Partial charge transfer and adsorption at metal electrodes. The underpotential deposition of Hg(I), Tl(I), Bi(III) and Cu(II) on polycrystalline gold electrodes

Multifunctional Polypeptide EQCN Sensors: Probing the Cysteamine-Glutathione Film Permeability with Hg(II) Ions

Multifunctional films are the basis of biosensors and play an important role in the emerging field of nanobioelectronics. In this work, films of a tripeptide glutathione (GSH) immobilized on a self-assembled monolayer of cysteamine (CA-SAM) on a quartz crystal Au piezosensor have been synthesized and characterized using electrochemical quartz crystal nanogravimetry (EQCN) with a Hg(II) ion probe. It has been found that in contrast to previously studied Au/GSH films, the Au/CA-GSH films strongly hinder the formation of Hg⁰ with bulk properties while still allowing for relatively easy permeation by Hg(II) ions. This results in complete disappearance of the sharp Hg⁰ electrodissolution peak which is observed on bare Au and Au/GSH piezosensors. The multiple-peak anodic behavior of Au/CA and bare Au is replaced by a single high-field anodic peak of mercury reoxidation in the case of Au/CA-GSH sensors. The mass-to-charge plots indicate predominant ingress/egress of Hg(II) to/from the film. The strong hindrance of CA-SAM to bulk-Hg⁰ formation is attributed to film-stabilizing formation of surface (CA)₂Hg²⁺ complexes with conformation evaluated by ab initio quantum mechanical calculations of electronic structure using Hartree-Fock methods. The associates CA-GSH provide an additional functionality of the side sulfhydryl group which is free for interactions, e.g. with heavy metals. It is proposed that in the film, the CA-GSH molecules can assume open (extended) conformation or bent hydrogen-bonded conformation with up to four possible internal hydrogen bonds.

In Situ Stress and Nanogravimetric Measurements during Underpotential Deposition of Bismuth on (111)-Textured Au

The surface stress associated with the underpotential deposition (upd) of bismuth on (111)-textured Au is examined, using the wafer curvature method, in acidic perchlorate and nitrate supporting electrolyte. The surface stress is correlated to Bi coverage by independent nanogravimetric measurements using an electrochemical quartz crystal nanobalance. The mass increase measured in the presence of perchlorate is consistent with the (2 x 2) and (p x square root 3)-2Bi adlayers reported in the literature. ClO(4)(-) does not play a significant role in the upd process. The complete Bi monolayer causes an overall surface stress change of about -1.4 N m(-1). We attribute this compressive stress to the formation of Bi-Au bonds which partially satisfy the bonding requirements of the Au surface atoms, thereby reducing the tensile surface stress inherent to the clean Au surface. At higher Bi coverage, an additional contribution to the compressive stress is due to the electrocompression of the (p x square root 3)-2Bi adlayer. In nitric acid electrolyte, NO(3)(-) coadsorbs with Bi over the entire upd region but has little fundamental impact on adlayer structure and stress.

1998 Alcan Award Lecture Surface electrochemistry - surface science with a joy stick

This lecture gives a review of thermodynamic, spectroscopic, STM and AFM imaging, and X-ray diffraction studies of molecular and ionic adsorption at Au(111) electrodes. In the first part, thermodynamics of adsorption of simple ions such as sulfate, chloride, bromide, and iodide will be discussed. At high coverages, anions adsorbed at a single crystal surface form ordered 2D adlayers. We show that the structure of these adlayers can be studied by STM and surface X-ray diffraction techniques. Next, the information concerning adsorption of simple ions is used to describe mixed adlayers formed by coadsorption of anions and metal adatoms. We demonstrate how to combine electrochemical experiments with in situ polarization-dependent Cu K-edge X-ray absorption spectroscopy to determine the composition and the structure of mixed films formed by deposition of Cu on Au(111) in the presence of SO42-. In the last part we review our efforts to describe coordination of organic molecules to gold electrodes. First, we will discuss adsorption of benzonitrile at the Au(111) electrode surface. We combine electrochemical methods with in situ infrared spectroscopy to describe (i) the energetics of molecular adsorption at the gold electrode surface, (ii) the character of the interaction of the adsorbed molecule with the metal substrate, and (iii) the influence of the electric field on the orientation of the adsorbed molecule. In the last section we describe surface aggregation phenomena. We apply AFM and STM to determine the structure of hemimicelles formed at the Au(111) electrode surface by adsorbed molecules of sodium dodecyl sulfate and we discuss the potential-controlled transformation of these hemimicelles into a condensed monolayer.Key words: surface electrochemistry, electrosorption, surface aggregation, gold electrodes, molecular adsorption, ionic adsorption.