Biomedical Engineering Reference
In-Depth Information
them to be active and alive; this area is discussed in detail in the following
chapter.
The particular orientation of the probe with respect to the plane of the
device surface is an important element. As an example, if the Fab region of
an antibody is 'hidden' from the target because of binding of this region to
the solid surface instead of the Fc component, a true immunochemical
interaction cannot be expected.
In terms of sensitivity it is crucial to maximize the density of probe
molecules per surface unit area with the caveat specified in the next point.
Such density may well be influenced by the roughness and surface area of
the device. Furthermore, there are cases where polymer films are employed
rather than plane surfaces but the same criterion holds in terms of overall
surface density. In this situation a key factor is the ability of the analyte to
diffuse into the film in order to bind to probes attached to polymer
strands.
The spatial characteristics of the probe with regard to the surface plane are
a factor that is important but not studied widely. If probes are in close
molecular proximity on the surface this may result in steric hindrance to
target binding. Another consideration is the possibility that the chemistry
of probe attachment may result in 'island' formation. In this case the
overall number of binding sites will be significantly reduced. As would be
anticipated, control of this chemistry is very dicult to achieve.
There are other more mundane factors such as the cost of reagents and the
potential longevity of the modified sensor surface. These are clearly critical
from a commercial standpoint.
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We take a concise look at the myriad of possible strategies for probe
attachment to devices. The particular surface modifications necessary to
impose cells on a device is considered in the following chapter.
1.3.1 Direct and Linker Adsorption
This constitutes the simplest method for placing a biomolecule onto a substrate,
although there are inherent dangers for the technique in terms of possible
undesirable conformational changes. The probe of interest is chemically or
physically adsorbed from solution directly, in most cases, on the substrate
surface of the device in use. Various protocols are employed to achieve such an
effect such as dip casting, painting, spraying and spin coating—these terms will
be self-explanatory to the reader. The interaction of the probe with the surface
is characterized primarily by hydrogen bonding, van der Waals forces and
various dipole electrostatic interactions. The energy of the attachment process
is of the order of 10-40 kJmol
1
and is highly dynamic in nature. The latter
may lead to a lack of stability in terms of longevity of sensor performance.
It has been considered that a major aspect of this type of chemistry, when it
comes to protein molecules, is the hydrophobic interaction with a hydrophobic
substrate. In this case protein molecules can change their conformation in order
