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

1152 Chemical Reviews, 2005, Vol. 105, No. 4 Love et al.
peptide (linked to mercaptopropionic acid) to study
cell adhesion. 720 The authors argue that the small
size (∼8 nm) <strong>of</strong> the gold dots allows only one integrin
(8-12 nm) to bind per site. By varying the spacing
<strong>of</strong> the gold dots on which the cells are cultured, they
estimate that between 58 and 73 nm is the farthest
spacing between integrins that still allows them to
cluster and activate the formation <strong>of</strong> focal adhesion
complexes.
9.3.2. Silver Tetrahedrons for Localized Surface Plasmon
Resonance (LSPR)
Nanosphere lithography uses a colloidal crystal
(either single or double layer) <strong>of</strong> hexagonally closepacked
silica or polystyrene spheres as a mask for
material deposition. 721 Using this technique, arrays
<strong>of</strong> silver nanoparticles with triangular shape and
regular size and spacing can be generated on different
surfaces (mica, fused silica, optical glass, and SF-
10). 722 Van Duyne and co-workers demonstrated that
the shape <strong>of</strong> the nanoparticles, the solvent in which
the measurement is performed, the supporting surface,
and the alkanethiols used to form the SAM on
the surface all affect the frequency and line shape <strong>of</strong>
the LSPR <strong>of</strong> these particles. 721,723,724 In addition, these
particles can be used as substrates for surfaceenhanced
Raman scattering (SERS). 725
Binding an alkanethiol to the surface <strong>of</strong> the silver
nanoparticles alters the local refractive index and
induces a shift in the surface plasmon frequency. 53
This shift is sensitive to the length <strong>of</strong> the alkyl chain
(∼3 nm to the red for every additional carbon) and
can be modeled with Mie theory. The sensitivity
makes the system a good candidate for molecular
sensors. As few as 60 000 1-hexadecanethiol molecules
(∼100 zmol) binding to one silver nanoparticle
results in a 40.7 nm shift to the red in the LSPR
(Figure 26). 726 The adsorption can be monitored in
real time using dark-field optical microscopy.
By forming SAMs <strong>of</strong> biologically relevant ligands
the silver nanoparticles also can be used to detect
the binding <strong>of</strong> proteins, such as antibodies, with high
sensitivities, using relatively low-cost instrumentation.
724,727,728 By choosing the appropriate substrate
for the silver nanoparticles these assays can be
performed in real time in physiological buffer. 728 Haes
et al. reported the construction <strong>of</strong> a sensor for antiamyloid
�-derived diffusible ligands antibodies (anti-
ADDL) using arrays <strong>of</strong> silver nanoparticles functionalized
with mixed SAMs <strong>of</strong> octanethiolates and ADDL
linked to mercaptoundecanoic acid. 724 A sensor <strong>of</strong> this
type might be developed into a diagnostic test for
Alzheimer’s disease.
9.4. Metallic Shells
9.4.1. Metallic Half-Shells
E-Beam deposition <strong>of</strong> thin layers <strong>of</strong> metal onto
arrays <strong>of</strong> colloidal spheres forms metallic half-shells.
If the colloidal spheres are dissolved, free-standing
metallic half-shells several nanometers thick remain
(Figure 27a). The half-shell shape is attractive since
these particles cannot consolidate. When gold halfshells
are functionalized with a hydrophobic SAM,
the wetting properties <strong>of</strong> the aggregated particles are
Figure 26. (a) Tapping mode AFM image <strong>of</strong> a silver
nanoparticle array on a glass substrate. (Reprinted with
permission from ref 53. Copyright 2001 American Chemical
Society.) (b) UV-vis spectra <strong>of</strong> an individual silver nanoparticle
before and after modification with 1-hexadecanethiol.
(c) Real-time LSPR response <strong>of</strong> a single silver
nanoparticle upon injection <strong>of</strong> 1.0 mM 1-octanethiol into
the flow cell. The experimental data (circles) are fitted to
a first-order response pr<strong>of</strong>ile with k ) 0.0167 s -1 . (Reprinted
with permission from ref 726. Copyright 2003
American Chemical Society.)
altered; this formation <strong>of</strong> a SAM converts a surface
covered with half-shells into a superhydrophobic
surface (Figure 27b and c). 729
Bao et al. left gold-coated colloidal particles intact
and functionalized the asymmetric gold spheres with
SAMs <strong>of</strong> thiolated single-stranded DNA. 730 The DNA
was used to direct the assembly <strong>of</strong> these spheres, gold
side down, onto patterned gold surfaces with the
complementary DNA sequence. After assembly a
second gold cap was deposited on the opposite side
<strong>of</strong> the sphere. This technique can be used to form
multiply functionalized spheres.
Particles with silver half-shells are also good
substrates for surface-enhanced Raman scattering
(SERS) because they generate high electromagnetic
field gradients. 731 SAMs formed on the surface are
used to concentrate the analyte <strong>of</strong> interest at the
metal surface within the zone <strong>of</strong> electromagnetic field
enhancement. 732,733 In a recent example a SAM <strong>of</strong> (1mercaptoundeca-11-yl)tri(ethylene
glycol) was used
to selectively concentrate glucose from a solution
containing serum proteins at the silver surface; this
concentration improved the SERS signal and made