Charge-Transfer Patterns for [Ru(NH3)6]3+/2+ at SAM Modified Gold Electrodes: Impact of the Permeability of a Redox Probe

Abstract

Electrochemical performance of a [Ru(NH 3 ) 6 ]3+/2+ redox couple at gold electrodes modified by alkanethiol self assembled monolayer (SAM) films of the type [-SH -(CH 2 ) n -CH 3 ] with different number of methylene units (n = 2 to 10) in the presence and absence of glucose additives has been studied using fast scan cyclic and steady-state voltammetry. Specific scatter of measured rate constants caused by enhanced sensitivity of this probe to minor defects of SAMs has been observed in a general agreement with the published data for thicker SAMs (n = 9 to 18). In addition, we have disclosed the anomalous viscosity-imposed drop of the heterogeneous rate constant for the case of Au electrodes modified by thinner n-alkanethiol SAMs (n = 2, 4). Taking into the account the fact of [Ru(NH 3 ) 6 ] 3+/2+ couple's capability to penetrate into the SAM interior, we ascribe the obtained results to the manifestation of the solvent-friction mechanism under the condition where the redox species presumably together with a few of solvating water molecules reside in a SAM's peripheral interior marked by much higher local viscosity (slower dielectric relaxation) compared to the electrolyte solution.

Electronic Processes within Quantum Dot-Molecule Complexes

“…The permeability and stability of SAMs on planar or nanoparticle metal substrates are usually inferred from the voltammetric behavior of the SAM-coated substrates in the presence of redox-active molecular species. Many careful studies, most often on SAMs of alkylthiolates or disulfides on planar gold, have mapped the electrical current produced through heterogeneous charge transfer from the conductive substrate to a molecular probe as a function of intra- and intermolecular structural characteristics of the SAM, including the length, bulkiness, and conjugation of these organic molecules, the charge of the chain’s terminal group, and the packing orders of ligands. − …”

Electron Transfer as a Probe of the Permeability of Organic Monolayers on the Surfaces of Colloidal PbS Quantum Dots

“…Efficient extraction of photogenerated charge carriers from colloidal semiconductor QDs is essential to their application as photovoltaically and photocatalytically active materials . The organic ligand shell that electronically passivates − and solubilizes colloidal QDs also presents a physical barrier that impedes a molecular redox partner’s approach to the QD surface and limits the number of available sites per QD for its adsorption. − The ligand shell therefore acts as a semipermeable self-assembled monolayer (SAM), the properties of which are traditionally investigated through the voltammetric behavior of SAM-coated planar or nanoparticulate metal substrates in the presence of electroactive molecular species. − Many careful studies, most often on SAMs of alkylthiols or disulfides on planar gold, have mapped the electrical current produced through heterogeneous charge transfer from the conductive substrate to a molecular probe to intra- and intermolecular structural characteristics of the SAM, including the length and saturation of alkyl chains, the charge of the chain’s terminal group, and the density of pinholes, “thin” regions, and adventitious adsorbates. ,− …”

Temperature-Dependent Permeability of the Ligand Shell of PbS Quantum Dots Probed by Electron Transfer to Benzoquinone

“…The permeability of an organic adlayer on a QD can be inferred from the rate and yield of charge transfer (CT) between the QD and a redox-active probe molecule because CT between the QD and a molecule is dominated by pathways that bypass the electrically insulating layer. In the great majority of cases, CT only occurs for molecules that permeate the ligand shell and are within the tunneling radius of the QD, typically a few angstroms from the inorganic surface. , Previous studies have used cyclic voltammetry (CV) and scanning tunneling microscopy to detect heterogeneous charge transfer from conductive substrates, most frequently planar gold, to molecular probes to study intra- and intermolecular structural characteristics of the SAM, such as the conformation and tilt-angle of the molecules, the charge distribution, and the density of pinholes, “thin” regions, and adventitious adsorbates. − ,− The relationship between the structure of an organic adlayer on a semiconductor QD and the QD’s redox activity is not, however, directly analogous to (or predictable from) that relationship for a planar metal surface, as the high curvature of nanoparticle surfacesand the presence of facets, edges, and verticesinfluence the organization and density of molecules on these surfaces. ,,− Furthermore, planar surfaces and metal NPs are amenable to CV, whereas QDs tend to undergo irreversible redox reactions and precipitate from solution under relevant applied voltages.…”