Growth of different morphologies (quantum dots to nanorod) of Ag-nanoparticles: Role of cysteine concentrations

This work describes an easy chemical method for the preparation of orange-red color silver sol by the cysteine reduction of silver (I) in the presence of cetyltrimethylammonium bromide (CTAB). The obtained sol was found to have very small particles in the order of quantum dots for the first time. Transmission electron microscope (TEM) images show that the silver sol consists of aggregated as well as cross-linking arrangement of spherical silver quantum dots (size in the range ca. ≤16 nm). In addition, we observed the changes in the morphologies of the Ag-nanoparticles from quantum dots to nanorod of diameter 60 nm and pearl-necklace shaped which occurred due to the cross-linking aggregation of silver quantum dots. For a certain reaction time, i.e., 100 min, the absorbance of reaction mixture first increased until it reached a maximum, then decreased with [cysteine]. The rate of Ag-nanoparticles formation decreases with the increase in [cysteine] whereas [CTAB] and [Ag+] have no effect on the reaction rate. Interestingly, at higher [cysteine] (≥20.0 × 10−4 mol dm−3), white precipitate was formed instead of transparent silver sol. Cysteine acts as a reducing, cross-linking, stabilizing and buffering agent during the growth of different shape and size of silver nanoparticles.