Interrogation of Single Synthetic Polymer Chains and Polysaccharides by AFM-Based Force Spectroscopy

Physicochemical surface properties of Chlorella vulgaris: a multiscale assessment, from electrokinetic and proton uptake descriptors to intermolecular adhesion forces

Given the growing scientific and industrial interests in green microalgae, a comprehensive understanding of the forces controlling the colloidal stability of these bioparticles and their interactions with surrounding aqueous microenvironment is required. Accordingly, we addressed here the electrostatic and hydrophobic surface properties of Chlorella vulgaris from the population down to the individual cell levels. We first investigated the organisation of the electrical double layer at microalgae surfaces on the basis of electrophoresis measurements. Interpretation of the results beyond zeta-potential framework underlined the need to account for both the hydrodynamic softness of the algae cells and the heterogeneity of their interface formed with the outer electrolyte solution. We further explored the nature of the structural charge carriers at microalgae interfaces through potentiometric proton titrations. Extraction of the electrostatic descriptors of interest from such data was obscured by cell physiology processes and dependence thereof on prevailing measurement conditions, which includes light, temperature and medium salinity. As an alternative, cell electrostatics was successfully evaluated at the cellular level upon mapping the molecular interactions at stake between (positively and negatively) charged atomic force microscopy tips and algal surface via chemical force microscopy. A thorough comparison between charge-dependent tip-to-algae surface adhesion and hydrophobicity level of microalgae surface evidenced that the contribution of electrostatics to the overall interaction pattern is largest, and that the electrostatic/hydrophobic balance can be largely modulated by pH. Overall, the combination of multiscale physicochemical approaches allowed a drawing of some of the key biosurface properties that govern microalgae cell-cell and cell-surface interactions.

Recent advances in fabricating injectable hydrogels via tunable molecular interactions for bio-applications

Hydrogels with three-dimensional structures have been widely applied in various applications because of their tunable structures, which can be easily tailored with desired functionalities. However, the application of hydrogel materials in bioengineering is still constrained by their limited dosage flexibility and the requirement of invasive surgical procedures. Compared to traditional hydrogels, injectable hydrogels, with shear-thinning and/or in situ formation properties, simplify the implantation process and reduce tissue invasion, which can be directly delivered to target sites using a syringe injection, offering distinct advantages over traditional hydrogels. These injectable hydrogels incorporate physically non-covalent and/or dynamic covalent bonds, granting them self-healing abilities to recover their structural integrity after injection. This review summarizes our recent progress in preparing injectable hydrogels and discusses their performance in various bioengineering applications. Moreover, the underlying molecular interaction mechanisms that govern the injectable and functional properties of hydrogels were characterized by using nanomechanical techniques such as surface forces apparatus (SFA) and atomic force microscopy (AFM). The remaining challenges and future perspectives on the design and application of injectable hydrogels are also discussed. This work provides useful insights and guides future research directions in the field of injectable hydrogels for bioengineering.

Optical single molecule characterisation of natural and synthetic polymers through nanopores

Nanopore techniques are now widely used to sequence DNA, RNA and even oligopeptide molecules at the base pair level by measuring the ionic current. In order to build a more versatile characterisation system, optical methods for the detection of a single molecule translocating through a nanopore have been developed, achieving very promising results. In this work, we developed a series of tools to interpret the optical signals in terms of the physical behaviour of various types of natural and synthetic polymers, with high throughput. We show that the measurement of the characteristic time of a translocation event gives access to the apparent molecular weight of an object, and allows us to quantify the concentration ratio of two DNA samples of different molecular weights in solution. Using the same tools for smaller synthetic polymers, we were able to obtain information about their molecular weight distribution depending on the synthesis method.

Cooperative Tridentate Hydrogen-Bonding Interactions Enable Strong Underwater Adhesion

Multidentate hydrogen-bonding interactions are a promising strategy to improve underwater adhesion. Molecular and macroscale experiments have revealed an increase in underwater adhesion by incorporating multidentate H-bonding groups, but quantitatively relating the macroscale adhesive strength to cooperative hydrogen-bonding interactions remains challenging. Here, we investigate whether tridentate alcohol moieties incorporated in a model epoxy act cooperatively to enhance adhesion. We first demonstrate that incorporation of tridentate alcohol moieties leads to comparable adhesive strength with mica and aluminum in air and in water. We then show that the presence of tridentate groups leads to energy release rates that increase with an increase in crack velocity in air and in water, while materials lacking these groups do not display rate-dependent adhesion. We model the rate-dependent adhesion to estimate the activation energy of the interfacial bonds. Based on our data, we estimate the lifetime of these bonds to be between 2 ms and 6 s, corresponding to an equilibrium activation energy between 23k_BT and 31k_BT. These values are consistent with tridentate hydrogen bonding, suggesting that the three alcohol groups in the Tris moiety bond cooperatively form a robust adhesive interaction underwater.

Unusual Aspects of Charge Regulation in Flexible Weak Polyelectrolytes

This article reviews the state of the art of the studies on charge regulation (CR) effects in flexible weak polyelectrolytes (FWPE). The characteristic of FWPE is the strong coupling of ionization and conformational degrees of freedom. After introducing the necessary fundamental concepts, some unconventional aspects of the the physical chemistry of FWPE are discussed. These aspects are: (i) the extension of statistical mechanics techniques to include ionization equilibria and, in particular, the use of the recently proposed Site Binding-Rotational Isomeric State (SBRIS) model, which allows the calculation of ionization and conformational properties on the same foot; (ii) the recent progresses in the inclusion of proton equilibria in computer simulations; (iii) the possibility of mechanically induced CR in the stretching of FWPE; (iv) the non-trivial adsorption of FWPE on ionized surfaces with the same charge sign as the PE (the so-called "wrong side" of the isoelectric point); (v) the influence of macromolecular crowding on CR.

Simultaneous Force and Fluorescence Spectroscopy on Single Chains of Polyfluorene: Effect of Intra-Chain Aggregate Coupling

Conjugated polymer chains in compact conformations or in films exhibit spectral features that can be attributed to interactions between individual conjugated segments of the chain, including formation of aggregates or excimers. Here, we use atomic force microscopy (AFM) on single chains of the conjugated polymer polyfluorene (PFO) to control the intersegment interactions by mechanically unfolding the chain. Simultaneously with the force spectroscopy we monitor fluorescence from the single PFO chains using a fluorescence microscope. We found that mechanical stretching of the chain causes disappearance of the green emission band. This observation provides evidence that the green emission originates from an intrachain aggregated state on the self-folded chain, which is decoupled by the stretching. In addition, the stretching upon laser irradiation leads to the appearance of additional features in the force spectra, small force peaks in the initial stages of the unfolding. These features are attributed to a combination of excitonic and van der Waals coupling of a ground-state intrachain aggregate.

Hierarchical Quatsome-RGD Nanoarchitectonic Surfaces for Enhanced Integrin-Mediated Cell Adhesion

The synthesis and study of the tripeptide Arg-Gly-Asp (RGD), the binding site of different extracellular matrix proteins, e.g., fibronectin and vitronectin, has allowed the production of a wide range of cell adhesive surfaces. Although the surface density and spacing of the RGD peptide at the nanoscale have already shown a significant influence on cell adhesion, the impact of its hierarchical nanostructure is still rather unexplored. Accordingly, a versatile colloidal system named quatsomes, based on fluid nanovesicles formed by the self-assembling of cholesterol and surfactant molecules, has been devised as a novel template to achieve hierarchical nanostructures of the RGD peptide. To this end, RGD was anchored on the vesicle's fluid membrane of quatsomes, and the RGD-functionalized nanovesicles were covalently anchored to planar gold surfaces, forming a state of quasi-suspension, through a long poly(ethylene glycol) (PEG) chain with a thiol termination. An underlying self-assembled monolayer (SAM) of a shorter PEG was introduced for vesicle stabilization and to avoid unspecific cell adhesion. In comparison with substrates featuring a homogeneous distribution of RGD peptides, the resulting hierarchical nanoarchitectonic dramatically enhanced cell adhesion, despite lower overall RGD molecules on the surface. The new versatile platform was thoroughly characterized using a multitechnique approach, proving its enhanced performance. These findings open new methods for the hierarchical immobilization of biomolecules on surfaces using quatsomes as a robust and novel tissue engineering strategy.

Probing and Manipulating Noncovalent Interactions in Functional Polymeric Systems

Noncovalent interactions, which usually feature tunable strength, reversibility, and environmental adaptability, have been recognized as driving forces in a variety of biological and chemical processes, contributing to the recognition between molecules, the formation of molecule clusters, and the establishment of complex structures of macromolecules. The marriage of noncovalent interactions and conventional covalent polymers offers the systems novel mechanical, physicochemical, and biological properties, which are highly dependent on the binding mechanisms of the noncovalent interactions that can be illuminated via quantification. This review systematically discusses the nanomechanical characterization of typical noncovalent interactions in polymeric systems, mainly through direct force measurements at microscopic, nanoscopic, and molecular levels, which provide quantitative information (e.g., ranges, strengths, and dynamics) on the binding behaviors. The fundamental understandings of intermolecular and interfacial interactions are then correlated to the macroscopic performances of a series of noncovalently bonded polymers, whose functions (e.g., stimuli-responsiveness, self-healing capacity, universal adhesiveness) can be customized through the manipulation of the noncovalent interactions, providing insights into the rational design of advanced materials with applications in biomedical, energy, environmental, and other engineering fields.

Drag force of polyethyleneglycol in flows of polymer solutions measured using a scanning probe microscope

The drag force of polyethyleneglycol thiol (mPEG-SH) attached to a cantilever probe in the flows of glycerol and polyethyleneglycol (PEG) solutions was measured. The effects of the molecular weights of mPEG-SH, solute, and molecular weights of PEGs in the flows on the drag force were investigated. The drag force of mPEG-SH with any molecular weight in the flows of glycerol solutions was described well by the stem and ellipsoidal-flower model proposed in a previous study. However, the drag force further increased in the flow of the PEG solutions. To describe the increment, an assumption of polymer entanglement with mPEG-SH attached to the probe in the flow was employed. The modified stem and ellipsoidal-flower model that employed polymer entanglements fit well to the drag force of mPEG-SH with any molecular weight in the flow of the polymer solution.

Structure and Dynamics of a Temperature-Sensitive Hydrogel

Polyisocyanotripeptides (TriPIC) are biomimetic polymers which consist of a β-helical backbone stabilized by hydrogen bonds between amide groups. Their oligoethylene glycol side chains give aqueous TriPIC solutions a thermoresponsive behavior: at 50 °C the solution becomes a hydrogel. In this paper we study the molecular structure and water dynamics of TriPIC aqueous solutions while undergoing gelation using FT-IR spectroscopy and polarization-resolved femtosecond infrared spectroscopy (fs-IR). We find evidence that the oligoethylene glycol side chains trap part of the water molecules upon gel formation, and we propose that the interaction between the oligoethylene glycol side chains and water plays an essential role in the bundling of the polymers and thus in the formation of a hydrogel.

On the relationship between cutting and tearing in soft elastic solids

Unique observations of cutting energy in silicone elastomers motivate a picture of soft fracture that qualitatively and quantitatively links far-field tearing with push cutting for the first time. For blades of decreasing tip radii, the cutting energy decreases until it reaches a plateau that suggests a threshold for failure. A super-molecular damage zone, necessary for new surface creation, is defined using the tip radius at the onset of this threshold. Modifying the classic Lake-Thomas theory, in which failure occurs within a molecular plane, to this super-molecular zone provides order-of-magnitude agreement with the cutting energy threshold. Together, the threshold fracture energy and damage length scale define criteria for failure that, when implemented in finite element simulation, quantitatively reproduce the increase in cutting energy with increasing blade radius outside of the plateau. The rate of increase depends on the constitutive response of the material, with more neo-Hookean solids requiring a larger failure force per incremental increase in blade radius as observed experimentally. This combination of a geometry-independent failure threshold (from the cutting energy plateau) and a need to account for the role of material deformability in the stress concentration found at the crack tip (from the rate of cutting energy increase with blade radius) align with the discovery of a new dimensionless group. This new parameter proportionally maps cutting energy to the energy required to tear a sample under far-field loading conditions by using ultimate properties obtained in uniaxial tension.

Lipid bilayers: Phase behavior and nanomechanics

Lipid membranes are involved in many physiological processes like recognition, signaling, fusion or remodeling of the cell membrane or some of its internal compartments. Within the cell, they are the ultimate barrier, while maintaining the fluidity or flexibility required for a myriad of processes, including membrane protein assembly. The physical properties of in vitro model membranes as model cell membranes have been extensively studied with a variety of techniques, from classical thermodynamics to advanced modern microscopies. Here we review the nanomechanics of solid-supported lipid membranes with a focus in their phase behavior. Relevant information obtained by quartz crystal microbalance with dissipation monitoring (QCM-D) and atomic force microscopy (AFM) as complementary techniques in the nano/mesoscale interface is presented. Membrane morphological and mechanical characterization will be discussed in the framework of its phase behavior, phase transitions and coexistence, in simple and complex models, and upon the presence of cholesterol.

100th Anniversary of Macromolecular Science Viewpoint: Single-Molecule Studies of Synthetic Polymers

Single polymer studies have revealed unexpected and heterogeneous dynamics among identical or seemingly similar macromolecules. In recent years, direct observation of single polymers has uncovered broad distributions in molecular behavior that play a key role in determining bulk properties. Early single polymer experiments focused primarily on biological macromolecules such as DNA, but recent advances in synthesis, imaging, and force spectroscopy have enabled broad exploration of chemically diverse polymer systems. In this Viewpoint, we discuss the recent study of synthetic polymers using single-molecule methods. In terms of polymer synthesis, direct observation of single chain polymerization has revealed heterogeneity in monomer insertion events at catalytic centers and decoupling of local and global growth kinetics. In terms of single polymer visualization, recent advances in super-resolution imaging, atomic force microscopy (AFM), and liquid-cell transmission electron microscopy (LC-TEM) can resolve structure and dynamics in single synthetic chains. Moreover, single synthetic polymers can be probed in the context of bulk material environments, including hydrogels, nanostructured polymers, and crystalline polymers. In each area, we highlight key challenges and exciting opportunities in using single polymer techniques to enhance our understanding of polymer science. Overall, the expanding versatility of single polymer methods will enable the molecular-scale design and fundamental understanding of a broad range of chemically diverse and functional polymeric materials.

Uncommon nonlinear rheological phenomenology in uniaxial extension of polystyrene solutions and melts

This study examines nonlinear rheological responses to uniaxial extension of two entangled polystyrene (PS) solutions and two PS melts. Several unusual characteristics are revealed. The pair of the PS solutions have the same number of entanglements per chain (because of the same concentration) but well separated effective glass transition temperatures Tg. When examined at a common effective rate of extension (e.g., the same Rouse-Weissenberg number WiR) and at a comparable distance from their respective Tg, the solution A with lower Tg, examined at a lower temperature, shows stronger stress responses when WiR > 1. At the same test temperature and a common WiR, the solution A is still found to display a stronger stress response than the solution B that is made of the same fraction of parent PS in a second solvent also made of oligomeric PS of higher molecular weight. Finally, there are two features intrinsic to each of the four PS samples. First, at the same WiR they show reduced stress level at a lower temperature. Second, at sufficiently high applied Hencky rates, they show limiting rate behavior, i.e., undergoing the same melt rupture independent of the applied rate. These remarkable rheological responses indicate major theoretical difficulties facing the subject of nonlinear extensional rheology of entangled polymers.

Environment-dependent single-chain mechanics of synthetic polymers and biomacromolecules by atomic force microscopy-based single-molecule force spectroscopy and the implications for advanced polymer materials

"The Tao begets the One. One begets all things of the world." This quote from Tao Te Ching is still inspiring for scientists in chemistry and materials science: The "One" can refer to a single molecule. A macroscopic material is composed of numerous molecules. Although the relationship between the properties of the single molecule and macroscopic material is not well understood yet, it is expected that a deeper understanding of the single-chain mechanics of macromolecules will certainly facilitate the development of materials science. Atomic force microscopy-based single-molecule force spectroscopy (AFM-SMFS) has been exploited extensively as a powerful tool to study the single-chain behaviors of macromolecules. In this review, we summarize the recent advances in the emerging field of environment-dependent single-chain mechanics of synthetic polymers and biomacromolecules by means of AFM-SMFS. First, the single-chain inherent elasticities of several typical linear macromolecules are introduced, which are also confirmed by one of three polymer models with theoretical elasticities of the corresponding macromolecules obtained from quantum mechanical (QM) calculations. Then, the effects of the external environments on the single-chain mechanics of synthetic polymers and biomacromolecules are reviewed. Finally, the impacts of single-chain mechanics of macromolecules on the development of polymer science especially polymer materials are illustrated.

Thin Polymer Film Force Spectroscopy: Single Chain Pull-out and Desorption

Atomic force microscopy (AFM) was utilized to investigate the force associated with chain pull-out and single chain desorption of poly(styrene-co-butadiene) random copolymer thin films on mica, silicon, and graphite substrates. Chain pull-out events were common and produced a force of 20-25 pN. The polymer desorption force was strongest on the graphite substrate and weakest on the mica, which agreed with the calculated work of adhesion for each system and the substrate hydrophobicity. Furthermore, it was demonstrated that there was a systematic order to when each of these phenomena occurred during the tip retraction from the surface, which provided information about the structure of the thin films.

Role of Charge Regulation and Fluctuations in the Conformational and Mechanical Properties of Weak Flexible Polyelectrolytes

This work addresses the role of charge regulation (CR) and the associated fluctuations in the conformational and mechanical properties of weak polyelectrolytes. Due to CR, changes in the pH-value modifies the average macromolecular charge and conformational equilibria. A second effect is that, for a given average charge per site, fluctuations can alter the intensity of the interactions by means of correlation between binding sites. We investigate both effects by means of Monte Carlo simulations at constant pH-value, so that the charge is a fluctuating quantity. Once the average charge per site is available, we turn off the fluctuations by assigning the same average charge to every site. A constant charge MC simulation is then performed. We make use of a model which accounts for the main fundamental aspects of a linear flexible polyelectrolyte that is, proton binding, angle internal rotation, bond stretching and bending. Steric excluded volume and differentiated treatment for short-range and long-range interactions are also included. This model can be regarded as a kind of “minimal” in the sense that it contains a minimum number of parameters but still preserving the atomistic detail. It is shown that, if fluctuations are activated, gauche state bond probabilities increase and the persistence length decreases, so that the polymer becomes more folded. Macromolecular stretching is also analyzed in presence of CR (the charge depends on the applied force) and without CR (the charge is fixed to the value at zero force). The analysis of the low force scaling behavior concludes that Pincus exponent becomes pH-dependent. Both, with and without CR, a transition from 1/2 at high pH-values (phantom chain) to 3/5 at low pH-values (Pincus regime) is observed. Finally, the intermediate force stretching regime is investigated. It is found that CR induces a moderate influence in the force-extension curves and persistence length (which in this force regime becomes force-dependent). It is thus concluded that the effect of CR on the stretching curves is mainly due to the changes in the average charge at zero force. It is also found that, for the cases studied, the effect of steric excluded volume is almost irrelevant compared to electrostatic interactions.

Characterizing effects of fast melt deformation on entangled polymers in their glassy state

Fast deformation of entangled melts is known to cause chain stretching due to affinelike straining of the entanglement network. Since the chain deformation may also result in perturbations of covalent bond angles and bond length, there are always possible enthalpic effects. In this study, we first subject polystyrene and PMMA of different molecular weights to either uniaxial melt extension or planar extension and subsequently impose rapid thermal quenching to preserve the chain deformation. Then, such pre-melt-deformed samples are annealed at various temperatures below the glass transition temperature T_g. During annealing, these samples can undergo appreciable contraction on a time scale much shorter than the alpha relaxation time. Significant retractive stress is observed when such contracting samples are held fixed during the annealing. The stress level can be nearly as high as the Cauchy stress produced during melt stretching. These observations not only allowed us to investigate glassy chain dynamics as well as the molecular nature of mechanical stress but may also suggest that pre-melt-stretched polymers can cause segmental mobilization in the glassy state. The available evidence indicates that the retractive stress is enthalpic in origin, associated with the conformational distortion at the bond level produced by melt stretching.

Indentation of Graphene-Covered Atomic Force Microscopy Probe Across a Lipid Bilayer Membrane: Effect of Tip Shape, Size, and Surface Hydrophobicity

Understanding the interaction of graphene with cell membranes is crucial to the development of graphene-based biological applications and the management of graphene safety issues. To help reveal the key factors controlling the interaction between graphene and cell membranes, here we adopt the dissipative particle dynamics method to analyze the evolution of interaction force and free energy as the graphene-covered atomic force microscopy (AFM) probe indents across a lipid bilayer. The simulation results show that the graphene-covered AFM probe can cause severe deformation of the cell membrane which drives the lipid molecule to adsorb and diffuse at the surface of graphene. The breakthrough force and free energy are calculated to study the effects of the tip shape, size, and surface hydrophobicity on the piercing behaviors of graphene-covered AFM. In addition, the deformation of cell membrane can decrease the dependency of the breakthrough force on the tip shape. The analysis of surface functionalization suggests that the horizontal patterns on graphene can change the preferred orientation in the penetration process, but the vertical patterns on graphene may disrupt the cell membrane. What's more, the bending stiffness of graphene has little influence on the penetration process as graphene pierces into the cell membrane. These results provide useful guidelines for the molecular design of graphene materials with controllable cell penetrability.

Angle-Dependent Atomic Force Microscopy Single-Chain Pulling of Adsorbed Macromolecules from Planar Surfaces Unveils the Signature of an Adsorption-Desorption Transition

The adsorption-desorption behavior of polymer chains is at the heart of macromolecular surface science and technology. With the current developments in atomic force microscopy (AFM), it has now become possible to address the desorption problem from the perspective of a single macromolecule. Here, we report on desorption of single polymer chains on planar surfaces by AFM-based single molecule force spectroscopy (SMFS) as a function of the pulling angle with respect to the surface-normal direction. SMFS experiments were performed in water with various substrates using different polymers covalently attached to the AFM probe tip. End-grafting at the AFM tip was achieved by surface-initiated polymerization using initiator functionalized tips. We found that the desorption force increases with a decreasing pulling angle, i.e., an enhanced adhesion of the polymer chain was observed. The magnitude of the desorption force shows a weak angular dependence at pulling angles close to the surface normal. A significant increase of the force is observed at shallower pulling from a certain pulling angle. This behavior carries the signature of an adsorption-desorption transition. The angular dependence of the normalized desorption force exhibits a universal behavior. We compared and interpreted our results using theoretical predictions for single-chain adsorption-desorption transitions.

Getting the feel of food structure with atomic force microscopy

This article describes the progress in the development of the atomic force microscope as an imaging tool and a force transducer, with particular reference to applications in food science. Use as an imaging tool has matured and emphasis is placed on the novel insights gained from the use of the technique to study food macromolecules and food colloids, and the subsequent applications of this new knowledge in food science. Use as a force transducer is still emerging and greater emphasis is given on the methodology and analysis. Where available, applications of force measurements between molecules or between larger colloidal particles are discussed, where they have led to new insights or solved problems related to food science. The future prospects of the technique in imaging or through force measurements are discussed.

Polymer on Top: Current Limits and Future Perspectives of Quantitatively Evaluating Surface Grafting

Well-defined polymer strands covalently tethered onto solid substrates determine the properties of the resulting functional interface. Herein, the current approaches to determine quantitative grafting densities are assessed. Based on a brief introduction into the key theories describing polymer brush regimes, a user's guide is provided to estimating maximum chain coverage and-importantly-examine the most frequently employed approaches for determining grafting densities, i.e., dry thickness measurements, gravimetric assessment, and swelling experiments. An estimation of the reliability of these determination methods is provided via carefully evaluating their assumptions and assessing the stability of the underpinning equations. A practical access guide for comparatively and quantitatively evaluating the reliability of a given approach is thus provided, enabling the field to critically judge experimentally determined grafting densities and to avoid the reporting of grafting densities that fall outside the physically realistic parameter space. The assessment is concluded with a perspective on the development of advanced approaches for determination of grafting density, in particular, on single-chain methodologies.

Polymers and biopolymers at interfaces

This review updates recent progress in the understanding of the behaviour of polymers at surfaces and interfaces, highlighting examples in the areas of wetting, dewetting, crystallization, and 'smart' materials. Recent developments in analysis tools have yielded a large increase in the study of biological systems, and some of these will also be discussed, focussing on areas where surfaces are important. These areas include molecular binding events and protein adsorption as well as the mapping of the surfaces of cells. Important techniques commonly used for the analysis of surfaces and interfaces are discussed separately to aid the understanding of their application.

Layered Structure and Complex Mechanochemistry Underlie Strength and Versatility in a Bacterial Adhesive

While designing synthetic adhesives that perform in aqueous environments has proven challenging, microorganisms commonly produce bioadhesives that efficiently attach to a variety of substrates, including wet surfaces. The aquatic bacterium Caulobacter crescentus uses a discrete polysaccharide complex, the holdfast, to strongly attach to surfaces and resist flow. The holdfast is extremely versatile and has impressive adhesive strength. Here, we used atomic force microscopy in conjunction with superresolution microscopy and enzymatic assays to unravel the complex structure of the holdfast and to characterize its chemical constituents and their role in adhesion. Our data support a model whereby the holdfast is a heterogeneous material organized as two layers: a stiffer nanoscopic core layer wrapped into a sparse, far-reaching, flexible brush layer. Moreover, we found that the elastic response of the holdfast evolves after surface contact from initially heterogeneous to more homogeneous. From a composition point of view, besides N-acetyl-d-glucosamine (NAG), the only component that had been identified to date, our data show that the holdfast contains peptides and DNA. We hypothesize that, while polypeptides are the most important components for adhesive force, the presence of DNA mainly impacts the brush layer and the strength of initial adhesion, with NAG playing a primarily structural role within the core. The unanticipated complexity of both the structure and composition of the holdfast likely underlies its versatility as a wet adhesive and its distinctive strength. Continued improvements in understanding of the mechanochemistry of this bioadhesive could provide new insights into how bacteria attach to surfaces and could inform the development of new adhesives.IMPORTANCE There is an urgent need for strong, biocompatible bioadhesives that perform underwater. To strongly adhere to surfaces and resist flow underwater, the bacterium Caulobacter crescentus produces an adhesive called the holdfast, the mechanochemistry of which remains undefined. We show that the holdfast is a layered structure with a stiff core layer and a polymeric brush layer and consists of polysaccharides, polypeptides, and DNA. The DNA appears to play a role in the structure of the brush layer and initial adhesion, the peptides in adhesive strength, and the polysaccharides in the structure of the core. The complex, multilayer organization and diverse chemistry described here underlie the distinctive adhesive properties of the holdfast and will provide important insights into the mechanisms of bacterial adhesion and bioadhesive applications.

Influence of ligand-receptor interactions on force-extension behavior within the freely jointed chain model

We study the influence of receptor-ligand interactions on the force response of single polymer chains theoretically. The extension of the chain is modeled in terms of freely jointed chain or elastic freely jointed chain (EFJC) models. The situation involving noninteracting bonds is solved exactly, while effects of interactions are treated within a mean-field approximation. The form with shorter bonds governs the low force situation, while the form with longer bonds is relevant in the high force regime. We further discuss the accuracy of approximate relations, which were used to describe the response of the EFJC model.

An Optical Tweezers Platform for Single Molecule Force Spectroscopy in Organic Solvents

Observation at the single molecule level has been a revolutionary tool for molecular biophysics and materials science, but single molecule studies of solution-phase chemistry are less widespread. In this work we develop an experimental platform for solution-phase single molecule force spectroscopy in organic solvents. This optical-tweezer-based platform was designed for broad chemical applicability and utilizes optically trapped core-shell microspheres, synthetic polymer tethers, and click chemistry linkages formed in situ. We have observed stable optical trapping of the core-shell microspheres in ten different solvents, and single molecule link formation in four different solvents. These experiments demonstrate how to use optical tweezers for single molecule force application in the study of solution-phase chemistry.

Single Molecule Study on Polymer-Nanoparticle Interactions: The Particle Shape Matters

The study on the nanoparticle-polymer interactions is very important for the design/preparation of high performance polymer nanocomposite. Here we present a method to quantify the polymer-particle interaction at single molecule level by using AFM-based single molecule force spectroscopy (SMFS). As a proof-of-concept study, we choose poly-l-lysine (PLL) as the polymer and several different types of polyoxometalates (POM) as the model particles to construct several different polymer nanocomposites and to reveal the binding mode and quantify the binding strength in these systems. Our results reveal that the shape of the nanoparticle and the binding geometry in the composite have significantly influenced the binding strength of the PLL/POM complexes. Our dynamic force spectroscopy studies indicate that the disk-like geometry facilitate the unbinding of PLL/AlMo₆ complexes in shearing mode, while the unzipping mode becomes dominate in spherical PLL-P₈W₄₈ system. We have also systematically investigated the effects of charge numbers, particle size, and ionic strength on the binding strength and binding mode of PLL/POM, respectively. Our results show that electrostatic interactions dominate the stability of PLL/POM complexes. These findings provide a way for tuning the mechanical properties of polyelectrolyte-nanoparticle composites.

Pulling angle-dependent force microscopy

In this paper, we describe a method allowing one to perform three-dimensional displacement control in force spectroscopy by atomic force microscopy (AFM). Traditionally, AFM force curves are measured in the normal direction of the contacted surface. The method described can be employed to address not only the magnitude of the measured force but also its direction. We demonstrate the technique using a case study of angle-dependent desorption of a single poly(2-hydroxyethyl methacrylate) (PHEMA) chain from a planar silica surface in an aqueous solution. The chains were end-grafted from the AFM tip in high dilution, enabling single macromolecule pull experiments. Our experiments give evidence of angular dependence of the desorption force of single polymer chains and illustrate the added value of introducing force direction control in AFM.

Structure and Nanomechanics of Model Membranes by Atomic Force Microscopy and Spectroscopy: Insights into the Role of Cholesterol and Sphingolipids

Biological membranes mediate several biological processes that are directly associated with their physical properties but sometimes difficult to evaluate. Supported lipid bilayers (SLBs) are model systems widely used to characterize the structure of biological membranes. Cholesterol (Chol) plays an essential role in the modulation of membrane physical properties. It directly influences the order and mechanical stability of the lipid bilayers, and it is known to laterally segregate in rafts in the outer leaflet of the membrane together with sphingolipids (SLs). Atomic force microscope (AFM) is a powerful tool as it is capable to sense and apply forces with high accuracy, with distance and force resolution at the nanoscale, and in a controlled environment. AFM-based force spectroscopy (AFM-FS) has become a crucial technique to study the nanomechanical stability of SLBs by controlling the liquid media and the temperature variations. In this contribution, we review recent AFM and AFM-FS studies on the effect of Chol on the morphology and mechanical properties of model SLBs, including complex bilayers containing SLs. We also introduce a promising combination of AFM and X-ray (XR) techniques that allows for in situ characterization of dynamic processes, providing structural, morphological, and nanomechanical information.

Dynamics of single-stranded DNA tethered to a solid

Tethering is used to deliver specific biological and industrial functions. For example, single-stranded DNA (ssDNA) is tethered to polymerases and long sequences of double-stranded DNA (dsDNA) during replication, and to solids in DNA microarrays. However, tethering ssDNA to a large object limits not only the available ssDNA conformations, but also the range of time-scales over which the mechanical responses of ssDNA are important. In this work we examine the effect of tethering by measurement of the mechanical response of ssDNA that is tethered at each end to two separate atomic force microscope cantilevers in aqueous solution. Thermal motion of the cantilevers drives the ends of the ssDNA chain at frequencies near 2 kHz. The presence of a tethered molecule makes a large difference to the asymmetric cross-correlation of two cantilevers, which enables resolution of the mechanical properties in our experiments. By analysis of the correlated motion of the cantilevers we extract the friction and stiffness of the ssDNA. We find that the measured friction is much larger than the friction that is usually associated with the unencumbered motion of ssDNA. We also find that the measured relaxation time, ∼30 μs, is much greater than prior measurements of the free-molecule relaxation time. We attribute the difference to the loss of conformational possibilities as a result of constraining the ends of the ssDNA.

Multiscale Model Describing Bacterial Adhesion and Detachment

Bacterial surfaces are complex structures with nontrivial adhesive properties. The physics of bacterial adhesion deviates from that of ideal colloids as a result of cell-surface roughness and because of the mechanical properties of the polymers covering the cell surface. In the present study, we develop a simple multiscale model for the interplay between the potential energy functions that characterize the cell surface biopolymers and their interaction with the extracellular environment. We then use the model to study a discrete network of bonds in the presence of significant length heterogeneities in cell-surface polymers. The model we present is able to generate force curves (both approach and retraction) that closely resemble those measured experimentally. Our results show that even small-length-scale heterogeneities can lead to macroscopically nonlinear behavior that is qualitatively and quantitatively different from the homogeneous case. We also report on the energetic consequences of such structural heterogeneity.

A strategy for analyzing bond strength and interaction kinetics between Pleckstrin homology domains and PI(4,5)P2 phospholipids using force distance spectroscopy and surface plasmon resonance

Phospholipids are important membrane components involved in diverse biological activities ranging from cell signaling to infection by viral particles. A thorough understanding of protein-phospholipid interaction dynamics is thus crucial for deciphering basic cellular processes as well as for targeted drug discovery. For any specific phospholipid-protein binding experiment, various groups have reported different binding constants, which are strongly dependent on applied conditions of interactions. Here, we report a method for accurate determination of the binding affinity and specificity between proteins and phospholipids using a model interaction between PLC-δ1/PH and phosphoinositide phospholipid PtdIns(4,5)P2. We developed an accurate Force Distance Spectroscopy (FDS)-based assay and have attempted to resolve the problem of variation in the observed binding constant by directly measuring the bond force. We confirm the FDS findings of a high bond strength of ∼0.19 ± 0.04 nN by Surface Plasmon Resonance (SPR) data analysis, segregating non-specific interactions, which show a significantly lower K(D) suggesting tight binding.

Molecular Mechanisms of Ultrafiltration Membrane Fouling in Polymer-Flooding Wastewater Treatment: Role of Ions in Polymeric Fouling

Polymer (i.e., anionic polyacrylamide (APAM)) fouling of polyvinylidene fluoride (PVDF) ultrafiltration (UF) membranes and its relationships to intermolecular interactions were investigated using atomic force microscopy (AFM). Distinct relations were obtained between the AFM force spectroscopy measurements and calculated fouling resistance over the concentration polarization layer (CPL) and gel layer (GL). The measured maximum adhesion forces (Fad,max) were closely correlated with the CPL resistance (Rp), and the proposed molecular packing property (largely based on the shape of AFM force spectroscopy curve) of the APAM chains was related to the GL resistance (Rg). Calcium ions (Ca(2+)) and sodium ions (Na(+)) caused more severe fouling. In the presence of Ca(2+), the large Rp corresponded to high foulant-foulant Fad,max, resulting in high flux loss. In addition, the Rg with Ca(2+) was minor, but the flux recovery rate after chemical cleaning was the lowest, indicating that Ca(2+) created more challenges in GL cleaning. With Na(+), the fouling behavior was complicated and concentration-dependent. The GL structures with Na(+), which might correspond to the proposed molecular packing states among APAM chains, played essential roles in membrane fouling and GL cleaning.

Mechanically induced cis-to-trans isomerization of carbon–carbon double bonds using atomic force microscopy

Single molecule force spectroscopy can be used to induce cis-to-trans isomerization in carbon–carbon double bonds.

Buckling transition in long α-helices

The treatment of bending and buckling of stiff biopolymer filaments by the popular worm-like chain model does not provide adequate understanding of these processes at the microscopic level. Thus, we have used the atomistic molecular-dynamic simulations and the Amber03 force field to examine the compression buckling of α-helix (AH) filaments at room temperature. It was found that the buckling instability occurs in AHs at the critical force f(c) in the range of tens of pN depending on the AH length. The decrease of the force f(c) with the contour length follows the prediction of the classic thin rod theory. At the force f(c) the helical filament undergoes the swift and irreversible transition from the smoothly bent structure to the buckled one. A sharp kink in the AH contour arises at the transition, accompanied by the disruption of the hydrogen bonds in its vicinity. The kink defect brings in an effective softening of the AH molecule at buckling. Nonbonded interactions between helical branches drive the rearrangement of a kinked AH into the ultimate buckled structure of a compact helical hairpin described earlier in the literature.