Dye-sensitized cascaded energy transfer for amplified 1525 nm luminescence in highly doped lanthanide nanoparticles

Fluorescence/luminescence imaging represents novel and promising visualization approaches. Nevertheless, the image quality is often compromised by tissue photon scattering, absorption, and autofluorescence, which introduce background interference and spectral crosstalk. The second near-infrared (NIR-II) window (1000-2000 nm) has emerged as a powerful solution to mitigate these limitations. Among the various excitation-emission pairs within this window, the combination of ~808 nm excitation and >1500 nm emission is desirable due to its deep tissue penetration, minimal water-heating, and the commercial availability of efficient 808 nm lasers, as well as the significantly reduced tissue scattering achieved in the NIR-IIb sub-window. In particular, Er ³⁺ -based nanostructures are premier candidates, prized for their characteristic, intense emission at ~1530 nm, which aligns perfectly with the NIR-IIb subwindow. However, the performance of the nanostructures is fundamentally constrained by the inherently small absorption cross-section of Er ³⁺ (σ ~10 ^-21 cm ² /ion@808 nm), a challenge common to many lanthanide nanoparticles due to f-f forbidden transitions. Dye sensitization, which employs organic dyes with absorption cross-sections orders of magnitude greater than those of lanthanide ions, presents a powerful strategy to overcome this absorption bottleneck. Nonetheless, the efficiency of dye sensitization is governed by stringent factors, primarily the distance between the dye donor and the lanthanide acceptor, with effective Förster resonance energy transfer (FRET) typically confined to a short range. Therefore, a fundamental challenge persists: how to maximize the energy transfer efficiency from the surface-bound dye to the entire heavily doped lanthanide core.

In a new paper published in Light: Science & Applications , a team of scientists, led by Professor Yulei Chang from State Key Laboratory of Luminescence Science and Technology, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, China, and co-workers have proposed an innovative active-shell engineering strategy. This system employs 808 nm excitation and uses a NaErF ₄ core as the luminescent center, thereby maximizing absorption of the 808 nm excitation. Meanwhile, a shell structure is introduced between the dye and the luminescent core to balance surface quenching and energy transfer efficiency. In the Yb ³⁺ -doped active-shell system, the luminescence intensity at 1525 nm is enhanced by nearly 2000-fold, providing a novel class of nanoprobes with superior performance for high-resolution imaging in deep tissues.

The research team designed a dye-sensitized core-shell structure, NaErF ₄ @NaYF ₄ : 50%Yb@ICG, utilizing Yb ³⁺ as an "energy relay" to achieve synergistic cascade energy transfer between ICG and the Er ³⁺ -rich core under 808 nm excitation. By introducing a Yb ³⁺ -mediated relay step into the direct ICG → Er ³⁺ energy transfer pathway, an ICG → Yb ³⁺ → Er ³⁺ cascade transfer route was established, allowing the energy that would otherwise be lost through nonradiative relaxation to be collected and harnessed for luminescence. In contrast to an inert shell that merely suppresses surface quenching, a 50% Yb ³⁺ doping concentration achieves an optimal balance between energy transfer efficiency and quenching. This cascade mechanism effectively populates the ⁴ I _13/2 energy level of Er ³⁺ and promotes the Er ³⁺ : ⁴ I _13/2 → ⁴ I _15/2 transition at 1525 nm, thereby endowing the structurally simple probe with superior downshifting luminescence performance.

To elucidate the microscopic mechanism of energy transfer, the team systematically measured the 980 nm lifetime evolution of Er@Y, Y@50Yb, and Er@50Yb (where Er represents NaErF ₄ , Y represents NaYF ₄ , and 50Yb represents 50%Yb) after conjugation with ICG. The results show that Er@Y@ICG (containing only Er ³⁺ ) exhibits the shortest 980 nm emission lifetime, Y@50Yb@ICG (containing only Yb ³⁺ ) displays the longest fluorescence lifetime, and Er@50Yb@ICG (containing both Er ³⁺ and Yb ³⁺ ) possesses a lifetime intermediate between the two, yet longer than that of Er@Y@ICG. These comparative results provide direct kinetic evidence for the Yb ³⁺ active shell-mediated ICG → Yb ³⁺ → Er ³⁺ cascade energy transfer. Furthermore, upon conjugation of ICG to Er@50Yb, the ICG excited-state lifetime decreases dramatically from 883 ps to 84 ps, corresponding to an energy transfer efficiency of nearly 90%. Using cyclooctatetraene (COT) as a triplet-state quencher, control luminescence measurements verify that the energy transfer occurs predominantly from the singlet excited state (S ₁ ) of ICG, which effectively circumvents triplet-state (T ₁ ) losses. The study also clarifies that the 50% Yb ³⁺ -doped active shell balances energy transfer efficiency and surface quenching, suppresses the concentration quenching inherent to Er ³⁺ -rich systems, and significantly enhances the electron population of the Er ³⁺ luminescent level. Consequently, the 1525 nm luminescence is enhanced by a factor of 1965 compared to the bare core, with brightness superior to that of conventional Nd ³⁺ -sensitized systems. This represents an order-of-magnitude enhancement rooted in luminescence physics, providing crucial support for the design of high-performance lanthanide-based near-infrared luminescent materials.

After PEGylation, the probe was used for high-contrast imaging of the mouse vascular system in the NIR-IIb window, achieving a full width at half maximum of 218 μm, a signal-to-background ratio of 3.09, and a blood circulation half-life of 53 minutes. This strategy provides new insights into the development of high-performance nanoprobe and into vascular science research.

Light: Science & Applications

10.1038/s41377-026-02302-9

Dye-sensitized cascaded energy transfer for amplified 1525 nm luminescence in highly doped lanthanide nanoparticles