Adsorption of Polyelectrolyte versus Surface Charge: in Situ Single-Molecule Atomic Force Microscopy Experiments on Similarly, Oppositely, and Heterogeneously Charged Surfaces

Boundary Lubrication Performance of Polyelectrolyte-Surfactant Complexes on Biomimetic Surfaces

Aqueous mixtures of oppositely charged polyelectrolytes and surfactants are useful in many industrial applications, such as shampoos and hair conditioners. In this work, we investigate the friction between biomimetic hair surfaces in the presence of adsorbed complexes formed from cationic polyelectrolytes and anionic surfactants in an aqueous solution. We apply nonequilibrium molecular dynamics (NEMD) simulations using the coarse-grained MARTINI model. We first developed new MARTINI parameters for cationic guar gum (CGG), a functionalized, plant-derived polysaccharide. The complexation of CGG and the anionic surfactant sodium dodecyl sulfate (SDS) on virgin and chemically damaged biomimetic hair surfaces was studied using a sequential adsorption approach. We then carried out squeeze-out and sliding NEMD simulations to assess the boundary lubrication performance of the CGG-SDS complex compressed between two hair surfaces. At low pressure, we observe a synergistic friction behavior for the CGG-SDS complex, which gives lower shear stress than either pure CGG or SDS. Here, friction is dominated by viscous dissipation in an interfacial layer comprising SDS and water. At higher pressures, which are probably beyond those usually experienced during hair manipulation, SDS and water are squeezed out, and friction increases due to interdigitation. The outcomes of this work are expected to be beneficial to fine-tune and screen sustainable hair care formulations to provide low friction and therefore a smooth feel and reduced entanglement.

Single-Molecule Desorption Studies of Poly(acrylic acid) at Electrolyte/Oxide/TiAlN Interfaces

The presented studies correlate the surface chemistry of electrochemically oxidized TiAlN hard coatings with the desorption forces of poly(acrylic acid) (PAA) at the electrolyte/oxide/TiAlN interface. Atomic force microscopy (AFM)-based single-molecule force spectroscopy (SMFS) was performed at different pH values to investigate surface chemistry-induced changes in desorption force. The chemical state was characterized by X-ray photoemission spectroscopy and electrochemical analysis. The results show that the desorption forces continuously decrease with increasing pH in the range from pH 5 to 9. The comparison of the desorption forces on rf-sputtered titanium dioxide and aluminum oxide films shows that the electrochemically oxidized surface of TiAlN, in agreement with the revealed surface composition, shows interfacial adhesive properties in contact with PAA and water that resemble a pure titanium oxide layer. Load rate-dependent measurements were performed to analyze both the free energy barrier and the transition state distance.

DNA Computing Systems Activated by Electrochemically-triggered DNA Release from a Polymer-brush-modified Electrode Array

An array of four independently wired indium tin oxide (ITO) electrodes was used for electrochemically stimulated DNA release and activation of DNA-based Identity, AND and XOR logic gates. Single-stranded DNA molecules were loaded on the mixed poly(N,N-di-methylaminoethyl methacrylate) (PDMAEMA)/poly-(methacrylic acid) (PMAA) brush covalently attached to the ITO electrodes. The DNA deposition was performed at pH 5.0 when the polymer brush is positively charged due to protonation of tertiary amino groups in PDMAE-MA, thus resulting in electrostatic attraction of the negatively charged DNA. By applying electrolysis at -1.0 V(vs. Ag/AgCl reference) electrochemical oxygen reduction resulted in the consumption of hydrogen ions and local pH increase near the electrode surface. The process resulted in recharging the polymer brush to the negative state due to dissociation of carboxylic groups of PMAA, thus repulsing the negatively charged DNA and releasing it from the electrode surface. The DNA release was performed in various combinations from different electrodes in the array assembly. The released DNA operated as input signals for activation of the Boolean logic gates. The developed system represents a step forward in DNA computing, combining for the first time DNA chemical processes with electronic input signals.

In Situ Infrared Ellipsometry for Protein Adsorption Studies on Ultrathin Smart Polymer Brushes in Aqueous Environment

The protein-adsorbing and -repelling properties of various smart nanometer-thin polymer brushes containing poly(N-isopropylacrylamide) and poly(acrylic acid) with high potential for biosensing and biomedical applications are studied by in situ infrared-spectroscopic ellipsometry (IRSE). IRSE is a highly sensitive nondestructive technique that allows protein adsorption on polymer brushes to be investigated in an aqueous environment as external stimuli, such as temperature and pH, are varied. These changes are relevant to conditions for regulation of protein adsorption and desorption for biotechnology, biocatalysis, and bioanalytical applications. Here brushes are used as model surfaces for controlling protein adsorption of human serum albumin and human fibrinogen. The important finding of this work is that IRSE in the in situ experiments in protein solutions can distinguish between contributions of polymer brushes and proteins. The vibrational bands of the polymers provide insights into the hydration state of the brushes, whereas the protein-specific amide bands are related to changes of the protein secondary structure.

Polyelectrolyte adsorption, interparticle forces, and colloidal aggregation

This review summarizes the current understanding of adsorption of polyelectrolytes to oppositely charged solid substrates, the resulting interaction forces between such substrates, and consequences for colloidal particle aggregation. The following conclusions can be reached based on experimental findings. Polyelectrolytes adsorb to oppositely charged solid substrates irreversibly up to saturation, whereby loose and thin monolayers are formed. The adsorbed polyelectrolytes normally carry a substantial amount of charge, which leads to a charge reversal. Frequently, the adsorbed films are laterally heterogeneous. With increasing salt levels, the adsorbed mass increases leading to thicker and more homogeneous films. Interaction forces between surfaces coated with saturated polyelectrolyte layers are governed at low salt levels by repulsive electric double layer interactions, and particle suspensions are stable under these conditions. At appropriately high salt levels, the forces become attractive, principally due to van der Waals interactions, but eventually also through other forces, and suspensions become unstable. This situation can be rationalized with the classical theory of Derjaguin, Landau, Verwey, and Overbeek (DLVO). Due to the irreversible nature of the adsorption process, stable unsaturated layers form in colloidal particle suspensions at lower polyelectrolyte doses. An unsaturated polyelectrolyte layer can neutralize the overall particle surface charge. Away from the charge reversal point, electric double layer forces are dominant and particle suspensions are stable. As the charge reversal point is approached, attractive van der Waals forces become important, and particle suspensions become unstable. This behaviour is again in line with the DLVO theory, which may even apply quantitatively, provided the polyelectrolyte films are sufficiently laterally homogeneous. For heterogeneous films, additional attractive patch-charge interactions may become important. Depletion interactions may also lead to attractive forces and suspension destabilization, but such interactions become important only at high polyelectrolyte concentrations.

Conformational transitions of flexible hydrophobic polyelectrolytes in solutions of monovalent and multivalent salts and their mixtures

Conformations of cationic polyelectrolytes (PEs), a weak poly(2-vinylpyridine) (P2VP) and a strong poly(N-methyl-2-vinylpyridinium iodide) (qP2VP), adsorbed on mica from saline solutions in the presence of counterions of different valences are studied using in situ atomic force microscopy (AFM). Quantitative characteristics of chain conformations are analyzed using AFM images of the adsorbed molecules. The results of the statistical analysis of the chain contour reveal collapse of the PE coils when ionic strength is in a range from tens to hundreds of millimoles per kilogram and re-expansion of the coils with a further increase of ionic strength up to a region of the saturated saline solutions. The competition between monovalent and multivalent counterions simultaneously present in solutions strongly affects conformations of PE chains even at a very small fraction of multivalent counterions. Shrinkage of PE coils is steeper for multivalent counterions than for monovalent counterions. However, the re-expansion is only incremental in the presence of multivalent counterions. Extended adsorbed coils at low salt concentrations and at very high concentrations of monovalent salt exhibit conformation corresponding to a 2D coil with 0.95 fraction of bound segments (segments in "trains") in the regime of diluted surface concentration of the PE. Shrunken coils in the intermediate range of ionic strength resemble 3D-globules with 0.8 fraction of trains. The incrementally re-expanded PE coils at a high ionic strength remain unchanged at higher multivalent salt concentrations up to the solubility limit of the salt. The formation of a strong PE complex with multivalent counterions at high ionic strength is not well understood yet. A speculative explanation of the observed experimental result is based on possible stabilization of the complex due to hydrophobic interactions of the backbone.

Facilitating high-force single-polysaccharide stretching using covalent attachment of one end of the chain

Single polysaccharide force spectroscopy has yielded particularly interesting data, the interpretation of which requires the marriage of statistical-mechanical theories of polymer physics to the complexities afforded by possible force-induced conformational transitions of the constituent sugar rings. However, the difficulty of designing handles for the specific attachment of the different ends of polysaccharide chains to substrates, such as piezoelectric scanners, cantilevers or microbeads has meant that the majority of studies to date have been carried out with the polymer physisorbed to the substrates between which it is stretched, or at best chemically attached via bonds formed at uncontrolled locations along the length of the molecule. This means that the lengths of obtained polysaccharide stretches, as well as the forces that can be placed on the molecule without generating detachment, are generally smaller than those obtainable for polymers that offer the ability to be covalently attached to substrates specifically at their ends. As a consequence it is troublesome and tedious to record a statistically significant number of force curves that extend chains to high enough forces to investigate certain conformational transitions, such as the boat-to-inverted chair, exhibited by polysaccharides such as pectin. Herein, single molecule force-extension curves have been measured for the several pectin samples using AFM. The results are compared when either (1) the polymers have been physisorbed between the cantilever and the surface of the piezo-electric scanner, under several different solvent conditions of pH and ionic strength, or (2) the polymer molecule has been chemically attached at one end to the piezo surface using a recently reported coupling procedure. In fact, using such a chemical attachment to tether the end of the polysaccharide, reduced the frequency of successful stretching events obtained in a particular location, confirming the role of surface diffusion in the physisorbed experiments. Nevertheless, when polymer stretches were successfully recorded, the force that could be applied before detachment was significantly increased, indicating that this methodology has great potential for improving the acquisition of data reporting on force-induced conformational transitions of the sugar ring that require the application of significant stresses.

AFM imaging of adsorbed Nafion polymer on mica and graphite at molecular level

Perfluorosulfonic acid ionomer (PFSA, specifically Nafion at EW = 975 g/mol) was visualized at the single molecule level using atomic force microscopy (AFM) in liquid. The diluted commercial Nafion dispersion shows an apparent M(w) = 1430 kg/mol and M(w)/M(n) = 3.81, which is assigned to chain aggregation. PFSA aggregates, imaged on mica and HOPG during adsorption from EtOH-H(2)O solvent at pH(e) 3.0 (below isoelectric point), showed a stable, segmented rod-like conformation. This structure is consistent with earlier NMR, SAXS/SANS, and TEM results that support a stiff helical Nafion conformation with long persistence length, a sharp solvent-polymer interface, and an extension of the sulfonated side chain into solution. Adsorption of Nafion structures on HOPG was observed at even higher pH(e) from EtOH due to screening of the repulsive electrostatic interaction in lower dielectric constant solvent, while the chain adopted an expanded coil conformation. These measurements provided direct evidence of the chain aggregation in EtOH-H(2)O solution and revealed their equilibrium conformations for adsorption on two model surfaces, highly ordered pyrolitic graphite (HOPG) and mica. The commercial Nafion dispersion was autoclaved at 0.10% w/w in nPrOH/H(2)O = 4:1 v/v solvent at 230 °C for 6 h to give a single-chain dispersion with M(w) = 310 kg/mol and M(w)/M(n) = 1.60. The autoclaved chains adopt an electrostatically stabilized compact globule conformation as observed by AFM imaging of the single PFSA molecules after rapid deposition on mica and HOPG at a low surface coverage.

Self-localization of polyacrylic acid molecules on polar ZnO(0001)-Zn surfaces

The adsorption of single polyacrylic acid (PAAc) molecules was investigated on stepped hydroxide-stabilized polar ZnO(0001)-Zn surfaces using atomic force microscope (AFM) topography and force distance spectroscopy. Stepped surfaces of ZnO(0001)-Zn were prepared by a wet chemical etching procedure and PAAc molecules were adsorbed from aqueous NaClO(4) solutions. AFM single molecule topography studies could be utilized to show that polyacrylic acid molecules specifically adsorb on the non-polar (10-10) step edge faces at low ionic strengths. The radius of gyration of the dissolved PAAc in aqueous solution was measured by means of static light scattering experiments yielding a radius of gyration of R(g)=136 nm at pH 7.4 in 50 mM NaClO(4)/NaOH solution, which is in good agreement with the size of the adsorbed PAAc molecules as measured using AFM. The obtained results could be rationalized in terms of binding-site configurations at step edges and the effect of the chemical environment on both local electric double layer charge and molecular conformation of the PAAc molecules. The point of zero charge of the ZnO(10-10) surface was measured with chemical force microscopy to be pH(PZC)=10.2 ± 0.2. The specific adsorption of polyacrylic acid at non-polar ZnO step-edges can be explained by coordinative bonds formed between the carboxylic acid group and the Zn-surface atoms. On the hydroxide stabilized polar surface only weak hydrogen bonds can be formed in addition to van-der-Waals forces. Thus a "diffusion and trapping" mechanism keeps the adsorbed PAAc molecules mobile on the ZnO(0001)-Zn surface terraces due to small interaction forces until they are trapped at the (10-10) step faces by stronger coordinative bonds from the carboxylic groups to zinc atoms located in the first atomic layer of the crystal structure.

Enhancement of surface wettability via the modification of microtextured titanium implant surfaces with polyelectrolytes

Micrometer- and submicrometer-scale surface roughness enhances osteoblast differentiation on titanium (Ti) substrates and increases bone-to-implant contact in vivo. However, the low surface wettability induced by surface roughness can retard initial interactions with the physiological environment. We examined chemical modifications of Ti surfaces [pretreated (PT), R(a) ≤ 0.3 μm; sand blasted/acid etched (SLA), R(a) ≥ 3.0 μm] in order to modify surface hydrophilicity. We designed coating layers of polyelectrolytes that did not alter the surface microstructure but increased surface ionic character, including chitosan (CHI), poly(L-glutamic acid) (PGA), and poly(L-lysine) (PLL). Ti disks were cleaned and sterilized. Surface chemical composition, roughness, wettability, and morphology of surfaces before and after polyelectrolyte coating were examined by X-ray photoelectron spectroscopy (XPS), contact mode profilometry, contact angle measurement, and scanning electron microscopy (SEM). High-resolution XPS spectra data validated the formation of polyelectrolyte layers on top of the Ti surface. The surface coverage of the polyelectrolyte adsorbed on Ti surfaces was evaluated with the pertinent SEM images and XPS peak intensity as a function of polyelectrolyte adsorption time on the Ti surface. PLL was coated in a uniform thin layer on the PT surface. CHI and PGA were coated evenly on PT, albeit in an incomplete monolayer. CHI, PGA, and PLL were coated on the SLA surface with complete coverage. The selected polyelectrolytes enhanced surface wettability without modifying surface roughness. These chemically modified surfaces on implant devices can contribute to the enhancement of osteoblast differentiation.

Fluorescent nanoparticles stabilized by poly(ethylene glycol) containing shell for pH-triggered tunable aggregation in aqueous environment

Fluorescent silica nanoparticles decorated with a responsive shell, a mixed polymer brush, were synthesized. Specifically, a poly(2-vinylpyridine), P2VP, and poly(ethylene glycol), PEG, binary polymer brush was synthesized on silica nanoparticles via the "grafting to" technique. The selection of the components (PEG and P2VP) for the responsive brush shell was motivated by potential biomedical applications. Poly(glycidyl methacrylate), PGMA, labeled with Rhodamine B, RhB, was used to form a reactive and fluorescent shell on the nanoparticle surface. The interaction between the particles themselves and the particles and their environment can be precisely tuned by a change in pH. At lower pH, aqueous dispersions of the particles are stable, since PEG and P2VP are water-soluble, extended and contribute to the steric and electrostatic mechanisms of colloidal stability. An increase of pH to 6 causes a slow aggregation as a consequence of the hydrophobic attraction between the collapsed and almost nonprotonated P2VP macromolecules. The aggregation was well controlled and occurred within 90-120 min of the pH change. The aggregation was fully reversible after the decrease in pH. The pH variation did not quench the fluorescence of the colloidal suspensions. The pH-tunable aggregation of the fluorescent nanoparticles could find diverse applications for labeling and contrasting of cells and tissues when the size of the label and the intensity of the optical signals can be tuned by and related to the pH of the host environment.

Specific DNA-protein interactions on mica investigated by atomic force microscopy

DNA processing by site-specific proteins on surface remains a challenging issue for nanobioscience applications and, in particular, for high-resolution imaging by atomic force microscopy (AFM). To obtain high-resolution conditions, mica, an atomically flat and negatively charged surface, is generally used. However, even though many specific DNA/protein interactions have already been observed by AFM, little is known about DNA accessibility to specific enzymes on mica. Here we measured the accessibility of adsorbed DNA to restriction endonucleases (EcoRI and EcoRV) using AFM. By increasing the concentration of divalent or multivalent salts, DNA adsorption on mica switches from weak to strong binding. Interestingly, while the accessibility of strongly bound DNA was inhibited, loosely adsorbed DNA was efficiently cleaved on mica. This result opens new perspective to study DNA/protein interaction by AFM or to modify specifically DNA on surface.

Single molecules as sensors for local molecular adhesion studies

An experimental approach is presented that allows the measurement of interactions of single macromolecules at the electrolyte/single-crystal interfaces under the continuous variation of electrolyte composition. Single polyelectrolyte desorption experiments with poly(acrylic acid) were performed on atomically defined single-crystal ZnO(0001)-Zn surfaces in aqueous electrolytes of varying pH and constant ionic strength. The corresponding characterized single-crystalline surface structures were proven to be stable in the pH range of 4-11, enabling the analysis of desorption forces on both surface terraces and step edges as a function of pH. Thereby, contributions of electrostatic and van der Waals forces as well as contributions of coordinative interfacial bonds could be distinguished. The results showed that carboxylic acid functionalities adsorb weakly to hydroxide-stabilized polar ZnO(0001)-Zn surfaces with forces in the range of 60-80 pN, whereas they strongly bind to the separating step-edges between the polar terraces, most probably via coordinative bonds exhibiting forces of up to 700 pN. Thus, by means of single-molecule desorption spectroscopy individual binding sites could be readily identified by distinct features in the force-distance profiles. Moreover, the measurement of desorption forces on the large atomically flat terraces at varying pH proved that a maximum molecular desorption force occurs at pH 7 as a result of increasing repulsive interactions at pH values above the surface point of zero charge and decreasing electrostatic interactions when shifting the pH in the direction of the pK(A) of the poly(acrylic acid).

Interaction of lipid membrane with nanostructured surfaces

Tiny details of the phospholipid (DMPC) membrane morphology in close vicinity to nanostructured silica surfaces have been discovered in the atomic force microscopy experiments. The structural features of the silica surface were varied in the experiments by the deposition of silica nanoparticles of different diameter on plane and smooth silica substrates. It was found that, due to the barrier function of the lipid membrane, only particles larger than 22 nm in diameter with a smooth surface were completely enveloped by the lipid membrane. However, nanoparticles with bumpy surfaces (curvature diameter of bumps as that of particles <22 nm) were only partially enveloped by the lipid bilayer. For the range of nanostructure dimensions between 1.2 and 22 nm, the lipid membrane underwent structural rearrangements by forming pores (holes). The nanoparticles were accommodated into the pores but not enveloped by the lipid bilayer. The study also found that the lipid membrane conformed to the substrate with surface structures of dimensions less than 1.2 nm without losing the membrane integrity. The experimental results are in accord with the analytical free energy model, which describes the membrane coverage, and numerical simulations which evaluate adhesion of the membrane and dynamics as a function of surface topology. The results obtained in this study are useful for the selection of dimensions and shapes for drug-delivery cargo and for the substrate for supported lipid bilayers. They also help in qualitative understanding the role of length scales involved in the mechanisms of endocytosis and cytotoxicity of nanoparticles. These findings provide a new approach for patterning supported lipid membranes with well-defined features in the 1.2-22 nm range.

Individual bottle brush molecules in dense 2D layers restoring high degree of extension after collapse-decollapse cycle: directly measured scaling exponent

We prepared dense films of adsorbed brush-like macromolecules on mica substrate by transfer of compressed Langmuir monolayers from water subphase. The main macromolecular contours in the dense films were clearly resolved by SFM. The films were subjected to successive treatments by ethanol and water vapours. In accordance with previous results for isolated macromolecules, the films underwent collapse and subsequent decollapse morphological transformations in the changing vapour environment. Statistical analysis of the macromolecular dimensions in the films allowed us to measure the values of the scaling exponent nu determining the correlation between mean lateral and linear dimensions of the macromolecules. The analysis showed that the macromolecular conformations in the film as transferred were similar to the previously described conformations of the same macromolecules deposited directly on mica as isolated chains at much lower surface densities. The determined nu was close to the 0.75 value corresponding to the 2D SAW statistics. We assumed that the molecules retained the high degree of extension during the compression step due to suppressed reorganisation of the side chains. Differently from previous observations for isolated macromolecules, the restored conformations in the dense films after collapse-decollapse cycle were more extended with the nu of about 0.73 value. A theoretical explanation of the high degree of re-extension is proposed.

Single nanoparticle plasmonic devices by the "grafting to" method

Hierarchically organized single-nanoparticle structures synthesized in this work consisted of a 200 nm silica core and a pH-responsive poly(2-vinylpyridine) shell decorated with 15 nm gold nanoparticles. pH changes in the range of 3-6 back and forth results in a swelling-shrinking polymer brush shell and, thus, in the tuning distance between noble nanoparticles. A change in the interparticle distance is accompanied by a very pronounced shift in the maximum wavelength of the surface plasmon absorption peak. The dispersion of the resulting composite nanoparticles reversibly changed color from red to purple-blue as the pH changed from 2.5 to 6. Such hierarchically assembled nanostructures can be used as free-standing single-particle sensors in various miniaturized analytical systems.