In situ elucidating phase transition and differentiated hydrogen-bonding nanoconfinement in single nano-hydrogel

Stimuli-responsive smart polymer hydrogels have garnered significant interest across multidisciplinary fields, with conventional time-resolved bulk characterization techniques extensively applied to probe their ensemble behavior. However, a critical microscopic profiling of their fundamental dynamics down to single nanoparticle level remains elusive, particularly for resolving transient volume phase transition and metastable intermediates triggered by thermal and hydrogen-bonding regulation. To bridge this gap, we demonstrated an in situ single-nanoparticle strategy integrating localized thermoregulation with surface plasmon resonance imaging by introducing programmable optical tweezer system. This platform enables real-time tracking of volume phase transitions within individual nano-hydrogels under localized thermoregulation. Crucially, we identify transient intermediate states and thermodynamics of this transition event. The intrinsic volume phase transition temperature of single nano-hydrogel invariant to thermal modulation rate is quantified, governed by the equilibrium of hydrogen-bond nanoconfinement within polymer structure. By tuning the crosslinking density of three-dimensional network polymers within a single nano-hydrogel, heterogeneous volume phase transition behavior through pH-controlled hydrogen bonding nanoconfinement is also decoded. This operando single-entity plasmonic imaging methodology, synergized with spatiotemporally controlled thermal fields, provides a platform for investigating stimulus-adaptive behavior of thermally responsive nanomaterials at the single nanoparticle level.