Charge transfer interactions of pyrazine with Ag12 clusters towards precise SERS chemical mechanism

Abstract

We have synthesized Ag₁₂ nanoclusters (NCs) with mercaptosuccinic acid (H₂SMA) as the ligand. This cluster is found to be water-soluble and has satisfactory stability with [Ag₁₂(HSMA)₆Na₆]²⁺, as determined by high-resolution mass spectrometry. Interestingly, it is noted that both the H₂SMA ligand and Ag₁₂ clusters do not display interference Raman signals, suggesting that this material is a good candidate as a substrate for surface-enhanced Raman spectroscopy (SERS). As a result, we observe enhanced Raman activity of pyrazine molecules adsorbed on Ag₁₂ NCs along with a large red-shift up to ∼27 cm⁻¹. To fully demonstrate the charge transfer interactions between pyrazine and Ag₁₂ clusters, by utilizing first-principles calculations, we estimate polarizability tensor and conduct electronic natural population analysis (NPA), natural bond orbital (NBO) analysis, deformation density analysis (DDA) and charge decomposition analysis (CDA). In view of the minimized contribution from local surface plasmon resonance (LSPR), such a comprehensive study of metal NCs, which are free of Raman interference, provides a modelling method towards the long-debated chemical mechanism in SERS theory.