Supramolecular Materials Based on Ionic Self‐Assembly: Structure, Property, and Application

The technique of ionic self‐assembly (ISA), on the basis of electrostatic interactions, is a powerful tool to create new material nanostructures and chemical objects due to its advantages of facility, reliability, cost saving, flexibility, and universality. It has attracted great attention because of its promising applications in catalysis, drug delivery, and molecular detection. This review focuses on recent advances in the construction of self‐assemblies with different morphologies on the basis of ISA strategy and its applications. The ISA method provides an opportunity to generate complex and hierar‐ chical assemblies with tunable properties, which is regarded as a very promising case of supramolecular chemistry.


Introduction
Supramolecular self-assembly, which makes use of molecules instead of atomic units, offers a bottom-up approach to the construction of new materials on multiple length scales without complex organic synthesis [1][2][3][4][5][6][7][8][9][10]. The driving forces for supramolecular chemistry are noncovalent interaction ranging from host-guest interaction [11], hydrogen bonding [12], van der Waals interaction [13], π─π stacking [14], electrostatic interactions [15] and hydrophobic effect [16]. Table 1 summarizes most of this noncovalent interaction, as well as of its structuredetermining properties [17]. Among them, the theme of ionic self-assembly (ISA) on the basis of electrostatic interactions was first described by Faul and Antonietti [17]. Different with the simple coulombic binding of salts, the ISA is usually accompanied by a cooperative binding (π─π stacking, hydrophobic effect, and van der Waals interaction) which propagates toward the final self-assembly structures [17]. The ISA strategy is very easily available, reliable, and flexible and is much broader in application than multiple hydrogen bonding or stable metal coordination.
In this chapter, we provide an overview of work that contributed to establishing functional soft materials on the basis of ISA strategy and advancing the useness in the several areas such as soft matter template, luminescent materials, and inorganic-organic hybrid materials.

Dye-surfactant ISA materials
Dye molecules are almost the ideal building blocks for supramolecular chemistry, because they are easily available and have multiple functional groups, which possess a defined and regular shape (extended π system) and can facilitate mutual interactions, inducing them to have photoelectric functions and can form more complex structures, such as soft gels, liquid crystals, as well as plate-like and needle-like single crystals [18,19]. Besides, the surfactant molecule plays a special role in surface chemistry. Especially, surface active ionic liquids (SAILs) such as imidazolium-, pyrrole-based ionic liquids have attracted widespread interest for their outstanding performance in biomaterials, photoelectricity, and environmentally friendly trades, since they possess good biocompatibility, low critical micellar concentrations, wide liquid range, and high thermal stability [20,21]. Thus, the introduction of charged dye molecules or surfactants as ISA components will generate novel properties and structures of the systems and enrich the ranges and contents of ISA.
However, structures of supramolecular materials constructed by dye-surfactant have been limited to 1D and 2D structures, such as fiber and flake morphologies, and mostly have no fluorescent property, which greatly restricts the application of the material [22,23]. For example, highly ordered fibers constructed by dyes and surfactants have been reported by Faul [24]. Zheng et al. also prepared highly ordered supramolecular microfibers through ISA strategy from complexes of the SAIL N-tetradecyl-N-methylpyrrolidinium bromide (C 14 MPB) and anionic dye methyl orange (MO) with the aiding of patent blue VF sodium salt (PB) (Figure 1a-g) [23]. The crystal structure of microfibers can be indexed as a typical hexagonal columnar mesophase by XRD (Figure 1h). The driving force for the formation of microfiber is hydrogen-bonding interactions, π─π stacking interactions, especially, the role of PB whose big disc-like structure favors the formation of a one-dimensional supramolecular material.
Based on the literature work, Xin and coworkers constructed giant vesicles (1-10 μm) via a facile ISA strategy using an anionic dye Acid Orange II (AO) and an oppositely charged ionic-liquid-type cationic surfactant 1-tetradecyl-3-methylimidazolium bromide (C 14 mimBr) (Figure 2a-c) [25]. It is concluded that the electrostatic interaction, hydrophobic effect, and π─π stacking interaction play key roles in this self-assembly process. Importantly, the giant vesicles can act as a smart microcarrier to load and release carbon quantum dots (CQDs) under control (Figure 2d, e). Besides, the giant vesicles could also be applied as a microrector to synthesize monodispersed Ag nanoparticles with diameter of about 5-10 nm which exhibited the ability to catalyze reduction of 4-nitroaniline (Figure 2f, g). Therefore, it is indicated that the AO/C 14 mimBr assemblies hold promising applications in the areas of microencapsulation, catalyst support, and light weight composites owing to their huge sizes, and large microcavities.
Moreover, giant vesicle could also be prepared by anionic dye MO and an oppositely charged SAILs C 14 mimBr (Figure 3a-d) [26]. The giant vesicle performs fluorescence property owing to the break of π─π stacking of MO molecules (Figure 3c), and the formation of giant vesicle was confirmed by the fusion of small vesicle with the trace of TEM and DLS (Figure 3d, i). The MO/C 14 mimBr system also shows an interesting pH chromism phenomenon. Consequently, the MO/C 14 mimBr system displays abundant aggregate morphologies with the changing of pH that complex fluorescent structures formed at pH > 4 and simple nonfluorescent structures formed at pH < 4 (Figure 3e-h). This is attributed to that MO moieties take a more conjugated coplanar state bridged by N=N under alkaline conditions, whereas MO takes a quinoid structure crossed by N-N resulting the break of conjugated configuration with the decreasing of pH (Figure 3j, k) [27]. Thus, the fluorescence behavior can be predicted with the color change directly visible to the naked eye by changing the pH. It is expected that the facile and innovative design of supramolecular material by the ISA strategy could be used as pH detection probes and microreactors. Recently, one kind of biomolecule, bile salts, has been developed as building blocks for various functional nanomaterials [28,29]. All of the bile salts possess a rigid, nearly planar hydrophobic steroid backbone and have polar hydroxyl groups on the concave α-face and methyl groups on the convex β-face [30,31]. The ionic head with a carboxyl group is linked to the steroid ring through a short alkyl chain. Sodium deoxycholate (NaDC) is a bile salt and also an important biological surfactant which widely exists in the body of vertebrate [32]. The interesting structure of NaDC leads to novel and abundant self-assembly behavior in solution. For example, Xin and coworkers chose NaDC and a cationic dye (RhB) to construct soft materials [33]. In this system, different morphologies with high hierarchies can be reversibly controlled by varying the ratio of the two components (NaDC and RhB) and that the morphologies can switch between porous microspheres and urchin-like structures (Figure 4a-c). The robust hierarchical nanostructure performs superhydrophobicity (water contact angle reach to 137.1° for porous microspheres and 134.2° for urchin-like structures) (Figure 4a, c), which can be used to fabricate an anti-wetting surface. More interestingly, the interaction between NaDC and RhB can restrict the intramolecular motion and intramolecular charge transfer state of RhB, minimize aggregation-caused quenching (Figure 4d, e), enhance the luminescent efficiency of the dye, and improve the luminescence performance, which open up a new way to build soft materials. Besides, the emitting color of system can also be adjusted by changing the molar ratio of NaDC and RhB (Figure 4f, g) and the, lifetimes of the precipitates increase greatly with an increase in c NaDC (Figure 4h). Moreover, they also used another bile salt sodium cholate (SC) and a cationic dye (MB) to obtain bundles of ultra long nanobelts through ISA approach, and the shape and length of the bundles of SC/MB nanobelts could be easily controlled by changing the SC concentration and the aging temperature. Besides, the bundles of ultra long SC/MB nanobelts exhibited efficient electrocatalytic activity toward ascorbic acid (AA) oxidation in phosphate buffer solution (pH = 7.0). This work provides an alternative way to design and fabricate the ultra long belt-like structures with tunable sizes which may also open up a way for the design and development of optical and electronic devices in the potential bioapplications. Different from conventional planar dyes that always undergo π─π stacking which results innotorious aggregation-caused quenching (ACQ), a new class of dyes with propeller-shaped structure displays aggregation-induced emission (AIE) [34]. However, the drawback of the propeller-shaped topology is that it disfavors self-assembly, so that fluorescent nanostructures based on AIE molecules are still very rarely up to date [35]. In Huang's group, propeller-shaped dye TPE-BPA and surfactant CTAB were chosen to build AIE system [36]. The fl uorescence performs the maximum emission at the molar ratio at 1:8 confirming that the optimal interaction between TPE-BPA and CTAB is 1:8 (Figure 5a). And in Cryo-TEM observation, it can be found the formation of vesicles with an average size of 145 nm (Figure 5b). The hydrophilic heads of TPE-BPA molecules are capable of coordinating with metal ions and with the addition of Zn 2+ to the TPE-BPA@8CTAB system. It can be found that the fluorescence intensity decreases with the addition of Zn 2+ and research the plateau at a Zn 2+ /TPE-BPA ratio of 2 ( Figure 5d, e), implying that every two coordinating heads share one Zn 2+ to satisfy the space of an octahedral field. Cryo-TEM reveals the retain of the vesicular structure with smaller size, which was formed by the fission of lager vesicles (Figure 5c). The membranes of cancer cells are highly charged compared to the normal ones, the present results indicate that the high electrical charge may accelerate the cell fission and generate a looser molecular packing and increases the releasing rates of hydrophilic drug DOX (Figure 5f). The report of this research may help to reveal the mystery behind the easy metastasis of the cancer cells and inspire a novel strategy for cancer therapy.

POM-biomolecule ISA materials
Polyoxometalates (POMs), a large group of metal oxide clusters, represent some of the largest inorganic molecules known so far and have broad applications as catalysts, photoelectronic/ magnetic materials, and biologically active materials [37][38][39]. The POMs are formed by linking metal oxide polyhedra with each other through corner-, edge-, or face-sharing manner, which enable chemists to build POMs with different topologies and sizes. Due to the excess of oxo ligands over metal ions, POMs are usually highly negatively charged, which could interact with positively charged materials by electrostatic interaction. Biomolecule, such as amino acid and polypeptide, is essential for our body [40,41]. Thus, the self-assembly behavior of POM/ biomolecule has potential application in biological field as novel compartments, artificial cell membranes, drug and gene delivery agents [42,43].
Wu et al. constructed multivalent peptide nanofibers by using short peptides with the synergistic effect of POM clusters [44]. The short peptide L1 adopts a random-coil conformation in aqueous solution, while with the addition of H 4 SiW 12 O 40 (HSiW), the conformation transition from a random-coil to a β-sheet state (Figure 6a, e). In TEM, one-dimensional (1D) nanofibers were observed after the mixing of L1 with HSiW in water (Figure 6b, c). The multivalent L1/ HSiW nanofibers exhibited significantly enhanced antibacterial efficacy, while the inhibitory ability of L1 or HSiW alone was poor (Figure 6d). The ensembly nature of POMs enables the enhancement of the antimicrobial efficacy and biological stability of short peptides in situ.
Xin et al. obtained inorganic-organic hybrid vesicles using POMNa 9 [EuW 10 O 36 ]·32H 2 O (EuW 10 ) and different amino acids (arginine, lysine, histidine, glutamic acid, aspartic acid, leucine, alanine, and phenylalanine) (Figure 7a, b, g) [45]. The electrostatic interaction between amino acid and EuW 10 plays an important role for the formation of vesicle. While the luminescent property is clear distinction among different amino acids that alkaline amino acids (Arg, Lys and His) enhanced the luminescence, acidic amino acids (Glu, Asp) quenched the luminescence and nonpolar amino acids (Leu, Ala, and Phe) have no obviously influence towards luminescence (Figure 7c, d). This is attributed to the strength differences of the electrostatic interactions between amino acids and EuW 10 cluster. Meaningfully, the Arg/EuW 10 fluorescent vesicles can be used to detect Dopamine selectively with the detection limit of 3.2 μM on the basis of competition mechanism that dopamine could substitute Arg to form an assembly with EuW 10 through hydrogen bond interaction between the ammonium group of DA and the oxygen atom of EuW 10 , leading to fluorescence quenching (Figure 7e, f).
Three-dimensional (3D) hierarchical nanostructures have generated large amounts of interests due to their unique peculiar properties and wide range of potential applications. Among them, nanoflower is a fantastic name of some of the nanomaterials which in microscopic images resemble the flowers. Due to the large surface-to-volume ratio compared with that of bulk materials, nanoflowers have many applications in catalysis, magnetism, nanodevices, sensing and biosensing, and medicine [46][47][48][49][50][51]. In Xin's group, inorganic-organic hybrid hierarchical nanoflowers structures were prepared by EuW 10 /DA on the basis of ISA strategy with the cooperation of H-bonding interaction (Figure 8a-c) [52]. The structure of hybrid can be controlled easily by adjusting the ratio of EuW 10 and DA, and the formation of EuW 10 /DA hierarchical nanoflowers was monitored by SEM: The protonated DA interact with EuW 10 by H-bonding and electrostatic interaction, and then, the cores of the flower-like nanostructures were initially formed (Figure 8d). Subsequently, more building blocks were stacked into the surface of the cores forming a rough surface and the building blocks aggregated to form the resulted nanoflower (Figure 8e). Finally, the hierarchical nanoflowers grew more compact, and the surfaces of the nanopetals became very smooth due to Ostwald ripening in the further growth stage (Figure 8f). POMs have been used as catalysts for the oxidation of a variety of compounds such as alkenes, alcohols, sulfides and dyes [53]. In this work, the author calcinated the EuW 10 /DA nanoflower at 350°C for 2 h to get a pure EuW 10 framework, and the porous EuW 10 performs excellent degradation ability for MO with the exist of H 2 O 2 , the catalyst exhibits only a little loss of photocatalytic activity after six recycles for the degradation of MO, implying that the calcinated porous EuW 10 can be used as an excellent photocatalyst for multiple cycles to catalyze MO and have potential application for sewage treatment (Figure 8g-i). The schematic illustration of the assembly process was shown in Figure 9.

Conclusion and outlook
In a conclusion, ISA strategy on the basis of electrostatic interaction is a facile and convenient method to prepared complex and hierarchical materials for a wide range of applications without tangle some covalent functionlization. Although many studies have focused on the design and preparation of advanced materials using ISA strategy in the past few years, the successful translation of these laboratory innovations to tackle specific problems in the real world remains a great challenge. It is anticipated that preparing functional materials through ISA strategy is a promising pathway to design and generate new nanoassemblies with unique properties and will certainly play a significant role in the future of material science.