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Size dependence of the upconverted luminescence of NaYF4:Er,Yb microspheres for use in ratiometric thermometry Bin Dong,*a Rui N. Hua,b Bao S. Cao,a Zhi P. Li,*c Yang Y. He,a Zhen Y. Zhanga and Otto S. Wolfbeisd The size-dependent temperature sensitivity is observed on the upconversion luminescence of NaYF4:Er,Yb microspheres with sizes between 0.7 and 2 mm that are prepared by a poly(acrylic acid)-assisted hydrothermal process. It is found that the fluorescence intensity ratio (FIR) of their green upconversion emissions (with peaks at 521 and 539 nm) is strongly size-dependent at temperatures between 223 and 403 K. As the size of the spheres increases from 0.7 to 1.6 mm, the maximum sensitivity decreases from 36.8 104 to 24.7 104 K1.
Received 7th May 2014, Accepted 1st August 2014
This effect is mainly attributed to the larger specific surface area of the smaller spheres where a relatively large
DOI: 10.1039/c4cp01966k
number of Er(III) ions are located at the surface. This results in an increase in the efficiency of the 4S3/2 - 2H11/2
www.rsc.org/pccp
the spheres by NIR light is also supposed to cause enhanced electron–phonon interactions in such particles.
population process of the Er(III) ions due to stronger electron–phonon interactions with increasing T. Heating of
Introduction The precise determination of temperature (T) in a living cell is essential in areas such as pathology and physiology and, in turn, for the optimization of therapeutic processes.1,2 Seemingly simple, it is heavily compromised by the lack of nanosized sensors. Much effort therefore has been made to design nanosized optical probes for sensing T.3–5 Conventional contact thermometers cannot be applied for reasons of size (which limits their applicability to objects with sizes of o10 mm)6 and their invasiveness. Nanosized non-contact thermometers have been described more recently that can measure (and image) T on the nanoscale, for example in the case of intracellular fluctuations of T, or in microcircuits and microfluids.7–9 Among the non-contract thermometers, optical sensing based on the measurement of the fluorescence intensity ratio (FIR) is particularly attractive because it can reduce undesired effects of instrumental parameters, can facilitate calibration, and can improve resolution.10–12 The application of the FIR technique is particularly easy in the case of the up-conversion (UC) emission in nanothermometers13,14 based on lanthanide-doped UC nanoparticles (UCNPs) which a
Institute of Nano-Photonics, School of Physics and Materials Engineering, Dalian Nationalities University, Dalian, 116600, P. R. China. E-mail: [email protected] b School of Life, Dalian Nationalities University, Dalian, 116600, P. R. China c Beijing Key Laboratory of Nano-Photonics and Nano-Structure (NPNS), Department of Physics, Capital Normal University, Beijing, 10004, P. R. China. E-mail: [email protected] d Institute of Analytical Chemistry, Chemo- and Biosensors, University of Regensburg, D-93040 Regensburg, Germany
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therefore represent a new class of fluorescent probes for in vivo imaging.15,16 UCNPs have obvious advantages in the field of in vivoimaging because (a) photoexcitation is quite long wave (980 nm), which increases the depth of tissue penetration by light, (b) the lack of photobleaching, and (c) the lack of any phototoxicity.17 It was suggested18 to use UCNPs as nanoheaters for hyperthermal treatment of cells in combination with simultaneous monitoring of T by exploiting the fact that the green and the red fluorescence emissions (note: we use the more common term fluorescence here in place of the more correct term luminescence) have a different T-dependence. Vetrone et al.2 introduced UCNPs as nanoprobes for fluorescence sensing of T in HeLa cervical cancer cells, and similar results were reported later.14,19 Other fields of applications include sensing of T in synthetic nanometer-sized structures and machines,16 measurement of T in micro- and nanofluidic systems,20 thermally induced drug release,21,22 and ratiometric imaging of T inside cells.1 We report here on NaYF4:Er,Yb microspheres prepared by a poly(acrylic acid)-assisted hydrothermal process, on the effects of T on the emission spectra and on the finding that size exerts a large effect on the ratio of the intensity of the two green emissions of such particles (at 521 and 539 nm), which makes them promising candidates as probes to sense T on the micro/nano-scale but may also contribute to better interpret some of the results obtained in the past with UCNPs of varying sizes.
Experimental section All chemical reagents were of analytical grade and used without further purification. The rare earth oxides including Y2O3 (99.999%),
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Er2O3 (99.999%) and Yb2O3 (99.999%) were purchased from Sinopharm Chemical Reagent Co., Ltd (Beijing, China; www.crc-bj. com). Poly(acrylic acid) (PAA; MW = 1800) was purchased from Aladdin Chemistry Co. Ltd. (Shanghai, China; www.aladdin-e.com), NaOH, NaCl and ethanol were obtained from Tianjin Kermel Chemical Reagent (Tianjin, China; www.chemreagent.com). NH4HF2 was purchased from Sinopharm Chemical Reagent Co., Ltd (Beijing, China; www.crc-bj.com). Y(NO3)36H2O, Yb(NO3)36H2O and Er(NO3)36H2O powders were obtained by dissolving the corresponding rare earth oxide in 6 M nitric acid, evaporating the solution to dryness, and then recrystallizing the residue four times from deionized water.
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Fig. 1 Schematic of the set-up used to measure the UC emissions of single particles depended on temperature.
Fabrication of b-NaYF4:Er,Yb microspheres b-NaYF4 microspheres doped with 20% Yb(III) and 2% Er(III) and functionalized with carboxy groups were synthesized as follows: sodium chloride (1.0 mmol), Y(NO3)36H2O (0.78 mmol), Yb(NO3)36H2O (0.20 mmol) and Er(NO3)36H2O (0.02 mmol) were mixed with 10 ml of distilled water in a 100 mL beaker under magnetic stirring until they were completely dissolved. A mixture of ethanol (30 mL) and 10 mL of a 5 wt% solution of poly(acrylic acid) was slowly dropped into the respective solution of a rare earth nitrate. The solution was stirred for 10 min. Subsequently, 2.0 M aqueous NH4HF2 (10 ml) solution was slowly added to the above solution, and the pH value of the solution was adjusted to 8–9 with 2 M NaOH under magnetic stirring at room temperature for 20 min. Finally, the mixture solution was transferred to a 100 mL stainless Teflon-lined autoclave and heated at 473 K for 8 h. When the autoclave was naturally cooled to roomtemperature, the precipitates were separated by centrifugation, and washed with ethanol/water (v/v = 1 : 1) four times. Finally, the carboxy-functionalized spheres obtained after final centrifugation were dried in the air at 353 K for 8 h. UC emission measurements The UC emission of single particles was measured using an inverted microscope (Olympus IX71) combined with a spectrometer (PI instrument). The excitation with a 980 nm laser passing a laser clean-up was reflected into the objective (50 Olympus) using a dichroic short pass filter. The UC emission was collected using the same objective and conducted into the spectrometer. The laser clean-up and the dichroic filter were used to purify the excitation light and eliminate the laser line before the emission was detected. A drop of UC colloid was dripped on ITO glass and dried by a stream of nitrogen. By marking the samples with an indexed grid, the particles identified in optical images were then investigated by scanning electron microscopy (SEM) imaging. The UC sample was then placed in a thermostat (Linkam TMS94) whose T can be adjusted from 200 K to 400 K with a precision of about 0.1 K (Fig. 1).
Fig. 2 (a, b) SEM pictures at different scales. (c) XRD patterns. (d) Size distribution of NaYF4:Er,Yb spheres.
be well assigned to a hexagonal cell with a space group of Ia3% d. This is in good agreement with the previous data on b-NaYF4 as in the JCPDS no. 16-0334. Fig. 2(d) shows the size distribution of NaYF4:Er,Yb spheres. It can be seen that the particles are virtually spherical, with an average size between 0.6 and 2.1 mm. Fig. 3(a) shows microscopic images of NaYF4:Er,Yb spheres (the inset figure) and green up-conversion emission spectra originating from the transitions of 2H11/2 - 4I15/2 and 4S3/2 4 I15/2 for the single spheres at 223 K, 293 K and 403 K. Fig. 3(b) shows the simplified energy diagram and the green up-conversion mechanism for the Er–Yb system,23 in which sequential sensitization and relaxation processes are responsible for the population of
Results and discussion Fig. 2(a and b) shows the SEM images of NaYF4:Er,Yb spheres. Fig. 2(c) shows the XRD patterns of the spheres. The peaks can
20010 | Phys. Chem. Chem. Phys., 2014, 16, 20009--20012
Fig. 3 (a) Upconversion emission spectra at 223 K, 293 K and 403 K. (b) Sensitization diagrams of spheres of the type NaYF4:Er,Yb.
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Paper Table 1
Diameter (mm)
Temp. (K)
Sensitivitya (slope)
0.7 1.1 1.4 1.6
363 273 262 263
36.8 30.3 29.4 24.7
a
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Summarized performance of NaYF4:Er,Yb spheres
In 104 K1 units.
Fig. 4 (a) Mono-logarithmic plot of the fluorescence intensity ratio (FIR) at 521 and 539 nm as a function of inverse absolute temperature, (b) FIR relative to the temperature range of 223–304 K of NaYF4:Er,Yb spheres of different sizes.
The coefficient C is obtained by fitting the curve of the experimental data. Sensitivity can be defined as eqn (3).11 dR DE ¼R 2 (3) dT kT
2
The resulting plot is shown in Fig. 5. At 363 K, the sensitivity of the sphere with a size of 700 nm reaches the maximal value of about 36.8 104 K1. Table 1 summarizes the sensitivities of NaYF4:Er,Yb spheres of varying sizes. One can see that, as their size increases from 0.7 to 1.6 mm, the maximum sensitivity decreases from 36.8 104 to 24.7 104 K1. One can see that – despite the same thermal conductance and thermal transmission velocity – smaller particles are more sensitive to T than bigger ones. There was no any modification in the sample and the green up-conversion emissions could be measured easily in the range from 50 1C to 130 1C, and good repeatability was obtained with several temperature cycles. The strong effect of T on the fluorescence of these particles probably resulted by surface defects that become increasingly important with decreasing size. The specific surface area of particles increases with decreasing particle size, and thus the contribution of fluorescent ions (mainly Er and Yb ions) located at surface sites becomes increasingly important for the smaller particles. The fluorescence originating from these ions is decreased by nonradiative relaxation channels which are not only related to lattice vibrational modes but also to high energy modes of residual carbonates (B1500 cm1) and hydroxy groups (bands at B3350 cm1) absorbed on and in the crystallites.24 The dependence of the nonradiative relaxation rate on T is determined by effective phonon mode of the host. In the case of small spheres, the population process of 4S3/2 - 2H11/2 increases because the Er(III) ions located at the surface undergo stronger
H11/2 and 4S3/2 levels of Er3+ for UC emissions. Yb3+ is excited to the 2F5/2 level (GSA) with the absorption of a 980 nm photon, the sequential ETs from Yb3+ to Er3+ (ET 1 and ET 2) populate the 4F7/2 level of Er3+, and then the nonradiative relaxation populates the states of 2H11/2 and 4S3/2. The radiative decays of 2 H11/2 - 4I15/2 and 4S3/2 - 4I15/2 result in green emissions. The peak positions of the green emissions at 521 and 539 nm do not change upon increasing T. However, the ratio of their intensities varies as shown in Fig. 4. The 2H11/2 and 4S3/2 states are in close proximity, essentially separated by only an energy gap of about 640 cm1, leading to a thermal equilibrium governed by the Boltzmann law. Considering the thermalization of population and ignoring the self-absorption effect of fluorescence, the intensity ratios of the green up-conversion emissions at 521 and 539 nm, respectively, can be described11 using eqn (1): I521 N 2 H11=2 g s o DE DE ¼ H H H exp ¼ C exp ¼ R kT kT I539 gS sS oS N 4 S3=2 (1) where I521 and I539 are the integrated intensities of the H11/2 4 I15/2 (514–535 nm) and 4S3/2 - 4I15/2 (535–565 nm) transitions, respectively. The terms N, g, s, and o are the number of ions, the degeneracy, the emission cross-section, the angular frequency of fluorescence transitions from the 2H11/2, 4S3/2 levels to the 4I15/2 level, respectively, DE is the energy gap between the 2 H11/2 and 4S3/2 levels, k the Boltzmann constant, T the absolute temperature, and C the pre-exponential constant that is defined as gHsHoH/gSsSoS. Fig. 4(a) shows a mono-logarithmic plot of the FIR for the emission lines centered at 521 and 539 nm versus the inverse absolute T in the range of 223–403 K. The plot reveals a linear dependence of the FIR on the inverse T. The data can be fit with a linear plot (eqn (2)) of the form 2
ln(I521/I539) = (a/T) + b
(2)
Table 1 summarizes the average values for adjustable parameters a and b taken from several sets of measurements. The slope of the linear range (parameter a) is related to sensitivity defined as the rate at which the intensity ratio changes with T. The parameter a decreases with an increase of particle size. The FIR of green up-conversion emissions at 521 and 539 nm relative to T in the range of 223–403 K is shown in Fig. 4(b).
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Fig. 5 Sensitivity (slope of response) as a function of the temperature of single NaYF4:Er,Yb spheres of varying sizes.
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electron–phonon interactions with changing T than the Er(III) ions located in the interior.25 Consequently, the effect of the intensity ratio of IH/IS on T is enhanced. In addition, heating of the UCNPs by NIR light is more pronounced in the case of smaller nano/ microparticles than in the case of the larger ones. This undesired heating effect is supposed to result from the increase in the efficiency of a nonradiative relaxation process, which leads to an enhanced electron–phonon interaction in nano/micro particles.
Conclusion This study on the optical T-sensing properties based on the fluorescence intensity ratio of single Er,Yb-doped NaYF4 spheres revealed an unexpected size-dependence of the sensitivity to T. Specifically, the maximum sensitivity of the ratioed intensities at 521 and 539 nm, respectively, decreases from 36.8 104 to 24.7 104 K1 upon increasing the size of the spheres from 0.7 to 1.6 mm. The more expressed sensitivity of the smaller spheres is attributed to a more effective population process of the 4S3/2 2 H11/2 transition that increases because the Er(III) ions located at the surface undergo stronger electron–phonon interactions than those located in the inner spheres. Also, heating of the smaller spheres by NIR light is more pronounced than in the case of the bigger ones. The increase in the efficiency of the nonradiative relaxation process leads to an enhanced electron–phonon interaction in nano/microparticles. Micro- and nanosensors based on the measurement of the ratioed fluorescence of single rare earth-doped upconversion materials are considered to represent useful and fully reversibly responding (rather than single-shot) probes. They can be photoexcited with low-cost diode lasers and do not require specific conditions in terms of their microenvironment. Numerous T cycles can be performed with good repeatability, and such particles are readily internalized by cells. This indicates that such micro/ nanoprobes are powerful tools for optical sensing of T inside cells.
Acknowledgements This work was supported by the 973 Program (2012CB626801), the National Science Foundation of China (Grant No. 11274057, 21173034, 11004021 and 11204024), Program for New Century Excellent Talents in University (Grant No. NCET-13-0702), Science and Technology Project of Liaoning Province (Grant No. 2012222009), Program for Liaoning Excellent Talents in University (LNET) (Grant No. LJQ2012112), the Fundamental Research Funds for the Central Universities (Grant No. DC12010117), and Science and Technique Foundation of Dalian (Grant No. 2012J21DW016 and 2013A14GX040).
Notes and references 1 L. H. Fischer, G. S. Harms and O. S. Wolfbeis, Angew. Chem., Int. Ed., 2011, 50, 4546.
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2 F. Vetrone, R. Naccache, A. Zamarron, A. J. Fuente, F. SanzRodriguez, L. M. E. Maestro, M. Rodriguez, D. Jaque, J. G. Sole and J. A. Capobianco, ACS Nano, 2010, 4, 3254. 3 H. S. Peng, S. H. Huang and O. S. Wolfbeis, J. Nanopart. Res., 2010, 12, 2729. 4 H. Peng, M. I. J. Sitch, J. Yu, L. N. Sun, L. H. Fischer and O. S. Wolfbeis, Adv. Mater., 2010, 22, 716. 5 X. Wang, O. S. Wolfbeis and R. J. Meier, Chem. Soc. Rev., 2013, 42, 7834. 6 C. D. S. Brites, P. P. Lima, N. J. O. Silva, A. Millan, V. S. Amaral, F. Palacio and L. D. Carlos, New J. Chem., 2011, 35, 1177. 7 G. A. Baker, S. N. Baker and T. M. Mccleskey, Chem. Commun., 2003, 2932. 8 G. Tessier, M. Bardoux, C. Boue and D. Fournier, Appl. Phys. Lett., 2007, 90, 171112. 9 T. Barilero, L. Saux, C. Gosse and L. Jullien, Anal. Chem., 2009, 81, 7988. 10 S. A. Wade, S. F. Collins and G. W. Baxter, J. Appl. Phys., 2003, 94, 4743. 11 B. Dong, D. P. Liu, X. J. Wang, T. Yang, S. M. Miao and C. R. Li, Appl. Phys. Lett., 2007, 90, 181117. 12 V. K. Rai, Appl. Phys. B: Lasers Opt., 2007, 88, 297. 13 B. Dong, B. S. Cao, Y. Y. He, Z. Liu, Z. P. Li and Z. Q. Feng, Adv. Mater., 2012, 24, 1987. 14 A. Sedlmeier, D. E. Achatz, L. H. Fischer, H. H. Gorris and O. S. Wolfbeis, Nanoscale, 2012, 4, 7090. 15 J. Zhou, Z. Liu and F. Y. Li, Chem. Soc. Rev., 2012, 41, 1323. 16 H. H. Gorris and O. S. Wolfbeis, Angew. Chem., Int. Ed., 2013, 52, 3584. 17 G. Y. Chen, J. Shen, T. Y. Ohulchanskyy, N. J. Patel, A. Kutikov, Z. P. Li, J. Song, R. K. Pandey, H. Argen, P. N. Prasad and G. Han, ACS Nano, 2012, 6, 2969. 18 V. K. Tikhomirov, K. Driesen, V. D. Rodriguez, P. Gredin, M. Mortier and V. V. Moshchalkov, Opt. Express, 2009, 17, 11794. 19 N. N. Dong, M. Pedroni, F. Piccinelli, G. Conti, A. Sbarbati, ´ndez, L. M. Maestro, M. C. I. Cruz, J. E. Ramı´rez-Herna F. Sanz-Rodriguez, A. Juarranz, F. Chen, F. Vetrone, J. A. ´, M. Bettinelli, D. Jaque and Capobianco, J. G. Sole A. Speghini, ACS Nano, 2011, 5, 8665. 20 S. W. Hao, G. Y. Chen and C. H. Yang, Theranostics, 2013, 3, 331. 21 Y. Dai, P. Ma, Z. Cheng, X. Kang, X. Zhang, Z. Hou, C. Li, D. Yang, X. Zhai and J. Lin, ACS Nano, 2012, 6, 3327. 22 Z. L. Jiang, B. Dong, B. T. Chen, J. Wang, L. Xu, S. Zhang and H. W. Song, Small, 2013, 9, 604. 23 R. Naccache, F. Vetrone, V. Mahalingam, L. A. Cuccia and J. A. Capobianco, Chem. Mater., 2009, 21, 717. 24 X. Wang, X. G. Kong, Y. Yu, Y. J. Sun and H. Zhang, J. Phys. Chem. C, 2007, 111, 15119. 25 M. A. R. C. Alencar, G. S. Maciel and C. B. De Araujo, Appl. Phys. Lett., 2004, 84, 4753.
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Cite this: Phys. Chem. Chem. Phys., 2014, 16, 20009
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Size dependence of the upconverted luminescence of NaYF4:Er,Yb microspheres for use in ratiometric thermometry Bin Dong,*a Rui N. Hua,b Bao S. Cao,a Zhi P. Li,*c Yang Y. He,a Zhen Y. Zhanga and Otto S. Wolfbeisd The size-dependent temperature sensitivity is observed on the upconversion luminescence of NaYF4:Er,Yb microspheres with sizes between 0.7 and 2 mm that are prepared by a poly(acrylic acid)-assisted hydrothermal process. It is found that the fluorescence intensity ratio (FIR) of their green upconversion emissions (with peaks at 521 and 539 nm) is strongly size-dependent at temperatures between 223 and 403 K. As the size of the spheres increases from 0.7 to 1.6 mm, the maximum sensitivity decreases from 36.8 104 to 24.7 104 K1.
Received 7th May 2014, Accepted 1st August 2014
This effect is mainly attributed to the larger specific surface area of the smaller spheres where a relatively large
DOI: 10.1039/c4cp01966k
number of Er(III) ions are located at the surface. This results in an increase in the efficiency of the 4S3/2 - 2H11/2
www.rsc.org/pccp
the spheres by NIR light is also supposed to cause enhanced electron–phonon interactions in such particles.
population process of the Er(III) ions due to stronger electron–phonon interactions with increasing T. Heating of
Introduction The precise determination of temperature (T) in a living cell is essential in areas such as pathology and physiology and, in turn, for the optimization of therapeutic processes.1,2 Seemingly simple, it is heavily compromised by the lack of nanosized sensors. Much effort therefore has been made to design nanosized optical probes for sensing T.3–5 Conventional contact thermometers cannot be applied for reasons of size (which limits their applicability to objects with sizes of o10 mm)6 and their invasiveness. Nanosized non-contact thermometers have been described more recently that can measure (and image) T on the nanoscale, for example in the case of intracellular fluctuations of T, or in microcircuits and microfluids.7–9 Among the non-contract thermometers, optical sensing based on the measurement of the fluorescence intensity ratio (FIR) is particularly attractive because it can reduce undesired effects of instrumental parameters, can facilitate calibration, and can improve resolution.10–12 The application of the FIR technique is particularly easy in the case of the up-conversion (UC) emission in nanothermometers13,14 based on lanthanide-doped UC nanoparticles (UCNPs) which a
Institute of Nano-Photonics, School of Physics and Materials Engineering, Dalian Nationalities University, Dalian, 116600, P. R. China. E-mail: [email protected] b School of Life, Dalian Nationalities University, Dalian, 116600, P. R. China c Beijing Key Laboratory of Nano-Photonics and Nano-Structure (NPNS), Department of Physics, Capital Normal University, Beijing, 10004, P. R. China. E-mail: [email protected] d Institute of Analytical Chemistry, Chemo- and Biosensors, University of Regensburg, D-93040 Regensburg, Germany
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therefore represent a new class of fluorescent probes for in vivo imaging.15,16 UCNPs have obvious advantages in the field of in vivoimaging because (a) photoexcitation is quite long wave (980 nm), which increases the depth of tissue penetration by light, (b) the lack of photobleaching, and (c) the lack of any phototoxicity.17 It was suggested18 to use UCNPs as nanoheaters for hyperthermal treatment of cells in combination with simultaneous monitoring of T by exploiting the fact that the green and the red fluorescence emissions (note: we use the more common term fluorescence here in place of the more correct term luminescence) have a different T-dependence. Vetrone et al.2 introduced UCNPs as nanoprobes for fluorescence sensing of T in HeLa cervical cancer cells, and similar results were reported later.14,19 Other fields of applications include sensing of T in synthetic nanometer-sized structures and machines,16 measurement of T in micro- and nanofluidic systems,20 thermally induced drug release,21,22 and ratiometric imaging of T inside cells.1 We report here on NaYF4:Er,Yb microspheres prepared by a poly(acrylic acid)-assisted hydrothermal process, on the effects of T on the emission spectra and on the finding that size exerts a large effect on the ratio of the intensity of the two green emissions of such particles (at 521 and 539 nm), which makes them promising candidates as probes to sense T on the micro/nano-scale but may also contribute to better interpret some of the results obtained in the past with UCNPs of varying sizes.
Experimental section All chemical reagents were of analytical grade and used without further purification. The rare earth oxides including Y2O3 (99.999%),
Phys. Chem. Chem. Phys., 2014, 16, 20009--20012 | 20009
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Published on 04 August 2014. Downloaded by Aston University on 29/08/2014 14:07:55.
Paper
Er2O3 (99.999%) and Yb2O3 (99.999%) were purchased from Sinopharm Chemical Reagent Co., Ltd (Beijing, China; www.crc-bj. com). Poly(acrylic acid) (PAA; MW = 1800) was purchased from Aladdin Chemistry Co. Ltd. (Shanghai, China; www.aladdin-e.com), NaOH, NaCl and ethanol were obtained from Tianjin Kermel Chemical Reagent (Tianjin, China; www.chemreagent.com). NH4HF2 was purchased from Sinopharm Chemical Reagent Co., Ltd (Beijing, China; www.crc-bj.com). Y(NO3)36H2O, Yb(NO3)36H2O and Er(NO3)36H2O powders were obtained by dissolving the corresponding rare earth oxide in 6 M nitric acid, evaporating the solution to dryness, and then recrystallizing the residue four times from deionized water.
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Fig. 1 Schematic of the set-up used to measure the UC emissions of single particles depended on temperature.
Fabrication of b-NaYF4:Er,Yb microspheres b-NaYF4 microspheres doped with 20% Yb(III) and 2% Er(III) and functionalized with carboxy groups were synthesized as follows: sodium chloride (1.0 mmol), Y(NO3)36H2O (0.78 mmol), Yb(NO3)36H2O (0.20 mmol) and Er(NO3)36H2O (0.02 mmol) were mixed with 10 ml of distilled water in a 100 mL beaker under magnetic stirring until they were completely dissolved. A mixture of ethanol (30 mL) and 10 mL of a 5 wt% solution of poly(acrylic acid) was slowly dropped into the respective solution of a rare earth nitrate. The solution was stirred for 10 min. Subsequently, 2.0 M aqueous NH4HF2 (10 ml) solution was slowly added to the above solution, and the pH value of the solution was adjusted to 8–9 with 2 M NaOH under magnetic stirring at room temperature for 20 min. Finally, the mixture solution was transferred to a 100 mL stainless Teflon-lined autoclave and heated at 473 K for 8 h. When the autoclave was naturally cooled to roomtemperature, the precipitates were separated by centrifugation, and washed with ethanol/water (v/v = 1 : 1) four times. Finally, the carboxy-functionalized spheres obtained after final centrifugation were dried in the air at 353 K for 8 h. UC emission measurements The UC emission of single particles was measured using an inverted microscope (Olympus IX71) combined with a spectrometer (PI instrument). The excitation with a 980 nm laser passing a laser clean-up was reflected into the objective (50 Olympus) using a dichroic short pass filter. The UC emission was collected using the same objective and conducted into the spectrometer. The laser clean-up and the dichroic filter were used to purify the excitation light and eliminate the laser line before the emission was detected. A drop of UC colloid was dripped on ITO glass and dried by a stream of nitrogen. By marking the samples with an indexed grid, the particles identified in optical images were then investigated by scanning electron microscopy (SEM) imaging. The UC sample was then placed in a thermostat (Linkam TMS94) whose T can be adjusted from 200 K to 400 K with a precision of about 0.1 K (Fig. 1).
Fig. 2 (a, b) SEM pictures at different scales. (c) XRD patterns. (d) Size distribution of NaYF4:Er,Yb spheres.
be well assigned to a hexagonal cell with a space group of Ia3% d. This is in good agreement with the previous data on b-NaYF4 as in the JCPDS no. 16-0334. Fig. 2(d) shows the size distribution of NaYF4:Er,Yb spheres. It can be seen that the particles are virtually spherical, with an average size between 0.6 and 2.1 mm. Fig. 3(a) shows microscopic images of NaYF4:Er,Yb spheres (the inset figure) and green up-conversion emission spectra originating from the transitions of 2H11/2 - 4I15/2 and 4S3/2 4 I15/2 for the single spheres at 223 K, 293 K and 403 K. Fig. 3(b) shows the simplified energy diagram and the green up-conversion mechanism for the Er–Yb system,23 in which sequential sensitization and relaxation processes are responsible for the population of
Results and discussion Fig. 2(a and b) shows the SEM images of NaYF4:Er,Yb spheres. Fig. 2(c) shows the XRD patterns of the spheres. The peaks can
20010 | Phys. Chem. Chem. Phys., 2014, 16, 20009--20012
Fig. 3 (a) Upconversion emission spectra at 223 K, 293 K and 403 K. (b) Sensitization diagrams of spheres of the type NaYF4:Er,Yb.
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Paper Table 1
Diameter (mm)
Temp. (K)
Sensitivitya (slope)
0.7 1.1 1.4 1.6
363 273 262 263
36.8 30.3 29.4 24.7
a
Published on 04 August 2014. Downloaded by Aston University on 29/08/2014 14:07:55.
Summarized performance of NaYF4:Er,Yb spheres
In 104 K1 units.
Fig. 4 (a) Mono-logarithmic plot of the fluorescence intensity ratio (FIR) at 521 and 539 nm as a function of inverse absolute temperature, (b) FIR relative to the temperature range of 223–304 K of NaYF4:Er,Yb spheres of different sizes.
The coefficient C is obtained by fitting the curve of the experimental data. Sensitivity can be defined as eqn (3).11 dR DE ¼R 2 (3) dT kT
2
The resulting plot is shown in Fig. 5. At 363 K, the sensitivity of the sphere with a size of 700 nm reaches the maximal value of about 36.8 104 K1. Table 1 summarizes the sensitivities of NaYF4:Er,Yb spheres of varying sizes. One can see that, as their size increases from 0.7 to 1.6 mm, the maximum sensitivity decreases from 36.8 104 to 24.7 104 K1. One can see that – despite the same thermal conductance and thermal transmission velocity – smaller particles are more sensitive to T than bigger ones. There was no any modification in the sample and the green up-conversion emissions could be measured easily in the range from 50 1C to 130 1C, and good repeatability was obtained with several temperature cycles. The strong effect of T on the fluorescence of these particles probably resulted by surface defects that become increasingly important with decreasing size. The specific surface area of particles increases with decreasing particle size, and thus the contribution of fluorescent ions (mainly Er and Yb ions) located at surface sites becomes increasingly important for the smaller particles. The fluorescence originating from these ions is decreased by nonradiative relaxation channels which are not only related to lattice vibrational modes but also to high energy modes of residual carbonates (B1500 cm1) and hydroxy groups (bands at B3350 cm1) absorbed on and in the crystallites.24 The dependence of the nonradiative relaxation rate on T is determined by effective phonon mode of the host. In the case of small spheres, the population process of 4S3/2 - 2H11/2 increases because the Er(III) ions located at the surface undergo stronger
H11/2 and 4S3/2 levels of Er3+ for UC emissions. Yb3+ is excited to the 2F5/2 level (GSA) with the absorption of a 980 nm photon, the sequential ETs from Yb3+ to Er3+ (ET 1 and ET 2) populate the 4F7/2 level of Er3+, and then the nonradiative relaxation populates the states of 2H11/2 and 4S3/2. The radiative decays of 2 H11/2 - 4I15/2 and 4S3/2 - 4I15/2 result in green emissions. The peak positions of the green emissions at 521 and 539 nm do not change upon increasing T. However, the ratio of their intensities varies as shown in Fig. 4. The 2H11/2 and 4S3/2 states are in close proximity, essentially separated by only an energy gap of about 640 cm1, leading to a thermal equilibrium governed by the Boltzmann law. Considering the thermalization of population and ignoring the self-absorption effect of fluorescence, the intensity ratios of the green up-conversion emissions at 521 and 539 nm, respectively, can be described11 using eqn (1): I521 N 2 H11=2 g s o DE DE ¼ H H H exp ¼ C exp ¼ R kT kT I539 gS sS oS N 4 S3=2 (1) where I521 and I539 are the integrated intensities of the H11/2 4 I15/2 (514–535 nm) and 4S3/2 - 4I15/2 (535–565 nm) transitions, respectively. The terms N, g, s, and o are the number of ions, the degeneracy, the emission cross-section, the angular frequency of fluorescence transitions from the 2H11/2, 4S3/2 levels to the 4I15/2 level, respectively, DE is the energy gap between the 2 H11/2 and 4S3/2 levels, k the Boltzmann constant, T the absolute temperature, and C the pre-exponential constant that is defined as gHsHoH/gSsSoS. Fig. 4(a) shows a mono-logarithmic plot of the FIR for the emission lines centered at 521 and 539 nm versus the inverse absolute T in the range of 223–403 K. The plot reveals a linear dependence of the FIR on the inverse T. The data can be fit with a linear plot (eqn (2)) of the form 2
ln(I521/I539) = (a/T) + b
(2)
Table 1 summarizes the average values for adjustable parameters a and b taken from several sets of measurements. The slope of the linear range (parameter a) is related to sensitivity defined as the rate at which the intensity ratio changes with T. The parameter a decreases with an increase of particle size. The FIR of green up-conversion emissions at 521 and 539 nm relative to T in the range of 223–403 K is shown in Fig. 4(b).
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Fig. 5 Sensitivity (slope of response) as a function of the temperature of single NaYF4:Er,Yb spheres of varying sizes.
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electron–phonon interactions with changing T than the Er(III) ions located in the interior.25 Consequently, the effect of the intensity ratio of IH/IS on T is enhanced. In addition, heating of the UCNPs by NIR light is more pronounced in the case of smaller nano/ microparticles than in the case of the larger ones. This undesired heating effect is supposed to result from the increase in the efficiency of a nonradiative relaxation process, which leads to an enhanced electron–phonon interaction in nano/micro particles.
Conclusion This study on the optical T-sensing properties based on the fluorescence intensity ratio of single Er,Yb-doped NaYF4 spheres revealed an unexpected size-dependence of the sensitivity to T. Specifically, the maximum sensitivity of the ratioed intensities at 521 and 539 nm, respectively, decreases from 36.8 104 to 24.7 104 K1 upon increasing the size of the spheres from 0.7 to 1.6 mm. The more expressed sensitivity of the smaller spheres is attributed to a more effective population process of the 4S3/2 2 H11/2 transition that increases because the Er(III) ions located at the surface undergo stronger electron–phonon interactions than those located in the inner spheres. Also, heating of the smaller spheres by NIR light is more pronounced than in the case of the bigger ones. The increase in the efficiency of the nonradiative relaxation process leads to an enhanced electron–phonon interaction in nano/microparticles. Micro- and nanosensors based on the measurement of the ratioed fluorescence of single rare earth-doped upconversion materials are considered to represent useful and fully reversibly responding (rather than single-shot) probes. They can be photoexcited with low-cost diode lasers and do not require specific conditions in terms of their microenvironment. Numerous T cycles can be performed with good repeatability, and such particles are readily internalized by cells. This indicates that such micro/ nanoprobes are powerful tools for optical sensing of T inside cells.
Acknowledgements This work was supported by the 973 Program (2012CB626801), the National Science Foundation of China (Grant No. 11274057, 21173034, 11004021 and 11204024), Program for New Century Excellent Talents in University (Grant No. NCET-13-0702), Science and Technology Project of Liaoning Province (Grant No. 2012222009), Program for Liaoning Excellent Talents in University (LNET) (Grant No. LJQ2012112), the Fundamental Research Funds for the Central Universities (Grant No. DC12010117), and Science and Technique Foundation of Dalian (Grant No. 2012J21DW016 and 2013A14GX040).
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