Porous and Nanostructured CuBr Networks for Selective Ammonia Sensing at Room-Temperature

Introduction Ammonia (NH3), a major industrial commodity (142 Mt in 2017 [1]), is toxic as well as a tracer for food spoilage detection and important breath marker for impaired kidney function [2]. As a result, there is a strong interest in selective sensors that detect NH3 concentrations over a large range: from 250 to 2900 ppb in mouth-exhaled breath down to few ppb in indoor air at high relative humidity. Also, it occurs usually in gas mixtures containing a myriad of compounds requiring also high NH3 selectivity. Most promising as a solid-state and low-cost sensor material is CuBr featuring high selectivity and sensitivity to low NH3 concentrations (e.g. 20 ppb [3]) even at room temperature. However, current fabrication methods usually result in micro-sized CuBr particles [3] and rather dense film morphologies [4] that could impede efficient NH3 diffusion in the film and interaction with the CuBr surface. Here, we introduce a novel fabrication route yielding highly porous and nanostructured CuBr films for improved NH3 sensitivity and fast response dynamics at room temperature. Methods Porous and nanostructured CuBr films are obtained by a flame-aerosol based method on interdigitated electrodes. Their sensing performance was evaluated at room temperature in gas mixtures prepared with high-resolution mass flow controllers, as described in detail elsewhere [5]. In brief, analyte gases (i.e. NH3, isoprene, ethanol, methane, acetone, hydrogen, acetic acid, methanol, formaldehyde and CO) were supplied from calibrated cylinders (10 or 50 ppm in synthetic air, PanGas) and dosed to dry synthetic air (Pangas 5.0, CnHm and NOx ≤ 100 ppb) with calibrated mass flow controllers. Humidity was added by bubbling dry synthetic air through distilled water and admixing it to the analyte flow. All transfer lines were made of inert Teflon to minimized analyte gas adsorption and heated to 55 °C to avoid water condensation. The sensor film resistance was continuously monitored with a multimeter (Keithley, Integra Series 2700, USA). Results and Conclusions The porous CuBr films were tested for sensing of 5 - 5000 ppb NH3 at 90% RH (Figure 1a). When exposed to 5000 ppb of NH3, the resistance rapidly increases from 47 kΩ to 13 MΩ corresponding to a response (S) of 276. When exposed to NH3, the Cu+ as charge carriers are immobilized by forming that results in the observed resistance increase. Remarkably, this interaction is rapid and reversible even at room temperature, as indicated by the full recovery of the initial resistance baseline (dashed line, Figure 1a) and in line with literature [3]. Most impressively, NH3 concentrations down to 5 ppb (see also inset of Figure 1a for higher magnification) are detected and can be distinguished clearly from 10 and 20 ppb. The signal-to-noise-ratio (SNR, > 70) is remarkable and should enable the detection of even lower concentrations with an extrapolated lower limit of detection (LOD) of 210 parts-per-trillion (ppt) considering a typical SNR of 3. This is superior to state-of-the-art room temperature NH3 sensors: For instance, polyaniline sensors detect 40 ppb NH3 at ≥ 90% RH [6], but these are usually for single-use only as they recover too slowly. The porous CuBr sensor was tested with isoprene, ethanol, methane, acetone, hydrogen, acetic acid, methanol, formaldehyde and carbon monoxide (CO), all at a concentration of 1 ppm and 90% RH (Figure 1b). Most remarkably, the sensor responds strongest to NH3 with selectivity > 30 with CO being the highest one (> 290). This is similar to thermally deposited CuBr films with a CeO2 overlayer [3] for some of these analytes. Other NH3 sensors feature lower selectivities to these analytes. In fact, polymer-based PEDOT:PSS nanowires [7] are quite sensitive to ethanol (NH3 selectivity ~15 vs. CuBr ~40). In conclusion, such nanostructured and porous CuBr films are quite attractive for wearable breath analyzers [8] and detectors for distributed air and food quality monitoring networks with stringent power requirements. References [1] United States Geological Survey: Mineral Commodity Summaries - Nitrogen Statistics and Information, 2019. [2] Davies, S.; Spanel, P.; Smith, D., Quantitative analysis of ammonia on the breath of patients in end-stage renal failure. Kidney Int., 52 (1997) 223-228. [3] Li, H.-Y.; Lee, C.-S.; Kim, D. H.; Lee, J.-H., Flexible Room-Temperature NH3 Sensor for Ultrasensitive, Selective, and Humidity-Independent Gas Detection. ACS Applied Materials & Interfaces, 10 (2018) 27858-27867. [4] Bendahan, M.; Lauque, P.; Seguin, J.-L.; Aguir, K.; Knauth, P., Development of an ammonia gas sensor. Sens. Actuators B, 95 (2003) 170-176. [5] Güntner, A. T.; Righettoni, M.; Pratsinis, S. E., Selective sensing of NH3 by Si-doped α-MoO3 for breath analysis. Sens. Actuators B, 223 (2016) 266-273. [6] Hibbard, T.; Crowley, K.; Kelly, F.; Ward, F.; Holian, J.; Watson, A.; Killard, A. J., Point of Care Monitoring of Hemodialysis Patients with a Breath Ammonia Measurement Device Based on Printed Polyaniline Nanoparticle Sensors. Anal. Chem., 85 (2013) 12158-12165. [7] Tang, N.; Zhou, C.; Xu, L.; Jiang, Y.; Qu, H.; Duan, X., A Fully Integrated Wireless Flexible Ammonia Sensor Fabricated by Soft Nano-Lithography. ACS Sens., 4 (2019) 726-732. [8] Güntner, A. T.; Abegg, S.; Königstein, K.; Gerber, P. A.; Schmidt-Trucksäss, A.; Pratsinis, S. E., Breath Sensors for Health Monitoring. ACS Sens., 4 (2019) 268-280. Figure 1