Nitrogen Positioning in Bicyclic Aromatic Ligands Governs Binding to Nano-Gold: A Curious Case of Quinolinethiol

Abstract

Understanding how small aromatic ligands bind to noble-metal surfaces is critical for engineering plasmonic nanomaterials for sensing, catalysis, and biomedical applications. Here, we systematically investigate how nitrogen heteroatoms─and their positional placement within fused bicyclic molecules (Bm)─govern anchoring, orientation, and functionalization on colloidal gold nanoparticles (AuNPs). Surface-enhanced Raman scattering (SERS) is used as an in situ probe of adsorption motifs, while density functional theory (DFT) calculations quantify adsorption energies, preferred geometries, and chemical enhancement contributions using a Au20 cluster model. The Bm library comprises (i) a non-nitrogen thiolated reference (2-naphthalenethiol, 2-NT), (ii) thiolated nitrogen-containing quinolinethiols (2-QTH and 8-QTH), and (iii) nonthiolated nitrogen-functionalized naphthols (AN, NN, and NNA). Combined theoretical and experimental analyses reveal how molecular architecture controls binding strength, adsorption geometry, colloidal stability, and SERS performance. Three distinct interaction regimes emerge: strong bidentate anchoring for 2-QTH via cooperative Au–S and Au–N coordination (∼2.3 Å), monodentate π-stacking for 2-NT and 8-QTH (∼2.5 Å), and weak physisorption for nonthiols (∼3.2 Å). We further introduce a transferable descriptor, D*χ, linking molecular energetics with surface affinity and functionalization density. Overall, the results demonstrate that nitrogen positioning critically determines molecule–gold interactions and provide quantitative design rules for robust ligand anchoring and stable plasmonic interfaces.