Research On The Fabrication Technologies And Sensing Propertiesof Nanosphere-based SERS Substrates

Surface enhanced Raman scattering(SERS)refers to the amplified Raman signal depending on electromagnetic mechanism(EM)and chemical mechanism(CM).SERS has been widely employed in biology,chemistry,environment,food safety and other fields,due to the rapid detection of ultra-sensitivity,highly selectivity and non-destructiveness.Preparation of the high-performance SERS substrate is essential for SERS detection.At present,the metal nanoparticles or core-shell structures prepared by chemical reduction,as well as the periodic metal nanostructures prepared by physical etching and template method are used as the popular SERS substrates.However,there are still some great technical challenges in the preparation of large-area low-cost SERS substrate with satisfactory sensitivity,repeatability and stability.In this paper,we prepare large-area SERS substrate with periodic noble nanostructures through template-assisted method combining with physical deposition.Meanwhile,the long-term stability of the SERS substrate is improved by covering two-dimensional materials over Ag.The main research contents and results are as follows:1.The SiO2 nanospheres of different diameters(300,360,500 nm)are self-assembled on Si substrate through inclined drop-coating.For the SiO2 nanospheres with specific diameter,large-area monolayer close-packed arrays are obtained by optimizing experimental parameters such as nanosphere dispersion concentration,the self-assembly environment temperature and the slant angle of the Si substrate.Finally,the nanosphere arrays are used as a periodic template for the preparation of SERS substrate.2.Different thickness of Ag(40,60,80,120 nm)is deposited by magnetron sputtering on the surface of the above template which shows the high-density Ag nanoclusters are formed on the SiO2 spherical cap.The nanogaps between adjacent Ag nanoclusters act as hot spots and produce a significantly enhanced electromagnetic field,making a dominant contribution to the high-performance SERS detection.The preparation process of SERS substrate is pure physical,which is convenient and time-efficient.Rhodamine 6G(R6G)and melamine are used as probe molecules to evaluate the performance of the SERS substrates.The results show that the SERS substrate prepared with 60 nm Ag deposited on the SiO2 nanosphere template with a diameter of 360 nm demonstrates the optimal enhancement with enhancement factor(EF)value of 4.1 ×1012 at 1650 cm-1 for R6G,allowing the detection limit as low as 10-16 M.Additionally,SERS signal of melamine was still strong even though its concentration was as low as 10-7 M.3.The graphene oxide(GO)protective layer is introduced on the surface of Ag nanoclusters by spin-coating to separate Ag from the air,which slows down the oxidation of Ag.Moreover,GO also provides a certain degree of chemical enhancement.Consequently,a GO/Ag nanocluster SERS substrate with improved stability has been prepared.The SERS measurement results of 10-6 M R6G solution show that the intensity of the Raman characteristic peak at 611 cm-1 decays to 69.5%of its original value after stored in air for 15 days,but there are no obvious changes in the positions and shapes of characteristic peaks.However,the Raman peak intensity of the SERS substrate without GO protection decays to 20%and some impurity peaks also appear.