Ger J. M. Koper

Transient assembly of active materials fueled by a chemical reaction

Nonequilibrium transient self-assembly In biology, the constant supply of energy can drive a system to be far from its equilibrium conditions and allow for useful work to be done. In contrast, in most synthetic systems, there is a drive toward lower energy states. Boekhoven et al. made a molecule that can switch between a nonassociating state and an associating state through the addition of a chemical fuel (see the Perspective by Van der Zwagg and Meijer). The lifetime, stiffness, and regenerative behavior of the self-assembled state could be controlled and tuned by the kinetics of fuel conversion. Science, this issue p. 1075; see also p. 1056 A synthetic chemical system exhibits transient self-assembly fueled by a chemical reaction. [Also see Perspective by Van der Zwagg and Meijer] Fuel-driven self-assembly of actin filaments and microtubules is a key component of cellular organization. Continuous energy supply maintains these transient biomolecular assemblies far from thermodynamic equilibrium, unlike typical synthetic systems that spontaneously assemble at thermodynamic equilibrium. Here, we report the transient self-assembly of synthetic molecules into active materials, driven by the consumption of a chemical fuel. In these materials, reaction rates and fuel levels, instead of equilibrium composition, determine properties such as lifetime, stiffness, and self-regeneration capability. Fibers exhibit strongly nonlinear behavior including stochastic collapse and simultaneous growth and shrinkage, reminiscent of microtubule dynamics.

Dissipative Self-Assembly of a Molecular Gelator by Using a Chemical Fuel†

The construction of energy-dissipating self-assembling systems, which, like self-assembled structures found in nature are formed transiently, far from equilibrium, and under the constant influx of chemical energy, still represents a frontier in nanoscale assembly. The self-assembly of small molecules, polymers, proteins, nanoparticles, colloids, and particles with sizes that approach the mesoscale under thermodynamic equilibrium conditions has been a powerful approach for the construction of a variety of structures of nanoto micrometer dimensions, like vesicles, capsules, and nanotubules. The reversible nature of selfassembly processes has been exploited in switchable, adaptive, and autopoietic self-assembling systems, which lead to novel responsive materials and artificial systems that are capable of self-replication and compartmentalization. Recently, there has also been a strongly growing interest in self-assembled materials obtained under non-equilibrium conditions. For instance, the formation of hierarchically structured membranes in a reaction-diffusion field, and the orthogonal self-assembly of molecular gels with surfactants, liquid crystals, or other components can be controlled through the processing conditions, thus leading to a much richer structural diversity compared to equilibriumprocessed materials. These self-assembled structures offer new and intriguing opportunities for functional materials and biomimetic cellular structures. Nevertheless, in all these cases, the final self-assembling systems reside in a (local) thermodynamic minimum state. Despite these advances, the permanent nature of these synthetic self-assembled structures does not compare well to the complex spatiotemporally confined self-assembly processes seen in natural systems, which for instance allow the dynamic compartmentalization of incompatible processes, responsiveness, and self-healing. Natural self-assembled structures such as the cytoskeleton and phospholipid membranes are formed by dissipative self-assembly (DSA). In general, DSA systems consist of non-assembling entities which, through activation by an energy source, assemble into ordered structures. Energy dissipation causes deactivation of the building blocks, hence leading to a collapse of the formed structures. A typical example is microtubule assembly that uses guanosine-5’-triphosphate (GTP) as an energy source, which in turn catalyzes the hydrolysis of GTP and therefore its own collapse. The microtubule assembly process is controlled by feedback loops that lead to self-organization, including oscillatory behavior and nonlinear responses of microtubule formation, which are essential for rapid morphogenic alterations, self-healing, and self-replication. These fascinating properties of natural DSA systems have motivated research on their artificial counterparts. Several artificial DSA systems based on natural building blocks have been reported. Examples of fully artificial DSA systems are most commonly found in the top-down engineered mesoscopic regime with hard inorganic or polymeric objects. The few examples that concern soft matter are mostly fueled by light, whereas the dissipation of “chemical fuels” has been used to drive mechanical motion. It remains a challenge to develop a DSA system that is chemically fueled. A first step towards the development of a self-organizing self-assembly system is the construction of a simple DSA system without feedback control loops. Such a simple system typically follows a sequence of processes. Firstly, an energy source activates the precursor building blocks so that selfassembly is favored. Upon self-assembly, the activated building block can dissipate its energy, thus resulting in the formation of the initial building block and disassembly of the architecture. A requirement is that the rate of energy dissipation (Pd) should be lower than the consumption of fuel (Pc) to allow the formation of self-assembled architectures (Figure 1). Herein we present a synthetic DSA fibrous network that uses chemical fuel as an energy source. A gelator precursor is converted into a gelator by reaction with a chemical fuel, thus leading to self-assembly. Hydrolysis of the gelator, which is labile under ambient conditions, leads to energy dissipation and disassembly of the formed structures. Reactive gels have been previously reported and the hydrolysis of ester functions has been exploited to achieve an enzymatically controlled gel–sol phase transition. The design of the dissipative self-assembling system presented here is based on dibenzoyl-(l)-cystine (DBC; Bz = benzoyl), a well-known pH-responsive hydrogelator . Above their pKa value (ca. 4.5), intermolecular repulsion occurs between the anionic carboxylic acid groups of DBC, and therefore DBC [*] J. Boekhoven, Dr. A. M. Brizard, K. N. K. Kowlgi, Dr. G. J. M. Koper, Dr. R. Eelkema, Prof. Dr. J. H. van Esch Department of Chemical Engineering Delft University of Technology Julianalaan 136, 2628 BL, Delft (The Netherlands) Fax: (+ 31)15-278-4289 E-mail: j.h.vanesch@tudelft.nl Homepage: http://www.dct.tudelft.nl/sas

Triggered Self-Assembly of Simple Dynamic Covalent Surfactants

A prototype surfactant system was developed with the unique feature that it can be switched between an aggregated, amphiphilic state and a nonaggregated, nonamphiphilic state using external stimuli. This switchable surfactant system uses the reversible formation of a dynamic covalent bond for pH- and temperature-triggered on/off self-assembly of micellar aggregates by reversible displacement of the equilibrium between nonamphiphilic building blocks and their amphiphilic counterparts. The potential for application in controlled-release systems is shown by reversible uptake and release of an organic dye in aqueous media.

Acid-base properties of poly(propylene imine)dendrimers

Abstract Potentiometric titration experiments of poly(propylene imine) dendrimers (up to the fifth generation) were carriedout at salt concentrations of 0.1, 0.5 and 1.0 M KCl and NaCl. The experiments were performed at two different locations on different instruments and were converted to titration curves using two different methods, resulting in a consistent experimental data set for the dendrimers measured. The titration curves feature two distinct steps around p H 6 and 10 with an intermediate plateau at 2/3 of the total ionizable groups. This protonation pattern reflects short-ranged repulsive interactions between ionizables sites and can be modeled using an Ising model with nearest-neighbour pair interactions. The intermediate plateau results from the stability of an onion-like structure where all odd shells of the dendrimer are protonated, while the even ones remain deprotonated. The Ising model permits a quantitative analysis of the titration curves. For larger dendrimers, this Ising approach is shown to be superior to the classical analysis in terms of successive protonation equilibria.

Electrode effects in dielectric spectroscopy of colloidal suspensions

We present a simple model to account for electrode polarization in colloidal suspensions. Apart from correctly predicting the ω−32 dependence for the dielectric permittivity at low frequencies ω, the model provides an explicit dependence of the effect on electrode spacing. The predictions are tested for the sodium bis(2-ethylhexyl) sulfosuccinate (AOT) water-in-oil microemulsion with iso-octane as continuous phase. In particular, the dependence of electrode polarization effects on electrode spacing has been measured and is found to be in accordance with the model prediction. Methods to reduce or account for electrode polarization are briefly discussed.

Ionization Processes and Proton Binding in Polyprotic Systems: Small Molecules, Proteins, Interfaces, and Polyelectrolytes

Binding of ions to various materials, such as small molecules, proteins, polymers, colloid particles, and membranes, represents a central theme in basic and applied chemistry. Particularly, the case of proton binding to these substances (i.e., their acid-base behavior) has been a focus of research in many branches of chemistry since the turn of the century. One important topic in physical, analytical, and inorganic chemistry is the measurement, compilation, and prediction of acid-base properties of simple molecules or solvated metal ions.(1–8) These topics remain of much relevance for the development of new analytical techniques and tailoring of buffering or complexing agents.(7,9) Accurately known ionization constants also represent a rather stringent testing ground of our ab initio simulation capabilities of simple molecules in water.(10) Acid-base properties of proteins have been also investigated from early on.(11–14) This field has now matured into an active area of modern biochemistry with implications to the current view of protein folding, enzyme action, and photosynthesis.(15–17) Similar studies of weak polyelectrolytes were initiated in polymer science almost simultaneously(18–21) These systems represent an ongoing challenge to our understanding of acid-base equilibria.(22–25) The substantial interest in polyelectrolytes is due to their use as complexing, flocculating, or stabilizing agents, and their importance in various applications in catalysis, material engineering, biochemistry, and water purification.(26–28)

Affinity distributions and acid-base properties of homogeneous and heterogeneous sorbents: exact results versus experimental data inversion

Ionization properties of simple polyprotic acids and bases (oligoprotic carboxylic acids, polyelectrolytes, latex particles, proteins) can be described quantitatively with Ising models. These mechanistic models incorporate relevant molecular details and allow determination of exact affinity distributions (pK spectra). At present, such a molecularly based approach is too involved for complex heterogeneous substances, but the affinity distributions of these materials can be calculated by direct numerical inversion of the experimental data. Even though this problem is ill-posed, special numerical techniques can be used to stabilize the procedure and to select distributions that are smooth or that consist of a small number of isolated discrete peaks, which can be interpreted as a small number of ionizable sites. The results of these inversion techniques are discussed for a number of examples of environmental relevance such as oxides and humic substances, and for the simple model systems. Based on such observations for simple systems, we conclude that it is essentially impossible to extract true affinity distributions of complex environmental sorbents.

All-aqueous core-shell droplets produced in a microfluidic device

We present a microfluidic method to compartmentalize aqueous polymer solutions within water-in-water microdroplets, which are continuously generated without using organic solvents or surfactants. Phase separation inside the drops yields all-aqueous core-shell structures (water-in-water-in-water), as we demonstrate using the aqueous two phase system of polyethylene glycol and dextran.

Slow growth of the Rayleigh-Plateau instability in aqueous two phase systems

This paper studies the Rayleigh-Plateau instability for co-flowing immiscible aqueous polymer solutions in a microfluidic channel. Careful vibration-free experiments with controlled actuation of the flow allowed direct measurement of the growth rate of this instability. Experiments for the well-known aqueous two phase system (ATPS, or aqueous biphasic systems) of dextran and polyethylene glycol solutions exhibited a growth rate of 1 s(-1), which was more than an order of magnitude slower than an analogous experiment with two immiscible Newtonian fluids with viscosities and interfacial tension that closely matched the ATPS experiment. Viscoelastic effects and adhesion to the walls were ruled out as explanations for the observed behavior. The results are remarkable because all current theory suggests that such dilute polymer solutions should break up faster, not slower, than the analogous Newtonian case. Microfluidic uses of aqueous two phase systems include separation of labile biomolecules but have hitherto be limited because of the difficulty in making droplets. The results of this work teach how to design devices for biological microfluidic ATPS platforms.

Uniform metal nanoparticles produced at high yield in dense microemulsions.

This article demonstrates that bicontinuous microemulsions are optimal templates for high yield production of metal nanoparticles. We have verified this for a variety of microemulsion systems having AOT (sodium bis (2-ethyhexyl) sulphosuccinate) or a fluorocarbon (perfluoro (4-methyl-3,6-dioxaoctane)sulphonate) as surfactant mixed with water and oils like n-heptane or n-dodecane. Several types of metal nanoparticles, including platinum, gold and iron, were produced in these microemulsions having a size range spanning 1.8-17 nm with a very narrow size distribution of ±1 nm. Remarkably high mass concentrations up to 3% were reached. Size and concentration of the nanoparticles could be varied with the stoichiometries of the reagents that constituted them. The optimization towards high yield while maintaining low size polydispersity is due to the decoupling of the time scales for the precipitation reaction and for coarsening. In actual fact, coalescence is essentially prevented by the immobilization of nanoparticles within the bicontinuous microemulsion structure.