Photoluminescence of Gold Nanoparticles and Their Applications to Sensing and Cell Imaging - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 184

for the preparation and characterization of luminescent Au NCs and
NPs have been demonstrated during the past decade. In this chapter,
we discuss the synthesis, optical properties, and applications of
luminescent Au NCs and NPs. Herein, we categorize the preparation
and optical properties of these luminescent Au NCs and NPs in
three parts: polymer-stabilized Au NCs, thiol-stabilized Au NCs, and
luminescent Au NPs. We then emphasize their analytical applications,
notably in the detection of biomarkers, cells, and bacteria. Finally, the
chapter concludes with a brief summary of the present and future
directions of research in luminescent Au NCs or NPs.

9.2 Preparation and Optical Properties of

Polymer-Stabilized Au NCs

Luminescent Au NCs are usually prepared in the presence of
polymers that act as capping agents. To provide Au-Au interactions,
OH- and NH 2 -terminated PAMAM dendrimers are used to initiate
and then stabilize the formation of few-atom NCs. 21 PAMAM
dendrimers are near-spherical, highly branched macromolecules
presenting symmetrically emanating dendrons of deined molecular
weight and size. Water-soluble, monodisperse, blue-emitting
(450 nm) Au 8 NCs that are stabilized by encapsulation in
biocompatible PAMAM dendrimers exhibit a stable and high
luminescence (luorescence) with a quantum yield of 41%. 21 One
general procedure for the synthesis of Au NCs is co-dissolving
PAMAM and HAuCl 4 in distilled water and then reducing the Au +
ions by slowly adding an equivalent amount of NaBH 4 . Simultaneous
formation of small luminescent Au NCs (i.e., PAMAM-encapsulated
NCs) and large Au NPs is common when using this approach. 37
To produce stable luminescent Au NCs, it is necessary to stir the
solutions at room temperature for at least 2 days to complete the
reaction and aggregation processes. The solutions are subsequently
subjected to puriication through centrifugation to remove the large
Au NPs, leaving clear, luminescent Au-NC solutions. The presence
of the luorescence signal is attributed to the Au NCs; the pure
dendrimer does not luoresce. Through slow reduction of gold salts
(e.g., AuC l 4

- or AuBr 3 ) with different concentrations of aqueous
PAMAM solutions, the sizes of the luminescent Au NCs are readily
controlled. Differently sized Au NCs exhibit discrete excitation and