A.M. de Jong

Monomer diffusion assisted preparation of polymer gratings: A nuclear microprobe study

Abstract Polymers with an ordered molecular structure can be applied in optical systems for e.g. data transport, data storage and displays. Patterned UV photo-polymerization is used to prepare polymer gratings from a mixture of two acrylate monomers. A 3 MeV proton microprobe is used to study these gratings, prepared from two different monomers, each containing a different easily detectable label element, e.g. Cl, Si or F. During the preparation process, the difference in reactivity and mobility of these two monomers in combination with polymer–monomer interaction results in diffusion of monomers. Since this diffusion process takes place on length scales of micrometers, a scanning ion microprobe is a powerful tool for the quantitative analysis of the polymer films, obtained after complete polymerization. The microprobe is equipped with PIXE, PIGE and RBS, to quantify both the label elements and C and O. This makes it possible to determine the concentration of monomer units as a function of position and thus to study the diffusion process. Two combinations of different monomers are studied. In the case of a 0.5:1 mixture of a monofunctional and a difunctional monomer, both monomers migrate to the illuminated areas and large thickness variations are observed. When a 1:1 mixture of two difunctional monomers is used, opposite migration of the two monomers is observed, while the film shows no variation in thickness.

Torsion Stiffness of a Protein Pair Determined by Magnetic Particles

We demonstrate the ability to measure torsion stiffness of a protein complex by applying a controlled torque on a magnetic particle. As a model system we use protein G bound to an IgG antibody. The protein pair is held between a magnetic particle and a polystyrene substrate. The angular orientation of the magnetic particle shows an oscillating behavior upon application of a rotating magnetic field. The amplitude of the oscillation increases with a decreasing surface coverage of antibodies on the substrate and with an increasing magnitude of the applied field. For decreasing antibody coverage, the torsion spring constant converges to a minimum value of 1.5 × 10(3) pN·nm/rad that corresponds to a torsion modulus of 4.5 × 10(4) pN·nm(2). This torsion stiffness is an upper limit for the molecular bond between the particle and the surface that is tentatively assigned to a single protein G-IgG protein pair. This assignment is supported by interpreting the measured stiffness with a simple mechanical model that predicts a two orders of magnitude larger stiffness for the protein G-IgG complex than values found for micrometer length dsDNA. This we understand from the structural properties of the molecules, i.e., DNA is a long and flexible chain-like molecule, whereas the antibody-antigen couple is orders of magnitude smaller and more globular in shape due to the folding of the molecules.

Working surface science model of CoMoS hydrodesulfurization catalysts

Model catalysts, consisting of a conducting substrate with a thin SiO2 or Al2O3 layer on top of which the active catalytic phase is deposited, were applied to study the sulfidation of Co–Mo catalysts and to test their catalytic behavior in the hydrodesulfurization of thiophene. CoMoS, the highly active cobalt promoted MoS2 in which Co is thought to decorate the edges of MoS2 slabs, can be synthesized by sulfiding nitrilotriacetic acid complexes of cobalt and molybdenum. These complexes are deposited on SiO2/Si(100) and Al2O3/Si(100) model supports by spin coating. X-ray photoelectron spectroscopy measurements on these Co–Mo catalysts provide detailed insight into the mechanism of sulfidation. It appears that Mo is sulfided first and then the Co; this is imperative to form the CoMoS phase. Thiophene hydrodesulfurization studies of CoMoS model catalysts yield activities and product distributions consistent with those obtained from their high surface area counterparts, proving that these models are realistic...

PIXE monitoring of diffusion during photo-polymerization

Patterned UV photo-polymerization is used to prepare polymer gratings from a mixture of mono- and di-functional acrylate monomers. The difference in reactivity and mobility of these two monomers induces a concentration gradient during the photo-polymerization process. Uniform illumination afterwards fixes the grating. Monomers with different easily detectable elements, e.g., Cl, Br, Si, F, enable the observation of lateral variations of concentration in these gratings with PIXE and PIGE using a scanning ion probe of 3 MeV protons. In addition, backscattering spectrometry is applied to correct for lateral thickness variations.

Mesoscopic Concentration Variations Analyzed by Secondary Ion Mass Spectrometry

ABSTRACT Secondary ion mass spectrometry (SIMS) was used to identify concentration gradients of polyacrylate samples. Applying discriminant function analysis (DFA) on the SIMS spectra resulted in visualized separations between different sample concentrations. We identified and separated concentrations of polyacrylate blends; from the separation we can construct calibration curves. The visual identification was followed in depth of a poly(fluoro-acrylate) film covered by poly(iso-bornylmethacrylate), which served as model sample of a photo-induced phase separated device. The combination of SIMS and DFA offers new possibilities to study the phase separation processes. Furthermore, it can predict how the phase separation creates mesoscopic layered electro-optical devices.

Preparation, Structure and Surface Chemical Properties of Hydrotreating Model Catalysts: A Surface Science Approach

The preparation of an active hydrodesulphurization catalyst involves the impregnation of the oxidic support with molybdenum and cobalt compounds, followed by drying and calcination to obtain well-dispersed oxides of these elements [1,2]. The preparation finishes with the conversion of the oxides into the catalytically active phase by sulphidation. Being an essential part of the preparation, it is important to know the mechanism of the sulphidation process and to identify the elementary reaction steps that constitute the conversion from oxides to sulphides, in dependence of how the HDS catalyst is prepared. The sulphidation of molybdenum and cobalt-promoted molybdenum catalysts forms the main subject of this paper.

Molecular interference in antibody– antigen interaction studied with magnetic force immunoassay

Molecular interferences are an important challenge in biotechnologies based on antibody-antigen interactions, such as sandwich immunoassays. We report how a sandwich immunoassay with magnetic particles as label can be used to probe interference by surfactants. Surfactants are often used to improve the performance of immunoassays, however the surfactants can affect the involved proteins and the mechanism of action of surfactant molecules on the antibody-antigen system is mostly unknown. As an example, we investigated molecular interference by a nonionic surfactant (Pluronic F-127) in a cardiac troponin (cTn) sandwich immunoassay with two monoclonal antibodies. The influence of the surfactant below the critical micelle concentration (0.00-0.04%) on dissociation properties was quantified in a magnetic tweezers setup, where a force is applied to the molecules via magnetic particle labels. The force-dependent dissociation curves revealed the existence of two distinct cTn-dependent bond types, namely a weak bond attributable to non-specific binding of cTn, and a strong bond attributable to the specific binding of cTn. The dissociation rate constant of the strong bonds increased with the surfactant concentration by about a factor of two. Circular dichroism spectroscopy data showed that the nonionic surfactant influences the conformation of cTn while not noticeably affecting the two monoclonal antibodies. This suggests that the surfactant-induced increase of the dissociation rate of the specific sandwich-type cTn binding may be related to a conformational change of the antigen molecule. The described methodology is an effective tool to study the influence of surfactants and other interferences on assays based on protein interactions.