Antibody-Functionalized Inorganic NPs: Mimicking Nature for Targeted Diagnosis and Therapy - Bioinspired Approaches for Human-Centric Technologies - page 11

and diamino PEG derivates has been attached simultaneously to polymer-coated
nanoparticles (Sperling et al. 2006 ; Quarta et al. 2012 ). Besides the tuning of
functional groups at the nanoparticle surface, many other parameters need to be
considered for the linkage of antibodies to the nanoparticles, and they will be
explained in depth in the next paragraph.
1.5 Surface Functionalization of Inorganic Nanoparticles
with Antibodies
A fundamental requisite for all the studies is that the antibody linked to the
nanoparticle surface must preserve its biofunctionality. Therefore, it is not surpris-
ing that many efforts were made in order to optimize the coupling chemistry and the
binding geometry of the bionanocomplex. In general, to attach the antibodies to the
nanoparticle surface, three main different strategies can be exploited: (1) the
physical adsorption, (2) the covalent linkage, or (3) the use of adapter complexes
that work as spacers in a sandwich configuration.
Physical Adsorption (Fig. 1.2a ) The functional groups, either basic or acid, intro-
duced at the nanoparticle surface, can drive the electrostatic interactions of the
nanoparticles with suitably charged antibodies. This allows the adsorption of bio-
molecules at the surface of the nanoparticles at pH values that must be different
from the isoelectric point of the protein (at this pH, the protein has a net charge
equal to zero) and obviously of the nanoparticles. In this type of linkage, however,
there is a strong dependence of the interaction on the charge, and therefore either
changes in the salinity of the media or variation in pH can strongly affect the
stability of the binding. Moreover, often in this configuration, the interaction of the
nanoparticle with the antibody occurs on more portions of the antibody molecules,
and thus the antibody rearranges in a geometry which is not the optimal configu-
ration for promoting the further binding with the antigen (the antibody, for instance,
spreads flat on the nanoparticle surface). The physical adsorption of the antibody on
top of the nanoparticles can occur also via hydrophobic interactions. In this case,
however, as the hydrophobic portions of the antibody are often not exposed to the
environment, the native three-dimensional structure of the antibody is modified, as
the antibody rearranges due to its interaction with the nanoparticle surface, with
consequent loss of its biological activity (Pavlickova et al. 2004 ).
Covalent Linkage (Fig. 1.2b ) In the covalently linkage strategy, the formation of
new chemical bonds which involves the sharing of electron pairs between a
functional group present on the nanoparticle and one present on the antibody is
considered safer. By looking at the antibodies, different functionalities can be
exploited. The choice of the functional groups present on the antibody and on the
nanoparticles to be coupled together by covalently linkage can be quite broad and