Frank Caruso

Nanoengineering of Particle Surfaces

The creation of core–shell particles is attracting a great deal of interest because of the diverse applicability of these colloidal particles; e.g., as building blocks for photonic crystals, in multi-enzyme biocatalysis, and in drug delivery. This review presents the state-of-the-art in strategies for engineering particle surfaces, such as the layer-by-layer deposition process (see Figure), which allows fine control over shell thickness and composition.

Novel Hollow Polymer Shells by Colloid-Templated Assembly of Polyelectrolytes

Exact control of the film thickness of polyelectrolyte shells (a transmission electron microscopy image is shown) is achieved by colloid-templated consecutive adsorption of polyanions and polycations followed by decomposition of the templating core. Possible areas of application for these shells range from the pharmaceutical, food, cosmetic, and paint industries to catalysis and microcrystallization.

Hollow capsule processing through colloidal templating and self-assembly.

Hollow capsules of nanometer to micrometer dimensions constitute an important class of materials that are employed in diverse technological applications, ranging from the delivery of encapsulated products for cosmetic and medicinal purposes to their use as light-weight composite materials and as fillers with low dielectric constant in electronic components. Hollow capsules comprising polymer, glass, metal, and ceramic are nowadays routinely produced by using various chemical and physicochemical methods. The current article focuses on a recent novel and versatile technique, based on a combination of colloidal templating and self-assembly processes, developed for synthesizing uniform hollow capsules of a broad range of materials. The strategy outlined readily affords control over the size, shape, composition, and wall thickness of the hollow capsules.

Stepwise polyelectrolyte assembly on particle surfaces: a novel approach to colloid design

Polyelectrolyte multilayers were deposited onto polystyrene and melamine formaldehyde latex particles by means of consecutive adsorption. Two different methods of multilayer growth were employed. First, adsorption of polyelectrolytes at a concentration exceeding saturation amounts was combined with the removal of the nonbound polyelectrolyte by means of centrifugation. Second, adsorption of polyelectrolyte was performed at a concentration just sufficient for saturation coverage. Both methods yielded continuous layer growth. The process of film formation was followed by electrophoresis, dynamic light scattering, single particle light scattering and fluorescence intensity measurements. Layer deposition onto partially crosslinked melamine resin latex particles, which were soluble at pH values of less than 1.6, resulted in the production of three-dimensional thin polyelectrolyte shells upon dissolving the core. The ultrathin shells were observed by means of scanning and transmission electron microscopy.

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.

Hollow titania spheres from layered precursor deposition on sacrificial colloidal core particles.

The preparation of monodisperse hollow titania spheres with defined diameter, wall thickness and crystal phase is reported. The hollow spheres have been produced by the layered deposition of a water-soluble titania precursor, e.g. titanium(IV) bis (ammonium lactato) dihydroxide (TALH), in alternation with poly(diallydimethylammonium chloride) (PDADMAC) onto submicrometer-sized template particles e.g. polystyrene (PS) particles, followed by calcination at elevated temperatures. the layer-by-layer growth of the coating on the colloid particles was observed by microelectrophoresis and transmission electron microscopy (TEM). Calcination of the TALH/PDADMAC-coated particles resulted in intact, hollow titania spheres, as confirmed by scanning electron microscopy (SEM) and TEM. Calcining the coated particles at 450 or 950 DEG C resulted in hollow sphere consisting of titania in anatase or rutile form, respectively. Nanometer-level control over the sphere wall thickness was achieved by varying the number of layers deposited on the PS particles. The hollow titania spheres produced can be used in photonic applications, where hollow spheres of high refractive index materials are desired, and in catalysis.

Spontaneous phase transfer of nanoparticulate metals from organic to aqueous media.

4-dimethylaminopyridine (DMAP) is the answer to the quest for an efficient transfer of metallic nanoparticles from organic to aqueous solutions. The picture shows the transfer of gold nanoparticles from toluene to water by the addition of DMAP (0.1 M, pH 10.5). This method enables the generation of high concentrations of nanoparticles with better monodispersity than those commonly prepared in water.

Next generation, sequentially assembled ultrathin films: beyond electrostatics

Over the last 15 years, the layer-by-layer (LbL) assembly technology has proven to be a versatile method for surface modification. This approach is likely to find widespread application because of its simplicity and versatility; however, the conventional use of highly charged materials with limited responsive behaviour presents some key limitations. In this tutorial review, the formation of multilayer thin films prepared through non-electrostatic interactions is reviewed. We discuss the assembly of films via a number of different methodologies, with particular emphasis on those that provide enhanced orientational control, stimuli-responsive behaviour, and improved film stability.

Quantum measurement and orientation tracking of fluorescent nanodiamonds inside living cells

Fluorescent particles are routinely used to probe biological processes. The quantum properties of single spins within fluorescent particles have been explored in the field of nanoscale magnetometry, but not yet in biological environments. Here, we demonstrate optically detected magnetic resonance of individual fluorescent nanodiamond nitrogen-vacancy centres inside living human HeLa cells, and measure their location, orientation, spin levels and spin coherence times with nanoscale precision. Quantum coherence was measured through Rabi and spin-echo sequences over long (>10 h) periods, and orientation was tracked with effective 1° angular precision over acquisition times of 89 ms. The quantum spin levels served as fingerprints, allowing individual centres with identical fluorescence to be identified and tracked simultaneously. Furthermore, monitoring decoherence rates in response to changes in the local environment may provide new information about intracellular processes. The experiments reported here demonstrate the viability of controlled single spin probes for nanomagnetometry in biological systems, opening up a host of new possibilities for quantum-based imaging in the life sciences.