Biomedical Engineering Reference
In-Depth Information
the membrane domains of these proteins are so hydrophobic that it makes them
irreversibly precipitate as soon as they are removed from this lipidic environment.
Their crystallization thus necessitates particular protocols in which the hydropho-
bic domain remains protected from water by surfactants molecules. These protocols
have to be finely tuned for each particular protein and, for this reason, their struc-
tures are still unknown for most of them whereas soluble proteins whose crystalli-
zation conditions can be better rationalized, are better known in that respect [18].
8.2.3 Chemical Modification of Surfaces
For these micronanoparticles that have a large surface/volume ratio, surface related
problems and surface chemistry are of paramount importance. The first method
that comes to mind to immobilize biomolecules on surfaces is to rely on nonspecific
adsorption. Although such a coupling can be efficient enough in some situations, a
better control is usually required.
Very few surfaces are truly inactive. They very often bear chemical groups that
can be used for further surface chemistry. Metal surfaces—in particular gold—and
oxide surfaces—in particular SiO
2
—are good templates for chemical modifications.
This last case is of particular interest because these surface treatments are also ap-
plied to glass by extension, although the chemistries of these two surfaces are not
strictly identical. Polymer surfaces such as the surface of latex beads can also include
a sophisticated surface chemistry by the right choice of the monomers used for their
synthesis. However, even in this case, direct coupling may not be possible because
some very reactive groups would readily hydrolyze in water where the coupling reac-
tion is to take place. Intermediate coupling molecules are thus needed. Generically,
these molecules have a reactive group at each extremity. One of them reacts on the
solid surface: in the case of gold, it is a thiol group, and in the case of silica, it is a
silane group. The end-group at the other extremity of these molecules is exposed
toward the exterior world and is used for coupling to the proteins for instance.
Chemical grafting on a plane surface and on a microparticle share some com-
mon features but also differ in a number of ways. On plane surfaces, the grafting
of these molecules results from a collective mechanism where they interact together
by Van der Waals interactions as they react on the surface [19]. The monolayer can
be reinforced by a lateral polymerization illustrated by the case of silanes on silica
where some of the silane groups react on the surface while the others react together
forming a “net.” While it is relatively easy to qualitatively modify a surface, achiev-
ing good monolayers, which is the first step to a good surface coverage, is a delicate
operation particularly in the silane/silica surface (even more so for the silane /glass
system because of the defects and the different chemistry of the glass surface). With
this strategy, it becomes possible to change the physical properties of the surfaces
such as transform hydrophilic surfaces to hydrophobic ones. With particles, there
is usually no need for a “perfect” monolayer and the grafting conditions are less
drastic.
A major use of this surface chemistry is to protect surfaces from nonspecific
adsorption. In that case, long polyethylene glycol (PEG) molecules can be used. To
adsorb on the surface, a protein would have to compress this layer, which is entropi-
cally very unfavorable [20].
