Enhanced Temperature-Sensing Behavior of Ho3+–Yb3+-Codoped

Apr 24, 2019 - (38,39) Figure 6b depicts the CIE chromaticity diagram. ... A part of populated photons also get relaxed to 5I7 level and results in NI...
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Cite This: ACS Omega 2019, 4, 7482−7491

Enhanced Temperature-Sensing Behavior of Ho3+−Yb3+-Codoped CaTiO3 and Its Hybrid Formation with Fe3O4 Nanoparticles for Hyperthermia Neha Jain,† Rajan K. Singh,† Bheeshma Pratap Singh,‡ Amit Srivastava,§ R. A. Singh,† and Jai Singh*,† †

Department of Physics, Dr. Hari Singh Gour Central University, Sagar, M. P. 470003, India Chemistry Division, Bhabha Atomic Research Centre, Trombay, Mumbai 400085, India § Department of Physics, TDPG College, VBS Purvanchal University, Jaunpur, Uttar Pradesh 222001, India Downloaded via 79.133.106.172 on April 25, 2019 at 05:50:37 (UTC). See https://pubs.acs.org/sharingguidelines for options on how to legitimately share published articles.

‡

ABSTRACT: In the present work, we describe the synthesis of (Ca0.99−xHo0.01Ybx)TiO3 (x = 0.05, 0.10, and 0.15) perovskite nanoparticles with a cubic structure by using a sol−gel method. As-synthesized particles have been found to be spherical in shape, as analyzed by transmission electron microscopy and scanning electron microscopy. Fourier transform infrared spectra of the product perovskite consist of distinct featured vibrational modes of Ti−O. Upconversion as well as power-dependent (500−4000 mW) spectra have been investigated at 980 nm excitation. It exhibits emission peaks at 547, 655, and 759 nm owing to 5F4/5S2 → 5I8, 5F5 → 5I8, and 5F4/5S2 → 5I7 transitions of Ho3+ ions, respectively. Optical thermometry behavior has been evaluated using fluorescent intensity ratio (FIR) of thermally and electronically coupled levels of Ho3+ ions. It reports a good sensitivity with 0.0229 and 3 × 10−3 K−1 as estimated through thermally and electronically coupled FIR, respectively, at room temperature (303 K). Upconversion nanoparticles of (Ca0.89Ho0.01Yb0.1)TiO3 have further been coupled with Fe3O4 as hybrid magnetic nanoparticles for hyperthermia study. It attains hyperthermia temperature (42 °C) in 3 min under ac magnetic field, with a specific absorption rate of 35 W/g. Therefore, it could be interpreted that the present hybrid upconversion nanoparticles may be very useful and paves the path for cancer therapy (hyperthermia) and in likewise biomedical applications.

1. INTRODUCTION Research on the development of rare-earth (RE) activated phosphors has received considerable interest because of their unique optical and magnetic properties. Usually, RE elements give various narrow emission bands because of their characteristic electronic 4f−4f transitions, which lead to their tremendous potential applications such as in solid-state lasers, optical fiber communication systems, luminescent lamps, color rendering, especially biomedical/temperature−sensing, and so forth.1−3 Fluorescent intensity ratio (FIR) in lanthanide-based materials has been observed to be an effective route than its existing conventional temperature sensor counterparts, as it specifically does not rely on the measurement conditions such as electrical, magnetic, flammable situation, etc.). In this approach, the ratio of two thermally/electronically coupled levels of RE ions has usually been used to measure the temperature sensitivity. The energy separation between electronic levels should be less than 2000 cm−1.4 However, efforts have constantly been made to explore simple new temperature-sensing strategies using RE activated nanophosphors. There have been several reports based on the temperature-sensing performance of Eu3+-, Ho3+−Yb3+-, Er3+− Yb3+-, and Tm3+−Yb3+-codoped phosphors.6−10 In Er3+−Yb3+codoped system, the FIR with two thermally coupled closely © 2019 American Chemical Society

spaced peaks falls in the green region and the difference in its thermal sensitivity is not observed differently. Moreover, the

(

( −ΔkTE )) reveals its FIR to

optical performance FIR = B exp

be good. Some reports suggest that Ho3+, an RE ion, has a combination of two thermally coupled levels and thermometry analysis reveals its activation to be mainly due to 3K8 → 5I8 (460 nm) and 5F3 → 5I8 (489 nm) while FIR vests in the blue region.11,12 Moreover, significant FIR could also be observed by employing near-infrared (NIR)/red to green emission ratio. Recently, Chai et al. employed this approach to measure the temperature-sensitive performance and observe FIR, thereby suggesting the NIR to green emission peak ratio to be suitable for low-temperature sensitivity (6.4 × 10−3 K−1 at 83 K) and the red to green ratio for high-temperature sensitivity (1.5 × 10−3 K−1 at 503 K).13 The temperature sensitivity and photoluminescence (PL) emission performance get affected by the doping in the host. Some hosts such as CaWO4, CaMoO4, YPO4, Y2O3, NaLuF4, BaZrO3, and so forth11−19 have been reported for Ho3+−Yb3+ codoping earlier. NoneReceived: January 21, 2019 Accepted: April 5, 2019 Published: April 24, 2019 7482

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theless, Ho3+−Yb3+-codoped CaTiO3 has not been reported so far. Its phonon energy is less than 700 cm−1, which makes it viable for efficient PL emission and bioimaging because of low scattering and high chemical and thermal stability.20 As it is the derivative of a perovskite family, its structure influences the PL emission interestingly. Recently, there has been surge on its biocompatibility and bioimaging studies too.21,22 Magnetic nanoparticles (MNPs) have shown their vast application in cancer therapy and can also be used as contrasting agents in magnetic resonance imaging. Hyperthermia therapy has been found very effective to be used in cancer treatment. In the hyperthermia-based therapy, the temperature of the particular part of the body (affected by cancer) gets increased.23 Cancer cells are known to be sensitive to the temperature and get killed at 40−43 °C (hyperthermia temperature). The realization of such treatment has been found to be superior to its chemotherapy and several other treatments counterparts.24−27,30 Moreover, MNPs have also known to be biocompatible. Therefore, after injecting these MNPs into the body and thereby applying the magnetic field, the temperature of MNPs increases and attains hyperthermia temperature (42 °C) in few moments, subsequently killing the cancer cells.28,29 After removing the magnetic field, no magnetization retains in MNPs showing a superparamagnetic character. In the present study, Fe3O4 has been taken as MNPs and attached to Ho3+- and Yb3+-codoped CaTiO3 perovskite. Till date, to the best of our knowledge, the proposed study has not been carried out on (Ca0.99−xHo0.01Ybx)TiO3 (x = 0.05, 0.1, and 0.15) perovskite nanoparticles, yet for hyperthermia application. In the present study, we have used a facile sol−gel approach for the preparation of Ho3+- and Yb3+-codoped CaTiO3 upconversion nanoparticles (UCNPs). Ho3+ has already been optimized by researchers, leading to the prominent PL emission at 1% concentration. However, varying Yb3+ concentration has been used here with concentrations 5, 10, and 15% to obtain its optimum. In order to elucidate the structural feature as well as morphological character, X-ray diffraction (XRD), transmission electron microscopy (TEM), and scanning electron microscopy (SEM) analysis techniques have been employed. Upconversion as well as powerdependent upconversion spectra have been recorded at 980 nm NIR laser excitation. Involved energy-transfer (ET) mechanism in the radiative and nonradiative processes has also been discussed. Temperature-dependent upconversion spectra reveal its better sensitivity than other recently reported works. The temperature-sensing performance clearly indicates the emission-tuned temperature and attains the hyperthermia temperature remarkably.

Figure 1. XRD patterns of (a) (Ca0.99−xHo0.01Ybx)TiO3 (x = 0.05, 0.10, 0.15) and (b) CaTiO3, Fe3O4, and (Ca0.89Ho0.01Yb0.10)TiO3@ Fe3O4. (Vertical drop lines represent standard JCPDS data of CaTiO3 and Fe3O4 from card no. 08-0092 and 89-4319.)

have been evaluated by the Scherer formula and found to be 24.26, 23.81, and 23.62 nm for 5, 10, and 15% Yb3+-doped samples, respectively. The variation in the intensities of the diffraction peaks shows the alteration in the lattice parameter, which are found to be 15.29, 15.21, and 15.24 Å with 5−15% doped Yb3+, respectively. Figure 1b illustrates the cubic phase of CaTiO3 and Fe3O4 with high crystallinity and its diffraction peaks (29.96° and 35.34°) overlapped to the respective diffraction peaks of CHY@Fe3O4. However, other peaks of Fe3O4 with the diffraction angle more than 50° have not been observed in CHY@Fe3O4 because of the low concentration of Fe3O4 in the total mass of the sample.32 Figure 2 depicts the Fourier transform infrared (FTIR) spectra of the as-synthesized nanoparticles, which shows Ti−O

2. RESULTS AND DISCUSSION Figure 1a,b represents the typical XRD patterns of CaTiO3, Fe3O4, CaTiO3:Ho3+, Yb3+, and (Ca0.89Ho0.01Yb0.10)TiO3@ Fe3O4 (CHY@Fe3O4) hybrid nanoparticles. The diffraction peaks of all samples have been found to be in good agreement with the cubic phase of CaTiO3 (JCPDS 08-0092 with the space group Pm3̅m). The intensity of the peak at 29.94° gradually increases with the concentration of Yb3+ codoping. It is enhanced because of the variations in lattice parameters and improvement in phase purity with Yb3+ codoping. Additionally, diffraction peaks become sharper when codoped with Yb3+, leading to the better crystallinity of the samples. Yang et al. studied a detailed substitution mechanism of Ca2+ with lanthanides.31 The crystallite sizes of the nanophosphors

Figure 2. FTIR spectra of (Ca0.89Ho0.01Yb0.10)TiO3, Fe3O4, and (Ca0.89Ho0.01Yb0.10)TiO3@Fe3O4 representing the vibrational mode of Ti−O.

bands at ∼547 and 594 cm−1 in (Ca0.89Ho0.01Yb0.10)TiO3 and CHY@Fe3O4, respectively.22,33 Moreover, another band at around 790 cm−1 corresponds to the C−Cl stretching vibration and 1455 cm−1 owes to the C−C band.22,34 Fe3O4 vibrational bands centered at 608 and 884 cm−1 feature the Fe−O and Fe−O−Fe bending vibrations, respectively.35 In addition, hybrid UCNP magnetic nanocomposites have been prepared using poly(ethylene glycol); therefore, some additional bands 7483

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Figure 3. SEM images of (a) (Ca0.89Ho0.01Yb0.10)TiO3 and (b) Fe3O4, (c) EDX spectra of Fe3O4, (d) EDX spectra of (Ca0.89Ho0.01Yb0.10)TiO3, and (e−i) elemental mapping of O, Ti, Ca, Yb, and Ho in (Ca0.89Ho0.01Yb0.10)TiO3 nanocomposites.

Figure 4. (a) SEM images of (Ca0.89Ho0.01Yb0.10)TiO3@Fe3O4, (b) its EDX spectra, and (c−h) elemental mapping of O, Ti, Ca, Ho, Yb, and Fe in hybrid MNPs.

at 2856 and 2920 cm−1 appeared owing to C−H vibration.22 The change in absorption intensity has also been observed because of the formation of nanocomposites of CaTiO3 and Fe3O4 and the overlapping of the vibrational bands has also been traced in the region 540−610 cm−1. XRD patterns and FTIR spectra clearly indicate the successful formation of nanocomposites of CaTiO3 and Fe3O4. Figure 3a−i represents the scanning electron micrographs, energy-dispersive X-ray (EDX) spectra, and elemental mapping of UCNPs (Ca0.89Ho0.01Yb0.10)TiO3 and Fe3O4. It is evident from SEM micrographs that all the UC nanoparticles are

spherical in shape and sizes of the particles have been found to be in the range of 50−150 nm. These particles are slightly agglomerated and distributed over the surface of the sphere. The shape of MNPs is rectangular and gets agglomerated in the absence of a capping agent. EDX spectra depict the presence of elements Ca, Ti, O, Ho, and Yb in (Ca0.89Ho0.01Yb0.10)TiO3 and Fe, O elements in Fe3O4. Moreover, Figure 4a−h illustrates the SEM micrographs, EDX spectra, and elemental mapping of hybrid CHY@Fe3O4 nanocomposites. From the SEM image, it is evident that both spherical UCNPs and rectangular Fe3O4 are present in the 7484

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Figure 5. TEM image of (a) (Ca0.89Ho0.01Yb0.10)TiO3 (b) (Ca0.89Ho0.01Yb0.10)TiO3@Fe3O4 and (c) SADP of hybrid MNPs.

Figure 6. (a) Diffuse reflectance spectrum of (Ca0.89Ho0.01Yb0.10)TiO3 and hybrid MNPs (Fe3O4) with (Ca0.89Ho0.01Yb0.10)TiO3, (b) CIE chromaticity diagram (inset zoomed CIE diagram for x = 0.05, 0.15), (c) upconversion emission spectra of (Ca0.99−xHo0.01Ybx)TiO3 (x = 0.05, 0.10, 0.15), and (d) MNPs (Fe3O4) attached (Ca0.89Ho0.01Yb0.10)TiO3.

phors excited by a 980 nm (NIR) laser have been shown in Figure 6c, which shows Ho3+ characteristic emission peaks at 547, 655, and 759 nm corresponding to 5F4/5S2 → 5I8, 5F5 → 5 I8, and 5F4/5S2 → 5I7 transitions, respectively.13,37 The UC emission intensity varies with Yb3+ doping concentration and attains maximum with 10% Yb3+ concentration and then decreases further with an increase in Yb3+ concentration. This decrement in intensity may be due to the effect of concentration quenching,13 and the appearance of variation in the intensities of Ho3+ ions with Yb3+ doping concentration may probably be due to the slight variation in the local field of Ho3+ ions. It has already been discussed that with Yb3+ codoping, crystallinity and phase purity have improved, which primarily affect the PL behavior. Next, UC emission spectra of CaTiO3:1%Ho3+,10%Yb3+@Fe3O4 (CHY@Fe3O4) have been illustrated in Figure 6d. It is evident that the PL intensity decreases with MNP (Fe3O4) attachment, leading to the quenched luminescence. As Fe3+ is a paramagnetic system (basically d5 configured) in which excited-state energy is utilized in a spin−spin domain of the MNPs. Thus, when Fe3O4 is incorporated with CaTiO3:Ho,Yb, most of the excited-state energy is being utilized by the spin−spin domain of the paramagnetic nanoparticles, thus quenches the photoluminescence intensity.38,39 Figure 6b depicts the CIE

hybrid nanocomposites. EDX spectra and elemental mapping confirm the presence of Ca, Ti, O, Ho, Yb, and Fe elements in the as-synthesized nanocomposites. Figure 5a−c displays the TEM micrographs of UCNPs and hybrid CHY@Fe3O4 nanocomposites, which further confirm the spherical morphology of the nanoparticles, having sizes in the range of 100−150 nm. Because of the formation of a composite, bit agglomeration has been observed for hybrid UCNPs (marked by circle). As spherical morphology of the particles has known to be good for bioimaging purpose, it may also be viably good for further application. In addition, selected area diffraction pattern (SADP) of (Ca0.89Ho0.01Yb0.10)TiO3@ Fe3O4 has been shown in Figure 5c revealing a polycrystalline circular ring pattern.36 The circular ring pattern has been indexed with a cubic CaTiO3 perovskite (JCPDS card no. 080092), indicating (444), (800), and (940) reflecting planes. It also indicates the existence of cubic Fe3O4 ring indexed to the (331) reflection plane (JCPDS card no. 89-4319). Diffuse reflectance spectra of (Ca0.89Ho0.01Yb0.10)TiO3 and CHY@Fe3O4 have been presented in Figure 6a. Absorbance peaks at 980 nm ascribed as a Yb3+ characteristic peak (2F7/2 → 2 F5/2) and the band lying in the range 200−350 nm might have aroused because of the host absorption.4 The UC emission spectra of as-synthesized (Ca0.99−xHo0.01Ybx)TiO3 nanophos7485

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two ions as compared to GSA process. Further, excited electrons from the 5I6 state have promoted to the next higher energy state of Ho3+ ions (5F4/5S2 or 5F5 state) by excited-state absorption.11,14 It can be seen from a schematic energy-level diagram that for a single visible photon emission, two or three photon absorptions are required. Therefore, the number of photons involved in emission has been calculated with the help of a power-dependent study and discussed in the later section. The energy levels 5F3 and 3K8 are populated by cross-relaxation (CR) processes 3K8 + 5I7 → 5F4/5S2 + 5I6 and 5F3 + 5I8 → 5F5 + 5I7. However, in UC emission spectra, we have not observed emission from the 3K8 level, which may be a nonradiative (through phonon vibrations) relaxation to the 5F3 level.37 Furthermore, the intensity of the transition 5F3 → 5I8 (485 nm) is less than the transition 5F4/5S2 → 5I7 (759 nm), which implies that the probability of ET process by CR is less than other ET processes.11 Thereafter, these populated electrons relax from 5F4/5S2 to the ground-state (GS) (5I8) and give green emission (547 nm). Maximum electrons relax from 5 F4/5S2 to the GS; therefore, green emission peak is likely to be dominant over other emissions. Some electrons relax from these excited states to 5F5 state by NR relaxation and further to the GS by emitting red photon (655 nm). A part of populated photons also get relaxed to 5I7 level and results in NIR emission (759 nm).11 Figure 8a describes the power-dependent upconversion spectra. Its emission intensity depends on the pumped power and gradually increases with the increase in the power of the laser.37 It can also be seen from the inset that on increasing the pump power, a minor red shift occurs (about 2 nm) in blue emission (486 nm), which may be due to the laser-induced heating effect.40 It basically helps to understand the absorption of photons in UC emission process and can be described by the relation

chromaticity diagram. The coordinates are (0.33, 0.43), (0.32, 0.66), and (0.34, 0.39) for 5, 10, and 15%, respectively. Moreover, the Fe3O4-attached UCNP CIE has a coordinate (0.31, 0.65) fall in the green region. The shift attributes that with increasing Yb3+ concentrations, ratio of 5F4/5S2 → 5I8 to 5 F5 → 5I8 varies. From a zoomed CIE diagram, it can be seen that the CIE coordinate (5 and 15% Yb3+) has been found in the light yellowish region because of mixing the color green and red (5F4/5S2 → 5I8 to 5F5 → 5I8). The ET between Ho3+ and Yb3+ ions can also be understood by the mechanism as illustrated in the Figure 7. Upon NIR

Figure 7. Schematic illustration of involved ET between Ho3+ and Yb3+ in CaTiO3.

excitation (980 nm), Ho3+ electron gets excited to 5I6 state through the ground-state absorption (GSA).14 Yb3+ also gets excited to 2F5/2 by absorbing a photon of 980 nm. As absorption cross section of Yb3+ ion is greater than its Ho3+ counterpart at 980 nm excitation, ET is effective between these

Iuc = P n

(1)

Figure 8. (a) Power-dependent upconversion spectra at 980 nm excitation for (Ca0.89Ho0.01Yb0.10)TiO3 and (b−d) logarithmic plot for 547, 655, and 759 nm. 7486

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Figure 9. (a) Temperature-dependent upconversion spectra of (Ca0.89Ho0.01Yb0.10)TiO3 from 303 to 503 K at λex = 980 nm, (b) fitted FIR (I547/ I759) with temperature (inset four level system diagram), (c) absolute and relative sensitivity graph for (I547/I759), and (d) FIR (I655/I547) as a function of temperature (303−503 K).

where Iuc defines the upconversion emission intensity, P represents the pump power of laser, and n is the number of pump photon. It is represented by a bilogarithmic plot between laser input power and intensity (integrated area under the curve) as represented in Figure 8b−d. The slopes n have the values 1.91 ± 0.08, 2.02 ± 0.07, and 1.95 ± 0.09 for 547, 655, and 759 nm, respectively, as obtained from the linear fitting of the data. The slope value is ∼2, which indicates the involvement of a two-photon process which populates the 5 S2/5F4 and 5F5 states. Among various applications of Ho3+-doped materials, optical sensing has been found to be very attractive. Herein, temperature-dependent PL spectra have been recorded for (Ca0.89Ho0.01Yb0.10)TiO3 in the temperature range 303−503 K with 980 nm excitation line. Figure 9a represents the UC spectra, indicating the decrease in the emission intensity with the rise in temperature. It may occur because of the enhanced nonradiative relaxation channel rate. The dominant intensity is found to be 547 nm, and no significant shift has been observed in the emission peaks. However, the intensity of 547 nm gets reduced with increasing temperature, which may be ascribed to the thermal quenching effect. Temperature sensitivity of Ln3+ ions has been examined by FIR of two thermally coupled levels. For Ho3+ ions, the separation between the energy levels 5 F4 and 5S2 was ∼120 cm−1.17,41 As in Ho3+ ions transition occurs from 5F4/5S2 to 5I7 and 5I8 states, it behaves as a fourlevel system. Therefore, for FIR estimation, thermally coupled levels at 547 and 759 nm have been considered. FIR of this emission should follow a Boltzmann-type distribution function and can be fitted with the equation given by Haro-González et al.17,41 FIR =

Here, N4 and N3 are populations of the excited state 4 and 3, and ω41/ω42 and ω31/ω32 define the spontaneous emission rate from 5F4/5S2 levels to the 5I8 and 5I7, respectively. Moreover, g3 and g4 are degeneracy of 5S2 and 5F4 levels and hv is the energy of the respective transitions. ΔE depicts the energy gap between 5F4 and 5S2, T is the absolute temperature, and k is the Boltzmann constant. Population of level 4 can be expressed as i −ΔE yz zz N4 = N3 expjjj k kT {

(3)

After substituting this value in eq 2, FIR can be written in the form −ΔE ( kT ) FIR = −ΔE C2 + C3 exp( kT )

1 + C1 exp

(4)

Here C1, C2, and C3 are constants and depend on degeneracy, energy, and spontaneous emission rates from 5F4/5S2 to 5I7 and 5 I8. Figure 9b represents the temperature-dependent FIR indicating the enhancement in FIR with temperature. The graph was allowed to be fitted in accordance with eq 4, and the solid line represents the fitted values of FIR. The fitted values of energy gap ΔE, C1, C2, and C3 have been found to be 159.38 cm−1, −1.46, 0.03, and 0.01, respectively. The absolute temperature sensitivity can be determined by using the equation42 i ΔE y S = FIR × jjj 2 zzz k kT {

(5)

Furthermore, relative sensitivity was calculated to measure the performance of the temperature sensor which, can be estimated by the following equation18

N4ω41g4 hv41 + N3ω31g3hv31 I547 I + I1′ = = 1 I759 I2 + I2′ N4ω42g4 hv42 + N3ω32g3hv32

SR =

(2) 7487

1 dFIR ΔE = FIR dT kT 2

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Table 1. Comparative Table for Optical Temperature-Sensing Properties of Ho3+−Yb3+-Doped Materials with Other Recently Reported Materials transitions

temperature range (K)

sensitivity (K−1)

CaMoO4:Ho−Yb−Mg CaWO4:Ho−Yb ZnWO4:Ho−Yb Y2O3:Ho−Yb NaLuF4:Ho−Yb Ba0.77Ca0.23TiO3:Ho−Yb Y2O3:Ho−Tb

K8, F3 → I8 5 G6/5F1 → 5I8 and 5F2,3/3K8 → 5I8 5 F4/5S2 → 5I8, 5I7 3 K8 → 5I8, 5F3 → 5I8 5 F1/5G6, 5F2,3/3K8 → 5I8 (5F4/5S2) → 5I8, 5I7 (5F4/5S2) → 5I8 and (5F4/5S2) → 5I7

303−543 303−923 83−503 300−673 390−780 93−300 10−300

Ho−Yb:SiO2−PbF2 glass ceramic CaTiO3:Ho−Yb

5

6 × 10−3 (353 K) 5 × 10−3 (923 K) 6.4 × 10−3 (83 K) 3.199 × 10−3 (673 K) 1.4 × 10−4 (780 K) 5.3 × 10−3 (93 K) 0.097(536/772) (85 K) 0.065(536/764) (84 K) 0.046(536/758) (90 K) 7.5 × 10−4 (643 K) 0.0229 (303 K)

materials 3

5

5

F2,3/3K8 and 5F1/5G6 → 5I8 F4/5S2 to 5I7 and 5I8

303−643 303−503

5

reference 11 12 13 14 15 16 17

43 present work

hyperthermia temperature in 3 min. Moreover, heat generation depends on frequency and applied magnetic field. Specific absorption rate (SAR) can be calculated through the equation28

Figure 9c represents the variation of temperature sensitivity with temperature and shows that the absolute sensitivity has the highest value of 0.0229 K−1 and relative sensitivity is 0.25% K−1 at 303 K, which further decreases with temperature up to 503 K. Therefore, the present Ho3+- and Yb3+-codoped phosphor has been found to be suitable for room-temperature optical sensors. The optical-sensing performance of the present phosphor has shown better results compared to other recently reported phosphors at room temperature. Chai et al. reported temperature-sensing measurement ability with the ratio of two electronically coupled levels.13 For such measurement in the present work, the ratio of 5F4/5S2 → 5I8 (green) and 5F5 → 5I8 (red) transitions has been used. Figure 9d represents the FIR plot of Ired/Igreen emission peaks and shows its linear dependency on the temperature with a slope 3.3 × 10−3 K−1, obtained through linear fitting. The sensitivity of the present as-synthesized material compared to the other reported material based on Ho3+−Yb3+-codoped materials has been presented in Table 1, which clearly indicates that the sensitivity of CaTiO3:Ho3+−Yb3+ is higher than other reported sensitivity at room temperature. Herein, subsequent experiment has been performed to measure the heating ability of CaTiO3:1%Ho3+,10%Yb3+@ Fe3O4 MNPs. Constant current (400A at frequency ≈ 285 kHz) has been applied, and subsequently, the heat capacity as a function of time has been measured and displayed in Figure 10. Here, 5 g of Fe3O4 has been attached to UC nanoparticles. The current was allowed to flow for 6 min, and it attains

SAR = c

ΔT 1 Δt mmagn

(7)

where c is the sample-specific heat capacity and mmagn represents the amount of magnetite or Fe in the 1 mL system. As specific heat capacity contribution has been found to be very low because of lesser weight of the sample, it can be changed with specific heat capacity of water (4.18 J g−1 k−1) for the sample. ΔT/Δt is the slope of the time-dependent temperature curve and evaluated to be 0.04194. Consequently, the SAR for UC MNPs has been found to be 35 W/g.

3. CONCLUSIONS In summary, the Yb3+-doped CaTiO3:Ho3+ nanophosphor has successfully been prepared by an inexpensive and facile sol−gel route. MNPs have been attached to the UCNPs to study the hyperthermia activity of hybrid nanoparticles. XRD study reveals the single-phase formation of the as-synthesized sample with Yb3+ and Ho3+ codoping. TEM and SEM images indicate the formation of spherical nanoparticles and confirm the presence of Fe3O4 nanoparticles. Moreover, optical properties have been investigated by diffuse reflectance spectrum and UC emission spectra. The UC emission spectra have shown a characteristic transition of Ho3+ ions and give emissions at 547, 655, and 759 nm with Yb3+ codoping. Among distinct emission peaks, dominant intensity has been observed for 547 nm. Moreover, strongest emission has observed for 10%Yb3+codoped UCNPs, an optimum concentration. Power-dependent upconversion spectra recorded for Yb3+-optimized sample illustrate the involvement of multiphotons in the upconversion process. Temperature-sensing performance has been measured for 10%Yb3+-codoped UCNPs by using FIR of thermally and electronically coupled levels of Ho3+ ions. The highest sensitivity has been found to be 0.0229 K−1 at 303 K, which is quite good at room temperature. It confirms that the present nanoparticles have good sensitivity at low temperature and can be used in temperature-sensing devices. Furthermore, upconversion emission properties of CHY@Fe3O4 hybrid MNPs have been investigated and found that intensity is reduced as Fe3O4 serves as a PL quencher. The hybrid nanoparticles attain the hyperthermia temperature quickly in ac magnetic field (285 kHz frequency) and its specific

Figure 10. Hyperthermia measurement plot dealing with temperature vs time for the Ca0.89Ho0.01Yb0.10TiO3@Fe3O4 nanocomposite. 7488

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microscope (TECNAI G220 operated at 200 kV) were used to observe the morphological insight of the sample. The optical properties were evaluated by diffuse reflectance spectrum using a double-beam spectrophotometer (PerkinElmer, USA Model Lamda 950) and upconversion emission spectra by a continuous wave diode laser of 980 nm wavelength were recorded using an SP2300 grating spectrograph (Princeton instruments, USA). The power of the laser was set at 15 W cm−2 and measured by a power meter (model S310C Thorlabs, USA). Temperature-dependent emission spectra were recorded with 980 nm excitation by using a thermocouple placed near the sample surface and the spectrum was recorded by a charge-coupled device spectrometer (Model: ULS2048X64, Avantes, USA). Induction heating of hybrid MNPs was carried out by employing an instrument (Easy Heat 8310, Ambrell, UK). Its measurement was carried out by a 6 cm-long (L) coil and having 4 turns (n), employed at applied current 400 A (i) and 285 kHz radiofrequency for 6 min. For induction heating, 5 mg of hybrid MNPs was dispersed in 1 mL of deionized water and the solution was collected in a 1.5 mL microcentrifuge tube. The magnetic field (335 Oe) was 1.257ni calculated by using the equation H = L . Further, the heat generated in nanoparticles was measured through a glass fiber optics probe (Photon R & D, Canada).

absorbance ratio has been estimated to be 35 W/g, which is fairly good for cancer treatment.

4. EXPERIMENTAL METHOD CaCl2 (HiMedia, 97.7%), Ho2O3 (Alfa Aesar, 99.995%), Yb2O3 (Alfa Aesar, 99.998%), HNO3 (Merck, 69%), citric acid (Merck, 99.5%), titanium (IV) bis (acetylacetonate), diisopropoxide (75% solution in 2-propanol, Merck), ethanol (99.9%), ammonia (25%), FeCl3 (Merck, 99.9%), and FeCl2 (Merck, 99.995%) were procured commercially. All the reagents were of analytical grade and used without any further purification. 4.1. Preparation of (Ca0.99−xHo0.01Ybx)TiO3 (x = 0.05, 0.1, and 0.15). Facile sol−gel method was used for the preparation of CaTiO3:Ho3+,Yb3+ samples while keeping the Ho3+ concentration constant (1 mol %) and varying the Yb3+ concentration (5, 10, and 15 mol %). First, the solutions of CaCl2, Ho2O3, and Yb2O3 were mixed in a beaker with 20 mL of deionized water followed by the addition of few drops of conc. HNO3. The resultant solution was heated at 80 °C under magnetic stirring at a rate of 600 rpm, which in turn transformed it into a transparent RE oxide solution. This was marked as solution A. In an another beaker, 3 mL of titanium(IV) bis (acetylacetonate) diisopropoxide was taken and 10 mL of ethanol and 0.4 M (0.768 g) citric acid were added to it. It was further stirred for 5 min to obtain a complete mixture of all these precursors which is marked as solution B. Furthermore, solutions A and B were mixed together under magnetic stirring. The resulting mixture was heated for 1 h, which led to the removal of excess alcohol. The pH of the resultant solution was adjusted to 9 by using ammonia solution, which leads to the formation of sol. The asformed sol was heated at 80 °C for 12 h and transformed into the gel. In a subsequent step, the gel was heated at 400 °C for 2 h, resulting into a dried black powder which was finally calcined at 900 °C. 4.2. Preparation of Fe3O4. In a typical synthesis, Fe3O4 was obtained by coprecipitation method in which 0.5 M FeCl2 (10 mL) and 0.5 M FeCl3 (in 20 mL) were mixed together in a beaker.30 Sodium hydroxide (20 mL, 0.2 M) was added drop by drop to the previous solution with continuous stirring. This solution was stirred at room temperature for 20 min, resulting in a black precipitate. The precipitate was separated from the solution by centrifuging at 10 000 rpm. The final precipitated product was washed gently with double-distilled water several times and dried in an oven at 80 °C. Finally, the dried Fe3O4 powder was collected for further study. 4.3. Preparation of Hybrid MNPs. Briefly, Fe3O4 and (Ca0.89Ho0.01Yb0.10)TiO3 nanoparticles were taken into 1:5 M ratio and dispersed well in deionized water. Further, 1 g of polyethylene glycol was added into the solution and the mixture was ultrasonicated for 30 min. This resultant hybrid MN of (Ca0.89Ho0.01Yb0.10)TiO3@Fe3O4 was allowed to dry in an oven and used for further characterization and measurements. 4.4. Characterization Techniques. The as-synthesized samples were characterized by a range of analytic techniques. The structural features and crystalline-phase studies of the sample were examined by ADVANCED D8 Bruker X-ray diffractometer in a 2θ range ∼10°−80° with a step size of 0.02° (source Ni-filtered Cu Kα having wavelength 1.5405 Å). Scanning electron microscope (FESEM, Model-Nova Nano FESEM 450, operated at 30 kV) and transmission electron

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +91 9424459805. ORCID

Jai Singh: 0000-0001-8293-8921 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors are thankful to the Sophisticated Instrument Centre (SIC) of the University for providing various characterization facilities. They are also thankful to Dr. Kaushal Kumar for providing temperature-dependent upconversion data. They express their gratitude to Dr. Raghumani Singh Ningthoujam (BARC, Mumbai) for helping in hyperthermia measurements. Further, Neha acknowledges the Maulana Azad National Fellowship (MANF) provided by the University Grants Commission (UGC), Govt. of India. One of the authors, B.P.S., acknowledges the financial support through the DST Inspire Faculty award (IFA17-MS109). J.S. would like to acknowledge UGC-India and DST for providing project under UGC Start-up grant FT30 [HYPHEN] 56/2014 (BSR) 3(A)a and DST Fast track grant no. SR/FTP/PS [HYPHEN] 144/2012.

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REFERENCES

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