Synthesis of Nucleobase‐Functionalized Carbon Nanotubes and Their Hybridization with Single‐Stranded DNA

For the first time ssDNA (25‐aptamer of mixed dA, dT, dG, and dC) was wrapped around functionalized single‐walled carbon nanotubes (SWCNTs), whose external surfaces were attached to multiple triazole‐(ethylene glycol)‐dA ligands. This method of hybridization involved the formation of hydrogen bonds between dT of ssDNA and dA of functionalized SWCNTs. It deviates from the reported π–π stacking between the nucleobases of DNA and the external sidewalls of nanotubes. The structural properties of the functionalized SWCNTs and its ssDNA complex were characterized by spectroscopic (including CD and Raman), thermogravimetric, and microscopic (TEM) methods. The results thus obtained establish a new platform of DNA delivery by use of nanotubes as a new vehicle with great potential in biomedical applications and drug development.     

Effect of normal and reversed polarizations on optical characteristics of ultraviolet-violet InGaN laser diodes

The optical characteristics of ultraviolet-violet InGaN laser diodes with different numbers of quantum wells under normal and reversed polarizations are numerically investigated. For the laser structures under normal polarization, the lowest threshold current is obtained when the number of quantum wells is two in the spectral range of 380-408 nm. For the laser structures under reversed polarization, the single quantum-well laser structure possesses the lowest threshold current. The simulation results suggest that the physical origin for these phenomena is caused by the sufficiently suppressed electron and hole leakage currents when the laser diode is under reversed polarization.     

Phosphite-based sialic acid donors in the synthesis of α(2→9) oligosialic acids

The combination of a phosphite donor and an anomeric thiocresol-protected acceptor, both with a TFA protecting group at C-5 of the sialic acid, provides good α-selectivity and yield in sialylation. Although the convergent synthetic strategy of using a phosphite disialo-donor and a disialo-acceptor assembles tetra-sialic acid efficiently, overcoming the low α-selectivity of α-anomer and purifying it remain to be achieved. Furthermore, mono- and di-sialic acids were, respectively, conjugated on carrier protein, keyhole limpet hemocyanin. The enzymatic hydrolysis method is recommended for estimating the amount of sialic acid on a protein conjugate.     

Comparison of optical pump-probe characterization of low-temperature-grown GaAs at well-above-bandgap and near-bandedge wavelengths

Annealed low-temperature-grown GaAs was studied by time-resolved photoreflectance measurement at well-above-bandgap photon energies and by photoreflectance as well as transmission measurements at near-bandedge wavelength. At near-bandedge wavelength, the initial changes in reflectivity and transmission were observed to relax at identical relaxation rate, which was attributed to the absence of carrier cooling and the domination of carrier trapping. All the measured photoreflectance traces were found to be well fitted by the previously proposed three-component decomposition procedure. Among the three components, the fast positive peak was attributed to absorption bleaching and its relaxation, that is, the scattering of the photo-carrier out of their initially excited states by carrier cooling and trapping. The decay times of the positive peak, combined with the carrier cooling times extracted from photoreflectance measurement on semi-insulating GaAs, give consistent estimate of carrier trapping time at all wavelengths within the spectral range. Our results verify that well-above-bandgap photoreflectance measurement combined with the three-component fitting procedure can be used to estimate the photo-carrier trapping time which are consistent with that obtained by near-bandedge photoreflectance and transmission techniques.     

Poly(p‐phenylene vinylene) derivatives containing triazole or oxadiazole segments: Connector effect in optical,electrochemical, and electroluminescent properties

To study the effect of connector structure between hole‐ and electron‐transporting segments, we synthesized and characterized new electroluminescent polymers P 1 – P 7 consisting of hole‐transporting 1,4‐bis(hexyloxy)‐2,5‐distyrylbenzene (DSB: P 1 and P 2 ) and electron‐transporting 4‐(4‐(hexyloxy)phenyl)‐3,5‐diphenyl‐4H‐1,2,4‐triazole (TAZ: P 3 and P 4 ) or 2‐(2,5‐bis(hexyloxy)‐4‐(5‐phenyl‐1,3,4‐oxadiazol)phenyl)‐5‐phenyl‐1,3,4‐oxadiazole (DIOXD: P 5 – P 7 ) segments linked by different connectors. The connectors between hole‐ and electron‐transporting segments are (1) 1,4‐phenylene in P 3 and P 5 , (2) 1,4‐divinylbenzene in P 4 and P 6 , and (3) 4,4′‐biphenyl in P 7 . Three corresponding end‐capped model polymers P 1‐M , P 2‐M , and P 3‐M were also synthesized to evaluate the effect of end groups. From optimized semiempirical MNDO calculations, the adjacent benzene rings between DSB and TAZ or DIOXD chromophores in P 3 , P 5 , and P 7 twist about 81°–89°. The effect of twisted architectures and connectors in optical and electrochemical properties for P 1 – P 7 have been discussed by comparing with copolymers P 1 and P 2 , which possess single bond or ether spacer as connectors. From cyclic voltammograms, the torsion in P 3 , P 5 , and P 7 confines electron delocalization and leads to simultaneously enhanced hole and electron affinity as compared to those of P 1 and P 2 . Furthermore, double‐layer light‐emitting diodes with a configuration of ITO/PEDOT:PSS/ P 1 – P 7 /Al all reveal green–yellow electroluminescence with maximum luminance at 8–320 cd/m² and their performances are greatly influenced by the connector's structure. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4514–4531, 2006     

Luminescent copoly(aryl ether)s with new electron‐transporting bis(3‐(trifluoromethyl)phenyl)‐1,3,4‐oxadiazole or bis(3‐(trifluoromethyl)phenyl)‐4‐(4‐hexyloxyphenyl)‐4H‐1,2,4‐triazole segments

To investigate the effect of trifluoromethyl groups in enhancing electron affinity of aromatic oxadiazole and triazole chromophores, we prepared four new copoly(aryl ether)s ( P1 – P4 ) consisting of bis(3‐(trifluoromethyl) phenyl)‐1,3,4‐oxadiazole (ETO) or bis(3‐(trifluoromethyl)phenyl)‐4‐(4‐hexyloxyphenyl)‐4H‐1,2,4‐triazole (ETT) segments and hole‐transporting segments [2,5‐distyrylbenzene (HTB) or bis(styryl)fluorine (HTF)]. Molecular spectra (absorption and photoluminescence) and cyclic voltammetry were used to investigate their optical and electrochemical properties. The emissions of P1 – P4 are dominated by the hole‐transporting fluorophores with longer emissive wavelengths around 442–453 nm via efficient excitation energy transfer. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of P1 – P4 , estimated from electrochemical data, are ?5.15, ?5.18, ?5.30, ?5.27, ?3.39, ?3.49, ?3.36, and ?3.48 eV, respectively. The LUMO levels of ETO and ETT segments are significantly reduced to ?3.39～?3.36 eV and ?3.48～?3.49 eV, respectively, as compared with ?2.45 eV of P5 containing a 2,5‐diphenyl‐1,3,4‐oxadiazole segment. Moreover, electron and hole affinity can be enhanced simultaneously by introducing isolated hole‐ and electron‐transporting segments in the backbone. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 42: 5900–5910, 2004     

Coplanarity analysis and validation of PBGA and T-BGA packages

Plastic BGA (PBGA) has recently been extensively used in microelectronics package. However, its heat-dissipating capacity is limited to below 2 or 3 W, and thus fails to satisfy the requirement of high heat dissipation and improved electrical performance. This work offers a finite element analytic methodology to predict the coplanarity of T²-BGA (Turbo thermal ball grid array) and PBGA, using ANSYS software. T²-BGA involves inserting a heat slug into the molding compound of a PBGA. The material of the heat slug can be either aluminum or copper. Surface mount technology (SMT) of BGA is limited mainly by the coplanarity due to coefficient of thermal expansion mismatch, since the structure of BGA is asymmetric. The coplanarity is limited below 0.2 mm (8 mil) by the Joint Electron Device Engineering Council (JEDEC) standard (JEDEC Design Standard, No. 95-1, Section 14, Ball Grid Array, June 2000). Coplanarity analysis is performed for both PBGA and T²-BGA, using ANSYS. Finally, a coplanarity experiment is conducted on the constructed specimens of PBGA and T²-BGA, to confirm the analytic results. The results in this work establish the effectiveness of T²-BGA in improving coplanarity. Moreover, the analytic results differ slightly from the experimental results, by 1.53–10.50%. The FEM model proposed here can be extended to optimize the structure of T²-BGA.     

Macrocyclic scaffold for the collagen triple helix

[structure: see text] Three strands of natural collagen are linked by covalent bonds prior to their folding into a triple helix. We report on a synthetic collagen in which the strands are pendent on a rigid macrocyclic scaffold of C(3) symmetry. The scaffold confers substantial conformational stability upon the collagen triple helix and makes its folding independent of concentration, both desirable attributes for exploring and exploiting synthetic collagens.     

Iron oxide nanopropellers prepared by a low-temperature solution approach

The alpha-Fe(2)O(3) (hematite) nanopropellers were synthesized via a low-temperature solution-based method using FeCl(2) as a precursor in the presence of urea and glycine hydrochloride. The formation of alpha-Fe(2)O(3) nanopropellers is strongly depended on the addition of glycine hydrochloride, which serves as a pH modulator and affects the oxidation rate of Fe(2+). The structural evolution of the propeller-structured hematite was found to follow dissolution and recrystallization processes. For the structural conformation, each nanopropeller presents a hexagonal central column closed by six equivalent surfaces of {(-)1100} and the six arrays of the nanopropeller structure are a result of growth along +/- [(-)1100], +/- [(-)1010], and +/-[0(-)110]. Preliminary results show that the magnetic maghemite (gamma-Fe(2)O(3)) nanopropellers could also be prepared by a reduction and reoxidation process from the alpha-Fe(2)O(3) (hematite) nanopropeller precursors.