Chemistry Reference
In-Depth Information
nanoclusters in solution. For example, by using diffusion-ordered NMR spectros-
copy (DOSY), Salorinne et al. [
41
] successfully estimated the size of Au
25
,Au
38
,
and Au
144
nanoclusters by determining the diffusion coefficient and hydrodynamic
radius from solution samples. The measured cluster sizes agree well with the
average diameters of the corresponding single crystal or theoretical structures
reported previously.
2.2 Composition Characterization
Since Whetten et al. [
29
] used innovative laser desorption ionization mass spec-
trometry (LDI-MS) to analyze the gold clusters, various types of MS techniques
have become the most powerful tools to determine the composition of metal
clusters. Recent studies on high-resolution mass spectrometry from Murray [
42
,
43
] and Tsukuda [
44
] groups have unraveled the molecular formulas of some gold
nanoclusters. Arnold and Reilly [
45
] analyzed the composition of gold nanoclusters
by using high-resolution TOF mass spectrometry. The matrix-assisted laser desorp-
tion ionization mass spectrometry (MALDI-MS) of Au
25
clusters showed the
presence of only 25 Au atom species and no any 24 or 26 or other adjacent core
sizes [
43
]. Recently, Dass et al. [
39
,
43
] demonstrated that almost no fragmentation
of Au nanoclusters was observed in the MALDI-TOF mass spectra of
Au
25
(SCH
2
CH
2
Ph)
18
clusters using
trans
-2-[3-(4-
tert
-butylphenyl)-2-methyl-2-
propenylidene] malononitrile (abbreviated as DCTB) as matrix with low laser
pulse intensity (just above the threshold intensity). Since then, more gold
nanoclusters with various compositions have also been characterized by the effi-
cient MALDI-TOF MS technique.
Size-exclusion chromatography (SEC) has been used recently for the separation
of clusters with different compositions and preliminary characterization of the
cluster purity. For instance, Jin's group used SEC to isolate Au
40
(SR)
24
from
Au
38
(SR)
24
nanoclusters [
46
] as well as Pd-doped 25-atom Pd
1
Au
24
(SR)
18
nanoclusters from homogold Au
25
(SR)
18
[
47
] and Pt
1
Au
24
(SR)
18
nanoclusters
from Au
25
(SR)
18
[
48
].
2.3 Structure Characterization
Structure of gold nanoclusters plays a decisive role in determining their unique
properties. A precise knowledge of the cluster atomic structure is important to fully
understand the physical and chemical properties of gold nanoclusters. Based upon
crystal structures, the relationship between the structure and the electronic, optical,
and catalytic properties as well as the size-dependent evolution can be ultimately
understood. Although the accurate compositions of various gold nanoclusters have
been analyzed by MS, the core structure (arrangement of gold atoms) and surface
