The surface of oxygen-plasma-treated polystyrene (PSox) was investigated using X-ray photoelectron spectroscopy (XPS), streaming potential measurements and a dynamic study of the wetting properties at different pH (Wilhelmy plate method). The PSox surface is functionalized with various oxygen-containing groups, including carboxyl functions, and must be viewed as covered by a polyelectrolyte which swells depending on pH. The wetting hysteresis, its evolution upon repeated cycles and the influence of pH are controlled by the dissolution of functionalized fragments and the retention of water upon emersion; the retained water may evaporate progressively and allow macromolecule compaction and/or reorientation. Modification of the PSox surface upon aging in dry atmosphere, humid atmosphere, and water was studied using XPS and dynamic wetting measurements. Aging in water provoked the dissolution of PSox macromolecular chains, as indicated by adsorption of released fragments on a check PS sample placed nearby. However, the concentration of functionalized molecules at the surface of water-aged PSox was still sufficient to allow swelling at pH 5.6 and 11.0. Hydrophobicity recovery was faster in humid air (R. H. 95%) compared to dry air (R. H. 5%), due to the plasticizing effect of water. Hydrophobicity recovery upon aging in air was reversed quickly by immersion at pH 5.6 or 11.0, due to deprotonation and swelling.

We used atomic force microscopy (AFM) to explore the antigen binding forces of individual Fv fragments of antilysozyme antibodies (Fv). To detect single molecular recognition events, genetically engineered histidine-tagged Fv fragments were coupled onto AFM tips modified with mixed self-assembled monolayers (SAMs) of nitrilotriacetic acid- and tri(ethylene glycol)-terminated alkanethiols while lysozyme (Lyso) was covalently immobilized onto mixed SAMs of carboxyl- and hydroxyl-terminated alkanethiols. The quality of the functionalization procedure was validated using X-ray photoelectron spectroscopy (surface chemical composition), AFM imaging (surface morphology in aqueous solution), and surface plasmon resonance (SPR, specific binding in aqueous solution). AFM force-distance curves recorded at a loading rate of 5000 pN/s between Fv- and Lyso-modified surfaces yielded a distribution of unbinding forces composed of integer multiples of an elementary force quantum of approximately 50 pN that we attribute to the rupture of a single antibody-antigen pair. Injection of a solution containing free Lyso caused a dramatic reduction of adhesion probability, indicating that the measured 50 pN unbinding forces are due to the specific antibody-antigen interaction. To investigate the dynamics of the interaction, force-distance curves were recorded at various loading rates. Plots of unbinding force vs log(loading rate) revealed two distinct linear regimes with ascending slopes, indicating multiple barriers were present in the energy landscape. The kinetic off-rate constant of dissociation (k(off) approximately = 1 x 10(-3) s(-1)) obtained by extrapolating the data of the low-strength regime to zero force was in the range of the k(off) estimated by SPR.

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