Spin-Coated Ag NPs SERS Substrate: Role of Electromagnetic and Chemical Enhancement in Trace Detection of Methylene Blue and Congo Red

This work demonstrates a simple low-cost and reproducible spin-coating method followed by thermal reduction for the uniform growth of silver nanoparticles (Ag NPs) as surface-enhanced Raman scattering (SERS) substrate. In this work, cationic dye MB and anionic dye CR are selected as two different kinds of SERS probe molecules to investigate the differences between electromagnetic field enhancement (EME) and chemical enhancement (CE) mechanisms. The results indicate that SERS spectra of MB molecule on Ag NPs SERS substrate exhibit the shift in the Raman peak positions and high sensitivity (limit of detection, LOD ~ 10−10 M) with large GSERS (~ 109) value, which can be attributed to the contribution from both EME and CE mechanisms. It is found that Cl− halide ions present in the MB molecule are forming surface complex (Ag NPs@Cl−) on the surface of Ag NPs SERS substrate which is confirmed by the XPS investigation, which might be responsible for the CE enhancement in MB molecule. Whereas Congo red being anionic dye showed no shift in Raman peak positions and less sensitivity (LOD ~ 10−8) with GSERS (~ 107) value on Ag NPs SERS substrate, which can be attributed to EME mechanism only. The obtained results suggest that Cl− halide ions present in the MB molecule play an important role by giving large SERS enhancement for MB molecule on Ag NPs SERS substrate. It is expected that this work will provide insight into role of EME and CE mechanisms for cationic dye MB and anionic dye CR.