Optical and magnetic nanocomposites containing Fe3O4@SiO2 grafted with Eu3+ and Tb3+ complexes
Graphical abstract
Novel sophisticated, bifunctional nanocomposites of Eu3+ and Tb3+ ions, co-assembling optical and magnetic features into new single discrete entity nanoparticles were synthesized. They can act as red and green light emitters and display paramagnetic properties. The Fe3O4@SiO2-(TTA-Eu-TTA), Fe3O4@SiO2-(TTA-Eu-TC) and Fe3O4@SiO2-(TTA-Eu-AB) nanomaterials emit in red under UV irradiation lamp at 365 nm as well as Fe3O4@SiO2-(TTA-Eu-AMB) one at 254 nm. In addition, the Fe3O4@SiO2-(TTA-Tb-AB or AMB) nanocomposites display green emission under UV irradiation at 254 nm. These all nanomaterials can be easily attracted to the external magnet in suspension and solid phase.
Introduction
Progress in design and fabrication of bifunctional nanosized materials containing magnetic and photonic features integrated into a single entity nanostructures has been advanced rapidly [1], because of their potential multimodal biomedical applications such as: drug delivery carriers [2], photothermal destruction of tumor cells [3], MRI contrast agents [4], [5], quantitative DNA analyses [6], [7] and magnetic hyperthermia for cancer therapies [8], [9]. Therefore, several reports have focused on preparation and characterization of multifunctional magnetic and fluorescent core-shell materials [10], [11] as well as magnetite and trivalent rare earth ions (RE3+) based magnetic and luminescent ones [1], [12], [13], [14].
Among the iron oxides nanoscale particles, the Fe3O4 shows stronger magnetization, however, due to the large surface to volume ratio and high magnetization; bare magnetite nanoparticles are usually prone to aggregation [15]. This difficulty makes them inadequate candidates for biological applications. To overcome this drawback, coating with organic ligands, polymers and silica is often employed [16]. Nevertheless, the superparamagnetism of the Fe3O4 nanoparticles are also dependent on the surface modification [17], [18], influencing the saturation magnetization, coercivity and blocking temperature [19], [20] of these nanoparticles.
The silica coating is a most promising approach, presenting not only the encapsulated magnetite nanoparticles a biocompatibility [21], but also a greater chemical and mechanical stability against variation in pH and temperature [22]. Besides, SiO2 coating retards the oxidation of Fe3O4 nanocrystals to α-Fe2O3 at high temperature [23]. The surface of silica shell can be further functionalized to label with fluorescent dye molecules [24] and also introduce specific ligand functional groups to graft with the RE3+ complexes [13] to produce multifunctional optical and magnetic nanomaterials.
The interest in luminescent materials containing RE3+ ions has been grown considerably due to their unique ability to exhibit well-defined narrow emission bands in different spectral ranges from visible to near-infrared with relatively long lifetimes and high quantum yields [25]. These features make RE3+ materials efficient candidates for multidisciplinary photonic applications, recently extended from laser physics to materials sciences, optical markers, agriculture, and medical diagnostics [26], etc.
The photoluminescence properties of the RE3+ ions are mainly due to the 4f energy level structures, which are only slightly affected by the chemical environment due to the effective shielding of the 4f electrons by the external filled 5s and 5p sub-shells [25]. Therefore, the absorption and emission spectra of the 4f intraconfigurational transitions of the RE3+ ions retain more or less their atomic character and similar irrespective of the host matrix or organic ligand [27].
The rare earth complexes can be remarkable candidates for light conversion molecular devices (LCMDs), since introducing the intramolecular energy transfer from the organic ligands to RE3+ ions (antenna effect) [28] and overcoming the problem of very low molar absorption coefficients (1.0 M−1 cm−1) [27] of 4f-4f transitions. Therefore, the designs of efficient luminescent RE3+ complexes have become an important research subject [29], being investigated extensively using different organic ligands as sensitizers [30].
The RE3+ ions are paramagnetic due to the presence of unpaired electrons with the exception of La3+, Lu3+ and Y3+ ones. Their magnetic properties are determined entirely by the ground state (except for the Sm3+ and Eu3+ ions), as the excited states are so well separated from the ground state due to the spin–orbit coupling and are thermally inaccessible [31]. The magnetic moment of the RE3+ ions is essentially independent of chemical environment and one cannot distinguish between different coordination geometries. However, the magnetic moments of the rare earth ions also contribute to the whole magnetization of the bifunctional nanomaterials [1].
In the present work, the syntheses, structural and morphological characterizations as well as optical and magnetic properties are reported for the bifunctional Fe3O4@SiO2-(TTA-RE-L) (RE: Eu3+ and Tb3+) nanocomposites with different organic ligands (L), where L: thenoyltrifluoroacetonate (TTA), Thioglycolate (TC), 4-aminobenzoate (AB) and 4-(aminomethyl)benzoate (AMB). The Small-angle X-ray Scattering (SAXS) data in support with Transmission Electron Microscopy (TEM) images were used to determine the structural features and morphology of the core-shell Fe3O4@SiO2-(TTA-RE-L) nanomaterials containing aggregation of Fe3O4 core nanoparticles. In addition, core mean size , shell thickness , cluster size ξ and fractal dimension DF for these nanocomposites were also calculated and discussed. Based on these structural and morphological data, the DC magnetic properties were studied at room (300 K) and low (2 K) temperatures in order to better understand the final structure of the optical and magnetic nanocomposites. The influences of amorphous silica phase and RE3+ complexes on the behavior of magnetization (M), coercive field (HC) and blocking temperature (TB) were presented and discussed. In addition, the magnetic contribution of the of RE3+ ions to the whole magnetization (M − H and ZFC/FC measurements) of the Eu3+ and Tb3+ nanomaterials was also studied. Though, the magnetite is usually a luminescence quencher, this difficulty was overcome by coating the Fe3O4 nanoparticles with silica shell using modified Stöber method [32]. The influence of the chemical structure of the ligand on the photoluminescence properties of the Eu3+ and Tb3+ ions was also studied to produce highly luminescent nanomaterials. The experimental intensity parameters (Ω2 and Ω4), radiative and non-radiative rates (Arad and Anrad) for the Eu3+ nanophosphors were calculated. Finally, the emission spectral features, the experimental emission quantum efficiencies (η) and the emission lifetimes for these nanomaterials are also discussed.
Section snippets
Reagents
The following commercially available chemical reagents were used without further purification. The FeCl3·6H2O and FeCl2·4H2O were purchased from Synth as well as Tetraethyl orthosilicate (TEOS), 3-Chloropropyl-triethoxysilane (CPTES), thenoyltrifluoroacetone (HTTA), 4-aminobenzoic acid (ABA) and 4-aminomethyl benzoic acid (AMBA) from Sigma-Aldrich. The Thioglycolic acid (TCA) was purchased from Merck as well as EuCl3·6H2O and TbCl3·6H2O were synthesized from the Eu2O3 and Tb4O7 (99.99% CSTARM)
Results and discussion
The bifunctional core-shell luminescent and magnetic nanocomposites were prepared by multi-step syntheses, utilizing Fe3O4 as precursor core particles [33], [34], which were further coated with silica using 3-chloropropyl-triethoxysilane (CPTES) and tetraethyl orthosilicate (TEOS) [32], [33], [34]. The silica shell formation on Fe3O4 with this method is a very slow process from 12 to 48 h of mechanical stirring at room temperature. Nevertheless, results in Fe3O4@SiO2 particles usually contain
Conclusion
The red-green emitting and magnetic nanocomposites containing Eu3+ and Tb3+ ions were successfully prepared by multistep syntheses, utilizing Fe3O4@SiO2 nanostructures grafted with Eu3+ and Tb3+complexes. The structural features and morphologies of these core-shell Fe3O4@SiO2-(TTA-RE-L) nanocomposites were studied using SAXS, XPD and TEM analyses, as well as the average crystallite size of the Fe3O4 core nanoparticles were found near 10 nm. The SAXS data suggest that the bifunctional
Acknowledgments
The authors are grateful for financial support from the Coordenação de Aperfeiçoamento de Pessoal de Nível Superior (CAPES, Brazil), Conselho Nacional de Desenvolvimento Científico e Tecnológico (CNPq, Brazil), Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP, Brazil), Instituto Nacional de Ciência e Tecnologia de Nanotecnologia para Marcadores Integrados (inct-INAMI, Brazil), CNPEM-LNLS synchrotron, Campinas-SP, Brazil under Proposal Nos. SAXS1−14355 (7959), CNPEM-LNNano,
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