Scanning Transmission Soft X-ray Microscopy Research Articles

Scanning Transmission Soft X-Ray Microscopy Probes Topical Drug Delivery of Rapamycin Facilitated by Microneedles.

Scanning Transmission X-ray microscopy (STXM) is a sensitive and selective probe for the penetration of rapamycin which is topically applied to human skin ex vivo and is facilitated by skin treatment with microneedles puncturing the skin. Inner-shell excitation serves as a selective probe for detecting rapamycin by changes in optical density as well as linear combination modeling using reference spectra of the most abundant species. The results indicate that mechanical damage induced by microneedles allows this drug to accumulate in the stratum corneum without reaching the viable skin layers. This is unlike intact skin which shows no drug penetration at all and underscores the mechanical impact of microneedle skin treatment. These results are compared to drug penetration profiles of other drugs highlighting the importance of skin barriers. High spatial resolution studies also indicate that the lipophilic drug rapamycin is observed in corneocytes. Attempts in data evaluation are reported to probe rapamycin also in the lipid layers between the corneocytes, which was not accomplished before. These results are compared to recent results on rapamycin uptake in skin where barrier impairment was induced by pre-treatment with the enzyme trypsin and drug formulations leading to occlusion.

In Situ Studies of Copper-Based CO2 Reduction Electrocatalysts by Scanning Transmission Soft X-ray Microscopy.

A microfluidic-enabled electrochemical device has been developed to investigate electrochemically active nanomaterials under reaction conditions using in situ scanning transmission soft X-ray microscopy (STXM). In situ STXM measurements were conducted on electrodeposited Cu catalysts under electrochemical CO2 reduction (CO2R) conditions. The study provides detailed, quantitative results about the changes in the morphology and chemical structure of the catalytic nanoparticles as a function of applied potentials. The deposited Cu nanoparticles initially contain both Cu(0) and Cu(I). As an increasingly cathodic potential is applied, the Cu(I) species gradually convert to Cu(0) over the potential range of +0.4 to 0 V versus the reversible hydrogen electrode (VRHE). During this process, Cu(I) particles of various sizes are converted to metallic Cu at different reaction rates and at slightly different potentials, indicating a degree of heterogeneity in the electrochemical response of discrete particles. At CO2R relevant potentials, only metallic Cu is observed, and the morphology of the particles is fairly stable within the spatial resolution limits of STXM (∼40 nm). We also report in situ STXM studies of a working electrode with relatively thick Cu-based electrodeposits. The spatially resolved chemical analysis identifies that Cu-oxide species can persist under CO2R conditions, but only when the catalytic nanoparticles are electronically isolated from the working electrode and therefore are catalytically irrelevant. In summary, in situ STXM is presented as a technique to gain advanced morphological and spatially resolved chemical structure insights into electrochemically active nanomaterials, which was used to provide improved understanding regarding Cu nanomaterial catalysts under CO2 reduction conditions.

Soft X-ray scanning transmission microscopy as a selective probe of topical dermal drug delivery: The role of petrolatum and occlusion

Selective and label-free probing of drugs by scanning transmission X-ray microscopy (STXM) is applied along with hyperspectral imaging to probe topically applied drugs in human skin explants ex vivo. A comparison of data evaluation strategies is reported, which includes changes in optical density at selected photon energies in the O 1 s regime, linear combination modeling using reference spectra, as well as principal component and cluster analyses. It is shown that consistent results are obtained from linear combination modeling of reference spectra and changes in optical density. The dermal penetration of the anti-inflammatory drug rapamycin is investigated with high spatial resolution using a petrolatum-based formulation. For such well-known formulation the result of occlusion is investigated leading to drug uptake into corneocytes. Additional changes in drug penetration induced by weakening the skin barrier by the serine protease trypsin are also included. These results are discussed in comparison with the dermal penetration of other drug formulations. Finally, alternatives to present data acquisition strategies in STXM are discussed with respect to sub-sampling in combination with mathematical approaches, so that the full chemical and spatial information can be retrieved and radiation damage is minimized.

Ionic liquid functionalized tin halide perovskite investigated by STXM and spectro-ptychography

The structural and compositional properties of tin halide perovskites (CH(NH2)2SnI3, FASnI3) with and without an ionic liquid of 1-Ethyl-3-methylimidazolium bis(trifluormethylsulfonyl) imide (EMITFSI) were investigated using the scanning transmission soft X-ray microscopy (STXM) and spectro-ptychography techniques at the Sn M5,4-edge, I M5,4-edge, O K-edge, C K-edge, and N K-edge. The study offers a comprehensive interpretation of the spectral characteristics of main components (C, N, Sn, and I) in Sn-based perovskites through STXM-based techniques and further reveals differences between the perovskite samples with and without adding EMITFSI ionic liquid. The chemical imaging of Sn and I shows that the perovskite sample without ionic liquid has Sn-rich regions in iodine cavities, while the ionic liquid-treated sample exhibits a more homogeneous perovskite phase. Furthermore, the C and N K-edge XANES resonance features support the interaction between the non-volatile ionic liquid and FA+, thus playing a protective role in the formation and stabilization of perovskite phase.

X-ray Microspectroscopy and Ptychography on Nanoscale Structures in Rock Varnish

X-ray microspectroscopy is a powerful analytical method in geoscientific and environmental research as it provides a unique combination of nanoscale imaging with high spectroscopic sensitivity at relatively low beam-related sample damage. In this study, “classical” scanning transmission soft X-ray microscopy (STXM) with X-ray absorption spectroscopy and the recently established soft X-ray ptychography are applied to the analysis of selected rock varnish samples from urban and arid desert environments. X-ray ptychography enhances the spatial resolution relative to STXM by up to 1 order of magnitude. With its high chemical sensitivity, it can resolve nanoscale differences in valence states of the key varnish elements manganese (Mn) and iron. Our results emphasize the complex nanoarchitecture of rock varnish as well as the diverse mineralogy of the Mn oxy–hydroxide matrix and its embedded dust grains. In contrast to the fast-growing urban varnish, the slow-growing arid desert varnish revealed a remarkable nanoscale stratification of alternating Mn valence states, providing hints on the layer-wise and still enigmatic growth process.

Three-dimensional fast elemental mapping by soft X-ray dual-energy focal stacks imaging.

The three-dimensional (3D) dual-energy focal stacks (FS) imaging method has been developed to quickly obtain the spatial distribution of an element of interest in a sample; it is a combination of the 3D FS imaging method and two-dimensional (2D) dual-energy contrast imaging based on scanning transmission soft X-ray microscopy (STXM). A simulation was firstly performed to verify the feasibility of the 3D elemental reconstruction method. Then, a sample of composite nanofibers, polystyrene doped with ferric acetylacetonate [Fe(acac)3], was further investigated to quickly reveal the spatial distribution of Fe(acac)3 in the sample. Furthermore, the data acquisition time was less than that for STXM nanotomography under similar resolution conditions and did not require any complicated sample preparation. The novel approach of 3D dual-energy FS imaging, which allows fast 3D elemental mapping, is expected to provide invaluable information for biomedicine and materials science.

Calculating absorption dose when X-ray irradiation modifies material quantity and chemistry.

X-ray absorption is a sensitive and versatile tool for chemical speciation. However, when high doses are used, the absorbed energy can change the composition, amount and structure of the native material, thereby changing the aspects of the absorption process on which speciation is based. How can one calculate the dose when X-ray irradiation affects the chemistry and changes the amount of the material? This paper presents an assumption-free approach whichcan retrieve from the experimental data all dose-sensitive parameters -absorption coefficients, composition (elemental molecular units), material densities - which can then be used to calculate accurate doses as a function of irradiation. This approach is illustrated using X-ray damage to a solid film of aperfluorosulfonic acid fluoropolymer in a scanning transmission soft X-ray microscope. This new approach is compared against existing dose models which calculate the dose by making simplifying assumptions regarding the material quantity, density and chemistry. While the detailed measurements used in thisapproach go beyond typical methods to experimental analytical X-ray absorption, they provide a more accurate quantitation of radiation dose, and help to understand mechanisms of radiation damage.

Megapixel scanning transmission soft X-ray microscopy imaging coupled with compressive sensing X-ray fluorescence for fast investigation of large biological tissues.

Soft X-ray microscopy coupled with low energy X-ray fluorescence is a powerful tool for investigating complex biological systems like cells and tissues. Due to certain characteristics of X-ray sources, sample stage motors, and detectors, the examination of large areas at high resolutions is very time consuming, often confining the analysis only to a restricted number of pre-selected representative regions. Here we propose and demonstrate a compressive sensing method that provides an alternative approach for overcoming such limitations and can be applied to different kinds of samples and other microscopy and analytical techniques.

Soft X-ray Scanning Transmission Microscopy Studies of Radiation Damage by Electron, Ion and X-ray Beams

Soft X-ray Scanning Transmission Microscopy Studies of Radiation Damage by Electron, Ion and X-ray Beams

Simulation study for the formation of alkaline efflorescence on bauxite residue disposal areas following the phosphogypsum addition

Bauxite residue is a solid, highly alkaline byproduct generated in the alumina refining process. The process that forms the fine white salt deposits on the surface of bauxite residue disposal areas (BRDAs) following prolonged dry periods, and which also forms on products made from reused residues (sintered bricks, etc.), is known as efflorescence. It not only seriously affects the utilization of bauxite residue in building materials, but also creates potential dust pollution around the BRDAs, which means it would require sustained and intensive resources to manage and transform its alkalinity. Through a laboratory simulation experiment using soil leaching columns, we show that the extent of bauxite residue efflorescence is affected by many factors, including porosity, initial water content, bauxite residue particle size. The main soluble mineral precipitates of efflorescence are Na2CO3 and NaHCO3. Addition of phosphogypsum, a common amendment to reduce dispersion and promote removal of Na+ through leaching, shifted the composition of effloresced salts to favor Na2SO4·10H2O. SEM and Na K-edge X-ray absorption near-edge structure (XANES)spectroscopy demonstrated that free alkali migrates to the residue surface to precipitate as a uniform agglomerate in the efflorescence process, which also changes the chemical form of Na-containing minerals in the bauxite residue and the mesoscale spatial distribution of Na (soft X-ray scanning transmission microscopy imaging). Free alkali migration with water is the main driver of efflorescence, and this work also demonstrated that phosphogypsum can reduce the generation of alkaline efflorescence for ameliorating alkaline dust pollution from bauxite residue.

Significant Enhancement in Light Output of Photonic-Crystal-Based YAG:Ce Scintillator for Soft X-Ray Detectors

Because of its high light output, Gd <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> S:Tb (P43) phosphor powder is widely used in scintillator detectors for scanning transmission soft X-ray microscopy. However, the long decay time (>1 ms) of P43 phosphor powder limits the image scanning speed. Inorganic scintillators such as YAG:Ce are very suitable for fast scanning imaging due to their short decay time (tens of nanoseconds). Nevertheless, its light output is lower than that of P43 phosphor powder. Patterning photonic crystals (PhCs) on the output surface of an inorganic scintillator is a simple and efficient method to enhance its light output. In this article, we use a soft X-ray interference lithography technique to produce a large-area pattern of nanoscale PhCs on a 200-μm -thickness and 1-in-diameter YAG:Ce scintillator. Measurement of the enhancement ratio was performed on the soft X-ray beamline (BL08U1A) at the Shanghai Synchrotron Radiation Facility. The results show that the light output of the PhC-patterned YAG:Ce scintillator with air coupling is approximately 2.8 times higher than that of the unpatterned YAG:Ce scintillator. Meanwhile, the time-resolved emission with optical excitation showed that the PhC structures did not affect the spontaneous emission rate in the internal of scintillator crystal. This enhancement of the light output indicates that the PhC-patterned YAG:Ce scintillator is a good candidate for soft X-ray detection.

Quantitative Distribution of DNA, RNA, Histone and Proteins Other than Histone in Mammalian Cells, Nuclei and a Chromosome at High Resolution Observed by Scanning Transmission Soft X-Ray Microscopy (STXM).

Soft X-ray microscopy was applied to study the quantitative distribution of DNA, RNA, histone, and proteins other than histone (represented by BSA) in mammalian cells, apoptotic nuclei, and a chromosome at spatial resolutions of 100 to 400 nm. The relative distribution of closely related molecules, such as DNA and RNA, was discriminated by the singular value decomposition (SVD) method using aXis2000 software. Quantities of nucleic acids and proteins were evaluated using characteristic absorption properties due to the 1s–π * transition of N=C in nucleic acids and amide in proteins, respectively, in the absorption spectra at the nitrogen K absorption edge. The results showed that DNA and histone were located in the nucleus. By contrast, RNA was clearly discriminated and found mainly in the cytoplasm. Interestingly, in a chromosome image, DNA and histone were found in the center, surrounded by RNA and proteins other than histone. The amount of DNA in the chromosome was estimated to be 0.73 pg, and the content of RNA, histone, and proteins other than histone, relative to DNA, was 0.48, 0.28, and 4.04, respectively. The method we present in this study could be a powerful approach for the quantitative molecular mapping of biological samples at high resolution.

Carbon-coated montmorillonite nanocomposite for the removal of chromium(VI) from aqueous solutions.

A carbon-coated montmorillonite nanocomposite (CMt), obtained by hydrothermal carbonization of montmorillonite suspension in glucose, was used to remove Cr(VI) from aqueous solutions. The distribution and speciation of Cr immobilized by CMt were assessed by transmission electron microscopy with energy-dispersive X-ray analysis, X-ray absorption near edge structure (XANES), and scanning transmission soft X-ray microscopy (STXM). The variation in the functional groups and molecular structures of CMt was also investigated. The capacity of CMt for adsorbing Cr(VI) was markedly superior to that of the parent montmorillonite, showing maximum uptake of 100 and 12.4 mg g-1 at pH 2 and 8, respectively. The Cr K-edge XANES and STXM analyses indicated that Cr(VI) was reduced to Cr(III) under both acidic and alkaline conditions, while a Cr(OH)3 precipitate and Cr(III)-acetate complex were the predominant species present on the CMt surface. The Fourier transform infrared spectroscopy and C K-edge XANES further suggested that the phenolic groups in CMt could serve as electron donors, facilitating Cr(VI) reduction. The combined results indicate that electrostatic attraction, Cr(VI) reduction, complexation, and precipitation are involved in the removal of Cr(VI) by CMt.

Discrimination of DNA and RNA distribution in a mammalian cell by scanning transmission soft X-ray microscopy.

Soft X-ray spectromicroscopy was applied to study the distribution of DNA and RNA in a mammalian cell at the spatial resolution of 400 nm. The relative distribution of DNA and RNA was examined by the SVD (singular value decomposition) method in aXis2000 program using combined full spectra of DNA and RNA at the absorption edge regions of carbon, nitrogen and oxygen. The absorption of nucleic acid was evaluated using 1s-π* transitions in the NEXAFS spectra at the nitrogen K absorption edge and distributed to DNA and RNA according to the relative level obtained above. The present results revealed the usefulness of the SVD method to discriminate closely related molecules such as DNA and RNA.

Redox-Chemistry of Environmental Biofilms Probed on the Submicron Scale by in-situ Electrochemistry Scanning Transmission (soft) X-ray Microscopy

Redox-Chemistry of Environmental Biofilms Probed on the Submicron Scale by in-situ Electrochemistry Scanning Transmission (soft) X-ray Microscopy

PEMFC Catalyst Layers: Recent Advances in Process-Structure-Property Correlations

Proton exchange membrane (PEM) fuel cells are being developed as alternative energy sources for both residential and automotive applications. In order for this technology to become fully commercial, reduction of cost and improvements in performance and durability of PEM fuel cells’ membrane electrode assemblies (MEAs) are still required. To address the requirements for further cost reduction, structure and composition of the cathode catalyst layer (CCL) has to be optimized in order to achieve optimum performance and durability while minimizing the platinum group metals (PGM) loading. In order to rationally design CCL structures that meet the performance and durability requirements, a better understanding of structure versus performance relationships is needed. This requires the capability to fabricate different CCL structures, to characterize the spatial distribution of all the phases within the catalyst layer1-3 (carbon, Pt, ionomer, and void), to measure the physico-chemical properties (both ex-situ and in-situ), and, finally, to use these experimental data as inputs for the development of a model-based understanding of the relationship between the CCL structure and CCL performance and durability. Recently, significant progress has been made in reliable measurement and modeling of key CCL properties such as thermal conductivity, electronic conductivity, and gas diffusivity4-7. Dependence of these properties on the CCL structure and manufacturing processes provides support for rational CCL design. References J. Wu, A.P. Hitchcock, M. Lerotic, D. Shapiro, V. Berejnov, D. Susac, J. Stumper, “4D imaging of polymer electrolyte membrane fuel cell cathodes by scanning X-ray microscopy”, Microsc. Microanal., 23, 1784 (2017).V. Berejnov, M. Saha, D. Susac, J. Stumper, M. West, A.P. Hitchcock, Advances in structural characterization using soft x-ray scanning transmission microscopy (STXM): Mapping and measuring porosity in PEM-FC catalyst layers, ECS Transactions, 80, 241 (2017).L.G.A. Melo, A.P. Hitchcock, J. Jankovic, J. Stumper, D. Susac, V. Berejnov, Quantitative mapping of ionomer in catalyst layers by electron and x-ray spectromicroscopy, ECS Transactions, 80, 275 (2017).M. Ahadi, M. Tam, M.S. Saha, J. Stumper, M. Bahrami, Thermal conductivity of catalyst layer of polymer electrolyte membrane fuel cells: Part 1 – Experimental study, J. Power Sources, 354, 207 (2017).M. Ahadi, A. Putz, J. Stumper, M. Bahrami, Thermal conductivity of catalyst layer of polymer electrolyte membrane fuel cells: Part 2 – Analytical modeling, J. Power Sources, 354, 215 (2017).M. Ahadi, M. Tam, A. Putz, J. Stumper, C. McCague, M. Bahrami, Electrical conductivity of PEM fuel cell catalyst layers: Through-plane vs. in-plane, in: European Hydrogen Energy Conference 2018, Costa del Sol, Spain, 14-16th March, paper no. (124), 206 (2018).S. Salari, C. McCague, M. Tam, M.S. Saha, J. Stumper, M. Bahrami, Accurate ex-situ measurements of PEM fuel cells catalyst layer dry diffusivity, ECS Transactions, 69, 419 (2015).

Quantitative analysis of mammalian chromosome by scanning transmission soft X-ray microscopy

Soft X-ray spectromicroscopy was applied to study the quantitative distribution of DNA and protein in a mammalian chromosome at the spatial resolution of 100 nm. The quantities of DNA and protein were evaluated using 1s-π* transition in the NEXAFS spectra at the nitrogen K absorption edge. DNA was not uniformly distributed in the chromosome and DNA/protein ratio was less than 0.497. The present analysis revealed the clues to identify other molecules that contribute to the absorption spectrum of the sample. The results suggested that accumulation of the absorption spectra of relevant molecules would support the refinement of the analysis.

Selenite removal from groundwater by zero-valent iron (ZVI) in combination with oxidants

We recently demonstrated common oxidants simply coupled with ZVI to continuously drive the accelerated Fe0 corrosion and hence achieve fast and very efficient removal of heavy metals and metalloids from groundwater. In this study, we aimed first to answer a basic question of the oxidant dosage theoretically required to sequester a certain amount of selenite. The specific ratio of oxidant dosage to Se(IV) removal, which reflected the theoretically minimal oxidant dosage required to sequester one mole of Se(IV), was almost independent of the initial Se(IV) concentration but significantly affected by the difference in oxidant species. To sequester one mole of Se(IV), the minimum dosage of the required oxidant was calculated to be 3.94–4.09 for NaClO, 3.90–4.33 for H2O2, and 3.29–3.54 for KMnO4, respectively. Simultaneous aeration increased the removal efficiency of Se(IV) and substantially reduced the required dosage of oxidants. To form a strong contrast with very limited Se(IV) removal by ZVI alone, the coupling of NaClO and H2O2 into the ZVI system remarkably enhanced the performance of Se(IV) removal during the long term fixed-bed experiments with a real groundwater background. The ZVI columns coupled by NaClO and H2O2 steadily treated 10,000 volumes (BV) of real groundwater. X-ray adsorption near edge structure (XANES) demonstrated that more than 85% of the selenium was reduced to Se0 and Se2−, with Se0 as the dominant selenium species sequestered in the solid phase. Synchrotron-based scanning transmission soft X-ray microscopy (STXM) explicitly revealed the ubiquitous spatial distribution of selenium in the corrosion products and hence demonstrated a high accessibility of these corrosion products to selenite.

First-principles X-ray absorption dose calculation for time-dependent mass and optical density.

A dose integral of time-dependent X-ray absorption under conditions of variable photon energy and changing sample mass is derived from first principles starting with the Beer-Lambert (BL) absorption model. For a given photon energy the BL dose integral D(e, t) reduces to the product of an effective time integral T(t) and a dose rate R(e). Two approximations of the time-dependent optical density, i.e. exponential A(t) = c + aexp(-bt) for first-order kinetics and hyperbolic A(t) = c + a/(b + t) for second-order kinetics, were considered for BL dose evaluation. For both models three methods of evaluating the effective time integral are considered: analytical integration, approximation by a function, and calculation of the asymptotic behaviour at large times. Data for poly(methyl methacrylate) and perfluorosulfonic acid polymers measured by scanning transmission soft X-ray microscopy were used to test the BL dose calculation. It was found that a previous method to calculate time-dependent dose underestimates the dose in mass loss situations, depending on the applied exposure time. All these methods here show that the BL dose is proportional to the exposure time D(e, t) ≃ K(e)t.

Advances in Structural Characterization Using Soft X-ray Scanning Transmission Microscopy (STXM): Mapping and Measuring Porosity in PEM-FC Catalyst Layers

Access of fuel and oxidant gases to the active catalyst sites in dispersive electrodes of polymer electrolyte membrane fuel cells (PEM-FC) is an important characteristic defining the performance of the whole catalyst coated membrane unit (CCM). The electrode porosity – the ratio of empty space to the whole volume of the electrode – is a metric widely used to characterize the ability of a particular electrode to provide gas transport. It is common practice to present electrode porosity as a number averaged over a macroscopic volume of the electrode [1]. In compliment to already established methods, soft X-ray scanning transmission X-ray microscopy (STXM) offers the possibility to visualize the microscopic network of the pore space in the electrode with ~30 nm spatial resolution. Thus, in addition to average porosity, STXM provides porosity distributions in electrodes, with a local value averaged per voxel of ~1003 nm3. Relative to studies of the electrode porosity visualized by the focused ion beam – scanning electron microscopy method (FIB-SEM) [2], STXM provides direct visualization of the pore-space in the electrode without destroying the sample and omitting the 3D reconstruction process. Here we present STXM porosity distribution for 2D projection of the thin cathode lamella. STXM pore space visualization relies on filling the pores with a material that STXM can discriminate spectroscopically relative to the other components (carbon support, ionomer, and metal catalyst-Pt) [3]. In these experiments the cathode catalyst decals are embedded with a custom epoxy (TTE, the product of reacting trimethylolpropane triglycidyl ether and 4,4’-methylenebis(2-methylcyclohexylamine in 1:1 wt ratio, which results in a final chemical formula (C10H10N3)n and density of 1.0 g/cm3). While the TTE epoxy is fluid it wets well the electrode inner interfaces filling most of the available space. 100 nm thick microtomed sections of the cathode embedded in TTE are measured by STXM at the C 1s edge at the Canadian Light Source (Saskatoon) or the Advanced Light Source (Berkeley). The C1s stacks, consisting of images at 50-70 energies over 280-330 eV with 100 nm pixels, are converted to optical density, and then fit using standard, quantitative reference spectra (OD1 scale) of carbon-support (Cs), ionomer (I), TTE and Pt components to construct 2D component maps of each component. The grayscale value at each pixel is the nm thickness of that component. The total volume is the sum of Cs, I, TTE, and Pt values at each pixel. The 2D porosity map is then the ratio of the TTE map to the total-volume map, where each pixel has a unit of part per volume (ppvol). STXM performed using BL 10ID1 at CLS and BL 5.3.2.2 at ALS. Research supported by NSERC. S. Shukla, et.al., 231 ECS 2017, Abst 97034, https://ecs.confex.com/ecs/231/webprogram/Paper97034.htmlM.Sabharwal, et al., Fuel Cell, 16 (2016) No.6, 734–753H. Ade, et al., Polymers, 49 (2008) 643-675 Figure 1