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

Area of Science:

• Biomedical Engineering
• Materials Science
• Optical Imaging

Background:

• Second near-infrared (NIR-II) optical imaging (1000-1700 nm) enables noninvasive *in vivo* diagnosis due to low photon scattering and deep tissue penetration.
• Current nanoprobes emit below 1600 nm, limiting their utility for advanced imaging applications requiring longer wavelengths.

Purpose of the Study:

• To develop a general strategy for boosting NIR-II luminescence beyond 1600 nm in NaLnF4:Yb/Tm nanocrystals.
• To create efficient NIR-IIb (>1600 nm) nanoprobes for high-resolution *in vivo* biomedical imaging.

Main Methods:

• A manganese (Mn) doping strategy was employed to induce crystal structure transformation (hexagonal [β] to cubic [α]) in NaLnF4:Yb/Tm nanocrystals.
• Tuning of phonon energy and establishment of efficient Thulium-Manganese (Tm-Mn) energy transfer were utilized.
• Inert coating and core-shell structure fabrication (α-NaLuF4:Yb/Tm/Mn@NaLuF4) were performed to enhance luminescence.

Main Results:

• The manganese doping strategy resulted in a ~100-fold enhancement in 1632 nm emission from cubic α-NaLnF4:Yb/Tm/Mn nanocrystals compared to hexagonal β-NaLnF4.
• Inert coating further increased luminescence by an additional 4-fold.
• High-resolution NIR-IIb *in vivo* imaging of the intestine, vasculature, and bone fractures was successfully achieved using the developed core-shell nanoprobes.

Conclusions:

• The proposed Mn doping strategy is effective in significantly enhancing NIR-IIb luminescence beyond 1600 nm.
• The developed α-NaLuF4:Yb/Tm/Mn@NaLuF4 core-shell nanoprobes demonstrate great potential for advanced *in vivo* biomedical imaging.
• This work offers a novel approach for designing efficient NIR-IIb nanoprobes for pathological diagnosis.