Poly(ionic liquid)-based monodisperse microgels as a unique platform for producing functional materials

Article abstract of DOI:10.1039/c4tc02487g

In this work, we report the microfluidic preparation of monodisperse imidazolium-based poly(ionic liquid) (PIL) microgels with a controlled size and morphology, and show that the imidazolium units in the microgel network can be exploited as reactive sites to efficiently access desired functional materials by a simple counteranion-exchange or conversion reaction. Moreover, based on the counteranion-exchange reaction, spatio-temporal engineering of the surface of the PIL microgels could also be realized, and a new and simple strategy for the fabrication of diverse anisotropic microgels (patchy particles) with great flexibility was developed. In addition, by exploiting the convenient generation of carbene units from the imidazolium moieties, as well as the carbonizable feature of PIL, the prepared PIL microgels could be further converted into stable carbene spheres and monodisperse carbon particles. All the results show that these monodisperse PIL-based microgels can serve as a very useful platform for facilely accessing various functional materials.

Full text of DOI:10.1039/c4tc02487g

Journal of
Materials Chemistry C
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PAPER
Poly(ionic liquid)-based monodisperse microgels as
a unique platform for producing functional
materials†
Cite this: J. Mater. Chem. C, 2015, 3,
23
6
Jiecheng Cui,‡ Ning Gao,‡ Jian Li, Chen Wang, Hui Wang, Meimei Zhou, Meng Zhang
and Guangtao Li*
In this work, we report the microfluidic preparation of monodisperse imidazolium-based poly(ionic liquid)
(PIL) microgels with a controlled size and morphology, and show that the imidazolium units in the microgel
network can be exploited as reactive sites to efficiently access desired functional materials by a simple
counteranion-exchange or conversion reaction. Moreover, based on the counteranion-exchange
reaction, spatio-temporal engineering of the surface of the PIL microgels could also be realized, and a
new and simple strategy for the fabrication of diverse anisotropic microgels (patchy particles) with great
flexibility was developed. In addition, by exploiting the convenient generation of carbene units from the
imidazolium moieties, as well as the carbonizable feature of PIL, the prepared PIL microgels could be
further converted into stable carbene spheres and monodisperse carbon particles. All the results
show that these monodisperse PIL-based microgels can serve as a very useful platform for facilely
accessing various functional materials.
Received 31st October 2014
Accepted 11th November 2014
DOI: 10.1039/c4tc02487g
www.rsc.org/MaterialsC
microgels. Thereby, developing a new strategy to efficiently
access monodisperse functional microgels with controlled sizes
Introduction
Polymer microgels are micrometer-sized particles, which have a and morphologies is highly desirable.
three-dimensional network structure and are inated with a
Ionic liquids (ILs), a class of salts composed of discrete
solvent. Due to their unique structural features, microgels have organic cations and anions, exhibit a series of unique proper-
1
attracted increasing attention over the past few years and have ties, including negligible vapor pressure, non-ammability,
been exploited for various applications in drug delivery, chem- high ionic conductivity, a wide electrochemical window, and
4
ical sensing, separation and purication techniques, the fabri- good chemical and thermal stability. Importantly, such favor-
cation of photonic crystals, and catalysis for organic and able properties can be individually manipulated through
2
inorganic synthesis. Recently, due to their promising applica- modulating the combination of cations and anions, thus
5
tion as smart nanoreactors, the introduction of metal nano- providing unprecedented tunability. Thereby, these distinct
particles into polymer microgels to produce hybrid microgels physiochemical properties and features make ILs ideal candi-
3
has attracted considerable attention. However, to achieve dates for the design and development of functional materials
microgels with the desired function and sufficient mechanical and chemical systems. Some studies have demonstrated the
strength, traditionally each fabrication method is strongly applications of ionic liquids or their polymers (PILs) in the
dependent on the screening and on an arduous synthesis of a fabrication of functional materials through the counteranion-
6,7
functional monomer, as well as on optimization of the poly- exchange reaction. However, few works have been reported on
merization conditions. Especially in the case of sensitive func- multifunctional microgels and patchy microparticles, which
tional moieties (e.g., bioactive units) incompatible with the presents an exciting new research area and have attracted an
conventional thermal- or photo-induced polymerization increasing attention due to their strongly anisotropic and
8
process, the traditional polymerization methods are inappro- directional interactions. In this respect, there are still some
priate or have difficultly in fabricating the corresponding challenges needed to be faced. More importantly, the properties
of ionic liquids are actually much more than just the counter-
anion-exchange feature. Especially, in the case of imidazolium-
based PILs, the imidazolium moiety is not only a “reactive site”
Department of Chemistry, Key Lab of Organic Optoelectronics
& Molecular
Engineering, Tsinghua University, Beijing 100084, P. R. China. E-mail: LGT@mail.
tsinghua.edu.cn; Fax: +86 10-6279-2905
for counteranion exchange, but it can also serve as carbene and
4,9
carbon precursors. It is conceivable that the exploitation of
these unique properties of the imidazolium moiety could open
†
1
‡
Electronic supplementary information (ESI) available. See DOI:
0.1039/c4tc02487g
These authors contribute equally to this paper.
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up an efficient avenue to access various functional microgels (u
d
), the ow velocity of the continuous phase (v
c
) and dispersed
and materials from PIL-based microgels.
phase (v ), and the interfacial tensions between the continuous
d
Inspired by the unique properties of ILs, herein, we report on and dispersed phases. Recently, C. Serra and coworkers estab-
the microuidic synthesis of monodisperse imidazolium-based lished that the diameter of the resulting droplets in microuidic
PIL microgels, and show that the imidazolium units in the emulsions is a function of the channel dimensions, the ow
resultant polymer network indeed can be exploited as func- rates, viscosities, and the interfacial tensions between the
11
tional sites to efficiently access desired functional materials by continuous and dispersed phases, as follows:
not only simple counteranion exchange, but also by the
M
D /D ¼ K(Ca /Ca )
conversion reaction. As demonstration, three types of isotropic
functional particles, including metal–polymer hybrid particles,
conductive composite particles, and catalytic particles, were
d
g
c
d
where K and M are constant, Ca
c
d
and Ca are the capillary
number for the continuous phase and the dispersed phase, and
Ca /g, Ca ¼ u /g, and g is the interfacial tension.
¼ u

rst evolved from the prepared PIL microgels. More impor-
7
c
c
v
c
d
d d
v
tantly, based on the “task-specic” concept of ILs, the surface
of the PIL microgels could be spatio-temporally engineered to
afford multifunctional, as well as patchy microparticles. Thus, a
new and efficient strategy was developed to realize the fabrica-
tion of multifunctional patchy microparticles with great exi-
bility. Moreover, due to the convenient generation of carbene
units from the imidazolium moieties, as well as the carbon-
izable feature of PIL, the prepared PIL microgels could be
further converted into stable carbene spheres and mono-
disperse carbon particles. These monodisperse PIL-based
microgels can serve as a useful platform for facilely accessing
various functional materials (Scheme 1).
Moreover, for a given emulsion system in a microuidic device,
the dimensions of the formed particles can be controlled by
tuning the ow rates of the continuous and dispersed phases.
In the present work, a co-owing microuidic device was
employed to produce droplets with a narrow size distribution,
as shown in Scheme S1.† Poly(dimethylsiloxane) with surfac-
tants was used as the continuous oil phase, and the dispersed
phase was an aqueous solution of 1-butyl-3-vinylimidazolium as
the monomer and 1,6-di(3-vinylimidazolium) hexane as the
cross-linker (Scheme 1). As previously reported in the litera-
11
ture, the diameter of the droplets increases when the ow rate
of the dispersed phase increases or when the ow rate of the
d
continuous phase decreases. In fact, because the velocity (v ) of
the dispersed phase is considerably lower compared to the
continuous phase, the control of the continuous phase velocity
is found to be a more practically useful means for generating
droplets with a good controlled bead size and high repeatability.
Thus, in this work, the control of the ow rates of the contin-
uous and dispersed phases was employed to realize the
formation of microgel particles with different sizes. Microgels
Results and discussion
Synthesis of the monodisperse PIL-based microgels
The microuidic synthesis of microgel particles begins with the
emulsication of a precursor solution in an immiscible liquid.
The resulting particle dimensions and their uniformity are pre-
determined in the microuidic emulsication stage. In
ꢀ
1
10
of 467 mm were obtained when v
d
¼ 0.5 mL h and v
c
¼ 25 mL
previous works, it was veried that the diameter of the formed
ꢀ1
h
were used (Fig. 1a). As mentioned above, the regulation of
precursor droplets (D ) (and hence, the corresponding solid
d
the ow rate of the continuous phase provides more effective
control over the size of the resulting microgel particles. When
the ow rate of the continuous phase was increased from 25 mL
particles) is sensitive to a number of preparation parameters,
including the caliber of the glass capillary microchannel (D
g
),
the viscosity of the continuous phase (u ) and dispersed phase
c
Scheme 1 Poly(ionic liquid)-based monodispersed microparticles as a unique platform for producing functional materials, including patchy
particles, metal-containing microgels, conductive composite particles, catalytic particles, carbene particles, and carbon particles.
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Fig. 2 (a) Size evolution of the swollen PIL microgels exposed to the
air (b). Swelling of the prepared PIL particles exposed to different
organic solvents. Scale bar is 100 mm.
Fig. 1 Optical images of the fabricated PIL microgels with a diameter Counteranion-exchange behavior of the PIL microgels
of 467 mm (a) and 233 mm (b). The scale bar is 450 mm; (c) relationship
As mentioned in the introduction, one of the most attractive
between the diameter of the droplets D
d
and the flow velocity of the
ꢀ1
continuous phase (v ) under the conditions: v
c
d
¼ 0.5 mL h , D
¼ 120 features of ILs is that their physicochemical properties and
g
mm and 50 cSt silicone oil as the continuous phase; (d) size distribution functionality can be easily tailored by simple counteranion
of microgels, the mean diameter and the coefficient of variation of the
exchange. Thus, the counteranion-exchange behavior of the
formed microgels (244 mm).
ꢀ
prepared PIL microgels (with Br as the counteranion) was rst
examined using three salts, namely, sodium tetrauoroborate
(
NaBF
4 2
), lithium bis(triuoromethane sulfonimide) (LiTf N),
ꢀ1
ꢀ1
h
to 100 mL h , microgels with an obvious smaller diameter and sodium hexauorophosphate (NaPF ) (Scheme S2†). As
6
(
233 mm) were obtained (Fig. 1b). Fig. 1c shows the correlation expected, upon exposure of the fabricated microgels to an
between the ow velocity of the continuous phase (v ) and the aqueous solution of the chosen salts, a gradual shrinkage of
sizes of the formed droplets (D ) under conditions as follows: v
c
d
d
their sizes was observed, indicative of the occurrence of the
ꢀ
1
ꢀ
¼
0.5 mL h , D
g
¼ 120 mm and 50 cSt silicone oil as the counteranion-exchange reaction. With the displacement of Br
continuous phase. Clearly, it can be seen that with an increased by hydrophobic anions, the prepared PIL microgels became
ow velocity of the continuous phase, the diameter of the smaller. The more hydrophobic is the used counteranion, the

formed droplets gradually decreases. Certainly, the viscosity of smaller the microgels became (Fig. 3). The counteranion-
the continuous phase also has an obvious effect on the droplet exchange reaction seems relatively fast and took about two
size. Generally, a high viscosity of the continuous phase leads to hours for the used microgels. Indeed, the FTIR analysis
a smaller particle diameter, due to the relative increase in the conrmed the occurrence of the anion exchange and the
shear force exerted on the dispersed phase by the continuous
phase over the interfacial force. In our case, the dispersion of
the particle sizes is quite low (Fig. 1d). Under optimized
conditions, the microuidic generation of polymer particles
with a polydispersity below 1–2% is possible. These results
indicate that using a simple microuidic system, monodisperse
PIL microgels with controlled sizes could be facilely fabricated.
In our case, the value of M in the above equation was deter-
mined to be ꢀ0.27 by analyzing the parameter such as D
, u , and u in the size control experiments of the microgels.
As expected, the prepared PIL particles exhibit the charac-
d g c
, D , v ,
v
d
c
d
teristic features of microgels. The original PIL particles (with
ꢀ
Br as the counteranion) exhibited considerable swelling in
water or water solution. Upon exposure to air atmosphere, the
swollen PIL particles gradually became smaller with the loss of
water molecules (Fig. 2a) and their size dramatically changed
from a swollen to dried state, for example from 850 mm to 230
mm. The swelling and deswelling process is completely revers-
ible. Moreover, it is found that upon exposure to different
organic solvents, the prepared PIL particles exhibit different
swelling (Fig. 2b).
Fig. 3 Optical images of the PIL microgels with different counter-
ꢀ
anions: (a) microgels with Br as counteranion (diameter: 370 mm); (b)
ꢀ
microgels with BF
4
as counteranion (diameter: 305 mm); (c) microgels
ꢀ
with PF
6
as counteranion (diameter: 236 mm); (d) microgels with
as counteranion (diameter: 189 mm). The scale bar is 200 mm.
ꢀ
Tf
2
N
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existence of the chosen anions. Fig. S1 in the ESI† shows the
comparison of the FTIR spectra of the PIL microgel particles
before and aer exposure to different anion aqueous solutions.
It is clearly seen that the characteristic absorption bands of the
ꢀ
1
ꢀ
ꢀ1
used anions were detected at 1083 cm
(BF ), 838 cm
4
ꢀ ꢀ
ꢀ1
12
6 2
(PF ), and 1356 cm (Tf N ).
To visualize the anion-exchange process, in this work, three
typical dye molecules (SPADNS, 9,10-anthraquinone-2-sulfonic
acid sodium salt, and indigo carmine in Scheme S2†) were
employed to further conrm the feasibility of the introduction
of a specic function in the PIL microgels by an anion-exchange
ꢀ
reaction. Similarly, the prepared PIL microgels (with Br as the
counteranion) were soaked in aqueous solution of the corre-
sponding dye molecule at room temperature for 12 h. Aer
being ltered off, the treated PIL microgels were thoroughly
washed to remove the physically trapped dye molecules until no
coloration was observed in the washing solution. As shown in
Fig. 4, the treated PIL microgels, exhibiting a clear color
throughout the microgels, display a different appearance from
the original PIL microgels, which do not exhibit any optical
Fig. 5 UV/vis spectra of the fabricated PIL microgels before and after
the treatment with an aqueous solution of different dye molecules: (a)
original PIL microgel; (b) treated with SSPADNS; (c) treated with indigo
carmine; (d) treated with 9,10-anthraquinone-2-sulfonic acid sodium
salt. For comparison the UV/vis spectra of the used dye molecules are
property (Fig. 1). Fig. 5 shows the UV-vis spectra of the used dye also includes, respectively.
molecules and the PIL microgels before and aer the counter-
anion-exchange treatment. Obviously, by simple anion-
1
4
printing (mCP) (Scheme S3†) method, the PIL microgels with
A–B, A–B–A, or A–B–C patchy structures were easily accessible
(
exchange the optical properties of the functional anions were
correctly introduced into the PIL microgels. These results
clearly indicate that the counteranion-exchange property of the
ionic-liquid units inside the PIL microgels is still retained, and
that the functional groups could be facilely introduced into the
prepared microgels via an anion-exchange reaction.
Fig. 6). In this work, for visual inspection, two uorescent
counteranions (Indigo carmine and SPADNS) were used as a
chemical “ink” and the PDMS elastomer was utilized as a
Preparation of patchy microgels or particles through spatio-
temporal engineering of the surface of the PIL microgels
Compared to the monofunctional microgel particles described
above, multifunctional microgels, as well as anisotropic patchy
particles, are another more interesting class of particles that
exhibit completely different properties and that hold promise
13
for use in numerous applications. In this respect, the surface
of PIL microgels were designed by spatio-temporal engineering
to achieve multifunctional microparticles, as well as patchy
particles. Due to the counteranion-exchange capability of ionic
liquids, the PIL also show great exibility in surface chemistry
engineering, and allow for producing various multifunctional
anisotropic structures. Using
a “sandwich” microcontact
Fig. 6 Optical images of the original PIL microgel (a) and the derivated
Fig. 4 Optical images of the fabricated PIL microgels after the patchy particles with an A–B structure (b), with an A–B–A structure (c)
exposure to indigo carmine (a), and SPADNS (b) aqueous solution, with an A–B–C structure (d and e); the microgels with tunable patchy
respectively. The scale bar is 500 mm.
size (f–i). The scale bar is 100 mm (a–d and f–i) and 300 mm (e).
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stamp. It was found that the printing process in our case pro- optical and TEM images of the prepared Au–PIL hybrid micro-
ꢀ
ceeded very fast at room temperature. It took only several gels. Compared to the original PIL microgels (with Br as the
minutes to transfer the “ink” to the PIL microgels via counter- anion), the resultant microgels show considerable shrinking of
anion exchange exclusively in the area of contact. The contact their particle size and exhibit a purple color, indicative of the
time and pressure could be used to tune the size of the formed formation of gold nanoparticles (Fig. 7a). Under TEM observa-
patchy microgels (Fig. 6f–i). Due to the unlimited combination tion, a large number of gold dark dots with a diameter of about
5
of cations and anions, although only uorescent anisotropic 20 nm were detected to be homogeneously distributed inside
PIL microgels were fabricated, principally unlimited PIL patchy the PIL microgels (Fig. 2b). Fig. S2 and S3† show the TGA and
particles could be easily accessible through a simple counter- EDX spectra of the prepared Au–PIL hybrid microgels. The TGA
anion-exchange reaction, including biomolecule-binding pat- measurement demonstrates that 2 wt% Au nanoparticles were
chy particles and magnetic patchy particles. Compared to the immobilized into the PIL microgels. Moreover, the content of
13
numerous developed approaches, the spatio-temporally engi- the Au nanoparticles can be up to 6 wt% by adjusting the
ꢀ
neered surface strategy described here is simple and versatile, amount of AuCl
4
introduced during the counteranion-
offering a broad design space for fabricating desired multi- exchange reaction. Interestingly, it was found that the trapped
functional materials (patchy microparticles) under mild Au nanoparticles in the PIL gels display a remarkable stability
conditions.
and that no leakage or aggregation of the Au nanoparticles was
detected with the prolonged storage of the hybrid microgels in
water. Recently, numerous works have reported the similar
phenomenon that some ionic liquids, especially imidazolium-
Fabrication of various functional microgels using PIL
microgels
based ionic liquids, can hold stable suspensions of metallic
7
15
On the basis of the concept of “task-specic” ionic liquids, a nanoparticles without additional surface-active agents. Using
variety of functional groups as anions can be readily introduced XPS and Raman analysis, several studies suggest that in the case
into the PIL-IOMS by a simple anion-exchange reaction, leading of imidazolium-based ionic liquids, a strong interaction
to new microgels or spheres with desired functions. Thus, the between metal particles with positively charged imidazolium
immobilization of metal nanoparticles in microgels through the rings is present, which is responsible for the solvation and
counteranion-exchange reaction followed by reduction of the stability of the nanoparticles in ionic liquids. On the basis of
incorporated metallic complex is possible. Recently, the these results, we believe that the same stabilization mechanism
uniform and homogenous metal–polymer hybrid microgels could be used to explain the observed phenomenon in our case.
ꢀ
ꢀ
have fascinated scientists, due to their potential applications in Additionally, aer the displacement of Br by AuCl
chemical sensing or as nanoreactors. Under this consideration, followed by reduction using NaBH , (Scheme S4†), it was found
in our work, Au–PIL hybrid microgels were synthetized by that the resulting Au-containing microparticles have a smaller
4
anions,
4
ꢀ
ꢀ
introducing a precursor AuCl4 anion into the PIL microgels via size compared to the original microgels (with Br as the anion).
counteranion exchange and subsequent reduction using NaBH
The exact reason for this phenomenon is still not clear. Prob-
4
as a reducing agent (Scheme S4†). Fig. 7a and b shows the ably, the strong interaction between gold particles with IL
moieties reduces the size of the resulting composite particles.
ꢀ
Interestingly, we found that the AuCl4 anions incorporated
into PIL microgels could also serve as reducing agents to
produce uniform conducting polymer spheres when the PIL
microgels were exposed to the solution of pyrrole. Fig. 7c
displays the optical image of the resulting conductive hybrid
particles, which shows a quit different morphology from the
original microgels. In particular, under SEM observation, the
obtained conducting polymer (polypyrrole) microparticles show
a typical granular morphology (Inset in Fig. 7c), and the poly-
pyrrole coating on the surface is very evident. Importantly,
electrochemical analysis of the formed particles provide direct
evidence for the formation of conductive polypyrrole inside the
particles. As shown in Fig. 7d, the redox behavior of the poly-
pyrrole composite spheres was investigated by cyclic voltam-
metry. Compared to the original microgels, which do not
exhibit any electrochemical activity in the range from ꢀ0.6 V to
0.6 V, the resulting composite spheres show the characteristic
redox feature of polypyrrole.
Fig. 7 (a) Optical image of the microgels containing Au nanoparticles;
b) TEM image of the microgels containing Au nanoparticles; (c) optical
image of the conductive microgels; inset is the SEM image of the
prepared conductive particles, (d) cyclic voltammograms of the orig-
inal PIL microgel and the resulting composite conductive particles.
As a further demonstration, polyoxometalates (POMs) were
chosen as task-specic building blocks and introduced into the
PIL microgels. POMs are a family of anionic, inorganic metal
oxides, and exhibit a wide range of topologies and a very rich
(
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redox chemistry that provides the basis for their high compounds have attracted signicant interest in various elds
16
19
catalytic activity in oxidation reactions. The combination of of chemistry. As molecules with divalent carbon atoms, NHCs
organic cations of the PIL microgels with POM anions could are not only of theoretical interest but also of practical relevance
not only lead to the formation of recoverable catalysts, but as versatile ligands and effective organocatalysts. In our work,
also the microgels with electrochemical activity are we found that the PIL microgels produced from the imidazo-
obtained (Fig. 8). In this study, the dinuclear peroxotungstate lium-based IL mononer could be readily transformed to stable
2
–17
[
{W(]O)(O
2
)
2
(H
2
O)}
2
(m-O)] was used as a catalytic anion, due N-heterocyclic carbine microspheres by deprotonation of the
utilization and high selectivity to imidazolium moieties with sodium hydride in tetrahydrofuran
the epoxides. The incorporation of the PIL microgels with in the presence of a catalytic amount of dimethyl sulfoxide
to its high efficiency of H
2 2
O
16
19
peroxotungstate was simply achieved through the exposure of (Fig. 9a). Because of their high basicity, N-heterocyclic car-
the PIL microgels to an aqueous solution of the potassium salt benes can rapidly react with CO to afford zwitterionic adducts
2
of peroxotungstate at RT for 12 h. FTIR measurements (designated as NHC–CO ), even at very low CO concentrations.
2
2
conrmed the correct loading of peroxotungstate active species Indeed, it is found that the resultant carbene spheres can be
into the interior of the PIL microgels. Compared to the FTIR used as a highly efficient adsorbent for the reversible xation-
spectrum of the original PIL microgels, the characteristic release of CO
2
. Fig. 9c shows the FTIR spectra of the micro-
spheres before and aer the reaction with CO . The appearance
and 812 cm were detected in the treated samples (POM@PIL) of the asymmetric n(CO ) vibrations of the NHC–CO adduct at
Fig. 8a). The catalytic performance of POM@PIL microgels was 1649 cm conrmed the presence of the NHC moieties in the
18
ꢀ1
ꢀ1
ꢀ1
bands of peroxotungstate at 1080 cm , 978 cm , 895 cm
,
2
ꢀ
1
2
2
ꢀ
1
(
2
0
evaluated using the oxidation reaction of olens. Consequently, microspheres. As expected, the produced carbene micro-
POM@PIL microgels were dispersed in an acetonitrile solution spheres show an excellent catalytic effect for transesterication
of cyclooctene with hydrogen peroxide as an eco-friendly and for acylation reactions. In our study, a simple trans-
oxidant. The oxidation reaction was carried out at 333 K with esterication of benzyl alcohol and vinyl acetate in tetrahydro-
stirring. At given time intervals of reaction, the product and furan was performed in the presence of the carbene spheres and
yield were determined by GC/MS analysis. It was found that the under a nitrogen atmosphere (Fig. 9b). Consequently, mono-
POM@PIL microgels exhibited excellent catalytic activity. Aer disperse carbene spheres were dispersed in a tetrahydrofuran
5
h, 93% of cyclooctene were converted into the corresponding solution of benzyl alcohol and vinyl acetate. The reaction was
epoxides. The above results indicate that the observed catalysis carried out at 298 K with stirring. At given time intervals of
indeed originates from the ionic bonded peroxotungstate reaction, the product and yield were determined by GC/MS
species, and that the catalysis of the POM@PIL microgels is analysis. Aer 10 min, the reactants were quantitatively con-
truly heterogeneous in nature. As a control experiment, the verted into product. This indicated that the carbene spheres
same reaction mixture only without POM@PIL microgels was
stirred at 333 K for 5 h. As expected, the conversion was as low as
1.77%. Moreover, the catalytic property of the POMs-incorpo-
rated microgels was compared with that of pure POMs under
the same conditions. It was found that 98% of cyclooctene was
converted into the corresponding epoxides using pure POMs.
This result indicates that the catalytic performance of the POMs
remain nearly unchanged aer their incorporation into
microgels.
Fabrication of uniform carbene spheres from PIL microgels
Since the rst isolation and characterization of stable
N-heterocyclic carbenes (NHCs) by Arduengo in 1991, these
Fig. 9 Model reaction for synthesis of crabene (a) and the evaluation
of the catalytic capability of the carbene microspheres (b); (c) FTIR
spectra of the prepared carbene microspheres before and after the
Fig. 8 (a) FT-IR spectra of the fabricated PIL microgels before and
after treatment with an aqueous solution of peroxotungstate; (b) cyclic
voltammograms of the original PIL microgels before and after the
counteranion exchange with POM.
2
reaction with CO .
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22
exhibited excellent catalytic activity. In contrast, the same temperature. Thus, the carbonization of the PIL microgels at
reaction did not occur without the assistance of the carbene higher temperatures can lead to further graphitized structures.
spheres. Probably due to the steric hindrance offered by the
polymeric network, the obtained carbene microspheres show
remarkable stability and could serve as recoverable catalysts. As
Conclusions
a control experiment, the catalytic property of pure NHCs was
In summary, using a microuidic approach, we successfully
also studied under the same conditions. It was found that the
produced monodisperse poly(ionic liquid) microgels with
catalytic performance of the NHCs nearly remain unchanged
controlled size and morphology. On the basis of the utilization
aer their incorporation into microgels.
of the unique properties of poly(ionic liquid), such as spatio-
temporal engineering of the microgels' surface through a
counteranion-exchange reaction, the imidazolium-based
conversion reaction, as well as the carbonizable feature it was
found that the poly(ionic liquid) microgels could be easily
evolved for conveniently fabricating functional materials with a
variety of potential applications. Due to the unlimited combi-
nation of cations and anions, principally unlimited PIL multi-
functional materials, as well as anisotropic patchy particles are
easily accessible. The imidazolium-based conversion reaction of
poly(ionic liquid) provides another opportunity to construct
unique carbene microspheres with remarkable stability, which
shows promising catalytic applications as heterogeneous cata-
lysts. Additionally, the carbonizable feature of poly(ionic liquid)
allows for efficiently producing monodisperse carbon particles.
All the obtained results clearly indicate that, as a unique plat-
form, these poly (ionic liquid) microgels could provide
tremendous opportunities, principally unlimited possibilities
for facilely accessing various functional materials.
Fabrication of uniform carbon spheres using PIL microgels
The unique features of ionic liquids mentioned in the intro-
duction section have led to the wide use of ILs in many areas, for
instance, as materials for fabricating different types of func-
tional materials through counteranion exchange, as solvents for
organic and inorganic synthesis, and as electrolytes for energy
applications. In an effort to extend new applications, ionic
liquids and their polymers have been found to be excellent
4b
precursors for producing functional carbon materials. Fig. 10
shows the optical image of carbon microspheres produced from
carbonizing the PIL microgels at 1073 K under a N
Obviously, all the carbon spheres preserve
2
atmosphere.
spherical
a
morphology and have a narrow size distribution. However,
considerable shrinkage of the particle size occurred during the
carbonization process, and thus the diameter of the resultant
carbon particles is considerably smaller compared to the parent
particles. For example, the carbonization of the PIL microgels
with a diameter of 420 mm afforded 80 mm carbon spheres.
Nevertheless, this result indicates that different monodisperse
carbon particles can be readily fabricated by choosing PIL
microgels with different particle sizes. The cross-linked polymer
Experimental section
Chemicals
structures and surface tension of the microgels are the principal All the solvents and chemicals are of reagent quality and were
reason for the retention of the spherical morphology under used without further purication unless otherwise specically
high-temperature annealing.
explained. 1-Vinylimidazole and 1-bromobutane 1,6-dibromo-
A Raman spectrum of the obtained carbon spheres reveals hexane were purchased from Alfa and used as received. Silicone
ꢀ1
ꢀ1
two distinct peaks at 1350 cm and 1580 cm (Fig. 10b), oil (20 cSt, 50 cSt) and 2,2-azobis(2-methylpropion-amidine)
conrming the presence of amorphous and graphitic carbon dihydrochloride (AIBA) as the photoinitiator were purchased
2
1
ꢀ1
domains. The 1580 cm band is associated with a graphitic from Aldrich. Polydimethylsiloxane (KF-96 10 cSt) was obtained
2
carbon with a sp electronic conguration. The band at 1350 from Shin-Etsu Chemical, Japan. Silicone oil and span80 were
cm arises from polycrystalline graphite, and can be attributed brought from Yunuo Chemicals Ltd, China.
to diamond-like carbon atoms with a sp conguration. Earlier
work in our laboratory suggested that the area ratio of Asp to
ꢀ
1
3
3
2
Instrumentation
Asp
decreased with increasing the carbonization
1
H NMR spectra were obtained using a JEOLJNMECS 300NMR
spectrometer. UV-vis spectra were obtained on a PerkinElmer
Lambda35 spectrometer. The photopolymerization was carried
out under UV light. The microstructures of the beads were
characterized by a scanning electron microscopy (SEM, HITA-
CHI, S-300N). Images of the beads were taken using an optical
microscope (OLYMPUS BXFM) equipped with a CCD camera.
The microuidic devices were homemade by glass capillary
tubes with an inner diameter of 75 mm and an outer diameter of
200 mm and a T-junction. The monomer dispersed phase and oil
continuous phase were delivered by syringe pumps. The
dispersed phase was injected via a glass capillary tube posi-
tioned along the main axis of the T-junction. The continuous
Fig. 10 (a) Optical image of the resulting carbon spheres; (b) Raman
spectrum of the resulting carbon spheres. The scale is 200 mm.
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phase was injected perpendicular to the main axis of the T-
junction.
(B) Au–polymer hybrid microgels. The Au–polymer hybrid
microgels were also conveniently completed through an anion-
exchange reaction and redox reaction. The ILs microgels were
added to an aqueous solution of chloroauric acid at room
temperature for 12 h. Then, the microgels were immersed into
the aqueous solution of sodium borohydride to reduce the
chloroauric acid. Then, the Au–polymer hybrid microgels were
Synthesis of the ionic liquid monomers
The ionic liquid monomer, 1-butyl-3-vinylimidazolium bromide
23
was synthesized according to the literature. 1-Vinylimidazole
6.00 g, 64 mmol) was mixed with 1-bromobutane (10.5 g, 76.8
(

ltered off, and puried by extensive washing with deionized
water several times.
C) Conductive hybrid microgels. The conductive hybrid
mmol), and the mixture was vigorously stirred at 343 K. Aer 3 h
reaction, the residual 1-bromobutane was removed under
vacuum at 333 K to afford a yellow solid. The dicationic 1,6-di(3-
vinylimidazolium)hexane bromide as cross-linker was synthe-
sized as follows: a mixture of 2 molar equivalents of 1-vinyl-
imidazole and 1 molar equivalent of 1,6-dibromohexane was
stirred at room temperature until the IL solidied. Then, the
bromide salt was dissolved in water, and the mixture was
washed three times with ethyl acetate to remove impurities. The
last residue was evaporated in vacuum to ensure complete dryness.
(
microgels were also conveniently obtained through anion-
exchange and redox reactions. The ILs microgels were added to
an aqueous solution of chloroauric acid at room temperature
for 12 h. Then, the microgels were immersed into the solution
of pyrrole to obtain a polypyrrole lm coating on the surface of
the microgels. Then, the conducting polymer hybrid microgels
were ltered off, and puried by extensive washing with
deionized water several times.
1
H NMR (300 MHz, DMSO-d6, d): 9.56(s, 2H, –NCHN–), 8.22(s, 2H,
(D) Catalytic microgels. The prepared PIL microgels were
2
–NCHC–), 7.95(s, 2H, –CCHN–), 7.31(m, 2H, –CH]CH ), 5.97(dd,
added to an aqueous solution of peroxotungstate (POM) at room
temperature for 12 h. Then, the microgels were ltered off,
washed several times with deionized water, and dried in
vacuum to afford PIL microgels loaded with peroxotungstate.
The potential of the POM-loaded microgels for catalytic appli-
cations was checked as follows: the PIL microgels loaded with
POM were dispersed in acetonitrile solution (1 mL) containing
cyclooctene (0.2307 g, 2.1 mmol) and hydrogen peroxide (30%
aq. solution, 0.0647 g). Then, the reaction mixture was stirred at
J ¼ 2.4 Hz, J ¼ 15.8 Hz, 2H, –HCH]CHN–), 5.44(dd, J ¼ 2.07 Hz,
J ¼ 6.54 Hz, 2H, –HCH]CHN–), 4.21(t, J ¼ 7.2 Hz, 4H,
–
–
NCH CH ), 1.84(t, J ¼ 6.87 Hz, 4H, NCH CH CH ), 1.31(m, 4H,
2
2
2
2
2
CH CH CH –).
2
2
2
Preparation of monodisperse PIL microgels
.2 g IL monomer and 0.4 g cross-linker were dissolved in 400
1
mL water containing 2,2-azobis(2-methylpropion-amidine)
dihydrochloride (AIBA) as photoinitiator. To produce mono-
disperse PIL microgels, the homemade microcapillary devices
were used as shown in Scheme 1. The oil phases was poly-
333 K for 5 h. Aer the completion of the reaction, the ILs
microgels were separated by ltration, washed with solvent, and
dried in vacuum prior to being recycled.
(polydimethylsiloxane) with span80 as the stabilizer. We pum-
(E) Carbene microspheres. The carbene microspheres were
ped the aqueous solution of ILs and oil phases into the inner
obtained by treatment of the imidazolium-based PIL microgels
with NaH in THF at 273 K (i.e., a catalytic amount of DMSO)
and outer capillaries at volumetric ow rates of about 0.5 mL
ꢀ1
ꢀ1
h
and 50 mL h , respectively. Then, the aqueous ow was
under N
F) Carbon particles. The PIL microgels were rst
immersed in aqueous solution of FeCl for 3 h, and then aer
2
atmosphere.
broken into droplets by the oil ow at the needle tip. The
generated droplets were solidied by photopolymerization
under UV exposure of less than 1 min and the resulting PIL
microgels were taken into the collection container lled with
the oil by the oil ow. The generated PIL microgels were rinsed
several times with hexane, and deionized water, to remove the
silicone oil and the residual surfactant.
(
3
washing and drying in air the treated microgels were heated
ꢀ1
under a N atmosphere to 623 K, at a heating rate of 1 K min
,
2
and then maintained at 623 K for 2 h, and subsequently heated
ꢀ1
to 1073 K at a heating rate of 1 K min and nally placed at
1073 K for 5 h. Then, the resulting carbon particles were taken
out and cooled to room temperature.
Monodisperse PIL microgels as a platform to produce
functional materials
Acknowledgements
(
A) Patchy microgels. The patchy PIL microgels were
fabricated with a versatile strategy using a “sandwich” contact We gratefully acknowledge the nancial support from the NSFC
printing (mCP) method. The PDMS stamps were loaded with a (21025311, 21121004, 21261130581, 91027016), Ministry of
ꢀ
8
few drops of the respective chemical ink solution (10 mM, Education
(2011Z01014),
MOST
(2011CB808403,
aqueous solution of uorescent dye molecules) and the excess 2013CB834502) and the transregional project (TRR61).
ink solution was removed under a stream of argon. Then, the
PIL microgels were loaded onto the stamp and a second stamp
loaded with the second ink (like the rst one) was placed on top
Notes and references
using a press. The samples were le to react at room tempera-
ture. It took several minutes to transfer the “ink” to the PIL
microgels via a counteranion-exchange only in the area of
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