Luminescent properties of biohybrid (kaolinite-proline) materials synthesized by a new boric acid catalyzed route and complexed to Eu3+

Abstract

The properties of Eu3+-biohybrid (kaolinite-proline) materials were investigated by powder X-ray diffraction, vibrational spectroscopy, scanning electron microscopy, nitrogen adsorption and photoluminescence to understand how Eu3+ interacts with the biohybrid matrices. Biohybrids were obtained through the conventional route and by two new catalytic routes for functionalization of clay minerals, and were then complexed to Eu3+ ions at a constant proline:Eu3+ molar ratio of 3:1, while the water molecules originally existing in the first coordination sphere of Eu3+ ions in the kaolinite-grafted complexes were replaced by 2-thenoyltrifluoroacetone (tta). The typical Eu3+ emission spectra for solids containing tta revealed the characteristic Eu3+ transitions from fundamental 5D0 state to excited 7FJ (J = 0, 1, 2, 3 and 4) states. The lifetime measurements also confirmed that water molecules were exchanged, increasing emission efficiency. The time-resolved spectra allowed to remove the matrix and ligand emissions and thus to evaluate their influence on the emission of the lanthanide ion. After incorporation of Eu3+, the thermal stability of the solids improved. The intercalated europium complexes did not show the same ligand/Eu3+ molar ratio as the free complexes, due to the restricted mobility of the ligands grafted within the interlayer spaces of kaolinite. On the contrary, these complexes showed high internal quantum yields, low luminescence suppression (after tta coordination), preserved the kaolinite layered structure. They also showed a low water molecules coordination number, which made them very attractive for luminescent applications, in addition to bringing high added value, due to a low cost, non-toxicity, non-polluting and high efficiency.