Superior Acetone Selectivity in Gas Mixtures by Catalyst-Filtered Chemoresistive Sensors

Abstract

Acetone is a toxic air pollutant and a key breath marker for non-invasively monitoring fat metabolism. Its routine detection in realistic gas mixtures (i.e., human breath and indoor air), however, is challenging, as low-cost acetone sensors suffer from insufficient selectivity. Here, a compact detector for acetone sensing is introduced, having unprecedented selectivity (>250) over the most challenging interferants (e.g., alcohols, aldehydes, aromatics, isoprene, ammonia, H₂, and CO). That way, acetone is quantified with fast response (<1 min) down to, at least, 50 parts per billion (ppb) in gas mixtures with such interferants having up to two orders of magnitude higher concentration than acetone at realistic relative humidities (RH = 30-90%). The detector consists of a catalytic packed bed (30 mg) of flame-made Al₂O₃ nanoparticles (120 m² g^-1) decorated with Pt nanoclusters (average size 9 nm) and a highly sensitive chemo-resistive sensor made by flame aerosol deposition and in situ annealing of nanostructured Si-doped ε-WO₃ (Si/WO₃). Most importantly, the catalytic packed bed converts interferants continuously enabling highly selective acetone sensing even in the exhaled breath of a volunteer. The detector exhibits stable performance over, at least, 145 days at 90% RH, as validated by mass spectrometry.

Keywords: breath analysis; environmental monitoring; semiconductors; solid‐state gas sensors; wearables.

Figure 1

Si/WO₃ sensor response to 1 ppm isoprene (green), acetone (blue), ethanol (red), H₂ (purple), ammonia (yellow), or CO (black) in air at 400 °C and 90% RH after passing through the catalytic Pt/Al₂O₃ packed bed at a) room temperature (i.e., inactive) and c) at 135 °C. b,d) Corresponding acetone selectivities, as measured with the sensor. Error bars indicate the standard deviations for three identically prepared packed beds and sensors.

Figure 2

Responses of the Si/WO₃ sensor with catalytic Pt/Al₂O₃ packed bed at 135 °C upon exposure to 5, 10, 25, 50, and 100 ppm of H₂ (purple) and CO (black) at 90% RH. The response to 1 ppm acetone is indicated (blue dashed line) for reference. Note the scale break of the ordinate.

Figure 3

a) Si/WO₃ film resistance upon exposure to 50–1000 ppb acetone as single analyte (blue dashed line) and with a mixture containing simultaneously 5000 ppb ammonia, isoprene, ethanol, CO, and H₂ (each 1000 ppb, red solid line) at 50% RH after passing through the Pt/Al₂O₃ packed bed at 135 °C. b) Analyte concentration profiles of the same gas mixtures after the Pt/Al₂O₃ packed bed, as measured by PTR‐ToF‐MS that, however, cannot detect CO and H₂. c) Acetone sensor responses without (circles) and with the above mixture of interferants (triangles). The overlapping symbols and high coefficient of determination (R ² > 0.99, n = 6) highlight the excellent acetone selectivity of the present detector.

Figure 4

a) XRD pattern of 0.2 mol% Pt/Al₂O₃ powder with reference peaks for cubic Al₂O₃ (circles), Pt (triangles), and tetragonal PtO (squares). The Al₂O₃ crystal size (d _XRD) and SSA are indicated. b) HRTEM image of such crystalline Pt/Al₂O₃ particles. c) HAADF‐STEM of Al₂O₃ particles (dark grey) and Pt clusters (white). EDXS analysis of d) a Pt cluster and e) Al₂O₃ support, as indicated in (c). f) Counted Pt cluster size distribution, as determined from HAADF‐STEM images. Equivalent lognormal fit from the counted distribution (dashed line) together with its geometric diameter (d _g), standard deviation (σ _g), and number count of clusters (N).

Figure 5

Catalytic conversion of 1 ppm ethanol (squares), isoprene (triangles), ammonia (stars), H₂ (diamonds, 50 ppm), and acetone (circles) over 0.2 mol% Pt/Al₂O₃ at 90% RH as a function of catalytic packed bed temperature. All analyte concentrations were measured at the catalyst outlet by PTR‐ToF‐MS, except for H₂ where a QuinTron Breath Tracker was used.

Figure 6

a) Conversion of 1 ppm ethanol (squares), isoprene (triangles), ammonia (stars), H₂ (diamonds, 50 ppm), or acetone (circles) over 0.2 mol% Pt/Al₂O₃ at optimal 135 °C as a function of RH, as measured by PTR‐ToF‐MS and the QuinTron Breath Tracker. b) Breath acetone ratio (normalized to initial acetone concentration) after 40 min intense exercise^{[ 4 ]} by a single volunteer measured with the PTR‐ToF‐MS (filled symbols) and the Pt/Al₂O₃‐Si/WO₃ detector (empty symbols) for n = 6 breath samples proving its superior selectivity. c) Packed bed stability for a mixture of 1 ppm ethanol, isoprene, and acetone over 145 days at 90% RH, as measured at the catalyst outlet by PTR‐ToF‐MS and the QuinTron Breath Tracker. Continuous variables are expressed as mean ± standard deviation (n = 9).