No abstract

The physical characteristics of composite materials are dictated by both the chemical composition and spatial configuration of each constituent phase. A major challenge in nanoparticle-based composites is developing methods to precisely dictate particle positions at the nanometer length scale, as this would allow complete control over nanocomposite structure-property relationships. In this work, we present a new class of building blocks called nanocomposite tectons (NCTs), which consist of inorganic nanoparticles grafted with a dense layer of polymer chains that terminate in molecular recognition units capable of programmed supramolecular bonding. By tuning various design factors, including the particle size and polymer length, we can use the supramolecular interactions between NCTs to controllably alter their assembly behavior, enabling the formation of well-ordered body-centered cubic superlattices consisting of inorganic nanoparticles surrounded by polymer chains. NCTs therefore present a modular platform that enables the construction of composite materials where the composition and three-dimensional arrangement of different constituents within the composite can be independently controlled.

Nanocomposite tectons (NCTs) are a recently developed building block for polymer–nanoparticle composite synthesis, consisting of nanoparticle cores functionalized with dense monolayers of polymer chains that terminate in supramolecular recognition groups capable of linking NCTs into hierarchical structures. In principle, the use of molecular binding to guide particle assembly allows NCTs to be highly modular in design, with independent control over the composition of the particle core and polymer brush. However, a major challenge to realize an array of compositionally and structurally varied NCT-based materials is the development of different supramolecular bonding interactions to control NCT assembly, as well as an understanding of how the organization of multiple supramolecular groups around a nanoparticle scaffold affects their collective binding interactions. Here, we present a suite of rationally designed NCT systems, where multiple types of supramolecular interactions (hydrogen bonding, metal complexation, and dynamic covalent bond formation) are used to tune NCT assembly as a function of multiple external stimuli including temperature, small molecules, pH, and light. Furthermore, the incorporation of multiple orthogonal supramolecular chemistries in a single NCT system makes it possible to dictate the morphologies of the assembled NCTs in a pathway-dependent fashion. Finally, multistimuli responsive NCTs enable the modification of composite properties by postassembly functionalization, where NCTs linked by covalent bonds with significantly enhanced stability are obtained in a fast and efficient manner. The designs presented here therefore provide major advancement for the field of composite synthesis by establishing a framework for synthesizing hierarchically ordered composites capable of complicated assembly behaviors.

Nanoparticle assembly can be controlled by multivalent binding interactions between surface ligands, indicating that more precise control over these interactions is important to design complex nanoscale architectures. It has been well-established in natural materials that the arrangement of different molecular species in three dimensions can affect the ability of individual supramolecular units to coordinate their binding, thereby regulating the strength and specificity of their collective molecular interactions. However, in artificial systems, limited examples exist that quantitatively demonstrate how changes to nanoscale geometry can be used to rationally modulate the thermodynamics of individual molecular binding interactions. As a result, the use of nanoscale design features to regulate molecular bonding remains an underutilized design handle to control nanomaterials synthesis. Here, we demonstrate a polymer-coated nanoparticle material where supramolecular bonding and nanoscale structure are used in conjunction to dictate the thermodynamics of their multivalent interactions, resulting in emergent bundling of supramolecular binding groups that would not be expected if considering the molecular structures alone. Additionally, we show that these emergent phenomena can controllably alter superlattice symmetry by using mesoscale particle arrangement to alter the thermodynamics of supramolecular bonding behavior. The ability to rationally program molecular multivalency via a systems-level approach therefore provides a major step forward in the assembly of complex artificial structures, with implications for future designs of both nanoparticle and supramolecular-based materials.

Assembling superparamagnetic particles into ordered lattices is an attractive means of generating new magnetically responsive materials, and is commonly achieved by tailoring interparticle interactions as a function of the ligand coating. However, the inherent linkage between the collective magnetic behavior of particle arrays and the assembly processes used to generate them complicates efforts to understand and control material synthesis. Here, we use a synergistic combination of a chemical force (hydrogen bonding) and magnetic dipole coupling to assemble polymer-brush coated superparamagnetic iron oxide nanoparticles, where the relative strengths of these interactions can be tuned to reinforce one another and stabilize the resulting superlattice phases. We find that we can precisely control both the dipole−dipole coupling between nanoparticles and the strength of the ligand−ligand interactions by modifying the interparticle spacing through changes to the polymer spacer between the hydrogen bonding groups and the nanoparticles' surface. This results in modulation of the materials' blocking temperature, as well as the stabilization of a unique superlattice phase that only exists when magnetic coupling between particles is present. Using magnetic interactions to affect nanoparticle assembly in conjunction with ligand-mediated interparticle interactions expands the potential for synthesizing predictable and controllable nanoparticle-based magnetic composites.