Detection of 4-nitrophenol by a naphthene carboxylic acid-based fluorescent dicationic ionic liquid in environmental waters and soils

Herein, a naphthene carboxylic acid-based dicationic ionic liquid ([C3(MIM)2][NA]2) was synthesized and characterized in detail. The [C3(MIM)2][NA]2 was used as a high-efficiency fluorescent probe for 4-nitrophenol (4-NP) quantification based on a "ON-OFF" strategy in environmental waters and soils. At the 330-nm excitation wavelength, this fluoroprobe exhibited a strong emission at 415 nm with an absolute quantum yield of 8.5 %. The fluorescence of [C3(MIM)2][NA]2 could be effectively quenched by 4-NP through the fluorescence resonance energy transfer (FRET) process. By DFT theoretical calculation, the band gap of the [C3(MIM)2][NA]2 + 4-NP system was reduced as low as 3.10 eV, suggesting the occurrence of ground state electron transfer from HOMO of 4-NP to LUMO of [C3(MIM)2][NA]2. A satisfactorily linear equation was obtained when plotting the fluorescence intensities of [C3(MIM)2][NA]2 vs the concentrations of 4-NP with a detection limit of 0.3 μM. The newly developed method was successfully used to detect 4-NP in environmental water and soil samples with satisfactory fortification recoveries of 97.2–104.2 %. Moreover, it is comparable with or even better than the China National Standard Method (HG-T-4296–2012), i.e., conventional titration method, in the aspects of detection limit and linear range. To sum up, the the as-constructed fluorescence sensor holds great potential application in routine monitoring of 4-NP in environmental waters and soils.