Electric Potential Stability and Ionic Permeability of SAMs on Gold Derived from Bidentate and Tridentate Chelating Alkanethiols

Abstract

The interfacial electrochemical properties of self-assembled monolayers (SAMs) on gold derived from a structurally tailored series of monodentate, bidentate, and tridentate chelating alkanethiols were investigated. Specific adsorbates included 1-hexadecanethiol (C16), 2-tetradecylpropane-1,3-dithiol (C16C2), 2-tetradecyl-2-methylpropane-1,3-dithiol (C16C3), 2,2-ditetradecylpropane-1,3-dithiol (C16C16), and 1,1,1-tris(mercaptomethyl)pentadecane (t-C16). Reductive desorption of the SAMs as a function of potential was probed by voltammetric measurements, which indicated the following relative order of electric potential stability: t-C16 > C16C2 ≈ C16C3 ≈ C16C16 > C16. The ionic permeability was investigated under various applied cathodic potentials by electrochemical impedance spectroscopy (EIS). An examination of SAMs prepared at room temperature and accessed by EIS at open-circuit potential showed that the ionic permeability increased in the order C16C2 < C16 < C16C3 < C16C16 < t-C16. The ionic permeability of films was further influenced by the electric potential of the metal substrate and the temperature at which the monolayers were assembled. The potential dependence of the ionic permeability was qualitatively rationalized by considering both the initial ionic permeability and the electric potential stability of the SAMs. Similarly, the ionic permeability of the SAMs prepared at elevated temperature showed contributions from both their thermal stability and their insulating properties at room temperature.

Factors governing the reversible change in ionic permeability of a low-density monolayer

“…Wang et al [28] have recently shown that methyl-terminated loosely-packed SAMs do not display a potential-dependent change in ionic permeability over the same potential range investigated in our study. The high (>10 8 X cm 2 ) and invariant resistance of these monolayers might stem from the lack of amphiphilicity exhibited by the adsorbates or the greater rigidity of their chelating thiol headgroup.…”

“…When the fitted value of R m is very large (>10 7 X cm 2 ), however, Fig. 1 resembles the simple RC circuit (consisting of a film capacitance in series with the solution resistance) that has been employed by others to model fully-packed SAMs in the stability region [28][29][30][31]. Adding complexity to the model through the inclusion of other circuit elements (e.g.…”

“…Non-faradaic EIS is used to probe molecular relaxation processes over a range of time-scales, including diffusion of ions and water molecules within SAMs, in the absence of electroactive species [26][27][28]30,31]. Representative impedance spectra obtained at 0 V and at +0.3 V for each type of SAM, along with the corresponding fit to the electrical circuit ( Fig.…”

Electroactive Self‐Assembled Monolayers of Unique Geometric Structures by Using Rigid Norbornylogous Bridges

“…In this study, the Bode phase plot (Figure 9) demonstrates that the L-NB SAM behaves as a close-to-ideal capacitive SAM with a phase angle of about 878, which indicates an insulating monoA C H T U N G T R E N N U N G layer; Wang et al suggest that phase angles of 80 to 888 confirm the presence of a highly insulating dielectric material on the electrode surface. [54] Conversely, the phase angle at low frequency for the S-NB SAM was about 778, which indicates that it is less insulating than the L-NB SAM. A more quantitative information about the structures of the monolayers was obtained by fitting the impedance data to the circuit presented in Figure 10, inset.…”

“…Deviation of the phase angle from 908 (at low frequencies) would suggest an electrical current leakage through the dielectric layer of the Helmholtz capacitor. [52][53][54] At higher frequencies, the total impedance is dominated by the solution resistance and the phase angle decreases towards zero. In this study, the Bode phase plot (Figure 9) demonstrates that the L-NB SAM behaves as a close-to-ideal capacitive SAM with a phase angle of about 878, which indicates an insulating monoA C H T U N G T R E N N U N G layer; Wang et al suggest that phase angles of 80 to 888 confirm the presence of a highly insulating dielectric material on the electrode surface.…”