Controlling the properties of silver nanoparticles deposited on surfaces using supercritical carbon dioxide for surface-enhanced Raman spectroscopy

Silver nanoparticles (AgNPs) have been deposited on silicon and glass surfaces via a supercritical carbon dioxide (sc-CO₂) synthesis route for application in surface-enhanced Raman spectroscopy (SERS). Arrhenius plots revealed that nucleation and growth processes in this system depend on both temperature and surface chemistry. Results also demonstrated that temperature and surface chemistry could be varied to control nanoparticle properties, such as the mean nanoparticle size, density, and surface coverage, providing two useful variables for manipulating the properties of AgNPs deposited on surfaces in this system. These data also provide scientific insight into the underlying mechanisms governing heterogeneous AgNP deposition on a substrate in a sc-CO₂ system in addition to engineering insight into the variables that can be used to manipulate AgNP characteristics. The mean particle size could be tuned over the range 20–200 nm, the interparticle distance could be tuned over the range 70 nm–1 μm, and the surface coverage could be tuned over the range 0.035–0.58. Products were analyzed by scanning electron microscopy with image analysis, transmission electron microscopy, X-ray diffraction, and SERS. The silver nanoparticle-coated substrates were successfully applied in SERS, detecting the model analyte Rhodamine 6G at a concentration of 1 μM, a three orders of magnitude improvement over SERS surfaces previously fabricated in sc-CO₂ systems. Such surfaces can find use in trace concentration analyte detection in biomedical, chemical, and environmental applications.     

Ultrahigh speed graphene diode with reversible polarity

The ability to tune the carrier-type concentration ratio with applied field along with a mean-free path length approaching 1 μm in graphene enables a new diode in which the diode polarity can be reversed. The diode consists of a thin graphene film with a geometric asymmetry that determines a preferred direction for charge-carrier transport, independent of whether the carriers are electrons or holes. We fabricated submicron geometric diodes by patterning and etching exfoliated graphene. Applying field-effect voltages to the substrate, we reversed the carrier type and demonstrated reversal of the diode polarity. The graphene geometric diodes exhibited rectification at 28 THz, opening the way to ultrahigh speed applications for these versatile devices.     

Rapid determination of four polycyclic aromatic hydrocarbons by HPTLC using nano plates

Summary Reversed phase nano plates (HPTLC) have been used for a rapid analysis of a number of PAH (benzo(ghi) perylene, benzo(a)pyrene, benzofluoranthenes and benz(a) anthracene) regularly present in the ambient air. A definite volume of extract is applied to a silica gel column and eluated with cyclohexane. The concentrated eluat is spotted on plates in streak form. A twofold development with acetonitrile/dichloromethane/water (911) results in a complete distinct separation of the four components in 7 min. Evaluation of sample spots is accomplished by measuring the fluorescence intensity registered as a peak at 366 nm. The evaluated peak against benzo(b)fluoranthene is characterized as benzfluoranthenes since the benzfluoranthene isomers are not separated by this procedure. This method has a good reproducibility (rel. standard error 4.4%) and can be easily adopted to the quantitative determination of the four PAH in ambient air and in indoor air involving short sampling periods. The minimum detectable limits on these plates for benz(a) anthracene is 2 ng and 0.2 ng for the three other PAH.

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Schnelle Bestimmung von vier polycyclischen aromatischen Kohlenwasserstoffen durch HPTLC mit Nano-Platten

Zusammenfassung Für eine schnelle Bestimmung von einigen PAH, die stÄndig in der Au\enluft auftreten, werden Nano-Platten (HPTLC) mit reversed phase verwendet. Ein bestimmtes Extraktvolumen wird auf eine KieselgelsÄule gegeben und mit Cyclohexan eluiert. Das konzentrierte Eluat wird strichförmig auf der Platte aufgetragen. Eine zweifache horizontale Entwicklung mit Acetonitril/Dichlormethan/ Wasser (911) bedingt eine deutliche Auftrennung der vier Komponenten nach 7 min. Die Auswertung erfolgt durch Messung der FluorescenzintensitÄt bei 366 nm. Da bei diesem Verfahren die Isomere des Benzo(b)fluoranthens nicht getrennt werden, mu\ die dem Standard Benzo(b)fluoranthen zugeordnete PeakflÄche als Benzfluoranthene bezeichnet werden. Die Methode zeigt eine gute Reproduzierbarkeit (rel. Standardabweichung 4,4%) und kann zur Bestimmung dieser vier PAH in der Au\enluft als auch in der Raumluft bei kurzen Probenahmezeiten Anwendung finden. Die Nachweisgrenzen auf der Nano-Platte betragen für Benz(a) anthracen 2 ng, für die drei übrigen PAH 0,2 ng/Fleck.

     

Base catalyzed intramolecular transamidation of 2-aminoquinazoline derivatives on solid phase

A novel intramolecular cyclo-elimination via transamidation on the Rink Amide AM resin under mild basic conditions is presented. The methodology led to the synthesis of an important class of cardiotonic agents: imdiazo- and pyrimido-quinazolines from the corresponding 2-aminoquinazoline hydrobromide salt under mild basic conditions. NMR based titration studies revealed the role of hydrobromide as a molecular switch, which on removal triggers the cyclisation of aminoquinazoline to tricyclic structures. The main advantage of transamidation under basic conditions over the TFA cleavage is the recyclability of the resin obtained after cyclo-elimination. This has been demonstrated by successive synthesis of four structurally diverse imidazoquianzolin-2-ones using the same batch of resin without any cross contamination.     

Absence of first order electronic transitions in liquid metals

High pressure shock wave data on a wide variety of metals indicates that electronic transitions are continuously distributed in the liquid phase and accompanied by melting maxima. A qualitative explanation for this behavior is suggested.     

Metal binding characteristics of tetracycline derivatives in DMSO solution

A previous study of the site of metal binding in tetracycline has been extended to several derivatives of tetracycline in an effort to determine which specific functional groups are involved in binding to metal ions in DMSO solution and to explore relationships between antibacterial activity and metal binding characteristics. Proton NMR experiments using paramagnetic and diamagnetic lanthanide series ions as binding site probes indicate that the ring A tricarbonylmethane group is the binding site for tetracycline, 5-hydroxytetracycline, 4-epitetracycline and tetracyclinemethiodide in DMSO. No. NMR evidence for metal binding is found for 4-dedimethylaminotetracycline. Binding to Mg²⁺ is also investigated by NMR for several of these compounds, and a discussion of conformational preferences of tetracycline derivatives in DMSO is presented.     

Host and impurity NMR studies in CoGa alloys - a cluster spin glass system

The Co_x Ga_1-x alloys having CsCl structure contain nonmagnetic Co and Ga atoms (host) and magnetic Co atoms (impurities) for x ? 0.50. We report here the simultaneous observation of the applied field NMR of host nuclei and the zero field resonance (ZFR) of impurity nuclei. From the ac susceptibility measurements and the field dependence of ZFR signals we conclude that for 0.50 ? × ? 0.64, this system is a “cluster spin glass”.     

Impurity induced Fe spin reorientation in CeFe2

CeFe₂ in a unique ferromagnetic system in which small nonmagnetic substitutions on the Fe sublattice induces a second transition into a state of near zero net magnetization (presumably in a antiferromagnetic state). The second transition can be suppressed and ferromagnetism restored by substitution of (10%) Y at the Ce site.     

Non-statistical behavior in the dissociative photoionization of the 1,3-butadiene/sulfur dioxide van der Waals complex

Dissociative photoionization of the van der Waals complex of 1,3-butadiene and SO₂ to yield the fragment ion C₄H₆-SO⁺ is reported. From