Molecular rectification and conductance switching in carbon-based molecular junctions by structural rearrangement accompanying electron injection

Molecular junctions were fabricated consisting of a 3.7 nm thick layer of nitroazobenzene (NAB) molecules between a pyrolyzed photoresist substrate (PPF) and a titanium top contact which was protected from oxidation by a layer of gold. Raman spectroscopy, XPS, and AFM revealed that the NAB layer was 2-3 molecules thick and was bonded to the two conducting contacts by C-C and N-Ti covalent bonds. The current/voltage behavior of the PPF/NAB(3.7)/Ti junctions showed strong and reproducible rectification, with the current at +2 V exceeding that at -2 V by a factor of 600. The observed current density at +3 V was 0.71 A/cm(2), or about 10(5) e(-)/s/molecule. The i/V response was strongly dependent on temperature and scan rate, with the rectification ratio decreasing for lower temperature and faster scans. Junction conductivity increased with time over several seconds at room temperature in response to positive voltage pulses, with the rate of increase larger for more positive potentials. Voltage pulses to positive potentials and back to zero volts revealed that electrons are injected from the Ti to the NAB, to the extent of about 0.1-1 e(-)/molecule for a +3 V pulse. These electrons cause an activated transition of the NAB into a more conductive quinoid state, which in turn causes an increase in conductivity. The transition to the quinoid state involves nuclear rearrangement which occurs on a submillisecond to several second time scale, depending on the voltage applied. The quinoid state is stable as long as the applied electric field is present, but reverts back to NAB within several minutes after the field is relaxed. The results are interpreted in terms of a thermally activated, potential dependent electron transfer into the 3.7 nm NAB layer, which brings about a conductivity increase of several orders of magnitude.     

Development of the resolution theory for electrophoretic exclusion

Electrophoretic exclusion, a technique that differentiates species in bulk solution near a channel entrance, has been demonstrated on benchtop and microdevice designs. In these systems, separation occurs when the electrophoretic velocity of one species is greater than the opposing hydrodynamic flow, while the velocity of the other species is less than that flow. Although exclusion has been demonstrated in multiple systems for a range of analytes, a theoretical assessment of resolution has not been addressed. To compare the results of these calculations to traditional techniques, the performance is expressed in terms of smallest difference in electrophoretic mobilities that can be completely separated (R = 1.5). The calculations indicate that closest resolvable species (Δμ_min) differ by approximately 10^?13 m²/Vs and peak capacity (n_c) is 1000. Published experimental data were compared to these calculated results.     

Sensitive biological detection with a soluble and stable polymeric paramagnetic nanocluster

We describe the design, synthesis, and biological characterization of manganese oxocluster-based "single molecule magnets (SMMs)". We demonstrate that polymeric micellar nanoparticles can serve as a carrier and help to stabilize delicate SMM molecules from breaking down easily and thus prevent their property loss. Concentrating thousands of Mn-clusters per micelle provided a high ionic and per-particle relaxivity allowing sensitive MR imaging in vivo. This reports one of the earliest examples of in vivo imaging of a rationally designed polymeric micelle that features SMM.     

A Validated GC-MS Assay for the Quantitation of Trichloroethylene (TCE) from Drinking Water

Trichloroethylene (TCE) is a common ground and surface water contaminant found in the United States. A validated GC-MS assay for the quantitation of TCE in drinking water is presented here. The limit of quantitation, 5 µg/L, is lower than current validated methods for the analysis of TCE from water. This assay requires a small sample volume, has simple sample preparation, fast run time, high recovery, and reproducible and accurate results.     

Recent developments in electrophoretic separations on microfluidic devices

Research combining the areas of separation science and microfluidics has gained popularity, driven by the increasing need to create portable, fast, and low analyte-consumption devices. Much of this research has focused on the developments in electrophoretic separations, which use the electrokinetic properties of analytes to overcome many of the problems encountered during system scale-down. In addition, new physical phenomenon can be exploited on the microscale not available in standard techniques. In this study, the innovative developments, including electrophoretic concentration, sample preparation/conditioning, and separation on-chip are reviewed, along with some introductory discussions, from January 2008 to July 2010.     

Quantification and validation of HPLC‐UV and LC‐MS assays for therapeutic drug monitoring of ertapenem in human plasma

Rapid and simple HPLC‐UV and LC‐MS methods were developed and validated for the quantification of ertapenem (Invanz?) in human plasma. Ertapenem is a unique drug in that current dosing recommendations call for a 1 g dose for normal renal function patients, despite body weight. These assays, which involve a protein precipitation followed by liquid–liquid extraction, allow for fast therapeutic drug monitoring of ertapenem in patients, which is especially useful in special populations. Both methods were sufficient to baseline resolve meropenem (internal standard) and ertapenem, and were validated over 3 days using a six‐point calibration curve (0.5–50 µg/mL). Validation was collected using four different points on the calibrations curve yielding acceptable precision (<15% inter‐day and intra‐day; <20% for lower limit of quantitation, LLOQ) as well as accuracy (<15% inter‐day and intra‐day; <20% for LLOQ). The lower limit of detection (LOD) was determined to be 0.1 and 0.05 µg/mL for the HPLC‐UV and LC‐MS methods, respectively. The developed HPLC‐UV and LC‐MS methods for ertapenem quantification are fast, accurate and reproducible over the calibration range and can be used to determine ertapenem plasma concentrations for monitoring clinical efficacy. Copyright © 2012 John Wiley & Sons, Ltd.     

Milk and serum standard reference materials for monitoring organic contaminants in human samples

Four new Standard Reference Materials (SRMs) have been developed to assist in the quality assurance of chemical contaminant measurements required for human biomonitoring studies, SRM 1953 Organic Contaminants in Non-Fortified Human Milk, SRM 1954 Organic Contaminants in Fortified Human Milk, SRM 1957 Organic Contaminants in Non-Fortified Human Serum, and SRM 1958 Organic Contaminants in Fortified Human Serum. These materials were developed as part of a collaboration between the National Institute of Standards and Technology (NIST) and the Centers for Disease Control and Prevention (CDC) with both agencies contributing data used in the certification of mass fraction values for a wide range of organic contaminants including polychlorinated biphenyl (PCB) congeners, chlorinated pesticides, polybrominated diphenyl ether (PBDE) congeners, and polychlorinated dibenzo-p-dioxin (PCDD) and dibenzofuran (PCDF) congeners. The certified mass fractions of the organic contaminants in unfortified samples, SRM 1953 and SRM 1957, ranged from 12 ng/kg to 2200 ng/kg with the exception of 4,4′-DDE in SRM 1953 at 7400 ng/kg with expanded uncertainties generally <14 %. This agreement suggests that there were no significant biases existing among the multiple methods used for analysis.

Polymorphism in yttrium molybdate Y2Mo3O12

Yttrium molybdate (Y₂Mo₃O₁₂) has been prepared by non-hydrolytic sol–gel chemistry. The phase evolution upon heating was investigated using in situ and ex situ heat treatments combined with powder X-ray diffraction. This method has led to the isolation of two orthorhombic phases with different atomic connectivity. Yttrium adopts 6- and 7-coordinate sites in the Pbcn and Pba2 structures, respectively. Cocrystallization of both phases was observed in a narrow temperature range, suggesting that crystallization kinetics play a major role in phase formation. It was found that the Pba2 phase is the stable polymorph below 550 °C, and converts to Pbcn at higher temperatures.