Synthesis of Fe3O4@Y2O3:Eu3+ core–shell multifunctional nanoparticles and their magnetic and luminescence properties

Optical Materials 35 (2013) 1685–1692

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Synthesis of Fe3O4@Y2O3:Eu3+ core–shell multifunctional nanoparticles and their magnetic and luminescence properties Genekehal Siddaramana Gowd 1, Manoj Kumar Patra, Manoth Mathew, Anuj Shukla, Sandhya Songara, Sampat Raj Vadera ⇑, Narendra Kumar Defence Laboratory, Jodhpur 342 011, India

a r t i c l e

i n f o

Article history: Received 10 November 2012 Received in revised form 26 April 2013 Accepted 27 April 2013 Available online 6 June 2013 Keywords: Core–shell Superparamagnetic Luminescence Multifunctional

a b s t r a c t A simple wet chemical route has been employed to synthesize multifunctional core–shell nanoparticles of Fe3O4@Y2O3:Eu3+ showing an interesting combination of magnetic and luminescent properties having potential for medical applications. The core–shell nanoparticles were synthesized in a two-step process wherein first step, the Fe3O4 nanoparticles were synthesized and subsequently they are coated with Y2O3:Eu3+. XRD and magnetization curves were successfully used to retrieve the particle size of Fe3O4 nanoparticles. Particle size (10 nm) extracted from XRD and magnetization curves have been found to be consistent with the measured size from AFM and TEM. Further, the XRD analysis reveals formation of pure cubic phases of magnetite as well as of Y2O3:Eu3+. It has been shown here that through simple chemistry it is possible to change the thickness of Y2O3:Eu3+ shell. From SEM and TEM studies, the size of core shell nanoparticles seen as 30 nm. In addition to bright red (612 nm) emission, these materials also show superparamagnetic behavior at room temperature. Emission intensity has been found to significantly increase with increase in annealing temperature. The synthesized materials have extensive for applications in the area of drug delivery and bio-imaging. Ó 2013 Elsevier B.V. All rights reserved.

1. Introduction Nanoparticles with appropriate size scale and facile surface chemistry show unique properties suitable for many technological applications. Further, their size range being close to many biological systems, the nanoparticles present great promise for biomedical applications. Targeted nanoparticles are highly specific and are expected to bring revolutionary change in medical diagnosis and therapy with minimal damage. Certain magnetic materials at nanoscale show superparamagnetic behavior that has no permanent magnetic dipoles. Keeping in view this advantage, the superparamagnetic nanoparticles have been explored for a number of medical applications which include magnetic bioseparation of labeled cells and biological entities [1,2], magnetic resonance imaging (MRI) contrast agents [3–6], targeted drug delivery [7,8], biosensing [9], etc. Further, certain luminescent inorganic nanoparticles with high luminescence efficiency together with excellent photostability can serve as optical and biorecognition probes for in vitro imaging [10]. Multifunctional nanomaterials, wherein nanomaterials having different phases and physical properties co-exist, can show not ⇑ Corresponding author. Tel.: +91 291 2567400; fax: +91 291 2511191. 1

E-mail address: [email protected] (S.R. Vadera). ACNS, AIMS Eranakulam.

0925-3467/$ - see front matter Ó 2013 Elsevier B.V. All rights reserved. http://dx.doi.org/10.1016/j.optmat.2013.04.029

only combined properties of the original components but also possess novel and collective performances not seen in the original components [11,12]. Particularly, magnetic–luminescent nanoparticles provide a new platform for both diagnostics and treatment of disease due to their enhanced functionality combining magnetism and luminescence in a single system. This has led to an entirely new concept in medical science where in single system does the job of detection as well as treatment. There are reports where researcher could achieve magnetic– luminescent properties in single system, such as the core–shell type nanoparticles consisting of superparamagnetic Fe3O4 nanoparticle core and a layer of luminescent CdSe/ZnS quantum dots (QDs) on their surface [13], the simultaneous encapsulation of luminescent CdTe nanoparticles and magnetic Fe3O4 nanoparticles in polymer microcapsules [14], chitosan nanobeads [15] and silica shell [16,17]. Another strategy to produce hybrid nanosystems is colloidal synthesis, where each nanocrystal could be made of any desired inorganic materials purposely assembled together for tailored applications. But most of the reported core shell magnetic–luminescent nanoparticles consist of semiconductor luminescent QDs made from heavy metal ions such as Cd2+ and Pd2+. It has been observed that these QDs are highly toxic to cells exposed to UV for longer time as the UV dissolves the QDs and release toxic Cd2+ and Pb2+ ions [18]. Rare earth doped nanocrystals can be promising materials in this regard due to their high photostability, non-toxicity and high

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quantum efficiency. There are very few reports on coating of rare earth nanophosphors on to surface of Fe3O4 nanoparticles. In these chemical methods, authors have used either intermediate silica coating [19] or surfactants [20,21] to connect between core Fe3O4 and outer luminescent layers. In the present paper, we report direct coating of Y2O3:Eu3+ (Europium doped Yttrium Oxide) nanophosphor on to surface of superparamagnetic Fe3O4 nanoparticles, all through wet chemical slow crystallization process. These materials are annealed at different temperatures and studied for their structural, luminescence and magnetic properties. Further, the effect of coating thickness of Y2O3:Eu3+ layers on luminescence as well as magnetic properties have also been studied. 2. Experimental details For the synthesis of core–shell nanoparticles, a simple aqueous wet chemical method has been utilized, wherein Fe3O4 nanoparticles were synthesized and then coated with Y2O3 nanophosphor. The whole synthesis process involves two steps: (i) preparation of Fe3O4 nanoparticles (core) and (ii) coating of Y2O3:Eu3+ shell on to Fe3O4 core. 2.1. Synthesis of Fe3O4 nanoparticles (core) In a typical synthesis, aqueous solution (50 ml) of aniline hydrochloride and formaldehyde in equimolar concentration (8 mmol) were prepared separately. Initially both the solutions were mixed together to form a copolymer of aniline formaldehyde. Similarly, 100 ml aqueous solution of FeCl3 (8 mmol) solution was prepared and added drop wise to copolymer of aniline formaldehyde solution. This reaction mixture was stirred for 5–10 min for homogenizing the solution. Further, NaOH/Hydrazine stock solution was prepared by dissolving NaOH (50 mmol) in Hydrazine (10 ml) in 100 ml of deionized water. NaOH/Hydrazine stock solution (3 ml) was added rapidly into the above mixture and stirred well. The pH of the solution was maintained at 10 by adding NaOH solution. The reaction solution turned black after 2 min and became slightly turbid. The resulting mixture was kept for 1 h to complete the reaction. The precipitate of Fe3O4 nanoparticles was magnetically separated from the supernatant by using a permanent magnet. This procedure was repeated several times by adding fresh distilled water to remove unreacted products from the precipitate. Thoroughly washed Fe3O4 nanoparticles were finally dispersed in deionized water by sonication resulting into formation of highly stable colloidal solution of Fe3O4. A part of the solution dried on Petri dish to obtain sample powder for XRD, AFM, TEM and VSM analysis. This sample hereafter named as sample C1. Further, a measured volume of the solution was dried and weighed to find  concentration of Fe3O4 nanoparticles in the colloidal solution.

dried in ambient condition. By following the same processes three samples were prepared with different Fe3O4AY2O3 ratios i.e. 1:200, 1:100 and 1:50. The resulting samples are named as CS1, CS2 and CS3, respectively. Further, these samples were annealed at different temperatures (300–800 °C) in an air atmosphere and studied their structural, magnetic and optical properties. Structural characterization of the samples was carried out by powder X-ray diffraction method performed on a Philips X’Pert Pro system by using Cu Ka1 (k = 1.543 Å) radiation. Morphology and EDAX elemental analysis of synthesized core shell nanoparticles were examined using Carl Zeiss SEM (EV0 MA15). Photoluminescence studies of samples were carried out at room temperature using a Spectrofluorometer, JASCO FP-6500, fitted with 150 W Xenon flash lamp and R938 Hamamatzu PMT detector. Magnetization measurements on the samples were carried out by using ADE Model EV-5 Vibrating Sample Magnetometer (VSM) at a field of 1.5 T at room temperature. Atomic Force Microscope (AFM) NTMDT Solver TS 150 in semi-contact mode has been used to determine the size of Fe3O4 nanoparticles. For AFM studies, the colloidal solution of iron oxide nanoparticles was fine sprayed on polished mica substrate. Transmission Electron Microscope (TEM, TECNAI 20 G) has been used to determine the size and shape of the synthesized nanoparticles. FTIR spectra of samples were recorded in KBr using Shimadzu DR-8101A spectrometer.

3. Results and discussion 3.1. Characterization of Fe3O4 Fig. 1 shows XRD spectra of as prepared sample Fe3O4 (C1). The XRD pattern obtained for the sample shows characteristic diffraction peaks corresponding to (2 2 0), (3 1 1), (4 0 0), (4 2 2), and (5 1 1) planes of cubic Fe3O4 phase. Further, broad XRD peaks suggest formation of nanosize particles. By using Debye Scherrer formula, the average crystallite size of Fe3O4 particles is estimated to be 11 nm. AFM and TEM images of sample confirm the formation of nearly spherical shape particle of size 10 nm (Fig. 2A and B). The room temperature magnetization curve of sample C1 shows superparamagnetic behavior (Fig. 3), indicating formation of single domain Fe3O4 particles. The saturation magnetization (Ms) value is found to be 45 emu/g with almost zero coercive field value (inset Fig. 3). These VSM results are in accordance with what should be

α

2.2. Synthesis of Fe3O4@Y2O3:Eu3+ core–shell nanoparticles For synthesis of core/shell materials, initially pre calculated amount of Fe3O4 nanoparticles was taken and dispersed in aqueous solution. This solution was sonicated for 10–15 min to obtain homogeneous dispersion and 1 M NaOH was added dropwise to obtain pH  10. A stock solution 0.1 M Y2(NO3)3 and 0.01 M Eu2(NO3)3 was prepared separately. This precursor solution was added slowly to alkaline solution of Fe3O4 dispersed nanoparticles and stirred continuously. In this process Y2O3 precipitates over the iron oxide nanoparticles. The as prepared colloids were separated by centrifuging at the speed of 4000 rpm (REMI-R23) and washed several times by distilled water to remove reaction remnants and then

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θ Fig. 1. XRD spectra of as prepared sample (C1) and the sample annealed at 400 °C and 600 °C in an air atmosphere.

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expected from 10 nm size spherical superparamagnetic Fe3O4 nanoparticles. 3.2. Characterization of core–shell nanostructures 3.2.1. FTIR Studies Representative FTIR spectra of as prepared sample CS1 and the sample annealed at different temperatures up to 800 °C are shown in Fig. 4. In case of as prepared sample, broad absorption peak at 3446 cm1 is observed due to OAH stretching vibrations. The other absorption peaks at 1515 cm1, 1386 cm1 and 845 cm1 in FTIR spectra are due to N@O asymmetric stretching vibration and NO 3 bending [22]. The intensity of these peaks are found to decrease with increase in annealing temperature and almost completely vanish in sample annealed at 800 °C. However, a new absorption peak at 442 cm1 appears in the sample annealed at 600 °C due to YAO stretching vibration of Y2O3 [23]. The intensity of this peak increased further when the sample was annealed at 800 °C. This may be attributed to crystallization of yttria cubic phase on annealing beyond 600 °C.

Fig. 2. (A) AFM image and (B) TEM image of as prepared Fe3O4 nanoparticles (C1).

3.2.2. Structural studies Fig. 5 shows XRD pattern of as prepared (CS1) samples and the samples annealed at different temperatures. A broad diffraction peak at 28° was observed for samples annealed up to 400 °C. On annealing the material at 600 °C, diffraction peaks corresponding to (2 2 2), (3 2 1), (4 1 1), (4 2 0), (3 3 2), (4 2 2), (4 4 0) and (1 4 5) planes of cubic yttria phase have been observed. Further, on annealing at 800 °C, the intensities of these peaks were found to increase indicating enhancement in the degree of crystallinity of the sample. However, it is interesting to note that diffraction peaks corresponding to core material (Fe3O4 cubic phase) were not observed in any XRD spectra shown in Fig. 5. Though the exact reason for non detection of XRD peak of iron oxide is not understood, however, it may be attributed to factors such as relatively very low concentration of iron oxide particles in comparison to Y2O3 and also they are covered by the shells of Y2O3. Further, we have tried to infer indirectly, the presence of core of iron oxide through effect of core material on

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Fig. 3. Room temperature M–H curve of as prepared Fe3O4 nanoparticles (C1) and the material annealed at 400 °C and 600 °C. Inset: zoomed M–H curves to show coercivity.

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Fig. 4. FTIR spectra of as prepared CS1and annealed CS1samples at different temperatures.

sample. The particle size of core–shell nanoparticles under a 90,000 magnification is observed as 30 nm. Bigger particles (>30 nm) are also found, which may be due to agglomerations of core–shell nanoparticles during SEM sample preparation. Fig. 7B shows representative TEM micrograph of sample CS1 annealed at 600 °C. Agglomeration of core–shell nanoparticles can also be seen in TEM image. It is thus inferred from SEM and TEM that magnetite nanoparticles were coated by Y2O3 shell and these core–shell nanoparticles may agglomerate during sample preparation of SEM and TEM. Further, EDAX analysis was employed to analyze the composition of core shell particles. The results are shown in Fig. 7C. The EDAX pattern shows characteristic peaks of elements Fe, Y, Eu and O are present in the measured sample. This result confirm the presence of oxides of yttrium and iron in the measured sample.

3.2.3. Scanning electron microscopy, TEM and EDAX studies A representative SEM micrograph of sample CS1 annealed at 600 °C is shown in Fig. 7A. The SEM image does not show any contrast between the core Fe3O4 and Y2O3:Eu3+ shell present in

3.2.4. Magnetic studies Field dependent magnetization (M–H) curves of as prepared samples CS1, CS2 and CS3 are shown in Fig. 8. These samples exhibit superparamagnetic behavior with negligible coercivity (inset

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structural properties of cubic yttria phase of shell. We have recorded the XRD spectra of peaks corresponding to (2 2 2) and (4 0 0) planes of annealed sample CS1 (Fe3O4@Y2O3:Eu3+ core–shell) and pure Y2O3:Eu3+ nanophosphor for longer duration to obtain higher resolution XRD spectra as shown in Fig. 6. The peak position of Fe3O4@Y2O3:Eu3+ core–shell materials was found shifted towards lower diffraction angle with respect to pure yttria phase. The observed shift resulted due to tensile stress that acts on Y2O3:Eu3+ layer due to its coating over Fe3O4 nanoparticles [24]. These results suggest the presence of iron oxide nanoparticles as core material. Similar observations have also been recorded for samples CS2 and CS3. Though, CS2 and CS3 sample has the higher concentration of Fe in comparison to CS1 but it is also covered by Y2O3 shell and may be this concentration of Fe is not high enough to get XRD peak.

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Fig. 8) at room temperature. Magnetization values of core–shell materials are in the range of 0.2–0.8 emu/g, which is smaller than core Fe3O4 nanoparticles and can be result of the magnetically dead layer of Y2O3:Eu3+ at the nanoparticles surface. The increase of magnetization values from sample CS1–CS3 can be attributed to the increase in relative concentration of Fe3O4 nanoparticles. The mean size of core–shell materials has been calculated from magnetization curve. The slope of magnetization curve near applied magnetic field H ? 0 is used to calculate magnetic particle diameter of superparamagnetic materials [25–28]. In order to

calculate the diameter of core shell particle, we have used the Eq. (1) as given below

      lH KBT lH  ¼ Ms  L M ¼ M s coth KBT KBT lH

ð1Þ

where M is magnetization of superparamagnetic, H is applied filed, L is the Langevin function, Ms is the saturation magnetization of the bulk material, l is the magnetic moment of the particles, KB is Boltzman’s constant, and T is the absolute temperature in Kelvin. The magnetic moment of the particle l is related to the particle volume (v) by l = Msv. If sample is measured in the superparamagnetic state then the magnetization is simply given by the integration of the Langevin function (Eq. (1)) for each particle size in the distribution. In the present case, we assumed Schulz size distribution of particles [29]. To fit the magnetization curves, we assume that all the particles are spherical without mutual interaction and each particle has an inner single-domain core with the spontaneous magnetization Msbulk of the bulk material and outer nonmagnetic shell of susceptibility vpm and thickness dshell. The final fitted magnetization (M) has the form of the following equation.

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 þ v s vpm H dv

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3 where v ¼ ðp=6Þdpartilce ¼ ð4=3ÞpR3 , vc = (4/3)p(R - dshell)3, and vs = (4/3)p{(R)3  (R  dshell)3} = v - vc are the volumes of particle,

Fig. 7. (A) SEM image (inset figure shows magnified image) (B) TEM image and (C) EDAX of CS1 (Fe3O4@Y2O3:Eu3+ core–shell) sample annealed at 600 °C.

core, and shell, respectively of diameter d particle with radius R and Ms bulk = 92.7 emu/g. Magnetization curves for as prepared sample C1 and 600 °C annealed samples of CS1, CS2 and CS3 were fitted by the integration of the Eq. (2) for each particle size in the distribution using a leastsquares routine. Magnetization curves along with fitted lines are shown in Fig. 9. The optimum fitted parameters are also shown in Fig. 9. The diameter of pure Fe3O4 nanoparticles estimated from the magnetization curve is 10.83 nm which is in well agreement with XRD, TEM and AFM results. And the mean size of core of all the core shell nanoparticles is 10 nm. Further, mean thickness of nonmagnetic shells has been estimated to be 0.83, 11 nm, 9.5 nm and 7 nm for samples C1, CS1, CS2 and CS3 respectively. Presence of nonmagnetic shell in magnetic seed particles (C1) can be attributed to the canted spins in the surface layers due to a decrease in the exchange coupling which is caused by the lack of oxygen mediating super-exchange mechanism between nearest iron ions at the surface. Thus magnetically dead surface layers can appear in magnetic nanoparticles [30]. Estimated nonmagnetic shell in bare magnetic nanoparticles (C1) is 0.83 nm, which is consistent with the reported nonmagnetic shell thickness of similar type of nanoparticles [28]. Further, the thick nonmagnetic layer Y2O3:Eu3+ in samples CS1, CS2, and CS3 can account for the decrease of saturation magnetization down to 0.2–0.8 emu/gram in comparison to the 45 emu/g of seed magnetic nanoparticles. Shell thickness is also consistent with thickness estimated from SEM. The schematic of Fe3O4@Y2O3:Eu3+ core shell particle is represented in Fig. 10. As shown earlier (Fig. 1), the as-synthesized Fe3O4 nanoparticle (C1) can be transformed easily into a-Fe2O3 by annealing at 600 °C in air. The magnetization curves of the representative samples (CS2) of Fe3O4@Y2O3:Eu3+ core shell nanoparticles annealed at different temperatures are shown in Fig. 11. Other two samples CS1 and CS3 also exhibit similar magnetic behavior while annealing. As shown in Fig. 11, sample (CS2) annealed up to 600 °C exhibit superparamagnetic behavior (inset1 Fig. 11) with almost constant saturation magnetization. Further increase in annealing temperature, the saturation magnetization value is found to decrease. Interestingly, sample annealed at 800 °C shows significant decrease of magnetization value and exhibit almost paramagnetic

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Fig. 9. Magnetization curves of (A) as prepared sample C1, with annealed samples at 600 °C (B) CS1, (C) CS2 and (D) CS3 are curve fitted by Eq. (2). Fitted curve is indicated by red line in all the samples.

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behavior (inset2 Fig. 11). The observed change in superparamagnetic to paramagnetic behavior of the samples by annealing in air atmospheres at 800 °C is due to O2 oxidation of cubic Fe3O4 to rhombohedral nonmagnetic a-Fe2O3 phase. There may be possibility of thermal diffusion of oxygen through shell material during annealing in air atmospheres. Although, in case of core magnetic nanoparticles (C1), complete phase transformation from Fe3O4 to a-Fe2O3 was observed at annealing temperature 600 °C. We believe that the Y2O3:Eu3+ layer protects the magnetite structure up to higher annealing temperatures due to non availability of oxygen within Fe3O4 core of core shell structure compared to bare Fe3O4 seed particle. This observation again suggests that coating of Y2O3:Eu3+ has been accomplished on Fe3O4 nanoparticles and the presence of Y2O3:Eu3+ shell causes the retention of superparamagnetic behavior even after annealing at 600 °C.

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Fig. 12. PL Emission spectra of as prepared and annealed samples (CS1) at different temperatures. Inset figure shows the PL Emission spectra of samples annealed upto 400 °C.

Samples annealed at 600 °C shows significant increase of emission intensity in comparison to as prepared samples. The increase of emission intensity has resulted due to the formation of proper cubic yttria phase on surface of Fe3O4 as observed in the XRD spectra reported earlier in the paper. The lower and higher side band peaks are found more resolved in samples annealed at 600 °C and 800 °C. The emission peaks observed in Y2O3:Eu3+ nanophosphors are due to transitions from the excited 5D0 level to 7FJ (J = 0–4) levels of the Eu3+ ion sitting at yttria crystal field [31]. The high intensity of strongest PL emission peak at 612 nm is due to forced electronic dipole(ED) 5D0—7F2 transitions of Eu3+ ions in C2 symmetry

3.2.5. Photoluminescence (PL) studies The PL emission spectra of as prepared and annealed CS1 samples are shown in Fig. 12. Samples annealed upto 400 °C are also shown in inset of Fig. 12. The as-prepared samples show weak emission at 612 nm, however, samples annealed at 300 °C and 400 °C show lower side peaks at 582 nm and 593 nm along with higher side emission peak at 624 nm. Further increase in annealing temperature causes intensities of all emission peaks to increase.

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Fig. 13. Photograph of bright red luminescence from Fe3O4@Y2O3 Core Shell (CS1) nanoparticles under 254 nm UV excitation. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

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by using wet chemical method. For the synthesis work, aqueous dispersible pure Fe3O4 core materials of size 10 nm have been prepared initially with superparamagnetic characteristics. Different thickness of luminescent material coating has been obtained by varying core material concentrations during the reaction. FTIR studies indicate the crystallization of luminescent shell material on annealing of as synthesized sample at 600 °C. Pure cubic phase of Y2O3:Eu3+ has been established from XRD spectra. It is inferred from SEM, TEM, and VSM that magnetite nanoparticles were coated by Y2O3. From EDAX analysis, it was established that all constituent elements O, Y, Fe and Eu are present in synthesized core–shell nanoparticles. VSM studies of these materials show superparamagnetic behavior at room temperature and their magnetic properties are changed after annealing at higher temperature. These materials also exhibit bright red emission with peak maximum at 612 nm under UV excitation. The studies thus provide a novel scalable route for the synthesis of core–shell nanoparticles showing interesting combination of magnetic and luminescent properties highly useful for multimodal applications in field of biomedicine. References

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Fig. 14. PL spectra of CS1, CS2 and CS3 samples annealed at 600 °C.

incorporated in Y2O3 lattice [32]. In addition, the other energy transitions due to europium ions corresponding to 5D0—7F0 (582 nm), 5 D0—7F1 (587 nm, 593 nm and 599 nm), 5D0—7F2 (near 628 nm) are observed in the emission spectra. The relative PL intensities among these above mentioned emission peaks depend on local symmetry of the Eu3+ ion in the host Y2O3, which can be described in terms of Judd–Ofelt theory [33,34]. The overall enhancement of emission intensity at higher annealing temperatures can be attributed to improved crystallinity of the sample since f–f transitions of Eu3+ is better protected due to improved crystal field of yttria [30,35]. Photograph of bright red luminescence from Fe3O4@Y2O3 Core Shell (CS1) nanoparticles under 254 nm UV excitation is shown in Fig. 13. In order to study the effect of Y2O3:Eu3+ coating thickness on luminescence properties of core shell materials, the emission and excitation spectra of all three samples (CS1, CS2 and CS3) annealed at 600 °C are shown in Fig. 14. The PL intensity of both excitation and emission decreases with increase in Fe3O4 concentrations. The decrease of emission intensity can be attributed to the reduction of coating thickness of Y2O3:Eu3+ layer over Fe3O4 nanophosphors as was shown from magnetization studies. 4. Conclusions In summary, we have successfully synthesized multifunctional magneto-luminescent Fe3O4@Y2O3:Eu3+ core–shell nanoparticles

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