Luminescent Quantum Clusters of Gold as Bio-Labels - Advanced Fluorescence Reporters in Chemistry and Biology - page 335

The quantized nature of electronic energy levels due to size confinement is ampli-
fied in this term. They show characteristic absorption features and can be distin-
guished from each other from their absorption profiles [ 2 ]. Quantum clusters
typically exhibit strong photoluminescence and their wavelength of emission can
be tuned from the near infra red (NIR) to ultra violet (UV) [ 1 ].
In this chapter, we present this new family of materials. The chapter mainly
focuses on the photoluminescence properties of gold clusters. Reference [ 3 ]describes
the corresponding silver analogues. After describing the synthetic approaches used
and after reviewing the various properties of these clusters with special reference to
their luminescent properties, we look at their bio-labeling applications. Although
many of their known properties, such as fluorescence resonance energy transfer,
(FRET) are not used yet in imaging in the biological context, the exploitation of
activities in this area suggests adaptation of several of their properties in diverse
aspects of bio-labeling. We hope that this chapter initiates the reader into a new
family of materials in the context of luminescence imaging and other purposes.
2 History of Clusters in the Condensed Phase
A very brief history of quantum clusters in the condensed state is presented below.
This area of research started with the synthesis of water-soIuble undecagold (Au 11 )
in 1978 [ 4 ]. Authors predicted the use of these gold clusters as high-resolution
electron-dense labels for immunoelectron microscopy because of their extraordi-
nary heavy atom density, as well as their use in biological studies. This was
followed by the synthesis of triphenyl phosphine capped Au 55 clusters [ 5 ]. Whetten
et al. [ 6 ] prepared a 10.4 kDa gold/glutathione cluster compound, Au 28 SG 16 (later
reassigned as Au 25 SG 18 by Tsukuda et al. [ 2 ]). Dickson et al. synthesized a series of
clusters such as Au 5 ,Au 8 ,Au 13 ,Au 23 , and Au 31 , which are encapsulated in the poly
(amidoamine) (PAMAM) cavity [ 7 ]. These clusters have very high quantum yields,
as high as 70%, which is comparable with strongly emitting organic dyes. A series
of glutathione thiolate (SG-thiolate) protected gold clusters with well-defined
compositions were synthesized and separated utilizing polyacrylamide gel electro-
phoresis (PAGE) by Tsukuda et al. [ 2 ]. The molecular compositions of these
clusters were characterized by electrospray ionization (ESI) mass spectrometry.
Following these important studies, several groups have explored the research area
of quantum clusters in detail. New clusters have been synthesized, crystal structures
of a few well-known clusters have been determined, and their applications in
different areas have been reported.
3 Synthetic Approaches
Quantum clusters of gold can be synthesized in different ways. All the synthetic
methods reported so far can be broadly classified into four categories.