Stabilizing Dye‐Sensitized Upconversion Nanosystems via Singlet Oxygen Spin Flipping

Abstract Near‐infrared (NIR) dye sensitization has been widely used to enhance the brightness of lanthanide‐doped upconversion nanoparticles (UCNPs). However, the stability of these dye‐sensitized upconversion nanosystems remains a significant challenge, primarily due to singlet oxygen ( ¹ O ₂ )‐induced damage to the NIR dyes. In this study, a simple strategy is presented to stabilize these nanosystems by flipping the spin of ¹ O ₂ through surface‐anchored 4‐aminotriphenylamine (4A‐TPA). The electron‐rich triphenylamine group adsorbs electrophilic ¹ O ₂ and flips its spin into ³ O ₂ via intersystem crossing of charge transfer states, while the amino group facilitates coordination with the UCNPs surface in parallel to the NIR sensitizing dyes. It is demonstrated that incorporating 4A‐TPA significantly enhances the stability of the commonly used IR806‐sensitized UCNPs system, improving its photostability by 27‐fold compared to the control without 4A‐TPA after 30 min of 808 nm irradiation. This stability improvement is further validated in aqueous environments with amphiphilic polymer coating, enabling stable cellular upconversion luminescence imaging under high laser power density (1 200 W cm ⁻² ). This work provides a simple yet powerful method to overcome the instability of dye‐sensitized upconversion systems, unlocking their potential for a variety of bioapplications ranging from bioimaging to photodynamic therapy.