From Solvation Dynamics to Tether Constraints: The Distinct Electron Transfer Energetics of Free vs DNA-Bound Ferrocene

Ferrocene (Fc) derivatives are classically used as redox reporters in electrochemical DNA biosensors; it is therefore critical to understand how tethering to DNA strands alters their intrinsic electrochemical behavior. Here, we investigate the electron transfer energetics of soluble ferrocenedimethanol at a mercaptohexanol-coated gold electrode to establish a reference model for DNA-bound labels. Using high-scan-rate and convolutive cyclic voltammetry analyzed via the Marcus-Hush-Levich-Chidsey formalism, we determine the heterogeneous rate constant and reorganization energy across varying temperatures and viscosities. We report a relatively low reorganization energy (0.4 eV), driven by a large, positive activation entropy attributable to reporter desolvation. Comparing these results with literature data reveals how electron transfer energetics fundamentally change between the free reporter in solution, rigid Fc-alkyl monolayers, and sensing Fc-DNA layers on electrodes. We suggest that the unique kinetic behavior of Fc-DNA, characterized by a large suppression of the reorganization energy, reflects a transition from a purely ergodic, solvation-dominated mechanism for the free reporter to a non-ergodic regime dictated by the restricted conformational sampling of the DNA tether.