Differential etching of ZnO native planes under basic conditions

The in situ dissolution of polished (0001), (101(-)0), and (0001(-)) surfaces of ZnO was studied using Atomic Force Microscopy under alkaline conditions. In aqueous NaOH solution the (0001) plane forms a stepped surface whereas the (0001(-)) plane converts into more stable {101(-)1(-)} planes. Dissolution of the (101(-)0) plane leaves a combination of (0001) and (101(-)1(-)) planes. Dissolution in solutions containing both NaOH and Na(3)citrate causes the (0001) plane steps to increase in number and reduce in height, and cause an overall increase in the rate of dissolution in the [101(-)0] directions. These observations are explained using a mechanism based on edgewise dissolution where the etching rate depends on the number of surface oxygen atoms per zinc atom. Large areas of single index faces (over 50 μm(2)) of (0001) and (0001(-)), suitable for surface chemistry studies, were also generated by chemical dissolution.     

Sensitivity and resolution in proton solid-state NMR at intermediate deuteration levels: quantitative linewidth characterization and applications to correlation spectroscopy

We present a systematic study of proton linewidths in rigid solids as a function of sample spinning frequency and proton density, with the latter controlled by the ratio of protonated and perdeuterated model compounds. We find that the linewidth correlates more closely with the overall proton density (rho(H)) than the size of local clusters of (1)H spins. At relatively high magic-angle spinning (MAS) rates, the linewidth depends linearly upon the inverse MAS rate. In the limit of infinite spinning rate and/or zero proton concentration, the linewidth extrapolates to a non-zero value, owing to contributions from scalar couplings, chemical shift dispersion, and B(0) field inhomogeneity. The slope of this line depends on the overall concentration of unexchangeable protons in the sample and the spinning rate. At up to 30% protonation levels ( approximately 2 (1)H/100A(3)), proton detection experiments are demonstrated to have a substantial (2- to 3-fold) sensitivity gain over corresponding (13)C-detected experiments. Within this range, the absolute sensitivity increases with protonation level; the optimal compromise between sensitivity and resolution is in the range of 20-30% protonation. We illustrate the use of dilute protons for polarization transfer to and from low-gamma spins within 5A, and to be utilized as both magnetization source and detection spins. The intermediate protonation regime enhances relaxation properties, which we expect will enable new types of (1)H correlation pulse sequences to be implemented with improved resolution and sensitivity.     

High resolution AFM images of the single-crystal alpha-Al2O3(0001) surface in water

The (0001) surface of alpha-Al(2)O(3) single crystals has been imaged by atomic force microscopy in water. The observed hexagonal lattice arrangement has a period of 4.7 A, in good agreement with the known bulk unit cell. The sample cleaning procedure was found to be crucial in obtaining clean terraces and achieving lattice resolution.     

Organic complexation and total dissolved trace metal analysis in estuarine waters: comparison of solvent-extraction graphite furnace atomic absorption spectrometric and chelating resin flow injection inductively coupled plasma-mass spectrometric analysis

The measured concentrations of cadmium, cobalt, copper, nickel, lead, zinc, and manganese in acidified (pH<2) estuarine water samples analyzed for total dissolved trace metal concentrations using on-line chelating resin column partitioning with inductively coupled plasma-mass spectrometry (CRCP-ICP-MS) were compared to those analyzed by graphite furnace atomic absorption spectrometry (GFAAS) after liquid-liquid extraction using a combination of 1-pyrrolidinedithiocarbamate/diethyldithiocarbamate (PDC/DDC). Although there was good agreement between the two sets of analyses for cadmium, lead, manganese, and zinc concentrations, those of cobalt, copper, and nickel determined by CRCP-ICP-MS were found to be 10-20% lower than those determined by solvent-extraction GFAAS. The different yields were positively correlated (R>0.961, simple linear regression) to the dissolved organic carbon (DOC) concentration of the samples. Good agreement between the two methods for cobalt and copper was achieved after ultraviolet (UV) digestion of the acidified samples. Samples collected from the South Bay of the San Francisco Estuary with high DOC showed the greatest difference for cobalt, copper, and nickel which is tentatively attributed to complexation with humic material for copper and cobalt and strong synthetic chelating agents such as ethylenediaminetetraacetic acid (EDTA) for nickel. This is consistent with previous studies on copper, nickel and cobalt complexation in this region. We recommend UV digestion of acidified estuarine samples prior to multi-element analysis by chelating resin flow injection ICP-MS methods.     

Self-diffusion and electron spin exchange of hydrophobic probes in dilute solution

Electron spin exchange rate constants have been measured by ESR spectroscopy for a nitroxide spin probe in a number of solvents, including water. The apparent collision rate constants (k _c ^′ ) calculated from the spin exchange rate constants showed marked deviations from the Smoluchowsky equation (k _c ^′ η=const), which were greatest in solvents of lowest viscosity. These effects are attributed to inefficiency of the spin exchange process. Self-diffusion coefficients (D) were measured for diamagnetic analogs of the nitroxide spin probe in similar solvent systems by pulsed field gradient NMR spectroscopy. TheD values gave reasonable agreement when corrected for viscosity (Dη=const). Collision rate constants calculated fromD were in good agreement with those measured by ESR in solvents of high viscosity. Thek _c ^′ value for the spin probe in water was significantly lower than that in isoviscous organic solvents. This effect is discussed in terms of a hydrophobic hydration shell for the spin probe which acts as an additional barrier to collision.     

Isolation and structure elucidation of a novel yellow pigment from the marine bacterium Pseudoalteromonas tunicata

The marine environment is a major source for many novel natural compounds. A new yellow pigment has been isolated from the marine bacterium P. tunicata and identified as a new member of the tambjamine class of compounds. The structural identification was achieved by a combination of 1D and 2D-NMR spectroscopy and high resolution mass spectrometry data.     

Collision efficiency factor for heteroaggregation: extension to soft interactions

An improved model for the collision efficiency factor of clusters of oppositely charged spheres has been developed, which accounts for repulsive and attractive interactions that occur at a finite distance from the colliding species, i.e., the so-called "soft" interactions. Trends in measured optimum dosages for rapid aggregation with increasing Debye length (a decrease at particle size ratios between 0.3 and 1 and an increase at particle size ratios less than 0.3) are explained qualitatively by employing the modified collision efficiency model. Several observations from the literature, specifically the formation of stringlike aggregates at low ionic strength and the uneven optimum dosage requirements of particles of equal size, are also explained in view of the model presented.     

RTD-1mimic containing γPNA scaffold exhibits broad-spectrum antibacterial activities

Macrocyclic peptides with multiple disulfide cross-linkages, such as those produced by plants and those found in nonhuman primates, as components of the innate immunity, hold great promise for molecular therapy because of their broad biological activities and high chemical, thermal, and enzymatic stability. However, for some, because of their intricate spatial arrangement and elaborate interstrand cross-linkages, they are difficult to prepare de novo in large quantities and high purity, due to the nonselective nature of disulfide-bond formation. We show that the disulfide bridges of RTD-1, a member of the θ-defensin subfamily, could be replaced with noncovalent Watson-Crick hydrogen bonds without significantly affecting its biological activities. The work provides a general strategy for engineering conformationally rigid, cyclic peptides without the need for disulfide-bond reinforcement.     

Temperature-responsive solid-liquid separations with charged block-copolymers of poly(N-isopropyl acryamide)

Temperature responsive charged block-copolymers of poly(N-isopropylacrylamide) (PNIPAM) have been used in the solid-liquid separation of alumina mineral particles from aqueous solution. The effects of temperature, polymer charge-sign and fraction of charged segment have been investigated. Batch settling and adsorption studies showed that rapid sedimentation results for suspensions with polymers of opposite charge-sign to the particle surface-charge (counterionic) at 50 °C. Cooling the suspensions after flocculation at 50 °C was found to increase the final solids volume fraction of the sediment beds formed through a mechanism related to partial desorption of polymer and the reduction of the hydrophobic attraction. Suspension stability results after dosing with polymers of similar charge-sign to the particle surface-charge (co-ionic) at both 25 and 50 °C. Increasing the amount of polymer charge increased the influence of polymer charge-sign on the adsorption and solid-liquid separation behavior. The performance of the charged block copolymers are compared to that of the random charged copolymer and neutral homopolymer PNIPAM structures.