Photoluminescence of Gold Nanoparticles and Their Applications to Sensing and Cell Imaging - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 200

Unlike semiconductive QDs, they are more compatible with
biological systems and are not prepared from toxic precursors
under vigorous conditions. In addition, bioconjugation of Au NCs
and luminescent Au NPs is quite simple when taking advantage of
strong Au-S bonding.
Polymer-stabilized Au NCs are not popular for bioassays, mainly
because of their low water-solubility and their dificult bioconjugation
with recognition molecules. On the other hand, thiol-stabilized Au
NCs have very low quantum yields, making them poor materials
for developing sensitive nanosensors. Nevertheless, a few NCs have
exhibited practical use in bioassays. Polyclonal, goat-derived anti-
human IgG antibody-modiied PAMAM-Au NCs have been used
for the detection of human IgG antigen. 71 Coupling of goat-derived
polyclonal anti-human IgG to the luorescent PAMAM-Au NCs
( λ em

max = 450 nm) occurred through electrostatic interactions. Upon
increasing the concentration of human IgG antigen, the luminescence
intensity of the bioconjugated PAMAM-Au NCs decreased. Through
interactions between the antigen and antibody in the form of
a 2:1 complex, aggregation of the Au NCs occurred, leading to
decreased luminescence intensity. The relationship between the
luminescence intensity and the concentration of the human IgG
was linear over the micromolar to nanomolar range. By conducting
ligand exchange reactions, the PAMAM groups of the PAMAM-Au
NCs can be exchanged by carboxylate-terminated n -alkanethiols,
such as 11-MUA, allowing simple functionalization of the NCs with
site-speciic leading peptides, such as SV40 nuclear-localization
signal (NLS). 72 To modify the surfaces of the 11-MUA-capped Au
NCs with NLS for nuclear transport, 1-[(3-dimethylamino)propyl]-
3-ethylcarbodiimide hydrochloride (EDC) is usually introduced to
activate the carboxylic acid unit of 11-MUA prior to adding NLS.
Figure 9.8 displays confocal microscopy images of the 11-MUA-Au
NCs (without NLS) and the NLS-11-MUA-Au NCs inside HeLa cells.
The one-color image features no luminescent signals (panel A)
from the cells treated with the 11-MUA-Au NCs, implying that 11-
MUA-Au NCs are not able to enter the cells eficiently, due, at least
in part, to their negative surface charge. In contrast, intense blue PL
(panels C and E) appears from the NLS-11-MUA-Au NCs, indicative
of the internalization of the 11-MUA-Au NCs associated with NLS by
HeLa cells. Counter-staining the cells with a speciic membrane dye,
wheat germ agglutinin conjugated Alexa 594 (WGA-Alexa 594), and