Self-assembly Of Colloidal Particles Research Articles

Preparation of uniform-sized colloidosomes based on chitosan-coated alginate particles and its application for oral insulin delivery.

Colloidosomes are microcapsules that consist of a hollow core coated by a shell composed of self-assembled colloidal particles. In recent years, they have attracted significant attention as potential vehicles for the controlled delivery of active ingredients. A key challenge in such applications is the production of uniform-sized colloidosomes for the encapsulation of active ingredients such that the actual delivery amount can be well controlled. Based on our previous study on an ethyl acetate-in-water emulsion stabilized by chitosan-coated alginate particles, we further prepared monodisperse colloidosomes with a high yield and low permeability by combining the premix membrane emulsification technique and polymer deposition method. The potential application of the formed colloidosomes as oral insulin delivery vehicles was investigated. Our results revealed that in comparison to conventional PLGA microspheres, the formulated colloidosomes showed high drug encapsulation efficiency (up to 96.7%) and an obvious pH sensitive release profile. Moreover, in animal testing, the colloidosomes formulation achieved a long-term hypoglycemic effect up to 6 h, which confirmed its application as an oral drug delivery system.

Tunable assembly of heterogeneously charged colloids.

The self-assembly of colloidal particles is a route to designed materials production that combines high flexibility, cost effectiveness, and the opportunity to create ordered structures at length scales ranging from nano- to micrometers. For many practical applications in electronics, photovoltaics, and biomimetic material synthesis, ordered mono- and bilayers are often needed. Here we present a novel and simple way to tune via external parameters the ordering of heterogeneously charged colloids into quasi two-dimensional structures. Depending on the charges of the underlying substrate and of the particles, a rich and versatile assembly scenario takes place, resulting from the complex interplay between directional attractive and repulsive particle–particle and particle–substrate interactions. Upon subtle variations of the relative charge of the system components, emerging via pH modification, reversible changes either from extended aggregates to a monomeric phase or from triangular to square domains are observed.

Structure and dynamics of self-assembling colloidal monolayers in oscillating magnetic fields

Many fascinating phenomena such as large-scale collective flows, enhanced fluid mixing, and pattern formation have been observed in so-called active fluids, which are composed of particles that can absorb energy and dissipate it into the fluid medium. For active particles immersed in liquids, fluid-mediated viscous stresses can play an important role on the emergence of collective behavior. Here, we experimentally investigate their role in the dynamics of self-assembling magnetically driven colloidal particles which can rapidly form organized hexagonal structures. We find that viscous stresses reduce hexagonal ordering, generate smaller clusters, and significantly decrease the rate of cluster formation, all while holding the system at constant number density. Furthermore, we show that time and length scales of cluster formation depend on the Mason number (Mn), or ratio of viscous to magnetic forces, scaling as t∝Mn and L∝Mn(-1/2). Our results suggest that viscous stresses hinder collective behavior in a self-assembling colloidal system.

POLYMERIC MICELLES: POTENTIAL DRUG DELIVERY DEVICES

Polymeric micelles (PMs) have been the most popular and promising topic of many researches in the field of drug delivery and targeting for the past two decades. Polymeric micelles are the selfassembled nano-sized colloidal particles which are made up of amphiphilic block copolymers i.e. polymers consisting of hydrophobic block and hydrophilic block. In this highlight, we give an overview of the structure of micelles and polymeric micelles followed by a summary of the methods used for their preparation. We then focus on several kinds of PMs based on intermolecular forces such as polyion complex micelles (PICMs), non-covalently connected micelles (NCCMs) and recently developed smart polymeric assemblies which can respond to the application of external stimuli such as a change in temperature, pH, redox and light to afford novel nanomaterials. The types of polymers used in the preparation of PMs have also been highlighted so as to facilitate its use in drug delivery and targeting. These polymeric micelles nanocarriers have applications in drug delivery primarily such as anticancer therapy, to the brain to treat neurodegenerative diseases, antifungal agents, stimuli-responsive nanocarriers for drug and gene delivery, ocular drug delivery. Targeted drugs will hopefully reduce adverse reactions by limiting their action to cancer tissue only. Finally, this review broadly presents the basic aspects of PMs which help in delivery and targeting of actives with its recent advancements and applications. Key words: micelles, polymeric micelles, block copolymer, stimuli sensitivity

Self-assembly of colloidal particles into strings in a homogeneous external electric or magnetic field

Colloidal particles with a dielectric constant (magnetic susceptibility) mismatch with the surrounding solvent acquire a dipole moment in a homogeneous external electric (magnetic) field. The resulting dipolar interactions can lead to aggregation of the particles into string-like clusters. Recently, several methods have been developed to make these structures permanent. However, especially when multiple particle sizes and/or more complex shapes than single spheres are used, the parameter space for these experiments is enormous. We therefore use Monte Carlo simulations to investigate the structure of the self-assembled string-like aggregates in binary mixtures of dipolar hard and charged spheres, as well as dipolar hard asymmetric dumbbells. Binary mixtures of spheres aggregate in different types of clusters depending on the size ratio of the spheres. For highly asymmetric systems, the small spheres form ring-like and flame-like clusters around strings of large spheres, while for size ratios closer to 1, alternating strings of both large and small spheres are observed. For asymmetric dumbbells, we investigate both the effect of size ratio and dipole moment ratio, leading to a large variety of cluster shapes, including chiral clusters.

High-Fidelity Optofluidic On-Chip Sensors Using Well-Defined Gold Nanowell Crystals

Recent advances in nanofabrication techniques have enabled the creation of various metallic nanostructures in order to engineer the location and properties of electromagnetic hot spots in a controlled manner. However, most previous methods usually require complicated and time-consuming techniques, and the integration of metallic nanostructures into simple, low-cost devices for chemical or biological sensing is still challenging. Here, we report a promising new strategy for the fabrication of large-area gold nanowell arrays with novel geometric features that makes use of the trapping of self-assembled colloidal particles on a polymer surface. Through both systematic experimental and theoretical analysis, we confirm that the strong plasmon resonances of the proposed nanowell structures are associated with localized surface plasmon resonance (LSPR) on the brims of the nanoholes in the top gold films as well as in the bottom gold disks. In addition, we demonstrate a novel optofluidic platform with built-in subwavelength nanowell arrays that exhibits strong plasmon resonances within microfluidic chips. In our optofluidic systems, the plasmon coupling between the brims and the disks of nanowells makes the plasmon resonance more sensitive to surrounding materials. The dependence of the plasmon resonance on the refractive index of the surrounding medium is found to be as high as 570 nm RIU(-1) (refractive index units). These data lead to a figure of merit (FOM), the slope of refractive index sensitivity in eV RIU(-1)/line width (eV), as high as 4.1.

Self-assembled SnO2 colloidal particles and their gas sensing performance to H2, C2H5OH and LPG

Using organo-tin Sn(OC4H9)4 as precursor, sodium dodecyl sulfonate (SDS) and SDS-gelatin (SDS-G) complex as template, two tin dioxide colloidal particles were prepared by a self-assembly method. Both SnO2 products were respectively labelled SnO2-B particles with SDS and SnO2-C particles with SDS-G, which are applied in fabricating SnO2 gas sensors corresponding to SnO2-B′ and SnO2-C′ sensors. X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and thermogravimetry and different thermal analysis (TG/DTA) were used for characterizations. The experimental results show that SnO2-B colloidal particles are composed of mesoporous piece-like particles, while SnO2-C particles mainly consist of spherical particles. Gas sensing measurements show that SnO2-B′ sensor performs the best sensing response to all target gases, including H2, C2H5OH and liquid petroleum gas (LPG). In particular, the sensing response of SnO2-B′ sensor is achieved at 32 in H2 atmosphere at the concentration of 1000×10−6 M. The gas sensing mechanism was purposely discussed from the electron transfer process and the microstructures of the as-prepared SnO2 products. It is found that serious agglomerations in SnO2-B′ particles facilitate the high gas sensing performance of SnO2-B′ sensor, while mesoporous structures in SnO2-C′ particles decrease the gas sensing response of SnO2-C′ sensor.

Electric-field-assisted self-assembly of colloidal particles

A method for formation of photonic crystals has been proposed. The method is based on convective deposition of colloidal particles onto vertical substrates in the presence of a direct-current electric field directed perpendicular to the surface of the formed film and an alternating-current electric field applied parallel to the substrate plane. The structure and optical properties of the prepared colloidal crystals have been investigated using scanning electron microscopy, high resolution small-angle X-ray diffraction, and optical spectroscopy.

SEM‐EDS technique employed in evaluating the aggregation behaviors of amphiphilic ABC‐type triblock copolymers in mixed solvents with tuned polarities

In this paper, a scanning electron microscopy-energy dispersive spectroscopy (SEM-EDS) technique has been developed for evaluating the aggregation structure of amphiphilic fluorinated ABC-type triblock copolymers MeOPEO(16)-PSt(220)-PFHEA(22) in mixed solvents with different polarities. The polarities of mixed solvents can be tuned by changing volume ratios of toluene, anhydrous ethanol, and distilled water, which leads to the changes in morphology and size of self-assembled colloidal particles of the copolymers in the system. The aggregation behaviors of the copolymers are revealed by SEM, transmission electron microscopy (TEM), and corresponding SEM-EDS techniques. The variations in concentrations of O and F elements over the thickness of copolymers particles give direct evidence for a better understanding of the arrangement of each block segment of copolymers in solution. And the technique can also help to explain the aggregation structure of micro- or nanomaterial with shell-core structure. Microsc. Res. Tech., 2011. © 2011 Wiley Periodicals, Inc.

Synthesis and characterization of a hybrid material from self-assembling colloidal particles and carbon nanotubes

The work describes the synthesis of a hybrid material starting from surface-modified colloidal particles of styrene (ST)-acrylic acid (AA) copolymer and carbon nanotubes (CNTs). Vinyl double bonds have been chemically grafted on the surface of the ST-AA copolymer particles in order to be able to copolymerize with acrylamide (AM). The hybrid material was obtained by reaction between the free radicals resulted from both copolymerization and AM homopolymerization and the superficial groups of modified CNTs. Due to the difference between the diameter of the polymer particles and the one of the CNTs, a change in the CNTs shape is to be expected (disentanglement due to steric effects). The products thus obtained have been characterized using IR, SEM, XPS, Raman, and AFM techniques.

Permeability control in stimulus-responsive colloidosomes

Self-assembly of colloidal particles in the liquid interface of double emulsion droplets can be used to fabricate “colloidosome” microcapsules, which have great potential as vehicles for the controlled delivery of drugs or other cargoes. Here we present a novel class of aqueous core colloidosomes that combine the benefit of low capsule permeability (good cargo retention) with the option of a stimulus-triggered fast release in a target environment. Complete or partial dissolution of the capsule walls in response to a mild pH change is achieved in each case through the use of responsive particles made from polymers with pH-switchable solubility. We demonstrate three methods of controlling the capsule permeability prior to release while maintaining the intended response to the release trigger.

Self-Assembly of Colloidosome Shells on Drug-Containing Hydrogels

Colloidosomes are composed of an aqueous or hydrogel corethat is coated by a semi-permeable colloidal shell. The properties of the shell can be varied to control the rate of release of encapsulated components such as drugs. Specifi-cally, the pores formed between the colloidal particles suppress transport of large components, while allowing diffusion of smaller ones. Self-assembly of colloidal particles on hydrogel films is a convenient method forcolloidosome synthesis, but to date little is known regarding the effect (if any) of the encapsulated drug on the shell packing density. In this paper we examined self-assembly of colloidal shells on alginate films containing four model drugs: aspirin, caffeine, theophylline and theobromine. We find that the packing density in the colloidal shells is low for all drugs, and ranges between 0.16 and 0.3. There is no clear correlation between drug properties (in particular, water solubility) and the packing density of the self-assembled colloidal shell.

Pickering emulsions stabilized by self-assembled colloidal particles of copolymers of P(St-alt-MAn)-co-P(VM-alt-MAn)

A new type of copolymer containing two alternating segments, poly-(styrene-alt-maleic anhydride)-co-poly (7-(4-vinylbenzyloxyl)-4-methylcoumarin-alt-maleic anhydride) P(St-alt-MAn)-co-P(VM-alt-MAn)(PSMVM), was prepared. The copolymer self-assembled into nanoparticles with internal microphase structures in water because the hydrophilicity of segment P(VM-alt-MAn) is higher than that of P(St-alt-MAn). The particle size, morphology, ζ potential and surface properties and their dependence on the pH and slat concentrations were studied with a combination of techniques. The nanoparticles of PSMVM showed surface activity and pH sensitivity for producing Pickering oil-in-water emulsions. The emulsion volume increased and the size of oil/water droplet decreased with increasing salt concentration. Furthermore, cross-linked nanoparticles (CLPs) were obtained by photo-dimerization of the pendent coumarin groups in PSMVM under UV irradiation. The emulsions produced by using the CLPs as emulsifiers showed even better stability upon standing. Solid oil-phase droplets were obtained by preparation of CLPs-stabilized Pickering emulsions with an oil phase of styrene containing the initiator AIBN followed by the polymerization of styrene. Thus, the enrichment and aggregation of the CLPs on the emulsion droplets was visible because the solid droplets remained unchanged during the SEM sample preparation.

Self-Assembly of Colloidal Particles from Evaporating Droplets: Role of DLVO Interactions and Proposition of a Phase Diagram

The shape of deposits obtained from drying drops containing colloidal particles matters for technologies such as inkjet printing, microelectronics, and bioassay manufacturing. In this work, the formation of deposits during the drying of nanoliter drops containing colloidal particles is investigated experimentally with microscopy and profilometry, and theoretically with an in-house finite-element code. The system studied involves aqueous drops containing titania nanoparticles evaporating on a glass substrate. Deposit shapes from spotted drops at different pH values are measured using a laser profilometer. Our results show that the pH of the solution influences the dried deposit pattern, which can be ring-like or more uniform. The transition between these patterns is explained by considering how DLVO interactions such as the electrostatic and van der Waals forces modify the particle deposition process. Also, a phase diagram is proposed to describe how the shape of a colloidal deposit results from the competition among three flow patterns: a radial flow driven by evaporation at the wetting line, a Marangoni recirculating flow driven by surface tension gradients, and the transport of particles toward the substrate driven by DLVO interactions. This phase diagram explains three types of deposits commonly observed experimentally, such as a peripheral ring, a small central bump, or a uniform layer. Simulations and experiments are found in very good agreement.

Obtaining of monodisperse particles through soap-free and seeded polymerization, respectively, through polymerization in the presence of C60

The present study reports on methods of synthesis of self-assembling monodisperse colloidal particles, obtained for the system styrene (St)–hydroxypropyl methacrylate, through emulsion polymerization, using no surfactant. The syntheses performed have led to the conclusion that the emulsion copolymerization in the presence of fullerene is responsible both for a significant decrease of the particles size as well as for a dimensional monodispersity of the system; the latter has been achieved also through seeded polymerization. The morphology and the size distribution of particles have been explored using scanning electron microscope and dynamic light scattering.

Obtaining of monodisperse particles through soap-free polymerization in the presence of C 60

The present study reports on the synthesis methods for self-assembling monodisperse colloidal particles, obtained from the system styrene (St)–hydroxyethyl acrylate (HEA), through emulsion polymerization, using no surfactant. The syntheses performed have led to the conclusion that the emulsion copolymerization in the presence of fullerene is responsible both for a significant decrease of the particles size, as well as for a dimensional monodispersity of the system. The morphology and the size distribution of particles have been explored using SEM and DLS.

Bottom-Up Photonic Crystal Approach with Top-Down Defect and Heterostructure Fine-Tuning

We combine the most efficient (chemical) approach toward three-dimensional photonic crystals with the most convenient (physical) technique for creating non-close-packed crystalline structures. Self-assembly of colloidal particles in artificial opals is followed by a carefully tuned plasma etching treatment. By covering the resulting top layer of more open structure with original dense opal, embedded defect layers and heterostructures can be conveniently designed for advanced photonic band gap and band edge engineering.

Facile Organization of Colloidal Particles into Large, Perfect One- and Two-Dimensional Arrays by Dry Manual Assembly on Patterned Substrates

The ability to rapidly and reproducibly assemble colloidal particles into large (>mm) one- (1D) and two-dimensional (2D) single crystals with perfect control of the particle networking pattern would open a new world rich with high quality novel materials, technologies, and sciences. However, current methods rely on self-assembly of colloidal particles in solution (wet self-assembly), which intrinsically makes the assembly of the colloidal particles into defect-free large 1D and 2D single crystals difficult. We now demonstrate a new paradigm of colloidal particle organization into 1D and 2D single crystals, a process we call 'dry manual assembly on nanolithographically patterned substrates', which enables facile and rapid organization of colloidal particles in dry states into 1D and 2D single crystals in the centimeter or larger scales with a well-defined particle networking pattern. We believe that this novel methodology will serve as a key to open a new era of particle organization.

Design and Synthesis of Model Transparent Aqueous Colloids with Optimal Scattering Properties

We demonstrate the synthesis and self-assembly of colloidal particles with independently controlled diameter and scattering cross section. We show that it is possible to prepare bulk colloidal suspensions that are nearly transparent in water, while the particles themselves can be individually resolved using optical microscopy. These particles may be ideal model colloids for real-space studies of self-assembly in aqueous media. Moreover, they illustrate the degree to which the optical properties of colloids can be engineered through straightforward chemistry.

Probing the Stability of Magnetically Assembled DNA-Linked Colloidal Chains

The self-assembly of colloidal particles using DNA linker molecules has led to novel colloidal materials. This article describes the development and characterization of a new class of colloidal structures based on the directed assembly of DNA-linked paramagnetic particles. A key obstacle to assembling these structures is understanding the fundamental chemistry and physics of the assembly processes. The stability of these cross-linked chain structures is the first step toward reliable assembly and thus important for its applications; however, chain stability has yet to be systematically studied. In this paper, we investigate both theoretically and experimentally, the stability of DNA-linked paramagnetic colloidal chains as a function of externally applied magnetic field strength and surface grafted DNA length and density. A total interparticle free energy potential model is developed accounting for all major forces contributing to chain stability, and a phase diagram is obtained from experiments to illustrate linked chain phases, unstable unlinked particle phases, and their transitions, which agree well with those predicted by the model. From this study, optimized parameters for successful linking and building stable linked chains are obtained.