Bioconjugation of Noble Metal Nanoparticles and Their Applications to Biolabeling and Bioimaging - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 17

nanoparticle surfaces and to form covalent metal-S bonds. Another
conjugation method is electrostatic interaction using the surface
charge of biomolecules to adsorb on the particle surfaces with
opposite charges. It should be noted that these two conjugation
methods may cause biological activities or functions to disappear
due to the randomly conjugated sites in the biomolecules. When
biomolecules are conjugated with nanoparticles through covalent
bonds or electrostatic interactions, they may undergo some changes
in their conformation. The conformation of biomolecules is an
important factor to affect the performance of activities and functions.
Sen et al. showed that bovine serum albumin (BSA) proteins
exhibited conformational changes that affected the eficiency
of surface energy transfer (SET) after conjugation with Au
nanoparticles. 40 If the reactive sites in the biomolecules are not
speciic but random, the orientation and conformation is hindered
and resulting in changes in eficiency. To decrease this effect,
conjugation at speciic sites in the biomolecules is required. For
example, Liao et al. successfully conjugated antibodies on the Au
nanorods and kept the orientation of antibodies. 39 They used (n-
hydroxysuccinimide-PEG-thiol) to incubate with the Au nanorods,
and thiol groups were then attached to the nanorod surfaces to
form Au-S bonds. The terminally functional groups, NHS groups,
were exposed on the nanorod surfaces, and the antibodies reacted
with the NHS groups of NHS-PEG-S-Au nanorods. 41 In this case,
antibodies only used their amine groups, not other groups, so their
orientation had no change after conjugating with NHS-PEG-S-Au
nanorods resulting in the antibodies kept their bio-activity for
speciic targeting. Figure 2.3 shows the commonly used strategies
to conjugate biomolecules on the noble metal nanoparticles. These
conjugations are suitable for most biomolecules such as proteins,
antibodies, lipases, peptides, DNA, and enzymes, and even chemical
compounds. In Fig. 2.3A, the surfaces of noble metal nanoparticles
expose the amine group, and the biomolecules contribute the
thiol group to form the conjugate using sulfosuccinimidyl-4-( N -
maleimidomethyl) cycholhexane-1-carboxylate (SMCC). 42 Similar
results are seen in EDC conjugation methods (Fig. 2.3B). The EDC
serves as bioconjugation linker to form covalent binding between
the amine-exposed nanoparticles and the carboxylic group of
biomolecules. In addition to EDC and SMCC conjugating methods,
there are still many useful chemical-conjugated methods to link
nanoparticles and biomolecules. For example, Brennan et al. used