Biomedical subjects

J Mieke Kleijn

Publications and source records attributed to J Mieke Kleijn.

3 recordsLinked to original sources

PubMed2004

Adsorption of charged macromolecules at a gold electrode.

Using an optical reflectometer with impinging-jet system, the adsorption from aqueous solution onto gold of three charged macromolecules has been studied: the strong linear-chain polyelectrolyte polyvinyl pyridine (PVP(+)), the fifth-generation poly(propylene imine) dendrimer DAB-64, which has a pH-dependent charge and a relatively fixed shape, and the protein lysozyme, of which both the charge and the structure-stability are dependent on solution composition. Experimental conditions that have been varied include the adsorbate concentration, electrolyte concentration, pH, and externally applied potential across the gold/solution interface. Making use of the earlier established dependency of the double layer potential of the gold substrate on solution conditions and externally applied potential, the results of measurements as a function of pH and as a function of external potential control are compared. The total set of results enables us to draw conclusions with respect to the relative importance of electrostatic interactions for the adsorption process. PVP(+) adsorption follows the electric potential of the gold/solution interface and is further determined by a rather strong nonelectrostatic affinity between segments and surface. The adsorption behavior of DAB-64 is not quite understood, but electrostatic interactions with the gold surface seem to play a minor role. For lysozyme, surface-induced conformational changes dominate the adsorption process. The extent of spreading of the molecules decreases with increasing polarity of the surface, resulting in a minimum in adsorbed amount around the point of zero potential of the gold.

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PubMed2002

Interactions between acid- and base-functionalized surfaces.

In this paper we present an AFM force study on interactions between chemically modified surfaces. Surfaces with terminal groups of either NH(2) or COOH were obtained by chemisorption of a silane-based compound (3-amino-propyltriethoxysilane) on silica or a thiol compound (11-mercapto undecanoic acid) on gold. The surfaces were characterized by streaming potential and contact angle measurements. For the NH(2) surfaces the density of functional groups strongly depends on the pretreatment of the silica. Both the NH(2) and COOH surfaces show a steady and ongoing loss of functional groups, but on the time scale of our force measurements they can be considered to be stable. Interaction curves for the various combinations of surfaces in aqueous solutions show a strong correlation with the ionization state of the surface groups. The approach curves can be explained on the basis of electrostatic interactions. On retraction a pH-dependent adhesion is found, the strongest being between NH(2) and COOH surfaces as a result of acid-base interactions. Between NH(2) layers and between COOH layers there is also adhesion, due to the formation of H bonds. The corresponding work of adhesion W was calculated using the DMT (Derjaguin, Muller, Toporov) equation. The values of W obtained are small compared to literature data and suggest that only a fraction of the functional groups in the contact area is involved in acid-base or H-bond interactions. This is attributed mainly to the roughness of the surfaces, which substantially reduces adhesion. A rough estimate is made for the rupture force of the NH(2)-COOH acid-base bond in water.

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PubMed2002

The electrical double layer on gold probed by electrokinetic and surface force measurements.

Gold surfaces, obtained by vacuum deposition of 15-nm gold films on glass and silica wafers, were studied in aqueous solutions by streaming potential measurements and colloidal-probe AFM force measurements. In the force measurements both a bare and a gold-coated silica particle (6 microm in diameter) have been used as colloidal probes. From the streaming potential measurements we determined the zeta-potential of the gold surface, while from the force measurements the diffuse double-layer potential psi(d) was obtained by fitting the data to the DLVO theory or to the nonlinear Poisson-Boltzmann equation. Measured interactions were found to be entirely due to overlap of electric double layers with no indication of attractive Van der Waals forces. Results of both types of measurements are in good agreement. The double layer potential strongly depends on the pH, probably as a result of the presence of oxide species on the gold surface. Insight in the double layer potential of polarizable interfaces such as the gold/electrolyte solution interface is the first step for understanding the effect of externally applied potentials on the adsorption behavior of charged species.

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