Thomas Geue

The Swiss spallation neutron source SINQ at Paul Scherrer Institute (PSI) has been in operation since 1996 using an optical guide system to transport low-energy neutrons from the source to the neutron beam lines. Clear signs of degradation in the guide system motivated us to redesign all neutron guides, which were then replaced in 2019–2020. All instruments received new tailored guides and SINQ operation was resumed in July 2020. Neutron flux measurements before and after the upgrade show that the expected instrument-dependent flux gain of a factor 2–10 was achieved. Furthermore, the fast-neutron background in the guide hall could be suppressed with an improved shielding in the guide bunker. The upgrade was completed with a slight delay due to the corona pandemic and in full compliance with the financial budget.

The formation of viscoelastic networks at fluid interfaces by globular proteins is essential in many industries, scientific disciplines, and biological processes.

The formation of electrostatic protein–polysaccharide multilayers has attracted attention for the design of fluid interfaces with enhanced stability and functionality. However, current techniques are often limited to measuring final multilayer properties. We present an interfacial shear rheology setup with simultaneous subphase exchange, allowing the transient measurement of biopolymer multilayers by their viscoelasticity. The successive and simultaneous adsorption of β-lactoglobulin (β-lg) and low-methoxyl pectin were investigated at the n-dodecane/water interface at pH 4. The successive injection of pectin increased the viscoelasticity of an adsorbed β-lg layer by electrostatic complexation. On the other hand, simultaneous adsorption impeded adsorption kinetics and interfacial layer strength due to complexation in the bulk phase prior to adsorption. Neutron reflectometry at the air–water interface confirmed the formation of an initial β-lg layer and electrostatic complexation of a secondary pectin layer, which desorbed upon pH-induced charge inversion. The layer formed by simultaneous adsorption mainly consisted of β-lg. We conclude that protein–polysaccharide complexes show limited surface activity and result in a lower effective protein concentration available for adsorption.

Cellulose nanocrystals (CNC) are an emerging natural material with the ability to stabilize fluid/fluid interfaces. Native CNC is hydrophilic and does not change the interfacial tension of the stabilized emulsion or foam system. In this study, rod-like cellulose particles were isolated from hemp and chemically modified to alter their hydrophobicity, i.e. their surface-activity, which was demonstrated by surface tension measurements of the particles at the air/water interface. The build-up and mechanical strength of the interfacial structure was investigated using interfacial shear and dilatational rheometry. In contrast to most particle or protein-based interfacial adsorption layers, we observe in shear flow a Maxwellian behavior instead of a glass-like frequency response. The slow and reversible build-up of the layer and its unique frequency dependence indicate a weakly aggregated system, which depends on the hydrophobicity and, thus, on the contact angle of the CNC particles at the air/water interface. Exposed to dilatational flow the weakly aggregated particles cluster and form compact structures. The interfacial structure generated by the different flow fields is characterized by the contact angle, immersion depth, and layer roughness obtained by neutron reflectometry with contrast variation while the size and local structural arrangement of the CNC particles was investigated by AFM imaging.

The binding of bovine serum albumin (BSA) to a 316L stainless steel surface from a buffer solution has been characterised using neutron reflectometry (NR) and quartz crystal microbalance (QCM) measurements; coverage at all concentrations up to a near-physiological concentration was found to be relatively low (< 20 %); the protein followed a two-step isotherm adsorption model type and the overall thickness at the higher concentrations (around 80 Å) suggested possible multilayering and/or protein unfolding. As it has been postulated that BSA may inhibit the further adsorption of another blood plasma protein—fibrinogen—the effects of pre-adsorbing BSA on fibrinogen adsorption were examined, firstly by prior physisorption of BSA to the stainless steel surface and secondly by pre-treating the stainless steel with a layer of SDS to render it more hydrophobic. Whilst the pre-adsorption of BSA to an untreated stainless steel surface did slightly decrease the amount of fibrinogen adsorbed initially, it had no inhibiting effect if a solution containing solely fibrinogen was subsequently flowed through. In contrast, the SDS-treated surface yielded both an increased BSA adsorption and consistently decreased fibrinogen adsorption.

Delayed fat digestion might help to fight obesity. Fat digestion begins in the stomach by adsorption of gastric lipases to oil/water interfaces. In this study we show how biopolymer covered interfaces can act as a physical barrier for recombinant dog gastric lipase (rDGL) adsorption and thus gastric lipolysis. We used β-lactoglobulin (β-lg) and thermosensitive methylated nanocrystalline cellulose (metNCC) as model biopolymers to investigate the role of interfacial fluid dynamics and morphology for interfacial displacement processes by rDGL and polysorbate 20 (P20) under gastric conditions. Moreover, the influence of the combination of the flexible β-lg and the elastic metNCC was studied. The interfaces were investigated combining interfacial techniques, such as pendant drop, interfacial shear and dilatational rheology, and neutron reflectometry. Displacement of biopolymer layers depended mainly on the fluid dynamics and thickness of the layers, both of which were drastically increased by the thermal induced gelation of metNCC at body temperature. Soft, thin β-lg interfaces were almost fully displaced from the interface, whereas the composite β-lg–metNCC layer thermogelled to a thick interfacial layer incorporating β-lg as filler material and therefore resisted higher shear forces than a pure metNCC layer. Hence, with metNCC alone lipolysis by rDGL was inhibited, whereas the layer performance could be increased by the combination with β-lg.

In this paper we study the effect of a highly hemolytic saponin, digitonin, on model lipids: cholesterol and dipalmitoylphosphatydylcholine (DPPC) using a combination of tensiometric (surface pressure and dilatational surface elasticity), spectroscopic (infrared reflection absorption spectroscopy, IRRAS), microscopic (fluorescence microscopy) and scattering techniques (neutron reflectivity, NR and grazing incidence X-ray diffraction, GIXD). The monolayers of individual lipids and their 10:9 (mol/mol) mixture were exposed to the aqueous solution of digitonin (10-4 M) by subphase exchange using a setup developed recently in our laboratory. The results confirm that digitonin can adsorb onto both bare and lipid-covered water-air interfaces. In the case of DPPC a relatively weak interaction can be observed, but the presence of cholesterol drastically enhances the effect of digitonin. The latter is shown to dissociate the weak cholesterol-DPPC complexes and bind cholesterol in an additional layer attached to the original lipid monolayer.