Ionic liquid as an efficient modulator on artificial enzyme system: Toward the realization of high-temperature catalytic reactions

Article abstract of DOI:10.1021/ja400280f

Herein, with the aid of ionic liquid, we demonstrate for the first time that highly stable Au/SiO₂ hetero-nanocomposites can serve as a robust and recyclable peroxidase mimic for realizing high-temperature catalytic reactions. Our findings pave the way to use nanomaterials for the design and development of efficient biomimetic catalysts and, more significantly, to apply ionic liquid as a positive modulator in catalytic reactions.

Full text of DOI:10.1021/ja400280f

Communication
pubs.acs.org/JACS
Ionic Liquid as an Efficient Modulator on Artificial Enzyme System:
Toward the Realization of High-Temperature Catalytic Reactions
Youhui Lin,†,‡ Andong Zhao,†,‡ Yu Tao,†,‡ Jinsong Ren,*,† and Xiaogang Qu*,†
†State Key laboratory of Rare Earth Resources Utilization and Laboratory of Chemical Biology, Changchun Institute of Applied
Chemistry, Chinese Academy of Sciences, Changchun 130022, China
‡Graduate School of the Chinese Academy of Sciences, Beijing 100039, China
S
* Supporting Information
Scheme 1. (a) Scheme Describing the Synthesis of Au/SiO2
ABSTRACT: Herein, with the aid of ionic liquid, we
demonstrate for the first time that highly stable Au/SiO2
hetero-nanocomposites can serve as a robust and
recyclable peroxidase mimic for realizing high-temperature
catalytic reactions. Our findings pave the way to use
nanomaterials for the design and development of efficient
biomimetic catalysts and, more significantly, to apply ionic
liquid as a positive modulator in catalytic reactions.
Catalyst; (b) Schematic Illustration for the Realization of
High-Temperature Catalytic Reaction by Utilizing
Thermally Stable Au/SiO2 Catalyst as a Peroxidase Mimic
and Ionic Liquid as a Modulator
atural enzymes, which possess high activity and high
N
substrate specificity under mild reaction conditions, have
been a constant source of inspiration for chemists in their
efforts to engineer synthetic structures that mimic their
complexities and functions.1 Very recently, the merging of
nanotechnology with biology has also ignited research efforts
for designing functional nanomaterials with enzyme-like
catalytic activities.2 For instance, magnetic nanoparticles,
metallic nanocomposites, and carbon nanomaterials have
been discovered to possess unique peroxidase-mimic activity
and shown promising potential in environmental detection and
biomedical applications.1d,2b,3 Although they usually exhibit
superior thermal stability to natural enzyme, none of them can
realize high-temperature catalytic reactions mainly because of
the thermally induced instability of enzymatic product.1d,3d,4
Ionic liquids (ILs) are attractive, environmentally acceptable
solvents because of their near-zero vapor pressure, thermal
stability, and widely tunable properties through appropriate
modification of the cations and anions.5 Recently, ILs have
been actively explored for a variety of applications including as
media for catalytic reactions,6 sensors,7 biomolecules stabiliza-
tion and biopreservation.8 For instance, some proteins and
DNA have been found that they can be stabilized at higher
temperatures in IL media as compared to aqueous solution.8
Additionally, in some cases, the reactive catalytic species or
reaction intermediates can also be stabilized in ILs, which may
significantly improve the catalytic performance or even make it
possible to achieve catalytic reactions that are not possible to
conduct in common solvents.9
played a key role in the design and development of our new
biomimetic catalyst. Initially, the encapsulation of AuNPs in
SiNP provided a high density of very small and homogeneously
dispersed AuNPs. Meanwhile, it was able to hinder the
aggregation of neighboring particles and allow the diffusion of
small active molecules in and out of the nanospheres. Here, the
Au/SiO2 nanocatalyst exhibited high peroxidase-like activity
that could catalyze the reaction of peroxidase substrate ABTS in
the presence of H2O2 to produce a colored product ABTS·+. In
contrast, commonly used AuNPs stabilized by charge or steric
repulsion were inactive in catalytic reactions. Furthermore, the
heterogeneous nanostructures-based artificial enzyme offered a
new opportunity to achieve catalysis with high operational
stability and reusability. More significantly, ILs could act as a
stabilizing agent for enhancing the thermal stability of
enzymatic product, thus enabling high-temperature catalytic
reactions. Such unexpected promotion of catalytic activity at
high temperature may eventually be used in practical catalysis.
To the best of our knowledge, the present work demonstrates
for the first time that peroxidase mimics can realize efficient
high-temperature reactions.
Inspired by these unique features, herein, we propose a
conceptually new approach: by utilizing IL as an efficient
modulator, novel highly stable Au/SiO2 hetero-nanocomposites
with remained surface accessibility can serve as a robust and
reusable peroxidase mimic for realizing high-temperature
reactions (Scheme 1). Porous silica (SiNP) as the skeleton
Received: January 10, 2013
Published: March 7, 2013
© 2013 American Chemical Society
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Scheme 1a illustrates the rather simple procedure of using
SiNP with large pores as porous support to encapsulate AuNPs.
Recently, ordered mesoporous materials with large pores are in
urgent demand for applications in the encapsulation of
functional nanoparticles without the possibility of causing
pore blocking.10 In this work, SiNP with large pores was first
synthesized according to a swelling agent incorporation
method.11 The degree of pore size expansion could be easily
controlled by adjusting the synthesis conditions such as
reaction temperature and time. Here, mesopores were
expanded to about 3.1 times the size (8.7 nm, mean pore
size) of the original one (2.8 nm), according to nitrogen
sorption data (Figure S1a,b). The obtained large-pore SiNP
was studied by scanning electron microscopy (SEM) and
transmission electron microscopy (TEM), indicating that the
prepared SiNP had outer diameter of about 100 nm, while
retaining spherical morphology and monodispersity (Figure
1a,b, Figure S2). Its surface was then functionalized with amine
Figure 2. (a) Schematic illustration for different types of Au catalysts
for mimicking peroxidase. (b) The corresponding stability of these
catalysts in the presence of various solutes: (1) control; (2) 1 M NaCl;
(3) 50% DMSO; (4) 20% [ch][dhp].
mental conditions, such as salt and ILs. Although PVP/citrate-
AuNPs via steric stabilization are stable, direct application of
these nanoparticles as catalysts also faces significant challenges
because the coated polymers will block the particle surface from
adsorbing other molecules.15 Interestingly, the immobilization
of AuNPs on the mesoporous support is able to hinder the
interaction between NPs and keep these gold catalysts stable,
while in the meantime, allow the diffusion of small active
molecules in and out of the nanospheres.10b
The peroxidase-mimicking activity of different Au nanoma-
terials was then evaluated in the IL. Although ILs have been
widely explored in various bioapplications in recent years,
surprisingly, no attempt to apply ILs to artificial enzyme system
has been reported so far. In this study, we carefully investigated
the interactions between ILs and artificial enzyme, which might
allow for broader applications. A representative IL, choline
dihydrogen phosphate ([ch][dhp]), was used as a model
system to provide the “proof-of-principle” verification of the
concept. The catalytic activities were characterized in the
ABTS-H2O2 reaction system. As shown in Figure 3 and Figure
Figure 1. TEM images of large pore silica nanoparticles (a, b) and Au/
SiO2 catalyst (c, d). (e) Size distribution histogram of AuNPs in Au/
SiO2 catalyst. (f) EDX pattern of Au/SiO2 catalyst. Inset shows the
dark-field TEM image, and corresponding TEM elemental mappings.
groups by treatment with APTES,12 and these groups present
on the wall structure can act as a stabilizing agent by providing
an anchoring surface.13 The next step is to adsorb AuCl4 to
−
the NH2-group-rich surface via electronic interactions. Finally,
AuNPs were immobilized onto the SiNP surface by in situ
reduction of auric chloride ions. TEM images and the
corresponding size distribution histogram revealed that a high
density of dispersed AuNPs with an average size of 2.1 nm
supported on the SiNP surface(Figure 1c−e), which were
expected to exhibit much higher catalytic activity.14 The
formation of AuNPs was also verified by the energy-dispersive
X-ray (EDX) spectra and the X-ray powder diffraction (XRD)
pattern (Figure 1f, Figure S3). Meanwhile, the corresponding
TEM elemental mappings indicated the uniform distribution of
Si and Au elements in the same particle.
Next, the stability of Au/SiO2 hetero-nanocomposites was
tested under different conditions, compared with that of
commonly used AuNPs stabilized by charge or steric repulsion
(Figure 2a). Figure 2b and Figure S4 show the behavior of
citrate-capped AuNPs, PVP/citrate-AuNPs and Au/SiO2
hetero-nanocomposites in the presence of salt, organic solvents,
and ILs. Aggregation of citrate-AuNPs can be easily observed
via a red-to-blue color change, whereas aggregation of PVP/
citrate-AuNPs and Au/SiO2 nanocatalyst does not occur under
these relatively hash conditions. These results indicate that
citrate-AuNPs via charge stabilization is sensitive to environ-
Figure 3. (a) Effect of stability and surface accessibility on the
peroxidase-mimicking activity of Au catalysts. Visual color changes (b)
and the corresponding time-dependent absorption spectra of different
Au catalysts (c−e) in the presence of 1 mM ABTS, 60 mM H2O2 and
20% [ch][dhp].
S5, the oxidation of ABTS produced a green color with major
absorbance peaks at 417 nm, indicating that our Au/SiO2
hetero-nanocomposites exhibit peroxidase-like catalytic activity
in the IL.1d,3d Control studies indicated the enzymatic reactions
did not ensue in the absence of H2O2 or catalyst (Figure S6).
Meanwhile, the catalytic activity is dependent on catalyst
concentration and pH (Figure S6, S7). Additionally, the
reaction rate decreased gradually with increasing concentrations
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of [ch][dhp] (Figure S8), which is probably associated their
viscosity6a,d and their impact on catalyst activity8b (Figure S9).
Above results demonstrate that Au/SiO2 catalyst can be served
as an effective artificial enzyme in ILs. In contrast, neither
citrate-capped AuNPs nor PVP/citrate-AuNPs could efficiently
oxidize ABTS (Figure 3b−d), further demonstrating that the
enzyme-like activity of AuNPs is highly dependent on their
colloidal stability and surface accessibility.15a Our finding could
offer a facile and efficient strategy for tuning catalysts from inert
to active in ILs.
One of the most important advantages for heterogeneous
catalyst is that this catalyst can be recovered and reused.16 To
test its reusability, we compared the fresh and recovered Au/
SiO2 catalyst during the 5-min peroxidization of ABTS in a
solution of 20% [ch][dhp]. The result shows that the amount
of product in the eighth run reaches 71% of that in the first run
(Figure S10). Another notable feature is its high thermal
stability, compared with natural enzyme (HRP). The activity of
natural enzyme is completely inhibited under heating (85 °C, 5
min), whereas the activity of Au/SiO2 catalyst after heating
almost remains the same as before (Figure S11). Therefore,
these attractive properties are particularly useful for industrial
applications.
ance of Au/SiO2 catalyst was far below expectations, which
attained its highest catalytic activity at ∼55 °C in buffer and
deactivated at ∼65 °C (Figure 4b). Previous work revealed that
enzymatic product (ABTS·+) is relatively unstable, and decays
to a colorless product through disproportionation.4 We
speculated that its insufficient stability might seriously restrict
further high-temperature applications. As seen in Figure 4c and
Figure S12, ABTS·+ is unstable and very rapidly degraded in
buffer when handled at relatively high temperature (demon-
strated by the color change and the fast decreasing of its UV−
vis signal). Therefore, the thermal stability of product plays an
important role in high-temperature catalysis, which has been
largely ignored. Although regulation of enzyme-catalyzed
reaction has always attracted great attention,17 the exploring
of efficient modulator that make peroxidase mimics possible to
achieve high-temperature reactions still remains a big challenge.
Inspired by the unique properties of ILs,8,9 we reasoned that IL
could serve as a stabilizing agent for improving thermal stability
of ABTS·+, and subsequently enabling high-temperature
reaction that are not working efficiently in buffer. As expected,
ABTS·+ are tremendously stabilized upon addition of 20%
[ch][dhp] under heating (Figure 4c). More importantly, the
activity of nanocomposites in the presence of [ch][dhp] was
much greater than that obtained in the buffer at high
temperature (Figure 4b), suggesting that IL imparts substantial
thermostability to ABTS·+. To further demonstrate the ability of
IL to inhibit ABTS·+ disproportionation, another experiment
was designed (Figure 4d), in which [ch][dhp] was added to
Au/SiO2 catalyst catalyzed ABTS/H2O2 system after the
reaction was performed in buffer for 10 min. Initially, the
absorption signal gradually intensified during the first 125 s,
whereas, after then, the decrease in absorption signal was
observed, which could be attributed to the disproportionation
of ABTS·+. Interestingly, when [ch][dhp] was added, no color
fade was observed; on the contrary, the absorption signal
increased again, demonstrating that ABTS·+ disproportionation
was efficiently inhibited by IL. In addition, control studies
indicated that another IL (20% [BMIm]BF4) could also have
positive effect on nanocatalyst activity and ABTS·+ stability at
high temperature (Figure 4e). Similarly, another peroxidase
substrate, TMB, also had high activity in these ILs compared
with that in the buffer (Figure S13). Significant enhancement of
the activities suggests that ILs play an important role in
catalysis, and their positive effect is possibly connected with
their greater solvation power (Figure S14),8e a large number of
cations and anions, and their weak interactions with substrate
and product. Taken together, although the detailed mechanism
remains unclear, these observations demonstrated that IL can
act as a stabilizing agent for enhancing thermostability, which
offers a strategy for realizing high-temperature catalysis (Figure
S15).
After have demonstrated that our Au/SiO2 nanocatalyst
exhibits high peroxidase-mimicking catalytic activity in the IL,
we then address the possibility of applying thermal stable
artificial enzyme to high-temperature reactions with the aid of
IL (Scheme 1b, Figure 4). Initially, we explore the possibility of
using our enzyme mimic to catalyze the substrate oxidation
reaction in the absence of IL since it possesses high thermal
stability. Unfortunately, similar to HRP and other peroxidase
mimic (e.g., Fe3O4, graphene oxid),1d,3d the catalytic perform-
In summary, we have demonstrated the first example that
AuNPs encapsulated in porous support could serve as a robust
and efficient peroxidase mimic at high temperature by utilizing
ILs as an efficient modulator. Porous silica as the skeleton
assisted the synthesis of a high density of very small and stable
AuNPs and facilitated the catalytic reaction by providing porous
diffusion channels. Furthermore, our new artificial enzyme
exhibits high thermal stability and excellent reusability. More
importantly, the positive effect of ILs on the artificial enzyme
system may eventually be utilized advantageously in practical
catalysis. Our findings pave the way to use hetero-nano-
composites for the design and development of highly stable
Figure 4. (a) Critical factors for realizing high-temperature reactions.
(b) Catalytic activities of Au/SiO2 catalyst as a function of incubation
temperature in buffer or 20% [ch][dhp]. (c) Thermal stability of
enzymatic product in different fluids at 75 °C; inset shows IL can act
as a stabilizing agent. (Product results from the oxidation of ABTS by
Au/SiO2/H2O2 at 37 °C.) (d) Time-dependent absorbance changes at
417 nm as a result of the catalyzed oxidation of ABTS at 75 °C:
(Black) absorption changes in buffer; (red) absorption changes after
addition of 20% [ch][dhp]. (e) The temperature-dependent catalytic
activities of Au/SiO2 catalyst in buffer; 20% [ch][dhp]; 20%
[BMIm]BF4.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Detailed experimental procedures and supplementary figures.
This material is available free of charge via the Internet at
http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
jren@ciac.jl.cn; xqu@ciac.jl.cn
Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS
■
We acknowledgment financial support from National Basic
Research Program of China (Grant 2012CB720602,
2011CB936004) and the National Natural Science Foundation
of China (Grants 91213302, 21210002, 21072182).
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