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.