Kelvin probe force microscopy in application to biomolecular films: Frequency modulation, amplitude modulation, and lift mode

Kelvin probe force microscopy (KPFM) is a powerful technique to visualize the differences of work function in metals and lateral surface potential distribution in thin organic films. Earlier we have shown that frequency modulated-Kelvin probe force microscopy has significant advantages in both sensitivity and resolution when applied to metal and inorganic interfaces in vacuum. High resolution, high sensitivity, and performance in ambient conditions are required in order to study biologically relevant samples. In this work we compared the resolution of frequency modulation (FM-KPFM), amplitude modulation (AM-KPFM), and lift modes KPFM for imaging the local electrical surface potential of complex biomolecular films and demonstrated that FM-KPFM mode has superior resolution for biological applications. The power of the method was illustrated on pulmonary surfactant films, revealing nm spatial resolution and mV potential sensitivity in ambient air. At this level of resolution this method can provide critical insight into the molecular arrangement and function of complex biosystems in a way that other KPFM modes cannot do. Based on the observed changes in the local surface potential we discovered that excess cholesterol produces nm size electrostatic domains and change the electric fields.