Materials and apparatus. Tetrachloroauric(III) acid (HAuCl 4 •3H 2 O, 99.99+%), bovine serum albumin (BSA, 66 kD), hydrogen peroxide (H 2 O 2 , 31 wt%), ammonia, hydrochloric acid, Rhodamine B, ethanol, various divalent metal chlorides (magnesium, calcium, iron, cobalt, nickel, copper, zinc, cadmium, mercury and lead), and pesticides (dithizone, 2,4-dichlorophenoxyacetic acid, paraoxon-ethyl and fenitrothion) were purchased from Sigma-Aldrich. Phosphate buffer powder (pH=7.4) and sodium hydride

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... (NaOH) were obtained from Honeywell and MERCK, respectively. All chemicals were used without further purification, and their solutions were prepared in distilled water unless stated otherwise. All glassware were washed with Aqua Regia, and rinsed thoroughly with distilled water prior to use. Absorbance and fluorescence spectra were collected with a Shimadzu UV-3150 spectrophotometer and a Shimadzu PF-5301PC spectrofluorophotomer, respectively. The molecular weights of various BSA-protected Au 25 clusters were analyzed with a matrix-assisted laser desorption/ionization time-of-flight (MALDI-TOF) mass spectrometer on a Bruker Daltonics Autoflex II TOF/TOF system. Optical photographs were taken under room light with a Canon 350D digital camera, and fluorescent images were obtained under a 365-nm UV lamp with a Canon 350D digital camera. The energy dispersive X-ray (EDX) spectra were recorded with a