A mini-review on recent progress of new sensitizers for luminescence of lanthanide doped nanomaterials

Trivalent lanthanide (Ln³⁺) doped luminescent nanocrystals are promising for applications ranging from biosensor, lasing, super-resolution nanoscopy, information security and so on. Although the utility prospect is of great attractions, the light absorption of these lanthanide doped nanocrystals is inherently weak due to the electric dipole-forbidden 4f → 4f transitions. Even worse, the quantum yields of upconverison nanocrystals are very low, which will unavoidably hinder their further applications. In a typical lanthanide luminescent nanosystem, both sensitizers as light absorption centers and activators as light emitting centers are necessary and important for desired luminescence properties. Among various sensitization systems, only Yb³⁺ and Nd³⁺ are considered as the most efficient sensitizers. Thus, the corresponding excitation wavelengths are strictly limited around 980 and 808 nm. To enrich excitation wavelengths and boost luminescence intensity, exploring more sensitization units that possess larger absorption cross section, higher efficiency of energy transfer process and independent excitation is imperative and beneficial for the demands of different applications, such as broadened absorption in near infrared (NIR) region for higher conversion efficiency of solar cells, prolonged excitation wavelength to second near infrared windows region (NIR II, 1,000-1,700 nm) for in vivo fluorescence imaging with deeper tissue depth and higher spatial resolution, more orthogonal excitations and emissions to improve optical multiplexing, and so on. Therefore, in the review, we primarily conclude several major energy transfer mechanisms from sensitizers to activators. Then we present three kinds of sensitizers, including lanthanide ions, organic dyes and quantum dots (QDs), and introduce the newly designed sensitization system that allows us to exploit superior excitation wavelength and amplify luminescence intensity. Finally, several future challenges and opportunities for the sensitizing strategies are discussed in hope of directing and broadening the applications of lanthanide nanosystem.