Tailoring Crystal Structure and Energy Modulation Pathways for Boosted NIR-II Luminescence Imaging beyond 1600 nm

Second near-infrared (NIR-II, 1000-1700 nm) optical imaging offers remarkable potential for noninvasive <i>in vivo</i> pathological diagnosis due to its low photon scattering and deep tissue penetration. However, current Nd-, Ho-, and Er-based nanoprobes emit within 1000-1550 nm, limiting efficient emission beyond 1600 nm. Herein, a general Mn doping strategy is proposed to boost NIR-II luminescence beyond 1600 nm in NaLnF₄:Yb/Tm nanocrystals by inducing crystal structure transformation (hexagonal [β] → cubic [α]), tuning phonon energy, and establishing efficient Tm-Mn energy transfer. Due to efficient energy transfer and multiphonon relaxation, α-NaLnF₄:Yb/Tm/Mn exhibited ∼100-fold-enhanced 1632 nm emission versus β-NaLnF₄. Further, inert coating yielded an additional 4-fold enhancement. Utilizing α-NaLuF₄:Yb/Tm/Mn@NaLuF₄ core-shell structures, high-resolution NIR-IIb (>1600 nm) <i>in vivo</i> imaging of intestine, vasculature, and bone fractures was achieved. This work provides a new strategy for designing efficient NIR-IIb nanoprobes for biomedical imaging.