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Abstract
Temperature controllable photothermal therapy (PTT) requires a nanoplatform in which the optical temperature sensor and photothermal calorifier are integrated together. To establish such a nanoplatform, in this work we designed a NaYF4:Er3+/Yb3+@NaYF4:Tm3+/Yb3+ core-shell structure, and studied on its temperature sensing and photothermal conversion. The core-shell nanoparticles were prepared via a thermal decomposition method; furthermore, their crystal structure and microscopic morphology were characterized by means of XRD and SEM/TEM. The properties of temperature sensing and excitation power dependent fluorescence branching ratios were investigated. It was found that the temperature sensing could be achieved based on the fluorescence intensity ratio of green emissions from Er3+, but could not be realized by using other fluorescence intensity ratios. The photothermal conversion was demonstrated under 808 and 980 nm co-excitation, and the dependences of photothermal conversion on the excitation power and irradiation time of 808 nm laser were observed. Moreover, it was also found that the core-shell particles could effectively accelerate the evaporation of anhydrous ethanol, thus implying the photothermal conversion under single 980 nm laser excitation could also be achieved.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
With the advent of PTT in recent years, practical applications in treatment for cancer or tumor prompt researchers from biology, medicine and materials science to experiment with the photothermal conversion nanomaterials and optical temperature measurement methods for PTT [1–4]. Compared with the widely-studied photothermal conversion materials including gold nanoparticles [5–7], carbon-based nanomaterials [8-9], quantum dots [10-11], etc., the trivalent lanthanide ions doped materials exhibit many advantages, for example, abundant choices of biocompatible inorganic hosts and doping ions, easy preparation in most cases, excellent match of infrared excitation wavelength with the absorption wavelength of trivalent lanthanide ions, and high accommodation of trivalent lanthanide ions in inorganic hosts [12–15]. Therefore, novel photothermal conversion platforms based on trivalent lanthanide ions doped inorganic nanomaterials perhaps can be established. The effective photothermal conversion can be achieved by lanthanide ions doping strategy. For instance, in the Yb3+/Sm3+ codoped system, Yb3+ was designed as a sensitizer to effectively absorb 980 nm excitation energy, and then the absorbed energy could be effectively transferred to Sm3+ which has abundant lower energy levels to generate heat via cascade nonradiative relaxations [16]. In addition, the strong photothermal effect in Yb3+/Tm3+ co-doped NaY(WO4)2 microstructures was also discovered by using Er3+ as temperature probe [17]. When the excitation wavelength was 808 nm, Nd3+ could transfer the absorbed light energy to Sm3+ through cross relaxation route, then the thermal energy was produced by the nonradiative relaxations of Sm3+ [18]. In another case, the nanoparticles doped with higher Nd3+ concentrations could produce more heat, which is demonstrated by injecting LaF3:Nd3+ aqueous solution into chicken breast tissues [19]. These investigations exhibit intriguing prospects of the lanthanide ions doped optical nanomaterials as photothermal conversion agents to meet the requirements of PTT.
Aiming at the difficulty of temperature measurement in cells and tissues, the non-contact thermal probe lying on the fluorescence intensity ratio (FIR) holds great promise due to its excellent accuracy and resolution of measurement as well as the minimal impact of fluorescence intensity fluctuation [20–29]. More importantly, the thermal probe currently carries the potential applications for real-time monitoring and controlling temperature change of deep-seated tumors in PTT process. As a typical example of the temperature sensing, Er3+/Yb3+ co-doped NaYF4 upconversion (UC) materials were widely investigated as optical temperature sensors based on the thermally coupled levels 2H11/2 and 4S3/2 of Er3+ [21,26–29]. The measurement in intracellular temperature of living cells was successfully achieved based on the temperature sensing property of NaYF4:Er3+/Yb3+ nanoparticles [26]. In addition, other host material doped with Er3+ or Er3+/Yb3+ also exhibit temperature sensing property [30–35]. Aside from Er3+ the other lanthanide ions such as Tm3+, Nd3+, Ho3+ and Dy3+ also exhibited temperature sensing behaviors at their intrinsic emission wavelengths [36]. It is already known that maximum absorption of tissue depends on its own type of the tissues and the radiation frequency [37]. Therefore, it is necessary to find a universal temperature sensing probe working at various wavelengths so as to satisfy its applications for different tissues.
The aim of this work is to develop a new nano-platform in which the optical temperature sensing and photothermal conversion are organically allied together in a NaYF4:Er3+/Yb3+@NaYF4:Tm3+/Yb3+ core-shell structure [38,39], meanwhile explore the feasibility of multi-wavelength temperature sensing. The core-shell structured particles were synthesized via a thermal decomposition method. The NaYF4:Tm3+/Yb3+ shell was designed as nanoheater for generating heat, and the NaYF4:Er3+/Yb3+ core was designed as nanothermometers for probing temperature. The photo-to-heat conversion was examined in the NaYF4:Er3+/Yb3+@NaYF4:Tm3+/Yb3+ core-shell particles under 808 or 980 nm excitation. The fluorescence intensity ratios between several emissions were also studied.
2. Experimental methods
2.1 Sample preparation
NaYF4:Er3+/Yb3+ nanoparticles were synthesized using lanthanide chlorides with crystal waters (YCl3⋅6H2O, ErCl3⋅6H2O and YbCl3⋅6H2O) as raw materials. 2 mmol lanthanide chlorides with crystal waters (molar ratio for Y3+/Er3+/Yb3+ was 78:2:20), 12 mL of oleic acid (OA) and 30 mL of 1-octadecene (ODE) were added into a 100 mL three-necked flask. The mixture was heated to 150 °C to form a yellow transparent solution under condition of vacuum and stirring, and next the solution was naturally cooled to room temperature. Then 10 mL methanol solution containing NH4F (8 mmol) and NaOH (5 mmol) was added into the flask and the resulting solution reacted at room temperature for 30 min. After that, the solution was kept at 100 °C for 60 min under nitrogen atmosphere to evaporate methanol. Later on, the solution was heated to 310 °C in a nitrogen flow for 90 min and naturally cooled down to room temperature. The final products were precipitated by ethanol and centrifugation, and washed several times with ethanol and cyclohexane. The nanoparticles were dispersed in 5 mL cyclohexane for the further synthesis. More detailed preparative procedures can be found elsewhere [40,41].
For the synthesis of NaYF4:Er3+/Yb3+@NaYF4:Tm3+/Yb3+ core−shell nanoparticles, the shell precursor solution was prepared with an identical procedure as described above except for the use of 2 mmol lanthanide chloride (Y3+/Tm3+/Yb3+ = 79.5:0.5:20). After dissolving, 2 mmol NaYF4:Er3+/Yb3+ core nanoparticles prepared above were added into the precursor solution at room temperature. Then, the cyclohexane was completely removed when the solution was heated to 100 °C and maintained for 60 min in nitrogen atmosphere. The following experiments were carried out repetitively according to the synthesis of the NaYF4: Er3+/Yb3+ core sample.
2.2 Sample characterization
The crystal structures of the samples were identified by powder X-ray diffraction (XRD) using a Shimadzu X-ray diffractometer equipped with Cu-Kα1 radiation source (λ = 0.15406 nm). The diffraction patterns were recorded in the range of 2θ from 10 to 75° with a resolution of 0.02°/step. The samples were dispersed in cyclohexane and treated ultrasonically for observing microscopic structure. The particle morphologies were recorded on a Rigaku field emission scanning electron microscopy (FE-SEM) and a Joel transmission electron microscopy (TEM). The UC emission spectra were obtained by a Hitachi F-4600 fluorescence spectrometer. For the temperature sensing measurement, the temperature-dependent spectra were monitored by using 980 nm fiber laser as excitation source. Water bath was introduced to heat the cyclohexane solution with core-shell particles in cuvette (10 m/m). The excitation power densities of 980 nm in this work were about 11.8, 39.9, 45.8, 75.4 and 116.8 W/cm2 when the laser working currents were 0.5, 0.9, 1.0, 1.5 and 2.2 A. In the part of photothermal conversion examination, the solution was heated by 808 nm fiber laser with different excitation currents, and concurrently the solution temperature was probed by 980 nm fiber laser. The working currents of 808 nm laser were set to be 1.0, 1.5 and 2.2 A at which the effective power densities were around 81.1, 137.9 and 217.7 W/cm2. In all experiments the laser beam area will not change, thus hereinafter the working currents for 980 and 808 nm lasers will be mentioned if without specific statement. As a control group, the temperature of cyclohexane was monitored by a thermocouple immersed into it.
3. Results and discussion
3.1 Structure and morphology analysis
To examine the crystal structure, XRD patterns of naked NaYF4:Er3+/Yb3+ core and NaYF4:Er3+/Yb3+@NaYF4:Tm3+/Yb3+ core-shell nanoparticles were measured. All the diffraction patterns of the samples match well with JCPDS card No.28-1192 for hexagonal phase NaYF4 as shown in Fig. 1(a), and no diffraction peaks of other compounds can be observed. After the shell growth, the diffraction peaks of the core-shell particles are relatively narrower than that of the naked core nanoparticles. These results reveal that shell coating did not cause any change of the crystal phase but improve crystallinity of the core-shell nanoparticles. The high crystallinity of core-shell nanoparticles is further confirmed by the high resolution TEM image, the clear lattice fringes with interplanar spacing of 0.52 nm can be observed in Fig. 1(b), corresponding to the (100) crystal plane of standard hexagonal NaYF4 [42].
Fig. 1 (a) XRD patterns of NaYF4:Er3+/Yb3+ naked core and NaYF4:Er3+/Yb3+@NaYF4:Tm3+/Yb3+ core-shell and the pattern of β-NaYF4 taken from JCPDS: NO. 28-1192 as reference, (b) High resolution TEM image of core-shell sample.
The SEM images can directly reflect microscopic morphology of the naked NaYF4:Er3+/Yb3+ core and NaYF4:Er3+/Yb3+@NaYF4:Tm3+/Yb3+ core-shell nanoparticles. It is observed from Fig. 2(a)-2(b) that both naked core and core-shell particles are highly uniform and spherical in shape. To further confirm the morphology and size change of the naked core and core-shell samples, TEM images are shown in Fig. 2(c)-2(d), respectively. In both cases, it can be seen that all particles are well monodispersed, whereas the difference is that the shape of naked core particles is still spherical but all the core-shell particles exhibit hexagon morphology. On the basis of TEM images, the mean size of naked core and core-shell samples are calculated by separately counting 100 particles using noncommercial software, and the statistical results are displayed in Fig. 2(e)-2(f), respectively. It is found that the particle sizes have narrow distribution range, and the mean particle sizes of naked core and core-shell samples are calculated to be about 18.2 and 23.0 nm, respectively. The increase of mean particle size from naked core to core-shell indicates that about 2.5 nm shell (NaYF4:Tm3+/Yb3+) has been successfully grown around the core. It should be mentioned that the single-dispersed nanoparticles are preferable in practical application of photothermal therapy, because the dispersed nanoparticles are benefit to the loading into biological body and also make the supersession or excretion of the nanoparticles from body easier.
Fig. 2 (a) and (b) SEM images of naked core and core-shell nanoparticles, (c) and (d) TEM images of naked core and core-shell nanoparticles, (e) and (f) Statistical histograms for the particle size distribution of naked core and core-shell samples.
3.2 Er3+ green emissions based temperature sensing [43]
It is well known that the optical temperature sensors of Er3+ doped luminescence materials are on the base of fluorescence intensity ratio technique. In this technique the thermal coupling energy level pairs, viz. 2H11/2 and 4S3/2 levels, with small energy gap exist in a thermal equilibrium, thus the population distribution associated with temperature can be reflected by fluorescence emission intensity ratio between the two green emissions. In this sense, the theoretical expression for this relation can be derived as below [16,17].
where, R is the fluorescence intensity ratio, IH and IS are the fluorescence emission intensities related to the transitions from excited state 2H11/2 and 4S3/2 to ground state 4I15/2, respectively, C is a constant relevant to optically active ion and host matrix, ΔE is the energy gap between the two thermally coupled levels, k is the Boltzmann constant, and T is the absolute temperature. In above equation, ΔE and C can be confirmed by fitting Eq. (1) to the experimental data of fluorescence intensity ratios at various temperatures. Once the parameters ΔE and C are determined, the Eq. (1) can be used to calculate the temperature by taking fluorescence intensity ratio R derived from a spectrum measured at an unknown temperature.To collect the UC spectra at different sample temperatures for the temperature calibration, the core-shell particles should be irradiated by 980 nm laser. The laser irradiation will elevate sample temperature, thus bringing deviation to temperature calibration line. To avoid this deviation, the laser power used for the spectral measurements should be as low as possible, and the laser-induced thermal effect should be checked. To check the thermal effect, a solution with mass fraction of 4% NaYF4:Er3+/Yb3+@NaYF4:Tm3+/Yb3+core-shell nanoparticles was prepared by considering the fact that the dispersibility of OA-capped core-shell particles in cyclohexane is excellent. The above solution of 2 mL was poured into a cuvette which was then placed in the sample holder in the spectrometer. The time scan UC spectra under excitation of a 980 nm fiber laser working at current of 1.0 A were measured. The fluorescence emission intensity ratios of 2H11/2→4I15/2 to 4S3/2→4I15/2 were calculated by using the time scan UC spectra and are displayed in Fig. 3. It is found that the ratio increases with irradiation time within 70 min, but the increment per unit time is small, thus indicating that the laser self-generated thermal effect is not remarkable [18]. It should be emphasized that the thermal effect cannot be completely eliminated as long as the absorption for the studied object is existent. Fortunately, the change of fluorescence intensity ratio within initial 5 min can be neglected, thus if the fluorescence intensity can be measured within the initial 5 min, the laser-induced thermal effect on the temperature calibration line can be excluded. Actually, 5 min is long enough for accomplishing the measurement of a UC spectrum; in fact, it needs less then 1 min, usually.
Fig. 3 Fluorescence intensity ratio R (IH/IS) of NaYF4:Er3+/Yb3+@NaYF4:Tm3+/Yb3+ core-shell sample excited upon 980 nm with the laser current of 1.0 A (24.1 W/cm2) for a 70 min continuous irradiation.
Based on the above discussion, the 1.0 A working current was used in the measurements of UC spectra of the core-shell particles in cyclohexane solution. The solution was heated by water bath to 100 °C, and then quickly transferred to the holder in the spectrometer for measurements under the naturally cooling condition. The solution temperature was monitored with a proportional-integral-differential (PID) controller equipped with a K-type thermocouple. With solution temperature falling naturally, the UC emission spectra were measured in a temperature region of 300-336 K as depicted in Fig. 4(a). It should be pointed out that the laser was switched off during the idle time of spectral measurements in order to depress the laser-induced thermal effect as far as possible. From Fig. 4(a) it is clearly observed that both green emission intensities of Er3+ decrease with elevating solution temperature, but the 4S3/2→4I15/2 emission intensity decreases more quickly for the reason that 4S3/2 level can be depopulated by thermal excitation to 2H11/2 level and by nonradiative transition to 4F9/2 [44]. From these emission spectra the dependence of fluorescence intensity ratio R (IH/IS) on the solution temperature was derived and is shown in Fig. 4(b). The black solid circles stand for the fluorescence intensity ratio R (IH/IS), and the solid line is the fitting curve to Eq. (1). In the fitting process the free parameters C and ΔE/k were confirmed to be 6.53 and 936.55, meanwhile the obtained temperature calibration formula is displayed in Fig. 4(b). Here it should be emphasized that in the practical applications of the studied core-shell structure the accuracy of measuring temperature will depend on both the spectral measurement and the spectral properties of core-shell particles, but it will not be affected by the heating of the shell.
Figure 4 (a) UC emission spectra in a temperature region of 300-336 K upon 980 nm excitation for the NaYF4:Er3+/Yb3+@ NaYF4:Tm3+/Yb3+ core-shell particles, (b) Relationship between the green emission intensity ratio (IR/IS) (●) and sample temperature; solid curve is the fitting curve to Eq. (1).
3.3 Temperature dependence of fluorescence branching ratio
Aside from the temperature sensing based on fluorescence intensity ratio of Er3+, in this section, we explore whether the other fluorescence intensity ratios of core-shell particles can be used for temperature sensing. To that end, at first we should validate that the fluorescence intensity ratio used for temperature sensing is independent of the excitation power in temperature region from normal body temperature (37.5 °C) to effective photothermal therapy temperature (47 °C) [1]. In doing so, the UC spectra for the 2 mL cyclohexane solution with 4% NaYF4:Er3+/Yb3+@NaYF4: Tm3+/Yb3+ core-shell nanoparticles were measured at various temperatures under excitation of 980 nm laser working at different currents. Analogically, the change of solution temperature was also realized by naturally cooling after water bath at 100 °C. Two sets of spectral data were collected when the 980 nm laser worked at 0.5 and 0.9 A. The reason why the working currents of the 980 nm laser were lower than 1.0 A is to avoid the laser-induced thermal effect. To obtain more emissions for examining the multi-wavelength temperature sensing, the UC emission spectra in the region from 300 to 900 nm were recorded and are shown in Fig. 5.
Fig. 5 Temperature-dependent UC emission spectra of NaYF4:Er3+/Yb3+@NaYF4:Tm3+/Yb3+ core-shell particles under 980 nm fiber laser working at (a) 0.5 A (11.8 W/cm2) and (b) 0.9 A (39.9 W/cm2).
From Fig. 5(a)-5(b) it can be seen that the both spectra include UC emissions corresponding to 2H9/2→4I15/2 (408 nm), 2H11/2→4I15/2 (525 nm), 4S3/2→4I15/2 (545 nm), 4F9/2→4I15/2 (660 nm) and 4S3/2→4I13/2 (843 nm) transitions from Er3+, as well as 1G4→3H6 (475 nm) and 3H4→3H6 (802 nm) transitions from Tm3+ [45–49]. The integrated intensities for 545, 660, 802 and 843 nm emissions measured at different solution temperatures when the laser worked at 0.5 and 0.9 A were calculated. It should be mentioned that any couple amongst these four emissions are not from a pair of thermally coupled levels, thus Eq. (1) cannot be directly used for theoretically expressing the dependence of fluorescence intensity ratio on the temperature, but this does not mean that the temperature sensing cannot be realized by using the fluorescence intensity ratio. On contrary, the temperature sensing based on the ratio between the above mentioned transitions can be achieved so long as the intensity ratio is independent of excitation condition. So as to check the excitation power dependence of the fluorescence intensity ratio, we calculated the fluorescence intensity ratio R1 = I540/I802, R2 = I660/I802 R3 = I843/I802, R4 = I545/I660 and R5 = I660/I843, and the solution temperature corresponding to each spectrum in Fig. 5(a)-5(b) were calculated by taking the fluorescence intensity ratio of 2H11/2→4I15/2 to 4S3/2→4I15/2 into temperature calibration formula obtained in Section 3.2. Thus, the dependence of fluorescence intensity ratio Ri (i = 1-5) on solution temperature at different laser working currents is separately displayed in Fig. 6(a)-6(e). R1, R2 and R3 present the intensity ratios of Er3+ emissions to Tm3+ emission, and all these intensity ratios depend on the excitation power (the working current), meanwhile R4 and R5 which are also dependent on the excitation power are the ratios between Er3+ emissions. Moreover, from Fig. 6 it can be found that the variation trend of intensity ratio toward the excitation power is similar, but the intensity ratios for different transitions can be very different as shown in Fig. 6(a)-6(e). From these results it can be concluded that it is impossible to achieve temperature sensing by using the intensity ratio between the emissions originating from non-thermally coupled levels in NaYF4:Er3+/Yb3+@NaYF4:Tm3+/Yb3+ core-shell structure.
Fig. 6 Dependence of the fluorescence intensity ratio Ri (i = 1-5) (a-e) on solution temperature at excitation current of 0.5 A (11.8 W/cm2, square) and 0.9 A (39.9 W/cm2, hexagon).
3.4 Photothermal conversion measurement under 808 and 980 nm co-excitation
The NaYF4:Tm3+/Yb3+ shell is designed as photothermal converter, thus an excitation wavelength which matches both the output of a commercial laser and the optical transition of Tm3+ should be chosen. It is known that Tm3+ possesses a relatively strong absorption at around 800 nm corresponding to 3H6→3H4 transition (sometimes it is also ascribed as 3H6→3F4), hence 808 nm laser was used to demonstrate the photothermal conversion of the NaYF4:Er3+/Yb3+@NaYF4:Tm3+/Yb3+ core-shell particles [50]. Meanwhile, the 980 nm laser was used for stimulating the green emissions from the NaYF4:Er3+/Yb3+ core to confirm the sample temperature via aforementioned fluorescence intensity ratio technique. It should be pointed out that in the measurements of photothermal conversion both 808 and 980 nm laser beams were focused on central position of the cuvette containing 2 mL solution with mass fraction of 18% core-shell nanoparticles. To investigate the influence of excitation power on the photothermal conversion, the working current of 808 nm laser was set to be 1.0, 1.5 and 2.2 A, respectively, and for each working current the 808 nm laser continuously irradiated for 60 min. Within this 60 min the spectra were recorded every 5 min under 980 nm laser excitation. To refrain from the photothermal conversion induced by 980 nm irradiation, the working current of 980 nm laser was set to be 1.0 A, and 980 nm laser was switched off during the idle time of spectral measurements.
As an example, Fig. 7(a) shows green UC emission spectra of the core-shell particles measured at different moments within 60 min when working current of 808 nm laser is 1.0 A. It can be seen from Fig. 7(a) that with the increase of 808 nm laser irradiation time the emission intensity of 2H11/2→4I15/2 changes slightly, but the emission intensity of 4S3/2→4I15/2 decreases obviously, thus implying the solution temperature rises. The temperature correlated to each spectrum in Fig. 7(a) is calculated by taking the intensity ratio of 2H11/2→4I15/2 to 4S3/2→4I15/2 into Eq. (1), and dependence of solution temperature on irradiation time is illustrated by solid circles in Fig. 7(b). In an analogical way, the dependences of solution temperature on the irradiation time of 808 nm laser working at 1.5 and 2.2 A were also displayed in Fig. 7(b) as up-triangle and down-triangle dots, respectively. It should be noted that the core-shell particles were dispersed in cyclohexane, thus the heat energy generated by pure cyclohexane under 808 nm irradiation should be checked. Therefore, the temperature of the pure cyclohexane under irradiation of 808 nm laser working at 1.0 A was monitored by the thermocouple, and the data are also included in Fig. 7(b) for comparison (see solid squares). As can be seen from Fig. 7(b), the temperature of pure cyclohexane increases slowly and reaches a thermal equilibrium after about 45 min. However, at the same working current of 808 nm laser, when the NaYF4:Er3+/Yb3+@NaYF4: Tm3+/Yb3+ core-shell particles were loaded into the cyclohexane, the solution temperature increases more quickly at the initial time within 15 min, and then tends to get a constant temperature. This result implies that the core-shell particles effectively convert the excitation light into heat since the amount of the core-shell particles introduced into the solution is as less as 18% mass fraction. It can also be found that as the excitation current increases up to 1.5 and 2.2 A, the solution displays more effective photothermal conversion due to more quick temperature increase at the initial time of the laser irradiation and higher final equilibrium temperature. These results mean that the higher the laser excitation power of 808 nm laser, the more the light energy converted into heat. Moreover, the temperature sensing property of Er3+ in the core can be utilized simultaneously for evaluating the photothermal conversion of the core-shell particles, and it also exhibits that the designed core-shell nanoparticles are featured with bi-function for optical temperature reading and photothermal conversion.
Fig. 7 (a) UC emission spectra of the solution excited by 808 nm laser working at 1.0 A (81.1 W/cm2) within 60 min, (b) Dependence of the temperature of pure cyclohexane solution and the cyclohexane solution containing core-shell nanoparticles on the irradiation time and working current of 808 nm laser.
3.5 Photothermal conversion under 980 nm laser irradiation
The conversion from light energy to heat energy in rare earth doped materials takes place through cascade nonradiative relaxations which exist in all the luminescence processes including down- and up- conversion processes. Therefore, the photothermal conversion of the core-shell particles could also be achieved under 980 nm excitation. In above section, we discussed the effect of excitation power and irradiation time of 808 nm laser on photothermal conversion. In this section, we attempt to observe the photothermal converting capability of the core-shell nanoparticles under 980 nm irradiation.
To evaluate the photothermal conversion, 0.02 g of the powder NaYF4:Er3+/Yb3+ @NaYF4:Tm3+/Yb3+ core-shell nanoparticles was put in the center of a small glass vessel and stacked together, then the vessel was transferred carefully to an electronic balance. The 980 nm laser was vertically focused on the core-shell sample. After setting the electronic balance to zero, 0.0490 g of anhydrous ethanol was dropped in the vessel with great care, meanwhile the laser current was adjusted to 1.0 A. The anhydrous ethanol weight, which displayed on the electronic balance screen and changed with laser irradiation time, was recorded by a video camera. Following the analogical procedure, the changes of anhydrous ethanol weight with irradiation time were also recorded when the working current of 980 nm laser was 1.5 and 2.2 A. To exclude the effect caused by the absorption of anhydrous ethanol, the identical experiments were carried out by using the pure anhydrous ethanol without core-shell particles. The time dependent weight changes of anhydrous ethanol on working current of 980 nm laser are shown in Fig. 8. In Fig. 8 the discontinuous symbols draw the experimental data, and the straight lines guide the variation trends.
Fig. 8 Dependence of the weight of anhydrous ethanol on the working currents and irradiation time of 980 nm laser. The symbols present the experimental data, and the straight lines indicate the variation trend.
It can be found from Fig. 8 that the weight of pure anhydrous ethanol decreases with irradiation time slowly and the influence of laser power on the weight change is unobvious in comparison with that when the core-shell particles are introduced. These results indicate the photothermal conversion of the core-shell particles can also be achieved under 980 nm irradiation.
4. Conclusions
In summary, the designed bifunctional NaYF4:Er3+/Yb3+@NaYF4:Tm3+/Yb3+ core-shell nanoparticles were successfully synthesized by a thermolysis reaction. XRD patterns and SEM/TEM images proved the β phased crystal structure and nanosized microscopic morphology of the obtained product. Based on the fluorescence intensity ratio technique, the optical temperature sensing behavior was demonstrated and employed in characterizing photothermal conversion under 808 and 980 nm co-excitation. Moreover, anhydrous ethanol evaporation experiment exhibited the photothermal conversion of the core-shell nanoparticles under single 980 nm excitation.
Funding
National Natural Science Foundation of China (grant Nos. 11774042 and 11704056); Fundamental Research Funds for the Central Universities (Grant No. 3132016333); China Postdoctoral Science Foundation (Grant No. 2016M591420); Teacher Development Project of Dalian Maritime University (Grant Nos.2017JFZ04); Natural Science Foundation of Zhejiang Province (Grant No. LZ17E020001); the Open Fund of the State Key Laboratory on Integrated Optoelectronics (IOSKL2015KF27);
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