handbook of modern sensors

Table 17.1. SAW Chemical Sensors
17.5 Complex Sensors 519
Compound Chemical Coating SAW Substrate
Organic vapor Polymer film Quartz
SO 2 TEA a Lithium niobate
H 2 Pd Lithium niobate, silicon
NH 3 Pt Quartz
H 2 S WO 3 Lithium niobate
Water vapor Hygroscopic Lithium niobate
NO 2 PC b Lithium niobate, quartz
NO 2 ,NH 3 ,NH 3 ,SO 2 , PC b Lithium niobate
CH 4
Vapor explosives, drugs Polymer Quartz
SO 2 , methane C c Lithium niobate
Source: Ref. [22].
a Triethanolamine.
b Phthalocyanine.
c No chemical coating used. Detection is based on changes in thermal conductivity
produced by the gas.
17.5.5 Biochemical Sensors
Biosensors are a special class of chemical sensors. The evolution of species by means
of natural selection led to extremely sensitive organs, which can respond to presence
of just few molecules. Man-made sensors, although generally not as sensitive,
employ biologically active materials in combination with several physical sensing
elements (e.g., amperometric or thermal). The biorecognition element is actually a
bioreactor on the top of the conventional sensor, so the response of the biosensor
will be determined by the diffusion of the analyte, reaction products, coreactants or
interfering species, and the kinetics of the recognition process. The following biological
elements may be detected qualitatively and quantitatively by the biosensors:
organisms, tissues, cells, organelles, membranes, enzymes, receptors, antibodies, and
nucleic acids [17].
In the fabrication of biosensor, one of the key issues is immobilization of analytes
on the physical transducer. The immobilization must confine the biologically active
material on a sensing element and keep it from leaking out over the lifetime of the
biosensor, allow contact to the analyte solution, allow any product to diffuse out of
the immobilization layer, and not denature the biologically active material. Most of
the biologically active materials used in biosensors are proteins or contain proteins in
their chemical structures. Therefore, to immobilize the proteins on the surface of the
sensor, two basic techniques are employed: binding or physical retention. Adsorption
and covalent binding are the two types of binding technique. The retention involves
separating the biologically active material from analyte solution with a layer on the
surface of the sensor, which is permeable to the analyte and any products of the
recognition reaction, but not to the biologically active material.