Dendrimers as ligands. Formation of a 2:1 luminescent complex between a dendrimer with a 1,4,8,11-tetraazacyclotetradecane (cyclam) core and Zn2+

We have investigated the formation of metal complexes between Zn2+ and two derivatives, 1 and 2, of the well-known 1,4,8,11-tetraazacyclotetradecane (cyclam) ligand. Compound 1 is 1,4,8,11-tetrakis(naphthylmethyl) cyclam, and compound 2 is a dendrimer consisting of a cyclam core with appended 12 dimethoxybenzene and 16 naphthyl units. Compound 1 exhibits an emission band with a maximum around 480 nm, assigned to the formation of exciplexes between amine and excited naphthyl units. Dendrimer 2 exhibits three types of weak emission bands, assigned to naphthyl localized excited states (lambdamax = 337 nm), naphthyl excimers (lambdamax ca. 390 nm), and naphthyl-amine exciplexes (lambdamax = 480 nm). In CH3CN-CH2Cl2 1:1 v/v, titration of ligand 1 with Zn2+ causes the disappearance of the exciplex emission and the appearance of a strong naphthyl localized fluorescence; the titration plot is linear and reaches a plateau for a 1:1 stoichiometry, showing that a highly stable [Zn(1)]2+ complex is formed. In the case of 2, titration with Zn2+ causes the disappearance of the exciplex band, with a concomitant increase in the excimer and naphthyl localized emissions; the titration plot is again linear, but in this case it reaches a plateau for a 2:1 stoichiometric ratio, showing the unexpected formation of a [Zn(2)2]2+ complex. Such an unexpected stoichiometry for the complex of the dendritic ligand has been fully confirmed by 1H NMR titrations. The results obtained show that the dendrimer branches not only do not hinder, but in fact favor coordination of cyclam to Zn2+.     

Liquid chromatography-mass spectrometry and liquid chromatography-tandem mass spectrometry of the Alternaria mycotoxins alternariol and alternariol monomethyl ether in fruit juices and beverages

Alternariol (AOH) and alternariol monomethyl ether (AME) are among the main mycotoxins formed in apples and other fruits infected by Alternaria alternata. For determination of AOH and AME by LC, apple juice and other fruit beverages were cleaned up on C18 and aminopropyl solid-phase extraction columns. Positive and negative ion mass spectra of AOH and AME under electrospray (ESI) and atmospheric pressure chemical ionization (APCI) conditions were obtained. Collision-induced dissociation of the [M+H]+ and [M-H]- ions for both compounds were also studied. The phenolic anions of both compounds are more stable with less fragmentation. In quantitative analysis, negative ion detection also offers lower background and better sensitivity. Sensitive LC-MS and LC-MS-MS confirmatory procedures based on APCI with negative ion detection were applied to confirm the natural occurrence of AOH in nine samples of apple juice and in single samples of some other clear fruit beverages--grape juice, cranberry nectar, raspberry juice, red wine, and prune nectar (which also contained 1.4 ng AME/ml)--at levels of up to 6 ng AOH/ml. Electrospray LC-MS-MS with negative ion detection and in multiple reaction monitoring mode offers higher sensitivity and specificity. Absolute detection was better than 4 pg per injection for both compounds.     

Reduction of the (001) surface of gamma-V2O5 compared to alpha-V2O5

The defect-free gamma-V(2)O(5)(001) surface and ordered structures of oxygen vacancies have been studied for a wide range of defect concentrations, Theta ((1)/(6) monolayer (ML) < or = Theta < or = 1 ML), combining density functional theory and statistical thermodynamics. The gamma polymorph of V(2)O(5) is characterized by two structurally different vanadium sites, V(A) and V(B). The V(A) sites having a weaker bond to an adjacent crystal layer are easier to reduce. Up to (1)/(2) ML, the V(A) defect structures with defects aligned along the [010] direction are increasingly more stable as in alpha-V(2)O(5)(001). At higher defect concentrations, the different coordination of the V(B) vanadium atoms at the gamma-V(2)O(5) surface causes an increase in the vacancy formation energy of approximately 0.8 eV/atom at Theta = 1.0 compared to Theta = (1)/(2). For alpha-V(2)O(5), this increase amounts to 0.2 eV/atom only. Under conditions (low oxygen partial pressures and high temperatures) at which the alpha-V(2)O(5)(001) surface would be fully reduced, the gamma-V(2)O(5)(001) surface is only partially reduced. The presence of surface vanadyl oxygen groups at V(B) sites may change the surface reactivity compared to that of alpha-V(2)O(5)(001).     

Recent Reviews. 42

Reviews are listed in order of appearance in the sources indicated. In multidisciplinary review journals, only those reviews which fall within the scope of this Journal are included. Sources are listed alphabetically in three categories: regularly issued review journals and series volumes, contributed volumes, and other monographs. Titles are numbered serially, and these numbers are used for reference in the index. Major English-language sources of critical reviews are covered. Encyclopedic treatises, annual surveys such as Specialist Periodical Reports, and compilations of symposia proceedings are omitted. This installment of Recent Reviews covers principally the middle part of the 1996 literature. Previous installment: J. Org. Chem.1996, 61(14), 4860-8. These Recent Review articles are now also available in a database form on the ACS Organic Division WWW site http://www.organic.emory.edu/acsorg/index.html.     

A Note on Local Asymptotics of Solutions to Singular Elliptic Equations via Monotonicity Methods

This paper concerns the asymptotic behavior of solutions and their gradients to linear and nonlinear elliptic equations with singular coefficients of fuchsian type.     

Photothermal and photocatalytic processes on TiO2 based materials prepared by sol-gel method

In the present work, Fe³⁺ doped TiO₂ coatings on glass substrates were prepared by dip-coating from a sol-gel solution. The influence of the dopant concentration on the structure, optical, photocatalytic and photothermal properties of the films was studied. The results obtained have shown a strong correlation between the catalytic properties and the amount of iron dopant and the temperature of the thermal treatment.     

On the weak IFR aging of bivariate lifetime distributions

A new notion of bivariate aging in a competitive risk framework is introduced. Aging properties of bivariate distributions are defined by aging properties of a series system with possibly dependent components. A case of exponential marginals is considered. Sufficient conditions for a weak IFR aging (weak DFR negative aging) are derived and a number of simple examples are considered. Copyright © 2005 John Wiley & Sons, Ltd.     

Protecting Physical Layer Secret Key Generation from Active Attacks

Lightweight session key agreement schemes are expected to play a central role in building Internet of things (IoT) security in sixth-generation (6G) networks. A well-established approach deriving from the physical layer is a secret key generation (SKG) from shared randomness (in the form of wireless fading coefficients). However, although practical, SKG schemes have been shown to be vulnerable to active attacks over the initial “advantage distillation” phase, throughout which estimates of the fading coefficients are obtained at the legitimate users. In fact, by injecting carefully designed signals during this phase, a man-in-the-middle (MiM) attack could manipulate and control part of the reconciled bits and thus render SKG vulnerable to brute force attacks. Alternatively, a denial of service attack can be mounted by a reactive jammer. In this paper, we investigate the impact of injection and jamming attacks during the advantage distillation in a multiple-input–multiple-output (MIMO) system. First, we show that a MiM attack can be mounted as long as the attacker has one extra antenna with respect to the legitimate users, and we propose a pilot randomization scheme that allows the legitimate users to successfully reduce the injection attack to a less harmful jamming attack. Secondly, by taking a game-theoretic approach we evaluate the optimal strategies available to the legitimate users in the presence of reactive jammers.     

Urea-Formaldehyde Microcapsules Containing an Epoxy Resin: Influence of Reaction Parameters on the Encapsulation Yield

In this work urea-formaldehyde microcapsules containing an epoxy resin are prepared by in situ polymerization of monomers in an oil-in-water emulsion. Scanning electronic microscopy (SEM) was performed to investigate on microcapsule size and surface morphology. Calorimetric and spectroscopic analyses were carried out with the aim of evaluate the encapsulation yield and the shell features. Factors determining the microencapsulability of the core material were described. In particular, our interest was devoted to a better understanding of the influence of the reaction parameters on the microcapsule properties. It was found that the encapsulation yield as well as the extent of urea-formaldehyde polymerization depends on the reaction temperature and the stirring speed.