Programmable hydrogels by combining persistent and transient dynamic bonds

Open AccessCCS ChemistryMINI REVIEW1 Mar 2022Multidimensional Mass Spectrometry Assisted Metallo-Supramolecular Chemistry Heng Wang, Chenxing Guo and Xiaopeng Li Heng Wang College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen 518073 Shenzhen University General Hospital, Shenzhen University Clinical Medical Academy, Shenzhen 518055 , Chenxing Guo College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen 518073 and Xiaopeng Li *Corresponding author: E-mail Address: [email protected] College of Chemistry and Environmental Engineering, Shenzhen University, Shenzhen 518073 Shenzhen University General Hospital, Shenzhen University Clinical Medical Academy, Shenzhen 518055 https://doi.org/10.31635/ccschem.021.202101408 SectionsAboutAbstractPDF ToolsAdd to favoritesDownload CitationsTrack Citations ShareFacebookTwitterLinked InEmail Metallo-supramolecular chemistry based on the reversible, directional, and predictable noncovalent interaction of metal–ligand coordination has been widely applied in constructing a myriad of two- and three-dimensional supramolecules with sophisticated structures. The development of metallo-supramolecular chemistry highly relies on facile characterization methods. For advancement of structural complexity with desired functions, however, conventional crystallographic and spectroscopic characterizations are limited by crystal growth, sample purity, and severe signal overlaps. With high sensitivity and low requirement for purity, multidimensional mass spectrometry (MS), including electrospray ionization-MS (ESI-MS), tandem MS (MS2), and ion mobility-MS (IM-MS), is emerging as an essential analytical tool for characterization of the chemical compositions, connectivity, stability, shape, and size of metallo-supramolecules, as well as kinetic and thermodynamic features of self-assembly processes. Thus, this review mainly focuses on the recent development of multidimensional MS in metallo-supramolecular characterization and discusses how ESI-MS, MS2, and IM-MS assist the development of metallo-supramolecular chemistry. Download figure Download PowerPoint Introduction Supramolecular self-assembly, inspired by the precise organization of bio-macromolecules in living systems, allows for the spontaneous formation of ordered complexes from a chaos state.1 Among various noncovalent driving forces, metal–ligand coordination plays a vital role in the self-assembly of structurally and functionally sophisticated metallo-supramolecules, benefitting from the high directionality and predictable feature of coordination bonds.2–6 As a result, metallo-supramolecular chemistry has witnessed an explosion in constructing a myriad of artificial supramolecular architectures, including polygons,7–11 knots,12–15 links,16–19 fractals,20–22 as well as Platonic,23–27 Archimedean,28–34 Goldenberg polyhedrons,35,36 and so on. And these complexes show great potentials among different fields, including catalysis,37–46 nano-electronics,47,48 stabilization of air- or water-sensitive molecules,49–51 disease diagnosis and therapy,52–57 and so on. Furthermore, various delicate assembling approaches, inter alia, directional bonding,2 symmetry interactions,58 molecular paneling,59 and sub-components,60 have been developed to assist the exploration of metallo-supramolecular chemistry. Compared to the flourish of self-assembly strategies, however, the lack of efficient characterization methods always limits the investigation of assembled complexes at equilibrium and the kinetic details of self-assembly processes in solution because of the weak interaction and dynamic nature of metallo-supramolecules. Among the conventional characterizations of metallo-supramolecules, single-crystal X-ray diffraction (SC-XRD) is a powerful and direct method that allows for detailed structural information at the atomic level.61 However, it remains a formidable challenge to obtain high-quality single crystals, particularly for metallo-supramolecules with large voids filled by disordered solvent molecules or counterions. In addition, the refinement of the crystal structures for large metallo-supramolecules is difficult because the current crystal refinement approaches were mainly developed for small molecules. Apart from those issues the structural information obtained through SC-XRD in solid phase may not correlate with the study of metallo-supramolecular systems in solution or gas phase. Another well-established characterization method is NMR spectroscopy, whereby various nuclei in metallo-supramolecular systems, such as 1H, 13C, 19F, 31P, and so on can be detected, allowing the chemical environment of these nuclei in metallo-supramolecules to be elucidated.62,63 Through two-dimensional (2D) NMR spectroscopic techniques, for example, 1H–1H correlation spectroscopy (COSY), nuclear Overhauser effect spectroscopy (NOESY), and diffusion ordered spectroscopy (DOSY), diverse structural information about the metallo-supramolecules can be further obtained, including metal–ligand coordinations, geometric distributions of nuclei, and details of the size and shape of assembled architectures. However, the application of NMR is also limited in analyzing samples with low purity, severe overlapping signals, or those containing paramagnetic metal ions. As complementary characterization techniques, various microscopic methods, including atomic force microscopy (AFM), scanning tunneling microscopy (STM), and transmission electron microscopy (TEM), directly image the analytes. AFM and STM, in which tips probe the surface of the target molecules on substrates, reveal the size and shape of an individual molecule64 and distinguish the metal ions by using scanning tunneling spectroscopy (STS).65 However, high image resolution relies on sharp tips, as well as high-quality samples without impurities, severe aggregations, or dissociations of the target supramolecules. TEM uses electrons as the "probe" and can offer atomic resolution of the entire molecular structure assisted by computer-assisted three-dimensional (3D) reconstruction, selected area electron diffraction (SAED), and energy dispersive X-ray spectroscopy (EDX) techniques.66 Unfortunately, in addition to the challenges of preparing a high-quality sample, TEM also suffers from the intrinsic instability of metallo-supramolecules under electron beam or during sample preparation. The mass-to-charge ratio (m/z) information of the intact metallo-supramolecular ions obtained by mass spectrometry (MS) measurements in the gas phase enables the determination of the chemical composition of analytes. Furthermore, MS characterization provides higher sensitivity but a lower sample purity requirement than crystallographic analysis and NMR. Such features along with high analytical speed make it possible to monitor the dynamic process of self-assembly before the system in question reaches equilibrium. Thus, MS is emerging as an essential analytical tool for determining structures and monitoring reaction processes. However, traditional MS approaches also suffer from the fragmentation of metallo-supramolecules bound by weak coordination interaction even when a soft ionization method, that is, electrospray ionization (ESI), is applied. Cold-spray ionization (CSI),67 equipped with an ESI source cooled by liquid nitrogen, was used to overcome the problem by lowering the internal energy gained during the ionization. Due to the labile connectivity, matrix-assisted laser desorption/ionization (MALDI) could cause very severe fragmentation and thus is not preferred for the characterization of metallo-supramolecules. In addition to the m/z information obtained from MS, tandem MS (MS2) and ion mobility-MS (IM-MS) can elucidate more detailed information of the analytes. First, as a controllable fragmentation technique, MS2 offers deeper insight into the analytes, such as the information about the stability and connectivity of the structures. Furthermore, as an orthogonal in situ separation method, IM-MS can identify different species with the same m/z; more importantly, it provides structural information in terms of the shape and size of selected ions. During the past two decades, multidimensional MS with the combination of MS, IM-MS, and MS2 has played an important role in the development of metallo-supramolecular chemistry. Here, we discuss how ESI-MS, MS2, and IM-MS can be applied in (1) determining chemical compositions, connectivity, shape, and size of metallo-supramolecules, (2) distinguishing the isomers and by-products present in a dynamic combinatorial library, and (3) investigating kinetic and thermodynamic features of self-assembly processes. More importantly, we anticipate that multidimensional MS would not only act as an analytical method, but also serve as a powerful tool to assist the design of next-generation metallo-supramolecules, and even advance the development of new synthetic strategies. ESI-MS Studies Charged coordination complexes are ideal analytes for ESI-MS studies because of the easy-to-ionize feature of these supramolecules in which the counterions are readily stripped off, especially in polar and volatile solvents. Another advantage of ESI-MS is its intrinsic capability of reducing the signals corresponding to the multiply charged analytes with high molar mass (<104 Da) to the mass range of the conventional mass analyzers, since it only measures the m/z values of these ensembles. Consequently, a series of peaks is usually observed, representing different charge states of the same intact analyte through successive loss of counterions. To minimize the fragmentation during ionization, it is crucial to ionize the fragile analytes under mild conditions, including careful selection of the focusing and ionization voltages, low operation temperature, and fast ion extraction process through increasing source pressure (or a low vacuum).68 Additionally, highly concentrated metallo-supramolecular samples are often required to inhibit the potential entropy-driven dissociation in solution, although the ESI source needs more extensive cleaning after each measurement. Without annoying fragmentation, ESI-MS is suitable for detecting the chemical compositions of the ionic species in the metallo-supramolecular analytes, as well as providing information regarding the following aspects of the self-assembly: (1) fast screening the assembled structures; (2) monitoring the dynamic processes during the self-assembly; and (3) detecting the minor products to guide further design. Acting as a fast-screening characterization approach for the self-assembly Fast screening of the analytes' structural information by ESI-MS does not require extensive purification processes that are otherwise inevitable for NMR measurement and single-crystal growth. In most cases, after simply removing the inorganic salts to reduce ion suppression, the resulting sample solution is ready for direct measurement. With careful control of stoichiometry, the solution containing the self-assembled species can be directly subjected to ESI-MS without any further purification. Additionally, its capability of detecting m/z values enables the characterization of multiple assemblies with different numbers of building blocks, paramagnetic metallo-supramolecular analytes, and host–guest adducts. Therefore, ESI-MS could serve as a complementary or even superior method to perform structural characterization compared to NMR. The first example showcasing the fast screening by ESI-MS is the characterization of different generations of metallo-dendrimers S1/ S2 (Figure 1a) comprised of polyazaaromatic Ru(II) complexes reported by Kirsch-De Mesmaeker et al.69 ESI-MS directly generated clear and simple patterns that allowed the unambiguous determination of the chemical formula, whereas the other spectroscopic characterization just provided nonconclusive data (Figure 1b). Second, in the characterization of metallo-macrocycles reported by Newkome and co-workers,701H NMR gave very similar chemical shifts for the hexagon, octagon, and decagon ( S3- S5) assembled from the Ru(II)-connected bitopic 2,2′:6′,2″-terpyridyl (TPY) subunit L3 (Figure 1c) because of the similar symmetry and chemical environments of the detected protons. In contrast, these analogs with distinct molar mass were unambiguously identified by ESI-MS, where three distinct sets of peaks were detected (Figures 1d and 1e).70 Figure 1 | ESI-MS acting as a fast-screening approach for the preparation of (a and b) metallo-dendrimers comprised of polyazaaromatic Ru(II) complexes S2 and its corresponding ESI-MS spectrum; (c–e) TPY-Ru(II)/Fe(II)-based metallo-hexagon S3, -octagon S4, and -decagon S5 with corresponding 1H NMR spectra and ESI-MS spectra; (f–h) self-assembly of supramolecular fractals S6 based on the metal complexation between a TPY-based ligand and paramagnetic metal ions with corresponding 1H NMR and ESI-MS spectrum of the Co(II) complex (S6-Co). (a and b) Adapted with permission from ref 69. Copyright 1996 American Chemical Society. (c–e) Adapted with permission from ref 70. Copyright 2011 Wiley-VCH. (f–h) Adapted with permission from ref 71. Copyright 2020 American Chemical Society. Download figure Download PowerPoint Metallo-supramolecules containing paramagnetic transition metal ions with unpaired electrons remain a formidable challenge for NMR characterization. In contrast, ESI-MS has no problem characterizing these samples. For example, Shionoya et al.25 constructed a series of metallo-supramolecular octahedra by mixing a tritopic pyridinyl building block with different transition metal ions. ESI-MS spectra of the paramagnetic analytes containing Cu(II), Ni(II), Co(II), and Mn(II) ions clearly showed a dominant set of peaks ascribed to the designed structures; whereas, NMR spectra only displayed broad peaks with limited information.25 Similarly, with the help of ESI-MS characterization, coordination-driven self-assembly based on a TPY-paramagnetic metal ion complex was applied to construct a series of supramolecular fractals reported by Li et al.,71 which not only expanded the library of metal ions for assembling TPY-based architectures but also set up an ideal platform for studying the kinetic and thermodynamic features of these metallo-supramolecules (Figures 1f–1h). When characterizing the paramagnetic Co(II) and Ni(II) cages assembled using the sub-component self-assembly strategy, the reliable ESI-MS method also played a vital role in identifying the formation of target structures.72,73 Investigation of host–guest complexation is an important subfield of metallo-supramolecular chemistry, which is related to the fundamental study of molecular recognition and the potential applications of these metal–organic containers. NMR spectroscopy is extensively applied to provide evidence for the interaction with diagnostic chemical shifts of the protons on guest molecules. However, it cannot always accurately address how many guest molecules are encapsulated in the containers. ESI-MS can properly examine the exact change in mass upon the encapsulation of the guest molecules, thus, providing an accurate m/z value for deducing the number of guest molecules. In a self-assembled M8L6 (M = the metal ion, L = the ligand) cube with porphyrin faces, Nitschke et al.74 described that large guests such as coronene and fullerene can be selectively encapsulated (Figure 2a). Compared with the complicated NMR spectra that only provided a 2:1 ratio of guests in two different environments, ESI-MS spectrum of the coronene-encapsulated cube directly provided the number of the guest molecules via deconvolution of the dominant set of peaks (Figures 2b and 2c). The ESI-MS spectrum also clearly confirmed that the cube allowed for selective encapsulation of C70, C76, C82, and C84 over C60 from the fullerene soot, as evidenced by the absence of any discernible peaks assignable to the C60-encapsulated or guest-free cube. Figure 2 | ESI-MS-based fast screening for studying the guest encapsulation of metallo-supramolecular cages. (a–c) Self-assembly of porphyrin-faced cube S8 for encapsulation of three coronene with corresponding characterization by 1H NMR and ESI-MS; (d and e) a PBI-edged tetrahedron capable of encapsulating two molecules of C60S9 and the corresponding ESI-MS spectrum; (f–h) self-assembly of a metallo-supramolecular peanut S10 for simultaneous encapsulation of one diamantane and two phenanthrenes (S11) with corresponding characterization by 1H NMR and ESI-MS. (a–c) Adapted with permission from ref 74. Copyright 2011 Wiley-VCH. (d and e) Adapted with permission from ref 76. Copyright 2013 American Chemical Society. (f–h) Adapted with permission from ref 83. Copyright 2017 Springer Nature. Download figure Download PowerPoint Thus, ESI-MS is also a feasible and rapid method for qualitatively studying the encapsulation selectivity of a certain metallo-supramolecule with different guests. In another example of selective encapsulation, when mixing a metallo-supramolecular tetragonal prism with a mixture of C60 and its derivatives, the selectivity could be determined qualitatively by comparing the relative abundance of the peaks corresponding to the host–guest complexes containing different guests. Among them, C60 and PCBM-C60 have higher affinity with the host than N-methylpyrrolidine-C60.75 However, it is still challenging to quantitatively determine the selectivity by ESI-MS because of the different ionization efficiency of those complexes, although the host molecules are the same. Ideally, quantitative data could be obtained if the ionization efficiencies of different complexes are calibrated. In addition to encapsulating one fullerene molecule, Würthner et al.76 applied ESI-MS to unequivocally prove the encapsulation of one or two molecules of C60 inside the cavity of perylene-bisimide (PBI) edged tetrahedron S9 (Figures 2d and 2e). Note that the C60 guest does not bear any protons so that it can only be identified by 13C NMR. Although the 13C NMR could confirm the encapsulation of the C60, it was unable to determine the number of guest molecules because conventional 13C NMR does not allow for quantitative analysis as opposed to 1H NMR. Metallo-supramolecular trigonal prisms, wherein C3-symmetrical tritopic pyridyl ligands, linear ditopic pyridyl ligands, and an end-capped Pd(II) moiety served as the base faces, pillars, and vertices, respectively, were found to be an excellent molecular container capable of encapsulating a variety of guest molecules, including, coronene,77 triphenylene,78 pyrene-4,5-dione,79,80 and metal–organic complexes.81,82 More intriguingly, the number of the encapsulated guest molecules can be precisely manipulated via tuning the length of the organic pillars. The accurate number of encapsulated guests could be unambiguously determined by ESI-MS. A more complicated situation was reported by Yoshizawa et al.83 The self-assembled metallo-supramolecular peanut S10 heteroleptically encapsulated two different types of guests, that is, one diamantane and two phenanthrenes, inside its two cavities (Figure 2f). Without an SC-XRD result or clear NMR spectrum (Figure 2g), such an unusual encapsulation was identified by ESI-MS (Figure 2h), where peaks with m/z values of the expected host–guest complex were found. Monitoring the dynamic processes for unveiling self-assembly details Metallo-supramolecules bonded via labile coordination interactions exhibit dynamic features in solution due to the dissociation and reassociation of the weak bonds. Thus, it is hard to investigate the kinetic processes by traditional characterization techniques in solution. Nevertheless, in an ESI-MS measurement, each complex ion is a separated species in gas phase. Therefore, the dynamic process is "frozen" to display the kinetic instead of thermodynamic feature of the system. As one type of dynamic process, ligand exchange involves the disassociation of a ligand from the assembled supramolecule and subsequent replacement by another ligand. Such a process is related to the coordination bond strength and the activation energies during the exchange processes. If the time scales of such dynamic exchanges are comparable for MS measurement, one should be able to study the exchange process of two kinds of pre-assembled supramolecules with distinct molar masses of ligands by MS. Once the exchange occurs, the m/z values corresponding to each possible re-assembled species as well as the initial ensembles would be different, all of which can be specifically recorded by ESI-MS measurement. For example, Fujita et al.84 reported the studies of ligand exchange between two M12L24 spheres via mixing two topologically identical spheres bearing different length of alkyl chains (–OC3H7 vs –OC6H13) and then monitoring under time-dependent ESI-MS (Figures 3a and 3b). Due to the difference in mass, all the peaks corresponding to the initial two spheres and the exchanged ones were easily identified in the spectra. Time-dependent measurements revealed a quite remarkable kinetic stability of the spheres regarding ligand exchange. The half-time for the first ligand exchange was measured to be approximately 20 days. After evaluating the ionization efficiency of two spherical supramolecules, the estimated kinetic ligand exchange rate was found to be 2 × 10−2 M−1·s−1, which is far less than the exchange rate between a free ligand with a sphere (half-time of 23 min). Therefore, the ESI-MS studies semi-quantitatively revealed that the fully closed framework slowed down the dynamic exchange processes through a cooperative effect.84 A similar method was also utilized to carefully study the dynamic nature of metallo-polygons assembled from organoplatinum motifs and pyridyl ligands with 1H/2D isotope labeling.85 In addition to the quantitative determination of the ligand exchange rates, further understanding about the kinetic processes in coordination-driven self-assembly was provided by time-dependent ESI-MS, which showed the ligand exchange rate highly depended on the experimental conditions such as temperature, solvent, and counterions. Figure 3 | ESI-MS of ligand exchange. (a) Ligand exchange between two structurally identical spheres (Pd12L724 and Pd12L824) with different alkyl chain lengths (-n-C3H7 vs -n-C6H13), and (b) the time-dependent ESI-MS spectra; (c) self-assembly of four types of supramolecular snowflakes (S12–S15) with different lengths of the ligands, and time-dependent ESI-MS spectra of the mixture of and (a and b) Adapted with permission from ref Copyright American Chemical Society. and e) with permission from ref Copyright Wiley-VCH. with permission from ref Copyright 2017 American Chemical Society. Download figure Download PowerPoint In addition to the pyridyl ESI-MS also provided insight into the ligand exchange processes of metallo-supramolecular systems based on analyzing the peaks to the complexes in ESI-MS it was revealed that the exchange processes a certain of to the size and shape of the ligand via the of the structure when the Through a time-dependent ESI-MS measurement, such a was estimated in the metallo-supramolecular snowflakes with different length of ligand The exchange rate was found to directly correlate with that is, the the the ligand exchange. When = the exchange equilibrium = required for whereas = in no exchange (Figures Furthermore, the ligand exchange rate was also found to correlate with the coordination metal ion in TPY-based To quantitatively these exchange rates, different metal ions (M = were applied. of these metal ions was used to two structures with the same molecular size and shape but different molar After mixing the of pre-assembled supramolecules, time-dependent ESI-MS was utilized to the relative abundance of each After the ionization efficiency of these supramolecules, ESI-MS data provided exchange of these metallo-supramolecules at with a of Mn(II) Co(II), whereas the and fractals were found to be to the ligand exchange. The exchange rate of fractals was determined to be × M−1·s−1, which is higher than that of fractals × Monitoring the in the self-assembly processes is also a kinetic investigation that can be carefully by ESI-MS. When using NMR to monitor the complexation of ligands with or Ni(II) shifts of the peaks were observed, the of an before the formation of However, the limited information provided by NMR spectra not elucidate the ESI-MS was then used to the and the that is, linear by the m/z value of the intact Thus, ESI-MS can reveal the reaction via monitoring the Such capability was also used in monitoring the formation of a supramolecular wherein ligand and self-assembly were in a The time-dependent the of the coordination and processes (Figures and Figure | (a) The formation of supramolecular via a ligand and self-assembly (b) time-dependent ESI-MS monitoring the (b) Adapted with permission from ref Copyright American Chemical Society. Download figure Download PowerPoint With m/z values of the more details about the self-assembly processes could be allowing for deeper understanding of the assembling For in the of ligands with ions to and an with metal ion in the ligand with instead of was through ESI-MS by increasing ligand or metal Such an that the complexation to the A similar of complexation was also in other systems by ESI-MS, its ESI-MS of the species during the formation of prism with a ligand revealed the of the The ESI spectra the of self-assembly and showed the formation of the base with a Such a the design of a building block with two types of two distinct geometric that is, as base and respectively, allowing for the self-assembly of a structures to further design With the reliable m/z ESI-MS can identify the desired before other and characterization. For based on the directional an tritopic ligand was designed to metallo-supramolecules by coordination with or to the assembled could be or Unfortunately, NMR characterization because of the Nevertheless, ESI-MS provided evidence for the formation of a which the of the ligand to the of the complex for structural on

Multi-network systems in elastomers using dynamic bonds and static covalent bonds “Coexistence between thermoplasticity and heat resistance”

There is often a trade-off between mechanical properties (modulus and toughness) and dynamic self-healing. Here we report the design and synthesis of a polymer containing thermodynamically stable whilst kinetically labile coordination complex to address this conundrum. The Zn-Hbimcp (Hbimcp = 2,6-bis((imino)methyl)-4-chlorophenol) coordination bond used in this work has a relatively large association constant (2.2 × 1011) but also undergoes fast and reversible intra- and inter-molecular ligand exchange processes. The as-prepared Zn(Hbimcp)2-PDMS polymer is highly stretchable (up to 2400% strain) with a high toughness of 29.3 MJ m−3, and can autonomously self-heal at room temperature. Control experiments showed that the optimal combination of its bond strength and bond dynamics is responsible for the material’s mechanical toughness and self-healing property. This molecular design concept points out a promising direction for the preparation of self-healing polymers with excellent mechanical properties. We further show this type of polymer can be potentially used as energy absorbing material.

Metal coordination bonds are widely used as the dynamic cross-linkers to construct self-healing hydrogels. However, it remains challenging to independently improve the toughness of metal coordinated hydrogels without affecting the stretchability and self-healing properties, as all these features are directly correlated with the dynamic properties of the same metal coordination bonds. In this work, using histidine–Zn2+ binding as an example, we show that the coordination number (the number of binding sites in each cross-linking ligand) is an important parameter for the mechanical strength of the hydrogels. By increasing the coordination number of the binding site, the mechanical strength of the hydrogels can be greatly improved without sacrificing the stretchability and self-healing properties. By adjusting the peptide and Zn2+ concentrations, the hydrogels can achieve a set of demanding mechanical features, including the Young’s modulus of 7–123 kPa, fracture strain of 434–781%, toughness of 630–1350 kJ m−3, and self-healing time of ~1 h. We anticipate the engineered hydrogels can find broad applications in a variety of biomedical fields. Moreover, the concept of improving the mechanical strength of metal coordinated hydrogels by tuning the coordination number may inspire the design of other dynamically cross-linked hydrogels with further improved mechanical performance.

Unique Ligand Exchange Dynamics of Metal–Organic Polyhedra for Vitrimer-like Gas Separation Membranes

Exploring dynamic bonds and their applications in fabricating dynamic materials has received great attention. A photoinduced [2]rotaxane-based dynamic mechanical bond (DMB) features visible-light-triggered dynamic bonding behavior that is essentially distinguished from conventional dynamic chemical bonds. In this DMB, a photoisomerizable ortho-fluoroazobenzene unit is introduced as a steric-controllable stopper, the visible-light-induced dynamic wagging movement of which enables the photoregulated threading of the macrocycle. This allows reversible in situ de-/reforming of the mechanical bond without involving dynamic chemical linkage. The DMB-cross-linked polymeric gel shows interesting photoinduced degradation behavior upon visible light irradiation. Benefiting from the distinctive dual dynamic nature of reversible bonding behavior and mechanical interlocked structure, this DMB is expected to serve as a new type of dynamic bond that can be applied in designing dynamic soft materials.

Previous strategies for controlling the surface morphologies of polyvinyl alcohol (PVA)-based hydrogels, including freeze-drying and electrospinning, require a post-treatment process, which can affect the final textures and properties of the hydrogels. Of particular interest, it is almost impossible to control the surface morphology during the formation of PVA hydrogels using these approaches. The strategy reported in this study used the novel vortex fluidic device (VFD) technology, which for the first time provided an opportunity for one-step fabrication of PVA hydrogel films. PVA hydrogels with different surface morphologies could be readily fabricated using a VFD. By also reducing the crosslinking agent concentration, a self-healing gel with enhanced fracture stress (60% greater than that of traditionally made hydrogel) was achieved. Interestingly, the associated self-healing property remained unchanged during the 260-s mechanical testing performed with the strain rate of 5% s−1. The VFD can effectively tune the surface morphologies of the PVA-based hydrogels and their associated properties, particularly the self-healing property.

Rheology and texture analysis of gelatin/dialdehyde starch hydrogel carriers for curcumin controlled release

Hydrogels are a class of soft and wet materials that typically consist of a three‐dimensional polymer network that swells yet does not dissolve in water. There are extensive research and development of hydrogel materials for diverse applications over the past two decades. More recently, hydrogel films with controlled thickness have been fabricated based on crosslinked natural or synthetic polymer networks. These films not only show the chemical/physical properties of bulk gels, such as biocompatibility and stimuli responsiveness, but also exhibit faster response speed and high flexibility/adaptivity, allowing for new functions and applications. In this minireview, we summarize the recent advances in hydrogel films with focuses on two aspects: (i) methods for facile fabrication of hydrogel films, and (ii) versatile applications of hydrogel films in biomedical and emerging fields. Hydrogel films with controllable thickness, fast response, good compliance, and tunable mechanical properties are ideal materials as artificial muscles and wound dressing, and to construct soft actuators, flexible electronics, and so forth. Current challenges and future perspectives of hydrogel films are also given at the end of this minireview.

207 - Redox Activity Induced by Metal Coordination with Prion Peptide

Integrating the self-healing property with the shape-memory effect is a strategy that extends the service lifetime of shape-memory materials. However, this strategy is inadequate to reshape and recycle through the self-healing property or liquid-state remoldability. For more types of damage, solid-state plasticity is needed as a complementary mechanism to broaden the reprocessing channels of smart materials. In this study, multifunctional thermoplastic polyureas cross-linked by urea hydrogen bonds are prepared, which possess the multipathway remodeling property. The shape transition can be triggered after heating above 65 °C. The synergistic effect of dynamic disulfide bonds and hydrogen bonds causes the thermoplastic polyureas to possess characteristics similar to those of associative covalent adaptable networks. Thus, the polyureas can repair the damage or reconfigure the shape at 75 °C in 15 min by solid-state plasticity, instead of going into a viscous flow state. Soft grippers with various shapes are prepared by integration of solid-state plasticity, and the structure and function of the grippers can be repaired. The integration of solid-state plasticity and the self-healing property broadens the paths of shape-memory polymers in recyclability and reshapability.

In the past few years, combining supramolecular and macromolecular chemistries has become of great interest to yield dynamic and responsive assemblies with self-restructuring abilities. Among them, polymer networks, that are held together by one or a combination of supramolecular interactions, offer new possibilities to scientists for the creation of artificial materials with selfhealing properties. In particular, incorporating coordination complexes into polymeric architectures opens up the possibility of imparting the physicochemical properties of both partners to the resulting material. Here, recent achievements in the field of supramolecular gels that are formed via self-assembly of oligo- and polymeric units through reversible metal–ligand interactions are reviewed. The different strategies and routes for the elaboration of those materials are reported as well as the properties that the coordination centers confer to the supramolecular assemblies.

Both forward and backward electron transfer kinetics at the sensitizer/NiO interface is critical for p-type dye-sensitized photocathodic device. In this article, we report the photoinduced electron transfer kinetics of a Ru(II) chromophore–acceptor dyad sensitized NiO photocathode. The dyad (O26) is based on a cyclometalated Ru(N∧C∧N)(N∧N∧N) (Ru[II]) chromophore and a naphthalenediimide (NDI) acceptor, where N∧C∧N represents 2,2′-(4,6-dimethyl-phenylene)-bispyridine and N∧N∧N represents 2,2′,6′,6″-terpyridine ligand. When the dyad is dissolved in a CH3CN solution, electron transfer to form the Ru(III)–NDI– occurs with a rate constant kf = 1.1 × 1010 s–1 (τf = 91 ps), and electron–hole pair recombines to regenerate ground state with a rate constant kb = 4.1 × 109 s–1 (τb = 241 ps). When the dyad is adsorbed on a NiO film by covalent attachment through the carboxylic acid group, hole injection takes place first within our instrument response time (∼180 fs) followed by the subsequent electron shift onto the NDI to produce the interfacial charge-separated state [NiO(h+)–Ru(II)–NDI–] with a rate constant kf = 9.1 × 1011 s–1 (τf = 1.1 ps). The recovery of the ground state occurs with a multiexponential rate constant kb = 2.3 × 109 s–1 (τb = 426 ps). The charge recombination rate constant is slightly slower than a reference cyclometalated ruthenium compound (O25) with no NDI group (τb = 371 ps). The fast formation of interfacial charge separated state is a result of ultrafast hole injection resulting in the reduced form of sensitizer, which provides a larger driving force for NDI reduction. The kinetic study suggests that Ru(II) chromophore–acceptor dyads are promising sensitizers for the NiO photocathode devices.

Cellulose nanofiber-reinforced antimicrobial and antioxidant multifunctional hydrogel with self-healing, adhesion for enhanced wound healing.

Block copolymers containing both hydrogen bonding and metal coordination sites have been synthesized by ring-opening metathesis polymerization and subsequently functionalized using noncovalent interactions. The resulting block copolymers can be viewed as “universal polymer backbones”, as a wide variety of polymers with varying functionalities can be prepared by altering the noncovalent functionalization strategy of the same polymer backbone. The effect of degree of polymerization, block copolymerization, block copolymer composition, and metal coordination on the hydrogen bonding interaction has been investigated. In general, none of these variables have a profound effect on the strength of the hydrogen bonding interactions along the polymer backbones, suggesting that the metal coordination and hydrogen bonding are orthogonal to each other in block copolymers. Finally, the effect of the noncovalent functionalization on the thermal properties of the polymers was investigated. We found that the noncovalent functionalization of all copolymers via hydrogen bonding and/or metal coordination reduced the glass-transition temperature and the thermal stability of all copolymers.