Preparation, characterization and luminescence properties of core–shell ternary terbium composites SiO2(600)@Tb(MABA-Si)•L

Two novel core–shell structure ternary terbium composites SiO2(600)@Tb(MABA-Si)·L(L:dipy/phen) nanometre luminescence materials were prepared by ternary terbium complexes Tb(MABA-Si)·L2·(ClO4)3·2H2O shell grafted onto the surface of SiO2 microspheres. And corresponding ternary terbium complexes were synthesized using (CONH(CH2)3Si(OCH2CH3)3)2 (denoted as MABA-Si) as first ligand and L as second ligand coordinated with terbium perchlorate. The as-synthesized products were characterized by means of IR spectra, 1HNMR, element analysis, molar conductivity, SEM and TEM. It was found that the first ligand MABA-Si of terbium ternary complex hydrolysed to generate the Si–OH and the Si–OH condensate with the Si–OH on the surface of SiO2 microspheres; then ligand MABA-Si grafted onto the surface of SiO2 microspheres. The diameter of SiO2 core of SiO2(600)@Tb(MABA-Si)·L was approximately 600 nm. Interestingly, the luminescence properties demonstrate that the two core–shell structure ternary terbium composites SiO2(600)Tb(MABA-Si)·L(dipy/phen) exhibit strong emission intensities, which are 2.49 and 3.35 times higher than that of the corresponding complexes Tb(MABA-Si)·L2·(ClO4)3·2H2O, respectively. Luminescence decay curves show that core–shell structure ternary terbium composites have longer lifetime. Excellent luminescence properties enable the core–shell materials to have potential applications in medicine, industry, luminescent fibres and various biomaterials fields.

Two novel core-shell structure ternary terbium composites SiO 2(600) @Tb(MABA-Si)·L(L:dipy/phen) nanometre luminescence materials were prepared by ternary terbium complexes Tb(MABA-Si)·L 2 ·(ClO 4 ) 3 ·2H 2 O shell grafted onto the surface of SiO 2 microspheres. And corresponding ternary terbium complexes were synthesized using (CONH(CH 2 ) 3 Si(OCH 2 CH 3 ) 3 ) 2 (denoted as MABA-Si) as first ligand and L as second ligand coordinated with terbium perchlorate. The as-synthesized products were characterized by means of IR spectra, 1 HNMR, element analysis, molar conductivity, SEM and TEM. It was found that the first ligand MABA-Si of terbium ternary complex hydrolysed to generate the Si-OH and the Si-OH condensate with the Si-OH on the surface of SiO 2 microspheres; then ligand MABA-Si grafted onto the surface of SiO 2 microspheres. The diameter of SiO 2 core of SiO 2(600) @Tb(MABA-Si)·L was approximately 600 nm. Interestingly, the luminescence properties demonstrate that the two core-shell structure ternary terbium composites SiO 2(600) Tb(MABA-Si)·L(dipy/phen) exhibit strong emission intensities, which are 2.49 and 3.35 times higher than that of the corresponding complexes Tb(MABA-Si)·L 2 ·(ClO 4 ) 3 ·2H 2 O, respectively. Luminescence decay curves show that core-shell 2018 The Authors. Published by the Royal Society under the terms of the Creative Commons Attribution License http://creativecommons.org/licenses/by/4.0/, which permits unrestricted use, provided the original author and source are credited.

Introduction
There has been extensive arousing of interest in core-shell structure nanomaterials because of the interesting properties that can be employed in optical, electrical, magnetic and biological applications [1][2][3][4][5][6][7]. It can be found that a nanometre material was covered with another nanometre material by chemical bond or another affinity. The functionality of their properties is a critical factor that promotes the development of nanocomposites materials [8][9][10][11]. In particular, luminescence applications of rare earth core-shell nanomaterials are very attractive because of their superior luminescence intensity. Nowadays, they are used or are being tested for use in such fields as medicine, industry, luminescent fibres and various biomaterials [12][13][14][15][16][17]. Now, such rare earth core-shell nanomaterials have attracted considerable attention due to possessing high photostability and thermal stability that they have potential applications in luminescent areas [18][19][20]. Rare earth core-shell nanometre composite was a research subject that would be a good choice for improving the luminescence intensity and lifetime.
In the rare earth core-shell nanomaterials field, SiO 2 as the core was popular in recent years. SiO 2 microspheres are regarded as ideal core materials with several advantages [21]. First, SiO 2 possesses strong physical stability, which can fix the organic functional groups. Furthermore, SiO 2 can be obtained at room temperature because of convenient reaction conditions. SiO 2 is considered an ideal low-cost material that has previously been used for various core microsphere. SiO 2 core-shell nanomaterials can be obtained by covalent bonds. Good results have been obtained with functionalized siloxanes, because they form strong covalent bonds with most SiO 2 surfaces due to the presence of hydroxyl groups. The large number of commercially available trialkoxysilanes with various functional groups offers unique possibilities for the task-specific surface modification of SiO 2 microsphere. For example, MABA-Si can act as a 'bridge molecule' that connects with rare earth and SiO 2 to enhance physical stability and decrease the energy loss. In this way, core-shell structure materials connected by covalent bond are stable so that the covalent bond is difficult to break. As a result, these kinds of core-shell structure nanometre composites have high luminescence intensity. Therefore, these materials have become an active field of research due to their physical and chemical properties, as well as their potential application. So far, silica has been investigated in the field of DNA, fluorescent probes and sensing [22,23].
In our work, the synthesis and structure as well as luminescence properties of the SiO 2(600) @Tb(MABA -Si)·L composites that have SiO 2 as the core and ternary terbium complex as the shell was studied. Functionalized organosilane (denoted as MABA-Si) not only graft onto the SiO 2 surface by covalently bonding but also coordinates with rare earth ions (Tb 3+ ). To improve the luminescent performance of rare earth ions (Tb 3+ ), another kind of small-molecule ligand with a conjugate system (phenanthroline (phen) or dipyridine (dipy)) was introduced, not only meeting the rare earth ions (Tb 3+ ) coordination number but also having more efficient energy absorption and energy transfer. Terbium core-shell composites possess high luminescence intensity and long lifetime. The study of core-shell structure terbium luminescent nanomaterial is more meaningful to dispose of the potential application.

Physical measurements
Elemental analysis was taken with a HANAU analyser. Infrared spectra (IR, υ = 4000-400 cm −1 ) was obtained by a Nicolet NEXUS-670 FT-IR spectrophotometer, which was determined by the KBr pellet technique. Luminescence excitation and emission spectra were performed on FLS980 spectrophotometer at room temperature (slit width was 0.5 nm). Luminescence lifetime measurements were recorded by FLS980 Combined Steady State and Lifetime Spectrometer (slit width was 0.5 nm). Scanning electron microscope (SEM) images were recorded with a Hitachi S-4800. Transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy were performed on a FEI Tecnai F20 operated at 200 kV. Conductivity measurement was made by using 1 × 10 3 mol l −1 solution in dimethylformamide (DMF) on a DDS-11D conductivity metre at room temperature. The terbium content of the complex was measured by EDTA titration using xylenol-orange as an indicator.

Synthesis of the ternary terbium complexes 2.3.1. Synthesis of silylated m-aminobenzoic acid (MABA-Si)
The synthesis scheme of MABA-Si is shown in figure 1. m-Aminobenzoic acid (2 mmol) dissolved in 40 ml chloroform, 4 mmol 3-(triethoxysilyl)-propyl isocyanate was added dropwise into above solution with stirring at 60°C for 12 h. The obtained white precipitate was isolated by centrifugation, washed with water and ethanol and dried in an oven overnight at 60°C [24,25]. Yield: 40%. The resultant sample was characterized by 1 HNMR and elemental analysis.

Synthesis of SiO 2(600) @MABA-Si
To make MABA-Si graft onto SiO 2 microspheres by the Si-O-Si bond, the SiO 2 microspheres were activated with ammonium hydroxide. SiO 2 (0.1 g) was dissoved in 10 ml water and 10 ml ethanol mixture solution. A certain amount of ammonium hydroxide was added into the above solution to adjust the pH to 9.2 with stirring for 12 h. The precipitate was washed with water and anhydrous ethanol three times. The obtained activated SiO 2 microspheres and 0.2 g ligand MABA-Si were redissolved in 20 ml ethanol solution. The mixture solution was dispersed for 10 min by ultrasonication, then 10 ml water was added drop by drop. The above mixture solution was stirred for 10 h. The obtained precipitate was separated by centrifugation, washed with water and ethanol and dried in an oven at 60°C. As a result, MABA-Si was grafted onto SiO 2 core by formation of Si-O-Si bond, SiO 2(600) @MABA-Si was obtained.

Results and discussion
3.1. IR spectra 3.1.1. IR spectra of ternary terbium complexes    to 1521 cm −1 , δ C-H red-shifted to 717 cm −1 and 847 cm −1 , respectively. It displayed that the Tb 3+ ions coordinated with double nitrogen atoms of phen [35].
In addition, the characteristic absorption peaks of perchlorate group can be found in the ternary terbium complex (figure 3c).The characteristic absorption peaks of perchlorate groups appeared around at 1148 cm −1 , 1119 cm −1 , 1085 cm −1 , 622 cm −1 , which indicated that perchlorate was involved in the coordination. One perchlorate group bonded with the Tb 3+ ion, which corresponded with the results of the molar conductivity. Based on the literature, the vibration of perchlorate group just appeared at 1090 and 623 cm −1 , which demonstrated that perchlorate group is responsible for Td symmetry, and perchlorate group did not coordinate with Tb 3+ ions. When perchlorate coordinated with Tb 3+ ions, the perchlorate appeared at approximately 1145 cm −1 , 1115 cm −1 , 1079 cm −1 , 925 cm −1 and 627 cm −1 ; it was assigned to the C 2v symmetry [36,37]. It showed that three perchlorates of ternary terbium complexes were not completely Td symmetry. Some of them were C 2v symmetry which indicated that perchlorate was involved in the coordination.
3.1.2. IR spectra of core-shell structure ternary terbium composites SiO 2(600) @Tb(MABA-Si)·L   at 1102 cm −1 , and Si-OH was identified at 955 cm −1 (figure 4a). Thus, the Si-OH groups were predominantly adsorbed on the surfaces of SiO 2 . Figure 4b shows the IR spectrum of SiO 2(600) @MABA-Si. The absorption peak located at 1695 cm −1 was attributed to stretching vibration of -COOH. The characteristic peaks located at 1656 cm −1 and 1560 cm −1 belonged to stretching vibration of -CONHgroup of SiO 2(600) @(MABA-Si). The broad bands located at approximately 1100 cm −1 and 801 cm −1 were assigned to the asymmetric stretching vibration and symmetric stretching vibration of Si-O-Si band, respectively. Furthermore, the peak of Si-O-Si intensity is greatly enhanced, which should result from the MABA-Si grafting onto the SiO 2 . It displays that the first ligand MABA-Si generates the Si-OH, and the Si-OH condensate with the Si-OH on the surface of SiO 2 microspheres. In the IR spectrum of SiO 2(600) @Tb(MABA-Si)·phen (figure 4c), the characteristic absorption peak of carboxylic group −COOH appeared at 1656 cm −1 . Furthermore, the -CONH-characteristic bands appeared at 1556 cm −1 , indicating that carbonyl group and amide group were coordinated with Tb 3+ ions. The peak that appeared at 1522 cm −1 was ascribed to stretching vibration of nitrogen atoms of phen in SiO 2(600) @Tb(MABA-Si)·phen. It showed an obvious red shift compared with free phen (figure 3a), which proved that phen successfully coordinated with the Tb 3+ ions. The IR spectrum of MABA-Si in SiO 2(600) @Tb(MABA-Si)·dipy (figure 4d) is similar to that in SiO 2(600) @Tb(MABA-Si)·phen. The characteristic peaks of dipy that appeared at 1522 cm −1 and 1453 cm −1 showed that dipy successfully coordinated with the Tb 3+ ions. IR spectra showed that the ternary terbium complexes formed on the surface of SiO 2 microspheres. The core-shell ternary terbium composites SiO 2(600) @Tb(MABA-Si)·L were synthesized.

The TEM and SEM of core-shell structure ternary terbium composites SiO 2(600) @Tb(MABA-Si)·L
Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to monitor the fabrication of the as-synthesized products. Figure 5 shows the SEM (a), TEM of the SiO 2 (b) and TEM of the SiO 2(600) @MABA-Si (c, d). From figure 5a and b, we can observe that the as-formed SiO 2 core has a smooth surface, and the diameter is approximately 600 nm. The typical TEM images of SiO 2(600) @MABA-Si are shown in figure 5c,d, which reveals that SiO 2(600) @(MABA-Si) has a core-shell structure and presents a uniform spherical morphology with thin layer. The images (figure 6a-d) of SiO 2(600) @Tb(MABA-Si)·phen and SiO 2(600) @Tb(MABA-Si)·dipy show that the surface of SiO 2(600) @Tb(MABA-Si)·L becomes much rougher and has an obvious layer, which might be caused by the ternary terbium complexes grafted onto the surface of SiO 2 core. Figure 6b
To further discuss the luminescence properties of the resulting core-shell structure ternary terbium composites and corresponding complexes, the typical decay curves were measured. The resulting lifetime data of core-shell structure ternary terbium composites are given in table 4    corresponding complex. It suggested that the rare earth complexes and core materials were connected by covalent bonds enhanced the luminescent stability.

Conclusion
In this paper, two novel core-shell structure ternary terbium composites luminescent materials SiO 2(600) @Tb(MABA-Si)·L were prepared by grafting the Tb(MABA-Si)·L 2 ·(ClO 4 ) 3 ·2H 2 O complexes onto the surfaces of SiO 2 core. In the reaction system, MABA-Si of Tb(MABA-Si)·L 2 ·(ClO 4 ) 3 ·2H 2 O and SiO 2 core formed Si-O-Si band by means of a molecule bridge that derived from the silylated hydrolysis and condensation. The luminescence properties indicate that core-shell structure ternary terbium composites SiO 2(600) @Tb(MABA-Si)·L have stronger emission intensity than the corresponding complexes. And the core-shell structure ternary terbium composites have longer lifetimes. The formation of core-shell structure can increase the luminescence properties and reduce the amount of rare earth. This study provided new guidance in the design and fabrication of innovative rare earth materials. The synthesized novel core-shell structure terbium ternary composites have new opportunities to develop the application of rare earth element terbium.