Effects of Pore Size and Tethering on the Diffusivity of Lipids Confined in Mesoporous Silica

Abstract

Incorporation of lipid assemblies on the surface and within pores of mesoporous silica particles provides for biomimetic approaches to analyte sensing and separations using high surface area platforms. This work investigates the effect of pore confinement on the location and the diffusivity of lipid assemblies in mesoporous silica spherical particles (SBAS) as a function of nanopore diameters (nonporous, 3.0, 5.4, and 9.1 nm), which span the range of the thickness of the 1,2‐dipalmitoyl‐sn‐glycero‐3‐phosphocholine lipid bilayer (≈4 nm). Large‐diameter SBAS are imaged with sufficient spatial resolution to distinguish lipids at the exterior surface and in the center of the particles. Lipids incorporated on the silica by evaporation deposition exist as exterior lipid bilayers on all particles and lipid assemblies in the pores of 5.4 and 9.1 nm pore diameter materials. Lipid diffusivity increases with pore size and decreases in the presence of bilayer tethering functional groups. Lipid diffusivity in the core of the particles is similar to the surface diffusivity, consistent with long‐range mobility in accessible, ordered (but randomly oriented) mesopores of SBAS materials. This work presents a framework for interpreting high density loading of lipid bilayers and their function within mesoporous materials.

Nanoconfinement Effects on Redox Probe Transport in Lipid Assemblies on and in Mesoporous Silica Thin Films

“…Deposition within the pores by solvent evaporation and rehydration (Figure 1b) is expected to provide many of the advantages of lipid bilayers as selective, temperature responsive barriers, but with greater stability and versatility toward chemical conditions. In our previous work, [ 12,18 ] we demonstrated the confinement of lipids in the pores of mesoporous silica particles and membranes, and demonstrated identical lipid mobility at the core (confined lipids) and the surface (unconfined lipids) of these particles for pore diameters of 5.4 and 9.1 nm. In the 8–10 nm pores used here, the lipid is expected to form a continuous tube‐like cylindrical bilayer that lines the inner pore surface (Figure 1b).…”

“…[ 7 ] The traditional method of supporting lipid bilayers on porous material is through vesicle fusion (illustrated in Figure a), resulting in an enveloping of the surface with bilayers that span the pores or reside in the surface area on the pores. [ 1,8 ] Porous materials including alumina (55–280 nm diameter pores), [ 8b,9 ] silicon nitride (200–700 nm), [ 10 ] Teflon filters (5 µm pores), [ 11 ] silicon (0.2–2 µm pores), [ 9b ] and mesoporous silica (3–12 nm pores) [ 7,12 ] have been reported as lipid bilayer supports. [ 13 ] Among these, mesoporous silica thin films are promising because their synthesis uses scalable coating technology that is highly tunable by the choice of surfactant templating agent and synthesis conditions.…”

“…[ 14 ] In addition, silica is highly biocompatible, and thus provide advantageous platforms to explore the functionality of either proteins or lipophilic solutes incorporated in SLBs. Through surfactant templating, highly ordered mesoporous thin films with tunable nanopores can be synthesized, [ 15 ] which can match the size of lipid bilayers [ 12 ] or transmembrane proteins. [ 7 ] Advantages of enveloped lipid bilayers deposited by vesicle fusion stem from mimicking the fluidity, ion channel properties, and protein supporting functions of cell membranes.…”

“…We previously showed that continuous lipid bilayers can be deposited by vesicle rupture onto mesoporous silica membranes [ 18 ] and particles, but that the pores reduce lipid diffusivity in supported bilayers significantly. [ 12 ]…”

“…In the 8–10 nm pores used here, the lipid is expected to form a continuous tube‐like cylindrical bilayer that lines the inner pore surface (Figure 1b). [ 12,18 ] Furthermore, the combination of mobile pore‐confined lipids and mesoporous silica thin films was demonstrated to create a highly selective barrier while ensuring significant mobility to transport a small model lipophilic carrier (a boronic acid derivative for carbohydrate binding) across the synthetic membrane to facilitate sugar purification. [ 18 ] The main disadvantage relative to lipid enveloped systems is that the barrier and transport properties of lipid pore‐filled silica thin films are less well known, leading to the need for further fundamental study of nanoconfined lipid assemblies.…”