Monodisperse bifunctional Fe₃O₄@NaGdF₄:Yb/Er@NaGdF₄:Yb/Er core–shell nanoparticles

Abstract

We report for the first time the synthesis of monodisperse, magnetic and up-conversion luminescent Fe₃O₄@NaGdF₄:Yb/Er@NaGdF₄:Yb/Er core–shell structured nanoparticles (NPs) by a seed-growth process. The growth of a NaGdF₄:Yb/Er shell on Fe₃O₄@NaGdF₄:Yb/Er NPs can significantly enhance the up-conversion emission intensity.

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In recent years, core–shell structured NPs have attracted extensive interest,¹ because the combined functionalities of cores and shells endow them with great potential application in various fields.² In particular, core–shell structured NPs with magnetism and fluorescence have been intensively pursued for their biomedical applications, such as contrast enhancement agents in magnetic resonance imaging (MRI), magnetic carriers for drug delivery,³ biological labeling,⁴ biological luminescent probes.⁵ However, conventional fluorescent organic dyes used in these core–shell composites are not suitable for sensitive detection and real-time monitoring because of the serious photobleaching,⁶ and quantum dots (QDs) generally suffer from the inherent toxicity.⁷ These drawbacks dramatically limit their biological applications. Rare earth based luminescent NPs show superior luminescent and chemical properties, such as high fluorescent efficiency,⁸ multicoloured emissions,⁹ high resistance to photobleaching, and especially low toxicity,¹⁰ making them highly suitable as alternatives to organic dyes and QDs in biological applications.¹¹

Despite the considerable potential of nanomaterials for applications to bioscience and other fields,¹² improvements are still needed to optimize the properties for potential commercial applications.¹³ One strategy to improve the luminescence efficiency of NPs is the use of core–shell architectures where a shell of material with similar lattice constants is grown around the luminescent nanoparticles to avoid the formation of defects at the core–shell interface.¹⁴ In this structure, the increase of the distance between the luminescent rare earth ions and the surface results in reducing the non-radiative pathway, and the quantum yield of the nanomaterials is consequently enhanced. Nowadays, many research teams are devoted to synthesizing this kind of nanocomposite.¹⁵ However, to the best of our knowledge, monodisperse core–shell NPs with magnetic and up-conversion luminescence have never been reported so far. Thus, designing an effective strategy to synthesize high-quality core–shell structured colloidal NPs is of significant value.

Herein, we first report a seed-growth procedure to synthesize a new type of core–shell structured NPs composed of magnetite core and up-conversion luminescent shell. Each of the components in the nanocomposites displays a unique function. Fe₃O₄ as the core endows nanocrystals with magnetic property, and the outer NaGdF₄:Yb/Er shell is to improve the luminescent efficiency. This straightforward route may be applicable to the preparation of other bifunctional core–shell NPs.

The Fe₃O₄@NaGdF₄:Yb/Er and Fe₃O₄@NaGdF₄:Yb/Er@NaGdF₄:Yb/Er core–shell NPs were synthesized by a modified high-temperature thermolysis process.¹⁶ The synthesis process is presented in Scheme 1. Oleic acid (OA) capped Fe₃O₄ core NPs were obtained from the thermal decomposition of the iron-oleate precursors. The as-prepared Fe₃O₄ NPs then served as the seeds for the shell growth in the presence of oleic acid and 1-octadecene, resulting in the formation of the core–shell NPs. A luminescence shell was further grown using the as-prepared Fe₃O₄@NaGdF₄:Yb/Er as seeds. Oleic acid was selected as the surfactant to control the particle growth and to prevent the NPs from aggregation. The detailed experimental procedures are given in the experimental section.†

Scheme 1 Schematic formation processes of Fe₃O₄@NaGdF₄:Yb/Er@NaGdF₄:Yb/Er NPs.

Fig. 1 shows the transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) images of as-prepared pure Fe₃O₄, Fe₃O₄@NaGdF₄:Yb/Er, and Fe₃O₄@NaGdF₄:Yb/Er@NaGdF₄:Yb/Er, respectively. In Fig. 1A, Fe₃O₄ seeds consist of monodisperse NPs with narrow size distribution (inset in Fig. 1A). Fig. 1B clearly reveals a highly ordered nanoarray made up of circular NPs with an average diameter of 9 nm. In Fig. 1C, monodisperse Fe₃O₄@NaGdF₄:Yb/Er core–shell structured NPs with a nearly spherical morphology are obtained and the average particle size is about 22 nm, which is much bigger than that of Fe₃O₄ NPs, suggesting that a NaGdF₄:Yb/Er shell is successfully grown on the surface of Fe₃O₄ nanospheres. Moreover, there is no obvious boundary between the shell and core, which may be due to the ultra-small sizes of the Fe₃O₄ core NPs. The obvious lattice fringes in the HRTEM image (Fig. 2D) indicate the crystalline nature of Fe₃O₄@NaGdF₄:Yb/Er NPs. The distances of 0.30 nm between the adjacent lattices agree well with the d₁₁₀ spacing (0.301 nm) of hexagonal-phased NaGdF₄ (JCPDS no. 27-0699). In Fig. 1E, it is found that the growth of further shell results in an obvious increase of the NPs size. The average particle diameter increases from 22 nm for Fe₃O₄@NaGdF₄:Yb/Er NPs to 32 nm for Fe₃O₄@NaGdF₄:Yb/Er@NaGdF₄:Yb/Er core–shell NPs. Similarly, the particle sizes are uniform with good monodispersity and narrow size distribution (inset in Fig. 1E). The HRTEM image (Fig. 1F) clearly reveals the well-resolved lattices fringes, indicating the high crystallinity of the as-prepared sample, which are well consistent with the XRD results (Fig. S1 in the ESI†). The distance of the adjacent lattice fringes is determined to be about 0.30 nm, which corresponds well to the d₁₁₀ spacing of hexagonal phase NaGdF₄.

Fig. 1 (A) Low-resolution, (B) high-resolution TEM image of Fe₃O₄ NPs; (C) low-resolution, (D) high-resolution TEM images of Fe₃O₄@NaGdF₄:Yb/Er NPs; (E) low-resolution, (F) high-resolution TEM images of Fe₃O₄@NaGdF₄:Yb/Er@NaGdF₄:Yb/Er core–shell NPs. Insets are their corresponding particle size distribution histograms.

Fig. 2 Up-conversion emission spectra of (A) Fe₃O₄@NaGdF₄:Yb/Er (black line), Fe₃O₄@NaGdF₄:Yb/Er@NaGdF₄:Yb/Er (green line) and the corresponding photograph under 980 nm irradiation (inset); (B) magnetizations of pure Fe₃O₄ and Fe₃O₄@NaGdF₄:Yb/Er@NaGdF₄:Yb/Er NPs (inset) as a function of applied magnetic field measured at room temperature.

The oleic acid (OA) plays an important role in controlling the particle growth, which is confirmed by the FT-IR spectrum, as shown in Fig. S2.† The absorption peaks at 2924 cm⁻¹ and 2852 cm⁻¹ can be ascribed to the asymmetric (ν_as) and symmetric (ν_s) stretching vibration of methylene (–CH₂) group. The peak at 1698 cm⁻¹, 1567 cm⁻¹ and 1467 cm⁻¹ can be assigned to C=O stretching vibration, asymmetric (ν_as) and symmetric (ν_s) stretching vibrations of –COOH,¹⁷ respectively. The peak at 1112 cm⁻¹ can be due to the C–F stretching vibration. The peak at 721 cm⁻¹ is assigned to the in-planar swing of (CH₂) n (n > 4).¹⁸ The EDS spectrum of Fe₃O₄@NaGdF₄:Yb/Er@NaGdF₄:Yb/Er core–shell NPs (Fig. S3 in the ESI†) confirms the presence of element F, O, Na, Gd, and Yb in the product. While the absence of element Fe may be due to the coated layer on the surface of Fe₃O₄ core.

Fig. 2 shows the up-conversion emission spectra of the Fe₃O₄@NaGdF₄:Yb/Er, Fe₃O₄@NaGdF₄:Yb/Er@NaGdF₄:Yb/Er and its corresponding photograph (inset) under 980 laser diode excitation, respectively. In both spectra, the typical emissions at 409, 521, 540, and 654 nm can be ascribed to the respective ²H_9/2 → ⁴I_15/2, ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2 and ⁴F_9/2 → ⁴I_15/2 transitions of Er³⁺ ions_.¹⁹ The green emission at 540 nm is much stronger than that of the red one (654 nm), resulting in the bright green emission (inset in Fig. 2A). Moreover, it should be noted that the emission intensity of the Fe₃O₄@NaGdF₄:Yb/Er@NaGdF₄:Yb/Er NPs (green line in Fig. 2A) is almost four times to that of Fe₃O₄@NaGdF₄:Yb/Er (black line), which may be caused by the significant elimination of non-radiative centers on the surface of Fe₃O₄@NaGdF₄:Yb/Er NPs by the shielding effect of protective shell. In addition, the increased emission intensity may also derive from the suppressed quenching by the outer shell and the inherent emission of luminescent NaGdF₄:Yb/Er shell. The magnetic hysteresis loops of pure Fe₃O₄ and Fe₃O₄@NaGdF₄:Yb/Er@NaGdF₄:Yb/Er NPs are shown in Fig. 2B. The field-dependent magnetization plots illustrate that Fe₃O₄ NPs are superparamagnetic with saturation magnetization value of 20.5 emu g⁻¹. While Fe₃O₄@NaGdF₄:Yb/Er@NaGdF₄:Yb/Er NPs have a saturation magnetization value of 1.1 emu g⁻¹ (inset in Fig. 2B). The markedly reduced value should be caused by the thick shells and organic OA molecules coated on magnetic NPs.

The up-conversion emission spectra of Fe₃O₄@NaGdF₄:Yb/Tm@NaGdF₄:Yb/Tm, Fe₃O₄@NaGdF₄:Yb/Ho@NaGdF₄:Yb/Ho and their corresponding photographs under 980 NIR excitation are presented in Fig. 3. As shown, the Tm³⁺ and Ho³⁺ based samples exhibit bright blue and yellow–green emissions upon 980 nm irradiation. In Fig. 3A, two intense blue emissions at 450 and 475 nm and a weaker red emission at 652 nm can be ascribed to the ¹D₂ → ³F₄, ¹G₄ → ³H₆ and ¹G₄ → ³F₄ transitions of Tm³⁺.²⁰ In Fig. 3B for the Fe₃O₄@NaGdF₄:Yb/Ho@NaGdF₄:Yb/Ho NPs, the dominating green emission at 541 nm can be associated with the ⁵F₄, ⁵S₂ levels to the ⁵I₈ ground state. The weak blue emission at 475 nm originating from ⁵F₃ → ⁵I₈ and red emission at 645 nm from ⁵F₅ → ⁵I₈ transition are obvious.¹⁵

Fig. 3 Up-conversion emission spectra of (A) Fe₃O₄@NaGdF₄:Yb/Tm@NaGdF₄:Yb/Tm, (B) Fe₃O₄@NaGdF₄:Yb/Ho@NaGdF₄:Yb/Ho and their corresponding photographs under 980 irradiation (insets).

In conclusion, monodisperse, multicoloured and magnetic Fe₃O₄@NaGdF₄:Ln@NaGdF₄:Ln core–shell NPs have been successfully prepared via a seed-growth high-temperature thermolysis method. The as-prepared core–shell structured colloidal NPs show good monodispersity, uniform particle sizes. Notably, the core–shell structure markedly enhances the up-conversion emission intensity and also endows them with magnetic properties. These favorable features of the bifunctional material make it highly potential in biological field.

We are grateful to the National High Technology Research Program of China (2011AA03A407), National Basic Research Program of China (2010CB327704), National Natural Science Foundation of China (NSFC 20871035), Research Fund for the Doctoral Program of Higher Education of China (20112304110021) and the Fundamental Research Funds for the Central Universities of China HEUCF201210009).

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Footnote

† Electronic supplementary information (ESI) available: Experimental detail, XRD patterns, EDS spectra, FT-IR spectrum. See DOI: 10.1039/c2ra20070h

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