Fast, High Resolution, and Wide Modulus Range Nanomechanical Mapping with Bimodal Tapping Mode.

Tapping mode atomic force microscopy (AFM), also known as amplitude modulated (AM) or AC mode, is a proven, reliable, and gentle imaging mode with widespread applications. Over the several decades that tapping mode has been in use, quantification of tip-sample mechanical properties such as stiffness has remained elusive. Bimodal tapping mode keeps the advantages of single-frequency tapping mode while extending the technique by driving and measuring an additional resonant mode of the cantilever.

Generalized Hertz model for bimodal nanomechanical mapping.

Bimodal atomic force microscopy uses a cantilever that is simultaneously driven at two of its eigenmodes (resonant modes). Parameters associated with both resonances can be measured and used to extract quantitative nanomechanical information about the sample surface. Driving the first eigenmode at a large amplitude and a higher eigenmode at a small amplitude simultaneously provides four independent observables that are sensitive to the tip-sample nanomechanical interaction parameters.

Calibration of higher eigenmodes of cantilevers.

A method is presented for calibrating the higher eigenmodes (resonant modes) of atomic force microscopy cantilevers that can be performed prior to any tip-sample interaction. The method leverages recent efforts in accurately calibrating the first eigenmode by providing the higher-mode stiffness as a ratio to the first mode stiffness. A one-time calibration routine must be performed for every cantilever type to determine a power-law relationship between stiffness and frequency, which is then stored for future use on similar cantilevers.

Contact resonance atomic force microscopy imaging in air and water using photothermal excitation.

Contact Resonance Force Microscopy (CR-FM) is a leading atomic force microscopy technique for measuring viscoelastic nano-mechanical properties. Conventional piezo-excited CR-FM measurements have been limited to imaging in air, since the "forest of peaks" frequency response associated with acoustic excitation methods effectively masks the true cantilever resonance. Using photothermal excitation results in clean contact, resonance spectra that closely match the ideal frequency response of the cantilever, allowing unambiguous and simple resonance frequency and quality factor measurements in air and liquids alike.

Mirror symmetry breaking and chiral amplification in foldamer-based supramolecular helical aggregates.

Spontaneous asymmetric generation of supramolecular chiral fibers was observed in the folding induced self-assembly of a lock-washer shaped foldamer. A secondary nucleation growth mechanism is proposed to explain the observed chiral amplification or deracemization of these supramolecular fibers.

Separating attoliter-sized compartments using fluid pore-spanning lipid bilayers.

Anodic aluminum oxide (AAO) is a porous material having aligned cylindrical compartments with 55-60 nm diameter pores, and being several micrometers deep. A protocol was developed to generate pore-spanning fluid lipid bilayers separating the attoliter-sized compartments of the nanoporous material from the bulk solution, while preserving the optical transparency of the AAO. The AAO was selectively functionalized by silane chemistry to spread giant unilamellar vesicles (GUVs) resulting in large continuous membrane patches covering the pores.

Preparation of solvent-free, pore-spanning lipid bilayers: modeling the low tension of plasma membranes.

Plasma membrane tension, produced by the underlying cytoskeleton, governs many dynamic processes such as fusion, blebbing, exo- and endocytosis, cell migration, and adhesion. Here, a new protocol is introduced to model this intricate and often overlooked aspect of the plasma membrane. Lipid bilayers spanning pores of 600 nm radius were prepared by adsorption and spreading of giant unilamellar vesicles (GUVs) on moderately hydrophilic porous substrates prepared by gold-coating and subsequent self-assembly of a mercaptoethanol monolayer.

Single molecule atomic force microscopy and force spectroscopy of chitosan.

Atomic force microscopy (AFM) and AFM-based force spectroscopy was used to study the desorption of individual chitosan polymer chains from substrates with varying chemical composition. AFM images of chitosan adsorbed onto a flat mica substrate show elongated single strands or aggregated bundles. The aggregated state of the polymer is consistent with the high level of flexibility and mobility expected for a highly positively charged polymer strand.