High Efficient Loading of Hydrophobic Molecules in Layer-by-Layer Assembled Microgel Films with the Assistance of Surfactant Micelles

Abstract

Layer-by-layer (LbL) assembly enables the fabrication of various kinds of functional films by sequential deposi- tion of building blocks of polyelectrolytes, biomacromolecules, oligo-charged dyes, colloids, nanoparticles, polymeric com- plexes and so forth. However, water-insoluble organic species without necessary functional groups are difficult to be incor- porated into multilayer films by conventional LbL assembly method. The present work aims to develop a facile method for efficient loading of hydrophobic molecules into LbL assembled films. To do so, PAH-D microgels, which are chemically cross-linked poly(allylamine hydrochloride) (PAH) and dextran, are LbL assembled with hyaluronic sodium (HA) to produce PAH-D/HA microgel films. Sodium dodecylsulfate (SDS) micelles are used as carriers to incorporate hydrophobic molecules of pyrene and spiropyran. The negatively charged SDS micelles loaded with hydrophobic molecules are successfully incor- porated into the PAH-D/HA microgel films by a post-diffusion process, which produces transparent PAH-D microgel films with a large number of loaded pyrene and spiropyran molecules. Fluorescence emission spectra support that the hydrophobic molecules in PAH-D/HA films locate in the hydrophobic cores of SDS micelles. Stable loading of SDS micelles into PAH-D/HA films is achieved because of the strong electrostatic interaction between SDS micelles and protonated amine groups of PAH-D microgels. The high loading capacity of PAH-D/HA films toward SDS micelles encapsulated with hydro- phobic molecules originates from the high swellability of the PAH-D/HA films and the abundance of unpaired amine groups in the PAH-D layers for binding of SDS micelles. The amount of hydrophobic molecules loaded into PAH-D/HA microgel films can be conveniently controlled by varying the deposition cycles of PAH-D/HA films and the concentration of hydro- phobic molecules encapsulated in SDS micelles. The spiropyran-loaded PAH-D/HA microgel films show reversible photo- chromic property upon ultraviolet and visible light irradiation. The present work opens a cost-effective and general way to incorporate non-charged hydrophobic molecules into LbL assembled microgel films by using surfactant micelles as carriers. Keywords layer-by-layer assembly; polymeric microgels; microgel films; surfactant micelles; polymeric films