Hartwig Modrow

Synthesis and Characterization

As compared to bulk materials, magnetic nanoparticles possess distinct magnetic properties and attempts have been made to exploit their beneficial properties for technical and biomedical applications, e.g. for magnetic fluids, high-density magnetic recording, or biomedical diagnosis and therapy. Early magnetic fluids (MFs) were produced by grinding magnetite with heptane or long chain hydrocarbon and a grinding agent, e.g. oleic acid [152]. Later procedures for MFs precipitated Fe 3+/Fe 2+ of an aqueous solution with a base, coated the particles by oleic acid, and dispersed them in carrier liquid [161]. However, besides the elemental composition and crystal structure of the applied magnetic particles, particle size and particle size distribution determine the properties of the resulting MF. Many methods for nanoparticle synthesis including the preparation of metallic magnetic particles have been described in the literature. However, there still remain important questions, e.g. concerning control of particle size, shape, and monodispersity as well as their stability towards oxidation. Moreover, peptization by suitable surfactants or polymers into stable MFs is an important issue since each application in engineering or biomedicine needs special MFs with properties adjusted to the requirements of the system.

Quantitative speciation of sulfur in bacterial sulfur globules: X-ray absorption spectroscopy reveals at least three different species of sulfur.

X-ray absorption near edge structure (XANES) spectroscopy at the sulfur K-edge was applied to probe the speciation of sulfur of metabolically different sulfur-accumulating bacteria in situ. Fitting the spectra using a least-square fitting routine XANES reveals at least three different forms of sulfur in bacterial sulfur globules. Cyclooctasulfur dominates in the sulfur globules of Beggiatoa alba and the very recently described giant bacterium Thiomargarita namibiensis. A second type of sulfur globules is present in Acidithiobacillus ferrooxidans: here the sulfur occurs as polythionates. In contrast, in purple and green sulfur bacteria the sulfur mainly consists of sulfur chains, irrespective of whether it is accumulated in globules inside or outside the cells. These results indicate that the speciation of sulfur in the sulfur globules reflects the different ecological and physiological properties of different metabolic groups of bacteria.

A size-selective synthesis of air stable colloidal magnetic cobalt nanoparticles

A novel, size-selective preparation route leads to air stable ‘monodisperse’ colloidal cobalt nanoparticles via the thermolysis of Co2(CO)8 in the presence of aluminum alkyls. X-ray absorption near edge structure measurements have proved that this preparation pathway provides long term stable zerovalent magnetic Cobalt particles. In addition, these measurements show that the chemical nature of the surfactant used exerts a significant influence on the stability and the local electronic and geometric structure of the analyzed nanoparticles.

Interaction between core and protection shell of N(butyl)4Cl- and N(octyl)4Cl-stabilized Pd colloids

Abstract We present X-ray absorption near edge structure (XANES) measurements on N(butyl)4Cl- and N(octyl)4Cl-stabilized Pd colloids at both the Pd LIII- and the Cl K-edge. Metastable impact electron spectroscopy (MIES) and ultraviolet photoelectron spectroscopy (HeI) results for these colloids, deposited on silica substrates, are also shown. The results provide detailed insight into the mechanism of bonding between the protection shell and the colloidal core. Both XANES and MIES suggest that the chlorine is present on the inside of the protection shell, located between palladium core and N(alkyl)4 groups forming the protection shell. Moreover, the XANES results suggest a dependence of the equilibrium position of the chlorine between the metal core and the alkyl chain on the length of the alkyl chains. The possible motivation for such an effect is discussed on the basis of different models. MIES, in addition, provides information on the thermal stability of the shell-stabilized Pd colloids.

Tuning Nanoparticle Properties—The X‐ray Absorption Spectroscopic Point of View

Abstract A key requirement for the realization of nanotechnologys technological potential is the ability to produce nanoparticles with well defined, tailored properties in a cheap and reproducible way. For numerous materials, wet‐chemical approaches with scale‐up potential exist which allow to synthesize stabilized particles of a given size, but it turns out that control of these two parameters is not sufficient to control particle properties completely. This suggests that the exact way of stabilization and the course of the synthesis may exert notable influence on particle properties. This work reviews experimental evidence for such influence with a focus on the perspective of x‐ray absorption spectroscopy. The thus obtained results indicate that not only size‐dependent phase transitions, but also chemical interaction between the core of the nanoparticle and its surfactant molecules are responsible for the observed spectral changes, which can be explained when constructing detailed models of core‐surfactant interaction. Also, it is observed that the surfactant can influence the course of the synthesis of a given nanoparticle notably. All of these results suggest a possibility to refine the finetuning of particle properties by exerting influence on synthesis and surface‐matrix interaction by suitable choice of surfactant molecules.

Surface engineering of Co and FeCo nanoparticles for biomedical application

Monodisperse Co, Fe, and FeCo nanoparticles are prepared via thermal decomposition of metal carbonyls in the presence of aluminium alkyls, yielding air-stable magnetic metal nanoparticles after surface passivation. The particles are characterized by electron microscopy (SEM, TEM, ESI), electron spectroscopy (MIES, UPS, and XPS) and x-ray absorption spectroscopy (EXAFS). The particles are peptized by surfactants to form stable magnetic fluids in various organic media and water, exhibiting a high volume concentration and a high saturation magnetization. In view of potential biomedical applications of the particles, several procedures for surface modification are presented, including peptization by functional organic molecules, silanization, and in situ polymerization.

X-ray absorption spectroscopy and its application in biological, agricultural and environmental research

X-ray absorption spectroscopy is aspectroscopic in situ technique whichcombines the high penetration strength inherentto X-rays with the advantages of local probetechniques, such as no need for long rangeorder and the ability to obtain information onselected sites of a given sample only.Consequently, this technique is applicable to abroad variety of scientific questions,including many applications in biological,agricultural and environmental sciences. Thefirst part of this review provides anintroduction to the method, whose applicationto a broad variety of problems is discussed indetail, especially XAS of sulfur in biologicalsystems. In the second part new ideas forfurther experiments using this versatile methodare presented.

Air-stable Co-, Fe-, and Fe/Co-nanoparticles and ferrofluids

Summary Air-stable Co, Fe, and Fe/Co nanoparticles are accessible by thermolysis of the metal carbonyl precursors in the presence of aluminium alkyls and subsequent “smooth oxidation”. The structure of the particles was investigated by transmission electron microscopy (TEM, HRTEM), X-ray absorption spectroscopy (XAS), X-ray and ultraviolet photoelectron spectroscopy (XPS, UPS), metastable impact electron spectroscopy (MIES), and small-angle neutron scattering (SANS). The peptization of the nanoparticles with suitable surfactants (oleic and lauric acid, sodium dioctylsulfosuccinate (AOT), LP-4 (a fatty acid condensation polymer), and KorantinSH (N-oleyl sarcosine)) yields magnetic fluids dispersed in carrier liquids such as toluene, kerosene, vacuum and mineral oils which are remarkably stable in air under ambient conditions. The resulting magnetic fluids show good magnetic properties. Several methods for the preparation of water-based MF are presented, e.g., formation of surfactant bilayers, using phase transfer agents, or surface modification with L-cysteine ethyl ester. Water-based metallic magnetic fluids have a high potential for a number of technical and biomedical applications. Technical applications of the Co-based ferrofluids in the field of positioning systems and magnetohydrostatic bearings were investigated. The results emphasize the scope of nanoparticulate ferrofluids having a metallic core.

X-ray absorption near edge spectroscopy investigations of valency and lattice occupation site of Fe in highly iron-doped lithium niobate crystals

The oxidation state of Fe in highly doped lithium niobate crystals (2 wt% Fe2O3 )i n the as-grown, reduced, and oxidized states is determined by the combination of differentiation and integration methods applied to their Fe Kedge XANES (x-ray absorption near edge spectroscopy) spectra. The obtained valences are confirmed further by absorption measurements in the IR/vis (infrared and visible) spectral range. It is shown that reduction and thermoelectric oxidization of as-grown Fe:LiNbO3 ,r espectively, leads to a noticeable lowering of the number of Fe 3+ and t oc omplete oxidization of all Fe 2+ to Fe 3+ ,r espectively. It is found that Fe in highly iron-doped lithium niobate samples (0.5, 2, and 4 wt% Fe2O3 )i s incorporated onto the Li site comparing the experimental XANES spectra to the FEFF8 calculations. The difference of the XANES spectrum of the 4 wt% Fe2O3 doped sample as compared to those of the 0.5 and 2 wt% doped samples is explained by a co-phase formation in the former one. The results obtained can help to tailor such crystals for non-linear optical applications as well as for photorefraction.

Thermal decomposition of (NH4)2[PdCl6] studied by in situ X-ray absorption spectroscopy

In situ X-ray absorption near edge structure (XANES) investigations were carried out at chlorine K edge and palladium L3 edge to study the mechanism of the thermal decomposition of ammonium hexachloropalladate. The spectra show a characteristic feature for the initial step that might be explained as the formation of the precursor via ligand exchange (Cl --> NH3). Multiple scattering calculations (Feff 8) for the Cl K, Pd and Rh L3 edges were successful in simulating the XANES spectra of the precursor as well as the reference compounds (NH4)2 [PdCl4] and (NH4)3[RhCl6].