Biomedical Engineering Reference
In-Depth Information
photoelectron collection angles is due to the approximation of a smooth surface while the
analyzed surface is rough at the scale of the inelastic mean free paths. The real organic
adlayer thickness should be between the values given in Table 4. Comparison between nat
and sil samples suggests that silane just adds up to the contaminants. A 3.0 nm thick adlayer
containing 25 wt.% silane corresponds to 4.5 molecules.nm
-2
. This value is consistent with a
monolayer of silane, however the retained silane is mixed with a much larger amount of
contaminants. The protein treatment of silanized substrates (compare sil+Gox and
sil+BS+Gox to sil) led to a significant decrease of the amount of hydrocarbon-like
compounds, while the adlayer thickness did not change appreciably. This suggests that the
protein adsorption caused the displacement of part of contamination present on the
silanized stainless steel surface in the form of hydrocarbon-like compounds.
5. Conclusion
The decomposition of the C 1s and O 1s peak provided a distinct evaluation of oxygen
present in inorganic oxide, inorganic hydroxide and organic compounds. This led to a
stoichiometry close to (Fe, Cr)OOH for iron and chromium species in the passive layer. The
elemental composition of the organic adlayer was converted into concentration of model
organic compounds: protein, silane and contaminants. For the latter, extreme poles of
hydrocarbons and polysaccharides were taken as models.
Silanization increased markedly the retention of glucose oxidase, whether or not using a
linker expected to couple the enzyme with the silane. Direct retention of the enzyme by the
silane may be attributed to electrostatic attractions with the protonated amine groups
(Jasienak et al., 2009). It is thus demonstrated, with the same biomolecule, that its retention
by using an APTES-treated substrate and a linker does not infer covalent grafting through
the linker. The occurrence of covalent binding might possibly be evaluated by examining the
retention upon aging in electrolytes, keeping in mind that the silane layer itself may alter in
these conditions (Dekeyser et al., 2008).
The thickness of the organic adlayer was of the order of 3 nm. In all cases, the
concentration of contaminants exceeded the concentration of silane and protein. Angle-
resolved measurements did not reveal any stratification in the organic adlayer, but this
was not conclusive since the effect of a stratification may be masked owing to the
roughness created by nanoparticles of inorganic oxyhydroxide present at the stainless
steel surface. The presence of organic contaminants is unavoidable when high energy
materials are exposed to air for a few hours or to vacuum for a very short time. However
the influence of contaminants on the desired surface reactions is not known and is
difficult to establish. The best to reduce contamination is to thoroughly clean the substrate
by oxidation, to minimize the time of contact of substrates with air at any stage, and to
strictly avoid outgassing. Water contact angle measurement is the best way to assess the
cleanliness of the native substrate but the information will not be unambiguous after
silanization and further treatments.
6. Acknowledgments
The authors thank Simon Degand for his help in statistics. They acknowledge the support of
the "Conseil Régional de Picardie" (France) and the National Fondation for Scientific
Research (F.N.R.S. - Belgium).
