1,510 research outputs found
Chemical and biochemical sensors based on silicon nanowire field-effect transistor arrays
No full textField-effect transistors (FETs) made from semiconducting nanowires have great potential as electronic biochemical sensors if they can be integrated as an array in a CMOS-compatible architecture together with microfluidic channels and interfacing electronics. Such nanoscale electronic transducers based on ion-sensitive field-effect transistors could be mass fabricated at reasonable costs. This, in combination with their small size, makes them ideal for personalized medicine and for future implanted sensing devices.
The sensing principle is based on adsorption of charged species on the sensor surface, leading to a change in surface potential and subsequently a change in current in the FET channel. Thereby, the high-impedance input signal is transformed into a low-impedance output signal, which is an advantage against classic ion-selective electrodes. The potential for downscaling and integration for the simultaneous detection of multiple parameters make silicon nanowire FETs a promising platform to meet the demand for cheap, multifunctional and scalable sensors. Even though many promising results on chemical and biochemical sensing have been achieved so far, a detailed understanding of the electrolyte surface interaction is still missing. Inconsistent outcomes regarding the effect of electrolyte concentrations and electrical noise, suggest that further quantitative studies are needed. The aspect of the size compatibility between the sensor unit and the analyte species is often emphasized to favor nanoscale FETs. Another aspect, often mentioned, is the surface to volume ratio. Hence smaller sensing units should enhance the sensitivity of the sensor, allowing the detection at ultra-low concentrations or a small number of molecules. Furthermore, the capacitances decrease for smaller sensing units, which could lead to faster response times. However, other aspects such as the intrinsic electronic noise, the analyte diffusion time and surface reaction kinetics have to be considered for the development of an applicable sensor.
This thesis was part of a research project aimed at developing a modular, scalable and integrateable sensor platform for the electronic detection of analytes in solution. The main focus lies on the sensor-solution interface and thus the thesis quantitatively compares the experimental data with analytical models.
In this work we have established a versatile sensor platform based on silicon nanowire arrays. The sensor functionality was changed by surface modification for the detection of various analytes such as pH, alkaline ions and even FimH proteins. We achieved an ideal pH sensor with a response close to the Nernst limit. Full surface passivation for protons was accomplished for the implementation of a nanoscale reference electrode. Using the differential signal from differently functionalized silicon nanowires we could detect sodium and potassium ions selectively. Ultimately we present the detection of protein-ligand interactions of the physiologically relevant protein FimH. An extended site binding model was derived to calculate the theoretical limits and assess the properties of the surface groups by evaluating the experimental results
Review of America Discovers C. S. Lewis: His Profound Impact
No full textA review of K. Alan Snyder, America Discovers C. S. Lewis: His Profound Impact (Eugene, Oregon: Wipf and Stock, 2016). ix + 202 pages. $27.00. ISBN 9781498298209
Synthesis of new phosphino-oxazoline ligands for asymmetric catalysis
No full textBorabox ligands proved to be efficient ligands for controlling the enantioselectivity of various
metal-catalyzed reactions. Therefore modification of an existing borabox backbone was
implemented and new borabox ligands modified on C(5) position of the oxazoline ring were
prepared and tested in the copper-catalyzed asymmetric cyclopropanation. In this study high
stereocontrol of the reaction was observed. However the presence of sterically demanding
groups at position C(5) did not improve the results compared to the C(5) non-substituted
analogs.
The synthesis of analogous boron-bridged phosphino-oxazolines was attempted via several
synthetic approaches in order to prepare new zwitterionic N,P-ligands. The simple stepwise
substitution by subsequent addition of lithiated oxazoline and phosphine was not possible. It
either led to borabox ligands or to undesired dimeric species, which were inert towards
reaction with other nucleophiles.
We decided to tune the electronic properties of the boron compound by variation of the
substituents in order to avoid multiple substitution or undesired dimer formation. Therefore
aminochloroborates were examined due to their lower reactivity compared to chloroboranes
or chloroborates. A derivative with a phosphine-aminoborate backbone was prepared but
unfortunately the decreased reactivity of the nitrogen-substituted boron center did not allow
another nucleophilic addition of the oxazoline moiety. In order to avoid dimer formation the
reactivity of potassium diaryldifluoroborates was investigated. These tetrasubstituted boron
compounds reacted with lithium oxazolines and provided products of nucleophilic
substitution at the boron center. The resulting oxazoline-substituted fluoroborates could be
isolated as zwitterions after protonation of the oxazoline nitrogen atom. However, the second
intended substitution with the phosphine moiety was not possible. In addition, quantum
chemistry calculations were carried out to support the experimental studies.
The synthesis of new NeoPHOX ligands derived from inexpensive chiral aminoacids L-serine
and L-threonine was developed. These chiral ligands were tested in the iridium-catalyzed
asymmetric hydrogenation and palladium-catalyzed allylic substitution. In both reactions the
enantioselectivities achieved were excellent for most of the substrates tested. In the iridium
catalyzed hydrogenation it was found that presence of an acid-stable protecting group of
tertiary alcohol (R2) is necessary in order to achieve full conversions. The enantioselectivities
obtained in the catalytic asymmetric hydrogenation and allylic substitution with the L-serine
and L-threonine derived ligands were almost identical to those reported for tert-butyl-
substituted NeoPHOX ligands, which are derived from very expensive amino acid tert-
leucine.
The use of Ir catalysts for the diastereoselective hydrogenation of Diels-Alder products was
investigated. The best results were obtained with a pyridine-phosphinite complex that
afforded the saturated cyclohexane derivatives with diastereoselectivities of up to 98:2 and
full conversion. The reaction is strongly catalyst-controlled, so it is possible to obtain each of
the two diastereomeric products with high selectivity using either (R)- or (S)-catalyst
Review of The Unknown Garden of Another’s Heart: The Surprising Friendship between C. S. Lewis and Arthur Greeves
No full textReview of Joseph A. Kohm Jr., The Unknown Garden of Another’s Heart: The Surprising Friendship between C. S. Lewis and Arthur Greeves (Eugene, OR: Wipf and Stock, 2022). 114 pages. $21.00. ISBN 9781666710403
Synthesis of the C(1)-C(18) Segment of Lophotoxin and Pukalide. Control of 2-Alkenylfuran (E/Z)-Configuration.
No full text[reaction: see text] The convergent synthesis of the fully functionalized C(1)-C(18) segment 24 of the furanocembranes lophotoxin and pukalide was accomplished in 11 steps and 10% overall yield. The key step was a stereoselective conversion of alkynoate 21 to trimethylsilyl 2-alkenylfuran 22.\ud
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Total Synthesis and Structure Validation of (+)-Bistramide C.
No full textThe convergent total synthesis of the marine natural product (+)-bistramide C confirms the a priori assignments of its relative and absolute configurations, which were originally based on the combined application of [alpha]D analysis, NMR, and synthesis.\ud
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Functions of PDGF-A and -C. Essential ligands for the PDGF alpha receptor
No full textAltered protein kinase activity is a contributing factor in many diseases including most formsof human malignancies, and there is reason to believe that protein kinases will prove to be majordrug targets in the treatment of cancer. Protein kinases mediate most of the signal transduction ineukaryotic cells, regulating cellular events such as transcription, metabolism, proliferation,cytoskeletal rearrangement, migration, differentiation and apoptosis. Protein phosphorylation alsoplays a critical role in intercellular communication during embryonic development, especiallythrough activation of receptor tyrosine kinases (RTK:s).This study focuses on the Platelet-derived growth factor (PDGF) family, secreted moleculeswhose functions are to bind to, and activate, two structurally related RTK:s, PDGF receptoralpha and beta. To gain insight into the developmental role of PDGF-A and -C, three lines ofmice were generated in which these genes were modified by gene targeting.As a common principle, PDGF-A and -C, secreted from epithelial cells, induced proliferation,and possibly migration, of mesenchymal progenitor cells expressing PDGF receptor alpha.Mice deficient for PDGF-A died perinatally and displayed defective lung development due tolack of alveolar formation. This phenotype was coupled to a loss of alveolar smooth muscle cellsand reduced parenchymal elastin, resulting in a picture resembling emphysema.The sixth exon in PDGF-A is normally alternatively spliced, and, when present, it confersbinding to extracellular matrix structures. The second line of mice generated carried a mutationin the sixth exon splice acceptor, so that only the short, freely diffusible, form of PDGF-A wasbeing produced. Analysis of these mice suggested overlapping functions for PDGF-A and -C, andalso revealed that extracellular retention of PDGF-A is important for normal development ofgastrointestinal villi.Depending on genetic background, PDGF-C negative mice died postnatally due to a cleftpalate accompanied by moderate spina bifida. Interestingly, PDGF-A / PDGF-C double deficientmice were phenotypically indistinguishable from mice carrying a null mutation in PDGFreceptor alpha. Taken together, the results of this study imply that PDGF-A and -C are thephysiologically important ligands for PDGF receptor alpha during development
Transition-Metal-Mediated Cascade Reactions: <i>C</i>,<i>C</i>-Dicyclopropylmethylamines by Way of Double C,C-σ-Bond Insertion into Bicyclobutanes
No full textThe organometallic multicomponent reaction of alkenyl zirconocene, alkynyl imine, and zinc carbenoid in the presence of dimethylzinc leads to novel C,C-dicyclopropylmethylamines. The formation of intermediate bicyclo[1.1.0]butanes represents the first synthetically useful example of a double C,C-σ-bond insertion, and the increase in structural complexity from starting materials is highlighted by the formation of nine new C,C-bonds in the final product
Transition-Metal-Mediated Cascade Reactions: <i>C</i>,<i>C</i>-Dicyclopropylmethylamines by Way of Double C,C-σ-Bond Insertion into Bicyclobutanes
No full textThe organometallic multicomponent reaction of alkenyl zirconocene, alkynyl imine, and zinc carbenoid in the presence of dimethylzinc leads to novel C,C-dicyclopropylmethylamines. The formation of intermediate bicyclo[1.1.0]butanes represents the first synthetically useful example of a double C,C-σ-bond insertion, and the increase in structural complexity from starting materials is highlighted by the formation of nine new C,C-bonds in the final product
Total Synthesis of (-)-Sessilifoliamide C and (-)-8-Epi-Stemoamide.
No full textA convergent route featuring [3,3]-sigmatropic rearrangements of a linchpin azepinopyrrolidine served to install two of the four contiguous stereocenters present in the tricyclic Stemona alkaloids sessilifoliamide and stemoamide. In addition to the first total synthesis of (-)-sessilifoliamide C, a potential biosynthetic relationship between the sessilifoliamides and previously reported Stemona alkaloids is presented.\ud
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