Chemistry Reference
In-Depth Information
that typically result in multifunctional surfaces. Three possible explanations shall be
given here. First, the aforementioned chemical selectivity of amino groups can make
the existence of other functionalities irrelevant. Second, the very large molecular
weight (size) of biological macromolecules does not require high surface densities
of coupling groups. Third, many biomolecules are characterized by the existence
of numbers of both acidic and basic functional groups, e.g., amino acids in pro-
teins. Biomaterials with mixed surface functionalization may be better adapted for
attachment of such molecules. This argumentation could serve as a (very general)
explanation why so-called amino-functionalized surfaces often perform much better
than standard tissue culture polystyrene, which represents one of the aforementioned
oxygen functionalized surfaces.
Namely, these surfaces exhibit a mixed functionalization. An illustration of the
conceivable appearance of such
amino-functionalized
surfaces is given in Figure 8.38
for the example of polystyrene. This rough imagination was developed preferentially
on the basis of Xray photoelectron spectroscopy (XPS) of polystyrene surfaces after
low pressure microwave excited ammonia plasma treatment for about 10 s [216,
217]. XPS allows limited quantification of chemical bond structure. The existence
of primary amino groups can be proven in conjunction with chemical derivatization
techniques [218].
After longer plasma treatments, a noticeable modification of the polymer base
structure takes place. In the case of polystyrene, this is visible by a marked loss
of aromatic ring structures. Ring opening and hydrogen abstraction by the var-
ious active plasma species lead to numerous and different active sites. Not all
of them can be occupied by amino groups or reoccupied by hydrogen atoms.
A larger part of nitrogen is bound in other than primary amino groups, typically
several percent of surface elemental composition. Also, the amount of oxygen,
which is distributed over different functionalities, is much greater than the nitro-
gen amount in amino groups. It strongly depends on the time of air contact after
plasma processing, indicating a preferential uptake by autoxidation reactions of atmo-
spheric oxygen with long-lived surface alkyl radicals and their oxygen-containing
derivatives [216,219]. Other postplasma effects are oxidation reactions and surface
adaptation, leading to the disappearance of amines from the surface. During and
immediately after plasma processing, oxygen concentrations less than 1% of surface
O-O*
H
O
C
O-O*
NH
2
H
CH
CH
C
NH
H
C
CH
CH
CH
CH
C
H
3
C
CH
CH
2
CH
CH
2
CH
2
N H
C
C
HC
HC
CH
CH
HC
CH
CH
H
2
C
HC
CH
CH
CH
HC
HC
H
2
C
H
2
C
CH
OH
CH
3
H
H
C
O
NH
2
FIGURE 8.38
Schematic illustration of characteristic changes of surface composition of
polymers due to ammonia plasma treatment for the example of polystyrene.
