Nobuaki Soh

Perylene bisimide as a versatile fluorescent tool for environmental and biological analysis: A review

Perylene bisimide (PBI) is a fluorescent dye which has strong emission and high photostability. Although PBI has been widely used for industrial materials, the application of PBI in analytical fields was limited mainly due to its high hydrophobicity. In recent years, however, unique and useful analytical methods based on PBI platform are being successfully developed by utilizing the characteristic features of this compound including its high hydrophobicity. In this article, the recent trend of environmental and biological analysis using PBI is reviewed.

Performance of an organic photodiode as an optical detector and its application to fluorometric flow-immunoassay for IgA.

The performance of an organic thin film photodiode (OPD), fabricated from a hetero-junction comprised of two layers of C(60) and a phthalocyanine-Cu(II) complex was evaluated by detecting the chemiluminescence generated from the reaction of luminol with horseradish peroxidase in the presence of H(2)O(2), and the fluorescence from resorufin, as an optical detector. The photocurrent of the OPD was linear with respect to the power of light from a commercial LED. The sensitivity of the OPD was sufficient for detecting chemiluminescence with a power 0.1μW/cm(2). The OPD was successfully used in a flow-immunoassay for IgA, a marker of human stress, in which a sandwich immunoassay was carried out on the microchip and the fluorescence from resorufin, produced by the enzymatic reaction, was detected. The detection limits for resorufin and IgA were 5.0μM and 16ng/mL, respectively. The photosensitivity of the OPD remained relatively constant for a minimum of one year.

Temperature-controlled reversible exfoliation-stacking of titanate nanosheets in an aqueous solution containing tetraalkylammonium ions

Exfoliation and stacking of titanate nanosheets dispersed in aqueous solutions containing tetraalkylammonium ions can be reversibly controlled by adjusting the solution temperature as a tunable physical parameter. That is, the transparent colloidal solution of exfoliated titanates is clouded on the basis of spontaneous stacking of multiple nanosheets when heated at a certain temperature, and cooling of the clouded solution causes regeneration of the original exfoliated state. This cycle repeatedly and rapidly occurs according to the temperature fluctuation. The origin of behavior is qualitatively interpreted with a temperature-dependent variation in the Debye screening length of negatively charged nanosheets that affects the dispersibility of nanosheets.

Enhanced catalytic activity of enzymes interacting with nanometric titanate nanosheets

The effect of the coexistence of titanate nanosheets (TNS) with nanometric lateral dimensions (ca. 3 nm), which were prepared through a hydrolysis reaction of titanium tetraisopropoxide, on the catalytic activity of horseradish peroxidase (HRP) was investigated as a function of solution pH. Especially in diluted HRP solutions with a pH range of 7–8, enzymatic reaction rate, i.e. maximum velocity (Vmax), in the conventional Michaelis–Menten equation, was significantly enhanced more than 2 times in the presence of TNS. In contrast, the increase in Vmax was not very large in acidic (pH = 4.0) and basic solutions (pH = 9.0). It was demonstrated that the TNS brought about peptization of aggregates composed of several HRP molecules in a diluted solution, causing an increase in the apparent HRP concentration participating in the enzymatic reaction. Moreover, the TNS activated superoxide dismutase (SOD) with O2− scavenging performance.

Preparation of an enzyme/inorganic nanosheet/magnetic bead complex and its enzymatic activity

A complex composed of enzyme (horseradish peroxidase, HRP), layered titanate (TiOx), and magnetic beads (MB) was prepared, and the enzymatic activity of the complex and the repeatable use for the enzyme in the complex were investigated. The HRP/TiOx/MB complex was formed in acetate buffer (pH 4) by electrostatic interaction among the components where HRP and MB have positive charge, whereas TiOx is negatively charged. The resultant complex was sufficiently stable in an aqueous solution, and HRP immobilized on the layered titanate in the complex kept a certain enzymatic activity in which typical enzymatic relationship was observed between substrate concentration and initial reaction rate. By giving a magnetic field using commercial magnets, the HRP/TiOx/MB complex was easily and rapidly recovered, and the recovered complex was able to be used for the next enzymatic reactions. The enzyme/inorganic nanosheet/MB complex proposed in this study is expected to be applied as a new tool in the bioengineering and nanobiotechnological fields.

Enhanced visible-light-induced photocatalytic activity of α-Fe2O3 adsorbing redox enzymes

Abstract We report fabrication of hybrid photocatalyst composed of an n-type semiconductor (α-Fe2O3) and a redox enzyme (horseradish peroxidase; HRP), and its performance for oxidation of luminol in an aqueous solution. The hybrid photocatalyst is simply formed via physical adsorption of HRP to an α-Fe2O3 sintered body. Under visible light irradiation, the bare α-Fe2O3 with a narrow bandgap photocatalytically oxidizes luminol along with blue emission that can be used as an indicator of the photocatalytic performance. The blue emission is largely strengthened after the adsorption of HRP, demonstrating that the presence of enzyme improves apparent photocatalytic activity of α-Fe2O3. The favorable effect is derived from synergistic oxidation of luminol by the biocatalysts (HRP) as well as by the photocatalyst (α-Fe2O3). In this paper, influence of excitation wavelength, adsorption amount of HRP, and reaction temperature on the overall photocatalytic activity are elucidated, and then a reaction mechanism of the proposed novel hybrid photocatalyst is discussed in detail.

Time-resolved Fluorescent Detection for Glucose Using a Complex of Luminescent Layered Titanates and Enzymes

Luminescent europium-doped layered titanates (Eu-TiOx) were synthesized and complexed with horseradish peroxidase (HRP) and glucose oxidase (GOx). The emission of a resultant Eu-TiOx/HRP/GOx complex decreased upon the addition of glucose in the presence of guaiacol. The emission decrease was dependent on the concentrations of glucose, and the detection limit for glucose was 3.1 μM. The proposed system would be promising as a new detection method for glucose.

Enhanced catalytic activity and thermal stability of lipase bound to oxide nanosheets

The present study reports the effects of binding of lipase, which is an inexpensive digestive enzyme (candida antarctica lipase) that catalyzes the hydrolysis reaction and is frequently utilized for artificial synthesis of a variety of organic molecules, to titanate nanosheets (TNSs) on their biocatalytic activities and stabilities under several lipase concentrations. TNSs were prepared through a hydrolysis reaction of titanium tetraisopropoxide (TTIP) with tetrabutylammonium hydroxide (TBAOH), resulting in formation of a colorless and transparent colloidal solution including TNSs with nanometric dimensions (hydrodynamic diameter: ca. 5.6 nm). TNSs were bound to lipase molecules through electrostatic interaction in an aqueous phase at an appropriate pH, forming inorganic-bio nanohybrids (lipase–TNSs). The enzymatic reaction rate for hydrolysis of p-nitrophenyl acetate (pNPA) catalyzed by the lipase–TNSs, especially in diluted lipase concentrations, was significantly improved more than 8 times as compared with free lipase. On the other hand, it was confirmed that heat tolerance of lipase was also improved by binding to TNSs. These results suggest that the novel lipase–TNSs proposed here have combined enhancements of the catalytic activity and the anti-denaturation stability of lipase.

Synergistic Functions of Enzymes Bound to Semiconducting Layers.

Synthesis and cooperative functions of hybrid materials composed of enzyme and semiconducting layers are described in this chapter. The hybrids were produced via a simple physical interaction between the components, that is, electrostatic interaction in an aqueous solution. To form interstratifying enzymes in the galleries, solution pH, which is a key parameter to decide surface potential, should be adjusted appropriately. In other words, enzymes should have an opposite charge when compared to that of the layers at an identical pH. Even though the intercalation slightly reduced enzymatic activity as compared to those of the free enzymes, stability under cruel conditions was drastically improved due to screening effect of semiconducting layers from extrinsic stimuli. In addition, photochemical control of redox enzymes sandwiched between semiconducting layers was accomplished. Light irradiation of the hybrids induced band gap excitation of the layers, and holes produced in the valence band activated the enzymes. It was revealed that the semiconducting layers with magnetic elements might be useful to magnetic application (separation) of enzymes as similar to conventional magnetic beads.