Aim and background
Polyelectrolyte multilayers (PEM) are known since the pioneerining work of Decher (Thin Solid Films, 210/211, 831 (1992)) in the early nineties. Since then scientific papers appeared with an exponentially growing number versus year. PEM can be easily deposited on nearly every material surface and shape (planar substrate, fibers, particles etc.) by the consecutive adsorption from polycation (PC) and polyanion (PA) solutions. Different fundamental issues are addressed by the polymer and colloid community like e.g. the growth mechanism, overcompensation, location of the counterions and “fuzzy” or ordered internal structure as well as applied issues like e.g. biomedical surface modification or generation of specific layered architectures for optical, electronic or sensor applications. In that context our research activities with PEM are going in three main directions:
- Variation of parameters concentration, pH and ionic strength to control deposition
- Creation of defined PEM nanostructures by incorporation of stiff PEL to generate oriented PEM nanostructures
- PEM/Protein and PEM/chiral probe interaction studies aiming at protein location (surface or bulk phase), protein selectivity and enantiospecifity.
Materials and Methods
Flexible and stiff polyelectrolytes (PEL) are used like branched poly(ethyleneimine) (PEI, Mw = 750.000 g/mol), poly(diallyldimethylammonium chloride) (PDADMAC, Mw = 250.000 g/mol), poly(L-lysine) (PLL, Mw = 20.000-300.000 g/mol), PAC poly(acrylic acid) (PAC, Mw= 50.000 g/mol), poly(vinylsulfate) (PVS, Mw = 160.000 g/mol), poly(styrene sulfonate) (PSS, Mw= 5.000-1.000.000 g/mol) and poly(L-glutamic acid) (PLG, Mw = 70.000 g/mol). Characterization of consecutive deposition of the polyelectrolytes and of the deposited PEMs is mostly performed by in-situ-ATR-FTIR spectroscopy (Fig. 1) using the SBSR concept (OPTISPEC, Zürich, Switzerland) to obtain well compensated ATR-FTIR spectra. Polarized IR light is used for dichroic measurements. AFM measurements are performed mostly in non contact mode on the Ultramiscroscope consisting of optical microscope and AFM attachment (SIS GmbH, Herzogenrath, Germany).
- Fig. 1. Principle of in-situ ATR-FTIR spectroscopy featuring exponentially decaying evanescent waves at the optically dense/rare medium interface to probe surface attached organic material.
Some Results
(1.) Fig. 2a shows consecutive deposition of PEI/PAC monitored by in-situ-ATR-FTIR spectroscopy and Fig. 2b shows the deposited amount in dependence of different pH settings [1].
- Fig. 2a. in-situ-ATR-FTIR spectra on the consecutive PEI/PAC deposition (c = 0.01 M, pH = 9 (PEI) 4 (PAC)) from solution onto Si-ATR crystals.
- Fig. 2b. Adsorbed amounts of the PEM-PEI/PAC at Si-supports in dependence on the adsorption step x.
(2.) Fig. 3 shows surface morphologies of PEM-5 of PLL/PVS (AFM images, 2 x 2 µm2, topography), which were deposited varying substrate texturisation, conformation and molecular weight of poly(L-lysine) PLL [2, 3].
- Fig. 3a: a-helical PLL-205.00, untexturized substrate
- Fig. 3b: a-helical PLL-205.00, texturized substrate
- Fig. 3c: random coiled PLL-205.00, texturized substrate
- Fig. 3d: a-helical PLL-25.700, texturized substrate
(3.) Fig. 4a shows a typical protein sorption series at PEM-5 of PEI/PAC and Fig. 4b the significant decrease of adsorbed amount with increasing PEM thickness pointing to surface rather than bulk sorption [4].
- Fig. 4a. Typical ATR-FTIR spectra recorded during HSA adsorption at PEM-5 of PEI/PAC (pH = 9/4, c = 0.01 M) at t = 5 –120 min (from bottom to top).
- Fig. 4b. Protein sorption kinetics rationalized by the course of Amide I integrals at PEM-5, PEM-7, PEM-9 and PEM-11 of PEI/PAC deposited from 0.01 M solutions at pH = 9/4 in dependence of the time.
Fig. 5 shows the high selectivity of PEM of PEI/PAC to a protein mixture (lysozyme/concanavalin A) varying outermost PEL type and pH value [5].
- Fig. 5. ATR-FTIR spectra on protein layers adsorbed from the mixed solution of LYZ and COA (1 mg/ml) on PEM-5 or PEM-6 at pH = 4 or 7.3, respectively (left), and the corresponding scenarios (right).
Literature
[1] M. Müller, J. Meier-Haack, S. Schwarz, H.M. Buchhammer, K.J. Eichhorn, A. Janke, B. Keßler, J. Nagel, M. Oelmann, T. Reihs, K. Lunkwitz, J. Adhesion, 80(6), 521-548 (2004)
[2] M. Müller, B. Kessler, K. Lunkwitz, J. Phys. Chem., 107 (32), 8189-8197 (2003)
[3] M. Müller, W. Ouyang, B. Keßler, Intern. J. Polymer. AC, 12, 1-11 (2007)
[4] M. Müller, B. Keßler, W. Ouyang, Z.Phys.Chem., 221 127-138 (2007)
[5] M. Müller, B. Keßler, N. Houbenov, K. Bohata, Z. Pientka, E. Brynda, Biomacromolecules, 7(4), 1285-1294 (2006)
