Facile synthesis of water-soluble luminescent mesoporous Tb(OH)3@SiO2 core-shell nanospheres
© Ansari et al.; licensee Springer. 2013
Received: 30 January 2013
Accepted: 12 March 2013
Published: 10 April 2013
Luminescent mesoporous Tb(OH)3@SiO2 core-shell nanospheres were synthesized through W/O microemulsion process at ambient temperature. The negatively charged silica favors a coating of the positively charged Tb3+ composite. Thus, silicon layer was adsorbed on the surface of Tb(OH)3 groups to form Tb-O-Si through electrostatic interaction. X-ray diffraction, field emission transmission electron microscopy (FE-TEM), energy-dispersive X-ray spectrometry, and Fourier transform infrared, UV/Visible, and photoluminescence spectroscopies were applied to examine the phase purity, crystallinity, surface morphology, and optical properties of the core-shell nanospheres. The FE-TEM results have revealed typically ordered mesoporous characteristics of the material with monodisperse spherical morphology in a narrow size distribution. The luminescent mesoporous core-shell nanospheres exposed remarkable splitting with broadening in the emission transition 5D4 → 7F5 (543 nm). In addition, the luminescent mesoporous core-shell nanospheres emit strong green fluorescence (from Tb3+) in the middle of the visible region under 325 nm (3.8) excitation. The luminescent mesoporous Tb(OH)3@SiO2 core-shell nanospheres can therefore be exploited as fluorescent probes in biomarkers or biolabeling, optical sensing, and drug delivery system. Further, these nanospheres could have potential use as scattering layers in dye-sensitized solar cells.
KeywordsLuminescent mesoporous Tb(OH)3@SiO2 core-shell nanospheres Optical properties Photoluminescence
During the past decade, great efforts have been devoted to the preparation of mesoporous core-shell nanomaterials due to their potential applications in drug-delivery carriers [1–3], optical bioprobes , biomarkers , and fluorescent biolabeling [6, 7]. These mesoporous core-shell nanomaterials possess attractive features such as well-defined and controllable pore size, high pore volume, large surface area, non-toxic nature, easily modified surface properties, and good biocompatibility . However, the use of bulk mesoporous silica in many applications suffers from many limitations, especially in the targeted drug delivery mechanisms as carrier and drug kinetics marker in the pharmacological research [9, 10]. Recently, luminescent metal-doped mesoporous materials, which can be tracked or monitored to evaluate the efficiency of the drug release, have become a research hotspot [1–3, 11–14]. The integration of luminescent metal-doped nanocrystals with mesoporous silica to form core-shell structures is undoubtedly of great value because mesoporous shells not only offer high surface area for derivation of numerous functional groups but also provide accessible large pore channels for the adsorption and encapsulation of biomolecules and even functional nanoparticles. Up to date, a lot of techniques have been reported for the synthesis of luminescent metal-doped mesoporous silica core-shell structures, such as mesoporous silica encapsulating quantum dots/nanoparticles [15, 16], luminescent metal nanoparticles , and luminescent lanthanide metal nanoparticles [18, 19]. However, all core particles are spherical. Among various luminescent metal ion-doped mesoporous core-shell nanoparticles, luminescent lanthanide-doped core-shell nanoparticles are promising because of their good chemical durability, thermal stability, and optical features. Moreover, such luminescent Ln3+-doped mesoporous core-shell nanoparticles have sharp emission lines, long lifetimes, superior photostablility, large Stokes shifts, good chemical/physical stability, and low toxicity . At present, there are only a few reports on the synthesis of luminescent lanthanide-doped mesoporous core-shell nanospheres. For example, Qian et al. have synthesized mesoporous-silica-coated upconversion fluorescent nanoparticles through water/oil (W/O) microemulsion process for photodynamic therapy . Yang et al. prepared mesoporous silica encapsulating upconversion luminescence rare-earth fluoride nanorods by using the surfactant-assisted sol-gel process . Lin and his coworkers have been synthesizing mesoporous upconversion luminescent NaYF4:Yb3+/Er3+@nSiO2@mSiO2-doped core-shell nanospheres via a simple two-step sol-gel process . Although it is well accepted that uniform spherical core-shell nanoparticles with lower surface defects are preferred to improve optical properties, little effort has been devoted to the synthesis of mesoporous core-shell nanospheres. However, in most of these mentioned approaches, the synthesis process of the core-shell nanoparticles involves a multistep high-temperature preparation and less biocompatibility, such as first preparation of core (seed spherical nanoparticles) and then coating a shell of silica on the surface of the nanoparticles. Therefore, it is desirable to develop a facile, low-cost, and large-scale approach to prepare water-soluble, luminescent, mesoporous core-shell and well-dispersed spherical nanoparticles. To the best of our knowledge, the luminescent lanthanide mesoporous core-shell nanospheres have been rarely fabricated. In the present work, a method for direct coating of β-diketonate stabilized the luminescent metal-chelating complex with silica shells by a seeded polymerization technique is proposed. The method does not require any coupling molecules and is based on a method that our group has used for the preparation of monodispersed mesoporous silica core-shell nanospheres [8, 20]. The concentrations of water, ammonia, luminescent metal-chelating complex, cetyltrimethyl-ammonium bromide (CTAB), and silicon alkoxide are important factors governing particle size and distribution in microemulsion reaction of alkoxides. Fine control of the amount of silicon alkoxide, ethanol, water, and ammonia (catalyst) is used to prevent secondary silica nucleus formation and to provide rapid shell growth.
Herein, we report a facile synthesis of water-soluble, luminescent Tb3+-doped mesoporous core-shell nanospheres via a modified W/O microemulsion process. We are employing Tb(acac)3·3H2O as doping chelating complex in the silica framework which shows green luminescence in visible region. In addition, the size of the nanospheres could be fine-tuned from 10 to 130 nm, which is very crucial for applications in the biofield.
Materials and methods
Terbium oxide (99.99%, Alfa Aesar, Karlsruhe, Germany), tetraethyl orthosilicate (TEOS, 99 wt.% analytical reagent A.R.), Cyclohexane (BDH, England, UK), C2H5OH, HNO3, NH4OH, n-hexanol, and Triton X-100 (Sigma-Aldrich, St. Louis, MO, USA) were used as starting materials without any further purification. Tb(NO3)3·6H2O were prepared by dissolving the corresponding oxides in diluted nitric acid, and nanopure water was used for preparation of solutions. Ultrapure deionized water was prepared using a Milli-Q system (Millipore, Bedford, MA, USA). All other chemicals used were of reagent grade.
One-pot synthesis of luminescent mesoporous Tb(OH)3@SiO2 core-shell nanospheres
Luminescent mesoporous Tb(OH)3@SiO2 core-shell nanospheres were prepared via a modified W/O microemulsion process as follows: before the nanoparticle preparation, the Tb(acac)3·3H2O chelating complex was prepared by a reported method . In a typical procedure, firstly, microemulsion was prepared by mixing 3.54 ml of Triton X-100, 15 ml of cyclohexane, and 4.54 ml of n-hexanol under constant stirring at room temperature. Then, 2 ml of an aqueous solution of Tb(acac)3·3H2O chelating complex (1 M) was added into the mixture. After that, a mixed solution containing TEOS (2 ml), H2O (5 ml), and CTAB (50 mg) was added. In the presence of TEOS, a polymerization reaction was initiated by adding 1 ml of NH4OH. The resulting reaction was allowed to continue for 24 h. After the reaction was completed, the luminescent mesoporous nanospheres were isolated by acetone followed by centrifuging and washing with ethanol and water several times to remove any surfactant molecules.
The X-ray diffraction (XRD) of the powder samples was examined at room temperature with the use of PANalytical X’Pert X-ray diffractometer (Almelo, The Netherlands) equipped with a Ni filter using Cu Kα (λ = 1.54056 Å) radiations as X-ray source. The size and morphology of the samples were inspected using a field emission transmission electron microscope (FE-TEM) equipped with an energy-dispersive X-ray spectrometer (EDX) (FE-TEM, JEM-2100F, JEOL, Akishima-shi, Japan) by operating at an accelerating voltage of 200 kV. EDX analysis was used to confirm the presence of the species. Samples for TEM were prepared by depositing a drop of a colloidal ethanol solution of the powder sample onto a carbon-coated copper grid. The FTIR spectra were recorded using a PerkinElmer 580B IR spectrometer (Waltham, MA, USA) using the KBr pellet technique in the range of 4,000 to 400 cm-1. The UV/vis absorption spectra were measured using a PerkinElmer Lambda-40 spectrophotometer, with the sample contained in a 1-cm3 stopper quartz cell of a 1-cm path length, in the range of 190 to 600 nm. Photoluminescence spectra were recorded on Horiba Synapse 1024x 256 pixels, size of the pixel 26 microns, detection range: 300 (efficiency 30%) to 1000 nm (efficiency: 35%) (Kyoto, Japan). In all experiments, a slit width of 100 microns is employed, ensuring a spectral resolution better than 1 cm-1. All measurements were performed at room temperature.
Results and discussion
The figure shows significant differences in the band shapes of the emission transitions such as 5D4 → 7F6, 5D4 → 7F4, and 5D4 → 7F3, and this is attributed to the differences in their structure and interaction of Si molecules with the 4f-electrons of the metal ions. These intensity enhancement effects may be related to the change in the strength and symmetry of the crystal field produced by the silica network . The broadening and splitting of spectral lines are also observed and are induced by the change in chemical environment of Tb3+ ions during the formation of new chemical bonds between silica network and terbium hydroxide. The luminescence spectrum displayed well-defined crystal-field splitting of the narrow luminescence lines, which are induced by the change in chemical environment of Tb3+ ions during the formation of new chemical bonds between silica network and terbium hydroxide. This shows that the crystal field is very similar for most Tb3+ ions as previously reported in the literature [32–34]. This is expected because the Tb3+surface sites are converted into volume sites by growing the silica core-shell, thereby reducing the number of different Tb3+ sites in the material. The highest branching ratio corresponds to the 5D4 → 7F5 transition (543 nm), and this transition may therefore be considered to be a possible laser transition.
In summary, luminescent mesoporous silica-coated terbium hydroxide core-shell nanospheres were synthesized through W/O microemulsion process. The FE-TEM, EDX, XRD, and FTIR techniques were used to characterize the morphology and composition of the core-shell nanospheres. The optical spectra of the core-shell nanospheres confirmed that the properties of the terbium ion were strongly affected by the doping procedure. The emission spectrum of Tb(OH)3@SiO2 nanospheres shows the characteristic emission peaks of Tb3+ and a weak background band of SiO2.
The luminescent intensity of the hypersensitive transition (5D4 → 7F5) in core-shell nanospheres is greatly enhanced because the non-radiative processes at or near the surface of the nanospheres is greatly reduced. The strong green emission of Tb3+ in core-shell nanospheres results from an efficient energy transfer from silica to Tb3+, in which the non-bridging oxygen atom is present between the metal ion and silica frameworks. The luminescent metal ion inside the nanospheres has two functional entities which allow optimizing their luminescence and aqueous solubility separately. The study of these novel composite nanospheres is of profound importance for the new applications in biomarkers and drug delivery, as well as in nucleic acid assay. The luminescent property of these materials as well as their reported light upconversion can have a potential use in dye-sensitized solar cells as a scattering layer for better harvesting of solar light, which will be subject for future investigation.
This study is supported by the NPST Program of the King Saud University, Riyadh, KSA under Project no. 11-ENE1474-02.
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