Syntheses of the novel acidic and basic ligands and superlattice formation from gold nanoparticles through interparticle acid-base interaction

Article abstract of DOI:10.1246/bcsj.77.1589

Novel acidic or basic aryl-alkyl disulfides having phenol, benzoic acid, benzenesulfonic acid, aniline, and benzylamine groups were synthesized as protective agents for gold (Au) nanoparticles. Monodisperse Au nanoparticles passivated by these ligands wer

Full text of DOI:10.1246/bcsj.77.1589

ꢀ 2004 The Chemical Society of Japan
Bull. Chem. Soc. Jpn., 77, 1589–1597 (2004) 1589
Syntheses of the Novel Acidic and Basic Ligands and Superlattice
Formation from Gold Nanoparticles through Interparticle Acid–Base
Interaction
y
y
ꢀ;1;2;
Masayuki Kanehara,1; Yasunori Oumi,1 Tsuneji Sano,1 and Toshiharu Teranishi
1School of Materials Science, Japan Advanced Institute of Science and Technology,
1-1 Asahidai, Tatsunokuchi, Nomi, Ishikawa 923-1292
2‘‘Organization and Function’’, PRESTO, Japan Science and Technology Agency
Received October 29, 2003; E-mail: teranisi@chem.tsukuba.ac.jp
Novel acidic or basic aryl-alkyl disulfides having phenol, benzoic acid, benzenesulfonic acid, aniline, and benzyl-
amine groups were synthesized as protective agents for gold (Au) nanoparticles. Monodisperse Au nanoparticles passi-
vated by these ligands were prepared using a chemical reduction technique, and self-assembled into well-ordered hexag-
onal close-packed (hcp) 2D superlattices on the substrate. We also present, for the first time, the formation of an aggre-
gated multilayer of Au nanoparticles at a water–organic solvent interface by mixing separately synthesized acidic and
basic Au nanoparticles. This film was definitely different from those obtained by the LB method and the conventional
self-assembly method through solvent evaporation.
Chemical techniques for the preparation and size separation
of organically protected metal1–3 and semiconductor nanoparti-
cles4,5 have advanced rapidly over the past several years. Such
nanoparticles can serve as building blocks for the creation of
extended one-,6–8 two-,9–15 and three-dimensional16,17 systems.
Recently, two- and three-dimensional superlattices of Au and
Ag nanoparticles have attracted the attention of many scientists
concerning their collective electronic18,19 and optical proper-
ties.20,21 In such systems, the interparticle spacings20 and perio-
dicity22 within the superlattices are the key determinant to con-
trol their physical properties. An acid–base interaction has been
effectively used to control both the interparticle spacings and
periodicity of metal nanoparticle superlattices. The acid–base
mediated fabrication of ordered monolayers of Au cluster com-
pounds was first reported by Schmid’s group.23 They utilized a
strong attractive interaction between sulfonic groups on cluster
surfaces and polyelectrolytes containing imino groups on a sub-
strate to promote the self-assembly of the acidic [Au55{PPh2(m-
C6H4SO3Na)12}Cl6] on a mica surface coated with poly(eth-
ylenimine). Rotello’s group has employed poly(amidoamine)
(PAMAM) dendrimers to assemble Au nanoparticles function-
alized with carboxylic acid through acid–base chemistry be-
tween the particle and the dendrimer.24 Five kinds of PAMAM
dendrimers with different generations (0, 1, 2, 4, 6) were used to
achieve a monotonic increase in the interparticle spacing from
4.1 to 6.1 nm with increasing the generation of dendrimers. Re-
cently, we demonstrated the novel concept based on an acid–
base interaction for controlling the structure of Au nanoparticle
superlattices.15 The amino-functionalized Au nanoparticles as
building blocks self-assembled into different symmetric 2D su-
perlattices using some organic acids. The 2D superlattices of
quasi-honeycomb and square structures were obtained by neu-
tralizing the amino-functionalized Au nanoparticles with ben-
zene-1,3,5-tricarboxylic acid and acetic acid, respectively.
The acid–base interaction between acidic metal nanoparticles
and basic ones seems to have a great potential for not only con-
trolling the structure of superlattices, but also for fabricating the
metal nanoparticle alloy structure, where the different kinds of
metal nanoparticles passivated by acidic and basic ligands are
controllably orientated alternatively (Fig. 1).
: Metal core
: Acidic ligand
: Basic ligand
y Present address: Graduate School of Pure and Applied Sciences,
University of Tsukuba, 1-1-1 Tennodai, Tsukuba, Ibaraki 305-
8571
Fig. 1. Schematic illustration of acid–base interaction
between acidic and basic metal nanoparticles to form the
alternatively arranged superlattice.
Published on the web August 6, 2004; DOI 10.1246/bcsj.77.1589

1590 Bull. Chem. Soc. Jpn., 77, No. 8 (2004)
Acidic and Basic Au Nanoparticle Superlattices
In this paper, we report on the syntheses of acidic and basic
ligands having various pKa values. Monodisperse Au nanopar-
ticles passivated by these ligands are prepared using a chemical
reduction technique, and being self-assembled into well-
ordered hexagonal close-packed (hcp) 2D superlattices on a
substrate. We also present, for the first time, the formation of
an aggregated multilayer of Au nanoparticles at a water–organ-
ic solvent interface by mixing separately synthesized acidic and
basic Au nanoparticles.
Experimental
General. The chemicals were bought from Sigma-Aldrich,
Wako Pure Chemicals, Kanto Kagaku, and Tokyo Chemical Indus-
try and used without further purification, unless otherwise noted.
Syntheses of Acidic and Basic Ligands. Various acidic (1–3
in Scheme 1) and basic (4, 5 in Scheme 1) ligands were synthesized
as follows:
(4-But-3-enyl)anisole (6). To a dispersion of p-bromoanisole
(3.0 g, 16.0 mmol) and [PdCl2(dppf)] (dppf: bis(diphenylphos-
OMe
OH
OMe
OMe
1, HBr, AcOH
reflux
AcSH, CH2Cl2, hν
MgBr
2, NaOH, H2O
(27%)
Pd(dppf), THF
(88%)
(97 %)
Br
S
Me
S
2
6
O
1
7
O
Me
COOH
AcCl, AlCl3,
CH2Cl2
AcSH, CH2Cl2, hν
KOH, H2O, CCl4
(18 %)
(100 %)
(99 %)
S
Me
S
Me
S
2
O
O
2
8
9
SO2Cl
SO3Na
KOH, 2-PrOH
HSO3Cl
(70 %)
NaOH
(95 %)
Me
MeCN, H2O
(88 %)
S
S
2
S
S
2
2
O
10
8
3
11
NMe2
NMe2
NMe2
NMe2
Br
AcSH, CH2Cl2, hν
NaOH, 2-PrOH
(87 %)
MgBr
Pd(dppf), THF
(91%)
(97 %)
S
Me
S
2
O
4
12
13
NEt2
NEt2
NEt2
Br
NEt2
AcSH, hν
NaOH, 2-PrOH
(68 %)
CH2Cl2
NHEt2
(97 %)
MgBr
Pd(dppf), THF
(89%)
(80 %)
Br
Br
S
Me
S
2
14
15
O
5
16
Scheme 1.

M. Kanehara et al.
Bull. Chem. Soc. Jpn., 77, No. 8 (2004) 1591
phino)ferrocene) (392 mg, 0.48 mmol) in dried THF (50 mL), a
dried THF solution (50 mL) of 4-butenylmagnesium bromide
(35.3 mmol) prepared from magnesium and 4-bromobutene was
added dropwise at room temperature; the solution was then re-
fluxed for 3 h. After removing the solvent under a vacuum, di-
chloromethane was added and the mixture was washed with a di-
lute aqueous NH4Cl solution, dried over Na2SO4, and evaporated
to concentration. The product was purified by column chromatog-
raphy (silica gel, hexane:dichloromethane = 2:1) to give 6 (2.30 g,
2.89 (t, 2H), 2.68 (t, 2H), 2.58 (s, 3H), 2.32 (s, 3H), 1.65 (m,
4H). GC–EI-MS 250 (Mþ).
Bis[4-(4-carboxyphenyl)butyl] Disulfide (2). To a mixture of
tetrachloromethane (20 mL) and potassium hydroxide (10 g) in wa-
ter (20 mL), 9 (1.80 g, 7.19 mmol) was added, and the solution was
vigorously stirred for 12 h at room temperature. The organic phase
was removed, and sulfuric acid was then added to the water phase.
The resulting precipitate was filtered off, washed with water and
1
hexane to give 2 (270 mg, 18%). H NMR (300 MHz, CD3OD)
ꢀ 7.96 (d, 4H), 7.32 (d, 4H), 2.71 (m, 8H), 1.73 (m, 8H). DI-EI-
MS 418 (Mþ).
1
88%). H NMR (300 MHz, CDCl3) ꢀ 7.10 (d, 2H), 6.83 (d, 2H),
5.85 (m, 1H), 5.00 (m, 2H), 3.79 (s, 3H), 2.65 (t, 2H), 2.34 (q,
2H). GC–EI-MS 162 (Mþ).
Bis(4-phenyl)butyl] Disulfide (10). 2-Propanol (20 mL) solu-
tion of 8 (3.0 g, 14.4 mmol) and KOH (660 mg, 10.0 mmol) was
stirred under ambient condition at room temperature for 17 h. After
removing the solvent under a vacuum, dichloromethane was added
and the mixture was then washed with a diluted aqueous NaOH so-
lution, dried over Na2SO4, and evaporated to concentration. The
product was purified by column chromatography (silica gel, hex-
ane:ethyl acetate = 2:1) to give 10 (2.25 g, 95%) as a colorless
oil. 1H NMR (300 MHz, CDCl3) ꢀ 7.27 (m, 4H), 7.18 (m, 6H),
2.66 (m, 8H), 1.71 (m, 8H). GC-EI-MS 330 (Mþ).
S-4-(4-Methoxyphenylbutyl) Thioacetate (7). A mixture of 6
(2.20 g, 13.6 mmol) and thioacetic acid (1.24 g, 16.3 mmol) in di-
chloromethane (50 mL) was irradiated with a 450 W medium-pres-
sure mercury lamp for 1.5 h. After removing the solvent under a
vacuum, dichloromethane was added, and the mixture was washed
with saturated aqueous NaHCO3 solution, dried over Na2SO4, and
evaporated to concentration. The product was purified by column
chromatography (silica gel, hexane:ethyl acetate = 2:1) to give 7
1
(3.14 g, 97%). H NMR (300 MHz, CDCl3) ꢀ 7.06 (d, 2H), 6.80
(d, 2H), 3.77 (s, 3H), 2.87 (t, 2H), 2.55 (t, 2H), 2.30 (s, 3H),
1.61 (m, 4H). GC–EI-MS 238 (Mþ).
Bis[4-(4-chlorosulfonylphenyl)butyl] Disulfide (11). Chloro-
sulfuric acid (8.7 g, 74.4 mmol) was added dropwise into a solution
of 10 (1.23 g, 3.72 mmol) in dry dichloromethane (20 mL) under
cooling with an ice-water bath; the solution was then stirred at
room temperature for 1.5 h. The solution was poured into crushed
ice, extracted with dichloromethane, dried over Na2SO4, and
evaporated to concentration. The product was purified by column
chromatography (silica gel, dichloromethane) to give 11 (1.38 g,
Bis[4-(4-hydroxyphenyl)butyl] Disulfide (1). An acetic acid
(20 mL) solution of 7 (2.0 g, 8.4 mmol) and conc. HBr (8.0 mL,
74 mmol) was refluxed under ambient condition for 2 h. After re-
moving the solvent under a vacuum, a 6 M aqueous NaOH solution
was added, and the mixture was washed with dichloromethane.
The aqueous solution was then stirred under ambient condition at
room temperature for 60 h. After adding sulfuric acid to the solu-
tion, the product was extracted with dichloromethane, dried over
Na2SO4, and evaporated to concentration. The product was puri-
fied by column chromatography (silica gel, dichloromethane:ethyl
acetate:methanol = 100:10:1) to give 1 (0.42 g, 27%) as a pale yel-
low solid. 1H NMR (300 MHz, CDCl3) ꢀ 7.02 (d, 4H), 6.74 (d, 4H),
5.17 (s, 2H), 2.66 (t, 4H), 2.54 (t, 4H), 1.67 (m, 8H). DI-EI-MS 362
(Mþ).
1
70%). H NMR (300 MHz, CDCl3) ꢀ 7.95 (d, 4H), 7.42 (d, 4H),
2.76 (t, 4H), 2.68 (t, 4H), 1.75 (m, 8H). DI-EI-MS 526 (Mþ).
Disodium Bis[4-(4-sulfonatophenyl)butyl] Disulfide (3). To
an acetonitrile solution (40 mL) of 11 (1.35 g, 2.56 mmol), a so-
dium hydroxide (0.56 g, 14.4 mmol) aqueous solution (30 mL)
was added, and the solution was then stirred at room temperature
for 16 h. The solution was concentrated under reduced pressure,
purified by column chromatography (ODS silica, water:methanol
= 1:1) to give 3 (1.20 g, 88%). 1H NMR (300 MHz, DMSO) ꢀ
7.44 (d, 4H), 7.07 (d, 4H), 3.53 (s, 4H), 2.61 (t, 4H), 2.49 (t,
4H), 1.54 (m, 8H).
S-4-Phenylbutyl Thioacetate (8). A mixture of 4-phenyl-1-
butene (4.40 g, 33.3 mmol) and thioacetic acid (3.81 g, 50.0 mmol)
in dichloromethane (50 mL) was irradiated with a 450 W medium-
pressure mercury lamp for 30 min. After removing the solvent un-
der a vacuum, dichloromethane was added, and the mixture was
washed with a saturated aqueous NaHCO3 solution, dried over
Na2SO4, and evaporated to concentration. The product was puri-
fied by column chromatography (silica gel, hexane:ethyl acetate
4-(4-But-3-enyl)-N,N-dimethylaniline (12). To a dispersion
of p-bromo-N,N-dimethylaniline (2.0 g, 10.0 mmol) and [PdCl2-
(dppf)] (82 mg, 0.1 mmol) in dried THF (30 mL), a dried THF so-
lution (50 mL) of 4-butenylmagnesium bromide (22.0 mmol), pre-
pared from magnesium and 4-bromobutene, was added dropwise at
room temperature. The solution was then refluxed for 6 h. After re-
moving the solvent under a vacuum, dichloromethane was added
and the mixture was washed with a dilute aqueous NH4Cl solution,
dried over Na2SO4, and evaporated to concentration. The product
was purified by column chromatography (silica gel, hexane:dichlo-
1
= 1:1) to give 8 (7.03 g, 100%). H NMR (300 MHz, CDCl3) ꢀ
7.28 (t, 2H), 7.16 (d, 3H), 2.88 (t, 2H), 2.62 (t, 2H), 2.31 (s, 3H),
1.64 (m, 4H). GC–EI-MS 208 (Mþ).
S-4-(4-Acetylphenylbutyl) Thioacetate (9). After trichloro-
aluminum anhydride (2.82 g, 21.1 mmol) was dissolved with dry
dichloromethane (20 mL), the mixture was cooled to 0 ꢁC with
an ice-water bath. Acetyl chloride (0.9 g, 11.5 mmol) was added
to the solution, and the mixture was stirred at 0 ꢁC for 10 min.
To the solution, 8 (2.0 g, 9.60 mmol) was added dropwise within
10 min at 0 ꢁC, and the solution was contentiously stirred at 0
ꢁC for 3 h. Water (10 mL) was carefully added dropwise to the so-
lution, and 6 M sulfuric acid (10 mL) was then added to dissolve
the aluminum hydroxide. The solution was extracted with
dichloromethane, dried over Na2SO4, and evaporated to concentra-
tion. The product was purified by column chromatography (silica
gel, dichloromethane:ethyl acetate = 100:1) to give 9 (2.39 g,
1
romethane = 1:1) to give 12 (1.60 g, 91%). H NMR (300 MHz,
CDCl3) ꢀ 7.06 (d, 2H), 6.68 (d, 2H), 5.86 (m, 1H), 5.00 (m, 2H),
2.89 (s, 6H), 2.61 (t, 2H), 2.33 (q, 2H). GC–EI-MS 175 (Mþ).
S-4-(4-Dimethylaminophenyl)butyl Thioacetate (13).
A
mixture of 12 (1.54 g, 8.79 mmol) and thioacetic acid (3.13 mL,
44.0 mmol) in dichloromethane (70 mL) was irradiated by a 450
W medium-pressure mercury lamp for 1.5 h. After removing the
solvent under a vacuum, dichloromethane was added and the mix-
ture was washed with a saturated aqueous NaHCO3 solution, dried
over Na2SO4, and evaporated to concentration. The product was
purified by column chromatography (silica gel, dichloromethane:
ethyl acetate = 10:1) to give 13 (2.15 g, 97%). 1H NMR (300
1
99%). H NMR (300 MHz, CDCl3) ꢀ 7.88 (d, 2H), 7.23 (d, 2H),

1592 Bull. Chem. Soc. Jpn., 77, No. 8 (2004)
Acidic and Basic Au Nanoparticle Superlattices
MHz, CDCl3) ꢀ 7.05 (d, 2H), 6.68 (d, 2H), 2.91 (s, 6H), 2.89 (t,
2H), 2.53 (t, 2H), 2.32 (s, 3H), 1.62 (m, 4H). GC–EI-MS 251 (Mþ).
Bis[4-(4-dimethylaminophenyl)butyl] Disulfide (4). A 2-
propanol solution (20 mL) of 13 (1.09 g, 4.34 mmol) and NaOH
(0.48 g, 12.0 mmol) was stirred under ambient condition for 34
h. After removing the solvent under a vacuum, dichloromethane
was added and the mixture was washed with a dilute aqueous
NaOH solution, dried over Na2SO4, and evaporated to concentra-
tion. The product was purified by column chromatography (silica
gel, dichloromethane:ethyl acetate = 20:1) to give 4 (0.79 g,
87%) as a pale yellow oil. 1H NMR (300 MHz, CDCl3) ꢀ 7.05
(d, 4H), 6.69 (d, 4H), 2.90 (s, 12H), 2.68 (t, 4H), 2.54 (t, 4H),
1.67 (m, 8H). GC–EI-MS 416 (Mþ).
ligand-protected Au nanoparticles were synthesized as follows:
Ligand 1-Protected Au (1–Au) Nanoparticles. Brust’s two-
phase reaction was modified to synthesize 1–Au nanoparticles.
To a mixture of o-dichlorobenzene (o-DCB, 30 mL), acetic acid
(0.1 mL), 1 (10.0 mmol), and HAuCl4 4H2O (10.0 mmol) in water
ꢂ
(15 mL), tetraoctylammonium bromide (11 mg, 20.0 mmol) was
added and the mixture was vigorously stirred for 5 min. Under vi-
gorous stirring, a NaBH4 (100 mmol) aqueous solution (5 mL) was
swiftly added to the solution, and continuously stirred for 30 min to
obtain 1–Au nanoparticles as a precipitate. The resulting precipi-
tate of 1–Au nanoparticles was filtered off, washed with water
and o-DCB, and then dissolved into a few mL of N,N-dimethyl-
acetamide (DMAc).
4-Bromo-N,N-diethylbenzylamine (14). 4-Bromobenzyl bro-
mide (10.0 g, 39.0 mmol) was added to diethylamine (50 mL) at
room temperature, and the solution was refluxed for 1 h. After re-
moving the solvent under a vacuum, dichloromethane was added
and the mixture was then washed with dilute aqueous NaOH solu-
tion, dried over Na2SO4, and evaporated to concentration. The
product was purified by column chromatography (activated alumi-
na, ethyl acetate) to give 14 (10.0 g, 97%). H NMR (300 MHz,
CDCl3) ꢀ 7.41 (d, 2H), 7.21 (d, 2H), 3.50 (s, 2H), 2.49 (q, 4H),
1.02 (t, 6H). GC–MS 241 (Mþ).
Ligand 2-Protected Au (2–Au) Nanoparticles. To a MeOH/
H2O (44 mL/1 mL) mixed solution of 2 (5.0 mmol) and
HAuCl4 4H2O (10.0 mmol), a NaBH4 (100 mmol) methanol solu-
ꢂ
tion (5 mL) was swiftly added under vigorous stirring. The solution
was continuously stirred for 30 min to give 2–Au nanoparticles.
Ligand 3-Protected Au (3–Au) Nanoparticles. To a MeOH/
DMAc/AcOH (44 mL/1 mL/0.1 mL) mixed solution of 3 (5.0
1
mmol) and HAuCl4 4H2O (10.0 mmol), a NaBH4 (100 mmol)
ꢂ
methanol solution (5 mL) was swiftly added under vigorous stir-
ring. The solution was continuously stirred for 30 min to give a
methanol solution of 3–Au nanoparticles. Water was added to pre-
cipitate the 3–Au nanoparticles and the 3–Au nanoparticles were
filtered off, followed by being redispersed in methanol (2.0 mM
in a concentration based on the Au atom).
4-(4-But-3-enyl)-N,N-diethylbenzylamine (15). To a disper-
sion of 14 (2.0 g, 8.3 mmol) and [PdCl2(dppf)] (67 mg, 83 mmol) in
dried THF (50 mL), a dried THF solution (50 mL) of 4-butenyl-
magnesium bromide (18 mmol), prepared from magnesium and
4-bromobutene, was added dropwise at room temperature. The so-
lution was then refluxed for 1.5 h. After removing the solvent under
a vacuum, dichloromethane was added and the mixture was wash-
ed with a dilute aqueous NH4Cl solution, dried over Na2SO4, and
evaporated to concentration. The product was purified by column
chromatography (silica gel, hexane:ethyl acetate = 1:1 then ethyl
Ligand 4-Protected Au (4–Au) Nanoparticles. To a DMAc
solution (45 mL) of 4 (10.0 mmol) and HAuCl4 4H2O (10.0 mmol),
ꢂ
NaBH4 (100 mmol) methanol solution (5 mL) was swiftly added
under vigorous stirring. The solution was continuously stirred for
30 min to give 4–Au nanoparticles. Water was added to precipitate
the 4–Au nanoparticles, and the 4–Au nanoparticles were then fil-
tered off, followed by being redispersed in DMAc (2.0 mM in a
concentration based on the Au atom).
1
acetate:diethylamine = 10:1) to give 15 (1.60 g, 89%). H NMR
(300 MHz, CDCl3) ꢀ 7.24 (d, 2H), 7.12 (d, 2H), 5.86 (m, 1H),
5.00 (m, 2H), 3.50 (s, 2H), 2.68 (t, 2H), 2.50 (q, 4H), 2.38 (q,
2H), 1.04 (t, 6H). GC–MS 217 (Mþ).
Ligand 5-Protected Au (5–Au) Nanoparticles. To a DMAc
solution (45 mL) of 5 (5.0 mmol) and HAuCl4 4H2O (10.0 mmol),
ꢂ
S-[4-(4-Diethylaminomethylphenyl)butyl] Thioacetate (16).
A mixture of 15 (1.60 g, 7.36 mmol) and thioacetic acid (2.8
mL, 39 mmol) in dichloromethane (50 mL) was irradiated with a
450 W medium-pressure mercury lamp for 4 h. After removing
the solvent under a vacuum, dichloromethane was added and the
mixture was washed with a saturated aqueous NaHCO3 solution,
dried over Na2SO4, and evaporated to concentration. The product
was purified by column chromatography (silica gel, hexane:ethyl
acetate = 1:1 then ethyl acetate:diethylamine = 100:1) to give
16 (1.73 g, 80%). 1H NMR (300 MHz, CDCl3) ꢀ 7.23 (d, 2H),
7.10 (d, 2H), 3.53 (s, 2H), 2.89 (t, 2H), 2.60 (t, 2H), 2.52 (q,
4H), 2.32 (s, 3H), 1.65 (m, 4H), 1.04 (t, 6H). GC–MS 293 (Mþ).
NaBH4 (100 mmol) methanol solution (5 mL) was swiftly added
under vigorous stirring. The solution was continuously stirred for
30 min to give 5–Au nanoparticles. After water was added to pre-
cipitate the 5–Au nanoparticles, the 5–Au nanoparticles were fil-
tered off, followed by being redispersed in ethyl acetate (2.0 mM
in concentration based on the Au atom).
Preparation of an Aggregated Network of Au Nanoparticles
through an Acid–Base Interaction. To a mixture of a 2.0 mM 5–
Au ethyl acetate solution (100 mL) and water (300 mL) in a poly-
propylene test tube, a 2.0 mM 3–Au methanol solution (100 mL)
was added, and the mixture was vigorously shaken. An aggregated
layer formed at an ethyl acetate–water interface was poured onto
100 mL of water. The resulting film was transferred onto a poly-
vinylformal-coated TEM grid, using a horizontal lifting method
in which a TEM grid was carefully contacted horizontally with
the film.
Measurements. Samples for transmission electron microscopy
(TEM) were prepared by dropping Au nanoparticle solutions onto
amorphous carbon-coated Cu grids. TEM images were taken at an
accelerating voltage of 100 kV on a Hitachi H-7100. The particle
sizes of Au nanoparticles were estimated by counting two hundred
particles from TEM images. The mean interparticle spacing was
obtained by a direct measurement of individual spacings within a
well-ordered hcp domain. NMR (1H, 300 MHz) spectra were ob-
tained in CDCl3, CD3OD or DMSO-d6 on a Varian 300 MHz spec-
Bis[4-(4-diethylaminomethylphenyl)butyl] Disulfide (5).
A
2-propanol solution (20 mL) of 16 (1.70 g, 5.79 mmol) and
KOH (0.66 g, 10.0 mmol) was stirred under ambient condition at
room temperature for 26 h. After removing the solvent under a vac-
uum, dichloromethane was added and the mixture was washed with
a dilute aqueous NaOH solution, dried over Na2SO4, and evaporat-
ed to concentration. The product was purified by column chroma-
tography (activated alumina, dichloromethane:diethylamine =
100:1) to give 5 (0.98 g, 68%) as a pale yellow oil. 1H NMR
(300 MHz, CDCl3) ꢀ 7.23 (d, 4H), 7.10 (d, 4H), 3.53 (s, 4H),
2.68 (t, 4H), 2.61 (t, 4H), 2.51 (q, 8H), 1.70 (m, 8H), 1.04 (t,
12H). GC–MS 501 (Mþ).
Preparation of Au Nanoparticles. Various acidic and basic

M. Kanehara et al.
Bull. Chem. Soc. Jpn., 77, No. 8 (2004) 1593
trometer model Gemini. The chemical shifts are reported in ppm,
relative to tetramethylsilane (ꢀ 0.00). GC and DI EI-mass analyses
were carried out on a Shimadzu QP-5050 mass spectrometer. UV–
vis spectra were obtained on a Hitachi U-3310 spectrophotometer
in the 350–800 nm range. Powder X-ray diffraction (XRD) of the
amine and a following similar introduction of alkyldisulfide
gave the benzylamine-derivative 5 in an overall yield of 47%.
In this case, two or more equivalents of thioacetic acid were re-
quired during the photo radical addition to olefin 15 due to salt
formation between benzylamine and thioacetic acid. For the
syntheses of aryl-alkyl disulfides, our three-step approach, that
is, the cross coupling, photo radical addition of thioacetic acid,
and alcoholysis, is a quite useful synthetic route.
ꢀ
Au nanoparticles was performed using Cu Kꢁ radiation (1.540 A)
on a MAC science M18XHF-SRA.
Results and Discussion
Formation and Characterization of Au Nanoparticles.
All of the Au nanoparticles protected by ligands 1–5 were syn-
thesized by the chemical reduction of Au(III) to Au(0) with
NaBH4 as a reducing agent in the presence of 0.5–1 equivalents
of ligands to Au species. The Au nanoparticles were purified
with poor solvents, and then dissolved into a proper solvent
to prepare a solution containing only pure Au nanoparticles.
The reaction conditions, mean diameters and standard devia-
tions of the obtained Au nanoparticles are given in Table 1.
The 1–Au nanoparticles were synthesized using a modified
Brust’s method.25 The addition of AcOH is requisite to produce
monodisperse Au nanoparticles, by preventing the formation of
phenolate.26 The Au nanoparticles obtained at a molar ratio of
1/Au = 1 were 2:1 ꢃ 0:3 nm in size. Although the phenol-de-
rivative 1 could be dissolved in o-DCB, the resulted 1–Au
nanoparticles were not soluble in both o-DCB and water, indi-
cating that the nanoparticles started to precipitate when their
size reached a certain diameter. This in-situ size-selective pre-
cipitation technique was quite effective for 1–Au nanoparticles.
The 1–Au nanoparticles could be dissolved into some polar sol-
vents, like MeOH and DMAc. When benzoic acid 2 was used as
a protective ligand, the monodisperse Au nanoparticles with
sizes of 2:3 ꢃ 0:3 nm were successfully obtained at a molar ra-
tio of 3/Au = 0.5 in a MeOH/H2O mixed solvent. The 2–Au
nanoparticles could be dissolved into some polar solvents, like
MeOH, ethyleneglycol, and DMAc. For the sulfonate-deriva-
tive 3, 3–Au nanoparticles prepared at a molar ratio of 3/Au
= 0.5 in water had a mean diameter of 1.8 nm with a large size
distribution, presumably because a micelle formation of the li-
gands in water confines the disulfide groups inside the micelles
and suppresses the uniform protection of nanoparticles. For the
amine-derivatives 4 and 5, monodisperse Au nanoparticles
were easily obtained in a MeOH/DMAc mixed solvent; their
sizes were 2:1 ꢃ 0:2 nm and 2:4 ꢃ 0:2 nm, respectively. Espe-
General Remarks on Ligand Syntheses. In order to dem-
onstrate our new concept for the formation of novel 2D nano-
particle superlattices based on the acid–base interaction, acidic
(1–3) and basic (4, 5) ligands of various pKa values were syn-
thesized. These ligands were designed to have three functional
groups, that is, disulfide groups to coordinate with the surface
Au atoms of the nanoparticles, benzene rings endowing solubil-
ity with Au nanoparticles, and acidic or basic groups to provide
the reactive sites.
Terminal olefin 6 was synthesized using a cross-coupling re-
action over a Pd catalyst between a Grignard reagent and p-bro-
moanisol (Scheme 1). The Ni(dppf)-catalyzed cross-coupling
reaction failed in this case. The photo radical addition of thio-
acetic acid to the olefin 6 proceeded smoothly in almost equiv-
alent yield to give thioester 7. Since this process did not require
any radical initiator reagent, the reaction was very clean and the
product was easily purified by short silica-gel column chroma-
tography. The thioester 7 was treated with boiling HBr in acetic
acid; then, the auto-oxidation of thiol gave the phenol-deriva-
tive 1 in an overall yield of 23%. Thioester 8 was synthesized
under the same condition as thioester 7. Then, the standard
Friedel–Crafts acylation provided acetophenone 9. A two-
phase haloform reaction in CCl4 and aqueous KOH gave the
benzoic acid-derivative 2 in an overall yield of 18%. Disulfide
10 was synthesized through mild methanolysis and auto-oxida-
tion of thioester 8. A subsequent chlorosulfuric acid treatment
at room temperature gave sulfonyl chloride 11, which was hy-
drolyzed to give 3 in an overall yield of 59%. Olefin 12 was
synthesized through a cross-coupling reaction over a Pd cata-
lyst under the same condition as that of 6. The subsequent photo
radical addition of thioacetic acid and mild alcoholysis gave
aniline-derivative 4 in an overall yield of 77%. The standard
substitution of p-bromomethylbromobenzene with diethyl-
Table 1. Reaction Conditions and Particle Sizes of the Au Nanoparticles Protected by the
Ligand 1–5
Ligand
No.
Ligand
/mmol
HAuCl4
/mmol
NaBH4
/mmol
Solvent
/mL
Particle size
/nm
o-DCB
H2O
AcOH
30
20
0.1
1
10
10
100
2:1 ꢃ 0:3
MeOH
H2O
49
1
2
3
4
5
5
10
10
10
100
100
100
2:3 ꢃ 0:3
1:8 ꢃ 0:6
2:1 ꢃ 0:2
H2O
50
DMAc
MeOH
45
5
10
DMAc
MeOH
45
5
5
5
10
100
2:4 ꢃ 0:2

1594 Bull. Chem. Soc. Jpn., 77, No. 8 (2004)
Acidic and Basic Au Nanoparticle Superlattices
Fig. 2. UV–vis spectra of (a) 1–Au, (b) 2–Au, (c) 3–Au, (d) 4–Au, and (e) 5–Au nanoparticles.
cially, the 5–Au nanoparticles were highly soluble in both polar
solvents, like DMAc, and non-polar solvents, like toluene.
Figure 2 shows UV–vis spectra of the Au nanoparticles. The
broad tailing absorptions were observed in the range from the
UV to visible regions in all cases, due to the formation of a band
structure, indicating the formation of Au nanoparticles. Small
surface plasmon resonances at ꢄ520 nm were observed from
all Au nanoparticles, which means the formation of very small
Au nanoparticles.27 Especially, small surface resonances of 1–
Au and 3–Au reflect their quite small sizes. The powder X-ray
diffraction patterns of 5–Au in Fig. 3 revealed that the 5–Au
have the typical face-centered-cubic (fcc) structure for Au,
and that the particle size calculated from the full width at half
maximum of the (111) peak with Scherrer’s equation is in good
agreement with the mean diameter obtained from the TEM ob-
servation. Other Au nanoparticles were also identified to have
fcc structures in a similar fashion.
Fabrication of Au Nanoparticle Superlattices. Generally,
the monodisperse nanoparticles have a strong tendency to form
self-assembled superstructures, so-called superlattices, during
evaporation of the solvent. The solutions of Au nanoparticles
were dropped on amorphous carbon-coated TEM grids, which
were immediately covered with glass lids to slowly evaporate
the solvent. Figure 4 shows TEM images of Au nanoparticles
protected by ligands 1, 2, 4, and 5. TEM observations revealed
that these monodisperse Au nanoparticles formed self-assem-
bled hcp 2D superlattices, where the mean interparticle spac-
ings were estimated to be ꢄ1:4 nm in any superlattices. By tak-
ing the length of these ligand molecules to be ꢄ1:3 nm into
consideration, this result suggests that each Au nanoparticle

M. Kanehara et al.
Bull. Chem. Soc. Jpn., 77, No. 8 (2004) 1595
R
(111)
R
S
S
R
S
R
S
S
(200)
S
(220)
(311)
R
S
R
R
30
40
50
60
(degrees)
70
80
90
Fig. 5. Schematic illustration of interligand ꢂ–ꢂ interac-
tion, where R stands for acidic or basic group.
2
θ
Fig. 3. XRD diffraction pattern of 5–Au nanoparticles.
Fig. 4. TEM image of (a) 1–Au, (b) 2–Au, (c) 4–Au, and (d) 5–Au nanoparticles. The inset in (d) stands for the FFT image.
is stabilized by the ligand monolayer, and that the protective li-
gands are interpenetrating each other as the result of an aromat-
ic interaction between the benzene rings of adjacent particles
(Fig. 5). Actually, the interparticle spacing is estimated to be
ꢄ1:4 nm, regardless of the kind of terminal functional groups
from Fig. 5, in which the alkyl chains are thought to have an
all-trans conformation in the dry state.8 Especially, we could
obtain nearly perfect hcp superlattices for ligand 5.
The pKa values for the representative acid compounds corre-
sponding to ligands 1, 2, 4, and 5 in water are given in Table 2.
In order to realize our concept involving superlattice formation
through an acid–base interaction between the adjacent nanopar-
ticles, acidic 2–Au and basic 5–Au nanoparticles were chosen
as candidates. The pKa values of benzoic acid (4.19) and ben-
zylammonium cation (9.35) imply that nearly perfect salt for-
mation can be achieved when acidic 2–Au and basic 5–Au

1596 Bull. Chem. Soc. Jpn., 77, No. 8 (2004)
Acidic and Basic Au Nanoparticle Superlattices
are mixed. When a methanol solution of 2–Au nanoparticles
and an ethyl acetate solution of 5–Au nanoparticles were
mixed, a black precipitate was formed, which had no solubility
for any neutral solvents. When 100 mL of a 20 mM methanol
solution of 2–Au nanoparticles was added to a mixture of 100
mL of a 20 mM ethylacetate solution of 5–Au nanoparticles
and 300 mL of water, vigorous shaking of the solution led to
the formation of a film consisting of an aggregation of 2–Au
and 5–Au nanoparticles at the organic solvent–water interface.
During vigorous stirring of a bi-phase mixture of an organic
solvent and water, small droplets of organic solvent were
formed in water, and the aggregates of 2–Au and 5–Au nano-
particles formed in the organic phase were gathered at the inter-
face between water and the organic phase in order to diminish
the interface free energy. The mixture was then poured into a
large amount of water to place the film at an air–water inter-
face. The film at an interface could be visibly recognized to
be spread in a few cm2 regime. The obtained film was transfer-
red onto a polyvinylformal-coated TEM grid for a TEM obser-
vation. Figure 6 presents a representative TEM image of the
film formed at an air–water interface, which indicates that the
film had a widely-spread network structure consisting of a mul-
tilayer of Au nanoparticles with many defects. The inset in
Fig. 6 shows a magnified image of aggregated layers. Besides
the nanoparticle monolayer around the edge of the aggregate,
many linear structures formed by placing the nanoparticles at
two-fold saddle sites of nanoparticle underlayer were observed.
Therefore, these aggregates are likely to consist of predominant
bi-layers with a small number of multilayers of Au nanoparti-
cles. The acidic 2–Au or basic 5–Au nanoparticles themselves
surely do not form the aggregates, and moreover when the
weaker acid–base interactions, except for the combination of
2–Au and 5–Au nanoparticles, were employed, film formation
was not observed. These results strongly indicate that this film
is definitely different from those obtained by the LB method
and the conventional self-assembly method through solvent
evaporation. This simple method would allow us to easily ob-
tain not only conductive film of metal nanoparticles, but also
an alloyed metal film containing different kinds of metal nano-
particles as novel non-linear optical materials. An investigation
on the detailed structure involving whether each particle is al-
ternatively placed within the film is in progress.
Conclusion
Here, we have reported on useful synthetic routes of various
acidic and basic ligands, 1–5, for Au nanoparticles. Monodis-
perse Au nanoparticles protected by ligands 1, 2, 4, and 5 were
successfully prepared through the chemical reduction of the Au
ion in the presence of the ligands. These nanoparticles had a
strong tendency to self-assemble into hcp 2D superlattices
due to their monodispersity. When the organic solutions of
acidic 2–Au and basic 5–Au nanoparticles were mixed with wa-
ter, a film consisting of the aggregated multilayer of Au nano-
particles was easily obtained at the organic solvent–water inter-
face by the acid–base interaction, which would enable us to
construct complex patterns of different kinds of nanoparticles.
Table 2. The pKa Values for the Representative Acidic
Compounds Corresponding to the Ligands 1, 2, 4, and 5
in Water
NH3
OH
COOH
NH3
Acid
compound
9.35
pKa
9.89
4.19
4.63
Fig. 6. TEM image of the aggregated multilayer of the 2–Au and 5–Au nanoparticles. The inset shows the magnified image of the
aggregated multilayer.

M. Kanehara et al.
Bull. Chem. Soc. Jpn., 77, No. 8 (2004) 1597
J. Phys. Chem. B, 107, 2719 (2003).
14 B.-H. Sohn, J.-M. Choi, S. I. Yoo, S.-H. Yun, W.-C. Zin,
J. C. Jung, M. Kanehara, T. Hirata, and T. Teranishi, J. Am. Chem.
Soc., 125, 6368 (2003).
The present work was partially supported by a Grant-in-Aid
for Young Scientists (A) (No. 15681009) from the Ministry of
Education, Culture, Sports, Science and Technology (T. T.).
The financial supports was also provided from The Murata
Science Foundation and The Nippon Sheet Glass Foundation
for Materials Science and Engineering.
15 M. Kanehara, Y. Oumi, T. Sano, and T. Teranishi, J. Am.
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