Single Molecules of Polymers

Single molecules of polymers are the objects, which study opens new possibilities to collect important information for polymer and colloidal science. Understanding of the behavior of single polymer chains in solutions and in adsorbed state is crucial for adsorption processes, chromatography and other separation methods, design and applications of new advanced materials and technologies such as drug delivery systems, sensors, responsive materials, catalysts, miniaturized devices, lab-on-chip and microfluidic technologies.

Polymer single molecules are studied in our group using the recent advances in atomic force microscopy (AFM). AFM, as one of the major constitutive parts of scanning probe microscopy (SPM), allows for the visualization of fine conformational details of polymer molecules under air atmosphere and under liquid medium. Recently, we have succeeded in a high resolution visualization of single chains of synthetic polymers on mica surface in situ under aqueous solutions. This experience allowed us to compare the conformations of polymer chains on the mica substrate using samples prepared in different conditions and also in applied electrical field. We obtain information about equilibrium conformations of the adsorbed polymer molecules under aqueous solutions:

 

Video 1. Polymer single molecule imaging under liquid medium.


and in air after the slow drying in the atmosphere at 99% humidity, as well as, after fast drying by nitrogen stream or spin-coating for “frozen” conformations of polymer molecules:

 

Video 2. Preparation of single molecule sample for AFM-imaging under air.


2D Single Molecule Software was developed in our group for the processing of recorded images of single polymer molecules. “2D Single Molecule Software” is a freeware available upon request. 

Major Directions:
  • Study of the conformations of single flexible polyelectrolyte molecules (PE) in controlled environments;
  • Investigation of phase transitions of PE visualized as single molecule events and in molecular assemblies;
  • Study of PE adsorption as single molecule events;
  • Study of single molecule conformations for PE of various molecular architecture;
  • PE-PE complexes, and PE-biomolecule complexes.

Single molecules under aqueous medium

Single MoleculesSingle Molecule
Figure 1. Single polymer molecule images recorded using a MultiMode microscope equipped with a fluid cell. A sharp coil-to-globule transition of a poly(2-vinylpyridine) molecule upon pH change has been observed in this AFM images. (Roiter, Y.; Minko, S. J. Am. Chem. Soc. 2005,127(45), 15688-15689. Details)

Single Molecule Adsorption
Figure 2. Single molecules adsorbed on mica at lower pH values (recorded in situ). The fraction of uncovered mica surface descreases as the quantity of surface negative charges reduces and quantity of positive charges increases. Thus, positively charged polyelectrolyte prefer to adsorb only to the areas enriched with remaining negative charges, avoiding the rest of mica. (Roiter, Y.; Minko, S. (Invited paper). Journal of Physical Chemistry B 2007,111(29), 8597-8604. Details)

AFM images of polymer single molecules recorded in air

Single Molecules; Slow Drying
Figure 4. Polymer single molecules recorded in air after slow drying of samples at 99% humidity. Images were produced in the course of the sample history study for poly(methacryloyloxyethyl dimethylbenzylammonium chloride) (PMB). Samples were prepared from 0.5 mg/L PMB solutions without salt (a) and containing trisodium orthophosphate: (b) 4 mM; (c) 8 mM; and (d) 24 mM. Molecules were able to equilibrate on the surface at slow sample drying, however, their radius of gyration decreased significantly with salt addition. (Roiter, Y.; Jaeger, W.; Minko, S. (Invited PaperPolymer 200647(7), 2493-2498.Details)

Single Molecules; Fast Drying
Figure 5. Single molecules recorded in air after fast drying of samples by spin-coating. Samples were prepared from 0.5 mg/L PMB solutions without salt (a) and containing trisodium orthophosphate (24 mM) leading to the formation of PMB compact globules in solution. Molecules were “frozen” or trapped in the conformation resembling their conformation in the solution. (Roiter, Y.; Jaeger, W.; Minko, S. (Invited PaperPolymer200647(7), 2493-2498. Details)

 

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