Development of a compact, IoT-enabled electronic nose for breath analysis

In this paper, we report on an in-house developed electronic nose (E-nose) for use with breath analysis. The unit consists of an array of 10 micro-electro-mechanical systems (MEMS) metal oxide (MOX) gas sensors produced by seven manufacturers. Breath sampling of end-tidal breath is achieved using a heated sample tube, capable of monitoring sampling-related parameters, such as carbon dioxide (CO2), humidity, and temperature. A simple mobile app was developed to receive real-time data from the device, using Wi-Fi communication. The system has been tested using chemical standards and exhaled breath samples from healthy volunteers, before and after taking a peppermint capsule. Results from chemical testing indicate that we can separate chemical standards (acetone, isopropanol and 1-propanol) and different concentrations of isobutylene. The analysis of exhaled breath samples demonstrate that we can distinguish between pre- and post-consumption of peppermint capsules; area under the curve (AUC): 0.81, sensitivity: 0.83 (0.59–0.96), specificity: 0.72 (0.47–0.90), p-value: <0.001. The functionality of the developed device has been demonstrated with the testing of chemical standards and a simplified breath study using peppermint capsules. It is our intention to deploy this system in a UK hospital in an upcoming breath research study

Nickel-oxide based thick-film gas sensors for volatile organic compound detection

In this paper, we report on the development of a highly sensitive and humidity-tolerant metal-oxide-based volatile organic compound (VOC) sensor, capable of rapidly detecting low concentrations of VOCs. For this, we successfully fabricated two different thicknesses of nickel oxide (NiO) sensors using a spin-coating technique and tested them with seven different common VOCs at 40% r.h. The measured film thickness of the spin-coated NiO was ~5 μm (S-5) and ~10 μm (S-10). The fastest response and recovery times for all VOCs were less than 80 s and 120 s, respectively. The highest response (Rg/Ra = 1.5 for 5 ppm ethanol) was observed at 350 °C for both sensors. Sensors were also tested in two different humidity conditions (40% and 90% r.h.). The humidity did not significantly influence the observed sensitivity of the films. Furthermore, S-10 NiO showed only a 3% drift in the baseline resistance between the two humidity conditions, making our sensor humidity-tolerant compared to traditional n-type sensors. Thus, we propose thick-film NiO (10 μm) sensing material as an interesting alternative VOC sensor that is fast and humidity-tolerant

Humidity dependence of commercial thick and thin-film MOX gas sensors under UV illumination

Enhancing the performance of a chemo-resistive gas sensor is often challenging due to environmental humidity influencing its sensitivity and baseline resistance. One of the most promising ways of overcoming this challenge is through ultraviolet (UV) illumination of the sensing material. Most research has focused on using UV with in-house developed sensors, which has limited their widespread use. In this work, we have evaluated if UV can enhance the performance of commercially available MOX-based gas sensors. The performance of five different MOX sensors has been evaluated, specifically SGX Microtech MiCS6814 (thin-film triple sensor), FIGARO TGS2620 (n-type thick film), and Alphasense VOC sensor (p-type thick film). These sensors were tested towards isobutylene gas under UV light at different wavelengths (UV-278 nm and UV-365 nm) to investigate its effect on humidity, sensitivity, baseline drift, and recovery time of each sensor. We found the response time of thin-film sensors for reducing gases was improved by 70 s under UV- 365 nm at normal operating temperatures. In addition, all the sensors were left in a dirty environment and the humid-gas testing was repeated. However, due to their robust design, the sensitivity and baseline drift of all the sensors remained the same. This indicates that UV has only limited uses with commercial gas sensors

ZnO/MoO3 Heterojunction thick films to detect ppb level Volatile Organic Compounds

The development of a fast, stable metal oxide-based VOC sensor for the detection of trace-level ppb concentrations, remains a challenge. Recently, many composite materials have been investigated to try and develop sensors with these characteristics. Here, we report on the development of ZnO/MoO3 heterojunction thick film devices, fabricated by a spin-coating technique, to detect a wide range of VOCs at application relevant ppb level concentrations. For comparison, pristine ZnO and pristine MoO3 devices were also fabricated. Sensors were tested at different temperatures and resulting in an optimum temperature of 380°C. The sensors were tested towards 11 different VOCs at ppb concentrations. Of all the sensors tested, the heterojunction device showed the highest response to VOCs, with the highest sensitivity towards 200 ppb of ethanol (Ra/Rg = 12.84). The results compared well with the pristine materials, with the response of the ZnO material being 6 times smaller and MoO3 10 times smaller compared to their heterojunction counterpart. The response times of the heterojunction device were also faster, at around 30 sec compared with 120 sec for pristine materials

Direct in situ spectroscopic evidence of the crucial role played by surface oxygen vacancies in the O2-sensing mechanism of SnO2

NAP-XPS characterisation of SnO2 under operando conditions shows that resistance change, band bending and surface O-vacancy concentration are correlated with ambient O2 concentration, challenging current preconceptions of gas sensor function

Facile synthesis of Ag nanoparticles-decorated WO3 nanorods and their application in O2 sensing

Here we describe a two-step aerosol-assisted chemical vapor deposition (AACVD) synthesis method for the fabrication of Ag nanoparticles (NPs) decorated WO3 nanorods (NRs), evaluating the use of different organometallic silver precursors. Physical property characterization techniques including XRD, SEM, TEM, and XPS were carried out to investigate the composition and morphology of the pristine WO3 NRs and functionalized WO3 NRs with Ag NPs. The results showed that uniform WO3 NRs were obtained with a length of 600 nm to several μm and a diameter of 100–200 nm, and Ag NPs were well-dispersed on the surface of WO3 NRs with the size of 6–20 nm. The nanostructured WO3 thin films were synthesized and integrated directly onto alumina platforms via the AACVD method to fabricate gas sensors. Gas sensing performance was investigated towards different O2 concentrations between 1% and 20% at various operating temperatures. The sensing response revealed that an increase in baseline resistance was observed for the Ag-decorated WO3 sensors fabricated by using organometallic silver precursors, and the decoration of Ag NPs on WO3 sensors improved sensing properties as compared to the undecorated ones. The possible formation process and sensing mechanism of the Ag NPs decorated WO3 NRs are proposed

Direct in situ spectroscopic evidence of the crucial role played by surface oxygen vacancies in the O2-sensing mechanism of SnO2

Conductometric gas sensors (CGS) provide a reproducible gas response at a low cost but their operation mechanisms are still not fully understood. In this paper, we elucidate the nature of interactions between SnO2, a common gas-sensitive material, and O2, a ubiquitous gas central to the detection mechanisms of CGS. Using synchrotron radiation, we investigated a working SnO2 sensor under operando conditions via near-ambient pressure (NAP) XPS with simultaneous resistance measurements, and created a depth profile of the variable near-surface stoichiometry of SnO2−x as a function of O2 pressure. Our results reveal a correlation between the dynamically changing surface oxygen vacancies and the resistance response in SnO2-based CGS. While oxygen adsorbates were observed in this study we conclude that these are an intermediary in oxygen transport between the gas phase and the lattice, and that surface oxygen vacancies, not the observed oxygen adsorbates, are central to response generation in SnO2-based gas sensors

Enhancing metal oxide-based gas sensors for the detection of VOCs

Metal oxide semiconductor (MOX) based gas sensors are very promising and widely commercialized gas sensing technology that has the potential to replace the conventional gas analysing instruments such as Gas chromatography and Mass spectroscopy. Though there are many advantages attributed to MOX sensors, there exists some limitations to the current technology – such as, humidity dependence, baseline drift, and poor sensitivity to trace level concentrations, and cross sensitivity. This thesis reports the investigations undertaken to overcome such limitations by using different strategies, while testing a wide range of relevant volatile organic compounds (VOCs) that are present in the environment. Investigations were carried on Nickel oxide (NiO) thick films to achieve the humidity tolerance. Spin coated NiO thick film (10 μm) showed a stable and higher response towards ethanol (1.27) among the other VOCs while the baseline drift due to humidity was least when compared with other NiO sensors. A heterojunction composite sensor was developed using ZnO and MoO3 materials to achieve fast, stable and high sensitivity towards trace levels of VOCs. The ZnO/MoO3 thick film sensor was able to detect different VOCs at ppb concentrations within 30 sec and the relative response was 12.84 and 9.27 for 200 ppb ethanol and methanol gases respectively, which was 4 times higher than its pristine counterparts. Ultraviolet (UV) light was illuminated onto different types of commercial MOX sensors and in-house fabricated MOX sensors with a focus to examine the effect of UV on the sensor performance – specifically, sensitivity, baseline drift due to humidity, response, and recovery time. The response time was reduced by 70 sec and 40 sec for the commercial MOX sensors, while the baseline drift due to humidity of AACVD deposited Chromium-Titanium Oxide was reduced by ~2% due to UV light. Finally, the fabricated MOX sensors were evaluated in a real-world application sensing the toxic VOCs that effuses from thermally abused Li-ion cells as an attempt to prevent from battery explosions

Comparative study of spin-coated and vapour deposited nickel oxides for detecting VOCs

Nickel oxide (p-type) sensors are developed to detect volatile organic compounds (VOCs). In the presented work, NiO sensors are fabricated and tested towards acetone, ethanol, toluene, hexane, methanol, and n-propanol vapours between 5 to 25 parts-per-million concentrations, under both dry and humid conditions. NiO films are deposited onto alumina substrates using both spin-coating (SC) and vapour deposition (AACVD) methods. The measured thickness of the spin coated and AACVD NiO films are approximately comparable at 10.3 μm and 6.7 μm, respectively. Both SC and AACVD sensors showed a maximum response at 350°C. No significant influence of humidity was observed on sensor response and baseline resistance for either SC or AACVD sensors. The sensitivity is found to be highest for ethanol, acetone, and methanol vapours than the rest