Influence of the crystallographic orientation on the reductive desorption of self-assembled monolayers on gold electrodes

Journal of Electroanalytical Chemistry 649 (2010) 164–170

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Influence of the crystallographic orientation on the reductive desorption of self-assembled monolayers on gold electrodes Th. Doneux, M. Steichen 1, A. De Rache, Cl. Buess-Herman * Chimie Analytique et Chimie des Interfaces, Faculté des Sciences, Université Libre de Bruxelles, Boulevard du Triomphe, 2, CP 255, B-1050 Bruxelles, Belgium

a r t i c l e

i n f o

Article history: Received 14 November 2009 Received in revised form 15 February 2010 Accepted 24 February 2010 Available online 1 March 2010 This work is dedicated to Prof. Jacek Lipkowski on the occasion of his 65th birthday, in recognition of his outstanding contribution to electrochemistry Keywords: Au(hkl) Reductive desorption Self-assembled monolayer DNA Decanethiol 2-mercaptobenzimidazole-5-sulphonate

a b s t r a c t The voltammetric behaviour of the self-assembled monolayers formed by distinct types of thiolated molecules, namely decanethiol, 2-mercaptobenzimidazole-5-sulphonate (MBIS) and a thiolated DNA sequence was examined at polycrystalline gold electrodes and at the low-index faces Au(1 1 1), Au(1 0 0) and Au(1 1 0) as well as at Au(2 1 0). For the three thiol compounds, the multiwave voltammetric responses at polycrystalline are discussed on the basis of the behaviours observed at well-defined single-crystal electrodes. The sequence of reductive desorption (RD) potentials follows qualitatively the sequence of the potential of zero charge (pzc) of the uncovered electrodes in the case of MBIS. Such a sequence may be altered when attractive lateral interactions between adsorbed molecules, leading to an enhancement of the self-assembled monolayer stability, can be established. The RD peaks are therefore no longer simply directly related to the pzc of the substrate since the amplitude of lateral interactions depends on the atomic roughness of the substrate. Ó 2010 Elsevier B.V. All rights reserved.

1. Introduction Besides the mercury electrode, the gold electrode and more specifically gold single crystals have been extensively used to examine the adsorption of organic molecules. Significant progress in understanding adsorption phenomena particularly in relation with the role played by the nature and atomic structure of the substrate surfaces has been made by Lipkowski and co-workers using diethylether and pyridine as model molecules [1–12]. In the case of pyridine, it has been shown that the strength of the pyridine–metal interaction progressively increases moving from sp to d metals [12]. At the gold electrode, the pyridine–gold interaction is stronger than the water–metal interaction so that a weak chemisorption governs the interfacial behaviour. Significant differences in the adsorption isotherms were however observed when different crystallographic faces were exposed to the pyridine solution [10]. In previous works [13], we have shown that the crystallographic orientation of the gold single-crystal electrodes affects markedly the

* Corresponding author. Tel.: +32 650 29 39; fax: +32 650 29 34. E-mail addresses: [email protected] (Th. Doneux), [email protected] (M. Steichen), [email protected] (A. De Rache), [email protected] (Cl. Buess-Herman). 1 Present address: Université du Luxembourg, Laboratoire Photovoltaïque 41, rue du Brill, L-4422 Belvaux, G.-D. Luxembourg. 1572-6657/$ - see front matter Ó 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jelechem.2010.02.032

formation of 2D condensed adlayers. Voltammetric curves recorded in the presence of uracil and uridine display sharp spikes associated with the occurrence of 2D phase transitions only when terraces are sufficiently large to permit the establishment of a long-range ordering of the adsorbate. Due to their great potentialities for sensing and biosensing devices, for molecular electronics and more generally for the elaboration of materials by the so-called ‘‘bottom-up” strategy, wellordered self-assembled monolayers (SAMs) formed by spontaneous adsorption of thiol-containing compounds on gold have more recently stimulated a lot of work regarding their properties and electrochemical stability [14]. Stability of the SAMs is a key issue for their use in many applications and it is now well established that the order and stability of SAMs is enhanced by attractive van der Waals interactions between the alkanethiol chains. Therefore, long alkyl chains lead to SAMs with greater order and higher stability, as evidenced easily by the position of the reductive desorption potential of the SAMs [15,16]. Although it is recognized that, similarly to other 2D ordered monolayers, the quality of SAMs is also connected to the characteristics of the gold substrate [17–19], it is surprising to note that very few papers are aimed at studying the influence of the substrate crystallography on the organisation and stability of the SAMs. Multiple voltammetric desorption waves of a SAM have been observed by several authors

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at gold films and have been interpreted by differences in the microscopic roughness of the substrates resulting from their preparation [20–22]. For some applications in catalysis or sensing, rough surfaces may be preferable to smooth surfaces and the formation of SAMs on gold nanostructured surfaces have been recently investigated by Cortés et al. [23]. In line with earlier studies [17,21,22,24], the authors linked the increase in stability against SAMs desorption at nanostructured surfaces to the presence of a large number of defects such as adatoms, vacancies and steps where the thiolate binding energy is stronger than at terraces. In the present paper, we examine the voltammetric behaviour of three distinct types of thiolated molecules, namely decanethiol, 2mercaptobenzimidazole-5-sulphonate (MBIS) and a thiolated DNA sequence, at polycrystalline gold electrodes, at the low-index faces Au(1 1 1), Au(1 0 0) and Au(1 1 0) as well as at the Au(2 1 0). The latter face was selected since for face centred cubic (fcc) crystals such as gold, it has the most open structure and constitutes the opposite extreme to the Au(1 1 1). Three markedly different surfactants were chosen in order to discuss both the influence of the substrate and that of the adsorbate. SAMs of n-alkanethiols on gold have been extensively studied [14,25], and decanethiol is thus a good reference compound for this investigation. Thiolated DNA is frequently used in the construction of hybridisation sensors; the electrochemical stability of DNA SAMs has been recently investigated by various groups [26–30] and selective desorption has been proposed [31,32] as a way to pattern polycrystalline gold electrodes for biosensing applications. Some studies on the reductive desorption of small heterocyclic molecules at polycrystal electrodes and their correlation with the behaviour at single-crystal electrodes have been published [33–35]. For MBIS, we have previously investigated in some detail the self-assembly of MBIS on Au(1 1 1) surfaces, both in single component- and mixed-monolayers [36–38]. The present work discusses the influence of the crystallographic orientation on the electrochemical stability of SAMs, and strengthens data published by different groups showing that the reductive desorption potentials of SAMs formed at low index single crystal faces can be correlated to the crystallinity of the substrate [15,16,20–22,34,35,39,40]. 2. Experimental 2.1. Reagents Aqueous solutions were prepared with ultrapure water from a Milli-Q system (Millipore). Perchloric acid (Merck SuprapurÒ) and potassium hydroxide (Carlo Erba) were used as electrolytes. Decanethiol was used as received from Aldrich. The 2-mercaptobenzimidazole-5-sulphonic acid sodium salt (MBIS) (98%, Sigma– Aldrich) was purified by recrystallisation in a mixture of absolute ethanol and water (9/1, v/v). The DNA oligonucleotide 50 -HS(CH2)6- CAA GAC GGA AAG ACC C-30 , was purchased from Eurogentec. The oligonucleotide was purified by RP-HPLC to remove failure sequences that could interfere during experiments. Thiol tethered oligonucleotide stock solutions were prepared with 100 mM sodium phosphate buffer solution (pH 7.4), containing 1 mM EDTA and kept frozen. The purity of the gold (Johnson Matthey) used for the counter and working electrodes was 99.9985%. 2.2. Electrochemical measurements Electrochemical experiments were performed in a three electrode cell connected to an Autolab PGSTAT 20 (Eco Chemie) potentiostat equipped with a Scangen module. The working electrodes were either polycrystalline substrates (wire or disks) or home-

165

Fig. 1. Cyclic voltammograms of the different gold single-crystal electrodes (as indicated) recorded in 0.01 M HClO4. Scan rate 50 mV s1.

made gold single-crystal electrodes. The single crystals were grown, oriented and polished in the lab according to the procedure developed by Hamelin [41]. Using the hanging meniscus technique, only the face of interest of the electrode was in contact with the electrolyte. Cyclic voltammograms were recorded in 0.01 M HClO4 to check the quality of the electrodes. The voltammograms, presented in Fig. 1, are in very good agreement with reference curves available in the literature [42,43]. Prior to each experiment, the gold single-crystal electrodes were annealed in a gas–oxygen flame, cooled down a few seconds in air and then quickly quenched in pure water. This treatment provides us a clean and well-ordered surface, thermally reconstructed in the case of the low-index faces. The polycrystalline gold wire was also cleaned in a flame prior to experiments. The polycrystalline disks were polished with alumina paste then electrochemically cleaned in 0.1 M perchloric acid until displaying steady-state voltammograms. The counter electrode consisted of a large area gold wire. The reference electrode was a KCl saturated calomel electrode connected to the cell through a salt bridge. All the potentials given in this paper refer to the saturated calomel electrode (SCE). All solutions were purged with highly purified nitrogen before the experiments and nitrogen was passed over the top of the solution during the measurements. 2.3. SAM preparation The self-assembled monolayers were formed by simple immersion of the electrode in the surfactants solutions. These solutions were deaerated with nitrogen prior to the immersion since it is known that thiols can be oxidized by atmospheric oxygen. The concentrations and immersion time were selected on the basis of the invariance of the CV curves to ensure that a full monolayer was obtained in each case. For decanethiol, a 1 mM ethanolic solution was used and the immersion time was fixed at 1 h. For MBIS,

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Fig. 2. A. Cyclic voltammogram recorded with a MBIS-covered gold polycrystalline electrode. B. Cyclic voltammograms recorded at the different gold single-crystal electrodes (as indicated) covered with a MBIS SAM. Electrolyte: 0.1 M KOH; scan rate 50 mV s1.

the self-assembly was performed by a 15 min immersion in a 1 mM aqueous solution. For thiolated DNA, the electrodes surfaces were put in contact with a 20 lM solution overnight (16 h).

3. Results and discussion 3.1. SAMs of 2-mercaptobenzimidazole-5-sulphonate The first compound under investigation is a small heterocyclic compound, the 2-mercaptobenzimidazole-5-sulphonate. The structure and reductive desorption (RD) of the self-assembled monolayer of 2-mercaptobenzimidazole-5-sulphonate was previously studied on Au(1 1 1) [36,37] and compared to the parent compound 2-mercaptobenzimidazole (MBI). The two SAMs were found to have a comparable surface coverage and to be reductively desorbed in alkaline solution at almost the same potential. This is interpreted as a predominance of the adsorbate–substrate (through the Au–S bond) interactions over the lateral adsorbate– adsorbate interactions. However, the shapes of the RD peak differed significantly, reflecting some differences in the desorption kinetics. The desorption peak of the MBIS SAM being much sharper than the corresponding one of MBI, its position could be determined with more accuracy and MBIS rather than MBI was chosen for the present study. Fig. 2A presents the cyclic voltammogram of a MBIS-covered polycrystalline gold electrode. Up to three cathodic peaks are discernible on the forward potential sweep (negative direction), associated with a reductive desorption process. Upon desorption, MBIS

diffuses in solution, resulting in the reverse sweep in a smaller, broad anodic response due to a partial oxidative readsorption of the surfactant [24,44,45]. Following the observations made on other systems [34,35,39], this behaviour was compared to the reductive desorption curves of SAMs formed on well-defined gold single-crystal surfaces. The cyclic voltammograms for Au(1 1 1), Au(1 0 0), Au(1 1 0) and Au(2 1 0) are shown in Fig. 2B. An excellent correlation is found between the positions of the cathodic peaks obtained at the polycrystal electrode and those recorded at the different single-crystal substrates, indicating that the polycrystal surface contains facets of distinct crystallographic orientations. The (1 1 1) and (1 0 0) surfaces are thermodynamically the most stable for face centred cubic crystals, and large areas having these two orientations are frequently found on polycrystalline gold surfaces. The two small peaks observed at the least negative potentials in Fig. 2A can be attributed to the reductive desorption of MBIS adsorbed on (1 1 1) and (1 0 0) terraces. Obviously, most of a polycrystal surface does not present a well-defined, atomically flat orientation, but rather contains a lot of steps and kink sites. As a result, the interfacial behaviour of polycrystalline electrodes has been shown to be satisfactorily represented as a weighted average of the individual characteristics obtained at the three low-index faces, (1 1 1), (1 0 0) and (1 1 0) [46]. In Fig. 2A, the peak at the most negative potentials, broader and more intense than those corresponding to the (1 1 1) and (1 0 0) domains, reflects the voltammetric response of a stepped surface area. This peak has been in some instances assigned to the desorption from domains of (1 1 0) orientation [34,35,39,40], but this view is disputable at the microscopic scale. In our case the peak potential is indeed close to that obtained at the Au(1 1 0) face, but an even better agreement exists with the Au(2 1 0) electrode. Considering that this latter is the roughest among the fcc crystallographic orientations, this correlation is not very surprising and the (2 1 0) face might be considered as representing well the microscopic environment encountered at the polycrystal surface, with many steps and kinks. However, attributing the peak to the existence of (2 1 0) domains would be as erroneous as attributing it unequivocally to (1 1 0) domains. Focusing more closely on the single-crystal electrodes, it is interesting to point out that the sequence of RD peak potentials at the different faces, Ep(2 1 0) < Ep(1 1 0) < Ep(1 0 0) < Ep(1 1 1), follows exactly the sequence of the potentials of zero charge (pzc, see Table 1) of the bare electrodes. For the three low-index faces, the same trend has been reported for SAMs of cysteine [35], 4-mercaptobenzoic acid [35], mercaptoacetic acid [39], 4-pyridinethiolate [33], 6-mercaptopurine [34] and with some alkanethiols [24]. Our results show that for MBIS, the sequence is also respected for the (2 1 0) face, for which no reductive desorption of selfassembled monolayers has been reported so far. The agreement between the sequences of pzc and the peak potentials suggests that the monolayer desorption is simply governed by the amplitude of the interfacial electric field. Nevertheless, a closer examination of the potential values (Table 1) shows that the difference Ep  Epzc varies from face to face. Such discrepancy was already reported in the case of SAMs of nonanethiol at Au(1 1 1) and Au(1 1 0) electrodes by Yang et al. [24], who pointed out that the different adsorption sites might be an essential factor. Attractive lateral interactions can also be an important parameter influencing the desorption potential. The direct comparison of the peak potentials relative to the pzc of the bare electrodes is thus oversimplistic. In the actual experiments, the pzc for a given crystallographic orientation is not known accurately, because the presence of hydroxide anions in solution as well as the presence of the monolayer at the surface probably affects its value significantly. Moreover, the RD peak potential is often affected by the kinetics of the desorption process [47].

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Th. Doneux et al. / Journal of Electroanalytical Chemistry 649 (2010) 164–170 Table 1 Potentials of zero charge and reductive desorption peak potentials. Bare Electrode a

Au(1 1 1) Au(1 0 0) Au(1 1 0) Au(2 1 0) a b

MBIS SAM

Decanethiol SAM

DNA SAM

Epzc /V vs. SCE

Ep/V vs. SCE

Ep  Epzc/V

Ep/V vs. SCE

Ep  Epzc/V

Epb/V vs. SCE

Ep  Epzc/V

0.23 0.08 0.02 0.1

0.556 0.836 0.943 0.955

0.786 0.916 0.923 0.855

1.112 1.258 1.318 1.3

1.342 1.338 1.298 1.2

0.748 – 1.036 1.086

0.978 – 1.016 0.986

Values in 0.01 M HClO4. The values correspond to the peak labelled P1 in Fig. 4.

3.2. SAMs of decanethiol A clear illustration of the shortcomings of a simple pzc – peak potential correlation is provided by the case of decanethiol, presented in Fig. 3. Here again, the voltammetric response of the (1 1 1) facets of the polycrystal can be clearly identified on the cyclic voltammogram. However, SAMs of decanethiol are reduced in the same potential range on the (1 0 0), (1 1 0) and (2 1 0) faces, giving rise at the polycrystalline surface to a single, broad peak in this region. In contrast to MBIS, the RD peak potentials do not fully follow the pzc sequence, decanethiol being reduced at potentials more negative on the Au(1 1 0) electrode than on the Au(2 1 0) one. While in the case of MBIS no evidence suggesting strong attractive lateral interactions was found, alkanethiols are known to be strongly stabilised by van der Waals interactions between alkane chains. It is well established that the amplitude of attractive interactions increases with the number of methylene units in the alkane chain, causing a shift of the reduction potential towards more neg-

ative values [16]. As a result, the peak potential value is frequently used as a measure of the stability of a self-assembled monolayer. Though relevant for well-characterised systems, such as n-alkanethiols on Au(1 1 1), this approach is limited when comparing SAMs made of structurally remote surfactants or adsorbed on different substrates (e.g. crystallographic orientation or material). For instance, simply changing the type of gold electrode (polycrystalline [15], nanostructured [23], annealed films [23], single-crystal [48]) employed in an experiment leads to different values of the variation of Ep per methylene unit. This is because the lateral attractive forces between adsorbates are short-range in nature, so the stabilisation energy strongly depends on the exact molecular organisation of the adlayer, which in turn is expected to be affected by the substrate surface structure. Diffraction and STM studies on alkanethiols [49–54], aromatic thiols [33] or cysteine [55,56] have shown that SAMs exhibit different superstructures on Au(1 1 1), Au(1 0 0), Au(1 1 0) and on the stepped surface Au(7 5 5) (no structural data have been reported for Au(2 1 0)), in terms of packing, unit cell dimensions or tilt angle. The potential where reductive desorption takes place is clearly the result of two antagonistic contributions: on one hand a more negative peak is observed at atomically rough surfaces since their pzc is more negative than smooth surfaces, but on the other hand rough surfaces can undermine the establishment of lateral attractions and lower the stability of the SAM. The net result is that the atomic structure of the surface is still an essential factor by acting directly and indirectly on the stability of the SAM at the electrified interface. In the present case, the specificity of each interfacial organisation is probably reflected not only in the position, but also in the shape of the RD peak, which depends on the crystallographic orientation. This is seen in Fig. 3 for decanethiol and Fig. 2 for MBIS, and even more clearly in the case of thiolated DNA, whose results are presented below.

3.3. SAMs of thiolated DNA

Fig. 3. A. Cyclic voltammogram recorded with a decanethiol-covered gold polycrystalline electrode. B. Cyclic voltammograms recorded at the different gold singlecrystal electrodes (as indicated) covered with a decanethiol SAM. Electrolyte: 0.1 M KOH; scan rate 50 mV s1.

Fig. 4 shows the linear sweep voltammograms recorded for a thiolated DNA SAM formed on the different gold substrates. In agreement with the previous systems, a small cathodic response assigned to the desorption from the (1 1 1) domains is observed at potentials around 0.65 V, followed at more negative potentials by a peak assigned to the desorption from the unstructured surface areas. For the single-crystal electrodes, each crystallographic orientation exhibits a distinctly different RD behaviour. A single narrow peak centred around 1.08 V is obtained at the Au(2 1 0) surface. Two main peaks are seen on Au(1 1 0) and Au(1 1 1); in both case, a narrow peak (labelled P1) is preceded by a broader response (labelled P2) at less negative potentials. As for Au(1 0 0), only one broad peak is observed around 0.65 V. A small contribution attributed to surface defects (steps, kinks, adatoms) is noticeable around 1.08 V on the (1 1 1) and (1 0 0) faces. The presence of defects originates from different processes. Even on single-crystal surfaces, a certain density of defects is always

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the former case, the peaks were associated with high and low-coverage layers, while distinct adsorption sites were proposed in the latter. It would be inappropriate to propose an adsorption scheme for thiolated DNA on each surface on the sole basis of the voltammetric measurements shown in Fig. 4. However, the above examples provide a good indication that the voltammograms reflect to some extent the SAMs structures. 3.4. General discussion The above results show a large diversity of RD behaviours, each of the investigated compounds having its own specificity. The reductive desorption of self-assembled monolayers in alkaline medium is often described by the following equation:

RS  Au þ e ¡RS þ Au

Fig. 4. A. Cyclic voltammogram recorded with a thiolated DNA-covered gold polycrystalline electrode. B. Linear sweep voltammograms recorded at the different gold single-crystal electrodes (as indicated) covered with a thiolated DNA SAM. Electrolyte: 0.1 M KOH; scan rate 50 mV s1.

present due to the thermodynamic balance between energetic minimisation and entropic maximisation. Moreover, it has been shown in many cases that the self-assembly of sulphured compounds [51,57,58] and DNA derivatives [29,59] lifts the thermally-induced reconstruction of single-crystal surfaces and some studies have reported significant changes in the surface topography upon adsorption [60], leading in both instances to additional defects in the surface structure. Interestingly, the potential at which the desorption from the defect sites takes place is similar to that obtained with the rough (2 1 0) face. The striking differences between the different faces suggest that thiolated DNA adopts a markedly different interfacial structure on each crystallographic orientation, with perhaps two adsorption configurations existing on Au(1 1 1) and Au(1 1 0). Structural information on thiolated DNA SAMs is relatively scarce and available almost exclusively on Au(1 1 1) substrates, but various adsorption structures have been reported, including ordered and disordered adlayers [27,29,61], or ‘‘flat-lying” and ‘‘upright” configurations [29,30,62], depending on the sequences composition and selfassembly conditions. It is worth mentioning that the nucleic bases can also be adsorbed on gold [13]. This property could favour the formation of interfacial structures in which the DNA strand lays ‘‘flat” at the surface because the bases are in direct contact with the electrode. In a combined electrochemical and STM study of thiolated DNA and LNA (locked nucleic acid) SAMs on Au(1 1 1), Wackerbath et al. [27] have obtained for LNA a RD voltammogram similar to us (on the same surface), with one narrow peak preceded by a broad shoulder. On the basis of STM images, they connected the occurrence of two voltammetric peaks to the coexistence at the surface of ordered and disordered structures. Two RD peaks have also been reported for other systems, such as thiolated compounds on mercury [63] or alkanethiols on Ag(1 1 1) [64]. In

ð1Þ

This equation is very simple and provides a convenient way to estimate the stability of the monolayer, via the RD peak potential, and the surface coverage of the surfactant. However, this description tends to reduce the formation-desorption of SAMs to the sole gold–sulphur bond and obscures all the physico-chemical specificities of any given surfactant–substrate system. The desorption potential can be considered as a measure of the electrochemical stability of the SAM, reflecting a global free energy change between the initial and final states. Many energetic contributions have thus to be taken into account: substrate–adsorbate, SAM-solvent, lateral interactions, substrate–solvent, surfactant solvation [65]. Lipkowski and co-workers [66,67] proposed a more realistic view of the reductive desorption process, described as a substitution reaction between the adsorbate and the solvent. The purpose of the authors was to determine the coverage and charge number per adsorbed molecule with high accuracy, which is not possible with the traditional method consisting in integrating the voltammetric peak according to Eq. (1). Nevertheless, their approach implicitly takes into account the influence of the substrate, and we propose to resort to a slightly modified version of their description by using the following equation:

RSðsurf; AuðhklÞÞ þ xH2 OðaqÞ þ ne ðAuðhklÞÞ¡RS ðaqÞ þ xH2 Oðsurf; AuðhklÞÞ

ð2Þ

where the aggregation states (aq) and (surf) indicate that the compound is in aqueous solution or at the electrode surface, respectively. As compared to Eq. (1), Eq. (2) gives a better chemical description of the initial and final states, making it easier to identify the important energetic contributions and how the crystallographic orientation can influence them: – The surfactant–surface interaction takes place essentially through the gold–sulphur binding. Depending on the crystallographic orientation, the sulphur atom can interact at different binding sites, affecting the strength of the gold–sulphur bond. The experimental and theoretical data of Cortés et al. [23] on nanostructured electrodes, corroborated by previous theoretical calculations [68], point to a larger bond strength on the more open surfaces. Additional contributions may arise from the direct contact between the electrode and surface active groups present in the surfactant (such as the nucleic bases in DNA). – Whether in the adsorbed or desorbed state, surfactant–solvent interactions exist that can affect the quality [69] and stability of the monolayer. For alkanethiols, the stability in organic solvents is usually lower than in water [70]. More recent results have shown that the reductive desorption behaviour of SAMs in ionic liquids is markedly different than in aqueous solutions [71].

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– Upon desorption of the surfactant, the electrode surface is in contact with the solvent. The energetic contribution arising from the gold–water interaction is directly dependent on the difference between the applied potential and the pzc of the electrode. – The lateral interactions between adsorbates can be attractive or repulsive. Attractive forces contribute to stabilise the monolayer, shifting the RD potential towards more negative values. By contrast, significant repulsive interactions tend to destabilise the monolayer, so the predominant driving force for the adsorption becomes the sole gold–sulphur bond. For MBIS and DNA, both negatively charged, some long-range repulsive interactions are expected, and it is noted in these cases that the trend in RD peak potentials follows nicely the trend of pzc (see Table 1), though for DNA the peak P1 was not observed on Au(1 0 0). It is worth pointing out that the amplitude of the electrostatic interaction is highly dependent on the composition of the electrolyte solution, because the repulsion between adsorbate is partially screened by the ions of the electrolyte [38]. It has also been proposed in various instances that high coverage, ordered monolayers of charged thiols can be induced by counter ion condensation [27,29,72]. For decanethiol monolayers, short-range attractive forces between the alkane chains stabilise the SAM, to an extent which depends on the distance between the nearest-neighbour adsorbates, hence on the crystallographic orientation of the underlying substrate. By comparing the most stable surface, Au(1 1 1), with the most open surface, Au(2 1 0), it can be seen in Table 1 that the difference between their respective RD peak potentials is much smaller than the difference between their pzc. It can thus be concluded that attractive interactions are much more important in the SAM of decanethiol on Au(1 1 1) than in the corresponding SAM on Au(2 1 0). The same holds true for the SAMs of decanethiol on Au(1 0 0) and Au(1 1 0), the RD peak being found in this latter case at potentials more negative than that of Au(2 1 0). 4. Conclusions Self-assembled monolayers are widely studied in relation to their application in molecular electronics or (bio) sensing. For this purpose, most of the work is conducted on polycrystalline substrates, and the electrochemical stability of the SAMs is usually evaluated by the reductive desorption method. The present work provides new insights on the important factors governing the reductive desorption process, by correlating the voltammetric behaviour displayed at polycrystal electrodes with the behaviours obtained at the well-defined Au(1 1 1), Au(1 0 0), Au(1 1 0) and Au(2 1 0) single-crystal surfaces. Although RD of SAMs at the Au(2 1 0) has never been reported before, our results demonstrate that this surface may represent well the actual microscopic environment encountered in some areas of polycrystalline surfaces. By studying three different compounds on four different single crystal faces, we have shown that each surfactant–substrate system holds some specificities and that straightforward comparison with other systems is hazardous. Of particular interest is the voltammetric response of thiolated DNA, which seems to adopt markedly distinct structures on the different faces. Besides the crystallographic orientation of the substrate, the main factor influencing the value of the reductive desorption peak potential is the amplitude of the lateral interactions between adsorbates. The value of the reductive desorption peak potential for a given thiolated molecule is dependent in a complex way on the atomic structure of the electrode surface, since the structure governs the electric field at the interface and the amplitude of the attractive interactions stabilizing the SAM at the electrified interface.

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