Quantitative membrane electrostatics with the atomic force microscope.
The atomic force microscope (AFM) is sensitive to electric double layer interactions in electrolyte solutions, but provides only a qualitative view of interfacial electrostatics. We have fully characterized silicon nitride probe tips and other experimental parameters to allow a quantitative electrostatic analysis by AFM, and we have tested the validity of a simple analytical force expression through numerical simulations. As a test sample, we have measured the effective surface charge density of supported zwitterionic dioleoylphosphatidylcholine membranes with a variable fraction of anionic dioleoylphosphatidylserine. The resulting surface charge density and surface potential values are in quantitative agreement with those predicted by the Gouy-Chapman-Stern model of membrane charge regulation, but only when the numerical analysis is employed. In addition, we demonstrate that the AFM can detect double layer forces at a separation of several screening lengths, and that the probe only perturbs the membrane surface potential by <2%. Finally, we demonstrate 50-nm resolution electrostatic mapping on heterogeneous model membranes with the AFM. This novel combination of capabilities demonstrates that the AFM is a unique and powerful probe of membrane electrostatics.
Yang
Yi
Y
Department of Physics & Astronomy, Rice University, Houston, Texas, USA.
Mayer
Kathryn M
KM
Hafner
Jason H
JH
D000465:Algorithms; D002462:Cell Membrane / Q000737:chemistry*; D004574:Electromagnetic Fields; D008051:Lipid Bilayers / Q000737:chemistry*; D008564:Membrane Potentials; D018625:Microscopy, Atomic Force / Q000379:methods*; D010743:Phospholipids / Q000737:chemistry*; D055672:Static Electricity
D008051:Lipid Bilayers; D010743:Phospholipids