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Stimuli-Responsive Gels
6 STIMULI-RESPONSIVE GELS Mario Jose Politi Laboratory of Photochemistry and Fast Kinetics, Institute of Chemistry of the University of Sa˜o Paulo, Sa˜o Paulo, Brazil
Abstract The formation of ordered nanostructures within the gel phase from colloidal systems can be driven by different types of external stimuli such as temperature, light, ultrasound, electric and magnetic fields, and through the processes of transformation, diffusion, and chemical sensitivity. This chapter illustrates some gains from this area of research and presents building blocks of enormous potential for the development of intelligent materials based on molecules that will bring improvements and improvements to systems based on nanotechnology. Keywords: Ordered nanostrucutures; solgel systems; responsive gels; external stimuli; molecular building blocks; nanotechnology CHAPTER OUTLINE 6.1 SolGel Process Under Different Stimuli 111 6.1.1 Case 1. Two (Multi-)Components Gels 116 6.1.2 Case 2. Gelators Coassembling 120 6.1.3 Case 3 Gelator Plus Nongelating Additive Materials 6.2 Examples of Interesting Stimuli-Related Gels 127 6.3 Concluding Remarks 134 References 135 Further Reading 138
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SolGel Process Under Different Stimuli
In this chapter, the solgel systems that respond to external stimuli to undergo the solgel (SG) process are spotlighted. The large horizon of applications and uses of solgel systems confer to the stimuli-responsive systems a special position given their quite large spectra of low molecular weight gelators (LMWG)
Nano Design for Smart Gels. DOI: https://doi.org/10.1016/B978-0-12-814825-9.00006-0 © 2019 Elsevier Inc. All rights reserved.
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and easy and extended ways to promote the gelation (Buerkle & Rowan, 2012; Qi & Schalley, 2014; Liu et al., 2014). A very wide range of materials have been recognized since the 1860s to form gels, often with quite different characteristics. In 1974 four main categories of gel were distinguished by Flory (1974): (1) well-ordered lamellar structures (e.g., phospholipids); (2) disordered covalent polymeric networks (e.g., vulcanized rubber and phenolic resins); (3) polymer networks formed through physical aggregation (e.g., gelatin); and (4) particulate, disordered structures (e.g., precipitates consisting of highly anisotropic particles or reticular networks of fibers). Since that time interest within the chemical and materials science communities has increasingly focused on the variety of gels in category (4) that arise from the self-assembly of low molecular weight compounds (often called LMWGs) into fibrous gel networks. These materials are classified as organogels in cases where the solvent is organic and hydrogels when the solvent is water. Organogels derived from LMWGs were first comprehensively reviewed by the Weiss group (Huang, Terech, Raghavan, & Weiss, 2005; Rogers et al., 2015; Terech & Weiss, 1997), while Hamilton and Estroff summarized those of hydrogels (Estroff & Hamilton, 2004). Examples of LMWGs (Steed, 2011) include ureas, amides, nucleobases, porphyrin derivatives, dendrimers, surfactants, sugars, fatty acids, and amino acids amongst many others known to form fibers. Although the SG process is difficult to fully predict, it is predictable within these types of compounds and thus has allowed systematic studies of structureproperty relationships, as will be exemplified below. Over the past decade, there have been a number of important reviews pertaining to particular classes of LMWGs, such as dendritic gels (Smith, 2006), nucleobases (Araki & Yoshikawa, 2005), amides ˇ and ureas (Zinic, Vo¨gtle, & Fages, 2005), and metallogels (Steed et al., 2010), as well as more general overviews that chart the tremendous progression in our understanding of supramolecular gels and the dramatically increased scope of the gelators that form them (Araki & Yoshikawa, 2005; Estroff & Hamilton, ˇ 2004; Steed, 2011; Zinic et al., 2005). Gels generated by the association of LMWGs can assemble into various nanostructures through noncovalent interactions that combine the elastic behavior of solids with the microviscous properties of fluids. The self-assembly is derived from hydrogen-bonding, π 2 π-stacking, electrostatic interactions, dipoledipole interactions, van der Waals interactions, and metal-to-ligand coordination between the gelators, while the gelatorsolvent interactions are able to modulate these
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interactions and open a horizon for the development of stimuliresponsive and smart soft materials (Pal & Banerjee, 2018). While there are many important fundamental issues related to the gelation process, such as the design of the gelator, synergism of various noncovalent interactions between gelators and gelatorsolvents, the balances between gelation and crystallization, and so on, the self-assembled nanostructures that form during gelation are very interesting. These nanostructures have many unique features, such as the flexibility to respond to external stimuli, morphological diversity, ease of fabrication in large quantities, and so on. The high order structures formed include nanospheres, nanofibers, nanoribbons, nanosheets, etc. as depicted in Figs. 6.1 and 6.2. Examples of supramolecular gels are found in three of the four categories of gels defined by Flory (1974): (1) category 1:
Figure 6.1 Typical representation of supramolecular gels from LMWGs followed by association and gelation. The top figure depicts possible applications of supramolecular gels. Reproduced from Christoff-Tempesta, T., Lew, A., & Ortony, J. (2018). Beyond covalent crosslinks: Applications of supramolecular gels. Gels, 4(2), 40 under the Creative Commons Attribution License (CC BY 4.0).
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Figure 6.2 Applications of supramolecular gels. Reproduced from Christoff-Tempesta, T., Lew, A., & Ortony, J. (2018). Beyond covalent crosslinks: Applications of supramolecular gels. Gels, 4(2), 40 under the Creative Commons Attribution License (CC BY 4.0).
high-order lamellar gels (Boekhoven et al., 2016); (2) category 3: semiordered physical networks (Feng et al., 2018); and (3) category 4: disordered particulate gels (Buerkle & Rowan, 2012). Interestingly, one kind of gelator molecule can sometimes form various morphologies, which could be controlled by the gelation environment. In addition, the nanostructures formed through gel formation are hierarchically formed, which could sometimes cover the molecular scale from nano- to meso- and microscales, which is closely related to the formation of biological self-assemblies. Finally, the gel nanostructures are relatively uniform and can be fabricated in larger quantities, thus making them templates for the fabrication of other functional materials. Returning to the solgel processes, it is convenient to recall that gels are soft materials that are continuous in structure and solid-like in rheological behavior. They can be categorized into chemical gels and physical gels. Usually chemical gels are covalently cross-linked polymers and are not able to be redissolved, whereas physical gels based on intermolecular interactions can be redissolved, showing cycled reversibility. Self-assembly of LMWGs, through weak intermolecular interactions such as H-bonding, π 2 π-stacking and van der Waals interactions, can lead to solvent gelation and the formation of physical gels. In the gel phase, molecules of LMWGs are self-assembled into entangled three-dimensional networks, which entrap and immobilize solvent molecules by capillary forces. These physical gels are characterized by the following unique features: (1) they are generally composed of hierarchically fiber-like structures, which cross-link to form a three-dimensional (3D) network; (2) they are thermally responsive, that is, the gels are transformed into solutions after heating and they can be
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Figure 6.3 Classification of the different stimuli: physical and chemical. (A) Type of stimuli (Segarra-Maset et al., 2013). (B) External stimuli (Yang, Zhang, & Zhang, 2012). Journal of materials chemistry by Royal Society of Chemistry (Great Britain) Reproduced with permission of Royal Society of Chemistry, in the format Book via Copyright Clearance Center.
recovered after cooling, since molecules of LMWGs within the fibril structures are associated through weak noncovalent interactions such as H-bonding, π 2 π-stacking, and van der Waals forces; (3) they are viscoelastic, owing to the fact that solvent molecules are macroscopically immobilized by surface tensions and capillary forces; and (4) they possess the respective critical gelation temperatures (Tgel), at which the gelsol transitions occur, and the respective critical gelator concentrations, above which gels form in certain solvents at room temperature (Pal & Banerjee, 2018). The kinds of stimuli on various gels are depicted in Fig. 6.3 which covers all kinds of studies up to the present (see Fig. 6.10; Segarra-Maset, Nebot, Miravet, & Escuder, 2013). Three main conditions were proposed for supramolecular gels formation (Buerkle & Rowan, 2012), namely, (1) two components LMWG gel phase; (2) two gelators; and (3) gelator plus no additive component (Fig. 6.4). Following this division, the examples given below represent the current state-of-the-art of SG formation. (1) Two-component or multicomponent gel-phase materials self-associate to yield a gel (Fig. 6.4A). (2) Two-gelator-component gels where (a) both gelators interact with each other to form cofibers or (b) the two gelators self-sort resulting in two different nanofibers within the gel (Fig. 6.4B). (3) Gelator plus additive component gels (Fig. 6.4C).
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Figure 6.4 Illustration of three main classes of two-component supramolecular gels. Chemical Society reviews by Royal Society of Chemistry (Great Britain) Reproduced with permission of Royal Society of Chemistry in the format Book via Copyright Clearance Center.
6.1.1
Case 1. Two (Multi-)Components Gels
Fig. 6.4 from Rowan’s review (Buerkle & Rowan, 2012), encompasses examples of hydrogen-bonded carboxylic acid dendrons mixed with diamines. They have shown that not only is the chain length of a linear aliphatic diamine 1 n important in controlling the thermal stability of the gel, but that changing the ratio of the aliphatic diamine (1 n) to 2 results in significant changes to the morphology and thermal stability of the assemblies. For details refer to Buerkle & Rowan (2012) (Fig. 6.5). Other examples involves an alkyl (C12) methyl viologen, an active redox center, and a polycyclic tetracarboxylate derivative as depicted in Fig. 6.6 (from Buerkle & Rowan, 2012). In the absence of the viologen’s positively charged group no gels are found, and carboxylate compound fibers are formed.
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Figure 6.5 (A) Structures of linear aliphatic diamine (1), dendron (2), and 1,4/1,3/ 1,2-diaminobenzene (3), (4), and (5). (B) Results from NMR analysis shows that the dendrons preferentially incorporate 3 over 4 and 5 (Brizard et al., 2008). Reproduced with permission from John Wiley and Sons.
In a parallel fashion, when mixing a class of surfactant-like LMWGs based on charge stabilization, researchers have shown that the absorption and fluorescence of the gel can be tailored, depending on the gel component composition, given the spectral properties of the naphthalimides and hemocyanine chromophores (Yi et al., 2008). Work from Moraes et al (2018) showed an interesting photochromic system using naphthalene diimides anchored to a (3-aminopropyl)triethoxysilane as a SG synthon that deserves further development as a photoredox center for use like viologen. This type of approach relates
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Figure 6.6 (A) Structures of the donor and acceptor molecules (9 and 10). (B) FE-SEM image of the spherical assemblies formed by 10 alone in water (1 3 1024 M). (C) AFM of fibers resulting from the combination of 9 and 10 in a 1:1 ratio. (Buerkle & Rowan, 2012; George et al., 2010). Reproduced with permission from John Wiley and Sons.
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directly to the kind of stimulus that the mixture responds to, and accordingly these gels can be classified as photo-, chemo-, electro-, or mechanoactive polymers, as presented in this chapter. Ditopic ligands constitute a special case of LWMGs for the obtaining of supramolecular gels having a metallocenter, as presented in Fig. 6.7 (Weng, Beck, Jamieson, & Rowan, 2006). Those materials contain gel features plus the properties of the metal center confined in the gel. A metallo-supramolecular based gel showed in Fig. 6.7 synthesized with Zn(ClO4)2 reacts in acetronitrile (A) (Fig. 6.7) react nicely as the properties of supramolecular gels cited above (see page 5) and shown in Fig. 6.8.
Figure 6.7 Schematic representation of the formation of metallo-supramolecular polymeric aggregates using ditopic ligand end-capped monomers with metal salts: (A) Zn(ClO4)2; (B) Zn(ClO4)2 and La(NO3)3; and (C) Zn2(ClO4)2 and La(ClO4)3, mixed with monomer 1 in acetonitrile as solvent (percentages of metal ion salts are calculated as mol % relative to 1). Reprinted (adapted) with permission from Weng, W., Beck, J. B., Jamieson, A. M., & Rowan, S. J. (2006). Understanding the mechanism of gelation and stimuli-responsive nature of a class of metallo-supramolecular gels. Journal of the American Chemical Society, 128(35), 1166311672. Copyright r 2006, American Chemical Society.
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Figure 6.8 Gel (A) (11 wt.% in acetonitrile) exhibits the typical multistimuli responsive behavior observed for this class of metallo-supramolecular gels (Weng et al., 2006). Reprinted (adapted) with permission from Weng, W., Beck, J. B., Jamieson, A. M., & Rowan, S. J. (2006). Understanding the mechanism of gelation and stimuli-responsive nature of a class of metallo-supramolecular gels. Journal of the American Chemical Society, 128(35), 1166311672. Copyright r 2006, American Chemical Society.
6.1.2
Case 2. Gelators Coassembling
In this condition, various gelators are combined to form gels that present typical viscosities changes at Tgel, an example is provided in Fig. 6.9 (Buerkle & Rowan, 2012) for polyurea derivatives. This data suggests that the viscosity transition (Fig. 6.9b) corresponds to the change in the assembly of the bisureas from thick filaments at high concentrations and low temperatures to thin filaments (of the width of a single molecule) at low concentrations and higher temperatures. The concentration and the temperature at which this transition occurs appears to depend on the nature of the alkyl groups attached to the bisurea binding motif with 19 and 20 transitioning at 43 C and 63 C, respectively. Mixing these two bisurea gelators results in a systematic change in this transition temperature. The transitions were followed by FT-IR analysis (Fig. 6.9c) and monitored by the ratio of the 3328 cm21 vibration peak (hydrogen-bonded NH groups) to the 3300 cm21 reference one. The data show that this transition temperature can be systematically varied by simply changing the molar ratio of the two components (Buerkle & Rowan, 2012). In the case of polyureas, an advantage can be taken from the
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Figure 6.9 (A) Chemical structures of bisurea 19 and 20. (B) The relative viscosity of 19 in toluene versus temperature at two different concentrations. (Heating run (open symbols); cooling run (full symbols)). (C) Ratio of the absorbance at 3328 and 3000 cm1 for 12.5 mM solution of 19/20 mixtures (E100/0; B75/25; and 50/50; m0/100) versus Temperature. (A)(B) Reprinted (adapted) with permission from Bouteiller, L., Colombani, O., Lortie, F., & Terech, P. (2005). Thickness transition of a rigid supramolecular polymer. Journal of the American Chemical Society, 127(24), 88938898. Copyright r 2005, American Chemical Society. (C) Reprinted from Isare, B., Bouteiller, L., Ducouret, G., & Lequeux, F. (2009). Tuning reversible supramolecular polymer properties through co-monomer addition. Supramolecular Chemistry, 21(5), 416421 by permission of the publisher (Taylor & Francis Ltd, http://www.tandfonline.com).
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ability of these compounds to coordinate with anions (Fig. 6.10) instead of cations. Crown ethers have been pursued (Fig. 6.11). Another striking example was observed by mixing different alkyl pyrenyl urethanes presented in Fig. 6.12. In these experiments sharp differences in Tgel and in the stress amplitude were observed. Data for carbamate derivatives exemplify systems that auto- or self-sort towards gelation (Fig. 6.12). These molecules contain the pyrenyl moieties that associate via π 2 π-stacking, whereas the carbamate groups associate through H-bonding.
Figure 6.10 Urea anion interaction model (Segarra-Maset et al., 2013). Chemical Society reviews by Royal Society of Chemistry (Great Britain) Reproduced with permission of Royal Society of Chemistry in the format Book via Copyright Clearance Center.
Figure 6.11 (A) OT-gelator 8 described by Sobczuk, Tamaru, and Shinkai (2011) (B) Structure of gelator 9 described by Edwards and Smith (2012) for cation selective organogels. (C) Gel response to Ag1 and Li1; the Na1 and K1 cations do not go through solgel transition (Edwards & Smith, 2012; Segarra-Maset et al., 2013). Chemical Society reviews by Royal Society of Chemistry (Great Britain) Reproduced with permission of Royal Society of Chemistry in the format Book via Copyright Clearance Center.
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Figure 6.12 (A) Structures of the three gelators, 16, 17, and 18. Comparison of (B) stress sweep, and (C) variable temperature rheology experiments of the dodecane gels (total conc. of gelator 30 mM) of 16 (squares), 16 doped with 10 mol% 17 (triangles), and 16 doped with 10 mol% of 18 (circles). Reprinted and adapted with permission from Das, R. K., Kandanelli, R., Linnanto, J., Bose, K., & Maitra, U. (2010). Supramolecular chirality in organogels: A detailed spectroscopic, morphological, and rheological investigation of gels (and xerogels) derived from alkyl pyrenyl urethanes. Langmuir, 26 (20). Copyright 2010 American Chemical Society.
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The study shows how the length of the alkyl group changes the thermal and mechanical properties (Das, Kandanelli, Linnanto, Bose, & Maitra, 2010). In Fig. 6.13, another self-sorting system made up of sugar amide derivatives of different alkyl lengths and, more interestingly, having different stereochemistry (D and L) form distinct types of fibers (Fig. 6.13b, Fuhrhop & Boettcher, 1990). Vinyl derivatives presented in Fig. 6.14A show that the gel of 24 (vinyl group present) is more stable than 23 (vinyl absent) at B 20 mM and the Tgel of 24 is higher. In the compound 24, the interaction is between amides H-bonds whereas in 23 the H-bonding is between carbamate groups; clearly amideamide H-bonds are stronger than those between carbamates. Images of samples of the gels show thinner fibers for the amide
Figure 6.13 (A) Chemical structures of D-Glu 8 (21) and L-Glu 12 (22). (B) TEM image of self-sorting of 21 and 22 (Bar 5 100 nm) (Fuhrhop & Boettcher, 1990; Segarra-Maset et al., 2013). Reprinted (adapted) with permission from Fuhrhop, J. H., & Boettcher, C. (1990). Stereochemistry and curvature effects in supramolecular organization and separation processes of micellar N-alkylaldonamide mixtures. Journal of the American Chemical Society, 112(5), 17681776. Copyright r 1990, American Chemical Society.
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Figure 6.14 (A) Chemical structures of carbamate gelator 23 and amide gelator 24. FEG-SEM image of dried gels from styrene-DVB (9:1) of (B) gelator 24, scale bar: 200 nm; (C) gelalor 23, scale bar: 300 nm; (D) self-sorting mixture of 23 1 24, scale bar: 200 nm (Moffat & Smith, 2009). Chemical communications by Royal Society of Chemistry (Great Britain) Reproduced with permission of Royal Society of Chemistry in the format Book via Copyright Clearance Center.
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Figure 6.15 (A) Chemical structures of the 1,3,5-cyclohexyltrisamide hydrogelators (25 and 26), the surfactant cetyltrimethylammonium tosylate (CTAT), and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). (B) and (C) CryoTEM pictures of the multicomponent system of CTAT (100 mM) and 25 (2 mM) in water. (D) The effect of CTAT concentration on the Tgelsol of 25 at 3,4 and 5 mM. (E) and (F) Cryo-TEM showing the deformation of the DOPC vesicles by the growth of fibers of 26. (Brizard et al., 2008; Brizard et al. 2009; Buerkle & Rowan, 2012). Reproduced with permission from John Wiley and Sons.
compounds (Figs. 6.14B and 6.14C). By mixing equimolar ratios of the two gelators a self-sorting gel is found (Fig. 6.14D) and it shows the presence of both types of fibers (Moffat & Smith, 2009).
6.1.3
Case 3 Gelator Plus Nongelating Additive Materials
The effect of adding nongelating compounds as surfactants can be appreciated by the inspection of data shown in Fig. 6.15. In this study (Buerkle & Rowan, 2012), two cyclohexyl derivatives
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presenting both H-bond and hydrophobic interactions show SG formation modulated by typical surfactants. Either by micelle effects (cetyl ammonium derivative) (Figs. 6.15B and 6.15C), or vesicles (di-oleoylcholine derivative) (Fig. 6.15D) (Brizard et al., 2008; Brizard, Stuart, & van Esch, 2009).
6.2
Examples of Interesting Stimuli-Related Gels
Use of surfactants with LWMGs can take advantage of the possible states achieved by surfactant association as depicted in Fig. 6.16 (Liu et al., 2014). An example of such structures is presented for glutamic acid gelators derivatives in Fig. 6.17. Another example is presented for sugar derivatives in Fig. 6.18. Fibers and sheets are produced as a function of the length of the alkyl group (Cui, Zheng, Shen, & Wan, 2010). In parallel amphiphile gelators can display several different gels as functions of their interaction with solvents, as depicted below (Fig. 6.19; Jin, Zhang, & Liu, 2013; Liu et al., 2014). An interesting example is found with the association of a tripeptide gelator with quantum dots (QD) (Xue et al., 2017). In this case special attention is paid towards the 3D confinement properties of the system in maintaining the QD as single particles. The gel fibers are seen in Fig. 6.20, and fluorescence from the QDs in this gel is nicely presented in Fig. 6.21.
Figure 6.16 Illustration of the hierarchical self-assembly of amphiphilic gelators to diverse nanostructures. Amphiphilic gelators formed the bilayer structure first, which could be differently packed depending on the molecular structure of the gelator, and further forms multibilayer structures and then self-assembles into various higher order structures (Liu et al., 2014). Reproduced with permission from John Wiley and Sons.
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Figure 6.17 Various tubular structures formed by L-glutamic acid-based amphiphilic gelators, 3(A), 3(C) (Duan, Zhu, & Liu, 2011; Liu et al., 2014; Zhan, Gao, & Liu, 2005) and; 3(B), 3(D), 3(E) (Cao Duan, Zhu, Jiang, & Liu, 2012; Duan, Qin, Zhu, & Liu, 2011; Liu, Chen, Guo, & Jiang, 2010; Liu et al., 2014). Images reproduced with permission from John Wiley and Sons. Chemical communications by Royal Society of Chemistry (Great Britain), reproduced with permission of Royal Society of Chemistry in the format Book via Copyright Clearance Center.
Figure 6.18 The structure of organogelator 5, and SEM images of 5 with different alkyl chain lengths: (A) n 5 5, (B) n 5 8, (C) n 5 11. Reprinted (adapted) with permission from Cui, J., Zheng, Y., Shen, Z., & Wan, X. (2010). Alkoxy tail length dependence of gelation ability and supramolecular chirality of sugar-appended organogelators. Langmuir, 26(19), 1550815515. Copyright r 2010, American Chemical Society.
Figure 6.19 The structure of organogelator 6 (PPLG) in different solvents 11 (Jin et al. 2013; Liu et al., 2014). Images reproduced with permission from John Wiley and Sons.
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Figure 6.20 TEM image of encapsulated nanocrystals in tripeptide gelator (β-AspFF) toluene gel-nanofibril network (Xue et al., 2017). Reprinted from Encapsulation of nanocrystals with responsive gels for spatial optical identification. Supramolecular Chemistry, 29(8), 627632. by permission of the publisher (Taylor & Francis Ltd, http://www.tandfonline.com).
Figure 6.21 Fluorescence emission spectra (excitation wavelength of 365 nm) of QD520 (A) free QDs in toluene solution and (B) QDs encapsulated in gel (Xue et al., 2017). Reprinted from Encapsulation of nanocrystals with responsive gels for spatial optical identification. Supramolecular Chemistry, 29(8), 627632. by permission of the publisher (Taylor & Francis Ltd, http://www.tandfonline.com).
Steroidal derivatives were also employed for the SG process, such as those reported in the review by Svobodova´ et al. (2012). The gels formed using the gelators, such as those in Fig. 6.22, show a photoresponsive feature that is highly adequate for the generation of new materials, as depicted in Fig. 6.23 for another photoactive compound represented in the figure as compound 1.
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Figure 6.22 Structure and photochromic process of bisthienylethene-bridged cholesteryl derivative (Svobodova´, Noponen, Kolehmainen, & Sieva¨nen, 2012; Wang, Shen, Feng, & Tian, 2006). RSC advances by RSC Publishing. Reproduced with permission of RSC Publishing Copyright Clearance Center.
Figure 6.23 Gel formation with 1 and tuning by the formation of charge transfer complexes and chemical or electrochemical oxidation and reduction (Wang et al., 2006; Yang et al. 2012). Journal of materials chemistry by Royal Society of Chemistry (Great Britain) Reproduced with permission of Royal Society of Chemistry, in the format Book via Copyright Clearance Center.
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In a similar fashion, cavities with cyclodextrines, calixarenes, rotaxanes, and similar were exploited for stimuli-responsive gels, as reported in the review by Foster and Steed (2010) and sketched in Fig. 6.24. A good example of these systems is given in Fig. 6.25 for a curcubituryl (CB) gelator.
Figure 6.24 Scheme of the formation of supramolecular polymers using (A) covalently linked β-cyclodextrin (β-CD) dimer and ditopic adamantane guest dimers (with various spacers); (B) α-CD functionalized with a p-t-butoxyaminocinnamoylamino group in the 3-position; (C) a [2]rotaxane capped by α-CD and a trinitrophenyl group between β-CD. Reprinted (adapted) with permission from Ohga, K., Takashima, Y., Takahashi, H., Kawaguchi, Y., Yamaguchi, H., & Harada, A. (2005). Preparation of supramolecular polymers from a cyclodextrin dimer and ditopic guest molecules: Control of structure by linker flexibility. Macromolecules, 38(14), 58975904. Copyright r 2005, American Chemical Society. Reprinted (adapted) with permission from Miyauchi, M., Takashima, Y., Yamaguchi, H., & Harada, A. (2005). Chiral supramolecular polymers formed by host 2 guest interactions. Journal of the American Chemical Society, 127(9), 29842989. Copyright r 2005, American Chemical Society. Reprinted (adapted) with permission from Miyauchi, M., Hoshino, T., Yamaguchi, H., Kamitori, S. Harada, A. (2005). A [2]Rotaxane capped by a cyclodextrin and a guest: Formation of supramolecular [2]Rotaxane polymer. Journal of the American Chemical Society, 127, 2034-2035. Copyright r 2005, American Chemical Society.
Figure 6.25 (Left) Supramolecular structure of curcubituryl [7] (CB[7]). (Right) Behavior of guest-induced stimuliresponsive CB[7] gel; photographs of CB[7] (5 wt.%) gel containing 0.1 equivalent of trans-4,40 -diaminostilbene dihydrochloride before (left) and after (right) UV irradiation (365 nm) for 2 h, on the right (Hwang et al., 2007). Images reproduced with permission from John Wiley and Sons.
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A review on macrocycles for stimuli-responsive gels was presented by Qi and Schalley (2014). This review highlights several systems, as shown in Fig. 6.26. These gelators act as scaffolds in the SG transition towards the final gel, as represented in Fig. 6.27. A good example of these macrocycles is depicted in Fig. 6.28 using a cyclodextrine, a polyacrylamide, and grafene oxide. Gel formation is easily observed (Qi & Schalley, 2014).
Figure 6.26 Macrocycles used in supramolecular gels. Reprinted (adapted) with permission from Qi, Z., & Schalley, C. A. (2014). Exploring macrocycles in functional supramolecular gels: From stimuli responsiveness to systems chemistry. Accounts of Chemical Research, 47(7), 22222233. Copyright r 2014, American Chemical Society.
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Figure 6.27 Representation of solgel process and the scaffold motifs from the gelators. Scheme of different roles of macrocycles in supramolecular gels: (A) Scaffold only (stacking). (B) Directly involved in solgel transition. (C) Appendix to gelators that are not directly involved in gelation. Reprinted (adapted) with permission from Qi, Z., & Schalley, C. A. (2014). Exploring macrocycles in functional supramolecular gels: From stimuli responsiveness to systems chemistry. Accounts of Chemical Research, 47(7), 22222233. Copyright r 2014, American Chemical Society.
6.3
Concluding Remarks
A large number of LMWGs were described in this chapter. The design of gelators, the inclusion of additives, scaffolds, and so on are becoming relatively simple. Their functionalization and control of the nano-, meso-, or microstructure have until now been carried out on an empirical (trial and error) basis. Biocompatible soft materials are a natural target for the development of stimuli-responsive systems, where these stimuli could be a pH gradient, the appearance of an antigen, or something else. Supramolecular gels formed from the assembly of LWMGs are thus easy to tailor, but it is still difficult to design a new gelator that forms a gel with a specific range of properties. The goal is the ability to tune LWMGs from the molecular scale to the gel scale with selected viscosities, morphologies, durabilities, and environmental safety, etc. with selected stimuli. Progress in this area can be easily observed by the exponential growth in the literature. Thus the formation and tailoring of supramolecular gels is just in its early stages and will be increasingly developed for further future applications.
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Figure 6.28 Supramolecular hydrogel hybrid cyclodextrin-based multiple gel 2 sol and sol 2 gel transitions prompted by different stimuli by Jiang’s group (Jiang et al., 2010; Liu et al., 2010; Liu, Chen, & Jiang, 2011; Qi & Schalley; 2014). Reprinted (adapted) with permission from Qi, Z., & Schalley, C. A. (2014). Exploring macrocycles in functional supramolecular gels: From stimuli responsiveness to systems chemistry. Accounts of Chemical Research, 47(7), 22222233. Copyright r 2014, American Chemical Society. Reprinted (adapted) with permission from Liu, J., Chen, G., Guo, M., & Jiang, M. (2010). Dual stimuli-responsive supramolecular hydrogel based on hybrid inclusion complex (HIC). Macromolecules, 43(19), 8086809. Copyright r 2010, American Chemical Society. Reprinted (adapted) with permission from Du, P., Liu, J., Chen, G., & Jiang, M. (2011). Dual responsive supramolecular hydrogel with electrochemical activity. Langmuir, 27(15), 96029608. Copyright r 2011, American Chemical Society. Reprinted (adapted) with permission from Liu, J., Chen, G., & Jiang, M. (2011). Supramolecular hybrid hydrogels from noncovalently functionalized graphene with block copolymers. Macromolecules, 44(19), 76827691. Copyright r 2011, American Chemical Society.
References Araki, K., & Yoshikawa, I. (2005). Nucleobase-containing gelators. Topics in Current Chemistry, 256, 133165. Available from https://doi.org/10.1007/ b107173. Boekhoven, J., Brizard, A. M., Stuart, M. C. A., Florusse, L., Raffy, G., Del Guerzo, A., & van Esch, J. H. (2016). Bio-inspired supramolecular materials by orthogonal self-assembly of hydrogelators and phospholipids. Chemical Science, 7(9), 60216031. Available from https://doi.org/10.1039/c6sc01021k.
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Bouteiller, L., Colombani, O., Lortie, F., & Terech, P. (2005). Thickness transition of a rigid supramolecular polymer. Journal of the American Chemical Society, 127(24), 88938898. Available from https://doi.org/10.1021/ja0511016. Brizard, A., Stuart, M., van Bommel, K., Friggeri, A., de Jong, M., & van Esch, J. (2008). Preparation of nanostructures by orthogonal self-assembly of hydrogelators and surfactants. Angewandte Chemie International Edition, 47 (11), 20632066. Available from https://doi.org/10.1002/anie.200704609. Brizard, A. M., Stuart, M. C. A., & van Esch, J. H. (2009). Self-assembled interpenetrating networks by orthogonal self-assembly of surfactants and hydrogelators. Faraday Discussions, 143, 345. Available from https://doi.org/ 10.1039/b903806j. Buerkle, L. E., & Rowan, S. J. (2012). Supramolecular gels formed from multicomponent low molecular weight species. Chemical Society Reviews, 41(18), 60816102. Available from https://doi.org/10.1039/c2cs35106d. Cao, H., Duan, P., Zhu, X., Jiang, J., & Liu, M. (2012). Self-assembled organic nanotubes through instant gelation and universal capacity for guest molecule encapsulation. Chemistry - A European Journal, 18(18), 55465550. Available from https://doi.org/10.1002/chem.201103654. Christoff-Tempesta, T., Lew, A., & Ortony, J. (2018). Beyond covalent crosslinks: Applications of supramolecular gels. Gels, 4(2), 40. Available from https://doi. org/10.3390/gels4020040. Cui, J., Zheng, Y., Shen, Z., & Wan, X. (2010). Alkoxy tail length dependence of gelation ability and supramolecular chirality of sugar-appended organogelators. Langmuir, 26(19), 1550815515. Available from https://doi. org/10.1021/la101494t. Das, R. K., Kandanelli, R., Linnanto, J., Bose, K., & Maitra, U. (2010). Supramolecular chirality in organogels: A detailed spectroscopic, morphological, and rheological investigation of gels (and xerogels) derived from alkyl pyrenyl urethanes. Langmuir, 26(20), 1614116149. Available from https://doi.org/10.1021/la1029905. Du, P., Liu, J., Chen, G., & Jiang, M. (2011). Dual responsive supramolecular hydrogel with electrochemical activity. Langmuir, 27(15), 96029608. Available from https://doi.org/10.1021/la201843z. Duan, P., Qin, L., Zhu, X., & Liu, M. (2011). Hierarchical self-assembly of amphiphilic peptide dendrons: Evolution of diverse chiral nanostructures through hydrogel formation over a wide pH range. Chemistry - A European Journal, 17(23), 63896395. Available from https://doi.org/10.1002/ chem.201003049. Duan, P., Zhu, X., & Liu, M. (2011). Isomeric effect in the self-assembly of pyridine-containing L-glutamic lipid: Substituent position controlled morphology and supramolecular chirality. Chemical Communications, 47(19), 5569. Available from https://doi.org/10.1039/c1cc10813a. Edwards, W., & Smith, D. K. (2012). Cation-responsive silver-selective organogel— exploiting silveralkene interactions in the gel-phase. Chemical Communications, 48(22), 2767. Available from https://doi.org/10.1039/c2cc17854k. Estroff, L. A., & Hamilton, A. D. (2004). Water gelation by small organic molecules. Chemical Reviews, 104(3), 12011218. Available from https://doi. org/10.1021/cr0302049. Feng, Q., Wei, K., Zhang, K., Yang, B., Tian, F., Wang, G., & Bian, L. (2018). Onepot solvent exchange preparation of non-swellable, thermoplastic, stretchable and adhesive supramolecular hydrogels based on dual synergistic physical crosslinking. NPG Asia Materials, 10(1), e455. Available from https://doi.org/ 10.1038/am.2017.208. Flory, P. J. (1974). Introductory lecture. Faraday Discussions of the Chemical Society, 57, 718. Available from https://doi.org/10.1039/DC9745700007.
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Foster, J. A., & Steed, J. W. (2010). Exploiting cavities in supramolecular gels. Angewandte Chemie International Edition, 49(38), 67186724. Available from https://doi.org/10.1002/anie.201000070. Fuhrhop, J. H., & Boettcher, C. (1990). Stereochemistry and curvature effects in supramolecular organization and separation processes of micellar N-alkylaldonamide mixtures. Journal of the American Chemical Society, 112 (5), 17681776. Available from https://doi.org/10.1021/ja00161a018. Huang, X., Terech, P., Raghavan, S. R., & Weiss, R. G. (2005). Kinetics of 5α-cholestan-3β-ylN-(2-naphthyl)carbamate/n-alkane organogel formation and its influence on the fibrillar networks. Journal of the American Chemical Society, 127(12), 43364344. Available from https://doi.org/10.1021/ja0426544. Hwang, I., Jeon, W. S., Kim, H.-J., Kim, D., Kim, H., Selvapalam, N., . . . Kim, K. (2007). Cucurbit[7]uril: A simple macrocyclic, pH-triggered hydrogelator exhibiting guest-induced stimuli-responsive behavior. Angewandte Chemie, 119(12), 214217. Available from https://doi.org/10.1002/ange.200603149. Isare, B., Bouteiller, L., Ducouret, G., & Lequeux, F. (2009). Tuning reversible supramolecular polymer properties through co-monomer addition. Supramolecular Chemistry, 21(5), 416421. Available from https://doi.org/ 10.1080/10610270802105106. Jin, Q., Zhang, L., & Liu, M. (2013). Solvent-polarity-tuned morphology and inversion of supramolecular chirality in a self-assembled pyridylpyrazolelinked glutamide derivative: Nanofibers, nanotwists, nanotubes, and microtubes. Chemistry - A European Journal, 19(28), 92349241. Available from https://doi.org/10.1002/chem.201300612. Liu, J., Chen, G., Guo, M., & Jiang, M. (2010). Dual stimuli-responsive supramolecular hydrogel based on hybrid inclusion complex (HIC). Macromolecules, 43(19), 80868093. Available from https://doi.org/10.1021/ ma101437k. Liu, J., Chen, G., & Jiang, M. (2011). Supramolecular hybrid hydrogels from noncovalently functionalized graphene with block copolymers. Macromolecules, 44(19), 76827691. Available from https://doi.org/10.1021/ ma201620w. Miyauchi, M., Hoshino, T., Yamaguchi, H., Kamitori, S., & Harada, A. (2005). A [2]Rotaxane capped by a cyclodextrin and a guest: Formation of supramolecular [2]Rotaxane polymer. Journal of the American Chemical Society, 127, 20342035. Available from https://doi.org/10.1021/ja042840. Miyauchi, M., Takashima, Y., Yamaguchi, H., & Harada, A. (2005). Chiral supramolecular polymers formed by host 2 guest interactions. Journal of the American Chemical Society, 127(9), 29842989. Available from https://doi. org/10.1021/ja043289j. Moffat, J. R., & Smith, D. K. (2009). Controlled self-sorting in the assembly of “multi-gelator” gels. Chemical Communications, 3, 316318. Available from https://doi.org/10.1039/b818058j. Moraes, T. F. B., Schimidt, M. F. R. A., Bacani, R., Weber, G., Politi, M. J., Castanheira, B., . . . Triboni, E. R. (2018). Polysilsesquioxane naphthalenediimide thermo and photochromic gels. Journal of Luminescence. Available from https://doi.org/10.1016/j.jlumin.2018.08.036, In Press. Ohga, K., Takashima, Y., Takahashi, H., Kawaguchi, Y., Yamaguchi, H., & Harada, A. (2005). Preparation of supramolecular polymers from a cyclodextrin dimer and ditopic guest molecules: Control of structure by linker flexibility. Macromolecules, 38(14), 58975904. Available from https://doi.org/10.1021/ ma0508606. Pal, K., & Banerjee, I. (2018). Polymeric gels - characterization, properties and biomedical applications (1st ed). Elsevier. Available from http://doi.org/ 10.15713/ins.mmj.3.
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Qi, Z., & Schalley, C. A. (2014). Exploring macrocycles in functional supramolecular gels: From stimuli responsiveness to systems chemistry. Accounts of Chemical Research, 47(7), 22222233. Available from https://doi. org/10.1021/ar500193z. Segarra-Maset, M. D., Nebot, V. J., Miravet, J. F., & Escuder, B. (2013). Control of molecular gelation by chemical stimuli. Chemical Society Reviews, 42(17), 70867098. Available from https://doi.org/10.1039/c2cs35436e. Smith, D. K. (2006). Dendritic supermolecules towards controllable nanomaterials. Chemical Communication, 1, 3444. Available from https:// doi.org/10.1039/b507416a. Sobczuk, A. A., Tamaru, S., & Shinkai, S. (2011). New strategy for controlling the oligothiophene aggregation mode utilizing the gel-to-sol phase transition induced by crown-alkali metal interactions. Chemical Communications, 47 (11), 3093. Available from https://doi.org/10.1039/c1cc00026h. Steed, J. W. (2011). Supramolecular gel chemistry: Developments over the last decade. Chemical Communications, 47(5), 13791383. Available from https:// doi.org/10.1039/c0cc03293j. Svobodova´, H., Noponen, V., Kolehmainen, E., & Sieva¨nen, E. (2012). Recent advances in steroidal supramolecular gels. RSC Advances, 2(12), 4985. Available from https://doi.org/10.1039/c2ra01343f. Terech, P., & Weiss, R. G. (1997). Low molecular mass gelators of organic liquids and the properties of their gels. Chemical Reviews, 97(8), 31333160. Available from https://doi.org/10.1021/cr9700282. Wang, S., Shen, W., Feng, Y., & Tian, H. (2006). A multiple switching bisthienylethene and its photochromic fluorescent organogelator. Chemical Communications, 14, 1497. Available from https://doi.org/10.1039/b515412j. Weng, W., Beck, J. B., Jamieson, A. M., & Rowan, S. J. (2006). Understanding the mechanism of gelation and stimuli-responsive nature of a class of metallosupramolecular gels. Journal of the American Chemical Society, 128(35), 1166311672. Available from https://doi.org/10.1021/ja063408q. Xue, P., He, T., Wu, H., Xie, H., Shen, R., Yue, F., . . . Zhang, Y. (2017). Encapsulation of nanocrystals with responsive gels for spatial optical identification. Supramolecular Chemistry, 29(8), 627632. Available from https://doi.org/10.1080/10610278.2017.1298763. Yang, X., Zhang, G., & Zhang, D. (2012). Stimuli responsive gels based on low molecular weight gelators. Journal of Materials Chemistry, 22(1), 3850. Available from https://doi.org/10.1039/c1jm13205a. Zhan, C., Gao, P., & Liu, M. (2005). Self-assembled helical spherical-nanotubes from an l-glutamic acid based bolaamphiphilic low molecular mass organogelator. Chemical Communications, 4, 462. Available from https://doi. org/10.1039/b413259a. ˇ Zinic, M., Vo¨gtle, F., & Fages, F. (2005). Cholesterol-based gelators. Topics in Current Chemistry, 256, 3976. Available from https://doi.org/10.1007/ b107171.
Further Reading Hirst, A. R., Miravet, J. F., Escuder, B., Noirez, L., Castelletto, V., Hamley, I. W., & Smith, D. K. (2008). Self-assembly of two-component gels: Stoichiometric control and component selection. Chemistry - A European Journal, 15(2), 372379. Available from https://doi.org/10.1002/chem.200801475.
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Lan, Y., Corradini, M. G., Weiss, R. G., Raghavan, S. R., & Rogers, M. A. (2015). To gel or not to gel: Correlating molecular gelation with solvent parameters. Chemical Society Reviews, 44(17), 60356058. Available from https://doi.org/ 10.1039/c5cs00136f. Piepenbrock, M.-O. M., Lloyd, G. O., Clarke, N., & Steed, J. W. (2010). Metal- and anion-binding supramolecular gels. Chemical Reviews, 110(4), 19602004. Available from https://doi.org/10.1021/cr9003067. Rao, K. V., Jayaramulu, K., Maji, T. K., & George, S. J. (2010). Supramolecular hydrogels and high-aspect-ratio nanofibers through charge-transfer-induced alternate coassembly. Angewandte Chemie International Edition, 49(25), 42184222. Available from https://doi.org/10.1002/anie.201000527. Shu, T., Wu, J., Lu, M., Chen, L., Yi, T., Li, F., & Huang, C. (2008). Tunable redgreenblue fluorescent organogels on the basis of intermolecular energy transfer. Journal of Materials Chemistry, 18(8), 886. Available from https://doi. org/10.1039/b715462c. Zhang, L., Wang, X., Wang, T., & Liu, M. (2014). Tuning soft nanostructures in self-assembled supramolecular gels: From morphology control to morphology-dependent functions. Small, 11(910), 10251038. Available from https://doi.org/10.1002/smll.201402075. Zhu, X., Li, Y., Duan, P., & Liu, M. (2010). Self-assembled ultralong chiral nanotubes and tuning of their chirality through the mixing of enantiomeric components. Chemistry - A European Journal, 16(27), 80348040. Available from https://doi.org/10.1002/chem.201000595.
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Abstract The formation of ordered nanostructures within the gel phase from colloidal systems can be driven by different types of external stimuli such as temperature, light, ultrasound, electric and magnetic fields, and through the processes of transformation, diffusion, and chemical sensitivity. This chapter illustrates some gains from this area of research and presents building blocks of enormous potential for the development of intelligent materials based on molecules that will bring improvements and improvements to systems based on nanotechnology. Keywords: Ordered nanostrucutures; solgel systems; responsive gels; external stimuli; molecular building blocks; nanotechnology CHAPTER OUTLINE 6.1 SolGel Process Under Different Stimuli 111 6.1.1 Case 1. Two (Multi-)Components Gels 116 6.1.2 Case 2. Gelators Coassembling 120 6.1.3 Case 3 Gelator Plus Nongelating Additive Materials 6.2 Examples of Interesting Stimuli-Related Gels 127 6.3 Concluding Remarks 134 References 135 Further Reading 138
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SolGel Process Under Different Stimuli
In this chapter, the solgel systems that respond to external stimuli to undergo the solgel (SG) process are spotlighted. The large horizon of applications and uses of solgel systems confer to the stimuli-responsive systems a special position given their quite large spectra of low molecular weight gelators (LMWG)
Nano Design for Smart Gels. DOI: https://doi.org/10.1016/B978-0-12-814825-9.00006-0 © 2019 Elsevier Inc. All rights reserved.
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and easy and extended ways to promote the gelation (Buerkle & Rowan, 2012; Qi & Schalley, 2014; Liu et al., 2014). A very wide range of materials have been recognized since the 1860s to form gels, often with quite different characteristics. In 1974 four main categories of gel were distinguished by Flory (1974): (1) well-ordered lamellar structures (e.g., phospholipids); (2) disordered covalent polymeric networks (e.g., vulcanized rubber and phenolic resins); (3) polymer networks formed through physical aggregation (e.g., gelatin); and (4) particulate, disordered structures (e.g., precipitates consisting of highly anisotropic particles or reticular networks of fibers). Since that time interest within the chemical and materials science communities has increasingly focused on the variety of gels in category (4) that arise from the self-assembly of low molecular weight compounds (often called LMWGs) into fibrous gel networks. These materials are classified as organogels in cases where the solvent is organic and hydrogels when the solvent is water. Organogels derived from LMWGs were first comprehensively reviewed by the Weiss group (Huang, Terech, Raghavan, & Weiss, 2005; Rogers et al., 2015; Terech & Weiss, 1997), while Hamilton and Estroff summarized those of hydrogels (Estroff & Hamilton, 2004). Examples of LMWGs (Steed, 2011) include ureas, amides, nucleobases, porphyrin derivatives, dendrimers, surfactants, sugars, fatty acids, and amino acids amongst many others known to form fibers. Although the SG process is difficult to fully predict, it is predictable within these types of compounds and thus has allowed systematic studies of structureproperty relationships, as will be exemplified below. Over the past decade, there have been a number of important reviews pertaining to particular classes of LMWGs, such as dendritic gels (Smith, 2006), nucleobases (Araki & Yoshikawa, 2005), amides ˇ and ureas (Zinic, Vo¨gtle, & Fages, 2005), and metallogels (Steed et al., 2010), as well as more general overviews that chart the tremendous progression in our understanding of supramolecular gels and the dramatically increased scope of the gelators that form them (Araki & Yoshikawa, 2005; Estroff & Hamilton, ˇ 2004; Steed, 2011; Zinic et al., 2005). Gels generated by the association of LMWGs can assemble into various nanostructures through noncovalent interactions that combine the elastic behavior of solids with the microviscous properties of fluids. The self-assembly is derived from hydrogen-bonding, π 2 π-stacking, electrostatic interactions, dipoledipole interactions, van der Waals interactions, and metal-to-ligand coordination between the gelators, while the gelatorsolvent interactions are able to modulate these
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interactions and open a horizon for the development of stimuliresponsive and smart soft materials (Pal & Banerjee, 2018). While there are many important fundamental issues related to the gelation process, such as the design of the gelator, synergism of various noncovalent interactions between gelators and gelatorsolvents, the balances between gelation and crystallization, and so on, the self-assembled nanostructures that form during gelation are very interesting. These nanostructures have many unique features, such as the flexibility to respond to external stimuli, morphological diversity, ease of fabrication in large quantities, and so on. The high order structures formed include nanospheres, nanofibers, nanoribbons, nanosheets, etc. as depicted in Figs. 6.1 and 6.2. Examples of supramolecular gels are found in three of the four categories of gels defined by Flory (1974): (1) category 1:
Figure 6.1 Typical representation of supramolecular gels from LMWGs followed by association and gelation. The top figure depicts possible applications of supramolecular gels. Reproduced from Christoff-Tempesta, T., Lew, A., & Ortony, J. (2018). Beyond covalent crosslinks: Applications of supramolecular gels. Gels, 4(2), 40 under the Creative Commons Attribution License (CC BY 4.0).
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Figure 6.2 Applications of supramolecular gels. Reproduced from Christoff-Tempesta, T., Lew, A., & Ortony, J. (2018). Beyond covalent crosslinks: Applications of supramolecular gels. Gels, 4(2), 40 under the Creative Commons Attribution License (CC BY 4.0).
high-order lamellar gels (Boekhoven et al., 2016); (2) category 3: semiordered physical networks (Feng et al., 2018); and (3) category 4: disordered particulate gels (Buerkle & Rowan, 2012). Interestingly, one kind of gelator molecule can sometimes form various morphologies, which could be controlled by the gelation environment. In addition, the nanostructures formed through gel formation are hierarchically formed, which could sometimes cover the molecular scale from nano- to meso- and microscales, which is closely related to the formation of biological self-assemblies. Finally, the gel nanostructures are relatively uniform and can be fabricated in larger quantities, thus making them templates for the fabrication of other functional materials. Returning to the solgel processes, it is convenient to recall that gels are soft materials that are continuous in structure and solid-like in rheological behavior. They can be categorized into chemical gels and physical gels. Usually chemical gels are covalently cross-linked polymers and are not able to be redissolved, whereas physical gels based on intermolecular interactions can be redissolved, showing cycled reversibility. Self-assembly of LMWGs, through weak intermolecular interactions such as H-bonding, π 2 π-stacking and van der Waals interactions, can lead to solvent gelation and the formation of physical gels. In the gel phase, molecules of LMWGs are self-assembled into entangled three-dimensional networks, which entrap and immobilize solvent molecules by capillary forces. These physical gels are characterized by the following unique features: (1) they are generally composed of hierarchically fiber-like structures, which cross-link to form a three-dimensional (3D) network; (2) they are thermally responsive, that is, the gels are transformed into solutions after heating and they can be
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Figure 6.3 Classification of the different stimuli: physical and chemical. (A) Type of stimuli (Segarra-Maset et al., 2013). (B) External stimuli (Yang, Zhang, & Zhang, 2012). Journal of materials chemistry by Royal Society of Chemistry (Great Britain) Reproduced with permission of Royal Society of Chemistry, in the format Book via Copyright Clearance Center.
recovered after cooling, since molecules of LMWGs within the fibril structures are associated through weak noncovalent interactions such as H-bonding, π 2 π-stacking, and van der Waals forces; (3) they are viscoelastic, owing to the fact that solvent molecules are macroscopically immobilized by surface tensions and capillary forces; and (4) they possess the respective critical gelation temperatures (Tgel), at which the gelsol transitions occur, and the respective critical gelator concentrations, above which gels form in certain solvents at room temperature (Pal & Banerjee, 2018). The kinds of stimuli on various gels are depicted in Fig. 6.3 which covers all kinds of studies up to the present (see Fig. 6.10; Segarra-Maset, Nebot, Miravet, & Escuder, 2013). Three main conditions were proposed for supramolecular gels formation (Buerkle & Rowan, 2012), namely, (1) two components LMWG gel phase; (2) two gelators; and (3) gelator plus no additive component (Fig. 6.4). Following this division, the examples given below represent the current state-of-the-art of SG formation. (1) Two-component or multicomponent gel-phase materials self-associate to yield a gel (Fig. 6.4A). (2) Two-gelator-component gels where (a) both gelators interact with each other to form cofibers or (b) the two gelators self-sort resulting in two different nanofibers within the gel (Fig. 6.4B). (3) Gelator plus additive component gels (Fig. 6.4C).
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Figure 6.4 Illustration of three main classes of two-component supramolecular gels. Chemical Society reviews by Royal Society of Chemistry (Great Britain) Reproduced with permission of Royal Society of Chemistry in the format Book via Copyright Clearance Center.
6.1.1
Case 1. Two (Multi-)Components Gels
Fig. 6.4 from Rowan’s review (Buerkle & Rowan, 2012), encompasses examples of hydrogen-bonded carboxylic acid dendrons mixed with diamines. They have shown that not only is the chain length of a linear aliphatic diamine 1 n important in controlling the thermal stability of the gel, but that changing the ratio of the aliphatic diamine (1 n) to 2 results in significant changes to the morphology and thermal stability of the assemblies. For details refer to Buerkle & Rowan (2012) (Fig. 6.5). Other examples involves an alkyl (C12) methyl viologen, an active redox center, and a polycyclic tetracarboxylate derivative as depicted in Fig. 6.6 (from Buerkle & Rowan, 2012). In the absence of the viologen’s positively charged group no gels are found, and carboxylate compound fibers are formed.
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Figure 6.5 (A) Structures of linear aliphatic diamine (1), dendron (2), and 1,4/1,3/ 1,2-diaminobenzene (3), (4), and (5). (B) Results from NMR analysis shows that the dendrons preferentially incorporate 3 over 4 and 5 (Brizard et al., 2008). Reproduced with permission from John Wiley and Sons.
In a parallel fashion, when mixing a class of surfactant-like LMWGs based on charge stabilization, researchers have shown that the absorption and fluorescence of the gel can be tailored, depending on the gel component composition, given the spectral properties of the naphthalimides and hemocyanine chromophores (Yi et al., 2008). Work from Moraes et al (2018) showed an interesting photochromic system using naphthalene diimides anchored to a (3-aminopropyl)triethoxysilane as a SG synthon that deserves further development as a photoredox center for use like viologen. This type of approach relates
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Figure 6.6 (A) Structures of the donor and acceptor molecules (9 and 10). (B) FE-SEM image of the spherical assemblies formed by 10 alone in water (1 3 1024 M). (C) AFM of fibers resulting from the combination of 9 and 10 in a 1:1 ratio. (Buerkle & Rowan, 2012; George et al., 2010). Reproduced with permission from John Wiley and Sons.
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directly to the kind of stimulus that the mixture responds to, and accordingly these gels can be classified as photo-, chemo-, electro-, or mechanoactive polymers, as presented in this chapter. Ditopic ligands constitute a special case of LWMGs for the obtaining of supramolecular gels having a metallocenter, as presented in Fig. 6.7 (Weng, Beck, Jamieson, & Rowan, 2006). Those materials contain gel features plus the properties of the metal center confined in the gel. A metallo-supramolecular based gel showed in Fig. 6.7 synthesized with Zn(ClO4)2 reacts in acetronitrile (A) (Fig. 6.7) react nicely as the properties of supramolecular gels cited above (see page 5) and shown in Fig. 6.8.
Figure 6.7 Schematic representation of the formation of metallo-supramolecular polymeric aggregates using ditopic ligand end-capped monomers with metal salts: (A) Zn(ClO4)2; (B) Zn(ClO4)2 and La(NO3)3; and (C) Zn2(ClO4)2 and La(ClO4)3, mixed with monomer 1 in acetonitrile as solvent (percentages of metal ion salts are calculated as mol % relative to 1). Reprinted (adapted) with permission from Weng, W., Beck, J. B., Jamieson, A. M., & Rowan, S. J. (2006). Understanding the mechanism of gelation and stimuli-responsive nature of a class of metallo-supramolecular gels. Journal of the American Chemical Society, 128(35), 1166311672. Copyright r 2006, American Chemical Society.
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Figure 6.8 Gel (A) (11 wt.% in acetonitrile) exhibits the typical multistimuli responsive behavior observed for this class of metallo-supramolecular gels (Weng et al., 2006). Reprinted (adapted) with permission from Weng, W., Beck, J. B., Jamieson, A. M., & Rowan, S. J. (2006). Understanding the mechanism of gelation and stimuli-responsive nature of a class of metallo-supramolecular gels. Journal of the American Chemical Society, 128(35), 1166311672. Copyright r 2006, American Chemical Society.
6.1.2
Case 2. Gelators Coassembling
In this condition, various gelators are combined to form gels that present typical viscosities changes at Tgel, an example is provided in Fig. 6.9 (Buerkle & Rowan, 2012) for polyurea derivatives. This data suggests that the viscosity transition (Fig. 6.9b) corresponds to the change in the assembly of the bisureas from thick filaments at high concentrations and low temperatures to thin filaments (of the width of a single molecule) at low concentrations and higher temperatures. The concentration and the temperature at which this transition occurs appears to depend on the nature of the alkyl groups attached to the bisurea binding motif with 19 and 20 transitioning at 43 C and 63 C, respectively. Mixing these two bisurea gelators results in a systematic change in this transition temperature. The transitions were followed by FT-IR analysis (Fig. 6.9c) and monitored by the ratio of the 3328 cm21 vibration peak (hydrogen-bonded NH groups) to the 3300 cm21 reference one. The data show that this transition temperature can be systematically varied by simply changing the molar ratio of the two components (Buerkle & Rowan, 2012). In the case of polyureas, an advantage can be taken from the
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Figure 6.9 (A) Chemical structures of bisurea 19 and 20. (B) The relative viscosity of 19 in toluene versus temperature at two different concentrations. (Heating run (open symbols); cooling run (full symbols)). (C) Ratio of the absorbance at 3328 and 3000 cm1 for 12.5 mM solution of 19/20 mixtures (E100/0; B75/25; and 50/50; m0/100) versus Temperature. (A)(B) Reprinted (adapted) with permission from Bouteiller, L., Colombani, O., Lortie, F., & Terech, P. (2005). Thickness transition of a rigid supramolecular polymer. Journal of the American Chemical Society, 127(24), 88938898. Copyright r 2005, American Chemical Society. (C) Reprinted from Isare, B., Bouteiller, L., Ducouret, G., & Lequeux, F. (2009). Tuning reversible supramolecular polymer properties through co-monomer addition. Supramolecular Chemistry, 21(5), 416421 by permission of the publisher (Taylor & Francis Ltd, http://www.tandfonline.com).
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ability of these compounds to coordinate with anions (Fig. 6.10) instead of cations. Crown ethers have been pursued (Fig. 6.11). Another striking example was observed by mixing different alkyl pyrenyl urethanes presented in Fig. 6.12. In these experiments sharp differences in Tgel and in the stress amplitude were observed. Data for carbamate derivatives exemplify systems that auto- or self-sort towards gelation (Fig. 6.12). These molecules contain the pyrenyl moieties that associate via π 2 π-stacking, whereas the carbamate groups associate through H-bonding.
Figure 6.10 Urea anion interaction model (Segarra-Maset et al., 2013). Chemical Society reviews by Royal Society of Chemistry (Great Britain) Reproduced with permission of Royal Society of Chemistry in the format Book via Copyright Clearance Center.
Figure 6.11 (A) OT-gelator 8 described by Sobczuk, Tamaru, and Shinkai (2011) (B) Structure of gelator 9 described by Edwards and Smith (2012) for cation selective organogels. (C) Gel response to Ag1 and Li1; the Na1 and K1 cations do not go through solgel transition (Edwards & Smith, 2012; Segarra-Maset et al., 2013). Chemical Society reviews by Royal Society of Chemistry (Great Britain) Reproduced with permission of Royal Society of Chemistry in the format Book via Copyright Clearance Center.
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Figure 6.12 (A) Structures of the three gelators, 16, 17, and 18. Comparison of (B) stress sweep, and (C) variable temperature rheology experiments of the dodecane gels (total conc. of gelator 30 mM) of 16 (squares), 16 doped with 10 mol% 17 (triangles), and 16 doped with 10 mol% of 18 (circles). Reprinted and adapted with permission from Das, R. K., Kandanelli, R., Linnanto, J., Bose, K., & Maitra, U. (2010). Supramolecular chirality in organogels: A detailed spectroscopic, morphological, and rheological investigation of gels (and xerogels) derived from alkyl pyrenyl urethanes. Langmuir, 26 (20). Copyright 2010 American Chemical Society.
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The study shows how the length of the alkyl group changes the thermal and mechanical properties (Das, Kandanelli, Linnanto, Bose, & Maitra, 2010). In Fig. 6.13, another self-sorting system made up of sugar amide derivatives of different alkyl lengths and, more interestingly, having different stereochemistry (D and L) form distinct types of fibers (Fig. 6.13b, Fuhrhop & Boettcher, 1990). Vinyl derivatives presented in Fig. 6.14A show that the gel of 24 (vinyl group present) is more stable than 23 (vinyl absent) at B 20 mM and the Tgel of 24 is higher. In the compound 24, the interaction is between amides H-bonds whereas in 23 the H-bonding is between carbamate groups; clearly amideamide H-bonds are stronger than those between carbamates. Images of samples of the gels show thinner fibers for the amide
Figure 6.13 (A) Chemical structures of D-Glu 8 (21) and L-Glu 12 (22). (B) TEM image of self-sorting of 21 and 22 (Bar 5 100 nm) (Fuhrhop & Boettcher, 1990; Segarra-Maset et al., 2013). Reprinted (adapted) with permission from Fuhrhop, J. H., & Boettcher, C. (1990). Stereochemistry and curvature effects in supramolecular organization and separation processes of micellar N-alkylaldonamide mixtures. Journal of the American Chemical Society, 112(5), 17681776. Copyright r 1990, American Chemical Society.
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Figure 6.14 (A) Chemical structures of carbamate gelator 23 and amide gelator 24. FEG-SEM image of dried gels from styrene-DVB (9:1) of (B) gelator 24, scale bar: 200 nm; (C) gelalor 23, scale bar: 300 nm; (D) self-sorting mixture of 23 1 24, scale bar: 200 nm (Moffat & Smith, 2009). Chemical communications by Royal Society of Chemistry (Great Britain) Reproduced with permission of Royal Society of Chemistry in the format Book via Copyright Clearance Center.
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Figure 6.15 (A) Chemical structures of the 1,3,5-cyclohexyltrisamide hydrogelators (25 and 26), the surfactant cetyltrimethylammonium tosylate (CTAT), and 1,2-dioleoyl-sn-glycero-3-phosphocholine (DOPC). (B) and (C) CryoTEM pictures of the multicomponent system of CTAT (100 mM) and 25 (2 mM) in water. (D) The effect of CTAT concentration on the Tgelsol of 25 at 3,4 and 5 mM. (E) and (F) Cryo-TEM showing the deformation of the DOPC vesicles by the growth of fibers of 26. (Brizard et al., 2008; Brizard et al. 2009; Buerkle & Rowan, 2012). Reproduced with permission from John Wiley and Sons.
compounds (Figs. 6.14B and 6.14C). By mixing equimolar ratios of the two gelators a self-sorting gel is found (Fig. 6.14D) and it shows the presence of both types of fibers (Moffat & Smith, 2009).
6.1.3
Case 3 Gelator Plus Nongelating Additive Materials
The effect of adding nongelating compounds as surfactants can be appreciated by the inspection of data shown in Fig. 6.15. In this study (Buerkle & Rowan, 2012), two cyclohexyl derivatives
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presenting both H-bond and hydrophobic interactions show SG formation modulated by typical surfactants. Either by micelle effects (cetyl ammonium derivative) (Figs. 6.15B and 6.15C), or vesicles (di-oleoylcholine derivative) (Fig. 6.15D) (Brizard et al., 2008; Brizard, Stuart, & van Esch, 2009).
6.2
Examples of Interesting Stimuli-Related Gels
Use of surfactants with LWMGs can take advantage of the possible states achieved by surfactant association as depicted in Fig. 6.16 (Liu et al., 2014). An example of such structures is presented for glutamic acid gelators derivatives in Fig. 6.17. Another example is presented for sugar derivatives in Fig. 6.18. Fibers and sheets are produced as a function of the length of the alkyl group (Cui, Zheng, Shen, & Wan, 2010). In parallel amphiphile gelators can display several different gels as functions of their interaction with solvents, as depicted below (Fig. 6.19; Jin, Zhang, & Liu, 2013; Liu et al., 2014). An interesting example is found with the association of a tripeptide gelator with quantum dots (QD) (Xue et al., 2017). In this case special attention is paid towards the 3D confinement properties of the system in maintaining the QD as single particles. The gel fibers are seen in Fig. 6.20, and fluorescence from the QDs in this gel is nicely presented in Fig. 6.21.
Figure 6.16 Illustration of the hierarchical self-assembly of amphiphilic gelators to diverse nanostructures. Amphiphilic gelators formed the bilayer structure first, which could be differently packed depending on the molecular structure of the gelator, and further forms multibilayer structures and then self-assembles into various higher order structures (Liu et al., 2014). Reproduced with permission from John Wiley and Sons.
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Figure 6.17 Various tubular structures formed by L-glutamic acid-based amphiphilic gelators, 3(A), 3(C) (Duan, Zhu, & Liu, 2011; Liu et al., 2014; Zhan, Gao, & Liu, 2005) and; 3(B), 3(D), 3(E) (Cao Duan, Zhu, Jiang, & Liu, 2012; Duan, Qin, Zhu, & Liu, 2011; Liu, Chen, Guo, & Jiang, 2010; Liu et al., 2014). Images reproduced with permission from John Wiley and Sons. Chemical communications by Royal Society of Chemistry (Great Britain), reproduced with permission of Royal Society of Chemistry in the format Book via Copyright Clearance Center.
Figure 6.18 The structure of organogelator 5, and SEM images of 5 with different alkyl chain lengths: (A) n 5 5, (B) n 5 8, (C) n 5 11. Reprinted (adapted) with permission from Cui, J., Zheng, Y., Shen, Z., & Wan, X. (2010). Alkoxy tail length dependence of gelation ability and supramolecular chirality of sugar-appended organogelators. Langmuir, 26(19), 1550815515. Copyright r 2010, American Chemical Society.
Figure 6.19 The structure of organogelator 6 (PPLG) in different solvents 11 (Jin et al. 2013; Liu et al., 2014). Images reproduced with permission from John Wiley and Sons.
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Figure 6.20 TEM image of encapsulated nanocrystals in tripeptide gelator (β-AspFF) toluene gel-nanofibril network (Xue et al., 2017). Reprinted from Encapsulation of nanocrystals with responsive gels for spatial optical identification. Supramolecular Chemistry, 29(8), 627632. by permission of the publisher (Taylor & Francis Ltd, http://www.tandfonline.com).
Figure 6.21 Fluorescence emission spectra (excitation wavelength of 365 nm) of QD520 (A) free QDs in toluene solution and (B) QDs encapsulated in gel (Xue et al., 2017). Reprinted from Encapsulation of nanocrystals with responsive gels for spatial optical identification. Supramolecular Chemistry, 29(8), 627632. by permission of the publisher (Taylor & Francis Ltd, http://www.tandfonline.com).
Steroidal derivatives were also employed for the SG process, such as those reported in the review by Svobodova´ et al. (2012). The gels formed using the gelators, such as those in Fig. 6.22, show a photoresponsive feature that is highly adequate for the generation of new materials, as depicted in Fig. 6.23 for another photoactive compound represented in the figure as compound 1.
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Figure 6.22 Structure and photochromic process of bisthienylethene-bridged cholesteryl derivative (Svobodova´, Noponen, Kolehmainen, & Sieva¨nen, 2012; Wang, Shen, Feng, & Tian, 2006). RSC advances by RSC Publishing. Reproduced with permission of RSC Publishing Copyright Clearance Center.
Figure 6.23 Gel formation with 1 and tuning by the formation of charge transfer complexes and chemical or electrochemical oxidation and reduction (Wang et al., 2006; Yang et al. 2012). Journal of materials chemistry by Royal Society of Chemistry (Great Britain) Reproduced with permission of Royal Society of Chemistry, in the format Book via Copyright Clearance Center.
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In a similar fashion, cavities with cyclodextrines, calixarenes, rotaxanes, and similar were exploited for stimuli-responsive gels, as reported in the review by Foster and Steed (2010) and sketched in Fig. 6.24. A good example of these systems is given in Fig. 6.25 for a curcubituryl (CB) gelator.
Figure 6.24 Scheme of the formation of supramolecular polymers using (A) covalently linked β-cyclodextrin (β-CD) dimer and ditopic adamantane guest dimers (with various spacers); (B) α-CD functionalized with a p-t-butoxyaminocinnamoylamino group in the 3-position; (C) a [2]rotaxane capped by α-CD and a trinitrophenyl group between β-CD. Reprinted (adapted) with permission from Ohga, K., Takashima, Y., Takahashi, H., Kawaguchi, Y., Yamaguchi, H., & Harada, A. (2005). Preparation of supramolecular polymers from a cyclodextrin dimer and ditopic guest molecules: Control of structure by linker flexibility. Macromolecules, 38(14), 58975904. Copyright r 2005, American Chemical Society. Reprinted (adapted) with permission from Miyauchi, M., Takashima, Y., Yamaguchi, H., & Harada, A. (2005). Chiral supramolecular polymers formed by host 2 guest interactions. Journal of the American Chemical Society, 127(9), 29842989. Copyright r 2005, American Chemical Society. Reprinted (adapted) with permission from Miyauchi, M., Hoshino, T., Yamaguchi, H., Kamitori, S. Harada, A. (2005). A [2]Rotaxane capped by a cyclodextrin and a guest: Formation of supramolecular [2]Rotaxane polymer. Journal of the American Chemical Society, 127, 2034-2035. Copyright r 2005, American Chemical Society.
Figure 6.25 (Left) Supramolecular structure of curcubituryl [7] (CB[7]). (Right) Behavior of guest-induced stimuliresponsive CB[7] gel; photographs of CB[7] (5 wt.%) gel containing 0.1 equivalent of trans-4,40 -diaminostilbene dihydrochloride before (left) and after (right) UV irradiation (365 nm) for 2 h, on the right (Hwang et al., 2007). Images reproduced with permission from John Wiley and Sons.
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A review on macrocycles for stimuli-responsive gels was presented by Qi and Schalley (2014). This review highlights several systems, as shown in Fig. 6.26. These gelators act as scaffolds in the SG transition towards the final gel, as represented in Fig. 6.27. A good example of these macrocycles is depicted in Fig. 6.28 using a cyclodextrine, a polyacrylamide, and grafene oxide. Gel formation is easily observed (Qi & Schalley, 2014).
Figure 6.26 Macrocycles used in supramolecular gels. Reprinted (adapted) with permission from Qi, Z., & Schalley, C. A. (2014). Exploring macrocycles in functional supramolecular gels: From stimuli responsiveness to systems chemistry. Accounts of Chemical Research, 47(7), 22222233. Copyright r 2014, American Chemical Society.
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Figure 6.27 Representation of solgel process and the scaffold motifs from the gelators. Scheme of different roles of macrocycles in supramolecular gels: (A) Scaffold only (stacking). (B) Directly involved in solgel transition. (C) Appendix to gelators that are not directly involved in gelation. Reprinted (adapted) with permission from Qi, Z., & Schalley, C. A. (2014). Exploring macrocycles in functional supramolecular gels: From stimuli responsiveness to systems chemistry. Accounts of Chemical Research, 47(7), 22222233. Copyright r 2014, American Chemical Society.
6.3
Concluding Remarks
A large number of LMWGs were described in this chapter. The design of gelators, the inclusion of additives, scaffolds, and so on are becoming relatively simple. Their functionalization and control of the nano-, meso-, or microstructure have until now been carried out on an empirical (trial and error) basis. Biocompatible soft materials are a natural target for the development of stimuli-responsive systems, where these stimuli could be a pH gradient, the appearance of an antigen, or something else. Supramolecular gels formed from the assembly of LWMGs are thus easy to tailor, but it is still difficult to design a new gelator that forms a gel with a specific range of properties. The goal is the ability to tune LWMGs from the molecular scale to the gel scale with selected viscosities, morphologies, durabilities, and environmental safety, etc. with selected stimuli. Progress in this area can be easily observed by the exponential growth in the literature. Thus the formation and tailoring of supramolecular gels is just in its early stages and will be increasingly developed for further future applications.
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Figure 6.28 Supramolecular hydrogel hybrid cyclodextrin-based multiple gel 2 sol and sol 2 gel transitions prompted by different stimuli by Jiang’s group (Jiang et al., 2010; Liu et al., 2010; Liu, Chen, & Jiang, 2011; Qi & Schalley; 2014). Reprinted (adapted) with permission from Qi, Z., & Schalley, C. A. (2014). Exploring macrocycles in functional supramolecular gels: From stimuli responsiveness to systems chemistry. Accounts of Chemical Research, 47(7), 22222233. Copyright r 2014, American Chemical Society. Reprinted (adapted) with permission from Liu, J., Chen, G., Guo, M., & Jiang, M. (2010). Dual stimuli-responsive supramolecular hydrogel based on hybrid inclusion complex (HIC). Macromolecules, 43(19), 8086809. Copyright r 2010, American Chemical Society. Reprinted (adapted) with permission from Du, P., Liu, J., Chen, G., & Jiang, M. (2011). Dual responsive supramolecular hydrogel with electrochemical activity. Langmuir, 27(15), 96029608. Copyright r 2011, American Chemical Society. Reprinted (adapted) with permission from Liu, J., Chen, G., & Jiang, M. (2011). Supramolecular hybrid hydrogels from noncovalently functionalized graphene with block copolymers. Macromolecules, 44(19), 76827691. Copyright r 2011, American Chemical Society.
References Araki, K., & Yoshikawa, I. (2005). Nucleobase-containing gelators. Topics in Current Chemistry, 256, 133165. Available from https://doi.org/10.1007/ b107173. Boekhoven, J., Brizard, A. M., Stuart, M. C. A., Florusse, L., Raffy, G., Del Guerzo, A., & van Esch, J. H. (2016). Bio-inspired supramolecular materials by orthogonal self-assembly of hydrogelators and phospholipids. Chemical Science, 7(9), 60216031. Available from https://doi.org/10.1039/c6sc01021k.
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Bouteiller, L., Colombani, O., Lortie, F., & Terech, P. (2005). Thickness transition of a rigid supramolecular polymer. Journal of the American Chemical Society, 127(24), 88938898. Available from https://doi.org/10.1021/ja0511016. Brizard, A., Stuart, M., van Bommel, K., Friggeri, A., de Jong, M., & van Esch, J. (2008). Preparation of nanostructures by orthogonal self-assembly of hydrogelators and surfactants. Angewandte Chemie International Edition, 47 (11), 20632066. Available from https://doi.org/10.1002/anie.200704609. Brizard, A. M., Stuart, M. C. A., & van Esch, J. H. (2009). Self-assembled interpenetrating networks by orthogonal self-assembly of surfactants and hydrogelators. Faraday Discussions, 143, 345. Available from https://doi.org/ 10.1039/b903806j. Buerkle, L. E., & Rowan, S. J. (2012). Supramolecular gels formed from multicomponent low molecular weight species. Chemical Society Reviews, 41(18), 60816102. Available from https://doi.org/10.1039/c2cs35106d. Cao, H., Duan, P., Zhu, X., Jiang, J., & Liu, M. (2012). Self-assembled organic nanotubes through instant gelation and universal capacity for guest molecule encapsulation. Chemistry - A European Journal, 18(18), 55465550. Available from https://doi.org/10.1002/chem.201103654. Christoff-Tempesta, T., Lew, A., & Ortony, J. (2018). Beyond covalent crosslinks: Applications of supramolecular gels. Gels, 4(2), 40. Available from https://doi. org/10.3390/gels4020040. Cui, J., Zheng, Y., Shen, Z., & Wan, X. (2010). Alkoxy tail length dependence of gelation ability and supramolecular chirality of sugar-appended organogelators. Langmuir, 26(19), 1550815515. Available from https://doi. org/10.1021/la101494t. Das, R. K., Kandanelli, R., Linnanto, J., Bose, K., & Maitra, U. (2010). Supramolecular chirality in organogels: A detailed spectroscopic, morphological, and rheological investigation of gels (and xerogels) derived from alkyl pyrenyl urethanes. Langmuir, 26(20), 1614116149. Available from https://doi.org/10.1021/la1029905. Du, P., Liu, J., Chen, G., & Jiang, M. (2011). Dual responsive supramolecular hydrogel with electrochemical activity. Langmuir, 27(15), 96029608. Available from https://doi.org/10.1021/la201843z. Duan, P., Qin, L., Zhu, X., & Liu, M. (2011). Hierarchical self-assembly of amphiphilic peptide dendrons: Evolution of diverse chiral nanostructures through hydrogel formation over a wide pH range. Chemistry - A European Journal, 17(23), 63896395. Available from https://doi.org/10.1002/ chem.201003049. Duan, P., Zhu, X., & Liu, M. (2011). Isomeric effect in the self-assembly of pyridine-containing L-glutamic lipid: Substituent position controlled morphology and supramolecular chirality. Chemical Communications, 47(19), 5569. Available from https://doi.org/10.1039/c1cc10813a. Edwards, W., & Smith, D. K. (2012). Cation-responsive silver-selective organogel— exploiting silveralkene interactions in the gel-phase. Chemical Communications, 48(22), 2767. Available from https://doi.org/10.1039/c2cc17854k. Estroff, L. A., & Hamilton, A. D. (2004). Water gelation by small organic molecules. Chemical Reviews, 104(3), 12011218. Available from https://doi. org/10.1021/cr0302049. Feng, Q., Wei, K., Zhang, K., Yang, B., Tian, F., Wang, G., & Bian, L. (2018). Onepot solvent exchange preparation of non-swellable, thermoplastic, stretchable and adhesive supramolecular hydrogels based on dual synergistic physical crosslinking. NPG Asia Materials, 10(1), e455. Available from https://doi.org/ 10.1038/am.2017.208. Flory, P. J. (1974). Introductory lecture. Faraday Discussions of the Chemical Society, 57, 718. Available from https://doi.org/10.1039/DC9745700007.
Chapter 6 STIMULI-RESPONSIVE GELS
Foster, J. A., & Steed, J. W. (2010). Exploiting cavities in supramolecular gels. Angewandte Chemie International Edition, 49(38), 67186724. Available from https://doi.org/10.1002/anie.201000070. Fuhrhop, J. H., & Boettcher, C. (1990). Stereochemistry and curvature effects in supramolecular organization and separation processes of micellar N-alkylaldonamide mixtures. Journal of the American Chemical Society, 112 (5), 17681776. Available from https://doi.org/10.1021/ja00161a018. Huang, X., Terech, P., Raghavan, S. R., & Weiss, R. G. (2005). Kinetics of 5α-cholestan-3β-ylN-(2-naphthyl)carbamate/n-alkane organogel formation and its influence on the fibrillar networks. Journal of the American Chemical Society, 127(12), 43364344. Available from https://doi.org/10.1021/ja0426544. Hwang, I., Jeon, W. S., Kim, H.-J., Kim, D., Kim, H., Selvapalam, N., . . . Kim, K. (2007). Cucurbit[7]uril: A simple macrocyclic, pH-triggered hydrogelator exhibiting guest-induced stimuli-responsive behavior. Angewandte Chemie, 119(12), 214217. Available from https://doi.org/10.1002/ange.200603149. Isare, B., Bouteiller, L., Ducouret, G., & Lequeux, F. (2009). Tuning reversible supramolecular polymer properties through co-monomer addition. Supramolecular Chemistry, 21(5), 416421. Available from https://doi.org/ 10.1080/10610270802105106. Jin, Q., Zhang, L., & Liu, M. (2013). Solvent-polarity-tuned morphology and inversion of supramolecular chirality in a self-assembled pyridylpyrazolelinked glutamide derivative: Nanofibers, nanotwists, nanotubes, and microtubes. Chemistry - A European Journal, 19(28), 92349241. Available from https://doi.org/10.1002/chem.201300612. Liu, J., Chen, G., Guo, M., & Jiang, M. (2010). Dual stimuli-responsive supramolecular hydrogel based on hybrid inclusion complex (HIC). Macromolecules, 43(19), 80868093. Available from https://doi.org/10.1021/ ma101437k. Liu, J., Chen, G., & Jiang, M. (2011). Supramolecular hybrid hydrogels from noncovalently functionalized graphene with block copolymers. Macromolecules, 44(19), 76827691. Available from https://doi.org/10.1021/ ma201620w. Miyauchi, M., Hoshino, T., Yamaguchi, H., Kamitori, S., & Harada, A. (2005). A [2]Rotaxane capped by a cyclodextrin and a guest: Formation of supramolecular [2]Rotaxane polymer. Journal of the American Chemical Society, 127, 20342035. Available from https://doi.org/10.1021/ja042840. Miyauchi, M., Takashima, Y., Yamaguchi, H., & Harada, A. (2005). Chiral supramolecular polymers formed by host 2 guest interactions. Journal of the American Chemical Society, 127(9), 29842989. Available from https://doi. org/10.1021/ja043289j. Moffat, J. R., & Smith, D. K. (2009). Controlled self-sorting in the assembly of “multi-gelator” gels. Chemical Communications, 3, 316318. Available from https://doi.org/10.1039/b818058j. Moraes, T. F. B., Schimidt, M. F. R. A., Bacani, R., Weber, G., Politi, M. J., Castanheira, B., . . . Triboni, E. R. (2018). Polysilsesquioxane naphthalenediimide thermo and photochromic gels. Journal of Luminescence. Available from https://doi.org/10.1016/j.jlumin.2018.08.036, In Press. Ohga, K., Takashima, Y., Takahashi, H., Kawaguchi, Y., Yamaguchi, H., & Harada, A. (2005). Preparation of supramolecular polymers from a cyclodextrin dimer and ditopic guest molecules: Control of structure by linker flexibility. Macromolecules, 38(14), 58975904. Available from https://doi.org/10.1021/ ma0508606. Pal, K., & Banerjee, I. (2018). Polymeric gels - characterization, properties and biomedical applications (1st ed). Elsevier. Available from http://doi.org/ 10.15713/ins.mmj.3.
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Qi, Z., & Schalley, C. A. (2014). Exploring macrocycles in functional supramolecular gels: From stimuli responsiveness to systems chemistry. Accounts of Chemical Research, 47(7), 22222233. Available from https://doi. org/10.1021/ar500193z. Segarra-Maset, M. D., Nebot, V. J., Miravet, J. F., & Escuder, B. (2013). Control of molecular gelation by chemical stimuli. Chemical Society Reviews, 42(17), 70867098. Available from https://doi.org/10.1039/c2cs35436e. Smith, D. K. (2006). Dendritic supermolecules towards controllable nanomaterials. Chemical Communication, 1, 3444. Available from https:// doi.org/10.1039/b507416a. Sobczuk, A. A., Tamaru, S., & Shinkai, S. (2011). New strategy for controlling the oligothiophene aggregation mode utilizing the gel-to-sol phase transition induced by crown-alkali metal interactions. Chemical Communications, 47 (11), 3093. Available from https://doi.org/10.1039/c1cc00026h. Steed, J. W. (2011). Supramolecular gel chemistry: Developments over the last decade. Chemical Communications, 47(5), 13791383. Available from https:// doi.org/10.1039/c0cc03293j. Svobodova´, H., Noponen, V., Kolehmainen, E., & Sieva¨nen, E. (2012). Recent advances in steroidal supramolecular gels. RSC Advances, 2(12), 4985. Available from https://doi.org/10.1039/c2ra01343f. Terech, P., & Weiss, R. G. (1997). Low molecular mass gelators of organic liquids and the properties of their gels. Chemical Reviews, 97(8), 31333160. Available from https://doi.org/10.1021/cr9700282. Wang, S., Shen, W., Feng, Y., & Tian, H. (2006). A multiple switching bisthienylethene and its photochromic fluorescent organogelator. Chemical Communications, 14, 1497. Available from https://doi.org/10.1039/b515412j. Weng, W., Beck, J. B., Jamieson, A. M., & Rowan, S. J. (2006). Understanding the mechanism of gelation and stimuli-responsive nature of a class of metallosupramolecular gels. Journal of the American Chemical Society, 128(35), 1166311672. Available from https://doi.org/10.1021/ja063408q. Xue, P., He, T., Wu, H., Xie, H., Shen, R., Yue, F., . . . Zhang, Y. (2017). Encapsulation of nanocrystals with responsive gels for spatial optical identification. Supramolecular Chemistry, 29(8), 627632. Available from https://doi.org/10.1080/10610278.2017.1298763. Yang, X., Zhang, G., & Zhang, D. (2012). Stimuli responsive gels based on low molecular weight gelators. Journal of Materials Chemistry, 22(1), 3850. Available from https://doi.org/10.1039/c1jm13205a. Zhan, C., Gao, P., & Liu, M. (2005). Self-assembled helical spherical-nanotubes from an l-glutamic acid based bolaamphiphilic low molecular mass organogelator. Chemical Communications, 4, 462. Available from https://doi. org/10.1039/b413259a. ˇ Zinic, M., Vo¨gtle, F., & Fages, F. (2005). Cholesterol-based gelators. Topics in Current Chemistry, 256, 3976. Available from https://doi.org/10.1007/ b107171.
Further Reading Hirst, A. R., Miravet, J. F., Escuder, B., Noirez, L., Castelletto, V., Hamley, I. W., & Smith, D. K. (2008). Self-assembly of two-component gels: Stoichiometric control and component selection. Chemistry - A European Journal, 15(2), 372379. Available from https://doi.org/10.1002/chem.200801475.
Chapter 6 STIMULI-RESPONSIVE GELS
Lan, Y., Corradini, M. G., Weiss, R. G., Raghavan, S. R., & Rogers, M. A. (2015). To gel or not to gel: Correlating molecular gelation with solvent parameters. Chemical Society Reviews, 44(17), 60356058. Available from https://doi.org/ 10.1039/c5cs00136f. Piepenbrock, M.-O. M., Lloyd, G. O., Clarke, N., & Steed, J. W. (2010). Metal- and anion-binding supramolecular gels. Chemical Reviews, 110(4), 19602004. Available from https://doi.org/10.1021/cr9003067. Rao, K. V., Jayaramulu, K., Maji, T. K., & George, S. J. (2010). Supramolecular hydrogels and high-aspect-ratio nanofibers through charge-transfer-induced alternate coassembly. Angewandte Chemie International Edition, 49(25), 42184222. Available from https://doi.org/10.1002/anie.201000527. Shu, T., Wu, J., Lu, M., Chen, L., Yi, T., Li, F., & Huang, C. (2008). Tunable redgreenblue fluorescent organogels on the basis of intermolecular energy transfer. Journal of Materials Chemistry, 18(8), 886. Available from https://doi. org/10.1039/b715462c. Zhang, L., Wang, X., Wang, T., & Liu, M. (2014). Tuning soft nanostructures in self-assembled supramolecular gels: From morphology control to morphology-dependent functions. Small, 11(910), 10251038. Available from https://doi.org/10.1002/smll.201402075. Zhu, X., Li, Y., Duan, P., & Liu, M. (2010). Self-assembled ultralong chiral nanotubes and tuning of their chirality through the mixing of enantiomeric components. Chemistry - A European Journal, 16(27), 80348040. Available from https://doi.org/10.1002/chem.201000595.
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