Fluorescence enhancement of europium nitrobenzoates by Ag@SiO2 nanoparticles in solution

Europium complexes with o-nitrobenzoic acid (o-NBA), m-nitrobenzoic acid (m-NBA) and p-nitrobenzoic acid (p-NBA) were synthesized. Elemental analysis, IR spectroscopy and UV-vis absorption spectroscopy show that the compositions of the complexes are Eu(o-NBA)3·4H2O, Eu(m-NBA)3·4H2O and Eu(p-NBA)3·4H2O, respectively, and the europium ions coordinate with the carboxyl oxygen atoms of nitro-benzoates. Ag@SiO2 nanoparticles with different core size (32 nm, 49 nm and 72 nm) and varied silica shell thickness (5 nm, 15 nm, 19 nm, 25 nm and 53 nm) were prepared. The influence of these nanoparticles with different addition amounts (50 μL, 100 μL, 150 μL, 200 μL and 250 μL) on the luminescence properties of the europium complexes in solution were investigated, respectively. The results show that the luminescence intensities of the europium complexes are enhanced remarkably after the introduction of Ag@SiO2 nanoparticles. The fluorescence enhancement factors depend on the addition amount, shell thickness and core size of the Ag@SiO2 nanoparticles. Moreover, the fluorescence enhancement factors vary for the europium complexes with different ligands. The enhancement factors for the complex Eu(o-NBA)3·4H2O can reach as high as 3.1. The investigations indicate that the enhancement effect on the europium complexes mainly results from the surface plasmon resonance of the Ag@SiO2 nanoparticles. Besides, the introduction of Ag@SiO2 nanoparticles is also able to elongate the fluorescence lifetimes and increase the quantum efficiencies of the europium complexes.