Intense red upconversion luminescence in Er3+-sensitized particles through confining the 1532 nm excitation energy

Highlights: • Luminescencequenching is mitigated by adding Ho³⁺ and Tm³⁺energy trapping centers. • Red UC luminescence is enhanced through confining 1532 nm excitation energy. • The deep penetration of 1532 nm NIR-II excitation and 660 nm red UC emission in biological tissue. -- Abstract: Lanthanide-doped upconversion (UC) materials are commonly excited by 700–1000 nm laser because of Yb³⁺ (980 nm excitation) and Nd³⁺ (808 nm excitation) are typically used as sensitizers. However, the excitation band located in the second near-infrared window (NIR-II, 1000–1700 nm) demonstrates the deeper penetration in biological tissue with minimum tissue scattering and autofluorescence. Herein, intense red UC emission in Er³⁺-sensitized NaLuF₄ particles under 1532 nm NIR-II excitation is presented. Red UC luminescence is significantly enhanced and purified through confining the 1532 nm excitation energy by Ho³⁺ and Tm³⁺ mediators, via an energy back transfer pathway (Er³⁺ → Ho³⁺ → Er³⁺ and Er³⁺ → Tm³⁺ → Er³⁺). Furthermore, when the particles injected into the fresh beef, red UC luminescence can be distinctly observed from the inside of beef under 1532 nm NIR-II excitation, implying the deep penetration of 1532 nm NIR-II excitation and 660 nm red UC emission. Our results suggest that bright red UC luminescence from Er³⁺-sensitized NaLuF₄ crystals with 1532 nm NIR-II excitation may be suitable for deep optical imaging.