image: Figure 1 | Probing UCL emission driven by topology-based ETNs in core–shell UCNPs. (A) Schematic showing 450‑nm UCL from Yb,Tm@Y, Yb@Tm@Y, and Tm@Yb@Y UCNPs under 980 nm laser light alone (left) and combined 980 nm and 808 nm laser light (right). (B) Illustration of how Yb3+ sensitizers and Tm3+ emitters are arranged in Tm@Yb@Y UCNPs to form energy transfer networks that guide energy toward the core–shell interface, boosting UCL for U-STED imaging. The intensity profile along the dashed line of a single nanoparticle is shown. Excitation/depletion intensities: 0.03 MW cm-2 (980 nm) and 1 MW cm-2 (808 nm). Pixel dwell time: 10 ms. Scale bar: 500 nm
Credit: Min Gu et al.
Super-resolution microscopy has dramatically advanced the life sciences by enabling visualization of biological structures at the nanoscale, far below the traditional optical diffraction limit. Among the most widely adopted super-resolution techniques, stimulated emission depletion (STED) microscopy offers exceptional spatial resolution, yet it faces significant challenges that limit its broader applicability. Conventional STED requires high-intensity pulsed lasers and organic fluorophores, which are prone to rapid photobleaching and chemical degradation. These constraints not only restrict long-term imaging but also introduce heat load and photodamage, limiting its use with sensitive biological specimens. In contrast, lanthanide-doped upconversion nanoparticles (UCNPs) represent a promising alternative, offering robust photostability, excitation in the near-infrared (NIR) range, and minimal photodamage. Despite these advantages, implementing highly efficient optical switching in UCNPs remains challenging because the nonlinear upconversion process relies on intricate energy transfer dynamics between sensitizer and emitter ions, which are difficult to control precisely.
In a breakthrough study now published in Light: Science & Applications, a research team led by Academician Min Gu at the School of Artificial Intelligence Science and Technology (SAIST) and Institute of Photonic Chips (IPC), University of Shanghai for Science and Technology (USST), reports a novel approach for engineering UCNPs to overcome these limitations. The researchers developed a topology-driven strategy for designing energy transfer networks (ETNs) within nanoparticles, dramatically reducing the laser powers required for upconversion STED (U-STED) microscopy. By carefully re-engineering the spatial distribution and interaction pathways of lanthanide ions, the team demonstrated that energy flow within UCNPs could be guided with remarkable precision, enhancing both emission intensity and optical switching efficiency.
The core of their approach involves 50-nm core–shell UCNPs in which sensitizer ions (Yb3+) and emitter ions (Tm3+) are confined to distinct nanoscale regions (Figure 1). This spatial separation forces excitation energy to traverse the core–shell interface, where Yb–Tm interactions are maximized. The resulting topology-driven architecture forms an extended ETN that promotes long-range energy migration among Yb3+ ions, leading to highly efficient energy transfer to Tm3+ emitters. Consequently, the nanoparticles generate intense 450-nm emission using just 0.03 MW cm-2 of 980-nm excitation, roughly ten times lower than the excitation intensity typically required in conventional U-STED systems.
In addition to enhancing emission, the ETN topology significantly strengthens cross-relaxation interactions among Tm3+ ions—critical for effective population depletion under STED illumination. By amplifying these pathways, the system achieves highly efficient optical switching at low laser powers, reducing both energy consumption and thermal stress during imaging. The researchers measured an exceptionally low saturation intensity of 0.06 MW cm-2 under 980-nm excitation and 808-nm depletion (Figure 2). In practical imaging experiments, the nanoparticles enabled a lateral resolution of 65 nm, achieved with just 0.03 MW cm-2 excitation and 1 MW cm-2 depletion intensity, representing a tenfold reduction in excitation power and a threefold reduction in depletion power compared with previous state-of-the-art U-STED methods, while maintaining excellent photostability (Figure 3).
“Our topology-driven design fundamentally restructures how energy migrates within upconversion nanoparticles,” explained Dr. Simone Lamon, the lead scientist of the study. “By directing energy flow through extended networks rather than relying solely on local ion-to-ion interactions, we achieve strong emission and highly efficient switching at unprecedentedly low laser powers.”
Professor Qiming Zhang, who supervised the project, emphasized the broad impact of the strategy: “This approach opens new pathways for low-power super-resolution imaging. The reduced heat load and minimal photodamage will be invaluable for long-term observation of delicate biological samples.”
Looking ahead, topology-driven UCNPs hold promise across multiple fields, including life sciences, nanophotonics, and optical memory technologies. The work establishes a versatile materials framework for designing bright, stable, and energy-efficient probes suitable for next-generation super-resolution microscopy
Journal
Light Science & Applications
Article Title
Topology-driven energy transfer networks for upconversion stimulated emission depletion microscopy