Laser-polarized Xe NMR and MRI at Ultralow Magnetic Fields

Laser-polarized ¹²⁹Xe and a high-T_csuperconducting quantum interference device (SQUID) are used to obtain magnetic resonance images in porous materials at a magnetic field of 2.3 mT, corresponding to a Larmor frequency of 27 kHz. Image resolution of 1 mm is obtained with gradients of only 1 mT/m. The resolution of xenon chemical shifts in different physicochemical environments at ultralow fields is also demonstrated. Details of the circulating flow optical pumping apparatus and the SQUID spectrometer are presented.     

Weakly parametric pseudodifferential operators and Atiyah-Patodi-Singer boundary problems

Oblatum 9-IX-1993 & 20-II-1995     

Optoelectronic and charge transport properties at organic-organic semiconductor interfaces: comparison between polyfluorene-based polymer blend and copolymer

We report detailed studies of optoelectronic and charge transport properties at the organic-organic semiconductor interfaces formed between polymer chains (interchain) and within a polymer chain (intrachain). These interfaces are fabricated using poly(9,9-di-n-octylfluorene-alt-N-(4-butylphenyl)diphenylamine) (TFB [f8-tfb]) (electron-donor) and poly(9,9-di-n-octylfluorene-alt-benzothiadiazole) (F8BT [f8-bt]) (electron-acceptor) conjugated polymers, by blending them together or by covalently attaching them via a main polymer backbone (copolymer). For optoelectronic properties, when a bulky and twisted tfb molecule is incorporated into a rigid F8BT conjugated backbone, it disturbs the conjugation of F8BT polymer, leading to a blue-shift in the lowest absorption transition. However, by acting as an effective electron donor, it assists the formation of an intrachain singlet exciton that has a strong charge-transfer character, leading to a red-shifted and longer-lived emission than that of F8BT. An extremely efficient and fast energy transfer from tfb donor to bt acceptor is observed in the copolymer (<1 ps) compared to transfer from TFB to F8BT in the blend (tens of ps). This efficient energy transfer in the copolymer is found to be associated with its low fluorescence efficiency (40-45% vs 60-65% for blend) because of the migration of radiative singlet excitons to low-energy states such as triplet and exciplex states that are nonemissive or weakly emissive. The presence of molecular-scale tfb-f8-bt interfaces in the copolymer, however, does not hinder an efficient transport of charge carriers at high drive voltages. Instead, it provides a better balance of charge carriers inside the device, which leads to slower decay of the device efficiency and thus more stable light-emitting diodes with increasing voltage than the blend devices. These distinctive optoelectronic and charge transport properties observed at different organic-organic semiconductor interfaces will provide useful input for the design rules of conjugated polymers required for improved molecular electronics.     

Characterization of the transmembrane serine receptor by capillary zone electrophoresis

Summary Capillary zone electrophoresis (CZE) was applied to the characterization of the transmembrane serine receptor in biosynthetic samples. The serine receptor, otherwise known as Tsr (taxis to serine and repellents), is a ∼ 60,000 Dalton intrinsic membrane protein whose periplasmic domain (ligand binding domain) reversibly binds the amino acid serine. In general, the electrophoresis of intrinsic membrane proteins is difficult due to severe solubility problems and adsorption which occurs during the electrophoretic run. This is due to the tendency of these types of proteins to undergo aggregation, self-aggregation and precipitation in aqueous environments. The addition of percentage levels of the surfactant, sodium dodecyl sulfate (SDS), to a tetraborate run buffer was shown to be effective both in enhancing the solubility of intact Tsr and in preventing the adsorption of intact Tsr to the fused-silica capillary wall during electrophoretic analysis. Critical separation parameters such as run buffer concentration, surfactant concentration and surfactant type were optimized to give the best separation profiles.