Preferential coordination of ruthenium complex as an electroactive self-assembled monolayer on gold substrate and its application in sensing of dopamine

Abstract

Self-assembled monolayers (SAM) are molecular assemblies ordered on a solid surface. In general, they are formed by the reaction of an alkyl-thiol molecule and a gold surface. Alkyl groups do not have additional functionalities in this case. On the other hand, the detailed design of electroactive layers able to be assembled on gold surface can transfer all electrochemical behavior to this surface and thus, an inert surface can have defined and modulated properties. Motivated by this approach, our interest was to evaluate the preferential coordination of the complex [Ru(H4dcbpy)2Cl2] with the 4‑mercapto‑pyridine ligand through the pyridine portion. To achieve this specificity, the ligand was previously attached to the surface of the gold electrode by the thiol portion. Moreover, the electrode modified with the electroactive self-assembled monolayer (E-SAM) was used in sensing of dopamine. Based on the electrochemical studies it was possible to observe that the gold electrode modified with the E-SAM of ruthenium complex (Ru‑4‑mpy‑Au) had an increase in the resistance to charge transfer (Rct = 274 Ω) when compared to the bare gold electrode (Rct = 19 Ω). On the other hand, the charge transfer rate constant is 104 times faster than for the bare gold electrode. These results suggest that the Ru‑4‑mpy‑Au electrode can be used for sensing of molecules through the electrocatalytic process of oxidation. The Ru‑4‑mpy‑Au electrode shows the best results for linear range (0–400 μmol L−1), and the lowest potential of detection (0.2 V vs Ag/AgCl) and limit of detection (3.3 μmol L−1) when compared to the other electrodes that were studied.