Epi-fluorescence microscopic characterization of potential-induced changes in a DOPC monolayer on a Hg drop

Characterization of the potential-induced changes of a lipid-coated Hg-0.1 M KCl interface through electrochemical techniques and newly developed in situ fluorescence microscopy is described. Fluorescence of a fluorophore-containing dioleoyl phosphatidylcholine (DOPC) layer deposited from the gas-solution interface was observed to be dependent upon the potential of the Hg surface. The largest changes occurred for potentials where the lipid layer was desorbed: the lipid moved away from the electrode surface, reducing the efficiency of metal-mediated quenching of the excited state resulting in an increase in fluorescence. Electric potential-induced changes in the morphology of the adsorbed or desorbed DOPC lipid monolayer were observed optically for the first time using this technique. The observed potential-dependent fluorescence was compared to previous studies on an octadecanol-coated Au(111) electrode. Fluorescence microscopy was also used to characterize the fusion of DOPC liposomes with a previously adsorbed DOPC layer. Large changes in fluorescence were observed for the DOPC layer after fusion with liposomes. The fusion was accomplished via potential-created defects in the adsorbed DOPC monolayer through which the liposomes interact. The integration of the liposomes into the adsorbed monolayer results in a hybrid layer in which some lipid exists further from the electrode surface, resulting in a large increase in fluorescence. Possibilities for the creation of a biomimetic adsorbed hybrid lipid layer on Hg are also discussed.