Effects of ionic liquids on fluorescence characteristics of 17α- and 17β-estradiol

Herein, we report the effects of six different room temperature ionic liquids (RTILs) on fluorescence spectra of 17α-estradiol (EE1) and 17β-estradiol (E2). The selected RTILs belonged to the compound classes of 1-alkyl-3-methylimidazolium tetrafluoroborate ([C(n)MIM]BF(4)) and 1-alkyl-3-methyl imidazolium hexafluorophosphate ([C(n)MIM]PF(6)). RTILs had a gradual quenching effect on fluorescence intensity (FI) of EE1 and E2, and the quenching process followed the well-known Stern-Volmer theory. The quenching mechanism of EE1 and E2 by RTILs was demonstrated to be dynamic quenching. Additionally, the overall quenching efficiency by [C(n)MIM]BF(4) was higher than [C(n)MIM]PF(6). The increased carbon chain length of RTILs did not lead to obvious differences in FI for EE1 and E2. The quenching efficiency showed irregular trend at three different temperatures (25, 35 and 45 °C). RTILs such as [C(4)MIM]PF(6) had the different fluorescent effects on organic chemicals with different fluorophores. The enhancing effects of [C(4)MIM]PF(6) were observed on strong fluorescence chemicals (dansyl chloride, rhrodamine B, 1,10-phenanthroline, norfloxacin), while quenching effect on weak fluorescence chemicals (EE1 and E2). In theory, these results provide a theoretical foundation for deep insight into their interaction mechanism between RTILs and estradiol.