Synthesis Of Dendritic Gold Nanostructures Assisted By Quaternary Ammonium Cationic Surfactants

Gold nanomaterials have attracted wide attention and have been researched extensively due to their excellent properties, such as high electron density, dielectric properties, catalytic activity, combine with most of macromolecules without affecting their biological activity, hence bring many potential applications in the areas of catalysis, electronics, optics etc. Gold nanoparticles have different patterns of morphologies which determine their different properties. Thus, morphology control of gold nanomaterials became the hot topics. Many different shapes of gold nanomaterials have been prepared by scientists, such as nanorods, polyhedrons, sheets, stars, and dendrites. Stars and dendrites are hard to prepare through simple way because of complicated structures. However, they gain great attention due to the sharp edges, tips, conjunction in nanoscale and huge surface which made them have potential applications in SERS based detection and catalysis. In recent years, most of the dendritic nanostructures were prepared by electrochemical or galvanic method. Others were prepared through colloidal method (wet methods) with certain assistants. however, those methods are complicated or expensive. In present paper, we developed an easy way to prepare dendritic gold nanostructures by colloidal method with commonly used quaternary ammonium cationic surfactants as capping agents; tested the SERS enhancement and catalytic activity of as-prepared dendritic gold nanostructures. The whole paper contains three sections, which are summarized as following:Section I, Briefly introduced the research background and applications of Au nanomaterials. Summarize the preparation methods and formation mechanism of dendritic nanostructures developed in recent years.Section II, The preparation of dendritic gold nanostructures in bolaform surfactant [mpy-C12-mpy]Br2aqueous solution was investigated. Two-fold symmetrical three-dimensional dendritic gold nanostructures were prepared in aqueous solution by one-step method, with surfactant [mpy-C12-mpy]Br2as capping agent and ascorbic acid as reducing agent to reduce HAuCl4. The results from TEM, SEM, and HRTEM confirmed the stems of gold nanostructures grew toward<211> direction. The results from XRD and SAED showed that as-prepared dendritic nanostructures were pure gold crystal with perfect fccf (face-centered cubic) structures. The effects of concentration of surfactant, reaction time, length of hydrocarbon chains, concentration of precursor, reducing agents were investigated. The formation mechanism of dendritic structures was proposed with "zipping" and "fractal" theory supported. In application tests, the SERS activity of as-prepared gold dendritic nanomaterials toward R6G were investigated, it’s turned out that dendritic nanomaterials performed strong enhancement. And also, they have very high catalytic activity toward reduction of p-nitroaniline when potassium borohydride as reducing agent.Section III:The preparation of dendritic gold nanostructures in commonly used surfactant DTAB aqueous solution was studied. Three-fold symmetrical three-dimensional dendritic gold nanostructures were obtained in aqueous solution with DTAB as capping agents. The as-prepared dendritic gold nanostructures were characterized by TEM, SEM, HRTEM, XRD and SAED pattern. The crystal direction and lattice fringes were identical with the nanostructures prepared in bolaform surfactant aqueous solution. The effect of surfactant with different headgroups (DTEAB, C12-MPB) was investigated. It’s turned out that the surfactants with the same hydrocarbon chain but different headgroups could affect the morphologies of the final products. The dendritic nanostructures obtained in C12-MPB only have two-fold symmetrical structures. The effects of hydrocarbon chain length(TTAB, CTAB and C14-MPB, C16-MPB) were also investigated, and the length of hydrocarbon chain could substantially affect the final morphologies of products, the longer hydrocarbon chain is unfavorable for formation of dendritic nanostructures. The SERS tests showed that three-fold dendritic nanostructures have stronger enhancement than two-fold dendrites, which is in good agreement with the common view.