Chemistry

Controlling and steering the growth of single walled carbon nanotubes is often believed to require controlling of the nucleation stage. Yet, little is known about the microscopic mechanisms governing the nucleation from hydrocarbon molecules. Specifically, we address here the dehydrogenation of hydrocarbon molecules and the formation of all-carbon graphitic islands on metallic nanoclusters from hydrocarbon molecules under conditions typical for carbon nanotube growth. Employing reactive molecular dynamics simulations, we demonstrate for the first time that the formation of a graphitic network occurs through the intermediate formation of vertically oriented, not fully dehydrogenated graphitic islands. Upon dehydrogenation of these vertical graphenes, the islands curve over the surface, thereby forming a carbon network covering the nanoparticle. The results indicate that controlling the extent of dehydrogenation offers an additional parameter to control the nucleation of carbon nanotubes.

The performance of the Ion-Selective Electrode (ISE) for in vitro dissolution testing using biorelevant media was evaluated in this study. In vitro dissolution was carried out using USP apparatus 2 (paddle method) with classical and with updated biorelevant media to simulate the pre-and postprandial states. The ISE was used as an analytical stand-alone system and in combination with a single-point HPLC-UV measurement. A modified method enabling the use of the ISE for very poorly soluble substances is also proposed.

The dissolution of formulations of the drugs dapoxetine, paliperidone, cinnarizine, tetrazepam, mebeverine, loperamide, galantamine and ibuprofen was studied by an in-line potentiometric measurement system. The transpose of a Nikolskii-Eisenman type function performed the conversion of potential to percentage of dissolution. A novel gradient membrane electrode was developed especially for dissolution, varying continuously in composition from an ionically conducting rubber phase to an electronically conducting solid state PVC/graphite composite. The gradient part had a thickness of 200 m. The electrodes life span exceeded 6 months. An ion exchange procedure was used to prepare them for one specific drug. This enabled us to use one universal electrode built to measure a wide array of drugs. The system parameters such as accuracy, reproducibility and linearity were presented with the data obtained for the drug dapoxetine. In dissolution, accurate measurements were possible from 10 −9 to 10 −3 M concentrations, for high log P drugs. The effect of t 90 response times on the measurement error was estimated. The t 90 response times of the electrodes were concentration dependent, and varied between 50 and 10 s for, respectively, 10 −6 and 10 −3 M concentrations. Potential drift was studied in detail. The measurements performed with these electrodes showed an accuracy of 1%, and inter-and intra electrode variabilities of 0.6 and 1.7%, respectively. The electrodes were successfully applied in colloidal media containing suspended matter, typically formed during dissolution of tablets. The advantages and pitfalls of potentiometry over the presently used techniques for dissolution testing are discussed. (L.J. Nagels).

This review covers the penetration of polymerbased electrode coatings for amperometric detection in continuous £ow systems of analysis. Redox hydrogels, loaded ionomers, and conductive electroactive polymers form the basis of such materials. When used as such, they can improve the detectability of electroactive substances with slow kinetics on classical electrode materials. Conductive electroactive polymers can detect electroinactive ions amperometrically, via an indirect mechanism. When combined with immobilized enzymes, the above materials can detect otherwise electroinactive substances in LC and FIA methods.

Potentiometric detection of clenbuterol, ambroxol and bromhexine in marketed pharmaceuticals was described in six isocratic HPLC systems. The podant-and macrocyclic-type neutral ionophores, N ,N ,N ?,N ?-tetracyclohexyl-oxybis(ophenyleneoxy)diacetamide (TOPA) and hexakis(2,3,6-tri-O -octyl)-a-cyclodextrin (OCD), were applied in poly(vinyl)chloride (PVC)-based liquid membrane electrodes. Both types of neutral ionophores improve the sensitivity for all mentioned drugs when compared with a tetrakis(p -chlorophenyl)borate (BOR)-based electrode as well as with single wavelength UV detection. Detection limits (S /N 0/3) of 2.6 )/10 (10 mol l (1 (injected concentration) for the highly hydrophobic bromhexine were achieved with the TOPA-based electrode and a cyano reversed-phase (RP)-HPLC with Uptisphere † UP5CN-25QS silica column (250 )/4.6 mm i.d.) eluted with acetonitrile (AcN) Á/ethanol Á/perchloric acid (1.66 mM) (60:2:38, v/v/v) (pH* 2.45). Comparable result was obtained with OCD-based electrodes and an XTerra TM RP18 hybrid silica Á/polymer column eluted with AcN Á/phosphoric acid (20 mM) (25:75, v/v) (pH* 2.60). In the mobile phases containing 60 Á/75% v/v AcN or methanol, stable and reproducible response of both types of neutral ionophorebased electrodes was observed for at least 3 days. The results of the validated procedure for reliable simultaneous determination of the drugs in fortified representative samples of pharmaceuticals were also presented. #

The application of atmospheric pressure plasmas in medicine is increasingly gaining attention in recent years, although very little is currently known about the plasma-induced processes occurring on the surface of living organisms. It is known that most bio-organisms, including bacteria, are coated by a liquid film surrounding them, and there might be many interactions between plasma species and the liquid layer before the plasma species reach the surface of the bio-organisms. Therefore, it is essential to study the behaviour of the reactive species in a liquid film, in order to determine whether these species can travel through this layer and reach the biomolecules, or whether new species are formed along the way. In this work, we investigate the interaction of reactive oxygen species (i.e. O, OH, HO 2 and H 2 O 2 ) with water, which is assumed as a simple model system for the liquid layer surrounding biomolecules. Our computational investigations show that OH, HO 2 and H 2 O 2 can travel deep into the liquid layer and are hence in principle able to reach the bio-organism. Furthermore, O, OH and HO 2 radicals react with water molecules through hydrogen-abstraction reactions, whereas no H-abstraction reaction takes place in the case of H 2 O 2 . This study is important to gain insight into the fundamental operating mechanisms in plasma medicine, in general, and the interaction mechanisms of plasma species with a liquid film, in particular.

In this paper, an overview is given of modeling investigations carried out in our research group for a better understanding of plasmas used for medical, environmental and nano applications. The focus is both on modeling the plasma chemistry and the plasma-surface interactions. The plasma chemistry provides the densities and fluxes of the important plasma species. This information can be used as input when modeling the plasma-surface interactions. The combination of plasma simulations and plasma -surface interaction simulations provides a more comprehensive understanding of the underlying processes for these applications.

Nonthermal atmospheric pressure plasmas are gaining increasing attention for biomedical applications. However, very little fundamental information on the interaction mechanisms between the plasma species and biological cells is currently available. We investigate the interaction of important plasma species, such as OH, H 2 O 2 , O, O 3 , as well as O 2 and H 2 O, with bacterial peptidoglycan by means of reactive molecular dynamics simulations, aiming for a better understanding of plasma disinfection. Our results show that OH, O, O 3 , and H 2 O 2 can break structurally important bonds of peptidoglycan (i.e., C−O, C−N, or C−C bonds), which consequently leads to the destruction of the bacterial cell wall. The mechanisms behind these breakups are, however, dependent on the impinging plasma species, and this also determines the effectiveness of the cell wall destruction.

Plasma medicine is a rapidly evolving multidisciplinary field at the intersection of chemistry, biochemistry, physics, biology, medicine and bioengineering. It holds great potential in medical, health care, dentistry, surgical, food treatment and other applications. This multidisciplinary nature and variety of possible applications come along with an inherent and intrinsic complexity. Advancing plasma medicine to the stage that it becomes an everyday tool in its respective fields requires a fundamental understanding of the basic processes, which is lacking so far. However, some major advances have already been made through detailed experiments over the last 15 years. Complementary, computer simulations may provide insight that is difficult-if not impossible-to obtain through experiments. In this review, we aim to provide an overview of the various simulations that have been carried out in the context of plasma medicine so far, or that are relevant for plasma medicine. We focus our attention mostly on atomistic simulations dealing with plasma-biomolecule interactions. We also provide a perspective and tentative list of opportunities for future modelling studies that are likely to further advance the field.

The application of nonthermal atmospheric pressure plasma is emerging as an alternative and efficient technique for the inactivation of bacterial biofilms. In this study, reactive molecular dynamics simulations were used to examine the reaction mechanisms of hydroxyl radicals, as key reactive oxygen plasma species in biological systems, with several organic molecules (i.e., alkane, alcohol, carboxylic acid, and amine), as prototypical components of biomolecules in the biofilm. Our results demonstrate that organic molecules containing hydroxyl and carboxyl groups may act as trapping agents for the OH radicals. Moreover, the impact of OH radicals on N-acetyl-glucosamine, as constituent component of staphylococcus epidermidis biofilms, was investigated. The results show how impacts of OH radicals lead to hydrogen abstraction and subsequent molecular damage. This study thus provides new data on the reaction mechanisms of plasma species, and particularly the OH radicals, with fundamental components of bacterial biofilms.

While plasma treatment of skin diseases and wound healing has been proven highly effective, the underlying mechanisms, and more generally the effect of plasma radicals on skin tissue, are not yet completely understood. In this paper, we perform ReaxFF-based reactive molecular dynamics simulations to investigate the interaction of plasma generated OH radicals with a model system composed of free fatty acids, ceramides, and cholesterol molecules. This model system is an approximation of the upper layer of the skin (stratum corneum). All interaction mechanisms observed in our simulations are initiated by H-abstraction from one of the ceramides. This reaction, in turn, often starts a cascade of other reactions, which eventually lead to the formation of aldehydes, the dissociation of ceramides or the elimination of formaldehyde, and thus eventually to the degradation of the skin barrier function.

A multi-species Monte Carlo (MC) model, combined with an analytical surface model, has been developed in order to investigate the general plasma processes occurring during the sputter deposition of complex oxide films in a dual-magnetron sputter deposition system. The important plasma species, such as electrons, Ar+ ions, fast Ar atoms and sputtered metal atoms (i.e. Mg and Al atoms) are described with the so-called multi-species MC model, whereas the deposition of MgxAlyOz films is treated by an analytical surface model. Target–substrate distances for both magnetrons in the dual-magnetron setup are varied for the purpose of growing stoichiometric complex oxide thin films. The metal atoms are sputtered from pure metallic targets, whereas the oxygen flux is only directed toward the substrate and is high enough to obtain fully oxidized thin films but low enough to avoid target poisoning. The calculations correspond to typical experimental conditions applied to grow these complex oxide...

In traditional scanning electron microscopy ⁄ energy dispersive X-ray analysis of gunshot residue (GSR), one has to cope more and more frequently with limitations of this technique due to the use of lead-free ammunition or ammunition lacking heavy metals. New methods for the analysis of the organic components of common propellant powder stabilizers were developed based on liquid chromatography coupled to tandem mass spectrometry (LC-MS ⁄ MS). A multiple reactions monitoring scanning method was created for the screening of akardite II, ethylcentralite, diphenylamine, methylcentralite, N-nitrosodiphenylamine, 2-nitrodiphenylamine, and 4-nitrodiphenylamine, present in standards mixtures. Five out of seven of these target compounds can be selectively identified and distinguished from the two others with a high accuracy. Samples from the hands of a shooter were collected by swabbing and underwent solid phase extraction prior to analysis. Detection limits ranging from 5 to 115 lg injected were achieved. Results from several firing trials show that the LC-MS ⁄ MS method is suitable for the detection of stabilizers in samples collected following the firing of 9 mm Para ammunitions.