Conditioning of self-assembled monolayers at two static immersion test sites along the east coast of Florida and its effect on early fouling development.

Among the first events after immersion of surfaces in the ocean is surface 'conditioning'. Here, the accumulation and composition of the conditioning films formed after immersion in the ocean are analyzed. In order to account for different surface chemistries, five self-assembled monolayers that differ in resistance to microfouling and wettability were used. Water samples from two static immersion test sites along the east coast of Florida were collected at two different times of the year and used for experiments. Spectral ellipsometry revealed that conditioning films were formed within the first 24 h and contact angle goniometry showed that these films changed the wettability and rendered hydrophobic surfaces more hydrophilic and vice versa. Infrared reflection adsorption spectroscopy showed that the composition of the conditioning film depended on both the wettability and immersion site. Laboratory and field assays showed that the presence of a conditioning film did not markedly influence settlement of microorganisms.

Three dimensional tracking of exploratory behavior of barnacle cyprids using stereoscopy.

Surface exploration is a key step in the colonization of surfaces by sessile marine biofoulers. As many biofouling organisms can delay settlement until a suitable surface is encountered, colonization can comprise surface exploration and intermittent swimming. As such, the process is best followed in three dimensions. Here we present a low-cost transportable stereoscopic system consisting of two consumer camcorders. We apply this novel apparatus to behavioral analysis of barnacle larvae (≈800 µm length) during surface exploration and extract and analyze the three-dimensional patterns of movement. The resolution of the system and the accuracy of position determination are characterized. As a first practical result, three-dimensional swimming trajectories of the cypris larva of the barnacle Semibalanus balanoides are recorded in the vicinity of a glass surface and close to PEG2000-OH and C(11)NMe(3)(+)Cl(-) terminated self-assembled monolayers. Although less frequently used in biofouling experiments due to its short reproductive season, the selected model species [Marechal and Hellio (2011), Int Biodeterior Biodegrad, 65(1):92-101] has been used following a number of recent investigations on the settlement behavior on chemically different surfaces [Aldred et al. (2011), ACS Appl Mater Interfaces, 3(6):2085-2091]. Experiments were scheduled to match the availability of cyprids off the north east coast of England so that natural material could be used. In order to demonstrate the biological applicability of the system, analysis of parameters such as swimming direction, swimming velocity and swimming angle are performed.

Investigation of friction and perceived skin feel after application of suspensions of various cosmetic powders.

The perceived skin feel during and after application of skin care products is highly important to the consumer and therefore to cosmetic formulators. Powder particles are commonly incorporated in cosmetic formulations to improve their sensory properties. Although a large variety of cosmetic powders is available, it is presently uncertain how the particles' properties affect the perceived skin feel. Well-trained panellists usually assess the perceived skin feel; however, these tests are time-consuming and by nature subjective. To address this complexity, the authors have systematically investigated various suspensions of cosmetic powders with regard to the perceived skin feel after application. Furthermore, an in vitro friction measurement set-up was developed which features a tribological contact similar to the mechanical properties and the topography of the contact between finger tip and human skin. A correlation was found between the friction coefficients determined in vitro and the perceived skin feel after sample application (as assessed by a descriptive panel). The results indicate that cosmetic powder particles should be small with a rather irregular shape to better lubricate the tribological contact between finger tip and skin surface, which leads to a more 'powdery' skin feel. It is suggested to carry out further tests with different powder particles or other skin care formulations to fully understand the underlying mechanisms of skin feel improvement and to validate or even partly replace the results of panel testing.

Settlement behavior of zoospores of Ulva linza during surface selection studied by digital holographic microscopy.

Settlement of the planktonic dispersal stages of marine organisms is the crucial step for the development of marine biofouling. Four-dimensional holographic tracking reveals the mechanism by which algal spores select surfaces suitable for colonization. Quantitative analysis of the three dimensional swimming trajectories of motile spores of a macroalga (Ulva linza) in the vicinity of surfaces functionalized with different chemistries reveals that their search strategy and swimming behavior is correlated to the number of settled spores found in spore settlement bioassays conducted over 45 min. The spore motility and exploration behavior can be classified into different motion patterns, with their relative occurrence changing with the surface chemistry. Based on the detailed motility analysis we derived a model for the surface selection and settlement process of Ulva zoospores.

Cosmetic powder suspensions in compliant, fingerprintlike contacts.

Cosmetic powders are regularly employed in skin creams and cosmetic formulations to improve performance and enhance skin feel. A previous study investigated the effect of particle concentration and size on the lubricating properties of powder suspensions in smooth, compliant contacts [Timm et al., Tribol. Int. (2011)]. In this paper the tribological properties of cosmetic powder suspensions are investigated in compliant contacts having model fingerprintlike surface topography. Friction coefficients were measured for a series of powder suspensions with varying particle size and concentration in a polydimethylsiloxane (PDMS)/PDMS contact. A commercial tribometer (MTM, PCS Instruments) was employed to measure friction as a function of rubbing time (20 min), under pure sliding (50 mm/s) and low load (0.5 N) conditions. Compared to results using smooth surfaces, it was clear that surface topography has a pronounced affect on the time-dependent tribological behavior of the cosmetic powder suspensions studied. A two-stage friction coefficient versus time curve was observed. By varying the particle size and concentration it was shown that the duration and magnitude of each stage can be controlled.

Digital in-line X-ray holography with zone plates.

Single pulse imaging with radiation provided by free-electron laser sources is a promising approach towards X-ray microscopy, which is expected to provide high resolution images of biological samples unaffected by radiation damage. One fully coherent imaging technique for this purpose is digital in-line holography. Key to its successful application is the creation of X-ray point sources with high photon flux. In this study we applied zone plates to create such point sources with synchrotron radiation provided by the storage ring BESSY II. The obtained, divergent light cone is applied to holographic microscopy of biological objects such as critical point dried Navicula perminuta diatoms and human cells using photons with an energy of 250 eV. Compared to conventional experiments employing pinholes, exposure times are reduced by two orders of magnitude.

X-ray holographic microscopy with zone plates applied to biological samples in the water window using 3rd harmonic radiation from the free-electron laser FLASH.

The imaging of hydrated biological samples - especially in the energy window of 284-540 eV, where water does not obscure the signal of soft organic matter and biologically relevant elements - is of tremendous interest for life sciences. Free-electron lasers can provide highly intense and coherent pulses, which allow single pulse imaging to overcome resolution limits set by radiation damage. One current challenge is to match both the desired energy and the intensity of the light source. We present the first images of dehydrated biological material acquired with 3rd harmonic radiation from FLASH by digital in-line zone plate holography as one step towards the vision of imaging hydrated biological material with photons in the water window. We also demonstrate the first application of ultrathin molecular sheets as suitable substrates for future free-electron laser experiments with biological samples in the form of a rat fibroblast cell and marine biofouling bacteria Cobetia marina.

BioRef: a versatile time-of-flight reflectometer for soft matter applications at Helmholtz-Zentrum Berlin.

BioRef is a versatile novel time-of-flight reflectometer featuring a sample environment for in situ infrared spectroscopy at the reactor neutron source BER II of the Helmholtz Zentrum Berlin für Materialien und Energie (HZB). After two years of design and construction phase the instrument has recently undergone commissioning and is now available for specular and off-specular neutron reflectivity measurements. BioRef is especially dedicated to the investigation of soft matter systems and studies at the solid-liquid interface. Due to flexible resolution modes and variable addressable wavelength bands that allow for focusing onto a selected scattering vector range, BioRef enables a broad range of surface and interface investigations and even kinetic studies with subsecond time resolution. The instrumental settings can be tailored to the specific requirements of a wide range of applications. The performance is demonstrated by several reference measurements, and the unique option of in situ on-board infrared spectroscopy is illustrated by the example of a phase transition study in a lipid multilayer film.

Ptychographic coherent x-ray diffractive imaging in the water window.

Coherent x-ray diffractive microscopy enables full reconstruction of the complex transmission function of an isolated object to diffraction-limited resolution without relying on any optical elements between the sample and detector. In combination with ptychography, also specimens of unlimited lateral extension can be imaged. Here we report on an application of ptychographic coherent diffractive imaging (PCDI) in the soft x-ray regime, more precisely in the so-called water window of photon energies where the high scattering contrast between carbon and oxygen is well-suited to image biological samples. In particular, we have reconstructed the complex sample transmission function of a fossil diatom at a photon energy of 517 eV. In imaging a lithographically fabricated test sample a resolution on the order of 50 nm (half-period length) has been achieved. Along with this proof-of-principle for PCDI at soft x-ray wavelengths, we discuss the experimental and technical challenges which can occur especially for soft x-ray PCDI.

Protein density profile at the interface of water with oligo(ethylene glycol) self-assembled monolayers.

We determined the density profile of a high-molecular-weight globular protein (bovine serum albumin, BSA) solution at the methoxy tri(ethylene glycol)-terminated undecanethiol SAM/protein solution interface by neutron reflectivity measurements. Information about the interactions between oligo(ethylene glycol) (OEG)-terminated self-assembled monolayers (SAMs) and proteins is derived from the analysis of the structure of the solid-liquid interface. The fitting results reveal oscillations of the protein density around the bulk value with decaying amplitude on a length scale of 4 to 5 nm. The amplitude, phase, period, and decay length are found to vary only slightly with temperature and the ionic strength of the protein solution. Adsorption is reversible within the limits of detection, which suggests that the hydrated ethylene glycol surface inhibits the protein from unfolding and irreversible bonding. The insensitivity of BSA adsorption toward the ionic strength of the solution contrasts with observations in surface force experiments with a fibrinogen-coated AFM tip, where electrostatic repulsion dominates theprotein/OEG SAM interaction. As reported previously, irreversible BSA adsorption takes place below 283 K, which we interpret as indicative of the presence of dynamic effects in the protein resistance of short-chain OEG-terminated surfaces.

Analysis of holographic microscopy data to quantitatively investigate three-dimensional settlement dynamics of algal zoospores in the vicinity of surfaces.

In this article we describe the technical aspects of digital in-line holographic microscopy to track multiple macrofouling Ulva linza zoospores simultaneously during their exploration of surfaces. Using an effective method of artefact suppression at the edges of holograms in combination with projection of volume reconstructions, a fast algorithm was developed which allows a reliable determination of a large number of subsequent spore positions. Thus, statistical analysis of swimming behaviour in the vicinity of surfaces becomes possible. Using glass surfaces as example, velocity and diving direction distributions are calculated and the swimming behaviour is statistically analysed. Diving direction analysis provides a straightforward way to determine segments within traces with surface contact. The presented method of data analysis allows high throughput analysis of holographic microscopy data and sets the basis for different applications including biofouling.

Coherent-pulse 2D crystallography using a free-electron laser x-ray source.

Coherent diffractive imaging for the reconstruction of a two-dimensional (2D) finite crystal structure with a single pulse train of free-electron laser radiation at 7.97 nm wavelength is demonstrated. This measurement shows an advance on traditional coherent imaging techniques by applying it to a periodic structure. It is also significant that this approach paves the way for the imaging of the class of specimens which readily form 2D, but not three-dimensional crystals. We show that the structure is reconstructed to the detected resolution, given an adequate signal-to-noise ratio.

Isotope and temperature effects in liquid water probed by x-ray absorption and resonant x-ray emission spectroscopy.

High-resolution x-ray absorption and emission spectra of liquid water exhibit a strong isotope effect. Further, the emission spectra show a splitting of the 1b1 emission line, a weak temperature effect, and a pronounced excitation-energy dependence. They can be described as a superposition of two independent contributions. By comparing with gas phase, ice, and NaOH/NaOD, we propose that the two components are governed by the initial state hydrogen bonding configuration and ultrafast dissociation on the time scale of the O 1s core hole decay.

Anisotropy studied by polarization-modulated fourier transform infrared reflection difference microspectroscopy.

We investigated anisotropic optical behavior in solid-state materials using Fourier transform infrared reflection microspectroscopy in combination with polarization modulation. For a Ca1.8Sr0.2RuO4 crystal with an isotropic optical surface, we found the reflection difference to be very close to zero, independent of the azimuthal angle of the sample. A Ca1.4Sr0.6RuO4 crystal with an anisotropic optical surface, however, exhibited a large anisotropic optical response with a strong angular dependence following a sinusoidal behavior. Furthermore, we examined the spatial distribution of the reflection difference in Bi0.17Ca0.83MnO3+delta using infrared synchrotron radiation and could clearly distinguish microscopic anisotropic domains having different optical axes. These results demonstrate that our experimental scheme can be used as a powerful tool to spectrally and spatially resolve anisotropy of solid-state materials in the mid-infrared region.

Vacuum-ultraviolet Gabor holography with synchrotron radiation.

We present the realization of high-resolution holographic microscopy using the original Gabor geometry and imaging with radiation in the vacuum-ultraviolet (VUV) spectral region. Synchrotron VUV radiation with a wavelength of 13.8 nm was focused on a small pinhole generating a highly divergent light cone suitable for digital in-line holography. Objects of different thickness and materials have been used to test the imaging properties of holographic microscopy in the VUV wavelength range. The effective numerical aperture was limited by the illuminated area of the detector, yielding a theoretical resolution below 1 microm and an experimental one of approximately 1 microm.

Density depletion at solid-liquid interfaces: a neutron reflectivity study.

Neutron reflectivity experiments conducted on self-assembled monolayers (SAMs) against polar (water) and nonpolar (organic) liquid phases reveal further evidence for a density reduction at hydrophobic-hydrophilic interfaces. The density depletion is found at the interface between hydrophobic dodecanethiol (C12) and hexadecanethiol (C16) SAMs and water and also between hydrophilic SAMs (C12/C11OH) and nonpolar fluids. The results show that the density deficit of a fluid in the boundary layer is not unique to aqueous solid-liquid interfaces but is more general and correlated with the affinity of the liquid to the solid surface. In water the variation of pH has only minor influence, while different electrolytes taken from the Hofmeister series seem to increase the depletion. On hydrophobic SAMs an increase in density depletion with temperature was observed, in agreement with Monte Carlo simulations performed on corresponding model systems. The increase in the water density depletion layer is governed by two effects: the surface energy difference between water and the substrate and the chemical potential of the aqueous phase.

Theoretical modeling and interpretation of X-ray absorption spectra of liquid water.

We report extensive calculations to examine the capability of theory to explain the XAS spectra of liquid water. Several aspects that enter the theoretical model are addressed, such as the quantum mechanical methods, the statistics and the XAS model. As input into our quantum mechanical calculations we will use structural information on liquid water obtained from first principles and from classical molecular dynamics simulations. As XAS models, we will examine the full core hole and the half core hole approximations to transition state theory. The quantum mechanics is performed on the basis of density functional theory. We conclude from this study that recent experimental results are fully consistent with, and can be completely explained by, present day theory, in particular, the pre-edge peak is reproduced. We also find that the average bond coordination in liquid water is 3.1 and that the assertion in a recent paper that the hydrogen bond number is much less than that cannot be substantiated. Our calculations emphasize that further advances in our understanding of water can only be made by more sophisticated spectroscopy with significantly increased resolution.

Spectroscopic characterization of omega-substituted biphenylthiolates on gold and their use as substrates for "on-top" siloxane SAM formation.

Self-assembled monolayers (SAMs) of omega-substituted biphenylthiolates (omega-MBP) on gold were characterized by spectral ellipsometry, X-ray photoelectron spectroscopy (XPS), infrared reflection absorption spectroscopy (IRRAS), and vibrational sum frequency generation spectroscopy (VSFG). The vibrational studies of the SAMs were supported by an ab initio frequency analysis at HF/6-31G and BP86/6-31G levels, yielding an assignment of all relevant spectral features in the range from 3500 to 1200 cm(-1). We were able to demonstrate that hydroxy-terminated MBP (HMBP) SAMs are basically featureless in the range of the CH stretching vibrations. Accordingly, the adsorption of a SAM of octadecyltrichlorosilane (OTS) on top of this model surface could be studied. A red shift of the C-O stretching vibration from 1281 to 1264 cm(-1) was observed during the chemisorption of OTS, thus allowing for a quantification of the number of OTS molecules involved in surface binding of OTS, which was found to be about 26% on average.

Odd-even effects in photoemission from terphenyl-substituted alkanethiolate self-assembled monolayers.

Self-assembled monolayers (SAMs) formed from 4,4'-terphenyl-substituted alkanethiols C6H5(C6H4)2-(CH2)nSH (TPn, n = 1-6) on polycrystalline (111) gold and silver substrates have been characterized by synchrotron-based high-resolution X-ray photoelectron spectroscopy. The intensities, binding energy positions, and width of most photoemission lines exhibited pronounced odd-even effects, i.e., systematic and periodic variation, depending on either odd or even number of the methylene units in the aliphatic linker of the TPn molecules. The detailed analysis of these effects provides important information on the bonding and arrangement of the chemisorbed sulfur headgroups in the TPn films and balance of the structural forces in alkanethiolate SAMs.