Impact of Surface Adsorption on Metal–Ligand Binding of Phenanthrolines

Phenanthrolines are a class of ligands known to bind with many different metal cations to form complexes. The aromatic backbone of phenanthroline also allows for preferential adsorption on few-layer graphene (FLG) films via π–π stacking. Here we investigate the effects of adsorption and the resulting steric restrictions on the binding ability of four different phenanthroline derivatives: phenanthroline, neocuproine, bathophen­anthroline, and bathocuproine. In solution, a wide range of metal cations tested formed complexes with these ligands, but only Cu²⁺ and Ag⁺ showed evidence of binding to ligands adsorbed onto FLG, as measured by the chemiresistive response of the films. The substituents present on each ligand affected the magnitude of the response in different ways. Raman and X-ray photoelectron spectroscopy (XPS) were used to study two different systems in more detail: iron with phenanthroline, which shows a response in solution (ferroin) but not on the FLG surface (purported monoligand complex), and copper with neocuproine, which responds both in solution (bis­[neocuproine]­copper­(I)) and on the FLG surface (monoligand complex). Raman and XPS data indicate complexing of copper by surface-bound ligands. Theoretical calculations show that the copper–neocuproine monocomplex has a higher adsorption energy and binding energy to a graphene surface than the iron–phenanthroline monocomplex. The reduction of copper­(II) to copper­(I) by the surface-bound neocuproine further results in a stronger charge transfer response from the sensor. The results of this study provide insights into the mechanisms of solid-state sensing of metal cations for water quality detection based on steric and electrochemical restrictions induced by surface adsorption.