Theory of Solvation-Controlled Reactions in Stimuli-Responsive Nanoreactors

Abstract

Metallic nanoparticles embedded in stimuli-responsive polymers can be regarded as nanoreactors since their catalytic activity can be changed within wide limits: the physicochemical properties of the polymer network can be tuned and switched by external parameters, e.g. temperature or pH, and thus allows a selective control of reactant mobility and concentration close to the reaction site. Based on a combination of Debye's model of diffusion through an energy landscape and a two-state model for the polymer, here we develop an analytical expression for the observed reaction rate constant k obs . Our formula shows an exponential dependence of this rate on the solvation free enthalpy change∆G sol , a quantity which describes the partitioning of the reactant in the network versus bulk. Thus, changes in∆G sol , and not in the diffusion coefficient, will be the decisive factor affecting the reaction rate in most cases. A comparison with recent experimental data on switchable, thermosensitive nanoreactors demonstrates the general validity of the concept.

Thermosensitive Cu2O–PNIPAM core–shell nanoreactors with tunable photocatalytic activity

“…The re-partitioning and enrichment of MO at the hydrogel/solvent interface, in turn, would lead to a reduced MO concentration adjacent to the nanoparticle. According to our theory 24 this depletion of reactants should effectively reduce the rate. Secondly, from the UV-vis spectra (see Figure s10), we found that the core-shell particles can absorb more light at 15 °C than at 40 °C, thereby signifying that the core-shell particles can produce more active OH radicals at the nanocube at the lower temperature.…”

“…As we find that the reaction is fully surfacecontrolled (i.e., the reactant diffusive transport is much faster than the surface reaction, see supporting information), MO enrichment at the nanocube surface would lead proportionally to a higher surface rate. 24 However, details on such a rate enhancement depend on the exact spatial partitioning of the MO molecules in the nanoreactor that is experimentally not easily accessible. Another reason for the enhanced rate is related to the stronger absorption of light and narrower band gap in case of the core-shell particles (see Figure s9).…”

“…We demonstrate that the photocatalytic activity of the Cu2O nanocubes is significantly enhanced by the PNIPAM shell, and can be further tuned by temperature via the thermosensitive shell, as suggested by theory. 24 The present work opens a new way for the surface modification of Cu2O nanocubes, which will have a great potential for applications of Cu2O nanoparticles. In addition, such core-shell "nanoreactor" system is essential to understand the effect of PNIPAM shell on properties of metal or metal oxide nanomaterials.…”

Reaction rate of a composite core–shell nanoreactor with multiple nanocatalysts

“…However, in realistic systems the mobility will be certainly slowed down to some extent but not vanish. ∆G may be even negative (attractive) if the reactant interacts favorably with the polymer as found for rather hydrophobic reactants and collapsed PNIPAM-based hydrogels [11,12]. Since ∆G enters Eq.…”

“…This theory is based on the well-known seminal paper by Debye [13] and considers a single nanoparticle located in the center of a hollow thermosensitive network [12]. Here, the substrate that reacts at the surface of the nanoparticle diffuses through a free-energy landscape created by the hydrogel environment.…”