Arrays of microdots of gold nanoparticles immobilized above gold surface probed by optical second-harmonic microscopy

Article abstract of DOI:10.1063/1.2181273

Gold nanoparticle arrays are fabricated on a planar gold surface obtained from UV-photopatterned self-assembled monolayer (SAM) films. The arrays are composed of microdots in which spherical gold nanoparticles are immobilized above a gold surface with a gap distance of a few nanometers supported by SAM films used as a spacer. Two kinds of preparation methods were examined: (1) to pattern a spacer SAM film that makes cross-linkage between the gold surface and the gold nanoparticles, and (2) to form ordered microholes in a SAM film, followed by deposition of the spacer SAM film that makes the cross-linkage in the microholes. Both methods provide us with the ordered microdots of gold nanoparticles on a gold surface. The formation of the microdot arrays were probed by optical second-harmonic microscopy.

Full text of DOI:10.1063/1.2181273

Arrays of microdots of gold nanoparticles immobilized above gold surface probed by
optical second-harmonic microscopy
Kazuma Tsuboi and Kotaro Kajikawa
Citation: Applied Physics Letters 88, 103102 (2006); doi: 10.1063/1.2181273
View online: http://dx.doi.org/10.1063/1.2181273
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APPLIED PHYSICS LETTERS 88, 103102 ͑2006͒
Arrays of microdots of gold nanoparticles immobilized above gold surface
probed by optical second-harmonic microscopy
Kazuma Tsuboi
PRESTO, Japan Science and Technology Agency (JST), 4-1-8, Honcho, Kawaguchi, Saitama
332-0012, Japan
Kotaro Kajikawaa͒
Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, Midori-ku,
Yokohama 226-8502, Japan, and PRESTO, Japan Science and Technology Agency (JST), 4-1-8,
Honcho, Kawaguchi, Saitama 332-0012, Japan
͑Received 1 April 2005; accepted 23 January 2006; published online 7 March 2006͒
Gold nanoparticle arrays are fabricated on a planar gold surface obtained from UV-photopatterned
self-assembled monolayer ͑SAM͒ films. The arrays are composed of microdots in which spherical
gold nanoparticles are immobilized above a gold surface with a gap distance of a few nanometers
supported by SAM films used as a spacer. Two kinds of preparation methods were examined: ͑1͒ to
pattern a spacer SAM film that makes cross-linkage between the gold surface and the gold
nanoparticles, and ͑2͒ to form ordered microholes in a SAM film, followed by deposition of the
spacer SAM film that makes the cross-linkage in the microholes. Both methods provide us with the
ordered microdots of gold nanoparticles on a gold surface. The formation of the microdot arrays
were probed by optical second-harmonic microscopy. © 2006 American Institute of Physics.
͓DOI: 10.1063/1.2181273͔
Surface immobilized spherical metallic nanoparticles lo-
cated above a planar metallic surface with a gap distance of
a few nanometers show interesting optical characteristics1–5
because of strong electromagnetic interaction between the
nanoparticles and the metallic surface. The resonance wave-
length is redshifted from that of isolated nanoparticles,2,5 and
enhanced optical fields are generated in the nanogap under
the resonance condition.1,4 It is expected that the large elec-
tric field can be applied to various spectroscopic techniques,
exemplified by Raman scattering6,7 and nonlinear optics.8
Our recent study actually showed that the gold surface cov-
ered with gold nanoparticles has activity of second-harmonic
generation ͑SHG͒ 102–103 times more than that of a gold
surface without any gold nanoparticles.8 For practical appli-
cation of this system to nanophotonic sensor devices, meth-
ods for patterning this structure should be developed.
This letter reports preparation of microdot arrays of
spherical gold nanoparticles immobilized above a gold sur-
face with a gap distance of a few nanometers. Two methods
were examined, which are schematically illustrated in Fig. 1:
to photopattern an aminoundecanethiol ͑AUT͒ self-
assembled monolayer ͑SAM͒ film for making cross-linkage
between the gold surface and the gold nanoparticles ͑sample
preparation A͒, and to photopatteren ordered microholes in
an octadecanethiol ͑ODT͒ SAM film, followed by deposition
of the SAM films that make the cross-linkage ͑sample prepa-
ration B͒. In sample preparation A, the surface area outside
the microdots is a bare gold surface, which occasionally
forms nonspecific binding to biological molecules such as
proteins and DNA. Hence it is unsuitable to use this array for
biological applications. In sample preparation B, in contrast,
the surface areas outside the microdots can be covered with a
SAM film that resists the nonspecific binding to the biologi-
cal molecules, such as hydroxy-terminated alkanethiol SAMs
and poly͑ethyleneglycol͒ SAMs. This method also gives a
merit that the UV-photopatterning condition is independent
of the SAM film for making the cross-linkage in the micro-
dots. In this study, we characterized the arrays by optical
second-harmonic ͑SH͒ microscopy, which is powerful as a
probe for the samples of this kind, because the gold surface
covered with gold nanoparticles shows large SHG activity.8
The gold nanoparticles were prepared by reduction of
NaAuCl4. An aqueous solution of 0.254 mM NaAuCl4
͑100 ml͒ was maintained at 95 °C in a water bath, and an
aqueous solution of 33.3 mM citrate ͑2.5 ml͒ was added
with stirring. The color of the solution had turned ruby-red
immediately. After further stirring for 10 min., the solution
was cooled to room temperature. The resulting gold nanopar-
ticles were found to be spherical with a diameter of 40 nm
according to transmission electron microscopy.
ODT was used for the cover of the gold surface outside
the microdots in sample preparation B. It was dissolved in
ethanol with a concentration of 1 mM. The AUT or mero-
a͒
Author to whom correspondence should be addressed; electronic mail:
FIG. 1. ͑Color online͒ Schematic illustration of the preparation of the
arrays.
kajikawa@ep.titech.ac.jp
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0003-6951/2006/88͑10͒/103102/3/$23.00 88, 103102-1 © 2006 American Institute of Physics
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103102-2
K. Tsuboi and K. Kajikawa
Appl. Phys. Lett. 88, 103102 ͑2006͒
FIG. 2. Optical setup for SHG microscopy.
FIG. 3. SH images of the gold-nanoparticle array. ͑a͒ The SH image of the
samples prepared by sample preparation A. The bright microdots correspond
to the gold surface covered with gold nanoparticles using AUT SAM films,
whereas the surrounding dark area is a bare gold surface. ͑b͒ The SH image
of the samples prepared by sample preparation B. The bright microdots
correspond to the gold surface covered with gold nanoparticles using AUT
SAM films, whereas the surrounding dark area is the gold surface covered
with an ODT SAM film. Each dot is 120 ␮m in diameter.
cyanine SAM films were used for the cross-linkage between
the gold nanoparticles and the gold surface. They were dis-
solved in ethanol at a concentration of 1 mM for AUT and at
a concentration of 5 ␮M for the merocyanine disulfide
͓͑HO͑C6H4͒CH2CH2͑C5H4N+͒C11H22S−͔2·Br2−͒.9 The mol-
ecules are self-assembled by immersion of the gold substrate
in the ethanol solution. The UV photopatterning was per-
formed with a low-pressure mercury lamp ͑L937-01:
Hamamatsu K. K., Japan͒,10 followed by rinse with ethanol.
The intensity of the UV light was 25 mW/cm2, and the irra-
diation time was 12 min for the AUT film and 40 min for the
ODT film. For the UV photopatterning, two kinds of masks
were used. One was a cupper grid for a transmission electron
microscope having circular holes 125 ␮m in diameter, or-
dered in a square lattice with a separation of 200 ␮m. This
mask was used in sample preparation B. The other, used in
sample preparation A, was a homemade chromium mask,
which was made by vacuum evaporation of a 100-nm-thick
thin chromium film on a fused quartz plate through the cup-
per grids.
that of usual linear microscopy. For instance, usual UV-
visible spectroscopy provides an extinction ratio of 0.6: 1.0
even at the resonance.
SHG images of the arrays prepared by sample prepara-
tion B at each step are shown in Fig. 4. Two kinds of samples
were examined. One is the array in which AUT SAM films
were used for the cross-linkage ͓Figs. 4͑a͒–4͑c͔͒ and the
other is the array in which merocyanine SAM films were
used ͓Figs. 4͑d͒–4͑f͔͒ for the cross-linkage.
As drawn in Fig. 1͑b͒, ODT SAM film having micro-
holes are patterned first. The SH image is shown in Fig. 4͑a͒.
The bright dots correspond to the bare gold surface where the
ODT SAM films were removed by UV irradiation for
40 min. The surrounding area outside the microdots was cov-
ered with the ODT SAM film, in which the SHG activity is
by 20%–30% smaller than that of a bare gold surface as
previously reported,14 resulting in the weak SH contrast be-
The SHG image was taken with an SH microscope.11–13
The optical setup is shown in Fig. 2. A mode-locked Ti:Sa
laser ͑Tsunami, Spectra Physics Inc. ͒ was used as a light
source with a pulse width of 130 fs running at a repetition
rate of 82 MHz at a wavelength of 800 nm. The polarization
direction of the incident light was chosen with a combination
of a half wave plate and a Glan–Taylor prism. The
p-polarized incident light was loosely focused on the sample
surface to be an energy density of the incident light 5.2
ϫ104 W/m2. The SHG was detected in the reflected direc-
tion after filtering the fundamental light with color filters
͑BG-40: CVI Inc͒. The sample surface was imaged on a
cooled charge coupled device ͑CCD͒ camera ͑DU434-BV:
ANDOR, UK͒, through an objective lens ͑ϫ2.5, numerical
aperture=0.17͒. The CCD has 1024ϫ1024 pixels. Each
pixel was 13 ␮mϫ13 ␮m. The CCD camera was used at a
temperature of −30 °C. The typical accumulation time for
the SHG imaging measurements was 900 s.
Figures 3͑a͒ and 3͑b͒ show SHG images of the arrays
prepared by sample preparation A and sample preparation B,
respectively. In the bright microdots, the gold nanoparticles
were immobilized above the gold surface by the AUT SAM
film whose thickness is about 2.0 nm, according to the mo-
lecular model, assuming that the molecular long axis is
aligning in the surface normal. In Fig. 3͑a͒, the dark region
corresponds to a bare gold surface without any gold nano-
particles, and in Fig. 3͑b͒ the dark region corresponds to a
gold surface covered with an ODT SAM film. The typical
SHG intensity in the bright microdots was about
0.43 cps/pixel, whereas the intensity of the dark region ͑bare
gold surface͒ was 0.14 for the image of Fig. 3͑a͒. The con-
FIG. 4. SH images of the array at each step in sample preparation B. Images
͑a͒–͑c͒ are the preparation of the array in which gold nanoparticles are
supported by AUT SAMs and images ͑d͒–͑f͒ are that by merocyanine
SAMs. ͑a͒ The patterned ODT SAM with microholes, ͑b͒ the patterned ODT
SAM with AUT SAMs in the microholes, and ͑c͒ the array of microdots in
which gold nanoparticles are immobilized on the AUT SAMs. ͑d͒ The pat-
terned ODT SAM with microholes, ͑e͒ the patterned ODT SAM with mero-
cyanine SAMs in the microholes, and ͑f͒ the array of microdots in which
gold nanoparticles are immobilized on the merocyanine SAMs. Each dot is
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trast is evaluated to be about 2.0–3.0, which is higher than
120 ␮m in diameter.
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103102-3
K. Tsuboi and K. Kajikawa
Appl. Phys. Lett. 88, 103102 ͑2006͒
tween them. By immersion of the substrate in an ethanol
solution of AUT for 1 h, the AUT SAM films were formed in
the microdots ͓Fig. 4͑b͔͒. The SHG intensity from the micro-
dots was similar to that from the gold surface covered with
an ODT SAM film, because the sulfonate ͑S–Au͒ bonds
cause the decrease of second-order susceptibilities for both
gold surfaces.14 Hence the SHG image of the microdots dis-
appeared. However, subsequent immersion of the array in the
aqueous solution of gold nanoparticles resulted in the selec-
tive adsorption of gold nanoparticles on the AUT SAM films
in the microdots, whereas the nanoparticles were not ad-
sorbed outside the microdots ͓Fig. 4͑c͔͒. Since the SHG in-
tensity in the microdots is a few times much more than SH
intensity in the surrounding area covered with an ODT SAM
film, clear SHG image was obtained.
to photopattern an inert SAM film to have microholes fol-
lowed by deposition of the SAM films for the cross-linkage
in the microholes ͑sample preparation B͒. Both preparation
methods allowed us to prepare the ordered microdots of
surface-immobilized gold nanoparticles by immersion of the
SAM film in a gold nanoparticle solution. In the array
formed through sample preparation B, surface areas outside
the microdots can be inert by covering the surface with a
certain SAM film that resists the nonspecific binding to pro-
teins. The arrays of the microdots of gold nanoparticles were
characterized by SH microscopy, which delivers the high
contrast images due to enhancement of the electric field in
the nanogap between the gold nanoparticles and the gold
surface.
SHG images for the arrays in which merocyanine SAM
film was used for the cross-linkage are shown in Figs.
4͑d͒–4͑f͒. Figure 4͑d͒ is the SHG image for the patterned
ODT SAM film having microholes. The merocyanine SAM
film was deposited in the microholes by immersion of the
pattered ODT SAM film for 1 h in the ethanol solution of the
merocyanine disulfide. The SHG image is shown in Fig. 4͑e͒.
The microdots are bright because of a large second-order
susceptibility of the merocyanine SAM film.9 Subsequent
immersion of the substrate in the gold nanoparticle solution
allowed selective adsorption of gold nanoparticles on the
merocyanine SAM films, resulting in the rise of the contrast
of the SHG image ͓Fig. 4͑f͔͒. The selective adsorption will
be due to the electrostatic interaction between the merocya-
nine ͑positively charged͒ and the gold nanoparticles ͑nega-
tively charged͒.
In summary, we have reported preparations of microdot
arrays of the surface-immobilized gold nanoparticles on a
gold surface using UV-photopatterned SAM films. Two
methods for the sample preparation were examined: to pho-
topattern microdots of the AUT SAM films used as a spacer
on a gold surface for making cross-linkage between the sur-
face and the gold nanoparticles ͑sample preparation A͒, and
The authors thank Professor Katsuhiko Fujita of Kyushu
University for providing the merocyanine disulfide. Part of
this work was supported by the Ministry of Education, Cul-
ture, Sports, Science, and Technology of Japan, Grant-in-Aid
for Scientific Research ͑C͒ ͑15560002͒.
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