| The field enhancement effect of the localized surface plasmon resonance（LSPR）of noble metal nanoparticles on the incident light field makes surface-enhanced Raman spectroscopy（SERS）a method of analyzing and detecting trace substances.The SERS technology is mostly decided by SERS subtrate,which is usually formed by the dense arrangement of metal nanoparticles.Through the coupling of LSPR between nanoparticles,a hot spot area of enhanced Raman signal is formed at the gap.There is a narrow hotspot area in this kind of substrate,which has limited effect on Raman signal enhancement.Considering that metal grating can excite surface plasmon resonance（SPR）on its surface,LSPR-SPR coupling can be achieved through the combination of metal nanoparticles and metal grating,thus generating hot spots in a wide area of the surface of the composite structure and improving the overall enhancement effect of SERS substrate.In this paper,two types of double resonance coupling SERS substrate,gold nanoparticles/gold grating（Au NPs/Au grating）and silver nanoparticles/gold grating（Ag NPs/Au grating）,were designed and prepared.The simulation and experimental results showed that the double resonance substrate has higher sensitivity than that of nanoparticle substrate when detecting analytes.The main research contents of this paper are as follows:First,in order to identify the parameters of grating and nanoparticles that compose the double-resonance coupling structure,the finite-difference time-domain method was used to optimize the structural parameters of gold grating and simulate the electric field enhancement of gold and silver nanoparticles（Au NPs,Ag NPs）.When grating period is 567 nm,ridge height is 30 nm,thickness is20 nm and duty cycle is 0.47,the electric field enhancement effect is the best.Considering the limited width of grid groove and the different electric field enhancement ability of gold and silver particles,gold particles with diameter of 25 nm and silver particles with diameter of 50 nm were selected to form two kinds of double resonance substrate.The absorption characteristics and electric field distribution of nanoparticle/grating composite structure are simulated.The results show that the coupling between LSPR and SPR can greatly improve the electric field intensity,and the strong electric field distribution area is wider.Secondly,grating and gold,silver nanoparticles were prepared according to the simulation results.The period of the grating was measured to be 569 nm,the height of the grating ridge 33 nm and the duty cycle 0.492.The structural parameters are basically consistent with those of simulation optimization.The gold particles were prepared by one-step synthesis with average diameter of 25.1nm.Silver particles with average diameter of 50 nm were prepared by optimizing the amount of reactant.In addition,based on the preparation of 50 nm silver particles,silver particles with average diameter of 85 nm were prepared as the blank group for subsequent testing of SERS enhanced substrate performance.Finally,taking R6G as a probe molecule,SERS measurement was conducted on the double resonance and nanoparticles SERS substrate.Under 532 nm excitation,the detection limit of Au NPs/Au grating composite substrate was 10-10 mol/L and the corresponding detection limit of gold particle was 10-8 mol/L.The detection limit of Ag NPs/Au grating substrate was 10-14 mol/L and the corresponding silver particle detection limit was 10-13 mol/L.Under 633 nm excitation,the detection limit of Au NPs/Au grating composite substrate was 10-9 mol/L and the corresponding detection limit of gold particle was 10-7 mol/L.The detection limit of Ag NPs/Au grating substrate was 10-12mol/L and the corresponding silver particle detection limit was 10-10 mol/L.With thiram as another probe molecule,under 785 nm excitation the detection limit of Ag NPs/Au grating substrate was 10-10 mol/L and the corresponding detection limit of silver particle was 10-9 mol/L.It can be seen that the detection limit of the NPs/Au grating substance is increased by 1～2 orders of magnitude compared to the nanoparticles substrate. |
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