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Liquid cell transmission electron microscopy is a powerful tool for visualizing nanoparticle (NP) assemblies in liquid environments with nanometer resolution. However, it remains a challenge to control the NP concentration in the high aspect ratio liquid enclosure where the diffusion of dispersed NPs is affected by the exposed surface of the liquid cell walls. Here, we introduce a semi-empirical model based on the 1D diffusion equation, to predict the NP loading time as they pass through the nanochannel into the imaging volume of the liquid cell. We show that loading of NPs into the imaging volume of the liquid cell may take several days if NPs are prone to attach to the surface of the mm-long nanochannel when using an industry-standard flat microchip. As a means to facilitate mass transport via diffusion, we tested a liquid cell incorporating a microchannel geometry resulting in a NP loading time in the order minutes that allowed us to observe the formation of a randomly oriented self-assembled monolayer in situ using scanning transmission electron microscopy.
Observing processes of nanoscale materials of low atomic number is possible using liquid phase electron microscopy (LP-EM). However, the achievable spatial resolution (d) is limited by radiation damage. Here, we examine a strategy for optimizing LP-EM experiments based on an analytical model and experimental measurements, and develop a method for quantifying image quality at ultra low electron dose De using scanning transmission electron microscopy (STEM). As experimental test case we study the formation of a colloidal binary system containing 30-nm diameter SiO2 nanoparticles (SiONPs), and 100-nm diameter polystyrene microspheres (PMs). We show that annular dark field (DF) STEM is preferred over bright field (BF) STEM for practical reasons. Precise knowledge of the material's density is crucial for the calculations in order to match experimental data. To calculate the detectability of nano-objects in an image, the Rose criterion for single pixels is expanded to a model of the signal to noise ratio obtained for multiple pixels spanning the image of an object. Using optimized settings, it is possible to visualize the radiation-sensitive, hierarchical low-Z binary structures, and identify both components.
It is of great technological interest to control the organization of nanoparticles (NPs) into functional devices that can make use of NP’s properties not found in the bulk form of the solid material. To this end, a major scientific challenge is to further elucidate inter-particle forces that govern spontaneous self-assembly processes in liquid suspensions. Liquid-phase electron microscopy (LPEM) can resolve morphological details of small objects in μm-thick liquid layers with nanometer resolution. The goal of this doctoral thesis has been to develop LPEM towards directly visualizing colloidal self-assembly processes in aqueous suspensions. As a model system, we used a colloidal binary system in which positively charged 30 nm nanoparticles (SiONP) form a shell around 100 nm, negatively charged polystyrene microspheres (PMS). Analytical calculations and Monte-Carlo simulations were performed to optimize experimental parameters and to validate contrast in data obtained with a scanning transmission electron microscope (STEM). The extent of radiolytic damage due to the electron beam (PMS) was directly analyzed from the image data and an acceptable dose range was defined. Within this range, the core-shell structure of the pre-assembled binary system was directly visualized. Finally, a novel liquid cell design was tested which enabled us to initiate colloidal assembly reactions in the confinement of the nanofluidic device.
General and selective deoxygenation by hydrogen using a reusable earth-abundant metal catalyst
(2019)
Chemoselective deoxygenation by hydrogen is particularly challenging but crucial for an efficient late-stage modification of functionality-laden fine chemicals, natural products, or pharmaceuticals and the economic upgrading of biomass-derived molecules into fuels and chemicals. We report here on a reusable earth-abundant metal catalyst that permits highly chemoselective deoxygenation using inexpensive hydrogen gas. Primary, secondary, and tertiary alcohols as well as alkyl and aryl ketones and aldehydes can be selectively deoxygenated, even when part of complex natural products, pharmaceuticals, or biomass-derived platform molecules. The catalyst tolerates many functional groups including hydrogenation-sensitive examples. It is efficient, easy to handle, and conveniently synthesized from a specific bimetallic coordination compound and commercially available charcoal. Selective, sustainable, and cost-efficient deoxygenation under industrially viable conditions seems feasible.
