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Nitrogen dioxide (NO2) is a major air pollutant resulting in respiratory problems, from wheezing, coughing, to even asthma. Low-cost sensors based on WO3 nanoparticles are promising due to their distinct selectivity to detect NO2 at the ppb level. Here, we revealed that controlling the thickness of highly porous (97%) WO3 films between 0.5 and 12.3 μm altered the NO2 sensitivity by more than an order of magnitude. Therefore, films of WO3 nanoparticles (20 nm in diameter by N2 adsorption) with mixed γ- and ε-phase were deposited by single-step flame spray pyrolysis without affecting crystal size, phase composition, and film porosity. That way, sensitivity and selectivity effects were associated unambiguously to thickness, which was not possible yet with other sensor fabrication methods. At the optimum thickness (3.1 μm) and 125 °C, NO2 concentrations were detected down to 3 ppb at 50% relative humidity (RH), and outstanding NO2 selectivity to CO, methanol, ethanol, NH3 (all > 105), H2, CH4, acetone (all > 104), formaldehyde (>103), and H2S (835) was achieved. Such thickness-optimized and porous WO3 films have strong potential for integration into low-power devices for distributed NO2 air quality monitoring.

Introduction Formaldehyde is a carcinogenic indoor air pollutant emitted from wood-based furniture, building materials, paints and textiles.[1] Yet, no low-cost sensor exists for on-site monitoring to fulfill stringent current and upcoming (e.g., 8 parts-per-billion by volume, ppb, in France by 2023) exposure guidelines. Here, we present an inexpensive and handheld formaldehyde detector with proven performance in real indoor air. Selectivity is achieved by a compact packed bed column of nanoporous polymer sorbent that separates formaldehyde from interferants present in ambient air. Downstream, a highly sensitive nanoparticle-based chemoresistive Pd-doped SnO2 sensor detects formaldehyde in the relevant concentration range down to 5 ppb within 2 min. Method The handheld formaldehyde detector consists of a separation column and a chemoresistive microsensor [2]. The Separation column consists of a packed bed of Tenax TA powder (500 mg poly(2,6-diphenyl-p-phenylene oxide), 60–80 mesh, ~35 m2 g-1, Sigma-Aldrich) packed inside a Teflon tube (4 mm inner diameters) and secured on both ends with silanized glass wool plugs [3]. The sensor is based on a Pd-doped SnO2 nanoparticle film prepared by flame spray pyrolysis (FSP) and directly deposited onto microsensor substrates [4]. The measurement of emissions from wood products was based on standard testing protocol EN-717-1 to ensure comparable results. Measurements with room air were performed by exposing the detector directly to air from the laboratory. To also test higher concentration, room air was spiked with additional formaldehyde from the gas standard on a dynamic gas mixing setup. Results and Conclusions The concept of the formaldehyde detector is illustrated in Figure 1a–c. Indoor air contains >250 different analytes that are separated by a packed bed separation column of commercial Tenax TA particles. Tenax TA offers a high surface area (25 m2 g-1) due to its macroporous structure (Type II adsorption isotherm, Figure 1d) with a volume-average pore size of 120 nm (inset in Figure 1d). This enables efficient adsorption and retention of analytes, similar to a gas chromatographic column, though much more compact (13 cm length), inexpensive (˂ $20 for the Tenax TA powder) and without the need for column heating. After the column, formaldehyde is detected separately from interferants (e.g., hydrogen, methanol), thus selectively, by a highly sensitive but non-specific chemoresistive microsensor (Figure 1e). It consists of a highly porous film of flame-made and directly deposited Pd-doped SnO2 nanoparticles (Figure 1f) that form fine networks with large surface area (54.5 m2 g-1) for formaldehyde detection down to 5 ppb.. We validated this detector on real indoor air samples from our laboratory (Figure 2a). First, it shows a large response from non-retained compounds that might be CH4, CO and H2. These gases are all usually present at hundreds of ppb in indoor air. Thereafter, a smaller peak is detected (SNR >40, see inset) at 107 s corresponding to formaldehyde at 45 ppb according to a proton-transfer-reaction time-of-flight mass spectrometer (PTR-TOF-MS). It is not interfered by methanol and ethanol, which are both eluted later, in line with synthetic gas mixtures. Figure 2b shows the scatter plot of the formaldehyde concentration measured by the detector and PTR-TOF-MS for pristine (squares, measured on two different days) and formaldehyde-spiked indoor air (circles) in the range of 14–475 ppb. Both instruments show excellent agreement with an R2 of 0.996, despite the challenging lab environment with high and varying background concentrations of acetone (up to 1,000 ppb) and ethanol (up to 4,000 ppb). Most importantly, the detector shows a low average error of only 10 ppb and can clearly differentiate safe levels from such above the WHO guideline (dashed line) and when immediate sensory irritation (dotted line) occurs. As a result, based on its compact size and low price, this device is promising for monitoring of formaldehyde in indoor air. When interconnected, such next-generation low-cost detectors could enable distributed chemical recognition (Figure 2c) for air quality in “smart” buildings and “future” cities [5]. References [1] T. Salthammer, S. Mentese, R. Marutzky, Formaldehyde in the indoor environment, Chemical Reviews. 110 (2010) 2536-2572. [2] J. van den Broek, D.K. Cerrejon, S.E. Pratsinis, A.T. Güntner, Selective formaldehyde detection at ppb in indoor air with a portable sensor, Journal of Hazardous Materials. 399 (2020) 123052. [3] J. van den Broek, S. Abegg, S.E. Pratsinis, A.T. Güntner, Highly selective detection of methanol over ethanol by a handheld gas sensor, Nature Communications. 10 (2019) 4220. [4] S. Abegg, L. Magro, J. van den Broek, S.E. Pratsinis, A.T. Güntner, A pocket-sized device enables detection of methanol adulteration in alcoholic beverages, Nature Food. 1 (2020) 351-354. [5] M. Mayer, A.J. Baeumner, A Megatrend Challenging Analytical Chemistry: Biosensor and Chemosensor Concepts Ready for the Internet of Things, Chemical Reviews. 119 (2019) 7996-8027. Figure 1

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