Visualization of Single Polyelectrolyte Molecules

INTRODUCTION The field of single molecule research offers possibilities to study physico-chemical processes at a level of single molecules and single molecule events, and to design surfaces and devices operating with single molecules. Most of our knowledge was collected from the study of ensembles of molecules when properties are averaged over large number of molecules. Single molecule study allows to prove our knowledge and to address problems which can not be solved with investigations of ensembles of molecules. Particular, it concerns conformations of individual macromolecules in solvent and adsorbed on a flat surface, conformational transitions, analytic, mechanical properties, polymer-polymer, polymer-biomolecules, polymer – colloid interactions, etc. Manipulating single molecules we get "unlimited" possibilities to design surface properties of materials. We use recent advances of atomic force microscopy (AFM) to study polymers at the scale of single molecules. We study conformations and conformational transitions for single polyelectrolyte molecules deposited on flat surfaces. We try to obtain the information about conformations of the molecules in solution and changes of the conformation introduced when the molecules adsorb on flat substrates. Polyelectrolytes (PE) attract continuously enhanced interest due to their importance in industry and nature. Gelation of PE in water and a reversible swelling or shrinking of PE gels responding on external stimuli is considered to be one of the most promising properties of PE. At the heart of this phenomenon lies the coil-to-globule transition (CGT). It is well known that in good solvent PE chains display an extended coil conformation due to the Coulomb repulsion between charged units. A decrease of the charge density results in a collapse transition of PE chains to a globule conformation due to the short range van der Waals interactions. Theoretical analysis predict intermediate necklace-like conformations of PE chains, when a weakly charged PE chain adopts a conformation resembling a sequence of polymer beads interconnected by narrow strings. The theory suggests that a decrease of the charge density leads to a cascade of abrupt transitions between necklaces with different numbers of beads. There are numerous attempts to confirm experimentally this prediction. However, it has been difficult to obtain fully conclusive results about mechanism of CGT in a single PE chain, because the competition between single-chain events and an aggregation of several chains was always present under experimental conditions. In addition, conventional methods, such as laser light and neutron scattering, sedimentation and conductivity provide the information referring to the ensemble average over many polymer chains.