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applications is to get bilayers separating two compartments for studying the
properties of cell membranes such as permeability, active transport or signal
transduction by transmembrane proteins. The ultimate goal is to probe
single molecules in nano-size compartments. In this section, we highlight
a few recent biosensors developments which have used AFM as the main
characterization tool, knowing that interest in this ield is increasing very
quickly (for a recent review, see Ref. 28).
One approach for making biosensors is to form a lipid membrane on top
of a planar support that can be used as an electrode, typically a hydrophilic
semiconductor or oxide material like gold. Direct fusion of lipid vesicles can
occur spontaneously on these surfaces to form a planar bilayer, but, most
often, tethered bilayer lipid membranes (tBLMs) are used to generate an
additional aqueous space between the support and the membrane. This space
can be useful in studying the function of transmembrane proteins.
However,
AFM imaging of these systems is sparsely documented.
56,57
More recently,
porous materials have emerged as good candidates for supporting lipid
membranes and also providing a reservoir of buffer below the membrane.
Porous silicon obtained by the electrochemical etching of crystalline silicon
wafers is especially interesting because it behaves as a photonic crystal
relector and can be used as a label-free optical biosensor. Deposition of a
continuous planar phospholipid bilayer using phosphatidylethanolamine
/
PC/Chl
or DOPC/DPPC mixture at the surface of porous silicon has validated
the proof of concept.
58,59
Under these conditions, membrane dynamics was
well preserved.
Another strategy to make biosensors is to use nano-size holes supporting
the lipid membrane. An elegant method of fabrication of SLB on top of porous
alumina by vesicle spreading has been developed by Steinem's group.
60
Briely, a porous alumina surface, having 50 nm hexagonal organized pores,
is coated by a gold layer and further functionalized by thiols. Membrane
bilayers that include positively charged lipids are formed on the surface and
pictured by AFM. Under these conditions, most of the surface is covered by
free-standing bilayers over the holes. This system has been used to study the
channel activity of several proteins. One problem encountered with aperture-
spanning membranes is their low stability in time and their tendency to
rupture. A scaffold composed of S-layer proteins of
55
Bacillus sphaericus
, pre-
coated on the support,
61
as well as a gelling solution bathing the membrane,
62
can increase the stability of free-spanning membranes.
Whatever the strategy used to form the lipid bilayer in membrane-inspired
biosensors, the main bottleneck remains the incorporation of functional
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