Dual-channel visible and NIR-III upconversion optical probe for bioimaging

Fluorescence bioimaging, owing to its low invasiveness and real-time feedback capabilities, holds significant promise for biomedical applications. However, conventional fluorescent probes suffer from photobleaching and limited tissue penetration. In this study, we developed a dual-channel upconversion optical probe based on lanthanide-doped core-shell NaErF₄:Tm³⁺@NaYF₄:Yb³⁺ nanoparticles, capable of simultaneously emitting visible light at 654 nm and third near-infrared window (NIR-III) luminescence at 1523 nm under 980 nm excitation. The Tm³⁺ ions serve as energy traps, facilitating the Er³⁺→Tm³⁺→Er³⁺ energy transfer pathway, significantly enhancing both red and NIR-III emissions from the transitions of ⁴F_9/2 → ⁴I_15/2 and ⁴I_13/2 → ⁴I_15/2. Moreover, coating the core NaErF₄:Tm³⁺ nanoparticles with an active NaYF₄:Yb³⁺ shell led to a remarkable 210-fold enhancement in red emission intensity and the emergence of effective NIR-III emission This significant improvement in luminescence is attributed to the suppression of surface quenching by the core-shell structure and the enhanced absorption of 980 nm photons by Yb³⁺ ion in the shell. This dual-channel red and NIR-III luminescent nanoprobe could be a potential ideal probe for deep-tissue bioimaging. The ex vivo penetration experiment using porcine tissue demonstrated a deep tissue penetration of up to 10 mm in the NIR-III window. The Daphnia magna and Zebrafish treated with the nanoprobes display bright red emission without any background, indicating the excellent in vivo luminescent imaging capacity of NaErF₄:Tm³⁺@NaYF₄:Yb³⁺ nanoparticles. Our findings present a new class of dual-channel optical probes with visible and NIR-III emissions, enabling multiscale high-resolution biomedical imaging applications.