We report a novel hybrid charge sensor realized by the deposition of phospholipid monolayers on highly doped n-GaN electrodes. To detect the binding of recombinant proteins with histidine-tags, lipid vesicles containing chelator lipids were deposited on GaN electrodes pre-coated with octadecyltrimethoxysilane monolayers. Owing to its optical transparency, GaN allows the confirmation of the fluidity of supported membranes by fluorescence recovery after photo-bleaching (FRAP). The electrolyte-(organic) insulator-semiconductor (EIS) setup enables one to transduce variations in the surface charge density ΔQ into a change in the interface capacitance ΔC _p and, thus, the flat-band potential ΔU _FB. The obtained results demonstrate that the membrane-based charge sensor can reach a high sensitivity to detect reversible changes in the surface charge density on the membranes by the formation of chelator complexes, docking of eGFP with histidine tags, and cancellation by EDTA. The achievable resolution of ΔQ ≥ 0.1 μC/cm² is better than that obtained for membrane-functionalized p-GaAs, 0.9 μC/cm², and for ITO coated with a polymer supported lipid monolayer, 2.2 μC/cm². Moreover, we examined the potential application of optically active InGaN/GaN quantum dot structures, for the detection of changes in the surface potential from the photoluminescence signals measured at room temperature.

In biological cells, various transmembrane enzymes function as highly effective chemical reactors confined in space with characteristic length scales of tens of nanometers to micrometer. However, it is still challenging to quantitatively confine membranes in compact reactor platforms without losing their biochemical functions. Here, a simple and straightforward strategy towards the fabrication of a new flow-through reactor by the functional coating of porous silica microparticles with sarcoplasmic reticulum membranes is described. After a short incubation, the membranes achieve the homogeneous, full coverage of the particle surface, spanning across pores with the diameter of about 100 nm. By using the underlying pores as cavity reservoirs, transmembrane enzyme (Ca²⁺-ATPase) in the membrane retains their capability of ATP hydrolysis. This enables us to confine 1.1 m² of native membranes containing a large amount of Ca²⁺-ATPase (approx. 10 nmol) in a column-packaged, flow-through reactor with merely 1.8 mL volume, which cannot be achieved by the reconstitution of proteins in artificial lipid membranes or condensation of membranes in suspensions. The distinct functional levels corresponding to different reaction buffers can be reproduced even after many buffer exchanges over 14 days, confirming the stability and reproducibility of the membrane-particle hybrid reactors.

Nature defines the spatial boundaries between different phases using membranes, and the interfacial interactions are mediated by soft biopolymer interlayers that contain various carbohydrates. This Review provides a comprehensive overview on the interplay of generic and specific interactions at cell–cell and cell–tissue interfaces. A focus will be put on the combination of defined model systems, experimental techniques in real- and reciprocal space, and theoretical simulations.

Hydrated polymer interlayers between planar lipid membranes and solid substrates provide a water reservoir and thus maintain a finite membrane–substrate distance. Linear polymer spacers attached to lipid head groups (lipopolymer tethers) can be used as a defined model of oligo- and polysaccharides covalently anchored on cell surfaces (glycocalyx). They can offer a unique advantage over membranes physisorbed on polymer films (called polymer-cushioned membranes), owing to their ability to control both the length and density of polymer chains. In this study, a lipopolymer tether composed of a stable ether lipid moiety and a hydrophilic poly(2-methyl-2-oxazoline) spacer with a length of 60 monomer units is used to fabricate supported membranes by the successive deposition of proximal (lower) and distal (upper) leaflets. Using specular X-ray reflectivity and ellipsometry, we systematically investigate how the lateral density of polymer chains influences the membrane–substrate interactions. The combination of two types of reflectivity techniques under various conditions enables the calculation of quantitative force–distance relationships. Such artificial membrane systems can be considered as a half-model of cell–cell contacts mediated via the glycocalyx, which reveals the influence of polymer chain density on the interplay of interfacial forces at biological interfaces.