Self-Assembled Monolayers of Thiolates on Metals as - Whitesides ...

1130 Chemical Reviews, 2005, Vol. 105, No. 4 Love et al.
Figure 8. (a) Schematic representation <strong>of</strong> the different
growth modes <strong>of</strong> PbS nanocrystals in the presence <strong>of</strong>
different surfactants. (Reprinted with permission from ref
78. Copyright 2003 Wiley-VCH.) (b) TEM image <strong>of</strong> rodbased
PbS multipods formed in the presence <strong>of</strong> dodecanethiol
at 140 °C for 5 min. (c) TEM image <strong>of</strong> star-shaped
nanocrystals <strong>of</strong> PbS formed in the presence <strong>of</strong> dodecanethiol
at 230 °C. (d) TEM image <strong>of</strong> cubic PbS nanocrystals formed
in the presence <strong>of</strong> dodecylamine. (b-d) (Reprinted with
permission from ref 77. Copyright 2002 American Chemical
Society.)
Scheme 4. Three Common Strategies for Tailoring
the Composition <strong>of</strong> the SAM on Nanoparticles a
a Each one is discussed in the text in the section indicated in
the scheme.
For example, depending on the concentration <strong>of</strong>
dodecanethiol present during formation and on the
temperature <strong>of</strong> the reaction, the shape <strong>of</strong> PbS (a
semiconductor) nanocrystals changes from the equilibrium
cubic habit (bounded by six {100} faces), to
starlike crystals, to elongated rods and branched
structures (bounded by {111} faces) (Figure 8). 77,78
This effect is specific to thiols: growth in the presence
<strong>of</strong> dodecylamine, a ligand with lower affinity for PbS,
yields only PbS cubes. These results suggest that the
thiols have a higher affinity for the {111} faces,
where the sulfur can be positioned equidistant from
three Pb(II) atoms, than for the {100} faces, where
the sulfur can only be positioned equidistant from
two Pb(II) atoms.
6.2. Strategies for Functionalizing Nanoparticles
with Ligands
There are three common strategies for tailoring the
composition <strong>of</strong> the SAM on nanoparticles and the
functional groups exposed at the SAM-solvent interface
(Scheme 4). They are (1) forming the nanoparticles
directly in the presence <strong>of</strong> ω-functionalized
thiols, (2) exchanging an existing ligand for an
ω-functionalized thiol, and (3) modifying the original
thiol covalently by an interfacial reaction. We address
each <strong>of</strong> these approaches in the following sections.
6.2.1. Formation <strong>of</strong> Nanoparticles in the Presence <strong>of</strong>
Thiols
The ω-functionalities <strong>of</strong> the thiols used to protect
nanoparticles determine what solvents (aqueous or
organic) can disperse the particles. Some alkanethiols
can tolerate the reductive conditions used to prepare
nanoparticles and, therefore, can be used to protect
the nanoparticles during formation (Scheme 4). For
example, in the two-phase Brust-Schiffrin method,
n-alkanethiols and other organic soluble thiols, including
a BINOL (1,1′-bi-2-naphthol) derivative, 453
have been used. 57,92,454
Water-soluble nanoparticles are desirable for biological
applications, and many preparations have
been developed that use thiols with hydrophilic, polar
headgroups. For example, mercaptosuccinic acid can
serve as a stabilizer during borohydride reduction <strong>of</strong>
HAuCl4 to give 1-3 nm, water-dispersible gold nanoparticles
that are stabilized by the charge-charge
repulsion <strong>of</strong> the carboxylate ions. 447 Glutathione, 455
tiopronin (N-2-mercaptopropionyl-glycine), 456 coenzyme
A (CoA), 456 trimethyl (mercaptoundecyl)ammonium,
457 and thiolated derivatives <strong>of</strong> PEG 458 have
all been used as thiol-based water-soluble stabilizers
during the formation <strong>of</strong> gold nanoparticles with a
variety <strong>of</strong> reductants.
6.2.2. Ligand-Exchange Methods
Displacement <strong>of</strong> one ligand for another is a second
strategy for modifying the organic surface <strong>of</strong> nanoparticles
after their formation (Scheme 4). 459,460 These
so-called “ligand-exchange” methods are particularly
useful if the desired ligand is not compatible with the
highly reductive environment required for forming
nanoparticles or if the desired ligand is particularly
valuable (or simply not commercially available) and
cannot be used in the excess necessary for stabilization
during synthesis. Simple thiols can be exchanged
for more complex thiols 459 or disulfides. 91,461 Ligand
exchange is <strong>of</strong>ten used to synthesize poly-hetero-ωfunctionalized
alkanethiol gold nanoparticles via
either simultaneous or stepwise exchange. 462,463 There
are also several recent reports <strong>of</strong> solid-phase placeexchange
reactions with thiol ligands displayed on
Wang resin beads. 464,465
Thiols can displace other ligands weakly bound to
gold (e.g., phosphines and citrate ions); procedures
based on exchange are used to functionalize large
gold nanoparticles (>5 nm) that cannot be formed
directly with a protective layer <strong>of</strong> thiols. 466 For
example, dodecanethiol has been used to extract gold
nanoparticles from water (where they were formed
via ascorbic acid reduction in the presence <strong>of</strong> CTAB)
into organic solvents. 438,446 Caruso and co-workers
also demonstrated the extraction <strong>of</strong> gold nanoparticles
from toluene into aqueous solutions; this method
relies on the displacement <strong>of</strong> hydrophobic n-alkanethiols
with water-soluble thiols. 435,446