Published papers

Electrochemical detection of mixture solutions of As3+ and As5+ has been investigated by stripping voltammetry at gold-modified diamond electrodes. The method was performed based on stripping oxidation of As0 deposited at the electrode surface. Whereas As3+ can be deposited by simple electrochemical reduction of As3+ to As0 at -0.4 V (vs. Ag/AgCl), much higher potential is required to overcome the activation energy of As5+ reduction. However, in such a high potential, hydrogen evolution also occurs. Consequently, one more step should be added to release the hydrogen gas adsorbed at the electrode surface during the reduction step. During the deposition of As5+, As3+ species was also simultaneously deposited. Therefore, to differentiate As3+ and As5+ quantification in a mixture solution, both stripping voltammetry methods should be performed and calculated mathematically. Comparison of stripping voltammograms of both methods for As3+ solution in the absence of As5+ demonstrated similar peak shapes and current intensities, confirming that error calculation of As5+ concentration in the mixture solution with As3+ can be avoided. Good linear responses were observed for each standard solution of As3+ and As5+. Linear calibration curve could also be achieved for a series concentrations of 100-1000 ppb As5+ in a mixture solution with 100 ppb As3+ (r2 = 0.99) and for a series concentrations of 5-30 ppb As3+ in a mixture solution with 100 ppb As5+. Detection limits of 5 and 100 ppb can be achieved for As3+ and As5+ in a mixture solution, respectively. The reproducibility was shown for stripping voltammetry of As3+ and As5+ with an RSD (n = 8) of 7.5% and 8.4%, respectively. Good stability of gold-modified diamond electrodes before and after arsenic detection was also evaluated by SEM image. Application of the method for real sample analysis was performed for arsenic detection in Yokohama tap water. © 2008 Elsevier B.V. All rights reserved.

Spiropyran-montmorillonite hybrid materials have been fabricated by the Langmuir-Blodgett (LB) and an intercalation method. The LB method using clay minerals could be applied to fabricate hybrid ultrathin films consisting of a non-amphiphilic spiropyran. It is confirmed that hybrid LB films possess the layered structure and exhibit "normal photochromism" without any changes in the layered structure. In case of the intercalation method, hybrid compounds also possess the layered structure. Interestingly, intercalation hybrid compounds exhibit "reverse photochromism". It is suggested that a different process in formation of the layered structure influences the observed photochromism. © 2007 Elsevier B.V. All rights reserved.

An alternative method of a protein immunosensor has been developed at boron-doped diamond (BDD) electrode material. In order to construct the base of the immunosensor, o-aminobenzoic acid (o-ABA) was electropolymerized at an electrode by cyclic voltammetry. The poly-o-ABA-modified BDD was characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS). The XPS result found that carboxyl groups were formed at the electrode surface. The carboxyl groups were then used to covalently attach protein probes. The amperometric sensing of mouse IgG (MIgG) was selected as the model at the pory-o-ABA-modified BDD to compare to the pory-o-ABA-modified glassy carbon (GC) at the same condition. An antimouse IgG from goat (GaMIgG) was covalently immobilized at a pory-o-ABA-modified BDD electrode which used a sandwich-type alkaline phosphatase (ALP) catalyzing amperometric immunoassay with 2-phospho-L-ascorbic acid (AAP) as substrate. The ALP enzyme conjugated at the immunosensor can generate AAP to the electroactive species of ascorbic acid (AA), which can be determined by amperometric detection. The signal was found to be proportional with the quantity of MIgG. The limits of detection (LODs) of 0.30 (3 SD) and 3.50 ng mL-1 (3 SD) for MIgG at BDD and GC electrodes were obtained. It also was found that the dynamic range of 3 orders of magnitude (1-1000 ng mL-1) was obtained at BDD, whereas at GC, the dynamic range was more narrow (10-500 ng mL-1). The method was applied to a real mouse serum sample that contains MIgG. © 2008 American Chemical Society.

The effect of CF4 plasma etching on diamond surfaces, with respect to treatment time, was investigated using scanning electron microscopy (SEM), Raman spectroscopy, X-ray photoelectron spectroscopy (XPS), and electrochemical measurements. SEM observations and Raman spectra indicated an increase in surface roughening on a scale of 10-20 nm, and an increase in crystal defect density was apparent with treatment time in the range of 10 s to 30 min. In contrast, alteration of the diamond surface terminations from oxygen to fluorine was found to be rather rapid, with saturation of the F/C atomic ratio estimated from XPS analysis after treatment durations of 1 min and more. The redox kinetics of Fe(CN)63-/4- was also found to be significantly modified after 10 s of CF4 plasma treatment. This behavior shows that C-F terminations predominantly affect the redox kinetics compared to the effect on the surface roughness and crystal defects. The double-layer capacitance (Cdl) of the electrolyte/CF4 plasma-treated boron-doped diamond interface was found to show a minimum value at 1 min of treatment. These results indicate that a short-duration CF4 plasma treatment is effective for the fabrication of fluorine-terminated diamond surfaces without undesirable surface damage. © 2007 Elsevier B.V. All rights reserved.

The electrochemical oxidation of procaine hydrochloride (PC·HCL, 2-diethylaminoethyl 4-aminobenzoate hydrochloride) was investigated at as-deposited boron-doped diamond (ad-BDD) electrode, anodically oxidized BDD (aoBDD) electrode and glassy carbon (GC) electrode using cyclic voltammetry (CV). Well-defined cyclic voltammograms were obtained for PC·HCL oxidation with high signal-to-background (S/B) ratio, low tendency for adsorption, good reproducibility and long-term stability at ad-BDD electrode, demonstrating its superior electrochemical behavior and significant advantages in contrast to ao-BDD and GC electrode. At 100 μM PC·HCL, the voltammetric S/B ratio was nearly one order of magnitude higher at an ad-BDD electrode than that at a GC electrode. In a separate set of experiments for oxidation of 100 μM PC·HCL, 96%, 92% and 84% of the initial oxidation peak current was retained at the ad-BDD, ao-BDD and GC electrode, respectively, by stirring the solution after the tenth cycle. The current response was linearly proportional to the square root of the scan rate within the range 10-1000 mV s-1 in 10 μM PC·HCL solutions, indicating that the oxidation process was diffusion-controlled with negligible adsorption at an ad-BDD surface. The good linearity was observed for a concentration range from 5 to 200 μM with a linear equation of y = 0.03517x + 0.65346 (r = 0.999), and the detection limit was 0.5 μM for oxidation of PC·HCL at the adBDD electrode. The ad-BDD electrode could maintain 100% of its original activity after intermittent use for 3 months. © 2008 Wiley-VCH Verlag GmbH & Co. KGaA.

Direct oxidation peaks relating to proteins have been clearly observed by exploiting H-terminated boron-doped diamond (BDD) electrodes. Hydrogen generation by reduction treatment was enabled to provide the possibility of using BDD. The pH dependence of bovine serum albumin (BSA) oxidation suggested that the oxidation involved of at least three redox-active amino acid residues (cysteine, tryptophan and tyrosine). Furthermore, the electrochemical detection of not only BSA but also immunosuppressive acidic protein (IAP, a cancer marker) was demonstrated by using cyclic voltammetry and flow injection analysis. A linear dynamic in the concentration range of 5 and 3000 mg/dl (r2 = 0.9) with a lower detection limit (LOD) of 5 mg/dl was achieved for BSA. In the case of IAP, the calibration curve was linear over the concentration range from 200 to 800 μg/ml, with an experimental LOD of 100 μg/dl. Excellent reproducibility was also demonstrated, suggested that direct electrochemical detection using BDD electrodes can be performed with advantages in terms of simplicity, sensitivity and stability. © 2007 Elsevier B.V. All rights reserved.

Preparations, XPS and electronic spectroscopy, and magnetism of seven new one-dimensional cyano-bridged coordination polymers, chiral [Cu(RR-chxn)2][Pd(CN)4] · 2H2O (1), [Cu(trans-chxn)2][M(CN)4] · 2H2O (2, 4, and 6 for M = Pd, Ni, and Pt), and [Cu(cis-chxn)2][M(CN)4] · 2H2O (3, 5, and 7 for M = Pd, Ni, and Pt) (RR-chxn = cyclohexane-(1R,2R)-diamine, trans-chxn = racemic trans-cyclohexane-(1,2)-diamine, and cis-chxn = racemic cis-cyclohexane-(1,2)-diamine) have been reported in view of tuning of their electronic properties by stereochemistry of chxn ligands and metal-substitution. Comparison of Cu 2p1/2 and 2p3/2 peaks of XPS and broad d-d bands around 18 000 cm-1 of electronic spectra are described systematically for 1-7. Variable-temperature magnetic measurement shows that complexes 1-7 indicate weak antiferromagnetic interactions via cyano-bridges. Because of semi-coordination coupled with pseudo Jahn-Teller elongation and electrostatic interaction for 1, the axial Cu-N coordination bond distances of 2.330(7) and 3.092(8) Å are considerably longer than those of equatorial ones in the range from 2.016(6) to 2.030(6) Å. The former bond distances of 1 are intermediate values among the related Ni (2.324(6) and 3.120(8) Å) and Pt (2.34(1) and 3.09(1) Å) complexes. © 2007 Elsevier B.V. All rights reserved.

The quantitative determination of free chlorine based on its oxidation current was studied using cyclic voltammetry (CV) with highly boron-doped diamond (BDD) electrodes. The voltammograms exhibited well-defined anodic peaks and a good linear relationship in the concentration range of 20-100 mg Cl L-1 using BDD electrodes without any pretreatment. Excellent reproducibility was demonstrated by an RSD of 2.56% (n = 10). The electrochemical behavior of free chlorine at different pH values suggests the optimum condition to be pH 8. Observations with the BDD electrodes terminated with either hydrogen or oxygen suggest that the active species in the anodic reaction was ClO-. Interference of limited concentration of chloride ion was not observed, whereas exceeded concentration was found to accelerate the electron transfer on chlorine oxidation. Flow injection analysis (FIA) with amperometric detection was used to estimate lower concentrations of free chlorine. A linear relationship (r2 = 0.96) was found for the concentration range of 0.1-2 mg L-1 and the low detection limit (S/N = 3) was estimated to be 8.3 μg L-1. The results suggest that BDD electrodes are more than adequate for determining the concentration of free chlorine. © 2007 Elsevier B.V. All rights reserved.

Fine-controlled metal-organic hybridization is strongly desired for advanced nano-materials. Dendrimers are highly branched organic macromolecules with successive layers or "generations" of branch units surrounding a central core. Metal-organic hybrid versions have also been produced by trapping metal ions or metal clusters within the voids of dendrimers. Here, we report a technique to form a dendritic organic structure, which includes various metals, with fine control over the number and location of the metal salts through a radial stepwise complexation. FeCl3, GaCl3,VCl 3, and SnCl2 were decorated on a phenylazomethine dendrimer with 4th generations at the first, second, third, and forth layers, respectively. These stepwise complexation and heterometal assembling were also supported by TEM observation, 57Fe Mössbauer spectroscopy, XPS measurement, and NMR spectroscopy. © 2007 The Chemical Society of Japan.

Highly boron-doped diamond (BDD) was deposited on chemically etched micrometer-sized tungsten wires using microwave plasma assisted chemical vapor deposition (MPCVD), and these were used to fabricate BDD microelectrodes. BDD microelectrodes with very small diameter (about 5 μm) and 250 μm in length could be made successfully. In addition to the unique properties of BDD electrodes, such as a very low background current, high stability, and selective oxidation of dopamine (DA) in the presence of ascorbic acid (AA), other superior properties of the microelectrodes, including a constant current response, an increase in the mass transport, and the ability for use in high resistance media were also shown. An application study was conducted for in vivo detection of DA in mouse brain, where the BDD microelectrode was inserted into the corpus striatum of the mouse brain. A clear signal current response following medial forebrain bundle (MFB) stimulation could be obtained with high sensitivity. Excellent stability was achieved, indicating that the BDD microelectrodes are very promising for future in vivo electroanalysis. © 2007 American Chemical Society.

An attempt was made to make use of chemical modification of the diamond surface to control the adsorption of Au nanoparticles (AuNPs) on this surface via self-assembly. AuNPs prepared with a citrate reduction method (c-AuNP) and mercaptoacetic acid (MAA)-modified AuNPs (MAA-AuNP) were not adsorbed onto hydrogen-terminated and oxygen-terminated boron-doped diamond (BDD) surfaces. On the other hand, c-AuNP and MAA-AuNP were found to be adsorbed in a homogeneous, well-dispersed fashion on NH2-terminated and SH-terminated BDDs prepared with the silane coupling method. It was also found that the adsorption of c-AuNP on SH-BDD was more stable than that on NH2-BDD versus immersion in various thiol aqueous solutions used to form self-assembled monolayers (SAMs) on the Au surface, without desorption of the AuNP from the BDD surface. Immersion of the c-AuNP/SH-BDD sample in a ferrocenylalkanethiol/ ethanol solution was found, by cyclic voltammetry, to result in chemisorption of the thiol to the Au surface. Facile electronic communication through the ferrocenyl moiety to the BDD substrate suggests that SAM/AuNP/BDD configurations based on SH-BDD fabricated by a self-assembly method can be applied to possible molecular electronic and electrochemical devices. © 2007 American Chemical Society.

New crystal structure of Eu(DMF)4(H2O) 3Co(CN)6•H2O (DMF = N,N′- dimethylformamide) (Eu-Co) has been determined to be monoclinic, P2(1)/n, a = 19.796(12) Å, b = 8.862(11) Å, c = 17.525(10) Å, β = 96.26(5)°, V = 3056(5) Å3, Z = 4. The Eu(III) ion adopts an antiprismatic eight-coordination and forms a cyano bridge with r(Eu-N) = 2.496(7) Å and Θ(Eu-N-C) = 165.7(7)° to the Co(III) ion. The complex exhibits some common features with the Eu-Fe complex. Diffuse reflectance electronic spectra and magnetic susceptibility of Eu-Cr, Eu-Mn, Eu-Fe, and Eu-Co complexes were compared. By substituting the metal ions, both electronic and structural features affected the charge transfer bands and superexchange interactions concerning cyanide ligands. In addition, only Eu-Co exhibited 5 D 0 → 7 F 2 and 5 D 0 → 7 F 1 luminescence bands at 16300 cm-1 and 16900 cm-1, respectively at 298 K (λex = 360 nm (27000 cm-1)), because quenching by cyano-bridged ions did not prevent Eu(III) ion from exhibiting emission. Thus, only Eu-Co may be suitable for verification of an assumption of mechanism concerning drastic photoinduced magnetic changes for Nd-Fe. Merely small decrease of magnetization was observed for Eu-Co after UV light irradiation at 2.0 K. This result was attributed to slight structural changes around cyano bridges without transitions of spin states. © Institute of Chemistry, Slovak Academy of Sciences 2007.

Two novel types of photomagnetic hybrid ultrathin film (film A and B) of metal cyanides have been fabricated by means of the modified Langmuir-Blodgett method using a smectite clay mineral. Film A is composed of an amphiphilic azobenzene cation, a montmorillonite, and Prussian Blue in which photocontrol in the magnetization was realized by the photoisomerization of azobenzene chromophore. The observed photomagnetic efficiency was large (ca. 11%) due to the well-organized structure of the ultrathin film. Film B is composed of a quaternary ammonium salt, a montmorillonite, and Co-Fe Prussian Blue in which the photoinduced magnetization caused by the electron transfer exhibited an anisotropic response with regards to the direction of the applied magnetic field. This phenomenon is ascribed to the unique structure of Co-Fe Prussian Blue formed onto the clay layer. © Springer-Verlag 2007.

A photomagnetic ultrathin film consisting of an azobenzene, a deoxyribonucleic acid, and Prussian Blue has been fabricated by the Langmuir-Blodgett method. Prussian Blue layers in the films were a two-dimensional structure with the height of 50 Å and the average coherence length of 210 Å. Reversible photoisomerization of the azobenzenes was observed even in the films. The films exhibit the long-range ferromagnetic order below 4.2 K and moreover, upon photoillumination at 2 K, reversible changes in the magnetization were realized with the value of ca. 1.0%. This photoswitching in the magnetization is due to changes in the electrostatic field induced by photoisomerization. © 2007 Elsevier B.V. All rights reserved.

Fe3+ doped polyaniline (PANI) films containing azobenzene derivatives in the side chain have been prepared. Photoisomerization of the azobenzene derivative in the PANI films was observed at room temperature. Furthermore, 57Fe Mössbauer measurements showed that the ligand-metal bond distance and the symmetry around the Fe3+ were influenced by the photoisomerization. © 2007 Elsevier B.V. All rights reserved.

There has been a great interest in developing photoswitchable magnetic materials because of their possible applications for future high-density information storage media. In fact, however, the examples reported so far did not show ferromagnetic behavior at room temperature. From the viewpoint of their practical application to magnetic recording systems, the ability to fix their magnetic moments such that they still exhibit room-temperature ferromagnetism is an absolute requirement. Here, we have designed reversible photoswitchable ferromagnetic FePt nanoparticles whose surfaces were coated with azobenzene-derivatized ligands. On the surfaces of core particles, reversible photoisomerization of azobenzene in the solid state was realized by using spacer ligands that provide sufficient free volume. These photoisomerizations brought about changes in the electrostatic field around the core-FePt nanoparticles. As a result, we have succeeded in controlling the magnetic properties of these ferromagnetic composite nanoparticles by alternating the photoillumination in the solid state at room temperature. © 2007 American Chemical Society.

Boron-doped diamond (BDD) thin film surfaces were modified by brief plasma treatment using various source gases such as Cl2, CF4, Ar and CH4, and the electrochemical properties of the surfaces were subsequently investigated. From X-ray photoelectron spectroscopy analysis, Cl and F atoms were detected on the BDD surfaces after 3 min of Cl2 and CF4 plasma treatments, respectively. From the results of cyclic voltammetry and electrochemical AC impedance measurements, the electron-transfer rate for Fe(CN)63-/4- and Fe2+/3+ at the BDD electrodes was found to decrease after Cl2 and CF4 plasma treatments. However, the electron-transfer rate for Ru(NH3)62+/3+ showed almost no change after these treatments. This may have been related to the specific interactions of surface halogen (C-Cl and C-F) moieties with the redox species because no electrical passivation was observed after the treatments. In addition, Raman spectroscopy showed that CH4 plasma treatment of diamond surfaces formed an insulating diamond-like carbon thin layer on the surfaces. Thus, by an appropriate choice of plasma source, short-duration plasma treatments can be an effective way to functionalize diamond surfaces in various ways while maintaining a wide potential window and a low background current. © 2006 Elsevier Ltd. All rights reserved.

Highly boron-doped diamond (BDD) electrode, have been examined for simultaneous detection of purine and pyrimidine bases in mild acidic media by using HPLC with amperometric detection. Cyclic voltammetry at as-deposited (AD) and anodically oxidized (AO) BDD were used to study the electrochemistry and to optimize the condition for HPLC applications. At AO BDD electrode, due to its higher overpotential of oxygen evolution reaction, well-defined anodic peaks were observed for the oxidation of purine and pyrimidine bases in acid medium, whereas at AD BDD the oxidation peak of thymine was overlapped with the anodic current of oxygen evolution. The chromatograms of adenine, guanine, cytosine, thymine and 5-methylcytosine mixture were well resolved by using a silica-based column and a solution of 5% acetonitrile in 100 mM ammonium acetate buffer (pH 4.25) as the mobile phase. The detection was carried out at AO BDD electrode at an applied potential of 1.6 V versus Ag/AgCl. Linear calibration curves were obtained within the concentration range from 0.1 to 10 μM with the limits of detection (S/N = 3) ranging from 26.3 to 162.1 nM, resulting in an order of magnitude higher sensitivities than those at conventional electrodes. HPLC analysis with diamond amperometric detector was successfully applied for determination of 5-methylcytosine in real DNA samples with high reproducibility. No deactivation of the electrode was found during cyclic voltammetric and HPLC measurements, indicating the high stability for analysis of biological samples. © 2006 Elsevier B.V. All rights reserved.

Synthesis, crystal structures, and spectroscopic and magnetic properties of new one-dimensional cyano-bridged bimetallic complexes, [CuII(N-Eten)2][MII(CN)4] (N-Eten = N-ethylethylenediamine; MII = NiII (1) and PtII (2)), have been reported. Both complexes consist of one-dimensional alternate chains of CuII and MII moieties. The Pt-C bond distances of 1.997(3) and 2.001(3) Å for 2 are considerably longer than the Ni-C bond lengths of 1.866(3) and 1.872(3) Å for 1. Because of pseudo Jahn-Teller distortion, the axial Cu-N bond distances of 2.554(2) and 2.550(3) Å for 1 and 2 are longer than those of equatorial ones of 2.008(2) and 2.056(2) Å for 1 and 2.010(2) and 2.054(2) Å for 2. In contrast to MII-C bond distances, the Cu-N ones of 1 are similar to those of 2 regardless of element-substitution. These complexes indicate weak antiferromagnetic interactions with Weiss constants = - 4.68 and -3.95 K for 1 and 2, respectively. The emission spectrum of 2 (λex = 360 nm) exhibits a broad band with peaks at 22 800 and 24 000 cm-1 at 298 K. The Cu 2p1/2 and 2p3/2 peaks of XPS spectra are compared systematically to various copper(II) complexes showing different bridging features or distorted coordination geometries as models for excited structures induced by external physical conditions. The spectroscopic properties are discussed from the viewpoint of magneto-optical properties. © 2006 Elsevier B.V. All rights reserved.

Direct detection of sodium azide was examined by using highly boron-doped diamond (BDD) electrodes in some physiological saline buffer solutions. Three important saline buffers, ADA buffer (N-(2-acetamido) iminodiacetic acid), sodium EDTA and imidazole, were investigated. Hydrogen-terminated BDD was fixed for the investigation since it showed much better sensitivity for azide oxidation in comparison with the oxygen-terminated one. Wide range of buffer concentration was examined. A well-separated peak of sodium azide was observed in its mixture with the buffers, resulting in selective sodium azide detection. The current responses were exactly total addition of sodium azide and buffer current responses. The present method was evidenced to be available for direct detection of sodium azide in two samples of commercial diluents for cell counting. A standard addition method was applied. Sodium azide concentrations of 502 and 1355 μg mL-1 were detected in both samples with the R.S.D.s of 0.07 and 2.37%, respectively. Validation was achieved by the comparison with the sample concentrations given by the manufacturer. Simple, selective, good sensitivity and excellent reproducible detection make the diamond electrodes very attractive for numerous electroanalytical applications. © 2006 Elsevier B.V. All rights reserved.