Ionic liquids as selective depositions on quartz crystal microbalances for artificial olfactory systems—a feasibility study

A room-temperature ionic liquid (RTIL), 1-butyl-3-methylimidazolium hexafluorophosphate, was investigated as a selective deposition for quartz crystal microbalances. RTILs constitute versatile matrices of tuneable physicochemical properties providing a high solute mobility compared to polymer matrices, and hence in principle short response and desorption times. This paper explores the feasibility of using RTIL-based quartz crystal microbalances as vapour sensors with particular focus on the physicochemical interactions between the RTIL and a model solute, ethyl acetate. Preliminary experiments were conducted and proved an excellent baseline recovery of the sensor. The transient sensor response was modelled using a basic mass transport equation which proved the simplicity of the system when compared to polymer coatings. The diffusion coefficient of ethyl acetate was calculated to be 10.8·10− 11 m2·s− 1, which was one order of magnitude higher than in a common polymeric deposition material, polydimethylsiloxane. However, it is pointed out that care must be taken when interpreting the sensor signal upon dissolution of a complex vapour; absorption of solutes into the RTIL can give rise to viscosity changes which affect the sensor signal and hence overlap with the response to the solute mass absorbed. Nevertheless, for simple sample vapours we believe that quartz resonators can also be a complementary and cheap method for investigating basic mass transport phenomena.