Iron oxides represent a substantial fraction of secondary minerals and particularly affect the reactive properties of natural systems in which they formed, e.g. in soils and sediments. Yet, it is still obscure how transient conditions in the solution will affect the properties of in situ precipitated Fe oxides. Transient compositions, i.e. compositions that change with time, arise due to predominant non-equilibrium states in natural systems, e.g. between liquid and solid phases in soils. In this study, we characterize Fe-OM co-precipitates that formed in pH-neutral exfiltrates from anoxic topsoils under transient conditions. We applied soil column outflow experiments, in which Fe 2+ was discharged with the effluent from anoxic soil and subsequently oxidized in the effluent due to contact with air. Our study features three novel aspects being unconsidered so far: 3

Structures of colloidal compounds in soil, including organo–mineral and mineral–mineral associations, are considered as composite building units (CBUs) that may combine into soil microaggregates. Despite the ubiquitous occurrence of CBUs, the major formation mechanisms are rather obscure and little is known about whether they form primarily during weathering of the parent rocks or by aggregation processes from the soil suspension. We studied the formation of CBUs from suspensions composed of minerals and organic matter typical for temperate soils (i.e. quartz, goethite, illite and extracellular polymeric substances [EPS]). Without EPS, we found CBUs formed as mineral–mineral associations by hetero coagulation of illite and quartz that is bridged by goethite. The presence of EPS, in contrast, led to the formation of a stable suspension of clay‐sized CBUs with no involvement of quartz. We explained this by the rapid formation of organo–mineral CBUs made of EPS‐associated goethite and EPS‐associated illite. The sorption of EPS to goethite screened its surface charge, thereby reducing the electrostatic attraction between goethite and illite. This interaction effectively impeded the formation of mineral–mineral CBUs. Moreover, interactions of EPS with goethite resulted in a marked decrease of the phosphorus/carbon ratio in the suspension. This suggested a preferred adsorption of phosphorus‐containing EPS constituents to goethite and in turn to a compositional fractionation of EPS constituents between the solid and liquid phase as shown by Fourier‐transform infrared spectroscopy with attenuated total reflection (FTIR–ATR). Laser light diffraction measurements revealed a shift from the fine silt fraction to that of the fine sand that also supports the role of EPS as a ‘binding’ agent. Highlights Composite building units (CBUs) form in suspensions with different mineral and organic components. Both, hetero mineral–mineral and organo–mineral CBUs were formed. The initial composition of the suspension controls type and properties of resulting CBUs. Depending on the mineral surfaces, EPS may serve as a separation or binding agent.

Motivated by the possible importance of OBrO in atmospheric photochemistry, multireference configuration interaction calculations of the low-lying excited states were carried out to obtain information about the electronic vertical spectrum up to excitation energies of about 6 eV from the ground state, including the transition dipole moments, and about possible photodissociation pathways, based on one-dimensional cuts through the potential energy surfaces for dissociation into BrO + O and Br + O2, respectively. In addition, for probing the angle dependence the bending potentials were also calculated. From all computed eight doublet states (two/four of each symmetry in C 2 v   C s ) only the 12A2 state at 2.7 eV possesses a large transition dipole moment with the 12B1 ground state, whereas for all other states this quantity is very small or zero. Therefore the 12A2 state should play a decisive role in OBrO photochemistry. Close to the 12A2 state two other states were found at 2.4 eV (12B2) and 2.5 eV (12A1) so that interactions of these three states should certainly influence possible dissociation processes. For this reason, besides direct adiabatic photodissociation of the 12A2 state into BrO + O also predissociation via these close-lying states can be expected, leading to a very complex photodissociation mechanism for excitation energies around 2.5 eV. Moreover, in this energy range photodissociation into Br + O2 is only possible through the 12B2 state (after initial excitation of the 12A2 state) because only for this state a small barrier of 0.7 eV relative to its minimum is estimated from the calculation of a simplified C 2 v minimum energy path. For the 12A1 and 12A2 states, rather large barriers are predicted. The next higher-lying states, with excitation energies of 3.9 eV (22A1) and 4.5 eV (22B2) are well separated from lower- and higher-lying states and from each other, but due to their small transition dipole moments, they should be probably of minor importance for the OBrO photochemistry. The last two states considered in our study are predicted to lie close together at 6.0 eV (22A2) and 6.1 eV (22B1) and are strongly repulsive upon dissociation into BrO + O. Finally, it should be noted that our calculations demonstrate the expected qualitative similarity to the results previously obtained for the corresponding OClO system.

Novel fluorescent nanosensors, based on a naphthyridine receptor, have been developed for the detection of guanosine nucleotides, and both their sensitivity and selectivity to various nucleotides were evaluated. The nanosensors were constructed from polystyrene nanoparticles functionalized by (N-(7-((3-aminophenyl)ethynyl)-1,8-naphthyridin-2-yl)acetamide) via carbodiimide ester activation. We show that this naphthyridine nanosensor binds guanosine nucleotides preferentially over adenine, cytosine, and thymidine nucleotides. Upon interaction with nucleotides, the fluorescence of the nanosensor is gradually quenched yielding Stern-Volmer constants in the range of 2.1 to 35.9 mM(-1). For all the studied quenchers, limits of detection (LOD) and tolerance levels for the nanosensors were also determined. The lowest (3σ) LOD was found for guanosine 3',5'-cyclic monophosphate (cGMP) and it was as low as 150 ng/ml. In addition, we demonstrated that the spatial arrangement of bound analytes on the nanosensors' surfaces is what is responsible for their selectivity to different guanosine nucleotides. We found a correlation between the changes of the fluorescence signal and the number of phosphate groups of a nucleotide. Results of molecular modeling and ζ-potential measurements confirm that the arrangement of analytes on the surface provides for the selectivity of the nanosensors. These fluorescent nanosensors have the potential to be applied in multi-analyte, array-based detection platforms, as well as in multiplexed microfluidic systems.

Site-selective emission spectra of Eu 3+ -doped CeO 2 nanoparticles up to the 5 D 0 − 7 F 5 transition were recorded under cryogenic conditions to identify the local structure around the Eu 3+ dopants in ceria. It is found that pretreatment conditions are crucial for the redistribution of dopants from a broad variety of environments to six well-defined lattice sites. The influence of the dopant and the host structure on the catalytic activity was investigated. A relationship between structure and reactivity is discussed. It is shown that oxygen transport is most efficient in particles with a pronounced amorphous character.