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

1150 Chemical Reviews, 2005, Vol. 105, No. 4 Love et al.
thiolates and subsequent desorption (see section 4.3).
Molecular interactions between neighboring chains,
such as networks <strong>of</strong> hydrogen bonds, could provide
one means for improving the stability <strong>of</strong> the monolayer.
283,695
9. Applications <strong>of</strong> SAMs on Nanostructures
Templated synthesis <strong>of</strong> nanostructures is an approach
to forming nanometer-scale, metallic objects
with nonequilibrium (and <strong>of</strong>ten nonspherical) morphologies.
Although the shapes themselves may
exhibit new and interesting physical properties (optical,
electronic, or magnetic), much <strong>of</strong> the interest in
these types <strong>of</strong> structures focuses on their potential
applications, such as sensors, selective filters, or
probes for biology. These nanostructures are smaller
than cells; this size makes them useful for probing
subcellular features. For example, magnetic nanostructures
can be used to create very high localized
field gradients or apply torque at subcellular levels. 696
Metallic nanostructures with uniformly functionalized
surfaces could be used to target cells and bind
to specific receptors. Without functionalization the
native metal surfaces are prone to nonspecific protein
adsorption and degradation (oxidation, decomposition).
42 For structures made from gold and other
metals on which SAMs can form (Table 1), alkanethiols
are used to impart new properties and functions.
This section discusses some examples <strong>of</strong> templated
nanostructures and applications <strong>of</strong> such structures
that use SAMs to tailor the composition <strong>of</strong> their
surfaces.
9.1. Electrodeposited Metal Rods
Electrodeposition <strong>of</strong> metals within mesoporous
polycarbonate or alumina membranes-a technique
pioneered by Martin697-can generate segmented
nanowires with sections composed <strong>of</strong> different metals
with controlled lengths (Figure 24a and b). There are
reviews <strong>of</strong> the many applications <strong>of</strong> these rods
including their use as circuit components, 698 their
applications in biology, 699 and their use as bar-coded
tags. 700 Rods containing nickel segments are particularly
useful because <strong>of</strong> their magnetic properties. 701,702
Many applications <strong>of</strong> these rods rely on the orthogonal
functionalization <strong>of</strong> different metallic sections
with different SAMs (section 7.3.4). For example,
gold-platinum rods can be patterned with
thiols on the gold segments and isocyanides on the
platinum segments. 124 Gold-nickel rods can also be
orthogonally functionalized using thiols to decorate
the gold segments and carboxylic acids, which have
a preference for the native oxide on the nickel, 39 to
decorate the nickel segments. 41,42,702 Poly-histidine
and proteins with poly-His tags have also been shown
to bind selectively to nickel segments <strong>of</strong> gold-nickel
rods. 701
In biological applications orthogonally functionalized
rods make it possible to localize different proteins
on individual segments. One application <strong>of</strong> rods
functionalized in this way is the delivery <strong>of</strong> DNA
plasmids to a cell. Transferrin (a cell-targeting
protein used for receptor-mediated gene delivery via
endocytosis) 703 was linked to the gold segments via
Figure 24. Optical (a) and FE-SEM (b) images <strong>of</strong> a
segmented nanorod with ∼550 nm gold segments and silver
segments <strong>of</strong> 240, 170, 110, and 60 nm (from top to bottom).
(Reprinted with permission from ref 704. Copyright 2001
AAAS.) (c) Fluorescence micrograph <strong>of</strong> gold nanowires
functionalized with ssDNA only on the tips. After removal
from the alumina membrane, the ssDNA was hybridized
with a rhodamine-modified complementary strand <strong>of</strong> ss-
DNA. (d) Optical micrograph corresponding to the rods
shown in c. (Reprinted with permission from ref 707.
Copyright 2001 Wiley-VCH.)
thiols and a DNA plasmid was localized on the nickel
segments via electrostatic interactions with the amino
group <strong>of</strong> 3-[(2-aminoethyl)dithio]propionic acid. 41 Similar
chemistry has been used to functionalize the gold
tips <strong>of</strong> nickel/gold rods with biotin; the rods were then
magnetically manipulated onto silver surfaces functionalized
with avidin. 702 Another approach to localizing
proteins to one segment <strong>of</strong> a rod is to treat the
gold segments with EG6-thiol and the nickel segments
with palmitic acid. The EG-coated gold resists
protein adsorption, while the hydrophobic nickel
segments readily adsorb proteins. 42
SAMs on metallic “barcodes”, rods patterned with
sections <strong>of</strong> different metals, have been used to
perform DNA hybridization assays and immunoassays.
704,705 The rods are first functionalized with
mercaptoundecanoic acid and subsequently linked to
the molecules or proteins <strong>of</strong> interest via peptide
coupling reactions. The combination <strong>of</strong> the inherent
reflectivities (Figure 24a) <strong>of</strong> the different metallic
stripes-silver is more reflective than gold-and the
fluorescence <strong>of</strong> a bound analyte is used as a readout
for binding. Depending on the choice <strong>of</strong> metals
and fluorescent labels, the barcode pattern can either
be obscured by fluorescence or remain visible.
The tips <strong>of</strong> nanorods (rather than their length) can
be functionalized with different SAMs by exposing
the nanorods to a thiol solution before removing them
from the membrane used for templating (Figure
24c). 706,707 This technique has been used to assemble
rods in an end-to-end fashion using complementary
single-stranded DNA. 707 It has also been used to form
molecular “junctions” in the nanorods by exposing the
growing rod to a solution <strong>of</strong> an ω-functionalized
alkanethiol in the middle <strong>of</strong> the growth process and