Femtosecond Laser Structured Surface And Its Application In SERS

Surface-enhanced Raman scattering（SERS）is one of the most promising analytical tools,emerging with the development of laser technology,nanophotonics,and plasmonics in recent years.SERS is generally a means of combining noble metal plasmonic nanostructures（NMPNSs）with Raman spectroscopy,which mitigates the shortcomings of the weak Raman scattering cross section of molecules in Raman spectroscopy by the electromagnetic enhancement and chemical enhancement mechanisms of NMPNSs,and amplifies the weak Raman signal effectively.Due to its advantages of high sensitivity,unique spectral fingerprint,non-destructive,easy operation and rapidity,SERS is particularly suitable for food safety detection,environmental hygiene analysis and biomedical diagnosis.However,the preparation of SERS substrates with low cost,high sensitivity,high reproducibility and good practicability is the key and difficult point for the development of SERS technology,and the development of reliable methods to prepare densely distributed plasmonic nanostructures has attracted much attention.As femtosecond laser micro/nano processing technology can induce micro/nano structures of various morphs on the surface of materials in a flexible,low-cost and easy operate way,our research group prepared solid SERS substrates and flexible SERS substrate respectively by using femtosecond laser structured solid substrate and Polydimethylsiloxane（PDMS）nano transfer using femtosecond laser structured solid substrate as a template.And good results were obtained in the detection.The specific research and innovation points of this paper are as follows:（1）The high/low spatial frequency laser induced periodic surface structures of nested distribution was induced on the stainless-steel surface by a femtosecond laser two-step backtrack scanning method.Compared with the previous orthogonal scanning method and single line scanning method,the nanostructures induced by laser two-step backtrack scanning have a smaller nanoscale with gaps as low as 50 nm between adjacent nanostripes,and these periodic nanostructures can be used as the structural basis for the formation of gold plasmonic nanostructure.In addition,the substrate exhibits good SERS detection capability with a low detection limit of 10^-9M for R6G,reaching a single molecule detection level.（2）SERS substrate of patterned silicon decorated with Au-nanoparticles（Au-NPs）was prepared by using femtosecond laser direct writing of silicon wafer combined with solid state dewetting of Au nanofilms.The composite structure of nanopores,nanogrooves and raised silicon islands induced by the femtosecond laser over a large area on silicon wafers not only serves as a template to control the solid-state dewetting of Au nanofilms,but also provides a larger surface area as a carrier for Au-NPs.Secondly,patterned silicon decorated with dense Au-NPs can build multiple coupled forms of localized surface plasmon resonance（LSPR）,including silicon nanogrooves with Au-NPs,silicon nanoislands with Au-NPs,Au dimer,Au trimer,and multimer,to enhance the local field intensity greatly.In addition,the substrate has good SERS detection performance with a low detection limit of 10^-10M for R6G molecules and an enhancement factor of 2.68×10⁷.（3）A method was proposed for the preparation of patterned flexible SERS substrates.The patterned stainless-steel processed by femtosecond laser single line scan was used as a template,whose negative structure was reproduced by PDMS nano transfer technique to fabricate patterned PDMS flexible SERS substrates.In the direct titration detection of R6G molecules,the detection limit was as low as 10^-8M.And the Raman intensity and logarithm of R6G concentration tended to be perfectly linear with a linear correlation of 0.97.In addition,since the PDMS with gold plating has a transmittance of less than 40%,no significant Raman signal could be obtained even for in situ detection of high concentrations of R6G molecules.Therefore,based on the good flexibility of PDMS,the results of extracting R6G molecules for detection by face-face contact method with other non-planar surfaces in conformal contact are close to those obtained by direct titration detection method.This makes up for the infeasibility of in situ detection,and this detection flexibility provides the application basis for instant detection of pesticide residues on the surface of fruits and vegetables.