Loading of Silica Nanoparticles in Block Copolymer Vesicles during Polymerization-Induced Self-Assembly: Encapsulation Efficiency and Thermally Triggered Release.

Charlotte J Mable,Steven P Armes,Rebecca R Gibson,Oleksandr O Mykhaylyk,Beulah E Mckenzie,Sylvain Prevost

doi:10.1021/jacs.5b10415

Charlotte J Mable, Steven P Armes + Show 4 more

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https://doi.org/10.1021/jacs.5b10415

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Abstract

Poly(glycerol monomethacrylate)-poly(2-hydroxypropyl methacrylate) diblock copolymer vesicles can be prepared in the form of concentrated aqueous dispersions via polymerization-induced self-assembly (PISA). In the present study, these syntheses are conducted in the presence of varying amounts of silica nanoparticles of approximately 18 nm diameter. This approach leads to encapsulation of up to hundreds of silica nanoparticles per vesicle. Silica has high electron contrast compared to the copolymer which facilitates TEM analysis, and its thermal stability enables quantification of the loading efficiency via thermogravimetric analysis. Encapsulation efficiencies can be calculated using disk centrifuge photosedimentometry, since the vesicle density increases at higher silica loadings while the mean vesicle diameter remains essentially unchanged. Small angle X-ray scattering (SAXS) is used to confirm silica encapsulation, since a structure factor is observed at q ≈ 0.25 nm–1. A new two-population model provides satisfactory data fits to the SAXS patterns and allows the mean silica volume fraction within the vesicles to be determined. Finally, the thermoresponsive nature of the diblock copolymer vesicles enables thermally triggered release of the encapsulated silica nanoparticles simply by cooling to 0–10 °C, which induces a morphological transition. These silica-loaded vesicles constitute a useful model system for understanding the encapsulation of globular proteins, enzymes, or antibodies for potential biomedical applications. They may also serve as an active payload for self-healing hydrogels or repair of biological tissue. Finally, we also encapsulate a model globular protein, bovine serum albumin, and calculate its loading efficiency using fluorescence spectroscopy.

Highlights

Microcompartmentalization is widely acknowledged to be a fundamental prerequisite for life on Earth.[1−4] Many intracellular processes require spatial separation of components via impermeable lipid membranes, with membrane proteins allowing the selective diffusion of various chemical species in and out of cells.[5]
Over the last five years or so, polymerization-induced selfassembly (PISA) has become established as a powerful tool for the rational design and efficient synthesis of a wide range of diblock copolymer nano-objects in either aqueous solution or non-aqueous media.[31−33] Of particular relevance to the present study, RAFT aqueous dispersion polymerization can be utilized to prepare block copolymer vesicles at copolymer concentrations of up to 25% w/v solids.[34−38] Periodic sampling during such syntheses has confirmed a progressive evolution in copolymer morphology, with transmission electron microscopy (TEM) studies revealing that the transformation of highly anisotropic worms into well-defined vesicles proceeds via a socalled “jellyfish” intermediate.[35]
RAFT aqueous dispersion polymerization of hydroxypropyl methacrylate (HPMA) was conducted in the presence of 5−35% w/w silica nanoparticles. 1H NMR studies indicated that >99% HPMA conversion was achieved within 2 h at 70 °C, regardless of the presence of silica nanoparticles

Summary

■ INTRODUCTION

Microcompartmentalization is widely acknowledged to be a fundamental prerequisite for life on Earth.[1−4] Many intracellular processes require spatial separation of components via impermeable lipid membranes, with membrane proteins allowing the selective diffusion of various chemical species in and out of cells.[5]. A superposition of X-ray scattering signals from the two populations used in the model produced good fits to the SAXS data obtained for vesicles synthesized in the presence of silica nanoparticles after removal of excess non-encapsulated silica (Figure 6 and Table S1). SAXS patterns recorded for such dispersions did not possess any peak at q ≈ 0.25 nm−1 corresponding to silica nanoparticles, indicating that no structure factor is required in this case (see Figure S7) These SAXS observations confirm beyond any reasonable doubt that the silica nanoparticles are encapsulated within the vesicles during these PISA syntheses. Compared to the present study, only limited characterization of the extent of encapsulation was undertaken

■ CONCLUSIONS

■ ACKNOWLEDGMENTS

■ REFERENCES