SAMs are ordered molecular assemblies that are formed spontaneously by the adsorption of a molecule with a specific affinity of its headgroup to a substrate [1,2]. The figure shows a schematic, including the constituents of a SAM-molecule (headgroup, chain or backbone, endgroup). A typical example are alkanethiols on Au substrates. |
|
One of the attractive features of SAMs is the fact that by means of the headgroup and the backbone of the molecule one can anchor virtually any functional group to the surface and thus engineer the properties of the surface. This makes a variety of technological applications possible, ranging from protective surface coatings to ultrathin electronic junctions and to interfacing biological systems.
As an example of a current project, we are studying the adsorption behaviour of proteins from aqueous solution on surfaces functionalised by SAMs. Materials which resist the adsorption of proteins from biological media are of crucial importance in biotechnology and biomedical applications [3]. The most outstanding protein resistant properties are exhibited by surfaces containing poly- (PEG) and oligo(ethylene glycol) (OEG), (-O-CH2-CH2-)n. The protein resistance of OEG grafted surfaces cannot be explained by unfavorable changes in the free energy upon the adsorption of a protein as it is found for surface-grafted PEG [4]. The short and rigid OEG terminated alkanethiolate SAMs do not undergo conformational changes and thus there has to be some other mechanism leading to their inertness [5]. Recent FT-IRRAS studies by Schwendel et al [6] have shown a considerable temperature dependence of the protein adsorption, and in particular it has been observed that significant amounts of protein can adsorb if the temperature is decreased below room temperature. While these experiments have been performed ex situ, neutron reflectivity experiments can provide in situ information of the unperturbed system under physiological conditions, i.e. of a OEG grafted sample in contact with an aqueous protein solution. (larger view of the animation)
The thiol based SAMs on
gold are prepared
from a 250-500 µM solution of the desired thiol. The mainly
used thiols are
HS-C11H22-(O-C2H4)
n with -OCH3 or -OH endgroups. n is in most
cases in the range of 3-6. These molecules will be referred to as
"EG3OMe" or "EG6OH".
The silicon or quartz substrates coated
with chromium or titanium as an adhesion promoter followed by
~500Å of gold are cleaned in ozone atmosphere and immersed
into the solution usually over night. The result is a densely
packed SAM of 20-30Å thickness which prevents the adsorption
of proteins on the surface.
In order to unravel the mechanisms leading to the protein resistance of
(oligo)ethylene glycol SAMs using neutron reflectivity measurements, the
OEG grafted substrates are exposed to solutions of proteins like
 |
 |
 |
Fibrinogen, |
Bovine Serum Albumin (BSA) |
or Lysozyme. |
Then subtle changes in the distribution of the proteins in the proximity of the SAM are studied while varying the environmental conditions, i.e. temperature or salt content of the solution.
[1] F. Schreiber, Prog. in Surf. Sci. 65 (2000) 151
(pdf-file, 2383 kB)
[2] F. Schreiber, J. Phys.: Cond. Matter 16 (2004) R881
(pdf-file, 581 kB)
[3] Harris, J. M., Ed., Poly(ethylene glycol) chemistry:
biotechnical and biomedical applications; Plenum Press, New
York, 1992, pp. 1-14.
[4] Jeon, S. I.; Lee, J. H.; Andrade, J. D.; De Gennes,
P. G., J. Colloid Interface Sci. 142 (1991) 149.
[5] Prime, K. L.; Whitesides, G. M., Science 252
(1991) 1164.
[6] Schwendel, D.; Dahint, R.; Grunze, M., Langmuir 19
(2001) 5717.
[7] Skoda, M.; Jacobs, R.; Willis J.; F. Schreiber,
Langmuir 23(3) (2007) 970.
For our recent work on SAMs, see list of publications.