Open Access
Add to BibSonomy
Abstract
In this article, nanocrystals were co-doped with polymer for the fabrication of microfibers by using a simple drawing process. The microstructural characteristics of tetragonal-LiYF4:Yb3+/Er3+ nanocrystals were probed by X-ray diffraction (XRD), scanning electron microscopy (SEM) and transmission electron microscopy (TEM) techniques. The optical properties such as transmission losses and temperature dependent luminescence of a microfiber were demonstrated under 980 nm excitation. In addition, the fluorescence intensity ratio method (FIR) was utilized to carry out the optical thermometry based on green up-conversion luminescence behavior of nanocrystals in a single microfiber under two thermally coupled levels (2H11/2→4I15/2, 4S3/2→4I15/2) at 522 and 542 nm respectively. The high sensitivity of a microfiber was derived from the FIR technique at 358 K around 0.00307 K−1 in the temperature range of 298-358 K. These results revealed that a single microfiber is promising candidate for applications in optical temperature sensor.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Photon up-conversion is a process of sequential absorption of two or more than two photons results in the emission of light having wavelength shorter than excitation wavelength [1]. Phosphors are those materials which are composed of a transparent host as well as activator and contain a small quantity of transition metal or rare earth ions [2]. These materials emit light in the ultraviolet (UV), visible (VIS) or infrared (IR) spectral regions after absorbing a specific form of energy. Rare earth (RE3+) ions doped materials have been extensively used as fluorescent, biological labels, lasers, optical displays and sensors [3–7]. Therefore, excitation and emission characteristics of phosphors change with temperature that has multiple applications in optical thermal sensing devices [8]. Phosphor thermometry manifest an optical technique due to its surface temperature measurements. In fact, fluorescence intensity ratio (FIR) is the paramount method because it has been considered as inspiring branch of optical temperature [9, 10]. This technique depends on the up-conversion luminescence (UCL) of two thermally coupling energy levels of RE3+ ions. It has many advantages of high detection accuracy with non-contact measurement [11, 12]. Moreover, micro-tubes and nanowires have been prepared in literature cited and have been used as temperature sensing. For example, fabrication and temperature sensing behavior of Tm3+/Yb3+ doped β-NaLuF4 was reported in a study [13]. An others literatures discussed the fabrication process as well as temperature sensing behaviors of nanowires [14, 15]. Additionally, such kind of techniques are expensive and fabrication process is difficult. Since, the fabrications process of nano-wires and micro-tubes from glass-ceramic and crystals are complicated as well as having high cost. Thus, their applications are highly restricted. Unfortunately, easy method for fabrication of nanowires and micro tubes is still a challenge. Here, we introduced a simple and cheaper technique for the fabrication of microfibers which almost have same optical properties as like nanowires and micro-tubes. Fortunately, this disadvantage may be compensated by co-doping of nanocrystals with polymer for the fabrication of microfibers. Therefore, it is inspiring to probe the up-conversion nanocrystals in a single microfiber (UCMF) to designate the idea that its an easier and cheaper option in current times. It is noticed that Er3+ and Yb3+ ions possess relatively high luminescent efficiency because these ions exhibit visible up-conversion luminescence pumped by near infrared (NIR) region. The doping of nanocrystals with Er3+/Yb3+ ions are commonly used for the investigation of optical properties but co-doping with polymer is a growing interest for the fabrication of microfibers. To the best of our knowledge, very few investigations for optical sensing have been studied so far, mainly concerning the fabrication of up-conversion microfibers in a simplistic way [16–19]. Therefore, we focused on the simple strategy for the fabrication of microfibers and investigated up-conversion nanocrystals in a single microfiber as an idea for temperature sensing. In this paper, tetragonal-LiYF4: Yb3+/Er3+ ions were prepared by thermal decomposition technique. Sequentially, microfibers were successfully fabricated from co-doping of nanocrystals with PMMA solution by applying simple drawing way. Under the excitation of 980 nm fiber laser, microfibers demonstrated the light transmission loss and green up-conversion luminescence. The wave guiding and optical temperature sensing properties of the microfibers are discussed systemically. The fluorescence intensity ratio (FIR) method was applied for two thermally coupling levels. The sensitivity was calculated in the temperature range from 298 K to 358 K by using FIR technique. The results evidence that a single microfiber can be used as an excellent candidate for optical thermometry.
2. Experimental section
2.1 Synthesis of Yb3+/ Er3+:LiYF4
We demonstrated synthesis of tetrogonal-LiYF4 with some modifications. Therefore, Er3+ were doped in LiYF4 ultra-small NCs were prepared as per modified literature procedure [20]. 1M YCl3, ErCl3 and YbCl3 methanol solutions; 0.5M LiOH and NH4F methanol solutions were first prepared. In this typical experiment, ErCl3, YbCl3 and YCl3 methanol solution with molar ratio of 2:18:80 were added to a 50 ml three-necked flask along with 10 ml oleic acid (OA) and 15 ml 1-octadecene (ODE). Before cooling down to 50 °C, the mixture was first heated at 180 °C for 40 minutes. Afterwards, 5 and 6.6 ml of LiOH as well as NH4F methanol solution was added and stirred for 45 minutes. After this reaction, the mixture was then heated at 110 °C for 15 minutes to remove water and methanol. Before cooling down to room temperature, the solution was heated up to 300 °C and was kept for 1 hour. Further, we added the ethanol to precipitate the as-prepared NCs. The precipitated solution was then collected by applying centrifugation at 5000 rpm for 3 minutes. Furthermore, it was washed with ethanol and methanol for multiple times and duration. The final product was then dispersed in 5 ml of cyclohexane to obtain the nanocrystals of 2Er/18Yb:LiYF4. Figure 1 depicts the synthesis process. Argon gas purging protection was used throughout the entire experimental procedure to allow the reaction process.
2.2 Preparation method of microfibers
Microfibers were fabricated from mixed solution of nanocrystals and PMMA by applying directly drawing process. Experimentally, we dissolved PMMA solution in chloroform and nanocrystals dispersed in cyclohexane. The nanocrystals and PMMA solutions were mixed together and was followed by ultra-sonicating for one hour to get a uniform solution. The weight ratio of PMMA to nanocrystals appeared to be 100:1. A fiber probe with the tip of several micrometers was fabricated by flame heated drawing process. Afterwards, the fiber tip was immersed into mixed solution and then pulled out quickly for the fabrication of microfibers. The direct drawing process is shown in Figs. 2(a)-2(c) with microscopic view. The double fiber tip was used for cutting the microfibers into small pieces. The diameter of a microfiber could be increased or decreased by changing the speed of the pulling out of direct current (D.C.) motor.
2.3 Optical characterizations
The characterization and optical properties of UCMF was carried out under an optical microscope. The experimental setup is schematically illustrated in Fig. 3. The fiber probe tip coupled to 980 nm laser source, was focused on the microfiber. The microscopic view indicated that a microfiber illuminated as it touched the fiber laser. The photoluminescence (PL) signals were collected by 10x (numerical aperture (NA) = 0.25), 20x (NA = 0.4) and 40x (NA = 0.65) objectives respectively. Thus, emission spectra were observed at 40x (NA = 0.65) objective while heater was used to change the temperature of a sample during collection of emission spectra. Moreover, lens was used to focus the laser on the target but a 980 nm filter was used to eliminate reflection of the laser. The beam splitter was applied to the spectrometer (Ocean optics QE Pro) and charge-coupled device (CCD) camera (Olympus DP 26) for splitting the emission spectra of a microfiber.
3. Experimental results and analysis
Figure 4(a) represents the X-ray diffraction (XRD) pattern of tetragonal-LiYF4:Yb3+/Er3+. CuKα radiations (λ = 1.5406 Ao, 40Kv, 40Ma) with X-ray diffractometer were used to collect the required pattern. It is obvious that relative intensity and position of all diffraction peaks have closely matched with standard peaks (JCPDS No. 17-0874). The sharp diffraction peaks indicate that synthesized nanoparticles were well crystallized and no raw materials were detected in as-synthesized sample approving that Er3+, Yb3+ were well doped into tetragonal-LiYF4 lattices. Figures 4(b)-4(c) are typical transmission electron microscopy (TEM) images of nanocrystals at 100 and 20 scales respectively, which indicates that the nanoparticles particles are distributed monodispersely. Figure 4(d) shows the scanning electron microscopy (SEM, SEI X 300) image of a microfiber (diameter ~10 μm). The smooth surface depicts that nano-crystals were dispersed uniformly in a microfiber. Further, microfibers are well-defined and homogeneous on closer inspection with diameter of 1 micrometer (µm). A small diameter of the microfiber was prepared and characterized via TEM to prove that β-LiYF4:Yb3+/Er3+ were doped in a microfiber. Figure 4(e) indicates that β-LiYF4:Yb3+/Er3+ are well-dispersed in a single microfiber.
Fig. 4 The characteristics of images. 4(a) XRD of nanocrystals. 4(b)-4(c) TEM of nanocrystals. 4(d) SEM of the microfiber after pulling out from nanocrystals and PMMA mixed solution. 4(e) TEM image of a microfiber.
In order to investigate the optical properties of an individual microfiber, 980 nm laser source was used to irradiate the microfiber at middle and top points. Figure 5(a) shows that a microfiber (diameter ~10 μm and length ~400 μm) was excited at middle point with dark background. The two brightened end points act as an optical wave-guide. It seems that a microfiber absorbs the light and uniformly propagates towards the end points [21]. Similarly, Fig. 5(b) shows the dark field PL image of a microfiber with 10x objective. A little part of a microfiber is magnified by 20x objective which can be seen in Fig. 5(c). One can see that the distribution of nanocrystals in a microfiber is uniform because there is no existence of clustering and breaking point in the brightness of a microfiber. Further, the wave guiding performance, Fig. 5(d) shows the propagation of light with different excited points in black background (diameter ~8μm) under the excitation of 980 nm fiber laser. We excited a single microfiber at different position to measure the normalized PL intensity of output spots but the brightness of intensities remained same along the position of excited point. Output spot images were converted from RGB (red, green and blue) into gray styles by using Adobe Photoshop. Furthermore, Matlab was used to calculate the values of gray styles for characterizing the corresponding intensities [22]. At the end faces, the normalization was considered in measured intensities against those measured at each excited location on a microfiber due to little fluctuation in excited power. We observed that self-absorption and Rayleigh scattering created obstacle for the propagation of light in microfibers and turned out to be the source of optical loss. Similarly, the light propagated with different excited points in dark background with diameter ~10 μm and ~6 μm, respectively as shown in Figs. 5(e)-5(f). It further represents the same phenomena as like Fig. 5(d). In addition, the relationship between guiding distance and PL intensity are shown in Figs. 6(a)-6(c), with inset dark images of microfibers (diameter ~8 μm, ~10 μm and ~6 μm respectively). The experimental data was plotted by using first order exponential decay curve which is given as follows [23].
where Iendpoint = Intensity of endpoint of a microfiber and I0 = Intensity of excited spot, d, α are the propagation distance as well as fitting parameter respectively. We noticed that when propagation distance increases, PL intensity of endpoints decreases exponentially as seen in Figs. 6(a)-6(c). Hence, the transmission loss coefficients (α) were calculated from fitting curves with different diameters (~8 μm, ~10 μm and ~6 μm) as ~99.835 cm−1, 158.389 cm−1, and 81.299 cm−1 respectively. The lanthanides (Ln3 + ) ions have great absorption cross-section area because it largely overlaps with the emission cross section. Therefore, it undergoes to serious fluorescence self-absorption [24]. In Rayleigh scattering the dimension of microfiber is less than wavelength and measured the loss coefficients of microfiber which is agreed with Cdse/ZnS-doped nanofibers [25]. Our results closely match with the wave guiding performance of micro-tube [26]. And microfibers behave as typical active and passive waveguides feature of nanowires [27, 28].Fig. 5 (a)-5(f). Photoluminescence (PL) images of a single microfiber, PL images were collected upon at different excitation positions.
Fig. 6 (a)-(c) Normalized intensity of the output PL dependent the guided distance. The lines are an exponential fitting curve to the data yielding loss coefficients for wave guides in a microfiber.
With the intention to describe the energy levels diagram of Yb3+/ Er3+ ions, Fig. 7 depicts with possible up-conversion (UC) mechanisms. Under the excitation of 980 nm fiber laser, Yb3+ ions from 2F7/2 state are promoted to 2F5/2 one though ground-state absorption process. Afterwards, the successive energy process takes place such as ET1 and ET2 which transfers energy from Yb3+ to the Er3+. The ground state levels (4I15/2) are excited to higher state (4F7/2), from which the green and red-emitting (2H11/2/4S3/2, 4F9/2) states are populated by non-radiative multi-phonon relaxations respectively [29]. Moreover, the energy transfers process (ET3) populates the 4F9/2 level such as 4I13/2(Er3+)→4F9/2(Er3+), 2F5/2(Yb3+)→2F7/2(Yb3+). But the 4I13/2 state can be populated from 4I11/2 state through non-radiative transition in Er3+. Thus, the temperature dependent fluorescence intensity ratios (I522/I542) are used to evaluate the optical thermal sensing properties.
In the study of up-conversion (UC) emission spectra, we explored the possible application of a single microfiber in optical thermometry. The UC emission spectra of the nanocrystals in a single microfiber were investigated under 980 nm laser source ranging from 400 to 750 nm. Figure 8(a) depicts the emission spectra at different thermal ranges i.e. from 298 to 358 K. We acquired the emission spectra after each 5 °C and used 0.64 mW laser pumping power to avoid the thermal effect. It was observed that the emission intensities were decreased with increase of temperature. In order to confirm the different decay rates of temperature changes, the UCMF was emitted until the temperature was reduced from 358K to 298K and emission intensities were returned to its original place. We utilized temperature range from 298 K to 358 K because fabricated microfibers had good spectral stability with such range of temperature. When we increased the temperature range above 358 K, the microfiber could not stand with higher temperature. The up-conversion nanocrystals (UCNCs) powder was pumped by using 980 nm laser source in temperature ranging from 100 K to 700 K and could make an effective optical thermometer [30]. It is noticed that these spectra exhibit two distinct emission bands about 522 nm and 542 nm. Therefore, it is assigned to the 2H11/2→4I15/2 and 4S3/2→4I15/2 transitions of Er3+ ion, respectively. These two UC emissions are used in fluorescent intensity ratio (FIR) technique as depicted in Fig. 8(b). FIR reports a remarkable dependence on the temperature which is owing to the thermal coupling between 2H11/2 and 4S3/2 states of Er3+ levels. Since, two thermally coupled states depends on Boltzmann distribution, it can be denoted in the following equation [31, 32].
here, IU and IL are upper and lower levels of intensities. Whereas FIR = Fluorescence intensity ratio, A = Radiative spontaneous transition rate, f = Emitted frequency of studying energy levels, C = Constant value, ΔE = Energy difference between two levels, kB = Boltzmann constant, T = Absolute temperature. The Fluorescence intensity ratio technique is just a temperature dependent variable because its value can reflect the temperature change of specific sample. There will be no change in sample temperature if FIR value does not change. The investigational data was fitted by using Eq. (2). The values of constant C and ΔE/kB can be seen in Fig. 8(b). Moreover, we consider the further application. Therefore, it is necessary to elaborate the temperature sensing performance of LiYF4:Yb3+/Er3+ in a single microfiber. The absolute sensitivity (Sa) for two thermally coupled levels can be defined as the variation of FIR with temperature, is also studied in the following equation [33].where Sa as a function of absolute temperature based on Eq. (3) is plotted and shown in Fig. 8(c). It was observed, when temperature increased then sensitivity was also increased. We achieved the maximum value of around 0.00307 K−1 at the temperature range of 358 K. The sensitivities of the optical temperature sensors in different host materials are listed in Table 1. It can be found that the value of maximum sensitivity of a microfiber obtained in this work is approximately close to those reported in others host materials, indicating the fabricated microfibers have good temperature sensing property and are suitable for temperature detection. Moreover, microfibers have numerous applications, including optoelectronic communication devices and color displays. Especially, they have shown great potentials for biological applications such as biomedical engineering (guiding cell growth, alignment and migration), clinical treatments, 3D cell culturing and cell encapsulation [34–39]. It is also worthy of mention that our sensing range (298-358 K) is limited due to PMMA cannot stand with higher ambient temperature, it still covers the range of operation temperature of most biological applications mentioned above. Therefore, our microfiber thermometer possesses wider prospects, especially for biological applications, although its sensitivity and range are slightly lower than some nanocrystals [18,19]. The results indicate that as-prepared microfibers from mixed solution of nanocrystals and polymer exhibited greatly UC performance. Hence, it is expected that such fabricated microfibers have good thermal sensing property and are fit for the purpose of temperature detection.Fig. 8 (a)-8(c). 8(a) The up-conversion emission spectra of single microfiber in the range from 400 nm to 750 nm at various temperature range from 298 K to 358 K. 8(b) The fluorescence intensity ratio vs. temperature ranges from 298 K to 358 K. 8(c) The sensitivity of a microfiber.
Table 1. Comparison of sensitivity and temperature sensing region of Er3+ in different host materials.
4. Conclusion
In summary, tetroganol-LiYF4:Yb3+/Er3+ were synthesized by a thermal decomposition method. Meanwhile, microfibers were fabricated after co-doping of nanocrystals with PMMA solution by applying a simple drawing process. We calculated the transmission losses which indicated that a single microfiber behaved as an active waveguide. An optical thermometry has been investigated which depends on the thermally coupled levels 2H11/2 and 4S3/2 of Er3+ in a single microfiber. The optical temperature sensing property of a microfiber was studied in the temperature range of 298-358 K by using FIR method. The effectively up-conversion emission spectra of a single microfiber with relatively high sensitivity was obtained through the utilization of different thermal behavior of Er3+ ions at the temperature range of 298 K ~358 K. The maximum sensitivity of a single microfiber was recorded as 0.00307 K−1 at 358 K, illustrating that device can achieve relatively high sensitivity, as well as it could be used easily as a temperature sensor. The results manifest that a single microfiber is a potential candidate for optical thermometry. Furthermore, microfiber may be used in flowing gas or fluid to excite micro-system. Thus, in case of photonic platform, this work will open some new ways for micro light emitting devices.
Funding
National Key R&D Program of China (2016YFF0200700 and 2017YFB0405502); National Natural Science Foundation of China (Nos. 61635007 and 61605031).
References and links
1. B. Zhou, B. Shi, D. Jin, and X. Liu, “Controlling upconversion nanocrystals for emerging applications,” Nat. Nanotechnol. 10(11), 924–936 (2015). [CrossRef] [PubMed]
2. J. Brübach, T. Kissel, M. Frotscher, M. Euler, B. Albert, and A. Dreizler, “A survey of phosphors novel for thermometry,” J. Lumin. 131(4), 559–564 (2011). [CrossRef]
3. F. Wang and X. Liu, “Upconversion multicolor fine-tuning: visible to near-infrared emission from lanthanide-doped NaYF4 nanoparticles,” J. Am. Chem. Soc. 130(17), 5642–5643 (2008). [CrossRef] [PubMed]
4. H. Eilers, “Effect of particle/grain size on the optical properties of Y2O3:Yb3+/Er3+,” J. Alloys Compd. 474(1-2), 569–572 (2009). [CrossRef]
5. F. Vetrone, J. C. Boyer, J. A. Capobianco, A. Speghini, and M. Bettinelli, “Significance of Yb3+ concentration on the up-conversion mechanisms in co-doped Y2O3:Er3+, Yb3+ nanocrystals,” J. Appl. Phys. 96(1), 661–667 (2004). [CrossRef]
6. X. X. Luo and W. H. Cao, “Up-conversion luminescence of holmium and ytterbium co-doped yttrium oxysulfide phosphor,” Mater. Lett. 61(17), 3696–3700 (2007). [CrossRef]
7. D. Jaque, J. G. Sole, L. Macalik, J. Hanuza, and A. Majchrowski, “A pump-power-controlled luminescent switcher,” Appl. Phys. Lett. 86(1), 011920 (2005). [CrossRef]
8. S. A. Wade, S. F. Collins, and G. W. Baxter, “Fluorescence intensity ratio technique for optical fiber point temperature sensing,” J. Appl. Phys. 94(8), 4743–4757 (2003). [CrossRef]
9. N. Ishiwada, T. Ueda, and T. Yokomori, “Characteristics of rare earth (RE = Eu, Tb, Tm)-doped Y2O3 phosphors for thermometry,” Luminescence 26(6), 381–389 (2011). [CrossRef] [PubMed]
10. P. Haro-Gonzalez, S. F. Leon-Luis, S. González-Pérez, and I. R. Martín, “Analysis of Er3+ and Ho3+ codoped fluoroindate glasses as wide range temperature sensor,” Mater. Res. Bull. 46(7), 1051–1054 (2011). [CrossRef]
11. H. Zheng, S. Y. Xiang, and B. J. Chen, “Laser irradiation induced temperature effect of NaY (WO4)2: Tm3+/Yb3+ as optical temperature sensors,” Faguang Xuebao 35(7), 800–806 (2014). [CrossRef]
12. D. Wawrzynczyk, A. Bednarkiewicz, M. Nyk, W. Strek, and M. Samoc, “Neodymium(III) doped fluoride nanoparticles as non-contact optical temperature sensors,” Nanoscale 4(22), 6959–6961 (2012). [CrossRef] [PubMed]
13. V. K. Rai, “Temperature sensors and optical sensors,” Appl. Phys. B 88(2), 297–303 (2007). [CrossRef]
14. D. O’Carroll, I. Lieberwirth, and G. Redmond, “Melt-processed polyfluorene nanowires as active waveguides,” Small 3(7), 1178–1183 (2007). [CrossRef] [PubMed]
15. H. Li, Y. Wang, J. Li, S. Liu, and J. Yang, “Wave guiding performance of rhodamine 6G doped polymer nanowire,” Optik (Stuttg.) 127(18), 7268–7273 (2016). [CrossRef]
16. S. Jiang, P. Zeng, L. Q. Liao, S. F. Tian, H. Guo, Y. H. Chen, C. K. Duan, and M. Yi, “Optical thermometry based on up-converted luminescence in transparent glass ceramics containing NaYF4:Yb3+/Er3+ nanocrystals,” J. Alloys Compd. 617, 538–541 (2014). [CrossRef]
17. C. Li, B. Dong, C. Ming, and M. Lei, “Application to temperature sensor based on green up-conversion of Er3+ doped silicate glass,” Sensors (Basel) 7(11), 2652–2659 (2007). [CrossRef] [PubMed]
18. L. Tong, X. Li, R. Hua, L. Cheng, J. Sun, J. Zhang, S. Xu, H. Zheng, Y. Zhang, and B. Chen, “Optical temperature sensing properties of Yb3+/Tm3+ co-doped NaLuF4 crystals,” Curr. Appl. Phys. 17(7), 999–1004 (2017). [CrossRef]
19. B. Dong, D. P. Liu, X. J. Wang, T. Yang, S. M. Miao, and C. R. Li, “Optical thermometry through infrared excited green up-conversion emissions in Er3+/ Yb3+ co-doped Al2O3,” Appl. Phys. Lett. 90(18), 181117 (2007). [CrossRef]
20. F. Wang, R. Deng, and X. Liu, “Preparation of core-shell NaGdF4 nanoparticles doped with luminescent lanthanide ions to be used as upconversion-based probes,” Nat. Protoc. 9(7), 1634–1644 (2014). [CrossRef] [PubMed]
21. Y. S. Zhao, A. Peng, H. Fu, Y. Ma, and J. Yao, “Nanowire waveguides and ultraviolet lasers based on small organic molecules,” Adv. Mater. 20(9), 1661–1665 (2008). [CrossRef]
22. B. Yan, L. Liao, Y. M. You, X. J. Xu, Z. Zheng, Z. X. Shen, J. Ma, L. M. Tong, and T. Yu, “Single crystalline V2O5 ultralong nanoribbon waveguides,” Adv. Mater. 21(23), 2436–2440 (2009). [CrossRef]
23. Beer, “Bestimmung der Absorption desrothen Lichts in farbigen Flussigkeiten (Determination of the absorption of red light in colored liquids),” Annalen der Physik and Chemie. 86, 78–88 (1852).
24. B. Piccione, L. K. van Vugt, and R. Agarwal, “Propagation loss spectroscopy on single nanowire active waveguides,” Nano Lett. 10(6), 2251–2256 (2010). [CrossRef] [PubMed]
25. R. Zhang, H. Yu, and B. Li, “Active nanowaveguides in polymer doped with CdSe-ZnS core-shell quantum dots,” Nanoscale 4(19), 5856–5859 (2012). [CrossRef] [PubMed]
26. H. Li, Y. Wang, H. Li, Y. Zhang, and J. Yang, “Dispersing upconversion nanocrystals in a single silicon microtube,” Sci. Rep. 6(1), 35941 (2016). [CrossRef] [PubMed]
27. G. Morello, M. Moffa, S. Girardo, A. Camposeo, and D. Pisignano, “Optical gain in the near infrared by light‐emitting electrospun fibers,” Adv. Funct. Mater. 24(33), 5225–5231 (2014). [CrossRef]
28. G. Brambilla, V. Finazzi, and D. Richardson, “Ultra-low-loss optical fiber nanotapers,” Opt. Express 12(10), 2258–2263 (2004). [CrossRef] [PubMed]
29. D. Chen, Z. Wana, Y. Zhou, P. Huang, J. Zhong, M. Ding, W. Xiang, X. Liang, and Z. G. Ji, “Bulk glass ceramics containing Yb3+/Er3+: beta-NaGdF4 nanocrystals: Phase-separation-controlled crystallization, optical spectroscopy and up-converted temperature sensing behavior,” J. Alloys Compd. 638, 21–28 (2015). [CrossRef]
30. X. Wang, J. Zheng, Y. Xuan, and X. Yan, “Optical temperature sensing of NaYbF4: Tm3+@SiO2 core-shell micro-particles induced by infrared excitation,” Opt. Express 21(18), 21596–21606 (2013). [CrossRef] [PubMed]
31. K. Z. Zheng, Z. Y. Liu, C. J. Lv, and W. P. Qin, “Temperature sensor based on the UV up-conversion luminescence of Gd3+ in Yb3+-Tm3+-Gd3+ co-doped NaLuF4 microcrystals,” J. Mater. Chem. C Mater. Opt. Electron. Devices 1(35), 5502–5507 (2013). [CrossRef]
32. B. Dong, B. Cao, Y. He, Z. Liu, Z. Li, and Z. Feng, “Temperature sensing and in vivo imaging by molybdenum sensitized visible upconversion luminescence of rare-earth oxides,” Adv. Mater. 24(15), 1987–1993 (2012). [CrossRef] [PubMed]
33. S. A. Wade, S. F. Collins, and G. W. Baxter, “The fluorescence intensity ratio technique for optical fiber point temperature sensing,” J. Appl. Phys. 94(8), 4743–4756 (2003). [CrossRef]
34. J. A. Kechele, C. Hecht, O. Oeckler, J. S. Auf Der Günne, P. J. Schmidt, and W. Schnick, “Ba2AlSi5N9, “A new host lattice for Eu2+-doped luminescent materials comprising a nitridoalumosilicate framework with corner- and edge-sharing tetrahedral,” Chem. Mater. 21(7), 1288–1295 (2009). [CrossRef]
35. E. Chelebaeva, J. Larionova, Y. Guari, R. A. S. Ferreira, L. D. Carlos, F. A. A. Paz, A. Trifonov, and C. Guérin, “Luminescent and magnetic cyano-bridged coordination polymers containing 4d-4f ions: toward multifunctional materials,” Inorg. Chem. 48(13), 5983–5995 (2009). [CrossRef] [PubMed]
36. C. M. Hwang, A. Khademhosseini, Y. Park, K. Sun, and S. H. Lee, “Microfluidic chip-based fabrication of PLGA microfiber scaffolds for tissue engineering,” Langmuir 24(13), 6845–6851 (2008). [CrossRef] [PubMed]
37. B. G. Chung, K. H. Lee, A. Khademhosseini, and S. H. Lee, “Microfluidic fabrication of microengineered hydrogels and their application in tissue engineering,” Lab Chip 12(1), 45–59 (2012). [CrossRef] [PubMed]
38. S. K. Tiwari, R. Tzezana, E. Zussman, and S. S. Venkatraman, “Optimizing partition-controlled drug release from electrospun core-shell fibers,” Int. J. Pharm. 392(1-2), 209–217 (2010). [CrossRef] [PubMed]
39. J. D. Caplin, N. G. Granados, M. R. James, R. Montazami, and N. Hashemi, “Microfluidic organ-on-a-chip technology for advancement of drug development and toxicology,” Adv. Healthc. Mater. 4(10), 1426–1450 (2015). [CrossRef] [PubMed]
