Luminescent Quantum Clusters of Gold as Bio-Labels - Advanced Fluorescence Reporters in Chemistry and Biology - page 342

will resolve to discrete levels. Because of this, small nanoparticles or quantum
clusters no longer support the plasmon excitation unlike the bigger, metallic
nanoparticles. The electronic structure and the corresponding absorption features
of quantum clusters can be explained by taking the example of Au 25 (SH) 18 [ 19 ].
On the one hand, the optical absorption spectrum of Au 25 is molecule-like, in
sharp contrast to that of metallic gold nanoparticles. It has several absorption
features in the 400-1,000 nm range. On the other hand, spherical nanoparticles
show one absorption peak (520 nm for a gold nanoparticle of

15 nm core
diameter). The optical absorption spectrum of Au 25 is given in Fig. 3 . Three
prominent features, marked as “a,” “b,” and “c” are present in the absorption
spectrum of Au 25 . These absorption features can be explained with the help of
Kohn-Sham orbital energy level diagram of Au 25 (SH) 18 . In the electronic struc-
ture of Au 25 (SH) 18 , the HOMO and the lowest three LUMOs are mainly com-
posed of 6sp atomic orbitals of gold; thus, these orbitals constitute the sp band. The
HOMO-1 through HOMO-5 are mainly constructed from the 5d 10 atomic orbitals of
gold and hence constitute the d band. The first absorption is at 1.52 eV (670 nm),
denoted by peak “a” in the figure corresponding to a HOMO-LUMO transition,
which is otherwise called an intraband ( sp to sp ) transition. The peak at 2.63 eV
(marked as “b” in figure) arises from mixed intraband ( sp to sp ) and interband ( d
to sp ) transitions. The peak at 2.91 eV (marked as “c” in figure) arises principally
from an interband transition ( d to sp ). The absorption peak due to intraband
transition shows a blue shift with decrease in core size. For example, glutathione
protected Au 39 clusters show absorption at 1.4 eV or 730 nm because of the
intraband transition while Au 18 shows the same feature at 570 nm [ 2 ].

Fig. 3 (a) Kohn-Sham orbital energy level diagram for Au 25 (SH) 18 . Each KS orbital is drawn to
indicate the relative contributions (line length with color labels) of the atomic orbitals of Au (6 sp )
in green ,Au(5 d )in blue ,S(3 p )in yellow , and others in grey . The left column of the KS orbitals
shows the orbital symmetry (g, u) and degeneracy (in parenthesis ); the right column shows the
HOMO and LUMO sets. (b) The absorption spectrum of Au 25 (SH) 18 [ 19 ]