Change in optical properties of the nanomaterials sensing layer because of
interaction with a chemical analyte can provide good sensing response by measuring
the absorbance using ultraviolet-visible spectroscopy, surface plasmon resonance,
Raman spectroscopy, fluorescence measurements, etc. According to the optical
properties of the analyte, the optical chemical sensors in general may be categorized
into two types: inherent optical properties and apparent optical properties. In an
inherent optical property sensor, the analyte is itself optically active and senses
directly some intrinsic optical property such as absorption or luminescence. In
apparent optical property sensing systems, a change in the optical response of an
analyte is detected by using some optically active analyte-sensitive layer. This latter
technique is useful particularly in the case where the analyte has no convenient
intrinsic optical property, which is the case for many analytes. In the case of apparent
optical property, the possibilities are there to discuss about the analyte-sensitive
reagents based on polymer. The optical detection of the apparent optical property can
be done by ultraviolet-visible spectroscopy, infrared, surface plasmon resonance, or
Raman spectra. Nowadays, the optical fiber sensing technology is the most exploited
platform in the development of optical chemical sensors. In fiber optic sensors that
utilize surface plasmon resonance techniques and quantum cascade laser spectroscopy as detection system. Fiber-optic sensors are classified according to the role
played by the fiber in the operation of the sensor, i.e., passive or active. The fiber’s
role is considered to be passive if the fiber only acts to transport the optical signal to
and from the sensing environment. An active fiber-optic sensor utilizes a fiber that
has been modified so as to impart intrinsic analyte sensitivity to the fiber, and here
the optical properties of the fiber are in some way changed during the interaction
with analyte. An interaction of analytes which oxidize/reduce or protonate/
deprotonate of organic nanomaterials or alter the plasmon resonance of inorganic
nanomaterials can be a simple principle in an optical sensor. In hybrid nanomaterials
the inherent optical property or apparent optical property may be imparted from
either of the inorganic or organic component or both the components.
Mass sensing, which is a popular method for chemical analysis, transform the
change of most fundamental physical property, i.e., mass, after the interaction with
analyte into a change of a property of the sensor detection element. The principles
used in the devices to measure the mass sensitivity during the interaction with
analyte are quartz crystal microbalances and surface acoustic wave resonators.
Quartz crystal microbalance sensors are a kind of piezoelectric quartz crystal with
a selective coating deposited on the surface to serve as an adsorptive surface capable
of measuring an extremely small mass change. Piezoelectric devices used mainly in
gaseous phase, but also in solutions, are based on the measurement of the frequency
change of the quartz oscillator plate caused by adsorption of a mass of the analyte at
the oscillator. If a piezoelectric substance is incorporated in an oscillating electronic
circuit, a surface acoustic wave is formed across the substance. Any change in
velocity of these waves, due to the change in mass of the coating on the sensor by
an absorbing species, will alter the resonant frequency of the wave. The oscillations
are applied to the sensor through a set of metallic electrodes formed on the piezoelectric surface, over which a selective coating is deposited. The change of mass is
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interaction with a chemical analyte can provide good sensing response by measuring
the absorbance using ultraviolet-visible spectroscopy, surface plasmon resonance,
Raman spectroscopy, fluorescence measurements, etc. According to the optical
properties of the analyte, the optical chemical sensors in general may be categorized
into two types: inherent optical properties and apparent optical properties. In an
inherent optical property sensor, the analyte is itself optically active and senses
directly some intrinsic optical property such as absorption or luminescence. In
apparent optical property sensing systems, a change in the optical response of an
analyte is detected by using some optically active analyte-sensitive layer. This latter
technique is useful particularly in the case where the analyte has no convenient
intrinsic optical property, which is the case for many analytes. In the case of apparent
optical property, the possibilities are there to discuss about the analyte-sensitive
reagents based on polymer. The optical detection of the apparent optical property can
be done by ultraviolet-visible spectroscopy, infrared, surface plasmon resonance, or
Raman spectra. Nowadays, the optical fiber sensing technology is the most exploited
platform in the development of optical chemical sensors. In fiber optic sensors that
utilize surface plasmon resonance techniques and quantum cascade laser spectroscopy as detection system. Fiber-optic sensors are classified according to the role
played by the fiber in the operation of the sensor, i.e., passive or active. The fiber’s
role is considered to be passive if the fiber only acts to transport the optical signal to
and from the sensing environment. An active fiber-optic sensor utilizes a fiber that
has been modified so as to impart intrinsic analyte sensitivity to the fiber, and here
the optical properties of the fiber are in some way changed during the interaction
with analyte. An interaction of analytes which oxidize/reduce or protonate/
deprotonate of organic nanomaterials or alter the plasmon resonance of inorganic
nanomaterials can be a simple principle in an optical sensor. In hybrid nanomaterials
the inherent optical property or apparent optical property may be imparted from
either of the inorganic or organic component or both the components.
Mass sensing, which is a popular method for chemical analysis, transform the
change of most fundamental physical property, i.e., mass, after the interaction with
analyte into a change of a property of the sensor detection element. The principles
used in the devices to measure the mass sensitivity during the interaction with
analyte are quartz crystal microbalances and surface acoustic wave resonators.
Quartz crystal microbalance sensors are a kind of piezoelectric quartz crystal with
a selective coating deposited on the surface to serve as an adsorptive surface capable
of measuring an extremely small mass change. Piezoelectric devices used mainly in
gaseous phase, but also in solutions, are based on the measurement of the frequency
change of the quartz oscillator plate caused by adsorption of a mass of the analyte at
the oscillator. If a piezoelectric substance is incorporated in an oscillating electronic
circuit, a surface acoustic wave is formed across the substance. Any change in
velocity of these waves, due to the change in mass of the coating on the sensor by
an absorbing species, will alter the resonant frequency of the wave. The oscillations
are applied to the sensor through a set of metallic electrodes formed on the piezoelectric surface, over which a selective coating is deposited. The change of mass is
10 Nanomaterials Based Sensors for Air Pollution Control
371