AFM Colloidal Probe Research Articles

Comparison of Multilayer Formation Between Different Cellulose Nanofibrils and Cationic Polymers

The multilayer formation between polyelectrolytes of opposite charge offers possibility for creating new tailored materials. Exchanging one or both components for charged nanofibrillated cellulose (NFC) further increases the variety of achievable properties. We explored this by introducing unmodified, low charged NFC and high charged TEMPO-oxidized NFC. Systematic evaluation of the effect of both NFC charge and properties of cationic polyelectrolytes on the structure of the multilayers was performed. As the cationic component cationic NFC was compared with two different cationic polyelectrolytes, poly(dimethyldiallylammoniumchloride) and cationic starch. Quartz crystal microbalance with dissipation (QCM-D) was used to monitor the multilayer formation and AFM colloidal probe microscopy (CPM) was further applied to probe surface interactions in order to gain information about fundamental interactions and layer properties. Generally, the results verified the characteristic multilayer formation between NFC of different charge and how the properties of formed multilayers can be tuned. However, the strong nonelectrostatic affinity between cellulosic fibrils was observed. CPM measurements revealed monotonically repulsive forces, which were in good correspondence with the QCM-D observations. Significant increase in adhesive forces was detected between the swollen high charged NFC.

Release Properties of Pressurized Microgel Templated Capsules

AbstractDirection‐specific release induced through osmotic pressure inside giant microcapsules is realized and monitored at high and low concentrations of encapsulated polymer. A clear correlation between the release kinetics upon opening the shell and encapsulated polymer concentration is observed. This has been independently confirmed by shell opening via nanoindentation and laser radiation. To quantify these observations, the internal pressure of the capsules is determined by analysis of mechanical tests performed via colloidal probe AFM. As expected, larger amounts of encapsulated material lead to increased internal pressures and enhanced release kinetics. The results show how drug release can be accelerated by encapsulation of osmotic pressure generating species. Such pressurized capsules systems show large ejection velocities and are envisioned as an inexpensive biolistic transfection device for in vitro applications.

Compact Poly(ethylene oxide) Structures Adsorbed at the Ethylammonium Nitrate−Silica Interface

The adsorption of poly(ethylene oxide) (PEO) and poly(propylene oxide) (PPO) onto silica from ethylammonium nitrate (a protic ionic liquid) has been investigated using colloid probe AFM force curve measurements. Steric repulsive forces were measured for PEO, confirming that PEO can compete with the ethylammonium cation and adsorb onto silica. The range of the repulsion increases with polymer molecular weight (e.g., from 1.4 nm for 0.01 wt % 10 kDa PEO to 40 nm for 0.01 wt % 300 kDa PEO) and with concentration (e.g., from 16 nm at 0.001 wt % to 78 nm at 0.4 wt % for 300 kDa PEO). Fits to the force curve data could not be obtained using standard models for a polymer brush, but excellent fits were obtained using the mushroom model, suggesting the adsorbed polymer films are compressed and relatively poorly solvated. No evidence for adsorption of 3.5 kDa PPO could be detected up to its solubility limit.

Solvent segregation and capillary evaporation at a superhydrophobic surface investigated by confocal Raman microscopy and force measurements

Wetting of water, a 1 : 1 water/ethanol mixture and an aqueous dodecylbenzene sulfonic acid surfactant solution on hydrophobic and superhydrophobic surfaces were studied using confocal Raman microscopy. The superhydrophobic surfaces were prepared by immersion of a glass substrate in a silica particle/fluoropolymer formulation followed by silanization. Preparation of hydrophobic surfaces was done in the same way with the exception that the silica particles were excluded from the formulation. The hydrophobic and superhydrophobic surfaces were characterized with respect to surface roughness using AFM, and by contact angle measurements using different liquids. Confocal Raman microscopy measurements in a 1 : 1 water/ethanol mixture showed an enrichment of ethanol close to the superhydrophobic surface, which could not be observed for the hydrophobic surface. Unexpectedly, the Raman spectrum of a pure water film in close proximity to the superhydrophobic surface displayed some differences compared to that of bulk water and indicated a stronger hydrogen-bonding close to the superhydrophobic surface. Evidence for capillary evaporation next to the superhydrophobic surface was also found, and this results in very long-range capillary attraction between one superhydrophobic surface and a hydrophobic colloidal probe as shown by AFM colloidal probe force measurements. Addition of a surfactant or ethanol suppresses capillary evaporation.

Protein interactions with bottle-brush polymer layers: Effect of side chain and charge density ratio probed by QCM-D and AFM

Silica surfaces were coated with a range of cationic bottle-brush polymers with 45 units long poly(ethylene oxide) side chains, and their efficiency in reducing protein adsorption was probed by QCM-D, reflectometry and AFM. Preadsorbed layers formed by bottle-brush polymers with different side chain to charge ratio was exposed to two proteins with different net charge, lysozyme and BSA. The reduction in protein adsorption was found to depend on both the type of protein and on the nature of the polyelectrolyte layer. The most pronounced reduction in protein adsorption was achieved when the fraction of charged backbone segments was in the range 0.25–0.5 equivalent to a fraction of poly(ethylene oxide) side chains of 0.75–0.5. It was concluded that these polymers have enough electrostatic attachment points to ensure a strong binding to the surface, and at the same time a sufficient amount of poly(ethylene oxide) side chains to counteract protein adsorption. In contrast, a layer formed by a highly charged polyelectrolyte without side chains was unable to resists protein adsorption. On such a layer the adsorption of negatively charged BSA was strongly enhanced, and positively charged lysozyme adsorbed to a similar extent as to bare silica. AFM colloidal probe force measurement between silica surfaces with preadsorbed layers of bottle-brush polymers were conducted before and after exposure to BSA and lysozyme to gain insight into how proteins were incorporated in the bottle-brush polymer layers.

Dynamic Force Spectroscopy of Oppositely Charged Polyelectrolyte Brushes

Ion pairing is the main driving force in the formation of polyelectrolyte complexes, which find widespread use in micellar assemblies, drug carriers, and coatings. In this paper we examine the actual ion pairing forces in a polyelectrolyte complex between two oppositely charged polyelectrolyte brushes. Using colloidal probe AFM, we study the effect of salt on the strength of the model complex. We find a strong dependence of attractive force on salt concentration up to a critical salt concentration beyond which no attraction can be measured. By changing the salt concentration, the time allowed for formation of the complex, and the rate at which the complex is disrupted, we are able to measure the kinetics of complex formation and to estimate the ion pairing forces in equilibrium polyelectrolyte complexes. We use a kinetic model for the dynamics of complex disruption as a function of disruption rate to find a typical force of ion pairing in the complex. The model shows that our model polyelectrolyte complex...

Interfacial tension between a complex coacervate phase and its coexisting aqueous phase

Complex coacervation is the associative phase separation in a solution of positively and negatively charged macroions. Despite the widespread use of coacervation in e.g. micellar assemblies (complex coacervate core micelles), drug carriers and thin films, there is virtually no experimental data on the interfacial tension between such coacervate phases (polyelectrolyte complexes) and their coexisting aqueous phases or on the influence of salt thereon. In this paper we use colloidal probe AFM measurements of capillary adhesion forces to obtain the interfacial tension between a complex coacervate phase of two polyelectrolytes with high charge density and its coexisting aqueous phase. We find that the interfacial tension is of order 100 µN/m, decreases with increasing salt concentration and vanishes at the critical point. Interestingly, we find that the critical scaling exponent for the interfacial tension found in segregative demixing also applies here.

Colloid probe AFM study of thermal collapse and protein interactions of poly(N-isopropylacrylamide) coatings

Graft coatings of poly(N-isopropylacrylamide) (pNIPAM) are of considerable interest for the reversible control of bio-interfacial interactions. In this study, graft coatings were prepared by free radical polymerisation from surface-bound polymerisable groups, on silicon wafers and quartz crystal microbalance (QCM) sensors. QCM with dissipation monitoring showed a gradual, extended phase change as the temperature increased. Colloid probe atomic force microscopy (CP-AFM) revealed a marked change in the compressibility of the coating below and above the lower critical solution temperature (LCST). Force curves showed an approximate 9-fold reduction in thickness between 24 °C and 38 °C, yet the collapsed coating at 38 °C still had a thickness significantly higher than the ellipsometrically determined dry thickness, indicating a residual extent of hydration above the LCST. At all temperatures, interaction force curves showed steric repulsion, though over different distances. There was little hysteresis between approach and retract force curves, which is evidence for almost instantaneous relaxation of the coating upon decompression. CP-AFM using a probe coated with bovine serum albumin (BSA) showed repulsive interactions with little approach/retraction hysteresis below the LCST, indicating lack of adhesion between the pNIPAM coating and the BSA-coated probe. In contrast, above the LCST the force curves on retraction were characteristic of adhesion, while the approach curves were still repulsive, and the adhesion increased in strength as the temperature was increased further beyond the LCST. Thus, QCM-D and CP-AFM data correlated well in showing a gradual nature of the phase transition and a concomitant gradual change in the interaction force with BSA.

Lubrication by organized soft matter

The AFM-colloidal probe technique has been used to explore surface interactions and friction forces between polyelectrolyte-coated surfaces immersed in aqueous solutions in the absence and presence ...

Interaction Forces between Microsized Silica Particles and Weak Polyelectrolyte Brushes at Varying pH and Salt Concentration

The AFM colloidal probe technique was used to measure the interaction between microsized silica spheres and annealed polyelectrolyte brushes made of poly(acrylic acid) (PAA) and poly(2-vinyl pyridine) (P2VP) in KCl solutions of various pH values and salt concentrations. The interaction energy showed a distance dependence that was affected strongly by the swelling and the electric properties of the brushes. Between PAA brushes and silica particles, a repulsive interaction has been observed for all pH values and salt concentrations reflecting the swelling of the brush with varying pH value and the transition from osmotic to salted brush regime with increasing KCl concentration. Force measurements between P2VP brushes and silica particles revealed a much more complex behavior: a steric repulsion by the swollen brush at low pH values, a complex interplay of attractive and repulsive forces at intermediate pH values and a short-ranged attraction between the collapsed brush and the silica particle at basic pH values and high salt concentrations. The results are interpreted in comparison with the Alexander de Gennes model and zeta potential and ellipsometric measurements.

Adsorption Behavior and Adhesive Properties of Biopolyelectrolyte Multilayers Formed from Cationic and Anionic Starch

Cationic starch (D.S. 0.065) and anionic starch (D.S. 0.037) were used to form biopolyelectrolyte multilayers. The influence of the solution concentration of NaCl on the adsorption of starch onto silicon oxide substrates and on the formation of multilayers was investigated using stagnation point adsorption reflectometry (SPAR) and quartz crystal microbalance with dissipation (QCM-D). The wet adhesive properties of the starch multilayers were examined by measuring pull-off forces with the AFM colloidal probe technique. It was shown that polyelectrolyte multilayers (PEM) can be successfully constructed from cationic starch and anionic starch at electrolyte concentrations of 1 mM NaCl and 10 mM NaCl. The water content of the PEMs was approximately 80% at both electrolyte concentrations. However, the thickness of the PEMs formed at 10 mM NaCl was approximately twice the thickness formed at 1 mM NaCl. The viscoelastic properties of the starch PEMs, modeled as Voigt elements, were dependent on the polyelectrolyte that was adsorbed in the outermost layer. The PEMs appeared to be more rigid when capped by anionic starch than when capped by cationic starch. The wet adhesive pull-off forces increased with layer number and were also dependent on the polyelectrolyte adsorbed in the outermost layer. Thus, starch PEM treatment has a large potential for increasing the adhesive interaction between solid substrates to levels higher than can be reached by a single layer of cationic starch.

Simple Method for Controlled Association of Colloidal-Particle Mixtures using pH-Dependent Hydrogen Bonding

We describe a simple method for the controlled mixing of particles that could be used to produce materials with new properties. We demonstrate the procedure with sets of silica particles that have each been coated with one of two different organic thin films. One set of particles is functionalized with carboxylic acid groups and the other with ethylene oxide. Each of these sets of particles is stable in solution. When mixed, heteroaggregation can be induced reversibly and on demand simply by changing the pH. We provide evidence that control over aggregation is achieved by the ability to alter the number of hydrogen bonds between different types of particles and thus the strength of the attraction between different particles. We provide support for this mechanism by measuring the forces between a plate coated in a thin film of carboxylic groups and particles coated in ethylene oxide using colloid probe AFM. At pH 9, where we expect most of the acidic groups to be deprotonated, there is a strong repulsion between the particle and plate. However, at pH 3, the force is attractive, which we assign to the hydrogen bonding between the ether oxygen of the PEO and the hydrogen of the carboxylic acid group. Heteroflocculation occurred in the pH range of 3-4.5. At pH 5 and above, no flocculation was observed. Because the number of hydrogen bonds per surface area and therefore the strength of binding between dissimilar particles can be titrated through control of the pH, we can control the surface forces and avoid rapid coagulation that produces low density and disorganized particle arrangements. The control of interaction forces and therefore the approach of particles should allow the production of composite materials having mixture or product properties.

高分子吸着多孔体担持ＴｉＯ２ナノ粒子の光触媒作用及び樹脂保護効果のコロイドプローブＡＦＭ法による解析

TiO2 nanoparticles supported on aluminosilicate particles modified with cationic surfactant by simply mixing them in water could protect resin from photocatalytic decomposition while decomposing acetaldehyde via photocatalic reaction. When anionic polymer dispersants with different structures and molecular weights were adsorbed on TiO2 nanoparticles for the improvement of decomposition efficiency, the TiO2 nanoparticles supported on aluminosilicate particles modified with cationic surfactant exhibited protection effect of resin, depending on the structure and molecular weight of anionic polymer dispersant. This trend was not observed for the TiO2 nanoparticles dispersed by anionic polymer dispersant. The interaction between TiO2 nanoparticles and aluminosilicate surface was analyzed with the colloid probe AFM method. The protection effect of resin appears when a decrease in the adhesive force and weak repulsive force were observed.

Particulate drug interactions with polymeric and elastomeric valve components in suspension formulations for metered dose inhalers

Purpose To characterise the adhesive interactions between three pulmonary active pharmaceutical ingredient (API) materials and the components of pressurised metered dose inhalers (pMDIs) obtained from two commercially available products (termed ‘Prod-1’ and ‘Prod-2’). This is of potential interest, as a greater understanding of the interactions between specific APIs and surfaces may aid manufacturers in component selection during pMDI system development. Methods The theoretical work of adhesion (Δ G 132) for each API–pMDI component interaction was calculated using the surface component analysis (SCA) approach. These results were correlated with corresponding API–pMDI component separation energy measurements determined using colloid probe AFM. Results Strong correlations existed between separation energy and the Δ G 132 parameters where the polar contribution was accounted for. This highlighted the adhesive influence of polar surface energy on each interaction in this study. Generally the largest adhesive interactions involved APIs and pMDI components which have a bipolar surface energy (i.e. both γ − and γ + >1 mJ m −2). Conclusions For each API–pMDI interaction in this study, the polar component of surface energy has the greater influence on adhesive events. The bipolar surface energetics of certain APIs and pMDI components were deemed responsible for the increased adhesive interactions observed with these materials. This study highlights that different materials can have different effects on the adhesive interactions with particulate APIs; information that could aid the manufacturer in producing more effective and efficient pMDI systems.

Effect of Polymer Architecture on the Adsorption Properties of a Nonionic Polymer

The adsorption of a linear- and bottle-brush poly(ethylene oxide (PEO))-based polymer, having comparable molecular weights, was studied by means of quartz crystal microbalance with dissipation monitoring ability (QCM-D) and AFM colloidal probe force measurements. The energy dissipation change monitored by QCM-D and the range of the steric forces obtained from force measurements demonstrated that linear PEO forms a more extended adsorption layer than the bottle-brush polymer, despite that the adsorbed mass is higher for the latter. Competitive adsorption studies revealed that linear PEO is readily displaced from the interface by the bottle-brush polymer. This was attributed to the higher surface affinity of the latter, which is governed by the number of contact points between the polymers and the interface, and the smaller loss of conformational entropy.

Normal and friction forces between mucin and mucin–chitosan layers in absence and presence of SDS

Employing the colloidal probe AFM technique we have investigated normal and friction forces between flat mica surfaces and silica particles coated with mucin and combined mucin–chitosan layers in presence and absence of anionic surfactant, SDS, in 30 mM NaCl solution. We have shown that the normal interactions between mucin coated mica and silica surfaces are dominated by long-range steric repulsion on both compression and decompression. Friction forces between such mucin layers are characterized by a low effective friction coefficient, μ eff = 0.03 ± 0.02 , which is lower than the value of 0.13 ± 0.02 observed when chitosan layers were adsorbed. Forces between combined mucin–chitosan layers have also been measured. Adsorption of chitosan on mucin results in considerable compaction of the layer, and development of attractive forces detectable on separation. Friction between mucin–chitosan layers in 30 mM NaCl solution is high, with μ eff ≈ 0.4 . Adsorption of additional mucin to this layer results in no improvement with respect to lubrication as compared to the mucin–chitosan layer, and μ eff ≈ 0.4 is observed. We argue that the layers containing both mucin and chitosan are not strictly layered but rather strongly entangled. As a result attractive interactions between oppositely charged moieties of sialic acid residues from mucin and amine groups from chitosan residing on the opposing surfaces contribute to the increased friction. The effects of SDS on normal and friction forces between combined mucin–chitosan layers were also investigated. The relation between surface interactions and friction properties is discussed.

Top-Down Grafting of Xyloglucan to Gold Monitored by QCM-D and AFM: Enzymatic Activity and Interactions with Cellulose

This study focuses on the manufacture and characterization of model surfaces consisting of end-grafted xyloglucan (XG), a naturally occurring polysaccharide, onto a gold substrate. The now well-established XET-technology was utilized for enzymatic incorporation of a thiol moiety at one end of the xyloglucan backbone. This functionalized macromolecule was subsequently top-down grafted to gold, forming a thiol-bonded xyloglucan brushlike layer. The grafting was monitored in situ with QCM-D, and a significant difference in the adsorbed/grafted amount between unmodified xyloglucan and the thiol-functionalized polymer was observed. The grafted surface was demonstrated to be accessible to enzyme digestion using the plant endo-xyloglucanase TmNXG1. The nanotribological properties toward cellulose of the untreated crystal, brush-modified surface, and enzyme-exposed surfaces were compared with a view to understanding the role of xyloglucan in friction reduction. Friction coefficients obtained by the AFM colloidal probe technique using a cellulose functionalized probe on the xyloglucan brush showed an increase of a factor of 2 after the enzyme digestion, and this result is interpreted in terms of surface roughness. Finally, the brush is shown to exhibit binding to cellulose despite its highly oriented nature.

Colloid Probe AFM and XPS Study of Time‐Dependent Aging of Amine Plasma Polymer Coatings in Aqueous Media

AbstractEffects of surface oxidation on the properties of amine plasma polymer (pp) films in aqueous media have not been widely studied, despite their use to control bio‐interfacial properties. Changes in the surface composition of allylamine and heptylamine pp films, on water exposure, were followed by XPS. The surface potential was monitored via AFM measurements and comparison to DLVO theory. Analysis of XPS data implied high initial oxidation rates (rapid quenching of surface radicals). Oxidation of the pp films followed evolution of carbon‐based rather than nitrogen‐based species. Analysis of AFM force data showed changes in the sign of the surface potential with aging from positive to negative. Implications, for example for surface immobilization of molecules in aqueous environments, are also discussed.magnified image

Interaction Forces Between a Glass Surface and Ceria-Modified PMMA-Based Abrasives for CMP Measured by Colloidal Probe AFM

Interaction forces between a glass surface and two types of ceria-coated poly(methyl methacrylate) (PMMA)-based terpolymer abrasive particles were investigated using colloidal probe atomic force microscopy and correlated with relevant chemical mechanical planarization (CMP) and post-CMP parameters. The composite particles were achieved by either creating chemical bonds by silane coupling agents (composite A) or tuning the pH in order to form electrostatic attractive interactions between the core and shell composite (composite B). Based on the average values of the pull-off force vs pH, a qualitative agreement between the measured adhesion forces and the material removal rate (MRR) was found. For pH 3, both the MRR and adhesion forces are larger for composite B with respect to composite A. Interestingly, for pH 10, composite B and ceria give almost the same MRR and similar adhesion force values. Nevertheless, in general, the adhesion forces measured at pH 10 are significantly smaller than those monitored at pH 3, whereas the MRR is significantly larger for pH 10. This suggests that factors other than adhesion, such as an enhanced silica dissolution rate, dominate and define the MRR at pH 10. The increase in repulsive force with increasing pH corresponds to a decrease in composite-glass adhesion. For pH between 2 and 7, the electrostatic attractive forces between polymer particles and the silica surface are responsible for high particle counts. To improve the particle removal efficiency after CMP with the ceria-based slurry, pH values higher than 7 are recommended.

Influence of wettability and surface charge on the interaction between an aqueous electrolyte solution and a solid surface

The hydrodynamic drainage force of a Newtonian aqueous electrolyte solution squeezed between two surfaces of different wettability was measured using the AFM colloidal probe technique. The surface hydrophobicity, roughness, polarity and approach velocity, and thus the shearing rate of the liquid, were controlled. A direct relationship between the mobility of the aqueous electrolyte solution close to the surfaces and the hydrophobicity of the surfaces was not established. We predict that the mobility of the liquid depends in a more complex fashion on the polarity and charge of the surfaces and on the properties of the electrolyte.