Daeyeon Lee

Designer emulsions using microfluidics

We describe new developments for the controlled fabrication of monodisperse emulsions using microfluidics. We use glass capillary devices to generate single, double, and higher order emulsions with exceptional precision. These emulsions can serve as ideal templates for generating well-defined particles and functional vesicles. Polydimethylsiloxane microfluidic devices are also used to generate picoliter-scale water-in-oil emulsions at rates as high as 10 000 drops per second. These emulsions have great potential as individual microvessels in high-throughput screening applications, where each drop serves to encapsulate single cells, genes, or reactants.

Double emulsion templated monodisperse phospholipid vesicles.

We present a novel approach for fabricating monodisperse phospholipid vesicles with high encapsulation efficiency using controlled double emulsions as templates. Glass-capillary microfluidics is used to generate monodisperse double emulsion templates. We show that the high uniformity in size and shape of the templates are maintained in the final phospholipid vesicles after a solvent removal step. Our simple and versatile technique is applicable to a wide range of phospholipids.

Biologically templated photocatalytic nanostructures for sustained light-driven water oxidation

Over several billion years, cyanobacteria and plants have evolved highly organized photosynthetic systems to shuttle both electronic and chemical species for the efficient oxidation of water. In a similar manner to reaction centres in natural photosystems, molecular and metal oxide catalysts have been used to photochemically oxidize water. However, the various approaches involving the molecular design of ligands, surface modification and immobilization still have limitations in terms of catalytic efficiency and sustainability. Here, we demonstrate a biologically templated nanostructure for visible light-driven water oxidation that uses a genetically engineered M13 virus scaffold to mediate the co-assembly of zinc porphyrins (photosensitizer) and iridium oxide hydrosol clusters (catalyst). Porous polymer microgels are used as an immobilization matrix to improve the structural durability of the assembled nanostructures and to allow the materials to be recycled. Our results suggest that the biotemplated nanoscale assembly of functional components is a promising route to significantly improved photocatalytic water-splitting systems.

Droplet Microfluidics for Fabrication of Non-Spherical Particles

We describe new developments for controlled fabrication of monodisperse non-spherical particles using droplet microfluidics. The high degree of control afforded by microfluidic technologies enables generation of single and multiple emulsion droplets. We show that these droplets can be transformed to non-spherical particles through further simple, spontaneous processing steps, including arrested coalescence, asymmetric polymer solidification, polymerization in microfluidic flow, and evaporation-driven clustering. These versatile and scalable microfluidic approaches can be used for producing large quantities of non-spherical particles that are monodisperse in both size and shape; these have great potential for commercial applications.

Shape-Changing and Amphiphilicity-Reversing Janus Particles with pH-Responsive Surfactant Properties

Janus particles are biphasic colloids that have two sides with distinct chemistry and wettability. Because of their amphiphilicity, Janus particles present a unique opportunity for stabilizing multiphasic fluid mixtures such as emulsions. Our work is motivated by one class of molecular amphiphiles that change their surfactant properties in response to environmental stimuli. Depending on the environmental conditions, these stimuli-responsive molecular amphiphiles are able to assemble into different structures, generate emulsions with different morphologies, and also induce phase inversion emulsification. We present a new synthesis method utilizing a combination of polymerization-induced phase separation and seeded emulsion polymerization, which allows for the bulk synthesis of highly uniform pH-responsive Janus particles that are able to completely reverse their surfactant properties in response to solution pH. One side of these Janus particles is rich in a hydrophobic monomer, styrene, whereas the other side is rich in a pH-sensitive hydrophilic repeating unit, acrylic acid. These Janus particles change their aggregation/dispersion behavior and also transform into different shapes in response to pH changes. Furthermore, we demonstrate that these Janus particles can stabilize different types of emulsions (oil-in-water and water-in-oil) and, more importantly, induce phase inversion of emulsions in response to changes in solution pH. The pH-responsive aggregation/dispersion behavior of these Janus particles also allows us to tune the interactions between oil-in-water emulsion droplets without inducing destabilization; that is, emulsion drops with attractive or repulsive interactions can be generated by changing the pH of the aqueous phase. Our study presents a new class of colloidal materials that will further widen the functionality and properties of Janus particles as dynamically tunable solid surfactants.

Enhanced release of small molecules from near-infrared light responsive polymer-nanorod composites.

Stimuli-responsive materials undergo structural changes in response to an external trigger (i.e., pH, heat, or light). This process has been previously used for a range of applications in biomedicine and microdevices and has recently gained considerable attention in controlled drug release. Here, we use a near-infrared (NIR) light responsive polymer-nanorod composite whose glass transition temperature (T(g)) is in the range of body temperature to control and enhance the release of a small-molecule drug (<800 Da). In addition to increased temperature and resulting changes in molecule diffusion, the photothermal effect (conversion of NIR light to heat) adjusts the composite above the T(g). Specifically, at normal body temperature (T < T(g)), the structure is glassy and release is limited, whereas when T > T(g), the polymer is rubbery and release is enhanced. We applied this heating system to trigger release of the chemotherapeutic drug doxorubicin from both polymer films and microspheres. Multiple cycles of NIR exposure were performed and demonstrated a triggered and stepwise release behavior. Lastly, we tested the microsphere system in vitro, reporting a ∼90% reduction in the activity of T6-17 cells when the release of doxorubicin was triggered from microspheres exposed to NIR light. This overall approach can be used with numerous polymer systems to modulate molecule release toward the development of unique and clinically applicable therapies.

Nonspherical Colloidosomes with Multiple Compartments from Double Emulsions

Colloidosomes are hollow capsules whose walls are composed of densely packed colloidal particles. Colloidosomes are typically prepared by creating particle-covered water-in-oil (W/O) emulsion droplets. These particle shells in the oil phase are subsequently transferred into an aqueous phase to generate the colloidosomes. We recently reported a new approach to fabricate monodisperse colloidosomes by using double emulsions as templates. Water-in-oil-in-water (W/O/W) double emulsions with a core–shell structure are created using a glass capillary microfluidic device. Hydrophobic SiO2 nanoparticles, suspended in the oil phase, become the wall of colloidosomes upon removal of the oil. The functionality and physical properties of colloidosomes such as permeability, selectivity, and biocompatibility can be precisely controlled by suitable choice of colloidal particles and processing conditions. Such versatility makes these colloidosomes attractive candidates for applications in encapsulation and delivery of foodstuffs, fragrances, and active ingredients. Further advantages could be obtained from colloidosomes and capsules with a non-spherical geometry, or with multiplecompartments. Nonspherical particles can pack more densely than spherical ones. Nonspherical capsules, therefore, can be used to store a larger amount of encapsulated materials compared to spherical capsules. Nonspherical capsules could also facilitate the delivery of encapsulated materials through constrictions; red blood cells (RBC) are one of many examples in nature where a nonspherical compartment is utilized. In addition to the nonspherical nature, multiple-compartment colloidosomes would enable encapsulation and storage of multiple types of cells andmaterials in a single capsule without risk of cross contamination. Colloidosomes that have been reported to date, however, are spherical in shape, and have only one compartment. Interfacial tension between the two immiscible phases favors minimizing the surface area and thus leads to the formation of spherical droplets. This tendency naturally limits the fabrication process to the generation of spherical colloidosomes. Moreover, the traditional method of using simple W/O emulsions to template colloidosomes leads to generation of spherical colloidosomes with only one compartment. In this report, we demonstrate the generation of nonspherical colloidosomes with multiple compartments. We use glass capillary microfluidics to prepare W/O/W double emulsions with different morphologies. These double emulsions have a different number of internal aqueous drops in the oil drop. Hydrophobic SiO2 nanoparticles, suspended in the oil phase, and poly(vinyl alcohol) (PVA), dissolved in the continuous aqueous phase, stabilize the double emulsions. The nanoparticles in the oil phase eventually become the shell of colloidosomes upon the removal of the oil. During the oil removal, the internal W/O interface retains their spherical shapes whereas the outer O/W interface deforms; this process leads to the generation of nonspherical colloidosomes with multiple compartments (Scheme 1). A glass capillarymicrofluidic device that combines a co-flow and a flow-focusing geometry is used to generate double emulsions with controlled morphology (see Supporting Information, Scheme S1). W/O/W double emulsions with a different number of internal aqueous drops per oil drop (n) were generated by controlling the flow rates of three phases independently as shown inFigure 1. The outerO/Winterface was stabilized by a partially hydrolyzed PVA in the continuous phase and the inner W/O interface was stabilized by 15-nm hydrophobic SiO2 nanoparticles; the nanoparticles dispersed in theoilphaseadsorbedto the twoW/Ointerfacesasevidencedby communications

Equilibrium Orientation of Nonspherical Janus Particles at Fluid–Fluid Interfaces

We study the equilibrium orientation of nonspherical Janus particles at an oil-water interface. Two types of nonspherical Janus particles are considered: Janus ellipsoids and Janus dumbbells. To find their equilibrium orientation, we calculate and minimize the attachment energy of each Janus particle as a function of its orientation angle with respect to the oil-water interface. We find that the equilibrium orientation of the interface trapped Janus particles strongly depends on the particle characteristics, such as their size, aspect ratio, and surface properties. In general, nonspherical Janus particles adopt the upright orientation (i.e., the long axis of ellipsoids or dumbbells is perpendicular to the interface) if the difference in the wettability of the two sides is large or if the particle aspect ratio is close to 1. In contrast, Janus particles with a large aspect ratio or a small difference in the wettability of the two regions tend to have a tilted orientation at equilibrium. Moreover, we find that Janus ellipsoids, under appropriate conditions, can be kinetically trapped in a metastable state due to the presence of a secondary energy minimum. In contrast, Janus dumbbells possess only a primary energy minimum, indicating that these particles prefer to be in a single orientation. The absence of a secondary minimum is potentially advantageous for obtaining particle layers at fluid-fluid interfaces with uniform orientation. Our calculation provides a detailed guidance for synthesizing nonspherical Janus particles that can be used as effective solid surfactants for the stabilization of multiphasic fluid mixtures and the modification of the rheological properties of fluid interfaces.

Amphiphilic Janus particles at fluid interfaces

Janus particles are colloids that have both hydrophilic and hydrophobic faces. Recent advances in particle synthesis enable the generation of geometrically and chemically anisotropic Janus particles with high uniformity and precision. These amphiphilic particles are similar to molecular surfactants in many aspects; they self-assemble in bulk media and also readily attach to fluid interfaces. These particles, just like molecular surfactants, could potentially function as effective stabilizers for various multiphasic systems such as emulsions and foams. In particular, just as the shape and chemical composition have a significant impact on the surfactancy of molecular amphiphiles, the ability to control the shape and wetting properties of Janus particles could provide a unique opportunity to control their surface activity. In this review, we first examine the recent developments in using amphiphilic Janus particles as colloid surfactants to stabilize multiphasic mixtures such as emulsions. These results have motivated a number of detailed investigations aimed at understanding the behaviour of Janus particles at fluid–fluid interfaces at the microscopic level, which we highlight. This review also discusses the importance of controlling the shape of Janus particles, which has a drastic impact on their behaviour at fluid interfaces. We conclude this review by presenting outlook on the future directions and outstanding problems that warrant further study to fully enable the utilization of Janus particles as colloid surfactants in practical applications.

Gel-immobilized colloidal crystal shell with enhanced thermal sensitivity at photonic wavelengths.

and liquid crystals [ 3 ] to impart desired mechanical strength, biocompatibility, permeability, and release properties to the microcapsules. Such microcapsules are often used for a variety of applications such as macromolecular delivery, [ 4 ] hazardous material handling, [ 5 ] oil recovery, [ 6 ] and food processing. [ 7 ] The choice of an appropriate shell material is important for the development of microcapsules with new functionalities for novel applications in pharmaceutical, food, cosmetic, and materials industries. Charge-stabilized colloidal crystals are three-dimensional periodic arrays of charged particles with low packing density in a liquid medium. [ 8 ] The spatial periodicity of the refractive index of the colloidal crystalline arrays results in an optical stop band effect, and, hence, they act as photonic crystals in the optical regime. [ 9 ] Since the charged colloids do not touch one another, the lattice constant can be conveniently tuned by changing the particle volume fraction. The wavelength of the optical stop band can be fi xed by immobilizing these crystals in a hydrogel. [ 10 ] The Bragg wavelength can be altered on-demand if a stimuli-sensitive hydrogel is used. By adjusting the volume of the hydrogel through an external stimulus, the lattice constant of the crystals can be tuned accordingly; this is useful for applications such as tunable photonic crystals, [ 11 ] tunable laser, [ 12 ] and biological and chemical sensors. [ 13 ] In particular, capsules with a shell of such colloidal crystals will enable labeling and monitoring of the encapsulated materials as well as the environment around the capsules through the wavelength of the optical stop band or diffraction color.