Self-organized perylene diimide nanofibers

A propeller-shaped perylene diimide trimer was synthesized and a simple evaporation method was used for the self-organization of trimer molecules into fluorescent nanofibers. The sizes of these fibers-from 4 to 150 nm in diameter-were measured by atomic force microscopy and can be controlled by adjusting the concentration of the initial solution. The aspect ratios (length/height) are around 500. The plane of the trimer was determined by polarized scanning confocal microscopy to be perpendicular to the axis of the fibers, in agreement with molecular mechanics calculations. UV/vis and NMR spectroscopies were used to monitor concentration-dependent pi-pi stacking in solution. Single-fiber fluorescence imaging and spectroscopy were performed using a total internal reflection fluorescence microscope equipped with a digital color camera and imaging CCD spectrometer. Strongly red-shifted fluorescence from these fibers indicates a high degree of electronic delocalization, and breaking up this delocalization by photobleaching blue-shifts the emission toward that of an isolated noninteracting molecule. The delocalization along these nanofibers and the ability to study the electronic structure using fluorescence make them potentially useful in nanoscale devices, such as field effect transistors and photoconductors.     

Using single-molecule fluorescence spectroscopy to study electron transfer.

Techniques in single-molecule fluorescence spectroscopy now allow sophisticated studies of photophysical processes in single molecules. As interest grows in the possibilities of molecular electronics, researchers have begun to turn these techniques to the study of electron transfer. Electron-transfer reactions have now been detected and measured at the single-molecule level in a variety of systems and on a variety of timescales by adapting techniques from previous single-molecule fluorescence studies.     

Surface adsorption effects in the inkjet printing of an aqueous polymer solution on a porous oxide ceramic substrate

The inkjet printing of a polymeric solution into a porous substrate was studied, with the focus on phenomena occurring within the pore space during infiltration. Lines of aqueous polyacrylic acid (PAA) solution were printed onto the surface of porous, high-green-density ceramic powder beds. The PAA is a binder for the ceramic particles, allowing removal of the printed line structure ("primitive") and characterization of the extent of polymer penetration. Large differences in cross section of the retrieved printed structure were observed between ceramic systems and for different specific surface area powders. A mechanism for "filtration" of the polymer by adsorption onto the ceramic particle surfaces during infiltration was proposed. The adsorption of PAA onto Al2O3, SiO2, and TiO2 was characterized via adsorption isotherms, and the trend of primitive cross section with PAA adsorption was consistent with the filtration hypothesis, as was the variation with powder-specific surface area. These results can be generalized to other systems where a solution is inkjet printed onto a porous substrate (e.g., inks on plain paper, porous coated papers, etc.) Utilization of the adsorption effects may allow confinement of the solute molecules (e.g., colorant) to a small region near the substrate surface.     

Application of multivariate outlier detection to fluid velocity measurements

A statistical-based approach to detect outliers in fluid-based velocity measurements is proposed. Outliers are effectively detected from experimental unimodal distributions with the application of an existing multivariate outlier detection algorithm for asymmetric distributions (Hubert and Van der Veeken, J Chemom 22:235–246, 2008). This approach is an extension of previous methods that only apply to symmetric distributions. For fluid velocity measurements, rejection of statistical outliers, meaning erroneous as well as low probability data, via multivariate outlier rejection is compared to a traditional method based on univariate statistics. For particle image velocimetry data, both tests are conducted after application of the current de facto standard spatial filter, the universal outlier detection test (Westerweel and Scarano, Exp Fluids 39:1096–1100, 2005). By doing so, the utility of statistical outlier detection in addition to spatial filters is demonstrated, and further, the differences between multivariate and univariate outlier detection are discussed. Since the proposed technique for outlier detection is an independent process, statistical outlier detection is complementary to spatial outlier detection and can be used as an additional validation tool.     

Experimental study of kinematic focusing. Comparison of electroinjection and sequential injection determination of copper

The recently predicted phenomenon of kinematic focusing was studied experimentally using copper ions and EDTA as reactants. Kinematic focusing occurs, in electroinjection analysis, when the detected reaction product moves at the same rate as the reagent present in excess. Thus, reaction product accumulates without dispersion at the front of the excess reagent. Cu-EDTA(2-) complex was observed at 254 nm to form an exceptionally sharp peak as the front of the EDTA zone passed by the detector. The concentrating effect of kinematic focusing was quantified by electroinjection of premixed Cu-EDTA(2-). Sensitivity was compared to that of sequential injection analysis using a 1 cm optical pathlength. Sensitivity was highest in the electroinjection mode, in spite of its 120 mum capillary pathlength, due to kinematic focusing.     

Limitations and improvements of trace element analysis with proton-induced X-rays

The technique of trace element analysis by the detection of characteristic X-rays induced by high energy proton bombardment is critically examined. A discussion of the restrictions imposed by detector resolution is made, the behavior of the cross-section for X-ray production as a function of energy and atomic number is discussed. The effect of this behavior upon analysis time is demonstrated. Background is discussed in terms of its effect on the total counting rate. The use of a silicon solid state detector to determine the overlappingK-lines of light elements andL-lines of heavy elements is shown to have severe limitations. A combination of crystal monochromators and solid state detectors is suggested as a means to circumvent some of the fundamental limitations encountered with a single detector. An experimental arrangement for the use of twenty-four crystal monochromators and two solid state detectors is shown. Work supported in part by the U.S. Atomic Energy Commission.