Supplementary Information
Chemical coating of large-area Au nanoparticle two dimensional arrays as
plasmon-resonant optics
Katsuhiro Isozaki1†*, Takao Ochiai1,2, Tomoya Taguchi1,2, Koh-ichi Nittoh1, Kazushi Miki1,2*
1
National Institute for Materials Science, 1-1 Namiki, Tsukuba 305-0044, Japan. 2Department of Pure and
Applied Sciences, University of Tsukuba, 1-1-2 Tennodai, Tsukuba 305-8571, Japan. †Current address:
Institute for Chemical Research, Kyoto University, Gokasho, Uji 611-0011, Japan.
Correspondence
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for
materials
should
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Isozaki(ISOZAKI.Katsuhiro@nims.go.jp) or Kazushi Miki(MIKI.Kazushi@nims.go.jp).
to
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Table of Contents
Table S1. Averaged coverage of 10Dod 2D array with several solvents and applied
voltages.
Table S2. Averaged coverage of 10Dod 2D array with several interelectrode distances.
Table S3.
Averaged coverage of 10Dod 2D array with several concentrations.
Figure S1. SEM images of cathode and anode after electrophoresis using dithiol-coated
substrates for both electrodes.
Figure S2. SEM images of several positions of the substrate.
Figure S3. SEM images of the air/solvent interface and inside solution after
electrophoresis.
Figure S4. SEM images of the dithiol-coated substrate with 10Dod particles fabricated by
the solvent evaporation method without a voltage applied.
Figure S5. SEM images of before and after sonication of the covalent bonded 10Dod AuNP
2D-arrays fabricated by our hybrid method and non-covalent bonded 10Dod
AuNP 2D-arrays fabricated by previously reported method.
Figure S6. SEM images of AuNP 2D-arrays fabricated by the hybrid method.
Figure S7. Relationship of interparticle gap distance of AuNP 2D-arrays with carbon chain
number.
Figure S8. Relationship of interdigitating ratio of alkylthiols between gold particles in each
AuNP 2D-array with carbon chain number.
Figure S9. Schematic illustrations of interdigitated alkyl chains of AuNPs in 2D-arrays.
Figure S10. Photographs of dye-coated glass and 10Dod 2D array substrates with the light
irradiation.
Figure S11. Excitation spectrum of MUF-coated 10Dod 2D array.
1
Materials. Gold nanoparticle solution was purchased from Funakoshi Corporation
and used as received. Thiols and solvents were commercially available (from Nakarai Tesque
or Wako Pure Chemical Industries, Ltd.) and used without further purification. PFC (plastic
formed carbon) electrodes were purchased from TMIL (Tsukuba Materials Information
Laboratory Ltd.) and used after cutting and cleaning by sonication in water and hexane. The
fluorescent dye, 4-methylumbelliferone, was purchased from Tokyo Chemical Industry Ltd.
and used without further purification.
Preparation of alkylthiol-coated gold particles. A colloidal aqueous solution (10.0
mL) containing Au nanoparticles (10.0 nm, 5.70×1012/mL) was mixed with a solution of
1-dodecanethiol (3.38×10-2 g, 1.67×10-1 mmol) in acetone (10.0 mL), and then stirred
vigorously for 30 min until the solution color disappeared. n-hexane (6.00 mL) was added to
the mixture solution, which was then stirred for a further 1 min. After leaving the solution to
stand for 1 h, the organic layer containing gold particles was transferred into vials for
centrifuging (MCF-1350, Laboratory and Medical Supplies). The solution was centrifuged
(1.35×104 rpm, 5 min) to cause the gold particles to precipitate. The upper homogeneous
solution was removed by decanting. The remaining particles were redispersed in acetone
(1.00 mL) by sonication (28.0 kHz, 1 min, Model USK-3A AS-ONE ultrasonic cleaner).
These centrifuging and decantation sequences were repeated twice. Finally, the precipitate
was dried under vacuum to yield alkylthiol-coated Au nanoparticles, which were added to
each solvent and dispersed by sonication (28.0 kHz, 1 min) to produce the colloidal solutions
used for the experiments.
Preparation of dithiol-coated substrates. Chromium (10 nm) and gold (40 nm)
metal were vapor-deposited onto quartz plates (10×10×0.6 mm) to produce gold substrates.
The gold substrates were immersed in 1,6-hexanedithiol solution in ethanol (3.27×10-1 M) and
then left to stand at RT for 24 h. The substrates were washed well with ethanol and acetone,
and then dried with flowing nitrogen to produce dithiol-coated substrates.
Hybrid method for fabricating AuNP 2D arrays. The dithiol-coated substrate and
PFC plate (10×10×1.0 mm) were connected to a DC source as the cathode and anode using
gold clips. The two electrodes were positioned in parallel at an interelectrode distance of 1.2
mm, immersed in the reaction vessel charged with AuNP colloidal solution, and then DC
voltage was applied at RT with the cap of the vessel open. After complete evaporation of the
solvent, the cathode was annealed at 50°C for 12 h (Model PC-420D AS-ONE hot plate).
After annealing, the substrate was washed by sonication in hexane for 1 min, and then dried
with flowing nitrogen to produce the AuNP 2D-array-coated substrate.
Characterization. Reflective UV-Vis-NIR spectra were measured using a JASCO
MSV-370 spectrometer with an aluminum plate as a reference. UV-Vis absorption spectra
were measured using a Shimadzu UV-2550 and UV-3600 spectrophotometer. SAXS
measurements were performed using beamline BL40B2 with radiant light at SPring-8, Japan.
SEM observations were carried out using a Hitachi S-4800 electron microscope without metal
sputtering. Excitation spectra were recorded on JASCO FP-6500.
Calculation of the coverage values of AuNP 2D-arrays from SEM images. SEM
images were analyzed with ImageJ 1.4lo software (National Institute of Health, USA;
http://rsb.info.nih.gov/ij/) to obtain the area value of AuNPs. The area value of AuNPs was
divided by the area of SEM images to give the coverage value of AuNP 2D-arrays.
SAXS measurements. In the case of measuring an AuNP 2D-array, the substrates
were set orthogonal to the X-ray beam (1.5 Å) and exposed directly. In the case of measuring
alkylthiol-coated AuNP colloidal solutions, capillary tubes made of quartz glass (Hilgenberg,
Φ: 2.0 mm, length: 80 mm) were charged with sample solutions, set into corresponding
holders, and exposed.
2
Electrostatic affinities of alkylthiol-coated AuNPs to the electrodes. To check the
electrostatic affinity of alkyl-coated AuNPs for the electrodes, electrophoresis of 10Dod
colloidal solution in n-hexane (5.7 × 1013 /mL) was carried out using dithiol-coated quartz
substrates for both electrodes. SEM measurements of both electrodes showed the high affinity
of alkyl-coated AuNPs for the cathode. The coverage of the cathode was 99%, although that
of the anode was less than 50%.
Optimization of the hybrid method for fabricating AuNP 2D-arrays. To optimize
the electrophoresis conditions, we tested various combinations of solvents, evaporation speeds,
applied voltages and interelectrode distances between the cathode and the anode, and
concentrations. Electrophoresis of 10Dod (5.7 × 1013 /mL) in polar solvents such as acetone,
ethanol, acetonitrile, and water (data of acetonitrile and water not shown because AuNPs did
not disperse in such solvents) did not give high coverages of 10Dod 2D-arrays because of the
aggregation of hydrophobic alkylthiol-coated AuNPs in such polar media (Table S1).
Although dichloromethane is a non-polar solvent having affinity for alkyl-coated AuNPs, the
AuNP 2D-array fabricated in dichloromethane showed low coverage (4.7–24%) because the
speed of evaporation was too fast to enable the AuNPs to assemble at the surface. Other
non-polar solvents such as toluene, cyclohexane, and n-hexane gave excellent AuNP
2D-arrays with high coverage (45–95%) and density. During the self-assembly of AuNPs the
evaporation speed of solvents act a key role. With the faster evaporation speed (2.5 × 10-2
mL/min) of n-hexane at the lower pressure (1.1 × 10-2 MPa) the coverage value was decreased
to lower 65% than that of 95% obtained from the slower evaporation with atomospheric
pressure. When a voltage of 0.50 V was applied between the cathode and the anode, the
coverage did not increase much because the electrostatic affinity between the AuNPs and the
cathode was not enough. Then, at the high applied voltage of 2.0 V, aggregations of AuNPs
inhibited further increase of the coverage. Among these applied voltages, the medium voltage
of 1.0 V showed the best coverage of monolayered AuNP 2D-arrays. Especially,
electrophoresis using n-hexane as a solvent with an applied voltage of 1.0 V showed the
highest coverage, therefore such condition is considered optimal. The interelectrode distance
is one of the most important conditions for electrophoresis because electrostatic field intensity
depends on the distance. Three interelectrode distances, 1.2, 5.0, and 10 mm, were used for
the hybrid method with 10Dod solution in n-hexane (5.7 × 1013 /mL). The results indicated
that the minimum interelectrode distance was the best for electrophoresis, as expected (Table
S2). The concentration of AuNPs is considered to be a key parameter for controlling the
coverage and density of AuNP 2D-arrays. The concentration dependency was examined using
10Dod solutions in n-hexane with different concentrations as shown in Table S3. Interestingly,
there was a critical concentration for accelerating the self-assembly of 10Dod onto the
substrate. The lower concentration of 5.7 × 1012 /mL gave lower coverage as expected, but the
higher concentration of 5.7 × 1014 /mL unexpectedly gave a lower coverage than the medium
concentration of 5.7 × 1013 /mL. The reason is thought to be that too high concentration could
accelerate the self-assembly of AuNPs into large clusters in the solution before the AuNPs
adsorb onto the substrate. These results suggest that the concentration of 5.7 × 1013 /mL is
close to optimal for the electrophoresis of AuNPs.
To understand the fabrication mechanism of the hybrid method, two independent
fabrication methods, electrophoresis without solvent evaporation and solvent evaporation
without electrophoresis, were investigated using 10Dod solution in n-hexane (5.7 × 1013 /mL).
Electrophoresis of 10Dod colloidal solution in n-hexane without solvent evaporation was
carried out in a highly sealed reaction vessel at RT for 12 h. SEM observations after
electrophoresis showed quite high coverage (> 90%) at the air–solution interface on the
substrate, despite the lower coverage (≈ 30%) over the whole area of the sample inside the
solution (Fig. S3). This result clearly indicated the efficient adsorption and self-organization
of alkylthiol-coated AuNPs onto the substrates with electrophoresis at the air–solvent
3
interface. This phenomenon can be explained by the meniscus force which increased the local
concentration of the solution and thus accelerated crystallization or self-assembly.1,2 The
solvent evaporation method was carried out at RT for 2 h to fabricate 2D-arrays from 10Dod
solution in n-hexane (5.7 × 1013 /mL) using the same reaction vessel both without a voltage
applied and without any seal. SEM observation exhibited a lower coverage (25.1%) over the
whole area of the substrate plane (Fig. S4). These results indicate that the combination of
electrophoresis and solvent evaporation methods is advantageous for accelerating the
self-assembly of alkylthiol-coated AuNPs onto the dithiol-coated substrate compared with
each method alone.
Structure Analysis. Although the interparticle gap distance is proportional to the
carbon chain number of alkylthiols, the interparticle gap distance is smaller than twice the
length of the alkylthiols which should be placed between the nearest neighboring gold
particles (Fig. S7). The decrease of the interparticle gap distance results in the interdigitation
of alkyl chains caused by hydrophobic interactions, which is in accordance with the
hydrocarbon chain lengths in micelles assumed by Tanford to be 1.5 Å per CH2 group.3 The
interdigitation ratio showed a logarithmic relationship with the carbon chain number of
alkylthiols (Fig. S8). The computer-aided calculation of model molecules showed the
interdigitation structures of alkylthiols between gold clusters (Fig. S9).
Reference
1.
2.
3.
C.-A. Dai, Y.-L. Wu, Y.-H. Lee, C.-J. Chang, W.-F. Su, J. Cryst. Growth 288, 128-136
(2006).
H. Wang, C. S. Levin, N. J. Halas, J. Am. Chem. Soc. 127, 14992-14993 (2005).
C. Tanford, The Hydrophobic Effect. Formation of Micelles and Biological Membranes,
2nd ed.; Wiley: New York, 1980.
4
A
Table S1.
Averaged coverage of
10Dod 2D-array with several solvents
and applied voltages.
Table S2.
Averaged coverage of
10Dod
2D-array
with
several
interelectrode distances.
Table S3.
Averaged coverage of
10Dod
2D-array
with
several
concentrations.
Figure S1. SEM images of cathode (a) and anode (b) after electrophoresis
using dithiol-coated substrates for both electrodes. The scale bars are 200 nm.
5
Figure S2. SEM images of the corresponding positions (a)–(g). The scale
bars are 1.0 μm.
6
Figure S3. (a) Schematic representation of the electrophoresis
set-up. The red and blue arrows indicate the air-solution interface
and inside places of the electrophoresis solution used for SEM
observations. (b) and (c) show SEM images at the interface and the
inside positions of the electrophoresis solution. Scale bars are 1.0
μm.
7
Figure S4.
SEM images of the dithiol-coated
substrate with 10Dod particles fabricated by the
combination
of
electrophoresis
and
solvent
evaporation methods (a) and solvent evaporation
method without a voltage applied (b). The scale bars
are 1.00 μm.
8
Figure S5. SEM images of covalent bonded 10Dod AuNP 2D-arrays fabricated by
our hybrid method (a), (c) and non-covalent bonded 10Dod AuNP 2D-arrays
fabricated by the previously reported method (b), (d). Before (a), (b) and after (c), (d)
sonication for 5 min at RT in n-hexane. The scale bars are 1.0 μm.
9
Figure S6. SEM images of 10Hex (a), 10Dod
(b), and 10Hexd (c) 2D-arrays fabricated by the
hybrid method. The scale bars are 50.0 nm.
10
(a)
(b)
Figure S7. (a) Relationship of interparticle gap
distances of 10Hex, 10Dod, and 10Hexd 2D-arrays
with carbon chain number. The black line shows twice
the values of the corresponding alkylthiols. (b) shows
the lengths of alkylthiols.
Figure S8. Relationship of interdigitating ratio of
alkylthiols between gold particles in each AuNP
2D-array with carbon chain number.
11
Figure S9. Schematic illustrations of interdigitated alkyl chains of (a) 10Hex, (b)
10Dod, and (c) 10Hexd particles in 2D-arrays. These structures were optimized by
molecular mechanics (Spartan ’08) using Au12 clusters instead of Au particles. Yellow:
gold, orange: sulfur, gray: carbon, white: hydrogen atoms.
12
Figure S10. Photographs of dye-coated glass and 10Dod
AuNP 2D-array substrates (a) as prepared, (b) irradiated with
400 nm light, and (c) irradiated with 422–750 nm light.
13
Figure S11. (a) Fluorescent spectrum of MUF-coated
10Dod 2D array. Fluorescent band at 460 nm corresponds
to the blue fluorescence from MUF in solid state. (b)
Excitation spectrum of MUF-coated 10Dod 2D array
(emission: 460 nm, red line) and extinction spectrum of
10Dod 2D array (blue line).
14