Potential controlled adsorption and lateral mobility of DOPC on polycrystalline gold – an EQCM and in situ fluorescence microscopy study

The adsorption of 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC) from solution of unilamellar vesicles on polarized gold surfaces is directed by the surface charge of the gold substrate. The electrochemical quartz crystal microbalance (EQCM) data indicate the existence of two different adsorption patterns. If the surface charge of the gold surface exceeds ca. 8 μC cm−2, the determined adsorbed phospholipid mass indicates the presence of a “bi-layer-like” system. The existence of an adsorbed layer composed of intact vesicles remains possible, if the adsorption proceeds at potentials close to the potential of zero charge (pzc). The lateral mobility of the phospholipid molecules in the adsorbed layers was investigated by fluorescence recovery after photobleaching (FRAP) approach. The diffusion coefficient of the DOPC molecules in the adsorbed layers ranged between 1.0 and 2.5 × 10−12 cm2 s−1, which is about 4 magnitudes smaller than in intact mica-supported DOPC bi-layers in the liquid-crystalline state. The diffusion coefficients found in the “bi-layer-like” system are ca. 3–5 times slower than in the layer adsorbed at pzc. The mobility of the phospholipid molecules decreases with increasing surface charge of the substrate.