Role of Counteranions in Sol-Gel-Derived Alkoxyl-Functionalized Ionic-Liquid-Based Organic-Inorganic Hybrid Coatings for SPME

Three alkoxyl-functionalized ionic liquids, 1-(3-triethoxysilyl propyl)-3-methyl imidazolium hexafluorophosphate (TESPMIM[PF6]), 1-(3-triethoxysilyl propyl)-3-methyl imidazolium tetrafluoroborate (TESPMIM[BF4]), and 1-(3-triethoxysilyl propyl)-3-methyl imidazolium bis(trifluoromethanesulfonyl)imide (TESPMIM[N(SO2CF3)(2)]), were synthesized and used as selective coating materials to prepare chemically bonded ionic-liquid-based organic-inorganic hybrid solid-phase microextraction (SPME) fibers by sol-gel technology. A possible mechanism of the sol-gel process is proposed, and the successful binding of ionic liquids to the formed silica substrate was confirmed by Fourier-transform infrared spectroscopy (FT-IR). These ionic-liquid-based sol-gel coatings have porous surface structure, high thermal stability, strong solvent resistance, wide pH application range, good coating preparation reproducibility, special selectivity, and extraction efficiency for both polar and nonpolar compounds, such as phenolic environmental estrogens, fatty acids, aromatic amines, alcohols, phthalate esters, and polycyclic aromatic hydrocarbons. The TESPMIM[PF6]- and TESPMIM[BF4]-coated fibers have much lower thermal stability (to 300 and 285 A degrees C, respectively) than TESPMIM[N(SO2CF3)(2)]-coated fiber (to 454 A degrees C). However, the selectivity of these two fibers is higher towards strong polar analytes, while lower towards medium polar or nonpolar analytes compared with TESPMIM[N(SO2CF3)(2)]-based fiber. This could be explained by the fact that different counteranions in ionic liquid structures have different steric hindrance, nucleophilicity, hydrophobicity, and ability to form hydrogen bonds, resulting in significant difference in the characteristics of the ionic-liquid-based SPME fibers, such as the surface morphology, thermal stability, selective extraction ability, etc. This work demonstrates that the performance of the ionic-liquid-based coatings can be simply tuned by changing the counteranions incorporated into the ionic liquid structures.