Janus Particles Research Articles

Two Sides of the Coin: Synthesis and Applications of Janus Particles

Two Sides of the Coin: Synthesis and Applications of Janus Particles

Formation of three-dimensional (3D) Self-Assembled Clusters of Anisotropic Janus Particles in Microgravity

Abstract The self-assembly of colloidal particles enables the creation of complex materials with tailored properties. This process, particularly involving anisotropic particles, can lead to the formation of structurally unique and complex assemblies that are not achievable with isotropic particles. On Earth, gravitational forces limit the investigation of these particles’ intrinsic motion and interactions, posing significant challenges to comprehensively understanding the fundamental forces governing their interactions. To overcome these limitations, this study, in collaboration with NASA’s Glenn Research Center (GRC), employs the Light Microscopy Module (LMM) aboard the International Space Station (ISS) to observe the self-assembly phenomena of anisotropic particles under microgravity conditions. Our investigation shows that anisotropic Janus particles with their distinctive properties can spontaneously organize into ordered structures under microgravity. This directional interaction among anisotropic particles is expected to enable control over assembly processes, forming three-dimensional (3D) clustered structures that are unattainable on Earth. Thus, this study not only advances our understanding of particle self-assembly in microgravity but also opens new avenues for synthesizing materials with novel functionalities through the unique assembly of anisotropic colloids.

Synthesis of silver-palladium Janus nanoparticles using co-sputtering of independent sources: experimental and theorical study.

Janus-type nanoparticles are important because of their ability to combine distinct properties and functionalities in a single particle, making them extremely versatile and valuable in various scientific, technological, and industrial applications. In this work, bimetallic silver-palladium Janus nanoparticles were obtained for the first time using the inert gas condensation technique. In order to achieve this, an original synthesis equipment built by Mantis Ltd. was modified by the inclusion of an additional magnetron in a second chamber, which allowed us to use two monometallic targets to sputter the two metals independently. With this arrangement, we could find appropriate settings at room temperature to promote the synthesis of bimetallic Janus nanoparticles. The structural properties of the resulting nanoparticles were investigated by transmission electron microscopy (TEM), and the chemical composition was analyzed by TEM energy dispersive spectroscopy (TEM-EDS), which, together with structural analysis, confirmed the presence of Janus-type nanostructures. Results of molecular dynamics and TEM simulations show that the differences between the crystalline structures of the Pd and Ag regions observed in the TEM micrographs can be explained by small mismatches in the orientations of the two regions of the particle. A density functional theory structural aims to understand the atomic arrangement at the interface of the Janus particle.

Orientational ordering and assembly of silica-nickel Janus particles in a magnetic field.

The orientation ordering and assembly behavior of silica-nickel Janus particles in a static external magnetic field were probed by ultra small-angle X-ray scattering (USAXS). Even in a weak applied field, the net magnetic moments of the individual particles aligned in the direction of the field, as indicated by the anisotropy in the recorded USAXS patterns. X-ray photon correlation spectroscopy (XPCS) measurements on these suspensions revealed that the corresponding particle dynamics are primarily Brownian diffusion [Zinn, Sharpnack & Narayanan (2023). Soft Matter, 19, 2311-2318]. At higher fields, the magnetic forces led to chain-like configurations of particles, as indicated by an additional feature in the USAXS pattern. A theoretical framework is provided for the quantitative interpretation of the observed anisotropic scattering diagrams and the corresponding degree of orientation. No anisotropy was detected when the magnetic field was applied along the beam direction, which is also replicated by the model. The method presented here could be useful for the interpretation of oriented scattering patterns from a wide variety of particulate systems. The combination of USAXS and XPCS is a powerful approach for investigating asymmetric colloidal particles in external fields.

Engineering light-driven micromotors with fluorescent dye coatings for easy detection and tracking.

Micromotors are the backbone of material research as they are small-sized, self-propelled, intelligent systems capable of performing multiple tasks ranging from biomedicine to environmental monitoring. One of the primary obstacles the field faces is the live detection and differentiation of individual units through a complex environment. In this study, we demonstrate a facile approach for designing light-activated dye-tagged micromotors on a large scale. The micromotors are titanium dioxide (TiO2)/copper oxide (Cu2O)-silica Janus spheres that are self-propelled under the illumination of low-intensity light in aqueous peroxide medium. The micromotors were modified with different dyes, such as Alq3, Alizarin, zinc phthalocyanine, etc. The fabrication of micromotors and coating with dyes were performed using a simple and versatile physical vapor deposition-based glancing angle deposition (GLAD) technique. The fluorescent dyes help to detect the motion and position of micromotors independently. Moreover, they also help to identify the swimming direction as well as differentiate the micromotors in a complex medium consisting of similar configurations of other particles (bacteria and passive fluorescent particles). Light provides full control over the dynamics as well as the fluorescence nature of micromotors. To present the versatility of our design scheme, micromotors of different shapes, materials, and dye coatings are designed and explored for fluorescence-based observations. The simplistic design approach with easy-to-load multiple fluorescent dyes is an interesting feature that makes the micromotors suitable candidates for various microfluidic and lab-on-a-chip studies, including biological or fluorescent samples.

Janus particles with tunable patch symmetry and their assembly into chiral colloidal clusters

Janus particles, which have an attractive patch on the otherwise repulsive surface, have been commonly employed for anisotropic colloidal assembly. While current methods of particle synthesis allow for control over the patch size, they are generally limited to producing dome-shaped patches with a high symmetry (C∞). Here, we report on the synthesis of Janus particles with patches of various tunable shapes, having reduced symmetries ranging from C2v to C3v and C4v. The Janus particles are synthesized by partial encapsulation of an octahedral metal-organic framework particle (UiO-66) in a polymer matrix. The extent of encapsulation is precisely regulated by a stepwise, asymmetric dewetting process that exposes selected facets of the UiO-66 particle. With depletion interaction, the Janus particles spontaneously assemble into colloidal clusters reflecting the particles’ shapes and patch symmetries. We observe the formation of chiral structures, whereby chirality emerges from achiral building blocks. With the ability to encode symmetry and directional bonding information, our strategy could give access to more complex colloidal superstructures through assembly.

Deciphering Long-Range Order in Active Matter: Insights from Swimming Bacteria in Quasi-2D and Electrokinetic Janus Particles

Deciphering Long-Range Order in Active Matter: Insights from Swimming Bacteria in Quasi-2D and Electrokinetic Janus Particles

Janus particles: features, synthesis and potential uses, including surface functionalisation

ABSTRACTAs the development of nanostructured materials continues to blossom, there is increasing attention to functionalised particles. The so-called Janus particles are anisotropic regions having different physicochemical properties. The recent development of their synthesis in a pH field during suspension as bipolar particles between electrodes has opened the door to improved scalability due to the ease of entraining particles as a slurry in a flowing electrolyte stream. Potential application areas span catalysis of chemical synthesis, sustainable energy conversion and pharmaceutical and biomedical sectors of industry. In view of their versatility, the ability to undergo surface modification and opportunities for facile scale-up, Janus particles might be deployed as building blocks to fabricate functional surfaces having tailored chemical, structural and surface anisotropy. They could prove to be valuable feedstock in 3-D printing and surface finishing of fast prototype components and devices.

Phase separation of a magnetic fluid: Asymptotic states and nonequilibrium kinetics.

We study self-assembly in a colloidal suspension of magnetic particles by performing comprehensive molecular dynamics simulations of the Stockmayer (SM) model, which comprises spherical particles decorated by a magnetic moment. The SM potential incorporates dipole-dipole interactions along with the usual Lennard-Jones interaction and exhibits a gas-liquid phase coexistence observed experimentally in magnetic fluids. When this system is quenched from the high-temperature homogeneous phase to the coexistence region, the nonequilibrium evolution to the condensed phase proceeds with the development of spatial as well as magnetic order. We observe density-dependent coarsening mechanisms-a diffusive growth law ℓ(t)∼t^{1/3} in the nucleation regime and hydrodynamics-driven inertial growth law ℓ(t)∼t^{2/3} in the spinodal regimes. [ℓ(t) is the average size of the condensate at time t after the quench.] While the spatial growth is governed by the expected conserved order parameter dynamics, the growth of magnetic order in the spinodal regime exhibits unexpected nonconserved dynamics. The asymptotic morphologies have density-dependent shapes which typically include the isotropic sphere and spherical bubble morphologies in the nucleation region, and the anisotropic cylinder, planar slab, cylindrical bubble morphologies in the spinodal region. The structures are robust and nonvolatile, and exhibit characteristic magnetic properties. For example, the oppositely magnetized hemispheres in the spherical morphology impart the characteristics of a Janus particle to it. The observed structures have versatile applications in catalysis, drug delivery systems, memory devices, and magnetic photonic crystals, to name a few.

On the Interfacial Assembly of Anisotropic Amphiphilic Janus Particles

AbstractIt has been shown both theoretically and experimentally that amphiphilic Janus particles are the most effective solid surfactants to stabilize interfaces. In most cases, the Janus particles investigated have uniform morphologies with Janus boundaries dividing the particle into halves. However, there are many examples of Janus particles where the hydrophilic and hydrophobic domains are not equally distributed. The effects of this uneven domain distribution on the mechanism and kinetics of Janus particle assembly, and final equilibrium state are not well‐understood. Dynamic pendant drop tensiometry offers a means to probe both the equilibrium assembly and the kinetics and mechanism of assembly. Here, the interfacial kinetics and assembly of spherical anisotropic Janus particles are investigated using dynamic pendant drop tensiometry. Systematic studies quantifying the time‐dependent interfacial behavior as a function of Janus particle morphology, chemical composition, particle concentration, and NaOH and HCl concentration are performed. These studies shed light on the assembly mechanism of more complex Janus particle morphologies and highlight their effectiveness as interface stabilizers.

Self-Assembly Pathways of Triblock Janus Particles into 3D Open Lattices.

The self-assembly of triblock Janus particles is simulated from a fluid to 3D open lattices: pyrochlore, perovskite, and diamond. The coarse-grained model explicitly takes into account the chemical details of the Janus particles (attractive patches at the poles and repulsion around the equator) and it contains explicit solvent particles. Hydrodynamic interactions are accounted for by dissipative particle dynamics. The relative stability of the crystals depends on the patch width. Narrow, intermediate, and wide patches stabilize the pyrochlore-, the perovskite-, and the diamond-lattice, respectively. The nucleation of all three lattices follows a two-step mechanism: the particles first agglomerate into a compact and disordered liquid cluster, which does not crystallize until it has grown to a threshold size. Second, the particles reorient inside this cluster to form crystalline nuclei. The free-energy barriers for the nucleation of pyrochlore and perovskite are ≈10kBT, which are close to the nucleation barriers of previously studied 2D kagome lattices. The barrier height for the nucleation of diamond, however, is much larger (>20kBT), as the symmetry of the triblock Janus particles is not perfect for a diamond structure. The large barrier is associated with the reorientation of particles, i.e., the second step of the nucleation mechanism.

Synthesis of Anisotropic Magnetic Polymeric Janus Particles by In Situ Separation of Magnetic Nanoparticles in a Microfluidic Device.

Magnetic Janus particles have been studied extensively for medical and biological applications owing to their controllable mobility in fluid media. In this work, we report a novel microfluidic device designed for the synthesis of magnetically anisotropic Janus particles made of poly(ethylene glycol) diacrylate and embedded with magnetic iron oxide nanoparticles. Our method consists of a droplet generation step followed by magnetic separation using an external magnetic field and ultraviolet polymerization. The synthesized particles exhibit a monodisperse size distribution with a standard deviation of less than 3.5%, which is among the best size distributions obtained in the literature for magnetic Janus particles. The anisotropic magnetic property of the particles enable them to rotate about their own axes in the presence of an external magnetic field, introducing another degree of freedom to their motion. This microfluidic technique is simple, one-step, and versatile, offering control over the size distribution to synthesize magnetically anisotropic Janus particles.

One-pot two-step production of nano-CaCO3 with tunable Janus balance decorated by self-crosslinked admicelle of conjugated linoleate

BackgroundIn mass production of Janus nanoparticles, tunable Janus balance, nanoscale decoration, environmental and biological compatibility, and response to environmental stimuli are challenges. MethodsBy manipulating the self-assembly and self-crosslinking of sodium conjugated linoleate (SCL), we established a facile one-pot synthesis with two-step addition of SCL under wax protection on partial surface of the microspheres to synthesize CaCO3 Janus nanoparticles with above features simultaneously. Significant findingsThe self-crosslinked admicelle (SCA) of SCL decorated on the nano-CaCO3 surface to produce SCA-CaCO3 Janus nanoparticles, and their Janus balance is tunable by simply regulating the primary SCL dosage in the preparation of molten wax emulsions. Afterwards, O/W Pickering emulsions are stabilized by 0.25 wt% of the selected SCA-CaCO3 Janus nanoparticle with a proper Janus balance at primary SCL dosage of 60 mg·L−1; the obtained high internal phase emulsion is reversibly switchable by pH or CO2−N2 triggers. This strategy of one-pot two-step production of Janus particles may provide a facile mass production tool to create hierarchical Janus structures for various inorganic nanoparticles with different sizes and chemical compositions, endowing variable Janus surfaces with tunable Janus balance, providing a nanoscale SCA decoration layer of SCL with biocompatibility, environmental friendliness and multi-stimuli responses.

Demixing behavior of Pickering emulsions stabilized by Janus particles and uniform Pickering particles at different pH values

This work investigates the demixing behavior of Pickering emulsions consisting of water, n-hexadecane, and two different types of Pickering particles. The first type is uniform, and the second consists of one-side modified Janus particles influencing the emulsion behavior significantly. Both particle types are able to stabilize the emulsions, but lead to clear differences in the emulsion drop size and size distribution. This results in different creaming times varying between minutes and hours and different clearance processes of the water excess phase. Large pH effects were found in addition. At some conditions, non-adsorbed particles were observed that sediment over time in addition to the creaming of the emulsion.

Waterproof and Flame-Retardant Fabric Coating with Nail-Tie Structure was Constructed by Janus Particles with Strong Mechanical, Physical, and Chemical Durability.

Oil spills are one of the most dangerous sources that cause serious environmental pollution and fire and explosion. In this work, multifunctional separator silica@polydivinylbenzene/poly 2,6-dimethyl-1, 4-phenyl ether (silica@PDVB/PPE) Janus particles were fabricated via seed emulsion polymerization, causing phase segregation as well as selective modification. The epoxy modified silica is partially covalently bonded to the fabric substrate surface by simple spraying to achieve a strong composite coating. The low surface energy PDVB/PPE forms a micronano rough layered surface, which can achieve a super hydrophobic and lipophile surface (WCA = 155°) and obtain a high flux separation of water and oil at 32,700 L·m-2·h-1. At the same time, the Janus composite fabric coating has the advantages of high heat resistance and flame retardant, which is realized by halogen-free flame-retardant unsaturated polyphosphate (PPE), making Janus fabric have potential value in separating oil-water mixtures and fire protection applications. In addition, the coating shows excellent chemical durability. After soaking in various aqueous solvents and organic solvents for 30 h, it can still maintain superhydrophobicity and flame retardant. The coating still has water repellency and flame retardant after 50 washings and mechanical wear and has good mechanical durability.

Hollow Superstructure In Situ Assembled by Single-Layer Janus Nanospheres toward Electromagnetic Shielding Flame-Retardant Polyurea Composites.

A dodecahedral superstructure consisting of a single layer of Janus spheres containing ZIF-67 nanodots is prepared by in situ polymerization, with ZIF-67 and bio-based phytic acid (PA) as templates and dopants. It is used to improve the flame retardant, electromagnetic (EMI) shielding, and thermal conductivity properties of polyurea (PUA). By adding 5wt% polyaniline@cobalt phytate-2.0 (PANI@Co-PA-2.0), the peak of heat release rate and the peak of smoke production rate are reduced by 54.9 and 59.9%, respectively. The peak of CO and CO2 production also decreased by 46.2 and 53.1%, respectively. A decrease in the absorption intensity of aliphatic and aromatic volatiles is also observed. The fire safety of PUA is greatly improved. In addition, PUA/PANI@Co-PA-2.0 exhibits an EMI shielding capability of 22.4dB with the help of reduced graphene oxide, which confirms the possibility of PUA material application in the field of electromagnetic shielding. The 5wt% filler increases the tensile strength of the PUA matrix to 6.3MPa, and the composite material obtains good thermal conductivity. This work provides a viable method for the preparation of a flame-retardant, conductive, and electromagnetic refractory PUA substrate.

Anisotropic Multitentacle Janus Particles Synthesized by Selective Asymmetric Growth.

Anisotropic colloidal particles with asymmetric morphology possess functionally rich heterogeneous structures, thus offering potential for intricate superstructures or nanodevices. However, it is a challenge to achieve controlled asymmetric surface partitioned growth. In this work, an innovative strategy is developed based on the selective adsorption and growth of emulsion droplets onto different regions of object which is controlled by wettability. It is found that the emulsion droplets can selectively adsorb on the hydrophilic surface but not the hydrophobic one, and further form asymmetric tentacle by the interfacial sol-gel process along its trajectory. Janus particles with an anisotropic shape and multitentacle structure are achieved via integration of emulsion droplet (soft) and seed (hard) templates. The size and number of tentacles exhibit tunability mediated by soft and hard templates, respectively. This general strategy can be expanded to a variety of planar substrates or curved particles, further confirming the correlation between tentacle growth and Brownian motion. Most interestingly, it can be employed to selectively modify one region of surface partitioned particles to achieve an ABC three-component Janus structure.

Preparation of amphiphilic Janus nanosheets based on thermally expandable microspheres and evaluation of their physical and chemical properties

Janus nanosheets can reduce the interfacial tension of oil and water and are of great significance for improving the recovery efficiency of remaining oil. A novel strategy for preparing amphiphilic Janus nanosheets is presented in this paper. Trialkoxysilane is adsorbed on the surface of thermally expandable microspheres (TEMS), and Janus spheres (TEMS@SiO2) are obtained by sol-gel treatment. Dodecyltrimethoxysilane (DTES) is used to modify TEMS@SiO2, which is combined with the thermal expansion of TEMS and the breaking of the outer shell of Janus spheres to prepare amino/alkyl composite silicon-based amphiphilic Janus nanosheets (ASAJN). Characterization of ASAJN is carried out via FTIR, TG, SEM, TEM, and contact angle tester, with the results showing that ASAJN boasts amphiphilicity and two-sidedness, and a sheet structure with a thickness of 210 nm. The stability, wettability evaluation, emulsification properties, and interface properties of ASAJN nanofluids have been evaluated, with the results showing that ASAJN nanofluids boast good stability without delamination after 30 days. ASAJN nanofluid can convert oil-wet core slices into water-wettable ones. ASAJN nanofluid manifests excellent emulsification, with the 240-hour emulsification index of ASAJN nanofluid and silica fluid respectively at0.40 and 0.021 under ultra-low concentration (1000 mg/L). ASAJN is obviously able to reduce oil–water interfacial tension. At 1000 mg/L (60 ℃) concentration, the oil-water interfacial tension is reduced to 2.6 mN/m, while that of nano-silica at the same concentration is 15.1 mN/m. ASAJN flooding can significantly improve the recovery efficiency of low-permeability reservoirs, increasing the recovery efficiency by 32.8 % at a permeability of 7.5 × 10-3 μm2.

Multimode microdimer robot for crossing tissue morphological barrier

Swimming microrobot energized by magnetic fields exhibits remotely propulsion and modulation in complex biological experiment with high precision. However, achieving high environment adaptability and multiple tasking capability in one configuration is still challenging. Here, we present a strategy that use oriented magnetized Janus spheres to assemble the microdimer robots with two magnetic distribution configurations of head-to-side configuration (HTS-config) and head-to-head configuration (HTH-config), achieving performance of multiple tasks through multimode transformation and locomotion. Modulating the magnetic frequency enables multimode motion transformation between tumbling, rolling, and swing motion with different velocities. The dual-asynchronization mechanisms of HTS-config and HTH-config robot dependent on magnetic dipole-dipole angle are investigated by molecular dynamic simulation. In addition, the microdimer robot can transport cell crossing morphological rugae or complete drug delivery on tissues by switching motion modes. This microdimer robot can provide versatile motion modes to address environmental variations or multitasking requirements.

Driven transport of active particles through arrays of symmetric obstacles.

We numerically examine the driven transport of an overdamped self-propelled particle through a two-dimensional array of circular obstacles. A detailed analysis of transport quantifiers (mobility and diffusivity) has been performed for two types of channels, channel I and channel II, that respectively correspond to the parallel and diagonal drives with respect to the array axis. Our simulation results show that the signatures of pinning actions and depinning processes in the array of obstacles are manifested through excess diffusion peaks or sudden drops in diffusivity, and abrupt jumps in mobility with varying amplitude of the drive. The underlying depinning mechanisms and the associated threshold driving strength largely depend on the persistent length of self-propulsion. For low driving strength, both diffusivity and mobility are noticeably suppressed by the array of obstacles, irrespective of the self-propulsion parameters and direction of the drive. When self-propulsion length is larger than a channel compartment size, transport quantifiers are insensitive to the rotational relaxation time. Transport with diagonal drives features self-propulsion-dependent negative differential mobility. The amplitude of the negative differential mobility of an active particle is much larger than that of a passive one. The present analysis aims at understanding the driven transport of active species like, bacteria, virus, Janus particle etc. in porous medium.