Synthesis of Janus particles via kinetic control of phase separation in emulsion droplets

Development of Janus Particles as Potential Drug Delivery Systems for Diabetes Treatment and Antimicrobial Applications

Janus particles have emerged as a novel and smart material that could improve pharmaceutical formulation, drug delivery, and theranostics. Janus particles have two distinct compartments that differ in functionality, physicochemical properties, and morphological characteristics, among other conventional particles. Recently, Janus particles have attracted considerable attention as effective particulate drug delivery systems as they can accommodate two opposing pharmaceutical agents that can be engineered at the molecular level to achieve better target affinity, lower drug dosage to achieve a therapeutic effect, and controlled drug release with improved pharmacokinetics and pharmacodynamics. This article discusses the development of Janus particles for tailored and improved delivery of pharmaceutical agents for diabetes treatment and antimicrobial applications. It provides an account of advances in the synthesis of Janus particles from various materials using different approaches. It appraises Janus particles as a promising particulate system with the potential to improve conventional delivery systems, providing a better loading capacity and targeting specificity whilst promoting multi-drugs loading and single-dose-drug administration.

Shape-Tunable Biconcave Disc-Like Polymer Particles by Swelling-Induced Phase Separation of Seeded Particles with Hydrophilic Shells

Anisotropic shape-tunable polymer particles have gained significant attention for their wide applications, and their performances are usually strongly correlated to their shapes. In contrast to convex particles, the synthesis of highly uniform concave polymer particles remains a great challenge. Here, we present a facile and effective route to synthesize biconcave polystyrene (PS) discs by swelling-induced phase separation of hydrophilically modified PS microspheres and report an unexpected finding that even a tiny amount of hydrophilic units that were incorporated into PS microspheres can significantly change the shape of phase interfaces, resulting in the transformation of disc shapes from convex to flat to concave. This is realized by several typical hydrophilic monomers, such as sodium styrene sulfonate (NaSS), acrylic acid (AA), or (2-(methacryloyloxy)ethyl)trimethylammonium chloride (METAC). The effect of the distribution of hydrophilic units in microspheres was investigated, and the mechanism of shape tuning has been discussed. The curvatures of the bottom surfaces of discs show a strong correlation to the content of hydrophilic units. In particular, we emphasize that the shape control method is general since it does not depend on specific hydrophilic units. This research paves the way for precisely structuring polymer particle shapes, which is important for polymer particles to be used for self-assembly, diffusion, rheology, transport, filler, and many other applications.

Preparation of asymmetric particles by controlling the phase separation of seeded emulsion polymerization with ethanol/water mixture

Alcohols are discovered for the first time to tune the morphology of poly(vinyl benzyl chloride)-poly(3-methacryloxypropyltrimethoxysilane) (PVBC-PMPS) composite particles through seeded emulsion polymerization within the alcohol/water mixture. Here, monodispersed linear PVBC particles was synthesized through the dispersion polymerization and employed as the seeds. The as-obtained PVBC-PMPS composite particles could be dramatically tuned from core-shell structures to snowman-like particles, to dumbbell-shaped particles, to inverse snowman-like particles when the ethanol content in reaction mixtures is only adjusted within a narrow range. The morphology of fresh PMPS bulges was observed after removing the linear PVBC seeds with N,N'-dimethyl formamide, and their formation mechanism was studied by monitoring the free radical polymerization and sol-gel process of 3-methacryloxypropyltrimethoxysilane. It has been confirmed that the sol-gel kinetics were the main factor on the particles' morphology. In addition, morphologies of PVBC-PMPS particles were also varied by the MPS feeding amount, types of the co-solvent and pH values of alcohol/water mixtures.

Janus Droplet Formation via Thermally Induced Phase Separation: A Numerical Model with Diffusion and Convection

Microscale Janus particles have versatile potential applications in many physical and biomedical fields, such as microsensor, micromotor, and drug delivery. Here, we present a phase-field approach of multicomponent and multiphase to investigate the Janus droplet formation via thermally induced phase separation. The crucial kinetics for the formation of Janus droplets consisting of two polymer species and a solvent component via an interplay of both diffusion and convection is considered in the Cahn-Hilliard-Navier-Stokes equation. The simulation results of the phase-field model show that unequal interfacial tensions between the two polymer species and the solvent result in asymmetric phase separation in the formation process of Janus droplets. This asymmetric phase separation plays a vital role in the establishment of the so-called core-shell structure that has been observed in previous experiments. By varying the droplet size, the surface tension, and the molecular interaction between the polymer species, several novel droplet morphologies are predicted in the development process of Janus droplets. Moreover, we stress that the hydrodynamics should be reckoned as a non-negligible mechanism that not only accelerates the Janus droplet evolution but also has great impacts on the coarsening and coalescence of the Janus droplets.

Janus particles and motors: unrivaled devices for mastering (bio)sensing

Janus particles are a unique type of materials combining two different functionalities in a single unit. This allows the combination of different analytical properties leading to new analytical capabilities, i.e., enhanced fluid mixing to increase sensitivity with targeting capturing abilities and unique advantages in terms of multi-functionality and versatility of modification, use, and operation both in static and dynamic modes. The aim of this conceptual review is to cover recent (over the last 5 years) advances in the use of Janus microparticles and micromotors in (bio)-sensing. First, the role of different materials and synthetic routes in the performance of Janus particles are described. In a second main section, electrochemical and optical biosensing based on Janus particles and motors are covered, including in vivo and in vitro methodologies as the next biosensing generation. Current challenges and future perspectives are provided in the conclusions section.

Chemical Imaging of Single Anisotropic Polystyrene/Poly (Methacrylate) Microspheres with Complex Hierarchical Architecture

Monodisperse polystyrene spheres are functional materials with interesting properties, such as high cohesion strength, strong adsorptivity, and surface reactivity. They have shown a high application value in biomedicine, information engineering, chromatographic fillers, supercapacitor electrode materials, and other fields. To fully understand and tailor particle synthesis, the methods for characterization of their complex 3D morphological features need to be further explored. Here we present a chemical imaging study based on three-dimensional confocal Raman microscopy (3D-CRM), scanning electron microscopy (SEM), focused ion beam (FIB), diffuse reflectance infrared Fourier transform (DRIFT), and nuclear magnetic resonance (NMR) spectroscopy for individual porous swollen polystyrene/poly (glycidyl methacrylate-co-ethylene di-methacrylate) particles. Polystyrene particles were synthesized with different co-existing chemical entities, which could be identified and assigned to distinct regions of the same particle. The porosity was studied by a combination of SEM and FIB. Images of milled particles indicated a comparable porosity on the surface and in the bulk. The combination of standard analytical techniques such as DRIFT and NMR spectroscopies yielded new insights into the inner structure and chemical composition of these particles. This knowledge supports the further development of particle synthesis and the design of new strategies to prepare particles with complex hierarchical architectures.

Frame-Guided Synthesis of Polymeric Colloidal Discs

Anisotropic colloidal particles are important building blocks for the studies of self-assembly, which are visualized models for basic research and can be used to construct structured materials. Discs are one of the most typical anisotropic colloids; however, the synthesis of monodisperse colloidal discs with well-defined shape remains a challenge. Here we report a novel strategy for synthesizing polymeric discs based on frame-guided droplet shrinkage. This was realized by creating frame/liquid core/shell rings and utilizing the shrinking instability of the liquid rings. The resulting disc's shape parameters are tunable. The method is general, is not limited to specific polymers, solvents, and frames, and therefore has the potential to afford a variety of polymer discs. We also demonstrate the possibility of tuning the surface chemistry of the discs through surface-initiated polymerization. The frame-guided droplet shrinkage method opens up a new way to design and fabricate colloidal particles.

Simple preparation of magnetic field-responsive structural colored Janus particles

We established a simple method for preparing Janus particles displaying different structural colors using submicron-sized fine silica particles and magnetic nanoparticles composed of Fe₃O₄.

Biocompatible Amphiphilic Hydrogel-Solid Dimer Particles as Colloidal Surfactants

Emulsions of two immiscible liquids can slowly coalesce over time when stabilized by surfactant molecules. Pickering emulsions stabilized by colloidal particles can be much more stable. Here, we fabricate biocompatible amphiphilic dimer particles using a hydrogel, a strongly hydrophilic material, and achieve large contrast in the wetting properties of the two bulbs, resulting in enhanced stabilization of emulsions. We generate monodisperse single emulsions of alginate and shellac solution in oil using a flow-focusing microfluidics device. Shellac precipitates from water and forms a solid bulb at the periphery of the droplet when the emulsion is exposed to acid. Molecular interactions result in amphiphilic dimer particles that consist of two joined bulbs: one hydrogel bulb of alginate in water and the other hydrophobic bulb of shellac. Alginate in the hydrogel compartment can be cross-linked using calcium cations to obtain stable particles. Analogous to surfactant molecules at the interface, the resultant amphiphilic particles stand at the water/oil interface with the hydrogel bulb submerged in water and the hydrophobic bulb in oil and are thus able to stabilize both water-in-oil and oil-in-water emulsions, making these amphiphilic hydrogel-solid particles ideal colloidal surfactants for various applications.

Core-Shell SiO2-PS Colloids with Controlled Eccentric Ratio

Precisely controlling microstructure of colloidal particles is crucial for their applications. Core-shell colloids have been extensively synthesized and used in past decades. However, controlling the location of cores in core-shell particles remains a challenge. To address this problem we explored the synthesis of SiO₂-PS core-shell colloids by using a simple system containing only core particles, monomer, initiator, and water/ethanol and found the increase of ethanol/water ratio can induce a structure transition sequence from eccentric to concentric to eccentric to concentric to eccentric. Furthermore, we illustrate that the eccentric ratios of SiO₂-PS core-shell colloids, that is, the location of SiO₂ cores in the whole particles, can be precisely controlled by a two-step polymerization procedure. It is anticipated that our results can widen the application of core-shell colloids, especially after the introduction of functionality for core or shell materials.

Precise synthesis of unique polydopamine/mesoporous calcium phosphate hollow Janus nanoparticles for imaging-guided chemo-photothermal synergistic therapy

A novel synthetic strategy has been developed for fabricating spherical polydopamine/mesoporous calcium phosphate (PDA/mCaP) hollow Janus nanoparticles (H-JNPs).

Multifunctional polymer–inorganic Janus nanoparticles (JNPs) that simultaneously have therapeutic and imaging functions are highly desired in biomedical applications. Here, we fabricated spherical polydopamine/mesoporous calcium phosphate hollow JNPs (PDA/mCaP H-JNPs) via a novel and facile approach. The obtained PDA/mCaP H-JNPs were further selectively functionalized with indocyanine green (ICG) and methoxy-poly(ethylene glycol)thiol (PEG-SH) on PDA domains to achieve a superior photoacoustic (PA) imaging capability and stability, while the other mCaP sides with hollow cavities served as storage spaces and passages for the anti-cancer drug, doxorubicin (DOX). The resultant PEG–ICG–PDA/mCaP H-JNPs possess excellent biocompatibility, a competent drug loading capability, high photothermal conversion efficiency, strong near-infrared (NIR) absorbance, and pH/NIR dual-responsive properties, enabling the H-JNPs to be applied for PA imaging-guided synergistic cancer chemo-phototherapy in vitro and in vivo. Furthermore, the synthetic approach could be extended to prepare PDA/various mesoporous inorganic H-JNPs with spherical shapes for specific applications.

Controllable microfluidic fabrication of Janus and microcapsule particles for drug delivery applications

Janus and microcapsule particles are very attractive for drug delivery applications due to their capability of targeted and/or programmed drug release.

Hierarchical self-assembly of 'hard-soft' Janus particles into colloidal molecules and larger supracolloidal structures

Here we report the self-assembly of 'hard-soft' micron-sized Janus particles into clusters in aqueous media. The assembly process is induced by the desorption of a polymeric stabiliser from the particles, that is polyvinylpyrrolidone (PVP). Upon contact through collision and coalescence of the soft polymeric lobes, the newly formed clusters adopt a minimized surface area to volume ratio, thereby forming distinct microscopic supracolloidal analogues of simple molecular valance shell electron pair repulsion (VSEPR) space-fill structures. To explain this behaviour, the colloidal stability of our particle suspensions were studied with and without an adsorbed steric surfactant. Simulations of expected cluster morphology, compared with those from cryo-SEM analysis support the mechanism of assembly driven by surface area minimization in the case of soft-soft interactions. Altering the soft lobe size with respect to the hard lobe indicates a moderate effect on number of primary particles per cluster. Additionally, higher order structures of clusters containing a number of primary particles exceeding what is possible for a 'solid' core cluster are observed. As such, we also investigated the formation of suprastructures using a high number of 'hard-soft' Janus particles and verified their effective Pickering stabilization of air bubbles.

Janus nanoparticles: materials, preparation and recent advances in drug delivery

INTRODUCTION

The term Janus particles was used to describe particles that are the combination of two distinct sides with differences in chemical nature and/or polarity on each face. Due to the exponential growth of interest on multifunctional nanotechnologies, such anisotropic nanoparticles are promising tools in the field of drug delivery.

AREAS COVERED

The main preparation processes and the materials used have been described first. Then a specific focus has been done on therapeutic and/or diagnostic applications of Janus particles.

EXPERT OPINION

Janus particles are demonstrated as interesting objects with advanced properties that combine features and functionalities of different materials in one single unit. Due to their dual structure, Janus particles are promising candidates for a variety of high-quality applications dealing with drug delivery purposes. Still, the main challenges for the future lie in the development of the preparation of shape-controlled and nano-sized particles with large-scale production processes and approved pharmaceutical excipients.

Asymmetric hetero-assembly of colloidal nanoparticles through "crash reaction" in a centrifugal field

Asymmetric hetero-assembly of two kinds of colloidal nanoparticles (NPs) was achieved by "crash reaction" in a density gradient centrifugation system using Au NPs as an example. Centrifugal force was applied to overcome the Brownian motion effect and cause NPs' directional movements. A water-oil interface was introduced to increase the effective collision probability.