Thiol-Functionalized, 1.5-nm Gold Nanoparticles
A R T I C L E S
Scheme 1. Ligands Used in the Ligand Exchange Reaction
organic- and water-soluble nanoparticles with various core sizes
and functional groups.8,18 However, this approach continues to
be limited by a number of challenges, including difficulties
incorporating charged ligands into the ligand shell,19 controlling
the core size independent of the ligand used, and driving
complete replacement of the original ligand shell.18 Direct
synthesis approaches have also been employed to prepare
functionalized nanoparticles8,14,20 but most of these methods
suffer from the incompatibility of functionalized ligands with
the reaction conditions and show a strong dependence of the
core size on the stabilizing ligand used during synthesis.8,21
We recently developed a novel approach for the preparation
of diverse libraries of ligand-stabilized metal nanoparticles that
addresses the challenges stated above. It consists of a straight-
forward two-step procedure, involving (i) preparation of well-
defined phosphine-stabilized precursor particles (dCORE ) 1.5
( 0.5 nm)14,22 and (ii) functionalization of these particles
through ligand exchange reactions with ω-functionalized thi-
ols.23,24 The small set of thiols previously used in this approach
suggested that the method might be extended to provide general,
convenient access to functionalized, thiol-stabilized gold nano-
particles with a controlled core size. However, the full scope
and the mechanism of the ligand exchange approach had not
been determined. We anticipate that by understanding the
mechanism of the ligand exchange between thiols and phos-
phine-stabilized gold nanoparticles, the rational development
of new synthetic strategies will be facilitated, and the approach
will be extended to other materials. Given the potential benefit
of such mechanistic studies, it is surprising that only a few
studies have been reported to date.25-28 These studies mainly
explore the dynamics of ligand exchange reactions between
thiol-stabilized gold nanoparticles and other thiols.25-27
characterized a wide range of 22 thiol-stabilized gold nanopar-
ticles, incorporating a surprisingly diverse range of functional
groups into the ligand shell. The approach provides a convenient
route to new materials that have been previously inaccessible
and will be a key approach toward the realization of a broad
range of applications. In addition, our mechanistic studies of
the ligand exchange reaction provide the first fundamental
insight into the progression of these types of ligand exchanges.
The studies show an unexpected three-stage mechanism that
provides valuable insight for further refining the ligand exchange
reaction. To this end, we demonstrate that this knowledge allows
one to optimize reaction conditions and provides access to novel
particles with mixed ligand shells of controlled compositions.
II. Results and Discussion
Herein, we present a synthetic and mechanistic investigation
of the ligand exchange reaction between small, PPh3-stabilized
precursor nanoparticles (dCORE ) 1.5 ( 0.5 nm) and ω-func-
tionalized thiols, demonstrating the general nature and full scope
of this method. Using this method, we have prepared and fully
In the following section, we describe first the scope and broad
utility of the ligand exchange method for the preparation of a
wide range of small (dCORE ∼ 1.5 nm), functionalized nano-
particles. We describe functionalization by organic- and water-
soluble alkyl- or arylthiols (with neutral or charged headgroups)
to illustrate both the ease of preparation and the surprisingly
high tolerance of our approach to a large variety of functional
groups (Scheme 1). We also discuss convenient purification
procedures, the results of complete characterization of these
materials, and the important characteristics of the thiol-stabilized
nanoparticles obtained by our approach.
In the second part of this section, we describe a mechanistic
investigation of the ligand exchange reaction that suggests an
unexpected three-stage mechanism for the ligand exchange.
Evidence for such a mechanism is derived from product analysis
of the ligand exchange reaction, 31P NMR spectroscopy, and
trapping experiments to probe for the presence of free PPh3.
The results of these experiments suggest that, initially, the
nanoparticles lose triphenylphosphine in the form of AuCl-
(PPh3). During the later stages of the exchange, the remaining
phosphines are removed as PPh3 assisted by gold complexes in
solution. We also demonstrate how the results from these studies
suggest an approach to controlling the extent of the ligand
exchange. This makes it possible to synthesize nanoparticles
with mixed ligand shells of defined, reproducible composition.
Scope and Utility I: Scope of the Ligand Exchange
Reaction. In our previous work, we demonstrated that 1.5-nm
triphenylphosphine-stabilized gold nanoparticles (1.5-nm Aun-
(16) (a) Xia, B.; Lenggoro, I. W.; Okuyama, K. Chem. Mater. 2002, 14, 2623-
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