is defined as the lowest concentration difference that can be distinguished by a sensor [2].
Response time The characteristic time it takes
for a sensor to respond to a perturbation [2].
Semiconductor A semiconductor is a crystalline
or amorphous substance with a fully occupied
valence band and an empty conduction band,
and an electrical conductivity that increases
with increasing temperature [1]. Semiconductors have a band gap between the valence and
conduction bands that can be overcome by
thermal excitation. There are two types of
semiconductors: intrinsic and extrinsic. An
intrinsic semiconductor has a naturally small
band gap. When an electron is excited into the
conduction band a hole is left in the valence
band; the electron and hole are mobile and
conduct current. In contrast, an extrinsic semiconductor is not semiconducting in its pure
state but becomes semiconducting through
the addition of impurities (“dopants”) with elements that may have a deficit or excess of
electrons relative to the main compound.
There are thus two types of extrinsic semiconductors, p-type (positive charge doped) and
n-type (negative charge doped). For example,
a P atom (group V) in a Si crystal (group VI)
has an extra electron that can enter the Si conduction band making a n-type extrinsic semiconductor; an Al dopant (Group III) would
yield a p-type. Semiconductors are used as
gas sensors because oxidizing or reducing
gases change the population of the conduction
band, changing the conductivity. Reducing
gases (e.g., NH 3 , CO, H 2 ) lead to an increase
in conductivity for n-type and a decrease for
p-type, and the reverse for oxidizing gases
(e.g., NO 2 , O 3 , Cl 2 ) [9].
Sensor A sensor is a device that transforms
chemical information into an analytically useful signal [10].
Sensor Node A sensor node is an instrument that
may contain several sensors, a control and data
processing system, and power management.
Space charge A high density of like charges has
a high space charge; Coulombic repulsion
limits the ultimate charge density in a given
space. The charge density poses a fundamental
limit to the performance of semiconductors and
sensors as there is only a certain concentration
of holes or free electrons that can be pressed
into a certain volume without paying an unacceptable energy penalty [11].
Definition
Low-cost sensors for monitoring indoor and outdoor air pollution have been developed and are
available commercially. This entry describes the
function and performance of the most common
low-cost sensor technologies. Target pollutants
include particulate matter (PM), CO 2 , NO 2 , O 3 ,
CO, VOC, SO 2 , and N 2 O. Three different types of
low-cost sensors are considered: Metal oxide
(MOx) sensors, optical sensors, and electrochemical cells. The basic principles, fields of application, and performance of the sensor types are
detailed in order to contrast and compare them.
The devices are compared on the basis of parameters such as sensitivity, selectivity, stability, cost,
power consumption, and life cycle. Examples of
installations in the field and for personal exposure
monitoring are described. Finally, we consider
future developments in the areas of smart cities
and new materials such as nanowires and MOFs.
Introduction
In the beginning of the nineteenth century, many
technologies we have today were not available. At
that time, canary birds (Fig. 1) were commonly
used in coal mines to sense when the concentrations of toxic gases in the air had reached a hazardous level. Canaries are well-known to be more
sensitive to dangerous gases than humans. The
canary birds are especially good at sensing carbon
monoxide (CO), which is colorless and odorless,
and if a bird died, the miners knew they should
evacuate from the mine immediately. The birds
became an integrated part of mining culture, and
some were treated like pets. In some cases, mice
were used to indicate the presence of harmful
gases. These animals most likely saved the lives
of many miners [12].
424
Low-Cost Sensors for Indoor and Outdoor Pollution
Response time The characteristic time it takes
for a sensor to respond to a perturbation [2].
Semiconductor A semiconductor is a crystalline
or amorphous substance with a fully occupied
valence band and an empty conduction band,
and an electrical conductivity that increases
with increasing temperature [1]. Semiconductors have a band gap between the valence and
conduction bands that can be overcome by
thermal excitation. There are two types of
semiconductors: intrinsic and extrinsic. An
intrinsic semiconductor has a naturally small
band gap. When an electron is excited into the
conduction band a hole is left in the valence
band; the electron and hole are mobile and
conduct current. In contrast, an extrinsic semiconductor is not semiconducting in its pure
state but becomes semiconducting through
the addition of impurities (“dopants”) with elements that may have a deficit or excess of
electrons relative to the main compound.
There are thus two types of extrinsic semiconductors, p-type (positive charge doped) and
n-type (negative charge doped). For example,
a P atom (group V) in a Si crystal (group VI)
has an extra electron that can enter the Si conduction band making a n-type extrinsic semiconductor; an Al dopant (Group III) would
yield a p-type. Semiconductors are used as
gas sensors because oxidizing or reducing
gases change the population of the conduction
band, changing the conductivity. Reducing
gases (e.g., NH 3 , CO, H 2 ) lead to an increase
in conductivity for n-type and a decrease for
p-type, and the reverse for oxidizing gases
(e.g., NO 2 , O 3 , Cl 2 ) [9].
Sensor A sensor is a device that transforms
chemical information into an analytically useful signal [10].
Sensor Node A sensor node is an instrument that
may contain several sensors, a control and data
processing system, and power management.
Space charge A high density of like charges has
a high space charge; Coulombic repulsion
limits the ultimate charge density in a given
space. The charge density poses a fundamental
limit to the performance of semiconductors and
sensors as there is only a certain concentration
of holes or free electrons that can be pressed
into a certain volume without paying an unacceptable energy penalty [11].
Definition
Low-cost sensors for monitoring indoor and outdoor air pollution have been developed and are
available commercially. This entry describes the
function and performance of the most common
low-cost sensor technologies. Target pollutants
include particulate matter (PM), CO 2 , NO 2 , O 3 ,
CO, VOC, SO 2 , and N 2 O. Three different types of
low-cost sensors are considered: Metal oxide
(MOx) sensors, optical sensors, and electrochemical cells. The basic principles, fields of application, and performance of the sensor types are
detailed in order to contrast and compare them.
The devices are compared on the basis of parameters such as sensitivity, selectivity, stability, cost,
power consumption, and life cycle. Examples of
installations in the field and for personal exposure
monitoring are described. Finally, we consider
future developments in the areas of smart cities
and new materials such as nanowires and MOFs.
Introduction
In the beginning of the nineteenth century, many
technologies we have today were not available. At
that time, canary birds (Fig. 1) were commonly
used in coal mines to sense when the concentrations of toxic gases in the air had reached a hazardous level. Canaries are well-known to be more
sensitive to dangerous gases than humans. The
canary birds are especially good at sensing carbon
monoxide (CO), which is colorless and odorless,
and if a bird died, the miners knew they should
evacuate from the mine immediately. The birds
became an integrated part of mining culture, and
some were treated like pets. In some cases, mice
were used to indicate the presence of harmful
gases. These animals most likely saved the lives
of many miners [12].
424
Low-Cost Sensors for Indoor and Outdoor Pollution