Synthesis and optical properties of a Y3(Al/Ga)5O12:Ce3+,Cr3+,Nd3+ persistent luminescence nanophosphor: a promising near-infrared-II nanoprobe for biological applications.

Persistent luminescence nanophosphors (PLNPs) emitting in the second near-infrared window (1000-1700 nm, NIR-II) are emerging as one promising class of in vivo bio-imaging agents due to their unique advantages including non-autofluorescence and low optical scattering in tissues. Currently, it remains a great challenge to synthesize nanosized lanthanide-doped inorganic NIR-II phosphors with a good persistent luminescence performance. Herein, we present a salt microemulsion method for synthesizing Ce3+, Cr3+, Nd3+ codoped Y3(Al/Ga)5O12 nanocrystals, which generate multi-wavelength persistent luminescence in the visible (∼508 nm, 5d1→ 4f of Ce3+), the first near-infrared window (∼890 nm, 4F3/2→4I9/2 of Nd3+) and NIR-II (∼1063 nm, 4F3/2→4I11/2 of Nd3+) regions. Under illumination of a 410 nm diode (3 W) for 10 min, the observed duration time of NIR-II persistent luminescence is as long as 60 min at room temperature. Moreover, the persistent luminescence can be excited efficiently by multiple excitation sources including a blue diode, white LEDs and an X-ray generator, which is crucial for deep tissue imaging applications. By comparing the penetration depth between NIR-I and NIR-II persistent luminescence through chicken breast, we prove that NIR-II photons exhibit a deeper optical penetration length (3.9 mm) than that of the NIR-I ones (2.5 mm). In addition, the NIR signals can still be detected 3 min after ceasing the excitation source by a small animal imaging system (InGaAs detector) when the thickness of the covering chicken breast is 20 mm. These results show great promise for Y3(Al/Ga)5O12:Ce3+,Cr3+,Nd3+ nanocrystals as a PLNP for bio-imaging applications with deep penetration depth and a high signal-to-noise ratio.