Reto Luginbühl

Critical phenomena of water bridges in nanoasperity contacts

This article discusses capillary forces measured by scanning force microscopy (SFM), which, as recently reported, show a discontinuous behavior at a low relative humidity between 20% and 40% depending on the solid surfaces. A capillary force discontinuity is very interesting in terms of a possible phase change or restructuring transition of bulk water in the interfacial solid–liquid region. Unfortunately, we have found that SFM measurements show an inherent weakness in the determination of the origin of the forces that are obtained during pull-off measurements. This article critically discusses the origin of the adhesive interactions as a function of relative humidity with chemically modified probing surfaces. Our measurements indicate that force discontinuities in pull-off measurements are strongly affected by the inability of the liquid to form capillary necks below a critical threshold in relative humidity. In the course of this article, we will discuss roughness effects on capillary forces and provide a modified capillary force equation for asperity nanocontacts.

Glass and Structural Transitions Measured at Polymer Surfaces on the Nanoscale

This paper reviews our recent progress in determining the surface glass transition temperature, Tg, of free and substrate confined amorphous polymer films. We will introduce novel instrumental approaches and discuss surface and bulk concepts of Tg. The Tg of surfaces will be compared to the bulk, and we will discuss the effect of interfacial interactions (confinements), surface energy, disentanglement, adhesion forces, viscosity and structural changes on the glass transition. Measurements have been conducted with scanning force microscopy in two different shear modes: dynamic friction force mode and locally static shear modulation mode. The applicability of these two nano-contact modes to Tg will be discussed.

Glow discharge plasma deposited hexafluoropropylene films: surface chemistry and interfacial materials properties

Fluoropolymer films prepared by radio frequency glow discharge (RF-glow discharge) are of interest as biomaterials coatings. Previous studies focused on hexafluoroethane (C2F6, HFE) and tetrafluoroethylene (C2F4, TFE) as the monomer precursor have shown such surfaces to exhibit unique protein binding capabilities. In this study, slow, surface directed deposition of hexafluoropropylene (C3F6, HFP) films is shown to confer surface functional group presentation that promotes high protein retention. The surface chemistry is controllable over a range of values, and C3F6 films prepared by RF-glow discharge are smooth, homogeneous, and defect free. Surface patterns of the fluoropolymer can be created by direct or photolithographic techniques. Scanning probe microscopic analysis indicates a surface modulus in the range of 1:2 , EHFP , 5:5 GPa. These results are discussed in terms of the applicability of C3F6 films for coating biomaterials. q 1999 Published by Elsevier Science S.A. All rights reserved.

Self-assembled monolayers for polymer and protein cationization with time-of-flight secondary ion mass spectrometry

Detection of cationized polymers and proteins with time-of-flight secondary ion mass spectrometry has been achieved using novel substrates consisting of carboxylic acid terminated self-assembled monolayers (SAMs) with silver ion substituted headgroups. However, other metal ions (Na, Cu, Tl, or Li) as SAM headgroups can be used as well. Polymers were deposited on the metal ion derivatized SAMs by spin coating while proteins were adsorbed onto the substrates. Control experiments carried out on nonsubstituted and methyl-terminated SAMs suggest that only metal ions in close proximity to the polymer can cationize the molecules and their fragments. Both cationized fragments and whole molecular species were observed for 1 kD polyethyleneoxide and low molecular weight proteins (<2 kD).

Quantitative interrogation of micropatterned biomolecules by surface force microscopy

Synthetic biomaterials are widely used in medical implants with success in improving and extending quality of life. However, these materials were not originally designed to interact with cells through specific signaling pathways. As a result, the interaction with the body is mediated through passive adsorption of a disorganized protein monolayer. Next generation biomaterials have been proposed to be active in modifying the biological response of the host through the incorporation of specific biorecognition moieties. An important tool in the development of these novel active biomaterials is the scanning force microscope (SFM). The SFM allows for interrogation of bioactive biomaterials in mapping or spectroscopic modes. In this work, micropatterned protein surfaces were prepared using biomolecules implicated in wound healing. The surfaces were imaged via SFM and the specific binding forces between surface associated biomolecules and antibody functionalized tips were quantified.

Comprehensive surface analysis of hydrophobically functionalized SFM tips

Tip-sample interactions have been of interest since the early development of the scanning force microscope. Investigations of interfacial interactions at the molecular level are of importance for fundamental studies of bi-molecular interactions and for possible applications in biomedical research and industrial settings. By engineering the surface chemical properties of the SFM probes, specific force interactions may be measured. However, as these modification schemes become more widely applied, detailed chemical analysis of the modified cantilever surfaces becomes crucial. In this paper, we describe two approaches to coat SFM cantilevers with hydrophobic coatings: a silanization protocol and ratio frequency plasma enhanced chemical vapor deposition.