The Synthesis And SERS Property Of Au NF@SiO₂

Functionalized core shell composite nanoparticles are promising materials for fundamental studies and technical application. Gold nanoparticles have been widely utilized in optics, biomedicine, catalysis and many other fields due to their good biocompatibility, precisely tuned structures and unique properties, especially the ones prepared by surfactant-free method with ’cleaner’ surface to be easily functionalized. However, gold nanoparticles are likely to ripen and aggregate for their unstability which limits their practical applications. Only when the aggregation and ripening tendency of nanoparticles is suppressed by strong and hermetic surface encapsulation can the unique optical, electronic, and catalytic properties be preserved permanently. The good chemical stability, biocompatibility and rich surface chemistry make silica a favorable choice for shell coatings. However, anisotropic gold nanoparticles, especially Au NF, can not be easily encapsulated with silica in normal conditions which restricts the utilization of their superior optical, electrical and magnetic properties. Therefore, it is of great importance to encapsulate Au NF with silica to enhance their stability. As the limits of the St?ber system, surface modification is necessary before surface coating. The indirect coating method introduces surfactant which may affect the surface properties of particles and the process is tedious. To solve this problem, we designed experiments for direct silica coating of Au NF with controlled shell thickness from 6 to 50 nm.The paper is based on the St?ber system for silica coating which involves the hydrolyzation and condensation of TEOS catalyzed by ammonia in water–ethanol mixtures. We changed the amount of the sodium citrate to control the stability of charged Au NF particles and successfully fulfill the direct silica coating in water–ethanol St?ber system. Meanwhile, we systematically investigated how the ratio of water–ethanol, reaction time and the amount of NH3 influenced the coating performance. The results showed that the as-prepared Au NF@SiO₂ are monodispersed with one core encapsulated in low concentration of Na3Ct（0.10 m M）. The stability of Au NF,as well as the uniformity of Au NF@SiO₂, in the coating system is largely dependent on the amount of Na3 Ct. The as prepared Au NF@SiO₂ with shell thickness of 6-20 nm are all demonstrated having SERS activity displaying accurate and intact information with non-distorting signals. Therefore, we come up with SHINERS based on Au NF@SiO₂ for the first time, and the specific electromagnetic properties of Au NF enlightens shell restriction on the design of SHINs but offers satisfactory enhancing results. Additionally, the Au NF@SiO₂ core shell structure also exhibit high stability under a wide range of p H, high salt and ion concentration environments, which plays a key role for the practical applications.In this paper, we optimized the St?ber reaction system by examining the stability of Au NF in solutions consisted of different concentrations of reactants and finally confirmed the concentration ranges of reactants. Moreover, we controlled the concentrations of TEOS and NH3 and obtained monodispersed Au NF@SiO₂ with the shell thickness ranging from 6 to 50 nm. We also investigated the colloidal stability of Au NF and Au NF@SiO₂ from the points of ripening and aggregation. The results indicated that Au NF is sensitive to the amount of Cl- and p H value of the environment and will aggregate in low concentration of salt（10 m M）. However, the stability is dramatically improved after silica coating. The Au NF@SiO₂ is not easily tended to get ripened by varying the amount of Cl- and p H value of the solutions and the salt tolerance is up to 100 m M.We investigated the application of the resulted uniform Au NF@SiO₂ nanoparticles as spectroscopic enhancers for Raman and fluorescence spectroscopy. We applied the well established silica coating system of Au NF@SiO₂ for the fabrication of silica coated p-MBA/Au NF and NB/Au NF, both of which are proved to be effective nano-tagged NIR-SERS platform. Subsequently, the performances for enhanced Raman signal of p-MBA are studied taking Au NF@SiO₂ with the shell thickness ranging from 6 to 50 nm. The results demonstrated that the 6 nm silica coated Au NF@SiO₂ offered rich spectrum information and non-distorting signals because the molecules do not contact the metal surface directly, which is believed to perform effectively as novel choice of SHINERS. Finally, we examined the enhancing stability of Au NF@SiO₂ for SERS and the spectrum showed no distinguished changes. We also explored the possibility to use the particles for detecting set penicillin V potassium in artificial urine.