Abstract
We have used X-ray propagation imaging to visualize a less than 5 nm thick native lipid bilayer membrane freely suspended in aqueous solution. Contrast is formed by free space propagation of hard X-rays, with the membrane illuminated by a nano-focused, partially coherent synchrotron beam, at a controllable distance (defocus) behind the focal spot. Quantitative fitting of the magnified Fresnel fringes shows the transition from membranes swollen with solvent to the native bilayer, containing structural information at near-molecular resolution along the dimension perpendicular to the bilayer. We show first applications of this hybrid technique of propagation imaging and near-field diffraction to the investigation of ultra-thin organic films formed in micro-fluidic devices, namely the formation of a lipid bilayer by the adhesion of two constitutive monolayers.
Highlights
As the basic building blocks for biological membranes and many soft composite materials, lipid bilayers and liquid crystalline surfactant phases have for a long time been studied on molecular length scales by X-ray and neutron diffraction.[1]
black lipid membranes (BLM) and micro-fluidic BLM (mfBLM) can be electrically excited by the implementation of electrodes into the aqueous channels
Black lipid membranes (BLMs) of 1,2-diphytanoyl-sn-glycero-3-phosphocholine (DPhyPC) (Avanti Polar Lipids; Alabaster, USA) were prepared using the painting method of Mu€ller and Rudin,[16] using n-decane (Sigma-Aldrich; Germany) (20 mg mLÀ1) as a solvent, see ref. 17 and Fig. 1(b). micro-fluidic bilayer membranes have been prepared from oily solutions consisting of either DPhyPC in n-decane or Monoolein (Avanti Polar Lipids; Alabaster, USA) in Squalene (Merck; Germany), both at a concentration of 10 mg mLÀ1
Summary
This journal is a The Royal Society of Chemistry 2012 membranes, not at the resolution needed to probe the molecular structure of the bilayer. It will close the gap between conventional scattering studies on the one hand, carried out over large ensembles, and conventional microscopy studies on the other hand, including X-ray microscopy, concerning resolution, interaction volume and complexity of the system studied Such an investigative tool would be appealing in micro fluidic sample environment as micro fluidics has emerged as a very powerful tool for the controlled investigation of complex biochemical and physical phenomena, soft interfacial phenomena.[10,11,12,13] The combination of the method with micro fluidics can yield structural information on the interfaces during (hydro)dynamic processes, such as the formation of bilayers, thinning, or bulging, as well as membrane fusion,[14] down to the length scale of a few nanometers. BLMs and mfBLMs can be electrically excited by the implementation of electrodes into the aqueous channels
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