The premise that long afterglow can be applied is its duration, and the persistent duration is closely related to the depth of the traps. Therefore, the stable deep traps are the key to obtain long persistent luminescence. Based on this, a strategy that X-ray excites high-gap phosphors to achieve long persistent luminescence is firstly proposed. Herein, rare earth (RE) ions doped YPO4 phosphor is adopted as the research object as RE ions can form stable and deeper defect centers or luminescent centers in high bandgap materials. Furthermore, the efficient method of enhancing persistent luminescence is designed so that introducing Tb3+ ions into YPO4:Sm3+ crystals forms tightly bound excitons, which modulates the depth of defect centers (Sm3+ ions), improving the afterglow behavior from Sm3+ ions for more than two days, which is approximately 14 times stronger than the afterglow of YPO4:Sm3+ phosphors itself. Finally, highly efficient in vivo deep tissue bioimaging was successfully achieved through mouse tail intravenous injection. The results indicate that the YPO4:Sm3+,Tb3+ phosphor possesses great promise in the field of in vivo imaging.
© 2020 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
Persistent luminescence is the phenomenon that the phosphors can still emit light persisting from several seconds to a few days after being excited by excitation source [1,2]. Afterglow materials are generally used for special lighting displays and information storage applications [3,4], other than this, benefit from the ability to circumvent the autofluorescence, red to near-infrared (NIR) persistent luminescence has potential application value in the biological field [5–11]. But so far, the afterglow mechanism remains unclear, which have limited the further development of novel higher efficiency afterglow materials. Due to application restrictions, the usual afterglow materials are charged with mercury lamps, only the band gap of the material is required to be lower than 3.4 ev (365 nm) to stimulate electrons of the valence band into the conduction band , and the afterglow of these materials is usually not long. In fact, most materials (even high bandgap materials) have the potential to produce afterglow if the excitation source is appropriate. Recent years have witnessed several advances in utilizing X-ray as a new type of excitation source for optical imaging in vivo [13–15], without the need for external excitation during in situ imaging. It is known that high energy X-ray photons excite the inner electrons of the atoms (no selectivity), while ultraviolet or visible photons excite the outer electrons of the matrix or luminescent centers (selectivety), with different charging transfer mechanisms. Thereby, using X-ray as an excitation source is expected to achieve better afterglow emission.
Although the X-ray excited afterglow phenomenon was discovered many years ago, it has not received sufficient attention from researchers because X-ray excited phosphors are mainly used in high speed imaging applications, where the afterglow is unacceptable. Undoubtedly, the findings of X-ray storage phosphors have provided the basis for the investigation of X-ray excited afterglow properties [16,17]. Unlike the ordinary afterglow materials excited by ultraviolet or visible light, X-ray can excite the materials with higher band gap, which can provide a prerequisite for the formation of stable deep traps. Hence, X-ray excited long afterglow materials may be obtained from the higher band gap materials. From the aspect of luminescent centers, the emission of transition metal ions and divalent RE ions can be modulated by the crystal field. Moreover, the emission wavelength changes with the change of host materials, so it is difficult to regulated the afterglow behaviour independently. The trivalent RE ions with their rich energy levels (emission across UV-to-near infrared wavelengths) shielded by their 5s5p shell layer are therefore unaffected by the crystal field, which are undoubtedly the ideal luminescent centers. Thus, this research focuses on trivalent RE ions-doped high band gap materials. In addition to the above reasons, the luminescent properties of high band gap materials doped with trivalent RE ions have a lot of intensive research results for Ref. [18–22]. However, there is no in-depth reports to date on the mechanism of the X-ray excited afterglow or how to improve the X-ray excited afterglow of trivalent RE ions-doped materials.
In this paper, the afterglow properties of a series of trivalent RE ions-doped high band gap YPO4 host were investigated and highly efficient red persistent YPO4: m3+ phosphors were synthesized. Inspired by isoelectronic doping enhanced luminescence of semiconductors [23–24], rare earth isoelectronic doping may also improve the afterglow behavior of YPO4:Sm3+ phosphors. After co-doping other trivalent RE ions, Sm3+ ions were found to be the main emitters of the persistent luminescence. Moreover, the co-doping of Tb3+ ions greatly improved the afterglow behavior of Sm3+ ions, and this interesting experimental result is of great significance for the preparation of other long persistent luminescence materials. The luminescence properties under different conditions [including X-ray excited luminescence, persistent luminescence, thermally stimulated luminescence (TSL) and photo stimulated luminescence (PSL)] were systematically studied. Furthermore, a feasible afterglow enhancement mechanism was proposed that can provide direction for achieving better afterglow materials. The synthesized red emitting persistent luminescence nanoparticles (PLNPs) were further evaluated for in vivo imaging with mice. The results of this work are expected to facilitate the development of improved optical probes for bioimaging in vivo.
2.1 Materials and methods
Micrometer-scale yttrium phosphate phosphor was synthesized by the conventional solid state reaction method. According to the designed composition, specific amounts of starting materials were weighed and mixed well together in a mortar and transferred into an aluminum oxide crucible. The mixtures were annealed in a reducing (nitrogen) atmosphere at 500°C for 2 h first, and then at 1300°C for 5 h. Then, the mixtures were cooled to room temperature (RT) immediately and crushed to obtain fine samples. All the raw materials used were obtained from Aladdin Industrial Inc. (Shanghai, China) and the purity of all RE oxides was 99.99%.
Nano-scale YPO4:Sm3+,Tb3+ (hereinafter referred to as YPNPs) was synthesized successfully by a conventional hydrothermal method. In a typical procedure, appropriate amounts of cation-containing Y(NO3)3•6H2O (99.99%), Sm(NO3)3•6H2O (99.99%) and Pr(NO3)3•6H2O (99.99%) were weighed according to the predetermined ratio and dissolved in 30 ml of deionized water to form a transparent nitrate solution (aqueous solution 1). Next, an appropriate amount of anion-containing Na2HPO4•12H2O was weighed and dissolved in 30 ml of deionized water to form a transparent phosphate solution (aqueous solution 2). The aqueous solution 2 was slowly poured into the aqueous solution 1 and stirred until it was thoroughly mixed. Then, the pH of the mixed solution was adjusted to neutrality and stirring was continued until sufficient mixing was achieved. The resulting colloidal solution was charged into a reaction vessel and reacted at 210°C for 10 h. The formed precipitate was washed three times with deionized water and ethanol, dried at 80°C, and then cooled naturally to room temperature (RT). It was then ground into a powder and calcined at 800°C for 1 h in an air atmosphere.
2.2 Surface modification of YPNPs
First, hydroxy-YPNPS were synthesized through alkaline wet milling of the as-obtained YPNPs in NaOH solution for 45 min and then dispersing the pulverized powder in NaOH solution overnight. The resulting samples were ultrasonically dispersed in an aqueous solution of PVP for 1 hour. Then, the suspension was allowed to stand for 3 hours to precipitate large particles. The polyvinyl pyrrolidone (PVP)-modified hydroxy-YPNPs in the supernatant were separated by centrifugation for 5 min. The resulting precipitate was ultrasonically dispersed in a mixture of ethanol, deionized water and concentrated aqueous ammonia (28 wt%) for 30 min. The suspension was placed in an ice-water bath and appropriate amounts of tetraethyl orthosilicate (TEOS) and triethoxysilane (APTES) diluted with ethanol were added dropwise to the dispersion under continuous magnetic stirring, followed by further stirring for 12 hours. The product obtained by centrifugation was washed three times with ethanol. The final product was dispersed in a phosphate buffer saline (PBS) (pH = 6) solution, and appropriate amounts of 1-(3-Dimethylaminopropyl) (EDC), N-Hydroxy succinimide (NHS) and HOOC-PEG-OMe were sequentially added and stirred at room temperature for 24 hours. The polyethylene glycol (PEG) modified YPNPs were thus obtained.
2.3 In vivo imaging of modified YPNPs
The modified YPNPs at a dose of 100 µL (approximately 4 mg/kg) were irradiated by X-ray (Cu Ka) at 20 mA/30 KV for 3 min and then injected into the mice (approximately 6 mg/kg) by tail vein injection. In vivo imaging was performed using a CCD camera for 60 seconds exposure to obtain the near-infrared afterglow signal.
Structural characterization of the synthesized phosphors was supported by X-ray diffraction (XRD) with Bruker D8 advance X-ray diffractometer (Bruker Optics, Ettlingen, Germany) using CuKα radiation at a scanning step of 0.02°. The excitation and emission spectra of the as-synthesized materials were recorded using a combined time resolved and steady state fluorescence spectrometer (Edinburgh FSP-920). The afterglow phosphors were excited using an XRad-320 X-ray irradiator (Precision X-ray, Inc., North Branford, CT) under 30mA, 50 kVp wolfram target. X-ray excited emission spectra, afterglow emission spectra and afterglow decay curves were recorded using an Andor SR-500i spectrometer (Andor Technology Co. Belfast, UK) equipped with a Hamamatsu R928 photomultiplier. All afterglows were obtained about ten minutes after ceasing the X-ray irradiation. The afterglow images were recorded using a digital SLR camera (EOS 5D Mark III) in a dark room. Thermoluminescence spectra were measured using a Self-assembling thermoluminescence system including high precision thermal Stages (THMS600) (British Linkam Scientific Instruments) and Andor SR-500i spectrometer (Andor Technology Co. Belfast, UK), with a fixed heating rate of 5 K/s within the range 83.15 K-600 K and a fixed heating rate of 3 K/s within the range RT-600 K. Photoluminescence were measured using 450 nm, 488 nm, 532 nm and 808 nm fiber lasers with collimating lens. The size and morphology of the products were observed using transmission electron microscope (TEM) (Techai G2 F20 S-TWIN, America). Hydrodynamic size distribution was measured by dynamic light scattering method (DLS) using a Zetasizer Nano-ZS (Malvern, UK). Infrared spectra were obtained on a Fourier transform infrared spectrometer (FTIR) over the range from 400 nm to 4000 nm with KBr pellet technique (Bruker Tensor 27, Germany). Optical images in mice were achieved in an imaging system (Caliper Life sciences, IVIS lumina II).
3. Results and discussion
3.1 Structure and luminescence properties of RE-doped materials
RE single-doped yttrium phosphate exhibits characteristic emission of each RE ions [Fig. 1(a)], and the afterglow from Sm3+ is the longest [Fig. 1(b)]. In order to achieve longer afterglow, RE isoelectronic traps co-doping mechanism is applied in YPO4:Sm3+ phosphors. Interestingly, by introducing different Ln3+ ions (Ln3+= Ce3+, Pr3+, Nd3+, Eu3+, Tb3+, Dy3+, Ho3+, Er3+, Tm3+, respectively) into YPO4:Sm3+ phosphors, the afterglow luminescence of these materials, after ceasing the X-ray irradiation, mainly came from the energy level transitions of Sm3+ ions [Fig. 1(c)]. It is exciting that the afterglow luminescence of Sm3+ ions was significantly enhanced by co-doping Tb3+ ions (or Pr3+ ions) [Fig. 1(d)]. Moreover, all the co-doped materials owned the same thermoluminescence peak as YPO4:Sm3+ phosphors, apart from the Sm3+/Pr3+ and Sm3+/Tb3+ co-doped YPO4 which appeared the second thermoluminescence peak with a higher peak temperature [Fig. 1(e)]. Based on these exciting finding, a more in-depth investigation of Tb3+/Sm3+ co-doped YPO4 was performed.
3.2 Afterglow properties of YPO4:Ln3+ (Ln = Sm, Tb) and YPO4:Sm3+,Tb3+ phosphors
Afterglow emission spectra of Tb3+ doped YPO4, Sm3+ doped YPO4 and Tb3+/Sm3+ co-doped YPO4 were compared, as shown in Fig. 2(a). The afterglow emission spectra of Tb3+ doped YPO4 and Sm3+ doped YPO4 can be attributed to the characteristic 4f transition of Tb3+ and Sm3+ ions, respectively. Interestingly, the afterglow emission spectrum of Tb3+/Sm3+ co-doped YPO4 not only had the emission of Sm3+ ion but also the emission of Tb3+ ion in the early stages. Thus, a strong golden yellow phosphorescence was emitted. Importantly, the afterglow of Tb3+/Sm3+ co-doped YPO4 was far longer than that of Tb3+ doped YPO4 and Sm3+ doped YPO4 [Figs. 2(b) and 2(c)]. And the afterglow spectrum of Tb3+/Sm3+ co-doped YPO4 can still be obtained after 50 hours decay [the upper inset of Fig. 2(b)]. Over time, the color of the afterglow emission changed from yellow to red for the Tb3+/Sm3+ co-doped YPO4 [Fig. 2(c)].
3.3 Thermally stimulated luminescence (TSL) and photo stimulated luminescence (PSL) of YPO4:Sm3+,Tb3+ phosphors
To gain a deeper insight into the afterglow mechanism of the YPO4:Sm3+,Tb3+ phosphors, TSL [Fig. 3(a)] and PSL [Fig. 3(b)] experiments were performed. From Fig. 3(a) we can judge that the TSL broad peak of 83.15 K to RT may come from continuous shallow traps caused by its own structure, while the latter two peaks are from discrete deep traps caused by doping of RE ions. The extra peak after co-doping is likely due to the coulomb interaction in the excitons, resulting in an increase in the depth of electron traps (Sm3+ ions). The trap depths (E) with respect to the CBM can be roughly estimated by the simple expression E = Tm/500, where Tm is the temperature for which the TSL peak is the maximum (in kelvin, K) . The activation energy of the shallow trap (E1) observed at 238 K is 0.476 eV, the activation energy of the two deep traps (E2 and E3) observed at 376 K and 460 K are 0.752 eV and 0.920 eV, respectively. As seen from Fig. 3(b), the electrons can be effectively released from electronic traps via irradiation with 450 nm, 488 nm, 532 nm and 808 nm fiber laser, the local luminescence appeared bright compared to the background after a short time of excitation and became darker with the increase in excitation time. The above phenomenon indicates that both thermal and photo excitation can release the electrons in the electron traps. The energy required for photo excitation is higher than the energy required for thermal excitation may due to the Stokes shift.
3.4 Discussion on the long afterglow mechanism of YPO4:Ln3+ (Ln = Sm, Tb) and YPO4:Sm3+,Tb3+ phosphors
Here, YPO4 was chosen as the substrate for the following reasons: (1) YPO4 host has higher band gap , in which stable deep traps can be formed and (2) it has one type of yttrium lattice site that can be occupied by the trivalent lanthanide ions without the need for charge compensating defects [26,27]. This would allow us to study the influence of the different RE dopants on the persistent luminescence properties of the materials. Since the 4f electron numbers of RE ions are different, they all have different electron accepting ability [Fig. 4(a)]. For instance, Ce3+ ions and Tb3+ ions are prone to lose the outer electrons to turn into Ce4+ ions and Tb4+ ions with stable shell structures, which act as deep hole traps in the forbidden band of YPO4 host. On the other hand, Eu3+ ions and Yb3+ ions tend to accept electrons to turn into Eu2+ ions and Yb2+ ions with stable shell structures, acting as deep electron traps in the forbidden band of YPO4 host. Similarly, Ho3+, Er3+ and Tm3+ ions tend to exist as shallow electronic traps in YPO4 host; Pr3+ ions tend to act as shallow hole traps; and Nd3+ and Dy3+ ions can either exist as shallow electronic traps or as shallow hole traps. Thus, it is obvious that the afterglow properties are closely related to defects from RE ions, and the trap depth is related to the position of RE ions in the YPO4 forbidden band [28,29]. In the afterglow mechanism of single doped trivalent RE ions, Nd3+, Dy3+, Ho3+, Er3+ and Tm3+ ions can be considered as electronic traps that combine with the holes bound by the coordinated oxygen ions at the adjacent position, resulting in the emission from RE ions (i.e. persistent luminescence). As a typical example, the afterglow process of Sm3+ ion is described as follows [Fig. 4(b) and blue part of Fig. 4(c)]. After ceasing the X-ray excitation, the electrons in the VB of YPO4 host are excited into the CB (i.e., free-state) [Progress ① in Fig. 4(c)]. Then, they lose their kinetic energy by collisions and are captured into the 3d orbital of Y located at the bottom of the CB, while the holes are bound into the 2p orbital of O at the top of the VB. Then, a portion of the electrons at the bottom of the CB are trapped by Sm3+ ions which act as electron traps [Progress ② in Fig. 4(c)]. After photo or thermal excitation, the electrons bound by Sm3+ and Y ions re-enter the CB and get transported in the crystal [Progress ③ in Fig. 4(c)]. Once their kinetic energy is exhausted, they are again bound by Y located at the bottom of the CB and finally trapped by Sm3+ ions which coordinate to oxygen ions attached holes [Progress ④ in Fig. 4(c)]. Thus, the recombination of electron-hole pairs occurs and the energy is translated into the charge transfer state of Sm3+ ions, resulting in the 4f emission from Sm3+ ions [Progress ➄ in Fig. 4(c)]. Undoubtedly, Sm3+ ions show the strong afterglow behavior due to their ability to act as deep electron traps. On the other hand, Ce3+, Tb3+ and Pr3+ ions act as hole trapping centers in single doped YPO4. Their specific afterglow process is similar to that of single-doped RE ions which act as electron traps. Just take Tb3+ ions as an example [Fig. 4(b) and green part of Fig. 4(c)], first, electrons bound to Y ions enter the CB upon photo or thermal excitation, and are subsequently captured by the Y ions adjacent to the Tb3+ ions which bind the holes. Then, recombination of electron-hole pairs occurs to generate 4f-4f luminescence from the Tb3+ ions. It is worth mentioning that, in theory, the energy can be transmitted to the RE ions (luminescent centers) through exciton recombination. The obtained data show that when other RE ions were co-doped with Sm3+ into YPO4, the persistent luminescence mainly came from Sm3+ ions [Fig. 1(c)], precisely because that Nd3+, Eu3+, Dy3+, Ho3+, Er3+ and Tm3+ exist as electron traps. Upon photo or thermal excitation, the electrons captured by these electron traps enter and migrate in the CB until they are again captured by the deeper electron traps (Sm3+ ions), and these electron traps exist as defects rather than as luminescence centers in the co-doped materials. Moreover, in Sm3+/Tb3+ co-dopants and Sm3+/Pr3+ co-dopants, Tb3+ ions and Pr3+ ions can also act as emitters in the afterglow progress. Consider the example of Sm3+/Tb3+ co-dopants, their persistent luminescence comes from the following three routes: (1) Recombination of electrons trapped by Sm3+ ions and holes trapped by coordinating oxygen ions [blue part of Fig. 4(c)]. (2) Recombination of electrons trapped by Y ions and holes trapped by Tb3+ ions [green part of Fig. 4(c)]. (3) Recombination of electrons trapped by Sm3+ ions and holes trapped by Tb3+ ions) [yellow part of Fig. 4(c)]. Only the third route can increase the afterglow duration.
3.5 Long afterglow for in vivo real-time imaging
We further successfully synthesized YPNPs which have afterglow emissions at 601 nm, 645 nm, and 705 nm, falling within the tissue transparency window [Figs. 5(a) and (b)]. Moreover, the decay time of afterglow monitored at 601 nm was longer than 4 h [Fig. 5(c)]. The strategy for real-time in vivo biodistribution imaging in mice by using persistent luminescence of low-dose X-ray-activatable YPNPs is illustrated in Fig. 6(a). The YPNPs are composed of nanosized particles characterized by regular and uniform spherical morphology with a size distribution between 25 nm and 50 nm in diameter [Fig. 5(d)]. The high-resolution TEM (HRTEM) image shows two lattice structures, with distances of 0.143 nm and 0.347 nm between the lattice fringes, which correspond to the lattice spacing of the (332) and (200) planes of cubic YPO4, respectively [Fig. 5(e)]. To confirm the successful modification, FTIR spectrum of the as-modified YPNPs was obtained [Fig. 5(f)]. The FTIR peaks of the sample at 1399, 1630 and 3427 cm-1 are attributed to the stretching vibration of -CH3, C = O and -COOH bonds, respectively, indicating the presence of PEG molecules on the surface of YPNPs [30,31]. YPNPs-PEG-OCH3 nanoparticles are easily dispersed in water and good biocompatibility could be achieved after the surface modification.
We also provided the relationship between the absorbed X-ray dose and its incident power as shown in illustration of Fig. 6(a). The calculated irradiance of the YPNPs aqueous solution is about 7.8 Gy. Then, the persistent luminescence YPNPs were employed to provide real-time in vivo biodistribution imaging in mice after systemic tail vein injection, as shown in Fig. 6(b). To the best of our knowledge, the first highly vascularized organ encountered after tail vein injection of YPNPs is the lung, due to the slower blood circulation in the capillaries . Thereby, non-specific interactions dominated all the forces involved, resulting in trapping of the YPNPs in the lungs. As demonstrated, after 15 min of X-ray pre-excitation and another 8 min operating time following the tail injection of YPNPs, significant NIR persistence signals were observed in the liver region, which gradually attenuated. Thus, the in vivo imaging in mice was successfully achieved with PEG modified YPNPs after X-ray excitation.
In this work, we for the first time discovered the X-ray excited red afterglow material YPO4:Sm3+. A strategy that introducing Tb3+ ions into YPO4:Sm3+ crystals was presented for improving the persistent luminescence behavior. The luminescence intensity and persistent duration from Sm3+ at RT after X-ray excitation were improved about 14 times. As the defects play a crucial role for persistent luminescence, TSL and PSL experiments of the afterglow decay process were conducted to determine the electron trap levels and explain the electronic transfer behavior specifically. The mechanism of afterglow as well as the mechanism of how to enhance afterglow were discussed in detail by analyzing the obtained data. Furthermore, low-dose X-ray-activated YPNPs-PEG-OCH3 nanoparticles were designed with efficient red persistent luminescence and biocompatibility, which were successfully applied for deep-tissue real-time optical imaging in mice. This will provide a new perspective in design of nanoprobes for bioimaging.
National Natural Science Foundation of China (11474083); Department of Education of Hebei Province (ZD2014069).
We gratefully thank College of Science and Technology, Hebei University and Institute of Urban Environment, Chinese Academy of Sciences for providing measuring and test instruments.
The authors declare no conflicts of interest.
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