AuroVist™1 is a novel nanotechnology contrast agent based on gold nanoparticles, the first of its kind to be commercially available. Low viscosity, similar to water; easy to inject with minimal trauma, even into small blood vessels.
AuroVist™ is particularly useful for MicroCT animal imaging, such as in vivo vascular casting,3 studies of tumors, stroke, atherosclerosis and other vascular conditions. Visualization of blood vessels by X-rays is best just after injection (Fig 1), but gives good imaging even hours later. High resolution vascular imaging of blood vessels in a live mouse after iv injection of AuroVist™-15. MicroCT section of mouse with two subcutaneous tumors (one one each leg, arrows) showing AuroVist™-15 nm uptake 20 hours after intraveneous injection. The absorption increases by a significant factor (jump ratio) above its L and K edges (Table 1). 80 kVp gives the greatest attenuation, but higher voltages, particularly with filtering can make use of the Au K edge. Here the resolution is increased (to even 2 microns), but the tube power is typically ~100 times less than a clinical unit. Scientists from University of Leeds (UK) and the XMaS CRG beamline have developed a new approach to studying complex immobilised catalysts using a combination of X-ray absorption spectroscopy at both hard and soft X-ray energies and lab-based physical organic chemistry. In situ X-ray absorption spectroscopy confirms catalyst deactivation by loss of chlorine (blue) and increase in potassium (red). Transfer hydrogenation is a ubiquitous class of catalytic reaction in chemical syntheses [1]. XAS studies on this catalyst and its deactivation were carried out at BM28 (XMaS, the UK-CRG). These preliminary results culminated in an in situ experiment employing a spectroscopic flow-cell (see Figure 2), in which the slow loss of catalytic activity over time was directly linked with the loss of chloride ligands from the iridium catalyst. Appropriate re-activation strategies have consequently been developed based on these findings to regenerate the catalyst to its original reactivity through reinstatement of chloride ligands in the catalyst.
Despite numerous studies, the surface structure and magnetism of ferrihydrite are still not clearly understood. Due to the poorly crystalline nature of ferrihydrite, its structure has still not been fully resolved, especially its surface structure and the origin of the disorder. To extend our knowledge of the dependence of the magnetic properties of ferrihydrites on their crystallite sizes and structure, the magnetic properties of four extremely fine-grained (?1–5 nm) ferrihydrite samples have been investigated in the present study.
The crystal structures of the ferrihydrite samples were identified through XRD using a Bruker D8 ADVANCE X-ray diffractometer equipped with a LynxEye detector using Ni-filtered Cu K? radiation (? = 0.15418 nm). Mossbauer spectra are commonly related to the magnetic behavior of Fe in a crystal structure, yielding information about the Fe valence state and coordination.44,45 Mossbauer measurements were performed at 297 K and 20 K in transmission mode using a constant-acceleration spectrometer (OXFORD MS-500, Britain).
A PPMS-9T vibrating sample magnetometer (QUANTUM, USA) was employed for magnetization measurements.
The results of fitting of the XRD data for the studied ferrihydrites (red lines) using the Rietveld method are shown in Fig. Table 1 The structural parameters obtained from PDF fitting for the studied ferrihydrite samples. The ferrihydrite samples also have similar Fe K-edge X-ray absorption near edge structure (XANES) spectra (Fig.
Table 2 Fe K-edge EXAFS structural parameters derived from the fits of the spectra of the ferrihydrite samples. The general conclusion based on the analysis results of the XRD, PDF and X-ray absorption spectra is that the studied samples possess a common long-, medium- and local-range structure.
At 295 K, the Mossbauer spectra of all ferrihydrite samples present a paramagnetic doublet (Fig.
Table 3 Room-temperature Mossbauer parameters of the four synthetic ferrihydrites fitted with one doublet and two doublets.
Using two doublets provides better fits of the spectra (Table 3) and may allow for a distinction between the core and surface regions of the particles.55–57 The range of Cs values is similar to that for one-doublet fitting and the variation in the Cs values is not correlated with the crystallite size of the ferrihydrites. The site populations of doublet 2 (core) are larger than those of doublet 1 (layer), and the difference between them increases with increasing crystallite size, which results from an increasing surface layer contribution with decreasing crystallite size. At 20 K, the Mossbauer spectra of the ferrihydrite samples show magnetic hyperfine splitting due to the weakness or even disappearance of the relaxation effect caused by their superparamagnetic behavior.51,58,59 The degree of splitting is related to the crystallite size (Fig. AuroVist™ enables greatly enhanced X-ray imaging of blood vessels, tumors, and other biological systems and organs. A typical volume for intravenous injection into a mouse (for example into the tail vein), is 0.2 mL. However, it has been documented that nanoparticles exit leaky tumor neovasculature through the "Enhanced Permeability and Retention" (EPR) effect. This strategy was successfully demonstrated with an immobilised hydrogen transfer catalyst, showing that the slow deactivation of this catalyst was due to gradual exchange of a chloride ligand on the Ir catalyst with an alkoxide. Not only do these reactions eliminate the need for specialised hydrogenation equipment when scaled up, they also proceed with excellent selectivity under mild conditions. This was achieved through a novel immobilisation strategy whereby the catalyst is linked to the support through a strongly bound Cp* ligand. A novel immobilised catalyst for transfer hydrogenation which can be recycled up to 30 times.
In contrast to the common practice of focusing on the metal centre, the team employed a holistic approach to investigate the bonding environment around Ir and Cl atoms in the ex situ catalysts (fresh, deactivated and HCl treated). Monitoring XANES (X-ray absorption near-edge spectroscopy) spectra of the catalyst (see Figure 3) led to the observation of a concurrent increase in potassium content over the same timescale with catalyst deactivation, which happened as the chloride ligands were replaced with alkoxides to generate a less active Cp*Ir-trialkoxide complex (see Figure 4). In addition, changes to the reaction conditions can potentially suppress the undesirable ligand exchange of chloride. The XRD, PDF and Fe K-edge XAS data of the ferrihydrite samples are all fitted well with the Michel ferrihydrite model, indicating similar local-, medium- and long-range ordered structures.

In this study, ferrihydrite samples with different crystallite sizes were found to share similar short-, medium- and long-range structures, but with decreasing size, the Fe–Fe coordination numbers decreased and the surface layer disorder increased.
An early structural model by Drits et al.16 suggested that ferrihydrite is a multiphase material.
It has been reported that both 2LFh31 and 6LFh32 display antiferromagnetism as well as weak ferromagnetic-like behavior at low temperature and superparamagnetic behavior, with a low-temperature transition between blocked and unblocked magnetic states. The key factor for such small particles is the large contribution of the surface layer to the particle behavior. The diffractometer was operated at a tube voltage of 40 kV and a tube current of 40 mA with a scanning rate of 1° min?1 and a step size of 0.02°. The measurement was performed using the rapid acquisition PDF method37 by employing a Perkin Elmer amorphous silicon detector. In addition, the coherent scattering domain (CSD) sizes of the ferrihydrite samples were fixed at the values obtained from the XRD fitting by assuming spherical particles.21Rw values, calculated between rmin = 1 A and rmax = 20 A, represent the goodness-of-fit of the ferrihydrite samples.
First, hysteresis loops for the studied ferrihydrite samples were measured at 10, 100 and 300 K, and the maximum applied field was set to 5 T. 1 XRD patterns of the four ferrihydrite samples (blue lines), Rietveld fitting results (red solid lines), and difference between the experimental data and the fit (black solid lines below the data). 1 and Table SI-1.† Small differences between the observed and fitted patterns (grey lines) indicate that the XRD data can be fitted well with the Michel model,21 implying that the long-range bulk structures of 2LFh_1 to 6LFh_4 are very similar.
2 Pair distribution functions [G(r) functions] of the four synthetic ferrihydrite samples.
The MS path parameters were correlated as described by Mikutta.50 The spectra and the fits are compared in Fig.
The Qs values of the ferrihydrite samples obtained with two-doublet fitting slightly decrease (for both sites) with increasing crystallite size.
The site population of doublet 1 is plotted against the crystallite size, wherein a gradually decreasing trend is observed for the site population with crystallite size. AuroVist™-15 nm consists of gold nanoparticles with a core diameter of 15 nm, stabilized with a highly water soluble organic shell.
The gold nanoparticles leak into tumors and are retained there, particularly at the growing edge tumor periphery, and build up over ~10-24 hours.
This led to an observable loss of chloride content and a build-up in potassium content in the catalyst as the reaction progressed.
A slow and partial deactivation of the catalyst was observed over 30 uses, which prompted a mechanistic study into its deactivation in order to further improve the catalyst for application in fine chemical and pharmaceutical syntheses. This led to the crucial identification of a significant amount of potassium cations in the deactivated catalyst, which formed KCl upon treatment with aqueous HCl. Importantly, the study showcased the potential of this new multi-element XAS approach in studying solution based catalysis and in this particular case even with an immobilised catalyst.
PDF and XAS fitting results indicate that, with increasing crystallite size, the average coordination numbers of Fe–Fe and the unit cell parameter c increase, while Fe2 and Fe3 vacancies and the unit cell parameter a decrease. An investigation of the magnetization indicated that magnetization decreases with increasing size. Therefore, the surface layer effects will be most clearly revealed with such particles, and the results should be well suited for elucidating whether and how surface layer defects affect the magnetic properties. The high-resolution XRD data of the ferrihydrite samples were subjected to Rietveld fitting to obtain the crystallize size using the program TOPAS 4.2 (SI-1†). The image plate was exposed for 1 s and the measurement was repeated 120 times for a total collection time of 120 s for each sample. Then, zero-field-cooled (ZFC) and field-cooled (FC) magnetization curves were measured to determine the unblocking temperature (TB), which is sensitive to the grain-size distribution. A G(r) reveals peaks at characteristic atom-pair distances and thus reflects the crystal structure. To quantify structural variations caused by crystallite size changes, the G(r) functions were modeled against the Michel model.21 The Rw values indicate that the simulated G(r) is in good agreement with the experimental data (Fig. The first set of average parameters was obtained by fitting the spectra with one doublet (Fig. Panels (a) and (b) show the results at 295 K and their fits with one Lorentzian doublet (blue curves) or with two Lorentzian doublets (blue curves); the red curves show the two contributions. According to Cornell and Schwertmann,2 relatively high Qs values correspond to a high degree of distortion of FeO6 octahedra.
The shape of the magnetization curves depends on both the measurement temperature and crystallite size. If it is hard to inject, or a large amount of gold accumulates at the point of injection, the vein was missed. The innovative measurements at the ESRF were crucial in identifying the catalyst deactivation pathway. However, the effectiveness of traditional analytical techniques for studying this immobilised catalyst was limited, due to the lack of an NMR active label on the catalyst and interference from the support resin.
Mossbauer results indicate that the surface layer is relatively disordered, which might have been caused by the random distribution of Fe vacancies. Fe1 and Fe2 (60% and 20% of the total Fe sites, respectively) are octahedrally coordinated, while Fe3 (20%) is tetrahedrally coordinated.
Alternatively, magnetization results may provide information on the surface layer structure and assist in resolving the structure dispute. The software Fit2D38 was used to integrate and convert the 2-D raw data to 1-D intensity versus wave vector (Q) data. The ZFC curves were obtained by cooling the sample under a zero field from 300 K to 10 K, and subsequently sweeping the temperature from 10 K to 300 K under a weak applied field of 20 mT.
However, the obtained unit cell parameters (Table SI-1†) are likely not entirely realistic, because there was no strict restriction for the atomic positions and other structural factors during fitting with the present method. In panel (b), the experimental (black blank squares) and calculated (red solid lines) PDFs for the ferrihydrite samples are compared in the r range of 1–20 A; the gray curves are the differences and Rw indicates the goodness of fit.

In addition, this also implies a weaker order of Fe–Fe arrangements for smaller ferrihydrites, whereas the location of the disorder could not be revealed. Panel (c) depicts the spectra at 20 K and their fits with doublets and sextets (blue curves); the red curves show the doublet and sextet contributions.
Therefore, the present results indicate that the degree of distortion of the FeO6 octahedra increases with decreasing crystallite size, similar to the results of one-doublet fitting. These results indicate that the contribution of the surface layer increases with decreasing crystallite size.
Overall, the magnetization curves are nearly linear at 300 K and have a sigmoidal shape at 100 K and 10 K. If the animal moves during collection of this data set, the back projection 3D reconstruction will be errant.
However, direct insights into deactivation of these catalysts, which are critical for maximising their efficiency, are rare. These results support Hiemstra's surface-depletion model in terms of the location of disorder and the variations of Fe2 and Fe3 occupancies with size. In contrast, for ordered, relatively large ferrihydrites, magnetic enhancement with size was found. To achieve our goal, the structure of the four ferrihydrite samples was characterized in detail first by using XRD, the PDF derived from high-energy X-ray total scattering data, and Fe K-edge X-ray absorption spectroscopy (XAS). Fe K-edge extended X-ray absorption fine structure (EXAFS) data were collected over an energy range of 6911–7864 eV in transmission mode using an Ar-filled Lytle detector.
Then, the sample was cooled under 20 mT from 300 K to 10 K to obtain the FC magnetization curves. Panel (c) shows the deconvolution of the total G(r) of 2LFh_1 in its calculated major atomic correlations. In the next section, it will be shown that the analysis of the Mossbauer spectra at 295 K provides further information. As the surface layer becomes more disordered (higher Qs) than the core, the disorder will progressively increase with decreasing crystallite size.
Therefore, the ferrihydrite samples have no definite magnetic-ordering temperature, but they order magnetically at varying temperatures, as determined by their crystallite size. A clear hysteresis loop is observed only at 10 K and none of the samples are saturated in magnetization at 5 T. Panel (b) shows the zero field-cooled (ZFC, black) and field-cooled (FC, red) induced magnetizations obtained with an applied field of 20 mT.
Since gold has a favorable absorption at the K and L energies, one would use ~3 times this for the kVp.
The elaborate nature of the most active catalysts also means that catalyst recovery is important in industrial application.
Magnetic data indicate that the ferrihydrite samples show antiferromagnetism superimposed with a ferromagnetic-like moment at lower temperatures (100 K and 10 K), but ferrihydrite is paramagnetic at room temperature. In principle, the structure of the particles has been analyzed on the basis of the structural model of Michel et al.21 Subsequently, the magnetic properties of ultra-fine nanoparticles were examined through magnetic susceptibility measurements and Mossbauer spectroscopy. The monochromator was calibrated by setting the first inflection point in the absorption spectrum of a Fe foil to 7112.0 eV. Due to the limited CSD size, G(r) shows attenuation with increasing interatomic distance (r), and the attenuation completes as r approaches the dimensions of the scattering body.47,48 From 2LFh_1 to 6LFh_4, the attenuation rate decreases, and the distances over which the attenuation completes (indicated by the arrows in Fig. The combination of this information with the PDF fitting results (a certain amount of Fe vacancies exists in the structure) indicates that the disordered surface structure is derived from the random arrangement of Fe vacancies at or near the surface as well as minor stacking faults. This is consistent with the report of Schwertmann et al.,35 who suggested that a ferrihydrite with a smaller crystallite size (formed at a higher Fe3+ hydrolysis rate) had a lower magnetic ordering temperature. A simple solution is to kill the animal some time after injection and then image, but live imaging has been accomplished if the region can be gated or immobilized during the imaging time. In addition, both the magnetization and coercivity decrease with increasing ferrihydrite crystallite size due to strong surface effects in fine-grained ferrihydrites. The present structure and magnetization results significantly contribute to the understanding of the ferrihydrite structure, its surface reactivity and its magnetic behavior in the environment.
According to this model, dehydration alters only the ferrihydrite surface layers without changing the structure of the mineral core, which is in good agreement with the experimental observation by Xu et al.27 that 2LFh was largely dehydrated to ordered ferrihydrite without significant changes in the mineral structure.
Special attention has been paid to signals that point to surface layer defects and a structure that may correspond to Hiemstra's model.17 The magnetization results are discussed in relation to the structural information.
The observation that the disorder of the surface layer increases with decreasing crystallite size is consistent with the variation of Fe vacancies as a function of size, as obtained from the PDF fitting (Table 1).
Smaller ferrihydrite samples show less magnetic hyperfine splitting and a lower unblocking temperature (TB) than larger samples. The above analysis indicates that the samples vary from two-line ferrihydrite to six-line ferrihydrite.
In other words, crystallites with a smaller size have a higher amount of Fe2 and Fe3 vacancies, which leads to a higher degree of disorder in the surface layer.
The hyperfine splitting and hyperfine magnetic-field intensity decrease with decreasing crystallite size (Table SI-2†).
The dependence of magnetic properties on grain size for nano-sized ferrihydrite provides a practical way to determine the crystallite size of ferrihydrite quantitatively in natural environments or artificial systems.
In addition, from 2LFh_1 to 6LFh_4, the full width at half maximum (FWHM) of the peaks gradually decreases, which implies that the crystallite size gradually increases.

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