Toward Rechargeable Persistent Luminescence for the First and Third Biological Windows via Persistent Energy Transfer and Electron Trap Redistribution

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 Cr to Er, we report a novel garnet persistent phosphor of YAlGaO codoped with Er and Cr (YAG G:Er-Cr), which shows intense Cr PersL (∼690 nm) in the deep red region matching well with the first biological window (NIR-I, 650-950 nm) and Er 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 Er can achieve higher spatial resolution and more accurate signal location than that of Cr due to the reduced light scattering at longer wavelengths. Furthermore, by utilizing two independent electron traps with two different trap depths in YAG G:Er-Cr, the Cr/Er 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.

Adding memory to pressure-sensitive phosphors

“…Clearly, this opens a new opportunity for information storage and retrieval. The mechanical stimulation provides a unique way to write information, in addition to the methods of writing by photoexcitation 41,42 . This information can be stored in the phosphor for an increased period of time because the reshuffling of electrons to deep traps minimizes the thermal detrapping at ambient temperature.…”

“…Achieving high imaging resolution and deep tissue penetration from phosphors with intense and long-duration persistent luminescence in excess of 1000 nm is still an issue. Recently, a reshuffling or transfer mechanism was reported for ZnGa 2 O 4 doped with Cr 3+ , LiGa 5 O 8 doped with Cr 3+ and Y 3 Al 2 Ga 3 O 12 codoped with Er 3+ and Cr 3+ , opening the possibility for excitation-free and background-free optical imaging 42,44,45 . In all three compounds, the reshuffling of charges from deep traps to shallow traps is realized by photostimulation (977 nm NIR lamp, white LED, 660 nm LED), observed in the TL glow curves as an increase in occupation at shallow trap levels.…”

Broadband NIR photostimulated luminescence nanoprobes based on CaS:Eu2+,Sm3+ nanocrystals

“…43,44 By Gaussian tting to the TL glow curves, three TL glow peaks at 261, 344 and 416 K for CaS:Eu 2+ NCs and three TL glow peaks at 361, 442 and 518 K for CaS:Eu 2+ ,Sm 3+ NCs were de-convoluted, corresponding to different trap depths in the lattice of the NCs. [45][46][47] Based on Chen's equation, 48 the trap depths were calculated to be 0.33, 0.63 and 0.92 eV for CaS:Eu 2+ NCs and 0.47, 0.76 and 1.08 eV for CaS:Eu 2+ ,Sm 3+ NCs (Fig. S14 †).…”

Broadband infrared LEDs based on europium-to-terbium charge transfer luminescence

“…Large efforts have already been undertaken to optimize IR luminescent materials for the above-mentioned applications, starting from the well-known luminescence transitions such as 3 d –3 d , 4 f –4 f , or 5 d –4 f 4 – 10 , 13 , 20 . On the contrary, charge transfer (CT) states have remained below the radar because they often cause luminescence quenching, rather than generating luminescence themselves.…”