Stephanie H. Lee

Anisotropic Magnetic Colloids: A Strategy to Form Complex Structures Using Nonspherical Building Blocks

Microscopic magnetic particles dispersed in a solvent—or dipolar colloidal fluids—commonly assemble into chains due to a directional attractive interparticle potential. These chained structures can impart optical anisotropy (i.e., birefringence) to dipolar fluids, and have been demonstrated as effective matrix materials in the rapid separation of DNA using microfluidic electrophoresis. In the special case of magnetorheological (MR) systems, which require external fields to induce dipoles in polarizable colloids, an abrupt microstructural transition from isolated (unpolarized) particles into oriented chains can produce dramatic changes in the viscous behavior. This responsive property makes MR fluids attractive as field-controllable damping fluids in hydraulic valves, shock absorbers, brakes, and so on. Dipolar fluids also display interesting ordering phenomena beyond the 1D case. For instance, nickel-coated glass microspheres were shown to pack onto square, oblique, triangular, and even quasicrystalline lattices with five-fold symmetry in 2D. Binary particle mixtures assembled into ‘‘flower’’-like aggregates and superlattices in a planar configuration. In 3D, dipolar fluids were predicted to form rich mesophases as a function of the dipole moment strength, particle volume fraction, and the relative ‘‘hard’’ or ‘‘soft’’ nature of the colloidal interactions. Examples include the body-centered-tetragonal (bct) and body-centered-orthorhombic (bco) crystals, along with a broad fluid–bct coexistence phase. Yethiraj and van Blaaderen demonstrated these phases experimentally using a system with tunable long-range

Magnetically responsive and hollow colloids from nonspherical core-shell particles of peanut-like shape

Monodisperse peanut-shaped particles having various chemical compositions, architectures (core–shell and hollow) and properties (optical and magnetic) were prepared from hematite templates. The colloids enable shape programming alone, optical manipulation, or magnetic field approaches to assembly.

Synthesis and assembly of nonspherical hollow silica colloids under confinement

Hard peanut-shaped colloids were synthesized and organized into a degenerate crystal (DC), a phase previously observed only in simulations. In this structure, particle lobes tile a triangular lattice while their orientations uniformly populate the three underlying crystalline directions.

Rotator and crystalline films viaself-assembly of short-bond-length colloidal dimers

Nonspherical particles of pear-like and spherocylinder shape were organized into diverse two-dimensional (2D) structures, including the orientationally disordered rotator. Dry films with hexagonal, oblique, and centered rectangular symmetry were obtained by using convective assembly to condense and confine the system in a thin meniscus region. Monte Carlo simulations confirmed the transition from fluid to rotator simply as a function of system density and short-bond-length particle morphology.

Restricted dislocation motion in crystals of colloidal dimer particles.

At high area fractions, monolayers of colloidal dimer particles form a degenerate crystal (DC) structure in which the particle lobes occupy triangular lattice sites while the particles are oriented randomly along any of the three lattice directions. We report that dislocation glide in DCs is blocked by certain particle orientations. The mean number of lattice constants between such obstacles is Z[over](exp)=4.6+/-0.2 in experimentally observed DC grains and Z[over](sim)=6.18+/-0.01 in simulated monocrystalline DCs. Dislocation propagation beyond these obstacles is observed to proceed through dislocation reactions. We estimate that the energetic cost of dislocation pair separation via such reactions in an otherwise defect free DC grows linearly with final separation, hinting that the material properties of DCs may be dramatically different from those of 2-D crystals of spheres.

Asymmetric Colloidal Dimers under Quasi-Two-Dimensional Confinement

The synthesis and assembly of mildly fused asymmetric polystyrene/silica dimers confined to gap heights intermediate to an in-plane monolayer and an out-of-plane monolayer are explored. Using real-space confocal microscopy, we show that structures evolve from an oblique two-dimensional (2D) phase to a quasi-2D rotator, and finally to an upright hexagonally close-packed monolayer. The existence of the novel quasi-2D state, where out-of-plane motion is allowed, highlights the critical role that confinement dimensionality plays on the nature of ordering in complex colloidal systems.

Fluorescent Core-Shell Silica Nanoparticles: An Alternative Radiative Materials Platform

We report on monodisperse fluorescent core-shell silica nanoparticles (C dots) with enhanced brightness and photostability as compared to parent free dye in aqueous solution. Dots containing either tetramethylrhodamine or 7-nitrobenz-2-oxa-1,3-diazole dyes with diameters ranging from tens of nanometers to microns are discussed. The benefits of the core-shell architecture are described in terms of enhanced fluorescent yield of the fluorophores in the quasi-solid-state environment within the particle as compared with parent free dye in water. Several applications of these particles in the fields of photonics and the life sciences are discussed. Specifically, fluorescent core-shell silica nanoparticles are investigated as an active medium for photonic building blocks assembled on zinc sulfide-based seed particles. Initial assembly results for these composite raspberry structures are shown. Finally, applications in the life sciences are explored, including targeting of specific antibody receptors using these single-emission nanoparticles. We expand on single-emission core-shell architecture to incorporate environmentally-sensitive fluorophores to create quantitative ratiometric nanoscale sensors capable of interrogating chemical concentrations on the sub-cellular to molecular levels and demonstrate initial results of intracellular pH imaging. The concept of a single particle laboratory (SPL) is introduced as an active investigator of its environment.