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Experimental and theoretical studies of l -cysteine adsorbed at Ag(1 1 1) electrodes
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Electrochimica Acta 53 (2008) 6807–6817
Experimental and theoretical studies of l-cysteine adsorbed at Ag(1 1 1) electrodes E. Santos a,∗ , L. Avalle a , K. P¨otting c , P. V´elez b , H. Jones c a
Facultad de Matem´atica, Astronom´ıa y F´ısica, Universidad Nacional de C´ordoba, 5000 C´ordoba, Argentina b Facultad de Ciencias Qu´ımicas, Universidad Nacional de C´ ordoba, 5000 C´ordoba, Argentina c University of Ulm, 89069 Ulm, Germany Received 30 October 2007; received in revised form 23 December 2007; accepted 31 December 2007 Available online 15 January 2008
Abstract We have investigated l-cysteine adsorbed on Ag(1 1 1) electrodes under different conditions. We have employed experimental and theoretical approaches to obtain a better understanding of the adsorbed layer. An estimation of the coverage from charge measurements and the second √ √ harmonic response shows C3v symmetry for the interface indicating a ( 3 × 3)R◦ 30 overlayer. The theoretical calculations show a variety of different structures with local adsorption energy minima. Particularly, under special initial conditions, zwitterionic structures adsorbed at different sites have been found. This can account for the multiplicity of redox processes observed experimentally below the potential of zero charge. The presence of an external field produces the stabilization of the zwitterion by interaction of the amino/carboxylic groups with the substrate. © 2008 Elsevier Ltd. All rights reserved. Keywords: l-Cysteine; Self-assembled monolayer; Second harmonic generation; Density functional theory; Adsorption
1. Introduction l-Cysteine is an interesting amino acid with particular characteristics: it possesses a thiol group, which can bond strongly to silver or gold electrodes and carboxylic and amino groups, which can interact with more complicated biological molecules. Thus, it is an ideal adsorbate to functionalize metal electrodes and investigate systematically biological systems in an electrochemical environment. The self-assembling process of cysteine on silver or gold substrates is more complex than the case of alkanethiols with a methyl end group. l-Cysteine is a small, highly polar molecule, where intermolecular and intramolecular interactions and also the formation of hydrogen bonds and interactions with other ions present in its environment play a crucial role in determining the conformation of the adsorbate and the structure of the overlayer. In aqueous solutions, at the isoelectric pH 6.1 of l-cysteine, the molecule possesses no net charge, and is considered to have a zwitterionic structure. Below or above this pH, the molecule is predominately cationic or
∗
Corresponding author. E-mail address: [email protected] (E. Santos).
0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.12.080
anionic. Although it is a simple molecule, it can adopt several different conformations at the surface depending on the experimental conditions (pH, presence of anions which adsorb specifically, potential, coverage, presence of defects, √ applied √ etc.). A ( 3 × 3)R◦ 30 overlayer (θ = 0.33) has been proposed by Dakkouri et al. [1] for l-cysteine adsorbed on Au(1 1 1) in perchloric acid solutions. In other electrolyte media other structures with lower coverage have also been claimed [2] for the self-assembled monolayer (SAM) on Au(1 1 1). Nevertheless, charge determinations for the desorption peak of l-cysteine from Au(1 1 1) in alkaline solutions [2] point to a less compact overlayer. This fact has been corroborated by STM observations of dimer arrangement forming a network-like cluster structure in ammonium acetate electrolytes [2]. Combination of SERS and SHG results of l-cysteine on silver polycrystalline electrodes [3] suggested a potential induced reorientation of the adsorbate. In this case the authors proposed that the positively charged amino group is stabilized by the coadsorption of chloride anions at more positive potentials and pointing toward the surface. Theoretical studies of the structure, the surface bonding, and the energetics of alkanethiols adsorbed on Cu(1 1 1), Ag(1 1 1), and Au(1 1 1) surfaces were performed by means of density func-
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tional theory calculations (DFT) with clusters under low and high coverages by Cometto et al. [4] and previously by Sellers et al. [5]. The adsorption of l-cysteine on Au(1 1 1) and Au(110) has been also investigated by means of (DFT) calculations [6–9]. Felice et al. [6,7] found that thiolate adsorption is favourable through two molecular fragments, the thiol and the amino functional groups, and in the most stable configuration the sulphur headgroup sits at the bridge site between two surface Au atoms. This position is about 0.36 eV more stable than the fcc hollow site. Nazmudtinov et al. [8] have performed DFT calculations to obtain the optimized geometry for the adsorption of four l-cysteine forms: the molecule, the anion, the neutral radical and its zwitterions adsorbed at top, bridge and threefold hollow site of a planar Au(1 1 1) Au12 cluster. They concluded that on top radical adsorption emerges as the best representation of l-cysteine adsorption on Au(1 1 1). In a previous work [10], we have investigated the electrochemical behaviour and non-linear optical properties of l-cysteine on Ag(1 1 1) single crystal electrodes in neutral solutions of potassium perchlorate. Desorption/adsorption (reduction/oxidation) processes have been identified at potentials well below the potential of zero charge (pzc = −0.735 V vs. SCE [11]). The origin of the multiplicity of peaks was still not clear. The aim of the present contribution is to gain a better understanding of the self-assembling process of l-cysteine on Ag(1 1 1). The role of the environment such as pH and electrical field at the interface are investigated. We have employed a combination of experimental and theoretical approaches to study the energetics of different possible structures and conformations that the adsorption of l-cysteine can adopt on Ag(1 1 1) electrodes. 2. Methodology 2.1. Experimental work Aqueous solutions of l-cysteine from Aldrich (>99.9%) were prepared at two different pH: in 10 mM of KClO4 (Merck 99.99%) and 10 mM of KOH (Merck 99.99%). The Ag(1 1 1) single crystal electrodes were treated with the same annealing/cooling procedure described elsewhere [11]. The SHG setup was similar to that used in the previous work [10,12–16]. The incident beam impinging at 45◦ on the electrode surface consisted of the fundamental of a pulsed Nd:YAG laser (Lumonics HY 1200, pulse duration ∼10 ns, frequency: 20 Hz, pulse energy ∼1–2 mJ, spot size ∼3.2 mm2 , wavelength: 1064 nm). The direction of polarization was controlled by a combination of Fresnel/Glan-Thomson polarizers. The polarization of the incident beam was oriented in the “p” direction, i.e., the electrical field of the electromagnetic wave was parallel to the plane of incidence. The reflected beam was analyzed in the same direction. At the start of each measurement sequence the coated crystal was positioned with the 2 1¯ 1¯ plane parallel to the incidence plane (φ = 0). The initial position of the silver single crystal to the incidence plane is very important for comparing the relative phases of covered and bare surfaces. The second harmonic anisotropy measurements as a
function of the position angle φ were performed by rotating the single crystal electrode around an axis perpendicular to the surface from this initial position. A digitally controlled potentiostat (model Zahner 5d) supplied the potential during the in situ SHG measurements. Impedance spectra have been recorded with an EG&G Potentiostat 263A and a Lock-in Amplifier 5210 in a frequency range of 100 mHz to 100 kHz. Ag/AgCl or saturated calomel (SCE) electrodes were employed as reference electrodes, but all potentials are given vs. SCE. 2.2. Theoretical work We have employed the generalized gradient approximation (GGA) in the version of Perdew–Burke–Ernzerhof (PBE) [17] to perform density functional theory (DFT) calculations as implemented in the SIESTA code [18,19]. It uses Troullier–Martins norm-conserving pseudopotentials [20] to represent the nucleus and core electrons of the considered species. The basis set used for the expansion of the Kohn-Sham eigenstates are composed of a set of numerical atomic orbitals including optionally polarization orbitals. Energy shift of 75 meV has been chosen as a compromise between accuracy and computational efficiency. We have taken an energy cut-off of 200 Ry and a double-zeta plus polarization orbital basis set (DZP). All geometries have been optimized until the force on each atom was less than ˚ The ground state energy of the single l-cysteine 0.04 eV/A. ˚ × 30 A ˚ × 30 A) ˚ unit molecule has been calculated using a (30 A cell. We have modelled our system employing repeated supercells consisting of four (1 1 1) layers of silver atoms and a ˚ The fcc structure of the metal has been vacuum width of 20 A. represented through a dense hexagonal packing (ABCA). We have used 12 k-points with respect to the surface unit cell for the Brillouin zone (BZ) and the calculated lattice constant for ˚ We have considered one l-cysteine radisilver was a = 4.19 A. cal adsorbed at one √ √ surface of the slab. Each layer of the slab forms a 3 × 3)R◦ 30 cell in the (1 1 1) plane (see Fig. 1a). Different positions (top, bridge, hollow-fcc and hollow-hcp) of the S headgroup have been investigated as shown in Fig. 1b. We have started the calculation with three different initial configurations for the l-cysteine radical (• S–C H2 –C␣ H–NH2 –COOH, see Fig. 1c–e). All these configurations have a protonated carboxylic group. As a consequence of the energy minimization, some of these structures evolve to the radical zwitterion, with the deprotonated carboxylic group and protonated amine group (• S–CH2 –C␣H–NH3 + –COO− , see below). The Ag–S distance has been allowed to relax only in the direction perpendicular to the metal surface. All other atoms of l-cysteine have been allowed to relax in all directions. The position of the Ag atoms has been kept fixed. The adsorption energy Eads has been calculated from: Eads = Esystem − (Eslab + ECys-Rad )
(1)
where Esys is the total energy of the system, Eslab is the energy of the metal slab without geometry optimization and ECys-Rad is the energy of the relaxed adsorbate in vacuum.
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√ √ Fig. 1. (a) Model of the surface with a ( 3 × 3)R◦ 30 overlayer. The unit cell used for the calculations is shown. (b) Top-view of the three outermost layers of a Ag(1 1 1) surface showing the different adsorption sites considered in the calculations: top, bridge, hollow-fcc and hollow-hcp. Different initial configurations of the radical adsorbed on the bridge position employed for the DFT calculations. Two different C␣ –S–Ag angles 120◦ (c) and 180◦ were considered. For the 180◦ angle two orientations of the carboxylic group were regarded: with the hydrogen pointing down (d) and up (e) relative to the surface plane. Similar conformations for the adsorbate have been employed for the other sites of adsorption. Yellow, red, blue, dark grey and white balls represent atoms of sulphur, oxygen, nitrogen, carbon and hydrogen, respectively. Silver atoms of the substrate are light grey. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
3. Experimental results Fig. 2 shows voltammograms obtained with Ag(1 1 1) electrodes in the presence of l-cysteine for two different conditions of pH. According to the ionization constants [21], the predominant species in the KClO4 solution has a zwitterionic structure with no net charge, while in the KOH solution the l-cysteine is predominately anionic with a deprotonated carboxylic group. The rational potential (E–Epzc ) is plotted as x-coordinate. Two potential regions with markedly different behaviour can be distinguished in both media. At potentials lower than the potential of zero charge (pzc) a multiplicity of redox processes are observed. They are better defined in KOH than in KClO4 solutions. Because of the shift into lower overpotentials of the onset of hydrogen evolution reaction in perchlorate solutions, the processes occurring at more negative potentials are masked in neutral solutions. In alkaline media it is possible to scan the potentials up to a limit of 100 mV more negative. Alkanethiols show reductive desorption evidenced by well-defined cathodic peaks in voltammograms [22–25]. The position of these peaks is determined by the molecular structure of the thiol, including its length and functional end groups and the nature of the substrate. Environmental conditions such as pH and the crystal plane of the metallic substrate also influence the position of this peak [22–23]. The multiplicity of peaks observed in
Fig. 2 for l-cysteine indicates that different adsorbates exist on Ag(1 1 1) with different bond energies. In the case of lcysteine adsorbed on Au single crystal surfaces, mainly only one desorption peak has been observed [1,26–28]. The position of this peak on a potential scale relative to the same reference electrode lies at more positive values on Au(1 1 1) than the desorption processes observed on Ag(1 1 1). Similar results have been obtained with alkanethiols [24]. However, the affirmation that this fact evidences a stronger bond between Ag and the sulphur atom of the l-cysteine than in the case of Au is not straightforward. If we consider the electroreduction reaction [24,25]: R–S–M + e− → R–S− + M
(2)
It is evident that the presence of solvent and ions at the interface are necessary to stabilize the thiolate formed during this process. Also during the desorption process, energy must be employed to overcome the Van der Waals and electrostatic interaction between neighbours. The electrical field at the interface plays an important role on all these interactions. Widrig et al. [24] have proposed for alkanethiols that the differences observed between Ag and Au in the position of the desorption peak are due to the different potential of zero charge of both metals. The pzc of gold is also at more positive values [29]. Thus, the electrical
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the Faraday law: Q θ = FQmax
Fig. 2. Cyclic voltammograms obtained for a Ag(1 1 1) electrode at 50 mV/s in 10 mM KClO4 + 150 M l-cysteine in solution (top) and in 10 mM KOH + 150 M l-cysteine (bottom). The dotted lines correspond to the voltammograms in absence of l-cysteine. The cyclic voltammogram in the presence of l-cysteine in solution was recorded after the electrode was stabilized at −0.3 V during 1 h. The charges indicated in the figure were obtained by integration of the current/potential curves discounting the base electrolyte contribution (pzc = −0.735 V vs. SCE).
field which drives the penetration of ions and solvent molecules in the hydrophobic adlayer should be different for these two metals. However, it is not clear in which way the position of the pzc is affected by the presence of the adsorbed layer [30,31]. In contrast to alkanethiols with long chains, cysteine is a small polar molecule and should not hinder the presence of ions or solvent molecules. We return to this point when we discuss the theoretical calculations. A common method to estimate the coverage by alkanethiols is to integrate the current passed during the desorption process [24,25]. In the case of a potential sweep, it is the charge associated with the cathodic peak given by: Q=
Ef
Ei
j dE v
(3)
where ν is the potential scan rate and Ef and Ei are the limits of the potential sweep. The coverage is then obtained according to
(4)
Qmax is the maximal possible charge assuming that the bonding (S–M) occurs on every metal atom of the surface. This procedure contains a series of implicit assumptions. First, it is assumed that the complete monolayer is reduced during the scan. Second, that the reduction process implies the transfer of one electron per adsorbate, i.e., the possibility of a partial charge transfer is neglected. Finally, some approximation must be considered to discount the contribution of the double layer charging. The current passing in the absence of adlayer is usually subtracted to take into account this latter process. However, the presence of l-cysteine at the interface can produce a shift of the potential of zero charge [30,31], and so an extra charge for the rearrangement of the double layer can be required. These limitations on the use of reductive peaks to calculate the thiols coverage has been pointed out by Schneider and Buttry [32]. Recently, a new method to measure packing densities of self-assembled monolayers of thiols has been proposed [33,34]. The method relies on chronocoulometry to measure the charge density at the SAM covered metal electrode surface. It is also shown [33] that the charge numbers per adsorbed molecule depend on the electrode potential and may assume values smaller than the number of electron participating in the reductive desorption step. However, the application of this method to the l-cysteine system is more complicated. It is difficult to reach the same initial conditions for each adsorption potential. It takes a long time for the film to recover from the desorption step. We are working to improve these measurements. Another method to estimate the coverage is the determination of the charge during the adsorption process. By keeping the potential at a constant value, l-cysteine is injected in the solution and the transient current integrated. We have employed this methodology in a previous work [10]. On the other hand, the reproducibility is low and the difficulties with the accounting of double layer charging contributions also appear with this method. In perchlorate solutions the average adsorption charge obtained was about 30 C cm−2 [10]. However, Widrig et al. have suggested that the surface of Ag substrate could be initially partially oxidized and the adsorption process should occur at some places as follows: Ag(OH) + R–SH → AgS–R + HO− + H+
(5)
Thus, we employ as a first approximation the integration of the current passed during the desorption process by applying a potential sweep. Nevertheless, we keep in mind, that the values obtained by this procedure are probably an overestimation of the real coverages. The charges involved in the cathodic potential sweep are higher in alkaline than in neutral solutions and not totally recovered during the reverse positive sweep. However, due to the earlier onset of the hydrogen evolution reaction in neutral solution, the more effective way to estimate the coverage of SAMs is from the reductive desorption charge in basic medium as suggested by Wong and Porter [35]. The potential sweep can
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Fig. 3. (Top) Capacity vs. potential plots at Ag(1 1 1) in 10 mM KClO4 . Single-frequency (20 Hz) differential curves obtained by potentiodynamic conditions (sweep rate: 20 mV/s) with 150 M l-cysteine during the negative (full line) and positive (dashed line) scans. The dotted line was obtained in the absence of l-cysteine. Also shown are calculated capacities from the complex permittivity plots displayed in the bottom, obtained under stationary conditions (waiting time at each potential: 10 min) with 150 M l-cysteine. (Bottom, left) Complex permittivity plots obtained at different potentials below the pzc: −0.75 V (open squares), −0.85 V (closed circles) and −1.0 V (closed up-triangles). (Bottom, right) Complex permittivity plots obtained at different potentials above the pzc: −0.75 V (open squares), −0.60 V (open circles), −0.30 V (open up-triangles), and −0.20 V (open down-triangles). All stationary measurements were started at the more positive potential. Frequency range: 100 mHz–100 kHz. Amplitude of the superimposed potential: 10 mV.
be extended to more negative potentials in the latter case. The lower values obtained during the reverse scan indicate that the readsorption process shows some irreversibility. Effectively, in a previous work [10] we have observed that the adsorption process involves a certain induction time. The value of 67 C cm−2 obtained √ in√KOH correlates well with the coverage of 0.33 for the ( 3 × 3)R◦ 30 structure. This structure has been suggested for the adsorption of l-cysteine on Au(1 1 1) in perchlorate solutions according to the analysis of STM images [1]. At potentials more positive than the pzc the current is substantially lower than in the absence of l-cysteine. Dynamic capacity curves obtained by superimposing an alternating potential of 20 Hz with a potential sweep at 20 mV/s have been compared with stationary values obtained from impedance spectroscopy measurements at constant potentials. Figs. 3 top and 4 top show the results for perchlorate and alkaline solutions, respectively. Particularly in perchlorate medium, a significant hysteresis between positive and negative dynamic sweeps at potentials below the pzc is observed. This potential range corresponds to the region where the multiple redox processes appear in the voltammograms (see Fig. 2). At least two peaks are observed in the dynamic values of the capacity in KOH solutions. Because of our interest to ana-
lyze the effect of l-cysteine on the interfacial capacitance, the dielectric complex permittivity (ε = (jωZ)−1 ) representation of the data is more convenient than the typically employed Nyquist plots with impedance Z [36]. The plots for the potential range below the pzc are shown in Figs. 3 and 4 bottom, left and for the potential range above the pzc in Figs. 3 and 4 bottom, right. The corresponding plots obtained without the presence of l-cysteine (not shown) give typical half circles which can be described by a simple model consisting of the double layer capacitance connected in series with the electrolyte resistance. A more complicated equivalent circuit including constant phase elements gives the same trend. Thus the stationary capacity values can be estimated from the extrapolation to the real axis. When l-cysteine is present at the interface, also capacitive or pseudocapacitive half circles are obtained. Nevertheless, a bend of the curves at lower frequency is observed, denoting the appearance of a second semicircle, which is more evident in alkaline media. Effectively, two different processes at potentials below the pzc with different time constants can be clearly distinguished in the complex permittivity plots with this electrolyte. The surface is partially covered by the film at this potential range. Then, this behaviour can be described by an equivalent circuit consisting
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Fig. 4. (Top) Capacity vs. potential plots for Ag(1 1 1) in 10 mM KOH. Single-frequency (20 Hz) differential curves obtained by potentiodynamic conditions (sweep rate: 20 mV/s) with 150 M l-cysteine during the negative (full line) and positive (dashed line) scans. The dotted line was obtained in the absence of l-cysteine. Also shown are calculated capacities from the complex permittivity plots displayed in the bottom, obtained under stationary conditions (waiting time at each potential: 10 min) with 150 M l-cysteine. (Bottom, left) Complex permittivity plots obtained at different potentials below the pzc: −0.75 V (open squares), −0.90 V (closed circles), −1.0 V (closed up-triangles) and −1.1 V (closed down-triangles). (Bottom, right) Complex permittivity plots obtained at different potentials above the pzc: −0.75 V (open squares), −0.60 V (open circles) and −0.50 V (open up-triangles). All stationary measurements were started at the more positive potential. Frequency range: 100 mHz–100 kHz. Amplitude of the superimposed potential: 10 mV.
of two parallel branches of resistances connected in series with capacitances corresponding to either covered or bare surface. At potentials higher than the pzc the capacity decreases markedly to values much lower than those observed in the absence of l-cysteine, and dynamic and stationary values coincide. This result correlates very well with the diminution of the current in the voltammograms (see Fig. 2). The adsorption of l-cysteine produces the displacement of water molecules from the interface, such as suggested by Laredo et al. [34], and consequently the decrease on the capacity. In a previous work [10] we have found that the second-order polarizability of the interface increases remarkably when lcysteine is added at a constant potential higher than the pzc. This effect has been also observed with other thiols films [13–16] and can be attributed to the effect of the sulphur–silver bond which produces a change in the electronic properties of the interface. Fig. 5 shows 3D-plots of the stationary second harmonic signal when the single crystal electrode is rotated around an axis perpendicular to the surface for different applied poten-
tials in the presence (Fig. 5a) and in the absence of l-cysteine (Fig. 5b) in perchlorate solution. Similar results are obtained in alkaline solutions. The features of the anisotropy showing three or six peaks reveal a C3v symmetry of the system as previously √ √observed. This result supports also the existence of a ( 3 × 3)R◦ 30 overlayer. Other geometry arrangements with different rotation√angles √ should reduce the surface symmetry. For example, a ( 7 × 7)R◦ 10 overlayer, which is frequently proposed for the adsorption of thiols at silver surfaces [22,23] produces the loss of the reflexion planes and the symmetry reduces to C3 . The second harmonic signal depends strongly on the potential. The change in the number of maxima between six and three, as also their relative positions, indicate phase changes between the isotropic and anisotropic contributions produced by resonances. Particularly, a sudden change has been observed near the pzc which separates two regions of different behaviour. At potential below the pzc six peaks have been observed when the l-cysteine was present and the intensity is comparable or somewhat lower than that of the bare surface. Near the pzc the intensity of the signal increases considerably and only three
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Fig. 5. 3D-plots of the stationary second harmonic response as a function of the potential and position of the single crystal electrode. φ = 0 corresponds to the orientation of the 2 1¯ 1¯ plane parallel to the incidence plane. The plots at the left correspond to the response in the presence and those at the right in the absence of l-cysteine in solution. At the bottom are shown the contour graphs projected on the plane formed by the potential and rotation angle coordinates. The arrows indicate the pzc for the bare surface.
peaks are observed above the pzc. This trend with the potential can be better seen in Fig. 6. The stationary SH intensity at two different positions of the crystal substrate relative to the incidence plane is plotted again the potential on the left. The corresponding dynamic response at the position φ = 0 is shown on the right side. While in the case of the bare surface for both positions an almost smooth increase of the SH signal with the potential is observed, in the presence of l-cysteine the SH signal increases suddenly. Brolo et al. [3] have observed with l-cysteine adsorbed on polycrystalline silver electrodes, that near the pzc a peak appears in the SH signal. They ascribed this result to changes in the electronic characteristics of the interface due to a reorientation of the
amino group of the adsorbate. They suggested that this variation of the conformation of the adsorbate is produced by desorption of chloride anions at more negative potentials. However, it is difficult to understand, why a transition between two different structures should give a peak instead of a step. Moreover, our experimental conditions are different: we are working with a single crystal surface, in the absence of chloride anions, and the wavelength of the incident beam is longer (1064 nm instead of 800 nm). Probably in our case, a transition between two conformations also occurred, but with different structures. At potentials below the pzc an adsorbate with the amino group oriented to the negatively charged substrate can be stabilized, while at poten-
Fig. 6. Dependence of the second harmonic response on the potential. (Left) Stationary signal at two different positions of the single crystal electrodes with respect to the incidence plane. (Right) Dynamic signal during a potential sweep at 50 mV/s from −0.5 V to −1.2 V and back to −0.5 V vs. SCE.
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tials above the pzc a conformation with the carboxylic group pointing to the positively charged surface is energetically more favourable. We will discuss in more detail these possibilities in the next theoretical section. Once more an hysteresis between forward and backward potential scan is observed. Comparing the dynamic responses obtained from voltammograms, capacity curves and SH signals, we can draw the conclusion that the desorption process occurs almost fast, while the readsorption during the positive scan show a delay. 4. Theoretical results In order to complement the experimental investigations, we have performed theoretical calculations of the energetics and possible structures of the species formed by the adsorption of l-cysteine on Ag(1 1 1). We have assumed that the bond occurs through the S headgroup as is the case for thiols compounds on silver and gold [22–23]. We have to distinguish between the different definitions of adsorption energy, binding energy, dissociation energy and the contributions from solvation in order to compare the results with those found in the literature and understand the energetics of the investigated systems. The adsorption energy defined by Eq. (1) corresponds to the bond formation between l-cysteine radical and the surface of the metal: Cys-S• (vacuum) + Ag(1 1 1) → Cys-S–Ag(1 1 1)
(6)
The values calculated by Felice et al. [6,7] as adsorption energy of cysteine on gold surfaces are different from our definition given in (6) and are for the reactions on different adsorption sites of Au(1 1 1): 2Cys-S–H (vacuum) + 2Au(1 1 1)br → Cys-S–2Au(1 1 1)br + H2
H = −0.841 eV
(7)
2Cys-S–H (vacuum) + 2Au(1 1 1)fcc → Cys-S–2Au(1 1 1)fcc + H2
ΔH = −0.481 eV
(8)
The dissociation energy for cysteine given by Felice et al. [6] is: Cys-S–H (vacuum) → Cys-S• (vacuum) + H• (vacuum) ΔH = 3.885 eV
(9)
Combining (7)–(9) and taking into account that the dissociation energy for a hydrogen molecule is about 4.52 eV, the corresponding adsorption values as defined by reaction (6) are −2.046 and −1.686 eV at bridge and fcc sites of Au(1 1 1), respectively. Fig. 7 shows the adsorption energy of different species which are√stable√at a coverage of 0.33 on a Ag(1 1 1) surface and form a ( 3 × 3)R◦ 30 overlayer. These values correspond to local minima obtained by the relaxation of the starting conformations shown previously in Fig. 1 to an optimized geometry at different surface sites. The line indicates the bond energy for Ag–S. The first insight of this plot is the existence of several different possibilities of structures with local minimum energies depending
Fig. 7. Adsorption energy calculated according to eq. (6) for different optimized geometries of l-cysteine at different sites of an Ag(1 1 1) surface. The dashed line indicates the bond enthalpy for the gaseous diatomic species of Ag–S.
on the starting configuration. When the final state is an adsorbed radical, the stability increase from the top, to bridge, to hollow (fcc and hcp) sites. A similar trend has been found for the adsorption of small thiols on Ag(1 1 1) and Au(1 1 1) [4]. We return now to the discussion about the relative position of the reduction peaks observed experimentally with silver and gold. We have observed that the adsorption energy defined by (1) and (6) strongly depends on the coverage. Then, we have performed also calculations at a lower coverage in order to compare with the results of Felice et al. mentioned above. The resulting values, −3.04 and −3.11 eV for silver bridge and fcc, respectively, indicate that the bonding of l-cysteine is much stronger with Ag than with Au. However, it is difficult to compare absolute values of computational calculations performed under different conditions. As a test, we have calculated the adsorption of the radical sulphur (S• ) on Au(1 1 1) and Ag(1 1 1) for different coverages. Thus, these results reflect directly the contribution of the bond S–Metal to the adsorption energy. We have observed a difference of about 0.3–0.4 eV between both metals, being more favourable the adsorption on silver. These values correlate very well with the results of Widrig et al. [24]. They have compared the peak potential for the reductive desorption of thiols with different chain-length n. The intercept of the plot of Epeak vs. n is 0.3 V more negative for desorption from silver compare to gold surfaces. However, the most important result is that in some special cases we have found that the geometry relaxation leads to the formation of the zwitterions via proton transfer between two adjacent adsorbates as can be observed in Fig. 8. The zwitterions conformation is about 0.7–0.8 eV energetically more favourable than the radical forms. Contrary to the radicals, the difference in energy between the zwitterions at the different sites is not higher than 0.06 eV for this form. We are showing as an example in Fig. 8 an on top and lateral view of the relaxation process from the radical form (c) of Fig. 1 to the zwitterion. On the bottom is also shown the evolution of the energy as a function of the optimization steps. As can be observed from the sequence between the initial and final stabilized state, the adsorbate stands up and a torsion of the carboxylic and amino group occurs. In this way the proton of the carboxylic group approaches the amino group of the neighbouring adsorbate. When they adopt a favourable
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Fig. 8. Optimization process for the adsorbate which leads to the formation of a zwitterion. Top-view of the initial and final state (upper part). Lateral view showing three conformations adopted by the adsorbate during the optimization process corresponding to the three different energy states as indicated in the bottom.
geometry the proton is transferred. It must be kept in mind that these DFT calculations are carried out in vacuum. Thus, only at high coverages, where the adsorbates are sufficiently close to each another, proton transfer can occur, leading to the formation of zwitterions. In real electrochemical systems, water effects are crucial. It is expected that the adsorbed zwitterions is still more stable than in vacuum and the presence of water molecules can facilitate the proton transfer process also at lower coverages. Also the interactions with the solvent can destabilize the adsorbate at negative potentials and produce its desorption as experimentally observed. In the present state of these investigations, we have not included solvent effects. We put the focus of our analysis on the contributions of adsorbate–adsorbate and adsorbate–substrate interactions. The pzc of Ag(1 1 1) lies about 1 V more negative than the pzc of the unreconstructed Au(1 1 1). The reduction processes of l-cysteine on Ag(1 1 1) occur at potentials about 0.35 V more negative referred to the same reference electrode. Thus, the electrical field at the interface are much more negative at Au(1 1 1) than Ag(1 1 1) assuming that the presence of the adsorbed film does not produce an extra shift of the pzc.
Then, another interesting aspect to be analyzed theoretically is the effect of the presence of an electrical field. In the case of alkane thiols, previous calculations [4] have shown that for negative fields there is a decrease in the stability of the adsorbate, while for positive fields the adsorbate is stabilized. Fig. 9 shows the changes in the total energy (left) and the adsorption energy defined by (1) (right) produced by the application of positive or negative fields when the zwitterion is adsorbed on top sites. Independent of the sign of the field the zwitterion is always stabilized. These results are different from those obtained with cluster calculations for short alkanethiols. However, the presence of the two groups, carboxylic and amino in the case of l-cysteine, provides different possibilities for the conformation of the adsorbate. The zwitterion is a neutral species but with two centers of opposite charge. The interaction of the field with these two groups in a complementary way can explain its stabilization. Preliminary results of a comparative analysis of the effect of electrical field on different species of adsorbed l-cysteine confirmed this statement [37]. In order to analyze this effect, we consider as a first approximation electrostatic interactions and expand the total energy as a second-order polynomial of the
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Fig. 9. Effect of an external electric field on the adsorption energy of the zwitterion resulting from an initial radical positioned at a top site with an initial C␣ –S–Ag angle of 120◦ . The energy for the relaxed zwitterion at zero electric field has been taken as reference. The full squares correspond to the conformation of the zwitterion relaxed in the absence of electric field. The open squares are for the zwitterion relaxed in the presence of the electric field.
strength of the electric field E: 1 2 − αE Energy(tot) = Energy(tot)E=0 − μperm E 2
(10)
where the permanent dipole moment μperm is due to zero bias charge transfer, and α is the polarizability of the interface, which is related to the induced dipole moment: α = μind E. The fit ˚ A (shown also in Fig. 9) gives a value of μperm = 0.0818 e A. similar value has been obtained with cluster calculations [37]. This is a rather small positive value, and it is due to the fact that the carboxylic group is closer to the metal surface than the ˚ ; dNH3+ /Ag = 5.50 A ˚ ). Thus amino group (dCOO− /Ag = 5.09 A a dipole occurs in the z direction pointing away from the surface. The existence of a non-zero permanent dipole causes the center of the curve in Fig. 9 (right) to be displaced from zero electrical field strength. The curve shown in Fig. 9 was obtained with a fixed geometry for the zwitterion, i.e., the same conformation as at zero field. If the zwitterion is allowed to relax, the more favourable geometry implies a slightly closer amino group to the surface for negative fields and a slightly closer carboxylic group to the metal for a positive field. The energy for these relaxed conformations decreases about 0.4 eV as can be observed from the open squares in Fig. 9. 5. Conclusions We have investigated the l-cysteine adsorbed on Ag(1 1 1) electrodes under different conditions. We have employed experimental and theoretical approaches to obtain a better understanding of the adsorbed layer. Two different potential regions of different behaviour have been observed at both pH values, neutral and alkaline. Below the pzc a multiplicity of redox processes have been observed, which are more clearly defined in KOH solutions. An estimation of the coverage from charge measurements together with the second harmonic response √ √showing a C3v symmetry for the interface pointed to a ( 3 × 3)R◦ 30 overlayer as has been proposed by Dakkouri et al. for Au(1 1 1) [1]. At potentials higher than the pzc the adlayer exhibits the behaviour of an insulator. The theoretical calculations show
a variety of different structures with local adsorption energy minima. Particularly, under special initial conditions different structures with a zwitterionic character have been found, which are more stable than radical adsorbates. These different structures could be the origin of the multiplicity of redox processes observed at low potentials. The presence of an external field produces the stabilization of the zwitterion for both directions of the electrical field. The amino and carboxylic groups change their positions relative to the substrate by the presence of an electrical field in a way that stabilizes the adsorbate. At this stage of the investigation, we have not included the effect of the solvent in the theoretical calculations. We have focus on the contributions of the adsorbate–adsorbate and adsorbate–substrate interactions. Obviously, for such a hydrophilic molecule like lcysteine it should be crucial to include the presence of water. Work is in progress to investigate these effects [37]. Finally, we have demonstrated that the combination of different experimental methods with theoretical calculations provides an effective tool to investigate these complex systems. Acknowledgements Financial support from CONICET (Argentina) and SeCyTDAAD is gratefully acknowledged. We thank Prof. Wolfgang Schmickler, Prof. Renat Nazmudtinov and Prof. Leiva for valuable and fruitful discussions. E.S. participates in the COST D36 Action project Structure and Reactivity Relationship of nanoarrays. References [1] A.S. Dakkouri, D.M. Kolb, R. Edelstein-Shima, D. Mandler, Langmuir 12 (1996) 2849. [2] J. Zhang, Q. Chi, J.U. Nielsen, E.P. Friis, J.E.T. Andersen, J. Ulstrup, Langmuir 16 (2000) 7229. [3] A.G. Brolo, P. Germain, G. Hager, J. Phys. Chem. B 106 (2002) 5982. [4] F.P. Cometto, P. Paredes-Olivera, V.A. Macagno, E.M. Patrito, J. Phys. Chem. B 109 (2005) 21737. [5] H. Sellers, A. Ulman, Y. Shnidman, J.E. Eilers, J. Am. Chem. Soc. 115 (1993) 9389.
E. Santos et al. / Electrochimica Acta 53 (2008) 6807–6817 [6] R. Di Felice, A. Selloni, E. Molinari, J. Phys. Chem. B 107 (2003) 1151. [7] R. Di Felice, A. Selloni, J. Chem. Phys. 120 (2004) 4906. [8] R.R. Nazmudtinov, J. Zhang, T.T. Zinkicheva, I.R. Manyurov, J. Ulstrup, Langmuir 22 (2006) 7556. [9] A. K¨uhnle, T.R. Linderoth, B. Hammer, F. Besenbacher, Nature 415 (2002) 891. [10] E. Santos, L.B. Avalle, R. Scurtu, H. Jones, Chem. Phys. 342 (2007) 236. [11] G. Beltramo, E. Santos, J. Electroanal. Chem. 556 (2003) 127. [12] G. Beltramo, E. Santos, W. Schmickler, J. Electroanal. Chem. 447 (1998) 71. [13] E. Santos, C. Sch¨urrer, A. Brunetti, W. Schmickler, Langmuir 18 (2002) 2771. [14] E. Santos, C. Sch¨urrer, G. Beltramo, W. Schmickler, J. Solid State Electrochem. 7 (2003) 567. [15] E. Santos, D.T. Sch¨ule, H. Jones, W. Schmickler, Langmuir 21 (2005) 6406. [16] E. Santos, A. L´opez-Cudero, H. Jones, Electrochim. Acta 50 (2005) 4837. [17] J.P. Perdew, et al., Phys. Rev. B 46 (1992) 6671. [18] J.M. Soler, E. Artacho, J.D. Gale, A. Garc´ıa, J. Junquera, P. Ordejon, D. S´anchez-Portal, J. Phys. Condens. Matter 14 (2002) 2745. [19] J.M. Soler, E. Artacho, J.D. Gale, A. Garc´ıa, J. Junquera, P. Ordejon, D. S´anchez-Portal, http://uam.es/departamentos/fismateriac/siesta/. [20] N. Troullier, J.L. Martins, Phys. Rev. B 43 (1991) 1993. [21] T.R. Ralph, M.L. Hitchman, J.P. Millington, F.C. Walsh, J. Electroanal. Chem. 375 (1994) 1. [22] A. Ulman, Chem. Rev. 96 (1996) 1533.
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[23] J.C. Love, L.A. Estroff, J.K. Kriebel, R.G. Nuzzo, G.M. Whitesides, Chem. Rev. 105 (2005) 1103. [24] C.A. Widrig, Ch. Chung, M.D. Porter, J. Electroanal. Chem. 310 (1991) 335. [25] M.M. Walczak, D.D. Popenoe, R.S. Deinhammer, B.D. Lamp, Ch. Chung, M.D. Porter, Langmuir 7 (1991) 2687. [26] W.R. Fawcett, M. Fedurco, Z. Kov´acˇ ov´a, Z. Borkowska, J. Electroanal. Chem. 368 (1994) 275. [27] G. Hager, A.G. Brolo, J. Electroanal. Chem. 550–551 (2003) 291. [28] K. Arihara, T. Ariga, N. Takashima, K. Arihara, T. Okajima, F. Kitamura, K. Tokuda, T. Ohsaka, Phys. Chem. Chem. Phys. 5 (2003) 3758. [29] D. Eberhardt, E. Santos, W. Schmickler, J. Electroanal. Chem. 419 (1996) 23. [30] V. Climent, R. G´omez, J.M. Orts, A. Aldaz, J.M. Feliu, in: C. Korzeniewski, B.E. Conway (Eds.), The Electrochemical Society Proceedings, vol. 97-17, The Electrochemical Society, Pennington, NJ, 1997, p. 222. [31] V. Climent, R. G´omez, J.M. Feliu, Electrochim. Acta 45 (1999) 629. [32] T.W. Schneider, D.A. Buttry, J. Am. Chem. Soc. 115 (1993) 12391. [33] J. Kunze, J. Leitch, A.L. Schwan, R.J. Faragher, R. Naumann, S. Schiller, W. Knoll, J.R. Dutcher, J. Lipkowski, Langmuir 22 (2006) 5509. [34] T. Laredo, J. Leitch, M. Chen, I.J. Burgess, J.R. Dutcher, J. Lipkowski, Langmuir 23 (2007) 6205. [35] S.-S. Wong, M.D. Porter, J. Electroanal. Chem. 485 (2000) 135. [36] J.R. Macdonald, Impedance Spectroscopy, John Wiley & Sons, Inc., 1987. [37] E. Santos, R.R. Nazmudtinov, K. P¨otting, P. V´elez (in preparation).
Electrochimica Acta 53 (2008) 6807–6817
Experimental and theoretical studies of l-cysteine adsorbed at Ag(1 1 1) electrodes E. Santos a,∗ , L. Avalle a , K. P¨otting c , P. V´elez b , H. Jones c a
Facultad de Matem´atica, Astronom´ıa y F´ısica, Universidad Nacional de C´ordoba, 5000 C´ordoba, Argentina b Facultad de Ciencias Qu´ımicas, Universidad Nacional de C´ ordoba, 5000 C´ordoba, Argentina c University of Ulm, 89069 Ulm, Germany Received 30 October 2007; received in revised form 23 December 2007; accepted 31 December 2007 Available online 15 January 2008
Abstract We have investigated l-cysteine adsorbed on Ag(1 1 1) electrodes under different conditions. We have employed experimental and theoretical approaches to obtain a better understanding of the adsorbed layer. An estimation of the coverage from charge measurements and the second √ √ harmonic response shows C3v symmetry for the interface indicating a ( 3 × 3)R◦ 30 overlayer. The theoretical calculations show a variety of different structures with local adsorption energy minima. Particularly, under special initial conditions, zwitterionic structures adsorbed at different sites have been found. This can account for the multiplicity of redox processes observed experimentally below the potential of zero charge. The presence of an external field produces the stabilization of the zwitterion by interaction of the amino/carboxylic groups with the substrate. © 2008 Elsevier Ltd. All rights reserved. Keywords: l-Cysteine; Self-assembled monolayer; Second harmonic generation; Density functional theory; Adsorption
1. Introduction l-Cysteine is an interesting amino acid with particular characteristics: it possesses a thiol group, which can bond strongly to silver or gold electrodes and carboxylic and amino groups, which can interact with more complicated biological molecules. Thus, it is an ideal adsorbate to functionalize metal electrodes and investigate systematically biological systems in an electrochemical environment. The self-assembling process of cysteine on silver or gold substrates is more complex than the case of alkanethiols with a methyl end group. l-Cysteine is a small, highly polar molecule, where intermolecular and intramolecular interactions and also the formation of hydrogen bonds and interactions with other ions present in its environment play a crucial role in determining the conformation of the adsorbate and the structure of the overlayer. In aqueous solutions, at the isoelectric pH 6.1 of l-cysteine, the molecule possesses no net charge, and is considered to have a zwitterionic structure. Below or above this pH, the molecule is predominately cationic or
∗
Corresponding author. E-mail address: [email protected] (E. Santos).
0013-4686/$ – see front matter © 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.electacta.2007.12.080
anionic. Although it is a simple molecule, it can adopt several different conformations at the surface depending on the experimental conditions (pH, presence of anions which adsorb specifically, potential, coverage, presence of defects, √ applied √ etc.). A ( 3 × 3)R◦ 30 overlayer (θ = 0.33) has been proposed by Dakkouri et al. [1] for l-cysteine adsorbed on Au(1 1 1) in perchloric acid solutions. In other electrolyte media other structures with lower coverage have also been claimed [2] for the self-assembled monolayer (SAM) on Au(1 1 1). Nevertheless, charge determinations for the desorption peak of l-cysteine from Au(1 1 1) in alkaline solutions [2] point to a less compact overlayer. This fact has been corroborated by STM observations of dimer arrangement forming a network-like cluster structure in ammonium acetate electrolytes [2]. Combination of SERS and SHG results of l-cysteine on silver polycrystalline electrodes [3] suggested a potential induced reorientation of the adsorbate. In this case the authors proposed that the positively charged amino group is stabilized by the coadsorption of chloride anions at more positive potentials and pointing toward the surface. Theoretical studies of the structure, the surface bonding, and the energetics of alkanethiols adsorbed on Cu(1 1 1), Ag(1 1 1), and Au(1 1 1) surfaces were performed by means of density func-
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tional theory calculations (DFT) with clusters under low and high coverages by Cometto et al. [4] and previously by Sellers et al. [5]. The adsorption of l-cysteine on Au(1 1 1) and Au(110) has been also investigated by means of (DFT) calculations [6–9]. Felice et al. [6,7] found that thiolate adsorption is favourable through two molecular fragments, the thiol and the amino functional groups, and in the most stable configuration the sulphur headgroup sits at the bridge site between two surface Au atoms. This position is about 0.36 eV more stable than the fcc hollow site. Nazmudtinov et al. [8] have performed DFT calculations to obtain the optimized geometry for the adsorption of four l-cysteine forms: the molecule, the anion, the neutral radical and its zwitterions adsorbed at top, bridge and threefold hollow site of a planar Au(1 1 1) Au12 cluster. They concluded that on top radical adsorption emerges as the best representation of l-cysteine adsorption on Au(1 1 1). In a previous work [10], we have investigated the electrochemical behaviour and non-linear optical properties of l-cysteine on Ag(1 1 1) single crystal electrodes in neutral solutions of potassium perchlorate. Desorption/adsorption (reduction/oxidation) processes have been identified at potentials well below the potential of zero charge (pzc = −0.735 V vs. SCE [11]). The origin of the multiplicity of peaks was still not clear. The aim of the present contribution is to gain a better understanding of the self-assembling process of l-cysteine on Ag(1 1 1). The role of the environment such as pH and electrical field at the interface are investigated. We have employed a combination of experimental and theoretical approaches to study the energetics of different possible structures and conformations that the adsorption of l-cysteine can adopt on Ag(1 1 1) electrodes. 2. Methodology 2.1. Experimental work Aqueous solutions of l-cysteine from Aldrich (>99.9%) were prepared at two different pH: in 10 mM of KClO4 (Merck 99.99%) and 10 mM of KOH (Merck 99.99%). The Ag(1 1 1) single crystal electrodes were treated with the same annealing/cooling procedure described elsewhere [11]. The SHG setup was similar to that used in the previous work [10,12–16]. The incident beam impinging at 45◦ on the electrode surface consisted of the fundamental of a pulsed Nd:YAG laser (Lumonics HY 1200, pulse duration ∼10 ns, frequency: 20 Hz, pulse energy ∼1–2 mJ, spot size ∼3.2 mm2 , wavelength: 1064 nm). The direction of polarization was controlled by a combination of Fresnel/Glan-Thomson polarizers. The polarization of the incident beam was oriented in the “p” direction, i.e., the electrical field of the electromagnetic wave was parallel to the plane of incidence. The reflected beam was analyzed in the same direction. At the start of each measurement sequence the coated crystal was positioned with the 2 1¯ 1¯ plane parallel to the incidence plane (φ = 0). The initial position of the silver single crystal to the incidence plane is very important for comparing the relative phases of covered and bare surfaces. The second harmonic anisotropy measurements as a
function of the position angle φ were performed by rotating the single crystal electrode around an axis perpendicular to the surface from this initial position. A digitally controlled potentiostat (model Zahner 5d) supplied the potential during the in situ SHG measurements. Impedance spectra have been recorded with an EG&G Potentiostat 263A and a Lock-in Amplifier 5210 in a frequency range of 100 mHz to 100 kHz. Ag/AgCl or saturated calomel (SCE) electrodes were employed as reference electrodes, but all potentials are given vs. SCE. 2.2. Theoretical work We have employed the generalized gradient approximation (GGA) in the version of Perdew–Burke–Ernzerhof (PBE) [17] to perform density functional theory (DFT) calculations as implemented in the SIESTA code [18,19]. It uses Troullier–Martins norm-conserving pseudopotentials [20] to represent the nucleus and core electrons of the considered species. The basis set used for the expansion of the Kohn-Sham eigenstates are composed of a set of numerical atomic orbitals including optionally polarization orbitals. Energy shift of 75 meV has been chosen as a compromise between accuracy and computational efficiency. We have taken an energy cut-off of 200 Ry and a double-zeta plus polarization orbital basis set (DZP). All geometries have been optimized until the force on each atom was less than ˚ The ground state energy of the single l-cysteine 0.04 eV/A. ˚ × 30 A ˚ × 30 A) ˚ unit molecule has been calculated using a (30 A cell. We have modelled our system employing repeated supercells consisting of four (1 1 1) layers of silver atoms and a ˚ The fcc structure of the metal has been vacuum width of 20 A. represented through a dense hexagonal packing (ABCA). We have used 12 k-points with respect to the surface unit cell for the Brillouin zone (BZ) and the calculated lattice constant for ˚ We have considered one l-cysteine radisilver was a = 4.19 A. cal adsorbed at one √ √ surface of the slab. Each layer of the slab forms a 3 × 3)R◦ 30 cell in the (1 1 1) plane (see Fig. 1a). Different positions (top, bridge, hollow-fcc and hollow-hcp) of the S headgroup have been investigated as shown in Fig. 1b. We have started the calculation with three different initial configurations for the l-cysteine radical (• S–C H2 –C␣ H–NH2 –COOH, see Fig. 1c–e). All these configurations have a protonated carboxylic group. As a consequence of the energy minimization, some of these structures evolve to the radical zwitterion, with the deprotonated carboxylic group and protonated amine group (• S–CH2 –C␣H–NH3 + –COO− , see below). The Ag–S distance has been allowed to relax only in the direction perpendicular to the metal surface. All other atoms of l-cysteine have been allowed to relax in all directions. The position of the Ag atoms has been kept fixed. The adsorption energy Eads has been calculated from: Eads = Esystem − (Eslab + ECys-Rad )
(1)
where Esys is the total energy of the system, Eslab is the energy of the metal slab without geometry optimization and ECys-Rad is the energy of the relaxed adsorbate in vacuum.
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√ √ Fig. 1. (a) Model of the surface with a ( 3 × 3)R◦ 30 overlayer. The unit cell used for the calculations is shown. (b) Top-view of the three outermost layers of a Ag(1 1 1) surface showing the different adsorption sites considered in the calculations: top, bridge, hollow-fcc and hollow-hcp. Different initial configurations of the radical adsorbed on the bridge position employed for the DFT calculations. Two different C␣ –S–Ag angles 120◦ (c) and 180◦ were considered. For the 180◦ angle two orientations of the carboxylic group were regarded: with the hydrogen pointing down (d) and up (e) relative to the surface plane. Similar conformations for the adsorbate have been employed for the other sites of adsorption. Yellow, red, blue, dark grey and white balls represent atoms of sulphur, oxygen, nitrogen, carbon and hydrogen, respectively. Silver atoms of the substrate are light grey. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of the article.)
3. Experimental results Fig. 2 shows voltammograms obtained with Ag(1 1 1) electrodes in the presence of l-cysteine for two different conditions of pH. According to the ionization constants [21], the predominant species in the KClO4 solution has a zwitterionic structure with no net charge, while in the KOH solution the l-cysteine is predominately anionic with a deprotonated carboxylic group. The rational potential (E–Epzc ) is plotted as x-coordinate. Two potential regions with markedly different behaviour can be distinguished in both media. At potentials lower than the potential of zero charge (pzc) a multiplicity of redox processes are observed. They are better defined in KOH than in KClO4 solutions. Because of the shift into lower overpotentials of the onset of hydrogen evolution reaction in perchlorate solutions, the processes occurring at more negative potentials are masked in neutral solutions. In alkaline media it is possible to scan the potentials up to a limit of 100 mV more negative. Alkanethiols show reductive desorption evidenced by well-defined cathodic peaks in voltammograms [22–25]. The position of these peaks is determined by the molecular structure of the thiol, including its length and functional end groups and the nature of the substrate. Environmental conditions such as pH and the crystal plane of the metallic substrate also influence the position of this peak [22–23]. The multiplicity of peaks observed in
Fig. 2 for l-cysteine indicates that different adsorbates exist on Ag(1 1 1) with different bond energies. In the case of lcysteine adsorbed on Au single crystal surfaces, mainly only one desorption peak has been observed [1,26–28]. The position of this peak on a potential scale relative to the same reference electrode lies at more positive values on Au(1 1 1) than the desorption processes observed on Ag(1 1 1). Similar results have been obtained with alkanethiols [24]. However, the affirmation that this fact evidences a stronger bond between Ag and the sulphur atom of the l-cysteine than in the case of Au is not straightforward. If we consider the electroreduction reaction [24,25]: R–S–M + e− → R–S− + M
(2)
It is evident that the presence of solvent and ions at the interface are necessary to stabilize the thiolate formed during this process. Also during the desorption process, energy must be employed to overcome the Van der Waals and electrostatic interaction between neighbours. The electrical field at the interface plays an important role on all these interactions. Widrig et al. [24] have proposed for alkanethiols that the differences observed between Ag and Au in the position of the desorption peak are due to the different potential of zero charge of both metals. The pzc of gold is also at more positive values [29]. Thus, the electrical
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the Faraday law: Q θ = FQmax
Fig. 2. Cyclic voltammograms obtained for a Ag(1 1 1) electrode at 50 mV/s in 10 mM KClO4 + 150 M l-cysteine in solution (top) and in 10 mM KOH + 150 M l-cysteine (bottom). The dotted lines correspond to the voltammograms in absence of l-cysteine. The cyclic voltammogram in the presence of l-cysteine in solution was recorded after the electrode was stabilized at −0.3 V during 1 h. The charges indicated in the figure were obtained by integration of the current/potential curves discounting the base electrolyte contribution (pzc = −0.735 V vs. SCE).
field which drives the penetration of ions and solvent molecules in the hydrophobic adlayer should be different for these two metals. However, it is not clear in which way the position of the pzc is affected by the presence of the adsorbed layer [30,31]. In contrast to alkanethiols with long chains, cysteine is a small polar molecule and should not hinder the presence of ions or solvent molecules. We return to this point when we discuss the theoretical calculations. A common method to estimate the coverage by alkanethiols is to integrate the current passed during the desorption process [24,25]. In the case of a potential sweep, it is the charge associated with the cathodic peak given by: Q=
Ef
Ei
j dE v
(3)
where ν is the potential scan rate and Ef and Ei are the limits of the potential sweep. The coverage is then obtained according to
(4)
Qmax is the maximal possible charge assuming that the bonding (S–M) occurs on every metal atom of the surface. This procedure contains a series of implicit assumptions. First, it is assumed that the complete monolayer is reduced during the scan. Second, that the reduction process implies the transfer of one electron per adsorbate, i.e., the possibility of a partial charge transfer is neglected. Finally, some approximation must be considered to discount the contribution of the double layer charging. The current passing in the absence of adlayer is usually subtracted to take into account this latter process. However, the presence of l-cysteine at the interface can produce a shift of the potential of zero charge [30,31], and so an extra charge for the rearrangement of the double layer can be required. These limitations on the use of reductive peaks to calculate the thiols coverage has been pointed out by Schneider and Buttry [32]. Recently, a new method to measure packing densities of self-assembled monolayers of thiols has been proposed [33,34]. The method relies on chronocoulometry to measure the charge density at the SAM covered metal electrode surface. It is also shown [33] that the charge numbers per adsorbed molecule depend on the electrode potential and may assume values smaller than the number of electron participating in the reductive desorption step. However, the application of this method to the l-cysteine system is more complicated. It is difficult to reach the same initial conditions for each adsorption potential. It takes a long time for the film to recover from the desorption step. We are working to improve these measurements. Another method to estimate the coverage is the determination of the charge during the adsorption process. By keeping the potential at a constant value, l-cysteine is injected in the solution and the transient current integrated. We have employed this methodology in a previous work [10]. On the other hand, the reproducibility is low and the difficulties with the accounting of double layer charging contributions also appear with this method. In perchlorate solutions the average adsorption charge obtained was about 30 C cm−2 [10]. However, Widrig et al. have suggested that the surface of Ag substrate could be initially partially oxidized and the adsorption process should occur at some places as follows: Ag(OH) + R–SH → AgS–R + HO− + H+
(5)
Thus, we employ as a first approximation the integration of the current passed during the desorption process by applying a potential sweep. Nevertheless, we keep in mind, that the values obtained by this procedure are probably an overestimation of the real coverages. The charges involved in the cathodic potential sweep are higher in alkaline than in neutral solutions and not totally recovered during the reverse positive sweep. However, due to the earlier onset of the hydrogen evolution reaction in neutral solution, the more effective way to estimate the coverage of SAMs is from the reductive desorption charge in basic medium as suggested by Wong and Porter [35]. The potential sweep can
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Fig. 3. (Top) Capacity vs. potential plots at Ag(1 1 1) in 10 mM KClO4 . Single-frequency (20 Hz) differential curves obtained by potentiodynamic conditions (sweep rate: 20 mV/s) with 150 M l-cysteine during the negative (full line) and positive (dashed line) scans. The dotted line was obtained in the absence of l-cysteine. Also shown are calculated capacities from the complex permittivity plots displayed in the bottom, obtained under stationary conditions (waiting time at each potential: 10 min) with 150 M l-cysteine. (Bottom, left) Complex permittivity plots obtained at different potentials below the pzc: −0.75 V (open squares), −0.85 V (closed circles) and −1.0 V (closed up-triangles). (Bottom, right) Complex permittivity plots obtained at different potentials above the pzc: −0.75 V (open squares), −0.60 V (open circles), −0.30 V (open up-triangles), and −0.20 V (open down-triangles). All stationary measurements were started at the more positive potential. Frequency range: 100 mHz–100 kHz. Amplitude of the superimposed potential: 10 mV.
be extended to more negative potentials in the latter case. The lower values obtained during the reverse scan indicate that the readsorption process shows some irreversibility. Effectively, in a previous work [10] we have observed that the adsorption process involves a certain induction time. The value of 67 C cm−2 obtained √ in√KOH correlates well with the coverage of 0.33 for the ( 3 × 3)R◦ 30 structure. This structure has been suggested for the adsorption of l-cysteine on Au(1 1 1) in perchlorate solutions according to the analysis of STM images [1]. At potentials more positive than the pzc the current is substantially lower than in the absence of l-cysteine. Dynamic capacity curves obtained by superimposing an alternating potential of 20 Hz with a potential sweep at 20 mV/s have been compared with stationary values obtained from impedance spectroscopy measurements at constant potentials. Figs. 3 top and 4 top show the results for perchlorate and alkaline solutions, respectively. Particularly in perchlorate medium, a significant hysteresis between positive and negative dynamic sweeps at potentials below the pzc is observed. This potential range corresponds to the region where the multiple redox processes appear in the voltammograms (see Fig. 2). At least two peaks are observed in the dynamic values of the capacity in KOH solutions. Because of our interest to ana-
lyze the effect of l-cysteine on the interfacial capacitance, the dielectric complex permittivity (ε = (jωZ)−1 ) representation of the data is more convenient than the typically employed Nyquist plots with impedance Z [36]. The plots for the potential range below the pzc are shown in Figs. 3 and 4 bottom, left and for the potential range above the pzc in Figs. 3 and 4 bottom, right. The corresponding plots obtained without the presence of l-cysteine (not shown) give typical half circles which can be described by a simple model consisting of the double layer capacitance connected in series with the electrolyte resistance. A more complicated equivalent circuit including constant phase elements gives the same trend. Thus the stationary capacity values can be estimated from the extrapolation to the real axis. When l-cysteine is present at the interface, also capacitive or pseudocapacitive half circles are obtained. Nevertheless, a bend of the curves at lower frequency is observed, denoting the appearance of a second semicircle, which is more evident in alkaline media. Effectively, two different processes at potentials below the pzc with different time constants can be clearly distinguished in the complex permittivity plots with this electrolyte. The surface is partially covered by the film at this potential range. Then, this behaviour can be described by an equivalent circuit consisting
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Fig. 4. (Top) Capacity vs. potential plots for Ag(1 1 1) in 10 mM KOH. Single-frequency (20 Hz) differential curves obtained by potentiodynamic conditions (sweep rate: 20 mV/s) with 150 M l-cysteine during the negative (full line) and positive (dashed line) scans. The dotted line was obtained in the absence of l-cysteine. Also shown are calculated capacities from the complex permittivity plots displayed in the bottom, obtained under stationary conditions (waiting time at each potential: 10 min) with 150 M l-cysteine. (Bottom, left) Complex permittivity plots obtained at different potentials below the pzc: −0.75 V (open squares), −0.90 V (closed circles), −1.0 V (closed up-triangles) and −1.1 V (closed down-triangles). (Bottom, right) Complex permittivity plots obtained at different potentials above the pzc: −0.75 V (open squares), −0.60 V (open circles) and −0.50 V (open up-triangles). All stationary measurements were started at the more positive potential. Frequency range: 100 mHz–100 kHz. Amplitude of the superimposed potential: 10 mV.
of two parallel branches of resistances connected in series with capacitances corresponding to either covered or bare surface. At potentials higher than the pzc the capacity decreases markedly to values much lower than those observed in the absence of l-cysteine, and dynamic and stationary values coincide. This result correlates very well with the diminution of the current in the voltammograms (see Fig. 2). The adsorption of l-cysteine produces the displacement of water molecules from the interface, such as suggested by Laredo et al. [34], and consequently the decrease on the capacity. In a previous work [10] we have found that the second-order polarizability of the interface increases remarkably when lcysteine is added at a constant potential higher than the pzc. This effect has been also observed with other thiols films [13–16] and can be attributed to the effect of the sulphur–silver bond which produces a change in the electronic properties of the interface. Fig. 5 shows 3D-plots of the stationary second harmonic signal when the single crystal electrode is rotated around an axis perpendicular to the surface for different applied poten-
tials in the presence (Fig. 5a) and in the absence of l-cysteine (Fig. 5b) in perchlorate solution. Similar results are obtained in alkaline solutions. The features of the anisotropy showing three or six peaks reveal a C3v symmetry of the system as previously √ √observed. This result supports also the existence of a ( 3 × 3)R◦ 30 overlayer. Other geometry arrangements with different rotation√angles √ should reduce the surface symmetry. For example, a ( 7 × 7)R◦ 10 overlayer, which is frequently proposed for the adsorption of thiols at silver surfaces [22,23] produces the loss of the reflexion planes and the symmetry reduces to C3 . The second harmonic signal depends strongly on the potential. The change in the number of maxima between six and three, as also their relative positions, indicate phase changes between the isotropic and anisotropic contributions produced by resonances. Particularly, a sudden change has been observed near the pzc which separates two regions of different behaviour. At potential below the pzc six peaks have been observed when the l-cysteine was present and the intensity is comparable or somewhat lower than that of the bare surface. Near the pzc the intensity of the signal increases considerably and only three
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Fig. 5. 3D-plots of the stationary second harmonic response as a function of the potential and position of the single crystal electrode. φ = 0 corresponds to the orientation of the 2 1¯ 1¯ plane parallel to the incidence plane. The plots at the left correspond to the response in the presence and those at the right in the absence of l-cysteine in solution. At the bottom are shown the contour graphs projected on the plane formed by the potential and rotation angle coordinates. The arrows indicate the pzc for the bare surface.
peaks are observed above the pzc. This trend with the potential can be better seen in Fig. 6. The stationary SH intensity at two different positions of the crystal substrate relative to the incidence plane is plotted again the potential on the left. The corresponding dynamic response at the position φ = 0 is shown on the right side. While in the case of the bare surface for both positions an almost smooth increase of the SH signal with the potential is observed, in the presence of l-cysteine the SH signal increases suddenly. Brolo et al. [3] have observed with l-cysteine adsorbed on polycrystalline silver electrodes, that near the pzc a peak appears in the SH signal. They ascribed this result to changes in the electronic characteristics of the interface due to a reorientation of the
amino group of the adsorbate. They suggested that this variation of the conformation of the adsorbate is produced by desorption of chloride anions at more negative potentials. However, it is difficult to understand, why a transition between two different structures should give a peak instead of a step. Moreover, our experimental conditions are different: we are working with a single crystal surface, in the absence of chloride anions, and the wavelength of the incident beam is longer (1064 nm instead of 800 nm). Probably in our case, a transition between two conformations also occurred, but with different structures. At potentials below the pzc an adsorbate with the amino group oriented to the negatively charged substrate can be stabilized, while at poten-
Fig. 6. Dependence of the second harmonic response on the potential. (Left) Stationary signal at two different positions of the single crystal electrodes with respect to the incidence plane. (Right) Dynamic signal during a potential sweep at 50 mV/s from −0.5 V to −1.2 V and back to −0.5 V vs. SCE.
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tials above the pzc a conformation with the carboxylic group pointing to the positively charged surface is energetically more favourable. We will discuss in more detail these possibilities in the next theoretical section. Once more an hysteresis between forward and backward potential scan is observed. Comparing the dynamic responses obtained from voltammograms, capacity curves and SH signals, we can draw the conclusion that the desorption process occurs almost fast, while the readsorption during the positive scan show a delay. 4. Theoretical results In order to complement the experimental investigations, we have performed theoretical calculations of the energetics and possible structures of the species formed by the adsorption of l-cysteine on Ag(1 1 1). We have assumed that the bond occurs through the S headgroup as is the case for thiols compounds on silver and gold [22–23]. We have to distinguish between the different definitions of adsorption energy, binding energy, dissociation energy and the contributions from solvation in order to compare the results with those found in the literature and understand the energetics of the investigated systems. The adsorption energy defined by Eq. (1) corresponds to the bond formation between l-cysteine radical and the surface of the metal: Cys-S• (vacuum) + Ag(1 1 1) → Cys-S–Ag(1 1 1)
(6)
The values calculated by Felice et al. [6,7] as adsorption energy of cysteine on gold surfaces are different from our definition given in (6) and are for the reactions on different adsorption sites of Au(1 1 1): 2Cys-S–H (vacuum) + 2Au(1 1 1)br → Cys-S–2Au(1 1 1)br + H2
H = −0.841 eV
(7)
2Cys-S–H (vacuum) + 2Au(1 1 1)fcc → Cys-S–2Au(1 1 1)fcc + H2
ΔH = −0.481 eV
(8)
The dissociation energy for cysteine given by Felice et al. [6] is: Cys-S–H (vacuum) → Cys-S• (vacuum) + H• (vacuum) ΔH = 3.885 eV
(9)
Combining (7)–(9) and taking into account that the dissociation energy for a hydrogen molecule is about 4.52 eV, the corresponding adsorption values as defined by reaction (6) are −2.046 and −1.686 eV at bridge and fcc sites of Au(1 1 1), respectively. Fig. 7 shows the adsorption energy of different species which are√stable√at a coverage of 0.33 on a Ag(1 1 1) surface and form a ( 3 × 3)R◦ 30 overlayer. These values correspond to local minima obtained by the relaxation of the starting conformations shown previously in Fig. 1 to an optimized geometry at different surface sites. The line indicates the bond energy for Ag–S. The first insight of this plot is the existence of several different possibilities of structures with local minimum energies depending
Fig. 7. Adsorption energy calculated according to eq. (6) for different optimized geometries of l-cysteine at different sites of an Ag(1 1 1) surface. The dashed line indicates the bond enthalpy for the gaseous diatomic species of Ag–S.
on the starting configuration. When the final state is an adsorbed radical, the stability increase from the top, to bridge, to hollow (fcc and hcp) sites. A similar trend has been found for the adsorption of small thiols on Ag(1 1 1) and Au(1 1 1) [4]. We return now to the discussion about the relative position of the reduction peaks observed experimentally with silver and gold. We have observed that the adsorption energy defined by (1) and (6) strongly depends on the coverage. Then, we have performed also calculations at a lower coverage in order to compare with the results of Felice et al. mentioned above. The resulting values, −3.04 and −3.11 eV for silver bridge and fcc, respectively, indicate that the bonding of l-cysteine is much stronger with Ag than with Au. However, it is difficult to compare absolute values of computational calculations performed under different conditions. As a test, we have calculated the adsorption of the radical sulphur (S• ) on Au(1 1 1) and Ag(1 1 1) for different coverages. Thus, these results reflect directly the contribution of the bond S–Metal to the adsorption energy. We have observed a difference of about 0.3–0.4 eV between both metals, being more favourable the adsorption on silver. These values correlate very well with the results of Widrig et al. [24]. They have compared the peak potential for the reductive desorption of thiols with different chain-length n. The intercept of the plot of Epeak vs. n is 0.3 V more negative for desorption from silver compare to gold surfaces. However, the most important result is that in some special cases we have found that the geometry relaxation leads to the formation of the zwitterions via proton transfer between two adjacent adsorbates as can be observed in Fig. 8. The zwitterions conformation is about 0.7–0.8 eV energetically more favourable than the radical forms. Contrary to the radicals, the difference in energy between the zwitterions at the different sites is not higher than 0.06 eV for this form. We are showing as an example in Fig. 8 an on top and lateral view of the relaxation process from the radical form (c) of Fig. 1 to the zwitterion. On the bottom is also shown the evolution of the energy as a function of the optimization steps. As can be observed from the sequence between the initial and final stabilized state, the adsorbate stands up and a torsion of the carboxylic and amino group occurs. In this way the proton of the carboxylic group approaches the amino group of the neighbouring adsorbate. When they adopt a favourable
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Fig. 8. Optimization process for the adsorbate which leads to the formation of a zwitterion. Top-view of the initial and final state (upper part). Lateral view showing three conformations adopted by the adsorbate during the optimization process corresponding to the three different energy states as indicated in the bottom.
geometry the proton is transferred. It must be kept in mind that these DFT calculations are carried out in vacuum. Thus, only at high coverages, where the adsorbates are sufficiently close to each another, proton transfer can occur, leading to the formation of zwitterions. In real electrochemical systems, water effects are crucial. It is expected that the adsorbed zwitterions is still more stable than in vacuum and the presence of water molecules can facilitate the proton transfer process also at lower coverages. Also the interactions with the solvent can destabilize the adsorbate at negative potentials and produce its desorption as experimentally observed. In the present state of these investigations, we have not included solvent effects. We put the focus of our analysis on the contributions of adsorbate–adsorbate and adsorbate–substrate interactions. The pzc of Ag(1 1 1) lies about 1 V more negative than the pzc of the unreconstructed Au(1 1 1). The reduction processes of l-cysteine on Ag(1 1 1) occur at potentials about 0.35 V more negative referred to the same reference electrode. Thus, the electrical field at the interface are much more negative at Au(1 1 1) than Ag(1 1 1) assuming that the presence of the adsorbed film does not produce an extra shift of the pzc.
Then, another interesting aspect to be analyzed theoretically is the effect of the presence of an electrical field. In the case of alkane thiols, previous calculations [4] have shown that for negative fields there is a decrease in the stability of the adsorbate, while for positive fields the adsorbate is stabilized. Fig. 9 shows the changes in the total energy (left) and the adsorption energy defined by (1) (right) produced by the application of positive or negative fields when the zwitterion is adsorbed on top sites. Independent of the sign of the field the zwitterion is always stabilized. These results are different from those obtained with cluster calculations for short alkanethiols. However, the presence of the two groups, carboxylic and amino in the case of l-cysteine, provides different possibilities for the conformation of the adsorbate. The zwitterion is a neutral species but with two centers of opposite charge. The interaction of the field with these two groups in a complementary way can explain its stabilization. Preliminary results of a comparative analysis of the effect of electrical field on different species of adsorbed l-cysteine confirmed this statement [37]. In order to analyze this effect, we consider as a first approximation electrostatic interactions and expand the total energy as a second-order polynomial of the
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Fig. 9. Effect of an external electric field on the adsorption energy of the zwitterion resulting from an initial radical positioned at a top site with an initial C␣ –S–Ag angle of 120◦ . The energy for the relaxed zwitterion at zero electric field has been taken as reference. The full squares correspond to the conformation of the zwitterion relaxed in the absence of electric field. The open squares are for the zwitterion relaxed in the presence of the electric field.
strength of the electric field E: 1 2 − αE Energy(tot) = Energy(tot)E=0 − μperm E 2
(10)
where the permanent dipole moment μperm is due to zero bias charge transfer, and α is the polarizability of the interface, which is related to the induced dipole moment: α = μind E. The fit ˚ A (shown also in Fig. 9) gives a value of μperm = 0.0818 e A. similar value has been obtained with cluster calculations [37]. This is a rather small positive value, and it is due to the fact that the carboxylic group is closer to the metal surface than the ˚ ; dNH3+ /Ag = 5.50 A ˚ ). Thus amino group (dCOO− /Ag = 5.09 A a dipole occurs in the z direction pointing away from the surface. The existence of a non-zero permanent dipole causes the center of the curve in Fig. 9 (right) to be displaced from zero electrical field strength. The curve shown in Fig. 9 was obtained with a fixed geometry for the zwitterion, i.e., the same conformation as at zero field. If the zwitterion is allowed to relax, the more favourable geometry implies a slightly closer amino group to the surface for negative fields and a slightly closer carboxylic group to the metal for a positive field. The energy for these relaxed conformations decreases about 0.4 eV as can be observed from the open squares in Fig. 9. 5. Conclusions We have investigated the l-cysteine adsorbed on Ag(1 1 1) electrodes under different conditions. We have employed experimental and theoretical approaches to obtain a better understanding of the adsorbed layer. Two different potential regions of different behaviour have been observed at both pH values, neutral and alkaline. Below the pzc a multiplicity of redox processes have been observed, which are more clearly defined in KOH solutions. An estimation of the coverage from charge measurements together with the second harmonic response √ √showing a C3v symmetry for the interface pointed to a ( 3 × 3)R◦ 30 overlayer as has been proposed by Dakkouri et al. for Au(1 1 1) [1]. At potentials higher than the pzc the adlayer exhibits the behaviour of an insulator. The theoretical calculations show
a variety of different structures with local adsorption energy minima. Particularly, under special initial conditions different structures with a zwitterionic character have been found, which are more stable than radical adsorbates. These different structures could be the origin of the multiplicity of redox processes observed at low potentials. The presence of an external field produces the stabilization of the zwitterion for both directions of the electrical field. The amino and carboxylic groups change their positions relative to the substrate by the presence of an electrical field in a way that stabilizes the adsorbate. At this stage of the investigation, we have not included the effect of the solvent in the theoretical calculations. We have focus on the contributions of the adsorbate–adsorbate and adsorbate–substrate interactions. Obviously, for such a hydrophilic molecule like lcysteine it should be crucial to include the presence of water. Work is in progress to investigate these effects [37]. Finally, we have demonstrated that the combination of different experimental methods with theoretical calculations provides an effective tool to investigate these complex systems. Acknowledgements Financial support from CONICET (Argentina) and SeCyTDAAD is gratefully acknowledged. We thank Prof. Wolfgang Schmickler, Prof. Renat Nazmudtinov and Prof. Leiva for valuable and fruitful discussions. E.S. participates in the COST D36 Action project Structure and Reactivity Relationship of nanoarrays. References [1] A.S. Dakkouri, D.M. Kolb, R. Edelstein-Shima, D. Mandler, Langmuir 12 (1996) 2849. [2] J. Zhang, Q. Chi, J.U. Nielsen, E.P. Friis, J.E.T. Andersen, J. Ulstrup, Langmuir 16 (2000) 7229. [3] A.G. Brolo, P. Germain, G. Hager, J. Phys. Chem. B 106 (2002) 5982. [4] F.P. Cometto, P. Paredes-Olivera, V.A. Macagno, E.M. Patrito, J. Phys. Chem. B 109 (2005) 21737. [5] H. Sellers, A. Ulman, Y. Shnidman, J.E. Eilers, J. Am. Chem. Soc. 115 (1993) 9389.
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