Biomedical Engineering Reference
In-Depth Information
pollutants in the environment, which is essential for their human and eco-
logical exposure and risk assessment and regulation.
7.3.1 Air Monitoring Systems
Analytical methods, such as mass spectrometry, gas chromatography, chemi-
luminescence, and infrared spectrometry are capable of precise air compo-
sition analyses; however, they are time consuming, relatively expensive,
and difficult to use in the field. The enormous pollution complexity and
its intensive dynamics required the application of new, more flexible, and
cost-effective systems, operating at higher spatial resolutions. This demand
determined the development of the solid-state gas sensors (SGSs), which are
able to analyze air samples faster and less expensive than their conventional
alternatives [12].
SGSs consist of one or more oxides of the transition metals (e.g., tin, alu-
minum, zinc, cobalt, tungsten). These materials are processed into paste,
which is then used to construct bead-type sensors. Another type is the film-
chip sensors, which are fabricated as the metal oxides are vacuum deposited
onto silica chips [13]. Nowadays, nanoparticles (NPs) and thin films of metal
oxides (<100 nm thick) are used to construct the sensors. This offers consider-
able advantages over the standard technology, increasing the sensitivity and
reducing the response time of the devices.
The SGSs operate in a simple fashion. As ambient gas molecules are cap-
tured within the sensor, the metal oxide causes the gas to dissociate into
charged ions or complexes, and electron transfer takes place [14]. This effect
changes the electrical conductivity of the semiconducting material in accor-
dance with the composition of the surrounding air. A pair of very thin elec-
trodes is attached to the metal oxide to measure the changes in conductivity
and report them as signals [14]. SGSs typically produce strong signals, as
their strength depends on the gas concentrations and the size of the particles
used to build the semiconductor [15].
Currently, nanostructured metal oxides are the main types of materi-
als used to fabricate gas sensors. That is why the devices are often called
“conductimetric nanosensors.” The reason to employ nanomaterials in the
SGSs is that they have very high surface-area-to-volume ratios (sa/vol) [16],
which allows that more ambient gas molecules are adsorbed on the material
interface [16]. The higher gas concentrations invoke stronger changes in con-
ductivity of the semiconductors and stronger electrical signals, respectively,
which increases the sensitivity of the devices [16].
Today, metal oxides, shaped as nanowires, nanobelts, and nanocombs,
are tested for use in SGSs. The results show that these morphologies sig-
nificantly increase the effective surface area of the semiconducting material
[16] and they may be integrated into the next generation of SGSs, which will
emerge on the market in the next 3 to 5 years.
