Biomedical Engineering Reference
In-Depth Information
and to the fact that protonated amine of silane favors adsorption by electrostatic attraction
(Jasienak et al., 2009). The pH of the Gox solution (6.8) is indeed higher than the isoelectric
point of glucose oxidase (4.9).
In the above computation, the concentration of the BS coupling agent could not be
evaluated. The reaction of BS with NH
2
transforms an amine function into amide. If only
one end of BS reacts with silane, the
N
400
concentration should be doubled, which is
consistent with the increase found in Table 2. However, converting it into protein-equivalent
gives a number with no physical meaning. When the second end of the coupling agent
reacts with the protein, no additional nitrogen is incorporated, the evaluation of the protein
concentration is correct but the suberate (CH
2
)
6
chain is counted in the HC concentration.
This has no important impact owing to the high concentration of hydrocarbon-like
compounds. Despite the limitations regarding the accuracy of the data in Table 4, it is clear
that prior silanization increases markedly the concentration of glucose oxidase (sil+Gox and
sil+BS+Gox compared to nat+Gox); however the treatment with the coupling agent does not
increase the amount of immobilized enzyme (sil+BS+Gox compared to sil+Gox).
4.3 State of stainless steel surface
It appears in Table 4 that the SS surface prior to and after silanization or enzyme
immobilization is bearing a high amount of organic contaminants. It may be argued that this
is due to improper cleaning protocols, inappropriate sample manipulation or contamination
in the XPS spectrometer. Actually, a clean stainless steel surface is getting quickly
contaminated in contact with the surrounding atmosphere, as revealed by water contact
angles, which can be measured quickly in the same environment. A nat sample showed a
water contact angle of 44° which increased to about 60° within a delay of a few hours
(Figure 10). When a nat sample was further treated with UV-ozone to oxidize organic
compounds, the water contact angle was lowered down to 12°. However it increased rapidly
in contact with the surrounding atmosphere (Figure 10) to reach values above 40° in a few
hours. Similar results were obtained with 304L stainless steel. Wet cleaning essentially
standardizes the surface contamination; further cleaning leaves a material with a high
surface energy, which adsorbs quickly significant amounts of contaminants (Caillou et al.,
2008).
80
60
40
20
0
0
4
8
12
Square root of time (h
1/2
)
Fig. 10. Contact angle measurements as a function of incubation time in ambient atmosphere
performed on (
z
) native stainless steel and (
{
) after UV-ozone treatment.
