Yoichi Otsuka

Control of electrical conduction in DNA using oxygen hole doping

Using oxygen adsorption experiments on poly (dG)-poly (dC) DNA molecules, we found that their conductance can be easily controlled by several orders of magnitudes using oxygen hole doping, which is a characteristic behavior of a p-type semiconductor. It also suggests that the conductance of the DNA under doping results from charge carrier transport, not from an ionic conduction. On the other hand, we will also show that the poly (dA)-poly (dT) DNA molecules behave as an n-type semiconductor. This letter demonstrates that the concentration and the type of carriers in the DNA molecules could be controlled using proper doping methods.

Influence of Humidity on the Electrical Conductivity of Synthesized DNA Film on Nanogap Electrode

We have studied the electrical conductivity of DNA film using nanogap electrodes. Current–voltage measurements and alternating current measurements were performed for analysis of conductivity. The electrical conductivity of the DNA films of poly(dG)poly(dC) are found to depend strongly on the humidity. The resistance of poly(dG)poly(dC) decreases dramatically with increasing relative humidity. The contact resistance between DNA film and Au electrodes is also examined by the conventional four-probe technique.

Point-contact current-imaging atomic force microscopy: Measurement of contact resistance between single-walled carbon nanotubes in a bundle

Conductance of bundled single-walled carbon nanotubes (b-SWCNTs) are measured by point-contact current-imaging atomic force microscopy (PCI–AFM). Simultaneous mapping of the topographic information and current through SWCNTs enable us to investigate the relationship between structure and conductance. Variation in resistance of a b-SWCNT indicates that the resistance between SWCNTs was higher than 107 Ω with strong voltage dependence. Because PCI–AFM measurement can obtain vertical conductance information, this approach appears to be a powerful technique for characterization of nanoscale electronic devices.

Conductance measurement of a DNA network in nanoscale by point contact current imaging atomic force microscopy

We measured the electrical conductivity of a DNA network as a spatially-resolved current-image using point-contact current imaging atomic force microscopy (PCI-AFM) under various humidity conditions. The simultaneous observation of topography and current image by PCI-AFM can provide information pertaining to the electrical properties of biological and∕or soft materials in the nano-scale range. Under dry condition (0% humidity), no difference was observed for the electrical current both of the DNA network and mica surface, whereas the electrical current along the DNA network was larger than that of the mica surface by 20pA at a bias voltage of 5V under high humidity conditions of 60%.

A simple fabrication method of nanogap electrodes for top-contacted geometry: application to porphyrin nanorods and a DNA network

We have developed a fabrication method for nanogap electrodes without employing photo- or electron-beam lithography to measure the electrical characteristics of nanostructured molecules. This angle-controlled shadow-masking method enables us to construct nanogap electrodes without a wet process after the molecules are positioned on the substrate. The proposed method makes it possible to measure electrical characteristics without structurally deforming or denaturing the molecules due either to the step edge of an electrode or to the organic solvents used in the wet process. The results demonstrate that a gap length between the electrodes of less than 100 nm can be fabricated reproducibly. We have measured the electrical characteristics of lambda DNA (λ-DNA) networks and molecular nanorods made of porphyrin-derivative molecules (TPPS: 5,10,15,20-tetraphenyl-21H,23H-porphyrine tetrasulfonic acid) in which J-aggregates are formed inside. Experimental findings reveal that the electrical conductivity of λ-DNA decreased under a vacuum condition, whereas that of TPPS nanorods decreased under oxygen and nitrogen gas-purged conditions.

A nano tester: A new technique for nanoscale electrical characterization by point-contact current-imaging atomic force microscopy

A new atomic force microscopy (AFM) technique, called point-contact current-imaging AFM (PCI-AFM)-which combines tapping mode (for mapping topographic image) and point-contact operation (for measuring current?voltage characteristics)-has been developed. This new AFM technique can simultaneously map high-resolution topographic image and measure spatially resolved I?V characteristics of materials (placed on an insulative substrate and connected to a gold electrode) on the nanoscale. The high performance of the PCI-AFM system was evaluated in experiments on single-walled carbon nanotubes (SWNTs).

Scanning probe electrospray ionization for ambient mass spectrometry

RATIONALE Ambient sampling and ionization techniques have been attracting attention in imaging mass spectrometry because they offer the advantage of rapid testing. We have developed a method which exploits the fluid motion of charged solvents for both local sampling and ionization with a single vibrating capillary probe. METHODS The capillary probe was used to supply solvents in order to form a liquid bridge between the probe and a sample surface. A bias voltage was applied to the solvents to generate electrospray ionization (ESI). The probe was also vibrated by either an ultrasonic transducer fixed at the back of the sample (contact-mode) or spontaneous vibration of probe itself (tapping-mode). The ions generated by ESI were detected by a triple quadrupole mass spectrometer. RESULTS Sampling of the specimens at the liquid bridge and ESI of the dissolved solutions both occurred around the probe apex. The sampling and ionization co-existed in contact-mode, while they were explicitly separated in the tapping-mode. The one-dimensional mapping of solid samples such as protein films and tissue sections was demonstrated. The results indicated that there was little cross-contamination during the operation. CONCLUSIONS The method, named scanning probe electrospray ionization (SPESI), promises to be a simple and unique approach toward direct sampling and ionization methodology.

Refinement of Conditions of Point-Contact Current Imaging Atomic Force Microscopy for Molecular-Scale Conduction Measurements

We have refined the measurement conditions of point-contact current imaging microscopy (PCI-AFM) to measure the electric properties along the long axes of one-dimensional structures. Using this refinement, the current image of the PCI-AFM can be used to distinguish individual single-walled carbon nanotubes in a bundled structure. The PCI-AFM will thus help further developments in nanoscience for conduction measurements in one-dimensional structures.

Printing electrode for top-contact molecular junction

We have developed a fabrication method of electrodes for molecular electronics based on nanotransfer printing lithography using a release agent layer and without any surface modification. A gold layer deposited on a release agent layer coating a mold can transfer to the nonmodified surfaces of SiO2, sapphire, or organic layer. The printed electrode has a resistivity of 4×10−6Ωcm which is close to the value of 2.1×10−6Ωcm for bulk gold. The application of this process to top-contact type molecular junctions is demonstrated using carbon nanotubes and the electric potential distribution of the junctions is observed by Kelvin force microscopy.

Synthesis and DNA cleaving activity of water-soluble non-conjugated thienyl tetraynes.

Water-soluble non-conjugated thienyl tetraynes (3-6) were synthesized and their DNA cleaving activity was evaluated using electrophoresis, atomic force microscopy (AFM) and Escherichia coli (E. coli) transformation techniques. The amino-functionalized compound 4 was shown to possess an activity to cleave plasmid DNA by both electrophoresis and E. coli transformation techniques. AFM also showed a cleavage of the circular DNA into a linear form with a formation of burst-star-shaped architectures, which were envisaged to be cross-linked DNA oligomers.