The Active‐Core/Active‐Shell Approach: A Strategy to Enhance the Upconversion Luminescence in Lanthanide‐Doped Nanoparticles

Abstract

Nanoparticles of NaGdF4 doped with trivalent erbium (Er3+) and ytterbium (Yb3+) are prepared by a modified thermal decomposition synthesis from trifluoroacetate precursors in 1‐octadecene and oleic acid. The nanoparticles emit visible upconverted luminescence on excitation with near‐infrared light. To minimize quenching of this luminescence by surface defects and surface‐associated ligands, the nanoparticles are coated with a shell of NaGdF4. The intensity of the upconversion luminescence is compared for nanoparticles that were coated with an undoped shell (inert shell) and similar particles coated with a Yb3+‐doped shell (active shell). Luminescence is also measured for nanoparticles lacking the shell (core only), and doped with Yb3+ at levels corresponding to the doped and undoped core/shell materials respectively. Upconversion luminescence was more intense for the core/shell materials than for the uncoated nanoparticles, and is greatest for the materials having the “active” doped shell. Increasing the Yb3+ concentration in the “core‐only” nanoparticles decreases the upconversion luminescence intensity. The processes responsible for the upconversion are presented and the potential advantages of “active‐core”/“active‐shell” nanoparticles are discussed.

RGD Peptide–Conjugated Multimodal NaGdF4:Yb3+/Er3+ Nanophosphors for Upconversion Luminescence, MR, and PET Imaging of Tumor Angiogenesis

“…4A). Our synthesized NaGdF4: Yb 31 /Er 31 UCNPs had a hexagonal structure that has been reported to show much higher fluorescent efficiency than does a cubic structure (26)(27)(28). Upconversion luminescence was found to be extremely resistant to photobleaching, and no photoblinking occurred.…”

Upconversion nanoparticle as a theranostic agent for tumor imaging and therapy

“…Xie et al 39 and Wang et al 40 reported the synthesis of monodispersed Nd 3þ -sensitized coreÀshell nanoparticles with narrow size distribution and regular morphology using thermal decomposition method. Vetrone et al 41 reported that the nanoparticles coated with a shell of NaGdF 4 could minimize the luminescence quenching caused by the surface defects and surface-associated ligands. They also found that the UCL of the coated nanoparticles was more intense for the core/shell structure.…”

Recent advances in the optimization and functionalization of upconversion nanomaterials for in vivo bioapplications

“…For example, to absorb more excitation photons at 980 nm, Capobianco and coworkers 39 41 have also been fabricated to enhance the upconversion emission intensities. In these core-shell structured UCNPs, the amount of Yb 3 þ in the active shell needs to be optimized because the Yb 3 þ can also provide a route to transfer the absorbed excitation energy to the surface quenching centers, which weakens the upconversion emission.…”