Surface selective binding of 2,5-dimercapto-1,3,4-thiadiazole (DMTD) on silver and gold nanoparticles: a Raman and DFT study

Abstract

The binding affinity of 2,5-dimercapto-1,3,4-thiadiazole (DMTD) on noble metal silver (Ag) and gold (Au) nanoparticles (NPs) was investigated using the surface-enhanced Raman scattering (SERS) technique in combination with density functional theoretical (DFT) calculations. DMTD contains different anchoring groups; firstly the thiadiazole ring nitrogen (N) atoms, secondly the ring sulphur (S) atom, thirdly the thiocarbonyl S atoms and fourthly the ring π electrons. SERS combined with DFT calculations provided useful insight into the interaction between the molecule and metal, thus, illustrating the active sites of DMTD and the metal substrates being directly involved in binding. Raman, SERS and DFT studies were further exploited to study the tautomeric conformations of DMTD present in solid, aqueous solution and on the Ag and Au NP surfaces. The Raman spectrum indicated the existence of mainly the dithiol tautomer in the solid state with little contribution from the dithione and the thione–thiol forms. The pH-dependent Raman study of DMTD in aqueous solution clearly demonstrated the existence of different tautomers of DMTD with the changes in acidity or basicity. The SERS study of DMTD on the Ag NP surface with support from DFT calculations clearly suggested the abundance of mainly the thiol–thiolate tautomer at neutral pH while the dithiol form was predominant at acidic pH. The concentration-dependent SERS study on the Ag NP surface at acidic pH, indicated the preferential existence of the dithione form at low concentrations while the dithiol form is predominant at higher concentrations. On the Au NP surface at acidic pH, the abundance of the dithiol tautomer with slight contributions from the thione–thiol and the dithione forms are indicated. The binding is mainly through the thiadiazole ring N atom in case of Ag NPs whereas it is from the thiocarbonyl or the thiadiazole ring S atom in case of Au NPs. This selective binding observed in case of DMTD may be further extended in designing novel plasmonic nanostructures.