Preparation And Characterization Of Large-area Flexible SERS Substrates Based On Transfer Printing

Flexible surface-enhanced Raman scattering(SERS)substrates have attracted widespread attention due to their outstanding versatility in food safety,trace analysis,biomedicine,and other fields.Benefiting from continuous research and related technological advancements,a large number of methods such as template transfer,plasma etching,and nanosphere(NS)monolayer self-assembly have been proposed to fabricate flexible SERS substrate and push the SERS technique toward practical applications.Among them,the use of polystyrene(PS),SiO2,Au,and other NS monolayer array to prepare flexible SERS substrate have been intensively studied.However,to the best of our knowledge,the self-assembly of large-area NS monolayer array requires precise control of a series of parameters such as temperature,colloid concentration,and spin speed to control the arrangement of NSs,which is not conducive to the facile fabrication of flexible SERS substrate.In the process of preparing flexible SERS substrate by self-assembly and transfer of NSs,we found that large-area SiO2 NS multilayer can be easily and reproducibly obtained by spin coating but has never been used in the flexible SERS field.In this study,we innovatively use the SiO2 NS multilayer obtained by spin coating to replace the traditional monolayer array to prepare large-area flexible SERS substrate,which further simplifies the preparation of flexible SERS substrate.The main work and results of our research are as follows:(1)With the assistance of liquid nitrogen,the 500nm SiO2 NS multilayer(2-7 layers)formed by simple spin-coating was completely cryo-transferred from the silicon wafer into the 2-inches waterborne polyurethane(WPU)flexible film.Moreover,the NS multilayer structure was embed in the film and remains intact during the transfer process,which enables the application of NS multilayer in the flexible field.(2)The surface of the WPU film was etched with air plasma to remove part of the WPU,so that the top layer NSs of the multilayer SiO2 NSs fully which were embedded in the WPU can be controllably exposed.Finally,A layer of Au was deposited by E-beam evaporation and formed cauliflower-like Au nanostructures on the exposed nanosphere crowns,which provided abundant "hot spots",thereby achieving the facile fabrication of the 2-inches flexible SERS substrate.(3)The effects of etching time and Au thickness on the performance of the prepared flexible SERS substrate were systematically studied.The results shows that when the etching time is 6 minutes and the Au thickness is 120 nm,the SERS performance of the substrate reaches the optimum.The detection results shows that the detection ability of the optimal substrate for Rhodamine 6G(R6G)is 10-9M,and the enhancement factor(EF)reaches 1.35×108.(4)The mechanical and temporal stability of the prepared flexible SERS substrates were tested.The SERS performance of the substrate decreased by 21%after 100 times of folding.The SERS performance of the substrates decreased by only 5%after one month of indoor storage,demonstrating the good stability of the substrate.The analysis shows that compared with the traditional method of sticking NS monolayer array on the surface of the flexible film,this scheme embeds hard multiple layers of SiO2 NSs in the flexible WPU film and then controls the exposure of the top layer of NSs by etching.This makes it difficult for the NSs on the surface of the substrate to fall off or sink,endowing it good stability.