Luminescence sensitization of terbium-loaded supramolecular gels by hydroxybenzoic acids and used for salicylates sensing

Abstract

The luminescent terbium (Tb³⁺)-loaded supramolecular gels were facilely prepared through the self-assembly of Fmoc-diphenylalanine (FmocPhePhe) at room temperature. Hydroxybenzoic acid (HA, the isomers are denoted as 2-HA, 3-HA, and 4-HA depending upon the positions of hydroxyl groups) was used as a sensitizer to Tb³⁺. The luminescence sensitization of Tb³⁺ in the gels was realized by the coordination with hydroxybenzoic acids. The spectra of luminescence, UV-vis, FT-IR, and ¹H NMR verified that this sensitization was attributed to the energy transfer from hydroxybenzoic acids to Tb³⁺. The results of XRD, SEM, and phase transfer temperature further indicated that the initial molecule arrangement of the gels was significantly changed by 2-HA, resulting in more ordered and more compact morphology of the gels. 2-HA exhibited more effective sensitization to Tb³⁺ in the gels than 3-HA and 4-HA. It was also found that 2-HA did not affect the self-assembly of FmocPhePhe. Due to the effective fluorescence sensitization by 2-HA, the as-prepared gels can be used for salicylic acid sensing with 6.8 μM of the detection limit. This strategy has been successfully used for the detection of salicylates in pharmaceuticals and cosmetics.

Keywords: Hydroxybenzoic acids; Luminescence sensitization; Salicylates sensing; Supramolecular gel; Terbium ion.