David J. Pine

Ordered macroporous materials by emulsion templating

Ordered macroporous materials with pore diameters comparable to optical wavelengths are predicted to have unique and highly useful optical properties such as photonic bandgaps and optical stop-bands. Tight control over the pore size distribution might also lead to improved macroporous materials (those with pores greater than approximately 50 nm) for application as catalytic surfaces and supports, adsorbents, chromatographic materials, filters, light-weight structural materials, and thermal, acoustic and electrical insulators. Although methods exist for producing ordered porous materials with pore diameters less than 10 nm (refs 10, 11), there is no general method for producing such materials with uniform pore sizes at larger length scales. Here we report a new method for producing highly monodisperse macroporous materials with pore sizes ranging from 50 nm to several micrometres. Starting with an emulsion of equally sized droplets (produced through a repeated fractionation procedure), we form macroporous materials of titania, silica and zirconia by using the emulsion droplets as templates around which material is deposited through a sol–gel process. Subsequent drying and heat treatment yields solid materials with spherical pores left behind by the emulsion droplets. These pores are highly ordered, reflecting the self-assembly of the original monodisperse emulsion droplets into a nearly crystalline array. We show that the pore size can be accurately controlled, and that the technique should be applicable to a wide variety of metal oxides and even organic polymer gels.

Rapidly recovering hydrogel scaffolds from self-assembling diblock copolypeptide amphiphiles

Protein-based hydrogels are used for many applications, ranging from food and cosmetic thickeners to support matrices for drug delivery and tissue replacement. These materials are usually prepared using proteins extracted from natural sources, which can give rise to inconsistent properties unsuitable for medical applications. Recent developments have utilized recombinant DNA methods to prepare artificial protein hydrogels with specific association mechanisms and responsiveness to various stimuli. Here we synthesize diblock copolypeptide amphiphiles containing charged and hydrophobic segments. Dilute solutions of these copolypeptides would be expected to form micelles; instead, they form hydrogels that retain their mechanical strength up to temperatures of about 90 °C and recover rapidly after stress. The use of synthetic materials permits adjustment of copolymer chain length and composition, which we varied to study their effect on hydrogel formation and properties. We find that gelation depends not only on the amphiphilic nature of the polypeptides, but also on chain conformations—α-helix, β-strand or random coil. Indeed, shape-specific supramolecular assembly is integral to the gelation process, and provides a new class of peptide-based hydrogels with potential for applications in biotechnology.

Colloids with valence and specific directional bonding

The ability to design and assemble three-dimensional structures from colloidal particles is limited by the absence of specific directional bonds. As a result, complex or low-coordination structures, common in atomic and molecular systems, are rare in the colloidal domain. Here we demonstrate a general method for creating the colloidal analogues of atoms with valence: colloidal particles with chemically distinct surface patches that imitate hybridized atomic orbitals, including sp, sp2, sp3, sp3d, sp3d2 and sp3d3. Functionalized with DNA with single-stranded sticky ends, patches on different particles can form highly directional bonds through programmable, specific and reversible DNA hybridization. These features allow the particles to self-assemble into ‘colloidal molecules’ with triangular, tetrahedral and other bonding symmetries, and should also give access to a rich variety of new microstructured colloidal materials.

Lock and key colloids

New functional materials can in principle be created using colloids that self-assemble into a desired structure by means of a programmable recognition and binding scheme. This idea has been explored by attaching ‘programmed’ DNA strands to nanometre- and micrometre- sized particles and then using DNA hybridization to direct the placement of the particles in the final assembly. Here we demonstrate an alternative recognition mechanism for directing the assembly of composite structures, based on particles with complementary shapes. Our system, which uses Fischer’s lock-and-key principle, employs colloidal spheres as keys and monodisperse colloidal particles with a spherical cavity as locks that bind spontaneously and reversibly via the depletion interaction. The lock-and-key binding is specific because it is controlled by how closely the size of a spherical colloidal key particle matches the radius of the spherical cavity of the lock particle. The strength of the binding can be further tuned by adjusting the solution composition or temperature. The composite assemblies have the unique feature of having flexible bonds, allowing us to produce flexible dimeric, trimeric and tetrameric colloidal molecules as well as more complex colloidal polymers. We expect that this lock-and-key recognition mechanism will find wider use as a means of programming and directing colloidal self-assembly.

Chiral colloidal clusters

Chirality is an important element of biology, chemistry and physics. Once symmetry is broken and a handedness is established, biochemical pathways are set. In DNA, the double helix arises from the existence of two competing length scales, one set by the distance between monomers in the sugar backbone, and the other set by the stacking of the base pairs. Here we use a colloidal system to explore a simple forcing route to chiral structures. To do so we have designed magnetic colloids that, depending on both their shape and induced magnetization, self-assemble with controlled helicity. We model the two length scales with asymmetric colloidal dumbbells linked by a magnetic belt at their waist. In the presence of a magnetic field the belts assemble into a chain and the steric constraints imposed by the asymmetric spheres force the chain to coil. We show that if the size ratio between the spheres is large enough, a single helicity is adopted, right or left. The realization of chiral colloidal clusters opens up a new link between colloidal science and chemistry. These colloidal clusters may also find use as mesopolymers, as optical and light-activated structures, and as models for enantiomeric separation.

A photoluminescence study of poly(phenylene vinylene) derivatives: The effect of intrinsic persistence length

We report the results of light scattering, absorption, excitation, and emission spectroscopy of three polyphenylene vinylene (PPV) derivatives; poly[2‐methoxy, 5‐(2’‐ethyl‐hexyloxy‐p‐phenylene‐ vinylene] (MEH‐PPV), poly[2‐butoxy, 5‐(2’‐ethyl‐hexyloxy‐p‐phenylene‐vinylene] (BEH‐PPV), and poly[2‐dicholestanoxy‐p‐phenylene‐vinylene] (BCHA‐PPV) in solution with p‐xylene. We find that increasing the size of the solubilizing side chains increases the intrinsic persistence length of the polyphenylene vinylene backbone and that this change in stiffness has dramatic effects on the photoluminescence of polyphenylene vinylene. We have determined the luminescence quantum efficiencies of the polyphenylene vinylene derivatives relative to a known standard, Rhodamine 6G, and find that the photoluminescence can be greatly enhanced by increasing the intrinsic stiffness of the polymer backbone. The stiffest polymer, poly[2‐dicholestanoxy‐p‐phenylene‐vinylene] (BCHA‐PPV), has a quantum efficiency of 0.66±0.05. The quantum e...

Ordered Macroporous Materials by Colloidal Assembly: A Possible Route to Photonic Bandgap Materials

that Me-LPPP, in addition to MEH-PPV, is hole-limited with a Ca cathode. For conjugated polymer films, such as PAni, the improvement in quantum efficiency is due to an increase in the anode work function to 5.1 ± 0.1 eV, which results in a nearly ohmic contact. For nanoparticle mono-layers, the improvement is due to an increase in the local electric field across the interface. This accelerating local electric field is induced by a net negative charge on the nanoparticle surface which results either from silicon hydroxyl groups on the SiO 2 surface or from electrons which are trapped at the interface between the conjugated polymer and nanoparticle. In conclusion, we have shown that modification of the ITO electrode with SiO 2 nanoparticles can dramatically improve electroluminescence properties of polymer light-emitting devices (PLED). The charged nanoparticle surface , which serves as a carrier trap at low current densities, can induce a dipole moment across the electrode interface, effectively increasing the local electric field and promoting carrier injection. This effect enables the ability to improve PLED efficiency with a single monolayer without including additional polymer layers or modifying the electrode work function. Understanding the nature of the nanoparticle surface will clearly be critical to controlling and optimizing the performance of polymer/nanoparticle composite materials , offering further promise for innovative optoelectronic applications. Ordered macroporous materials with pore sizes in the sub-micrometer range have elicited much interest recently because of their applications in separations processes, ca-talysis, low dielectric constant materials, and lightweight structural materials. Macroporous oxides such as silica, tita-nia, and zirconia as well as polymers such as polyacryl-amide and polyurethane with well-defined pore sizes in the sub-micrometer regime have been successfully synthesized. [1±8] Apart from uses in structural and catalytic materials , the length scales of the pores confer these materials with unique optical properties. For instance, ordered macropores in a high refractive index matrix such as titania can be used to make photonic crystals with a photonic bandgap (PBG). [9] Applications of PBG materials include omnidirectional mirrors, waveguides, and suppression or enhancement of spontaneous emission. The key to making PBG materials is the requirement to make an ordered dielectric lattice of materials with a high refractive index contrast, n 2 /n 1 , where n 2 and n 1 refer to the larger and smaller refractive indices in the structure. The minimum contrast required depends on the lattice type and varies from …

Multiple Light-Scattering Probes of Foam Structure and Dynamics

The structure and dynamics of three-dimensional foams are probed quantitatively by exploiting the strong multiple scattering of light that gives foams their familiar white color. Approximating the propagation of light as a diffusion process, transmission measurements provide a direct probe of the average bubble size. A model for dynamic light scattering is developed that can be used to interpret temporal fluctuations in the intensity of multiply scattered light. The results identify previously unrecognized internal dynamics of the foam bubbles. These light-scattering techniques are direct, noninvasive probes of bulk foams and therefore should find wide use in the study of their properties.

Cubic crystals from cubic colloids

We have studied the crystallization behavior of colloidal cubes by means of tunable depletion interactions. The colloidal system consists of novel micron-sized cubic particles prepared by silica deposition on hematite templates and various non-adsorbing watersoluble polymers as depletion agents. We have found that under certain conditions the cubes self-organize into crystals with a simple cubic symmetry, which is set by the size of the depletant. The dynamic of crystal nucleation and growth is investigated, monitoring the samples in time by optical microscopy. Furthermore, by using temperature sensitive microgel particles as depletant it is possible to fine tune depletion interactions to induce crystal melting. Assisting crystallization with an alternating electric field improves the uniformity of the cubic pattern allowing the preparation of macroscopic (almost defect-free) crystals that show visible Bragg colors.

Shaping colloids for self-assembly

The creation of a new material often starts from the design of its constituent building blocks at a smaller scale. From macromolecules to colloidal architectures, to granular systems, the interactions between basic units of matter can dictate the macroscopic behaviour of the resulting engineered material and even regulate its genesis. Information can be imparted to the building units by altering their physical and chemical properties. In particular, the shape of building blocks has a fundamental role at the colloidal scale, as it can govern the self-organization of particles into hierarchical structures and ultimately into the desired material. Herein we report a simple and general approach to generate an entire zoo of new anisotropic colloids. Our method is based on a controlled deformation of multiphase colloidal particles that can be selectively liquified, polymerized, dissolved and functionalized in bulk. We further demonstrate control over the particle functionalization and coating by realizing patchy and Janus colloids.