Controlled synthesis and luminescence properties of core-shell-shell structured SiO2@AIPA-S-Si-Eu@SiO2 and SiO2@AIPA-S-Si-Eu-phen@SiO2 nanocomposites

Two novel core-shell structured SiO2@AIPA-S-Si-Eu and SiO2@AIPA-S-Si-Eu-phen nanocomposites have been synthesized by a bifunctional organic ligands ((HOOC)2C6H3NHCONH(CH2)3Si(OCH2CH3)3) (defined as AIPA-S-Si) connected with Eu3+ ions and silica via covalent bond. And the corresponding core-shell-shell structured SiO2@AIPA-S-Si-Eu@SiO2 and SiO2@AIPA-S-Si-Eu-phen@SiO2 nanocomposites with enhanced luminescence have been synthesized by tetraethyl orthosilicate (TEOS) hydrolysis co-deposition method. The composition and micromorphology of the nanocomposites were characterized by means of Fourier-transform infrared spectroscopy (FT-IR), thermal gravimetric analysis (TG), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDX) and X-ray photoelectron spectroscopy (XPS). The as-synthesized core-shell and core-shell-shell structured nanocomposites have excellent luminescence intensity and long lifetime. The nanocomposites show bright red light under ultraviolet lamp. However, the core-shell-shell structured nanocomposites have stronger luminescence intensity than the corresponding core-shell structured nanocomposites. Meanwhile, the core-shell-shell structured nanocomposites still exhibit good luminescence stability in aqueous solution. In addition, a large number of Si-OH on the surface of the core-shell-shell structured nanocomposites can be attached to many biomacromolecules. Therefore, they have potential applications in the fields of biology and luminescence.

two novel core-shell structured Sio 2 @AipA-S-Si-eu and Sio 2 @AipA-S-Si-eu-phen nanocomposites have been synthesized by a bifunctional organic ligands ((Hooc) 2 c 6 H 3 nHconH(cH 2 ) 3 Si(ocH 2 cH 3 ) 3 ) (defined as AIPA-S-Si) connected with Eu 3+ ions and silica via covalent bond. And the corresponding core-shell-shell structured Sio 2 @AipA-S-Si-eu@Sio 2 and Sio 2 @AipA-S-Si-eu-phen@Sio 2 nanocomposites with enhanced luminescence have been synthesized by tetraethyl orthosilicate (teoS) hydrolysis co-deposition method. the composition and micromorphology of the nanocomposites were characterized by means of fourier-transform infrared spectroscopy (ft-iR), thermal gravimetric analysis (TG), X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), energy-dispersive X-ray spectrometry (EDX) and X-ray photoelectron spectroscopy (XpS). the as-synthesized core-shell and core-shell-shell structured nanocomposites have excellent luminescence intensity and long lifetime. the nanocomposites show bright red light under ultraviolet lamp. However, the core-shell-shell structured nanocomposites have stronger luminescence intensity than the corresponding core-shell structured nanocomposites. Meanwhile, the core-shell-shell structured nanocomposites still exhibit good luminescence stability in aqueous solution. in addition, a large number of Si-oH on the surface of the core-shell-shell structured nanocomposites can be attached to many biomacromolecules. Therefore, they have potential applications in the fields of biology and luminescence.
In recent years, core-shell structured nanocomposites are gaining increasing attention due to their unique structures and properties. Core-shell structured nanocomposites not only have core properties but also shell properties, or new properties depend on the interaction between the core and the shell. This unique property also makes it a very important emerging nanomaterial that is widely used in various fields. For example, bio-nanotechnology, optics, electroluminescence devices, bio-imaging energy storage materials, etc. [1][2][3][4][5][6][7][8] .
Silica is commonly used as core and shell material in core-shell structured nanocomposites due to its high mechanical stability, low cost, low cytotoxicity, ease of preparation, and bioconpatibility [9][10][11][12] . So far, many research groups have studied core-shell structured nanocomposites using silica as core and shell, which have potential applications in different fields. Yan's group had reported the SiO 2 @lanthanide complex@MOF composites with excellent properties for metal ion sensing 13 . Li's group had reported the SiO 2 @Tb/GMP as fluorescence sensor probe 14 . Tobias's group had reported that SiO 2 @Pt@SiO 2 is used as a new type of optical component 15 . In previous studies, our group reported that the core-shell structured nanocomposite SiO 2 @MABA-Si-Tb was synthesized by using MABA-Si as a ligand. It is found that SiO 2 can improve the luminescence intensity and stability of rare earth complexes, which has potential application prospects in the field of luminescence 16 . www.nature.com/scientificreports www.nature.com/scientificreports/ average diameter of about 180 nm. SEM and TEM of SiO 2 @AIPA-S-Si in Fig. 1(d,e) shows that the surface of the SiO 2 microspheres were slightly rough. Nevertheless, it was worth noting that the size distribution of Fig. 1(c,f) shows that there was a significant difference in diameter between SiO 2 and SiO 2 @AIPA-S-Si, and the diameter of SiO 2 @AIPA-S-Si increases by about 10 nm compared with SiO 2 microspheres. The results showed that the organic ligand was successfully grafted onto the surface of the SiO 2 microspheres. teM analysis of core-shell and core-shell-shell structured nanocomposites. The TEM images of SiO 2 @AIPA-S-Si-Eu and SiO 2 @AIPA-S-Si-Eu@SiO 2 nanocomposites were presented in Fig. 2(a,b,d,e). Accordingly, the TEM images of SiO 2 @AIPA-S-Si-Eu-phen and SiO 2 @AIPA-S-Si-Eu-phen@SiO 2 nanocomposites were shown in Fig. S1(a,b,d,e). It can be observed from Fig. 2(a,b) and Fig. S1(a,b) that the SiO 2 @AIPA-S-Si-Eu and SiO 2 @AIPA-S-Si-Eu-phen nanocomposites have a rougher surface than SiO 2 @AIPA-S-Si, but still have good dispersibility. Furthermore, the surface roughness of core-shell-shell structured nanocomposites are very different from that of the corresponding core-shell structured nanocomposites. It can be seen from Figs. 2 and S1(d,e) that the gray layer was attached to the surface of the core-shell structured nanocomposites. Specifically, the amorphous silica layer approximately of 8-10 nm thick was successfully coated onto the SiO 2 @AIPA-S-Si-Eu and SiO 2 @AIPA-S-Si-Eu-phen, respectively. In addition, the EDX analysis that was performed on the core-shell and core-shell-shell structured nanocomposites suggested that the existence of N, O, Si, Cl and Eu peaks in the spectra (Figs. 2 and S1(c,f)). The strong peaks of Cu and C that can be observed from the spectra are due to the carbon coated copper grid. It can be found that the contents of Eu in SiO 2 @AIPA-S-Si-Eu, SiO 2 @AIPA-S-Si-Eu@ SiO 2 , SiO 2 @AIPA-S-Si-Eu-phen and SiO 2 @AIPA-S-Si-Eu-phen@SiO 2 nanocomposites were 3.47%, 0.17%, 4.09% and 0.22%, respectively. The content of Eu in the core-shell-shell structured nanocomposite is lower than that of the corresponding core-shell structured nanocomposite, which further proves that the amorphous silica is successfully coated on the surface of the core-shell structured nanocomposite. In addition, TEM mapping measurements of SiO 2 @AIPA-S-Si-Eu@SiO 2 (Fig. 3) and SiO 2 @AIPA-S-Si-Eu-phen@SiO 2 (Fig. S2) nanocomposites were performed, which exhibited uniform distributions of N, O, Si, Cl, and Eu components throughout the nanocomposites. The above data further demonstrated the successful synthesis of core-shell and core-shell-shell structured nanocomposites.
the ft-iR spectra analysis of AipA-S-Si. The FT-IR spectra further confirmed the synthesis of ligand AIPA-S-Si. Figure S3 shows the FT-IR spectra of the AIPA (a) and AIPA-S-Si (b). For the FT-IR spectra of AIPA (a), bands located at 3425 cm −1 and 1695 cm −1 were assigned to the stretching vibration of hydroxyl (ν OH ) and carbonyl groups (ν C=O ), respectively. The emergence of band located at 1629 cm −1 belongs to the stretching vibration of NH (ν N-H ). In the FT-IR spectrum of AIPA-S-Si (b), the appearance of the new bands located at around 1573 cm −1 and 1635 cm −1 due to the absorption of amide groups (-CONH-). Besides, the presence of stretching vibrations of methyl (-CH 3 ) and methylene groups (-CH 2 ) were located at 2977 cm −1 , 2931 cm −1 and 2885 cm −1 from TEPIC. In addition, the stretching vibration (ν Si-O ) located at 1103 cm −1 and the stretching vibration (ν Si-C ) located at 956 cm −1 suggested that TEPIC has been successfully grafted onto AIPA [27][28][29][30][31][32] . The above data indicates that the ligand AIPA-S-Si has been successfully synthesized. the ft-iR spectra analysis of Sio 2 @AipA-S-Si-eu and Sio 2 @AipA-S-Si-eu@Sio 2 . The FT-IR spectra was conducted to confirm the presence of chemical groups in the nanocomposites. At the same time, the synthesis of core-shell and core-shell-shell structured nanocomposites were further confirmed. The FT-IR spectra of SiO 2 , SiO 2 @AIPA-S-Si, SiO 2 @AIPA-S-Si-Eu and SiO 2 @AIPA-S-Si-Eu@SiO 2 were shown in Fig. S4. In the spectrum of SiO 2 (Fig. S4a)   www.nature.com/scientificreports www.nature.com/scientificreports/ the SiO 2 microspheres. In the FT-IR spectrum of SiO 2 @AIPA-S-Si-Eu (Fig. S4c), the characteristic absorption bands of -CONH-were located at 1627 cm −1 and 1558 cm −1 , respectively. Obviously, compared with SiO 2 @ AIPA-S-Si (Fig. S4b), the absorption band of amide groups was red shifted by 8 cm −1 , indicating that the first ligand AIPA-S-Si coordinated with the Eu 3+ ions by carboxyl oxygen atom. Fig. S4d shows the FT-IR spectrum of SiO 2 @AIPA-S-Si-Eu@SiO 2 . After coating a layer of amorphous SiO 2 on the core-shell structured nanocomposite, the characteristic absorption band of SiO 2 appeared at 1095 cm −1 (ν Si-O-Si ) and 952 cm −1 (ν Si-OH ), and the absorption bands of -CONH-and -C=O-(COOH) were obviously weakened. This further confirmed the synthesis of the SiO 2 @AIPA-S-Si-Eu@SiO 2 nanocomposite. the ft-iR spectra analysis of Sio 2 @AipA-S-Si-eu-phen and Sio 2 @AipA-S-Si-eu-phen@ Sio 2 . The FT-IR spectra of SiO 2 , phen, SiO 2 @AIPA-S-Si-Eu-phen and SiO 2 @AIPA-S-Si-Eu-phen@SiO 2 were shown in Fig. S5. In the FT-IR spectrum for phen ( Fig. S5b), the stretching vibration of -C=N− appeared at 1589 cm −1 , and the bending vibration of C-H appeared at 740 cm −1 and 856 cm −1 35,36 . In the FT-IR spectrum for SiO 2 @AIPA-S-Si-Eu-phen ( Fig. S5c), the corresponding characteristic absorption bands are red shifted to 1566 cm −1 (ν C=N ), 725 and 848 cm −1 (δ C-H ), respectively. The above results demonstrate the success of the coordination of the Eu 3+ ions with the nitrogen atoms in the second ligand phen. As can be observed in the FT-IR spectrum of SiO 2 @AIPA-S-Si-Eu-phen@SiO 2 nanocomposite (Fig. S5d), the absorption band of -C=N− is significantly weakened and the characteristic absorption band of SiO 2 appears in 1095 cm −1 (v Si-O-Si ) and 950 cm −1 (v Si-OH ), which further demonstrated that amorphous SiO 2 is successfully coated on the surface of core-shell structured nanocomposite. XRD analysis. In order to study the structure of nanocomposites, the x-ray diffraction (XRD) patterns of all samples were tested from 5° to 80° at room temperature. As shown in Fig. S6, all samples exhibited similar wide diffraction peak at approximately 23° without any shift, except for a slight difference in intensity. The wide diffraction peak belongs to the amorphous SiO 2 37,38 . The SiO 2 @AIPA-S-Si ( Fig. S6 (A,B)b) was the same as the pure SiO 2 (Fig. S6 (A,B)a) peak shape at about 23°, indicating that the first ligand (AIPA-S-Si) was grafted on the SiO 2 surface without changing the structure of the SiO 2 . The peak shape of the core-shell structured nanocomposites SiO 2 @AIPA-S-Si-Eu (Fig. S6Ac) and SiO 2 @AIPA-S-Si-Eu-phen (Fig. S6Bc) were different from SiO 2 @ AIPA-S-Si. It was noticed that a new diffraction peak appears at about 10°, which may be attributed to the formation of the Eu complex. Fig. S6Bc indicates that SiO 2 @AIPA-S-Si-Eu-phen nanocomposite shows some small diffraction peaks at 5-30°, while the diffraction peaks of amorphous SiO 2 at about 23° were significantly weakened, further indicating that the rare earth complexes coated on the surface of SiO 2 microspheres. Moreover, the peak shape of the core-shell-shell structured nanocomposites SiO 2 @AIPA-S-Si-Eu@SiO 2 (Fig. S6Ad) and SiO 2 @ AIPA-S-Si-Eu-phen@SiO 2 (Fig. S6Bd) are similar to the pure SiO 2 peak shape, indicating that amorphous SiO 2 was successfully coated on the surface of the core-shell structured nanocomposite. www.nature.com/scientificreports www.nature.com/scientificreports/ tG analysis. In order to investigate core-shell and core-shell-shell structured nanocomposites have good thermal stability. The thermogravimetric analysis (TGA) curves of the core-shell and core-shell-shell structured nanocomposites were performed from 30 °C to 950 °C at a rate of 20 °C/min under N 2 atmosphere. The thermogravimetric curve of core-shell structured SiO 2 @AIPA-S-Si-Eu-phen (a) and core-shell-shell structured SiO 2 @ AIPA-S-Si-Eu-phen@SiO 2 (b) were shown in Fig. S7. It can be observed that two nanocomposites have similar trends in weight loss and there were two major degradation steps. The first stage exhibited a weight loss of 3% and 6% for core-shell and core-shell-shell structured nanocomposites, respectively, which were attributed to the loss of water molecules physically adsorbed on the surface of the nanocomposites 39 . In the second stage, the weight loss of core-shell and core-shell-shell structured nanocomposites were 28% and 12%, respectively, which were due to the decomposition of organic complex. In general, the total weight loss percentage of core-shell and core-shell-shell structured nanocomposites were 31% and 18% from 34 °C to 670 °C. It was indicated that the percentage of organic complex in the core-shell structured nanocomposite is higher than that of the core-shell-shell structured nanocomposite, which further proves that amorphous SiO 2 was coated on the surface of the core-shell structured nanocomposite. In addition, the second stage organic complex of the core-shell and core-shell-shell structured nanocomposites began to decompose at temperatures of 125 °C and 161 °C, respectively. The result indicates that the decomposition temperature of the core-shell-shell structured nanocomposite is higher than the core-shell structured nanocomposite. Therefore, the thermal stability of the core-shell-shell structured nanocomposite is higher than that of the core-shell structured nanocomposite. At the same time, because the core-shell-shell structured nanocomposite has the protection of SiO 2 shell, it has a good promotion effect on thermal stability of nanocomposite.
XpS analysis. The surface chemical compositions of the core-shell structured nanocomposites SiO 2 @ AIPA-S-Si-Eu and SiO 2 @AIPA-S-Si-Eu-phen were further confirmed by XPS testing. The survey scan ranging from 0 to 1200 eV was shown in Fig. 4, the results demonstrated that core-shell structured nanocomposites were mainly composed by the elements, including C, N, O,Si, Cl and Eu. As shown in the XPS spectra (Fig. S8) [40][41][42][43] . However, since XPS is a technique for surface detection and its detection depth is less than 10 nm, the presence of an element of N, O, Si, Cl and Eu indicates that the Eu complex was coated on the surface of the silica microspheres 44 . The above data further indicates that the core-shell structured nanocomposites SiO 2 @AIPA-S-Si-Eu and SiO 2 @AIPA-S-Si-Eu-phen have been successfully synthesized.
Solid state luminescence properties. The excitation and emission spectra of the core-shell and core-shell-shell structured nanocomposites were measured in solid state by FLS-980 luminescence spectrometer at room temperature. The slit width of the nanocomposites SiO 2 @AIPA-S-Si-Eu, SiO 2 @AIPA-S-Si-Eu-phen, SiO 2 @AIPA-S-Si-Eu@SiO 2 and SiO 2 @AIPA-S-Si-Eu-phen@SiO 2 were 1, 0.7, 1, and 0.7 nm respectively. The luminescence intensity comparison of the core-shell and core-shell-shell structured nanocomposites were shown in Fig. 5. All nanocomposites exhibit similar luminescence spectra except for changes in intensity. The excitation spectra of all nanocomposites were recorded by monitoring at emission wavelength of 615 nm. The spectra were dominated by a broad excitation band centered at about 270-370 nm in ultraviolet region. It was designated as ligands that are absorbed into their π→π* transitions 37 . As shown in Fig. 5, five characteristic emission peaks were observed at 579 nm, 591 nm, 615 nm, 652 nm and 701 nm which represented 5 D 0 → 7 F J (J = 0, 1, 2, 3, 4) transitions of Eu 3+ ions within 4f 6 configuration, respectively. The strongest emission peak at 615 nm was attributed to the 5 D 0 → 7 F 2 electrical dipole transition of Eu 3+ ion, which was caused by the lack of inversion symmetry at Eu 3+ ions site 45,46 . So, the strong red luminescence can be observed in the emisssion spectra. It can be seen from Fig. 5a that the luminescence intensities of core-shell structured nanocomposite SiO 2 @AIPA-S-Si-Eu was 1397610 a.u. Figure 5b shows that the luminescence intensity of the core-shell structured nanocomposite SiO 2 @AIPA-S-Si-Eu-phen was 2072052 a.u., which was 1.48 times higher than that of the core-shell structured www.nature.com/scientificreports www.nature.com/scientificreports/ nanocomposite SiO 2 @AIPA-S-Si-Eu. This is due to the addition of the second ligand (phen) can coordinate with the Eu 3+ ion and improve the energy transfer efficiency, so that the nanocomposite luminescence intensity was significantly improved. On the other hand, the luminescence intensity of the core-shell-shell structured nanocomposite was stronger than that of the corresponding core-shell structured nanocomposite. From the data in Table 1, it was indicated that the two core-shell-shell structured nanocomposites increase by 1.67 times and 2.21 times compared with the corresponding core-shell structured nanocomposites, respectively. Since the core-shell structured nanocomposites were coated with a silica shell, which the high vibration energies loss of the ligand molecules (AIPA-S-Si, phen) or other quenching sites located at the surface of the core-shell structured nanocomposite could largely reduce. The silica shell can protect the rare earth complexes from quenching by the external environment 21 . Thus, the luminous intensity of the core-shell-shell structured nanocomposite is stronger than that of the corresponding core-shell structured nanocomposite. Luminescence intensity comparison data for all nanocomposites were shown in Table 1.
Aqueous solution luminescence properties. In order to further research the potential application of core-shell-shell structured nanocomposites in the biological field, we have tested the stability of core-shell-shell structured nanocomposites in aqueous solution. The core-shell-shell structured nanocomposites were dissolved in distilled water to prepare a suspension having a concentration of 0.1 g/L. The prepared solution was placed for 0 h, 24 h, 48 h and the luminescence intensity was measured. Figure 6 shows that the luminescence intensity of the nanocomposites in the aqueous still has strong luminescence intensity and no significant change after being placed for different time. Therefore, the core-shell-shell structured nanocomposites have excellent luminescence stability in aqueous solution. Fig. S9 shows the calculated CIE chromaticity coordinates of all nanocomposites investigated. SiO 2 @ AIPA-S-Si-Eu, SiO 2 @AIPA-S-Si-Eu@SiO 2 , SiO 2 @AIPA-S-Si-Eu-phen and SiO 2 @AIPA-S-Si-Eu-phen@SiO 2 show a red emission located at (0.5152, 0.2655), (0.4893, 0.2536), (0.5736, 0.3124) and (0.5917, 0.3212) respectively. The results of chromaticity coordinates show that all the nanocomposites are located in the red or pink region. Figure 7 exhibits, as an example, the red light image of SiO 2 @AIPA-S-Si-Eu@SiO 2 (a,b) and SiO 2 @ AIPA-S-Si-Eu-phen@SiO 2 (e,f) nanocomposites without and under UV irradiation. In addition, the image also show the pink light image of a solution of SiO 2 @AIPA-S-Si-Eu@SiO 2 (c,d) and SiO 2 @AIPA-S-Si-Eu-phen@ SiO 2 (g,h) nanocomposites dispersed in water without and under UV irradiation. Under ultraviolet irradiation, core-shell-shell structured nanocomposites exhibit bright red or pink light in solid or aqueous solution, which indicates that core-shell-shell structured nanocomposites have excellent luminous properties. the luminescence lifetime. To further study the luminescence properties, the lifetime curve of nanocomposites was also measured at room temperature. As shown in Fig. S10, all curves follow a double exponential fit (1) 38 :  Table 1. Luminescence emission spectra data of core-shell and core-shell-shell nanocomposites.
Low temperature phosphorescence spectra. To further explain the luminescence mechanism of the nanocomposites. The low phosphorescence spectra of the ligands were tested on a FLS-980 phosphorescence spectrometer at 77 K. The excitation wavelengths of the ligands AIPA-S-Si and phen were 355 nm and 372 nm, respectively. The corresponding slit width was 1 nm and 10 nm, respectively. The triplet energy level of the ligand was determined by the half width of the band. In the phosphorescence spectra of the first ligand AIPA-S-Si (Fig. S11a), there was an emission band of 455 nm to 530 nm, indicating that the triplet energy level was 21978 cm −1 to 18868 cm −1 . The excited state energy level 5 D 0 of Eu 3+ ion was calculated to be 17277 cm −1 .  www.nature.com/scientificreports www.nature.com/scientificreports/ Therefore, the triplet energy level of the first ligand AIPA-S-Si was higher than the excitable energy level of the Eu 3+ ion, and the ligand can effectively transfer the energy absorbed from the ultraviolet region to the central Eu 3+ ions, thereby sensitizing the luminescence of the Eu 3+ ion. In the phosphorescence spectrum of the second ligand phen, the emission band from 503 nm to 618 nm can be observed from the Fig. S11b. The corresponding triplet energy level was 19881 cm −1 to 16181 cm −1 . The highest triplet energy level of phen was also higher than the excitable energy level of Eu 3+ ions, so the luminescence of Eu 3+ ions can also be sensitized. The schematic diagram of the triplet energy levels and Eu 3+ ions excited state levels of the ligands AIPA-S-Si and phen was shown in Fig. S12. Since the first ligand AIPA-S-Si and the second ligand phen can synergistically sensitize the Eu 3+ ions. Therefore, the luminescence intensity of the SiO 2 @AIPA-S-Si-Eu-phen nanocomposite was superior to that of the SiO 2 @AIPA-S-Si-Eu nanocomposite. In summary, the presence of the ligands AIPA-S-Si and phen can sensitize Eu 3+ ions to have excellent luminescent properties.

conclusions
The four novel nanocomposites SiO 2 @AIPA-S-Si-Eu, SiO 2 @AIPA-S-Si-Eu-phen, SiO 2 @AIPA-S-Si-Eu@SiO 2 and SiO 2 @AIPA-S-Si-Eu-phen@SiO 2 have been successfully prepared; All nanocomposites have excellent luminescence properties, and the core-shell-shell structured nanocomposites have stronger luminescence intensity than the corresponding core-shell structured nanocomposites; The aqueous solution of core-shell-shell structured nanocomposites have excellent luminescence stability, and the luminescence intensity is almost unchanged after being placed for 0 h, 24 h and 48 h at room temperature; Besides, the core-shell and core-shell-shell structured nanocomposites can also save rare earth resources and reduce production cost in the actual production process; More importantly, the silanol groups on the surface of the core-shell-shell structured nanocomposites are easily attached to biomolecule. Consequently, the research of core-shell-shell structured nanocomposites not only has potential applications in biology, but also excellent luminescent properties provide new research ideas for other luminescent materials.

Materials and Methods
Materials. 5
Synthesis of Sio 2 microspheres. The SiO 2 microspheres were prepared according to the Stöber method 47 .
Specifically, 1.7 mL ammonia water and 4 mL distilled water were added to 35 mL absolute ethanol, and then 1.7 mL TEOS was added dropwise to the above solution under magnetic stirring. Then, a white suspension was obtained after continuing the reaction for 6 h at room temperature. The resulting product was centrifuged, washed with ethanol and distilled water several times, respectively. Finally, SiO 2 microspheres were dried at 50 °C for 5 h.
Synthesis of Sio 2 @AipA-S-Si. Briefly, 0.1 g as-prepared SiO 2 microspheres were accurately weighed and dissolved in a mixed solution of 10 mL absolute ethanol and 10 mL distilled water, and then the as-prepared ligand (AIPA-S-Si) was ultrasonically dissolved in 20 mL 1,4-dioxane. Subsequently, the ligand (AIPA-S-Si) solution was gradually added dropwise to the silica solution, and the reaction was stirred at room temperature for 36 h. The product was washed three times with absolute ethanol and collected by centrifugation at 8000 r. min −1 . Finally, the product was dried at 50 °C for 10 h. The product is defined as SiO 2 @AIPA-S-Si.
Synthesis of Sio 2 @AipA-S-Si-eu and Sio 2 @AipA-S-Si-eu@Sio 2 . Typically, 0.0550 g as-prepared SiO 2 @AIPA-S-Si was uniformly dispersed in 15 mL absolute ethanol. Subsequently, 0.0550 g Eu(ClO 4 ) 3 .nH 2 O was dissolved in 5 mL absolute ethanol, which was added dropwise to the above solution, and fully reacted under magnetic stirring at 50 °C for 24 h. The obtained product was centrifuged, washed with absolute ethanol several times, and the white product was dried at 50 °C for 5 h. The product is defined as SiO 2 @AIPA-S-Si-Eu. 0.02 g SiO 2 @AIPA-S-Si-Eu and 0.01 g CTAB were dissolved in 20 mL of a mixed solvent (5 mL distilled water, 15 mL ethanol), then add the appropriate amount of ammonia water and 15 μL TEOS. Then, the mixture was continuously stirred at room temperature for 24 h. The resulting suspension was washed six times with ethanol and water. After centrifugation at 8000 r.min −1 , the final product was dried at 50 °C for 5 h. The product is defined as SiO 2 @AIPA-S-Si-Eu@SiO 2 .