ABSTRACT:

 

Harnessing silica surface chemistry to engineer particle-stabilized microcapsules for protein encapsulation

 

Functional protein molecules have evolved to perform numerous crucial tasks within living organisms. The highly specialized properties of these biomolecules make them uniquely well suited for a variety of applications that extend beyond their usage in vivo. However, employing proteins in applications such as pharmaceuticals or energy conversion requires robust synthetic host structures that preserve their active state in non-native environments. Siliceous materials
combine favorable mechanical properties with tunable surface chemistry, which could enable efficient optimization to meet the needs of different proteins of interest if assembled into the right architecture. Although silica has long garnered significant interest in materials research, few studies have attempted to bridge the gap between atomic-scale structures and macroscopic properties in the design of silica-based protein host materials. In this work, I introduce a composite material platform in which silica nanoparticle-polyelectrolyte microcapsules serve as stable hosts capable of encapsulating proteins. Multiscale characterization of this system reveals how molecular interactions propagate through mesoscale particle assembly to shape macroscopic
behavior, providing a foundation for the targeted adjustment of material properties for specific protein applications.
 

Silica nanoparticles are highly versatile colloidal building blocks whose tailorable surface chemistry enables precise control over their interactions. Adjusting solution pH modulates silica surface charge, and chemical modifications to the surfaces are routinely used to control colloidal stability and wettability. Nonetheless, silica surface chemistry can be fairly dynamic; even covalently grafted functional groups can undergo surface migration and, in some cases, exchange between particles under ambient conditions. Complementary rheology and advanced dynamic nuclear polarization (DNP) NMR analyses reveal key insights into the kinetics of hydrophilic surface group redistribution, which leads to significant increases in colloidal stability. This deeper
understanding of the effect of silica surface chemistry on macroscopic properties informs our design of particle-stabilized microcapsules as host materials for water-soluble cargo molecules. Two-dimensional NMR analyses show the nature of interactions within silica-polyelectrolyte complexes, which are expected to offer hybrid material properties for microcapsules prepared from
particle-stabilized emulsions. Cryo-TEM reveals dense silica layers at oil-water interfaces, while interfacial rheology demonstrates highly adjustable interfacial mechanical properties based on the ratio of spherical to elongated nanoparticles as well as their surface chemistry. These insights provide multiscale physicochemical guidelines for designing silica–polyelectrolyte microcapsule
systems that can be tailored to a wide range of applications.

One particularly compelling use for these microcapsules is as host structures for reflectin, a water-soluble protein central to the dynamic color-changing and light-scattering capabilities of cephalopods. In response to several different stimuli, reflectin proteins undergo reversible assembly that drives osmotic water flux and produces pronounced, controllable changes in volume. The porous, viscoelastic structure of the silica–polyelectrolyte microcapsules suggests compatibility with such assembly-induced osmotic responses, potentially enabling photo-triggered
size changes within a synthetic material. Fluorescence microscopy confirms successful encapsulation of labeled reflectin within the microcapsule interior, and Langmuir–Blodgett deposition allows controlled assembly of microcapsules on water surfaces. Subsequent transfer of the microcapsules into polyacrylamide hydrogel films yields stable, water-in-water emulsions
capable of supporting reversible water uptake and expulsion. Overall, these results represent a substantial step toward creating biologically inspired optical materials that mimic the dynamic photonic behaviors of cephalopods.