Scanning Tunneling Microscopy, Fourier Transform Infrared Spectroscopy, and Electrochemical Characterization of 2-Naphthalenethiol Self-Assembled Monolayers on the Au Surface:  A Study of Bridge-Mediated Electron Transfer in Ru(NH₃)₆²⁺|Ru(NH₃)₆³⁺ Redox Reactions

Abstract

We have studied the structure, adsorption kinetics, and barrier properties of self-assembled monolayers of 2-naphthalenethiol on Au using electrochemical techniques, grazing-angle Fourier transform infrared (FTIR) spectroscopy, and scanning tunneling microscopy (STM). The results of cyclic voltammetric and impedance measurements using redox probes show that 2-naphthalenethiol on Au forms a stable and reproducible, but moderately blocking, monolayer. Annealing of the self-assembled monolayer (SAM)-modified surface at 72 +/- 2 degrees C remarkably improves the blocking property of the monolayer of 2-naphthalenethiol on Au. From the study of kinetics of SAM formation, we find that the self-assembly follows Langmuir adsorption isotherm. Our STM and FTIR results show that the molecules are adsorbed with the naphthalene ring tilted from the surface normal by forming a square root 3 x 3 R30 degrees overlayer structure. From our studies, we conclude that the electron-transfer reaction of ferro/ferricyanide in the freshly formed monolayer occurs predominantly through the pinholes and defects present in the monolayer. However, in the case of thermally annealed specimen, although the ferro/ferricyanide reaction is almost completely blocked, the electron-transfer reaction of hexaammineruthenium(III) chloride is not significantly inhibited. It is proposed that the electron-transfer reaction in the case of the ruthenium redox couple takes place by a tunneling mechanism through the high-electron-density aromatic naphthalene ring acting as a bridge between the monolayer-modified electrode and the ruthenium complex.

Electrochemical characterization of a 1,8-octanedithiol self-assembled monolayer (ODT-SAM) on a Au(111) single crystal electrode

“…In this sense, electrochemical impedance spectroscopy is particularly valuable for pinhole characterization because the effects of solution resistance, double-layer charging and currents due to diffusion or to other processes occurring in the SAM can be separated [37,38]. This approximation has then been used to characterized the pinhole size and distribution together with the surface coverage of different SAMs [32,37,[39][40][41][42][43].…”

Electrochemical determination of calf thymus DNA on Zr(IV) immobilized on gold–mercaptopropionic-acid self-assembled monolayer

“…3) and CV measurements in the presence of [Ru(NH 3 ) 6 ] 3+/2+ during layer-by-layer assembly process. Kinetic of the electron transfer for [Ru(NH 3 ) 6 ] 3+/2+ is very fast [40], specially on the short chain thiol (MPA) that was used in this work. Thus, we expected almost no isolation effect.…”

Electrocatalytic determination of hydroquinone with Mn+ modified gold-5-amino-2-mercaptobenzimidazole self-assembled monolayer electrodes

“…This behavior is explained as follows. Some factors like hydrogen bonding [44] and ʌ-ʌ interaction between aromatic rings [45] can impose a barrier to the charge transfer between the probe and metal base of the electrode. Consequently, larger values have been obtained for surface coverages (ș) by methods a, b, c ( Table 1).…”