Preparation And Bioimaging Research Of Silica-coated Gold Nanoclusters

In recent years, more and more fluorescent probes have been used in biological research and clinical diagnostics. Metal nanoclusters as a relatively novel fluorescent material has its unique advantages. High quantum yield, low cytotoxicity and high biocompatibility make gold nanoclusters?AuNCs? having more prominent advantages in cell recognition, cell tracking and in vivo fluorescence imaging. Due to the particle size and the different synthetic ligands the fluorescence emission wavelength could be adjusted from visible to near infrared range. Silica has advantages of easily surface functional modification, a larger surface area, low toxicity and high stability characteristics. Because of their unique structural characteristics and special physical and chemical properties, silica-coated Shell/ Core composite nanomaterials have wide applications in biological imaging, cell labeling, drug delivery and biosensors. Therefore, this paper will combine the advantages of gold nanoclusters and silica to prepare silica-coated gold nanoclusters with more stronger chemical and optical stability, and study its applications in biological imaging.Firstly, this paper studied the preparation of gold clusters. In this paper, we selected three processes for the preparation of gold nanoclusters: glutathione?GSH? as a template to synthesize AuNCs; histidine?His?as a stabilizer and reducing agent to produce AuNCs and bovine serum albumin?BSA? as a green template, folic acid as a reducing agent and a degradation agent to prepare AuNCs conveniently. GSH-AuNCs showed poor stability for biological imaging. The AuNCs with His and BSA as a reducing agent and stabilizer displayed perfect fluorescence properties and stability. But fluorescence signal of BSA-AuNCs revealed higher energy. And its maximum emission wavelength of 624 nm was up to near-infrared band which was easier to penetrate biological tissue for better biological imaging applications. Therefore we selected the BSA-AuNCs for the following silica-coating experiments.Secondly, in order to further improve the stability of fluorescent characteristic of AuNCs we coated AuNCs with silica?AuNCs@SiO₂?. With this step we could separate, purify and modify AuNCs for further biomedical imaging easily. This paper adopted two programs for silica wrapping process with different concentrations of reagents and reaction conditions by improving the Stober method. This process could be described as that a certain amount of gold nanoclusters disperses in the solution of anhydrous ethanol.Then add ammonia to form an alkaline environment. By adding TEOS twice as the source of silica the TEOS ishydrolyzed to form monodispersed silica in the alkaline environment. Last, SiO₂ deposites around the surface of gold nanoclusters by condensation resulting in the formation of Shell/Core struction of AuNCs.In the end, we studied the biological imaging of AuNCs and AuNCs@SiO₂. Firstly we verified the cytotoxicity of AuNCs and AuNCs@SiO₂ by MTT assays. The results showed that gold nanomaterials had a good biocompatibility and low cytotoxicity. Secondly, we used dark field microscopy and fluorescence microscopy to detect the in vitro cell imaging of AuNCs and AuNCs@SiO₂. Both dark field imaging and fluorescence imaging testified that gold nanomaterials could bind to cancer cells effectively and the cell morphology displayed clearly with perfect imaging effect. Finally, we prepared two different concentrations of AuNCs@SiO₂ solution to inject into the back and the right hind leg of the mice by subcutaneous and intramuscular injection respectively. With the help of CCD camera, we captured good in vivo fluorescence imaging in mice. Gold nanomaterials in mice could emit bright fluorescence with clear distinction from background. And the higher the concentration of the gold nanomaterials was, the better the image was. This also explained that silica-coating not only improved the stability of AuNCs, but also had an apparent fluorescence characteristics for biomedical imaging.