Structure and thermodynamics of lipid bilayers on polyethylene glycol cushions: fact and fiction of PEG cushioned membranes.

In developing well hydrated polymer cushioned membranes, structural studies are often neglected. In this work, neutron and X-ray reflectivity studies reveal that hybrid bilayer/polyethylene glycol (PEG) systems created from mixtures of phospholipids and PEG conjugated lipopolymers do not yield a hydrated cushion beneath the bilayer unless the terminal ends of the lipopolymers are functionalized with reactive end groups and can covalently bind (tether) to the underlying support surface. While reactive PEG tethered systems yielded bilayers with near complete surface coverage, a bimodal distribution of heights with sub-micrometer lateral dimensions was observed consisting of cushioned membrane domains and uncushioned regions in close proximity to the support. The membrane fraction cushioned by the hydrated polymer could be controlled by adjusting the molar ratio of lipopolymer in the bilayer. A general phase diagram based on the free energy of the various configurations is derived that qualitatively predicts the observed behavior and the resulting structure of such systems a priori. As further evidenced by ellipsometry, atomic force and fluorescence microscopy, the tethered system provides a simple means for fabricating small cushioned domains within a membrane.

Fabrication and characterization of rare-earth-doped nanostructures on surfaces.

This article presents a simple and practical means to produce rare-earth-based nanostructures, as well as a combined characterization of structure and optical properties in situ. A nanosphere lithography strategy combined with surface chemistry enables the production of arrays of ß-NaYF(4):Yb,Er nanorings inlaid in an octadecyltrichlorosilane matrix. These arrays of nanorings are produced over the entire support, such as a 1 cm(2) glass coverslip. The dimension of nanorings can be varied by changing the deposition conditions. A home-constructed, multifunctional microscope integrating atomic force microscopy, near-field scanning optical microscopy, and far-field optical microscopy and spectroscopy is utilized to characterize the nanostructures. This in situ and combined characterization is important for rare-earth-containing nanomaterials in order to correlate local structure with upconversion photoluminescence. Knowledge gained from the investigation should facilitate materials design and optimization, for instance, in the context of photovoltaic devices and biofluorescent probes.

pH responsive polymer cushions for probing membrane environment interactions.

A robust and straightforward method for the preparation of lipid membranes upon dynamically responsive polymer cushions is reported. Structural characterization demonstrates that complete, well-packed membranes with tunable mobility can be constructed on the polymeric cushion. With this system, membrane conformational changes induced by cellular cytoskeleton interactions can be modeled. The membrane can be tailored to screen the cushion from changes in pH or allow rapid response to the pH environment by incorporation of protein ion channels. This elementary system offers a means to replicate the conformational changes that occur with the cellular cytoskeleton and has great potential for fundamental biophysical studies of membrane properties and membrane-protein interactions decoupled from the underlying solid support.

A stripe-to-droplet transition driven by conformational transitions in a binary lipid-lipopolymer mixture at the air-water interface.

We report the observation of an unusual stripe-droplet transition in precompressed Langmuir monolayers consisting of mixtures of poly(ethylene) glycol (PEG) amphiphiles and phospholipids. This highly reproducible and fully reversible transition occurs at approximately zero surface pressure during expansion (or compression) of the monolayer following initial compression into a two-dimensional solid phase. It is characterized by spontaneous emergence of an extended, disordered stripe-like morphology from an optically homogeneous phase during gradual expansion. These stripe patterns appear as a transient feature and continuously progress, involving gradual coarsening and ultimate transformation into a droplet morphology upon further expansion. Furthermore, varying relative concentrations of the two amphiphiles and utilizing amphiphiles with considerably longer ethylene glycol headgroups reveal that this pattern evolution occurs in narrow concentration regimes, values of which depend on ethylene oxide headgroup size. These morphological transitions are reminiscent of those seen during a passage through a critical point by variations in thermodynamic parameters (e.g., temperature or pressure) as well as those involving spinodal decomposition. While the precise mechanism cannot be ascertained using present experiments alone, our observations can be reconciled in terms of modulations in competing interactions prompted by the pancake-mushroom-brush conformational transitions of the ethylene glycol headgroup. This in turn suggests that the conformational degree of freedom represents an independent order parameter, or a switch, which can induce large-scale structural reorganization in amphiphilic monolayers. Because molecular conformational changes are pervasive in biological membranes, we speculate that such conformational transition-induced pattern evolution might provide a physical mechanism by which membrane processes are amplified.

Polystyrene-b-poly(tert-butyl acrylate) and polystyrene-b-poly(acrylic acid) dendrimer-like copolymers: two-dimensional self-assembly at the air-water interface.

The two-dimensional self-assembly at the air/water (A/W) interface of two dendrimer-like copolymers based on polystyrene and poly(tert-butyl acrylate) (PS-b-PtBA) or poly(acrylic acid) (PS-b-PAA) was investigated through surface pressure measurements (isotherms, isochores, and compression-expansion hysteresis experiments) and atomic force microscopy (AFM) imaging. The two dendrimer-like block copolymers have an 8-arm PS core (Mn = 10 000 g/mol, approximately 12 styrene repeat units per arm) with a 16-arm PtBA (Mn = 230 000 g/mol, approximately 112 tert-butyl acrylate repeat units per arm) or PAA (Mn = 129 000 g/mol, approximately 112 acrylic acid repeat units per arm) corona. The PS-b-PtBA sample forms stable Langmuir monolayers and aggregates into circular surface micelles up to a plateau observed in the corresponding isotherm around 24 mN/m. Beyond this threshold, the monolayers collapse above the interface, resulting in the formation of large and irregular desorbed aggregates. The PS-b-PAA sample has ionizable carboxylic acid groups, and its A/W interfacial self-assembly was therefore investigated for various subphase pH values. Under basic conditions (pH = 11), the carboxylic acid groups are deprotonated, and the PS-b-PAA sample is therefore highly water-soluble and does not form stable monolayers, instead irreversibly dissolving in the aqueous subphase. Under acidic conditions (pH = 2.5), the PS-b-PAA sample is less water-soluble and becomes surface-active. The pseudoplateau observed in the isotherm around 5 mN/m corresponds to a pancake-to-brush transition with the PAA chains dissolving in the water subphase and stretching underneath the anchoring PS cores. AFM imaging revealed the presence of circular surface micelles for low surface pressures, whereas the biphasic nature of the pseudoplateau region was confirmed with the gradual aggregation of the micellar PS cores above the PAA chains. The aggregation numbers for both samples were estimated around 3-5 dendrimer-like copolymers per circular surface micelle. These rather low values confirmed the tremendous influence of molecular architecture on the two-dimensional self-assembly of block copolymers.