Few-Atom Silver Clusters as Fluorescent Reporters - Advanced Fluorescence Reporters in Chemistry and Biology - page 308

5 Silver Clusters as Fluorescent Probes for Molecular Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325
5.1 Quenching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 325
5.2 Analyte-Induced Synthesis of Fluorescent Silver Clusters . . . . . . . . . . . . . . . . . . . . . . . . . . 326
5.3 Wavelength-Shifting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 327
6 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 328
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 329

1 Introduction

Luminescence of silver by quantum confinement has been known for several decades
e.g., silver clusters in zeolites [ 1 - 3 ], in cryogenic noble gas matrices [ 4 , 5 ], in
inorganic glasses [ 6 - 8 ] and in silver oxide films [ 9 - 11 ]. In the 1980s and 1990s, silver
clusters could be prepared in aqueous solution, often using radiolysis, however they
were rather unstable and their luminescence was not reported [ 12 - 14 ]. For example,
Henglein wrote about “long-lived” silver clusters that live “for many hours” [ 14 , 15 ].
Obviously, for many practical applications a lifetime of hours is not sufficient. Stable
aqueous solutions containing small silver clusters and their luminescence were
reported in 2002 in the pioneering work by Zheng and Dickson [ 16 ]. Meanwhile,
silver clusters have gained importance [ 17 ], and basically two kinds of applications
have been explored: (1) their use as fluorescent labels for microscopic imaging, and (2)
their use as fluorescent probes in molecular sensing.
In general, silver clusters in solution are prepared by reduction of silver ions.
Proper scaffolds, e.g., DNA, proteins, dendrimers and polymers, are essential to
prevent the aggregation of clusters to larger nanoparticles. Although it is clear that
the emission originates from few-atom silver clusters, many aspects of this exciting
class of nanoscopic metals are not yet fully understood.
Current fluorescence applications mostly involve organic fluorophores (e.g.,
rhodamine dyes) or semiconductor quantum dots (e.g., CdSe), both have their
own weaknesses and strengths (see Table 1 ). For example organic fluorophores
exist in a wide range of chemical structures and spectral properties; however, their
main weakness is that they are prone to photobleaching. On the other hand,
semiconductor quantum dots are photostable; however, their large physical size
may hinder their use as fluorescent reporters of binding events and, in addition, they
are toxic, which may compromise their use for in-vivo applications. Silver clusters
combine their positive properties. They are extremely bright, photostable, and
nontoxic, have a subnanometer size, and do not blink at time scales relevant for
biological applications (0.1 ms-1 s) [ 18 , 19 ]. These properties allow their use even
in single-molecule studies [ 19 ]. Recently, Ras et al. demonstrated that silver

Table 1 Comparison of silver clusters with conventional fluorophores [ 18 ]
Silver clusters Organic dyes Semiconductor quantum dots

Size 1nm 1 nm 10-20 nm

<

Photostability Stable Bleaches Stable

Toxicity Nontoxic Toxic/nontoxic Toxic

Blinking No Yes Yes