Christoph Langhammer

Optical microscopy that employs intensity measurements of scattered, transmitted, or emitted light is critical to study micro- and nanofluidic systems widely used in, e.g., biology, chemistry, and materials science. Since such fluidic systems often are transparent, simple bright-field microscopy observations are bound to yield low-contrast images. Fluorescence labeling or Raman imaging can be used to overcome this limitation and yield high contrast. However, these approaches are invasive or complex and slow and do not yield direct quantitative information about a liquid inside a micro- or nanofluidic structure. Quantitative phase microscopy techniques on the other hand are very efficient in enhancing the contrast of transparent samples, in a noninvasive manner. However, they have never been applied to nanofluidic systems. To fill this gap, we demonstrate how cross-grating wavefront microscopy enables the quantitative characterization of liquids in terms of concentration and refractive index, as well as solute diffusion, inside a single nanofluidic channel embedded in a SiO2 matrix in general and from diffraction-limited nanofluidic channel volume fractions as small as ∼45 aL. In a wider perspective, this study thus demonstrates the applicability of cross-grating wavefront microscopy to the characterization of processes taking place in volumes smaller than the diffraction limit of light, such as, e.g., nanofluidic systems, nanopores, or nanocavities.

Nanofluidics enables the high-precision control of fluid flow, as well as of tiny particles and molecules, at the nanoscale. Combined with optical microscopy and spectroscopy, this has generated an experimental platform for the study of single nanoentities by means of light, often enhanced by nanoscale interference effects. However, in the realm of metal nanoparticles inside nanofluidic systems, contradictory light scattering properties have been reported, and the underlying physics is poorly understood. Here, we systematically investigate Pt and Au nanodisks nanofabricated into nanotrenches in a poly(methyl methacrylate) matrix to emulate nanofluidic systems and find that their light scattering signature can appear both brighter and darker than the trench alone, or become completely invisible, depending on the specific interplay between trench and nanodisk diameter. An analytical model based on the electrostatic approximation allows us to qualitatively understand the fundamental physics of this effect based on the interference of light scattered by a metal nanodisk and a nanofluidic structure. To corroborate the model and demonstrate an application in a fully functional nanofluidic system, we show that the optical appearance of single metal nanodisks inside a fully enclosed nanofluidic channel inside an SiO2 matrix can be dynamically tuned from dark to bright to completely invisible by adjusting the refractive index of a liquid inside the channel. We predict that this effect will find application in optical sensing in nanofluidic systems for volumes smaller than the diffraction limit of light.

Nanofluidic scattering microscopy characterizes single molecules in subwavelength nanofluidic channels label-free, using the interference of visible light scattered by the molecule and nanochannel. It determines a molecule’s hydrodynamic radius by tracking its diffusion trajectory and its molecular weight by analyzing its scattering intensity along that trajectory. However, using standard analysis algorithms, it is limited to characterization of proteins larger than ≈ 60 kDa. Here, we push this limit by one order of magnitude to below ≈ 6 kDa molecular weight and ≈ 1.5 nm hydrodynamic radius — as we exemplify on the peptide hormone insulin — by using ultrasmall nanofluidic channels and by analyzing the data with a hierarchical vision transformer. When we benchmark this approach against the theoretical limit set by the Cramér–Rao Lower Bound, we find that it can be approached with sufficiently long molecular trajectories. This enables quantitative label-free single-molecule microscopy for biologically relevant families of sub-10-kDa molecules, such as cytokines, chemokines and peptide hormones.

This study investigates the impact of sulfuric acid (H ₂ SO ₄ ) poisoning on pristine and annealed Pd NPs to identify the influence of structure on the density and stability of poisoners.

Fluidics on the micro- and nanoscale have been revolutionary for the fields of biology and medicine, and they are gaining a strong foothold in chemistry with the rise of micro and nanoscale reactors. These systems are based on fluidic platforms crafted into polymer or silicon-based substrates, and are comprised of channels with different functions and sizes that span from the micro- to the nanoscale. However, to fully capitalize on the possibilities offered by such highly integrated fluidic systems, the periphery that connects the fluidic chip to the macroscopic world, and thereby makes it accessible for the envisioned functions and applications, is equally important but receives much less attention. Such periphery needs to be versatile and enable accurate control of pressures and flow of liquids or gases, of sample temperature, and for certain applications even electric fields. Here, we report the development of a temperature-controlled fluidic chip holder for heating and cooling that is integrated with electrodes for the creation of electric fields across the fluidic system. It interfaces 1 cm ² silicon-based nanofluidic chips with up to 12 fluidic connection points and optically transparent lid, that makes them compatible with optical microscopy techniques. We demonstrate the different functionalities of the sample holder by using nanofluidic scattering spectroscopy (NSS) to monitor the on-chip mixing of two different dyes, the diffusion of fluorescein into water at different temperatures, and the diffusion of fluorescein into water at different strengths of an electric field applied along a nanochannel.

The detection of hydrogen gas in humid air environments is a key unresolved challenge for hydrogen safety in the rapidly growing hydrogen energy technologies, which hold a key to abate the CO₂ emissions from particularly challenging sectors that together represent more than 20% of man-made CO₂. In this work, we introduce a catalytic-plasmonic optical hydrogen sensor that employs nanofabricated and plasmonically active Pt nanoparticles as transducer elements for hydrogen detection in highly humid environments in air. Leveraging the combination of the Pt nanoparticles' intrinsic high activity in the catalytic hydrogen oxidation reaction with their high sensitivity in plasmonic dielectric sensing, we show that this catalytic-plasmonic sensor is able to operate in the entire humidity range of 0-80% relative humidity accessible in our test setup and exhibits a measured limit of detection of 30-50 ppm hydrogen in air at 100 °C and 80 °C sensor operating temperatures, respectively, and that it delivers consistent and constant response to hydrogen during a 143 h long continuous measurement in 80% relative humidity. We also show that above a given hydrogen concentration, the sensor response magnitude to a specific hydrogen concentration increases with increasing humidity, which is the distinct opposite of any other known hydrogen sensing technology, whose response deteriorates or is entirely suppressed in high humidity. This advertises catalytic-plasmonic sensors as a new paradigm in the hydrogen sensor arena with particular promise for hydrogen detection in high-humidity environments.

Pd nanoparticles, together with bulk and thin film Pd, constitute the archetype model system for metal-hydrogen interactions. The density of defects in Pd nanoparticles, such as grain boundaries and dislocations, combined with their size, shape, composition and lattice strain, dictate their hydrogen sorption kinetics and thermodynamics. Despite decades of research and its relevance in applications, such as solid-state hydrogen storage, hydrogen sensors, hydrogen embrittlement, and hydrogen separation membranes, a coherent picture of the intricate interplay between defects, strain and Pd nanoparticle hydrogen sorption properties is missing. Here, we employ a combination of single particle nanocompression, single particle plasmonic nanoimaging and high-resolution cross-sectional single particle TEM imaging to investigate hydrogen absorption kinetics and hydride phase formation pressures in a nanofabricated array of Pd nanoparticles on sapphire substrate with systematically varied levels of plastic deformation – and thus defects and strain. We not only show a clear deformation-level dependent trend of both the kinetics and the hydride formation pressure, but also reveal their complex evolution upon hydrogen cycling. We discuss how these results provide a quantitative view of the impact of plastic deformation on nanoscale metal hydrides, and how they reveal the surface and bulk morphology of Pd nanoparticles upon repeated hydrogen cycling.

Hydrogen gas is rapidly approaching a global breakthrough as a carbon-free energy source. In such a hydrogen economy, safety sensors for hydrogen leak monitoring will be an indispensable element due to the high flammability of hydrogen-air mixtures. Palladium-based nanoparticles function as optical hydrogen sensors due to their ability to reversibly absorb hydrogen and undergo a phase transition to palladium hydride, which induces a spectral shift in their localized plasmon resonance. However, the effectiveness of palladium-based nanoparticles as hydrogen sensors is compromised in realistic environments due to surface poisoning from various contaminants, including sulfur-containing compounds (SO_x), which block active sites required for hydrogen dissociation. In this study, we use atomic force microscopy, infrared nanospectroscopy, and Kelvin probe force microscopy, in addition to density functional theory (DFT) calculations, to investigate the impact of SO_x poisoning on the hydrogen sorption dynamics of single Pd nanoparticles. It is demonstrated that SO_x preferentially adsorbs on the particle's rim, significantly altering the kinetics of hydrogen (de)sorption and lowering the total sorption capacity. Single particle analysis revealed that poisoning leads to slower (de)sorption kinetics due to blocking of highly reactive surface sites that are located on the particle's rim. DFT calculations show that SO_x binds significantly less strongly to the flat palladium hydride surface compared to the flat palladium surface and the rough surface found at the nanoparticle rim. These calculations rationalize the selective desorption of SO_x from the center of the nanoparticle following exposure to hydrogen and its persistent binding to the particle rim.

Extracting weak signals from noise is a generic challenge in experimental science. In catalysis, it manifests itself as the need to quantify chemical reactions on nanoscopic surface areas, such as single nanoparticles or even single atoms. Here, we address this challenge by combining the ability of nanofluidic reactors to focus reaction product from tiny catalyst surfaces towards online mass spectrometric analysis with the high capacity of a constrained denoising auto-encoder to discern weak signals from noise. Using CO oxidation and C₂H₄ hydrogenation on Pd as model reactions, we demonstrate that the catalyst surface area required for online mass spectrometry can be reduced by ≈ 3 orders of magnitude compared to state of the art, down to a single nanoparticle with 0.0072 ± 0.00086 μm² surface area. These results advocate deep learning to improve resolution in mass spectrometry in general and for online reaction analysis in single-particle catalysis in particular.

Nanofluidic scattering microscopy characterizes single molecules in subwavelength nanofluidic channels label-free, using the interference of visible light scattered by the molecule and nanochannel. It determines a molecule’s hydrodynamic radius by tracking its diffusion trajectory and its molecular weight by analyzing its scattering intensity along that trajectory. However, using standard analysis algorithms, it is limited to characterization of proteins larger than ≈ 60 kDa. Here, we push this limit by one order of magnitude to below ≈ 6 kDa molecular weight and ≈ 1.5 nm hydrodynamic radius — as we exemplify on the peptide hormone insulin — by using ultrasmall nanofluidic channels and by analyzing the data with a hierarchical vision transformer. When we benchmark this approach against the theoretical limit set by the Cramér-Rao Lower Bound, we find that it can be approached with sufficiently long molecular trajectories. This opens interesting prospects for quantitative label-free single-molecule microscopy to characterize biologically relevant families of sub-10-kDa molecules, such as cytokines, chemokines and peptide hormones.