Self-Sorting Assembly in Multicomponent Self-Assembled Low Molecular Weight Hydrogels

Liangchun Li; Renlin Zheng; Yi Huang; Rongqin Sun

doi:10.7536/PC220716

Progress in Chemistry ›› 2023, Vol. 35 ›› Issue (2) : 274-286 DOI: 10.7536/PC220716

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Self-Sorting Assembly in Multicomponent Self-Assembled Low Molecular Weight Hydrogels

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Abstract

Multicomponent assembly includes multiple components that can form self-assemblies, which is a common phenomenon in natural processes. We can analyze the structural characteristics embedded in the natural supramolecular structure and design innovative materials according to the predicted molecular interactions. However, due to the limited understanding of the nature of the molecules, the design of low molecular weight hydrogels (LMWGs) with controllable hierarchical structure is still facing certain difficulties, and it is far from the naturally formed high-level complex living system. In the field of multicomponent supramolecular chemistry, it is necessary for us to study the structure and function of multicomponent self-assembly networks by using the method of system theory. In addition to understanding the properties of component molecular monomers, we also need to study the chemical networks formed by component molecules for the better understanding of nature. When self-assembly is triggered in a multicomponent system, there are usually three assembly pathways, namely, co-assembly, self-sorting and multidimensional hierarchical combination of assemblies or heterojunction. These three assembly systems compete with each other but may also coexist, resulting in the complexity and multiple responsiveness of the multicomponent assembly system. Therefore, the design and assembly structure prediction of the multicomponent assembly building blocks or assembly systems are also very challenging. The high-level and multicomponent assembly process allows multiple self-assemblies to operate cooperatively and orthogonally, and has precise spatial control. The self-sorting is the basis of many related (biological) chemical processes (such as phase separation, kinetic analysis or self-replication), which can be narcissistic or social. The study of self-sorting assembly in multicomponent assembly system is of great significance for deepening the understanding of the relationship between components and molecules, realizing the control of the network, and realizing the controllable construction of high-level complex assembly system. In this paper, the characteristics and research methods of multicomponent assembly are reviewed, and the research results of self-sorting in the fields of self-assembly characteristics and control of multicomponent assembly are displayed and discussed, in order to promote the understanding and in-depth research in this field.

Contents

1 Introduction

2 Characteristics of multicomponent assembly system

3 Study on multicomponent self-assembly from the viewpoint of system theory

4 Study on self-sorting assembly of multicomponent self-assembly hydrogels and their control factors

4.1 Utilizing the difference and complementarity of structures to achieve control

4.2 Utilizing the chiral inductions to achieve control

4.3 Utilizing the dynamic process to achieve control

5 Image research method in the researches of self-sorting assembled hydrogels

6 Conclusion and outlook

Graphical abstract

Keywords

multicomponent self-assembly / low molecular weight hydrogels / self-sorting / co-assembly / systems chemistry

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Liangchun Li, Renlin Zheng, Yi Huang, Rongqin Sun. Self-Sorting Assembly in Multicomponent Self-Assembled Low Molecular Weight Hydrogels. Progress in Chemistry, 2023, 35(2): 274-286 DOI:10.7536/PC220716

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1 Introduction

Some small organic molecules, at very low concentrations (mass fraction can be less than 1%), can form a supramolecular network structure in water through the process of molecular self-assembly, thus forming a non-flowing colloidal structure, which is called hydrogel^[1,2]. These small organic molecules, which can form hydrogels, mainly form ordered aggregation through non-covalent intermolecular interactions such as hydrogen bonds, π-π interactions, electrostatic forces, van der Waals forces, and further self-assemble to form multi-level structures^[3⇓~5]. Individual noncovalent interactions are generally weak and reversible; However, when they act together, robust self-assembled structures are formed. Different small molecules can self-assemble to form different basic structures such as nanofibers, nanotubes and nanosheets, which determine the basic physical and chemical properties of the hydrogels formed by them, and then determine their applications^[6⇓~8]^[9,10]. Since the network in the hydrogel structure is a fragile and reversible network formed by non-covalent forces, these gels can react to external stimuli, such as temperature, pH, or ions, and can endow the gel with interesting mechanical properties, such as reversible healing^{[11⇓⇓~14]}^[15]. Therefore, small-molecule hydrogels have attracted increasing attention in the past few decades in the fields of environmental remediation, biosensors, drug delivery, optical switches, and self-healing materials^[16⇓~18]^[19,20]^[21⇓~23]^[24,25]^[26,27].

Usually, in the research papers of small molecular hydrogels, you can see a picture of an inverted test tube to judge whether the hydrogel is formed or not. From the appearance point of view, the difference of hydrogel is mainly reflected in transparency and color. However, from the molecular level, even the hydrogel systems with the same appearance can show great differences, for example, the molecular assembly units can be arranged from 1D to 3D, from single order to diversity, which reflects the diversity of the final gel network structure caused by the diversity of molecular design^[28⇓~30]. Because the self-assembly process is easily affected by various factors, and the combination of various factors may produce new properties, it is difficult to design and predict whether small organic molecules can form hydrogels and the size and morphology of self-assembled nanomaterials that form hydrogels^{[19,31⇓~33]}. From the analysis of the components of self-assembled small molecular gels forming hydrogels, the current research is mainly reflected in two aspects:

(1) Single-component self-assembled hydrogel. The method is prepare by self-assembly of a single micromolecular gel except a solvent, and self-assembled nanostructures with similar size are obtained by changing preparation parameters, such as a phase equilibrium method, a pH drive method and that like, so as to obtain an assembled nanotube structure with adjustable size^[34]^[35]. In this method, the hydrophilicity and hydrophobicity of the side chains of the assembled molecules and the charge (pH-driven or stimuli-responsive) are mainly considered, and sometimes a small amount of hydrophilic organic solvent (such as DMSO, DMF or small molecular alcohol) is introduced to promote the formation of the hydrogel^[36,37]^[38⇓~40].

(2) Multicomponent self-assembled hydrogel. The self-assembly process of assembled molecules is influenced by multi-component molecules, so the interaction between different added molecules and molecular side chains (such as the recognition of ligands and receptors) can be realized to control the self-assembly process^[4,41⇓~43]. Some impressive reviews discuss the effectiveness of different techniques for modeling and characterization of multicomponent assemblies^[12,44,45]. The performance of an assembly depends not only on this primary assembly, but also on how the assembly, such as fibers, is wound, cross-linked, and interacted. Therefore, to fully understand these systems, it is necessary to characterize multicomponent assemblies at multiple length scales, from molecular interactions to bulk properties.

In this paper, the characteristics and research methods of multi-component assembly systems will be reviewed, and the research results in the fields of the characteristics and control of the self-sorting assembly of multi-component assembly systems will be presented, and the research progress of small molecule gels in the multi-component self-sorting assembly hydrogel assembly system will be discussed, so as to promote the understanding and further research in this field.

2 Characteristics of multicomponent assembly system

Multicomponent self-assembly is one of the hot topics in supramolecular research. Multi-component self-assembly contains multiple components (molecular building blocks, or building blocks) that can form self-assemblies, which is a common process in nature. Many highly ordered and complex structures in nature are formed by the interaction of multiple components^[46,47]. Such multicomponent formation structures tend to be thermodynamically more favorable than single component formation structures. Biological species, such as phospholipid cell walls and DNA, are formed by multifunctional non-covalent self-assembly of various components, in which small molecules or macromolecules are aggregated alone or cooperatively to form biomaterials with multiple functions^[48]. These supramolecular assemblies are the result of coordinated noncovalent interactions, providing adaptability, dynamism, and responsiveness. Unlike gene-encoded biomolecular organization, abiotic self-assembly of multicomponent systems often exhibits complex pathways, and the complexity of self-assembly depends mainly on the surrounding environment, the interaction between components, and the structural similarity of components^[31,49,50].

We can design innovative materials based on predicted molecular interactions by analyzing structural features embedded in natural supramolecular structures. These design strategies are typically developed using well-defined molecular building blocks, including β-sheet fibrotic peptides, peptide amphiphiles, engineered peptides, and low molecular weight species, among others^[51,52]. Many artificial nanostructures consist of a single class of molecular building blocks, which limits structural and functional complexity, diversity, and tunability. Multicomponent self-assembly can generate more extensive and complex structures, enhance modularity and provide spatiotemporal control of self-assembly, such as peptide-peptide, protein-peptide, peptide-polysaccharide, protein-protein and protein/peptide-DNA^[53]^[4]. For these multi-component assemblies, since two or more different interactable molecular building blocks are used, the structures and properties produced by these systems can provide new opportunities for more complex and functional materials design by taking advantage of the synergistic properties of these multi-component molecules. Multi-component self-assembly is a general strategy to design new materials and supramolecular hydrogels with molecular complexity, enhanced interfacial area between nanofibers, tunable mechanical properties, and in-situ formation of various morphologies^[54]. In addition, the multi-component gel has more advantages compared with the single-component system. For example, multi-component gels can display optoelectronic properties that cannot be achieved with a single component, or systems can be prepared with unusual behaviors, such as delayed gelation, two-stage rheology control, or improved cell behavior^[55,56]^[57]^[58]^{[59⇓⇓~62]}. New adaptive materials can also be designed by multi-component self-assembly of mixed systems using the different stimulus responsiveness of different self-assembly gelators. For example, Hamachi et al. Designed a hybrid hydrogel consisting of self-sorting dual networks of self-assembled nanofibers of different structures (peptide-type gelator and phospholipid-type gelator), where each network responds independently to an applied external stimulus^[63]. Hydrogels can be made to change their mechanical properties and the release rate of encapsulated proteins by the addition of Na₂S₂O₄ or bacterial alkaline phosphatase, and it is worth noting that the sequence of application of external stimuli achieves the regulation of gel properties.

In general, when Self-assembly is triggered in a multicomponent system, three competitive assembly systems can be expected, namely, Co-assembly, self-sorting, and Multidimensional hierarchical combination of assemblies or heterojunction^[48]. A schematic diagram of the assembly of these three forms is shown in Figure 1. Co-assembly means that when the interaction between different components is relatively strong, they can randomly or specifically produce mixed supramolecular arrangements, and multi-component molecules cooperate to integrate at the molecular scale and form highly ordered assemblies on this basis. self-sorting shows selective recognition of a single component and gives orthogonal or parallel aggregation, which can be divided into Narcissistic self-sorting and Social self-sorting^[64]. Narcissistic self-classification shows that multi-component assemblies are independent of each other and there is almost no interaction^[65]. Social self-classification, on the other hand, shows specific recognition characteristics between different component molecules, and multi-component molecules form a specific way of co-assembly^[66,67]. Self-sorting can endow assemblies with rich skeletons, resulting in highly ordered materials with complex structures and functions. Co-assembly allows materials integration at the molecular scale, which can show specific advantages over physical mixing or micro/nanoscale mixing prepared by self-sorting^[68,69]. For example, liposomes and polymer vesicles are self-assembled vesicles of lipid and polymer amphiphiles, often combined with targeting ligands, drugs, and bioimaging agents as payloads. If prepared using the co-assembly pathway, the payload would be much more efficient and might fulfill pH, enzyme, light, temperature, or other stimulus-responsive functions^[70]. In the multi-component assembly system, the formation of heterogeneous multi-dimensional assembly can be conceived as a similar top-down assembly result, in which one component is pre-assembled as a seed and then self-assembled with another component (or assembly) to form a superstructure, and this method has a good guiding significance for the development of multi-dimensional multi-level assemblies. For example, Manners et al. Select different hydrophobic or polar components, use different solvents to mix, stack side by side or combine end-to-end micelles to form co-micelles, and prepare a variety of stable three-dimensional superstructures^[71]. In fact, these three assembly systems compete with each other but may coexist, resulting in the complexity and multiple responsiveness of multi-component assembly systems, so the design and assembly structure prediction of multi-component assembly building blocks or assembly systems are very challenging.

3 A systems-theoretic approach to multicomponent self-assembly

Although the formation of single-component self-assembled nanostructures and multi-component co-assembled supramolecular structures is controlled by non-covalent interactions, the kinetic and thermodynamic parameters are particularly important for the controlled assembly of multi-component hydrogels^[72]. Environmental or external stimuli such as temperature, pH, and solvent interactions have been shown to influence the kinetic and thermodynamic properties that determine the final formation of supramolecular hydrogels^[73,74]. In order to selectively form controllable multicomponent supramolecular hydrogels, the kinetics and thermodynamics of the whole assembly system must be considered simultaneously. Because these complex mixtures can produce interesting and desirable emergent properties that result from the interaction of the components and cannot be attributed to any one of them alone. Therefore, for most multicomponent systems, it is difficult to predict and control the outcome of molecular assembly as determined by various external (solvent composition, temperature, polarity, etc.) and internal (chemical structure, solubility, interaction strength, etc.) factors. While analytically understanding the elements of these self-assembly and co-assembly pathways is complex, the manipulation of kinetics and thermodynamics can lead to greater hierarchical control over the formation of complex synthetic biomaterials^[75,76].

Traditionally, we study complex chemical systems by a reductionist approach, that is, their individual components or subsystems are analyzed separately under certain control conditions. However, a complex chemical system is not simply the sum of the properties of its parts. In the field of multi-component supramolecular chemistry, it is necessary for us to study the structure and function of multi-component self-assembly networks by using system theory^[77]. System theory focuses on thinking and exploring the origin of things from the whole system. Contrary to the reductionist method, the system theory method needs to study multiple variables at the same time^[78]. The multi-component self-assembly system is a dynamic combinatorial chemical system, which allows a more refined control of the chemical network by establishing controllable interactions between molecules, so that different self-sorting network formations are observed under kinetic and thermodynamic control^[79]. For example, for a multicomponent supramolecular hydrogel system generated by non-covalent interactions, changes in the environment or external stimuli will lead to structural and functional responses, as well as the adaptation of the system under study to the new environment, resulting in new assembly results^[80,81]. These changes are closely related to the chemical network formed by the component molecules, and the control of the network (such as the introduction of specific molecular control network nodes) will affect the interaction between the components^[78].

Therefore, the research method of system theory is helpful to better understand the organizational principles of self-assembly networks and the relationship between these principles and functions^[52]. Of course, with the increase of system complexity and research variables, the research methods of system theory also show some limitations, and more interference factors may affect the research results, but we believe that with the development of science and technology and the updating of research methods, these problems will be overcome^[82]. Chemists have a unique ability to design and create new molecules, and extending this creativity from isolated molecules to molecular networks will surely lead to many new molecular systems with unique properties. The in-depth study of multi-component assembly systems will promote the development of supramolecular hydrogel functional chemistry, which can successfully simulate complex biological networks by using relatively simple chemical networks.

4 Study on Self-sorting Self-assembling Hydrogel and Its Controlling Factors

Supramolecular chemistry is now moving towards more and more complex systems. In order to mimic some of the characteristics of biological processes, it is essential to study and understand the nature of multi-component molecular interactions^[83,84]. The multi-level multi-component assembly process allows multiple self-assemblies to operate synergistically and orthogonally with precise spatial and temporal control^[85]. The phenomenon of self-sorting, which underlies a variety of related (bio) chemical processes such as phase separation, kinetic resolution, or self-replication, among others, defines the collective behavior of a group of molecules that are highly capable of maintaining the specific assembly they form and that do not interfere with other molecules^[86]. The phenomenon of self-classification can be narcissistic, that is, it has a strong tendency to self-identify and thus self-integrate. It can also be social, that is, when a molecule undergoes a self-discrimination process, it shows a high affinity for other molecules, such as host-guest molecules^[79]. For a multi-component gel system to achieve self-sorting assembly, the individual components should not only assemble in the same environment (matched solvent, pH, and temperature), but also display a high degree of specificity, preventing co-assembly in their non-covalent interactions.

4.1 Control by using the difference and complementarity of structures

Orthogonal self-assembly, which relies on the self-sorting assembly phenomenon, is a promising systematic strategy to realize the construction of biomimetic materials, which shows the dynamic and multifunctional properties of integrated systems. Multicomponent gels are usually composed of two or more independent gelling agents, providing an opportunity to prepare useful materials that are not available using a single component. It is found that different self-assembly interactions and strong driving force are the key factors to control the orthogonal self-assembly.Their molecular structures (spatial structure, hydrophilic and hydrophobic properties of side chains, etc.) Or supramolecular structures after assembly should have greater specificity, but at the same time, there should be certain interactions between molecules to coexist in the system and achieve adjustable control^[87].

van Esch et al. First reported self-sorting assembly of hybrid supramolecular hydrogels by screening hydrogelators in combination with phospholipids, which can encapsulate enzyme-containing vesicles orthogonally, showing that orthogonal self-assembly can occur only when the component molecules are assembled through a set of strong and unique interactions^[87]. Mata et al. Utilized peptide amphiphile (PA-E3) and 1,3: 2,4-dibenzylidene-d-sorbitol (DBS-COOH) gelling agent mixing (Fig. 2) to achieve synergistic multi-component self-assembly, generating hydrogels with tunable nanoscale morphology, improved hardness, and enhanced self-healing and enzymatic degradation stability^[54]. It was found that PA-E3 formed self-sorting self-assembly in the multi-component system, while DBS-COOH gel was used as an additive modifier for PA-E3 nanofibers. Compared with the single-component system, the structure of PA-E3 and DBS-COOH was quite different, and co-assembly did not occur, but the supramolecular interaction between the molecular assemblies improved the volumetric properties and cytocompatibility of the two-component hydrogel.

Adams et al. Studied the structural relationship of pH-controlled self-sorting peptide hydrogels^[58,88]. They found that hydrogels with similar structures tend to assemble with each other, even though the pKa values of the hydrogels are different. Smith found that two dendrimers with similar structures but different peripheral groups could form self-sorting assembly under certain conditions^[89]. These pioneering studies demonstrate that structurally distinct building blocks and driving forces are essential for achieving self-sorting assembly. Srivatsan et al. Used amphiphilic purine and pyrimidine ribonucleoside-fatty acid conjugates (nucleolipids) to construct multi-level assemblies of two-component systems, showing an interesting self-sorting and co-assembly behavior^[90]. The formation of the gel can be controlled by coupling fatty acid acyl chains of different lengths at the 2 '-O and 3' -O positions of the ribose,Different mixtures of supramolecular synthons and complementary ribonucleosides exhibit cooperative and destructive self-sorting and co-assembly behavior, and obtain gels with significantly enhanced mechanical strength over single-component systems.

Adams et al. Recently reported pH-regulated self-sorting assembly systems of p-type and n-type gelators (see Figure 3 for chemical structures), such as hydrogels based on symmetrically functionalized diketopyrrole aromatic nuclei DPP-1 and PBI-A (an n-type gelator), which have complementary electron-withdrawing and electron-donating interactions when they are close to each other^[91]. The results showed self-sorting self-assembly of DPP-1 and PBI-A by rheology, pH change, UV-visible absorption spectroscopy, and NMR spectroscopy. For example, compared with the UV-visible absorption spectrum of a single component, the UV-visible absorption spectrum of a mixed system looks like the addition of two spectra, rather than a new peak appearing in the co-assembled system. However, all of these data do not rule out that a small amount of co-assembly occurs. Adams also reported other two-component gel systems based on n-type perylene bisimide (PBI) and p-type distyryl (or p-styrylene), and found that the self-sorting assembly system can be formed by slow pH change. Small angle neutron scattering showed that the self-sorting assembly system is sensitive to pH, and pH also affects the electron transfer between components^[92⇓~94].

The physical and chemical properties such as hydrophilicity, hydrophobicity, charge, aromaticity and spatial structure of different amino acid sequences and different side chains in the peptide chain vary a lot, which leads to the formation of the assembly network with various mechanisms.Therefore, the use of short peptides to study the assembly control of multi-component gels is one of the current hotspots in this field. Roy et al. studied the self-assembly of two short peptides with different structures based on IKVAV and YIGSR and their composite supramolecular gels (Fig. 4), and analyzed the structures of the two peptides. The hydrophobicity of the two peptides is quite different. Fmoc-YIGSR has water solubility, while Fmoc-IKVAV has poor water solubility, and its assembly performance after mixing is greatly affected by its solubility^[95]. This composite gel can mimic the essential function of the extracellular matrix protein laminin. They used a unique strategy of simply mixing two short peptides, which tend to form a self-sorting self-assembled nanofiber network, with two functional peptides forming two mutually entangled networks, and this entanglement strengthened the recovery properties of the gel, making it suitable for development as an injectable gel. Webber et al. Reported the self-assembly of two oppositely charged tetrapeptides (DWDW and KWKW), which produced highly different fibrillar and spherical assembled nanostructures^[96]. The different nanostructures of the parent peptide remained after mixing at different ratios. Interestingly, while these assemblies appear to be self-sorting, the surface-mediated interaction between the spherical assembly and the fiber assembly translates into improved mechanical properties through enhanced fiber bundling. Moreover, the observed self-sorting assembly is a thermodynamic product rather than a kinetically trapped pre-existing structure.

Wakabayashi et al. Successfully regulated the self-sorting and co-assembly network of this class of peptides by modifying peptide amphiphiles with a pair of immiscible hydrophobic substituents. Through standard heating-cooling treatment, peptide amphiphiles with alkyl chains and peptide amphiphiles with fluoroalkyl chains formed self-sorting assembly fibers.But before the cooling process, the pair of peptide amphiphiles with longer hydrophobic tails were converted from self-sorting assembly to co-assembly network by additional high temperature cultivation (363 K), and the co-assembly morphology depended on the cultivation time^[97].

It is an effective control strategy to realize multi-component self-sorting assembly by using the complementary structural differences of components and the resulting electrostatic drive. For example, Adams et al. Designed a multi-component hydrogel system (Figure 5) using different ionized side chains on the dipeptide gelling agent backbone, which can show gel-to-gel transition when the pH value is reversed, and the system shows cooperative hydrogen bonding reaction behavior between the components^[42]. At low pH, the system mainly showed self-sorting assembly, but when the pH increased, it showed co-assembly behavior, and the different assembly results could be found by transmission electron microscopy. For another example, Escuder and van Esch et al. Prepared two different hydrogels by mixing SucVal8 and two different L-proline derivatives (ProValDoc and ProVal8) (fig. 6)^[98]. The large structural difference between SucVal8 and ProValDoc leads to their orthogonal self-assembly into two self-sorting networks, while SucVal8 and ProVal8 with similar backbone structures form a single co-assembly network.

4.2 Using chirality-induced control

Subtle changes in molecular structure, including changes in chirality, also play an important role in the control of assembly. Dendrimers with similar geometric structures and peripheral functions are generally considered to tend to form co-assemblies rather than self-sorting assemblies, but Wang et al. Achieved the control of self-sorting assemblies of this kind of two-component dendrimers by using the kinetic control of chiral-induced recognition.In this composite gel, chirality-induced recognition on dendrimers acts as a "booster" for the efficient fast gelation process, facilitating the recognition of hydrogen bonds between dendrimers and inhibiting the heterogeneous self-assembly of each individual^[63]^[99].

Pal et al. Realized self-sorting self-assembly by chiral induction in amphiphilic minimalist peptide (C10-L/D-VFFAKK) nanofibers constructed based on β-amyloid protein, and used peptide fluorophores with donor and acceptor FRET pairs to study the competition relationship between self-sorting assembly fibers and co-assembly fibers in detail^[100]. Meijer et al. Used trans-1,2-disubstituted cyclohexane as a model system to compare the gelation tendency of individual enantiomers and racemates, and found that the racemates substituted with formamide, thioamide and their combinations adopted network structures such as self-sorting, co-assembly and mixed organization, respectively (Fig. 7)^[101]. Remarkably, the assembly modes of these racemates are also preserved in the solid state, and these results suggest that studying solution-phase assembly is a key link between molecular structure and assembly in the solid state, even for racemates. Kraatz et al. Reported a class of self-assembled organogel fibers based on the tripeptide Phe-Phe-Leu, which was modified with ferrocene or pyrene groups, and the self-assembled organogel fibers were observed only when a specific pair of tripeptides were mixed (chirality: SSS and RRR, respectively)^[102].

In addition, the chirality of the polymer or supramolecular fiber also governs the assembly behavior. For example, Wu et al. Reported the self-sorting assembly behavior of helical polyisocyanurium in liquid and gel States^[103]. Zhao et al. Designed two gelators containing the same chiral phenylalanine core but different achiral peripheral substituents (LPF and LPFEG, Fig. 8) to obtain hydrogels with opposite supramolecular chirality. When the two hydrogels were mixed, double-network nanofibers with opposite chirality were formed through the high-level self-sorting assembly of the two gelators^[104]. The chiral activity of the double-network hydrogel can be tuned by changing the molar ratio of LPF and LPFEG in the mixture, thus indicating that the two gelators are highly independent of each other. When the molar ratio of LPF/LPFEG was 3:7, the mechanical properties of the interpenetrating network were observed to be enhanced, and the storage (G ′) and loss modulus (G '') were increased by more than four times compared with those of the individual hydrogels.

4.3 Process control by dynamics

In addition to structural factors, self-sorting multicomponent supramolecular gels with similar chemical structures can be prepared by kinetically controlled self-sorting pathways. For example, self-sorting assembly control can be achieved by using the kinetic control of different assembly (nucleation) rates of individual molecules in a multi-component molecule, but aging induction has an important impact on this process. Recently, van Esch et al. Reported a hydrazone-based multicomponent hydrogel system, in which hydrazine (H) and an aldehyde composed of neutral (A) and negatively charged (A^-) species are coupled together via in situ hydrazone bond formation to form a neutral (G) and negatively charged hydrogel (G^-) (Figure 9 A)^[105,106]. The generated G and G^-, because of their different nucleation rates, can self-assemble into thermodynamically more stable heterogeneous hydrogels (pathway I, Fig. 9 B) through a multistage self-sorting assembly process, consisting of separate microdomains rich in neutral fibers (F) or negatively charged fibers (F^-). However, if the H solution is subjected to aging treatment, it can induce the formation of H aggregates in the seed self-assembly process, interfere with the nucleation step of hydrogel assembly, and guide it to self-assemble along a kinetically controlled path, thus forming a hydrogel composed of F^- (pathway II, fig. 9b). Interestingly, these hydrogels are in a metastable transient state and can change to a thermodynamically more stable hydrogel state with time at room temperature. Therefore, aging induction of self-assembled components in multicomponent supramolecular systems is also potentially important for controlling the self-assembly process. In addition, Wang and van Esch further found that the use of aniline catalysis can effectively promote the formation rate of hydrazone-based gels, thus affecting the supersaturation, thereby affecting the nucleation of gels.Thus, catalytically controlled self-assembly of molecular gelling agents allows the preparation of kinetically more preferred hydrogels, displaying network morphologies and mechanical properties that are quite distinct from thermodynamically more stable products^[107]. These methods can be used to prepare non-equilibrium supramolecular materials with path-dependent properties for high-tech applications.

Another strategy to achieve a higher level of control of the hydrogel network by modulating the nucleation process is through the regulation of dynamic covalent bonds. Hamachi et al. selectively constructed two different self-sorting network modes, namely interpenetrating and parallel self-sorting assembly networks, by controlling the formation kinetics of hydrogel seeds through dynamic covalent oxime chemistry (Fig. 10).Moreover, through the oxime exchange reaction, the conversion of interpenetrating self-sorting assembly network to parallel self-sorting assembly network is realized, which will help to rationally design the next generation of adaptive soft materials with hierarchical organization^[108].

5 Image Research Method of Self-sorting Assembly Hydrogel

It is very difficult to evaluate self-assembled supramolecular nanofibers by transmission electron microscopy or scanning electron microscopy if the molecular structures of multi-component systems are similar^[109]. Super-resolution microscopy (SRM) can be used to directly observe the self-sorting assembly structure and realize the image study by combining specific fluorescent groups with component molecules^[110,111]. For example, Pal et al. Verified the chiral recognition of L/D peptide nanofibers by combining with thioflavin-T (ThT), and directly observed the chirality-driven self-sorting peptide fibers with super-resolved structural microscope (SR-SIM) for the first time^[100]. Using different fluorescent probes combined with different similar hydrogels, Hamachi et al. Used super-resolution microscopy, such as STED and Airyscan technology, to show the 3D structure of the self-sorting network, and realized the evaluation of the self-sorting assembly behavior of peptide and lipid nanofibers based on in situ imaging^[112]. In situ time-lapse imaging showed that the two fibers formed at different rates and their respective physicochemical properties remained unchanged in the gel. Therefore, image-based screening methods make it possible to study self-sorting or co-assembly nanostructures in a high-throughput manner^[113,114]. However, it is worth noting that for small molecule systems, fluorescent labeling may affect their assembly, and direct morphological characterization is not suitable for assemblies with similar morphology^[115]. Computational chemistry, such as DFT quantification or molecular dynamics simulation, should be able to partially solve these problems and provide important assembly information^[116,117]^{[118⇓⇓~121]}.

Recently, Zhang and Cao et al. Used UV and visible light to achieve self-sorting assembly of patterned bioactive molecules (i.e., cyclic arginine-glycine-aspartic acid (cRGD) and osteogenic growth peptide (OGP)) using the photoreactive sites contained in peptides^[60]. In this study, the designed self-sorting double-network supramolecular hydrogel mimicking extracellular matrix (ECM) peptides can be directly observed, which realizes the direct observation of cell culture microenvironment. Smith et al. Monitored the time-dependent spectral characteristics of the self-assembly of mixed gelators (DBS-CONHNH₂ and DBS-COOH) by nuclear magnetic resonance hydrogen spectroscopy (¹H NMR), which could quantify the immobilization of individual gelators (after assembly into a "solid-like" nanofiber network, the nuclear magnetic resonance signal was invisible), thus judging that they formed a self-sorting assembly system^[122].

6 Conclusion and prospect

In summary, the common strategies for controlling the self-assembly of small molecular hydrogels and the imaging characterization methods are discussed. At present, although nanostructures or microstructures with various structures and functions can be prepared by different interactions, they are used in many fields, such as drug delivery, catalysts, microreactors, etc^[123]^[124]^[125]^[126]. However, self-sorting assembly is still a potential and promising approach in integrating the performance and advantages of different functional nanostructures. Recently, some interesting applications have been achieved by using self-sorting assembly, which expand the research of self-sorting assembly, such as drug targeted delivery, tissue engineering, manufacturing hierarchical microstructures, simulating the structure of complex extracellular matrix, controlling the final products in reversible reactions, and forming complex catalytic cascades^[127]^[128]^[129]^[60]^[130]^[113].

Of course, although most self-sorting assembly gels show some excellent and unique properties, the research on their assembly mechanism and application is not deep enough. To achieve gelation, an equilibrium must be reached between the tendency of molecules to dissolve or aggregate, and for most self-sorting assembly systems, the components remain in thermodynamic equilibrium at all times^[131,132]. However, the self-sorting assembly units existing in nature can easily switch between kinetic control and thermodynamic equilibrium, and realize self-sorting assembly or co-self-assembly when necessary to form multi-level complex assemblies according to the change of environment (stimulus response)^[133,134]. From the analysis of the current literature, there are many studies on the self-sorting and self-assembly system of two components, and few studies on the self-sorting and self-assembly system of three components, four components or even more components.Consciously designing controllable hydrogels still faces difficulties, often with unexpected results, knowing that a small change in the gelling agent often changes the entire assembly system^[135]^[45,115,136]. Therefore, the multi-component system in nature is obviously more complex, and more exploration and efforts are needed to realize the artificial construction of self-sorting assembly system simulating nature and achieve controllable and high-level assembly, especially in the synthesis of artificial building blocks, control methods and comprehensive application of multi-strategies.

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