Tailored Macroporous Hydrogels with ... - ACS Publications

Letter Cite This: ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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Tailored Macroporous Hydrogels with Nanoparticles Display Enhanced and Tunable Catalytic Activity Teodora Gancheva and Nick Virgilio* CREPEC, Department of Chemical Engineering, Polytechnique Montréal, C.P. 6079 Succursale Centre-Ville, Montréal, Québec H3C 3A7, Canada

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S Supporting Information *

ABSTRACT: This work demonstrates that a model system of poly(N-isopropylacrylamide) (PNIPAam) macroporous hydrogels, with tailored microstructures and comprising gold (Au) or silver (Ag) nanoparticles, display enhanced and tunable catalytic activity. These nanocomposites are prepared using polymer templates obtained from melt-processed cocontinuous polymer blends. The reaction rate, controlled by both hydrogel porosity and the PNIPAam lower critical solution temperature, increases by more than an order of magnitude as compared to nonporous gels, and is comparable to micro- or nanocarrier-based systems, with easier catalyst recovery. The fabrication process is scalable, and is compatible with broad choices of polymer blend, gel, and nanoparticle chemistries. KEYWORDS: nanoparticles, catalysis, macroporous hydrogels, monoliths, cocontinuous polymer blends, nanocomposites, poly(N-isopropylacrylamide)

I

embedded in macroporous stimuli-sensitive hydrogels with precise control over the average pore diameter, specific surface area, pore volume fraction and interconnectivity. We use a model system based on temperature-sensitive poly(N-isopropylacrylamide) (PNIPAam) hydrogels with gold (Au) nanoparticles, which catalyze the reduction of 4-nitrophenol to 4-aminophenol.13,14,18,19 Porous PNIPAam gels were prepared by using porous polymer templates, based on a method illustrated in Scheme 1 and described in previous publications.16,20,21 The method consists in (1) preparing a cocontinuous polymer blend by melt-processing,22−27 herein comprised of ethylene propylene diene monomer and poly(ε-caprolactone) (EPDM/PCL 50/ 50 vol %, Tprocessing = 180 °C, tmixing = 7 min, see Experimental Details in the Supporting Information); (2) annealing the polymer blend under quiescent conditions to allow microstructure coarsening (Tanneal = 180 °C, tanneal = 60 or 240 min);22−26 (3) extracting selectively the EPDM phase with cyclohexane to obtain porous PCL molds (Figure 1a−c, SEM observations), herein trimmed into 1 cm3 cubes; (4) injecting the solution containing the monomers, cross-linker, and other necessary reagents, within the molds, followed by in situ gelling; (5) extracting selectively the PCL molds with toluene to obtain macroporous PNIPAam hydrogels (Figure 1d−f, Xray microcomputed tomography (microCT)). NPs are subsequently synthesized within the hydrogel phase following

n the last 20 years, noble metal nanoparticles (NPs) have demonstrated a broad array of possibilities when used as heterogeneous catalysts.1−3 Embedding catalytic NPs in gels (i.e., micro/nanogels or hydrogels) has emerged as a clever strategy to design functional materials because (1) gels can efficiently immobilize NPs,4−7 ensure their long-term stability,4−7 and facilitate their recovery and reuse; (2) their high permeability, comparable to liquids,8 allows the diffusion of reactants (and products) to (and from) NPs catalytic sites; (3) NPs can be synthesized within the gels following simple protocols;4,6 (4) catalyst activity can be tuned by using stimulisensitive gel chemistries.9−12 When embedded in soft micro/ nanoparticles (e.g., core−shell structures,10,13 micro/nanogels,12,14 dendrimers,15 etc.), the combination of high specific surface and short diffusion path to the NPs reactive sites results in high reaction rates; however, an elaborate separation process is still required to recover the particles. On the other hand, when NPs are embedded in macroscopic gels (or hydrogels), the separation process is simple, but the reaction rate slows down considerably because of the limited surface of exchange between the gel and surrounding medium, and diffusionlimited mass transfer.5 As a result, the use of macroporous hydrogels has recently emerged as an interesting approach to significantly enhance the transport and catalytic properties of macroscopic gels.16,17 However, control over key porosity parameters such as pore interconnectivity and average size remain a significant challenge, resulting in the absence of correlations between material processing, gel microstructure, reaction kinetics and experimental flow parameters (such as pressure drop as a function of flow rate). Herein, NPs are © XXXX American Chemical Society

Received: April 22, 2018 Accepted: June 13, 2018

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DOI: 10.1021/acsami.8b06560 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

Letter

ACS Applied Materials & Interfaces Scheme 1. Steps Followed to Prepare Porous Hydrogels Embedded with NPs: (a) Cocontinuous Polymer Blend of EPDM and PCL; (b) Porous PCL Mold Obtained after EPDM Extraction; (c) Porous PCL Mold Filled with Hydrogel; (d) Porous Hydrogel Embedded with NPs, Obtained after PCL Extraction and Subsequent NP Synthesis

well established protocols (demonstrated herein for Au and silver (Ag) NPs).4,6 Figure 2a−d displays the initial polymer blend and PCL mold, and the resulting porous gel, before and after AuNPs synthesis. This technique possesses the following features and advantages: (1) the polymer molds and gels show nearly identical macroscopic dimensions (Figure 2b, c); (2) the average pore diameter can be tuned from ∼1 to 1000 μm; (3) the pores are fully interconnected; (4) several choices of polymer blends, gels and NPs chemistries can be selected;20,21,23 (5) scale up is possible by polymer meltextrusion;20 (6) the resulting macroporous gels possess enhanced stimuli-sensitive properties due to coupling between mass and energy transfers.16 The polymer blend morphology significantly coarsens with increasing annealing time tanneal (Figure 1a−c), the process being driven by the polymer pair interfacial tension,22−24,26

Figure 1. Morphology of porous PCL materials obtained after EPDM phase extraction: (a) tanneal = 0 min (unannealed blend); (b) tanneal = 60 min; (c) tanneal = 240 min; Morphology of porous PNIPAam gels as a function polymer blend annealing time (gel phase in black, pores in white): (d) tanneal = 0 min; (e) tanneal = 60 min, and f) tanneal = 240 min; (g) evolution of the specific surface area with annealing time for porous PCL molds and porous PNIPAam hydrogels.

and slowed down by viscosity.23 During annealing, both phases remain fully interconnected (≈ 100% cocontinuous), as measured by gravimetric analysis (Table S1). The average pore diameter dpores increases nearly linearly from 19 ± 2 μm (tanneal = 0 min, unannealed blend) to 310 ± 4 μm (tanneal = 240 min), while the specific surface area S decreases from 1180 ± 145 cm−1 (tanneal = 0 min) to 65 ± 1 cm−1 (tanneal = 240 min) (Figure 1g and Table S1). B


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ACS Applied Materials & Interfaces

Figure 2. Successive steps for the preparation of macroporous PNIPAam gels embedded with AuNPs (illustrated for a blend annealed during 240 min): (a) polymer blend of EPDM and PCL; (b) porous PCL mold; (c) macroporous PNIPAam hydrogel; (d) macroporous PNIPAam hydrogel with AuNPs. Figure 3. AuNPs characterization: (a) TEM micrograph showing AuNPs size and distribution (illustrated for a G0 gel, sample obtained close to the gel surface, arrows pointing AuNPs); (b) HRTEM image of single AuNP, with corresponding lattice spacing; (c) AuNPs size distribution histogram (calculated from the TEM micrographs by measuring ∼200 NPs, including G0 and G60 gels); (d) UV−visible spectra of NG gels with AuNPs (dash/dot line: gel analyzed close to the surface; dashed line: gel analyzed in the inner region; dotted line: NG gel without AuNPs).

The PNIPAam gels microstructures, analyzed by X-ray microtomography (Figure 1d−f), reveal well-preserved, fully cocontinuous gel phases and pore networks. For the macroporous gels, the average pore diameter dpores increases from 23 ± 9 μm (for tanneal = 0 min) to 262 ± 34 μm (for tanneal = 240 min), and the specific surface area S decreases from 850 ± 180 cm−1 (for tanneal = 0 min) to 77 ± 10 cm−1 (for tanneal = 240 min) (Figure 1g and Table S2); these results are quite comparable with the PCL molds values. From this point, porous hydrogels are designated as G0, G60, and G240, following the polymer blend annealing time, whereas NG corresponds to bulk nonporous hydrogels. Porous PNIPAam hydrogels were next immersed in a solution of 5 mM HAuCl4 for 15 h and then were plunged in a 50 mM solution of NaBH4 for 30 min. The gels almost instantaneously darkened (Figure 2d),11 indicating the formation of AuNPs. The reaction occurs much faster in porous gels, as compared to NG gels (see Videos S1 and S2). Similar results were obtained when synthesizing AgNPs (Figure S1). Transmission electron microscopy (TEM) was used to analyze AuNPs morphology and size distribution. As Figure 3a shows, well-dispersed NPs with an average diameter of 1.5 (±0.5) nm were synthesized. Note that AuNPs aggregates are also observed (albeit much less frequently) in the gels, causing their dark color (Figure S2). High-resolution TEM on a single AuNP clearly shows a crystal lattice with an interplanar distance of about 2.36 Å (Figure 3b), attributed to the [111] plane in the cubic Au crystalline structure.9,19 Figure 3c, d display NPs size distribution and UV−visible spectra. The plasmon absorption spectra position depends on AuNPs distribution within the gels (Figure 3d). Near the gel surface, a broad absorption spectrum appears around 525 nm, due to the AuNPs aggregates. In the inner region, a nonsignificant plasmon absorption band appears around 450−600 nm, characteristic for AuNPs with an average size of ∼2 nm.15,28 These results

correlate well with the color distribution within the NG gels caused by AuNPs plasmon absorption (Figure S3).29 The amount of AuNPs within the gels was determined by thermogravimetric analysis (TGA). Porous gels possess a higher AuNPs content (G0 = 4.6 wt %, G60 = 3.6 wt %, G240 = 4.7 wt %), compared to nonporous gels (NG = 2.7 wt %), when the measure is based on the initially dried material. However, in their hydrated state, nearly 50% of a porous gel volume is occupied by free water within the macropores. As a result, based on the hydrated volume, the AuNP content is comparable or lower in macroporous gels per unit volume. Furthermore, AuNPs distribution throughout the gels depends on gel porosity: G60 and G240 are completely filled with NPs, whereas the centers of G0 and NG gels do not contain any (Figure S4).11 The catalytic performance of macroporous PNIPAam gels with AuNPs was evaluated with a model reduction reaction of 4-nitrophenol to 4-aminophenol.13,14,18,19 The absorption spectra at 400 nm of 4-nitrophenol was used to monitor the reaction progress (Figure S5). Figure 4a displays the relative concentration of 4-nitrophenol as a function of time and average pore diameter, compared to nonporous gels with AuNPs. G60 gels show the fastest reaction rate, and an almost complete reaction after ∼10 min, whereas G0 and G240 both display slower kinetics, completing the reaction in ≈25 min. In comparison, NG gels display a much slower rate, and the full conversion of 4-nitrophenol takes ∼80 min. Similar results were obtained with gels containing AgNPs (Figure S6). As Figure 4a shows, an induction period (≈ 10 min) in which no reaction takes place was observed for NG gels. Several studies C


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ACS Applied Materials & Interfaces

treated as a pseudo-first-order reaction.9,11,13,18 In our case, a nearly linear correlation between ln(Ct/C0) (Ct is the concentration of 4-nitrophenol at time t and C0 at t = 0) and time t was observed for porous gels (Figure 4a). In comparison, NG gels deviate from linearity since kapp gradually increases with time, but remains on average much lower compared to porous gels. This deviation is most probably caused by the diffusion-limited mass transfer in the bulk of the material and to the AuNPs surface. Figure 4b shows the apparent rate constant kapp as a function of the average pore diameter, compared to NG gels. kapp is strongly dependent on gel microstructure and pore size. Porous gels G60 display the fastest reaction rate (= 0.29 min−1), which decreases for G0 and G240 to 0.12 min−1. In comparison, kapp for NG gels is much lower (= 0.03 min−1) − this is an order of magnitude inferior to G60. Since the concentration of AuNPs based on the hydrated gel volume is relatively comparable between the various materials, the presence of an open and interconnected macroporous structure significantly enhances the reaction kinetics and confirms the importance of microstructure control when designing such materials. Fully interconnected pores allow efficient mass transfer within the gels,16,20 and decreases the diffusion path to the NPs reactive sites, as compared to NG gels. Also, as Figure 4a, b demonstrates, the catalytic activity of macroporous hydrogels strongly depends on pore size− allowing tuning of the reaction rate. G240 gels, with an average gel domain size dgel = 280 ± 65 μm (Table S2) possess a relatively long diffusion path in the gel phase for the reactants to reach the catalytic sites−consequently, slower kinetics are observed. In comparison, G0 gels, with dgel = 31 ± 15 μm, possess a short diffusion path, but also a very fine and tortuous macroporous structure, which slows mass transfer (as demonstrated in previous publications)16,20 and the reaction rate. G60 gels represent an optimum, combining relatively short diffusion path (dgel = 135 ± 5 μm) with less tortuosity. Dividing kapp by the AuNPs total surface per unit volume allows comparison with other systems reported in the literature (k′app in Table 1). G60 gels show relatively close reaction kinetics compared to certain micro/nanometer size particulate carrier systems. This is interesting since the distribution in solution of microcarrier based systems is typically more homogeneous compared to monolith materials. This illustrates that our macroporous gel-based materials enhance mass Figure 4. (a) Catalytic reduction of 4-nitrophenol at 20 °C as a function of time and average pore diameter, as compared to nonporous gels NG (each data point for G0 and G240 corresponds to the average of 4 samples. Data point for G60 and NG gels correspond to the average of 6 and 3 samples, respectively. See standard deviations in Figure S7); (b) evolution of kapp at 20 °C, as a function of the average pore diameter, as compared to nonporous gels. The dashed line is a guide for the eye. kapp was calculated from the linear fittings, considering only the nearly linear part for NG gels; (c) reduction of 4-nitrophenol with G60 gels, as a function of time and temperature, including the linear fittings. Note: (a, c) illustrate ln(Ct/ C0) vs t to better evaluate if the reaction is pseudo-first-order.

Table 1. Reduction of 4-Nitrophenol with AuNPs Immobilized in Different Carrier Systems system G60 G0 G240 NG Fe3O4-polymer microgel AuNPs dumbbell-like Au-Fe3O4 NPs polymer micelle AuNPs polymer brush AuNPs yolk−shell AuNPs polyelectrolyte brush AuNPs

have attributed this feature to a substrate-caused surface restructuring required to render the NPs active.13,18 Because of mass transfer limitations, the substrate adsorption rate to the AuNPs surface is much slower in NG gels, as compared to porous G0, G60, and G240 gels. When AuNPs are immobilized in microcarriers and NaBH4 is in excess compared to 4-nitrophenol, the kinetics can be

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Corresponds to the average AuNPs diameter. bApparent rate constant. cValues calculated from the presented data in the cited reference. D


Letter

ACS Applied Materials & Interfaces Notes

transport, with the additional advantage of simple recovery and reuse. However, a similar comparison with other gel monolithbased systems was difficult to realize due to incomplete or partial information in the literature. Note finally that we observed catalytic deactivation when reusing the materials after a certain number of days or weeks, which we suspect is due to the strong adsorption of aminophenols onto the AuNPs surface after reaction completion, as reported recently.19 Because PNIPAam possesses a lower critical solution temperature (LCST) at ∼33 °C, the catalytic activity of G60 gels was monitored at 20, 33, and 50 °C. Figure 4c shows the relative concentration of 4-nitrophenol as a function of time and temperature. At 20 °C, below the LCST, the network is swollen and the NPs are fully accessible for catalytic reduction, resulting in the fastest reaction rate (kapp = 0.29 min−1). At 33 °C, just above the LCST, the reaction rate decreases significantly (kapp = 0.008 min−1) due to the collapse of the gel network that limits mass transfer. Further rise in temperature overcompensates the diffusion barrier and accelerates the reaction rate (kapp = 0.06 min−1); such a trend was also observed for other gel-based catalytic systems.9,10,12 In summary, this article demonstrates how melt-processed cocontinuous polymer blends, with tunable morphological features, can be used to prepare tailored macroporous gel nanocomposites displaying enhanced and tunable catalytic activity. By controlling pore and gel domain sizes, tortuosity, and interconnectivity, an optimum apparent reaction rate is obtained which is more than an order of magnitude superior to nonporous gel materials. This method constitutes an interesting technological platform for the development of functional macroporous gels for a variety of applications, because broad arrays of polymer blends, gels, and nanoparticle chemistries can be used.

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The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the National Sciences and Engineering Research Council of Canada (NSERC Discovery Grant) for funding; Jean-Philippe Masse from the Centre for characterization and microscopy of materials (CM)2 at Polytechnique Montréal for the TEM observations; Dr. Benoı̂t Liberelle for fruitful discussions and technical support; and Mario ArayaMarchena, Dr. Davood Bagheri, Jonathan Lavoie, and Matthieu Gauthier for their help with the glovebox experiments.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.8b06560.

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REFERENCES

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Experimental procedures, Tables S1 and S2 reporting porous polymer molds and gels microstructural features, Figures S1−S7 illustrating a Ag/hydrogel nanocomposite, AuNP size and distribution within the gels, and catalysis results with Ag and AuNPs nanocomposites (PDF) Video S1, AuNP formation inside the porous gel, G60 (AVI) Video S1, AuNP formation inside the nonporous gel, NG (AVI)

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: 1-514-340-4711, #4524. Fax: 1-514-340-4159. ORCID

Nick Virgilio: 0000-0002-5629-4867 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. E


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