Contents of Research

Contents of Research 

Example of recent studies

Example of recent studies (1)
New analysis of electroabsorption spectra (Integral method analysis)

K. Awasthi, T. Iimori, N. Ohta: J. Phys. Chem. C 118, 18170 (2014); 119, 4351 (2015)

Usually electroabsorption (E-A) spectra are analyzed as follows: 1) absorption spectra are divided into each of the absorption bands; 2) a linear combination of the zeroth, first and second derivatives of each absorption band is considered, and E-A spectra are simulated by a superposition of these linear combinations of different bands. Based on the coefficient of each derivative of each absorption band, the change in electric dipole moment and/or molecular polarizability following absorption into each electronic state is determined. The above-mentioned analysis, which may be called as the differential method, is applicable, only when the absorption bands as well as the corresponding E-A bands are fully separated to each other. If the absorption bands cannot be separated to each other or the absorption band which corresponds to the observed E-A signals cannot be identified, the differential method is inapplicable. In fact, we met with this kind of situation in the analysis of the E-A spectra of semiconductor quantum dots (QDs) of PbSe and PbS. Then, the new analysis (integral method analysis) has been applied to the E-A spectra of PbSe and PbS QDs.

In the integral method, the first integral and second integral of the E-A spectra are taken along wavenumber. Then, the first integral is simulated by a linear combination of the zeroth and first derivatives and the first integral of the absorption bands, and the second integral of the E-A spectrum is simulated by a linear combination of the zeroth derivative and the first and second integrals of the absorption bands. By using the integral method, therefore, each of the E-A spectrum and its first and second integrals can be checked by comparing the simulated spectrum with the experimental data. If the Stark shift is so large, the absorption band may be confirmed, even when the absorption intensity is negligibly small. Thus, the advantage of the integral method analysis is that the E-A spectra can be confirmed even when the absorption intensity is too weak to be detected, in contrast with the differential method analysis. Another advantage of the integral method is that the simulation of the E-A spectra can be done more precisely because not only the observed E-A spectrum but also its first and second integrals are compared with the simulated spectrum.

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Example of recent studies (2) Control of intracellular function by nanosecond pulse electric field (Induction of apoptosis and measurement of fluorescence lifetime imaging)

K. Awasthi, T. Nakabayashi, N. Ohta, J. Phys. Chem.B 116, 11159 (2012)
Introduction by Hokkaido Medical Newspapers     October 19, 2012

By combining the fluorescence lifetime imaging system with a metal electrode microchamber, a system was developed for observing the cellular change induced by application of a nanosecond pulsed electric field at a single cell level (Fig. 1). Cells cultured in the gap of a comb-shaped Au electrode were observed on the spot using a microscope.

By applying a nanosecond pulse electric field, apoptosis could be induced in a cell. Fig. 2 shows an example of the effect of the nanosecond pulse electric field on a HeLa cell. Here (A) shows the observed images before the electric field is applied and (B) shows the observed images after applying the pulsed electric field of 4 MVm-1 with a repetition of 1 kHz for 60 seconds. In the figures, left-hand side images are of fluorescence intensity of enhanced green fluorescence protein (EGFP) expressed in the HeLa cell and right-hand side images are the corresponding fluorescence lifetime images. Fig. 2C shows the histogram of the fluorescence lifetime images observed before and after application of the electric field. The images of the fluorescence intensity in Fig. 2B reveals that by applying an electric field multiple projections are generated in the cell, which is the typical feature of apoptotic cell death.

In addition, the fluorescence lifetime of EGFP is found to be shortened by the electric field (refer to Fig. 2C). It is shown that apoptosis is induced by the pulse electric field and intracellular environment around the EGFP is changed.

It usually takes several hours to induce apoptosis in the cell using a reagent but our result shows that apoptosis can be induced in a short time without a reagent using a nanosecond pulsed electric field, suggesting that malignant cells can be perished swiftly and diseases can be cured.

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Example of recent studies (3) Insulator-metal transition induced by photoirradiation and electric field

F. Sabeth, T. Iimori, N. Ohta, J. Am. Chem. Soc. 134, 6984 (2012)
Introduction by the Science News (Ltd.) May 18, 2012

Changes in electrical conductivity by photoirradiation and an electric field are extremely interesting not only as basic research but also as applied research. We prepared two electrodes with a separation of approximately 400 μm on the surface of the single crystal of deuterated κ-(BEDT-TTF)2Cu[N(CN)2]Br, which is known as the organic Mott insulator. Then, by applying a pulsed voltage having a width of 40 to 50 milliseconds and measuring the value of the electrical current, electrical conductivity was measured. In addition, change in the time profiles of the electric current was traced by radiating a visible pulse laser beam (synchronized with the applied pulsed voltage) between the electrodes.

The resistance of the sample was found to increase as the temperature decreases (Fig. 1). The crystal shows transition to a Mott insulator at 15K. Under this state, the pulse voltage of various magnitudes was applied and the electrical current profiles were measured (Fig. 2). As a result, when the voltage is smaller than a certain critical value (the threshold voltage), only negligibly small current is observed. However, when the applied voltage is increased up to the threshold value, a current begins to flow suddenly and an abrupt change (switching) in the electrical conductivity is observed. This is insulator-metal transition induced by an electric field. As shown in Fig. 3, hysteresis is also observed, where the switching voltage is different for the reverse direction of the applied voltage. The switching is found to change depending on the temperature.

By irradiating visible pulse laser (a wavelength of 470 nanometers) synchronized with the applied pulsed voltage, non-linear conduction characteristics are found to change (Fig. 4). When the applied voltage is smaller than the threshold voltage (17 V in Fig. 4), conductivity switching is not observed without photoirradiation. However, when a laser beam is irradiated on the sample, switching is found to be induced at the same applied voltage. Thus, it is found that electrical conductivity can be controlled as a synergic effect between light and the electric field.

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Example of recent studies (4) Measurement of electric field absorption and electroluminescence in liquid solution

H.-C. Chiang, T. Iimori, T. Onodera, H. Oikawa, N. Ohta, J. Phys. Chem. C 116, 8230-8235 (2012)
H.-C. Chiang, N. Ohta, J. Phys. Chem. B 117, 3861 (2013)

Electroabsorption (E-A) spectrum and electroluminescence spectrum in a liquid solution have been successfully measured in combination with polarization experiment. For example, polarized electroabsorption spectra were measured by changing angle (χ) defined by the polarization direction of excitation light and electric field for a sample, in which microcrystal (approximately 0.5 mm) of DAST, which is known to exhibit a significantly large nonlinear optical effect, was solved in a non-polar solvent (Fig. 1). In comparison with luminescence excitation spectrum, it is found that the absorption spectrum of the microcrystal is composed of three components (Fig. 2). It is found that when χ is 54.7° (a magic angle), the absorption spectrum exhibits a shape which closely resembles to the first differential of the absorption spectrum of each component, and when χ is 90°, the absorption spectrum closely resembles to the absorption spectrum of each component. It means that DAST microcrystal is oriented in the direction of the electric field because of the existence of electric dipole moment. Analysis shows that the magnitude of the electric dipole moment of the microcrystal is as large as approximately 40,000 debye although the electric dipole moment of an ordinary molecule is about 10 debye. Thus one can compare how large electric dipole moment of the DAST microcrystal is. A sample in which pyrene was solved in a non-polar solvent, an E-A spectrum (Fig. 3) and an E-PL spectrum (Fig. 4) could be measured. Pyrene is known to form an excited complex (excimer). Besides monomer luminescence from a locally excited state of the excited pyrene itself, broad excimer fluorescence was observed at the longer wavelength side. From contribution of a first differential contained in an E-PL spectrum, molecular polarizability of excimer was found to be larger than the ground state of pyrene by 270 angstroms. The intensity of monomer fluorescence and excimer fluorescence are found to increase with application of an electric field. It is considered that monomer and excimer are in equilibrium and non-radiation process of excimer is suppressed by the electric field. These results are significantly different from the case in which the sample is embedded in a high polymer (see Bull. Chem. Soc. Jpn. 75, 1637 (2002)).

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Example of recent studies (5) Success in visualization of hydrogen-ion concentration in an unstained single cell

J. Phys.Chem. B 115, 10385 (2011)
Introduction by the Science News (Ltd.)             September 2, 2011
Introduction by Nikkan Kogyo Shimbun             September 7, 2011

A certain level of hydrogen-ion concentration (pH) is extremely important for a well functioning of human body. Although an adjusting function operates so as to make the hydrogen-ion concentration constant in human body, disruption of balance leads to vomiting and breathing difficulty. In a cancer cell, the hydrogen-ion concentration in the cell increases rapidly. The hydrogen-ion concentration in a liquid solution can easily be checked by using litmus paper, but it is not possible to use the litmus paper in a cell and living tissue. Generally, cells are loaded with a fluorescent dye and the hydrogen-ion concentration in the cell is obtained by the intensity of fluorescence emitted by the material. However, since the method by dyeing introduces a foreign material, that is a fluorescent material, into the cell, the original state of the cell is changed. In addition, it takes time for dyeing and rapid determination cannot be performed for example during a surgical operation. Beside this the fluorescent material may have poisonous nature which is necessary to investigate before the clinical demostration.

We started to develop a fluorescence lifetime imaging method for measuring the hydrogen-ion concentration in a cell and in living tissues under the unperturbed state of the cell and had succeeded in measuring the hydrogen-ion concentration without using a dye no dyeing using for a cultured cell. A sensitive response from the fluorescence lifetime of nicotinamide adenine dinucleotide (NADH) contained in a cell to the hydrogen-ion concentration was used (refer to Fig. 1). NADH is a compound serving as a coenzyme and known to exist in almost all the cells. Observed luminescence is called autofluorescence because it originates from the cell itself. Since, being dependent on various experimental conditions, it is difficult to quantitatively obtain fluorescent intensity in the cell and tissues by intensity measurement. Our method achieved high sensitivity as it depends on fluorescence lifetime measurement instead of the fluorescence intensity measurement. The fluorescence lifetime of NADH is found to extend with the increase in the hydrogen-ion concentration in the cell. It is shown that the hydrogen-ion concentration in the cultured cell can be obtained using the fluorescence life of NADH by the fluorescence lifetime imaging microscopy (refer to Fig. 2). Since luminescent species originally existing in the cell are used, the hydrogen-ion concentration in the cell can be obtained on the spot without dyeing the cell, and swift determination is possible. Application to early detection of disease and cancer diagnosis can be expected.

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Example of recent studies (6) Optical response of electrical conductivity of organic conductor

J. Phys. Chem. C 113, 4654 (2009);J. Phys. Chem. C 114, 9070 (2010), CHEMISTRY TODAY, June 2011

Optical response of electrical conductivity of organic superconductor

We could clarify a specific photoirradiation effect in the vicinity of a superconducting transition temperature for the crystal κ-(BEDT-TTF)2Cu[N(CN)2]Br. According to the theory on phase transition and critical phenomena, a relaxation time is predicted to be divergently extended as the temperature approaches a critical (phase transition) temperature(Tc). When comparing the case of approaching to the critical temperature from a higher temperature side to a lower temperature side, the temperature dependency of the relaxation time is considered to symmetrically diverge against a critical point. An observed relaxation time was not at Tc but remarkably extended at a temperature lower than the critical temperature, being asymmetrical to the critical point. That shows peculiarity of an organic superconductor in a critical phenomenon.

Insulator to metal transition by photoirradiation and memory effect of electrical conductivity

The crystal of α-(BEDT-TTF)2I3 is an insulator at 115K and almost no current flows without photoirradiation. On the other hand, it is found that when light is irradiated on the sample, the magnitude of current increases hundred times, resulting in switching to a metallic state having high electrical conductivity. Further, a memory effect was observed depending on a pulse width of an electric field applied during optical illumination. (If the time width of the electric field is small, the crystal returns to an insulating state when light is turned off. If the pulse width is increased, the metallic state is maintained even when light is turned off.)

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Example of recent studies (7) Fluorescence characteristics shows synergy effect between electric field and magnetic field

J. Photochem. Photobiol. A 221, 1 (2011) (Invited Feature Article)

Fluorescence characteristics shows magnetic field effect by hyperfine interaction in the case where a donor (D, electron donor) and an acceptor (A, electron acceptor) showing photoinduced electron transfer reaction are relatively separated, since exchange interaction is so small that the difference in energy between a spin-singlet state and a spin-triplet state of radical ion pairs is small. By simultaneously applying an electric field and a magnetic field to a photoinduced electron transfer reaction system, a significant magnetic field effect is observed on the electric field effect even when almost no magnetic field effect would be observed. That is, synergy effect between an electric field and a magnetic field on photoexcited dynamics was found to exist.

It is conceivable that the level of radical ion pairs is shifted by an electric field, and the length of ion pairs varies by the movement of holes or electrons. Synergy effect between an electric field and a magnetic field enables fluorescence from a locally excited state which is not usually observed as shown in the figure to show a magnetic effect. (To be exact, a magnetic field can influence the electric field effect because quenched fluorescence by the electric field varies with the application of the magnetic field.)

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