Nanoscale Forces Research Articles

Regular Motifs in Xylan Modulate Molecular Flexibility and Interactions with Cellulose Surfaces.

Xylan is tightly associated with cellulose and lignin in secondary plant cell walls, contributing to its rigidity and structural integrity in vascular plants. However, the molecular features and the nanoscale forces that control the interactions among cellulose microfibrils, hemicelluloses, and lignin are still not well understood. Here, we combine comprehensive mass spectrometric glycan sequencing and molecular dynamics simulations to elucidate the substitution pattern in softwood xylans and to investigate the effect of distinct intramolecular motifs on xylan conformation and on the interaction with cellulose surfaces in Norway spruce (Picea abies). We confirm the presence of motifs with evenly spaced glycosyl decorations on the xylan backbone, together with minor motifs with consecutive glucuronation. These domains are differently enriched in xylan fractions extracted by alkali and subcritical water, which indicates their preferential positioning in the secondary plant cell wall ultrastructure. The flexibility of the 3-fold screw conformation of xylan in solution is enhanced by the presence of arabinofuranosyl decorations. Additionally, molecular dynamic simulations suggest that the glycosyl substitutions in xylan are not only sterically tolerated by the cellulose surfaces but that they increase the affinity for cellulose and favor the stabilization of the 2-fold screw conformation. This effect is more significant for the hydrophobic surface compared with the hydrophilic ones, which demonstrates the importance of nonpolar driving forces on the structural integrity of secondary plant cell walls. These novel molecular insights contribute to an improved understanding of the supramolecular architecture of plant secondary cell walls and have fundamental implications for overcoming lignocellulose recalcitrance and for the design of advanced wood-based materials.

Adhesion properties of warm-modified bituminous binders (WMBBs) determined using pull-off tests and atomic force microscopy

The aggregate-binder bond is one of the main factors that affect the durability of asphalt mixtures. This can be investigated based on the energy required to fracture the adhesive bond between binder and aggregate. In this study, the effects of Sasobit, Rediset WMX and Rediset LQ on the adhesive bond strength of an aggregate-binder system is investigated using the pull-off test. Test data are compared with the nano-scale adhesion force determined using atomic force microscopy (AFM) using the PeakForce Quantitative Nanomechanical Mapping (PFQNM) method. The impact of warm mix additives, test temperature and binder grade on the practical work of fracture was investigated. It was found that Sasobit, Rediset WMX and Rediset LQ increased the practical work of fracture by 170%, 100% and 143%, respectively, for the aggregate-binder system produced using 40/60 Pen binder, and 70%, 25% and 50%, respectively, for the system produced using 100/150 Pen binder. The contribution of warm mix additives in improving the practical work of fracture has been linked to the adhesion force determined using AFM. AFM offers great advantage in characterising the nano-scale properties which were shown to include co-localisation of nano-topography with adhesion, ease of sample preparation and reduction in experimental time relative to the direct tension pull-off test.

Nanoparticle Superlattices: The Roles of Soft Ligands.

Nanoparticle superlattices are periodic arrays of nanoscale inorganic building blocks including metal nanoparticles, quantum dots and magnetic nanoparticles. Such assemblies can exhibit exciting new collective properties different from those of individual nanoparticle or corresponding bulk materials. However, fabrication of nanoparticle superlattices is nontrivial because nanoparticles are notoriously difficult to manipulate due to complex nanoscale forces among them. An effective way to manipulate these nanoscale forces is to use soft ligands, which can prevent nanoparticles from disordered aggregation, fine‐tune the interparticle potential as well as program lattice structures and interparticle distances – the two key parameters governing superlattice properties. This article aims to review the up‐to‐date advances of superlattices from the viewpoint of soft ligands. We first describe the theories and design principles of soft‐ligand‐based approach and then thoroughly cover experimental techniques developed from soft ligands such as molecules, polymer and DNA. Finally, we discuss the remaining challenges and future perspectives in nanoparticle superlattices.

Temperature dependence of plastic instability in Al alloys: A nanoindentation study

An elevated temperature nanoindentation based method for characterizing the thermal dependence of plastic instability and assessing the activation energies associated with the phenomenon in Al alloys is presented in this work. The method exploits the nanoscale force–displacement resolution capabilities of the Nanoindenter, precludes the ambiguities inherent in the uniaxial testing based methods and offers increased reliability because of the statistical significance of the data achieved. The activation energies estimated for an Al—Mg and an Al—Li alloy with the proposed method were found to be 0.59±0.07eV and 0.72±0.01eV, respectively, and are consistent with values derived with other methods. The rate controlling mechanisms associated with these activation energies are described in terms of existing models for plastic instability in these alloy systems.

Dissection of mechanical force in living cells by super-resolved traction force microscopy.

Cells continuously exert or respond to mechanical force. Measurement of these nanoscale forces is a major challenge in cell biology; yet such measurement is essential to the understanding of cell regulation and function. Current methods for examining mechanical force generation either necessitate dedicated equipment or limit themselves to coarse-grained force measurements on the micron scale. In this protocol, we describe stimulated emission depletion traction force microscopy-STED-TFM (STFM), which allows higher sampling of the forces generated by the cell than conventional TFM, leading to a twofold increase in spatial resolution (of up to 500 nm). The procedure involves the preparation of functionalized polyacrylamide gels loaded with fluorescent beads, as well as the acquisition of STED images and their analysis. We illustrate the approach using the example of HeLa cells expressing paxillin-EGFP to visualize focal adhesions. Our protocol uses widely available laser-scanning confocal microscopes equipped with a conventional STED laser, open-source software and common molecular biology techniques. The entire STFM experiment preparation, data acquisition and analysis require 2-3 d and could be completed by someone with minimal experience in molecular biology or biophysics.

Tuning underwater adhesion with cation-π interactions.

Cation-π interactions drive the self-assembly and cohesion of many biological molecules, including the adhesion proteins of several marine organisms. Although the origin of cation-π bonds in isolated pairs has been extensively studied, the energetics of cation-π-driven self-assembly in molecular films remains uncharted. Here we use nanoscale force measurements in combination with solid-state NMR spectroscopy to show that the cohesive properties of simple aromatic- and lysine-rich peptides rival those of the strong reversible intermolecular cohesion exhibited by adhesion proteins of marine mussel. In particular, we show that peptides incorporating the amino acid phenylalanine, a functional group that is conspicuously sparing in the sequences of mussel proteins, exhibit reversible adhesion interactions significantly exceeding that of analogous mussel-mimetic peptides. More broadly, we demonstrate that interfacial confinement fundamentally alters the energetics of cation-π-mediated assembly: an insight that should prove relevant for diverse areas, which range from rationalizing biological assembly to engineering peptide-based biomaterials.

Calcium Microdomains and Modulation in Mechanosensory Hair Cells

Hair cells from the cochlea and vestibular system contain highly ordered rows of actin-based stereocilia that translate nanoscale physical force from sound, head movement or gravity, into an electrical neural code, through mechanoelectrical transduction (MET). Here we report on calcium signaling in hair cells and neurons of the inner ear using a transgenic mouse line Gad2-Cre::GCaMP5G-tdTomato. Swept field confocal microscopy was used to image [Ca2+] modulation in the inner ear of transgenic mice, in response to physiological hair bundle deflection from either micromechanical movement or chemical stimuli. Images were collected through the intact membranous labyrinth or, in some cases, through a fenestration in the membrane. In semicircular canals, spatially localized [Ca2+]i transients were observed and quantified at the extreme tips of stereocilia, the apex of hair cells, in microdomains at the base of hair cells, and in postsynaptic terminals. Hair bundles were either stimulated with a fluid jet directly attached to a picospritzer or by piezoelectric micromechanical indentation applied to mimic physiological head rotation. Fluorescence were imaged using a custom upright Scientifica microscope configured with Olympus optics and a fast swept field confocal imaging system. In the semicircular canals, ∼1µm deflection of the intact cupula revealed punctate increases in GCaMP5G fluorescence (ΔF/F) at the tips of stereocilia. [Ca2+]i transients were restricted to the tips of stereocilia where MET channels are located and were not present in kinocilia. Cupula deflection also evoked large [Ca2+]i transients just below the cuticular plate, putatively colocalizing with the striated organelle. Depolarization with application of KCl revealed [Ca2+] transients in microdomains at the base of hair cells, likely reflecting voltage gated calcium channels modulating synaptic release. Preliminary results using adult vestibular organs of Gad2-Cre::GCaMP5G-tdTomato transgenic mice reveal highly localized time-resolved [Ca2+] modulation at the tips of stereocilia, in microdomains of hair cells, and at synaptic terminals.

An AFM Investigation of the Nanoscale Forces that Govern the Interactions between Actin and Proteins of Tropomodulin Family

Tropomodulin family of proteins includes several isoforms of tropomodulins (Tmod) and leiomodins (Lmod), which are differentially expressed in various tissues. They bind to the pointed end of an actin filament and regulate dynamics at that end. By binding to G-actin they sequester actin or nucleate actin polymerization. Of Tmod isoforms, Tmod2 is the best nucleator and Tmod3 is the best in sequestering. Although these properties are isoform-dependent, knowledge on how they vary in their strength of interactions to actin is missing.

Nanoscale imaging and force probing of biomolecular systems using atomic force microscopy: from single molecules to living cells.

Due to the lack of adequate tools for observation, native molecular behaviors at the nanoscale have been poorly understood. The advent of atomic force microscopy (AFM) provides an exciting instrument for investigating physiological processes on individual living cells with molecular resolution, which attracts the attention of worldwide researchers. In the past few decades, AFM has been widely utilized to investigate molecular activities on diverse biological interfaces, and the performances and functions of AFM have also been continuously improved, greatly improving our understanding of the behaviors of single molecules in action and demonstrating the important role of AFM in addressing biological issues with unprecedented spatiotemporal resolution. In this article, we review the related techniques and recent progress about applying AFM to characterize biomolecular systems in situ from single molecules to living cells. The challenges and future directions are also discussed.

Conduction Channel Formation and Dissolution Due to Oxygen Thermophoresis/Diffusion in Hafnium Oxide Memristors.

Transition-metal-oxide memristors, or resistive random-access memory (RRAM) switches, are under intense development for storage-class memory because of their favorable operating power, endurance, speed, and density. Their commercial deployment critically depends on predictive compact models based on understanding nanoscale physicochemical forces, which remains elusive and controversial owing to the difficulties in directly observing atomic motions during resistive switching, Here, using scanning transmission synchrotron X-ray spectromicroscopy to study in situ switching of hafnium oxide memristors, we directly observed the formation of a localized oxygen-deficiency-derived conductive channel surrounded by a low-conductivity ring of excess oxygen. Subsequent thermal annealing homogenized the segregated oxygen, resetting the cells toward their as-grown resistance state. We show that the formation and dissolution of the conduction channel are successfully modeled by radial thermophoresis and Fick diffusion of oxygen atoms driven by Joule heating. This confirmation and quantification of two opposing nanoscale radial forces that affect bipolar memristor switching are important components for any future physics-based compact model for the electronic switching of these devices.

Spatially resolved organic coating on clay minerals in bitumen froth revealed by atomic force microscopy adhesion mapping

Mineral solids in oil sands are often coated with organic matter. These organically-modified mineral solids hinder bitumen aeration and stabilize water-in-oil emulsions, leading to low bitumen recovery and poor bitumen product quality. The study of these organic coating on the fine solids has been elusive due to the nanometer length scale and the unsuitability of sampling in high vacuum sample chambers. In the present work, we report the first attempt of using PeakForce Quantitative Nanomechanical Mapping (QNM) Atomic Force Microscopy (AFM) to study the organic matter adsorption on fine mineral solids extracted from bitumen froth. Taking advantage of the simultaneous nano-scale resolution topographic imaging and adhesion force mapping enabled by PeakForce QNM-AFM, the mineral and organic components in the bitumen froth fine solids were clearly distinguished due to their variations in shapes and mechanical properties. The inhomogeneous spatial distribution of adhesion force on surfaces of platy particles indicated the existence of organic patches on clay minerals. The area percentage of surface organic matter coverage on the clay basal faces was calculated to be 17±6%, and the average thickness of the organic coating was estimated to be 1.4nm based on the adhesion force maps. The organic matter associated with the mineral solids, which cannot be washed off by toluene, was different from asphaltenes in terms of mechanical properties. More specifically, the irreversibly-adsorbed organic matter on clay minerals was found to be softer than the asphaltene fraction of oil sands bitumen.

Light-induced actuating nanotransducers

Nanoactuators and nanomachines have long been sought after, but key bottlenecks remain. Forces at submicrometer scales are weak and slow, control is hard to achieve, and power cannot be reliably supplied. Despite the increasing complexity of nanodevices such as DNA origami and molecular machines, rapid mechanical operations are not yet possible. Here, we bind temperature-responsive polymers to charged Au nanoparticles, storing elastic energy that can be rapidly released under light control for repeatable isotropic nanoactuation. Optically heating above a critical temperature [Formula: see text] = 32 °C using plasmonic absorption of an incident laser causes the coatings to expel water and collapse within a microsecond to the nanoscale, millions of times faster than the base polymer. This triggers a controllable number of nanoparticles to tightly bind in clusters. Surprisingly, by cooling below [Formula: see text] their strong van der Waals attraction is overcome as the polymer expands, exerting nanoscale forces of several nN. This large force depends on van der Waals attractions between Au cores being very large in the collapsed polymer state, setting up a tightly compressed polymer spring which can be triggered into the inflated state. Our insights lead toward rational design of diverse colloidal nanomachines.

The Effects of Noncellulosic Compounds on the Nanoscale Interaction Forces Measured between Carbohydrate-Binding Module and Lignocellulosic Biomass.

The lack of fundamental understanding of the types of forces that govern how cellulose-degrading enzymes interact with cellulosic and noncellulosic components of lignocellulosic surfaces limits the design of new strategies for efficient conversion of biomass to bioethanol. In a step to improve our fundamental understanding of such interactions, nanoscale forces acting between a model cellulase-a carbohydrate-binding module (CBM) of cellobiohydrolase I (CBH I)-and a set of lignocellulosic substrates with controlled composition were measured using atomic force microscopy (AFM). The three model substrates investigated were kraft (KP), sulfite (SP), and organosolv (OPP) pulped substrates. These substrates varied in their surface lignin coverage, lignin type, and xylan and acetone extractives' content. Our results indicated that the overall adhesion forces of biomass to CBM increased linearly with surface lignin coverage with kraft lignin showing the highest forces among lignin types investigated. When the overall adhesion forces were decoupled into specific and nonspecific component forces via the Poisson statistical model, hydrophobic and Lifshitz-van der Waals (LW) forces dominated the binding forces of CBM to kraft lignin, whereas permanent dipole-dipole interactions and electrostatic forces facilitated the interactions of lignosulfonates to CBM. Xylan and acetone extractives' content increased the attractive forces between CBM and lignin-free substrates, most likely through hydrogen bonding forces. When the substrates treated differently were compared, it was found that both the differences in specific and nonspecific forces between lignin-containing and lignin-free substrates were the least for OPP. Therefore, cellulase enzymes represented by CBM would weakly bind to organosolv lignin. This will facilitate an easy enzyme recovery compared to other substrates treated with kraft or sulfite pulping. Our results also suggest that altering the surface hydrophobicity and the surface energy of lignin that facilitates the LW forces should be a priori to avoid nonproductive binding of cellulase to kraft lignin.

Heterogeneous-surface-mediated crystallization control

The crystallization of matter at interfaces has long been a significant issue in science and technology, but surface-directed crystallization with controlled kinetics remains a matter of challenge. Here, we demonstrate a conceptual novel mechanism to steer liquid–solid phase transformation at interfaces by tailoring the chemical and structural inhomogeneity of a glass substrate through self-limited nanocrystallization of the glassy phase. Importantly, this approach enables large-scale development of metastable crystallization products, such as nanowire membranes. The thorough studies of the intermediate stages of crystallization reveal a unique cooperative mechanism in which the intricate interplays between inherent nanoscale forces and unique heterogeneous surfaces contribute to the mesoscale structural transformation from isolated units to superstructures. We further show that the constructed superstructures offer unprecedented opportuntities for the development of functional membrane systems possessing the combination of robust trace-detection performance and molecular trapping function. These findings not only present clear technological implications, but also provide an improved understanding of the fundamental mechanisms of surface-induced geological and biological processes. A glassy surface embedded with zones of crystalline order can spur materials to form nanowires and molecule-trapping membranes on demand. Surface-induced crystallization plays important roles in processes ranging from bone biomineralization to transistor fabrication. Shifeng Zhou from South China University of Technology in Guangzhou and co-workers have reported a great discovery that glass-ceramics surface can help to control liquid-to-solid transformations at interfaces without resorting to chemical treatments or specialized templates. Using thermal treatments, the team precipitated titanium dioxide nanocrystals into an otherwise homogenous inorganic oxide glass. The resulting non-uniform interface created periodic potential field that enabled strict control over crystal nucleation and aggregation. Additional charge imbalances between tiny aggregations guided materials to grow into remarkable superstructures. As a practical example, microporous silver membranes that detect trace amounts of hazardous aromatic vapours were demonstrated. A new surface-directed crystallization mechanism enabled by the chemical and structural inhomogeneity of the glass substrate through self-limited nanocrystallization of glassy phase was proposed and demonstrated. Benefiting from the intricate interplays between nanoscale force and heterogeneous surface, the strategy offers new opportunities in the large-scale development of various metastable crystallization products and also serves practical purpose.

Modulation of the contractility of micropatterned myocardial cells with nanoscale forces using atomic force microscopy.

The ability to modulate cardiomyocyte contractility is important for bioengineering applications ranging from heart disease treatments to biorobotics. In this study, we examined the changes in contraction frequency of neonatal rat cardiomyocytes upon single-cell-level nanoscale mechanical stimulation using atomic force microscopy. To measure the response of same density of cells, they were micropatterned into micropatches of fixed geometry. To examine the effect of the substrate stiffness on the behavior of cells, they were cultured on a stiffer and a softer surface, glass and poly (dimethylsiloxane), respectively. Upon periodic cyclic stimulation of 300 nN at 5 Hz, a significant reduction in the rate of synchronous contraction of the cell patches on poly(dimethylsiloxane) substrates was observed with respect to their spontaneous beat rate, while the cell patches on glass substrates maintained or increased their contraction rate after the stimulation. On the other hand, single cells mostly maintained their contraction rate and could only withstand a lower magnitude of forces compared to micropatterned cell patches. This study reveals that the contraction behavior of cardiomyocytes can be modulated mechanically through cyclic nanomechanical stimulation, and the degree and mode of this modulation depend on the cell connectivity and substrate mechanical properties.

The Mendeleev-Meyer force project.

Here we present the Mendeleev-Meyer Force Project which aims at tabulating all materials and substances in a fashion similar to the periodic table. The goal is to group and tabulate substances using nanoscale force footprints rather than atomic number or electronic configuration as in the periodic table. The process is divided into: (1) acquiring nanoscale force data from materials, (2) parameterizing the raw data into standardized input features to generate a library, (3) feeding the standardized library into an algorithm to generate, enhance or exploit a model to identify a material or property. We propose producing databases mimicking the Materials Genome Initiative, the Medical Literature Analysis and Retrieval System Online (MEDLARS) or the PRoteomics IDEntifications database (PRIDE) and making these searchable online via search engines mimicking Pubmed or the PRIDE web interface. A prototype exploiting deep learning algorithms, i.e. multilayer neural networks, is presented.

Pressure sensitive microparticle adhesion through biomimicry of the pollen-stigma interaction.

Many soft biomimetic synthetic adhesives, optimized to support macroscopic masses (∼kg), have been inspired by geckos, insects and other animals. Far less work has investigated bioinspired adhesion that is tuned to micro- and nano-scale sizes and forces. However, such adhesive forces are extremely important in the adhesion of micro- and nanoparticles to surfaces, relevant to a wide range of industrial and biological systems. Pollens, whose adhesion is critical to plant reproduction, are an evolutionary-optimized system for biomimicry to engineer tunable adhesion between particles and micro-patterned soft matter surfaces. In addition, the adhesion of pollen particles is relevant to topics as varied as pollinator ecology, transport of allergens, and atmospheric phenomena. We report the first observation of structurally-derived pressure-sensitive adhesion of a microparticle by using the sunflower pollen and stigma surfaces as a model. This strong, pressure-sensitive adhesion results from interlocking between the pollen's conical spines and the stigma's receptive papillae. Inspired by this behavior, we fabricated synthetic polymeric patterned surfaces that mimic the stigma surface's receptivity to pollen. These soft mimics allow the magnitude of the pressure-sensitive response to be tuned by adjusting the size and spacing of surface features. These results provide an important new insight for soft material adhesion based on bio-inspired principles, namely that ornamented microparticles and micro-patterned surfaces can be designed with complementarity that enable a tunable, pressure-sensitive adhesion on the microparticle size and length scale.

Nanoscale adhesion forces between the fungal pathogen Candida albicans and macrophages.

The development of fungal infections is tightly controlled by the interaction of fungal pathogens with host immune cells. While the recognition of specific fungal cell wall components by immune receptors has been widely investigated, the molecular forces involved are not known. In this Communication, we show the ability of single-cell force spectroscopy to quantify the specific adhesion forces between the fungal pathogen Candida albicans and macrophages. The Candida-macrophage adhesion force is strong, up to ∼3000 pN, and corresponds to multiple cumulative bonds between lectin receptors expressed on the macrophage membrane and mannan carbohydrates on the fungal cell surface. Adhesion force signatures show constant force plateaus, up to >100 μm long, reflecting the extraction of elongated tethers from the macrophage membrane, a phenomenon which may increase the duration of intercellular adhesion. Adhesion strengthens with time, suggesting that the macrophage membrane engulfs the pathogen quickly after initial contact, leading to its internalization. The force nanoscopy method developed here holds great promise for understanding and controlling the early stages of microbe-immune interactions.

Implications for directionality of nanoscale forces in bacterial attachment.

Adhesion and friction are closely related and play a predominant role in many natural processes. From the wall-clinging feet of the gecko to bacteria forming a biofilm, in many cases adhesion is a necessity to survive. The direction in which forces are applied has shown to influence the bond strength of certain systems tremendously and can mean the difference between adhesion and detachment. The spatula present on the extension of the feet of the gecko can either attach or detach, based on the angle at which they are loaded. Certain proteins are known to unfold at different loads, depending on the direction at which the load is applied and some bacteria have specific receptors which increase their bond strength in the presence of shear. Bacteria adhere to any man-made surface despite the presence of shear forces due to running fluids, air flow, and other causes. In bacterial adhesion research, however, adhesion forces are predominantly measured perpendicularly to surfaces, whereas other directions are often neglected. The angle of shear forces acting on bacteria or biofilms will not be at a 90° angle, as shear induced by flow is often along the surface. Measuring at different angles or even lateral to the surface will give a more complete overview of the adhesion forces and mechanism, perhaps even resulting in alternative means to discourage bacterial adhesion or promote removal.

Interfacial Study To Suppress Charge Carrier Recombination for High Efficiency Perovskite Solar Cells.

We report effects of an interface between TiO2-perovskite and grain-grain boundaries of perovskite films prepared by single step and sequential deposited technique using different annealing times at optimum temperature. Nanoscale kelvin probe force microscopy (KPFM) measurement shows that charge transport in a perovskite solar cell critically depends upon the annealing conditions. The KPFM results of single step and sequential deposited films show that the increase in potential barrier suppresses the back-recombination between electrons in TiO2 and holes in perovskite. Spatial mapping of the surface potential within perovskite film exhibits higher positive potential at grain boundaries compared to the surface of the grains. The average grain boundary potential of 300-400 mV is obtained upon annealing for sequentially deposited films. X-ray diffraction (XRD) spectra indicate the formation of a PbI2 phase upon annealing which suppresses the recombination. Transient analysis exhibits that the optimum device has higher carrier lifetime and short carrier transport time among all devices. An optimum grain boundary potential and proper band alignment between the TiO2 electron transport layer (ETL) and the perovskite absorber layer help to increase the overall device performance.