Self-regulated supramolecular assembly driven by a chemical-oscillating reaction

Article abstract of DOI:10.1039/c2cc36069a

A novel self-regulated supramolecular assembly (SSA) system driven by a chemical-oscillating reaction is constructed based on dynamic supramolecular interactions and the rhythm of the SSA process can be controlled by temperature.

Full text of DOI:10.1039/c2cc36069a

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ARTICLE TYPE
Self-regulated Supramolecular Assembly Driven by a
Chemical-Oscillating Reaction
Hongwei Zhou, a,b Enxiang Liang, a,b Xiaobin Ding,* a Zhaohui Zheng,* a and Yuxing Penga
Received (in XXX, XXX) Xth XXXXXXXXX 20XX, Accepted Xth XXXXXXXXX 20XX
5
DOI: 10.1039/b000000x
40 itself without any external stimuli. Therefore, we describe
A novel self-regulated supramolecular assembly (SSA) system
driven by a chemical-oscillating reaction is constructed based
on dynamic supramolecular interaction and the rhythm of
such SSA process can be controlled by temperature.
herein the design and construction of such systems by
utilizing BZ reaction solution as the periodical driving source
and metal-ligand complexation as the supramolecular
interaction.
45
A
prefabricated assembly with terpyridine-ruthenium
complexes (Ru(II)(tpy)2) was first designed and prepared
through “combined protocol” of free radical
copolymerization and following coordination process
(Scheme 1). Reactive groups in the vacant ligands were
10 How far can we push chemical self-assembly? It remains a
question because supramolecular assembly has been highly
attractive as an efficient and useful tool for understanding
exquisitely arranged biological processes and constructing
controllable assembly systems.1 Various systems, especially
15 those based on relatively strong but responsive
supermolecular interactions, such as metal-ligand interaction
and host-guest recognition, are extensively studied.2 Despite
the inherent responsive functions of these systems, “on/off”
a
a
a
50 specially designed for further functionalization or grafting
modification with other linear polymers or dendrimers to
obtain various topological structures.6 Detailed preparation
and characterization procedures can be found in Figure S1-S9
in ESI†.
switching of external stimuli is essential to trigger
20 transformation of the equilibrium states,3 and construction of
completely self-regulated assembly system is still
challenge.
a
a
a
As a pioneering attempt to develop self-regulated intelligent
systems, Yoshida group achieved rhythmically pulsatile
25 mechanical motion of a hydrogel by introducing a pH
oscillating reaction in a continuous-flow stirred tank reactor.4
Afterwards, this method was further developed to design a
new kind of polymers and gels exhibiting similar oscillating
behaviors utilizing Belousov–Zhabotinsky (BZ) reaction as a
30 driving source.5 Accordingly, it can be inferred that if the
chemical-oscillating reaction is properly coupled with a respo-
55
Scheme 2 Prefabrication of the assembly.
Theoretically, the construction of the proposed SSA
systems requires careful consideration of both the reversibility
of the supramolecular interaction and the ability of stable
60 assembly in the highly acidic BZ reaction environment.
Therefore, the assembly/de-assembly/re-assembly behavior
under separate stimulating conditions was first studied. Based-
on the principle that assembled bis-complex Ru(II)(tpy)2
exhibits characteristic metal to ligand charge transfer (MLCT)
65 absorption near 490 nm but de-assembled mono-complex
Scheme 1. Concept of the self-regulated supramolecular assembly. The
three pictures from left to right show the SSA system, the SSA process,
35 and on-line detection of the SSA behaviour.
nsive but reversible supramolecular assembly process, a self-
regulated supramolecular assembly (SSA) system should be
realized (Scheme 1). In this way, the transformation of
assembly states could be completely controlled by the system
Ru(III)(tpy) has no peak absorption at this wavelength,7
a
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reversibility study was first carried out by utilizing UV-vis
become slightly opaque due to the known “salting-out effect”,
spectrophotometry through the absorbance change at the
characteristic wavelength. It was found that the Ru metal ions
can be successfully oxidized by ceric sulfate (Ce(SO4)2) and
in reverse reduced by sodium sulfite (Na2SO3) in an acidic
condition, which triggers the de-assembly and re-assembly
process, respectively. As revealed from Figure 1, in all the
ten tested cycles, reversible assembly/de-assembly/re-
assembly behaviours were realized and monitored by UV-vis,
which may also prevent the system from re-assembling.
Therefore, a proper [H+] no more than 2.0 mol∙l-1 should be
chosen for the construction of SSA systems.
To gain insight into de-assembly/re-assembly process, a
gradual reduction and oxidation process was carried out
(Figure S13). By stepwise increasing the amount of stimuli,
the absorption near 490 nm shows simultaneous decrease for
the process of oxidation (Figure S13-A) and increases in the
5
50
10 as can also be supported by the transformed colour (Movie 1), 55 case of reduction process (Figure S13-C). Meanwhile, the
variation of average molecular weight (Figure S10), and the
reversible sol-gel transition (Figure S11) simultaneously with
the oxidizing and reducing process. For the state 1, the
assembly was in its branch-like assembled state with a colour
15 of orange red, which is indicated by the MLCT absorption at
494 nm. By adding Ce(SO4)2 solution, the system transformed
to light yellow green and the characteristic absorption
disappeared, demonstrating the de-assembled state -1. The
system re-assembled again (state 2) when the reducing agent
colour of the tested systems also showed gradual change
(Figure S14), which is similar to that observed in moive 1.
This result clearly shows a continuous de-assembly and re-
assembly process under separated stimulating conditions.
According to the external stimuli-regulated assembly/de-
assembly/re-assembly behaviour, it can be expected that a
SSA system can be constructed if ruthenium ion changes
autonomously between a reduced Ru(II) state and an oxidized
Ru(III) state. BZ reaction, in which the ruthenium ion works
60
20 were introduced and the further investigations of the left eight 65 as a catalyst and periodically changes between a reduced
cycles also exhibited efficient reversibility. It is inevitable to
have dilution effect from accumulated addition of
Ru(II) state and an oxidized Ru(III) state, should be a good
choice.8 By dissolving proper amount of prefabricated branch-
like assembly into a system of BZ reaction solution containing
certain concentrations of sodium bromate (NaBrO3), malonic
70 acid (MA) and sulfuric acid (H2SO4), an autonomous
transformation between assembled state and de-assembled
state was demonstrated, as can also be vividly revealed by the
periodical colour change (Figure 2 and Movie 2), which is
comparable to the assembly/de-assembly behaviour under
75 external stimuli conditions. Meanwhile, the change of colour
intensity of red (R), green (G) and blue (B) all exhibit an
oscillating profile, further indicating the occurrence of SSA
behaviour (Figure 2-C). What is more interesting, when SSA
process was carried out in a stirred system, a homogeneous
80 and faster transformation between assembled state to de-
assembled state was observed, while a heterogeneous and
slower change was observed in an unstirred system, as can be
macroscopically observed in Figure 2 A-B and Moive 3-4.
a
Ce(SO4)2/Na2SO3 solutions and the possible incomplete
conversion of Ru ions. That is why the absorbance at 494 nm
25 in the ten cycles decreases with increased testing cycles.
Figure 1. Reversible assembly/de-assembly/re-assembly behaviour under
external stimuli. UV-vis spectra of the assembled states (1 to 10) and de-
assembled states (-1 to -10) in water. The ordered cycles were carried out
30 as follows, 1/-1/2/-2/……9/-9/10/-10. Initial solution of 1 are composed
of H2SO4 (0.2 mol∙l-1), [assembly]=0.59 mg∙ml-1. From -1 to -10, a 50 ul
solution of Ce(SO4)2 (0.02 mol∙l-1) was stepwise added. From 2 to 10,
Na2SO3 solution with equal molar weight was stepwise added.
Because the possible protonation of terpyridine ligand in
35 acidic condition may lead to a failure of re-assembly process,
the assembly ability under different concentrations of H+
([H+]) was then studied. This study was carried out by adding
Na2SO3 solution to an already de-assembled system to induce
a re-assembly process under different [H+] (Figure S12). As
40 is revealed, the system exhibited stable re-assembly behaviors
under a condition with [H+] from 0.4 mol∙l-1 to 2.0 mol∙l-1.
When the [H+] increased higher than 2.0 mol∙l-1, the peak at
494 nm become asymmetric and smaller, which indicate that a
protonation of terpyridine ligand occurred and the re-assembly
45 process is to some extent restrainted. Meanwhile, the system
85 Figure 2. Photographs of the self-regulated supramolecular assembly
systems under a stirred (A) and an unstirred (B) condition and the R/G/B
value changes of the marked position in A against time (C). The initial
substrate concentrations were fixed to [MA]=50 mmol∙l-1, [NaBrO3]=300
mmol∙l-1, [H2SO4]=1 mol∙l-1, and [assembly]=0.5 wt.%.
90 The different SSA behaviours are originated from the fact that
a BZ reaction is controlled by both chemical reaction itself
and matter diffusion process.9 Under an unstirred condition,
matter diffusion process was difficult to proceed and the SSA
behaviour was not homogeneous and slower, while in the case
2
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of a stirred condition, mechanical stir promoted the matter
together the thermosensitivity and “salting-out effect”, a non-
diffusion process, which led to the homogeneous and faster 45 thermosensitive assembly system may exhibit a larger SSA
SSA behaviour.
amplitude by elevating the assembly concentration and further
optimizing the BZ reaction system.
In conclusion, a novel kind of supramolecular assembly
systems in which the transformation of assembly states was
Further study revealed that temperature played an important
role in the SSA systems. Due to the thermosensitive poly(N-
isopropyl acrylamide) component, a hydrophilic–hydrophobic
5
transition occurred when the temperature was higher than the 50 completely controlled by the system itself was presented.
lower critical solution temperature (LCST) of the assembly
and the SSA process was almost stopped. However, when the
Control of supramolecular assembly by nonlinear chemical
kinetics paves way for studying and understanding
a
10 temperature was fixed below the LCST of the assembly, a
SSA behavior can be successfully observed. Figure 3 shows
supermolecular assembly from a new perspective. In a broader
content, this system may be regarded as a template for other
the SSA-induced transmittance change under different 55 systems with potential self-regulated function, including
constant temperatures when the other influences factors,
including the concentrations of BZ substrates and the amount
15 of the assembly, were fixed. As is shown, in all the studied
systems, the transmittance exhibits periodical change, which
amphiphilic assembly system, assembly on the surface of
nanoparticle or nanosheet and hydrogel systems (in such cases,
as dynamic cross-linking sites).
The work was supported by the National Natural Science
60 Foundation of China (51073161).
Notes and references
aChengdu Institute of Organic Chemistry, Chinese Academy of
Sciences, Chengdu 610041, P. R. China. E-mail: xbding@cioc.ac.cn;
bGraduate School of the Chinese Academy of Sciences, Beijing 100049, P.
65 R. China
† Electronic Supplementary Information (ESI) available: [Sntheses of
relevant materials and supplementary moives of SSA]. See
DOI: 10.1039/b000000x/
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Figure 3. Transmittance change (494 nm) of the SSA systems as a
function of time under different constant temperatures. The initial
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mmol∙l-1, [H2SO4]=1 mol∙l-1, and [assembly]=1 wt.%.
indicates the cyclic assembly and de-assembly process.
Because the assembled state has a peak absorption at the
detecting wavelength but the de-assembled has slight
25 absorption at this wavelength, the transmittance increases with
the gradual de-assembly process and decreases with the re-
assembly process. This is well in accordance with the result
obtained under external stimuli conditions. Moreover, it can
also be seen from Figure 3 that the rhythm of the SSA
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decreased SSA periods with the increasing temperature
(Figure S15), but the transmittance value, which reflects the
extent of assembly process, decreased from ca. 90 for 14 °C to
ca. 81 for 23 °C. This is because the assembly exhibited slight
35 phase transition when the temperature was increased, although
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A novel self-regulated supramolecular assembly (SSA) system was constructed. The assembly/de-sassembly/re-assembly
behaviour was completely controlled by the system itself without any external stimuli and the rhythm of such SSA behaviours
could be controlled by temperature.