Yoko Iizumi

Coaxially Stacked Coronene Columns inside Single-Walled Carbon Nanotubes†

One of the most interesting features of molecular materials is the fact that their physical properties change with the arrangement of the molecules as well as with the properties of the molecules themselves. Self-organization is an efficient pathway through which organic molecules assemble to form well-ordered nanometer-scale objects that are hardly synthesized by conventional chemical reactions. In these systems, two or more molecules are held together and are assembled by means of intermolecular (noncovalent) bonding such as ion–dipole or dipole–dipole interactions, hydrogen bonding, hydrophobic interactions, or p–p stacking. Such molecular self-organization and recognition processes are among the most practical and effective means to facilitate a “bottom-up” approach in nanotechnology. In general, however, fabrication of well-defined organic nanowires or other types of onedimensional (1D) nanostructures with controllable size and morphology is not as far advanced as for their inorganic counterparts. 2] Single-walled carbon nanotubes (SWCNTs) can offer a suitable interior space for the accommodation of molecules. Nanostructures produced by the incorporation of such molecules into SWCNTs are expected to exhibit several superior features. For example, because the diameter of SWCNTs can be adjusted to the size of the molecules, wellordered molecular arrangements more than a micrometer in length can be easily produced. The synthesized molecular arrangements are also expected to be strong and flexible under mechanical strain because the nanotube templates sustain the structure. Furthermore, the already synthesized nanostructures are isolated from reactive species by the tube wall, which leads to the superior durability of the encapsulated molecules. Herein we demonstrate such a 1D SWCNT-templated nanostructure using planar p-conjugated molecules (coronenes). Encapsulated coronenes form nano-scale columns in a way that differs from 3D solid coronenes, thus resulting in electronic and optical properties peculiar to the 1D structure. The production of well-ordered molecular assemblies in SWCNTs can be expected to inspire novel approaches for the synthesis of low-dimensional molecular materials with unique physical properties. The self-organized 1D structure of coronenes inside SWCNTs (coronenes@SWCNTs) was achieved through vapor-phase doping (Figure 1). Coronene confinement in nanometer spaces produces a characteristic columnar structure. Figure 1b,d and Figure S2a (see the Supporting Information) show high-resolution TEM (HRTEM) images of the coronene columns inside SWCNTs; coronene molecules are positioned along the tube axis at almost regular intervals. The formation of the columnar structure is due to two dominant factors: p–p stacking between coronenes and the interaction between the encapsulated coronene and the host SWCNT. In the monoclinic crystal, coronenes have an interplanar distance of 0.34 nm with a tilt angle of about 468 relative to the stacking axis. On the other hand, within SWCNTs, statistical analysis of the HRTEM images shows that the distance between the molecular planes is 0.35 0.03 nm (Figure S2b) and the angle between the molecular plane and the tube axis (q) is approximately 778 (Figure S2c, 1c). Although the interplanar distances are identical between 1D and solid coronenes within the experimental error, a substantial difference is observed for the tilt angle. The observed structure that involves coronene columns was supported by total energy calculations for zig-zag (n, 0) SWCNTs. Figure 1e shows the stabilization energy (DE) of coronenes inside SWCNTs as a function of tube diameter (d), where the molecular plane of coronenes was set to be [*] Prof. T. Okazaki, Y. Iizumi, Dr. S. Okubo, Dr. Z. Liu, Dr. K. Suenaga, Y. Tahara, Dr. M. Yudasaka, Prof. S. Iijima Nanotube Research Center, National Institute of Advanced Industrial Science and Technology (AIST) Tsukuba 305-8565 (Japan) Fax: (+ 81)29-861-6241 E-mail: toshi.okazaki@aist.go.jp

Single-molecule sensing electrode embedded in-plane nanopore

Electrode-embedded nanopore is considered as a promising device structure for label-free single-molecule sequencing, the principle of which is based on nucleotide identification via transverse electron tunnelling current flowing through a DNA translocating through the pore. Yet, fabrication of a molecular-scale electrode-nanopore detector has been a formidable task that requires atomic-level alignment of a few nanometer sized pore and an electrode gap. Here, we report single-molecule detection using a nucleotide-sized sensing electrode embedded in-plane nanopore. We developed a self-alignment technique to form a nanopore-nanoelectrode solid-state device consisting of a sub-nanometer scale electrode gap in a 15 nm-sized SiO2 pore. We demonstrate single-molecule counting of nucleotide-sized metal-encapsulated fullerenes in a liquid using the electrode-integrated nanopore sensor. We also performed electrical identification of nucleobases in a DNA oligomer, thereby suggesting the potential use of this synthetic electrode-in-nanopore as a platform for electrical DNA sequencing.

Immunoassay with single-walled carbon nanotubes as near-infrared fluorescent labels.

The intrinsic photoluminescence of single-walled carbon nanotubes (CNTs) in the near-infrared (NIR) above 1000 nm makes them promising candidates for biological probes owing to low interference by bioorganic molecules and deep tissue penetration. We here demonstrate an immunoassay by using a NIR CNT labels conjugated to immunoglobulin G (IgG) antibodies. Most of the CNT-conjugated IgG was successfully immunoprecipitated with protein G-attached magnetic beads and eluted from them, which was confirmed by the NIR emission of the conjugated CNTs at 1000-1200 nm. The photoluminescence intensity of the CNT labels was strong enough to detect antigens at 600 pM by our simple procedures.

Host-guest interactions in azafullerene (C59N)-single-wall carbon nanotube (SWCNT) peapod hybrid structures

The effect of azafullerene encapsulation on the electronic states of single-wall carbon nanotubes (SWCNTs) is investigated; UV-vis-NIR absorption and photoluminescence spectroscopy shows that the interaction between SWCNTs and the encapsulated azafullerenes is originated from the weak intermolecular forces, which suggests a lack of strong doping effect such as electron transfer between them.

Preparation of small-sized graphene oxide sheets and their biological applications

By using carbon nanohorns as starting materials, small- and uniform-sized graphene oxide (S-GO) sheets can be prepared in high yields via an oxidation method. The obtained S-GO sheets have a band-like structure with a length of 20-50 nm, a width of 2-10 nm, and a thickness of 0.5-5 nm. S-GO sheets are hydrophilic due to abundant oxygenated groups on the surfaces and edges; hence, this nanomaterial is highly dispersive in aqueous solutions and some hydrophilic organic solvents. Additionally, like other S-GO samples, the S-GO sheets prepared here are strongly fluorescent over the visible light wavelength region. These characteristics underscore the high potential of S-GO sheets for nanomedical and diagnostic applications. In proof-of-concept experiments, the S-GO sheets were conjugated with an arginine-glycine-aspartic acid derivative for tumour-targeting drug delivery applications, and with an immunoglobulin G antibody for immunoassay applications.

Energetics and Electronic Structures of Carbon Nanotubes Encapsulating Polycyclic Aromatic Hydrocarbon Molecules

We report total-energy electronic structure calculations that provide energetics of the encapsulation of polycyclic aromatic hydrocarbon (PAH) molecules coronene, sumanene, and corannulene into carbon nanotubes (CNTs) and electronic structures of the resulting carbon hybrid structures. Our calculations elucidate that the encapsulation of these PAHs into CNTs is an exothermic reaction for nanotubes with indexes of \((16,0)\), \((17,0)\), and \((18,0)\) or thicker for coronene, sumanene, and corannulene molecules, respectively, and that the energy gain upon encapsulation is up to 1 eV per molecule. We also find that the stacking arrangement of encapsulated PAH molecules depends on the molecular species and inner spacing of the CNTs: coronene is tilted to the CNT axis in its stable conformation, sumanene is stacked normal to the CNT axis, and corannulene is randomly arranged along the CNT axis. The electron states of the PAH–CNT hybrids depend on both the space inside the CNTs and the tilting angle of the PA...

Microwave assisted covalent functionalization of C60@SWCNT peapods

The covalent functionalization of the external wall of C(60)@SWCNT peapods, by in situ generated aryl diazonium salts, assisted by microwave irradiation is reported. Spectroscopic, thermal and microscopy characterization was performed. Electrochemistry revealed the three reversible reductions of encapsulated C(60), however, shifted towards positive potentials when compared with those of intact C(60).

Single atom spectroscopy: Decreased scattering delocalization at high energy losses, effects of atomic movement and X-ray fluorescence yield

Single atom localization and identification is crucial in understanding effects which depend on the specific local environment of atoms. In advanced nanometer scale materials, the characteristics of individual atoms may play an important role. Here, we describe spectroscopic experiments (electron energy loss spectroscopy, EELS, and Energy Dispersed X-ray spectroscopy, EDX) using a low voltage transmission electron microscope designed towards single atom analysis. For EELS, we discuss the advantages of using lower primary electron energy (30 keV and 60 keV) and higher energy losses (above 800 eV). The effect of atomic movement is considered. Finally, we discuss the possibility of using atomically resolved EELS and EDX data to measure the fluorescence yield for X-ray emission.

Spectroscopic Characterization of Nanohybrids Consisting of Single-walled Carbon Nanotubes and Fullerodendron

Hydrogen gas, which can be used in fuel cells to generate electricity, is considered the ultimate clean energy source. Recently, it was reported that a photo-induced electron transfer system consisting of single-walled carbon nanotubes (SWCNTs) and fullerodendrons shows photo-catalytic activity with a very high quantum yield for splitting water under visible light irradiation. However, the mechanism of high efficiency hydrogen generation is not yet clearly understood. We report here the spectroscopic characterizations of the SWCNT-fullerodendron composites. The results indicate two important fundamental properties of the composite system. First, fullerodendrons preferentially interact with the semiconducting SWCNTs instead of with their metallic counterparts. Second, the photo-induced electron transfer process from the C60 moiety of fullerodendrons to SWCNTs occurs more efficiently with an increasing tube diameter.

Origin of the n-type transport behavior of azafullerene encapsulated single-walled carbon nanotubes

The transport properties of C59N encapsulated semiconducting single-walled carbon nanotubes (SWCNTs) (C59N-peapod) are investigated. Transport measurements of the peapods in field effect transistors (FETs) reveal that ∼14% of the C59N-peapod sample shows n-type behavior even though the electronic properties of the host SWCNTs are similar to those of C60-peapods that exhibit only p-type property. First-principles electronic-structure calculations reveal that the unique transport behavior originates from the monomer form of C59N encapsulated in SWCNTs. The singly occupied (SO) state of C59N lies in the energy gap of the SWCNT and the energy of this state increases substantially when electrons are injected. Because of this shift to higher energy, the SO state acts as a shallow donor state for the conduction band of the nanotube, which leads to n-type behavior in FET measurements.