Design and fabrication of highly sensitive graphene-based sensors for environmental and health monitoring applications

Abstract

Gas sensing technology has emerged as an invaluable tool for real-time monitoring and detection of gases in different settings, ranging from environmental monitoring to agriculture, industrial processes, healthcare, and automotive industry. Gas sensors have numerous and significant practical uses. For example, in environmental monitoring gas sensor are used to identify contaminants and to evaluate indoor and outdoor air quality. Gas sensors in industrial settings are utilised to identify harmful and combustible gases to protect and maintain workplace safety. Applications in healthcare span from breath analysis for disease detection to anaesthesia monitoring during operations. In agriculture, gas sensors are used for determining the condition of the soil and identifying hazardous emissions from cattle.

Recent developments in gas sensor technology have resulted in advancements in several areas. Real-time monitoring in both industrial and personal contexts is made possible by the development of portable and wearable sensors. This is mainly due to miniaturisation and integration with microelectronics. Metal oxide (MOX) semiconductor gas sensors are widely used as the sensing element due to their cost-effective large-scale production and ability to detect a wide range of targeted gases. But, due to their limitations such as cross-selectivity, requirement for periodic calibration, signal drift, and operation at high temperature (between 100 ℃ and 400 ℃), research has been carried out in identifying other candidate materials for gas sensing applications to replace MOX sensors. Additionally, MOX based gas sensors show higher sensitivity towards variety of gases, resulting in poor selectivity. Similarly, the limit of detection of MOX sensors is insufficient to meet the criteria for sub ppm (parts per million) and ppb (parts per billion) gas detection levels. For example, the majority of gases exhaled by humans require ppb level detection sensitivity. The well-established MOX sensors (e.g. tin oxide, zinc) show response times between 5 seconds to 50 seconds, which are not fast enough to monitor human respiration patterns that requires very fast response rate.

Graphene, a two-dimensional carbon allotrope has emerged as a promising material for revolutionising gas sensing systems, thanks to its remarkable electrical, mechanical, and chemical properties. Every atom in graphene can be considered as a surface atom and it shows capability of interacting with molecules of target gas, resulting in an ultrasensitive sensor response. There are different methods utilized for production of graphene. The first graphene based chemiresistive gas sensor was studied in 2007 using a mechanically exfoliated graphene, it showed varying resistance when exposed to various gases. Other appealing properties of graphene include its compatibility with complementary metal oxide semiconductor (CMOS) fabrication techniques, possibility of growing high-quality graphene on a large area, its commercial availability, and low optical absorption. It is also reported that graphene sensors could deliver response rates between several hundred milliseconds to a few seconds making it a suitable sensing material to study applications that require fast response. The main emphasis of this thesis is to study the possibility of using commercially available CVD monolayer graphene film for chemiresistive and optical readout-based gas sensors for environmental and respiratory monitoring applications while utilising CMOS compatible fabrication techniques.

One of the main challenges in utilising graphene as a gas sensor is its very sensitive electronic and surface properties. Due to its high surface to volume ratio, water, oxygen, and other environmental moieties adsorb on the surface of graphene, which leads to reducing the adsorption site for target analyte. One method of cleaning graphene surface is by ultraviolet (UV) irradiation, which leads to photo desorption of the adsorbates and hence improving the response by creating more sites for target gas adsorption. Previously, studies on graphene-based gas sensors have been carried out inside systems with large measurement chambers of volume of several litres (1-10) and equipped with multiple mass flow controllers. In the current study (Chapter 4), we have integrated a three-dimensional (3D) printed glass microchamber (50 µl) on the graphene chemiresistive sensor for improving long-term stability of the gas sensor. This integrated chamber design enabled cleaning graphene surface via UV irradiation and hence laid down the possibility of developing a portable graphene-based system for gas sensing applications. We studied the sensor response towards acetone vapour with UV irradiation and without UV irradiation (UV power 7.5 mW/cm²). The stability of the sensor was studied for 70 days and stable response of ~ 1% was obtained towards the same concentration (20 ppm) of acetone after UV treatment for 70 days. We also used our gas sensor to measure the concentration of acetone from an artificially spiked soil sample. The response of the sensor was as expected, the soil sample spiked with a higher weight percentage of acetone showed a higher response while for a lower weight percentage showed a diminished response. The corresponding concentration of acetone emitted from the spiked soil sample was calculated from the calibration curve.

Following this study, a Wheatstone bridge type sensor configuration was designed and manufactured (Chapter 5). In this design, all four resistors are made up of patterned graphene for gas sensing. In the sensor array, two resistors are encapsulated, and the remaining two resistors are non-encapsulated (acting as sensing resistors). This chemiresistive graphene sensor was used to study human respiratory patterns owing to graphene’s good response towards humidity (with response time of 900 milliseconds and recovery time of 1.5 seconds). The sensor was able to identify fast breathing pattern with a fall time of 500 ms and a rise time of 550 ms which confirms the fast response and recovery rate of graphene sensors. Unlike the previous study where graphene had to be treated under UV for improving response, the Wheatstone bridge graphene sensor with two encapsulated and two non-encapsulated graphene resistors, demonstrated an excellent response towards different human respiratory patterns without UV treatment, thus only using a low power of 300 µW. Six volunteers were studied, and the device showed excellent stability for 140 days.

In the literature different types of graphene sensors for sensing a variety of gases are reported, however achieving a lower device-to-device variability is an ongoing fabrication challenge. For example, in the current study we were able to fabricate and measure portable graphene-based sensor for mainly two applications (acetone sensing and human respiratory sensing). For graphene chemiresistive acetone sensing, three devices were studied, and the devices showed a variation in response of~ 0.7%, ~0.9%, and ~1% towards 20 ppm of acetone gas. In our second study, device-to-device variation within the same die was <5% and between two different dies was found to be ~25%. In a nutshell, this thesis provides a clear study on using commercially available CVD graphene films, CMOS compatible fabrication process, and the main challenges associated with using graphene as a sensing material.

Date of Award	Jul 2024
Original language	English
Awarding Institution	Queen's University Belfast
Sponsors	Northern Ireland Department for the Economy
Supervisor	David McNeill (Supervisor) & Hamza Shakeel (Supervisor)