Control and Analysis of Oriented Thin Films of Lipid Inverse Bicontinuous Cubic Phases Using Grazing Incidence Small-Angle X-ray Scattering

Abstract

Lipid cubic phases are complex nanostructures that form naturally in a variety of biological systems, with applications including drug delivery and nanotemplating. Most X-ray scattering studies on lipid cubic phases have used unoriented polydomain samples as either bulk gels or suspensions of micrometer-sized cubosomes. We present a method of investigating cubic phases in a new form, as supported thin films that can be analyzed using grazing incidence small-angle X-ray scattering (GISAXS). We present GISAXS data on three lipid systems: phytantriol and two grades of monoolein (research and industrial). The use of thin films brings a number of advantages. First, the samples exhibit a high degree of uniaxial orientation about the substrate normal. Second, the new morphology allows precise control of the substrate mesophase geometry and lattice parameter using a controlled temperature and humidity environment, and we demonstrate the controllable formation of oriented diamond and gyroid inverse bicontinuous cubic along with lamellar phases. Finally, the thin film morphology allows the induction of reversible phase transitions between these mesophase structures by changes in humidity on subminute time scales, and we present time-resolved GISAXS data monitoring these transformations.

Aligned platinum nanowire networks from surface-oriented lipid cubic phase templates

“…There have been several reported methods of how to produce a Q II phase with an overall alignment by constraining sample dimensions [25][26][27] , controlled hydration 20 and by shear flow [28][29][30] .While it might be expected that in order to template an orientated metal structure the lipid template must also feature an overall alignment, we demonstrate in this paper that it is possible to produce an orientated metal nanostructure using a polydomain lipid template with no overall orientation. By depositing our templated platinum structures onto thin gold foil which can be measured in both perpendicular and parallel configuration to the SAXS beam we are able to collect both in-plane (defined as plane parallel to the substrate) and out-of-plane (plane perpendicular to the substrate) data which typically requires both transmission SAXS and grazing incidence (GISAXS) or reflection SAXS measurements 25,27,31,32 .…”

“…The 3.3 µm film was found to be more disordered with no defined orientation. Interestingly a film measured to be 2.5 ± 0.2 µm thickness was observed to feature the (110) orientation in addition to the (111) orientation; in earlier GISAXS work, lipid films produced by allowing a drop of lipid solution in chloroform to run down an angled substrate displayed both the (111) and (110) orientations; such films were less uniform, and likely to have thicker regions 25 . These results are consistent with a Q II lipid template that is overall polydomain and unoriented, but with a region a few microns thick close to the lipid/substrate interface oriented with (111) parallel to the substrate.…”

“…Phases were identified by assigning peaks measured to Bragg reflections for a given phase. Simulated spot patterns were produced using previously published calculations 25,27 as detailed in S1.…”

“…In order to explain this surprising result, we suggest that while overall the lipid is polydomain, the lipid close to the substrate/lipid interface exhibits an overall orientation. Theoretical calculations have shown that for an interface where the Q II lipid bilayer remains continuous and closed, the energy is minimised when parallel to the (111) planes 25,27 . Our grazing-incidence SAXS studies of different spin-coated Q II films between 0.2 and 1 μm thick all confirmed this orientation 27 .…”

“…By depositing our templated platinum structures onto thin gold foil which can be measured in both perpendicular and parallel configuration to the SAXS beam we are able to collect both in-plane (defined as plane parallel to the substrate) and out-of-plane (plane perpendicular to the substrate) data which typically requires both transmission SAXS and grazing incidence (GISAXS) or reflection SAXS measurements 25,27,31,32 . Through this technique, we demonstrate that we can reproducibly fabricate platinum nanostructures with an Fd3m morphology aligned with the (111) plane parallel to the substrate.…”