Microwave assisted solvothermal method has been employed to synthesize multifunctional upconverting β-NaGdF4:Ln3+ and magnetic-upconverting Fe3O4/γ-Fe2O3@NaGdF4:Ln3+ (Ln = Yb and Er) nanoparticles. The powder x-ray diffraction data confirms the hexagonal structure of NaGdF4:Ln3+ and high resolution transmission electron microscopy shows the formation of rod shaped NaGdF4:Ln3+ (∼ 20 nm) and ovoid shaped Fe3O4/γ-Fe2O3@NaGdF4:Ln3+ (∼ 15 nm) nanoparticles. The magnetic hysteresis at 300 K for β-NaGdF4:Ln3+ demonstrates paramagnetic features, whereas iron-oxide@β-NaGdF4:Ln3+ exhibits superparamagnetic behavior along with a linear component at large applied field due to paramagnetic NaGdF4 matrix. Both nanoparticle samples provide an excellent green emitting [(2H11/2, 4S3/2)→4I15/2 (∼ 540 nm)] upconversion luminescence emission under excitation at 980 nm. The energy migration between Yb and Er in NaGdF4 matrix has been explored from 300-800 nm. Intensity variation of blue, green and red lines and the observed luminescence quenching due to the presence of Fe3O4/γ-Fe2O3 in the composite has been proposed. These kinds of materials contain magnetic and luminescence characteristics into single nanoparticle open new possibility for bioimaging applications.

For the last two decades, scientific and industrial interest in developing materials has culminated multiple times especially in the preparation of quality materials with enhanced multifunctionality at nanoscale.1–4 For instance, the nanocomposites containing luminescent and magnetic characteristics are trending in a wide range of applications, such as bioimaging, diagnostic, and therapeutics. Meanwhile, magnetic iron oxide nanoparticles have also been proved promising hence allowed in a wide range of biomedical applications.2,4 However, a small application oriented work has been performed combining iron oxide with luminescent materials due to the so called quenching effect induced by semimetallic Fe3O4 during simultaneous optical excitation/emission with applied external magnetic field.5–7 These multifunctional nanoparticles can serve as luminescent markers; they can also be controlled by an external magnetic field.8 Several parameters such as magnetic, electrostatic, hydrophobic and many chemical interaction creates a boundary and limitation for the real application of multifunctional nanoparticles as these issues generates low chemical stability and aggregation at nanoscale.2,9,10 To resolve above mentioned quenching issue, aggregation, and agglomeration etc., few approaches are in use such as: a magnetic core coated with silica, polymer or lipid containing fluorescent components; covalently bounded to a fluorophore via a spacer; a fluorescent shell; and/or quantum dots encapsulated in a polymer or silica matrix.

Lanthanide (Ln3+) doped rare earth fluoride upconversion (UC) nanoparticles absorb near–infrared (NIR) photons and emit higher energy photons, either in the ultraviolet–visible-NIR regions and are being potentially used in industry and biomedical applications.11,12 They are preferred nanoluminescent materials because of high penetration depth and signal–to–noise ratio, a large anti-Stokes shift, a long luminescence lifetime, good chemical and photochemical stability, resistance to photo bleaching, low auto–fluorescence and excellent detection sensitivity. Additionally, Gd3+ containing UC nanoparticles exhibit paramagnetism at room temperature and they can efficiently alter the spin–spin relaxation time of surrounding water protons because Gd possesses seven unpaired electrons.13 Therefore, Gd3+ containing rare earth fluoride UC luminescence nanomaterials have been developed as potential T1–weighted MR imaging contrast agents in biomedical applications and are being explored.5,14

In this article, we present a facile microwave solvothermal method for the synthesis of bifunctional β-NaGdF4:Ln3+ nanoparticles and direct coating of the same over surface of iron-oxide core nanoparticles. We focus on the changes of the intensity of emission peaks with lifetime variation in these two bifunctional nanomaterials and provide direct evidence of quenching induced by magnetite/maghemite phase. The nanocomposite particles have been thoroughly characterized using XRD, HRTEM, photoluminescence and dc magnetization measurements. These investigating techniques suggest a comparative approach to understand two different templates of magnetic-luminescent nanoparticles.

The iron-oxide nanoparticles were prepared using method described in Ref. 15. A facile microwave assisted solvothermal reaction has been proposed for the synthesis of NaGdF4:Ln3+ and iron-oxide@NaGdF4:Ln3+ (Ln3+ = 20%Yb, 2% Er) nanoparticles. The shell NaGdF4:Ln3+ was prepared in following way: firstly, 0.78 mmol GdCl3.6H2O, 0.20 mmol YbCl3.6H2O and 0.02 mmol ErCl3.6H2O was added into solution containing 16 mL oleic acid to form rare earth oleate and 8 mL of 1-octadecene under continuous stirring at ambient temperature for 20 minutes. Meanwhile, 2.5 mmol NaOH was dissolved in 10 mL methanol and then 8 mL oleic acid was added by stirring to prepare Na-oleate translucent solution. The above two solutions were mixed quickly and stirred for additional 5 minutes. A separate solution of iron oxide nanoseeds prepared in 1-octadecene was poured in above solution slowly and stirred for 10 minutes at room temperature. Another solution of 4 mmol fluoride source (NH4F) in 15 mL methanol was mixed properly in above solution. The mixture was transferred into the reaction vessel of a commercial microwave reactor working at 1000 W. The working cycle of the microwave reactor was set as (i) 200C/minute rapid heating until 1500C from room temperature; and (ii) 40 minute at 1500C. The system was allowed to cool quickly to room temperature and obtained samples were washed sequentially with methanol and ethanol, and then dried in an air oven at 50°C for 6 hours.

The phase purities of all the samples were checked by using X-ray diffraction (XRD) measurements on a Brucker`s D8 Advance diffractometer using Cu Kα radiation (λ= 1.54 A°). The size and shape morphology were characterized using transmission electron microscopy (TEM, FEI Tecnai, 200 keV). The MPMS3 Quantum Design SQUID magnetometer was used for the dc magnetic measurement, whereas a spectrophotometer (Horiba NanoLog spectrofluorimeter) equipped with an external excitation laser source of 980 nm was used for the upconversion experiments.

The phase structure of synthesized iron-oxide, NaGdF4:Ln3+ and iron-oxide@NaGdF4:Ln3+ nanoparticles were determined by XRD patterns (Figure 1). The XRD pattern of iron-oxide was indexed to the face-centered cubic Fe3O4 spinel structure (Fd3m space-group) (ICDD#019-0629). Moreover, the as-prepared Fe3O4 was partially reduced into γ-Fe2O3 phase (ICDD#039-1346) (highlighted by star in Figure 1). Furthermore, by comparing with the ICDD#027-0699, the XRD pattern for NaGdF4:Yb,Er nanoparticles has been indexed for hexagonal structure with space group P6¯ For iron-oxide@NaGdF4:Ln3+ nanocomposites, besides the characteristic diffractions of Fe3O4/γ-Fe2O3, the obvious diffraction peaks were indexed to NaGdF4:Ln3+ (ICDD#027-0699). The weakening of the Fe3O4/γ-Fe2O3, diffraction peaks in the nanocomposites indicates that the structure of Fe3O4/γ-Fe2O3@NaGdF4:Ln3+ was formed successfully, although, lattice energy mismatch between iron oxide and NaGdF4:Yb, Er suggest the formation of composite type structure.8 The crystallite size was calculated by using Debye-Scherrer’s equation16,17 and the mean particle size of Fe3O4/γ-Fe2O3 nanoparticles was ∼ 5 nm, whereas for NaGdF4:Yb,Er, it was ∼20 nm. After coating of NaGdF4:Ln3+ on Fe3O4/γ-Fe2O3, the calculated size was ∼ 15 nm, which is subsequently confirmed by particle size distribution obtained through TEM imaging (Figure 2).

FIG. 1.

X-ray powder diffraction patterns of cubic Fe3O4/γ-Fe2O3, hexagonal NaGdF4:Yb, Er and Fe3O4/γ-Fe2O3@NaGdF4:Yb, Er nanoparticles.

FIG. 1.

X-ray powder diffraction patterns of cubic Fe3O4/γ-Fe2O3, hexagonal NaGdF4:Yb, Er and Fe3O4/γ-Fe2O3@NaGdF4:Yb, Er nanoparticles.

Close modal
FIG. 2.

High resolution transmission electron microscopy images of (a) spherical Fe3O4/γ-Fe2O3, (b) elongated rod like NaGdF4: Yb, Er and (c) ovoid shaped Fe3O4/γ-Fe2O3@NaGdF4:Yb, Er nanoparticles. (d) Particle Size distribution of Fe3O4/γ-Fe2O3@NaGdF4:Yb,Er obtained by high resolution transmission electron microscopy.

FIG. 2.

High resolution transmission electron microscopy images of (a) spherical Fe3O4/γ-Fe2O3, (b) elongated rod like NaGdF4: Yb, Er and (c) ovoid shaped Fe3O4/γ-Fe2O3@NaGdF4:Yb, Er nanoparticles. (d) Particle Size distribution of Fe3O4/γ-Fe2O3@NaGdF4:Yb,Er obtained by high resolution transmission electron microscopy.

Close modal

Figure 2 shows the TEM images for Fe3O4/γ-Fe2O3, NaGdF4:Ln3+, and Fe3O4/γ-Fe2O3@NaGdF4:Ln3+ nanoparticles. The spherical iron oxide nanoparticles of 5-6 nm were observed (Figure 2a). In Figure 2b, most of the particles have a rod like morphology and few particles appear to be close to a spherical shape. During coating process of iron-oxide with NaGdF4:Yb, Er, the overall heterogeneous nucleation was slowed down and ovoid shaped morphology has been observed as shown in Figure 2c.

The green emitting visible UC luminescence spectra of NaGdF4:Yb, Er and Fe3O4/γ-Fe2O3@NaGdF4:Yb,Er nanoparticles have been shown in Figure 3a. On the basis of energy matching conditions the synthesized nanoparticles are discussed by observing the emission spectra (Figure 3, a & b). Under excitation at 980 nm, the green luminescence assigned to the (2H11/2, 4S3/2)→4I15/2 (∼ 540 nm) transition, red emission originating from the 4F9/24I15/2 (∼ 660 nm) transition and the blue emissions attributed to the 4G11/24I15/2 (∼ 378 nm), 2H9/24I15/2 (∼ 411 nm) transitions of Er3+ ions were observed. The overall green emission line intensity of Fe3O4/γ-Fe2O3@NaGdF4:Yb, Er is approximately 30% of NaGdF4:Yb, Er green line. Furthermore, the surface properties and crystallinity of NaGdF4:Yb, Er nanoparticles can affect the different emitting levels of doped ions, and hence result in the different emission intensity of red, green and blue light and the ratio of these are also different in both samples. Hence, the reduction of intensities (green, blue and red) can be ascribed due to direct energy mismatch of cubic Fe3O4/γ-Fe2O3 and hexagonal NaGdF4 matrix; reduction of radiative luminescence centers over the surface of luminescence shell of NaGdF4:Ln3+ when iron-oxide provides a leakage path for radiative centers (Er3+); and induced arrangement of magnetic domain ordering at the surface of iron oxide.

FIG. 3.

(a) Upconverting luminescence (UCL) emission spectra of NaGdF4:Yb,Er and Fe3O4/γ-Fe2O3@NaGdF4:Yb,Er nanoparticles recorded in range from 300 nm to 800 nm, upon excitation of 980 laser. The inset figure shows the bright green emission for NPs dispersed in hexane at excitation of 980 nm. (b) Proposed energy transfer mechanism of photons in between Yb3+ and Er3+ ions doped in NaGdF4 matrix.

FIG. 3.

(a) Upconverting luminescence (UCL) emission spectra of NaGdF4:Yb,Er and Fe3O4/γ-Fe2O3@NaGdF4:Yb,Er nanoparticles recorded in range from 300 nm to 800 nm, upon excitation of 980 laser. The inset figure shows the bright green emission for NPs dispersed in hexane at excitation of 980 nm. (b) Proposed energy transfer mechanism of photons in between Yb3+ and Er3+ ions doped in NaGdF4 matrix.

Close modal

The energy transfer (ET) in these samples can be explained in following steps under 980 nm of excitation: (i) Yb3+ ions are excited from the 2F7/2 level to the 2F5/2 level and then transfer their energies to the nearby Er3+ ions; (ii) the upper levels 4G11/2, 4F7/2, and 4F9/2 of Er3+ ions are mainly populated by the ETs between Yb3+ and Er3+ (2F5/22F7/2 (Yb); 4I15/24I11/2 (Er), 4I13/24F9/2 (Er), and 4I11/24F7/2 (Er)), (iii) the excited state absorption of the pump radiation from the 4I11/2 and 4I13/2 levels, and (iv) the cross relaxation between Er3+ ions, as depicted in Figure 3b.

The magnetic hysteresis loops recorded at 300 K for Fe3O4/γ-Fe2O3, NaGdF4:Yb, Er and Fe3O4/γ-Fe2O3@NaGdF4:Yb, Er nanoparticles are shown in Figure 4. NaGdF4:Yb, Er nanoparticles were found paramagnetic with a susceptibilities of 0.8 x 10-2 of emu g-1Oe-1, whereas Fe3O4/γ-Fe2O3, and Fe3O4/γ-Fe2O3@NaGdF4:Yb, Er clearly showed superparamagnetic behavior with negligible coercive field. The saturation magnetization (Ms) values were found to be 30.4 emu g-1 and 7.8 emu g-1 for Fe3O4/γ-Fe2O3 and Fe3O4/γ-Fe2O3@NaGdF4:Yb, Er, respectively. These reductions in MS were attributed to the variance in particle size distribution, prolonged existence relaxation of nanostructures and non-collinear spin arrangement/disorder spin on the surface of very small nanoparticles (∼ 5 nm) of Fe3O4/γ-Fe2O3. The partial reduction of Fe3O4 into γ-Fe2O3 also affects the MS (25% of Fe3O4) in the nanoparticles. Also, the small particle size and crystallinity of nanoparticles leads to a lower magnetization value. The MS of Fe3O4/γ-Fe2O3@NaGdF4:Yb, Er was decreased due to the contribution of a low mass fraction of the NaGdF4:Yb, Er with Fe3O4/γ-Fe2O3. Further, the M-H curve for Fe3O4/γ-Fe2O3@NaGdF4:Ln3+ nanoparticles consists of two components, a superparamagnetic component with magnetic moment ∼ 8 emu g-1, and a paramagnetic component (linear slope at large applied fields). However, the overall Ms value of Fe3O4/γ-Fe2O3@NaGdF4:Yb,Er showed satisfactory response to the external applied magnetic field (Inset, Figure 4).

FIG. 4.

Field dependent magnetization (M-H curves) of iron-oxide, paramagnetic NaGdF4:Yb, Er, and magnetic luminescence Fe3O4/γ-Fe2O3@NaGdF4:Yb,Er nanoparticles. Inset diagram shows that Fe3O4/γ-Fe2O3@NaGdF4:Yb,Er nanoparticles attracted towards a bar magnet with sufficient magnetic characteristics for several applications.

FIG. 4.

Field dependent magnetization (M-H curves) of iron-oxide, paramagnetic NaGdF4:Yb, Er, and magnetic luminescence Fe3O4/γ-Fe2O3@NaGdF4:Yb,Er nanoparticles. Inset diagram shows that Fe3O4/γ-Fe2O3@NaGdF4:Yb,Er nanoparticles attracted towards a bar magnet with sufficient magnetic characteristics for several applications.

Close modal

The paramagnetic contributions in Fe3O4/γ-Fe2O3@NaGdF4:Yb,Er and Ln3+-doped NaGdF4 nanocrystals featured mainly arising from Gd3+ ions. The seven unpaired electrons in the inner 4f subshells of the Gd3+ ions are tightly bound to the nucleus and shielded by the outer closed shell 5d16s2 electrons from the crystal fields and are responsible for the magnetic characteristics of the Gd3+ ions. The magnetic moments related to the Gd3+ ions are all localized and non-interacting, which led to paramagnetism of the Gd3+ ions. Hence, under magnetic field the samples containing matrix of NaGdF4 nanoparticles can induce relative magnetism and such samples recovers its initial state before applied field, when magnetic field is removed. This kind of effect is called magneto-optical interaction in NaGdF4:Yb, Er nanoparticles and obtained under certain magnetic fields and the relative luminescence modulation may be accessible with a magnetic field. Generally, modulating luminescence of materials via magnetic fields needs the coupling of optical and magnetic interaction and it is believed that simultaneous realization of optical and magnetic properties in a single phase will avail optical and magnetic interaction within the crystal lattice and hence promising modulation luminescence via magnetic fields.18 

To summarize, we have reported a microwave assisted solvothermal synthesis of multifunctional NaGdF4:Ln3+ and Fe3O4/γ-Fe2O3@NaGdF4:Ln3+ (Ln = Yb, Er) nanoparticles. This synthesis suggested morphological difference for both the samples as observed through XRD and HRTEM. The magnetic and luminescent properties of Fe3O4/γ-Fe2O3@NaGdF4:Ln3+ nanocomposites were compared with paramagnetic NaGdF4:Ln3+ and discussed in context to the possible luminescence quenching due to Fe3O4/γ-Fe2O3. The magnetic hysteresis at 300 K for β-NaGdF4:Ln3+ showed paramagnetic features; whereas Fe3O4/γ-Fe2O3@β-NaGdF4:Ln3+ exhibited superparamagnetic behavior with negligible coercive field. The luminescent properties with strong magnetic response in an external magnetic field and better stability of the nanocomposites can be used for the development of certain luminescence−magnetic composites for biomedical industry and may open up new opportunities in this field.

This work was supported by the jointly CAPES, FAPEMA, and FAPEAL in Brazil and CONICET in Argentina.

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