Reversible sol-to-gel transformation of uracil gelators: Specific colorimetric and fluorimetric sensor for fluoride ions

Article abstract of DOI:10.1021/la304920j

A new type of anthracene organogelator based on uracil was obtained using organic aromatic solvents, cyclohexane, DMSO, ethanol, and ethyl acetate. It was further characterized by field-emission scanning electron microscopy and transmission electron micro

Full text of DOI:10.1021/la304920j

Article
pubs.acs.org/Langmuir
Reversible Sol-to-Gel Transformation of Uracil Gelators: Specific
Colorimetric and Fluorimetric Sensor for Fluoride Ions
Ling-Bao Xing,† Bing Yang,† Xiao-Jun Wang,† Jiu-Ju Wang,† Bin Chen,† Qianhong Wu,‡ Hui-Xing Peng,†
Li-Ping Zhang,† Chen-Ho Tung,† and Li-Zhu Wu*,†
†Key Laboratory of Photochemical Conversion and Optoelectronic Materials, Technical Institute of Physics and Chemistry and
University of Chinese Academy of Sciences, Chinese Academy of Sciences, Beijing 100190, P. R. China
‡Department of Mechanical Engineering, Villanova University, Villanova, Pennsylvania 19085, United States
S
* Supporting Information
ABSTRACT: A new type of anthracene organogelator based on uracil was obtained using organic aromatic solvents,
cyclohexane, DMSO, ethanol, and ethyl acetate. It was further characterized by field-emission scanning electron microscopy and
transmission electron microscopy. Specifically, the resulting organogels were demonstrated to be promising colorimetric and
fluorescent sensors toward fluoride ions with high sensitivity and selectivity, accompanying the disruption of the gelators.
Spectroscopic study and 1H NMR titration experiment revealed that the deprotonation of the hydrogen atom on the N position
of uracil moiety by fluoride ions is responsible for the recognition events, evidenced by immediate transformation from the sol
phase to the gel state upon adding a small amount of a proton solvent, methanol. The process is reversible, with zero loss in
sensing activity and sol-to-gel transformation ability even after five runs.
subunits.17−20 The receptor bears one or more hydrogen-
INTRODUCTION
■
bonding donor sites for selective binding and sensing of specific
Gelation is an intriguing phenomenon characterized by the
̌
́
anions. Recently, Zinic and co-workers reported fluoride
formation of supramolecular architectures in nano/micrometer
dimensions, from molecules in aqueous or organic solvents.1,2
Self-assembled gelation is generally organized by intermolecular
interactions, such as hydrogen bonding, π−π interactions, van
der Waals forces of long alkyl chains, and electrostatic
forces.3−5 As a soft matter, gelators have important applications
in tissue engineering and drug delivery.6−8 As a light-sensitive
matter, they are also widely applied in sensors, light harvesting,
and photoresponsive systems.9−12 Recently, supramolecular
structures composed of low molecular weight gelators
(LMWGs) attracted much attention from the chemistry
community. There are extensive reports on the gelation of
small molecules coordinated with cations.13 On the other hand,
considerable efforts have been made for the development of
anion-responsive (e.g., fluoride ions) gels, which is of particular
importance in revealing a number of biological processes,
determining various disease states, and detecting environmental
pollution.14−16
sensing using a gel of oxalamide-derived anthraquinone.21 As
fluoride ions were present to deprotonate the amide group, the
gel showed color change and gel-to-sol phase transition. Lee
and co-workers reported fluoride sensing using a gel based on
benzoxazole derivative.22 In the presence of fluoride ions, the
translucent colorless gel was transformed to a solution with
strong greenish fluorescence. Wei and co-workers designed a
gelator bearing phenol O−H and acyl-hydrazone N−H groups.
The organogel in N,N-dimethylformamide is very stable and
23
−
contains two channels responsive to F−, AcO−, and H2PO4 .
Very recently, Ju and co-workers designed a novel function-
alized tweezer of uracil-appended glycyrrhetinic acid.24 The
tweezer possesses excellent gelation ability and responds to
fluoride ions and mercury ions by undergoing a gel-to-sol phase
transition. Review of the literature indicates that the recognition
of fluoride ions has been realized in organogelator systems,21−32
It is generally accepted that a synthetic anion sensor involves
an optical-signaling chromophore covalent-linked to a neutral
anion receptor, like urea, thiourea, amide, phenol, or pyrrole
Received: December 12, 2012
Revised: February 7, 2013
© XXXX American Chemical Society
A
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a
Scheme 1
a
(a) K2CO3, DMF; (b) 2-methyl-3-butyn-2-ol, [Pd(PPh3)4], CuI, Et3N, THF; (c) NaH, toluene; (d) [Pd(PPh3)4], CuI, Et3N, THF; (e) BuLi, I2,
THF; (f) [Pd(PPh3)4], CuI, Et3N, THF.
26.14, 22.84, 14.27; EI-MS (m/z) calcd for C18H29IO 388.13, found
388.20 [M]+.
but the selectivity of fluoride detection over other anions has
yet to be improved.
Synthesis of 3. To the mixture of 9,10-dibromoanthracene (1 g, 3
mmol), CuI (5.7 mg, 0.03 mmol), and Pd(PPh3)4 (69.3 mg, 0.06
mmol) were added anhydrous THF (40 mL) and TEA (20 mL) under
argon. While stirring, 2-methyl-3-butyn-2-ol (0.50 g, 6 mmol) was
injected through a syringe. The reaction mixture was stirred at 75 °C
overnight under argon atmosphere and was monitored by TLC. Upon
completion, the solution was evaporated in vacuo to dryness. The
crude product was purified by silica gel flash column chromatography
(CH2Cl2/EtOAc, 4/1) to give compound 3 (0.6 g, 17.7 mmol, 60%)
as a yellow solid: 1H NMR (400 MHz, CDCl3) δ 8.64−8.45 (m, 4H),
7.67−7.57 (m, 4H), 1.85 (s, 6H); 13C NMR (101 MHz, CDCl3) δ
132.96, 130.12, 128.19, 127.38, 127.02, 126.82, 124.22, 117.57, 106.33,
78.71, 66.34, 31.88; EI-MS (m/z) calcd for C19H15BrO 338.03, found
338.03 [M]+.
Synthesis of 4. To a toluene (30 mL) solution of 3 (0.6 g, 1.8
mmol) was added NaH (0.22 g, 9 mmol), and the mixture was stirred
at 80 °C for over 20 min and monitored by TLC. Upon completion,
the solution was evaporated in vacuo to dryness. The crude product
was purified by silica gel flash column chromatography (petroleum
ether) to give compound 4 (0.47 g, 1.67 mmol, 93%) as a yellow solid:
1H NMR (400 MHz, CDCl3) δ 8.68−8.61 (m, 2H), 8.59−8.54 (m,
2H), 7.70−7.56 (m, 4H), 4.06 (s, 1H); 13C NMR (101 MHz, CDCl3)
δ 133.67, 130.26, 128.37, 127.57, 127.18, 125.03, 117.08, 89.46, 80.16;
EI-MS (m/z) calcd for C16H9Br 279.99, found 279.98 [M]+.
Synthesis of 5. To the mixture of compound 2 (0.56 g, 2 mmol),
4 (0.67 g, 2.4 mmol), CuI (3.8 mg, 0.02 mmol), and Pd(PPh3)4 (46.2
mg, 0.04 mmol) were added anhydrous THF (40 mL) and TEA (20
mL) under argon. The reaction mixture was stirred at 75 °C overnight
under argon atmosphere and was monitored by TLC. Upon
completion, the solution was evaporated in vacuo to dryness. The
crude product was purified by silica gel flash column chromatography
(petroleum ether/EtOAc, 10/1) to give compound 5 (1.0 g, 1.85
mmol, 93%) as a red solid: 1H NMR (400 MHz, CDCl3) δ 8.70 (dd, J
= 6.9, 2.5 Hz, 2H), 8.57 (dd, J = 7.1, 2.4 Hz, 2H), 7.69 (dd, J = 9.2, 2.3
Hz, 2H), 7.67−7.55 (m, 4H), 6.97 (d, J = 8.8 Hz, 2H), 4.03 (t, J = 6.6
Hz, 2H), 1.82 (m, 2H), 1.52−1.45 (m, 2H), 1.32 (m, 16H), 0.89 (t, J
= 6.8 Hz, 3H); 13C NMR (101 MHz, CDCl3) δ 159.81, 133.25,
132.96, 130.42, 128.30, 127.51, 127.02, 126.73, 123.66, 118.96, 115.31,
114.90, 102.33, 84.83, 68.33, 32.08, 29.80, 29.75, 29.55, 29.51, 29.36,
26.18, 22.85, 14.27; EI-MS (m/z) calcd for C34H37BrO 540.20, found
540.20 [M]+.
In the present work, we report a low molecular weight
gelator 1 composed of an anthracene unit, a uracil unit, and
long alkyl chains. The aromatic anthracene unit serves as a
signaling chromophore, the long alkyl chains were selected for
their gelation ability, and the amidic N−H in the uracil unit
serves as the recognition site for its hydrogen-bonding ability
(Scheme 1). The group of nitrogen-based molecules necessary
for hydrogen bonding of complementary DNA and RNA
strands enables nuclebases to assemble elegant macroscopic
structures using hydrogen-bonding interactions.33,34 However,
reports of gelators based on uracil, especially for recognition
events, remain scarce. It is anticipated that with the cooperative
multiple intermolecular interactions, including π−π interaction,
hydrogen-bonding interaction, and van der Waals force in these
components, compound 1 can self-assemble into long fibers in
organic solvents and entangle further to form gels. This
expectation was found indeed to be the case. Compound 1 is
capable of forming organogelators in aromatic solvents,
cyclohexane, DMSO, ethanol, and ethyl acetate. Significantly,
the resulting organogels, upon the disruption of the gelators,
present intriguing characteristics, making them promising
colorimetric and fluorescent sensors in response to fluoride
ions with high sensitivity and selectivity. Other anions,
including Cl−, Br−, I−, ClO4 , AcO−, HSO4 , and H2PO4 ,
however, do not cause any alternation in either the color,
fluorescence, or in the phase state of the organogelators. This
unique feature of 1 allows for so-called “naked-eye” detection of
fluoride ions without interference of other anions. The detailed
results are presented below.
−
−
−
EXPERIMENTAL SECTION
■
Synthesis of 2. To a solution of 4-iodophenol (3 g, 13.6 mmol) in
DMF (30 mL) were added 1-bromododecane (4.1 g, 16.5 mmol) and
dry K2CO3 (11.3 g, 81.8 mmol), and the mixture was stirred at 70 °C
overnight. Then the solution was poured into water (100 mL) and
extracted with CH2Cl2 (3 × 50 mL). The combined organic layer was
washed with 1 M HCl (1 × 100 mL) and brine (3 × 150 mL), dried
over Na2SO4, and evaporated in vacuo to dryness. The crude product
was purified by silica gel flash column chromatography (petroleum
ether) to give compound 2 (5.1 g, 13.1 mmol, 96%) as a white solid:
1H NMR (400 MHz, CDCl3) δ 7.61−7.48 (m, 2H), 6.73−6.60 (m,
2H), 3.91 (t, J = 6.6 Hz, 2H), 1.85−1.67 (m, 2H), 1.39−1.17 (m,
18H), 0.88 (t, J = 6.8 Hz, 3H); 13C NMR (101 MHz, CDCl3) δ
159.17, 138.28, 117.08, 82.52, 68.27, 32.07, 29.79, 29.73, 29.51, 29.29,
Synthesis of 6. To a THF solution (40 mL) of 5 (1.0 g, 1.85
mmol) was added BuLi (2.3 mL of a 1.6 M solution, 3.7 mmol)
dropwise, and the resulting solution was stirred under argon at −78
°C. For 1.5 h, I2 (1.88 g, 7.4 mmol) was added and the reaction
mixture was stirred for 2 h. The solution was allowed to warm to room
temperature. The organic phase was diluted with CH2Cl2 (30 mL),
washed with Na2SO3 (3 × 30 mL) and brine (3 × 30 mL), dried over
B
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Na2SO4, and then evaporated in vacuo to dryness. The crude product
was purified by silica gel flash column chromatography (petroleum
ether/EtOAc, 10/1) to give compound 6 (0.94 g, 1.59 mmol, 86%) as
Table 1. Gelation Experimental Results of Compound 1 in
Various Solvents at Room Temperature
a,b
1
solvent
hexane
gelation result
solvent
gelation result
a yellow solid: H NMR (400 MHz, CDCl3) δ 8.73−8.62 (m, 2H),
8.58−8.47 (m, 2H), 7.70 (d, J = 8.5 Hz, 2H), 7.67−7.55 (m, 4H), 6.97
(d, J = 8.6 Hz, 2H), 4.03 (t, J = 6.6 Hz, 2H), 1.88−1.78 (m, 2H),
1.52−1.45 (m, 2H), 1.28 (s, 16H), 0.88 (t, J = 6.6 Hz, 3H); 13C NMR
(101 MHz, CDCl3) δ 159.80, 134.04, 133.61, 133.24, 132.72, 127.95,
127.68, 126.67, 120.32, 115.30, 114.87, 106.62, 102.76, 84.89, 68.30,
32.08, 29.80, 29.76, 29.55, 29.52, 29.36, 26.18, 22.85, 14.28; EI-MS
(m/z) calcd for C34H37IO 588.19, found 588.19 [M]+.
I
m-xylene
pyridine
THF
OG (7.2)
cyclohexane
dichloromethane
chloroform
acetone
TG (9.6)
S
P
S
P
1,4-dioxane
ethyl acetate
ethanol
PG
I
OG (18.5)
benzene
OG (12.4)
P
OG (14.4)
chlorobenzene
toluene
DMSO
OG (8.6)
Synthesis of 7. This is described in the Supporting Information.
Synthesis of 1. To the mixture of compound 6 (0.88 g, 1.49
mmol), 7 (0.54 g, 1.79 mmol), CuI (5.68 mg, 0.03 mmol), and
Pd(PPh3)4 (17.3 mg, 0.015 mmol) were added anhydrous THF (40
mL) and TEA (20 mL) under argon. The reaction mixture was stirred
at 75 °C overnight under argon atmosphere and was monitored by
TLC. Upon completion, the solution was evaporated in vacuo to
dryness. The crude product was purified by silica gel flash column
chromatography (CH2Cl2/EtOAc, 10/3) to give compound 1 (1.09 g,
1.43 mmol, 96%) as a red solid: 1H NMR (400 MHz, CDCl3) δ 8.84−
8.62 (m, 2H), 8.44−8.33 (m, 2H), 7.80−7.60 (m, 6H), 6.99 (d, J = 8.1
Hz, 2H), 6.26 (s, 1H), 4.34−4.19 (m, 2H), 4.04 (t, J = 6.5 Hz, 2H),
2.00−1.78 (m, 4H), 1.58−1.06 (m, 36H), 0.87 (dt, J = 13.7, 6.8 Hz,
6H); 13C NMR (101 MHz, CDCl3) δ 160.21, 150.72, 138.99, 133.48,
132.95, 132.12, 131.65, 128.08, 127.94, 126.98, 126.02, 122.75, 114.99,
114.78, 113.61, 106.98, 105.12, 98.58, 92.19, 85.09, 68.39, 47.26,
46.49, 32.03, 29.70, 29.53, 29.47, 29.34, 29.21, 26.92, 26.16, 22.80,
14.23, 8.78; MALDI-TOF-MS (m/z) calcd for C52H64N2O3 764.50,
found 764.50 [M]+.
OG (6.8)
OG (5.4)
DMF
P
I
o-xylene
acetonitrile
a
TG, transparent gel; OG, opaque gel; PG, partial gel; P, precipitation;
b
S, soluble; I, insoluble when heated. The critical gelation
concentrations (CGC, mg mL−1) of gelators are shown in the
parentheses.
dimensional fibrous network. TEM images (Figure 1d−f) also
indicate that molecules of 1 are piled up to form cross-linking
network structures.
The interaction of 1 with anions (using their tetrabutylam-
monium salts as the sources) in the gel state was investigated in
DMSO solution. When excess tetrabutylammonium fluoride
was added to the hot DMSO solution of 1, the mixture was
cooled to a clear solution at room temperature instead of
forming a gel as it would form in its original state (Figures 2
and S1, Supporting Information). The transformation of gel-to-
sol is faster for compound 1 at high temperature than that at
the low temperature. Under the same condition, addition of
RESULTS AND DISCUSSION
■
The synthesis of compound 1 started from 9,10-dibromoan-
thracence, which was reacted with 2-methyl-3-butyn-2-ol to
produce compound 3. In the presence of toluene, compound 3
was deprotected with NaH to give compound 4 in a yield of
93%. Then compound 4 was reacted with 2 to afford
compound 5 as a yellow solid, which was iodinated to give
compound 6 in 86% yield. Compound 6 and 7, synthesized
according to the literature35, were dissolved in THF and Et3N
and catalyzed by CuI and Pd(PPh3)4 to give compound 1 as the
product in 96% yield (the details are in the Supporting
−
−
−
other anions (Cl−, Br−, I−, ClO4 , AcO−, HSO4 , H2PO4 ) did
not trigger the gel-to-sol transition (Figure S1, Supporting
Information). More strikingly, the addition of fluoride ions
caused solution color changes from salmon pink to dark red
and fluorescence changes from yellow to green (Figure 2).
Other anions have no obvious effect on the absorption and
fluorescence spectra or the gel-to-sol transition (Figure S1,
Supporting Information), indicating the high selectivity of gel 1
toward fluoride ions by naked-eye sensing.
1
Information). All of the compounds were characterized by H
To examine the binding site of gel 1 and fluoride ions, we
determined the spectral change at a low concentration (1 ×
10−5 M) in DMSO. Compound 1 is strongly fluorescent in
DMSO solution with a maximum at 530 nm. Upon addition of
fluoride ions, however, 1 becomes weakly fluorescent and blue-
shifted to 498 nm (Figure S2, Supporting Information), and at
the same time the UV−vis absorption is blue-shifted from 465
to 455 nm accompanied by enhancement at 320 nm and
decrease at 340 nm (Figure 3a). After 20 equiv of fluoride ions
were added to the DMSO solution, the fluorescence spectra
and absorption spectra of compound 1 were saturated. The
decreased fluorescence intensity and blue-shifted absorption are
as a function of the concentration of fluoride ions. In the case of
other anions, alterations in the absorption and fluorescence
spectra were hardly observed (Figures 3b and S3, Supporting
Information). Similar results were also observed in CH2Cl2
(Figures S4−S6, Supporting Information). Clearly, only
fluoride ions could lead to the color and fluorescent changes
of the solution (Figure S7, Supporting Information).
and 13C NMR spectroscopy, as well as mass spectrometry.
The gelation ability of compound 1 was examined in various
solvents by means of the “stable to inversion of a test tube”
method (for more details see the Supporting Information). The
corresponding critical gelation concentrations (CGC) at room
temperature were also measured. As shown in Table 1,
compound 1 can gel aromatic solvents, such as benzene,
toluene, o-xylene, and m-xylene, while it is insoluble in hexane,
acetone, and acetonitrile, even after heating. These gels were
found to be stable in closed tubes for at least 3 months at 25
°C. The better gelation for aromatic solvents relative to the
hydrocarbon or halogenated hydrocarbons is probably due to
the enhancement of π−π interactions in aromatic solvents.5
The morphology of xerogels obtained from different solvent
(after evaporation of the solvent) was characterized by field-
emission scanning electron microscopy (FE-SEM). Most of the
gels show an entanglement of fibrous aggregates. Figure 1a−c
shows the typical FE-SEM images of the xerogels upon
depositing the gels from cyclohexane, ethanol, and DMSO on
silicon wafers under identical conditions. Note that the
molecules in the gel phase were self-assembling into one-
dimensional fibers with a few micrometers long and 50−100
nm wide that further extended to form a large three-
The dramatic color changes may be interpreted by the fact
that the hydrogen atom on the N position of uracil moiety was
deprotonated by fluoride ions20 (Scheme 2), which sub-
sequently modulated intramolecular charge transfer from the
anthracene unit to the amide moiety of compound 1. This
C
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Figure 1. FE-SEM and TEM images of xerogels of compound 1 prepared from (a, d) cyclohexane, (b, e) ethanol, and (c, f) DMSO solution,
respectively.
Figure 2. Illustration of the gel-to-sol transformation, as well as the color and fluorescence changes, of compound 1 (10 mg/mL) in DMSO with the
addition of fluoride ions (1 equiv) under ambient conditions.
Figure 3. (a) UV−vis absorption of compound 1 upon addition of tetrabutylammonium fluoride (0−20 equiv), and (b) fluorescent changes of
compound 1 upon addition of tetrabutylammonium anions (20 equiv) in DMSO at room temperature; [1] = 1.0 × 10−5 M, excitation was at 450
nm.
speculation was confirmed by the identical spectral changes of
compound 1 when tetrabutylammonium hydroxide was used to
replace fluoride ions (Figure S8, Supporting Information),
suggestive that the deprotonation occurs. As a result, the
absorption and fluorescence were blue-shifted, and at the same
time the negatively charged compound would decrease the
hydrogen-bonding interactions of the amide moiety, leading to
the disruption of gelators in the presence of fluoride ions or
tetrabutylammonium hydroxide.
in DMSO-d6 was also carried out. As shown in Figure 4, the
amide N−H proton signal (11.7 ppm) on the uracil unit of
compound 1 disappeared completely with the addition of
excess fluoride ions, and a new weak broad signal appeared at
−
16.6 ppm, the typical signal of HF2 . Evidently, the
deprotonation of N−H of uracil in compound 1 took place.
As a consequence, the proton signal resonance of the uracil unit
shifted upfield.
More interestingly, the organogels of compound 1 could be
re-formed with the aid of CH3OH. For example, when fluoride
ions were added to the organogels of 1 in DMSO, the gel-to-sol
Furthermore, the 1H NMR titration experiment of
compound 1 before and after addition of excess fluoride ions
D
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Scheme 2. Representative Deprotonation of Compound 1 by
Fluoride Ions
Figure 5. The reversibility of gel-to-sol transition processes of 1 (10
mg/mL) in DMSO monitored at 630 nm in the presence of
tetrabutylammonium fluoride (13 mM) and CH3OH (0.6 M).
transformation was clearly observed. Meanwhile, the solution
color changed from salmon pink to dark red and fluorescence
changed from yellow to green (Figure 2). When a small amount
of CH3OH (0.6 M) was introduced into the system, the
organogels were regenerated immediately (Figure 2). The
influence of methanol is attributed to the reprotonation of
compound 1. The equilibrium of protonation and deprotona-
tion depends on not only the basicity but also the
concentrations of methanol and fluoride, respectively. The
higher concentration of methanol solution turned back into a
gel in spite of the presence of fluoride. Furthermore, the better
solvation of fluoride in a polar protic solvent makes it a less
efficient competitor for the gelator amide NH groups. Taking
the fluorescent maximum at 630 nm for gel state (Figure S9,
Supporting Information), we inferred that the whole process is
reversible with no loss in sensing activity and sol-to-gel
transformation ability even after five runs (Figure 5).
formation of organogelators in aromatic solvents, cyclohexane,
DMSO, ethanol, and ethyl acetate. Of particular significance,
the resulting organogels can detect fluoride ions with high
sensitivity and selectivity, accompanying with the disruption of
the gelators. The addition of fluoride ions in DMSO of 1, for
example, results in either absorption color change from vivid
salmon pink to dark red or fluorescence change from yellow to
green, respectively. A small amount of CH3OH can regenerate
the organogels immediately. The whole process is reversible,
with no loss in sensing activity and sol-to-gel transformation
ability even after five runs. Other anions, including Cl−, Br−, I−,
ClO4 , AcO−, HSO4 , and H2PO4 , caused no alternation
either in the color, fluorescence, or the phase state of the
organogelators. The experimental study presented herein could
have important potentials for the fabrication of smarter uracil
supramolecular systems for anion sensing applications.
−
−
−
CONCLUSION
ASSOCIATED CONTENT
* Supporting Information
Synthesis of 7, UV−vis absorption and fluorescence spectra,
and color changes of 1 after addition of various anions. This
■
■
S
In summary, we have developed a specific colorimetric and
fluorimetric sensor 1 for detecting fluoride ions. Benefiting
from the unique structure of uracil, this compound enables the
1
−
Figure 4. Partial H NMR spectra of 1 (8 mM) in DMSO-d6 upon addition of excess fluoride ions. Inset: the new typical signal of HF2 .
E
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AUTHOR INFORMATION
■
Corresponding Author
*E-mail: lzwu@mail.ipc.ac.cn.
Notes
̌
́ ́
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̌
Fluoride-Responsive Organogelator Based on Oxalamide-Derived
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1482.
ACKNOWLEDGMENTS
■
We are grateful for financial support from the Ministry of
Science and Technology of China (2013CB834505 and
2009CB22008), the National Natural Science Foundation of
China (91027041 and 20973189), and the Bureau for Basic
Research of Chinese Academy of Sciences.
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dx.doi.org/10.1021/la304920j | Langmuir XXXX, XXX, XXX−XXX