Electrochemistry of Biomimetic Membranes - Applications of Electrochemistry and Nanotechnology in Biology and Medicine - page 162

IV. FORMATION OF LIPID FILMS IN BIOMIMETIC
MEMBRANES
Methodologies for the fabrication of biomimetic membranes vary
somewhat from one biomimetic membrane to another. However, a
number of experimental procedures for the formation of lipid
monolayers and bilayers on solid supports are common to several
biomimetic membranes. The most popular procedures are vesicle
fusion, Langmuir-Blodgett and Langmuir-Schaefer transfers, and
rapid solvent exchange. The formation of lipid monolayers and
bilayers on gold and silver substrates is commonly monitored by
surface plasmon resonance (SPR). Therefore, a brief description of
this surface-sensitive technique seems appropriate.
1. SurfacePlasmonResonance
Surface plasmons are collective electronic oscillations in a metal
layer, about 50 nm in thickness, excited by photons from a laser
beam. 9 The beam is reflected by the back surface of the metal lay-
er, while its front surface supports a dielectric film (e.g., a lipid
bilayer), usually in contact with an aqueous solution. The evanes-
cent electromagnetic field thereby generated in the metal layer can
couple with the electronic motions in the dielectric film. The inten-
sity of the electromagnetic field associated with surface plasmons
has its maximum at the metal/(dielectric film) surface and decays
exponentially into the space perpendicular to it, extending into the
metal and the dielectric. This makes SPR a surface-sensitive tech-
nique particularly suitable for the measurement of the optical
thickness of ultrathin films adsorbed on metals. As a rule, the inci-
dent angle of the laser beam is varied with respect to the back sur-
face of the metal layer, and the reflected light intensity is meas-
ured. At a certain angle of incidence there exists a resonance con-
dition for the excitation of surface plasmons, which causes the
energy of the incident laser light to be absorbed by the surface
plasmon modes and the reflectivity to attain a minimum (see Fig.
7 ).