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.

Introduction

There has been extensive aroun class="Chemical">sing of interest in core–shell structure nanomaterials because of the interesting properties that can be employed in optical, electrical, magnetic and biological applications [1-7]. 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 [8-11]. 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 [12-17]. 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 [18-20]. 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, SiO2 as the core was popular in recent years. n class="Chemical">SiO2 microspheres are regarded as ideal core materials with several advantages [21]. First, SiO2 possesses strong physical stability, which can fix the organic functional groups. Furthermore, SiO2 can be obtained at room temperature because of convenient reaction conditions. SiO2 is considered an ideal low-cost material that has previously been used for various core microsphere. SiO2 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 SiO2 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 SiO2 microsphere. For example, MABA-Si can act as a ‘bridge molecule’ that connects with rare earth and SiO2 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 SiO2(600)@Tb(MABA-Si)·L composites that have SiO2 as the core and ternary terbium complex as the shell was studied. Functionalized organosilane (denoted as MABA-Si) not only graft onto the SiO2 surface by covalently bonding but also coordinates with rare earth ions (Tb3+). To improve the luminescent performance of rare earth ions (Tb3+), another kind of small-molecule ligand with a conjugate system (phenanthroline (phen) or dipyridine (dipy)) was introduced, not only meeting the rare earth ions (Tb3+) 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.

Material and methods

Chemicals

The starting materials for the preparation of SiO2 were tetraethoxysilane (TEOS), ammonium hydroxide, water and anhydrous ethanol. Tb4O7 (99.999%) was dissolved in perchloric acid to prepare Tb(ClO4)3·nH2O. 3-(triethoxysilyl)-propyl isocyanate (TEPIC, 96%, Aldrich), pure phen, dipy, m-aminobenzoic acid were also used. All other chemical reagents were analytical grade.

Physical measurements

Elemental analysis was taken with a HAn class="Chemical">NAU 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 × 103 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

Synthesis of silylated m-aminobenzoic acid (MABA-Si)

The synthesis scheme of n class="Chemical">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 1HNMR and elemental analysis. 1HNMR: δ0.56 ppm(4H), δ1.04–1.51 ppm(18H), δ2.93 ppm(4H), δ3.33–3.47 ppm(12H), δ3.73 ppm(4H), δ7.43–7.57 ppm(2H) and δ12.4 ppm(1H). Anal. Calcd. of C28H49N3O10Si2 (M = 631 g mol−1): C, 51.35%; H, 7.76%; N, 6.66%; found: C, 50.80%; H, 7.82%; N, 6.55%. m.p.: 46–48°C.

Figure 1.

The synthesis scheme of MABA-Si.

Synthesis of ternary terbium complexes of Tb(MABA-Si)·L2·(ClO4)3·2H2O

MABA-Si (1 mmol) and 2 mmol n class="Chemical">phen (or dipy) were dissolved in 10 ml anhydrous ethanol, and then 1 mmol Tb(ClO4)3·nH2O was added in the above solution with stirring at 60°C for 2 h, the Tb3+: MABA-Si: L molar ratio of 1 : 1 : 2. The white precipitate was isolated by centrifugation and washed with anhydrous ethanol. The elemental analysis and molar conductivities of the products were measured (table 1). Anal. Calcd. of Tb(MABA-Si)·dipy2·(ClO4)3·2H2O (M = 1509.26 g mol−1): C, 38.33; H, 6.44; N, 4.79; Tb, 10.21; found: C, 37.40; H, 6.49; N, 4.57; Tb, 10.53; and Anal. Calcd. of Tb(MABA-Si)·phen2·(ClO4)3·H2O (M = 1557.7 g mol−1): C, 40.02; H, 6.37; N, 4.28; Tb, 9.85, found: C, 39.90; H, 6.29; N, 4.43; Tb, 10.02. The corresponding molar conductivities were 148 S cm2 mol−1 and 160 S cm2 mol−1. The ternary terbium complexes formulated 1 : 2 electrolytes [26].

Table 1.

Composition (%) and molar conductivities (S cm2 mol−1) of ternary terbium complexes. Calculated value in brackets.

ternary terbium complexes	M(g mol−1)	C	N	H	RE	λm
Tb(MABA-Si)·dipy2·(ClO4)3·2H2O	1509.26	38.33 (37.40)	6.44 (6.49)	4.79 (4.57)	10.21 (10.53)	148
Tb(MABA-Si)·phen2·(ClO4)3·2H2O	1557.78	40.02 (39.90)	6.37 (6.29)	4.28 (4.43)	9.85 (10.02)	160

Preparations of core–shell structure ternary terbium composites SiO2(600)@Tb(MABA-Si)·L

Synthesis of SiO2 monodisperse

Spherical SiO2(600) was prepared by outstanding Stober method [27]. A total of 50 ml anhydrous ethanol, 2.6 ml ammonium hydroxide, 5 ml tetraethyl orthosilicate and 15 ml water were mixed with stirring for 6 h at room temperature (table 2). A white silica colloidal suspension was formed. The solid product was washed thoroughly with water and anhydrous ethanol [28].

Table 2.

The volume of materials to prepare silica and reaction time.

EtOH (ml)	H2O (ml)	TEOS	NH3H2O (ml)	time (h)	size (nm)
50	15	5.0	2.6	5	600

Synthesis of SiO2(600)@MABA-Si

To make MABA-Si graft onto SiO2 microspheres by the Si–O–Si bond, the SiO2 microspheres were activated with ammonium hydroxide. SiO2 (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 SiO2 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 SiO2 core by formation of Si–O–Si bond, SiO2(600)@MABA-Si was obtained.

Synthesis SiO2(600)@Tb(MABA-Si)·L

Core–shell structure ternary n class="Chemical">terbium composite SiO2(600)@Tb(MABA-Si)·dipy was prepared as follows: 0.1 g SiO2(600)@MABA-Si and 0.2 g dipy were dissolved in 10 ml anhydrous ethanol, then the 5 ml anhydrous ethanol solution of 0.1 g Tb(ClO4)3·nH2O was added into the above mixture solution. The mixture solution was stirred at room temperature for 12 h, then the white precipitate was obtained. The obtained precipitate was separated by centrifugation, washed with distilled water and ethanol and dried in an oven at 60°C. The core–shell structure ternary terbium composite of SiO2(600)@Tb(MABA-Si)·dipy was synthesized successfully. The synthesis procedure for SiO2(600)@Tb(MABA-Si)·phen was similar to that of SiO2(600)@Tb(MABA-Si)·dipy except dipy was replaced by phen.

Results and discussion

IR spectra

IR spectra of ternary terbium complexes

Figure 2a–c shows the IR spectra of MABA-Si, n class="Chemical">dipy and Tb(MABA-Si)·dipy2·(ClO4)3·2H2O. In the spectrum of MABA-Si, the characteristic peaks located at 1639 cm−1 (νC=O) and 1558 cm−1 (δNH) were attributed to stretching vibration and bending vibration of –CONH– (figure 2a). The characteristic absorption of amide group (–CONH–) suggested that MABA-Si has been successfully synthesized by the amidation reaction with MABA and 3-(triethoxysilyl)-propyl isocyanate. The characteristic peaks located at 1700 and 1417 cm−1 belonged to stretching vibration and bending vibration of –COOH. Figure 2b shows IR spectrum of dipy. The stretching vibration of C = N appeared at 1578 and 1455 cm−1. Futhermore, in the IR spectrum of ternary terbium complex Tb(MABA-Si)·dipy2·(ClO4)3·2H2O (figure 2c), the stretching vibration and bending vibration of the –COOH were red-shifted to 1689 and 1400 cm−1. In addition, the stretching vibration and bending vibration of –CONH– groups were shifted to 1618 cm−1 and 1557 cm−1, respectively. It indicated that MABA-Si coordinated with the Tb3+ ions by carboxylic group and amide groups [29-33]. The stretching vibration of C = N shifted low wavenumber, which appeared at 1557 and 1439 cm−1 in the ternary terbium complex Tb(MABA-Si)·dipy2·(ClO4)3·2H2O. The stretching vibration of C = N had obvious red shift, which showed that Tb3+ ions coordinated with two nitrogen atoms of dipy [34].

Figure 2.

IR spectra of the MABA-Si (a), dipy (b) and Tb(MABA-Si)·dipy2·(ClO4)3·2H2O (c).

The IR spectra of MABA-Si, n class="Chemical">phen, Tb(MABA-Si)·phen2·(ClO4)3·2H2O are shown in figure 3a–c. Comparing the spectrum of Tb(MABA-Si)·phen2·(ClO4)3·2H2O with MABA-Si, the characteristic absorption peaks of carboxylic group appeared at 1689 cm−1, indicating that the carbonyl group of MABA-Si was coordinated with Tb3+ ions. The stretching vibration C = N group located at 1587 cm−1, bending vibration of C–H located at 735 and 851 cm−1 in the IR spectrum of phen (figure 3b). In the IR spectrum of ternary terbium complex Tb(MABA-Si)·phen2·(ClO4)3·2H2O (figure 3c), νC=N red-shifted to 1521 cm−1, δC–H red-shifted to 717 cm−1 and 847 cm−1, respectively. It displayed that the Tb3+ ions coordinated with double nitrogen atoms of phen [35].

Figure 3

IR spectra of the MABA-Si (a), phen (b) and Tb(MABA-Si)·phen2·(ClO4)3·2H2O (c).

In addition, the characteristic absorption peaks of perchlorate group can be found in the ternary n class="Chemical">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 Tb3+ 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 Tb3+ ions. When perchlorate coordinated with Tb3+ 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 C2v symmetry [36,37]. It showed that three perchlorates of ternary terbium complexes were not completely Td symmetry. Some of them were C2v symmetry which indicated that perchlorate was involved in the coordination.

IR spectra of core–shell structure ternary terbium composites SiO2(600)@Tb(MABA-Si)·L

Figure 4a–d gives IR spectra of SiO2, n class="Chemical">SiO2(600)@MABA-Si, SiO2(600)@Tb(MABA-Si)·phen and SiO2(600)@Tb(MABSi)·dipy. In the spectrum of SiO2, the characteristic absorption peaks of Si–O–Si located 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 SiO2. Figure 4b shows the IR spectrum of SiO2(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 –CONH– group of SiO2(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 SiO2. 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 SiO2 microspheres. In the IR spectrum of SiO2(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 Tb3+ ions. The peak that appeared at 1522 cm−1 was ascribed to stretching vibration of nitrogen atoms of phen in SiO2(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 Tb3+ ions. The IR spectrum of MABA-Si in SiO2(600)@Tb(MABA-Si)·dipy (figure 4d) is similar to that in SiO2(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 Tb3+ ions. IR spectra showed that the ternary terbium complexes formed on the surface of SiO2 microspheres. The core–shell ternary terbium composites SiO2(600)@Tb(MABA-Si)·L were synthesized.

Figure 4.

IR spectra of SiO2 (a), SiO2(600)@MABA-Si (b), SiO2(600)@Tb(MABA-Si)·phen (c) and SiO2(600)@Tb(MABA-Si)·dipy (d).

The TEM and SEM of core–shell structure ternary terbium composites SiO2(600)@Tb(MABA-Si)·L

Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to monitor the fabrication of the as-synthen class="Chemical">sized products. Figure 5 shows the SEM (a), TEM of the SiO2 (b) and TEM of the SiO2(600)@MABA-Si (c, d). From figure 5a and b, we can observe that the as-formed SiO2 core has a smooth surface, and the diameter is approximately 600 nm. The typical TEM images of SiO2(600)@MABA-Si are shown in figure 5c,d, which reveals that SiO2(600)@(MABA-Si) has a core–shell structure and presents a uniform spherical morphology with thin layer. The images (figure 6a–d) of SiO2(600)@Tb(MABA-Si)·phen and SiO2(600)@Tb(MABA-Si)·dipy show that the surface of SiO2(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 SiO2 core. Figure 6b indicates that the Tb(MABA-Si)·dipy and Tb(MABA-Si)·phen surface layer has a diameter of approximately 5 nm. EDX analysis of SiO2(600)@Tb(MABA-Si)·L (figure 7) confirms the existence of Cl, Tb, N, O and Si, which gives experimental evidence for the existence of the core–shell structure ternary terbium composites. The formation mechanism of core–shell structure composite is inferred (figure 8). In this process, Si–O–Si chemical bond can be formed by hydrolysis-polycondensation method from ethoxy of MABA-Si and hydroxyl groups of the SiO2 surface. Then the carboxyl oxygen atoms of MABA-Si groups can be coordinated with Tb3+ ions. It is quite possible that the ternary terbium complex grafted onto the surface of the SiO2 at the early stage of the reactions, due to the synthesized Si–O–Si band.

Figure 5.

The SEM of SiO2 (a), TEM of SiO2 (b) and SiO2(600)@MABA-Si (c,d).

Figure 6.

The TEM images of core–shell structures SiO2(600)@Tb(MABA-Si)·phen (a,b), SiO2(600)@Tb(MABA-Si)·dipy (c,d).

Figure 7.

The EDX spectrum of core–shell structure ternary terbium composites.

Figure 8.

The formation mechanism of core–shell structures SiO2(600)@Tb(MABA-Si)·L.

Luminescence properties

The luminescence properties of ternary terbium complexes and core–shell structure ternary terbium composites SiO2(600)@Tb(MABA-Si)·L have been investigated. The excitation spectra of Tb(MABA-Si)·dipy2·(ClO4)3·2H2O and SiO2(600)@Tb(MABA-Si)·dipy were measured by monitoring the emission of Tb3+ at 543 nm. As shown in figure 9, a broad excitation band extending from 200 to 400 nm presented the main peak centred at 298 nm. The corresponding emission spectra of the products are shown in figure 10. The emission peaks of the core–shell structure ternary terbium composite SiO2(600)@Tb(MABA-Si)·dipy and corresponding complex were located at 489, 543, 583 and 621 nm, which correspond to the (J = 3–6) transitions of Tb3+ ions. The strongest emission peak located at 543 nm was attributed to transitions of Tb3+ ions. The strongest emission intensity of SiO2(600)@Tb(MABA-Si)·dipy and Tb(MABA-Si)·dipy2·(ClO4)3·2H2O is 10 894 848 and 4 370 973 arb. units, respectively. It suggests that the very effective energy transfer from the ligand to Tb3+ ion in the complexes and core–shell structure ternary terbium composites and corresponding ternary terbium complexes. Two kinds of materials exhibit excellent characteristic green luminescence. It is worth noting that the core–shell structure ternary terbium composite SiO2(600)@Tb(MABA-Si)·dipy shows approximately 2.49 times stronger emission than the corresponding ternary terbium complex (table 3). Figure 11 presents typical excitation spectra of ternary terbium complex Tb(MABA-Si)·phen2·(ClO4)3·2H2O and core–shell structure ternary terbium composite SiO2(600)@Tb(MABA-Si)·phen. The obtained products possessed broad excitation bands with maxima at 306 nm and were recorded by monitoring the emission of Tb3+ ions at 543 nm. The emission spectra of SiO2(600)@Tb(MABA-Si)·phen and corresponding complex are shown in figure 12. SiO2(600)@Tb(MABA-Si)·phen and corresponding complex exhibited characteristic emission peaks of Tb3+ (J = 6, 5, 4, 3) transition at 489 nm (), 543 nm (), 583 nm (), 621 nm (). The strongest emission peak located at 543 nm was attributed to transitions of Tb3+ ions. The strongest emission intensity of Tb(MABA-Si)·dipy2·(ClO4)3·2H2O is 12 029 100 arb. units. It is more remarkable that strongest emission intensity of core–shell structure ternary terbium composite SiO2(600)Tb(MABA-Si)·phen is 40 339 128 arb. units, which is 3.35 times higher than that of the corresponding complex (table 3). The emission intensity of the obtained core–shell structure composites was increased, compared with the corresponding ternary complexes. We contribute this enhancement to the unique core–shell structure, in which the SiO2 cores greatly enhance the physical stability of ternary terbium complexes and decrease the energy loss of ternary terbium complexes molecular vibration. The luminescence emission intensity was increased. SiO2 core and organic ligands played a mutually synergistic part in the energy transfer process of the ligands to Tb3+ ions.

Figure 9.

Excitation spectra of SiO2(600)@Tb(MABA-Si)·dipy (a) and Tb(MABA-Si)·dipy2·(ClO4)3·2H2O (b).

Figure 10.

Emission spectra of SiO2(600)@Tb(MABA-Si)·dipy (a) and Tb(MABA-Si)·dipy2·(ClO4)3·2H2O (b).

Table 3.

Emission spectra data of the complexes and core–shell structure composites.

complexes	slit width (nm)	λEX (nm)	λEM (nm)	I (arb. units)	energy transition	intensity changes
Tb(MABA-Si)·phen2·(ClO4)3·2H2O	0.5	298	543	12 029 100	^5D_4→^7F_5	—
SiO2(600)@Tb(MABA-Si)·phen	0.5	298	543	40 339 128	^5D_4→^7F_5	3.35
Tb(MABA-Si)·dipy2·(ClO4)3·2H2O	0.5	306	543	4 370 973	^5D_4→^7F_5	—
SiO2(600)@Tb(MABA-Si)·dipy	0.5	306	543	10 894 848	^5D_4→^7F_5	2.49

Figure 11.

Excitation spectra of SiO2(600)@Tb(MABA-Si)·phen (a) and Tb(MABA-Si)·phen2·(ClO4)3·2H2O (b).

Figure 12.

Emission spectra of SiO2(600)@Tb(MABA-Si)·phen (a), Tb(MABA-Si)·phen2·(ClO4)3·2H2O (b).

To further discuss the luminescence properties of the resulting core–shell structure ternary n class="Chemical">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. The lifetime of the Tb3+ ions can be expressed by and where I(t) is the luminescence intensity varying with time t, and τ1 and τ2 are lifetime. The lifetimes of ternary terbium complex Tb(MABA-Si)·phen2·(ClO4)3·2H2O and corresponding composite are 0.57714 and 1.08274 ms, respectively. The lifetimes of ternary terbium complex Tb(MABA-Si)·dipy2·(ClO4)3·2H2O and corresponding composite are 1.03614 and 1.26420 ms, respectively. It is found that the core–shell structure ternary terbium composites present longer luminescent lifetimes than corresponding complex. It suggested that the rare earth complexes and core materials were connected by covalent bonds enhanced the luminescent stability.

Table 4.

The lifetime of ternary terbium complexes and corresponding core–shell composites.

terbium complexes and core–shell composites	excited state	lifetime (ms)	X2
Tb(MABA-Si)·phen2·(ClO4)3·2H2O	5D4	0.57714	0.9993
SiO2(600)@Tb(MABA-Si)·phen	5D4	1.08274	0.9987
Tb(MABA-Si)·dipy2·(ClO4)3·2H2O	5D4	1.03614	0.9989
SiO2(600)@Tb(MABA-Si)·dipy	5D4	1.26420	0.9972

Conclusion

In this paper, two novel core–shell structure ternary n class="Chemical">terbium composites luminescent materials SiO2(600)@Tb(MABA-Si)·L were prepared by grafting the Tb(MABA-Si)·L2·(ClO4)3·2H2O complexes onto the surfaces of SiO2 core. In the reaction system, MABA-Si of Tb(MABA-Si)·L2·(ClO4)3·2H2O and SiO2 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 SiO2(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.