Rolling circle amplification (RCA) is an elegant biochemical method by which long single-stranded DNA molecules with a repeating sequence motif can be readily synthesized. In RCA, small circular single-stranded oligonucleotides serve as templates for the polymerization of the complementary strand. A DNA polymerase with an efficient strand displacement activity can copy the circular template without stopping. This results in a long DNA strand with periodic sequence. We here demonstrate that this method, using DNA recognition and biotin-streptavidin binding, provides a simple procedure for DNA-directed nanoscale organization of matter. As an example, a 74 nucleotide (nt) long circular DNA molecule is amplified into a sequence-periodic single strand with a length up to several micrometers. Hybridization of this long periodic DNA template to the biotinylated complement of the sequence motif results in a long DNA duplex with a periodic arrangement of biotin binding sites. On this duplex, streptavidin-coated particles can be organized into one-dimensional arrays. The resulting DNA constructs are characterized by gel electrophoresis and atomic force microscopy.

DNA-templated polyaniline nanowires and networks are synthesized using three different methods. The resulting DNA/polyaniline hybrids are fully characterized using atomic force microscopy, UV–vis spectroscopy and current–voltage measurements. Oxidative polymerization of polyaniline at moderate pH values is accomplished using ammonium persulfate as an oxidant, or alternatively in an enzymatic oxidation by hydrogen peroxide using horseradish peroxidase, or by photo-oxidation using a ruthenium complex as photo-oxidant. Atomic force microscopy shows that all three methods lead to the preferential growth of polyaniline along DNA templates. With ammonium persulfate, polyaniline can be grown on DNA templates already immobilized on a surface. Current–voltage measurements are successfully conducted on DNA/polyaniline networks synthesized by the enzymatic method and the photo-oxidation method. The conductance is found to be consistent with values measured for undoped polyaniline films.

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As a bottom-up approach toward spintronics, a network structure of gold nanoparticles connected with spin-polarized wire molecules has been studied. A spinless network is prepared as a reference system. The network of gold nanoparticles with an average diameter of 4 nm form granules ͑average diameter of 100 nm͒, which in turn, connect themselves with each other to bridge 2 m-gap gold electrodes. Since the charging energy of a 4-nm gold nanoparticle amounts to 160 meV, it works as a Coulomb island and the conduction through the network is dominated by Coulomb blockade effect at room temperature. Thermal-activation-type conduction is found in a temperature range of 300 K-30 K, below which cotunneling is suggested to dominate. Important findings reported here are as follows: ͑1͒ The cotunneling occurs at elevated temperatures as high as T = 30 K due to the small size of gold nanoparticles: Nonlinear characteristics featured by I-V 3 are found, suggesting that the number of tunnel junctions relevant to the cotunneling is two. ͑2͒ The cotunneling current is substantially smaller in spin-polarized network than in spinless network, suggesting that spin-flip scattering caused by localized spins on wire molecules suppresses cotunneling process: The interpretation is supported by negative magnetoresistance observed in spin-polarized networks.