Chemical force microscopy (CFM) in water was used to map the surface hydrophobicity of UV/ozone-treated poly(dimethylsiloxane) (PDMS; Sylgard 184) as a function of the storage/recovery time. In addition to CFM pull-off force mapping, we applied indentation mapping to probe the changes in the normalized modulus. These experiments were complemented by results on surface properties assessed on the micrometer scale by X-ray photoelectron spectroscopy and water contact-angle measurements. Exposure times of < or = 30 min resulted in laterally homogeneously oxidized surfaces, which are characterized by an increased modulus and a high segmental mobility of PDMS. As detected on a sub-50-nm level, the subsequent "hydrophobic recovery" was characterized by a gradual increase in the pull-off forces and a decrease in the normalized modulus, approaching the values of unexposed PDMS after 8-50 days. Lateral imaging on briefly exposed PDMS showed the appearance of liquid PDMS in the form of droplets with an increasing recovery time. Longer exposure times (60 min) led to the formation of a hydrophilic silica-like surface layer. Under these conditions, a gradual surface reconstruction within the silica-like layer occurred with time after exposure, where a hydrophilic SiOx-enriched phase formed < 100-nm-sized domains, surrounded by a more hydrophobic matrix with lower normalized modulus. These results provide new insights into the lateral homogeneity of oxidized PDMS with a resolution in the sub-50-nm range.

The synthesis and characterization of a novel nanocomposite is reported that was developed as an efficient adsorbent for the removal of toxic methylene blue (MB) and methyl violet (MV) from aqueous solution. The nanocomposite comprises hydrolyzed polyacrylamide grafted onto xanthan gum as well as incorporated nanosilica. The synthesis exploits the saponification of the grafted polyacrylamide and the in situ formation of nanoscale SiO2 by a sol-gel reaction, in which the biopolymer matrix promotes the silica polymerization and therefore acts as a novel template for nanosilica formation. The detailed investigation of the kinetics and the adsorption isotherms of MB and MV from aqueous solution showed that the dyes adsorb rapidly, in accordance with a pseudo-second-order kinetics and a Langmuir adsorption isotherm. The entropy driven process was furthermore found to strongly depend on the point of zero charge (pzc) of the adsorbent. The remarkably high adsorption capacity of dyes on the nanocomposites (efficiency of MB removal, 99.4%; maximum specific removal Qmax, 497.5 mg g(-1); and efficiency of MV removal, 99.1%; Qmax, 378.8 mg g(-1)) is rationalized on the basis of H-bonding interactions as well as dipole-dipole and electrostatic interactions between anionic adsorbent and cationic dye molecules. Because of the excellent regeneration capacity the nanocomposites are considered interesting materials for the uptake of, for instance, toxic dyes from wastewater.

The adsorption of phosphatidylcholine (PC) vesicles (30, 50, and 100 nm nominal diameters) and of dye-labeled PC vesicles (labeled with 6% Texas Red fluorophore (TR) and encapsulated carboxy fluorescein (CF)) to glass surfaces was studied by contact mode atomic force microscopy in aqueous buffer. These studies were performed in part to unravel details of the previously observed isolated rupture of dye-labeled PC vesicles on glass (Johnson, J. M.; Ha, T.; Chu, S.; Boxer, S. G. Biophys. J. 2002, 83, 3371-3379), specifically to differentiate partial rupture, that is, pore formation and leakage of entrapped dye, from full rupture to form bilayer disks. In addition, the adhesion potential of PC vesicles on glass was calculated based upon the adhesion-driven flattening of adsorbed vesicles and a newly developed theoretical model. The vesicles were found to flatten considerably upon adsorption to glass (width-to-height ratio of approximately 5), which leads to an estimate for the adhesion potential and for the critical rupture radius of 1.5 x 10(-4) J/m2 and 250 nm, respectively. Independent of vesicle size and loading with dye molecules, the adsorption of intact vesicles was observed at all concentrations below a threshold concentration, above which the formation of smooth lipid bilayers occurred. In conjunction with previous work (Johnson, J. M.; Ha, T.; Chu, S.; Boxer, S. G. Biophys. J. 2002, 83, 3371-3379), these data show that 6% TR 20 mM CF vesicles adsorb to the surface intact but undergo partial rupture in which they exchange content with the external buffer.

We have characterized the structure, molecular orientation, and crystallization kinetics of isothermally crystallized thin (film thickness d < 500 nm) and ultrathin films (d < 100 nm) of poly(ethylene oxides) on oxidized silicon substrates by a combination of microscopic and spectroscopic methods. In situ hot stage atomic force microscopy (AFM) reveals a preferred flat-on orientation of lamellar crystals in films thinner than ca. 300 nm. The mean orientation of the polymer molecules, as measured by transmission and grazing angle reflection FT-IR spectroscopy, fully agrees with the preferred orientation of the PEO helices parallel to the surface-normal direction, as inferred from the AFM data. In addition to a strong film thickness dependence of this preferred chain orientation, the FT-IR data indicate that the degree of crystallinity decreases steadily when the film thickness becomes smaller than ∼200 nm. The local environment of pyrene end-labels in derivatized PEO was characterized by steady-state fluorescence spectroscopy, and the excimer/monomer emission ratio was found to be very sensitive to both film thickness and crystallization temperature. The latter relationship could be described by an Arrhenius equation and yielded an excimer-forming-site energy of 17 ± 2 kJ/mol. Finally, the isothermal crystallization of PEO in ultrathin films was followed spectroscopically in situ. Both fluorescence and FT-IR spectroscopy indicated that the crystallization kinetics are progressively slowed down for decreasing film thickness, presumably due to the increased glass transition temperature of ultrathin PEO films on interactive substrates.

We report on the isothermal crystallization behavior of thin (film thickness d < 500 nm) and ultrathin (d < 100 nm) films of poly(ethylene oxide) (PEO), as well as pyrene end-labeled PEO, on native silicon studied by in situ hot stage atomic force microscopy (AFM). Individual lamellae were imaged during crystallization and melting. Using AFM, we have directly measured lamellar growth rates, lamellar thicknesses, and melting ranges as a function of film thickness (ca. 15->500 nm), crystallization temperature (40-62 °C), and molar mass (11-100 kg/mol). On the basis of the Hoffman-Weeks extrapolation, the Gibbs-Thomson equation, and the Hoffman-Lauritzen theory, we show that the crystallization of PEO in thin and ultrathin films can be described with the same laws as the bulk crystallization. In addition, we find that the equilibrium melting points and surface free energies of the fold surfaces agree quantitatively with literature data for bulk crystallization and hence are not altered due to confinement in ultrathin films. However, there is a monotonic decrease of lamellar growth rates with decreasing film thickness for films thinner than ca. 250 nm. The growth rates decrease to below 1% of their bulk value in the thinnest films; this is attributed to an increase in glass transition temperature of up to 30 °C for the confined PEO and the concomitant reduction of molecular mobility.

The rupture forces of individual host-guest complexes between beta-cyclodextrin (beta-CD) heptathioether monolayers on Au(111) and several surface-confined guests were measured in aqueous medium by single molecule force spectroscopy using an atomic force microscope. Anilyl, toluidyl, tert-butylphenyl, and adamantylthiols (0.2-1%) were immobilized in mixed monolayers with 2-mercaptoethanol on gold-coated AFM tips. For all guests and for all surface coverages, the force-displacement curves measured between the functionalized tips and monolayers of beta-CD exhibited single, as well as multiple, pull-off events. The histograms of the pull-off forces showed several maxima at equidistant forces, with force quanta characteristic for each guest of 39 +/- 15, 45 +/- 15, 89 +/- 15, and 102 +/- 15 pN, respectively. These force quanta were independent of the loading rate, indicating that, because of the fast complexation/decomplexation kinetics, the rupture forces were probed under thermodynamic equilibrium. The force values followed the same trend as the free binding energy Delta G degrees measured for model guest compounds in solution or on beta-CD monolayers, as determined by microcalorimetry and surface plasmon resonance measurements, respectively. A descriptive model was developed to correlate quantitatively the pull-off force values with the Delta G degrees of the complexes, based on the evaluation of the energy potential landscape of tip-surface interaction.

Supramolecular host-guest interactions in aqueous environment were studied by dynamic single molecule force spectroscopy. The unbinding between ferrocene moieties immobilized on atomic force microscopy tips and -cyclodextrin receptors in highly ordered self-assembled monolayers on Au(111) was studied. The rupture force of individual ferrocene--cyclodextrin complexes was found to be 56 ( 10 pN. The value of this unbinding force was independent of the unloading rate. This was attributed to the fast (de)complexation kinetics of the host-guest complex.

Shock wave induced cavitation experiments and atomic force microscopy measurements of flat polyamide and hydrophobized silicon surfaces immersed in water are performed. It is shown that surface nanobubbles, present on these surfaces, do not act as nucleation sites for cavitation bubbles, in contrast to the expectation. This implies that surface nanobubbles are not just stable under ambient conditions but also under enormous reduction of the liquid pressure down to −6MPa. We denote this feature as superstability.