Metal Nanospheres/SiO₂Composite Structure:a SERS Platform For Ultrasensitive Detection

In agriculture and industry, dithiocarbamates are a group of organosulfur compounds and it is extensively used for a wide range of applications. Thiram (bis-(dimethyldithiocarbamoyl) disufide) which is one of them, a dithiocarbamate fungicide, is one of the most largely used worldwide in agriculture as a fungicide and animal repellent. However, the toxicity induced by exposuring to thiram can cause serious eye and skin illnesses, simultaneously hurt the liver due to the carbon disulfide which is released from thiram. Moreover, its derivative products have carcinogenic agent that causes human malformation. Melamine is an organic chemical material which mainly used to produce melamine-formaldehyde resins for glues, adhesives, and plastics. However, recently melamine has been illegally added to dairy products to boost the apparent protein contents due to its high nitrogen content (66%by mass) and low cost. As we know melamine is not inherently a toxic chemical, but large-dose ingestion can lead to kidney malfunction and even infant death. Organic pollutants, such as organophosphate parathion which can accumulate in living organisms and lead to negative effects including carcinogenicity and acute toxicity is also serious. We concern the technique which can detect this class of harmful compounds. SERS technique has a wide range of applications in trace analysis due to its ultrasensitive. It is importment to prepare the substrate highly efficient and stable to achieve good surface plasmon resonance and high-quality enhanced Raman spectra. Our research is focused on preparation of the substrate which is ultrasensitive and highly reproducible. This study investigate the preparation and application of SERS substrates. We prepare different nano-sized materials of SiO2@Ag, and then the material was assembled onto the wafer surface to form a regular array. Through the construction of the noble metal nanoparticles and silica composites, we used this nano-structure as SERS substrate and achieved greatly enhanced Raman signal of the analytes. We developed a simple and highly sensitive detection method.In the first chapter, we introduce the surface-enhanced Raman scattering (SERS) phenomenon, mechanism and its application in different field, but also introduce the method of preparation of SERS substrate.In the second chapter, we report a simple and fast method for synthesis of SERS substrate. We prepare the composite microspheres of SiO2@Ag based on its efficient and environmentally friendly and then they are self-assembled on SH-modified silicon surface to form an regular array. By using this two-dimensional regular array as SERS active substrate, we achieve largely enhanced Raman signal of the analytes due to the large specific surface area of SiO2and the orderly arrangement of SiO2@Ag leading to a large amount of Raman hotspots. We investigate the impact of the particle size of SiO2and Ag respectively, and the load concentration of SiO2on SERS enhancement factor. Moreover, based on the repeatability、stability and sensitivity of the SERS signal, we discover the indicators are better than the SERS substrate of Ag nanoparticles. This specific nanostructure can be applied to detect pesticide of thiram in water and juice、melamine in food and ethy parathion. All the detection limit are far below than the maximum residue environmental defense. Furthermore, the SiO2@Ag array enhanced Raman spectroscopic technique provides potential practical applications for the on-site detection and assessment of harmful chemicals in agricultural products and foods.In the third chapter, we develop a new method to detect the mixture of thiram and melamine in water by using several different SERS-active substrates. Specially by using the substrate of SiO2@Ag array and Ag nano-flowers, when the concentration of the mixture is lower than the maximal residue limit (MRL), each characteristic peaks of thiram and melamine are clearly appeared in the SERS spectra. We can observe the characteristic peaks of harmful substances in the mixture for qualitative analysis. This method is highlighting advantageous and we improve the sensitivity and reproducibility in detection of the mixture.