Mechanism of 2-Mercaptoethanesulphonate Adsorption onto Sputtered Palladium Films: Influence of Surface Oxide Species Gavin Macfie,* Louise V. Simpson, David McColl, and Marco F. Cardosi LifeScan Scotland Ltd., Beechwood Park North, Inverness IV2 3ED, United Kingdom ABSTRACT: A method is presented whereby the adsorption of 2-mercaptoethanesulphonate (MESA) onto Pd may be quantified using the peak separation of a cyclic voltammogram of potassium ferricyanide measured following sequential treatment of the Pd surface with MESA and 1-dodecanethiol. The observed kinetics of MESA adsorption onto sputtered Pd were slower than those observed on sputtered Au by 3-4 orders of magnitude. The rate of MESA adsorption onto a freshly polished Pd disk electrode was comparable to, but slower than, that onto sputtered Au. The lower rate of MESA coating observed on sputtered Pd as compared with sputtered Au was attributed to the presence of oxide species on the former. The rate of MESA coating on Pd was found to decrease with increasing oxide surface coverage. Rate constants were calculated using the method of initial rate as 4 × 10 -2 s -1 for Au and 8 × 10 -5 and 8 × 10 -6 s -1 for Pd with 0.5 and 0.7 fractional surface coverage of PdO, respectively. The kinetics of MESA coating onto Pd were rationalized in terms of the removal of surface oxide species. Specifically, linear sweep voltammetry revealed that the amount of metallic Pd at the surface increased with coating time through two distinct mechanisms. First, metallic Pd was formed through oxide dissolution. Second, metallic Pd was formed through reaction of adsorbed oxygen species with MESA. Measurements of Pd concentration in the coating solution using ICP- MS were consistent with the oxide films on the sputtered Pd films possessing both crystalline and amorphous character. In the case of sputtered Pd films, an increase in the crystalline character of the film may occur coincidently with an increase in oxide surface coverage. 1. INTRODUCTION The adsorption of thiols onto palladium surfaces has received relatively little attention in comparison with gold, silver, or platinum. 1-3 Nonetheless, thiol-treated Pd surfaces find applications as etch resists, 4 in the manufacture of function- alized surfaces, 5 as electrode surfaces for biosensors, 6 and in biotechnology. 7 In recent years, a number of studies have demonstrated that thiol compounds form self-assembled monolayers (SAMs) on Pd. 8-10 To date, the studies of thiol SAMs on Pd have focused on the desorption behavior of the SAMs from the metal surface. In contrast to Au, 11 it is not possible to directly measure thiolate desorption from Pd by integration of the current transient measured during either oxidative or reductive desorption following a voltage sweep. In the case of oxidative desorption, the current arising from thiol desorption is overlaid with that arising from the oxidative dissolution of Pd. In the case of reductive desorption, the cathodic limit of the voltammetric window occurs at a potential more positive than that required for thiol desorption. This behavior contrasts with that observed during equivalent experiments using Au electrodes where integration of the current transients arising from both oxidative and reductive desorption have been employed for the quantification of thiol desorption. Specifically, in the case of oxidative desorption of thiols from Au electrodes, the Au surface is sufficiently stable to oxidizing potentials that interpretation of the current transient is not complicated by current arising from oxidative dissolution of the metal; 11 in the case of reductive desorption from Au surfaces, the cathodic limit of the voltammetric window occurs at a potential more negative than that required for thiol desorption. 12 Previous studies of thiol desorption from Pd surfaces have used the approach of holding the electrode at a negative potential such that the thiols desorb reductively. These studies have not then been able to quantify the amount of thiol that desorbed through interrogation of the current transient, as is possible in the case of reductive desorption of thiols from Au surfaces. Rather, these studies have employed less direct means, inferring the degree of coverage by using the Faradaic response of ferricyanide 9 or by using XPS 8,10 to quantify the amount of sulfur, and hence thiol, on the surface before and after the reductive desorption. We are not aware of any studies into the kinetics of SAM formation on Pd. In contrast, SAM formation on Au has been shown by other investigators to proceed rapidly. For example, based on RAIRS spectra, an immersion time of 45 s in micromolar C 22 thiol solutions was judged to be sufficient to produce an ordered monolayer on polycrystalline Au deposited on a Ti-precovered glass slide; 13 monolayers of octanethiol were formed on sputter grown Au(111) in 0.2-60 min. 14 Received: November 3, 2011 Revised: February 29, 2012 Published: April 3, 2012 Article pubs.acs.org/JPCC © 2012 American Chemical Society 9930 dx.doi.org/10.1021/jp2105715 | J. Phys. Chem. C 2012, 116, 9930-9941