![]() In 8.5 ± 2% of all F–D curves we find a constant force plateau (type I). The measured interactions due to polyelectrolytes come in two characteristic types. Figure 1a shows some examples of force curves for a salt concentration of 1.0 M. ![]() When we repeat experiments with tips without any modification or with only physically adsorbed polymers, we find such interactions in <0.2% of all F–D curves. The overall fraction of force–distance (F–D) curves in which we find an attractive interaction outside the region of contact adhesion (closer than 30 nm) is 16 ± 5% for all conditions. We measure the strength of these bonds in aqueous solutions at various salt concentrations. ![]() (17, 18) Upon contact, the oppositely charged polyelectrolytes form complexes consisting of one or multiple ionic bonds. We then bring this cantilever in contact with a substrate on which an anionic poly(3-sulfopropyl methacrylate) brush (PSPMA 40) is grown, according to a previously described and calibrated procedure. (16) The polymer is covalently bound to the AFM tip with its PEO-end, which acts as a neutral, flexible linker, while the PM2VP block is positively charged. To assess the strength of ionic bonds between single molecules by AFM, we covalently attach a cationic diblock copolymer (poly( N-methyl-2-vinylpyridinium iodide)- block-poly(ethylene oxide), PM2VP 249- b-PEO 134) to a silicon nitride AFM cantilever, following an adapted method of Hinterdorfer et al.
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