Two-Dimensional Nanomaterials for Chemiresistive Gas Sensors: Towards Development of Breath based Diagnostics

Abstract

Breath based Diagnostics (BbD) can enable a paradigm shift in the Point-of-Care Diagnostic (PoCD) devices. Exhaled human breath has been demonstrated to contain over 2000 volatile organic and inorganic compounds, some of which report marked change in concentration under diseases conditions. A sensitive, selective, cost effective and portable gas sensing system could thus non-invasively diagnose multiple diseases from a single breath sample. However, there is a need to develop highly sensitive gas sensors with very low limit of detection (LLoD) down to ppb to ppt and high selectivity to meet this requirement.

This thesis focuses on developing such gas sensors based on novel 2D nanomaterials and their hybrids while using a simple, scalable synthesis route. This is in contrast to the conventional choice of sensing materials (Metal Oxides, polymers, CNT’s etc.) and expensive fabrication methods. Here, we explored layered materials namely Transition Metal Dichalcogenides (TMDC) and Layered Transition metal oxides (TMO) and their hybrids for the detection of Ammonia (NH3), Hydrogen Sulphide (H2S) and Nitrogen Dioxide (NO2), three important constituents of exhaled breath. The synthesis of these layered materials was carried out at room temperature via the liquid phase exfoliation (LPE) technique using low boiling point solvents. This technique is attractive because it is simple, scalable and does not require sophisticated instrumentation.

The key findings from this work can be summarized as follows. Layered Transition metal oxide (TMO) namely 2D MoO3 based devices demonstrated reasonable response to NH3 at room temperature but only down to 300 ppb which was not sufficient for our intended application. Further, we observed that the layered TMD’s WS2, WSe2 and its hybrid with Fe3O4 demonstrate remarkable ammonia sensing. WS2 demonstrated high sensitivity towards NH3 (detection down to 50 ppb) with fair selectivity but at an elevated operating temperature of 250oC. On the other hand, WSe2/fe3O4 hybrid-based devices demonstrated enhanced sensitivity and selectivity towards ammonia, that too at room temperature, with a 50 ppb LLoD. Another notable observation was the similar response of pristine WSe2 nanosheets towards NO2 as NH3. Hence, we enhanced the NO2 sensing performance of WSe2 based sensors by functionalizing their surface with noble metals such as Au and Pt using a simple wet chemical route. Interestingly, we obtained highly sensitive (down to 100 ppb) and selective response towards NO2 at room temperature. More importantly, the complete recovery to the original baseline without any external energy source was remarkable since it is known to be challenging.

While exploring other inorganic TMO’s, we observed that 2D V2O5 based devices detect H2S non-selectively at 350oC and down to only 500 ppb. Further improvement in H2S sensing is helped by TMD’s again as we modified the surface of WS2 in such a manner that it suppressed NH3 sensing, by using low temperature microwave irradiation assisted synthesis technique. Thus, it demonstrated highly selective, sensitive, and prompt H2S detection, though at an elevated temperature of 250oC. Later, we observed that a novel material of this same class (1T-TiS2) could provide similar attributes at room temperature. This material was not investigated before for gas sensing; hence we conducted a theoretical study and presented a plausible mechanism based on vdW interaction, substantiating physisorption between adsorbate and adsorbent. Thus, this thesis investigates novel materials, hybrids, and methods for scalable production of ultrasensitive, selective, stable, and low-cost sensors for NH3, H2S and NO2, which can potentially find applications for field-usable breath-based diagnostics in the future