Spectroscopic and microscopic investigation of gold nanoparticle nucleation and growth mechanisms using gelatin as a stabilizer

A microscopic and spectroscopic investigation of the synthesis of gold nanoparticles (AuNPs) within gelatin is reported. The AuNPs were synthesized first by reducing tetrachloraurate ions (AuCl4(-)) by 2-[4-(2-hydroxyethyl)-1-piperazinyl] ethanesulfonic acid (HEPES), mixing the HAuCl4/HEPES solution with gelatin solution and heating at different temperatures. The polymeric structure of gelatin stabilized the HAuCl4/HEPES/gelatin system and slowed the synthesis of AuNPs, enabling a time-dependent investigation. Based on the results of transmission electron microscopy (TEM) analysis and UV-Vis spectra, we identified three distinct stages involved in the synthesis of AuNPs. First, during the initial stage, the reduction of gold precursor occurred along with nucleation and growth, which resulted in a red-shift phenomenon of the localized surface plasmon resonance (LSPR) peak of AuNPs in UV-Vis spectra (size and size dispersion increase). Second, the LSPR peaks showed red-shift first and then blue-shift during the growth of AuNPs. The blue-shift might result from the diffusion-limited Ostwald-ripening mechanism. Third, as the supply of the growth species became lower, during the growth of AuNPs, a diffusion-limited Ostwald-ripening mechanism along with a blue-shift only phenomenon in UV-Vis spectra was observed. We also determined that slowing the synthesis process during the nucleation stage can prolong the nucleation time, which can generate larger AuNPs. The TEM analysis showed that higher heating temperature and longer heating time can lead to larger particles. By controlling the reduction (nucleation) time, heating time and temperature, AuNPs of size ranging from 5 to 17 nm can be synthesized.