Mechanical manifestations of rare atomic jumps in dynamic force microscopy.

The resonance frequency and the excitation amplitude of a silicon cantilever have been measured as a function of distance to a cleaved KBr(001) surface with a low-temperature scanning force microscope (SFM) in ultrahigh vacuum. We identify two regimes of tip-sample distances. Above a site-dependent critical tip-sample distance reproducible data with low noise and no interaction-induced energy dissipation are measured. In this regime reproducible SFM images can be recorded. At closer tip-sample distances, above two distinct atomic sites, the frequency values jump between two limiting curves on a timescale of tens of milliseconds. Furthermore, additional energy dissipation occurs wherever jumps are observed. We attribute both phenomena to rarely occurring changes in the tip apex configuration which are affected by short-range interactions with the sample. Their respective magnitudes are related to each other. A specific candidate two-level system is also proposed.

Rapid and label-free nanomechanical detection of biomarker transcripts in human RNA.

The availability of entire genome sequences has triggered the development of microarrays for clinical diagnostics that measure the expression levels of specific genes. Methods that involve labelling can achieve picomolar detection sensitivity, but they are costly, labour-intensive and time-consuming. Moreover, target amplification or biochemical labelling can influence the original signal. We have improved the biosensitivity of label-free cantilever-array sensors by orders of magnitude to detect mRNA biomarker candidates in total cellular RNA. Differential gene expression of the gene 1-8U, a potential marker for cancer progression or viral infections, has been observed in a complex background. The measurements provide results within minutes at the picomolar level without target amplification, and are sensitive to base mismatches. This qualifies the technology as a rapid method to validate biomarkers that reveal disease risk, disease progression or therapy response. We foresee cantilever arrays being used as a tool to evaluate treatment response efficacy for personalized medical diagnostics.

Energy level alignment at metal-octaethylporphyrin interfaces.

We studied the electronic structure of copper-octaethylporphyrin (CuEOP) adsorbed on three metal surfaces--Ag(001), Ag(111), and Cu(111)--by means of ultraviolet photoelectron spectroscopy (UPS). The adsorption-induced work function shifts saturate roughly beyond two monolayers. The saturation values are substrate dependent, negative, and range from -1.30 to -0.85 eV. This shift is larger than that for tetraphenylporphyrins. The two highest occupied molecular orbitals (HOMO and HOMO-1) of the organic are clearly resolved in the UPS spectra. The origin of the negative work function shift is discussed.

Study of the mechanical properties of myomesin proteins using dynamic force spectroscopy.

Myomesin is the most prominent structural component of the sarcomeric M-Band that is expressed in mammalian heart and skeletal muscles. Like titin, this protein is an intracellular member of the Ig-fibronectin superfamily, which has a flexible filamentous structure and which is largely composed of two types of domain that are similar to immunoglobulin (Ig)-like and fibronectin type III (FNIII) domains. Several myomesin isoforms have been identified, and their expression patterns are highly regulated both spatially and temporally. Particularly, alternative splicing in the central part of the molecule gives rise to an isoform, EH (embryonic heart)-myomesin, containing a serine and proline-rich insertion with no well-defined secondary structure, the EH segment. EH-myomesin represents the major myomesin isoform at embryonic stages of mammalian heart and is rapidly down-regulated around birth, but it is re-expressed in the heart of patients suffering from dilated cardio-myopathy. Here, in order to facilitate a better understanding of the physiological, and possibly pathological, functions of myomesin proteins, we explore the mechanical stability, elasticity and force-driven structural changes of human myomesin's sub-molecular segments using single-molecule force spectroscopy and protein engineering. We find that human myomesin molecules are composed of modules (Ig and FNIII), that are designed to withstand force and we demonstrate that the human cardiac EH segment functions like an additional elastic stretch in the middle part of the EH-myomesin and behaves like a random coil. Consequently myomesin isoforms (proteins with or without the EH segment) have different elastic properties, the EH-myomesin being the more compliant one. These findings imply that the compliance of the M-band increases with the amount of EH-myomesin it contains. So, we provide the evidence that not only titin but also other sarcomeric proteins have complicated visco-elastic properties depending on the contractile parameters in different muscle types.

Octyl-decorated Fréchet-type dendrons: a general motif for visualisation of static and dynamic behaviour using scanning tunnelling microscopy?

A detailed STM study of monolayers of 3,5-bis[(3,5-bisoctyloxyphenyl)methyloxy]benzaldehyde and 3,5-bis[(3,5-bisoctyloxyphenyl)methyloxy]benzyl alcohol adsorbed on graphite is presented. Very highly resolved scanning tunnelling microscopy images are observed at room temperature in air allowing the analysis of the conformation of the adsorbed molecules. These long-chain alkyl-decorated Fréchet-type dendrons are a powerful assembly motif and initially form a pattern based on trimeric units, assembled into hexagonal host structures with a pseudo-unit cell of seven molecules, one of which remains highly mobile. Over time, the supramolecular ordering changes from a trimeric into a dimeric pattern. The chirality arising from the adsorption onto a surface of the dendrons is discussed.

Sublattice identification in scanning force microscopy on alkali halide surfaces.

We propose and apply to the KBr(001) surface a new procedure for species recognition in scanning force microscopy (SFM) of ionic crystal surfaces which show a high symmetry of the charge arrangement. The method is based on a comparison between atomistic simulations and site-specific frequency versus distance measurements. First, by taking the difference of force-distance curves extracted at a few judiciously chosen surface sites we eliminate site-independent long-range forces. The obtained short-range force differences are then compared with calculated ones assuming plausible tip apex models. This procedure allows for the first time identification of the tip apex polarity and of the positive and negative sublattices in SFM images of the (001) cleavage surface of an ionic crystal with the rock salt structure.

Melting of isolated tin nanoparticles

The melting of isolated neutral tin cluster distributions with mean sizes of about 500 atoms has been investigated in a molecular beam experiment by calorimetrically measuring the clusters' formation energies as a function of their internal temperature. For this purpose the possibility to adjust the temperature of the clusters' internal degrees of freedom by means of the temperature of the cluster source's nozzle was exploited. The melting point of the investigated tin clusters was found to be lowered by 125 K and the latent heat of fusion per atom is reduced by 35% compared to bulk tin. The melting behavior of the isolated tin clusters is discussed with respect to the occurrence of surface premelting.

A cantilever array-based artificial nose

We present quantitative and qualitative detection of analyte vapors using a microfabricated silicon cantilever array. To observe transduction of physical and chemical processes into nanomechanical motion of the cantilever, swelling of a polymer layer on the cantilever is monitored during exposure to the analyte. This motion is tracked by a beam-deflection technique using a time multiplexing scheme. The response pattern of eight cantilevers is analyzed via principal component analysis (PCA) and artificial neural network (ANN) techniques, which facilitates the application of the device as an artificial chemical nose. Analytes tested comprise chemical solvents, a homologous series of primary alcohols, and natural flavors. First differential measurements of surface stress change due to protein adsorption on a cantilever array are shown using a liquid cell.