Monodispersed Ag nanoparticles as catalyst: Preparation based on crystalline supramolecular hybrid of decamethylcucurbit[5]uril and silver ions

Article abstract of DOI:10.1021/ic500464v

Monodispersed silver nanoparticles (Ag⁰ NPs) have been first prepared on the basis of a postsynthesis via mild reduction from a new crystalline supramolecular hybrid solid assembled from Ag⁺ ions and decamethylcucurbit[5]uril (Me₁₀CB[5]). Uniform growth of nearly spherical Ag⁰ NPs with an average size of ca. 4.4 nm was observed on the organic Me₁₀CB[5] support to form Ag@Me₁₀CB[5] composite material. The as-synthesized composite material was characterized by a range of physical measurements (PXRD, TGA, XPS, ICP, TEM, etc.) and was further exploited as a heterogeneous catalyst for the reduction of various nitrophenols in the presence of NaBH₄. The kinetics of the reduction process was monitored under various experimental conditions. The Ag@Me₁₀CB[5] composite material showed excellent catalytic performance over the reduction reactions and remained active after several consecutive cycles.

Full text of DOI:10.1021/ic500464v

Article
pubs.acs.org/IC
Monodispersed Ag Nanoparticles as Catalyst: Preparation Based on
Crystalline Supramolecular Hybrid of Decamethylcucurbit[5]uril and
Silver Ions
Hong-Fang Li, Jian Lu, Jing-Xiang Lin, and Rong Cao*
̈
State Key Laboratory of Structural Chemistry, Fujian Institute of Research on the Structure of Matter, Chinese Academy of Sciences,
Fuzhou, Fujian 350002, People’s Republic of China
S
* Supporting Information
ABSTRACT: Monodispersed silver nanoparticles (Ag0 NPs) have been
first prepared on the basis of a postsynthesis via mild reduction from a new
crystalline supramolecular hybrid solid assembled from Ag+ ions and
decamethylcucurbit[5]uril (Me10CB[5]). Uniform growth of nearly
spherical Ag0 NPs with an average size of ca. 4.4 nm was observed on
the organic Me10CB[5] support to form Ag@Me10CB[5] composite
material. The as-synthesized composite material was characterized by a
range of physical measurements (PXRD, TGA, XPS, ICP, TEM, etc.) and was further exploited as a heterogeneous catalyst for
the reduction of various nitrophenols in the presence of NaBH4. The kinetics of the reduction process was monitored under
various experimental conditions. The Ag@Me10CB[5] composite material showed excellent catalytic performance over the
reduction reactions and remained active after several consecutive cycles.
example as catalysts, were greatly limited. It was not until very
recently that successful syntheses of Pd0 and Pt0 NPs as
catalysts were reported by Cao’s group.10 In these cases,
palladium and platinum chloride were treated with sodium
borohydride in the presence of CB[6] and the as-prepared
composite material catalysts exhibited excellent catalytic
activity, stability, and reusability. However, fine control of the
morphology and particle size of the as-prepared metal NPs was
somewhat difficult. In the reported synthetic strategy of metal
NPs in the presence of CB[n]s, strong reduction agents, e.g.
sodium borohydride, were generally included, which easily led
to the growth of large metal particles and particle aggregations
(especially for the systems of Au0 and Ag0 NPs). Therefore,
exploring a simple and mild synthetic method for the
preparation of metal NPs with manageable morphology and
size favored by CB[n]s has great theoretical and practical
significance.
INTRODUCTION
■
Metal nanoparticles (NPs) have been an attractive subject of
research in the realm of nanoscience and technology, owing to
features distinct from those of their bulk counterparts. Research
attention has thus been focused on the preparation and
characterization of metal NPs in terms of both fundamental and
practical research.1,2 Among the various metal NPs, silver (Ag0)
NPs have received significant attention, because of their
extraordinary properties suitable for applications in antibacte-
rial, anti-inflammatory, antiviral, wound healing, and catalysis.3,4
Cucurbit[n]urils (CB[n]s) are a family of barrel-shaped
macrocyclic molecules featuring a hydrophobic cavity with two
identical carbonyl-fringed portals.5 Because of their unique
structures and outstanding multiple recognition locations,
CB[n]s have been widely studied in coordination and
supramolecular chemistry, as well as host−guest recognition.6
The combination of the concepts of synthetic chemistry and
materials science, applying CB[n] molecules to the fabrication
of metal NPs, offers great opportunities to produce unique
organic−inorganic composite materials. In recent years,
phenomenal progress has been achieved in using CB[n]s as
precursors to prepare metal NPs (especially Au0 and Ag0
Herein, we report the preparation of well-dispersed Ag0 NPs
supported on decamethylcucurbit[5]uril (Me10CB[5]) via a
green and facile postsynthetic method from the crystalline
supramolecular hybrid of Me10CB[5] and Ag+ ions ({[Ag-
(H2O)]2(H2O@Me10CB[5])}·2NO3·2H2O, complex 1). Com-
plex 1 is, to the best of our knowledge, the first example of a
crystalline supramolecular hybrid solid based solely on noble
metal ions and CB[n], without secondary structure inducers. By
means of mild thermal treatment of complex 1 precursors
under an H2 atmosphere, spherical Ag0 NPs with a average size
of ca. 4.4 nm were produced in situ and grew on the organic
Me10CB[5] support to form Ag@Me10CB[5] composite
NPs).7−10 In 2007, Garcia et al. reported the stabilization of
́
Au0 NPs with CB[n]s upon tetrachloroauric acid reduction
using sodium borohydride.7 A series of subsequent studies
targeted at Au0 NP preparation in the presence of CB[n]s were
carried out.8 By means of a similar method, Ag0 NPs could be
prepared upon reduction of silver nitrate with sodium
borohydride and sodium hydroxide in the presence of CB[8]
or CB[7], respectively.9 Nevertheless, the as-prepared metal
NPs were dispersed in solution and could hardly be separated
as solid materials by a facile method; thus applications, for
Received: February 27, 2014
Published: May 12, 2014
© 2014 American Chemical Society
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(n-AP) in the presence of sodium borohydride. For a standard
procedure, a certain amount (3.75 × 10−4 mmol, 0.7 mg) of Ag@
Me10CB[5] composite was added to the reaction mixture of n-NP (15
mL, 0.01 M), water (100 mL), and NaBH4 (1.5 mL, 1 M) solution at
25 °C. The evolution of the absorption spectra due to the reduction of
n-NP to n-AP was recorded using a UV2550 UV−visible (UV−vis)
spectrophotometer. UV−vis spectra of blank reactions without
catalysts were also recorded as references. The reusability of the
catalyst was studied by separating the Ag@Me10CB[5] composite
from the solution after each cycle of the reaction, washing with water,
drying in air, and reusing for the next run.
material. The as-prepared Ag@Me10CB[5] composite material
exhibited excellent catalytic activity and reusability in the
reduction reactions of various nitrophenols.
EXPERIMENTAL SECTION
■
General Considerations. All chemicals were commercially
purchased and used without purification. Me10CB[5] was synthesized
according to the reported process.11 Elemental analyses (C, H, and N)
were carried on an Elementar Vario EL III analyzer. Ag loading was
determined with a Jobin Yvon Ultima 2 inductively coupled plasma
emission spectrometer (ICP). Powder X-ray diffractions (PXRD) were
carried out with a Rigaku DMAX 2500 diffractometer. Thermogravi-
metric analysis (TGA) was performed under a flow of nitrogen by
using a TA SDT-Q600 instrument. X-ray photoelectron spectra (XPS)
were collected at a takeoff angle of 45° using a PHI Quantum 2000
scanning ESCA microprobe (Physical Electronics, USA) with an Al Kα
X-ray line (1486.6 eV). Transmission electron microscope (TEM)
images were taken on an FEI TECNAI G2 F20 microscope at an
accelerating voltage of 200 kV. The Ag@Me10CB[5] composite
material was ultrasonically dispersed in ethanol and deposited on the
holey carbon film on a copper grid prior to the TEM observation.
Synthesis of Ag@Me10CB[5] Composite Material. The
crystalline supramolecular hybrid {[Ag(H2O)]2(H2O@Me10CB[5])}·
2NO3·2H2O (complex 1) was synthesized and used as a precursor to
produce Ag@Me10CB[5] composite material. In a typical recipe,
AgNO3 (1.6 g, 10 mmol) and Me10CB[5] (0.97 g, 1 mmol) were
dissolved in 75 mL of ultrapure water with magnetic stirring. The
resulting white precipitate was removed by filtration, and the mother
liquor was placed at room temperature for several days in the dark.
Colorless crystals of complex 1 (0.53 g, 37.8% yield based on
Me10CB[5]) were collected by filtration and dried in air. Anal. Calcd
for C40H60N22O21Ag2: C, 34.3; H, 4.32; N, 22.0. Found: C, 34.6; H,
4.38; N, 22.0. The hybrid solid precursor was then transferred to a
tubular furnace and heated to 200 °C for 4 h under an H2 atmosphere.
Ag@Me10CB[5] composite material was readily produced through in
situ reduction of the precursor complex 1. The overall synthetic
procedure of the composite material is illustrated in Scheme 1.
RESULTS AND DISCUSSION
■
Characterization of the Crystalline Hybrid Precursor
Complex 1. The crystalline hybrid solid {[Ag(H2O)]2(H2O@
Me10CB[5])}·2NO3·2H2O (complex 1) was synthesized from
an aqueous solution of Me10CB[5] and AgNO3. The crystal
structure of complex 1 was confirmed by the combination use
of single-crystal X-ray diffraction, elemental analysis, and
thermal gravimetric analyses. In the structure of complex 1,
two Ag+ cations are capped to the portals of a Me10CB[5] unit
through the coordination of three carbonyl oxygen atoms and
one aqua ligand to a Ag+, giving rise to a “half-open” molecular
capsule (Figure 1). One water molecule is encapsulated inside
Scheme 1. Proposed Synthetic Route of the Composite
Material Ag@Me10CB[5]
Figure 1. View of the “half-open” molecular capsule of Me10CB[5] and
Ag+ cations in complex 1.
the cavity of a molecular capsule. Moreover, two uncoordinated
water molecules and two NO3 anions, which play the role of
−
X-ray Crystallography. Single-crystal X-ray diffraction data of
complex 1 were collected on a Rigaku Saturn 70 (4 × 4 bin mode)
diffractometer equipped with graphite-monochromated Mo Kα
radiation (λ = 0.71073 Å) and a CCD area detector. Empirical
absorption corrections were applied to the data using the Crystal Clear
program.12 Structural solutions and full matrix least-squares refine-
ments based on F2 were performed with the SHELXS-97 and
SHELXL-97 program packages, respectively.13 All of the non-
hydrogen atoms were refined anisotropically. Hydrogen atoms on
the disassociated water molecules (O2W and O3W) were not located
from a difference Fourier map or by theoretical methods, due to the
limited quality of the crystal data. Crystal data and structure
refinement parameters are given in Table S1 (Supporting
Information). CCDC-971702 contains supplementary crystallographic
data for this paper. These data can be obtained free of charge from
The Cambridge Crystallographic Data Centre via www.ccdc.cam.ac.
uk/data_request/cif.
charge balancing, appear in the crystal lattice. Adjacent
molecular capsules are interconnected through the hydrogen
bonds (H-bonds) among the aqua ligand, portal carbonyl
oxygen atoms, and uncoordinated water molecules (H-bond
distances 2.986 and 2.990 Å, respectively), giving rise to a one-
dimensional (1D) supramolecular chain structure (Figure 2).
−
NO3 anions are anchored on each side of the supramolecular
chain through weak H-bond interactions (H-bond distance
3.355 Å) (Figure 2). It is well-known that the portal carbonyl
groups of CB[n] are capable of coordinating with alkali metals,
alkaline-earth metals, and lanthanide ions.14 Transition-metal
ions normally form hydrates or complex fragments and interact
with CB[n] through H-bonding interactions.14a Only a few
exceptions in which transition-metal ions attach to the portal
carbonyl groups of CB[n] via coordinative bonds have been
documented, even though extra transition-metal complex
moieties were observed in the crystal lattice as structure
Catalytic Experiments. The catalytic activity of the composite
material Ag@Me10CB[5] was investigated in reduction reactions of n-
nitrophenols (n-NP, n = 4−2) to the corresponding n-aminophenols
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Figure 2. View of the 1D supramolecular chain structure in complex 1.
inducers.15 To the best of our knowledge, complex 1 represents
the first example of a crystalline supramolecular hybrid solid
based solely on noble-metal ions and CB[n].
The phase purity and homogeneity of the crystalline hybrid
precursor complex 1 were identified by powder X-ray
diffraction (PXRD) at room temperature (Figure 3). The
ment at 200 °C for 4 h under a H2 atmosphere, the colorless
crystalline bulky compound became gray, indicating a phase
change of the sample. The TGA curve of the postsynthesized
composite material Ag@Me10CB[5] shows continuous weight
loss before 300 °C (Figure S1, Supporting Information),
corresponding to the release of adsorbed water. The organic
support of the composite began to decompose at lower
temperature in comparison with the original complex 1 due to
the weaker interaction between Ag NPs and the Me10CB[5]
support. The PXRD pattern of the Ag@Me10CB[5] composite
matches fairly well with the PXRD of the pure Me10CB[5]
sample, which confirms the major content of the organic
support. Characteristics of the Ag0 NPs were missing from the
PXRD pattern, presumably due to the weak diffraction intensity
of Ag0 NPs in comparison with that of the Me10CB[5] support
(Figure 3). However, direct evidence for the formation of
metallic Ag0 was provided by XPS measurements (Figure 4).
Figure 3. Simulated and experimental PXRD patterns of the complex
1 (red and black, respectively), the reduced composite material Ag@
Me10CB[5] (blue), and the pure organic support Me10CB[5]
(purple).
experimental PXRD pattern and that simulated from single-
crystal X-ray diffraction data of complex 1 are nearly identical,
which clearly indicates the good purity and homogeneity of the
complex precursor. Thermogravimetric analysis (TGA) of the
hybrid precursor complex 1 was recorded in the temperature
range 20−900 °C to reveal the thermal stability as well as to
determine the water molecules present. The TGA curve shows
continuous weight loss before 200 °C (Figure S1, Supporting
Information), corresponding to the release of uncoordinated
and coordinated water molecules (five water molecules per
formula unit; calcd 6.4%, found 6.6%). A plateau is observed
between 200 and 300 °C, followed by further decomposition of
the material. A rational reduction temperature of 200 °C to
postsynthesize the composite material Ag@Me10CB[5] was
optimized, which ensured the complete reduction of Ag+ ions
into Ag0 NPs without destruction of the Me10CB[5] organic
support.
Figure 4. XPS spectra of the composite material Ag@Me10CB[5],
indicating the presence of Ag0 species.
The Ag species exhibited two characteristic peaks with binding
energies of 368.1 and 374.1 eV due to the 3d5/2 and 3d3/2
electrons of Ag0, respectively. These peaks suggested the
successful in situ formation of Ag0 NPs from the postsynthetic
reduction of the crystalline supramolecular hybrid precursor.
The ICP result showed that the loading content of Ag0 NPs was
11.6 wt %. The morphology and particle size of Ag0 NPs in the
composite material Ag@Me10CB[5] were studied directly from
TEM images. As shown in Figure 5, monodispersed spherical
Ag0 NPs with a diameter of ca. 4.4 nm have grown on site on
the Me10CB[5] support. A high-resolution TEM image
Characterization of the Composite Material Ag@
Me10CB[5]. Upon postsynthetic reduction by thermal treat-
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6, which was red-shifted immediately to ca. 400 nm as a result
of the formation of 4-nitrophenolate ions under alkaline
Figure 6. UV−vis spectral changes of characteristic bands of 4-NP in
aqueous solution (red) and in the reduction reactions with composite
material Ag@Me10CB[5] as catalyst (multiple colors). Inset:
corresponding pseudo-first-order plot of −ln A400 versus time.
conditions upon addition of a freshly prepared ice-cold aqueous
solution of NaBH4. In the absence of Ag@Me10CB[5] catalyst,
the characteristics at ca. 400 nm changed slightly with time
(Figure S3, Supporting Information), suggesting an extremely
slow process of the reduction reaction, which was in agreement
with observations in previous reports.17 Upon addition of Ag@
Me10CB[5] composite, the absorption peaks at ca. 400 nm
decreased gradually with time and a new peak at 295 nm,
characteristic of 4-AP, appeared alongside (Figure 6). During
the catalytic reduction, the originally bright yellow solution
became visually colorless after around 35 min, indicating
completion of the reaction. Since NaBH4 was used in excess,
the reaction could be pseudo first order and the apparent rate
constant was calculated by plotting −ln A400 vs time. The plot
appeared nearly linear (Figure 6, inset), and the apparent rate
constant value was calculated to be 7.87 × 10−2 min−1 with a
normalized rate constant (knor) of 1.75 mmol−1 s−1, which is
comparable to a recent report of Au NPs with a knor value of
2.04 mmol−1 s−1.18 Furthermore, the catalytic reduction
reaction of 4-NP was studied at different temperatures and
the corresponding rate constants were calculated as given in
Table 1 (entries 1−5). The activation energy was calculated to
Figure 5. TEM micrograph (a) and size distribution (b) of the
composite material Ag@Me10CB[5], showing Ag0 NPs supported on
Me10CB[5].
(HRTEM, Figure 5a inset) indicates that the Ag0 NPs are
highly crystalline with a plane distance of 2.38 Å, which
corresponds to the [111] lattice plane of Ag0 NPs. Me10CB[5]
plays an important role in stabilizing Ag NPs. The Ag NPs
aggregated severely to form large particles when the Me10CB-
[5] support was removed (Figure S2, Supporting Information).
Catalytic Performance in Organic Reduction Reac-
tions. The nitrophenols are among the most common organic
pollutants in industrial and agricultural wastewater. However,
aminophenols are important intermediates for the manufacture
of analgesic and antipyretic drugs.16 In this context, the
reduction of nitrophenols to aminophenols becomes much
more fascinating for pollution abatement as well as industrial
applications. The reduction reaction of 4-nitrophenol (4-NP) in
the presence of NaBH4 at 25 °C was selected as a model system
to demonstrate the catalytic performance of the composite
material Ag@Me10CB[5]. Specifically, the catalytic reduction of
nitrophenols with NaBH4 was spectrophotometrically per-
formed using Ag@Me10CB[5] as a catalyst in aqueous
medium. A maximum absorption (λmax) at ca. 317 nm was
observed for the aqueous solution of 4-NP, as shown in Figure
Table 1. Rate Constant Values of the Reduction Reactions of
n-NP in the Presence of Ag@Me10CB[5] Catalyst
entry
substrate
4-NP
T (°C)
k (min−1
)
Knor (mmol−1 s−1
)
1
2
10
25
40
50
60
25
25
25
25
25
0.0195
0.0787
0.172
0.43
1.75
3.82
7.31
14.1
1.19
0.25
0.78
0.64
1.01
4-NP
3
4-NP
4
4-NP
0.329
5
4-NP
0.635
6
2-NP
0.0535
0.0111
0.0352
0.0288
0.0454
7
3-NP
8
2-CH3-4-NP
3-CH3-4-NP
2-Cl-4-NP
9
10
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be 53 kJ mol−1 by applying the Arrhenius equation to the
kinetic data of entries 1−5 (Table 1 and Figure S4 (Supporting
Information)).
has been tested for up to five consecutive cycles for the
reduction reaction of 4-NP and monitored by UV−vis spectra.
The calculated apparent rate constants, as plotted in Figure 8,
For heterogeneous catalysis, the rate constant of the reaction
generally increases linearly with the amount of catalyst.19 These
parameters have been determined for the current system. As
shown in Figure 7, the rate constants are plotted against the
Figure 8. Rate constant values (k) of the catalytic reduction reaction of
4-NP in five consecutive cycles.
Figure 7. Plot of rate constants against the amount of Ag@
Me10CB[5] catalyst used for the reduction of 4-NP.
shows that the composite catalyst preserves more than 80% of
the initial catalytic ability after it is recycled five times. The
rational decrease of catalytic ability might account for the slight
growth and aggregation of Ag0 NPs on the Me10CB[5] support
after several times of use, as confirmed by a TEM image (Figure
S5, Supporting Information), as well as the insignificant loss of
catalyst in the multiple separation processes.
amount of catalyst used. In the current system, it has been
found that the rate constant also increases with the increasing
amount of Ag@Me10CB[5] catalyst. The role of metal NPs is
crucial in terms of electrochemical current potential in the
reduction reactions.20 In such reactions, electron transfer from
−
CONCLUSION
BH4 to 4-NP occurs only when both are absorbed onto the
■
surface of Ag0 NPs. Thus, increasing the amount of Ag@
Me10CB[5] catalyst will provide a larger accessible substrate
surface area for Ag0 NPs, which in turn favors substrate
adsorption and increases the rate constant.
In this work, a new crystalline supramolecular hybrid,
{[Ag(H2O)]2(H2O@Me10CB[5])}·2NO3·2H2O (complex 1),
was successfully synthesized from Me10CB[5] and Ag+ ions as
the first example of a crystalline solid based solely on noble-
metal ions and CB[n]. Complex 1 was further used as a
precursor for postsynthetic reduction under a H2 atmosphere,
which gave rise to a composite material (Ag@Me10CB[5]) of
monodispersed Ag0 NPs with an average size of 4.4 nm
supported on Me10CB[5]. In comparison with the traditional
reduction method for NP−CB[n] systems, we presented here a
much greener and milder method, which prevented the growth
of large Ag0 NPs and particle aggregations. The morphology
and the particle size of the as-prepared Ag0 NPs were easily
manageable. The Ag@Me10CB[5] composite material was
investigated as an efficient catalyst for the reduction of
nitrophenols. Moreover, the Ag@Me10CB[5] catalyst could
be easily recycled several times, through simple centrifugation,
without dramatic loss of catalytic activity. The Ag@Me10CB[5]
composite material, prepared through in situ reduction of the
crystalline solid precursor (complex 1), is promising as a
catalyst for the conversion of nitro to amino compounds on a
large scale.
In addition, the Ag0 NP loaded composite material Ag@
Me10CB[5] is applicable to other nitrophenol reduction
systems. As shown in Table 1 (entries 6−10), catalytic
reductions with different reaction rates were successful with
other nitrophenol precursors in the presence of Ag@Me10CB-
[5] catalyst. It is well-known that 4- and 2-nitrophenolate ions
are much more stable than 3-nitrophenolate ion.21,22 For 4-
nitrophenolate ion, the “negative” charge on oxygen is
delocalized throughout the benzene ring (conjugation effect)
and becomes resonance stabilized. For 2-nitrophenolate ion,
the “negative” charge on oxygen is also delocalized throughout
the benzene ring but not as effectively as in 4-nitrophenolate
ion. For 3-nitrophenolate ion, however, there is no conjugation
effect, only the -I effect (inductive effect) of the nitro group is
valid. Moreover, the inductive effect of the nitro group is less
effective due to steric hindrance and therefore it is less stable
than other isomers. The rate of reduction of the nitrophenols is
expected to follow the order 4-NP > 2-NP > 3-NP, which is in
good agreement with the experimental results (Table 1).
Moreover, the rate constants decrease greatly with the insertion
of either electron-accepting (Cl) or -donating (CH3)
substituent groups in the ortho and meta positions of 4-NP
due to the reduction of the conjugation effect.
ASSOCIATED CONTENT
■
S
* Supporting Information
Figures, a table, and a CIF file giving TGA, UV−vis, TEM, and
crystallographic data. This material is available free of charge via
the Internet at http://pubs.acs.org.
Stability and reusability are important parameters for the
practical applications of catalysts. The Ag@Me10CB[5] catalyst
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Article
Z.; Zhu, Q.-J.; Zhang, Y.-Q.; Liu, J.-X. Inorg. Chem. 2010, 49, 7638−
7640.
(16) Dotzauer, D. M.; Dai, J. H.; Sun, L.; Bruening, M. L. Nano Lett.
AUTHOR INFORMATION
Corresponding Author
*E-mail for R.C.: rcao@fjirsm.ac.cn.
Notes
■
2006, 6, 2268−2272.
(17) (a) Lu, Y.; Mei, Y.; Drechsler, M.; Ballauff, M. Angew. Chem., Int.
Ed. 2006, 45, 813−816. (b) Mei, Y.; Sharma, G.; Lu, Y.; Drechsler, M.;
Ballauff, M.; Irrgang, T.; Kempe, R. Langmuir 2005, 21, 12229−12234.
(c) Hayakawa, K.; Yoshimura, T.; Esumi, K. Langmuir 2003, 19,
5517−5521. (d) Lu, Y.; Mei, Y.; Ballauff, M.; Drechsler, M. J. Phys.
Chem. B 2006, 110, 3930−3937.
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
We thank the 973 Program (2011CB932504 and
2012CB821705), NSFC (21331006, 21221001, 21101155
and 21203199), Fujian Key Laboratory of Nanomaterials
(2006L2005), and the Key Project from CAS for financial
support.
(18) Jana, D.; Dandapat, A.; De, G. Langmuir 2010, 26, 12177−
12184.
(19) Spiro, M. In Essays in Chemistry; Bradley, J. N., Gillard, R. D.,
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(20) (a) Freund, P. L.; Spiro, M. J. Phys. Chem. B 1985, 89, 1074−
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