Chemistry Reference
In-Depth Information
1
Introduction
One of the major goals in nanoparticle research is to investigate their unique
properties not seen in bulk materials or small molecules. By tailoring the size or
shape of nanoparticles, their physical and chemical properties exhibit significant
changes compared to bulk materials [
1
]. In terms of size control, there have been
major advances in the last decade, and a wide range of monodisperse nanoparticles
(e.g., 3-100 nm diameter) are now accessible.
Monodispersity is the most important criterion in terms of the quality of nano-
particles and is typically assessed by transmission electron microscopy (TEM)
(Fig.
1a
, b). The ultimate control over nanoparticles is to obtain atomically precise
particles [
2
]. While such atomic monodispersity has not been realized for regular sized
nanoparticles (e.g.,
3 nm diameter), ultrasmall nanoparticles (1-3 nm, equivalent to
a few tens to hundreds of atoms) are now possible to achieve atomic precision, for
example, 25-gold-atom nanoparticles (1 nmmetal core diameter (Fig.
1c
). The atomic
monodispersity of nanoparticles is assessed by mass spectrometry (Fig.
1d
). These
ultrasmall nanoparticles are often called nanoclusters to distinguish from regular
nanoparticles.
The term “monodispersity” used in nanochemistry is not as precise as the term
“purity” in molecular chemistry. A pure compound should be free of impurities and
also with a definite chemical formula. For molecularly pure nanoclusters, all the
particles should have the same molecular weight and the same formula, i.e. “atomic
precision.” It has long been a major dream for nanochemists to prepare atomically
precise nanoparticles. Such nanoparticles will be absolutely monodisperse and
uniform at the atomic scale and thus can be treated as giant molecules. When all
the nanoparticles in a sample are atomically monodisperse, mass spectrometry
analysis will show a single molecular weight (Fig.
1d
). Hence, the atomic precision
is a stricter and more accurate criterion than the conventional term “mono-
dispersity” used for regular nanoparticles, and correspondingly, mass spectrometry
is a more accurate characterization tool than TEM and is indeed indispensible in
nanocluster characterization.
To make atomically precise nanoclusters is of paramount importance for under-
standing the fundamental science of nanoclusters [
3
]. For molecularly pure
nanoclusters, many well-established characterization tools in the traditional molec-
ular chemistry can be applied and provide in-depth characterization. For example,
one can employ mass spectrometry (e.g., electrospray ionization mass spectrome-
try, ESI-MS, and matrix-assisted laser desorption ionization mass spectrometry,
MALDI-MS) to unambiguously determine the molecular weight of nanoclusters
[
3
-
6
], single-crystal X-ray crystallography to determine the total structure of
nanoclusters [
7
-
13
], nuclear magnetic resonance (NMR) spectroscopy to probe
organic ligand environment and metal core chirality [
14
,
15
], and so forth. These
molecular characterization tools lead to fundamental understanding of the physical
and chemical properties of atomically precise nanoclusters.
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