Few-Atom Silver Clusters as Fluorescent Reporters - Advanced Fluorescence Reporters in Chemistry and Biology - page 321

Fig. 10 (a) Fluorescence image of methanol-fixed NIH3T3 cells loaded with peptide encapsulated
silver clusters for 1 h at room temperature. (b) Time profile of the time series images of cell stained
with silver nitrate showing the fast silver cluster emission centered in the nucleus at short times
with a maximum at 320 nm. Note that black indicates an intermediate intensity level in this color
scheme [ 57 ]

clusters in living cells. Living cells can be loaded with peptide-encapsulated
silver clusters even at low temperature, and the cells show evenly distributed silver
clusters emission indicating that clusters do not penetrate in the cells by endocyto-
sis. In fixed cells, there is an additional strong staining in the nuclear region
(Fig. 10a ). The lifetime of silver clusters protected by peptides is similar to the
lifetime of fluorescent silver clusters formed in cells, and has two components, one
fast of 220 ps (33%) and one slow of 1,760 ps (67%). The fast component allows
high signal-to-noise ratio, minimizing contributions from autofluorescence or other
added dyes when using picosecond-gated microscopy (Fig. 10b ). As a disadvan-
tage, the peptide-encapsulated silver clusters have still relatively low fluorescence
quantum yield, about 3% [ 50 ].

4.3 Polymers and Dendrimers

The first reported water-soluble fluorescent silver clusters were prepared in den-
drimers, by Dickson et al. The combination of second-generation OH-terminated
poly(amidoamine) (PAMAM) dendrimer (16 OH per dendrimer) and irradiation
with blue light (30 W/cm 2 , 6 s) transforms a 1:3 dendrimer:Ag + (OH:Ag + 1:0.19)

solution into a highly luminescent solution containing silver clusters encapsu-
lated in dendrimers [ 16 ]. The silver clusters after more than 30 min of continuous
excitation (

500 nm, 300 W/cm 2 ) keep still about 80% of their fluorescence
intensity providing a good photostability. Further studies showed that silver clusters
prepared in dendrimers have the similar level of cytotoxicity than that of the
corresponding dendrimers. For instance, the NH 2 -terminated PAMAM and its
succinamic acid-derivative show cytotoxicity only at high concentrations (1

M),

m

and cells incubated with their silver nanocomposites show intracellular