Dissipative out-of-equilibrium assembly of man-made supramolecular materials

ATP-fuelled self-assembly to regulate chemical reactivity in the time domain

Here, we exploit a small biomolecule - ATP - to gain temporal control over chemical reactivity in a synthetic system composed of small self-assembling molecules and reactants. The approach relies on the capacity of ATP to template the formation of amphiphile-based assemblies. The presence of the enzyme alkaline phosphatase causes a gradual decrease in the ATP-concentration in time and, consequently, a spontaneous dissociation of the assemblies. The uptake of apolar reactants in the hydrophobic domain of the assemblies leads to an enhancement of the reaction rate. It is shown that ATP-triggered self-assembly causes the selective upregulation of one out of two hydrazone-bond formation reactions that take place concurrently in the system. This leads to an inversion in the product ratio, which, however, is transient in nature because the upregulated reaction spontaneously reverts to its basal low reaction rate once the ATP has been consumed by the enzyme. Overall, the results demonstrate the potential of chemically-fuelled self-assembly under dissipative conditions to gain temporal control over reactivity in complex chemical systems.

Revising Complex Supramolecular Polymerization under Kinetic and Thermodynamic Control

Pathway complexity, hierarchical organization, out of equilibrium, and metastable or kinetically trapped species are common terms widely used in recent, high-quality publications in the field of supramolecular polymers. Often, the terminologies used to describe the different self-assembly pathways, the species involved, as well as their relationship and relative stability are not trivial. Different terms and classifications are commonly found in the literature, however, in many cases, without clear definitions or guidelines on how to use them and how to determine them experimentally. The aim of this Minireview is to classify, differentiate, and correlate the existing concepts with the help of recent literature reports to provide the reader with a general insight into thermodynamic and kinetic aspects of complex supramolecular polymerization processes. A good comprehension of these terms and concepts should contribute to the development of new complex, functional materials.

Evaluation of Charge-Regulated Supramolecular Copolymerization to Tune the Time Scale for Oxidative Disassembly of β-Sheet Comonomers

A multistimuli-responsive supramolecular copolymerization is reported. The copolymerization is driven by hydrogen bond encoded β-sheet-based charge co-assembly into 1D nanorods in water, using glutamic acid or lysine residues in either of the peptide comonomers. The incorporation of methionine as hydrophobic amino acid supports β-sheet formation, but oxidation of the thioether side-chain to a sulfoxide functional group destabilizes the β-sheet ordered domains and induces disassembly of the supramolecular polymers. Using H₂ O₂ as reactive oxygen species, the time scale and kinetics of the oxidative disassembly are probed. Compared to the charge neutral homopolymers, it is found that the oxidative disassembly of the charged ampholytic copolymers is up to two times faster and is operative at neutral pH. The strategy is therefore an important addition to the growing field of amphiphilic polythioether containing (macro)molecular building blocks, particularly in view of tuning their oxidation induced disassembly which tends to be notoriously slow and requires high concentrations of reactive oxygen species or acidic reaction media.

Photon Upconverted Circularly Polarized Luminescence via Triplet-Triplet Annihilation

Circularly polarized luminescent materials are of increasing attention due to their potential applications in advanced optical technologies, such as chiroptical devices and optical sensing. Recently, in all reported circularly polarized luminescent materials, high-energy excitation results in low-energy or downconverted circularly polarized luminescence (CPL) emission. Although photon upconversion-i.e., the conversion of low-energy light into higher-energy emission, with a wide variety of applications-has been widely reported, the integration of photon upconversion and CPL in one chiral system to achieve higher-energy CPL emission has never been reported. Herein, a brief review is provided of recent achievements in photon-upconverted CPL via the triplet-triplet annihilation mechanism, focusing on the amplified dissymmetry factor g_lum through energy transfer process and dual upconverted and downconverted CPL emission through chirality and energy transfer process.

Adaptive Polymeric Assemblies for Applications in Biomimicry and Nanomedicine

Dynamic and adaptive self-assembly systems are able to sense an external or internal (energy or matter) input and respond via chemical or physical property changes. Nanomaterials that show such transient behavior have received increasing interest in the field of nanomedicine due to improved spatiotemporal control of the nanocarrier function. In this regard, much can be learned from the field of systems chemistry and bottom-up synthetic biology, in which complex and intelligent networks of nanomaterials are designed that show transient behavior and function to advance our understanding of the complexity of living systems. In this Perspective, we highlight the recent advancements in adaptive nanomaterials used for nanomedicine and trends in transient responsive self-assembly systems to envisage how these fields can be integrated for the formation of next-generation adaptive stimuli-responsive nanocarriers in nanomedicine.

Fuel-Driven Transient Crystallization of a Cucurbit[8]uril-Based Host-Guest Complex

Transient self-assembling systems often suffer from accumulation of chemical wastes that interfere with the formation of pristine self-assembled products in subsequent cycles. Herein, we report the transient crystallization of a cucurbit[8]uril-based host-guest complex, preventing the accumulation of chemical wastes. Base-catalyzed thermal decarboxylation of trichloroacetic acid that chemically fuels the crystallization process dissolves the crystals, and produces volatile chemical wastes that are spontaneously removed from the solution. With such self-clearance process, no significant damping in the formation of the crystals was observed. The morphology and structural integrity of the crystals was also maintained in subsequent cycles. The concept may be further extended to obtain other temporally functional materials, quasicrystals, etc., based on stimuli-responsive guest molecules.

A chemically fueled non-enzymatic bistable network

One of the grand challenges in contemporary systems chemistry research is to mimic life-like functions using simple synthetic molecular networks. This is particularly true for systems that are out of chemical equilibrium and show complex dynamic behaviour, such as multi-stability, oscillations and chaos. We report here on thiodepsipeptide-based non-enzymatic networks propelled by reversible replication processes out of equilibrium, displaying bistability. Accordingly, we present quantitative analyses of the bistable behaviour, featuring a phase transition from the simple equilibration processes taking place in reversible dynamic chemistry into the bistable region. This behaviour is observed only when the system is continuously fueled by a reducing agent that keeps it far from equilibrium, and only when operating within a specifically defined parameter space. We propose that the development of biomimetic bistable systems will pave the way towards the study of more elaborate functions, such as information transfer and signalling.

Thermodynamic efficiency in dissipative chemistry

Chemical processes in closed systems inevitably relax to equilibrium. Living systems avoid this fate and give rise to a much richer diversity of phenomena by operating under nonequilibrium conditions. Recent experiments in dissipative self-assembly also demonstrated that by opening reaction vessels and steering certain concentrations, an ocean of opportunities for artificial synthesis and energy storage emerges. To navigate it, thermodynamic notions of energy, work and dissipation must be established for these open chemical systems. Here, we do so by building upon recent theoretical advances in nonequilibrium statistical physics. As a central outcome, we show how to quantify the efficiency of such chemical operations and lay the foundation for performance analysis of any dissipative chemical process.

Kinetic asymmetry allows macromolecular catalysts to drive an information ratchet

Molecular machines carry out their function by equilibrium mechanical motions in environments that are far from thermodynamic equilibrium. The mechanically equilibrated character of the trajectories of the macromolecule has allowed development of a powerful theoretical description, reminiscent of Onsager’s trajectory thermodynamics, that is based on the principle of microscopic reversibility. Unlike the situation at thermodynamic equilibrium, kinetic parameters play a dominant role in determining steady-state concentrations away from thermodynamic equilibrium, and kinetic asymmetry provides a mechanism by which chemical free-energy released by catalysis can drive directed motion, molecular adaptation, and self-assembly. Several examples drawn from the recent literature, including a catenane-based chemically driven molecular rotor and a synthetic molecular assembler or pump, are discussed.

The mechanism by which macromolecular catalysts use energy from exergonic reactions to move, adapt, and assemble has been unclear. In this Perspective article, R. Dean Astumian shows that in addition to disequilibrium of the catalyzed reaction, kinetic asymmetry is the essential feature required to drive non-equilibrium response by an information ratchet mechanism.

Gel to gel transitions by dynamic self-assembly

Dynamic systems are of great interest from the perspective of mimicking biology through to preparing useful and exciting materials. Transient supramolecular gels are potentially useful, but there are limited applications that require a gel that only exists for a short length of time. Here, we show how a dynamic system can be designed to prepare materials with properties that cannot be directly accessed using the same gelator.

Dissipative Catalysis with a Molecular Machine

We report on catalysis by a fuel-induced transient state of a synthetic molecular machine. A [2]rotaxane molecular shuttle containing secondary ammonium/amine and thiourea stations is converted between catalytically inactive and active states by pulses of a chemical fuel (trichloroacetic acid), which is itself decomposed by the machine and/or the presence of additional base. The ON-state of the rotaxane catalyzes the reduction of a nitrostyrene by transfer hydrogenation. By varying the amount of fuel added, the lifetime of the rotaxane ON-state can be regulated and temporal control of catalysis achieved. The system can be pulsed with chemical fuel several times in succession, with each pulse activating catalysis for a time period determined by the amount of fuel added. Dissipative catalysis by synthetic molecular machines has implications for the future design of networks that feature communication and signaling between the components.

Programmable dynamic steady states in ATP-driven nonequilibrium DNA systems

Inspired by the dynamics of the dissipative self-assembly of microtubules, chemically fueled synthetic systems with transient lifetimes are emerging for nonequilibrium materials design. However, realizing programmable or even adaptive structural dynamics has proven challenging because it requires synchronization of energy uptake and dissipation events within true steady states, which remains difficult to orthogonally control in supramolecular systems. Here, we demonstrate full synchronization of both events by ATP-fueled activation and dynamization of covalent DNA bonds via an enzymatic reaction network of concurrent ligation and cleavage. Critically, the average bond ratio and the frequency of bond exchange are imprinted into the energy dissipation kinetics of the network and tunable through its constituents. We introduce temporally and structurally programmable dynamics by polymerization of transient, dynamic covalent DNA polymers with adaptive steady-state properties in dependence of ATP fuel and enzyme concentrations. This approach enables generic access to nonequilibrium soft matter systems with adaptive and programmable dynamics.

Pathway Dependence in the Fuel-Driven Dissipative Self-Assembly of Nanoparticles

We describe the self-assembly of gold and iron oxide nanoparticles regulated by a chemical reaction cycle that hydrolyzes a carbodiimide-based fuel. In a reaction with the chemical fuel, the nanoparticles are chemically activated to a state that favors assembling into clusters. The activated state is metastable and decays to the original precursor reversing the assembly. The dynamic interplay of activation and deactivation results in a material of which the behavior is regulated by the amount of fuel added to the system; they either did not assemble, assembled transiently, or assembled permanently in kinetically trapped clusters. Because of the irreversibility of the kinetically trapped clusters, we found that the behavior of the self-assembly was prone to hysteresis effects. The final state of the system in the energy landscape depended on the pathway of preparation. For example, when a large amount of fuel was added at once, the material would end up kinetically trapped in a local minimum. When the same amount of fuel was added in small batches with sufficient time for the system to re-equilibrate, the final state would be the global minimum. A better understanding of pathway complexity in the energy landscape is crucial for the development of fuel-driven supramolecular materials.

Typical Stochastic Paths in the Transient Assembly of Fibrous Materials

When chemically fueled, molecular self-assembly can sustain dynamic aggregates of polymeric fibers-hydrogels-with tunable properties. If the fuel supply is finite, the hydrogel is transient, as competing reactions switch molecular subunits between active and inactive states, drive fiber growth and collapse, and dissipate energy. Because the process is away from equilibrium, the structure and mechanical properties can reflect the history of preparation. As a result, the formation of these active materials is not readily susceptible to a statistical treatment in which the configuration and properties of the molecular building blocks specify the resulting material structure. Here, we illustrate a stochastic-thermodynamic and information-theoretic framework for this purpose and apply it to these self-annihilating materials. Among the possible paths, the framework variationally identifies those that are typical-loosely, the minimum number with the majority of the probability. We derive these paths from computer simulations of experimentally-informed stochastic chemical kinetics and a physical kinetics model for the growth of an active hydrogel. The model reproduces features observed by confocal microscopy, including the fiber length, lifetime, and abundance as well as the observation of fast fiber growth and stochastic fiber collapse. The typical mesoscopic paths we extract are less than 0.23% of those possible, but they accurately reproduce material properties such as mean fiber length.

Temporally-Programmed Transient Supramolecular Gels

In living systems, self-assembly processes are driven by the consumption of chemical fuels. Synthetic adaptation of living systems can be achieved by coupling of competing pathways that drive the assembly and disassembly, respectively, under the influence of chemical fuels. Here, a pH-responsive transient gel system is created by simultaneous incorporation of two triggers, of which one is responsible for the initiation of the self-assembly by increasing the pH and the second trigger drives the disassembly by reducing the pH. This method allows us to prepare transient gels with a high degree of control over the self-assembly lifetime as well as the mechanical properties of the transient gels.

Bio-inspired temporal regulation of ion-transport in nanochannels

Temporal regulation of mass transport across the membrane is a vital feature of biological systems. Such regulatory mechanisms rely on complex biochemical reaction networks, often operating far from equilibrium. Herein, we demonstrate biochemical reaction mediated temporal regulation of mass transport in nanochannels of mesoporous silica sphere. The rationally designed nanochannels with pH responsive electrostatic gating are fabricated through a hetero-functionalization approach utilizing propylamine and carboxylic acid moieties. At basic pH, cationic small molecules can diffuse into the nanochannels which release back to the solution at acidic pH. The transient ion transport is temporally controlled using a base as fuel along with esterase enzyme as the mediator. The slow enzymatic hydrolysis of a dormant deactivator (ethyl acetate) determines the lifetime of transient encapsulated state, which can be programmed easily by modulating the enzymatic activity of esterase. This system represents a unique approach to create autonomous artificial cellular models.

Chiral Metamolecules with Active Plasmonic Transition

Energy-dissipating self-assembly is at the basis of many important cellular processes, such as cell organization, proliferation, and morphogenesis. Beyond equilibrium self-assembled molecular systems and materials, it is increasingly recognized that the control of assembly kinetics provides great opportunity for the next generation of molecular materials with intelligent behavior including programmed spatiotemporal organization. Here we show the transient self-assembly of active chiral plasmonic metamolecules (CPMs), which is controlled by the proton flux generated from a positive-feedback chemical reaction network. The fuel-conversion kinetics allows for temporal control and adaptive tuning of multiple structures of plasmonic metamolecules (PMs). This approach enables autonomous tuning of chiroptical properties of metamolecules with dynamic behavior. Moreover, we show that 11 types of spatial configurations of PMs are assembled, and 9 types of temporal configurations of CPMs are differentiated.

Temperature-controlled repeatable scrambling and induced-sorting of building blocks between cubic assemblies

Separation of a homogeneous mixture of different components to reach an ordered out-of-equilibrium state in solution has attracted continuous attention. While this can be achieved using external chemical fuels or photo energy, an alternative energy source is heat. Here we realize a temperature-controlled cycle of transitions between ordered and disordered states based on a mixture of two kinds of building blocks that self-assemble into cubic structures (nanocubes). An almost statistical mixture of nanocubes (disordered state) is thermodynamically most stable at lower temperature (25 °C), while homoleptic assemblies composed of single components are preferentially produced at higher temperature (100 °C) followed by rapid cooling. The scrambling of the building blocks between the nanocubes takes place through the exchange of free building blocks dissociated from the nanocubes. Based on this mechanism, it is possible to accelerate, retard, and perfectly block the scrambling by the guest molecules encapsulated in the nanocubes.

In this paper, the authors study the temperature-controlled dynamic behavior of a system of nanocubes self-assembled from two different building blocks. Non-intuitively, the disordered, equilibrium state (a mixture of heteroleptic cubes) and the ordered, out-of-equilibrium state (a mixture of homoleptic cubes) are cycled by heating and subsequent rapid cooling.

A chemically fuelled self-replicator

The continuous consumption of chemical energy powers biological systems so that they can operate functional supramolecular structures. A goal of modern science is to understand how simple chemical mixtures may transition from non-living components to truly emergent systems and the production of new lifelike materials and machines. In this work a replicator can be maintained out-of-equilibrium by the continuous consumption of chemical energy. The system is driven by the autocatalytic formation of a metastable surfactant whose breakdown products are converted back into building blocks by a chemical fuel. The consumption of fuel allows the high-energy replicators to persist at a steady state, much like a simple metabolic cycle. Thermodynamically-driven reactions effect a unidirectional substrate flux as the system tries to regain equilibrium. The metastable replicator persists at a higher concentration than achieved even transiently in a closed system, and its concentration is responsive to the rate of fuel supply.

Supramolecular Emulsion Interfacial Polymerization

A new method of supramolecular emulsion interfacial polymerization is developed, which can be used to fabricate supramolecular polymeric nanospheres. We designed a water-soluble monomer containing two maleimide end groups, acting as both building block and surfactant, and an oil-soluble supramonomer bearing two thiol groups connected by quadruple hydrogen bonds. With the assist of ultrasonication, hollow nanospheres can be controllably prepared by thiol-maleimide reaction of two monomers at the emulsion interface, which exhibit good stability and dynamic property. In addition, the encapsulated guest molecules could be controllably released from the supramolecular polymeric nanospheres, owing to their stimuli-responsiveness. It is anticipated that this approach will enrich the methodology of supramolecular polymerization and can be applied to constructing supramolecular materials with controllable structures and functions.

Annealing multicomponent supramolecular gels

Annealing is widely used as a means of changing the physical properties of a material. The rate of heating and cooling used in the annealing process controls the final properties. Annealing can be used as a means of driving towards the, or at least a, thermodynamic minimum. There is surprisingly little information on annealing kinetically-trapped supramolecular gels. Here, we show that annealing multicomponent gels can be used to prepare materials with tunable mechanical properties. We show that annealing in a two-component gel leads to a self-sorted network, which has significantly different mechanical properties to the as-prepared gels. Whilst the fibres are self-sorted, we show that the annealing of this system leads to significant change in the network level of assembly, and it is this that leads to the increase in storage modulus. We also show that it is possible to selectively anneal only a single component in the mixture.

Chemically fueled covalent crosslinking of polymer materials

Non-equilibrium covalently crosslinked hydrogels are synthesized using carbodiimide fueled coupling of carboxylic acids to anhydrides which eventually dissipate by hydrolysis.

Self-assembling outside equilibrium: emergence of structures mediated by dissipation

Self-assembly under non-equilibrium conditions may give rise to the formation of structures not available at equilibrium.

Stimuli-Responsive Supramolecular Hydrogels and Their Applications in Regenerative Medicine

Supramolecular hydrogels are a class of self-assembled network structures formed via non-covalent interactions of the hydrogelators. These hydrogels capable of responding to external stimuli are considered to be smart materials due to their ability to undergo sol-gel and/or gel-sol transition upon subtle changes in their surroundings. Such stimuli-responsive hydrogels are intriguing biomaterials with applications in tissue engineering, delivery of cells and drugs, modulating tissue environment to promote innate tissue repair, and imaging for medical diagnostics among others. This review summarizes the recent developments in stimuli-responsive supramolecular hydrogels and their potential applications in regenerative medicine. Specifically, various structural aspects of supramolecular hydrogelators involved in self-assembly, the role of external stimuli in tuning/controlling their phase transitions, and how these functions could be harnessed to advance applications in regenerative medicine are focused on. Finally, the key challenges and future prospects for these versatile materials are briefly described.

Substrate-Induced Self-Assembly of Cooperative Catalysts

Dissipative self-assembly processes in nature rely on chemical fuels that activate proteins for assembly through the formation of a noncovalent complex. The catalytic activity of the assemblies causes fuel degradation, resulting in the formation of an assembly in a high-energy, out-of-equilibrium state. Herein, we apply this concept to a synthetic system and demonstrate that a substrate can induce the formation of vesicular assemblies, which act as cooperative catalysts for cleavage of the same substrate.

Substrate-Induced Self-Assembly of Cooperative Catalysts

Dissipative self-assembly processes in nature rely on chemical fuels that activate proteins for assembly through the formation of a noncovalent complex. The catalytic activity of the assemblies causes fuel degradation, resulting in the formation of an assembly in a high-energy, out-of-equilibrium state. Herein, we apply this concept to a synthetic system and demonstrate that a substrate can induce the formation of vesicular assemblies, which act as cooperative catalysts for cleavage of the same substrate.

Acylhydrazone functionalized benzimidazole-based metallogel for the efficient detection and separation of Cr3

Chromium(iii) is a kind of microelement and can be converted to the more toxic chromium(vi), which is a carcinogen, by redox cycling. Thus, the development of novel materials for the detection and removal of Cr³⁺ is a very important issue. A novel metallogel chemosensor (BMG-Fe) based on functionalized benzimidazole (BM) and Fe³⁺ was constructed, which could fluorescently detect and separate Cr³⁺. The detection limit of BMG-Fe for Cr³⁺ is 2.62 × 10^-8 M, and it exhibited high sensitivity and selectivity for Cr³⁺. Meanwhile, the absorbing percentage of BMG-Fe for Cr³⁺ is 96.36%, indicating a high separation rate. Interestingly, the sensitivity and ingestion capacity of BMG-Fe for Cr³⁺ are better than that of the simple organogel (BMG). So, the metallogel BMG-Fe could be utilized for the efficient removal of heavy metal ions from waste water.

The hallmarks of living systems: towards creating artificial cells

Despite the astonishing diversity and complexity of living systems, they all share five common hallmarks: compartmentalization, growth and division, information processing, energy transduction and adaptability. In this review, we give not only examples of how cells satisfy these requirements for life and the ways in which it is possible to emulate these characteristics in engineered platforms, but also the gaps that remain to be bridged. The bottom-up synthesis of life-like systems continues to be driven forward by the advent of new technologies, by the discovery of biological phenomena through their transplantation to experimentally simpler constructs and by providing insights into one of the oldest questions posed by mankind, the origin of life on Earth.

Energy consumption in chemical fuel-driven self-assembly

Nature extensively exploits high-energy transient self-assembly structures that are able to perform work through a dissipative process. Often, self-assembly relies on the use of molecules as fuel that is consumed to drive thermodynamically unfavourable reactions away from equilibrium. Implementing this kind of non-equilibrium self-assembly process in synthetic systems is bound to profoundly impact the fields of chemistry, materials science and synthetic biology, leading to innovative dissipative structures able to convert and store chemical energy. Yet, despite increasing efforts, the basic principles underlying chemical fuel-driven dissipative self-assembly are often overlooked, generating confusion around the meaning and definition of scientific terms, which does not favour progress in the field. The scope of this Perspective is to bring closer together current experimental approaches and conceptual frameworks. From our analysis it also emerges that chemically fuelled dissipative processes may have played a crucial role in evolutionary processes.