Enhanced catalytic activity of self-assembled-monolayer-capped gold nanoparticles

Enhanced Catalytic Activity of Self-Assembled-Monolayer-

Capped Gold Nanoparticles

Tomoya Taguchi, Katsuhiro Isozaki,* and Kazushi Miki*

Natural metalloenzymes mediate highly selective and efﬁcient

catalytic reactions on the basis of molecular recognition (i.e.,

highly ordered and selective non-covalent encapsulation of a

substrate molecule within the enzyme’s active reaction space

and subsequent transformation of the substrate to the target

product by metal-complex-based catalysis).^[1]Much effort has

been devoted to mimic molecular-encapsulation-based catalysis

since the mechanism of this catalytic reaction provides an ideal

and promising model to develop highly active catalysts. In this

sense, semiartiﬁcial approaches based on the use of natural

proteins (e.g., apoenzyme), wherein a metal-complex catalyst is

incorporated into the reaction sites as a cofactor,^[2,3]and artiﬁcial

approaches, in which large molecules such as polymers,^[4–6]den-

drimers,^[7]supramolecular hosts,^[8,9]and mesoporous materials^[10]

provide the protein-like reaction space required for enhancing

the catalytic activity of the metal complex, have been adopted.

As a result of the growing interest in metallic nanoparticle

(MNP) catalysts,^[11,12]there are numerous reports on the incor-

poration of MNPs into a reaction space constructed within

different architectures such as polymers,^[13]dendrimers,^[14]

micelles,^[15]and mesoporous materials.^[16,17]One key feature

of MNPs is their surface-functionalizing property, which has

allowed for the design of recyclable catalyst supports,^[18]sen-

sors,^[19]and drug-delivery materials.^[20]Among the different

surface functionalization strategies developed for MNPs, crea-

tion of self-assembled monolayers (SAMs) offers the possibility

of mimicking biological structures such as micelles and lipid

bilayers,^[21]thereby affording a molecular recognition ﬁeld to

encapsulate molecules. However, with a few exceptions,^[22]SAM

functionalization has not been used in catalysis since capping

of the catalyst surface generally degrades the catalytic activity.

Recently, it has been reported that alkanethiol-SAMs improve

the reaction selectivity at the Pd(111) surface, although thiols

are generally known to act as catalyst poisons; nevertheless, the

catalytic activity in this case decreased to 40% of that observed

when using unmodiﬁed Pd(111).^[23]Consequently, the simulta-

neous realization of improved catalytic activity and molecular

recognition via surface functionalization of MNPs remains a

challenge.

Herein, we report an unprecedented catalytic-activity

enhancement effect of alkanethiol-SAMs on Au nanoparticles

(AuNPs). In this system, the alkanethiol-SAM acts as a soft

interface that shows molecular-recognition properties based

on intermolecular hydrophobic interactions. This arrangement

allows for the encapsulation of substrate molecules inside the

alkanethiol-SAM space, thereby facilitating interactions with

the AuNPs and accelerating catalytic reactions at the AuNP sur-

face (Figure 1). The activity of the SAM-capped AuNP catalytic

system is maximized (turnover frequency, TOF = 55 000 h⁻¹;

turnover number, TON = 200 000, on the basis of the surface

Au atoms) when the SAM-capped AuNPs are immobilized on

Au-thin-ﬁlm-coated quartz substrates to form dense two-dimen-

sional arrays (Figure 1a,b).^[24]On the other hand, the activity

enhancement is only moderate in the colloidal state because of

self-aggregation issues. These results are valuable in that they

indicate the possibility of developing highly active catalysts with

excellent enantio-, stereo-, or substrate selectivities by designing

SAM interfaces on MNPs.

Various Au materials were investigated to study the cata-

lytic enhancement effect of the alkanethiol-SAM system in a

probe reaction, n-butanolysis of dimethylphenylsilane (DMPS).

A homogeneous Au catalyst such as tetrachloroauric acid

(HAuCl₄·3H₂O) showed very low alcoholysis activity (2.8%

yield, Table 1, entry 2), while a blank experiment (Table 1, entry

1) conﬁrmed the catalytic nature of the selected reaction. A Au-

thin-ﬁlm-coated quartz substrate showed higher catalytic activity

than the homogeneous Au salt catalyst (6.7%, Table 1, entry

3). A commercial citrate-capped 10 nm AuNP catalyst (10Cit-

AuNP, Funakoshi Corp.) and a mercaptotetraethyleneglycol-

SAM-capped 10 nm AuNP catalyst (10TEG-AuNP) could be

efﬁciently dispersed in the reaction solution, and the silylether

products were obtained in 14.7% and 12.8% yields, respectively

(Table 1, entry 6 and 7). Dodecanethiol-SAM-capped 10 nm

AuNPs (10Dod-AuNP) displaying noticeable self-aggregation

properties in the reaction solution showed unexpectedly higher

activity (12.5% yield, entry 8) as compared with the well-dis-

persed 10Cit-AuNP.

T. Taguchi, Dr. K. Isozaki, Prof. K. Miki

Polymer Materials Unit

National Institute for Materials Science

1-1 Namiki, Tsukuba, Ibaraki, 305-0044, Japan

E-mail: MIKI.Kazushi@nims.go.jp

T. Taguchi, Prof. K. Miki

Department of Pure and Applied Sciences

University of Tsukuba, 1-1-1 Tennodai

Tsukuba, Ibaraki 305-8571, Japan

Dr. K. Isozaki

Institute for Chemical Research

Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan

E-mail: kisozaki@scl.kyoto-u.ac.jp

The speciﬁc catalytic activity (TOF) of the Au-based materials

was calculated on the basis of the surface area of the AuNPs.^[25]

In the case of the 10Dod-AuNP, the effective surface area

was calculated from the mean diameter of the self-aggregated

DOI: 10.1002/adma.201202979

Adv. Mater. 2012,

DOI: 10.1002/adma.201202979

2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Table 1, entry 8) than did the

well-dispersed 10Cit-AuNP and

10TEG-AuNP catalysts. This

unprecedented catalytic enhance-

ment was presumably due to

the surface-capping alkanethiol-

SAM. Unlike a previous study

that reported

noticeable

decrease (to 40%) in the activity

of an alkanethiol-capped Pd(111)

catalyst at high temperatures,

the results herein revealed a rate

enhancement even under room-

temperature conditions.^[23]An

SAM-coated Au ﬁlm (Dod-Au

thin ﬁlm) exhibited higher cata-

lytic activity than the parent bare

Au thin ﬁlm (8.7% yield vs. 6.7%

yield, TOF = 5650 vs. 4350 h⁻¹,

Table 1, entries 3 and 4), despite

the low accessibility of the cata-

lyst surface by the substrate

molecules. These results sug-

gest that the alkanethiol-SAM

either: 1) acts as an enzyme-like

reaction space showing catalytic

activity similar to porous co-

Figure 1. a) Schematic illustration of a AuNP array. b) SEM image of a 10Dod-array. c) Schematic illustra-

tion of the metalloenzyme-like catalytic-enhancement mechanism based on molecular encapsulation in the

alkanethiol-SAM interface around the AuNPs.

ordination compounds,^[8,27–30]or 2) favors the encapsulation

and subsequent access of substrate molecules via co-operative

intermolecular interactions, as in the case of metalloenzymes

(Figure 1c).^[1]To investigate the intrinsic catalytic activity of

dodecanethiol-SAM, we developed a dodecanethiol-SAM-coated

quartz substrate (Dod-quartz) by means of the self-assembly and

a silane-coupling reaction^[31]of dodecyltrimethoxysilane with a

piranha-cleaned quartz substrate. Even though the Dod-quartz

showed measurable catalytic activity (1.8%, Table 1, entry 5),

the catalytic activity of the Dod-Au thin ﬁlm was conﬁrmed to

originate mainly from the Au surface. Furthermore, because of

particles (ca. 60 nm) determined by dynamic light scattering

(DLS) measurements (Supporting Information, Figure S14).

Interestingly, the speciﬁc activities of the well-dispersed 10Cit-

AuNP and 10TEG-AuNP catalysts were consistent with that

of a ﬂat Au thin-ﬁlm material (4900 and 4300 vs. 4350 h⁻¹,

Table 1, entries 3, 6, and 7). These results are consistent with

those of previous studies, which state that the size effect is not

observed when AuNPs larger than 10 nm are used.^[26]Surpris-

ingly, when the number of surface metal atoms was taken into

consideration, the self-aggregated 10Dod-AuNP catalyst showed

approximately three times higher activity (TOF = 13 900 h⁻¹,

Table 1. Heterogeneously catalyzed n-butanolysis of DMPS.^a)

Entry

Catalyst

Yield

[%]

surface area

[cm²]

Surface Au atoms

[× 10⁻⁹mol]

TOF of surface Au

[h⁻¹

]

no catalyst

HAuCI₄·3H₂O

Au thin ﬁlm

–

2.8

–

29.8^b)

2.31

–

140

4350

5650

–

6.7

1.0

Dod-Au thin ﬁlm

Dod-quartz

8.7

1.0

1.8

1.0

10Cit-AuNP

14.7

12.8

12.5

82.8

2.04

0.93

1.02

4.49

1.35

2.24

4900

4300

13 900

55 000

10TEG-AuNP

10Dod-AuNP

10Dod-array

^a)Reaction conditions: DMPS (150 μmol) in n-BuOH (3.0 mL) with Au catalysts (10Cit-AuNP, 10TEG-AuNP, 10Dod-AuNP, 10Dod-array: 2.67 × 10¹¹mL⁻¹, 2.98 × 10⁻⁸mol

for Au atom) at 25 °C for 1 h; ^b)Homogeneous solution.

2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Adv. Mater. 2012,

DOI: 10.1002/adma.201202979

their spherical shape, the AuNPs seemed to favor effective the

encapsulation of the substrates, in contrast to the comparatively

rigid SAMs on ﬂat surfaces, as suggested by the drastic catalytic

enhancement observed for the AuNP-containing materials after

alkanethiol-SAM capping (Table 1, entries 3, 4, 6, 8).

the 10Dod-AuNP showed poor catalytic activity, especially in the

case of the bulkier triisopropylsilane and tris(trimethylsiloxy)

silane substrates, thereby suggesting steric-hindrance effects.

The 10Dod-array showed a similar trend, although the activity

values were far higher for this sample. These results strongly

suggest that the alkanethiol-SAM interface around the AuNPs

acts as a hydrophobic encapsulating nanospace in which linear

and long-alkyl-chain-substituted molecules are better accommo-

dated, resulting in higher reactivity for these substrates. The aro-

matic silanes showed a higher reactivity than the aliphatic silanes

for all the AuNP catalysts (Table 2, entries 5, 6, 8), with DMPS

being the most reactive within 1 h of the reaction (Table 2, entry

8). When using 10Cit-AuNP, primary alcohols such as ethanol,

n-butanol, n-hexanol, and benzyl alcohol showed similar reac-

tivities (Table 2; 13.5%, 14.7%, 14.1%, and 13.2% yields, entries

7, 8, 11, and 12, respectively), in contrast to sterically bulky sec-

ondary (s-butanol: 1.9% yield) and tertiary alcohols (t-butanol:

0% yield) (Table 2, entries 9, 10). The 10Dod-AuNP and the

10Dod-array showed comparatively higher catalytic activities for

s-butanol (7.1% and 50.3% yields, respectively), although exces-

sive steric hindrance led to poor activities for t-butanol. Interest-

ingly, the 10Dod-AuNP and the 10Dod-array afforded reactivities

different from those afforded by the 10Cit-AuNP for primary

alcohols, depending on their alkyl chain lengths. In this sense,

the 10Dod-AuNP and the 10Dod-array showed the highest reac-

tivities for n-butanol (12.5% and 82.8% yields, respectively),

followed by ethanol (9.4% and 65.2% yields), n-hexanol (5.9%

and 37.6% yields), and benzyl alcohol (1.1% and 31.2% yields).

This trend suggests that the hydrophobic nanospace in the

alkanethiol-SAM allows for molecular recognition on the basis

of the relative afﬁnity between the alkanethiol-SAM and the

substrate molecules, which ultimately determines the catalytic

activity of the SAM-capped AuNPs.

Most interestingly, the 10Dod-array showed a remarkably

high catalytic activity, affording the product in an 82.8% yield

(Table 1, entry 9). In order to calculate the effective number of

Au atoms in the 10Dod-array, we assumed that the upper half

surface of each AuNP was exposed to the reaction solution (Sup-

porting Information, Figure S8). Following this assumption, the

TOF of the 10Dod-array was calculated to be 55 000 h⁻¹, which

was nearly four times that of the 10Dod-AuNP (Table 1, entries

8, 9). Although the origin of this catalytic enhancement remains

unknown, a similar effect has been reported for AuNP catalysts

supported on porous metallic oxides or polymer supports.^[32–41]

From these results, the high activity of the 10Dod-array could be

attributed to a synergistic combination of the positive effect of the

support and the catalytic enhancement effect of the alkanethiol-

SAMs. Moreover, the 10Dod-array was highly stable as it could

be reused thrice without appreciable loss of catalytic activity

(96.1% and 94.8% yields after two and three reuses, respectively,

Table S2). The TON was calculated under the same reaction con-

ditions using a smaller 10Dod-array substrate catalyst (substrate

size: 0.23 × 1.00 cm², AuNPs: 1.84 × 10¹¹particles, Au atoms:

6.85 × 10⁻⁸mol). The calculated TON values were 15 000 (total

Au basis) and 200 000 (surface Au basis), indicating that the

10Dod-array showed a much more stable catalytic activity than

the homogeneous HAuCl₄3H₂O catalyst (TON: 19 000).

In order to investigate the catalytic activity of alkanethiol-SAM-

capped AuNPs for a variety of substrates, a series of silanes, alco-

hols, and AuNP catalysts such as the 10Cit-AuNP, the 10Dod-

AuNP, and the 10Dod-array were tested in the silane alcoholysis

reaction at room temperature (Table 2). In the reactions with the

aliphatic silanes (Table 2, entries 1−4), both the 10Cit-AuNP and

Further insight into the molecular-recognition properties of

alkanethiol-SAM was obtained from the relationship between

Table 2. Alcoholysis of trialkylsilanes catalyzed by AuNP catalysts.^a)

Entry

R¹

R²

R³

Time [h]

Yield [%]

10Cit-AuNP

10Dod-AuNP

10Dod-array

24.9

0.9

i-Pr

n-Hex

Me₃SiO

i-Pr

n-Bu

1.2

0.3

0.2

0.1

0.7

n-Hex

Me₃SiO

63.6

0.9

0.6

13.5

14.7

1.9

0.7

0.5

9.4

12.5

7.1

0.1

5.9

1.1

95.5

91.4

65.2

82.8

50.3

2.3

n-Bu

s-Bu

t-Bu

n-Hex

Bzl

14.1

13.2

37.6

31.2

^a)Reaction conditions: Silane (150 μmol) in alcohol (3.0 mL) with 10Cit-AuNP, 10Dod-AuNP (2.67 × 10¹¹mL⁻¹), or 10Dod-array at 25 °C.

Adv. Mater. 2012,

DOI: 10.1002/adma.201202979

2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

showed lower reaction rates as compared

with ethanol derivatives. The activity of the

long-chain molecules increased with the

chain length of the alkanethiol-SAM. These

results clearly showed that in the case of

the long-alkyl-chain-substituted alkanethiol-

SAM, thermodynamic control prevails as the

hydrophobic interaction between the SAM

and the substrate molecules contributes to

an increase in the reactivity. Interestingly, in

the case of the n-dodecanethiol- and n-hex-

adecanethiol-capped AuNP array catalysts,

the moderately long-alkyl-chain-substituted

n-butanol gave the highest reactivity (85.9%

and 78.8% yields, respectively) because of

the competing kinetic control and ther-

modynamic control. The important role of

hydrophobic afﬁnity was further supported

by a catalytic alcoholysis reaction in a mixed

alcohol solvent (EtOH:n-BuOH:n-HexOH

= 1:1:1) with the 10Dod-array, where the

catalytic reaction proceeded slowly to afford

silylether products in the following yield:

n-hexyloxysilylether > n-butoxysilylether >

n-ethoxysilylether (Supporting Information,

Figure S10). These results clearly indicate

that the alkanethiol-SAM acts as a reac-

tion ﬁeld showing molecular-recognition

properties.

The behavior of alkanethiol-SAM as a

soft-interface was identiﬁed from the tem-

perature dependence of the catalytic activity

of the 10Dod-array and the self-aggregation

property of the 10Dod-AuNP colloid. Catalytic

alcoholysis reactions of DMPS were carried

out at various temperatures in n-BuOH using

the 10Dod-array as the catalyst. The yield vs.

temperature plot showed a characteristic non-

linear proﬁle, with the maximum catalytic

activity at around 25 °C (Figure 2b). A sim-

ilar temperature dependence was observed

with the Dod-Au ﬁlm and the 10Dod-AuNP,

but not with the 10Cit-AuNP (Supporting

Information, Figure S15). Remarkably, this

temperature dependence resembled that of

natural enzymes with speciﬁc optimum temperatures origi-

nating from their soft structures based on non-covalent bonding

interactions.^[1]In order to examine the origin of this tempera-

ture dependence, the self-aggregation property of the parent

10Dod-AuNP colloids in n-BuOH was investigated by localized

surface plasmon resonance (LSPR), ¹H NMR spectroscopy, and

DLS measurements. The LSPR band of the 10Dod-AuNP red-

shifted gradually from 550 nm with increasing concentration,

saturating at 557 nm over 6.0 × 10¹¹mL⁻¹(Supporting Infor-

mation, Figure S12). In contrast, the hydrophilic 10Cit-AuNP

and 10TEG-AuNP did not show such a concentration depend-

ency (Supporting Information, Figure S16). This concentra-

tion dependency revealed the self-aggregation property of the

10Dod-AuNP, as the above-mentioned red-shift was caused

Figure 2. a) Alcoholysis activity for different substrates with varying alkyl chain lengths.

b) Alcoholysis yield when using the 10Dod-array (blue plot) and LSPR absorption shift of col-

loidal 10Dod-AuNP solution (in n-BuOH, 2.67 × 10¹¹mL⁻¹) as a function of temperature.

the reactivities and alkyl chain lengths of the alkanethiols and

alcohol substrates. Thus, catalytic alcoholysis reactions of DMPS

were carried out at room temperature (RT) for 1 h using three

different alcohols (ethanol, n-butanol, and n-hexanol) and AuNP-

array catalysts capped with three different alkanethiols such as

n-hexanethiol, n-dodecanethiol, and n-hexadecanethiol (Figure

2a). When ethanol was used, the reactivity decreased with an

increase in the alkyl chain length of the alkanethiol-SAM, in the

following order: n-hexanethiol > n-dodecanethiol > n-hexade-

canethiol. Since ethanol is a short-alkyl-chain alcohol, it shows

much lower hydrophobic afﬁnity for the alkanethiol-SAM, and

therefore, the reaction proceeds with kinetic control. This trend

was conﬁrmed for the rest of the alcohol substrates. Owing

to steric repulsion, longer-alkyl-chain-substituted substrates

2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Adv. Mater. 2012,

DOI: 10.1002/adma.201202979

by the close packing of the AuNPs (Supporting Information,

Figure S4).^[42]The DLS measurements also showed a similar

concentration dependency for the formation of larger aggre-

gated particles of the 10Dod-AuNP (diameter: ca. 130 nm) over

6.0 × 10¹¹mL⁻¹(Supporting Information, Figure S14). In the

¹H NMR spectra, a new triplet signal due to the terminal methyl

proton of the dodecanethiols bound to AuNPs appeared in the

up-ﬁeld region, and the peaks due to the internal methylene

protons broadened with increasing concentration (Supporting

Information, Figure S17), clearly indicating the formation of

self-aggregated polymer-like particles with a bilayer structure of

interdigitated alkanethiol-SAMs.^[43]This concentration depend-

ence is typical of self-aggregation behavior based on non-cov-

alent bonding interactions, which is evidence for the molec-

ular encapsulation property of the alkanethiol-SAM around

the AuNPs through intermolecular hydrophobic interactions.

Most importantly, the temperature-dependent UV–vis extinc-

tion spectra of the 10Dod-AuNP dispersed in n-BuOH (2.7 ×

10¹¹mL⁻¹) indicated that dissociation of the self-aggregated par-

ticles started at around 25°C (Figure 2b and Supporting Infor-

mation, Figure S13), which is close to the temperature at which

the 10Dod-array shows a high non-linear catalytic activity. This

coincidence strongly suggests that the observed temperature

dependence of the catalytic activity for the 10Dod-array is due

to the thermal stability of the dodecanethiol-SAM around the

AuNPs. This relationship between catalytic activity and thermal

stability is similar to the origin of the optimal temperature in

the case of enzymes and results from the competition between

two opposite effects: the kinetic effect, which increases the reac-

tion rate with increasing temperature, and the thermodynamic

effect, which decreases the catalytic activity by thermal denatur-

ation of the protein backbone.^[1]Therefore, the non-linear high

catalytic activity of the 10Dod-array probably resulted from the

competition between the kinetic and thermodynamic driving

forces affecting the molecular encapsulation property of the

alkanethiol-SAM. The combination of such a catalytic enhance-

ment effect of the alkanethiol-SAM at room temperature and

the linear temperature dependence of the background catalytic

activity of the AuNPs affords the characteristic temperature-

dependent catalytic activity of the 10Dod-array, as depicted in

Figure 2b.

In conclusion, we found an unprecedented catalytic-

enhancement effect of alkanethiol-SAMs on AuNP systems.

The product yields observed in a series of catalytic silane alco-

holysis reactions strongly suggested that the alkanethiol-SAMs

around the AuNPs act as a reaction space for encapsulating

substrate molecules. This behavior was based on molecular

recognition through intermolecular hydrophobic interactions,

because of which catalytic reactions between the encapsulated

molecules and the AuNPs were highly accelerated. Because of

the synergistic combination of the catalytic enhancement effect

of the alkanethiol-SAM and the NP immobilization effect, the

10Dod-array exhibited an excellent catalytic activity for a variety

of silane alcoholysis reactions, with a maximum TOF of 55 000

h⁻¹and TON of 200 000. The effect of temperature and con-

centration on the self-aggregation of the colloidal 10Dod-AuNP

revealed signiﬁcant thermal instability of the hydrophobic reac-

tion space constructed of the alkanethiol-SAM. As a result, the

10Dod-array showed a high catalytic activity at low temperatures

(even at room temperature), in contrast with the other AuNP

catalysts, which required higher temperatures.^[35,40]This paper

is the ﬁrst to report on the catalytic-rate-enhancement effect

of alkanethiol-SAMs through hydrophobic-interaction-based

molecular encapsulation, which is opposite to the conventional

effect (i.e., decrease in the catalytic activity after complete cap-

ping of the catalyst surface). This new concept enables us to

design highly active and selective nanoparticle catalysts based

on the molecular-recognition properties of SAM interfaces.

Experimental Section

Fabrication of Alkanethiol-SAM-Capped AuNP-Arrays: AuNP arrays

such as 10Hex-array, 10Dod-array, and 10Hexd-array were fabricated by

our previously reported hybrid method: electrophoresis under solvent-

evaporation conditions and chemical immobilization of AuNPs on the

thiol-terminated ﬂat surface of Au-thin-ﬁlm-coated quartz substrates

(10 × 10 × 0.6 mm³) (Supporting Information, Figure S1).^[24]The

synergistic combination of electrophoresis and solvent evaporation gave

almost complete coverage (90%) of the AuNP arrays on the substrates.

Annealing enhanced the substitution of thiols between the substrates

and the AuNPs in the ﬁrst layer. After sonication cleaning for 1 min

in n-hexane, the upper AuNP layers were removed, and chemically

immobilized monolayer AuNP arrays were obtained. Scanning electron

microscopy (SEM) measurements and small-angle X-ray scattering

(SAXS) analysis revealed the formation of regular-hexagonal AuNP arrays

on the entire substrate (Supporting Information, Figure S2–S4).

Reaction Procedures for Immobilized AuNP Catalysts: A AuNP-array

substrate (1.0 × 1.0 cm²) was suspended from a poly(tetraﬂuoroethylene)

(Teﬂon)-coated wire in a reaction vessel to immerse the substrate

completely inside solution. Alcohol (3.0 mL) and silane (150 μmol) were

added, and the reaction mixture was stirred in a temperature-ﬁxed water

bath. The yield was determined by gas chromatography (GC) analysis.

All of the reaction yields reported were averaged values from 3 time

experiments.

Reaction Procedures for Colloidal AuNP Catalysts: A colloidal solution

of AuNPs dispersed in n-hexane (10Dod-AuNP) or water (10Cit-AuNP)

was prepared to have a concentration of 2.7 × 10¹¹mL⁻¹, with the same

number of particles immobilized on the 10Dod-array, by checking the

concentration by UV–vis extinction spectroscopy. The concentration-

ﬁxed colloidal AuNP solution was centrifuged and dried under vacuum

to remove solvent. The dried AuNP was dispersed into alcohol (3.0 mL)

by sonication and silane (150 μmol) was added into the solution. The

colloidal solution was stirred in a temperature-ﬁxed water bath. The yield

was determined by GC analysis. All of the reaction yields reported were

averaged values from 3 time experiments.

Supporting Information

Supporting Information is available from the Wiley Online Library or

from the author.

Acknowledgements

We thank the Japan Society for the Promotion of Science (JSPS) and the

Ministry of Education, Culture, Sports, Science and Technology of Japan

(MEXT) for ﬁnancial support; a Grant-in-Aid for Challenging Exploratory

Research (K.M., 21656007 & 24656040); a Grant-in-Aid for Scientiﬁc

Research on Priority Areas “Strong Photon-Molecule Coupling Fields”

(K.M., 21020036); a Grant-in-Aid for Scientiﬁc Research on Innovative

Areas “Integrated Organic Synthesis” (K.M., 22106545 & 24106746);

and a Grant-in-Aid for Young Scientists (K.I., 30455274). Also a part of

Adv. Mater. 2012,

DOI: 10.1002/adma.201202979

2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

our research was supported by Research Foundation for Opto-Science

and Technology. T.T. thanks the Japan Society for the Promotion of

Science for the Young Scientist Fellowship. We are grateful to Prof. M.

Nakamura and Prof. H. Takaya (Kyoto University) for their support in the

analysis and helpful discussions. DLS measurements were carried out

with assistance from Prof. Y. Sakka (NIMS). The SAXS measurements

were performed at BL40B2, SPring-8, with the approval of the Japan

Synchrotron Radiation Research Institute (JASRI) (Grant No. 2009B1471,

2010B1744, 2012A1575).

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Received: July 24, 2012

Published online:

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