Seed-mediated Growth And Properties Of Sea Urchin-like Gold Nanoparticles Facilitated By Tannic Acid

Gold nanomaterials have unique optical, electrical and catalytic properties and are themost active areas of material sciences. In recent years, the study interests havetransferred from spherical structure to anisotropic structures. The most widely usedway to synthesize water phase gold nanoparticles by sodium citrate is very popularamong researchers because of its simplicity and small error. However, due to theweak reducing ability of sodium citrate, this method is usually used to synthesizequasi-spherical nanoparticles rather than anisotropic nanoparticles. Our group haspreviously developed a way to synthesize gold nanoflowers by hydroxylaminehydrochloride. Nevertheless, due to the strong reducing ability of hydroxylaminehydrochloride, this reaction is finished with several seconds and thus is difficult tocontrol. For this reason, in this work we chose a moderate reducing agent-tannic acid,which is strong enough to reduce chloroauric acid under room temperature andmeanwhile is not so strong to finish the reaction in several minutes. This gives usopportunity to better control this reaction and get more ideal outcomes. In our work,we studied the controllable synthesis of anisotropic gold nanoparticles. We tried toexplore the growing mechanism and the affecting factors in this system and studiedthe application of these materials in SERS and catalyzing.In the first part of this work, we synthesized sea-urchin-like gold nanoparticles byseeding approach. Using15nanometer gold nanoparticles as seed, tannic acid asreducing agent, and chloroauric acid as gold precursor, we synthesized sea-urchin-likegold nanoparticle under room temperature. We studied how the molar ratio of tannicacid and chloroauric acid (R) and pH affect this system. The results showed that whenthe molar ratio of tannic acid and chloroauric acid (R) is relatively low(R<2.0), the system tended to form quasi-spherical gold nanoparticles (about50nm); and when Ris relatively high (R>2.0), the system tended to form anisotropic gold nanoparticles.Besides, when the pH is between3.0-8.0, the anisotropic degree increased as the pHincreased; however, when the pH is higher than8.0, the anisotropic degree decreasedas the pH increased and the monodisperse of the particles also decreased.The second part of this work studied the properties of the gold nanoparticlessynthesized in part one. Based on their structural feature, we first explored thestability of these materials. Judging from the facts that they could be stable underroom temperature for48hours, under70℃and under5.0mM NaCl, we could saythat these materials have relatively stable property. We also studied their applicationin the surface enhanced Raman shift. It turned out that their SERS property isrelatively higher than the same size quasi-spherical gold nanoparticles. Besides, themost anisotropic nanoparticles synthesized under pH=5.0has the strongest SERSeffect. Moreover, we studied the catalytic property of these materials by using thereaction of NaBH4reduced p-nitroaniline as model. The results showed that they havecertain degree activity in catalyze.