Robert J. Macfarlane

Nanoparticle superlattices enable programmable material structure through ligand-mediated interactions, but attractive ligand interactions typically limit assembly to densely packed lattices that maximize nearest-neighbour bonding. Intentionally programmed directional bonding and valency are therefore typically assumed to require particle-level anisotropy in shape or surface patterning. Here, using assemblies of flexible, electrostatically active isotropic nanoparticles, we show that valency need not be encoded by particle geometry, but can instead emerge spontaneously during assembly. When ligand coronas are sufficiently deformable, ligands reconfigure to align along interparticle interfaces, generating an effective valency. Using a combination of experiment and simulation, we show that increasing ligand deformability lowers lattice coordination numbers, and identify design considerations for assembling isotropic building blocks into lattices with lower coordination, packing density and symmetry than would be predicted from hard-sphere packing. The work presented here therefore establishes ligand flexibility as a design parameter for programming coordination in nanoparticle assembly.

Mucus is a critical component of the human immune system, creating a barrier against pathogenic infections and providing an environment that supports beneficial microbial communities. Mucins, the glycoproteins that form complex gel networks in mucus, leverage complex, multivalent interactions to tune their physicochemical properties, which impact their interaction with both commensal and pathogenic microbes. Because of the diversity of roles that mucus plays in biological systems, mucins themselves are so varied that it is impossible to fully replicate them. This limitation has hindered our ability to study mucosal structure–function–property relationships in vitro, especially as they relate to pathogenesis, and to deploy mucins for therapeutic applications. To overcome these barriers, we have developed a tunable, scalable platform that mimics key physical properties of mucus, supporting the standardization of studies and the application of a mucus-inspired polymer as a drug-delivery vehicle. Inspired by biological mucin synthesis, we report a bottlebrush polymer hydrogel platform capable of mimicking the macromolecular structure and function of these glycoproteins via redox-responsive cross-links and glycan presentation. The mechanical properties of this hydrogel are tunable and can be matched to the viscoelasticity of the mucus from diverse sources. Furthermore, no inherent microbial cytotoxicity is observed, enabling this platform to support in vitro studies of pathogen transport and growth in mucus-like systems. The ease of synthesis, structural control, and tunability afforded by this hydrogel will enable previously inaccessible studies of the mucosal environment and rational design of biosimilar hydrogels to support human health.

Understanding how molecular-scale interactions govern the assembly and processing of nanocomposite tectons (NCTs) is essential for the design of functional nanoparticle superlattices. Here, we present a multiscale simulation framework that connects explicit microscale descriptions of polymer-grafted, supramolecularly functionalized NCTs to mesoscale predictions of superlattice formation and cold sintering. Potentials of mean force (PMFs) extracted from microscale simulations show that the graft length and nanoparticle diameter tune both the depth and range of effective attraction through the interplay of hydrogen bond-mediated association, brush entropy, and core curvature. These interactions predict the emergence of body-centered-cubic order during annealing, with a clear crossover from disordered aggregation to crystallization as the attractions strengthen. Slower annealing and higher concentration both promote larger, more ordered crystallites, whereas an excessive graft length frustrates long-range order. Simulated cold sintering further reveals how compression merges crystalline grains into continuous networks while introducing structural defects. By directly linking NCT chemistry to assembly kinetics and final microstructure, this framework provides mechanistic insights and quantitative guidance for engineering nanoparticle superlattices with tailored order and connectivity.

Nanoparticle superlattices enable programmable material structure through ligand-mediated interactions, but attractive ligand interactions typically limit assembly to densely packed lattices that maximize nearest neighbor bonding. Intentionally programmed directional bonding and valency are therefore typically assumed to require particle-level anisotropy in shape or surface patterning. Using assemblies of flexible isotropic nanoparticles, we show here that valency need not be encoded by particle geometry, but can instead emerge spontaneously during assembly. When ligand coronas are sufficiently deformable, ligands reorganize to align along interparticle interfaces, generating an effective valency. Using a combination of experiment and simulation, we show that increasing ligand deformability lowers lattice coordination numbers, and are able to establish design principles for assembling isotropic building blocks into lattices with lower coordination, packing density, and symmetry than would be predicted from hard-sphere packing. The work presented here therefore establishes ligand flexibility as a design parameter for programming coordination in nanoparticle assembly.

Nanoparticle assembly enables bottom-up synthesis of ordered materials with precise control over their nanoscale structure. However, existing methods typically require either complex ligands or long assembly time scales, meaning that scalability and speed of assembly remain key challenges for the development of functional materials. In this work, we demonstrate that polymer brushes can finely tune the chemical potentials between colloidal particles as a simple function of solvent content. Thus, solvent-induced destabilization of particles presents a rapid and scalable method for assembling nanoparticles that allows ordered superlattices to be obtained in gram-scale quantities in minutes. We systematically elucidate how factors like particle concentration, solvent identity, polymer brush architecture, and nanoparticle size affect the crystal quality and crystallographic symmetry of the assemblies. A computational model is presented that describes how these factors affect the chemical interactions between particles, providing insight into crystallographic phase selection in these systems. Finally, we demonstrate the generalizability of this approach to a variety of nanoparticle compositions, including gold, indium tin oxide, and manganese oxide, enabling the formation of multiple crystal symmetries (e.g., FCC, BCC, smectic liquid crystals). Since our method is compatible with polymers prepared via different synthetic routes and bearing different end-group functionalities─including commercially available polymers─it significantly lowers the technical barrier to producing ordered nanocomposite assemblies. Thus, the approach presented here and the fundamental chemical insight into these particles' assembly behavior provide a pathway toward the large-scale production of ordered hybrid materials from a diverse array of colloidal building blocks.

Significant advances in science and engineering often emerge at the intersections of disciplines. Nanoscience and nanotechnology are inherently interdisciplinary, uniting researchers from chemistry, physics, biology, medicine, materials science, and engineering. This convergence has fostered novel ways of thinking and enabled the development of materials, tools, and technologies that have transformed both basic and applied research, as well as how we address critical societal challenges. In this Nano Focus, we pose and explore 33 questions whose answers could profoundly impact fields such as energy, electronics, the environment, optics, and medicine. These questions highlight the need for deeper foundational understanding, improved tools and techniques, and innovative applications─each with significant societal relevance. Together, they represent a global call-to-action for the scientific community.

Nanocomposites assembled from polymer-grafted plasmonic nanoparticles (PGNs) can combine strong light-matter interactions with soft-matter functionalities and a high degree of translational symmetry. This work explored the potential of gold nanoparticles (16 nm diameter) grafted with polystyrene chains (degree of polymerization, N ≈ 63) as building blocks for acoustoplasmonic metasurfaces. We have decorated inorganic surfaces─crystalline silicon and SiO₂ glass─with PGN monolayers and explored their surface acoustic waves with micro-Brillouin Light Scattering (μ-BLS) at various photon energies. Aided by finite-element-method calculations of acoustic phonons, plasmons and optomechanics, we show that PGNs sustain coupled sphere modes with rattling, torsional, and quadrupolar features. The coupled sphere modes exhibit plasmon-enhanced BLS and form a wide acoustic band gap below the line of sound. In the long wavelength limit, the coupling to the substrate leads to the emergence of shear-horizontal and Sezawa waves, whose dispersion relationships yield the local scale elasticity of ultrathin PGN monolayers.

Vat photopolymerization (VP) additive manufacturing (AM) is valued for its speed, precision, and material versatility. However, its requirement for support structures limits printable geometries, complicates post‐processing, and generates non‐recyclable waste when typical thermoset resins are used. Here, a wavelength‐selective resin system for VP that enables single‐vat, multi‐material printing with dissolvable supports is introduced. Exposure to visible light produces a rigid, dissolvable thermoplastic, while UV light forms a crosslinked thermoset resistant to dissolution. This process, termed selective solubility vat photopolymerization (SSVP), eliminates the geometric constraints imposed by conventional VP methods, facilitating the creation of complex objects with supports that are removable using green and food‐safe solvents such as D‐limonene and ethyl acetate, as well as mineral oil. Post‐print heat treatment tunes crosslink density and solubility. Dissolved supports can be recycled into fresh resin and reprinted without mechanical property loss, offering a practical, scalable route to reducing waste. Additionally, SSVP provides spatial control of dissolution kinetics, enabling programmable 3D dissolution profiles. By enabling the integration of dissolvable and insoluble regions in a single print, SSVP sets the stage for fully automated and more sustainable AM workflows.

Nanoparticle superlattices present transformative opportunities for material design by enabling precise control over both nanoscale organization and composition; however, translating these assemblies into macroscopic constructs while preserving nanoscale order remains a critical challenge due to the incompatibility of traditional processing techniques with colloidal systems. This study introduces "colloidal metallurgy," a framework for understanding and controlling defect evolution and densification in nanoparticle superlattices during colloidal sintering. We investigate the effects of pressure and temperature to elucidate mechanisms of particle transport, defect annealing, and densification as single-crystal colloidal assemblies coalesce into polycrystalline superlattices. Pressure-driven crystallite fracture is identified as the primary mode of densification, while temperature enhances particle mobility, enabling defect reduction and grain growth. A multistage sintering strategy employing high temperature annealing to grow grains and restore fracture-based capacity for densification was developed to produce dense (∼1% porosity) polycrystals with low defect counts, demonstrating a pathway for processing nanoparticle superlattices. By exploring the parallels and distinctions between atomic and colloidal sintering, this work establishes critical insights into the mechanisms governing colloidal material processing. These findings lay the groundwork for defect engineering in colloidal systems, offering a scalable approach to design macroscopic materials with tailored properties.