Green Synthesis, Characterization and Catalytic Activity of Gold Nanoparticles Prepared Using Rosmarinic Acid.

Green strategies to synthesize gold nanoparticles have attracted a substantial amount of attention because global sustainability is a focal issue in many research areas. In the present study, rosmarinic acid was utilized as a reducing agent to reduce gold ions to gold nanoparticles. The characteristic surface plasmon resonance of gold nanoparticles was observed at 532 nm with a pink-colored colloidal solution. High-resolution transmission electron microscopy (HR-TEM) revealed the presence of spherical nanoparticles along with triangular nanoplates. The average nanoparticle size was determined as 30.46 ± 6.25 nm from HR-TEM images, and the hydrodynamic size was measured as 31.90 nm. An evaluation of the shelf stability indicated that the colloidal solution retained an excellent stability for two weeks at room temperature. A large negative value for the zeta potential (-24.09 ±3.97 mV) also suggested the excellent stability of the colloidal solution. The face-centered cubic structure of gold nanoparticles was confirmed by the strong diffraction peaks in the high-resolution X-ray diffraction analysis. Hydroxyl and ketone functional groups in rosmarinic acid were most likely involved in the synthesis of gold nanoparticles. Catalytic activity was evaluated in 4-nitrophenol, methylene blue and methyl orange reduction reactions in the presence of sodium borohydride. The order of catalytic activity for reduction reactions was methylene blue > methyl orange > 4-nitrophenol. These three reactions were conducted at four different reaction temperatures (room temperature, 30 °C, 40 °C and 50 °C). The catalytic activity was temperature-dependent, and the highest rate constant was obtained in the methylene blue reduction reaction at 50 °C.