794 nm excited core–shell upconversion nanoparticles for optical temperature sensing

Abstract

Hexagonal core–shell NaYF₄ upconversion nanoparticles (UNCPs) based on Nd³⁺ sensitization for optical temperature sensing were successfully synthesized by a solvothermal method using oleic acid and octadecene as coordinating solvents. Compared to the conventional Yb³⁺ sensitized UNCPs, the usage of Nd³⁺ as the sensitizers can shift the excitation wavelength from 975 nm to 794 nm where the optical absorption of water decreased dramatically, and thus make UNCPs more suitable for biological application. The upconversion (UC) luminescence intensity of the 794 nm-excitation UNCPs is comparable to that of the conventional 975 nm excitation, showing that Nd³⁺ sensitized UNCPs are efficient. The efficiently successive Nd³⁺ → Yb³⁺ → Er³⁺ energy transfer processes in this UNCP were demonstrated by excitation spectra and time-resolved spectra. The temperature dependence of the fluorescence intensity ratios (FIR) for the two green emissions (525 nm and 545 nm) from the thermally coupled levels of Er³⁺ was studied in the temperature range from 25 to 60 °C under 808 nm excitation, and the temperature mapping of a device was acquired according to this technique. These indicate that Nd³⁺ sensitized core–shell UNCPs are promising candidates for application in optical temperature sensors.

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1 Introduction

In the past few years, lanthanide doped upconversion nanoparticles (UCNPs) have been extensively investigated because of their potential applications in security labels, LEDs, photovoltaics, lasers, biological imaging,^1–4 biosensing,^5–8 and photodynamic therapy.^9–12 Upconversion is capable of emitting ultraviolet, visible and near-infrared (NIR) light when excited with an NIR light source with low-energy photons. As well known, UCNPs are usually co-doped with Yb³⁺ as sensitizer and Er³⁺ (Tm³⁺, Ho³⁺) as activators.^13–19 Under 975 nm excitation, Yb³⁺ ions absorb NIR photons and excited to the ²F_5/2 state, then transfer the energy to Er³⁺ (Tm³⁺, Ho³⁺) ions and emit various ultraviolet, visible and NIR lights. UCNPs have many advantages compared to current biological labels such as fluorescent proteins,²⁰ organic dyes,²¹ and quantum dots (QDs)²² because of their very low phototoxicity, high photochemical stability, and minimal autofluorescence background. They also exhibit narrow emission spectra, long luminescence lifetimes, and large anti-Stokes shifts. Moreover, the emission can be tuned by varying the lanthanide dopants and their doping concentration in the matrix.^23,24 However, the 975 nm light can be strongly absorbed by water,^25,26 which is the most significant component of cell, animal, and human body. In addition, the absorbed light energy would be transformed into heat, which often causes overheating issues and results in serious cell death and tissue damages. Therefore, to make UCNPs more suitable for biological applications, it is essential to change the excitation source other than 980 nm light source within the biological window.

Recently, it was found that 800 nm NIR laser as excitation source can minimize the overheating problem and improve the penetration depth for biological applications because of low absorption coefficients by water and biological tissues at 800 nm.²⁵ Nevertheless, the conventional UNCPs, which are simultaneously doped with sensitizers (Yb³⁺) and activators (Er³⁺, Tm³⁺, or Ho³⁺), could not be excited by 800 nm light. To overcome this problem, it is necessary to combine conventional UNCPs with another sensitizer with a large absorption cross-section at 800 nm. Luckily, Nd³⁺ ion has such unique properties. Nd³⁺ doped UNCPs can realize upconversion emission with excitation at 800 nm,^27–33 due to a sharp absorption band of Nd³⁺ at around 800 nm and the efficient energy transfer from Nd³⁺ to Yb³⁺.^34,35 However, Nd³⁺ ion as sensitizer has serious quenching effect caused by energy back-transfer from activators to sensitizers and only allows to be doped at very low concentrations (less than 2%), which leads to weak absorption at 800 nm and thus weak upconversion emissions. To resolve this issue, spatially separating the activator and the sensitizer (Nd³⁺) is necessary. Therefore, a core–shell structure was designed to separate the activator Er³⁺ and the sensitizer Nd³⁺ into core and shell layers, respectively, which can suppress energy back-transfer from Er³⁺ to Nd³⁺ and increase sensitizer (Nd³⁺) concentration for better absorbing 800 nm photons. In addition, Er³⁺ ion, which owns two thermally coupled ²H_11/2 and ⁴S_3/2 levels, can be used for temperature sensing based on the fluorescence intensity ratio (FIR) technique.^36–42 Here, we report a type of NaYF₄:Yb³⁺/Er³⁺@NaYF₄:Yb³⁺/Nd³⁺ active-core/active-shell UNCP for optical temperature sensing, which can be effectively excited at 794 nm. The UC emission mechanisms were studied, and sequential Nd³⁺ → Yb³⁺ → Er³⁺ energy transfer processes were proved through excitation spectra and luminescence decay curves. To show the feasibility of these Nd³⁺ sensitized core–shell UNCPs, we demonstrate the temperature mapping of a heated glass plate, which is coated by the UNCPs.

2 Experimental

2.1 Materials

LnCl₃·6H₂O (Ln = Y, Yb, Er and Nd; 99.9%, Aldrich), oleic acid (OA; 90%, Alfa), 1-octadecene (ODE; 90%, Aldrich), cyclohexane, methanol, NH₄F (99.9%), and NaOH (≥97%) were used as purchased without further purification.

2.2 Synthesis of NaYF₄:18 mol% Yb³⁺, 2 mol% Er³⁺ core UCNPs

The synthesis of core NPs is similar to the previously reported protocol with some minor modifications.^3,43 YCl₃·6H₂O (1.6 mmol), YbCl₃·6H₂O (0.36 mmol), and ErCl₃·6H₂O (0.04 mmol) were mixed in a 100 mL three-neck round-bottom flask with 12 mL oleic acid and 30 mL octadecene. This mixture was heated to 125 °C and maintained for 40 minutes with vigorous stirring under vacuum to obtain an optically clear solution. When the mixture was cooled down to room temperature, a methanol (∼20 mL) solution containing NaOH (0.2 g, 5 mmol) and NH₄F (296 mg, 8 mmol) was added dropwise into the three-neck flask. The mixed solution was stirred for 40 minutes at room temperature followed by slow evaporation of methanol. Once methanol was completely evaporated, the reaction mixture was rapidly heated to 300 °C and maintained at that temperature for 1 hour under argon atmosphere. After the solution was cooled down to room temperature, the product was mixed with some amount of anhydrous ethanol, ensued by centrifugation to obtain the core NPs. The obtained core NPs were washed with ethanol for three times and then dispersed in 20 mL cyclohexane.

2.3 Synthesis of NaYF₄:18 mol% Yb³⁺, 2 mol% Er³⁺@NaYF₄:10 mol% Yb³⁺, 10 mol% Nd³⁺ core–shell UCNPs

The procedure of core–shell synthesis is also very similar to previously reported procedure.³ YCl₃·6H₂O (0.4 mmol), YbCl₃·6H₂O (0.05 mmol), and NdCl₃·6H₂O (0.05 mmol) were added to a 100 mL three-neck round-bottom flask containing 3 mL oleic acid and 7.5 mL octadecene. The solution was then heated to 125 °C under vacuum with vigorous stirring for 20 min. When the temperature was cooled down to 80 °C under a steady flow of argon, a dispersion (5 mL) containing NaYF₄:Yb³⁺/Er³⁺ core NPs in cyclohexane was added. After evaporating the cyclohexane, the solution was brought down to room temperature, after which 5 mL of methanol solution containing NaOH (1.25 mmol) and NH₄F (2 mmol) was added dropwise into the mixture. The cloudy solution obtained was stirred for 30 min at room temperature. The mixture was then slowly heated to evaporate the methanol. After the methanol was removed, the solution was rapidly heated to 300 °C under an argon atmosphere and kept at this temperature for 80 min. After cooling back to room temperature, the core–shell UCNPs were separated from the reaction mixture by precipitating them in anhydrous ethanol, washed with ethanol thrice and then dispersed in 5 mL cyclohexane.

2.4 Characterization

The X-ray diffraction (XRD) patterns of the samples were recorded with an X-ray diffractometer (MAC Science Co. Ltd MXP18AHF) by using nickel-filtered Cu K_α radiation (λ = 0.15418 nm). The morphology and size of the nanoparticles were obtained using a JEOL JEM-2010 high resolution transmission electron microscope (HRTEM). The dilute dispersion of nanoparticles in cyclohexane was drop-cast on the carbon-coated copper grid and dried in air at room temperature. The average size of the UNCPs was acquired by measuring a group of 100 particles from the image. The absorption spectra were recorded on a SHIMADZU SolidSpec-3700 spectrophotometer. For the excitation and emission spectra measurements, a tune laser system (Opolette 355 LD OPO system) was used as the excitation source with the wavelength tuning range of 410–2200 nm, the spectral linewidth of 4–7 cm⁻¹, the pulse duration of 7 ns, and the repetition rate of 20 Hz. In addition, an 808 nm diode laser was used as the excitation source for temperature sensing experiments. The upconversion emission spectra were obtained using a Jobin-Yvon HRD1 double monochromator equipped with a Hamamatsu R928 photomultiplier. The signal was analyzed by an EG&G 7265 DSP lock-in amplifier and stored into computer memories. The decay curve was measured with a Tektronix TDS2024 digital storage oscilloscope. Temperature of the sample (in a 1 cm path length quartz cuvette) merged into water was controlled over the range of 25 to 60 °C by an IKA RET basic safety control hot plate stirrer with an ETS-D4 electronic contact thermometer. For thermal field mapping, a glass plate coated by UNCPs was heated by a small ceramic wafer equipped with a built-in wire. The method of mapping thermal field was as followed: upconversion spectra of serial space points on the glass were first measured. Following the fluorescent intensity ratios I₅₂₅/I₅₄₅ of these upconversion spectra were calculated. After that, the temperature of space points can be acquired acceding to the linear dependence of ln(I₅₂₅/I₅₄₅) and the inverse of absolute temperature. All emission curves obtained were corrected for the detector sensitivity.

3 Results and discussion

3.1 Structure and morphology of UCNPs

The as-obtained UCNPs were characterized using various techniques, including XRD, TEM, and HRTEM. The XRD patterns of NaYF₄ UCNPs (Fig. 1(a)) match very well with the standard data of β-NaYF₄ (JCPDS no. 16-0334), and the absence of impurity peaks imply that we have synthesized high purity hexagonal phase NaYF₄ nanoparticles. The TEM images show that the particles are of uniform size and shape with the size of the Yb³⁺ and Er³⁺ doped core UCNPs being 28 ± 1 nm and the core–shell UCNPs triply doped with Yb³⁺, Er³⁺, and Nd³⁺ ions being 35 ± 4 nm (Fig. 1(b and c)), implying a shell thickness of about 7 nm. The high-resolution TEM (HRTEM) images (Fig. 1(d and e)) show that the core–shell UCNPs have similar lattice fringes with that of the core UCNPs, so that all the core–shell UCNPs are highly crystallized and maintain the same hexagonal crystal structure as the core UCNPs, which is also confirmed by the results of XRD. Moreover, a typical d-spacing around 0.52 nm is observed in the HRTEM image, corresponding to the (100) plane of β-NaYF₄.

Fig. 1 (a) XRD patterns, (b and c) TEM images, and (d and e) HRTEM images of core NaYF₄:Yb³⁺/Er³⁺ and core/shell NaYF₄:Yb³⁺/Er³⁺@NaYF₄:Yb³⁺/Nd³⁺ UNCPs.

3.2 Optical property of UCNPs

It is well known that overheating issue is a barrier for using 975 nm excited UNCPs in biological applications, which may cause damages to the biological tissues. Nd³⁺ ion has multiple NIR excitation bands, such as 740, 794 and 864 nm. All of these wavelengths for water absorption are lower than that of 975 nm, and the typical absorption coefficient is 0.039 at 794 nm, in contrast to 0.636 at 975 nm (Fig. S1†). Moreover, it is reported that the absorption cross section for Nd³⁺ ion at 794 nm is about 10 times higher than that of Yb³⁺ ion at 975 nm.^28,29 Therefore, we introduce the Nd³⁺ ion to sensitize the conventional 975 nm-excitation UNCPs for minimizing heating effect.

The upconversion luminescence properties of core NaYF₄:Yb³⁺/Er³⁺ and core–shell NaYF₄:Yb³⁺/Er³⁺@NaYF₄:Yb³⁺/Nd³⁺ UNCPs were obtained under 975 nm and 793.5 nm excitations with the same power density (0.2 W cm⁻²), as shown in Fig. 2. The emission peaks at 520, 540, and 653 nm come from intra 4f transitions of Er³⁺ ions involving ²H_11/2 → ⁴I_15/2, ⁴S_3/2 → ⁴I_15/2, and ⁴F_9/2 → ⁴I_15/2 transitions, respectively. Upon the 975 nm excitation, the UC intensity for core–shell UNCPs is about 2 times higher than that of core UNCPs, which is attributed to the shell shielding the dopants in the core from quenching, arising from ligands and solvents in the surface of UNCPs.^44–46 This mechanism is further verified by the decay curves in Fig. S2.† The decay lifetime of the green emission for the core–shell UNCPs (235 μs) is longer than that of the core UNCPs (116 μs). In addition, the UC intensity of the 793.5 nm-excitation core–shell UNCPs is comparable to that of the conventional 975 nm-excitation core–shell UNCPs (Fig. 2), which indicates that Nd³⁺ sensitized UNCPs are highly efficient. We also measured the power-dependent upconversion emission intensities under 793.5 nm and 975 nm excitations. As shown in Fig. S3,† two slopes both close to 2 were acquired, revealing both are two-photon UC processes. All those results show that the energy transfer from Nd³⁺ ions to Yb³⁺ ions does not influence the subsequent Yb³⁺ → Er³⁺ UC process.

Fig. 2 Upconversion emission spectra of (a) core NaYF₄:Yb³⁺/Er³⁺ UNCPs under 975 nm excitation, (b and c) core/shell NaYF₄:Yb³⁺/Er³⁺@NaYF₄:Yb³⁺/Nd³⁺ UNCPs under 975 nm and 793.5 nm excitation dispersed in cyclohexane (1 wt%).

To clearly demonstrate the UC process in Nd³⁺ sensitized UNCPs, we measured the excitation spectra (Fig. 3(a)) and the decay curves (Fig. 3(b)) of UNCPs. Compared to the excitation spectrum of core NaYF₄:Yb³⁺/Er³⁺, three new excitation peaks were found in the NIR excitation spectrum of core–shell NaYF₄:Yb³⁺/Er³⁺@NaYF₄:Yb³⁺/Nd³⁺, which locate at 740, 794, and 864 nm due to the transitions of Nd³⁺ from the ground state ⁴I_9/2 to excited states ⁴F_7/2, ⁴F_5/2, and ⁴F_3/2, respectively. This result is consistent with the absorption spectra (Fig. S1†). These three new excitation peaks are also observed in the excitation spectrum of Yb³⁺ (λ_em = 975 nm), which indicates the efficient energy transfer from Nd³⁺ → Yb³⁺. Therefore, the UC process in Nd³⁺ sensitized UNCPs is the energy transfer through Nd³⁺ → Yb³⁺ → Er³⁺. This UC process can be testified by the decay curves of UNCPs. As shown in Fig. 3(b), under 793.5 nm excitation, the emission intensity of Nd³⁺ at 1059 nm (⁴F_3/2 → ⁴I_13/2) first reached its maximum at 28 μs. Following a transfer of energy to Yb³⁺ ion, thus the emission of Yb³⁺ at 975 nm (²F_5/2 → ²F_7/2) was clearly observed. Later, the emission from Yb³⁺ comes to its maximum at 94 μs, along with a decay of Nd³⁺ emission. After that, the emission of Yb³⁺ decreases. Meantime, the emission of Er³⁺ at 540 nm (⁴S_3/2 → ⁴I_15/2) increases, and reaches its maximum at 195 μs. This successive rise and decay of transition of these three dopants (Nd³⁺, Yb³⁺, and Er³⁺) demonstrated sequential Nd³⁺ → Yb³⁺ and Yb³⁺ → Er³⁺ energy transfer process under 793.5 nm excitation in Nd³⁺ sensitized UNCPs.

Fig. 3 (a) Excitation spectra of core NaYF₄:Yb³⁺/Er³⁺ and core/shell NaYF₄:Yb³⁺/Er³⁺@NaYF₄:Yb³⁺/Nd³⁺ UNCPs dispersed in cyclohexane, (b) decay curves of core/shell UCNPs dispersed in cyclohexane under 793.5 nm excitation.

To confirm 794 nm-excited Nd³⁺ sensitized UNCPs have deeper penetration depth for detecting the interior, we measured upconversion spectra of core–shell NaYF₄:Yb³⁺/Er³⁺@NaYF₄:Yb³⁺/Nd³⁺ UNCPs under 793.5 and 975 nm excitations with different penetration depth of water (Fig. 4). After penetrating through 3 cm of water, the 540 nm emission intensities of core–shell NaYF₄:Yb³⁺/Er³⁺@NaYF₄:Yb³⁺/Nd³⁺ UNCPs under 793.5 and 975 nm excitations were absorbed 26% and 89%, respectively, which indicates strong water-penetration ability of 793.5 nm laser. In addition, the use of Nd³⁺ as the sensitizers for 794 nm excitation could minimize overheating in biological application compared to conventional Yb³⁺ sensitization for the corresponding 975 nm excitation, due to lower water and biological tissue absorptions at 794 nm.^25,28–31

Fig. 4 Upconversion spectra of core/shell NaYF₄:Yb³⁺/Er³⁺@NaYF₄:Yb³⁺/Nd³⁺ UNCPs under (a) 793.5 nm and (b) 975 nm excitation with different penetration depth of water. (c) The 540 nm emission intensities of core/shell UNCPs with different penetration depths of water.

3.3 Temperature measurements

Fig. 5(a) presents upconversion green emission spectra of core–shell NaYF₄:Yb³⁺/Er³⁺@NaYF₄:Yb³⁺/Nd³⁺ UCNPs in the temperature range from 25 to 60 °C under 808 nm excitation. The green emission consists of two distinct bands centered at 525 nm (515–534 nm) and 545 nm (534–565 nm), coming from ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions of Er³⁺ ions, respectively. These two excited states (²H_11/2 and ⁴S_3/2) are in close proximity, essentially separated by only a few hundred wavenumbers, leading to a thermal equilibrium governed by the Boltzmann's law. According to the theory by Wade et al.,³⁶ the fluorescence intensity ratio (FIR) for the emissions from the thermally coupled levels of Er³⁺ ions is modified as:where I₅₂₅ and I₅₄₅ are the integrated intensities of the ²H_11/2 → ⁴I_15/2 and ⁴S_3/2 → ⁴I_15/2 transitions, respectively, A is a constant that depends on the degeneracy, spontaneous-emission rate, and photon energies of the emitting states, ΔE is the energy difference between these two thermally coupled levels, k is the Boltzmann constant, and T is the absolute temperature. According this, we can obtain a thermometric scale for nano-thermometers using Nd³⁺ sensitized core–shell UNCPs. Fig. 5(b) shows the dependence of ln(I₅₂₅/I₅₄₅) on the inverse of absolute temperature (1/T). Based on eqn (1), a linear expression of ln(I₅₂₅/I₅₄₅) = 1.79 − (993.2 ± 18.8)/T was acquired. The fitted value of the energy difference between the ²H_11/2 and ⁴S_3/2 levels is about 691 cm⁻¹, which is close to the reported result of about 700 cm⁻¹.^37–39 In addition, this demonstrates that we can obtain the temperature reading in the physiological temperature range by accurately measuring the fluorescent intensity ratio of 525 and 545 nm emissions of Er³⁺ ions. Compared to the conventional Yb³⁺ sensitized UNCPs for optical temperature sensing,³⁹ the use of Nd³⁺ as the sensitizers can shift the excitation wavelength from 975 nm to 794 nm, thus minimize the overheating problem and improve the penetration depth for biological applications.

Fig. 5 (a) Upconversion emission spectra of core/shell NaYF₄:Yb³⁺/Er³⁺@NaYF₄:Yb³⁺/Nd³⁺ UCNPs in the temperature range from 25 to 60 °C under 808 nm excitation. (b) ln(I₅₂₅/I₅₄₅) as a function of inverse absolute temperature to calibrate temperature for core/shell UCNPs (R² = 0.9949).

The temperature mapping of a glass plate heated by a small ceramic wafer equipped with a current built-in wire was displayed in Fig. 6. This temperature mapping was acquired by monitoring the fluorescent intensity ratio I₅₂₅/I₅₄₅ of Er³⁺ ions that have a well-defined temperature dependence. We can see that the temperature distribution in the heated glass plate varied from 30 to 70 °C. The center temperature of the heating point is close to about 77 °C. While the temperature of the area far from the heating point is about 30 °C, which is close to the room temperature. Thus, the ability of the Nd³⁺ sensitized core–shell UNCPs to record thermal imaging of device is substantiated. Utilizing this, we can gain the temperature mapping in multifarious microdevices such as microchip, microsensor, and miniature photoelectric device, thereby improving their features and performance.

Fig. 6 Temperature mapping of a glass plate heated by a small ceramic wafer equipped with a built-in wire.

4 Conclusions

In summary, we have successfully synthesized Nd³⁺ sensitized core–shell NaYF₄:Yb³⁺/Er³⁺@NaYF₄:Yb³⁺/Nd³⁺ UNCPs, which can be used for non-contact optical temperature sensing. The sequential Nd³⁺ → Yb³⁺ and Yb³⁺ → Er³⁺ energy transfer processes in this UNCP were systematically studied by analyzing photoluminescence spectra and time-resolved spectra. As a main advantage, we can use 794 nm NIR laser instead of 975 nm laser to excite Nd³⁺ sensitized core–shell UNCPs, thereby improving the penetration depth and minimizing the overheating effect. Additionally, the dependences of FIR for the emissions from the thermally coupled levels of Er³⁺ ions on temperature were investigated in the range of 25 to 60 °C. As an application example of this UNCP for thermometer, we showed the temperature mapping of a glass plate with nonuniform heating, which can be extended to measure the temperature distinction in the interior of biological tissues.

Acknowledgements

This work was financially supported by the National Key Basic Research Program of China (No. 2013CB632900), the National Natural Science Foundation of China (No. 11274299, 11204292, 11374291, 11404321, 11574298, and 51572056), and the Fundamental Research Funds for the Central Universities and Program for Innovation Research of Science in Harbin Institute of Technology (No. Q201509).

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Footnote

† Electronic supplementary information (ESI) available: Absorption spectra of core nanoparticles, core/shell nanoparticles and water. Decay curves of the core and core/shell nanoparticles. Power dependence curves of core/shell for 540 nm emission under 975 nm and 793.5 nm excitation. See DOI: 10.1039/c5ra27203c

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