Accepted Manuscript Influence of silica surface coating on optical properties of Er3+-Yb3+:YMoO4 upconverting nanoparticles Manisha Mondal, Vineet Kumar Rai, Chandan Srivastava PII: DOI: Reference:
S1385-8947(17)31118-X http://dx.doi.org/10.1016/j.cej.2017.06.166 CEJ 17247
To appear in:
Chemical Engineering Journal
Received Date: Revised Date: Accepted Date:
18 January 2017 13 May 2017 28 June 2017
Please cite this article as: M. Mondal, V. Kumar Rai, C. Srivastava, Influence of silica surface coating on optical properties of Er3+-Yb3+:YMoO4 upconverting nanoparticles, Chemical Engineering Journal (2017), doi: http:// dx.doi.org/10.1016/j.cej.2017.06.166
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Influence of silica surface coating on optical properties of Er3+Yb3+:YMoO4 upconverting nanoparticles Manisha Mondal1, Vineet Kumar Rai1*, Chandan Srivastava2 1
Laser and Spectroscopy Laboratory Department of Applied Physics
Indian Institute of Technology (Indian School of Mines) Dhanbad-826004, Jharkhand, India 2
Department of Materials Engineering, Indian Institute of Science (IISc), Bangalore- 560012, Karnataka, India
* Corresponding Author: Email address:
[email protected];
[email protected] Phone no.:+91-326-223 5404/5282
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Abstract Er3+-Yb3+:YMoO4@SiO2 core@shell upconversion (UC) nanoparticles (NPs) with its particle size ~ 35 nm and shell thickness ~ 3 nm synthesized via Stöber method have explored its thermal stability, particle morphology and optical properties. The optical band gap energy clearly shows the effect of surface coating of silica layer across the surface of core@shell NPs due to increase in the crystallite size. Due to presence of silica layer across the core NPs, the UC emission intensity corresponding to the 2H11/2, 4S3/2 → 4I15/2 transition has been increased about ~ 10 times in the core@shell NPs than the core NPs. Temperature dependent study using thermally coupled 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions in the biological temperature range (300 – 368 K) has been performed. The maximum sensitivity ~ 2.36% K-1 at 368 K, which is ~ 12 times greater than the core NPs of the core@shell UC NPs has been reported. Thermal heat gain by the core@shell NPs within 311-343 K temperature range is suitable for hyperthermia based cancer treatment has been reported. The core@shell NPs show good dispersibility in different biological solvents such as water, methanol and DMSO. Moreover, the core@shell NPs dispersed with biological solvents show good luminescence stability without interactions with solvent molecules which could minimize the thermal effect caused by the 980 nm diode laser excitation. As a consequence, Er3+Yb3+:YMoO4@SiO2 core@shell NPs have suitable applications for making optical devices namely upconvertors, LEDs, optical bioprobes, etc.
Keywords: Core@shell nanoparticles, HRTEM, FFT, Temperature sensing, Heat gain.
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1. Introduction Frequency upconversion (UC), a nonlinear optical process followed by the successive absorption of multiple photons is capable of emitting high energy photons, when excited with a light source of low energy photons [1]. The rare earth doped UC luminescent nanoparticles (NPs) have attracted much attention now a day’s due to their various applications in different fields such as solid state lighting devices, security purposes, optical amplifiers, drug delivery, biomedical diagnostics, optical bioprobes, fluorescent bioimaging, therapeutic purposes, temperature sensing, photoactivation reactions, photo voltaic devices, sensing and detection, etc. [2-18]. UC luminescent nanomaterials have several advantages over organic dyes and quantum dots because of their higher chemical stability, lower toxicity and higher signal to noise ratio [19]. Organic dyes and quantum dots have not been used widely because of their weak photostability, broad absorption and emission bands, low solubility in water and moderate quantum yield [2, 7]. Due to low solubility in water and unsuitable surface property they donot have functional groups or appropriate sites for attachment of biomolecules [20, 21]. To resolve these drawbacks the development of lanthanide doped UC NPs, which exhibit unique luminescent properties, sharp absorption and emission bands, high quantum yield, long lifetimes and good photostability, are in great demand [7, 22-25]. The emission arising from the lanthanides doped UC NPs can also be tuned by changing the dopants and their doping concentration in the hosts. Several processes have been explored to improve the efficiency of the frequency UC luminescent NPs. Among them the coating of NPs with SiO2 layer may be an effective strategy to improve the luminescence efficiency. Now a days, silica coated core@shell nanostructured materials have great demand in the materials community [7, 26-30]. As a surface modifier silica coating is very important due to its excellent biocompatibility, stability, nontoxicity and it can easily conjugate with various functional groups so that they enable the coupling and lebeling the biotargets [3136]. In our daily lives, temperature has become one of the most important parameter because many properties of material depend on it. Thus, for controlling the quality of the developed materials it has become very important to monitor the temperature during the preparation of materials [37]. In case of contact thermometry methods, so many difficulties like invasiveness and big sizes occur but these problems could be overcome by noncontact thermometry based on fluorescence intensity ratio (FIR) techniques [38, 39]. Luminescence thermometry based on the FIR
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techniques is an effective and reliable approach for accurate measurement of temperature, because it is free of errors arising from pump power fluctuations, inhomogeneities of the material, etc. [40]. The FIR method between two closely spaced energy levels which is based on Boltzmann distribution constitutes self-referencing method to compute the absolute temperature. The rare earth doped UC NPs are the most promising candidate in the luminescence thermometry because they have long emission lifetimes, deep penetration depth and good chemical and physical stability [41-43]. The local environment around the rare earth ions has greatly influenced the UC emission intensity as well as the sensing ability of the rare earths doped UC NPs. Moreover, the rare earths doped phosphor materials are more suitable in the field of temperature sensors over inorganic or ceramic materials due to their resistiveness for oxidation in high temperature environment [44]. Among the various rare earths doped materials, viz, Er3+/Er3+-Yb3+ doped/codoped nanomaterials have potential applications in luminescence nanothermometry [45-50]. Er3+ ions show efficient green UC emission corresponding to the 2
H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions. These transitions are used in the thermometric
applications because it has two thermally coupled levels 2H11/2 and 4S3/2 with very small energy gap of about ~ 800 cm-1. To get better sensing medium the materials must have minimum effect of nonradiative relaxation in the UC emission study upon excitation at 980 nm and also the FIR should vary even in high temperature range at high pump power density. For this, Yb3+ can be used as a codopant sensitizer because it has higher absorption crosssection corresponding to the 2
F7/2 → 2F5/2 absorption transitions at 980 nm. This allows the Yb3+ ions to transfer its excitation
energy efficiently to the Er3+ through energy transfer process [51]. Generally, the luminescent ions in the rare earth doped UC NPs of smaller sizes show poor UC emission because in the core surface they suffer mostly the nonradiative relaxations caused by the surface defects. The UC emission of the rare earth doped UC NPs can be improved by designing core@shell nanoarchitectures in which the shell plays a vital role in passivating the core surfaces [52-54]. Keeping this in mind, we report the successful synthesis of well defined Er3+Yb3+:YMoO4@SiO2 core@shell UC NPs by Stöber method. To the best of our knowledge such a core@shell UC NPs have been prepared for the first time till date. The crystalline nature, morphology and the optical properties of the core and core@shell NPs have been investigated. The solubility, thermal stability and optical thermometric property of the core@shell UC NPs,
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which are promising for its use as luminescent probes in biological applications and temperature sensor, have also been studied. 2. Experimental section 2.1. Chemicals Yttrium oxide (99.9%, Central Drug House, India), ammonium hepta molybdate (E-Merck, India), erbium oxide (GR 99.99%, Otto Chemica-Biochemica reagents), ytterbium oxide (99.9%, Central Drug House, India), aqueous ammonia (E-Merck, India), tetraethyl orthosilicate (TEOS, 99.0%, Sigma Aldrich, USA) and ethanol (99.9%, Analytical regents A.R.) were used as received without purification. The water used was purified through a Millipore system. 2.2. Synthesis 2.2.1. Synthesis of Er3+-Yb3+:YMoO4 core NPs Er3+/ Er3+-Yb3+ doped/ codoped YMoO4 NPs have been prepared by chemical coprecipitation method. Y2O3, (NH4)6Mo7O24, Er2O3 and Yb2O3 were taken as starting materials. The host matrix YMoO4 has been prepared by the following balanced reaction, Y2O3+6HNO3→2Y(NO3)3+3H2O ……. (1) 7 Y(NO3)3 + (NH4)6Mo7O24.4H2O → 7 YMoO4 + 2 NH3 + 25 NO2 + 13 H2O ……. (2) The dopants were taken as follows, (100-x)YMoO4 + xEr2O3 …… (3) where x= 0.0, 0.05, 0.1, 0.2 0.3, 0.4 and 0.5 mol%. After optimizing the Er3+ ions concentration at 0.3 mol%, the codoped (Er3+-Yb3+:YMoO4) nanoparticles were prepared by the following reaction, (100-x-y)YMoO4+xEr2O3+yYb2O3 …… (4) where x= 0.3 mol%, y=0.5, 1.0, 3.0, 5.0, 7.0 mol%. At first the oxide materials were mixed with nitric acid (HNO3) to form their nitrates. After formation of Y(NO3)3, it was dissolved in ammonium hepta molybdate to form yttrium molybdate (YMoO4). After that the dopants Er3+/ Yb3+ were mixed with the host material. The solution was then heated at 65ºC. The ammonium solution was then added to the transparent
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solution to get the precipitate and kept it for 20 h at room temperature. The precipitate was then filtered and washed with distilled water and ethanol for 2 times. The obtained precursor was then kept in the furnace for 15 min at 550ºC to get the powder form. Further, the NPs were kept for two hours at 800ºC for annealing to get better crystalline NPs. 2.2.2. Synthesis of Er3+-Yb3+:YMoO4@SiO2 core@shell NPs Er3+/Yb3+:YMoO4@SiO2 core@shell NPs were prepared by Stöber method [55-57]. The synthesized core NPs (50.0 mg) were well dispersed in a mixed solution of distilled water (50.0 ml), ethanol (70.0 ml) and concentrated aqueous ammonia (1.0 ml, pH=8.0) in a three neck round bottom flask. After that 1.0 ml TEOS was added within 2.0 minutes, and the reaction was allowed to proceed for 6.0 hours under continuous stirring at room temperature. After 6.0 hours of continuous stirring at room temperature, the silica coated core@shell NPs were separated by centrifugation and then washed with ethanol for 3 times and dried at room temperature. 2.3. Material Characterization For X-ray diffraction (XRD) measurements, the prepared samples were kept in a smooth surface sample holder at an angle of 45 degree. A Bruker D8 advanced X-ray diffractometer with CuKα radiation (λ= 1.54Å) has been used for the XRD study of the prepared NPs. For thermogravimetric analysis (TGA), 10 mg of the prepared samples were taken at stable temperature environment. Argon gas was provided from horizontal at flow rate of 60 ml/min and for vertical the flow rate was 20 ml/min. The instrument NETZSCH STA 409 (with PROTEUS software) has been used for TGA/DTA measurements at heating rate of 30K/min from ambient to 800ºC temperature. For transmission electron microscopy (TEM) imaging, high resolution TEM (HRTEM) imaging and energy dispersive X-Ray spectroscopy (EDS) analysis, 20 mg of the samples were dissolved into 2 ml of acetone with the help of an ultrasonic bath for 2 minutes. After dissolving completely a small amount of obtained solution was dropped onto the Cu grid and then dried in vacuum. A FEI Tecnai F30 transmission electron microscope (TEM) operating at 300 keV was used for obtaining the TEM, HRTEM and EDS analysis. For FTIR study, 5 mg of the prepared samples were mixed well into 200 mg of potassium bromide (KBr) powder and grinded into fine powder. The pressure of ~ 5.0 tons was applied for 5 minutes for making the pellets (~10 mm diameter). Fourier transform infrared (FTIR) spectra of the samples have been recorded in the 400–4000 cm-1 spectral region using a Perkin-Elmer spectra-2 FTIR using KBr pellet technique. The optical absorption spectra have been performed on UV-Vis-NIR
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spectrophotometer (Model no- Agilent Cary 5000). The frequency UC emission and thermal stability study has been performed with a Princeton triple turret grating monochromator connected with a photomultiplier tube (PMT) upon excitation with continuous wave (CW) laser diode lasing at 980 nm. The temperature dependent UC emission study and laser induced heating effect has been performed by placing the samples in a homemade oven and temperature was measured with the help of a thermocouple located close (~ 2.0 mm) to the laser focus spot on the sample. The UC luminescence study in different solvents {water, methanol and Dimethyl sulfoxide (DMSO)} at different depths (i.e., 2.0, 4.0 and 6.0 cm of the solvents) have been performed by dispersing the 20 mg of prepared NPs in a tube of radius 7.0 mm into the said solvents. All the measurements were performed at room temperature. 3. Results and Discussion 3.1. Thermal Degradation To examine the thermal properties of the developed core and core@shell NPs, thermogravimetric analysis (TGA) and differential thermal analysis (DTA) have been utilized under argon gas with heating rate at 30K/min was carried out from ambient to 800ºC. In the thermogram {Fig. 1}, the first stage weight loss of ~ 5% associated with exothermic peak at 161 ºC was observed. This is due to the evaporation of physically absorbed or bonding water molecules present on the surface of the core@shell NPs. The second stage of weight loss starts at 166 ºC with continuous decomposition ending at 800 ºC with ~ 10% weight loss. From the thermo-gravimetric analysis of the core and core@shell NPs, almost similar behavior has been observed and the second stage weight loss for both the core and core@shell NPs are assigned to the continuous deposition of the crystallization of scheelite tetragonal phase of YMoO4, which is further confirmed by XRD and TEM results. For the second stage of weight loss in the core@shell NPs, a small difference in the TGA curve occurs compared to the core NPs due to the condensation of surface silanols groups.
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Fig. 1. Thermo-gravimetric analysis of (a) core (b) core@shell NPs.
3.2. Crystal structure and morphology of the core and core@shell NPs The phase composition and the crystalline nature of the prepared core and core@shell NPs have been identified by employing the XRD pattern. The XRD patterns of Er3+-Yb3+:YMoO4 core and Er3+-Yb3+:YMoO4@SiO2 core@shell NPs are matching well with JCPDS file no. 35-1470 {Fig. 2 (a)}. The diffraction peaks of the prepared NPs are found at 29.46º, 32.91º, 34.45º, 48.40º, 49.55º, 56.23º, 58.26º, 60.83º, 78.44º, 82.43º and 88.87º corresponding to (112), (004), (200), (204), (220), (116), (312), (224), (316), (404) and (228) planes of YMoO4 respectively. No second phase has been detected with codoping with Er3+/ Yb3+ ions, indicating that Er3+/ Yb3+ are completely dissolved in the YMoO4 host matrix by substituting Y3+ ions. This may be due to the similar ionic radii of Er3+ (0.89 Å for coordination number VI), Yb3+ (0.86 Å for coordination number VI) and Y3+ (0.90 Å for coordination number VI). D. Errandonea et al. have reported that yttrium molybdate (YMoO4) has scheelite type structure to the tetragonal space group I41/a [58]. In YMoO4 host lattice, the molybdenum atoms are tetrahedrally coordinated to the oxygen atoms in highly distorted lattice. In this case, the interactions between Mo5+ ions with tetrahedral coordination have taken into account. In YMoO4 with schellite type structure, Mo5+ ions in the [MoO4]-3 groups are coordinated tetrahedrally with O-2 ion and Y3+ ions are coordinated dodecahedrally with O-2 ions. The overall structure of YMoO4 is composed of alternate edge sharing of YO8 and MoO4 tetrahedra. Consequently, the substitution of Y3+ ions with Er3+ and Yb3+ ions result UC emissions. The diffraction peaks positions of the core@shell NPs matches
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well with that of the core NPs. This implies that the structure of the Er3+-Yb3+:YMoO4 NPs is not influenced by SiO2 shell which is amorphous in nature [20, 59]. The relative change in the diffraction intensity occurs due to the amorphous nature of the SiO2 layer. To investigate the direct formation of the silica layer across the UC NPs TEM techniques have been carried out. The average particle size of the core NPs and the thickness of the coating layer (i.e., shell) is found to be ~ 35 nm {Fig. 2 (b) and (c)} and ~ 3 nm respectively {Fig. 2 (d)}. From Fig. 2 (b), (c) and (d) it is clear that the NPs are irregular in shapes with nanosize distribution. These irregular shapes of the NPs resulted from the coprecipitation method. From the HRTEM image shown in Fig. 2 (e), the darker interior and the surrounding lighter interior corresponds to the Er3+-Yb3+:YMoO4 core and their amorphous silica coating. Typical d-spacing is observed to be ~ 2.7 Å, which corresponds to the (004) plane. The HRTEM image shows the (204) and (220) planes of YMoO4 lattice consistent with the XRD result. The Fast Fourier Transform (FFT) pattern shows the single crystalline nature of the developed NPs with highly crystalline nature of the prepared core@shell NPs.
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Fig. 2 (a) X-ray Diffraction patterns of core and core@shell NPs, (b), (c) TEM images of core NPs at 0.2 µm and 100 nm resolution (inset shows the statistical distribution of particle size), (d) TEM image of core@shell NPs at 100 nm resolutions (inset shows the statistical distribution of shell thickness) (d) HRTEM image of core@shell NPs (inset shows the FFT patterns). The chemical compositions of the core NPs have been confirmed by EDS analysis which reveals the presence of all the elements (Y, Mo, O, Er and Yb) in the NPs respectively. Fig. 3 (a) shows the Scanning Tunneling Electron Microscopy-High Angle Annular Diffraction Fringes (STEMHAADF) of the NPs from which the compositional line profile data was obtained {shown in Fig. 3 (b)}. In the YMoO4 NPs, the atomic concentration of the Y and Mo is similar. But from the compositional line profile of the Er3+-Yb3+:YMoO4 core NPs it has been seen that the amount of Mo is greater than Y. As the dopants Er and Yb have ionic radii similar to Y therefore the rare earth dopants occupy the Y sites. Due to this the amount of Y is less than Mo. The representative EDS compositional profile of all the present elements is shown in Fig. 3 (c).
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Fig. 3 (a) STEM-HAADF image (b) Compositional line profile across line XY (c) Representative compositional profile of the elements across the core NPs. 3.3. Fourier Transform Infrared Spectroscopy The surface chemistry of the Er3+-Yb3+:YMoO4 core and silica modified core@shell NPs have been examined by the Fourier Transform Infrared Spectroscopy (FTIR) technique {Fig. 4}. The absorption bands around ~ 851 cm-1 and ~ 529 cm-1 arise due to the Mo-O stretching vibrations and asymmetric bonding vibrations involved in the O-Mo-O bonds [60, 61]. The existence of the extra peaks at ~ 1615 cm-1 and ~ 3371 cm-1 are assigned to the O-H bending and stretching vibration of water molecules that may be present due to the absorption of moisture present in KBr during the pellet formation [62]. The Er3+-Yb3+:YMoO4@SiO2 core@shell NPs exhibit peaks at ~ 464 cm-1, ~ 524 cm-1, ~ 788 cm-1, ~ 863 cm-1, ~ 1060 cm-1, ~ 1615 cm-1 and ~ 3371 cm-1. The peaks arise at ~ 468 cm-1, ~ 788 cm-1 and ~ 1060 cm-1 occur due to the Si-O out of plane deformation, Si-O bending and Si-O-Si stretching bonds respectively. The developed silica encapsulated NPs play important role in the biocompatibility in biological systems and also high colloidal stability under different conditions. Further, the characteristic bands near ~ 529 cm-1
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and ~ 851 cm-1, which arises due to MoO4 group has a shift in the core@shell NPs, occurs at ~ 524 cm-1 and ~ 873 cm-1. This shifting is due to change of surface ligands and the variation of coordination modes of the core NPs [37]. The XRD, TEM, HRTEM, EDS and FTIR spectral analysis results confirm that the silica has been successfully coated on the surface of the Er3+Yb3+:YMoO4 core NPs.
Fig. 4. FTIR spectra of core and core@shell NPs. 3.4. Optical absorption spectra To use the core@shell NPs for applications in biomedical fields it is essential to disperse the NPs in biological compatible solvents [63-66]. Silica surface modification increases the solubility as well as dispersibility of the NPs which plays an important role for using them in biomedical applications [67, 68]. The UV-Vis absorption spectra of the core and core@shell samples are recorded in both the protic (such as water, methanol) and aprotic {such as dimethayl sulfoxide (DMSO)} polar solvents within 200-750 nm spectral region {Fig. 5 (a)}. The absorption spectra of the core and core@shell NPs show similar features but a little difference occurs in the band edge at longer wavelength. The occurrence of such type of shift is correlated with a change in the
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coordination geometry and symmetry of the material due to the formation of thinner amorphous silica layer across the surface of the Er3+-Yb3+:YMoO4 core NPs. The Si-OH donor group interacts with the f-electrons of the lanthanide ions [59]. Differences in the intensity of the absorption spectra and deviation observed at longer wavelength for the core@shell NPs dispersed in three kinds of solvents (water, methanol, DMSO) occur due to the different scattering behavior of the NPs in different solvents. The sharp spectrum in the UV region with high absorption band indicates the formation of a stable colloidal suspension. The absorption band of the core@shell NPs in methanol is higher than water and DMSO. Furthermore, the absorption band at ~ 206 nm is due to an oxygen 2p electron which moves into one of the empty molybdenum 4d orbitals [69]. The silica surface modified core@shell NPs show excellent solubility as well as colloidal stability in both the protic and aprotic polar solvents. Because the coating of silica layer establishes hydrogen bonds with protic polar solvents and van der Walls’ weak interactions with aprotic polar solvents [37]. For preparing the colloidal solvents, 15.0 mg of core@shell NPs is dispersed in 15.0 ml of water, methanol and DMSO respectively and kept in the ultrasonic bath for few minutes. The absorption band of the core@shell NPs in these three kinds of solvents shows similar trend {Fig. 5 (a)}. It is observed that the core@shell NPs show good stability in these three solvents but after one week it was reduced in DMSO. Due to high colloidal stability and solubility, silica surface modified core@shell NPs can be used in the biorelated applications such as drug delivery, fluorescent imaging, etc. and also because of its small size (~ 38 nm) it can be escaped from phagocytes in the reticuloendotheliel system. Many of the researchers have used the inorganic inert shell materials but they are hydrophobic in aqueous environment so that the solubility in aqueous solvents is very poor. Therefore, these inorganic inert core@shell materials cannot be used directly in biological applications [57]. So, among various surface coating processes the important advantage of the silica layer is that the presence of silinol groups can conjugate with various biomolecules for further bioapplications of the core@shell NPs. The occurrence of the shift at longer wavelength in the absorption band edge can be further explained by calculating the band gap energies of the core and core@shell NPs {Fig. 5 (b)}. The optical band gap energies have been calculated by Tauc and Menths method by the following relation,
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ℎ = (ℎ − ) ……… (i) where ‘α’ is optical absorption coefficient, ‘h’ is Planck’s constant, ‘ν’ is the frequency, ‘Eg’ is the optical band gap and ‘n’ is a constant that characterizes the nature of the band transition. According to Tauc and Menths law the optical energy band gap Eg is determined from the sharply increasing absorption region. The band gap of the core and core@shell NPs is found to be ~ 3.15 eV and ~ 3.68 eV respectively {Fig. 5 (b)}. The increment in the energy band gap occurs on increasing the grain size after shelling with silica layer [69].
Fig. 5 UV-Vis absorption spectra of (a) core and core@shell NPs suspended in DMSO, methanol and water (b) A plot of (αhν)2 vs. photon energy of the core and core@shell NPs. (c) Upconversion emission spectra under 980 nm excitation at 40.14 W/cm2 pump power density of core and core@shell NPs. (d) Schematic representation of energy level diagram of Er3+-Yb3+ system with the possible transitions.
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3.5. Spectroscopic Characterization The frequency UC emission spectra of the core and surface modified core@shell NPs under excitation of 980 nm laser diode at 40.14 W/cm2 pump power density in the 400-900 nm wavelength range at room temperature is shown in Fig. 5 (c). The UC emission spectral profiles are similar but they differ in emission intensity. The identical transition positions confirm that local crystalline environments are identical before and after growth of silica shell. The UC emission spectra of these samples consist of two main green emission bands centered at ~ 531 nm and ~ 552 nm which corresponds to the 2H11/2 → 4I15/2 and 4S3/2 → 4I15/2 transitions of the Er3+ ion, respectively. The emission intensity of the green band for the core@shell NPs is enhanced about ~ 10 times than that of the core NPs. Additionally, two low intense emission bands are observed in the blue and red regions of the UC emission spectrum. The red emission band corresponds to the 4F9/2 → 4I15/2 transition of the Er3+ ions whereas the emission band at ~ 476 nm corresponding to the 2F5/2 → 2F7/2 transition of the Yb3+ ions is due to the cooperative emission from a pair of Yb3+ ions [44]. The UC luminescence intensity in the core NPs has been increased after shelling the core with silica layer due to reduction in the surface defects on the core@shell NPs because of the presence of SiO2 layer [37]. The increase in the UC luminescence intensity for the Er3+-Yb3+:YMoO4@SiO2 core@shell NPs is accredited to the presence of the silica shell which protects the dopant ions present in the core, especially to those which are near the surface, from the nonradiative deexcitation [6]. The quantum efficiency in the core@shell NPs was improved due to the reduction of quenching of UC emission intensity [57]. Based on the pump power dependence versus UC emission intensity study the involvement of two NIR photons for the green band has been calculated. Under 980 nm NIR diode laser excitation, Yb3+ ions are sensitized from ground state (2F7/2) to the excited state (2F5/2) via ground state absorption process. Afterwards, the 4F7/2 state of the Er3+ ions is populated through energy transfer process from Yb3+ ions to Er3+ ions. Consequently, nonradiative relaxation from the 4F7/2 level to the 2H11/2 and 4S3/2 levels results intense green emission. These two emission bands (2H11/2 → 4I15/2 and 4S3/2 → 4I15/2) exhibit a dependence on temperature owing to the narrow energy gap between the 2H11/2 and 4S3/2 states of the Er3+ ions. The processes responsible for the UC emissions in the codoped NPs are shown in Fig. 5 (d) [44, 51].
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3.6. Luminescence thermometry and laser induced heating The temperature sensing capabilities have been measured for the core and core@shell NPs in the biological range (300-368 K) at a fixed excitation pump power density (18.51 W/cm2) under 980 nm diode laser excitation. Fig. 6 (a) represents the UC emission intensity at room temperature and 368 K for the core@shell NPs in their powder form. It is seen that the energy difference between the 2H11/2 and 4S3/2 level of the Er3+ ion is ~ 800 cm-1 and follows the Boltzmann’s distribution. With increasing the external temperature of the NPs the UC emission bands (~ 531 nm and ~ 552 nm) positions don’t change but the intensity of the bands changes which causes a change in the fluorescence intensity ratio (FIR) of the two closely spaced transitions 2H11/2 → 4
I15/2 and 4S3/2 → 4I15/2. At room temperature (300 K) the intensity of both the bands (~ 531 nm
and ~ 552 nm) is nearly equal whereas at 368 K the intensity of the 2H11/2 → 4I15/2 transition is more than that of the 4S3/2 → 4I15/2 transition. However, the overall integrated intensity has been decreased with increasing the temperature due to increase in the nonradiative relaxation rate. The thermal characterization of the 2H11/2 and 4S3/2 level has been performed by noncontact thermometry based on the FIR technique followed by the equation [70-72],
= = = exp = !exp …….. (1)
where I531 and I552 is the integrated intensity corresponding to the 2H11/2→ 4I15/2 and 4S3/2→ 4I15/2 transitions respectively, w1 and w2 is the radiative transition probabilities of the two transitions of Er3+ ions, g1 and g2 are the degeneracy of the 2H11/2→ 4I15/2 and 4S3/2→ 4I15/2 transitions
respectively {g1= g2= (2J+1)}. "E is the energy band gap between the two energy states considered, kB is the Boltzmann’s constant and ‘T’ is the absolute temperature. Equation (1) can also be expressed as, ln( ) = ln(!) + ……. (2)
where the contant ! = .
A plot of ln(FIR) versus 1/T for the core@shell NPs is shown in the Fig. 6 (b). The slope ( )
and the intercept ln(B) can be determined via the linear fitting of the experimental data and the
best fit for that linear observation is ln(FIR) = (4.34±0.24)-(1274.46±80.89)/T. From this value
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"E comes out to be ~ 884 cm-1. This deviation of ∆E value from the actual value (~800 cm-1)
occurs because of the fluctuations in the laser power and the absorption caused by the host NPs during the recording of the spectra [50]. As the UC emission of Er3+ ions is generated from 4f-4f transitions and the 4f orbitals of Er3+ ions are shielded from the surroundings of the 5s2 and 5p6 filled shells [40, 71, 72]. The observed result clearly shows that the temperature can be easily monitored by the ratio of UC emission intensity of the emission bands at ~ 531 nm and ~ 552 nm in the temperature range related to most biological systems.
Fig. 6 (a) Temperature dependent UC emission spectra of green emission bands of the core@shell NPs (in powder form) (b) The monolog plot of the FIR (I531/I552) as a function of inverse absolute temperature (c) Variation of sensitivity as a function of temperature (d) Temperature variation as a function of pump power density upon 980 nm excitation of the core@shell NPs in powder form.
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Moreover, the sensitivity can be understood as the rate of alteration in the FIR with response to the change of temperature. The sensitivity can be defined as, &=
'(()) '
= ( )
……… (3)
The maximum sensitivity of the Er3+-Yb3+:YMoO4@SiO2 core@shell NPs has been found to be ~ 23.58x10-3 K-1 at 368 K with thermal resolution ~ 0.5 K {Fig. 6 (c)} whereas the sensitivity of the core NPs is found to be ~ 1.98x10-3 K-1. Savchuk et al. have observed the thermal sensitivity enhancement of about ~ 2 times in core@shell NPs with respect to that of the core NPs. This was the maximum enhancement from core NPs to the core@shell NPs in the biological range [37]. In the present case, the enhancement about ~ 12 times from core NPs to core@shell NPs is observed. This is the maximum enhancement reported from core NPs to core@shell NPs in the biological range till date [6, 37, 42, 73-76]. In comparison with other Er3+ doped materials the thermal sensitivity of the Er3+-Yb3+:YMoO4@SiO2 core@shell NPs lies in the upper thermal sensitivity range. Table 1. Comparison of Er doped materials used for luminescence nanothermometry using noncontact FIR technique in the green region of the electromagnetic spectrum. Material
Temperature
Excitation
Wavelength
Relative
Thermal
range
wavelength
ratio
sensitivity
Resolution
-1
(%K ) Er3+-
298–323 K
920 nm
I538/I522
2.3
301–368 K
980 nm
I531/I552
2.4
References
(K) 73
Yb3+:CaF2 Er3+-
0.5
Yb3+:YMoO4
Present work
@SiO2 Er3+-
303–523 K
980 nm
I531/I552
2.2
300–1050 K
980 nm
I510/I565
1.51
74
Yb3+:BaMoO4 Er3+3+
Yb :Gd2O3/
1
75
19
Au NPs Yb/Er:
303–333 K
976 nm
I528/I540
1.20
6
303–593 K
980 nm
I523/I551
1.03
72
299–336 K
920 nm
I525/I545
1.0
42
297–343 K
980 nm
I520/I550
0.94
322–466 K
980 nm
I547/I526
0.52
NaGdF4@ NaYF4 Er3+-Eu3+3+
Yb :Y2O3 Er3+Yb3+:NaYF4 Er3+-
0.4
37
3+
Yb :GdVO4 @SiO2 Er3+:BaTiO3
76
Furthermore, we have measured the internal temperature gain by the core@shell NPs by varying the pump power density within 18.51 W/cm2-34.88 W/cm2 range upon 980 nm laser diode excitation {Fig. 6 (d)}. The laser beam was focused on the sample surface within 1.54 mm2 area. UC emission intensity ratio corresponding to the green emission bands varies with temperature as well as with excitation pump power density. The optical heating generation with laser excitation pump power density can be calculated by *=
+
,
2 ….. (4)
0 - (.)- / 1 0
where all the terms have their usual meanings. Using equation (4), the temperature gain at different pump power densities has been calculated. The calculated temperature was found to vary from 311 K to 343 K {Fig. 6 (d)}. The pump power density dependence temperature gain is due to nonradiative relaxation and the crystalline nature of the developed NPs [6]. The internal temperature gain is in the required range for hyperthermia based cancer treatment and the heat generated by the developed core@shell NPs find suitable applications in cancer therapy.
20
3.7. UC luminescence intensity of core@shell NPs in different dispersible solvents To realize the possible applications of the core@shell NPs in biological fields the UC emission intensity of the core and core@shell NPs in different kinds of biocompatible solvents (such as water, methanol and DMSO) under 980 nm CW diode laser excitation have been analyzed. The time evaluation sedimentation of the core@shell NPs in these solvents is shown in Fig. 7 (a). From the figure it is clearly visible that the colloidal stability of the core@shell NPs in DMSO is reduced after 72 hours than the other solvents (water and methanol) which further confirm the reduction in the UC luminescence intensity {Fig. 7 (b)}. The reduction observed in the UC emission intensity for the core@shell NPs dispersed in the three solvents (water, methanol and DMSO) is due to difference in the colloidal stability and hence the difference in the concentration of the NPs in the three solvents {Fig. 7 (b)}. The UC emission intensity (for the green band) of the core@shell NPs dispersed in the water, DMSO and methanol at 6.0 cm depth has been absorbed by ~ 53%, ~ 54% and ~ 14% respectively whereas for the core NPs at the same depth the UC luminescence intensity has been absorbed by ~ 63%, ~79% and ~ 53% respectively compared to that of the powder NPs. Thus the core@shell NPs are more suitable than the core NPs to minimize the overheating problem. The UC luminescence intensity for core and core@shell NPs in its powder form is higher. This is due to the higher sample concentration in powder form compared to those dispersed in three different solvents. The reduction in the UC luminescence intensity for suspensions takes place due to the efficient nonradiative relaxation process that arises from the interactions established with the solvation molecules. Moreover, the difference in the UC luminescence intensity in different suspensions may be due to the difference in the dipole moment of the solvents [37].
21
Fig. 7 (a) Time evaluation sedimentation of the core@shell NPs dispersed in water, methanol and DMSO (b) Variation of green emission intensity of core and core@shell NPs dispersed in different solvents at 6.0 cm of penetration depth (c) Variation of intensity ratio of green bands (I531/I552) of the core@shell NPs dispersed in different solvents (d) Variation of UC emission intensity of green band with temperature of core and core@shell NPs (in powder form). Moreover, the temperature response of core NPs in different solvents that has affected the FIR value (I531/I552) corresponding to the 2H11/2→ 4I15/2 and 4S3/2→ 4I15/2 transitions under 980 nm diode laser excitation {Fig. 7 (c)} has been analyzed. From the UC emission spectra it can be seen that the FIR is higher when the NPs are in powder form but when it was dispersed in different kinds of solvents, the FIR value is decreased. This indicates that the growth of silica layer on the core NPs greatly reduces the thermal effect which is more suitable for biological
22
applications. The smaller FIR value obtained for the core@shell NPs in water and DMSO is due to the higher dipolar moment of the solvents [37]. 3.8. Thermal Stability Thermal stability is one of the main characteristics of the developed phosphors for home and industrial appliances. Generally, when the phosphors are used in the conventional fluorescent lamps, small amount of energy flux density is required but when the phosphors are used in LED packaging higher energy flux density are required [77]. Thus the requirement of novel phosphor materials having lower thermal quenching is in demand. The thermal quenching phenomena occurs due to the lattice vibrations and activation energy (i.e., when the temperature increases the interaction of electron-phonon also increases as a result the release in energy occurs by the formation of lattice vibrations). The variation of UC emission intensity with temperature {shown in Fig. 7 (d)} indicates that the core@shell NPs (in powder form) have less thermal quenching than the core NPs (in powder form), thereby indicating its good stability for LED packaging. Further, a reduction of ~ 43 % of initial intensity at 150º C for the Er3+-Yb3+:YMoO4@SiO2 core@shell NPs is lower than that of the commercial YAG:Ce3+ (~ 45%) [78]. While the intensity of the core NPs have been reduced to ~ 65% at 150º C than the initial intensity. Thus better thermal stability in the core@shell NPs causes reduction in the surface defects which occurs due to presence of silica layer across the core NPs. The Si-O bridge stabilizes the structure which causes better thermal stability such that the energy of the electrons in the excited state is quite insignificant to discharge the electrons via lattice vibrations i.e., higher activation energy is needed. The activation energy can be estimated by the Arrhenius equation [77, 79], =
3
9 ,45 678 : ;
3 − 1 vs ,
