BUNSEKI KAGAKU Abstracts

The modification of gold nanoparticles on the surfaces of conducting materials usually develops interesting electrochemical properties that are different from those of conducting materials. Especially, unique electrochemical and catalytic properties of nanostructured gold, which are not available in the bulk state, permit the construction of novel modified electrodes, while realizing some significant contributions to electroanalytical chemistry. Our group has been studying the fabrications and applications of gold nanoparticles-modified indium tin oxide electrodes since 2001. In this account, I introduce some developments of our research on gold-nanoparticle-modified electrodes after 2010, including some achievements in collaborative research as well as research results obtained using platinum nanoparticles. As relatively new topics, the combination of gold nanoparticles with a palladium base electrode and the utilization of non-conducting supporting materials for modifying the vicinity of the electrode surface with gold nanoparticles are described.

In order to further demonstrate the oxidation ability of diluted aqueous nitric acid containing abundant amounts of salts, the dissolution of precious metals (Au, Pt, and Pd), especially of gold, has been examined in 0.1 – 2 mol dm⁻³ HNO₃ accompanied by alkali metal, alkaline earth metal, and aluminum chlorides. The complete dissolution time of a pure gold plate (20 ± 2 mg, 0.1 mm thickness) in 2.0 mol dm⁻³ HNO₃ accompanied by 1.0 mol dm⁻³ AlCl₃ was shortened remarkably with a temperature increase from 15 to 80°C. The dissolution rate constants, log (k/s⁻¹), of a piece of gold wire (19.7 ± 0.5 mg) in 20 mL of 2.0 mol dm⁻³ HNO₃ accompanied by the metal chlorides, in general, increased with increasing salt concentrations at 40 and 60°C. For instance, the log (k/s⁻¹) values linearly increase from −4.15 via −3.77, −3.45 to −3.14 by the addition of 1.0, 2.0, 3.0, and 4.0 mol dm⁻³ LiCl in a 2.0 mol dm⁻³ HNO₃ solution at 60°C. The complete dissolution experiments of the gold wire in much lower HNO₃ concentrations (down to 0.1 mol dm⁻³) with the chloride salts have also been successfully performed. Gold could be dissolved in a solution of 1.0 mol dm⁻³ HNO₃ and 1.0 mol dm⁻³ HCl, i.e. a “dilute aqua regia”. We have achieved a total dissolution of five pieces (0.10 g) of the gold wire in 100 mL of a 1 : 1 mixture between seawater and 2.0 mol dm⁻³ HNO₃ at ca. 100°C, with a complete dissolution time of within 17 hours and log (k/s⁻¹) = −4.52. The distortion of bulk water and the alternation of water properties are discussed based on the Raman spectra with increasing HCl concentrations.

We have developed a new analytical method for measuring VOCs in air by using a headspace method. In this method, 2-hydroxypyridine (HP) and tetrabutylphosphonium bromide (TBPB) are used as an eluate for the desorption of VOCs. VOCs in air are collected with a passive sampler containing activated carbon. Activated-carbon particles adsorbing VOCs and the mixed solvent of HP and TBPB are introduced into a 22 mL vial. This vial is sealed with a septum seal cap, and is then heated by a headspace sampler. VOCs collected in the activated carbon are desorbed by the mixed solvent. TBPB enhances the desorption efficiency because it is an ionic liquid, and shows a salting-out effect. The vapor-liquid phase equilibrium of VOCs is observed in the head-space vial, and VOCs in the gas-phase is introduced to GC/MS. The optimized analytical conditions, such as the mixed-solvent ratio, the amount of mixed solvent, the heating temperature and the heating time were investigated in this study. Moreover, the headspace methods were compared with the traditional solvent-desorption method. The sensitivities by the headspace method were about 20 – 40 fold higher than those by solvent desorption, except for tetrachloromethane and styrene. In addition, there were no solvent peaks in the chromatogram because HP and TBPB have very high boiling points.

To evaluate the oxidative deterioration of typical olefinic rubber materials, the determination of the oxidative depolymerization of cis-1,4-polyisoprene (IR) was developed by fluorescence labeling and size-exclusion chromatography (SEC). IR was oxidized under the lipid peroxidation of linoleic acid. Lipid peroxidation generates free radicals that attack the IR polymer chain. The oxidation of IR was shown from the decrease in the molecular weight of IR and the formation of carboxy groups on the molecular chain. The treated IR was labeled with a fluorescent compound at their carboxy group, followed by analysis with SEC attached to a fluorescent detector and a differential refractometer in series. In this method, the distributions of the molecular weight and the amount of the carboxy group are simultaneously obtained against the molecular weight, implying that the degree of oxidation occurring at the middle of the polymer chain without chain cleavage can be observed by comparing with those accompanied by cleavage and the formation of carboxy groups at the newly formed terminal end. Under lipid peroxidation, the treatment time and the exposure condition of oxygen were changed, and then the correlation between the molecular weight and the carboxy group amount of treated IR was analyzed. The resulting oxidation reaction was shown; in the initial stage, the oxidation occurred by a polymer cleavage reaction, but in the latter stage the continuous oxidation was followed without a cleavage reaction. By using FTIR, no carbonyl group was detected during the entire employed treatment period. It was also indicated that an excess of oxygen exposure with Parwel-G (CaO₂) enhanced both the oxidation reactions with and without any cleavage reaction. Thus, in this method, the oxidation reaction with or without the polymer cleavage reaction could be identified and analyzed simultaneously from the early stage of oxidation, even when oxidation doesn’t proceed sufficiently to allow for the detection of carbonyl groups with FTIR. Furthermore, the method could be applied to other polyolefin rubber materials.

A spectrophotometric method was developed and validated for choline in enteral nutrition. The method involves an enzymatic conversion of choline by choline oxidase to hydrogen peroxide, which reacts with 3-(N-ethyl-3-methylanilino)-2-hydroxypropanesulfonic acid (TOOS) and 4-aminoantipyrine in the presence of peroxidase to form a purple colored product with the optimal absorption at 554 nm. The test conditions were optimized for avoiding interference by any active ingredient other than choline in a commercial drug product (ELENTAL^®). This method was validated for the determination of choline bitartrate in the ELENTAL^®, and demonstrated good specificity, linearity, accuracy and precision. The proposed method was successfully applied to assays of choline in commercial drug products (ELENTAL^® and HEPAN ED^®). The proposed method is simple, specific and useful for the determination of choline in enteral nutrition and supplements.

A quantitative method for the determination of urea in cosmetics was studied. An ion chromatograph (IC) with a conductivity detector was used in this method, where the chromatograph was modified by placing a cation exchanger column and an immobilized urease column on the injection loop with a 6-way valve in IC. A sulfo-type cation exchanger based on cedar sawdust had an ability to remove oils dispersing in a cosmetics sample solution. Therefore, by using the above-mentioned reactor, the urea concentration in commercial cosmetics, such as cream, skin lotion shampoo, and rinse could be determined. The time required for measuring one sample was within 7 minutes after developing a cosmetics sample, and the correlation coefficients between the cosmetics urea concentration obtained by this reactor (IC) and by the spectrophotometric method (diacetylmonooxime) were 0.999.

The determination of 2-phenoxyethanol (PE) and six kinds of parabens [methyl paraben (MP), ethyl paraben (EP), isopropyl paraben (IPP), propyl paraben (PP), isobutyl paraben (IBP), butyl paraben (BP)] in cosmetics and foods by high-performance liquid chromatography (HPLC) with an ultraviolet detector (UV) on a biphenyl stationary phase column has been developed. This analytical method provides high linearity of the calibration curve as well as repeatability. The correlation coefficient of the working curve of the calibration were estimated to be from 0.9998 to 0.9991 for 2-phenoxyethanol and six kinds of parabens in the concentration range from 5 mg L⁻¹ to 150 mg L⁻¹. The limits of detection (LOD) calculated on 3 σ at 10 mg L⁻¹ were 0.438 mg L⁻¹ for PE, 0.241 mg L⁻¹ for MP, 0.946 mg L⁻¹ for EP, 0.844 mg L⁻¹ for IPP, 1.093 mg L⁻¹ for PP, 1.008 mg L⁻¹ for IBP, and 1.108 mg L⁻¹ for BP. The limits of quantification (LOQ) calculated on 10 σ at 10 mg L⁻¹ were 1.460 mg L⁻¹ for PE, 0.803 mg L⁻¹ for MP, 3.153 mg L⁻¹ for EP, 2.813 mg L⁻¹ for IPP, 3.643 mg L⁻¹ for PP, 3.360 mg L⁻¹ for IBP, and 3.693 mg L⁻¹ for BP. As good results, the recoveries were 87.1 – 111.2% and the relative standard deviations (RSD) of peak area were 0.3 – 1.7%, obtained in recovery tests by using hand cream and soy sauce. This proposed method could be successfully applied to the determination of 2-phenoxyethanol and six kinds of parabens in cosmetics and foods.

Trace elements were determined across the annual ring structures of a ferromanganese nodule by laser ablation/inductively coupled plasma mass spectrometry (LA-ICP-MS). The determined values were obtained from 30 points by spatial-resolution analysis using LA-ICP-MS. The ferromanganese nodule was also sampled by hand-picking, and the samples were determined by ICP-MS in order to compare the determined values by LA-ICP-MS. With internal standard corrections using Mg or Mo, the determined values by LA-ICP-MS were in good agreement with those by ICP-MS. LA-ICP-MS verified that the Fe concentration increased with increasing Co and REE concentration in a ferromanganese nodule. This fact seemed to indicate that the determined values by LA-ICP-MS were acceptable. Furthermore, the variation of a cerium anomaly across the annual ring structure was verified by LA-ICP-MS. A positive cerium anomaly was observed at the points where Fe was enriched. From these results, the determination method for a spatial-resolution analysis of a ferromanganese nodule using LA-ICP-MS was developed and achieved.