Synthesis and Characterization of Colloidal Gold Nanoparticles Controlled by the pH and Ionic Strength

Colloidal gold nanoparticles (GNPs) were prepared according to the Lee-Meisel method based on the chemical reduction of gold chloride trihydrate in aqueous solution. Different techniques were employed to characterize the gold nanoparticles, such as UV-Vis spectrometry, which was used to obtain absorption spectra of the colloidal samples. These spectra showed that the gold nanoparticles have an absorption peak at 520 nm, which corresponds to the surface plasmon resonance (SPR) of gold nanoparticles. Another technique is the use of a transmission electron microscope (TEM) to obtain images of the prepared GNPs to ensure their nano-size as well as their spherical shape. According to the obtained images, the inverse proportionality between the pH value of the solution and the size of the gold nanoparticles was clear. Moreover, for pH values less than 8.4 the particles were aggregated in the form of clusters, while above lids value they were well dispersed in the solution. X-ray diffraction analysis was performed to study the crystal structure of the GNPs. The aggregation of GNPs as a result of adding chemical compounds to the colloidal solution was investigated. As the concentration of the added compound is increased, a gradual decay of the surface plasmon resonance peak accompanied by the formation of a new peak in the long wavelength region was observed.