Effect of Solvent on the Red Luminescence of Novel Lanthanide

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Effect of Solvent on the Red Luminescence of Novel Lanthanide NaEu(WO4)2 Nanophosphorfor Theranostic Applications Archana K Munirathnappa, Ananda K, A. K Sinha, and Nalini G. Sundaram Cryst. Growth Des., Just Accepted Manuscript • DOI: 10.1021/acs.cgd.7b01177 • Publication Date (Web): 15 Nov 2017 Downloaded from http://pubs.acs.org on November 18, 2017

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Crystal Growth & Design

Effect of Solvent on the Red Luminescence of Novel Lanthanide NaEu(WO4)2 Nanophosphor for Theranostic Applications Archana K. Munirathnappaa,d, Ananda Kb, A.K Sinhac and Nalini G. Sundarama*

a.

Materials Science Division, Poornaprajna Institute of Scientific Research, Bidalur, Near Devanahalli, Bengaluru, Karnataka, India562110 and Manipal University, Manipal-576104

b.

Biological Science Division, Poornaprajna Institute of Scientific Research, Bidalur, Near Devanahalli, Bengaluru, Karnataka, India562110.

c. Hard d.

X-ray Application Laboratory, SUS,RRCAT, Indore-452013

Manipal University, Manipal-576104

*corresponding author: E-mail: [email protected]

KEYWORDS: Synchrotron Diffraction, Red nanophosphor, Biocompatible and Bio-imaging.

ABSTRACT Investigation of the red luminescence in NaEu(WO4)2 nanoparticles synthesized using water and ethylene glycol and its biocompatibility on HeLa cells, E. coli, S. aureus, C. albicans has been evaluated for the first time. The Scheelite like pure tetragonal NaEu(WO4)2 nanoparticles have been synthesized by a simple solvothermal method by using water (NaEuW I) and ethylene glycol (NaEuW II) as reaction medium. Detailed particle size and crystal structural analysis of both the samples is carried out using HRTEM and Synchrotron powder diffraction. Enhanced red fluorescence was observed on increase in calcination temperature in both the samples signifying their greater stability and increased crystallinity on thermal treatment. The blue shift of charge transfer (CT) band in NaEuW II nanoparticles is explained by the nature of the Eu-O bond lengths obtained by Rietveld refinement using Synchrotron diffraction data. Chromaticity diagram revealed the possibility of tuning the red emission of as prepared and annealed samples against particle size, choice of solvent and calcination temperature. The calculated Commission Internationale de l’Eclairage (CIE) coordinates of NaEuW II at 600° C matches the CIE values

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of the commercial red phosphor Y2O3S: Eu3+ standards of NSTC (National Standards of Television Commission). This indicates the potential applications of nano NaEuW [NaEu(WO4)2]

to generate red emission. Investigation of solvent effect on particle size

dependent biocompatibility of NaEu(WO4)2 via in-vitro cytotoxicity studies showed no significant toxicity towards HeLa cells, E. coli, S. aureus and C. albicans. More interestingly, bio imaging studies show excellent bio distribution and localization of luminescent nano NaEuW II. Hence, our preliminary studies could demonstrate a new strategy to design nano luminescent NaEu(WO4)2 for theranostic applications.

1. INTRODUCTION Inorganic oxide materials have attracted tremendous interest in the field of optoelectronics including phosphors for lighting and display systems due to their unique luminescent properties1. These properties have been majorly characterized by their chemical composition, stability, phase, surface morphology and particle size2, 3. In this regard lot of research has been focused on the morphology controlled synthesis of inorganic oxide materials for photoluminescent applications4, 5. There are many studies in literature6-8 that focus on the ratio of two solvents on the particle size and consequently on the luminescence properties of rare earth doped materials. Owing to their novel optical and magnetic properties, lanthanide doped oxide materials play a significant role in solid state lighting (SSL) applications as well as in biophotonics 9, 10. Presently due to the lower phonon energy and concentration quenching, lanthanide doped alkali fluorides and oxides have attracted significant attention in biomedical applications; such as drug delivery 11

vehicles, photo activated up converted nanoparticles etc. However, they do suffer from

poor mechanical stability and high toxicity1,

12, 13

. Among rare earth compounds, alkali

rare earth double tungstates represented as ARE(WO4)2,where A=monovalent cation (LiCs) and RE=rare earth ion, have recently generated considerable interest due to their high luminescence efficiency, high absorption, narrow emission, high critical concentration and quantum yield in phosphor based SSL applications 3, 10, 14-16. It is well studied that the luminescent properties of these materials significantly rely on host composition, the type of dopants and its concentration in the lattice14,

17

. Previously ARE(WO4)2 were mostly

synthesized via conventional solid state method18-20 and studied as a diode pumped


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tunable laser for solid state lighting applications. Currently, due of their intense luminescence properties and non toxicity they have also been investigated as active biological probes in theranostic applications such as bio imaging, photo activated drug delivery systems and optical bio probes21-24. To realize their practical performance, lot of work has been dedicated to enhance their unique properties such as non-toxicity, water solubility, and surface modification in order to increase their responsiveness and biocompatibility in in-vivo and in-vitro applications24, 25. In this context recently, M. Yang et.al, investigated NaLa(MoO4)2:Eu3+/Tb3+ biocompatibility on ARPE-19 cell lines for invitro applications21, due to the lower toxicity and tunable emitting properties. It has been observed in literature that particle size and surface charge play a significant role in the luminescent color and intensities16. More importantly, for biological applications it is imperative that, the nanoparticles be non-toxic and possesses higher colloidal stability under physiological conditions. Thus presence of functional groups such as (-COOH, OH, -NH2) usually enhance nanoparticles colloidal stability via surface charge modulation26 and also facilitate bio conjugation with the molecule of interest. Quite a lot of work is dedicated for nanoparticles surface fuctionalization with polymer, as they offer relatively good solubility, biocompatibility and also provide specific anchoring for targeted bio conjugation24,

27

. More recently, Laguna et al investigated the optical and

MRI imaging applications of citrate coated poly (L-lysine) functionalized Eu-doped NaGd(MoO4)216. However, to the best of our knowledge, there has been no work on the application of the potential optical properties of nano alkali rare earth double tungstates in biological systems.

This study presents for the first time the synthesis and biocompatibility of fluorescent nano NaEu(WO4)2 (NaEuW) by controlled soft chemical method using water and ethylene glycol under hydrothermal/solvothermal conditions. The main focus is on the investigation of the effect of solvent on the particle size, thereby on the fluorescence properties and biocompatibility of NaEuW nanoparticles for theranostic applications. Detailed crystal structure analysis of both the samples was carried out using synchrotron diffraction data. In addition, we have also systematically studied the structural aspects, particle size, surface charge and luminescent properties of as prepared NaEuW I and NaEuW II nanoparticles was found to be particle size


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dependent. Additionally, preliminary biocompatible studies were carried out in the presence NaEuW I and II on several microorganisms to assess their toxicity. Furthermore, non-toxicity and bio-imaging of NaEuW I and NaEuW II were carried out on HeLa cells. Hence, NaEuW is found to be non-toxic even under UV excitation and could be potentially used for invitro/in-vivo theranostic applications. Additionally, with suitable fuctionalization and crystal structure modification, it could also be excited under NIR region which is safer and leads to lesser photo damage.

2. EXPERIMENTAL SECTION 2.1 Materials and methods Stochiometeric amounts of Eu(NO3)3.5H2O and Na2WO4.2H2O were used as starting precursors to synthesize NaEu(WO4)2. Nutrient agar (NA) and Nutrient broth (NB) media were utilized for maintaining microbial culture. DMSO, MTT (3-(4,5-Dimethylthiazol-2yl)-2,5-diphenyltetrazolium bromide), Dulbecco’s modified Eagle’s medium (DMEM), Fetal bovine serum (FBS), phosphate-buffered saline (PBS), penicillin and streptomycin were used in pure form for MTT assay. Milli-Q water was obtained from an ultrapure water purification system and ethylene glycol (EG) were utilized as solvent under hydrothermal condition as reaction particle growth media. All the materials are purchased from Alfa Aesar, Sigma Aldrich and HiMedia of analytical grade and used as such without further purification.

2.1.1 Hydrothermal synthesis of NaEuW I In a typical synthesis 1 mmol of Eu(NO3)3.5H2O was dissolved in 10 ml milli-Q water (solution-A) and 2 mmol of Na2WO4.2H2O was dissolved in another 10 ml milli-Q water (solution-B) and stirred continuously. Further solution-A was added drop wise to solution-B and stirred at room temperature for a half an hour. The resulting milky white suspension was made up to 40 ml and poured into a Teflon lined auto-calve of 50 ml capacity sealed and subsequently heated at 200° C for 24 hours. After cooling to room temperature, the formed precipitate was separated via centrifugation and washed several times with milli-Q water and finally dried at 90° C for 4 hours, white color powder of


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NaEuW I was obtained. The reaction parameters such as temperature, time and pH have been optimized for NaEuW I (water) and similar conditions were followed for NaEuW II (EG) (SI Figure 1-3).

2.1.2 Solvothermal Synthesis of NaEuW II

Similar procedure as described above was followed using ethylene glycol (EG) instead of water as the solvent. Light grayish color powder of NaEuW II was obtained.

2.2 Characterization Phase purity and the crystallinity of NaEuW I and NaEuW II was confirmed by Bruker, D2 phaser X-ray powder diffractometer using Cu-Kα radiation of wavelength (λ=1.5418 Å). Particle size distribution were observed from High Resolution Transmission Electron Micrographs(HRTEM) obtained using JEOL JEM Electron microscope operated at 200 kV accelerating voltage. Lattice fringes and d-spacing was calculated from the selected area electron diffraction (SAED) pattern and HRTEM image using Gatan digital micrograph and Image J software. Furthermore, elemental composition was estimated by energy dispersed X-ray analysis (EDAX). Zeta potential measurements were carried out on NaEuW I and NaEuW II using Brookhaven Zeta PALS (Phase analysis light scattering sensitive) instrument at different pH values. Synchrotron diffraction data was collected for both the samples at Indus-2 synchrotron source (2.5 GeV, 200 mA electron storage ring) using Image plate (Mar 345 Dtb) area detector. Angle Dispersive X-ray Diffraction beamline (BL-12) was used for the measurements. Wavelength (λ=0.8252 Å) was accurately calibrated using XRD pattern of LaB6 NIST standard. Rietveld refinement was carried out on NaEuW I and NaEuW II using the GSAS & EXPGUI program suite. The Diffuse reflectance spectra (UV-DRS) of both the samples were measured with Perkin Elmer UV-Visible spectrophotometer using tungsten halogen lamp source in the range 200-1200 nm. The functional group on the surface of NaEuW I and NaEuW II were analyzed with Bruker Alpha FTIR Spectrophotometer in the range of 500-4000 cm-1. Room temperature fluorescence properties such as excitation and emission spectra were


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measured and recorded using in-house Agilent Cary Eclipse Spectrophotometer with Xenon flash lamp source of 5 nm excitation and emission slit respectively. Quantum yield of both the samples were calculated by an absolute integrated sphere method using Edinburg instrument equipped with the 450W Xenon lamp source on FLSP920 spectrophotometer. NaEuW non toxicity studies were carried out on E. coli, C. albicans, S. aureus and HeLa cells. Cell imaging studies were carried out on human mammalian HeLa cells and bio images were collected with the help of EVOS® FL inverted microscope.

2.3. Microbial toxicity assay

Microbial toxicity and susceptibility of nanomaterials was studied using gram-positive (bacteria) Staphylococcus aureus (MTCC 3160), gram-negative (bacteria) Escherichia coli (MTCC 1687) and fungi Candida albicans (MTCC 7253). All the strains were purchased from the microbial type culture collection, IMTECH and maintained as per the standard protocol.

2.3.1 Agar diffusion method

All the microbial strains were sub cultured in sterile nutrient broth under aseptic condition at 37° C for 4 hrs. Luminescent NaEuW I and NaEuW II test samples were suspended in sterile milli-Q water using ultra sonicator. Different concentrations of sample (2.5, 5.0 and 10.0 μg/ml) was directly placed on the surface of agar which was previously inoculated with the different microbial strains independently. Later the agar plates were incubated at room temperature for 24 hr and microbial toxicity was evaluated by measuring the zone of inhibition formed around the test samples. Similarly antimicrobial standards (AMS) ciprofloxacin and fluconazole served as positive control during the experiment. Experiment was repeated for several times to obtain good reliability.


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2.3.2 Microbial cell viability method

Microbial cell viability of NaEuW I and NaEuW II was analyzed against 12 hr old bacterial and fungal cultures. 100 μL of diluted microbial culture was suspended in 96well microtiter wells and different concentrations of test sample (ranging from 31.25 to 500 µg/mL) were loaded to each well separately. The total volume of the micro wells was made up to 200 µL and optical density (OD) was recorded at 600 nm using microplate reader. Later on the plate was incubated at 37°C for 24 hours and OD was taken at different time intervals. The percent microbial cell viability was calculated using initial and final OD. The experiment was repeated in triplicates and the mean standard deviation was calculated. Microbial culture grown without sample was considered as a standard (control).

2.3.3 Cytotoxicity by MTT assay Cytotoxicity assay was carried out using HeLa cell lines grown in Dulbecco’s modified Eagle’s medium (DMEM) with 10% fetal bovine serum (FBS), 60 μg/ml penicillin, 100 μg/ml streptomycin in an humidified incubator at 37°C and 5% CO 2 with reference to the standard procedure reported with slight modification16. Cells were transferred in each well of 96 well micro titer plates and incubated in DMEM medium for 24 hr. Later the cell medium was replaced and different concentrations of test sample NaEuW I and II (31.12, 62.5, 125, 250, 375 and 500 μg/mL) were added separately in triplicates and further incubated for 24-48 hr. After the completion of incubation period, media was removed and cells were washed with sterile PBS. Additionally, 20 μl of MTT reagent (3, (4,5-dimethyl-2-yl)-2,5-diphenyltetrazolium bromide) was introduced and incubated for 4hr. Further, DMEM medium containing different concentration of luminescent nanomaterials was replaced with 200 μl of DMSO (dimethyl sulphoxide) and stabilized for 5 minutes. Optical absorbance of solubilized Formosan was measured at 570 nm. Average absorbance of treated cells was compared with the control (cell culture without sample) and negative control (DMSO).


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2.3.4 Bioimaging of HeLa cells incubated with NaEuW I and II Trypsinised HeLa cells were cultured for 24 hrs by seeding 2X10 4 cell per well at 37 °C in 5% CO2 incubator with DMEM media. Then the cultured HeLa cells were transferred to the 4-well chambered slide and further incubated for 24 hours. Finally the adherent cells were incubated with the 50 μg/ml concentrated NaEuW I and II to interact with cells for 24hr. Later on the adherent cells were washed with fresh phosphate buffer to circumvent excess nanoparticles and cells were viewed under EVS® FL color inverted fluorescence microscope.

3. RESULTS AND DISCUSSION 3.1 Powder X-ray diffraction NaEuW was prepared by facile solvothermal method in water and ethylene glycol and the phase purity was confirmed by powder X-Ray patterns (figure 1). The observed XRD patterns are in good agreement to the Scheelite like tetragonal phase with I4 1/a space group of ICSD No: 184023. Broadened peaks observed in NaEuW II compare to NaEuW I suggesting the formation of smaller crystallites in ethylene glycol and the calculated crystallite size from Scherer formula is found to be 26 nm and 102 nm for NaEuW I and II respectively.

3.1.1 Growth mechanism of NaEu(WO4)2 Polyols such as ethylene glycol (EG) and glycerol found to play crucial role in determining particle size and morphology28. The plausible mechanism for the formation of NaEuW nanoparticles can be explained on the basis of Einstein-Stokes theorem as reported by Nuria28 and Gibbs-Thomson Law explained by X. Liu29. When EG is utilized as full reaction medium one could except diffusion controlled growth mechanism wherein, EG molecules might have migrated from the solution to the surface of nuclei formed as a result of (WO4)2- and cation interactions. EG molecules could have capped the surface of primary nuclei which in turn inhibits the growth in all directions and thus results in smaller particles as it shown by TEM characterization. The high viscosity and the complexing behavior of EG could be responsible for the formation of smaller particles which in turn determined the particle growth rate compared to


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water6. However, when water molecules are used the onset of precipitation was faster than that of EG which is mainly attributed to the lower viscosity of water. This in turn accelerates the nucleation process and eventually resulted in agglomerated larger particles in water. The irregular particle size and morphology of NaEuW could be due to the growth of nuclei along the direction of high surface energy. We observed under uniform reaction temperature, time, pH and the solvent ethylene glycol are found to have significant role in slowing down the rate of particle growth in NaEuW II.

Figure 1a) and b) Powder X-ray diffraction pattern of NaEuW I and NaEuW II. Inset images illustrate the red fluorescence from NaEuW I and II under ordinary and UV light illumination. Figure c) and d) Observed (red), calculated (green) and difference between observed and calculated (pink) obtained from the Rietveld refinements using Synchrotron diffraction data of NaEuW I and II respectively.


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3.2 High Resolution Transmission Electron (HRTEM) analysis

The HRTEM measurements [Figure 2a), b) and c)] illustrate bright field micrograph, selective area electron diffraction (SAED) pattern, HRTEM image of NaEuW I. A typical bright field image revealed that formation of agglomerated irregular shaped particles of average size 200-300 nm and the calculated crystallite size is found to be ~102 nm from Scherrer equation. The intense bright spots in SAED pattern indicates the formation of single crystalline nature of NaEuW I. FFT (Fast Fourier transform) of HRTEM micrograph confirms the growth of particle along plane of d=4.7 Å, which is indexed to the spacing of (011) plane. Bright field image of NaEuW II (Figure 3a) revealed the formation of agglomerated sphere like small particles of an average size 10-20 nm and the corresponding calculated crystallite size is found to be 26 nm. Intense bright spots in diffused multiple rings like manner in SAED pattern [figure 3b)] reveal the formation of much smaller particles of NaEuW II than NaEuW I. FFT (Fast Fourier transform) of HRTEM micrograph [figure 3 c)] shows the particle along a plane with d=3.1 Å, indexed as the (112) plane and another particle along a plane of d=4.6 Å indexed as (011) plane. All the hkl and d values of NaEuW I and NaEuW II are in good agreement with the pXRD results. The EDS analyses [SI Figure4a) and 1b)] confirmed the atomic fractions of Na, Eu, W and O as prepared NaEuW I and II.

Figure2.a) Bright field micrograph b) SAED pattern and c) HRTEM image of as prepared NaEuW I.


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Figure 3. a) Bright field micrograph b) SAED pattern and c) HRTEM image of as prepared NaEuW II.

3.3 Zeta potential study

Zeta potential analysis was carried out to measure the net surface charge on the NaEuW particles. The samples were dispersed in double distilled water and diluted to the required volume. The temperature was maintained at 34° C and subsequent surface charge was measured at different pH. Measured Zeta potential as a function of pH is shown in figure 4 clearly indicates -15 mV and -32 mV for NaEuW I and NaEuW II at pH=7.4 and error bars are indicated on y-axis. Presence of negative surface charge on both the samples is attributed to the presence of surface –OH group from the solvent used during synthesis (supported by FTIR analysis SI Figure5). It is noteworthy that NaEuW II exhibits higher negative surface charge as compared to NaEuW I which is attributed to the capping action of the EG and also due to smaller particle size of NaEuW II. Furthermore, negative charges is reported to provide platform for surface modification by anchoring functional group for further bio conjugation with the molecule of interest 16, 30.


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Figure 4. A plot of zeta potential versus pH of NaEuW I and NaEuW II

3.4 Synchrotron X-ray diffraction study

The Profile of both the samples was fitted using Pseudo-Voigt function and a shifted Chebyshev function was applied to refine the background. Figure 1c) and d) depicts the observed, calculated and difference plots obtained from Rietveld refinement using GSAS program& EXPGUI interface31. Table 1 shows the room temperature crystallographic data of NaEuW I and II respectively. Since heavy atoms such as W, Na and Eu were in special position only the occupancy and thermal parameters of Na and Eu atoms were refined alternately keeping overall occupancy of the site is 1 [Table 2 and 3]. Further, the occupancy and thermal of parameters of W was also refined alternately to convergence. Since it is high resolution synchrotron data, the thermal parameters of oxygen atoms could also be refined to convergence. From figure 6, it is clear that Na and Eu atoms occupy the same position forming regular polyhedra of EuO8 with eight oxygen atoms. Each tungsten atom bonds forms a forming regular tetrahedron with four oxygen atoms and connects to Na/Eu-O8 polyhedra via single oxygen atoms. Even though the reaction medium is changed to EG from water, the tetragonal phase with I4 1/a space group was retained. It is well known17 that the luminescent properties of an emitter ion is mainly decided by its local environment [figure 7 a) and b)] i.e., any changes around its local


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structure could directly influence the luminescence properties. Therefore it is essential to calculate bond lengths around Eu3+ in NaEuW samples. From Table 3, Figure 7 a) and b), it can be observed that there is a wider distribution of Eu-O bonds in NaEuW II compared to NaEuW I indicating the Eu-O environment is more asymmetric in NaEuW II and this observation could have implications on the fluorescent properties of the samples. Crystallographic data

NaEuW I

NaEuW II

Space group

I41/a

I41/a

a=b (Å)

5.216 (1)

5.232 (1)

c (Å)

11.345 (1)

11.340 (2)

α, β, γ (°)

90°

90°

Unit cell volume (Å3)

308.75 (3)

310.42 (2)

RWp(%)

3.3

1.3

Rp (%)

2.0

1.0

R I(hkl)

3.4

2.1

Table 1. Room temperature Crystallographic data of NaEuW I and NaEuW II

Figure 6.Structural representation of NaEu(WO4)2 (NaEuW) along b-direction.


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Figure 7.Local environment of luminescent Eu3+ in a) NaEuW I and b) NaEuW II.

Atom

Wyckoff site

X

y

z

Occupancy

U(Å2)

Na

4a

0.000

0.250

0.125

0.50 (1)

0.009 (1)

Eu

4a

0.000

0.250

0.125

0.50 (1)

0.009 (1)

W

4b

0.500

0.750

0.125

1.00 (2)

0.016 (2)

O

16f

0.7785 (3)

0.5946 (3)

0.0482(3)

1.00 (2)

0.026 (3)

Table 2. Atomic coordinates, occupancy and isothermal parameters of Na, Eu, W and O in NaEuW I

Atom

Wyckoff site

x

y

Z

Occupancy

U(Å2)

Na

4a

0.000

0.250

0.125

0.50 (1)

0.005 (2)

Eu

4a

0.000

0.250

0.125

0.50 (1)

0.005 (2)

W

4b

0.500

0.750

0.125

1.00 (2)

0.014 (3)

O

16f

0.7477 (3)

0.5891 (3)

0.0388(3)

1.00 (2)

0.029 (3)

Table 3. Atomic coordinates, occupancy and isothermal parameters of Na, Eu, W and O in NaEuW II


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Selected bond distances Bond type

NaEuW I

NaEuW II

Bond length in Å

Bond length in Å

(Na/Eu-O1)x4

2.427 (1)

2.421 (1)

(Na/Eu-O2)x4

2.412 (1)

2.313 (1)

(W-O)x4

1.825 (1)

1.882 (1)

Table 4.Selected bond distances of NaEuW I and NaEuW II by Rietveld refinements using Synchrotron data. 3.5 UV-DRS studies

The peak around 250 nm-300 nm [SI Figure 6] in the reflectance spectra of NaEuW samples is attributed to the Eu-O charge transfer band (CTB). The other weak peaks at 396 nm, 465 nm and at 538 nm are assigned to the intra configurational f-f transitions which are the metastable energy states of Eu3+. Furthermore in the reflectance spectra, due to the quantum size effect, a shift in the CTB band towards higher energy is observed for NaEuW II as it has smaller particle size compared to NaEuW I. Interestingly, the position of intra configurational f-f transition of Eu3+ remains unchanged in the wavelength range 350-600 nm indicating similar crystallographic site occupancy of Eu 3+ in both the samples [SI Figure 6]. The importance of solvent and its effect on the particle size and consequently on the band shifting is deduced from the DRS measurements. The optical band gap values are calculated according to Kubelka-Munk function (Tauc plot) using DRS spectra. From the Tauc plot (figure 8) it is observed that the calculated indirect band gap values of NaEuW I and NaEuW II are 3.93 and 4.40 eV, respectively. The significant increase in band gap value for NaEuW II is mainly attributed to the smaller nano particle size in EG solvent which is in accordance with the HRTEM and p-XRD results.


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Figure 8.Tauc plot of NaEuW I and NaEuW II obtained from the diffuse reflectance spectra using Kubelka-Munk function.

3.6 Fluorescence studies 3.6.1 Effect of solvent on fluorescence properties of NaEu(WO 4)2 Excitation spectra monitored at 614 nm emission [Figure 9a)] clearly indicates that NaEuW I and NaEuW II can be excited at several wavelength ranging from UV (270-280 nm) near UV (396 nm) and visible region (466 nm-538 nm) suggesting the presence of band-band as well as the CT band to achieve red emission (SI Figure 7). The most intense peak around 260-300 nm is assigned to the ligand to metal charge transfer band of Eu-O ie., electron transfer from the filled oxygen 2p shell to partially filled Eu 3+4f shell. Several other well resolved weak peaks of the forbidden f-f transitions are also observed at 396 nm, 466 nm and 538 nm and assigned to

7

F 0- 5L 6,

7

F0-5D2 and

7

F0-5D1

respectively18, 19, 32. Blue shift of 10 nm is attributed to the smaller particle size in NaEuW II compared to NaEuW I33 and also dictates the nature of Eu-O bond as studied by J.H. Lin34. Additionally, blue shift is attributed to increase in optical electro negativity of central Eu3+ which means increase in ionic strength and decreased covalency between Eu3+ and surrounding ligand facilitating CTB towards higher energy 35. Additionally, the decreased Eu-O bond length in NaEuW II (table 4) compared to NaEuW I calculated via


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Rietveld refinements using Synchrotron data also suggests the increased energy difference between 4f orbital’s of Eu3+ and 2p O orbital’s. Hence the shorter Eu-O bond could be responsible for the shift of the CTB towards higher energy which is mainly attributed to the surrounding environment of the central Eu 3+.

Measured emission spectra under CTB excitation show typical characteristics transitions of europium ion at 592 nm, 614 nm and 652 nm in both the samples (figure 9b), and the normalized emission spectra of as prepared NaEuW I and II (SI Figure 8). These are attributed to the radiative transitions of Eu3+ where J= 1, 2 and 3. Emission at 614 nm is assigned to 5D0-7F2, followed by 5D0-7F1 at 594 nm and 5D0-7F3 at 652 nm, respectively. In general it is well known that when Eu3+ occupies a site without an inversion symmetry, electric dipole transitions are more favorable and dominant whereas, in a site with inversion symmetry, magnetic transitions are dominant 36. In the above nanomaterials, 5

D0-7F2 transition due to the electric dipole transition is hypersensitive to the local site

(4a) of Eu3+ ion ie., S4 symmetry without inversion centre whereas, 5D0-7F1 magnetic transition is insensitive to site symmetry of Eu 3+ and is generally allowed by selection rules37,

38

. The coefficient of fluorescence intensity also called as asymmetry ratio was

calculated for both the samples. It is the ratio of electronic dipole transition to the magnetic transitions which provides information about crystal structure/local environment around Eu3+ ion39. The strong electric dipole transition at 614 nm was compared to the magnetic transition at 594 nm in NaEuW samples demonstrates that Eu 3+ion occupies a lattice site without inversion symmetry even though the overall space group of NaEuW is centrosymmetric I41/a40. Hence integrated intensity ratio of red 5D0-7F2 (represented as IED) to orange 5D0-7F1 (represented as IMD)4 is calculated. The increase in calculated asymmetry ratio of NaEuW II (2.30) indicates greater distortion of the Eu-O bond compared to NaEuW I (7.13). These results are also supported by the calculated Eu-O bond length via Rietveld refinement (selected bond length table 4). Significant variation in peak intensities is observed in emission spectra monitored at 270 and 280 nm (figure 9b) excitation of NaEuW I and II respectively which again can be attributed to the particle size effect. The decreased PL intensity could be due the presence of surface organic moieties such as –OH and CH functional groups [SI figure5] on NaEuW II surface which


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collectively enhances the surface defects resulting in increased non radiative transition. The calculated quantum yield values of both the samples were given in SI table 1. Thus, our investigation of fluorescence emission clearly dictates the critical role of solvent on the particle size.

Figure 9 optical a) excitation spectra of NaEuW I and NaEuW II monitored at 614 nm emission and b) emission spectra monitored at CTB excitation

3.6.2 Effect of thermal treatment on fluorescence red emission. On increasing calcination temperatures, the intensity of the red emission significantly increased up to 1000° C in both NaEuW I and NaEuW II [Figure 10 a) and b) ] signifying the thermal stability of the material and moreover phase was retained even at higher calcination temperature. As it is well known, due to Ostwald ripening 1, 41 the particle size increases with increase in calcination temperature, thereby improves crystallinity and fluorescence intensity. Additionally, the reduction of non-radiative transitions due to the reduction in micro strain and surface defects as well as the disappearance of -OH, -CH and other organic moieties from the calcined samples might have also increased the PL intensity1, 42.


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Figure 10. Optical emission spectra of calcined a) NaEuW I and b) NaEuW II at different temperatures 3.7 CIE (Commission Internationale de l’Eclairage)

Chromaticity studies: CIE chromaticity coordinates: The calculated CIE-X and CIX-Y were found to be 0.66, 0.34 and 0.68, 0.32 for the as prepared NaEuW I and NaEuW II respectively. Thermally treated NaEuW I exhibited 0.66 and 0.34 at all the temperatures whereas NaEuW II showed 0.68, 0.32 at 300° C, 900° C and 1000° C, however, at 600° C the values were found to be 0.67 and 0.33 which is closer to the commercially used red phosphor. Interestingly, it is observed from fig 11 that CIE-X and CIE-Y coordinates are in the red region of the visible spectrum indicating the color purity or hue of these materials. Color coordinates near red regions are the spectral characteristics of Eu3+and the calculated coordinates show the possibility of tuning the red color emission. The values of x and y coordinates corresponds exactly to the standard coordinate NTSC (National Television standard committee) values of the commercially available red Y2O3.S:Eu3+ phosphor (0.67, 0.33). More interestingly our materials shows quite higher CIE values compared to any other similar kind of materials reported so far19. These results suggest that these phosphors would be the promising materials that can be applied for solid state lighting applications.


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Figure 11.CIE chromaticity diagram of NaEuW I and NaEuW II.

3.8 Microbial toxicity studies 3.8.1 Agar diffusion method Diffusion assay of luminescent NaEuW I [SI Figure 9a)] and NaEuW II [SI Figure 9b)] against two bacteria E. coli, S. aureus and a fungus C. albicans clearly revealed no zone of inhibition compared to the antimicrobial standard drug used [SI Figure 9c)]. As compared to the other metal oxide nanoparticles such as ZnO reported so far43-45 it is noteworthy that there is also no size dependent toxicity of NaEuW. Furthermore, the assay suggests that NaEuW nano particles have wide range of non toxicity and could be applicable for variety of microbes such as gram negative E. coli, gram positive S. aureus and fungi C. albicans.

3.8.2 Microbial cell viability method

Even though there is no significant toxicity, diffusion assay could not explain if there is size dependent toxicity induced by NaEuW nanoparticles as smaller particles has greater efficacy in inhibiting cell growth by producing reactive oxygen species (ROS) which


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might play critical role in in-vitro and in-vivo applications46. Hence to circumvent certain limitations in quantifying NaEuW non-toxicity by diffusion assay, microbial cell viability technique was employed as shown in SI figure 10a)and b). S. aureus and C. albicans have shown 100% cell viability for both NaEuW I and NaEuW II whereas, slight decrease in cell viability of 5-10% was observed in case of E. coli. This observation indicates that gram negative E. coli bacteria has less resistance compared to gram positive S. aureus and C. albicans. More interestingly, it is also noticed that as the particle size decreases from NaEuW I(200-300 nm) to NaEuW II (10-20 nm), slight decrease in cell viability was observed in all microbial strains indicating the importance of particle size in determining the viability. The reasons could either be attributed to the penetration of the smaller NaEuW II nanoparticles in to the cell via endocytosis or by any other pathway or due to the release of ROS46-48. Nevertheless it is noteworthy to point out that, incubation of microorganism with NaEuW nanoparticles not only proved their biocompatibility but also stimulated the S. aureus growth even at 500 μg/ml. Additionally, calculated % cell viability was more than 100% in case of S. aureus for both NaEuW I and II compare to E. coli and C. albicans suggesting a wide range of potential applications.

3.8.3 MTT assay Cytotoxicity studies on HeLa cells with the different concentration of NaEuW I and II test samples revealed the non toxicity of NaEuW irrespective of the concentration used. Both the materials have shown no inhibition instead, increase in cell viability was observed [Figure 12e)] even at higher concentrations, indicating simulative response of NaEuW towards cell growth. Moreover, the observed non toxicity of NaEuW is much higher compared to the similar studies on Eu 3+ doped NaGd(MoO4)2nanomaterials16. On other hand they also found significant reduction in cell viability upon PLL (poly L-lysine) surface modification and their studies revealed the importance of negative surface charge in in–vitro applications49,

50

. As shown in figure 12 both the materials exhibits good

biocompatibility even higher concentration compared to the earlier reports on NLM:Ln 3+ and Eu3+ doped CaMoO4-Fe3O321,

51

. Marginal decline in cell viability in the case of

NaEuW II compared to NaEuW I can be attributed to the particle size effect as witnessed by TEM and PL analysis and also due to the internalization of nanoparticles via receptor


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mediated endocytosis. This is the first study to investigate the biocompatibility of NaEuW luminescent materials and to demonstrate the importance of surface charge and particle size in determining cytotoxicity of HeLa cells via MTT assay.

3.8.4 Cell imaging studies:

Cell imaging of luminescent nanoparticles offers noninvasive diagnostic visualization of physiological process at cellular and sub cellular level. Due to the poor stability, low signal-tonoise ratio, higher cytotoxicity and the limited penetration depths of organic dyes and biomarkers, luminescent nanoparticles are widely used in bio imaging52. As the biocompatibility of NaEuW nanoparticles is established for the first time, the application of their intrinsic red fluorescence was carried out to investigate the internalization of NaEuW nanoparticles.The red fluorescence image of internalized NaEuW II was obtained under 538 nm excitation.

In figure 12 it is observed that even at higher concentrations of the nanoparticles, cells were grown to more confluence in the case of NaEuW I [Figure 12 a) and c)] where as cells were in still fibroblast stage (80% confluence) with NaEuW II [Figure 12b) and d). On the other hand one can also notice the adsorption and the cellular uptake of NaEuW II nanoparticles by HeLa cell indicates the applications of NaEuW in bio-imaging52. Cellular up take and the adsorption can be attributed to the presence of attractive negative forces and smaller particle size13. Internalization of NaEuW II nanoparticles also indicates the potential application of NaEuW II as drug delivery carrier to target local delivery which can be featured by facile surface modification for certain selective type of cancer cells compared to free drug delivery50. More recently the application of Europium doped oxide materials in bio-imaging applications as markers in quantifying the concentration of physiological oxidants such as H2O2, ClO- and NO has been reported53. Hence the detection of intracellular signaling molecules such as H2O2 could be possible with the NaEuW provided specific surface medications as they exhibit an intrinsic red emission which plays important role in in-situ tracking and pathophysiological conditions such as malignant diseases, atherosclerosis and neurodegenerative diseases53. However, more studies are required to accurately investigate the internationalization which could play critical role in biomedical application of NaEuW phosphor. Finally our investigation on fluorescent


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NaEuW towards microorganism and HeLa cells confirms the possibility of tuning the particle size for advanced in-vitro theranostic applications up on efficient surface modification with the molecule of interest for bioconjugation.

Figure 12. a) and b) Transmitted images of NaEuW I and NaEuW II c) and d) Merged bio images of red fluorescence with the trasnmitted image of the same. e) A plot of in-vitro cell viability of HeLa cells incuabted with the various concentrations of NaEuW I and II for a period of 48 hours.

4. CONCLUSION Solvothermal method was employed to design NaEu(WO4)2 red nano phosphor using water and EG as solvents. NaEuW I and II were well characterized by synchrotron XRD, HRTEM, Zeta potential, UV-DRS, FTIR and room temperature photoluminescence. Both NaEuW I and II show bright red emission upon CT band excitation. Declined fluorescence intensity observed in NaEuW II has been attributed due to the smaller particle size and to the presence of fluorescence quenchers such as O-H and C-H on its surface. Detailed crystal structure analysis using synchrotron diffraction data revealed that the shorter Eu-O bond in NaEuW II could be responsible for the blue shift of CT band. Investigation of the fluorescence properties along with the corresponding CIE coordinates of as prepared and calcined NaEu(WO4)2 suggests the that the red emission of


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NaEu(WO4)2 can be tuned to match the commercial phosphors used in LEDs. Additionally, non-toxicity of NaEu(WO4)2 was evaluated by agar diffusion, microbial cell viability and MTT assay using several microorganisms and HeLa cell lines for the first time. Thus the study dictates the biocompatibility of NaEuW I and NaEuW II irrespective of test organism and NaEuW I and II concentration used. The intense red luminescence, biocompatibility and the effect of solvent on nano NaEu(WO4)2 particle growth highlights the novelty of this work for bio imaging, cell labeling and tracing rare earth in-situ at real time imaging. Nevertheless, further studies are need to be carried out to understand the mechanism of NaEu(WO4)2 nano particle, cell interactions and bio accumulation/distribution to realize their importance in in-vitro and in-vivo applications. SUPPORTING INFORMATION: SI Figure 1. P-XRD pattern at various temperature indicating optimized temperature of 200° C for the formation of phase pure NaEuW in water, SI Figure 2. P-XRD pattern of NaEuW I nanoparticle phase formation for different time duration at 200° C, SI Figure 3. P-XRD pattern at 200° C for 24hr and the reaction was carried out at different pH values indicating optimized neutral pH for the formation of phase pure NaEuW. SI Figure 4 FTIR spectra, SI Figure 5a) and b) Energy dispersive spectrum of NaEuW I and II respectively, SI Figure 6 Diffuse reflectance spectra, SI figure 7 Schematic energy level diagram representing CT (charge transfer band) and Eu3+ intra-configurational electronic transitions in NaEuW, SI figure 8 Normalized fluorescence emission spectra, SI Figure 9a) Diffusion assay of C. albicans, E. coli and S. aureus with NaEuW I, SI Figure 9b) Diffusion assay of C. albicans, E. coli and S. aureus with NaEuW II, SI Figure 9c) Diffusion assay of C. albicans, E. coli and S. aureus with AMS (Antimicrobial standard, ciprofloxacin for bacteria and fluconazole for fungus), SI Figure 10a) Microbial cell viability assay of NaEuW I against E. coli, S. aureus and C. albicans, SI Figure 10b) Microbial cell viability assay of NaEuW II E. coli, S. aureus and C. albicans. SI table 1: Calculated quantum yield of NaEuW I and II


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ACKNOWLEDGMENT: A.K.M thank to PPISR and UGC-DAE for fellowship. A.K.M and N.G.S. are thankful to Prof. Jagidhar, IPC, IISc, Bengaluru, India, for HRTEM facility. NOTES: The authors declare no competing financial interest. REFERENCES: (1) Dai, Q.; Foley, M. E.; Breshike, C. J.; Lita, A.; Strouse, G. F., Ligand-passivated Eu:Y2O3 nanocrystals as a phosphor for white light emitting diodes. J. Am. Chem. Soc 2011, 133, 15475-86. (2) Liu, X.; Hou, W.; Yang, X.; Liang, J., Morphology controllable synthesis of NaLa(WO4)2: the morphology dependent photoluminescent properties and single-phased white light emission of NaLa(WO4)2: Eu3+/Tb3+/Tm3+. CrystEngComm 2014, 16, 1268-1276. (3) Suzuki, S.; Ryo, M.; Yamamoto, T.; Sakata, T.; Yanagida, S.; Wada, Y., Preparation of luminescent nanosized NaEu(MoO4)2 incorporated in amorphous matrix originated from zeolite. J. Mater. Sci 2007, 42, 5991-5998. (4) Maheshwary; Singh, B. P.; singh, j.; Singh, R. A., Luminescence properties of Eu3+ -activated SrWO4 nanophosphors- Concentration and Annealing effect. RSC Adv 2014, 4, 32605-32621. (5) Jia, G.; Tian, J.; Lin, P.; Liu, S.; Sun, Y.; Chen, L.; Zhang, M.; Yao, R.; Zheng, Y.; Zhang, C., Hydrothermal synthesis of NaTb(MoO 4 ) 2 hierarchical architectures with novel morphology and luminescence properties. Cerams Int 2016, 42, 17936-17940. (6) Marques, V. S.; Cavalcante, L. S.; Sczancoski, J. C.; Alcântara, A. F. P.; Orlandi, M. O.; Moraes, E.; Longo, E.; Varela, J. A.; Siu Li, M.; Santos, M. R. M. C., Effect of Different Solvent Ratios (Water/Ethylene Glycol) on the Growth Process of CaMoO4Crystals and Their Optical Properties. Cryst Growth Des 2010, 10, 4752-4768. (7) Wang, W.; Hu, Y.; Goebl, J.; Lu, Z.; Zhen, L.; Yin, Y., Shape-and size-controlled synthesis of calcium molybdate doughnut-shaped microstructures. J. Phys Chem C 2009, 113, 16414-16423. (8) Phaomei, G.; Singh, W. R., Effect of solvent on luminescence properties of re-dispersible LaF3:Ln3+(Ln3+=Eu3+, Dy3+, Sm3+ and Tb3+) nanoparticles. J. Rare Earths 2013, 31, 347-355. (9) Esteban-Betegón, F. t.; Zaldo, C.; Cascales, C. n., Hydrothermal Yb3+-Doped NaGd(WO4)2Nano- and Micrometer-Sized Crystals with Preserved Photoluminescence Properties. Chem. Mater 2010, 22, 2315-2324. (10) Liu, Y.; Wang, Y.; Wang, L.; Gu, Y.-Y.; Yu, S.-H.; Lu, Z.-G.; Sun, R., General synthesis of LiLn(MO4)2:Eu3+(Ln = La, Eu, Gd, Y; M = W, Mo) nanophosphors for near UV-type LEDs. RSC Adv 2014, 4, 4754-4762.


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(11) Liu, X.; Hou, W.; Yang, X.; Liang, J., Morphology controllable synthesis of NaLa (WO 4) 2: the morphology dependent photoluminescent properties and single-phased white light emission of NaLa (WO 4) 2: Eu 3+/Tb 3+/Tm 3+. CrystEngComm 2014, 16, 1268-1276. (12) Chunyan Liu, Z. G., Jianfeng Zeng, Yi Hou, Fang Fang, Yilin Li, Ruirui Qiao, Lin Shen,; Hao Lei, W. Y., and Mingyuan Gao, Magnetic/Upconversion FluorescentNaGdF4:Yb,Er NanoparticleBasedDual-Modal Molecular Probes forImaging Tiny Tumors in Vivo. ACS Nano 2013, 7, 7227-40. (13) Ajithkumar, G.; Yoo, B.; Goral, D. E.; Hornsby, P. J.; Lin, A. L.; Ladiwala, U.; Dravid, V. P.; Sardar, D. K., Multimodal bioimaging using rare earth doped Gd2O2S: Yb/Er phosphor with upconversion luminescence and magnetic resonance properties. J. Mater. Chem. A, Materials for energy and sustainability 2013, 1, 1561-1572. (14) Loiko, P. A.; Vilejshikova, E. V.; Mateos, X.; Serres, J. M.; Dashkevich, V. I.; Orlovich, V. A.; Yasukevich, A. S.; Kuleshov, N. V.; Yumashev, K. V.; Grigoriev, S. V.; Vatnik, S. M.; Bagaev, S. N.; Pavlyuk, A. A., Spectroscopy of tetragonal Eu:NaGd(WO4)2 crystal. Opt. Mater 2016, 57, 1-7. (15) Xiaolin Liu, W. H., Xiaoyan Yang, Jiyuan Liang, Morphology Controllable Synthesis of NaLa(WO4)2: Morphology Dependent Photoluminescent Property and Single-phased White Light Emission of NaLa(WO4)2: Eu3+/Tb3+/Tm3+CrystEngComm 2014, 16, 1268-1276.. (16) Laguna, M.; Nunez, N. O.; Rodriguez, V.; Cantelar, E.; Stepien, G.; Garcia, M. L.; de la Fuente, J. M.; Ocana, M., Multifunctional Eu-doped NaGd(MoO4)2 nanoparticles functionalized with poly(llysine) for optical and MRI imaging. Dalton Trans 2016, 45, 16354-16365. (17) Bhat, S. S.; Huq, A.; Swain, D.; Narayana, C.; Sundaram, N. G., Photoluminescence tuning of Na1-xKxNdW2O8 (0.0

Effect of Solvent on the Red Luminescence of Novel Lanthanide

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