2D Colloidal Crystals with Anisotropic Impurities.

We report a study of 2D colloidal crystals with anisotropic ellipsoid impurities using video microscopy. It is found that at low impurity densities, the impurity particles behave like floating disorder with which the quasi-long-range orientational order survives and the elasticity of the system is actually enhanced. There is a critical impurity density above which the 2D crystal loses the quasi-long-range orientational order. At high impurity densities, the 2D crystal breaks into polycrystalline domains separated by grain boundaries where the impurity particles aggregate. This transition is accompanied by a decrease in the elastic moduli, and it is associated with strong heterogeneous dynamics in the system. The correlation length vs impurity density in the disordered phase exhibits an essential singularity at the critical impurity density.

2D isotropic-nematic transition in colloidal suspensions of ellipsoids.

Liquid crystals are important condensed matter systems for technological applications, as well as for fundamental studies. An important unresolved issue is the nature of the phase transition in a two-dimensional (2D) liquid crystal system. In contrast to numerous computational studies reported in the last few decades, there have been no convincing experiments to verify these numerical results. Anisotropic colloids provide an excellent experimental model system to study phase transitions, such as crystallization and glass transition in condensed matter physics with single particle resolution. However, using colloids to probe the two-dimensional liquid crystal transition remains a challenge, since the condensed anisotropic colloids usually become stuck in the metastable glassy state rather than approaching their equilibrium liquid crystal phase. Here we report a method of using an external magnetic field to assist a colloidal system of super-paramagnetic anisotropic particles to overcome the local free energy barriers in the metastable states and approach the equilibrium phase. The experiments demonstrate a 2D isotropic-nematic phase transition with increasing packing density. The effects of the anisotropy of the colloidal particles on the 2D isotropic-nematic transition are explored. Our experimental results are compared with those from previous computational work, and quantitative agreements are reached.

Dynamical processes of interstitial diffusion in a two-dimensional colloidal crystal.

In two-dimensional (2D) solids, point defects, i.e., vacancies and interstitials, are bound states of topological defects of edge dislocations and disclinations. They are expected to play an important role in the thermodynamics of the system. Yet very little is known about the detailed dynamical processes of these defects. Two-dimensional colloidal crystals of submicrometer microspheres provide a convenient model solid system in which the microscopic dynamics of these defects can be studied in real time using video microscopy. Here we report a study of the dynamical processes of interstitials in a 2D colloidal crystal. The diffusion constants of both mono- and diinterstitials are measured and found to be significantly larger than those of vacancies. Diinterstitials are clearly slower than monointerstitials. We found that, by plotting the accumulative positions of five- and sevenfold disclinations relative to the center-of-mass position of the defect, a sixfold symmetric pattern emerges for monointerstitials. This is indicative of an equilibrium behavior that satisfies local detailed balance that the lattice remains elastic and can be thermally excited between lattice configurations reversibly. However, for diinterstitials the sixfold symmetry is not observed in the same time window, and the local lattice distortions are too severe to recover quickly. This observation suggests a possible route to creating local melting of a lattice (similarly one can create local melting by creating divacancies). This work opens up an avenue for microscopic studies of the dynamics of melting in colloidal model systems.

Rapid fabrication of solid-state nanopores with high reproducibility over a large area using a helium ion microscope.

The fabrication of solid-state nanopores in an insulating membrane has attracted much attention for biomolecule analysis such as DNA sequencing and detection in recent years. For practical applications and device integration, the challenges include precise size control for sub 10 nm nanopores, excellent repeatability and rapid fabrication over a large area to reduce the cost for mass production. A helium ion beam could provide an effective fabrication approach to produce such solid-state nanopores. It is easy to control the nanopore size and reach sub 10 nm pore size with a simple change in the milling time with an appropriate ion beam current. Here we report new results in a set of experiments demonstrating that with a small range of stage automatized motions and equal mill times one can obtain good fabrication reproducibility in nanopore sizes (<10% variation in size). The automation in the stage motion and milling time opens a door for the rapid mass production of nanopore chips over a wafer size of several inches.

On the distribution of DNA translocation times in solid-state nanopores: an analysis using Schrödinger's first-passage-time theory.

In this short paper, a correction is made to the recently proposed solution of Li and Talaga to a 1D biased diffusion model for linear DNA translocation, and a new analysis will be given to their data. It was pointed out by us recently that this 1D linear translocation model is equivalent to the one that was considered by Schrödinger for the Ehrenhaft­Millikan measurements on electron charge. Here, we apply Schrödinger's first-passage-time distribution formula to the data set in Li and Talaga. It is found that Schrödinger's formula can be used to describe the time distribution of DNA translocation in solid-state nanopores. These fittings yield two useful parameters: the drift velocity of DNA translocation and the diffusion constant of DNA inside the nanopore. The results suggest two regimes of DNA translocation: (I) at low voltages, there are clear deviations from Smoluchowski's linear law of electrophoresis, which we attribute to the entropic barrier effects; (II) at high voltages, the translocation velocity is a linear function of the applied electric field. In regime II, the apparent diffusion constant exhibits a quadratic dependence on the applied electric field, suggesting a mechanism of Taylor-dispersion effect likely due the electro-osmotic flow field in the nanopore channel. This analysis yields a dispersion-free diffusion constant value of 11.2 nm2 µs-1 for the segment of DNA inside the nanopore, which is in quantitative agreement with the Stokes­Einstein theory. The implication of Schrödinger's formula for DNA sequencing is discussed.

Detection of DNA hybridizations using solid-state nanopores.

We report an experimental study of using DNA translocation through solid-state nanopores to detect the sequential arrangement of two double-stranded 12-mer hybridization segments on a single-stranded DNA molecule. The sample DNA is a trimer molecule formed by hybridizing three single-stranded oligonucleotides. A polystyrene bead is attached to the end of the trimer DNA, providing a mechanism in slowing down the translocation and suppressing the thermal diffusion, thereby allowing the detection of short features of DNA by standard patch-clamp electronics. The electrical signature of the translocation of a trimer molecule through a nanopore has been identified successfully in the temporal traces of ionic current. The results reported here represent the first successful attempt in using a solid-state nanopore as an ionic scanning device in resolving individual hybridization segments (or 'probes') on a DNA molecule.

Reverse DNA translocation through a solid-state nanopore by magnetic tweezers.

Voltage-driven DNA translocation through nanopores has attracted wide interest for many potential applications in molecular biology and biotechnology. However, it is intrinsically difficult to control the DNA motion in standard DNA translocation processes in which a strong electric field is required in drawing DNA into the pore, but it also leads to uncontrollable fast DNA translocation. Here we explore a new type of DNA translocation. We dub it 'reverse DNA translocation', in which the DNA is pulled through a nanopore mechanically by a magnetic bead, driven by a magnetic-field gradient. This technique is compatible with simultaneous ionic current measurements and is suitable for multiple nanopores, paving the way for large scale applications. We report the first experiment of reverse DNA translocation through a solid-state nanopore using magnetic tweezers.

The potential and challenges of nanopore sequencing.

A nanopore-based device provides single-molecule detection and analytical capabilities that are achieved by electrophoretically driving molecules in solution through a nano-scale pore. The nanopore provides a highly confined space within which single nucleic acid polymers can be analyzed at high throughput by one of a variety of means, and the perfect processivity that can be enforced in a narrow pore ensures that the native order of the nucleobases in a polynucleotide is reflected in the sequence of signals that is detected. Kilobase length polymers (single-stranded genomic DNA or RNA) or small molecules (e.g., nucleosides) can be identified and characterized without amplification or labeling, a unique analytical capability that makes inexpensive, rapid DNA sequencing a possibility. Further research and development to overcome current challenges to nanopore identification of each successive nucleotide in a DNA strand offers the prospect of 'third generation' instruments that will sequence a diploid mammalian genome for approximately $1,000 in approximately 24 h.

Statics and dynamics of 2D colloidal crystals in a random pinning potential.

We report the first experimental study of a model system of a two-dimensional colloidal crystal in a random pinning potential. The colloidal crystal consists of monodispersed charged polystyrene microspheres suspended in deionized aqueous media and confined near a rough charged surface. It is found that the static orientational correlation function g6(r) decays exponentially for intermediate and strong pinning, in agreement with theories. The driven depinning is dominated by thermally activated creep motion along 1D-like channels between regions with short-range order. A coexistence model is proposed for describing the observations.

Lithography-free formation of nanopores in plastic membranes using laser heating.

Synthetic nanopores are a new class of single-molecule sensors capable of electronically detecting, counting, and characterizing biomolecules. There have been studies of nanopore formation in solid-state materials. This paper reports a novel lithography-free method of nanopore formation in plastic membranes fluidized using laser heating. It was found that the pore shrinking dynamics follows a universal behavior with the diameter of a pore decreasing linearly with time similar to that found in fluidized SiO(2). A theoretical model based on a surface-tension-driven mass flow mechanism is proposed to successfully explain the observed universality in the pore shrinking dynamics. We demonstrate the potential of this lithography-free nanofabrication technique in biomolecular sensing with a lambda-DNA detection experiment.