Joseph J. Richardson

One-Step Assembly of Coordination Complexes for Versatile Film and Particle Engineering

One-Step Coverage Controllable formation of thin films often requires slow deposition conditions or multiple rounds of coating. Ejima et al. (p. 154; see the Perspective by Bentley and Payne) report a simple and versatile method for coating surfaces with thin biocompatible films made from the condensation of Fe3+ ions and a natural polyphenol, tannic acid, from aqueous solutions. Flat surfaces, colloidal particles, and even bacterial cells could be coated, and the coats could subsequently be degraded by changing the pH. Thin adherent films formed from ferric ions and a natural polyphenol, tannic acid, can coat a wide variety of surfaces. [Also see Perspective by Bentley and Payne] The development of facile and versatile strategies for thin-film and particle engineering is of immense scientific interest. However, few methods can conformally coat substrates of different composition, size, shape, and structure. We report the one-step coating of various interfaces using coordination complexes of natural polyphenols and Fe(III) ions. Film formation is initiated by the adsorption of the polyphenol and directed by pH-dependent, multivalent coordination bonding. Aqueous deposition is performed on a range of planar as well as inorganic, organic, and biological particle templates, demonstrating an extremely rapid technique for producing structurally diverse, thin films and capsules that can disassemble. The ease, low cost, and scalability of the assembly process, combined with pH responsiveness and negligible cytotoxicity, makes these films potential candidates for biomedical and environmental applications.

Technology-driven layer-by-layer assembly of nanofilms

Thin-film fabrication The deposition of thin films from multiple materials is essential to a range of materials fabrication processes. Layer-by-layer processes involve the sequential deposition of two or more materials that physically bond together. Richardson et al. review some of the techniques and materials that are used to make thin films, including sequential dip coating, spraying, and electrochemical deposition. Despite the versatility of the methods and the range of materials that can be deposited, the techniques remain mostly confined to the lab because of challenges in industrial scaling. But because there is tremendous scope for fine-tuning the structure and properties of the multilayers, there is interest in broadening the use of these techniques. Science, this issue 10.1126/science.aaa2491 BACKGROUND Over the past few decades, layer-by-layer (LbL) assembly of thin films has been of considerable interest because of its ability to exert nanometer control over film thickness and its extensive choice of usable materials for coating planar and particulate substrates. The choice of materials allows for responsive and functional thin films to be engineered for various applications, including catalysis, optics, energy, membranes, and biomedicine. Furthermore, there is now a growing realization that the assembly technologies substantially affect the physicochemical properties and, ultimately, the performance of the thin films. ADVANCES Recent advances in LbL assembly technologies have explored different driving forces for the assembly process when compared with the diffusion-driven kinetics of classical LbL assembly, where a substrate is immersed in a polymer solution. Examples of different assembly technologies that are now available include: dipping, dewetting, roll-to-roll, centrifugation, creaming, calculated-saturation, immobilization, spinning, high gravity, spraying, atomization, electrodeposition, magnetic assembly, electrocoupling, filtration, microfluidics, and fluidized beds. These technologies can be condensed into five broad categories to which automation or robotics can also be applied—namely, (i) immersive, (ii) spin, (iii) spray, (iv) electromagnetic, and (v) fluidic assembly. Many of these technologies are still new and are actively being explored, with research shedding light on how the deposition technologies and the underlying driving forces affect the formation, properties, and performance of the films, as well as the ease, yield, and scale of the processing. OUTLOOK Layer-by-layer assembly has proven markedly powerful over the past two decades and has had a profound interdisciplinary effect on scientific research. Scaling up the process is crucial for furthering real-world applications, and moving forward, an understanding of how to carefully select assembly methods to harness the specific strengths of different technologies has the potential to be transformative. Comprehensive comparisons between the technologies still need to be conducted, especially in regard to coating particulate substrates, where comparisons are limited but crucial for advancing fundamental research and practical applications. Layer-by-layer assembly of nanofilms for preparing functional materials. The properties and performance of the resulting films depend on the substrate and layer material choices, as well as the assembly technology. ILLUSTRATION CREDIT: ALISON E. BURKE AND CASSIO LYNM Multilayer thin films have garnered intense scientific interest due to their potential application in diverse fields such as catalysis, optics, energy, membranes, and biomedicine. Here we review the current technologies for multilayer thin-film deposition using layer-by-layer assembly, and we discuss the different properties and applications arising from the technologies. We highlight five distinct routes of assembly—immersive, spin, spray, electromagnetic, and fluidic assembly—each of which offers material and processing advantages for assembling layer-by-layer films. Each technology encompasses numerous innovations for automating and improving layering, which is important for research and industrial applications. Furthermore, we discuss how judicious choice of the assembly technology enables the engineering of thin films with tailor-made physicochemical properties, such as distinct-layer stratification, controlled roughness, and highly ordered packing.

Engineering Multifunctional Capsules through the Assembly of Metal–Phenolic Networks

Metal-organic coordination materials are of widespread interest because of the coupled benefits of inorganic and organic building blocks. These materials can be assembled into hollow capsules with a range of properties, which include selective permeability, enhanced mechanical/thermal stability, and stimuli-responsiveness. Previous studies have primarily focused on the assembly aspects of metal-coordination capsules; however, the engineering of metal-specific functionality for capsule design has not been explored. A library of functional metal-phenolic network (MPN) capsules prepared from a phenolic ligand (tannic acid) and a range of metals is reported. The properties of the MPN capsules are determined by the coordinated metals, allowing for control over film thickness, disassembly characteristics, and fluorescence behavior. Furthermore, the functional properties of the MPN capsules were tailored for drug delivery, positron emission tomography (PET), magnetic resonance imaging (MRI), and catalysis. The ability to incorporate multiple metals into MPN capsules demonstrates that a diverse range of functional materials can be generated.

pH-Responsive Capsules Engineered from Metal–Phenolic Networks for Anticancer Drug Delivery

A new class of pH-responsive capsules based on metal-phenolic networks (MPNs) for anticancer drug loading, delivery and release is reported. The fabrication of drug-loaded MPN capsules, which is based on the formation of coordination complexes between natural polyphenols and metal ions over a drug-coated template, represents a rapid strategy to engineer robust and versatile drug delivery carriers.

Engineering poly(ethylene glycol) particles for improved biodistribution.

We report the engineering of poly(ethylene glycol) (PEG) hydrogel particles using a mesoporous silica (MS) templating method via tuning the PEG molecular weight, particle size, and the presence or absence of the template and investigate the cell association and biodistribution of these particles. An ex vivo assay based on human whole blood that is more sensitive and relevant than traditional cell-line based assays for predicting in vivo circulation behavior is introduced. The association of MS@PEG particles (template present) with granulocytes and monocytes is higher compared with PEG particles (template absent). Increasing the PEG molecular weight (from 10 to 40 kDa) or decreasing the PEG particle size (from 1400 to 150 nm) reduces phagocytic blood cell association of the PEG particles. Mice biodistribution studies show that the PEG particles exhibit extended circulation times (>12 h) compared with the MS@PEG particles and that the retention of smaller PEG particles (150 nm) in blood, when compared with larger PEG particles (>400 nm), is increased at least 4-fold at 12 h after injection. Our findings highlight the influence of unique aspects of polymer hydrogel particles on biological interactions. The reported PEG hydrogel particles represent a new class of polymer carriers with potential biomedical applications.

Preparation of Nano- and Microcapsules by Electrophoretic Polymer Assembly†

Nanoand microcapsules are of significant interest for application in biomedicine, especially in diagnostics and therapeutic delivery. The fabrication of polymer capsules is often accomplished by the particle-mediated assembly of polymer films, whereby films are formed on particles through polymerization or by depositing multiple polymer layers through layer-by-layer (LbL) assembly. In particular, LbL assembly enables the use of particles of various types, shapes, and sizes as templates and enables a suite of polymers with different properties (functionality, responsiveness, and degradability) to be used to engineer the physical, chemical, and biological properties of the capsules. The versatility of LbL assembly with particle templates has led to the development of a range of coated particles and capsules for diverse applications, including drug and vaccine delivery. 4a–f] Despite the significant progress in the field of LbLengineered particles, the film-deposition process typically requires numerous centrifugation and rinse steps to separate the coating material (e.g., polymer) and the particles. Furthermore, it is generally limited to particles either dense and/or large enough for centrifugal sedimentation. Other methods to generate LbL capsules have also been reported: for example, atomization techniques, filter membranes, and microfluidic systems have been used; however, each of these approaches reduces the diversity of the types of particles and polymers that can be employed. These limitations highlight the necessity for the development of alternative rapid and robust approaches for the formation of LbLassembled coatings on particles to generate engineered core– shell particles and capsules. Herein, we report an electrophoretic polymer assembly (EPA) technique for depositing a range of polymers on particles of different sizes. In this approach, electrophoresis is used to generate particles coated with multiple polymer layers and, following core removal, polymer multilayer capsules. An inherent requirement of EPA is the immobilization of particles in a porous hydrogel; in this study, namely, the biologically derived polysaccharide agarose. Agarose has historically been used with electrophoresis to separate biopolymers (e.g., proteins and nucleic acids) because it gelates under ambient conditions, shows low reactivity, is easy to prepare, and has various, tunable pore sizes. In the present system, agarose acts as a “natural immobilizing microfluidic system”. However, we did not use electrophoresis to simply separate free polymer from deposited polymer, but also to deposit the polymers on immobilized particles (Figure 1). This technique reduces handling times, minimizes

Modular assembly of superstructures from polyphenol-functionalized building blocks

The organized assembly of particles into superstructures is typically governed by specific molecular interactions or external directing factors associated with the particle building blocks, both of which are particle-dependent. These superstructures are of interest to a variety of fields because of their distinct mechanical, electronic, magnetic and optical properties. Here, we establish a facile route to a diverse range of superstructures based on the polyphenol surface-functionalization of micro- and nanoparticles, nanowires, nanosheets, nanocubes and even cells. This strategy can be used to access a large number of modularly assembled superstructures, including core-satellite, hollow and hierarchically organized supraparticles. Colloidal-probe atomic force microscopy and molecular dynamics simulations provide detailed insights into the role of surface functionalization and how this facilitates superstructure construction. Our work provides a platform for the rapid generation of superstructured assemblies across a wide range of length scales, from nanometres to centimetres.

Biomimetic Replication of Microscopic Metal–Organic Framework Patterns Using Printed Protein Patterns

It is demonstrated that metal-organic frameworks (MOFs) can be replicated in a biomimetic fashion from protein patterns. Bendable, fluorescent MOF patterns are formed with micrometer resolution under ambient conditions. Furthermore, this technique is used to grow MOF patterns from fingerprint residue in 30 s with high fidelity. This technique is not only relevant for crime-scene investigation, but also for biomedical applications.

Lead(II) uptake by aluminium based magnetic framework composites (MFCs) in water

The recent combination of Metal-Organic Frameworks (MOFs) and magnetic nanoparticles has shown their potential as a composite material in practical applications including drug delivery, catalysis and pollutant sequestration. Here, we report for the first time the preparation of a robust magnetic nanocomposite material based on an aluminium MOF (MIL-53) and iron oxide nanoparticles for the uptake of lead(II) ions. Different aminofunctionalized MIL-53 MOFs were prepared by increasing the 2-aminoterephthalic/terephthalic acid ratio. The composite materials were tested to determine the sequestration capability of heavy metals from various solvents (methanol, DMSO and water), pH (2, 7, 12) and a range of Pb(II) concentrations (10–8000 ppm). The magnetic composite based on MIL-53 showed remarkable capacity to sequester Pb(II) ions from water (up to 492.4 mg g−1 of composite), the highest recorded for a MOF sorbent system to date. While the MOF played a crucial role in the efficient heavy metal uptake, the magnetic nanoparticles allowed the prompt collection of the sorbent from solution. The triggered release of Pb(II) was investigated using an alternating magnetic field. The exceptional adsorption capacity and the response to the magnetic field make this class of innovative functional material a promising candidate for environmental remediation technologies.

Endocytic pH-Triggered Degradation of Nanoengineered Multilayer Capsules

The synthesis of cross-linker free layer-by-layer (LbL) capsules that solely utilize cellular pH variations as a trigger to specifically deconstruct and subsequently release cargo in cells is reported. These capsules demonstrate retention of water-soluble therapeutic molecules as small as 500 Da at extracellular pH. Triggered capsule degradation and release of cargo is observed within 30 min of cell uptake.