transducers commonly used in the fabrication of biosensors are thermistors and
thermopiles and are discussed belowThermistors These types of transducers detect changes in resistance, which is
proportional to a change in ambient temperature. Two types of thermistors exist,
one that has a positive temperature coefficient and the other is a negative temperature coefficient. A key element of the thermistor is the enzyme, which serves as a
bio-recognition element for the measurement. An important support material on
which the enzyme is immobilized is also important for keeping the catalytic nature
of the enzyme. Controlled pore glass works as a support material [52].
Thermopile These types of transducers work on the principle of Seebeck effect to
detect changes in temperature. As per the Seebeck effect, two dissimilar metals in
contact generate a potential difference at the junction. Thermocouples are fabricated
based on the principle of the Seebeck effect. When several thermocouples are
connected in series, the arrangement is called a thermopile. For example, an array of
p-type silicon or aluminum thermocouple connected in series may serve as a
thermopile. Thermopile serves as an amplifier of signals generated in a thermal
process. The electrochemical activity of the enzyme immobilized on the thermopile
results in an induced voltage over the thermopile. The induced voltage depends on
the number of thermocouples used as they actively take part in the amplification of
the induced voltage [53].
3.2.3 Optical Biosensors
An optical biosensor is a device containing a sensing element, which may be any
biological material such as enzymes, antibodies, cells, tissues. The sensing element
is conjoined with a transducer used for sensing purposes. A change in mass or
concentration of the sensing element is observed when it comes in contact with the
analyte. This change is sensed by the transducer unit in the form of change in light
intensity. Optical biosensor finds application in the detection of organic materials,
heavy metals in water-based solutions and is a good candidate for detecting the
water pollutants. The transducer section of the biosensor generally comprise of a
surface plasmon resonance based chip, interferometer, resonator, gratings or
refractometers [54].
Depending upon the type of transducer element, some of the optical biosensors
are discussed belowSurface plasmon resonance based biosensor: Surface plasmon resonance phenomenon is observed on the surface of any conducting material such as the metal at
the interface of two mediums having a difference in their refractive indices. The
interface, when illuminated by a polarized light source, emits surface plasmons
followed by a change in intensity of the reflected light at a specific angle known as
the resonance angle. Interaction of an analyte with the bio-sensing element may
then be directly correlated with the change in intensity of reflected light.
Materials in Bio-Sensing of Water Pollutants
197
thermopiles and are discussed belowThermistors These types of transducers detect changes in resistance, which is
proportional to a change in ambient temperature. Two types of thermistors exist,
one that has a positive temperature coefficient and the other is a negative temperature coefficient. A key element of the thermistor is the enzyme, which serves as a
bio-recognition element for the measurement. An important support material on
which the enzyme is immobilized is also important for keeping the catalytic nature
of the enzyme. Controlled pore glass works as a support material [52].
Thermopile These types of transducers work on the principle of Seebeck effect to
detect changes in temperature. As per the Seebeck effect, two dissimilar metals in
contact generate a potential difference at the junction. Thermocouples are fabricated
based on the principle of the Seebeck effect. When several thermocouples are
connected in series, the arrangement is called a thermopile. For example, an array of
p-type silicon or aluminum thermocouple connected in series may serve as a
thermopile. Thermopile serves as an amplifier of signals generated in a thermal
process. The electrochemical activity of the enzyme immobilized on the thermopile
results in an induced voltage over the thermopile. The induced voltage depends on
the number of thermocouples used as they actively take part in the amplification of
the induced voltage [53].
3.2.3 Optical Biosensors
An optical biosensor is a device containing a sensing element, which may be any
biological material such as enzymes, antibodies, cells, tissues. The sensing element
is conjoined with a transducer used for sensing purposes. A change in mass or
concentration of the sensing element is observed when it comes in contact with the
analyte. This change is sensed by the transducer unit in the form of change in light
intensity. Optical biosensor finds application in the detection of organic materials,
heavy metals in water-based solutions and is a good candidate for detecting the
water pollutants. The transducer section of the biosensor generally comprise of a
surface plasmon resonance based chip, interferometer, resonator, gratings or
refractometers [54].
Depending upon the type of transducer element, some of the optical biosensors
are discussed belowSurface plasmon resonance based biosensor: Surface plasmon resonance phenomenon is observed on the surface of any conducting material such as the metal at
the interface of two mediums having a difference in their refractive indices. The
interface, when illuminated by a polarized light source, emits surface plasmons
followed by a change in intensity of the reflected light at a specific angle known as
the resonance angle. Interaction of an analyte with the bio-sensing element may
then be directly correlated with the change in intensity of reflected light.
Materials in Bio-Sensing of Water Pollutants
197