Probing the Organization of Charged Self-Assembled Monolayers by

J. Phys. Chem. C 2009, 113, 2405–2416

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Probing the Organization of Charged Self-Assembled Monolayers by Using the Effects of pH, Time, Electrolyte Anion, and Temperature, on the Charge Transfer of Electroactive Probes Jose´ M. Campinã,* Ana Martins,†,† and Fernando Silva‡ Centro de InVestigaçaõ em Quı´mica, Departamento de Quı´mica, Faculdade de Cieˆncias, UniVersidade do Porto, Rua do Campo Alegre 687, 4169-007 Porto, Portugal ReceiVed: September 17, 2008; ReVised Manuscript ReceiVed: October 29, 2008

The effect of the solution pH and the electrolyte anion on the capacitive behavior observed for a self-assembled monolayer (SAM) of the ionizable 11-amino-1-undecanethiol (AUT) deposited on polycrystalline Au was checked by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) using pure electrolyte solutions. The double layer capacitance (Cdl) decreased with increasing the pH and was shown to be dependent on the electrolyte anion, suggesting that the equilibria controlling the degree of charge at the monolayer may affect the order of the thiol chains at the outermost part of the film. The effect of these parameters, plus the temperature and the time of electrode-solution contact, were also checked by monitoring the electron transfer (ET) process of electroactive Ru(NH3)6+3 and Fe(CN)6-3 redox probes in solution. The results obtained at short contact times showed that while the structure of the monolayer was not shown to affect significantly the charge transfer process for attractive probe-end group interactions, Fe(CN)6-3 probes, the process seemed to be controlled by the probe access into the layer when they were repulsive, Ru(NH3)6+3. The results indicated that the anion, the pH, and the temperature can affect the initial structure of the monolayer. The evolution observed in the redox response when the contact time was increased was associated with the establishment of a disordering process affecting the thiol organic chains the kinetics of which was determined from the impedance data. An apparent rate constant (k) of the order of 10-4 s-1 was obtained, agreeing reasonably well with the previously reported value. Additional results obtained for the unionizable 1-DT film suggest that the access of both probes into the monolayer was the limiting step, which was already expected from the absence of electrostatic interactions. The rate constant fell within the same order of magnitude of that found for AUT. 1. Introduction Self-assembly of thiols and derivatives on solid substrates (and/or nanoparticles) has become a usual step during the preparation of nanostructured and functionalized materials with new promising properties for catalysis, sensing, molecular electronics, corrosion protection, cell adhesion, and many other applications. The early works that characterized the structure and the electron-transfer (ET) properties of self-assembled monolayers (SAMs)1-4 stated the easy and fast formation of well-packed, compact, rigid, and electrical insulating structures of long chain n-alkanethiols (n > 9) on Au substrates. A (3 × 3) R30° structure was accepted for the films formed by n-alkanethiols on Au(111) from the data obtained in ultra high vacuum (UHV) and ambient conditions by low energy atom diffraction (LEAD), grazing-incidence X-ray diffraction (GIXD), infrared spectroscopy (IR), and scanning tunneling microscopy (STM).6-15 It seems to be widely accepted that the high degree of structural order observed for this archetypal case is kept in different experimental conditions and is extended to other derivative thiol molecules which stem from these. However new evidence reported in the last years indicates that the structure of such films could be slightly more disordered * To whom correspondence should be addressed. E-mail: [email protected]. Tel: 00351960081254. Fax: 00351220402659. † E-mail: [email protected]. Tel: 00351220402643. ‡ E-mail: [email protected]. Tel: 00351220402613.

than this prototypical structure without the need to call for the existence of a significant amount of extrinsic pinholes (those related with the characteristics of the substrate and the purity of the adsorbate solutions: pit-like defects, step defects, defects at grain boundaries, etc). On one hand, the crystalline order of these films can be affected by the substitution of the terminal methyl by other bulky or charged endgroups. Stirling et al.16 and Schreiber, in an excellent review,17 claimed that the (3 × 3) R30° structure can be achieved unless it was sterically prohibited and/or disrupted by endgroup-endgroup interactions. These conclusions are based on the evidence found, among others, by Himmel18,19 and Allara et al.20 using near-edge X-ray absorption fine structure spectroscopy (NEXAFS) and X-ray photoelectron spectroscopy (XPS) to study the SAMs formed by hydroxyl (-OH), carboxylic acid (-COOH), and nitrile (-CN) terminated alkanethiols on Au. Tilt angles between 39 and 44° were found there, in contrast with the 28° found by Chidsey2 for unsubstituted thiols, which was in conflict with the maximum packing given by the van der Waals radii if the well packed (3 × 3) R30° structure is assumed. Additional NMR evidence supporting this view has been reported by Badia et al.21 On the other hand, it seems that even the films formed by unsubstituted n-alkanethiols can loose its crystalline structural order when they are put in contact with a liquid phase. Ex situ as well as in situ STM and AFM studies have visualized ordered (3 × 3) R30° domains22-28 but in situ IR indicated some conformational disorder near the alkyl chain terminus.29-32

10.1021/jp808278r CCC: $40.75  2009 American Chemical Society Published on Web 01/15/2009

2406 J. Phys. Chem. C, Vol. 113, No. 6, 2009 Capitań et al. reported recently how the crystalline order of a monolayer of 1-nonanethiol on Au(111) (represented by the diffraction peak observed at the 1/3,1/3,1 reciprocal-space position) in contact with a continuous flow of helium was disrupted after 30 min contact with an ethanol solution.33 The crystallographic order was shown to recover after removing the ethanol and flushing the surface again with a helium flow upon 8 h. The results suggested that, although the thiol heads can be periodically arranged on the substrate, the alkyl chains do not display long-range order, especially along the surface normal where the scanning probe techniques can not supply much information. The chain-chain interactions (van der Waals, dipole-dipole) are less energetic than the S-Au bond (∼160 kJ/mol) which turns the structure sensitive to temperature. This fact and other dynamic phenomena as mass loading and/or liquid dragging, allow for the establishment of a variety of thermally driven gauche conformations and tilt-order phase transitions even at room temperature. It is due to this chain dynamic behavior that the films have been often described in the literature as liquid-like structures at high enough temperatures.3-5 In our previous work, we characterized a SAM of 11-amino1-undecanthiol (AUT) on polycrystalline Au.34,35 The ET properties through the monolayer in the presence of highly charged electroactive species were governed by the electrostatic interactions established in the SAM-solution interface between the charged amino groups and these probes. A mechanism of selective ion permeation into the monolayer allowed by the slightly disordered outer structure of the film and driven by the electrostatic interactions explained the ET properties observed, initially, in the presence of such species. The binding of the electrolyte anion to the charged endgroups was proposed to explain the shift registered in those properties with the increase of the electrode-solution contact time. However it seems that the kinetics of such a process should be much faster than those the observed there. In this work, the initial disorder in the outermost part of an AUT film has been investigated by cyclic voltammetry (CV) and electrochemical impedance spectroscopy (EIS) at different pH. In order to check our previous hypothesis and to evaluate the existence of order-disorder chain phase transitions (analogous to that observed by Capitań et al.) that could explain the shift found in the ET properties, we have studied this phenomenon in the presence of different electrolyte anions and at different temperatures. The results are contrasted with the ones obtained using a monolayer of the unsubsituted 1-dodecanethiol (1-DT). 2. Experimental Section 2.1. Chemicals and Solutions. AUT (Dojindo Laboratories), 1-DT (Aldrich, >98%), KCl (Merck, p.a.), NaOH (Merck Suprapur), HClO4 (Merck Suprapur; 70%), NaClO4 · H2O (Merck p.a.), NaF (Merck, suprapur), K3Fe(CN)6 (Merck, p.a.), Ru(NH3)6Cl3 (Aldrich, 98%), ethanol (Aga, 96% vol.), N2 (N45, Air Liquid), and all other chemicals were analytical grade and were used as received. An AUT 1 mM and a 1-DT 1 mM solutions in pure ethanol were used for the preparation of the AUT and the 1-DT modified electrodes. A group of solutions constituted by: a NaClO4 0.3 M, a NaClO4 0.3 M +KH2PO4 10 mM, and a NaClO4 0.3 M + Na2HPO4 10 mM + KH2PO4 0.15 mM, were prepared in order to study the effect of pH in the insulating properties of the layer. A pH value of 5.85 was obtained for the first while the other two were fine adjusted to 3 and 9 by adding droplets of HClO4 0.1 M and NaOH 0.5 M, respectively. A Methrom 654 pH meter

Campinã et al.

Figure 1. CVs obtained for a 24 h AUT-modified electrode in the different NaClO4 0.3 M solutions: Not buffered (blue), pH 3 (red), and pH 9 (green). The profile for the bare Au in the not buffered solution (black) is shown for comparison. Scan rate: 50 mV s-1

was used for pH measurements in all cases. The pKa of a selfassembled monolayer of AUT on Au has been estimated as ≈7.5 from the results reported by Degefa et al. (Figure 1.I in 36). Consequently we should have, at least, a partially charged monolayer for the not buffered solution, a fully charged monolayer at pH 3, and a neutrally charged at pH 9. Other solutions, KCl 0.2 M, NaF 0.2 M, and NaNO3 0.3 M, were prepared to elucidate the electrolyte anion role in the capacitive properties of the film. The pH values obtained for these solutions were 5.42, 7.16, and 5.33 respectively. Other solutions, KCl 0.2 M + Ru(NH3)6Cl3 2 mM; NaF 0.2 M + Ru(NH3)6Cl3 2 mM; NaNO3 0.3 M + Ru(NH3)6Cl3 2 mM; KCl 0.2 M + K3Fe(CN)6 2 mM; NaF 0.2 M + K3Fe(CN)6 2 mM; and NaNO3 0.3 M + K3Fe(CN)6 2 mM (showing pH values of 5.04, 7.01, 4.64, 5.46, 7.57, and 5.72, respectively) were used to study the effect of the electrolyte anion and the temperature in the electron-transfer properties through the film. All of the aqueous solutions were prepared in ultrapure water from a Milli-RO 3 Plus, coupled with a Milli-Q water purification system, with a resistivity value of 18 MΩ · cm. 2.2. SAM Preparation. AUT and 1-DT films were prepared on a gold polycrystalline rod-shaped electrode, in order to apply electrochemical methods. Following the procedure described in ref 35, first the electrode was cycled up to +1.3 V in HClO4 0.1 M for more than 2 h in order to promote the anodic removal of the residual molecules on surface. Then it was annealed in a butane-air flame, according to the Clavilier method,37 with the purpose of achieving the cleanest and smoothest possible surface. This method was applied until a CV matching the one for the bare Au fingerprint in HClO4 was obtained (see the Supporting Information). After this treatment, the electrode was rinsed with ultrapure water, later with ethanol, and then quickly immersed in the corresponding thiol solutions. It was kept there for 24 h in order to obtain the best possible organized and compact film.34 Finally, it was removed and copiously rinsed in ethanol and ultrapure water. This procedure was conducted prior to every electrochemical experiment. 2.3. Cyclic Voltammetry. Experiments were performed in a three-electrode cell under N2 flow using a Voltalab PGZ301 Potentiostat (from Radiometer Analytical) controlled by VoltaMaster 4 software. A calomel saturated electrode (Hg/Hg2Cl2/ NaCl sat., SCE), in a separated compartment containing the electrolyte (and connected by a salt bridge coupled with a luggin

Permeation of SAMs by Electroactive Probes

J. Phys. Chem. C, Vol. 113, No. 6, 2009 2407

tip to the main cell), and a helix-shaped gold wire were used as reference electrode and counter electrode respectively. A bulk rod-shaped gold polycrystalline electrode with a surface area of 0.11 ( 0.05 cm2, estimated by the method of the electrochemical reduction of an AuO monolayer,38-41 was used as working electrode in all cases. The contact between the electrode surface and the solution was made by means of the hanging meniscus method.42 Unless otherwise indicated, CV was performed at 50 mV/s scan rate and at room temperature. A Grant LTD 6 thermostat was used to control the solution temperature in those cases by pumping a water stream into the main cell jacket that, at the desired temperature, gets back to the device at a constant rate. 2.4. Electrochemical Impedance Spectroscopy. Impedance spectra were registered using a sine wave of 10 mV amplitude, in the 10 kHz to 500 mHz frequency range, using the same Voltalab PGZ301 potentiostat at room temperature unless otherwise indicated. The data obtained were fitted using the Eco Chemie Frequency Response Analyzer (FRA) 4.9 software to: (a) A RQ equivalent circuit for experiments in supporting electrolyte (i.e., a resistor R, electrolyte resistance, in series with a constant phase element; CPE); (b) a Randles-type circuit in the presence of electroactive species.43 The CPE is a powerlaw dependent interfacial capacity accounting for the topological imperfections originated by the different crystal facets, and the surface roughness. The admittance of a CPE is commonly described as

YCPE)Y o(iω)n

(1)

In our previous work it was shown how the values obtained from the fittings using FRA should be treated to obtain Y° in their main units of sn Ω-1 (as FRA considers a different admittance definition).34 As described there, the double-layer capacitance (Cdl) can be obtained from the admittance values using

Cdl)(Y °)1/n(RΩ)1-n/n

(2)

Cdl ) Y °(ωmax)n-1

(3)

Equation 2 is useful in the case of an ideal polarized electrode and was employed to obtain the capacitance in electrolyte solution.44 For the case of a depressed semicircle (as observed in the presence of electroactive species), eq 3 is most appropriate.45 RΩ represents the solution resistance and ωmax the frequency at which the imaginary component of the impedance reaches a maximum in the semicircle region of the Nyquist plot. The capacitance values obtained using eqs 2 and 3 were divided by the working electrode area (A). The final capacitance values presented in this work consist of the average of 5 independent measurements (with their corresponding standard deviation) performed at each potential. The capacitance data were used for the estimation of the film thickness considering the Helmholtz model of electrical doublelayer. According to this model, the double-layer is described as an ideal parallel plate condenser34

C ) Aεε0/d

(4)

A corresponds to the plate geometrical surface area, ε0 is the free space permittivity constant (8.84 × 10-12 F/m), d is the plate distance, and ε is the dielectric constant for the medium. Under the reasonable assumption that the dielectric constant for AUT films is not very different from 11-mercaptoundecanoic acid (MUA) films, a value of ε ) 3.946 was used in eq 4 to estimate the thickness (d) of the AUT film.

Several works in the literature47,48 have shown that the apparent rate for electron transfer across a SAM monolayer (kapp) can be estimated from the value of the transfer resistance (R2) values resulting from the fittings of the impedance data as follows:

kapp)[RT/(nF)2AC](1/R2) ) k(1/R2)

(5)

R is the ideal gas constant, T the temperature, F the Faraday′s constant, n the number of electrons transferred in the redox reaction (one for the probes used in this work), A the area of the electrode, C the concentration of the electroactive probes, and R2 the charge transfer resistance provided, mostly, by the film on surface. This rate constant is given in units of cm s-1 always that A is in cm2, C in mol/cm3, and R2 in Ω. 3. Results and Discussion 3.1. Studies in Pure Electrolyte. 3.1.1. pH Effect. With the purpose of investigating the influence of the establishment of amino-amino repulsive interactions on the chain order at the outermost part of the film, the effect of the pH has been studied in the absence of electroactive species using pure electrolyte solutions. Figure 1 shows the cyclic voltammograms (CVs) obtained for a 24 h AUT-modified electrode (prepared as described in the experimental section) immersed in NaClO4 0.3 M electrolyte solutions at different pHs. The potential window was kept, in all cases, within (0.4 V in order to ensure the stability of the film.34 The comparison of these profiles with that obtained for the bare Au electrode in a not buffered solution (black line) showed the great insulating effect of the film. Significant differences between the voltammetric profiles obtained for the AUT-modified electrode were only observed at pH)3. Figure 2 presents the EIS data obtained at 0 V in the same solutions. The stronger capacitor character of the AUT-covered electrode was also clearly deduced from the admittance (YR, -Yi) plots reported in Figure 2A (depressed semicircles) and the Bode and Bode phase plots in Figure 2B (log Z for the AUT-modified electrodes is higher than for the bare Au in the whole frequency domain and the phase angle (Ψ) remains closer to 90° in a wider range of frequencies: values higher than 88° are considered in practical terms to correspond to pure capacitor behavior49). Figure 2A shows that the pH induced variations on the capacitive behavior, which are also suggested after the close observation of the CVs, are enhanced in the admittance plots (semicircle radius depending on the electrolyte pH). The impedance data were fitted to an RQ equivalent circuit. The double layer capacitance (Cdl) (obtained as described in the experimental section) for the AUT-modified electrode at the different pHs are reported in Table 1. These capacitance data were used for the estimation of the film thickness (d) according to eq 4 (Table 1). CV, EIS, and the derived Cdl, concur to support that the film capacity is pH dependent, being lower at high pH values. Decreasing pH makes the protonation of the end groups increase, thus, increasing the charge density at the surface of the monolayer. This vision agrees with the theoretical treatment developed by Smith et al.23 and Aoki et al.24 to describe the capacitive behavior of surface-confined layers terminated with acid-basic endgroups. This increase in the charge density may increase the intensity of the protonated amino-amino repulsive interactions at the outer part of the monolayer which suggests that the film permeation may be enhanced as a consequence of the disorder introduced at the SAM-solution interface. The estimated geometrical thickness agrees with the conclusions made from other techniques18,19,25-28 supporting the earlier suggestions that

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Figure 2. (A) Admittance (YR, -Yi) plots registered at 0 V (10 mV amplitude) for the bare Au electrode (b) and for the 24 h AUT-modified electrode (0) in a NaClO4 0.3 M not buffered solution. The plots obtained for the modified electrode in NaClO4 0.3 M buffered at pH 3 (O) and pH 9 (4) are also included. (B) Corresponding Bode plots and Bode phase plots for the bare Au electrode (b, s) and for the 24 h modified electrode: Not buffered (0, - -), pH 3 (O, - - -), pH 9 (4, - - -).

TABLE 1: Double-Layer Capacitance Values (Cdl)a and Film Thickness (d)b for a 24 h AUT-Modified Electrode Obtained from Impedance Data in Figure 2c solution

Cdl/µF cm-2

d/nm

not buffered pH 3 pH 9 bare Au (not buff)

1.9 ( 0.1 2.5 ( 0.2 1.60 ( 0.02 23 ( 7

1.9 ( 0.1 1.4 ( 0.1 2.16 ( 0.03

a

Obtained using eq 2. b Values obtained from capacitance using eq 4. c Capacitance for a bare Au electrode in NaClO4 0.3 M at 0 V is shown for comparison.

the archetypical structures formed by n-alkanethiols on Au become slightly disrupted at its outermost part, although keeping unmodified the array of sulfur atoms on the Au substrate (as previously observed by in situ scanning probe techniques29-32,50,51), due to the presence of bulky or charged endgroups. 3.1.2. Electrolyte Anion Effect. The presence of different anions in solution could affect the intensity of the endgroupendgroup and/or probe-endgroup electrostatic interactions established at the SAM-solution interface. Thereby, these effects should be detected using electrochemical techniques. In order to check this point, the response of the AUT-modified electrode to the presence of different anions was studied both in the absence (3.1.2) and the presence of electroactive species (3.2). The CVs reported in Figure 3 are similar to those obtained in the study of the pH effect; that is, the current is drastically reduced after the modification of the surface in the presence of the three anions studied. However in chloride solution, the current is higher than in NO3- and F-, which may be related to the higher adsorbability of the Cl- ions. It should be remarked that the pH was not controlled in these experiments to avoid the interfering effect of buffer anions. The admittance (YR, -Yi) plots obtained at 0 V for the AUT-modified surfaces in the presence of each of these electrolytes (Figure 4A), plus the previously obtained in NaClO4 0.3 M, support these observations and enhance the visual comparison. The data confirms the similarity of the interfacial structure in the presence of NO3and ClO4- but shows considerable differences for F- and Cl-. It may be suggested that while the polyanions do not seem to affect significantly the structure of the modified surfaces, the small Cl- anion may permeate through the relatively compact AUT monolayer. The data for F- is not conclusive since it would be expected to behave like the polyanions due to its hydrated

Figure 3. CVs obtained for a 24 h AUT-modified electrode right after contact with a KCl 0.2 M (blue line), a NaF 0.2 M (red), and a NaNO3 0.3 M solution (green). The profile corresponding to the bare Au in KCl 0.2 M (black line) has been included for comparison. The scan rate was 50 mV s-1 in all cases.

Figure 4. (A) Admittance (YR, -Yi) plots and (B) Bode and Bode phase plots registered at 0 V for the 24 h AUT-modified electrode since the first contact with a KCl 0.2 M (O, s), NaF 0.2 M (0, - -), NaNO3 0.3 M (4, - - -), and NaClO4 0.3 M (3, - - -) solutions, respectively. The perturbation amplitude was 10 mV.

size but the results are also affected by the high solution pH (7.16), for which the modified electrode is expected to exhibit a lower capacity due to the absence of endgroup-endgroup electrostatic interactions at the film surface. These conclusions were additionally supported by the evidence obtained in the Bode and Bode phase plots (Figure 4B). The magnitude of log Z increased in the order: Cl- < NO3- e ClO4- < F-. The inset in Figure 4B clearly revealed that the highest values for this parameter were obtained in the presence of the latter anion.

Permeation of SAMs by Electroactive Probes TABLE 2: Double-Layer Capacitance Values (Cdl)a Obtained from the Impedance Data in Figures 2 and 4 for a 24 h AUT-Modified and the Bare Au Electrodes in Different Electrolytes Cdl/µF cm-2 electrolyte -

ClO4 ClFNO3a

pH

bare Au

24 h AUT-modified

5.85 5.42 7.16 5.33

23 ( 7 53 ( 3 18 ( 2 10 ( 2

1.8 ( 0.1 2.0 ( 0.2 1.32 ( 0.02 1.7 ( 0.1

Obtained using eq 2.

Indeed, the phase angle value (Ψ) remained around 75-80° in the medium-low frequency region in the presence of Cl- while it was much closer to 90° in the other electrolytes. The EIS data were fitted to an RQ circuit and the double layer capacitance (Cdl) was calculated using eq 2. The values obtained are reported in Table 2 and agree with the previous description. 3.2. Studies in the Presence of Electroactive Probes. In the previous sections, it was demonstrated that the capacity of an Au electrode modified with an AUT monolayer on top can be affected by the charge distribution resulting from the protonation of the amino endgroups and, to a small extent, by the adsorbability of the electrolyte anions. The effect of pH on the ET of electroactive charged redox probes in solution through a SAM of AUT was previously reported by Degefa et al.36 The redox response was shown to be governed by the intensity and nature (repulsive or attractive) of the endgroup-probe electrostatic interactions at the SAM-solution interface, which were clearly dependent on the solution pH. In order to assess whether the adsorbability of the anions alter the structure of the film and/or have an effect on the ET of redox probes in solution across the AUT monolayer, the electrochemical response of Ru(NH3)6+3/+2 and Fe(CN)6-3/-4 couples and its evolution with increasing the electrode-solution contact time were measured in solutions of the previously studied electrolytes (perchlorate was not included due to the low solubility of the hexaamineruthenium complex at the concentrations studied). The effect of the temperature on these properties was also investigated. 3.2.1. Ru(NH3)6+3. Figure 5 reports the results obtained in three different electrolytes containing the Ru(NH3)6+3 redox probe at 25 °C. The alternate registration of CVs and impedance spectra started right after the first electrode-solution contact, and was kept during contact times higher than 4 hours (further details in the Supporting Information). Each CV in Figure 5A was followed by the recording of one of the EIS spectra in Figure 5B. This is extensive to the other graph couples in the figure (Figure 5C,D and Figure 5E,F). Agreeing with the behavior previously reported,34 the redox response of this probe at short contact times was strongly blocked in all solutions. As it was explained there, the establishment of repulsive electrostatic interactions between the equally charged amino endgroups (-NH3+) and the positive Ru(NH3)6+3 cations, hinders the electron transfer process. The strongly blocked initial voltammograms (Figure 5, panels A, C, and E) support this suggestion as well as the wide semicircles in the medium-low frequency region of the resistance more sensitive (ZR, -Zi) Nyquist plots obtained at short contact times (Figure 5, panels B, D, and F). Further analysis of the data was carried out by fitting the EIS spectra to a Randles-type equivalent circuit (as described in the experimental section) where R2 is a pseudo charge transfer resistance. Table 3 reports the values obtained for this parameter from the fittings of the first impedance spectrum in each

J. Phys. Chem. C, Vol. 113, No. 6, 2009 2409 electrolyte (R02). The results seem to follow the tendency already observed for the capacitance measurements (section 3.1.2): R20 decreased in the order: F- > NO3- > Cl-. The lower R2 value of Cl-, compared with the value for NO3-, is possibly the reflection of the higher adsorbability of the Cl- anions at the charged terminal endgroups of the AUT film which leads to a reduction in the intensity of the endgroup-probe repulsive interactions. The behavior of F- containing solutions, both in the presence or absence of redox probes, can be ascribed to the low intensity of the endgroup-endgroup repulsive interactions (pH effect) which may increase the initial compactness of the film. Consequently, it seems that the variations of R20 induced by changes in the pH and the electrolyte anion may account, as well as those found for the capacitance data in pure electrolyte, for structural changes in the outermost part of the monolayer. Further measurements on the effect of increasing the electrolyte-solution contact time on the CVs and impedance spectra are also given in Figure 5. As shown earlier,34 the i-E profile changed with time from an initial blocking-type voltammogram to a profile similar to those obtained for an array of microelectrodes. This is common to the three anions studied and may be taken as evidence of a time dependent process affecting the blocking properties of the SAM. EIS spectra demonstrate the decrease of the semicircle diameter with time, thus, confirming the existence of changes in the properties of the AUT monolayer: either related to changes in the whole film structure and/or changes in the electrostatic interactions at the electrode-solution interface. Additional CV and EIS measurements were performed, at the end of the experiments described in Figure 5, for these modified electrodes in KCl 0.2 M showing no significant differences respect to those previously reported in Figures 3 and 4. The decrease in the values of R2 with time in the three electrolytes, suggests that it results from a disordering process of the monolayer which facilitates the redox process. These changes are further evidenced by the appearance, at long contact times, of a linear contribution (in the low frequency region) that may be ascribed to diffusion. Using the intensity of the diffraction peak associated to crystalline order, Capitań et al. studied the kinetics of ordering of a 1-nonanethiol SAM on Au (111) after removing the film from an ethanol liquid phase and flushing the surface with helium upon 8 h.33 A kinetic law was proposed linking the surface coverage of ordered domains (θ) with the drying time. In an analogous way, the kinetics of the structural changes observed for the AUT monolayer could be followed by means of the variations in R2. Assuming a first order kinetics

V ) -dθ/dt ) kθ

(6)

where θ corresponds to the coverage of ordered domains at a time t and k is the effective rate constant for the process. Considering that R2 is directly proportional to θ (a high value of R2 corresponds to a high θ), eq 6 can be transformed into

V ) -dθ/dt ∝ -dR2/dt ) kR2

(7)

Integrating, reorganizing in terms of the kapp for the redox process (which is proportional to 1/R2 according to eq 5) and taking natural logarithms

ln(1/R2) ) ln(1/R02) + kt

(8)

The kinetic law proposed in eq 8 supposes that the changes in the structure involving the pH, anion, and other factors affecting the initial value of R20 are characterized by a rate constant (k0) that is much faster than the disorganization of the layer (k0 . k). Scheme 1 illustrates the two step mechanism


Campinã et al.

Figure 5. Selected CVs (A, C, and E) and Nyquist plots (B, D, and F) alternately obtained since the first electrode-solution contact for a 24 h AUT-modified Au electrode in Ru(NH3)6Cl3 2 mM + KCl 0.2 M (A and B), + NaF 0.2 M (C and D), and NaNO3 0.3 M (E and F) at 25 °C during contact times longer than 4 h. The scan rate was 50 mV s-1 and the amplitude 10 mV in all cases.

underlying this model. The values of ln(1/R2) were plotted versus the contact time in the three different electrolytes studied (Figure 6). These data were fitted to eq 8 in order to determine the rate constant (k) for the disordering process (Table 4). Values in the order of 1 × 10-4 s-1 were obtained for Cl- and NO3- which suggests that the kinetics of the disordering process is not strongly affected by the anion nature. Nevertheless, k was slightly higher in F- solution. Whether this small difference can be ascribed to pH effects it is not possible to assess. These results are 1 order of magnitude higher than the rate constant measured by Capitań et al. (2.20 × 10-5 s-1).33

The energy of the chain-chain intermolecular interactions involved in SAM formation is of the order of tens of kilojoules per mole weaker than the sulfur-gold binding energy suggesting that the monolayer structure may be sensitive to temperature (T). With the purpose of assessing the effect of this parameter, a set of experiments analogous to those performed previously were conducted in KCl 0.2 M + Ru(NH3)6Cl3 2 mM solution at different T. The range was chosen between 5 and 45 °C. The bottom limit was fixed in order to avoid problems such as solvent freezing or low conductivity. On the other hand, the upper value falls slightly over the value of T at which the



SCHEME 1: Model Illustrating the Changes Performed in the Structure and Charge of the AUT Film since the Moment Previous to the First Electrode-Solution Contact (t ) 0) until Times Higher than 4 h in the Presence of Ru(NH3)6+3a

a The electrolyte and the redox probes are represented as red and lilac spheres. The amino endgroups are represented by grey spheres when uncharged and blue when they are positively charged (+). The size of the endgroups has been highlighted for better clarity.

Figure 6. The natural logarithm of the apparent charge transfer rate constant, obtained from the EIS data in Figure 5, for a 24 h AUTmodified electrode in the presence of Ru(NH3)6Cl3 2 mM + KCl 0.2 M (O), NaF 0.2 M (0), and NaNO3 0.3 M (g) is plotted versus the time spent since the first electrode-solution contact. The experimental data was fitted to equation 8 (solid lines).

TABLE 3: Initial Values of the Charge Transfer Resistance (R20)a through a 24 h AUT-Modified Electrode Obtained in the Different Electrolyte Solutions at 25 °C R02/kΩ electrolyte ClFNO3-

-3

Fe(CN)6

Ru(NH3)6+3

0.02 0.04 0.08

156 478 276

a Obtained by fitting the impedance data in Figures 5 and 7 to a Randles-type equivalent circuit.

TABLE 4: Rate Constant Values (k)a Associated with the Time Transition Observed for the Charge Transfer Resistance of Ru(NH3)6+3 Electroactive Solution Species through a 24 h AUT-Modified Electrode at 25 °C in Different Electrolytes electrolyte

k × 10-4/s-1

ClFNO3-

1.3 2.6 1.1

a

Values obtained fitting the data in Figure 6 to the kinetic law described by eq 8.

formation of 2D liquid phases of alkylthiols maybe expected.21,52 Figure 8 shows the CVs and EIS spectra alternately obtained just after the first contact of a 24 h AUT-modified electrode with the Ru(NH3)6+3 containing solution. For clarity the representations are only shown for three values of T: 5, 25, and 45 °C. Figure 8, panels A, C, and E, shows the time evolution

of the CVs for the redox process of Ru(NH3)6+3 species at these temperatures. At each temperature, it is clear that the film undergoes structural changes becoming less and less blocking as the time of contact with the electrolyte increases which agrees with the results previously observed at 25 °C. However, at constant elapsed time of contact, the change in the CVs may contain both the effect of T on the structure of the monolayer and the effect on the kinetics of the ET. In order to assess whether the observed effect is the result of a disordering process initiated after contact with the electrolyte solution, EIS spectra were also obtained as a function of the time and the temperature, and are shown in Figure 8, panels B, D, and F. The data clearly shows that the diameter of the semicircle diminishes when the time is increased which confirms the increased facility for the charge transfer process at each temperature. Furthermore, the value of the diameter at the initial time decreases with the temperature indicating that the initial monolayer properties may be temperature dependent. The values of R2 were obtained by fitting to the same equivalent circuit. The pseudo charge transfer resistance for the first spectrum (R20) was obtained at each temperature (Table 5). As expected R02 decreases markedly with increasing the temperature. As observed in Table 5, the decrease is much stronger than the observed for the bare Au (the R20AUT/ R20Au ratio decreases with temperature) indicating that changes in this parameter can be also induced by the effect of the temperature in the SAM structure. The difference in the absolute values of R20 between bare and AUT-covered Au may be taken as further evidence indicating that the values of this parameter are controlled by the AUT monolayer properties, being this one of the reasons why it has been proposed to call R2 pseudo charge transfer resistance. Under the earlier assumption that R2 reflects the kinetics of the disordering process of the monolayer, ln(1/R2) data were plotted versus the contact time for every T and fitted to eq 8 (Figure 9). Although the slope obtained at 25 °C is unexpectedly those obtained at 5 and 45 °C the values obtained for k (Table 6), fell in the order of 1.3-2.5 × 10-4 s-1 which suggests that the kinetics of disorganization of the SAM may be slightly accelerated with increasing the temperature. 3.2.2. Fe(CN)6-3. As already shown,34 the charge of the probe seems to have a key role on the charge transfer response across an AUT SAM. Figure 7 shows the time evolution of the CVs and impedance spectra for the Fe(CN)6-3 probe in Cl-, F-, and NO3- electrolytes at 25 °C. The data was obtained in an analogous way to that reported in Figure 5. Contrasting with the results obtained for the positively charged probes, the CVs in Figure 7 display almost diffusion controlled profiles in all cases and its evolution with the contact time did not show significant changes (Figure 7, panels A, C, and E). The variations observed in the three electrolytes are so small that they could be even due to normal changes expected in the shape and height of a hanging meniscus kept during such long times. The EIS spectra obtained in the same media are also shown in the Figure 7, panels B, D, and F). The impedance values achieved under these conditions were considerably lower than those obtained for the Ru(NH3)+3 probe. The initial (ZR, -Zi) Nyquist plots for each electrolyte showed a very small semicircle (almost negligible) and a dominating contribution of the linear Warbourg impedance in the medium-low frequency region. A small increase of the semicircle diameter (almost negligible in NO3-) was further observed with increasing the contact time. The initial values of R2 obtained in the different electrolytes were very similar and lower than 100 Ω (Table 3). The contrasting impedance plots obtained for both probes are clear evidence of


Campinã et al.

Figure 7. Selected CVs (A, C, and E) and Nyquist plots (B, D, and F) alternately obtained since the first electrode-solution contact for a 24 h AUT-modified Au electrode in K3Fe(CN)6 2 mM + KCl 0.2 M (A and B), + NaF 0.2 M (C and D), and NaNO3 0.3 M (E and F) during contact times longer than 4 h. The scan rate was 50 mV s-1 and the amplitude 10 mV in all cases.

a relation between SAM properties and the charge of the redox probe on the overall charge transfer process. The results can be explained considering the opposite nature of the electrostatic interactions established at the SAM-solution interface. In the case of ruthenium, the repulsive nature of the interactions keeps a great part of the positive cations outside the film at the initial contact times, when it is still considerably ordered. However, in the case of Fe(CN)6-3, the attractive probe-endgroups interactions guide the species into the monolayer at any time. Differences in the initial pH and the electrolyte anion could be

affecting the order at the outermost part of the SAM, however the results show that the effect of these parameters on the redox response is very small, or even negligible, due to the strong attractive interactions. It may be suggested that electrostatic effects are the dominant factor. In these conditions, the access into the monolayer is not the step controlling the process. Consequently, the time evolution of R2 can not reflect any disorganization process. It seems that the presence of an organized or disorganized AUT film does not affect significantly the charge transfer kinetics of the Fe(CN)6-3 probes. Therefore,



Figure 8. Set of selected CVs and EIS spectra obtained since the first electrode-solution contact for a 24 h AUT-modified electrode in a KCl 0.2 M + Ru(NH3)6Cl3 2 mM solution at 5 °C (A and B), 25 °C (C, and D), and 45 °C (E and F) during contact times longer than 4 h. Scan rate: 50 mVs-1. The oscillation amplitude was 10 mV in all cases.

the charge transfer of Fe(CN)6-3 across the AUT monolayer is not sensitive to the SAM structure. If any effect may be ascribed to the disorganization of the monolayer, it should be the slow down of the kinetics (difficult to detect from the CVs) indicated by the small increase in R2 with the contact time. The effect of increasing the temperature was checked by CV and EIS (Supporting Information) in an analogous way as it

was done in section 3.2.1. At constant temperature, the results followed the same trend than those obtained at 25 °C: small values of R20 (10-20 Ω; Table 5) that were slightly increased with the contact time. At constant elapsed time, the initial CVs and EIS showed that the process was slightly facilitated with increasing the temperature: decrease in the size of the semicircle diameter (i.e., R20) and CVs closer to the one for the bare Au.


Campinã et al.

TABLE 5: Initial Values of the Initial Charge Transfer Resistance (R20)a through a 24 h AUT-Modified, the Bare Au Electrode and Its Ratio at Different Temperatures Fe(CN)6-3

Ru(NH3)6+3

T (°C)

R0AUT /Ω 2

R0Au 2 /Ω

R0AUT /R0Au 2 2

R0AUT /Ω 2

5 25 45

19 23 9

249 158 15

0.08 0.15 0.59

601000 156400 27120

0AUT 0Au R0Au /R2 2 /Ω R2

494 134 78

1217 1166 349

a Obtained by fitting the impedance data in Figure 8 (and Supporting Information) to a Randles-type equivalent circuit.

Figure 9. The natural logarithm of the apparent charge transfer rate constant, obtained at different temperatures for a 24 h AUT-modified electrode in the presence of Ru(NH3)6Cl3 2 mM + KCl 0.2 M, is plotted versus the time since the first electrode-solution contact. The data obtained at the different temperatures are represented by: (O) 5 °C, (4) 25 °C, and (g) 45 °C and fitted to eq 8 (solid lines).

TABLE 6: Rate Constant (k)a Values Associated with the Time Transition Observed for the Charge Transfer Resistance of Ru(NH3)6+3 Electroactive Species in KCl 0.2 M Solutions, through a 24 h AUT-Modified Electrode at Different Temperatures

a

T (°C)

k × 10-4/s-1

5 25 45

1.8 1.3 2.5

Values obtained by fitting the data in Figure 9 to eq 8.

The results confirm that, at each temperature, the charge transfer is less affected by time than in the case of the positively charged hexaamineruthenium probes confirming that the charge transfer kinetics of negatively charged probes is not greatly affected by the organization of the AUT layer. Obviously, both contrasting effects are a confirmation of the establishment of electrostatic interactions between the layer and the probe charges. 3.3. 1-DT Monolayer. So far the results allowed proposing that charged monolayers undergo disorganization with time assisted by pH, anion, and temperature. Probing this disorganization could only be achieved when the charge of the probe was equal to the charge in the ionized SAM which may obtain support by studies with an uncharged monolayer formed by an unionizable molecule. To maintain as close as possible the thickness and structure of the molecules, the choice was made to 1-dodecanethiol (1-DT), a methyl terminated thiol with a chain length (1.6406 nm; estimated by computer MM2 calculations of energy minimization, Chem3D Ultra9.0 CambridgeSoft Corporation, for a single molecule in a fully extended configuration) similar to AUT (1.6352 nm 34). CVs and impedance spectra were collected at 25 °C for a 24 h 1-DT modified electrode since the first contact with

Ru(NH3)6+3 (Figures 10, panels A and B) and Fe(CN)6-3 (Figure 10, panels C and D) containing solutions during times longer than 4 h. In contrast with the results obtained for AUT, the initial blocking properties of the 1-DT monolayer can be seen for both probes from the CVs (Figure 10, panels A and C) as well as by the large value of the semicircle diameter in the impedance spectra (Figure 10, panels B and D). The initial resistance (R20; Table 7) achieved a high value in the case of both probes as expected for the formation of an ordered and compact SAM. The evolution of both CVs and EIS spectra with increasing the contact time, showed that the film becomes less blocking for both probes irrespective of their charge. The minor differences found in the data may only reflect the specific kinetic details for the ET or the mass transport for both redox couples. These results are clear evidence of the disorganization of the film with the contact time, as suggested by Capitań et al. and proposed earlier. In the absence of endgroups susceptible to get charged, electrostatic interactions are not established with the electractive species and, consequently, the whole process may be controlled by the access of both probes into the SAM. The assessment of the kinetics of the disorganization may be, therefore, possible using the law described by eq 8 under the assumption that it is the rate determining step. The plots of ln(1/R2) versus t obtained in the presence of both probes were fitted to this equation. The data obtained from the slope of the adjusted straight lines show that k fell, for both probes, within the same order of magnitude than for AUT (Table 7). 4. Conclusions The results obtained for AUT, in the absence of redox probes, have shown the influence of the solution pH on the structure of the monolayer. Thereby, pH seems to determine the intensity of the amino-amino repulsive electrostatic interactions between neighbor endgroups by controlling the degree of charge at the film-solution interface. These interactions may be responsible for the slight distortion, at the outermost part of film, of the archetypical structures found for unsubstituted SAMs of nalkanethiols in the low index planes. In addition, the results obtained for different electrolytes and temperatures have shown that these parameters can also influence the initial structure of charged monolayers (Figure 11). The initial charge transfer properties of electroactive probes in solution and its evolution with increasing the time of electrode-solution contact have been shown to be dependent on both the probe and the SAM charge. While the changes in the organization of the layer seem to play a minor role in the kinetics of the process when attractive endgroup-probe electrostatic interactions are established, it is dominant when they are repulsive or negligible (as seen for 1-DT). In the latter cases, the disorganization of the layer can be, thus, monitored by EIS measurements. The kinetics of the process was obtained by applying a model consisting of (a) a fast step including the effect of pH, electrolyte anion, and temperature on the structure of the SAM and (b) a subsequent and slower step accounting for the progressive disorganization of the monolayer. The values obtained for the effective rate constant have been shown to be relatively independent of the ionizing properties of the thiol and of the charge and nature of the probes, which allows the observed behavior to be ascribed to the disorganization of the monolayer after contact with a liquid phase (as proposed by Capitań). The kinetic data has also suggested a small acceleration of the process by increasing the temperature. The conclusion may be that, in the presence of a liquid phase, the thiol chains undergo a process leading to the decrease in



Figure 10. Set of selected CVs and EIS spectra obtained at 25 °C since the moment of electrode-solution contact, and during contact times longer than 4 h, for a 24 h 1-DT modified electrode in KCl 0.2 M + Ru(NH3)6Cl3 2 mM (A and B) and KCl 0.2 M + K3Fe(CN)6 2 mM (C and D) solutions. The scan rate was 50 mV s-1 and the amplitude 10 mV in all cases.

TABLE 7: Initial Values of the Charge Transfer Resistance (R20)a and the Rate Constant (k)b Associated with the Time Transition Observed for the Charge Transfer Resistance of Fe(CN)6-4 and Ru(NH3)6+3 Electroactive Species in KCl 0.2 M Solutions through a 24 h 1-DT Modified Electrode at 25 °C probe

R02/kΩ

k × 10-4/s-1

Fe(CN)6-3 Ru(NH3)6+3

54 479

1.37 1.97

a Obtained by fitting the impedance data in Figure 10, panles B and D, to a Randles equivalent circuit. b Obtained using eq 8.

the long-range order even if the thiol heads can remain periodically arranged on the Au substrate. This process is probably induced by the establishment of solvent-chain interactions that may compete with the low energy chain-chain intermolecular interactions (which provide the high order and compactness exhibited by these films) and act, at least initially, at the outermost part of the monolayer as has been previously suggested.25 The approximately one order difference found in the rate constant with respect to the results obtained by Capitań et al. may be justified by the fact that in the present study a polar solvent (water) containing ions was used, which may contribute to accelerate the process by means of enhanced solvent-endgroup or ion-endgroup interactions. Furthermore, while Capitań et al. reported kinetic results for the organization

Figure 11. Model illustrating the variables with influence on the initial structure of the film and the evolution followed with increasing the SAM-solution contact time.

of the SAM, the data reported here concerns with disorganization and both processes may have different rate limiting steps. Acknowledgment. This work was carried out within the line 4 of the CIQ (Centro de Investigaçaõ em Quı´mica da Universidade do Porto) research program. J. M. Campinã gratefully acknowledges FCT (Fundaçaõ para a Cieˆncia e a Tecnologia de Portugal) for the concession of a Ph.D. grant (Contract SFRH/ BD/23851/2005). Supporting Information Available: Additional figures and schemes which give a better description of the experiments

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