Nanoparticle Superlattices with Nonequilibrium Crystal Shapes.

Nanoparticle assembly is a material synthesis strategy that enables precise control of nanoscale structural features. Concepts from traditional crystal growth research have been tremendously useful in predicting and programming the unit cell symmetries of these assemblies, as their thermodynamically favored structures are often identical to atomic crystal analogues. However, these analogies have not yielded similar levels of influence in programming crystallite shapes, which are a consequence of both the thermodynamics and kinetics of crystal growth. Here, we demonstrate kinetic control of the colloidal crystal shape using nanoparticle building blocks that rapidly assemble over a broad range of concentrations, thereby producing well-defined crystal habits with symmetrically oriented dendritic protrusions and providing insight into the crystals' morphological evolution. Counterintuitively, these nonequilibrium crystal shapes actually become more common for colloidal crystals synthesized closer to equilibrium growth conditions. This deviation from typical crystal growth processes observed in atomic or molecular crystals is shown to be a function of the drastically different time scales of atomic and colloidal mass transport. Moreover, the particles are spherical with isotropic ligand grafts, and these kinetic crystal habits are achieved without the need for specifically shaped particle building blocks or external templating or shape-directing agents. Thus, this work provides generalizable design principles to expand the morphological diversity of nanoparticle superlattice crystal habits beyond the anhedral or equilibrium polyhedral shapes synthesized to date. Finally, we use this insight to synthesize crystallite shapes that have never before been observed, demonstrating the ability to both predict and program kinetically controlled superlattice morphologies.

Nanocrystal Assemblies: Current Advances and Open Problems.

We explore the potential of nanocrystals (a term used equivalently to nanoparticles) as building blocks for nanomaterials, and the current advances and open challenges for fundamental science developments and applications. Nanocrystal assemblies are inherently multiscale, and the generation of revolutionary material properties requires a precise understanding of the relationship between structure and function, the former being determined by classical effects and the latter often by quantum effects. With an emphasis on theory and computation, we discuss challenges that hamper current assembly strategies and to what extent nanocrystal assemblies represent thermodynamic equilibrium or kinetically trapped metastable states. We also examine dynamic effects and optimization of assembly protocols. Finally, we discuss promising material functions and examples of their realization with nanocrystal assemblies.

Rationally Designing the Supramolecular Interfaces of Nanoparticle Superlattices with Multivalent Polymers.

In supramolecular materials, multiple weak binding groups can act as a single collective unit when confined to a localized volume, thereby producing strong but dynamic bonds between material building blocks. This principle of multivalency provides a versatile means of controlling material assembly, as both the number and the type of supramolecular moieties become design handles to modulate the strength of intermolecular interactions. However, in materials with building blocks significantly larger than individual supramolecular moieties (e.g., polymer or nanoparticle scaffolds), the degree of multivalency is difficult to predict or control, as sufficiently large scaffolds inherently preclude separated supramolecular moieties from interacting. Because molecular models commonly used to examine supramolecular interactions are intrinsically unable to examine any trends or emergent behaviors that arise due to nanoscale scaffold geometry, our understanding of the thermodynamics of these massively multivalent systems remains limited. Here we address this challenge via the coassembly of polymer-grafted nanoparticles and multivalent polymers, systematically examining how multivalent scaffold size, shape, and spacing affect their collective thermodynamics. Investigating the interplay of polymer structure and supramolecular group stoichiometry reveals complicated but rationally describable trends that demonstrate how the supramolecular scaffold design can modulate the strength of multivalent interactions. This approach to self-assembled supramolecular materials thus allows for the manipulation of polymer-nanoparticle composites with controlled thermal stability, nanoparticle organization, and tailored meso- to microscopic structures. The sophisticated control of multivalent thermodynamics through precise modulation of the nanoscale scaffold geometry represents a significant advance in the ability to rationally design complex hierarchically structured materials via self-assembly.

Controlling Colloidal Crystal Nucleation and Growth with Photolithographically Defined Templates.

Colloidal crystallization provides a means to synthesize hierarchical nanostructures by design and to use these complex structures for nanodevice fabrication. In particular, DNA provides a means to program interactions between particles with high specificity, thereby enabling the formation of particle superlattice crystallites with tailored unit cell geometries and surface faceting. However, while DNA provides precise control of particle-particle bonding interactions, it does not inherently present a means of controlling higher-level structural features such as the size, shape, position, or orientation of a colloidal crystallite. While altering assembly parameters such as temperature or concentration can enable limited control of crystallite size and geometry, integrating colloidal assemblies into nanodevices requires better tools to manipulate higher-order structuring and improved understanding of how these tools control the fundamental kinetics and mechanisms of colloidal crystal growth. In this work, photolithography is used to produce patterned substrates that can manipulate the placement, size, dispersity, and orientation of colloidal crystals. By adjusting aspects of the pattern, such as feature size and separation, we reveal a diffusion-limited mechanism governing crystal nucleation and growth. Leveraging this insight, patterns are designed that can produce wafer-scale substrates with arrays of nanoparticle superlattices of uniform size and shape. These design principles therefore bridge a gap between a fundamental understanding of nanoparticle assembly and the fabrication of nanostructures compatible with functional devices.

Ultrafast dense DNA functionalization of quantum dots and rods for scalable 2D array fabrication with nanoscale precision.

Scalable fabrication of two-dimensional (2D) arrays of quantum dots (QDs) and quantum rods (QRs) with nanoscale precision is required for numerous device applications. However, self-assembly-based fabrication of such arrays using DNA origami typically suffers from low yield due to inefficient QD and QR DNA functionalization. In addition, it is challenging to organize solution-assembled DNA origami arrays on 2D device substrates while maintaining their structural fidelity. Here, we reduced manufacturing time from a few days to a few minutes by preparing high-density DNA-conjugated QDs/QRs from organic solution using a dehydration and rehydration process. We used a surface-assisted large-scale assembly (SALSA) method to construct 2D origami lattices directly on solid substrates to template QD and QR 2D arrays with orientational control, with overall loading yields exceeding 90%. Our fabrication approach enables the scalable, high fidelity manufacturing of 2D addressable QDs and QRs with nanoscale orientational and spacing control for functional 2D photonic devices.

Nanoparticle Brushes: Macromolecular Ligands for Materials Synthesis.

ConspectusColloidal nanoparticles have unique attributes that can be used to synthesize materials with exotic properties, but leveraging these properties requires fine control over the particles' interactions with one another and their surrounding environment. Small molecules adsorbed on a nanoparticle's surface have traditionally served as ligands to govern these interactions, providing a means of ensuring colloidal stability and dictating the particles' assembly behavior. Alternatively, nanoscience is increasingly interested in instead using macromolecular ligands that form well-defined polymer brushes, as these brushes provide a much more tailorable surface ligand with significantly greater versatility in both composition and ligand size. While initial research in this area is promising, synthesizing macromolecules that can appropriately form brush architectures remains a barrier to their more widespread use and limits understanding of the fundamental chemical and physical principles that influence brush-grafted particles' ability to form functional materials. Therefore, enhancing the capabilities of polymer-grafted nanoparticles as tools for materials synthesis requires a multidisciplinary effort, with specific focus on both developing new synthetic routes to polymer-brush-coated nanoparticles and investigating the structure-property relationships the brush enables.In this Account, we describe our recent work in developing polymer brush coatings for nanoparticles, which we use to modulate particle behavior on demand, select specific nanoscopic architectures to form, and bolster traditional bulk polymers to form stronger materials by design. Distinguished by the polymer type and capabilities, three classes of nanoparticles are discussed here: nanocomposite tectons (NCTs), which use synthetic polymers end-functionalized with supramolecular recognition groups capable of directing their assembly; programmable atom equivalents (PAEs) containing brushes of synthetic DNA that employ Watson-Crick base pairing to encode particle binding interactions; and cross-linkable nanoparticles (XNPs) that can both stabilize nanoparticles in solution and polymer matrices and subsequently form multivalent cross-links to strengthen polymer composites. We describe the formation of these brushes through "grafting-from" and "grafting-to" strategies and illustrate aspects that are important for future advancement. We also examine the new capabilities brushes provide, looking closely at dynamic polymer processes that provide control over the assembly state of particles. Finally, we provide a brief overview of the technological applications of nanoparticles with polymer brushes, focusing on the integration of nanoparticles into traditional materials and the processing of nanoparticles into bulk solids.

Tunable Mechanical Response of Self-Assembled Nanoparticle Superlattices.

Self-assembled nanoparticle superlattices (NPSLs) are an emergent class of self-architected nanocomposite materials that possess promising properties arising from precise nanoparticle ordering. Their multiple coupled properties make them desirable as functional components in devices where mechanical robustness is critical. However, questions remain about NPSL mechanical properties and how shaping them affects their mechanical response. Here, we perform in situ nanomechanical experiments that evidence up to an 11-fold increase in stiffness (∼1.49 to 16.9 GPa) and a 5-fold increase in strength (∼88 to 426 MPa) because of surface stiffening/strengthening from shaping these nanomaterials via focused-ion-beam milling. To predict the mechanical properties of shaped NPSLs, we present discrete element method (DEM) simulations and an analytical core-shell model that capture the FIB-induced stiffening response. This work presents a route for tunable mechanical responses of self-architected NPSLs and provides two frameworks to predict their mechanical response and guide the design of future NPSL-containing devices.

Reversible Diffusionless Phase Transitions in 3D Nanoparticle Superlattices.

Nanocomposite tectons (NCTs), polymer brush-grafted nanoparticles that use supramolecular interactions to drive their assembly, form ordered nanoparticle superlattices (NPSLs) with well-defined unit cell symmetries when thermally annealed. In this work, we demonstrate that appropriate assembly and processing conditions can also enable control over the microstructure of NCT lattices by balancing the enthalpic and entropic factors associated with ligand packing and supramolecular bonding during crystallization. Unary systems of NCTs are assembled via the addition of a small molecule capable of binding to multiple nanoparticle ligands; these NCTs initially form face-centered-cubic (FCC) structures in solvents that are favorable for the particles' polymer brushes. However, the FCC lattices undergo a reversible, diffusionless phase transition to body-centered-cubic (BCC) lattices when transferred to a solvent that induces polymer brush collapse. The BCC superlattices maintain the same crystal habit as the parent FCC phase but exhibit significant transformation twinning similar to that seen in martensitic alloys. This previously unseen diffusionless phase transformation in NPSLs enables unique microstructural features in the resulting assemblies, suggesting that NPSLs could serve as models for the investigation of microstructural evolution in crystalline systems and extend our understanding of NPSLs as atomic material analogues.

Self-Assembling Systems for Optical Out-of-Plane Coupling Devices.

Micromirrors are used in integrated photonics to couple extraplanar light into the planar structure of a device by redirecting light via specular reflection. Compared with grating or prism-based couplers, micromirrors allow for coupling of light over a broader range of wavelengths, provided that the micromirror is fabricated with a specific 3D shape to ensure proper reflection angles. In principle, self-assembly methods could enable reliable, parallelizable fabrication of such devices with a high degree of precision by designing self-assembling components that produce the desired microscale geometry as their thermodynamic products. In this work, we use DNA-functionalized nanoparticles to assemble faceted crystallites with predetermined crystal shapes, and demonstrate with microscale retroreflectance measurements that these self-assembled nanoparticle arrays do indeed behave like optically flat mirrors. Furthermore, we show that the tilt angle of the micromirrors can be intentionally controlled by altering the crystallographic symmetry and preferred crystal orientations as a function of the self-assembly process, thereby altering the resulting specular angle in a programmable manner. Measurements of optical coupling from normal incidence into the substrate plane via an optical fiber confirm that the faceted structures can function as optical out-of-plane coupling devices, and coating these structures with reflective materials allows for high efficiency of light reflection in addition to the angular control. Together, these experiments demonstrate how self-assembled nanoparticle materials can be used to generate optically relevant architectures, enabling a significant step in the development of self-assembly as a materials fabrication tool for integrated optical devices.

Improving nanoparticle superlattice stability with deformable polymer gels.

The self-assembly of colloidal nanoparticles into ordered superlattices typically uses dynamic interactions to govern particle crystallization, as these non-permanent bonds prevent the formation of kinetically trapped, disordered aggregates. However, while the use of reversible bonding is critical in the formation of highly ordered particle arrangements, dynamic interactions also inherently make the structures more prone to disassembly or disruption when subjected to different environmental stimuli. Thus, there is typically a trade-off between the ability to initially form an ordered colloidal material and the ability of that material to retain its order under different conditions. Here, we present a method for embedding colloidal nanoparticle superlattices into a polymer gel matrix. This encapsulation strategy physically prevents the nanoparticles from dissociating upon heating, drying, or the introduction of chemicals that would normally disrupt the lattice. However, the use of a gel as the embedding medium still permits further modification of the colloidal nanoparticle lattice by introducing stimuli that deform the gel network (as this deformation in turn alters the nanoparticle lattice structure in a predictable manner). Moreover, encapsulation of the lattice within a gel permits further stabilization into fully solid materials by removing the solvent from the gel or by replacing the solvent with a liquid monomer that can be photopolymerized. This embedding method therefore makes it possible to incorporate ordered colloidal arrays into a polymer matrix as either dynamic or static structures, expanding their potential for use in responsive materials.

Facial Nerve Function Outcome and Risk Factors in Resection of Large Cystic Vestibular Schwannomas.

Objective Cystic vestibular schwannomas (VS) in contrast to solid VS tend to have accelerated growth, larger volume, rapid/atypical presentation, lobulated/adherent surface, and unpredictable course of the cranial nerves. Cystic VS are surgically challenging, with worse clinical outcomes and higher rate of subtotal resection (STR). Methods We retrospectively analyzed postoperative outcomes of 125 patients with cystic VS, operated between years 2005 and 2019 in our center. We confronted the extent of the resection and House-Brackmann (HB) grade of facial palsy with the results of comparable cohort of patients with solid VS operated in our center and literature review by Thakur et al. 1 Results Translabyrinthine approach was preferred for resection of large, cystic VS (97.6%). Gross-total resection (GTR) was achieved in 78 patients (62.4%), near-total resection (NTR) with remnant (<4 × 4 × 2 mm) in 43 patients (34.4%), and STR in 4 patients (3.2%). NTR/STR were significantly associated with higher age, tumor volume >5 cm 3 , retrosigmoid approach, high-riding jugular bulb, tumor adherence to the brain stem, and facial nerve ( p = 0.016; 0.003; 0.005; 0.025; 0.001; and <0.00001, respectively). One year after the surgery, 76% of patients had HB grades 1 to 2, 16% had HB grades 3 to 4, and 8% had HB grades 5 to 6 palsy. Worse outcome (HB grades 3 to 6) was associated with preoperative facial palsy, tumor volume >25 cm 3 , and cyst over the brain stem ( p = 0.045; 0.014; and 0.05, respectively). Comparable solid VS operated in our center had significantly higher HB grades 1 to 2 rate than our cystic VS (94% versus 76%; p = 0.03). Comparing our results with literature review, our HB grades 1 to 2 rate was significantly higher (76% versus 39%; p = 0.0001). Tumor control rate 5 years after surgery was 95.8%. Conclusion Our study confirmed that microsurgery of cystic VS has worse outcomes of facial nerve preservation and extent of resection compared with solid VS. Greater attention should be paid to the above-mentioned risk factors.

A 2 and 5-Year Longitudinal Analysis of 671 Consecutive Patients Diagnosed with Unilateral Vestibular Schwannoma.

OBJECTIVE: To analyse the 2 and 5-year outcomes of a cohort of patients with newly diagnosed vestibular schwannoma patients. STUDY DESIGN: Longitudinal cohort study. SETTING: Tertiary skull base referral unit. PATIENTS: Six hundred and seventy-one consecutive patients referred to the Cambridge Skull Base Unit between 2005 and 2010. MAIN OUTCOME MEASURES: 1) Treatment outcomes, 2) hearing preservation, 3) facial nerve function, and 4) gross tumor control. RESULTS: At initial diagnosis, 440/671(65.6%) of patients entered a watch, wait and rescan (WWR) pathway, 208/671 (31%) underwent primary microsurgical excision and 23/671 (3.4%) primary radiotherapy. Of patients who entered WWR, 144/440 (32.7%) exhibited tumor growth necessitating secondary intervention at a mean of 22.7 months; 67.4% had radiotherapy and 32.6% surgery. Radiotherapy was successful at achieving tumor control in 95% of cases. Primary surgery via the translabyrinthine route was performed in the majority of cases allowing for a radiologically clear surgical resection in 93% of cases. CONCLUSION: The Cambridge philosophy of treating the newly diagnosed vestibular schwannoma aims at preserving functional status where possible. In patients with small tumors it is recommended that they are placed on to a WWR pathway, of which 67.3% will not exhibit any growth by 5 years. If small tumors grow more than 2 mm by serial scanning, radiotherapy offers a high rate of tumor control. In patients with larger tumors more than 20 mm we would advocate microsurgical excision by the translabyrinthine route which offers excellent functional outcomes and gross tumor control.

Polymer Grafted Nanoparticle Composites with Enhanced Thermal and Mechanical Properties.

The distribution of filler particles within a polymer matrix nanocomposite has a profound influence on the properties and processability of the material. While filler aggregation and percolation can significantly enhance particular functionalities such as thermal and electrical conductivity, the formation of larger filler clusters and networks can also impair mechanical properties like strength and toughness and can also increase the difficulty of processing. Here, a strategy is presented for the preparation of functional composites that enhance thermal conductivity over polymer alone, without negatively affecting mechanical performance or processability. Thermal cross-linking of self-suspended polymer grafted nanoparticles is used to prepare highly filled (>50 vol %) macroscopic nanocomposites with homogeneously dispersed, non-percolating alumina particles in an organic matrix. The initial composites use low glass transition temperature polymer grafts and thus are flexible and easily shaped by thermoforming methods. However, after thermal aging, the resulting materials display high stiffness (>10 GPa) and enhanced thermal conductivity (>100% increase) and also possess mechanical strength similar to commodity plastics. Moreover, the covalent bonding between matrix and filler allows for the significant elevation of thermal conductivity despite the extensive interfacial area in the nanocomposite. The thermal aging of polymer grafted nanoparticles is therefore a promising method for producing easily processable, mechanically sturdy, and macroscopic nanocomposites with improved thermal conductivity.

Controlling the dynamics of elastomer networks with multivalent brush architectures.

Herein, we report a design strategy for developing mechanically enhanced and dynamic polymer networks by incorporating a polymer with multivalent brush architecture. Different ratios of two types of imidazole functionalized polymers, specifically poly(n-butyl acrylate) (PnBA) and poly(poly(n-butyl acrylate)) (PPnBA) were blended with Zn(II) ions, thereby forming a series of elastomers with consistent composition but varying network topologies. As the weight fraction of PPnBA increased, the melting temperature, plateau modulus, and relaxation time of the melt increased because of the increase in the crosslinking density and coordination efficiency. Remarkably, however, the activation energy of the flow, Ea, decreased with increasing amounts of PPnBA despite the observed increases in mechanical properties. This unique behavior is attributed to the multivalent nature of the brush polymer, which allows the PPnBA to generate a higher crosslinking density than networks of linear PnBA, even though the brush polymers contain a lower weight fraction of the imidazole crosslinks. This method of lowering Ea, while improving the mechanical properties of the elastomers has great potential in the development of various soft materials such as self-healing or 3D-printable elastomeric structures.

Impact of the National Lockdown Due to the COVID-19 Pandemic On Upper Limb Trauma Workload in Central London: A Multi-Centre Longitudinal Observational Study During Implementation and Ease of National Lockdown.

Background: This study assessed the impact of the COVID-19 pandemic on acute upper limb referrals and operative case-mix at the beginning and ease of British lockdown. Methods: A longitudinal multicentre observational cohort study was conducted for both upper limb trauma referrals and operative case-mix over a 12-week period (6 weeks from the beginning and 6 weeks from the ease of the national lockdown). Statistical analysis included median (± median absolute deviation), risk and odds ratios, and Fisher's exact test to calculate the statistical significance, set at p ≤ 0.05. Results: There was a 158% (n = 456 vs. 177) increase in upper limb referrals and 133% (n = 91 vs. 39) increase in the operative trauma caseload at the ease of lockdown compared with its commencement. An increase in sporting injuries was demonstrated (p=0.02), specifically cycling (p=0.004, OR=2.58). A significant increase in COVID-19 testing was demonstrated during the ease of lockdown (p=0.0001) with more patients having their management changed during the beginning of the pandemic (9.6% vs. 0.7%, p=0.0001). Of these patients, 47% went on to have delayed surgery within 6 months. No patients who underwent surgery tested positive for COVID-19 infection within 14 days post-operatively and no mortalities were recorded at 30 days. Conclusion: The ease of lockdown has seen upper limb referrals and operations more than double compared to early lockdown. With no patients testing positive for COVID-19 within 14 days of the procedure, this demonstrates that having upper limb surgery during the current pandemic is safe.

Nanocomposite tectons as unifying systems for nanoparticle assembly.

Nanocomposite tectons (NCTs) are nanocomposite building blocks consisting of nanoparticle cores functionalized with a polymer brush, where each polymer chain terminates in a supramolecular recognition group capable of driving particle assembly. Like other ligand-driven nanoparticle assembly schemes (for example those using DNA-hybridization or solvent evaporation), NCTs are able to make colloidal crystal structures with precise particle organization in three dimensions. However, despite the similarity of NCT assembly to other methods of engineering ordered particle arrays, the crystallographic symmetries of assembled NCTs are significantly different. In this study, we provide a detailed characterization of the dynamics of hybridizations through universal (independent of microscopic details) parameters. We perform rigorous free energy calculations and identify the persistence length of the ligand as the critical parameter accounting for the differences in the phase diagrams of NCTs and other assembly methods driven by hydrogen bond hybridizations. We also report new experiments to provide direct verification for the predictions. We conclude by discussing the role of non-equilibrium effects and illustrating how NCTs provide a unification of the two most successful strategies for nanoparticle assembly: solvent evaporation and DNA programmable assembly.

Nanoparticle Assembly as a Materials Development Tool.

Nanoparticle assembly is a complex and versatile method of generating new materials, capable of using thousands of different combinations of particle size, shape, composition, and ligand chemistry to generate a library of unique structures. Here, a history of particle self-assembly as a strategy for materials discovery is presented, focusing on key advances in both synthesis and measurement of emergent properties to describe the current state of the field. Several key challenges for further advancement of nanoparticle assembly are also outlined, establishing a roadmap of critical research areas to enable the next generation of nanoparticle-based materials synthesis.

Crosslinking of Pressure-Sensitive Adhesives with Polymer-Grafted Nanoparticles.

Nanocomposite filler particles provide multiple routes to mechanically reinforce pressure-sensitive adhesives (PSAs), as their large surface area to volume ratios provide a means of effectively crosslinking multiple polymer chains. A major advancement could therefore be enabled by the design of a particle architecture that forms multiple physical and chemical interactions with the surrounding polymer matrix, while simultaneously ensuring particle dispersion and preventing particle aggregation. Understanding how such multivalent interactions between a nanoparticle crosslinking point and the PSA polymer affect material mechanical performance would provide both useful scientific knowledge on the mechanical structure-property relationships in polymer composites, as well as a new route to synthesizing useful PSA materials. Herein, we report the use of polymer-grafted nanoparticles (PGNPs) composed of poly(n-butyl acrylate-co-acrylic acid) chains grafted to SiO2 nanoparticle (NP) surfaces to cohesively reinforce PSA films against shear stress without compromising their adhesive properties. The use of acrylic acid-decorated PGNPs allows for ionic crosslinking via metal salt coordination to be used in conjunction with physical entanglement, yielding 33% greater shear resistance and up to 3-fold longer holding times under static load. In addition, the effects of material parameters such as PGNP/crosslinker loading, polymer graft length, and core nanoparticle size on mechanical properties are also explored, providing insights into the use of PGNPs for the rational design of polymer composite-based PSAs.