The role of the pH conditions of growth on the bioadhesion of individual and lawns of pathogenic Listeria monocytogenes cells

The work of adhesion that governs the interactions between pathogenic Listeria monocytogenes and silicon nitride in water was probed for individual cells using atomic force microscopy and for lawns of cells using contact angle measurements combined with a thermodynamic-based harmonic mean model. The work of adhesion was probed for cells cultured under variable pH conditions of growth that ranged from pH 5 to pH 9. Our results indicated that L. monocytogenes cells survived and adapted well to the chemical stresses applied. For all pH conditions investigated, a transition was observed in the generation time, physiochemical properties, biopolymer grafting density and bioadhesion for cells cultured in media adjusted to pH 7 of growth. In media with pH 7, the generation time for the bacterial cells was lowest, the specific growth rate constant was highest, the cells were the most polar, cells displayed the highest grafting density of surface biopolymers and the highest bioadhesion to silicon nitride in water represented in terms of the work of adhesion. When compared, the work of adhesion values quantified between silicon nitride and lawns of L. monocytogenes cells were linearly correlated with the work of adhesion values quantified between silicon nitride and individual L. monocytogenes cells.     

New insights on growth mechanisms of protein clusters at surfaces: an AFM and simulation study

Despite its relevance to a wide range of technological and fundamental areas, a quantitative understanding of protein surface clustering dynamics is often lacking. In inorganic crystal growth, surface clustering of adatoms is well described by diffusion-aggregation models. In such models, the statistical properties of the aggregate arrays often reveal the molecular scale aggregation processes. We investigate the potential of these theories to reveal hitherto hidden facets of protein clustering by carrying out concomitant observations of lysozyme adsorption onto mica surfaces, using atomic force microscopy, and Monte Carlo simulations of cluster nucleation and growth. We find that lysozyme clusters diffuse across the substrate at a rate that varies inversely with size. This result suggests which molecular scale mechanisms are responsible for the mobility of the proteins on the substrate. In addition the surface diffusion coefficient of the monomer can also be extracted from the comparison between experiments and simulations. While concentrating on a model system of lysozyme-on-mica, this 'proof of concept' study successfully demonstrates the potential of our approach to understand and influence more biomedically applicable protein-substrate couples.     

Platinum ion uptake by dendrimers: an NMR and AFM study

The reaction of generation 2 and generation 4 poly(amidoamine) (PAMAM) dendrimers with K2PtCl2 was studied by several NMR methods. The time dependency of the Pt(II) complexation was followed with 195Pt NMR for both dendrimers and the equilibrium product was further characterized with (1)H NMR, and indirectly detected 13C NMR, in the case of the generation 2 dendrimer. After 2 days, a black precipitate of Pt(0) was observed, half the original 195Pt signal was lost, and approximately 20% of the initial Pt(II) was coordinated to the tertiary and secondary nitrogens of the generation 2 dendrimer. The uptake of Pt(II) by the generation 4 dendrimer was much slower, consistent with the steric crowding of the surface groups on the generation 4 dendrimer compared to the more open generation 2. After 10 days, 80% of the Pt(II) was deep within the generation 4 dendrimer; the remaining 20% was unreacted or bound near the surface nitrogens of a single dendrimer. The location and time course of the platinum ion uptake by the dendrimers provides valuable insight into the formation of Pt(0) nanoparticles made in the presence of dendrimers as stabilizers, visualized by atomic force microscopy.     

The mechanics of contact and adhesion of periodically rough surfaces

An analytical model based on the Johnson–Kendall–Roberts (JKR) theory of adhesion was used to study the contact mechanics and adhesion of periodically rough surfaces. The relation of the applied load to the contact area and the work of adhesion W was found in closed form for arbitrary surface profiles. Our analysis showed that when the parameter [where α* is a numerical constant of order one, β is the aspect ratio of a typical surface profile (or asperity), and ρ is the number of asperities per unit length], the surfaces will jump into contact with each other with no applied load, and the contact area will continue to expand until the two surfaces are in full contact. The theory was then extended to the non‐JKR regime in which the region where the surface forces act is no longer confined to a small region near the contact zone. Exact solution was also obtained for this case. An exact analysis of the effect of entrapped air on the mechanics of adhesion and contact was also enacted. The results showed that interaction between asperities should be taken into consideration in contact‐mechanics models of adhesion or friction. © 2001 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 39: 1195–1214, 2001     

Initial stages of growth of poly(p-xylylene) coatings: AFM study

The morphology of poly(p-xylylene) ultrathin films prepared by vapor deposition polymerization on the surface of single-crystal silicon (100) and on the cleaved surface of mica at a substrate temperature of 20°C has been studied by atomic force microscopy. At the initial stage, the growth of the poly(p-xylylene) coating follows the island mechanism. Within the framework of pyramidal model of island growth, the mean diffusion length for monomer p-xylylene is calculated: For the single-crystal silicon, this parameter is 15 ± 3 nm; for the cleaved surface of mica, 9 ± 2 nm. The nature of the substrate and defects on its surface show a peculiar effect on the structure of the poly(p-xylylene) coating. Thus, at a low monomer flow, nucleation of polymer islands on the surface of silicon is predominantly homogeneous, whereas on the cleaved surface of mica, it is heterogeneous. A change in the monomer flow significantly affects the rate of nucleation of polymer islands.     

The role of few-asperity contacts in adhesion

The surface roughness of a few asperities and their influence on the work of adhesion is of scientific interest. Macroscale and nanoscale adhesion data have seemingly given inconsistent results. Despite the importance of bridging the gap between the two regimes, little experimental work has been done, presumably due to the difficulty of the experiment needed to determine how small amounts of surface roughness might influence adhesion data lying in between the two scales. To investigate the role of few-asperity contacts in adhesion, the pull-off force was measured between different sized atomic-force microscope (AFM) tips (with different roughnesses) and sample surfaces that had well-controlled material properties. There were seventeen tips of four different types, with radii from 200 nm to 60 microm. The samples were unpatterned single crystal silicon with a chemical silicon dioxide surface resulting from a standard silicon wafer clean. Some of the samples were treated with a few angstroms of vapor deposited diphenylsiloxane. We observed that the uncorrected (for surface roughness) pull-off force was independent of the radius of the AFM tip, which was contrary to all continuum-mechanics model predictions. To explain this behavior, we assumed that the interactions between the AFM tip and sample were additive, material properties were constant, and that the AFM tip, asperities, and sample surfaces were of uniform density. Based on these assumptions, we calculated a simple correction due to the measured root mean square (RMS) surface roughness of the AFM tips. The simple correction for the RMS surface roughness resulted in the expected dependence of the pull-off force on radius, but the magnitudes were higher than expected. Commercial and heat-treated AFM tips have minimal surface roughness and result in magnitudes that are more reliable. The relative uncertainty for the pull-off force was estimated to be 10%. In this paper, we derive how the cantilever and tip parameters contribute to the measured pull-off force and show how the corrected results compare with theory. Although much work is still needed, the work presented here should advance the understanding of adhesion between the macroscale and nanoscale regimes.     

A real-time AFM study on crystal nucleation and growth in nanodroplets of isotactic polypropylene

Isotactic polypropylene (iPP) nanodroplets were prepared by using the classical droplet method in this study. The formation of nanodroplets allowed the controlled observation of polymer nucleation as well as access to crystal growth at exceptionally high supercooling in iPP. Three cases including the heterogeneous nucleation and fast crystallization in iPP droplets, the formation of multiple independent homogeneous nuclei within a single droplet and a single nucleus within a single droplet were detected by using atomic force microscopy (AFM) during gradually cooling after remelting the nanodroplets. Moreover, it is found that when the volume of droplet is larger than the value of ca. 130000 nm3, the first case was observed. Otherwise, the latter two cases appeared. The temperature at which the onset of nucleation was observed in individual droplets was found to be mainly dependent on height of the droplets when the size scale of the droplet is comparable to the size of the critical nucleus in at least one dimension, which indicates the nucleation behavior under confinement.     

Homophilic interactions between cadherin fragments at the single molecule level: an AFM study

We report measurements of the adhesion forces between single E-cadherin fragments anchored on solid surfaces. These fragments consist of the two outermost extracellular domains of the protein. The specificity of the measured rupture forces was demonstrated by Ca2+ exchange experiments. Two series of experiments were performed using two linkers of different rigidity and length. We find that the pull-off force is distributed with a maximum value independent of the linker and logarithmically dependent on the velocity of separation of the two surfaces. Our dynamical results are compatible with previous flow chamber experiments performed with the same fragments and can be compared from a different perspective with previously reported AFM experiments on the full-length extracellular domain of the VE-cadherin. Interestingly, using a rigid linker, we have been able for the first time to evidence the deformation of the cadherin molecule under mechanical stress, a piece of information not accessible with more classical grafting strategies.     

The role of the hydrophobic force in bilayer adhesion and fusion

The Surface Forces Apparatus technique was used for measuring the adhesion, deformation, and fusion of bilayers supported on mica. The technique allows the molecular rearrangements to be followed in real time during the fusion process, and the most important forces involved to be identified. The adhesion between two bilayers can be increased by two orders of magnitude if they are thinned so as to expose more hydrophobic groups. For all the bilayer systems studied a single basic fusion mechanism was found in which the bilayers do not “overcome” the short-range repulsive steric-hydration forces; instead, local bilayer deformations allow these repulsive forces to be “bypassed”. The results further indicate that the most important force leading to the direct fusion of bilayers is the hydrophobic attraction acting between the hydrophobic interiors of bilayers (1, 2).     

The role of molecular diffusion in the adhesion of elastomers

Butyl rubber (polyisobutylene-co-isoprene) mixed with polyisobutylene was crosslinked to yield elastomeric macromolecular networks containing dissolved linear macromolecules. Adhesion of these materials to themselves (self-adhesion) and to an inert substrate was investigated over a wide range of peel rates and test temperatures. Greatly enhanced self-adhesion was found when linear polyisobutylene molecules of high molecular weight were present, but the strength of adhesion to a rigid inert substrate was hardly affected. The enhancement of self-adhesion is attributed to interdiffusion of polyisobutylene molecules. It was greatest at intermediate peel rates and temperatures, becoming insignificant at extremely low rates, probably because the diffusing species can then migrate readily, and at high effective rates of peel when the polymer approaches the glassy state and the strength of adhesion is high in all cases. A transition to somewhat lower levels of adhesion at relatively high rates of peel is tentatively ascribed to the onset of molecular fracture in place of pullout. The presence of large amounts of low-molecular-weight polyisobutylene (M?_v = 50,000 g/mol) increased the level of self-adhesion and of adhesion to an inert substrate to a similar degree, over a broad range of peel rates. This effect is attributed primarily to enhanced viscous losses in the elastomeric layer during separation. Application of these results to crack and weld-line healing in glassy plastics is discussed.     

The role of the pressing-on in the adhesion of elastic particles

One has carried out an analysis of the influence of the preliminary pressing-on on components of the adhesion force of different natures for the case of the contact of elastic particles with a rigid substrate. Two pressing-on variants are considered; as, - those realized under the effect of a dynamic and a static load. It has been shown that an increase in the adhesion of elastic particles to the surface resulting from the preliminary pressing-on is attributable to an increase in the adhesion force component due to a double electric layer in contact.     

The role of polymer spacers in specific adhesion

We study the role of flexible spacers in specific adhesion from the point of view of polymer reaction--diffusion theory. By assuming that the interactions between complementary adhesion moieties occur on a length scale much smaller than the size of the polymer spacer, we describe in detail binding and rupture between two opposing surfaces. Predictions are given for the physical properties of interest such as the time evolution of bond density and the ranges of attraction and unbinding. We also discuss the dynamic crossover between reversible and irreversible bridging.     

The role of temperature on the structure and dimensionality of MOFs: an illustrative study of the formation of manganese oxy-bis(benzoate) structures

Three new manganese oxy-bis(benzoate) compounds, synthesized by increasing the reaction temperature, with increasing dimensionality and decreasing Mn-Mn distances, indicate a possible pathway through an entropy driven dehydration route.     

Protection of photosystem II against UV-A and UV-B radiation in the cyanobacterium Plectonema boryanum: the role of growth temperature and growth irradiance

Plectonema boryanum UTEX 485 cells were grown at 29 degrees C and 150 mumol m-2 s-1 photosynthetically active radiation (PAR) and exposed to PAR combined with ultraviolet-A radiation (UV-A) at 15 degrees C. This induced a time-dependent inhibition of photosystem II (PSII) photochemistry measured as a decrease of the chlorophyll a fluorescence ratio, Fv/Fm, to 50% after 2 h of UV-A treatment compared to nontreated control cells. Exposure of the same cells to PAR combined with UV-A + ultraviolet-B radiation (UV-B) caused only a 30% inhibition of PSII photochemistry relative to nontreated cells. In contrast, UV-A and UV-A + UV-B irradiation of cells cultured at 15 degrees C and 150 mumol m-2 s-1 had minimal effects on the Fv/Fm values. However, cells grown at 15 degrees C and lower PAR irradiance (6 mumol m-2 s-1) exhibited similar inhibition patterns of PSII photochemistry as control cells. The decreased sensitivity of PSII photochemistry of P. boryanum grown at 15 degrees C and 150 mumol m-2 s-1 to subsequent exposure to UV radiation relative to either control cells or cells grown at low temperature but low irradiance was correlated with the following: (1) a reduced efficiency of energy transfer to PSII reaction centers; (2) higher levels of a carotenoid tentatively identified as myxoxanthophyll; (3) the accumulation of scytonemin and mycosporine amino acids; and (4) the accumulation of ATP-dependent caseinolytic proteases. Thus, acclimation of P. boryanum at low temperature and moderate irradiance appears to confer significant resistance to UV-induced photoinhibition of PSII. The role of excitation pressure in the induction of this resistance to UV radiation is discussed.     

Lubrication properties of bottle-brush polyelectrolytes: an AFM study on the effect of side chain and charge density

The effect of side chain to charge ratio on the frictional properties of adsorbed layers formed by bottle-brush polyelectrolytes with poly(ethylene oxide) side chains has been investigated. The brush polyelectrolytes were preadsorbed from 0.1 mM NaNO(3) solutions onto mica and silica surfaces; the interfacial friction was then measured in polyelectrolyte-free solutions via AFM (with the silica surface acting as the colloidal probe). It was concluded that the decisive factor for achieving favorable lubrication properties is the concentration of nonadsorbing poly(ethylene oxide) side chains in the interfacial region. However, contrary to what may be expected, the results showed that an ideal brush layer structure with the adsorbed polymers adopting comb-like conformation is not necessary for achieving a low coefficient of friction in the asymmetric mica-silica system. In fact, the lowest coefficient of friction (<0.01) under applied pressures as high as 30 MPa was observed for a system with a side chain to charge ratio of 9:1, incapable of forming brush-like layers.     

The role of molecular diffusion in the adhesion of EPDM and EPR elastomers

Adhesion of lightly crosslinked sheets of EPDM (ethylene–propylene–diene terpolymer) to themselves and to a Mylar substrate has been investigated over wide ranges of peel rate and test temperature. The effect of incorporating ethylene–propylene copolymer (EPR) before crosslinking, to yield a loose macromolecular network containing dissolved linear EPR macromolecules, was also studied. The self-adhesion of these materials was found to be much greater than their adhesion to Mylar, over a wide range of effective peel rates. This is attributed to interdiffusion of EPR and EPDM molecular strands. At extremely low peel rates the enhancement of adhesion was smaller, probably because of back-diffusion, and at high rates, the strength of adhesion became high in all cases. These results are compared to those obtained previously for polyisobutylene-co- isoprene networks containing linear polyisobutylene molecules. The enhancement of self-adhesion at intermediate rates of peel was considerably greater for the EPDM-based materials, probably because of a lower degree of crosslinking and a greater tendency to form molecular entanglements.     

AFM studies of metal deposition: instantaneous nucleation and the growth of cobalt nanoparticles on boron-doped diamond electrodes.

In situ atomic force microscopy (AFM) is used to study the growth of cobalt nuclei on a boron doped diamond electrode under potentiostatic control. The rate of growth of the nuclei at the electrode surface is monitored using AFM as a function of time at different deposition potentials. The nucleation of cobalt nuclei is found to be "instantaneous" and the growth of the nuclei is shown to be kinetically rather than diffusionally controlled over periods of tens and hundreds of seconds. At very short times (<10 seconds) the kinetics of nucleation are apparent.