Emergent stoichiometric host-guest recognition on ligated colloidal surfaces via dynamic ligand reorientation

Physical adsorption on ligand-protected colloidal nanoparticles is central to their properties and applications, yet adsorption events on dynamic ligand shells are rarely defined, counted, or quantified with molecular precision. Here we show that on a native ligand shell without pre-designed receptor-like motifs, molecular adsorption can trigger stoichiometric host-guest recognition through dynamic ligand reorientation. In aqueous dispersion, atomically precise [Au25(p-MBA)18]− (p-MBA = para-mercaptobenzoic acid) nanoclusters accommodate tetraalkylammonium ions within transient aromatic pockets formed by three to four neighboring p-MBA ligands. NMR titration and diffusion-ordered spectroscopy reveal an apparent saturated binding number of six guests per nanocluster, with an apparent association constant of ~3.5 × 104 M−1 and ΔG of ~−25 kJ•mol−1 for tetrapropylammonium binding. Molecular simulations and free-energy analysis rationalize this stoichiometry through a cooperative yet self-limiting mechanism, in which ligand reorientation creates bowl-like pockets stabilized by multiple CH···π interactions while steric constraints limit further guest accommodation. The guest-size- and surface-dependent binding behaviors further reveal the adaptive nature of emergent pockets. These findings change the role of the ligand shell from a stabilizing layer or diffuse adsorption medium to an active supramolecular interface capable of producing molecularly defined recognition, paving the way for adaptive nanomaterials, selective sensing, and stimuli-responsive colloidal systems. Physical adsorption on ligand-protected colloidal nanoparticles is central to their properties and applications, yet adsorption events on dynamic ligand shells are rarely characterized with molecular precision. Here the authors show that molecular adsorption can trigger stoichiometric host-guest recognition through dynamic ligand reorientation.