Biomedical Engineering Reference
In-Depth Information
2.5
2.5
a
= 0
a
= 2
2.0
2.0
1.5
1.5
1.0
1.0
0.5
0.5
0.0
0.0
0.0
0.5
1.0
1.5
2.0
2.5
0.0
0.5
1.0
1.5
2.0
2.5
at θ = 0°
at θ = 0°
Fig. 8. Plots of OH
inorg
/ O
529.7
ratio measured by XPS at θ = 60°
vs
θ = 0° on native (open
symbols) or silanized stainless steel (closed symbols), as such (
¡
,
) or further treated with
coupling agent BS (
S
,
U
), glucose oxidase (
z
,
{
) or coupling agent followed by glucose
oxidase (
,
). The dashed lines represent a
y/x
ratio of 1:1.
4.2 Chemical speciation of the organic adlayer
Table 2 reveals an increase of
N
tot
concentration as a result of surface treatments, and an
increase of
Si
org
concentration for samples prepared with APTES treatment. However, the
concentration of carbon is high and remains almost unchanged, suggesting that organic
contaminants, mainly hydrocarbon-like compounds, are always dominating in the organic
adlayer. If nitrogen was exclusively due to amide functions (N-C=O) as in the peptide link
of proteins, and if the C 1s component at 287.8 eV was exclusivley due to amide, a 1:1
correlation would be found between the concentrations of
C
287.8
and
N
tot
. This is indeed
observed (Figure 9a) for the set of samples involving the silanized substrate. As nitrogen is
partly in the form of silane, relevant alternatives for the abscissa scale may be the
concentration of
N
400
or the difference between the concentrations of
N
tot
and
Si
org
. If
polysaccharides were present with protein, the
C
287.8
concentration should be corrected by
subtracting the contribution of acetal and thus replaced by
[C
287.8
− (C
286.3
− N
400
)/5]
(Ahimou
et al., 2007, Landoulsi et al., 2008a) or [
C
287.8
− (C
286.3
− N
tot
+ Si
org
)/5].
A comparison between
different plots in Figure 9 shows that the dots representative of samples prepared with non-
silanized substrate remain clustered. The shift of the cluster along the ordinate scale
according to the plot indicates that
C
287.8
concentration is higher than what can be attributed
to amide. On the other hand, the samples prepared with silanized substrate preserve a unit
slope whatever the plot, with much higher values of the coordinates for samples exposed to
the enzyme, with or without the linker. This reveals an excellent agreement between the
increases of concentrations of nitrogen and of carbon attributed to peptidic links (N−C=O),
which result from the Gox treatment. It also validates the C 1s peak decomposition and
component attribution.
The meaning of the surface composition appears more clearly if it is summarized in terms of
concentration of model molecular compounds. This approach was already used for
microbial surfaces (Dufrêne & Rouxhet, 1996; Tesson et al., 2009), for food products
(Rouxhet et al., 2008) and for stainless steel aged in different conditions (Landoulsi et al.,
