Multifunctional Hybrid Nanomaterials from Water Dispersible CaF2

Article pubs.acs.org/JPCC

Multifunctional Hybrid Nanomaterials from Water Dispersible CaF2:Eu3+, Mn2+ and Fe3O4 for Luminescence and Hyperthermia Application Laishram Priyobarta Singh,† Sri Krishna Srivastava,*,† Ratikant Mishra,‡ and Raghumani Singh Ningthoujam*,‡ †

Department of Chemistry, Manipur University, Imphal-795003, India Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai-400085, India

‡

S Supporting Information *

ABSTRACT: CaF2 nanoparticles doped with 1 at. % Eu3+and codoped with Mn2+ ions (1, 2, and 3 at. %) were synthesized at 180 °C by using ethylene glycol as a capping agent and solvent medium. The emission spectra of these nanomaterials show two main peaks at 590 and 612 nm after excitation at 240 or 394 nm. Enhancement in luminescence is found upon codoping of Mn2+. Parts of Eu3+ ions are substituted into Ca2+ sites (having a symmetry environment), which is reflected in the higher probability of magnetic dipole transition over that of electric dipole transition. The polymer film of polyvinyl alcohol incorporated with CaF2:1Eu shows the red emission. The prepared nanomaterials are highly water dispersible. The hybrid of CaF2:1Eu and Fe3O4 shows the heating ability up to 42 °C under an AC magnetic field. This is suitable for cancer therapy through hyperthermia. A very high specific absorption rate (SAR) of 283 W/g is observed. This can be ascribed to the increased magnetocrystalline anisotropy and Brownian relaxation. Biocompatibility of hybrid nanoparticles up to 75% in HeLa cells is observed. Also, this hybrid shows the red emission, and thus, it will be useful in tracing of magnetic nanoparticles through in vivo and in vitro optical imaging applications. Interestingly, the new exciton band at 300 nm is found in such a hybrid material.

T

he lanthanide (Ln3+)-doped nanoparticles are used in many applications, such as optical telecommunications, display, biolabeling, catalysts, sensors, etc.1−5 Among Ln3+ ions, Eu3+ ions are used as the source of the red emitter. However, the improvement of luminescence intensity is important so that the amount of materials can be reduced economically. Two ways to improve the luminescence are (1) the use of a host having low phonon vibrations and (2) codoping. The fluorides usually as compared to oxides have low phonon vibrations; e.g., CaF2, YF3, and GdF3 have 328, 490, and 350 cm−1,6−8 respectively, whereas CaO, Y2O3, and Gd2O3 have 960, 600, and 550 cm−1, respectively.9−11 All Ca2+ precursors are very cheap, whereas Y3+ and Gd3+(rare-earth ions) precursors are very costly. CaF2 is used as a window/cell in infrared spectrometers. There are reports on enhancement of the luminescence of ZnS:Mn2+ by codoping of Eu2+,12 and also the improvement of luminescence by codoping of Li+ and Bi3+ through the process of energy transfer or the improvement of crystallinity or the increase in optical dipole-moment.13,14 The nanoparticles (NPs) capped by ligands having suitable functional groups are dispersible in water. Such water dispersible particles are useful in biolabeling through optical imaging and can be incorporated in polymers for formation of film devices. The water dispersible CaF2:Ln3+ NPs were reported.15 In the initial stage, NPs are capped by oleic acids © 2014 American Chemical Society

and thus, NPs are not dispersible in water. Then NPs are made water dispersible after substitution of oleic acids by 2aminoethyl dihydrogen phosphates. This process is very lengthy and also complete ligand exchange does not take place. Oleic acid and DMF (dimethylformamide) were used as capping agent and solvent, respectively, but both are toxic to living cells above a certain concentration. However, a simple route for water dispersible nanoparticles employing ethylene glycol (EG) as both capping agent and solvent will be interesting. EG is nontoxic up to a high concentration in our living body. If luminescence nanoparticles are made hybrid with magnetic nanoparticles, the hybrid material will have multifunctional properties. There are a few reports on YPO4:Eu/Fe3O417 and YPO4:Tb/Fe3O4.18 Fe3O4 NPs are useful in both diagnosis through magnetic resonance imaging (MRI) and therapy through hyperthermia.19 In this study, CaF2 nanoparticles have been prepared by an ethylene glycol route. So far, there is no report on CaF2:1Eu nanoparticles functionalized with ethylene glycol (EG) to the best of our knowledge. Received: March 21, 2014 Revised: July 3, 2014 Published: July 24, 2014 18087

dx.doi.org/10.1021/jp502825p | J. Phys. Chem. C 2014, 118, 18087−18096

The Journal of Physical Chemistry C

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Boltzmann’s constant. The τB becomes slow with the increase of rh and η at a particular temperature. In a superparamagnetic regime, the particles are considered as a single domain. Each domain has a spin (↑). Domains are assumed as noninteracting. Each domain orients from an easy direction to the opposite direction with relaxation (τN) of 10−9 s. This is known as Néel’s relaxation. Overall, the net moment becomes zero. It is shown in Figure 2a−d. In Figure 2a,b, the

The occupancy of Eu3+ ions in Ca2+ sites is studied on the basis of the electronic and magnetic allowed transitions of Eu3+. Enhancement in luminescence of Eu3+ is found by codoping of Mn2+. The hybrid CaF2:1Eu/Fe3O4 has been prepared, and its luminescence and magnetic properties have been studied. Neither CaF2:1Eu nor Fe3O4 exhibits an absorption band at 300 nm, but interestingly, their hybrid shows such an absorption band. This exhibits the red emission, which is included in biological windows. The high specific absorption rate (SAR) of 283 W/g is observed, and this will be useful in cancer therapy. Biocompatibility of a hybrid in HeLa cells is studied.

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THEORETICAL MODEL FOR THE HEAT GENERATION FROM MAGNETIC FLUIDS UNDER AC MAGNETIC FIELD (AMF) OR INDUCTION COIL The heat generation from magnetic nanoparticles dispersed in a fluid medium under an AC magnetic field (AMF) or simply an induction coil is due to the following phenomena:16−19 (1) Brownian particle rotation after collision of particle in a fluid medium, (2) Néel’s spin relaxation, (3) hysteresis loss, and (4) eddy current. However, the first and second factors mentioned above contribute significantly to heat generation. The third factor also contributes in a small amount, but contribution of the fourth factor is almost negligible. These are explained here: Figure 1a−d shows the Brownian particle rotation in a fluid medium. In Figure 1a, the particles are approaching; Figure 1b Figure 2. Behaviors of domains dispersed in a fluid medium at different times and with or without a magnetic field (H): (a) H = 0 Oe, t = 0 s, M = 0; (b) H = 0 Oe, t = 10−9 s, M = 0; (c) H = 0 Oe, t = 10−5 s, M ≠ 0. (d) In Néel’s relaxation, thermal energy (kBT) overcomes the particles’ energy (KV) and the domain rotates from 0° to 180°.

net moment is zero when measured at time = 0, 10−9 s, and in the absence of a magnetic field. The coercivity (Hc) is zero. This τN is given by

τN = τ0eΔE / kBT = τ0e KV / kBT

(2) −9

where τ0 is on the order of 10 s and ΔE is the anisotropic energy barrier. ΔE is the product of the anisotropic energy constant (K) and the volume (V) of the particle. During the relaxation of spin, the heat is generated to the surrounding medium. τN becomes slow with the increase of K and V at a particular temperature. Thermal energy (KBT) dominates over ΔE when K and V values are so small. In such a situation, the spin of the domain rotates from 0° to 180° (Figure 2d). In a DC magnetic field, significant heat could not be produced from magnetic nanoparticles (MNPs) dispersed in a fluid medium. Here, the measured time of 0.1−1 s cannot determine spin relaxation in a superparamagnetic regime. In an AC magnetic field, frequency is associated, and thus, it can change the direction of the current over time. Here, applied frequency ( f) is 265 kHz and its reciprocal gives 4 × 10−6 s, which is assumed as the measurement times. Within t = 4 × 10−6 s, the relaxation of spin of the domain could be observed. The net moment can be recognized. Hysteresis loss in an AC magnetic field is important in heat generation. When the current changes over time, a loop can be generated. The area of the loop is given by

Figure 1. (a) Particles are approaching, (b) collision, (c) particles after collision, and (d) a larger view of a particle that shows the particle radius (rc) and hydrodynamic radius (rh).

shows the collision between particles. In Figure 1c, the particles rebound after collision and relax. The relaxation produces heat to the surrounding medium. These particles rotate to the equilibrium position before collision with another one. This is known as Brownian relaxation (τB). The collision produces heat also. In AMF, this collision is different from those without it due to the magnetic field experienced by magnetic nanoparticles. This relaxation (τB) at a particular temperature (T) is given by τB =

4πηrh 3 kBT

(1)

where η is the dynamic viscosity of the carrier fluid, rh is the hydrodynamic radius (sum of core radius of the particles and surfactant coating layer), which is shown in Figure 1d. kB is the 18088



Area = f

∫ MdH

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water, was repeated three times. In this way, the excess HCl acid was removed. The obtained solution was treated with 30 mL of ethylene glycol and heated in a round-bottom flask at 40 °C for 30 min. Then, 0.5 g of NH4F was added, and it was allowed to reflux at 60 °C for 2 h. The resulting white precipitate was collected by centrifugation at 10 000 rpm (rpm = revolutions per minute) after washing with methanol. Since ethylene glycol has a boiling point at 195 °C, the reaction temperature can be monitored below this. In a similar way, samples were prepared at 120 and 180 °C. Synthesis of Mn2+ Codoped CaF2:1Eu. In the synthesis process of Mn2+ (at. %) codoped CaF2:1Eu (Mn2+ at. % = 1, 2, and 3), stoichiometric amounts of Eu2O3, MnCl2, and CaCO3 were dissolved in concentrated HCl acid. The excess acid was removed. The obtained solution was treated with 30 mL of ethylene glycol and heated in a round-bottom flask at 40 °C for 30 min. Then, 1 g of NH4F was added, and it was allowed to reflux at 180 °C for 2 h. The resulting white precipitate was collected by centrifugation at 10 000 rpm after washing with methanol. Typical CaF2 formation is given below.

(3)

where M is magnetization at the applied magnetic field (i.e., current is proportional to H) Heat/power dissipation (P) of the whole system is given by

P = μ0 πχ ″f Ho2

(4)

where μo is the permeability of free space. χ′′ is the imaginary part of susceptibility (χ). Susceptibility is the magnetization divided by the applied magnetic field (χ = M/H). χ has two parts: real (χ′) and imaginary (χ′′). χ=

(χ ′)2 +

(χ ″)2

(5)

The (χ′′) is related to the heat dissipation of the system, which is defined as ωτ χ″ = χ 1 + (ωτ )2 (6)

Ca 2 +(aq) + 2F−(aq) + 2Cl−(aq) + 2NH4 +(aq) → CaF2 ↓(s) + 2NH4 +(aq) + 2Cl−(aq)

where τ is the total relaxation contributed by Brownian (τB) and Néel’s (τN). 1 1 1 = + τ τB τN (7)

Preparation of PVA Polymer Film. About 5 mg of sample (CaF2:1Eu or Mn2+ codoped CaF2:1Eu) was dispersed in 10 mL of deionized water separately, and the dispersion remained for 2 days. A 5 mL aliquot of the dispersed solution was mixed with 10 mL of 4% PVA (polyvinyl alcohol) solution (i.e., 4 g of PVA in 100 mL of deionized water). After being thoroughly mixed in an ultrasonic bath, it was treated with 1 mL of 0.1 M solution of borax and warmed up to 60 °C. Then, the polymer composite film was prepared over a glass slide with a film thickness of ∼1 mm. Here, borax helps in crosslinking between PVA molecules. Synthesis of Hybrid Nanoparticles. In the synthesis process of hybrid CaF2:1Eu/Fe3O4, materials were prepared in a 4:1 wt % ratio; 2, 4, and 6 mg of Fe3O4 were mixed with 8, 16, and 24 mg of CaF2:1Eu, respectively. Fe3O4 nanoparticles are prepared by hydrolysis of Fe2+ and Fe3+ ions in NH4OH solution. Detailed studies of the preparation and magnetization of Fe3O4 MNPs were reported elsewhere.19 Each mixture was treated with 1 mL of PEG solvent (PEG solution was prepared by dissolving 100 mg of 6000 polyethylene glycol in 100 mL of distilled water). It was ultrasonicated for 30 min. Cytotoxicity Study. Cell Culture. The in vitro cytotoxicity study of the hybrid sample was carried out on HeLa (Human Epitheloid Cervix Carcinoma) cell lines obtained from the National Centre for Cell Sciences, Pune, India, and the detailed toxicity study was done in the National Toxicology Centre, Pune, India (ISO 10993/USP 32 NF 27) by MTT assay. The HeLa cells were grown in MEM + 10% FBS + antibiotics at 37 °C in a 5% CO2 atmosphere. MTT Assay. The cells (2 × 105 cells mL−1) were incubated in the respective medium for 24 h in a 96-well microtiter plate. The old medium was replaced by fresh medium after 24 h. The HeLa cells were treated with different concentrations of 0.2, 0.4, 0.6, and 0.8 μg per mL of cultured media and incubated at 37 °C in a 5% CO2 atmosphere for 24 h. The microtiter plates were observed to confirm the absence of contamination using an inverted microscope. A 10 μL aliquot of MTT solution was added into each well, including control wells. These were incubated for 3 h at 37 °C in a 5% CO2 atmosphere. Then, the medium was removed and the cells were washed with PBS. The formazan formed was dissolved in 200 μL of acidic isopropanol, and finally absorbance was read at 570 nm. The cell viability was calculated [(absorbance value for the test divided by that for control) × 100]. The control sample was prepared by the same process in which PEG solution was used instead of hybrid nanoparticles. Characterization. A Philips X-ray diffractometer (PW 1071) with Cu Kα (1.5405 Å) radiation having a Ni filter was used for X-ray diffraction (XRD) study. All patterns were recorded over the angular range 10 ≤ 2θ/deg ≤ 90 with a step size of Δ2θ = 0.02. The powder samples were ground and dispersed in methanol on a glass slide and allowed to dry. The average crystallite size (t) was calculated using the

The fourth possibility of the heat generation is from eddy current (ED), which suggests that the induced current is generated in the metallic rod. The ED is defined as ED =

(μπdf Ho)2 20ρ

(8)

where μ is the permeability of a material, d is the diameter of the particle, and ρ is the resistivity of the material. Since particles are so small and many particles are existing in a fluid medium, they nullify each other even if eddy currents are associated in particles. Also, these MNPs are insulating and ρ is high. Thus, the overall ED is negligible in the case of MNPs. The density of the particle relates to the loss power density P or specific absorption rate (SAR), which can be calculated using following relation SAR = C

ΔT 1 Δt mmagn

(9)

where C is the specific heat capacity of the sample. Usually, it is calculated by considering both the sample weight and the weight of the water. Comparing to the weight of the water, sample weight is negligible, so only the specific heat capacity of water is contributed, and its value is 4.18 J g−1 K−1. ΔT/Δt is the slope of the time-dependent temperature curve. mmagn is the amount of magnetite or Fe in the 1 mL system.

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METHODS

Materials. Calcium carbonate (CaCO3, 99.99%), ammonium fluoride (NH4F, 99.99%), manganese chloride (MnCl2, 99.99%), and europium oxide (Eu2O3, 99.99%) from Sigma-Aldrich, ethylene glycol and sodium hydroxide from Merck, and methanol were used as received without further purification. Double distilled water was used throughout the experiment. Synthesis of CaF2:1Eu Nanoparticles. CaF2:1Eu nanoparticles were synthesized by an ethylene glycol route. In a typical synthesis procedure, 0.118 g of Eu2O3 and 0.668 g of CaCO3 were dissolved in 3 mL of concentrated HCl. The solution was heated slowly, and the amount of solution changes from 3 to about 1 mL. Then, we added 3 mL of double distilled water into it. The heating, followed by adding 18089



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Scherrer relation, t = 0.9λ/β cos θ, where λ is the wavelength of the Xray and β is the full width at half-maximum (fwhm). The lattice parameter was calculated from the least-squares fitting of the diffraction peaks. For the characterization of the nanoparticles, FTIR (Fourier transform infrared) spectra were recorded on a PerkinElmer Spectrum 400 FT-IR spectrometer. Powder samples were studied by making thin pellets with KBr. A JEOL 2000 FX transmission electron microscope was used for recording TEM images. For the TEM measurement, the powder samples were ground and dispersed in methanol. A drop of the dispersed particles was put over the carbon-coated copper grid and evaporated to dryness at room temperature. All the luminescence spectra were recorded using a PerkinElmer LS55 fluorescence spectrophotometer. The decays were recoded using an Edinburgh Instrument FLS920 having a μs-Flash lamp (100 W). A thin film of samples was spread on a thin glass slide with the help of methanol and dried before starting the reading. The heat generation from the magnetic nanoparticles was studied using an Easy Heat 8310 instrument, Ambrell, U.K. The induction coil had four turns, and the diameter of the coil was 6 cm. The applied frequency was fixed at 265 kHz. In order to maintain ambient temperature, the coil was provided with normal water circulation. A 2−10 mg amount of the samples was dispersed in 1 mL of PEG solution. Then, it was transferred into a 1.5 mL micro centrifuge tube, and this was placed at the center of the coil. The center of the coil experienced the maximum magnetic field. The sample was heated using a current of 200, 300, and 400 A up to 10 min. The resultant magnetic field generated at different applied currents was calculated by the following relation

H=

1.257ni L

Figure 3. XRD patterns of (a) CaF2:1Eu nanoparticles at different synthesis temperatures of 60, 120, and 180 °C and (b) Mn2+ (1, 2, and 3 at. %) codoped prepared at 180 °C.

the Supporting Information). Here, samples are prepared at 180 °C. Ionic size of Mn2+ (0.83 Å) is less than that of Ca2+ (1.2 Å) based on 8 CN (coordination number).20,21 Then, there will be a decrease of d-spacing in the case of Mn2+ codoping to Ca2+ sites. The d111 values of 1, 2, and 3 Mn2+ codoping CaF2:1Eu are 1.624, 1.623, and 1.619 Å, respectively, based on the relation

(10)

2d(111) sin θ = λ

where H is the magnetic field, n is the number of turns in the coil, i is the applied current, and L is the diameter of the turn in centimeters. The calculated values of the magnetic fields with respect to the applied currents of 200, 300, and 400 A were 168, 251, and 335 Oe (equivalent to 13, 20, and 27 kA m−1), respectively. The temperature was recorded using an optical temperature sensor (Photon R &D, Canada) with the accuracy of ±0.01 °C by keeping an optical fiber wire in the medium (sample). DC magnetization versus magnetic field at room temperature on all the compositions was carried out using a commercial 9 T PPMS based vibrating sample magnetometer (made by Quantum Design).

d(111) =

1.5405 Å 2 sin θ

The crystalline sizes of CaF2:1Eu (180 °C prepared sample) and 1, 2, and 3 Mn2+ codoped CaF2:1Eu are 33 and 27, 25, and 20 nm, respectively, based on the (111) plane. The hybrid material of CaF2:1Eu and Fe3O4 (4:1 wt %) is prepared. Its XRD pattern is found to consist of two phases CaF2 and Fe3O4 (with JCPDS no. 77-1445), which is shown in Figure S2a (see the Supporting Information), and its expanded view is shown in Figure S2b (see the Supporting Information). Figure 4 (spectra a−d) shows the FTIR spectra of CaF2:1Eu nanoparticles prepared at 60, 120, and 180 °C temperatures. FTIR spectra of ethylene glycol are attributed by O−H stretching and bending vibrations around 3408 and 1641 cm−1, respectively. Vibrations corresponding to C−O and CH2 bending are observed around 1350−1450 and 700−1200 cm−1, respectively. The intensity of the CH2 bending vibration is less than that of the C−O vibration. The peaks around 2900 and 2860 cm−1 correspond to the CH2 asymmetric and symmetric stretching vibrations, respectively. The observation of peaks of O−H, C−C, C−O, and C−H indicates that the nanoparticles are capped with ethylene glycol molecules. From the figure (Figure 4), we observe that, with the increasing synthesis temperature, the peak intensities at 3300−3600 and 1500−1650 cm−1 decrease. This is related to the decrease of water content at higher temperature. The CaF2:1Eu/Fe3O4 (4:1 wt % ratio) hybrid shows the similar FTIR spectrum of CaF2:1Eu, but there is an extra peak at 600 cm−1, which can be attributed to the Fe−O bond of Fe3O4 (Figure 4d).17,22 Nanoparticles are dispersible in polar media such as water and ethanol due to the PEG/EG coated over the nanoparticles. A proposed dispersion mechanism is shown in

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RESULTS AND DISCUSSION Figure 3 shows the XRD patterns of (a) CaF2:1Eu nanoparticles at different synthesis temperatures of 60, 120, and 180 °C and (b) Mn2+ (at. %) codoped CaF2:1Eu (at. % = 1, 2 and 3) prepared at a synthesis temperature of 180 °C. All the peaks can be indexed to the well crystalline pure phase of CaF2 (JCPDS 77-2245) with a cubical structure belonging to space group Fm3m. There is no extra peak from the impurity. The reported unit cell volume and lattice parameter of CaF2 are V = 162.95 (Å)3 and a = 5.46 Å, respectively (JCPDS 77-2245). In our prepared nanoparticles, CaF2:1Eu has a = 5.461 Å, V = 162.82 (Å)3. The expansion of the peak in 2θ = 27−29° for all samples of CaF2:1Eu is shown in Figure S1a (see the Supporting Information). There are slight variations of the peak. The shift in the peak should not happen in the case of the ordered CaF2:1Eu lattice. It is suggested that, when Eu3+ ions are incorporated in the lattice of CaF2, such distortion/defect arises due to charge imbalance upon incorporation of Eu3+ ions in Ca2+ sites. However, there is a slight improvement in crystallinity with the increase of synthesis temperature. In the case of Mn2+ codoping, the peak shifts to the higher 2θ value with the increase of Mn2+ concentration (Figure S1b; see 18090



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Figure 5. TEM image of (a) CaF2:1Eu nanoparticles synthesized at 180 °C and (b) its SAED pattern. TEM image of (c) CaF2:1Eu:3Mn and (d) its SAED pattern. Inset shows the respective HRTEM.

Figure 4. FTIR spectra of doped CaF2:1Eu nanoparticles at different synthesis temperatures of (a) 60, (b) 120, and (c) 180 °C and (d) CaF2:1Eu/Fe3O4 nanoparticles (4:1 wt %). (e) Hydrogen bonding between nanoparticles and water.

Figure 6. (a) Excitation spectra of CaF2:1Eu nanoparticles prepared at different temperatures (λem = 612 nm) and (b) emission spectra of the sample prepared at 180 °C (λex = 240 and 394 nm).

and 180 °C monitored at an emission wavelength of 612 nm. The excitation spectra can be divided into two regions: One region around 220−276 nm is related to the charge transfer transition (CT transition) from the 2p orbital of the F− ion to the empty 4f orbital of the Eu3+ ion, and sharp peaks above 350 nm are due to 4f−4f transitions of Eu3+.23 The 4f6 electrons of Eu3+ are shielded by 5s25p6 shells. The intraelectronic transitions are not disturbed by the environment, thus transitions are sharp, but their absorption cross section is very weak (∼0.1 cm−1) due to forbidden transitions.24 The peaks are observed at 318, 362, 376, and 394 nm, which correspond to 7F0 → 5H3,6, 7F0,1 → 5D0, 7F0,1 → 5G1, and 7F0 → 5 L6 transitions of Eu3+, respectively.25 From the excitation spectrum, we observed that the intensity of excitation spectra increases with increase of synthesis temperature, which is related to the decrease of a luminescent quencher like a OH molecule from the surface of the nanoparticles. Figure 6b shows the emission spectra of CaF2:1Eu nanoparticles after excitation through the CT band (240 nm) and direct f−f transition of the Eu3+ ion (394 nm). Peaks at 590 and 612 nm are observed, and these correspond to the 5D0 → 7 F1 (magnetic dipole) and 5D0 → 7F2 (electric dipole)

Figure 4e in which H2O can make hydrogen bonding with EG molecules coated over nanoparticles. Figure 5a shows the TEM image of CaF2:1Eu synthesized at 180 °C. The particles are found to be spherical in shape, having sizes of 25 nm. The SAED pattern (Figure 5b) shows the crystallinity of the sample, which is indicated by clear spots in the ring. Reflections are assigned in the pattern. The HRTEM image (inset) gives the d = 3.15 Å, which is assigned to the (111) plane. In the case of CaF2:1Eu:3Mn, the particles are still spherical in shape, having sizes of 20 nm (Figure 5c), and its SAED shows the crystallinity of the samples (Figure 5d). The HRTEM image (inset) gives the d = 3.12 Å, which is assigned to (111) plane. EDAX (energy-dispersive analysis of X-ray) spectra of CaF2:1Eu and of CaF2:1Eu:2Mn are shown in Figure S3 (see the Supporting Information). The presence of Ca, F, and Eu could be identified, but the presence of Mn could not be detected within instrument error. It may be due to the light element and that the amount is 2 at. %. Figure 6a shows the excitation spectra of CaF2:1Eu nanoparticles at different synthesis temperatures of 60, 120, 18091



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transitions of the Eu3+ ion.24 The peaks at 651 and 685 nm correspond to 5D0 → 7F3 and 5D0 → 7F4 transitions of the Eu3+ ion. In general, the symmetry of the crystal site in which the Eu3+ ions are located is determined by the ratio of the 5D0 → 7 F1 to 5D0 → 7F2 transitions. The intensity of the peak at 612 nm (5D0 → 7F2 electric dipole transition) is greatly affected by the center of symmetry around the Eu3+ ion (strongly depends on the environment of Eu3+ ion), and it is allowed only on the condition that the Eu3+ occupies a site without an inversion center. On the other hand, the 5D0−7F1 (magnetic dipole transition) is relatively insensitive to the local symmetry. In a site with inversion symmetry, the 5D0 → 7F1 transition is dominating, whereas, in a site with no inversion symmetry, the 5 D0 → 7F2 transition is predominating.24 Upon excitation at 240 nm, the peak intensity at 612 nm is predominant over that at 590 nm; i.e., the intensity of 5D0 → 7F2, the electric dipole transition, is predominant over the intensity of 5D0−7F1, the magnetic dipole transition. A small peak (hump) at 575 nm is observed corresponding to 5D0 → 7F0 transition of the Eu3+ ion. However, in the case of excitation at the direct 4f−4f transition of the Eu3+ ion, i.e., 394 nm, the emission peak at 590 nm (5D0 → 7F1, magnetic dipole) is more intense than the peak at 612 nm (5D0 → 7F2, electric dipole). Variation in intensities of 5D0 → 7F1 and 5D0 → 7F2 at 240 and 394 nm excitation is due to association of the energy transfer process from Eu-F CTB to Eu3+ at the former excitation. Indirect excitation at 240 nm could not determine the symmetry environment of the Eu3+ ion, but direct excitation at 394 nm could do. Upon excitation at 394 nm, the intensity of the magnetic dipole transition is more than that of the electric dipole transition. It is suggested that Eu3+ has a more symmetric environment. This can be possible only when the Eu3+ ion occupies a Ca2+ site. However, upon substitution of Ca2+ sites by Eu3+ ions, there will be some distortion in EuF8 symmetry and a vacancy in the F− ion. Small distortion of EuF8 symmetry can be seen in emission spectra in which the magnetic dipole transition intensity is slightly more than the electric dipole transition. This is similar to the XRD results in which peaks of the reflection change with temperature of preparation. Ca2+ has 8F− surrounding to form CaF8 with an inversion center (i). The point group is Oh, and its space group is Oh5 (Fm3m). Thus, the higher intensity of the magnetic dipole transition over the electric dipole transition indicates that more Eu3+ ions occupy Ca2+ lattice sites and fewer Eu3+ ions occupy the grain boundary or are in the defect lattice. Figure S4a (see the Supporting Information) shows the emission spectrum of (a) CaF2:1Eu nanoparticles at different synthesis temperatures monitored at an excitation wavelength of 240 nm. The intensity of samples prepared at 180 °C is highest as compared to the other two samples prepared at temperatures of 60 and 120 °C. This is related to the improved crystallinity with the increase of heat treatment. The lower crystalline sample can have more surface water molecules/ dangling bonds over particles as compared to the higher crystalline particles. The intensity of the peak at 612 nm is predominant over the 590 nm emission peak, but by exciting at 394 nm, the peak at 590 nm is more intense than that at 612 nm (Figure S4b; see the Supporting Information). Figure 7a,b shows the emission spectra of the Mn2+ ion (1, 2, and 3 at. %) codoped CaF2:1Eu. Variation of electric and magnetic dipole transitions is the same as that of CaF2:1Eu upon excitation at 240 and 394 nm. However, luminescence is more for 1 at. % of Mn2+ ion codoped CaF2:1Eu as compared to CaF2:1Eu. Luminescence intensity increases with codoping

Figure 7. Emission spectra of Mn2+ ion (0, 1, 2, and 3 at. %) codoped CaF2:1Eu nanoparticles monitored at (a) 240 and (b) 394 nm excitation wavelengths. Samples are prepared at 180 °C.

of Mn2+ up to 3 at. %. It means that codoping of Mn2+ improves luminescence. Improvement of luminescence is due to substitution of Ca2+sites by Mn2+ ions. It is likely that the dipole moment of the transition increases upon codoping of Mn2+ ions. Improvement of luminescence intensity with increasing synthesis temperature from 60 to 180 °C in the case of CaF2:1Eu is due to the decrease of nonradiative (Ro) rate as compared to radiative rate. Ro is expressed as R o = α e−(ΔE − 2hνmax)β

(11)

where α and β are constants. ΔE is the difference in the energy between excited and ground states of the Eu3+ ions, and vmax are the highest available vibrational modes of the surroundings of the rare-earth ion. In the Eu3+ion, ΔE is about 10000−15000 cm−1, and the value is comparable with the third overtone stretching vibration of the −OH group (3500 cm−1). This OH functional group arises from the water molecules absorbed or associated during the synthesis of the nanomaterials. Here, an aqueous medium and EG solvent are used and are the source of water. The Ro value becomes large when ΔE ∼ 2hνmax. The significant extent of nonradiative transfer of energy from the excited state of the Eu3+ ions to the different vibrational modes of OH species occurs, leading to a reduction in Eu 3+ luminescence intensity. During excitation at 240 and 394 nm, part of the excited photon is used for excitation of the nearby surrounding phonons of the OH group. The process is termed as multiphonon relaxation.14,26 Figure S5 (see the Supporting Information) shows the emission spectra of the samples CaF2:1Eu and Mn2+ ion codoped CaF2:1Eu incorporated into the PVA polymer. Excitation is fixed at (a) 240 and (b) 394 nm. The variation of intensity ratio of electric and magnetic dipole transitions is the same as those without PVA. The intensity of the samples incorporated in the polymer is lower than that of the samples not incorporated in the polymer. This can be explained as follows: The PVA polymer has many C−H and O−H bonds that can absorb the incoming excitation light, and thus, it reduces the intensity of the emission peaks. There is a broad peak with a maximum at 525 nm that is related to the contribution from the PVA sample interface. Polymer incorporated with luminescence nanoparticles is mainly used in display, telecommunication, and sensor applications.24 Powder samples of CaF2:1Eu and CaF2:1Eu:Mn are dispersible in methanol/ethanol/PEG. 18092



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Figure 8 shows the decay curves for the 5D0 level of Eu3+ in CaF2:1Eu and CaF2:1Eu:3Mn prepared at a 180 °C synthesis

6.63 ms (52) and the average lifetime is 3.81 ms. The lifetime is longer in the case of Mn codoping as compared to that without codoping. This is related to the enhancement of luminescence by Mn codoping. Figure 10 shows the plot of temperature achieved by samples of Fe3O4 and hybrid CaF2:1Eu/Fe3O4 vs time. With time, the

Figure 8. Decay curves monitored at 615 nm emission and 394 nm excitation for CaF2:1Eu and CaF2:1Eu:3Mn nanoparticles prepared at 180 °C. Y axis is expressed in log scale. The data are fitted with a biexponential decay.

temperature. Excitation and emission are fixed at 394 and 615 nm, respectively. The y axis is expressed in log scale. Decay data are fitted well by biexponential eq 12. In biexponential equation, I1 and I2 are the intensities at different time intervals; and τ1 and τ2 are their corresponding lifetimes.12 I = I1(e−t / τ1) + I2(e−t / τ2)

Figure 10. Temperature achieved by (a) Fe3O4 and (b) CaF2:1Eu/ Fe3O4 with time at different concentrations (current = 400 A) and (c) that achieved by CaF2:1Eu/Fe3O4 with time at different currents (concentration = 6 mg/mL). Samples are dispersed in 1 mL of PEG solution (100 mg of PEG is dissolved in 100 mL of water). (d) Linear fit to data of temperature achieved by sample (6 mg of CaF2:1Eu/ Fe3O4) with H2.

(12)

By assuming spherical particles, a particle can be divided into two shells of equal volume (Figure 9). The inner shell has a

temperature increases when either the concentration of the sample or the current increases. Here, the applied frequency is fixed at 265 kHz. The samples are Fe3O4 and CaF2:1Eu/Fe3O4 hybrid nanoparticles. When the concentration of a sample increases at a particular current (400 A or 335 kOe), the alternating magnetic field (AMF) experienced by the sample increases. Thus, the higher heat is generated, whereas, when the current increases from 200 to 400 A (168 to 335 kOe), at a particular concentration of sample (6 mg), a similar increase in heat generation occurs because of the increase of AMF experienced by the sample. In AMF, the heat can be generated by the Brownian particle rotation, Néel’s spin relaxation, hysteresis loss, and eddy current. Brownian particle rotation and Néel’s spin relaxation contribute more to heat generation. There is also a contribution to heat generation from hysteresis loss because, at frequency f = 265 kHz, the time for the change of spin of the domain is 10−6 s, whereas Néel’s spin relaxation is 10−9 s. At least, some moment can be seen during the relaxation phenomenon. However, heat generation from eddy current is negligible. In Figure 10a, the temperatures achieved by 2, 4, 6, and 10 mg of pure Fe3O4 are 39, 44, 50, and 62 °C, respectively, at a fixed current of 400 A for 10 min each. Two milligrams of sample could not reach hypothermia temperature (42 °C). Four milligrams of sample reached 42 °C in just 550 s, whereas 6 and 10 mg of sample are able to get to 42 °C in 60 and 20 s, respectively. In Figure 10b, 2, 4, and 6 mg of CaF2:1Eu/Fe3O4 are able to get to 42 °C in 200, 50, and 40 s, respectively. It is to

Figure 9. Availability of Eu3+ activators in the CaF2 lattice (inner and outer shells). Inner Eu3+ has inversion symmetry (symbol i).

longer lifetime than the outer shell. The average lifetime in the case of biexponential decay can be calculated using the following equation τav =

I1τ12 + I2τ22 I1τ1 + I2τ2

(13)

From the calculation, we get τ1 = 1.08 ms (47%) and τ2 = 5.81 ms (53%) for CaF2:1Eu samples and the average lifetime is 3.44 ms, and for CaF2:1Eu:3Mn, τ1 = 0.99 ms (48%) and τ2 = 18093



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will be useful in tracing of magnetic nanoparticles when injected into a living body or in vitro study. Recently, CaF2:Ln3+ nanoparticles were used in bioassays for cancer diagnosis.15 Figure S7a−c (see the Supporting Information) shows the dispersing behavior of samples in PEG medium. The CaF2:1Eu are well dispersed in PEG, as shown in Figure S7a. The hybrid materials, i.e., CaF2:1Eu/Fe3O4, are also well dispersed in PEG, as shown by the uniform brown color (Figure S7b). By the application of a magnetic field, the brown dispersed materials are attracted toward the applied field, leaving the solution transparent (Figure S7c), indicating that not only the magnetic part but also CaF2:1Eu are attracted toward the applied field. Magnetization-field data (MH) of Fe3O3 and hybrid CaF2:1Eu/Fe3O4 (4:1 wt. ratio) are shown in Figure S8 (see the Supporting Information) and respective M values at 4 T are 62.2 and 13.3 emu/g, respectively. M per 1 g of Fe3O4 for the hybrid is 66.6 emu. Figure 11 shows the percentage viability of HeLa cells treated with PEG solvent (control) and the CaF2:1Eu/Fe3O4 nano-

be noted that 2, 4, and 6 mg of CaF2:1Eu/Fe3O4 refer to 2, 4, and 6 mg of Fe3O4 content, respectively, and these are mixed with 8, 16, and 24 mg of CaF2:1Eu, respectively. The heating experienced by the hybrid is significantly more than that of pure Fe3O4. The increase in heating may be related to the following possibilities: (1) The formation of Fe3O4/Eu3O4, since Eu3O4 is also antiferromagnetic, it will enhance magnetization. However, we could not detect the formation of Eu3O4 in the XRD pattern since the amount of Eu3+ is 1 at. % with respect to Ca2+. This is a very low concentration. (2) Some kind of exchange interaction among magnetic spins takes place, and the anisotropy energy increases because f-orbitals of Eu3+ in Eu3O4 are more anisotropic than d-orbitals of Fe2+ in Fe3O4. (3) The f-orbitals of Eu3+ (4f6 electron) act as spin-pinning, and thus, magnetocrystalline anisotropy increases. In order to see the current effect (or magnetic field) on 6 mg of CaF2:1Eu/ Fe3O4, (Figure 10c), temperature achieved vs time data are recorded. It is observed that heat generation increases with an increase of current from 200 to 400 A. Times taken by the sample to achieve 42 °C are 213, 52 and 36 s, respectively, for 200, 300, and 400 A current. The SAR (specific absorption rate) values are given Table 1. The reported SAR values for Fe3O4 nanoparticles were 35−100 W/g at the same AC frequency and magnetic field.17,19 Table 1. SAR (W/g) Values of Pure Fe3O4 and CaF2:1Eu/ Fe3O4 at Different Currents SAR (W/g) at different currents Sl. No. 1 2 3 4 1 2 3

sample (mg) Fe3O4 2 4 6 10 CaF2:1Eu/Fe3O4 2 4 6

200 A

300 A

400 A 94 69 81 71

111 120 86

186 170 99

283 191 122

Figure 11. Percentage viability of HeLa cells treated with PEG solution (control) and CaF2:1Eu/Fe3O4 nanohybrid at different concentrations. PEG solution is prepared by dissolving 100 mg of PEG-6000 in 100 mL of distilled water. Nanoparticles are dispersed in 1 mL of PEG solution.

In order to prove the square dependence field (H) on the heat dissipation, temperature vs H2 is plotted in Figure 10d, which shows a linear dependence (T vs H2) in accordance with eq 4. The hybrids of CaF2:1Eu and Fe3O4 (4:1 and 6:1 wt %) are prepared. Their excitation spectra after keeping emission at 615 nm are shown in Figure S6a (see the Supporting Information). There are strong absorption bands at 300 and 325 nm for 6:1 and 4:1, respectively. There are peaks at 395, 465, and 525 nm due to the Eu3+ ion, and positions of these peaks are found to be unchanged with variation of CaF2:1Eu and Fe3O4 ratios. The absorption band shifts from 300 to 325 nm with the increase of Fe3O4. The similar band was observed in SnO2:Eu-Y2O327 and SnO2:TiO228 hybrid materials, and this was ascribed to exciton formation in two semiconductors interface by Ningthoujam et al. The intensity decreases with the increase of Fe3O4 amount with respect to CaF2:1Eu. The emission spectra of 4:1 and 6:1 CaF2:1Eu and Fe3O4 wt % are shown in Figure S6b,c (see the Supporting Information) after excitation at 394 and 300 nm. We see the emission peak at 575, 590, and 615 nm due to the Eu3+, and this range is included in the biological window. Thus, these hybrid materials

hybrid at different concentrations. It has 75% viability even at a concentration of 100 μg/mL. Here, PEG-600 solution (100 mg/100 mL) is used as the medium/control.

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CONCLUSIONS Highly water dispersible CaF2:1Eu nanoparticles are prepared. The observed luminescence peaks at 590 and 612 nm are related to the magnetic and electric dipole transitions, respectively. The probability of magnetic dipole transition is more than that of electric dipole transition upon excitation at 394 nm, which indicates that more Eu3+ ions occupy Ca2+ sites having an inversion center (i), and few Eu3+ ions occupy the grain boundary/surface or are present in the defect lattice. Improvement of luminescence is found after codoping of Mn2+, and this can be ascribed to the increase of optical dipole transition. Prepared PVA polymer films also show the 18094



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(8) Rotereau, K.; Daniel, P.; Gedland, J. Y. Vibrational and Electronic Properties of the Lanthanide Trifluorides GdF3, TbF3, ErF3 and YbF3 Studied by Raman Spectroscopy. J. Phys. Chem. Solids 1998, 59, 969− 980. (9) Gourley, J. T.; Runciman, W. A. Multiphonon Infrared Absorption Spectra of MgO and CaO. J. Phys. C: Solid State Phys. 1973, 6, 583−592. (10) Singh, L. R.; Ningthoujam, R. S.; Sudarsan, V.; Srivastava, I.; Singh, D. S.; Dey, G. K.; Kulshreshtha, S. K. Luminescence Study on Eu3+ Doped Y2O3 Nanoparticles: Particle Size, Concentration and Core−Shell Formation Effects. Nanotechnology 2008, 19, 055201− 055207. (11) Guo, H.; Yang, X.; Xiao, T.; Zhang, W.; Lou, L.; Mugnier, J. Structure and Optical Properties of Sol−Gel Derived Gd 2O 3 Waveguide Films. Appl. Surf. Sci. 2004, 230, 215−221. (12) Marius, H.; Roger, O. S.; Lun, M.; Wei, C.; Yongbin, Z.; Ramaswami, S.; Alan, G. J. On the Luminescence Enhancement of Mn2+ by Co-Doping of Eu2+ in ZnS:Mn,Eu. Opt. Mater. 2013, 35, 1513−1519. (13) Parchur, A. K.; Ningthoujam, R. S. Preparation, Microstructure and Crystal Structure Studies of Li+ Co-Doped YPO4:Eu. RSC Adv. 2012, 2, 10854−10858. (14) Luwang, M. N.; Ningthoujam, R. S.; Srivastava, S. K.; Vatsa, R. K. Disappearance and Recovery of Luminescence in Bi3+, Eu3+ CoDopedYPO4 Nanoparticles Due to the Presence of Water Molecules Up to 800 °C. J. Am. Chem. Soc. 2011, 133, 2998−3004. (15) Zheng, W.; Zhou, S.; Chen, Z.; Hu, P.; Liu, Y.; Tu, D.; Zhu, H.; Li, R.; Huang, M.; Chen, X. Sub-10 nm Lanthanide-Doped CaF2 Nanoprobes for Time-Resolved Luminescent Biodetection. Angew. Chem., Int. Ed. 2013, 52, 6671−6676. (16) Ningthoujam, R. S.; Vatsa, R. K.; Kumar, A.; Pandey, B. N. Functionalized Magnetic Nanoparticles: Concepts, Synthesis and Application in Cancer Hyperthermia. In Functional Materials: Preparation, Processing and Applications; Banerjee, S., Tyagi, A. K., Eds.; Elsevier Inc.: Waltham, MA, 2012; Chapter 6, pp 229−260. (17) Prasad, A. I.; Parchur, A. K.; Juluri, R. R.; Jadhav, N.; Pandey, B. N.; Ningthoujam, R. S.; Vatsa, R. K. Bi-Functional Properties of Fe3O4@YPO4:Eu Hybrid Nanoparticles: Hyperthermia Application. Dalton Trans. 2013, 42, 4885−4896. (18) Luwang, M. N.; Chandra, S.; Bahadur, D.; Srivastava, S. K. Dendrimer Facilitated Synthesis of Multifunctional Lanthanide Based Hybrid Nanomaterials for Biological Applications. J. Mater. Chem. 2012, 22, 3395−3403. (19) Ghosh, R.; Pradhan, L.; Yensenbam, P. D.; Meena, S. S.; Tewari, R.; Amit, K.; Sachil, S.; Gajbhiye, N. S.; Vatsa, R. K.; Badri, N. P.; Ningthoujam, R. S. Induction Heating Studies of Fe3O4 Magnetic Nanoparticles Capped with Oleic Acid and Polyethylene Glycol for Hyperthermia. J. Mater. Chem. 2011, 21, 13388−13398. (20) Strobel, P.; Ibarra-Palos, A.; Pernet, M.; Zouari, S.; CheikhRouhou, W.; Cheikh-Rouhou, A. Crystal Chemistry of Non-Perovskite Manganese Oxides − Implications for Magnetic Properties. Phys. Status Solidi C 2004, 7, 1625−1630. (21) Gejihu, D.; Weiping, Q.; Zhang, J.; Jishuang, Z.; Yan, W.; Chunyan, C.; Yang, C. Synthesis and Optical Characterizations of Undoped and Rare- Earth-Doped CaF2 Nanoparticles. J. Solid State Chem. 2006, 179, 955−958. (22) Kemp, W. Organic Spectroscopy; Macmillan Educational Ltd.: Basingstoke, England, 1987. (23) Gejihu, W.; Qin, J.; Zhang, J.; Zhang, Y.; Wang, C.; Yang, C. Synthesis and Photoluminescence of Single Crystals Europium IonDoped BaF2 Cubic Nanorods. J. Solid State Chem. 2006, 179, 955− 958. (24) Ningthoujam, R. S. Enhancement of Luminescence by Rare Earth Ions Doping in Semiconductor Host. In Synthesis, Characterization and Applications of Multifunctional Materials; Rai, S. B., Dwivedi, Y., Eds.; Nova Science Publishers Inc.: Hauppauge, NY, 2012. (25) Luwang, M. N.; Ningthoujam, R. S.; Jagannath, S.; Srivastava, S. K.; Vatsa, R. K. Effects of Ce3+Co-doping and Annealing on Phase

luminescence. When hybrid material with Fe3O4 is prepared, extra properties are found, i.e., red emission and heating under an AC magnetic field. We achieved a hyperthermia temperature of 42 °C with a 2 mg amount of hybrid material at 265 kHz and 335 Oe. These will be useful in diagnosis through optical imaging and therapy through hyperthermia. The SAR value of 283 W/g is found. Biocompatibility (75%) of the hybrid material in HeLa cells is observed for concentrations up to 100 μg/mL.

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ASSOCIATED CONTENT

S Supporting Information *

Figures of XRD patterns, EDAX spectra, emission spectra of CaF2:1Eu nanoparticles at different synthesis temperatures, emission spectra of Mn2+ ion (0, 1, 2, and 3 at. %) codoped CaF2:1Eu nanoparticles incorporated in PVA polymer, the excitation/emission spectra of CaF2:1Eu/Fe3O4, and magnetization vs applied magnetic field. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected], [email protected]. Phone: +91-22-25592321. Fax: +91-22-25505151 (R.S.N.). *E-mail: [email protected]. Phone: 0385-2435049. Fax: 0385-2435145 (S.K.S.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank Dr. L. R. Singh and A. I. Prasad, NEHU, for providing the TEM facility. R.S.N. thanks Dr. R. K. Vatsa, Dr. V. K. Jain, Chemistry Division, and Dr. B. N. Jagatap, Chemistry Group, BARC, for encouragement and Dr. P. D. Babu and UGC-DAE Consortium for Scientific Research, Mumbai Centre, for help in magnetization measurements.

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REFERENCES

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Multifunctional Hybrid Nanomaterials from Water Dispersible CaF2

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