Selective Permeation of a Liquidlike Self-Assembled Monolayer of 11-Amino-1-undecanethiol on Polycrystalline Gold by Highly Charged Electroactive Probes

Abstract

Self-assembled monolayers (SAMs) of 11-amino-1-undecanethiol (AUT) have been prepared on polycrystalline Au by immersion of the corresponding surfaces in an AUT 1 mM solution in pure ethanol. The films were studied by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), atomic force microscopy (AFM), and quartz crystal microbalance (QCM). Results of CV and EIS experiments in NaClO 4 solution agree with the fast formation of a well-packed film with low current density (hundreds of nA/cm 2 ) and capacitance values around 2 µF/cm 2 . The films were stable between -0.7 and 0.7 V (vs Ag/AgCl/NaCl sat.). Average values of 1.6 nm and 26°were obtained for the film thickness and the tilt chain angle in the potential region under study. The kinetic analysis of the adsorption process, monitored in situ by the QCM technique, showed that it occurs in two stages: a fast Langmuir type adsorption step (k 1 ) 0.1047 min -1 ), followed by a much slower process of molecular rearrangement (k 2 ) 0.0020 min -1 ). AFM operated in tapping mode did not reveal any morphological changes on the surface after immersion in AUT discarding multilayer growth. The electron transfer (ET) of Fe(CN) 6 -3 and Ru(NH 3 ) 6 +3 species in solution through the AUT layer was investigated by CV and EIS. A mechanism of selective permeation of the electroactive species across the monolayer, controlled by the nature of the electrostatic interactions established at the SAM-solution interface, explains the experimental data obtained and previously reported in the literature for ET processes through substituted SAMs. Analysis of the film structure according to theoretical models (commonly used for SAM characterization) to our experimental data, led to contradictory results clearly affected by the nature of the unconsidered electrostatic interactions (values of θ ) 0.80 and 0.99 were obtained in the presence of Fe(CN) 6 -3 and Ru(NH 3 ) 6 +3 , respectively, in KCl electrolyte using the Amatore model).

Effect of van der Waals interactions in the DFT description of self-assembled monolayers of thiols on gold

“…The 6-aminohexane-1-thiol and 11-aminoundecane-1-thiol SAMs are characterized by thiols adsorbed on bridge positions through their S atoms and being the thiol chains tilted with respect to the surface normal. The tilt angles are similar to those obtained for the alkanethiol SAMs, i.e., 16.3°, 46.1°, 43.0° and 46.0°, with GGA-PBE, GGA-PBE-D2, GGA-PBE-TS and optB86b-vdW, respectively, in the case of the 6-aminohexane-1-thiol chains and 20.9°, 43.9°, 40.9° and 41.0° in the case of the 11-aminoundecane-1-thiol chains, being in both cases the GGA-PBE values closer to the experimental value (i.e., 26°) [82]. As in the case of simple alkanethiol SAMs, the inclusion of the van der Waals corrections seems to overestimate the tilt angles and it also leads to the differentiation between the adsorption energies for long and short aminoalkanethiol SAMs.…”

“…[81]). The tilted SAMs' structures are adopted to maximize the interaction between chains, and normally thiol SAMs exhibit tilt angles between 20° and 35° depending on the type of chains [15,33,40,47,53,64,82,83]. In addition, it has been suggested that the tilt angle of the thiol chains depends on parameters such as the temperature, the nanoparticle size, the chain length (only for short thiols, <20 CH 2 groups) [69] and the deposition of metals on the surface [84].…”

Electrochemiluminescent detection of glyphosate using electrodes modified with self-assembled monolayers

“…33,34 In addition, an electrostatic attractive interaction between the negatively charged carboxylate terminal groups and the positive Ru(bpy) 3 2+ cation can produce a pre-concentration effect. [35][36][37] On the other hand, the GlyP oxidation was not detected under the current electrochemical conditions. However, as mentioned above, this was not an essential condition to generate ECL.…”

Electrochemical Performance of Self‐Assembled Monolayer Gold Nanoparticle‐Modified Ultramicroelectrode Array Architectures

“…In this context, the area of the peaks ascribed to the thiol reductive desorption process is a useful parameter to make an estimation of the thiol surface coverage, which may depend on the SAM composition, while the peak position gives a rough idea of the metal-sulfur bond strength and stability of the formed monolayer. The mechanism proposed for this cathodic desorption was a single-electron reduction of the surface-bound species to form a thiolate in solution [42]. Figure 2 shows the LSV scans carried out in a deoxygenated 0.5 M KOH solution to desorb the corresponding thiolated species from the UMEA surface.…”

“…There are studies of such effect reported in the literature [14]. Indeed, comparative studies carried out with charged species showed a different behavior depending on whether both the electroactive species and the thiolated molecules possessed the same or opposite charge [42,50]. Size exclusion could also influence permeation of species across the SAMs [50].…”