Understanding charge transport in molecular electronics.

For molecular electronics to become a viable technology the factors that control charge transport across a metal-molecule-metal junction need to be elucidated. We use an experimentally simple crossed-wire tunnel junction to interrogate how factors such as metal-molecule coupling, molecular structure, and the choice of metal electrode influence the current-voltage characteristics of a molecular junction.

Metal-molecule contacts and charge transport across monomolecular layers: measurement and theory.

Charge transport studies across molecular length scales under symmetric and asymmetric metal-molecule contact conditions using a simple crossed-wire tunnel junction technique are presented. It is demonstrated that oligo(phenylene ethynylene), a conjugated organic molecule, acts like a molecular wire under symmetric contact conditions, but exhibits characteristics of a molecular diode when the connections are asymmetric. To understand this behavior, we have calculated current-voltage (I-V) characteristics using extended Huckel theory coupled with a Green's function approach. The experimentally observed I-V characteristics are in excellent qualitative agreement with the theory.

Explosives detection in soil using a field-portable continuous flow immunosensor.

A field method for quantitative analysis of explosives in contaminated soil samples is described. The method is based on a displacement immunoassay performed in a commercial instrument, the FAST 2000, engineered by Research International Inc. The method can be used on-site to measure 2,4,6-trinitrotoluene (TNT) and hexahydro-1,3,5-trinitro-1,3,5-triazine (RDX) within 5min. For this study, replicate analyses were performed on soil extracts prepared from each field sample as well as appropriate controls, blanks, and laboratory standards. Statistical analyses were done to assess accuracy, bias, and predictability of the method. The results demonstrated that the immunosensor could be used effectively to screen environmental samples for the presence or absence of explosives. In most samples, the method also provided quantitative values that were in good agreement with standard laboratory analyses using HPLC. A limited number of sample matrices interfered with the immunoassay and produced results that varied significantly from the laboratory data. In each case, the compounds causing the problem have been identified and efforts are being made to minimize these matrix interferences in future field evaluations.

Continuous flow displacement immunosensors: a computational study.

Numerical modeling has been used to investigate the disparity in performance and sensitivity that has been reported for flow displacement immunosensors based on bead-packed columns, membranes, and capillary tubes. The simulations strongly suggest that the high surface areas in the porous media systems may actually be detrimental to sensor performance because of large numbers of free antibody binding sites. Since the free antibody sites are created during the wash step in which the baseline is established, wash protocols are critical in optimizing the sensitivity for a given displacement sensor.

The remote electron beam-induced current analysis of grain boundaries in semiconducting and semi-insulating materials.

When no charge collecting p-n junction or Schottky barrier is present in the specimen, but two contacts are applied, conductive mode scanning electron microscope (SEM) observations known as remote electron beam-induced current (REBIC) can be made. It was described as "remote" EBIC because the contacts to the specimen can lie at macroscopic distances from the beam impact point. In recent years, REBIC has been found to be useful not only for studies of grain boundaries in semiconducting silicon and germanium, but also in semi-insulating materials such as the wider bandgap II-VI compounds and electroceramic materials like varistor ZnO and positive temperature coefficient resistor (PTCR) BaTiO3. The principles of this method are outlined. Accounts are given of the five forms of charge collection and resistive contrast that appear at grain boundaries (GBs) in REBIC micrographs. These are (1) terraced contrast due to high resistivity boundary layers, (2) peak and trough (PAT) contrast due to charge on the boundary, (3) reversible contrast seen only under external voltage bias due to the beta-conductive effect in a low conductivity boundary layer, (4) dark contrast due to enhanced recombination, and (5) bright contrast apparently due to reduced recombination. For comparison, the results of the extensive EBIC studies of GBs in Si and Ge are first outlined and then the results of recent REBIC grain boundary studies in both semiconducting and semi-insulating materials are reviewed.

Fabrication of resorbable microporous intravascular stents for gene therapy applications.

The authors have produced resorbable, microporous endoluminal stents from Poly-L-lactic acid (PLLA)/Poly epsilon-caprolactone (PCL) blends. Both helical and tube stent designs have been obtained by solvent casting and flotation-precipitation fabrication techniques. A range of PLLA/PCL blend ratios and process variables were employed to investigate their influence on mechanical properties, porosity, and degradation rate. Polymer blends with higher PLLA proportions exhibit higher elastic moduli and ultimate tensile strength, and lower elongation, porosity, and degradation rates than do materials with higher PCL content. Stents with suitable mechanical properties for deployment and support of the vessel wall were obtained. Poly(ethylene oxide) was incorporated into these devices using an acid swelling technique, opening the pore structure and improving the hydrophilic character, thereby enabling the uptake of recombinant adenoviral vectors. The 50:50 PLLA/PCL blended stents were impregnated with recombinant adenovirus (AdCMB beta Gal, encoding a nuclear localizing variant of Escherichia coli beta-galactosidase). Cultured CV-1 cells incubated with stents impregnated with the recombinant virus expressed nuclear localized beta-galactosidase activity, confirming that absorbed virus is released from the matrix in an infectious form, with kinetics suggesting that genetically enhanced endovascular devices of this design are feasible.

Endothelial cell binding to Dacron modified with polyethylene oxide and peptide.

Polyethylene oxide (PEO) was incorporated into the surface of Dacron (PET) vascular prosthetic material (crimped Bionit I, BNl) followed by covalent attachment of an endothelial cell (EC) adhesion peptide; Gly-Arg-Glu-Asp-Val-Tyr (GREDVY). This procedure provides the possibility of a surface selective for EC adherence. Optimal PEO incorporation with minimal fiber damage was achieved from 78% (vol) trifluoroacetic acid (TFA) as characterized by scanning electron microscopy, nuclear magnetic resonance, bromphenol blue staining, and tensile testing. By weight, there was 4.4 times as much 18.5kD PEO as 1.5kD PEO incorporated into PET, but there were 2.8 times as many molecules of low molecular weight PEO. Attachment of 125I-GREDVY to substrates increased as follows: BNJ < BNI - 18.5 kD PEO < BNI - 1.5 kD PEO. Polyethylene oxide size affected EC binding with high molecular weight material decreasing binding and low molecular weight PEO increasing attachment. GREDVY modification of BNI or either of the two BNI-PEOs gave small but consistently increased EC binding compared with the same preparation without GREDVY. In contrast, human fibroblasts cultured from umbilical vein showed decreased binding when GREDVY was attached to BNI or PEO modified BNI. These results indicate that selective EC binding to PET vascular prostheses can be achieved.

Developing an in-house physician advisor program.

The rapid rise in health care costs during the 1980s has led to a growing demand for utilization management companies, supported by teams of physician advisors. The increasing involvement of physician advisors in day-to-day case review has also led to a growing necessity for their being hired on an in-house basis. This article attempts to show a basic process for developing a functional and efficient in-house physician advisor program.