Doped ZnO for Selective Acetic Acid Sensing

Introduction Acetic acid is a key tracer in chocolate and coffee processing defining their final taste. Additionally, it is a promising breath marker for cystic fibrosis and gastro-oesophageal reflux disease as breath concentrations are significantly increased with 170 ppb [1] and 85 ppb [2], respectively, compared to healthy humans (48 ppb). However, current sensors lack ppb to ppm level sensitivity with high selectivity to detect relevant acetic acid concentrations accurately in gas mixtures at high humidity (90%). Here, we present an acetic acid selective gas sensor able to detect lowest ppb-level concentrations. It consists of doped ZnO nanoparticles whereas doping level, particle & crystal size as well as film morphology were subsequently optimized in a flame spray pyrolysis (FSP) reactor. Method ZnO nanoparticles with different (0 - 10 mol%) doping contents were produced by flame spray pyrolysis (FSP) and directly deposited [3] onto water-cooled Al2O3 sensor substrates forming a highly porous sensing network. The mechanical stability of the nanoparticles on the Al2O3 was fortified by in-situ annealing [4] with a particle-free xylene flame followed by annealing at 500 °C for 5 h in an oven. The sensors were mounted onto Macor holders, placed in a sensing chamber and installed in an evaluation setup described elsewhere [5]. The sensors were heated to 250 - 450 °C and tested on 1 ppm acetic acid, acetone, ethanol and ammonia at relative humidity ranging from 10 to 90%. Results and Conclusions Figure 1a shows a top view SEM of the sensing film for ZnO doped with 1 mol% transition metals. The doping thermally stabilizes the particles during high temperature treatment and preserves the highly porous nanostructured sensing network typical for FSP. In specific, it consists of aggregates and agglomerates of individual nanoparticles with particle sizes ~35 nm (Figure 1a, inset), as determined by nitrogen adsorption from filter-collected powders and ideal for gas sensing due to the high available surface area. Figure 1b shows the sensor response to 1 ppm acetic acid, ethanol, acetone and H2 as a function of the doping content. Additionally, the doping with 1 mol% increases the sensor response to acetic acid and selectivity to ethanol and acetone compared to pure ZnO (Figure 1b). That way, even concentrations down to 50 ppb were detectable at 50% relative humidity (RH), unprecedented by other acetic acid sensors. Note that at higher doping contents, the responses to all analytes decrease again, most likely due to the isolating character of segregated transition metal phases. As a result, an inexpensive acetic acid detector has been developed that could be easily incorporated into a portable breath analyzer, used for food processing monitoring or incorporated into orthogonal arrays [6]. References [1] Smith, D.; Sovova, K.; Dryahina, K.; Dousova, T.; Drevinek, P.; Spanel, P., Breath concentration of acetic acid vapour is elevated in patients with cystic fibrosis. J Breath Res (2016). [2] Dryahina, K.; Pospisilova, V.; Sovova, K.; Shestivska, V.; Kubista, J.; Spesyvyi, A.; Pehal, F.; Turzikova, J.; Votruba, J.; Spanel, P., Exhaled breath concentrations of acetic acid vapour in gastro-esophageal reflux disease. J Breath Res (2014). [3] Mädler, L.; Roessler, A.; Pratsinis, S. E.; Sahm, T.; Gurlo, A.; Barsan, N.; Weimar, U., Direct formation of highly porous gas-sensing films by in situ thermophoretic deposition of flame-made Pt/SnO2 nanoparticles. Sensor Actuat B-Chem (2006), 283-295. [4] Tricoli, A.; Graf, M.; Mayer, F.; Kuhne, S.; Hierlemann, A.; Pratsinis, S. E., Micropatterning layers by flame aerosol deposition-annealing. Adv Mater (2008), 3005-3010. [5] Güntner, A. T.; Righettoni, M.; Pratsinis, S. E., Selective sensing of NH3 by Si-doped a-MoO3 for breath analysis. Sensor Actuat B-Chem (2016), 266-273. [6] Pineau, N. J.; Kompalla, J. F.; Güntner, A. T.; Pratsinis, S. E., Orthogonal gas sensor arrays by chemoresistive material design. Microchim Acta (2018). Figure 1