Light-controlled switching of the self-assembly of ill-defined amphiphilic SP-PAMAM

Abstract

Light-responsive amphiphilic spiropyrans-decorated polyamidoamine (SP-P3) with ill-defined structure was prepared by using 3.0G-PAMAM as the scaffold and introducing the spiropyrans to the periphery of it randomly. Under visible light illumination, the ill-defined structure SP-P3 could form an adaptive amphiphilic macromolecule by rearranging dynamically the peripheral amino and SP groups on the surface of PAMAM. The resultant adaptive amphiphilic SP-P3 could hierarchically self-assemble into uniform macrorods about 800–1100 nm in width and 50–80 μm in length. When irradiated with UV light (365 nm), hydrophobic SP-P3 would isomerise into hydrophilic MC-P3, and induced the disassembly of rod-like aggregates. Irradiation with visible light transformed the MC-P3 back to the SP-P3 and then it could re-self-assemble into the rod-like aggregates. These results demonstrated that these macrorods could reversibly disassemble and re-self-assemble in aqueous solution under alternative UV and visible light irradiation. Our experiments not only provide a novel strategy for preparing responsive dynamic materials, but also support the concept that ill-defined amphiphilic macromolecules could also self-assemble to form well-shaped supramolecular structures.

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Introduction

Molecular self-assembly has attracted extensive attention and become a hot issue in recent years.^1–4 A variety of molecules with well-defined structures have been designed and proposed for self-assembly in numerous reports.^5–8 Compared to these well-defined molecules, which are promising candidates for self-assembly, the self-assembly behaviors of molecules with irregular structures have been neglected for a long time because people held the attitude that molecules with irregular, random branched structures could not form regular supramolecular structures.^9,10 Nevertheless, there are many reports showing that well-shaped supramolecular structures could be assembled from these irregular macromolecules. For instance, Yan et al.¹¹ reported that an amphiphilic hyperbranched polymer with a hyperbranched hydrophobic core and ligated hydrophilic arms could self-assemble into macroscopic tubes in a selective solvent. At the same time, Tsukruk et al.^12,13 demonstrated that long, uniform nanofibers could be assembled from amphiphilic-functionalized hyperbranched molecules. Zhou and co-workers¹⁴ synthesized a novel amphiphilic homopolymer HPHDP and realized its aqueous self-assembly to form various supramolecular structures, including micelles, vesicles, tubes, fibers and films. The fact that ill-defined amphiphilic dendritic molecules can self-assemble to form uniform supramolecular structures provides a new way for the development of supramolecular chemistry, and adds a new method for progressing materials science.

Though self-assembly of molecules with ill-defined structures has gradually attracted attention, little has been reported about the construction of environment-stimuli responsive dynamic materials based on these ill-defined amphiphilic molecules. Dynamic materials have multiple advantages over their static counterparts: selected properties of interest can be reversibly turned “on” and “off” at will and the ability to reconfigure these materials imparts many uses upon them.¹⁵ Among different stimulations that can change the state of materials, light is one good form of external input.¹⁶ It has many advantages, such as no chemical contaminants are introduced, closed systems can be actuated, and light with specific wavelengths can be delivered.¹⁷ Various photoswitchable molecules, like azobenzenes and spiropyrans, have been widely investigated and employed for the construction of light-responsive systems and materials.¹⁸ Spiropyrans (SPs) are a family of photosensitive molecules. It is well known that under UV light irradiation, hydrophobic SPs can isomerize to hydrophilic merocyanines (MC), and the MC form can revert to SP form again under visible light irradiation.¹⁹ Because of the large difference between SP and MC, various dynamic materials based on SP have been explored. SPs have been investigated for optical memories, cell images, logic gates and so on.^20–24 For instance, the light-induced reversible formation of polymeric micelles has been reported.²⁵ Qu et al.²⁶ utilized spiropyrans-conjugated nanophosphors to successfully prepare NIR/visible light tuned interfacially active nanoparticles with reversible inversion properties. Meanwhile, using SP-functionalized dendrons to construct new photoswitchable supramolecular materials is also fantastic. For example, the light-triggered formation of nano- or micro-meter-sized particles from SP-functionalized dendrons has been reported²⁷ and light-responsive micelles of spiropyran-initiated hyperbranched polyglycerol have been explored for smart drug delivery.²⁸ However, to the best of our knowledge, reports concerning the use of SP-functionalized dendrons to construct dynamic materials remain rare.

Here, we present a new kind of SP-functionalized dendron (SP-P3) with a percentage of the spiropyrans reacting on the periphery of the 3.0G-PAMAM randomly (Scheme 1A). 3.0G-PAMAM with 32 terminal amino groups around the surface was an optimal “soft nanoparticle” with a diameter of 4.0 nm and can be a good scaffold for self-assembly. SP-P3 was an amphiphilic macromolecule with ill-defined structure initially. On visible light illumination, it could form uniform macrorods by rearranging dynamically on the peripheral groups during their self-assembly from solution (Scheme 1C). Under UV light irradiation (365 nm), the photoisomerization of hydrophobic SP-P3 to hydrophilic MC-P3 occurred (Scheme 1B) and amphiphilic molecules lessened, resulting in the dissolution of self-assembly. Regeneration of self-assembly was observed as a result of irradiation with visible light, returning to its amphiphilic form as MC-P3 transformed to SP-P3 (Scheme 1B). These results provide new light-stimuli responsive dynamic materials and support the concept that ill-defined amphiphilic macromolecules could also self-assemble to form well-shaped supramolecular structures.

Scheme 1 (A) Synthesis of spiropyran-conjugated 3.0G-PAMAM (SP-P3). (B) Light-induced isomerization between SP-P3 and MC-P3 by UV (365 nm) and visible light irradiation. (C) The suggested hierarchical self-assembly of SP-P3 and light-induced disruption and regeneration of the self-assembly.

Experimental section

Materials

All the solvents were purchased from Beijing Chemical plant. Dichloromethane (DCM), acetonitrile (MeCN), diethyl ether, chloroform (CHCl₃) and dimethyl formamide (DMF) were used with further purification. All the reagents were purchased from Energy Chemical plant.

Instruments

¹H-NMR spectra were measured by Bruker 510 spectrometer (500 MHz); the DLS instrument used was a Malvern Instruments Zetasizer Nano ZS. Optical microscopy images and fluorescence microscope images were obtained by using an Olympus BX61. SEM images were recorded on a scanning electron microscope (JEOL JSM 6700F). TEM images were recorded on a JEM-2100F. UV/vis spectra were obtained using a Shimadzu 3100. Fluorescence spectra were obtained using a fluorescence spectrophotometer (RF-5301PC). UV irradiation was carried out with a xenon lamp (300 W; Asahi Spectra Co. Ltd.; MAX-302). Visible light irradiation was carried out with a fluorescent lamp.

Synthesis of 3.0G-PAMAM

The synthesis of 3.0G-PAMAM was carried out according to our previous work.²⁹ The ¹H-NMR spectrum of PAMAM is shown in Fig. S1.† ¹H-NMR (500 MHz, D₂O, 25 °C, TMS): δ = 2.43 (120H, –CCH₂CONH–), 2.62 (60H, –NCH₂CH₂N–, –NHCCH₂N–), 2.72 (64H, –CCH₂NH₂), 2.82 (120H, –CH₂CCONH–), 3.24 (64H, –CONHCH₂CNH₂), 3.29 (56H, –CONHCH₂CN–).

Synthesis of spiropyran (SP)

The SP was synthesized according to previous work³⁰ and is shown in Scheme S1.† During the synthesis, all the reaction vessels were wrapped in aluminum foil to ensure the reaction was performed in the dark. A solution of 2,3,3-trimethyl-3H-indole (3.18 g, 20.00 mmol) and 2-bromoethanol (3.12 g, 25.00 mmol) in MeCN (20 mL) was heated for 24 h under reflux and N₂. After cooling to ambient temperature, the solvent was distilled off under reduced pressure. The residue was suspended in hexane (20 mL) and the mixture was then sonicated and filtered. The resulting solid was crystallized from CHCl₃ (30 mL) to afford 1-(2-hydroxyethyl)-2,3,3-trimethyl-3H-indolium bromide (1) (4.19 g, 73.70%). The solution of bromide (4.19 g, 14.40 mmol) and KOH (1.10 g, 19.20 mmol) in H₂O (60 mL) was stirred at ambient temperature for 20 min, and then it was extracted with Et₂O (3 × 25 mL). The organic phase was concentrated under reduced pressure to afford 9,9,9a-trimethyl-2,3,9,9a-tetrahydro-oxazo[3,2-a]indole (2) (2.39 g, 81.49%). The solution of 2 (2.39 g, 11.76 mmol) and 2-hydroxy-5-nitrobenzaldehyde (2.95 g, 17.64 mmol) in EtOH (15 mL) was heated for 3 h under reflux and N₂. The mixture was filtered after cooling to room temperature. The resulting solid was washed with EtOH (3 mL) and dried to afford SP (3) (3.47 g, 83.82%). The ¹H-NMR spectrum of SP is shown in Fig. S2.† ¹H-NMR (500 MHz, DMSO, 25 °C, TMS): δ = 1.22–1.32 (6H, –C(CH₃)₂), 3.34–3.52 (2H, –NCH₂CH₂–), 3.73–3.85 (2H, –OCH₂CH₂–), 5.90–6.0 and 6.69–6.71 (2H, –CCHCH–), 6.78–8.06 (7H, –ArH).

Synthesis of acryl-modified SP derivative (SPA)

The acryl-modified SP derivative (SPA) was synthesized according to previous work³¹ as shown in Scheme S2.† SP (0.51 g, 1.45 mmol) was dissolved in CH₂Cl₂ (40 mL). Acryloyl chloride (1 mL, 12.46 mmol) dissolved in CH₂Cl₂ (100 mL) was added dropwise to the solution stirred at 0 °C under N₂. Upon completion, the system was stirred at 40 °C under N₂ for 4 h. After cooling to ambient temperature, the resulting solution was washed with 0.1 M HCl (3 × 25 mL) and saturated NaHCO₃ solution (3 × 25 mL), then dried over Na₂SO₄. The acryl-modified SP derivative (SPA) was afforded by reducing evaporation (0.32 g, 55.2%). The ¹H-NMR spectrum of SPA is shown in Fig. S3.† ¹H-NMR (500 MHz, CDCl₃, 25 °C, TMS): δ = 1.22–1.32 (6H, –C(CH₃)₂), 3.34–3.52 (2H, –NCH₂CH₂–), 3.73–3.85 (2H, –OCH₂CH₂–), 5.86 (1H, –CHCHH), 5.90 (1H, –CCHCH–), 6.00–6.10 (1H, –CHCH₂), 6.39–6.58 (1H, CHCHH), 6.66–6.75 (1H, –CCHCH–), 6.78–8.06 (7H, ArH).

Synthesis of spiropyran-conjugated 3.0G-PAMAM (SP-P3)

The spiropyran-conjugated 3.0G-PAMAM (SP-P3) was synthesized as shown in Scheme 1A. A solution of 3.0G-PAMAM (71 mg, 0.01 mmol) and triethylamine (1 mL) in anhydrous methanol was stirred at ambient temperature under N₂. SPA (110 mg, 0.25 mmol) dissolved in DMF (10 mL) was added to the solution dropwise. The system was stirred at 40 °C under N₂ for 36 hours, after which time the reaction finished. After cooling to room temperature, the solution was rinsed with deionized water several times in order to remove unreacted SPA and other impurities to afford SP-P3 (100 mg). The ¹H-NMR spectrum of SP-P3 is shown in Fig. S4-1.† ¹H-NMR (500 MHz, DMSO, 25 °C, TMS): δ = 2.10–2.40 (120H, –CCH₂CONH–), 3.10–3.30 (120H, –CONHCH₂CNH₂ and –CONHCH₂CN–), 6.90–8.06 (71H, ArH).

We also synthesized a series of SP-P3 with proper SP ratios of 9%, 18%, 25%, 50% and 63% according to the above method. Their ¹H-NMR spectra are shown in the ESI from Fig. S4-2 to S4-6.†

Results and discussion

The spiropyran-conjugated 3.0G-PAMAM (SP-P3) was synthesized by general addition reaction (Scheme 1A). We estimated from the ¹H-NMR spectrum that the number of SP conjugated to the surface of PAMAM was approximately 12. The number of peripheral aminos after reaction (α) was about 20. The total number of peripheral groups (γ) was 32 (α + β = 32). The percentage of SP was about (δ) 38% (β/γ = 12/32 = 38%). We should notice that the numbers presented and discussed here should be considered as averages for the ill-defined structure of the macromolecules. Under visible light, the SP-P3 had no visible absorption at 550–600 nm wavelength. UV (365 nm) irradiation of SP-P3 gave rise to the open-ring isomer (MC-P3) and it showed strong absorption at 550–600 nm and an intense emission band at 600–700 nm. Compared with the MCA, the largest emission peak wavelength of MC-P3 did not change (Fig. S5†). It indicated that PAMAM had no effect on the properties of SP, including the photo-isomerization and ultraviolet spectrum.

Different quantities of SP-P3 were dispersed in deionized water (the solution pH varied between 7.0 and 8.0) and sonicated with water bath at 30 °C for 1 hour and then left to stand for 1 day. We found that uniform structures can only be formed in the concentration between 0.1 mg mL⁻¹ and 1.0 mg mL⁻¹. Giant rod-like self-assembly aggregates of multiple-length scale were observed under a microscope (Fig. 1A and C). Irradiated with UV light (365 nm) for 10 s (here, we irradiated the samples for 10 s to induce a small number of SP-P3 to isomerize into MC-P3, which showed strong red fluorescence; at the same time, we ensured that the UV irradiation would not cause the disassembly of the rods), it was found that these aggregates showed strong red fluorescence under a fluorescence microscope (shown in Fig. 1B and D). Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis were used to investigate the morphology and characterization of the self-assembly aggregates (shown in Fig. 1). From SEM images (Fig. 2A and B), it was observed that these rod-like aggregates distributed with thickness and length unevenness and the surface was not smooth. We calculated that the sizes of the rod-like aggregates were about 800–1100 nm in width and 50–80 μm in length. Transmission TEM analysis revealed that rod-like aggregates were huge micro-scale rods (Fig. 2C and D). It clearly shows that the surface of them was not smooth, though it has the tendency to grow smooth. To further illustrate the assembly, we designed a series of SP-P3 with percentages of SP of about 10%, 18%, 25%, 33%, 50% and 63%. Under the same experimental environment and conditions, we found that 3.0G-PAMAM itself could not self-assemble (Fig. S6A†) and if a high percentage of SP conjugated to the PAMAM (with 63% SP up), self-assembly did not occur, only the aggregation of clusters (Fig. S6B†). The assembly formation could only occur for SP-P3 with a proper SP ratio of 18% to 50%. All the above experiments indicated that SP-P3 with irregular architecture self-assembled into supramolecular aggregates with uniform structures. These aggregates showed a large scale and the surface of them was not smooth.

Fig. 1 Images of self-assembled aggregates generated from solution (0.3 mg mL⁻¹). (A) and (C) Optical microscopy images; (B) and (D) fluorescence microscope images (after exposure to UV light (365 nm) for 10 s). We should note here that (A), (B), (C) and (D) are the same samples. The samples were prepared by dropping the solutions onto a slide and then air-drying.

Fig. 2 Images of the self-assembly aggregates stemmed from solution (0.3 mg mL⁻¹). (A) and (B) Scanning electron microscopy images. (C) and (D) Transmission electron microscopy images.

To understand the formation process and mechanism of self-assembly of SP-P3 thoroughly, we prepared the samples and continued to observe them during the assembly process. We used TEM analysis to observe the change of the samples every 4 hours. The TEM images at different times are shown in Fig. 3. We can see that multi-micelles of about 300 nm were formed at the beginning (Fig. 3A), and we could clearly see that these multi-micelles consist of small particles (Fig. S7†) after 8 h. These multi-micelles aggregated to form rod-like micelles (Fig. 3B and C) after 12 h. After standing for a period of time, these large micelles hierarchically assembled to form these large scale rods (Fig. 3D). Finally, these rods grew more smoothly.

Fig. 3 Transmission electron microscopy images of self-assembly from solution (0.3 mg mL⁻¹). (A), (B), (C) and (D) are the same sample at different times. Every 4 hours, we observed the change of the sample. (A) After 4 h. (B) After 8 h. (C) After 12 h. (D) After 16 h.

All of the above experiments inspired us to suggest a proposal mechanism of macrorod formation shown in Scheme 1C. It was suggested that the self-assembly of SP-P3 was a multi-micelles aggregating process and, during the self-assembly process, the spontaneous organization into hydrophilic and hydrophobic domains at the surface of the PAMAM dendrimer, that is, the ability of ill-defined SP-P3 to flexibly rearrange its structure, played an essential role in the self-assembly process. SP could rearrange dynamically on the surface of PAMAM from solution to form hydrophobic “patches” (Scheme 1C) for its hydrophobic interactions distinguished from the hydrophilicity of the peripheral amino and the flexibility of the dendrimer. The adaptive amphiphilic macromolecules with the sizes of 4–5 nm self-assembled gradually into multi-micelles (Scheme 1C) of about 300 nm through H bonding and hydrophobic interaction. Then these micelles aggregated into large micelles. These large micelles further aggregated together to finally form rod-like self-assembly aggregates (Scheme 1C). Although why these large scale micelles could aggregate together to form uniform rod-like aggregates remains unclear, we supposed that these multi-micelles may be gradually fused together by weak interactions in one direction.

The photo-switching of disruption and regeneration of the macrorods was conducted by alternating irradiation of UV and visible light. When the assembly solution was exposed to 365 nm UV light irradiation, the well-defined characteristic macrorods disappeared but smaller particles of about 200–700 nm formed (Fig. 4A). We found that these smaller particles were ill-defined aggregates (Fig. S8†) and clusters. After being exposed to visible light illumination for 24 h, macrorods of about 700–900 nm in width emerged (Fig. 4B), which indicated the regeneration of the rod aggregates. The light-induced reversible formation of self-assembly was also demonstrated by fluorescence spectroscopy and dynamic light scattering (DLS) analysis. The initial self-assembly solution had no emission at 620 nm (upon excitation at 550 nm) (Fig. 4C line 1). The original size of the aggregates was about 1100 nm by the size distribution of volume with few small particles at 200 nm (Fig. S9† blue line). When the solution was irradiated with UV (365 nm) light for 10 min, an emission at 620 nm appeared (Fig. 4C line 2) and the aggregates decreased to 400 nm (Fig. S9† red line), which was characteristic of the disruption of the self-assembly form. Irradiation with visible light isomerized the MC-P3 form back to the SP-P3 form, as confirmed by the spectra reverting to the original no emission profile after 24 h (Fig. 4C line 3) and the system returning to the initial aggregate size distribution (Fig. S9† green line). After five UV-vis cycles, the rod-like structures still remained though the reversible fluorescent intensity showed a little decay (Fig. 4D). By fluorescence spectroscopy analysis and dynamic light scattering analysis, we can also evidently find that the light switching exhibited an excellent effect on the reversible disruption and regeneration of the self-assembly based on SP-P3 through controlling the transitions between the macrorods and small particles.

Fig. 4 (A) and (B) Scanning electron microscopy images. (C) Fluorescent spectra with three different conditions, λ_exc = 550 nm. Line 1 is the original condition. Line 2 is after UV irradiation condition. Line 3 is the regeneration condition. (D) Reversible fluorescence (lem = 620 nm) switching upon exposure to UV and vis light (5 cycles). (A), (B), (C) and (D) are the same sample (0.3 mg mL⁻¹). (E) Scheme of light-induced isomerization between SP-P3 and MC-P3 by UV (365 nm) and visible light irradiation. R represented PAMAM.

The light-induced disruption and regeneration behaviours of the macrorods should be attributed to the reversed isomerization of SP-P3 (see Fig. 4E). On visible light illumination, SP-P3 was an ill-defined structure amphiphilic macromolecule initially and it could rearrange dynamically on the peripheral groups to form an adaptive amphiphilic macromolecule by rearranging dynamically on the peripheral groups during their self-assembly from solution (Scheme 1B). The adaptive macromolecules could self-assemble into large scale supramolecular structures. Irradiated with UV light, hydrophobic SP-P3 would isomerize into hydrophilic MC-P3 (Fig. 4E), which induced the disassembly of rod-like aggregates (Scheme 1B). Irradiation with visible light transformed the MC-P3 back to the SP-P3 and then it self-assembled into the rod-like aggregates again.

Conclusions

In conclusion, we have described the synthesis, self-assembly and light-controlled switching of the self-assembly and disassembly of SP-P3. The results show that the SP-P3 with irregular structure could self-assemble to form a uniform supramolecular structure by adapting the structure and rearrangement of the surface. These self-assembly structures were successfully characterized by optical microscopy, fluorescence microscopy, scanning electron microscopy (SEM) and transmission electron microscopy (TEM) analysis. In addition, we have shown that UV/vis light could induce disruption and regeneration of the macrorods. These results not only support the opinion that ill-defined amphiphilic macromolecules can also self-assemble to form well-shaped supramolecular structures, but provide a new light-stimuli responsive dynamic material, which may have potential applications in the light-stimuli responsive field.

Acknowledgements

The authors acknowledge the financial support for this work received from the Natural Science Foundation of China (No. 21234004, 21420102007, 21221063, 21574056), 111 project (B06009) and the Chang Jiang Scholars Program of China.

Notes and references

1. D. E. Discher and A. Eisenberg, Science, 2002, 297, 967–973.
2. A. Travesset, Science, 2011, 334, 183–184.
3. K. Zhang, M. Jiang and D. Y. Chen, Prog. Polym. Sci., 2012, 37, 445–486.
4. M. Shen and X. Shi, Nanoscale, 2010, 2, 1596–1610.
5. J. C. M. V. Hest, D. A. P. Delnoye, M. W. P. L. Baars, M. H. P. V. Genderen and E. W. Meijer, Science, 1995, 268, 1592–1595.
6. G. R. Newkome and C. N. Moorefield, Chem. Soc. Rev., 2015, 44, 3954–3967.
7. H. J. Sun, S. Zhang and V. Percec, Chem. Soc. Rev., 2015, 44, 3900–3923.
8. I. N. Kurniasih, J. Keilitz and R. Haag, Chem. Soc. Rev., 2015, 44, 4145–4164.
9. A. Sunder, M. F. Quincy, R. Mulhaupt and H. Frey, Angew. Chem., Int. Ed., 1999, 38, 2928–2930.
10. S. E. Stiriba, H. Kautz and H. Frey, J. Am. Chem. Soc., 2002, 124, 9698–9699.
11. D. Yan, Y. Zhou and J. Hou, Science, 2004, 303, 65–67.
12. M. Ornatska, S. Peleshanko, K. L. Genson, B. Rybak, K. N. Bergman and V. V. Tsukruk, J. Am. Chem. Soc., 2004, 126, 9675–9684.
13. M. Ornatska, S. Peleshanko, B. Rybak, J. Holzmueller and V. V. Tsukruk, Adv. Mater., 2004, 16, 2206–2212.
14. H. Jin, W. Huang, X. Zhu, Y. Zhou and D. Yan, Chem. Soc. Rev., 2012, 41, 5986–5997.
15. R. Klajn, Chem. Soc. Rev., 2014, 43, 148–184.
16. M. Irie, Chem. Rev., 2000, 100, 1685–1716.
17. H. M. D. Bandarab and S. C. Burdette, Chem. Soc. Rev., 2012, 41, 1809–1825.
18. G. Berkovic, V. Krongauz and V. Weiss, Chem. Rev., 2000, 100, 1741–1753.
19. V. I. Minkin, Chem. Rev., 2004, 104, 2751–2776.
20. Z. Y. Tian and A. D. Q. Li, Acc. Chem. Res., 2013, 46, 269–279.
21. F. M. Raymo, Adv. Mater., 2002, 14, 401–414.
22. F. M. Raymo and S. Giordani, Proc. Natl. Acad. Sci. U. S. A., 2002, 99, 4941–4944.
23. L. Florea, S. Scarmagnani, F. Benito-Lopez and D. Diamond, Chem. Commun., 2014, 50, 924–926.
24. Y. Niu, Y. Li, Y. Lu and W. Xu, RSC Adv., 2014, 4, 58432–58439.
25. H. Lee, W. Wu, J. K. Oh, L. Mueller, G. Sherwood, L. Peteanu, T. Kowalewski and K. Matyjaszewski, Angew. Chem., Int. Ed., 2007, 46, 2453–2457.
26. Z. Chen, L. Zhou, W. Bing, Z. Zhang, Z. Li, J. Ren and X. Qu, J. Am. Chem. Soc., 2014, 136, 7498–7504.
27. Q. Chen, Y. Feng, D. Zhang, G. Zhang, Q. Fan, S. Sun and D. Zhu, Adv. Funct. Mater., 2010, 20, 36–42.
28. S. Son, E. Shin and B. S. Kim, Biomacromolecules, 2014, 15, 628–634.
29. H. C. Sun, L. Miao, J. X. Li, S. Fu, G. An, C. Si, Z. Y. Dong, Q. Luo, S. J. Yu, J. Y. Xu and J. Q. Liu, ACS Nano, 2015, 9, 5461–5469.
30. F. M. Raymo and S. Giordani, J. Am. Chem. Soc., 2001, 123, 4651–4652.
31. Y. Shiraishi, R. Miyamoto and T. Hirai, Org. Lett., 2009, 11, 1571–1574.

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Footnote

† Electronic supplementary information (ESI) available: Synthesis schemes and characterization of new compounds, and spectroscopic studies (UV, NMR) and some images (TEM, SEM). See DOI: 10.1039/c5ra17264k

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