Janus Triad: Three Types of Nonspherical, Nanoscale Janus Particles from One Single Triblock Terpolymer

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.

Highly Porous Magnetic Janus Microparticles with Asymmetric Surface Topology

Monodispersed magnetic Janus particles composed of a porous polystyrene portion and a nonporous poly(vinyl acetate) portion with embedded oleic acid-coated magnetic nanoparticles were generated using microfluidic emulsification followed by two distinct phase separation events triggered by solvent evaporation. The template droplets were composed of 2 wt % polystyrene, 2 wt % poly(vinyl acetate), and 0.5-2 wt % n-heptane-based magnetic fluid dissolved in dichloromethane (DCM). The porosity of polystyrene compartments was the result of phase separation between a nonvolatile nonsolvent (n-heptane) and a volatile solvent (DCM) within polystyrene-rich phase. The focused ion beam cross-sectioning and scanning electron microscopy (SEM) imaging revealed high surface porosity of polystyrene compartments with negligible porosity of poly(vinyl acetate) parts, which can be exploited to increase the wettability contrast between the two polymers and enhance bubble generation in bubble-driven micromotors. The porosity of the polystyrene portion was controlled by varying the fraction of n-heptane in the dispersed phase. The particle composition was confirmed by scanning electron microscopy-energy-dispersive X-ray spectroscopy, Fourier transform infrared spectroscopy, and differential scanning calorimetry. The fabricated particles were successfully magnetized when subjected to an external magnetic field, which led to their aggregation into regular 2D assemblies. The particle clusters composed of two to four individual particles could be rotated with a rotating magnetic field. Microfluidic generation of highly porous Janus particles with compositional, topological, and magnetic asymmetry provides a cost-effective, easy-to-implement yet highly robust and versatile strategy for the manufacturing of multifunctional smart particles.

Formation and assembly of amphiphilic Janus nanoparticles promoted by polymer interactions

Almost three decades after de Gennes have introduced the term Janus for particles possessing two faces with different chemical nature, Janus particles are currently a hot topic in itself. Although de Gennes was not concerned with the size of particles, due to the advent and perspectives of nanotechnology, nanosized Janus particles have particularly received great attention. The capacity of having two antagonistic properties within the same particle has attracted interest on Janus nanoparticles for innumerous potential applications. It took some years for the studies about Janus nanoparticles to finally see great advances, mainly due to the progress in nanoparticle synthesis. What de Gennes might have not predicted (or at least he did not mention it during his speech) is that intermolecular interactions between polymers would be of immense importance to the actual achievement of Janus nanoparticles. Moreover, these interactions can also have large effects on the assembly process of amphiphilic Janus nanoparticles, which is important to form hierarchical structures and new materials at different scales. Hence, it is interesting to notice that de Gennes' contribution for the polymer field has been influencing the preparation and the controlled assembly of Janus nanoparticles. This article attempts to summarize empirical studies where noncovalent forces between polymers played a role, either on the production of Janus nanoparticles or on their assembly.

Janus Nanostructures from ABC/B Triblock Terpolymer Blends

Lamella-forming ABC triblock terpolymers are convenient building blocks for the synthesis of soft Janus nanoparticles (JNPs) by crosslinking the B domain that is "sandwiched" between A and C lamellae. Despite thorough synthetic variation of the B fraction to control the geometry of the sandwiched microphase, so far only Janus spheres, cylinders, and sheets have been obtained. In this combined theoretical and experimental work, we show that the blending of polybutadiene homopolymer (hPB) into lamella morphologies of polystyrene-block-polybutadiene-block-polymethylmethacrylate (SBM) triblock terpolymers allows the continuous tuning of the polybutadiene (PB) microphase. We systematically vary the volume fraction of hPB in the system, and we find in both experiments and simulations morphological transitions from PB-cylinders to perforated PB-lamellae and further to continuous PB-lamellae. Our simulations show that the hPB is distributed homogeneously in the PB microdomains. Through crosslinking of the PB domain and redispersion in a common solvent for all blocks, we separate the bulk morphologies into Janus cylinders, perforated Janus sheets, and Janus sheets. These studies suggest that more complex Janus nanostructures could be generated from ABC triblock terpolymers than previously expected.

Confinement Assembly of ABC Triblock Terpolymers for the High-Yield Synthesis of Janus Nanorings

Block copolymers are versatile building blocks for the self-assembly of functional nanostructures in bulk and solution. While spheres, cylinders, and bilayer sheets are thermodynamically preferred shapes and frequently observed, ring-shaped nanoparticles are more challenging to realize due to energetic penalties that originate from their anisotropic curvature. Today, a handful of concepts exist that produce core-shell nanorings, while more complex ( e. g., patchy) nanorings are currently out of reach and have only been predicted theoretically. Here, we demonstrate that confinement assembly of properly designed ABC triblock terpolymers is a general route to synthesize Janus nanorings in high purity. The triblock terpolymer self-assembles in the spherical confinement of nanoemulsion droplets into prolate ellipsoidal microparticles with an axially stacked lamellar-ring ( lr)-morphology. We clarified and visualized this complex, yet well-ordered, morphology with transmission electron tomography. Blocks A and C formed stacks of lamellae with the B microdomain sandwiched in-between as nanorings. Cross-linking of the B-rings allowed disassembly of the microparticles into Janus nanorings carrying two strictly separated polymer brushes of A and C on the top and bottom. Decreasing the B volume leads to Janus spheres and rods, while an increase of B results in perforated and filled Janus disks. The confinement assembly of ABC triblock terpolymers is a general process that can be extended to other block chemistries and will allow to synthesize a large variety of complex micro- and nanoparticles that inspire studies in self-assembly, interfacial stabilization, colloidal packing, and nanomedicine.

Hybrid Janus Particles: Challenges and Opportunities for the Design of Active Functional Interfaces and Surfaces

Janus particles are a unique class of multifunctional patchy particles combining two dissimilar chemical or physical functionalities at their opposite sides. The asymmetry characteristic for Janus particles allows them to self-assemble into sophisticated structures and materials not attainable by their homogeneous counterparts. Significant breakthroughs have recently been made in the synthesis of Janus particles and the understanding of their assembly. Nevertheless, the advancement of their applications is still a challenging field. In this Review, we highlight recent developments in the use of Janus particles as building blocks for functional materials. We provide a brief introduction into the synthetic strategies for the fabrication of JPs and their properties and assembly, outlining the existing challenges. The focus of this Review is placed on the applications of Janus particles for active interfaces and surfaces. Active functional interfaces are created owing to the stabilization efficiency of Janus particles combined with their capability for interface structuring and functionalizing. Moreover, Janus particles can be employed as building blocks to fabricate active functional surfaces with controlled chemical and topographical heterogeneity. Ultimately, we will provide implications for the rational design of multifunctional materials based on Janus particles.

Precise Self-Assembly and Controlled Catalysis of Thermoresponsive Core-Satellite Multicomponent Hybrid Nanoparticles

The construction of multicomponent hybrid nanomaterials with well-controlled architecture, especially bearing an ordered homogeneity and distribution of the subunits with tunable functions, is a key challenge in chemistry and material science. Herein, we reported a versatile and novel strategy to fabricate core-satellite multicomponent nanostructures with tunable interparticle distances and catalysis properties by the combination of surface-initiated reversible addition-fragmentation chain transfer (SI-RAFT) polymerization and self-assembly. The arrangement and interparticle distance of gold satellites could be precisely tuned by the SI-RAFT polymerization process and the feeding ratio of gold nanoparticles (AuNPs) and the core nanoparticle. It is worth to note that multilayered core-satellite nanostructures have been fabricated by a high-feeding ratio of AuNPs and magnetite NP (MNP)@SiO₂-PNIPAm. Notably, the core-satellite MNP@SiO₂-PNIPAm-Au nanoparticles exhibited excellent thermoresponsive behaviors with the change of temperature. Furthermore, the catalytic efficiency of MNP@SiO₂-PNIPAm-Au nanoparticles via the reduction of 4-nitrophenol to 4-aminophenol can be well modulated by the nanoparticle size, temperature, and polymer feed ratio. This strategy for precise construction of core-satellite nanostructures would open a new pathway to construct multicomponent functional nanostructures.

Magneto-thermochromic coupling Janus sphere for dual response display

This work demonstrates a simple microfluidic device to synthesize a magneto-thermochromic sphere with Janus inner structure. The Janus sphere is composed of Fe₃O₄ microspheres, thermochromic particles, and polyacrylamide matrix. Because the Fe₃O₄ microspheres are assembled together in one pole, the Janus sphere can turn around by varying the direction of the external magnetic field. Originating from the temperature-dependent property of the thermochromic particles, the final Janus sphere can change its color from red to pale blue when the temperature is increased from 5 to 45 °C. The detailed formation process and the magneto-thermochromic mechanism are carefully investigated. Due to the magnetic switch and thermochromism, these Janus spheres can be applied as colorful displays by controlling the magnetic field and temperature. The results demonstrate that the dual responsive Janus spheres possess broad application potential in temperature sensors and displays.

This work demonstrates a simple microfluidic device to synthesize a magneto-thermochromic sphere with Janus inner structure.

Solution-Based Thermodynamically Controlled Conversion from Diblock Copolymers to Janus Nanoparticles

Nanosized polymeric Janus particles (NPJPs) have important applications in a variety of theoretical and practical research fields. However, the methods that are versatile and can prepare NPJPs highly efficiently are very limited. Herein, we reported a two-step thermodynamically controlled preparation of NPJPs with a high yield using a diblock copolymer as the precursor. At the first step, A-b-B coassembled in the solution with a partner diblock copolymer C-b-B to form the mixed shell micelles (MSMs) with B core and A/C mixed shell. Then, intramicellarly covalently cross-linking the A block chains resulted in the complete phase separation of A and C chains in the mixed shell, leading to the direct conversion of the MSMs into NPJPs. The first step, diblock copolymer micellization, is known as a thermodynamically controlled process, and we also made the second step, conversion from MSMs to NPJPs, be thermodynamically controlled due to the application of covalent cross-linking. As the result, the conversion efficiency is close to 100%. Besides, it was further confirmed that the method can be applied to different systems and used to tune the Janus balance. Therefore, this conversion should be very significant for the fabrication and application of the NPJPs.

Design Advances in Particulate Systems for Biomedical Applications

The search for more efficient therapeutic strategies and diagnosis tools is a continuous challenge. Advances in understanding the biological mechanisms behind diseases and tissues regeneration have widened the field of applications of particulate systems. Particles are no more just protective systems for the encapsulated drugs, but they play an active role in the success of the therapy. Moreover, particles have been explored for innovative purposes as templates for cells growth and as diagnostic tools. Until few years ago the most relevant parameters in particles formulation were the chemistry and the size. Currently, it is known that other physical characteristics can remarkably affect the performance of particulate systems. Particles with non-conventional shapes exhibit advantages due to the increasing circulation time in blood stream, less clearance by the immune system and more efficient cell internalization and trafficking. Creation of compartments has been found useful to control drug release, to tune the transport of substances across biological barriers, to supply the target with more than one bioactive agent or even to act as theranostic systems. It is expected that such complex shaped and compartmentalized systems improve the therapeutic outcomes and also the patient's compliance, acting as advanced devices that serve for simultaneous diagnosis and treatment of the disease, combining agents of very different features, at the same time. In this review, we overview and analyse the most recent advances in particle shape and compartmentalization and applications of newly designed particulate systems in the biomedical field.

Hierarchical Micelles via Polyphilic Interactions: Hydrogen-Bonded Supramolecular Dendrons and Double Immiscible Polymers

We report a simple strategy to form three-phase segregated hierarchical micelles via a counterbalanced phase segregation/self-assembly process. Our methodology relies on a cooperative polyphilic phase segregation, paralleled by a self-assembly process induced by hydrogen-bonds to afford the generation of supramolecular multicompartment dendrons. The versatile preparation of such hierarchical morphologies is evidenced on the basis of a series of supramolecular dendrons, composed of semifluorinated copolymers, homopolymers, or nonfluorinated polymers. We do have designed and prepared mid- and α,ω-barbiturate (Ba) functionalized poly(n-butyl acrylates), Ba-(PnBuA-Ba)2, together with a series of heterocomplementary α,ω-Hamilton wedge (HW) functionalized polymers via reversible addition-fragmentation chain transfer (co)polymerization. To enable subtle phase segregation processes, the semifluorinated homo- and copolymers HW-P(nBuA-co-PFPA)-HW (prepared via copolymerization of nBuA with 2,2,3,3,3-pentafluoropropyl acrylate (PFPA)) and HW-PPFPA-HW, as well as the nonfluorinated polymer HW-PnBuA-HW and HW-PI-HW (PI, polyisoprene), have been generated. Selective intermolecular complexation between Ba-(PnBuA-Ba)2 and the complementary polymers (such as HW-P(nBuA-co-PFPA)-HW, HW-PPFPA-HW or HW-PI-HW) leads to the successful generation of supramolecular dendrons as evidenced by (1)H NMR and diffusion-ordered NMR spectroscopy, together with the formation of well-defined disc-like nano-objects as demonstrated by microscopy investigations. Transmission electron microscopy demonstrates a unique, uncommon phase behavior showing remarkable three-phase segregated hierarchical micelles, indicative of the desired micellar multicompartments.

Magnetic interaction of Janus magnetic particles suspended in a viscous fluid

We studied the magnetic interaction between circular Janus magnetic particles suspended in a Newtonian fluid under the influence of an externally applied uniform magnetic field. The particles are equally compartmentalized into paramagnetic and nonmagnetic sides. A direct numerical scheme is employed to solve the magnetic particulate flow in the Stokes flow regime. Upon applying the magnetic field, contrary to isotropic paramagnetic particles, a single Janus particle can rotate due to the magnetic torque created by the magnetic anisotropy of the particle. In a two-particle problem, the orientation of each particle is found to be an additional factor that affects the critical angle separating the nature of magnetic interaction. Using multiparticle problems, we show that the orientation of the particles has a significant influence on the dynamics of the particles, the fluid flow induced by the actuated particles, and the final conformation of the particles. Straight and staggered chain structures observed experimentally can be reproduced numerically in a multiple particle problem.

Self-assembly concepts for multicompartment nanostructures

Compartmentalization is ubiquitous to many biological and artificial systems, be it for the separate storage of incompatible matter or to isolate transport processes. Advancements in the synthesis of sequential block copolymers offer a variety of tools to replicate natural design principles with tailor-made soft matter for the precise spatial separation of functionalities on multiple length scales. Here, we review recent trends in the self-assembly of amphiphilic block copolymers to multicompartment nanostructures (MCNs) under (semi-)dilute conditions, with special emphasis on ABC triblock terpolymers. The intrinsic immiscibility of connected blocks induces short-range repulsion into discrete nano-domains stabilized by a third, soluble block or molecular additive. Polymer blocks can be synthesized from an arsenal of functional monomers directing self-assembly through packing frustration or response to various fields. The mobility in solution further allows the manipulation of self-assembly processes into specific directions by clever choice of environmental conditions. This review focuses on practical concepts that direct self-assembly into predictable nanostructures, while narrowing particle dispersity with respect to size, shape and internal morphology. The growing understanding of underlying self-assembly mechanisms expands the number of experimental concepts providing the means to target and manipulate progressively complex superstructures.

Hairy Core-Shell Polymer Nano-objects from Self-Assembled Block Copolymer Structures

Fabrication of core-shell polymer nano-objects with well-defined shape and hairy shell has been a subject of immense interest in polymer chemistry for more than two decades now. Different approaches such as those involving synthesis (grafting approaches) and block copolymer self-assembly (solution as well as bulk) have been used for the preparation of such nano-objects. Of these approaches that involving bulk self-assembled structures of block copolymers have been of special interest because of the simplicity and range of shape and structures possible. The present review focuses on the advances which have been made in this direction using diblock and triblock self-assembled structures. It will be shown that this approach allows to fabricate hairy nano-objects of not only simple shapes such as spheres, rods, and sheets but also those with more complex shape and morphology such as multicompartment micelles, which are not possible to obtain with synthetic or solution self-assembly approaches. Furthermore, interesting structures such as Janus nano-objects could also be fabricated using this approach. The review further highlights the use of such nano-objects for templating applications.

Origami with ABC Triblock Terpolymers Based on Glycopolymers: Creation of Virus-Like Morphologies

Morphologies, that resemble viruses, were created using a single ABC triblock terpolymer poly(2-acryloylethyl-α-d-mannopyranoside)-b-poly(n-butyl acrylate)-b-poly(4-vinylpyridine) (PAcManA₇₀-b-PBA₃₆₉-b-PVP₃₇₀). Morphologies ranging from flower-like micelles, cylindrical micelles, raspberry-like morphologies to nanocaterpillars were obtained by adjusting the pH value during the self-assembly process. The resulting nanoparticles had an abundance of mannose on the surface, which were recognized by the mannose receptors of RAW264.7, a macrophage cell line that can be used as a model for virus entry.

Preparation of cylindrical multi-compartment micelles by the hierarchical self-assembly of ABC triblock polymer in solution

Amphiphilic linear ABC triblock copolymer PnBA₂₈-b-PS₃₇-b-P2VP₇₃ was prepared by the RAFT method. Spherical patchy micelles and cylindrical MCMs were formed in different steps of its two-step hierarchical self-assembly in selected solvents.

RAFT polymerization of linear ABC triblock copolymer PtBA-b-PS-b-P2VP and regulation of its hierarchical self-assembly structure in solution

Linear ABC triblock copolymer PtBA

Janus Polymer Single Crystal Nanosheet via Evaporative Crystallization

We show that liquid/liquid interface can guide polymer chain folding during crystallization. Evaporation-induced crystallization of telechelic dicarboxyl end-functionalized poly(ε-caprolactone) (COOH-PCL-COOH) at a water/pentyl acetate interface produced millimeter-scale, uniform polymer single crystal (PSC) films. Due to the asymmetric nature at the interface, the PSC nanosheets exhibited a Janus structure: the two surfaces of the crystal showed distinct water contact angle, which are quantitatively confirmed by in situ nanocondensation using environmental scanning electron microscopy (ESEM).

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.

Emulsions with unique properties from proteins as emulsifiers

Many proteins are surface active molecules and form stable emulsions. In these emulsions, the protein covered oil droplets behave as sticky droplets even when they are ionically charged. As a result of the stickiness of the droplets the emulsions have gel-like properties. The stickiness is due to the multipolar nature of the proteins in contrast to the bipolar nature of surfactants or other amphiphilic compounds that form emulsions with repulsive droplets. Stable emulsions are also formed from particles like clays to which proteins are adsorbed. These hybrid compounds form even more stable emulsions with stronger elastic properties than clays and proteins on their own. These so called pickering emulsions have paste-like properties and do not flow. The scaffolding network of the crosslinked protein bilayers on the droplets is so strong that both the water and the oil can be removed from the emulsions by freeze drying without collapse of the scaffold. The resulting sponge can be used again for the uptake of both water and oil. Emulsions which are prepared from different proteins differ mainly in their elastic properties.

Structural characterization of amphiphilic homopolymer micelles using light scattering, SANS, and cryo-TEM

We report the aqueous solution self-assembly of a series of poly(N-isopropylacrylamide) (PNIPAM) polymers end-functionalized with a hydrophobic sulfur-carbon-sulfur (SCS) pincer ligand. Although the hydrophobic ligand accounted for <5 wt% of the overall homopolymer mass, the polymers self-assembled into well-defined spherical micelles in aqueous solution, and these micelles are potential precursors to solution-assembled nanoreactors for small molecule catalysis applications. The micelle structural details were investigated using light scattering, cryogenic transmission electron microscopy (cryo-TEM), and small angle neutron scattering (SANS). Radial density profiles extracted from the cryo-TEM micrographs suggested that the PNIPAM chains formed a diffuse corona with a radially decreasing corona density profile and provided valuable a priori information about the micelle structure for SANS data modeling. SANS analysis indicated a similar profile in which the corona surrounded a small hydrophobic core containing the pincer ligand. The similarity between the SANS and cryo-TEM results demonstrated that detailed information about the micelle density profile can be obtained directly from cryo-TEM and highlighted the complementary use of scattering and cryo-TEM in the structural characterization of solution-assemblies, such as the SCS pincer-functionalized homopolymers described here.

Influence of Janus particle shape on their interfacial behavior at liquid-liquid interfaces

We investigate the self-assembly behavior of Janus particles with different geometries at a liquid-liquid interface. The Janus particles we focus on are characterized by a phase separation along their major axis into two hemicylinders of different wettability. We present a combination of experimental and simulation data together with detailed studies elucidating the mechanisms governing the adsorption process of Janus spheres, Janus cylinders, and Janus discs. Using the pendant drop technique, we monitor the assembly kinetics following changes in the interfacial tension of nanoparticle adsorption. According to the evolution of the interfacial tension and simulation data, we will specify the characteristics of early to late stages of the Janus particle adsorption and discuss the effect of Janus particle shape and geometry. The adsorption is characterized by three adsorption stages which are based on the different assembly kinetics and different adsorption mechanisms depending on the particle shape.