Exploring Tunneling Decay Coefficient of Self-Assembled Monolayers of Alkanethiolates via Electrochemical Impedance Spectroscopy

Understanding the charge transfer and dielectric properties of organic monolayers in electrochemical systems is still challenging because these two properties are intertwined by variables including monolayer structure, electrolyte type, and concentration. The tunneling decay coefficient (β) reflects the evolution of molecular resistance vs length in a circuit. In this work, we applied electrochemical impedance spectroscopy (EIS) to characterize self-assembled monolayers (SAMs) of S(CH₂)_<i>n</i>-1CH₃ (<i>n</i>_C = 4, 6, 8, 10, and 12) on Au electrodes in both redox Fe(CN)₆^3-/4- and nonredox NaClO₄ electrolytes. We found that decreasing the concentration of Fe(CN)₆^3-/4- (<i>C</i>_Fe) from 100 to 1.0 mM led to increases in solution resistance (<i>R</i>_S) from ∼3.3 to 17.0 Ω·cm², log-resistance of SAM from 1.62 to 5.04 Ω·cm² for <i>n</i>_C = 12, and a 63% increase in β from 0.51 to 0.83 <i>n</i>_C^-1; the dielectric constant of the SAM (ε_r) decreased with <i>n</i>_C while remaining almost constant at different <i>C</i>_Fe. EIS in the nonredox NaClO₄ electrolyte showed a low β value of 0.62 <i>n</i>_C^-1. Low <i>C</i>_Fe of 1.0-10 mM was optimal for precise electrochemical characterizations of short and compact SAMs with a 2.5 Å variation in molecular thickness. Our study contributes to a better understanding of how the electrolyte and monolayer-modified barrier affect the charge transfer across SAM-electrode interfaces and the dielectric response of SAM, in electrochemical and biological sensing applications.