Gert Frens

How to develop globular proteins into adhesives.

To make globular proteins suitable for application in adhesives, the specific bonds and interactions which shape their structure have to broken. Only then, a layer of relatively large, flexible and interwoven polymer chains, which are firmly attached to the solid surface by adsorption, can be created. Such a network layer is essential to save the adhesive bond under an applied force, because it can distribute the concentration of stresses generated at the interface into the bulk. Unfolding and swelling of a protein can be achieved by changing the solvent quality. For the globular whey protein beta-lactoglobulin, the optimal conditions for unfolding and swelling is found with 98% formic acid as a solvent. In formic acid, beta-lactoglobulin looses its amphoteric character (it is protonated, probably for approximately 20%). In addition, formic acid is less polar than water and thus a better solvent for the apolar parts of the protein. The swelling and unfolding behaviour of beta-lactoglobulin is studied by viscosity and CD-spectroscopy measurements. For the interpretation of the results we apply the Kuhn formalism that the conformation of a protein can be described in terms of a statistical chain which consists of segments of an average persistence length P. The statistical segment length P, which varies with the experimental conditions, is directly related to the adsorption energy required for a strong adhesion between coil and surface. It determines the depletion energy kT P(-2) m(-2) which must be overcome by specific attraction between side groups of the protein chain and the surface. For beta-lactoglobulin in 98% formic acid, we find a P value of approximately 2.2 nm, pointing at a relatively flexible chain. The minimum net adsorption energy kT P(-2) is then approximately 1 mJ m(-2), a relatively small value to be exceeded. Preliminary results of destructive adhesion tests on beech wood lap-shear joints reveal promising tensile strengths of approximately 2.9+/-1.1 N mm(-2), indeed.

Electroless Silver Deposition in 100 nm Silicon Structures

A new and simple method is described to plate silicon structures with metallic silver for ultralarge-scale integration in dimensions down to 100 nm at an aspect ratio of 4.25. The silver deposition is initiated by an exchange reaction of silicon with silver ions, and the subsequent layer growth of the activated wafers occurs by electroless plating from supersaturated aqueous silver salt solutions at pH ∼ 11. No extra reducing agents are needed since silver ions are reduced at the catalytic silver surface by hydroxyl ions. The spontaneous ion-metal transition only proceeds at pH ∼ 11 and is likely mediated by the formation of subnanometer-sized [Ag 4 (OH) 2 ] 2+ clusters. The silver plating proceeds more easily in smaller structures and yields void-free, crystallized deposits.

Surface properties of plastic materials in relation to their adhering performance

Adhesion between polymeric phases like adhesives and plastic surfaces is critical in many technological and industrial applications. In the last decades much progress has been made to understand the impact of the surface properties of both phases on the adhesive strength between them. These surface properties influence processes like wetting, molecular adsorption and inter-diffusion which determine how an interface develops into an interphase after the two materials have been brought into contact. Ultimately, the properties of this interphase determine the overall adhesion strength of an assembly. In this paper important parameters in the adhesion process will be reviewed, including methods to engineer these parameters in order to attain adhesion strengths ranging from complete release to irreversible bonding.

The degree of aggregation in solution controls the adsorbed amount of mussel adhesive proteins on a hydrophilic surface

Abstract The adsorption behaviour of the mussel adhesive protein Mefp-1 on a hydrophilic surface was studied by Surface Plasmon Resonance (SPR) at pH values of 4.5 and 6.5 under aerobic conditions and at an ion strength of 0.1 M NaCl. In this environment Mefp-1 molecules aggregate by crosslinking, likely via Dopa–Dopa quinone charge transfer interactions. The initial rate of aggregation increases with increasing pH, as could be derived from Photon Correlation Spectroscopy measurements. The degree of aggregation determines the adsorption plateau value of Mefp-1. Step-like adsorption curves have been found at pH 6.5, as well as at pH 4.5, which can be interpreted as the adsorption of an ad-layer of Mefp-1 aggregates onto the initially adsorbed Mefp-1 layer on the surface. The rate of formation of this second layer increases with increasing pH and Mefp-1 concentration. The affinity of the ad-layer for the first adsorbed layer appears to be much smaller than the affinity of the first layer for the surface (Poly Vinyl Alcohol). Probably, also the ad-layer formation proceeds by the establishment of specific crosslinks with the first layer of adsorbed Mefp-1.

Electrocatalytic oxidation of formaldehyde on gold studied by differential electrochemical mass spectrometry and voltammetry

The electrocatalytic oxidation of normal formaldehyde I CH 2 O) and deuterated formaldehyde (CD 2 O) has been studied on gold in aqueous, alkaline solution as a function of pH, concentration, potential, and temperature by voltammetry, chronoamperometry, and differential electrochemical mass spectrometry. The H 2 . D 2 . and CO 2 gas evolution kinetics depend to great extent on the pH, potential, and temperature but play a minor role in the overall rate of the electro-oxidation reaction. The evolution of hydrogen at the open-circuit potential and the current efficiencies larger than 100% pointed toward the occurrence of a nonelectrochemical dehydrogenation reaction parallel to the electro-oxidation reaction, The kinetic isotope effects and activation energies suggested that the overall rate of the electro-oxidation reaction is determined by the hydroxyl catalyzed, enthalpy-driven, chemisorption of the enolate anion at low potentials, by the entropy-driven desorption of the formate anion at higher potentials, and by diffusion at the highest potentials. The apparent activation energies (E a ) ranged in value between -25 and 60 kJ mol 1 confirming the highly catalytic properties of gold in the overall rate of the reaction.

Active thin sections to study syneresis

Abstract Thin sections are made from hardened products. To achieve information on the very early hydration process of cementitious composites, one would need an “active thin section,” i.e., a thin section of a cement paste that is hardening while under investigation. In our research, it has been possible to construct such active thin sections. Using ordinary light microscopy it is possible to observe water movement and volume changes on hydrating specimens of about 100-μm thickness. This research is part of a larger project to investigate the formation of the interfacial transition zone. We believe that a process called syneresis might play an important role. Syneresis is the contraction of a gel under the expulsion of a liquid. This is addressed in the second part of this paper. This phenomenon is studied using active thin sections.