Luminescence enhancement of europium(III) originating from self-assembled supramolecular hydrogels

Luminescence enhancement of the rare-earth ion Eu(III) in supramolecular hydrogels formed by self-assembly of the hydrogelator N,N-dibenzoyl-L-cystine (DBC) was observed by fluorescent optical microscopy and steady-state luminescence. The spectra of varying temperature luminescence showed that luminescence intensities of Eu(III) in DBC gels decreased with an increase of temperature and a dramatic decrease observed in the range 60-70 degrees C, which can be ascribed to the dissociation of DBC aggregates. This implied that the luminescence enhancement of Eu(III) in supramolecular hydrogels is related to the self-assembled DBC. The morphology of DBC aggregates in the presence of Eu(III) shows highly entangled and thin fibrillar aggregates compared with less entangled and thick aggregates in the absence of Eu(III). The phase transition temperature (T(SG)) of DBC gels increased with an increase of Eu(III) concentration. This suggests that interaction occurs between Eu(III) and DBC aggregates. The UV and FT-IR spectra of the DBC gels in the presence of Eu(III) indicated the formation of complexation between Eu(III) and oxygen atoms of DBC aggregates. The results reveal the mechanism of the luminescence enhancement of Eu(III) in supramolecular hydrogels, which most likely involve the interaction between Eu(III) and DBC aggregates.