Low-Cost Sensors for Indoor and Outdoor Pollution

A solid will have a density of electronic states as a function of energy. For example, a certain solid may have a band of fully occupied states, a gap or energy range in which there are no states, and at higher energies, an unoccupied band. This gap is called the bandgap. Electrons may be thermally excited from the fully occupied band into the unoccupied band, resulting in limited conductivity; the solid is then a semiconductor. A solid with a large bandgap is an insulator and with no bandgap, a conductor. The top of the occupied states is called the Fermi level [1].

The range of performance of a sensor: the analyte concentration range between the highest observable concentration and the detection limit [2].

The explosion range specifies the concentration range of a species in air, over which a given compound can burn or explode. This range extends from the lower to the upper explosive limit and is often given in % v/v. The lower limit is defined as the lowest concentration where a vapor or gas will burn or explode, whereas the upper limit is the highest concentration at which the gas or vapor will burn or explode [3, 4].

The specified threshold of peak concentration, or average concentration for a certain period, for a hazardous substance, that a person can be safely exposed to without endangering health. The exposure limit assures workers’ occupational health and is often expressed as an 8 hour time-weighted average concentration [3‐5].

Metal-organic frameworks (MOFs) are crystalline compounds consisting of metal ions or metal clusters, which are linked by organic ligands in a regular pattern to form 3D structures with pores [6].

A nanowire is a highly conducting nanostructure with a diameter in nanometers [7, 8].

Resolution can be used in several senses. First, the temporal resolution of a sensor is not the data sampling rate but rather the minimum time for a meaningful measurement, whether defined by the response time or the averaging time to get a reasonable signal to noise. Spatial resolution refers to the density of measurement sites. Measurement resolution is defined as the lowest concentration difference that can be distinguished by a sensor [2].

The characteristic time it takes for a sensor to respond to a perturbation [2].

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., NH₃, CO, H₂) lead to an increase in conductivity for n-type and a decrease for p-type, and the reverse for oxidizing gases (e.g., NO₂, O₃, Cl₂) [9].

A sensor is a device that transforms chemical information into an analytically useful signal [10].

A sensor node is an instrument that may contain several sensors, a control and data processing system, and power management.

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].