Sensitivity of lipid bilayer sensing of pore-forming toxins relies on electrical properties of nanoscale-thick submembrane reservoir separating solid support and phospholipid membrane

The comparative study of two types of tethered bilayer membranes (tBLMs), one formed on atomically smooth gold films and the other on fluorine-doped tin oxide (FTO) films on glass is reported. These solid substrates exhibited different surface morphology and roughness. After tailoring with thiol or silane compounds, both formed intact tBLMs as demonstrated by electrochemical impedance spectroscopy (EIS). In contrast to the flat Au surfaces, the FTO substrates showed the presence of local water pockets, which caused a lower electrical resistance of the submembrane space separating the bilayers and the solid substrate.

The EIS data analysis showed that the residual lateral defects, with an average radius between 10 and 20 nm, are heterogeneously distributed over the surface of the tBLMs. The concentration of defects on the surface is very low, ranging from 10 to 30 defects per 1000 μm². Both types of tBLMs responded to the pore-forming toxin, the α-hemolysin (αHL) from Staphylococcus aureus in a concentration dependent manner. The EIS response and the sensitivity to the toxin were stronger for tBLMs on FTO than on Au. We show that, independent of the solid substrate, the property that affects the sensitivity of tBLM sensors to membrane-damaging toxins is related to the resistance of the submembrane reservoir. The lower the resistance, the higher the sensitivity. Although we cannot exclude other factors such as membrane composition, buffer salts, and pH, we conclude that, in general, submembrane resistance is the fundamental factor determining the sensitivity of tBLM sensors.