A Multi‐Stimuli‐Responsive Pd ₂ L ₄ Metallacage for Amino Acid Sensing

ABSTRACT Self‐assembled metallacages with stimuli‐responsive structural transformation and optical tunability present great potential in sensing and detection applications. Herein, the design and synthesis of a multi‐stimuli‐responsive Pd 2 L 4 ‐type metallacage ( MC ) are reported, which is constructed through the coordination‐driven self‐assembly of triphenylamine‐based dipyridyl ligand and Pd(II) ions. MC undergoes reversible disassembly upon interaction with specific basic organic ligands and reassembles in the presence of acidic reagents. MC demonstrates an apparent fluorescence resonance energy transfer (FRET) emission enhancement in the presence of naphthalene disulfonate (NDS) isomers and highly selective binding towards four NDS isomers, where, only upon binding with 2,6‐NDS, a solution‐to‐gel transition is observed, due to the specific electrostatic and π–π interactions between MC and 2,6‐NDS. Significantly, MC enables highly selective and rapid detection of thiol‐containing amino acids with a detection limit of 1.22 × 10 −6 M via a self‐destructive fluorescence detection mechanism. A facile test strip based on this cage has also been developed to detect cysteine visually. This work widens the application scopes of self‐assembled metallacages and opens new perspectives for building stimuli‐responsive supramolecular coordination complexes.