Biology Reference
In-Depth Information
Another approach to form SLBs on a solid support is the use of the
Langmuir-Blodgett or Langmuir-Schaefer techniques. Both consist in the
transfer of a lipid monolayer (inner lealet) on a hydrophilic support by
pulling it vertically through a lipid monolayer at the air-water interface. The
outer lealet is then transferred using either another vertical immersion of
the support through the lipid monolayer at the air-water interface or by
horizontally dipping the support into the lipid monolayer at the air-water
interface. In theory, the advantage of the double transfer methods is that
asymmetrical bilayers can be formed. However, it appears that the lipid
composition of each lealet is often very far from the expected composition. 25
Moreover, thinning of the water layer between the mica and the inner lealet,
during the lag time before the second monolayer transfer, often results in
a change in the diffusion properties of this inner lealet. 26 In addition, this
technique cannot be used to incorporate transmembrane proteins during
bilayer assembly since the protein could be exposed to air during the creation
of the second lealet.
To minimize the membrane-support interaction mentioned above,
polymer-supported bilayers (PSBs) have also been developed. 27,28 They can
be composed of a soft polymer cushion with typically less than 100 nm
thickness to act as a lubricating layer between the support and the bilayer.
Alternatively, lipopolymer tethers can also be used to separate membrane
components from the support. Generally, PSBs are obtained by the Langmuir-
Blodgett technique, vesicle fusion or a combination of both techniques which
involves the fusion of LUVs on a pre-deposited monolayer. 29 They have been
successfully used for incorporating proteins, preserving their functions,
and this technique has now been extended to the biosensors ield. However,
getting free diffusion of proteins in cushion-supported membranes is not so
straightforward, and it seems that protein mobility is strongly dependent on
the method of fabrication. 30
1.3 AFM IMAGING OF SUPPORTED LIPID BILAYERS
Artiicial supported membranes are very soft materials, meaning that the
tip-sample interaction has to be inely tuned to minimize the force applied
during tip scanning, thus preventing the membrane to be swept away (the
force between tip and sample can be simpliied as a combination of the effects
of van der Waals attraction and electrostatic repulsion due to the so-called
double layer of counterions).
To do so, the spring constant of the cantilever
should be low, generally in the 1-100 mN/m range, and force-distance
curves should be performed to adjust the force. The pH and buffer (mainly
31
 
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