Tethered Lipid Bilayers on Ultraflat Gold Surfaces

Abstract

Tethered lipid bilayers (tBLMs) were obtained by the fusion of liposomes from diphytanoylphosphatidylcholine (DPhyPC) with self-assembled monolayers (SAMs) of a newly designed archaea analogue thiolipid, 2,3-di-O-phytanyl-sn-glycerol-1-tetraethylene glycol-d,l-α-lipoic acid ester (DPTL) on template stripped gold (TSG) films from silicon wafer as a template. SAMs, as characterized by reflection absorption infrared spectroscopy (RAIRS), show a mixture of different conformations of the tetraethylene segment in air, which appears to rearrange into the fully extended conformation when the SAM is immersed into an aqueous electrolyte solution, as deduced from thickness measurements by surface plasmon resonance spectroscopy (SPR). The fusion of liposomes was followed by SPR, quartz crystal microbalance (QCM), and fluorescence microscopy. Highly resistive tBLMs were obtained, as demonstrated by electrochemical impedance spectroscopy (EIS) results, which are equivalent to those for the BLM. This large resistivity is attributed to the ultraflat surface of TSG, as well as to the distinctive architecture of the newly designed molecule. The roughness of the TSG obtained from mica and silicon wafer as template was determined by AFM and compared to that of a Au(111) surface on mica. The largest roughness features of TSG are shown to be 0.5−1 nm, which is small compared to the vertical dimension of the DPTL molecules. This is regarded to be crucial for the self-assembly process, particularly in the case of amphiphilic molecules.

Formation of tethered bilayer lipid membranes probed by various surface sensitive techniques

“…28 This could be confirmed by using QCM-D techniques ͑see supporting information͒. A slow frequency decrease and a dissipation increase were observed after exchange of ethanol by 0.1M NaCl solution.…”

“…28,33 48 nm of gold are evaporated onto a clean silicon wafer ͑CrysTec, Berlin, Germany͒, glued with an epoxy glue ͑Epo-Tek 353ND4, Polytec, Waldbronn, Germany͒ to a glass slide, and cured at 150°C for 1 h. Stripping of the silicon wafer reveals an extremely flat gold surface that is used immediately for the experiments. 26 If not stripped, the samples can be stored for weeks before use.…”

“…C SC represents the space charge capacitance due to the spacer region combined with the capacitive effects of the electrochemical double layer at the gold interface. 28,35 In the case of low monolayer resistances, an additional resistance attributed to the spacer region can be seen; therefore, an R͑RC͒͑RC͒ circuit has been used. 36 EIS data can be visualized in Bode plots ͑see Fig.…”

“…A Sauerbrey conversion of frequency shift led to a thickness increase of 1 nm, corresponding to the value reported previously. 28 These changes in frequency and dissipation could be explained by the relaxation of the poly͑ethylene͒glycol ͑PEG͒ chains in the electrolyte solution; the frequency shift could be attributed to the increase in the mass of water coupled into the SAM and the variation of dissipation to the relative increase in the flexibility of the PEG chains.…”

“…Membranes based on the anchorlipid 2,3-di-O-phytanyl-glycerol-1-tetraethylene glycol-D,L-lipoic acid ester lipid ͑DPhyTL͒ have been shown to form dense layers with high electrical sealing properties. [26][27][28][29][30][31] In order to study the detailed assembly processes of the system, a combination of different techniques has been used. The high spatial resolution of the AFM was used to probe topographic changes during the assembly processes.…”