Photoresponsive supramolecular soft materials in aqueous media
Chen, Shaoyu
DOI:
10.33612/diss.107818650
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Publication date: 2019
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Chen, S. (2019). Photoresponsive supramolecular soft materials in aqueous media. Rijksuniversiteit Groningen. https://doi.org/10.33612/diss.107818650
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Supramolecular Self-Assembly of
Photoresponsive Molecular
Abstract: Photoresponsive molecular amphiphiles, with light responsive functionality and designed with both hydrophobic and hydrophilic moieties, are able to self-assemble spontaneously into well-defined and stimuli-responsive structures at interfaces and in solutions. This has been regarded as a promising bottom-up approach to develop smart materials for applications in a wide range of areas, such as nanotechnology and biological systems. Supramolecular self-assembly of photoresponsive molecular amphiphiles in aqueous media, an emerging area of material science, is employed in creating photoresponsive well-defined structures at air-water interfaces and in solutions, ranging from one-dimensional nanostructures to isotropic entangled three-one-dimensional networks and anisotropic three-dimensional structures, which are biocompatible in nature. In this chapter, we summarize and discuss recent progress in supramolecular self-assembled structures of photoresponsive molecular amphiphiles, including their molecular design, recognition and responsive functions, to provide insight into the fascinating supramolecular self-assemblies and highlight the amplification of molecular motions to enable macroscopic functions.
1.1 Introduction
The word amphiphile, derived from the Greek roots of αμφις, is a term referred to chemical compounds that consist of a polar part and a nonpolar moiety, possessing both hydrophilic (water-loving, polar) and lipophilic (fat-loving) properties. Common amphiphiles include surfactants (e.g. detergents) and diglycerides (e.g. phospholipids).1 The coexistence of both hydrophobic and hydrophilic moieties in
the same molecular structure, i.e., amphiphilic molecule, acting as small molecular building blocks, allows spontaneous self-assembly into well-defined structures at interfaces and in solutions based on non-covalent interactions.2–10 The hydrophilic
part of an amphiphile interacts with a polar phase, and as a result, the hydrophobic component interacts with a nonpolar environment or in air, forming self-assembled structures in the solution (e.g., micelle and bilayer) or at the interface (e.g., monolayer).1,11,12 The current supramolecular bottom-up approach allows to build
natural and synthetic supramolecular structures of molecular amphiphiles over multiple length scales.13–15
Nature has provided the most elegant examples of self-assembled systems derived from amphiphiles. Natural phospholipids, a typical class of amphiphiles, self-assemble into bilayer biological membranes in living organisms.16,17 The presence of
functional proteins endows the living organisms with “smart” self-assembled structures, allowing to automatically respond to subtle variations in the surrounding environment.11,18 As inspired by nature, a variety of stimuli-responsive synthetic
amphiphiles have been designed and synthesized to fabricate well-defined supramolecular self-assemblies whose structures are manipulated by the control of external stimuli, such as pH, CO2, heat and light.19–25 Light, as a renewable energy
source, offers multiple advantages such as tunable wavelength and intensity as well as temporal and spatial controllability.26,27 The structural modifications of
amphiphilic molecules by the attachment of a photoresponsive unit or the introduction of a photoresponsive component via noncovalent interactions provide synthetic photoresponsive amphiphiles.5 The supramolecular structures of
photoresponsive amphiphiles can be controlled by light, which provides important opportunities towards adaptive materials in various research areas, ranging from the nanotechnology, biological systems to material science.1,26,28,29
Virtually, all functional supramolecular self-assembly processes found in living systems take place in aqueous media.30,31 Water is a critical and unique medium for
self-assembly in natural biological processes, allowing for complexity, adaptability and robustness. Supramolecular self-assembly of photoresponsive molecular amphiphiles in aqueous media provides a possibility to create biocompatible systems.31–33 In this context, we summarize and discuss in this chapter the recent
amphiphiles in aqueous media, ranging from assembly at air-water interfaces to that in solutions, including their molecular structures and recognition functions. The following topics are described: (1) molecular design of photoresponsive amphiphiles, (2) dynamic functions of photoresponsive molecular amphiphiles at air-water interfaces and (3) functional supramolecular self-assembly of photoresponsive molecular amphiphiles in solutions, ranging from one-dimensional nanostructures to isotropic entangled three-dimensional networks and anisotropic hierarchical structures. We focus on structures and functions of photoresponsive molecular amphiphiles and aim to provide insight into the fascinating supramolecular assemblies from interfaces to solutions, highlighting the amplification of molecular motions to achieve macroscopic responsive functions.
1.2 Photoresponsive molecular amphiphiles
Amphiphilic molecular structures consist of both lipophilic and hydrophilic moieties. The lipophilic moiety is typically a large hydrocarbon structure. Based on the chemical nature of the hydrophilic moiety, amphiphiles are commonly classified into ionic (i.e., anionic, cationic), zwitterionic and nonionic amphiphiles.2 According to
the number and mode of connections of the hydrophilic moiety (polar head) and the lipophilic moiety (hydrophobic tail), amphiphiles are also classified as conventional single head/single tail amphiphiles, bolaamphiphiles, gemini amphiphiles and double tails amphiphiles (Figure 1.1).1
Figure 1.1 Schematic illustration of typical types of molecular amphiphiles. Adapted with permission from ref. 1. Copyright 2013, Royal Society of Chemistry.
Among them, conventional single head/single tail amphiphiles, bolaamphiphiles and gemini amphiphiles are the common types in both natural and synthetic systems. Bolaamphiphiles containing two hydrophilic heads connected by a hydrophobic chain were found commonly in cell membranes of thermophilic bacteria.34,35
Bolaamphiphiles usually show high thermal resistance.11 Gemini amphiphiles
aggregate at very low concentrations and also can significantly reduce the surface tension.1,11,36 The self-assembled structures of molecular amphiphiles vary
significantly with high dependences on molecular structures. In this regard, a target self-assembled structure, featuring the nature of the specific functions, can be obtained through a rational molecular design and synthesis.
Photoresponsive molecular amphiphiles are designed by the attachment of a photoresponsive unit either in the headgroup or the hydrophobic chain. A series of photoresponsive molecular structures can be potentially employed to produce amphiphiles, e.g., azobenzene, stilbenes, dithienylethene, spiropyran, molecular motor and donor-acceptor stenhouse adducts (DASAs). The molecular structures and the related photoisomerization processes of these photoresponsive systems are shown in Figure 1.2.
Figure 1.2 Photoresponsive structures and their photoisomerization processes of (a) azobenzene, (b) stilbene, (c) dithienylethene, (d) spiropyran, (e) molecular motor and (f) donor-acceptor stenhouse adducts (DASAs).
1.3 Dynamic functions of photoresponsive molecular amphiphiles
at air-water interfaces
Amphiphilic molecules, which are composed of hydrophobic and hydrophilic moieties to induce microphase separation, allow the formation of structurally well-defined self-assembled systems. When amphiphiles are introduced to aqueous media, the molecules spread over the entire interface with the hydrophilic heads orient towards the polar phase and the hydrophobic tails orient towards the non-polar air phase, allowing for the formation of self-assembled monolayers at air-water interfaces. According to the solubility of the amphiphiles in aqueous media, the self-assembled monolayers are classified as Langmuir monolayers and Gibbs monolayers (adsorption monolayers).37 Langmuir monolayers are formed by insoluble or
formed by amphiphiles which are soluble in aqueous media. In this section, we will focus on the Gibbs monolayers of photoresponsive molecular amphiphiles at air-water interfaces.
In 1999, Shin and Abbott38 reported an active control of dynamic surface tension of
mixed surfactants containing sodium dodecyl sulfate (SDS) and a photoresponsive azobenzene bolaamphiphile 1 (Figure 1.3a). The positive charged 1 showed an improved intermolecular interaction to the negative charged SDS. This complementary Coulombic interaction allows a closer packing of the assembled structure at the air-water interfaces. The geometrical transformation of azobenzene, from trans to cis, induced a large change of dynamic surface tension before and after light irradiation (up to 25 mM/m) at the concentration of 1 below the critical micelle concentration (CMC, Figure 1.3b). This study demonstrated a new principle for dynamic control of surface tension of aqueous solutions, instead of change the surface tension at equilibrium state, based on a photoresponsive amphiphile. However, the adsorption behaviors of azobenzene amphiphiles in the trans and cis conformations at the air-water interfaces, which induced the change of surface tension, are still unclear.
Figure 1.3 (a) Molecular structures and photoisomerization of 1. (b) Surface tension of aqueous solutions of 1 and SDS (1.6 mM) mixture as a function of the concentration of 1 before (filled circles) and after (open circles) illumination with UV light. The surface tension was measured by du Nouy ring method39. Adapted with permission from ref. 38. Copyright 1999, American Chemical Society.
In 2011, Monteux and co-workers40 designed and synthesized a photoresponsive
amphiphile 2 which was composed of a cationic head and an azobenzene-based hydrophobic tail (Figure 1.4) to investigate dynamic adsorption/desorption behaviors of 2 and the resulting photoresponsive air-water interface with tunable surface tension. Using a kinetically limited model taking into account the electrostatic barrier to adsorption, they reported that the cis-2 adsorbed 10 times faster than the trans-2 but desorbed 300 times faster, resulting in the monolayers packed almost exclusively with trans-2 under equilibrium conditions. The
competition between trans and cis isomers occurred in a few seconds, allowing to trigger rapid variations of interfacial properties. In order to observe the surface flux, a solution containing 2 (1.15 mM) and talc particles was placed under a microscope. When a UV light spot (λ = 365 nm) was employed on the surface of the solution, a majority of trans-2 was adsorbed outside the light spot, while inside the light spot, the trans-2 converted to cis-2 and immediately desorbed from the interface, leading to a rapid increase of surface tension. This surface tension gradient resulted in a Marangoni flow from the outside to the inside of the light spot, which induced the movements of talc particles towards the light spot. Therefore, a particle concentrating behavior in the light spot was observed in a few seconds after exposure to UV light (Figure 1.5). Interestingly, such particle concentrating behavior not only can be induced by UV light but also by blue light (λ = 436 nm). This work demonstrated a distinct difference of adsorption/desorption behavior of trans-2 and cis-2, highlighting a new way to stimulate the air-water interface of an azobenzene amphiphile solution and achieve light-induced Marangoni flows.
Figure 1.4 Molecular structures and photoisomerization of azobenzene amphiphile 2.
Figure 1.5 (a) Photograph of a particle concentrating behavior under a UV or a blue light spot. (b) Schematic illustration of the particles concentrating behavior mechanism. The open arrows represent the photoconversion and the filled arrows represent the transfer of molecules via Marangoni flows and adsorption/desorption fluxes. Adapted with permission from ref. 40. Copyright 2011, Royal Society of Chemistry.
Taking advantage of dynamically active interfacial properties of aqueous solutions of the azobenzene amphiphile 2, Monteux’s group reported photoresponsive foams with in situ photo-controlled stability and breakage. These were prepared from aqueous solutions of 2, providing a noninvasive clean method to remote control the stability of foams (Figure 1.6a).41 Stable foams were obtained from aqueous
solutions of 2 predominantly in the trans state, which ruptured in a short time by exposure to UV light (Figure 1.6b,c). Notably, after irradiating with blue light for several minutes on the solutions obtained from the broken foams, it was possible to prepare stable foams again, indicating a reversible control of foams by alternating UV light and blue light irradiation. Upon UV light irradiation of the stable foams, the trans-2 converted into cis-2, which subsequently desorbed from the air-water interfaces to the bulk solutions, providing dynamic adsorption/desorption interfaces. The authors proposed that due to the dynamic adsorption/desorption behavior of 2, a resulting out-of-equilibrium surface tension gradient induced Marangoni flows, which might lead to the breakage of foams upon UV light irradiation. However, the correlation between these light-induced flows and the foam destabilization mechanisms remained unclear.
Figure 1.6 (a) Schematic illustration of photoresponsive foams prepared from a solution of 2. (b) Stable foams are produced by trans-2. (c) Unstable foams are obtained by UV light irradiation. Adapted with permission from ref. 41. Copyright 2012, American Chemical Society.
To identify the key correlation between the light-induced flow and the foam destabilization mechanisms, Monteux and co-workers investigated the linkage of light-induced flow of 2 and foam breakage at a micrometer-length-scale (thin-liquid films of bubbles) and at a millimeter-length-scale (macroscopic foams).42 In
thin-liquid film, a tunable velocity of Marangoni flow was obtained by controlling the photoisomerization of 2 at the air-water interfaces with variable UV light intensity. The results obtained from experiments in macroscopic foams showed that the light-induced flow can slow down the foam drainage under stimulation at the beginning of UV light irradiation, while a rapid rupture of foams was subsequently observed. Next, they investigated the influence of illumination on disjoining pressures of thin liquid films in identical foam systems, suggesting that the UV-light-induced rupture
of foams could be explained by the decrease of electrostatic repulsion in the thin foam film as well as the oscillation of the disjoining pressure isotherm.43 In summary,
Monteux and co-workers provided a systematic studies of the amphiphile-based photoresponsive air-water interfaces and its application in remote control of foam stability.
Subsequently, Jiang and co-workers modified the azobenzene amphiphile 2 with a tertiary amine group to provide a novel dual-stimuli responsive amphiphile 3 (Figure 1.7).44 Firstly, the amphiphile 3 could be reversibly transferred between a
hydrophobic tertiary amine and an amphiphilic ammonium bicarbonate by alternately purging a solution of 3 with CO2 and N2, which was confirmed by
conductivity measurements. Secondly, a reversible photoisomerization of 3 between trans and cis configurations, induced by UV light (λ =365 nm) and blue light irradiation, were studied by UV-vis and 1H NMR spectroscopies. The ratio of
trans-3/cis-3 was determined as 4/96 at the photostationary state (PSS). Due to the drastic structural geometrical transformations of 3 triggered by both CO2/N2 and light
stimuli, dual-stimuli responsive foams were obtained. Stable foams prepared from trans-3 solutions bubbled with CO2 were ruptured in 10 min after purging with N2,
alternatively, the breakage of stable foams also could be induced by UV light irradiation, providing a promising strategy for fabricating multi-stimuli responsive systems (Figure 1.7).
Figure 1.7 Schematic illustration of dual-stimuli responsive foams prepared from solutions of 3. Adapted with permission from ref. 44. Copyright 2017, ScienceDirect, Elsevier B.V.
Except for the modification of the hydrophilic moiety of azobenzene amphiphiles, a more rigid azobenzene amphiphile 4 was designed and synthesized by Cui’s group, which was anticipated to provide photoresponsive foams with a more significant differences of foam stability before and after light stimuli (Figure 1.8).45 The
the amount of cis-4 obtained after exposure to UV light was estimated to be 95% (from 1H NMR spectra) at the photostationary state. With a more rigid hydrophobic
moiety, a drastic variation of critical micelle concentration (CMC) between trans-4 and cis-4 was observed, indicating a significant molecular geometrical transformation between trans-4 and cis-4. Additionally, stable foams with a half-life of 975 min were obtained from a trans-4 (2 mmol L-1) solution, which were much
more stable than those prepared from other reported azobenzene amphiphiles. In sharp contrast, all the stable foams collapsed within 4 min after irradiation with UV light, providing photoresponsive foams with a dramatic variation of foam stability triggered by light.
Figure 1.8 Schematic illustration of photoisomerization and photoresponsive foams prepared from a solution of 4. Adapted with permission from ref. 45. Copyright 2017, American Chemical Society.
The modification of the cationic azobenzene amphiphile 2 by replacing the bromide counterion by an anion of bis-(trifluoromethanesulfonimide), [BTF], provided a new strategy to control foam stability by incorporating with cucurbit[7]uril (CB[7]) and spermine moieties (Figure 1.9), as reported by Shen’group.46 The foamability of the
obtained azobenzene amphiphile 5 was significantly improved after an addition of CB[7], which was attributed to a closer packing of [Azo] tails at the air-water interfaces. The formation of the host-guest complex between 5 and CB[7], confirmed by 1H NMR spectroscopy and conductivity measurements, might shield the
electrostatic interaction between [Azo] and [BTF] to induce the desorption of [BTF] from the interfaces, allowing for the closer packing of [Azo] tails. With an addition of spermine, a stronger host-guest interaction between spermine and CB[7] than that between CB[7] and 5 resulted in a removal of CB[7] from the complex of 5/CB[7], providing a reversible switching back to a loose packing monolayer of 5 at the
air-water interfaces and hence a decrease of foamability was observed. The results demonstrated an alternative strategy to reversible control the foamability of the photoresponsive amphiphile by adding CB[7] and spermine.
Figure 1.9 The structural transformation at the air-water interface of aqueous solutions of (a) 5 and (b) complex of 5/CB[7]. Adapted with permission from ref. 46. Copyright 2016, Royal Society of Chemistry.
The dynamic photoresponsive air-water interfaces, derived from the isomerization of amphiphiles, also provide other macroscopic functions, such as optical particle depositions with predefined patterns47,48 and liquid marble transports.49 Similar to
the concept of the particle concentrating behavior in the solution of amphiphile 2 by the light-induced Marangoni flow, reported by Monteux et.al. in 2012,40 a strategy
to accumulate particles at predefined positions with complex patterns was reported, derived from the light-induced Marangoni flow in an evaporating droplet containing a cationic azobenzene amphiphile 6 (Figure 1.10a).47 Using photomasks, a broad
variety of complex patterns can be obtained in arbitrary particle systems, ranging from model suspensions to complex, real-world formulations such as commercial coffee suspensions. Meanwhile, a solution of the identical amphiphile 6 was also employed to develop a light-driven transport of floating liquid marbles based on the Marangoni flow at the air-water interfaces (Figure 1.10b).49 Interestingly, two motion
direction of the liquid marbles, including Marangoni motion (i.e., in the same direction of the Marangoni flow) and anti-Marangoni motion, were observed by controlling the thickness of the liquid substrate, proving an alternative macroscopic function of photoresponsive amphiphiles at the air-water interface.
Figure 1.10 (a) Optical particle depositions with predefined patterns. (b) Optical actuations of a floating liquid marble. Adapted with permission from ref. 47. Copyright 2016, American Chemical Society. Adapted with permission from ref. 49. Copyright 2016, Wiley-VCH.
Recently, Baigl and co-workers, by employing micromolar amounts of 6 in a suspension of anionic polystyrene microparticles, developed a novel photoresponsive dissipative self-assembling system. The particle organization can be reversibly and dynamically actuated between a highly crystalline assembly (under UV or blue light) and a disordered state (in the dark) with a fast response time at the air-water interfaces (Figure 1.11).50 The controllable crystallization and disassembly
of the particles were driven by the light-induced dynamic adsorption/desorption behavior of 6 at the air-water interface. In the absence of light irradiation, the suspension composed 100% of trans-6, and the particles formed the disordered structure. After irradiating with UV light or blue light for 1 min, the suspension at a photostationary state (PSS) composed 95% of cis-6 or 45% of cis-6, respectively. Although the photostationary states obtained by UV light or blue light irradiation were significantly different, the response of the colloidal assembly at the air/water interface was strikingly similar, i.e., the formation of highly crystalline colloidal assemblies after ~10 s, indicating that the crystallization and the disassembly were not controlled by the bulk composition of 6. The authors proposed that the continuous light-induced desorption of 6 maintained the system out-of-equilibrium and allowed the crystallization process to occur. When the light was switched off, the crystals were transferred to the disordered phase. This was the first report that process aqueous solutions of a photoresponsive amphiphile allowed a large colloidal assembly at the air-water interfaces, expanding the realm of currently known dissipative systems.
Figure 1.11 Light-induced two-dimensional colloidal crystallization at the air-water interfaces. Top: set-up for particle adsorptions at the air-water interfaces and light irradiations. A mixture of particles and amphiphile 6 was turned upside down for two hours to let particles sediment towards the interface. The sample cell was then flipped back and left overnight for particles adsorptions to accumulate at the center of the interfaces. Bottom: transmission microscopy images before (left) and after (right) UV irradiation for 30 s. UV and blue light irradiation were performed at 365 nm and 440 nm, respectively. Scale bar: 100 mm. Adapted with permission from ref. 50. Copyright 2019, Wiley-VCH.
It is clear that the properties and applications of azobenzene amphiphiles at the air-water interfaces have been well investigated. However, to identify the key processes for amplification from molecular photoisomerization at air-water interfaces to macroscopic functions, such as macroscopic photoresponsive foams and particle collections, remains highly challenging. Additionally, amphiphiles containing other photoresponsive systems applied at air-water interfaces remain largely unexplored.
1.4 Functional supramolecular self-assembly of photoresponsive
molecular amphiphiles in solutions
The self-aggregation of amphiphilic molecules has long been known to yield a rich variety of assembled structures, including micelles (spherical, rod-like, and wormlike) and bilayers structures (vesicles, tubules and planar lamellae), in organic, aqueous or organic-aqueous media. Since the supramolecular self-assembled structures of amphiphiles in aqueous media allow compatibility to natural systems as well as potential for biological relevant applications, we focus on the self-assembled microstructures and the structural transformations of molecular amphiphiles in aqueous media. The self-assembled systems depend on the molecular structures and experimental conditions, such as concentration, temperature, pH and
ionic strength.51 According to Israelachvili et al.,2 the shape and size of
self-assembled structures of amphiphiles in aqueous media can be predicted by using the packing parameter, P (Table 1.1). The packing parameter was defined as: P = v/a0l0,
where v is the volume of the amphiphile tail, a0 and l0 are the area of the hydrophilic
groups and the length of tail in the amphiphile, respectively. Therefore, a geometrical change of amphiphiles would affect the packing parameter P, leading to transformations of self-assembled structures. In this regard, photoresponsive amphiphiles, whose molecular structures can be changed by light stimuli, provide potential applications of light-induced self-assembly transformations in aqueous media. In this section, we focus on the self-assembled structures of photoresponsive amphiphiles in aqueous media, ranging from one-dimensional nanostructures to isotropic entangled three-dimensional networks and anisotropic three-dimensional structures.
Table 1.1 Different self-assembled structures predicted by the packing parameter P. Adapted with permission from ref. 12. Copyright 2014, Royal Society of Chemistry.
1.4.1 Isotropic self-assembly of photoresponsive molecular amphiphiles Self-assembly of molecules into well-defined and dynamic supramolecular structures is vital for correct functioning of biological systems. Inspired by fascinating examples of complex self-assembled structures created by nature, the formation of well-defined nanoscale objects in aqueous media that are responsive to external stimuli have been a longstanding goal of many research groups. In this connection, photoresponsive amphiphiles, featuring with self-assembling and light-responsive characteristics, are promising to build well-defined and smart supramolecular architectures.
Self-assembly of photoresponsive molecular amphiphiles into one-dimensional nanostructures offers many potential applications in biological system. In biomaterials, the single-handed helical structure has always been an attractive target. The Stupp group reported a self-assembly transformation of a water-soluble
photoresponsive peptide amphiphile 7a from a quadruple helical fiber into single fibers (Figure 1.12).52 The peptide amphiphile 7a contained a palmitoyl tail, a
2-nitrobenzyl protecting group, and an oligopeptide segment GV3A3E3, in which the
2-nitrobenzyl group can be cleaved by irradiation at 350 nm to afford 7b (Figure 1.12a). Due to the lack of hydrogen bonding on the amide closest to the alkyl segment and the bulkiness of the 2-nitrobenzyl group, different supramolecular architectures of 7a and 7b were observed by transmission electron microscopy (TEM). The TEM and AFM images of 7a revealed rare quadruple helix assemblies with a nearly uniform width and helical pitch of 33 2 and 92 4 nm, respectively (Figure 1.12b-d). After irradiating with 350 nm light for 5 min, the photocleavage of the 2-nitrobenzyl group, studied by UV-vis and circular dichroism (CD) spectroscopies (Figure 1.12f), resulted in the dissociation of the quadruple helix assemblies into single non-helical fibrils though a non-switchable photodeprotection strategy (Figure 1.12e). This study suggested novel strategies to create functional and photoresponsive helical supramolecular architectures, providing potential applications in sensing or actuation.
Figure 1.12 (a) Upon irradiation of 7a, the 2-nitrobenzenzyl group is cleaved to afford 7b. (b) TEM and (c) AFM images of 7a (7.4 × 10-4 M) in water (pH 11) containing NH4OH. (d) TEM image of a quadruple fiber: the quadruple strand (yellow arrow) uncoils into double helices (blue arrows) and further into single fibers (red arrows). (e) TEM image of a solution of photoirradiated 7a (350 nm, 250 W, 5 min). (f) UV-vis (top) and CD (bottom) spectral changes of 7a (7.4 × 10-4 M) in water upon 350 nm light irradiation at 25 °C. Adapted with permission from ref. 52. Copyright 2008, American Chemical Society.
Later, Stupp and co-workers replaced the oligopeptide segment GV3A3E3 of 7a by a
fibronectin epitope Arg-Gly-Asp-Ser (RGDS), synthesizing a new bioactive photoresponsive peptide amphiphile 8a (Figure 1.13a).53 According to the previous
report, by Hargerink et. al.,54 the molecular change in the β-sheet domains affected
the self-assembled structures. Indeed, 8a with a weaker β-sheet-forming sequence (compared to 7a) remained as a clear solution under the self-assembling conditions (4.0 × 10-4 M in 0.1 M CaCl
2 aqueous solution, Figure 1.13b), while in the identical
conditions, the quadruple-helix-forming 7a generated nanofibers and gels. However, 8b, obtained from irradiation of 8a with 350 nm light, generated a transparent gel under the same self-assembling conditions, demonstrating a three-dimensional sol-to-gel transition in response to light (Figure 1.13c). Transmission electron microscopy (TEM) images of 8a and 8b revealed a transformation from nanospheres (12 2 nm in diameter) in 8a to nanofibers (11 1 nm in diameter) in 8b (Figure 1.13d,e). Furthermore, they demonstrated that 8a before and after photoirradiation was not toxic to cells and the light-triggered gelation of 8a increased the bioactivity, providing a pathway to develop biomaterials that exhibit photoresponsive bioactivity.
Figure 1.13 (a) Upon irradiation of 8a, the 2-nitrobenzyl group is cleaved to afford
8b. The sol-to-gel transformation of 8a (4.0 × 10-4 M) in a 0.1 M CaCl
2 solution (b) before and (c) after irradiation with 350 nm light for 45 min at 25 oC. TEM images of structures formed by depositions of 8a (4.0 × 10-4 M) in a 0.1 M CaCl
2 solution (d) before and (e) after irradiation with 350 nm light for 45 min at 25 oC. Adapted with permission from ref. 53. Copyright 2009, Wiley-VCH.
Except for supramolecular architectures with multiple helices, the design of dynamic, complex self-assembled structures with adaptability in response to external stimuli is also one of the challenges in artificial nanostructures. In 2011, a photoresponsive self-assembled vesicle-capped nanotube system in water of a photoresponsive amphiphile 9, containing an overcrowded alkene core, was firstly developed by our group (Figure 1.14).55 In the presence of common phospholipids, unique
vesicle of which can then be chemically altered or removed and reattached by adding/removing Triton X-100, without affecting the nanotubes (Figure 1.14b). Alternatively, the vesicle-capped nanotubes can also be selectively disassembled by photoirradiation in an irreversible process (Figure 1.14c). The fluorescent nature of the overcrowded alkene core allowed the observation of the self-assembly transformations in real time using fluorescence microscopy. The reversible vesicle removal and light-induced disassembly processes of the vesicle-capped nanotube provided a strategy to trigger and control supramolecular self-assembly architectures in a complex multicomponent system by using a combination of chemical composition and light-stimuli function.
Figure 1.14 (a) Schematic illustration of assembly and disassembly of vesicle-capped nanotubes. (b) Cryo-TEM images demonstrating the effect of addition of detergent Triton X-100 on DOPC-capped amphiphile nanotubes: dissolution and regeneration of the phospholipid capping vesicle. (c) Structural transformation of amphiphile 9 and Cryo-TEM images demonstrating the self-assembly change under laser irradiation at a wavelength of 400.8 nm. Cryo-TEM images were taken at t = 0 min, 20 min and 120 min. Scale bars, 100 nm. Adapted with permission from ref. 55. Copyright 2011, Macmillan Publishers Limited, part of Springer Nature.
UV light was widely used in most of the light-stimulated systems. However, the poor light penetration and potential harmful nature of UV light limit in vivo applicability
of UV-sensitive materials. In this connection, photoresponsive systems, triggered by NIR (near infrared) light at a long wavelength, lower cytotoxicity and deeper penetrating depth, provide a promising solution for in vivo applicability. Recently, Wang’s group used coumarin to modify lipid molecules and synthesized a photoresponsive lipid amphiphile, enabling the preparation of NIR-responsive liposomes for drug delivery56. However, the self-assembly transformation of the
obtained liposomes was not studied in depth. Subsequently, they synthesized a new UV/NIR responsive amphiphile 10a, containing a coumarin group, which co-assembled with a common surfactant, tetradecyldimethylamine oxide (C14DMAO),
developing a dual-responsive self-assembly system (Figure 1.15).57 Depending on
the concentration of 10a, it can induce the formation of wormlike micelles (30 mmol L−1) or vesicles (50 mmol L−1) in C
14DMAO solutions. Furthermore, both the
wormlike micelles and vesicles can be transferred into spherical micelles triggered by NIR light irradiation because of the degradation of 10a.
Figure 1.15 (a) Schematic of the photo-induced microstructure transformation. Cryo-TEM images of solutions containing C14DMAO (150 mmol L−1) and 10a (30 mmol L−1) (b) before and (c) after NIR light (λ = 808 nm) irradiation from wormlike micelles to spherical micelles (black arrows). Cryo-TEM images of solutions containing C14DMAO (150 mmol L−1) and 10a (50 mmol L−1) (d) before and (e) after NIR light (λ = 808 nm) irradiation from vesicles to spherical micelles (black arrows). Adapted with permission from ref. 57. Copyright 2017, Royal Society of Chemistry. Based on the aforementioned systems, irreversible photoresponsive amphiphiles showed controllable supramolecular self-assembled structures in aqueous media, from one-dimensional nanostructures transformations to isotropic entangled three-dimensional sol-gel transformations, providing potential applications in artificial nanostructures and biological systems. However, reversible transformation of self-assembled structures could provide more interesting and sophisticated applications
in smart materials. In early research on self-assembled structures of photoresponsive molecular amphiphiles, the Engberts group have reported a series of investigations about the co-assembly behaviors and molecular interactions in aqueous solutions of common surfactants and azobenzene amphiphiles (on the basis of azo dyes, e.g., methyl orange and ethyl orange), which is vital for providing a fundamental and systematic understanding of molecular amphiphilic co-assembly systems.58–61
Recently, various supramolecular assemblies formed by reversible photoresponsive molecular amphiphiles have been employed to control nanostructure transformations in solutions. For instance, the Stupp group reported supramolecular nanofibers with reversibly tunable helix pitch formed by a photoresponsive peptide amphiphile 11 containing an azobenzene group (Figure 1.16a).62 A suspension of nanofibers made
of 11 in cyclohexyl chloride, as solvent, was irradiated with 360 nm light to induce a photoisomerization from trans-11 to cis-11. As the cis isomer is less planar than the trans isomers, the isomerization should increase the sterically induced torque, thus leading to a reduction in the helical pitch. The helix pitch of nanofibers formed by trans-11 decreased from 78 4 nm to 56 4 nm upon 365 nm light irradiation, as shown in AFM images (Figure 1.16b,c). The self-assembly of 11 occurred in organic media, which limited the application in biological systems.
Figure 1.16 (a) Molecular structure of photoresponsive peptide amphiphile 11. (b) AFM images demonstrating reversible control of helix pitch of nanofibers by trans-cis photoisomerization of 11. Adapted with permission from ref. 62. Copyright 2007, Wiley-VCH.
The azobenzene moiety, as the one of the most common units in molecular photoresponsive amphiphiles, has been widely employed in controlling self-assembled structures in aqueous media. The development of photo-controlled multi-states and multiple length-scales molecular self-assemblies from photoresponsive amphiphiles in aqueous media remains a challenging task. In 2010, Huang and co-workers reported photoresponsive nanostructures with multi-states by a co-assembly of an azobenzene amphiphile 12 containing sodium (4-(phenylazo)-phenoxy) acetate
and a common amphiphile CTAB (Figure 1.17).63 The co-assembly structures of 12
and CTAB exhibit reversible transformations from wormlike micelles, vesicles, lamellar structure to small micelles in aqueous solutions controlled by the irradiation time. Based on the photoisomerization of 12, the transformations of molecular self-assembly structures resulted in significant macroscopic properties changes in solutions. According to the rheology behavior of the solution of 12 and CTAB, i.e., a transformation from a transparent, gel-like solution to biphasic solution and homogenous solution upon prolonging 365 nm light irradiation, the co-assembly structures have been classified into four states: (1) wormlike micelle (89% of trans-12), (2) bilayer vesicle and planar lamellae (68% of trans-trans-12), (3) wormlike micelle (37% of trans-12), and (4) micelle (17% of trans-12). The ratio of trans-12 was determined by NMR and UV-vis spectroscopies. However, systematic structural analyses, e.g., cryo-TEM images, to confirm the self-assembled structures of these four states, were not provided. Subsequently, the same azobenzene amphiphile 12 was co-assembled with a surface active ionic liquid to investigate photoresponsive self-assembly behaviors, as reported by Yu et. al.64 The co-assembly structures of
wormlike micelles became longer and more entangled (as observed by cryo-TEM images) after 365 nm light irradiation, allowing to form a solution with a higher viscosity. Self-assembly systems based on ionic liquids have attracted increasing attention due to their extraordinary properties, such as negligible vapor pressure, low melting point and thermal stability.65,66 However, no significant transformation of
these self-assembled systems was observed in Yu’s work. In 2016, Zheng’s group synthesized a novel photoresponsive surface active ionic liquid with azobenzene in the headgroup, i.e. amphiphile 13, to develop a reversible transformation from vesicles to spherical micelles in an aqueous solution of 13, without co-assembly with other amphiphiles (Figure 1.18a).67 The photoisomerization of 13 was investigated
by 1H NMR and UV−vis spectroscopies, demonstrating a trans-13/cis-13 ratio of
39/61 at the photostationary state after UV light irradiation for 30 min. Density functional theory (DFT) calculations showed that trans-13 potentially formed vesicles, while the geometrical structural transformations from trans-13 to cis-13 induced the transformations from vesicles into spherical micelles. Indeed, a reversible transformation from vesicles (50 to 500 nm in diameter) to spherical micelles triggered by UV and visible light was observed in cryo-TEM images of the solution of trans-13 (0.15 mM, Figure 1.18c-e). Furthermore, a transformation from spherical micelles to vesicles was also observed when the concentration of trans-13 increased from 0.05 mM to 0.15 mM (Figure 1.18b,c), demonstrating the concentration effect on the self-assembled structures. This work provided a strategy to achieve controlled self-assembly from a single component, i.e. a photoresponsive surface active ionic liquid.
Figure 1.17 Schematic illustration of photo-controlled self-assembled systems with multi-states at multiple length-scales. Adapted with permission from ref. 63. Copyright 2010, Royal Society of Chemistry.
Figure 1.18 (a) Schematic illustration of molecular structure of 13 and proposed packing structures of self-assembly of 13. Cryo-TEM images of solutions of 13 at a concentration of (b) 0.05 mM or (c) 0.15 mM before UV light irradiation, (d) the solution of 13 (0.15 mM) irradiated with UV light was (e) subsequently exposed to visible light. Adapted with permission from ref. 67. Copyright 2016, American Chemical Society.
According to the aforementioned systems, photoresponsive self-assembly transformations were observed at equilibrium state after the isomerization of amphiphiles. To provide a systematic understanding of the dynamic self-assembly
transformation, the morphologies and self-assembly properties of photoresponsive carbohydrate-based amphiphilic micelles undergoing reversible photoisomerizations were monitored by using time-resolved small-angle neutron scattering (TR-SANS), as reported by the Wilkinson group.68 Later, a dual stimuli-responsive entangled
three-dimensional sol-gel transformations formed by a co-assembly of a cationic azobenzene amphiphile 14 and a commercially available organic salt, sodium azophenol (AzoONa), was developed by Jiang and co-workers (Figure 1.19).69 The
reversible structural transformation between AzoONa and AzoOH was controlled alternately through purging with CO2 and N2 (studied by 1H NMR). As a result, a
gel-to-sol transformation was observed. Furthermore, this gel-to-sol transformation can also be induced by UV/Vis light irradiations, derived from the photoisomerization of amphiphile 14. The dual stimuli-responsive gel-to-sol transformation system was expected to have potential applications in microfluidics and tertiary oil recovery.
Figure 1.19 (a) Molecular structures and structural transformations of 14 and AzoONa. (b) Schematic illustration of a dual stimuli-responsive entangled three-dimensional sol-gel transformation. Adapted with permission from ref. 69. Copyright 2018, ScienceDirect, Elsevier B.V.
Molecular amphiphiles, containing other photoresponsive units, such as diarylethenes, molecular motors and spiropyrans, have been employed in the development of photoresponsive self-assembly systems. In 2006, Irie’s group synthesized a diarylethene-based amphiphile 15 containing hexa(ethylene glycol) (Hxg) side chains and chiral methyl groups (Figure 1.20a), allowing for a photo-controlled chirality transformation between the open-ring-15 and the closed-ring-15 isomers in an aqueous solution (studied by CD spectroscopy, Figure 1.20b).70 The
CD spectral changes indicated that the self-assembly nanostructures of photogenerated closed-ring-15 showed supramolecular helicity (Figure 1.20c). The asymmetric supramolecular helicity transformation between the open-ring-15 and the closed-ring-15 isomers enabled a new strategy for photoswitching chiroptical properties in aqueous media.
Figure 1.20 (a) Schematic illustration of molecular structures and photoisomerization of diarylethene-based amphiphile 15 containing hexa(ethylene glycol) (Hxg) side chains and chiral methyl groups. (b) CD spectral change of 15 upon photoirradiation in an aqueous solution demonstrating that only the structure composed of the closed-ring isomers exhibited a supramolecular helicity. (c) Schematic illustration of the self-assembled nanostructure of 15 in aqueous media. A nanostructure of the (left) open-ring isomer and that of the (right) closed-ring isomer. Adapted with permission from ref. 70. Copyright 2006, American Chemical Society.
On the basis of our reported amphiphile 9, we designed and synthesized a novel photoresponsive amphiphile 16 containing a molecular motor core, to develop a system for reversible self-assembly transformation in water (Figure 1.21a).71 The
photoisomerization and thermal helix inversion (THI) of amphiphile 16 were studied by UV-vis and NMR spectroscopies, which showed an unstable-16/stable-16 ratio of 95/5 at a photostationary state with a half-life of 270 h at 20 °C and 4.3 h at 50 °C in the THI step. Tubular structures, co-assembled by the motor amphiphile 16 and DOPC, were observed in the cryo-TEM images, which disappeared completely after 365 nm irradiation for 15 min, and only the resulting bilayered vesicles were observed (Figure 1.21b,c). UV−vis absorption measurements on the identical sample showed a hypsochromic shift upon irradiation, indicating the formation of unstable-16, thus it was concluded that the isomerization of the motor amphiphile 16 induced the morphological transformations of the self-assembled structures. The sample, irradiated with 365 nm light for 15 min, was heated at 50 oC for 16 h. The vesicles
remained unchanged (Figure 1.21d). Tubular structures were observed when the heated sample was treated by freeze-thawing process for 3 times (Figure 1.21e),
demonstrating a reversible self-assembly transformation between well-defined nanotubes and vesicles induced by light and heat. This was the first example, using a molecular motor amphiphile, showed a reversible self-assembly transformation in aqueous media, providing a new generation of water-soluble photoresponsive amphiphile, i.e., with molecular motor core, and paving the way to increasingly complex, highly dynamic artificial nanosystems in aqueous media.
Figure 1.21 (a) Schematic illustration of molecular structures of motor amphiphile 16 and a reversible supramolecular self-assembly transformation of 16 in aqueous media between nanotubes and vesicle upon isomerization. Cryo-TEM images of co-assembled structures of motor amphiphile 16 and DOPC (1:1) in water at a total concentration of 1 mg/mL. The consecutive processes were (b) before irradiation (stable-16); (c) after 365 nm light irradiation for 15 min (unstable-16); (d) after heating at 50 °C for 16 h (stable-16); (e) after freeze−thawing 3 times. Adapted with permission from ref. 71. Copyright 2016, American Chemical Society.
Recently, Kudernac and co-workers designed and synthesized a visible light sensitive photoresponsive amphiphile 17, containing a spiropyran core, developing a reversible light-induced expansion of vesicles (Figure 1.22).72 The photoresponsive
amphiphile 17 was comprised of an oligoether dendron as the hydrophilic moiety, and a bent aromatic unit containing two spiropyran moieties as the hydrophobic moiety (Fig. 1.22a). The photoresponsive amphiphile 17 can be dissolved in acidic water, adopting an open protonated merocyanine form (MCH+). This MCH+ form
can be transiently converted into a closed and neutral spiropyran form (SP) by irradiation with visible light (Fig. 1.22a). Transmission electron microscopy (TEM) showed that both the 17(MCH+) and 17(SP) isomers self‐assembled into vesicles
(Fig. 1.22b). The 17(MCH+) isomer formed small vesicles with a constant average
17(SP) isomer formed larger vesicles. Upon visible light irradiation, an increase of vesicle diameter from 20 3 nm (17(MCH+)-vesicle) to 35 4 nm (17(SP)-vesicle)
was observed in TEM images (Figure 1.22b). The out-of-equilibrium state of 17(SP)-vesicle shrank back following the thermal relaxation from 17(SP) to 17(MCH+). This work provided a reversible expansion of vesicles in water formed
by a spiropyran-based amphiphile triggered by visible light, which showed promising potentials in biological systems.
Figure 1.22 Schematic illustration of (a) molecular structures and structural transformations of a spiropyran-based amphiphile 17, and a reversible expansion of vesicles in water. (b) Light‐induced reversible expansions from 17(MCH+)-vesicles to
17(SP)‐vesicle in water (pH 2) determining by related size distributions in TEM images. Top: initial 17(MCH+)-vesicles, middle: at PSS after irradiation with visible
light, and bottom: relaxation after heating at 60 °C for 30 min. Adapted with permission from ref. 72. Copyright 2018, Royal Society of Chemistry.
1.4.2 Anisotropic self-assembly of photoresponsive molecular amphiphiles
Anisotropic three-dimensional and responsive supramolecular hierarchical assembled structures have much potential in adaptive materials for biomedical and soft actuator applications. Numerous well-organized and complicated supramolecular structures are found in biological systems,73–78 and collagen is one
of the well-known examples. Triple-stranded helices are formed by folding of three polypeptide chains and then the microfibrils assemble into higher hierarchical collagen fibrils. Another illustrative example is actin filaments, which provides structural stability to cells and as the contractile apparatus in muscle cells. The helical ribbon of actin filaments composed two parallel strands held together tightly by
multiple supramolecular interactions, in which each strand is a linear array of protein monomer. As inspired by natural hierarchical supramolecular assembled structures, the manipulation of synthetic molecular amphiphiles assembling into precisely organized and anisotropic macroscopic structures becomes a significant field in supramolecular chemistry. At macroscopic length-scales, anisotropic three-dimensional hierarchical supramolecular structures generate exciting opportunities towards applications in regenerative biomedical materials, anisotropic actuators, electronic and optoelectronic materials. Recently, the Stupp group demonstrated that macroscopic supramolecular assembled tubes of peptide amphiphiles could be applied as templates for the formation of anisotropically aligned thermal responsive polymers, providing anisotropic macroscopic actuations upon heating.79 Taking
inspiration from molecular motions in muscle tissues, a photoresponsive hierarchical self-assembled structure of a cationic gemini azobenzene amphiphile 18 was developed by Liu and co-workers (Figure 1.23).80 The azobenzene amphiphile 18
assembled hierarchically from nanorods into crystalline helical twisted bundles (observed in polarized optical microscopy) by organic solvent evaporation over 2 days. The resulting bundled helices (~3 mm in length and ~25 µm in diameter) bent towards incident light source (302 nm light, 120 min) from an initial angle of 36° to a saturated flexion angle of 50° with an actuation speed of 1.9 x 10–3 degree/s,
providing a millimeter-length-scale anisotropic actuation based on the supramolecular polymer interactions from the azobenzene amphiphile 18. Although actuators over various length scales in a supramolecular system were achieved, this system worked in the organic media and a clear mechanistic study on the actuation mechanism is lacking.
Figure 1.23 Schematic illustration of the hierarchical self-assembled structures of 18. Adapted with permission from ref. 80. Copyright 2015, Macmillan Publishers Limited, part of Springer Nature.
To provide macroscopic anisotropic self-assembled structures from photoresponsive amphiphiles in aqueous media based on a clear photoactuation mechanism, recently our group reported the first photo-controlled unidirectionally aligned hierarchical supramolecular structure from a molecular motor amphiphile 19 in aqueous media to realize a photo-controlled macroscopic muscle-like action in both water and air (Figure 1.24).81 We designed a molecular motor amphiphile 19 with a dodecyl chain
formed the upper half, while two carboxyl groups linked with two alkyl moieties formed the lower half (Figure 1.24b). The highly amphiphilic nature of 19 allowed the microphase separation and the molecular ordering in aqueous media forming as fibers. The photoisomerization from stable-19 to 19 and THI from unstable-19 to stable-unstable-19 were studied using UV-vis and 1H NMR spectroscopies,
demonstrating an unstable-19/stable-19 ratio of 9:1 at PSS and a half-life of 128 h at 20 °C and 2.7 h at 50 °C in the THI step. Macroscopic strings were prepared from a nanofiber-containing solution of 19 by a shear-flow method in a calcium chloride solution, affording unidirectionally aligned structures (characterized with POM and SEM). The macroscopic strings bent towards the light source from an initial angle of 0° to a saturated flexion angle of 90° within 60 s in water (Figure 1.24c). Furthermore, the macroscopic string of stable-19 could be pulled out from aqueous media and performed photoactuations in air (Figure 1.24d), which allowed the attachment and motion of a 0.4 mg piece of paper (Figure 1.24e). The photoactuation process in air allowed in-situ SAXS measurements to exclude the scattering effects from aqueous media. According to the SAXS results (Figure 1.24f), a mechanism for the photoactuation was proposed that the photoisomerization from stable-19 to unstable-19 resulted in an increase of excluded volume around the motor unit and disturbance of local packing arrangement in the motor amphiphiles. In the meantime, the diameter of nanofibers expanded with total volume of the macroscopic string remained unchanged, the long axis of string contracted. Considering the light penetration to ~300 µm thickness of the string, a light intensity gradient was expected to illustrate the bending of the macroscopic string towards incident light source. This work clearly demonstrated that the motor amphiphile assembled hierarchically from individual nanofibers to bundled nanofibers, more importantly, the nanofibers were aligned unidirectionally, allowing for an effective energy conversion, accumulation, and amplification of molecular motion from the nanoscale up to macroscopic dimensions.
Figure 1.24 Schematic illustration of (a) hierarchical supramolecular assembled structures with photoactuated property of 19 and (b) molecular structures and photoisomerization of 19. A nanofibre-containing solution of stable-19 was manually drawn from a pipette into a CaCl2 solution to achieve unidirectional alignment in bundles, generating a string that was able to bend upon exposure to UV irradiation in (c) aqueous media, air (d) without weight and (e) with 0.4 mg paper as weight, scale bar: 0.5 cm. (f) In-situ SAXS of a stable-19 string before and after actuation, demonstrating the photoactuation mechanism. Adapted with permission from ref. 81. Copyright 2018, Macmillan Publishers Limited, part of Springer Nature.
According to the aforementioned artificial muscle, the electrostatic interaction between carboxylate groups of motor amphiphile 19 and Ca2+ allowed the
stabilization of nanofiber of 19 and the formation of a macroscopic string. By the nature of the cationic counterion effect on the hierarchical supramolecular assembled structure, we investigated the ionic effect on the nanofiber formation, aggregation of nanofiber, structural order of macroscopic string and its photoactuation speed (Figure 1.25).82 The 2D-SAXS images of stable-19 macroscopic strings, prepared
from a series of metal chlorides, demonstrated the cationic counterion effect on the orientational order, i.e., the order of counteractions for increasing the degree of unidirectional alignment in stable-19 macroscopic strings: Ca2+ > Mg2+ > Be2+ ≈ Sr2+
≈ Sc3+ > Ba2+ ≈ Li+ ≈ Na+ ≈ K+. Except for tuning the orientational order, the cationic
counterion also affected the actuation speed of stable-19 macroscopic strings, i.e., the order of counteractions for accelerating actuation speed: Ca2+ > Mg2+ > Sr2+ Sc3+ >
Be2+. The stable-19 strings, prepared from solutions of BaCl
2, LiCl, NaCl, and KCl,
revealed no alignment, and no actuation was observed upon photoirradiation. The results indicated that the structural order and orientation order of the string, as well as the actuation speed, can be fine-tuned by selection of a particular metal chloride for preparing the stable-19 macroscopic string. In addition to the counteraction effect, the alkyl-linker of motor amphiphile, connecting the carboxyl group to the lower half of the motor unit, was modified to various lengths (amphiphile 19-22) to provide a systematic modification of the packing in the resultant motor amphiphile strings and their actuation functions (Figure 1.25). The results showed that stable-19 was the optimal structure to allow a high structural order and a fast actuation speed. This study successfully demonstrated the three-dimension unidirectionally aligned hierarchical supramolecular structure and its actuation function can be controlled and modified simply by the nature of the cationic counterions, without covalent modification of the motor amphiphile.
Figure 1.25 Schematic illustration of molecular structures of amphiphiles 19-22 with various lengths of alkyl-linkers and the hierarchical organization and photoactuation process of their assembled structures by the addition of different cationic counterions. Adapted with permission from ref. 82. Copyright 2018, American Chemical Society.
Recently, we reported the first dual-controlled macroscopic actuation and cargo carrier from a supramolecular hierarchical assembled structure of motor amphiphiles.83 The motor amphiphile 23 was designed with two additional histidine
moieties from 19, which acted as the nucleation site for iron nanoparticles (FeNP) formation (Figure 1.26a). A higher structurally orientated macroscopic string was prepared from a combination of a nanofiber-containing solution of stable-19 and FeNP-23 (the nanofibers of stable-23 indirect contact with iron nanoparticles on the
surface), which were observed in POM, SEM, and SAXS measurements. The resulting string bent towards the incident light source (365 nm light) from an initial angle of 0° to a saturated flexion angle of 90° within 25 s. Furthermore, the macroscopic string of FeNP-23/19 was able to move towards a magnetic field within 2 s (Figure 1.26b), allowing for the application in a cargo transport process (Figure 1.26c-g). By sequential control of light/magnetic stimuli, the macroscopic string of FeNP-23/19 could carry a piece of paper away out ~2 cm distance from the original position through a series of cargo capture, transfer and release processes.
Figure 1.26 Schematic illustration of (a) the hierarchical organization from a combination of FeNP-23 and 19 and (b) a FeNP-23/19 string in a CaCl2 solution (150 mM) moved towards a magnetic field from the right. Snapshots of a dual-controlled cargo process in a CaCl2 solution (150 mM): (c) a FeNP-23/19 string in position B was (d) changed to a curved-shape upon photoirradiation, (e) carrying a piece of paper to position C guided by a magnetic field, (f) followed by changing to a linear-shape upon photoirradiation, (g) unloading the paper and moving to position D. Adapted with permission from ref. 83. Copyright 2019, Wiley-VCH.
The anisotropic hierarchical supramolecular assembled structures of molecular motor amphiphiles in water with controllable structural order and actuation speed
presented muscle-like functions by using external stimuli, i.e., light and a magnetic field. This allowed the applications of cargo transport and weight lifting, which were the first experimental demonstrations of molecular energy conversion, accumulation of mechanical strain, and amplification to macroscopic actuation using such supramolecular system. However, this system was limited by the slow thermal helix inversion which hindered a reversible macroscopic actuation under ambient conditions, whereas the biocompatibility of these materials remains unexplored. Furthermore, so far limited examples demonstrate anisotropic hierarchical supramolecular assemblies of photoresponsive molecular amphiphiles in aqueous media. We envision major potential of anisotropic self-assembled systems based on photoresponsive molecular amphiphiles with various photoresponsive units to design biocompatible materials triggered by visible light, controlling cell growths and alignments, and out-of-equilibrium soft materials.
1.5 Summary and outlook
Supramolecular self-assembled systems of amphiphiles are found in many living systems. For example, natural phospholipids, a typical class of amphiphiles, self-assemble into bilayer biological membranes in living organisms. Due to the presence of functional proteins, the self-assembled membranes in living systems automatically respond to external stimuli with correct functioning. Inspired by nature, stimuli-responsive molecular amphiphiles in aqueous media provide promising synthetic systems in order to create biomimetic well-defined supramolecular self-assembled structures with “smart response” functions, allowing for applications in nanotechnology and biomaterials. Compared to the existing external stimuli, light shows unique advantages. The coexistence of both hydrophobic and hydrophilic moieties in the same molecular structure, photoresponsive molecular amphiphiles, acting as small molecular building blocks, are able to self-assemble spontaneously into well-defined structures at interfaces and in solutions. Supramolecular self-assembly of photoresponsive molecular amphiphiles in aqueous media hold major potential in developing biocompatible soft materials.
Photoresponsive monolayers are developed by self-assembly at the air-water interface of molecular amphiphiles containing light-active units. At a molecular level, the dynamic adsorption and desorption of isomers generated by light at air-water interfaces result in light-induced flows, e.g., Marangoni flow, allowing for macroscopic length-scale photoresponsive functions, such as photoresponsive foams, optical particle depositions, liquid marble transports and crystallizations. However, the properties and applications of self-assembled monolayers at air-water interfaces of photoresponsive molecular amphiphiles mainly contain the azobenzene motif, whereas molecular amphiphiles containing other photoresponsive units remain
largely unexplored. Furthermore, to identify the key processes for amplification from molecular photoisomerization at the air-water interface to macroscopic functions remains highly challenging.
Photoresponsive molecular amphiphiles show controllable supramolecular self-assembled structures in aqueous media, from one-dimensional nanostructures transformations to isotropic entangled three-dimensional networks and anisotropic hierarchical structures, providing potential applications in artificial nanostructures and biological systems. As for the self-assembly of photoresponsive amphiphiles in aqueous media, extensive potential applications of molecular amphiphiles with various photoresponsive units remain largely unexplored. This offers prospects for biocompatible materials triggered by visible light, delivery systems, cell growth and alignments, and out-of-equilibrium soft materials.
1.6 Outline of this thesis
As discussed in the previous sections, supramolecular self-assembled systems of photoresponsive molecular amphiphiles at air-water interfaces and in aqueous solutions provide adaptive soft materials with correct functioning, showing promising potentials in nanotechnology and biological systems. However, the development of soft materials based on molecular amphiphiles containing other photoresponsive units, not only limited to azobenzene, and the applications of macroscopic functional soft materials in industrial processes, as well as related investigations in order to provide insight into key processes for amplification from molecular photoisomerization to macroscopic structural transformation remain challenging. In this thesis, some of these challenges are addressed such as applications of soft materials based on the photoresponsive molecular amphiphiles in industrial textile coloring processes (chapter 2 to 5, carried out in Jiangnan University), investigations of key processes for amplification from nanometer length-scale motions to macroscopic structural transformations (chapter 6, carried out in University of Groningen) and the development of photoresponsive biocompatible materials (chapter 7, carried out in University of Groningen). Chapter 2 describes the design, synthesis and properties of a series of nonionic azobenzene amphiphiles with different hydrophobic chain lengths. Due to the nonionic characteristic, the corresponding amphiphiles are used as dopants in foaming solutions of sodium dodecyl sulfate (SDS) to generate photoresponsive foams, which are controlled by the photoisomerization of the azobenzene amphiphiles. The results in chapter 2 provide insight into the correlation between the molecular design, focused on the hydrophobic chain length, and the properties of photoresponsive foams. The potential application in recycling residual foams is
