Alloyed semiconductor quantum dots: tuning the optical properties without changing the particle size

Alloyed semiconductor quantum dots (cadmium selenium telluride) with both homogeneous and gradient internal structures have been prepared to achieve continuous tuning of the optical properties without changing the particle size. Our results demonstrate that composition and internal structure are two important parameters that can be used to tune the optical and electronic properties of multicomponent, alloyed quantum dots. A surprising finding is a nonlinear relationship between the composition and the absorption/emission energies, leading to new properties not obtainable from the parent binary systems. With red-shifted light emission up to 850 nm and quantum yields up to 60%, this new class of alloyed quantum dots opens new possibilities in band gap engineering and in developing near-infrared fluorescent probes for in vivo molecular imaging and biomarker detection.     

The synthesis and structure of bis(acetonitrile)nitrosyl tris(3,5-dimethylpyrazolyl)borato molybdenum hexafluorophosphate

Summary Treatment of [Mo{HB(3,5-Me₂C₃HN₂)₃}(NO)I₂] with one or two moles of AgPF₆ in acetonitrile afforded the paramagnetic (one unpaired electron) complex [Mo{HB(3,5-Me₂C₃NH₂)₃}(NO)(NCMe)₂][PF₆]. The structure of this complex was determined crystallographically, and the six-coordinate geometry of the complex cation confirmed.     

Characterization of acridone dyes for use in four-decay detection in DNA sequencing

Four acridone dyes and dye-labeled primers were characterized for use in four-decay DNA sequencing. In the four-decay scheme, fluorescence lifetime replaces spectral ("color") selectivity for distinguishing between four base-specific labels in a single-lane capillary electrophoresis (CE) separation of the DNA fragments. Prior to the introduction of the acridone dyes, a major obstacle to four-decay detection was the lack of four suitable dyes with resolvable lifetimes. The four acridone dyes, whether free in solution or tethered to DNA primer, exhibit significant differences among their lifetimes and are well-suited to use together in four-decay sequencing. The lifetimes of the four dye-labeled DNA primers that were sequentially injected and detected on-the-fly in a 2% POP6 sequencing gel were 4, 6, 11 and 14 ns. A 405 nm violet laser diode provides optimal excitation of the four dyes.     

Improved method for the determination of the major neutral steroids and unconjugated bile acids in human faeces using capillary gas chromatography

An improved method has been developed for the determination of the major neutral steroids (cholesterol and 5 beta-cholestan-3 beta-ol) and unconjugated bile acids (deoxycholic acid and lithocholic acid) in human faeces, using capillary gas chromatography with flame ionization detection. The freeze-dried faecal sample was subjected to a two-stage Soxhlet extraction followed by an aqueous alkali-organic solvent partition step to separate neutral steroids from bile acids. The neutral steroids were analysed as their trimethylsilyl ether derivatives on an OV-1 capillary column. The bile acids were further purified on a Sep-Pak C18 cartridge and then fractionated on a Sep-Pak SIL cartridge. Unconjugated bile acids were analysed as their methyl ester-trimethylsilyl ether derivatives also on an OV-1 capillary column. Quantitation of neutral steroids and unconjugated bile acids was achieved by reference to appropriate internal standards, added to the faecal extract immediately after the Soxhlet extraction stage. The method is being used in a study of the effect of diet on the metabolic activity of human gut flora.     

An analysis of avidin, biotin and their interaction at attomole levels by voltammetric and chromatographic techniques

The electroanalytical determination of avidin in solution, in a carbon paste, and in a transgenic maize extract was performed in acidic medium at a carbon paste electrode (CPE). The oxidative voltammetric signal resulting from the presence of tyrosine and tryptophan in avidin was observed using square-wave voltammetry. The process could be used to determine avidin concentrations up to 3 fM (100 amol in 3 l drop) in solution, 700 fM (174 fmol in 250 l solution) in an avidin-modified electrode, and 174 nM in a maize seed extract. In the case of the avidin-modified CPE, several parameters were studied in order to optimize the measurements, such as electrode accumulation time, composition of the avidin-modified CPE, and the elution time of avidin. In addition, the avidin-modified electrode was used to detect biotin in solution (the detection limit was 7.6 pmol in a 6 l drop) and to detect biotin in a pharmaceutical drug after various solvent extraction procedures. Comparable studies for the detection of biotin were developed using HPLC with diode array detection (HPLC-DAD) and flow injection analysis with electrochemical detection, which allowed biotin to be detected at levels as low as 614 pM and 6.6 nM, respectively. The effects of applied potential, acetonitrile content, and flow rate of the mobile phase on the FIA-ED signal were also studied.     

Use of an automated image processing program to quantify recombinant adenovirus particles.

Electron microscopy has a pivotal role as an analytical tool in pharmaceutical research. However, digital image data have proven to be too large for efficient quantitative analysis. We describe here the development and application of an automated image processing (AIP) program that rapidly quantifies shape measurements of recombinant adenovirus (rAd) obtained from digitized field emission scanning electron microscope (FESEM) images. The program was written using the macro-recording features within Image-Pro Plus software. The macro program, which is linked to a Microsoft Excel spreadsheet, consists of a series of subroutines designed to automatically measure rAd vector objects from the FESEM images. The application and utility of this macro program has enabled us to rapidly and efficiently analyze very large data sets of rAd samples while minimizing operator time.     

Enantiomeric resolution of underivatized small peptides by HPLC with a chiral crown ether stationary phase

Factors influencing the stereoisomeric resolution of underivatized dipeptides and a representative tripeptide on Crownpak (CR) columns have been investigated. The elution order and relative retention suggest that a combination of chiral, steric, and hydrophobic interactions effects the extent of chiral recognition and the retention achieved during separations. Some dipeptides whose amine terminus is located three atoms from the asymmetric center (such as dipeptides of D ,L -glycine) were resolved, but the elution order was the opposite of that expected for the type of Crownpak column used (CR(+)). Peptides containing hydrophobic substituents were strongly retained, but their retention times could be significantly reduced, and detectability improved, by use of gradient elution. Analysis of a commercial sample of D ,L -leucine-D ,L -alanine revealed the stereoisomers to be present in an unexpected quantitative ratio and demonstrated the utility of these separations for quality assurance and quantitative analyses.     

Determination of uranium in electro-refined plutonium by a combined ion-exchange and x-ray fluorescence technique

Uranium is separated from plutonium by adsorption on an anion-exchange resin from a concentrated hydrochloric acid solution and is eluted from the resin, collected on a disc of cation-exchange paper, and determined by X-ray fluorescence. The lower limit of detection is 2mug of uranium, and 50mug may be determined with a recovery of 98% and a standard deviation of 3.2mug. In plutonium metal, 19ppm of uranium were found, with a standard deviation of 2 ppm (6 results).     

The Role of Multiple Teaching Strategies in Promoting Active Learning in Organic Chemistry

We have introduced four alternative teaching strategies into our yearlong organic chemistry course and have assessed changes in student performance relative to the same course taught by the same instructors using a more traditional lecture format. These strategies, which include reading worksheets, dialogues, in-class worksheets, and role-playing, allow the professor to move through a learning cycle that may effectively accommodate the students needs and multiple learning styles. The reading worksheets guide students through the concept phase and dialogues help students identify the importance of the concepts as they articulate these ideas for themselves. Group worksheets and role-playing provide opportunities for peer-interaction, application of knowledge, creativity, and self-expression. Others have shown that active learning strategies neither enhance nor diminish a students ability to retain factual information, a finding that is supported by our study. Our data from this one study show, however, that students taught by a more traditional approach demonstrate a greater variation in final exam performance between first and second semesters than those taught using a combination of techniques. This result reflects a shift in emphasis from the professor as an information source to the actively engaged student taking responsibility for his or her own learning. This study suggests that these methods, when used in a consistent fashion in conjunction with interactive lecturing, provide a broad base to facilitate student learning and aid in the development of higher order thinking skills.