Electrochemistry of Biomimetic Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 199

silane-coated glass substrate during the process of hydrogel
polymerization, sealing the solvent annulus. This allowed the ca-
pacitance of the membrane to maintain its value of about 0.4 PF
cm -2 , typical of a solvent-containing bilayer, for 12 days. In a dif-
ferent approach, a lipid bilayer was protected on both sides by pre-
cast gel slabs, and the electrical properties of nicotinic acetylcho-
line receptor 119 and of the peptides valinomycin and gramicidin 120
were examined, upon incorporating them in the bilayer. An appre-
ciable decrease in the noise level was achieved by Kang et al. 121 by
fabricating a chip in which a bilayer containing a single D-
hemolysin channel was incapsulated by forming an agarose gel
around it. A Teflon septum with a 100 Pm diameter aperture was
clamped between the cis and trans compartments of a chamber. A
buffer solution in liquid agarose at 45°C was added to both com-
partments, maintaining the liquid level below the orifice of the
septum. After spreading a DPhPC solution in pentane on the sur-
face of the liquid agarose and allowing pentane to evaporate, the
agarose level in the two compartments was raised by adding fur-
ther warm agarose solution. By adopting this Montal and Mueller
procedure (cf. Fig. 13 ), a lipid bilayer was obtained with a capaci-
tance of 0.8-1 PF cm 2 , typical of a solvent-free bilayer. The subse-
quent insertion of a preformed heptameric D-hemolysin channel
into the lipid bilayer was monitored by an abrupt current jump.
The chamber was then cooled to allow the agarose to form a gel,
and the single nanopore chip was cut away from the chamber. The
chip was stable and storable for at least three weeks at 4°C and
endured mechanical disturbance.
With BLMs, the transmembrane potential can be readily ad-
justed and varied by applying a given potential difference across
two identical reference electrodes (say, two Ag/AgCl electrodes)
immersed in the bulk solutions bathing the two sides of the mem-
brane. This procedure is not usually applicable to vesicles, due to
their small size. However, a sophisticated and elegant electrophys-
iological technique, called patch-clamping , allows an ultrami-
cropipette, whose tip has an inner diameter of a few micrometers,
to penetrate both cells 122 and giant unilamellar vesicles (GUVs).
The ultramicropipette contains an electrolyte solution with a refer-
ence Ag/AgCl electrode inside, while an identical reference elec-
trode is immersed in the external solution. This allows the trans-