LSPR effects in a magnetic–luminescent heterostructure for efficient enhanced luminescence performance

Abstract

The major challenges faced by magnetic–luminescent bi-functional nanoparticles include poor luminescence intensity and severe magnetic–luminescence quenching. Introduction of an intermediate layer with localized surface plasmon resonance (LSPR) effects in magnetic–luminescent bi-functional nanoparticles plays a major role in solving these challenges. Herein, a fascinating process of plasmonic energy conversion in Fe₃O₄@WO₃@GdF₃:Eu³⁺ heterostructures, which can enhance the luminescence by LSPR effects of WO₃, is demonstrated. It is proposed that the LSPR of WO₃ is the primary reason for the enhanced luminescence of Fe₃O₄@GdF₃:Eu³⁺. Moreover, saturation magnetization did not significantly decline on introducing WO₃ with LSPR effect. This research conclusion can solve the problems faced by magnetic–luminescent bimodal imaging materials and promote the application of this type of composite materials in tumor therapy and optical confocal microscopy technology.

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1. Introduction

In recent years, researchers have found that magnetic–luminescent bi-functional nanoparticles can provide high-resolution, high-contrast images for precision medicine, and are crucial for imaging applications.^1–3 Among them, the design and synthesis of Fe₃O₄@REL (Rare Earth Luminescence) magnetic–luminescent bi-functional nanoparticles has become the focus of current research.^4–8 For example, a literature⁹ report has described magnetic resonance and photo-thermal bimodal imaging materials based on Fe₃O₄@NaYF₄:Yb,Er nanoparticles. Fe₃O₄@NaYF₄:Yb,Tm nanoparticle composite has been used as a three-mode imaging agent for magnetic resonance, fluorescence, positron emission tomography and single photon emission computed tomography.¹⁰ Although the research on magnetic–luminescent bi-functional Fe₃O₄@REL nanoparticles has made some progress, there is still a problem of weak luminescence intensity, which limits its further application.

Motivated by this situation, extensive efforts have been made to enhance the emission intensity, such as modulating host materials and dopant concentrations, coating protective shells and coupling with noble metals. Unfortunately, the magnetic core and the luminescent shell layer are still in direct contact, the magneto-optical quenching effect is difficult to avoid and the actual enhancement effect of the luminescence intensity is not significant. In addition, the target position of the doped element (Gd³⁺,¹¹ Mn^2+ 12 and Li^+ 13) is difficult to control. Inert intermediate barrier materials (such as SiO₂,¹⁴ C¹⁵ and PMMA¹⁶) were introduced into the Fe₃O₄@REL magnetic–luminescent bi-functional nanocomposites to prevent the direct contact between the magnetic core and the luminescent shell, so that the magneto-optical quenching effect was reduced and the luminescence intensity of the nanoparticles was enhanced to a certain extent. However, the enhancement effect of luminescence intensity is limited because the inert isolation layer lacks chemical activity, and it is difficult to meet the application requirements of clinical medicine.

By introducing protective shells such as noble metals (Ag/Au), luminescence can be enhanced due to localized surface plasmon resonance (LSPR).¹⁷ However, as the common surface plasma materials, the precious metals (such as Au and Ag) are limited due to the energy band structure of the metal itself, and the high carrier concentration. The regulation of the LSPR peak can only be realized by changing the morphology of the metal nanoparticles, and hence, the practical application of metal nanoparticles is difficult.¹⁸ Limited precious metal resources and high price limits its application in clinical medicine, and hardly satisfies biological applications.

Recently, some researches have reported that the LSPR phenomena can also occur on various nonmetal nanostructures of heavily doped nonstoichiometric semiconductors, such as Cu_2−xS, WO_3−x, and MoO_3−x.^19–21 LSPR band and intensity of semiconductors can be easily controlled via adjusting the stoichiometric ratios, vacancy, or dopant concentrations, as well as phase structures.²² Among the plasmonic semiconductor nanostructures, tungsten oxide nanoparticles (WO₃), made via a facile solvothermal method, possess an intense LSPR band across the visible and NIR regions due to abundant oxygen vacancies on their surface.²³ Such a broad plasmonic absorption of WO₃ offers a unique opportunity to gain deeper understanding on the influence of LSPR on the photon absorption and emission processes of the luminescent nanoparticles within the same plasmonic nanostructure.

For the first time, we demonstrate the nonmetallic plasmon induced enhancement of luminescence in a core–shell structure consisting of Fe₃O₄ as the core, GdF₃:Eu³⁺ as the luminescence layer and WO₃ as the plasmonic layer. Compared with Fe₃O₄@GdF₃:Eu³⁺, Fe₃O₄@WO₃@GdF₃:Eu³⁺ exhibits enhancement on the emission originating from the ⁵D₀–⁷F₁/⁷F₂ transition of Eu³⁺ ions. We propose that this enhancement of luminescence is caused via LSPR-induced plasmonic energy conversion. In this process, WO₃ converts the NIR energy of incident photons into LSPR oscillation energy that subsequently transfers to the adjacent GdF₃:Eu³⁺, thus substantially enhancing the luminescence intensity based on the plasmon-enhanced localized electric fields and the photo-thermally improved electron population (Fig. 1). This finding provides new ideas for the emission intensity of magnetic–luminescent bifunctional nanocomposites to be further enhanced through the LSPR effect.

Fig. 1 Synthesis procedure for Fe₃O₄@WO₃@GdF₃:Eu³⁺ core–shell structured nanocomposites.

2. Experimental section

2.1 Chemicals and materials

Rare earth oxides (Gd₂O₃, Eu₂O₃ 99.99%) were purchased from Science and Technology Parent Company of Changchun Institute of Applied Chemistry. Ferric chloride hexahydrate (purity ≥ 99.0%, FeCl₃·6H₂O), sodium acetate (purity ≥ 99.0%, CH₃COONa), ethylene glycol (purity 96.0%, EG), and poly(ethylene glycol) (PEG M_w = 20 000) were purchased from Beijing Chemical Reagent Limited China. Sodium tungstate dihydrate (Na₂WO₃·2H₂O, purity 99%) was purchased from Tianjin Kemiou Chemical Reagent Co. Ltd (Tianjin, China). Hexadecyltrimethyl ammonium bromide (C₁₉H₄₂BrN, CTAB, purity 99%), sodium hydroxide (NaOH, purity 99%), concentrated hydrochloric acid (HCl, 38%), and ammonium hydroxide (NH₃·H₂O, AR) were purchased from Kaixin Chemical Reagent Co. Ltd (Hunan, China). Ammonium fluoride (NH₄F) was purchased from Beijing Chemical Works. All the above chemical reagents were of analytical grade and used without further purification. Rare earth salts (Gd(NO₃)₃, Eu(NO₃)₃) were prepared by dissolving the corresponding rare earth oxides (Gd₂O₃, Eu₂O₃, 99.99%) in nitric acid and evaporating their solutions.

2.2 Characterization

X-ray diffraction (XRD) patterns of the samples were measured using an AXS D8 Advance diffractometer (Bruker, Bremen, Germany) with Cu K_α radiation (λ = 0.15406 nm) at 40 kV and 40 mA. Scanning electronic microscopy (SEM) images were recorded on a Hitachi S-4800 microscope. The morphologies and structures of the as-prepared samples were inspected using a JEM 2010 transmission electron microscope (TEM). The photoluminescence (PL) excitation and emission spectra were recorded on a Hitachi F-4500 spectrofluorimeter equipped with a 150 W xenon lamp as the excitation source. Magnetization measurements were performed on an MPMSSQUID XL superconducting quantum interference device (SQUID) magnetometer at 300 K. Absorbance by treated cells in the MTT assay was measured using an RT-6000 microplate reader (Rayto, USA).

2.3 Synthesis of the Fe₃O₄@WO₃ core–shell structured nanocomposites

Magnetic Fe₃O₄ nanoparticles were prepared by the solvothermal method.²⁴ WO₃ was layered on the surface of the magnetic Fe₃O₄ nanoparticles using a direct precipitation method. In a typical procedure, 0.1 g of the as-prepared Fe₃O₄ nanoparticles was dispersed in 75 mL of CTAB (0.364 g) solution. The mixture was sonicated for 15 min, followed by the addition of 50 mL of Na₂WO₃·2H₂O (3.3 g) solution. Then, hydrochloric acid and ammonia solution were added with constant stirring to adjust the pH value to 9, followed by stirring for 4 h at room temperature. The resulting products were separated with a magnet, thoroughly washed several times with ethanol and deionized water, and dried at 60 °C overnight. The dried particles were then calcinated for 2 h at 300 °C. The resulting products were the Fe₃O₄@WO₃ core–shell structured nanoparticles.

2.4 Synthesis of the Fe₃O₄@WO₃@GdF₃:Eu³⁺ core–shell-structured nanoparticles

GdF₃:Eu³⁺ luminescent shell was coated on Fe₃O₄@WO₃ nanoparticles by a direct precipitation method. In a typical procedure, 0.1 g of the as-prepared Fe₃O₄@WO₃ nanoparticles were dispersed in 3.2 mL of Gd(NO₃)₃ and 1.2 mL of Eu(NO₃)₃ solutions. The mixture was sonicated for 20 min, followed by the addition of 10 mL of 0.6 mol L⁻¹ NH₄F solution and then, this mixture was heated to 75 °C under vigorous mechanical stirring. After 2 h, the resultant products were separated with a magnet, thoroughly washed with ethanol and deionized water several times, and further dried at 60 °C overnight. Finally, Fe₃O₄@WO₃@GdF₃:Eu³⁺ nanocomposites were obtained.

2.5 MTT assays

To test the cytotoxicity of Fe₃O₄@WO₃@GdF₃:Eu³⁺ nanoparticles, we performed an MTT assay using MCF-7 cells, which are common representative cancer cells.¹⁴ MCF-7 cells were cultured in 96-well microtiter plates and incubated at 37 °C in a 5% CO₂ atmosphere for 24 h. MCF-7 cells were incubated with culture medium containing Fe₃O₄@WO₃@GdF₃:Eu³⁺ (0, 0.5, 1, 5, 10, and 15 mg mL⁻¹) for 24 h. Next, we added 100 μL of MTT solution (0.5 mg mL⁻¹) to each well. After 4 h, the remaining MTT solution was removed, and 150 μL of dimethyl sulfoxide was added to each well to dissolve the formazan crystals. Absorbance was measured at 490 nm with the RT 6000 microplate reader. The % cell viability was determined as follows:

% cell viability = (absorbance of treated cells)/(absorbance of control) × 100.

The experiments were conducted in triplicate and data were represented as mean ± standard deviation.

3. Results and discussion

To clarify the influence of WO₃ LSPR on the luminescent behavior of magnetic–luminescent bi-functional nanocomposites, a Fe₃O₄@WO₃@GdF₃:Eu³⁺ heterostructure nanoparticle was designed. First, hydroxyl-functional magnetic Fe₃O₄ hydrophilic nanoparticles as the core were synthesized by a solvothermal method. Then, the as-prepared cores were used as seeds to fabricate core–spacer nanoparticles of Fe₃O₄@WO₃via homogeneous coating. Following this, CTAB on the surface of core–spacer nanoparticles of Fe₃O₄@WO₃ was washed off using a faintly acidic solution to realize the modulation of surface chemical ligands. To introduce nanocrystalline GdF₃:Eu³⁺ core–spacer, the nanoparticles of Fe₃O₄@WO₃ were functionalized with amino groups (–NH₂) by adding CTAB, which has the ability to concentrate Gd³⁺ on the Fe₃O₄@WO₃ surface. Then, the GdF₃ shell was fabricated on the Fe₃O₄@WO₃ surface by adding NH₄F under the help of CTAB, which acts as a dispersant in aqueous solution.

The X-ray diffraction (XRD) pattern of the Fe₃O₄@WO₃@GdF₃:Eu³⁺ nanoparticles shows three sets of characteristic peaks, belonging to cubic spinel Fe₃O₄, hexagonal WO₃, and orthorhombic GdF₃, respectively (Fig. 2). Scanning electron microscopic (SEM) images display pure Fe₃O₄ (Fig. 3a) with a mean particle size of 150 nm and a rough surface. After surface modification by adding the WO₃ shell, the Fe₃O₄@WO₃ nanoparticles retained their spherical morphology with a lack of aggregation and smoother and very dense surface (Fig. 3b).

Fig. 2 XRD patterns of (a) Fe₃O₄, (b) Fe₃O₄@WO₃, (c) Fe₃O₄@WO₃@GdF₃:Eu³⁺, and (d) GdF₃.

Fig. 3 SEM images and the scale bar of Fe₃O₄ (a and d); Fe₃O₄@WO₃ (b and e); Fe₃O₄@WO₃@GdF₃:Eu³⁺ (c and f) (insets: photographs of Fe₃O₄, Fe₃O₄@WO₃, and Fe₃O₄@WO₃@GdF₃:Eu³⁺ powders).

Compared with Fe₃O₄, the particle size of Fe₃O₄@WO₃ increased significantly and was about 250 nm. As shown in the insets of Fig. 3(d–f), the color of Fe₃O₄@WO₃ powder is lighter than that of Fe₃O₄ powder. These results suggested that the WO₃ spacer layer was uniformly deposited on the Fe₃O₄ nanoparticles. After surface modification by adding the GdF₃:Eu³⁺ shell layer, the Fe₃O₄@WO₃@GdF₃:Eu³⁺ nanoparticles retained their spherical morphology with a lack of aggregation and a rough surface (Fig. 3(c)). The color of the Fe₃O₄@WO₃@GdF₃:Eu³⁺ nanoparticles became lighter. Compared with Fe₃O₄@WO₃, the particle size of Fe₃O₄@WO₃@GdF₃:Eu³⁺ increased significantly and was about 400 nm. These results suggested that the GdF₃:Eu³⁺ phosphor layer was uniformly deposited on the Fe₃O₄@WO₃ nanoparticles. The analysis results of XRD, SEM, and photographs of the powders proved that Fe₃O₄@WO₃@GdF₃:Eu³⁺ nanoparticles were successfully produced.

TEM images of the Fe₃O₄@WO₃@GdF₃:Eu³⁺ nanoparticles are shown in Fig. 4(a and b). TEM observations revealed that the as-prepared Fe₃O₄@WO₃@GdF₃:Eu³⁺ nanoparticles appear spherical. The core–shell structure could be clearly distinguished because of the different color contrast between the cores and shells. The average diameter of the Fe₃O₄@WO₃@GdF₃:Eu³⁺ nanoparticles was about 400 nm, and the shell showed a gray color. Moreover, the distinct lattice fringes in the high-resolution TEM image (Fig. 4i) confirmed the successful crystallization of GdF₃ and WO₃ on the surface of Fe₃O₄, which was in good agreement with the wide-angle XRD results (Fig. 1). The spacing in the shell region was about 0.3800 nm and 0.1487 nm, for the (002) and (420) planes of WO₃ and GdF₃, respectively, versus about 0.2518 nm in the core region, corresponding to the (311) plane of Fe₃O₄. To further identify the structure of the Fe₃O₄@WO₃@GdF₃:Eu³⁺ nanocomposites, the corresponding EDX mapping of Fe₃O₄@WO₃@GdF₃:Eu³⁺ sample was recorded to analyze the elements, as shown in Fig. 4(c–h). Fig. 4(c–h) represent the mapping of Fe, O, W, Gd, F and Eu elements, respectively. It can be seen that all the elements are distributed homogeneously in the sample. The selected-area electron diffraction (SAED, the inset of Fig. 4i) reveals the polycrystalline feature of the as-prepared product. From EDX mapping and TEM, we further confirmed the formation of Fe₃O₄@WO₃@GdF₃:Eu³⁺ nanocomposites.

Fig. 4 (a and b) TEM images of Fe₃O₄@WO₃@GdF₃:Eu³⁺ nanocomposites and high magnification (i); (c–h) energy dispersive X-ray (EDX) mapping of Fe₃O₄@WO₃@GdF₃:Eu³⁺ nanocomposites.

Fig. 5 presents the comparison UV-vis absorption spectra of the Fe₃O₄@WO₃ and Fe₃O₄@WO₃@GdF₃:Eu³⁺. The UV-vis absorption spectra of plasmonic Fe₃O₄@WO₃ and Fe₃O₄@WO₃@GdF₃:Eu³⁺ nanoparticles exhibit distinct absorption bands located at around 691 nm and 700 nm, respectively. These observations are in accordance with the intrinsic optical property of WO₃ reported in the literature,²³ originating from intense LSPR absorption of the WO₃. The LSPR of WO₃ can be maintained for at least three months at room temperature and atmospheric pressure, even though this property originates from the oxygen vacancies.

Fig. 5 UV-vis absorption spectra of (a) Fe₃O₄@WO₃ and (b) Fe₃O₄@WO₃@GdF₃:Eu³⁺.

To investigate the luminescence properties of the nanoparticles, we recorded these spectra at room temperature. In the excitation spectra (Fig. 6), which were monitored with the emission of Eu³⁺ at an excitation wavelength of 595 nm, we observed a strong excitation peak at 395 nm that can be assigned to the energy level transition of ⁷F₀ → ⁵L₆ of Eu³⁺. In the emission spectra (Fig. 6) obtained by excitation at 395 nm, the strongest emission (at 595 nm) was due to the ⁵D₀ → ⁷F₁ forced electric dipole transition, and the other emission bands observed at 616 nm were due to ⁵D₀ → ⁷F₂ transition. Additionally, as shown in Fig. 6, we can see the increase in intensity for Fe₃O₄@WO₃@GdF₃:Eu³⁺, which may be due to the following reasons: first, the shielding effect of the homogeneously coated WO₃ spacer prevents the direct contact between magnetic materials and luminescent materials, and thus effectively reduces the quenching effect of magnetic–luminescence. In addition, when there is external light radiation, the WO₃ spacer layer produces the plasma resonance,^21,22 so that the electromagnetic field of WO₃ is redistributed. The surface plasma resonance of WO₃ is represented by the absorption of ultraviolet visible light.²³ Notably, the Fe₃O₄@WO₃@GdF₃:Eu³⁺ nanoparticles still displayed an intense LSPR absorption band at 400–700 nm (Fig. 5). Upon 395 nm excitation, WO₃ serves as the “plasmonic antenna” to locally concentrate the UV energy near the WO₃ spacer layer, and then resonantly transfers this energy to the adjoining GdF₃:Eu³⁺ nanoparticles (Fig. 6).²⁴ Therefore, the GdF₃:Eu³⁺ near the Fe₃O₄@WO₃@GdF₃:Eu³⁺ interface would experience a far more intense excitation electric field based on the surface enhancement effect, which could promote electron population on the excited-state energy levels of Eu³⁺ ions, thus resulting in enhanced luminescence.²⁴ Moreover, the LSPR-enhanced localized electric field can interact with the emission electric field of GdF₃:Eu³⁺ and enhance the emission electric field of WO₃ and GdF₃:Eu³⁺ interface.²⁴ From all the above discussions, we have faith in speculating that the enhancement in luminescence intensity obtained in our study comes from the shielding effect of the homogeneously coated WO₃ spacer. LSPR of WO₃ shell also greatly contributes to the enhancement mechanisms owing to local electric field amplification. The quantum yield of Fe₃O₄@GdF₃:Eu³⁺ and Fe₃O₄@WO₃@GdF₃:Eu³⁺ was also measured, which was 0.23 and 1.36, respectively. This further proves that the luminescence intensity of Fe₃O₄@GdF₃:Eu³⁺ is enhanced by WO₃. In addition, luminescence was also demonstrated by the photos taken by a digital camera with and without an external magnetic field. Fig. 6 (inset) shows the corresponding luminescence photos of the solutions before applying magnetic field (left) and in the presence of a magnet (right), confirming the photoluminescence upon irradiation by light (395 nm). From these results it can be concluded that the Fe₃O₄@WO₃@GdF₃:Eu³⁺ nanocomposites exhibit visible light emission that can be used for fluorescent imaging of cells and tissues.

Fig. 6 Emission and excitation spectra of the samples: GdF₃:Eu³⁺, Fe₃O₄@WO₃@GdF₃:Eu³⁺, Fe₃O₄@GdF₃:Eu³⁺ and dark field under 395 nm excitation, (inset) photographs of aqueous solution of the Fe₃O₄@WO₃@GdF₃:Eu³⁺ before applying a magnetic field (left) and in the presence of a magnet (right).

Magnetic measurements showed that pure Fe₃O₄, Fe₃O₄@WO₃, and Fe₃O₄@WO₃@GdF₃:Eu³⁺ had magnetization saturation values of 71.4, 39.1 and 24.5 emu g⁻¹, respectively (Fig. 7). It is noteworthy that the Fe₃O₄@WO₃@GdF₃:Eu³⁺ retained strong magnetization, indicating its suitability for magnetic targeting and separation as a drug carrier. Saturation magnetization of the Fe₃O₄@WO₃@GdF₃:Eu³⁺ nanocomposites occurred at 24.5 emu g⁻¹, which is smaller than that of the Fe₃O₄@WO₃ nanoparticles. This is likely due to the decreased proportion of Fe₃O₄ in the nanocomposites. Moreover, the Fe₃O₄@WO₃@GdF₃:Eu³⁺ nanocomposites showed fast response to the external magnet (Fig. 7). When an external magnetic field (magnet) was applied, the magnetic nanoparticles rapidly concentrated on the wall of the bottle on the side of the magnet, and the solution quickly cleared. Compared with Fe₃O₄ and Fe₃O₄@WO₃ nanoparticles, the magnetic saturation intensity of Fe₃O₄@WO₃@GdF₃:Eu³⁺ nanoparticles was low; hence, within the same time, Fe₃O₄@WO₃@GdF₃:Eu³⁺ nanoparticles slowly concentrated on the wall of the bottle on the side of the magnet, and the solution clarified slowly. The solution of Fe₃O₄@WO₃@GdF₃:Eu³⁺ was brown even after the addition of external magnet. These results indicate that the Fe₃O₄@WO₃@GdF₃:Eu³⁺ nanocomposites possess excellent magnetic responsivity, which is important for their practical application.

Fig. 7 Magnetic hysteresis loops of pure Fe₃O₄ (a), Fe₃O₄@WO₃ (b), and Fe₃O₄@WO₃@GdF₃:Eu³⁺ (c). Photographs of Fe₃O₄@WO₃@GdF₃:Eu³⁺ nanoparticles dispersed in aqueous solution.

Fig. 8 shows the percentage viability of MCF-7 cells treated with the PEG solvent and Fe₃O₄@WO₃@GdF₃:Eu³⁺ nanoparticles at different concentrations. The viability is 89% up to 15 mg mL⁻¹ concentration. These results indicate that the nanocarrier almost has no cytotoxicity or side effects in living cells (Fig. 8) because Fe₃O₄, WO₃ and GdF₃:Eu³⁺ have good biocompatibility.

Fig. 8 The percentage viability of MCF-7 treated with the Fe₃O₄@WO₃@GdF₃:Eu³⁺ nanoparticles at different concentrations.

4. Conclusions

In summary, we report a facile method for the preparation of core–shell structured Fe₃O₄@WO₃@GdF₃:Eu³⁺ bifunctional nanocomposites. The nanocomposites have high emission intensity and magnetisation saturation value. Compared with Fe₃O₄@GdF₃:Eu³⁺, the luminescence intensity of Fe₃O₄@WO₃@GdF₃:Eu³⁺ nanocomposites increased significantly. Therefore, the as-prepared Fe₃O₄@WO₃@GdF₃:Eu³⁺ core–shell structured bifunctional nanocomposites can be feasibly applied to simultaneous cell imaging and target drug delivery.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

This study was supported by National Natural Science Foundation of China (51704116), Hunan Province Natural Science Foundation of China (2018JJ3252, 2018JJ3250), the Scientific Research Project of Hunan Province Department of Education (16B136) and China Postdoctoral Science Foundation (2017M 612582).

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