Thermodynamic, Kinetic, Surface pKa, and Structural Aspects of Self

Article pubs.acs.org/Langmuir

Thermodynamic, Kinetic, Surface pKa, and Structural Aspects of SelfAssembled Monolayers of Thio Compounds on Gold Tércio de F. Paulo,† Héctor D. Abruña,*,‡ and Izaura C. N. Diógenes*,† †

Departamento de Química Orgânica e Inorgânica, Universidade Federal do Ceará, Cx. Postal 6021, Fortaleza, Ceará, Brasil, 60455-970 ‡ Department of Chemistry and Chemical Biology, Baker Laboratory, Cornell University, Ithaca, New York 14853-1301, United States S Supporting Information *

ABSTRACT: The thermodynamic and kinetic aspects of the formation of self-assembled monolayers (SAMs) of thio compounds on gold have been studied via electrochemical and quartz crystal microbalance (QCM) techniques. The data indicate that the adsorption process involves a significant free energy of adsorption (ΔG° = −36.43 kJ/mol) and that there are slight repulsive interactions between adjacent molecules on the surface. A method for the calculation of surface pKa values of molecules containing more than one protonation site is proposed and used for the determination of the pKa values of SAMs derived from thioisonicotinamide, thionicotinamide, 5-(4-pyridyl)-1,2,4-oxadiazole-2-thiol, and 4-mercaptopyridine (pyS) on gold. Structural aspects of the SAMs were studied by using impedance with [Fe(CN)6]4−/3− as redox probe. Evidence of faster kinetics for an oxidative decomposition of pyS SAM in the presence of [Fe(CN)6]3− is discussed based on electrochemical and impedance data.

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INTRODUCTION The spontaneous adsorption of organic and/or inorganic molecules on noble metals has been the focus of intense research1−9 due to the large range of applications of so-called self-assembled monolayers (SAMs). Among these, SAMs of sulfur-containing molecules on gold have been among the most extensively studied systems because of the convenient and simple way to form organized structures on surfaces by simple immersion of a substrate in solutions containing the adsorbate. In addition, given that the adsorption site is the sulfur atom, which has a high affinity for the gold surface, specific functionalities can be attached to the interface, allowing the modified electrode to be used in numerous ways. A multidisciplinary approach, therefore, is an inherent condition for the understanding of the formation of SAMs and the way in which they alter/modulate interfacial reactivity. This can include applications as diverse as the protection against corrosion, and the assessment of electron transfer reactions of metalloproteins, to the modification of the work function of electrodes in electronic devices. This clearly involves the juncture of chemistry, physics, biochemistry, materials science, and other fields. Whatever the application, it is essential to have a clear understanding of the physical and chemical aspects involved in the modification process of a given surface.4,10−12 In this sense, a knowledge of the thermodynamic and kinetic aspects of the adsorption process of a given molecule are particularly important if we are to establish design criteria to explain or even predict the reactivity of an interface for a specific application. © 2012 American Chemical Society

An additional and relevant aspect relates to the surface stability of SAMs of thiolate molecules on gold.10,13−16 For example, SAMs of 4-mercaptopyridine (pyS) on gold have been the subject of intense and, at times, controversial research. This SAM was first used in 1985 to assess the heterogeneous electron transfer (hET) reaction of cytochrome c (cyt-c) metalloprotein.17 In such work, electrochemistry was used to study the hET reaction of cyt c, and SERS spectra were acquired to provide insights about the orientation of the molecule on the surface. However, it was not until the late 1990s that Lamp et al. addressed the stability of the monolayer.18 Using XPS and FTIRRAS, they showed that pyS SAMs suffer a conversion to an adlayer composed of atomic and/or oligomeric sulfur forms due to an oxidative cleavage of the CS bond. According to the authors, this process increases with an increase in the immersion time of the gold electrode in an ethanolic solution of pyS. In addition, they observed a decrease in the ability of the modified electrode to facilitate the hET reaction of cyt c with an increase in the immersion time. Following this report, a work based on STM images suggested that the pyS molecules dimerize at the sulfur to form a disulfide and that no conversion to sulfur forms on the surface occurs at all.19 However, the immersion time of the Au(111) in the pyS solution was kept at 1 min which, according to Lamp’s paper,18 is not sufficient to allow for the conversion Received: August 26, 2012 Revised: November 5, 2012 Published: November 26, 2012 17825

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Table 1. Values of Experimental (Γs) and Simulated (Γ*s) Saturated Surface Coverage, Adsorption Coefficient (β), Adsorption Free Energy (ΔGads), and Interaction Parameter (g) for the Formation of SAMs Derived from Hpyt, TNA, and iTNA on Gold at 20 °C species

Γ*s (10−10 mol cm−2)a

Γs (10−10 mol cm−2)

β (104 L mol−1)

ΔGads (kJ mol−1)

gb

reference

Hpyt Hpyt TNA TNA iTNA

6.9

4.37

2.43

−0.45

9.4

8.33

3.90

9.4

8.87

2.38

−35.9 ± 2.4 −39.9 −38.5 ± 1.7 −45.74 −34.9 ± 1.5

this work 26 this work 25 this work

a

−0.30 −0.10

b

Values were simulated assuming a compact monolayer adsorbed in a direction normal to the surface. Interaction parameters were determined based on the Frumkin isotherms (see Figure S2 in the Supporting Information). Electrochemical impedance measurements were carried out by using the AUTOLAB model PGSTAT 30 connected to a PC microcomputer and controlled by FRA software. The impedance experiments were performed in 0.1 M KCl solution containing 8.0 mM Na3[Fe(CN)6]/Na4[Fe(CN)6] under potentiostatic control at open circuit potential over the frequency range from 10 kHz to 1 mHz. An amplitude perturbation of 5 mV rms was used. For the impedance measurements, a gold electrode of 0.0314 cm2 of geometric area was used. AT-cut quartz crystals (5 MHz) of 24.5 mm diameter with Au electrodes deposited over a Ti adhesion layer (Maxtek Co.) were used for QCM measurements. An asymmetric keyhole electrode arrangement was used, in which the circular electrode geometrical areas were 1.370 cm2 (front side) and 0.317 cm2 (backside). The electrode surfaces were overtone polished. The quartz crystal resonator was set in a probe (TPS-550, Maxtek) made of Teflon, in which the oscillator circuit was included. The probe was immersed in several aqueous electrolyte solutions which were thermostatted at 25.0 °C by a waterjacketed beaker connected to a thermostatted bath (Digital Temperature Controller 9101, Fisher Scientific). The solutions were degassed with N2 gas before use and the nitrogen atmosphere was kept during experiments. The frequency was measured with a plating monitor (PM-740, Maxtek) that was interfaced to a desktop computer. Once the frequency reached a steady state, electrochemical experiments were performed. The frequency (ΔF) and mass (Δm) changes are related by the Sauerbrey equation,28 Δm = −CfΔF, where Cf (17.7 ng Hz−1 cm−2) is the proportionality constant for the 5 MHz crystals. Quartz crystal resonators were also used as the working electrode in voltammetric experiments of reductive desorption. The potential of the working electrode was controlled with a potentiostat (CV-27 or EC Epsilon, BAS). Electronic spectra in the ultraviolet and visible (UV−vis) regions were acquired with a Hitachi model U-2000 spectrophotometer. Electrode Modification. Gold polycrystalline electrodes were mechanically polished with a 0.05 μm alumina slurry, rinsed with water, sonicated for 10 min, immersed in fresh “piranha” solution (3H2SO4:1H2O2) for 2 min, rinsed with water, and sonicated again (CAUTION: Piranha solution is a strong oxidant solution that reacts violently with organic compounds). Following this cleaning procedure, the electrode was pretreated by continuous cycling in 0.5 mol L−1 H2SO4 until the characteristic voltammetric curve29 for a clean polycrystalline gold surface was obtained. From the integration of the area under the oxide formation wave, the electrochemically active area was determined using a conversion factor30 of 390 μC cm−2. The clean gold electrode was then immersed in aqueous solutions containing the modifier species at different concentrations and different immersion times and temperatures, rinsed with water, and dried under an argon flux. For the QCM measurements, the samples were dissolved in water and injected into the cell containing water using a gastight syringe. The injected volumes were varied according to the desired final concentration. Although the injection sometimes gave rise to sudden variations in the frequency, the signal (frequency) stabilized in less than 1 min (see Figure S1 of the Supporting Information).

reaction. In addition, it was found that this molecule yields an ordered structure with a rectangular (5 × 31/2) unit cell19 with a molecular image very different from that reported by Taniguchi’s group,20 although the long-range structure was the same. The main difference lies in the orientation of the pyridine ring which is perpendicular in Taniguchi’s report20 while it is nearly parallel to the surface in Wan’s work.19 Later, Taniguchi suggested21 that a very small amount (1 mol %) of sulfide impurity in pyS modifier solution is sufficient to completely replace a pyS monolayer on Au(111) and that it could be responsible for the poor electrochemical response of cyt-c. Some of our work, based on the coordination of pyS to metal centers, reinforces the idea of Lamp by showing that the stability of pyS SAMs is greatly enhanced by π back-bonding interactions which strengthen the CS bond.22−24 In this work, we present kinetic and thermodynamic aspects of the adsorption of the thio compound thioisonicotinamide (iTNA) on gold based on quartz crystal microbalance (QCM) and electrochemical results. The data are presented in comparison and in relation to results previously reported by our group25−27 on the adsorption of thionicotinamide (TNA), 5-(4-pyridyl)-1,2,4-oxadiazole-2-thiol (Hpyt), 4-mercaptopyridine (pyS), and 1,4-dithiane (1,4-dt) on gold. Impedance data and pKa surface values are also presented to better describe the structural and chemical nature of these SAMs. In addition, and with the intent of providing further insights into the stability of pyS SAMs on gold, results are discussed, focusing on the oxidative cleavage of the CS bond.

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EXPERIMENTAL SECTION

Chemicals. Thioisonicotinamide, thionicotinamide, 5-(4-pyridyl)1,2,4-oxadiazole-2-thiol, 1,4-dithiane, 4-mercaptopyridine, KOH, KCl, H3PO4, K2HPO4, and KH2PO4 were purchased from Aldrich and used as received. K4[Fe(CN)6] and K3[Fe(CN)6] were purchased from Merck and used without further purification. Aqueous solutions were prepared using Millipore water of at least 18 MΩ cm resistance. KOH or H3PO4 were used to adjust the pH values of phosphate buffer solutions. Apparatus. Cyclic voltammetric measurements were carried on a computer-controlled EC Epsilon potentiostat (Bioanalytical Systems BAS, Inc., West Lafayette, IN). Electrochemical cells of conventional design were employed, and potentials were measured against a silver/ silver chloride electrode (Ag/AgCl/3.5 mol L−1 KCl, BAS). Modified polycrystalline gold surfaces (BAS, A = 0.0314 cm2) and coiled platinum wires were used as working and auxiliary electrodes, respectively. The supporting electrolyte was purged with high purity nitrogen for 20 min prior to experiments, and a N2 atmosphere was maintained during all the experiments. For acquisition of the reductive desorption curves, a Teflon cell was used to prevent KOH chemical attack. Polycrystalline gold surfaces (BAS, A = 0.0314 cm2) modified with Hpyt, iTNA, and TNA species and a gold flag were used as working and auxiliary electrodes, respectively. 17826

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Γt/Γs = K (C*/Γs)(Dt )1/2

RESULTS AND DISCUSSION Thermodynamic and Kinetic Studies. The surface coverage, Γ, of the SAMs was determined via reductive desorption31,32 in 0.5 M KOH according to Γ = Q/nFA, where Q is the charge under the reductive peak, n is the number of electrons, F is the Faraday constant, and A is the electroactive area. Linear sweep voltammetry was used to acquire the curves which, in most cases, presented only one wave assigned to the electrode reaction Au−SR + e− → Au0 + SR−. The saturation surface coverage, Γs, and the adsorption coefficient, β, were determined based on the linearized Langmuir equation,33 and the values are presented in Table 1. Accordingly, a plot of [RS]/Γ vs [RS] yields 1/Γs and 1/Γsβ as slope and intercept, respectively. The value of Γs for Hpyt is approximately half of those observed for TNA and iTNA, consistent with the larger volume of this molecule which results in a higher repulsion between adjacent species as seen through the g (interaction) parameter (−0.45). For TNA and iTNA, although there is a slight difference in Γs values, the g parameters (repulsive interactions because g < 0) are considerably different, suggesting that TNA is more tilted than iTNA relative to the surface. Following this reasoning, Hpyt molecules should be even more tilted relative to the surface normal. In fact, all experimental surface coverage data are lower than those obtained assuming a compact monolayer adsorbed in a direction normal to the surface (Γ*s), indicating that the adsorbed molecules are, indeed, at an angle relative to the surface normal. The free energy of adsorption, ΔGads, which gives a quantitative measure of the adsorption strength, was calculated based on the equation ΔGads = −RT ln(asβ) = −RT ln(55.5β), where as is the activity of the solvent and/or ions in solution, and values are also presented in Table 1. These values are indicative of strong interactions, compatible with chemisorptive processes. Values of ΔGads previously reported for Hpyt26 and TNA25 are also shown in Table 1. The difference between the previously reported values and the data presented in this work is assigned to the fact that in the former, the solvent activity was not taken into account in the adsorption equilibrium. Thermodynamic and kinetic parameters for the adsorption processes of pyS and 1,4-dt species on gold were not determined because of experimental aspects related to stability and reproducibility. The desorption curves of 1,4-dt were not reproducible even when keeping the concentration and the immersion time constant. Although somewhat speculative on our part, we believe that this is due, at least in part, to the volatility of 1,4-dt which makes it difficult to maintain a constant concentration in solution. With regards to pyS monolayers, the problem is related to the stability of the molecule upon adsorption. As discussed later, the pyS SAM on gold may undergo decomposition as a result of an oxidative cleavage of the CS bond as proposed by Lamp et al. and discussed above (vide supra).18 The adsorption kinetics of iTNA, TNA, and Hpyt on gold were also studied by following changes in the surface coverage as a function of time, Γt, at different temperatures and concentrations in solution. To ascertain whether the process is controlled12 by diffusion34 or activation,35 plots of Γt vs t and Γt vs t1/2 were obtained according to eqs 1 and 2 which relate, respectively, to activation and diffusion models Γt = Γs(1 − exp(−k′C*t ))

(2)

where k′ is the adsorption rate constant, C* and D are the bulk concentration and diffusion coefficient of the adsorbate, respectively, and K is a constant equal to 2π1/2. Plots of Γt/ΓS vs t1/2 (Figure 1a) did not exhibit the linear relationship anticipated for diffusional control (simulated

Figure 1. Γt/ΓS (a) and Γt (b) as a function of immersion time of Au electrode in solutions containing 0.2 mmol L−1 of modifiers at 20 °C. Experimental data for iTNA (□), TNA (▲), and Hpyt (★). Simulated curves (solid lines) were obtained from eqs 1 and 2 with k′ and D as adjustable parameters, respectively.

curves in solid lines). On the other hand, the agreement of the experimental data to the fits was excellent for plots of Γt vs t (Figure 1 b), indicating that a kinetically controlled model is applicable to the adsorption processes of iTNA, TNA, and Hpyt on gold. Thus, assuming an activation mechanism, the rate constants for the adsorption of iTNA, TNA, and Hpyt were calculated as 90.9, 65.0, and 6.9 L mol−1 s−1, respectively. Comparatively, the adsorption process of iTNA and TNA is about ten times faster than that of Hpyt on gold. Quartz Crystal Microbalance. Figure 2 presents the frequency changes as a function of time during the adsorption of iTNA, TNA, Hpyt, pyS, and 1,4-dt on gold. In such

Figure 2. Frequency shift as a function of time for gold quartz crystal resonators at 25 °C in 0.10 × 10−3 mol L−1 aqueous solution of TNA, iTNA, Hpyt, pyS, and 1,4-dt.

(1) 17827

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through which water molecules can penetrate. Also, Abruña et al.,42 on the basis of a combination of QCM and cyclic voltammetry techniques, found that the formation of monolayers of an Os complex on Pt is accompanied by water molecules. Assuming that the mass difference between QCM and reductive desorption is due solely to water, the numbers of coadsorbed water molecules per molecule of adsorbate were calculated and are presented in Table 2. For the pyS-modified crystal, the reductive desorption curve obtained after QCM measurements presented two waves at −0.9 and −0.56 V assigned, respectively, to the desorption of sulfur species (likely elemental sulfur) and the pyS molecule. Although the former presented a relatively lower value of charge, relative to the latter, it suggests the decomposition of the pyS SAM and, as a result, the presence of sulfur species on the surface. Because of the hydrophobic character of sulfur, a smaller amount of water would be expected on the surface. Surface pKa. Surface pKa values of SAMs can be estimated by measuring the peak current (ip) of a given redox couple in different pH buffers. Zhao et al.43 proposed a simple method in which cyclic voltammetry is used to measure ip values of [Fe(CN)6]3− as a function of pH. However, using the experimental conditions of Zhao (1.0 mmol L−1 [Fe(CN)6]3− in phosphate buffer 0.1 mol L−1 and 0.01 mol L−1 KCl at different pH and v = 100 mV s−1), only one pKa value was observed for TNA and iTNA, which is inconsistent with the existence of two protonation sites (NH2 and N of the pyridine ring). To our knowledge, there is no method in the literature to determine more than one surface pKa value of SAMs. In this work, we propose a modification of Zhao’s method which involves a lower concentration of the redox couple (10.0 μmol L−1 [Fe(CN)6]3− in 0.01 mol L−1 phosphate buffer and 0.01 mol L−1 KCl at various pH values) and lower scan rate (v = 15 mV s−1). The decrease in the ionic strength and the concentration of the redox couple decrease the repulsion among the ions in solution, favoring the interaction between the monolayer and the redox couple in solution. In addition, the charge, instead of current, was used because of difficulties in determining the baseline at certain pH values. The relationship of charge with pH is expressed in eqs 3 and 4 for systems with one and two protonation sites, respectively:

measurements, a closed apparatus was used to avoid changes in the solution concentrations, particularly for 1,4-dt for the reasons mentioned previously. For the in situ QCM analysis, a baseline was first obtained with water for 10 min. After that, a steep change in frequency was observed following injection of the samples containing the adsorbate. Following the analysis of Nuzzo,36 this first step is likely related to the surface-headgroup reaction with the surface which occurs over a very short time. There was a subsequent second step, which presented much slower kinetics and which we ascribed to a surface reorganization/crystallization process likely involving chain disorder, different chain−chain interactions, and surface mobility of molecules. For the species under study, two different behaviors were observed for this second step. For iTNA and TNA, there was a gradual increase in ΔF with time while for Hpyt, pyS, and 1,4-dt a decrease in ΔF was observed. Although somewhat speculative on our part, we believe that the difference in behavior could be due to differences in the polarity of the molecules involved. In addition to the R-NH2S group, which allows hydrogen bond formation, iTNA and TNA have the highest dipole moments (see Figure S3 in Supporting Information) among the species studied so that they are likely to have a higher degree of solvation, especially in the early stages of adsorption. As the adsorbed molecules rearrange (second step) on the surface, some of the solvent molecules could be released, giving rise to the observed increase in frequency. After a steady state was achieved, the mass change (Δm in g cm−2) was calculated based on the Sauerbrey28 equation. It is well-known, however, that frequency changes are caused not only by changes in mass but also by changes in solution and film properties.37−41 Due to this reason, surface coverages of the gold crystals modified with the molecules under study were determined by reductive desorption right after QCM measurements as follows. After QCM measurements, the modified crystals were transferred to an electrochemical cell containing 0.5 mol L−1 KOH where reductive potential scans were run. From the obtained curves, the reductive desorption charges were converted to a mass change (ΔmRD). Table 2 presents Δm values calculated from QCM (ΔmQCM) and electrochemistry (ΔmRD).

⎡⎛ Q − Q A− ⎞ ⎤ Q = ⎢⎜ pAH ⎟ + Q A−⎥ K a − pH ⎣⎝ 10 ⎦ + 1⎠

Table 2. Values of ΔmQCM and ΔmRD and Number of Water Molecules Coadsorbed with TNA, iTNA, Hpyt, pyS, and 1,4dt Species

⎡⎛ Q AH − Q AH− ⎞ ⎛ Q − − Q 2− ⎞ ⎤ A ⎟ + ⎜ AH Q = ⎢⎜ pK2 − pH + Q ⎟ 2 −⎥ A ⎢⎣⎝ 10 a1 ⎥⎦ + 1 ⎠ ⎝ 10 pKa2 − pH + 1 ⎠

Δm, μg cm−2 SAM TNA iTNA Hpyt pyS 1,4-dt

QCM 0.48 0.65 0.49 0.35 0.39

± ± ± ± ±

0.05 0.07 0.03 0.15 0.07

RD 0.10 0.12 0.09 0.07 0.11

± ± ± ± ±

0.005 0.003 0.022 0.036 0.057

water molecules 29 34 28 17 17

± ± ± ± ±

(3)

8 4 7 6 3

(4)

QA−

where QAH and are charges of the probe on the SAM formed by protonated and deprotonated structures, respectively. Similarly, QAH2, QAH−, and QA2− are charges of the probe on the SAM for the fully protonated, half-protonated, and fully deprotonated sites and pKa1 and pKa2 are the values for the first and second protonation sites, respectively. The derivation of the equations above can be found in the Supporting Information. The surface pKa values of the SAMs formed with iTNA, TNA, Hpyt, and pyS were determined from the inflection points in the charge vs pH and current vs pH (Zhao’s method) curves which are presented in Figure 3. For pyS, only Zhao’s method was used because there is only one protonation site.

As can be ascertained, the values of ΔmQCM are greater than those for ΔmRD. Knowing that the surface coverages calculated from the integrated charge under the reductive peak are a direct measure of the adsorbed molecules, because it is based on the electrode reaction Au−SR + e− → Au0 + SR−, this difference is believed to be due to the presence of coadsorbed water molecules. In previously reported studies on the adsorption of Hpyt,26 TNA,25 and 1,4-dt27 on gold, we found that the monolayers formed with these molecules present defects 17828

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discrepancy observed in solution is attributed to a tautomeric equilibrium.50 After adsorption, however, this equilibrium is no longer operative, indicating changes in the thermodynamics of the dissociation reaction. In fact, the value of 5.3 is almost the same as that reported51 for pyridine in solution (5.2). In this work, the surface pKa value of the N atom of the pyridine ring was determined to be 7.5, higher than the data cited above. To evaluate the integrity of the monolayer after the acquisition of the voltammetric curves in solution containing [Fe(CN)6]3−, the reductive desorption potential was verified in an electrochemical cell containing 0.5 M KOH. In such an experiment, only one wave at −0.9 V was observed instead of at −0.56 V, which is the value assigned for the desorption of pyS from gold. This result suggests that there was a modification in the pyS monolayer after the acquisition of the voltammetric curves. In addition, a bare gold electrode was immersed in a saturated aqueous solution of Na2S for 1 h, and the surface pKa value was determined to be 7.0, a value close to that determined for the pyS monolayer in this work. This result is in agreement with the study of Lamp et al.18 in which the decomposition of a pyS monolayer on gold was followed by infrared reflection and XPS spectra as a function of the immersion time. In that work, the authors assigned the decomposition to the conversion to an adlayer composed of various forms of atomic and oligomeric sulfur, the presence of which increased with an increase in the immersion time. In comparison to the desorption curve acquired after QCM measurements, this result suggests that the oxidative cleavage of the CS bond of pyS is facilitated by the [Fe(CN)6]3− complex which is itself reduced. To verify this hypothesis, 1.0 mmol L−1 aqueous solutions of [Fe(CN)6]3− and pyS were mixed and successive UV−vis spectra, which are shown in Figure 4, were acquired and compared to a control

Figure 3. Charge vs pH (○) and current vs pH (●) curves for the SAMs formed with iTNA, TNA, Hpyt, and pyS on gold. Charge and current were obtained by cyclic voltammetry in solution containing [Fe(CN)6]3− as redox couple under different experimental conditions according to the employed method (as described in the text).

As ascertained from Figure 3, the charge and/or the current indicates a slower hET reaction with increasing pH. As reported for similar pyridyl-containing SAMs,25,43 this observation is ascribed to the repulsion between the modified surface and the negative complex in solution. The data obtained from the curves above are summarized in Table 3 where pKa values in solution are also shown, for comparison. Table 3. Surface and Solution pKa Values for iTNA, TNA, Hpyt, and pySa pKa in solution44−47

surface pKa SAM

1b

2c

1b

2c

iTNA TNA Hpyt pyS

4.5 5.0 4.2 7.5

7.9 8.5 n n

3.8 3.2 n 1.4

10.6 n n n

a

n = nonassigned or nonreported values. bN of pyridine ring. cNH2 group.

The surface pKa values assigned to the N atom of the pyridine ring are higher than those reported in solution. Similar trends have been previously reported by the Whitesides48 and Crooks groups.44,49 According to these authors, removal of acidic protons results in a close-packed monolayer of negative charge which is a thermodynamically unfavorable situation. According to the surface pKa values determined for TNA and iTNA molecules, a conformation in which the NH2 group is close to the surface is favored upon adsorption (as derived from SERS measurements), allowing the interaction of N with gold and making the deprotonation easier. In fact, in situ SERS (surface-enhanced Raman scattering) spectra of TNA SAMs on gold indicate a dependence of the closeness of the NH2 group to the surface on the applied potential.25 For iTNA SAMs, preliminary SERS results also indicate that the NH2 group is close to the surface, being possibly involved in the adsorption process. The behavior observed for pyS seems to be quite different. Values of 5.3 and 4.6 were reported for the surface pKa of the N atom of the pyridine ring, based on SERS45 and capacitance measurements, respectively.44 In addition, the pKa value reported for this molecule in solution is 1.446 while that expected based on theoretical calculations is 6.0.47 The

Figure 4. UV−vis spectra of 0.5 × 10−3 mol L−1 pyS aqueous solutions without (a) and with 1.0 × 10−3 mol L−1 [Fe(CN)6]3− (b) at t = 0 h (solid line) and t = 1 h (dashed line). Inset: Absorbance vs time curves for the solutions (black) without and (grey) with [Fe(CN)6]3−.

sample (0.5 mmol L−1 aqueous solution of pyS). After 1 h, the solution containing [Fe(CN)6]3− exhibited a significant decrease in the intensity of the absorption at 326 nm, which is assigned to an intraligand transition of pyS.46 For the spectra obtained in a solution without [Fe(CN)6]3−, there was almost no change observed after 1 h. This result supports our above hypothesis in the sense that the decomposition kinetics of pyS is faster in the presence of [Fe(CN)6]3−. 17829

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kapp = RT /(F 2R ctC*)

Electrochemical Impedance. Electrochemical impedance plots for bare and modified gold with iTNA, TNA, and Hpyt molecules were obtained in 0.1 mol L−1 KCl containing [Fe(CN)6]4−/[Fe(CN)6]3− as the redox probe (see Figure S5 in the Supporting Information). The charge-transfer resistances (Rct*) were determined from the extrapolation of the highfrequency capacitive loop to the impedance real axis and are summarized in Table 4. Assuming that all faradaic current of

where C* is the concentration of the redox couple in solution. As shown in Table 4, the values of kapp present, as expected, the opposite tendency relative to θ and Rct* parameters, i.e., an increase in θ and Rct* results in a decrease in kapp, indicating the difficulty of the hET reaction to take place after longer immersion times in the solutions of Hpyt, iTNA, and TNA. On the other hand, for SAMs derived from pyS, the value obtained after 10 min of immersion is close to that calculated for the bare gold electrode. Taking into account that [Fe(CN)6]3− is present in solution and in accordance with the previous discussion, two possible alternatives can be considered: (i) the structural conversion of the pyS SAM leads to an adlayer in which the adjacent species are sufficiently far apart to allow the redox probe to access the surface or (ii) the hET reaction occurs through the involvement of the orbitals of the adsorbed sulfur species.

Table 4. Values of Charge-Transfer Resistance (Rct*), Fractional Coverage (θ), and Apparent Charge-Transfer Rate Constant (kapp) of [Fe(CN)6]3−/4− as a Function of the Immersion Time of the Gold Electrode in Solution of iTNA and Hpyta SAM iTNA

TNA25

Hpyt

pyS52

1,4-dt27

immersion time (min)

Rct* (Ω cm−2)

θ

kapp × 103 (cm s−1)

0 5 30 1440 0 5 30 1440 0 5 30 1440 0 1 5 10 0 1 30 1440

8.2 40.9 73.5 136.4 15.4 38.2 80.9 217.9 9.2 37.1 39.2 71.7 4.6 11.5 8.7 5.5 23.8 55.0 75.0 180.0

− 0.79 0.88 0.94 − 0.60 0.81 0.93 − 0.75 0.77 0.88 − 0.60 0.48 0.16 − 0.58 0.68 0.87

4.04 0.81 0.45 0.24 2.18 0.88 0.41 0.15 3.60 0.89 0.85 0.46 7.42 3.01 3.93 6.35 1.45 0.56 0.44 0.18

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CONCLUSIONS Thermodynamic data obtained for the adsorption of iTNA, TNA, and Hpyt on gold indicate a spontaneous process with an average ΔG ads value of −36.43 kJ/mol and repulsive interactions between adjacent molecules. The kinetics data indicate that a kinetically controlled model is applicable to the adsorption process with rate constants of 90.9, 65.0, and 6.9 L/ mol s for iTNA, TNA, and Hpyt on gold, respectively. QCM studies are consistent with a two-step process in which the first one is associated with the surface-headgroup reaction (fast) while the second step, which is much slower, is ascribed to a surface crystallization process. A comparison of the values of adsorbed material calculated by QCM and electrochemistry indicated that the adsorption of these molecules is accompanied by water molecules. Surface pKa values of NH2 and the nitrogen atom of the pyridine ring of iTNA (4.5 and 7.9) and TNA (5.0 and 8.5) were determined based on a method proposed in this work, which is a modification on the voltammetric method of Zhao.43 With regards to stability of pyS SAMs on gold, our results are consistent with an oxidative decomposition, dependent on the time of immersion in the modifier solution, as previously suggested by Lamp et al.18

a

Reported values of TNA, pyS, and 1,4-dt are also presented. Values of Rct* were divided by the active area of the gold electrode. T = 25 °C.

the redox probe is due to pinholes within the SAMs, the fractional coverage (θ) values of the monolayers as a function of the immersion time were calculated based on eq 5: θ = 1 − (R ct /R ct*)

(6)

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

(5)

Additional figures and information. This material is available free of charge via the Internet at http://pubs.acs.org.

where Rct is the charge-transfer resistance of the bare gold electrode. As can be ascertained, the Rct* values for [Fe(CN)6]4−/3− increase with an increase in the immersion time, indicating a decrease in the density of the active sites on the gold surface. This result is consistent with QCM data which suggested that the kinetics of adsorption might involve a second step of surface crystallization that results in a more densely packed monolayer. Similar behavior was observed for SAMs derived from 1,4-dt on gold.27 For SAMs derived from pyS on gold, the Rct* values for [Fe(CN)6]4−/3− decreased with an increase in the immersion time, reinforcing the suggestion of a decomposition of the monolayer. The impedance data also allow for an evaluation of the apparent electron-transfer rate constant, kapp, in accordance with eq 6:

Corresponding Author

*E-mail: [email protected]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS T. F. Paulo thanks FAPESP for a grant. I. C. N. Diógenes thanks CNPq (307534/2011-1) for the grants. REFERENCES

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