Detection of a Single Molecule Adsorption Structure of Poly(ethylenimine) Macromolecules by AFM

In this study, tapping mode scanning force microscopy was applied to characterize the distribution and adsorption structure of poly(ethylenimine) (PEI) macromolecules adsorbed onto negatively charged polystyrene latexes as colloidal model systems and onto molecularly flat mica surfaces as reference systems. On both surfaces, PEI macromolecules can be reproducibly mapped by tapping mode AFM, yielding molecular resolution without sample degradation despite the only relatively weak noncovalent coupling of the polyelectrolytes to the substrate surface. We are able to quantify their lateral dimensions as a function of the corresponding molecular weight. The lateral dimension of the adsorbed PEI macromolecules (60 down to 20 nm) on both types of substrates are in fair agreement with the diameters as measured by dynamic light scattering for the respective molecules in solution. Their adsorption structure is patch-like flat in the dried state under ambient air. However, mica and polystyrene surfaces result in a large difference in the height of the adsorbed macromolecules, which we interpret as being due to the grossly different surface charge densities of the substrates. Quasi-elastic light scattering (QELS) on PEI-covered polystyrene latexes in solution yields essentially the same heights of the adsorbed macromolecules as found by AFM in the dried state in ambient air. This indicates that there is no appreciable collapse upon drying at ambient conditions and further backs the notion of a dense patchlike adsorption structure in solution. These findings are discussed with respect to implications for the flocculation mechanism relevant for PEI.