Identification of Cadmium Compounds in a Solution Using Graphene-Based Sensor Array.

Rapid detection of heavy metals in solution is necessary to ensure human health and environmental protection. Some heavy-metal compounds are present in solution as compounds instead of as ions owing to their low ionization. Therefore, the development of sensor devices for the detection of heavy-metal compounds is important. In this study, as a proof of concept, we propose a sensor device using graphene and a chelating agent, which were used to develop an identification technique for three types of cadmium compounds. Pristine-graphene and two types of chelator-modified graphene-based sensors were successfully used to detect cadmium compounds at concentrations ranging from 50 to 1000 µM. The detection time was less than 5 min. The three type of graphene-based sensors responded differently to each cadmium compound, which indicates that they detected cadmium as a cadmium compound instead of as cadmium ions. Furthermore, we successfully identified cadmium compounds by operating these three types of sensors as a sensor array on the same substrate. The results indicate that sensors that focus on heavy-metal compounds instead of heavy-metal ions can be used for the detection of heavy metals in solution.

Iridium(III)-Catalyzed Asymmetric Site-Selective Carbene C-H Insertion during Late-Stage Transformation.

C-H functionalization has recently received considerable attention because C-H functionalization during the late-stage transformation is a strong and useful tool for the modification of the bioactive compounds and the creation of new active molecules. Although a carbene transfer reaction can directly convert a C-H bond to the desired C-C bond in a stereoselective manner, its application in late-stage material transformation is limited. Here, we observed that the iridium-salen complex 6 exhibited efficient catalysis in asymmetric carbene C-H insertion reactions. Under optimized conditions, benzylic, allylic, and propargylic C-H bonds were converted to desired C-C bonds in an excellent stereoselective manner. Excellent regioselectivity was demonstrated in the reaction using not only simple substrate but also natural products, bearing multiple reaction sites. Moreover, based on the mechanistic studies, the iridium-catalyzed unique C-H insertion reaction involved rate-determining asynchronous concerted processes.

Development of an odorant sensor with a cell-free synthesized olfactory receptor and a graphene field-effect transistor.

Animals sense odorants using olfactory receptors. Many trials have been conducted to develop artificial odorant sensors using olfactory receptors. However, the development has been hindered by the difficulty in obtaining olfactory receptors. In this study, we expressed an olfactory receptor, cOR52, using a wheat germ cell-free synthesis system. The functionality of the expressed cOR52 was confirmed by ligand concentration-dependent interactions with the mini-G protein. The expressed cOR52 was immobilized on a graphene field-effect transistor. The cOR52-modified graphene field-effect transistor exhibited a ligand-specific response between 100 nM and 100 µM. This approach seems to be applicable for other olfactory receptors. Therefore, it will be possible to develop an odorant sensor equipped with various olfactory receptors by this method.

Electrical Detection of Molecular Transformations Associated with Chemical Reactions Using Graphene Devices.

This study proposes a method to electrically detect chemical reactions that involve bond changes through reactions on graphene surfaces. To achieve a highly sensitive detection, we focused on the thiol-ene reaction that combines the maleimide and thiol groups. Graphene field-effect transistors (FETs) were used to detect the binding changes of the modified molecules. Graphene has high carrier mobility and is sensitive to changes in the electronic state of its surface. Graphene has been used as a sensor to detect low-concentration targets with high sensitivity. N-(9-Acridinyl)maleimide (NAM) was chosen as the modified molecule to immobilize maleimide on graphene through π-interaction, and methanethiol (MeSH) was set as the target thiol. The modification of NAM to graphene was first confirmed by attenuated total reflection Fourier transform infrared spectroscopy, and the modification density was 0.5 ± 0.1/nm2 through cyclic voltammetry. Owing to a bond exchange, the transfer characteristics of the graphene FET shifted by 2 V to the negative direction after being exposed to MeSH at 10 parts per billion (ppb), equivalent to 0.2 ng, under ultraviolet irradiation. With 5000 ppb of acetic acid, it only shifted 0.7 V. With 1000 ppb of ethanol and 10,000 ppb of methanol, it shifted to the positive direction by 0.4 and 0.6 V, respectively. Because the nontarget molecule showed only a slight response, a thiol-ene chemical reaction was detected. The proposed method can detect the bond-change reaction using an ultralow concentration of MeSH, which indicates that at least 10 ppb (or 0.2 ng) of MeSH was detected by the graphene FET.

Electrical detection of ppb region NO2 using Mg-porphyrin-modified graphene field-effect transistors.

The trace detection of NO2 through small sensors is essential for air quality measurement and the health field; however, small sensors based on electrical devices cannot detect NO2 with the desired selectivity and quantitativity in the parts per billion (ppb) concentration region. In this study, we fabricated metalloporphyrin-modified graphene field-effect transistors (FETs). Mg-, Ni-, Cu-, and Co-porphyrins were deposited on the graphene FETs, and the transfer characteristics were measured. With the introduction of NO2 in the ppb concentration region, the FETs of pristine graphene and Ni-, Cu-, and Co-porphyrin-modified graphene showed an insufficient response, whereas the Mg-porphyrin-modified graphene exhibited large voltage shifts in the transport characteristics. This indicates that Mg-porphyrin acts as an adsorption site for NO2 molecules. An analysis of the Dirac-point voltage shifts with the introduction of NO2 indicates that the shifts were well-fitted with the Langmuir adsorption isotherm model, and the limit of detection for NO2 was found to be 0.3 ppb in N2. The relationship between the mobility and the Dirac-point voltage shift with the NO2 concentration shows that the complex of NO2 and Mg-porphyrin behaves as a point-like charge impurity. Moreover, the Mg-porphyrin-modified graphene FETs show less response to other gases (O2, H2, acetic acid, trimethylamine, methanol, and hexane), thus indicating high sensitivity for NO2 detection. Furthermore, we successfully demonstrated the quantitative detection of NO2 in air, which is near the environmental standards. In conclusion, the results of the Mg-porphyrin-modified graphene FETs enable a rapid, easy, and selective detectability.

Selective Detection of Cu2+ Ions by Immobilizing Thiacalix[4]arene on Graphene Field-Effect Transistors.

Highly accurate quantitative detection of heavy metals is essential for environmental pollution monitoring and health safety. Here, for selective detection of Cu2+ ions with high sensitivity, thiacalix[4]arene (TCA) immobilized on graphene field-effect transistors (G-FETs) are developed. Our proposed TCA-immobilized G-FETs are successfully used to detect Cu2+ ions at concentrations ranging from 1 µM to 1 mM via changes in their transfer characteristics. Moreover, the measured transfer characteristics clearly shift only when Cu2+ ions are introduced in the buffer solution despite it containing other metal ions, including those of Na+, Mg2+, Ni2+, and Cd2+; this selective detection of Cu2+ ions is attributed to the planar arrangement of TCA on graphene. Therefore, TCA-immobilized G-FETs selectively detect Cu2+ with high sensitivity.

Properties of electrostatic correcting systems with annular apertures.

Image formation in electron microscopes with circular hole and annular apertures is studied theoretically. The apertures-the circular hole aperture being negative with respect to the annular aperture-produce an additional electrostatic field that exerts a force on the electrons directed toward the optical axis. The resulting deflection angle decreases with increasing distance from the optical axis. This electrostatic field results in a correcting effect of the unavoidable spherical aberration of round electron lenses; the deflection toward the optical axis increases stronger than linearly with increasing distance from the optical axis. Analytical formulae are given for the correcting effect of circular hole and annular apertures. The expressions are based on the Davisson-Calbick formula, which is used to calculate focal length of a simple electrostatic lens.

Photosensing System Using Photosystem I and Gold Nanoparticle on Graphene Field-Effect Transistor.

In this study, a light sensor is fabricated based on photosystem I (PSI) and a graphene field-effect transistor (FET) that detects light at a high quantum yield under ambient conditions. We immobilized PSI on a micrometer-sized graphene FET using Au nanoparticles (AuNPs) and measured the I-V characteristics of the modified graphene FET before and after light irradiation. The source-drain current (Isd) increased upon illumination, exhibiting a photoresponsivity of 4.8 × 102 A W-1, and the charge neutrality point of graphene shifted by -12 mV. This system represents the first successful photosensing system based on proteins and a solution-gated graphene FET. The probable mechanism of this negative shift can be explained by the increase in negative charge carriers in graphene induced by a hole trap in the AuNP resulting from electron transfer from the AuNP to PSI. Photoresponses were only observed in the presence of two surface-active agents, n-hexyltrimethylammonium bromide and sodium dodecylbenzenesulfonate, because they caused the formation of a hydrophobic environment on the surface of the graphene. The lipid layer of these agents caused dissociation of ascorbate ions from the graphene sheet, thereby expanding the Debye screening length of the electrolyte solution. The hydrophobic environment above graphene also enhanced hole storage in the AuNP through electron transfer from the AuNP to PSI.

Graphene Surface Acoustic Wave Sensor for Simultaneous Detection of Charge and Mass.

We have combined a graphene field-effect transistor (GFET) and a surface acoustic wave (SAW) sensor on a LiTaO3 substrate to create a graphene surface acoustic wave (GSAW) sensor. When a SAW propagates in graphene, an acoustoelectric current (IA) flows between two attached electrodes. This current has unique electrical characteristics, having both positive and negative peak values with respect to the electrolyte-gate voltage (VEg) in solution. We found that IA is controlled by VEg and the amplitude of the SAW. It was also confirmed that the GSAW sensor detects changes of electrical charge in solution like conventional GFET sensors. Furthermore, the detection of amino-group-modified microbeads was performed by employing a GSAW sensor in a phthalate buffer solution at pH 4.1. The hole current peak shifted to the lower left in the IA-VEg characteristics. The left shift was caused by charge detection by the GFET and can be explained by an increase of amino groups that have positive charges at pH 4.1. In contrast, the downward shift is thought to be due to a reduction in the amplitude of the propagating SAW because of an increase in the mass loading of microbeads. This mass loading was detected by the SAW sensor. Thus, we have demonstrated that the GSAW sensor is a transducer capable of the simultaneous detection of charge and mass, which indicates that it is an attractive platform for highly sensitive and multifunctional solution sensing.

Room-temperature discrete-charge-fluctuation dynamics of a single molecule adsorbed on a carbon nanotube.

Detection and use of physical noise fluctuations in a signal provides significant advantages in the development of bio- and neuro-sensing and functional mimicking devices. Low-dimensional carbon nanomaterials are a good candidate for use in noise generation due to the high surface sensitivity of these materials, which may themselves serve as the main building blocks of these devices. Here, we demonstrate that the addition of a molecule with high redox activity to a carbon nanotube (CNT) field-effect transistor provides tunable current fluctuation noise. A unique charge-trap state in the vicinity of the CNT surface due to the presence of the single molecule is the origin of the noise, which generates a prominent and unique slow discrete random telegraph signal in the device current. The power spectral density reveals the peculiar frequency limit of the fluctuation for different types of molecules depending on their redox activity and adsorption configuration. These results indicate that the detected noise will provide new opportunities to obtain electronic information for a single molecule combined with a nanotube surface, and that controllability of the noise may contribute to the expansion of noise utilization in future bio-inspired devices.

Quantitative evaluation of annular bright-field phase images in STEM.

A phase reconstruction method based on multiple scanning transmission electron microscope (STEM) images was evaluated quantitatively using image simulations. The simulation results indicated that the phase shift caused by a single atom was proportional to the 0.6th power of the atomic number Z. For a thin SrTiO3 [001] crystal, the reconstructed phase at each atomic column increased according to the specimen thickness. The STEM phase images can quantify the oxygen vacancy concentration if the thickness is less than several nanometers.

Phase reconstruction in annular bright-field scanning transmission electron microscopy.

A novel technique for reconstructing the phase shifts of electron waves was applied to Cs-corrected scanning transmission electron microscopy (STEM). To realize this method, a new STEM system equipped with an annular aperture, annularly arrayed detectors and an arrayed image processor has been developed and evaluated in experiments. We show a reconstructed phase image of graphite particles and demonstrate that this new method works effectively for high-resolution phase imaging.

Development of a parallel detection and processing system using a multidetector array for wave field restoration in scanning transmission electron microscopy.

A parallel image detection and image processing system for scanning transmission electron microscopy was developed using a multidetector array consisting of a multianode photomultiplier tube arranged in an 8 x 8 square array. The system enables the taking of 64 images simultaneously from different scattered directions with a scanning time of 2.6 s. Using the 64 images, phase and amplitude contrast images of gold particles on an amorphous carbon thin film could be separately reconstructed by applying respective 8 shaped bandpass Fourier filters for each image and multiplying the phase and amplitude reconstructing factors.