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An atomic force microscope (AFM) operating in force modulation mode is used to study solvation forces at the interface between a graphite (HOPG) surface and the liquids octamethylcyclotetrasiloxane (OMCTS) and n-dodecanol. Simple analytical models are used that adequately describe the response of the cantilever as the modulation frequency and tip−sample interaction change. The analysis of AFM force curves yields the tip−sample interaction stiffness and damping. Hydrodynamic damping is significant for all the levers used and at present this limits the sensitivity of detecting weak tip−surface damping effects. The main results are: (i) Confinement of liquid between two surfaces can lead to oscillatory structural forces even when one of the surfaces has very high curvature. This could influence topographic images at the atomic level in liquids. In these experiments the typical radius for a sharp AFM tip is measured as Rtip ≈ 14 nm. (ii) The effective viscosity increases by ∼4 orders of magnitude for a sharp...

We have investigated the reliability of a variety of metal coated and semiconductor tips for use in conducting atomic force microscopy experiments in an ultrahigh vacuum (UHV) environment. In order to obtain reliable conduction data we find it necessary to first clean the tips using a short argon ion sputter. Scanning transmission electron microscopy is used to image tips after the conductivity experiments and found to be very useful for assessing tip wear and interpreting conductivity data. Tip reliability is found to be strongly dependent on the sample and the experimental conditions. Wear and contamination of the tip are found to be severe problems which are related to the tip-sample adhesion. We illustrate these effects and highlight some of the common reliability problems which we encountered using specific examples. In general, we find that metal coated tips are not reliable enough to obtain repeatable data, especially if lateral forces are exerted on the tip. Homogeneous semiconductor tips, once cl...

Photon emission from the scanning tunneling microscope (STM) has been shown to produce spectra that are dependent on the dielectric function of the surface. This presents the exciting possibility of chemical information from the STM, previously impossible albeit with a few exceptions. However, the spectra are also critically dependent on the geometry of the tunnel junction---changes in either tip or sample curvature can have dramatic effects on the spectra. A model is presented here that allows for curvature in the sample as well as the tip, and the results compare favorably with existing experimental data. It predicts that under certain conditions, spectra can be made almost invariant to changes in the geometry, leaving them dependent solely on the sample material---making the STM a chemical probe. The model also predicts that gold and silver particles may be differentiated by comparing their variations of photon intensity with bias, and this prediction is confirmed experimentally. Finally, a strategy for chemical identification of any metal surface is presented.

We used an ultrahigh-vacuum atomic force microscope (AFM) to measure lateral forces and conductivity simultaneously as a function of the applied normal force for nanometer-sized elastic contacts. Metal-coated or bare Si AFM tips are used on cleaved ${\mathrm{NbSe}}_{2}$ or graphite surfaces. Results are used to compare various means of obtaining the tip-sample contact area ${(A}_{0}).$ We find that simple continuum models can give a reasonable description of the mechanical behavior of the contact. Specifically, the Maugis-Dugdale model [D. Maugis, J. Colloid Interface Sci. 150, 243 (1992)] provides a good basis for describing the elastic contact between an AFM tip and a smooth sample. The theoretical variation in contact radius with load is in good agreement with the experimental variation in friction force, conductivity, and lateral tip-sample contact stiffness. To find the contact area ${A}_{0},$ the best approaches appear to be either to fit the applied force data to an appropriate continuum model (provided the contact is elastic) or to measure the lateral tip-sample contact stiffness. In principle we show that conduction AFM methods can be used to find ${A}_{0}$ for Ohmic contacts. However, the uncertainty in the conduction properties of available AFM tips means that at present the absolute value of ${A}_{0}$ cannot be found with confidence. In this regard the use of metal-coated tips can often be misleading for conduction and mechanical measurements because metal wears rapidly off all or some of the tip apex.

Reversible, controllable optical nanocontraction and dilatation in a chalcogenide glass film was induced by polarized light, and a direct correlation of this optomechanical effect with the reversible optical-induced optical anisotropy (dichroism) also exhibited by the chalcogenide glass was observed. A microscopic model of the photoinduced, reversible structural phenomenon responsible for the optomechanical behavior is presented. The ability to induce an anisotropic optomechanical effect could form the basis of a number of applications, including polarized light-dependent optical nanoactuators, optomechanical diaphragm micropumps, and even motors driven by polarized light.

Poly(methylmethacrylate) (PMMA) which is commonly used as a positive resist can also be used in a negative manner with exposure at higher dose levels. In this paper we investigate the full potential of this resist for high-resolution pattern definition. We show that although the point spread exposure distribution is similar to that for positive PMMA, features of the order of 10 nm are easily achieved. These resist structures can be transferred into the underlying materials using plasma etch techniques to a similar degree of resolution. The density of resist features is also greater than for positive PMMA with features of nm in width being possible on a 30 nm pitch. The instability of the resist structures at extreme linewidths has been identified as a potential problem in utilizing the process.

We have used an ultrahigh vacuum atomic-force microscope to study the variation in contact radius and friction with applied force between a silicon tip and a ${\mathrm{NbSe}}_{2}$ sample. The data are compared to the Maugis-Dugdale theory, which is the appropriate continuum mechanics model for the properties and size of the tip-sample contact. The lateral stiffness of the tip-sample contact is related to the radius of the tip-sample contact through the shear moduli of the materials and we have used this relationship to measure directly the variation in contact radius with applied load. The contact radius measured in this way is found to be in agreement with the Maugis-Dugdale theory using the bulk values of the shear moduli. We also measured the variation in friction force with applied load using the same silicon tip. The variation in friction force with applied normal force is found to follow the variation of the contact area as predicted by the Maugis-Dugdale theory [D. Maugis, J. Colloid Interface Sci. 150, 243 (1992)], which supports the hypothesis that for a single asperity contact, the frictional shear stress \ensuremath{\tau} is constant. The value of the shear stress is found to be \ensuremath{\tau}\ensuremath{\approx}6\ifmmode\times\else\texttimes\fi{}${10}^{8}$ N/${\mathrm{m}}^{2}$, which is comparable to the estimated theoretical shear strength of ${\mathrm{NbSe}}_{2}$.

A new directionally sensitive velocity measurement system based upon a single oscillating hot-wire probe has been developed. The vibration of the hot-wire was effected by a piezoelectric oscillator that could be driven at different amplitudes and frequencies. The sign of the local flow velocity was determined by the phase shift between the motion and the output from the hot-wire. The system was used to measure mean and fluctuating velocities profiles of the separated flow behind a backward facing step.

Nanobodies are single-domain fragments of camelid antibodies that are emerging as versatile tools in biotechnology. We describe here the interactions of a specific nanobody, NbSyn87, with the monomeric and fibrillar forms of α-synuclein (αSyn), a 140-residue protein whose aggregation is associated with Parkinson's disease. We have characterized these interactions using a range of biophysical techniques, including nuclear magnetic resonance and circular dichroism spectroscopy, isothermal titration calorimetry and quartz crystal microbalance measurements. In addition, we have compared the results with those that we have reported previously for a different nanobody, NbSyn2, also raised against monomeric αSyn. This comparison indicates that NbSyn87 and NbSyn2 bind with nanomolar affinity to distinctive epitopes within the C-terminal domain of soluble αSyn, comprising approximately amino acids 118–131 and 137–140, respectively. The calorimetric and quartz crystal microbalance data indicate that the epitopes of both nanobodies are still accessible when αSyn converts into its fibrillar structure. The apparent affinities and other thermodynamic parameters defining the binding between the nanobody and the fibrils, however, vary significantly with the length of time that the process of fibril formation has been allowed to progress and with the conditions under which formation occurs, indicating that the environment of the C-terminal domain of αSyn changes as fibril assembly takes place. These results demonstrate that nanobodies are able to target forms of potentially pathogenic aggregates that differ from each other in relatively minor details of their structure, such as those associated with fibril maturation.

There is increasing evidence for the involvement of lipid membranes in both the functional and pathological properties of α-synuclein (α-Syn). Despite many investigations to characterize the binding of α-Syn to membranes, there is still a lack of understanding of the binding mode linking the properties of lipid membranes to α-Syn insertion into these dynamic structures. Using a combination of an optical biosensing technique and in situ atomic force microscopy, we show that the binding strength of α-Syn is related to the specificity of the lipid environment (the lipid chemistry and steric properties within a bilayer structure) and to the ability of the membranes to accommodate and remodel upon the interaction of α-Syn with lipid membranes. We show that this interaction results in the insertion of α-Syn into the region of the headgroups, inducing a lateral expansion of lipid molecules that can progress to further bilayer remodeling, such as membrane thinning and expansion of lipids out of the membrane plane. We provide new insights into the affinity of α-Syn for lipid packing defects found in vesicles of high curvature and in planar membranes with cone-shaped lipids and suggest a comprehensive model of the interaction between α-Syn and lipid bilayers. The ability of α-Syn to sense lipid packing defects and to remodel membrane structure supports its proposed role in vesicle trafficking.

Ure2p is the protein determinant of theSaccharomyces cerevisiaeprion state [URE3]. Constitutive overexpression of the HSP70 family memberSSA1cures cells of [URE3]. Here, we show that Ssa1p increases the lag time of Ure2p fibril formationin vitroin the presence or absence of nucleotide. The presence of the HSP40 co-chaperone Ydj1p has an additive effect on the inhibition of Ure2p fibril formation, whereas the Ydj1p H34Q mutant shows reduced inhibition alone and in combination with Ssa1p. In order to investigate the structural basis of these effects, we constructed and tested an Ssa1p mutant lacking the ATPase domain, as well as a series of C-terminal truncation mutants. The results indicate that Ssa1p can bind to Ure2p and delay fibril formation even in the absence of the ATPase domain, but interaction of Ure2p with the substrate-binding domain is strongly influenced by the C-terminal lid region. Dynamic light scattering, quartz crystal microbalance assays, pull-down assays and kinetic analysis indicate that Ssa1p interacts with both native Ure2p and fibril seeds, and reduces the rate of Ure2p fibril elongation in a concentration-dependent manner. These results provide new insights into the structural and mechanistic basis for inhibition of Ure2p fibril formation by Ssa1p and Ydj1p.

We have prepared single crystalline SnO2 and ZnO nanowires and polycrystalline TiO2 nanotubes (1D networks) as well as nanoparticle-based films (3D networks) from the same materials to be used as photoanodes for solid-state dye-sensitized solar cells. In general, superior photovoltaic performance can be achieved from devices based on 3-dimensional networks, mostly due to their higher short circuit currents. To further characterize the fabricated devices, the electronic properties of the different networks were measured via the transient photocurrent and photovoltage decay techniques. Nanowire-based devices exhibit extremely high, light independent electron transport rates while recombination dynamics remain unchanged. This indicates, contrary to expectations, a decoupling of transport and recombination dynamics. For typical nanoparticle-based photoanodes, the devices are usually considered electron-limited due to the poor electron transport through nanocrystalline titania networks. In the case of the nanowire-based devices, the system becomes limited by the organic hole transporter used. In the case of polycrystalline TiO2 nanotube-based devices, we observe lower transport rates and higher recombination dynamics than their nanoparticle-based counterparts, suggesting that in order to improve the electron transport properties of solid-state dye-sensitized solar cells, single crystalline structures should be used. These findings should aid future design of photoanodes based on nanowires or porous semiconductors with extended crystallinity to be used in dye-sensitized solar cells. © 2013 The Royal Society of Chemistry.

Near‐field imaging is a means of exceeding the diffraction limit in optical microscopy to yield subwavelength resolution optical images of a sample surface. In order to achieve such high resolution, it is necessary to scan the measurement probe above the surface at a height of only a few nanometers which requires careful control of the separation between tip and sample. In the implementation of the near‐field optical microscope (NFOM) reported here, the distance regulation scheme is based on an inverted noncontact atomic force microscope (AFM) in which a cantilever is used as the sample substrate and imaging is performed with a fiber optic tip. In this way, both the benefits of AFM and NFOM are realized simultaneously.

An atomic force microscope cantilever has been used as a bending-beam sensor to measure surface stress changes which occur during electrochemical processes. The mechanical properties of the lever and the sensitivity of the detection system mean that the sensor is both fast and sensitive. Surface stress changes presented for the electrochemical deposition and stripping of a Pb monolayer on an Au(111) surface show features which match peaks in the cyclic voltammogram and can be understood by reference to the known surface structure determined by STM, AFM, and grazing incidence XRD. There is a pronounced reduction in the stress derivative at the potential corresponding to the rotational phase transition of the lead monolayer. In the electrocompression region which follows monolayer formation, there is an essentially linear increase in compressive stress which may be modeled to within 50% accuracy by a simple linear elastic model.

A scanning tunneling microscope in ultrahigh vacuum has been used to investigate the growth, morphology, and surface atomic structure of ultrathin titanium silicide films on Si(111) substrates. Microstructural considerations have been used to identify various stages of the silicide growth. Atomic resolution images of a titanium silicide crystallite facet, formed at 850 °C, have been identified as a 2×2 silicon termination of a C54‐TiSi2(010) surface. Possible epitaxial silicide/silicon relationships are provided. Theoretical consideration has been given to the interatomic bonding in the C54‐TiSi2 lattice and the dangling bond density of ideally terminated silicide planes has been calculated. The highly reconstructed atomically flat surface of a large crystallite, formed at 1200 °C, has been assigned as a C54‐TiSi2(311) plane giving the epitaxial relation C54‐TiSi2(311)∥Si(111). The presence of pairs and linear chains of defects, with common orientations, is attributed to the decomposition of a diatomic ga...

In-situ high temperature scanning tunnelling microscopy (STM) measurements were performed on vacuum-annealed, boron-doped Si(111) wafers. The samples were p-type with an atomic boron concentration of ∼ 10 19 cm −3 which were chosen such that after ultra-high vacuum processing at up to 1250°C and annealing at 600°C they gave rise to co-existence of both 7 × 7 and √3 × √3R30° surface terminations. After the initial “flashing” of the sample to 1250°C, we observe pinning of a variety of reconstructions due to the random out-segregation of boron to the surface. Prolonged annealing at the measurement temperature of 600°C was observed to give rise to the agglomeration of the boron into well-defined √3 × √3 regions. In the measurements, we observe preferential directions for the in-plane boundaries between √3 × √3 and 7 × 7 regions, showing clearly the importance of dimerisation as a strain relief mechanism between these regions. Detailed observations reveal the lateral diffusion of boron and silicon in the √3 × √3 regions as a function of time. The data are discussed in terms of the various controlling processes at the surface.

The Na-stabilised Si(111)3 × 1 phase has been investigated upon annealing. At temperatures between 410 and 540°C the Na desorbs gradually, resulting in a structural transformation to the 7 × 7 reconstruction. The use of a high-temperature STM enabled the in situ observation of this morphological transition, revealing a variety of intermediate Si(111) reconstructions such as 2 × 2, c4 × 2, √3 × √3R30°, 5 × 5, 7 × 7 and 9 × 9 on Na-free areas, before the entire surface reconstructed to the 7 × 7 structure at higher temperatures (800°C). The transition is accompanied by a pronounced mass transport and can be explained in terms of the removal of excess Si atoms from local Na-free 3 × 1 formations.

The conductivity of a variety of atomic force microscopy tips was investigated by measuring both current‐voltage characteristics and force profile data for the tip in contact with graphite. This allows the conductivity to be studied as a function of tip‐sample distance. It was found that etched gold wires or levers coated with a conducting diamond film were the best tips to use for experimentation on hard (SiO2) surfaces. Metal‐coated levers, particularly gold, were found to wear rapidly such that the very end of the tip became insulating.

The molecular ordering of coronene (C 24 H 12 ) obtained by vacuum-deposition onto predominantly Ag(111) on mica has been investigated using the scanning tunnelling microscope. Real-space topographic images reveal that in certain regions we obtain layer-by-layer ordered growth of the molecules on this substrate which agrees with previous indirect measurements (the growth did not display this ordering in other regions). In our experiments on the ordered regions, we observe the best imaging contrast at a voltage bias of −0.28 V which may correspond to a resonant tunnelling process through the molecules.

A scanning tunneling microscope (STM) in ultrahigh vacuum has been used to investigate the growth, morphology, and surface atomic structure of ultrathin titanium silicide films on Si(100) substrates. Microstructural considerations have been used to identify various stages of the silicide growth. Methods for STM crystallography have been developed and used to identify possible epitaxial silicide/silicon relationships based on morphological considerations. Atomic resolution images of a titanium silicide crystallite have identified a 2×2 silicon termination of a C54‐TiSi2(111) surface. It is shown that unambiguous identification of epitaxial relationships requires images of the atomic structure of the silicide crystallite surfaces in addition to morphological information.

Gold clusters were deposited on graphite substrates by evaporation in an argon atmosphere. The low-temperature electronic behaviour of these samples was examined by scanning tunnelling microscopy. Isolated islands showed clear examples of the Coulomb blockade. It was possible to alter the size of the blockade and the residual charging of the clusters by adjusting the tip-sample spacing. These results may be explained in terms of the semiclassical theory of charging effects in the double tunnel junction.

Protein adsorption plays a crucial role in biomaterial surface science as it is directly linked to the biocompatibility of artificial biomaterial devices. Here, elucidation of protein adsorption mechanism is effected using dual polarization interferometry and a quartz crystal microbalance to characterize lysozyme layer properties on a silica surface at different coverage values. Lysozyme is observed to adsorb from sparse monolayer to multilayer coverage. At low coverage an irreversibly adsorbed layer is formed with slight deformation consistent with side-on orientation. At higher coverage values dynamic re-orientation effects are observed which lead to monolayer surface coverages of 2-3 ng/mm² corresponding to edge-on or/and end-on orientations. These monolayer thickness values ranged between 3 and 4.5 nm with a protein density value of 0.60 g/mL and with 50 wt% solvent mass. Further increase of coverage results formation of a multilayer structure. Using the hydration content and other physical layer properties a tentative model lysozyme adsorption is proposed.

We report the use of near-field electrospinning (NFES) as a route to fabricate composite electrodes. Electrodes made of composite fibers of carbon nanotubes in polyethylene oxide (PEO) are formed via liquid deposition, with precise control over their configuration. The electromechanical properties of free-standing fibers and fibers deposited on elastic substrates are studied in detail. We then examine the elastic deformation limit of the resulting free-standing fibers and find, similarly to bulk PEO composites, that the plastic deformation onset is below 2% of tensile strain. In comparison, the apparent deformation limit is much improved when the fibers are integrated onto a stretchable, elastic substrate. It is hoped that the NFES fabrication protocol presented here can provide a platform to direct-write polymeric electrodes, and to integrate both stiff and soft electrodes onto a variety of polymeric substrates.

Kinetic measurements of amyloid growth provide insight into the free energy landscape of this supramolecular process and are crucial in the search for potent inhibitors of the main disorders with which it is associated, including Alzheimer's and Parkinson's diseases and Type II diabetes. In recent years, a new class of surface-bound biosensor assays, e.g., those based on surface plasmon resonance (SPR) and the quartz crystal microbalance (QCM) have been established as extremely valuable tools for kinetic measurements of amyloid formation. Here we describe detailed protocols of how QCM techniques can be used to monitor the elongation of amyloid fibrils in real time and to study the influence of external factors on the kinetics of amyloid growth with unprecedented accuracy.

Kinetic measurements of amyloid growth provide insight into the free energy landscape of this supramolecular process and are crucial in the search for potent inhibitors of the main disorders with which it is associated, including Alzheimer's and Parkinson's diseases and Type II diabetes. In recent years, a new class of surface-bound biosensor assays, e.g., those based on surface plasmon resonance (SPR) and the quartz crystal microbalance (QCM) have been established as extremely valuable tools for kinetic measurements of amyloid formation. Here we describe detailed protocols of how QCM techniques can be used to monitor the elongation of amyloid fibrils in real time and to study the influence of external factors on the kinetics of amyloid growth with unprecedented accuracy.

We describe a highly sensitive new type of calorimeter based on the deflection of a ‘‘bimetallic’’ micromechanical sensor as a function of temperature. The temperature changes can be due to ambient changes, giving a temperature sensor or, more importantly, due to the heat absorbed by a coating on the sensor, giving a heat sensor. As an example we show the results of using the sensor as a photothermal spectrometer. The small dimensions and low thermal mass of the sensor make it highly sensitive and we demonstrate a sensitivity of roughly 100 pW. By applying a simple model of the system the ultimate sensitivity is expected to be of the order of 10 pW. The thermal response time of the cantilever can also be determined, giving an estimate of the minimum detectable energy of the sensor. This we find to be 150 fJ and again from our model, expect a minimum value of the order of 20 fJ.

The domain structure of sputtered Co‐Pd multilayer films of varying thickness has been investigated by magnetic force microscopy. The domains appear as stripe domains, typical of perpendicularly oriented films. The size of the domains was strongly influenced by the thickness of the film. The domain repetition lengths give an additional experimental parameter which has been used to provide a stronger test of a theoretical model developed for ferromagnetic multilayer films [H. J. G. Draaisma and W. J. M. de Jonge, J. Appl. Phys. 62, 3318 (1987)]. It is found that the experiment and theory are broadly in agreement provided that the increased magnetization of the multilayer caused by polarization of the Pd is accounted for. There is a noticeable difference between the variation of the measured and theoretical domain repetition lengths with film thickness. This is attributed to the effects of domain‐wall pinning which is not considered in the model. It is estimated that the characteristic length of the films i...

A magnetic force microscope is used as a highly localized probe of the magnetic properties of Co‐Pd multilayer films. The stray field from the magnetic tip is used to induce magnetization changes in the sample on a scale of the order of ∼100 nm. These changes are characterized by discontinuous jumps in the force or force gradient acting on the tip due to its interaction with the magnetostatic stray field from the sample. Both reversible and irreversible domain‐wall motion is seen.

A modified atomic force microscope (AFM) is used to directly measure the local compliance of ordered liquid layers at solid—liquid interfaces. Measurements of the compliance of the solvation structure for octamethylcyclotetrasiloxane and n-dodecanol near graphite and mica surfaces are presented. We show that reasonable correlation decay lengths, molecular sizes, and material rigidities can be determined. The new method is based on a force modulation technique and can be more sensitive to weak longer range interactions compared with the more direct measurement of forces using the static deflection of the AFM cantilever.

Since the invention of the scanning tunnelling microscope (STM) by Gerd Binnig and Heinrich Rohrer in 1982, both the number and diversity of applications of the technique have expanded dramatically – a success that was recognized promptly with the 1986 Nobel prize for physics. In particular it has provided new ways of measuring a variety of surface properties with high spatial resolution.

An inevitable step in the evolution of a substantive new research tool or topic is the publication of a book giving a detailed overview of developments. For scanning tunnelling and related probe microscopies several such books have appeared in the last year. Although there is commonality of content between these contributions, Dawn Bonnell claims here to combine a presentation of basic concepts with up-to-date examples from the research literature. There is always a difficulty in deciding when a technique has matured adequately for a book of this type to be written; too early and it can be entirely out of date within months of release, too late and it ceases to be as valuable. Given that scanning tunnelling microscopy (STM) was invented nearly 14 years ago and is now in widespread use both commercially and as a research tool, the publication of a solid reference book appears apposite.

Light emission with a blackbody-like spectrum was observed during current flow through atomic-size metallic contacts formed in the scanning tunneling microscope. Within the contact, the electron temperature rises above the lattice temperature as electron–phonon energy transfer vanishes. Electron temperatures of up to 9000 K were deduced from optical spectroscopy of stable contacts. An important consequence of greatly reduced electron energy losses is that these atomic-size metal contacts have maximum current densities of ∼1015 A m−2, several orders of magnitude greater than for macroscopic wires.

We describe measurements of the stiffness and Young’s modulus, Y of single crystals of molybdenum-based compounds. Atomic force microscopy is used first to image, and then to perform stiffness measurements on crystals which are growing up out of a substrate. Y is extracted by comparing the measured stiffness with that calculated from a continuum elasticity model of the crystals, whose experimentally indeterminate parameters are the Young’s modulus and the geometry. We find a value for Y in the range 0.8–1.4 TPa, depending on the assumed geometry of the nanocrystal. As these crystals are essentially perfectly ordered on the micron scale, this opens the possibility of forming composite materials of immense strength.

We have investigated the magnetization reversal process of a single chain of identical nanomagnetic dots fabricated from 30 nm thick Ni80Fe20. The structures consist of two 5 μm wide support wires bridged with a single chain of identical dots of diameter δ in the range 100–250 nm. For fields applied perpendicular to the single chain, we observed an unusual size dependent hysteretic behavior in the magnetoresistance curve at high field. This is due to the magnetostatic interaction arising from the proximity of the magnetic charges. We are able to deduce from a simple micromagnetic simulation that the reversal process in the chain of dots with δ=100 nm is mediated by a collective process of nearly coherent spin rotation. The magnetotransport measurements along the chain reveal a complex magnetization reversal process in the chain of nanomagnets.

By combining the spatial resolution of a scanning-tunneling microscope (STM) with the electronic spin sensitivity of electron-spin resonance, we show that it is possible to detect the presence of localized spins on surfaces. The principle is that a STM is operated in a magnetic field, and the resulting component of the tunnel current at the Larmor (precession) frequency is measured. This component is nonzero whenever there is tunneling into or out of a paramagnetic entity. We have succeeded in obtaining spectra from free radical molecules from which the g factor of a spin entity may be inferred. For the molecules studied here, α,γ-bisdiphenylene-β-phenylallyl, g was found to be 2±0.1.

Solubility is a key requirement for the functioning of a protein within the complex network of cellular components. A class of highly debilitating disorders, including Alzheimer s and Parkinson s diseases, is related to the loss of solubility of peptides and proteins that is accompanied by their aggregation into ordered amyloid fibrils. It has been found that, under at least some physiological conditions, these aggregates are thermodynamically more stable than the native forms of biological polypeptides. This finding raises questions as to the factors governing the crucial ability of native proteins to remain soluble even under conditions where they do not necessarily correspond to global minima on free energy landscapes. In order to address this question, we have studied in detail the kinetics of elongation of amyloid fibrils formed by a wide range of polypeptides. The formation of amyloid fibrils from soluble protein molecules involves at least a primary nucleation step, an elongation step and, in general, a secondary nucleation process such as fibril fragmentation. In addition, multiple interconverting oligomeric intermediates can be involved. Measurements of amyloid growth in bulk solution often reflect all of these processes, and it can therefore be extremely challenging to determine accurately the concentrations of the different species and the rate constants for the individual elementary steps. In order to overcome these difficulties, surface-based sensing techniques, notably those based on quartz crystal microbalance (QCM) measurements, have been developed in recent years, by which the growth of a constant, surfacebound ensemble of fibrils can be monitored. These methods make use of the fact that in the presence of preexisting fibrils, aggregation can be highly accelerated through seeding. This seeding process corresponds to the elongation of existing fibrils and can be well described as diffusional motion over a single free energy barrier, involving no intermediate species between monomeric and fibrillar peptide. The elongation of the fibrils is monitored through the increase in hydrodynamic mass bound to the quartz crystal, as the rate of change of the resonant frequency is proportional to the average elongation rate of the fibrils. The opportunity to image the sensor surface enables an estimation of the surface density of fibrils, an important factor in the determination of the rate constants in this bimolecular reaction, the overall rate of which depends on both the concentration of soluble protein and the number of available fibril ends. In addition, the lengths of the fibrils before and after an experiment can be compared and therefore an independent measurement of the length increase can be made and used to calibrate the frequency response of the microbalance. The covalent irreversible attachment of the preformed fibrils to the sensor surface and the subsequent passivation of the remaining surface, as well as the short duration of individual experiments, ensure that only fibril elongation is measured, and that primary and secondary nucleation events can be neglected; this selectivity is confirmed by the high reproducibility of the data obtained from QCM measurements. The starting point of a systematic study of the energy barriers that separate the soluble from the fibrillar states of a protein is the measurement of the temperature dependence of the fibril elongation rate; such an approach allows for the determination of the enthalpy of activation from an Arrhenius plot. The temperature dependence of amyloid growth has already been measured for a range of amyloidogenic peptides and proteins, and where possible these literature data are included in the analysis described herein. These published data have been acquired with a range of different techniques, mainly involving small-molecule labels such as Thioflavin-T. In such experiments, the exclusive study of the elementary elongation reaction is challenging and therefore the published values on energy barriers may in some cases refer to a combination of different elementary steps. We have, however, used the QCM approach, which is particularly suited for such measurements, to increase substantially the size of the available dataset by studying peptides and proteins of very diverse sequence that form amyloid fibrils under varied solution conditions. Figure 1 shows, as an example, raw QCM data for the temperature dependence of PI3K-SH3 amyloid fibril elongation, as well as AFM images of the QCM sensor surface. Similar experiments were performed for a range of other peptides and proteins, and the resulting Arrhenius plots are shown in Figure 2. No pronounced curvature is apparent, unlike that sometimes observed for protein folding; this [*] A. K. Buell, M. E. Welland Nanoscience Centre, University of Cambridge 11 JJ Thomson Avenue, West Cambridge CB3 0FF (UK) E-mail: mew10@cam.ac.uk

Much effort has focussed in recent years on probing the interactions of small molecules with amyloid fibrils and other protein aggregates. Understanding and control of such interactions are important for the development of diagnostic and therapeutic strategies in situations where protein aggregation is associated with disease. In this perspective article we give an overview over the toolbox of biophysical methods for the study of such amyloid-small molecule interactions. We discuss in detail two recently developed techniques within this framework: linear dichroism, a promising extension of the more traditional spectroscopic techniques, and biosensing methods, where surface-bound amyloid fibrils are exposed to solutions of small molecules. Both techniques rely on the measurement of physical properties that are very directly linked to the binding of small molecules to amyloid aggregates and therefore provide an attractive route to probe these important interactions.

Amyloid nanofibers derived from hen egg white lysozyme were processed into macroscopic fibers in a wet-spinning process based on interfacial polyion complexation using a polyanionic polysaccharide as cross-linker. As a result of their amyloid nanostructure, the hierarchically self-assembled protein fibers have a stiffness of up to 14 GPa and a tensile strength of up to 326 MPa. Fine-tuning of the polyelectrolytic interactions via pH allows to trigger the release of small molecules, as demonstrated with riboflavin-5'-phophate. The amyloid fibrils, highly oriented within the gellan gum matrix, were mineralized with calcium phosphate, mimicking the fibrolamellar structure of bone. The formed mineral crystals are highly oriented along the nanofibers, thus resulting in a 9-fold increase in fiber stiffness.

We demonstrate a controllable formation process of wave-like patterns in thermally unstable surface-capped polymer films on a rigid substrate. Self-ordered wave-like structures over a large area can be created by applying a small lateral tension to the film, whereupon it becomes unstable. A clear mode selection process which includes creation, decay and interference between coexisting waves at different annealing conditions has been observed, which makes it possible to restrain the patterns which are formed finally. Our results provide a clear and new evidence of spinodal behaviour in such a film due to thermal instability. Furthermore, we show that the well-controlled patterns generated in such a process can be used to fabricate nanostructures for various applications.

The supra-molecular self-assembly of peptides and proteins is a process which underlies a range of normal and aberrant biological pathways in nature, but one which remains challenging to monitor in a quantitative way. We discuss the experimental details of an approach to this problem which involves the direct measurement in vitro of mass changes of the aggregates as new molecules attach to them. The required mass sensitivity can be achieved by the use of a quartz crystal transducer-based microbalance. The technique should be broadly applicable to the study of protein aggregation, as well as to the identification and characterisation of inhibitors and modulators of this process.

The self-assembly of proteins and peptides into polymeric amyloid fibrils is a process that has important implications ranging from the understanding of protein misfolding disorders to the discovery of novel nanobiomaterials. In this study, we probe the stability of fibrils prepared at pH 2.0 and composed of the protein insulin by manipulating electrostatic interactions within the fibril architecture. We demonstrate that strong electrostatic repulsion is sufficient to disrupt the hydrogen-bonded, cross-β network that links insulin molecules and ultimately results in fibril dissociation. The extent of this dissociation correlates well with predictions for colloidal models considering the net global charge of the polypeptide chain, although the kinetics of the process is regulated by the charge state of a single amino acid. We found the fibrils to be maximally stable under their formation conditions. Partial disruption of the cross-β network under conditions where the fibrils remain intact leads to a reduction in their stability. Together, these results support the contention that a major determinant of amyloid stability stems from the interactions in the structured core, and show how the control of electrostatic interactions can be used to characterize the factors that modulate fibril stability.

International audience; Soils contain an abundance of nano-size particles. Because of their tendency to aggregate and associate with organic colloids, however, soil nanoparticles are difficult to obtain and characterize. Here we report on a simple and rapid method of extracting mesoporous nanomaterials from the clay fraction of an Andisol with narrow size distribution. The clay and nanoclay were characterized by elemental analysis, pyrolysis GC/MS, electron and atomic force microscopy, infrared spectroscopy, and electrophoresis. The nanoclay dominantly consists of hollow allophane spherules forming globular aggregates of about 100 nm in diameter. The nanoclay contains more organic matter (carbon and nitrogen) with a larger proportion of polysaccharides and nitrogen containing compounds, and has a lower isoelectric point, than the clay. Treatment with hydrogen peroxide causes a large decrease in the organic matter contents of both nanoclay and clay. The aggregates of allophane nanoparticles retain a significant amount (similar to 12%) of carbon against intensive peroxide treatment. Thus, besides playing an important role in carbon stabilization, these naturally occurring nanomaterials are potentially useful for developing a low-cost carbon sequestration technology. (C) 2010 Elsevier B.V. All rights reserved.

Peptides and proteins possess an inherent propensity to self-assemble into generic fibrillar nanostructures known as amyloid fibrils, some of which are involved in medical conditions such as Alzheimer disease. In certain cases, such structures can self-propagate in living systems as prions and transmit characteristic traits to the host organism. The mechanisms that allow certain amyloid species but not others to function as prions are not fully understood. Much progress in understanding the prion phenomenon has been achieved through the study of prions in yeast as this system has proved to be experimentally highly tractable; but quantitative understanding of the biophysics and kinetics of the assembly process has remained challenging. Here, we explore the assembly of two closely related homologues of the Ure2p protein from Saccharomyces cerevisiae and Saccharomyces paradoxus, and by using a combination of kinetic theory with solution and biosensor assays, we are able to compare the rates of the individual microscopic steps of prion fibril assembly. We find that for these proteins the fragmentation rate is encoded in the structure of the seed fibrils, whereas the elongation rate is principally determined by the nature of the soluble precursor protein. Our results further reveal that fibrils that elongate faster but fracture less frequently can lose their ability to propagate as prions. These findings illuminate the connections between the in vitro aggregation of proteins and the in vivo proliferation of prions, and provide a framework for the quantitative understanding of the parameters governing the behavior of amyloid fibrils in normal and aberrant biological pathways.

The supra-molecular self-assembly of peptides and proteins is a process which underlies a range of normal and aberrant biological pathways in nature, but one which remains challenging to monitor in a quantitative way. We discuss the experimental details of an approach to this problem which involves the direct measurement in vitro of mass changes of the aggregates as new molecules attach to them. The required mass sensitivity can be achieved by the use of a quartz crystal transducer-based microbalance. The technique should be broadly applicable to the study of protein aggregation, as well as to the identification and characterisation of inhibitors and modulators of this process.

We describe photon-emission scanning-tunneling-microscopy experiments on polycrystalline silver surfaces. By performing a method of simultaneous photon emission and tunneling spectroscopy measurements, we are able to show that the photon emission process and, in particular, the intergranular contrast in the photon maps depend upon interactions between individual grains in the silver film. Factors known to affect photon emission, such as grain shape and surface morphology, are shown to be inconsistent with the detailed structure of the photon maps. We observe that certain grains behave as isolated entities while others radiate in a more collective manner. This is indicative that for these materials the photon emission behavior is strongly affected by grain-boundary effects and we discuss a recent model which takes these into account.

In this work we simultaneously measure both the friction and applied force acting during imaging on graphite with an atomic force microscope (AFM). We show that: (1) The scan direction with respect to the cantilever is important for imaging at atomic resolution. (ii) In order to minimise the forces acting on the tip it may not be sufficient to simply reduce the applied force because if strong adhesive forces are present then both the friction forces and the contact area may be significant even at zero applied load. (iii) Adhesive forces may also influence the imaging of adsorbates which are only weakly bound to the substrate. At low scanning speeds in air adsorbed material tends to be dragged along by the tip whereas at higher speeds the adsorbates remain fixed. (iv) The friction data suggest that very high shear forces (~1 GPa) can be sustained at the tip-sample contact.