Immobilization of bioaffinity agents can be via noncovalent bonding onto charged surfaces thereby negating
the need for chemical coupling and retaining high activity
of the enzyme or antibody. For non-covalent binding the
nanobeads of interest are zirconia that are essentially inert
but readily absorbs proteins. Functionalized silica beads
also provide a charged surface by which proteins can be absorbed via ionic bounds. Iron or zinc oxide beads can also
be used to absorb protein bioaffinity agents by virtue of carrying a net negative charge at neutral pH (Gomez & Tigli,
2013). However, as with all non-covalent bonding, the
attachment of proteins is sensitive to pH, ionic strength
and/or temperature. Therefore, covalent linkage is preferred
for immobilization of bioaffinity agents onto the surfaces of
the nanoparticles. The most common example of covalent
bonding is with gold nanoparticles which are amenable to
coupling via thiol bonds. The relatively simple chemistry
of gold surfaces makes it suitable to immobilize a diverse
range of bioaffinity agents such as antibodies, oligonucleotides and enzymes. With respect to the latter, the high electrical conductivity of gold nanoparticles enhances electron
transfer and hence the sensitivity of the assay (Majdalawieh
et al., 2014). Gold nanoparticles also exhibit novel optical
properties which alter with interaction of target (antigen
or oligonucleotide) with immobilized bioaffinity agent