Capillary aging of the contacts between glass spheres and a quartz resonator surface

The strength of the contacts between small glass spheres and the surface of a quartz crystal resonator has been probed based on the increase of resonance frequency induced upon sphere contact. The acoustic interaction between the sphere and the plate is modeled as a low-frequency coupled resonance; the dependence of the resonant parameters on overtone order lends support to this model. After exposing the sample to humid air and drying it again, the contact strength increases at least tenfold due to capillary forces--we observe a hysteretic form of the sand-castle effect. Repeated wet-dry cycles reveal logarithmic capillary aging with time. The experiments suggest that the drying of the liquid bridges leads to a contraction of small voids in the contact zone, subsequently increasing cohesion.

Synthesis of well-defined hydrogel networks using Click chemistry

New PEG-based hydrogel materials have been synthesized by Click chemistry and shown to result in well-defined networks having significantly improved mechanical properties; the selectivity of the azide/acetylene coupling reaction also allows for the incorporation of various additives and functional groups leading to chemical tailoring of the hydrogels.

The dramatic effect of architecture on the self-assembly of block copolymers at interfaces

Dramatic morphological changes are observed in the Langmuir-Blodgett (LB) film assemblies of poly(ethylene glycol)-b-(styrene-r-benzocyclobutene) block copolymer (PEG-b-(S-r-BCB)) after intramolecular cross-linking of the S-r-BCB block to form a linear-nanoparticle structure. To isolate architectural effects and allow direct comparison, the linear block copolymer precursor and the linear-nanoparticle block copolymer resulting from selective intramolecular cross-linking of the BCB units were designed to have exactly the same molecular weight and chemical composition but different architecture. It was found that the effect of architecture is pronounced with these macromolecular isomers, which self-assemble into dramatically different surface aggregates. The linear block copolymer forms disklike surface assemblies over the range of compression states, while the linear-nanoparticle block copolymer exhibits long (>10 microm) wormlike aggregates whose length increases as a function of increasing cross-linking density. It is shown that the driving force behind the morphological change is a combination of the altered molecular geometry and the restricted degree of stretching of the nanoparticle block because of the intramolecular cross-linking. A modified approach to interpret the pi-A isotherm, which includes presence of the block copolymer aggregates, is also presented, while the surface rheological properties of the block copolymers at the air-water interface provide in-situ evidence of the aggregates' presence at the air-water interface.

Near-infrared optical-absorption behavior in high-beta nonlinear optical chromophore-polymer guest-host materials. II. Dye spacer length effects in an amorphous polycarbonate copolymer host

In the second of a three-part series, spectral absorption behavior of nonlinear optical (NLO) dyes incorporated into amorphous polycarbonate, comprised of a homologous series of dialkyl spacer groups extending from the midsection of the dye molecule, is characterized by UV-Vis and photothermal deflection spectroscopy. The dyes are structural analogs of the NLO dye FTC [2-(3-cyano-4-{2-[5-(2-{4-[ethyl-(2-methoxyethyl)amino]phenyl}vinyl)-3,4-diethylthiophen-2-yl]vinyl}-5,5-dimethyl-5H-furan-2-ylidene)malononitrile]. Previous Monte Carlo calculations [B. H. Robinson and L. R. Dalton, J. Phys. Chem. A 104, 4785 (2000)] predict a strong dependence of the macroscopic nonlinear optical susceptibility on the chromophore waist: length aspect ratio in electric-field-poled films arising from interactions between chromophores. It is expected that these interactions will play a role in the absorption characteristics of unpoled films, as well. The spacer groups range in length from diethyl to dihexyl, and each dye is studied over a wide range of concentrations. Among the four dyes studied, a universal dependence of near-IR loss on inhomogeneous broadening of the dye main absorption peak is found. The inhomogeneous width and its concentration dependence are seen to vary with spacer length in a manner characteristic of the near-IR loss-concentration slope at transmission wavelengths of 1.06 and 1.3 mum, but not at 1.55 mum. The lower wavelength loss behavior is assigned to purely Gaussian broadening, and is described by classical mixing thermodynamic quantities based on the Marcus theory of inhomogeneous broadening [R. A. Marcus, J. Chem. Phys. 43, 1261 (1965)], modeled as a convolution of dye-dye dipole broadening and dye-polymer van der Waals broadening. The Gaussian dipole interactions follow a Loring dipole-broadening description [R. F. Loring, J. Phys. Chem. 94, 513 (1990)] dominated by the excited-state dipole moment, and have a correlated homogeneous broadening contribution. The long-wavelength loss behavior has a non-Gaussian dye-dye dipole contribution which follows Kador's broadening analysis [L. Kador, J. Chem. Phys. 95, 5574 (1991)], with a net broadening described by a convolution of this term with a Gaussian van der Waals interaction given by Obata et al. [M. Obata, S. Machida, and K. Horie, J. Polym. Sci. B 37, 2173 (1999)], with each term governed by the dye spacer length. A minimum in broadening and loss-concentration slope at a spacer length of four carbons per alkyl at all wavelengths has important consequences for practical waveguide devices, and is of higher aspect ratio than the spherical limit shown by Robinson and Dalton to minimize dipole interactions under a poling field.

Vesicle adsorption and lipid bilayer formation on glass studied by atomic force microscopy

The adsorption of phosphatidylcholine (PC) vesicles (30, 50, and 100 nm nominal diameters) and of dye-labeled PC vesicles (labeled with 6% Texas Red fluorophore (TR) and encapsulated carboxy fluorescein (CF)) to glass surfaces was studied by contact mode atomic force microscopy in aqueous buffer. These studies were performed in part to unravel details of the previously observed isolated rupture of dye-labeled PC vesicles on glass (Johnson, J. M.; Ha, T.; Chu, S.; Boxer, S. G. Biophys. J. 2002, 83, 3371-3379), specifically to differentiate partial rupture, that is, pore formation and leakage of entrapped dye, from full rupture to form bilayer disks. In addition, the adhesion potential of PC vesicles on glass was calculated based upon the adhesion-driven flattening of adsorbed vesicles and a newly developed theoretical model. The vesicles were found to flatten considerably upon adsorption to glass (width-to-height ratio of approximately 5), which leads to an estimate for the adhesion potential and for the critical rupture radius of 1.5 x 10(-4) J/m2 and 250 nm, respectively. Independent of vesicle size and loading with dye molecules, the adsorption of intact vesicles was observed at all concentrations below a threshold concentration, above which the formation of smooth lipid bilayers occurred. In conjunction with previous work (Johnson, J. M.; Ha, T.; Chu, S.; Boxer, S. G. Biophys. J. 2002, 83, 3371-3379), these data show that 6% TR 20 mM CF vesicles adsorb to the surface intact but undergo partial rupture in which they exchange content with the external buffer.

In situ formation and characterization of poly(ethylene glycol)-supported lipid bilayers on gold surfaces

Inclusion of a polymer cushion between a lipid bilayer membrane and a solid surface has been suggested as a means to provide a soft, deformable layer that will allow for transmembrane protein insertion and mobility. In this study, mobile, tethered lipid bilayers were formed on a poly(ethylene glycol) (PEG) support via a two-step adsorption process. The PEG films were prepared by coadsorbing a heterofunctional, telechelic PEG lipopolymer (1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-poly(ethylene glycol)-2000-N-[3-(2-(pyridyldithio)propionate]) (DSPE-PEG-PDP) and a nonlipid functionalized PEG-PDP from an ethanol/water mixture, as described in a previous paper (Munro, J. C.; Frank, C. W. Langmuir 2004, 20, 3339-3349). Then a two-step lipid adsorption strategy was used. First, lipids were adsorbed onto the PEG support from a hexane solution. Second, vesicles were adsorbed and fused on the surface to create a bilayer in an aqueous environment. Fluorescence recovery after photobleaching experiments show that this process results in mobile bilayers with diffusion coefficients on the order of 2 microm2/s. The mobility of the bilayers is decreased slightly by increasing the density of tethered lipids. The formation of bilayers, and not multilayer structures, is also confirmed by surface plasmon resonance, which was used to determine in situ film thickness, and by fluorimetry, which was used to determine quantitatively the fluorescence intensity for each 18 by 18 mm sample. Unfortunately, fluorescence microscopy also shows that there are large defects on the samples, which limits the utility of this system.

Fluid biomembranes supported on nanoporous aerogel/xerogel substrates

Planar supported lipid bilayers have attracted immense interest for their properties as model cell membranes and for potential applications in biosensors and lab-on-a-chip devices. We report the formation of fluid planar biomembranes on hydrophilic silica aerogels and xerogels. Scanning electron microscopy results showed the presence of interconnected silica beads of approximately 10-25 nm in diameter and nanoscale open pores of comparable size for the aerogel and grain size of approximately 36-104 nm with approximately 9-24 nm diameter pores for the xerogel. When the aerogel/xerogel was prehydrated and then allowed to incubate in l-alpha-phosphatidylcholine (egg yolk PC) unilamellar vesicle (approximately 30 nm diameter) solution, lipid bilayers were formed due to the favorable interaction of vesicles with the hydroxyl-abundant silica surface. Lateral mobility of labeled lipid N-(7-nitrobenz-2-oxa-1,3-diazol-4-yl)-1,2-dihexadecanoyl-sn-glycero-3-phosphoethanolamine was retained in the membranes. A diffusion coefficient of 0.61 +/- 0.22 microm(2)/s was determined from fluorescence recovery after photobleaching analysis for membranes on aerogels, compared to 2.46 +/- 0.35 microm(2)/s on flat glass. Quartz crystal microbalance-dissipation was utilized to monitor the kinetics of the irreversible adsorption and fusion of vesicles into bilayers on xerogel thin films.

Langmuir and Langmuir-Blodgett films of amphiphilic bistable rotaxanes

A series of amphiphilic bistable [2]rotaxanes--in which a ring-shaped component, the tetracationic cyclophane, cyclobis(paraquat-p-phenylene), has been assembled around two recognition sites, a tetrathia-fulvalene (TTF) unit and a 1,5-dioxynaphthalene (DNP) ring system, situated apart at different strategic locations within the central polyether section of an amphiphilic dumbbell component that is terminated by a hydrophobic tetraarylmethane-based stopper (near the TTF unit) at one end and by a hydrophilic tetraarylmethane-based stopper (near the DNP ring system) at the other end--has been designed and synthesized. The effects of systematic changes in the constitutions of the three ethylene glycol tails (diethylene or tetraethylene glycol) and end groups (hydroxyl or methoxyl functions) attached to the hydrophilic stoppers on Langmuir film balance and surface rheology experiments at 20 degreesC were examined to determine the monolayer stabilities and co-conformations of the [2] rotaxanes and their free dumbbell counterparts. These experiments allow us to propose a model for the rotaxane's structures at different surface pressures. All the [2]rotaxanes form stable Langmuir films. These films typically pass from a liquid-expanded region to a liquid-condensed region. The transition between the two regions was either directly observed or ascertained using film stability experiments. Film balance and surface rheology experiments showed that the addition of the tetracationic cyclophane component and hydroxyl end groups markedly increased the stabilities and viscoelasticity of the films.

Adsorption of lipid-functionalized poly(ethylene glycol) to gold surfaces as a cushion for polymer-supported lipid bilayers

Inclusion of a polymer cushion between a lipid bilayer membrane and a solid surface has been suggested as a means to provide a soft, deformable layer that will allow for transmembrane protein insertion and mobility. In this study, the properties of a heterofunctional, telechelic PEG lipopolymer (1,2-distearoyl-sn-glycero-3-phosphoethanolamine-N-poly(ethylene glycol)-2000-N- [3-(2-(pyridyldithio)propionate]) (DSPE-PEG-PDP) adsorbed from ethanol and water solutions onto gold surfaces were studied using a variety of surface-sensitive techniques. X-ray photoelectron spectroscopy showed that the PEG molecules are tethered to the gold surface via thiolate bonds. When adsorbed from water, ethanol, or their mixtures, reflection-absorption infrared spectroscopy showed that amorphous PEG layers with disordered DSPE alkyl chains were formed, independent of adsorption time or solution concentration. On the basis of advancing and receding water and hexadecane contact angles on the lipopolymer films, the DSPE lipid groups appear to segregate from the PEG layer and become exposed at the surface of the polymer films. Swelling observed in surface plasmon resonance experiments and the large contact angle hysteresis observed indicate that highly swellable, mobile films capable of molecular rearrangements are formed. The self-assembling and amorphous properties of these PEG layers make them ideal candidates as polymer cushions for polymer-supported lipid bilayers. The DSPE surface concentration can be controlled, to a limited degree, by varying the adsorption time of DSPE-PEG-PDP from ethanol. A more effective strategy is to coadsorb DSPE-PEG-PDP with a non-lipid-functionalized PEG-PDP from an ethanol/water mixture, which allows the PEG thickness and density to remain constant while decreasing the density of DSPE groups.

Hindered diffusion in polymer-tethered membranes: a monolayer study at the air-water interface

Polymer-tethered phospholipid bilayers, which are based on a phospholipid-lipopolymer mixture, represent a very promising approach to stabilize complex biomimicking composite membranes. Furthermore, they are interesting model systems to study problems of hindered diffusion in two-dimensional liquids. Here, we present fluorescence recovery after photobleaching experiments (FRAP) on mixed phospholipid-lipopolymer monolayers of DMPC and DSPE-EO(45) at the air-water interface. In contrast to recent polymer-tethered bilayer experiments where the hydrophobic lipopolymer anchors behaved as immobile obstacles within the fluid phospholipid matrix,(1) this paper investigates the influence of mobile lipopolymer obstacles on the lateral diffusion of phospholipids. We found that the lateral diffusion of phospholipids with D = 7.1 +/- 0.5 microm(2)/s is independent of the lipopolymer obstacle concentration if adjacent polymer chains do not interact with each other. However, the diffusion coefficient of nontethered phospholipids gradually decreases from D = 7.1 +/- 0.5 microm(2)/s to D = 3.4 +/- 0.1 microm(2)/s in the case of increasing polymer-polymer interactions based on frictional coupling. This can be understood by a slowing down of the obstacle mobility. While phospholipids still show a significant lateral diffusion as long as the polymer moieties interact with each other only via frictional coupling, they become rather immobile (D = 0.9 +/- 0.1 microm(2)/s) if lipopolymers form a two-dimensional physical network.