Few-Atom Silver Clusters as Fluorescent Reporters - Advanced Fluorescence Reporters in Chemistry and Biology - page 318

Fig. 7 (a)-(c) Fluorescence images of NIH 3T3 cells stained with Avidin-dC 24 -Ag. (a) Fixed
cells, biotinylated. (b) Fixed cells, nonbiotinylated. (c) Live cells, biotinylated. Images were
recorded on a Zeiss Axiovert 200 microscope with a CoolSNAP CCD camera (Roper Scientific).
Scale bar 30 m. (d)-(f) Fluorescence images of live NIH 3T3 cells stained with anti-HS-dC 24 -Ag.

Live cells incubated with anti-HS-dC 24 -Ag at 37 C for 6 min (d, bright field; e, silver clusters;
f, merge). Images were recorded on Zeiss LSM 10 confocal microscope. The fluorescence images
were taken at 543 nm excitation. Scale bar 25 m m[ 50 ]

m

ions and peptides cannot be easily extracted. In addition to the silver binding amino
groups, some authors have demonstrated higher degree of silver binding in peptides
rich in proline and hydroxyl residues [ 55 ], whereas others showed a preferential
affinity of silver for methionine-containing peptides compared to their nonmethio-
nine containing counterparts [ 56 ].
In 2007, Dickson et al. found that it is possible to stain fixed cells with
fluorescent silver clusters instead of silver nanoparticles by tuning the staining
conditions [ 57 ]. The new approach consists of staining fixed cells with a low
concentrated silver nitrate solution 20-100 mM, within 20 h at ambient conditions,
and reducing the silver by photoactivation, with the result of small silver clusters
that present a broad emission band between 500 and 700 nm (Fig. 8a-d ). The
discovery that fluorescent silver clusters can be generated by photoactivation of
cells fed with silver salt, opens up new paths for the application of silver clusters in
biological systems.
Besides the formation of luminescent silver clusters in fixed cells, likely due to
the presence of proteins, fluorescent silver clusters have been synthesized using
proteins as templates. In 2008, Pal et al. reported on the use of an enzyme, bovine
pancreatic

-chymotrypsin (CHT) as biotemplate during the chemical reduction of

a