Lanthanide (Ln³⁺)-doped luminescent nanoparticles (NPs) with emission in the second near-infrared (NIR-II: 1000-1700 nm) biological window have shown great promise in various bioapplications, but they are currently limited by the low absorption efficiency of Ln³⁺ due to the parity-forbidden 4f→4f electronic transition. 

In a study published in Angew. Chem. Int. Ed., a research group led by Prof. CHEN Xueyuan from Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences developed a novel strategy for the controlled synthesis of a new class of NIR-II luminescent nanoprobes based on Ln³⁺ co-doped CaS NPs.  

Through photoluminescence (PL) and PL decay analyses, researchers comprehensively surveyed the effect of NP size and doping concentration on the optical properties as well as the energy transfer processes in Ce³⁺ singly doped and Ce³⁺/Er³⁺ and Ce³⁺/Nd³⁺ co-doped CaS NPs.  

Besides, through sensitization by the allowed 4f→5d transition of Ce³⁺, they achieved intense NIR-II luminescence from Er³⁺ and Nd³⁺ with quantum yields of 9.3% and 7.7%, respectively, under blue light emitting diode (LED) chip excitation. 

After surface modification with a layer of amphiphilic phospholipids, these NPs can be stably dispersed in aqueous solution and explored as sensitive NIR-II luminescent nanoprobes for sensitive and selective detection of an important disease biomarker, xanthine, in human serum with a limit of detection (LOD) of 32.0 nM. 

This study provides a general approach for the development of highly efficient Ln³⁺-doped NIR-II luminescent nanoprobes through intrinsic antenna sensitization, thereby opening up a new avenue for the exploration of NIR-II luminescent nanoprobes towards versatile bioapplications, such as chip-based point-of-care testing for clinical bioassays.  

Schematic illustration of blue-LED-excitable NIR-II luminescent CaS:Ce³⁺/Er³⁺ and CaS:Ce³⁺/Nd³⁺ nanoprobes and their application for xanthine detection. (Image by Prof. CHEN’s group)