Tetrachloroauric 3 acid

Au₂₅(Capt)₁₈ clusters were synthesized according to a previously described method.19 ,21 (link) Tetrachloroauric (III) acid (0.20 mmol, Wako Pure Chemical Industries Ltd., Osaka, Japan) and tetraoctylammonium bromide (0.23 mmol, Wako Pure Chemical Industries Ltd.) were dissolved in 10 mL methanol and stirred for 20 min. Subsequently, captopril (1 mmol, Tokyo Chemical Industry Co. Ltd., Tokyo, Japan) was dissolved in 5 mL methanol, injected in the reaction mixture, and further stirred for 30 min. Sodium borohydride (2 mmol) was dissolved in 5 mL of cold water and added to the mixture with stirring and kept under stirring for 8–12 h at room temperature. The resultant mixture was centrifuged to remove insoluble Au (I) polymer. The supernatant was collected and concentrated by rotary evaporation and then Au₂₅(Capt)₁₈ clusters were precipitated by ethanol and dried in a vacuum. The generation of Au₂₅(Capt)₁₈ clusters was confirmed by a ultraviolet-visible (UV-vis) spectrophotometer (V-670 UV-VIS-NIR Spectrophotometer, Jasco, Tokyo, Japan) and a spectrofluorometer (FP-6300 Spectrometer, Jasco).