Nanomaterial coatings: controlling and analyzing thin films of lipid cubic phase

Figure 1. Structures shown in order of increasing relative humidity (left to right): lamellar; gyroid LCP; diamond LCP.

Figure 2. Top row: gyroid LCP with the (110) plane highlighted (left) and diamond LCP with the (111) plane highlighted (right). Bottom row: corresponding x ray scattering patterns with predicted x-ray reflections shown as small black circles. Figure adapted with permission from M. Rittman et al, Langmuir 29, pp. 9874-9880 (2013). Copyright 2013 American Chemical Society.
Abstract: We have studied new forms of a nanomaterial known as a LCP (Lipid Cubic Phase), preparing them as thin films rather than the bulk gels or liquids typically used. LCPs are of interest for biological research as cell membrane models, and for technological applications including drug delivery and nanomaterial production. Using its new thin film form, we show that we can use the surrounding humidity to control the nanostructure, suggesting a new type of huminity-responsive nanomaterial. We also show that we can induce order into the LCP, with the surface making it lie in a specific direction.

Lipid cubic phases (LCPs) are complex three-dimensional nanostructures, containing networks of water channels billionths of a meter in size, separated by a lipid “bilayer” similar to a cell membrane. The material forms spontaneously when particular lipid molecules (typically plant-derived, low-cost, biocompatible and often edible) are mixed with water. These properties make them of interest for biological membrane research, and for technological applications such as in the production of high-performance platinum electrodes, or as vehicles for drug and food flavor delivery. Here, their structure can be made to respond to external stimuli such as temperature and acidity, for controlled or triggered release. LCPs are usually prepared in bulk gel or liquid forms, by mixing the lipid with the required amount of water. In our approach, we instead began with a thin film of lipid spread on a flat silicon surface. We introduced water by changing the humidity of the air above the sample. The lipid molecules absorb water from the air, and assemble themselves into a variety of different shapes shown in Fig. 1, as we gradually increase humidity: first, a “lamellar phase” forms, consisting of a stack of flat bilayers, on the left side; then, at relative humidity values above 83%, the molecules start to rearrange into an LCP known as the gyroid cubic phase; finally, at still higher humidity values, another LCP structure known as the diamond cubic phase appears. We know this by analyzing the film using a technique known as grazing-incidence small-angle x-ray scattering (GISAXS), which we carry out at the Austrian SAXS beamline at Elettra..

Because our LCP films are so thin – much thinner than a human hair – it only takes a few minutes for the water to penetrate into the lipid layer. In this way, we can use our thin film approach to make a new type of smart LCP material, which responds to humidity rather than temperature or acidity. A second new feature of LCPs studied as thin films is their orientation. Traditionally prepared gels or liquids contain many grains of LCP, millionths of a metre in size, all facing in different directions, like grains in a powder. These scatter x-rays equally in all directions, giving so-called “powder-like” x-ray scattering patterns consisting of rings. In contrast, when we prepared thin films of LCP samples, we found that the films were all aligned, with the same surface within the LCP sample lying parallel to the plane of the film throughout the sample. These surfaces in the gyroid and diamond LCP are technically referred to as the (110) and (111) surfaces, respectively, and are shown in Fig. 2, which also shows the scattering patterns from the two LCP structures; the fact that the x rays (white) give spots rather than rings shows that, rather than being powder-like, the samples are highly aligned; the small
 black circles show the predicted positions for gyroid and diamond LCPs aligned parallel to the (110) and (111) planes respectively, confirming the orientation. More recently, we have worked with professor W.T. Gozdz at the Polish Academy of Sciences, who has calculated the most favourable surface for the LCPs to adopt, and has shown that the (110) and (111) are indeed the orientations we would expect for these two LCP structures.

For an x ray scattering scientist, an oriented sample is more like a single crystal and less like a powder, and the arrangement of spots gives much more structural information than the rings from a powder pattern. Over sixty years ago, the structure of DNA was solved by Crick and Watson, in a large part due to the information that could be obtained using x ray patterns from highly oriented DNA, produced by Rosalind Franklin. In the same way, at a more modest level, we hope that new structural insights into LCPs and the molecules they interact with will be gained using the highly oriented LCPs in our thin films..

Retrieve article

Control and Analysis of Oriented Thin Films of Lipid Inverse Bicontinuous Cubic Phases Using Grazing Incidence Small-Angle X-ray Scattering;
M. Rittman, H. Amenitsch, M. Rappolt, B. Sartori, B.M.D. O’Driscoll, and A.M. Squires;
Langmuir 29, 9874-9880 (2013). doi: 10.1021/la401580y

Last Updated on Tuesday, 14 May 2019 17:05