chemical reduction, using sodium borohydride as reducing agent, of the attached Ag
ions into a specific DNA sequence. This work opened up the possibilities for
tailoring the optical and electronic properties of sub-nanometer silver clusters by
controlling the stoichiometry of the initial reactants. From this study, several reports
related to the fabrication, characterization, and applications of luminescent
DNA-templated silver clusters appeared during the past decade [31–34]. These
new emerging materials have found multiple applications, mostly as single molecule
fluorophores [16], biolabels [35], intracellular staining agents [36], and (bio)chemical sensors [30]. At present, the structure of the silver clusters encapsulated in DNA
strands is mostly studied by combinatorial approaches using HPLC-mass spectrometry, single crystal XRD, XAFS, IR, DFT, and UV-Vis spectroscopy [30]. However,
up to now, there is no consensus about the correlation between the different emission
colors observed in these samples and their structures. In a recent study [37] carried
by Copp and collaborators, a dependence of fluorescent wavelength emissions on the
number of silver atoms confined in DNA templates was proposed. They suggested
that a magic number of silver atoms resulted in “magic emission bands.” Green
emission was correlated to Ag 4
n+ species, whereas Ag 6
m+ clusters were suggested as
being responsible of the red emission. These assumptions should be taken with
caution, since, as already demonstrated by several groups [15, 16, 36], the functionality of DNA-templated silver clusters is influenced by several factors. Nevertheless,
this pioneering work [37] might set the basis of a more rational approach for
studying luminescent DNA-templated silver clusters.
1.3 Peptide-Protected Silver Clusters
Analogous to the production of luminescent silver clusters in DNA templates, the
use of peptides to stabilize and generate luminescent silver clusters was introduced
by Makarava and collaborators [38]. In this work, the fabrication of water-soluble
organic-inorganic hybrid nanoclusters composed of silver and thioflavin T with
remarkable fluorescent properties was reported. Different types of peptides were
then explored, for instance, luminescent silver nanoclusters protected by glutathione
spanning the entire visible range (blue-, green-, yellow-, and red-emitting species),
and luminescence quantum yields over 60% were reported by Le Guevel and
co-workers [39]. In a different study, the photoemission mechanism of
carboxylate-protected silver nanoclusters was elucidated by Chen and collaborators
[40]. They attributed the emission observed in Ag-carboxylate nanoclusters to
ligand-to-metal charge transfer from silver in the carboxylate complex to the Ag
atoms in the core of the clusters and the subsequent radiative relaxation. Based on
these findings, the authors proposed a molecular-level design of luminescent
Ag-carboxylate probes for different applications, such as in optoelectronics. On
the other hand, the development of luminescent thiolate-capped silver nanoclusters
has been explored [41]. Yang and collaborators [42] were able to elucidate the
molecular structure of a yellow-emitting thiolate-protected Ag 14 cluster by using
80
E. Coutino-Gonzalez et al.
ions into a specific DNA sequence. This work opened up the possibilities for
tailoring the optical and electronic properties of sub-nanometer silver clusters by
controlling the stoichiometry of the initial reactants. From this study, several reports
related to the fabrication, characterization, and applications of luminescent
DNA-templated silver clusters appeared during the past decade [31–34]. These
new emerging materials have found multiple applications, mostly as single molecule
fluorophores [16], biolabels [35], intracellular staining agents [36], and (bio)chemical sensors [30]. At present, the structure of the silver clusters encapsulated in DNA
strands is mostly studied by combinatorial approaches using HPLC-mass spectrometry, single crystal XRD, XAFS, IR, DFT, and UV-Vis spectroscopy [30]. However,
up to now, there is no consensus about the correlation between the different emission
colors observed in these samples and their structures. In a recent study [37] carried
by Copp and collaborators, a dependence of fluorescent wavelength emissions on the
number of silver atoms confined in DNA templates was proposed. They suggested
that a magic number of silver atoms resulted in “magic emission bands.” Green
emission was correlated to Ag 4
n+ species, whereas Ag 6
m+ clusters were suggested as
being responsible of the red emission. These assumptions should be taken with
caution, since, as already demonstrated by several groups [15, 16, 36], the functionality of DNA-templated silver clusters is influenced by several factors. Nevertheless,
this pioneering work [37] might set the basis of a more rational approach for
studying luminescent DNA-templated silver clusters.
1.3 Peptide-Protected Silver Clusters
Analogous to the production of luminescent silver clusters in DNA templates, the
use of peptides to stabilize and generate luminescent silver clusters was introduced
by Makarava and collaborators [38]. In this work, the fabrication of water-soluble
organic-inorganic hybrid nanoclusters composed of silver and thioflavin T with
remarkable fluorescent properties was reported. Different types of peptides were
then explored, for instance, luminescent silver nanoclusters protected by glutathione
spanning the entire visible range (blue-, green-, yellow-, and red-emitting species),
and luminescence quantum yields over 60% were reported by Le Guevel and
co-workers [39]. In a different study, the photoemission mechanism of
carboxylate-protected silver nanoclusters was elucidated by Chen and collaborators
[40]. They attributed the emission observed in Ag-carboxylate nanoclusters to
ligand-to-metal charge transfer from silver in the carboxylate complex to the Ag
atoms in the core of the clusters and the subsequent radiative relaxation. Based on
these findings, the authors proposed a molecular-level design of luminescent
Ag-carboxylate probes for different applications, such as in optoelectronics. On
the other hand, the development of luminescent thiolate-capped silver nanoclusters
has been explored [41]. Yang and collaborators [42] were able to elucidate the
molecular structure of a yellow-emitting thiolate-protected Ag 14 cluster by using
80
E. Coutino-Gonzalez et al.