Influence of pH Value and Anion on Surface-Enhanced Raman Scattering of 2,6-Pyridinedicarboxylic Acid on Gold Nanoparticle Surface

Surface-enhanced Raman scattering (SERS) was used for the detection of 2,6-pyridinedicarboxylic acid (DPA), a biomarker for bacterial spores. By immobilizing gold nanoparticles of diameter 60 nm on a polished Au electrode using PVP as an adhesive layer, the steady and highly sensitive SERS substrates were fabricated. By using strong acid and alkali to adjust the pH value of DPA molecules, the molecular adsorption configuration of DPA via the SERS characteristics under various pH values were studied and the changes of SERS intensity were compared when HCl, H2SO4 and HNO3 were separately added to the DPA solution to get a pH value of 1.3. The changes of the adsorption form were also analyzed when NaCl, MgSO4 and NaNO3 were separately added to the DPA solution. The results showed that the anion of acids could play a role as a bridge when DPA molecules were protonated by the addition of acids, which could greatly enhance the SERS signal; however, when the pH value is greater than the second dissociation constant of DPA, because of steric effect and the disappearance of the bridge, the SERS signal decreases gradually. Compared with NO3− and SO42-, Cl− has a much stronger binding force with protonated N on DPA; consequently, the addition of HCl can get the maximum SERS signal enhancement.