Many scientific papers have been written concerning gas sensors for different sensor applications using several sensing principles. This review focuses on sensors and sensor systems for gaseous ammonia. Apart from its natural origin, there are many sources of ammonia, like the chemical industry or intensive life-stock. The survey that we present here treats different application areas for ammonia sensors or measurement systems and different techniques available for making selective ammonia sensing devices. When very low concentrations are to be measured, e.g. less than 2 ppb for environmental monitoring and 50 ppb for diagnostic breath analysis, solid-state ammonia sensors are not sensitive enough. In addition, they lack the required selectivity to other gasses that are often available in much higher concentrations. Optical methods that make use of lasers are often expensive and large. Indirect measurement principles have been described in literature that seems very suited as ammonia sensing devices. Such systems are suited for miniaturization and integration to make them suitable for measuring in the small gas volumes that are normally available in medical applications like diagnostic breath analysis equipment.

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Ammonia sensors and their applications - a review

Conventional approaches to chemical sensors have
traditionally made use of a “lock-and-key” design,
wherein a specific receptor is synthesized in order to
strongly and highly selectively bind the analyte of
interest.1-6 A related approach involves exploiting a
general physicochemical effect selectively toward a
single analyte, such as the use of the ionic effect in
the construction of a pH electrode. In the first
approach, selectivity is achieved through recognition
of the analyte at the receptor site, and in the second,
selectivity is achieved through the transduction
process in which the method of detection dictates
which species are sensed. Such approaches are appropriate
when a specific target compound is to be
identified in the presence of controlled backgrounds
and interferences. However, this type of approach
requires the synthesis of a separate, highly selective
sensor for each analyte to be detected. In addition,
this type of approach is not particularly useful for
analyzing, classifying, or assigning human value
judgments to the composition of complex vapor
mixtures such as perfumes, beers, foods, mixtures of
solvents, etc.

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Cross-reactive chemical sensor arrays.

Metal oxide semiconductor gas sensors are utilised in a variety of different roles and industries. They are relatively inexpensive compared to other sensing technologies, robust, lightweight, long lasting and benefit from high material sensitivity and quick response times. They have been used extensively to measure and monitor trace amounts of environmentally important gases such as carbon monoxide and nitrogen dioxide. In this review the nature of the gas response and how it is fundamentally linked to surface structure is explored. Synthetic routes to metal oxide semiconductor gas sensors are also discussed and related to their affect on surface structure. An overview of important contributions and recent advances are discussed for the use of metal oxide semiconductor sensors for the detection of a variety of gases—CO, NOx, NH3 and the particularly challenging case of CO2. Finally a description of recent advances in work completed at University College London is presented including the use of selective zeolites layers, new perovskite type materials and an innovative chemical vapour deposition approach to film deposition.

https://www.mdpi.com/1424-8220/10/6/5469/pdf

Metal Oxide Semi-Conductor Gas Sensors in Environmental Monitoring

This article extensively reviews the recent development of semiconductor metal oxide gas sensors for environmentally hazardous gases including NO2, NO, N2O, H2S, CO, NH3, CH4, SO2 and CO2. The gas sensing properties of differently-prepared metal oxides and loaded metal oxides towards nine environmentally hazardous gases have been individually compared and digested. Promising materials for sensitive and selective detection of each hazardous gas have been identified. For instance, unloaded WO3 nanostructures are the most promising candidates for NO2 sensing while metal catalyst loaded WO3 and gold-loaded SnO2 sensors are among the most effective for NO and N2O sensing, respectively. Moreover, related gas-sensing mechanisms are comprehensively discussed.

Semiconducting metal oxides as sensors for environmentally hazardous gases

Research on the luminescent solar concentrator (LSC) over the past thirty-odd years is reviewed. The LSC is a simple device at its heart, employing a polymeric or glass waveguide and luminescent molecules to generate electricity from sunlight when attached to a photovoltaic cell. The LSC has the potential to find extended use in an area traditionally difficult for effective use of regular photovoltaic panels: the built environment. The LSC is a device very flexible in its design, with a variety of possible shapes and colors. The primary challenge faced by the devices is increasing their photon-to-electron conversion efficiencies. A number of laboratories are working to improve the efficiency and lifetime of the LSC device, with the ultimate goal of commercializing the devices within a few years. The topics covered here relate to the efforts for reducing losses in these devices. These include studies of novel luminophores, including organic fluorescent dyes, inorganic phosphors, and quantum dots. Ways to limit the surface and internal losses are also discussed, including using organic and inorganic-based selective mirrors which allow sunlight in but reflect luminophore-emitted light, plasmonic structures to enhance emissions, novel photovoltaics, alignment of the luminophores to manipulate the path of the emitted light, and patterning of the dye layer to improve emission efficiency. Finally, some possible ‘glimpses of the future’ are offered, with additional research paths that could result in a device that makes solar energy a ubiquitous part of the urban setting, finding use as sound barriers, bus-stop roofs, awnings, windows, paving, or siding tiles.

Thirty Years of Luminescent Solar Concentrator Research: Solar Energy for the Built Environment

A model has been developed for the sensing mechanism of metal oxide-based thick-film gas sensors. The model explains the behaviour of the sensor conductance as a function of the concentration of test gas and the operating temperature of the sensor. Using the Schottky-barrier conduction mechanism through grain boundaries, a relationship between the degree of surface coverage θ and the conductance G has been obtained. To relate the conductance with the concentration of the gas, the Freundlich adsorption isotherm for gases and vapours on a solid surface has been used. The isotherm relates the degree of surface coverage θ with the partial pressure of the gas (concentration). By eliminating θ, an expression relating the variation of G with concentration has been obtained. To study the validity of the model, a thick-film Pd-doped tin oxide gas sensor has been fabricated and tested with propanol (C3H7OH). The variation in the conductance with changes in concentration and temperature has been observed. The observed data show an excellent fit with the developed model. Using the experimental data, the constants of the theoretical equation have also been evaluated.

Sensing mechanism in tin oxide-based thick-film gas sensors

The tin oxide (SnO 2 ) thick film gas sensor is fabricated by employing screen-printing technology. This pure SnO 2 thick film is doped with 1 or 2 wt% of cadmium sulphide (CdS) by its weight and, thereby, the effect of dopant is presented. X-ray diffraction (XRD) analyses are administrated, which suggest that CdS dopant inhibits the crystallite growth leading to nanometric reduction in grain size. The fabricated gas sensor is responsively studied on exposure to liquid petroleum gas (LPG), methanol, and acetone. It is observed that CdS (2 wt%) doped structure exhibited highest response and is more selective to methanol (70 for 5000 ppm) over LPG and acetone at the operating temperature 200 °C. The CdS-doping improved response- and recovery-time from 90 s and 200 s, for undoped-film, to 40 s and 110 s for methanol (5000 ppm at 200 °C).

Sensing properties of CdS-doped tin oxide thick film gas sensor

This paper deals with the performance of a palladium-gate MOS hydrogen sensor studied by conductance method. Structure of the device was fabricated on a n-type 〈100〉 silicon wafer having resistivity of 1–6 Ω cm using plasma technology. Sensitivity and response–recovery time of the fabricated sensor have been studied for different concentration (1480–11 840 ppm) of hydrogen with varying signal frequency (500 Hz, 10 and 100 kHz) at room temperature. Hydrogen-induced interface-trapped density ( N it ) has been also evaluated as a function of gas concentration using a bias scan conductance method. Obtained results show that device performance is improved (i.e., high sensitivity and low response recovery time) and further it has been concluded that implementation of plasma technology (i.e., dry plasma cleaning of Si surface and in-situ RF anodization of Silicon in oxygen plasma near room temperature) may be a future step towards development of MOS-based sensors and integrated arrays with improved performance at room temperature.

R. Dwivedi

Papers

Sensing mechanism in tin oxide-based thick-film gas sensors

Sensing properties of CdS-doped tin oxide thick film gas sensor

Sensing properties of palladium-gate MOS (Pd-MOS) hydrogen sensor-based on plasma grown silicon dioxide

Structural and micro structural studies of PbO-doped SnO2 sensor for detection of methanol, propanol and acetone

Sensing mechanism of Pd-doped SnO2 sensor for LPG detection