Karsten Reihs

Valinomycin-mediated transport of alkali cations through solid supported membranes

Abstract Highly flexible lipid bilayers were immobilized on gold surfaces via self-assembly of thiolipids on gold in order to obtain an appropriate matrix for the carrier valinomycin. The bilayers were created by chemisorption of thiolipids to form the first hydrophobic monolayer on the gold substrate and subsequent fusion of unilamellar 1-palmitoyl-2-oleoyl- sn -glycero-3-phosphocholine vesicles resulted in the second physisorbed monolayer. The synthesis of the thiolipid is based on the functionalization of phosphatidylethanolamine with a linker consisting of succinic acid hooked on a thiolated tetraethyleneglycol serving as the hydrophilic linker responsible for the flexibility of the monolayer and the anchor group. The solid supported membranes were characterized by X-ray photoelectron spectroscopy and impedance spectroscopy. The latter technique revealed that the bilayers form a considerable barrier against ions in solution. The capacitance and resistivity of the prepared bilayers amount to C m =1.0±0.2 μ F/cm 2 and R m =11 000±1000 Ω cm 2 in 10 mM Tris, 50 mM N(CH 3 ) 4 Cl, pH 7.0. Ion transport of sodium and potassium ions through the bilayers in the absence and presence of valinomycin was investigated by impedance spectroscopy in the frequency range of 10 −1 –10 6 s −1 . Valinomycin was dissolved in dimethyl sulfoxide and added to the bilayer. Data evaluation was performed using a modified model established by de Levie for carrier-mediated ion transport through free-standing lipid bilayers making use of the continuum equation. The membrane resistance showed the expected linear relation to the reciprocal of the valinomycin concentration in solution. The conductivity of the membrane in the presence of valinomycin corrected for the conductivity in the absence of the carrier vs. the concentration of alkali ions in solution showed a tenfold larger slope for potassium than for sodium ions.

Ar plasma treated and Al metallised polycarbonate: a XPS, mass spectroscopy and SFM study

Abstract Ar plasma etched and Al metallised bisphenol A carbonate was analysed by mass spectroscopy, photoelectron spectroscopy (XPS), and scanning force microscopy (SFM). We mainly used a technical polymer (Makrolon 2808, Bayer) made by injection-moulding, as well as spin coated bisphenol A carbonate ( n =1) and polycarbonate (PC) ( n =115). The mass spectroscopy during the etching process shows the degradation of the PC in the form of carbon monoxide, carbon dioxide and methyl groups. The photoelectron spectroscopy shows in detail the surface modification after Ar plasma treatment and metallisation. The plasma induces a reduction of the carboxylic carbon (C 1s), a strong reduction of singly bonded oxygen (O 1s) and also a slight reduction of doubly bonded oxygen. After Al metallisation, a reaction of Al with the oxygen groups and an interaction with the aromatic system is documented. Ar plasma etching increases the chemical interaction of Al mainly with the aromatic carbon. The X-ray photoelectron spectroscopy of metallised PC under different initial conditions shows a strong influence of incorporated water in the PC bulk that cannot be seen by XPS on uncoated PC. The O 1s signal increases during metallisation and results in an oxidation of Al probably caused by the fact that the hydrophobic surfaces becomes hydrophillic. Temperature-dependent XPS was done on technical PC samples and on spin coated samples ( n =1, n =115) and supports the influence of the bulk state for the Al–PC interface. For n =1 carbonate, a diffusion of Al into the PC volume was observed. The SFM measurements showed a roughening effect on the nanometer scale even after short treatment times. Al can be seen as a weakly bound cluster on the virgin PC, and if a pre-etching is done, Al seems to grow as a good wetting film. The adhesion force of Al films on PC without any influence of the volume can be explained by the chemical bonding of Al to the carboxylic and aromatic systems. The adhesion can be increased by plasma pre-treatment. A breakdown of the adhesion on technical PC is probably induced by a reaction of Al with mobile intercalated gas, that is enriched near the surface after Al coating.

Aluminium deposition on SF6 plasma‐treated polycarbonate: an AFM, XPS and mass spectroscopy study

A systematic investigation was made of the chemical and morphological influences of SF 6 plasma on polycarbonate and the influence of plasma treatment on Al metallization. Mass and ion spectroscopy were used for characterization of the plasma and the etching process. X-ray photoelectron spectroscopy (XPS) measurements were applied for the chemical characterization, while atomic force microscopy (AFM) (static and dynamic mode) served to inspect the surface morphology. All analytical techniques were performed in an ultrahigh vacuum system, in order to prevent the polycarbonate sample from being exposed to ambient air after the plasma treatment. During the etching process we used mass difference spectra to demonstrate the removal of masses 19, 28 and 32 corresponding to HF, CO (N 2 ) and CF. Additionally, the inclusion of fluorine was also observed by this technique. The XPS spectra of polycarbonate surfaces show a significant inclusion of fluorine (C-F, C-F 2 ) and a reduction of the oxygen content after the plasma treatment. Aluminium metallization leads to the formation of an A<F interlayer; metallic growth of Al is only observed when the metallic layers become thicker than a few nanometres. The AFM investigations have shown that even a short plasma treatment causes changes in morphology (structures with an extension of 20-40 nm). After extended plasma exposure the surface becomes very rough, resulting in poor Al adhesion. On untreated polycarbonate, Al grows in the form of weakly bound clusters, which can only be imaged in the dynamic AFM mode. After plasma treatment, Al grows in the form of well-adhering flat layers without clustering.

Chemisorption of poly(methylhydrogensiloxane) on oxide surfaces: a quantitative investigation using static SIMS

Abstract Monolayers of poly(methylhydrogensiloxane) (PMHS) were prepared on oxide powder surfaces consisting mostly of SiO 2 . The polymer is covalently bonded via SiOSi bonds formed by reaction of SiH groups with surface SiOH groups. The large powder surface area allows the determination of the average number of surface bonds by classical chemical titration. Depending on the coverage of PMHS the average number of surface bonds ranges from 15 to 25 for a linear polymer chain with an average length of 30 repeat units. Static SIMS spectra of the samples show peak patterns similar to those obtained from PMHS prepared on silver targets. However, the fragment ion intensity distributions strongly depend on the average number of surface bonds per polymer chain. These intensity distributions are quantitatively analyzed using a simple statistical fragmentation model. The model assumes that fragments having the structure of the polymer backbone with different lengths are released from nonbonded polymer sections only. The probability of bond cleavage in the backbone is assumed to be constant. The model predicts the experimentally obtained intensity distributions very well. The average numbers of surface bonds calculated from fragment ion intensities are in very good agreement with the independent data obtained by chemical titration. Some information about the population distribution of surface bonds can also be obtained. This example shows that appropriate physical models of the secondary ion emission process in static SIMS can be used to obtain very detailed quantitative information about the molecular structure of polymer surfaces.