Aromatic Gain in a Supramolecular Polymer

Modulating self-assembly and polymorph transitions in bisdendronized squaramides

Supramolecular self-assembly is an advanced approach for constructing ordered nanoscale architectures with broad applications. While the principles of supramolecular polymerization have been thoroughly explored in artificial small molecules, polymer transformations remain barely explored, likely due to the lack of suitable reference models presenting well-defined and reversible transitions between aggregates. In this study, we introduce a series of bisdendronized squaramides (SQs) 1-3, showcasing complex self-assembly behaviours involving four distinct aggregates, three different interaction patterns, and various thermodynamically controlled polymorph transformations. Notably, SQ 3, with ethyl spacers between the SQ cores and the dendrons, exhibits a concentration and temperature-dependent equilibrium among three polymorphs: the particle-like Agg-A and fibrillar Agg-C, formed by slipped hydrogen bonds, and the fibrillar Agg-B, formed by head-to-tail hydrogen bonds. Additional solid-state experiments revealed that these SQs also form columnar liquid crystals, assembled by π-π interactions in SQ 1 and hydrogen bonding in SQ 2 and SQ 3. This work positions SQ units as valuable models for understanding polymorph equilibrium in solution and solid-state, which is crucial for developing stimuli-responsive supramolecular polymers.

Cooperative intra- and intermolecular hydrogen bonding in scaffolded squaramide arrays

The structural, self-assembly and binding properties of oligo-(phenylene-ethynylene) (OPE) rigid rods linked to squaramides (SQs) have been studied and correlated with rod length. In the solid-state, OPE-SQ conjugates form indefinite arrays of head-to-tail hydrogen bonded SQs, arrays that include both intra- and intermolecular hydrogen bonds. In dichloromethane solution, intramolecularly hydrogen bonded SQ chains show cooperative polarisation, an effect that increases with OPE-SQ length. Appending powerful hydrogen bonding groups to the OPE-SQ termini further increases this intramolecular polarisation. Greater end-to-end polarisation leads to stronger intermolecular interactions, with longer OPE-SQs showing stronger hydrogen bonding to DMSO as well as stronger self-association. These studies show how cooperative hydrogen bond polarisation in a hydrogen bonded array can be strengthened and how this polarisation can continue intermolecularly.

Influence of Polymer Side Chain Size and Backbone Length on the Self-Assembly of Supramolecular Polymer Bottlebrushes

Hydrogen bonds are a versatile tool for creating fibrous, bottlebrush-like assemblies of polymeric building blocks. However, a delicate balance of forces exists between the steric repulsion of the polymer chains and these directed supramolecular forces. In this work we have systematically investigated the influence of structural parameters of the attached polymers on the assembly behaviour of benzene trisurea (BTU) and benzene tris(phenylalanine) (BTP) conjugates in water. Polymers with increasing main chain lengths and different side chain sizes were prepared by reversible addition-fragmentation chain-transfer (RAFT) polymerization of hydroxyethyl acrylate (HEA), tri(ethylene glycol) methyl ether acrylate (TEGA) and oligo(ethylene glycol) methyl ether acrylate (OEGA). The resulting structures were analyzed using small angle X-ray scattering (SAXS) and transmission electron microscopy (TEM). Both BTU and BTP formed fibres with PHEA attached, but a transition to spherical morphologies was observed at degrees of polymerisation (DP) of 70 and above. Overall, the main chain length appeared to be a dominating factor in inducing morphology transitions. Increasing the side chain size generally had a similar effect but mainly impeded any aggregation as is the case of POEGA. Interestingly, BTP conjugates still formed fibres, suggesting that the stronger intermolecular interactions can compensate partially for the steric repulsion.

Thermoreversible Polymorph Transitions in Supramolecular Polymers of Hydrogen-Bonded Squaramides

Hydrogen-bonded squaramide (SQ) supramolecular polymers exhibit uncommon thermoreversible polymorph transitions between particle- and fiber-like nanostructures. SQs 1-3, with different steric bulk, self-assemble in solution into particles (AggI) upon cooling to 298 K, and SQs 1 and 2, with only one dendronic group, show a reversible transformation into fibers (AggII) by further decreasing the temperature to 288 K. Nano-DSC and UV/Vis studies on SQ 1 reveal a concentration-dependent transition temperature and ΔH for the AggI-to-AggII conversion, while the kinetic studies on SQ 2 indicate the on-pathway nature of the polymorph transition. Spectroscopic and theoretical studies reveal that these transitions are triggered by the molecular reorganization of the SQ units changing from slipped to head-to-tail hydrogen bonding patterns. This work unveils the thermodynamic and kinetic aspects of reversible polymorph transitions that are of interest to develop stimuli-responsive systems.

Head vs. Tail Squaramide–Naphthalimide Conjugates: Self-Assembly and Anion Binding Behaviour

The syntheses of two squaramide–naphthalimide conjugates (SN1 and SN2) are reported; the structures of SN1 and SN2 differ by the attachment of a squaramide—either at the ‘head’ or the ‘tail’ of the naphthalimide fluorophore. Both compounds displayed weak fluorescence due to the inclusion of a nitro-aromatic squaramide which efficiently quenches the emission of the naphthalimide. Both compounds were also shown to undergo self-aggregation as studied by 1H NMR and scanning electron microscopy (SEM). Furthermore, SN1 and SN2 gave rise to stark colourimetric changes in response to basic anions such as AcO−, SO42− HPO42−, and F−. The observed colour changes are thought to be due to deprotonation of a squaramide NH. The same basic anions also result in a further quenching of the naphthalimide emission. No colour change or emission modulations were observed in the presence of Cl−; however, 1H NMR studies suggest that moderate H-bonding occurs between this anion and both SN1 and SN2.

Spatial and Temporal Modulation of Cell Instructive Cues in a Filamentous Supramolecular Biomaterial

Supramolecular materials provide unique opportunities to mimic both the structure and mechanics of the biopolymer networks that compose the extracellular matrix. However, strategies to modify their filamentous structures in space and time in 3D cell culture to study cell behavior as encountered in development and disease are lacking. We herein disclose a multicomponent squaramide-based supramolecular material whose mechanics and bioactivity can be controlled by light through co-assembly of a 1,2-dithiolane (DT) monomer that forms disulfide cross-links. Remarkably, increases in storage modulus from ∼200 Pa to >10 kPa after stepwise photo-cross-linking can be realized without an initiator while retaining colorlessness and clarity. Moreover, viscoelasticity and plasticity of the supramolecular networks decrease upon photo-irradiation, reducing cellular protrusion formation and motility when performed at the onset of cell culture. When applied during 3D cell culture, force-mediated manipulation is impeded and cells move primarily along earlier formed channels in the materials. Additionally, we show photopatterning of peptide cues in 3D using either a photomask or direct laser writing. We demonstrate that these squaramide-based filamentous materials can be applied to the development of synthetic and biomimetic 3D in vitro cell and disease models, where their secondary cross-linking enables mechanical heterogeneity and shaping at multiple length scales.

Supramolecular Biomaterials in the Netherlands

Synthetically designed biomaterials strive to recapitulate and mimic the complex environment of natural systems. Using natural materials as a guide, the ability to create high performance biomaterials that control cell fate, and support the next generation of cell and tissue-based therapeutics, is starting to emerge. Supramolecular chemistry takes inspiration from the wealth of non-covalent interactions found in natural materials that are inherently complex, and using the skills of synthetic and polymer chemistry, recreates simple systems to imitate their features. Within the past decade, supramolecular biomaterials have shown utility in tissue engineering and the progress predicts a bright future. On this 30th anniversary of the Netherlands Biomaterials and Tissue Engineering society, we will briefly recount the state of supramolecular biomaterials in the Dutch academic and industrial research and development context. This review will provide the background, recent advances, industrial successes and challenges, as well as future directions of the field, as we see it. Throughout this work, we notice the intricate interplay between simplicity and complexity in creating more advanced solutions. We hope that the interplay and juxtaposition between these two forces can propel the field forward.

Supramolecular grafting of stimuli-responsive, carrier-free, self-deliverable nanoparticles of camptothecin and antisense DNA for combination cancer therapy

A supramolecular approach for the crafting of self-deliverable nanoparticles of antisense DNA and camptothecin for combination cancer therapy is reported.

AIE Featured Supramolecular Tubisomes for Imaging-Guided Drug Delivery

Polymeric cylinders, a fascinating type of nanostructures with high surface area, internal volume and rigidity, have been exploited as novel drug delivery vehicles over the past decade. However, it's still an open challenge to afford cylindrical nanostructures using polymeric building blocks via traditional self-assembly processes. Herein, we report a hierarchical self-assembly strategy of preparing cylindrical aggregates (tubisomes) from an amphiphilic supramolecular bottlebrush polymer in which cyclic peptide nanotube is employed as the noncovalent backbone. Additionally, aggregation induced emission effect was introduced into the tubisomes to endow them with excellent fluorescent property. Intriguingly, encapsulation of anticancer drug doxorubicin (DOX) can inactivate the fluorescence of both tubisome and DOX due to the energy transfer relay (ETR). The release of DOX can interrupt the ETR effect and light up the silenced fluorescence, thereby permitting the in-situ visualization of drug release. The supramolecular tubisomes described here paves an alternative way for fabricating polymeric cylindrical nanostructures, and holds great potential in imaging-guided drug delivery.

σ-Electrons Responsible for Cooperativity and Ring Equalization in Hydrogen-Bonded Supramolecular Polymers

We have quantum chemically analyzed the cooperative effects and structural deformations of hydrogen-bonded urea, deltamide, and squaramide linear chains using dispersion-corrected density functional theory at BLYP-D3(BJ)/TZ2P level of theory. Our purpose is twofold: (i) reveal the bonding mechanism of the studied systems that lead to their self-assembly in linear chains; and (ii) rationalize the C-C bond equalization in the ring moieties of deltamide and squaramide upon polymerization. Our energy decomposition and Kohn-Sham molecular orbital analyses reveal cooperativity in all studied systems, stemming from the charge separation within the σ-electronic system by charge transfer from the carbonyl oxygen lone pair donor orbital of one monomer towards the σ* N-H antibonding acceptor orbital of the neighboring monomer. This key orbital interaction causes the C=O bonds to elongate, which, in turn, results in the contraction of the adjacent C-C single bonds that, ultimately, makes the ring moieties of deltamide and squaramide to become more regular. Notably, the π-electron delocalization plays a much smaller role in the total interaction between the monomers in the chain.

Galactose Grafted Two-Dimensional Nanosheets as a Scaffold for the In Situ Synthesis of Silver Nanoparticles: A Potential Catalyst for the Reduction of Nitroaromatics

Two major hurdles in NP-based catalysis are the aggregation of the NPs and their recycling. Immobilization of NPs onto a 2D support is the most promising strategy to overcome these difficulties. Herein, amphiphilicity-driven self-assembly of galactose-hexaphenylbenzene-based amphiphiles into galactose-decorated 2D nanosheet is reported. The extremely dense decoration of reducing sugar on the surface of the sheets is used for the in situ synthesis and immobilization of ultrafine catalytically active AgNPs by using Tollens' reaction. The potential of the system as a catalyst for the reduction of various nitroaromatics is demonstrated. Enhanced catalytic activity is observed for the immobilized AgNPs when compared to the corresponding discrete AgNPs. Recovery of the catalytic system from the reaction mixture by ultrafiltration and its subsequent recycling for several cycles without dropping its activity is shown. This is the first report demonstrating the in situ synthesis and immobilization of ultrafine AgNPs onto a 2D nanosheet that exhibits excellent catalytic performance for the reduction of nitroaromatics.

Squaramide-Based Self-Associating Amphiphiles for Anion Recognition

The synthesis and characterisation of two novel self-assembled amphiphiles (SSAs) SQS-1 and SQS-2 are reported. Both compounds, based on the squaramide motif, were fully soluble in a range of solvents and were shown to undergo self-assembly through a range of physical techniques. Self-assembly was shown to favour the formation of crystalline domains on the nanoscale but also fibrillar film formation, as suggested by SEM analysis. Moreover, both SQS-1 and SQS-2 were capable of anion recognition in DMSO solution as demonstrated using 1 H NMR and UV/Vis absorption spectroscopy, but displayed lower binding affinities for various anions when compared against other squaramide based receptors. In more competitive solvent mixtures SQS-1 gave rise to a colourimetric response in the presence of HPO42- that was clearly visible to the naked eye. We anticipate that the observed response is due to the basic nature of the HPO42- anion when compared against other biologically relevant anions.

Enhanced visible-light photocatalytic activity of perylene diimide (PDI) supramolecular nanorods with Pt QDs deposited in situ

Well-dispersed Pt quantum dots (QDs) were the first to be successfully deposited onto a PDI supramolecular nanorods surface via a simple in situ chemical reduction. Under visible light irradiation, Pt QDs/PDI composites displayed excellent photocatalytic property in the degradation of phenol. The optimum 1 wt% Pt QDs/PDI composite was found to be 6.2 times greater than pure PDI supramolecular nanorods for the degradation rate constant (k). The enhanced photocatalytic performance can be attributed to the rapid transfer and efficient separation of photogenerated carriers, originating from the effective trapping and transporting of electrons by Pt QDs. At the same time, Pt QDs were also loaded as active sites during the photocatalytic reaction. Moreover, the 1 wt% Pt QDs/PDI composite was found to have high photocatalytic stability and cycle utilization, suggesting its great potential in the area of water environmental purification.

Influence of the Z/E Isomerism on the Pathway Complexity of a Squaramide-Based Macrocycle

The rising interest on pathway complexity in supramolecular polymerization has prompted the finding of novel monomer designs able to stabilize kinetically trapped species and generate supramolecular polymorphs. In the present work, the exploitation of the Z/E (geometrical) isomerism of squaramide (SQ) units to produce various self-assembled isoforms and complex supramolecular polymerization pathways in methylcyclohexane/CHCl₃ mixtures is reported for the first time. This is achieved by using a new bissquaramidic macrocycle (MSq) that self-assembles into two markedly different thermodynamic aggregates, AggA (discrete cyclic structures) and AggB (fibrillar structures), depending on the solvent composition and concentration. Remarkably, UV-vis, ¹ H NMR, and FT-IR experiments together with quantum-chemical calculations indicate that these two distinct aggregates are formed via two different hydrogen bonding patterns (side-to-side in AggA and head-to-tail in AggB) due to different conformations in the SQ units (Z,E in AggA and Z,Z in AggB). The ability of MSq to supramolecularly polymerize into two distinct aggregates is utilized to induce the kinetic-to-thermodynamic transformation from AggA to AggB, which occurs via an on-pathway mechanism. It is believed that this system provides new insights for the design of potential supramolecular polymorphic materials by using squaramide units.

Overcoming the Necessity of a Lateral Aggregation in the Formation of Supramolecular Polymer Bottlebrushes in Water

The assembly of supramolecular polymer bottlebrushes in aqueous systems is, in most cases, associated with a lateral aggregation of the supramolecular building blocks in addition to their axial stacking. Here, it is demonstrated that this limitation can be overcome by attaching three polymer chains to a central supramolecular unit that possesses a sufficiently high number of hydrogen bonding units to compensate for the increased steric strain. Therefore, a 1,3,5-benzenetrisurea-polyethylene oxide conjugate is modified with different peptide units located next to the urea groups which should facilitate self-assembly in water. For a single amino acid per arm, spherical micelles are obtained for all three tested amino acids (alanine, leucine, and phenylalanine) featuring different hydrophobicities. Only a slight increase in size and solution stability of spherical micelles is observed with increasing hydrophobicity of amino acid unit. In contrast, introducing two amino acid units per arm and thus increasing the number of hydrogen bonds per unimer molecule results in the formation of cylindrical structures, that is, supramolecular polymer bottlebrushes, despite a suppressed lateral aggregation. Consequently, it can be concluded that the number of hydrogen bonds has a more profound impact on the resulting solution morphology than the hydrophobicity of the amino acid unit.

Thiosquaramide-Based Supramolecular Polymers: Aromaticity Gain in a Switched Mode of Self-Assembly

Despite a growing understanding of factors that drive monomer self-assembly to form supramolecular polymers, the effects of aromaticity gain have been largely ignored. Herein, we document the aromaticity gain in two different self-assembly modes of squaramide-based bolaamphiphiles. Importantly, O → S substitution in squaramide synthons resulted in supramolecular polymers with increased fiber flexibility and lower degrees of polymerization. Computations and spectroscopic experiments suggest that the oxo- and thiosquaramide bolaamphiphiles self-assemble into "head-to-tail" versus "stacked" arrangements, respectively. Computed energetic and magnetic criteria of aromaticity reveal that both modes of self-assembly increase the aromatic character of the squaramide synthons, giving rise to stronger intermolecular interactions in the resultant supramolecular polymer structures. These examples suggest that both hydrogen-bonding and stacking interactions can result in increased aromaticity upon self-assembly, highlighting its relevance in monomer design.

Glycosyl squaramides, a new class of supramolecular gelators

Glycosyl squaramides were synthesised and evaluated as low molecular weight gelators. Amphiphilic glycosyl squaramides 5 and 6, with a C-16 aliphatic chain, formed thermoreversible gels in polar organic solvents and 1 : 1 ethanol/water mixtures with high efficiency. Rheological analysis showed these gels achieve their structural stability 120 h after gelation and were robust, making them particularly suitable for biomedical applications. The interactions between solvent and gelator strongly influence SAFiN (Self-Assembled Fibrillar Network) formation, critical gelation concentration (CGC) and subsequent gel structure, as evidenced by SEM imaging of xerogels. Spectroscopic studies indicate that H-bonding is involved in the self-assembly of the glycosyl squaramides in organic solvents, while hydrophobic interactions are the major driving force for gel formation in the presence of water. The compounds described herein are the first reported examples of carbohydrate-squaramide conjugates capable of forming supramolecular gels.

Non-conventional Hydrogen Bonding and Aromaticity: A Systematic Study on Model Nucleobases and Their Solvated Clusters

The conceptual development of aromaticity is essential to rationalize and understand the structure and behavior of aromatic heterocycles. This work addresses for the first time, the interconnection between aromaticity and sulfur/selenium centered hydrogen bonds (S/SeCHBs) involved in representative heterocycle models of canonical nucleobases (2-Pyridone; 2PY) and its sulfur (2-Thiopyridone; 2TPY) and selenium (2-Selenopyridone; 2SePY) analogs. The nucleus-independent chemical shift (NICS) and gauge induced magnetic current density (GIMIC) values suggested significant reduction of aromaticity upon replacement of exocyclic carbonyl oxygen with sulfur and selenium. However, we observed two-fold (57 %) and three-fold (80 %) enhancement in the aromaticity for 2TPY dimer, and 2SePY dimer, respectively which are connected through S/SeCHBs. Aromaticity enhancement was also noticed in 1 : 1 H-bonded complexes (heterodimers), micro hydrated clusters and for bulk hydration. It is expected that exocyclic S and Se incorporation into heterocycles without compromising aromatic loss would definitely reinforce to design new supramolecular building blocks via S/SeCH-bonded complexes.

Hydrogen bond design principles

Hydrogen bonding principles are at the core of supramolecular design. This overview features a discussion relating molecular structure to hydrogen bond strengths, highlighting the following electronic effects on hydrogen bonding: electronegativity, steric effects, electrostatic effects, π-conjugation, and network cooperativity. Historical developments, along with experimental and computational efforts, leading up to the birth of the hydrogen bond concept, the discovery of nonclassical hydrogen bonds (C-H…O, O-H…π, dihydrogen bonding), and the proposal of hydrogen bond design principles (e.g., secondary electrostatic interactions, resonance-assisted hydrogen bonding, and aromaticity effects) are outlined. Applications of hydrogen bond design principles are presented.

Supramolecular polymer bottlebrushes

The field of supramolecular chemistry has long been known to generate complex materials of different sizes and shapes via the self-assembly of single or multiple low molar mass building blocks. Matching the complexity found in natural assemblies, however, remains a long-term challenge considering its precision in organizing large macromolecules into well-defined nanostructures. Nevertheless, the increasing understanding of supramolecular chemistry has paved the way to several attempts in arranging synthetic macromolecules into larger ordered structures based on non-covalent forces. This review is a first attempt to summarize the developments in this field, which focus mainly on the formation of one-dimensional, linear, cylindrical aggregates in solution with pendant polymer chains - therefore coined supramolecular polymer bottlebrushes in accordance with their covalent equivalents. Distinguishing by the different supramolecular driving forces, we first describe systems based on π-π interactions, which comprise, among others, the well-known perylene motif, but also the early attempts using cyclophanes. However, the majority of reported supramolecular polymer bottlebrushes are formed by hydrogen bonds as they can for example be found in linear and cyclic peptides, as well as so called sticker molecules containing multiple urea groups. Besides this overview on the reported motifs and their impact on the resulting morphology of the polymer nanostructures, we finally highlight the potential benefits of such non-covalent interactions and refer to promising future directions of this still mostly unrecognized field of supramolecular research.

Influences of different sugar ligands on targeted delivery of liposomes

Ligands are an important part of targeted drug delivery systems. Optimised lignads not only improve the target efficiency, but also enhance therapeutical effect of drugs. In our research, five sugar molecules (Mannose, Galactose, Glucose, Malt disaccharide, and Maltotriose) conjugated PEG₆₀₀-DSPE were synthesised, of which polysaccharides were first discovered by us as sugar ligands to modify liposomes, which interacts with over expressive GLUT on cancer cells. DiO was encapsulated as fluorescent probe to evaluate their cellular uptake abilities of targeting C6 glioma cells, and the distribution in different visceral organs of rats. The results demonstrated that Malt disaccharide and Glucose-PEG₆₀₀-DSPE had the strong efficiency of cellular uptake by C6 glioma cells. The distribution and accumulation of liposomes showed that different sugars modified liposomes could target different visceral organs in rats. It has provided a novel idea for ligand selectivity and optimisation of nanocarriers for tumour targeted therapy.

Impact of Positional Isomerism on Pathway Complexity in Aqueous Media

Pathway complexity has become an important topic in recent years due to its relevance in the optimization of molecular assembly processes, which typically require precise sample preparation protocols. Alternatively, competing aggregation pathways can be controlled by molecular design, which primarily rely on geometrical changes of the building blocks. However, understanding how to control pathway complexity by molecular design remains elusive and new approaches are needed. Herein, we exploit positional isomerism as a new molecular design strategy for pathway control in aqueous self-assembly. We compare the self-assembly of two carboxyl-functionalized amphiphilic BODIPY dyes that solely differ in the relative position of functional groups. Placement of the carboxyl group at the 2-position enables efficient pairwise H-bonding interactions into a single thermodynamic species, whereas meso-substitution induces pathway complexity due to competing hydrophobic and hydrogen bonding interactions. Our results show the importance of positional engineering for pathway control in aqueous self-assembly.

Galactose-Grafted 2D Nanosheets from the Self-Assembly of Amphiphilic Janus Dendrimers for the Capture and Agglutination of Escherichia coli

High aspect ratio, sugar-decorated 2D nanosheets are ideal candidates for the capture and agglutination of bacteria. Herein, the design and synthesis of two carbohydrate-based Janus amphiphiles that spontaneously self-assemble into high aspect ratio 2D sheets are reported. The unique structural features of the sheets include the extremely high aspect ratio and dense display of galactose on the surface. These structural characteristics allow the sheet to act as a supramolecular 2D platform for the capture and agglutination of E. coli through specific multivalent noncovalent interactions, which significantly reduces the mobility of the bacteria and leads to the inhibition of their proliferation. Our results suggest that the design strategy demonstrated here can be applied as a general approach for the crafting of biomolecule-decorated 2D nanosheets, which can perform as 2D platforms for their interaction with specific targets.

Impact of amino acids on the aqueous self-assembly of benzenetrispeptides into supramolecular polymer bottlebrushes

The choice of the amino acid unit in benzenetrispeptide-PEO conjugates allows to fine-tune the self-assembly strength and to control the resulting solution morphologies in water.

Hydrophobic domain flexibility enables morphology control of amphiphilic systems in aqueous media

In this work, we unravel the impact of hydrophobic domain flexibility on the self-assembly pathways and aggregate morphology of amphiphilic systems in aqueous media.

Self-assembly of amphiphilic aryl-squaramides in water driven by dipolar π–π interactions

Amphiphilic aryl-squaramides self-assemble via unprecedented dipolar π–π interactions forming well-defined supramolecular aggregates and self-consistent hydrogels in water

Water-Soluble Pillar[n]arene Mediated Supramolecular Self-Assembly: Multi-Dimensional Morphology Controlled by Host Size

We report tunable supramolecular self-assemblies formed by water-soluble pillar[n]arenes (WPns, n=5-7) and bipyridinium-azobenzene guests. Nanoscale or microscale morphology of self-assemblies in water was controlled by the host size of WPn. Supramolecular self-assemblies could undergo morphology conversion under irradiation with UV light.

Shape modulation of squaramide-based supramolecular polymer nanoparticles

We report the synthesis and self-assembly of a library of squaramide-based bolaamphiphiles with variable hydrophobic and hydrophilic domain sizes to understand their effect on the formation of supramolecular polymer nanoparticles.

Strain-Stiffening in Dynamic Supramolecular Fiber Networks

The cytoskeleton is a highly adaptive network of filamentous proteins capable of stiffening under stress even as it dynamically assembles and disassembles with time constants of minutes. Synthetic materials that combine reversibility and strain-stiffening properties remain elusive. Here, strain-stiffening hydrogels that have dynamic fibrous polymers as their main structural components are reported. The fibers form via self-assembly of bolaamphiphiles (BA) in water and have a well-defined cross-section of 9 to 10 molecules. Fiber length recovery after sonication, H/D exchange experiments, and rheology confirm the dynamic nature of the fibers. Cross-linking of the fibers yields strain-stiffening, self-healing hydrogels that closely mimic the mechanics of biological networks, with mechanical properties that can be modulated by chemical modification of the components. Comparison of the supramolecular networks with covalently fixated networks shows that the noncovalent nature of the fibers limits the maximum stress that fibers can bear and, hence, limits the range of stiffening.

Molecular bionics - engineering biomaterials at the molecular level using biological principles

Life and biological units are the result of the supramolecular arrangement of many different types of molecules, all of them combined with exquisite precision to achieve specific functions. Taking inspiration from the design principles of nature allows engineering more efficient and compatible biomaterials. Indeed, bionic (from bion-, unit of life and -ic, like) materials have gained increasing attention in the last decades due to their ability to mimic some of the characteristics of nature systems, such as dynamism, selectivity, or signalling. However, there are still many challenges when it comes to their interaction with the human body, which hinder their further clinical development. Here we review some of the recent progress in the field of molecular bionics with the final aim of providing with design rules to ensure their stability in biological media as well as to engineer novel functionalities which enable navigating the human body.

Reversible Loading of Nanoscale Elements on a Multicomponent Supramolecular Polymer System by Using DNA Strand Displacement

Nucleic acids are excellent building blocks to enable switchable character in supramolecular polymer materials because of their inherent dynamic character and potential for orthogonal self-assembly. Herein, DNA-grafted squaramide bola-amphiphiles are used in a multicomponent supramolecular polymer system and it is shown that they can be addressed by DNAlabeled gold nanoparticles (5 and 15 nm) through sequence complementarity. These nanoparticles can be selectively erased or rewritten on-demand by means of DNA-strand displacement.

Strain Stiffening Hydrogels through Self-Assembly and Covalent Fixation of Semi-Flexible Fibers

Biomimetic, strain-stiffening materials are reported, made through self-assembly and covalent fixation of small building blocks to form fibrous hydrogels that are able to stiffen by an order of magnitude in response to applied stress. The gels consist of semi-flexible rodlike micelles of bisurea bolaamphiphiles with oligo(ethylene oxide) (EO) outer blocks and a polydiacetylene (PDA) backbone. The micelles are fibers, composed of 9-10 ribbons. A gelation method based on Cu-catalyzed azide-alkyne cycloaddition (CuAAC), was developed and shown to lead to strain-stiffening hydrogels with unusual, yet universal, linear and nonlinear stress-strain response. Upon gelation, the X-ray scattering profile is unchanged, suggesting that crosslinks are formed at random positions along the fiber contour without fiber bundling. The work expands current knowledge about the design principles and chemistries needed to achieve fully synthetic, biomimetic soft matter with on-demand, targeted mechanical properties.