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Cite this: Chem. Commun., 2014, 50, 1447 Received 15th November 2013, Accepted 26th November 2013
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Tunable, luminescent, and self-healing hybrid hydrogels of polyoxometalates and triblock copolymers based on electrostatic assembly† Haibing Wei,a Shuming Du,a Yang Liu,a Hongxiang Zhao,a Chongyi Chen,b Zhibo Li,b Jun Lin,c Yang Zhang,c Jie Zhang*a and Xinhua Wan*a
DOI: 10.1039/c3cc48732f www.rsc.org/chemcomm
Hybrid hydrogels based on electrostatic co-assembly of polyoxometalates and ABA triblock copolymers were readily prepared and exhibit excellent luminescence and self-healing performance.
Self-healing materials, which possess the ability to repair themselves after damage, are emerging as a novel class of smart materials.1 The supramolecular assembly through non-covalent bonds, such as p–p stacking, hydrogen bonding, and host–guest interactions, is considered to be the most promising way to construct healable materials, owing to their intrinsic and autonomic self-healing nature.2–6 Inspired by supramolecular self-healable hybrid nanocomposites in nature, such as nacre and bones, recent advances have resulted in synthetic organic–inorganic healable materials consisting of two constituents at the nanometer or molecular level, which not only resemble natural healable materials, but also can be endowed with exceptional integrated functions and tunable supramolecular adaptive characteristics.7,8 A remarkable high-modulus supramolecular self-healing hydrogel prepared using inorganic clay with a dendritic molecular binder at high water content can immobilize enzymes and transport biological catalytic activities.7 Incorporation of Fe3O4 nanoparticles into PEG-grafted chitosan produced a magnetic self-healing hydrogel.8 Polyoxometalates (POMs), as a type of nanosized inorganic transition-metal oxide clusters, possess diverse functionalities in catalytic, photoelectrical, magnetic, and biological fields.9 Introducing POMs into the self-healing materials would be promising to extend the application scope of POM materials. For instance, selfhealing POM hydrogels could be injectable materials, i.e. the broken
hydrogel fragments after injection can in situ reform an integral gel at the target position under physiological conditions, which may result in enhanced delivery efficiency of POM medical cargos or low toxicity and high luminescence efficiency for bio-imaging in the presence of POM luminescent probes.10 However, as far as we know, there still is no report on the POM-based self-healing materials, due to challenges in exploration of facile supramolecular approaches to hybridize POMs into the self-healable materials. Recently, we have demonstrated that self-assembly is a powerful technique for constructing hierarchical POM nanomaterials with enhanced luminescence by co-assembly of Na9EuW10O36 (EuW10) with diblock copolymers.11 Herein, we report a facile method to prepare self-healable supramolecular luminescent hydrogels from macroanionic POMs and cationic-neutral-cationic ABA triblock copolymers. The supramolecular strategy is illustrated in Fig. 1a. We envisioned that the micelle networks should be constructed, driven by the electrostatic interaction between the cationic A block and the anionic EuW10. To test the aforementioned idea, we first synthesized a series of novel ABA triblock copolymers poly(2-(2-guanidinoethoxy)ethyl methacrylate)-b-poly(ethylene oxide)-b-poly(2-(2-guanidinoethoxy)ethyl methacrylate) (PGn-b-PEO230-b-PGn; Scheme 1) with various block lengths of cationic PG segments with polydispersity indices
a
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China. E-mail: [email protected], [email protected] b Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China c Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China † Electronic supplementary information (ESI) available: Syntheses and characterization of monomers and polymers, and supplementary figures and videos. See DOI: 10.1039/c3cc48732f
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Fig. 1 (a) Schematic representation of the formation of hybrid hydrogels based on charge-driven co-assembly. Cryo-TEM micrographs of hybrid hydrogels prepared from (b) PG9-b-PEO230-b-PG9 and (c) PG34-bPEO230-b-PG34. Insets show magnified images.
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Scheme 1
Synthetic route to ABA triblock copolymers.
of 1.18–1.25 and n = 6–34. Scheme 1 shows the synthetic route to copolymers, and all synthesis procedures are described in the ESI.† As the guanidinium ion is a strong electrolyte with a pKa value of B12.5, the copolymers are completely positively charged in water (pH = 7). The aqueous solutions of PGn-b-PEO230-b-PGn and EuW10 were mixed together under vigorous stirring at a charge stoichiometric point and appropriate solid concentrations above 5.0 wt%, and the hydrogels were formed spontaneously (Fig. S3, ESI†). Interestingly, the minimum gelation concentration (MGC) of the hydrogels is highly dependent on the length of the positively charged PG blocks of the copolymers. For the copolymer PGn-b-PEO230-b-PGn (n Z 9), the MGC of the hybrid hydrogels decreases with an increased length of the cationic PG blocks, while the copolymer, PG6-b-PEO230-b-PG6, did not cause gelation when mixed with the EuW10 solution even at a high concentration of 20 wt%. Cryogenic transmission electron microscopy (cryo-TEM) (Fig. 1b and c) reveals that the hydrogels are composed of well-separated isotropic micelles with a small size distribution. The micelles of hybrid hydrogels from PG34-b-PEO230-b-PG34 have an average radius of B10 nm, and those of PG9-b-PEO230b-PG9 with a shorter PG chain have a smaller average radius of less than 5 nm. Furthermore, a generalized indirect fourier transformation12 analysis of small-angle X-ray scattering (SAXS) measurement of the PG34-b-PEO230-b-PG34–EuW10 complex in aqueous dispersion indicates that the scattering objects, the micellar cores, have a globular, almost spherical, shape with a Rg value of about 10 nm (Fig. S4, ESI†), in good agreement with the cryo-TEM results. We reasonably postulate the possible structural model of the cross-linked network as shown in Fig. 1a: the visible micelles constitute the flower-like core–shell micelles with the dense complex coacervate core of PG–EuW10 and the folded PEO corona, whereas the extended PEO linkers which are invisible under cryogenic TEM because of their relatively low electron density connect adjacent micelles to afford the cross-linked network. The hydrogel exhibits a well linear viscoelastic response with the storage modulus G 0 greater than the loss modulus G00 at a fixed frequency (o = 2 rad s1) before the critical strain region (g = 2.0%), confirming its gel state (Fig. S5, ESI†). At a fixed solid concentration (15 wt%), G 0 of the hydrogel increased by a magnitude more than 26-fold when the PG block length increased from
1448 | Chem. Commun., 2014, 50, 1447--1450
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Fig. 2 Rheological properties (25 1C) of hydrogels. (a) G 0 values of PGn-bPEO230-b-PGn–EuW10 hydrogels on a frequency sweep and (b) G 0 values of PG20-PEO230-PG20–EuW10 hydrogels at various salt concentrations on a frequency sweep. All hydrogel samples were prepared at a concentration of 15 wt%.
9 to 34 (Fig. 2a). In accordance with the cryo-TEM results, as the longer cationic PG blocks participate in complex coacervation, the micellar core becomes larger, resulting in a higher crosslinking density and thereby a higher mechanical strength. The G 0 value of the toughest hydrogels derived from PG34PEO230-PG34–EuW10, is 3.2 104 Pa, which is one order higher than hydrogels based on oppositely charged organic triblock copolyelectrolytes under similar conditions.13 Furthermore, to examine a dynamic response to the ionic strength, the hydrogels were exposed to additional NaCl salt at a concentration of 0–1.6 M. The mechanical properties of the hybrid hydrogel (Fig. 2b) remain intact as the NaCl concentration increases to 0.4 M, demonstrating its high stability. Even if the NaCl concentration increases to 1.6 M, the hydrogel network could be maintained well while the G 0 value decreases to about 50%. These results suggest that the hydrogels are quite stable against ionic strengths, even though the interaction between PG and EuW10 within the coacervate domains was gradually weakened by increased NaCl screening. In our hybrid hydrogels, we envisioned that the dynamic nature of electrostatic interactions which drive the formation of coacervate core micelles should afford self-healing capacity. To evaluate our speculation, a hydrogel based on PG34-b-PEO230b-PG34–EuW10 was moulded into a free-standing column. The self-healing test was carried out by cutting the column-shaped self-standing hydrogel into three parts with a razor blade, and then the fresh surfaces contacted together through light pressing. After about 3 minutes, the three pieces healed to a single one and the scars almost disappeared. The resulting hydrogel was strong enough to hold itself when suspended horizontally between two posts (Fig. 3) and slantwise (Movie S1, ESI†), demonstrating its high self-healing efficiency. Furthermore, the self-healing feature of the hydrogel was also confirmed by rheological measurements (Fig. 3c). After a large-amplitude force (g = 50%, o = 2 rad s1) was applied, the G 0 value decreased from 30 kPa to 0.4 kPa and the tan d (tan d = G00 |G 0 ) value increased from 0.15 to 4.0, indicative of the collapse of the hydrogel to a quasi-liquid state. By the removal of the large strain, G 0 fully recovered to its initial value (30 kPa) within 20 s and returned to its original gel state. Moreover, the recovery behavior was repeated for three cycles without any distinct degradation. This fast recovery observed in our hybrid hydrogels
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Fig. 3 Photographs of self-healing behavior of a free-standing, luminescent hydrogel under (a) visible light and (b) ultraviolet illumination (lex = 254 nm). (c) G 0 and G00 values of the hydrogel in continuous step strain measurements. (o = 2 rad s1, g = 50% (600 s) - 0.8% (600 s) - 50% (600 s) - 0.8% (600 s) 50% (600 s) - 0.8% (1200 s).)
Table 1 Summary of photophysical properties of the EuW10 crystal and the hydrogels based on EuW10 and copolymers with diverse block lengths
Lifetimes and amplitudesb Samples
Copolymer
jPLa
Gel-1 Gel-2 Gel-3 Gel-4 Gel-5 EuW10 EuW10 (aq)
PG9-PEO230-PG9 PG13-PEO230-PG13 PG20-PEO230-PG20 PG25-PEO230-PG25 PG34-PEO230-PG34 — —
15.5 16.7 18.5 19.2 22.3 26.6 0.51
(%) t1 (ms)/f1 3.67/0.908 3.85/0.930 4.07/0.931 3.96/0.916 4.16/0.941 3.06/1.000 1.79/1.000
t2 (ms)/f2
qc
0.76/0.092 0.99/0.070 1.17/0.069 0.84/0.084 1.16/0.059 — —
— — 0.09 0.10 — 0.02 0.39
a
Apart from determining 8 wt% EuW10 (aq) by the relative method with tryptophan as a standard, quantum yields were measured by an integrating sphere (lex = 280 nm). b The fluorescence decay intensity I(t) as a function of time t was fitted by the equation consisting of two exponential terms, I(t) = f1 exp(t/t1) + f2 exp(t/t2), where the amplitude of each lifetime component fi represents the fractional contribution of the fluorophore in a specific environment to the total fluorescence decay. c q is the number of water molecules coordinated to Eu3+ in the micellar core of the hydrogels associated with the long lifetime t1.
may be attributed to the disassociation and reformation of the coacervate core arising from quickly responsive and nondirectional nature of electrostatic attraction. It is known that in aqueous solutions, coordination with water leads to a severe quenching of the EuW10 luminescence, owing to radiationless deactivation of the 5D0 excited state through coupling with O–H vibrations.14 In the present work, the hydrogels show the typical emission spectra of EuW10 (Fig. S6, ESI†), similar to the EuW10 crystals, and the EuW10 emission of hybrid hydrogels in an aqueous milieu was greatly enhanced (Table 1). The absolute photoluminescence quantum yields of the hydrogels increase from 15.5% to 22.3% as the PG block lengths of the copolymers increase from 9 to 34, independent of the concentration and ionic strength (Table S2 and S3, ESI†). The hybrid hydrogels exhibit a high quantum yield of 22.3% which is close to that of the EuW10 crystals (26.6%). According to the time-resolved fluorescent measurements, all hybrid hydrogels display double-exponential emission decays (Fig. S7, ESI†). As summarized in Table 1, hybrid hydrogels have a long lifetime (t1 E 3.67–4.12 ms) with a dominant fraction of f1 > 0.9 and a short lifetime (t2 E 0.76– 1.17 ms) with a much lower fraction of f2 o 0.1. Similar to our previous reports,11 the longer one can be attributed to EuW10
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species located in the hydrophobic core of hybrid micelles, while the shorter one, which is comparable to 1.79 ms of EuW10 in 8 wt% aqueous solution, could be associated with the free EuW10 molecules outside the micelles. It is noteworthy that the decay lifetime t1 of the hydrogels was even longer than that of the EuW10 crystal (3.1 ms), which is unusual among EuW10 hybrid materials. Meanwhile, the number of water molecules (q) coordinated to Eu3+ in aqueous solutions can be estimated by eqn (1): q = 1.05 (1/tH2O 1/tD2O)
(1)
where 1/tH2O and 1/tD2O are the reciprocal experimental lifetimes in H2O and D2O solutions in ms1, respectively.15 For luminescent EuW10 species with super long lifetimes in hydrogels, the q values are less than 0.1, suggesting that there is almost no water molecules coordinated to the Eu3+ ion. Therefore, the luminophore EuW10 confined within the hydrophobic cores effectively shielded the coordination with water molecules, contributing to the enhanced luminescence and prolonged decay lifetime. In summary, we reported the facile preparation of hybrid luminescent hydrogels by assembling triblock copolymers with polyoxometalates through electrostatic interactions in an aqueous solution. The mechanical strength of the hydrogel is exceptionally high and can be easily tuned by altering the copolymer composition, concentrations, and ionic strengths. The hydrogels exhibit excellent self-healable properties. Owing to the hydrophobic nature of the coacervate core, the hybrid hydrogels exhibit significantly enhanced emission with comparably high quantum yields to the EuW10 crystals. Such a novel modular assembly strategy as well as the diversity of POMs offers us an opportunity in the realms of catalytic, electrochromic hybrid gels, etc. This work was supported by the NNSFC (No. 51003001, No. 51373001, No. 21322404, No. 20834001), the Beijing Natural Science Foundation (No. 2122024), and the Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, CIAC, CAS (No. 110000R041).
Notes and references 1 (a) G. L. Fiore, S. J. Rowan and C. Weder, Chem. Soc. Rev., 2013, 42, 7278; (b) S. R. White, N. R. Sottos, P. H. Geubelle, J. S. Moore, M. R. Kessler, S. R. Sriram, E. N. Brown and S. Viswanathan, Nature, 2001, 409, 794; (c) X. X. Chen, M. A. Dam, K. Ono, A. Mal, H. B. Shen, S. R. Nutt, K. Sheran and F. Wudl, Science, 2002, 295, 1698; (d) Y. Amamoto, H. Otsuka, A. Takahara and K. Matyjaszewski, Adv. Mater., 2012, 24, 3975. 2 K. Imato, M. Nishihara, T. Kanehara, Y. Amamoto, A. Takahara and H. Otsuka, Angew. Chem., Int. Ed., 2012, 51, 1138. 3 S. Burattini, H. M. Colquhoun, J. D. Fox, D. Friedmann, B. W. Greenland, P. J. F. Harris, W. Hayes, M. E. Mackay and S. J. Rowan, Chem. Commun., 2009, 6717. 4 Y. Chen, A. M. Kushner, G. A. Williams and Z. Guan, Nat. Chem., 2012, 4, 467. 5 M. Nakahata, Y. Takashima, H. Yamaguchi and A. Harada, Nat. Commun., 2011, 2, 511. 6 M. Zhang, D. Xu, X. Yan, J. Chen, S. Dong, B. Zheng and F. Huang, Angew. Chem., Int. Ed., 2012, 51, 7011. 7 Q. Wang, J. L. Mynar, M. Yoshida, E. Lee, M. Lee, K. Okuro, K. Kinbara and T. Aida, Nature, 2010, 463, 339. 8 Y. Zhang, B. Yang, X. Zhang, L. Xu, L. Tao, S. Li and Y. Wei, Chem. Commun., 2012, 48, 9305.
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11 12 13 14 15
E. Pereira, Eur. J. Inorg. Chem., 2013, 2877; (c) H. K. Yang, Y. X. Cheng, M. M. Su, Y. Xiao, M. B. Hu, W. Wang and Q. Wang, Bioorg. Med. Chem. Lett., 2013, 23, 1462. J. Zhang, Y. Liu, Y. Li, H. X. Zhao and X. H. Wan, Angew. Chem., Int. Ed., 2012, 51, 4598. B. Weyerich, J. Brunner-Popela and O. Glatter, J. Appl. Crystallogr., 1999, 32, 197. J. N. Hunt, K. E. Feldman, N. A. Lynd, J. Deek, L. M. Campos, J. M. Spruell, B. M. Hernandez, E. J. Kramer and C. J. Hawker, Adv. Mater., 2011, 23, 2327. M. Sugeta and T. Yamase, Bull. Chem. Soc. Jpn., 1993, 66, 444. W. D. Horrocks and D. R. Sudnick, Acc. Chem. Res., 1981, 14, 384.
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Cite this: Chem. Commun., 2014, 50, 1447 Received 15th November 2013, Accepted 26th November 2013
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Tunable, luminescent, and self-healing hybrid hydrogels of polyoxometalates and triblock copolymers based on electrostatic assembly† Haibing Wei,a Shuming Du,a Yang Liu,a Hongxiang Zhao,a Chongyi Chen,b Zhibo Li,b Jun Lin,c Yang Zhang,c Jie Zhang*a and Xinhua Wan*a
DOI: 10.1039/c3cc48732f www.rsc.org/chemcomm
Hybrid hydrogels based on electrostatic co-assembly of polyoxometalates and ABA triblock copolymers were readily prepared and exhibit excellent luminescence and self-healing performance.
Self-healing materials, which possess the ability to repair themselves after damage, are emerging as a novel class of smart materials.1 The supramolecular assembly through non-covalent bonds, such as p–p stacking, hydrogen bonding, and host–guest interactions, is considered to be the most promising way to construct healable materials, owing to their intrinsic and autonomic self-healing nature.2–6 Inspired by supramolecular self-healable hybrid nanocomposites in nature, such as nacre and bones, recent advances have resulted in synthetic organic–inorganic healable materials consisting of two constituents at the nanometer or molecular level, which not only resemble natural healable materials, but also can be endowed with exceptional integrated functions and tunable supramolecular adaptive characteristics.7,8 A remarkable high-modulus supramolecular self-healing hydrogel prepared using inorganic clay with a dendritic molecular binder at high water content can immobilize enzymes and transport biological catalytic activities.7 Incorporation of Fe3O4 nanoparticles into PEG-grafted chitosan produced a magnetic self-healing hydrogel.8 Polyoxometalates (POMs), as a type of nanosized inorganic transition-metal oxide clusters, possess diverse functionalities in catalytic, photoelectrical, magnetic, and biological fields.9 Introducing POMs into the self-healing materials would be promising to extend the application scope of POM materials. For instance, selfhealing POM hydrogels could be injectable materials, i.e. the broken
hydrogel fragments after injection can in situ reform an integral gel at the target position under physiological conditions, which may result in enhanced delivery efficiency of POM medical cargos or low toxicity and high luminescence efficiency for bio-imaging in the presence of POM luminescent probes.10 However, as far as we know, there still is no report on the POM-based self-healing materials, due to challenges in exploration of facile supramolecular approaches to hybridize POMs into the self-healable materials. Recently, we have demonstrated that self-assembly is a powerful technique for constructing hierarchical POM nanomaterials with enhanced luminescence by co-assembly of Na9EuW10O36 (EuW10) with diblock copolymers.11 Herein, we report a facile method to prepare self-healable supramolecular luminescent hydrogels from macroanionic POMs and cationic-neutral-cationic ABA triblock copolymers. The supramolecular strategy is illustrated in Fig. 1a. We envisioned that the micelle networks should be constructed, driven by the electrostatic interaction between the cationic A block and the anionic EuW10. To test the aforementioned idea, we first synthesized a series of novel ABA triblock copolymers poly(2-(2-guanidinoethoxy)ethyl methacrylate)-b-poly(ethylene oxide)-b-poly(2-(2-guanidinoethoxy)ethyl methacrylate) (PGn-b-PEO230-b-PGn; Scheme 1) with various block lengths of cationic PG segments with polydispersity indices
a
Beijing National Laboratory for Molecular Sciences, Key Laboratory of Polymer Chemistry and Physics of Ministry of Education, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, China. E-mail: [email protected], [email protected] b Institute of Chemistry, Chinese Academy of Sciences, Beijing 100190, China c Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, China † Electronic supplementary information (ESI) available: Syntheses and characterization of monomers and polymers, and supplementary figures and videos. See DOI: 10.1039/c3cc48732f
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Fig. 1 (a) Schematic representation of the formation of hybrid hydrogels based on charge-driven co-assembly. Cryo-TEM micrographs of hybrid hydrogels prepared from (b) PG9-b-PEO230-b-PG9 and (c) PG34-bPEO230-b-PG34. Insets show magnified images.
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Scheme 1
Synthetic route to ABA triblock copolymers.
of 1.18–1.25 and n = 6–34. Scheme 1 shows the synthetic route to copolymers, and all synthesis procedures are described in the ESI.† As the guanidinium ion is a strong electrolyte with a pKa value of B12.5, the copolymers are completely positively charged in water (pH = 7). The aqueous solutions of PGn-b-PEO230-b-PGn and EuW10 were mixed together under vigorous stirring at a charge stoichiometric point and appropriate solid concentrations above 5.0 wt%, and the hydrogels were formed spontaneously (Fig. S3, ESI†). Interestingly, the minimum gelation concentration (MGC) of the hydrogels is highly dependent on the length of the positively charged PG blocks of the copolymers. For the copolymer PGn-b-PEO230-b-PGn (n Z 9), the MGC of the hybrid hydrogels decreases with an increased length of the cationic PG blocks, while the copolymer, PG6-b-PEO230-b-PG6, did not cause gelation when mixed with the EuW10 solution even at a high concentration of 20 wt%. Cryogenic transmission electron microscopy (cryo-TEM) (Fig. 1b and c) reveals that the hydrogels are composed of well-separated isotropic micelles with a small size distribution. The micelles of hybrid hydrogels from PG34-b-PEO230-b-PG34 have an average radius of B10 nm, and those of PG9-b-PEO230b-PG9 with a shorter PG chain have a smaller average radius of less than 5 nm. Furthermore, a generalized indirect fourier transformation12 analysis of small-angle X-ray scattering (SAXS) measurement of the PG34-b-PEO230-b-PG34–EuW10 complex in aqueous dispersion indicates that the scattering objects, the micellar cores, have a globular, almost spherical, shape with a Rg value of about 10 nm (Fig. S4, ESI†), in good agreement with the cryo-TEM results. We reasonably postulate the possible structural model of the cross-linked network as shown in Fig. 1a: the visible micelles constitute the flower-like core–shell micelles with the dense complex coacervate core of PG–EuW10 and the folded PEO corona, whereas the extended PEO linkers which are invisible under cryogenic TEM because of their relatively low electron density connect adjacent micelles to afford the cross-linked network. The hydrogel exhibits a well linear viscoelastic response with the storage modulus G 0 greater than the loss modulus G00 at a fixed frequency (o = 2 rad s1) before the critical strain region (g = 2.0%), confirming its gel state (Fig. S5, ESI†). At a fixed solid concentration (15 wt%), G 0 of the hydrogel increased by a magnitude more than 26-fold when the PG block length increased from
1448 | Chem. Commun., 2014, 50, 1447--1450
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Fig. 2 Rheological properties (25 1C) of hydrogels. (a) G 0 values of PGn-bPEO230-b-PGn–EuW10 hydrogels on a frequency sweep and (b) G 0 values of PG20-PEO230-PG20–EuW10 hydrogels at various salt concentrations on a frequency sweep. All hydrogel samples were prepared at a concentration of 15 wt%.
9 to 34 (Fig. 2a). In accordance with the cryo-TEM results, as the longer cationic PG blocks participate in complex coacervation, the micellar core becomes larger, resulting in a higher crosslinking density and thereby a higher mechanical strength. The G 0 value of the toughest hydrogels derived from PG34PEO230-PG34–EuW10, is 3.2 104 Pa, which is one order higher than hydrogels based on oppositely charged organic triblock copolyelectrolytes under similar conditions.13 Furthermore, to examine a dynamic response to the ionic strength, the hydrogels were exposed to additional NaCl salt at a concentration of 0–1.6 M. The mechanical properties of the hybrid hydrogel (Fig. 2b) remain intact as the NaCl concentration increases to 0.4 M, demonstrating its high stability. Even if the NaCl concentration increases to 1.6 M, the hydrogel network could be maintained well while the G 0 value decreases to about 50%. These results suggest that the hydrogels are quite stable against ionic strengths, even though the interaction between PG and EuW10 within the coacervate domains was gradually weakened by increased NaCl screening. In our hybrid hydrogels, we envisioned that the dynamic nature of electrostatic interactions which drive the formation of coacervate core micelles should afford self-healing capacity. To evaluate our speculation, a hydrogel based on PG34-b-PEO230b-PG34–EuW10 was moulded into a free-standing column. The self-healing test was carried out by cutting the column-shaped self-standing hydrogel into three parts with a razor blade, and then the fresh surfaces contacted together through light pressing. After about 3 minutes, the three pieces healed to a single one and the scars almost disappeared. The resulting hydrogel was strong enough to hold itself when suspended horizontally between two posts (Fig. 3) and slantwise (Movie S1, ESI†), demonstrating its high self-healing efficiency. Furthermore, the self-healing feature of the hydrogel was also confirmed by rheological measurements (Fig. 3c). After a large-amplitude force (g = 50%, o = 2 rad s1) was applied, the G 0 value decreased from 30 kPa to 0.4 kPa and the tan d (tan d = G00 |G 0 ) value increased from 0.15 to 4.0, indicative of the collapse of the hydrogel to a quasi-liquid state. By the removal of the large strain, G 0 fully recovered to its initial value (30 kPa) within 20 s and returned to its original gel state. Moreover, the recovery behavior was repeated for three cycles without any distinct degradation. This fast recovery observed in our hybrid hydrogels
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Fig. 3 Photographs of self-healing behavior of a free-standing, luminescent hydrogel under (a) visible light and (b) ultraviolet illumination (lex = 254 nm). (c) G 0 and G00 values of the hydrogel in continuous step strain measurements. (o = 2 rad s1, g = 50% (600 s) - 0.8% (600 s) - 50% (600 s) - 0.8% (600 s) 50% (600 s) - 0.8% (1200 s).)
Table 1 Summary of photophysical properties of the EuW10 crystal and the hydrogels based on EuW10 and copolymers with diverse block lengths
Lifetimes and amplitudesb Samples
Copolymer
jPLa
Gel-1 Gel-2 Gel-3 Gel-4 Gel-5 EuW10 EuW10 (aq)
PG9-PEO230-PG9 PG13-PEO230-PG13 PG20-PEO230-PG20 PG25-PEO230-PG25 PG34-PEO230-PG34 — —
15.5 16.7 18.5 19.2 22.3 26.6 0.51
(%) t1 (ms)/f1 3.67/0.908 3.85/0.930 4.07/0.931 3.96/0.916 4.16/0.941 3.06/1.000 1.79/1.000
t2 (ms)/f2
qc
0.76/0.092 0.99/0.070 1.17/0.069 0.84/0.084 1.16/0.059 — —
— — 0.09 0.10 — 0.02 0.39
a
Apart from determining 8 wt% EuW10 (aq) by the relative method with tryptophan as a standard, quantum yields were measured by an integrating sphere (lex = 280 nm). b The fluorescence decay intensity I(t) as a function of time t was fitted by the equation consisting of two exponential terms, I(t) = f1 exp(t/t1) + f2 exp(t/t2), where the amplitude of each lifetime component fi represents the fractional contribution of the fluorophore in a specific environment to the total fluorescence decay. c q is the number of water molecules coordinated to Eu3+ in the micellar core of the hydrogels associated with the long lifetime t1.
may be attributed to the disassociation and reformation of the coacervate core arising from quickly responsive and nondirectional nature of electrostatic attraction. It is known that in aqueous solutions, coordination with water leads to a severe quenching of the EuW10 luminescence, owing to radiationless deactivation of the 5D0 excited state through coupling with O–H vibrations.14 In the present work, the hydrogels show the typical emission spectra of EuW10 (Fig. S6, ESI†), similar to the EuW10 crystals, and the EuW10 emission of hybrid hydrogels in an aqueous milieu was greatly enhanced (Table 1). The absolute photoluminescence quantum yields of the hydrogels increase from 15.5% to 22.3% as the PG block lengths of the copolymers increase from 9 to 34, independent of the concentration and ionic strength (Table S2 and S3, ESI†). The hybrid hydrogels exhibit a high quantum yield of 22.3% which is close to that of the EuW10 crystals (26.6%). According to the time-resolved fluorescent measurements, all hybrid hydrogels display double-exponential emission decays (Fig. S7, ESI†). As summarized in Table 1, hybrid hydrogels have a long lifetime (t1 E 3.67–4.12 ms) with a dominant fraction of f1 > 0.9 and a short lifetime (t2 E 0.76– 1.17 ms) with a much lower fraction of f2 o 0.1. Similar to our previous reports,11 the longer one can be attributed to EuW10
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species located in the hydrophobic core of hybrid micelles, while the shorter one, which is comparable to 1.79 ms of EuW10 in 8 wt% aqueous solution, could be associated with the free EuW10 molecules outside the micelles. It is noteworthy that the decay lifetime t1 of the hydrogels was even longer than that of the EuW10 crystal (3.1 ms), which is unusual among EuW10 hybrid materials. Meanwhile, the number of water molecules (q) coordinated to Eu3+ in aqueous solutions can be estimated by eqn (1): q = 1.05 (1/tH2O 1/tD2O)
(1)
where 1/tH2O and 1/tD2O are the reciprocal experimental lifetimes in H2O and D2O solutions in ms1, respectively.15 For luminescent EuW10 species with super long lifetimes in hydrogels, the q values are less than 0.1, suggesting that there is almost no water molecules coordinated to the Eu3+ ion. Therefore, the luminophore EuW10 confined within the hydrophobic cores effectively shielded the coordination with water molecules, contributing to the enhanced luminescence and prolonged decay lifetime. In summary, we reported the facile preparation of hybrid luminescent hydrogels by assembling triblock copolymers with polyoxometalates through electrostatic interactions in an aqueous solution. The mechanical strength of the hydrogel is exceptionally high and can be easily tuned by altering the copolymer composition, concentrations, and ionic strengths. The hydrogels exhibit excellent self-healable properties. Owing to the hydrophobic nature of the coacervate core, the hybrid hydrogels exhibit significantly enhanced emission with comparably high quantum yields to the EuW10 crystals. Such a novel modular assembly strategy as well as the diversity of POMs offers us an opportunity in the realms of catalytic, electrochromic hybrid gels, etc. This work was supported by the NNSFC (No. 51003001, No. 51373001, No. 21322404, No. 20834001), the Beijing Natural Science Foundation (No. 2122024), and the Open Research Fund of State Key Laboratory of Polymer Physics and Chemistry, CIAC, CAS (No. 110000R041).
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