Synthesis , and Characteristic Luminescence of Core-Shell SiO 2 @ Eu ( MABA-Si ) · ( phen ) Microspheres

Monodispersed SiO2@Eu(MABA-Si)·(phen) core-shell microspheres were synthesized by silane coupling agent method. The core-shell microspheres was used the europium complex Eu(MABA-Si)·(phen)2·(ClO4)3·2H2O as shell and SiO2 as the inorganic core. The europium complex shell has grafted to silica surface through forming a Si-O-Si bond. The europium complex was synthesized by HOOCC6H4N(CONH(CH2)3Si(OCH2CH3)3)2 (denoted as MABA-Si) and phen coordinated europium perchlorate. The europium complex has been characterized by element analysis, molar conductivity, 1HNMR and IR. The TEM and SEM showed that the diameter of SiO2 core was about 400 nm and the thickness of the europium complex shell was about 20 nm. Fluorescence spectra illustrated that the core-shell microspheres have stronger fluorescence intensity than the europium complex which was 1.87 times.


Introduction
Core-shell nanometer microspheres are a kind of nanomaterial with a core-shell structure that can be widely applied in making functional fluorescent materials, magnetic materials, electronics, medicine, and so on.By coating a layer of organic alkoxysilane complexes, the luminescence properties of core-shell materials can be changed including luminescence intensity, fluorescence quantum efficiency, stability and fluorescence lifetime [1][2][3].
Silica probably is the most popular inorganic material because of its good mono-disperse, flexibility of controlling the particle size, low cost, low toxicity, and more mechanical stability.Owing to the external hydrogen bonds of its surface, SiO2 is easy to bond with different materials through chemical bonds or electrostatic interactions [4][5][6].At present, silane coupling agent is used to prepare the coating structure nanomaterials that was generally described as Y(CH2)nSiX3.Y group is the organic functional group that can combine with the rare earth ions.X represents the hydroxyl groups which combined with Si-OH of the surface of SiO2.As a result, RE 3+ and SiO2 can be incorporated through silane coupling agent which plays a role of the "molecular bridge".
In this article, we have controlled prepared ternary europium complex Eu(MABA-Si)•(phen)2•(ClO4)3•2H2O.The MABA-Si is a kind of silane coupling agent, which can be easily coordinated with Eu 3+ ions through the carboxyl oxygen atoms of MABA-Si groups, and the second ligand phen also can be coordinated with Eu 3+ .Furthermore, the monodispersed SiO2@Eu(MABA-Si)•(phen) core-shell microspheres synthesized by the europium complex grafting to silica surface through forming a Si-O-Si bond.The luminescent properties of europium complex and core-shell microspheres SiO2@Eu(MABA-Si)•(phen) was discussed in detail.

Physical measurements.
Elemental analysis was carried on PF-2400 Elemental Analyzer.Conductivity was measured by DDS-11D Conductivity Meter.The europium ion content was measured by EDTA titration, Xylenol-Orange as indicator.The IR was determined by Nicolet NEXUS-670 FT-IR spectrophotometer.The 1 HNMR spectra was determined in the DMSO-d6 by Bruker AC-500 nuclear magnetic resonance spectrophotometer.The morphology of the products was observed by Scanning Electronic Microscope (Hitachi S-4800) and Transmission Electronic Microscope (FEI Tecnai F20).Fluorescence properties were determined via FLS980 (slit width: 1 nm).Fluorescent decay curves were recorded by FLS980 Combined Steady State and Lifetime Spectrometer.

Preparation of the europium ternary complex.
The ligand MABA-Si was synthesized according to the method of the literature [7]. 1 mmol MABA-Si and phen were added into 1 mmol Eu(ClO4)3•nH2O, anhydrous ethanol solution drop by drop under the magnetic stirring.White precipitate was obtained by washing for several times with anhydrous ethanol and distilled water.

IR spectra analysis of core-shell microspheres
Fig. 2 displays the IR spectra of SiO2, MABA-Si, phen, and SiO2@Eu(MABA-Si)•(phen).The absorption band Si-O-Si was appeared at 1096 and 800 cm -1 (Fig. 2a and 2d), which confirmed that the europium complex shell has grafted to silica surface through forming a Si-O-Si bond.In Fig. 2b, 2c and 2d, it was similar to above complex that the stretching frequency red shifted to 1656, 1619, 1553 cm -1 (MABA-Si) and 1568 cm -1 (phen).Compared with 1700, 1639 and 1558 cm -1 (MABA-Si) and 1587 cm -1 (phen).The above information indicated that europium ion coordinated with through C=O oxygen atom of MABA-Si and double nitrogen of phen.

The SEM and TEM analysis of core-shell microspheres.
The morphology was revealed by SEM and TEM images (Fig. 3a and 3b).The d3iameter of SiO2 core was about 400 nm.The TEM images of the core-shell SiO2@Eu(MABA-Si)•(phen) microspheres were presented in Fig. 3c and 3d, the thickness of coating layer was about 20 nm.The core-shell microspheres showed rougher surface than pure SiO2 particles.In addition, characteristic peaks of europium was found in the EDX spectrum of core-shell microspheres, which confirmed that Eu(MABA-Si)•(phen) exist in the cladding layer of core-shell microspheres (Fig. 4).The emission spectra of the europium complex (Fig. 6b) and core-shell microsopheres SiO2@Eu(MABA-Si)•(phen) (Fig. 6a) were recorded under excitation wavelength at λex=272 nm that exhibited the characteristic transitions 5 D0→ 7 FJ (J=0-4) of Eu 3+ , and the peaks are appeared at 579, 593, 612, 652 and 699 nm respectively.The strongest emission peak appears at 612 nm, which was assigned to 5 D0→ 7 F2 transition of Eu 3+ .The emission intensity of the core-shell microspheres (36282456 a.u.) stronger than the europium complex (19425658 a.u).The fluorescent lifetime value of the europium complex and core-shell microspheres (Fig. 5a) were calculated by the double exponential mode.The core-shell microspheres lifetime was 1009.67 μs which langer than 389.55 μs of the europium complex (Fig. 5b).

Conclusions
The novel europium complex of Eu(MABA-Si)(phen)2•(ClO4)3•2H2O and core-shell SiO2@Eu(MABA-Si)(phen) microspheres were prepared.The core-shell microspheres was used the europium complex as shell and SiO2 as the inorganic core.The europium complex shell has grafted to silica surface through forming a Si-O-Si bond.The emission intensity of core-shell microspheres was enhanced 1.87 times as great as the europium complex.And the core-shell microspheres have longer lifetime.The core-shell microspheres have a potential application as red luminescence nanomaterials, which will expand their applications in optical or electronic field.