Magnetic and fluorescent materials are of great importance in the fields of chemistry, biology, and medical sciences as well as in biotechnology, such as biomolecule detection, bioimaging, controlled drug release, targeted temperature sensing applications, and so on[1-5]. Recently, there has been growing research interest in magnetic and fluorescent materials with the combination of magnetic and fluorescent nanoparticles(NPs) into new bifunctional nanoclusters(NCs)[6-12]. In contrast to their limited single component counterparts, the designed bifunctional NCs allow not only for external manipulation with a magnetic field but also for real time visualization with fluorescence imaging techniques.
Ferroferric oxide(Fe3O4) is a preferable magnetic material owing to its superparamagnetism, low toxicity, and good biocompatibility for biological applications, such as cell separation, gene or drug delivery, stem cell labeling, and magnetic-resonance imaging[13-16]. Fluorescent materials, such as organic dyes and semiconductor quantum dots(QDs), have been extensively used in multimodal imaging, in vitro tracking of cells, bio-imaging applications, and so forth[17-21]. However, organic dyes which exhibit potential toxicity, are pregnable to photo-bleaching and photolysis, and suffer from poor photochemical stability[22]. QDs are issues of toxicity resulting from the release of heavy metallic ions, less chemically stable, and show fluorescence intermittence[7]. These disadvantages would seriously hinder their application in the biomedical field, especially for the use in the human body. Lanthanide-doped NPs have attracted growing attention owing to their admirable properties, such as excellent chemical stability, low toxicity, large Stokes shifts, narrow line-width emission bands, long lifetimes and superior photostability[23-27]. Compared with above-mentioned Ln3+-doped inorganic NPs, ZrO2, as an oxide host for Ln3+ doping, which has low phonon energy(470 cm-1) and high host absorption coefficient[28-29], is considered to be a potential host matrix for the design and fabrication of efficient photosensitive materials. Ninjbadgar et al. prepared fluorescent ZrO2∶Eu3+ NPs through a one-step solvothermal process with a mean size of 4 nm which were observed at an optimum dopant molar fraction of 11% could be used for the holographic fabrication of volume diffraction elements with light emissive properties[30]. Liu et al. reported a new inorganic oxide bioprobe based on sub-5 nm amine-functionalized tetragonal ZrO2∶Ln3+ NPs synthesized via a facile solvothermal method combined with a ligand exchange procedure, which were used as luminescent bioprobes in a TR-FRET assay of avidin with a detection limit down to 3 nmol/L[31].
To the best of our knowledge, there is no report on the combination of magnetic particles with ZrO2∶Ln3+ NPs. In this work, we focused our attention towards the design of magnetic and fluorescent NPs consisting of magnetic cores of silica coated ferroferric oxide(Fe3O4@SiO2) and fluorescent shells of terbium-doped zirconia(ZrO2∶Tb3+) as illustrated in Scheme 1. To begin with, Fe3O4 was synthesized by the facile solvothermal method. In order to reduce fluorescent quenching, SiO2 layer was encapsulated on the Fe3O4 NPs via a typical modified St ber sol gel process. Afterwards, ZrO2∶Tb3+ was coated through the in-situ solvothermal method to prepare water-soluble Fe3O4@SiO2@ZrO2∶Tb3+ NCs.
Zirconium(IV) propoxide propanol(mass fraction of 70%) was purchased from Aladdin. Diethylene glycol(DEG) and tetraethyl orthosilicate(TEOS) were purchased from Sinopharm Chemical Reagent Co., Ltd. FeCl3·6H2O, ethylene glycol(EG) and aqueous ammonia(25%~28%) were purchased from Beijing Chemical Reagent Co. Benzyl alcohol, Na3Cit and anhydrous NaOAc were purchased from Beijing Yili Fine Chemicals Co., Ltd. All of this chemicals were of analytical grade reagents and used directly without further purification. Tb(CH3COO)3·6H2O was prepared by dissolving corresponding oxides in ethanoic acid and heating.
The morphology of the samples was observed with a scanning electron microscope(SEM, S-4800, Hitachi, Japan). X-ray powder diffraction(XRD) measurements were performed on a D8 Focus diffractometer(Bruker, German) at a scanning rate of 2.5°/min in the 2θ range from 20° to 70°, with using of Cu-Kα radiation(λ=0.154 05 nm). FTIR spectra was measured using a Nicolet 6700 IR Fourier spectrometer(FT-IR, Thermo Fisher Scientific, America) equipped with smart iTR FTIR attachment in the range from 500 to 4000 cm-1.
The magnetization curves of the products were recorded on a Quantum Design MPMS-XL7 SQUID magnetometer of America equipped with a 7 T magnet. The magnetization was measured on the magnetic field range of -7000~7000 Oe at 300 K.
The photoluminescence(PL) excitation and emission spectra were recorded with a Hitachi F-7000 spectrophotometer of Japan equipped with a 150 W xenon lamp as the excitation source.
Fe3O4 NPs were prepared by the solvothermal method. FeCl3·6H2O(0.54 g) was dissolved in a mixture solvent consisted of EG(5 mL) and DEG(5 mL) to form a clear solution. Then, a certain amount of Na3C it was added, stirring for a time to form transparent solution. A given mass of anhydrous NaOAc was dissolved in the above solution. The homogeneous mixture was transferred into a Teflon-lined autoclave and heated at 230 ℃ for 10 h. The synthesized mixture was then allowed to cool naturally to room temperature. The black products were washed with ethanol and distilled water for three times separately. At last, the Fe3O4 NPs were redispersed in ethanol for further use.
Fe3O4@SiO2 NPs were composited by a typical modified St ber sol gel process. To investigate the thickness of SiO2 layer on the Fe3O4 is tunable, the cladding process was given into three groups. (i)Above-mentioned Fe3O4 particles were dispersed in the mixture solution of H2O(40 mL) and ethanol(136 mL). The mixture solution was sonicated for 15 min. After that, 4.5 mL aqueous ammonia was added dropwisely to the solution under sonication. After sonication for about 15 min, 0.80 mL TEOS dispersed in ethanol(24 mL) was then added drop by drop to the solution under ultrasonication. The resulting solution was sonicated for 90 min at room temperature. The obtained particles were washed with deionized water and ethanol for three times separately. At last, the synthetic Fe3O4@SiO2 NPs were redispersed in ethanol for further use. (ii)The procedures were similar except that the amount of aqueous ammonia and TEOS were replaced by 6.0 mL and 0.72 mL separately. (iii)The amount of aqueous ammonia and TEOS were replaced by 6.0 mL and 0.80 mL separately.
Fe3O4@SiO2@ZrO2∶Tb3+ NPs were synthesized according to the in-situ solvothermal method. In brief, zirconium(IV) propoxide propanol(0.0785 mL) was added in the benzyl alcohol(10 mL). Then, according to stoichiometric of Tb(CH3COO)3·6H2O was dissolved in the above mixed solution. A balanced amount of Fe3O4@SiO2 NPs were put in the above-mentioned transparent solution under ultrasonication to form homogeneous solution. Subsequently, the homogeneous solution was transferred into a Teflon-lined autoclave and heated at 230 ℃ for 72 h. When the solution cooled to room temperature naturally, the gray-black granules were washed with ethanol for three times. The obtained Fe3O4@SiO2@ZrO2∶Tb3+ NPs were redispersed in ethanol.
The morphology of the composites was characterized by SEM images in Fig. 1. As shown in Fig. 1A, the Fe3O4 NPs which can be dispersed perfectly in polar solvent such as ethanol have a mean diameter(D) of about 200 nm, possess a rough surface, and behave well monodispersity. The Fe3O4 microspheres were dispersed in a mixed solution containing TEOS, followed by the formation of a SiO2 shell (Fig. 1D~F) as a result of the hydrolysis of alkyl silicates. The mean diameter of Fe3O4@SiO2 in Fig. 1D~F is 225 nm, 250 nm, and 300 nm, respectively. Hence, in a certain concentration range, the larger of the amount of NH4OH and TEOS, the thicker of the silica layer under the same Fe3O4 NPs concentration. The iron oxide cores have a serious quenching effect on the ZrO2∶Tb3+ fluorescence NPs[32]. Hence, thicker silica could enhance the fluorescent intensity by enlarging the distance between the ZrO2∶Tb3+ NPs and iron oxide, whereas the increase in silica would result in weakened magnetic property due to the diamagnetic contribution of the silica layer[33-35] (inset Ⅰ of Fig. 5). At last, the diameter of the thickness of silica layer was adjusted to 25 nm(Fig. 1B). The surface of Fe3O4@SiO2 NPs is smooth relatively. Thus, it can be seen that, the Fe3O4 NPs were encapsulated excellently to obtain Fe3O4@SiO2 NPs. Fig. 1C shows the SEM image of Fe3O4@SiO2@ZrO2∶Tb3+ NPs synthesized by the in-situ solvothermal method. It is proposed that the surface of the Fe3O4@SiO2 microspheres act as nucleation sites during the initial stage of forming ZrO2∶ Tb3+, and the as-formed ZrO2∶ Tb3+ nuclei subsequently act as nucleation sites for further deposition of ZrO2∶ Tb3+ nanoparticles, which leads to a layer-by-layer assembly[36]. The even diameter of Fe3O4@SiO2@ZrO2∶Tb3+ NPs is about 300 nm which is slightly increased in contrast with Fe3O4@SiO2 NPs. The surface of Fe3O4@SiO2@ZrO2∶Tb3+ NPs is crude compared with the surface of Fe3O4@SiO2 NPs. We can preliminary conclude that ZrO2∶ Tb3+ is cladded onto the Fe3O4@SiO2 NPs to obtain Fe3O4@SiO2@ZrO2∶Tb3+ NPs. The EDX imaging(Fig. 2E) reveals the presence of the elements Fe, O, Tb, and Zr for Fe3O4@SiO2@ZrO2∶Tb3+, showing that, the ZrO2∶Tb3+ particles are distributed onto the surface of Fe3O4@SiO2. To further investigate their microstructures, elemental mapping was employed to investigate the elemental distributions (Fig. 2A~D) in the Fe3O4@SiO2@ZrO2∶Tb3+ NPs. The Fe element stays in the core region, and the Tb and Zr elements are detected on the surface of the NCs, while the O element can be observed in all the regions.
Fig. 3 exhibits the XRD patterns of Fe3O4, Fe3O4@SiO2, and Fe3O4@SiO2@ZrO2∶Tb3+, respectively. There are six major diffraction peaks at 2θ=30.04°, 35.46°, 43.04°, 53.42°, 56.94°, and 62.48° observed for the Fe3O4 nanocrystals(Fig. 3a), which can be assigned to (220), (311), (400), (422), (511), and (440) planes of the Fe3O4 crystal, respectively. All of the identified peaks in the XRD pattern(Fig. 3a) mean that the product is pure Fe3O4 nanocrystals. The wide-angle XRD pattern(Fig. 3b) of the Fe3O4@SiO2 sample shows almost the same feature as pure Fe3O4, except the broad band from 21.5 to 26.0 corresponding to the amorphous SiO2 shell. In the case of Fe3O4@SiO2@ZrO2∶Tb3+(Fig. 3c), the obvious XRD diffraction peaks at 2θ=30.12°, 35.52°, 49.84°, and 60.03° corresponding to (101), (110), (112), and (211) of ZrO2∶Tb3+ crystals, so ZrO2∶Tb3+ crystals are coated successfully on the surface of Fe3O4@SiO2.
The FT-IR spectra of Fe3O4, Fe3O4@SiO2, and Fe3O4@SiO2@ZrO2∶Tb3+ NPs are given in Fig. 4. As depicted in Fig. 4a, the absorption bands at 1652 cm-1 and 1575 cm-1 are assigned to the stretching vibrations of carboxyl salt, which is because of Na3Cit was added in the synthesis of Fe3O4. The peak at 1050 cm-1 is probably due to the hydroxyl group from Na3Cit. The absorption peak at 544 cm-1 corresponds to the Fe—O vibration related to the magnetite phase. Compared with Fe3O4, the Fig. 4b presents an intense adsorption peak at 1060 cm-1 and two weak peaks at 951 cm-1 and 794 cm-1, which could be ascribed to the vibrations of Si—O—Si, Si—OH, and Si—O groups in the SiO2 shell. The bands observed at 1635 cm-1 and 3381 cm-1 are on account of the vibrations of H2O and OH. The FT-IR spectra(Fig. 4c) of Fe3O4@SiO2@ZrO2∶Tb3+ shows almost the same adsorption peaks as Fe3O4@SiO2 except that the absorption peaks at 951 cm-1 and 794 cm-1 are weaker than those of Fe3O4@SiO2, which is because of the forming of Si—O—Zr. It confirms the successful coating of ZrO2∶Tb3+ on the surface of Fe3O4@SiO2.
The magnetic hysteresis loops of Fe3O4@SiO2 samples with different thickness of SiO2 are shown in the inset Ⅰ of Fig. 5. It is concluded that the magnetization intensity of Fe3O4@SiO2 decreases with the increase of the thickness of SiO2 layer. Taking account of that increasing the thickness of SiO2 layer could enhance the fluorescent intensity by increasing the distance between the ZrO2∶Tb3+ and Fe3O4 NPs, the thickness of silica layer was adjusted to 25 nm(Fig. 1B). Fig. 5 demonstrates magnetic hysteresis loops of Fe3O4, Fe3O4@SiO2(D=250 nm), and Fe3O4@SiO2@ZrO2∶Tb3+, which shows the saturation magnetization values of Fe3O4, Fe3O4@SiO2(D=250 nm), and Fe3O4@SiO2@ZrO2∶Tb3+ are 66 emu/g, 40 emu/g, and 36 emu/g, respectively. All the samples have stronger magnetism with negligible coercivity and remanence at room temperature. No hysteresis loop can be observed, which shows a superparamagnetic characteristic at 300 K. Their saturation magnetizations decrease evidently when Fe3O4 are coated with SiO2 and father Fe3O4@SiO2 are coated with ZrO2∶Tb3+. The low saturation magnetization is mainly attributed to the decrease in the interparticle interactions, originating from the increased distance between the magnetic cores and the coated shells. The reduction in saturation magnetization value can also be attributed to the diamagnetic contribution of the SiO2 and ZrO2∶Tb3+ to dilute the concentration of Fe3O4 NPs. Moreover, the multifunctional NCs with homogeneous dispersion show fast response to the external magnetic field due to its high magnetization, and no residual magnetism is detected. A good magnetic separation performance of the Fe3O4@SiO2@ZrO2∶Tb3+ NPs was observed by using a magnet for only 30 s in aqueous solution. After removal of the magnetic field, the Fe3O4@SiO2@ZrO2∶Tb3+ NPs can be redispersed readily in water with a slight shaking to form a brown-colored suspension (inset Ⅱ in Fig 5). The result reveals that the particles exhibit good magnetic separability and redispersibility, indicating a potential application for targeting and separation.
The excitation and emission spectra of the ZrO2∶Tb3+ and Fe3O4@SiO2@ZrO2∶Tb3+(26% molar fraction, the optimized Tb3+ doping concentration to the zirconia) are shown in Fig 6. The characteristic excitation and emission bands of Fe3O4@SiO2@ZrO2∶Tb3+ are similar to ZrO2∶Tb3+, which suggests that the fluorescent properties are maintained in the product. Compared with the ZrO2∶Tb3+, the fluorescent intensity of the Fe3O4@SiO2@ZrO2∶Tb3+ is decreased by coating ZrO2∶Tb3+ on the Fe3O4@SiO2. The magnetic Fe3O4 has strong quenching effect on the ZrO2∶Tb3+ caused by the decrease of absorption UV energy of ZrO2∶Tb3+. Therefore, the decreased volume for the photon-solid interaction on the surface of Fe3O4@SiO2@ZrO2∶Tb3+ may be a main reason for the decreasing luminescent intensities[32]. Even so, the fluorescence property is still preserved when the fluorescence material is coated on the magnetic NPs. The PL excitation spectra of Tb3+ show an intense broad excitation band centre at 239 nm(inset of Fig. 6), which is measured at 549 nm(5D4→7F5). The broad excitation band is due to the host absorption[29]. In this energy transfer process, the excitation energy was first absorbed by the bandgap of ZrO2, after that, transferred via the ZrO2 lattice to the emitters, and then released as the visible emissions of Tb3+. The energy level transitions from 5D4 to 7F6, 7F5, 7F4, and 7F3 are corresponding to the four emission bands peaked at approximately 494 nm, 549 nm, 587 nm, and 625 nm (Fig. 6). The surface properties and crystallinity of ZrO2∶Tb3+ NPs have a certain impact on the different energy level transition of doped ions, resulting in the different emission intensity of green light.
In summary, we have successfully prepared water-soluble magnetic-fluorescent Fe3O4@SiO2@ZrO2∶Tb3+ NPs by the in-situ solvothermal method. The properties of Fe3O4@SiO2@ZrO2∶Tb3+ NPs have been further investigated. The results show that the NCs have both magnetism and fluorescence. The as-prepared magnetic and fluorescent NPs are water-soluble, showing great potential applications in biomedical fields in the future. The method we used can be extended to the synthesis of other NPs based on lanthanide-doped materials and metal oxides.
2016,
Vol. 33
Issue (8): 968-976
PDF
2016,
Vol. 33
Issue (8): 968-976
PDF