Determination of gold nanoparticles sizes from their plasmon resonance within the optical spectrum

Nanoparticles have exciting properties that can be tailored by altering their size, density, and shape. A number of important properties of the nanoparticles have been investigated for various applications. One such property that is strongly affected by nanoparticle size is localised surface plasmon resonance (LSPR). The resonance from metal nanoparticles has been used in dye-sensitised solar cells to improve their performance. In this work, the dependence of plasmonic properties on nanoparticle sizes is shown. The gold nanoparticles were prepared using a reduction process where hydrogen tetrachloroaurate acid was used as the base gold salt and reduced by sodium citrate at different molarities ranging from 0.015 to 0.035 mol/L. The method produces monodispersed nanoparticles whose sizes are sensitive to the concentration of chemicals used and the completeness of the reduction process. The process took approximately 18 minutes, and the colour changed from pale yellow to wine-red. The absorbance of the resulting gold nanoparticles was determined using a UV-Vis spectrophotometer within the range of 300 nm to 800 nm. The LSPR peaks were found to occur within 518 nm to 520 nm, and from a Gaussian fit, the FWHM ranged from 45.5 to 51.0 nm. The absorption peaks had a narrow range of 14 nm over the range of molarity of sodium citrate. A high molarity concentration of 0.035 mol/L produced a small particle with a diameter of 17.04 nm, while a low concentration of 0.015 mol/L produced a size of 26.55 nm. The interaction of electrons in the specific orbitals, sp- and d-, of nanoparticles exhibited pronounced multiple resonances with the reduction of nanoparticle sizes.