Determination of accessible amino groups on surfaces by chemical derivatization with 3,5-bis(trifluoromethyl)phenyl isothiocyanate and XPS/NEXAFS analysis

The determination of amino groups on surfaces capable of binding biomolecules is important for the understanding and optimization of technologically relevant coupling processes. In this study, three different types of amino-functionalized model surfaces, amino thiolate on Au, amino siloxane on Si, and polyethylene (PE) foils and films reacted with 1,2-diaminoethane (DAE) were derivatized with 3,5-bis(trifluoromethyl)phenyl isothiocyanate. Subsequently, these samples were analyzed by chemical derivatization X-ray photoelectron spectroscopy (CD-XPS) and near-edge X-ray absorption fine structure spectroscopy (NEXAFS). The determination of amino groups by this analytical approach allows gaining insight into the availability of groups on surfaces that can actually serve as attachment sites for biomolecules in technical applications. In the case of the amino thiolate on Au, almost 90% of the expected amino groups were detected by CD-XPS. Investigation of the amino siloxane films revealed lower yields for the derivatization reaction in the order of 30%. The lowered reaction yields are thought to be due to interactions between the amino siloxane’s amino and silanol groups or the underlying substrate, making them inaccessible to the derivatization agent. The aminated PE samples are characterized by a complex surface chemistry and structure, and reaction yields of the derivatization reaction cannot be unequivocally derived. However, 1–3% of the total carbon atoms in the surface layer were found to be bound to amino groups accessible to the derivatization agent. It can be concluded that, depending on the detailed character of the investigated amino-terminated surface, the amount of amino groups accessible to CD-XPS can be substantially lower than the total amount of amino groups present at the surface.     

Local structure of amorphous ice as revealed by O K-edge EXAFS.

The oxygen K-edge extended X-ray absorption fine structure (EXAFS) spectrum of an ice film prepared by deposition of water vapor on a substrate at 100 K was measured in the surface-sensitive Auger yield mode. Five distinct peaks are revealed in the Fourier transform spectrum of the EXAFS data. The peaks are attributed to O-H bonds (with overlapping contributions from intramolecular covalent and intermolecular hydrogen bonds) as well as to intermolecular O...O scattering paths in the distance range of 1-7 A. The pattern of the longer O...O distances resembles that of a high-pressure crystalline modification of ice (ice II).     

A survey of the parallel performance and accuracy of Poisson solvers for electronic structure calculations

We present an analysis of different methods to calculate the classical electrostatic Hartree potential created by charge distributions. Our goal is to provide the reader with an estimation on the performance—in terms of both numerical complexity and accuracy—of popular Poisson solvers, and to give an intuitive idea on the way these solvers operate. Highly parallelizable routines have been implemented in a first‐principle simulation code (Octopus ) to be used in our tests, so that reliable conclusions about the capability of methods to tackle large systems in cluster computing can be obtained from our work. © 2013 Wiley Periodicals, Inc.