Abstract
Metal-enhanced luminescence of lanthanide complexes by noble metal nanoparticles has attracted much attention because of its high efficiency in improving the luminescent properties of lanthanide ions. Herein, nine kinds of europium and terbium complexes—RE(TPTZ)(ampca)3·3H2O, RE(TPTZ)(BA)3·3H2O, RE(phen)(ampca)3·3H2O, RE(phen)(PTA)1.5·3H2O (RE = Eu, Tb) and Eu(phen)(BA)3·3H2O (TPTZ = 2,4,6-tri(2-pyridyl)-s-triazine, ampca = 3-aminopyrazine-2-carboxylic acid, BA = benzoic acid, phen = 1,10-phenanthroline, PTA = phthalic acid)—have been synthesized. Meanwhile, seven kinds of core-shell Ag@SiO2 nanoparticles of two different core sizes (80–100 nm and 40–60 nm) and varied shell thicknesses (5, 12, 20, 30 and 40 nm) have been prepared. The combination of these nine types of lanthanide complexes and seven kinds of Ag@SiO2 nanoparticles provides an opportunity for a thorough investigation of the metal-enhanced luminescence effect. Luminescence spectra analysis showed that the luminescence enhancement factor not only depends on the size of the Ag@SiO2 nanoparticles, but also strongly relates to the composition of the lanthanide complexes. Terbium complexes typically possess higher enhancement factors than their corresponding europium complexes with the same ligands, which may result from better spectral overlap between the emission bands of Tb complexes and surface plasmon resonance (SPR) absorption bands of Ag@SiO2. For the complexes with the same lanthanide ion but varied ligands, the complexes with high enhancement factors are typically those with excitation wavelengths located nearby the SPR absorption bands of Ag@SiO2 nanoparticles. These findings suggest a combinatorial chemistry strategy is necessary to obtain an optimal metal-enhanced luminescence effect for lanthanide complexes.
Highlights
Luminescent lanthanide compounds have been widely used in various fields, such as fluorescence materials [1,2,3], electroluminescence devices [4,5], fluorescence probes and labels in biological systems [6,7,8,9], because they have the advantages of narrow emission bands, long fluorescence lifetimes and large Stokes shifts [10,11,12,13,14]
The other series of core-shell Ag@SiO2 nanoparticles with core sizes of 80–100 nm and shell thicknesses of 5, 12, 20 and 30 nm were prepared by varying the amounts of tetraethyl orthosilicate (TEOS) added, by 5, 7, 10 and 15 mL, respectively
For the complex Eu(phen)(PTA)1.5 ·3H2 O, the stretching vibration peak of the carbonyl group in PTA at 1689 cm−1 disappears after coordinated with the europium ions and new bands at 1610 cm−1 and 1518 cm−1 appeared in the complex, which can be ascribed to the anti-symmetric and symmetric stretching vibration of carboxyl group, indicating that the europium ions have coordinated with PTA [48]
Summary
Luminescent lanthanide compounds have been widely used in various fields, such as fluorescence materials [1,2,3], electroluminescence devices [4,5], fluorescence probes and labels in biological systems [6,7,8,9], because they have the advantages of narrow emission bands, long fluorescence lifetimes and large Stokes shifts [10,11,12,13,14]. Because of the parity rule, the f–f transition of lanthanide ions is forbidden, which leads to their weak luminescence intensities [1,2] To enhance their luminescence intensities, one traditional strategy is to coordinate with various organic ligands. The luminescent properties of the lanthanide complexes depend strongly on their compositions, i.e., both the kind of lanthanide ions and the coordinated ligands. We synthesize nine kinds of Eu and Tb ternary complexes with both anion and neutral ligands, and prepare seven types of core-shell Ag@SiO2 nanoparticles of distinct size. The mechanism investigation indicates that the excitation enhancement and emission enhancement may lead to distinct enhancement factors for complexes with different lanthanide ions and organic ligands
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