Magnetically Responsive Optical Modulation: from Anisotropic Nanostructures to Emerging Applications

Magnetically responsive optical modulation has emerged as a promising application in various fields such as smart windows, anti‐counterfeiting, and colorimetric sensors, due to its unique advantages in remote and nondestructive control, as well as precise and high‐contrast response. Compared to isotropic counterparts, anisotropic magnetic nanostructures (AMNs) take advantage of the interplay among magnetic dipoles, magnetocrystalline anisotropy, shape anisotropy, and crystal facets to control diverse dimensions of light propagation. By manipulating frequency, amplitude, polarization, and plasmonic resonance, AMNs can provide a variety of functional applications. A comprehensive overview of recent developments in optical modulation based on AMNs with a focus on their emerging applications and design strategies is presented. It starts with a brief introduction of typical AMN building blocks, highlighting various optical modulation modes, including tunable transmission, diffraction, polarization, and plasmonic resonance. The primary focus is to discuss design strategies for various applications, i.e., smart windows, anti‐counterfeiting labels, colorimetric sensors, and multifunctional‐driven devices. It concludes with the challenges and perspectives for magneto‐optical modulation based on anisotropic structures. Overall, this review provides in‐depth insight into building blocks, magnetic modulation strategies, and optical property modulation of AMNs, thereby accelerating practical applications of these structures in functional devices.


Introduction
3][4][5][6] They can undergo substantial changes in their optical properties in response to external stimuli, including electric, light, humidity, mechanical stress, magnetic field (H), and chemical stimuli. [7][15][16][17] As a typical magnetically optical modulation system, photonic crystal (PC) structures constructed from 1D magnetic arrays on the scale of visible light wavelengths have been extensively investigated. [18]Generally, the assemble of ferromagnetic/ superparamagnetic (SPM) nanoparticles (NPs) into 1D ordered chain-like structures by an external H enables dynamic modulation of interparticle distances and resulting diffracted light. [19]y carefully controlling the strength and spatial distribution of the H, the motion of individual NPs or their collective large assemblies can be precisely manipulated, leading to localized optical modulation or macroscopic visual changes. [20][32][33][34] Particularly, anisotropic magnetic nanostructures (AMNs), to minimize magnetocrystalline anisotropy energy, exhibit preferential alignment along their long axis, demonstrating an anisotropic response to H and offering additional spatial functionality for diverse optical applications. [35][38][39] Exploiting their anisotropic nature, these particles and assemblies surpass the limitations of general isotropic particles and enable the development of diverse functions.Shape anisotropy allows for different orientations of the particles under H, providing control over light transmittance. [40][54][55][56] Herein, we attempt to decipher a comprehensive understanding of how AMNs can be designed for various optical applications under magnetic fields by highlighting the recent advances in AMNs, focusing on emerging applications.Our review aims to inspire and advance technologies across multiple domains.We commence with the construction of anisotropic structures and subsequently explore several methods for modulating light propagation using magnetic fields, including field-tunable light transmission, diffraction, polarization, and plasmonic resonance and coupling.We elaborate on various applications, such as smart windows, anti-counterfeit labels, information encryption, sensing, and multifunctional optical nanorobots, highlighting the design strategies and optical properties based on their specific anisotropic behavior (Figure 1).

Anisotropic Structures
Anisotropic structures could be divided into several types, including nanocubes, nanorods, solid nanochains, and hydrogel-based nanochains.This section focuses on the preparation methods of anisotropic nanocrystals and magnetic assemblies.

Anisotropic Crystal Fabrication
Magnetic nanoparticles typically adopt a spherical shape due to their isotropic crystalline structure and the minimization of surface energy through traditional seeded-growth techniques.However, for asymmetric shapes, template-assisted methods are predominantly used, which confine particle growth within a template of specified geometry.Yin's group first reported the fabrication of magnetic magnetite (Fe 3 O 4 ) NRs using sacrificial templates (Figure 2a). [65]Iron oxyhydroxide (FeOOH) NRs were synthesized as sacrificial templates, coated with silica though the sol-gel process, and then transformed into FeOOH into Fe 3 O 4 nanostructures by reducing them while maintaining a similar morphology.Similarly, Fe 3 O 4 nanocubes were synthesized by adopting Prussian blue (PB) nanocubes as templates. [58]The silica layer coating plays a crucial role in retaining the morphology during reduction, preventing aggregation of the as-reduced magnetic Fe 3 O 4 , and facilitating the surface group modification for further functionalization (Figure 2b).For example, plasmonic tuning of Au nanorods was achieved by attaching them to magnetic nanorods through grafted amino groups on SiO 2 . [66]urthermore, magnetic-plasmonic hybrid nanorods were developed via a space-confined seed-mediated process, involving the growth of Au nanorods and encapsulation of reduced iron oxide in resorcinol phenol. [67]Similarly, through electrostatic selfassembly, cationic polyethyleneimine functionalized Fe 3 O 4 NPs deposited on the anionic bovine serum albumin functionalized Au NRs, forming magnetic Au NRs coating with magnetic satellite NPs. [59]A sacrificial template was primarily used for oxide AMNs, while other reviews can be referred to for different fabrication techniques, including controlled crystallization with shaping agents and natural anisotropic crystal growth methods. [37]

Magnetic Assembly
An alternative approach to the fabrication of AMNs involves the magnetic assembly of colloidal nanoparticles (NPs), enabling the creation of ordered arrangements with collective properties. [68,69]igure 2c illustrates this process, where magnetic NPs acquire a magnetic dipole moment m along the direction of the H.Over long distances, the magnetic dipole-dipole (D-D) force comes into Figure 1. Outline for the review: magnetically responsive optical modulation, anisotropic nanostructures, and applications.Left: Reproduced under the terms of the CC BY-NC 4.0 license. [57]Copyright 2021, The Authors, published by the American Association for the Advancement of Science.Reproduced with permission. [58]Copyright 2019, American Chemical Society.Reproduced under the terms of the CC BY 4.0 license. [59]Copyright 2022, The Authors, published by Wiley-VCH.Reproduced with permission. [60]Copyright 2020, Elsevier.Reproduced with permission. [39]Copyright 2011, WILEY-VCH.Reproduced with permission. [61]Copyright 2020, American Chemical Society.Reproduced with permission. [62]Copyright 2019, WILEY-VCH.Right: Reproduced with permission. [44]Copyright 2020, American Chemical Society.Reproduced with permission. [63]Copyright 2013, Royal Society of Chemistry.Reproduced with permission. [61]Copyright 2020, American Chemical Society.Reproduced with permission. [64]Copyright 2023, The Author(s), published by UESTC and John Wiley & Sons Australia, Ltd.
play, leading to the spontaneous self-assembly of particles into 1D chains aligned with the external magnetic field.This process results in the creation of the fundamental photonic structure. [20,70]o preserve their 1D shape anisotropy and superparamagnetism, the particles can be encapsulated in situ during the assembly of magnetic nanoclusters with silica.Due to their high sensitivity to the field, they may align even at weak magnetic fields ranging from 0.4 to 5 mT. [37]n the magnetic assembly process, anisotropic primitives such as nanocubes and nanorods could also serve as the building blocks (Figure 2d).Generally, an anisotropic NP is more readily magnetized along its long axis than along its short axis.Moreover, the anisotropically shaped NPs tend to aggregate with the configuration of minimized energy associated with D-D interaction. [37]he asymmetric nature of magnetic NPs leads to the formation of magnetic nanochains along specific directions through the combination of favorable magnetization and D-D coupling.Previous studies have demonstrated that nanocubes can overcome spatial configuration limitations and assemble into 1D nanochains in a [109] edge-to-edge manner. [58]In the case of magnetite nanorods with a large aspect ratio, shape anisotropy induces two distinct attractive domains and a repulsive center domain within each nanorod, directing the assembly of body-centered tetragonal (BCT) colloidal crystals. [57]Conversely, nanorods with a small aspect ratio (≈2) tend to form stretched hexagonal closed packing (HCP) structure due to interparticle electrostatic repulsion. [40,44]dditionally, anisotropy also comes from surface modifications.By metal vapor deposition of a thin magnetic film on the particle surface, AMNs such as Janus spheres, and one-sided metal-coated cubes could be constructed. [71,72]Localized dipole interactions on the coated patchy face may serve as structural directors for the assembly, driving particles in a specific direction and determining the assembled structures, which eventually enables the assembly of a wide variety of complex morphologies. [73]atchy microcubes with one face Co coating tend to assemble into stretched chain structures oriented along the field under the dipole-dipole attraction between the magnetic patches on the microcubes.When the field is removed, spatial constraints guide its remaining magnetically interacting assemblies to become capable of storing or releasing magnetic energy to realize self-folding and wrapping. [74]n addition to silica-coated solid NCs, soft NCs composed of isotropic particles and responsive hydrogels represent another important class of AMNs, with significant applications in visualization sensors. [61]As depicted in Figure 2e, hydrogel monomers, such as N-isopropylacrylamide (NIPAM), 2-hydroxyethyl methacrylate (HEMA), hydroxyethyl acrylate (HEA), acrylic acid (AA), acrylamide (AM), etc., are initially concentrated around Fe 3 O 4 @polyvinylpyrrolidone (PVP) NPs  [61] Copyright 2020, American Chemical Society.
through hydrogen bonding.Subsequently, under the influence of an H, the NPs assemble into 1D lines.Upon UV polymerization of the monomers and crosslinkers, the NPs are in situ immobilized, resulting in flexible 1D photonic nanochains (PNCs).
Moreover, in contrast to linear assembly within a stationary magnetic field, intricate chiral superstructures can be magnetically assembled by harnessing the quadrupole chiral field generated through the continuous rotation of the permanent magnet. [28]It is worth mentioning that by incorporating guests, including metals, polymers, semiconductors, and dyes into magnetic NPs, chiral superstructures can be rapidly formed by magnetically assembled materials of any chemical composition at all scales from molecular to nano and micrometer structures.

Modulation Mode
Based on the above AMNs, the primary modulations of light by magnetic field could be reflected in various aspects, including transmission, diffraction, polarization, and plasmonic properties.The distinguishing feature of the AMNs lies in the dis-tinct projection region against the incident light.Consequently, the orientation of anisotropic nanostructures, such as magnetic NRs, solid NCs, and microrods (Figure 3a), has successfully utilized magnetically modulated transmittance. [40,44]ight diffraction typically takes place in colloidal photonic crystals composed of chain-like 1D structures with an ordered arrangement (Figure 3b). [75,14]Bragg diffraction occurs due to the existence of a photonic forbidden band, and the diffraction wavelengths are determined by the equation m = 2ndsin, where m represents the diffraction order,  is the reflectance wavelength, n and d are the effective refractive index and periodicity, and  is the glancing angle of the incident light. [14]The spacing lattice of the formed 1D structures changes in response to variations in the strength of the H, thereby modifying the diffraction of light.Building upon this mechanism, photonic structures can be constructed using fixed chain structures, solid/soft nanochains, and anisotropic elements, exhibiting additional features such as strong angular dependence. [57,58,67]dditionally, AMNs with large aspect ratios, such as NRs, magnetic cellulose, exhibit orientational order, where the majority of anisotropic particles align in parallel to minimize the magnetic potential energy once the concentration surpasses a critical threshold. [42,60,62,76]Magnetic liquid crystals (LCs), similar to the nematic phase of organic LCs, are fabricated to exhibit analogous behavior.These magnetic LCs allow for instant and reversible control of optical polarization through external H (Figure 3c).By placing them between cross polarizers, the field direction within the plane parallel to the polarizers and perpendicular to the light can be manipulated.Light cannot pass through when the H direction is parallel or perpendicular to the polarizer, but can transmit when the field direction forms an angle with the polarizer.By rotating the LC orientation using the H, varying intensities of light can be transmitted through the orthogonal polarizers. [60]][79] For example, magnetic gold NRs exhibit highly angular-dependent excitations of transverse and longitudinal modes, which are determined by the angle between the long axis of the NRs and the polarization of incident light.The plasmonic excitation of the nanorods can be directly controlled by the magnetic field through changes in their orientation. [66]Similarly, in assemblies of magnetic plasmonic nanospheres, the magnetic field can be used to tailor the nearfield plasmonic coupling between nanoparticles in the secondary structure. [80]Based on the aforementioned mechanisms of magnetic modulation of light propagation, various applications have been developed, including smart windows, anti-counterfeit tags, sensors, and composite functional devices.

Smart Window
[83][84][85] The development of magnetically responsive systems with tunable transparent and reflective properties presents an intriguing alternative to SWs due to their ability to remotely and instantly control the structural arrangement.Particularly, AMNs with large shape anisotropy can exhibit varying degrees of scattering and reflectance depending on their orientations in the H, thereby effectively modulating light transmission.
Early research on magnetically responsive SWs involved composite colloid systems comprising anisotropic micromirrors and magnetic fluid, where the fluid was utilized to align stable dispersions of metal micromirrors (metal flakes with high aspect ratio). [86]Similarly, Yin's group developed a field-controlled switch with tunable transmittance of 20% under the field of 2800 Oe using Ag nanoplates and Fe 3 O 4 nanoparticle-based ferrofluids (Figure 4a).When a H 1 perpendicular to the incident light is applied, the Ag nanoplates show the complete or tilted triangular facets, effectively blocking the incident light and reducing transmission intensity (low luminous transmittance (LT) mode).When the field orientation (H 2 ) is switched to be parallel to the incident light, all Ag nanoplates align their edges along the light path, minimizing the absorption, scattering, or reflection of Ag nanoplates and increasing the transmittance accordingly (high LT mode). [87]However, this method generally requires a large H for optical modulation, relying on the directed rotation of anisotropic building blocks through the pulsed collision of magnetic components, which severely hindered practical implementation.
To address this challenge, a direct field-responsive system was fabricated by attaching amine-terminated -Fe 2 O 3 nanoparticles to the surface of Au microplates through strong coordination between the amine and the Au surface, enabling the manipulation of transmittance through H (Figure 4b).When a H parallel to the viewing direction ( = 0°) was applied, the microplates rapidly aligned with the H, resulting in a transparent solution with a faint brown tint due to the minimized cross-section of the microplates.Conversely, when the microplates aligned perpendicular to the viewing angle ( = 90°), the window exhibited reflective by maximizing the projected cross-section of the plates to reduce transmitted light. [88]Reproduced with permission. [87]Copyright 2012, American Chemical Society.b) Magnetic optical switch of magnetic Au microplates. is the angle between the applied H and the viewing direction.Reproduced with permission. [88]Copyright 2016, Royal Society of Chemistry.c) Nanorod-based SWs under H of different directions.Reproduced with permission. [62]Copyright 2019, Wiley-VCH GmbH.d) Scheme of the optical switch in 1D NC-based SW with the approaching of a magnet.Insets are the distribution and arrangement of NCs in different states.Reproduced with permission. [44]Copyright 2020, American Chemical Society.e) Schematic illustration of the formation of the magnetic microrods by applying an H. Reproduced with permission. [89]opyright 2021, Royal Society of Chemistry.f) i 1D NCs assembled from nanodisks, and solution switch between the transparent and opaque states.ii Transmission spectra of aqueous NCs rotated in the x-y plane under different H directions.The light incident along the y-axis.Reproduced under the terms of the CC BY 4.0 license. [40]Copyright 2019, The Authors, published by Wiley-VCH.
To enhance magnetic responsiveness, AMNs made entirely of magnetic NPs have recently been used as building blocks for the creation of magnetically responsive SWs.Zhang et al. designed a magnetically responsive optical system by employing a crystalline colloidal array of magnetic Fe 3 O 4 @SiO 2 nanorods, which assemble into LCs (see details in 4.2.3) and demonstrates a smart window display with different optical states under varying H orientations (Figure 4c). [62]Generally, AMNs with a large aspect ratio (i.e., large length of 1D structure) are preferred for achieving significant optical modulation.However, these structures often face the challenge of coercivity and remanence due to irreversible magnetization.To reconcile the contradiction between optical modulation capacity and magnetic response, 1D NCs can be employed, which preserve the SPM characteristics of NPs and possess significant shape anisotropy within their 1D structure.Figure 4d illustrates the design of SWs based on the 1D Fe 3 O 4 @SiO 2 NCs, where the proximity of the magnet allows for easy and rapid switching between opaque-translucenttransparent states by controlling the orientation of the NCs. [44]ost of the incident light reflected by randomly arranged NCs leads to an opaque state of the window, while the transmittance is significantly improved as NCs tend to be oriented along the vertical field direction.A similar approach is employed in the fabrication of SWs using 1D microrods assembled from Fe 2 O 3 nanoparticles under an H and stabilized with polyethylene glycol (PEG) (Figure 4e).PEG adheres to the NP surfaces and coats the entire surface of the microrods, ensuring their stability even after the H is removed.The direction of the microrods and resulting transmittance could be switched by controlling the H. [89] Furthermore, Yin's group extensively studies the dynamic tuning of optical transmission in 1D colloidal assemblies of nanostructures.They demonstrate that 1D nanodisc nanochains assembled in an "edge-to-edge" manner can be switched between the transparent and opaque states (Figure 4f (i)) by altering the direction of the chains. [40]When the incident light is along the yaxis, the optical transmittance of the nanodisc NCs decreases as the angle between the NCs and light increases from 0°to 45°and then remained unchanged from 45°to 90°in the visible range (Figure 4f (ii)).

Anti-Counterfeiting
[92][93] One promising approach involves the use of AMNs, which have the ability to exhibit color changes or pattern displays under specific conditions such as magnetic fields or polarized light. [22,94,95][98] In this section, we explore recent research focusing on anti-counterfeiting labels based on AMNs, which encompasses various strategies such as magnetic assembly, orientation of anisotropic structures, magnetic tuning of polarization, and magnetic tuning of plasmonic resonance.

Magnetic Assembly
Magnetic assembly offers initial possibilities for anticounterfeiting by providing tunable diffracted color under the H (Figure 5a). [99]However, the reliance of NP assembly and motion on a liquid environment presents challenges for direct application.To address this, Yin's group pioneered the embedding of Fe 3 O 4 @SiO 2 colloid particles into glycol (EG) liquid droplets within the polydimethylsiloxane (PDMS) matrix, enabling magnetic assembly and response in a nonaqueous environment (Figure 5b). [100]Building upon this work, Chen et al. developed an invisible photonic printing technique (Figure 5c) that enables precise structural printing of latent watermarks. [63,97]In their study, droplets in the patterned area are shielded from UV irradiation, allowing the watermark to appear and exhibit structural colors under the field.Conversely, the exposed area loses its photonic response due to the rupture of PDMS networks.Wang et al. integrated 3D printing to fabricate colorful magneto-responsive photonic crystal (MRPC) inks by emulsifying the PDMS precursor with Fe 3 O 4 @SiO 2 EG droplets for printing complex smart devices.As shown in Figure 5d, a butterfly pattern was designed and 3D-printed.With increasing field intensity, the distance between adjacent NPs was reduced within the droplets, reflecting shorter-wavelength light according to Bragg's law. [98]nother effective approach involves dispersing MRPC particles directly into networking hydrogels to construct magnetically responsive information decoding labels (Figure 5e).Zhao et al. dispersed monodisperse SPM Fe 3 O 4 @poly (4-styrenesulfonic acid-co-maleic acid)@SiO 2 colloidal nanocrystal clusters (CNCs) into acrylamide gel, fixing PC chain-like structure with uniform interparticle spacing through UV polymerization of the gel substrate under the field (Figure 5f).This arrangement with highly ordered NPs imparts controlled motion to hydrogels.As seen in Figure 5f (i), the dolphin-shaped hydrogel film experienced a bend angle ranging from 0°to 75°in response to different external H strengths.This change in orientation resulted in a corresponding shift in structural color, demonstrated by the reflection spectra presented in Figure 5f (ii).Notably, the reflection peak of the dolphin's tail progressively shifted from 530 to 653 nm. [101]Furthermore, Yu et al. fabricated photonic hydrogel microparticles (HMPs) by encapsulating magnetic colloids into microdroplets through microfluidics, where HMPs serve as isolated microreactors for magnetic assembly.Taking the HMPs as pixels of colored patterns, a dynamic displaying pattern with the number "1 2 3″" could be modulated by magnetic fields through the colloidal assembly (Figure 5g).Moreover, these HMPs possess enhanced mechanical performance (shear thinning and selfhealing) originating from dynamic borate bond-based interparticle linkage, allowing for the creation of customized structural colored objects using extrusion 3D printing. [102]

Orientation of Anisotropic Structures
Unlike magnetic assembly, the anti-counterfeiting labels based on the orientation of anisotropic structures no longer rely on an external H (Figure 5h).Instead, a switchable color distribution or dynamic color halo can be achieved through mechanical bending of the matrix or variations in the observation angle.This characteristic offers greater convenience for practical applications.Kim et al. fabricated a flexible polycarbonate PC thin film by immobilizing NPs into chain-like structures along the field using UV irradiation, which exhibits color changes corresponding to variations in curvature. [96]Similarly, PC film with specific patterns can be fabricated through lithographic photopolymerization (Figure 5i), where chain-like structures of different orientations are fixed within the matrix.By changing the viewing angles, color switching between the pattern and background can be observed due to the different lattice spacing. [103]To advance the application of magnetic security labels, we integrated the high-throughput screen printing technique (Figure 5j) into the label fabrication process.Lipophilic Fe 3 O 4 @PVP@poly (glycerol dimethacrylate) PNCs were synthesized via a selective concentration polymerization method.These PNCs were then mixed with solvent-free photopolymerizable epoxy resins, enabling the fabrication of durable and robust anti-counterfeiting labels. [104]he label shows gradually blueshifting colors in the digital pattern "100" from left to right.With the label tilting increasing from 5°to 60°, the left side of the pattern exhibits the blue-shift colors, while the right one exhibits the red-shift.In contrast, when the label tilted from -15°to -50°, all the PNCs monotonously decreased the diffraction angles, corresponding to the blue shift of the structural colors.
Adopting anisotropic primitives such as nanocubes and nanorods as secondary structures for security labels enhances, optical features and anti-counterfeiting performance can be Solid matrix embedded with microdroplets of assembled NPs.c) Invisible photonic print process for anti-counterfeiting watermarks.Reproduced with permission. [97]Copyright 2013, The Author(s), published by Springer Nature.d) 3D-printing of the customized pattern.Reproduced with permission. [98]opyright 2021, Royal Society of Chemistry.e) Net-structure hydrogel with field-assembled NPs.f) i Dolphin-patterned PC hydrogels at a bending angle ranging from 0°to 75°.ii The corresponding spectra of the dolphin's tail.Reproduced with permission. [101]Copyright 2019, Wiley-VCH GmbH.g) Photonic hydrogel microparticles and colored pattern triggered by the field.Reproduced with permission. [102]Copyright 2022, Wiley-VCH GmbH.h) Scheme of color switching with orientation change.i) Photonic printing with different orientational AMNs for anti-counterfeiting labels.Reproduced with permission. [103]Copyright 2011, American Chemical Society.j) Screen printing with lipophilic magnetic PNCs and the anti-counterfeiting label taken at different tilting angles.Reproduced with permission. [104]Copyright 2022, Wiley-VCH GmbH.k) Photonic nanocube NCs and thus based encryption film.i Scheme showing the nanocube NCs parallel to the surface and diffracting light out of the plane.ii Photomusk and the printed pattern against a dark background.Digital images of the pattern with counterclockwise rotation from 0°to 90°, 180°, and 270°.The pattern rotates counterclockwise, but it appears to rotate clockwise.Reproduced with permission. [58]Copyright 2019, American Chemical Society.l) Photonic pigments based on BCT colloidal crystals.i Scheme of the alignment of the pigments for mechanochromic responses.ii Digital photos of two photonic films showing the displayed patterns and their color changes in response to mechanical rotation.The incident angle is 40°in the top panel and 30°in the bottom.Reproduced with permission. [48]Copyright 2021, Wiley-VCH GmbH.
enhanced by introducing diverse photonic structures generated by shape anisotropy.The 1D nanocube NCs exhibit vivid structural colors across a wide range of viewing angles, enabling inplane periodic arrangement (Figure 5k (i)) while displaying the resulting structural colors in the out-of-plane direction."Magic" patterns can be thus fabricated, where the nanochains in the black areas are vertically aligned and horizontally aligned in the white regions (Figure 5k (ii)).When the film was illuminated from the front and viewed against a dark background, no obvious structural color was observed as demonstrated.The hidden  (a-c) Reproduced with permission. [60]Copyright 2020, Elsevier.d) SEM images of building blocks of magnetic LCs: Fe 3 O 4 @SiO 2 nanorods.e) i Bright-field images of two polarization-modulated patterns, POM images of the same patterns under cross polarizers ii before and iii after shifting the direction of the transmission axis of the polarizers for 45°.(d,e) Reproduced under the terms of Standard ACS AuthorChoice/Editors' Choice usage agreement. [95]Copyright 2014, American Chemical Society.f) Schematic illustration of the ORD measurement. denoted as the angle between the polarization direction of the analyzer and the horizontal direction.g) Digital images of the dispersion without (w/o mag) and with (w/ mag) a magnetic field.The  is 3°.h) Photos of the dispersion under different field conditions. switched from -30 to 30°during the measurement.(f-h) Reproduced with permission. [28]Copyright 2023, The Authors, published by the American Association for the Advancement of Science.
pattern with an orientation-dependent contrast appears when the light is illuminated from the backside.At the original orientation (0°), only the regions with vertical photonic chains exhibited a deep blue color.When the film was rotated 90°counterclockwise, the orientations of the embedded NCs in different regions reversed, leading to a color switch in these regions.The film appeared to be rotated clockwise, exhibiting a visually opposite rotation effect to the actual rotational direction of the film. [58]CT colloidal crystals assembled from NRs possess highly tunable structural colors, and multicolor photonic pigments could be thus fabricated (Figure 5l). [48,57]As shown in Figure 5l (i), The photonic pigments in the two domains were aligned symmetrically, with the same tilting angle (≈30°).Rotating such a patterned film in the x-y plane led to two different visual effects depending on the incident angle (Figure 5l (ii)).When the incident angle was ≈40°, the upper "triangle" pattern showed blue, whereas the lower "square" domain showed red due to the tilting-induced change in incident angles.Just rotating the film by 180°in the x-y plane caused color switching in the two domains due to the variations in their incident geometry (top panel).If the incident angle decreased to 30°, the diffraction in the two domains would blueshift, with red "square" turning to green and blue "triangle" disappearing.Simple in-plane rotation of the pattern would transform its observable pattern from a "square" to a "triangle" (bottom panel). [48]

Magnetic Tuning of Polarization
Polarization modulation induced by the magnetic field is observed in LCs and chiral superstructures, offering a novel approach for anti-counterfeiting applications. [62,60,95,105,28]Figure 6a illustrates the optical switching property of LCs in response to the changes in the direction of the magnetic field.When there is an angle between the LCs and the orthogonal polarizers, the light passes through the whole system, while when the LCs are parallel to the linear polarizer (P) or analyzer (A) direction, the light is blocked.Taking advantage of the magnetic orientation of the LCs, their polarization states can be permanently fixed within a solid matrix, enabling the design of high-level security devices that are imperceptible to the naked eye.The fabrication process for LC anti-counterfeiting labels is illustrated in Figure 6b, where AMNs are oriented long the field direction and regionally fixed using photolithographic techniques. [60]For instance, the magnetic cellulose microcrystals through the coordination between iron ions and hydroxyl groups of cellulose can serve as building blocks to construct transparent films with polarized patterns (Figure 6c).The latent pattern is invisible under normal light and only appears when sandwiched between cross-polarizers.The contrast can be reversed by adjusting the positions of the polarizers.This approach holds promise for high-level anti-counterfeiting due to its exceptional transparency. [60]Additionally, LCs composed of Fe 3 O 4 @SiO 2 NRs (Figure 6d) with different aspect ratios can also be used to design the anti-counterfeiting label. [95]Predesigned patterns with distinct optical polarizations are created, exhibiting a contrast pattern when illuminated with linear polarized light (Figure 6e).In a pattern consisting of two sets of NRs oriented at a 45°angle, the pattern is barely visible under normal light (bright-field images, Figure 6e (i)).However, when placed between cross polarizers, the latent pattern (the dot and panda patterns) emerges.The region cured with a parallel arrangement of NRs relative to the transmission axis of P appears dark under the Polarized optical microscope (POM), while the areas are bright with NRs oriented 45°(Figure 6e (ii)).Rotating the polarizer can completely reverse the dark and bright regions (Figure 6e (iii)).
Chiral superstructures comprising magnetic Au NRs demonstrate field-tunable optical rotatory dispersion (ORD), entailing the assessment of the polarization rotation exhibited by linearly polarized light. [28]Linearly polarized light is comprised of two circularly polarized light components of equal magnitude but opposing handedness.The substantial correlation between these components gives phase difference, leading to the polarization rotation observed in the incident light beam (Figure 6f).As shown in Figure 6g, in the absence of H, the solution appears dark because only minimal light passes through the analyzer.While a field is applied in the z-direction, light with a specific wavelength can traverse the entire system at a particular polarization angle (denoted as "").The upper region transitions to yellow, while the lower region turns red, suggesting the formation of a chiral superstructure with opposite handedness in these two regions.
The ORD effect results from superstructure rotational chirality and "", as seen in Figure 6h.Under a z-direction magnetic field, the upper region shifts through red, orange, yellow, and green, while the lower region exhibits the reverse color change.Shifting the magnetic field to the y-axis indicates a field chirality shift.Conversely, the absence or presence of an x-direction magnetic field yields no CD response, with dispersion solely changing contrast.This advancement holds significant promise for anticounterfeiting technology, enabling the authentication of objects or documents through their concealed chiral patterns.These pat-terns remain invisible to the naked eye but become discernible through the use of polarized lenses.

Magnetically Tunable Plasmonic Excitation
Similar strategies for hiding/showing variable patterns under polarized light have been successfully extended to anisotropic magnetic plasmonic nanostructures through the hybridization of optically active NPs.In the case of magnetic assembled chainlike plasmonic structures (Figure 7a), the near-field interparticle plasmonic coupling can be tailored by H.This coupling effect is maximized when the polarization directions (E) are parallel (|0°>) to the oriented chain-like structures (i.e., the direction of H), while almost disappearing when E is perpendicular to the H (|90°>).
The extinction spectra (Figure 7b) show two distinct resonant peaks appear in the hybrid Fe 3 O 4 @Au core-shell 1D magnetic assembly, i.e., an intrinsic resonant scattering peak (≈540 nm) and an inter-particle plasmon coupling peak (≈725 nm) of Au nanoshells. [106]The intensity of the coupling peak gradually decreases as the angle between H and E increases.By fixing the plasmonic chains with pre-designed orientations perpendicular to each other in a photocurable polymer, color-changing anticounterfeiting devices responsive to polarized light could be designed.Certain patterns (circle array, octagon in Figure 7c) appear red under horizontally polarized light due to the coupled resonant scattering of Au nanoshells, while the remaining area appears brown.When rotating the polarizer to a vertical position, the color in these two regions switched.Moreover, the color contrast in the two regions disappears at a relative angle of 45°or in the absence of a polarizer because the plasmonic excitation of the 1D structure is the same in both cases.The color contrast of perceived patterns could be simply altered by switching the polarization of the incident light, which can be potentially used for anticounterfeiting or information encryption.Additionally, due to localized surface plasmon resonance (LSPR), plasmonic nanoshells efficiently scatter light of a particular wavelength ≈540 nm, while being "transparent" at off-resonance wavelengths.Therefore, the film containing Fe 3 O 4 @Au NPs (Figure 7d) is almost transparent under ambient lighting, while selectively scattering green light under white light projection and showing letters of "UCR" (top row).Under monochromic illumination, the film only displays the letter "C" (bottom) because the projected light matches the nanoshell plasmonic band (≈540 nm), indicating its potential for transparent displays and anti-counterfeiting devices.
Furthermore, magnetically responsive plasmonic photonic crystals have been developed through the integration of fieldresponsive plasmonic scattering and optical diffraction of PC into one system (Figure 7e). [80]The film with 1D plasmonic PC structures exhibits two distinct color states when viewed from different sides: angular-dependent diffractive color from the front (0°-45°) and plasmonic scattering color (blue) from the back (-90°-0°).By stacking two patterned films together, such as a "panda head" pattern on the top and a "deer" pattern at the bottom (Figure 7f), functional photonic films can be achieved.When light is irradiated from top to bottom, the "panda head" pattern is disclosed (Figure 7f (i)) for front  b-d) Reproduced with permission. [106]Copyright 2020, American Chemical Society.e) Magnetically responsive plasmonic PCs with angular-dependent reflective colors due to the backward optical diffraction (front) and the angular-independent transmissive color caused by the forward plasmonic scattering (back).f) Double-layer anti-counterfeiting film with a top "panda head" pattern and a bottom "deer" pattern.Light irradiation from top gives the "panda head" pattern for i frontal observation due to optical diffraction and the "deer" pattern for ii back observation caused by plasmonic scattering.The situation is switched iii, iv when light is irradiated from the bottom.(e,f) Reproduced with permission. [80]Copyright 2023, American Chemical Society.g) Illustration of plasmonic excitation in Au NRs under different directions of the field.h) Extinction spectra of Au NRs under polarized light and corresponding solution images.i) Patterned plasmonic film under i normal light, ii vertical, iii horizontal, and iv 45°polarization, indicated by the white arrow.j) Mechanochromic film containing butterfly pattern with 45°aligned Au NRs to the surface.When subjected to pressure, the top plasmonic film expands upward or downward and exhibits an asymmetric mechanochromic response in the two wings of the butterfly due to the excitation of different plasmon modes of AuNRs.(h-j) Reproduced with permission. [67]Copyright 2020, The Author(s), published by Springer Nature.
observation due to the dominated optical diffraction, whereas the "deer" pattern is apparent for backward observation (Figure 7f (ii)) due to plasmonic scattering.This situation is inverted when the light is illuminated from the back, revealing only the diffractive color of the "deer" pattern (Figure 7f (iii)) from the front view and the plasmonic scattering color of the "panda head" (Figure 7f (iv)) from the back view.More intricate information patterns and expanding anti-counterfeiting applications could be expected by employing the integration of these different optical effects.
On the other hand, reversible magnetic tuning of plasmonic properties can be achieved in elongated plasmonic nanostructures, such as AuNRs attached to the magnetic nanorods, Fe 3 O 4 /Au NRs@resorcinol phenol hybrid nanostructures, and Au NRs coated with magnetic satellite NPs, by controlling their alignment relative to light polarization. [59,66,67]These structures exhibit two distinct resonance modes (Figure 7g) due to the dependence of electron oscillation on shape, i.e., the longitudinal mode when the H is applied parallel to the E (|0°>) and the transverse mode when the H is vertical to the E (|90°>).One of the modes could be completely suppressed by linearly polarized light.The extinction spectra shown in Figure 7h demonstrate the excitation of the longitudinal mode in Au NR at a longer wavelength (at |0°>), resulting in a green color in the Au NRs solution.Conversely, the transverse mode (at |90°>) at 525 nm is excited, causing the Au NRs solution to turn red.
By aligning the hybrid NRs along desired directions and fixing them in defined regions of the polymer matrices, polarized anti-counterfeiting labels could be fabricated (Figure 7i).The latent pattern is unnoticeable under normal light irradiation (Figure 7i (i)).Under vertical polarized light, latent patterns appear, with horizontal Au NRs appearing red and vertical ones appearing green (Figure 7i (ii)).A 90°rotation of the polarization reverses the color contrast, with green and red areas interchanging due to the conversion of excitation modes (Figure 7i (iii)).When illuminated with 45°polarized light, the pattern disappears again because both transverse and longitudinal modes are excited in the entire pattern area (Figure 7i (iv)).Furthermore, a mechanochromic film containing uniformly aligned Au NRs at a 45°angle in a "butterfly" pattern was fabricated.As the film expands upward or downward in response to the air pressure, the excitation of different plasmon modes of Au NRs leads to a color change in the wings (Figure 7j), which could also be potentially used for a simple colorimetric pressure indicator. [67]

Sensors with Visual Detection
[109] The utilization of 1D chain-like MRPC through instant magnetic assembly holds great potential for sensor design, as they can visualize ambient stimuli and translate signals into brilliant colors by modulating their photonic band gaps and reflected light properties (Figure 8a).

Bulk Sensors with Visual Detection
Currently, the primary strategy for sensors with visual detection involves modulating diffracted light by altering the lattice spacing of the PC through the physical expansion/contraction of the responsive gel in response to environmental changes.For example, Ge et al. fabricated a humidity sensor using a PC film by assembling Fe 3 O 4 @SiO 2 NPs in a poly (ethylene glycol) acrylates matrix (Figure 8b). [110]The matrix has a strong affinity for water, causing it to swell when exposed to humid air.As the room humidity (RH) increases from 33% to 97%, the lattice constant increases, resulting in a red shift of the diffraction peak and a color change in the film from blueish-green to red.Similarly, we develop a flexible, free-standing thermochromic film by incorporating Fe 3 O 4 @PVP NPs into chain-like PC structures within PNIPAM networks (Figure 8c).As the temperature decreases from 35 to 10 °C, the temperature-induced hydrophobichydrophilic transition causes the gradual expansion of the circle matrix, accompanied by the redshifts of the diffraction peak. [25]dditionally, microsphere PCs could be employed as colorimetric sensors by utilizing the diffraction of structural colors to detect H, T, etc. Yin.et al reported magnetochromatic microspheres by fixing 1D MRPC in UV-curable resin.These microspheres freely rotate under H (Figure 8d), aligning the PCs along the direction of H.The diffraction can be reversibly switched between off (H along the in-plane direction) and on (H along the out-of-plane direction) states. [112]Moreover, temperaturesensitive microspheres could be developed by replacing the matrix with PNIPAM hydrogel (Figure 8e).Typically, temperature responsive PC microspheres are fabricated using a micro syringe, where the ethylene glycol precursor solution of Fe 3 O 4 @PVP colloidal particles is formed into droplets through extrusion.Subsequently, a chain-like PC structure is formed within the droplet upon the application of a H, followed by UV polymerization to retain the PC structure.This design expands the response dimension of the microspheres, enabling the detection of both the direction of H and changes in ambient temperature.Distinct colors can be observed in the temperature range of 10-35 °C (Figure 8f). [111]

NC Sensors with Visual Detection
To improve response time and sensing resolution, we have developed a range of chain sensors based on soft hydrogel-based PNCs. [61]The external stimuli, such as pH and temperature, trigger noticeable changes in the volume of the hydrogel shell and the resulting interparticle spacing (Figure 9a), leading to a significant variation in the structural color of the PNCs, allowing for precise detection and accurate measurement of the applied stimulus.
As a typical example, Figure 9b (i) shows the reflection spectra of the Fe 3 O 4 @PVP@poly (HEA-co-AA) PNCs under varying pH. [61]As pH increases from 3.6 to 7.2, the hydrogel shells expand due to deprotonation of the carboxyl groups, resulting in carboxylates with higher solubility and larger interparticle distance.This leads to a red shift up to 210 nm in the reflection spectra of the PNCs.Then, a Y-shaped microfluidic device (Figure 9b  [110] Copyright 2011, Royal Society of Chemistry.c) Reflection spectra and digital photographs of the free-standing thermochromic film sensor.Reproduced with permission. [25]Copyright 2015, Royal Society of Chemistry.d) Schematic illustrations of the PC ball sensor, the diffraction state is turned on with NPs vertically aligned along out-of-plane H, and the diffraction color varies with T, H.The diffraction is off when NPs assembled along the in-plane direction.e) Synthetic procedures for the temperature responsive PC microspheres using a micro syringe and UV photo-polymerization. f) Digital photos of the PC microball sensors at various temperatures and the "off"/ "on" switch.(e,f) Reproduced with permission. [111]Copyright 2017, Royal Society of Chemistry.
(ii)) was utilized to measure the response time, where a neutral aqueous solution comprising PNCs and an HCl aqueous solution were injected respectively from the upper and bottom inlets with the same flow.This design ensured that the pH distribution remained static over time.As a result, a horizontal color stripe spanning 100 μm, ranging from red to blue, appears due to the diffusion of protons.Based on this observation, the response time of PNCs could be estimated as the distance (100 μm) divided by the speed (2.5 mm −1 s), which equals 40 ms.By analyzing the color change images captured by the dark-field optical microscope (OM) (Figure 9b (iii)), the lateral resolution of the sensor is estimated to be ≈2 μm, which is 2-3 orders of magnitudes higher than film or microsphere counterparts.Dual-responsiveness to both field strength and temperature is realized by substituting the hydrogel shell with thermoresponsive PNIPAM. [113]With increasing temperature, the diffracted wavelength gradually shifted from red to green due to the hydrophobic shrinkage of the PNIPAM shell (Figure 9c (i)).Under the influence of magnetic dipole interactions, the soft PNCs elongate as the H strength increases (Figure 9c (ii)).Therefore, the field could be visually detected according to the color brightness originating from the periodic structure of the PNCs (Figure 9c (iii)).
A real-time glucose monitor was also demonstrated using Fe 3 O 4 @PVP NPs with a poly (3-acrylamido phenylboronic acidco-N-2-hydroxyethyl acrylamide) (poly (AAPBA-co-HEAAm)) shell (Figure 9d). [114]With increasing glucose concentration ([glucose]), the thickness of hydrogel layers increases due to the coordination interaction between the phenylboronic acid functional groups on the hydrogel and glucose molecules.The PNC solution thus exhibited a color change from green to red (Figure 9d Different from the previously mentioned film and microsphere sensors (section 4.3.1)which have dimensions of at least tens of micrometers in three dimensions and are primarily suitable for bulk samples, these soft PNC sensors offer a distinct advantage.With the submicron size of the entire photonic crystal structure, they are well-suited for sensing and imaging microscale zones or heterogeneous microenvironments.The rapid iii Microenvironment imaging with dark-field OM.Reproduced with permission. [61]Copyright 2020, American Chemical Society.c) Temperature and field dual responsive PNCs.i The reflection spectra of PNCs under different temperatures.ii Bright-field OM images of PNCs at different H. iii Dark-field OM images with different color brightness under varying H. Reproduced with permission. [113]Copyright 2022, The Authors, published by Wiley-VCH.
response of the PNC sensor to stimuli is facilitated by the hydrogel shell, which has a thickness of only tens of nanometers, allowing for swift diffusion of the analyte to reach equilibrated states.Consequently, soft PNCs exhibit superior sensing capabilities with improved resolution and real-time response, surpassing the previous counterparts by 2-3 orders of magnitude.This design strategy enables the incorporation of diverse responsiveness and functionality in these hydrogel shell-coated PNCs, leveraging the versatility of hydrogels with different functional groups.

Multi-Functional Devices
In recent years, significant progresses have been made in the development of multi-functional smart systems by leveraging the rational design of spatial structures and visual features of AMNs, along with their unique mechanical characteristic arising from shape anisotropy.These advancements have led to the integration of multiple optical properties, such as transmittance, reflectance, within a single material or system, resulting in composite optical functionalities. [62]or instance, Wu et al. reported an orthogonal response (Figure 10a (i)) to both light and H in a smart photonic gel composed of photoresponsive organogelator (Azo-Ch) and SPM Fe 3 O 4 @SiO 2 NPs. [115]In this reversible system, the Azo-Ch exhibited sol-gel transitions that could be reversibly controlled through the cis-trans photoisomerization of the photo-responsive azobenzene group, while the SPM nanoparticles could form chain-like photonic structures under magnetic control.The system exhibited four distinct states and their mutual switching, enabling multiple functions, particularly in the gel-H and sol-H states (states 3 and 4).One application of this system is the fabrication of smart windows by sandwiching the gels between glass ii Photographs of the SW switch between the sol and gel states (3,4) with an in-plane H. iii Reshaping composite gel with the UV/vis light through the sol-gel transition.iv photographs of writing and erasing a QR code on a composite gel.Reproduced with permission. [115]Copyright 2023, The Author(s), published by Springer Nature.b) i Fabrication of SMPNCAs made of poly (AA-co-HEMA) hydrogel-based on amino-group-modified glass slide with the aid of H and UV irradiation.ii Schematic illustration of pH sensing of the SMPNCAs.iii Schematic illustration of the fluid pumping by rotating SMPNCAs.iv Image of the trajectories of the tracers near the tips of SMPNCAs under a rotating H, showing the directional fluid pumping.Reproduced with permission. [116]Copyright 2020, Wiley-VCH GmbH.c) i A "rolling" RPNR under a rotating H and collective motion of swarming RPNRs passing through a microchannel.ii Microenvironmental mapping of the PH-RPNRs when moving toward ager gel with pH 4.4.iii Schematic demonstration of mapping-guided photothermal therapy by swarming pH-RPNRs.
layers.The photoinduced reversible sol-gel transition of Azo-Ch (states 3 and 4), induced by light irradiation, modulates the scattering, resulting in a change in transparency as seen in Figure 10a (ii).Furthermore, the sol-gel transitions make the gel reconfigurable, allowing it to be shaped into different forms using visible light/UV irradiation (Figure 10a (iii)).Another interesting aspect is the creation of rewritable multicolor patterns (Figure 10a (iv)).By utilizing a photomask, the gel-to-sol transition can be selectively induced in irradiated areas, producing a QR pattern.The pattern can then be erased by irradiating the entire sample with UV light followed by visible light.Meanwhile, the assembled PC structures in the Azo-Ch gel enable both reshaping of the gel and the generation of rewritable patterns with variable colors in response to H.
Furthermore, the unique advantages of remote actuation and mechanical anisotropy in AMNs have positioned them as highly advantageous materials in the field of nanorobotics.Based on the soft hydrogel-coated NCs, self-adaptive magnetic photonic nanochain cilia arrays (SMPNCAs) were successfully developed (Figure 10b (i)).The fabrication process involves anchoring the Fe 3 O 4 @PVP@ poly (AA-co-HEMA) to a glass substrate via hydrogen bonding and subsequent UV polymerization. [116]With the swelling or contracting of the poly (AA-co-HEMA) hydrogel shell as a result of the reversible ionization of carboxyl groups in response to local pH variations, the SMPNCAs exhibit diffracted color depending on the change in lattice constant (Figure 10b (ii)).When a rotating magnet was positioned off the center of SMPNCAs, the cilia synchronize with the rotating H, causing them to rotate at a tilted angle (Figure 10b (iii)).This unique behavior allows the delivery of "cargos" (tracers) by the adjacent cilia.The spiral trajectories in Figure 10b (iv) with obvious net displacement demonstrate the directional pumping of the printed SMPNCAs as a micro/nanopump.
Exploiting the collective out-of-plane rolling property of PNCs, swarming responsive photonic nanorobots (RPNRs) with multiple integrated functions have been fabricated (Figure 10c).These RPNRs exhibit magnetically actuated collective motions, responsive sensors with visual detection, and photothermal conversion. [117]When subjected to a rotating H, the PNCs rotate under the interaction of magnetic torque.Hydrodynamic symmetry of the rotating RPNR is disrupted when it rotates near a substrate, causing the top and bottom portions of the RPNR to experience distinct viscous fluidic drag forces.As a result, the RPNP exhibits a "rolling" translational motion (Figure 10c (i)) owing to the coupling of the rotational motion around the bottom and the translational motion of the bottom.Through hydrodynamic coupling between neighbors, these PNCs self-organize into microswarms, showing a collective effect that enables them to navigate actively through microchannels and traverse complex environments.Leveraging their swarming behaviors, the RPNPs achieve high collective velocity and map environmental physicochemical conditions, such as pH (Figure 10c (ii)), thereby indicat-ing the proximity of abnormal targets (e.g., tumor lesion).Taking advantage of the photothermal conversion of the Fe 3 O 4 component, the RPNRs can also guide external near-infrared irradiation to initiate targeted photothermal treatment (Figure 10c (iii)).
Furthermore, photonic-crystal microrobots (PC-bots) capable of spontaneously realizing real-time visual pH detection and selfregulated drug delivery have been fabricated by integrating pHresponsive poly (AA-co-AM)) hydrogel microspheres with 1D PC assemblies of Fe 3 O 4 NPs (Figure 10d (i)). [64]Similar to the magnetic propulsion and PC properties of aforementioned RPNRs, the PC-bots with in-plane translational motion self-organize into large swarms, collectively moving toward the capillary (pH 4.4) under rotating H and perform on-the-fly visual pH detection (Figure 10d (ii)).Furthermore, pH-modulated drug/release could be realized through the integration of the poly (AA-co-AM) scaffold and antitumor drugs (e.g., doxorubicin (DOX)), which generally exists in a protonated cation form and could be electrostatically anchored within the PC-bots through the carbonyl sites of the hydrogel scaffold (Figure 10d (iii)).When approaching a target with acidic environment (e.g., an ulcerated superficial tumor lesion), the anchored DOX molecules can be released due to the deswelling of the PH-responsive PC-bots s after the protonation of carboxylate sites.

Conclusions and Outlook
In this review, we systematically reviewed the recent progress in magnetically responsive AMNs for advanced functional applications.The combination of shape anisotropy and magnetic anisotropy provides complex multi-functionality, enabling modulation of light propagation multi-dimensionally and facilitating the development of emerging application designs.One notable application is the development of smart windows, which utilize the orientation change of AMNs to achieve adjustable solar transmission.Additionally, anti-counterfeiting labels based on the MRPC, magnetically responsive LCs, and field tunable plasmonic resonance have been successfully designed, providing enhanced security features.Moreover, colorimetric sensors based on soft photonic nanochains (PNCs) have been developed.These sensors exploit the response of AMNs to detect and quantify changes in environmental factors, such as pH or temperature, by monitoring variations in the reflected or diffracted colors.Furthermore, the mechanical response of AMNs has been ingeniously incorporated into the design of multifunctional nanorobots.These nanorobots exhibit collective motions under the influence of a magnetic field and possess responsive sensors with visual detection.
Despite the significant advancement, the development of AMNs for optical applications still faces a set of challenges and simultaneously unveils promising opportunities.To navigate this complex landscape effectively, it is imperative to establish a Reproduced under the terms of the Creative Commons CC BY license. [117]Copyright 2023, The Author(s), published by Springer Nature.d) i Magnetic PC-bots with encapsulated 1D Fe 3 O 4 NP assemblies in poly (AA-co-AM)) hydrogel microspheres.ii Scheme of the pH sensing of a PC-bot swarm approaching a targeted capillary with pH 4.4 buffer.iii Diagram depicting the DOX release from the PC-bot as a result of the pH-responsive electrostatic interaction decrease between DOX and the hydrogel scaffold.Reproduced under the terms of the CC BY 4.0 license. [64]Copyright 2023, The Author(s), published by UESTC and John Wiley & Sons Australia, Ltd. comprehensive understanding of anisotropic architecture and macroscopic performance.Achieving homogeneous assembly within a controllable uniform H, encompassing aspects like chain length and inter-particle spacing is crucial.This uniformity prevents magnetic separation or targeting and facilitates high-quality optical modulation.Furthermore, it is conceivable that intricate architectures endowed with enhanced characteristics could be meticulously engineered.These structures can be assembled, manipulated, and guided not exclusively by magnetic field, but also by electric fields, light stimulation, capillary interactions, and a spectrum of alternative methodologies or their combinations.For instance, capillary interaction in conjunction with magnetic fields can be employed to shape and manipulate soft matter systems, facilitating the development of magnetically responsive foams, gels, and 3D-printed pastes. [73]Similarly, the application of electric and magnetic fields can enable both static and dynamic assembly of colloids and the self-propulsion of active particles. [118]This collective momentum is positioned to advance scientific progress.
In terms of application, while the orientation change of AMNs provides a new solution for SW, the current focus is primarily on visible modulation for privacy protection and glare prevention.Energy-saving characteristics in the near-infrared and long-wave infrared need to be further considered for architectural windows.Iron oxide and other magnetic particles with high refractive indices typically exhibit strong light absorption, making them potentially suitable for efficient solar energy harvesting through rational design.Nevertheless, their inherent darkness results in relatively restricted incident light transmission, presenting a challenge in augmenting modulation capability.Therefore, innovative strategies must be devised to overcome this limitation.Moreover, the use of block magnets to trigger the switch is impractical, and the key issue for future applications lies in controlling the window through subtle structural design with a controllable H. Particularly in large-area practical implementations, ensuring a uniform magnetic field distribution, encompassing field intensity and direction, is paramount to achieving consistent and high-quality light transmission management.It is conceivable that the incorporation of electronic switch electromagnet systems may offer a more effective and reliable means of regulation.For colorimetric devices, e.g., anti-counterfeiting labels and sensors, the hydrogel matrix gel poses challenges for durability due to its inherent chemical instability and lack of mechanical robustness.Moreover, compatibility with the working environment, such as combining temperature sensors with textile structures with washability and deformability is challenging.The ultimate goal in anti-counterfeiting is to achieve the highest security level using the simplest methods.By combining different optical properties and synergizing other functions (e.g., shape memory, shape morphing, and self-healing), gel-based anti-counterfeiting systems can attain more complex and diversified forms in the encryption/decryption process with potential compromise in durability.
Finally, nanorobots based on AMNs are still in their infancy but hold great promise for achieving multi-functional sensing and actuating in robotics.Further advancements are required in design principles, fabrication methods, control mechanisms, and sensing modalities to unlock their full potential.Last but not least, the surface-functionalization of AMNs or their build-ing blocks holds significant potential as an effective strategy to enable the generation of multifunctional capabilities.By modifying the surface properties of AMNs, such as introducing specific chemical groups or functional molecules, it becomes possible to impart additional functionalities to meet specific requirements or to develop highly versatile and adaptable systems that can simultaneously perform multiple tasks.We envision that AMNsbased systems, with their remote response, shape and mechanical anisotropy, high optical refractive indices, and immense potentials, could be increasingly impactful across various fields, including magnetism, optics, biology, environment, energy, and further exploited as functional materials in diverse applications such as camouflage, photonic printing systems and inks, personal wearable device, biomedicine, energy conversion, catalysis, and sensing.
It is worth mentioning that the potential interaction between magnetic materials and light within magnetic fields, such as the magneto-optical effect, optical signals, particularly polarized ones may undergo distortions. [119,120]The AMN systems in general are relatively less susceptible to magneto-optical effects.It remains a concern worthy of consideration in future developments due to the inherent interaction of light with magnetic materials.In addition, when optical systems are deployed in large-scale applications, careful system design, material selection, shielding, and maintenance are all issues that need to be considered.

Figure 2 .
Figure 2. Schematic diagram of the fabrication for the anisotropic magnetic nanostructures.a) Synthesis of anisotropic nanoparticles by templateassisted method.b) Fabrication of the plasmonic AMNs.c) Magnetic assembly of isotropic primitives for the synthesis of AMNs.d) Assembly of asymmetric primitives.e) Formation of responsive soft nanochains via the general hydrogen bond-guided template polymerization method.Monomers first concentrate around Fe 3 O 4 @PVP nanoparticles via hydrogen bonds.Then, the NPs are aligned linearly into dynamic 1D periodical structures under an H, and immobilized in cross-linked hydrogel to obtain responsive 1D PNCs by UV polymerization of monomers and cross-linker.Reproduced with permission.[61]Copyright 2020, American Chemical Society.

Figure 3 .
Figure 3. Scheme for magnetically responsive optical modulation.a) Tuning of transmittance.b) Tuning of diffraction.c) Tuning of polarization.d) Tuning of plasmonic resonance.

Figure 4 .
Figure 4. Magnetically responsive smart windows based on orientation change of AMNs.a) Transmittance modulation of the colloidal solution by changing the orientation of Ag nanoplates and Fe 3 O 4 nanoparticle-based ferrofluids.H 1 , H 2 implying the different fields applied to orient nanoplates.Reproduced with permission.[87]Copyright 2012, American Chemical Society.b) Magnetic optical switch of magnetic Au microplates. is the angle between the applied H and the viewing direction.Reproduced with permission.[88]Copyright 2016, Royal Society of Chemistry.c) Nanorod-based SWs under H of different directions.Reproduced with permission.[62]Copyright 2019, Wiley-VCH GmbH.d) Scheme of the optical switch in 1D NC-based SW with the approaching of a magnet.Insets are the distribution and arrangement of NCs in different states.Reproduced with permission.[44]Copyright 2020, American Chemical Society.e) Schematic illustration of the formation of the magnetic microrods by applying an H. Reproduced with permission.[89]Copyright 2021, Royal Society of Chemistry.f) i 1D NCs assembled from nanodisks, and solution switch between the transparent and opaque states.ii Transmission spectra of aqueous NCs rotated in the x-y plane under different H directions.The light incident along the y-axis.Reproduced under the terms of the CC BY 4.0 license.[40]Copyright 2019, The Authors, published by Wiley-VCH.

Figure 5 .
Figure 5. Anti-counterfeiting labels based on magnetically responsive photonic crystals.a) Scheme of color display based on magnetic assembly.b)Solid matrix embedded with microdroplets of assembled NPs.c) Invisible photonic print process for anti-counterfeiting watermarks.Reproduced with permission.[97]Copyright 2013, The Author(s), published by Springer Nature.d) 3D-printing of the customized pattern.Reproduced with permission.[98]Copyright 2021, Royal Society of Chemistry.e) Net-structure hydrogel with field-assembled NPs.f) i Dolphin-patterned PC hydrogels at a bending angle ranging from 0°to 75°.ii The corresponding spectra of the dolphin's tail.Reproduced with permission.[101]Copyright 2019, Wiley-VCH GmbH.g) Photonic hydrogel microparticles and colored pattern triggered by the field.Reproduced with permission.[102]Copyright 2022, Wiley-VCH GmbH.h) Scheme of color switching with orientation change.i) Photonic printing with different orientational AMNs for anti-counterfeiting labels.Reproduced with permission.[103]Copyright 2011, American Chemical Society.j) Screen printing with lipophilic magnetic PNCs and the anti-counterfeiting label taken at different tilting angles.Reproduced with permission.[104]Copyright 2022, Wiley-VCH GmbH.k) Photonic nanocube NCs and thus based encryption film.i Scheme showing the nanocube NCs parallel to the surface and diffracting light out of the plane.ii Photomusk and the printed pattern against a dark background.Digital images of the pattern with counterclockwise rotation from 0°to 90°, 180°, and 270°.The pattern rotates counterclockwise, but it appears to rotate clockwise.Reproduced with permission.[58]Copyright 2019, American Chemical Society.l) Photonic pigments based on BCT colloidal crystals.i Scheme of the alignment of the pigments for mechanochromic responses.ii Digital photos of two photonic films showing the displayed patterns and their color changes in response to mechanical rotation.The incident angle is 40°in the top panel and 30°in the bottom.Reproduced with permission.[48]Copyright 2021, Wiley-VCH GmbH.

Figure 6 .
Figure 6.Anti-counterfeiting labels based on magnetic tuning of polarization.a) Schematic illustration of the magnetically-actuated optical switching process.b) Scheme of lithography process for the film that contains magnetic LCs, B 1 B 2 implying the different fields applied to orient LCs in different regions.c) Scheme of magnetic cellulose microcrystals and thus fabricated transparent anti-counterfeiting film, images of the film sandwiched between cross polarizers before and after shifting the direction of the transmission axis of the polarizers for 45°.(a-c)Reproduced with permission.[60]Copyright 2020, Elsevier.d) SEM images of building blocks of magnetic LCs: Fe 3 O 4 @SiO 2 nanorods.e) i Bright-field images of two polarization-modulated patterns, POM images of the same patterns under cross polarizers ii before and iii after shifting the direction of the transmission axis of the polarizers for 45°.(d,e) Reproduced under the terms of Standard ACS AuthorChoice/Editors' Choice usage agreement.[95]Copyright 2014, American Chemical Society.f) Schematic illustration of the ORD measurement. denoted as the angle between the polarization direction of the analyzer and the horizontal direction.g) Digital images of the dispersion without (w/o mag) and with (w/ mag) a magnetic field.The  is 3°.h) Photos of the dispersion under different field conditions. switched from -30 to 30°during the measurement.(f-h) Reproduced with permission.[28]Copyright 2023, The Authors, published by the American Association for the Advancement of Science.

Figure 7 .
Figure 7. Magnetic tuning of plasmonic properties in AMNs.a) Schematic illustration of near-field plasmon coupling of plasmonic chains with respect to different light polarization directions.b) Extinction spectra of the plasmonic chain-like structure composed of Fe 3 O 4 @Au NPs under different orientations.c) Digital images of a pattern under horizontal, vertical, and 45°polarization, indicated by the white arrow.d) Photographs of the composite film containing hybrid NPs.The letters were created by the projector and then illuminated on the films with three white letters on the top row and three primary colored (blue, green, and red) letters light at the bottom row.(b-d) Reproduced with permission.[106]Copyright 2020, American Chemical Society.e) Magnetically responsive plasmonic PCs with angular-dependent reflective colors due to the backward optical diffraction (front) and the angular-independent transmissive color caused by the forward plasmonic scattering (back).f) Double-layer anti-counterfeiting film with a top "panda head" pattern and a bottom "deer" pattern.Light irradiation from top gives the "panda head" pattern for i frontal observation due to optical diffraction and the "deer" pattern for ii back observation caused by plasmonic scattering.The situation is switched iii, iv when light is irradiated from the bottom.(e,f) Reproduced with permission.[80]Copyright 2023, American Chemical Society.g) Illustration of plasmonic excitation in Au NRs under different directions of the field.h) Extinction spectra of Au NRs under polarized light and corresponding solution images.i) Patterned plasmonic film under i normal light, ii vertical, iii horizontal, and iv 45°polarization, indicated by the white arrow.j) Mechanochromic film containing butterfly pattern with 45°aligned Au NRs to the surface.When subjected to pressure, the top plasmonic film expands upward or downward and exhibits an asymmetric mechanochromic response in the two wings of the butterfly due to the excitation of different plasmon modes of AuNRs.(h-j) Reproduced with permission.[67]Copyright 2020, The Author(s), published by Springer Nature.

Figure 8 .
Figure 8. Sensors with visual detection based on the AMNs.a) Working scheme of the film sensor in response to external stimulus and the mechanism of color switching.b) Reflection spectra and digital photographs of humidity sensitive PC film in different humidity environments.Reproduced with permission.[110]Copyright 2011, Royal Society of Chemistry.c) Reflection spectra and digital photographs of the free-standing thermochromic film sensor.Reproduced with permission.[25]Copyright 2015, Royal Society of Chemistry.d) Schematic illustrations of the PC ball sensor, the diffraction state is turned on with NPs vertically aligned along out-of-plane H, and the diffraction color varies with T, H.The diffraction is off when NPs assembled along the in-plane direction.e) Synthetic procedures for the temperature responsive PC microspheres using a micro syringe and UV photo-polymerization. f) Digital photos of the PC microball sensors at various temperatures and the "off"/ "on" switch.(e,f) Reproduced with permission.[111]Copyright 2017, Royal Society of Chemistry.
(i)), with a distinct transition occurring at ≈7 mM of glucose, indicating the potential for diabetes diagnosis.The cyclic variations of [glucose] resulted in corresponding repeatable variations in the diffraction wavelength (Figure 9d (ii)), demonstrating the good reversibility of soft PNCs.The nearly synchronous change of the equilibrated diffracted wavelength and [glucose] changes further confirmed the real-time response capability (Figure 9d (iii)).

Figure 9 .
Figure 9. High-resolution NC sensor based on the soft nanochains.a) Working scheme of the color changing process of PNC sensors in response to external stimuli.b) pH responsive soft PNCs.i 2D contour maps of the reflection spectra under different pH.ii Scheme of response time measurement with a Y-shaped channel, where PNCs flow across rectangular area at a constant rate of 2.5 mm −1 s and lead to a horizontal color strip of 100 μm.H = 400 Oe.iii Microenvironment imaging with dark-field OM.Reproduced with permission.[61]Copyright 2020, American Chemical Society.c) Temperature and field dual responsive PNCs.i The reflection spectra of PNCs under different temperatures.ii Bright-field OM images of PNCs at different H. iii Dark-field OM images with different color brightness under varying H. Reproduced with permission.[113]Copyright 2021, Royal Society of Chemistry.d) Glucose responsive PNCs.i Digital photograph of the PNC solutions with different [glucose].ii Diffracted wavelength of PNCs during a cyclic sweep of [glucose].iii Response time of the diffracted wavelength with [glucose] changing.Reproduced under the terms of the CC BY 4.0 license.[114]Copyright 2022, The Authors, published by Wiley-VCH.
Figure 9. High-resolution NC sensor based on the soft nanochains.a) Working scheme of the color changing process of PNC sensors in response to external stimuli.b) pH responsive soft PNCs.i 2D contour maps of the reflection spectra under different pH.ii Scheme of response time measurement with a Y-shaped channel, where PNCs flow across rectangular area at a constant rate of 2.5 mm −1 s and lead to a horizontal color strip of 100 μm.H = 400 Oe.iii Microenvironment imaging with dark-field OM.Reproduced with permission.[61]Copyright 2020, American Chemical Society.c) Temperature and field dual responsive PNCs.i The reflection spectra of PNCs under different temperatures.ii Bright-field OM images of PNCs at different H. iii Dark-field OM images with different color brightness under varying H. Reproduced with permission.[113]Copyright 2021, Royal Society of Chemistry.d) Glucose responsive PNCs.i Digital photograph of the PNC solutions with different [glucose].ii Diffracted wavelength of PNCs during a cyclic sweep of [glucose].iii Response time of the diffracted wavelength with [glucose] changing.Reproduced under the terms of the CC BY 4.0 license.[114]Copyright 2022, The Authors, published by Wiley-VCH.

Figure 10 .
Figure 10.Multi-functional smart optical materials based on the AMNs.a) Orthogonally photo-and magnetic-responsive composite gel, i Schematic illustration of four states, i.e., 1 gel without H, 2 sol without H, 3 gel with H, and 4 sol with H.ii Photographs of the SW switch between the sol and gel states(3,4) with an in-plane H. iii Reshaping composite gel with the UV/vis light through the sol-gel transition.iv photographs of writing and erasing a QR code on a composite gel.Reproduced with permission.[115]Copyright 2023, The Author(s), published by Springer Nature.b) i Fabrication of SMPNCAs made of poly (AA-co-HEMA) hydrogel-based on amino-group-modified glass slide with the aid of H and UV irradiation.ii Schematic illustration of pH sensing of the SMPNCAs.iii Schematic illustration of the fluid pumping by rotating SMPNCAs.iv Image of the trajectories of the tracers near the tips of SMPNCAs under a rotating H, showing the directional fluid pumping.Reproduced with permission.[116]Copyright 2020, Wiley-VCH GmbH.c) i A "rolling" RPNR under a rotating H and collective motion of swarming RPNRs passing through a microchannel.ii Microenvironmental mapping of the PH-RPNRs when moving toward ager gel with pH 4.4.iii Schematic demonstration of mapping-guided photothermal therapy by swarming pH-RPNRs.