Abstract
Recently developed sub-resonance tapping modes (such as Digital Pulse, Peak Force Tapping, HybriD, etc.) of atomic force microscopy (AFM) allow imaging of compositional contrast of (bio)materials and biological cells down to the nanoscale. Here we report on a powerful extension of those modes, “ringing” mode, which more than doubles the number of non-trivial physical channels that can be collected with a regular sub-resonance tapping. It can simultaneously record five new additional compositional parameters related to adhesive and viscoelastic properties of the sample surface: the restored (averaged) adhesion, adhesion height, pull-off neck height, detachment distance, and detachment energy losses. Ringing mode can be up to 20 times faster and showing fewer artifacts compared to the existing sub-resonance tapping modes. Ringing mode is based on an analysis of ringing signal of the AFM cantilever after detaching the AFM probe from the sample surface (this signal is currently treated as noise, and typically filtered out in the existing modes). We demonstrate that this new mode allows recording robust and unique information on fixed human epithelial cells, corneocyte skin flakes, and polymers used for bioimplants.
Highlights
Traditional atomic force microscopy (AFM) is used to visualize sample surfaces, including biomaterials, living and fixed cells down to the nanoscale[1,2]
We demonstrate realization of this method using an example of PeakForce Tapping mode
The AFM deflection signal is recorded and processed in real time. It brings the maps of sample height, values of the pull-off force, mechanical stiffness, and viscoelastic losses during the sample deformation by the AFM probe at each cycle
Summary
Traditional atomic force microscopy (AFM) is used to visualize sample surfaces, including biomaterials, living and fixed cells down to the nanoscale[1,2]. The sub-resonance ramping modes allow recording of the sample height, adhesion, sample surface stiffness, viscoelastic dissipation energy. Such information is important for biomedical applications. In all sub-resonance modes, the AFM probe ramps vertically (while the sample moves in lateral direction) oscillating at the frequencies much smaller than the resonant frequency of the AFM cantilever These modes have been introduced to combine quantitative advantages of the Force-Volume mode while retaining the fast speed of typical imaging scanning. Epithelial cells with HarmoniX and PeakForce Tapping allowed to segregate cancerous and normal cells with unprecedented accuracy It took about 1.5 hours to obtain a map of one cell, which makes it impractical for a potential clinical use. Some particular orientations of the semi-transparent samples can create interference artifacts which are a serious issue for quantitative interpretation of results
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