Jul 26, 2011
Double-pass force-distance mapping simplifies nanomechanical characterization
Characterizing the mechanical properties of nanostructures is an important research activity, particularly in the fields of polymer science and biomaterials. Polymer blends, for example, may show distinct domains of varying mechanical and surface adhesion properties. Biomaterials exhibit significant changes in adhesion properties due to the presence of functional groups on their surfaces, which can be probed selectively using functionalized tips. A variety of characterization techniques have emerged including multi-frequency techniques, or methods that use special cantilevers and electronics to measure interaction forces during imaging.
Now, researchers from Bilkent University, Turkey, have demonstrated a simple method for obtaining high-resolution mechanical maps. The technique applies the widely available double-pass scheme of the force-microscope to force-distance measurements, and can be applied using standard cantilevers. Non-resonant measurements used in this scheme inherently reject ambiguities and complications that may arise due to coupling of elasticity information with dissipation or adhesion information.
The scientists, based at the Institute of Materials Science and Nanotechnology, used the double-pass scheme of an atomic force microscope to acquire nanomechanical maps of soft biomolecular structures. Double-Pass methods are commonly used for the characterization of long-distance interactions, such as electrostatic and magnetic forces. Force-distance mapping, on the other hand, is a valuable tool to study the adhesion and elasticity properties of surfaces.
The application of nanomechanical characterization in polymer science and biomaterials provides valuable information on a rich variety of samples. Key requirements in non-destructive nanomechanical characterization are keeping the peak force to a minimum, ease of application of the technique and artifact free interpretation of data.
Trace and retrace
The double-pass force-distance mapping measures the topography in the first-pass using dynamic-mode imaging, and this topography information is used to fly the tip with a few nanometers separation over the surface in the retrace. During the retrace, the tip-sample separation is modulated at a frequency (about 2 KHz) much lower than the cantilever resonance frequency (about 100 KHz), and rapid force-distance curves are recorded. The elimination of feedback in the force-distance acquisition allows high-density force-distance mapping.
The method is shown to provide high-resolution and high-speed nanomechanical characterization, which provides a large number of measurement points that can be processed into histograms of elastic properties and adhesion. In the study, the researchers demonstrate the method on self-assembled peptidic nanofibres. Recently, they have used nanomechanical characterization to study the factors affecting the mechanical properties of biocompatible gels made of such nanofibres (See Dagdas et al. Soft Matter 2011 7 3524).
More information on the force-mapping technique can be found in the journal Nanotechnology.
About the author
The study was conducted in collaboration by three research groups at the UNAM Institute of Materials Science and Nanotechnology, Bilkent University, Turkey. Dr Dana's group is more oriented towards engineering and physics and develops novel techniques for surface characterization. Dr Guler and Dr Tekinay's groups synthesize self-assembled nanostructures and study the properties of these materials, as well as their applications in molecular biology and medicine.