye trapped in a gel ma-
trochemical potential or faradaic current associated with trix in a cell at the tip of a fiber-optic system. Ratio meth-
redox reactions at an electrode. They are particularly suit- ods of quantitative analysis and multiwavelength analy-
able for enzyme–substrate receptor systems by virtue of sis for selective observation of two or more species are
the ionic products often produced in such reactions. The preferred for elimination of background drift and interfer-
sensing membranes of the ion-selective electrodes previ- ence problems. A form of evanescent wave spectroscopy
ously described have been combined with semiconductor in which the electromagnetic field is coupled into the con-
devices for miniaturization, low-impedance output, signal duction band of a thin metal film is currently being com-
amplification, and capability of on-chip processing. The mercialized. This technique, known as Surface Plasmon
ion-sensitive field effect transistor (ISFET) is based on re- Resonance Spectroscopy (SPR), is very sensitive to any
placement of the conventional transistor gate with the ion- organic reagents located on the exterior of the metal film