Time-resolved X-ray Absorption Near-edge Structure Research Articles

Intracellular biotransformation and disposal mechanisms3d cancer cells.

Magnetosomes are magnetite nanoparticles biosynthesized by magnetotactic bacteria. Given their potential clinical applications for the diagnosis and treatment of cancer, it is essential to understand what becomes of them once they are within the body. With this aim, here we have followed the intracellular long-term fate of magnetosomes in two cell types: cancer cells (A549 cell line), because they are the actual target for the therapeutic activity of the magnetosomes, and macrophages (RAW 264.7 cell line), because of their role at capturing foreign agents. We show that cells dispose of magnetosomes using three mechanisms: splitting them into daughter cells, excreting them to the surrounding environment, and degrading them yielding less or non-magnetic iron products. A deeper insight into the degradation mechanisms by means of time-resolved X-ray absorption near-edge structure (XANES) spectroscopy has allowed us to follow the intracellular biotransformation of magnetosomes by identifying and quantifying the iron species occurring during the process. In both cell types there is a first oxidation of magnetite to maghemite and then, earlier in macrophages than in cancer cells, ferrihydrite starts to appear. Given that ferrihydrite is the iron mineral phase stored in the cores of ferritin proteins, this suggests that cells use the iron released from the degradation of magnetosomes to load ferritin. Comparison of both cellular types evidences that macrophages are more efficient at disposing of magnetosomes than cancer cells, attributed to their role in degrading external debris and in iron homeostasis. This article is protected by copyright. All rights reserved.

Tracking structural solvent reorganization and recombination dynamics following e- photoabstraction from aqueous I- with femtosecond x-ray spectroscopy and scattering.

We present a sub-picosecond resolved investigation of the structural solvent reorganization and geminate recombination dynamics following 400nm two-photon excitation and photodetachment of a valence p electron from the aqueous atomic solute, I-(aq). The measurements utilized time-resolved X-ray Absorption Near Edge Structure (TR-XANES) spectroscopy and X-ray Solution Scattering (TR-XSS) at the Linac Coherent Light Source x-ray free electron laser in a laser pump/x-ray probe experiment. The XANES measurements around the L1-edge of the generated nascent iodine atoms (I0) yield an average electron ejection distance from the iodine parent of 7.4 ± 1.5 Å with an excitation yield of about 1/3 of the 0.1M NaI aqueous solution. The kinetic traces of the XANES measurement are in agreement with a purely diffusion-driven geminate iodine-electron recombination model without the need for a long-lived (I0:e-) contact pair. Nonequilibrium classical molecular dynamics simulations indicate a delayed response of the caging H2O solvent shell and this is supported by the structural analysis of the XSS data: We identify a two-step process exhibiting a 0.1ps delayed solvent shell reorganization time within the tight H-bond network and a 0.3ps time constant for the mean iodine-oxygen distance changes. The results indicate that most of the reorganization can be explained classically by a transition from a hydrophilic cavity with a well-ordered first solvation shell (hydrogens pointing toward I-) to an expanded cavity around I0 with a more random orientation of the H2O molecules in a broadened first solvation shell.

Tracking multiple components of a nuclear wavepacket in photoexcited Cu(I)-phenanthroline complex using ultrafast X-ray spectroscopy

Disentangling the strong interplay between electronic and nuclear degrees of freedom is essential to achieve a full understanding of excited state processes during ultrafast nonadiabatic chemical reactions. However, the complexity of multi-dimensional potential energy surfaces means that this remains challenging. The energy flow during vibrational and electronic relaxation processes can be explored with structural sensitivity by probing a nuclear wavepacket using femtosecond time-resolved X-ray Absorption Near Edge Structure (TR-XANES). However, it remains unknown to what level of detail vibrational motions are observable in this X-ray technique. Herein we track the wavepacket dynamics of a prototypical [Cu(2,9-dimethyl-1,10-phenanthroline)2]+ complex using TR-XANES. We demonstrate that sensitivity to individual wavepacket components can be modulated by the probe energy and that the bond length change associated with molecular breathing mode can be tracked with a sub-Angstrom resolution beyond optical-domain observables. Importantly, our results reveal how state-of-the-art TR-XANES provides deeper insights of ultrafast nonadiabatic chemical reactions.

Evolution of Pt and Pd species in functionalized UiO-67 metal-organic frameworks

Abstract Functionalization of metal-organic frameworks (MOFs) with noble metals is a promising way for producing new versatile catalysts that will combine the outstanding porosity and specific surface area of MOFs with high catalytic activity of metals. Here, we present a comparative study of two metal-organic frameworks with UiO-67 topology, functionalized with palladium and platinum moieties. The initial structure of all studied samples contained palladium or platinum atoms grafted into MCl2bpydc (M = Pd, Pt) linkers of MOFs. The materials were further activated by heating in inert and H2-containing atmospheres. Both Pd- and Pt- functionalized materials exhibited high thermal stability upon heating in these atmospheres. The evolution of Pt and Pd species during the activation procedure was monitored by in situ time-resolved X-ray absorption near-edge structure (XANES) spectroscopy. We applied multivariate curve resolution alternating least squares (MCR-ALS) approach to XANES to unravel the intermediates which can be formed during the activation procedure. For UiO-67-Pd, only simple one-step transformation from PdCl2bpydc to Pd nanoparticles (NPs) was observed. For UiO-67-Pt, two additional intermediate states were observed, which behave differently depending on the activation procedure. Theoretical calculation of XANES spectra allowed us to suggest the 3D-atomic structures corresponding to each of the pure spectra determined by MCR-ALS. In addition, reaction enthalpies for different possible reaction routes were calculated within a density functional theory approach. Based on the experimental and theoretical results showed that Pd nanoparticles (NPs) tend to be formed in UiO-67-Pd samples irrespective of the activation procedure, while either Pt NPs or isolated PtII active sites, grafted in the MOF framework may be formed in UiO-67-Pt samples depending on the activation temperature and atmosphere.

Growth Behavior of Gold Nanorods Synthesized by the Seed-Mediated Method: Tracking of Reaction Progress by Time-Resolved X-ray Absorption Near-Edge Structure, Small-Angle X-ray Scattering, and Ultraviolet–Visible Spectroscopy

Gold nanorods (GNRs) are attractive and important nanomaterials that show enormous promise for a wide range of applications. To investigate the formation process of GNRs generated by the seed-mediated method, we tracked the growth of GNRs with aspect ratios of 2 (GNR2s), 4 (GNR4s), and 6 (GNR6s) using time-resolved X-ray absorption near-edge structure (XANES). Moreover, for GNR6s, additional measurements by small-angle X-ray scattering and ultraviolet–visible spectroscopy were carried out for longer reaction times of up to 20 000 s. Cetyltrimethylammonium bromide (CTAB) was used as the surfactant in the generation of GNR2s and GNR4s, whereas benzyldimethylhexadecylammonium chloride (BDAC) and CTAB were used for GNR6s. The three analysis methods used provided consistent and compensatory results. It was found that GNR2s and GNR4s finish growing by 2000 s, consuming all the Au in the solutions, and that GNR6s keeps growing for more than 20 000 s by a different formation process. From these comprehensive resu...

Polarized XANES Monitors Femtosecond Structural Evolution of Photoexcited Vitamin B12.

Ultrafast, polarization-selective time-resolved X-ray absorption near-edge structure (XANES) was used to characterize the photochemistry of vitamin B12, cyanocobalamin (CNCbl), in solution. Cobalamins are important biological cofactors involved in methyl transfer, radical rearrangement, and light-activated gene regulation, while also holding promise as light-activated agents for spatiotemporal controlled delivery of therapeutics. We introduce polarized femtosecond XANES, combined with UV-visible spectroscopy, to reveal sequential structural evolution of CNCbl in the excited electronic state. Femtosecond polarized XANES provides the crucial structural dynamics link between computed potential energy surfaces and optical transient absorption spectroscopy. Polarization selectivity can be used to uniquely identify electronic contributions and structural changes, even in isotropic samples when well-defined electronic transitions are excited. Our XANES measurements reveal that the structural changes upon photoexcitation occur mainly in the axial direction, where elongation of the axial Co-CN bond and Co-NIm bond on a 110 fs time scale is followed by corrin ring relaxation on a 260 fs time scale. These observations expose features of the potential energy surfaces controlling cobalamin reactivity and deactivation.

Ce K edge XAS of ceria-based redox materials under realistic conditions for the two-step solar thermochemical dissociation of water and/or CO2.

X-ray absorption spectroscopy was used to characterise ceria-based materials under realistic conditions present in a reactor for solar thermochemical two-step water and carbon dioxide splitting. A setup suitable for in situ measurements in transmission mode at the cerium K edge from room temperature up to 1773 K is presented. Time-resolved X-ray absorption near-edge structure (XANES) data, collected for a 10 mol% hafnium-doped ceria sample (Ce0.9Hf0.1O2-δ) during reduction at 1773 K in a flow of inert gas and during re-oxidation by CO2 at 1073 K, enables the quantitative determination of the non-stoichiometry δ of the fluorite-type structure. XANES analysis suggests the formation of the hexagonal Ce2O3 phase upon reduction in 2% hydrogen/helium at 1773 K. We discuss the experimental limitations and possibilities of high-temperature in situ XAS at edges of lower energy as well as the importance of the technique for understanding and improving the properties of ceria-based oxygen storage materials for thermochemical solar energy conversion.

Rate Enhancements in Structural Transformations of Pt–Co and Pt–Ni Bimetallic Cathode Catalysts in Polymer Electrolyte Fuel Cells Studied by in Situ Time-Resolved X-ray Absorption Fine Structure

In situ time-resolved X-ray absorption fine structure spectra of Pt/C, Pt3Co/C, and Pt3Ni/C cathode electrocatalysts in membrane electrode assemblies (catalyst loading: 0.5 mgmetal cm–2) were successfully measured every 100 ms for a voltage cycling process between 0.4 and 1.0 V. Systematic analysis of in situ time-resolved X-ray absorption near-edge structure and extended X-ray absorption fine structure spectra in the molecular scale revealed the structural kinetics of the Pt and Pt3M (M = Co, Ni) bimetallic cathode catalysts under polymer electrolyte fuel cell operating conditions, and the rate constants of Pt charging, Pt–O bond formation/breaking, and Pt–Pt bond breaking/re-formation relevant to the fuel cell performances were successfully determined. The addition of the 3d transition metals to Pt reduced the Pt oxidation state and significantly enhanced the reaction rates of Pt discharging, Pt–O bond breaking, and Pt–Pt bond re-forming in the reductive process from 1.0 to 0.4 V.

Direct Observation of 4f Intrashell Excitation in Luminescent Eu Complexes by Time-Resolved X-ray Absorption Near Edge Spectroscopy

We report time-resolved X-ray absorption near edge structure (TR-XANES) measurements at the Eu L3 edge upon photoexcitation of several Eu(III)-based luminescent lanthanide complexes. We find an unambiguous signature of the 4f intrashell excitation that occurs upon energy transfer from the photoactive organic antennas to the lanthanide species. Phenomenologically, this observation provides the basis for direct investigation of a crucial step in the energy transfer pathways that lead to sensitized luminescence in lanthanide-based dyes. Interestingly, the details of the TR-XANES feature suggest that the degree of 4f-5d hybridization may itself vary depending on the excited state of the Eu(III) ion.

Phase transition kinetics of LiNi0.5Mn1.5O4 electrodes studied by in situ X-ray absorption near-edge structure and X-ray diffraction analysis

In situ time-resolved X-ray absorption near-edge structure (XANES) and X-ray diffraction (XRD) measurements are applied to track the phase transition processes of LixNi0.5Mn1.5O4, which is one of the most promising positive electrode materials for lithium ion batteries with its high redox potential of 4.7 V vs. Li/Li+ and good rate capability. Thanks to the high time resolution brought by a strong synchrotron X-ray beam, the XANES and XRD measurements separately capture the three phases involved in nearly the same potential region, namely LiNi0.5Mn1.5O4 (Li1), Li1/2Ni0.5Mn1.5O4 (Li1/2) and Ni0.5Mn1.5O4 (Li0), and the phase transition kinetics under fast charging–discharging conditions is elucidated. The detailed kinetic analysis shows that the phase transitions are apparently expressed by the first-order kinetics and that the transition between the Li1 and Li1/2 phases is faster than that between the Li1/2 and Li0 phases, which leads to kinetically asymmetric behavior of the LiNi0.5Mn1.5O4 electrode. The obtained rate constants can be used to characterize and optimize the rate capability of the electrode.

In Situ Time-Resolved X-ray Near-Edge Absorption Spectroscopy of Selenite Reduction by Siderite

The reduction-oxidation reaction between aqueous selenite (SeO(3)(2-)) and siderite (FeCO(3(s))) was monitored by in situ, time-resolved X-ray absorption near-edge structure (XANES) spectroscopy at the selenium K edge in a controlled electrochemical environment. Spectral evolutions showed that more than 60% of selenite was reduced at the siderite surface after 20 h of experiment, at which time the reaction was still incomplete. Fitting of XANES spectra by linear combination of reference spectra showed that selenite reaction with siderite is essentially a two-step process, selenite ions being immobilized on siderite surface prior to their reduction. A kinetic model of the reduction step is proposed, allowing to identify the specific contribution of surface reduction. These results have strong implications for the retention of selenite by corrosion products in nuclear waste repositories and in a larger extent for the fate of selenium in the environment.

Time-Resolved XANES Studies on Used Silica Supported Cobalt Catalysts

In order to investigate the phases of used Co/SiO 2 catalysts, one of the most commonly used catalysts for Fischer-Tropsch synthesis, time-resolved X-ray absorption near edge structure (TR-XANES) technique was introduced. The catalysts were prepared by incipient wetness impregnation method with %Co loading of 15% and 20% and used for Fischer-Tropsch synthesis at the reaction temperature of 190 o C and the pressure of 10 and 20 bar, called 15%Co/Aerosil_wi_used_10_bar, 15%Co/Aerosil_wi_used_20_bar, and 20%Co/Aerosil_wi_used_20_bar. TR-XANES showed the edge energy of 7721 eV for all used catalysts and by looking at the feature of their spectra, the results implied that the major phase was CoO. To further investigate their ability of being oxidized at elevated temperatures, the catalysts were oxidized by heating from ambient to 450 o C with the heating rate of 8 o C/min at the pressure of 1 bar with the O 2 :N 2 flow rate of 70:30 ml/min. Once reaching 450 o C, the temperature was held at 450 o C for 90 min before cooling down to room temperature. During heating, holding, and cooling, the catalyst properties were measured by TR-XANES. When all catalysts heating up from 300 to 400 o C, the edge energy of 15%Co/Aerosil_wi_used_10_bar, 15%Co/Aerosil_wi_used_20_bar, and 20%Co/Aerosil_wi_used_20_bar at 7719, 7717, and 7718 eV, respectively, showed the mixed phases of Co 3 O 4 and CoO. There were no significant changes in phase while holding the temperature at 450 o C. Once cooling from 450 o C to room temperature, the edge energy of 15%Co/Aerosil_wi_used_10_bar at 7724 eV showed the main Co 3 O 4 phase, the ones of 15%Co/Aerosil_wi_used_20_bar and 20%Co/Aerosil_wi_used_20_bar at 7717 and 7718 eV showed the mixed phase of Co 3 O 4 and CoO. All results would be confirmed by further studies on temperature programmed oxidation (TPO).

Insight into Copper‐Based Catalysts: Microwave‐Assisted Morphosynthesis, In Situ Reduction Studies, and Dehydrogenation of Ethanol

Herein, we report results of three distinct controlled CuO morphologies, urchin-like (CuO-UC), fiber-like (CuO-FB) and nanorods (CuO-NR), prepared by MWA method. These well-designed structures were then dispersed in silica (SiO2) and reduced in a H2 flow that produced the following catalysts: CuO1� xUC, CuO1� xFB, and CuO1� xNR (x = 0, 1, and/or 2); their subsequent activity for the dehydrogenation of ethanol is also discussed herein. The activity and selectivity of these catalysts depend on the physical and chemical structure of the active components, which are largely influenced by the catalyst preparation method. [13] In particular, the use of small-sized CuO crystals produced a highly active and selective catalyst for the dehydrogenation of ethanol into acetaldehydes with high turnover frequencies (TOF). The structural characterization of catalysts was carried out by using several ex situ techniques; highresolution field-emission scanning electron microscopy (FESEM), transmission electron microscopy (TEM), X-ray diffraction (XRD), conventional temperature-programmed reduction under H2 (TPR-H2), X-ray photoelectron spectroscopy (XPS), Brunauer–Emmett–Teller (BET) method for surface area determination, and in situ time-resolved X-ray absorption near-edge structure (XANES) spectroscopy at the Cu K-edge during TPR under H2 experiments (XANES–TPR-H2). Complete experimental details are provided in the Supporting Information. In addition, the Cu metal surface after the reduction treatment of the samples was subjected to TPR measurements with N2O oxidation. [14] FESEM images of CuO assemblies with different morphologies obtained by efficient MWA synthesis are shown in Figure 1 . Our results show that both the solvent and the base influence the final shape of crystals. By using copper(II) chloride dihydrate in deionized water and ammonium hydroxide as the base, CuO-UC structures were obtained with an average size of 3 mm in diameter and nanostructured spines (10 nm of

In Situ X-ray Absorption Near-Edge Structure Spectroscopy of ZnO Nanowire Growth During Chemical Bath Deposition

Chemical bath deposition (CBD) offers a simple and inexpensive route to deposit semiconductor nanostructures, but lack of fundamental understanding and control of the underlying chemistry has limited its versatility. Here we report the first use of in situ X-ray absorption spectroscopy during CBD, enabling detailed investigation of both reaction mechanisms and kinetics of ZnO nanowire growth from zinc nitrate and hexamethylenetetramine (HMTA) precursors. Time-resolved X-ray absorption near-edge structure (XANES) spectra were used to quantify Zn(II) speciation in both solution and solid phases. ZnO crystallizes directly from [Zn(H2O)6]2+ without long-lived intermediates. Using ZnO nanowire deposition as an example, this study establishes in situ XANES spectroscopy as an excellent quantitative tool to understand CBD of nanomaterials.

In Situ Spectroscopic Investigation of Low-Temperature Oxidation of Methane over Alumina-Supported Platinum during Periodic Operation

Methane oxidation over Pt/Al2O3 at transient inlet-gas conditions was studied in situ using synchronous energy dispersive X-ray absorption spectroscopy, Fourier transform infrared spectroscopy, and mass spectrometry. The employed combination of experimental techniques allows for simultaneous analysis of the electronic state of platinum, surface coverage of reaction intermediates/products, and catalytic activity/selectivity, respectively. By cycling of the feed gas composition between net-oxidizing and net-reducing conditions, the activity for methane oxidation can be increased as compared to continuous net-oxidizing conditions. Using the white-line area of time-resolved X-ray absorption near-edge structure spectra, a quantitative estimation of the surface O/Pt ratio indicates the formation of an inhomogeneous surface oxide on the platinum crystallites during reaction. The obtained temporary high activity can be explained through Langmuir-Hinshelwood kinetics and may result either from the formation of a partially oxidized platinum surface that is more effective for methane dissociation or, more likely, from a period with more reactive chemisorbed oxygen prior to oxide formation.

Transient electronic structure of the photoinduced phase ofPr0.7Ca0.3MnO3probed with soft x-ray pulses

We use time-resolved x-ray absorption near-edge structure spectroscopy to investigate the electronic dynamics associated with the photoinduced insulator-to-metal phase transition in the colossal magnetoresistive manganite Pr{sub 0.7}Ca{sub 0.3}MnO{sub 3}. Absorption changes at the O K and Mn L edges directly monitor the evolution of the density of unoccupied states in the transient photoinduced phase. We show that the electronic structure of the photoinduced phase is remarkably similar to that of the ferromagnetic metallic phase reached in related manganites upon cooling below the Curie temperature.

Solving the structure of reaction intermediates by time-resolved synchrotron x-ray absorption spectroscopy

We present a robust data analysis method of time-resolved x-ray absorption spectroscopy experiments suitable for chemical speciation and structure determination of reaction intermediates. Chemical speciation is done by principal component analysis (PCA) of the time-resolved x-ray absorption near-edge structure data. Structural analysis of intermediate phases is done by theoretical modeling of their extended x-ray absorption fine-structure data isolated by PCA. The method is demonstrated using reduction and reoxidation of Cu-doped ceria catalysts where we detected reaction intermediates and measured fine details of the reaction kinetics. This approach can be directly adapted to many time-resolved x-ray spectroscopy experiments where new rapid throughput data collection and analysis methods are needed.

Electronics and sample cell for the collection of time-resolved X-ray absorption spectra

We have recently obtained time-resolved X-ray absorption near-edge structure (XANES) spectra of iron in the protein myoglobin during recombination with CO proceeding laser photolysis, with a temporal resolution of hundreds of microseconds. In order to obtain this data, a computer interface and flow cell had to be developed. These will be described and the initial data collected with this equipment will be presented.