Article pubs.acs.org/IC
Cite This: Inorg. Chem. XXXX, XXX, XXX−XXX
Toward Rechargeable Persistent Luminescence for the First and Third Biological Windows via Persistent Energy Transfer and Electron Trap Redistribution Jian Xu,*,† Daisuke Murata,† Jumpei Ueda,† Bruno Viana,‡,§ and Setsuhisa Tanabe† †
Graduate School of Human and Environmental Studies, Kyoto University, Kyoto 606-8501, Japan Chimie ParisTech, Institutde Recherche de Chimie Paris, CNRS, PSL Research University, Paris 75005, France § Chimie ParisTech, CNRS, Paris cedex F-75231, France ‡
S Supporting Information *
ABSTRACT: Persistent luminescence (PersL) imaging without real-time external excitation has been regarded as the next generation of autofluorescence-free optical imaging technology. However, to achieve improved imaging resolution and deep tissue penetration, developing new near-infrared (NIR) persistent phosphors with intense and long duration PersL over 1000 nm is still a challenging but urgent task in this field. Herein, making use of the persistent energy transfer process from Cr3+ to Er3+, we report a novel garnet persistent phosphor of Y3Al2Ga3O12 codoped with Er3+ and Cr3+ (YAGG:Er−Cr), which shows intense Cr3+ PersL (∼690 nm) in the deep red region matching well with the first biological window (NIR-I, 650−950 nm) and Er3+ PersL (∼1532 nm) in the NIR region matching well with the third biological window (NIR-III, 1500−1800 nm). The optical imaging through raw-pork tissues (thickness of 1 cm) suggests that the emission band of Er3+ can achieve higher spatial resolution and more accurate signal location than that of Cr3+ due to the reduced light scattering at longer wavelengths. Furthermore, by utilizing two independent electron traps with two different trap depths in YAGG:Er−Cr, the Cr3+/Er3+ PersL can even be recharged in situ by photostimulation with 660 nm LED thanks to the redistribution of trapped electrons from the deep trap to the shallow one. Our results serve as a guide in developing promising NIR (>1000 nm) persistent phosphors for long-term optical imaging.
1. INTRODUCTION Over ten years have passed since le Masne de Chermont et al. first demonstrated that persistent phosphors emitting nearinfrared (NIR) light without in situ external excitation can be utilized as a highly promising optical contrast agent for longterm in vivo bioimaging.1 The persistent luminescence nanoparticles (PLNPs) with intense and long duration persistent luminescence (PersL) after ceasing ultraviolet (UV) or visible light charging can totally remove the autofluorescence from organisms, avoid the heating effects by real-time laser excitation in the conventional photoluminescence (PL) imaging, and thus improve the signal-to-noise ratio (SNR) remarkably. 2−4 Till now, Cr 3+ singly doped ZnGa 2 O 4 (ZGO:Cr) deep red persistent phosphor first developed by Bessière et al.5 together with its transformation by partial substitution with germanium and/or tin first proposed by Pan et al.6 and Allix et al.,7 respectively, has been widely studied in biomedical fields and proved to be successful for this new generation of excitation-free and background-free imaging technique. One of the unique characteristics of this spinel compound lies in the fact that its PersL from Cr3+ could be © XXXX American Chemical Society
charged by orange/red light, well below its band gap energy and close on/within the first biological window (NIR-I, 650− 950 nm).8−10 Therefore, in vivo rechargeable ability by orange/ red light or even white light-emitting diode (LED) shown in ZGO:Cr makes it quite popular in the past five years.5−14 However, since the Rayleigh scattering (varying as λ−4, where λ is the wavelength) decreases with increasing wavelength, it has been proved that the wavelength region longer than 1000 nm, which is named as the second (NIR-II, 1000−1350 nm) and third (NIR-III, 1500−1800 nm) biological windows, is highly advantageous over the shorter wavelength NIR-I one because of its much lower optical scattering coefficient result in deeper tissue penetration and improved contrast quality.15−17 Therefore, shifting the emission band of the optical probe from the NIR-I to NIR-II and/or NIR-III windows is definitely required.18−20 Fortunately, some lanthanide ions with typical sharp 4f−4f emission bands in the NIR region over 1000 nm, such as Nd3+, Pr3+, Ho3+, Tm3+, and Er3+, are considered to be Received: January 26, 2018
A
DOI: 10.1021/acs.inorgchem.8b00218 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry
Figure 1. (a) Response sensitivities of the Si and InGaAs photodiodes as well as the wavelength region of three biological windows. (b) The diffuse reflectance of the YAGG:Er ceramic sample (pink-dotted line area is the detector change region from PbS to PMT) plotted with PL spectra (λex = 590 nm) of the YAGG:Er−Cr and YAGG:Cr ceramic samples. (c) PersL spectrum of the YAGG:Er−Cr ceramic sample at 30 s after ceasing UV illumination. (d) Energy-level diagrams of Cr3+ and Er3+ in garnet host. (e) Fluorescence decay curves of the YAGG:Cr and YAGG:Er−Cr ceramic samples monitoring Cr3+ emission (λex = 590 nm; λem = 690 nm).
highly promising candidates to realize this purpose.21−23 However, there are no distinct sites for the large-sized lanthanide doping ions in the lattice of ZGO spinel host,9,11,24 and lanthanide ions are usually adopted as sensitizers with very low doping concentrations to enhance the Cr3+ PersL in the NIR-I window by increasing the distortion of GaO6 octahedra, rather than directly be used as emitting centers.25,26 As a consequence, it limits the practical application of ZGO spinel persistent phosphors for the longer wavelength biomedical imaging, especially in the NIR-II and NIR-III windows. On the contrary, garnet structure in the form of A3B2C3O12 is more favorable than spinel, since it presents three types of cation sites able to accommodate different cation ions:27 (i) the A site, 24(c) dodecahedral site, (ii) the B site, 16(a) octahedral site, and (iii) the C site, 24(d) tetrahedral site. Considering the high tolerability and flexibility of these three cation sites, versatile doping strategies by lanthanide ions at A sites and/or transition-metal ions at B/C sites can be easily employed for various photonic applications.27−30 Recently, we developed a series of garnet persistent phosphors with tunable and intense PersL from UV to NIR region by the combination of lanthanide and/or transition-metal doping ions.31−34 Furthermore, by utilizing the efficient persistent energy transfer (ET) from Ce3+ to Er3+ in garnet host, we also successfully developed the first persistent phosphor with long PersL of the Er3+: 4I13/2 → 4I15/2 transition at ∼1.53 μm suitable for the NIR-III window.35 Herein, we step forward again and report the first rechargeable garnet persistent phosphor of Er3+/Cr3+ codoped Y3Al2Ga3O12 (YAGG:Er−Cr), which exhibits long (>10 h)
PersL in the broad range from 1450 to 1670 nm due to the typical Er3+: 4I13/2 → 4I15/2 transition in garnet for the NIR-III window as well as from 650 to 950 nm due to the Cr3+: 2E (2G)/4T2 (4F) → 4A2 (4F) transitions for the NIR-I window. Moreover, different from the rechargeable mechanism via storing the orange/red excitation light by the generation of shallow traps due to antisite defects in ZGO:Cr spinel,8,9,11,24 we make full use of two independent electron traps with two different trap depths in YAGG:Er−Cr. Both Cr3+ and Er3+ PersL can be recharged in situ by photostimulation (PS) with 660 nm LED thanks to the redistribution of trapped electrons from the deep trap to the shallow one, which is called “photostimulated redistribution (PSR) of electron traps”.
2. EXPERIMENTAL SECTION 2.1. Materials and Synthesis. The YAGG:Er−Cr ceramic sample with the composition of Y2.94Er0.06Al1.99Cr0.01Ga3O12 was fabricated by a conventional solid-state reaction method. Y2O3 (99.99%), Er2O3 (99.99%), Al2O3 (99.99%), Ga2O3 (99.99%), and Cr2O3 (99.9%) were used as raw materials. The starting powders were mixed by a ball milling method (Premium Line P-7, Fritsch) with anhydrous ethanol for several hours. The mixed powder was dried at 80 °C for 36 h, compacted to form a ceramic green body (ϕ20 mm, 2 mm thickness) under uniaxial pressing of 50 MPa, and finally sintered at 1600 °C for 10 h in air. The YAGG:Er (Y2.94Er0.06Al2Ga3O12) and YAGG:Cr (Y3Al1.99Cr0.01Ga3O12) ceramic samples prepared by the same experimental procedure were used as references. All the prepared samples were double-face polished (thickness of 1.5 ± 0.1 mm) and confirmed to be single phases (ICSD: No. 089-6660) by X-ray diffraction (XRD) measurements (Figure S1). 2.2. Characterization. The diffuse reflectance spectrum of the YAGG:Er ceramic sample was measured by a spectrophotometer (UVB
DOI: 10.1021/acs.inorgchem.8b00218 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 2. PLE spectra (λem = 690 nm) of the (a) YAGG:Cr and (λem = 1532 nm) of the (b) YAGG:Er and (c) YAGG:Er−Cr ceramic samples. (d) PersLE spectrum of the YAGG:Er−Cr sample at 5 min after ceasing illumination by monochromatic light for 1 min (magnify 10 times from 300 to 700 nm). (e) Excitation (λ)−emission (λ) 2D contour mapping of the PersL spectra as a function of excitation wavelengths; the broadening of Cr3+ R-line emission is due to large width of the optical slit (wavelength resolution: ∼20 nm). Xe lamp with the UV module was measured at 25 °C using a photomultiplier tube (PMT; R11041, Hamamatsu Photonics) or the InGaAs PD detectors (the schematic illustration of the measurement setup shown in Figure S2). PS was induced by a 660 nm red LED (LED660/L-STND, Optocode) with a power density of ∼0.14 W/cm2 or an 850 nm NIR LED (LED850/L-STND, Optocode) with a power density of ∼0.14 W/cm2. The PersL imaging after charging by a 254 nm mercury lamp (6 W output) for 10 min of the raw-pork-tissue covered YAGG:Er−Cr and YAGG:Cr samples were taken by a VisNIR bioimaging machine (Clairvivo-OPT, Shimadzu) equipped with a Si CCD camera (cooled to 208 K) and a prototype NIR bioimaging machine (NIS-OPT, Shimadzu) equipped with an InGaAs camera (Xeva-1.7−320-TE3, cooled to 223 K) using a “thermal blue” imaging mode. Both of the cameras were fixed on a constant position during the operation.
3600, Shimadzu) equipped with an integrating sphere. Fluorescence lifetimes of the YAGG:Er−Cr and YAGG:Cr samples were measured by a compact lifetime spectrometer (Quantaurus-Tau-C11367, Hamamatsu Photonics) using a pulse Xe lamp connected with a 590 nm band-pass filter as the excitation source for Cr3+ emission. PL and PersL spectra of the YAGG:Er−Cr and YAGG:Cr samples were measured by a Si CCD spectrometer (QE65-Pro, Ocean Optics) from 600 to 1050 nm and an InGaAs spectrometer (NIR-Quest512, Ocean Optics) from 1050 to 1700 nm connected with UV−vis or NIR optical fibers at room temperature (RT). All the PL and PersL spectra were calibrated using a standard halogen lamp (SCL-600, Labsphere). The photoluminescence excitation (PLE) spectra of the YAGG:Er−Cr and YAGG:Er samples monitoring Er3+ emission (1532 nm) were recorded by a monochromator (G250, Nikon) equipped with an InGaAs photodiode (PD) detector (IGA-030-H, Electro-Optical System Inc.), and a 300 W Xe lamp (MAX-302, Asahi Spectra) was used as the excitation source. The persistent luminescence excitation (PersLE) spectrum and related two-dimensional (2D) plot of excitation and emission wavelength of the YAGG:Er−Cr sample after ceasing the illumination by monochromatic light for 1 min was measured by a fluorescence spectrophotometer (RF-5000, Shimadzu) using a homemade automatic operation program by an external control mode. Every PersL spectrum was recorded at 5 min after ceasing the corresponding monochromatic light, the charging wavelength was changed between 200 and 700 nm at 10 nm intervals, and the emission wavelength was monitored in the range from 600 to 900 nm. The 300 W Xe lamp with a UV mirror module (250−380 nm) was used as the excitation source for thermoluminescence (TL)-2D mapping measurements. The ceramic sample was set in a cryostat (Helitran LT3, Advanced Research Systems) to control its temperatures and first illuminated by UV light at 100 K for 10 min, then heated at 10 min after ceasing the illumination up to 600 K at a rate of 10 K/min. The Si/InGaAs spectrometers were operated simultaneously with the TL measurement to monitor the TL emission spectra at different temperatures. The persistent luminescence decay curve of the YAGG:Er−Cr sample after being excited for 5 min by the 300 W
3. RESULTS AND DISCUSSION 3.1. Spectroscopy. Figure 1b gives the PL spectra (λex = 590 nm, selectively pumping only Cr3+ ions) of the YAGG:Cr and YAGG:Er−Cr samples, also the diffuse reflectance spectrum of the YAGG:Er sample. The broad emission band ranging from 650 to 950 nm in YAGG:Cr is ascribed to the mixing of sharp spin-forbidden 2E (2G) → 4A2 (4F) (R-line) transition peaking at 690 nm together with its thermal vibration induced anti-Stokes/Stokes phonon sidebands (PSBs) and the broad spin-allowed 4T2 (4F) → 4A2 (4F) transition of Cr3+ at RT.36,37 Comparing the PL spectrum of YAGG:Cr with the diffuse reflectance spectrum of YAGG:Er, the absorption band of Er3+: 4I15/2 → 4I9/2 is overlapped with the emission band of Cr3+ indicating the possible ET process from Cr3+ to Er3+. It is confirmed by the reabsorption “dips” located at ∼788 nm in the PL spectrum of YAGG:Er−Cr due to the radiative ET from Cr3+ to Er3+ and the presence of split emission bands peaking at 966 and 1532 nm due to the Er3+: 4I11/2 → 4I15/2 and 4I13/2 → C
DOI: 10.1021/acs.inorgchem.8b00218 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 3. Persistent luminescence decay curves (a) in unit of radiance (mW·sr−1·m−2) and (b) in unit of photon emission rate (cps·sr−1·m−2) of the YAGG:Er−Cr ceramic sample monitoring Cr3+ and Er3+ emission [YAGG:Cr and ZnGa2O4:Cr3+ (ZGO:Cr) ceramic samples as references]; photographs of the YAGG:Cr and YAGG:Er−Cr ceramic samples after 254 nm (6 W output) illumination for 5 min (c) taken by a Si CCD camera (integrating time: 1.0 s, the tail in the 5 min image is due to the intensity saturation of the camera) and (d) taken by an InGaAs camera (integrating time: 0.04 s).
Cr3+ and Er3+ in the two regimes. Since the ET process from Cr3+ to Er3+ is confirmed from emission profiles and lifetime values as shown in Figure 1b,e, persistent ET process is assumed to occur in YAGG:Er−Cr as previously reported in the Ce3+-Er3+ and Ce3+-Nd3+ codoped garnets.35,43 Moreover, compared with the wavelength regions of three biological windows19,20,23 and response curves of commercial Si and InGaAs detectors (see Figure 1a), almost the whole Cr3+ PersL band is located within the NIR-I window suitable for the highsensitivity region of the Si detector, while the longer wavelength of Er3+ PersL matches well with the NIR-III window presenting the highest sensitivity region of the InGaAs detector. The PLE spectra monitoring the Er3+ emission (λem = 1532 nm) of the YAGG:Er and YAGG:Er−Cr samples are given in Figure 2b,c, respectively, in which the PLE spectrum monitoring the Cr3+ emission (λem = 690 nm) of the YAGG:Cr sample is also plotted as a reference44 in Figure 2a. Besides several sharp 4f−4f excitation bands of Er3+ from the 4I15/2 ground state to the 4G11/2 (383 nm), 2H9/2 (409 nm), 2F3/2, 5/2 (452 nm), 2F7/2 (489 nm), 2H11/2 (526 nm), 4S3/2 (544 nm), and 4F9/2 (649 nm) excited states, the typical two broad excitation bands of Cr3+ attributed to the 4A2 (4F) → 4T1 (4F) (400−500 nm) and 4A2 (4F) → 4T2 (4F) (540−700 nm) transitions are also observed in Figure 2c. It again confirms the ET process from Cr3+ to Er3+. To clarify the charging efficiency under different excitation wavelengths for the YAGG:Er−Cr compound, the PersLE spectrum and related 2D contour mapping are shown in Figure 2d,e. Compared with the PLE spectra respectively monitoring the Er3+ and Cr3+ emission in the YAGG:Er−Cr and YAGG:Cr compounds, the efficient charging wavelength region shown in the PersLE spectrum of YAGG:Er−Cr is mainly located at ∼210−280 nm (peaking at ∼240 nm). On the one hand, it indicates that as for PersL, the high-energy charging in the UV
4
I15/2 parity-forbidden 4f−4f transitions, respectively. Taking into account the large Stark splitting of the terminal Er3+: 4I15/2 level due to the crystal field effect on garnet hosts,35,38 the Er3+: 4 I13/2 → 4I15/2 emission band also extends from 1450 to 1670 nm in the YAGG host. As the ET process from the Cr3+: 2E (2G) excited state to the Er3+: 4I9/2 state is energetically favorable according to the energy level diagrams of Cr3+ and Er3+ (see Figure 1d), this should affect the fluorescence decay curves of Cr3+ emission in the codoped sample. Decay profiles of both YAGG:Cr and YAGG:Er−Cr samples are shown in Figure 1e. Since the decay profiles are not exponential, a mean lifetime value can be calculated by eq 1 ∞
τ=
∫0 tI(t ) dt ∞
∫0 I(t ) dt
(1)
where I(t) is the emission intensity at time t. As a consequence, the mean lifetime of Cr3+ varies from 0.46 ms in YAGG:Cr to 0.37 ms in YAGG:Er−Cr, suggesting extra decay pathways due to the nonradiative ET from Cr3+ to Er3+ in the YAGG:Er−Cr sample. It is worth noting that the real steady/transient-state ET processes from Cr3+ to Er3+ at RT are assumed to be much more complicated involving three principal contributions: (i) ET from the Cr3+: 4T2 (4F) state, (ii) ET from the anti-Stokes PSB transition of the Cr3+: 2E (2G) state, (iii) ET from the zero phonon line (ZPL) and Stokes PSB transitions of the Cr3+: 2E (2G) state.39 All these ET processes may exhibit various temperature-dependent behaviors, the study of which is out of the scope of this paper, so that detailed quantitative models (e.g., Inokuti-Hirayama,40 Yokota-Tanimoto,41 or Burshtein42 models) are required for the further understanding of ET dynamics from Cr3+ to Er3+ in YAGG. The PersL spectrum after UV illumination of the YAGG:Er− Cr sample is given in Figure 1c. The identical shape of PL and PersL spectra suggests the same emission components from D
DOI: 10.1021/acs.inorgchem.8b00218 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry region mainly pumping the Cr3+: 4T1 (4P) level and/or the mixed levels with the conduction band (CB) is more favorable than visible light for the delocalization of excited electrons. On the other hand, compared with the UV light, the charging efficiency below 500 nm (e.g., 460 nm blue light) is much lower, but still exhibits PersL signals even at 20 K with very low thermal-assisted energy, which suggests that a trappingdetrapping tunneling process independent of temperature may occur in the forbidden band between the Cr3+: 4T1 (4F) level44 and the electron trap (see Figures S3 and S4 and the mechanism of trapping-detrapping process in Section 3.4). 3.2. Persistent Luminescence Decay. The persistent luminescence decay curves with the absolute intensity unit (mW·sr−1·m−2) monitoring Cr3+ (580−750 nm) and Er3+ (>1450 nm) emission bands of the YAGG:Er−Cr sample are shown in Figure 3a, in which the decay curves of the standard YAGG:Cr45 and ZGO:Cr46 persistent phosphors under the same experimental condition are also presented as references. It suggests that for the NIR-I window, the persistent radiance of the YAGG:Er−Cr sample monitoring Cr3+ emission (0.13 × 10−1 mW·sr−1·m−2) at 60 min after ceasing the UV illumination is much weaker than that of the YAGG:Cr compound (0.67 × 10−1 mW·sr−1·m−2), due to the quenching effect of deep red Cr3+ emission by the persistent ET to Er3+ as discussed in the previous part. However, the persistent radiance value is still comparable with that of the ZGO:Cr (0.15 × 10−1 mW·sr−1· m−2) persistent phosphor. On the contrary, the Er3+ PersL at longer wavelengths gives relatively smaller radiance value (0.03 × 10−1 mW·sr−1·m−2). Taking into account the totally different photon response of Si and InGaAs detectors in the different wavelength regions, as well as the large wavelength difference between Cr3+ and Er3+ emission bands, photon emission rate (in units of cps·sr−1·m−2) can be adopted to fairly describe the absolute intensity as shown in Figure 3b. The NIR photon emission rate of Er3+ in YAGG:Er−Cr (2.64 × 1016 cps·sr−1· m−2) at 60 min for the NIR-III window is comparable with that of Cr3+ in ZGO:Cr (5.41 × 1016 cps·sr−1·m−2) for the NIR-I window. It proves that this persistent phosphor exhibits intense multiwavelength PersL suitable for both the NIR-I and NIR-III biological windows. The PersL images from two garnet pellets obtained by commercial Si and InGaAs cameras are also given in Figure 3c,d. Although the YAGG:Cr sample shows higher emission intensity than that of the YAGG:Er−Cr sample using the Si camera in the Vis−NIR region (1000 nm) monitored by the InGaAs detector (Figure 5b), except intense blackbody radiation from the YAGG:Cr sample and partially from the measurement setup, no contributed emission can be recorded. According to the TL 2D contour mappings of the YAGG:Er−Cr sample in Figure 5c,d, besides the emission band from Cr3+, the NIR 4f−4f emission band from Er3+ appears at the same temperature range. The intense TL glow peaks of both Cr3+ singly and Cr3+-Er3+ codoped samples lie in almost the same temperature range (315 and 406 K shown in Figure S7 to compare with 315 and 408 K in Figure S6) indicating similar electron trapping−detrapping processes for Cr3+ and Cr3+/Er3+ PersL in YAGG. 3.4. Persistent Luminescence and Photostimulation Mechanism. On the basis of the results shown in Sections 3.1−3.3, the PersL mechanism at RT of the YAGG:Er−Cr persistent phosphor can be briefly explained via the vacuum
Figure 5. Contour mappings of the thermoluminescence intensity after UV illumination as a function of emission wavelength (λ) and temperature (T) of the (a, b) YAGG:Cr and (c, d) YAGG:Er−Cr ceramic samples.
YAGG:Cr and YAGG:Er−Cr samples to determine which emission band contributes to the TL glow peak at different temperatures. From the contour mapping of the YAGG:Cr sample recorded by the Si CCD (see Figure 5a), it can be seen that with increasing temperature, the TL spectrum is composed of only Cr3+ emission band indicating that except Cr3+ ions, no additional emission centers (e.g., impurities introduced during sample preparation procedures) contribute to the TL glow curves. Besides, the TL glow curve exhibits two broad bands
Figure 6. Mechanism of the persistent luminescence and photostimulation-induced persistent luminescence using the VRBE diagram including selected energy levels of Cr3+ and Er3+ in the Y3Al2Ga3O12 (YAGG) host: (a) charging step by UV or blue light excitation. (b) Persistent luminescence at RT. (c) Shallow trap cleaning after detrapping process at RT. (d) Photostimulation. (e) Trap redistribution after photostimulation. (f) Photostimulation induced persistent luminescence at RT. F
DOI: 10.1021/acs.inorgchem.8b00218 Inorg. Chem. XXXX, XXX, XXX−XXX
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Inorganic Chemistry referred binding energy (VRBE) diagram with energy levels of Cr3+, Er3+, CB, and valence band (VB) in the YAGG host34,52 in Figure 6a−c. (a) In the charging step, when the YAGG:Er−Cr sample is excited by UV light, the electron located at the Cr3+: 4 A2 (4F) ground state is excited to the 4T1 (4P) state in the vicinity of the CB. Hence, on the one hand, the excited electron can be captured by the electron trapping centers (Trap-I and Trap-II) with two different trap depths. On the other hand, although the charging efficiency is much lower than UV light, the 4A2 (4F) → 4T1 (4F) transition by blue light charging can also give rise to PersL through tunneling process feeding the deep Trap-II.47 (b) After the excitation ceased, the detrapping process occurs with thermal release of the trapped electrons from the shallow Trap-I assisted by the thermal activation energy at RT. After the released electron is captured by Cr3+ in the recombination process, the radiative relaxation of the excited state (Cr3+)* gives the typical Cr3+ luminescence, and the Cr3+ → Er3+ persistent ET occurs at the same time populating the Er3+: 4I9/2 level. The rapid multiphonon relaxation (MPR) from 4I9/2 down to the 4I11/2 and 4I13/2 excited levels takes place followed by the PersL of Er3+: 4I11/2 → 4 I15/2 (966 nm) and 4I13/2 → 4I15/2 (1532 nm) in the NIR region. (c) After a long fading time at RT, nearly all of the trapped electrons in Trap-I will be thermally released (trap cleaning) accompanied by the intensity decrease of both Cr3+ and Er3+ PersL, while those captured by Trap-II are almost unchanged, since the thermal activation energy at RT is not sufficient to empty Trap-II. As mentioned in the Introduction, in vivo rechargeable ability is a key point to solve the problem of quick PersL fading within several hours in most NIR persistent phosphors.53−55 Therefore, in the case of YAGG:Er−Cr, a proper light source such as red or NIR light (in the NIR-I window) could photostimulate the deeper trap (Trap-II) as demonstrated in Figure 6d. Then, Trap-II can be redistributed toward Trap-I through CB (Figure 6e) during in situ PS. As a consequence, the refilled or recharged Trap-I after PS can again contribute to the Cr3+/Er3+ PersL as shown in Figure 6f, which is the so-called “PSR of electron traps”. 3.5. Photostimulation-Induced Trap Redistribution and Thermoluminescence. To clarify the feasibility of the proposed “trap redistribution” principle, TL glow curves monitoring Cr3+ emission in the YAGG:Er−Cr sample performed under different conditions are given in Figure 7. The TL glow curve of the same sample recorded after UV charging at 100 K (black dotted line) is also plotted as a reference. We assume that TL glow curves of Er3+ emission exhibit the same behavior with those of Cr3+ emission in YAGG, since the ground states of Er2+/Er3+ are located within CB/VB,34,35 and Er3+ is only playing a role as energy acceptor in the Cr3+ → Er3+ ET process as discussed in Figure 5c,d, rather than taking part in the trapping−detrapping process as the trap center. Compared with the TL glow curve [curve (a)] after UV charging at 100 K, the glow curve obtained after UV charging at 300 K [curve (b)] gives a different shape in which the low-temperature part, below 300 K, belonging to Trap-I almost disappeared because of the immediate electron detrapping during the charging process. When the waiting time was extended to 2 h at 300 K [curve (c)], the Trap-I related glow curve disappeared as predicted in the mechanism (Figure 6c), which proves that most of the trapped electrons in Trap-I are cleaned up by thermal activation energy at RT, while those are remained in Trap-II. However, when a 660 nm LED
Figure 7. TL glow curves monitoring Cr3+ emission of the YAGG:Er− Cr sample (a) after UV charging at 100 K (black dotted line), (b) after UV charging at 300 K without waiting (red line) down to 100 K, (c) after UV charging at 300 K with 2 h waiting (blue line) down to 100 K and photostimulation by (d) 660 nm LED (power density: ∼0.14 W/ cm2, fwhm: 20 nm) for 30 min at 100 K (green line) (e) 850 nm LED (power density: ∼0.14 W/cm2, fwhm: 48 nm) for 30 min at 100 K (magenta line).
lamp (∼0.14 W/cm2) with a full width at half-maximum (fwhm) of 20 nm (Figure S8) is used as a PS light source at 100 K [curve (d)], it can be clearly seen that the TL glow curve at lower temperature belonging to Trap-I rises again, while that at higher temperature belonging to Trap-II decreases, which confirms the redistribution of trapped electrons from Trap-II to Trap-I after PS. The shift on the TL glow peaks can be explained by different charging temperatures and equilibrium between trap levels.49 It is also worth noting, since trap depths in the energy-level diagram are estimated from the TL results, the phonon-assisted thermal energy required to reach upper levels should be smaller than the required optical energy.56 To obtain intense PS-induced PersL through “trap redistribution” in YAGG, higher-energy photons at 660 nm, for instance, are therefore desirable and much more efficient to liberate the trapped electrons from Trap-II to CB than lower-energy ones such as 808 and 977 nm lasers reported in our previous work,44 or the 850 nm LED lamp demonstrated here, as an NIR PS light source in curve (e). 3.6. Photostimulation-Induced Persistent Luminescence and Optical Imaging. The rechargeable ability of the YAGG:Er−Cr sample by the 660 nm red LED is further investigated by the PS-induced PersL decay curves (red line) separately monitoring the Cr3+ and Er3+ emission shown in Figure 8a,b, in which the corresponding normal decay (black line) without PS is also plotted as a reference. After ceasing initial UV illumination and following 20 min of natural decay, the YAGG:Er−Cr sample is then triggered by the red LED for 1 min with 20 min interval for four cycles. Although it is very difficult to totally avoid the intensity saturation during PS (“on” mode) from the normal Cr3+ or Er3+ PL by absorbing the excitation light from LED, the enhancement of both Cr3+ and Er3+ PersL can be clearly observed after ceasing the LED PS light (“off” mode) due to the contribution of repopulated shallow Trap-I through trap redistribution from Trap-II. On the contrary, when the same sample is photostimulated by the 850 nm NIR LED as presented in Figure 8c,d, the enhancement of Cr3+/Er3+ PersL intensity is rather smaller than that by the 660 G
DOI: 10.1021/acs.inorgchem.8b00218 Inorg. Chem. XXXX, XXX, XXX−XXX
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Figure 8. Persistent luminescence decay curves of the YAGG:Er−Cr ceramic sample (a, c) monitoring Cr3+ emission (b, d) monitoring Er3+ emission (black line: normal decay; red line: with PS by 660 nm/850 nm LED for 1 min every 20 min). Figure 9. Optical imaging through raw-pork tissues with 1 cm thickness of the YAGG:Er−Cr ceramic sample after 254 nm (6 W output) illumination for 10 min (a) Cr3+ emission monitored by the Si CCD camera (integrating time: 1 s, the tail in the 1 min image is due to the intensity saturation of the camera). (b) Er3+ emission monitored by the InGaAs camera (integrating time: 5 s). The ceramic sample is in situ photostimulated by 660 nm LED for 1 min every 20 min after the initial 20 min natural decay.
nm red LED (emphasized by the blue zone), which further proves that relatively high-energy photons are desirable to give rise to efficient trap redistribution by overcoming the large energy barrier between Trap-II and CB. Optical imaging through pork tissues with 1 cm thickness by the in situ 660 nm PS under the similar experimental condition as presented in Figure 8a,b is also given in Figure 9a,b monitored by commercial Si and InGaAs cameras, respectively. The ceramic sample is first charged by 254 nm light for 10 min before covering by the raw-pork tissue, then after natural decay for 20 min (1st cycle), it is in situ photostimulated by 660 nm LED for 1 min followed by another 20 min of natural decay (second cycle), etc. Because of the enhanced PersL by PS, both Cr3+ and Er3+ emission can be monitored over 104 min, and the SNR of Cr3+ PersL at 104 min after ceasing the initial UV excitation can reach ∼31 (Figure S9) thanks to this unique autofluorescence-free imaging technology. Moreover, compared with the Cr3+ emission, the Er3+ emission can achieve higher spatial resolution and more accurate signal localizations owing to the reduced light scattering at longer wavelengths.
emitting centers in persistent phosphors for long wavelength optical imaging.59,60 Efficient persistent energy transfer from intense PersL sensitizers such as Ce3+, Eu2+, Cr3+, and Mn2+ with broad emission bands generally in the visible light region, to target lanthanide ions such as Nd3+, Ho3+, Tm3+, and Er3+ with relatively sharp emission bands in the NIR region (>1000 nm) can be a very useful tool to achieve this goal. Optimization of such lanthanide-doped PLNPs working in the NIR-II/III biological windows is currently under work.
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4. CONCLUSIONS AND OUTLOOK In summary, with combination of the persistent energy transfer from Cr3+ to Er3+ and so-called trap redistribution by photostimulation with 660 nm LED, we have successfully developed a new YAGG:Er−Cr persistent phosphor with multiwavelength NIR persistent luminescence suitable for both the NIR-I and NIR-III biological windows. Prospectively, on the one hand, the rechargeable ability by a cheap red LED demonstrated in this garnet compound overcomes the quick PersL fading within initial hours and encourages making full use of deep traps (detrapping temperature over ∼400 K), which are usually considered to be useless especially for most persistent phosphors working at RT. On the other hand, unique 4f−4f transitions from lanthanide ions, which have been widely used for biophotonics,57,58 should be again focused on as main
ASSOCIATED CONTENT
* Supporting Information S
The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.inorgchem.8b00218. XRD patterns of as-prepared YAGG ceramic samples; Measurement setup of PersL and PS induced PersL decay; Low temperature decay at 20 K after 460 nm blue light charging; Photochromic phenomenon after UV (254 nm) charging; Calculation of signal-to-noise (SNR) in the optical imaging by Region of Interest (ROI) method; TL 2D contour plots, TL glow curves and TL emission spectra of YAGG:Cr and YAGG:Er−Cr samples; Emission spectra of the 660 and 850 nm LEDs (PDF) H
DOI: 10.1021/acs.inorgchem.8b00218 Inorg. Chem. XXXX, XXX, XXX−XXX
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AUTHOR INFORMATION
Corresponding Author
*E-mail:
[email protected]. ORCID
Jian Xu: 0000-0002-1040-5090 Jumpei Ueda: 0000-0002-7013-9708 Funding
This work was financially supported by a Grant-in-Aid for JSPS Fellows (No. 16J09849), JSPS-CNRS Bilateral Program (No. 146161100001), and JSPS Grant-in-Aid for Scientific Research on Innovative Areas “Mixed anion” (No. JP16H6441). Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS We gratefully thank, for the support of NIR PersL imaging by the prototype NIS-OPT in vivo bioimaging machine, Dr. M. Kamimura and Prof. K. Soga from Tokyo Univ. of Science. We also thank, for the Vis-NIR PersL imaging by the Clairvivo OPT in vivo bioimaging machine, Dr. L. Zhao and Prof. N. Komatsu from Kyoto Univ. and Dr. T. Amano and Dr. F. Yoshino from Shiga Univ. of Medical Science. J.X. personally acknowledges Dr. Y. Katayama from the Univ. of Tokyo for fruitful discussions on photostimulation-induced persistent luminescence and Prof. M. G. Brik who stayed at Kyoto Univ. as a visiting professor for fruitful discussions on group theory and spectroscopy of transition-metal/lanthanide ions.
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