A New Class of Blue-LED-Excitable NIR-II Luminescent Nanoprobes Based on Lanthanide-Doped CaS Nanoparticles

Lanthanide (Ln³⁺ )-doped luminescent nanoparticles (NPs) with emission in the second near-infrared (NIR-II) biological window have shown great promise but their applications are currently limited by the low absorption efficiency of Ln³⁺ owing to the parity-forbidden 4f→4f electronic transition. Herein, we developed a strategy for the controlled synthesis of a new class of NIR-II luminescent nanoprobes based on Ce³⁺ /Er³⁺ and Ce³⁺ /Nd³⁺ co-doped CaS NPs, which can be effectively excited by using a low-cost blue light-emitting diode chip. Through sensitization by the allowed 4f→5d transition of Ce³⁺ , intense NIR-II luminescence from Er³⁺ and Nd³⁺ with quantum yields of 9.3 % and 7.7 % was achieved, respectively. By coating them with a layer of amphiphilic phospholipids, these NPs exhibit excellent stability in water and can be exploited as sensitive NIR-II luminescent nanoprobes for the accurate detection of an important disease biomarker, xanthine, with a detection limit of 32.0 nm.