Faculty Bibliography

The development of a chip-based sensor array composed of individually addressable polystyrene-poly(ethylene glycol) and agarose microspheres has been demonstrated. The microspheres are selectively arranged in micromachined cavities localized on silicon wafers. These cavities are created with an anisotropic etch and serve as miniaturized reaction vessels and analysis chambers. A single drop of fluid provides sufficient analysis media to complete approximately 100 assays in these microetch pits. The cavities possess pyramidal pit shapes with trans-wafer openings that allows for both fluid flow through the microreactors/analysis chambers and optical access to the chemically sensitive microspheres. Identification and quantitation of analytes occurs via colorimetric and fluorescence changes to receptor and indicator molecules that are covalently attached to termination sites on the polymeric microspheres. Spectral data are extracted from the array efficiently using a charge-coupled device allowing for the near-real-time digital analysis of complex fluids. The power and utility of this new microbead array detection methodology is demonstrated here for the analysis of complex fluids containing a variety of important classes of analytes including acids, bases, metal cations, metabolic cofactors, and antibody reagents.

This paper provides the first direct structural evidence describing conjugated polymer self-assembly at the air-water interface. Grazing-incidence X-ray diffraction (GIXD) and X-ray reflectivity measurements on a number of derivatives of amphiphilic regioregular polythiophenes (e.g., poly(3'-dodecyl-3-(2,5,8-trioxanonyl)-2',5-bithiophene), polymer 1) show that these conjugated polymers self-assemble as 2-dimensional polycrystalline monolayers at the air/water interface with the amphiphilic polymers preorganized into rigid boards standing edge-on on the water surface. The monolayer consists of highly ordered (similar to 70% crystalline) domains, with a centered rectangular unit cell having the polymer backbone along the a-axis and the thiophene, pi-stack along the b axis with a distance of 3.85-3.94 A depending on the applied surface pressure. These domains are connected by soft, more disordered boundaries. This is evidenced by the macroscopic compressibility of the entire LB film (C(macro) approximate to 4-7 m/N) being one order of magnitude larger than the microscopic compressibility (C(micro) approximate to 0.6 m/N) of the polycrystalline domains. The alkyl chains in the 3-position of the thiophene rings are in a crystallographically disordered state due to their cross-sectional mismatch with the packing of the thiophenes. The importance of having the side chains coupled in a regioregular fashion to the 3-position of the thiophene rings is evidenced by a dramatic increase in the coherence length of the crystalline domains for highly regioregular samples (>95% head-tail couplings) as compared to less regioregularly coupled polymers (similar to 80% head-tail couplings). Transfer to solid support by the Langmuir-Blodgett technique induces an overall orientation of the domains in the film, giving rise to a dichroic ratio of up to 4. Reflection-absorption infrared spectroscopy (RAIRS) shows that the alkyl chains of transferred films are in an all-trans conformation with a locally ordered environment, having only few gauche defects.

Thin films of (001)-oriented Sr2FeMoO6 have been epitaxially deposited or LaAlO3 and SrTiO3 (001) substrates by pulsed laser deposition. The deposition conditions were optimized. Single-phase Sr2FeMoO6 was obtained in 100 mTorr 99.999% Ar gas at 825 degrees C. Transport and magnetic data showed a metallic temperature dependence and a saturation magnetization M-s at 10 K of 3.2 mu(B)/f.u. However, the Curie temperature T(C)approximate to 380 K was reduced from 415 K found for tetragonal polycrystalline best ceramics, which lowers M-s at 300 K in the thin films to 1.5 mu(B)/f.u. compared to 2.2 mu(B)/f.u. in the ceramics. A low remanence was attributed to the presence of antiphase boundaries. A Wheatstone bridge arrangement straddling a bicrystal boundary was used to verify that spin-dependent electron transfer through a grain boundary and not an antiphase boundary is responsible for the low-field magnetoresistance found in polycrystalline samples below T-C. (C) 2000 American Institute of Physics. [S0021-8979(00)55908-8].

A micromachined fluidic sensor array for the rapid characterization of multiple analytes in solution has been developed. A simple micromachined fluidic structure for this biological and chemical agent detection system has been designed and fabricated, and the system has been tested. Sensing occurs via optical changes to indicator molecules that are attached to polymeric microspheres (beads). A separate charged-coupled-device (CCD) is used for the simultaneous acquisition of the optical data from the selectively arranged beads in micromachined etch cavities. The micromachined bead support structure has been designed to be compatible with this hybrid optical detection system. The structure consists of four layers: cover glass, micromachined silicon, dry film photoresist, and glass substrate. The bottom three layers are fabricated first, and the beads are selectively placed into micromachined etch cavities. Finally, the cover glass is applied to confine the beads. This structure utilizes a hydrophilic surface of the cover glass to draw a liquid sample into the sensor array without moving components, producing a compact, reliable, and potentially low-cost device. We have initially characterized fluid flow through a complete chip, showing complete filling of the sample chamber in approximately 2 seconds. The test results show that this system may be useful in micro total analysis systems (mu - TAS), especially in single-use biomedical applications.

Thin films of (001)-oriented Sr2FeMoO6 have been epitaxially deposited on LaAlO3 and SrTiO3 (001) substrates. Comparison of their transport and magnetic properties with those of polycrystalline ceramic samples shows a metallic versus semiconductor temperature dependence and a saturation magnetization M-s at 10 K of 3.2 mu(B)/f.u. in the film as against 3.0 for a tetragonal polycrystalline sample. However, the Curie temperature T(C)approximate to 389 K is reduced from 415 K found for the tetragonal ceramic, which lowers M-s at 300 K in the thin films to 2.0 mu(B)/f.u. compared to 2.2 mu(B)/f.u. in the ceramics. A Wheatstone bridge arrangement straddling a bicrystal boundary has been used to verify that spin-dependent electron transfer through a grain boundary is responsible for the low-field magnetoresistance found in polycrystalline samples below T-C. (C) 1999 American Institute of Physics. [S0003-6951(99)00244-2].