Diffusion of guest molecules within sensitive core–shell microgel carriers

• The diffusivity of oligomers inside a core–shell microgel is quantified by two-focus fluorescence correlation spectroscopy.
• Deswelling the microgel shell has little impact on the tracer mobility in the core.
• Entrapment of the tracers in the microgel only by topological constraints.

The diffusion of payloads within core–shell carrier particles is of major relevance for drug-delivery applications. We use spatially resolved two-focus fluorescence correlation spectroscopy to quantify the diffusivity of different dextran molecules and colloids within carrier particles composed of a temperature-responsive poly(N-isopropylacrylamide) (PNIPAM) shell that surrounds a temperature-insensitive polyacrylamide core. The deswelling of the shell that occurs upon heating above the lower critical solution temperature of PNIPAM slightly slows down the diffusion of these tracer oligomers near the core–shell interface. By contrast, the mobility of the tracers inside the core is not affected by deswelling of the shell. This finding assures absence of artifacts such as adsorption of the guests to the amphiphilic shell polymer, supporting the utility of these microgel carriers in encapsulation and controlled release applications.

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