In order to enable prepared colloidal nanomaterials to form MRPCs under EMFs, the MNPs have to be equipped with homogenous particle size and monodispersing in solution. Nonetheless, the aggregation caused by the inherent mutual attraction between magnetic particles (MPs) could be the obstacle to monodispersing. In this regard, similar to the traditional self-assembling methods, MRPCs are required to be prepared in suitable particle sizes for visible spectral segments and narrow particle size distribution. Accordingly, recent studies have found that surface-active agents, such as polyacrylic acid (PAA) (Ge et al., 2007) (Figures 1A,B), polyvinylpyrrolidone (PVP) (Ma et al., 2017) (Figures 1C,D) and oleic acid (OA) (You et al., 2017) (Figures 1E,F) were usually coated on MNPs or a carbon nanolayer with a large number of carboxyl groups (Wang et al., 2010) (Figures 1G,H) so as to introduce a large amount of charge on the MNPs surface with a large amount of charge to balance the attraction between MNPs so that it could disperse evenly in the solvent. Basically, in the absence of EMF, these synthesized SMNs are randomly dispersed and scattered distribution in an aqueous solution which is known as Brownian motion. However, when an EMF is applied, the forces between the particles are mainly the electrostatic force or steric repulsion force provided by the surface coating and the magnetic dipole force between EMF and MNPs (Liu H. et al., 2019). The magnetic dipole force (He et al., 2010) was described as Eq. 1: F = 3 μ ( 1 − 3 cos 2 θ ) / d 4 (1) Where μ and θ is the magnetic moment and the angle between the center of the particle and the magnetic field; while d is the distance from the center of the adjacent particle.

Therefore, the dipole-dipole interaction of two adjacent particles along the MF is gravity, which is perpendicular to the direction of the MF is the repulsion force. The magnetic dipole repulsion along the MF is in balance with the electrostatic force (or long-range gravitational force), and MNPs will move in the direction of the weakening of the MF, which allows NPs to form a nano-chain structure along the MF direction. In this case, the magnetic dipole repulsion and electrostatic repulsion (or long-range repulsion) between the nano-chains can be separated from each other; thereby eventually forming a periodic configuration of the PCs on a plane perpendicular to the MF. This periodic structure has a specific photonic bandgap, where the transmission of light in the photonic bandgap could be restrained and reflected. It can be visible as bright color by naked eyes, and the optical fiber spectrometer could be used to capture its specific reflection peak. In the magnetic field, the diffraction of one-dimensional nano-chain accords with the Bragg diffraction law (Ma et al., 2017) can be expressed by Eq. 2: λ = 2 n d sin θ (2)

Where λ is the spectral peak, n is the refractive index of the solvent, d is the distance between two adjacent particles, namely, the PCs cycle, and θ is the angle of incidence. Therefore, the factors affecting the structural color of PCs mainly include the refractive index of the solvent, periodicity, and the angle of incidence. Under a condition of constant refractive index and the incident angle, the PCs period d of the nano-chain will become small, and along with the blueshift of the diffraction peak, if the strength of the MF is increased, and the moving range is related to the particle size.

He’s research group (Yang et al., 2016) proved that Fe₃O₄@PAA colloidal aqueous solution with a diameter of 123 nm has 310 nm reflection peak movement in the MF including most of the visible spectrum. However, Fe₃O₄@PAA colloidal aqueous solutions with a larger diameter and smaller diameter have only about 200 nm reflection peak movement because the size of the MNPs limits the variation of the PCs cycle under varying MFs (Figure 2).

The random-stated and assembly SMNs can be formed into ordered chain-like 1D structures and then evolve into the multi-chains and the long-range 2D planar/sheet structures (Liu J. et al., 2019). Further increasing the SMNs concentration and the interparticle potential may lead to coalescence of the 2D planar structures and eventually assemble to 3D crystals (Wang and Yin, 2016). Originally, in the absence of EMF, these SMNs were of randomly dispersed and scattered distribution (Brownian motion) in aqueous solution by the strong electrostatic repulsive interactions and the color in the visual field turned to almost brown (left panel of Figure 3A). The forces imposed on SMNs were illustrated in the right panel of Figure 3A. The MRPCs diffraction was correspondingly red-shift from left to right as blue, green, and red with the SMNs-magnet distance gradually increased (Figure 3B). The key point for the assembling of MRPCs in an aqueous solution is to establish a balance among the dipole-dipole attractive force, exclusion force, and dipole-dipole electrostatic repulsive forces (Wang and Yin, 2016).

Specific structural colors could be generated by the periodic arrangement of MRPCs under MFs, even of an ordinary magnet. The induced color could be randomly controlled by MF size, particle size, and solvent; and the structural color does not easily fade. These unique properties grant MRPCs good application prospects in the fields of anti-counterfeiting, secret information storage, real-time MF detection, etc. Asher’s group (Xu et al., 2001a) set an important precedent by reporting Fe₃O₄@PS ferromagnetic fluid as early as 2001 (Figure 7). Under the MF, a regular PCs structure is formed under the drive of the kernel MNPs. Because of the low saturation magnetization of the colloidal MNPs, the response to EMF is relatively slow, which would take a few minutes or even 1 h, so that the wavelength of MRPCs prepared is narrow and the response time was relatively longer.

Yin’s group (Ge et al., 2007) has advanced a big step in the research of MRPCs. The electrostatic repulsion provided by a large amount of charge on the surface of the Fe₃O₄ MNPs coated with PAA could separate MNPs from each other. In this way, a paramagnetic chain structure is formed rapidly (<1 s) under the MF. After coating a silicon ioxide shell (SiO₂) on the surface, a large number of silica oxygen bonds can be well dispersed into organic solvents by controlling the thickness of the silicon shell, and the diameter of Fe₃O₄@SiO₂ could be adjusted randomly. The ethanol solutions of Fe₃O₄ and Fe₃O₄@SiO₂ with different particle sizes have different structural colors and diffraction peaks; therefore, the increase of particle size could result in the redshift of the diffraction peak.

Similarly, due to the different refractive indices in different solvents, MNPs with the same particle size in different solvents also showed different structural colors and diffraction peaks in different solvents under the equivalent EMF. However, the charge on the surface of NPs is difficult in the long-term storage in the water, and the surface charge will be reduced due to the separation of PAA on its surface, resulting in a narrow diffraction range of the PCs response. In the subsequent studies, it could be solved by hot reflux closure polyelectrolytes. Subsequently, the Yin’s group (Ge et al., 2009) combined the microfluidic technology with MRPCs, using microfluidic channels to polymerize the nano-level Fe₃O₄@SiO₂ into tens of micron-sized photon microspheres (Figure 8A), and the color changes were clearly observed under the light microscope when the MF was applied (Figures 8B–G). The micron grade of material is very stable, well compatible with dispersion medium, and capable of rapid color change with the MF applied, which could be very suitable for color display, signal identification, biological and chemical detection, and MF sensor.

Liu’s group (Tang et al., 2018) fabricated a hydrophilic and size-controllable Fe₃O₄@poly (4-styrenesulfonic acid-co-maleic acid) (PSSMA)@SiO₂ MRPC by orderly assembling of the core-shell colloidal nanocrystal clusters (CNC) via a two-step facile synthesis approach under EMF in aqueous solution. Figure 9A demonstrates a rainbow-like color effect within 1 s when the prepared MRPC was placed in a circular shaped disk-like NdFeB magnet. Figure 9B displays the intensity distribution of the EMF simulated by COMSOL Multiphysics 5.3. This was the verification of MRPC formation.

MRPCs relying on the long-range steric hindrance effect were firstly reported by Guan’s group (Luo et al., 2014). The force is provided by the macromolecular PVP which wraps the surface of MNPs. In the organic solvent dimethylformamide, with the increase of the MF, the reflection peak was blue-shifted with a width of 280 nm which has been considered as the widest diffraction peak displacement of the MRPCs reported in the organic phase. Whereas the diffraction peaks of NPs with the same particle size only shifted 238 and 200 nm in ethanol and dichloromethane. Both the electric field and MF are similar referring to the regulation of MNPs arrangement. Joo (Joo et al., 2014) reported PCs that can be either induced by MF and electric field, the hydrophobic group, and the positive charge modified Fe₂O₃@SiO₂, which could be well distributed in polar and non-polar solvents. The electrostatic repulsive force can be balanced with the magnetic dipole attraction between particles, and a bright color response would occur under EMF. When the same magnetic colloid solution was injected into the conductive glass interlayer, the structure colors were similar to those in EMF which could be assembled under the electric field due to the repulsive force of the electric field. As same as the MF, along with the increasing of the electric field, the diffraction peak also takes a blueshift.

Induced by MF, the MNPs of MRPCs with different particle sizes are characterized as periodic permutations of dielectric constants. Chen’s group (Hu et al., 2012) mixed Fe₃O₄@C MNPs with two different particle sizes in EG solution, and then sealed the result in polydimethylsiloxane gel film in the form of micron droplets. After self-assembling by magnetic inducing, MRPCs containing a two-photon bandgap structure were prepared and successfully achieved photon toning, which is difficult for ordinary pigments or chemical dyes to imitate. It could be used as a new type of anti-counterfeiting identification, and also provides the idea of multi-channel detection for sensing technology.

Compared with the general magnetic materials, MRPCs not only have good magnetic response effect but are also characterized as a monodispersing and periodic arrangement under MF as well as having good application prospects when combined with other technology. It was reported liquid fluorescence enhanced substrates consisting of suspensions of Fe₃O₄ NPs could assemble 3D PCs under EMF (Hu et al., 2016). Combining the dye molecules whose incident wavelength overlapping with the PCs photonic bandgap on the surface of MRPCs, the periodic PCs can provide a continuous and highly ordered substrate, whereby enhancing the collection efficiency of fluorescence incident, and reaching each up to 12.3-fold fluorescence enhancement without the utilization of metal particles (Hu et al., 2016). They also emphasized that the dynamic modulation and precise control of the photonic bandgap of the liquid Fe₃O₄ PC system may significantly contribute to more accurate and flexible applications of fluorescence enhancement. Kwon’s group (Kim et al., 2012) modified amino on the surface of Fe₃O₄ wrapped in silicon shell and further coated the shell structure of Au NPs. It also reported that, under EMF, the magnetic core could cause the promotion of Au NPs to form a periodic one-dimensional chain structure, and the introduction of the Au NPs has expanded the application of MRPCs in localized surface plasmon resonance, such as the surface-enhanced Raman matrix construction (Liu et al., 2017), whereby the orderly structure makes the MRPCs difference from general magnetic materials, and can significantly improve the sensing Raman signal by the coupling of the core-shell structure owing to the high surface to volume ratio of PC beads as well.

Besides the spherical magnetic materials, the non-spherical materials, such as elliptic and fusiform, also can be quickly assembled into PCs under magnetic induction. Clays’s group (Ding et al., 2009) prepared the elliptical and densely packed Fe₂O₃@SiO₂ by direct convective self-assembly with the aid of EMF (Figures 10A–E). The PCs of this cube lattice have a bright structural color, and the color takes a blueshift when the axial proportion of ellipsoid is larger. Ellipsoidal NPs have special properties under MF due to their unique structure which could respond to the direction of the MF. Accordingly, Fe@SiO₂ nano-ellipsoids as anisotropic building blocks. Wang (Wang M. et al., 2015) employed the ellipsoidal FeOOH@SiO₂ colloidal solution to respond to the direction of applying the MF. When the direction of the MF which used to be perpendicular to the incident angle becomes parallel to the incident angle, the diffraction peak takes a blueshift up to 210 nm. Compared with the spherical magnetic colloidal solution which only has two colors on the Halbach magnet array (Wang et al., 2013), due to its response to the different MF directions of ellipsoidal special structure, three different color responses could occur on the Halbach array (Figure 10F).

Klöckner (Klöckner et al., 2017) modified the surfactant outside the fusiform Fe₃O₄ nano-rod to enable a large amount of charge on its surface. The space resistance repulsion generated by the outer wrapping polymer could effectively reduce the aggregation of NPs and show good dispersibility so that it can be very well dispersed in any solvent. With the incident angle of 45°, the “meteor” shape could be observed in an optical microscope under the MF, which intuitively shows that the fusiform magnetic sphere could also be arranged in the direction of MF so as to form a periodic PCs structure (Figures 10G,H).

He’s group developed, tailored, and assembled ellipsoidal particles into PC supraparticles (PCSs) with unusual non-close-packed structures with a spatially confined magnetic assembly strategy (Liu J. et al., 2019) which exhibits anisotropic optical properties and multiple magneto-optical responses. Based on the above-mentioned, the periodic structure of PCs is not confined to the spheres, and the formation of PCs by non-spherical magnetic material under the MF also further have validated the regulation of MNPs by the MF. This non-spherical material has shape-dependent physical and chemical properties, which could provide new ideas for the composition and display of non-spherical MRPCs photons, such as a color display that can be used to detect the direction of MFs.

Yin’s group (Li et al., 2021) reported the construction of magnetite nanorods by magnetic assembly into tetragonal colloidal crystals and demonstrated that the assembly method caused the nanorods to be assembled along a size-dependent critical angle rather than the simple end-on attachment. The unconventional assembly manner was determined by the coupled shape and magnetic anisotropy and it led to the non-close-packed and hard-contact phase. The novelty of the work lies in preparing the body-centered tetragonal colloidal crystals by manipulating magnetic interactions of various anisotropically shaped nanostructures which are actively tuned by changing the magnetic field direction.

Non-magnetic materials in magnetic fluids have similar properties to magnetic materials under MFs, therefore not only magnetic materials but also non-magnetic materials could be arranged and assembled into ordered structures under EMF so as to form PCs structures. Non-magnetic liquid mixed with magnetic colloids could become anti-magnetic liquids, this concept was first proposed by Skjeltorp (Skjeltorp, 1983). It was proposed that the non-MPs incorporated into the magnetic fluid were named magnetic holes forming a variety of different lattices. He (He et al., 2010) investigated the phase transitions of the diamagnetic liquid containing the PS microspheres using EMF. Non-magnetic polymer beads measuring 185 nm were assembled into PC structures, from 1D chains to 3D assemblies as determined by the interplay of magnetic dipole force and packing force.

The structural color could be significantly blue-shifted with increasing MF (Figures 11A,B). Also, the intensity of the diffraction peak continuously keeps rising within 3 min and 30 s after the application of a strong MF (2500 Gs), and even continued to 6 min after the removal of MF, could be up to a maximum of 83%, which is far beyond the maximum of magnetically induced material and then slowly decreases within 20 min (Figures 11C,D). In short, this is the entire process of magnetization and demagnetization of PS microspheres with negative charges on the surface of the magnetic fluid. They also demonstrate the magnetic assembly of Janus PCSs with increased complexity from a colloidal mixture of nonmagnetic spheres and magnetic ellipsoids (Xiao et al., 2021). According to these studies, PS microspheres can also respond well under EMF, which provides new technical means for the magnetic control of non-MPs.

Asher’s group (Xu et al., 2002) employed a 134 nm PS sphere and a mixed colloidal solution of Fe₃O₄ or Co₃O₄ to form a PC under MF, but the assembly time was as long as 60 min due to its low contents of magnetic material. Besides, Deng (Deng and Li, 2013) observed the dynamics of the formation of different structures by assembly of non-magnetic spheres, i.e., micro-sized PS microspheres and nano-sized Fe₃O₄ magnetic colloid under an EMF. When MF is parallel to the direction of the incident light, PS microspheres were arranged from the original random distribution to the chain-type arrangement under MF. Along with the repulsion shown by the adjacent chains this structure has a certain threshold value for the MF strength; when applying a vertical MF, PS microspheres attracted each other along the direction of the MF to form a short chain structure. Subsequently, Ge (Liu et al., 2012) synthesized PS with different particle sizes (311–477 nm). Under the weaker MF (650 Gs), PC structural color changes from blue to purple with the increase of particle size (Figure 11E). The peak intensity of non-magnetic PS first increased and then decreased with the increase of the concentration of the magnetic fluid solution, reaching the highest value at 3% (Figure 11F), and the wavelength decreased continuously with the increase of the concentration of the magnetic fluid solution (Figure 11G). The non-magnetic PCs of this system are relatively stable as they still exhibit good reproducibility after multiple repetitions, and they could also maintain consistent peak intensity under a continuous 60-min MF.

The assembly of non-magnetic materials in magnetic fluids is fast and efficient. However, due to the natural adsorption of MNPs by MFs, it could only remain stable in liquids for a certain period. Recently, Ge’s group (Zhang et al., 2017) prepared micro-scale SiO₂ inverse opal by emulsion polymerization and UV-curing on the previous basis and then mixed it with the magnetic fluid (Figure 12A). Under the MF, the response time decreased with the increase of the MF strength, showing an exponential correlation (Figure 12B); whereas the reflectivity increases with increasing MF intensity which is linearly correlated (Figure 12C). This system could simultaneously detect the relevant MF strength of both the indicator response time and reflectivity. The minimum detectable MF is 5.4 mT. The principle of this magnetometer is only magnetic tuning instead of magnetic assembly; therefore, it has subtly avoided the shortcomings of traditional magnetic fluids in measuring magnetic intensity by using magnetic assembly optical signals in the past, that is, avoiding the PCs structural colors and reflection peaks being too sensitive to EMF and also avoiding the reflectivity varies with the applied MF time (He et al., 2012).

The combination of MRPCs with microfluidic technology and the artificial aggregation of nanometer PCs into micron scales have achieved the free control of micron-size PCs. Lee (Lee et al., 2017) synthesize the “Janus” microspheres microfluidic devices to include the PS and magnetic balls in equal proportion by the microfluidic devices. Since half of them were made of magnetic materials, when the incident light and MF direction was changed from 0° to 60°, the diffraction wavelength of the other half-formed PCs would shift accordingly. When the rotation angle is 180°, the orderly arrangement of PCs disappears instantaneously. This material not only has a stable PC structure but also has MF responsiveness; therefore, it could store photon signals and served as a carrier for biological materials (Shin et al., 2017). The magnetically induced non-magnetic material has a stable response signal and responds in a weak MF. Therefore, it has a good prospect in PCs magnetometer and photon color display.

The responsiveness of PCs refers to the influence of external environmental changes on its photonic bandgap structure. This responsiveness endows PCs with unique sensing characteristics, enabling the conversion of various external environmental responses into optical signals. MRPCs have the advantage of rapid assembly, which attracted much attention in the study of the responsiveness of PCs. The responsiveness of MRPCs could be easily classified as physical (temperature, humidity, pressure, etc.) and chemical (salt ion concentration and small molecule concentration) responsiveness. Mixing the magnetic material with a gel (such as acrylamide electrophoresis gel) could immobilize the MNPs; therein, MPs are induced to self-assemble into PCs. With the other external conditions such as strength and direction of the MF are not changed, according to Bragg’s diffraction law, the periodical changes of PCs caused by external conditions would lead to the change of λ. If changing one of the external conditions, it also would be altered of the diffraction structural color which was visible to the naked eye in the macroscopic. Due to its special and rapid assembly method, this kind of MRPCs is widely used in the response of PCs.

Physical parameters such as humidity, temperature, pressure, etc., greatly impact the periodic structure of PCs. Yin’s group (Wang et al., 2014) reported a light-printable rewritable paper based on color switching of commercial redox dyes which were introduced as the imaging layer and used titanium oxide-assisted photocatalytic reactions (Figure 13).

Fe₃O₄@SiO₂ colloidal NPs were immobilized in a PEGDA array and rapidly magnetically induced to self-assemble into PCs. After contacting the “ink” of different humidity represented by the saturated salt solution, the interval diameter of the particles in the polymer matrix in the contact area increases, contributing to a redshift of the diffraction peak, and it’s very stable at RT and normal humidity. In addition, the residual salts on the surface of the ink-free light-printable rewritable paper could be washed away by using the distilled water, and the “ink” can also be wiped off. After it dried, the light-printable rewritable paper would be returned to its original state and could be recycled at least 20 times (Figures 13A–F). As the poly-N-isopropyl acrylamide (NIPAM) gels have hydrophilic amide and hydrophobic isopropyl on the macromolecular chain at the same time, the cross-linked PNIPAM is characterized as temperature-sensitive.

Guan’s group (Luo et al., 2017) have developed the first 1-D magnetic PC balls with tunable lattice constants by fixing collectively oriented periodical 1-D magnetic nanochain-like structures in responsive polymer PNIPAM hydrogel balls under magnetic field (H) and UV irradiation. They employed PNIPAM as the crystal structure to build the matrix, i.e., the MNPs and the gel mixture were then injected into the microinjection solution, and the microsphere-sized polymer spheres were subsequently prepared by dropwise addition to induce into PCs in a fixed MF. When the temperature rises from 10 to 40°C, the structural color changes from red to yellow, and then to green, and the temperature is inversely correlated with the wavelength of the reflected peak. Similarly, Wang (Wang et al., 2016) immobilized the Fe₃O₄@C in PNIPAM gel. In this case, the temperature changed in the range of 15–26°C which initiated a blueshift of 100 nm. The change from green to dark blue was visible by the naked eye, and the temperature-sensitive test paper was made with 5-time repeatability.

You (You et al., 2015) mixed the product of polymerization of Fe₃O₄@OA with monomer and crosslinker with polyacrylamide gel, and polymerization is initiated by using EMF, and the diffraction peak intensity of the obtained product film could take a blueshift with the stretch of external force and generate a redshift with extrusion because stretching reduced the interval between adjacent particles in a self-assembled single chain. Accordingly, Wang (Wang et al., 2017) offered simultaneous multicolor displayed on the photopolymerization of hydrogel under MF. Under a tensile force of 0–12 kPa, with the tensile force increased the structural color changed from brown to blue, whereby, a blueshift could reach 250 nm, and a gradient color card was also made by the change of color with tension. The color fixed by PCs would not fade over time, with long-term stability which not only provides a good prospect for real-time pressure sensing but also broadens the application of PCs hydrogels in fast-response sensors. Research on MRPCs’ temperature, humidity, and pressure response properties is an interesting exploration of the physical response of MRPCs. These rapid, reversible, and visualized features will broaden the application of physicochemical sensors, and renewable photonic papers and inks.

The uniform and orderly arrangement of MNPs colloids in an MF is a dynamic equilibrium system. Changes in conditions in the system, such as salt ion concentration and compound concentration, would destroy this dynamic equilibrium and cause it to be rearranged into a new equilibrium, which would result in changes in the PCs cycle. The nano-colloids such as Fe₃O₄@PAA could build the MRPCs which rely on the interaction between the surface electrostatic repulsion and the magnetic dipole force. The electrons in the surface layer become thinner as the ion concentration increases and the electrostatic repulsion would also decrease or even disappear. As a result, when the concentration of salt ions increases to a certain concentration, MRPCs’ suspension would lose its color response. While MNPs colloids rely on the long-range repulsive provided by MNPs surface macromolecule flexible polymer PVP (Luo et al., 2014), the salt ions have little effect on their surface spatial conformation, and therefore the long-range repulsion of the surface remains unchanged. And the suspension of the MF could be tuned to a wavelength of 700–460 nm with a salt concentration of 0–100 mM. Chen’s group (Wang et al., 2016) put the PNIPAM gel immobilized with Fe₃O₄@C into the concentration gradient of 200–1,200 mM NaCl solution. They suggested that with increasing salt ion concentration, the gel color was gradually changed from green to dark blue and could be observed by the naked eyes along with the blueshift of the responsive diffracted wavelength, and the color response to salt concentration could be completed in a matter of seconds with good repeatability. The salt ion concentration test strip manufactured on this principle has the advantages of fast response and portability and is suitable for rapid on-site detection.

You (You et al., 2015; You et al., 2016) combined MRPCs with molecularly imprinted technology with specific recognition to achieve an optical response to melamine and L-phenylalanine. The surface of Fe₃O₄@OA was coated with a molecularly imprinted polymer, and the holes left after removing the template molecule had a specific recognition effect on the template molecule. Under the action of a certain MF, the specific recognition of template molecules with different concentrations could be expressed directly through the optical signal and could be visible to the naked eye. The study was able to realize the visual detection of 10⁻²–10^–5 mg/ml melamine and 6.0 × 10⁻⁷–6.0 × 10^–4 ml-phenylalanine, which only took 5 min with a diffraction shift of 200 nm. Although the response system is fast and reversible, its stability, specificity, and responsiveness to actual samples are a long way to go.

At present, although the application of MRPCs’ responsiveness still focuses on some simple physical and chemical properties, the advantages of MRPCs have been fully reflected and laid the foundation for their application development. In the future research of MRPCs, the on-site rapid detection of environmental pollutants, clinical targets, and biological macromolecules will be the focus of researchers’ attention and development.