Energy-Relay-Engineered Upconversion Emission with Intrinsic Environmental Responsiveness

Lanthanide-based upconversion nanoparticles (UCNPs) offer excellent photostability and large anti-Stokes shifts but are often limited by poor environmental responsiveness, hindering sensing applications. To address this challenge, we developed an energy-relay strategy by integrating near-infrared (NIR) dyes with Er³⁺/Tm³⁺ doped UCNPs. Under 980 nm excitation, Yb³⁺ transfers energy to Tm³⁺ and Er³⁺, excited Tm³⁺ ions then pass their energy to NIR dyes such as Cy7.5, which subsequently relay it back to Yb³⁺ ions, enabling a second Yb³⁺-to-Er³⁺ energy transfer. This cascade process amplifies Er³⁺ emission while quenching Tm^{3 +} emission. In Cy7.5-modified Er³⁺/Tm³⁺ doped UCNPs (NaYbF₄:Er@NaYbF₄@NaYbF₄:Tm@NaYbF₄@NaLuF₄), the energy-relay effect enhances the Er³⁺/Tm³⁺ ratiometric signal by two orders of magnitude. The process is highly dependent on the dopant architecture, as swapping Er³⁺ and Tm³⁺ positions disrupts the relay. The generality of this strategy is supported by similar energy-relay observed with Cy7 and IR806. Despite the incorporation of dyes, our energy-relay design retains the excellent photostability characteristic of lanthanide upconversion while introducing environmental responsiveness. As a proof of concept, the energy-relay nanoprobe demonstrated high sensitivity in both thermal and chemical sensing. Our findings establish energy-relay engineering as a versatile design principle for designing stable, efficient, and responsive upconversion nanoprobe platforms.