Recognition-Induced Polymersomes: Structure and Mechanism of Formation

Polymersomes at the solid-liquid interface: Dynamic morphological transformation and lubrication

Polymersomes are hollow spheres self-assembled from amphiphilic block copolymers of certain molecular architecture. Whilst they have been widely studied for biomedical applications, relatively few studies have reported their interfacial properties. In particular, lubrication by polymersomes has not been previously reported. Here, interfacial properties of polymersomes self-assembled from poly(butadiene)-poly(ethylene oxide) (PBD-PEO; molecular weight 10,400 g mol^-1) have been studied at both hydrophilic and hydrophobic surfaces. Their morphology at silica and mica surfaces was imaged with quantitative nanomechanical property mapping atomic force microscopy (QNM AFM), and friction and surface forces they mediate under confinement between two surfaces were studied using colloidal probe AFM (CP-AFM). We find that the polymersomes remained intact but adopted flattened conformation once adsorbed to mica, with a relatively low coverage. However, on silica these polymersomes were unstable, rupturing to form donut shaped residues or patchy bilayers. On a silica surface hydrophobized with a 19 nm polystyrene (PS) film, the polymer vesicles formed a more stable layer with a higher surface coverage as compared to the hydrophilic surface, and the interfacial structure also evolved over time. Moreover, friction was greatly reduced on hydrophobized silica surfaces in the presence of polymersomes, suggesting their potential as effective aqueous lubricants.

Organic chemistry meets polymers, nanoscience, therapeutics and diagnostics

The atom-by-atom control provided by synthetic organic chemistry presents a means of generating new functional nanomaterials with great precision. Bringing together these two very disparate skill sets is, however, quite uncommon. This autobiographical review provides some insight into how my program evolved, as well as giving some idea of where we are going.

Straightforward synthesis route to polymersomes with simple molecules as precursors

We demonstrate an easy-to-implement experimental emulsion polymerization protocol whose outcome is an amphiphilic copolymer capable of forming vesicles in an aqueous phase. The protocol does not require prior purification of chemicals or the exclusion of oxygen. Using n-butyl acrylate as the monomer, we employ a redox initiation system composed of cerium(IV) ions and poly(ethylene glycol) (PEG), optimizing the performance of this redox couple such that the reaction can be conducted in air. The PEG-based chain radicals produced during initiation attack the monomer molecules, resulting in an amphiphilic product, which brings the synthesis of a vesicle-forming polymer to a level where no complicated equipment is required and may have implications for origins of life research.

Self-assembly and morphology control of new L-glutamic acid-based amphiphilic random copolymers: giant vesicles, vesicles, spheres, and honeycomb film

New amphiphilic random copolymers containing hydrophobic dodecyl (C12) chain and hydrophilic L-glutamic acid were synthesized, and their self-assembly in solution as well as on the solid surfaces was investigated. The self-assembly behavior of these polymers are largely dependent on their hydrophilic and hydrophobic balances. The copolymer with a more hydrophobic alkyl chain (∼90%) self-assembled into giant vesicles with a diameter of several micrometers in a mixed solvent of ethanol and water. When the hydrophobic ratio decreased to ca. 76%, the polymer self-assembled into conventional vesicles with several hundred nanometers. The giant vesicles could be fused in certain conditions, while the conventional vesicles were stable. When the content of the hydrophilic part was further increased, no organized structures were formed. On the other hand, when the copolymer solutions were directly cast on solid substrates such as silicon plates, films with organized nanostructures could also be obtained, the morphology of which depended on solvent selection. When ethanol or methanol was used, spheres were obtained. When dichloromethane was used as the solvent, honeycomb-like morphologies were obtained. These results showed that through appropriate molecular design, random copolymer could self-assemble into various organized structures, which could be regulated through the hydrophobic/hydrophilic balance and the solvents.

Stimuli-responsive polymersomes for programmed drug delivery

In the past decade, polymersomes (also referred to as polymeric vesicles) have attracted rapidly growing interest based on their intriguing aggregation phenomena, cell and virus-mimicking dimensions and functions, as well as tremendous potential applications in medicine, pharmacy, and biotechnology. Unlike liposomes self-assembled from low molecular weight lipids, polymersomes are in general prepared from macromolecular amphiphiles of various architectures including amphiphilic diblock, triblock, graft and dendritic copolymers. Polymersomes exhibit very unique features highlighted with high stability, tunable membrane properties, versatility, and capacity of transporting hydrophilic as well as hydrophobic species such as anticancer drugs, genes, proteins, and diagnostic probes. Recently, much effort has been directed to the development of intelligent polymersomes that respond to internal or external stimuli, in particular, pH, temperature, redox potential, light, magnetic field, and ultrasound, either reversibly or nonreversibly. Stimuli-sensitive polymersomes have emerged as novel programmable delivery systems in which the release of the encapsulated contents can be readily modulated by the stimulus. The stimuli-responsive release may result in significantly enhanced therapeutic efficacy and minimized possible side effects. It is also feasible to form and disassemble polymersomes in water simply by applying an appropriate stimulus. In this article, recent advances in stimuli-sensitive polymersomes have been reviewed, and perspectives on future developments have been discussed.

Solid-supported block copolymer membranes through interfacial adsorption of charged block copolymer vesicles

The properties of amphiphilic block copolymer membranes can be tailored within a wide range of physical parameters. This makes them promising candidates for the development of new (bio)sensors based on solid-supported biomimetic membranes. Here we investigated the interfacial adsorption of polyelectrolyte vesicles on three different model substrates to find the optimum conditions for formation of planar membranes. The polymer vesicles were made from amphiphilic ABA triblock copolymers with short, positively charged poly(2,2-dimethylaminoethyl methacrylate) (PDMAEMA) end blocks and a hydrophobic poly( n-butyl methacrylate) (PBMA) middle block. We observed reorganization of the amphiphilic copolymer chains from vesicular structures into a 1.5+/-0.04 nm thick layer on the hydrophobic HOPG surface. However, this film starts disrupting and dewetting upon drying. In contrast, adsorption of the vesicles on the negatively charged SiO2 and mica substrates induced vesicle fusion and formation of planar, supported block copolymer films. This process seems to be controlled by the surface charge density of the substrate and concentration of the block copolymers in solution. The thickness of the copolymer membrane on mica was comparable to the thickness of phospholipid bilayers.

Tuning the properties of PS-block copolymers and their assembly into polymersomes

The diblock copolymer polystyrene-b-polyisocyanoalanine(2-thiophene-3-yl-ethyl)amide (PS-PIAT) was prepared by reacting the isocyanide monomer (1) with a Ni(ii) initiator complex prepared from polystyrene amine (PS40NH2), either obtained by atom transfer radical polymerization (ATRP) or anionic polymerization (AP). It was found that polymerization of optically pure 1 followed first-order kinetics in monomer concentration and resulted in the formation of insoluble block copolymers, whereas the rate of polymerization of optical mixtures of 1 was retarded and yielded block copolymers that were better soluble. Furthermore, PS-PIAT polymersomes of which the PS-block was prepared by AP were more stable than polymersomes of which the PS-block was prepared by ATRP, as was indicated by combined turbidity and dynamic light scattering (DLS) measurements on the aggregate solutions.

H-bonding-directed self-assembly of synthetic copolymers containing nucleobases: organization and colloidal fusion in a noncompetitive solvent

The self-organization of random copolymers composed of a nucleobase monomer (either 1-(4-vinylbenzyl)thymine or 9-(4-vinylbenzyl)adenine) and dodecyl methacrylate (DMA) was studied in dilute chloroform solutions. The balance between the molar fractions of the nucleobase monomer (leading to intermolecular H-bonding) and DMA (soluble moiety in chloroform) in the polymer chains was found to be the parameter that principally influences the self-organization. DMA-rich copolymers are molecularly soluble in chloroform, whereas nucleobase-rich copolymers are insoluble in this solvent. Copolymers possessing an equimolar comonomer composition self-assemble into micrometer-sized particles physically cross-linked by intermolecular H-bonds (either thymine-thymine or adenine-adenine interactions, depending on the studied copolymer). Nevertheless, when mixed together, thymine- and adenine-based colloids fuse into thermodynamically stable microspheres cross linked by adenine-thymine interactions.