A growing number of companies have started commercializing low-cost sensors (LCS) that are said to be able to monitor air pollution in outdoor air. The benefit of the use of LCS is the increased spatial coverage when monitoring air quality in cities and remote locations. Today, there are hundreds of LCS commercially available on the market with costs ranging from several hundred to several thousand euro. At the same time, the scientific literature currently reports independent evaluation of the performance of LCS against reference measurements for about 110 LCS. These studies report that LCS are unstable and often affected by atmospheric conditions—cross-sensitivities from interfering compounds that may change LCS performance depending on site location. In this work, quantitative data regarding the performance of LCS against reference measurement are presented. This information was gathered from published reports and relevant testing laboratories. Other information was drawn from peer-reviewed journals that tested different types of LCS in research studies. Relevant metrics about the comparison of LCS systems against reference systems highlighted the most cost-effective LCS that could be used to monitor air quality pollutants with a good level of agreement represented by a coefficient of determination R² > 0.75 and slope close to 1.0. This review highlights the possibility to have versatile LCS able to operate with multiple pollutants and preferably with transparent LCS data treatment.

Polyaniline (PANI) is a conducting polymer, widely used in gas-sensing applications. Due to its classification as a semiconductor, PANI is also used to detect reducing ammonia gas (NH₃), which is a well-known and studied topic. However, easier, cheaper and more straightforward procedures for sensor fabrication are still the subject of much research. In the presented work, we describe a novel, more controllable, synthesis approach to creating NH₃ PANI-based receptor elements. The PANI was electrochemically deposited via cyclic voltammetry (CV) on screen-printed electrodes (SPEs). The morphology, composition and surface of the deposited PANI layer on the Au electrode were characterised with electron microscopy, Fourier-transform infrared spectroscopy and profilometry. Prior to the gas-chamber measurement, the SPE was suitably modified by Au sputtering the individual connections between the three-electrode system, thus showing a feasible way of converting a conventional three-electrode electrochemical SPE system into a two-electrode NH₃-gas detecting system. The feasibility of the gas measurements’ characterisation was improved using the gas analyser. The gas-sensing ability of the PANI-Au-SPE was studied in the range 32–1100 ppb of NH₃, and the sensor performed well in terms of repeatability, reproducibility and sensitivity.

Hybrid composites based on tin chloride and the conductive polymers, polyaniline (PAni) and poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), were integrated into high-performance hydrogen sulphide (H2S) gas sensors working at room temperature. The morphology and chemical properties were studied by scanning and transmission electron microscopy (SEM, TEM), energy dispersive spectroscopy (EDS) and Fourier-transform infrared (FTIR). The composites demonstrated a slightly porous nanostructure and strong interactions between the polymers and the metal salt, which slightly dopes PAni. The hybrid sensors exhibited a very low detection limit (<85 ppb), fast response, repeatability, reproducibility and stability over one month. Moreover, this work presents how calibration based on the derivative of the signal can give hybrid sensors the ability to quantify the concentration of targeted gas, even during continuous variation of the analyte concentration. Finally, the effect of interfering species, such as water and ammonia, is discussed.

The use of multi-sensor array systems has become commonplace in indoor air quality or individual exposure applications. Nevertheless, there is a lack of reliable data, as qualification protocols are sparsely applied by end-users to multi-sensor commercial systems prior to field or lab implementation. This is especially the case with regard to VOC monitoring, although they are ubiquitous species of indoor environment. The aim of the present work is to propose an all-encompassing qualification protocol, suitable for a wide range of multi-sensor systems, able to assess the metrological performance regarding indoor VOCs. Furthermore, it provides data on metrological performance for 45 replicas of 5 types of sensors, 4 metal oxides and 1 electrochemical, embedded in a single system, under controlled conditions. Multi-sensor systems were exposed to 6 single VOCs, with their mixture representative of indoor pollution (formaldehyde, acetaldehyde, acetone, ethanol, limonene) in a 1:1 scale experimental room. Metrological performances were retrieved from the correlation curves established by comparison with the responses of a reference analyzer. The results show that the electrochemical sensor, supposedly dedicated to total VOCs, is acetone-blind and underestimates the effective quantity of TVOCs by a factor of 3. Metal oxide sensors, chosen for their reliability in establishing pollution source fingerprints, indicate effectively differentiated sensitivities by type of VOC, but without homogeneity by chemical family. A pre-processing of the data is required to overcome their non-linear responses depending on the concentration range. These results provide crucial insights for the interpretation of field campaign data retrieved from multi-sensor arrays.

Hydrogen sulfide (H₂S) gas poses a significant risk and thus demands continuous monitoring using reliable sensors. Chemiresistive sensors can serve this purpose, and the utilization of conducting polymeric materials, particularly polyaniline (PAni), has demonstrated significant potential. To fabricate chemiresistors based on PAni, the materials must be dispersed in an organic solvent like dimethylformamide (DMF), which has several hazardous properties. Fortunately, dimethyl sulfoxide (DMSO) is a safer alternative with similar properties to DMF, and may be used instead of DMF. In this study, we aim to compare the efficacy of DMF and DMSO solvents in fabricating PAni and metal chloride composite films for detecting H₂S gas.