Potential-Dependent Interaction of DOPC Liposomes
with an Octadecanol-Covered Au(111) Surface Investigated Using Electrochemical
Methods Coupled with in Situ Fluorescence Microscopy
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Abstract
The
potential-controlled incorporation of DOPC liposomes (100 nm
diameter) into an adsorbed octadecanol layer on Au(111) was studied
using electrochemical and in situ fluorescence microscopy. The adsorbed
layer of octadecanol included a small amount of a lipophilic fluorophoreoctadecanol
modified with BODIPYto enable fluorescence imaging. The deposited
octadecanol layer was found not to allow liposomes to interact unless
the potential was less than −0.4 V/SCE, which introduces defects
into the adsorbed layer. Small increases in the capacitance of the
adsorbed layer were measured after introducing the defects, allowing
the liposomes to interact with the defects and then annealing the
defects at 0 V/SCE. A change in the adsorbed layer was also signified
by a more positive desorption potential for the liposome-modified
adsorbed layer as compared to that for an adsorbed layer that was
porated in a similar fashion but without liposomes present in the
electrolyte. These subtle changes in capacitance are difficult to
interpret, so an in situ spectroscopic study was performed to provide
a more direct measure of the interaction. The incorporation of liposomes
should result in an increase in the fluorescence measured because
the fluorophore should become further separated from the gold surface,
reducing the efficiency of fluorescence quenching. No significant
increase in the fluorescence of the adsorbed layer was observed during
the potential pulses used in the poration procedure in the absence
of liposomes. In the presence of liposomes, the fluorescence intensity
was found to depend on the potential and time used for poration. At
0 V/SCE, no significant change in the fluorescence was observed for
defect-free adsorbed layers. Changing the poration potential to −0.4
V/SCE caused significant increases in the fluorescence and the appearance
of new structural features in the adsorbed layers that were more easily
observed during the desorption procedure. The extent of fluorescence
changes was found to be strongly dependent on the nature of the adsorbed
layer under investigation, which suggests that the poration and liposome
interaction are dependent on the quality of the adsorbed layer and
its ease of poration through changes in the electrode potential