Synthesis and Characterization of Photoluminescent Eu(III) Coordination Halloysite Nanotube-Based Nanohybrids

Multicarboxyl polymer-functionalized halloysite nanotubes (HNTs) were synthesized by controlled surface-initiated atom transfer radical polymerization (SI-ATRP) of methyl methacrylate (MMA) and/or hydroxyethyl methacrylate (HEMA) to form HNT-(PMMA-b-PHEMA) or HNT-PHEMA hybrids, followed by esterification of the hydroxyl groups on the PHEMA blocks using excessive succinic anhydride in pyridine. The obtained products, HNT-(PMMA-b-PSEMA) or HNT-PSEMA with multicarboxyl groups on the external layers, were coordinated with triplet europium ions (Eu3+) in the presence of 1,10-phenanthroline (phen). For comparison purposes, PSEMA and PMMA-b-PSEMA copolymer were also synthesized via the same procedure and used as macromolecular ligands for coordination with Eu3+. The microstructure and fluorescence properties of the four kinds of Eu3+ coordination complexes were characterized via FTIR, FESEM, EDS, 1H NMR, TGA, and fluorescence spectroscopy. The conversion of hydroxyl groups to carboxyl groups during the esterification reactions was up to 100% for the synthesized polymers and modified HNT-based hybrids. The Eu3+ coordination HNT-based hybrids exhibited efficient narrow bandwidth emission of red light with high spectral purity when excited at 266 nm. The HNT-(PMMA-b-PSEMA)-Eu complex even gave 1.63 and 1.85 times higher emission intensities than those of HNT-PSEMA-Eu and (PMMA-b-PSEMA)-Eu complexes, respectively. The improved luminescence properties of both of the Eu3+ coordination HNT-based hybrids are attributable to the efficient intramolecular energy transfer from the hybrid ligands and phen to Eu3+ ions, and the rigid HNTs framework also plays a positive role in the enhancement of the emission intensity. Such photoluminescent Eu3+ coordination HNT-based hybrids are expected for spectroscopy probes, fluorescent plastics, and fluoroimmunoassay applications.