Upconversion nanoparticles: two infrared photons in, one visible photon out

Lanthanide upconversion written as a sum: two near-infrared photons absorbed by ytterbium ions in a NaYF₄ core are relayed to erbium, which emits one visible photon. Under the sum, the energy ladder, how a shell lifts quantum yields from fractions of a percent towards 13%, and uses written as small equations: hidden inks, 28 nm nanoscopy and zebrafish embryos.

1. Two photons in: Yb³⁺ ions absorb near-infrared light (980 nm excitation) and pass the energy to Er³⁺ in two transfers (Jung et al. 2015). 2. One photon out: Er³⁺ emits in two green bands and one red band; the red mostly via ⁴I₁₁/₂ → ⁴I₁₃/₂ → ⁴F₉/₂ (Jung et al. 2015). 3. A shell keeps the light: Bare 10–100 nm particles: 0.005–0.3% quantum yield, bulk 3% (Boyer & van Veggel 2010); a core in an ultrathick shell: up to 13% (Liu & Jiang 2026). 4. Uses: Inks read under 980 nm (Xu et al. 2019), STED nanoscopy at 28 nm (Liu et al. 2017), particles in zebrafish embryos with minimal toxicity (Mirmajidi et al. 2024); little autofluorescence background (Liu & Chen 2026).

Starting materials: NaYF₄ nanocrystals doped with Yb³⁺ (sensitizer) and Er³⁺ (activator), grown inside a shell. Product: Nanoparticles that turn 980 nm near-infrared light into visible green and red light.

Jung et al., “The preferred upconversion pathway for the red emission of lanthanide-doped upconverting nanoparticles, NaYF4:Yb3+,Er3+”, Phys. Chem. Chem. Phys. 2015 (doi:10.1039/c5cp01634g); Boyer & van Veggel, “Absolute quantum yield measurements of colloidal NaYF4: Er3+, Yb3+ upconverting nanoparticles”, Nanoscale 2010 (doi:10.1039/c0nr00253d); Liu & Jiang, “Brightening Upconversion Nanoparticles”, Acc. Chem. Res. 2026 (doi:10.1021/acs.accounts.5c00910); Liu & Chen, “Photophysics-Guided Upconversion Nanosystems for Sensing”, Acc. Chem. Res. 2026 (doi:10.1021/acs.accounts.5c00916); Liu et al., “Amplified stimulated emission in upconversion nanoparticles for super-resolution nanoscopy”, Nature 2017 (doi:10.1038/nature21366); Xu et al., “Dual-Mode, Color-Tunable, Lanthanide-Doped Core-Shell Nanoarchitectures for Anti-Counterfeiting Inks and Latent Fingerprint Recognition”, ACS Appl. Mater. Interfaces 2019 (doi:10.1021/acsami.9b10989); Mirmajidi et al., “Nano-bio interactions of Gum Arabic-stabilized lanthanide-based upconverting nanoparticles: in vitro and in vivo study”, J. Mater. Chem. B 2024 (doi:10.1039/d4tb01579g); Wang et al., “Tuning upconversion through energy migration in core-shell nanoparticles”, Nat. Mater. 2011 (doi:10.1038/nmat3149).

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