Exploring energy distribution of photon upconversion and downconversion luminescence in lanthanide-doped nanoparticles

Lanthanide-doped nanoparticles exhibiting short-wavelength infrared (SWIR) luminescence have emerged as promising luminescent probes for advanced in vivo imaging with enhanced tissue penetration, sensitivity and resolution. However, a key bottleneck remains in the limited luminescence efficiency, which fundamentally stems from the complicated energy levels and transition pathways possessed by lanthanide emitters. This work aims to perform a spectroscopic study on several lanthanide-doped nanoparticles to illustrate their detailed photon transfer and energy distribution processes. A luminescence spectroscopy system is first set up, allowing characterisation of broad emission spectra spanning the ultraviolet to the SWIR region. Then, Er- and Tm-activated nanoparticles are examined, and the energy transition processes corresponding to individual characteristic emission peaks are interpreted based on their measured energy level diagrams alongside the excitation-emission relations. Finally, the influence of doping concentrations as well as additional co-dopants are investigated to explore potential approaches to regulating the energy distribution. The results obtained here may be exploited to engineer lanthanide-doped nanoparticles with concentrated SWIR emission for efficient in vivo optical imaging in the future.