A new water-soluble pillar[5]arene: Synthesis and application in the preparation of gold nanoparticles

Article abstract of DOI:10.1039/c2cc31962d

A new water-soluble pillar[5]arene containing ten imidazolium groups was prepared. It can be used as a stabilizer to fabricate gold nanoparticles smaller than 6 nm in water.

Full text of DOI:10.1039/c2cc31962d

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COMMUNICATION
A new water-soluble pillar[5]arene: synthesis and application in the
preparation of gold nanoparticlesw
Yong Yao,a Min Xue,a Xiaodong Chi,a Yingjie Ma,a Jiuming He,b Zeper Ablizb and
Feihe Huang*a
Received 17th March 2012, Accepted 4th May 2012
DOI: 10.1039/c2cc31962d
A new water-soluble pillar[5]arene containing ten imidazolium
groups was prepared. It can be used as a stabilizer to fabricate
gold nanoparticles smaller than 6 nm in water.
Over the past decade, there has been widespread interest in
nanomaterials’ syntheses and applications.1 Among various
nanomaterials, gold nanoparticles (GNPs) have made signifi-
cant progress in different scientific areas due to their unique
properties and applications in electronics, photonics and
catalysis.2 However, like other metal nanoparticles, small
GNPs are only kinetically stable and easily aggregate to form
larger particles owing to the large surface area and high surface
energy, which decreases their activity.3 Therefore, the controlled
preparation of gold materials in the nanometer range is attracting
increasing research efforts.4 For the fabrication of GNPs,
stabilizers play an essential role in controlling the formation
of nanoparticles as well as their dispersion stability.5–7
On the other hand, as a new class of supramolecular hosts,
pillar[n]arenes are versatile macrocyclic compounds that pre-
sent a hydrophobic core sandwiched between two functiona-
lizable rims.8 They can be easily modified chemically and such
modification is of interest for promising applications in nano-
materials and molecular recognition.9
Scheme 1 Syntheses of pillar[5]arene 1 and noncyclic monomeric
analog 4.
Here we designed and synthesized a new water-soluble
pillar[5]arene stabilizer 1 (Scheme 1) with ten imidazolium
and transmission electron microscopy (TEM). Hence, their
groups for the preparation of GNPs, in which the pillar[5]arene
catalytic activity for the reduction of 4-nitroaniline in the
framework was used as the platform, and the imidazolium
presence of NaBH4 was also studied (Fig. 2).
groups contain weakly coordinating cations and anions that
bind feebly to the gold surface.6b This new type of stabilizer is less
The synthetic route to the stabilizer pillar[5]arene 1 in three
steps is shown in Scheme 1. Condensation of 2 with boron
deactivating than the commonly employed capping or protective
trifluoride etherate as the catalyst in CH2ClCH2Cl yielded 3
(52%).9j Then compound 1 was obtained by refluxing a
ligands, and thus can enhance the catalytic activity (Table S1, ESIw).
The fabrication of GNPs in the presence of trace pillar[5]arene
solution of 3 and N-methylimidazole in toluene (91%).
The structure of 1 was characterized by 1H NMR, 13C NMR
in aqueous media was investigated in detail by ultraviolet
visible (UV/Vis) spectroscopy, X-ray diffraction (XRD),
and HRESIMS (ESIw). HRESIMS of 1 revealed peaks at m/z
1170.65, 754.13, 545.37, 420.51, 336.93 and 277.53, corre-
a Department of Chemistry, Zhejiang University, Hangzhou 310027,
P. R. China. E-mail: fhuang@zju.edu.cn; Fax: +86-571-8795-1895;
Tel: +86-571-8795-3189
sponding to [M À 2Br]2+, [M À 3Br]3+, [M À 4Br]4+
,
[M À 5Br]5+, [M À 6Br]6+and [M À 7Br]7+, respectively
(Fig. S8, ESIw). It is shown that 1 can lose different numbers of
bromide anions to form a series of fragments containing
different numbers of positive charges.
b Institute of Materia Medica, Chinese Academy of Medical Sciences
and Peking Union Medical College, Beijing 100050, P. R. China
w Electronic supplementary information (ESI) available: Compound
characterization, synthesis details, preparation of gold nanoparticles,
catalytic reduction of 4-nitroaniline, and other materials. See DOI:
10.1039/c2cc31962d
Because of the presence of ten imidazolium groups on the
macrocyclic ring, pillar[5]arene 1 can be considered as a cluster
c
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of imidazolium groups and bromide anions, which could
provide a shell of anions and cations around the gold nano-
particles, so gold nanoparticles can be stabilized in aqueous
solution. For comparison, noncyclic monomeric analog 4
(Scheme 1) was also prepared and used to stabilize gold
particles under the same experimental conditions. Unfortunately,
with 4 as the stabilizer, we obtained dark purple particles
instead of wine red nanoparticles with 1 as the stabilizer
(Fig. 1). Therefore, it was indicated that the pillar[5]arene
framework played an important role in stabilizing the gold
nanoparticles.
Representative UV/Vis spectra of gold colloids stabilized
by 1 at different concentrations revealed that all of the
pillar[5]arene-protected GNPs displayed the characteristic
optical signal of gold colloids: a surface-plasmon resonance in
the visible region within the 500 nm to 560 nm range (Fig. S10,
ESIw).10 When the concentration of 1 was 2 mM, 4 mM, 10 mM,
50 mM and 200 mM, the corresponding maximum absorp-
tion wavelength of the gold hydrosols was 560 nm, 545 nm,
534 nm, 519 nm and 512 nm, respectively. The surface-
plasmon absorption band undergoes a slight blue shift from
560 to 512 nm with increasing concentration of 1. This shift
might be due to the decreasing size of the GNPs formed.
Additionally, the intensity of GNPs’ absorption peak decreased
with the decreasing concentration of pillar[5]arene, which
suggested that larger-sized GNPs were obtained at lower
concentrations of 1.
Fig. 2 Successive UV/Vis absorption of the reduction of 4-nitroaniline
by excess NaBH4 using the pillar[5]arene-stabilized gold nanoparticles
(0.08 mol%, 1.88 Æ 0.58 nm) as the catalyst.
narrow particle size distribution were also observed. With
decreasing concentration of the stabilizer, spherical GNPs
together with a few triangular GNPs appeared. In addition,
when 1 was used as a dispersing agent at a very low concen-
tration (10À6 mol LÀ1), although some polydisperse GNPs
were observed, the average particle size was still smaller than
6 nm (Fig. S12–S14, ESIw).
The high resolution TEM image revealed that the GNPs
had well-defined crystalline planes (Fig. S16, ESIw). The
marked inter-planar d spacings are 0.235 nm and 0.203 nm,
which are consistent with the inter-planar distance of (111)
and (200) planes, respectively, for face-centered-cubic (fcc)
gold.11 The powder XRD pattern of GNPs shows that five
diffraction peaks positioned at Bragg angles (2y) between 301
and 901 can be attributed to the diffractions from five different
gold lattice planes [(111), (200), (220), (311) and (222)]
(Fig. S15, ESIw).
When the concentration of 1 was 200 mM, a typical trans-
mission electron microscopy (TEM) image and histogram of
size distribution of GNPs were obtained (Fig. S11, ESIw). The
average diameter and standard deviation determined from the
histogram were 1.88 Æ 0.58 nm. The TEM images revealed
that the nanoparticles were spherical in shape. As anticipated
from the absorption data, spherical GNPs with a relatively
One of the important applications of GNPs is to catalyze
some reactions that are otherwise not feasible. Hence, in the
present study, we have examined the performance of these
pillar[5]arene stabilized GNPs as catalysts for the borohydride
reduction of 4-nitroaniline as a model reaction. In the absence
of GNPs, the mixture of 4-nitroaniline and NaBH4 shows an
absorption peak at 400 nm (Fig. 2). This peak remains
unaltered for a long duration suggesting the inability of the
reducing agent NaBH4 itself to reduce 4-nitroaniline.
However, the addition of a very small amount of 1-stabilized
GNPs to the reaction system caused fading and ultimate
bleaching of the yellow colour of the reaction mixture in quick
succession, suggesting the occurrence of the reduction reac-
tion. This discoloration was quantitatively monitored spectro-
photometrically with time and noted visually as well; this was
due to the reduction of 4-nitroaniline to 1,4-diaminobenzene.
Fig. 2 shows that the absorption band of 4-nitroaniline at
400 nm decreases and disappears within six minutes after the
addition of the GNPs, with the concomitant appearance
of new peaks at 300 nm and 240 nm, which are attributed to
1,4-diaminobenzene.
À
In our reaction system, the concentration of BH4 greatly
Fig. 1 Gold particles stabilized by 1 and 4.
6506 Chem. Commun., 2012, 48, 6505–6507
exceeded that of 4-nitroaniline, so it is reasonable to consider
c
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its concentration to be constant during the reaction.12 As
expected, a good linear correlation of ln(A) versus time was
obtained (Fig. S17, ESIw), and the kinetic reaction rate con-
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stant was estimated to be 8.34 Â 10À3
s
À1. The catalytic
activity of the GNPsÀis possibly due to efficient electron
transfer from the BH4 anion to nitro compounds mediated
by the large Fermi level shift of nanoparticles.13 As shown in
Fig. S21 (ESIw), the catalytic reduction is carried out on the
surface of the GNPs: the nanoparticles react with the boro-
hydride ions to form the metal hydride. Subsequently,
nitroaniline molecules adsorb onto the metal surface. The
adsorption/desorption of both reagents on the surface is fast
and can be modelled in terms of a Langmuir isotherm.14
When the same experiment was conducted with GNPs of
3.09 Æ 0.79 nm, 3.86 Æ 0.91 nm and 5.95 Æ 1.64 nm, the rate
constants were estimated to be 5.73 Â 10À3 sÀ1, 2.72 Â 10À3 sÀ1
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and 8.17 Â 10À4
s
À1, respectively, indicating that the
smaller GNPs were more efficient in catalyzing the reaction
(Fig. S18–S20, ESIw).
In conclusion, we have successfully synthesized a new water-
soluble pillar[5]arene 1 with ten imidazolium groups at its two
rims. When 1 was used as the stabilizer at very low concentra-
tions, GNPs smaller than 6 nm were prepared in aqueous
solution. It was found that the average particle size of the
AuNPs stabilized by 1 decreased with increasing molar ratio of
[pillar[5]arene]/[Au3+], and their standard deviations also
became smaller. Since this water-soluble pillar[5]arene con-
tains only weakly coordinating cations and anions that bind
feebly to the gold surface, the plasmon resonance of the gold
nanoparticles stabilized by it shows greater sensitivity than
that of the gold nanoparticles stabilized by the commonly
employed capping or protective ligands, and thus the obtained
gold nanoparticle catalysts are of superior activity (Table S1,
ESIw). The present study provided a potential application of
pillar[n]arenes in the catalysis industry.
This work was supported by the National Natural Science
Foundation of China (20834004, 91027006, and 21172166),
the Fundamental Research Funds for the Central Universities
(2012QNA3013), National Basic Research Program
(2009CB930104), and Zhejiang Provincial Natural Science
Foundation of China (R4100009).
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Chem. Commun., 2012, 48, 6505–6507 6507