Yoshitaka Morimoto

Doping graphene films via chemically mediated charge transfer

Transparent conductive films (TCFs) are critical components of a myriad of technologies including flat panel displays, light-emitting diodes, and solar cells. Graphene-based TCFs have attracted a lot of attention because of their high electrical conductivity, transparency, and low cost. Carrier doping of graphene would potentially improve the properties of graphene-based TCFs for practical industrial applications. However, controlling the carrier type and concentration of dopants in graphene films is challenging, especially for the synthesis of p-type films. In this article, a new method for doping graphene using the conjugated organic molecule, tetracyanoquinodimethane (TCNQ), is described. Notably, TCNQ is well known as a powerful electron accepter and is expected to favor electron transfer from graphene into TCNQ molecules, thereby leading to p-type doping of graphene films. Small amounts of TCNQ drastically improved the resistivity without degradation of optical transparency. Our carrier doping method based on charge transfer has a huge potential for graphene-based TCFs.

Detection of Magnetic Nanobeads by Self-Assembly of Superparamagnetic Microbeads for Biosensing

Detection of magnetically labeled biomolecules is a promising method for fast and inexpensive medical diagnosis. Recently, there has been an increasing interest in using magnetic labels with diameters of 20-150 nm-dimensions that are similar to actual biomolecules. However, detection of small numbers of nanometer-scale magnetic beads (labels) distributed over large surface areas by magnetoresistive devices is extremely challenging due to the intrinsic noise and design limitations of giant magnetoresistance (GMR) and Hall-type of biosensors. Here, we describe proof of principle experiments on exploiting magnetically induced self-assembly of superparamagnetic beads for detecting superparamagnetic nanobeads with diameters of less than 150 nm. We successfully detected low areal densities of ~ 130 nm diameter superparamagnetic ldquotarget beadsrdquo immobilized over millimeter-sized substrates, by optically monitoring the ldquocapturerdquo of easily visible, micrometer-sized superparamagnetic beads by the targets. Our procedure could readily be extended for the detection of magnetically labeled biomolecules.

Temperature Dependence of the Resistance of AlGaN/GaN Heterostructures and Their Applications as Temperature Sensors

AlGaN/GaN heterostructures with a two-dimensional electron gas (2DEG) exhibit unique transport properties that could potentially be used for novel applications that are not related to conventional modulation-doped field effect transistor devices. Here, we describe the fabrication of high sensitivity temperature sensors exploiting temperature induced resistance changes of AlGaN/GaN-2DEG heterostructures. We observed a monotonous change in the resistance of the 2DEG from 3 to 1000 K. The temperature dependence of the resistance above 180 K fitted the Callender–Van Dusen equation. The sensitivity of the AlGaN/GaN temperature sensors was more than 2 times higher than conventional resistance temperature detectors near room temperature and 5 times higher at about 900 K. These new AlGaN/GaN temperature sensors may find niche applications in extreme environments, such as space exploration, as well as where high sensitivity is required over wide temperature ranges.

Compact electromagnetically operated microfluidic system for detection of sub-200-nm magnetic labels for biosensing without external pumps

The combination of small sample analyte volumes, high sensitivity, ease of use, high speed, and portability is an important factor for the development of protocols for point of care biodiagnosis. Currently, handling small amounts of liquids is achieved using microfluidic systems but it is challenging to satisfy the remaining factors using conventional approaches based on biosensors employing detection of fluorescent labels. Thus to resolve the other requirements, biosensing systems based on the detection of functionalized superparamagnetic beads acting as “magnetic labels” are being studied as an alternative approach. Notably, for greater quantification, there are increasing demands for the use of sub-200-nm magnetic labels, which are comparable in size to actual biomolecules. However, detection of small numbers of sub-200-nm diameter magnetic beads by magnetoresistive device-based platforms is extremely challenging due to the intrinsic noise of the electronic devices. In order to overcome the limitation,...

Planar Microfluidic System Based on Electrophoresis for Detection of 130-nm Magnetic Labels for Biosensing

Superparamagnetic beads (SPBs) used as magnetic labels offer potential for the realization of high sensitivity and low cost biosensors for point of care treatment (POCT). For better biomolecular affinity and higher sensitivity, it is desirable to use sub-200-nm-diameter SPBs comparable in size to actual biomolecules. However, the detection of small concentrations of such SPBs by magnetoresistive devices is extremely challenging due to small magnetic response of SPBs. As a solution to these limitations, we describe a simple detecting procedure where the capture of micro-SPBs by immobilized nano-target SPBs due to self-assembly induced by an external magnetic field, which was monitored under an optical microscope. Here we describe biosensing system based on self-assembly of micro-SPBs by nanoSPBs targets using a system without external pumps, thereby enabling greater miniaturization and portability.

Patterning of Two-Dimensional Graphene Oxide on Silicon Substrates

Chemically synthesized graphene is promising for device applications because the chemical approach enables ease of mass production and chemical modification of its properties. However, a major drawback of graphene based devices is that it is difficult to integrate the small flakes of graphene into device architectures. In order to overcome this limitation, we describe a simple procedure for patterning graphene oxide (GO) flakes onto predefined locations of silicon substrates. We exploited the negatively charged surface of GO flakes, and successfully patterned GO flakes onto photolithographically defined positively charged regions on silicon substrates. We demonstrate the simultaneous fabrication of multiple GO flakes device structures by controlling the surface chemistry of substrates. Our procedure for the precise positioning of GO flakes will be an important step in the fabrication of graphene devices.

Rapid Biosensing Platform based on Monitoring Changes in the Optical Reflectance of Porous Silicon due to Penetration by Functionalized Superparamagnetic Beads

Monitoring the highly efficient optical interference spectra of light scattered from the surface of porous silicon (PSi) shows potential as means of using Psi as a material for biosensing applications [1]. Reports show the spectral positions of the Fabry-Pérot fringes to shift as a function of the refractive index of the material filling the pores of PSi. Notably, the penetration of biomolecules into pores of PSi due to either non-specific adsorption or specific binding, is observed as a shift of the Fabry-Pérot fringes to longer wavelengths [2]. This effects is due to an increase in the refractive index of the PSi layer as biomolecules displace aqueous solution occupying the pores. The optical properties of PSi, which is fabricated by electrochemical anodization, are precisely tunable by control of the geometric orientation and size of the pores [3]. Thus PSi could potentially be used for applications such as optical devices and chemical and/or biosensors. We are interested in exploiting the aforementioned optical interference properties of PSi for biosensing. Todate PSi-based biosensors have been based on a large surface area matrix covered with immobilized biomolecules such as enzymes, proteins, or DNA fragments. However, such an approach is slow, taking hours for molecules to deeply penetrate inside pore walls where complementary molecules are immobilized. Furthermore, a small concentrations of molecules and/or small molecules only produce a vanishingly small change in the scattered optical signal. Thus, a means of additional amplification is essential for higher sensitivity. Here we report on a novel protocol utilizing functionalized superparamagnetic nanoparticles (beads) for not only amplifying the spectrum shift of the reflected optical signal, but also for enhancing the efficiency of the penetration of biomolecules into pores. Superparamagnetic beads of ~130 nm in diameter, penetrate porous silicon with pore diameters of about 500 nm. The spectrum shift of the scattered light caused by the dramatic change in index refraction of the pores due to penetration by superparamagnetic beads was readily observed by spectroscopic measurements. More importantly, due to the functionalized superparamagnetic beads, wavelength shifts of the reflection spectrum due to chemical binding reactions between the beads and the PSi was amplified and clearly observed although such reactions are extremely difficult to detect by conventional methods.

Compact Electro-Magnetically Operated Microfluidic System for Detection of sub-200 nm Magnetic Labels for Biosensing without External Pumps

1 Department of Electrical and Electronic Engineering, Tokyo Institute of Technology, 2-12-1 O-okayama, Meguro-ku, Tokyo 152-8552, Japan Telephone/facsimile. +81-3-5734-2807, E-mail : takamura.t.ab@m.titech.ac.jp 2 Tokyo Tech Global COE Program on Evolving Education and Research Center For Spatio-Temporal Biological Network, Tokyo, 152-8552, Japan 3 Electronics-Inspired Interdisciplinary Research Institute (EIIRIS), Toyohashi University of Technology, 1-1 Hibarigaoka, Tempaku-cho, Toyohashi, Aichi 441-8580, Japan