Luminescent Quantum Clusters of Gold as Bio-Labels - Advanced Fluorescence Reporters in Chemistry and Biology - page 334

7.2 Luminescence Enhancement During Aqueous to Organic Phase Transfer . . . . . . . . . . 345
7.3 Fluorescence Resonance Energy Transfer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 346
7.4 Two-Photon Emission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 347
7.5 Photon Antibunching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348
7.6 Photoreactivity at Single-Cluster Level . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 348
7.7 Quantum Clusters as Metal Ion Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 350
7.8 Quantum Clusters for In Vitro Imaging . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 350
8 Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 352

1 Introduction

Synthesis of novel materials with desired and tunable physical and chemical
properties continues to draw wide interest. Nanomaterials with a variety of shapes
and sizes have been synthesized as they offer numerous possibilities to study size
and shape-dependent variations of electronic, optical, and chemical properties.
Nanomaterials of a particular element show drastic differences in physical and
chemical properties when compared with the bulk state. For example, bulk gold, a
metal that is insoluble in water can be made dispersible when it is in the nanoparti-
cle form. There are drastic changes in the optical properties as well. Bulk gold
appears yellow in color, but when it is in the nanoparticle form with an average core
diameter of 16 nm, it appears wine red. Likewise, the chemistry of gold, such as
catalysis, also shows a drastic change when the constituent units are in the nano-
meter range.
When we study the variation in chemical and physical properties from atoms to
nanoparticles consisting of hundreds to thousands of atoms, one wonders about the
properties of materials composed of only a few tens of atoms. This question led to
the discovery of a new class of nanomaterials called quantum clusters or sub-
nanoclusters. Quantum clusters can be considered as the missing link between the
nanoparticle and atomic behavior, and they are entirely different from these two
size regimes [ 1 ]. They consist of only a few atoms with a core size in the
subnanometer regime. This extremely small size changes the entire electronic
structure of the particles and hence they show different physico-chemical proper-
ties. Quantum clusters are generally represented in terms of the number of core
atoms and the protecting ligands, unlike the conventional metallic nanoparticles,
which are represented by their core diameter. For example, Au 25 SG 18 , a well known
water soluble quantum clusters of gold, consists of a core of 25 gold atoms
protected with 18 glutathione ligands [ 2 ]. Having subnanometer size, these clusters
cannot possess surface plasmon resonance as the density of states is insufficient to
create metallicity. They show discrete energy levels and exhibit distinct, but
different, optical absorption and emission features. In other words, quantum clus-
ters show “molecule-like” optical transitions in absorption and emission and are
termed also as molecular clusters. A range of names such as gold molecules, super
atoms, molecular clusters and subnanoclusters can be used to describe these
materials. However, we follow the term quantum clusters (QC) in this chapter.