Formation and properties of phospholipid bilayers on fluorine doped tin oxide electrodes

Commercially available fluorine doped tin oxide (FTO) films on glass were functionalized by the octadecyltrichlorosilane (OTS) self-assembled monolayers (SAM), which upon exposure to a multilamellar phospholipid vesicle solutions trigger vesicle fusion and formation of a tethered phospholipid bilayer membrane (tBLM). The formation of tBLMs was monitored in real-time by the Fast Fourier Transform (FFT) electrochemical impedance spectroscopy (EIS), which attested for the completion of the fusion process within 2000 s. We established that highly insulating complete bilayers can be accomplished only in some specific cases of OTS functionalized FTO films. In particular, only OTS functionalized FTO films exhibiting double semicircular features on the Cole – Cole EIS plots produced tBLMs with the specific capacitance below 1 μF cm⁻² comparable to capacitances of the free-standing phospholipid bilayers. The double-semicircular features of EIS spectra can be used as a diagnostic tool for predicting the suitability of silanized FTO substrates for the complete vesicles fusion leading to a formation of the tBLMs. The average value of the capacitance of DOPC bilayers containing 40% (mol) cholesterol was of 0.82 ± 0.10 μF cm⁻² consistent with the 3.02 nm thick dielectric film with the relative dielectric constant of 2.8. In contrast to the gold substrates, tBLMs on OTS functionalized FTO films can be regenerated at least 3 times by washing off lipid layer and re-assembling tBLM via the vesicle fusion process. EIS provided direct evidence of the susceptibility of the tBLMs to phospholipase A₂, which triggered degradation of the films. However, transmembrane reconstitution of the pore-forming toxin vaginolysin (VLY) proteins was not detected. Surface constructs described in this study may be utilized in peripheral protein studies or in applications in which planar phospholipid layers supported on a semiconductor metal oxide surfaces are required.