Chemical Structure and Distribution in Nickel-Nitrogen-Carbon Catalysts for CO2 Electroreduction Identified by Scanning Transmission X-ray Microscopy.

Atomically dispersed metal-nitrogen-carbon (M-N-C) materials are a class of electrocatalysts for fuel cell and electrochemical CO2 reduction (CO2R) applications. However, it is challenging to characterize the identity and concentration of catalytically active species owing to the structural heterogeneity of M-N-C materials. We utilize scanning transmission X-ray microscopy (STXM) as a correlative spectromicroscopy approach for spatially resolved imaging, identification, and quantification of structures and chemical species in mesoscale regions of nickel-nitrogen-carbon (Ni-N-C) catalysts, thereby elucidating the relationship between Ni content/speciation and CO2R activity/selectivity. STXM results are correlated with conventional characterization approaches relying on either bulk average (X-ray absorption spectroscopy) or spatially localized (scanning transmission electron microscopy with electron energy loss spectroscopy) measurements. This comparison illustrates the advantages of soft X-ray STXM to provide spatially resolved identification and quantification of active structures in Ni-N-C catalysts. The active site structures in these catalysts are identified to be atomically dispersed NiN x /C sites distributed throughout entire catalyst particles. The NiN x /C sites were notably demonstrated by spectroscopy to possess a variety of chemical structures with a spectroscopic signature that most closely resembles nickel(II) tetraphenylporphyrin molecules. The quantification and spatial distribution mapping of atomically dispersed Ni active sites achieved by STXM address a target that was elusive to the scientific community despite its importance in guiding advanced material designs.

Effect of UV radiation damage in air on polymer film thickness, studied by soft X-ray spectromicroscopy.

The thicknesses of thin films of polystyrene (PS), poly(methyl methacrylate) (PMMA), and perfluorosulfonic acid (PFSA) were measured by Ultraviolet Spectral Reflectance (UV-SR) and Scanning Transmission X-ray Microscopy (STXM). At high doses, the UV irradiation in air used in the UV-SR method was found to modify the chemical structures of PS and PMMA (but not PFSA), leading to thinning of these polymer films. The chemical changes caused by UV/air radiation damage were characterized by STXM. When UV and X-ray radiation are applied using no-damage conditions, the film thicknesses measured with the two techniques differ by less than 15% for PS and PMMA and less than 5% for PFSA. This is an important result for verifying the quantitation capabilities of STXM. The chemical damage to PS and PMMA is explained by oxygen implantation from air with formation of ozone. The thickness depletion caused by UV/air radiation for PS and PMMA films is exponential with exposure time. Different rates of depletion are linked to surface or bulk driven photo-chemical product erosion. The initial rate of material erosion was found to be constant and non-specific to the studied polymers.

Iron Biochemistry is Correlated with Amyloid Plaque Morphology in an Established Mouse Model of Alzheimer's Disease.

A signature characteristic of Alzheimer's disease (AD) is aggregation of amyloid-beta (Aß) fibrils in the brain. Nevertheless, the links between Aß and AD pathology remain incompletely understood. It has been proposed that neurotoxicity arising from aggregation of the Aß1-42 peptide can in part be explained by metal ion binding interactions. Using advanced X-ray microscopy techniques at sub-micron resolution, we investigated relationships between iron biochemistry and AD pathology in intact cortex from an established mouse model over-producing Aß. We found a direct correlation of amyloid plaque morphology with iron, and evidence for the formation of an iron-amyloid complex. We also show that iron biomineral deposits in the cortical tissue contain the mineral magnetite, and provide evidence that Aß-induced chemical reduction of iron could occur in vivo. Our observations point to the specific role of iron in amyloid deposition and AD pathology, and may impact development of iron-modifying therapeutics for AD.

An X-ray spectromicroscopy study of protein adsorption to polystyrene-poly(ethylene oxide) blends.

Synchrotron-based X-ray photoemission electron microscopy (X-PEEM) and atomic force microscopy (AFM) were used to characterize the composition and surface morphology of thin films of a polystyrene-poly(ethylene oxide) blend (PS-PEO), spun cast from dichloromethane at various mass ratios and polymer concentrations. X-PEEM reveals incomplete segregation with ∼30% of PS in the PEO region and vice versa. Protein (human serum albumin) adsorption studies show that this partial phase separation leads to greater protein repellency in the PS region, whereas more protein is detected in the PEO region compared to control samples.

Quantitative evaluation of radiation damage to polyethylene terephthalate by soft X-rays and high-energy electrons.

The chemical changes and absolute rates in radiation damage to polyethylene terephthalate (PET) caused by soft X-rays and energetic electrons have been measured using a scanning transmission X-ray microscope (STXM). Electron beam damage at two different dose rates and a range of doses was performed in an 80 keV transmission electron microscope (TEM). The STXM beam was used to create damage patterns with systematically varied doses of monochromatic soft X-rays on an adjacent piece of the same PET sample. NEXAFS spectroscopy at the C 1s and O 1s edges was used to study the chemistry of the radiation damage and to determine quantitative critical doses for PET damage by both types of radiation. The spectral changes were similar for damage by electrons and X-rays, indicating the radiation chemistry is dominated by secondary processes, not the primary event. The critical dose for chemical changes determined from C 1s spectral features is 4.2(6) x 10(8) Gy and was the same for soft X-rays and electrons within measurement uncertainties. The critical dose for specific damage processes (as defined by changes in several different, bond-specific spectral features) was found to be similar in the C 1s region and was comparable between C 1s and O 1s edges for electron beam damage. There were statistically different critical doses for soft X-ray damage as probed by changes in O 1s spectral features related to carbonyl and ester bonds.

Mapping the speciation of iron in Pseudomonas aeruginosa biofilms using scanning transmission X-ray microscopy.

An important feature of microbial communities is the spatial heterogeneity of extracellular chemistry. Scanning transmission X-ray microscopy (STXM) was used to map the spatial distribution of iron speciesthroughout Pseudomonas aeruginosa biofilms to assess the influence of chemical heterogeneity on biomineralization. P. aeruginosa biofilms were treated with Fe(III)-amended media. Speciation and quantitative mapping using STXM image sequences in the Fe 2p(3/2) (L3) absorption edge region revealed both Fe(II) and Fe(III) in localized microenvironments. Fe(III) was mainly associated with cell surfaces, while small amounts of Fe(II) was found in the extracellular space. Biofilms were also characterized using C 1s edge STXM image sequences. Anaerobic growth assays and confocal microscopy revealed the inability of P. aeruginosa to directly reduce Fe(III), implicating indirect iron reduction mechanisms in the formation of fine-grained, multivalent minerals. These studies suggest that geochemical microenvironments found throughout microbial communities are even more complex than previously believed.

Using intrinsic X-ray absorption spectral differences to identify and map peptides and proteins.

The intrinsic variation in the near-edge X-ray absorption fine structure (NEXAFS) spectra of peptides and proteins provide an opportunity to identify and map them in various biological environments, without additional labeling. In principle, with sufficiently accurate spectra, peptides (<50 amino acids) or proteins with unusual sequences (e.g., cysteine- or methionine-rich) should be differentiable from other proteins, since the NEXAFS spectrum of each amino acid is distinct. To evaluate the potential for this approach, we have developed X-SpecSim, a tool for quantitatively predicting the C, N, and O 1s NEXAFS spectra of peptides and proteins from their sequences. Here we present the methodology for predicting such spectra, along with tests of its precision using comparisons to the spectra of various proteins and peptides. The C 1s, N 1s, and O 1s spectra of two novel antimicrobial peptides, Indolicidin (ILPWKWPWWPWRR-NH2) and Sub6 (RWWKIWVIRWWR-NH2), as well as human serum albumin and fibrinogen are reported and interpreted. The ability to identify, differentiate, and quantitatively map an antimicrobial peptide against a background of protein is demonstrated by a scanning transmission X-ray microscopy study of a mixture of albumin and sub6.