2. Experimental
The gold nanoparticle solutions were prepared by the solution plasma sputtering method in quart cell (100 mm × 30 mm × 110 mm) with stirring. A pulsed high voltage was applied between two opposing electrodes with a gap of 1 mm, to generate solution plasma by glow like discharge using an AC power supply (voltage, approximately 2.4 kV; repetition frequency, 20 kHz; Kurita). We used gold rods (1.0 mm diameter, NILACO, 99.9%) as the electrodes and 200 mL of deionized water as the solvent. Sodium chloride (NaCl, ≥99%) was supplied by Wako Pure Chemical. Pulse widths were 1.5, 1.35 and 1.2 μs in solutions which had concentrations of 3, 5 and 10 mmol/L, respectively. These pulse widths were selected to prepare same concentrations of atomic gold in solutions. The adjustment of pulse width can change the numbers of particles per time without change of the initial particles diameter [12]. The gold nanoparticles in 5 mmol/L were also prepared with 1.2 and 1.5 μs to discuss about effect of concentration of gold. The optical property of the solutions was monitored during the solution plasma process by in situ UV–vis spectroscopy. The in situ absorption spectra were recorded at regular time intervals using a Xe lamp and a multichannel spectrophotometer (Hamamatsu Photonics PMA-12). The length of light path was 24 mm. The particle diameters and concentration of atomic gold were estimated by absorption spectra. For the purpose of reference, the distributions of particle diameter of the sample which were prepared in 5 mmol/L NaCl solution were obtained by observation of transmission electron microscopy (TEM) (JEOL JEM-250 at 200 kV). The TEM samples were prepared by drops of gold nanoparticles solution on cupper TEM grids (NISSHIN EM). The atomic gold concentration was also measured by inductively coupled plasma atomic emission spectroscopy (ICP-AES) (Thermo Scientific ICAP6500 Duo).