Tim Salditt

We study how the lipid membrane structure changes upon interaction with ‐synuclein (aSyn) at low and physiological concentrations, using three different X‐ray scattering methods and two well‐established model systems, the Langmuir film balance and small unilamellar vesicles in bulk solution. While aSyn is highly membrane active, we find that at physiological protein concentration it induces only rather small changes in lipid membrane structure and vesicle shape. The vertical density profile of Langmuir lipid monolayers shows no observable changes for DPPC:DPPS and DOPC:DOPS, and moderate changes only for a model system containing gangliosides. Contrarily, the protein induces more pronounced changes in the bilayer density profile of highly curved anionic lipid vesicles, but without rupture and major remodeling of vesicular shapes as reported previously for pathological concentrations. In fact, the shape of the vesicles showed even a slight reduction in ellipticity. In addition to aSyn only, we also study the structure of the monolayer and vesicular membranes in the presence of two major synaptic proteins, aSyn and synapsin 1, at concentrations where the latter induces lipid‐protein condensates.

X-ray absorption near-edge structure (XANES) spectroscopy is key to understanding functional materials. We present, to our knowledge, the first use of hard x-ray near-field holography (NFH) for three-dimensional (3D), spatially resolved XANES. While NFH provides lensless imaging with an adjustable field of view, it entails complex reconstruction. Our method jointly reconstructs NFH data acquired at multiple energies and enforces a low-dimensional spectral model based on the physically motivated assumption that each voxel spectrum is a linear combination of a small number of constituents. We demonstrate the approach on a mixed metal oxide catalyst particle, achieving accurate spectral reconstruction and revealing 3D chemical heterogeneity. XANES in NFH mode has the potential to enable nano-scale in-operando XANES microscopy with a large field of view.

When hearing fails, cochlear implants (CIs) partially restore auditory perception. Yet, poor coding of spectral information remains a bottleneck as each electrode broadly activates the auditory nerve. As light can be more conveniently confined, optical (o)CIs present a promising alternative. Here, we combined expression of the potent channelrhodopsin ChReef in spiral ganglion neurons (SGNs) and oCIs based on 5-10 green LED in gerbils. We characterized the oCI encoding of intensity and spectral information by ChReef-SGNs using recordings from the central nucleus of the inferior colliculus (ICC). ChReef aligned light sensitivity of SGNs well with the radiant fluxes provided by individual LEDs: ICC-activity had thresholds 1 spectral peaks for stronger intensities. Representational Similarity Analysis and Linear Discriminant Analysis of ICC activity indicated improved channel discriminability of optical over electrical stimulation. In summary, µJ oCI stimulation achieves near-physiological spectral resolution. The Paper Explained: Problem: Electrical cochlear implants (eCIs) partially restore speech comprehension in most of >1 million otherwise deaf users, who still face challenges hearing in daily situations. This is primarily due to poor spectral selectivity of electrical sound encoding. Spatially more confined optogenetic activation of the auditory nerve by optical cochlear implants (oCI) promises to overcome this limitation. However, a thorough characterization of bionic coding of sound information by multichannel oCI is needed to evaluate the potential for improved hearing restoration. Results: Here, we combine the potent channelrhodopsin ChReef and 10-channel oCI based on green LEDs in gerbils and characterize their utility for encoding of spectral and intensity information by multielectrode array recordings from the midbrain. ChReef enabled activation of the auditory pathway with nano-joule thresholds and up to high levels of midbrain activity with low µJ radiant energy. The cochlear spread of excitation and channel discriminability for low to medium activity levels were close to what we observed with acoustic stimulation. Impact: Our work demonstrates great potential of multichannel optogenetic stimulation for encoding sound frequency information.

The phase shift of an electromagnetic wave, imprinted by its interaction with atomic scatterers, is a central quantity in optics and photonics. In particular, it encodes information about optical resonances and photon–matter interaction. Although being a routine task in the optical regime, interferometric measurements of phase shifts in the X-ray frequency regime are notoriously challenging due to the short wavelengths and associated stability requirements. As a result, existing X-ray interferometers are ill-suited for light confined to the nanometre scale. Here we demonstrate a nanoscale interferometer, inspired by Young’s double-slit experiment, to measure the dispersive phase shift due to the 14.4-keV nuclear resonance of the Mössbauer isotope ⁵⁷ Fe coupled to an X-ray waveguide. From the single-photon interference patterns, we precisely extract the phase shifts in the vicinity of the nuclear resonance resolved in photon energy by using Bayesian inference. We find that the combined information from phase shift and absorbance reveals microscopic coupling parameters, which are not accessible from the absorption measurements alone. The demonstrated principle lays the basis for integrated X-ray interferometric sensors.

Wetting of solid surfaces by a liquid is important for many natural and industrial processes, such as printing, painting, and coating. However, a quantitative description of the dynamic receding and advancing contact angle is still debated, in particular for aqueous solutions. One reason for our lack of quantitative understanding is the limited spatial resolution of currently used optical methods. We therefore present a new approach to access the submicroscopic region. We use X-ray phase contrast imaging to measure the dynamic receding contact angle on a moving glass fiber of 17 μm diameter. The fiber was pulled out of a liquid bath, which was filled with a mixture of glycerol and Milli-Q water. The dynamic receding contact angle decreased with increasing contact line velocity for all mixtures. In the holograms, we achieved a resolution of 50 nm/pixel with a spatial error of 450 nm. This spatial error is due to an extended surface region of the fiber and the liquid surface in the holograms. Our results demonstrate the feasibility of X-ray holography as a method to investigate dynamic contact angle phenomena and thereby open pathways to higher spatial and temporal resolution.

Structure analysis of supra-molecular assemblies is of considerable interest in a wide range of research fields, but also extremely challenging. Biomolecular colloidal particles, such as biological vesicles or small unilamellar lipid vesicles, for example, measure only a few tens of nanometers, and need to be probed in hydrated and functionalized environments. Small-angle X-ray scattering (SAXS) is a well established technique offering high resolution in solution and at room temperature. However, due to the average over an extremely large ensemble, SAXS yields information only about the average structure (size and electron density profile). Many structural details are lost or screened by polydispersity, as well as by powder averaging. To overcome these limitations, we study single vesicles sequentially by coherent diffractive X-ray imaging using femtosecond X-ray free-electron laser (XFEL) pulses. For these experiments, single vesicles surrounded by a thin water layer are delivered into a nano-focused XFEL beam by an aerosol injector. Following the ‘diffract-before-destroy’ principle, the individual vesicles are probed on time scales where radiation damage has not yet set in. This approach leads to the measurement of thousands of diffraction patterns that can now be analyzed without limits associated with ensemble averaging.

Here we report on recent progress in X-ray waveguide optics for full-field coherent imaging at the Göttingen Instrument for Nano-Imaging with X-rays (GINIX), installed at the P10 coherence beamline of the PETRA III storage ring at DESY, Hamburg. We describe fabrication methods including new materials and the corresponding characterization in terms of transmission, exit intensity distribution, far-field intensity distribution and coherence properties. In addition to single channels, which are currently used for holographic imaging, we include results on tapered waveguides and off-axis waveguide interferometers. We also address optimization of waveguide optics with respect to the novel super-resolution holography method presented by Soltau et al. [ Optica (2021), 8 , 818–823]. Finally, we discuss the further development of high-resolution nano-holography in view of the planned storage ring upgrade to PETRA IV.

X-ray microtomography at synchrotron sources is fundamentally limited by the high radiation dose applied to the samples, which restricts investigations to non-native tissue states and thereby compromises the biological relevance of the resulting data. The limitation stems from inefficient indirect detection schemes that require prolonged exposures. Efforts to extract additional contrast through multi-modal techniques, like modulation-based imaging, worsen the problem by requiring multiple tomographic scans. In addition, the techniques suffer from low modulator pattern visibility, which reduces measurement efficiency and sensitivity. We address both the detection efficiency and modulation visibility challenges using a setup that combines an X-ray waveguide, a structured phase modulator, and a photon-counting detector. Our approach simultaneously achieves near-theoretical limits in both visibility (95%) and quantum efficiency (98%), thereby enabling dose-efficient multi-modal microtomography at single-micrometer resolution. This advance represents an important step toward routine multi-modal microtomography of native-state biological specimens with the potential to advance biomedical research and disease diagnosis.

X-ray holo-tomography is a powerful tool for high-resolution three-dimensional imaging, but it faces inherent limitations in resolution caused by detector constraints, numerical aperture restrictions, and the empty beam division, which is a ubiquitous step of raw data correction in conventional inline holography and (phase contrast) μ CT. To address these challenges, we extend the technique of super-resolution holography (SRH), an iterative phase reconstruction scheme that achieves high resolution by leveraging a complex illumination model, compatible with low flux areas in the beam tail and far-field diffraction components. We include a realistic pixel block constraint and represent the fields with moderate upsampling in the effective parallel beam geometry. We then demonstrate the improvements gained in resolution and sampling efficiency experimentally, using X-ray waveguide illumination and a single photon counting detector. Our results highlight the benefits of combining computational development with experimental design and showcase the potential of SRH to improve resolution and contrast at holo-tomography beamlines.