Ionic liquid-based amperometric microarray gas sensors

Gas sensors have been applied for a wide variety of applications in environmental and occupational health and safety (OHS) sectors. Current commercial amperometric gas sensors often suffer from short lifetime due to evaporation of electrolyte (e.g. H2SO4/H2O mixtures or organic solvents such as acetonitrile or propylene carbonate). Non-volatile ionic liquids (ILs) are promising electrolytes which provide a pathway to solve the evaporation problem in amperometric gas sensor. However the IL-based amperometric gas sensor suffers from low sensitivity and slow response time as a result of intrinsic high viscosity of IL. This thesis focuses on the development of IL-based amperometric gas sensor using microelectrode arrays prepared by microcontact printing (μCP) of a passive layer of self-assembled monolayer (SAM) onto gold electrodes to obtain enhanced mass transport in ILs. Thus the thesis is consisted of following content:i) The stability of SAMs such as 1-hexadecanethiol (HDT) and 11-mercaptoundercanoic acid (MUA) at gold and platinum electrodes are established in protic and aprotic room temperature ionic liquids (RTILs). SAMs in ILs show wider potential window compared to that in aqueous solution. Interestingly, it is found that the SAMs prepared by using ionic liquid as solvent are electrochemically more stable than that prepared from organic solvents.ii) An oxygen sensor system has been designed and established using microelectrode array fabricated by μCP of HDT on gold electrodes. The micropatterned gold electrode is immersed into a thin layer of hydrophobic IL electrolyte (e.g. 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide [BMIM][NTf2]). The results demonstrate that the microelectrode array sensor has better sensitivity and higher current density compared to macroelectrode-based gas sensor.iii) Finally, a high performance IL microarray gas sensor is developed using individual IL microdroplets as sensing units for oxygen sensing. Benefiting from non-volatility of ILs, small volume of droplet size (short diffusion distance), large surface-to-volume ratio, the new sensor design demonstrates significantly enhanced sensitivity and response time, compared to a conventional macroelectrode based gas sensor