Continuous multi-channel sensing of volatile acid and organic amine gases using a fluorescent self-assembly system

Abstract

A new gelator (1) was designed and synthesized through a Knoevenagel condensation reaction between 4-dimethylaminocinnamaldehyde and a gallic acid derivative containing a carbanion. This gelator could form stable organogels in hexane, petroleum ether, DMSO and acetonitrile. The self-assembly processes of 1 in four solvents were carefully investigated via UV-vis absorption, fluorescence, FTIR, XRD, water contact angles and field emission scanning electron microscopy (FESEM). Nanofibre and microsphere structures were observed in their self-assembly systems. The xerogel 1 film surfaces exhibited different hydrophobicities with water contact angles of 113.5–155°. The J-type aggregation mode was employed in the self-assembly process, and hydrogen bonding and π–π stacking were the main driving forces for organogel formation. Fluorescence emission of organogel 1 from acetonitrile was shifted from 558 nm of its corresponding diluted acetonitrile solution to 601 nm with a red-shift of 43 nm. Interestingly, compound 1 could sensitively respond to volatile acid, and further to organic amine gases along with obvious color changes. The detection limit for trifluoroacetic acid (TFA) by solution 1 in acetonitrile was 1.03 × 10⁻⁸ M with a corresponding association constant (K) of 6.85 × 10⁴ M⁻¹. The detection limit for triethylamine (TEA) by solution 1 in acetonitrile with 1.8 eq. of TFA was 1.27 × 10⁻⁸ M with a corresponding association constant (K) of 4.027 × 10⁴ M⁻¹. The yellow color of solution 1 could be reversibly changed to colorless in the titration process. At the same time, xerogel 1 exhibited sensitive response abilities for volatile acids and organic amines. The fluorescence of xerogel 1 could be quenched when in contact with a volatile acid within 7 s and recovered under further contact with a volatile amine within 15 s. The detection limit of xerogel film 1 towards trifluoroacetic acid (TFA) gases was 3.2 ppb. Organogel 1 exhibited sensitive response abilities towards TFA and TEA, and then rapidly expressed fluorescence, a gel state and a color change. This research would provide a new window into fast and sensitive detection of volatile acids and organic amines.