Sensors - Application and Multidisciplinary Aspects of Wireless Sensor Networks - page 23

The surface hydrolization of OH groups of the gate materials varies in aqueous
solutions due to pH value. Typical gate materials are Si 3 N 4 , Al 2 O 3 , and Ta 2 O 5 .
ISFET sensors are used in devices for continuous measurements like those for
continuous measurement of pH value and gases in blood (O 2 , CO 2 ).

2.7.5 Piezoelectric Chemical Sensors

Piezoelectric effect is the generation of an electric charge in a crystalline material upon
subjecting it to stress. A piezoelectric chemical sensor is a piezoelectric oscillator that
responds to changes in the chemical composition of its environment with changes of
the resonant frequency, or wave speed [ 4 ].
The complex nature of these sensors makes them unsuitable for a brief overview
of operating principals, as is suitable for this topic. However, as these sensors are
undergoing a rapid expansion, readers are encouraged to turn to references for more
detailed explanations.

2.8 Biosensors

Detection of presence and concentrations of bacteria, viruses, or molecules and
molecular complexes like proteins, enzymes, antibodies, DNA, etc. is essential to a
wide range of applications. Traditionally, this has been done through time-consuming
chemical analysis methods that require laboratory conditions and employ expensive
reagents and equipment. Technological advancements and introduction of micro-
sensor technology to this field has led to development of biosensors.
Much like a chemical sensor, biosensor consists of three parts: a sensitive layer,
transducer, and electronic circuitry to process the signal from the transducer.
Sensitive layer in a biosensor is a biosensitive biological component, like enzymes,
antibodies, cell membrane receptors, tissue slices, etc.
One example of biosensors is imunno-sensors. These sensors use specific anti-
bodies as the sensitive layer. These immobilized antibody molecules bond with
antigen molecules in the substance. Concentration of antigens can then be measured
using, for example, interferometric method.
Biosensors are under constant and rapid development. Currently, design and
production of a biosensor face serious difficulties, most related to immobilization
of proteins and long-term stability of proteins.

2.8.1 Radiation Sensors

Ionizing radiation consists of subatomic particles or waves that are energetic enough
to detach electrons from atoms or molecules, ionizing them. Exposure to radiation