Supramolecular control of two-dimensional phase behavior

We have used directed two-component self-assembly to "pattern" organic monolayers on the nanometer scale at the liquid/solid interface. The ability of the scanning tunneling microscope to investigate structural details in these adlayers was used to gain insight into the two-component two-dimensional phase behavior. The components are symmetrically alkylated bisurea derivatives (R1-urea-spacer-urea-R2; R1, R2=alkyl, spacer=alkyl or bisthiophene). The bisthiophene unit acts as a marker and its bisurea derivative (T2) is a component in all the mixtures investigated. By varying the position of the hydrogen-bond forming urea groups along the molecule and the length of the alkyl chains of the other components, the effect of 1) hydrogen bonding, 2) molecule length, 3) odd-even effects, and 4) shape complementarity on the two-dimensional phase behavior was investigated. Insight into the effect of these parameters leads to the control of the two-dimensional patterning: from randomly intermixed systems to phase separation.     

Calibration transfer for excitation-emission fluorescence measurements

The main part of the wide array of different calibration transfer methods found in literature is dedicated to two-way data arrangements (m×n matrices). Less work has been done within the area of calibration transfer for three-way data structures (m×n×l tensors) such as calibrations made for excitation-emission-matrix (EEM) fluorescence spectra. There are two possible ways to attack the problem for EEM transfer. Either the tensors are unfolded to two-way data, whereby the existing methods can be applied, or new methods dedicated to three-way calibration transfer have to be developed. This paper presents and compares both. It was possible to make a local linear pixel-based model that could be used for transfer of EEM's. This new method has a similar performance to the classical methods found in literature, direct- and piecewise direct standardization. The three-way advantages made it possible to use as few as four samples to build useable transfer models. Care has to be taken though when choosing the samples. When subset recalibration of the systems is compared to calibration transfer, better performance is seen for the transferred calibrations. Overall the three-way calibration transfer methods have a slightly better performance than the two-way methods.     

Reversed-phase high-performance liquid chromatography prefractionation prior to two-dimensional difference gel electrophoresis and mass spectrometry identifies new differentially expressed proteins between striate cortex of kitten and adult cat

Two-dimensional difference gel electrophoresis (2-D DIGE), in combination with mass spectrometry, is a highly effective method for the rapid and reproducible detection of differentially expressed proteins. This approach, however, has the unfortunate drawback that it preferentially displays rather abundantly expressed proteins. Nevertheless, comparison of the protein expression levels of the striate cortex of adult cats and 30-day-old kittens, resulted in the identification of several proteins related to postnatal brain development and possibly age-dependent plasticity as well (Van den Bergh et al., J. Neurochem. 2003, in press). The goal of the present study was the selective enrichment and identification of less abundant proteins within the same paradigm. Hereto, we performed a reversed-phase chromatography prefractionation of our tissue lysate to separate the proteins in four fractions based on their hydrophobicity prior to 2-D DIGE analysis. This approach not only confirmed the differential expression levels of a number of proteins from the previous study, but also identified three additional proteins preferentially expressed in kitten visual cortex and five additional proteins with higher expression levels in adult cat visual cortex. These spots were not visible on the total tissue lysate protein maps, indicating that the high-performance liquid chromatography (HPLC) prefractionation enabled us to visualize additional, less abundant proteins.     

Self-assembly at the liquid/solid interface: STM reveals

The liquid/solid interface provides an ideal environment to investigate self-assembly phenomena, and scanning tunneling microscopy (STM) is the preferred methodology to probe the structure and the properties of physisorbed monolayers on the nanoscale. Physisorbed monolayers are of relevance in areas such as lubrication, patterning of surfaces on the nanoscale, and thin film based organic electronic devices, to name a few. It's important to gain insight in the factors which control the ordering of molecules at the liquid/solid interface in view of the targeted properties. STM provides detailed insight into the importance of molecule-substrate (epitaxy) and molecule-molecule interactions (hydrogen bonding, metal complexation, and fluorophobic/fluorophilic interactions) to direct the ordering of both achiral and chiral molecules on the atomically flat surface. By controlling the location and orientation of functional groups, chemical reactions can be induced at the liquid/solid interface, via external stimuli, such as light, or by controlled manipulation with the STM tip. The electronic properties of the self-assembled physisorbed molecules can be probed by taking advantage of the operation principle of STM, revealing spatially resolved intramolecular differences within these physisorbed molecules.     

Cavity enhanced absorption spectroscopy for N_2O detection at 2.86 μm using a continuous tunable color center laser

The cavity enhanced absorption technique is applied to N2O detection around 2.86 μm using a continuous-wave color center laser. A high-finesse triangular ring cavity is used in this technology. Transmission through the cavity is obtained by jittering the cavity-length with a piezo on one of the cavity mirrors. A minimum detectable absorption coefficient of 2 × 10-6 cm-1 is achieved with a mirror reflectivity of 99.24%, corresponding to a N2O detection limit of 600 parts per billion.     

Photophysics and Fluorimetric Titrations of Fluorescent Ion Indicators

A test based on time-resolved fluorescence experiments (Anal. Biochem. 245, 28–37, 1997) allows one to assess the interference of the excited-state association with the fluorimetric determination of the ground-state dissociation constant K _d of fluorescent ion:indicator complexes. If an inflection point occurs in the plot of the fluorescence signal vs – log[ion] in the ion concentration range where both decay times are invariant, this inflection point can be associated with the correct K _d. Here we apply this test to the fluorescent ion indicators SBFO (for Na⁺), Mag-fura-2 (for Mg²⁺), and APTRA-BTC (for Ca²⁺). In all three cases the decay times are invariant in the concentration ranges of the respective ions where the fluorescence titrations show unique inflection points, indicating that the fluorimetrically determined K _d values are the true K _d values.     

Interfacially induced additional damping peaks in dynamic mechanical spectra – micromechanical transitions in multipolymeric materials

The viscoelastic properties of several hypothetical multiphase polymeric materials were investigated in relation to their phase-property dependencies and microstructures. Theoretical mechanical considerations based on the self-consistent interlayer model were performed to point out that specific geometrical arrangements into phases of a set of properties of the pure constituents can lead to interfacially induced damping peaks in dynamic mechanical spectra (DMS). Such additional contributions in DMS were referred to micromechanical transitions to distinguish them from ordinary molecular transitions.