Charged Mica Research Articles

Growth processes of poly methylmethacrylate particles investigated by atomic force microscopy

The growth processes of poly methylmethacrylate (PMMA) particles prepared by soap-free emulsion polymerization at a reaction temperature 70°C are observed using scanning electron microscopy and atomic force microscopy to clarify the growth mechanisms at the molecular scale. Here, the use of a cationic initiator, V-50, enables us to synthesize positively charged PMMA, which then adsorbs electrostatically on the negatively charged mica plate with molecular scale smoothness. As a result, the following results are found. If the monomer concentration is high, PMMA particle size increases with increasing initiator concentration, otherwise particle size decreases with increasing initiator concentration. In the former case, the growth mechanism is very similar to that of the polystyrene particles we proposed; in the latter case, the phenomenon is thought to follow the LaMer diagram.

Two-Dimensional Densely Packed DNA Nanostructure Derived from DNA Complexation with a Low-Generation Poly(amidoamine) Dendrimer

One of the keys for using deoxyribonucleic acid (DNA) as a nanomaterial relies on how the individual DNA chain can be aligned and how a multitude of DNA chains can be packed into ordered nanostructures. Here we present a simple method for constructing a 2-D densely packed DNA nanostructure using the electrostatic complex of DNA with a poly(amidoamine) (PAMAM) dendrimer of generation two. Ordered DNA arrays are formed by drop-casting an aqueous solution containing positively overcharged complexes onto mica followed by a prolonged incubation. During the incubation, the complexes tend to adsorb onto the negatively charged mica surface through electrostatic attraction. The rodlike complexes organize to form ordered arrays to increase the surface density of the adsorbed complexes and hence the attractive free energy of adsorption. The densely packed nanostructure obtained here is distinguished from the previously reported spheroid or toroid structure derived from DNA complexations with the higher-generation dendrimers.

The EcoRI−DNA Complex as a Model for Investigating Protein−DNA Interactions by Atomic Force Microscopy

Atomic force microscopy (AFM) is a technique widely used to image protein-DNA complexes, and its application has now been extended to the measurements of protein-DNA binding constants and specificities. However, the spreading of the protein-DNA complexes on a flat substrate, generally mica, is required prior to AFM imaging. The influence of the surface on protein-DNA interactions is therefore an issue which needs to be addressed. For that purpose, the extensively studied EcoRI-DNA complex was investigated with the aim of providing quantitative information about the surface influence. The equilibrium binding constant of the complex was determined by AFM at both low and high ionic strengths and compared to electrophoretic mobility shift assay measurements (EMSA). In addition, the effect of the DNA length on dissociation of the protein from its specific site was analyzed. It turned out that the AFM measurements are similar to those obtained by EMSA at high ionic strengths. We then advance the idea that this effect is due to the high counterion concentration near the highly negatively charged mica surface. In addition, a dissociation of the complexes once they are adsorbed onto the surface was observed, which is weakly dependent on the ionic strength contrary to what occurs in solution. Finally, a two-step mechanism, which describes the adsorption of the EcoRI-DNA complexes on the surface, is proposed. This model could also be extended to other protein-DNA complexes.

Real-time and Single Fibril Observation of the Formation of Amyloid β Spherulitic Structures

In Alzheimer disease, amyloid beta, a 39-43-residue peptide produced by cleavage from a large amyloid precursor protein, undergoes conformational change to form amyloid fibrils and deposits as senile amyloid plaques in the extracellular cerebral cortices of the brain. However, the mechanism of how the intrinsically linear amyloid fibrils form spherical senile plaques is unknown. With total internal reflection fluorescence microscopy combined with the use of thioflavin T, an amyloid-specific fluorescence dye, we succeeded in observing the formation of the senile plaque-like spherulitic structures with diameters of around 15 microm on the chemically modified quartz surface. Real-time observation at a single fibrillar level revealed that, in the absence of tight contact with the surface, the cooperative and radial growth of amyloid fibrils from the core leads to a huge spherulitic structure. The results suggest the underlying physicochemical mechanism of senile plaque formation, essential for obtaining insight into prevention of Alzheimer disease.

Adsorption of dimethyldodecylamine- N-oxide at the mica-solution interface studied by ellipsometry

The surface excess of dimethyldodecylamine- N-oxide (DDAO) surfactant at the mica–water interface has been determined using phase modulated ellipsometry. Surface excess versus bulk surfactant concentration isotherms were constructed at different pH values. The plateau adsorbed amount reaches a maximum in the pH range 6–7.5, which includes the native pH of DDAO solutions at the cmc (pH 7.5). At low pH values, where the surfactant is essentially cationic, the surface excess is reduced due to decrease of the mica surface charge, and due to that electrostatic repulsion between charged headgroups prevents formation of tightly packed surfactant bilayers. Adsorption from solutions at pH 9 also renders a lower plateau surface excess than that obtained at neutral pH. In this case the surfactant–substrate affinity is greatly reduced due to complete deprotonation of the headgroup of the surfactant in bulk solution. The results suggest that the degree of protonation of DDAO in the adsorbed layer differs from its bulk value. Close to the negatively charged mica surface the protonation of the headgroup is increased in order to adjust its charge to that of the surface, whereas the uncharged form is expected to be enriched in the outer part of the bilayer.

Adhesion of Spherical Polyelectrolyte Brushes on Mica: An in Situ AFM Investigation

We demonstrate that the adsorption of cationic spherical polyelectrolyte brushes (SPB) on negatively charged mica substrates can be controlled in situ by the ionic strength of the suspension. The SPB used in our experiments consist of colloidal core particles made of polystyrene. Long cationic polyelectrolyte chains are grafted onto these cores that have diameters in the range of 100 nm. These particles are suspended in aqueous solution with a fixed ionic strength. Atomic force microscopy (AFM) in suspension as well as in air was used for surface characterization. In pure water the polymer particles exhibit a strong adhesion to the mica surface. AFM investigations of the dry samples show that the particles occupy the identical positions as they did in liquid. They were not removed by the capillary forces within the receding water front during the drying process. The strong interaction between the particles and the mica surface is corroborated by testing the adhesion of individual particles on the dried surface by means of the AFM tip: after a stepwise increase of the force applied to the surface by the AFM tip, the polymer particles still were not removed from the surface, but they were cut through and remained on the substrate. Moreover, in situ AFM measurements showed that particles which adsorb under liquid in a stable manner are easily desorbed from the surface after electrolyte is added to the suspension. This finding is explained by a decreasing attractive particle-substrate interaction, and the removal of the particles from the surface is due to the significant reduction of the activation barrier of the particle desorption. All findings can be explained in terms of the counterion release force.

Negatively charged hyperbranched polyether-based polyelectrolytes.

The preparation of carboxylated hyperbranched polyglycerols of narrow polydispersity was achieved by modification (78–90%) of the hydroxyl end groups via Michael addition of acrylonitrile, followed by hydrolysis. High conversion could only be achieved for low molecular weight starting materials (520 and 1,030 g mol−1). The solution properties of the resulting materials were investigated by dynamic light scattering (DLS), showing the formation of large aggregates with size depending on the pH value. After deposition on a negatively charged mica surface, the structures observed by atomic force microscope (AFM) show the coexistence of aggregates and single macromolecules. Most interesting, in the case of the lower molecular weight sample (PG 520 g mol−1), extended and ordered terrace structures were formed, which are unprecedented for hyperbranched polymers and are of interest for surface modification in general.

Adsorption and flocculation behaviors of polydiallyldimethylammonium (PDADMA) salts: Influence of counterion

Two modified polymer flocculants of PDADMA salts, PDADMA nitrate (PDADMAN) and PDADMA sulfate (PDADMAS), were prepared from the most widely used commercial product of polydiallyldimethylammonium chloride (PDADMAC). Solution properties such as conductivity and viscosity were investigated, showing that the conductivity follows the order of PDADMAS > PDADMAC > PDADMAN, while the reduced viscosity increases in the order of PDADMAS < PDADMAC < PDADMAN. The results indicate that, compared with PDADMAC, PDADMAN has a more cationic density and a more extended polymer chain, whereas PDADMAS exhibits a smaller coiled polymer size because of the stronger charge affinity of sulfate ion to the polycation of PDADMA. Flocculation experiments were performed on kaolin suspensions. It was found that PDADMAN is the most efficient in turbidity removal, while PDADMAS has wider optimum dosages, larger floc size and more rapid settling rate. Atomic force microscopy (AFM) was employed for imaging the adsorption structures of the three polymers on the negatively charged mica surface. Remarkable differences in molecular conformation were observed. Compared with the pearl necklace-like aggregation of hemispheroids of PDADMAC polymer, PDADMAN takes a sponge-like thin layer structure and PDADMAS exhibits a uniform hemispherical structure. The above differences in both the solution property and the adsorption and flocculation behaviors are attributable to counterion effects. Different counterions may lead to different adsorption and flocculation mechanisms that PDADMAN adopts a flat and layer–layer adsorption without loops and tails extended from the surface, implying that a charge neutralization mechanism plays more roles, while adsorbed PDADMAS is in the form of coiled spheres with loops and tails so that a bridging mechanism is more involved. Namely, it is the adsorption structure and morphology of the polymer flocculants that is the main factor influencing the flocculation mechanism.

Generation of nanostructures of mica supported lysozyme molecules in aqueous solution by atomic force microscopy

Nanostructures of lysozyme molecules adsorbed to mica were generated by the tip of an atomic force microscope in contact, tapping, and force-distance mode in aqueous solution. In contact mode at high ionic strength and adjusted lysozyme concentration a monolayer of defined pattern and orientation could be formed by the scan process of the tip. A lysozyme monolayer with minimal pattern size of about 60 nm was achieved by line scan. At larger loading forces besides a monolayer also 3D-aggregates of lysozyme molecules could be generated. In force-distance mode the volume of 3D-aggregates grows with increasing generation time, lysozyme concentration in the bulk phase, loading force, and frequency of up- and down-movement of the substrate toward the fixed cantilever. In tapping mode 3D-aggregates could be generated as well. It is postulated that reduction of electrostatic interaction between the oppositely charged lysozyme molecules and mica surface by sufficient high ionic strength is essential for monolayer formation. It is discussed that for the underlying mechanism of monolayer generation in contact mode lysozyme molecules of the bulk phase adsorb to the tip, become pulled off and attach to the mica surface by the scan process of the tip.

Kaolin-Coating of Stylolites, Effect on Quartz Cementation and General Implications for Dissolution at Mineral Interfaces

Abstract The deeply buried (4 km, 160°C) quartz-rich sandstones of the Middle Jurassic Garn Formation in well 6507/7-1 offshore mid-Norway typically contain 20–30% quartz cement and only 3–9% porosity. The exception is a less than 1.5 m-thick interval at the top of the formation where quartz cement volumes typically drop to 8–16% and porosities are in the range 7–19%. The uppermost part of the Garn Formation is not isolated from the rest of the formation by any type of impermeable barrier; the sandstones are water-filled indicating that the change in cementation does not correspond to a hydrocarbon contact; and, there are no significant changes in grain size, grain coating abundance, grain mineralogy or other factors that could give a lower quartz surface area for development of quartz overgrowths within the uppermost 1.5 m of the sandstones. However, in the samples from the less cemented interval, illitization of authigenic kaolin is either not detectable or very restricted compared to deeper down in the cores where illitization of kaolin is usually complete. We propose that the cause of reduced quartz cementation in the top of the Garn Formation may be the coating of stylolites developed from primary illitic clay-rich and micaceous laminae with authigenic kaolin as quartz cementation proceeds and the authigenic kaolin-rich rock adjacent to the stylolites is dissolved. Although we do not have evidence indicating that this may be a common phenomenon of major importance for preservation of reservoir quality, our observations may help to unravel the mechanisms of quartz dissolution at stylolites. Specifically, we suggest that the lack of significant surface charge on kaolin compared to illite and mica may lead to a reduced thermodynamic drive for quartz dissolution at kaolin-quartz contacts. Similarly, lack of dissolution at clean quartz-quartz surfaces may be due to smaller and equal surface charges setting up oppositely directed electric fields that cancel each other, whereas the positive surface charge of carbonate minerals may actually enhance the electric fields from negatively charged illitic clay and mica, which may explain the extensive dissolution often seen at stylolites in limestones.

Nucleation and Growth Process of Polystyrene Particle Investigated by AFM

The nucleation and growth process of the polystyrene particles prepared by soap-free emulsion polymerization at the reaction temperature of 70°C are observed by dynamic light scattering (DLS), scanning electron microscopy (SEM) and atomic force microscopy (AFM). Here, the use of a cationic initiator enables us to synthesize the positively charged polystyrene, which then adsorbs electrostatically on the negatively charged mica plate with molecular scale smoothness. As a result, the followings are found.1. In the nucleation process, the monomers are polymerized by initiators and form polymers, and the polymers are coagulated and probably make micelle-like embryos, and then nuclei are generated. The nuclei absorb monomers in the bulk to form particles, such that the surfaces of the formed particles increase hydrophobicity. The hydrophobic particles are coagulated by their hydrophobic attractions. Through these processes, mono-dispersed particles are formed.2. In the growth of the mono-dispersed particles, they absorb the monomers in the bulk and swelling, and then become extremely soft. As the reaction time increases, the rigidity of the particles changes from soft to hard by their polymerization. In the present experimental system, polymeric materials are not born in the bulk continuously because of a small amount of initiators. That is why the growth by their deposition to the particle surface, so called hetero coagulation, can not be expected.

Fate of Prions in Soil: Adsorption Kinetics of Recombinant Unglycosylated Ovine Prion Protein onto Mica in Laminar Flow Conditions and Subsequent Desorption

Prions can be disseminated in soils. Their interaction with soil minerals is a key factor for the assessment of risks associated with the transport of their infectivity. We did not examine here the infectivity itself but the adsorption kinetics of an ovine recombinant prion protein (ovine PrPrec), as a noninfectious model protein, on muscovite mica, a phyllosilicate with surface properties analogous to soil clays, in conditions of laminar flow through a channel. The protein was labeled with (125)I, and its adsorption examined between pH 4.0 and 9.0. At wall shear rate 100 s(-1), we found the process to be controlled mainly by transport at the beginning of the adsorption process. Additional experiments at 1000 s(-1) (pH 5 and 6) determined that the diffusion coefficient was in accordance with the hydrodynamic radius measured by size exclusion chromatography. The pseudo-plateau of the interfacial concentration with time was compatible with more than a monolayer and suggests the presence of aggregates. Desorption was not observed in the presence of buffer between pH 4 and 9 and sheep plasma, while the addition of alkaline detergent or 10(-1) M NaOH allowed an almost complete removal from the interface. The ensemble of results suggests that the largely irreversible adsorption of the ovine PrPrec onto mica is mainly due to electrostatic attraction between the protein and the highly negatively charged mica surface. Possible consequences for the mode of dissemination of prion proteins in soils are indicated.

New synthetic method and ionic conductivity of Na-4-mica

If interlayer cations of micas can move in the interlayer, the micas show ionic conductivity. However, K-type fluorophlogopite (KMg3AlSi3O10F2) which is the representative synthetic mica and the main crystal of mica ceramics is known as an electrical insulator. If interlayer cations of micas are smaller than K+ ion which is the interlayer cation of K-type fluorophlogopite and a larger amount of cation is stuffed in the interlayer, the micas have a higher possibility as ionic conductors. As one of such micas, highly charged sodium fluorophlogopite mica (Na4Mg6Al4Si4O20F4·xH2O) which is called “Na-4-mica” is nominated. But there are no studies on its ionic conductivity. In this study, Na-4-mica could be synthesized by new and simple method which consisted of mixing of MgO, Al2O3, SiO2 and NaF, calcination, sieving, compaction and firing in a sealed platinum container. The conductivity of the obtained Na-4-mica increased with an increase in temperature and reached 4.3×10−4 S/cm at 650 °C.

Mucin−Chitosan Complexes at the Solid−Liquid Interface: Multilayer Formation and Stability in Surfactant Solutions

The adsorption of a biologically important glycoprotein, mucin, and mucin-chitosan complex layer formation on negatively charged surfaces, silica and mica, have been investigated employing ellipsometry, the interferometric surface apparatus, and atomic force microscopy techniques. Particular attention has been paid to the effect of an anionic surfactant sodium, dodecyl sulfate (SDS), with respect to the stability of the adsorption layers. It has been shown that mucin adsorbs on negatively charged surfaces to form highly hydrated layers. Such mucin layers readily associate with surfactants and are easily removed from the surfaces by rinsing with solutions of SDS at concentrations > or =0.2 cmc (1 cmc SDS in 30 mM NaCl is equal to 3.3 mM). The mucin adsorption layer is negatively charged, and we show how a positively charged polyelectrolyte, chitosan, associates with the preadsorbed mucin to form mucin-chitosan complexes that resist desorption by SDS even at SDS concentrations as high as 1 cmc. Thus, a method of mucin layer protection against removal by surfactants is offered. Further, we show how mucin-chitosan multilayers can be formed.

Molecular-scale observation of the surface of polystyrene particles by AFM

The surface morphology of polystyrene particles prepared by soap-free emulsion polymerization was observed in situ using the tapping mode of an atomic force microscope (AFM). Here, the use of a cationic initiator enabled us to synthesize positively charged polystyrene, which then adsorbed electrostatically onto the negatively charged mica plate with molecular-scale smoothness. The following was found from AFM measurements in water. The surface of polystyrene particles changed from smooth to rough as the polymerization proceeded in the experimental condition, where the bulk generated tiny particles continuously, because of the heterocoagulation in the growth process between the newborn and tiny particles and the existing particles. However, it is easy to prepare the particles with smooth surfaces on the condition of rich monomers because the monomers in the bulk dissolve the polymers which form the surface morphology to be deformable and reduce the interfacial area.

Direct atomic force microscopy observations of monovalent ion induced binding of DNA to mica.

Multivalent ions in solution are known to mediate attraction between two like-charged molecules. Such attraction has proved useful in atomic force microscopy (AFM) where DNA may be immobilized to a mica surface facilitating direct imaging in liquid. Theories of DNA immobilization suggest that either 'salt bridging' or fluctuation in the positions of counter ions about both the mica surface and DNA backbone secure DNA to the mica substrate. Whilst both theoretical and experimental evidence suggest that immobilization is possible in the presence of divalent ions, very few studies identify that such immobilization is possible with monovalent ions. Here we present direct AFM evidence of DNA immobilized to mica in the presence of only monovalent ions. Our data depict E. coli plasmid pBR322 adsorbed onto the negatively charged mica both after short (10 min) and long (24 h) incubation periods. These data suggest the need to re-explore current theories of like-charge attraction to include the possibility of monovalent interactions. We suggest that this DNA immobilization strategy may offer the potential to image natural processes with limited immobilization forces and hence enable maximum conformational freedom of the immobilized biomolecule.

Effect of Sodium Dodecyl Sulfate on Adsorbed Layers of Branched Polyethylene Imine

The effect of an anionic surfactant, sodium dodecyl sulfate, SDS, on the properties of adsorbed layers of a cationic highly branched polyelectrolyte, polyethylene,imine, PEI, has been investigated using surface force measurements, atomic force microscopy, and ESCA. The strategy used was to preadsorb PEI on negatively charged muscovite mica surfaces from a 20 ppm polyelectrolyte solution in 10 mM NaCl. The adsorption of PEI under these conditions gives rise to a strong recharging of the surface, i.e. after adsorption of PEI the surface carries a net positive charge. This conclusion was supported by ESCA measurements. Despite the high molecular weight of PEI (1.8 x 10(5) g/mol), the adsorbed layer is found to be very flat, and a repulsive double-layer force dominates at large separations. At smaller separation a strong attractive force is the main feature. Addition of SDS to a concentration of 0.01 cmc results in a nearly complete removal of the double-layer force, an increase in compressed layer thickness, and an increase in the magnitude of the adhesion force. Hence, significant SDS incorporation in the adsorbed layer occurs already at this low surfactant concentration. A recharging of the layer occurs at higher surfactant concentrations. At 0.5 cmc, the compressed thickness of the adsorbed layer is increased significantly and the adhesion between the surfaces has disappeared. We argue that each layer now is coated with an outer SDS layer oriented with the polar groups toward solution. In fact, there are striking similarities between the interfacial association between SDS and PEI and the adsorption of an ionic surfactant to an oppositely charged mineral surface. AFM imaging reveals that the adsorbed PEI layer is somewhat nonhomogeneous and contains polyelectrolyte patches. These surface features are smoothed out by addition of SDS to a concentration of 6 mM (1 cmc in 10 mM NaCl). ESCA measurements showed that exposure of the preadsorbed PEI layer to SDS concentrations around the cmc resulted in a limited desorption; about 15% of the initially adsorbed polyelectrolyte was removed after 16 h.

Molecular-Scale Observation of Formation of Nuclei in Soap-Free Polymerization of Styrene

To clarify the mechanism of the nucleation process in the soap-free polymerization of styrene in water, the polymerization process of styrene mainly at a temperature 46 degrees C was investigated on a molecular scale using an atomic force microscope (AFM) as well as a scanning electron microscope (SEM) and a dynamic light scattering apparatus (DLS). A cationic initiator was employed to make polymerized styrene adsorb electrostatically on the negatively charged mica plate with molecular-scale smoothness, and the mechanism was estimated from their AFM measurements in situ in water. The following was found. There exist two streams to produce PSL particles in the polymerization: one is the polymerization in monomer droplets fragmented by mixing from the main monomer reservoir floating above the solution, and the other is the polymerization of monomers dissolved in the solution by initiators. The former is normally neglected or not recognized because of the small contribution. The latter is the main production process of PSL particle, which is investigated in detail on a molecular scale, and a possible mechanism was proposed.

Preparing Contamination-free Mica Substrates for Surface Characterization, Force Measurements, and Imaging

Due to its perfect cleavage that provides large areas of molecularly smooth, chemically inert surfaces, mica is the most commonly used natural substrate in measurements with the surface forces apparatus (SFA), in atomic force microscopy (AFM), and in many adsorption studies. However, preparing mica surfaces that are truly clean is not easy since mica is a high-energy surface that readily adsorbs water, organic contaminants, and gases from the atmosphere. Mica can also become charged on cleaving, which makes it prone to picking up oppositely charged particles or mica flakes from the surroundings. High refractive index particles, such as metals, will adhere to mica through van der Waals forces. Recent articles have demonstrated that particle contamination is obtained when inappropriate cutting and handling procedures for the mica are used. In this paper, we show that both particle and other critical contamination is easy to detect and provide proper steps to take during the sample preparation process.

Normal and Shear Forces between Mica and Model Membrane Surfaces with Adsorbed Hyaluronan

Using a surface forces apparatus, we have measured the normal and shear forces between surfaces containing physisorbed hyaluronic acid (HA), which is a major component of synovial fluid and which is believed to play a role in joint lubrication. The adsorption mechanisms involved the ionic binding of negatively charged HA (i) to positively charged surfactant bilayers supported on mica substrates and (ii) to negatively charged mica surfaces via Ca2+ bridges. Our results show that physisorbed HA fails as a lubricant when surfaces are pressed strongly together and/or sheared due to the squeezing out of the HA from between the surfaces. On the basis of these and previous studies, we conjecture that HA might function as an effective boundary biolubricant, exhibiting both low friction and low wear, but only if it is chemically or specifically bound exclusively to each surface, but not to both surfaces.