Europium activated gadolinium sulfide nanoparticles

Abstract

In the present work, we report the synthesis and characterization of Eu³⁺ doped Gd₂S₃ nanoparticles, a potential candidate for T₁-weighted MRI contrast agents and molecular markers. Eu³⁺ doped Gd_2(1−x)S₃:Eu_x (x = 0.0, 0.02, 0.04 and 0.06) were synthesized by a chemical route by varying the Eu concentration. X-ray diffraction and scanning electron microscopy were performed to extract the information about structure and surface morphology of prepared nanoparticles, respectively. The oxidation state of the Eu ions was elucidated from X-ray absorption near edge structure (XANES) spectra, which indicate the presence of only Eu³⁺ ions without any signature of Eu²⁺ ions. 120 MeV Ag⁹⁺ ions were used for ionoluminescence (IL) measurements to study the optical properties. The IL results show that the luminescence intensity increases with increasing Eu doping and no saturation or degradation of the luminescence were observed for the as-prepared nanoparticles. The nanoparticles doped with 2 and 4% Eu showed an increase in the IL intensity initially before decreasing to saturation at higher fluences. On the other hand, IL intensity from 6% doped samples decreased exponentially and saturated at higher fluences.

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1. Introduction

Luminescent magnetic nanoparticles have attracted researchers due to their numerous uses in in vivo applications such as medical diagnostics, contrast enhancement, hyperthermia, magnetic resonance imaging (MRI), therapeutics and tissue repair.^1–6 The common magnetic nanoparticles based on super paramagnetic iron oxides (SPIO) have high transverse and longitudinal relaxivity ratio because of their superparamagnetic nature and are used as negative contrast agents. On the other hand, the paramagnetic contrast agents, which exhibited rather low transverse and longitudinal relaxivity ratio and impose the shortening of the longitudinal relaxation time T₁ (known as T₁-weighted positive contrast agents), have several important advantages over negative contrast agents. However, there is a high demand for innovative and extra proficient T₁-weighted contrast agents because of their low relaxivity and higher signal enhancing effect with magnetic homogeneity without perturbation of the magnetic field. This has led researchers to develop various types of T₁ contrast agents. Among them, rare-earth chalcogenides, such as sulfides of europium or gadolinium have drawn much attention because of their intrinsic magneto-optical properties. It is a great challenge in MR and imaging field to develop a positive contrast agent^7,8 with lanthanides based luminescence.⁹ To solve such questions, recently, lanthanides ions (Eu³⁺, Tb³⁺, Yb³⁺ or Er³⁺) doped magnetic host nanoparticles were developed and it was shown that these particles are valuable for the advanced dual-mode imaging.⁶ Among them, Eu³⁺ has attracted to most as dopant because of large Stokes shifts, long lifetime and narrow emission lines and moreover it has potential to optimize the intensity of red emission for cell imaging. On the other hand, the Eu³⁺ ion, with an even number of 4f electrons, has a great advantage over rest of the lanthanide ions as the starting levels of the transitions are non-degenerate in both the absorption and the luminescence spectrum. While, Gd³⁺ forms the basis of the most widely used T₁ contrast agents due to advantage of its seven unpaired electrons and become a suitable platform for doping with luminescent lanthanide ions to achieve dual mode MRI contrast agent. Owing to these properties, Eu doped Gd₂O₃, GdPO₄ and GdS/Gd₂S₃ magnetic luminescent nanoparticles have been prepared and tested. Europium(III) ion doped host matrix exhibit typical red emission due to its characteristic transitions of ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₂ with good color purity resulting from sharp emission by well shielded 4f electrons. We have extensively studied the photoluminescence properties of rare earth doped in different matrix.^9–14 As far as gadolinium based nanomaterials are concerned, our group also reported synthesis of Eu doped Gd₂S₃,¹² Gd₂O₃,¹³ GdS¹⁴ magnetic nanoparticles by chemical methods along with special reference to their properties viz. optical, structural and potential candidate for MRI contrast agent.

In the present report, we have synthesized the Eu doped Gd₂S₃ nanoparticles and studied its electronic and ionoluminescence (IL) properties. IL is a characterization technique used for study of defects and materials characterization using beam of swift heavy ions responsible for light emission from solids due to electronic excitation and relaxation.^15–17 It is an ion beam analysis (IBA) technique complementary to other IBA techniques such as particles induced X-ray emission (PIXE), nuclear reaction analysis (NRA), elastic recoil detection analysis (ERDA), Rutherford back scattering (RBS) spectrometry etc. It is a suitable technique to probe the impurities, doped ions and defect in the matrix, colour centres formation and it can also be used to understand the fundamental processes associated with the damage creation and its kinetic evolution under applied ion fluences.¹⁸ IL is nothing but a type of luminescence in which accelerated energetic ion beam is used as excitation source. IL signals are associated with the energy levels of their external valence band electrons and thus are highly sensitive to the chemical states, surrounding crystal field of emitting atoms and its local symmetry.^19,20 IL can also be used to excite higher electronics levels under dense electronic excitation due to the higher energy of the ion beam which cannot be probed by conventional luminescence methods. The fascinating use of IL is its in situ applications during ion beam induced materials engineering and modification^21,22 which enables good imaging capabilities.²³ As far as the transition metal is concerned, IL is especially sensitive to the presence of the d-transition metal ions and rare earths. IL studies on Eu³⁺ doped in CaSiO₃,²⁴ Zn₂SiO₄,²⁵ Mg₂SiO₄ (ref. 26) matrix have been reported, but, to the best of our knowledge, this is the first report on IL investigations on Eu³⁺ doped Gd₂S₃ nanoparticles. To get the information about the structure and surface morphology, X-ray diffraction (XRD) and scanning electron microscopy (SEM) were also performed, respectively. To reveal valence state of metal ions, X-ray absorption near edge structure (XANES) spectra of Gd and Eu L₃-edge were recorded.

2. Experimental

Gd₂S₃:Eu³⁺ nanoparticles were prepared using same protocol used in previous reports by our group¹² in a clean room of class 1000 under ambient conditions. The analytical grade europium(III) nitrate hexahydrate, Eu(NO₃)₃·6H₂O (99.99%), gadolinium(III) nitrate hexahydrate, Gd(NO₃)₃·6H₂O, (99.9%), sodium sulphide (Na₂S), ethanol and de-ionized water were used to synthesize Gd₂S₃:Eu³⁺ nanoparticles. Stock solutions of metal nitrates were prepared by dissolving an appropriate amount of metal nitrates in de-ionized water. 10 mM aqueous solution of Na₂S was prepared separately. Then, 40 mM of gadolinium nitrate solution and 4 mM of europium nitrate solution (for 10% Eu doping) were mixed together homogeneously for 60 min with constant stirring. Thereafter, 40 mM of Na₂S solution was added to the above metal nitrate solution. After 30 min of stirring at room temperature the solution was heated at 60 °C with constant stirring until visible precipitate appeared. Precipitate was centrifuged, and washed several times with absolute ethanol and de-ionized water. Obtained product was kept at 60 °C in a vacuum oven for 24 h to get solid powder.

The prepared materials were characterized by XRD, SEM, XANES and IL to get information about structural and optical properties. XRD was performed on Rigaku D/max-2200 PC diffractometer operated at 40 kV/20 mA, using Cu Kα₁ radiation with wavelength of 1.54 Å in the region of 23–60° on 2θ scale. The surface morphology was observed by field emission scanning electron microscope MIRA II LMH from TESCAN. The energy of electrons was kept at 25 keV and images were captured using the secondary electron detector. The oxidation states of metal ions were elucidated by XANES on the L₃-edge of Gd and Eu at BL-9, Scanning EXAFS Beamline of Indus-2. The beamline mainly consist of Rh/Pt coated meridional cylindrical mirror for collimation and Si (111) based double crystal monochromator to select excitation energy. IL properties of Gd₂S₃:Eu³⁺ nanoparticles were studied under bombardment of 120 MeV Ag⁹⁺ ion. For IL measurements, pellets of nanoparticles with 10 mm diameter and 1 mm of thickness were prepared using a pelletizer. IL measurement were performed at room temperature in high vacuum chamber of Materials Science beam line at Inter University Accelerator Centre, New Delhi. The data were collected for 5 s for each fluence with an interval of 6.4 s in the range of 7 × 10¹² to 1 × 10¹⁴ ions per cm² ion beam fluence. Under excitation, Eu³⁺ ions showed different emission band in the range of 570–840 nm. An overview of the transitions²⁷ reported in emission spectra of europium(III) compounds are shown in Fig. 1. Due to an even number of 4f electrons, starting levels of the transitions of the Eu³⁺ ion are non-degenerate in both the absorption and the luminescence spectrum. The number of lines observed for the ⁵D₀ → ⁷F₁ transitions in the emission spectrum allows determining the site symmetry of the Eu³⁺ ion. The fine structure and the relative intensities of the bands in the luminescence spectra of Eu³⁺ can be used to probe the local environment of the Eu³⁺ ion.

Fig. 1 The transitions reported in emission spectra of europium(III).

3. Results and discussion

Fig. 2 shows XRD patterns of nanomaterials for 2 and 6% Eu doped Gd₂S₃. No significant change is observed in the XRD patterns for 2 and 6% Eu doping. XRD spectra show broad peaks at the positions of 27.88, 29.32, 32.4, 41.1, 43.38, 49.64, 50.36, 52.87 and 58.27° corresponding to (210), (211), 212), (411), (105), (503), (600), (222) and (116) diffraction planes, respectively, which is in good agreement with standard JCPDS (76-0265). There is a slight change in the peak position for some of the diffraction lines, but they are close enough to the standard one. The shift in the peak position towards lower angle may be due to the development of strain etc. in the doped material. Absence of any additional/impurity peak indicates complete doping of Eu ion the matrix. The crystallize size, d, was estimated by Scherrer's formula,²⁸ d = 0.9λ/β cos θ, where d is the crystallite size, λ is the wavelength of radiation used, 2θ is the peak position of lattice planes, and β is the full width at half maximum (FWHM) on 2θ scale. The average size of the crystallites were found to be about 13.8 and 14.3 nm for 2 and 6% Eu doping, respectively.

Fig. 2 XRD patterns of nanomaterials for 2 and 6% Eu doped Gd₂S₃ nanoparticles.

Fig. 3 presents SEM micrographs of (a) 2%, (b) 4% and (c) 6% Eu doped Gd₂S₃ nanomaterials at two different magnifications. Idea of SEM measurement was to see the morphological pattern such as nanowire, rods or some special formation etc., but no specific structure was observed. Surface morphology of the materials is nearly similar for all the prepared samples as seen in low magnification images. While, agglomerated particles are observed on the surface in the micrographs of high magnification.

Fig. 3 SEM micrographs of (a) 2%, (b) 4% and (c) 6% doped Gd₂S₃ nanoparticles at two different magnifications.

Fig. 4 presents the XANES spectra of Gd L₃-edge and Eu L₃-edge. XANES measurements were carried out in transmission mode for Gd L₃-edge and in fluorescence mode for Eu L₃-edge. The energy range was calibrated simultaneously by measurements on a commercial Fe foil. Eu₂O₃ and Gd₂O₃ were used as reference compounds for Eu³⁺ and Gd³⁺ ions, respectively. The XANES data has been then analysed using ATHENA software.²⁹ Fig. 4(a) and (c) show the XANES spectra of Eu and Gd L₃-edges for doped nanoparticles, whereas Fig. 4(b) and (d) show 1^st derivative spectra of Eu₂O₃ (Eu³⁺) standard, Eu doping with 2, 4 and 6% and Gd₂O₃ (Gd³⁺) standard, Eu doping in Gd₂S₃, respectively. From XANES spectra and from 1^st derivative of XANES spectra of Eu and Gd, it is clear that Eu and Gd both are present in 3+ oxidation state only, and these are nearly similar as in case of standard Eu₂O₃ and Gd₂O₃. Being more favourable during synthesis, the doping of Eu³⁺ ions results in co-existence of Eu³⁺ and Eu²⁺ ions in the system due to the reduction of Eu³⁺ into Eu²⁺. But, absence of any signature of Eu²⁺ in XANES spectra confirms that we have successfully doped solo Eu³⁺ ion in Gd₂S₃ nanoparticles. Generally, L₃ edge XANES spectra exhibit a white line (WL) [as seen feature “A” in Fig. 4(a)] that results due to 2p_3/2 → 5d transition. The Eu L₃-edge WL intensity has been observed to decrease with higher Eu content. This reduction in WL intensity suggests that unoccupied states of Eu are more populated with increase in Eu doping in Gd₂S₃ matrix.

Fig. 4 (a) Eu L₃-edge XANES spectra of Eu doping, inset shows zoomed view of white line variation as a function of doping percentage of Eu (c) 1^st derivative of XANES spectra. (b) Gd L₃-edge XANES spectra of Eu doped, (d) 1^st derivative of XANES spectra. Dashed lines in (b & d) are drawn to highlight the edge position of Eu³⁺ and Gd³⁺.

Fig. 5 shows IL spectra for undoped and Eu³⁺ doped Gd₂S₃ as-prepared nanoparticles. For clarity, IL spectrum of undoped nanoparticles is shifted downward in the intensity. The IL spectra consist of mainly three peaks at peak positions of 590, 612 and 700 nm (orange-red emission), while no peak is present in IL spectrum of the undoped sample. 2% Eu doped sample shows very weak IL signal and only peak at 612 nm is clearly visible. The most intense emission peak exhibited at 612 nm is attributed to ⁵D₀ → ⁷F₂ electric dipole transition of Eu³⁺ ion. The other relatively weak observed IL emission peaks are at 590 and 700 nm and are attributed to the characteristic luminescence centers activated by Eu³⁺ ions corresponding to ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₄ transitions, respectively. From the IL spectra, it is confirmed that all the IL bands are associated with Eu³⁺ ion and it is in agreement with the XANES results that the dopant oxidation state is 3+ only. IL intensity increases with increasing Eu concentration in the host matrix. Appearance of the shoulders at 613, 619 and 692, 700 nm indicates the splitting of transitions corresponding to j = 2 and 4, respectively. Fig. 6 shows deconvoluted IL spectrum for 6% Eu doping to demonstrate the deconvolution of these shoulders luminescence. It was also observed that the band widths are narrow and the full width at half maxima (FWHM) decreases with increase in the Eu concentration in the Gd₂S₃ matrix. The FWHMs of the band at 612 nm are 16, 14 and 11 nm for 2, 4, and 6% Eu doped Gd₂S₃ as-prepared nanoparticles, respectively.

Fig. 5 Primary IL spectra of undoped and Eu doped as-prepared nanoparticles. For clarity, IL spectrum of undoped nanoparticles is shifted downward in intensity.

Fig. 6 IL spectrum of 6% Eu doped Gd₂S₃ to demonstrate the deconvolution of different luminescence peaks.

The dependences of IL intensities on Eu³⁺ doping concentrations in Gd₂S₃ phosphors are shown in Fig. 7. IL intensity increases with the increase in the Eu³⁺ doping ratio and no concentration quenching was observed up to the highest doping concentration of 6%. It is pretty established fact that the origin of ⁵D₀ → ⁷F₁ transition is from a magnetic dipole transition and its intensity hardly varies with the crystal field strength acting on the Eu³⁺ ion. On the other hand, the lack of inversion symmetry at the Eu³⁺ site induces electric dipole ⁵D₀ → ⁷F₂ transition. The ⁵D₀ → ⁷F₂ transition dominates the emission spectra if Eu³⁺ ion localizes at the site without inversion symmetry, while its intensity increases with increase in the distortion of the local field around the Eu³⁺ ion.^27,29–31 On the other hand, if Eu occupies site with inversion symmetry, the ⁵D₀ → ⁷F₁ transition is the dominant transition. The intensity ratio of (⁵D₀ → ⁷F₂)/(⁵D₀ → ⁷F₁) is defined as the asymmetry ratio and therefore, can be used to determine the degree of distortion from the inversion symmetry of the local environment of Eu ions in the host matrix.

Fig. 7 Dependence of IL intensity on Eu ion doping in Gd₂S₃ system, measured at room temperature (RT) and low temperature (LT).

The fluence dependence of IL properties was studied for all the doped systems as presented in Fig. 8 for various fluences. The variation of ⁵D₀ → ⁷F₂ IL intensity for 4 and 2% (in the inset) of Eu doped nanomaterials are shown in Fig. 9, while Fig. 10 presents the variation of ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₄ band's IL intensities for 6% Eu doped system. The inset in Fig. 10 shows the intensity ratio of the bands at 612 and 590 nm i.e. the degree of distortion from the inversion symmetry of the local environment of Eu ion in the host matrix. As ⁵D₀ → ⁷F₁ band is not present in the case of 2% Eu doping and the dominance of ⁵D₀ → ⁷F₂ band in all the other samples can be seen from Fig. 8, it is clear that the Eu ion locates the site with noninversion symmetry. The sample with 4% Eu doping showed increase in the IL intensity initially and then exponentially decreases to saturation. Although we measured only few fluences for 2% doping, but it clearly shows the same behaviour of increase in the intensity followed by decrease. On the other hand, IL intensity from 6% doped samples decreases exponentially with fluence and saturates at higher fluences. The ⁵D₀ → ⁷F₂ band disappeared after bombardment of first fluence of 2 × 10¹² ions per cm² for 4% doping samples while it disappeared at 2 × 10¹³ ions per cm² for 6% Eu doped Gd₂S₃ sample. Furthermore, the ⁵D₀ → ⁷F₄ band vanished out at a fluence of 2 × 10¹³ ions per cm² for 4% Eu doped sample. Increase in the ion fluence excites more and more number of Eu ions followed by radiative decay resulting in increase in IL intensity. After some fluence, disorder causes quenching of IL intensity and at higher fluence when the system enriched with defects, the intensity saturates. For 6% doping system, the intensity is highest for lower fluence due to excitation of large number of the Eu ions. However, increase in the non-radiative transition due to disordered system results in the decrease in the IL intensity with further applied ion fluence leading to saturation at higher fluence. On the other hand, thermal quenching also reduces the radiative transition. The disorder in the system and the thermal quenching are produced by the dense electronic excitation of the system and can be explained on the basis of the inelastic thermal spike model.^32–35 According to the inelastic thermal spike model, the energy is deposited by the ions in the electronic subsystem of the target materials. This energy is shared among the electrons by electron–electron coupling mechanism and transferred subsequently to the lattice atoms via electron–lattice interactions resulting in a transient increase in the temperature along and in the vicinity of the ion path in a nano-dimensional cylinder region. Rapid quenching of thermal energy creates ion damage zone, known as ion track, of excited electron cloud resulting in system disorder and defect formation. The excited system neutralize by emitting its energy via radiative emission, from excited state to ground level, of doped ions located in the matrix or by the non-radiative emission of the phonons from the lattice system. It is well established fact that the radiative transition rate decreases due to high defect concentration, resulting in the reduction of integrated IL intensity from the sample as discussed in case of color centers induced photoluminescence from LiF thin films.³⁶ The non-radiative emission, due to continuous bombardment of the SHIs, results in the rise of temperature in the solid causing thermal quenching and producing disorder in the system. The damage cross section, σ, was estimated by the exponential fitting of the experimental data of ⁵D₀ → ⁷F₂ band for 4 and 6% Eu doped Gd₂S₃ systems using expression I(ϕ) = I₀ exp(−σϕ) and the track radius, R, was deduced by the relation, R = (σ/π)^1/2, where I₀ is the IL intensity for as-prepared nanomaterials, while I(ϕ) is the IL intensity at a fluence of ϕ.³⁵ The damage cross sections and track radii were 6.2 nm², 1.4 nm and 3.6 nm², 1.14 nm for 4 and 6% Eu doped Gd₂S₃ systems, respectively.

Fig. 8 IL spectra of (a) 2%, (b) 4% and (c) 6% Eu doped Gd₂S₃ nanoparticles collected at different ion fluences.

Fig. 9 Variation of areal intensity of the band at 612 nm for 2 and 4% doped Gd₂S₃ system. Solid lines are just to guide the eyes, while dashed line shows the exponential fit of the decay response data for 4% Eu doped Gd₂S₃ system.

Fig. 10 Variation of areal intensity of the bands at 612 and 700 nm for 6% Eu doped Gd₂S₃ nanoparticles. The solid lines are exponential fit to the respective data. The inset shows the intensity ratio of the bands at 612 and 590 nm i.e. the degree of distortion from the inversion symmetry of the local environment of Eu ion in the host matrix.

Based on the above results, IL measurement was also performed at low temperature (LT, ∼77 K) for 6% Eu doped Gd₂S₃, as it showed the highest IL intensity among the doped samples with maximum number of bands which starts disappearing at least after 2 × 10¹³ ions per cm² applied ion fluence as presented in Fig. 11. The LT data revealed that IL intensity is very poor as compared to the data collected at room temperature. The intensity observed at 77 K is nearly 6 times lower than that for RT (300 K) measurement. Also, it showed only presence of ⁵D₀ → ⁷F₂ band, while other bands, ⁵D₀ → ⁷F₁ and ⁵D₀ → ⁷F₄ are absent. The intensity of the band decreases with fluence and nearly disappeared at a fluence of 3 × 10¹² ions per cm² and only signature of the band is present.

Fig. 11 IL spectra of 6% Eu doped Gd₂S₃ nanoparticles performed at low temperature (77 K) at different ion fluences.

The detailed analysis of IL properties with different Eu concentration and ion fluence at different temperature indicates the importance of red emission at 612 nm from Gd₂S₃:Eu nanoparticles. One can tune the emission intensity by doping concentration, ion fluence or irradiation temperature. Although, low temperature results in the reduction of IL intensity, but it can be used to get single band emission. In the view of the present findings, it is essential to examine the possibility of the nanoparticles for MRI contrast agent applications. It is also worth to mention that the irradiated materials have to be encapsulated or coated with biocompatible material for clinical testing. Further studies are in progress to examine these nanoparticles for biological marker and MRI contrast agents' applications after encapsulation of the desired nanoparticles with some biocompatible material followed by irradiation at different ion fluence and temperature.

4. Conclusions

Electronic and luminescence properties of chemically synthesized Eu³⁺ doped Gd₂S₃ nanoparticles are investigated. The 3+ oxidation states of the Eu ions is determined by XANES, which is also confirmed by the IL spectra of the nanoparticles as the prominently observed IL emission peaks centered at 590, 612 and 700 nm are attributed to the luminescence centers activated by Eu³⁺ ions corresponding to ⁵D₀ → ⁷F₁, ⁵D₀ → ⁷F₂ and ⁵D₀ → ⁷F₄ transitions, respectively. From the intensity ratio of (⁵D₀ → ⁷F₂)/(⁵D⁰ → ⁷F₁) bands, it is clear that the Eu ion locates the site with noninversion symmetry. Results show that the luminescence intensity increases with increasing Eu concentration in host matrix and 6% Eu doping results in highest IL intensity. Reduction in the IL intensity was observed with increase in the ion fluence and saturates at higher fluence. On the other hand, the IL measurement at low temperature shows reduction in intensity and disappearance of the bands after certain fluence.

Acknowledgements

NAC is thankful to DST, India for providing funds under Nano-mission HFIBF project (IR/S2/PF/0001/2009). We also thank to Dr Subodh and Pelletron Group for their support during in situ measurements.

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Footnote

† Present address: K. N. Govt P. G. College, Gyanpur SRN, Bhadohi 221304, India.

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