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. 2019 Dec 2;5(1):344–349. doi: 10.1021/acsomega.9b02785

Reversible Transformation of a Supramolecular Hydrogel by Redox Switching of Methylene Blue—A Noncovalent Chain Stopper

Suneesh C Karunakaran 1,*, Brian J Cafferty 1, Kyan S Jain 1, Gary B Schuster 1, Nicholas V Hud 1
PMCID: PMC6964268  PMID: 31956781

Abstract

graphic file with name ao9b02785_0008.jpg

The simple and reversible control of the degree of polymerization, and thereby the bulk material properties, of a supramolecular polymer is reported. Noncovalent capping agents (chain stoppers) modulate the length of supramolecular polymers by stacking on the surfaces of the polymer’s ends. Methylene blue (MB) is a positively charged, planar polycyclic dye that acts as a chain stopper. It can be reversibly switched between its colored, planar, cationic state and a colorless, nonplanar, neutral state (leucomethylene blue, LMB) by reduction with ascorbic acid and then reoxidized to MB by O2. LMB does not act as a chain stopper. This behavior was utilized to reversibly trigger the gel to sol transformation of supramolecular polymers formed by the self-assembly of hexameric rosettes comprising 2,4,6-triaminopyrimidine and a hexanoic acid-substituted cyanuric acid (CyCo6) in aqueous media. The results of our experiments highlight the ability of this approach to reversibly switch between the gel and solution states of materials formed from supramolecular polymers and thereby control their bulk properties.

Introduction

Supramolecular polymers are highly responsive to environmental stimuli as a result of reversible—and dynamic—noncovalent interactions between their monomers.1−5 Materials formed from supramolecular polymers, in particular hydrogels, that can be switched between their solution and gel phases are of interest for biomedical applications such as drug delivery and cellular growth scaffolding.6−9 A number of controlling factors have been exploited to modulate the structure and assembly of supramolecular polymers including changes in temperature, pH, ion binding, and the presence of enzymes.10−17,36 Practical applications of these switchable materials typically require specially designed and chemically complex monomers that, for example, incorporate ligand-binding sites.6,11,14,18 We report herein that the gels formed by supramolecular polymers in aqueous solution from 2,4,6-triaminopyridine (TAP) and a hexanoic acid-substituted cyanuric acid (CyCo6) (Figure 1A) can be easily and reversibly switched by coupling with redox reactions of methylene blue (MB).

Figure 1.

Figure 1

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(A) Structures of TAP and CyCo6 within the hexad assembly formed by H-bonding of three of each monomer and the proposed mechanism of reversible transformation of short and long fibers in the presence of MB and LMB, respectively. (B) Reversible reduction of MB to LMB and a side view of the energy-minimized structures showing MB and LMB as planar and bent, respectively. The geometry optimizations of MB and LMB structures were performed at the long-range corrected ωB97XD/6-31G(d,p) level of theory; omega was kept at the default value of 0.2. All calculations were performed in the Gaussian09.D01 package.

TAP and CyCo6 assemble to form hydrogen-bonded hexad structures (rosettes) that stack into linear, noncovalent hydrogel-forming supramolecular polymers (Figure 1A).10,19,20 We previously reported that positively charged planar heterocyclic molecules can act as noncovalent chain stoppers. These molecules can be used to control the length of TAP–CyCo6 supramolecular polymers and thus the bulk properties of their hydrogels.21 The chain stoppers previously examined could not be reversibly switched by external chemical or physical means.22−25

It is well known that MB exists in two stable oxidation states: a colored, planar cationic dye (MB) and a colorless, electrically neutral, bent structure, leucomethylene blue (LMB)26−28 (Figure 1B). The reversible reduction of MB has previously been used to switch stimuli responsive materials.29,30 This application takes advantage of the relatively slow reduction of MB to LMB by organic reductants and its much faster reversal by O2.31 We utilized a large excess of ascorbic acid (AA) to reduce MB to LMB.26 This reaction is readily reversed by exposure of LMB solutions to air as O2 rapidly oxidizes LMB to MB and in the absence of a metal catalyst, the oxidation of AA by O2 proceeds at an immeasurably slow rate.32

Results and Discussion

Noncovalent Chain Termination Using MB

At concentrations above 15 mM, the minimum assembly concentration (MAC), in 200 mM phosphate buffer solution, equimolar mixtures of TAP and CyCo6 form gels of very long supramolecular polymers (>10 μm). Detailed analyses of the stability of TAP–CyCo6 supramolecular hydrogels and the role of various chain stopper molecules in controlling the length and viscoelastic properties of these hydrogels were previously reported.10,21 The effect of MB on these TAP–CyCo6 gels (30 mM TAP–CyCo6) was investigated by the vial inversion test, nuclear magnetic resonance (NMR) spectroscopy, atomic force microscopy (AFM), and dynamic light scattering (DLS) techniques. The vial inversion test is a simple way to observe sol–gel transitions. A gel does not flow when the vial containing it is inverted, whereas a solution does. Addition of MB to gelled solutions of TAP–CyCo6 results in gradual loss of elastic strength and fluidizing of the system as the concentration of MB is increased to ca. 1 mM (Figure 2A). AFM imaging was used to confirm the chain termination of TAP–CyCo6 assemblies with MB. In the absence of MB, AFM images reveal micron length linear fibers of TAP–CyCo6 with heights of approximately 2 nm (Figure 2B). The addition of MB results in a concentration-dependent reduction of the length of these supramolecular polymers (Figure 2C).

Figure 2.

Figure 2

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(A) Inverted vial test illustrating the influence of MB (1 mM) on the bulk viscoelastic properties of a solution of 30 mM TAP–CyCo6 supramolecular assemblies. (B,C) AFM images of TAP–CyCo6 assemblies without and with 0.25 mM MB, respectively. All samples were prepared in 200 mM phosphate buffer at pH 7.

1H NMR spectroscopy was used to determine the MAC of the TAP–CyCo6 supramolecular assemblies in the absence and presence of MB. The 1H NMR resonances of unassembled monomers are present in the spectra, but monomers incorporated into the supramolecular polymers are “invisible” because their slow tumbling results in their resonances being broadened to baseline.20,21,331H NMR measurements of TAP–CyCo6 assemblies in the absence and presence of MB indicate that the MAC (15 mM for each TAP and CyCo6 monomers at 20 °C in this case) is unchanged by the addition of MB (Figure S1, Supporting Information). This shows that the number of assembled TAP and CyCo6 monomers is not affected by the presence of MB, indicating that MB shortens the length of TAP–CyCo6 supramolecular polymers without affecting the total number of assembled monomers.

DLS reveals the effect of MB on the dynamic viscosity of TAP–CyCo6 polymers. The square electric field autocorrelation was measured at 20 °C as a function of the correlation time for solutions containing 20 mM TAP–CyCo6 at various concentrations of MB. At MB concentrations up to 0.25 mM, the autocorrelation function was found to change from a clear two-step function that indicates two well-defined dynamic time scales to a single-step function and faster decay (Figure 3). These results show that the supramolecular polymers are shortened by addition of MB in a concentration dependent manner and suggest that MB binds to and reduces the length of TAP–CyCo6 supramolecular polymers in accord with the mechanism that was previously reported for noncovalent chain stoppers.21

Figure 3.

Figure 3

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Electric field autocorrelation functions for a TAP–CyCo6 sample (20 mM in each monomer) in the absence and presence of various concentrations of MB (0–0.25 mM).

The interaction of MB with TAP–CyCo6 supramolecular polymers was investigated by absorption spectroscopy. Figure 4A shows the absorption spectra of 0.25 mM of MB in buffer solution and in a solution containing 30 mM TAP–CyCo6 (15 mM free monomers). The absorption spectrum of MB in phosphate buffer (distinct peaks at 606, 662 nm) and in the presence of TAP–CyCo6 assembly (peaks at 619, 650 nm) differ and neither corresponds to its spectrum in methanol (Figure S2, Supporting Information), which shows a single peak at 654 nm corresponding to monomeric MB. MB is known to aggregate in aqueous solution, which accounts for the observed changes in the absorption spectra.34 When the concentration of MB in the phosphate buffer is increased from 0.062 to 3.75 mM (Figure 4B), the amount of absorption attributed to aggregates with respect to the monomer increases gradually and then reaches a plateau at ca. 1 mM (Figure 4D). Significantly, the absorption spectrum of MB in the presence of TAP–CyCo6 shows only negligible changes in the relative intensity of aggregates to the monomer as the MB concentration is increased (Figure 4C,D), which is consistent with the proposed dynamic equilibrium between MB-capped TAP–CyCo6 rosettes and free monomeric TAP and CyCo6 in solution. Increasing the concentration of MB in the presence of TAP–CyCo6 leads to the formation of a greater number of shorter fibers as a result of MB binding to the surface of TAP–CyCo6 rosettes and thus competing with the self-aggregation of MB.

Figure 4.

Figure 4

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(A) Absorption spectra of 0.25 mM MB alone (blue) and with 30 mM TAP–CyCo6 (red) (15 mM assembled monomers). Concentration-dependent absorption spectral changes of 0.25 mM MB (B) alone and (C) with TAP–CyCO6 assemblies. (D) Relative changes in absorption of 0.25 mM MB monomer and aggregate peaks alone (red) and with TAP–CyCo6 assembly (black), plot showing the stabilization of MB monomers by TAP–CyCo6 assembly. All solutions are buffered with phosphate at pH 7.

The inhibition of MB aggregation by TAP–CyCo6 was confirmed by temperature-dependent absorption studies. The absorption spectrum of a 0.25 mM solution of MB in 200 mM phosphate buffer shows an increase in the monomer peak at 650 nm as the temperature is increased (Figure 5A). In contrast, increasing the temperature of a sample containing 0.25 mM of MB and 30 mM of TAP–CyCo6 results in no change to the MB spectrum up to 25 °C. Above 25 °C, the MB absorption band at 650 nm corresponding to the monomer begins to increase with an accompanying bathochromic shift to 666 nm. The monomer band continues to grow in intensity up to 35 °C, beyond which there is no change in the spectrum (Figure 5B). The relative changes in intensity of the MB aggregate peak at 664 nm is shown in Figure 5C. These results indicate that at elevated temperature, where the gel is unstable, MB is released into the solution. This finding supports the conclusion that MB aggregation is inhibited by association with TAP–CyCo6 monomers and supramolecular polymers.

Figure 5.

Figure 5

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Changes in the absorption spectra with temperature of 0.25 mM MB in (A) 200 mM phosphate buffer at pH 7 and (B) 30 mM TAP–CyCo6 assembly. (C) Relative changes in absorbance with temperature of 0.25 mM MB at 664 nm in phosphate buffer (black) and in the presence of 30 mM TAP–CyCo6 assembly (red).

Reversible Reduction of MB to LMB

MB behaves as a noncovalent chain stopper, in part, because it is positively charged and planar. LMB is nonplanar and neutral, and for these reasons, it is not expected to behave as a noncovalent chain stopper. We examined the redox reactions of AA and O2 in the presence of TAP–CyCo6 assemblies on MB and LMB. Addition of 50 mM of AA to a solution containing 0.25 mM MB (a 200-fold excess of AA compared to MB) and 30 mM of TAP–CyCo6 assemblies results in the nearly complete loss of the MB absorption signal (Figure 6A). The MB spectrum reappears at almost its previous intensity when the solution is purged with O2 for 5 min.

Figure 6.

Figure 6

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Reversible gel to sol transformation of TAP–CyCo6 with MB, AA, and O2 (A) Absorption spectra of 0.25 mM MB in 30 mM TAP–CyCo6 (black), after adding 25 mM of AA (red), and after purging with O2 (blue). (B) Plot showing the reversibility of redox switching. The process was repeated for 11 cycles. (C) Vial inversion test of solutions of 30 mM of TAP–CyCo6 alone (gel state), with 0.25 mM of MB (solution state), after adding AA (gel sate), and after purging with O2 (solution state).

Gel to Sol Transformation of Supramolecular Polymer by Redox Switching of MB and LMB

The reversible conversion of MB to LMB causes the reversible transition of the TAP–CyCo6 gel to a solution. The vial inversion test was used to observe sol–gel transitions (Figure 6C). This test was performed on vials containing solutions of 30 mM TAP–CyCo6 alone, those containing TAP–CyCo6 and 0.25 mM MB before and after the addition 50 mM AA, and finally after purging the AA-containing vials with O2 for 5 min. The results are shown in Figure 6. Addition of MB to the gel results in its conversion to a solution. This transition is reversed by the addition of AA, and then purging the solution with O2 regenerates the MB and converts the gel into a free-flowing solution. Deoxygenation enables the competitive reduction of MB to LMB by AA and regenerates the gel. This was accomplished either by heating the solution of TAP–CyCo6 and LMB to 60 °C, which rapidly consumes all of the O2, or by flowing N2 through the mixture.

The sol–gel transition of TAP–CyCo6 is pH dependent.10TAP and CyCo6 form their most stable supramolecular assemblies and thereby the hydrogels when the pH of the solution equals their pKa (ca. 7); however, the hydrogel dissolves at pH 6 and pH 8. Consequently, it was necessary to separate the effect of pH change from the reduction of MB that is caused by addition of AA to solutions of TAP–CyCo6 (Figure S3, Supporting Information). The gradual addition of various concentrations of AA (0–100 mM) to 200 mM phosphate buffer lowers the pH from ca. 7 to ca. 5.7. The pH of a sample containing 20 mM of TAP–CyCo6 in the phosphate buffer solution was measured before and after the addition of 50 mM of AA. The addition of AA results in a change in pH from 7.0 to 6.5. This change causes a ca. 30% decrease in the amount of assembled monomers compared with that at pH 7. In contrast to the effect of lowered pH alone, the addition of AA to solutions of 0.25 mM MB and TAP–CyCo6 assemblies lowers the pH, forms LMB, and increases the length of the assemblies, eventually reforming the gel (Figure 6C). The effects of AA on pH and on LMB formation operate on the gel in opposite directions. Thus, the reformation of TAP–CyCo6 assemblies is a direct consequence of MB reduction to LMB by AA.

Reversibility of MB-Mediated Redox Switching

We examined reversibility of the MB- to LMB-induced sol–gel transition of TAP–CyCo6 polymers. We compared the rates of reduction of 0.25 mM solutions of MB and AA in a solution containing only 200 mM phosphate buffer and in a solution that also contained 20 mM of TAP–CyCo6 assembly. The rate of the reduction of MB to LMB is reduced by the supramolecular polymer (Figure S4, Supporting Information). This decreased rate of reduction complicates the bleaching of MB because diffusion of O2 from the air interface competitively reoxidizes the LMB to MB. To eliminate this problem, the reduction of MB by AA was carried out at 60 °C, where it is rapid because the TAP–CyCo6 assemblies are not stable under these conditions. The gel is reformed when the solution is cooled to room temperature.

The reversibility of the redox-coupled sol–gel transition was examined. A sample containing 0.25 mM of MB and of 30 mM TAP–CyCo6 assembly was reduced with 25 mM AA at 60 °C, allowed to cool to room temperature, and then purged for 5 min with O2. This constitutes one cycle that included conversion of the MB-containing solution to a LMB-containing gel and then back to a MB-containing solution. The process was monitored by absorption spectroscopy and repeated for 11 cycles (Figure 6B). The results demonstrate that the reversible redox cycling of MB to LMB is robust and in the presence of a large excess of AA, it can be used to repeatedly control the gel to sol transition. There is an initial reduction in the amount of MB in solution caused by the precipitation of LMB, which is less soluble than MB, but the system stabilizes after three cycles. The reversible gel to sol transformation of TAP–CyCo6 assembly and the difference in viscoelastic properties of the solutions containing MB and shorter fibers, and the decolorized gel containing LMB and longer fibers was recorded in a video that is included in the Supporting Information (Video S1).

The reversible gel to solution transformation of TAP–CyCo6 supramolecular assemblies by the reduction of MB to LMB was further investigated by AFM (Figure 7A,B). Micron length linear fibers of untreated TAP–CyCo6 assemblies show a concentration-dependent decrease in their length in the presence of MB. The reduction of MB to LMB by AA results in the reformation of the micron length polymers (Figure 7B).

Figure 7.

Figure 7

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(A) AFM image of TAP–CyCo6 with MB (short fibers) and (B) after adding AA (long fibers). All samples contain phosphate buffer, pH 7 (200 mM).

Conclusions

In summary, TAP–CyCo6 supramolecular polymers interact with MB. The spectroscopic and physical data are all consistent with MB acting as a noncovalent chain stopper that binds electrostatically to polymer ends leading to shorter fibers and the concomitant loss of gelation. The reduction of MB by AA to LMB, which does not act as a noncovalent chain stopper for TAP–CyCo6 polymers because it is uncharged and nonplanar, results in the reformation of the gel phase. The reoxidation of LMB to MB by O2 regenerates the solution phase. This reversible redox coupled sol–gel transition can be repeated at least a dozen times. Notably, this switchable hydrogel system does not require synthetic modification of the monomers. This finding provides a simple, noninvasive approach to control the assembly of these and related supramolecular polymers, which may aid in the development of gel-based materials.

Experimental Details

Sample Preparation

Supramolecular polymers for this work were prepared by mixing equimolar amounts of CyCo6 and TAP in 200 mM sodium phosphate buffer at pH 7. MB was added to a preformed TAP–CyCo6 supramolecular assembly, and the mixture was heated to 60 °C and cooled to ensure complete mixing of MB with TAP–CyCo6 hexads. Unless otherwise noted, the preparation of assemblies and all experiments were carried out at 20 °C. CyCo6 was synthesized according to the previously reported method and its purity was determined by NMR spectroscopy and liquid chromatography–mass spectrometry (LCMS). pH was measured with a VWR 8100 pH meter equipped with an InLab semimicro combination electrode.

NMR and UV Absorption Analyses

NMR analyses were performed in D2O on a Bruker DRX-500 NMR spectrometer. The spectra were summed over 32 transients. UV–vis analyses were carried out on an Agilent 8453 spectrophotometer equipped with an 89090A temperature controller. Cells of 1 mm path length were used for absorption measurements. Assembly solutions investigated by NMR spectroscopy contained 90% H2O and 10% D2O and were observed using the WATERGATE pulse sequence. 3-(Trimethylsilyl)propionic-2,2,3,3-d4 acid sodium salt was used as an internal standard, which did not show any indication of interacting with or being incorporated within the assemblies.

AFM Imaging

AFM imaging was performed on freshly cleaved mica. The mica substrate was rinsed with water and dried under N2. A 3 μL sample of the assembly solution was spread over the mica using N2 flow and was dried with N2 gas. AFM imaging was performed with a Nanoscope IIIa (Digital Instruments) in the tapping mode, using Si tips (Vistaprobes, 48 N/m).

Dynamic Light Scattering

DLS experiments were carried out using a DynaPro MS/X dynamic light scattering instrument (Proterion, Piscataway, NJ) with a laser wavelength of 830 nm and a constant scattering collection angle of 90° at room temperature. Each measurement reported is the average of 20 s scattering intensity accumulations.

Materials

2,4,6-Triaminopyrimidine (TAP) and methylene blue (MB) were purchased from Acros Organic and Sigma-Aldrich, respectively, and were both used as received. Synthesis of 1-(5-carboxypentyl)-1,3,5-triazin-2,4,6-trion (CyCo6) was performed by following the procedure reported by Hager et al35 with minor modifications.10 CyCo6: 1H NMR (500 MHz, DMSO-d6): δ 1.27 (m, 2H; CH2), 1.50 (m, 4H; CH2), 2.19 (t, J = 7.5 Hz, 2H; CH2CO), 3.61 (t, J = 7.5 Hz, 2H; CH2N), 11.63 ppm (br m, NH); 13C NMR (125 MHz, DMSO-d6): δ 24.1, 25.6, 27.05, 33.5, 40.2, 148.6, 149.8, 174.4 ppm; HRMS m/z: calcd for [M – H]−, 242.0777; found, 242.0785 (first reported in Cafferty et al. 2014).10

Acknowledgments

This work was supported by the NSF and the NASA Astrobiology Program, under the NSF Center for Chemical Evolution (CHE-1504217).

Glossary

Abbreviations

TAP

2,4,6-triaminopyrimidine

CyCo6

1-(5-carboxypentyl)-1,3,5-triazin-2,4,6-trion

MB

methylene blue

LMB

leucomethylene blue

AA

ascorbic acid

Supporting Information Available

The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acsomega.9b02785.

  • MAC of TAP–CyCo6 assembly without and with MB; absorption spectra of MB in methanol and in phosphate buffer; changes in pH of the solutions of MB in buffer or in TAP–CyCo6 assemblies with added AA; kinetics of AA-mediated decoloration of MB in buffer and in TAP–CyCo6 assembly (PDF)

  • Reversible gel to sol transformation of TAP–CyCo6 assembly with MB and LMB (MOV)

Author Contributions

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

The authors declare no competing financial interest.

Supplementary Material

ao9b02785_si_001.pdf (275KB, pdf)
ao9b02785_si_002.mov (9.2MB, mov)

References

  1. Petkau-Milroy K.; Sonntag M. H.; Brunsveld L. Modular columnar supramolecular polymers as scaffolds for biomedical applications. Chemistry 2013, 19, 10786–10793. 10.1002/chem.201301324. [DOI] [PubMed] [Google Scholar]
  2. Brunsveld L.; Vekemans J. A. J. M.; Hirschberg J. H. K. K.; Sijbesma R. P.; Meijer E. W. Hierarchical formation of helical supramolecular polymers via stacking of hydrogen-bonded pairs in water. Proc. Natl. Acad. Sci. U.S.A. 2002, 99, 4977–4982. 10.1073/pnas.072659099. [DOI] [PMC free article] [PubMed] [Google Scholar]
  3. Brunsveld L.; Folmer B. J. B.; Meijer E. W.; Sijbesma R. P. Supramolecular polymers. Chem. Rev. 2001, 101, 4071–4098. 10.1021/cr990125q. [DOI] [PubMed] [Google Scholar]
  4. Babu S. S.; Praveen V. K.; Ajayaghosh A. Functional π-gelators and their applications. Chem. Rev. 2014, 114, 1973–2129. 10.1021/cr400195e. [DOI] [PubMed] [Google Scholar]
  5. Yang L.; Tan X.; Wang Z.; Zhang X. Supramolecular polymers: historical development, preparation, characterization, and functions. Chem. Rev. 2015, 115, 7196–7239. 10.1021/cr500633b. [DOI] [PubMed] [Google Scholar]
  6. Wang D.; Tong G.; Dong R.; Zhou Y.; Shen J.; Zhu X. Self-assembly of supramolecularly engineered polymers and their biomedical applications. Chem. Commun. 2014, 50, 11994–12017. 10.1039/c4cc03155e. [DOI] [PubMed] [Google Scholar]
  7. Wei M.; Gao Y.; Li X.; Serpe M. J. Stimuli-responsive polymers and their applications. Polym. Chem. 2017, 8, 127–143. 10.1039/c6py01585a. [DOI] [Google Scholar]
  8. Du X.; Zhou J.; Shi J.; Xu B. Supramolecular hydrogelators and hydrogels: from soft matter to molecular biomaterials. Chem. Rev. 2015, 115, 13165–13307. 10.1021/acs.chemrev.5b00299. [DOI] [PMC free article] [PubMed] [Google Scholar]
  9. Freeman R.; Boekhoven J.; Dickerson M. B.; Naik R. R.; Stupp S. I. Biopolymers and supramolecular polymers as biomaterials for biomedical applications. MRS Bull. 2015, 40, 1089–1101. 10.1557/mrs.2015.270. [DOI] [PMC free article] [PubMed] [Google Scholar]
  10. Cafferty B. J.; Avirah R. R.; Schuster G. B.; Hud N. V. Ultra-sensitive pH control of supramolecular polymers and hydrogels: pKa matching of biomimetic monomers. Chem. Sci. 2014, 5, 4681–4686. 10.1039/c4sc02182g. [DOI] [Google Scholar]
  11. Steed J. W. Supramolecular gel chemistry: developments over the last decade. Chem. Commun. 2011, 47, 1379–1383. 10.1039/c0cc03293j. [DOI] [PubMed] [Google Scholar]
  12. Lloyd G. O.; Steed J. W. Anion-tuning of supramolecular gel properties. Nat. Chem. 2009, 1, 437–442. 10.1038/nchem.283. [DOI] [PubMed] [Google Scholar]
  13. Yang Z.; Liang G.; Xu B. Enzymatic hydrogelation of small molecules. Acc. Chem. Res. 2008, 41, 315–326. 10.1021/ar7001914. [DOI] [PubMed] [Google Scholar]
  14. Peng F.; Li G.; Liu X.; Wu S.; Tong Z. Redox-responsive gel–sol/sol–gel transition in poly(acrylic acid) aqueous solution containing Fe(III) ions switched by light. J. Am. Chem. Soc. 2008, 130, 16166–16167. 10.1021/ja807087z. [DOI] [PubMed] [Google Scholar]
  15. Kim D. S.; Chang J.; Leem S.; Park J. S.; Thordarson P.; Sessler J. L. Redox- and pH-responsive orthogonal supramolecular self-assembly: An ensemble displaying molecular switching characteristics. J. Am. Chem. Soc. 2015, 137, 16038–16042. 10.1021/jacs.5b06524. [DOI] [PubMed] [Google Scholar]
  16. Ma X.; Tian H. Stimuli-responsive supramolecular polymers in aqueous solution. Acc. Chem. Res. 2014, 47, 1971–1981. 10.1021/ar500033n. [DOI] [PubMed] [Google Scholar]
  17. Frisch H.; Unsleber J. P.; Lüdeker D.; Peterlechner M.; Brunklaus G.; Waller M.; Besenius P. pH-switchable ampholytic supramolecular copolymers. Angew. Chem., Int. Ed. Engl. 2013, 52, 10097–10101. 10.1002/anie.201303810. [DOI] [PubMed] [Google Scholar]
  18. Yagai S.; Nakajima T.; Kishikawa K.; Kohmoto S.; Karatsu T.; Kitamura A. Hierarchical organization of photoresponsive hydrogen-bonded rosettes. J. Am. Chem. Soc. 2005, 127, 11134–11139. 10.1021/ja052645a. [DOI] [PubMed] [Google Scholar]
  19. Karunakaran S. C.; Cafferty B. J.; Weigert-Muñoz A.; Schuster G. B.; Hud N. V. Spontaneous symmetry breaking in the formation of supramolecular polymers: implications for the origin of biological homochirality. Angew. Chem., Int. Ed. Engl. 2019, 58, 1453–1457. 10.1002/anie.201812808. [DOI] [PubMed] [Google Scholar]
  20. Cafferty B. J.; Gállego I.; Chen M. C.; Farley K. I.; Eritja R.; Hud N. V. Efficient self-assembly in water of long noncovalent polymers by nucleobase analogues. J. Am. Chem. Soc. 2013, 135, 2447–2450. 10.1021/ja312155v. [DOI] [PubMed] [Google Scholar]
  21. Karunakaran S. C.; Cafferty B. J.; Peláez-Fernández M.; Neselu K.; Schmidt-Krey I.; Fernandez-Nieves A.; Schuster G. B.; Hud N. V. Exquisite regulation of supramolecular equilibrium polymers in water: chain stoppers control length, polydispersity and viscoelasticity. Polym. Chem. 2018, 9, 5268–5277. 10.1039/c8py01146j. [DOI] [Google Scholar]
  22. Smulders M. M. J.; Nieuwenhuizen M. M. L.; Grossman M.; Filot I. A. W.; Lee C. C.; de Greef T. F. A.; Schenning A. P. H. J.; Palmans A. R. A.; Meijer E. W. Cooperative two-component self-assembly of mono- and ditopic monomers. Macromolecules 2011, 44, 6581–6587. 10.1021/ma201255q. [DOI] [Google Scholar]
  23. Knoben W.; Besseling N. A. M.; Cohen Stuart M. A. Chain stoppers in reversible supramolecular polymer solutions studied by static and dynamic light scattering and osmometry. Macromolecules 2006, 39, 2643–2653. 10.1021/ma0518914. [DOI] [Google Scholar]
  24. Knoben W.; Besseling N. A. M.; Bouteiller L.; Cohen Stuart M. A. Dynamics of reversible supramolecular polymers: independent determination of the dependence of linear viscoelasticity on concentration and chain length by using chain stoppers. Phys. Chem. Chem. Phys. 2005, 7, 2390–2398. 10.1039/b503463a. [DOI] [PubMed] [Google Scholar]
  25. Sijbesma R. P.; Beijer F. H.; Brunsveld L.; Folmer B. J. B.; Hirschberg J. H. K. K.; Lange R. F. M.; Lowe J. K. L.; Meijer E. W. Reversible polymers formed from self-complementary monomers using quadruple hydrogen bonding. Science 1997, 278, 1601–1604. 10.1126/science.278.5343.1601. [DOI] [PubMed] [Google Scholar]
  26. Mowry S.; Ogren P. J. Kinetics of methylene blue reduction by ascorbic acid. J. Chem. Educ. 1999, 76, 970. 10.1021/ed076p970. [DOI] [Google Scholar]
  27. Impert O.; Katafias A.; Kita P.; Mills A.; Pietkiewicz-Graczyk A.; Wrzeszcz G. Kinetics and mechanism of a fast leuco-Methylene Blue oxidation by copper(ii)-halide species in acidic aqueous media. Dalton Trans. 2003, 348–353. 10.1039/b205786g. [DOI] [Google Scholar]
  28. Liu Y.-N.; Zhou X.; Wang X.; Liang K.; Yang Z.-K.; Shen C.-C.; Imran M.; Sahar S.; Xu A.-W. Hydrogenation/oxidation induced efficient reversible color switching between methylene blue and leuco-methylene blue. RSC Adv. 2017, 7, 30080–30085. 10.1039/c7ra04498d. [DOI] [Google Scholar]
  29. Wang W.; Xie N.; He L.; Yin Y. Photocatalytic colour switching of redox dyes for ink-free light-printable rewritable paper. Nat. Commun. 2014, 5, 5459. 10.1038/ncomms6459. [DOI] [PubMed] [Google Scholar]
  30. Anwar N.; Vagin M.; Naseer R.; Imar S.; Ibrahim M.; Mal S. S.; Kortz U.; Laffir F.; McCormac T. Redox switching of polyoxometalate-methylene blue-based layer-by-layer films. Langmuir 2012, 28, 5480–5488. 10.1021/la3004068. [DOI] [PubMed] [Google Scholar]
  31. Cook A. G.; Tolliver R. M.; Williams J. E. The blue bottle experiment revisited: how blue? how sweet?. J. Chem. Educ. 1994, 71, 160. 10.1021/ed071p160. [DOI] [Google Scholar]
  32. Dekker A. O.; Dickinson R. G. Oxidation of ascorbic acid by oxygen with cupric ion as catalyst. J. Am. Chem. Soc. 1940, 62, 2165–2171. 10.1021/ja01865a070. [DOI] [Google Scholar]
  33. Hirst A. R.; Coates I. A.; Boucheteau T. R.; Miravet J. F.; Escuder B.; Castelletto V.; Hamley I. W.; Smith D. K. Low-molecular-weight gelators: elucidating the principles of gelation based on gelator solubility and a cooperative self-assembly model. J. Am. Chem. Soc. 2008, 130, 9113–9121. 10.1021/ja801804c. [DOI] [PubMed] [Google Scholar]
  34. Patil K.; Pawar R.; Talap P. Self-aggregation of methylene blue in aqueous medium and aqueous solutions of Bu4NBr and urea. Phys. Chem. Chem. Phys. 2000, 2, 4313–4317. 10.1039/b005370h. [DOI] [Google Scholar]
  35. Hager K.; Franz A.; Hirsch A. Self-assembly of chiral depsipeptide dendrimers. Chemistry 2006, 12, 2663–2679. 10.1002/chem.200501300. [DOI] [PubMed] [Google Scholar]
  36. Vantomme G.; ter Huurne G. M.; Kulkarni C.; ten Eikelder H. M. M.; Markvoort A. J.; Palmans A. R. A.; Meijer E. W. Tuning the length of cooperative supramolecular polymers under thermodynamic control. J. Am. Chem. Soc. 2019, 141, 18278–18285. 10.1021/jacs.9b09443. [DOI] [PMC free article] [PubMed] [Google Scholar]

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