Sub-20\nnm Core–Shell–Shell Nanoparticles\nfor Bright Upconversion and Enhanced Forster Resonant Energy\nTransfer

Abstract

Upconverting nanoparticles provide\nvaluable benefits as optical\nprobes for bioimaging and Förster resonant energy transfer\n(FRET) due to their high signal-to-noise ratio, photostability, and\nbiocompatibility; yet, making nanoparticles small yields a significant\ndecay in brightness due to increased surface quenching. Approaches\nto improve the brightness of UCNPs exist but often require increased\nnanoparticle size. Here we present a unique core–shell–shell\nnanoparticle architecture for small (sub-20 nm), bright upconversion\nwith several key features: (1) maximal sensitizer concentration in\nthe core for high near-infrared absorption, (2) efficient energy transfer\nbetween core and interior shell for strong emission, and (3) emitter\nlocalization near the nanoparticle surface for efficient FRET. This\narchitecture consists of β-NaYbF₄ (core) @NaY_{0.8–<i>x</i>}Er_<i>x</i>Gd_0.2F₄ (interior shell) @NaY_0.8Gd_0.2F₄ (exterior shell), where sensitizer\nand emitter ions are partitioned into core and interior shell, respectively.\nEmitter concentration is varied (<i>x</i> = 1, 2, 5, 10,\n20, 50, and 80%) to investigate influence on single particle brightness,\nupconversion quantum yield, decay lifetimes, and FRET coupling. We\ncompare these seven samples with the field-standard core–shell\narchitecture of β-NaY_0.58Gd_0.2Yb_0.2Er_0.02F₄ (core) @NaY_0.8Gd_0.2F₄ (shell), with sensitizer and emitter ions codoped in\nthe core. At a single particle level, the core–shell–shell\ndesign was up to 2-fold brighter than the standard core–shell\ndesign. Further, by coupling a fluorescent dye to the surface of the\ntwo different architectures, we demonstrated up to 8-fold improved\nemission enhancement with the core–shell–shell compared\nto the core–shell design. We show how, given proper consideration\nfor emitter concentration, we can design a unique nanoparticle architecture\nto yield comparable or improved brightness and FRET coupling within\na small volume.