Fabrication of bifunctional core-shell Fe3O4 particles coated with ultrathin phosphor layer

Bifunctional monodispersed Fe3O4 particles coated with an ultrathin Y2O3:Tb3+ shell layer were fabricated using a facile urea-based homogeneous precipitation method. The obtained composite particles were characterized by powder X-ray diffraction, transmission electron microscopy (TEM), quantum design vibrating sample magnetometry, and photoluminescence (PL) spectroscopy. TEM revealed uniform spherical core-shell-structured composites ranging in size from 306 to 330 nm with a shell thickness of approximately 25 nm. PL spectroscopy confirmed that the synthesized composites displayed a strong eye-visible green light emission. Magnetic measurements indicated that the composite particles obtained also exhibited strong superparamagnetic behavior at room temperature. Therefore, the inner Fe3O4 core and outer Y2O3:Tb3+ shell layer endow the composites with both robust magnetic properties and strong eye-visible luminescent properties. These composite materials have potential use in magnetic targeting and bioseparation, simultaneously coupled with luminescent imaging.


Background
In modern materials science, considerable attention has been paid to the precise manipulation and development of new user-friendly methods for fabricating a range of inorganic systems in the nanoscale region. Among these inorganic systems, bifunctional magnetic-luminescent composites are particularly attractive because of their unique magnetic and luminescent properties in combination in a single particle. Each bifunctional particle normally has a paramagnetic core structural domain covered by a luminescent shell domain. Unique paramagnetic properties of iron oxides have been studied intensively for many technological applications, such as jet printing, magnetic storage media, MRI contrast enhancement, hyperthermia treatment, targeted drug delivery, and cell separation [1][2][3][4][5][6][7][8]. Therefore, iron oxides (such as γ-Fe 2 O 3 or Fe 3 O 4 ) have been considered ideal candidates for core-shell structures owing to their strong paramagnetic properties. The formation of core-shell structures is followed conventionally by an encapsulation process, where the paramagnetic core is encapsulated by the silica shell layer with embedded organic dyes [9,10] or quantum dots [11,12]. On the other hand, the direct linking of a fluorescent moiety to a magnetic core normally requires the use of a sufficiently long molecular linker to bypass any possible quenching by the ferro/paramagnetic core. Furthermore, the photobleaching and quenching of organic dyes and the instability and toxicity of QDs have seriously limited the broad applications of such core-shell structures, particularly in biomedicine. Another class of a luminescent material is lanthanide-doped inorganic composites. Lanthanide-doped composites are quite promising owing to their large Stokes shift, sharp emission spectra, high luminescence quantum yield, superior photostability, and low toxicity [13,14]. Therefore, lanthanide-doped composites have become a new generation of optical probes with great potential in biomedical imaging [13].
A combination of magnetic and luminescent properties of different ceramic materials into a single composite system might enhance their application range significantly. A unique magneto-optical composite composed of a magnetite core and coated phosphor material would have great potential in both nano-and biotechnology. Up to now, there are few reports on the preparation of multifunctional composites consisting of a magnetite core with a sol-gel-coated YVO 4 :Eu 3+ shell layer and directly linked NaYF 4 :Yb 3+ , Er 3+ nanoparticles [14,15]. Therefore, the development of a simple and reliable synthetic method for the fabrication of bimodal nanostructures with controlled morphologies and designed chemical components is still a challenge. Moreover, magneto-optical nanostructures can provide an all-in-one diagnostic and therapeutic tool, which can be used to visualize and treat various diseases simultaneously. Another exciting application of bimodal nanocomposites is in cytometry and magnetic separation, which can be controlled and monitored easily by fluorescent microscopy.
This paper proposes a facile strategy for the fabrication of bimodal nanocomposites using Fe 3 O 4 spheres as a core and a thin Y 2 O 3 :Tb 3+ layer phosphor coating as the shell structure. Morphological, structural, and chemical analyses of the synthesized nanocomposites were performed using a range of microscopy and energy-dispersive X-ray analysis techniques. As the main focus of this study, the magnetic and optical properties of synthesized nanocomposites are also discussed in detail. Moreover, the simple approach presented in this paper can be applied to the fabrication of Y 2 O 3 thin layers doped with other rareearth ions or even for different rare-earth host oxides.  Therefore, the synthesized bimodal magneto-optical system appears to be promising for magnetic separation and the diagnostic targeting and tracking of drug delivery.

Methods
Synthesis of core-shell Fe 3 O 4 @Y 2 O 3 :Tb 3+ particles All chemical reagents used in this study were of analytical grade (Sigma-Aldrich, St. Louis, MO, USA) and used as received. Spherical magnetic Fe 3 O 4 particles were prepared using a solvothermal method according to reported protocols [15,16]. Core-shell Fe 3 O 4 @Y 2 O 3 : Tb 3+ particles were further prepared using a facile ureabased homogeneous precipitation method [17][18][19]. In a typical process, rare-earth nitrates (0.0005 mol, Y/Tb = 99:1 mol%) were added to 40 ml of deionized (DI) water. Subsequently, 0.3 g of urea was dissolved in the solution with vigorous stirring to form a clear solution.
The as-prepared Fe 3 O 4 particles (50 mg) were then added to the above solution under ultrasonic oscillation for 10 min. Finally, the mixture was transferred to a 50-ml flask, sealed and heated to 90°C for 1.5 h. The resulting colloidal precipitates were centrifuged at 4,000 rpm for 30 min. The precipitates were washed three times each with ethanol and DI water and dried at 70°C for 24 h under vacuum. The dried precipitates were calcined in air at 700°C for 1 h.

Physical characterization
The structure of the samples was examined by X-ray diffraction (XRD;D8 Discover, Bruker AXS GmbH, Karlsruhe, Germany) with Cu Kα radiation (λ = 0.15405 nm) and with a scan range of 20°to 60°2θ. The morphology of the particles was characterized by field emission transmission electron microscopy (FETEM;JEM-2100 F, JEOL Ltd., Tokyo, Japan). The elemental properties of the samples were characterized by energy-dispersive X-ray spectroscopy (EDX;EMAX 6853-H, Horiba Ltd., Kyoto, Japan). Photoluminescence (PL;F-7000, Hitachi High-Tech, Tokyo, Japan) excitation and emission measurements were performed using a spectrophotometer equipped with a 150-W xenon lamp as the excitation source. Size measurements were performed using the Malvern Zetasizer Nano ZS machine (Malvern, UK). Magnetization measurements were performed using a quantum design vibrating sample magnetometer (QD-VSM option on a physical property measurement machine, PPMS 6000). All measurements were performed at room temperature.

Results and discussion
Morphology and structural properties Tb 3+ composite particles were slightly larger (approximately 325 nm) than the bare Fe 3 O 4 microspheres because of the additional coated layer of Y 2 O 3 :Tb 3+ , as shown in Figure 3. Moreover, the core-shell structure can also be observed clearly due to the small gap between the cores and shells. In addition, EDX analysis of the Fe 3 O 4 @Y 2 O 3 :Tb 3+ composite particles revealed the presence of yttrium (Y), terbium (Tb), iron (Fe), and oxygen (O) in the final composite particles.
XRD was used to investigate the structure and composition of the synthesized particles. Figure 4 shows XRD patterns of the bare Fe 3 O 4 and Fe 3 O 4 @Y 2 O 3 :Tb 3+ composite particles. The bare magnetite cores were indexed to the face-centered cubic (Fd3m space group) magnetite structure (JCPDS no. 19-0629) [15,16]. In the case of Fe 3 O 4 @Y 2 O 3 :Tb 3+ composite particles, in addition   to the characteristic diffraction peaks of the cubic Fe 3 O 4 structure, there were obvious diffraction peaks indexed to the cubic phase of Y 2 O 3 (JCPDS no. 86-1107, marked with •), which suggests the successful crystallization of a Y 2 O 3 :Tb 3+ thin layer on the surface of Fe 3 O 4 particles. In addition, no additional peaks for other phases were detected, indicating that no reaction had occurred between the core and shell during the annealing process.
Optical and magnetic properties of core-shell Fe 3 O 4 @Y 2 O 3 :Tb 3+ particles According to Li et al. [20] for the Y/Tb binary systems, homogeneous nucleation of Tb(OH)CO 3 occurs in priority and then the precipitation of Y(OH)CO 3 largely proceeds via heterogeneous nucleation on already-formed Tb(OH)CO 3 layer. Therefore, it was assumed that Tb (OH)CO 3 was firstly fully deposited (1 mol%) on a Fe 3 O 4 surface and then doped into the Y 2 O 3 structure (after the annealing process).
The PL properties of the core-shell Fe 3 O 4 @Y 2 O 3 :Tb 3+ composite particles were characterized further by excitation and emission spectroscopy, as shown in Figure 5. The excitation peak of the Fe 3 O 4 @Y 2 O 3 :Tb 3+ particles monitored at 544 nm ( 5 D 4 → 7 F 5 transition of Tb 3+ ) consisted of a double-charge-transfer band, which was assigned to charge transfer from the 2p orbital of oxygen to the 4f orbital of Tb 3+ between 250 and 320 nm [21,22]. The emission spectra of the Fe 3 O 4 @Y 2 O 3 :Tb 3+ composite particles consisted of three easily distinguishable f-f transitions within the terbium ions. The strong green emission band with a maximum at 544 nm corresponds to the 5 D 4 → 7 F 5 transition. The blue emission at 480 to 510 nm is another characteristic of the 5 D 4 → 7 F 6 transition in Tb ions. The feeble yellow-near-red band in the range of 577 to 600 nm was assigned to the 5 D 4 → 7 F 4 transition. The characteristic emission and excitation peaks were similar to those observed in previous studies for pure Y 2 O 3 :Tb 3+ nanocrystals, which suggest that the luminescent properties are maintained in the final composite particles [21,22].
To examine the magnetic properties of the bare Fe 3 O 4 and core-shell Fe 3 O 4 @Y 2 O 3 :Tb 3+ particles, the magnetization curves were measured by QD-VSM with a magnetic field cycle between −10 and +10 kOe at 300 K, as shown in Figure 6. The saturation magnetization value of the Fe 3 O 4 @Y 2 O 3 :Tb 3+ particles was 15.12 emu/g. This value is much lower than that (34.97 emu/g) of the bare Fe 3 O 4 due to diamagnetic Y 2 O 3 :Tb 3+ thin shell coating. The coercivity at 300 K was negligible, indicating typical superparamagnetic behavior. Although thin shell coating reduces the magnetization of the bare Fe 3 O 4 significantly, the Fe 3 O 4 @Y 2 O 3 :Tb 3+ composites still showed strong magnetization, which suggests their suitability for magnetic targeting and separation. The inset in Figure 6 shows that bifunctional Fe 3 O 4 @Y 2 O 3 :Tb 3+ composites can be attracted easily by an external magnet and show strong eye-visible green luminescence upon the excitation of a commercially available 254-nm UV lamp. Therefore, bifunctional Fe 3 O 4 @Y 2 O 3 :Tb 3+ composites exhibit good magnetic and optical properties and have potential applications in targeting and bioseparation.

Conclusions
Bifunctional Fe 3 O 4 @Y 2 O 3 :Tb 3+ composites were prepared using a facile urea-based homogeneous precipitation method. These composite particles offer two distinct functionalities: an inner Fe 3 O 4 core, which gives the composites strong magnetic properties, making them easy to manipulate magnetically, and an outer Y 2 O 3 :Tb 3+ shell with strong luminescent properties. A similar approach can be used to develop certain bifunctional composites with different core-shell structures. In addition, the simple design concept for bifunctional composites might open up new opportunities in bioanalytical and biomedical applications.