Probing the dimensionality of confinement in soft nanoreactors through plasmonic nanoparticles nucleation

Soft nanoscale confinement fundamentally alters the thermodynamic and kinetic landscape of chemical reactions, yet a predictive understanding of how nanoreactor geometry dictates reactivity remains elusive. In this work, we demonstrate that the dimensionality of soft confinement governs the onset of nanomaterial formation. By systematically tuning the water-to-surfactant molar ratio () in self-assembled lecithin reverse micelles, we decoupled the geometry of the host template from its chemical composition. We utilized the in situ nucleation of gold nanoparticles (AuNPs) as an active optical and structural probe across isotropic, anisotropic, and bulk-like spatial domains. Correlating UV-Vis spectroscopy with small-angle X-ray scattering (SAXS) analyses, we reveal that confinement dimensionality acts as an ON/OFF switch for the reduction of gold ions. Under extreme isotropic confinement (), where the interfacial water network is severely restricted, AuNP formation is suppressed for at least 30 days. Conversely, a hydration-induced structural transition toward anisotropic, wormlike domains () coincides with the onset of AuNP nucleation and yields the highest reaction extent. Finally, further expansion into bulk-like regimes () dilutes the gold precursor within the aqueous domains, attenuating the reaction yield. We rationalize these trends within a phenomenological framework based on classical nucleation theory, highlighting confinement dimensionality as a design parameter for soft nanoreactors and tailored plasmonic nanomaterials.