Effects of sonication on electrode surfaces and metal particles

Ultrasonic irradiation at a frequency of 20 kHz has varying effects on electrode surfaces. Non-metals such as glassy carbon and Ebonex™ are severely pitted after only a few minutes of sonication in aqueous media. By contrast, metals such as Pt, Au, W and Pd remain largely undamaged after 120 s, as observed by scanning electron microscopy. The extent of damage does not appear to be related to the melting point of the material. By contrast, when electrodes are sonicated in suspensions of metal powders, particles are deposited onto electrode surfaces. The deposits were subsequently observed by scanning electron microscopy and by voltammetry. It is concluded that the ability to deposit particles on an electrode depends on both the melting point of the particles and the electrode, whereas surface damage is more closely related to the hardness of the material.     

Fluorine-containing natural products

It is now more than 50 years since the first fluorinated natural product was identified. In that time only about a dozen fluorinated natural products have been isolated, the last one over a decade ago. Very little is known about the mechanism of biological fluorination although significant progress has been made in elucidating the pathway by which biosynthesis of fluoroacetate and 4-fluorothreonine occurs in the bacterium Streptomyces cattleya. In this article we review the fluorinated natural products and the current status of our understanding of fluorometabolite biosynthesis.     

The Instability of Nonmonotonic Wave Solutions of Parabolic Equations

We consider the class of equations u_t=f(u_xx, u_x, u) under the restriction that for all a,b,c. We first consider this equation over the unbounded domain ? ∞ < x < + ∞, and we show that very nearly every bounded nonmonotonic solution of the form u(t, x)=?(x?ct) is unstable to all nonnegative and all nonpositive perturbations. We then extend these results to nonmonotonic plane wave solutions u(t, x, y)=?(x?ct) of u_t = F(u_xx, u_xy, u_x, u_y, u). Finally, we consider the class of equations u_t=f(u_xx, u_x, u) over the bounded domain 0 < x < 1 with the boundary conditions u(t, x)=A at x=0 and u(t, x)=B at x=1, and we find the stability of all steady solutions u(t, x)=?(x).     

Investigation of two-photon absorption properties in branched alkene and alkyne chromophores

Novel alkene and alkyne branched structures have been synthesized, and their two-photon absorption (2PA) properties are reported. This series of alkene and alkyne trimer systems tests the mechanistic approach for enhancing the 2PA process which is usually dictated by the pi-bridging, delocalization length, and corresponding charge transfer on the 2PA cross sections. The results suggest that alkene branched systems have higher 2PA cross sections. While steady-state absorption and emission measurements were not successful in predicting the observed trend of 2PA cross sections, time-resolved measurements have explained the trends observed. It was found that, upon photoexcitation, there is an ultrafast charge localization to an intramolecular charge-transfer (ICT) state, followed by the presence of a solvent and conformationally relaxed ICT state in these branched systems.     

Accounting for threshold uncertainty in extreme value estimation

Tail data are often modelled by fitting a generalized Pareto distribution (GPD) to the exceedances over high thresholds. In practice, a threshold is fixed and a GPD is fitted to the data exceeding . A difficulty in this approach is the selection of the threshold above which the GPD assumption is appropriate. Moreover the estimates of the parameters of the GPD may depend significantly on the choice of the threshold selected. Sensitivity with respect to the threshold choice is normally studied but typically its effects on the properties of estimators are not accounted for. In this paper, to overcome the difficulties of the fixed-threshold approach, we propose to model extreme and non-extreme data with a distribution composed of a piecewise constant density from a low threshold up to an unknown end point and a GPD with threshold for the remaining tail part. Since we estimate the threshold together with the other parameters of the GPD we take naturally into account the threshold uncertainty. We will discuss this model from a Bayesian point of view and the method will be illustrated using simulated data and a real data set.     

The effect of coordinating and non-coordinating additives on the transport properties in ionic liquid electrolytes for lithium batteries

In the present study we expand our analysis of using two contrasting organic solvent additives (toluene and THF) in an ionic liquid (IL)/Li NTf(2) electrolyte. Multinuclear Pulsed-Field Gradient (PFG) NMR, spin-lattice (T(1)) relaxation times and conductivity measurements over a wide temperature range are discussed in terms of transport properties and structuring of the liquid. The conductivity of both additive samples is enhanced the most at low temperatures, with THF slightly more effective than toluene. Both the anion and lithium self-diffusivity are enhanced in the same order by the additives (THF > toluene) while that of the pyrrolidinium cation is marginally enhanced. (1)H spin-lattice relaxation times indicate a reasonable degree of structuring and anisotropic motion within all of the samples and both (19)F and (7)Li highlight the effectiveness of THF at influencing the lithium coordination within these systems.     

Tuning the cavity of cyclodextrins: altered sugar adaptors in protein pores

Cyclodextrins (CDs) have been widely used in host-guest molecular recognition because of their chiral and hydrophobic cavities. For example, β-cyclodextrin (βCD) lodged as a molecular adaptor in protein pores such as α-hemolysin (αHL) is used for stochastic sensing. Here, we have tuned the cavity and overall size of βCD by replacing a single oxygen atom in its ring skeleton by a disulfide unit in two different configurations to both expand our ability to detect analytes and understand the interactions of βCD with protein pores. The three-dimensional structures of the two stereoisomeric CDs have been determined by the combined application of NMR spectroscopy and molecular simulation and show distorted conformations as compared to natural βCD. The interactions of these synthetic βCD analogues with mutant αHL protein pores and guest molecules were studied by single-channel electrical recording. The dissociation rate constants for both disulfide CDs from the mutant pores show ～1000-fold increase as compared to those of unaltered βCD, but are ～10-fold lower than the dissociation rate constants for βCD from wild-type αHL. Both of the skeleton-modified CDs show altered selectivity toward guest molecules. Our approach expands the breadth in sensitivity and diversity of sensing with protein pores and suggests structural parameters useful for CD design, particularly in the creation of asymmetric cavities.     

A modular microfluidic system for deoxyribonucleic acid identification by short tandem repeat analysis

Microfluidic technology has been utilized in the development of a modular system for DNA identification through STR (short tandem repeat) analysis, reducing the total analysis time from the ∼6 h required with conventional approaches to less than 3 h. Results demonstrate the utilization of microfluidic devices for the purification, amplification, separation and detection of 9 loci associated with a commercially-available miniSTR amplification kit commonly used in the forensic community. First, DNA from buccal swabs purified in a microdevice was proven amplifiable for the 9 miniSTR loci via infrared (IR)-mediated PCR (polymerase chain reaction) on a microdevice. Microchip electrophoresis (ME) was then demonstrated as an effective method for the separation and detection of the chip-purified and chip-amplified DNA with results equivalent to those obtained using conventional separation methods on an ABI 310 Genetic Analyzer. The 3-chip system presented here demonstrates development of a modular, microfluidic system for STR analysis, allowing for user-discretion as to how to proceed after each process during the analysis of forensic casework samples.