Controlled fabrication of bi-functional Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanoparticles and their magnetic, up-conversion luminescent properties

Abstract

We develop a facile layer-by-layer deposition process to create bi-functional Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanostructures composed of magnetic Fe₃O₄ cores, variable SiO₂ mid-layers, and up-converting Gd₂O₃ shells. The synthetic process is well-controlled and the obtained Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanoparticles show relative monodispersity and exhibit tunable magnetic and up-conversion luminescence depending on the thickness of SiO₂ mid-layers. The influences of SiO₂ mid-buffer-layers on morphology, magnetism, and up-conversion luminescence are well addressed. The obtained bi-functional Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanoparticles may be potentially applicable in magnetic, fluorescent, and biological applications. The synthetic route may be employed for fabricating other multifunctional nanostructures.

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1. Introduction

Magnetic and luminescent bi-functional nanomaterials have attracted great attention in recent years owing to their potential application in biological fields such as in fluorescent targeting, multimodal imaging, and drug delivery.^1–3 In the choice of luminescent nanomaterials for labeling, targeting and imaging, up-conversion nanoparticles (UCNPs) possess lots of advantages such as high fluorescence quantum yields, low toxicity, long lifetimes, and high stability in comparison with quantum dots and organic dyes.² UCNPs can convert near-infrared wavelength radiation to shorter wavelength radiation through a two-photon or multi-photon mechanism.^4–8 Because UCNPs can be luminous under the excitation of NIR radiation, the strong background autofluorescence from biological entities, which may decrease the sensitivity of detection, can be largely avoided.^9,10 Thus, the biological detection and imaging based on UCNPs with NIR radiation may have deeper penetration depth in tissues, enabling better detection and imaging sensitivity and resolution.¹¹ To realize the magnetic feasibility simultaneously in UCNPS, magnetic Fe₃O₄ nanoparticles with large magnetic moment are expected to be integrated into such UCNPs to achieve bi-functional entities through various synthetic strategies of core–shell architecture, surface decoration, and composite integration for multimodal applications.

To date, there have been some reports for constructing bi-functional nanostructures that were made up of Fe₃O₄ and UCNPs. The Fe₃O₄ and UCNPs could be combined with each other by a cross-linker anchoring process or via a facile aqueous-based co-precipitation approach under mild conditions.^3,12–15 In these demonstrations, if the UCNPs are chosen as cores,¹⁶ their luminescent intensity may be suppressed to some extent due to the coating of outer layers. Meanwhile, if the UCNPs are direct in contact with Fe₃O₄, their luminescence may be decreased as the direct contact can cause fluorescence quenching.¹⁷ Therefore, a SiO₂ mid-layer between Fe₃O₄ and UCNPs is needed and the core–shell structure is widely endowed in such bi-functional nanostructures. These multifunctional nanostructures can be used not only in MRI and UCL imaging, but also in CT imaging due to their large atomic number and high X-ray absorption coefficient of lutetium. Besides, amino-modified surface and mesoporous silica outer shell can also endow them positive aspects for loading and controlled release of ibuprofen (IBU). It is revealed that in such core–shell nanostructures, SiO₂ layer has a significant influence on their structure, morphology and property. Herein, we fabricate bi-functional core–shell Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanostructures composed of Fe₃O₄ cores, SiO₂ mid-layers and Gd₂O₃ shells through a facial layer-by-layer deposition approach. The obtained bi-functional core–shell Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanostructures exhibit uniform spherical morphology and excellent magnetic and fluorescent properties. The thickness of SiO₂ mid-layers is found to have great effects on the magnetic and luminescent properties of Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanoparticles. An appropriate thickness of SiO₂ mid-buffer-layers may result in higher luminescent intensity of bi-functional core–shell Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanoparticles and make them retain good magnetic responsive property simultaneously. The obtained bi-functional Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanoparticles may have potential values in magnetic, fluorescent, and biological applications. The synthetic route may also be applicable for fabricating other multifunctional nanostructures.

2. Experimental

2.1. Chemicals

Tetraethyl orthosilicate (TEOS) was purchased from Sinopharm Chemical Reagent Co., Ltd. FeCl₃·6H₂O, sodium acetate, trisodium citrate, ammonia solution (25–28 wt%), urea, ethylene glycol, and absolute ethanol were obtained from Tianjin Windship Chemistry Technological Co., Ltd. Gd(NO₃)₃·6H₂O, Yb(NO₃)₃·5H₂O, Er(NO₃)₃·5H₂O and 3-aminopropyl-triethoxysilane (APTES) were purchased from Aladdin. All chemicals were used as received without further treatment. Deionized water was used for all experiments.

2.2. Preparation of materials

2.2.1. Synthesis of uniform Fe₃O₄ nanoparticles. Fe₃O₄ nanoparticles were prepared according to the literature reported previously.¹⁸ Briefly, FeCl₃·6H₂O (1.3 g), sodium acetate (2 g), and trisodium citrate (0.5 g) were dissolved in 40 mL ethylene glycol with continuous stirring for 1 h. Then, the solution was transferred into a Teflon-lined stainless-steel autoclave, the autoclave was heated to 200 °C and maintained for 10 h. After it was cooled to room temperature, the obtained product was washed with ethanol and deionized water several times.

2.2.2. Synthesis of Fe₃O₄@SiO₂ nanoparticles. The Fe₃O₄@SiO₂ nanoparticles were prepared by a versatile Stöber sol–gel method.¹⁸ In a typical synthesis, 0.1 g as-prepared Fe₃O₄ nanoparticles were dispersed in a mixture of ethanol (187 mL), deionized water (47 mL), and ammonia solution (3.5 mL) under ultrasonication for 30 min. Then, different amount of TEOS precursor was added to the solution dropwise, the reaction was proceeded under mechanical stirring for 10 h at room temperature. After that, the product was collected by a magnet and washed by ethanol and deionized water several times. To obtain the samples with different thicknesses of SiO₂ mid-layers, 0.53, 2.67, 5.34, 13.35, and 18.69 mL of TEOS were used.

2.2.3. Synthesis of Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanoparticles. To incorporate Gd₂O₃:Yb, Er onto SiO₂ layers,¹⁹ the synthetic process was carried out and described below: 0.701 g of Gd(NO₃)₃·6H₂O, 0.079 g Yb(NO₃)₃·5H₂O, and 0.006 g Er(NO₃)₃·5H₂O and 2.241 g urea were mixed and dissolved in 38 mL deionized water. 0.15 g Fe₃O₄@SiO₂ was then added and the solution was hold for 4 h at 85 °C. The final product was separated by a magnet and washed with deionized water several times. The product was then dried at 80 °C for 12 h and calcined at 800 °C for 2 h under the protection of argon. The samples with different thickness of SiO₂ mid-layers prepared using the TEOS precursor of 0.53, 2.67, 5.34, 13.35, and 18.69 mL were denoted as S1, S2, S3, S4, and S5.

2.2.4. Surface modification of Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanoparticles. 0.1 g Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er, 0.1 mL APTES were mixed in 50 mL ethanol, the solution was refluxed at 80 °C for 12 h. The product was separated by a magnet and washed with ethanol several times.

2.3. Characterization

XRD was used to characterize the phase structure of the samples. Measurements were performed using a PANalitical EMPYREAN X-ray diffractometer operated at 40 kV and 40 mA using Cu Kα radiation (k = 0.15406 nm). Scanning electron micrographs were recorded with a JEOL-JSM 6700-F field-emission scanning electron microscope (FE-SEM). Samples for SEM measurements were deposited on silicon substrates and coated with a 5 nm Pt for characterization. TEM characterization was performed on a Hitachi H-800 system operated at an acceleration voltage of 200 kV. Samples for TEM analysis were prepared by drying a drop of microspheres dispersion on an amorphous carbon coated copper grid for observation. Scanning transmission electron microscopy (STEM) was performed on a FEI Tecnai F20 field-emission transmission electron microscope (FE-TEM). The up-conversion luminescence spectra were recorded on an Edinburgh FLS-920 instrument with a 980 nm diode laser as the excitation source. The magnetization curves of different samples were measured with a vibrating sample magnetometer (Nanjing university Co., Ltd, HH-15).

3. Result and discussion

The synthetic procedures for Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanostructures are presented in Scheme 1. As illustrated in Scheme 1, magnetic Fe₃O₄ nanoparticles were firstly prepared and used as starting cores. Then, a layer of SiO₂ was coated on Fe₃O₄ cores to achieve Fe₃O₄@SiO₂ nanostructures. Depending on the amount of TEOS precursor used for SiO₂ coating, the obtained Fe₃O₄@SiO₂ nanostructures have variable shell thickness and morphologies. Then, by coating a layer of up-converting Gd₂O₃:Yb,Er, the bi-functional Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanostructures were obtained. The synthetic process was monitored by XRD characterization. Fig. 1 shows the XRD patterns of Fe₃O₄, Fe₃O₄@SiO₂ (the amount of TEOS precursor fixed at 5.34 mL) and Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er (S3). The characteristic diffraction peaks in Fig. 1 can be well indexed to face centered cubic Fe₃O₄ (JCPDS no: 19-0629), cubic Gd₂O₃ (JCPDS no: 88-2165), and SiO₂ (JCPDS no: 29-0085). Obviously, the diffraction peaks corresponding to Fe₃O₄ become weaker after depositing the SiO₂ layer. When Gd₂O₃:Yb,Er layer was subsequently coated on Fe₃O₄@SiO₂, the diffraction peaks indexed to Fe₃O₄ can be hardly seen and the diffraction peaks ascribed to SiO₂ become weaker as well. This evidently proves the successful construction of core–shell Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanostructures. From the XRD data in Fig. 2, the full width at half maximum of Gd₂O₃ gradually narrows as the amount of added TEOS increases to 5.34 mL, then broadens as added amount further increases indicating the good crystallinity of Gd₂O₃ in S2, S3, and S4. The crystallinity of Gd₂O₃ in S3 is better than that in S2 and S4, while they are similar in S2 and S4. However, for S1 and S5, the characteristic diffraction peaks of Gd₂O₃ are relatively weak. Besides, there are also some diffraction peaks (they are noted as *) appeared in S1, which can be indexed to gadolinium iron oxide (JCPDS no: 78-0451 and no: 72-0141). The distinctions in crystallinity of Gd₂O₃ in S1 and S5 (Fig. 2a and e) will be discussed below combined with the corresponding TEM characterization.

Scheme 1 Schematic illustration shows the procedures for synthesis of Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanoparticles.

Fig. 1 XRD patterns of (a) Fe₃O₄, (b) Fe₃O₄@SiO₂ (the amount of TEOS fixed at 5.34 mL) and (c) Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanoparticles (S3).

Fig. 2 XRD patterns of Fe₃O₄@SiO₂@Gd₂O₃:Yb, Er with different thickness of SiO₂ mid-layer: (a) S1, (b) S2, (c) S3, (d) S4, and (d) S5 obtained using 0.53, 2.67, 5.34, 13.35, and 18.69 mL TEOS as precursor.

The size and morphology of Fe₃O₄, Fe₃O₄@SiO₂, and Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanostructures are shown in Fig. 3 and 4. From the SEM images in Fig. 3a–f, it can be seen that Fe₃O₄ nanoparticles are quite uniform and possess spherical morphology. The Fe₃O₄@SiO₂ nanoparticles show relative monodispersity with little aggregation. After coating Gd₂O₃:Yb,Er on Fe₃O₄@SiO₂ nanoparticles, the morphology of the obtained Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er (Fig. 4) is similar as the corresponding Fe₃O₄@SiO₂ nanoparticles, but the size is largely influenced by the quality of subsequent coating.²⁰

Fig. 3 SEM images of Fe₃O₄ (a) and Fe₃O₄@SiO₂ nanoparticles with different thickness of SiO₂ shells obtained using (b) 0.53, (c) 2.67, (d) 5.34, (e) 13.35, and (f) 18.69 mL TEOS as precursor.

Fig. 4 SEM images of Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er with different thickness of SiO₂ mid-layers obtained using (a) 0.53, (b) 2.67, (c) 5.34, (d) 13.35, and (e) 18.69 mL TEOS as precursor.

TEM characterization is conducted to further investigate the interior structure of the nanoparticles (Fig. 5). The mid-layer of SiO₂ and the outer layer of Gd₂O₃:Yb,Er can be clearly observed, combined with the XRD data, indicating that the successful synthesis of core–shell Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanoparticles. The sizes of the nanoparticles increase when more TEOS precursor is used in the coating process. High angle annular dark field-scanning transmission electron microscopy (HAADF-STEM) and elemental mapping have also been performed to reveal the elemental distribution in the microspheres. The HAADF-STEM images indicate that the Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er (S3) possesses well defined core–shell structure (Fig. 6a–c). The EDX elemental mapping of Fe, O, Si, Gd, Er, and Yb (Fig. 6d–i) further confirm the structure as these elements are well dispersed at specific location in the core–shell structure. The line scanning and mapping results (Fig. 7A) indicate that the elements Fe, O, Si, Gd, Er and Yb are spread evenly throughout the whole spheres. Meanwhile, the Si, Gd, Er and Yb are almost completely located on the outermost surface of the microspheres. The energy-dispersive spectrum (EDS) result (Fig. 7B) also reveals that Fe, O, Si, Gd, Er and Yb are contained in the sample. The size and morphology of Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanoparticles with different thickness of SiO₂ mid-layer can be tuned. From the XRD data (Fig. 2), the crystallinity of Gd₂O₃ in S1 and S5 are relatively low. A very thin layer of SiO₂ which cannot act as an effective spacer means that some Fe₃O₄ are still exposed (S1). As a result, a large portion of Gd₂O₃ react with Fe₃O₄ at 800 °C during the synthesis of Gd₂O₃:Yb, Er.²¹ The severe aggregation and the breakdown of spherical morphology in TEM image (Fig. 5b) may also indicate the occurrence of this reaction. However, with a thick SiO₂ layer (S5), the size of Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanoparticles is about 200 nm, which is twice as large as S1. More aggregated particles will form at the same time. Like cerium oxide, there may also occur a chemical reaction between gadolinium oxide particles or gadolinium precursor and SiO₂.²² Larger surface area of SiO₂ makes more gadolinium take part in the reaction under the same molar ratio of (Gd₂O₃)/(Fe₃O₄@SiO₂), finally leading to the low crystallinity of Gd₂O₃ (S5). This phenomenon can be proven by HADDF-STEM (Fig. 8A) and EDX elemental mapping images (Fig. 8B) of Fe₃O₄@Gd₂O₃:Yb,Er after removing SiO₂ from Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanoparticles (S5). There still exists some Si element close to gadolinium element even after removing SiO₂ by 2 M NaOH twice. This left part of Si could be the section that reacts with gadolinium. Because the reacted Si does not exist in the type of SiO₂, they cannot be removed by NaOH. This is a powerful proof for the chemical reaction between gadolinium oxide particles or gadolinium precursor and SiO₂. That is to say too thin or too thick SiO₂ layer is not helpful for crystallization of Gd₂O₃.

Fig. 5 TEM images of Fe₃O₄ (a) and Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanoparticles with different thickness of SiO₂ mid-layers obtained using (b) 0.53, (c) 2.67, (d) 5.34, (e) 13.35, and (f) 18.69 mL TEOS as precursor.

Fig. 6 (a)–(c) HAADF-STEM images of Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er (S3) and (d)–(i) EDX elemental mapping of Fe, O, Si, Gd, Er and Yb in Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er (S3).

Fig. 7 (A) line scanning profiles of Fe, O, Si, Gd, Er and Yb in Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er (S3) recorded along the line shown in Fig. 6c, (B) EDS spectrum of Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er (S3) recorded for the given point that the arrow indicated in Fig. 6c.

Fig. 8 (a and b) HAADF-STEM images of Fe₃O₄@Gd₂O₃:Yb,Er (S5) and (c) EDX elemental mapping of Si in Fe₃O₄@Gd₂O₃:Yb,Er (S5).

The up-conversion luminescence (UCL) spectra of Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er with different thicknesses of SiO₂ layer under 980 nm NIR excitation are shown in Fig. 9A. There are three emission band appeared in the spectra. Fig. 9B displays the energy level diagrams of Er³⁺ and Yb³⁺ ions and possible mechanism accounting for the four emissions under 980 nm excitation. The green emission band in 525 nm and 560 nm are attributed to the transitions ²H_11/2 to ⁴I_15/2 and ⁴S_3/2 to ⁴I_15/2 and the red emission band in 662 nm corresponds to the ⁴F_9/2 to ⁴I_15/2 transition.¹⁵ As the molar ratio of Yb/Er in Gd₂O₃:Yb,Er is 10 : 1, the dominant emission is in red region, which can be seen by naked eye as well.²³ The inset in Fig. 9A shows the relationship between the thickness of SiO₂ mid-layer and luminescence intensity at 662 nm of Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er (S1–S5). The intensity of luminescence is low at first (S1), then increases rapidly (S2) and reaches the highest point (S3), then decrease a little (S4), at last it decreases to nearly initial level (S5). There are two reasons accounting for this phenomenon: (1) direct contact between Fe₃O₄ and Gd₂O₃:Yb,Er will cause strong quenching effect on luminescence, this negative effect is weakened to some extent when Fe₃O₄ is coated by SiO₂, so the luminescent intensities from S1 to S3 increase with the increase of the thickness of SiO₂ layer; (2) Gd₂O₃ acts as matrix material for luminescence, so better crystallinity is significant for well-defined optical properties.^24,25 The crystallinity of Gd₂O₃ in S1, S2 and S3 follows this order: S3 > S2 > S1. However, for S3 to S5, the luminescent intensities from S3 to S5 decrease with the further increase of the thickness of SiO₂ layer, a thicker SiO₂ mid-layer do not mean a better optical performance as before. The quenching effect is negligible, the crystallinity of Gd₂O₃ is the main reason as the crystallinity of Gd₂O₃ in S3, S4 and S5 follows this order: S3 > S4 > S5. The large particle size and serve aggregation, suppresses the luminescence as well.

Fig. 9 Up-conversion luminescent spectra of (A) Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er with different thickness of SiO₂ mid-layers obtained using (a) 0.53, (b) 2.67, (c) 5.34, (d) 13.35, and (e) 18.69 mL TEOS as precursor for preparation. The inset shows the relationship of the thickness of SiO₂ mid-layer with luminescence intensity at 662 nm for Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanoparticles. (B) The proposed energy transfer mechanism for up-conversion process of Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er under 980 nm excitation.

The magnetic property of Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanoparticles is also examined as shown in Fig. 10. The magnetization values are measured to be 42.90, 4.15, and 4.12 emu g⁻¹ for Fe₃O₄, Fe₃O₄@SiO₂ (the amount of TEOS precursor is fixed at 5.34 mL), and Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er (S3). The huge decrease in Ms value (42.90 to 4.15 emu g⁻¹) is caused by coating a thick nonmagnetic SiO₂ layers (curve b in Fig. 10A).¹⁵ However, Ms value of Fe₃O₄@SiO₂ and Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er (4.15 and 4.12 emu g⁻¹) are nearly the same. A thin Gd₂O₃:Yb,Er shell which can be clearly seen from the TEM image (Fig. 5d) is formed on the outer layer, causing little influence on the Ms value. The Ms values of Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er with different thickness of SiO₂ mid-layers are 12.89, 4.82, 4.12, 3.09, and 2.22 emu g⁻¹, respectively (Fig. 10B). This is consistent with the thicknesses of SiO₂ mid-layers in S1–S5. Thick SiO₂ mid-layers may cause the decrease of Ms value. To exhibit a better performance in biological applications, higher luminescent intensity and excellent magnetic property are needed. The dual properties of Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er are demonstrated by digital photographs in Fig. 11. The Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanoparticles can be easily collected by an external magnetic field near a vessel after several minutes. The red up-converting emitting light of the aqueous solution with dispersed Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanoparticles collected by a magnetic field can be easily seen. Therefore, the bi-functional Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanoparticles with good magnetic and luminescent properties may find a wide range of applications in magnetic, fluorescent, and biological applications.

Fig. 10 (A) Magnetization curves of (a) Fe₃O₄, (b) Fe₃O₄@SiO₂, and (c) Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er (S3) nanoparticles and (B) magnetization curves of Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanoparticles with different thickness of SiO₂ mid-layers obtained using obtained using (a) 0.53, (b) 2.67, (c) 5.34, (d) 13.35, and (e) 18.69 mL TEOS for preparation.

Fig. 11 Photographs showing (A) aqueous solution of Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er (S3) (a) before and (b) after magnetic separation and (B) red emission of the aqueous solution of Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er (S3) under 980 nm excitation.

To further expand its application, surface modification with amino group is an excellent tool as it can be linked to specific drug or proteins. Infrared spectroscopy of the sample S3 before and after surface modification is shown in Fig. 12. Bands at 3406 cm⁻¹ and 1621 cm⁻¹ are attributed to bending vibration of absorbed molecular water. The Bands at 1093 cm⁻¹, 626 cm⁻¹ and 428 cm⁻¹ correspond to Si–O bond, Gd–O bond and Fe–O bond. The carboxyl salts belonged to citric acid which was located on the surface of Fe₃O₄ has a absorption band at 1395 cm⁻¹, After surface modification with amino groups (Fig. 12b), the absorption of asymmetrical and symmetrical stretching of –CH₂– which was linked to –NH₂ are presented at 2912 cm⁻¹ and 2837 cm⁻¹, suggesting that the nanoparticle is modified with amino groups successfully. As described above, these amino group modified nanoparticles may be applied for drug loading and further move to a specific area to under an external magnetic field.³ Moreover, Another significant application is cell imaging such as the HeLa cells.²⁶ Specific protein is attached onto the nanoparticles and then through the recognition to the specific cells cell-imaging is achieved.

Fig. 12 FTIR spectrums of (a) Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er and (b) Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er–NH₂.

4. Conclusion

In summary, bi-functional core–shell Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanoparticles composed of Fe₃O₄ cores, SiO₂ mid-layers and Gd₂O₃ shells were synthesized through a relatively simple layer-by-layer deposition process. The obtained Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanoparticles show relative monodispersity and possess magnetic and up-conversion luminescent properties with good water solubility. It is revealed that the SiO₂ mid-layer has a great effect on the size, morphology and magnetic and luminescent properties of final bi-functional Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanoparticles. A suitable SiO₂ mid-layer may help to gain higher luminescence while maintaining its magnetic property simultaneously. The bi-functional core–shell Fe₃O₄@SiO₂@Gd₂O₃:Yb,Er nanoparticles may have potential application in MRI, biolabeling, and bioimaging.

Acknowledgements

This work was supported by NSFC (21261011), NSFC (21467019), NSFC (21201097), Program for New Century Excellent Talents in University (NCET-10-0907), Application Program from Inner Mongolia Science and Technology Department (2011401), Inner Mongolia Talent Development Fund, and Inner Mongolia “Grassland Talent” Program (12101619) and Natural Science Foundation of Inner Mongolia Autonomous Region of china (2014MS0506).

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