DSpace at VNU: Luminescent core-shell Fe3O4 Gd2O3:Er3+, Li+ composite particles with enhanced optical properties

J Sol-Gel Sci Technol DOI 10.1007/s10971-014-3382-9 ORIGINAL PAPER Luminescent core–shell Fe3O4@Gd2O3:Er3+, Li+ composite particles with enhanced optical properties Hong Ha Thi Vu • Timur Sh Atabaev • Nang Dinh Nguyen • Yoon-Hwae Hwang Hyung-Kook Kim • Received: January 2014 / Accepted: 29 April 2014 Ó Springer Science+Business Media New York 2014 Abstract Bifunctional magneto-optical nanocomposites with Fe3O4 nanoparticles as a core and erbium and lithium codoped gadolinium (Gd2O3:Er3?, Li?) as the shell were synthesized successfully using a simple urea homogeneous precipitation method The fabricated Fe3O4@Gd2O3:Er3?, Li? particles were characterized by X-ray diffraction, field emission scanning electron microscopy, transmission electron microscopy, photoluminescence spectroscopy and quantum design vibrating sample magnetometry The upconversion emission intensity was enhanced significantly comparing to that without Li? ions These bifunctional composites are expected to be potentially applied for drug delivery, cell separation and bioimaging Keywords Luuminescence Á Bifunctional nanocomposites Á Core–shell structure H H T Vu Á T Sh Atabaev Á Y.-H Hwang (&) Á H.-K Kim (&) Department of Nanomaterials Engineering and BK 21 Nano Fusion Technology Division, Pusan National University, Miryang 627-706, Republic of Korea e-mail: yhwang@pusan.ac.kr H.-K Kim e-mail: hkkim@pusan.ac.kr N D Nguyen Department of Semiconducting Nanomaterials and Devices, Faculty of Engineering Physics and Nanotechnology, University of Engineering and Technology, Vietnam National University Hanoi, 144 Xuan Thuy Street, Cau Giay, Hanoi, Vietnam Introduction Over the past decade, bifunctional composite materials with both fluorescent and magnetic properties have been applied widely in biological and chemical fields, such as biomedical multitasking, magnetic biological separation, targeted-drug delivery and cell imaging [1–7] Iron oxide is considered as an ideal candidate for bio-applications due to their special magnetic properties, lack of toxicity and good biocompatibility [8–10] Recently, a range of bifunctional magnetic properties with luminescence nanophosphors with Fe3O4 nanoparticles as the core and the phosphor materials as the shell have been studied [10–16] Yu.et al [17] synthesized bifunctional magnetic-optical Fe3O4@SiO2/Y2O3:Yb3?, Er3? particles using a layer-by-layer method Wang et al [18] prepared Fe/Fe3O4@Y2O3:Eu nanocomposites and Yang et al [19] synthesized multifunctional Fe@C@Y2O3:Eu3? nanoparticles using a solvothermal method The synthesis process in these methods, however, was relatively complex and expensive Down-conversion materials display some intrinsic limitations (e.g the quantum dots are toxic and organic dyes exhibit broad absorption and emission bands and low photostability), whereas, up-conversion (UPC) materials have superior photostability and low toxicity Among the various of UPC materials, gadolinium oxide (Gd2O3)doped Er3? ions have attracted increasing attention Gd2O3 is a good host matrix for the UPC luminescence process because of its interesting physical properties, such as chemical durability, high melting point (*2,320 °C), thermal stability and low phonon energy (phonon cutoff *600 cm-1) Also the Gd2O3 doped with Er3? has long been reported to be a strong green phosphor [20] On the other hand, Gd2O3 based phosphors show a weaker stability against atmospheric H2O and CO2, which are known 123 J Sol-Gel Sci Technol as luminescence killers [21] Therefore, it is important to enhance the luminescent properties of Gd2O3-based phosphors It is well know that the UPC emission intensity of luminescent materials is dependent on their intra 4f transition probabilities, which are significantly affected by their crystal field symmetry [22] Therefore, incorporation of additional metal ions to the phosphors can be considered an effective way to improve the luminescence Among them, the Li? ion has a smaller cationic radius than other ions Therefore, it can be doped easily into the host lattice, either substitutionally or interstitially, and break the symmetry of the crystal field around the rare-earth element, leading to an increase in photoluminescence intensity [22–24] For example, Chen et al reported the enhancement of UPC radiation in Y2O3:Yb3?/Er3? nanocrystals by codoping with Li? ions [25] Qian Cheng et al reported a 47 fold increase in visible green UPC emission in Yb3? and Er3? codoped NaGdF4 nanoparticles by introducing Li? ions [26] This study reports the synthesis and properties of bifunctional optical-magnetic Fe3O4@Gd2O3:Er3?:Li? particles with Fe3O4 nanoparticles as the core, and Gd2O3 codoped Er3? and Li? as the shell using a facile urea homogeneous precipitation method The relationship between fluorescent and magnetic properties is discussed The results revealed a significant increase in the UPC emission in shell Gd2O3:Er3? layer by introducing Li? ions The combined fluorescent and magnetic properties of the nanoparticles highlight their potential applications in bioseparation, drug delivery and luminescence labeling Experimental 2.1 Reagents Ferrous chloride tetrahydrate (FeCl2Á4H2O), ferric chloride (FeCl3, 97 %), polyethylene glycol (PEG, MR = 4,600), sodium acetate (CH3COONa, 99.0 %), ammonium hydroxide (28–30 % NH3 basis), gadolinium (III) oxide (Gd2O3, 99.99 %), erbium oxide (Er2O3, 99.99 %), nitric acid (HNO3, 70 %), urea ((NH2)2CO, 99–100.5 %), and ethanol were purchased from Sigma-Aldrich and used as received Purified deionized (DI) water was used in all experiments 2.2 Synthesis of Fe3O4 nanoparticles Fe3O4 nanoparticles were prepared using the reported coprecipitation method [26] Firstly, FeCl3 (1.625 g) and FeCl2Á4H2O (0.955 g) at 2:1 molar ratio were dissolved in 50 ml of DI water at 70 °C PEG can displace H2O, [OH-] on the surface of particles, generating a sticky layer by 123 adhering to them through coordination bond or hydrogen bond, reducing the surface free energy, preventing agglomeration from small particles to big particles, and dispersing particles stably in base solution Therefore, PEG (1 g) was then added as a surfactant and the resulting mixture was stirred of 30 until complete dissolution The iron solution source was added slowly to a NH4OH solution with vigorously stirring for 45 The black Fe3O4 precipitates were separated and rinsed several times with DI water The precipitates were dried in a vacuum oven at 50°C for 24 h, and Fe3O4 nanoparticles were finally obtained 2.3 Synthesis of bifunctional Fe3O4@Gd2O3:Er3?:Li? nanocomposites Gd2O3 codoped with Er3? and Li? phosphor was coated on the magnetic Fe3O4 using an urea homogeneous precipitation method [27, 28] In a typical procedure, appropriate amount of Gd2O3, Er2O3 and LiOHÁ4H2O were dissolved in nitric acid to form 0.001 mol of sample with composition of Gd2O3:1 % Er3?,10 % Li? At the same time 0.001 mol of another sample with composition of Gd2O3:1 % Er3? was prepared and used as a reference The solutions were dried at 70°C for day to remove the excess of nitric acid After cooling to room temperature, samples were mixed with DI water (40 ml) and urea (0.5 g), and stirred vigorously for 10 to form a clear solutions Subsequently, Fe3O4 nanoparticles (70 mg for each sample) were then added to the above solutions The mixtures were sonicated for 30 and heated to 90 °C for h under vigorous stirring The resulting suspensions were centrifuged at 7,000 rpm for 45 The precipitates were washed times with ethanol and DI water and dried overnight at 75 °C under vacuum Finally, the precipitates were calcined at 700 °C in air for h to produce final structures 2.4 Characterization The crystal phase of the prepared samples was characterized by X-ray power diffraction (XRD, Bruker D8 Discover) using Cu Ka (k = 0.15405 nm) radiation within the range of 20–60° 2h The morphology and composition of the samples were examined by transmission electron microscopy (TEM, JEOL JEM-2100F) The UPC emission spectra of the samples were recorded using a Hitachi F-7000 spectrophotometer using a commercially available power adjustable NIR (975 nm) diode laser The magnetization measurements were performed using a quantum design vibrating sample magnetometer (QD-VSM option on a physical properties measurement machine PPMS 6000) All measurements were carried out at room temperature J Sol-Gel Sci Technol Scheme Illustration of the formation of Fe3O4@Gd2O3:1 % Er3?, 10 % Li? NPs 3? Fig The XRD patterns of Fe3O4, Fe3O4@Gd2O3:1 % Er and Fe3O4@Gd2O3:1 % Er3?, 10 % Li? composites calcinated at 700 °C Results and discussion 3.1 Crystal structure and morphology The synthetic procedure is schematically illustrated at Scheme The Fe3O4 nanoparticles were firstly prepared by using a co-precipitation method as the core, and then Gd2O3 codoped with Er3? and Li? phosphors shell layer was coated on surface of the Fe3O4 nanoparticles The structure of the synthesized samples was examined by XRD Figure shows the XRD patterns of Fe3O4, Fe3O4@Gd2O3:1 % Er3? and Fe3O4@Gd2O3:1 % Er3?, 10 % Li? nanocomposites after calcination at 700 °C The XRD peaks for Fe3O4 were indexed to the face-centered (Fd3m space group) cubic structure of Fe3O4 (JCPDS 19-0629) [29] In the case of Fe3O4@Gd2O3:1 % Er3? and Fe3O4@Gd2O3:1 % Er3?, 10 % Li? nanocomposites exhibited the characteristic Gd2O3 diffraction peaks (JCPDS no 88-2165) and additional peaks that coincide with the strongest {311} reflection peak of the cubic face-centered Fe3O4 (marked with black star) No additional peaks from the doped components were detected due to the low concentration of codopant ions Furthermore, all the diffraction peaks of the samples were intense and sharp, indicating that the final product with high Fig TEM images of Fe3O4@Gd2O3:1 % Er3?:10 % Li? composites calcinated at 700 °C crystallinity was obtained using this method, which is very important for the luminescence properties of the composites The well-known Debye–Scherrer’s equation was used to calculate the mean crystallite sizes of the samples The calculated mean crystallite sizes of the composites without and with Li? codoping were approximately 46.77 and 51.47 nm, respectively, indicating an increase in crystallite size upon Li? codoping The morphology and structure of the obtained products were examined by TEM study Figure revealed a core– shell structure of the Fe3O4@Gd2O3:1 % Er3?, 10 % Li? particles The Fe3O4 cores were black spheres with a mean size of approximately 30 nm, and the Gd2O3:1 % Er3?, 10 % Li? shell showed a gray color with a mean thickness of 15 nm The similar core–shell structure was also observed in the case of Fe3O4@Gd2O3:1 % Er3? composites (not shown) 3.2 Optical properties Figure presents the room temperature up-conversion UPC luminescence spectra of Fe3O4@Gd2O3:1 % Er3? and Fe3O4@Gd2O3:1 % Er3?, 10 % Li? nanocomposites under a 975 nm NIR laser with a 125 mW excitation power Emission bands at 482–494 nm (blue), 512–581 nm (green) and 123 J Sol-Gel Sci Technol Fig Up-conversion luminescence spectra of Fe3O4@Gd2O3:1 % Er3? and Fe3O4@Gd2O3:1 % Er3?, 10 % Li? composites excited with a 975 nm NIR laser at 125 mW pump power Fig Dependence of upconversion emission intensity of Fe3O4@Gd2O3:1 % Er3?, 10 % Li? sample on the excitation power 650–691 nm (far-red) were assigned to the 4F7/2 ? 4I15/2, H11/2; 4S3/2 ? 4I15/2, and 4F9/2 ? 4I15/2 transitions, respectively The emission intensity of sample codoped with Li? was enhanced strongly (3.3 times) compared to that without Li? ions The enhanced UPC emission intensity may be because doping with a small radius of Li? ion can diffuse easily to sites in/near Er3? or to the interstitial sites in the Gd2O3 host lattice Thus, the banned electric-dipole transitions of Er3? can be allowed for Li?-tailored phosphors since the inversion symmetry can be destroyed easily by the interstitial Li? ions in the Gd2O3 host lattice [26] On the other hand, Li? codoping improves the crystallinity significantly and produces a larger crystallite size [22] Thus, the distortion of local symmetry around Er3? and enhanced crystallinity of the composites favors the enhanced luminescence emission Figure shows the room temperature up-conversion UPC luminescence emission spectra of Fe3O4@Gd2O3:1 % Er3?, 10 % Li? composites in a powder under a 975 nm NIR laser at excitation powers ranging from 20 to 160 mW The overall emission intensity was increased with increasing of the laser output power It is well-known that the intensity of UPC photoluminescence, IUPC, for an unsaturated mechanism is proportional to the power of n of the excitation intensity, IEx: respectively, which are close to This suggests that two-photons were needed to populate the higher lying levels of Er3? n IUPC / IEx where n = 2, 3,… is the number of pump photons absorbed per upconverted photon emitted A plot of logIUPC versus logIEx yielded a straight line with a slope n The slopes were equal to 1.57, 1.84, and 1.56 for blue (4F7/2 ? 4I15/2), green (2H11/2, 4S3/2 ? 4I15/2) and red emission (4F9/2 ? 4I15/2), 123 3.3 Magnetic properties The magnetic properties of the pure Fe3O4 and Fe3O4@Gd2O3:1 % Er3?:10 % Li? composite particles were examined using a QD-VSM at room temperature as it shown in Fig Both samples displayed super-paramagnetic properties The saturation magnetization (Ms) value of pure Fe3O4 was equal to 44 emu/g After coating with the phosphor shell layer, the saturation magnetic value decreased to 4.89 emu/g This value was much lower than for pure Fe3O4 due to paramagnetic nature of the shell coating The inset in Fig 5b shows the magnetic separation of an aqueous dispersion of the Fe3O4@Gd2O3:1 % Er3?, 10 % Li? nanocomposites The sample can be quickly directed and accumulated to the side of the glass vial near the magnet This suggests that the nanocomposite particles exhibit both magnetic and optical properties Therefore, sinthesized Fe3O4@Gd2O3:1 % Er3?, 10 % Li? nanocomposites have promising potential in many areas, including targeting, separation, visual tracking, etc Conclusions Bifunctional nanocomposites with Fe3O4 particles as the core and Er3?, Li? codoped Gd2O3 as the shell were synthesized successfully using a facile and inexpensive urea homogeneous precipitation method The overall UPC J Sol-Gel Sci Technol Fig Magnetic hysteresis loops of Fe3O4 (a) and Fe3O4@Gd2O3:1 % Er3?:10 % Li? (b) nanoparticles measured at 300 K The insets show the photographs of magnetic Fe3O4@Gd2O3:1 % Er3?:10 % Li? nanocomposites dispersed in aqueous solution (left) without and (right) with external magnetic field luminescence was enhanced significantly due to Li? codoping Strong green emission due to the 2H11/2 ? 4I15/2 transition in Er3? was observed upon 975 nm NIR laser excitation of the sample The magnetic properties of the Fe3O4@Gd2O3:1 % Er3?, 10 % Li? composites were also evaluated These bifunctional nanocomposites are expected to have promising applications in fluorescence labeling, drug delivery, cell separation and diagnostic analysis 10 Leng Y, Sato K, Shi Y, Li J-G, Ishigaki T, Yoshida T, Kamiya H (2009) J Phys Chem C 113:16681–16685 11 Peng H, Liu G, Dong X, Wang J, Yu W, Xu J (2012) Powder Technol 215–216:242–246 12 Atabaev TS, Kim HK, Hwang YH (2013) Nanoscale Res Lett 8:357 13 Liu D, Zhao D, Shi F, Zheng K, Qin W (2012) Mater Lett 85:1–3 14 Shi J, Tong L, Ren X, Li Q, Ding H, Yang H (2013) Mater Chem Phys 139:73–78 15 Shi J, Tong L, Ren X, Li Q, Yang H (2013) Ceram Int 39:6391–6397 16 Peng H, Cui B, Li L, Wang Y (2012) J Alloys Comp 531:30–33 17 Yu XG, Shan Y, Li GC, Chen KZ (2011) J Mater Chem 21:8104–8109 18 Wang Q, Yang X, Yu L, Yang H (2011) J Alloy Comp 509:9098–9104 19 Yang J, Yang X, Yang H (2012) J Alloy Comp 512:190–194 20 Xu L, Yu Y, Li X, Somesfalean G, Zhang Y, Gao H, Zhang Z (2008) Opt Mater 30:1284–1288 21 Li J-G, Li X, Sun X, Ishigaki T (2008) J Phys Chem C 112:11707–11716 22 Atabaev TS, Piao Z, Hwang Y-H, Kim H-K, Hong NH (2013) J Alloy Comp 572:113–117 23 Huang Q, Yu J, Ma E, Lin K (2010) J Phys Chem C 114:4719–4724 24 Ajmal M, Atabaev TS (2013) Opt Mater 35:1288–1292 25 Chen G, Liu H, Liang H, Somesfalean G, Zhang Z (2008) J Phys Chem C 112:12030–12036 26 Cheng Q, Sui J, Cai W (2012) Nanoscale 4:779–784 27 Thi H-HV, Atabaev TS, Kim Y-D, Lee J-H, Kim H-K, Hwang Y-H (2012) J Sol–Gel Sci Technol 64:156–161 28 Atabaev TS, Vu H-HT, Kim Y-D, Lee J-H, Kim H-K, Hwang Y-H (2012) J Phys Chem Solids 73:176–181 29 Guo S, Li D, Zhang L, Li J, Wang E (2009) Biomaterials 30:1881–1889 Acknowledgments This work was supported by the National Research Foundation of Korea (Grant No 2012R1A1B3001357) References Corr SA, O’Byrne A, Gun’ko YK, Ghosh S, Brougham DF, Mitchell S, Volkov Y, Prina-Mello A (2006) Chem Commun 43:4474–4476 Hu J, Xie M, Wen C-Y, Zhang Z-L, Xie H-Y, Liu A-A, Chen Y-Y, Zhou S-M, Pang DW (2011) Biomaterials 32:1177–1184 Chen K, Conti PS (2010) Adv Drug Deliv Rev 62:1005–1022 Levy L, Sahoo Y, Kim K-S, Bergey E, Prasad PN (2002) Chem Mater 14:3715–3721 Wang Z, Wu L, Chen M, Zhou S (2009) J Am Chem Soc 131:11276–11277 Hu S-H, Tsai C-H, Liao C-F, Liu D-M, Chen S-Y (1818) Langmuir 24(2008):11811–11818 Shen J-M, Xu L, Lu Y, Cao H-M, Xu Z-G, Chen T, Zhang H-X (2012) Int J Pharm 427:400–409 Sun P, Zhang H, Liu C, Fang J, Wang M, Chen J, Zhang J, Mao C, Xu S (2010) Langmuir 26:1278–1284 Selvan ST, Tan TTY, Yi DK, Jana NR (2010) Langmuir 26:11631–11641 123 ... solution with vigorously stirring for 45 The black Fe3O4 precipitates were separated and rinsed several times with DI water The precipitates were dried in a vacuum oven at 50°C for 24 h, and Fe3O4. .. carried out at room temperature J Sol-Gel Sci Technol Scheme Illustration of the formation of Fe3O4@ Gd2O3:1 % Er3?, 10 % Li? NPs 3? Fig The XRD patterns of Fe3O4, Fe3O4@ Gd2O3:1 % Er and Fe3O4@ Gd2O3:1... displayed super-paramagnetic properties The saturation magnetization (Ms) value of pure Fe3O4 was equal to 44 emu/g After coating with the phosphor shell layer, the saturation magnetic value decreased