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in a polymer matrix was studied at the single-cluster level [
24
]. The formation of
individual clusters can be viewed in real-time by a single molecule luminescence
spectroscopy (SMS) image. The sample for SMS was fabricated by spin coating a
solution of the polymer containing a radical precursor (2-hydroxy-4
0
-(2-hydro-
xyethoxy)-2-methylpropiophenone) and HAuCl
4
. Upon photoexcitation, the radical
precursor yields radicals via a Norrish-type-I R-cleavage. These radicals work as
reducing agents for the Au ions to generate gold clusters. Initially, no luminescence
was observed, with successive laser excitation, an increase in individually blinking
fluorescent species with a long off-time was observed. A reversible and irreversible
quenching of luminescence from this ligand-free gold clusters using O
2
was
observed. Number of fluorescent species decreased dramatically when the samples
were exposed to O
2
. The observed decrease was recovered by 50% by removing O
2
.
Electron transfer played an important role in the luminescence quenching of
photoluminescence of gold clusters. The luminescence spectra and the histogram
of the luminescence maxima did not change significantly after the recovery.
Therefore, it was concluded that there were two different quenching processes,
that is, a reversible process and an irreversible process. Luminescence quenching
by O
2
was prohibited by capping the clusters with octadecanethiol, inferring that
access of O
2
to the surface of gold clusters played a key role in quenching. Relevant
data are presented in Fig.
10
.
Fig. 10 (a) SMS image under excitation of a 405-nm laser for 60 s showing the photofabricated
gold clusters. Inset shows the typical luminescence trajectories observed for a single gold cluster.
(b) Time-dependent change in the number of fluorescent species. (c) Single-cluster luminescence
spectra. Inset shows the histogram of single-cluster emission peak. Emission maxima for typical
single gold cluster are ca. 2.2 and
2.4 eV. (d) Dependence of the change in the number of
fluorescent species on O
2
concentration ([O
2
]) [
24
]
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