Synthesis and characterization of 12 nm spherical citrate capped AuNPs
The 12nm spherical AuNPs were prepared through citrate mediated reduction of HAuCl4 (Grabar et al., 1995; Storhoff et al., 1998). Aqueous HAuCl4 (200 mL) of 1 mM was heated to a vigorous boiling status while stirring in a round-bottom flask fitted with a reflux condenser. Trisodium citrate (20 mL) of 38.8 mM was added rapidly to the solution. It could be found that the solution turned to an intense dark violet firstly then quickly changed to red. The solution was reflexed for an additional 15 min then cooled to room temperature with continuous stirring. The size of the AuNPs was characterized by transmission electron microscope (TEM) analysis (H7650, Hitachi High-Technologies Corporation, Tokyo, Japan) and showed as monodisperse nanoparticles with an average size of 12 nm. The plasma absorption was measured in a double-beam UV–vis spectrophotometer (Varian, Cary 300 Conc). The concentration of the AuNPs can be calculated according to Beer’s law using an extinction coefficient of 2.4 × 108 M−1 cm−1 at 520 nm for the 12 nm AuNPs