Lanthanide ion-doped rare-earth fluorides have gained considerable attention because of their potential applications in the fields such as photodynamic therapy, three-dimensional displays, catalysis, solid-state lasers, low-intensity IR imaging and other optical devices arising from intra f-f transitions [1-5]. Among the various host matrices for Ln3+ ions, fluoride host lattices have high chemical and thermal stability and also possess low phonon energies to reduce non-radiative relaxations, thereby improving the luminescence of the optically active dopants [6-8]. Low-energy phonons and high ionicity are highly desirable properties for efficient luminescence. Multifunctional nanocrystals possessing both magnetic and fluorescent properties have received increasing attraction in the past decade [9-12].The synthesis of magnetic nanoparticles (MNPs) have attracted much attention due to their excellent physico-chemical properties. During the last few years, efficient routes have been devised for the synthesis of shape-controlled and highly stable functionalized polyethylenimine (PEI), β‐cyclodextrin and Succinate grafted PEI nanocomposites with magnetic ferrites [13-17]. These functionally modified MNPs found their applications in dye degradations [18, 19], drug delivery [20-22] and as magnetically separable efficient organic catalyst, used for green oxidation of alcohols [23-27], oxidation of hydrocarbons in the presence of H2O2 , anti-oxidant  and synthesis of polyhydroquinoline derivatives [30, 31].Paramagnetic Gd3+ doped host metal fluoride, when combined with luminescent lanthanide metal ion gives a wide range of applications including magnetic resonance imaging, drug target, various medical diagnostics, cancer therapy, recording material, catalysis and magneto-optics devices [32-38]. NaLuF4 is considered as one of the promising host fluoride nanomaterials based on the physical, chemical and optical properties of the Lu3+ ion [39-41]. Although the Lu-based host nanomaterials exhibit promising photoluminescence behaviour , limited reports are there on the study of the bi-functional optical and magnetic NaLuF4 nanomaterials. In addition, the optical, magnetic, physical, and chemical properties of nanocrystals depend highly on their structure, size, and shape [43-45]. Therefore, it is very important to synthesize nanocrystals with well controlled size and structure. Lanthanide doped luminescent materials have been synthesized using various methods such as thermal decomposition , co-precipitation , hydro(solvo)thermal [48,49], ionic liquid-based synthesis , microemulsion assisted  and microwave-assisted synthesis . Among these methods, hydrothermal synthesis allows excellent control over particle size, shape, distribution and crystallinity of material. The synthesis is conducted in a stainless autoclave using water as a solvent and nanocrystal formation process occurs under high autogenous pressure at a synthesis temperature above the boiling point of the solvent or mixed solution.
In this paper, we report the development of a hydrothermal method for the preparation of multifunctional magnetic-fluorescent lanthanide doped sodium lutetium fluoride (NaLuF4:Gd3+/Yb3+/Er3+) nanocrystals. Luminescence efficiency and paramagnetic behaviour of doped NaLuF4 nanocrystals have been studied which are promising for use as luminescent probes in biological labeling and imaging technology.
Materials and instrumentation
Lutetium(III) nitrate hydrate Lu(NO3)3.H2O (99.9%), gadolinium(III) nitrate hexahydrate Gd(NO3)3.6H2O (99.9%), ytterbium(III) nitrate hexahydrate Yb(NO3)3.6H2O (99.9%) and erbium(III) nitrate hexahydrate Er(NO3)3.6H2O (99.9%) were purchased from sigma Aldrich. Sodium hydroxide (NaOH), trisodium citrate (Na3C6H5O7) and ammonium tetrafluoroborate (NH4BF4)were purchased from Alfa Aesar and used as received without further purification. Doubly distilled water was used for preparing aqueous solutions.
The phase structure and size of the as-prepared samples were determined from powder X-ray diffraction (PXRD) using D8 X-ray diffractometer (Bruker) at a scanning rate of 12ο min-1 in the 2θ range from 10ο to 70ο, with Cu Kα radiation (λ=0.15405 nm). Scanning electron microscopy (SEM) analysis of the samples was recorded on FEI Nova NanoSEM 450. High resolution transmission electron microscopy (HRTEM) was recorded on Tecnai G2 20 S-TWIN Transmission Electron Microscope with a field emission gun operating at 200 kV. Samples for TEM measurements were prepared by evaporating a drop of the colloid onto a carbon-coated copper grid. The energy spectra were obtained by the energy-dispersive X-ray spectrum equipped on a Transmission Electron Microscope. The particle size of each compound was determined by DLS technique using Zetasizer Nano ZS-90 (Malvern Instruments Ltd., Worcestershire, UK). The photoluminescence excitation and emission spectra were recorded at room temperature using Agilent Cary Eclipse Fluorescence Spectrophotometer equipped with a Xenon lamp that was used as an excitation source. The magnetization as a function of an applied field for Gd3+ doped in core/shell nanoplates was recorded using vibrating sample magnetometer (VSM), Lakeshore 7410. All the measurements were performed at room temperature.
The pure/undoped NaLuF4 and NaLuF4:20%Yb3+ /2%Er3+/xGd3+ (x = 0%, 10%, 20%, 30%, and 45%) nanocrystals were synthesized by a hydrothermal method using trisodium citrate as a structure directing agent. The typical synthesis involved the addition of 10 mL of ethanol to 2 mL of an aqueous solution containing 1.2 g of NaOH under stirring to form a homogeneous solution. Then, 10 mL of trisodium citrate was added into the above solution under continuous stirring. Subsequently, 2 mmol RE(NO3)3 (RE = Lu, Yb, Er and Gd with designed molar ratios) and 5 mL of 4 mmol aqueous NH4BF4 solutions were added under constant vigorous stirring for 30 minutes. The resulting solution was transferred into a 50 mL stainless Teﬂon-lined autoclave, which was operated at 170 °C for 24 hours. As autoclave was cooled to room temperature naturally, the precipitates were separated by centrifugation, washed with deionized water and ethanol in sequence, and then collected nanocrystals were dried at 60 οC for 12 hours.
RESULTS AND DISCUSSION
Crystalline structure and morphology
The crystal structures and the phase purity of the as-prepared nanocrystals were examined by powder X-ray diffraction (PXRD) analysis. The diffraction peaks of the samples depicted in Fig. 1correspond to the hexagonal and cubic phase (marked with stars) of NaLuF4 (JCPDS card no. 27-0726). It can also be seen that the diffraction peaks of the NaLuF4 samples are very sharp and strong, indicating that products with high crystallinity have been obtained. High crystallinity is important for phosphors, because it generally means less traps and stronger luminescence. It should be noted that the distinctly strengthen intensities of typical peaks indicate the ideal growth orientation of the samples. The lattice parameters calculated using indexing method are in good agreement with those reported in the literature for bulk NaLuF4; a =5.901 Å, c = 3.453 Å (JCPDS card no. 27-0726) are shown in Table 1 . The slight change in the value of lattice constants may be caused by the addition of the dopant metal ion. The diffraction peaks gradually broaden on increasing the Gd3+ concentration, indicating a reduction of particle size, which is further confirmed by modified Scherrer’s equation. The broadening of the diffraction peaks indicates that the sizes of the undoped and doped NaLuF4 nanocrystals are at the nanoscale.
The average crystallite size of these nanocrystals was calculated according to the modified Scherrer’s equation
where, L is the crystallite size, λ is the wavelength of the Cu Kα radiant, λ=0.15405 nm, β is the full-width at half-maximum (fwhm) of the diffraction peak, θ is the diﬀraction angle and K is the Scherrer constant equals to 0.89. If we plot the results of lnβ against ln1/cosθ, then a straight line with the slope of around one and the intercept of lnK/L was obtained. After getting the intercept, the exponential of the intercept was obtained:
elnKλ/L = Kλ(nm)/L(nm)
Having the value of K and λ, a single value of L in nanometer was calculated. All the major peakswere used to calculate the average crystallite size of the synthesized nanocrystals.
The morphology of the synthesized undoped NaLuF4 and NaLuF4:20%Yb3+/2%Er3+/xGd3+ (x = 0%, 10%, 20%, 30%, and 45%) nanocrystals was investigated by using electron microscope studies such as scanning electron microscopy (SEM) and transmission electron microscopy (TEM). Fig. 2 shows SEM images of the synthesized undoped, 20%Yb3+/2%Er3+ doped and 20%Yb3+/2%Er3+/x%Gd3+ doped NaLuF4 nanocrystals. It is evident from the SEM images that the synthesized particles have flower-shaped morphology and do not show any aggregation of particles. SEM images also indicate that the surface morphology of nanocrystals do not cause any obvious change with the nature of dopant. Fig. 3 shows the high resolution TEM images of the synthesized undoped, 20%Yb3+/2%Er3+ doped and 20%Yb3+/2%Er3+/x%Gd3+ doped NaLuF4 nanocrystals.
Energy dispersive X-ray analysis spectroscopy was performed to investigate the presence of dopant in the doped NaLuF4 nanocrystals. Fig. 4 show the energy dispersive spectra of the Yb3+/Er3+ doped NaLuF4 nanocrystals and Yb3+/Er3+/x%Gd3+ doped NaLuF4 nanocrystals. It has been found that the elements ytterbium, erbium, fluorine, sodium and lutetium exist in the Yb3+/Er3+ doped NaLuF4 nanocrystals whereas gadolinium presents in the respective synthesized Yb3+/Er3+/Gd3+ doped NaLuF4 nanocrystals. These results confirm the doping in NaLuF4 nanocrystals. There is no impurity phase detected in the EDS spectra showing the formation of pure product.
Particle size by DLS technique
The particle size of the synthesized undoped NaLuF4 and NaLuF4:20%Yb3+/2%Er3+/xGd3+ (x = 0%, 10%, 20%, 30%, and 45%) nanocrystals was also examined using DLS technique. The nanocrystals were suspended in aqueous medium as colloidal solution after mild sonication for 15 minutes. As observed from Fig. 5, the DLS measurements show mean particle size of 156, 137, 123, 98, 90 and 85 nm for the synthesized undoped NaLuF4 and NaLuF4:20%Yb3+/2%Er3+/xGd3+ (x = 0%, 10%, 20%, 30%, and 45%) nanocrystals, respectively. The size calculated by DLS technique is usually larger than that calculated from PXRD data. The anomaly in sizes is due to surface solvation and agglomeration of the particles.
The controlling and tuning of band edge emission and surface traps state emission of NaLuF4:20%Yb3+/2%Er3+ and NaLuF4:20%Yb3+/2% Er3+/xGd3+ nanocrystals are very important to realize the tunable optical properties and laser emissions. Fig. 6 shows the UV–Vis absorption spectra of the synthesized NaLuF4:20%Yb3+/2%Er3+ and NaLuF4:20%Yb3+/2%Er3+/x%Gd3+ (x = 20% and 45%) nanocrystals. For recording the absorption spectra, the as-prepared doped NaLuF4 nanocrystals were dispersed in deionized water by ultrasonication for 15 minutes. The prominent absorption edge for as-prepared nanocrystals was observed at around 980 nm and some other less intense absorption edges were also observed lying in the range of 350- 900 nm. The optical band gap of the synthesized nanocrystals was calculated according to the relationship between the optical band gap (Eg) and wavelength (λ) (i.e., Eg = 1240/λ). The band gap thus calculated was found to be 5.1 eV.
The down-conversion emission spectra of NaLuF4:20%Yb3+/2%Er3+/xGd3+ (x = 0%, 10%, 20%, 30% and 45%) nanocrystals are shown in Fig. 7. Ytterbium acts as a sensitizer to enhance the luminescent property of erbium dopant ion . Upon excitation at 378 nm, the obtained emission spectrum originates due to transition from the 4S3/2 excited state to the 4I15/2 ground state of the Er3+ ions. With the increase in Gd3+ content, the down-conversion (DC) emission intensity of NaLuF4:20%Yb3+/2%Er3+/xGd3+ decreases evidently, which is mainly attributed to a signiﬁcant reduction in the size of NaLuF4 nanocrystals as evident from PXRD and DLS analyses. Decrease in the size of nanocrystals leads to the larger surface quenching sites, hence smaller nanocrystals may suppress DC luminescence by enhanced nonradiative energy transfer processes of the luminescent lanthanide ions [55, 56].
Besides the efficient optical property, Gd3+ doped NaLuF4 nanocrystals exhibit magnetic properties due to the large magnetic moment of Gd3+ at room temperature. Measurement of the magnetization as a function of the applied ﬁeld (-15 kOe to 15 kOe) for NaLuF4 nanocrystals doped with different Gd3+ contents, demonstrate that all the samples present typical paramagnetic behaviour (Fig. 8). The paramagnetic behaviour is mainly attributed to the seven unpaired inner 4f electrons, which are closely bound to the nucleus and effectively shielded by the outer closed shell electrons (5s25p6) from the crystal field . The magnetization value of the as-prepared NaLuF4 nanocrystals doped with 10%, 20%, 30%, and 45% Gd3+ ions are found to be 4.83 × 10-3, 6.2× 10-3, 7.04× 10-3 and 8.24× 10-3 emu g-1 at 15 kOe, respectively. The magnetization of the NaLuF4 nanocrystals can therefore be modified from 4.83 × 10-3 emu g-1 to 8.24× 10-3 emu g-1 at 15 kOe with increasing the Gd3+ doping content from 10% to 45%. These results indicate that these multifunctional NaLuF4 nanocrystals may have promising potential applications in bio-separation  and magnetic resonance imaging .
In summary, monodispersed Ln3+ (Ln = Gd, Yb and Er) doped NaLuF4 nanocrystals were synthesized via a simple hydrothermal method. PXRD analysis reveals that the size of NaLuF4 nanocrystals can be tailored by doping with Gd3+. Increase in the concentration of Gd3+ dopant ion can reduce the size of nanocrystal. Besides the efficient optical properties, Gd3+ doped NaLuF4 nanocrystals exhibit paramagnetic behaviour at room temperature with magnetization of up to 8.24× 10-3 emu g-1 at 15 kOe, which provides a simple approach for combining two functions into a single phase material. Therefore, the Gd3+ doped NaLuF4 nanocrystals not only can control the size but also can integrate additional magnetic functionality into these optical nanomaterials. It is expected that these monodispersed bi-functional NaLuF4 nanocrystals may have potential applications in in vitro and in vivo dual-modal fluorescent, magnetic bio-imaging and bio-separation.
We are thankful to Human Resource Department (HRD) Group of Council of Scientific & Industrial Research (CSIR) for funding the research fellowship. We would also like to acknowledge Indian Institute of Technology Mandi and Indian Institute of Technology Guwahati for their technical support. We thank SAIF, Panjab University, Chandigarh for powder X-ray diffraction study and we are thankful to Dr. Vinay Kumar, Assistant Professor, School of Physics, Shri Mata Vaishno Devi University (SMVDU) for photoluminescence studies.
CONFLICT OF INTEREST
The authors declare that there is no conflict of interests regarding the publication of this manuscript.