Physikalische Chemie I: Forschung

Spherical Polyelectrolyte Brushes: Survey

If long chains of polyelectrolytes are affixed to planar or curved surfaces, a polyelectrolyte brush results. The term “brush” denotes a polymeric layer where the radius of gyration of the individual chains is much greater than the average distance of the grafting points.

Spherical polyelectrolyte brushes (SPB) consist of a solid core of colloidal dimensions (ca. 100nm diameter) onto which long polyelectrolyte chains have been grafted. These particles that are dispersed in water are prepared by photoemulsion polymerization [1-3]: In a first step polystyrene latex particles with typical sizes in the order of 100 nm and narrow polydispersity are prepared by conventional emulsion polymerization. In a second step a thin layer of a photo-initiator is affixed to the poly(styrene) particles (seeded emulsion polymerization). This second emulsion polymerization is carried out under starved conditions to avoid formation of new particles and to ensure a defined core shell morphology. In the third and final step the polyelectrolyte brushes are generated on the surface of the particles by photoinitiation using the surface-bound initiators in the presence of water-soluble monomer. Shining light on the latex particles lead to the radical polymerization of water-soluble monomers on the surface of these core particles ("grafting-from").

Although the latex with the surface-attached initiator presents a turbid system, the strong light scattering in such systems is almost completely elastic, so that photoinitiation can be used for such systems with satisfactory efficiency. Sufficient dilution increases the interparticle distances and ensures that no recombination of radicals growing on two different particles occurs. This is important because such a process would ultimately lead to cross-linking of the complete system [2].

In our investigations of these systems we obtained the following results:

- We demonstrated that the surface layer consisting of the weak polyelectrolyte poly(acrylic acid) swells if the pH is raised [2,4]. The swelling is less pronounced at higher ionic strength as expected from theory [3]. However, strong polyelectrolyte like poly(styrene sulfonic acid) are swollen independent of the pH in the system [4].

The counterions are mostly confined within the brush layer as predicted by theory (see [5,6] and further references given there). This can be shown by direct osmotic measurements [7];

- Cationic polyelectrolyte brushes exhibit a strong interaction with negatively charged Mica surfaces in water as shown by AFM whereas anionic systems exhibit only a weak adhesion. [6]. Polyelectrolyte chains hence can be used to modify the interaction of latex particles with given surfaces.

- Spherical polyelectrolyte brushes can be used as “nano-reactors” [9-11]. The counterions of the polyelectrolyte chains are confined to the surface layer. Hence, chemical reactions of counterions can be done directly in this layer. Reduction of e.g. AuCl₄^- ions lead to metallic nano-particles that can be used in catalysis.

- Anionic spherical polyelectrolyte brushes may be used to immobilize proteins in aqueous solution [12-13]. The adsorption takes place despite the fact that the brush particles and the proteins have an overall negative charge. The analysis demonstrates that the adsorption takes place only if the ionic strength is low in the system, virtually no adsorption is seen at high ionic strength.

- The protein molecules enter deeply into the brush layer and are evenly distributed within the brush [14]. This could be shown directly by analyzing the SPB before and after the adsorption of proteins by small-angle X-ray scattering (SAXS). These findings have been qualitatively corroborated by cryogenic transmission electron microscopy [15].

- The secondary and the tertiary structure of the adsorbed proteins is hardly disturbed [16-18]. This can be also demonstrated by monitoring the enzymatic activity of adsorbed glucoamylase [19,20].

[1] X. Guo, A. Weiss, M. Ballauff; Synthesis of Spherical Polyelectrolyte Brushes by Photo-Emulsion Polymerization, Macromolecules 1999, 32, 6043.

[2] X. Guo, M. Ballauff, The Spatial Dimensions of Colloidal Brushes As Determined by Dynamic Light Scattering, Langmuir 2000, 16, 8719.

[3] Y. Lu, A. Wittemann, M. Ballauff, M. Drechsler, Preparation of poly (styrene)-poly(N-isopropylacrylamide) (PS-PNIPA) core-shell particles by photoemulsion polymerization, Macromol. Rapid. Commun. 2006, 27, 1137.

[4] X. Guo, M. Ballauff; Spherical polyelectrolyte brushes: A comparison between annealed and quenched brushes, Phys. Rev. E 2001, 64, 051406.

[5] A. Jusufi, C. N. Likos, M. Ballauff, Counterion distribution and effective interaction in spherical polyelectrolyte brushes, Colloid Polym. Sci. 2004, 282, 910.

[6] A. Jusufi, CN. Likos, M. Ballauff, Counterion distribution and effective interaction in spherical polyelectrolyte brushes, Colloid and Polymer Science 2004, 282, 910.

[7] B. Das, X. Guo, M. Ballauff, The osmotic coefficient of spherical polyelectrolyte brushes in aqueous salt-free solution; Progr. Colloid Polym. Sci. 2002, 121, 34.

[8] Y. Mei, A. Wittemann, G. Sharma, M. Ballauff, Th. Koch, H. Gliemann, J. Horbach, Th. Schimmel; Engineering the Interaction of Latex Spheres with Charged Surfaces: AFM Investigations of Spherical Polyelectrolyte Brushes on Mica, Macromolecules 2003, 36, 3452.

[9] G. Sharma, M. Ballauff, Cationic Spherical Polyelectrolyte Brushes as Nanoreactors for the Generation of Gold Particles, Macromol. Chem. Rapid Comm. 2004, 25, 547.

[10] Y. Mei, G. Sharma, Y. Lu, M. Ballauff, M. Drechsler, T. Irrgang, R. Kempe, High Catalytic Activity of Platinum Nanoparticles Immobilized on Spherical Polyelectrolyte Brushes, Langmuir 2005, 21, 12229.

[11] Y. Lu, Y. Mei, R. Walker, M. Ballauff, M. Drechsler; "Nano-tree"-type Spherical Polymer Brushes as Templates for Metallic Nanoparticles, Polymer 2006, 47, 4985.

[12] A. Wittemann, B. Haupt, M. Ballauff; Adsorption of Proteins on Spherical Polyelectrolyte Brushes in Aqueous Solution, Phys. Chem. Chem. Phys. 2003, 5, 1671.

[13] A. Wittemann, B. Haupt, M. Ballauff, Polyelectrolyte-mediated protein adsorption, Progr. Colloid Polym. Sci. 2006, 133, 58.

[14] S. Rosenfeldt, A. Wittemann, M. Ballauff, E. Breininger, J. Bolze, N. Dingenouts, Interaction of proteins with spherical polyelectrolyte brushes in solution as studied by small-angle X-ray scattering, Physical Review E 2004, 70, 061403.

[15] A. Wittemann, M. Drechsler, Y. Talmon, M. Ballauff, High Elongation of Polyelectrolyte Chains in the Osmotic Limit of Spherical Polyelectrolyte Brushes: A Study by Cryogenic Transmission Electron Microscopy, J. Am. Chem. Soc. 2005, 127, 9688.

[16] A. Wittemann, B. Ballauff, Analysis of the secondary structure of proteins embedded in spherical polyelectrolyte brushes by FT-IR spectroscopy, Anal. Chem. 2004, 76, 2813.

[17] G. Jackler, A. Wittemann, M. Ballauff, Spherical polyelectrolyte brushes as carrier particles for proteins: An investigation of the structure of adsorbed and desorbed Bovine Serum Albumine, Spectroscopy 2004, 18, 1671.

[18] K. Anikin, C. Röcker, A. Wittemann, J. Wiedenmann, M. Ballauff; U. Nienhaus, Fluorescent Protein Binding to Individual Polyelectrolyte Nanospheres, J. Phys. Chem. B. 2005, 109, 5418.

[19] T. Neumann, B. Haupt, M. Ballauff, High Activity of Enzymes Immobilized in Colloidal Nanoreactors, Macromol. Bioscience 2004, 4, 13

[20] B. Haupt, T. Neumann, A. Wittemann, M. Ballauff, Activity of Enzymes Immobilized in Colloidal Spherical Polyelectrolyte Brushes, Biomacromolecules 2005, 6, 948.