Electrochemical investigations on the reduction of short chain SAMs from a Au(111) electrode




Hager, Gabriele

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Self-assembled monolayers (SAMs) derived from long chain alkanethiols are known to exhibit generalized trends as a function of chain length where n denotes the number of methylene units (CH2). For n 3, these trends are no longer manifest. It can be shown that SAMs of short chain lengths are much more affected by the presence and type of functional group. The reduction of electrochemically induced SAMs derived from cysteine (cys), cystine ((cys)2), mercaptopropionic acid (MPA) and mercaptoethylamine (MEA) from Au(111) highlight the effect of the two functional groups evaluated (R-CO2 - and R-NH2). The reductive desorption of these species was monitored by cyclic voltammetry and electrochemical impedance spectroscopy (EIS) in 0.1 M KClO4 and 0.1 M NaOH. The work presented herein demonstrates that under short time frames of immobilization, the presence of NH2 provides a stabilizing effect to the SAM. Cys and (cys)2 SAMs that maintain both functional groups are generally found to provide the lowest surface coverage under the short term conditions of assembly. The thiol derived monolayers (cys) are consistently higher packed than the disulfide SAMs iv from (cys)2 in both media evaluated. In 0.1 M NaOH however, cys coverage is consistent with coverages obtained from very long incubation times. In the presence of the strong base the disulfide species, (cys)2, desorbs at potentials that are always more positive than those of the thiol species (cys), further supporting poor monolayer formation. Additionally, these monolayers also exhibit the presence of two separate processes in 0.1 M KClO4, whereas only desorption is noted in 0.1 M NaOH. It is likely that a deprotonation of the amine group occurs prior to the desorption of the SAM. The SAM desorption occurs near -0.65 V vs. SCE, and the de-protonation at about -0.50 V vs. SCE. Since the monolayers formed from cys are better formed than those from (cys)2, this deprotonation is much more pronounced in the cys SAMs. The presence of only the CO2 - group (MPA) on the SAM, yields surface coverage that is intermediate compared to the bi-functionalized SAMs formed from cys and (cys)2 and the NH2 containing SAMs of MEA. In the potential region up to and prior to desorption, only one process is noted in both media. SAMs derived from MEA provide the highest surface coverage of the four species, approximating theoretical values. The presence of two surface species is observed in both media, as a result of trans and gauche binding. Of the four species evaluated, MEA appears to be most suitable for rapid SAM formation. The disulfide species, (cys)2, is found to be unsuitable for short-term preparation of SAMs.



SAM, gold, electrochemistry, reduction, cysteine, impedance spectroscopy