Colloidal Probe Atomic Force Microscopy Research Articles

Characterization of local hydrophobicity on sapphire (0001) surfaces in aqueous environment by colloidal probe atomic force microscopy

Sapphire (0001) surfaces exhibit a phase-separation into hydrophobic and hydrophilic domains upon high-temperature annealing, which were previously distinguished by the thickness of adsorbed water layers in air using atomic force microscopy (AFM). To characterize their local surface hydrophobicity in aqueous environment, we used AFM equipped with a colloidal probe and measured the local adhesive force between each sapphire domain and a hydrophilic SiO2 probe surface, or a hydrophobic polystyrene one. Two data acquisition modes for statistical analyses were used: one is force measurements at different positions of the surface and the other repeated measurement at a fixed position. We found that adhesive force measurements using the polystyrene probe allow us to distinctly separate the hydrophilic and hydrophobic domains. The dispersion in the force measurement data at different positions of the surface is larger than that in the repeated measurements at a fixed position. It indicates that the adhesive force measurement is repeatable although their data dispersion for the measurement positions is relatively large. From these results, we can conclude that the hydrophilic and hydrophobic domains on the sapphire (0001) surfaces are distinguished by a difference in their hydration degrees.

Enhanced specific ion effects in ethylene glycol-based thermoresponsive polymer brushes

The thermoresponse of poly(di(ethyleneglycol) methyl ether methacrylate) (PMEO2MA) brushes has been investigated in the presence of monovalent anions at either end of the Hofmeister series using ellipsometry, neutron reflectometry (NR) and colloid probe atomic force microscopy (AFM). NR measurements in deuterium oxide showed no evidence of vertical phase separation perpendicular to the grafting substrate with a gradual transition between a block-like, dense structure at 45°C and an extended, dilute conformation at lower temperatures. All three techniques revealed a shift to a more collapsed state for a given temperature in kosmotropic potassium acetate solutions, while more swollen structures were observed in chaotropic potassium thiocyanate solutions. No difference was observed between 250mM and 500mM thiocyanate for a 540Å brush studied by ellipsometry, while the lower molecular weight ∼200Å brushes used for NR and AFM measurements continued to respond with increasing salt concentration. The effect of thiocyanate on the temperature response was greatly enhanced relative to PNIPAM with the shift in temperature response at 250mM being five times greater than a PNIPAM brush of similar thickness and grafting density.

Molecular insight into the nanoconfined calcite–solution interface

Little is known about the influence of nanoconfinement on calcium carbonate mineralization. Here, colloidal probe atomic force microscopy is used to confine the calcite-solution interface with a silica microsphere and to measure Derjaguin-Landau-Verwey-Overbeek (DLVO) and non-DLVO forces as a function of the calcium concentration, also after charge reversal of both surfaces occurs. Through the statistical analysis of the oscillatory component of a strong hydration force, the subnanometer interfacial structure of the confined atomically flat calcite is resolved in aqueous solution. By applying a mechanical work, both water and hydrated counterions are squeezed out from the nanoconfined solution, leaving the calcite surface more negatively charged than the analogous unconfined surfaces. Layer size and applied work allow a distinction between the hydration states of the counterions in the Stern layer; we propose counterions to be inner- and outer-sphere calcium ions, with a population of inner-sphere calcium ions larger than on unconfined calcite surfaces. It is also shown that the composition of the nanoconfined solution can be tuned by varying calcium concentration. This is a fundamental study of DLVO and hydration forces, and of their connection, on atomically flat calcite. More broadly, our work scrutinizes the greatly unexplored relation between surface science and confined mineralization, with implications for diverse areas of inquiry, such as nanoconfined biomineralization, CO2 sequestration in porous aquifers, and pressure solution and crystallization in confined hydrosystems.

Modular assembly of superstructures from polyphenol-functionalized building blocks.

The organized assembly of particles into superstructures is typically governed by specific molecular interactions or external directing factors associated with the particle building blocks, both of which are particle-dependent. These superstructures are of interest to a variety of fields because of their distinct mechanical, electronic, magnetic and optical properties. Here, we establish a facile route to a diverse range of superstructures based on the polyphenol surface-functionalization of micro- and nanoparticles, nanowires, nanosheets, nanocubes and even cells. This strategy can be used to access a large number of modularly assembled superstructures, including core-satellite, hollow and hierarchically organized supraparticles. Colloidal-probe atomic force microscopy and molecular dynamics simulations provide detailed insights into the role of surface functionalization and how this facilitates superstructure construction. Our work provides a platform for the rapid generation of superstructured assemblies across a wide range of length scales, from nanometres to centimetres.

Zero-Dimensional and Highly Oxygenated Graphene Oxide for Multifunctional Poly(lactic acid) Bionanocomposites

The unique strengths of 2D graphene oxide nanosheets (GONSs) in polymer composites are thwarted by nanosheet agglomeration due to strong intersheet attractions. Here, we reveal that shrinking the planar size to 0D graphene oxide quantum dots (GOQDs), together with the intercalation of rich oxygen functional groups, reduces filler aggregation and enhances interfacial interactions with the host polymer. With poly(lactic acid) (PLA) as a model matrix, atomic force microscopy colloidal probe measurements illustrated that a triple increase in adhesion force to PLA was achieved for GOQDs (234.8 nN) compared to GONSs (80.4 nN), accounting for the excellent exfoliation and dispersion of GOQDs in PLA, in contrast to the notable agglomeration of GONSs. Although present at trace amount (0.05 wt %), GOQDs made a significant contribution to nucleation activity, mechanical strength and ductility, and gas barrier properties of PLA, which contrasted the inferior efficacy of GONSs, accompanied by clear distinction in film...

Relating Nanoparticle Shape and Adhesiveness to Performance as Flotation Collectors

Cationic polystyrene-core-poly(n-butyl methacrylate)-shell (PS–PB) nanoparticles perform as flotation collectors as they spontaneously adsorb onto 43 μm glass beads in water, promoting glass bead attachment to air bubbles. Under our flotation conditions at room temperature, polystyrene is a hard plastic, whereas, with glass transition near room temperature, poly(n-butyl methacrylate) is a soft polymer. Colloidal probe atomic force microscopy measurements revealed that the pull-off forces and the work of adhesion of PS–PB nanoparticles to glass were significantly higher than observed with harder PS particles. Glass bead recovery in laboratory flotation experiments increased significantly with thickness of the soft PB shells on the PB–PS core/shell nanoparticles. Ninety-two nm Janus particles consisting of one PS and one PB lobe were also very effective collectors. We propose that high nanoparticle/glass bead adhesion minimizes nanoparticle removal by bead/bead collisions (nanoscale ball milling) during mix...

Colloidal Properties of Recombinant Spider Silk Protein Particles

Colloidal particles have been prepared from polyanionic and polycationic recombinant spider silk protein. The amino acid sequences of these spider silk proteins are identical except for 16 residues bearing either a cationic or an anionic ionizable group. Electrophoretic titration showed that protonation of the acidic and basic amino acids had significant impact on the electrophoretic mobility of the protein particles and, in particular, on their point of zero mobility (PZM). The experimentally determined PZMs are in good agreement with the theoretical values evaluated on the basis of the relevant amino acid sequences. A comprehensive description of the electrokinetic properties of the recombinant spider silk protein particles as a function of pH and solution ionic strength was provided from adequate application of electrokinetic theory for soft particles. Within the framework of this formalism, spider silk protein particles are viewed as porous colloids penetrable for ions and characterized by a finite penetration length for the electroosmotic flow. The differentiated electrokinetic properties of the particles were shown to be solely governed by the electrohydrodynamic features of their poorly charged outer peripheral layer with a thickness of about 10–20 nm. This finding was further corroborated experimentally by demonstrating that electrokinetics of particles bearing an additional outer layer consisting of oppositely charged spider silk proteins is entirely dominated thereby. The presence of a fuzzy, ion-permeable particle interface with an extension of several tenths of a nanometer was confirmed by direct measurement of the resulting steric forces using the colloidal probe atomic force microscopy (AFM) technique.

Interaction forces between coal and kaolinite particles measured by atomic force microscopy

A fundamental investigation was carried out on coal–coal, kaolinite–kaolinite, and coal–kaolinite interactions using the atomic force microscopy colloidal probe technique at different pH and calcium concentrations to explore the effect of kaolinite on fine coal flotation in the present study. The results showed that the kaolinite particles seem to be always detrimental to the flotation process. In deionized water, a repulsive force was always detected in the kaolinite–kaolinite interaction while kaolinite-coating phenomenon was not observed on a coal surface under the neutral pH. However, the repulsive force made it more difficult for the ultra-fine kaolinite particles to settle in the tailing thickener and the unsettled particles returned to the flotation flowsheet, which decreased the flotation efficiency because of the water entrainment. With the addition of Ca2+, the repulsive electrostatic force between kaolinite and kaolinite decreased. The attractive van der Waals force could dominate the kaolinite–kaolinite interactions, enhancing the sedimentation classification performance of fine kaolinite in tailing thickener. However, the interaction forces between coal and kaolinite changed from being weakly repulsive to strongly attractive and increased monotonously, resulting in the occurrence of the kaolinite-coating phenomenon during flotation.

Comparison of the cohesion-adhesion balance approach to colloidal probe atomic force microscopy and the measurement of Hansen partial solubility parameters by inverse gas chromatography for the prediction of dry powder inhalation performance

The abilities of the cohesive-adhesive balance approach to atomic force microscopy (AFM) and the measurement of Hansen partial solubility parameters by inverse gas chromatography (IGC) to predict the performance of carrier-based dry powder inhaler (DPI) formulations were compared. Five model drugs (beclometasone dipropionate, budesonide, salbutamol sulphate, terbutaline sulphate and triamcinolone acetonide) and three model carriers (erythritol, α-lactose monohydrate and d-mannitol) were chosen, giving fifteen drug-carrier combinations. Comparison of the AFM and IGC interparticulate adhesion data suggested that they did not produce equivalent results. Comparison of the AFM data with the in vitro fine particle delivery of appropriate DPI formulations normalised to account for particle size differences revealed a previously observed pattern for the AFM measurements, with a slightly cohesive AFM CAB ratio being associated with the highest fine particle fraction. However, no consistent relationship between formulation performance and the IGC data was observed. The results as a whole highlight the complexity of the many interacting variables that can affect the behaviour of DPIs and suggest that the prediction of their performance from a single measurement is unlikely to be successful in every case.

Micromechanical Analysis of the Hyaluronan-Rich Matrix Surrounding the Oocyte Reveals a Uniquely Soft and Elastic Composition

The cumulus cell-oocyte complex (COC) matrix is an extended coat that forms around the oocyte a few hours before ovulation and plays vital roles in oocyte biology. Here, we analyzed the micromechanical response of mouse COC matrix by colloidal-probe atomic force microscopy. We found that the COC matrix is elastic insofar as it does not flow and its original shape is restored after force release. At the same time, the COC matrix is extremely soft. Specifically, the most compliant parts of in vivo and in vitro expanded COC matrices yielded Young’s modulus values of 0.5 ± 0.1 Pa and 1.6 ± 0.3 Pa, respectively, suggesting both high porosity and a large mesh size (≥100 nm). In addition, the elastic modulus increased progressively with indentation. Furthermore, using optical microscopy to correlate these mechanical properties with ultrastructure, we discovered that the COC is surrounded by a thick matrix shell that is essentially devoid of cumulus cells and is enhanced upon COC expansion in vivo. We propose that the pronounced nonlinear elastic behavior of the COC matrix is a consequence of structural heterogeneity and serves important functions in biological processes such as oocyte transport in the oviduct and sperm penetration.

Development of in situ soft colloidal probe atomic force microscopy for probing the adhesion between wood extractives and model surfaces

A new method to attach colloidal particles in situ directly onto atomic force microscopy (AFM) cantilevers was developed and used as colloidal probes. In this method, cantilever surfaces were hydrophobically functionalised and the colloids adhered without glue in an aqueous environment. Experiments measuring the interaction forces between colloidal probes, composed of either wood extractives or a range of model compounds normally present in wood extractives, and glass, cellulose, and chromium surfaces were carried out. Wood extractive colloids exhibited the strongest adhesion to glass control surface (7.0 mN m−1), followed in decreasing order by triolein, abietic acid and oleic acid colloids (3.9 mN m−1, 2.2 mN m−1 and 1.2 mN m−1 respectively). Adhesion onto a cellulose surface was strongest for triolein colloids (8.1 mN m−1) followed by oleic acid colloids (6.1 mN m−1), while adhesion forces for abietic acid and wood extractives colloids were similar (4.9 mN m−1 and 4.3 mN m−1, respectively). When attached to a chromium surface adhesion forces of abietic acid and wood extractives were strongest and similar (3.6 mN m−1 and 3.9 mN m−1, respectively); the weakest interaction at this surface was recorded for oleic acid colloids (0.8 mN m−1). Similar adhesion forces for wood extractive colloids and abietic acid colloids (onto cellulose and chromium surfaces) reinforces the proposal that abietic acid occupies the outer layer of wood extractive colloids. The differences in the interaction between the colloids and the cellulose and chromium surface helps to explain the transfer of resin acid exclusively over triolein or oleic acid from a cellulose paper surface onto chromium plated printing cylinders.

Near-second-order transition in confined living-polymer solutions.

We analyze a near-second-order transition occurring in solutions of living polymers confined by two parallel surfaces in equilibrium with a reservoir solution. The molecular self-consistent field theory in the regime of weak adsorption or depletion is mapped to phenomenological Landau theory, where the order parameter is the average degree of polymerization or, equivalently, the normalized chain-end concentration. The distance between two surfaces at which the transition occurs scales as ℓ(c)(2)|c| where ℓ(c) is the correlation length of the polymer solution in the reservoir and c(-1) is de Gennes adsorption length. In the second half of the paper we focus on experimentally observable features. The predicted transition can be detected experimentally by probing the living-polymer mediated disjoining potential between surfaces by means of, e.g., colloidal probe atomic force microscopy. To facilitate experimental investigations we derive simple explicit expressions for the disjoining potential for several regimes. By comparison with full numerical calculations it was verified that these are quite accurate.

Poly(ethylene oxide) Mushrooms Adsorbed at Silica-Ionic Liquid Interfaces Reduce Friction.

The adsorbed layer conformation and lubricity of 35, 100, and 300 kDa PEO adsorbed to ionic liquid (IL)-silica interfaces from 0.01 wt % solutions have been investigated using colloid probe atomic force microscopy. The ILs used were propylammonium nitrate (PAN) and 1-butyl-3-methylimidazolium tetrafluoroborate ([BMIM][BF4]), which are protic and aprotic ILs, respectively. Normal force curves reveal steric interactions consistent with adsorbed polymer layers which are best fit using the mushroom model. Friction measurements show that the adsorbed polymer layer markedly reduces friction compared to surfaces sliding in the pure ILs and that lubricity increases with polymer length. When polymer is adsorbed to the sliding surfaces, friction is controlled by the creation and disruption of intermolecular interactions between entangled chains and the dragging of polymer chains through the interpenetration region. These experiments show that added polymer can reduce friction while maintaining the useful properties of ILs as lubricants.

Boundary flow on end-grafted PEG brushes.

We investigated the boundary conditions for flow of a Newtonian liquid over soft interfaces by measuring hydrodynamic drainage forces with colloid probe atomic force microscopy in a viscous liquid. The investigated soft surfaces are end-grafted brushes of thiolated poly(ethylene glycol) (PEG), of molecular weight 1k and 30k, grafted-to gold. The conditions for brush preparation were optimized as to meet the stringent conditions required for surface force measurements, namely reproducible and uniform surface composition and roughness. The fit of a slip model to the experimental data returned a slip length of 16 nm on the PEG 1k brush and 25 nm on the PEG30k brush. The slip length can be interpreted as a penetration length, which accounts for flow within the top half of the brush for the PEG30k case, and within the brush and surface roughness for the PEG1k case. These findings confirm earlier simulation studies by our group on the flow of liquids within polymer brushes.

周波数変調原子間力顕微鏡を用いた溶液環境下のナノスケール電位・電荷密度計測

Surface charges on nanometer-scale structures in liquids play an essentially important role in various physical, chemical and biochemical phenomena such as ionic adsorptions, microscopic electrode reactions and specific ligand bindings. These molecular-scale interactions are mediated through local electric double layers (EDLs) formed by the counter ions in electrolyte solution. Although static-mode atomic force microscopy (AFM) including colloidal probe AFM is a powerful technique for surface charge density measurements and EDL analysis on a submicron scale in liquids, precise surface charge density analysis with molecular resolution has not been realized because of its limitation of the resolution and the electrical force detection sensitivity. Here recent molecular-scale surface charge measurements of self-assembled micellar structures and biomolecules by three-dimensional (3D) force mapping based on frequency modulation AFM are presented.

Experimental and computational assessment of F-actin influence in regulating cellular stiffness and relaxation behaviour of fibroblasts

In biomechanics, a complete understanding of the structures and mechanisms that regulate cellular stiffness at a molecular level remain elusive. In this paper, we have elucidated the role of filamentous actin (F-actin) in regulating elastic and viscous properties of the cytoplasm and the nucleus. Specifically, we performed colloidal-probe atomic force microscopy (AFM) on BjhTERT fibroblast cells incubated with Latrunculin B (LatB), which results in depolymerisation of F-actin, or DMSO control. We found that the treatment with LatB not only reduced cellular stiffness, but also greatly increased the relaxation rate for the cytoplasm in the peripheral region and in the vicinity of the nucleus. We thus conclude that F-actin is a major determinant in not only providing elastic stiffness to the cell, but also in regulating its viscous behaviour. To further investigate the interdependence of different cytoskeletal networks and cell shape, we provided a computational model in a finite element framework. The computational model is based on a split strain energy function of separate cellular constituents, here assumed to be cytoskeletal components, for which a composite strain energy function was defined. We found a significant influence of cell geometry on the predicted mechanical response. Importantly, the relaxation behaviour of the cell can be characterised by a material model with two time constants that have previously been found to predict mechanical behaviour of actin and intermediate filament networks. By merely tuning two effective stiffness parameters, the model predicts experimental results in cells with a partly depolymerised actin cytoskeleton as well as in untreated control. This indicates that actin and intermediate filament networks are instrumental in providing elastic stiffness in response to applied forces, as well as governing the relaxation behaviour over shorter and longer time-scales, respectively.

Effect of surface hydrophobicity on short-range hydrophobic attraction between silanated silica surfaces

The interaction forces between silanated silica surfaces without nanobubbles were measured using colloidal probe atomic force microscopy (AFM). To obtain hydrophobic surfaces without nanobubbles, an aqueous solution was introduced between the surfaces following an exchange process involving several solvents. In the obtained approaching force curves, an attractive force was observed from a distance of 10–20nm, which is an additional attractive force stronger than typical van der Waals attractions. When the surface hydrophobicity decreased, the range of this attraction decreased slightly; the attraction disappeared when the surface contact angle was below 90°. In contrast, measurements in the water–ethanol mixtures revealed that the attraction persisted even when the contact angle was well below 90°. The possible origin of this force was discussed on the basis of the obtained results.

Structure Governs the Deformability of Polymer Particles in a Microfluidic Blood Capillary Model.

Particle stiffness is a design parameter that affects bionano interactions, including biodistribution kinetics and cellular processing. Herein, we develop soft polysaccharide (hyaluronic acid, HA) replica particles and capsules with tunable stiffness and sizes similar to human red blood cells (RBCs) via atom transfer radical polymerization-mediated continuous assembly of polymers (CAPATRP) and investigate their stiffness and deformability using colloidal-probe atomic force microscopy (CP-AFM) and a microfluidic blood capillary model, respectively. We demonstrate that HA replica particles and capsules with comparable nanoscale stiffness exhibit significantly different behaviors in a microfluidic blood capillary model. HA capsules behaved as RBCs, while HA replica particles had difficulty passing through the capillaries. These results (i) demonstrate how flow-based deformability measurements can be used to complement nanoscale stiffness measurements and (ii) provide important insight into the role of particle structure on the flow-based deformability of soft replica particles and capsules in a physiologically relevant microfluidic model.

Abstracts from The Aerosol Society Drug Delivery to the Lungs 25Edinburgh International Conference Centre Edinburgh, Scotland, UK December 10–12, 2014

Abstracts from The Aerosol Society Drug Delivery to the Lungs 25Edinburgh International Conference Centre Edinburgh, Scotland, UK December 10–12, 2014

Role of hydrophobicity on the adsorption of synovial fluid proteins and biolubrication of polycarbonate urethanes: Materials for permanent meniscus implants

Abstract Both absorption and release of synovial fluid components lubricate the porous natural meniscus, whereas only adsorption can lubricate non-porous meniscus prostheses. The aim of this study was to establish the adsorption characteristics of the synovial fluid proteoglycan 4 (PRG4) and albumin on modified and unmodified polycarbonate urethane (PCU) and determine the effects on the coefficient of friction. PCU was modified with surface-tethered C18 chains (mPCU). Self-assembled monolayers (SAM) on gold were also used to generate higher and lower hydrophobicities. Protein adsorption and coefficients of friction were measured by quartz crystal microbalance and colloidal probe atomic force microscope. PRG4 formed a thick viscoelastic layer and significantly decreased the coefficient of friction on PCU and mPCU, with an exceptionally low coefficient of friction measured on mPCU (0.02 ± 0.02) due to its soft surface. Albumin formed a thin rigid layer with a much higher coefficient of friction on mPCU (1.14 ± 0.19). Albumin blocked PRG4 adsorption when simultaneously added to PCUs, and coefficients of friction of 0.48 ± 0.24 (PCU) and 0.49 ± 0.17 (mPCU) were measured. Albumin adsorption on hydrophobic substrates (water contact angle ⩾70° ± 4°) dramatically increased the coefficient of friction (3.41 ± 1.21 on hydrophobic SAM), indicating that increased hydrophobicity through hydrocarbon surface modification of PCU carries tribological risks.