We report and demonstrate biomedical applications of a new technique – ‘living’ PEGylation – that allows control of the density and composition of heterobifunctional PEG (HS-PEG-R) on gold nanoparticles (AuNPs). We first establish ‘living’ PEGylation by incubating HS-PEG5000-COOH with AuNPs (~20 nm) at increasing molar ratios from zero to 2000. This causes the hydrodynamic layer thickness to differentially increase up to 26 nm. The controlled, gradual increase in PEG-COOH density is revealed after centrifugation, based on the ability to re-suspend the pellet and increase the AuNP absorption. Using a fluorescamine-based assay we quantify differential HS-PEG5000-NH2 binding to AuNPs, revealing it is highly efficient until AuNP saturation. Furthermore, the zeta potential incrementally changes from −44.9 to +52.2 mV and becomes constant upon saturation. Using ‘living’ PEGylation we prepare AuNPs with different ratios of HS-PEG-RGD and incubate them with U-87 MG and non-target cells, demonstrating that targeting ligand density is critical to maximizing the targeting efficiency of AuNPs to cancer cells. We also sequentially control the HS-PEG-R density to develop multifunctional nanoparticles, conjugating positively-charged HS-PEG-NH2 at increasing ratios to AuNPs containing negatively-charged HS-PEG-COOH to reduce uptake by macrophage cells. This ability to minimize non-specific binding/uptake by healthy cells could further improve targeted nanoparticle efficacy.

The water-induced disproportionation of the electrogenerated superoxide ion (O2 -) in acetonitrile, dimethylformamide, and dimethyl sulfoxide media containing various concentrations of water as a Brønsted acid has been examined by UV−vis spectroscopy. Analysis of the kinetics as a function of O2 - and water concentrations and of the measurement time demonstrated that the disproportionation of O2 - by water in these media obeys a common mechanism:  O2 - + H2O ⇄ HO2 • + OH- (k 1, k -1) HO2 • + O2 - → HO2 - + O2 (k 2) (HO2 •, hydroperoxyl radical; HO2 -, hydrogen peroxide anion). The solvent dependence of the obtained kinetic parameters of (k 1/k -1)k 2, k 1 and k -1/k 2 is discussed in terms of the solvation of O2 - and H2O as well as the effective acidities of H2O in different aprotic solvents.

No abstract

Antimony-doped p-type ZnO films epitaxially grown on (0001) sapphire substrates were fabricated by pulsed laser deposition at 400–600°C in 5.0×10−2Torr oxygen without postdeposition annealing. The films grown at 600°C have among the highest reported hole concentration of 1.9×1017cm−3 for antimony doping, Hall mobility of 7.7cm2∕Vs, and resistivity of 4.2Ωcm. Transmission electron microscopy reveals that the p-type conductivity closely correlates to the high density of defects which facilitate the formation of acceptor complexes and the compensation of native shallow donors. The thermal activation energy of the acceptor was found to be 115±5meV and the corresponding optical ionization energy is ∼158±7meV.

The process of particle generation during ultrafast pulsed laser ablation of nickel is investigated. Two types of particles with different sizes depending on the laser fluence are found, indicating different particle generation mechanisms. By limiting the laser fluence below a threshold of strong plasma formation, the large dropletlike particles can be eliminated. In addition, by supplying different background gases, various crystalline structures are obtained for the particles, including Ni∕NiO core/shell spheres and NiO cubes. This study provides evidence that ultrafast laser ablation can be a room temperature physical method for generating nanocrystals with a narrow particle size distribution.

We report on intrinsic p-type ZnO thin films by plasma-assisted metal-organic chemical vapor deposition. The optimal results give a resistivity of 12.7 ⍀ cm, a Hall mobility of 2.6 cm 2 / V s, and a hole concentration of 1.88ϫ 10 17 cm −3 . The oxygen concentration is increased in the intrinsic p-type ZnO, compared with the n-type layer. Two acceptor states, with the energy levels located at 160 and 270 meV above the valence band maximum, are identified by temperature-dependent photoluminescence. The origin of intrinsic p-type behavior has been ascribed to the formation of zinc vacancy and some complex acceptor center.

Zinc oxide is a wide bandgap semiconductor with potential applications in optoelectronic devices. The greatest challenge for these applications, however, remains the fabrication of reliable and stable p-type ZnO thin films. Here we report stable phosphorus-doped p-type ZnO thin films grown on (0001) sapphire substrates by pulsed laser ablation. While as-deposited films all show n-type conductivity, films grown at 600°C become p-type after annealing in oxygen atmosphere with a resistivity of 4.9 × 10 1 X cm, a Hall mobility of 1 cm 2 V -1 s -1 , and a hole concentration of 1.3 × 10 17 cm -3 . Such p-type films have been stable under ambient conditions for 16 months so far without apparent degradation. Transmission electron microscopy reveals that the p-type films consist of a high density of dislocations, which enhance both the solubility of phosphorus and the formation of Zn vacancies to facilitate the n-to-p conversion of electrical conductivity. These studies provide microscopic evidence of the amphoteric nature of the phosphorus dopant in ZnO. There has recently been an increasing interest in ZnO for applications in optoelectronics such as light emitting diodes, ultraviolet (UV) lasers, and UV light detectors because of its wide bandgap (3.37 eV). In comparison with GaN, ZnO has some obvious advantages for optoelectronic applications due to the availability of single crystal substrates, relatively low growth temperatures (T G ), and a large exciton binding energy (∼ 60 meV).[1] Optically pumped excitonic lasing of ZnO thin films at room temperature (RT) has been reported. [2,3] Lasing effects in ZnO nanowire arrays have been demonstrated, [4] and electroluminescence (EL) has been observed at room temperature in thin-film ZnO homojunctions. [5][6][7] Although p-type ZnO thin films were reported by several groups, they showed high resistivity and/or poor stability and reproducibility. Thus, the greatest remaining challenge for ZnO optoelectronics is the reproducible fabrication of stable p-type ZnO thin films. Like many other II-VI semiconductors, ZnO has asymmetric doping limits: [8] it can be easily doped n-type, [9] but remains strongly resistant to p-type doping.[10] Though nitrogen is theoretically the most promising acceptor for ZnO, its low solubility and compensation by donors such as hydrogen [11] and Zn interstitials [12] are major obstacles.As alternatives to N, larger-size group V elements such as P, As, Sb and Bi have been widely studied. Puzzling observations of p-type conductivity in such materials have stimulated theoretical investigations into the electronic structure of the defects induced by P, As or Sb in ZnO. Limpijumnong et al. [13] predicted that under oxygen-rich growth conditions, a complex involving a group V antisite and two zinc vacancies (V Zn ) would have a low formation energy, and behave as a shallow acceptor with an ionization energy of 150-160 meV. Lee et al. used the same concept to study phosphorus complexes in ZnO.[14] One of the most important conclusions from these studies ...

No abstract