Magnetic Assembly of Nanocubes for Orientation-Dependent Photonic Responses

Tunable and Non-Invasive Printing of Transmissive Interference Colors with 2D Material Inks

Interference colors hold significant importance in optics and arts. Current methods for printing interference colors entail complex procedures and large-scale printing systems for the scarcity of inks that exhibit both sensitivity and tunability to external fields. The production of highly transparent inks capable of rendering transmissive colors has presented ongoing challenges. Here, a type of paramagnetic ink based on 2D materials that exhibit polychrome in one magnetic field is invented. By precisely manipulating the doping ratio of magnetic elements within titanate nanosheets, the magneto-optical sensitivity named Cotton-Mouton coefficient is engineerable from 728 to a record high value of 3272 m-1 T-2, with negligible influence on its intrinsic wide optical bandgap. Combined with the sensitive and controllable magneto-responsiveness of the ink, modulate and non-invasively print transmissive interference colors using small permanent magnets are precised. This work paves the way for preparing transmissive interference colors in an energy-saving and damage-free manner, which can expand its use in widespread areas.

Hollow mesoporous cubic silica self-assembling into photonic crystals with rhombohedral lattices and vivid structural colors for anti-counterfeiting

Colloidal photonic crystals (PCs) feature face-centered cubic (FCC) lattices since spherical particles are usually used as building blocks; however, constructing structural colors originating from PCs with non-FCC lattices is still a big challenge due to the difficulty in preparing non-spherical particles with tunable morphologies, sizes, uniformity, and surface properties and assembling them into ordered structures. Here, uniform, positively charged, and hollow mesoporous cubic silica particles (hmc-SiO2) with tunable sizes and shell thicknesses prepared by a template approach are used to self-assemble into PCs with rhombohedral lattice. The reflection wavelengths and structural colors of the PCs can be controlled by altering the sizes or the shell thicknesses of the hmc-SiO2. Additionally, photoluminescent PCs have been fabricated by taking the advantage of the click chemistry between amino silane and isothiocyanate of a commercial dye. The PC pattern achieved by a hand-writing way with the solution of the photoluminescent hmc-SiO2 instantly and reversibly shows the structural color under visible light but a different photoluminescent color under UV illumination, which is useful for anticounterfeiting and information encryption. The non-FCC structured and photoluminescent PCs will upgrade the basic understanding of the structural colors and facilitate their applications in optical devices, anti-counterfeiting, and so forth.

Theoretical Investigation on the Metamaterials Based on the Magnetic Template-Assisted Self-Assembly of Magnetic-Plasmonic Nanoparticles for Adjustable Photonic Responses

The assembly of artificial nano- or microstructured materials with tunable functionalities and structures, mimicking nature's complexity, holds great potential for numerous novel applications. Despite remarkable progress in synthesizing colloidal molecules with diverse functionalities, most current methods, such as the capillarity-assisted particle assembly method, the ionic assembly method based on ionic interactions, or the field-directed assembly strategy based on dipole-dipole interactions, are confined to focusing on achieving symmetrical molecules. But there have been few examples of fabricating asymmetrical colloidal molecules that could exhibit unprecedented optical properties. Here, we introduce a microfluidic and magnetic template-assisted self-assembly protocol that relies mainly on the magnetic dipole-dipole interactions between magnetized magnetic-plasmonic nanoparticles and the mechanical constraints resulting from the specially designed traps. This novel strategy not only requires no specific chemistry but also enables magnetophoretic control of magnetic-plasmonic nanoparticles during the assembly process. Moreover, the assembled asymmetrical colloidal molecules also exhibit interesting hybridized plasmon modes and produce exotic optical properties due to the strong coupling of the individual nanoparticle. The ability to fabricate asymmetrical colloidal molecules based on the bottom-up method opens up a new direction for the fabrication of novel microscale structures for biosensing, patterning, and delivery applications.

Stimulus-responsive nonclose-packed photonic crystals: fabrications and applications

Stimulus-responsive photonic crystals (PCs) possessing unconventional nonclosely packed structures have received growing attention due to their unique capability of mimicking the active structural colors of natural organisms (for example, chameleons' mechanochromic properties). However, there is rarely any systematic review regarding the progress of nonclose-packed photonic crystals (NPCs), involving their fabrication, working mechanisms, and applications. Herein, a comprehensive review of the fundamental principles and practical fabrication strategies of one/two/three-dimensional NPCs is summarized from the perspective of designing nonclose-packed structures. Subsequently, responsive NPCs with exciting functions and working mechanisms are sorted and delineated according to their diverse responses to physical (force, temperature, magnetic, and electric fields), chemical (ions, pH, vapors, and solvents), and biological (glucose, organophosphate, creatinine, and bacteria) stimuli. We then systematically introduced and discussed the applications of NPCs in sensors, printing, anticounterfeiting, display, optical devices, etc. Finally, the current challenges and development prospects for NPCs are presented. This review not only concludes the design principle for NPCs but also provides a significant basis for the exploration of next-generation NPCs.

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.

Permanent irreversible structural color based on core-shell chemically bonded SiO2@P(St-BA) particles

Novel core-shell chemically bonded SiO₂@P(St-BA) particles were designed and self-assembled to prepare photonic crystals. Due to the irreversible collapse of polymer shells during hot-pressing, SiO₂@P(St-BA) particles could provide new ideas for high-stability and bright red-shifted structural color patterns.

Magnetic photonic crystals for biomedical applications

Magnetic photonic crystals (PhCs), as a representative responsive structural color material, have attracted increasing research focus due to merits such as brilliant refraction colors, instant responsiveness, and excellent manipuility, thus having been widely applied for color displaying, three-dimensional printing, sensing, and so on. Featured with traits such as contactless manner, flexible orientations, and adjustable intensity of external magnetism, magnetic PhCs have shown great superiority especially in the field of biomedical applications such as bioimaging and auxiliary clinical diagnosis. In this review, we summarize the current advancements of magnetic PhCs. We first introduce the fundamental principles and typical characteristics of PhCs. Afterward, we present several typical self-assembly strategies with their frontiers in practical applications. Finally, we analyze the current situations of magnetic PhCs and put forward the prospective challenges and future development directions.

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.

Active control of equilibrium, near-equilibrium, and far-from-equilibrium colloidal systems

The development of top-down active control over bottom-up colloidal assembly processes has the potential to produce materials, surfaces, and objects with applications in a wide range of fields spanning from computing to materials science to biomedical engineering. In this review, we summarize recent progress in the field using a taxonomy based on how active control is used to guide assembly. We find there are three distinct scenarios: (1) navigating kinetic pathways to reach a desirable equilibrium state, (2) the creation of a desirable metastable, kinetically trapped, or kinetically arrested state, and (3) the creation of a desirable far-from-equilibrium state through continuous energy input. We review seminal works within this framework, provide a summary of important application areas, and present a brief introduction to the fundamental concepts of control theory that are necessary for the soft materials community to understand this literature. In addition, we outline current and potential future applications of actively-controlled colloidal systems, and we highlight important open questions and future directions.

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.

Magnetic/Plasmonic Hybrid Nanodisks with Dynamically Tunable Mechano-Chiroptical Responses

Chiral plasmonic nanostructures have promising applications in optoelectronics due to their chiroptical responses. However, achieving active tuning of optical chirality remains challenging. Here, we develop stretchable chiroptical films with mechanically tunable extrinsic chirality by assembling hexagonal magnetic/plasmonic hybrid nanodisks in magnetic fields. The nanodisks, synthesized using a space-confined growth method, display three distinct plasmonic resonance modes at the UV-vis-NIR region, which red shift with increasing size as demonstrated by simulation and experimental results. The coupled magnetic and plasmonic anisotropy allows convenient control over the plasmonic resonance modes by altering the strength or direction of external magnetic fields. Further, magnetically aligning the nanodisks in a stretchable polymer film produces superstructures with extrinsic chirality, displaying selective absorption of circularly polarized light and inverted circular dichroism due to the linear dichroism-linear birefringence effect. Reversible mechanical stretching allows for continuous switching of circular dichroism in a wide range (from -1° to +1°). The efficient magnetic alignment of hybrid nanodisks in the hydrogel provides a simple and effective strategy for designing stretchable optical devices with tunable extrinsic chirality.

Study on the Preparation and Optical Properties of Graphene Oxide@Fe3O4 Two-Dimensional Magnetically Oriented Nanocomposites

In this work, graphene oxide@Fe3O4 (GO@Fe3O4) two-dimensional magnetically oriented nanocomposites were prepared through the co-precipitation approach using graphene oxide as the carrier and FeCl3·6H2O and FeSO4·7H2O as iron sources. The samples were characterized and tested by X-ray diffraction, a transmission electron microscope, Fourier-transform infrared spectroscopy, a vibrating-specimen magnetometer, a polarized optical microscope, an optical microscope, etc. The effects of material ratios and reaction conditions on the coating effects of Fe3O4 on the GO surface were investigated. The stable GO@Fe3O4 sol system was studied and constructed, and the optical properties of the GO@Fe3O4 sol were revealed. The results demonstrated the GO@Fe3O4 two-dimensional nanocomposites uniformly coated with Fe3O4 nanoparticles were successfully prepared. The GO@Fe3O4 two-dimensional nanocomposites exhibited superparamagnetic properties at room temperature, whose coercive force was 0. The stable GO@Fe3O4 sol system could be obtained by maintaining 1 < pH < 1.5. The GO@Fe3O4 sol showed magneto-orientation properties, liquid crystalline properties, and photonic crystal properties under the influence of the external magnetic field. The strength and direction of the magnetic field and the solid content of the GO@ Fe3O4 sol could regulate the aforementioned properties. The results suggest that GO@Fe3O4 two-dimensional magnetically oriented nanocomposites have potential applications in photonic switches, gas barriers, and display devices.

Magnetic nanofluids (Ferrofluids): Recent advances, applications, challenges, and future directions

Impelled by the need to find solutions to new challenges of modern technologies new materials with unique properties are being explored. Among various new materials that emerged over the decades, magnetic fluids exhibiting interesting physiochemical properties (optical, thermal, magnetic, rheological, apparent density, etc.) under a magnetic stimulus have been at the forefront of research. In the initial phase, there has been a fervent scientific curiosity to understand the field-induced intriguing properties of such fluids but later a plethora of technological applications emerged. Magnetic nanofluid, popularly known as ferrofluid, is a colloidal suspension of fine magnetic nanoparticles, has been at the forefront of research because of its magnetically tunable physicochemical properties and applications. Due to their stimuli-responsive behaviour, they have been finding more applications in biology and other engineering disciplines in recent years. Therefore, a critical review of this topic highlighting the necessary background, the potential of this material for emerging technologies, and the latest developments is warranted. This review also provides a summary of various applications, along with the key challenges and future research directions. The first part of the review addresses the different types of magnetic fluids, the genesis of magnetic fluids, their synthesis methodologies, properties, and stabilization techniques are discussed in detail. The second part of the review highlights the applications of magnetic nanofluids and nanoemulsions (as model systems) in probing order-disorder transitions, scattering, diffraction, magnetically reconfigurable internal structures, molecular interaction, and weak forces between colloidal particles, conformational changes of macromolecules at interfaces and polymer-surfactant complexation at the oil-water interface. The last part of the review summarizes the interesting applications of magnetic fluids such as heat transfer, sensors (temperature, pH, urea detection, cations, defect detection sensors), tunable optical filters, removal of dyes, dynamic seals, magnetic hyperthermia-based cancer therapy and other biomedical applications. The applications of magnetic nanofluids in diverse disciplines are growing day by day, yet there are challenges in their practical adaptation as field-worthy or packaged products. This review provides a pedagogical description of magnetic fluids, with the necessary background, key concepts, physics, experimental protocols, design of experiments, challenges and future directions.

Site-Selective Assembly of Centimeter-Scale Arrays of Precisely Oriented Magnetic Nanoellipsoids

The precise organization and orientation of anisotropic nanoparticles (NPs) on substrates over a large area is key to the application of NP assemblies in functional optical, electronic, and magnetic devices, but achieving such high-precision NP assembly still remains challenging. Here, we demonstrate the site-selective assembly of magnetic nanoellipsoids into large-area precisely positioned, orientationally controlled arrays via a combination of chemical patterning and magnetic manipulation. Magnetic ellipsoidal NPs are selectively positioned on predetermined chemical patterns with high fidelity through electrostatic interactions and aligned uniformly in line with an applied magnetic field. The position, orientation, and interparticle spacing of the ellipsoids can be precisely tuned by controlling the chemical patterns and magnetic field. This approach is simple to implement and can generate centimeter-scale arrays in high yield (up to 99%). The arrays exhibit collective magnetic responses that are dependent on the orientation of the ellipsoids. This work offers a tool for the fabrication of precisely engineered arrays of anisotropic NPs for applications such as metasurface and artificial spin ice.

Design and Self-Assembly of Polyhedron Particles to Construct Iridescent Structural Colors

Polyhedron particles exhibit unique physical properties in constructing novel materials. Here, the polystyrene (PS) polyhedron particles were fabricated via dispersion polymerization, and their morphologies can be controlled by tuning the divinylbenzene (DVB) content and polarity of the reaction medium. The possible formation mechanism is the asymmetric distribution of cross-linked networks during the phase separation process. In addition, the large-scale iridescent structural colors based on polyhedrons were obtained and further explored their applications in smart displays. This presented method guides the fabrication of anisotropic particles and their further assembly to construct novel materials.

Smart Nanostructured Materials for SARS-CoV-2 and Variants Prevention, Biosensing and Vaccination

The coronavirus disease 2019 (COVID-19) pandemic, caused by the severe acute respiratory syndrome coronavirus 2 (SARS-CoV-2), has raised great concerns about human health globally. At the current stage, prevention and vaccination are still the most efficient ways to slow down the pandemic and to treat SARS-CoV-2 in various aspects. In this review, we summarize current progress and research activities in developing smart nanostructured materials for COVID-19 prevention, sensing, and vaccination. A few established concepts to prevent the spreading of SARS-CoV-2 and the variants of concerns (VOCs) are firstly reviewed, which emphasizes the importance of smart nanostructures in cutting the virus spreading chains. In the second part, we focus our discussion on the development of stimuli-responsive nanostructures for high-performance biosensing and detection of SARS-CoV-2 and VOCs. The use of nanostructures in developing effective and reliable vaccines for SARS-CoV-2 and VOCs will be introduced in the following section. In the conclusion, we summarize the current research focus on smart nanostructured materials for SARS-CoV-2 treatment. Some existing challenges are also provided, which need continuous efforts in creating smart nanostructured materials for coronavirus biosensing, treatment, and vaccination.

Magnetic Field-Modulated Plasmonic Scattering of Hybrid Nanorods for FFT-Weighted OCT Imaging in NIR-II

We report a method for fast Fourier transform (FFT)-weighted optical coherence tomography (OCT) in the second biological tissue transparency window by actively modulating the plasmonic scattering of Fe₃O₄@Au hybrid nanorods using magnetic fields. Instead of tracking the nanoparticles' lateral displacement in conventional magnetomotive OCT imaging, we monitor the nanorod rotation and optical signal changes under an alternating magnetic field in real time. The coherent rotation of the nanorods with the field produces periodic OCT signals, and the FFT is then used to convert the periodic OCT signals in the time domain to a single peak in the frequency domain. This allows automatic screening of nanorod signals from the random biological noises and reconstruction of FFT-weighted images using a computer program based on a time-sequence image set. Compared with conventional magnetomotive OCT, the FFT-weighted imaging technique creates enhanced OCT images with dB-scale contrast over an order of magnitude higher than the original images.

Construction of magnetic nanochains to achieve magnetic energy coupling in scaffold

Background

Fe₃O₄ nanoparticles are highly desired for constructing endogenous magnetic microenvironment in scaffold to accelerate bone regeneration due to their superior magnetism. However, their random arrangement easily leads to mutual consumption of magnetic poles, thereby weakening the magnetic stimulation effect.

Methods

In this study, magnetic nanochains are synthesized by magnetic-field-guided interface co-assembly of Fe₃O₄ nanoparticles. In detail, multiple Fe₃O₄ nanoparticles are aligned along the direction of magnetic force lines and are connected in series to form nanochain structures under an external magnetic field. Subsequently, the nanochain structures are covered and fixed by depositing a thin layer of silica (SiO₂), and consequently forming linear magnetic nanochains (Fe₃O₄@SiO₂). The Fe₃O₄@SiO₂ nanochains are then incorporated into poly l-lactic acid (PLLA) scaffold prepared by selective laser sintering technology.

Results

The results show that the Fe₃O₄@SiO₂ nanochains with unique core–shell structure are successfully constructed. Meanwhile, the orderly assembly of nanoparticles in the Fe₃O₄@SiO₂ nanochains enable to form magnetic energy coupling and obtain a highly magnetic micro-field. The in vitro tests indicate that the PLLA/Fe₃O₄@SiO₂ scaffolds exhibit superior capacity in enhancing cell activity, improving osteogenesis-related gene expressions, and inducing cell mineralization compared with PLLA and PLLA/Fe₃O₄ scaffolds.

Conclusion

In short, the Fe₃O₄@SiO₂ nanochains endow scaffolds with good magnetism and cytocompatibility, which have great potential in accelerating bone repair.

Voltage-tunable elastomer composites that use shape instabilities for rapid structural color changes

Structurally colored materials can switch colors in response to external stimuli, which makes them potentially useful as colorimetric sensors, dynamic displays, and camouflage. However, their applications are limited by the angular dependence, slow response, and absence of synchronous control in time and space. In addition, out-of-plane deformation from shape instability easily occurs in photonic films, leading to inhomogeneous colors in photonic-crystal materials. To address these challenges, we combine structurally colored photonic glasses and dielectric elastomer actuators. We use an external voltage signal to tune color changes quickly (much less than 0.1 s). The photonic glassses produce colors with low angular dependence, so that their colors are homogeneous even when they become curved due to voltage-triggered instabilities (buckling or wrinkling). As proof of concept, we present a pixelated display in which segments can be independently and rapidly turned on and off. This wide-angle, instability-tolerant, color-changing platform could be used in next-generation soft and curved color displays, camouflage with both shape and color changes, and multifunctional sensors.

Toward High Sensitivity: Perspective on Colorimetric Photonic Crystal Sensors

The sensitivity of colorimetric photonic crystal (PC) sensors have been significantly improved with the advancement of deformable structural color materials, structures design, sensing signal analysis methods, and fabrication strategies. In this perspective, the strategies toward high-sensitivity colorimetric PC sensors are discussed, from the perspectives of molecular design, single sensor construction, and multisensor assembly, which include incorporation of flexible polymer chains, construction of strong sensor-analyte interactions, incorporation of more soft materials, construction of stimuli-angle/orientation relationship, design of colorimetric sensors in series, and assembly of colorimetric PC sensors in parallel. Based on these strategies, progress of high-sensitivity colorimetric PC sensors in recent years is summarized, in terms of mechano-sensors and chemo-/biosensors. Specifically, PC based optical-electrical dual-signal sensing devices are included. Finally, the future development and challenges of high-sensitivity colorimetric PC sensors are presented, in regards to deformable properties, optical properties, analysis methods, and fabrication strategies.

Aptamer-Functionalized Barcodes in Herringbone Microfluidics for Multiple Detection of Exosomes

Tumor-derived exosomes are vital for clinical dynamic and accurate tumor diagnosis, thus developing sensitive and multiple exosomes detection technology has attracted remarkable attention of scientists. Here, a novel herringbone microfluidic device with aptamer-functionalized barcodes integration for specific capture and multiple detection of tumor-derived exosomes is presented. The barcodes with core-shell constructions are obtained by partially replicating the periodically ordered hexagonal close-packaged colloidal crystal beads. As their inverse opal hydrogel shell possesses rich interconnected pores, the barcodes could provide abundant surface area for functionalization of DNA aptamers to realize specific recognition of target exosomes. Besides, the encoded structure colors of the barcodes can be maintained stably during the detection events as their hardish cores are with sufficient mechanical strength. It is demonstrated that by embedding these barcodes in herringbone groove microfluidic device with designed patterns, the specific capture efficiency and synergetic detection of multiple tumor-derived exosomes in peripheral blood can be significantly improved due to enhanced resistance of turbulent flow. These features make the aptamer-functionalized barcodes and herringbone microfluidics integrated platform promising for exosomes extraction and dynamic tumor diagnosis.

Photonic crystal sensor for melamine based on magnetic molecularly imprinted nanoparticles self-assembled with an amphiphilic random copolymer

Magnetic molecularly imprinted nanoparticles (MMINPs) were obtained with a one-step process through miniemulsion self-assembly using an amphiphilic random copolymer as both an emulsifier and MMINP coating, oleic acid-modified magnetite nanoparticles as magnetic cores, and melamine (MEL) as the template molecule. MMINPs were assembled under an external magnetic field to construct photonic crystal (PC) sensor for naked-eye detection of MEL. The MMINPs were characterized by FT-IR, TEM, TGA, and VSM. The analytical performances of the magnetic molecularly imprinted PC sensor for MEL (MEL-MMIPCs) were investigated with respect to sensitivity, response time, selectivity, and stability. As the MEL concentration increases from 1.0 to 1.0 × 10⁶ μg/l, the reflection wavelength of MEL-MMIPCs shifted from 497 to 709 nm, and was linear with the logarithm of MEL concentration in this range. The detection limit was 0.21 μg/l (S/N = 3) and response time was within 30 s. The MEL-MMIPC sensor had an imprinting factor of 5.09, and selectivity factors for the analogs cyanuric acid and atrazine were 8.76 and 5.75, respectively, indicating the high sensitivity and selectivity. After 10 cycles of elution/response, MEL-MMIPCs still had a good ability to recognize MEL.

Reversible Mechanochromisms via Manipulating Surface Wrinkling

Mechanochromic structural-colored materials have promising applications in various domains. In this Letter, we report three types of reversible mechanochromisms in simple material systems by harnessing mechano-responsive wrinkling dynamics including (i) brightness mechanochromism (BM), (ii) hue change mechanochromism (HCM), and (iii) viewable angle mechanochromism (VAM). Upon stretching, the BM device exhibits almost a constant hue but reduces light brightness due to the postbuckling mechanics-controlled deformation, while the HCM device can change the hue from blue to red with almost constant intensity because of the linear elastic mechanics-controlled deformation. The VAM device shows a constant hue because of the thin film interference effect. However, the viewable angles decrease with increasing applied strain owing to the light scattering of wrinkles. All of the mechanochromic behaviors exhibit good reversibility and durability. We clearly elucidated the underlying mechanisms for different mechanochromisms and demonstrated their potential applications in smart displays, stretchable strain sensors, and antipeeping/anticounterfeiting devices.

Field-Induced Assembly and Propulsion of Colloids

Electric and magnetic fields have enabled both technological applications and fundamental discoveries in the areas of bottom-up material synthesis, dynamic phase transitions, and biophysics of living matter. Electric and magnetic fields are versatile external sources of energy that power the assembly and self-propulsion of colloidal particles. In this Invited Feature Article, we classify the mechanisms by which external fields impact the structure and dynamics in colloidal dispersions and augment their nonequilibrium behavior. The paper is purposely intended to highlight the similarities between electrically and magnetically actuated phenomena, providing a brief treatment of the origin of the two fields to understand the intrinsic analogies and differences. We survey the progress made in the static and dynamic assembly of colloids and the self-propulsion of active particles. Recent reports of assembly-driven propulsion and propulsion-driven assembly have blurred the conceptual boundaries and suggest an evolution in the research of nonequilibrium colloidal materials. We highlight the emergence of colloids powered by external fields as model systems to understand living matter and provide a perspective on future challenges in the area of field-induced colloidal phenomena.

Multicolor Photonic Pigments for Rotation-Asymmetric Mechanochromic Devices

Photonic crystals are extensively explored to replace inorganic pigments and organic dyes as coloring elements in printing, painting, sensing, and anti-counterfeiting due to their brilliant structural colors, chemical stability, and environmental friendliness. However, most existing photonic-crystal-based pigments can only display monochromatic colors once made, and generating multicolors has to start with designing different building blocks. Here, a novel photonic pigment featuring highly tunable structural colors in the entire visible spectrum, made by the magnetic assembly of monodisperse nanorods into body-centered-tetragonal photonic crystals, is reported. Their prominent magnetic and crystal anisotropy makes it efficient to generate multicolors using one photonic pigment by magnetically controlling the crystal orientation. Further, the combination of angle-dependent diffraction and magnetic orientation control enables the design of rotation-asymmetric photonic films that display distinct patterns and encrypted information in response to rotation. The efficient multicolor generation through precise orientational control makes this novel photonic pigment promising in developing high-performance structural-colored materials and optical devices.

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.

Flux and separation of magneto-active superballs in applied fields

The term "active matter" describes a class of out-of-equilibrium systems, whose ability to transform environmental to kinetic energy is sought after in multiple fields of science. A challenge that still remains is to craft nanometer-sized active particles, whose motion can be efficiently directed by externally applied bio-noninvasive stimuli. Adding a magnetic component and therefore being able to direct the motion of active nanoparticles with an applied magnetic field is one of the promising solutions in the field. In this study, we employ molecular dynamics simulations to predict an external field-induced flow that arises in mixtures of magneto-active nanosized cubic and spherical particles with distinct mutual orientations between magnetization and propulsion. We explain why the flux of the suspended particles in the field direction does not only depend on the angle between the active force, driving a particle forward, and the orientation of its magnetization, but also on particle shape and inter-particle interactions. Our results show that by tuning those parameters, one can achieve complete separation of particles according to their magnetization orientation. Based on our findings, along with optimizing the cargo properties of magneto-active nano-units, the actual composition of the magneto-active particle suspension can be characterized.

Ophthalmic sensing technologies for ocular disease diagnostics

Point-of-care diagnosis and personalized treatments are critical in ocular physiology and disease. Continuous sampling of tear fluid for ocular diagnosis is a need for further exploration. Several techniques have been developed for possible ophthalmological applications, from traditional spectroscopies to wearable sensors. Contact lenses are commonly used devices for vision correction, as well as for other therapeutic and cosmetic purposes. They are increasingly being developed into ocular sensors, being used to sense and monitor biochemical analytes in tear fluid, ocular surface temperature, intraocular pressure, and pH value. These sensors have had success in detecting ocular conditions, optimizing pharmaceutical treatments, and tracking treatment efficacy in point-of-care settings. However, there is a paucity of new and effective instrumentation reported in ophthalmology. Hence, this review will summarize the applied ophthalmic technologies for ocular diagnostics and tear monitoring, including both conventional and biosensing technologies. Besides applications of smart readout devices for continuous monitoring, targeted biomarkers are also discussed for the convenience of diagnosis of various ocular diseases. A further discussion is also provided for future aspects and market requirements related to the commercialization of novel types of contact lens sensors.

Coupling morphological and magnetic anisotropy for assembling tetragonal colloidal crystals

Morphological and magnetic anisotropy can be combined in colloidal assembly to create unconventional secondary structures. We show here that magnetite nanorods interact along a critical angle, depending on their aspect ratios and assemble into body-centered tetragonal colloidal crystals. Under a magnetic field, size-dependent attractive and repulsive domains develop on the ends and center of the nanorods, respectively. Our joint experiment-computational multiscale study demonstrates the presence of a critical angle in the attractive domain, which defines the equilibrium bonding states of interacting rods and leads to the formation of non–close-packed yet hard-contact tetragonal crystals. Small-angle x-ray scattering measurement attributes the perfect tetragonal phase to the slow assembly kinetics. The crystals exhibit brilliant structural colors, which can be actively tuned by changing the magnetic field direction. These highly ordered frameworks and well-defined three-dimensional nanochannels may offer new opportunities for manipulating nanoscale chemical transformation, mass transportation, and wave propagation.

A dual responsive photonic liquid for independent modulation of color brightness and hue

Responsive chromic materials are highly desirable in the fields of displays, anti-counterfeiting, and camouflage, but their advanced applications are usually limited by the unrealized delicate and independent tunability of their three intrinsic attributes of color. This work achieves the separate, continuous, and reversible modulation of structural color brightness and hue with an aqueous suspension of dual-responsive Fe₃O₄@polyvinylpyrrolidone (PVP)@poly(N-isopropyl acrylamide) (PNIPAM) flexible photonic nanochains. The underlying modulation mechanism of color brightness was experimentally and numerically deciphered by analyzing the morphological responses to stimuli. When an increasing magnetic field was applied, the random worm-like flexible photonic nanochains gradually orientated along the field direction, due to the dominant magnetic dipole interaction over the thermal motion, lengthening the orientation segment length up to the whole of the nanochains. Consequently, the suspension displays increased color brightness (characterized by diffraction intensity). Meanwhile, the color hue (characterized by diffraction frequency) could be controlled by temperature, due to the volume changes of the interparticle PNIPAM. The achieved diverse color modulation advances the next-generation responsive chromic materials and enriches the basic understanding of the color tuning mechanisms. With versatile and facile color tunability and shape patterning, the developed responsive chromic liquid promises to have attractive potential in full-color displays and in adaptive camouflages.

Structurally coloured contact lens sensor for point-of-care ophthalmic health monitoring

Point-of-care (POC) diagnosis is of great significance in offering precise and personalized treatment for patients with eye diseases. Contact lenses, as a kind of popular wearable device on the eye, provide a suitable platform for the integration of biosensors for the POC diagnosis of eye diseases. However, existing contact lens sensors usually involve complex electronics and circuits, the manufacturing of which is complicated and signal readout requires additional instruments. To realize the instrument-free detection of pathologically relevant signals of eye diseases, we successfully established a structurally coloured contact lens sensor with a tunable colour in this investigation, which can directly report changes in moisture and pressure that are critical signs for xerophthalmia and glaucoma diagnosis, respectively, by altering colours. Importantly, this structurally coloured contact lens sensor is made solely from a biocompatible hydrogel, without the addition of any chemical pigments, therefore exhibiting superior biosafety and wearing comfort for wearable applications. With both excellent biocompatibility and sensing capabilities, this structurally coloured contact lens sensors thus holds great promise for instrument-free ophthalmic health monitoring, which will benefit a large proportion of the population that have a high risk of eye disease.

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.

Hierarchically Molecular Imprinted Porous Particles for Biomimetic Kidney Cleaning

Blood purification by adsorption of excessive biomolecules is vital for maintaining human health. Here, inspired by kidney self-purification, which removes a number of biomolecules with different sizes simultaneously, hierarchical molecular-imprinted inverse opal particles integrated with a herringbone microfluidic chip for efficient biomolecules cleaning are presented. The particle possesses combinative porous structure with both surface and interior imprints for the specific recognition of small molecules and biomacromolecules. Additionally, the presence of the herringbone mixer largely improve the adsorption efficiency due to enhanced mixing. Moreover, the inverse opal framework of the particles give rise to optical sensing ability for self-reporting of the adsorption states. These features, together with its reusability, biosafety, and biocompatibility, make the platform highly promising for clinical blood purification and artificial kidney construction.

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.

Photonics in nature and bioinspired designs: sustainable approaches for a colourful world

Biological systems possess nanoarchitectures that have evolved for specific purposes and whose ability to modulate the flow of light creates an extraordinary diversity of natural photonic structures. In particular, the striking beauty of the structural colouration observed in nature has inspired technological innovation in many fields. Intense research has been devoted to mimicking the unique vivid colours with newly designed photonic structures presenting stimuli-responsive properties, with remarkable applications in health care, safety and security. This review highlights bioinspired photonic approaches in this context, starting by presenting many appealing examples of structural colours in nature, followed by describing the versatility of fabrication methods and designed coloured structures. A particular focus is given to optical sensing for medical diagnosis, food control and environmental monitoring, which has experienced a significant growth, especially considering the advances in obtaining inexpensive miniaturized systems, more reliability, fast responses, and the use of label-free layouts. Additionally, naturally derived biomaterials and synthetic polymers are versatile and fit many different structural designs that are underlined. Progress in bioinspired photonic polymers and their integration in novel devices is discussed since recent developments have emerged to lift the expectations of smart, flexible, wearable and portable sensors. The discussion is expanded to give emphasis on additional functionalities offered to related biomedical applications and the use of structural colours in new sustainable strategies that could meet the needs of technological development.

Assembly of nanocube super-structures directed by surface and magnetic interactions

We study the stabilisation of clusters and lattices of cuboidal particles with long-ranged magnetic dipolar and short-ranged surface interactions. Two realistic systems were considered: one with magnetisation oriented in the [001] crystallographic direction and the other with magnetisation along the [111] direction. We have studied magnetic nanocube clusters first in the limit of T = 0 K intending to elucidate the structural genesis of low energy configurations and then analysed finite-temperature behaviour of the same systems in simulations. Our results demonstrate that dipolar coupling can stabilise nanoparticle assemblies with cubic, planar, and linear arrangements seen previously in experiments. While attractive surface energy supports the formation of super-cubes, repulsion results in the elongated structures in the form of rods and chains. We observe the stabilisation of the ferromagnetic planar arrangements of the cubes standing on their corners and in contact over edges. We illustrate that minimal energy structures depend only on the size of the assembly and balance of surface repulsion and magnetic dipolar coupling. The presented results are scalable to different particle sizes and material parameters.

Recent advances in self-assemblies and sensing applications of colloidal photonic crystals

Colloidal photonic crystals (PCs), consisting of highly ordered monodisperse nanoparticles, have been carried out a great deal of research in recent decades due to the attributes of readable signal, easy modification and low cost. With these unique features, colloidal PCs have also gradually become a focus of candidates applied in sensing fields. In this review, an overview of recent advances in colloidal PCs including self-assemblies and sensing applications is illustrated. With respect to the development in self-assemblies of colloidal PCs, the review concentrates on the summary of responsive mechanisms, detection methods, responsive materials, unit cells and fabrication methods. In terms of advances in sensing application of colloidal PCs, various types of sensors are summarized based on the kinds and applications of target analytes. Furthermore, the current limitations and potential future directions of colloidal PCs in self-assemblies and sensing areas are also discussed.

Self-assembly of anisotropic nanoparticles into functional superstructures

Self-assembly of colloidal nanoparticles (NPs) into superstructures offers a flexible and promising pathway to manipulate the nanometer-sized particles and thus make full use of their unique properties. This bottom-up strategy builds a bridge between the NP regime and a new class of transformative materials across multiple length scales for technological applications. In this field, anisotropic NPs with size- and shape-dependent physical properties as self-assembly building blocks have long fascinated scientists. Self-assembly of anisotropic NPs not only opens up exciting opportunities to engineer a variety of intriguing and complex superlattice architectures, but also provides access to discover emergent collective properties that stem from their ordered arrangement. Thus, this has stimulated enormous research interests in both fundamental science and technological applications. This present review comprehensively summarizes the latest advances in this area, and highlights their rich packing behaviors from the viewpoint of NP shape. We provide the basics of the experimental techniques to produce NP superstructures and structural characterization tools, and detail the delicate assembled structures. Then the current understanding of the assembly dynamics is discussed with the assistance of in situ studies, followed by emergent collective properties from these NP assemblies. Finally, we end this article with the remaining challenges and outlook, hoping to encourage further research in this field.

Coupling magnetic and plasmonic anisotropy in hybrid nanorods for mechanochromic responses

Mechanochromic response is of great importance in designing bionic robot systems and colorimetric devices. Unfortunately, compared to mimicking motions of natural creatures, fabricating mechanochromic systems with programmable colorimetric responses remains challenging. Herein, we report the development of unconventional mechanochromic films based on hybrid nanorods integrated with magnetic and plasmonic anisotropy. Magnetic-plasmonic hybrid nanorods have been synthesized through a unique space-confined seed-mediated process, which represents an open platform for preparing next-generation complex nanostructures. By coupling magnetic and plasmonic anisotropy, the plasmonic excitation of the hybrid nanorods could be collectively regulated using magnetic fields. It facilitates convenient incorporation of the hybrid nanorods into polymer films with a well-controlled orientation and enables sensitive colorimetric changes in response to linear and angular motions. The combination of unique synthesis and convenient magnetic alignment provides an advanced approach for designing programmable mechanochromic devices with the desired precision, flexibility, and scalability.

Rotational Periodicity Display of the Tunable Wettability Pattern in a Photoswitch Based on a Response Bilayer Photonic Crystal

Although the forward diffraction of the three-dimensional (3D) photonic crystal is easily applied to a photoswitch, backward diffraction rainbows are rarely reported. The first rotational photoswitch based on a bilayer 3D photonic crystal with backward diffractions similar to those of two-dimensional photonic crystals was fabricated by vertically combining different thicknesses of nanoparticle templates. When rotating the bilayer photonic crystal, the opening or closing of the rotational photoswitch shows periodic reproducibility values of 30 and 60°. Different periods are regulated by the thickness and scattering effect of the top layer. Moreover, invisible patterns can be encoded and erased by changing the wettability via pH. Because of the decreasing of the refractive index differentials, it will be revealed rapidly when immersed in water. The revealed pattern can be periodically turned on and off by rotating the bilayer photonic crystal. It has great application prospects in optical prism, warning board, anti-counterfeiting, steganography, watermarking, and complex information coding.