Mapping the ionic fingerprints of molecular monolayers

Abstract

We have previously proposed, and experimentally resolved, an ionic charge relaxation model for redox inactive self-assembled monolayers (SAMs) on metallic electrodes in contact with a liquid electrolyte. Here we analyse, by capacitance spectroscopy, the resistance and capacitance terms presented by a range of thiolated molecular films. Molecular dynamics simulations support a SAM-specific energy barrier to solution-phase ions. Once surmounted, the entrapped ions support a film embedded ionic capacitance and non-faradaic relaxation, which can be assigned as a particular case of general electrochemical capacitance.

Charge transport and energy storage at the molecular scale: from nanoelectronics to electrochemical sensing

“…47,48 Traditionally, non-faradaic capacitive ion sensing has been interpreted by solely considering perturbations of a conventional double layer. 49 This is a rather simplistic and incomplete picture and, furthermore, in the presence of moderately dense molecular films (see above), double layer effects are much reduced 36,39 and the measured capacitance C t , though still dominated by the Debye 'electrostatics', as discussed in the preceding sections, includes film characteristics itself, such as those related to non-specific ionic ingress. 36,39 In an analysis of designed ion sensors we have shown that a modulation of C m i , an ion binding specific form of C t , (Fig.…”

“…If one presents an electrode-surface-confined and electrolyteexposed molecule with an applied potential, V, and there exists no redox activity at V, then one would expect (only) a field driven ionic ingress. 36 This can be monitored through both the resistance and capacitance of the film 37,38 (which in this case is primarily ionic in nature) as measured by IS. To understand how the above discussed concepts of charge conductance G and C m apply here, we need to consider eqn (2) under such (ionic charging) conditions.…”

“…This in turn is dependent on chemical potential gradients between solution and film phase ions. Under such circumstances we have a series combination of a resistance R t = 1/G t and a film capacitance C t 36,39 both representing the film ionic (non-faradaic)…”

“…5. 36,39 These (non-faradaic) resistive and capacitive terms represent, then, a fingerprint of film structure after equilibration with the electrolyte and follow the principles of eqn (3) -but in a non-faradaic fashionwhere, now k = G t /C t , and G t = 1/R t and C t (these being classical terms). Physically, this k rate reports on the dynamical dipolar relaxation response of the ions within the molecular layer to the field perturbation (and not to any electronic mobility).…”

“…The nature of C t does align with eqn (2) and the rate, G t and C t phenomena align with eqn (3). 36,39 Electrochemical capacitance, conductance and charge transfer rate in redox molecules…”