Atomic Force Microscopy (AFM) is a popular characterization technique which uses a very sharp probe (radius of curvature as low as a few nm) mounted at the end of a cantilever to “feel” a surface. Many different imaging modes are available depending on the type of sample (e.g., hard or soft, conductor or insulator) and information desired (e.g., topography, friction, surface charge).
For many soft materials, such as cells or fluid droplets, the high contact pressure (∼GPa) exerted by the probe can cause deformation or destruction; many “non-contact” imaging modes have been developed for these soft samples. One set of non-contact modes applies an AC voltage to the cantilever, which then vibrates due to electrical interactions with the surface. By monitoring and controlling these vibrations it is possible to obtain both topography and surface potential via non-contact imaging. These techniques, called Scanning Polarization Force Microscopy (SPFM) and Kelvin Probe Force Microscopy (KFM), have been used to study everything from hard disk lubricants to charged biomolecules.
Thus far, these techniques have only been used in vacuum or gas environments, since an aqueous environment would shield the electrical forces that drive the cantilever vibration. Non-contact imaging of topography and surface potential in solution would be useful for studying cells and biomolecules in situ. One exciting example is imaging the change in cell structure and surface charge of fungus F. oxysporum as it attacks the plant A. thaliana. This work is in collaboration with Prof. Seogchan Kang in Penn State Plant Pathology department.
We have recently developed a system which allows simultaneous imaging of both surface topography and potential in non-contact mode in solution. We monitor the amplitude of the cantilever vibration and the phase lag between the applied electric bias and the vibration; both the amplitude and phase change depending on the probe-surface interaction forces. The nature of the interaction forces is complicated due to the aqueous environment. Possible contributing forces are dielectrophoresis, electrophoresis, electrohydrodynamics, electrostatics and van der Waals. We are currently investigating which forces are acting and how they affect the vibrational amplitude and phase signals.