Rapid formation of highly stretchable and notch-insensitive hydrogels

Abstract

Highly stretchable and notch-insensitive hydrogels were rapidly prepared using redox initiators. The hydrogels had a large number of non-covalent crosslinking points, which come from hydrophobic side chains and metal ion coordination without any chemical crosslinking agents. The non-covalent crosslinking points in the hydrogels were stochastic and dynamic, compared to the covalent crosslinking points. When the notched sample was stretched, the hydrophobic segments would contribute to elongation of the hydrogels via molecular stretching. When the hydrogels had a notch, new crosslinking points could be formed via metal ion coordination, even if some previous metal ion–ligand bonds were destroyed. As a result, the sample notch was blunted, widened and merged into the edge gradually under tensile stress. The hydrogels could be elongated up to 12 times their original length and show notch-insensitive properties. Moreover, the metal ion coordination interaction between the amide groups and Fe³⁺ was proved using Fourier transform infrared spectroscopy (FTIR). The rheological properties and internal morphology of the hydrogels were also measured with different dosages of hydrophobic segments and metal ions. Thus, it was envisioned that the non-covalent bonds would make it possible to create hydrogels with more unexpected properties.

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Introduction

Hydrogels, comprising a considerable amount of water and three-dimensional polymer networks, are used as smart materials for biomedical applications in sensors,^1,2 artificial muscles,^3–5 drug delivery carriers,^6,7 wound dressings,^8,9 and model extracellular matrices.¹⁰ Moreover, other scientists have focused on the applications of hydrogels from biomedicine to catalysis,¹¹ adsorption,^12,13 electrochemistry,^14,15 oil–water separation^16,17 and so on. However, the poor mechanical properties of hydrogels significantly limit the application range of hydrogels. For example, pure polyacrylamide hydrogels with N,N′-methylenebisacrylamide as a chemical crosslinker exhibited a brittle fracture under the action of external forces. Therefore, many efforts have been focused on exploring new theories for improving the mechanical strength of hydrogels. These tough hydrogels include slide-ring hydrogels,^18,19 double network hydrogels,^20,21 organic–inorganic nano-composite gels,^22,23 macromolecular microsphere composite hydrogels,^24,25 multi-armed hydrogels^26,27 and hydrophobic association hydrogels.^28–30 Although the mechanical strength of these hydrogels was significantly enhanced in compression or tensile tests, the hydrogels were disrupted facilely along the cracks and the strength decreased rapidly if there was a little notch or flaw in the bulk. As a result, the notch sensitivity of hydrogels obviously reduces their sustainability and reliability for practical applications. Therefore, notch-insensitive tough hydrogels have attracted great attention. Suo et al. reported an alginate–polyacrylamide hybrid hydrogel, which was formed by the combination of weak ionic effects and strong covalent crosslinking.³¹ Tong et al. reported notch-insensitive nano-composite hydrogels, derived from the introduction of more hydrophilic acrylamide segments into clay-crosslinked copolymers.³² Moreover, Chen et al. reported a tough and notch-insensitive magnetic hydrogel which had dispersed alginate-coated Fe₃O₄ nanoparticles in the interpenetrating polymer networks of alginate and polyacrylamide.³³ These advances could greatly expand the application range of hydrogels. However, the rapid formation of notch-insensitive hydrophobic association hydrogels has barely been reported.

Here, we report the rapid formation of highly stretchable and notch-insensitive hydrogels using redox initiation systems (Fe²⁺/K₂S₂O₈). The hydrogels were prepared by free radical copolymerization of acrylamide (AAm) and hydrophobic vinyl groups. Sodium chloride and sodium dodecyl sulfate were added to increase the solubility of the hydrophobic groups.²⁹ The crosslinking points of the hydrogels were attributed to the combined effects of hydrophobic association and metal ion coordination. Moreover, the hydrogen bonds between amide groups in the hydrogels³⁴ and the entanglement of molecular chains were also conducive to crosslinking. These crosslinking points were dynamic, compared to covalently crosslinking hydrogels. The hydrogels were formed immediately after adding ferrous chloride and exhibited fantastic notch-insensitive and excellent mechanical properties. Subsequently, the hydrogels were measured using tensile tests, rheological tests, and scanning electron microscopy (SEM) to investigate the influence of different concentrations of FeCl₂ and hydrophobic groups on their mechanical properties.

Materials and methods

Materials

Acrylamide (99.0%), N,N′-methylenebisacrylamide (97%), sodium dodecyl sulfate (SDS, ≥97%), potassium persulfate (99.5%), ferrous chloride tetrahydrate (≥97%), methyl methacrylate (99.0%), butyl methacrylate (99.5%), sodium chloride (99.5%), and lauryl methacrylate (96%) were supplied by Aladdin (Shanghai, China). Hexadecyl methacrylate (95%) was supplied by Zhejiang Kangde New Materials Co., Ltd, China. Deionized water (18.2 Ω cm resistivity at 25 °C) was used in the experiment.

The hydrophobic association hydrogels without metal ion coordination

The hydrophobically modified polyacrylamide hydrogels were prepared by dissolving NaCl (0.32 g), and SDS (0.8 g) at room temperature in deionized water (10 mL) with constant stirring. After obtaining a transparent solution, hexadecyl methacrylate (0.3 g) was added into the solution and stirred for 3 h to make the hydrophobic groups disperse completely. When the hydrophobic groups were fully dissolved, KPS (0.01 g) and AAm (2 g) were added into the beaker and stirred until completely dissolved. Then, the solution was kept at 70 °C for 3 h and the hydrophobic association polyacrylamide hydrogel was formed.

The highly stretchable and notch-insensitive hydrophobic association hydrogels

The highly stretchable and notch-insensitive hydrophobic association hydrogels were synthesized by free radical copolymerization. In order to describe the synthesis process clearly, we give details for the preparation of the hydrogels with hexadecyl methacrylate (15 wt% in AAm) and FeCl₂ (15 wt% in AAm):

Firstly, NaCl (0.32 g), SDS (0.8 g) and deionized water (10 mL) were added into a beaker and stirred for 2 h at room temperature to obtain a transparent solution. After that, hexadecyl methacrylate (0.3 g) was added into the solution and stirred for 3 h to ensure the hexadecyl methacrylate had dispersed completely. Then KPS (0.01 g) and AAm (2 g) were added into the beaker and stirred until completely dissolved. Finally FeCl₂ (0.007 g) was added into the solution to obtain the hydrogels immediately. The resulting samples would be analysed using the techniques listed below. All the following components of the samples were fixed: NaCl (0.32 g), SDS (0.8 g), deionized water (10 mL), and KPS (0.01 g).

Chemical structure by FTIR

Fourier transform infrared spectroscopy (FTIR) was used to identify the metal ion coordination interaction between the amide groups and Fe³⁺. The samples of the PAAm hydrogel and the PAAm-co-Fe³⁺ hydrogel (the PAAm-co-Fe³⁺ hydrogel was obtained by immersing the PAAm hydrogel in a saturated solution of ferric chloride for 4 hours) with a weight of 1.5 mg were ground and dispersed in potassium bromide (KBr), followed by compression to form the sheet. FTIR spectra were obtained in the wave number range from 400 cm⁻¹ to 4000 cm⁻¹.

Mechanical properties

In order to explore the mechanical properties of the hydrogels, tensile tests on the hydrogels were carried out using a mechanical tester (SHIMADZU, model AGS-X, 100N, Japan). According to ASTM D638, dumbbell-shaped tensile samples were fixed with two tensile clamps at each end and elongated at a tensile rate of 40 mm min⁻¹. The tensile strength of each sample was analysed at least three times. Moreover, the samples were cut to 50% of their original width for the notch tensile test.

Rheological measurement

Frequency sweep measurements were used to investigate the viscoelastic properties of the hydrogels. All the samples were tested using a rheometer (AR 2000ex TA America) with a parallel plate geometry (25 mm diameter rotating top plate), and the plate-to-plate distance was 3 mm. The storage modulus (G′) and loss modulus (G′′) were recorded over the frequency range of 0.1–100 rad s⁻¹ at 25 °C.

Morphology observation

The internal morphology of the hydrogels was observed using a Scanning Electron Microscope (SEM) (JSM 6510). The samples were first swollen for 12 h and freeze-dried in a freeze vacuum drier (FDU-2110, EYELA). Before measurement with the SEM, all samples were sputtered with platinum. The magnification factor was 500 times.

Results and discussion

Design of highly stretchable and notch-insensitive hydrophobic association hydrogels

In this investigation, a redox initiation system (Fe²⁺/K₂S₂O₈) was employed to initiate the polymerization process of the hydrogels. Compared to traditional thermal initiation systems, the redox initiation systems will immediately release a large number of free radicals at low temperature and enhance the formation velocity of the hydrogels (Movie S1 in ESI†).³⁵ Moreover, with the increase in the concentration of FeCl₂, the formation time of the hydrogels decreased obviously. When the concentration of FeCl₂ in AAm was 0.55%, the formation time was only 8 s at 25 °C. In contrast, the formation time of the hydrogels required several hours or even dozens of hours via other traditional thermal initiation systems (Table 1).

Table 1 The formation time of hydrogels by the redox initiation system and other traditional thermal initiation systems^a

FeCl2 (wt% in AAm)	Temperature (°C)	Forming time
a The rapid formation hydrogels in the table contained water (10 mL), NaCl (0.32 g), SDS (0.8 g), AAm (2 g) and hexadecyl methacrylate (15 wt% in AAm).
0.25	25	19 s
0.35	25	13 s
0.45	25	10 s
0.55	25	8 s
—	25	24 h (ref. 28)
—	50	12 h (ref. 30)
—	65	6 h (ref. 36)
—	70	3 h (ref. 25)

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During polymerization, hydrophobic alkyl segments were also introduced into the side chains of PAAm, which self-assembled into micelles as crosslinking points and the hydrophobic segments could contribute to elongation of the hydrogels via molecular stretching.²⁸ Moreover, the Fe³⁺ ions, which were derived from KPS (K₂S₂O₈) and FeCl₂ as redox initiators, would be coordinated with the polyacrylamide (PAAm) as crosslinking points due to the complex interaction between metal ions with unoccupied orbitals and electron-rich amide groups. The hydrophobic association hydrogels exhibited notch-sensitivity when the metal ions were removed from the hydrogels (Fig. 1A). Also, the hydrogels were not formed with only metal ions due to the weak effect of metal ion coordination (Fig. 1B). However, the hydrogels could be efficiently toughened owing to the combined effects of hydrophobic association and metal ion coordination. Moreover, hydrogen bonding between the amide groups in the hydrogels³⁴ and the entanglement of molecular chains was also conducive to the crosslinking of hydrogels.

Fig. 1 (A) The images for notch stretching of the hydrophobically modified PAAm hydrogels without the metal ion coordination (15 wt% hexadecyl groups in AAm); (B) the acrylamide solution with ferrous chloride (0.35 wt% FeCl₂ in AAm); (C) the schematic illustration of the highly stretchable and notch-insensitive hydrophobic association hydrogels.

In order to demonstrate the metal ion coordination interaction between the amide groups and Fe³⁺, the FTIR spectra of PAAm hydrogels and PAAm-co-Fe³⁺ hydrogels were obtained and are shown in Fig. 2. The spectrum of the PAAm-co-Fe³⁺ hydrogels exhibited a bending vibration peak at 1571.03 cm⁻¹ for N–H and a stretching vibration peak at 1640.41 cm⁻¹ for C=O, compared with the spectrum for PAAm hydrogels. This indicates that metal ion coordination interaction occurs between amide groups and Fe³⁺.

Fig. 2 FTIR spectra of PAAm hydrogels and PAAm-co-Fe³⁺ hydrogels.

When the hydrogels with a notch were stretched in the tensile tests, the notch was gradually blunted, widened and merged into the edge. As a result, the hydrogels with a notch weren’t disrupted even when they were stretched to similar lengths as the original hydrogels, shown in Fig. 3. The highly stretchable and notch-insensitive properties of the hydrogels were attributed to the effects of hydrophobic association and metal ion coordination. The non C–C covalent crosslinking PAAm hydrogels, different from C–C covalent crosslinking hydrogels, had metal ion coordination and hydrophobic association as the temporary and dynamic crosslinking points. When the hydrogels were drawn in the tensile tests, the curly alkyl chains would slide and extend along with the deformation of micelles until the molecular chains were disrupted. When the sample with a notch was stretched, the metal ions could form new crosslinking points with surrounding amide groups of polyacrylamide even though some previous metal ion ligand bonds were destroyed. As a result, the hydrogels via the effect of non C–C covalent bonding had highly stretchable and notch-insensitive properties. Even if there were several notches in the sample, the hydrogels still had high tensile strength.

Fig. 3 The images for notch stretching of PAAm hydrogels by the effects of hydrophobic association and metal ion coordination (0.35 wt% FeCl₂ in AAm, 15 wt% hexadecyl groups in AAm).

Effect of ferrous chloride (FeCl₂) on the properties of hydrogels

In this investigation, KPS and FeCl₂ were used as a couple of redox initiators. Fe²⁺ would reduce the activation energy of KPS and release a large number of free radicals at room temperature, initiating monomers to form hydrogels immediately. Subsequently the transformed Fe³⁺, which came from Fe²⁺ in the redox initiator, could coordinate with PAAm as a dynamic crosslinking point for the hydrogels. Therefore, the properties of the hydrogels were significantly influenced by different concentrations of FeCl₂. It was found that when the concentration of FeCl₂ in acrylamide was lower than 0.25%, the hydrogels could not be formed due to the lower concentration of free radicals produced by the initiator. However, the hydrogels exhibited the maximum tensile strength when the concentration of FeCl₂ was approximately 0.25%. With the concentration of Fe²⁺ increased, the hydrogels exhibited poor mechanical properties in the stress–strain curves. When the concentration of FeCl₂ in AAm was 0.55%, the elongation was only 8 times and the sample was disrupted after the tensile stress reached 26 kPa (Fig. 4A), and the tensile stress of hydrogels with a notch exhibited the same trend (Fig. 4B). But the notch insensitive properties of hydrogels were enhanced with the increasing concentration of FeCl₂ (Fig. 4C). The high concentration of Fe²⁺ ions produced the high concentration of free radicals which were released immediately for redox initiation in the reaction systems. The high concentration of free radicals would initiate the fast polymerization of AAm, subsequently, resulting in the short molecular chain for PAAm. The short PAAm segments could easily move and rearrange, and the high concentration of metal ions could enhanced the notch-insensitivity of hydrogels due to dynamic coordination effects. However, the short PAAm segments did not have the ability to curl over and entangle, inducing a decrease in the toughness of the hydrogels.

Fig. 4 The mechanical properties of hydrogels with different concentrations of FeCl₂ (the mass fraction is calculated by FeCl₂/AAm × 100%): (A) the stress–strain curves of hydrogels without a notch; (B) the tensile stress of hydrogels with a notch. All samples had 15 wt% hexadecyl methacrylate groups in AAm; (C) the notch-insensitive properties of hydrogels (the notch-insensitive properties are the elongation of hydrogels with a notch/the elongation of hydrogels without a notch × 100%). All samples had 15 wt% hexadecyl methacrylate groups in AAm.

The viscoelasticity of the hydrogels with different concentrations of FeCl₂ is shown in Fig. 6. It was found that the hydrogels with higher concentrations of FeCl₂ had a lower storage modulus (G′), despite little difference in their loss modulus (G′′) (Fig. 5A). For the reaction systems with low concentrations of FeCl₂, the molecular weight of PAAm became high. That is, the longer molecular chains increased the entanglement between different chains as additional crosslinking, leading to better elastic properties of the hydrogels and higher storage modulus (G′). Moreover, the loss factor (tan δ) of hydrogels exhibited a trend of decline with an increase of shear frequency (Fig. 5B). This may be due to the inability of the movement rate of the molecular chains to keep up with the shear frequency increasing.

Fig. 5 The rheological curves of hydrogels with different concentrations of FeCl₂ (the mass fraction was calculated by FeCl₂/AAm × 100%): (A) storage moduli G′ and loss moduli G′′; (B) the loss factor (tan δ = G′′/G′). All samples had 15 wt% hexadecyl methacrylate groups in AAm.

Fig. 6 The SEM images of hydrogels with different concentrations of FeCl₂ (the mass fraction was calculated by FeCl₂/AAm × 100%): (A) 0.25%; (B) 0.35%; (C) 0.45%; (D) 0.55%. All samples had 15 wt% hexadecyl methacrylate groups in AAm.

The microscopic morphology of the hydrogels with different concentrations of FeCl₂ is exhibited in Fig. 6. With the increase of FeCl₂ in the hydrogels, the thickness of the cavity walls became thinner, suggesting that the hydrogels were easier to stretch. As a result, the hydrogels with 0.55% FeCl₂ exhibited the poorest fracture elongation performance in the tensile test but had the best notch insensitive properties. Besides, an interesting phenomenon was found; there were some fibers appearing on the surface of the cavities when the concentration of FeCl₂ was lower than 0.35% in the hydrogels. The fiber structure could significantly enhance the toughness of the hydrogels.³⁷

Effect of hydrophobic groups on the properties of hydrogels

Hydrophobic association interactions act as one of the important toughening mechanisms for hydrogels. Okay had discussed that some factors which could affect the mechanical properties of hydrophobic association hydrogels, such as the concentrations of NaCl, SDS and hydrophobic groups, etc.²⁹ In our investigation, the different types and concentrations of alkyl segments were also considered to affect the mechanical properties of PAAm hydrogels. Fig. 7A shows the stress–strain curves for hydrogels with different hydrophobic alkyl groups. When the lengths of the alkyl segments were shorter, the formed micelles were weaker crosslinking points and more easily disrupted. For example, the PAAm with methyl side groups couldn't form hydrogels, as demonstrated in the inserted image in Fig. 7A. With the increase of the length of the alkyl side chains, PAAm with butyl groups could form hydrogels, but the mechanical strength was so weak that the sample couldn't be tested in the tensile test. Only when the length of the alkyl groups was enough long that the hydrophobic segments could form stable micelles as crosslinking points, could the hydrogels display excellent mechanical strength in the tensile test. As can be seen, the hydrogels with hexadecyl groups exhibited the greatest elongation, breaking at 13 times their original length and possessing a tensile strength of 320 kPa, larger than the hydrogels with lauryl groups. Although the longer length of the alkyl segments enhanced the mechanical properties of the hydrogels (Fig. 7B), it reduced the notch-insensitive properties of the hydrogels, as shown in Fig. 7C. This may be because of the limited movement of molecular chains due to the close entanglement of hexadecyl groups, resulting in the PAAm main chains being rigid. As a result, the rearrangement ability of molecular chains becomes poor and the hydrogels were easy to break when the samples had a gap. Fig. 7D and E show the stress–strain curves of the hydrogels with different concentrations of hydrophobic groups. When the concentration of hydrophobic groups was 5% in the hydrogels, the elongation was only 9.5 times and the sample disrupted at even less than 25 kPa. With the concentration of the hexadecyl groups increasing, however, the mechanical properties of the hydrogels were enhanced obviously. When the concentration of hexadecyl groups was 20%, the fracture strength could reach 400 kPa. When the concentration of hydrophobic groups was low, the hydrophobic groups in the micelles were entangled weakly and easily disconnected under the tensile conditions. With the increasing number of hydrophobic groups, the molecular entanglements were in closer proximity in the micelles and this means that the accumulated molecules need more external force to separate under the same elongation conditions. Also, the notch-insensitive properties of hydrogels with different concentrations of hydrophobic groups is shown in Fig. 7F. It shows that the notch-insensitive properties of hydrogels decreased due to the close entanglement of the hexadecyl groups with the concentration of the hexadecyl groups increasing. Moreover, the sample with 10% hydrophobic groups exhibited better notch-insensitive properties compared with the sample with 5%, which may be because the low concentration of hexadecyl groups could not provide effective crosslinking in the hydrogels.

Fig. 7 (A) The stress–strain curves of hydrogels with different hydrophobic alkyl groups (without the notch), (a) lauryl groups and (b) hexadecyl groups (the inserted images were PAAm with methyl groups and hexadecyl groups, respectively) (the hydrophobic group was 15 wt% in AAm); (B) the tensile stress of hydrogels with different hydrophobic alkyl groups (have the notch); (C) the notch-insensitive properties of hydrogels with different hydrophobic alkyl groups (the notch-insensitive properties are the elongation of hydrogels with a notch/the elongation of hydrogels without a notch × 100%); (D) the stress–strain curves of hydrogels with different concentrations of hexadecyl groups (without the notch) (the mass fraction was calculated by hexadecyl methacrylate/AAm × 100%); (E) the tensile stress of hydrogels with different concentrations of hexadecyl groups (have the notch); (F) the notch-insensitive properties of hydrogels with different concentrations of hexadecyl groups. All samples had the 0.35 wt% FeCl₂ in AAm.

The viscoelastic behavior of hydrogels with lauryl and hexadecyl groups, and with different concentrations of hexadecyl groups as side chains was also investigated and are shown in Fig. 8. Although the tensile properties of the hydrogels with the hexadecyl groups were better than those of the hydrogels with the lauryl groups, the loss modulus (G′′) of the hydrogels with lauryl groups was lower than that of the hydrogels with hexadecyl groups at a low shear frequency and higher at a shear frequency over 40 rad s⁻¹ (Fig. 8A), and the loss factor (tan δ) exhibits a similar trend in Fig. 8B. It may be due to the length of the lauryl groups being shorter than that of the hexadecyl groups, and the lauryl hydrogels had less frictional resistance at a shear frequency lower than 40 rad s⁻¹, leading to a lower loss modulus (G′′) and loss factor (tan δ). When studying the viscoelastic properties of hydrogels with lauryl groups, the curve of the loss modulus (G′′) firstly decreased and then increased, indicating that the accumulated lauryl groups in the micelles may become disentangled partially at a shear frequency of over 16 rad s⁻¹. As a result, the lauryl chains could move more flexibly and the loss modulus (G′′) became higher. The viscoelastic results for the hydrogels with different concentrations of hexadecyl groups revealed that the hydrogels had a higher storage modulus (G′) and lower loss modulus (G′′) (Fig. 8C and D) when the hydrophobic groups were tangled more closely with each other in micelles. This may be due to the more rigid polyacrylamide main chains leading to weakened hysteresis, resulting in a decrease of internal friction when the molecules moved.

Fig. 8 (A) Storage moduli G′ and loss moduli G′′ of hydrogels with different hydrophobic alkyl groups; (B) the loss factor (tan δ = G′′/G′) of hydrogels with different hydrophobic alkyl groups. The hydrophobic group of (A) and (B) was 15 wt% in AAm; (C) the storage moduli G′ and loss moduli G′′ of hydrogels with different concentrations of hydrophobic groups (the mass fraction was calculated by hexadecyl methacrylate/AAm × 100%); (D) the loss factor (tan δ = G′′/G′) of hydrogels with different concentrations of hydrophobic groups. All samples had 0.35 wt% FeCl₂ in acrylamide.

To observe the internal morphology, a scanning electron microscope (SEM) was utilized to measure the fracture surface of the hydrogels. As can be seen from Fig. 9, the SEM images showed the morphology of the hydrogels with different concentrations of hexadecyl groups from 5% to 20%. With the increase of hydrophobic segments in the hydrogels, the thickness of the cavity walls increased. It also deserves to be mentioned that fibers were also observed on the surface of the cavities when the concentration of the hexadecyl groups was higher than 15%.

Fig. 9 The SEM images of hydrogels with different concentrations of hydrophobic groups (the mass fraction is calculated by hexadecyl methacrylate/AAm × 100%): (A) 5%; (B) 10%; (C) 15%; (D) 20%. All samples had 0.35 wt% FeCl₂ in AAm.

Conclusions

In conclusion, we successfully realized the rapid formation of hydrophobic association hydrogels by using a redox initiation system. It was found that the crosslinking points of hydrogels, which were derived from hydrophobic association and metal ion coordination, were stochastic and dynamic. As a result, the hydrogels could be elongated up to 12 times their original length and show notch-insensitive properties. The test results indicate that when the crosslinking points of hydrogels were rigid or the molecular chain of hydrogels was longer, the mechanical properties and elasticity of the hydrogels were better. Moreover, we found that the structure of fiber-attached cavities could significantly enhance the mechanical properties of the hydrogels. It was envisioned that more interactions, including hydrogen bonds, coulombic forces, electrostatic effects and so on, would make it possible to design hydrogels with more unexpected properties (self-healing, biological glue…).

Acknowledgements

This research was supported by a grant from the National Natural Science Foundation of China (NSFC) (No. 51473023 and 51103014).

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra27306d

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