Vesicle-to-micelle oscillations and spatial patterns

Consequences of Vibrational Strong Coupling on Supramolecular Polymerization of Porphyrins

Supramolecular polymers display interesting optoelectronic properties and, thus, deploy multiple applications based on their molecular arrangement. However, controlling supramolecular interactions to achieve a desirable molecular organization is not straightforward. Over the past decade, light-matter strong coupling has emerged as a new tool for modifying chemical and material properties. This novel approach has also been shown to alter the morphology of supramolecular organization by coupling the vibrational bands of solute and solvent to the optical modes of a Fabry-Perot cavity (vibrational strong coupling, VSC). Here, we study the effect of VSC on the supramolecular polymerization of chiral zinc-porphyrins (S-Zn) via a cooperative effect. Electronic circular dichroism (ECD) measurements indicate that the elongation temperature (Te) of the supramolecular polymerization is lowered by ∼10 °C under VSC. We have also generalized this effect by exploring other supramolecular systems under strong coupling conditions. The results indicate that the solute-solvent interactions are modified under VSC, which destabilizes the nuclei of the supramolecular polymer at higher temperatures. These findings demonstrate that the VSC can indeed be used as a tool to control the energy landscape of supramolecular polymerization. Furthermore, we use this unique approach to switch between the states formed under ON- and OFF-resonance conditions, achieved by simply tuning the optical cavity in and out of resonance.

Assessment of Stoichiometric Autocatalysis across Element Groups

Autocatalysis has been proposed to play critical roles during abiogenesis. These proposals are at odds with a limited number of known examples of abiotic (and, in particular, inorganic) autocatalytic systems that might reasonably function in a prebiotic environment. In this study, we broadly assess the occurrence of stoichiometries that can support autocatalytic chemical systems through comproportionation. If the product of a comproportionation reaction can be coupled with an auxiliary oxidation or reduction pathway that furnishes a reactant, then a Comproportionation-based Autocatalytic Cycle (CompAC) can exist. Using this strategy, we surveyed the literature published in the past two centuries for reactions that can be organized into CompACs that consume some chemical species as food to synthesize more autocatalysts. 226 CompACs and 44 Broad-sense CompACs were documented, and we found that each of the 18 groups, lanthanoid series, and actinoid series in the periodic table has at least two CompACs. Our findings demonstrate that stoichiometric relationships underpinning abiotic autocatalysis could broadly exist across a range of geochemical and cosmochemical conditions, some of which are substantially different from the modern Earth. Meanwhile, the observation of some autocatalytic systems requires effective spatial or temporal separation between the food chemicals while allowing comproportionation and auxiliary reactions to proceed, which may explain why naturally occurring autocatalytic systems are not frequently observed. The collated CompACs and the conditions in which they might plausibly support complex, "life-like" chemical dynamics can directly aid an expansive assessment of life's origins and provide a compendium of alternative hypotheses concerning false-positive biosignatures.

Oscillatory dynamics in a reaction network based on imine hydrolysis

We have built an autocatalytic reaction network, based on the hydrolysis of certain imines, which exhibits bistability in an open system. The positive feedback originates from the interplay of fast acid-base equilibria, leading to hydroxide ion production, and pH-dependent hydrolysis rates. The addition of a first-order removal of the autocatalyst can result in sustained pH oscillations close to physiological conditions. The unit-amplitude pH oscillations are accompanied by the stoichiometric conversion of imine into amine back and forth. A systematic parameter search is carried out to characterize the rich observable dynamics and identify the evolving bifurcations.

Non-autonomous zinc-methylimidazole oscillator and the formation of layered precipitation structures in a hydrogel

Oscillations are one of the intrinsic features of many animate and inanimate systems. The oscillations manifest in the temporal periodic change of one or several physical quantities describing the systems. In chemistry and biology, this physical quantity is the concentration of the chemical species. In most chemical oscillatory systems operating in batch or open reactors, the oscillations persist because of the sophisticated chemical reaction networks incorporating autocatalysis and negative feedback. However, similar oscillations can be generated by periodically changing the environment providing non-autonomous oscillatory systems. Here we present a new strategy for designing a non-autonomous chemical oscillatory system for the zinc-methylimidazole. The oscillations manifested in the periodic change of the turbidity utilizing the precipitation reaction between the zinc ions and 2-methylimidazole (2-met) followed by a partial dissolution of the formed precipitate due to a synergetic effect governed by the ratio of the 2-met in the system. Extending our idea spatiotemporally, we also show that these precipitation and dissolution phenomena can be utilized to create layered precipitation structures in a solid agarose hydrogel.

Insights into Chemically Fueled Supramolecular Polymers

Supramolecular polymerization can be controlled in space and time by chemical fuels. A nonassembled monomer is activated by the fuel and subsequently self-assembles into a polymer. Deactivation of the molecule either in solution or inside the polymer leads to disassembly. Whereas biology has already mastered this approach, fully artificial examples have only appeared in the past decade. Here, we map the available literature examples into four distinct regimes depending on their activation/deactivation rates and the equivalents of deactivating fuel. We present increasingly complex mathematical models, first considering only the chemical activation/deactivation rates (i.e., transient activation) and later including the full details of the isodesmic or cooperative supramolecular processes (i.e., transient self-assembly). We finish by showing that sustained oscillations are possible in chemically fueled cooperative supramolecular polymerization and provide mechanistic insights. We hope our models encourage the quantification of activation, deactivation, assembly, and disassembly kinetics in future studies.

pH Oscillating System for Molecular Computation as a Chemical Turing Machine

It has previously been demonstrated that native chemical Turing machines can be constructed by exploiting the nonlinear dynamics of the homogeneous oscillating Belousov-Zhabotinsky reaction. These Turing machines can perform word recognition of a Chomsky type 1 context sensitive language (CSL), demonstrating their high computing power. Here, we report on a chemical Turing machine that has been developed using the H₂O₂-H₂SO₄-SO₃ ^2--CO₃ ^2- pH oscillating system. pH oscillators are different to bromate oscillators in two key ways: the proton is the autocatalytic agent, and at least one of the reductants is always fully consumed in each turnover-meaning the system has to be operated as a flow reactor. Through careful design, we establish a system that can also perform Chomsky type 1 CSL word recognition and demonstrate its power through the testing of a series of in-language and out-of-language words.

Design of non-autonomous pH oscillators and the existence of chemical beat phenomenon in a neutralization reaction

The beat in physical systems is a transparent and well-understood phenomenon. It may occur in forced oscillatory systems and as a result of the interference of two waves of slightly different frequencies. However, in chemical systems, the realization of the latter type of the beat phenomenon has been lacking. Here we show that a periodic titration of acid and alkaline solutions with each other using programmable syringe pumps in a continuous stirred-tank reactor exhibits the beat phenomenon in the temporal pH oscillation pattern if the time periods of sinusoidal inflow rates of the reagents are slightly different. Interestingly, the frequency of the chemical beat pattern follows the well-known relationship from physics, namely the frequency of the beat is equal to the absolute value of the difference of the two wave frequencies. Based on our strategy, we can design and engineer non-autonomous pH oscillatory systems, in which the characteristics of the temporal oscillations (amplitude, time period) can easily and precisely be controlled by the experimental conditions such as the inflow rates and feed concentrations. The demonstrated phenomena can be exploited in practical applications, we use the non-autonomous pH oscillators to drive the reversible assembly and disassembly of pH-sensitive building blocks (oleic acid and gold nanoparticles), both highly relevant in nanotechnology and biomedical applications.

Concurrent self-regulated autonomous synthesis and functionalization of pH-responsive giant vesicles by a chemical pH oscillator

The semibatch BrO3--SO32- pH oscillator serves as the radical source for the in situ polymerization of the pH-responsive 2-(diisopropylamino)-ethyl methacrylate monomer on poly(ethylene-glycol)-macroCTA chain and generates an amphiphilic block copolymer. These building blocks concurrently self-assemble to micelles and then transforms to vesicles as the chain length of the hydrophobic block growths. Large amplitude oscillations in the concentration of H+ by the semibatch BrO3--SO32- are provoked when the conditions in the system are favorable. The oscillations control the protonation state of the tertiary amine group in the core segment of the block copolymer. Rhythmic assembly-disassembly of the polymer structures is observed. All processes, from the time- regulated autonomous formation of the building blocks, their self-assembly and the rhythmic disassembly-reassembly are governed by the same simple chemical system, in the same reaction vessel, without complicated multi step procedures and are fueled and kept out of equilibrium by the uniform inflow of SO32-.

Using reactive dissipative particle dynamics to understand local shape manipulation of polymer vesicles

Biological cells have long been of interest to researchers due to their capacity to actively control their shape. Accordingly, there is significant interest in generating simplified synthetic protocells that can alter their shape based on an externally or internally generated stimulus. To date, most progress has been made towards controlling the global shape of a protocell, whereas less is known about generating a local shape change. Here, we seek to better understand the possible mechanisms for producing local morphological changes in a popular protocell system, the block copolymer vesicle. Accordingly, we have combined Dissipative Particle Dynamics (DPD) and the Split Reactive Brownian Dynamics algorithm (SRBD) to produce a simulation tool that is capable of modeling the dynamics of self-assembled polymer structures as they undergo chemical reactions. Using this Reactive DPD or RDPD method, we investigate local morphological change driven by either the microinjection of a stimulus or an enzymatically-produced stimulus. We find that sub-vesicle-scale morphological change can be induced by either a solvent stimulus that swells the vesicle membrane, or by a reactant stimulus that alters the chemistry of the block polymer in the membrane corona. Notably, the latter method results in a more persistent local deformation than the former, which we attribute to the slower diffusion of polymer chains relative to the solvent. We quantify this deformation and show that it can be modulated by altering the interaction parameter of the parts of the polymer chain that are affected by the stimulus.

Carbon Dioxide-Driven Coupling in a Two-Compartment System: Methyl Red Oscillator

Strategies for designing autonomous oscillatory systems have gained much attention in the past few decades. A broadly accepted and used strategy for the generation of forced oscillations in the originally non-oscillatory subsystems is to couple a pH (driving) oscillator to a pH-sensitive substance (forced oscillatory subsystem) in a one-compartment system. The forced oscillatory subsystem comprises pH-sensitive components, which inevitably generate negative feedback and affect the characteristics of the driving oscillatory system. Here, we present a different approach by separating the driving and forced oscillatory systems into a two-compartment system using a silicone membrane, and the forced oscillations of the absorbance of a pH-sensitive chemical species (methyl red dye) were realized by the transport of carbon dioxide through the membrane generated periodically by the driving pH oscillator. The transported carbon dioxide produced the pH change in the separated compartment by carbon dioxide-hydrogen carbonate-carbonate equilibria and created forced oscillations of a pH-sensitive chemical species manifested in the oscillation of its absorbance at a fixed wavelength. This approach avoids any feedback from the forced oscillatory system to the driving system via the cross-membrane transport of the chemical species from the forced to the driving oscillatory system. Additionally, we present that this carbon dioxide coupling to the methyl red dye can be used to estimate the carbon dioxide content in both liquid and gas phases.

A simple hydrogel device with flow-through channels to maintain dissipative non-equilibrium phenomena

The development of autonomous chemical systems that could imitate the properties of living matter, is a challenging problem at the meeting point of materials science and nonequilibrium chemistry. Here we design a multi-channel gel reactor in which out-of-equilibrium conditions are maintained by antagonistic chemical gradients. Our device is a rectangular hydrogel with two or more channels for the flows of separated reactants, which diffuse into the gel to react. The relative position of the channels acts as geometric control parameters, while the concentrations of the chemicals in the channels and the variable composition of the hydrogel, which affects the diffusivity of the chemicals, can be used as chemical control parameters. This flexibility allows finding easily the optimal conditions for the development of nonequilibrium phenomena. We demonstrate this straightforward operation by generating diverse spatiotemporal patterns in different chemical reactions. The use of additional channels can create interacting reaction zones.

Nucleotide-Selective Templated Self-Assembly of Nanoreactors under Dissipative Conditions

Nature adopts complex chemical networks to finely tune biochemical processes. Indeed, small biomolecules play a key role in regulating the flux of metabolic pathways. Chemistry, which was traditionally focused on reactions in simple mixtures, is dedicating increasing attention to the network reactivity of highly complex synthetic systems, able to display new kinetic phenomena. Herein, we show that the addition of monophosphate nucleosides to a mixture of amphiphiles and reagents leads to the selective templated formation of self-assembled structures, which can accelerate a reaction between two hydrophobic reactants. The correct matching between nucleotide and the amphiphile head group is fundamental for the selective formation of the assemblies and for the consequent up-regulation of the chemical reaction. Transient stability of the nanoreactors is obtained under dissipative conditions, driven by enzymatic dephosphorylation of the templating nucleotides. These results show that small molecules can play a key role in modulating network reactivity, by selectively templating self-assembled structures that are able to up-regulate chemical reaction pathways.

Chemical Resonance, Beats, and Frequency Locking in Forced Chemical Oscillatory Systems

Resonance, beats, and synchronization are general and fundamental phenomena in physics. Their existence and their in-depth understanding in physical systems have led to several applications and technological developments shaping our world today. Here we show the existence of chemical resonance, chemical beats, and frequency locking phenomena in periodically forced pH oscillatory systems (sulfite-hydrogen peroxide and sulfite-formaldehyde-gluconolactone pH oscillatory systems). Periodic forcing was realized by a superimposed sinusoidal modulation on the inflow rates of the reagents in the continuous-flow stirred tank reactor. The dependence of the time period of beats follows the relation known from classical physics for forced physical oscillators. Our developed numerical model describes qualitatively the resonance and beat phenomena experimentally revealed. Application of periodic forcing in autonomously oscillating systems can provide new types of oscillators with a controllable frequency and new insight into controlling irregular chemical oscillation regimes.

Modulation Effects of Permanganate and Manganese(II) Ions on the Oscillatory Dynamics in a Bromate-Sulfite Reaction System

It is important to study nonlinear dynamical systems showing pH and temperature oscillations simultaneously. Here, we systematically investigated the bromate-sulfite reaction in its coupled system. Large-amplitude temperature oscillations could be measured accompanied by the pH oscillations with or without permanganate and manganese(II) ions. The modulation effects on the oscillatory dynamics of the bromate-sulfite reaction system produced by permanganate and manganese(II) ions were investigated in detail. On the one hand, with permanganate, an additional negative pH feedback process between permanganate and bisulfite occurs, leading to weakening the pH positive feedback. The above opposite effects make the period length change unmonotonically when adjusting the permanganate concentration and flow rate. On the other hand, with Mn²⁺ as the feedback agent, the nonmonotonic change of period was not obvious because it only contained one feedback loop, which can only reinforce negative feedback without affecting positive feedback.

Autonomous Chemical Modulation and Unidirectional Coupling in Two Oscillatory Chemical Systems

Controlling and coupling of out-of-equilibrium reaction networks have great importance in chemistry and biology. We provide an example for the ideal master-slave coupling between two pH oscillators (the sulfite-bromate and the hydrogen peroxide-sulfite pH oscillators operated in continuous-flow stirred tank reactors). The coupling between the reactors was realized by transport of carbon dioxide through a silicon membrane, which is a common chemical species in both systems. We showed that by using this strategy, the master system can generate forced pH oscillations in the slave system. We could control the amplitude and frequency of the oscillations in the slave system and reversibly drive the transition in the oscillations between the regular and chaotic regimes. Using this coupling strategy, we could present an example of amplitude modulation in a coupled chemical system.

Evolving polymersomes autonomously generated in and regulated by a semibatch pH oscillator

pH-O-PISA: a semibatch pH oscillator drives polymerization by generating radicals periodically while simultaneously regulating the evolution of the self-assembled polymersomes.

Oscillations, travelling fronts and patterns in a supramolecular system

Supramolecular polymers, such as microtubules, operate under non-equilibrium conditions to drive crucial functions in cells, such as motility, division and organelle transport¹. In vivo and in vitro size oscillations of individual microtubules^2,3 (dynamic instabilities) and collective oscillations⁴ have been observed. In addition, dynamic spatial structures, like waves and polygons, can form in non-stirred systems⁵. Here we describe an artificial supramolecular polymer made of a perylene diimide derivative that displays oscillations, travelling fronts and centimetre-scale self-organized patterns when pushed far from equilibrium by chemical fuels. Oscillations arise from a positive feedback due to nucleation-elongation-fragmentation, and a negative feedback due to size-dependent depolymerization. Travelling fronts and patterns form due to self-assembly induced density differences that cause system-wide convection. In our system, the species responsible for the nonlinear dynamics and those that self-assemble are one and the same. In contrast, other reported oscillating assemblies formed by vesicles⁶, micelles⁷ or particles⁸ rely on the combination of a known chemical oscillator and a stimuli-responsive system, either by communication through the solvent (for example, by changing pH^7-9), or by anchoring one of the species covalently (for example, a Belousov-Zhabotinsky catalyst^6,10). The design of self-oscillating supramolecular polymers and large-scale dissipative structures brings us closer to the creation of more life-like materials¹¹ that respond to external stimuli similarly to living cells, or to creating artificial autonomous chemical robots¹².

Surfactant and Cyclodextrin Induced Vesicle to Micelle to Vesicle Transformation in Aqueous Medium

The physicochemical behavior and characteristics of lipid vesicles and micelles in aqueous medium are greatly tuned by changing the ambient physical parameters, such as temperature, pH, and ionic strength. The process is also controlled by external additives and the nature of the surfactants. In this work, we have used water-soluble surfactant and cyclodextrin to transform lipid vesicles to micelles to vesicles without changing the physical ambience. In this regard, we have used a special pyrene-tagged guest compound that readily forms excimer in water and thus acts as a reporter for the process. Giant lipid vesicles (biological cell mimics) are disrupted by cationic surfactants to form mixed elongated micelles that transform to vesicles on applying a cyclodextrin host.

Small Angle Scattering for Pharmaceutical Applications: From Drugs to Drug Delivery Systems

The sub-nanometer scale provided by small angle neutron and X-ray scattering is of special importance to pharmaceutical and biomedical investigators. As drug delivery devices become more functionalized and continue decreasing in size, the ability to elucidate details on size scales smaller than those available from optical techniques becomes extremely pertinent. Information gathered from small angle scattering therefore aids the endeavor of optimizing pharmaceutical efficacy at its most fundamental level. This chapter will provide some relevant examples of drug carrier technology and how small angle scattering (SAS) can be used to solve their mysteries. An emphasis on common first-step data treatments is provided which should help clarify the contents of scattering data to new researchers. Specific examples of pharmaceutically relevant research on novel systems and the role SAS plays in these studies will be discussed. This chapter provides an overview of the current applications of SAS in drug research and some practical considerations for selecting scattering techniques.

Fuel-Driven Dissipative Self-Assembly of a Supra-Amphiphile in Batch Reactor

Dissipative self-assembly is an intriguing but challenging research topic in chemistry, materials science, physics, and biology because most functional self-assembly in nature, such as the organization and operation of cells, is actually an out-of-equilibrium system driven by energy dissipation. In this article, we successfully fabricated an I₂-responsive supra-amphiphile by a PEGylated poly(amino acid) and realize its dissipative self-assembly in batch reactor by coupling it with the redox reaction between NaIO₃ and thiourea, in which I₂ is an intermediate product. The formation and dissipative self-assembly of the supra-amphiphile can be repeatedly initiated by adding the mixture of NaIO₃ and thiourea, which herein acts as "chemical fuel", while the lifetime of the transient nanostructures formed by the dissipative self-assembly is easily tuned by altering thiourea concentration in the "chemical fuel". Furthermore, as an application demo, the dissipative self-assembly of the supra-amphiphile is examined to control dispersion of multiwalled carbon nanotubes in water, exhibiting a good performance of organic pollutant removal.

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

The use of dissipative self-assembly driven by chemical reaction networks for the creation of unique structures is gaining in popularity. In dissipative self-assembly, precursors are converted into self-assembling building blocks by the conversion of a source of energy, typically a photon or a fuel molecule. The self-assembling building block is intrinsically unstable and spontaneously reverts to its original precursor, thus giving the building block a limited lifetime. As a result, its presence is kinetically controlled, which gives the associated supramolecular material unique properties. For instance, formation and properties of these materials can be controlled over space and time by the kinetics of the coupled reaction network, they are autonomously self-healing and they are highly adaptive to small changes in their environment. By means of an example of a biological dissipative self-assembled material, the unique concepts at the basis of these supramolecular materials will be discussed. We then review recent efforts towards man-made dissipative assembly of structures and how their unique material properties have been characterized. In order to help further the field, we close with loosely defined design rules that are at the basis of the discussed examples.

From Master-Slave to Peer-to-Peer Coupling in Chemical Reaction Networks

Design strategy through linking a driving pH oscillator (master system) to a pH sensitive complexation, precipitation, or protonation equilibrium (slave system) has been widely used to create and control concentration oscillations of chemical entities (e.g., monovalent cations, DNA, nanoparticles) not participating in the pH oscillatory system. No systematic investigation has been carried out on how the components of these equilibria affect the characteristics of the driving pH oscillators, and this feedback effect has been often neglected in previous studies. Here we show that pH sensitive species (hydrogen carbonate, EDTA) through a pH-dependent equilibrium could significantly affect the characteristics (time period and amplitude) of the driving pH oscillators. By varying the concentration of those species we are able to control the strength of the chemical feedback from slave system to master system thus introducing a transition from master-slave coupling to peer-to-peer coupling in linked chemical systems. To illustrate this transition and coupling strategies we investigate two coupled chemical systems, namely, the bromate-sulfite pH oscillator and carbonate-carbon dioxide equilibrium and the hydrogen peroxide-thiosulfate-copper(II) and EDTA complexation equilibrium. As a sign of the peer-to-peer coupling the characteristics of the driving oscillatory systems can be tuned by controlling the feedback strength, and the oscillations can be canceled above a critical value of this parameter.

Chemomechanical oscillations with a non-redox non-oscillatory reaction

Periodic length changes, over 20%, were sustained in a pH-responsive gel by associating the methylene glycol–sulphite OH-producing clock-reaction with variations of the exchange time induced between the core of the gel and a steady chemical environment.

Effect of substrate concentrations on the aggregation behavior and dynamic oscillatory properties of self-oscillating block copolymers

The effect of substrate concentrations of the BZ reaction as well as specific salts on the dynamic properties of self-oscillating block copolymers was studied in detail.

Materials learning from life: concepts for active, adaptive and autonomous molecular systems

A broad overview of functional aspects in biological and synthetic out-of-equilibrium systems.

Thiosulfate-Hydrogen Peroxide Redox Oscillator as pH Driver for Ribozyme Activity in the RNA World

The RNA world of more than 3.7 billion years ago may have drawn on thermal and pH oscillations set up by the oxidation of thiosulfate by hydrogen peroxide (the THP oscillator) as a power source to drive replication. Since this primordial RNA also must have developed enzyme functionalities, in this work we examine the responses of two simple ribozymes to a THP periodic drive, using experimental rate and thermochemical data in a dynamical model for the coupled, self-consistent evolution of all reactants and intermediates. The resulting time traces show that ribozyme performance can be enhanced under pH cycling, and that thermal cycling may have been necessary to achieve large performance gains. We discuss three important ways in which the dynamic hydrogen peroxide medium may have acted as an agent for development of the RNA world towards a cellular world: proton gradients, resolution of the ribozyme versus replication paradox, and vesicle formation.

The fabrication of a supra-amphiphile for dissipative self-assembly

Dissipative self-assembly is a challenging but attractive field of supramolecular science, because it generally concerns complex systems but is more close to the self-assembly of living bodies. In this article, we realized dissipative self-assembly by coupling a supra-amphiphile with a chemical oscillator. The supra-amphiphile was fabricated with iodine and a double hydrophilic block copolymer containing PEG segments, as the non-covalent interaction between PEG and iodine could turn PEG hydrophobic, leading to the formation of the supra-amphiphile. The self-assembly and disassembly of the supra-amphiphile could be controlled by varying the concentration of iodine. Therefore, the dissipative self-assembly of the supra-amphiphile was realized when it was coupled with the IO3--NH3OH+-OH- chemical oscillator, which was able to produce iodine periodically. Meanwhile, the kinetic data of the self-assembly and disassembly of the supra-amphiphile could be estimated by the theoretical simulation of the chemical oscillations. This line of research promotes the self-assembly of supra-amphiphiles one step forward from thermodynamic statics to a dissipative system, and also suggests a new strategy to investigate the kinetics of stimuli-responsive molecular self-assembly.

Reversible morphological changes of assembled supramolecular amphiphiles triggered by pH-modulated host–guest interactions

Acid–base modulated host–guest binding at the micellar–water interface triggers reversible oblate ellipsoid-to-lamellar morphological transitions revealing the relationship between and morphology.

Self-Assembly of Charged Nanoparticles by an Autocatalytic Reaction Front

In this work we present that aggregation of charged and pH sensitive nanoparticles can be spatiotemporally controlled by an autonomous way using the chlorite-tetrathionate autocatalytic front, where the front regulates the electrostatic interaction between nanoparticles due to protonation of the capping (carboxylate-terminated) ligand. We found that the aggregation and sedimentation of nanoparticles in liquid phase with the effect of reversible binding of the autocatalyst (H(+)) play important roles in changing the front stability (mixing length) and the velocity of the front in both cases when the fronts propagate upward and downward. Calculation of interparticle interactions (electrostatic and van der Waals) with the measurement of front velocity revealed that the aggregation process occurs fast (within a few seconds) at the front position.

Celebrating Soft Matter's 10th Anniversary: Approaches to program the time domain of self-assemblies

Self-regulating reconfigurable soft matter systems are of great interest for creating adaptive and active material properties. Such complex functionalities emerge from non-linear and interactive behavior in space and time as demonstrated by a plethora of dynamic, self-organizing biological structures (e.g., the cytoskeleton). In man-made self-assemblies, patterning of the spatial domain has advanced to creating hierarchical structures via precise molecular programming. However, orchestration of the time domain of self-assemblies is still in its infancy and lacks universal design principles. In this Emerging Area article we outline major strategies for programming the time domain of self-assemblies following the concepts of regulatory reaction networks, energy dissipation and kinetic control. Such concepts operate outside thermodynamic equilibrium and pave the way for temporally patterned, dynamic, and autonomously acting functional materials.

Generic concept to program the time domain of self-assemblies with a self-regulation mechanism

Nature regulates complex structures in space and time via feedback loops, kinetically controlled transformations, and under energy dissipation to allow non-equilibrium processes. Although man-made static self-assemblies realize excellent control over hierarchical structures via molecular programming, managing their temporal destiny by self-regulation is a largely unsolved challenge. Herein, we introduce a generic concept to control the time domain by programming the lifetimes of switchable self-assemblies in closed systems. We conceive dormant deactivators that, in combination with fast promoters, enable a unique kinetic balance to establish an autonomously self-regulating, transient pH-state, whose duration can be programmed over orders of magnitude-from minutes to days. Coupling this non-equilibrium state to pH-switchable self-assemblies allows predicting their assembly/disassembly fate in time, similar to a precise self-destruction mechanism. We demonstrate a platform approach by programming self-assembly lifetimes of block copolymers, nanoparticles, and peptides, enabling dynamic materials with a self-regulation functionality.

pH-regulated chemical oscillators

The hydrogen ion is arguably the most ubiquitous and important species in chemistry. It also plays a key role in nearly every biological process. In this Account, we discuss systems whose behavior is governed by oscillations in the concentration of hydrogen ion. The first chemical oscillators driven by changes in pH were developed a quarter century ago. Since then, about two dozen new pH oscillators, systems in which the periodic variation in pH is not just an indicator but an essential prerequisite of the oscillatory behavior, have been discovered. Mechanistic understanding of their behavior has grown, and new ideas for their practical application have been proposed and, in some cases, tested. Here we present a catalog of the known pH oscillators, divide them into mechanistically based categories based on whether they involve a single oxidant and reductant or an oxidant and a pair of reductants, and describe general mechanisms for these two major classes of systems. We also describe in detail the chemistry of one example from each class, hydrogen peroxide-sulfide and ferricyanide-iodate-sulfite. Finally, we consider actual and potential applications. These include using pH oscillators to induce oscillation in species that would otherwise be nonoscillatory, creating novel spatial patterns, generating periodic transitions between vesicle and micelle states, stimulating switching between folded and random coil states of DNA, building molecular motors, and designing pulsating drug delivery systems. We point out the importance for future applications of finding a batch pH oscillator, one that oscillates in a closed system for an extended period of time, and comment on the progress that has been made toward that goal.

pH Oscillator Stretched in Space but Frozen in Time

Chemical oscillations are studied using a continuous-flow microfluidic system transforming the time domain of chemical oscillators into a spatial domain. This system allows one (i) to monitor the dynamics of chemical oscillators with the accuracy of vigorously stirred batch reactors but with the ease and speed of CSTRs and (ii) to rapidly screen the phase space of chemical oscillators in just one experiment versus a traditional series of batch measurements.