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Microfluidics fabrication of self-oscillating microgel clusters with tailored temperature-responsive properties using polymersomes as ‘microreactors’ Yuandu Hu, and Juan Pérez-Mercader Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b03166 • Publication Date (Web): 14 Nov 2017 Downloaded from http://pubs.acs.org on November 15, 2017

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Microfluidics fabrication of self-oscillating microgel clusters with tailored temperature-responsive properties using polymersomes as ‘microreactors’

Yuandu Hu†* and Juan Pérez-Mercader†‡* † Department of Earth and Planetary Sciences, Harvard University, Cambridge, Massachusetts, United States ‡ Santa Fe Institute, Santa Fe, New Mexico, United States *To whom correspondence should be addressed. E-mail: [email protected] [email protected]

ABSTRACT

Poly(N-isopropyl acrylamide)-based microgel clusters were successfully prepared using polymersomes as ‘microreactors’, which were fabricated through microfluidics. The clusters were formed from the crosslinking reaction between ruthenium/amino group dual functionalized poly(N-isopropyl acrylamide) microgels and linear poly(N-isopropyl acrylamide)-r-(N-acryloxysuccinimide)-based polymer linkers under neutral pH conditions. By simply adjusting the ratio of N-isopropyl acrylamide to Nacryloxysuccinimide in the polymer crosslinkers, the internal structures of the clusters can be controlled; hence, the temperature response of the clusters can be regulated. It was demonstrated that these different microgel clusters showed various degrees of chemomechanical oscillations when the clusters were exposed to a catalyst-free solution containing Belousov-Zhabotinsky reaction substrates. KEYWORDS: Microgel clusters, Ruthenium, Polymersomes ‘microrectors’, Temperature response, BelousovZhabotinsky reaction

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INTRODUCTION

Supra-structures based on particle building blocks, such as colloids, have attracted wide attention from the fields of physics, chemistry, materials science and biology.1-3 Soft supra-structures are of particular interest because of their diverse applications in many fields, such as light-controllable materials,4 photonic crystals,5 and drug delivery and release vehicles.6-7 Among the types of supra-structures, suprastructures constructed from active particles, particularly self-oscillating microgels, have gained increasing attention because they are intrinsically endowed with unique active properties.8-9 Self-oscillating microgels are polymer particles that possess chemical or mechanical oscillation behavior or even the combination of both under the effect of the classical Belousov-Zhabotinsky reaction (B-Z reaction), which has been intensively studied by many means.10-12 These gel particles are mostly based on polymer backbones composed of poly(N-isopropyl acrylamide) analogs and generally have a three-dimensional network structure in which their hydrodynamic diameters range from hundreds of nanometers to hundreds of micrometers.13-14 In addition to the polymer-based skeleton, metal-based catalyst moieties, such as ruthenium-based complexes, and ferroin-based complexes, are usually incorporated into the polymer network in the polymerization process or through a post modification process.15-17 Prior to our recent report, the traditional emulsification/precipitation polymerization method was the most commonly used and results in the formation of monodisperse, self-oscillating, spherical gel particles with diameters of approximately 200 nm to 1 µm, 18 which can only be characterized by indirect methods, such as dynamic light scattering (DLS) or electronic microscopy, and thus, the in situ dynamic behavior cannot be directly visualized.10, 19 Moreover, the mechanical oscillation amplitude of these gel particles induced by chemical oscillation was too low and is difficult to detect under optical microscopy because the original particle size is very small. Two typical efforts can be taken to control the size and oscillation amplitude of such auto-oscillating gel particles: one example is to place the microfluidics-generated doughnut-shaped gel particles under the flow of a solution of the chemical substrate, which can quickly remove Br- inhibitors


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and therefore induce a large volume oscillation of the gel particles;14, 20 the other effort is to construct a so-called bulk gel actuator through the linking of sub-micrometer-sized self-oscillating gel particles inside an Eppendorf tube.21 However, both of these gel particles have limitations in further applications; for example, the lack of size control and feasible maneuverability limits the former, while for the latter, the poor controllability over the size and shape is a challenge, and a high microgel concentration is required to induce the close packing of microgels since the linker molecules are small molecules.21 Particularly, the construction of free-standing and well-defined microgel-based superstructures, such as opal photonic crystals using microgels as building blocks, have been an attractive topic because this kind of particles can be feasibly endowed with multiple functional properties.22-23 Hence, in this report, we describe a simple yet robust method to fabricate a microgel-based superstructure (see the microgel cluster below) through the organization and further linking of individual microgels in polymersome ‘microreactors’. We first used a microfluidics technique to fabricate polymer vesicles evolved from core/shell droplets.24-25 The core part of the droplets included a phosphate buffer solution of a mixture of ruthenium/NH2 dual functionalized

poly(N-isopropylacrylamide)

microgels

and

poly(N-isopropylacrylamide-r-N-

acryloxysuccinimide)-based linear polymer crosslinkers. The spacing among the microgels can be feasibly adjusted by simply controlling the osmolality difference inside and outside of the core/shell droplets, which favorably advances the linking reaction between the microgels and linear linker molecules. This linking reaction, as well as the slow evaporation of solvent in the shell part of the droplets, finally results in the formation of microgel clusters within the polymer vesicles.26 The morphology and stiffness of microgel cluster was maintained after the polymer vesicles were ruptured under mild sonication or osmotic pressure regulation. Then, the clusters were utilized as catalysts for the B-Z reaction, and the oscillation behavior of this microgel cluster was studied. This approach allows the versatile use of polymersomes as “microreactors” and can be extended to applications requiring long reaction periods (several hours or longer) as well as mild reaction conditions, which is crucial for biologyrelated materials (like cell culture matrices).27 Additionally, this approach will also inspire the design of a variety of giant three-dimensional superstructures based on larger building blocks, such as polymer


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precursors and even various colloids with chemically modifiable surfaces, rather than small molecules.2829

MATERIALS AND METHODS Materials: N,N’-Methylenebisacrylamide (BIS, 99%), ammonium persulfate (APS, ≥ 98%), malonic acid (MA, 99%), sodium bromate (NaBrO3, 99%), tris(2,2’-bipyridyl)dichlororuthenium(II) hexahydrate (99.95%), methanol (anhydrous, > 99.8%), poly(vinyl alcohol) (PVA, Mw 13000~23000, 87~89% hydrolysis), lithium bromide (LiBr, >99.0%), ammonium hexafluorophosphate (NH4PF6, ≥ 98.0%), 0.1 M phosphate buffer solution (pH=7.5), dimethylformamide (> 99.8%, DMF), N-(3-aminopropyl)methacrylamide hydrochloride (APMA, 98%), and 2,2′-azobis(2-methylpropionitrile) (AIBN, 98%) were all obtained from Sigma-Aldrich and used without further purification. Nitric acid (6.0 Normal, Ricca) was purchased from VWR International. Methacryloxyethyl thiocarbamoyl rhodamine B (denoted MRB below) was obtained from Polysciences, Inc. 4-Vinyl-4’-methyl-2,2’-bipyridine (99%) was purchased from Ark Pharm, Inc. NIsopropyl acrylamide (NIPAAm, 97%) and N-acryloxysuccinimide (NAS, 99%) were obtained from Acros Organics.

Ruthenium(II)(4-vinyl-4’-methyl-2,2’-bipyridine)bis(2,2’-bipyridine)bis(hexafluorophosphate)

[denoted Ru(bpy)3 monomer below] was synthesized according to existing literature and with minor modifications30-31. The amphiphilic diblock copolymer poly(butadiene)65-b-(ethylene oxide)35 (PB65-PEO35, PDI=1.04) was purchased from Polymer Source. Distilled water was produced from a reverse osmosis system. Synthesis and characterization of ruthenium and N-(3-aminopropyl) methacrylamide hydrochloride (APMA) dual functionalized microgels: Microgel particles were synthesized using a controlled precipitation polymerization method based on previous references.32,

18, 33

In short, NIPAAm(2.5 g, 22 mmol), BIS(0.053 g, 0.34 mmol) and N-(3-

aminopropyl)methacrylamide hydrochloride(0.126 g, 0.71 mmol) were dissolved in 50 mL of distilled


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water. Before being transferred into a 250 mL three-neck flask, the solution was then filtered with a 0.22 µm-pore-size filter. The solution was degassed under nitrogen for at least 30 min. Then, 30 mL of the solution was removed from the flask with a glass syringe, and 20 mL of distilled water was added into the remaining solution in the flask. Thus, the remaining amount of NIPAAm in the flask was ~0.8 g. The solution in the flask was kept under nitrogen. In another clean glass vial, the Ru(bpy)3 monomer(0.0988 g, 0.11 mmol) of was dissolved in a 5 mL mixed solution of acetone and ethanol (the volume ratio of acetone/ethanol was 1:1). Then, ~2 mL of distilled water was added to the mixed acetone/ethanol solution to obtain a clear Ru(bpy)3 monomer solution. The filtered Ru(bpy)3 monomer solution was then separated into two halves: one half was put into the NIPAAm solution in the flask, and the other half was mixed with the NIPAAm solution in the syringe. The final liquid volume in the syringe was ~33 mL. After another 30 min of degassing in the flask, the flask was heated to 80 ℃ and maintained at this temperature using a hot plate equipped with an oil bath. Then, 2 mL of APS (0.011 g, 0.05 mmol) solution was added into the hot monomer solution in the flask to initiate the polymerization. Then, approximately 4 min after the addition of the APS solution, the monomer solution in the syringe was injected into the flask at an injection rate of 600 µL/min. The total injection time was ~50 min. The flask was instantly cooled in an ice bath after completion of the injection process. The microgel solution turned from turbid to clear and transparent after the cooling process. For the purification steps, a stainless-steel mesh with a pore size of 25 µm was employed to first filtrate larger aggregates. The filtered solution was then purified by repeated centrifugation (9000 rpm for 50 mins) and redispersed via sonication. After three cycles of centrifugation/redispersion, the microgel solution was dialyzed in distilled water for three days with frequent water changes. Finally, the dialyzed microgel solution was freeze-dried for 24 hours, and the resulting powder was used for the subsequent quantifications. For the DLS experiment, 1 mL of 0.24 wt% microgel solution was used. The equilibrium time for all measurements was 30 min.


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Syntheses and characterization of linear polymer linkers poly(NIPAAm-r-NAS), poly(NIPAAm-r-NAS-rRu) and poly(NIPAAm-r-NAS-r-MRB): Linear poly(NIPAAm-r-NAS) was synthesized by conventional free radical polymerization, according to previous reports.34-35 In short, NIPAAm (5.0 g, 44 mmol) and NAS (0.37 g, ~ 2.2 mmol) were dissolved in 24 mL of methanol in a 50 mL Schlenk flask charged with a magnetic stir bar. Then, AIBN (0.04 g, 0.24 mmol) was dissolved in 10 mL of methanol and added into the Schlenk flask. The mixed solution in the flask was bubbled with N2 for at least 30 min before beginning the polymerization. Then, the flask was placed in an oil bath with a temperature of 60 ℃ to initiate the polymerization. The polymerization reaction was maintained for 36 hours under N2 protection. After the reaction, the product solution was concentrated by rotatory evaporation and redissolved in 10 mL of a mixed solvent (toluene and acetone with volume ratio of 3:7). Then, the product was precipitated from 200 mL of cold n-hexane. Finally, the resulting polymer was dried under vacuum for 24 hours. The syntheses of poly(NIPAAm-r-NAS-r-Ru) and poly(NIPAAm-r-NAS-r-MRB) followed the same procedure, only varying the initial amount of NIPAAm, NAS and ruthenium monomer (or MRB). The initial molar ratios of NIPAAm to NAS for these syntheses were fixed at 20:1 (for pNIPAAm-r-NAS), 13.7:1 (for pNIPAAm-r-NAS-Ru) and 10:1 (for pNIPAAm-rNAS-r-MRB), while the ruthenium monomer and MRB monomers were both tiny amount, 0.100 g (~ 0.11 mmol) and 0.0056 g(~0.09 mmol), respectively. 1H NMR data were obtained using CDCl3 (for pNIPAAmr-NAS and pNIPAAm-r-NAS-r-MRB) or deuterated DMSO (for pNIPAAm-r-NAS-r-Ru) as the solvent. GPC measurements were performed on an Agilent SEC/GPC equipment (Agilent 1260 Infinity II LC) using a DMF solution of LiBr (10 mg/mL) as solvent. Calibration was carried out using poly(styrene) standards provided by Polymer Standards Service. The concentrations of the polymer linkers were fixed at 5 mg/mL. The temperature for the eluent fluids for all of the GPC measurements was 50 ℃. UV-Vis spectrum measurements were conducted on an Agilent Cary 60 spectrophotometer equipped with a temperature controller (TC 1 temperature controller, Quantum Northwest, Inc.). The polymers were all dissolved in aqueous solution with concentration of 10 mg/mL. The heating rate for all the measurements


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was 0.1 ℃/min and data collection rate was 0.1 second/point. The LCST was taken as the point where absorption was half the maximum value. Preparation of the buffer solution of microgels and polymer linkers: For the buffer solution of microgels, 0.1110 g microgels powder was dispersed in 2.5 mL of buffer solution to obtain ~ 44.4 mg/mL of microgels’ buffer solution. At least two days of magnetic stirring plus sonication to make the dispersion solution more homogeneous. For the linear polymer solutions, 0.2140 g polymer linkers were dissolved in 5 mL of buffer solution to obtain ~ 44.35 mg/mL of polymer linker solutions. For each microfluidics experiment, ~ 400 µL of microgels dispersion was mixed with ~ 400 µL of polymer linker solution gave the final concentrations of microgels and polymer linkers both ~22.2 mg/mL. Construction of microgel clusters within polymersome ‘microreactors’ fabricated using microfluidics: We used polymersomes fabricated by microfluidics as templates for the formation of the microgel clusters. A 3 wt% PVA solution, 5 mg/mL PB-b-PEO in a mixed solvent of cyclohexane/chloroform and a mixed buffer solution of microgels/polymer linkers were used as the outer phase, middle phase and inner phase, respectively, for injection into the microfluidics device, as noted previously.26, 36 These three phases were emulsified inside a glass capillary-based microfluidic device to generate core/shell droplets. The core/shell droplets included 5 mg/mL of PB-b-PEO in a mixed solvent of cyclohexane/chloroform as the shell and a mixed solution of microgels/pNIPAAm-co-NAS as the core. The droplets were collected in a 0.1 M phosphate buffer solution containing 0.1 M NaBr. Temperature-response experiments of the microgel clusters: Temperature stimulus-response experiments of the microgel clusters were performed on a Zeiss optical microscope equipped with a temperature controller (TC-202A Bipolar Temperature Controller, Harvard Apparatus, with a resolution of 0.1 ℃). For the optical microscopy characterization, the microgel cluster sample was placed in an ad-hoc container with 400 µL of 0.1 M phosphate buffer solution. The container


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was mounted on the temperature controller to precisely control the temperature of the cluster. At each temperature increment (1 ℃), the system was allowed to equilibrate for 30 min. Chemomechanical behavior of the microgel clusters: In the chemomechanical experiments, the microgel clusters were placed in a mixed solution of malonic acid, NaBrO3 and HNO3 with concentrations of 84 mM, 62.5 mM and 0.3 M, respectively. The chemomechanical behaviors were recorded under the bright field of Zeiss microscope and analyzed using software developed in our group. RESULTS AND DISCUSSION Two steps were involved in the construction of the microgel clusters. First, monodisperse core/shell droplets were fabricated using a glass capillary-based microfluidic technique, where the core and shell of the droplets were a 0.1 M buffer solution of Ru-pNIPAAm microgels/linear pNIPAAm-r-NAS and the amphiphilic polymer PB-b-PEO in a mixed solvent of chloroform/cyclohexane, respectively (this process is shown in Figure S1 and S2). Second, the droplets were incubated in a loosely closed glass vial (containing 1 mL of the buffer solution containing 0.1 M NaBr) to slowly evaporate the solvent in the shell. Owing to the osmolality imbalance inside and outside of the droplets (Os-i < Os-o), the core of the droplets tended to decrease in size.26 Meanwhile, unlike small linker molecules, such as glutaraldehyde, polymer crosslinkers provide multiple reaction sites for microgels and hence make it easier to induce crosslinking between the microgels and linker molecules, compared with using glutaraldehyde as the crosslinking agent (see Figure S3). Two main roles were involved in the utilization of polymersomes as ‘microreactors’ in the formation of the final clusters: on one hand, the volume fraction of the microgel solution in the core adjusted accordingly to the osmolality difference, inducing close packing in the microgels, which favors the reaction between the amino groups (-NH2) on the microgels and the succinimide groups in the linear pNIPAAm-co-NAS under neutral pH (pH ~ 7.4) conditions;34, 37 on the other hand and more importantly, the polymer membrane can act as a barrier to suppress the self-assembly of microgels at the interface,38-39 which usually occurs in single emulsions and eventually induces the


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formation of hollow structures (such as colloidosomes) rather than solid structures. The linking reaction was allowed to continue overnight at 25 ℃. After the crosslinking reaction, the membrane of the polymersomes was removed by slight sonication or shaking. The overall process for the formation of microgel clusters is illustrated in Figure 1 (a). Figure 1 (b) shows the chemical structure of the microgel building blocks. Figure 1 (c) indicates the chemical reaction between the ruthenium/NH2 dual functionalized microgels and the linear polymer linker pNIPAAm-co-NAS. Polymersomes, as well as inter-crosslinked microgel clusters, were completely formed after incubation overnight, as shown in Figure 2 (a). The orange color of the polymersomes is attributed to the ruthenium moiety in the microgels. We also observed irregular-shaped clusters outside of the polymersomes (as illustrated by the red arrows in Figure 2 (a) and also shown in Figure S4). These structures are probably due to the rupture of some polymersomes prior to completion of the crosslinking reaction, which failed to maintain the spherical structure. As is well known, one of the most important properties of pNIPAAmbased hydrogels is their temperature-responsive property, which has been widely and intensively studied.40-43 To take advantage of this property and further confirm the formation of microgel clusters through the inter-particle crosslinking reaction, we heated the polymersomes to 40 ℃. Figure 2 (b) and (c) shows the polymersomes, including the inside, at 40 ℃ and 21 ℃, respectively. The overall inside area clearly shrank when the temperature was higher than the lower critical solution temperature (LCST) of pNIPAAm (~32 ℃, depending on the comonomers) and swelled again when the temperature was lower than the LCST, indicating the formation of the entire entity from the microgel building blocks. All of the microgel building blocks were functionalized with ruthenium monomer, which shows a characteristic fluorescence signal when excited by blue light at a peak value of 488 nm.44 Figure 2 (d) shows the typical fluorescence signal of the polymersomes composed of microgel clusters. Furthermore, the scanning electron microscopy (SEM) images indicate the close packing of the microgels covered by the polymer membrane, as shown in Figure 2 (e) and (f), respectively. To obtain pure clusters, the polymer membrane from the ‘microreactors’ can be simply removed by slight sonication (the bare clusters are shown in Figure


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S5). Figure 2 (g) shows the SEM image of the cluster after purification, and the arrows show the microgels on the cluster (also shown in Figure S6). From the above result, it is clear that the microgel cluster displays temperature-dependent size-variation behavior; however, the temperature-dependent behavior of these clusters is different from the behavior of the microgels themselves since both the network and building blocks of the clusters are based on pNIPAAm. In principle, two key factors contribute to the temperature-dependent behavior of these clusters: one is the composition of microgels and the other is the linking density between the microgels and linking polymers. The amount of NH2 in the microgels is constant because the microgels are the same, and the only variable parameter is the number of succinimide groups in the linear polymer linkers. Therefore, except for pNIPAAm-co-NAS, the two other linear polymer linkers, pNIPAAm-r-NAS-r-Ru and pNIPAAm-r-NAS-r-MRB, with different molar ratios of NIPAAm and NAS (pNIPAAm/NAS, calculated from NMR measurements) were introduced to crosslink the microgel building blocks (the chemical formulas, synthetic routes and characterization of the polymer linkers are shown in Figure S7, S9-S11). Ruthenium and MRB were incorporated to label and distinguish the polymers. Figure 3 shows the temperature-dependent behavior of the clusters containing these three different polymer linkers. Figure 3 (a) and (b) displays the actual behavior of the microgel clusters (denoted cluster 1) using pNIPAAm-rNAS as the crosslinker and the corresponding change in diameter of the clusters as the temperature increases. The diameter of the cluster decreased from ~137 µm to ~49 µm as the temperature increased from 20 ℃ to 45 ℃. Clusters containing pNIPAAm-r-NAS-r-Ru (denoted cluster 2) or pNIPAAm-r-NASr-MRB (denoted cluster 3) as the polymer linker also displayed temperature-dependent behavior, but the temperature-dependent trends are different, as shown in Figure 3(c)~(d) and Figure 3 (e)~(f), respectively. Though the initial diameters of the three different clusters are different, the initial diameter only has a minor effect on the temperature-dependent behavior (for example, see data of cluster 2 in Figure S8). The temperature-dependent behavior of the clusters formed from the latter two linkers displayed almost linear trends (Figure 3 (d) and (f)), which were plausibly attributed to the higher crosslinking density caused by


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the higher amount of NAS in these two polymer linkers (see supporting information Figure S9). To quantitatively compare the temperature-responsive properties of the microgel clusters, the cluster diameter

ratio at 20 ℃ and 45 ℃ is defined, which is expressed as = ℃, where ℃ and ℃ represent the ℃

diameters at 20 ℃ and 45 ℃, respectively. Figure 3 (g) shows the change in for the three different clusters as a function of time. The initial value of was 2.8, 2.10 and 1.45 for cluster 1, cluster 2 and cluster 3, respectively. This indicates that the order of crosslinking density of the clusters is as follows: cluster 1 < cluster 2 < cluster 3, which is in accordance with the increase in the molar percentage of NAS in the overall polymer chain of the three polymer linkers of 7.72% to 9.76% to 13.14% (calculated from the NMR data in the supporting information Figure S9). Interestingly, the microgel building blocks themselves displayed different temperature behaviors. For example, ≈ 6.0 for the initial state of the microgel building blocks, which indicates that the microgel cluster sacrificed part of its temperatureresponsive property after crosslinking (as shown in Figure S12). Indeed, we observed clusters with more complex structures, which were due to the instability of the microfluidic emulsification process, which induced the formation of irregular-shaped ‘microreactors’ and eventually induced the formation of irregular-shaped clusters (as shown in Figure S13). Such clusters typically show catalytic activity in the classical B-Z reaction, since all of the microgel building blocks were functionalized with ruthenium.45 Herein, we studied the catalytic properties of these clusters when exposed to a solution of B-Z reaction substrates without catalyst. The catalytic reaction follows the classical Field, Körös, and Noyes (FKN) model (as shown in Figure S14). It was found that all three microgel clusters displayed chemomechanical oscillation behavior, as the reaction induced the hydrophilic/hydrophobic switch of the polymer chains, as confirmed by previous reports.46 To compare their chemomechanical behaviors, two parameters were introduced to evaluate their properties: the maximum volume oscillation percentage (∆Vos%).47 These two parameters are defined as follows: ∆ % =

!"#$%$&'% (!)'%*+'% , !)'%*+'%

= , -. ,


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where /0 , , and r represent the volume of the cluster in the oxidized state, the volume of the cluster in the reduced state and the radius of the cluster, respectively. The results are displayed in Figure 4 (also as shown in Figure S15 and supplementary video 1~3). Cluster 2 clearly showed the highest ∆Vos% compared with the other two clusters, which is probably due to the higher amount of incorporated ruthenium, as not only the microgels but also crosslinker 2 contained ruthenium moieties, and thus, both components enhanced the hydrophilic/hydrophobic effect. Cluster 1 had a lower ∆Vos% than cluster 3, which was likely caused by the reduced amount of microgel building blocks in cluster 1 compared with cluster 3. However, in the current stage, the oscillation profiles were not perfect, which made it difficult to compare the oscillation period. We plan to continue to optimize the experimental parameters in our ongoing and future work. CONCLUSIONS In this report, we developed a novel method to fabricate microgel superstructures using polymersomes as ‘microreactors’, which were prepared using glass capillary-based microfluidics. To generate the superstructures, we first synthesized monodisperse pNIPAAm-based ruthenium/NH2 dual functionalized microgels and pNIPAAm-r-NAS-based linear polymer crosslinkers with various component ratios. Second, a buffer solution of the microgels and linear crosslinkers were simultaneously encapsulated into the polymersome precursors to produce core/shell droplets through glass capillary-based microfluidics. The polymersomes were slowly formed via the evaporation of solvent in the shell and served as ‘microreactors’. The microgel-based clusters were formed inside these ‘microreactors’ through a mild reaction between NH2 groups and succinate groups. The internal microstructures of these clusters were controlled by adjusting the contents of NAS component in the polymer linkers, and as a result, the temperature-responsive properties of the clusters were consequently regulated. Finally, the catalytic properties of the ruthenium-loaded clusters for the B-Z reaction were studied using microscopy. Owing to the catalytic properties of the ruthenium moiety, all three types of clusters displayed chemomechanical features to varying degrees, likely because of the different internal microstructures.


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ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website.

Notes: The authors declare no competing financial interest. The software we used for the automated tracking the edge of the clusters is available through contacting the authors.

ACKNOWLEDGMENT We thank Dr. Alec Pawling for providing the basic engine for automated tracking the edge of the clusters. This work was funded by Repsol, S.A. Spain. The funders had no role in the study design, data collection and analysis, decision to publish, or preparation of the manuscript. NMR was conducted at the Chemistry and Chemical Biology Department of Harvard University. The UV-Vis spectrum, DLS and SEM measurements were performed at the Center for Nanoscale Systems (CNS) in Harvard University, a member of the National Nanotechnology Coordinated Infrastructure Network (NNCI), which is supported by the National Science Foundation under NSF ECCS award no. 1541959.

REFERENCES (1) Service, R. F. How far can we push chemical self-assembly? Science 2005, 309,95. (2) Shen, J.; Ye, T.; Chang, A.; Wu, W.; Zhou, S. A colloidal supra-structure of responsive microgels as a potential cell scaffold. Soft Matter 2012, 8,12034. (3) Mansson, L. K.; Immink, J. N.; Mihut, A. M.; Schurtenberger, P.; Crassous, J. J. A new route towards colloidal molecules with externally tunable interaction sites. Faraday Discuss 2015, 181,49-69. (4) Klajn, R.; Bishop, K. J.; Grzybowski, B. A. Light-controlled self-assembly of reversible and irreversible nanoparticle suprastructures. Proc Natl Acad Sci U S A 2007, 104,10305-10309. (5) Chen, M.; Zhou, L.; Guan, Y.; Zhang, Y. Polymerized microgel colloidal crystals: photonic hydrogels with tunable band gaps and fast response rates. Angew. Chem. 2013, 52,9961-9965.


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(6) Li, S.; Moosa, B. A.; Croissant, J. G.; Khashab, N. M. Electrostatic assembly/disassembly of nanoscaled colloidosomes for light-triggered cargo release. Angew.Chem. 2015, 54,6804-6808. (7) Morimoto, N.; Winnik, F. M.; Akiyoshi, K. Botryoidal assembly of cholesteryl-pullulan/poly(Nisopropylacrylamide) nanogels. Langmuir 2007, 23,217-223. (8) Shen, J.; Pullela, S.; Marquez, M.; Cheng, Z. Ternary phase diagram for the BelousovZhabotinsky reaction-induced mechanical oscillation of intelligent PNIPAM colloids. J Phys Chem A 2007, 111,12081-12085. (9) Kuksenok, O.; Dayal, P.; Bhattacharya, A.; Yashin, V. V.; Deb, D.; Chen, I. C.; Van Vliet, K. J.; Balazs, A. C. Chemo-responsive, self-oscillating gels that undergo biomimetic communication. Chem Soc Rev 2013, 42,7257-7277. (10) Suzuki, D.; Sakai, T.; Yoshida, R. Self-flocculating/self-dispersing oscillation of microgels. Angew. Chem. 2008, 47,917-920. (11) Tamate, R.; Mizutani Akimoto, A.; Yoshida, R. Recent advances in self-oscillating polymer material systems. Chem Rec 2016, 16,1852-1867. (12) Epstein, I. R.; Xu, B. Reaction-diffusion processes at the nano- and microscales. Nat Nanotechnol 2016, 11,312-319. (13) Sakai, T.; Yoshida, R. Self-oscillating nanogel particles. Langmuir 2004, 20,1036-1038. (14) Zhang, Y.; Zhou, N.; Li, N.; Sun, M.; Kim, D.; Fraden, S.; Epstein, I. R.; Xu, B. Giant volume change of active gels under continuous flow. J. Am. Chem.Soc. 2014, 136,7341-7347. (15) Kramb, R. C.; Buskohl, P. R.; Dalton, M. J.; Vaia, R. A. Belousov–Zhabotinsky hydrogels: relationship between hydrogel structure and mechanical response. Chem. Mater. 2015, 27,5782-5790. (16) Zhao, F.; Ding, Y.; Lu, Y.; Liu, X.; Zhang, G. Microcalorimetric studies on chemical oscillation of microgels. J Phys Chem B 2009, 113,6661-6665. (17) Zhang, Y.; Li, N.; Delgado, J.; Zhou, N.; Yoshida, R.; Fraden, S.; Epstein, I. R.; Xu, B. Structural modulation of self-oscillating gels: changing the proximity of the catalyst to the polymer backbone to tailor chemomechanical oscillation. Soft Matter 2012, 8,7056. (18) Hu, Y.; Perez-Mercader, J. Controlled Synthesis of Uniform, Micrometer-sized rutheniumfunctionalized poly(N-Isopropylacrylamide) gel particles and their Application to the Catalysis of the Belousov-Zhabotinsky Reaction. Macromol Rapid Commun 2017, 38,1600577. (19) Royall, C. P.; Poon, W. C. K.; Weeks, E. R. In search of colloidal hard spheres. Soft Matter 2013, 9,17-27. (20) Zhang, Y.; Zhou, N.; Akella, S.; Kuang, Y.; Kim, D.; Schwartz, A.; Bezpalko, M.; Foxman, B. M.; Fraden, S.; Epstein, I. R.; Xu, B. Active cross-linkers that lead to active gels. Angew. Chem. 2013, 52,11494-11498. (21) Suzuki, D.; Kobayashi, T.; Yoshida, R.; Hirai, T. Soft actuators of organized self-oscillating microgels. Soft Matter 2012, 8,11447-11449. (22) Gao, Y.; Li, X.; Serpe, M. J. Stimuli-responsive microgel-based etalons for optical sensing. RSC Adv. 2015, 5,44074-44087. (23) Han, K.; Go, D.; Hoenders, D.; Kuehne, A. J. C.; Walther, A. Switchable supracolloidal coassembly of microgels mediated by host/guest interactions. ACS Macro Letters 2017, 6,310-314. (24) Martino, C.; Kim, S. H.; Horsfall, L.; Abbaspourrad, A.; Rosser, S. J.; Cooper, J.; Weitz, D. A. Protein expression, aggregation, and triggered release from polymersomes as artificial cell-like structures. Angew. Chem. 2012, 51,6416-6420. (25) Lee, T. Y.; Choi, T. M.; Shim, T. S.; Frijns, R. A.; Kim, S. H. Microfluidic production of multiple emulsions and functional microcapsules. Lab on a chip 2016, 16,3415-3440. (26) Hu, Y.; Perez-Mercader, J. Microfluidic fabrication of polymersomes enclosing an active Belousov-Zhabotinsky (BZ) reaction: Effect on their stability of solute concentrations in the external media. Colloids Surf B Biointerfaces 2016, 146,406-414. (27) Postma, S. G.; Vialshin, I. N.; Gerritsen, C. Y.; Bao, M.; Huck, W. T. Preprogramming complex hydrogel responses using enzymatic reaction networks. Angew. Chem. 2017, 56,1794-1798.


Page 14 of 20

Page 15 of 20

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Langmuir

(28) Seiffert, S.; Weitz, D. A. Controlled fabrication of polymer microgels by polymer-analogous gelation in droplet microfluidics. Soft Matter 2010, 6,3184. (29) Chiang, M. Y.; Hsu, Y. W.; Hsieh, H. Y.; Chen, S. Y.; Fan, S. K. Constructing 3D heterogeneous hydrogels from electrically manipulated prepolymer droplets and crosslinked microgels. Sci Adv 2016, 2,e1600964. (30) Fang, Z.; Keinan, S.; Alibabaei, L.; Luo, H.; Ito, A.; Meyer, T. J. Controlled electropolymerization of ruthenium(II) vinylbipyridyl complexes in mesoporous nanoparticle films of TiO(2). Angew. Chem. 2014, 53,4872-4876. (31) Pushpito K. Ghosh, T. G. S. Photoelectrochemistry of Tris( bipyridyl)ruthenium( 11) covalently attached to n-type SnO2 J. Am. Chem. Soc. 1980, 102,5543-5549. (32) Meng, Z.; Smith, M. H.; Lyon, L. A. Temperature-programmed synthesis of micron-sized multiresponsive microgels. Colloid Polym. Sci 2009, 287,277-285. (33) Still, T.; Chen, K.; Alsayed, A. M.; Aptowicz, K. B.; Yodh, A. G. Synthesis of micrometer-size poly(N-isopropylacrylamide) microgel particles with homogeneous crosslinker density and diameter control. J. Colloid Interface Sci 2013, 405,96-102. (34) Chang, C.; Wei, H.; Wu, D. Q.; Yang, B.; Chen, N.; Cheng, S. X.; Zhang, X. Z.; Zhuo, R. X. Thermo-responsive shell cross-linked PMMA-b-P(NIPAAm-co-NAS) micelles for drug delivery. Int J Pharm 2011, 420,333-340. (35) Tamate, R.; Ueki, T.; Yoshida, R. Evolved Colloidosomes undergoing cell-like autonomous shape oscillations with buckling. Angew. Chem. 2016, 55,5179-5183. (36) Hu, Y.; Wang, J.; Li, C.; Li, Z.; Liang, R.; Wang, Q.; Wang, H.; Zhu, J.; Yang, Y. Non-Spherical hollow microgels with uniform sizes and tunable shapes from microfluidic-assisted approach. Sci. Adv. Mater. 2015, 7,902-909. (37) Liu, X.; Zhou, P.; Huang, Y.; Li, M.; Huang, X.; Mann, S. Hierarchical Proteinosomes for Programmed Release of Multiple Components. Angew. Chem. 2016, 55,7095-7100. (38) Shah, R. K.; Kim, J. W.; Weitz, D. A. Monodisperse stimuli-responsive colloidosomes by selfassembly of microgels in droplets. Langmuir 2010, 26,1561-1565. (39) Sun, J.; Wang, W.; He, F.; Chen, Z.-H.; Xie, R.; Ju, X.-J.; Liu, Z.; Chu, L.-Y. On-chip thermotriggered coalescence of controllable Pickering emulsion droplet pairs. RSC Adv. 2016, 6,64182-64192. (40) Pelton RH, C. P. Preparation of aqueous latices with N-Isopropylacrylamide. Colloids Surf. 1986, 20,247-256. (41) Kim, J. W.; Utada, A. S.; Fernandez-Nieves, A.; Hu, Z.; Weitz, D. A. Fabrication of monodisperse gel shells and functional microgels in microfluidic devices. Angew. Chem. 2007, 46,18191822. (42) Clinton D. Jones, L. A. L. Synthesis and characterization of multiresponsive core-shell microgels. Macromolecules 2008, 33,8301-8306. (43) Halperin, A.; Kroger, M.; Winnik, F. M. Poly(N-isopropylacrylamide) phase diagrams: fifty years of research. Angew. Chem. 2015, 54,15342-15367. (44) Byrne, A.; Burke, C. S.; Keyes, T. E. Precision targeted ruthenium(ii) luminophores; highly effective probes for cell imaging by stimulated emission depletion (STED) microscopy. Chem Sci 2016, 7,6551-6562. (45) Guzowski, J.; Gizynski, K.; Gorecki, J.; Garstecki, P. Microfluidic platform for reproducible selfassembly of chemically communicating droplet networks with predesigned number and type of the communicating compartments. Lab chip 2016, 16,764-772. (46) Yoshida, R.; Sakai, T.; Ito, S.; Yamaguchi, T. Self-oscillation of polymer chains with rhythmical soluble-insoluble changes. J. Am. Chem. Soc. 2002, 124,8095-8098. (47) Herranz-Blanco, B.; Arriaga, L. R.; Makila, E.; Correia, A.; Shrestha, N.; Mirza, S.; Weitz, D. A.; Salonen, J.; Hirvonen, J.; Santos, H. A. Microfluidic assembly of multistage porous silicon-lipid vesicles for controlled drug release. Lab chip 2014, 14,1083-1086.


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Pathway for the fabrication of active microgel clusters using giant polymersomes and illustration of their activity Microgel clusters were prepared using polymersomes as ‘microreactors’, which were fabricated through microfluidics. The clusters were formed through a crosslinking reaction between ruthenium/NH2 dual functionalized microgels and poly(N-acryloxysuccinimide)-based polymer linkers. By varying the component ratios of the polymer linkers, the internal structure and temperature-dependent behaviors of the clusters can be controlled. The microgel clusters showed chemomechanical oscillations when exposed to a catalyst-free solution containing Belousov-Zhabotinsky reaction substrates.


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Figure 1.(a) Illustration showing the process for the formation of microgel clusters using polymersomes as ‘microreactors’. (b) Schematic figure of the microgel and its chemical structure. The red circle on the right indicates the functional groups used in the following linking reaction. (c) Mechanism of the reaction between the amino groups from the microgels and the succinimide groups from the linear polymer linker pNIPAAmcoNAS. 164x248mm (300 x 300 DPI)


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Figure 2. (a) Optical microscopy image of the polymersomes containing the microgels and polymer linker at 21 °C. (b), (c) Comparison of an individual polymersome containing a mixture of microgels/crosslinker at 40 °C and 21 °C, respectively. (d) Fluorescence microscopy image of the polymersome composed of microgel building blocks and polymer linkers. (e) SEM image of the microgel cluster within a polymersome; the red arrow indicates the membrane of the polymersome. (f) Partial enlargement of the microgel cluster from (e),the red arrows indicate the gel particles. (g) SEM image of the bare cluster. 412x183mm (300 x 300 DPI)


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Figure 3. (a), (c), (e) Optical microscopy images of the temperature-dependent behaviors of the microgel clusters containing pNIPAAm-r-NAS, pNIPAAm-r-NAS-r-Ru or pNIPAAm-r-NAS-r-MRB as the polymer linker, respectively. (b), (d), (f) Change in the diameter of these microgel clusters as a function of temperature; the inset figures show the fluorescence microscopy images of the corresponding microgel cluster at different temperatures. (g) Change in the hydrodynamic diameters of the microgel clusters as a function of temperature. (h) Schematic figure illustrating the possible internal structures of the clusters formed from the three different polymer linkers. 385x257mm (300 x 300 DPI)


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Figure 4. Top: Optical microscopy images of the chemomechanical behaviors of the clusters when exposed to B-Z reaction solutions without any catalyst. Bottom: Table indicating the different extent of chemomechanical oscillations of the clusters under the B-Z reaction. 98x82mm (300 x 300 DPI)


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Microfluidics Fabrication of Self-Oscillating ... - ACS Publications

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