Remarks on product

Well known results related to the compactness of Hankel operators of one complex variable are extended to little Hankel operators of two complex variables. Critical to these considerations is the result of Ferguson and Lacey (2002) characterizing the boundedness of the little Hankel operators in terms of the product BMO of S.-Y. Chang and R. Fefferman (1985), (1980).

     

Electrochemical sensors and biosensors based on redox polymer/carbon nanotube modified electrodes: A review

The aim of this review is to present the contributions to the development of electrochemical sensors and biosensors based on polyphenazine or polytriphenylmethane redox polymers together with carbon nanotubes (CNT) during recent years. Phenazine polymers have been widely used in analytical applications due to their inherent charge transport properties and electrocatalytic effects. At the same time, since the first report on a CNT-based sensor, their application in the electroanalytical chemistry field has demonstrated that the unique structure and properties of CNT are ideal for the design of electrochemical (bio)sensors. We describe here that the specific combination of phenazine/triphenylmethane polymers with CNT leads to an improved performance of the resulting sensing devices, because of their complementary electrical, electrochemical and mechanical properties, and also due to synergistic effects. The preparation of polymer/CNT modified electrodes will be presented together with their electrochemical and surface characterization, with emphasis on the contribution of each component on the overall properties of the modified electrodes. Their importance in analytical chemistry is demonstrated by the numerous applications based on polymer/CNT-driven electrocatalytic effects, and their analytical performance as (bio) sensors is discussed.     

Ultra-high surface area and mesoporous N-doped carbon derived from sheep bones with high electrocatalytic performance toward the oxygen reduction reaction

A nitrogen (N)-doped mesoporous carbon material exhibiting ultra-high surface area was successfully synthesized from sheep bones via a facile and low-cost method. The obtained carbon material had an ultra-high specific surface area of 1961 m² g^?1 and provided rich active sites for the oxygen reduction reaction (ORR), which in turn resulted in high electrocatalytic activity. It was found that the pore size distribution for the newly prepared carbonaceous material fell in the range of 1–4 nm. Benefiting from its high surface area and the presence of pyridine-N and quaternary-N species, the as-prepared carbon material exhibited excellent ORR activity in an oxygen-saturated 0.1 M KOH solution, compared to commercial Pt/C (10 wt%). Due to its high ORR catalytic activity, stability and low-cost, using sheep bone as C and N precursors to produce N-doped carbon provides an encouraging step toward the goal of replacing commercial Pt/C as fuel cell cathode electrocatalyst.     

Elements of Design‐Based Science Activities That Affect Students' Motivation

The primary purpose of this study was to examine the ways in which a 12‐week afterschool science and engineering program affected middle school students' motivation to engage in science and engineering activities. We used current motivation research and theory as a conceptual framework to assess 14 students' motivation through questionnaires, structured interviews, and observations. Students reported that during the activities they perceived that they were empowered to make choices in how to complete things, the activities were useful to them, they could succeed in the activities, they enjoyed and were interested in the hands‐on activities and some presentations, they felt cared for by the facilitators and received help when they were stuck or confused, and they put forth effort. Based on our examination of data across our three data sources, we identified motivating opportunities that were provided to students during the activities. These motivating opportunities can serve as examples to help both formal and informal science educators better connect motivation theory to practice so that they can create motivating opportunities for students. Furthermore, this study provides a methodological example of how students' motivation can be examined during the context of authentic science and engineering instruction.     

Theoretical Study of the Reaction Formalhydrazone with Singlet Oxygen. Fragmentation of the C=N Bond,Ene Reaction and Other Processes

Photobiologic and synthetic versatility of hydrazones has not yet been established with ¹O₂ as a route to commonly encountered nitrosamines. Thus, to determine whether the “parent” reaction of formalhydrazone and ¹O₂ leads to facile C=N bond cleavage and resulting nitrosamine formation, we have carried out CCSD(T)//DFT calculations and analyzed the energetics of the oxidation pathways. A [2 + 2] pathway occurs via diradicals and formation of 3‐amino‐1,2,3‐dioxazetidine in a 16 kcal/mol^?1 process. Reversible addition or physical quenching of ¹O₂ occurs either on the formalhydrazone carbon for triplet diradicals at 2–3 kcal mol^?1, or on the nitrogen (N(3)) atom forming zwitterions at ~15 kcal/mol^?1, although the quenching channel by charge‐transfer interaction was not computed. The computations also predict a facile conversion of formalhydrazone and ¹O₂ to hydroperoxymethyl diazene in a low‐barrier ‘ene’ process, but no 2‐amino‐oxaziridine‐O‐oxide (perepoxide‐like) intermediate was found. A Benson‐like analysis (group increment calculations) on the closed‐shell species are in accord with the quantum chemical results.     

Methods and options for the heterologous production of complex natural products

This review will detail the motivations, experimental approaches, and growing list of successful cases associated with the heterologous production of complex natural products.     

Centrifugally-driven sample extraction, preconcentration and purification in microfluidic compact discs

Centrifugally-driven microfluidic compact discs (μ-CDs) have attracted significant interest within the analytical science community in the past decade, with the primary focus on the potential of such platforms for performing parallel and/or multiplex biological assays and further application in biomedical diagnostics. More recently, μ-CD-based devices were also applied to environmental analysis as platforms for multi-sample extraction and transportation, prior to off-disc analysis in the laboratory. This review critically summarizes recent developments in μ-CD platforms for sample extraction, preconcentration, fractionation and purification in bioanalytical and environmental applications. We also summarize the common methods employed in the fabrication of μ-CD platforms. Further, we discuss preparation of stationary phases in microfluidic channels embedded in μ-CDs, as applications of μ-CDs in sample extraction are generally based on enclosed series of extraction phases and microcolumns.     

Characterising lone-pair activity of lead(II) by 207Pb solid-state NMR spectroscopy: coordination polymers of [N(CN)2]- and [Au(CN)2]- with terpyridine ancillary ligands

A series of lead(II) coordination polymers containing [N(CN)₂]^? (DCA) or [Au(CN)₂]^? bridging ligands and substituted terpyridine (terpy) ancillary ligands ([Pb(DCA)₂] ( 1 ), [Pb(terpy)(DCA)₂] ( 2 ), [Pb(terpy){Au(CN)₂}₂] ( 3 ), [Pb(4′‐chloro‐terpy){Au(CN)₂}₂] ( 4 ) and [Pb(4′‐bromo‐terpy)(μ‐OH₂)_0.5{Au(CN)₂}₂] ( 5 )) was spectroscopically examined by solid‐state ²⁰⁷Pb MAS NMR spectroscopy in order to characterise the structural and electronic changes associated with lead(II) lone‐pair activity. Two new compounds, 2 and [Pb(4′‐hydroxy‐terpy){Au(CN)₂}₂] ( 6 ), were prepared and structurally characterised. The series displays contrasting coordination environments, bridging ligands with differing basicities and structural and electronic effects that occur with various substitutions on the terpyridine ligand (for the [Au(CN)₂]^? polymers). ²⁰⁷Pb NMR spectra show an increase in both isotropic chemical shift and span (Ω) with increasing ligand basicity (from δ_iso=?3090 ppm and Ω=389 ppm for 1 (the least basic) to δ_iso=?1553 ppm and Ω=2238 ppm for 3 (the most basic)). The trends observed in ²⁰⁷Pb NMR data correlate with the coordination sphere anisotropy through comparison and quantification of the Pb? N bond lengths about the lead centre. Density functional theory calculations confirm that the more basic ligands result in greater p‐orbital character and show a strong correlation to the ²⁰⁷Pb NMR chemical shift parameters. Preliminary trends suggest that ²⁰⁷Pb NMR chemical shift anisotropy relates to the measured birefringence, given the established correlations with structure and lone‐pair activity.