Sergei Magonov

An atomic force microscope tip designed to measure time-varying nanomechanical forces

Tapping-mode atomic force microscopy (AFM), in which the vibrating tip periodically approaches, interacts and retracts from the sample surface, is the most common AFM imaging method. The tip experiences attractive and repulsive forces that depend on the chemical and mechanical properties of the sample, yet conventional AFM tips are limited in their ability to resolve these time-varying forces. We have created a specially designed cantilever tip that allows these interaction forces to be measured with good (sub-microsecond) temporal resolution and material properties to be determined and mapped in detail with nanoscale spatial resolution. Mechanical measurements based on these force waveforms are provided at a rate of 4 kHz. The forces and contact areas encountered in these measurements are orders of magnitude smaller than conventional indentation and AFM-based indentation techniques that typically provide data rates around 1 Hz. We use this tool to quantify and map nanomechanical changes in a binary polymer blend in the vicinity of its glass transition.

A nanomechanical device based on linear molecular motors

An array of microcantilever beams, coated with a self-assembled monolayer of bistable, redox-controllable [3]rotaxane molecules, undergoes controllable and reversible bending when it is exposed to chemical oxidants and reductants. Conversely, beams that are coated with a redox-active but mechanically inert control compound do not display the same bending. A series of control experiments and rational assessments preclude the influence of heat, photothermal effects, and pH variation as potential mechanisms of beam bending. Along with a simple calculation from a force balance diagram, these observations support the hypothesis that the cumulative nanoscale movements within surface-bound “molecular muscles” can be harnessed to perform larger-scale mechanical work.

True molecular resolution in tapping-mode atomic force microscopy with high-resolution probes

True molecular resolution was demonstrated in tapping-mode atomic force microscopy study of polydiacetylene crystal using carbon probes with an extremity of ∼1 nm. Images of the bc plane of the crystal, which were obtained at ambient conditions, reproduce the crystallographic molecular arrangement. The image features directly correspond to the edges of the individual side groups of the polymer chains, which form the crystal surface. In the consecutive images, the molecular-size defects have been observed on this surface as an additional proof of the true molecular resolution achieved with the probes.

The atomic force microscope: A tool for science and industry

Abstract Images of graphite and RuCl 3 show that the atomic force microscope (AFM) is capable of imaging rigid samples with atomic resolution. Images of photographic film showing the emulsion demonstrate the potential of the microscope for industrial quality control. An image of a stoma on a leaf shows that the microscope is gentle enough not to damage surfaces, even of soft biological samples.

Single-pass Kelvin force microscopy and dC/dZ measurements in the intermittent contact: applications to polymer materials.

Summary We demonstrate that single-pass Kelvin force microscopy (KFM) and capacitance gradient (dC/dZ) measurements with force gradient detection of tip–sample electrostatic interactions can be performed in the intermittent contact regime in different environments. Such combination provides sensitive detection of the surface potential and capacitance gradient with nanometer-scale spatial resolution as it was verified on self-assemblies of fluoroalkanes and a metal alloy. The KFM and dC/dZ applications to several heterogeneous polymer materials demonstrate the compositional mapping of these samples in dry and humid air as well as in organic vapors. In situ imaging in different environments facilitates recognition of the constituents of multi-component polymer systems due to selective swelling of components.

Exploring writhe in supercoiled minicircle DNA

Using λ-Int recombination in E. coli, we have generated milligram quantities of supercoiled minicircle DNA. Intramolecular Int recombination was efficient down to lengths ~254 bp. When nicked and religated in the presence of ethidium bromide, 339 bp minicircles adopted at least seven unique topoisomers that presumably correspond to ΔLk ranging from 0 to -6, which we purified individually. We used these minicircles, with unique ΔLk, to address the partition into twist and writhe as a function of ΔLk. Gel electrophoresis and atomic force microscopy revealed progressively higher writhe conformations in the presence of 10 mM CaCl(2) or MgCl(2). From simplistic calculations of the bending and twisting energies, we predict the elastic free energy of supercoiling for these minicircles to be lower than if the supercoiling was partitioned mainly into twist. The predicted writhe corresponds closely with that which we observed experimentally in the presence of divalent metal ions. However, in the absence of divalent metal ions only limited writhe was observed, demonstrating the importance of electrostatic effects on DNA structure, when the screening of charges on the DNA is weak. This study represents a unique insight into the supercoiling of minicircle DNA, with implications for DNA structure in general.

Atomic force microscopy in analysis of rubber materials

Abstract Atomic force microscopy (AFM) and electric force microscopy (EFM) have been applied for compositional mapping of a number of elastomers and related multicomponent materials. Several aspects of optimizing AFM experiments on polymers are discussed. AFM images revealed changes of EPDM morphology caused by crosslinking and by loading with fillers [carbon black (CB) and silica particles] and oil. It was shown that the morphology of isotactic polypropylene (iPP)/EPDM vulcanizates, which were studied with AFM and EFM, depends on the ratio of components, degree of cure and processing conditions. Diffusion of oil from the elastomer component to the matrix is evidenced in the AFM images. Selective distribution of CB in the iPP matrix is responsible for the electric conductivity of the thermoplastic vulcanizate.

Single molecule transcription profiling with AFM

Established techniques for global gene expression profiling, such as microarrays, face fundamental sensitivity constraints. Due to greatly increasing interest in examining minute samples from micro-dissected tissues, including single cells, unorthodox approaches, including molecular nanotechnologies, are being explored in this application. Here, we examine the use of single molecule, ordered restriction mapping, combined with AFM, to measure gene transcription levels from very low abundance samples. We frame the problem mathematically, using coding theory, and present an analysis of the critical error sources that may serve as a guide to designing future studies. We follow with experiments detailing the construction of high density, single molecule, ordered restriction maps from plasmids and from cDNA molecules, using two different enzymes, a result not previously reported. We discuss these results in the context of our calculations.

Topography and surface potential in Kelvin force microscopy of perfluoroalkyl alkanes self-assemblies

Self-assemblies of two perfluoroalkyl alkanes [F(CF2)n(CH2)mH with n=14, m=20 and n=12, m=8] on different substrates (mica, Si, and graphite) were examined with Kelvin force microscopy (KFM) by detecting either the electrostatic tip-sample force or its gradient. Both measurements were realized in the single pass operation using different frequencies for monitoring topography and surface potential changes. Furthermore, the KFM imaging was performed in the intermittent contact regime that allowed high-resolution mapping of surface potential changes without noticeable cross-talk between signals related to mechanical and electric forces. The use of force gradient for electrostatic force measurements led to high-resolution surface potential images of the perfluoroalkyl alkane adsorbates with strong contrast of the particular self-assemblies (spirals, ribbons, and toroids). The obtained potential data are close to ones recorded with macroscopic Kelvin probe and to theoretical estimates. The surface potential co...

Novel diamond/sapphire probes for scanning probe microscopy applications

Novel diamond/sapphire probes are introduced for different scanning probe microscopy applications: from nanoindentation to high-resolution imaging of soft samples. These probes have sharp diamond tips with apex size below 10 nm and their stiffness/resonant frequency and tip shape can be adjusted for customer needs. Several examples of imaging with novel probes are given. Probes with conducting diamond tips exhibited high-sensitivity in electric force microscopy study.