Article pubs.acs.org/JPCC
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 ICPMS 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. 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 ferricyanide9 or by using XPS8,10 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 C22 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
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 functionalized 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 © 2012 American Chemical Society
Received: November 3, 2011 Revised: February 29, 2012 Published: April 3, 2012 9930
dx.doi.org/10.1021/jp2105715 | J. Phys. Chem. C 2012, 116, 9930−9941
The Journal of Physical Chemistry C
Article
SAMs on Pd have been variously reported to be more10 or less5 stable to reductive desorption than equivalent SAMs on Au. Indeed, the structure of alkanethiolate monolayers on Pd are far from being fully understood.10 The mechanism proposed by the XPS studies involves the formation of a metastable Pd sulphide interlayer.8,10 However, no appreciable influence of such an interlayer was detected on the redox kinetics of ferricyanide.9 Furthermore, a recent study treated thiol adsorption on Pd(111) from first principles without taking into account any such interlayer.15 Pd surfaces coated with the specific thiol compound used in this study, 2-mercaptoethanesulphonate (MESA), are industrially important due to their application in glucose biosensors.16,17 The purpose of the coating is to increase and to standardize the hydrophilicity of the metal surface, allowing the controlled and reproducible deposition onto the Pd surface of an aqueous ink containing the reagents required to perform the glucose assay. The current study presents a method whereby the surface coverage of thiol on a Pd surface may be quantified using electrochemical methods only, obviating the need for additional, specialized surface analysis techniques, such as XPS, employed by previous studies. Specifically, the surface coverage of MESA on a Pd surface may be quantified by means of the peak separation of a cyclic voltammogram of potassium ferricyanide, following treatment of the MESA-coated Pd surface with 1-dodecanethiol. This technique was developed using Au electrodes. Specifically, Au electrodes with a range of MESA surface coverages were prepared. These MESA-coated Au electrodes were then subjected to two electrochemical tests. First, the MESA was desorbed oxidatively by sweeping the voltage to a positive potential. Integration of the resulting current transient and comparison with literature values permitted quantification of the MESA surface coverage on the Au electrodes. This technique is made possible by the stability of the Au surface; in the case of Au, interpretation of the current transient is not complicated by current arising from oxidative dissolution of the metal, as it is in the case of Pd. Second, the MESA-coated Au electrodes were treated with the long-chain thiol, 1-dodecanethiol. The alkane chain on 1-dodecanethiol is sufficiently long that it acts as a barrier to electron transfer between solution species and the metal surface. 1-Dodecanethiol may only bond onto those surface sites not already occupied by MESA. Hence, where the initial surface coverage of MESA is low, the electrode surface may be substantially deactivated by 1-dodecanethiol adsorption. This deactivation results in an increase in the peak separation of a cyclic voltammogram of potassium ferricyanide. The peak separation of a cyclic voltammogram of potassium ferricyanide measured following sequential treatment with MESA and dodecanethiol is thus a qualitative indicator of the extent of MESA surface coverage. By applying both techniques to Au electrodes with a range of MESA surface coverages, the relationship between the peak separation of a cyclic voltammogram of potassium ferricyanide measured following sequential treatment with MESA and dodecanethiol was correlated with the surface coverage of MESA. Peak separations of cyclic voltammograms of potassium ferricyanide measured following sequential treatment of Pd electrodes with MESA and dodecanethiol were then measured, permitting the direct electrochemical quantification of MESA surface coverage on the Pd electrodes.
This method may, in principle, be used to quantify the surface coverage of any thiol in which the combined length of the hydrocarbon chain and any end groups is not sufficiently long as to allow it to act as a barrier to electron transfer between solution species and the metal surface. In the current study, this method is applied to a study of the relative rates of MESA adsorption onto the surface of sputtered Pd and Au films.
2. EXPERIMENTAL METHODS 2.1. Materials. Pd and Au films used in this study were commercially available and were manufactured to a resistance specification of 8−10 Ω/square. The substrate onto which the metals were sputtered was Melinex 329 (DuPont). Two lots of sputtered Pd films that had been “aged” under ambient conditions for 103 and 188 days were used. The starting palladium oxide (PdO) surface coverages were estimated using a previously reported method18 to be 0.5 and 0.7, respectively. These surface coverages were calculated with respect to the true surface area, defined as 1.1 times the geometrical area.18 Potassium ferricyanide (Fluka Ultra, ≥99.5%), sodium hydroxide (NaOH) (SigmaUltra, min. 98%) and potassium chloride (KCl) (puriss, ≥99.5%), 2-mercaptoethansulfonate (MESA) (Biochimika, >98%), 1-dodecanethiol (≥98%), sulphuric acid (ACS Reagent, 95−98%), hydrochloric acid (HCl) (ACS Reagent, 37%), and ethanol (for HPLC, ≥99.8%) were obtained from Sigma-Aldrich and used as supplied. All solutions were made using Analar water obtained from VWR. Solutions were degassed using oxygen-free nitrogen (
