Regulating the upconversion luminescence of lanthanide-doped nanoparticles through multilayered structure

The concept of upconversion luminescence (UCL) was first proposed by Bloembergen in 1959. As a nonlinear optical process, photon upconversion in lanthanide-doped upconversion nanoparticles (UCNPs) features by sequential absorption of multiple low-energy photons via the long-lived intermediate energy states of lanthanide ions and emission of highenergy photons. Unlike common luminescent materials such as organic dyes and quantum dots, lanthanide-doped UCNPs exhibit large anti-Stokes shift, excellent stability against photobleaching, long fluorescence lifetime and sharp multiline emissions, enabling a variety of applications, such as bioimaging, anti-counterfeiting, diagnosis, photodynamic therapy, background-free biosensing, solar energy harvesting and super-resolution nanoscopy. However, it is still challenging for these UCNPs to be utilized practically because of limited UCL brightness. Firstly, the UCL is severely quenched by surface quenchers like ligands and defects. Moreover, the cross-relaxation between lanthanide-doped ions suppresses upconversion efficiency to a large extent. What's worse, the limited excitation light harvesting due to the parity-forbidden nature of 4f-4f electronic transitions still remains a hindrance for UCL enhancement. Design of the core-shell (C/S) structure, via coating a protecting shell on the luminescent core, can significantly suppress the energy transfer from core to surface quenchers (defects, ligands, etc.) for lanthanide-doped UCNPs. Therefore, the C/S engineering of lanthanide-doped UCNPs can enhance UCL significantly and has been widely adopted and investigated since it was first proposed in 2007. Recently, multilayered structures have attracted more and more attention because these structures offer versatile modulation ways on their optical properties and make them highly potential for application. In this review, we summarize the merits of these multilayered UCNPs and their practical applications in various fields. First of all, we introduce the flexible modulation on UCL performance via multilayered structure, such as full-color emissions and orthogonal excitations-emissions, which can be applied in full-color display and image-guided photodynamic therapy. Secondly, we emphasize the controllable energy upconversion and migration in multilayered UCNPs, enriching sensitizer-activator pairs with an efficient upconversion. Subsequently, we describe the significant enhancement in brightness and upconverting efficiency for heavily doped UCNPs via designing multilayered structure, with minimizing the energy loss and cross-relaxation. Last but not the least, we present the distinguished lifetime tunability of multilayered UCNPs, which are favorable for multiplexing, deep-tissue imaging and multi-level anti-counterfeiting. The multilayered structures make it more possible for us to design and synthesize bright, versatile lanthanide-doped UCNPs to meet the demands of various applications. However, challenges still remain in practical utilization of these UCNPs. On the one hand, the absorption cross-section of lanthanide ions is relatively small for harvesting excitation light. Introduction of organic antenna, construction of photonic crystals and plasma enhancement by composition with metal nanostructures may also be applicable for these multilayered UCNPs. On the other hand, it is urgent to develop a simple and controllable method for constructing multilayered UCNPs. The great progress of artificial intelligence may be helpful for us to synthesize UCNPs with complex structures more easily and controllably.

Automated summary

Design of core-shell structure significantly enhances upconversion luminescence (UCL) of lanthanide-doped UCNPs and multilayered structures offer versatile modulation ways for their optical properties and potential applications. Challenges still remain in practical utilization of these UCNPs, but progress in artificial intelligence may help develop simple methods for constructing complex structures of UCNPs.