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Abstract
X-ray combined with short-wavelength infrared (SWIR) phosphors, that can make full advantage of the deeper tissue penetration of SWIR light, can be used as a fluorescent probe to realize biological imaging of deep tissues; they are, however, limited by their lower luminescence efficiency. Here, we describe a strategy to synthesize highly efficient SWIR luminescence phosphor based on the efficient energy transfer process between charge transfer state (CTS) of Yb3+ and the 6IJ levels of Gd3+ as well as Gd3+-Gd3+. This allows us to achieve 813.8 mW/m2 of SWIR luminescence power in Yb3+-BaGd0.6Y0.4ZnO5 (BGYZ). Our results highlight that this approach to enhance SWIR luminescence may provide new opportunities for the deep-tissue biological imaging.
© 2019 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Photoluminescence (PL)-based biomedical imaging as a non-contact biomedical imaging method, owing to the advantages of the fast response, high sensitivity and resolution, has received great interest for in vivo imaging in the past few years [1,2]. However, the biomedical imaging technology reported before was limited by the lower tissue penetration depth of the traditional light, unable to realize deeper tissue imaging [3–7]. The reason that hinders the propagation of light in the biological tissues comes from the absorption and scattering of light by the biological tissue. As the biological tissues are partially transparent to radiation wavelengths incident on them, well-known biological windows have been proposed, such as NIR-I (650-950 nm) and NIR-II (1000-1700 nm) [8,9]. Short-wavelength infrared (SWIR, 900-1700 nm) light correspond to the “second near infrared biological optical transmission window”, has smaller tissue scattering coefficient so that it has higher penetration depth and imaging resolution compared with the first biological window light [9–11]. Furthermore, SWIR imaging has a higher signal-to-noise ratio than visible imaging in living tissue because its low photon energy unable to stimulate auto-fluorescence in the tissue [9]. Considering that the SWIR luminescence is emitted from a lower-levels radiation transition where the multi-phonon non-radiative relaxation rate is large, so it is difficult to achieve through up-conversion process (Anti-stokes process). Therefore, PL-based imaging using SWIR light generally adopts a down-conversion process (Stokes process), producing SWIR luminescence by using higher energy light as an excitation source such as the near infrared light, visible light or ultraviolet light [12–14]. However, the tissue penetration depth of these lights is much lower than that of SWIR light, which causes the failure to excite the sample at the depth where the SWIR light can penetrate [15,16]. In that case, a high penetrability excitation source, which can excite SWIR luminescence probes in the deeper tissues, is the priority in the development of deep-tissue PL-based biomedical imaging technology.
X-ray, owing to its superiority of unlimited penetration depth in biological tissues, has widely applied in biomedical imaging science such as computed tomography, chest X-ray and cardiac angiography etc [17–19]. Scientists often refer to fluorescent materials, which can convert high-energy rays into efficiently detectable light, as scintillators which are commonly used to detect X-ray and other high-energy radiation [20–22]. However, the most reported scintillators are visible emitting materials, infrared emitting scintillators are less studied [17,22,23]. Recently, X-ray excited phosphors has already been used in optical imaging in several reports [4,7,10,24]. Undoubtedly, X-ray combined with SWIR scintillator can be used as a fluorescent probe to realize imaging of deep tissues, which has a good application prospect in the field of biomedical imaging. These good properties can only be achieved with efficient fluorescence emission. Therefore, the design and fabrication of X-ray excited efficient SWIR phosphors is required.
Yb3+ ion has received considerable attention among trivalent rare earth (RE) ions for its efficient SWIR luminescence since the lack of intermediate levels from its unique f-layer electronic arrangement reduce the probability of a non-radiative decay [25,9]. However, this limited energy levels structure also limit the choice of excitation wavelength and is difficult to be effectively excited by high energy light such as X-rays. Although most of RE ions can sensitize Yb3+, these RE ions have many intermediate levels, resulting in only part of the energy being transferred to Yb3+ [26–30]. Gd3+ ion has less energy levels and can effectively absorb X-ray photons. But the minimum transition energy of Gd3+ from 6P7/2→8S7/2 is 3 times that of Yb3+ from 2F5/2→2F7/2, no matching energy levels between Gd3+ and Yb3+. Here, we propose a strategy to synthesize highly efficient SWIR luminescence phosphor based on the efficient energy transfer process between charge transfer state (CTS) of Yb3+ and the 6IJ levels of Gd3+ as well as Gd3+-Gd3+. A novel X-ray excited phosphor with efficient SWIR luminescence is achieved and the potential for deep tissue biological imaging applications is demonstrated.
2. Materials and methods
2.1 Synthesis and characterization of BaGdxY1-xZnO5:Yb3+
Yb3+, Gd3+ and Y3+ doped BaGd2ZnO5 phosphors are prepared by solid state method. Stoichiometric amount of BaCO3 (AR), ZnO (AR), Gd2O3 (4N), Y2O3 (4N), Yb2O3 (4N) are mixed in mortar by slurrying in ethanol adequately. After drying in the air, the mixtures are powdered in mortar and fully ground, subsequently, heated in muffle stove at 500 °C for 1 h and 1400 °C for 3 h. At last, the resulting samples are taking out and reground, preparing for spectral testing.
The excitation and emission spectra of these samples are recorded by fluorescence spectrometer (Fluorolog-3, HORIBA Jobin-Yvon). All measurements are performed at room temperature. Scanning electron microscope (SEM) (Nova NanoSEM450, FEI) and Energy dispersive spectroscopy (EDS) are used to character the morphology and composition of the sample.
2.2 X-ray excited SWIR luminescence spectra and the luminescence power experimental scheme
The experimental scheme of X-ray luminescence test is shown in Fig. 1(a). The phosphors are irradiated using a benchtop X-ray source (JF-2000, Dandong Aolong Radiographic Instrument Group Co., Ltd.) with 50 kV, 30 mA tungsten target. The light signal is modulated by a chopper (SR540, Stanford research systems, Inc.) and detected by a spectrometer (SR-500i, Andor Technology Co.) equipped a thermoelectric cooling (at −40 °C) single point InGaAs (800-1900 nm) detector (ACC-SR-ASM-0044, Andor technology Co.). The output electronic signal is transported to a phase lock-in amplifier (Model SR810 DSP, Stanford research systems, Inc.) for signal amplification, then converted to a digital signal by a digital-to-analog converter and output through a computer.
Fig. 1 Experimental scheme of (a) X-ray irradiated SWIR emission spectra (b) the luminescence powers (c) Transmission parameter curve of filter in light path.
The Fig. 1(b) shows the test scheme for SWIR luminescence powers under X-ray excitation. Simple process is that SWIR emission passes a filter (FEL0900, Thorlabs, Detailed parameters in Fig. 1(c)) and is received by the laser power meter (Proto-Si, Gentec-EO, Square sensor, 10 mm × 10 mm). In the experiment we have designed, the emission light of the sample can be expressed as a light-emitting point. Therefore, the luminescence power per unit area of the sample can be expressed as Eq. (1):
where S1 = 2πd2, d is the distance between sensor and sample, S2 = l2, l is the side length of sensor, S3 = πr2, r is the radius of sample cell, η is the average transmission rate of the filter, x is the value shown by laser power meter.3. Results and discussion
3.1 Structure and morphology of BGYZ: Yb3+
The powder X-ray diffraction patterns result of BaGd1.2Y0.8ZnO5 (BGYZ):3% Yb3+ are displayed in Fig. 2(a), which agrees well with the standard orthorhombic phase BaGd2ZnO5 (ICSD-88602) as shown in Fig. 2(b) and indicates that Y3+, Yb3+ are successfully incorporated in the host matrix lattice and occupy the lattice site of Gd3+ due to the same valence state of Gd3+, Y3+ and Yb3+. The main elements of samples are accorded with the chemical formula as demonstrated in Fig. 2(c). The SEM images shows that synthesis sample is of a few microns’ shapes (Fig. 2(d)).
Fig. 2 (a) The XRD patterns of BaGd1.2Y0.8ZnO5(BGYZ):3% Yb3+ and standard ICSD card. (b) The crystal structure of BaGd2ZnO5. (c) The EDS of BGYZ. (d) SEM imaging of the as-synthesized BGYZ.
3.2 Spectral properties of Yb3+ doped BGYZ phosphor
The Fig. 3(a) shows the excitation and emission spectra of Yb3+ doped BaGd2ZnO5. Two distinct excitation peaks can be seen, one is a narrow band located at 273 nm and another is a broadband located at 261 nm. The emission spectra have only one emission band located at 979 nm, which is derived from the typical 2F5/2→2F7/2 transition of Yb3+ ions. The optimal doping concentration of Yb3+ ions is 3%. Figure 3(b) shows the excitation and emission spectra of different Y3+ and Gd3+ doping concentrations. The optimal doping concentration is confirmed as 40% Y3+, 60% Gd3+. The excitation spectra consist of two peaks at 273 nm and 261 nm, respectively in the phosphors Y3+, Gd3+ co-doped BGYZ phosphors, which agrees well with that in Yb3+ doped BaGd2ZnO5. It is worth mentioning that only one broad peak at 261 nm can be seen when the Gd3+ ion is absent. The above data allows us to identify the excitation peak of 273 nm coming from the 8S7/2→6IJ absorption of Gd3+, the peak of 261 nm is attributed to the O2--Yb3+ charge transfer state (CTS) absorption. Due to the unique f-layer electronic arrangement of Gd3+ and Yb3+, their energy levels are more limited than those of other RE ions and no matching energy levels between Gd3+ and Yb3+. The most reasonable explanation is that the Gd3+ ion transfers the absorbed energy to the CTS of Yb3+, eventually producing the 2F5/2→2F7/2 emission of Yb3+ ions. In addition, the concentration of Gd3+ ions are not as high as possible. If the concentration of Gd3+ ions is too high, it will cause concentration quenching and reduce the energy transfer from Gd3+ to Yb3+. The effect is that the fluorescence emission of Yb3+ ions is weakened. The Figs. 3(c) and 3(d) show the luminescence spectra of the above samples under X-ray excitation and strong SWIR luminescence at 979 nm from the 2F5/2→2F7/2 transition of Yb3+ ions can be seen, and the optimal doping concentration of RE ions is the same as that obtained for xenon lamps.
Fig. 3 Excitation and emission spectra of (a) BaGd2ZnO5: x% Yb3+ (x = 0.5, 1, 2, 3, 4, 5) and (b) BaGdxY2-xZnO5: 3% Yb3+ (x = 0, 0.4, 0.8, 1.2, 1.6, 2), the broken lines show the integral intensity of luminescence from 2F5/2→2F7/2 transition of Yb3+ ions. X-ray excited luminescence spectra of (c) BaGd2ZnO5: x% Yb3+ (x = 0.5, 1, 2, 3, 4, 5) and (d) BaGdxY2-xZnO5: 3% Yb3+ (x = 0, 0.4, 0.8, 1.2, 1.6, 2), the inside histograms show the integral intensity of luminescence from 2F5/2→2F7/2 transition of Yb3+ ions.
3.3 SWIR luminescence power of Yb3+, Gd3+ co-doped phosphors
According to the above discussion, the 2F5/2→2F7/2 emission of Yb3+ ions comes from the energy transfer process between the 6IJ levels of Gd3+ ions and the CTS of Yb3+ ions. Therefore, in order to generate a highly efficient SWIR emission of Yb3+ ions, it is required that the CTS of Yb3+ ions overlaps with the 6IJ levels of Gd3+ ions. Considering that the main dependent factors on CTS of RE ions are the coordination number of elements, the covalentity of RE ions and ligands; the polarizability of chemical bond volume and the charge of the ligand in the chemical bond, it is therefore possible to predict CTS energy through a comprehensive analysis of these most factors [31]. Until now, the CTS energies of Yb3+ ions in lots of phosphors have been reported, which allows us to select representative materials for verification [32–38].
To validate our hypothesis, Fig. 4(a) shows the excitation and emission spectra of Yb3+ ions in representative materials. It can be seen that Gd(PO3)3 and GdPO4 can hardly produce Yb3+ emission. As we expected, the CTS energy of Yb3+ ions in these matrices are high, no absorption peak of the CTS of Yb3+ ions in the excitation spectra (Fig. 4(a)) [33–35]. Yb3+ doped NaGdF4 can produce the Yb3+ emission, however, its emission intensity is very weak. In the excitation spectra, there are two absorption peaks located at 310 nm and 273 nm from the 8S7/2→6P7/2 and the 8S7/2→6IJ transitions of Gd3+ respectively, no absorption peak of the CTS of Yb3+ ions can be seen. Considering that the higher coulomb interaction between two nearby-located Yb3+ ions leads to the multiple Yb3+ clusters, the most reasonable explanation is that the Gd3+ ion transfers the absorbed energy to the excited state levels of Yb3+ through the quantum cutting process [39–43]. However, a high-energy photon of Gd3+ ion change into three lower-energy photons of Yb3+ ions, the absorption cross section is very small, resulting in lower emission efficiency. In the excitation spectra in BGYZ and Gd2O2S, Yb3+ has a strong emission whose CTS energy matches the 6IJ levels of Gd3+ ions. And the excitation spectra contain the absorption peak of Gd3+ ions and CTS absorption peak of Yb3+ ions as shown in Fig. 4(a) [32,35]. Although the CTS of Yb3+ doped Gd2O2S has a broader absorption band than that in BGYZ, the emission of Yb3+ ions in BGYZ is the strongest one under X-ray excitation (Fig. 4(b)), which indicates that the sensitization of Gd3+ to Yb3+ is the main process under X-ray excitation. The luminescence intensity of the X-ray scintillators has a linear relationship with the intensity of the X-ray, that is, the stronger the intensity of the X-ray, the stronger the luminescence intensity. The linear relationship between the luminescence intensity under X-ray and the currents of X-rays is as shown Fig. 4(c). We tested the emission power of these samples under same X-ray (50 kV, 25 mA), the power of Yb3+ doped BGYZ reaches 813.8 mW/m2 (Figs. 4(c) and 4(d)).
Fig. 4 (a) Excitation and emission spectra of Yb3+ ions in different kinds of materials. (b) The luminescence spectra of Yb3+-doped different kinds of materials under X-ray excited. (c) The luminescence power of BGYZ under different X-ray controlling currents. (d)The luminescence power of these materials under same measurement environment.
3.4 Mechanism of SWIR luminescence emission of Yb3+ doped BGYZ phosphor
To further demonstrate the energy transfer process of Yb3+ and Gd3+, we give the emission spectra of all samples with a wavelength range of 200nm to 400nm excited by X-rays (Fig. 5(a)). It is found that Yb3+ doped Gd(PO3)3 and GdPO4 phosphors, in which Yb3+ emission is very weak under X-ray excitation, have strong Gd3+ emission, while Yb3+ doped BGYZ and Gd2O2S phosphors, in which the emission of Yb3+ ions are very strong, have almost no emission of Gd3+ ions. In Yb3+-missing BGYZ, the Gd3+ luminescence is higher than that in Yb3+ doped BGYZ (Inset of Fig. 5(a)). We test the lifetime of Gd3+ in these materials and find that Gd3+ has a longer lifetime in Gd(PO3)3, GdPO4 and NaGdF4 hosts and shorter lifetime in BaGd2ZnO5 and Gd2O2S hosts (Figs. 5(b) and 5(c)). These data show that Gd3+ transfers energy to Yb3+ and leads to a decrease in the lifetime of Gd3+ and an increase in the emission of Yb3+ in BaGd2ZnO5 and Gd2O2S. Because of the smaller concentration of Yb3+, only the Gd3+-Yb3+ energy transfer process through the neighbor is insufficient to produce a strong emission. Therefore, in addition to the efficient transmission of Gd3+-Yb3+ in the neighborhood, there is a need for a more efficient energy transfer process between neighboring Gd3+-Gd3+. According to Förster-Dexter theory, the rate of energy transfer is inversely proportional to the nth power of the distance (n = 6, 8, 10) when the distance between Gd3+-Gd3+ is not very close [44,45]. We can approximately calculate the Gd3+-Gd3+ distance by Eq. (2) [46–51]:
where V is the volume of the unit cell, X is the concentration, and N is the number of available crystallographic sites occupied by the activator ions in the unit cell. From Table 1, we can see that the R in GdPO4, NaGdF4 and Gd(PO3)3 is larger than that in BaGd2ZnO5 and Gd2O2S. which is an evidence of an efficient energy transfer process from Gd3+ to Gd3+ in BaGd2ZnO5 and Gd2O2S.Fig. 5 (a) The X-ray luminescence spectra of Gd3+ ions in different substrates. The inset shows X-ray luminescence spectra of Gd3+ ions in Yb3+-missing BGYZ and Yb3+ doped BGYZ. (b) Logarithmic scale decay curves of Gd3+ in different hosts. The decay curves of Gd3+ in BaGd2ZnO5 and Gd2O2S is too short to be given. (c) The decay curves as well as the fitting lines of BGYZ: Yb3+ and Gd2O2S: Yb3+. (d) Three luminescence mechanism exist in Yb3+ doped BGYZ under X-ray excitation. (1) Directly excite the electrons to the CTS of Yb3+. (2) Energy transfer process from Gd3+ 6IJ levels to CTS of Yb3+. (3) Quantum cutting process change a Gd3+ photons into three Yb3+ photons.
Table 1. Related crystal parameters (V, Z, and N) and the corresponding Gd3+-Gd3+ distances (R) for Gd(PO3)3, GdPO4, NaGdF4, BaGd2ZnO5 and Gd2O2S
To further demonstrate the existence of an energy transfer process between Gd3+-Yb3+ as well as Gd3+-Gd3+, we have tested the lifetime of Gd3+ in Yb3+ doped in BGYZ and Gd2O2S. If there is a migration process between donors, it will be reflected in the life curve. Based on the work related to his predecessors, Martín have proposed the following life formula (Eq. (3)) based on the energy transfer process between donors [52,53]:
where τ is the lifetime of the 8S7/2 energy levels of Gd3+, S = 6, 8, 10 depending on the interaction character (dipole-dipole, dipole-quadrupole, quadrupole-quadrupole), CA is the Yb3+ concentration, Γ(y) is the gamma function, CDA is the donor-acceptor energy transfer parameter, and D is the diffusion parameter which characterizes the energy transfer between donors. a1, a2 and b1 are the parameters related to S. Figure 5(c) shows that the lifetime curves of Gd3+ ions in Yb3+ doped BGYZ and Gd2O2S can be well fitted by the formula, which proves the energy transfer process of Gd3+ to Yb3+ again and gives dipole-dipole interaction between Gd3+ and Yb3+ (Detail parameters see Table 2).Table 2. The parameters of BGYZ and Gd2O2S lifetime fitting curve
The above results allow us to give the following luminescence mechanism for Yb3+ in BGYZ under X-ray excitation. The simple process is as follows: high-energy X-ray photons are absorbed by electrons in the inner layer of a high atomic number to produce Auger electrons and secondary electrons. These electrons constantly collide with other electrons, producing more secondary electrons until their energies are below the energy gap. A part of the electrons is absorbed by the O2--Yb3+ CTS, and the energy is transferred to the f-shell of Yb3+ to generate the typical 2F5/2→2F7/2 transition of Yb3+ ions. Another part of the electrons is absorbed by the f-layer electrons of the Gd3+ ion, and the energy is transferred to the O2--Yb3+ CTS through the energy transfer process, and finally the 2F5/2→2F7/2 transition of Yb3+ ions is generated. Considering that the concentration of Gd3+ is much higher than the concentration of Yb3+, the latter is the main process. In addition, we cannot rule out that Gd3+-Yb3+ have a quantum cutting process in the BGYZ similar to NaGdF4 (Fig. 5(d)). However, we believe that it is secondary to compared with the energy transfer process. It is no doubt that Yb3+-doped BGYZ phosphor is not necessarily the most efficient SWIR phosphors, but this mechanism allows us to find a way to obtain high-efficiency SWIR fluorescent materials.
3.5 SWIR luminescence imaging of X-ray excited Yb3+ doped BGYZ phosphor in vivo
In order to prove that Yb3+ doped BGYZ phosphors can be used as fluorescent probes for living organisms, we have designed an in vivo experiment in mice and the simple experiment instrument is shown in Fig. 6(a). Yb3+ doped BGYZ phosphors dissolved in normal saline are injected into the skin and the stomach of mouse. The mouse is fixed on the platform with a horizontal angle of 45 degrees, an EMCCD infrared camera (DU-888U3-CS0-BV, Andor Technology Co.) directly above it and an X-ray source (50 kV, 30 mA) directly in front of it, the angle between the camera and X-ray source is a right angle. The Fig. 6(b) shows quantum efficiency of EMCCD infrared camera, which is less than 15% at 980 nm. Although the camera is very inefficient, the SWIR luminescence imaging can be clearly seen as shown in Fig. 6(c). Since X-rays have certain hazards to biological tissues, the total radiation dose should be monitored during the relevant experiments for in vivo imaging. On radiation safety aspect, depending on the scan mode, the commonly used micro-CT system currently produces a total radiation dose of about 10 cGy for rodents such as mice, while the SPECT and PET systems output whole body dose no more than 100 cGy [10,54,55]. We give the relationship between the output power of the instrument and the amount of radiation, as shown in Fig. 6(d). According to the data, we can calculate that the amount of radiation during imaging time is about 42.5 cGy, which is much lower than the LD50/30 dose (≈7 Gy) to mice [56,57].
Fig. 6 (a) Schematic diagram of in vivo luminescence experiment. (b) Quantum efficiency curve of EMCCD camera. (c) X-ray excited SWIR luminescence imaging of mice injected phosphors into skin (1) and stomach (2), the exposure time for imaging is 5 seconds. (d) The absorbed dose of X-ray irradiation versus different controlling power.
4. Conclusion
Due to the unique f-layer electronic arrangement of Gd3+ and Yb3+, their energy levels are more limited than those of other RE ions and no matching energy levels between Gd3+ and Yb3+. In order for Gd3+ to effectively transfer the absorbed energy to Yb3+, it is most desirable that the energy levels of Gd3+ matches the CTS energy of Yb3+, and the f-layer transition of Yb3+ is realized by CTS energy transfer. In addition, in order to obtain a higher infrared emission efficiency of Yb3+, not only Gd3+-Yb3+ is required to have a high transfer efficiency, but also a small atomic pitch of Gd3+-Gd3+ is required. Based on the above factors, we have designed an efficient X-ray excited SWIR phosphor: Yb3+- BGYZ which can be used as a fluorescent probe to realize imaging of deep tissues in the field of biomedical imaging.
Funding
National Natural Science Foundation of China (11474083); Natural Science Foundation of Hebei province (A2015201192); Department of Education of Hebei Province (ZD2014069).
Acknowledgment
We gratefully thank College of Science and Technology, Hebei University for providing measuring and test instruments. We also express our gratitude to M. Ran who is serving on Opcrown Company for providing Proto-Si power meter.
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