Aggregation and self-assembly of amphiphilic block copolymers in aqueous dispersions of carbon nanotubes

The self-assembly (SA) of amphiphilic block copolymers (poly(ethylene oxide)-poly(propylene oxide)-poly(ethylene oxide)) was investigated in dispersions of single-walled and multiwalled carbon nanotubes (SWNT and MWNT, respectively) as a function of temperature. Differential scanning calorimetry (DSC) was used for characterization of the thermal behavior of the combined polymers-nanostructures system, and spin-probe electron paramagnetic resonance (EPR) was employed for probing the local dynamic and polarity of the polymer chains in the presence of nanostructures. It was found that SWNT and MWNT modify the temperature, enthalpy, and dynamic behavior of polymer SA. In particular, SWNT were found to increase the cooperativity of aggregating chains and dominate aggregate dynamics. MWNT reduced the cooperativity, while colloidal carbon black additives, studied for comparison, did not show similar effects. The experimental observations are consistent with the suggestion that dimensional matching between the characteristic radius of the solvated polymer chains and the dimensions of additives dominate polymer SA in the hybrid system.     

Topological ferrimagnetism and superparamagnetic-like behaviour in a disordered homometallic coordination network

Crystals of a cobalt(II) 3-amino-1,2,4-triazolate; Co(2)Cl(C(2)N(4)H(3))(3), 1 can be formed from a hydrothermal synthesis. X-Ray crystallography shows an extended 3-dimensional network structure within a hexagonal space group with a=9.9655(7) and c=7.7523(7) A. The data reveal an orientational disorder to the ligand. Structural considerations suggest that the ligand orientation is strongly correlated in 1-dimension, but the nature and length scale of the 2-dimensional order is not obvious. The structure is discussed in terms of three key structural models, two of which are crystallographically ordered, while the third is disordered in 2-dimensions. The effect of ligand orientation on the network topology has profound effects on the expected magnetic behaviour, with the two fully ordered models having either antiferromagnetic or ferrimagnetically ordered ground states. The magnetism of 1 is complex. It shows an antiferromagnetically coupled paramagnetic phase above approximately 5.3 K. Below approximately 4.5 K we see a ferrimagnetically ordered state (H(coer)=50 Oe, M(rem)=1830 Oe cm(3) mol(-1), M(sat) = 3450 Oe cm(3) mol(-1) at 2 K). In the intermediate temperature range 4.5 to 5.3 K we see unusual behaviour with evidence of rapid, low-energy relaxation of the magnetisation. The unusual relationship between the structure and the magnetism of 1 is discussed in detail.     

An unexpected directing effect in the asymmetric transfer hydrogenation of α,α-disubstituted ketones

α,α-Disubstituted ketones containing an aromatic ring or alkene are reduced in high enantiomeric excess using an asymmetric transfer hydrogenation catalyst. The sense of reduction indicates that the unsaturated region of the ketone adopts a position adjacent to the Ru-bound η(6)-arene ring in the reduction transition state.     

Application of a modified Pechini method for the synthesis of Ln<Subscript>2</Subscript>MGe<Subscript>4</Subscript>O<Subscript>12</Subscript> (Ln = Y,Eu; M = Ca,Zn, Mn) optical hosts

A modified Pechini method followed by conventional and microwave heating was used to synthesise the new promising Ln₂MGe₄O₁₂ (Ln = Y, Eu; M = Ca, Zn, Mn) optical hosts. Comparison between solid-state and Pechini synthesis methods showed that the latter reduces the temperature required for cyclo-tetragermanate formation. The highest yield of cyclo-tetragermanates for both methods is observed at 1,000–1,100 °C, with significantly shorter time of annealing in the case of the Pechini synthesis. Compositional, structural and morphological characterisations of the samples obtained by both routes were carried out using X-Ray powder diffraction, thermogravimetry, electron spin resonance spectroscopy, energy-dispersive X-Ray spectroscopy and scanning electron microscopy.     

Paradoxes as a window to infinity

This study examines approaches to infinity of two groups of university students with different mathematical background: undergraduate students in Liberal Arts Programmes and graduate students in a Mathematics Education Master's Programme. Our data are drawn from students’ engagement with two well-known paradoxes – Hilbert's Grand Hotel and the Ping-Pong Ball Conundrum – before, during, and after instruction. While graduate students found the resolution of Hilbert's Grand Hotel paradox unproblematic, responses of students in both groups to the Ping-Pong Ball Conundrum were surprisingly similar. Consistent with prior research, the work of participants in our study revealed that they perceive infinity as an ongoing process, rather than a completed one, and fail to notice conflicting ideas. Our contribution is in describing specific challenging features of these paradoxes that might influence students’ understanding of infinity, as well as the persuasive factors in students’ reasoning, that have not been unveiled by other means.     

On the content-dependence of prospective teachers’ knowledge: a case of exemplifying definitions

In this article, we demonstrate that prospective teachers’ content knowledge related to defining mathematical concepts is dependent on content area. We use the example of generation (a research tool we developed in a previous study) to investigate prospective teachers’ knowledge. We asked prospective secondary mathematics teachers to provide multiple examples of definitions of concepts from different areas of mathematics. We examined teacher-generated examples of concept definition and analysed individual and collective example spaces, focusing on their correctness and richness. We demonstrate differences in prospective teachers’ knowledge associated with defining mathematical concepts in geometry, algebra and calculus.     

Effect of Polystyrene Microsphere Surface to Fluorescence Lifetime Under Two-Photon Excitation

Molecular assays such as immunoassays are often performed using solid carriers and fluorescent labels. In such an assay format a question can be raised on how much the fluorescence of the label is influenced by the bio-affinity binding events and the solid carrier surface. Since changes in fluorescence intensity as labels bind to surfaces are notoriously difficult to quantify other approaches are preferred. A good indicator, independent of the fluorescence intensity of the label, is the fluorescence lifetime of the marker fluorophore. Changes in fluorescence lifetime reliably indicate the presence of dynamic quenching, energy transfer or other de-excitation processes. A microsphere based assay system is studied under two-photon excitation. Changes in fluorescence lifetime are studied as labeled protein conjugates bind on microsphere surfaces – both direct on the surface and with a few nanometer distance from the surface. Fluorescence signal is measured from individual polystyrene microspheres and the fluorescence lifetime histogram is simultaneously recorded. The results indicate that self-quenching and quenching by the polystyrene surface are both present in such a system. However, the effect of the surface can be avoided by increasing the distance between the surface and the label. Typical distances achieved by a standard sandwich type of assay, are already sufficient to overcome the surface induced quenching in fluorescence detection.     

Concise and stereoselective synthesis of 2,5- and 2,4-disubstituted thiazole amino acid subunits for synthesizing thiazole-containing peptides

A concise, highly stereoselective synthesis of 2,4- and 2,5-disubstituted thiazole amino acids was developed. These are important building blocks for various biologically active thiazole-containing natural peptides and their regioisomeric analogues. The fundamental reactions are diastereoselective addition of (4- or 5-bromothiazol-2-yl)lithium to N-tert-butanesulfinyl imine with subsequent Pd-catalyzed phenoxycarbonylation.     

Synthesis of gentianine N-oxide by enzymatic hydrolysis of swertiamarin in the presence of hydroxylamine and reaction pathway to gentianine and gentianol

Gentianine is a metabolite of gentiopicroside and swertiamarin. Several biological activities have been reported for gentianine, such as antiinflammatory and antidiabetic activity, and hypotensive effect. Gentiopicroside is found in 0.9–9.8% content in Gentian root or Gentian scabra root, and Swertiamarin is contained in Swertia herb in 2–10%. These natural products can be potential starting materials for the synthesis of gentianine. This study describes the β-glucosidase-catalyzed hydrolysis of gentiopicroside and swertiamarin in the presence of hydroxylamine to afford gentianine N-oxide, which can be a synthetic precursor of gentianine derivatives. Enzymatic hydrolysis of swertiamarin selectively afforded gentianine N-oxide in 81% yield, whereas gentiopicroside afforded gentianine N-oxide and gentianol N-oxide. Plausible reaction pathways leading to gentianine, gentianol, and their N-oxides were also investigated.