Synthesis and Design of Aggregation-Induced Emission Surfactants: Direct Observation of Micelle Transitions and Microemulsion Droplets

Article abstract of DOI:10.1002/anie.201507236

The direct visualization of micelle transitions is a long-standing challenge owing to the intractable aggregation-caused quenching of light emission in the micelle solution. Herein, we report the synthesis of a surfactant with a tetraphenylethene (TPE) co

Full text of DOI:10.1002/anie.201507236

.
Angewandte
Communications
International Edition: DOI: 10.1002/anie.201507236
German Edition: DOI: 10.1002/ange.201507236
Fluorescent Micelles
Synthesis and Design of Aggregation-Induced Emission Surfactants:
Direct Observation of Micelle Transitions and Microemulsion Droplets
Weijiang Guan, Wenjuan Zhou, Chao Lu,* and Ben Zhong Tang*
[
4]
Abstract: The direct visualization of micelle transitions is
a long-standing challenge owing to the intractable aggregation-
caused quenching of light emission in the micelle solution.
Herein, we report the synthesis of a surfactant with a tetraphe-
nylethene (TPE) core and aggregation-induced emission
(CMC). In principle, the formation of micelles could be
directly visualized by confocal fluorescence microscopy
(CFM) if the micellar aggregates could emit light. Although
the unimers of well-known fluorescent surfactants emit
intensely, their luminescence is quenched upon the formation
of micelles as a result of the intrinsic aggregation-caused-
(
AIE) characteristics. The transition processes of surfactant
[
5]
micelles and the microemulsion droplets (MEDs) formed by
the surfactant with a TPE core were clearly visualized by
a high-contrast fluorescence imaging method. The fluorescence
intensity of the MEDs decreased as the size of MEDs increased
as a result of weakening of the restriction of intramolecular
rotation (RIR). The results of this study deepen our under-
standing of micelle-transition processes and provide solid
evidence in favor of the hypothesis that the AIE phenomenon
has its origin in the RIR of fluorophores in the aggregate state.
quenching (ACQ) effect. Therefore, the direct visualization
of micelle transitions seems to be a great challenge.
An important class of aggregation-induced emission
(AIE) materials have emerged: molecules that are non-
emissive in the solution state but are induced to emit intensely
upon aggregate formation. Since then, a large number of
new AIE-active materials, in which tetraphenylethene (TPE)
is a typical structural unit, have been explored for their
potential application in a wide range of fields, such as optical
[
6]
[7]
sensing, bioimaging, and optoelectronic devices. Therefore,
it is reasonable to anticipate that the direct visualization of
surfactant-micelle-transition processes may be enabled by the
design of TPE-based surfactants, thus avoiding the wide-
spread occurrence of ACQ in micelles of conventional
fluorescent surfactants.
S
urfactants have been widely used in many areas, from basic
technologies in daily products to advanced applications in
[
1]
biotechnology and nanomedicine. Although an ancient
subject, transition processes of surfactant micelles are still
a topic of intense interest in both academic and technological
research. Kinetic monitoring of transition processes of
micelles from spherical to wormlike micelles by in situ
small-angle X-ray scattering (SAXS) has led to an impressive
amount of data. The development of a visualization tech-
nology would open new opportunities for the interrogation of
the world of micelles. However, the direct visualization of
micelle transitions by a high-contrast fluorescence imaging
method has not yet been described.
A micelle is an aggregation of surfactant unimers in
aqueous solution as a result of the inevitable process of
surfactant self-assembly when the concentration of the
surfactant is greater than the critical micelle concentration
[2]
In this study, we incorporated TPE units into sodium
dodecyl sulfonate (SDS) molecules to generate luminescent
surfactant (denoted as TPE-SDS) with AIE characteristics.
Fluorescence microscopy was used for the direct visualization
of the as-prepared TPE-SDS micelles in aqueous solution
with excellent imaging contrast. More interestingly, the
micellar transition that occurred during the continuous
addition of a salt could also be visualized directly. We found
that the original spherical micelles are first fused into bigger
rodlike micelles, and that these rods continue to grow into
wormlike micelles. Furthermore, the microemulsion droplets
(MEDs) formed from TPE-SDS could also be observed
directly, and the fluorescence intensity of MEDs was found to
be inversely proportional to their size, because the restriction
of intramolecular rotation (RIR) of TPE units is gradually
weakened.
[
3]
[
*] Dr. W. Guan, Dr. W. Zhou, Prof. Dr. C. Lu
State Key Laboratory of Chemical Resource Engineering
Beijing University of Chemical Technology
P.O. Box 79, 100029, Beijing (China)
The synthetic route to the amphipathic TPE-SDS surfac-
tant is outlined in Figure 1A. The fluorescent TPE-2OH core
(1) was prepared readily through McMurry coupling of 4-
E-mail: luchao@mail.buct.edu.cn
Prof. Dr. B. Z. Tang
Department of Chemistry
The Hong Kong University of Science and Technology
Clear Water Bay, Kowloon, Hong Kong (China)
E-mail: tangbenz@ust.hk
[
8]
hydroxybenzophenone. Its molecular structure was verified
1
by H NMR spectroscopy (see Figure S1 in the Supporting
Information). TPE-2OH was treated with NaH (1 equiv) to
activate one of the two hydroxy groups for the installation of
a hydrophobic tail (À(CH ) CH ) to provide 2. The structure
Supporting information for this article is available on the WWW
under http://dx.doi.org/10.1002/anie.201507236.
2
7
3
1
of compound 2 was confirmed by H NMR spectroscopy,
C NMR spectroscopy, and mass spectrometry (see Fig-
ꢀ
2015 The Authors. Published by Wiley-VCH Verlag GmbH & Co.
13
KGaA. This is an open access article under the terms of the Creative
Commons Attribution Non-Commercial License, which permits use,
distribution and reproduction in any medium, provided the original
work is properly cited and is not used for commercial purposes.
ures S2–S4). It was necessary to purify compound 2 by silica-
gel column chromatography to assure the complete removal
of unreacted TPE-2OH. Next, compound 2 was transformed
1
5160
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The surface activity of the as-prepared surfactant TPE-
SDS in aqueous solution was evaluated through the mea-
[9]
surement of surface tension. A plot of surface tension (g)
versus TPE-SDS concentration (log(C); Figure 2A) shows
Figure 2. A) Plot of surface tension versus log(concentration) for TPE-
SDS. B) Plot of conductivity versus the concentration of TPE-SDS.
C) Absorption spectra of TPE-SDS at different concentrations. D) Plot
of fluorescence intensity at 490 nm versus the corresponding TPE-SDS
concentration.
that at low concentrations, the TPE-SDS unimers were
preferentially adsorbed at the air–water interface, thus
leading to a sharp reduction in the surface tension of water.
When the concentration of TPE-SDS continued to increase,
the adsorption of TPE-SDS at the air–water interface reached
a saturation state, and thus a relatively constant surface
tension was observed. An inflection point appears at around
5
0 mm, indicative of the CMC of the surfactant TPE-SDS.
The CMC value of TPE-SDS could be measured readily
[
10]
by the method of conductivity variation. The conductivity
(
(
k) of TPE-SDS in water is plotted against its concentration
C) in Figure 2B. The conductivity increased linearly as the
concentration of TPE-SDS increased up to 30 mm; however,
when the concentration was higher than 30 mm, the plot was
linear with a lower slope, thus indicating an increase in the
mass-per-unit charge of TPE-SDS. This break between the
Figure 1. A) Synthetic route to the amphipathic surfactant TPE-SDS
1
6
(3). B) H NMR spectrum of TPE-SDS in [D ]dimethyl sulfoxide (the
solvent peaks are marked with asterisks). C) Negative-ion ESIMS
spectrum of TPE-SDS. DMF=N,N-dimethylformamide.
[1b]
two slopes indicates the formation of TPE-SDS micelles.
These results, along with those obtained by surface-tension
measurements, strongly demonstrated that the as-prepared
TPE-SDS was a typical anionic surfactant. Furthermore,
a transmission electron microscopy (TEM) image of 40 mm
TPE-SDS showed a spherical structure of the TPE-SDS
micelles with a radius of (3.5 Æ 0.5) nm (see Figure S6A),
which is almost equal to the extended length of a TPE-SDS
molecule (i.e., 2.97 nm).
into a sodium salt in a basic environment and treated with 1,4-
butanesultone to introduce
a
sulfonate substituent
À
(
À(CH ) SO ). The structure of the product TPE-SDS (3)
2
4
3
was confirmed by the presence of resonance peaks in the
1
H NMR spectrum for all alkyl hydrogen atoms (Figure 1B).
A mass-to-charge (m/z) ratio of 611.2842 in the MS spectrum
further proved the formation of TPE-SDS (Figure 1C). This
value is consistent with the exact mass generated for this
compound by the software ChemDraw Ultra 12.0. The
The optical properties of the surfactant TPE-SDS were
studied by UV/Vis absorption and fluorescence spectroscopy.
The absorption spectra (Figure 2C) showed two peaks, at 250
and 318 nm. They were assigned to the absorption of the
1
3
structure of TPE-SDSA was also verified by C NMR
spectroscopy (see Figure S5).
[11]
phenyl groups and the conjugated TPE, respectively. Their
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intensity increased linearly with the TPE-SDS concentration
in the range from 2.5 to 80 mm. The peak position at 318 nm
remained unchanged as the TPE-SDS concentration
increased; in contrast, a clear peak shift occurred for reported
[12]
aggregates of TPE derivatives.
This difference between
TPE-SDS and other AIE molecules with a TPE core
indicated that the conformation of the TPE-SDS molecules
did not depend on the micelle aggregates.
In general, the luminescence of AIE molecules is trig-
[13]
gered by aggregate formation.
Herein, the fluorescence
excitation and emission spectra of TPE-SDS in aqueous
solutions were studied (see Figure S7). The fluorescence
intensity at 490 nm versus the corresponding TPE-SDS
concentration is plotted in Figure 2D. The fluorescence of
the surfactant TPE-SDS gradually increased as the TPE-SDS
concentration increased. One inflection point appeared at
around 30 mm (equal to the CMC obtained from conventional
electrical conductivity method), thus suggesting a change in
[14]
the aggregation state from surfactant unimers to micelles.
The decrease in the slope after the inflection point is possibly
due to the increased freedom of the TPE units in the nonpolar
interior of the micelles as compared to the aqueous environ-
[1b]
ment.
The structure transition of the TPE-SDS micelles in the
solution state was directly visualized by CFM. Fluorescence
microscopy images of the TPE-SDS micelles in pure water,
0
.5m aqueous NaCl, and 1.0m aqueous NaCl were taken with
a 405 nm laser. Although the micelle size was outside the
resolution of the CFM measurements, diffraction blur ena-
bled the observation of the luminescent dots of the TPE-SDS
micelles (Figure 3A,D). The spherical structure of the TPE-
SDS micelles was further confirmed by TEM (see Fig-
ure S6A). The formation of the spherical structure was
attributed to the strong electrostatic repulsion between the
anionic head groups, thus leading to a high curvature of the
Figure 3. Fluorescence microscopy images of the TPE-SDS micelles in
A) pure water, B) 0.5m NaCl solution, and C) 1.0m NaCl solution. D–
F) Magnification of the indicated regions in (A–C). TPE-SDS concen-
tration: 80 mm. Scale bars: 5 mm. All images were taken with a 405 nm
laser.
[4]
TPE-SDS micelle core. Furthermore, the electrical double
layer formed at the micelle–water interface has a z-potential
value of greater than À50 mV, thus demonstrating good
dispersion stability of the resulting TPE-SDS micelles.
Generally, the effective thickness of the electrical double
layer can be decreased in a controlled manner by the addition
to rodlike and finally wormlike micelles (see Figure S6). DLS
measurements indicated that the average diameters of the
spherical, rodlike, and wormlike micelles were 7.62, 214.2,
and 2991 nm, respectively (see Figure S8). Rheology meas-
urements provided additional information about the micelle
[15]
of neutral electrolytes (e.g., NaCl). Rodlike micelles were
directly observed when 0.5m aqueous NaCl was added to the
solution of the TPE-SDS micelles (Figure 3B,E) because the
repulsion between anionic heads in the micelle is reduced
when the electrical double layer surrounding the anionic
transitions. We measured the zero-shear viscosity (h ) of
0
80 mm solutions of TPE-SDS as a function of the NaCl
concentration and found that at a low salt content (< 0.5m),
the viscosity increased slowly as the NaCl concentration
increased, thus indicating the formation of rodlike micelles.
When the concentration of NaCl continued to increase above
0.5m, the rodlike micelles grew continuously longer to form
wormlike micelles, thus resulting in a sharp increase in
[
4]
heads is compressed. Closer packing of the anionic heads
leads to a transformation of micelle structure from spherical
to rodlike. More interestingly, as the NaCl concentration was
increased to 1.0m, the tendency of the micelles to become
elongated became more obvious until wormlike micelles
could be clearly seen (Figure 3C,F). In conclusion, direct
visualization clearly disclosed the processes of micelle tran-
sition from spherical, to rodlike, and finally to wormlike
structures.
[16]
viscosity (see Figure S9). In conclusion, the processes of
micelle transition as determined by these measurement
techniques were in good agreement with those found by
CFM.
The micelle-transition processes were also investigated by
other techniques, such as dynamic light scattering (DLS),
TEM, and rheology measurements. TEM showed that the size
of the micelles increased during the transition from spherical
Although the origins of the AIE phenomenon are still
under debate, the most widely accepted explanation involves
the RIR of fluorophores in the aggregate state. However,
there is currently no solid evidence to support the claim. In
[17]
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this study, we synthesized a water-in-oil (W/O) microemulsion
system containing TPE-SDS, 1-butanol, isooctane, and water
to measure the shortest distance between TPE units for the
emission of light. Interestingly, we found that the fluorescence
intensity decreased as the size of the MEDs increased
The number of water pools (N ) in the microemulsion
system is then obtained from Equation (3):
w
Nw ¼ Vw,t=Vw
ð3Þ
in which Vw,t is the total volume of water in the microemulsion
system. The average aggregation number of TPE-SDS
molecules associated with a water pool is calculated by
Equation (4):
(
Figure 4A). The spherical structure of the formed MEDs
was confirmed by CFM (Figure 4B).
N ¼ n N =N
ð4Þ
s
s
A
w
in which n and N are the total number of moles of TPE-SDS
s
A
and the Avogadro constant, respectively. The effective
volume (V ) of a single TPE-SDS molecule as a cylinder can
s
be written as:
2
V ¼ p r
s
l
ð5Þ
x
The volume of a single MED is equal to the sum of the
volume of the TPE-SDS layer and the water pool:
V ¼ Vw þ N Vs
ð6Þ
s
The value of r can be calculated by combining the above
x
equations. We estimated the corresponding distance between
two adjacent TPE units attached at water pools with different
sizes. (Note that the amount of TPE-SDS used to synthesize
the microemulsion system was 2.0 nmol.) For example, if
0
.5 mL of water is added, the radius of the water pool is
calculated to be 11.76 nm. The estimated distance between
two adjacent TPE units is 0.54 nm, which is equal to half the
TPE length. In this case, the TPE units could be effectively
restricted so that the formed microemulsion could emit
intense light. If 20 mL of water is added, the fluorescence
intensity of the microemulsion is close to zero, possibly
because the rotation of the two free phenyl groups on the TPE
core can not be effectively restricted. In this case, the radius of
the water pool is increased to 66.00 nm, with a distance of
Figure 4. A) Fluorescence spectra of microemulsion solutions with
differing water content (0.5, 5, 10, 15, and 20 mL; TPE-SDS: 2.0 nmol,
isooctane volume: 3.2 mL, 1-butanol volume: 0.8 mL). B) Fluorescence
microscopy image of a TPE-SDS microemulsion. C) Schematic illustra-
tion of a microemulsion droplet, and the TPE-SDS molecular model
generated with the ACD Labs software.
1
.10 nm between two adjacent TPE units. Therefore, we
We calculated the structural parameters of the W/O
conclude that MEDs cannot be directly visualized by CFM if
the distance between two adjacent TPE units is above
1.10 nm.
microemulsion by using
a
mathematical model (Fig-
[18]
ure 4C).
The TPE-SDS and 1-butanol molecules are
arranged closely in an orderly fashion around the water
pools so that the water pools are separated from the oil phase.
In this model, we assume that the size of spherical MEDs is
uniform. A TPE-SDS molecule is considered as a cylinder:
The height of the cylinder is equal to the length of TPE-SDS
In conclusion, we have presented the synthesis and
micelle characteristics of a novel kind of anionic surfactant,
TPE-SDS, with an AIE effect. Micelle-transition processes
(involving spherical micelles, rodlike micelles, and wormlike
micelles) and TPE-SDS MEDs were directly visualized by
CFM. In comparison to other currently available techniques,
the high-resolution imaging method offers a clear view into
the world of micelles. Furthermore, the fluorescence intensity
of MEDs indicated their size. This study not only establishes
a feasible method for the imaging of micelle-transition
processes, but also deepens our understanding of the AIE
effect. This facile method may open viable opportunities and
inspiration for the direct visualization of other self-assembly
processes of materials related to surfactants. The results of
our current efforts toward these objectives will be reported in
due course.
(
l), and the radius of the circle (r ) is equal to half the
x
estimated distance between two adjacent TPE units. The
water-pool radius (r ) is calculated by subtracting the TPE-
w
SDS length (l) from the MED radius (r). The size of the MED
was measured with a z-potential analyzer. The spheroidal
volume of the MED (V) and the water pool (V ) is given by
w
Equations (1) and (2), respectively:
3
V ¼ 4=3 p r
ð1Þ
3
3
Vw ¼ 4=3 p rw ¼ 4=3 p ðrÀlÞ
ð2Þ
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[
[
8] C. M. Yu, Y. L. Wu, F. Zeng, X. Z. Li, J. B. Shi, S. Z. Wu,
Biomacromolecules 2013, 14, 4507 – 4514.
9] P. C. Yin, P. F. Wu, Z. C. Xiao, D. Li, E. Bitterlich, J. Zhang, P.
Cheng, D. V. Vezenov, T. B. Liu, Y. G. Wei, Angew. Chem. Int.
Ed. 2011, 50, 2521 – 2525; Angew. Chem. 2011, 123, 2569 – 2573.
Acknowledgements
This research was supported by the National Basic Research
Program of China (973 Program, 2014CB932103), the
National Natural Science Foundation of China (21375006
and 21575010), and the Innovation and Promotion Project of
Beijing University of Chemical Technology.
[
[
[
10] L. Shi, D. Lundberg, D. G. Musaev, F. M. Menger, Angew. Chem.
Int. Ed. 2007, 46, 5889 – 5891; Angew. Chem. 2007, 119, 5993 –
5995.
11] H. Tong, Y. N. Hong, Y. Q. Dong, M. Häussler, Z. Li, J. W. Y.
Lam, Y. P. Dong, H. H.-Y. Sung, I. D. Williams, B. Z. Tang, J.
Phys. Chem. B 2007, 111, 11817 – 11823.
12] a) S.-B. Han, H.-J. Kim, D. Jung, J. Kim, B.-K. Cho, S. Cho, J.
Phys. Chem. C 2015, 119, 16223 – 16229; b) A. Rananaware, R. S.
Bhosale, K. Ohkubo, H. Patil, L. A. Jones, S. L. Jackson, S.
Fukuzumi, S. V. Bhosale, S. V. Bhosale, J. Org. Chem. 2015, 80,
Keywords: aggregation-induced emission · fluorescence ·
micelles · microemulsions · surfactants
How to cite: Angew. Chem. Int. Ed. 2015, 54, 15160–15164
Angew. Chem. 2015, 127, 15375–15379
3
832 – 3840; c) J. Wang, J. Mei, R. R. Hu, J. Z. Sun, A. J. Qin,
[
1] a) D. Hu, K. C. Chou, J. Am. Chem. Soc. 2014, 136, 15114 –
5117; b) M. J. Rosen, J. T. Kunjappu, Surfactants and Interfacial
B. Z. Tang, J. Am. Chem. Soc. 2012, 134, 9956 – 9966.
1
[
13] a) J. W. Li, Y. Li, C. Y. K. Chan, R. T. K. Kwok, H. K. Li, P.
Zrazhevskiy, X. H. Gao, J. Z. Sun, A. J. Qin, B. Z. Tang, Angew.
Chem. Int. Ed. 2014, 53, 13518 – 13522; Angew. Chem. 2014, 126,
Phenomena, Wiley, Hoboken, NJ, 2012; c) K. Holmberg, B.
Jçnsson, B. Kronberg, B. Lindman, Surfactants and Polymers in
Aqueous Solution, Wiley, Chichester, 2002; d) E. Soussan, S.
Cassel, M. Blanzat, I. Rico-Lattes, Angew. Chem. Int. Ed. 2009,
1
3736 – 13740; b) Y. Okazawa, K. Kondo, M. Akita, M. Yoshi-
zawa, J. Am. Chem. Soc. 2015, 137, 98 – 101.
4
8, 274 – 288; Angew. Chem. 2009, 121, 280 – 295.
2] a) H. Hoffmann, G. Ebert, Angew. Chem. Int. Ed. Engl. 1988, 27,
02 – 912; Angew. Chem. 1988, 100, 933 – 944; b) S. Ezrahi, E.
Tuval, A. Aserin, Adv. Colloid Interface Sci. 2006, 128 – 130, 77 –
02; c) G. V. Jensen, R. Lund, J. Gummel, M. Monkenbusch, T.
Narayanan, J. S. Pedersen, J. Am. Chem. Soc. 2013, 135, 7214 –
222.
[
[
[
14] Y. J. Xia, L. Dong, Y. Z. Jin, S. Wang, L. Yan, S. C. Yin, S. X.
Zhou, B. Song, J. Mater. Chem. B 2015, 3, 491 – 497.
[
[
9
15] a) D. C. Grahame, Chem. Rev. 1947, 41, 441 – 501; b) J. J. López-
García, J. Horno, C. Grosse, Langmuir 2011, 27, 13970 – 13974.
16] a) J.-H. Mu, G.-Z. Li, X.-L. Jia, H.-X. Wang, G.-Y. Zhang, J.
Phys. Chem. B 2002, 106, 11685 – 11693; b) P. A. Hassan, S. R.
Raghavan, E. W. Kaler, Langmuir 2002, 18, 2543 – 2548.
17] a) N. B. Shustova, B. D. McCarthy, M. Dinc a˘ , J. Am. Chem. Soc.
1
7
3] G. V. Jensen, R. Lund, J. Gummel, T. Narayanan, J. S. Pedersen,
[
Angew. Chem. Int. Ed. 2014, 53, 11524 – 11528; Angew. Chem.
2
011, 133, 20126 – 20129; b) N. B. Shustova, A. F. Cozzolino,
V. K. Michaelis, R. G. Griffin, M. Dinc a˘ , J. Am. Chem. Soc. 2012,
34, 15061 – 15070; c) N. B. Shustova, A. F. Cozzolino, S. Rein-
eke, M. Baldo, M. Dinc a˘ , J. Am. Chem. Soc. 2013, 135, 13326 –
3329; d) J. Zhao, D. Yang, Y. X. Zhao, X.-J. Yang, Y.-Y. Wang,
2
014, 126, 11708 – 11712.
[
[
4] B. Kronberg, K. Holmberg, B. Lindman, Surface Chemistry of
Surfactants and Polymers, Wiley, Chichester, 2014.
1
5] a) F.-Y. Tsai, H.-L. Tu, C.-Y. Mou, J. Mater. Chem. 2006, 16, 348 –
1
ˇ
3
8
50; b) M. St eˇ pµnek, K. Prochµzka, Langmuir 1999, 15, 8800 –
806.
B. Wu, Angew. Chem. Int. Ed. 2014, 53, 6632 – 6636; Angew.
Chem. 2014, 126, 6750 – 6754.
[
6] J. D. Luo, Z. L. Xie, J. W. Y. Lam, L. Cheng, H. Y. Chen, C. F.
Qiu, H. S. Kwok, X. W. Zhan, Y. Q. Liu, D. B. Zhu, B. Z. Tang,
Chem. Commun. 2001, 1740 – 1741.
[
18] a) S. Bardhan, K. Kundu, G. Chakraborty, S. K. Saha, B. K. Paul,
J. Surfactants Deterg. 2015, 18, 547 – 567; b) S. K. Hait, S. P.
Moulik, Langmuir 2002, 18, 6736 – 6744.
[
7] a) Y. N. Hong, J. W. Y. Lam, B. Z. Tang, Chem. Soc. Rev. 2011,
4
0, 5361 – 5388; b) D. Ding, K. Li, B. Liu, B. Z. Tang, Acc. Chem.
Res. 2013, 46, 2441 – 2453; c) R. R. Hu, N. L. C. Leung, B. Z.
Tang, Chem. Soc. Rev. 2014, 43, 4494 – 4562; d) J. Mei, Y. N.
Hong, J. W. Y. Lam, A. J. Qin, Y. H. Tang, B. Z. Tang, Adv.
Mater. 2014, 26, 5429 – 5479.
Received: August 3, 2015
Revised: September 18, 2015
Published online: October 16, 2015
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