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2.2.2 Force curve acquisition and analysis

All AFM experiments were carried out with silicon nitride probes (DNP, cantilever C,
Veeco Probes). Both tip and sample were immersed in 0.5 mM Tris buffer (pH 7)
throughout the experiment. To record force curves over lipids, the AFM tip was situated
over a lipid membrane by first imaging the topography and then positioning the tip over
the lipid region. For reference measurements, force curves were recorded over the silicon
nitride chip of a probe from the same wafer as the tip. The gold coating on this chip was
first etched with aqua regia to reduce interference from the reflected AFM laser beam.
Force curves were recorded with the Nanoscope software (version 5.30rl) with 10,240
data points over an 800 nm scan range at 1.4 Hz, with tip retraction triggered for a
maximum cantilever deflection corresponding to 5 nN. The raw force curves (cantilever
deflection voltage on the y axis versus z piezo position on the x axis) were exported and
read into MatLab where they were converted to force versus tip-sample separation, D, by
the following methods which are similar to those described previously [1, 38, 47]:

1. A у-axis offset was applied to set the deflection voltage equal to zero for large tip-
sample separation.

2. An interference intensity function was fit to the large tip-sample separation region and
the resulting function was subtracted from the entire force curve to compensate for
optical interference between reflections from the cantilever and the sample surface.

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