Magnetic assembly of plasmonic chiral superstructures with dynamic chiroptical responses.

Plasmonic nanostructures exhibiting dynamically tunable chiroptical responses hold great promise for broad applications such as sensing, catalysis, and enantioselective analysis. Despite the successful fabrication of chiral structures through diverse templates, creating dynamic chiroptical materials with fast and reversible responses to external stimuli is still challenging. This work showcases reversible magnetic assembly and active tuning of plasmonic chiral superstructures by introducing a chiral magnetic field from a cubic permanent magnet. Manipulating the strength and direction of the magnetic field controls both the chiral arrangement and plasmonic coupling of the nanoparticle assembly, enabling fast and reversible tunability in not only the handedness of the superstructures but also the spectral characteristics of their chiroptical properties. The dynamic tunability further enables the fabrication of color-changing optical devices based on the optical rotatory dispersion effect, showcasing their potential for application in anti-counterfeiting and stress sensors.

Correlating chemistry and mass transport in sustainable iron production.

Steelmaking contributes 8% to the total CO2 emissions globally, primarily due to coal-based iron ore reduction. Clean hydrogen-based ironmaking has variable performance because the dominant gas-solid reduction mechanism is set by the defects and pores inside the mm- to nm-sized oxide particles that change significantly as the reaction progresses. While these governing dynamics are essential to establish continuous flow of iron and its ores through reactors, the direct link between agglomeration and chemistry is still contested due to missing measurements. In this work, we directly measure the connection between chemistry and agglomeration in the smallest iron oxides relevant to magnetite ores. Using synthesized spherical 10-nm magnetite particles reacting in H2, we resolve the formation and consumption of wüstite (Fe1-xO)-the step most commonly attributed to whiskering. Using X-ray diffraction, we resolve crystallographic anisotropy in the rate of the initial reaction. Complementary imaging demonstrated how the particles self-assemble, subsequently react, and grow into elongated "whisker" structures. Our insights into how morphologically uniform iron oxide particles react and agglomerate in H2 reduction enable future size-dependent models to effectively describe the multiscale aspects of iron ore reduction.

Dual-Responsive Fe3O4@Polyaniline Chiral Superstructures for Information Encryption.

Incorporating stimuli-responsive mechanisms into chiral assemblies of nanostructures offers numerous opportunities to create optical materials capable of dynamically modulating their chiroptical properties. In this study, we demonstrate the formation of chiral superstructures by assembling Fe3O4@polyaniline hybrid nanorods by using a gradient magnetic field. The resulting superstructures exhibit a dual response to changes in both the magnetic field and solution pH, enabling dynamic regulation of the position, intensity, and sign of its circular dichroism peaks. Such responsiveness allows for convenient control over the optical rotatory dispersion properties of the assemblies, which are further integrated into the design of a chiroptical switch that can display various colors and patterns when illuminated with light of different wavelengths and polarization states. Finally, an optical information encryption system is constructed through the controlled assembly of the hybrid nanorods to showcase the potential opportunities for practical applications brought by the resulting responsive chiral superstructures.

Magnetically Induced Anisotropic Interaction in Colloidal Assembly.

The wide accessibility to nanostructures with high uniformity and controllable sizes and morphologies provides great opportunities for creating complex superstructures with unique functionalities. Employing anisotropic nanostructures as the building blocks significantly enriches the superstructural phases, while their orientational control for obtaining long-range orders has remained a significant challenge. One solution is to introduce magnetic components into the anisotropic nanostructures to enable precise control of their orientations and positions in the superstructures by manipulating magnetic interactions. Recognizing the importance of magnetic anisotropy in colloidal assembly, we provide here an overview of magnetic field-guided self-assembly of magnetic nanoparticles with typical anisotropic shapes, including rods, cubes, plates, and peanuts. The Review starts with discussing the magnetic energy of nanoparticles, appreciating the vital roles of magneto-crystalline and shape anisotropies in determining the easy magnetization direction of the anisotropic nanostructures. It then introduces superstructures assembled from various magnetic building blocks and summarizes their unique properties and intriguing applications. It concludes with a discussion of remaining challenges and an outlook of future research opportunities that the magnetic assembly strategy may offer for colloidal assembly.

A magnetic assembly approach to chiral superstructures.

Colloidal assembly into chiral superstructures is usually accomplished with templating or lithographic patterning methods that are only applicable to materials with specific compositions and morphologies over narrow size ranges. Here, chiral superstructures can be rapidly formed by magnetically assembling materials of any chemical compositions at all scales, from molecules to nano- and microstructures. We show that a quadrupole field chirality is generated by permanent magnets caused by consistent field rotation in space. Applying the chiral field to magnetic nanoparticles produces long-range chiral superstructures controlled by field strength at the samples and orientation of the magnets. Transferring the chirality to any achiral molecules is enabled by incorporating guest molecules such as metals, polymers, oxides, semiconductors, dyes, and fluorophores into the magnetic nanostructures.

Magnetically Tunable One-Dimensional Plasmonic Photonic Crystals.

Integrating plasmonic resonance into photonic bandgap nanostructures promises additional control over their optical properties. Here, one-dimensional (1D) plasmonic photonic crystals with angular-dependent structural colors are fabricated by assembling magnetoplasmonic colloidal nanoparticles under an external magnetic field. Unlike conventional 1D photonic crystals, the assembled 1D periodic structures show angular-dependent colors based on the selective activation of optical diffraction and plasmonic scattering. They can be further fixed in an elastic polymer matrix to produce a photonic film with angular-dependent and mechanically tunable optical properties. The magnetic assembly enables precise control over the orientation of the 1D assemblies within the polymer matrix, producing photonic films with designed patterns displaying versatile colors from the dominant backward optical diffraction and forward plasmonic scattering. The combination of optical diffraction and plasmonic properties within a single system holds the potential for developing programmable optical functionalities for applications in various optical devices, color displays, and information encryption systems.

Photothermal heating of titanium nitride nanomaterials for fast and uniform laser warming of cryopreserved biomaterials.

Titanium nitride (TiN) is presented as an alternative plasmonic nanomaterial to the commonly used gold (Au) for its potential use in laser rewarming of cryopreserved biomaterials. The rewarming of vitrified, glass like state, cryopreserved biomaterials is a delicate process as potential ice formation leads to mechanical stress and cracking on a macroscale, and damage to cell walls and DNA on a microscale, ultimately leading to the destruction of the biomaterial. The use of plasmonic nanomaterials dispersed in cryoprotective agent solutions to rapidly convert optical radiation into heat, generally supplied by a focused laser beam, proposes a novel approach to overcome this difficulty. This study focuses on the performance of TiN nanoparticles (NPs), since they present high thermal stability and are inexpensive compared to Au. To uniformly warm up the nanomaterial solutions, a beam splitting laser system was developed to heat samples from multiple sides with equal beam energy distribution. In addition, uniform laser warming requires equal distribution of absorption and scattering properties in the nanomaterials. Preliminary results demonstrated higher absorption but less scattering in TiN NPs than Au nanorods (GNRs). This led to the development of TiN clusters, synthetized by nanoparticle agglomeration, to increase the scattering cross-section of the material. Overall, this study analyzed the heating rate, thermal efficiency, and heating uniformity of TiN NPs and clusters in comparison to GNRs at different solution concentrations. TiN NPs and clusters demonstrated higher heating rates and solution temperatures, while only clusters led to a significantly improved uniformity in heating. These results highlight a promising alternative plasmonic nanomaterial to rewarm cryopreserved biological systems in the future.

Surface Engineering and Controlled Ripening for Seed-Mediated Growth of Au Islands on Au Nanocrystals.

Engineering the nucleation and growth of plasmonic metals (Ag and Au) on their pre-existing seeds is expected to produce nanostructures with unconventional morphologies and plasmonic properties that may find unique applications in sensing, catalysis, and broadband energy harvesting. Typical seed-mediated growth processes take advantage of the perfect lattice match between the deposited metal and seeds to induce conformal coating, leading to either simple size increases (e.g., Au on Au) or the formation of core-shell structures (e.g., Ag on Au) with limited morphology change. In this work, we show that the introduction of a thin layer of metal with considerable lattice mismatch can effectively induce the nucleation of well-defined Au islands on Au nanocrystal seeds. By controlling the interfacial energy between the seed and the deposited material, the oxidative ripening, and the surface diffusion of metal precursors, we can regulate the number of islands on the seeds and produce complex Au nanostructures with morphologies tunable from core-satellites to tetramers, trimers, and dimers.

Polarization-Modulated Multidirectional Photothermal Actuators.

Photothermal actuators have attracted increasing attention due to their ability to convert light energy into mechanical deformation and locomotion. This work reports a freestanding, multidirectional photothermal robot that can walk along a predesigned pathway by modulating laser polarization and on-off switching. Magnetic-plasmonic hybrid Fe3 O4 /Ag nanorods are synthesized using an unconventional templating approach. The coupled magnetic and plasmonic anisotropy allows control of the rod orientation, plasmonic excitation, and photothermal conversion by simply applying a magnetic field. Once the rods are fixed with desirable orientations in a bimorph actuator by magnetic-field-assisted lithography, the bending of the actuator can be controlled by switching the laser polarization. A bipedal robot is created by coupling the rod orientation with the alternating actuation of its two legs. Irradiating the robot by a laser with alternating or fixed polarization synergistically results in basic movement (backward and forward) and turning (including left-, right-, and U-turn), respectively. A complex walk along predesigned pathways can be potentially programmed by combining the movement and turning modes of the robots. This strategy provides an alternative driving mechanism for preparing functional soft robots, thus breaking through the limitations in the existing systems in terms of light sources and actuation manners.

Magnetically Tunable Plasmon Coupling of Au Nanoshells Enabled by Space-Free Confined Growth.

We report the unconventional space-free confined growth of Au nanoshells with well-defined plasmonic properties and active tuning of their plasmon coupling by the nanoscale magnetic assembly. The seeded growth of Au exclusively occurred at the hard-soft interfaces between the Fe3O4 core and phenolic resin without the need of creating a limiting space, which represents a general and elegant approach to various core-shell nanostructures. The deformability of permeable phenolic layers plays an essential role in regulating the interfacial growth of Au nanoshells. While the polymer elasticity suppresses the radial deposition of Au atoms, their high deformability can afford enough spaces for the formation of conformal metallic shells. The coupled magnetic-plasmonic properties allow active tuning of the plasmon coupling and the resonant scattering of Au nanoshells by the magnetic assembly of the hybrid nanoparticles into plasmonic chains, whose potentials in applications have been demonstrated in designing transparent displays and anticounterfeiting devices.

Tannic-Acid-Coated Polypropylene Membrane as a Separator for Lithium-Ion Batteries.

To solve the wetting capability issue of commercial polypropylene (PP) separators in lithium-ion batteries (LIBs), we developed a simple dipping surface-coating process based on tannic acid (TA), a natural plant polyphenol. Fourier transform infrared and X-ray photoelectron measurements indicate that the TA is coated successfully on the PP separators. Scanning electron microscopy images show that the TA coating does not destroy the microporous structure of the separators. After being coated with TA, the PP separators become more hydrophilic, which not only enhances the liquid electrolyte retention ability but also increases the ionic conductivity. The battery performance, especially for power capability, is improved after being coated with TA. It indicates that this TA-coating method provides a promising process by which to develop an advanced polymer membrane separator for lithium-ion batteries.