Nanotechnology-Enabled Sensors

414 Chapter 7: Organic Nanotechnology Enabled Sensors
the epitopes and paratopes are brought together. The major reasons for the
attractions are probably electrostatic interactions and still several available
hydrophobic sites on antibodies and antigens.
For developing biosensors which are sensitive to specific antibodies or
antigens their corresponding antibodies or antigens should be either immobilized
on the active area of the affinity-type sensors or embedded within
the bulk of the bulk-type sensors.
The concept of immobilization has already been discussed earlier in this
chapter. It was shown that different types of functional groups can be created
on the surface of transducers which interact with corresponding functional
groups in proteins.
For successful antibody or antigen immobilizations the functional sites
of the proteins should be recognized. Consequently, a procedure should be
developed to realize the binding of the proteins on the surface that keeps
them active. In the immobilization of antibodies the epitopes and paratopes
should remain active and available while the other site (the stem) should
perform the immobilization task.
Protein immobilization can be carried out using hydrogen bonds, covalent
bonds, via Van der Waals forces, ionic bonds, etc. If the protein adsorption
occur on a hydrophobic surface, it is often followed by a small
unfolding of a protein structure (e.g. from a quaternary which causes the
amount of hydrophobic polypeptide chain to increase). In many cases, this
phenomenon is undesired, as it leads to the denaturing and deactivation of
the protein. Fortunately, many antibodies are quite resistant to being unfolded
during the adsorption process, as they have a very rigid tertiary
structure. The hydrophobic adsorption method is very popular for welldeveloped
antibody-based immunoassays such as radioimmunoassay (RIA)
and enzyme-linked immunosorbent assay (ELISA). Another problem with a
hydrophobic surface is that any protein in the solution will tend to bind to
the surface, which is also referred to as non-specific binding (NSB). For a
sensor this is problematic as it causes a high background signal. Proteins
can bind to a charged surface due to ion-pair interactions, as some proteins
have ionized surface groups which can interact with the charges on the surface.
This interaction occurs on many different types of surfaces, even
those that are weakly charged.
As described previously, semi-permeable polymeric and non-polymeric
membranes can be employed for trapping proteins such as enzymes and
antibodies. Generally the proteins which are captured this way are small
proteins, having molecular weights less that 10 kDa. Several well studied
membranes include polycarbonate and nylon. Proteins may also become
physically entrapped within the volume of a hydrogel. A hydrogel can be
made of a polymer which is dissolvable in warm water and gels when