Hexagonal-phase NaYF₄ (β-NaYF₄) has been acknowledged to be one of the most efficient doping hosts to prepare bright lanthanide-doped luminescent nano-bioprobes for various biomedical applications. However, to date, it remains a great challenge to synthesize ultra-bright lanthanide-doped β-NaYF₄ nano-bioprobes under a low reaction temperature by using conventional synthetic methods. Herein, we first develop an acetic acid (HAc)-mediated coprecipitation method for the preparation of ultra-bright lanthanide-doped β-NaYF₄ nanoprobes under a low reaction temperature at 200 °C. Based on a series of comparative spectroscopic investigations, we show that the use of HAc in the reaction environment can not only promote the rapid α–β phase transformation of NaYF₄ host at 200 °C within 1 h but also boost the absolute photoluminescence quantum yield (PLQY) of NaYF₄ nanocrystals to 30.68% for near-infrared emission and to 3.79% for upconversion luminescence, both of which are amongst the highest values for diverse lanthanide-doped luminescent nanocrystals ever reported. By virtue of their superior near-infrared luminescence, we achieve optical-guided dynamic vasculature imaging in vivo of the whole body at a high spatial resolution (23.8 µm) under 980 nm excitation, indicating its potential for the diagnosis and treatment evaluation of vasculature-related diseases.

Self-trapped excitons (STEs) emission from halide perovskites with strong exciton-phonon coupling has attracted considerable attention due to the widespread application in optoelectronic devices. Nevertheless, the in-depth understanding of the relationship between exciton-phonon coupling and luminescence intensity remains incomplete. Herein, a doping-enhanced exciton-phonon coupling effect is observed in Cs₃Cu₂I₅ nanocrystals (NCs), which leads to a remarkable increasement of their STEs emission efficiency. Mechanism study shows that the hetero-valent substitution of Cu⁺ with alkaline-earth metal ions (AE²⁺) causes a greater degree of Jahn–Teller distortion between the ground state and excited state structures of [Cu₂I₅]³⁻ clusters as evidenced by our spectral analysis and first-principles calculations. As a consequence, an X-ray detector based on these Cs₃Cu₂I₅:AE NCs delivers an X-ray imaging resolution of up to 10 lp·mm⁻¹ and a low detection limit of 0.37 μGy_air·s⁻¹, disclosing the potential of doping-enhanced exciton-phonon coupling effect in improving STEs-emission and practical application for X-ray imaging.