Intrinsic Redox‐Responsive NIR‐II Lanthanide Luminescent Nanoprobes Enabled by Phonon‐Assisted Cooperative Quenching for Reversible Redox Imaging In Vivo

ABSTRACT Lanthanide nanoparticles (LnNPs) are photostable second near‐infrared window (NIR‐II) emitters well suited to long‐term redox imaging. However, responsive LnNP nanoprobes generally require surface‐installed molecular recognition components to transduce chemical changes into luminescence signals, which may introduce additional functionalization complexity and compromise long‐term signal reliability. Here, we identify a redox‐regulated phonon‐assisted cooperative quenching (PACQ) phenomenon in NaCeF 4 :Nd, where Ce(III)/Ce(IV) conversion modulates Nd(III) emission through lattice‐mediated interactions. To elucidate this intrinsic response, a model NaLuF 4 @NaCeF 4 :Nd architecture was constructed to expose the NaCeF 4 :Nd shell and reveal the underlying PACQ mechanism. Oxidation‐induced changes in Ce electronic configuration and local lattice environment weaken Ce‐mediated quenching and restore NIR‐II Nd(III) emission. Based on this mechanism, we further develop a ratiometric NaErF 4 :Yb@NaLuF 4 @NaCeF 4 :Nd nanoprobe (RPACQ), where the Er(III) emission provides a stable internal reference. RPACQ exhibits reversible H 2 O 2 /glutathione responses, with second‐scale activation, concentration‐dependent ratiometric responses in vitro, and excellent photostability. In a cerebral ischemia‐reperfusion injury (CIRI) model, RPACQ provides proof‐of‐concept for in vivo redox imaging. Collectively, this work establishes an intrinsic lanthanide nanoparticle strategy for reversible NIR‐II luminescence regulation without external molecular reporters, potentially opening new avenues for engineering responsive lanthanide nanomaterials.