Luminescence enhancement of CaF2:Nd3+ nanoparticles in the second near-infrared window for in vivo imaging through Y3+ doping

In vivo luminescent imaging in the second biological window (1000–1400 nm, NIR-II) has attracted increasing attention since it can provide high sensitivity to deep tissue in vivo imaging. Herein, we synthesized approximately 10–15 nm-sized NIR-II luminescent nanoparticles (CaF2:Nd3+ NPs). Furthermore, co-doped Y3+ was utilized to enhance the NIR-II luminescence of the CaF2:Nd3+ NPs via breaking the aggregation of Nd3+. The appearance of a (200) diffraction peak and the broadening of the interplanar spacing of the (111) plane both showed that the incorporated Y3+ can dissolve in CaF2 by occupying the Ca2+ sites to form a CaF2–YF3 solid solution. In particular, the addition of Y3+ can greatly enhance the of the NIR-II luminescence of CaF2:Nd3+ NPs. When the Y3+ doped concentration reached 0.30, the luminescence intensity of CaF2:Y3+,Nd3+ NPs was about 65 times that of CaF2:Nd3+ NPs. In addition, the quantum yield of Ca0.68Y0.30Nd0.02F2.32 NPs was 9.30% under the excitation of an 808 nm laser with 483 mW cm−2 power, which was about 3 times higher than that of CaF2:Nd3+ NPs (3.10%). The in vivo imaging results revealed that the in vivo imaging intensity of Ca0.68Y0.30Nd0.02F2.32 NPs was about 2.38-fold stronger than that of Ca0.98F2.02:Nd3+0.02 NPs. All of these results indicated that CaF2:Y3+,Nd3+ NPs can be regarded as potential in vivo imaging probes for biological imaging.