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Abstract
Continuous monitoring and real-time detection of gas molecules in the environment is very important in every aspect of modern society. There is an increasing demand to develop robust amperometric gas sensors with extended lifetimes and enhanced overall performance (sensitivity, selectivity, response time and limit of detection. Current commercial amperometric gas sensors often suffer from short lifetime due to evaporation of conventional electrolyte (e.g. aqueous or organic solvents). Even though membranes were introduced in gas sensor design to reduce the evaporation rate of the electrolyte, but it is unable to solve the problem completely.
In this thesis, we described strategies to fabricate robust and “membrane-less” electrochemical gas sensors with improved overall performance, using ionic liquids (ILs) as substitute electrolytes. First, microcontact printing was utilised to fabricate IL-based microchannels electrode gas sensors. The fabricated sensor was firstly applied for carbon dioxide sensing and compared with ionic liquid-based macroelectrode gas sensor. The enhanced performance of the IL-based microchannels electrode gas sensors was observed by using long-term chronoamperometry technique for carbon dioxide sensing. The IL-based microchannels electrode gas sensor is then further studied for toxic gases detection including ammonia and hydrogen chloride. On the timescale of the electrochemical experiments, no unusual behaviour or detrimental effects were observed on the Au modified microchannels electrode. Two different electrochemical techniques, linear sweep voltammetry (LSV) and chronoamperometry been utilised for toxic gas sensing with two different IL. Very low LODs were obtained from 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C2mim][NTf2]) by LSV (4.1 ppm for ammonia and 3.6 ppm for hydrogen chloride). The [C2mim][NTf2]-based microchannels electrode sensors not only demonstrate a fast response (t90: 15-30 s) but also excellent linearity in real-time sensing with a sensitivity of 1.8 nA ppm−1 (for ammonia) and 25.5 nA ppm−1 (for hydrogen chloride), respectively.
Furthermore, the IL-based microchannels electrode gas sensor was electrodeposited with platinum nanoparticles (PtNPs) and demonstrated for hydrogen gas sensing for the first time. A linear trend line and a ultrafast response (less than 2 s) were achieved in [C2mim][NTf2] using the proposed sensor design to measure hydrogen gas in the range from 10-100% and 1-10% with a sensitivity of 1.2 × 10−7 A %−1 and 2.1 × 10−7A %−1, respectively. Moreover, the IL-based microchannels electrode was also electrodeposited with copper nanoparticles (CuNPs) and presented for carbon dioxide sensing. Benefiting from non-volatility of ILs, a small volume of droplet size (short diffusion distance), large surface-to-volume ratio, more electroactive sites on Cu nanoparticles, this IL-based CuNPs-microchannels gas sensor demonstrates improved sensitivity and response time (about 10-20 s faster) for CO2 sensing, compared to IL-based microchannels gas sensor design without Cu catalyst deposition. In addition, IL-based CuNPs-microchannels gas sensor is able to detect carbon dioxide molecules at lower potentials whereas no signal response can be obtained on IL-based microchannels gas sensor.