Water distribution at solid/liquid interfaces visualized by frequency modulation atomic force microscopy

Interfacial phenomena at solid/water interfaces play an important role in a wide range of industrial technologies and biological processes. However, it has been a great challenge to directly probe the molecular-scale behavior of water at solid/water interfaces. Recently, there have been tremendous advancements in frequency modulation atomic force microscopy (FM-AFM), enabling its operation in liquids with atomic resolution. The high spatial and force resolutions of FM-AFM have enabled the visualization of one-dimensional (1D) profiles of the hydration force, two-dimensional (2D) images of hydration layers and three-dimensional (3D) images of the water distribution at solid/water interfaces. Here I present an overview of the recent advances in FM-AFM instrumentation and its applications to the study of solid/water interfaces.     

Revealing molecular-level surface structure of amyloid fibrils in liquid by means of frequency modulation atomic force microscopy

We have investigated the surface structure of islet amyloid polypeptide (IAPP) fibrils and α-synuclein protofibrils in liquid by means of frequency modulation atomic force microscopy (FM-AFM). ?ngstr?m-resolution FM-AFM imaging of isolated macromolecules in liquid is demonstrated for the first time. Individual β-strands aligned perpendicular to the fibril axis with a spacing of 0.5?nm are resolved in FM-AFM images, which confirms cross-β structure of IAPP fibrils in real space. FM-AFM images also reveal the existence of 4?nm periodic domains along the axis of IAPP fibrils. Stripe features with 0.5?nm spacing are also found in images of α-synuclein protofibrils. However, in contrast to the case for IAPP fibrils, the stripes are oriented 30° from the axis, suggesting the possibility of β-strand alignment in protofibrils different from that in mature fibrils or the regular arrangement of thioflavin T molecules present during the fibril preparation aligned at the surface of the protofibrils.     

Interfacial atomic diffusion in AF/Fe/Cu/Fe (AF = Fe50Mn50 and Ir50Mn50) multilayer systems

Spin valve like AF/Fe/Cu/Fe (AF = Fe₅₀Mn₅₀ and Ir₅₀Mn₅₀) multilayer systems have been prepared by molecular beam epitaxy. Thin tracer layers enriched in the ⁵⁷Fe isotope were artificially grown at the AF/Fe and Fe/Cu interfaces and the interfacial atomic diffusion was observed via ⁵⁷Fe conversion electron Mössbauer spectroscopy. The results show that the atomic interdiffusion at all involved interfaces is lower in the IrMn based structures as compared to the FeMn based ones.     

Characterizing the Influence of Water on Charging and Layering at Electrified Ionic‐Liquid/Solid Interfaces

The importance of water on molecular ion structuring and charging mechanism of solid interfaces in room temperature ionic liquid (RTIL) is unclear and has been largely ignored. Water may alter structures, charging characteristics, and hence performance at electrified solid/RTIL interfaces and is utilized in various fields including energy storage, conversion, or catalysis. Here, atomic force microscopy and surface forces apparatus experiments are utilized to directly measure how water alters the interfacial structuring and charging characteristics of [C₂mim][Tf₂N] on mica and electrified gold surfaces. On hydrophilic and ionophobic mica surfaces, water‐saturated [C₂mim][Tf₂N] dissolves surface‐bound cations, which leads to high surface charging and strong layering. In contrast, layering of dry RTIL at weakly charged mica surfaces is weakly structured. At electrified, hydrophobic, and ionophilic gold electrodes, significant water effects are found only at positive applied electrochemical potentials. Here, the influence of water is limited to interactions within the RTIL layers, and is not related to a direct electrosorption of water on the polarized electrode. More generally, the results suggest that effects of water on interfacial structuring of RTIL strongly depend on both (1) surface charging mechanism and (2) interfacial wetting properties. This may greatly impact utilization/design of RTILs and surfaces for interface‐dominated processes.     

Structural comparisons of passivated SI(100) by atomic and molecular oxygen

Abstract

We compared the structural characteristics of a silica layer formed on Si(100) by oxidation in hyperthermal atomic oxygen and molecular oxygen at 493K. The laser detonation method was used to create primarily neutral atomic oxygen with kinetic energy of 5.1eV. The silicon oxides were characterized by High resolution transmission electron microscopy (HRTEM), atomic force microscopy (AFM), rutherford backscattering spectrometry (RBS), scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). We determined that atomic oxygen forms amorphous silica that is almost twice as thick and nearly double the surface roughness as compared to molecular oxygen – formed silica at the same temperature and time conditions     

Hydrophobin Vmh2–glucose complexes self-assemble in nanometric biofilms

Hydrophobins are small proteins secreted by fungi, which self-assemble into amphipathic membranes at air–liquid or liquid–solid interfaces. The physical and chemical properties of some hydrophobins, both in solution and as a biofilm, are affected by poly or oligosaccharides. We have studied the interaction between glucose and the hydrophobin Vmh2 from Pleurotus ostreatus by spectroscopic ellipsometry (SE), atomic force microscopy (AFM) and water contact angle (WCA). We have found that Vmh2–glucose complexes forms a chemically stable biofilm, obtained by drop deposition on silicon, 1.6 nm thick and containing 35 per cent of glucose, quantified by SE. AFM highlighted the presence of nanometric rodlet-like aggregates (average height, width and length being equal to 3.6, 23.8 and 40 nm, respectively) on the biofilm surface, slightly different from those obtained in the absence of glucose (4.11, 23.9 and 64 nm). The wettability of a silicon surface, covered by the organic layer of Vmh2–glucose, strongly changed: WCA decreased from 90° down to 17°.     

Molecular Ordering at the Interface Between Liquid Water and Rutile TiO2(110)

The pivotal importance of TiO₂ as a technological material involves most applications in an aqueous environment, but the single‐crystal TiO₂/bulk‐water interfaces are almost completely unexplored, since up to date solid/liquid interfaces are more difficult to access than surfaces in ultrahigh vacuum (UHV). Only a few techniques (as scanning probe microscopy) offer the opportunity to explore these systems under realistic conditions. The rutile TiO₂(110) surface immersed in high‐purity water is studied by in situ scanning tunneling microscopy. The large‐scale surface morphology as obtained after preparation under UHV conditions remains unchanged upon prolonged exposure to bulk water. Moreover, in contrast to UHV, atomically resolved images show a twofold periodicity along the [001] direction, indicative of an ordered structure resulting from the hydration layer. This is consistent with density‐functional theory based molecular dynamics simulations where neighboring interfacial molecules of the first water layer in contact with the bulk liquid form dimers. By contrast, this dimerization is not observed for a single adsorbed water monolayer, i.e., in the absence of bulk water.     

Novel amplitude and frequency demodulation algorithm for a virtual dynamic atomic force microscope

Frequency-modulated atomic force microscopy (FM-AFM; also called non-contact atomic force microscopy) is the prevailing operation mode in (sub-)atomic resolution vacuum applications. A major obstacle that prohibits a wider application range is the low frame capture rate. The speed of FM-AFM is limited by the low bandwidth of the automatic gain control (AGC) and frequency demodulation loops. In this work we describe a novel algorithm that can be used to overcome these weaknesses. We analysed the settling times of the proposed loops and that of the complete system, and we found that an approximately 70-fold improvement can be achieved over the existing real and virtual atomic force microscopes. We show that proportional-integral-differential controllers perform better in the frequency demodulation loop than conventional proportional-integral controllers. We demonstrate that the signal to noise ratio of the proposed system is 5.7 × 10(-5), which agrees with that of the conventional systems; thus, the new algorithm would improve the performance of FM-AFMs without compromising the resolution.     

Potential of interferometric cantilever detection and its application for SFM/AFM in liquids

We have developed an optical cantilever deflection detector with a spot size <3?μm and fm?Hz(-1/2) sensitivity over a>10?MHz bandwidth. In this work, we demonstrate its potential for detecting small-amplitude oscillations of various flexural and torsional oscillation modes of cantilevers. The high deflection sensitivity of the interferometer is particularly useful for detecting cantilever oscillations in aqueous solutions, enabling us to reach the thermal noise limit in scanning or atomic force microscopy experiments with stiff cantilevers. This has resulted in atomic-resolution images of solid-liquid interfaces and submolecular-resolution images of native membranes.     

Hydration of Nafion® studied by AFM and X-ray scattering

Nafion® is a commercially available perfluorosulphonate cation exchange membrane commonly used as a perm-selective separator in chlor-alkali electrolysers and as the electrolyte in solid polymer fuel cells. This usage arises because of its high mechanical, thermal and chemical stability coupled with its high conductivity and ionic selectivity, which depend strongly on the water content. The membrane was therefore studied in different states of hydration with two complementary techniques: atomic force microscopy (AFM) and small angle X-ray scattering (SAXS) combined with a maximum entropy (MaxEnt) reconstruction. Tapping mode phase imaging was successfully used to identify the hydrophobic and hydrophilic regions of Nafion. The images support the MaxEnt interpretation of a cluster model of ionic aggregation, with spacings between individual clusters ranging from 3 to 5 nm, aggregating to form cluster agglomerates with sizes from 5 to 30 nm. Both techniques indicate that the number density of ionic clusters changes as a function of water content, and this explains why the bulk volumetric swelling in water is observed to be significantly less than the swelling inferred from scattering measurements.     

Direct observation of epitaxial alignment of Au on MoS2 at atomic resolution

The morphology and structural stability of metal/2D semiconductor interfaces strongly affect the performance of 2D electronic devices and synergistic catalysis. However, the structural evolution at the interfaces has not been well explored particularly at atomic resolution. In this work, we study the structural evolution of Au nanoparticles (NPs) on few-layer MoS₂ by high resolution transmission electron microscope (HRTEM) and quantitative high-angle annular dark field scanning TEM. It is found that in the transition of Au from nanoparticles to dendrites, a dynamically epitaxial alignment between Au and MoS₂ lattices is formed, and Moiré patterns can be directly observed in HRTEM images due to the mismatch between Au and MoS₂ lattices. This epitaxial alignment can occur in ambient conditions, and can also be accelerated by the irradiation of high-energy electron beam. In situ observation clearly reveals the rotation of Au NPs, the atom migration inside Au NPs, and the transfer of Au atoms between neighboring Au NPs, finally leading to the formation of epitaxially aligned Au dendrites on MoS₂. The structural evolution of metal/2D semiconductor interfaces at atomic scale can provide valuable information for the design and fabrication of the metal/2D semiconductor nano-devices with desired physical and chemical performances.

     

Morphologies and optical properties of nanostructures self-assembled from asymmetrical, amphiphilic perylene derivatives

Two asymmetrical, amphiphilic perylene derivatives, N-Decyl-perylene-3,4:9,10-tetracaboxylic-3,4-di(methoxyethoxyethyl)ester-9,10-imide (D1E2) and N-(1-Decylundecyl)-perylene-3,4:9,10-tetracaboxylic-3,4-di(methoxyethoxyethyl)ester-9,10-imide (D2E2), have been synthesized and characterized. These compounds contain one long hydrophobic chain on one end and two hydrophilic ethoxy chains on the other end. Self-assembly of these molecules in a variety of solvents has been demonstrated. Scanning electron microscopy images showed that these compounds self-assembled to various nanostructures in different solvents. The most well-defined structure was flexible nanoribbons obtained from D1E2 precipitation in methanol. The UV–vis absorption and fluorescence spectra of these compounds in solution and solid form are also reported. The self-assembled nanostructures have potential applications in optoelectronics.     

Superlubricity between MoS2 Monolayers

The ultralow friction between atomic layers of hexagonal MoS₂, an important solid lubricant and additive of lubricating oil, is thought to be responsible for its excellent lubricating performances. However, the quantitative frictional properties between MoS₂ atomic layers have not been directly tested in experiments due to the lack of conventional tools to characterize the frictional properties between 2D atomic layers. Herein, a versatile method for studying the frictional properties between atomic‐layered materials is developed by combining the in situ scanning electron microscope technique with a Si nanowire force sensor, and the friction tests on the sliding between atomic‐layered materials down to monolayers are reported. The friction tests on the sliding between incommensurate MoS₂ monolayers give a friction coefficient of ≈10^?4 in the regime of superlubricity. The results provide the first direct experimental evidence for superlubricity between MoS₂ atomic layers and open a new route to investigate frictional properties of broad 2D materials.     

Probing the morphology and nanoscale mechanics of single poly(N-isopropylacrylamide) microgels across the lower-critical-solution temperature by atomic force microscopy

Submicrometer-sized particles of poly(N-isopropylacrylamide) (PNIPAM) are synthesized by surfactant-free radical polymerization. The morphology and nanomechanical properties of individual, isolated PNIPAM microgel particles at the silicon/air and silicon/water interfaces, below and above the PNIPAM volume-phase-transition temperature (VPTT), are probed by atomic force microscopy. In air, and in water below the VPTT, the PNIPAM spheres are flattened and adopt a pancakelike shape. Interestingly, above the VPTT the microgels adopt a more spherical form with increased height and decreased width, which is attributed to reduced interactions of the particles with the substrate. The elastic modulus calculated from force-indentation curves obtained for individual microgel spheres reveals that the stiffness of the particle's surface decreases by two orders of magnitude upon swelling in water. Additionally, the modulus of the PNIPAM spheres in water increases by one order of magnitude when crossing the VPTT from the swollen to the collapsed states, indicating a more compact chain packing at the particle surface.     

Computer prediction of microsegregation in peritectic alloy systems

Abstract

A numerical model for the prediction of solidification and the accompanying microsegregation in peritectic alloys is described. Several finite difference schemes have been tested, based on one-dimensional computational cells for microsegregation calculations. The method selected as the most efficient is that of a fixed computational grid with both solid/solid and solid/liquid boundaries able to move freely between the nodal planes, using a Lagrangian interpolation procedure due to Crank in order to determine solute gradients at the interfaces. The solution method is essentially explicit, although the composition and position of the solid/solid interface have to be floated in an implicit manner to obtain consistent, unique, solutions in multicomponent alloys. Microsegregation and non­equilibrium solidus temperatures have been computed for a number of alloy steel systems using the proposed model and satisfactory agreement obtained with experimental results.     

The Role of Moisture in Protein Stability

Abstract

The importance of water sorption on solid state stability of proteins can be addressed through an understanding of properties of the sorbed water and its impact on the properties of the protein. Most decomposition reactions are minimal at or below the monolayer level of hydration. Sorption of water beyond that of the monolayer generally results in increased rates of decomposition due to the increased conformational flexibility of the protein and the ability of the less tightly bound water to mobilize reactants. In many cases the rates of decomposition can be influenced by the addition of excipients. An understanding of how such excipients can influence water sorption and stability will allow for improved development strategies to minimize the decomposition of proteins in the solid state.     

Optical interferometry at ultra low temperatures

We have developed optical, interferometric methods for investigations of interfaces at ultra low temperatures. In our scheme conventional optical windows are avoided: laser illumination (He-Ne) is guided into the cryostat via a single-mode optical fiber and images are taken using a CCD sensor mounted inside the 4-K vacuum can. A real-time video camera has been successfully used in investigations of superfluid³He down to 0.6 mK whereas a slow-scan camera has been employed for optimal contrast in low-intensity imaging of liquid/solid interfaces (reflection coefficient 10^–6). The investigated topics include (1) superfluid³He surface in rotation and during rapid deceleration, (2) hydrodynamics of thin superfluid³He layers, (3) superfluid/solid interface in⁴He, and (4) wetting of superfluid⁴He by normal³He in phase separated mixtures. A vertical resolution of 10 nm and even below has been achieved in these studies.     

Phase morphology of Pb–Sn–Cd ternary eutectic

Abstract

A method for viewing the detailed solid/liquid interface morphology of metallic eutectic systems has been developed. It is based on an apparatus designed to allow the unidirectional solidification of metallic samples having low thermal mass at considerably lower, and more stable, growth rates than have previously been possible with conventional equipment. As a result, high resolution quench interfaces can be produced which in quality rival those of transparent organic analogue systems. Application of this technique to the solid/liquid interface of the Pb–Sn–Cd ternary eutectic has revealed a number of new facts. It was discovered that the Cd phase forms a facet plane coplanar with its basal plane and with the Pb/Cd lamellar habit plane. Qualitative observations of the differing behaviour of the Pb and Sn phases in contact with the Cd facet plane allowed the magnitudes of the three solid/solid interfacial free energies to be compared and a mechanism for the formation and stabilization of the ABCBA lamellar structure to be postulated.

MST/352     

Atomic force microscopy of the morphology and mechanical behaviour of barnacle cyprid footprint proteins at the nanoscale

Barnacles are a major biofouler of man-made underwater structures. Prior to settlement, cypris larvae explore surfaces by reversible attachment effected by a ‘temporary adhesive’. During this exploratory behaviour, cyprids deposit proteinaceous ‘footprints’ of a putatively adhesive material. In this study, footprints deposited by Balanus amphitrite cyprids were probed by atomic force microscopy (AFM) in artificial sea water (ASW) on silane-modified glass surfaces. AFM images obtained in air yielded better resolution than in ASW and revealed the fibrillar nature of the secretion, suggesting that the deposits were composed of single proteinaceous nanofibrils, or bundles of fibrils. The force curves generated in pull-off force experiments in sea water consisted of regions of gradually increasing force, separated by sharp drops in extension force manifesting a characteristic saw-tooth appearance. Following the relaxation of fibrils stretched to high strains, force–distance curves in reverse stretching experiments could be described by the entropic elasticity model of a polymer chain. When subjected to relaxation exceeding 500 ms, extended footprint proteins refolded, and again showed saw-tooth unfolding peaks in subsequent force cycles. Observed rupture and hysteresis behaviour were explained by the ‘sacrificial bond’ model. Longer durations of relaxation (>5 s) allowed more sacrificial bond reformation and contributed to enhanced energy dissipation (higher toughness). The persistence length for the protein chains (L_P) was obtained. At high elongation, following repeated stretching up to increasing upper strain limits, footprint proteins detached at total stretched length of 10 µm.     

Stability of macrodefect-free cement

Abstract

The effect of water immersion on the strength and dimension stability of macrodefect-free (MDF) cement has been studied. The results show that the polymer phase in the MDF matrix is unstable in water. Prolonged immersion resulted in substantial expansion and leaching of the matrix. Microstructural changes were examined by scanning electron microscopy, X-ray diffraction, and infrared spectroscopy. The cause of weakening of the matrix is thought to be the modification of the cement/polymer interface through the hydration of anhydrous grains. Methods of improving the performance of MDF pastes in water have been considered.

MST/692