Selecting the Swimming Mechanisms of Colloidal Particles: Bubble Propulsion versus Self-Diffusiophoresis

Bubble propulsion and self-diffusiophoresis are two common mechanisms that can drive autonomous motion of microparticles in hydrogen peroxide. Although microtubular particles, when coated with platinum in their interior concave surfaces, can propel due to the formation and release of bubbles from one end, the convex Janus particles usually do not generate any visible bubble. They move primarily due to the self-diffusiophoresis. Coincidentally, the platinum films on those particles were typically coated by physical evaporation. In this paper, we use a simple chemical deposition method to make platinum–polystyrene Janus dimers. Surprisingly, those particles are propelled by periodic growth and collapse of bubbles on the platinum-coated lobes. We find that both high catalytic activity and rough surface are necessary to change the propulsion mode from self-diffusiophoresis to bubble propulsion. Our Janus dimers, with combined geometric and interfacial anisotropy, also exhibit distinctive motions at the respective stages of bubble growth and collapse, which differ by 5−6 orders of magnitude in time. Our study not only provides insight into the link between self-diffusiophoresis and bubble propulsion but also reveals the intriguing impacts of the combined geometric and interfacial anisotropy on self-propulsion of particles

Bicompartmental Phase Transfer Vehicles Based on Colloidal Dimers

Colloidal particles have been used extensively for stabilizing oil–water interfaces in petroleum, food, and cosmetics industries. They have also demonstrated promising potential in the encapsulation and delivery of drugs. Our work is motivated by challenging applications that require protecting and transporting active agents across the water–oil interfaces, such as delivering catalysts to underground oil phase through water flooding for in situ cracking of crude oil. In this Research Article, we successfully design, synthesize, and test a unique type of bicompartmental targeting vehicle that encapsulates catalytic molecules, finds and accumulates at oil–water interface, releases the catalysts toward the oil phase, and performs hydrogenation reaction of unsaturated oil. This vehicle is based on colloidal dimers that possess structural anisotropy between two compartments. We encapsulate active species, such as fluorescent dye and catalytic molecules in one lobe which consists of un-cross-linked polymers, while the other polymeric lobe is highly cross-linked. Although dimers are dispersible in water initially, the un-cross-linked lobe swells significantly upon contact with a trace amount of oil in aqueous phase. The dimers then become amphiphilic, migrate toward, and accumulate at the oil–water interface. As the un-cross-linked lobe swells and eventually dissolves in oil, the encapsulated catalysts are fully released. We also show that hydrogenation of unsaturated oil can be performed subsequently with high conversion efficiency. By further creating the interfacial anisotropy on the dimers, we can reduce the catalyst release time from hundred hours to 30 min. Our work demonstrates a new concept in making colloidal emulsifiers and phase-transfer vehicles that are important for encapsulation and sequential release of small molecules across two different phases

Nanocomposite Teflon AF 2400 Films as Tunable Platforms for Selective Transport

The performance of polymeric film-based sensors, separations, including extractions, depends on solute transport rates and selectivity. The membrane’s chemical composition, its state (e.g, crystalline, glassy, rubbery), and its fractional free volume are all important in defining performance attributes. Other properties of films important in sensors are robustness in the environment, chemical inertness and biocompatibility, thermal stability, and optical transparency. With the long-term goal of selective transport/extraction based on molecular recognition, we have focused on fluorous media such as Teflon AF 2400. We present a novel approach to create nanocomposite Teflon AF 2400 films with the polymer in different states to facilitate permeation and fluorous selectivity in liquid phase transport. Films cast from stable suspensions of the fluorocarbon polymer Teflon AF 2400 (<i>T</i>_g ∼ 240 °C), fluoroalkylsilane-modified solid, low polydispersity silica nanoparticles (FNPs: 116 nm diameter), and with or without a plasticizer (perfluorotripentylamine, FC-70) are macroscopically homogeneous. The nanocomposite films with glassy polymer absorb considerable solvent, CHCl₃, when in contact with solutions. Thus, the films are very permeable to solutes (toluene and octafluorotoluene) from CHCl₃ solution with poor selectivity for the fluorinated solute. Plasticized Teflon AF nanocomposite films show very low solvent sorption, improved fluorocarbon/hydrocarbon selectivity, and excellent transport rates. This is an unprecedented example demonstrating the effect of a plasticizer to create polymer nanocomposites with different chemical and barrier properties. The state of the polymer in the nanocomposites dictates chemical properties. The chemical properties dictate the transport behavior. In all cases, the films are dimensionally and thermally quite stable, making them ideal materials for applications in separations and sensors

Photocontrollable Intermittent Release of Doxorubicin Hydrochloride from Liposomes Embedded by Azobenzene-Contained Glycolipid

Azobenzene-contained glycolipids GlyAzoC<i>n</i>s, newly structured azobenzene derivatives, which have an azobenzene moiety between the galactosyl and carbon chains of various sizes, have been synthesized. The GlyAzoC<i>n</i>s undergo reversible photoinduced isomerization in both ethanol solution (free state) and liposomal bilayer (restricted state) upon irradiation with UV and vis light alternately. The drug release of Liposome@Gly induced by isomerization was found to be an instantaneous behavior. The photoinduced control of DOX release from liposome was investigated in various modes. The Liposome@Glys have been found to keep the entrapped DOX stably in the dark with less than 10% leakage in 10 h but release nearly 100% of cargos instantaneously with UV irradiation. The molecular structure of GlyAzoC<i>n</i>s and the property of the liposomal bilayer were considered as important factors influencing drug release. Among the synthesized GlyAzoC<i>n</i>s, GlyAzoC7 was shown to be the most efficient photosensitive actuator for controlling drug release. A lower proportion of cholesterol in Liposome@Glys was conducive to promote the release amount. Results indicated that the synthesized GlyAzoC<i>n</i>s could act as a role of smart actuators in the liposome bilayer and control the drug to release temporarily and quantitatively

Bulk Synthesis of Metal–Organic Hybrid Dimers and Their Propulsion under Electric Fields

Metal–organic hybrid particles have great potential in applications such as colloidal assembly, autonomous microrobots, targeted drug delivery, and colloidal emulsifiers. Existing fabrication methods, however, typically suffer from low throughput, high operation cost, and imprecise property control. Here, we report a facile and bulk synthesis platform that makes a wide range of metal–organic colloidal dimers. Both geometric and interfacial anisotropy on the particles can be tuned independently and conveniently, which represents a key advantage of this method. We further investigate the self-propulsion of platinum-polystyrene dimers under perpendicularly applied electric fields. In 1 × 10^–4 M KCl solution, the dimers exhibit both linear and circular motion with the polystyrene lobes facing toward the moving direction, due to the induced-charge electroosmotic flow surrounding the metal-coated lobes. Surprisingly, in deionized water, the same dimers move in an opposite direction, i.e., the metallic lobes face the forward direction. This is because of the impact of another type of electrokinetic flow: the electrohydrodynamic flow arising from the induced charges on the conducting substrate. The competition between the electrohydrodynamic flow along the substrate and the induced-charge electroosmotic flow along the metallic lobe dictates the propulsion direction of hybrid dimers under electric fields. Our synthetic approach will provide potential opportunities to study the combined impacts of the geometric and interfacial anisotropy on the propulsion, assembly, and other applications of anisotropic particles

Neighbor-Net of 21 Native American populations.

<p>The color of the squares indicates the geographic relationship of the populations (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0044788#pone-0044788-g005" target="_blank">Figure 5</a> for more details).</p

Moving from the two-population model to a four-population model.

<p>Moving from the two-population model to a four-population model.</p

Disruption of Tumor Cells Using a pH-Activated and Thermosensitive Antitumor Lipopeptide Containing a Leucine Zipper Structure

Antitumor peptides may potentially alleviate the problem of chemoresistance but do not yet target tumor cells and would be cytotoxic to normal cells. Here, we designed a pH-activated and thermosensitive lipopeptide (C6-Pep) containing a leucine zipper and an alkyl chain and assessed the ability of C6-Pep to kill cancer cells. Pep, the same sequence without the N-terminal hexanoic acid moiety, was generated as a less hydrophobic control. First, lipopeptide adsorption into lipid monolayers was studied using Langmuir–Blodgett and polarization modulation infrared reflection adsorption spectroscopy. Under weakly acid conditions, electrostatic interactions between C6-Pep and negatively charged phospholipids increased the adsorption/insertion of C6-Pep (vs Pep) into lipid monolayers. Cargo leakage from liposomes was assayed to model lipopeptide-induced lipid membrane disruption. The ability of C6-Pep to disrupt liposomes depended on the peptide molecular structure/hydrophobicity, solution pH, and temperature-induced uncoiling of the zipper structure; the greatest cargo leakage from the liposome with negative charge was observed for C6-Pep at pH 5.5 under mildly hyperthermic conditions (45 °C). In vitro, C6-Pep was significantly more cytotoxic toward HeLa cells at pH 5.5 under hyperthermic conditions than at pH 7.4 and/or 37 °C. Overall, this study demonstrates that amphipathic C6-Pep can insert into cell membranes in the low-pH tumor microenvironment, whereas the application of heat promotes the uncoiling of the zipper structure, leading to the disruption of tumor cell membranes and cell death. pH-activated and thermosensitive C6-Pep represents a promising tool to kill cancer cells via a strategy that does not invoke chemoresistance and may have low side effects

Geographic locations of 21 Native American populations.

<p>The populations are categorized into four different geographic clusters, i.e., Northern American, Central American I, Central American II, and Southern American.</p

A four-population model with demographic parameters.

<p>Population PopD was the reference population with known divergence time when the model was used in our study.</p