A R T I C L E S
Wang et al.
difficult to perform for semiconductor nanocrystals. Excess free
ligands were found necessary for the gel electrophoresis of
semiconductor and large-sized gold nanocrystals, and the
nanocrystals were often not recoverable.
to control the size of gold nanocrystals by the different
generation dendrons.27 The results indicate that the gold
nanocrystals synthesized with high generation dendrons have a
strong tendency to aggregate, which is different from the results
of this work. We think their results may be caused by the rigid
branches of their dendrons,27 which cannot provide sufficient
steric crowding on the surface of nanocrystals and have difficulty
possessing inter- and intramolecular chain tangling.23 A series
of similar structured dendrons with thiol as the binding group,
whose branches are also quite rigid, was reported recently for
the synthesis of gold nanocrystals.28
Dendrimers have already been used for the synthesis of
different types of colloidal nanocrystals.30 In this case, nano-
crystals are encapsulated inside dendrimers, which possess a
different type of structure in comparison to that of the dendron-
nanocrystals discussed in this paper. To our best knowledge,
the quality of the resulting nanocrystals synthesized using
dendrimers or dendrons, at least for semiconductor nano-
crystals,31-33 is not comparable to the ones synthesized by the
green chemical methods at high temperatures.2,3 For stabilizing
nanocrystals as the surface ligands, dendrimers are not as ideal
as dendrons. The cone-shaped structural feature and the single
binding site of a dendron ligand should provide a better packing
in the ligand shell and an unambiguous orientation of the ligands
on the surface of nanocrystals, respectively.
Evidently, the dendron ligands reported here have dramati-
cally improved the photochemical stability of both types of
nanocrystals. For other types of nanocrystals, such as magnetic
oxides and metals, it would be worth to apply dendron ligands
since their stability against a required environment is also a
problem. In principle, uncross-linked dendrons as the ones
described in this paper can stabilize the related nanocrystal/
ligand complexes, if the stability is controlled by the diffusion
of some small molecules into the interface between a nanocrystal
and its ligand shell.
There is still some room to improve (see Figure 3) if one
wants absolute stability for CdSe and Au nanocrystal/ligand
complexes. If the resulting ligands generated by the photocata-
lytic oxidation occurred on the surface of nanocrystals were
designed to be insoluble in the solvent, the oxidized ligands
would still surround the inorganic core to form a micelle
structure although there were no chemical bonds between the
ligands and the inorganic cores.14 If the inorganic core was stable
against photooxidation, the resulting nanocrystal/ligand complex
would remain soluble and processable. In practice, an insoluble
ligand shell may be achievable for the dendron ligands by
intermolecular cross-linking between the chains by multiple
hydrogen bonds, covalent bonds, or other relatively strong
interactions. For the inorganic core, active semiconductor and
metal nanocrystals can be coated by another inorganic compo-
nent29 prior to the surface modification by dendron ligands to
convert them into being photooxidation inactive.
Conclusion
In conclusion, dendron ligands are used for stabilizing CdSe
and Au nanocrystals. The experimental results confirmed that
the photochemical stability of semiconductor and noble metal
nanocrystal/ligand complexes is the key for the development
of reliable processing chemistry for these nanocrystals. The
surface-modification chemistry of the nanocrystals with dendron
ligands is simple and straightforward. The thickness of the ligand
layer of the dendron-nanocrystals can be as thin as about 1 nm
to achieve substantial stability for those dendron-nanocrystals
to be manipulated as standard chemical reagents. The chemistry
related to CdSe dendron-nanocrystals can be immediately
applied for developing photoluminescence-based labeling re-
agents using semiconductor nanocrystals for biomedical ap-
plications.12,13 The chemistry presented also provides an alter-
native path to apply noble metal nanocrystals for chemical34
and biomedical applications.16,17 The concept should also create
many new opportunities in the field of colloidal nanocrystals
and related materials since it carries the possibility to develop
simple and affordable processing chemistry. For example, stable
magnetic dendron-nanocrystals may represent a new avenue for
using magnetic nanocrystals for drug delivery and enhanced
magnetic resonance imaging.35,36
Certain types of dendron ligands can presumably be used for
the synthesis of high-quality semiconductor nanocrystals, pro-
vided the recent discovery of many alternative routes toward
high-quality semiconductor nanocrystals.2,3 With rationally
designed ligands, one may directly prepare stable nanocrystals
with desired functionality, such as being water soluble and
chemically accessible. It should be pointed out that the thiol-
based ligands described in this paper cannot be used for the
synthesis of high-quality semiconductor nanocrystals because
thiols were found to be not compatible with the existing
synthetic schemes.2,3 We are currently designing and synthesiz-
ing dendron ligands using carboxylic acid, amine, phosphine
oxide, or phosphonic acid groups as the coordinating sites for
the direct synthesis of high-quality semiconductor nanocrystals
and magnetic nanocrystals. The thiol-based dendrons presented
in this paper may be applied for the synthesis of stable noble
metal nanocrystals using the existing methods.25
The results indicate that the inter- and intramolecular chain
tangling between the branches of dendrons have played an
important role for stabilizing the semiconductor nanocrystals,
in addition to the steric crowding feature of the dendrons.23
Dendron ligands have recently been used for the synthesis of
gold nanocrystals using different generations of quinone-based
(-CdO) hydrophobic dendrons with rigid branches in the hope
Supporting Information Available: Synthesis and spectra of
parent dendrimers (PDF). This material is available free of
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