Figure 2.

UCNP characteristics.

Notes: (A) A cross-sectional schematic representation of a typical UCNP. This includes an optically inert host crystal where optically active Ln³⁺ activator and sensitizer ions are embedded by replacing the cations of host matrix in the crystallization process. (B) Lanthanide-based UCNPs in colloidal solution are able to convert NIR light into different wavelengths in the VIS region of the EM. Reproduced from Zhong Y, Tian G, Gu Z, et al. Elimination of Photon Quenching by a Transition Layer to Fabricate a Quenching-Shield Sandwich Structure for 800 nm Excited Upconversion Luminescence of Nd³⁺-Sensitized Nanoparticles. Adv Mater. 2013;26(18):2831–2837.²⁹ Publisher: John Wiley and Sons. © 2013 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. (C) Ln³⁺ ions exhibit different emission spectral bands under NIR excitation, spanning from NIR to VIS and UV regions as each Ln³⁺ has its unique energy level structure.

Abbreviations: EM, electromagnetic; NIR, near infrared; UCNP, upconverting nanoparticle; UV, ultraviolet; VIS, visible.