Photoluminescence of Gold Nanoparticles and Their Applications to Sensing and Cell Imaging - From Bioimaging to Biosensors: Noble Metal Nanoparticles in Biodetection - page 188

activate the Au atoms and to subtract Au 8 NCs from the NC surface.
The formation of Au 8 NCs probably results from the extraordinary
stability of this magic NC with closed electronic shells. In the other,
Au(I) ions are possibly released from the Au NCs during the ligand
exchange reaction, similar to that of triphenylphosphine-protected
1.5 nm Au NPs when using thiol compounds. 49 Also, it is known
that Au(I) species tend to aggregate through strong aurophilic
interactions, due to hybridization of the empty 6s/6p orbital and
illed 5d orbital. 30 With its high density of binding groups, PEI can
facilitate the formation of aggregates from Au(I) atoms.

9.3 Preparation and Optical Properties of

Thiol-Stabilized Au NCs

HAuCl 4 , NaBH 4 , and thiols are commonly used for preparation of
thiolate monolayer-protected Au NCs. 49 The as-prepared thiol-
stabilized Au NCs possess molecule-like PL properties, with their
emission wavelengths centered in the NIR region upon excitation
in the visible region. Voltammetric data reveal that Au NCs capped
with alkanethiolate and phosphine ligands possess redox-like
and quantized double layer charging properties. 50-56 Table 9.1
summarizes the optical properties of the luminescent Au NCs,
along with those of other ligand-protected Au NCs, 50-56 as well as
assignments of the corresponding transitions for the PL bands of
the Au NCs. One feature of these Au NCs is that their luorescence
emission wavelengths are all longer than 600 nm, making them
useful for bioassays because sample matrix interference is usually
low in this region. For example, tiopronin-coated NCs (Au 201 Tio 85 )
feature a broad emission from 650 to 1250 nm and glutathione
(GSH)-passivated Au 28 NCs luoresce over the range 840-1080
nm. 57-59 One disadvantageous property of these Au NCs is their low
emission quantum eficiency (from 10 -6 to 10 -2 ), which limits their
practical use in assays.
There are several explanations for the PL properties of thiol-
stabilized Au NCs. The interband transition (from d band electrons
to the sp conduction band) and the relaxed radiative recombination
across the energy gap between the highest and lowest occupied
molecular orbitals (HOMO-LUMO gap) within the sp conduction