Mirjam E. Leunissen

Ionic colloidal crystals of oppositely charged particles.

Colloidal suspensions are widely used to study processes such as melting, freezing and glass transitions. This is because they display the same phase behaviour as atoms or molecules, with the nano- to micrometre size of the colloidal particles making it possible to observe them directly in real space. Another attractive feature is that different types of colloidal interactions, such as long-range repulsive, short-range attractive, hard-sphere-like and dipolar, can be realized and give rise to equilibrium phases. However, spherically symmetric, long-range attractions (that is, ionic interactions) have so far always resulted in irreversible colloidal aggregation. Here we show that the electrostatic interaction between oppositely charged particles can be tuned such that large ionic colloidal crystals form readily, with our theory and simulations confirming the stability of these structures. We find that in contrast to atomic systems, the stoichiometry of our colloidal crystals is not dictated by charge neutrality; this allows us to obtain a remarkable diversity of new binary structures. An external electric field melts the crystals, confirming that the constituent particles are indeed oppositely charged. Colloidal model systems can thus be used to study the phase behaviour of ionic species. We also expect that our approach to controlling opposite-charge interactions will facilitate the production of binary crystals of micrometre-sized particles, which could find use as advanced materials for photonic applications.

Electrostatics at the oil–water interface, stability, and order in emulsions and colloids

Oil–water mixtures are ubiquitous in nature and are particularly important in biology and industry. Usually additives are used to prevent the liquid droplets from coalescing. Here, we show that stabilization can also be obtained from electrostatics, because of the well known remarkable properties of water. Preferential ion uptake leads to a tunable droplet charge and surprisingly stable, additive-free, water-in-oil emulsions that can crystallize. For particle-stabilized (“Pickering”) emulsions we find that even extremely hydrophobic, nonwetting particles can be strongly bound to (like-charged) oil–water interfaces because of image charge effects. These basic insights are important for emulsion production, encapsulation, and (self-)assembly, as we demonstrate by fabricating a diversity of structures in bulk, on surfaces, and in confined geometries.

A new colloidal model system to study long-range interactions quantitatively in real space

We present quantitative three-dimensional real space measurements by confocal microscopy on fluorescently labelled and sterically stabilized dispersions of polymethylmethacrylate spheres dispersed in index and density-matched solvent mixtures with a relative dielectric constant 5 <e r < 10. In this new model system Debye screening lengths (κ −1 ) comparable to the particle size (diameter σ ) can be realized even for particles with sizes of several micrometres. Moreover, by addition of salt (tetrabutylammonium chloride) κ −1 can be varied and the surface charge of the particles can be set roughly in between the values +100 and −100 mV, as determined by electrophoresis. By a comparison of radial distribution functions and displacements from lattice positions with Monte Carlo computer simulations we found that both the structure in the liquid and the crystallization volume fraction could be described with a Yukawa potential characterized by one set of parameters, a surface potential of 36 mV and κσ = 5, where the particle diameter σ = 2 µm. Anomalous (‘phase’) behaviour such as extreme long-range repulsions, ‘coexistence’ of high-density and low-density colloidal crystals and void formation, previously observed for deionized dispersions in water, was observed as well, and can now be studied in ad ifferent system without ion exchange resin. These anomalous effects are seen relatively soon after preparing the systems and are absent or short-lived in systems with grounding and at higher salt concentrations.

Self-replication of information-bearing nanoscale patterns

DNA molecules provide what is probably the most iconic example of self-replication—the ability of a system to replicate, or make copies of, itself. In living cells the process is mediated by enzymes and occurs autonomously, with the number of replicas increasing exponentially over time without the need for external manipulation. Self-replication has also been implemented with synthetic systems, including RNA enzymes designed to undergo self-sustained exponential amplification. An exciting next step would be to use self-replication in materials fabrication, which requires robust and general systems capable of copying and amplifying functional materials or structures. Here we report a first development in this direction, using DNA tile motifs that can recognize and bind complementary tiles in a pre-programmed fashion. We first design tile motifs so they form a seven-tile seed sequence; then use the seeds to instruct the formation of a first generation of complementary seven-tile daughter sequences; and finally use the daughters to instruct the formation of seven-tile granddaughter sequences that are identical to the initial seed sequences. Considering that DNA is a functional material that can organize itself and other molecules into useful structures, our findings raise the tantalizing prospect that we may one day be able to realize self-replicating materials with various patterns or useful functions.

Re-entrant melting and freezing in a model-system of charged colloids

We studied the phase behavior of charged and sterically stabilized colloids using confocal microscopy in a low polarity solvent (dielectric constant 5.4). Upon increasing the colloid volume fraction we found a transition from a fluid to a body centered cubic crystal at 0.0415+/-0.0005, followed by reentrant melting at 0.1165+/-0.0015. A second crystal of different symmetry, random hexagonal close packed, was formed at a volume fraction around 0.5, similar to that of hard spheres. We attribute the intriguing phase behavior to the particle interactions that depend strongly on volume fraction, mainly due to the changes in the colloid charge. In this low polarity system the colloids acquire charge through ion adsorption. The low ionic strength leads to fewer ions per colloid at elevated volume fractions and consequently a density-dependent colloid charge.

Numerical study of DNA-functionalized microparticles and nanoparticles: explicit pair potentials and their implications for phase behavior.

DNA-coated colloids have great potential for the design of complex self-assembling materials. In order to predict the structures that will form, knowledge of the interactions between DNA-functionalized particles is crucial. Here, we report results from Monte Carlo simulations of the pair-interaction between particles coated with single-stranded DNA sticky ends that are connected to the surface by relatively short and stiff surface tethers. We complement our calculations with a study of the interaction between two planar surfaces coated with the same DNA. Based on our simulations we propose analytical expressions for the interaction potentials. These analytical expressions describe the DNA-mediated interactions well for particle sizes ranging from tens of nanometers to a few micrometers and for a wide range of grafting densities. We find that important contributions to both the repulsive and attractive parts of the free energy come from purely entropic effects of the discrete tethered sticky ends. Per bond, these entropic contributions have a magnitude similar to the hybridization free energy of a free pair of sticky ends in solution and they can thus considerably change the effective sticky-end binding strength. Based on the calculated interaction potentials, we expect that stable gas-liquid separation only occurs for particles with radii smaller than a few tens of nanometers, which suggests that nanoparticles and micrometer-sized colloids will follow different routes to crystallization. Finally, we note that the natural statistical nonuniformities in the surface distribution of sticky ends lead to large variations in the binding strength. This phenomenon may compromise the reliability of tests that aim to detect specific DNA targets in diagnostics. In addition to guiding the design of novel self-assembling materials and gene-detection assays, the insights presented here could also shed more light on (multivalent) interactions in other systems with tethered binding groups, for instance in the areas of supramolecular chemistry or ligand-receptor mediated biorecognition.

Towards self-replicating materials of DNA-functionalized colloids

We report the first results of ongoing research that involves the creation of a new class of non-biological materials designed to self-replicate and, as a result, to grow exponentially. We propose a system design that exploits the strong specificity and thermal reversibility of the interactions between colloidal particles functionalized with complementary single-stranded DNA ‘sticky ends’. Here, we experimentally test the fundamentals of the different steps that constitute the self-replication scheme. First of all, we quantitatively study the equilibrium and kinetic aspects of the aggregation–dissociation behavior of the particles. We find that the dissociation transition is very sharp (∼1 °C) and that it occurs at unexpectedly low temperatures, with the dissociation temperature shifting further down when the fraction of sticky ends becomes smaller. The sharpness of the transition and its sensitivity to the sticky end fraction are important control parameters in our self-replication scheme. We further find that for our present purposes it is best to use a DNA construct with a double-stranded backbone, as this largely prevents unwanted hybridization events, such as secondary structure formation. The latter is seen to lead to peculiar aggregation kinetics, due to a competition between inter- and intra-particle hybridization. Finally, we show how one can obtain dual recognition at different temperatures by functionalizing a single particle species with two distinct DNA sequences and we demonstrate the formation of permanent bonds, using the chemical intercalator psoralen and long-wavelength UV exposure.

Gel Formation in Suspensions of Oppositely Charged Colloids: Mechanism and Relation to the Equilibrium Phase Diagram

We study gel formation in a mixture of equally-sized oppositely charged colloids both experimentally and by means of computer simulations. Both the experiments and the simulations show that the mechanism by which a gel is formed from a dilute, homogeneous suspension is an interrupted gas-liquid phase separation. Furthermore, we use Brownian dynamics simulations to study the relation between gel formation and the equilibrium phase diagram. We find that, regardless of the interaction range, an interrupted liquid-gas phase separation is observed as the system is quenched into a state point where the gas-liquid separation is metastable. The structure of the gel formed in our experiments compares well with that of a simulated gel, indicating that gravity has only a minor influence on the local structure of this type of gel. This is supported by the experimental evidence that gels squeezed or stretched by gravity have similar structures, as well as by the fact that gels do not collapse as readily as in the case of colloid-polymer mixtures. Finally, we check whether or not crystallites are formed in the gel branches; we find crystalline domains for the longer ranged interactions and for moderate quenches to the metastable gas-liquid spinodal regime.

Out-of-equilibrium processes in suspensions of oppositely charged colloids: liquid-to-crystal nucleation and gel formation

We study the kinetics of the liquid-to-crystal transformation and of gel formation in colloidal suspensions of oppositely charged particles. We analyse, by means of both computer simulations and experiments, the evolution of a fluid quenched to a state point of the phase diagram where the most stable state is either a homogeneous crystalline solid or a solid phase in contact with a dilute gas. On the one hand, at high temperatures and high packing fractions, close to an ordered-solid/disordered-solid coexistence line, we find that the fluid-to-crystal pathway does not follow the minimum free energy route. On the other hand, a quench to a state point far from the ordered-crystal/disordered-crystal coexistence border is followed by a fluid-to-solid transition through the minimum free energy pathway. At low temperatures and packing fractions we observe that the system undergoes a gas-liquid spinodal decomposition that, at some point, arrests giving rise to a gel-like structure. Both our simulations and experiments suggest that increasing the interaction range favors crystallization over vitrification in gel-like structures.

Controlling the temperature sensitivity of DNA-mediated colloidal interactions through competing linkages

We propose a new strategy to improve the self-assembly properties of DNA-functionalised colloids. The problem that we address is that DNA-functionalised colloids typically crystallize in a narrow temperature window, if at all. The underlying reason is the extreme sensitivity of DNA-mediated interactions to temperature or other physical control parameters. We propose to widen the window for colloidal crystallization by exploiting the competition between DNA linkages with different nucleotide sequences, which results in a temperature-dependent switching of the dominant bond type. Following such a strategy, we can decrease the temperature dependence of DNA-mediated self assembly to make systems that can crystallize in a wider temperature window than is possible with existing systems of DNA functionalised colloids. We report Monte Carlo simulations that show that the proposed strategy can indeed work in practice for real systems and specific, designable DNA sequences. Depending on the length ratio of the different DNA constructs, we find that the bond switching is either energetically driven (equal length or ‘symmetric’ DNA) or controlled by a combinatorial entropy gain (‘asymmetric’ DNA), which results from the large number of possible binding partners for each DNA strand. We provide specific suggestions for the DNA sequences with which these effects can be achieved experimentally.