Luminescence Studies and Judd–Ofelt Analysis on SiO2@LaPO4:Eu@SiO2 Submicro-spheres with Different Size of Intermediate Shells

The novel submicro-spheres SiO2@LaPO4:Eu@SiO2 with core-shell-shell structures were prepared by connecting the SiO2 submicro-spheres and the rare earth ions through an organosilane HOOCC6H4N(CONH(CH2)3Si(OCH2CH3)3 (MABA-Si). The as-prepared products were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), X-ray photoelectron spectroscopy (XPS), and infrared spectroscopy (IR). It is found that the intermediate shell of the submicro-spheres was composed by LaPO4:Eu nanoparticles with the size of about 4, 5–7, or 15–34 nm. A possible formation mechanism for the SiO2@LaPO4:Eu@SiO2 submicro-spheres has been proposed. The dependence of the photoluminescence intensity on the size of the LaPO4:Eu nanoparticles has been investigated. The intensity ratios of electrical dipole transition 5D0 → 7F2 to magnetic dipole transition 5D0 → 7F1 of Eu3+ ions were increased with decreasing the size of LaPO4:Eu nanoparticles. According to the Judd-Ofelt (J-O) theory, when the size of LaPO4:Eu nanoparticles was about 4, 5–7 and 15–34 nm, the calculated J-O parameter Ω2 (optical transition intensity parameter) was 2.30 × 10−20, 1.80 × 10−20 and 1.20 × 10−20, respectively. The increase of Ω2 indicates that the symmetry of Eu3+ in the LaPO4 lattice was gradually reduced. The photoluminescence intensity of the SiO2@LaPO4:Eu@SiO2 submicro-spheres was unquenched in aqueous solution even after 15 days.

and S3 can be attributed to the monoclinic phase of LaPO 4 (JCPDS No. 84-600), while the broad band at 2θ = 22° might result from the amorphous SiO 2 (Fig. 1b,c). The peaks at 18 The structure and morphology of the S1, S2, and S3 products were also identified by TEM and high-resolution TEM (HRTEM). Figure 2 shows the TEM and HRTEM images of the SiO 2 @LaPO 4 :Eu@SiO 2 submicro-spheres and the particle size distribution of the intermediate shell LaPO 4 :Eu nanoparticles. Figure 2a,f,k are the low-resolution images of the products S1, S2, and S3, indicating that the three products have "core-shell-shell" structures. Moreover, the "core-shell-shell" structures were composed of uniform submicro-spheres with smooth surfaces. The high-resolution images of the products S1, S2 and S3 showed that the size of LaPO 4 :Eu nanoparticles was 4-34 nm. These nanoparticles were uniformly distributed on the surface of the SiO 2 core, and the average diameter of the SiO 2 core was about 200 nm (Fig. 2b,c,g,f,l,m). For the product S1, the size of the intermediate shell LaPO 4 :Eu nanoparticles was about 4 nm and the average thickness of SiO 2 shell was about 25 nm. The size of the intermediate shell LaPO 4 :Eu nanoparticles was 5-7 nm in the product S2, and the average thickness of the SiO 2 shell was about 25 nm. However, the intermediate shell LaPO 4 :Eu of the product S3 showed single spherical particles with 15-34 nm in size, and the average thickness of the SiO 2 shell was about 10 nm. The HRTEM images of the products S1, S2, and S3 shows clear lattice fringes with the lattice spacing of 0.33, 0.33, and 0.34 nm, respectively. This result well agrees with the (220) crystal plane of the monoclinic phase LaPO 4 (Fig. 2d,i,n).
Figures 2e,j,o are size distribution images of the intermediate shell LaPO 4 :Eu nanoparticles in the products S1, S2, and S3, respectively. Figure 3 shows the EDX mapping image of the S1 product. The STEM image of the SiO 2 @ LaPO 4 :Eu@SiO 2 submicro-spheres indicates that the construction of the product is an obvious "core-shell-shell" structure. The elemental mapping result revealed that the La, Eu, P, O, and Si were distributed over the whole range of submicro-spheres.
To further investigate the amount of SiO 2 submicro-spheres effects on the size of the LaPO 4 :Eu nanoparticles, the morphology of the products at the different stages were investigated by TEM (Figs S1 and S2). In this reaction system, only the amount of SiO 2 submicro-spheres was changed and the other conditions were kept constant.
When the bridging ligand organosilane MABA-Si connected with different amount of SiO 2 submicro-spheres, the thickness of MABA-Si grafting on the surface of SiO 2 core was different. The thickness of the coating shell was about 2, 4, and 10 nm with decreasing the amount of SiO 2 (Fig. S1a-l). Therefore, when different sizes of SiO 2 @ MABA-Si connected with the LaPO 4 :Eu, we can obtain the SiO 2 @LaPO 4 :Eu submicro-spheres with different sizes of core-shell (the products N1, N2, and N3, Fig. S2). The TEM images (Fig. S2a-d) illustrate that LaPO 4 :Eu nanoparticles with about 4 nm in diameters could be uniformly coated on the surface of SiO 2 submicro-spheres. www.nature.com/scientificreports www.nature.com/scientificreports/ By contrast, the thickness of LaPO 4 :Eu were increased to about 6 nm for the product N2 ( Fig. S2e-h), and it was about 18 nm in the product N3 ( Fig. S2i-l).
It is found that the thickness of SiO 2 @MABA-Si and SiO 2 @LaPO 4 :Eu would be changed by adding different amounts of SiO 2 submicro-spheres in the reaction system. As the amount of SiO 2 submicro-spheres decreased, we found that the thickness of SiO 2 @MABA-Si was proportionally increased. In addition, there would be higher content -COOH groups existing on the surface of the SiO 2 submicro-spheres. The surface -COOH groups play an important role in the formation of LaPO 4 :Eu shell on the SiO 2 core surfaces. Enough -COOH groups would coordinate with more rare earth ions, and the thickness of LaPO 4 :Eu coated on the surface of SiO 2 core would be increased. After the core-shell-shell structured SiO 2 @LaPO 4 :Eu@SiO 2 submicro-spheres were calcined at 900 °C, LaPO 4 :Eu particles crystallized and grew to nanoparticles with different sizes. In short, the larger thickness of the intermediate shell LaPO 4 :Eu was, the larger LaPO 4 :Eu nanoparticles might be obtained. the growth mechanism of Sio 2 @Lapo 4 :eu@Sio 2 submicro-spheres. To better understand the growth mechanism of the SiO 2 @LaPO 4 :Eu@SiO 2 submicro-spheres, the products of S1 at different stages were studied by TEM, IR, EDX, and XPS. A possible growth mechanism for the SiO 2 @LaPO 4 :Eu@SiO 2 submicro-spheres was proposed, as Fig. 4a-g shows. First, the SiO 2 submicro-spheres were obtained from the hydrolysis of TEOS. The SiO 2 submicro-spheres presented a uniform and smooth spherical morphology. The average diameter of the SiO 2 submicro-spheres was about 200 nm ( Fig. 4b). At 1104, 950, and 450 cm −1 , the IR absorption peaks of SiO 2 submicro-spheres are observed. They should be attributed to the vibration of Si-O-Si, Si-OH, and Si-OH stretching (Fig. S3a). Second, the bridging ligand MABA-Si was grafted on the surface of the SiO 2 core through Si-O-Si bond that derived from the hydrolysis and the condensation of silane coupling agent. The as-synthesized SiO 2 @MABA-Si exhibited a relatively rough surface with a thin layer of about 2 nm (Fig. 4c). In the IR spectrum of SiO 2 @MABA-Si, the -COOH stretching vibrations of MABA-Si appeared at 1720 cm −1 (Fig. S3b). When the -COOH groups exposed onto the surface of SiO 2 core to coordinate with La 3+ and Eu 3+ ions, the peak of -COOH was found at 1705 cm −1 , which appeared an obvious red shift (Fig. S3c). As Fig. 4d shows, a shell was coated on the surface of SiO 2 core with a thickness of about 4 nm. Third, the PO 4 3− would react with rare earth ions, and the LaPO 4 :Eu nanoparticles were formed on the surface of SiO 2 @MABA-Si. It was found that the surface of submicro-spheres became rough and its thickness was about 6 nm ( Fig. 4e). Furthermore, the stretching and bending vibration of PO 4 3− appeared at 872 and 578 cm −1 in the IR spectrum of SiO 2 @LaPO 4 :Eu (Fig. S3d). EDX analysis was also used to investigate the composition of the SiO 2 @LaPO 4 :Eu (Fig. S4). The appeared peaks demonstrated that the product was composed of Si, O, P, La and Eu elements. The La and Eu elements were 19% and 9%, respectively. According to the XPS analysis results (Fig. 5), the signals of 1164, 1135, 853, 836, 134, 1300, 105, and 288 eV were assigned to the binding energies of La 3d, Eu 3d, P 2p, O 1 s, and C1s, respectively (Fig. 5a). The presence of peaks at 852 and 935 eV, associated with La elemental (Fig. 5b). The two peaks of Eu 3d were located at 1165 and 1134 eV, which was attributed to 3d 5 and 3d 3 (Fig. 5c). The peak of P 2p was at 134 eV (Fig. 5d). The EDX and XPS results indicated that the LaPO 4 :Eu shell structure was formed on the surface of the SiO 2 core. Fourth, in order to form a SiO 2 shell on the surface of SiO 2 @LaPO 4 :Eu submicro-spheres, www.nature.com/scientificreports www.nature.com/scientificreports/ the TEOS should be hydrolyzed slowly. After calcination, the core-shell-shell structured SiO 2 @LaPO 4 :Eu@SiO 2 submicro-spheres were finally obtained. As shown in Fig. 4f,g, a uniform silica shell was coated onto the surface of the submicro-spheres. The SiO 2 @LaPO 4 :Eu@SiO 2 was an obvious "core-shell-shell" structured submicro-sphere, which has a ∼25 nm outermost shell, a ∼4 nm intermediate shell, and a ∼200 nm core. The selected area electron diffraction pattern (SAED) clearly shows several diffraction points, suggesting excellent purity of the intermediate shell LaPO 4 :Eu (inset Fig. 4f). In the corresponding IR spectrum, the characteristic absorption peaks of Si-O-Si (1100 cm −1 ), the typical PO 4 3− symmetrical stretching and bending vibrations (879 and 567 cm −1 ) are observed (Fig. S3e). Finally, the core-shell-shell structured SiO 2 @LaPO 4 :Eu@SiO 2 submicro-spheres were controllably synthesized. photoluminescence properties. The photoluminescence of core-shell-shell structured products with different size of LaPO 4 :Eu nanoparticles were investigated by the room-temperature photoluminescence (PL) spectra. The excitation spectra of the products S1, S2, and S3 are presented in Fig. 6. For the S1 sample, the broad band centered at 282 nm should be assigned to the charge transfer (CTB) of Eu 3+ ions. The other four peaks at 317, 361, 375, and 393 nm are attributed to the direct excitation of the f-f shell transitions of Eu 3+ ions 29 . The CTB for the products S2 and S3 were displayed at 271 nm and 269 nm, respectively. Generally, the CTB position depends on the Eu-O bond length. If the length of Eu-O bond is long, the CTB usually has a longer wavelength 30,31 . When the size of the LaPO 4 :Eu nanoparticles is about 4 nm, the CTB position band would show a red shift. It indicates that the Eu-O bond distance become longer and the ratio of surface Eu 3+ ions is increased as the particle size shrinks 32 . It is found that emission peaks at 587, 612, 650, and 685 nm would excite with a 393 nm wavelength. This result should be originated from the 5 D 0 → 7 F J (J = 1-4) transitions of Eu 3+ (Fig. 7) 33 . The emission peaks at 612 nm and 587 nm should correspond to the electric dipole transition 5 D 0 → 7 F 2 and the magnetic dipole transition 5 D 0 → 7 F 1 of Eu 3+ ions. The S1-S3 samples have the same peak positions in the emission spectra. However, the intensity patterns of these products are different. The strongest peak for the S1 product is found at 612 nm, but at 587 nm for the products S2 and S3. Additionally, the intensity ratios of the electric dipole transition 5 D 0 → 7 F 2 to magnetic dipole transition 5 D 0 → 7 F 1 in the products are different, and the calculated intensity ratios are 1.40, 0.98 and 0.45 for the products S1, S2, and S3, respectively. With the size of the LaPO 4 :Eu nanoparticles decreases, the intensity ratios of the 5 D 0 → 7 F 2 transition to the 5 D 0 → 7 F 1 transition increases gradually, and the intensity of the electric dipole transition 5 D 0 → 7 F 2 becomes stronger. These results indicate that the PL properties depend on the size of LaPO 4 :Eu nanoparticles. Generally, the electric-dipole transitions are strictly forbidden, and the magnetic-dipole transitions are permitted due to the parity selection rules. The electric-dipole transition 5 D 0 → 7 F 2 is very sensitive to the local environment 34 . When the Eu 3+ ions do not lie on an inversion center of the crystal, the forbiddance of electric-dipole transition is resolved to some extent, and the electric-dipole transition 5 D 0 → 7 F 2 may become www.nature.com/scientificreports www.nature.com/scientificreports/ stronger. It is believed that the crystal field around the Eu 3+ ions should not much affect the magnetic dipole transition 5 D 0 → 7 F 1 35 . When the size of the nanoparticles decreased, the ratio of Eu 3+ ions in the surface of nanoparticles would be increased. Therefore, the decrease of the symmetry around the Eu 3+ ions would lead to the enhancement of the 5    www.nature.com/scientificreports www.nature.com/scientificreports/ a decrease in site symmetry. The variation of the Ω 2 value often relate to the change in local symmetry around the Eu 3+ ions due to the hypersensitivity of the electric dipole transition 5 D 0 → 7 F 2 to the local environment 40 . Here, in order to further understand the local symmetry properties of the Eu 3+ ions, J-O theory was used to calculate Ω 2 and Ω 4 by analyzing the emission spectrum of the products S1-S3. As the J-O theory formula (1) shows, the transition rate of the energy level is in proportion with the integral strength of the emission spectrum. Thus, the magnetic dipole transition 5 D 0 → 7 F 1 of Eu 3+ ion is independent of the environment and can be used as a reference 41 . We can calculate Ω 2 and Ω 4 values by calculating the integrated intensity of the electric-dipole transitions 5 D 0 → 7 F 2 and 5 D 0 → 7 F 4 . The ratio of the electric dipole transition rate to magnetic dipole transition rate can be expressed as: values of the products S1, S2 and S3 into the formula (1), we can get the value of Ω 4 is 1.00 × 10 −20 , 1.08 × 10 −20 , and 1.05 × 10 −20 . The calculation results are shown in Table 1.
In our study, when the size of LaPO 4 :Eu nanoparticles is about 4 nm (Ω 2 is 2.30 × 10 −20 ), the intensity ratio of the 5 D 0 → 7 F 2 to 5 D 0 → 7 F 1 is greater than 1.0. This result indicates that the excited RE 3+ was not in the symmetric center of the lattice. Moreover, the result also shows that the forbiddance of electric-dipole transition was resolved to some extent because of the perturbation of the crystal field. When the size of LaPO 4 :Eu nanoparticles is 5-34 nm (Ω 2 ≤ 1.80 × 10 −20 ), the intensity ratios of the 5 D 0 → 7 F 2 to 5 D 0 → 7 F 1 transitions is less than 1.0, indicating that the Eu 3+ ion locates in the symmetric center of the LaPO 4 lattice. www.nature.com/scientificreports www.nature.com/scientificreports/ The results were consistent with the CIE chromaticity diagram of the products S1, S2, and S3, which were estimated from their emission spectra (Fig. S5). The CIE chromaticity coordinates of the product S1 are closer to the red light than those of the product S2 or S3. Moreover, the parameter Ω 4 was hardly affected by the symmetry of the Eu 3+ ions in the LaPO 4 lattice 45 . The relative intensity ratio (R) of 5 D 0 → 7 F 2 to 5 D 0 → 7 F 1 can be calculated by the formula (4). According to the emission spectra of the products, we calculated the R values of the S1-3 products. When the size of the nanoparticles decreased, the value of R would increase, and the symmetry of the Eu 3+ ions in the LaPO 4 lattice was decreased (Table 1) The photoluminescence lifetime of the products with different sizes of LaPO 4 :Eu nanoparticles was also measured. The photoluminescence fitting curves of the products S1, S2, and S3 were shown in Fig S6. The calculated average lifetimes (τ) were calculated to be 1.53, 2.99, and 1.17 ms for the products S1, S2, and S3, respectively. Simultaneously, the absolute quantum yields were 40.23%, 11.26%, and 11.68% for the products S1, S2, and S3, respectively.
In order to investigate the possibility of SiO 2 @LaPO 4 :Eu@SiO 2 submicro-spheres in the biological application, we studied the relation of the concentration, the placement time, and the PL properties of SiO 2 @LaPO 4 :Eu@ SiO 2 submicro-spheres in aqueous solution. Figure 8 displays the emission intensity vs. the concentration of S1 in 5 mL H 2 O. At first, the emission intensity of SiO 2 @LaPO 4 :Eu@SiO 2 would increase with the increase of the concentration, but it comes to stable at last. When 5 mL H 2 O contained 0.033 g product S1, the strongest emission intensity could be detected. Because of the rate -OH quenching effect, the PL emission intensity in water gives a slight decrease with respect to that in the solid state. However, the SiO 2 @LaPO 4 :Eu@SiO 2 submicro-spheres in aqueous solution shows a stable emission property. As Fig. 9 shows, the emission of SiO 2 @LaPO 4 :Eu@SiO 2 would not be quenched even after 15 days. This good photoluminescence stability in aqueous solution might offer many opportunities for their applications in fluorescent bio-labeling/bioimaging and drug delivery.

conclusions
In summary, the core-shell-shell structured rare earth phosphates luminescent materials (SiO 2 @LaPO 4 :Eu@SiO 2 ) were controllably synthesized by a simple co-precipitation method using silane coupling agent (MABA-Si). The SiO 2 shell played a key role in perfecting the solubility and improving the photoluminescence properties of the products. By varying the thickness of MABA-Si grafting on the SiO 2 core and selecting the appropriate substitution reaction of phosphate, the intermediate shell LaPO 4   www.nature.com/scientificreports www.nature.com/scientificreports/ LaPO 4 :Eu@SiO 2 exhibited strong red luminescence, which would correspond to the 5 D 0 → 7 F 2 transition of the Eu 3+ ions. If the sizes of LaPO 4 :Eu nanoparticles were 5-34 nm (Ω 2 ≤ 1.80 × 10 −20 ), the Eu 3+ ion would locate in the symmetric center of the LaPO 4 lattice. Even over 15 days, the PL emission intensity of SiO 2 @LaPO 4 :Eu@SiO 2 was stable in aqueous solution. These studies might expand the application of submicro-spheres in the field of the fluorescent bio-label/bio-image.
Synthesis of Sio 2 @Lapo 4 :eu@Sio 2 submicro-spheres. The SiO 2 @LaPO 4 :Eu@SiO 2 submicro-spheres are core-shell-shell structures, which were synthesized according to our earlier report 46 . First, the synthesis progress of SiO 2 @LaPO 4 :Eu was briefly described as follows: (1) The Stöber method was employed to synthesize the SiO 2 submicro-spheres 47 . (2) The MABA-Si (bridging ligand organosilane) was prepared by using the method reported in literature 48  and SiO 2 submicro-spheres (0.067-0.200 g) were mixed with 25 mL ethanol. The pH value of the mixture was adjusted to ~8.0 by adding ammonia water, and the solution was magnetic-stirred for 4 h. The obtained SiO 2 @ MABA-Si solution should be washed by ethanol three times. The obtained depositions were dispersed in 10 mL ethanol followed by a slowly addition of RE(NO 3 ) 3 (95% La 3+ and 5% Eu 3+ ) ethanol solution (0.066 mol/L) under stirring for 4 h. Finally, the suitable (NH 4 ) 2 HPO 4 was added into the mixture under continuous stirring for 6 h and a precipitation of core-shell structured SiO 2 @LaPO 4 :Eu submicro-spheres were synthesized. The materials were further washed by water and ethanol, and were dried under air at 60 °C for 8 h. The details of obtained core-shell structured SiO 2 @LaPO 4 :Eu submicro-spheres were displayed in Table 2.
Second, the synthesis of the SiO 2 @LaPO 4 :Eu@SiO 2 submicro-spheres can be described as following. The above synthesized core-shell structured SiO 2 @LaPO 4 :Eu submicro-spheres (N1, N2, or N3, 0.100 g) were dispersed into 20 mL mixture solution of deionized water and ethanol. Subsequently, 0.1 g CTAB was introduced into the above mixture followed by dripping 1.0 mL aqueous ammonia with a concentration of 2.0 mol·dm −3 . The suitable tetraethoxysilane (TEOS) was dropwise added to this solution and magnetic-stirred for 6 h. When the white solid precipitation was found, it was filtered and dried at 60 °C for 8 h. Finally, the above products synthesized from N1, N2, and N3 were further calcined in muffle furnace at 900 °C for 4 h, which were defined as S1, S2, and S3, respectively.
Characterization. The scanning electronic microscopy (SEM; Hitachi S-4800, Japan) and the transmission electron microscopy (TEM; FEI Tecnai F20, USA) were used to characterize the structure and morphology of the products. In addition, XRD data were investigated by a X-ray diffractometer (Model M21XVHF22, MAC science  Table 2. Prepared conditions of the core-shell structured SiO 2 @LaPO 4 :Eu submicro-spheres.