Luminescent Quantum Clusters of Gold as Bio-Labels - Advanced Fluorescence Reporters in Chemistry and Biology - page 337

trapped intermediates of the growing Au cores. These clusters can be separated
from each other by PAGE.
3.2 Formation of Quantum Clusters Inside the Cavities of
Biologically Relevant Molecules as well as Biomolecules
Biologically important molecules also act as templates. For example, dendrimers
assist in the synthesis of gold clusters. This method produces clusters of cores
ranging from Au 5 to Au 33 , by changing the Au 3+ /dendrimer ratio [ 7 ]. Assignment of
these clusters to various cluster cores was largely based on fluorescence spectro-
scopy [ 1 ]. Limited mass spectral data have also been reported. G2-OH and G4-OH
PAMAM dendrimers (second and fourth generation dendrimers with -OH func-
tionality) are used for the synthesis. In this method, gold ions are sequestered into
the cavities of dendrimers and can be reduced into gold clusters by slow reduction.
Proteins are also used to make clusters. A simple, one-pot, green synthetic route
was developed for the synthesis of gold clusters based on the reduction capability of
the protein, bovine serum albumin (BSA) [ 9 ]. The cluster produced in this method
is said to have a core of Au 25 . This process is similar to the bio-mineralization
behavior of organisms in nature. Upon addition of Au(III) ions to the aqueous BSA
solution, the protein molecules sequestered Au ions and entrapped them. The
reduction ability of BSA molecules was then activated by adjusting the reaction
pH to 12 and the entrapped ions underwent progressive reduction to form gold
clusters in-situ.
3.3 Synthesis of Quantum Clusters by the Core Etching or Core
Size Reduction of Metallic Nanoparticles
In this method, a metallic nanoparticle is synthesized first. The metallic nanopar-
ticles are then treated with large amounts of suitably selected molecules, resulting
in the formation of quantum clusters.
The mechanism of formation of the clusters by this method is not well under-
stood. We present below our tentative suggestions regarding the formation of
clusters [ 10 ]. There are two possible routes for etching. In the first route, gold
atoms are removed from the surface of the nanoparticles by excess ligands as a gold
(I)-ligand complex. The gold(I) complex may undergo strong aurophilic inter-
actions as there is a tendency for gold(I) compounds to form dimers, oligomers,
chains, or layers via gold(I)-gold(I) interactions because of the hybridization of the
empty 6s/6p and filled 5d orbitals to form quantum clusters. In the second possible
route, ligands may etch the surface gold atoms of the nanoparticles leading to a