Magnetically Responsive Photonic Nanochains

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.

Self-Assembly of Magnetic Nanochains in an Intrinsic Magnetic Dipole Force-Dominated Regime

Magnetic nanoparticle chains offer the anisotropic magnetic properties that are often desirable for micro- and nanoscale systems; however, to date, large-scale fabrication of these nanochains is limited by the need for an external magnetic field during the synthesis. In this work, the unique self-assembly of nanoparticles into chains as a result of their intrinsic dipolar interactions only is examined. In particular, it is shown that in a high concentration reaction regime, the dipole-dipole coupling between two neighboring magnetic iron cobalt (FeCo) nanocubes, was significantly strengthened due to small separation between particles and their high magnetic moments. This dipole-dipole interaction enables the independent alignment and synthesis of magnetic FeCo nanochains without the assistance of any templates, surfactants, or even external magnetic field. Furthermore, the precursor concentration ([M] = 0.016, 0.021, 0.032, 0.048, 0.064, and 0.096 m) that dictates the degree of dipole interaction is examined-a property dependent on particle size and inter-particle distance. By varying the spinner speed, it is demonstrated that the balance between magnetic dipole coupling and fluid dynamics can be used to understand the self-assembly process and control the final structural topology from that of dimers to linear chains (with aspect ratio >10:1) and even to branched networks. Simulations unveil the magnetic and fluid force landscapes that determine the individual nanoparticle interactions and provide a general insight into predicting the resulting nanochain morphology. This work uncovers the enormous potential of an intrinsic magnetic dipole-induced assembly, which is expected to open new doors for efficient fabrication of 1D magnetic materials, and the potential for more complex assemblies with further studies.

Core-shell Fe3O4@SnO2 nanochains toward the application of radar-infrared-visible compatible stealth

Multiband-compatible stealth materials play an increasingly crucial role in the field of modern military defence because they can enable the targeted objects to dodge advance detection technologies. In this study, chain-like Fe₃O₄@poly(ethyleneglycol dimethacrylate-co-methacrylic acid) nanocomposites were constructed as precursors through the magnetic field-induced distillation precipitation polymerisation. Then, the liquid-phase seed-mediated growth method, together with subsequent calcination, was applied to introduce SnO₂ shells and remove poly(ethyleneglycol dimethacrylate-co-methacrylic acid) shells, which led to the successful preparation of innovative core-shell Fe₃O₄@SnO₂ nanochains. The unique microstructure and appropriate components endowed nanochains with multiple functional applications. The minimum reflection loss value was approximately -39.4 dB (5.67 GHz), exhibiting excellent microwave absorption performance. The possible microwave absorption mechanisms involve interfacial polarisation, space charge polarisation, natural resonance, and multiple reflections and scatterings. The optimal infrared reflectivity reached 0.64, 0.51, and 0.37 in three atmospheric windows, indicating outstanding infrared stealth performance, which was attributed to the intense infrared reflection of SnO₂ shells. Furthermore, three nanochains showed different colours (dark green, brick red, and bright orange), revealing selection absorption for visible light. This can be attributed to the combined effect of visible responses of SnO₂ shells along with Bragg diffraction from the periodic arrangement of Fe₃O₄ particles in a single nanochain. Thus, core-shell Fe₃O₄@SnO₂ nanochains can be considered as promising radar-infrared-visible compatible stealth materials. This discovery opens a new means to exploit multiband-compatible stealth materials.

Colloidal Self-Assembly Approaches to Smart Nanostructured Materials

Colloidal self-assembly refers to a solution-processed assembly of nanometer-/micrometer-sized, well-dispersed particles into secondary structures, whose collective properties are controlled by not only nanoparticle property but also the superstructure symmetry, orientation, phase, and dimension. This combination of characteristics makes colloidal superstructures highly susceptible to remote stimuli or local environmental changes, representing a prominent platform for developing stimuli-responsive materials and smart devices. Chemists are achieving even more delicate control over their active responses to various practical stimuli, setting the stage ready for fully exploiting the potential of this unique set of materials. This review addresses the assembly of colloids into stimuli-responsive or smart nanostructured materials. We first delineate the colloidal self-assembly driven by forces of different length scales. A set of concepts and equations are outlined for controlling the colloidal crystal growth, appreciating the importance of particle connectivity in creating responsive superstructures. We then present working mechanisms and practical strategies for engineering smart colloidal assemblies. The concepts underpinning separation and connectivity control are systematically introduced, allowing active tuning and precise prediction of the colloidal crystal properties in response to external stimuli. Various exciting applications of these unique materials are summarized with a specific focus on the structure-property correlation in smart materials and functional devices. We conclude this review with a summary of existing challenges in colloidal self-assembly of smart materials and provide a perspective on their further advances to the next generation.

One-dimensional necklace-like assemblies of inorganic nanoparticles: Recent advances in design, preparation and applications

One-dimensional (1D) necklace-like assembly of inorganic nanoparticles exhibits unique collective properties, which are critical to open up new and remarkable opportunities in the field of nanotechnology. This review focuses on the recent advances in the production of these types of assemblies employing two strategies: colloidal synthesis and self-assembly procedures. After a brief description of the forces guiding nanoparticles towards the assembly, the main features of both strategies are discussed. Examples of approaches, typically involved in colloidal synthesis, are highlighted. The peculiar properties of 1D nanostructures are strictly associated with the nanoparticle arrangement in the form of highly ordered assemblies, which are attained during the synthesis both in the solution and using a template, as well as under the action of an external force. The various 1D necklace-like structures, created through nanoparticle self-assembly, demonstrate aligned, oriented nanoparticle organization. Diverse nature, size and shape of preformed particles as building blocks, along with utilizing different linkers, templates or external field lead to fabrication of 1D chain nanostructures with properties responsible for their wide applications. The unique structure-property relationship, both in colloidal synthesis, and self-assembly, offers broad spectrum of 1D necklace-like nanostructure implementations, illustrated by their use in photonics, electronics, electrocatalysis, magnetics.

Bioinspired Magnetic Nanochains for Medicine

Superparamagnetic iron oxide nanoparticles (SPIONs) have been widely used for medicine, both in therapy and diagnosis. Their guided assembly into anisotropic structures, such as nanochains, has recently opened new research avenues; for instance, targeted drug delivery. Interestingly, magnetic nanochains do occur in nature, and they are thought to be involved in the navigation and geographic orientation of a variety of animals and bacteria, although many open questions on their formation and functioning remain. In this review, we will analyze what is known about the natural formation of magnetic nanochains, as well as the synthetic protocols to produce them in the laboratory, to conclude with an overview of medical applications and an outlook on future opportunities in this exciting research field.

Nanomaterial Shape Influence on Cell Behavior

Nanomaterials are proven to affect the biological activity of mammalian and microbial cells profoundly. Despite this fact, only surface chemistry, charge, and area are often linked to these phenomena. Moreover, most attention in this field is directed exclusively at nanomaterial cytotoxicity. At the same time, there is a large body of studies showing the influence of nanomaterials on cellular metabolism, proliferation, differentiation, reprogramming, gene transfer, and many other processes. Furthermore, it has been revealed that in all these cases, the shape of the nanomaterial plays a crucial role. In this paper, the mechanisms of nanomaterials shape control, approaches toward its synthesis, and the influence of nanomaterial shape on various biological activities of mammalian and microbial cells, such as proliferation, differentiation, and metabolism, as well as the prospects of this emerging field, are reviewed.

Structural Color Materials for Optical Anticounterfeiting

The counterfeiting of goods is growing worldwide, affecting practically any marketable item ranging from consumer goods to human health. Anticounterfeiting is essential for authentication, currency, and security. Anticounterfeiting tags based on structural color materials have enjoyed worldwide and long-term commercial success due to their inexpensive production and exceptional ease of percept. However, conventional anticounterfeiting tags of holographic gratings can be readily copied or imitated. Much progress has been made recently to overcome this limitation by employing sufficient complexity and stimuli-responsive ability into the structural color materials. Moreover, traditional processing methods of structural color tags are mainly based on photolithography and nanoimprinting, while new processing methods such as the inkless printing and additive manufacturing have been developed, enabling massive scale up fabrication of novel structural color security engineering. This review presents recent breakthroughs in structural color materials, and their applications in optical encryption and anticounterfeiting are discussed in detail. Special attention is given to the unique structures for optical anticounterfeiting techniques and their optical aspects for encryption. Finally, emerging research directions and current challenges in optical encryption technologies using structural color materials is presented.

Bifunctional nanoscale magnetic chains with high saturation magnetization and catalytic activity

The conventional stirring method cannot be employed for chip-on-lab reactions such as microfluidic and microdroplet reactions as well as nanoscale reactions. Therefore, it is necessary to design a nanoscale magnetic stirrer with a high magnetic response towards the external magnetic field. In this work, one dimentional core@shell structured Fe-Fe₂O₃@poly(cyclotriphosphazene-co-4,4'-sulfonyldiphenol) magnetic nanochains modified by nickel hydroxide (denoted as Fe-Fe₂O₃@PZS@Ni(OH)₂ NCs) as nanoscale magnetic stirrer and recyclable self-mixing nanocatalysts are successfully prepared through three steps: synthesis of Fe-Fe₂O₃ nanochains (NCs) with high saturation magnetization, coating with poly (cyclotriphosphazene-co-4,4'-sulfonyldiphenol) (PZS), and nickel hydroxide's anchoring on the surface of Fe-Fe₂O₃@PZS NCs. The cross-linked polymer PZS is used to protect Fe-Fe₂O₃ NCs from chemical corrosion and as a platform for subsequent immobilization of nickel hydroxide. Characterization results show that the Fe-Fe₂O₃@PZS@Ni(OH)₂ NCs own chainlike structure and high saturation magnetization of 103 emu g^-1 at room temperature, exhibiting high magnetic response to the external rotating magnetic field. In the macro-reaction system for 4-nitrophenol (4-NP) reduction, the as-prepared Fe-Fe₂O₃@PZS@Ni(OH)₂ NCs show an apparent rate constant of about 0.60 min^-1. Furthermore, the Fe-Fe₂O₃@PZS@Ni(OH)₂ catalyst is reused ten times while no obvious loss of catalytic activity was observed. In the micro-reaction system, the Fe-Fe₂O₃@PZS@Ni(OH)₂ NCs also display good magnetic response and favorable catalytic activity for the hydrogenation of methylene blue. These results indicate that the bifunctional Fe-Fe₂O₃@PZS@Ni(OH)₂ NCs with high saturation magnetization have great potential as excellent nanocatalysts and as promising nanoscale magnetic stirrers.

Patterned Colloidal Photonic Crystals

Colloidal photonic crystals (PCs) have been well developed because they are easy to prepare, cost-effective, and versatile with regards to modification and functionalization. Patterned colloidal PCs contribute a novel approach to constructing high-performance PC devices with unique structures and specific functions. In this review, an overview of the strategies for fabricating patterned colloidal PCs, including patterned substrate-induced assembly, inkjet printing, and selective immobilization and modification, is presented. The advantages of patterned PC devices are also discussed in detail, for example, improved detection sensitivity and response speed of the sensors, control over the flow direction and wicking rate of microfluidic channels, recognition of cross-reactive molecules through an array-patterned microchip, fabrication of display devices with tunable patterns, well-arranged RGB units, and wide viewing-angles, and the ability to construct anti-counterfeiting devices with different security strategies. Finally, the perspective of future developments and challenges is presented.

Precise Assembly of Particles for Zigzag or Linear Patterns

Precise control of particles assembly has tremendous potential for fabricating intricate structures and functional materials. However, it is still a challenge to achieve one-dimensional assembly with precisely controlled morphology. An effective strategy is reported to precisely assemble particles into well-defined patterns by liquid confinement through controlling the viscosity of the assembly system. It is found that high viscosity of the system impedes particles rearrangement and facilitates the generation of zigzag or twined zigzag assembly structures, while low viscosity of the system allows particles to rearrange into linear or zipper structures driven by lowering the surface deformation of the liquid. As a result, precise control of different assembly patterns can be achieved through tuning the viscosity of solvent and size confinement ratios. This facile approach shows generality for particles assembly of different sizes and materials.