The ionic insulating properties of n-alkylthiol self-assembled monolayers (SAMs) and a related fluorinated SAM are characterized by electrochemical ac impedance spectroscopy. CH3(CH2) n S/Au (n = 7−15) SAMs prepared via prolonged incubation in a thiol solution obey the Helmholtz ideal capacitor model and thus are virtually defect-free. The electrochemical properties of SAM-coated electrodes are compared with those of a bare gold electrode. The phase angle at an ion-diffusion-related frequency (1 Hz) is ≥ 88° and is independent of the electrolyte concentration. The thiol chains act as a dielectric material whose impedance (modulus) is 105 Ω cm2 at 1 Hz. Both the phase angle and the complex dielectric constant in the low-frequency region are good indicators of the film's ability to act as an ionic insulator.
The ionic permeability of alkanethiol self-assembled monolayers (SAMs) chemisorbed on gold is studied using ac impedance spectroscopy in the absence of redox active species. CH 3 (CH 2 ) n S/Au (n ) 7-15) SAMs behave as ionic insulators until a critical potential, V c , is reached or exceeded. At potentials more cathodic than V c , SAMs are no longer ionic insulators and a significant change in the phase angle is associated with ion penetration in the low-frequency region. V c is chain length dependent and is observed at potentials (-0.15 to -0.35 V vs Ag/AgCl) that are considerably more anodic than the alkanethiol electrodesorption potential. The relaxation frequency of trans-SAM ion migration (4-100 Hz) can be calculated from fitting of the impedance data to an appropriate equivalent circuit or from Bode phase plots.
Ion penetration into a series of ω-functionalized X(CH2) n S/Au (X = CH3, OH, or CO2H, and n = 15) self-assembled monolayers (SAMs) and a partially fluorinated (CF3(CF2)7(CH2)2S/Au) SAM has been investigated by electrochemical ac impedance spectroscopy in the absence of a redox probe. SAM permeability is revealed by the behavior of the phase angle at frequencies of less than ∼50 Hz, the frequency domain characteristic of diffusion processes. The permeability of these ω-functionalized SAMs, as a function of an applied potential, falls into two regimes. One regime corresponds to a state where the SAM is an ionic insulator and is well described by the Helmholtz capacitor model. The second regime begins when a critical applied potential, V c, is exceeded. V c corresponds to the applied potential at which ion penetration into the SAM is activated. For a chain length of 15 carbon atoms, the chemical nature of the terminal group X greatly influences the value of V c, where V c is +0.25 V (vs Ag/AgCl) for X = OH, +0.15 V for X = CO2H, −0.35 V for X = CH3, and −0.25 V for the fluorinated SAM. A hydrophilic SAM/electrolyte interface, rather than a hydrophobic one, is more readily transformed into a form which favors ion/water penetration into the SAM. The potential-induced transformation described here is of importance to the application of SAMs in biosensors and molecular electronic devices.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
customersupport@researchsolutions.com
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
This site is protected by reCAPTCHA and the Google Privacy Policy and Terms of Service apply.
Copyright © 2025 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.