Microchimica Acta

 Extra-ordinary mechanical features and high corrosion resistance are essential qualities of CrNi-steels and nickel base alloys. The corrosion resistance of these materials fundamentally depends on their surface conditions. Tarnish colours and welding scale, formed during thermal treatment or joining, are removed by pickling components made of these alloys with sulphuric acid, nitric acid and hydrofluoric acid as well. By this kind of surface refinement several million tons per annum of aggressive waste waters are produced, that cause high environmental risks and not acceptable employee endangerings. Therefore, a project was promoted by the DECHEMA e.V. to replace hydrofluoric acid by organic complexing agents. The related surface analytical examinations are reported in this publication.

The authors describe a dual (colorimetric and fluorometric) detection scheme for microRNA. The method is based on the use of fluorescent DNA-modified silver nanoclusters (DNA-AgNCs) and gold nanoparticles (AuNPs). The DNA-AgNCs were linked to AuNPs via the interaction of ssDNA-AgNC probes with AuNPs. This led to quenching of the fluorescence of the DNA-AgNCs (best measured at excitation/emission peaks of 370/450 nm) and also prevents salt-induced aggregation of the AuNPs. Upon addition of microRNA, the DNA on the AgNCs hybridizes with microRNA. This led to the formation of a DNA-AgNC/miRNA hetero-duplex and increase of the distance to the AuNPs. Quenching was suppressed and fluorescence was restored. The presence of microRNA also affected salt-induced particle aggregation as reflected by a color change from red to purple that is visible with bare eyes. The visual detection limit for microRNA is 0.6 nM, and the fluorometric detection limit is 0.4 pM.

We describe an electrochemical sensor for melamine based on a glassy carbon electrode (GCE) modified with reduced graphene oxide that was decorated with gold nanoparticles (AuNP/rGO). The AuNPs/rGO nanocomposite was synthesized by co-reduction of Au(III) and graphene oxide and characterized by transmission electron microscopy, Raman spectroscopy, X-ray diffraction and X-ray photoelectron spectroscopy. The response of the modified GCE to melamine was investigated by using hexacyanoferrate as an electrochemical reporter. It is found that the electrochemical response to hexacyanoferrate is increasingly suppressed by increasing concentration of melamine. This is attributed to competitive adsorption of melamine at the AuNP/rGO composite through the interaction between the amino groups of melamine and the AuNPs. The presence of rGO, in turn, provides a platform for a more uniform distribution of the AuNPs and enhances the electron transfer rate of the redox reaction. The findings were used to develop a sensitive method for the determination of melamine. Under optimized conditions, the redox peak current of hexacyanoferrate at a working voltage of 171 mV (vs. SCE) is linearly related to the concentration of melamine in 5.0 to 50 nM range. The method was successfully applied to the determination of melamine in food contact materials.

 Fibre/matrix interfaces in δ-Al2O3/Mg8Li and δ-Al2O3/Mg composites have been investigated using XPS and SIMS analysis of extracted δ-Al2O3 fibres in context with previous XRD observations. Results obtained indicate that in MgLi based composites lithium enters preferentially the interfacial redox reactions producing Li+ ions that occupy vacant cation positions in the defect δ-Al2O3 lattice which results in a strong fibre/matrix interfacial bond. On the other hand, in Mg matrix composites the magnesium oxide appears to be the final reaction product that does not enter the solid state reaction with adjacent δ-Al2O3 fibre during the melt infiltration process, so that only relatively weak interfacial bond is created.