Synthesis And Properties Of Metal Clusters Stabilized In Silica Nanospheres

Metal clusters have unique catalytic properties and specific optical, electrical, and magnetic properties. However, the small size, excess surface free energy, poor thermostability and high chemical activity make the stabilization of metal clusters a premise for research and applications. Organic polymers and ligands can stabilize metal clusters, but their instability and effect on the electron structure of clusters limit the research and applications of metal clusters. silica has good thermostability, mechanical stability, and biocompatibility. More importantly, Silica is chemically inert and optically transparent. Thus dispersing metal clusters in porous silica can improve their thermal and chemical stability without influencing the quantum confinement effect.Copper clusters which are about2nm and covered by triethanolamine (TEOA) are obtained by reducing Cu(Ⅱ)-TEOA in reverse microemulsions which contain Brij(?)58and cyclohexane. Then in the "sol-gel" process to form silica particles, these copper clusters were in situ dispersed in porous hollow silica spheres whose external diameter and inner diameter were about40nm and15nm, respectively. After removing organics by calcination and reduction in H2atmosphere, Cu@SiO2with unique hollow structure was obtained. A series of characterization methods, for example, TEM, XRD, and XPS, were used to prove the presence of metal clusters in Cu@SiO2, and the fluorescence of Cu@SiO2was also proved.Silver nanoparticles with size of about5nm were obtained when Ag(Ⅰ)-EOA complex was reduced by NaBH4in the same microemulsion. Then, silver particles were transformed to silver clusters by air etching in the presence of "trace amount" of chloride. With the same method used in synthesizing Cu@SiO2, silver clusters were dispersed in porous silica and Ag@SiO2was fabricated. TEM, UV-Vis, XRD, and so on were used to investigate this material. Ag@SiO2can exclusively detect nitro explosives in aqueous solution. The detection sensitivity to1μM nitro explosives is more than10%in contrast with less than5%to other polar organic compounds. Ag@SiO2is also an effective antibacterial to Gram-negative and Gram-positive bacteria.A kind of gold particles with size of<10nm was obtained via reducing gold complex with NaBH4in the microemulsion, and then mercaptoethanol was used to etch gold nanoparticles to gold clusters with size of about2nm. These clusters can be enriched on the interface of micelles via forming hydrogen bonds between bifunctional ligands and the polyether chain while a stable Pickering emulsion was formed. During the "sol-gel" process, these gold clusters were in situ dispersed in silica and Au@SiO2was obtained. This material is investigated by TEM, XRD, and XPS to prove the presence of gold clusters in the hollow silica spheres. Ag@SiO2, Pt@SiO2, and Pd@SiO2with the similar structure were also synthesized, when different metal ions and metal cluster bifunctional ligands were selected based on the soft hard acid base rule. Au@SiO2can be used as a catalyst for4-nitrophenol hydrogen transfer reduction. The apparent rate coefficient kapp=0.15min-1, is about30times greater than that of the gold particles supported on silica spheres. All of the four kinds of noble metal clusters confined in porous silica exhibit fluorescence. Pd@SiO2exhibits stable fluorescence and could be used for the fluorescent image of cancer cell like MCF-7.