The Construction Of Hollow Au-Ag Bimetallic Nanospheres SERS Substrates And Their Application In Nitrite Detection

As a new technology of spectrum analysis, Surface Enhanced Raman Scattering technology has been well applied in biology, chemistry and medicine, mainly attributing to its characteristics of fingerprint identification, stability, resistance to photo bleaching, noninvasive detection and the weak Raman signal of water. With the development of nano technology, various metal nanostructures with SERS activity have been synthesized. Taking advantages of the higher SERS activity of silver and the homogeneous superiority of gold, researchers have synthesized multicompoent architectures which own better SERS enhancement effect, stability and biocompatibility. At the same time, the electromagnetic field has been greatly enhanced owning to the porous structure of metal layer formed in the preparation process of hollow nanostructures, also the "hot spots" are more than solid nanostructure, thus the SERS enhancement effect has been greatly effectively enhanced. Therefore, the hollow gold and silver bimetallic structure has received more attention and extensive application in recent years. Excellent SERS effect can be obtained through assembling the hollow gold and silver bimetallic structures by the liquid drop self assembly method. The SERS substrate with excellent signal enhancement has great application potential in the field of inorganic ion detection. In this work, taking advantages of the higher SERS activity of silver, the homogeneous superiority of gold and the hot spots generated from the hollow structure, we synthesized hollow Au-Ag bimetallic nanospheres. Also, HGNs@Ag substrates with higher and homogeneous enhancement effect were constructed to get higher and homogeneous SERS signal. Then a Griess-based SERS sensor was designed for detection of nitrite anions down to the subpicomolar regime.The details are as follows:1. Preparation and characterization of Hollow Au-Ag Bimetallic Nanospheres(HGNs@Ag). Firstly, GNSs were fabricated by seed-mediated methods. Briefly, small gold nanoparticles are pre-formed on the surface of the monodispersed SiO2 to form SiO2/GNPs nanocomposite particles as deposition sites for gold produced by reduction, then, as more gold is reduced by H2O2, the GNPs grow larger and formation of a continuous shell layer. Then the GNPs ware used as the precursor, and the silver shell was formed on the surface of the GNPs through the method of reduction of silver nitrate. By HF corroding the silica core of GNSs@Ag, monodispersed and high SERS enhance HGNs@Ag are obtained. The morphology and optical properties of GNSs?GNSs@Ag and HGNs@Ag were characterized by SEM and Raman spectrometer. The results indicated the great uniformity, repeatability, stability and excellent SERS enhancement effect of HGNs@Ag.2. Preparation and characterization of HGNs@Ag substrates. Assemble GNSs arrays with crystalline structure by droplet self-assembly method with controlled concentration, temperature and angle with the horizontal plane. From the SEM images of the dried drop, three patterns of HGNs@Ag substrates have been obtained:the randomly-stack?the relatively closely ordered-stack and the closely ordered-stack HGNs@Ag substrates. The optical properties and homogeneity of these three HGNs@Ag substrates were characterized by SEM and Raman spectrometer. The results demonstrated that the closely ordered-stack HGNs@Ag substrates can be used as a SERS substrate with good sensitivity and excellent reproducibility. It has potential to produce this kind of HGNs@Ag substrates in mass production since droplet self-assembly method is an easy assemble method with little consumption of raw material.3. Determination of Nitrite based on HGNs@Ag substrates. Based on these, a high-performance nitrite-sensing platform has been developed by combining the classical colorimetric Griess reaction and SERS. Nitrite is first treated with a 4-aminothiophenol (4-ATP) in acidic media to form a transient diazonium salt, then the intermediate is allowed to react with -naphthylamine (NA) to form a stable azo-compound. The color change of the product is the key for monitoring nitrite at high concentration of nitrite. However at low concentration of nitrite, the color of the reaction process has no obvious change. Nitrite can be detected by SERS through the SERS signals changes of 4-ATP and diazocompound which was derived from the diazo reaction of 4-ATP with NA in acidic nitrite solution. This approach was rapid and highly sensitive, and will have great potential in science research.