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.     

Concentrating colloids with electric field gradients. II. Phase transitions and crystal buckling of long-ranged repulsive charged spheres in an electric bottle

We explored the usefulness of electric field gradients for the manipulation of the particle concentration in suspensions of charged colloids, which have long-ranged repulsive interactions. In particular, we studied the compression obtained by "negative" dielectrophoresis, which drives the particles to the regions of lowest field strength, thus preventing unwanted structural changes by induced dipole-dipole interactions. We used several sample cell layouts and suspension compositions, with a different range of the interparticle repulsions. For these systems, we obtained sufficient compression to observe a transition from the initial fluid phase to a random hexagonal close-packed crystal, as well as a body-centered cubic crystal. The heterogeneous dielectrophoretic crystallization mechanism involved an intriguing "pluglike" motion of the crystal, similar to what we have previously reported for hard-sphere suspensions. In this way, remarkably large single crystals were formed of several millimeters wide and a couple of centimeters long. Moreover, we found that these crystals could be compressed to such an extent that it led to an anisotropic deformation ("buckling") and, upon subsequent relaxation, a reorientation of the lattice, while stacking errors disappeared. These striking differences with the compressed hard-sphere crystals that we studied before [M. E. Leunissen et al., J. Chem. Phys. 128, 164508 (2008).] are likely due to the smaller elastic moduli of the present lower-density soft-sphere crystals.     

Concentrating colloids with electric field gradients. I. Particle transport and growth mechanism of hard-sphere-like crystals in an electric bottle

This work concerns the use of electric field gradients to manipulate the local particle concentration in a hard-sphere-like suspension. Inside a specially designed "electric bottle," we observed our colloids to collect in the regions of lowest field strength ("negative dielectrophoresis"). This allows for the use of larger field gradients and stronger dielectrophoretic forces than in the original electric bottle design, which was based on positive dielectrophoresis [M. T. Sullivan et al., Phys. Rev. Lett. 96, 015703 (2006)]. We used confocal scanning laser microscopy to quantitatively follow the time-dependent change in the particle density and the suspension structure. Within a few days, the dielectrophoretic compression was seen to initiate a heterogeneouslike growth of large single crystals, which took place far out-of-equilibrium. The crystals had a random hexagonal close-packed structure and displayed an intriguing growth mechanism, during which the entire crystal was continuously transported, while growing both on the "high-field" and the "low-field" sides, although at different rates. After switching off the electric field, the compressed crystals were found to relax to a lower packing fraction and melt, at a much slower rate than the crystal growth. Besides revealing the particular (far out-of-equilibrium) crystal growth mechanism in these electric bottles, our observations also shed light on the role of the different particle transport processes in the cell and some of the relevant tuning parameters. This is useful for different types of experiments, for instance, focusing more on melting, homogeneous crystallization, or the glass transition.     

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.     

CuAu structure in the restricted primitive model and oppositely charged colloids

We study the phase behavior of oppositely charged equal-size hard spheres both theoretically and experimentally, using Monte Carlo simulations and confocal microscopy. In the simulations, two systems are considered: the restricted primitive model (RPM) and a system of screened Coulomb particles. We construct the phase diagrams of both systems by computer simulations and predict a novel solid phase that has the CuAu structure. In addition, the CuAu structure is observed experimentally in a system of oppositely charged colloids. The qualitative agreement between the RPM, the screened Coulomb system, and the experiments shows that colloids form a suitable model system to study phase behavior in ionic systems.     

Magnetization reversal in exchange biased Co/CoO patterns

The influence of patterning on exchange bias has been investigated using arrays of micron-sized Co/CoO dots with different lateral confinement and length-to-width ratio. The patterned samples show higher coercive and exchange bias fields than a continuous Co/CoO bilayer. As in unpatterned film, magnetization reversal mechanisms on opposite sides of the hysteresis loops of the microstructured samples are different. However, with the increase of lateral confinement and shape anisotropy of the dots, the asymmetry in the magnetization reversal starts to differ from that observed in continuous Co/CoO films.     

Ion partitioning at the oil-water interface as a source of tunable electrostatic effects in emulsions with colloids

We present a combined experimental and theoretical investigation of the surprisingly strong electrostatic effects that can occur in mixtures of low- and high-polar liquids (e.g. oil-water emulsions), here in the presence of colloidal particles. For our experiments, we used confocal microscopy imaging, supplemented with electrophoresis and conductivity measurements. Theoretically, we studied our systems by means of a modified Poisson-Boltzmann theory, which takes into account image charge effects and the electrostatic self-energies of the micro-ions in the different dielectric media. Our results show that the unequal partitioning of micro-ions between the two liquid phases is the common driving force behind most of the observed electrostatic effects. The structural signatures of these effects typically develop on a time scale of hours to days and are qualitatively well-described by our theory. We demonstrate how the partitioning process and its associated phenomena can be controlled by shifting the balance of the interlocked ionic dissociation and partitioning equilibria. Moreover, we present strong experimental proof that the two-dimensional colloidal crystals at the oil-water interface are due to long-ranged Coulombic repulsion through the oil phase. The acquired insight in the role of electrostatics in oil-water emulsions is important for understanding the interactions in particle-stabilized ('Pickering') and charge-stabilized emulsions, emulsion production, encapsulation and self-assembly.     

The influence of finite size and shape anisotropy on exchange bias: A study of patterned Co/CoO nanostructures

The influence of finite dimensions on the exchange bias effect in patterned polycrystalline Co/CoO ferromagnet/antiferromagnet exchange bias systems was studied. Magnetization measurements on the smallest structures reveal that the exchange bias shift increases as the structure size becomes smaller. Off-specular neutron scattering experiments were used to study the asymmetric magnetization reversal behaviour.     

Magnetization and polarized neutron reflectivity experiments on patterned exchange bias structures

Patterned Co/CoO thin film structures have been investigated by magnetization and polarized neutron reflectivity measurements in order to study the influence of finite size and shape anisotropy effects on the magnetization reversal. An anomaly was found in the upper branch of the hysteresis loops, probably caused by incomplete bias in the patterned structures. The asymmetry in magnetization reversal mechanism commonly found in the two branches of the hysteresis loops of unpatterned Co/CoO layers is altered in the patterned structures, consistent with the existence of interfacial domains.     

Enhanced asymmetric magnetization reversal in nanoscale Co/CoO arrays: competition between exchange bias and magnetostatic coupling

Magnetization reversal was studied in square arrays of square Co/CoO dots with lateral size varying between 200 and 900 nm. While reference nonpatterned Co/CoO films show the typical shift and increased width of the hysteresis loop due to exchange bias, the patterned samples reveal a pronounced size dependence. In particular, an anomaly appears in the upper branch of the magnetization cycle and becomes stronger as the dot size decreases. This anomaly, which is absent at room temperature in the patterned samples, can be understood in terms of a competition between magnetostatic interdot interaction and exchange anisotropy during the magnetic switching process.