The synthesis and modification of supramolecular building blocks (polymers, nanoparticles, and proteins) to create functional materials in a "bottom up" self-assembly approach. For example, as shown above, we present a polymer-mediated 'bricks and mortar' strategy for the ordering of nanoparticles into structured assemblies. This methodology allows monolayer-protected gold particles to self-assemble into structured aggregates while thermally controlling the
ir size and morphology. Using 2-nm gold particles as building blocks, we show that spherical aggregates of size 97 ± 17 nm can be produced at 23 °C, and that 0.5–1 µm spherical assemblies with (5–40) ´ 10 5 individual subunits form at -20 °C. Intriguingly, extended networks of 50-nm subunits are formed at 10 °C, illustrating the potential of our approach for the formation of diverse structural motifs such as wires and rods. These findings demonstrate that the assembly process provides control over the resulting aggregates, while the modularity of the 'bricks and mortar' approach allows combinatorial control over the constituents, providing a versatile route to new materials systems. 
Reversible supramolecular interactions of pendant polymers side chains can be used to assemble polymer strands into higher order structures such as vesicles and micelles. Specific molecular recognition processes provide a tool for obtaining molecular-level control over higher-order assembly processes that is complementary to more traditional phase segregation behavior. Combination of complementary diacyldiamidopyridine- and thymine-functionalized polystyrenes in non-polar media results spontaneously in the formation of giant vesicular aggregates (micrograph on left), or Recognition-Induced Polymersomes (RIPs). In contrast to reports of diblock copolymer-based polymersomes that rely upon solvophobicity and phase segregation for self-assembly, these RIPs are formed through highly specific interactions between the complementary pendant sidechains. This assembly process is quite general: the more rigid polynorbornene scaffold also self-assembles in non-polar media to form RIPs (micrograph on right), when derivatized with diacyldiamidopyridine and thymine
Control of particle-particle spacing is a key determinant of optical, electronic, and magnetic properties of nanocomposite materials. We have used poly(amidoamine) (PAMAM) dendrimers to assemble carboxylic acid-functionalized mixed monolayer protected clusters (MMPCs) through acid/base chemistry between the particle and dendrimer. Small angle X-ray scattering was then used to establish average inter-MMPC distances. Five generations of PAMAM dendrimer (0, 1, 2, 4, 6) were investigated, with a monotonic increase in interparticle spacing from 4.1 to 6.1 nm observed with increasing generation.
Polymers containing recognition units designed to participate in specific three-point hydrogen bonding were adsorbed onto modified gold surfaces. Self-assembled monolayers (SAMs) containing complementary recognition units were used to direct the adsorption process. The renewable nature of these recognition unit functionalized surfaces was demonstrated by reversible binding of polymers. Adsorptions onto fresh surfaces, followed by desorption and subsequent readsorption of monoblock and diblock copolymers was investigated.
More recent projects (new concepts and applications):
Controlled assembly of protein-nanoparticle composites through complementary protein surface recognition is demonstrated. Interaction of an unstable protein (chymotrypsin) with a gold nanoparticle results in close interparticle spacing, while a stable protein (cytochrome C) that retains its structure upon binding produces a hybrid material with a larger interparticle distance.
The construction and fixation of ordered nanoparticle arrays allow not only the exploitation of the array's collective physical properties but also the possibility of manipulating discrete aggregates within a larger-scale assembly scheme. A simple approach utilizing block copolymer thin films as templates and coordination chemistry as a mild crosslinking mechanism is described by the image (by Nicholas Fischer) showing terpyridine-functionalized gold nanoparticles organized in stripes on top of a microphase-separated thin film of polystyrene-block-poly(methyl methacrylate) and crosslinked thorough the formation of iron-bisterpyridine complexes.
Recently, we reported the patterning of silicon substrates with thymine and positively charged N-methylpyridinium containing polymers using photolithography and the subsequent orthogonal modification of these surfaces using diaminopyridine-functionalized polystyrene and carboxylate derivatized CdSe/ZnS core-shell nanoparticle through diamidopyridine-thymine three-point hydrogen bonding and pyridinium-carboxylate electrostatic interactions, respectively. This recognition-induced orthogonal self-assembly provides high specificity and selectivity in both sequential and one-step functionalization of surfaces. The extension of this process to other interactions and building blocks, the shrinking of patterned size to nanometer scales and the buildup of complex 3-D functional materials are under investigation now.
We recently reported a method for selective modification of specific domain properties of block copolymer films using guests with dendritic substituents of varying size for obtaining different equilibrium morphologies from a single block copolymer scaffold. As shown in above, in the Thy-Gx mixtures, the morphology changed from lamellar (Thy-G0) to cylindrical (Thy-G1 and Thy-G2) to spherical (Thy-G3). In comparison, mixing with MeThy-Gx induced a slower morphology change: the lamellar morphology persisted through MeThy-G0 and MeThy- G1, and cylinders were observed with Methy-G2 and with MeThy-G3. This study highlights the difference in selectivity in the solid state when specific versus non- specific interactions are employed, and emphasizes the crucial role of specific interactions in achieving controlled, non-covalent polymer functionalization.
Cationic superparamagnetic iron oxide nanoparticles were assembled using a series of anionic polyamidoamine dendrimers. The resulting assemblies featured systematically increasing average interparticle spacing over a 2.4 nm range with increasing dendrimer generation. This increase in spacing modulated the collective magnetic behavior by effective lowering of the dipolar coupling between particles. The results obtained in these studies deviate from the predicted dependence of collective behavior on interparticle spacing, suggesting that a dense assembly of magnetically "free" particles can exist with a surprisingly small space between particles.
Molecular recognition dyad self-assembles in hydrocarbon solvents to form helically stacking structures. Combination of non-covalent bond interactions including specific hydrogen bonding and solvophobic pi-stacking, and absolute configuration of tails dictates the formation of self-assembled structures. The chirality in the tails is crucial to control the chiroptical response of the self-assembled structures. Using (S)-citronellols in combination with the same (S)-configurations in both recognition units provides efficient chiroptical switching through cooperative process. This cooperative translation results enhancement of structural stability as well as switching of chiroptical properties.
