Picoplus afm

Force measurements were performed on a PicoPlus AFM (Agilent Technologies) operated under PicoView 1.6.8 (Agilent Technologies). Force distance cycles were acquired using silicon cantilevers with a spring constant of 0.01 N/m or 0.6 N/m (Veeco) at pulling velocities of 0.14–1.33 µm/sec. Commercially available silicon-nitride AFM cantilevers with silicon tips (MSNL-10) were amine-functionalized as described in Tip- and Surface-Chemistry.
The effective spring constant was determined via thermal noise analysis³⁸ . For Fig. 3 we show the tip-support separation Δz of an AFM tip indenting into a supported DOPC bilayer. The distance of the tip from the surface of the mica-support was calculated from $\mathrm{\Delta }z=z-z_0-F/kS$ , where F denotes the force and z the cantilever position recorded during the approach curve, k the effective spring constant of the cantilever, and S the sensitivity taken from the approach curve upon bilayer penetration. The position of the mica surface $z_0$ was determined from the plateau regions observable in Fig. 3 at high forces, equivalent to full penetration of the tip through the bilayer.
F was plotted versus Δz for multiple approach curves, which allows for extracting the thickness of the elastically deformed fluid bilayer, 2d, by fitting with Equation 1^{39 (link)}: $$F=\frac{\pi {\kappa }_{A}R}{4}{\left(\frac{2\left(2d-\mathrm{\Delta }z\right)}{\mathrm{\Delta }z}\right)}^2$$
κ_A denotes the membrane compressibility and R the tip radius.