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.

open Scientific RepoRtS | (2020) 10:3522 | https://doi.org/10.1038/s41598-020-60538-w www.nature.com/scientificreports www.nature.com/scientificreports/ Over the years, though the core-shell structured nanocomposite with silica coating inorganic materials were successive studied [17][18][19] , but their weaker luminescent properties limit their application in many fields. However, the rare earth organic complexes have excellent luminescence properties, which the energy of the organic ligands was efficiently transferred into the rare earth ions by "antenna effect" 20 . Besides, rare earth organic complexes have strong and narrow emission bands and are widely used in many optical fields. Unfortunately, rare earth organic complex themselves are still restricted from being utilized in practical applications due to their poor thermal and mechanical stability 13 . If the rare earth organic complex were immobilized on the surface of the silica, the silica core can reduce the skeleton vibration of the ligand, thereby effectively improving the stability and luminescence intensity of the complex [21][22][23] . The nanocomposites with silica as the core and rare earth complexes as the shell have good luminous properties, which make them more widely used. However, in the core-shell structured nanocomposites, the rare earth organic complex of the outer shell layer being exposed to the environment, which tends to quench the luminescence. In order to overcome this problem, the core-shell structured nanocomposite is coated with an amorphous silica to form a core-shell-shell structured nanocomposite, which can improve the luminescent properties of nanocomposites. The core-shell-shell structured nanocomposites designed in this way has excellent luminescence properties and good stability. In addition, the SiO 2 shell coated on the surface of the core-shell structured nanocomposite is easily condensed with the -OH group on other molecules to form a Si-O-Si network, and various functional groups are easily introduced into the silica shell, which is widely used in biomedicine [24][25][26] .Up to now, there are two main methods for preparing core-shell structured nanocomposites: direct precipitation method and silane coupling agent method. Direct precipitation method is to deposit functional shell materials directly on the surface of SiO 2 core. The method is simple to operate, but the prepared nanocomposites are unstable and prone to collapse. The silane coupling method is to join the SiO 2 core and the shell by a covalent bond. What' more, the shell layer thickness of nanocomposites is easy to control and structurally stable via this method. The realization of this method is challenging. It was have a silane coupling agent called a "molecular bridge" to bond rare earth ions and SiO 2 . Accordingly, it is important to choose a suitable silane coupling agent.In this work, firstly, we synthesized the bifunctional organic ligands ((HOOC) 2 C 6 H 3 NHCONH(CH 2 ) 3 Si(OCH 2 CH 3 ) 3 , it is obtained by single-substitution of hydrogen atom ...

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Preparation and properties of core–shell and core–shell–shell structure composites with good luminescence properties.