Immobilizing Ionic Liquids onto Functionalized Surfaces for Sensing Volatile Organic Compounds.

Herein, a highly sensitive volatile organic compound (VOC) gas sensor is demonstrated using immobilized ionic liquid (IL), 1-butyl-3-methylimidazolium hexafluorophosphate, onto surfaces functionalized by the quaternary ammonium group -NR, -COOH, and -NH, , N-IL, COOH-IL, and NH-IL, respectively. These functional groups ensure highly tunable interactions between the IL and surfaces, efficiently modulating the electrical resistance of the immobilized IL upon exposure to acetone and toluene. The immobilized IL to both acetone and toluene displays significant electronic resistance changes at a concentration of 150 ppm, falling in the order NH-IL > N-IL > COOH-IL for acetone while COOH-IL > NH-IL > N-IL for toluene. A better gaseous sensing ability is achieved in COOH-IL for toluene than acetone, while this does not hold in the case of NH-IL and N-IL surfaces because of the completely different ion structuring of the IL at these functionalized surfaces. The accelerated ion mobility in the IL that is immobilized onto functionalized surfaces is also responsible for the strong gaseous sensing response, which is demonstrated further by the atomic force microscopy-measured smaller friction coefficient. This is highly encouraging and suggests that ILs can be immobilized by a network formed by surface functionalization to easily and cheaply detect VOCs at ppm concentrations.