recorded [16]. Accurate, wearable sensors have
the potential to give much higher quality data in
this respect. Increased temporal and spatial resolution mean specific exposure modes (e.g.,
“morning commute”) can be distinguished. The
advent of low-cost sensor technologies also
means that new applications can be developed.
These could be in areas such as adding sensors
to drones for remote sensing campaigns, and the
production of “smart” billboards that can measure
and display pollution levels, increasing awareness
for citizens [17–19].
The United Nations (UN) has defined its vision
for the future in the Sustainable Development
Goals [20]. The 11th goal is “Sustainable Cities
and Communities.” Low-cost sensors can help create more sustainable environments by providing
information that can promote sustainable government policies and healthy individual behaviors in
daily life. Government policies could include emissions controls on vehicles and factories, planting
green zones, choosing energy infrastructure, and
changing traffic patterns. Individuals could choose
to ride a bicycle instead of driving car, insulate their
homes, drive low emissions vehicles, and avoid
candles and wood stoves. Low-cost sensors can
contribute by being used to detect wildfires, gas
leaks, and other public safety events, giving an
early warning to authorities and helping to locate
the source of the event. Sustainable Development
Goal number 13 is “Climate Action” and point
13.2 says that society needs to “Integrate climate
change measures into national policies, strategies
and planning.” By supplying leaders and citizens
with information about air quality, sensors are a
powerful tool for identifying the problem that
needs to be solved, for example, by characterizing
fugitive emissions, and documenting that solution.
The data provided can be implemented in political
goals and strategies [20].
Humans are adversely affected by exposure to
air pollution, resulting in exposure limits and legislation for specific pollutants. Exposure limits
take different forms, for example, peak exposure,
or average exposure over an 8 hour period or a
year. Specific examples of uses for low-cost sensors with individual pollutants and their main
effects are detailed in Tables 1 and 2.
The Three ‘S’s: Sensitivity, Stability, and
Selectivity
The sensitivity of a gas sensor is defined as the
change of measured signal per change of analyte
concentration. Sensitivity is sometimes confused
with the limit of detection, which is the lower limit
of analyte concentration the sensor can detect under
certain defined conditions [38]. The stability is the
ability to produce consistent results over a defined
period or change in conditions, for example, temperature or humidity. (A related metric is the recovery
time, the time the sensor signal uses to return to its
initial value after a perturbation [2].) A further distinction is made between “active” and “conservative”
stability. Active stability depends on the sensor’s
ability to give reproducible results during operation
under variable conditions. Conservative stability
describes the ability of the sensor to retain its sensitivity and selectivity after storage [38]. The final “S,”
selectivity, is based on whether the sensor responds
to a group of analytes or a specific species [38].
We will use the example of metal oxide gas
sensors to illustrate sensitivity, selectivity, and stability. These sensors typically contain a sensitive
layer, a substrate, electrodes, and a heater. Transduction of the chemical signal to an electronic signal
occurs by the following process. In granular metal
oxides, Schottky barriers (an electrostatic depletion
layer formed at the junction) form at grain boundaries as a result of the depletion layer [38]. The
depletion layer is a region created when n-type and
p-type materials are placed in contact. The concentration of current carriers is reduced in this region,
and their electrical conductivity becomes very sensitive to minor perturbations, for example, the presence of an electron donor (Lewis base) or electron
withdrawing (Lewis acid) species adsorbed on the
surface [10], or space-charge effects [11]. The
Schottky barrier height describes the resistance to
electron transport across the interface [39].
For example, suppose that molecular oxygen
adsorbs on the surface. By doing so, it traps an
electron, changing the concentration of current
carriers in the depletion region. At higher temperatures, between 100
C and 500
C, ionized
molecular oxygen and atomic oxygen are present
at the surface, with unique effects on the depletion
region.
426
Low-Cost Sensors for Indoor and Outdoor Pollution
the potential to give much higher quality data in
this respect. Increased temporal and spatial resolution mean specific exposure modes (e.g.,
“morning commute”) can be distinguished. The
advent of low-cost sensor technologies also
means that new applications can be developed.
These could be in areas such as adding sensors
to drones for remote sensing campaigns, and the
production of “smart” billboards that can measure
and display pollution levels, increasing awareness
for citizens [17–19].
The United Nations (UN) has defined its vision
for the future in the Sustainable Development
Goals [20]. The 11th goal is “Sustainable Cities
and Communities.” Low-cost sensors can help create more sustainable environments by providing
information that can promote sustainable government policies and healthy individual behaviors in
daily life. Government policies could include emissions controls on vehicles and factories, planting
green zones, choosing energy infrastructure, and
changing traffic patterns. Individuals could choose
to ride a bicycle instead of driving car, insulate their
homes, drive low emissions vehicles, and avoid
candles and wood stoves. Low-cost sensors can
contribute by being used to detect wildfires, gas
leaks, and other public safety events, giving an
early warning to authorities and helping to locate
the source of the event. Sustainable Development
Goal number 13 is “Climate Action” and point
13.2 says that society needs to “Integrate climate
change measures into national policies, strategies
and planning.” By supplying leaders and citizens
with information about air quality, sensors are a
powerful tool for identifying the problem that
needs to be solved, for example, by characterizing
fugitive emissions, and documenting that solution.
The data provided can be implemented in political
goals and strategies [20].
Humans are adversely affected by exposure to
air pollution, resulting in exposure limits and legislation for specific pollutants. Exposure limits
take different forms, for example, peak exposure,
or average exposure over an 8 hour period or a
year. Specific examples of uses for low-cost sensors with individual pollutants and their main
effects are detailed in Tables 1 and 2.
The Three ‘S’s: Sensitivity, Stability, and
Selectivity
The sensitivity of a gas sensor is defined as the
change of measured signal per change of analyte
concentration. Sensitivity is sometimes confused
with the limit of detection, which is the lower limit
of analyte concentration the sensor can detect under
certain defined conditions [38]. The stability is the
ability to produce consistent results over a defined
period or change in conditions, for example, temperature or humidity. (A related metric is the recovery
time, the time the sensor signal uses to return to its
initial value after a perturbation [2].) A further distinction is made between “active” and “conservative”
stability. Active stability depends on the sensor’s
ability to give reproducible results during operation
under variable conditions. Conservative stability
describes the ability of the sensor to retain its sensitivity and selectivity after storage [38]. The final “S,”
selectivity, is based on whether the sensor responds
to a group of analytes or a specific species [38].
We will use the example of metal oxide gas
sensors to illustrate sensitivity, selectivity, and stability. These sensors typically contain a sensitive
layer, a substrate, electrodes, and a heater. Transduction of the chemical signal to an electronic signal
occurs by the following process. In granular metal
oxides, Schottky barriers (an electrostatic depletion
layer formed at the junction) form at grain boundaries as a result of the depletion layer [38]. The
depletion layer is a region created when n-type and
p-type materials are placed in contact. The concentration of current carriers is reduced in this region,
and their electrical conductivity becomes very sensitive to minor perturbations, for example, the presence of an electron donor (Lewis base) or electron
withdrawing (Lewis acid) species adsorbed on the
surface [10], or space-charge effects [11]. The
Schottky barrier height describes the resistance to
electron transport across the interface [39].
For example, suppose that molecular oxygen
adsorbs on the surface. By doing so, it traps an
electron, changing the concentration of current
carriers in the depletion region. At higher temperatures, between 100
C and 500
C, ionized
molecular oxygen and atomic oxygen are present
at the surface, with unique effects on the depletion
region.
426
Low-Cost Sensors for Indoor and Outdoor Pollution