Aggregates of fluorescent gels assembled by interfacial dynamic bonds

Life, defined as the specific form of substance, is an integration of aggregates at various scales, ranging from single molecules to tissues. However, these building blocks of common aggregates are usually recognized as confining at the microscopic level, while there are few studies focusing on macroscopic building blocks for aggregates. Fluorescent gels, as the important macroscopic building blocks, are drawing researchers’ attention on account of their extraordinary fluorescence as well as soft material properties. Inspired by nature, fluorescent gels can be aggregated through interfacial adhesion. According to the driving forces for interfacial adhesion, a series of aggregates of fluorescent gels (AFGs) was summarized, including H‐bond, metal coordinations, host‐guest interactions, hydrophobic interactions, electrostatic interactions, dynamic covalent bonds as well as multiple driving forces. These AFGs own dynamic assembled behaviors and rich stimuli responsiveness, which could be applied to information storage, sensing, biomedical systems, and so on. The authors anticipate this review can accelerate the development of aggregate science, especially based on macroscopic building blocks.


INTRODUCTION
Life has existed for billions of years since the single-cell organism was discovered, which remains a topical issue all along. [1,2] Understanding life could be started from its definition. According to the composition of matter, life could be defined as a specific substance structure. As is known to all, substance structure is able to be divided into distinct types in accordance with scales. Therein, an elementary particle is considered the basic building block of a substance, more than that, it acts as the material base of objects. [3,4] However, it reminds us that how to achieve the construction of substance structure by employing elementary particles. Aggregation is apparently the best answer, conducted as the vital linkage between macroscopic substance structures and elemental particles. [5] For instance, atoms bond together in order to form molecules via aggregation. [6,7] Molecules are recognized to be the smallest building unit of objects. [8] S C H E M E 1 Illustration of aggregates of fluorescent gels based on interfacial dynamic bonds studies of aggregates have focused on microscopic building blocks, only a few research is pivoted to macroscopic building blocks. [15][16][17] Among these reports about aggregates, fluorescent gels, as the important kind of macroscopic building blocks, are gaining extensive attention owing to their extraordinary fluorescence as well as soft material properties. Fluorescence, which is defined as the significant type of photoluminescence (PL), means the light emission exhibited by molecules or substances excited through another light with higher energy, possessing numerous merits of easy detection, high sensitivity, and low interferences. [18][19][20] Traditional fluorescent molecules are based on aggregation-caused quenching (ACQ) effect, namely, they are non-emissive at aggregation states. [21,22] In 2001, Tang and co-workers reported a type of molecule showing aggregation-induced emission (AIE) phenomenon that meant molecules were non-emissive at monomeric state while showing bright fluorescence at aggregation state, which could be a better candidate to fabricate fluorescent gels, [23][24][25][26][27][28][29][30][31][32][33][34][35][36][37][38][39] and this discovery helped to overcame the shortcomings brought by the traditional ACQ effect of fluorescent molecules. Since fluorescent gels are prepared via incorporating fluorescence substance into the network structure of gel, they also possess appealing soft material properties, like a highly stretchable nature, biodegradability, self-healing, and three-dimensional matrix characteristics. [41,42] Toward this end, these mentioned attractive properties allow fluorescent gels to be widely utilized in optical devices, information storage systems, tissue engineering, drug delivery, wound healing, and bioinspired actuators. [43,44] To obtain the aggregates of fluorescent gels (AFGs), we can get inspiration from numerous natural phenomena. For instance, the gecko shows extraordinary climbing capabilities, like crawling on vertical walls as a result of characteristics of van der Waals interactions. [45,46] Similarly, tree frogs are able to climb smooth vertical as well as overhanging surfaces on account of possessing analogous physiological structures. [47,48] Moreover, mussels have the ability to adhere to the stone firmly, since their secreted adhesion proteins enable themselves tightly against smooth rocks mainly ascribing to hydrogen bonds formed between proteins and surface. [49,50] Following these fascinating adhesion phenomena, numerous wonderful synthetic supramolecular adhesive materials were reported by scientists including Shi, Chen, and so on. [51,52] Then, AFGs can be also produced through interfacial assembly using dynamic bonds, which mainly encompass supramolecular interactions as well as dynamic covalent bonds, taking the advantage of the reversibility and stimuli-responsiveness (Scheme 1). [53,54] The involved supramolecular interactions include H-bond, [55,56] metal coordination interactions, [35,57,58] host−guest interactions, [59][60][61][62][63] hydrophobic interactions, [64][65][66] electrostatic interactions. [67][68][69] In addition to that, dynamic covalent bonds are also employed for interfacial adhesion, principally including the acyl hydrazone bond and imine bond. [70,71] To date, the studies concerning dynamic bondbased AFGs flourished, hence, this review summaries a series of AFGs in accordance with different interfacial dynamic bonds, also shedding light on their applications in distinct realms. Finally, we propose the challenging scientific issues that need to be tackled, then envisage a full picture for AFGs in terms of future research.

AFGs BASED ON INTERFACIAL H-BOND
H-bonding, first proposed in 1912, has been widely known for more than a hundred years. The concept of H-bonding is basically defined as a directional dipole-dipole attraction between an electronegative atom and a hydrogen atom, located on an electronegative atom possessing a lone pair of electrons, like oxygen, nitrogen, or fluorine. [72] Compared with other types of chemical bonds, the binding energy of a single H-bonding was weaker undoubtedly, within the scope of 5-30 kJ/mol. [73] Anyhow, when the multiple H-bond is arrayed together, thus the strength is increased substantially.  [74] Notwithstanding, the existence of the H-bond has a vital impact on materials' properties, such as bulk viscoelasticity and thermoreversibility owing to the directionality of binding as well as reversibility. Additionally, H-bond also exhibits rich responsiveness under external stimuli, including pH, temperature, and solvent polarity. Due to these extraordinary properties, H-bond is allowed to be integrated into materials, offering diverse useful functions. The ongoing utilization fields of H-bond have been pioneered by researchers in-depth as well as in breadth, involving material science, supramolecular chemistry, and biochemistry. On the basis of multiple H-bond, numerous AFGs serving various functions are developed by researchers. For instance, some common groups, such as ureidopyrimidinone (UPy) units are always being introduced into gels for providing H-bond, which promotes the interfacial adhesion between gels. Then AFGs based on interfacial H-bond would be discussed in the following parts.
Distinctive properties of supramolecular gels enable them to be strong candidates for building mutifluorescent materials. As for constructing AFGs showing multiple fluorescent colors, it is naturally to think of incorporating different fluorescent molecules into gels, however, this conventional mixing approach is recognized as infeasible due to the occurring interactions among these fluorescent molecules, causing only the single fluorescence exhibited by gels. Herein, to address this problem, Ji et al. completed the construction of AFGs through a supramolecular approach, which was achieved by interfacial assembly of discrete fluorescent gels based on multiple H-bond. [74] First, three kinds of polymers P1, P2, and P3 own the same poly(methyl methacrylate) (PMMA) main chain as well as pendant UPy units, various specific chromophores were fabricated shown in Figure 1A. Owing to the presence of UPy groups, the respective polymers 1, 2, and 3 were able to self-assemble into corresponding supramolecular polymeric gels, driven by intramolecular H-bond. As a consequence of the incorporation of diverse chromophores, the resulting gels G1, G2, and G3 displayed distinct fluorescence colors including yellow, blue, and green. Also, due to the existence of an H-bond on the surface, when these supramolecular polymeric gels were contacted together, thus interfacial adhesion occurred. Consequently, a composite gel, AFGs could be generated through the assembly of these three types of discrete fluorescent gels, which were considered as the building blocks.
Owing to the independence of three types of chromophores, AFGs could show multiple fluorescence colors with a single excitation wavelength. Through pushing these gels together, 1D AFGs were thus generated. Apart from producing 1D AFGs, these discrete fluorescent gels could also assemble into AFGs of more complicated structures. By using the same method, authors created more shapes for building AFGs, thus 2D, as well as 3D AFGs, were also obtained benefitting from the structural stability of polymeric backbones, as shown in Figure 1B. This work has realized a goal that AFGs displayed distinct fluorescent colors at the macroscopic level visually, which is considered as creating the fields of AFGs through a supramolecular approach, offering vast possibilities for developing further applications.
In the era of the information explosion, a great many media is emerging as new tools to realize information storage, however, these conventional approaches have the characteristics of storing merely a class of information. Thus, to enlarge the capacity of information media, the development of novel information code materials is in great demand. Ji and coworkers developed a supramolecular multicolor hydrogel, which was prepared by the aggregation of four different AIE poly(vinyl alcohol) (PVA) supramolecular hydrogels through the interfacial hydrogen-bonding adhesion. [75] The four types of gels were prepared by adding different AIE-gens including AIE-1, AIE-2, and AIE-3 into PVA, mixing in the water. Due to the incorporation of AIE-gens, these generated hydrogels exhibited different fluorescence colors under ultraviolet (UV) light, such as red (G4), yellow (G5), and blue (G6). Then G7 was obtained by covering a black tape on the surface of G6, which displayed no fluorescence, showing as black ( Figure 2A). Additionally, owing to the presence of hydroxyl groups on the gel surfaces, AFGs 2-1 showing as a 3D pattern were prepared via the adhesion of four single hydrogels together, driven by H-bond. AFGs 2-2 displayed diverse fluorescence colors under the irradiation of UV light,  [75] in which information was stored. However, in this pattern, only single 3D information could be stored, namely, the storage types of information were limited ( Figure 2B). For the sake of achieving multiple information storage, other types of information could be incorporated into AFGs 2-2, which were divided into a 1D barcode, 2D code as well as 3D code according to the pattern types. Through engraving the AFGs 2-2, thus 1D barcodes, as well as 2D codes, were embedded, displaying various patterns under UV light. First, some 1D barcodes were introduced into AFGs 2-2 to produce more new patterns including AFGs 2-3, AFGs 2-4, and AFGs 2-5, which could be scanned by mobile phone, to obtain different kinds of information. In this way, these patterns stored not only 3D information but also lots of 1D information. Apart from this, 2D codes could also be introduced into the AFGs 2-2, achieving information storage of 3D as well as 2D information. Then when both 1D barcodes and 2D codes were incorporated into patterns, causing the formation of AFGs 2-9 and AFGs 2-10. In these patterns, three kinds of information were acquired at the same time, which realized multiple information storage ( Figure 2C). As a novel type of code material, these AFGs initiate the boost of information storage development. The utilization of the "Codes in code" approach enlarges the information capacity of AFGs, which makes multiple information become accessible. This method provides more promising applications in the realms of industrial production, commerce, taxation, and so on.
To keep pace with the development of information technology, the materials applied in chemical-based encryption strategies for improved information security still have a broad developing space. Ji and co-workers fabricated an assembled stimuli-responsive gel which could be identified as AFGs 3-2 (QR code), achieving various encryption functions. [76] Three kinds of polymers P4, P5, and P6 were prepared, composed of the same main chains (PMMA), 2-ureido-4-pyrimidone (UPy) units as well as distinct chromogens. This work also employed UPy units as providing the hydrogen-bonding interactions. Due to the existence of the UPy unit, these polymers were able to self-assemble into gels G8, G9, and G10 correspondingly driven by quadruple H-bond. Therein, P4 could transform from colorless to colored, because P4 contained the rhodamine derivative which was non-fluorescent as well as colorless in its lactam form, while it became fluorescent at the corresponding open amide form under the exposure to acetic acid vapor. Thus, the resulting gel G8 would exhibit pink fluorescence via using this stimulus. Besides, a spirobenzopyran derivative was incorporated into P5, and UV light could induce the ring-opening process, that is, when P5 was photoirradiated by using UV light, it showed purple fluorescence (P2'). Similarly, the colorless G9 could be transformed into purple G9'. Then P6 contained a rhodamine derivative which would change colors as a consequence of the opening of this lactam ring in the presence of ions, rendering itself to change from colorless to yellow (P6) under exposure toward a Fe 3+ source. In this way, G10 displayed a yellow fluorescence when it was exposed to methanolic solutions of FeCl 3 ( Figure 3A). Driven by the H-bond provided by UPy groups on the gel surfaces, G8, G9, and G10 were allowed to be assembled together to produce AFGs 3-1. After being engraved, the new AFGs 3-2 were generated showing a QR code pattern ( Figure 3B). Then AFGs 3-2 were cut into pieces, which was divided into corresponding QR pattern version of G8-G10. These partial patterns laid bare towards to respective single stimulus, which only exhibited one-third of the original patterns. Later after the self-healing process, AFGs 3-3, AFGs 3-4, and AFGs 3-5 showing incomplete patterns were obtained, thus the stored information could not be acquired ( Figure 3C). Besides, AFGs 3-2 were sliced into three pieces, which were placed under two distinct stimuli. Consequently, these pieces are assembled together to generate AFGs 3-6, AFGs 3-7, and AFGs 3-8, merely displaying two-thirds of the patterns. Thus, the involved information was still inaccessible as well ( Figure 3D). However, after cutting AFGs 3-2 into three pieces, then three stimuli were utilized for these pieces. The assembled AFGs 3-9 with an intact pattern were generated, therefore, information A was decrypted by scanning by the mobile phone ( Figure 3E). Generally, the colors "turn-on" regulated by three stimuli "keys" approach enables the information being encrypted, providing a novel strategy for the development of chemical-based information encryption at a higher level of security.
Inspired by nature, biomimetic multi-responsive multicolor behaviors for display, camouflage, and sensing have been shown to be possible through artificial multicolor soft materials such as responsive multicolor polymeric hydrogels, among which fluorescent hydrogels remain of intense interest on account of their exploitable potential. Chen and co-workers designed a sort of dynamic polymer network assembled into plentiful AFGs via a dynamic H-bond. [77] Orthogonally Responsive red/green/blue luminogens within the hydrogel system were blue AIE-4: 4-phenoxy-N-allyl-1,8-naphthalimide (PhAN), red La-gens: Eu 3+ -potassium 6-acrylamidopicolinate (K6APA) and green La-gens: Tb 3+ -K6APA respectively. Following the blue Poly(PhAN-NIPAM-HEMA) (PPN) polymers integrated with blue AIE-4 mixing with poly(K6APA-NAGA) (P6N) polymer, Eu 3+ or Tb 3+ were coordinated with the PPN/P6N system leading to the resulting Eu/Tb-PPN/P6N network, a stable multicolor fluorescent hydrogel cross-linked by vast hydrogen bonds. The powerful multicolor fluorescence adjustability of scope covering almost the entire visible light and sensitive, due to the orthogonal responsiveness between blue AIE-4 and red/green La-gens, had been demonstrated by the color change of the hydrogel treated with multiple stimuli such as Tb 3+ /Eu 3+ ratios, temperature, pH, solvent and so on ( Figure 4A). The initial red emission color of Eu-PPN/P6N turned yellow with the addition of less amount of Tb 3+ ions, turned green with excessive Tb 3+ ions, turned blue in response to NaOH based on pH-controlled lanthanide coordination decomposition, and then turned purple upon increasing temperature. Combining it with satisfying selfhealing properties and remolding abilities, a series of AFGs showing as smart robots, artificial hydrogel, tree frogs and butterflies were adhered by the Eu/Pb-PPN/P6N hydrogel system through dynamic H-bond ( Figure 4B). A colorful hydrogel butterfly AFGs 4-2 was made up of the hydrogel Tb-PPN/P6N, PPN/PN, and Eu-PPN/P6N exhibiting colorchanging behaviors in response to multiple stimuli. Besides, the Eu-PPN/P6N hydrogel was tailored as the bionic treefrog main body while the PPN/PN hydrogel without the La-gens displayed blue emission color as its skin spots, adhered into AFGs 4-3 which turned from purple to green with prolonged immersion in 0.001 M Tb 3+ solution. Therefore, these prepared soft biomimetic color-changing skins designed by the AFGs are expected to be applied as fluorescent materials in the field of visual human-machine interactive technology, optical sensing, soft camouflaging robots, and so forth.
The high cost and complexity of experimental steps hinder the further development of multicolor fluorescent materials prepared by the introduction of multiple fluorophores. By contrast, taking advantage of a single AIRE (aggregationinduced ratiometric emission)-active fluorophore, the manufacture of multicolor fluorescent materials takes a step forward. For example, Ji and co-workers explored a kind of AFG system, exhibiting multiple colors with one single fluorophore via supramolecular adhesion driven by the interfacial hydrogen bonds. [78] The four polymers from P7 to P10 were all synthesized by PMMA chains, functionalized 2-ureido-4-pyrimidone (UPy) units, and the single fluorophore pyrene benzohydrazonate (PBHZ), whose fluorescence colors ranged from blue to bright green regulated by AIRE effect via increasing the concentration of PBHZ groups successively. Research on the AIRE effect showed that the intramolecular through-space conjugation affected the fluorescence colors through the change of the intermolecular aromatic stacking distances caused by the concentration of PBHZ. Then the four gel blocks G11, G12, G13, and G14 were prepared by the self-assembly of polymers P7, P8, P9, and P10 respectively. In the presence of UPy recognition groups, the quadruple H-bond played a key role in the formation of the multicolor AFGs from the gel blocks ( Figure 5A). Surprisingly, the AFGs system had been modified into diverse encrypted patterns, giving from AFGs 5-1 to AFGs 5-9. The design of AFGs 5-1, AFGs 5-2, and AFGs 5-3 with multi-dimension realized the recognition of the 1D code, 2D code, and 3D code under UV light for information encryption. Moreover, AFGs 5-4 and AFGs 5-6 appeared elegant biomimetic fish, flowers, and ladybirds respectively. With regard to the data pattern encryption, the letter "Z", the number "4" and the symbol "!" hidden in AFGs showed up when exposed to the UV light ( Figure 5B). More multicolor fluorescent materials prepared by this approach are proposed to be applied in the field of bionics and information storage.

AFGs BASED ON INTERFACIAL METAL COORDINATION INTERACTIONS
In nature, metal coordination interactions are an important type of noncovalent interactions provided by forming metal-ligand coordination bonds. Metal-ligand coordination bonds are extensively employed for offering structural support, which usually occur through a chelation process between metallic ions and multiple organic ligands. [79][80][81] [77] As such, a variety of metal-ligand coordination bonds are thus produced. With regard to its binding strength, it is basically up to the included ligand-ion pair, which can be enhanced via using multidentate ligands as well as multivalent metal ions. Generally, the corresponding k eq value is ranging from 106.5 M −1 (histidine−Zn 2+ ) to 1045 M −1 (catechol−Fe 3+ ). [82][83][84] Also, the type of ligand-ion pair has a great influence on the reversibility and solubility of the resulting materials incorporated in metal-coordination bonds. Benefitting from the intrinsic magnetic, electronic, optical, and catalytic potential of metals, metal-coordination bonds present an emerging capability in preparing AFGs. For constructing fluorescent gels, the employment of metal coordinations could certainly help to promote the interfacial adhesion of gels. For instance, lanthanide ions could interact with diarylethene photochromophores to form metalcoordination bonds, capable of being employed to generate gels, meanwhile, the mutual adhesion between gels could be better realized. Thereby, these obtained AFGs combine the traditional mechanical properties of macromolecules with the remarkable properties brought from the metal center: bioactivity, molecular magnetism, conductivity, nonlinear optical behavior, ferroelectricity, or sensitization. [85][86][87][88] Benefitting from the inherent advantages, stimulusresponsive luminescent materials are considered as possessing the great potential to be applied in many fields, especially confidential information protection. In 2019, Li and coworkers designed a series of photoresponsive luminescent polymeric hydrogels through in situ copolymerization of three building blocks: diarylethene photochromophore as the photoresponsive unit, lanthanide complex constructed by metal-coordination interactions as the luminescent center and acrylamide (AAm) monomers ( Figure 6A). [89] The integration of these units endowed the hydrogel with the behavior of photoresponsive ON-OFF switchable luminescence, that is, the switch was on under visible light (>450 nm) irradiation as a result of the absence of the spectral overlapping between the absorption of open form diarylethene and the emission of lanthanide ions with the inactivation of the fluorescence resonance energy transfer (FRET) process, as a contrast, the switch was off under 300 nm UV light due to the presence of the perfect spectral overlapping between the absorption of close form diarylethene and the emission of lanthanide ions with the activation of the FRET process. Except for the control over the reversible luminescence from the light irradiation, the emission colors of the hydrogel followed the change of the molar ratio of the lanthanide ions. Eu 3+containing red hydrogel, Tb 3+ -containing hydrogel, codoped yellow hydrogel, and hydrogel without luminescence (nonhydrogel) were arranged in order through interfacial metal coordination, leading to the formation of AFGs 6-1 as well as AFGs 6-2 showing code patterns where the hidden infor-  [78] mation A (Info A) could be picked up under 254 nm UV light. The Info A was erased under 300 nm UV light while >450 nm visible light was turned on the fluorescent switch to recover it ( Figure 6C). The reassembled AFGs 6-1 had good experimental reproducibility. Herein, this special kind of AFGs with photoresponsive ON-OFF switch luminescence, contain different coding information composed of gel blocks of diverse sizes, morphologies, and emission colors fulfilling the reversible confidential information encryption or decryption, which present a promising strategy for the smart anti-counterfeiting structures.

AFGs BASED ON INTERFACIAL HOST-GUEST INTERACTIONS
Among multitudinous noncovalent interactions, host-guest interactions act as an indispensable part of supramolecular chemistry that has been widely studied. [90,91] Therein, a host such as cyclodextrins (CDs), cucurbiturils (CBs), crown ethers, calixarenes, and pillararenes, usually means a molecule possessing a large cavity volume to encapsulate the guests, while a guest typically refers to general organic compounds owning a complementary shape which allows it to be incorporated into the cavity of host. [92] Besides, owing to the internal features of cavities, hosts usually interact with guests via hydrophobic interactions, H-bond, ionic bonds, electrostatic interactions, and so on. Through host-guest inclusion, various chemical moieties combine all together, thus the host-guest systems are endowed with selectivity and rich environmental responsiveness, which offer vast possibilities for building diverse novel supramolecular structures. On the whole, as a result of the dynamic, reversible, and adaptive nature, the introduction of host-guest interactions could be utilized to construct various AFGs, here, for example, the employment of classic host-guest interactions between calix [4]pyrroles (C4Ps) and pendant imidazolium-Xsubunits were highlighted. Meanwhile, macrocycle/anion recognition could also contribute to establishing AFGs, providing new insights into the development of sensing, gene, and drug delivery.
Compared with immutable storage methods, dynamic transformable recording media become more and more favored for abundant encoded information from flexible programming codes.
Ji and co-workers realized the diverse transformation of the encoded information using AFGs stabilized through interfacial host-gest interactions. [93] In the first step, four fluorescent hydrogel blocks were synthesized on the basis of the nonfluorescent hydrogel which was a poly(acrylamide) (pAAm) network combined with N,N′methylenebis(acrylamide) (MBAAm), pendant alkyl sulfonate subunit and tethered tetracationic macrocyclic anion receptors through the introduction of the different fluorescence centers into it. As shown in Figure 7A, blue-fluorescent gel G14, green-fluorescent gel G15, and red-fluorescent gel G16-1 were all grafted with single chromophores in contrast to another red-fluorescent gel G16-2 containing coumarin  [89] units as well as rhodamine B domains. Three types of AFGs showing 3D patterns came into being by the physical adhesion of the four gel blocks through macrocycle/anion interactions on a black nitrile substrate. Only structured AFGs 7-1 under UV illumination scanned by application software ("app") COLORCODE allowed the readout of Info A, but nothing else with the condition of the natural light. By removing random individual hydrogel blocks or treating with CuCl 2 powder, AFGs 7-1 exhibited incomplete patterns, which rendered the information inaccessible, leading to the function of the system for the protection, specificity, and eraser of information. Through the physical approach of cutting as well as reinsertion of hydrogel blocks, AFGs 7-2 could be changed to AFGs 7-1. Not only that, the transformation from AFGs 7-3 to AFGs 7-1 upon treating with ammonia vapor demonstrated the possibility of reconstituting the patterns via chemical means, which was actually illustrated by the quenched fluorescence of ring-opening rhodamine B spirolactam derivatives quenched by the base in the gel G16-2. The reversible interfacial macrocycle/anion interactions accounted for the changeable gel system applied in the masking, decrypting, and modifying information, and at the same time, the newly developed smart wearable materials may get inspired by it.
Even though diverse classical detection methods have been widely used as identification tools of anionic analytes, for example, nuclear magnetic resonance, the development in the availability of laboratory environment and operating portability are not proportional to the sensitivity. Ji and coworkers reported a double-layer AFGs based on host-guest interactions as a novel chloride anion sensor. [94] Both the bottom and top layers were polystyrene networks consisting of C4Ps as anion receptors, p-divinylbenzene, and pendant imidazolium-X -(X = F -, Cl -, Br -, and I -) subunits, but the differences between the two layers were the number of C4P and the X -. The lower layer integrated with imidazolium-Fhad approximately twice as many C4P receptors compared to the upper layer with excessive imidazolium-Br -. Correspondingly, the gel blocks G17-G20 exhibited blue, green, red, and black colors under UV irradiation due to the incorporation of fluorophores as coumarin, BODIPY, rhodamine B, and Cu(CH 3 COO) 2 powder were assembled into AFGs 8-1 via interfacial C4P/imidazolium-Finteractions ( Figure 8B), surely, in terms of the above specific structure settings, while the gel blocks G21-G24 contributed to the upper AFGs 8-2 via interfacial C4P/imidazolium-Brinteractions ( Figure 8C). The info A and info B were respectively hidden in AFGs 8-1 and AFGs 8-2. Then the resulting "sandwich structure" could be produced through the adhesion of the two layers based on the weak C4P-bromide anion interactions as a stimulus-responsive system ( Figure 8D). With the help of smart phone "app", chloride anion was like the key to unlocking the encoded information hidden in the bottom AFGs 8-2 rather than the info A. The micro-mechanism of the smart structure benefited from the exquisite design. The intermediate affinity of C4Ps towards the chloride anion decided the selective layer-by-layer delamination of the two layers: when exposed to the specific Clconcentration, competitive host-guest interactions between C4P and Clunmasked the top layer, and the more stable bottom layer appeared. In  [93] summary, it's an effective strategy for the recognition of the chloride anion using bar code-based polymeric gels avoiding the limitations as well as a new idea in the downstream information processing within the area of soft materials.

AFGs BASED ON INTERFACIAL HYDROPHOBIC INTERACTIONS
In addition to the noncovalent interactions mentioned above, hydrophobic interactions are also recognized as a key driving force to fabricate AFGs. The "hydrophobic interactions" are defined as the interaction of nonpolar molecules/groups with aqueous solutions, which plays a critical role in numerous phenomena in nature, such as cell membrane and vesicle formation, and protein folding. [95,96] The architecture and dynamics of diverse native or artificial biological systems are up to hydrophobic interactions. For example, in aqueous solutions, the three-dimensional stability of proteins is a consequence of hydrophobic interactions. [97][98][99][100] Aside from this, the active membranes of biology systems are produced via the self-connection of phospholipids and other lipids. Therefore, hydrophobic interactions can be utilized for constructing various complex formations. Here, a special kind of AFGs was discussed, which were prepared on the basis of engineered polypeptides PC 10 A and CdSe@ZnS QDs. Driven by the formation of hydrophobic coiled-coil  [94] aggregates of the terminal P and A domains, AFGs showing multiple fluorescences were produced.
The cooperation of semiconductor quantum dots (QDs) and polypeptides bring hope to the multifluorescent hydrogels with great properties owing to their own unique nature such as the biocompatibility and biodegradability of the polypeptides and the surprising spectral properties of the latter. Liu and co-workers developed injectable self-healing multifluorescent hydrogels, a kind of AFGs assembled through PC 10 A-QD hydrogels of three emission wavelengths through interfacial hydrophobic interactions. [101] Three PC 10 A-QD hydrogels blocks were prepared by PC 10 A polypeptide mixed with QD-PC 10 A hybrid nanogels originating from the phase transfer of hydrophobic CdSe@ZnS QDs into PC 10 A nanogels ( Figure 9A). On the surface of PC 10 A-QD hydrogels, PC 10 A polypeptides as cross-linkers facilitated the assembly of the three PC 10 A-QD hydrogels, leading to construct AFGs 9-1 through interfacial hydrophobic interactions ( Figure 9B). Both hydrophilic area and hydrophobic areas could be found in the PC 10 A nanogel based on the hydrophobicity of the terminal P and A domains and the hydrophilicity of the C 10 fragment. Therein, different from the usually weak fluorescent intensity of the watersoluble QDs, the hybrid nanogel integrated with oil-soluble CdSe@ZnS QDs presented strong fluorescence under UV light of 365 nm due to the shift of CdSe@ZnS QDs into the hydrophobic area. Self-repairing properties of PC 10 A-QD hydrogel could be demonstrated by its reversible gel-to-sol transition. Also, this hydrogel possessed injectability based on shear-thinning behaviors was confirmed by the reduced viscosity with increased shear rate. These two properties were both illustrated by the written word "HUST" (abbreviation of Huazhong University of Science and Technology) by employing the injected AFGs 9-1. Besides, the number of QDs had a positive effect on the strength and the elasticity of hydrogel, which enabled the control of hydrogel, and the participation of PC 10 A polypeptide also weakened the cytotoxicity obviously. The collaboration of the CdSe@ZnS QDs with PC 10 A polypeptide realizes the resulting multifluorescent hydrogel as expected instead of the simple combination of the organic dyes and self-healed polymers. The excellent capabilities render the hydrogels a bright prospect for bioimaging, in vivo research and tissue engineering so on.

AFGs BASED ON INTERFACIAL ELECTROSTATIC INTERACTIONS
Electrostatic interactions are a vital type of noncovalent interactions, which play an important role in managing a F I G U R E 9 Cartoon representation of (A) the preparation of an injectable self-assembling PC10A-QD hydrogel, (B) multifluorescent AFGs 9-1 through an interfacial assembly of PC10A-QD hydrogels driven by hydrophobic interaction. (A, B) Reproduced with permission. Copyright 2019, Wiley-VCH. [101] (C) Proposed structures, preparative route, as well as photoinduced sol-gel phase transition of photoresponsive hybrid luminescent supramolecular hydrogels driven by electrostatic interactions, (D) AFGs 9-2 assembled through hydrogel blocks. (C, D) Reproduced with permission. Copyright 2018, Wiley-VCH [107] diversity of colloidal and interfacial phenomena, such as protein folding, heterocoagulation, rheological behavior, and so forth. [102][103][104][105][106] Different organic monomers carrying opposite charges are able to generate complexes in water, recognized as a key feature that is distinguished from other noncovalent interactions. Electrostatic interactions have the ability to determine the structures as well as the flexibility of polymers, especially biopolymers. For instance, in protein, electrostatic interactions reside in the ionizable side chains including aspartate, glutamate, lysine, arginine, and histidine. Protein phosphorylation adjusts the charge state of side chains after undergoing the posttranslational modification, in this way, the structures of protein could be altered. [69] When opposite charges are incorporated into fluorophores as supramolecular interaction sites, a variety of AFGs can be constructed based on electrostatic interactions. Meanwhile, due to the sensitivity to pH, temperature, and competitive ions, the resulting AFGs allow themselves to be good candidates for applying to biosystems and medical fields. Next, a particular kind of AFGs showing different fluorescent colors was discussed, therein, the electrostatic interactions occurred between the guanidinium groups and the oxyanion groups, which enabled the hydrogels to stick together.
Among the photoresponse puzzles, remote control over the phase change of luminescent supramolecular hybrid hydrogels under light irradiation remains great interest on account of the contradiction between photochromic units and luminescent centers. Li and co-workers put forth a set of AFGs through electrostatic interactions between guanidinium functional groups and the negatively charged surface of the Laponite matrix to realize the remote control. [107] Compound 1 was prepared by α-cyclodextrins (α-CDs) functionalized with 2,6-pyridinedicarboxylic acid (PDA) groups and then coordinated with lanthanide metal ions (Tb 3+ , Eu 3+ ) into the Ln⋅1 3 complexes for luminescent emission. Meanwhile, guanidinium-azobenzene (Guazo) units utilized as the photoswitches were integrated with α-CD-Ln complexes through host-guest interactions. The subsequent addition of the finished Guazo⊂Ln⋅1 3 consisting of photoswitches and luminescent centers to sodium polyacrylate exfoliated Laponite nanosheets (SPLNs) produced threedimensional hybrid supramolecular networks, where the electrostatic interactions between oxyanion groups on the Laponite matrix surface and the guanidinium groups from Guazo units played the leading role ( Figure 9C). The fluorescent emission broad-spectrum colors of the transparent luminescent supramolecular gels could be precisely tuned by the changes in the molar ratio of Tb 3+ and Eu 3+ ions. As for the phase transition, the robust and stable hybrid hydrogels completed the gel-to-sol transformation in the presence of excessive α-CD or Guazo molecules. Besides that, the phase change from the gel to a sol state occurred upon UV irradiation, and the reversible light-induced phase change could be achieved via alternating UV and visible light without obvious mechanical strength loss, leading to the realization of the remote control in the entire gel monolith. Through constructing AFGs 9-2, self-healing behaviors of hydrogels were also demonstrated ( Figure 9D). The hidden micro-mechanisms behind the photoresponsive sol↔gel transition originated from the dissociation or association of the Guazo⊂Ln⋅1 3 complexes caused by the UV-induced isomerization of Guazo units. Through organic-inorganic hybrid self-assemblies, the approach affords a range of photoresponsive supramolecular hydrogels with outstanding mechanical properties, high water content and reversible phase-transition performance under remote control. The smart luminescent soft materials are expected to be further developed.  [114] 7

AFGs BASED ON INTERFACIAL DYNAMIC COVALENT BONDS
Since the great achievement made by the Lehn group, [108] dynamic covalent bonds have attracted extensive attention on account of their intriguing properties. The definition of 'dynamic covalent bond' can be interpreted as any chemical covalent bond owning the ability to be formed or broken under equilibrium control. [109][110][111][112][113] Under certain conditions, dynamic covalent bonds form at a static state, but cleave showing responses to distinct environment stimuli, including temperature, light, catalyst, and pH. Apart from that, another crucial characteristic of dynamic covalent bonds is the ability to undergo exchange processes by associative or dissociative pathways. As for the associative pathway, the breaking, as well as the reforming process of dynamic covalent bonds, occurs at the same time, however, in a dissociative way, linkage dissociates at the beginning with a new linkage produced after a while. There are lots of dynamic covalent bonds that have been commonly investigated, embodying imine bonds, disulfide bonds, boronate esters bonds, and so forth. These inherent appealing properties make dynamic covalent bonds an approach to developing a variety of AFGs, which can find vast application in contemporary fields, like information storage, diagnostics, and gene delivery. Herein, a series of AFGs based on dynamic covalent bonds were summarized. For instance, the acylhydrazone bonds and imine bonds were introduced to gels for providing strong interfacial adhesion.
Updatable and changeable patterns satisfied by array systems of the colored materials are helpful to the development of information storage and display capabilities. Ji and coworkers reported a "Rubik's Cube" (RC) analog adhered by fluorescent hydrogel blocks based on controllable dynamic covalent bonds. [114] The design of dynamic covalent bonds was crucial to the construction of the RC. If the dynamic covalent bonds were too strong that the layers of RC could not be rotated easily, while the dynamic bonds were too weak, then causing RC to become not stable. Six thin cuboids of AIE hydrogels (G25-G31) and one cube of hydrogel (G25) without AIE dots, employed respectively as the multicolor building support of a single block within the RC (AFGs 10-1), were synthesized by acylhydrazine-terminated PEO (AH-PEO-AH) and tetraaldehyde-terminated PEO (TAPEO) via the formation of acylhydrazone bonds ( Figure 10A). Following the integral cube AFGs 10-1 adhered through interfacial dynamic covalent interactions between the acylhydrazone bonds of the seven hydrogels, such 27 cube blocks of hydrogels built up in 3 × 3 × 3 arrangement constructed AFGs 10-2. The introduction of the six AIE dots with different emission colors endowed the AFGs 10-2 with a brightly colored look. The balance between hardness and softness through the time-dependent control was perfectly exhibited in an-all hydrogel-based RC aggregate, where the relatively weak dynamic covalent adhesion gifted the 1 × 3 × 3 layer free horizontal or vertical 90 • rotation for producing new patterns, at the same time, the strong interactions of G25 ensured the stability ( Figure 10D). Different from the ex situ change of conventional RC cube, incomplete AFGs 10-2 caused by taking a single building block would revert to a complete one after the block was inserted back ( Figure 10E). In the presence of the acid, AFGs 10-2 were renewed by the updated fluorescent color (from blue to orange) of AIE-9 under the chemical stimulus. The change of matrix sequence on account of horizontal and vertical rotation as well as the replacement of the colors with ex-situ and chemical stimulus makes the facile creation of multiple patterns possible. The easy-to-manipulate programming is anticipated to be applied in the preparation of soft materials.
Figuring out mechanisms of self-healing gels through the visible microscope processes plays a key role in the design of more complex self-healing gels utilized in different areas. Tang and co-workers proposed a fluorescence  [115] turn-on approach to the covalent bond-induced emission of AIEgens embedded in the self-healing gels. [115] AIEgencontaining polydimethylsiloxane (PDMS) gel for in situ monitoring of the self-healing process, as well as gelation, was prepared by the blend of AIE-active molecules like TPE−4CHO, bis(amino)-terminated poly(dimethylsiloxane) (NH 2 −PDMS−NH 2 ) and 1,3,5-triformylbenzene. Within the gel, AIEgen as a probe turned on fluorescence keeping pace with the dynamic imine bonds formed in the Schiff base reaction between the amine and aldehyde of TPE−4CHO and NH 2 −PDMS−NH 2 by the mechanism of restriction of intramolecular motions (RIM) ( Figure 11A). It is worth noting that the superior properties of low background and high contrast are shown in the real-time visualization through the self-healing PDMS gels. Small square pieces of PDMS gels linked with fluorophores with different emission colors (red, white, blue, green) were fabricated into AFGs 11-1 by Jigsaw-like coding driven by the dynamic imine bonds ( Figure 11B). The encrypted information stored in AFGs 11-1 was only invisible upon UV light irradiation with the help of the identification software. It was sure that a variety of realigned coding matrixes could be constructed by the different fluorescent self-healing PDMS gel blocks. The properties of self-healing, reusability as well as superiorities of stealth benefit the establishment of the big information repository. In summary, AFGs 11-1 adhered by PDMS gel blocks based on dynamic covalent bonds storing information can be recognized by the software of "COLORCODE" under UV light, leading to the reveal of the hidden information, which not only provides structural insights into the development of more new multifunctional self-healing gels but also broadens the scope of applications for anticounterfeiting.
Nowadays, the fabrication of soft fluorescent pattern systems gains tremendous attention, over which AIE-active fluorescent gels were a hit having the potential to realize the spatio-temporal control. Tang and co-workers developed a gel/metal-ion system loaded with AIEgens via forming hydrazone-based dynamic covalent bonds. [116] TPE-4SAH was prepared by 4-methoxybenzhydrazide (AH) and TPEcored-salicylaldehyde (TPE-4SA) functioned as an AIEgen, binding with the different metal ions based on coordination chemistry ( Figure 12A). The implementation of UV irradiation contributed to the formation of the aggregate, endowing the AIE-active TPE-4SAH/metal-ion system with the enhanced PL intensity due to the RIM effect of the AIEgen, which emerged as the phototriggered AIE (PTAIE) process ( Figure 12B). The UV-mediated fluorescence change (fluorescence emission colors and intensity) was not only regulated by the UV irradiation time but also the metal ions species. Then the AIE-active gel block PTPEG was prepared by the introduction of the PTAIE system into polyethylene glycol (PEG)-based polymeric networks covalently ( Figure 12C). The spatio-temporal control originated from the time-dependent PTAIE process as well as the self-healing property. Therefore, in the manipulation of luminescence behaviors, the mask-free programmed direct optical writing with a UV laser pen could be used to write characters "H" and "K" for mask-mediated information encoding. Besides, transformable time-gated AFGs could be supplied by repatterning the disassembled gels and healing in diverse arrays through dynamic covalent bonds. The multicolor encrypted AFGs 12-1 prepared by PTPEG/metal-ion systems containing Cd 2+ , Zn 2+ , and Al 3+ were constantly updated with the extension of irradiation time. The transition from AFGs 12-2 to AFGs 12-3 and then to AFGs 12-4, meant the timegated 4D patterning, giving the credit to the PTAIE process ( Figure 12D). The recombination via self-healing behaviors after the disassembly of the AFGs realized the multiple pattern changes. The applications of security display soft materials and other optoelectronic materials derived from the gel system will further inspire related progress in high-tech fields.  [116] 8

AFGs BASED ON INTERFACIAL MULTIPLE DRIVING FORCES
As mentioned above, a wide variety of dynamic bonds is employed to construct AFGs mainly due to their reversibility and sensitivity, realizing facile preparation, recycling, self-healing, and multifunctionality. Nevertheless, these systems usually embody a single kind of dynamic bond which is perceived as weak, and inadequate for being a feasible alternative to covalent systems. [117][118][119][120][121][122][123] Notably, there is undoubted that a single type of dynamic bond is inferior, but what if multiple dynamic bonds are combined together? For example, the folding polypeptide chain, the building block of proteins, is a result of joint working for H-bond, hydrophobic interactions, and π-π donor-acceptor effect. [124] Inspired by the natural systems, multiple noncovalent interactions are utilized to fabricate functional materials. Some polymeric materials possessing robust strength, such as Kevlar (poly(1,4-phenylene terephthalamide)), [125] can be ascribed to multiple noncovalent interactions including π-π donoracceptor effect and H-bond. Herein, rational designs of AFGs involving complex bonding modes are highlighted.
Selective self-healing property is intensively pursued to narrow the gap between artificial biomimetic materials and muscles in biological systems. Considering the limitation set by the uniform distribution of the interfacial dynamic interactions, Ji and co-workers present a multicolor AFGs 13-1, composed of three AIE-active gel blocks, driven by multiple interfacial interactions including dynamic covalent bonds, Hbond, and ionic interactions. [126] One of the AIE-active gel building blocks G32 modified with AIE-6 dot was generated by mixing AH-PEO-AH and TAPEO by forming a dynamic acylhydrazone bond ( Figure 13A). And another block G33 with an AIE-10 dot exhibiting green was constructed by the poly(N-isopropyl acrylamide) (PNIPA)/inorganic clay network cross-linked by ionic interactions ( Figure 13B). Then G34 prepared by PVA hydrogel via H-bond turned orange owing to the integration of AIE-11 ( Figure 13C). The three fluorescent gel blocks showing different self-healing mechanisms and colors were integrated with the covalently cross-linked hydrogel G35 to prepare the AFGs 13-1 through multiple interfacial forces using a bilayer hydrogel technique ( Figure 13E). It was worth mentioning that tensile tests demonstrated that the best healing efficiency occurred under Copyright 2020, The Author, published by Chinese Chemical Society [126] self-contact of the three fluorescent gels but decreased even disappeared efficiency under mutual contact with each other. AFGs 13-1 were divided into two parts: a flexible upper part for rotation and a fixed separated part. Seven broken cylindrical were compared and observed in terms of the different rotation angles of the upper part: 0 • , 60 • , 120 • , 180 • , 240 • , 300 • , and 360 • ( Figure 13F). With the help of the molecular visualization from AIEgens, AFGs 13-1 manipulated with rotation angles of 360 • , 300 • , 60 • , and 0 • appeared intact while the samples with other rotation angles still remained broken, coincided with subsequent quantitative experiments on the self-healing efficiency. The maxima of the breaking strain values belonged to the AFGs 13-1 with rotation angles of 360 • and 0 • , the medium values belonged to the rotation angles of 300 • and 60 • , while the minima belonged to the rotation angles of 120 • , 180 • , and 240 • . Both heterogeneous internal structure instead of the homogeneous structure and the recognition specificity of the three component gels pushed forward the interfacial dynamic interactions distribution on the rupturing surfaces, which was regarded as the mechanism of the selective self-healing property of the composite AFGs 13-1. The research opens the door to advanced materials with selective self-healing manners. Besides providing higher accuracy when healing misplaced wounds, the design has a bright prospect in the precise repair and reuse of materials.

CONCLUSION
In this review, we have classified AFGs on the basis of various interfacial dynamic bonds, including H-bond, metal coordination interactions, host-guest interactions, hydrophobic interactions, dynamic covalent bonds, and multiple driving forces. Owing to the inherent appealing properties, these dynamic bonds have been a powerful and versatile tool to promote interfacial adhesion and control the sizes and morphologies of AFGs. Benefitting from the multiple Hbond, the obtained AFGs own the properties of self-healing, remolding capacities, and pH-responsiveness. Moreover, taking advantage of metal coordination interactions, the fluorescent colors of resulting AFGs can be adjusted by using a distinct molar ratio of metal ions. The employment of host-guest interactions renders AFGs possessing reversible arrangement behaviors. Furthermore, hydrophobic interactions afford AFGs with shear-thinning characteristics. Except for these noncovalent interactions, a dynamic covalent bond is another effective approach to form AFGs as well as regulate the fluorescence patterns. When multiple dynamic bonds are combined together, this type of AFGs is endowed with higher adhesion strength. As such, with these great advances, the ongoing development of AFGs has been witnessed, which is of significance in vast applications, including information storage, molecular imaging, sensors, probes, and electronic devices.
Despite the great progress that has been made, there are still some challenges to be tackled in developing novel AFGs for practical applications. First, some important noncovalent interactions have not been reported for fabricating AFGs, including other classic macrocyclic-based host-guest interactions (like crown ethers, CDs, CBs, calixarenes, and pillararenes), ionic bonding, and π-π donor-acceptor effect. Beyond that, the employment of dynamic covalent bonds is confined to the Schiff-base bonds, so it is necessary to expand to the boronate esters bond, disulfide bond, and so forth. Secondly, the external stimuli controlled assembly/disassembly and assembly/reassembly of AFGs are needed to be developed. Then, currently, the single stimulus enables one building block of current AFGs showing responses with fluorescent color changing. Therefore, it's essential to introduce more stimuli for altering the fluorescent colors of all building blocks, offering new possibilities to achieve abundant fluorescent patterns. Besides, another single fluorescent molecule is also available to be incorporated into AFGs to realize multiple fluorescence colors, such as 2-(dimesitylboraneyl)-5-(dimethylamino)benzaldehyde and triphenylamine-modified bodily derivatives. What's more, aside from the fluorescent gels, other types of fluorescent materials possess great potential to be utilized for constructing AFGs as well, including carbon dots (CDs), semiconductor QDs, perovskites, and Ln 3+ -doped nanoparticles. Finally, diverse functional molecules serving distinct roles are available for being incorporated into AFGs to realize multiple functions like electroresponsiveness, thermoresponsiveness, and so on. Given the summary of AFGs assembled by diverse interfacial dynamic bonds, it will be able to speed up the development of vast fields, embodying assembly materials, soft materials, fluorescent materials, supramolecular chemistry, polymer science, information science, life sciences, aggregate science and so forth. Also, by highlighting the importance of AFGs, we anticipate that this review will present a set of rational guidelines for the development of aggregate science, especially the aggregates based on macroscopic building blocks, besides, offering valuable insights into more promising applications, such as drug delivery, tissue engineering, bioimaging, as well as biosensors.

C O N F L I C T O F I N T E R E S T
The authors declare that they have no conflict of interest.