Recent single-macromolecule observations revealed that the force/extension characteristics of single-stranded DNA (ssDNA) are closely related to solution ionic concentration and DNA sequence composition. To understand this, we studied the elastic property of ssDNA through the Monte Carlo implementation of a modified freely jointed chain (FJC), with electrostatic, base-pairing, and base-pair stacking interactions all incorporated. The simulated force-extension profiles for both random and designed sequences have attained quantitative agreements with the experimental data. In low-salt solution, electrostatic interaction dominates, and at low forces, the molecule can be more easily aligned than an unmodified FJC. In high-salt solution, secondary hairpin structure appears in ssDNA by the formation of base pairs between complementary bases, and external stretching causes a hairpin-coil structural transition, which is continuous for ssDNA made of random sequences. In designed sequences such as poly(dA-dT) and poly(dG-dC), the stacking potential between base pairs encourages the aggregation of base pairs into bulk hairpins and makes the hairpin-coil transition a discontinuous (first-order) process. The sensitivity of elongation to the base-pairing rule is also investigated. The comparison of modeling calculations and the experimental data suggests that the base pairing of single-stranded polynucleotide molecules tends to form a nested and independent planar hairpin structure rather than a random intersecting pattern.

Metallic membranes of about 2.5 nm thick but with macroscopic lateral dimensions have been successfully fabricated from ultrathin gold nanowires. Such metallic nanomembranes are transparent, conductive and mechanically strong, with an optical transmittance of 90-97%, an electrical resistance of ∼1142 kΩ sq(-1) , and a breaking strength of ∼14 N m(-1) with a typical atomic force microscope probe.

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Three Ag-based oxides, AgAlO 2 , AgCrO 2 , and Ag 2 CrO 4 , were prepared by the cation exchange method. Powder X-ray diffraction analysis showed that AgAlO 2 and Ag 2 CrO 4 crystallized in orthorhombic structures and AgCrO 2 crystallized in a hexagonal structure. The band gaps of AgAlO 2 , AgCrO 2 , and Ag 2 CrO 4 were estimated to be 2.95, 1.68, and 1.75 eV, respectively. The photooxidation of Methyl Orange and gaseous benzene were performed to test the activities of the three materials. In the two kinds of reactions, the activity orders were both Ag 2 CrO 4 > AgAlO 2 > AgCrO 2 , and it was found that the novel photocatalyst, Ag 2 CrO 4 , showed activity in the visible region up to 570 nm. The correlation among crystal structures, electronic structures, and photocatalytic properties in the three Ag-based oxides is discussed in detail.