Patterned Adhesion Layer Enables Rugged Pd-MIS Hydrogen Sensors.

Rugged Pd-metal-insulator-semiconductor (Pd-MIS) hydrogen sensors for detecting charge-exchange particles in fusion reactors have been constructed by utilizing a novel patterned adhesion layer. Poor adhesion at the interface between Pd and SiO is a common failure mode for Pd-MIS devices, severely limiting the Pd thickness and their usefulness as hydrogen sensors. The mechanical integrity of the Pd coatings is of particular importance in magnetic fusion energy research where the Pd-MIS diodes are used to measure hydrogen charge-exchange neutral fluence at the wall in tokamaks.

Hydrogen Storage in Partially Exfoliated Magnesium Diboride Multilayers.

Metal boride nanostructures have shown significant promise for hydrogen storage applications. However, the synthesis of nanoscale metal boride particles is challenging because of their high surface energy, strong inter- and intraplanar bonding, and difficult-to-control surface termination. Here, it is demonstrated that mechanochemical exfoliation of magnesium diboride in zirconia produces 3-4 nm ultrathin MgB nanosheets (multilayers) in high yield.

Mechanistic insight into biopolymer induced iron oxide mineralization through quantification of molecular bonding.

Microbial production of iron (oxyhydr)oxides on polysaccharide rich biopolymers occurs on such a vast scale that it impacts the global iron cycle and has been responsible for major biogeochemical events. Yet the physiochemical controls these biopolymers exert on iron (oxyhydr)oxide formation are poorly understood. Here we used dynamic force spectroscopy to directly probe binding between complex, model and natural microbial polysaccharides and common iron (oxyhydr)oxides.

How nanoscale surface steps promote ice growth on feldspar: microscopy observation of morphology-enhanced condensation and freezing.

Ice in the atmosphere affects Earth's radiative properties and initiates most precipitation. Growing ice often requires a solid surface, either to catalyze freezing of supercooled cloud droplets or to serve as a substrate for ice deposited from water vapor. There is evidence that this surface is typically provided by airborne mineral dust; but how chemistry, structure and morphology interrelate to determine the ice-nucleating ability of mineral surfaces remains elusive.

Surface Morphology and Electrical Properties of CuBTC Thin Films Before and After Reaction with TCNQ.

HKUST-1 or CuBTC (BTC = 1,3,5-benzenetricarboxylate) is a prototypical metal-organic framework (MOF) that holds a privileged position among MOFs for device applications, as it can be deposited as thin films on various substrates and surfaces. Recently, new potential applications in electronics have emerged for this material when HKUST-1 was demonstrated to become electrically conductive upon infiltration with 7,7,8,8-tetracyanoquinodimethane (TCNQ). However, the factors that control the morphology and reactivity of the thin films are unknown.

Elucidating the Variable-Temperature Mechanical Properties of a Negative Thermal Expansion Metal-Organic Framework.

We report the first experimental study into the thermomechanical and viscoelastic properties of a metal-organic framework (MOF) material. Nanoindentations show a decrease in the Young's modulus, consistent with classical molecular dynamics simulations, and hardness of HKUST-1 with increasing temperature over the 25-100 °C range. Variable-temperature dynamic mechanical analysis reveals significant creep behavior, with a reduction of 56% and 88% of the hardness over 10 min at 25 and 100 °C, respectively.

Quantifying the free energy landscape between polymers and minerals.

Higher organisms as well as medical and technological materials exploit mineral-polymer interactions, however, mechanistic understanding of these interactions is poorly constrained. Dynamic force spectroscopy can probe the free energy landscape of interacting bonds, but interpretations are challenged by the complex mechanical behavior of polymers. Here we restate the difficulties inherent to applying DFS to polymer-linked adhesion and present an approach to gain quantitative insight into polymer-mineral binding.

Imaging latex-carbon nanotube composites by subsurface electrostatic force microscopy.

Electrostatic modes of atomic force microscopy have shown to be non-destructive and relatively simple methods for imaging conductors embedded in insulating polymers. Here we use electrostatic force microscopy to image the dispersion of carbon nanotubes in a latex-based conductive composite, which brings forth features not observed in previously studied systems employing linear polymer films. A fixed-potential model of the probe-nanotube electrostatics is presented which in principle gives access to the conductive nanoparticle's depth and radius, and the polymer film dielectric constant.

Analytic descriptions of stochastic bistable systems under force ramp.

Solving the two-state master equation with time-dependent rates, the ubiquitous driven bistable system, is a long-standing problem that does not permit a complete solution for all driving rates. Here we show an accurate approximation to this problem by considering the system in the control parameter regime. The results are immediately applicable to a diverse range of bistable systems including single-molecule mechanics.

Nanomechanical force transducers for biomolecular and intracellular measurements: is there room to shrink and why do it?

Over the past couple of decades there has been a tremendous amount of progress on the development of ultrasensitive nanomechanical instruments, which has enabled scientists to peer for the first time into the mechanical world of biomolecular systems. Currently, work-horse instruments such as the atomic force microscope and optical/magnetic tweezers have provided the resolution necessary to extract quantitative force data from various molecular systems down to the femtonewton range, but it remains difficult to access the intracellular environment with these analytical tools as they have fairly large sizes and complicated feedback systems. This review is focused on highlighting some of the major milestones and discoveries in the field of biomolecular mechanics that have been made possible by the development of advanced atomic force microscope and tweezer techniques as well as on introducing emerging state-of-the-art nanomechanical force transducers that are addressing the size limitations presented by these standard tools.

Practical single molecule force spectroscopy: how to determine fundamental thermodynamic parameters of intermolecular bonds with an atomic force microscope.

Single molecule force spectroscopy involves loading a chemical bond using an atomic force microscope and measuring the rupture forces required to break that bond. In 20 years since its inception this technique developed into a robust way to extract a nearly complete set of the information about the bond that includes the bond energy, the kinetic parameters of the bond, and the geometry of the transition state. In this article we review the basic physics of the measurements, the model that is used for data interpretation, and go over the ways to extract the bond information from the experimental data.

Interpreting the widespread nonlinear force spectra of intermolecular bonds.

Single molecule force spectroscopy probes the strength, lifetime, and energetic details of intermolecular interactions in a simple experiment. A growing number of these studies have reported distinctly nonlinear trends in rupture force with loading rate that are typically explained in conventional models by invoking complex escape pathways. Recent analyses suggested that these trends should be expected even for simple barriers based on the basic assumptions of bond rupture dynamics and thus may represent the norm rather than the exception.

How the overlapping timescales for peptide binding and terrace exposure lead to non-linear step dynamics during growth of calcium oxalate monohydrate.

Using in situ atomic force microscopy (AFM), we investigate the inhibition of calcium oxalate monohydrate (COM) step growth by aspartic acid-rich peptides and find that the magnitude of the effect depends on terrace lifetime. We then derive a time dependent step-pinning model in which average impurity spacing depends on the terrace lifetime as given by the ratio of step spacing to step speed. We show that the measured variation in step speed is well fit by the model and allows us to extract the characteristic peptide adsorption time.

Subnanometer atomic force microscopy of peptide-mineral interactions links clustering and competition to acceleration and catastrophe.

In vitro observations have revealed major effects on the structure, growth, and composition of biomineral phases, including stabilization of amorphous precursors, acceleration and inhibition of kinetics, and alteration of impurity signatures. However, deciphering the mechanistic sources of these effects has been problematic due to a lack of tools to resolve molecular structures on mineral surfaces during growth. Here we report atomic force microscopy investigations using a system designed to maximize resolution while minimizing contact force.

Single-molecule approach to understanding multivalent binding kinetics.

Multivalency results from the simultaneous binding of multiple ligands with multiple receptors. Understanding the effect of multivalency on binding kinetics in molecular and cellular systems may aid the development of new types of therapeutics or countermeasures to pathogen infection. Here, we describe a method using single-molecule dynamic force spectroscopy to determine the binding strength of antibody-protein complexes as a function of binding valency in a direct and simple measurement.

Single functional group interactions with individual carbon nanotubes.

Carbon nanotubes display a consummate blend of materials properties that affect applications ranging from nanoelectronic circuits and biosensors to field emitters and membranes. These applications use the non-covalent interactions between the nanotubes and chemical functionalities, often involving a few molecules at a time. Despite their wide use, we still lack a fundamental understanding and molecular-level control of these interactions.

Unified model of dynamic forced barrier crossing in single molecules.

Thermally activated barrier crossing in the presence of an increasing load can reveal kinetic rate constants and energy barrier parameters when repeated over a range of loading rates. Here we derive a model of the mean escape force for all relevant loading rates-the complete force spectrum. Two well-known approximations emerge as limiting cases, one of which confirms predictions that single-barrier spectra should converge to a phenomenological description in the slow loading limit.

Controlled electrostatic gating of carbon nanotube FET devices.

Carbon nanotube transistors are a promising platform for the next generation of nonoptical biosensors. However, the exact nature of the biomolecule interactions with nanotubes in these devices remains unknown, creating one of the major obstacles to their practical use. We assembled alternating layers of oppositely charged polyelectrolytes on the carbon nanotube transistors to mimic gating of these devices by charged molecules.

Strength of multiple parallel biological bonds.

Multivalent interactions play a critical role in a variety of biological processes on both molecular and cellular levels. We have used molecular force spectroscopy to investigate the strength of multiple parallel peptide-antibody bonds using a system that allowed us to determine the rupture forces and the number of ruptured bonds independently. In our experiments the interacting molecules were attached to the surfaces of the probe and sample of the atomic force microscope with flexible polymer tethers, and the unique mechanical signatures of the tethers determined the number of ruptured bonds.

Dynamic force spectroscopy of parallel individual Mucin1-antibody bonds.

We used atomic force microscopy to measure the binding forces between Mucin1 (MUC1) peptide and a single-chain variable fragment (scFv) antibody selected from a scFv library screened against MUC1. This binding interaction is central to the design of molecules used for targeted delivery of radioimmunotherapeutic agents for prostate and breast cancer treatment. Our experiments separated the specific binding interaction from nonspecific interactions by tethering the antibody and MUC1 molecules to the atomic force microscope tip and sample surface with flexible polymer spacers.

Mechanism of DNA compaction by yeast mitochondrial protein Abf2p.

We used high-resolution atomic force microscopy to image the compaction of linear and circular DNA by the yeast mitochondrial protein Abf2p, which plays a major role in packaging mitochondrial DNA. Atomic force microscopy images show that protein binding induces drastic bends in the DNA backbone for both linear and circular DNA. At a high concentration of Abf2p DNA collapses into a tight nucleoprotein complex.

Packaging of single DNA molecules by the yeast mitochondrial protein Abf2p.

Mitochondrial and nuclear DNA are packaged by proteins in a very different manner. Although protein-DNA complexes called "nucleoids" have been identified as the genetic units of mitochondrial inheritance in yeast and man, little is known about their physical structure. The yeast mitochondrial protein Abf2p was shown to be sufficient to compact linear dsDNA, without the benefit of supercoiling, using optical and atomic force microscopy single molecule techniques.