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DOI: 10.1039/C4CC06988A
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Redox-responsive organometallic hydrogels for in-situ metal nanoparticle synthesis B. Zoetebier, M. A. Hempenius,* G. J. Vancsoa
Received 00th January 2012, Accepted 00th January 2012 DOI: 10.1039/x0xx00000x www.rsc.org/
A new class of redox active hydrogels composed of poly(ferrocenylsilane) polyanion and poly(ethylene glycol) chains was assembled, using a copper-free azide-alkyne Huisgen cycloaddition reaction. These organometallic hydrogels displayed reversible collapse and reswelling upon chemical oxidation and reduction, respectively, and formed relatively well-defined, unaggregated Pd(0) nanoparticles (8.2±2.2 nm) from K2PdCl4 salts. Stimulus-responsive gels are attracting much attention due to their interesting behavior as well as to many potential applications.1,2 Such gels can e.g. be used for controlled release of drugs or cosmetics, upon employing a specific stimulus.3 They can also be useful as actuators, artificial muscles or microfluidic valves.4,5 Typical stimuli used to date include changes in temperature,6 pH,7 ionic strength,8 electric field strength,9 light intensity,2d and variations in magnetic field strength.10,11 Gels responsive to redoxstimuli usually incorporate transition metals into network-forming polymer chains, either as main chain constituents or included in side groups, or are composed of (semi)conducting polymers.12,13 Their addressability by redox stimuli allows one to alter properties such as polarity, which generally has a large influence on polymer properties such as chain stiffness and extension, and on interactions with solvent. Redox responsiveness therefore is of interest for actuator applications. Until now, relatively few examples of transition-metalbased redox-active gels and hydrogels have appeared in the literature.13 For example, copolymerization of vinylferrocene with acrylamide and N,N'-methylenebis(acrylamide) led to a random copolymer gel with potential use as a glucose biosensor.14 A poly[(N-isopropylacrylamide)-co-vinylferrocene] gel was produced which displayed electrochemically and thermally controllable phase transitions.15 A self-oscillating redox-active gel was obtained by inducing the Belousov-Zhabotinsky (BZ) reaction within a ruthenium-bearing N-isopropylacrylamide gel.16 Other redoxresponsive gels reported in the literature include a ferrocenyl surfactant system showing controlled fluid viscoelasticity,17 and
This journal is © The Royal Society of Chemistry 2012
polyaniline-based gels as e.g. actuators for controlled release18 or catalysis.19 Self-healing, supramolecular materials using PAA modifed with β-cyclodextrin as a host polymer and PAA with ferrocene pendant groups as a guest polymer were also reported. By using chemical oxidation and reduction, a reversible sol–gel phase transition could be induced in this system.20 Organometallic polymers possessing skeletal transition metals show great promise as constituents of redox-responsive polymer gels.13,21 Among this class of organometallic polymers, poly(ferrocenylsilanes), PFS, are raising much interest due to the useful chemical, redox, optical, thermal, and magnetic properties of these materials.22,23A variety of polymerization methods have been developed, including living anionic ring-opening polymerization yielding homopolymers and block copolymers, transition metalcatalyzed ring-opening polymerization and photocontrolled living polymerization, broadening the range of accessible PFSs.22 While the crosslinking chemistries of poly(ferrocenylsilane) networks based on PFS chains soluble in regular organic solvents have been reported,21,24 few accounts have appeared on crosslinking approaches for water-soluble PFSs.25 We reported the first examples of metal nanoparticle fabrication mediated by main-chain redoxactive polymer hydrogels.21b,c PFS-based hydrogels were shown to provide a confined, reducing environment, allowing one to convert silver salts into defined, unaggregated metal nanoparticles without employing external reducing agents.21b,c,26 The production of metal nanoparticles often requires the use of organic ligands, salts, surfactants or other capping agents,27 which may significantly reduce their (catalytic) activity.27b Nanoparticle fabrication inside a polymer network circumvents these issues and suppresses excessive growth of the particles.28 Here we describe the synthesis of a PFS polyanion hydrogel, where crosslinks were formed between strained alkynes and azide groups in a copper-free Huisgen cycloaddition reaction,29 and its use in the in-situ formation of palladium nanoparticles. Palladium nanoparticles are highly important catalytic materials and continue to attract growing interest due to their catalytic properties
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combined with a high surface-area to volume ratio.27a,30 Palladium is used extensively in hydrogenations,31 oxidations,32 carbon–carbon bond formation,33 electrochemical reactions in fuel cells,34 hydrogen storage,35 and sensing applications.36 In order to form a polyanionic poly(ferrocenylsilane) hydrogel with controlled degrees of crosslinking, a PFS polyanion with tunable amounts of crosslinkable azide groups was synthesized by side group modification of PFS 1 (Scheme 1). Negatively charged side groups were introduced using the carbon nucleophile α-lithioisobutylethanesulfonate.39 Sulfonate ester carbanions enable one to attach alkanesulfonate ester moieties e.g. to polymer backbones which, after deprotection, are turned into alkanesulfonate functionalities. Sulfonate ester carbanions have been shown to react for instance with carbonyl groups or with alkyl halides. Alkyl iodides react quickly with these carbanions, corresponding chlorides however are much less reactive.37 The large difference in reactivity was exploited by forming PFS 1, which contains both chloropropyl and iodopropyl moieties, in a halogen exchange reaction starting from poly(ferrocenyl(3chloropropyl)methylsilane).39 Treatment of PFS 1 with αlithioisobutylethanesulfonate selectively displaced the iodo groups, giving PFS 2. Sulfonate ester deprotection and the introduction of azide groups by displacement of the unreacted chloro groups was conducted in a single step, yielding the targeted PFS 3.
Journal Name DOI: 10.1039/C4CC06988A completely remove MeOH, the gel was placed in Milli-Q water, which was replaced several times. Hydrogel 5 was subsequently freeze-dried for FTIR analysis.
H C
O
N
O 28
C
3
O
O
4
4 PEG PFS
5
Scheme 2 Formation of a PFS polyanion hydrogel 5 by a copperfree azide-alkyne Huisgen cycloaddition reaction between PFS 3 and BCN-terminated four-arm star PEG 4. Fig. 1 displays the FTIR spectra for PFS 3 and network 5. The absence of an azide stretching vibration absorption at 2094 cm−1 in the spectrum of hydrogel 5 provides evidence for completion of the click reaction. Furthermore, the spectrum displays characteristic peaks of both PFS (774, 1035 cm−1), PEG (1098 cm−1) and a carbonyl stretch absorption of the tetra-arm PEG at 1710 cm−1.
Fig. 1 FTIR spectra of PFS 3 and network 5, showing the disappearance of the azide stretching vibration at 2094 cm−1 as the network is formed. Scheme 1 Synthesis of a crosslinkable poly(ferrocenylsilane) polyanion. PFS 1 was converted into a PFS featuring isobutylsulfonate side groups (2) using α-lithioisobutylethanesulfonate. Treatment of PFS 2 with NaN3 gave the water soluble PFS 3. Polyanion 3 and bicyclononyne (BCN) derivatized tetra-arm PEG solutions in MeOH were mixed, employing equimolar amounts of BCN and azide moieties in the gel-forming mixture (Scheme 2). The viscosity of the solution increased rapidly and gel formation was confirmed by the vial tilting method already within 2 minutes after mixing 3 and 4. When water was used as a medium for crosslinking, gelation occurred within 30 seconds, thus providing less time for mixing. Gelation was allowed to proceed overnight. In order to
2 | J. Name., 2012, 00, 1-3
Upon swelling in deionized water, the anionic PFS network 5 became an amber transparent elastic hydrogel. Water-uptake and rheological measurements were performed from which the effective crosslink density could be calculated (Table S1).
Fig. 2 Response of hydrogel 5 to chemical oxidation (50 mM Fe(ClO4)3) and reduction (50 mM sodium ascorbate).
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Journal Name The hydrogel could be reversibly oxidized and reduced electrochemically. For cyclic voltammetry, gels were built up layerby-layer from PFS 3 and PEG 4 on a gold substrate, which served as working electrode, using an azidobutyl thioacetate self-assembled monolayer40 for surface anchoring. Cyclic voltammograms obtained after swelling of network 5 in aqueous NaClO4 showed the two oxidation and reduction waves typical of poly(ferrocenylsilanes) at Eox1 = 324 mV and Eox2 = 464 mV, Ered1 = 397 mV and Ered2 = 208 mV, respectively (Fig. S1). Ferrocene units at alternating positions are oxidized first, followed by the oxidation of the ferrocene units located between ferrocenium sites at a higher potential.23 Peak currents showed a square root dependence on scan rate, indicating that the electrochemical processes are diffusion controlled (Fig. S2). Upon chemical oxidation, the hydrogel changed color from amber to green-blue and turned back to amber upon reduction (Fig. 2). Oxidation of the anionic hydrogel was accompanied by a clear change in mechanical behavior: the gel collapsed and lost its elastic nature (Fig. S3). Upon reduction the network reswelled and regained its elasticity. The observed collapse, shown in Fig. 2, can be ascribed to electrostatic attraction between the positively charged ferrocenium units in the PFS main chain and the negatively charged sulfonate side groups of the polymer. The redox activity of the hydrogel was employed for the in-situ formation of palladium nanoparticles. Standard potentials for the reduction of noble metal salts such as H2AuCl4 (1.002 V), K2PdCl4 (0.915 V) and K2PtCl4 (1.188 V) to the corresponding metals indicate that PFSs may be used as reducing agents. When hydrogel 5 was immersed in a solution of potassium tetrachloropalladate (5 mM), it turned from orange to black within minutes, after which the reaction was left overnight in the dark. The hydrogel was washed with Milli-Q water to remove unreacted salts and was freeze-dried for analysis.
COMMUNICATION DOI: 10.1039/C4CC06988A TEM measurements show well-dispersed palladium nanoparticles inside the gel, with a narrow size distribution (Fig. 3). Images captured over larger areas demonstrate the uniformity and lack of aggregation of the formed nanoparticles. The dimensions and size distribution of the Pd nanoparticles (Fig. S4) was evaluated by measuring the diameter of 250 particles by TEM, yielding a diameter of (8.2±2.2 nm). EDX was used to confirm the chemical nature of the particles and the hydrogel (Fig. 4, S5). Diffraction patterns obtained from the particles proved that Pd(0) nanoparticles were produced. Au and Pt metal nanoparticles could also be produced from the mentioned salts using hydrogel 5. Experiments employing the Pd nanoparticles in the model reduction of 4-nitrophenol to 4aminophenol are in progress.
Fig. 4 EDX spectrum of the formed Pd nanoparticles. The Cu signals in the spectrum originate from the TEM grid.
Conclusions In summary, hydrogels of anionic poly(ferrocenylsilane) strong polyelectrolytes, with controlled degrees of crosslinking, were obtained by "Click" chemistry. The formed networks showed a strong swelling in water and could be reversibly oxidized and reduced chemically and electrochemically. Chemical oxidation and reduction led to a reversible collapse and reswelling of the network. The redox activity of these hydrogels was exploited successfully for the in-situ formation of rather well-defined palladium nanoparticles.
Notes and references a
Department of Materials Science and Technology of Polymers, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands. † This work is supported by NanoNextNL, a micro and nanotechnology consortium of the Government of the Netherlands and 130 partners. Electronic Supplementary Information (ESI) available: Experimental procedures, moduli and swelling characteristics, cyclic voltammetry, nanoparticle size distribution, spectra. See DOI: 10.1039/c000000x/
Fig. 3 TEM images of Pd nanoparticles formed inside hydrogel 5 at various magnifications.
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COMMUNICATION 1998, 23, 1019; (c) N. A. Peppas, J. Z. Hilt, A. Khademhosseini and R. Langer, Adv. Mater., 2006, 18, 1345. (a) J. F. Mano, Adv. Eng. Mater., 2008, 10, 515; (b) S. H. Gehrke, Adv. Polym. Sci., 1993, 110, 81; (c) J. M. G. Swann and A. J. Ryan, Polym. Int., 2009, 58, 285; (d) S.-K. Ahn, R. M. Kasi, S.-C. Kim, N. Sharma and Y. Zhou, Soft Matter, 2008, 4, 1151; (e) M. A. Cohen Stuart, W. T. S. Huck, J. Genzer, M. Müller, C. Ober, M. Stamm, G. B. Sukhorukov, I. Szleifer, V. V. Tsukruk, M. Urban, F. Winnik, S. Zauscher, I. Luzinov and Sergiy Minko, Nat. Mater., 2010, 9, 101. (a) O. Franssen, L. Vandervennet, P. Roders and W. E. Hennink, J. Control. Release, 1999, 60, 211; (b) P. D. Thornton, R. J. Mart, S. J. Webb and R. V. Ulijn, Soft Matter, 2008, 4, 821. Y. Zhang, S. Kato and T. Anazawa, Smart Mater. Struct., 2007, 16, 2175. Z. Liu and P. Calvert, Adv. Mater., 2000, 12, 288. Q. Luo, S. Muthu, Y. B. Gianchandani, F. Svec and J. M. J. Fréchet, Electrophoresis, 2003, 24, 3694. (a) D. J. Beebe, J. S. Moore, J. M. Bauer, Q. Yu, R. H. Liu, C. Devadoss and B.-H. Jo, Nature, 2000, 404, 588; (b) H. R. Allcock and A. M. A. Ambrosio, Biomaterials, 1996, 17, 2295. H. Li, F. Lai and R. Luo, Langmuir, 2009, 25, 13142. Y. Osada, H. Okuzaki, and H. Hori, Nature 1992, 355, 242. G. Filipcsei, I. Csetneki, A. Szilágyi and M. Zrínyi, Adv. Polym. Sci., 2007, 206, 137. (a) G. Y. Zou and J. R. Li, Smart Mater. Struct., 2003, 12, 859; (b) A. M. Schmidt, Macromol. Rapid Commun., 2006, 27, 1168. F. Peng, G. Li, X. Liu, S. Wu and Z. Tong, J. Am. Chem. Soc., 2008, 130, 16166. X. Sui, X. Feng, M. A. Hempenius and G. J. Vancso, J. Mater. Chem. B, 2013, 1, 1658. (a) H.-Z. Bu, S. R. Mikkelsen and A. M. English, Anal. Chem., 1995, 67, 4071; (b) H.-Z. Bu, A. M. English and S. R. Mikkelsen, J. Phys. Chem. B, 1997, 101, 9593. T. Tatsuma, K. Takada, H. Matsui and N. Oyama, Macromolecules, 1994, 27, 6687. (a) R. Yoshida, T. Takahashi, T. Yamaguchi and H. Ichijo, J. Am. Chem. Soc., 1996, 118, 5134; (b) S. Maeda, Y. Hara, T. Sakai, R. Yoshida and S. Hashimoto, Adv. Mater., 2007, 19, 3480. K. Tsuchiya, Y. Orihara, Y. Kondo, N. Yoshino, T. Ohkubo, H. Sakai and M. Abe, J. Am. Chem. Soc., 2004, 126, 12282. L.-M. Low, S. Seetharaman, K.-Q. He and M. J. Madou, Sens. Actuators B, 2000, 67, 149. N. Mano, J. E. Yoo, J. Tarver, Y.-L. Loo and A. Heller, J. Am. Chem. Soc., 2007, 129, 7006. M. Nakahata, Y. Takashima, H. Yamaguchi and A. Harada, Nat. Commun., 2011, 2, Article number 511. (a) X. Sui, L. van Ingen, M. A. Hempenius and G. J. Vancso, Macromol. Rapid Commun., 2010, 31, 2059; (b) X. Sui, X. Feng, A. Di Luca, C. A. van Blitterswijk, L. Moroni, M. A. Hempenius and G. J. Vancso, Polym. Chem., 2013, 4, 337; (c) X. Sui, L. Shui, J. Cui, Y. Yanbo Xie, J. Song, A. van den Berg, M. A. Hempenius and G. J. Vancso, Chem. Commun., 2014, 50, 3058; (d) A. C. Arsenault, D. P. Puzzo, I. Manners and G. A. Ozin, Nature Photonics, 2007, 1, 468. For reviews on poly(ferrocenylsilanes) see: (a) G. R. Whittell and I. Manners, Adv. Mater., 2007, 19, 3439; (c) V. Bellas and M. Rehahn, Angew. Chem. Int. Ed., 2007, 46, 5082; (d) Frontiers in Transition
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ChemComm Accepted Manuscript
This article can be cited before page numbers have been issued, to do this please use: M. A. Hempenius, B. Zoetebier and G. J. Vancso, Chem. Commun., 2014, DOI: 10.1039/C4CC06988A.
This is an Accepted Manuscript, which has been through the Royal Society of Chemistry peer review process and has been accepted for publication. Accepted Manuscripts are published online shortly after acceptance, before technical editing, formatting and proof reading. Using this free service, authors can make their results available to the community, in citable form, before we publish the edited article. We will replace this Accepted Manuscript with the edited and formatted Advance Article as soon as it is available. You can find more information about Accepted Manuscripts in the Information for Authors. Please note that technical editing may introduce minor changes to the text and/or graphics, which may alter content. The journal’s standard Terms & Conditions and the Ethical guidelines still apply. In no event shall the Royal Society of Chemistry be held responsible for any errors or omissions in this Accepted Manuscript or any consequences arising from the use of any information it contains.
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DOI: 10.1039/C4CC06988A
Journal Name
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Cite this: DOI: 10.1039/x0xx00000x
Redox-responsive organometallic hydrogels for in-situ metal nanoparticle synthesis B. Zoetebier, M. A. Hempenius,* G. J. Vancsoa
Received 00th January 2012, Accepted 00th January 2012 DOI: 10.1039/x0xx00000x www.rsc.org/
A new class of redox active hydrogels composed of poly(ferrocenylsilane) polyanion and poly(ethylene glycol) chains was assembled, using a copper-free azide-alkyne Huisgen cycloaddition reaction. These organometallic hydrogels displayed reversible collapse and reswelling upon chemical oxidation and reduction, respectively, and formed relatively well-defined, unaggregated Pd(0) nanoparticles (8.2±2.2 nm) from K2PdCl4 salts. Stimulus-responsive gels are attracting much attention due to their interesting behavior as well as to many potential applications.1,2 Such gels can e.g. be used for controlled release of drugs or cosmetics, upon employing a specific stimulus.3 They can also be useful as actuators, artificial muscles or microfluidic valves.4,5 Typical stimuli used to date include changes in temperature,6 pH,7 ionic strength,8 electric field strength,9 light intensity,2d and variations in magnetic field strength.10,11 Gels responsive to redoxstimuli usually incorporate transition metals into network-forming polymer chains, either as main chain constituents or included in side groups, or are composed of (semi)conducting polymers.12,13 Their addressability by redox stimuli allows one to alter properties such as polarity, which generally has a large influence on polymer properties such as chain stiffness and extension, and on interactions with solvent. Redox responsiveness therefore is of interest for actuator applications. Until now, relatively few examples of transition-metalbased redox-active gels and hydrogels have appeared in the literature.13 For example, copolymerization of vinylferrocene with acrylamide and N,N'-methylenebis(acrylamide) led to a random copolymer gel with potential use as a glucose biosensor.14 A poly[(N-isopropylacrylamide)-co-vinylferrocene] gel was produced which displayed electrochemically and thermally controllable phase transitions.15 A self-oscillating redox-active gel was obtained by inducing the Belousov-Zhabotinsky (BZ) reaction within a ruthenium-bearing N-isopropylacrylamide gel.16 Other redoxresponsive gels reported in the literature include a ferrocenyl surfactant system showing controlled fluid viscoelasticity,17 and
This journal is © The Royal Society of Chemistry 2012
polyaniline-based gels as e.g. actuators for controlled release18 or catalysis.19 Self-healing, supramolecular materials using PAA modifed with β-cyclodextrin as a host polymer and PAA with ferrocene pendant groups as a guest polymer were also reported. By using chemical oxidation and reduction, a reversible sol–gel phase transition could be induced in this system.20 Organometallic polymers possessing skeletal transition metals show great promise as constituents of redox-responsive polymer gels.13,21 Among this class of organometallic polymers, poly(ferrocenylsilanes), PFS, are raising much interest due to the useful chemical, redox, optical, thermal, and magnetic properties of these materials.22,23A variety of polymerization methods have been developed, including living anionic ring-opening polymerization yielding homopolymers and block copolymers, transition metalcatalyzed ring-opening polymerization and photocontrolled living polymerization, broadening the range of accessible PFSs.22 While the crosslinking chemistries of poly(ferrocenylsilane) networks based on PFS chains soluble in regular organic solvents have been reported,21,24 few accounts have appeared on crosslinking approaches for water-soluble PFSs.25 We reported the first examples of metal nanoparticle fabrication mediated by main-chain redoxactive polymer hydrogels.21b,c PFS-based hydrogels were shown to provide a confined, reducing environment, allowing one to convert silver salts into defined, unaggregated metal nanoparticles without employing external reducing agents.21b,c,26 The production of metal nanoparticles often requires the use of organic ligands, salts, surfactants or other capping agents,27 which may significantly reduce their (catalytic) activity.27b Nanoparticle fabrication inside a polymer network circumvents these issues and suppresses excessive growth of the particles.28 Here we describe the synthesis of a PFS polyanion hydrogel, where crosslinks were formed between strained alkynes and azide groups in a copper-free Huisgen cycloaddition reaction,29 and its use in the in-situ formation of palladium nanoparticles. Palladium nanoparticles are highly important catalytic materials and continue to attract growing interest due to their catalytic properties
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combined with a high surface-area to volume ratio.27a,30 Palladium is used extensively in hydrogenations,31 oxidations,32 carbon–carbon bond formation,33 electrochemical reactions in fuel cells,34 hydrogen storage,35 and sensing applications.36 In order to form a polyanionic poly(ferrocenylsilane) hydrogel with controlled degrees of crosslinking, a PFS polyanion with tunable amounts of crosslinkable azide groups was synthesized by side group modification of PFS 1 (Scheme 1). Negatively charged side groups were introduced using the carbon nucleophile α-lithioisobutylethanesulfonate.39 Sulfonate ester carbanions enable one to attach alkanesulfonate ester moieties e.g. to polymer backbones which, after deprotection, are turned into alkanesulfonate functionalities. Sulfonate ester carbanions have been shown to react for instance with carbonyl groups or with alkyl halides. Alkyl iodides react quickly with these carbanions, corresponding chlorides however are much less reactive.37 The large difference in reactivity was exploited by forming PFS 1, which contains both chloropropyl and iodopropyl moieties, in a halogen exchange reaction starting from poly(ferrocenyl(3chloropropyl)methylsilane).39 Treatment of PFS 1 with αlithioisobutylethanesulfonate selectively displaced the iodo groups, giving PFS 2. Sulfonate ester deprotection and the introduction of azide groups by displacement of the unreacted chloro groups was conducted in a single step, yielding the targeted PFS 3.
Journal Name DOI: 10.1039/C4CC06988A completely remove MeOH, the gel was placed in Milli-Q water, which was replaced several times. Hydrogel 5 was subsequently freeze-dried for FTIR analysis.
H C
O
N
O 28
C
3
O
O
4
4 PEG PFS
5
Scheme 2 Formation of a PFS polyanion hydrogel 5 by a copperfree azide-alkyne Huisgen cycloaddition reaction between PFS 3 and BCN-terminated four-arm star PEG 4. Fig. 1 displays the FTIR spectra for PFS 3 and network 5. The absence of an azide stretching vibration absorption at 2094 cm−1 in the spectrum of hydrogel 5 provides evidence for completion of the click reaction. Furthermore, the spectrum displays characteristic peaks of both PFS (774, 1035 cm−1), PEG (1098 cm−1) and a carbonyl stretch absorption of the tetra-arm PEG at 1710 cm−1.
Fig. 1 FTIR spectra of PFS 3 and network 5, showing the disappearance of the azide stretching vibration at 2094 cm−1 as the network is formed. Scheme 1 Synthesis of a crosslinkable poly(ferrocenylsilane) polyanion. PFS 1 was converted into a PFS featuring isobutylsulfonate side groups (2) using α-lithioisobutylethanesulfonate. Treatment of PFS 2 with NaN3 gave the water soluble PFS 3. Polyanion 3 and bicyclononyne (BCN) derivatized tetra-arm PEG solutions in MeOH were mixed, employing equimolar amounts of BCN and azide moieties in the gel-forming mixture (Scheme 2). The viscosity of the solution increased rapidly and gel formation was confirmed by the vial tilting method already within 2 minutes after mixing 3 and 4. When water was used as a medium for crosslinking, gelation occurred within 30 seconds, thus providing less time for mixing. Gelation was allowed to proceed overnight. In order to
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Upon swelling in deionized water, the anionic PFS network 5 became an amber transparent elastic hydrogel. Water-uptake and rheological measurements were performed from which the effective crosslink density could be calculated (Table S1).
Fig. 2 Response of hydrogel 5 to chemical oxidation (50 mM Fe(ClO4)3) and reduction (50 mM sodium ascorbate).
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Journal Name The hydrogel could be reversibly oxidized and reduced electrochemically. For cyclic voltammetry, gels were built up layerby-layer from PFS 3 and PEG 4 on a gold substrate, which served as working electrode, using an azidobutyl thioacetate self-assembled monolayer40 for surface anchoring. Cyclic voltammograms obtained after swelling of network 5 in aqueous NaClO4 showed the two oxidation and reduction waves typical of poly(ferrocenylsilanes) at Eox1 = 324 mV and Eox2 = 464 mV, Ered1 = 397 mV and Ered2 = 208 mV, respectively (Fig. S1). Ferrocene units at alternating positions are oxidized first, followed by the oxidation of the ferrocene units located between ferrocenium sites at a higher potential.23 Peak currents showed a square root dependence on scan rate, indicating that the electrochemical processes are diffusion controlled (Fig. S2). Upon chemical oxidation, the hydrogel changed color from amber to green-blue and turned back to amber upon reduction (Fig. 2). Oxidation of the anionic hydrogel was accompanied by a clear change in mechanical behavior: the gel collapsed and lost its elastic nature (Fig. S3). Upon reduction the network reswelled and regained its elasticity. The observed collapse, shown in Fig. 2, can be ascribed to electrostatic attraction between the positively charged ferrocenium units in the PFS main chain and the negatively charged sulfonate side groups of the polymer. The redox activity of the hydrogel was employed for the in-situ formation of palladium nanoparticles. Standard potentials for the reduction of noble metal salts such as H2AuCl4 (1.002 V), K2PdCl4 (0.915 V) and K2PtCl4 (1.188 V) to the corresponding metals indicate that PFSs may be used as reducing agents. When hydrogel 5 was immersed in a solution of potassium tetrachloropalladate (5 mM), it turned from orange to black within minutes, after which the reaction was left overnight in the dark. The hydrogel was washed with Milli-Q water to remove unreacted salts and was freeze-dried for analysis.
COMMUNICATION DOI: 10.1039/C4CC06988A TEM measurements show well-dispersed palladium nanoparticles inside the gel, with a narrow size distribution (Fig. 3). Images captured over larger areas demonstrate the uniformity and lack of aggregation of the formed nanoparticles. The dimensions and size distribution of the Pd nanoparticles (Fig. S4) was evaluated by measuring the diameter of 250 particles by TEM, yielding a diameter of (8.2±2.2 nm). EDX was used to confirm the chemical nature of the particles and the hydrogel (Fig. 4, S5). Diffraction patterns obtained from the particles proved that Pd(0) nanoparticles were produced. Au and Pt metal nanoparticles could also be produced from the mentioned salts using hydrogel 5. Experiments employing the Pd nanoparticles in the model reduction of 4-nitrophenol to 4aminophenol are in progress.
Fig. 4 EDX spectrum of the formed Pd nanoparticles. The Cu signals in the spectrum originate from the TEM grid.
Conclusions In summary, hydrogels of anionic poly(ferrocenylsilane) strong polyelectrolytes, with controlled degrees of crosslinking, were obtained by "Click" chemistry. The formed networks showed a strong swelling in water and could be reversibly oxidized and reduced chemically and electrochemically. Chemical oxidation and reduction led to a reversible collapse and reswelling of the network. The redox activity of these hydrogels was exploited successfully for the in-situ formation of rather well-defined palladium nanoparticles.
Notes and references a
Department of Materials Science and Technology of Polymers, MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands. † This work is supported by NanoNextNL, a micro and nanotechnology consortium of the Government of the Netherlands and 130 partners. Electronic Supplementary Information (ESI) available: Experimental procedures, moduli and swelling characteristics, cyclic voltammetry, nanoparticle size distribution, spectra. See DOI: 10.1039/c000000x/
Fig. 3 TEM images of Pd nanoparticles formed inside hydrogel 5 at various magnifications.
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