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

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.

Time-Resolved SAXS Studies of the Kinetics of Thermally Triggered Release of Encapsulated Silica Nanoparticles from Block Copolymer Vesicles

“…40 In order to study the extent of silica release under such conditions, this “vesicle plus sphere” two-population model was modified to include two new fitting parameters. For the vesicle model, the effective scattering length density of the lumen, ξ lum , was included as a fitting parameter in order to assess the rate of silica release from the vesicles at 0 °C.…”

“…40 These vesicles were synthesized at 10% w/w solids in the presence of 0–35% w/w silica nanoparticles followed by extensive purification via six centrifugation–redispersion cycles. Small-angle X-ray scattering (SAXS), disk centrifuge photosedimentometry, and cryo-TEM studies confirmed successful silica encapsulation within the vesicle lumen.…”

A Critical Appraisal of RAFT-Mediated Polymerization-Induced Self-Assembly

“…In closely related work, we have shown that the aqueous RAFT dispersion polymerization of HPMA using a PGMA macro-CTA in the presence of silica nanoparticles enabled their in situ encapsulation inside the resulting PGMA–PHPMA vesicles, as confirmed by TEM and SAXS studies (Figure 5). 173 This was followed by thermally-triggered release of the silica payload on cooling to 0–10 °C, since this induces a vesicle-to-sphere transition. Furthermore, BSA could be encapsulated intact by conducting the polymerization at 37 °C using a low-temperature initiator, thus avoiding its denaturation.…”

Controlling Nanomaterial Size and Shape for Biomedical Applications via Polymerization‐Induced Self‐Assembly

“…To that end, various chemistries have been explored to induce the release of large cargos from PISA particles . For example, Armes’ group carried out RAFT aqueous dispersion polymerization of HPMA in the presence of 5–35 wt% silica nanoparticles as a model of large payloads . After confirming encapsulation of the silica nanoparticles within purified vesicles, the feasibility of controlled release of the encapsulated silica nanoparticles was explored.…”

“…TEM images of A) silica‐loaded P(GMA)‐ b ‐P(HPMA) vesicles, and after cooling, B) P(GMA)‐ b ‐P(HPMA) spheres and released silica nanoparticles. Reproduced under the terms of the Creative Commons Attribution license . Copyright 2015, American Chemical Society.…”