Colloidal Flying Carpets

DNA plays a special role in polymer science not just because of the highly selective recognition of complementary single DNA strands but also because bacteria can express DNA chains that are very long yet perfectly monodisperse. The latter reason makes long DNA molecules widely used as model systems in polymer science. Here, we report the unusual self-assembly that takes place in systems of colloids coated with very long double-stranded DNA. In particular, we find that colloids coated with such long DNA can assemble into unique floating crystalline monolayers. Floating colloidal structures have potentially interesting applications as such ordered structures can be assembled in one location and then deposited somewhere else. This would open the way to the assembly of multi-component, layered colloidal crystals.Comment: submitted to PR

DNA-functionalized colloids: Physical properties and applications

The specificity and reversibility of the hydrogen-bonding between two complementary strands in DNA make this bio-molecule a unique binding agent. When DNA is grafted to nano-and micrometer sized colloids it can lead to specific binding between particles coated with complementary strands of single-stranded DNA. DNA-coated colloids hold great promise as the building blocks of a new generation of complex, self-assembling colloidal materials. This brief review sketches the recent developments and present status of the research on DNA-coated colloids with special emphasis on their role as potential building blocks in complex, self-assembling materials and as highly sensitive bio-sensors. Although the present review cannot be comprehensive, it hopefully highlights the promise of DNA-coated colloids as versatile and still largely unexplored form of soft matter

Direct observation of size fractionation during colloidal crystallization

We present a confocal microscopy study of the quasi-two-dimensional crystallization of a binary mixture of spherical colloids coated with long DNA strands. Our experiments show that in the crystalline phase the two colloidal species are completely demixed. Analysis of the lattice spacings in the two types of colloidal crystal shows that the diameters of the two species of colloids differ by 10%. We argue that the demixing in the crystalline phase is due to size segregation during crystallization. This phenomenon had been predicted in several theoretical studies. To our knowledge, the present study provides the first 'real-space' experimental confirmation of this effect

DNA-Mediated Two-Dimensional Colloidal Crystallization above Different Attractive Surfaces

We explore the formation of “floating” two-dimensional colloidal crystals above weakly attractive surfaces that are either positively or negatively charged. In particular, we studied crystal formation above positively charged poly-l-lysine−poly(ethylene glycol) surfaces with and without short single-stranded DNA and above negatively charged bovine albumin serum−streptavidin multilayers. Confocal microscopy revealed the evolution of crystals several micrometers above all three surfaces. Interestingly, the “flying height” of crystals was found to depend on the surface coating. All crystalline structures remained remarkably stable over weeks, even under high salt conditions. Neither lifting the crystals nor lowering them by means of buoyancy forces destroyed them

Molecular cooking: physical transformations in Chinese ‘century’ eggs

Over two thousand years ago the Chinese developed a method to preserve eggs such that they remain edible for many months. The room temperature, physico-chemical preservation process that is used to prepare century eggs transforms the egg white into a yellow, transparent gel with optical and mechanical properties that are very different to those of the familiar white protein aggregate that forms upon boiling a raw egg. Here we show that boiled egg white gels can be further transformed into an elastic and transparent gel using the Chinese preservation method. We demonstrate that the resulting protein gel is made of fine-stranded globular assemblies of partially denatured protein, and resembles the aggregates formed by colloidal particles interacting through long-range electrostatic repulsion combined with short-range attraction. These gels are not only highly deformable but are also very stable, maintaining their structure even when boiled. We suggest that the mechanism responsible for gelation in century eggs illustrates a non-specific aggregation pathway available to globular proteins

Molecular cooking: physical transformations in Chinese ‘century’ eggs

Over two thousand years ago the Chinese developed a method to preserve eggs such that they remain edible for many months. The room temperature, physico-chemical preservation process that is used to prepare century eggs transforms the egg white into a yellow, transparent gel with optical and mechanical properties that are very different to those of the familiar white protein aggregate that forms upon boiling a raw egg. Here we show that boiled egg white gels can be further transformed into an elastic and transparent gel using the Chinese preservation method. We demonstrate that the resulting protein gel is made of fine-stranded globular assemblies of partially denatured protein, and resembles the aggregates formed by colloidal particles interacting through long-range electrostatic repulsion combined with short-range attraction. These gels are not only highly deformable but are also very stable, maintaining their structure even when boiled. We suggest that the mechanism responsible for gelation in century eggs illustrates a non-specific aggregation pathway available to globular proteins

Using DNA-Driven Assembled Phospholipid Nanodiscs as a Scaffold for Gold Nanoparticle Patterning

Recently, a new class of materials emerged with the assembly of DNA-coated phospholipid nanodiscs into columnar BioNanoStacks. Within these stacks, lipid discs are periodically incorporated, resulting into quasi-one-dimensional superstructures. With each disc surrounded by two recombinant scaffolding proteins, we decided to examine whether the polyhistidine tags of these proteins could be utilized to bind additional molecules or particles to these BioNanoStacks. Here we demonstrate that patterning of gold nanoparticles onto these BioNanoStacks is indeed possible. Binding occurs via a nickel-mediated interaction between the nanogolds nitrilotriacetic acid and the histidine tags of the scaffold proteins surrounding the nanodiscs. Using Monte Carlo simulations, we determine that the binding of the nanogold particles to the stacks is not a random event. By comparing the simulation and experimental results, we find that there are preferred binding sites, which affects the binding statistics