drawback with this method is that there is no
official standard for “zero air.” Some studies use
pure nitrogen or pure synthetic air, and others use
ambient air when they consider the area to be
clean. Yet others use ambient air that has been
filtered to remove some impurities. This study
chose to use clean ambient air to calibrate the
SO 2 sensor, while synthetic air was used to calibrate the CO sensor [97].
Another method used to calibrate sensors is the
least-square method. The study in Ref. [98] uses
this method. The sensor node was placed in a clear
plastic sealed container with a sensor probe, the
GrayWolf Direct Sense IAQ 610. The concentration of gas inside the container was varied by
injecting a certain gas. Using the least-square
method, the collected data were calibrated against
the GrayWolf system [98].
The Future: Smart Cities and New
Materials
There are several definitions of a smart city. One
description is that a smart city is an advanced city
that uses information and built elements to create a
more connected and sustainable city and provide for
the needs of citizens. Ideally, the smart city improves
the quality of life for people using the city [99].
Low-cost sensors are often touted as a technology that could enable smart cities. Such sensors
could be used for many purposes, for example,
they could act as a monitoring network to indicate
fire, explosion, or elevated levels of dangerous
pollutants, and they can provide real-time data to
monitor the concentrations of the pollution. Data
from the sensors can support decisions and give
the public access to air quality information. The
sensors in a smart city could be integrated in
wireless network [100].
One real-world example is a study which
investigated the performance of a network of
wireless sensors for air pollution monitoring.
The main goal of the study was to monitor outdoor
air pollution levels at a school. The sensors had a
low cost and were able to measure pollution in real
time. The different sensor nodes in the system
measured air quality simultaneously, recording
CO, NO 2 , relative humidity, and temperature.
The next step is to integrate the Internet of Things
(IoT) in smart cities. The internet of Things is the
integration of the connectivity of the internet with
objects around us, allowing them to send and
receive data. In smart cities, sensors would be
connected to a cloud web interface enabling
access to sensor data in real-time [101]. The aim
of the Internet of Things is to make the internet
more extensive and engaged in day-to-day life.
The paradigm has many applications including
using sensors to monitor air pollution. The IoT is
likely to play an important role in realizing the
smart city dream [102].
There are three layers to the Internet of Things:
the perception layer, the network layer, and the
application layer. The perception layer consists of
objects that can gather information, measure different elements, and exchange information with
other objects such as sensors for measuring air
pollution. The network layer is responsible for
transferring data from the perception layer to the
application layer. In the application layer, the
information is gathered and treated, and it aims
to create smart cities, among other things [103].
Nanowires
Nanowires are an emerging technology for sensing
nitrogen oxides. A nanowire as the name implies is
a wire with a very small diameter typically tens or
hundreds of nanometers. The wire has a low conductivity due to its small cross section, and the
conductivity is sensitive to the presence of oxidizing or reducing gases which can donate or withdraw electrons in analogy to their effect on
semiconductors discussed above. To illustrate the
subject, we will summarize a study of the ability of
nanowires made of In 2 O 3 and WO 3 to sense NO 2 ,
NO, and N 2 O. The study found that all three nitrogen oxides can be sensed by the nanowires. The
In 2 O 3 nanowire showed the best performance in
terms of sensitivity and operating temperature; it
had a diameter of approximately 20 nm. The WO 3
nanowire was also shown to work although it had a
lower sensitivity than the In 2 O 3 nanowire. Both
In 2 O 3 and WO 3 nanowires can reliably sense
N 2 O, NO, and NO 2 [104].
Low-Cost Sensors for Indoor and Outdoor Pollution
447
official standard for “zero air.” Some studies use
pure nitrogen or pure synthetic air, and others use
ambient air when they consider the area to be
clean. Yet others use ambient air that has been
filtered to remove some impurities. This study
chose to use clean ambient air to calibrate the
SO 2 sensor, while synthetic air was used to calibrate the CO sensor [97].
Another method used to calibrate sensors is the
least-square method. The study in Ref. [98] uses
this method. The sensor node was placed in a clear
plastic sealed container with a sensor probe, the
GrayWolf Direct Sense IAQ 610. The concentration of gas inside the container was varied by
injecting a certain gas. Using the least-square
method, the collected data were calibrated against
the GrayWolf system [98].
The Future: Smart Cities and New
Materials
There are several definitions of a smart city. One
description is that a smart city is an advanced city
that uses information and built elements to create a
more connected and sustainable city and provide for
the needs of citizens. Ideally, the smart city improves
the quality of life for people using the city [99].
Low-cost sensors are often touted as a technology that could enable smart cities. Such sensors
could be used for many purposes, for example,
they could act as a monitoring network to indicate
fire, explosion, or elevated levels of dangerous
pollutants, and they can provide real-time data to
monitor the concentrations of the pollution. Data
from the sensors can support decisions and give
the public access to air quality information. The
sensors in a smart city could be integrated in
wireless network [100].
One real-world example is a study which
investigated the performance of a network of
wireless sensors for air pollution monitoring.
The main goal of the study was to monitor outdoor
air pollution levels at a school. The sensors had a
low cost and were able to measure pollution in real
time. The different sensor nodes in the system
measured air quality simultaneously, recording
CO, NO 2 , relative humidity, and temperature.
The next step is to integrate the Internet of Things
(IoT) in smart cities. The internet of Things is the
integration of the connectivity of the internet with
objects around us, allowing them to send and
receive data. In smart cities, sensors would be
connected to a cloud web interface enabling
access to sensor data in real-time [101]. The aim
of the Internet of Things is to make the internet
more extensive and engaged in day-to-day life.
The paradigm has many applications including
using sensors to monitor air pollution. The IoT is
likely to play an important role in realizing the
smart city dream [102].
There are three layers to the Internet of Things:
the perception layer, the network layer, and the
application layer. The perception layer consists of
objects that can gather information, measure different elements, and exchange information with
other objects such as sensors for measuring air
pollution. The network layer is responsible for
transferring data from the perception layer to the
application layer. In the application layer, the
information is gathered and treated, and it aims
to create smart cities, among other things [103].
Nanowires
Nanowires are an emerging technology for sensing
nitrogen oxides. A nanowire as the name implies is
a wire with a very small diameter typically tens or
hundreds of nanometers. The wire has a low conductivity due to its small cross section, and the
conductivity is sensitive to the presence of oxidizing or reducing gases which can donate or withdraw electrons in analogy to their effect on
semiconductors discussed above. To illustrate the
subject, we will summarize a study of the ability of
nanowires made of In 2 O 3 and WO 3 to sense NO 2 ,
NO, and N 2 O. The study found that all three nitrogen oxides can be sensed by the nanowires. The
In 2 O 3 nanowire showed the best performance in
terms of sensitivity and operating temperature; it
had a diameter of approximately 20 nm. The WO 3
nanowire was also shown to work although it had a
lower sensitivity than the In 2 O 3 nanowire. Both
In 2 O 3 and WO 3 nanowires can reliably sense
N 2 O, NO, and NO 2 [104].
Low-Cost Sensors for Indoor and Outdoor Pollution
447