Daisuke Nii

Controlling the adsorption and desorption of double-stranded DNA on functionalized carbon nanotube surface

We demonstrated controlled adsorption and desorption of double-stranded DNA (dsDNA) on single-walled carbon nanotube (SWNT) surface functionalized with polyethyleneglycol (PEG SWNT). First, when dsDNA molecules were mixed with the PEG SWNT solution, the DNA molecules spontaneously adsorbed onto the PEG SWNT surface and formed dsDNA-PEG SWNT conjugates without sonication. Next, we succeeded in detaching the dsDNA adsorbed on PEG SWNT by annealing at 95°C for 30 min. These results were confirmed using atomic force microscopy, agarose gel electrophoresis, and micro-Raman spectroscopy. In contrast, when we used the usual SWNT produced by the high-pressure carbon monoxide method (HiPco SWNT), the DNA molecules were fragmented during the adsorption process as sonication was necessary for the hybridization of DNA-SWNT conjugates. Furthermore, detachment of DNA molecules from HiPco SWNT by annealing was impossible. Our method may be useful for developing DNA devices using SWNTs as substrates when it is combined with previously established various biochemical techniques.

Selective binding of single-stranded DNA-binding proteins onto DNA molecules adsorbed on single-walled carbon nanotubes

Single-stranded DNA-binding (SSB) proteins were treated with hybrids of DNA and single-walled carbon nanotubes (SWNTs) to examine the biological function of the DNA molecules adsorbed on the SWNT surface. When single-stranded DNA (ssDNA) was used for the hybridization, significant binding of the SSB molecules to the ssDNA-SWNT hybrids was observed by using atomic force microscopy (AFM) and agarose gel electrophoresis. When double-stranded DNA (dsDNA) was used, the SSB molecules did not bind to the dsDNA-SWNT hybrids in most of the conditions that we evaluated. A specifically modified electrophoresis procedure was used to monitor the locations of the DNA, SSB, and SWNT molecules. Our results clearly showed that ssDNA/dsDNA molecules on the SWNT surfaces retained their single-stranded/double-stranded structures.

Biomolecular recognition ability of RecA proteins for DNA on single-walled carbon nanotubes

We examined the biomolecular recognition ability of RecA proteins using single-walled carbon nanotubes (SWNTs) wrapped with a single-stranded DNA (ssDNA) molecule as a mimic for the usual ssDNA molecules. The ssDNA-SWNT hybrids showed larger diameters compared to those of the usual ssDNA molecules. As a result, RecA molecules bound to the ssDNA-SWNTs, as observed using atomic force microscopy and agarose gel electrophoresis. On the other hand, when carboxymethylcellulose (CMC) was used rather than ssDNA, the RecA molecules did not bind to the CMC-SWNT hybrids. Our results indicate that RecA molecules recognize ssDNA on SWNT surfaces as DNA molecules through their biomolecular recognition ability.

Gigabits-per-Second Signal Transmission from Single-Mode Vertical-Cavity Surface-Emitting Laser via Thin-Film Waveguide for Wavelength-Division-Multiplexing Optical Interconnect Board

A wavelength-division-multiplexing (WDM) optical interconnect board providing broad-band transmission from a two-dimensional (2D) array of vertical-cavity surface-emitting lasers (VCSELs) to a 2D array of photodetectors is one of the attractive candidates for constructing future ultrahigh performance signal processing systems. Focusing grating couplers and distributed Bragg reflectors were integrated into a thin-film glass waveguide to form free-space-wave optical add/drop multi/demultiplexers required for the WDM optical interconnect board. A single-mode VCSEL was placed at the focal point of a focusing grating coupler, and a diverging wave from VCSEL was coupled and collimated into the waveguide. After 15 mm propagation, the guided wave was dropped by an optical drop demultiplexer to a focusing wave in free space. VCSEL was driven directly by a pulse pattern generator and the transmitted signal was detected. An open eye diagram of a 1 Gbps pseudo-random-binary-sequence non-return-to-zero signal was confirmed.

Conjugates between photosystem I and a carbon nanotube for a photoresponse device.

Photosystem I (PS I) is a large pigment–protein complex embedded in the thylakoid membranes that performs light-driven electron transfer across the thylakoid membrane. Carbon nanotubes exhibit excellent electrical conductivities and excellent strength and stiffness. In this study, we generated PSI–carbon nanotube conjugates dispersed in a solution aimed at application in artificial photosynthesis. PS I complexes in which a carbon nanotube binding peptide was introduced into the middle of the PsaE subunit were conjugated on a single-walled carbon nanotube, orienting the electron acceptor side to the nanotube. Spectral and photoluminescence analysis showed that the PS I is bound to a single-walled carbon nanotube, which was confirmed by transmission electron microscopy. Photocurrent observation proved that the photoexcited electron originated from PSI and transferred to the carbon nanotube with light irradiation, which also confirmed its orientated conjugation. The PS I–carbon nanotube conjugate will be a useful nano-optoelectronic device for the development of artificial systems.

Peptide aptamer-assisted immobilization of green fluorescent protein for creating biomolecule-complexed carbon nanotube device

Carbon nanotubes are a novel material for next-generation applications. In this study, we generated carbon nanotube and green fluorescent protein (GFP) conjugates using affinity binding peptides. The carbon nanotube-binding motif was introduced into the N-terminus of the GFP through molecular biology methods. Multiple GFPs were successfully aligned on a single-walled carbon nanotube via the molecular recognition function of the peptide aptamer, which was confirmed through transmission electron microscopy and optical analysis. Fluorescence spectral analysis results also suggested that the carbon nanotube–GFP complex was autonomously formed with orientation and without causing protein denaturation during immobilization. This simple process has a widespread potential for fabricating carbon nanotube–biomolecule hybrid devices.

Structures of hybrids of DNA and carbon nanotubes in air and in liquids

We investigated single-walled carbon nanotubes (SWNT) and DNA-SWNT hybrids by atomic force microscopy (AFM). From the AFM observation of several different types of SWNTs and DNA-SWNT hybrids in air, we found several specific differences in morphology among the samples. Longer SWNT molecules were observed when the SWNT was dispersed using a bath type sonicator. When a probe type sonicator was used, the SWNT became short obviously. The phenomenon was common in all of our experiments, thus, the phenomenon was independent on the types SWNTs. SWNT functionalized with polyethyleneglycol (PEG SWNT), amino group (NH2 SWNT), and carboxyl group (COOH SWNT) showed individual specific features in AFM images. Although NH2 SWNT is typically soluble in organic solvents, uniform distribution was observed when DNA molecules were mixed with NH2 SWNT. Finally, we observed DNA-SWNT hybrids by AFM in liquids for the first time. DNA-SWNT hybrids were significantly swollen in the aqueous solution even though the sample was dried once. This is helpful information for considering biological applications of the DNA-SWNT hybrids.

Continuous Emission-Point Shift in Vertical-Cavity Surface-Emitting Laser Controlled by Optical Feedback

Emission intensity patterns and spectra of a two-transverse-mode vertical-cavity surface-emitting laser (VCSEL) were observed and measured while the position and polarization of an optical feedback light were controlled. Lasing behavior was discussed in relation to linear polarization modes. The observed emission point shift with unchanged wavelengths was explained by the superposition of two newly appeared modes.

Signal Transmission from VCSEL in Thin-Film-Waveguide WDM Optical Interconnects Board

Diverging light from vertical cavity surface emitting laser was coupled by free-space-wave add/drop multiplexer consisting of focusing grating coupler and distributed Bragg reflector into thin-film waveguide and was transmitted with 1 Gbits/s NRZ signal.