Effects of ω-Functional Groups on pH-Dependent Reductive

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Effects of ω-Functional Groups on pH-Dependent Reductive Desorption of Alkanethiol Self-Assembled Monolayers Hirokazu Munakata, Daisuke Oyamatsu, and Susumu Kuwabata* Department of Materials Chemistry, Graduate School of Engineering, Osaka University, 2-1 Yamada-oka, Suita, Osaka 565-0871, Japan Received May 6, 2004. In Final Form: August 6, 2004 Self-assembled monolayers (SAMs) of alkanethiols having various terminal groups on their ω-positions were formed on a Au(111) electrode, and their reductive desorption was studied by linear sweep voltammetry, focusing on effects of solution pH on the desorption behavior. The peak potentials (Ep) of cathodic waves representing reductive desorption were found to be reflected by the pKa value of the thiol group and were negatively shifted with an increase in pH of the electrolyte solution. The magnitude of the pH dependency of Ep was greatly influenced by the hydrophobicity of the terminal groups. In the cases of alkanethiol SAMs having pH-sensitive terminal groups such as carboxyl and amino groups, their basicity was estimated from bending points appearing in the pH titration profile of Ep. This method allows direct determination of not only the pKa value of the arrayed groups but also that of the groups dissolved in solution simultaneously. The pKa values of the arrayed carboxyl groups in SAMs were larger by ca. 3 pH units than their original ones, while those for amino groups were smaller by ca. 2 pH units.

Introduction Organosulfur compounds such as alkanethiols, dialkyl disulfides, and dialkyl sulfides chemisorb strongly on metal surfaces and provide a highly ordered monolayer called a self-assembled monolayer (SAM).1-4 Ease of preparation of such a stable monolayer with high reproducibility facilitates investigation of the monolayer properties. If alkanethiols substituted by functional groups at the ω-position are used, one can introduce desired properties on metal surfaces.5-8 The SAMs of alkanethiols possessing reactive groups should be ideal models for understanding the properties of solid surfaces. An example of effects due to closely arraying functional groups is that the basicity of the arrayed functional groups is different from that of the original ones. Such behavior has been first investigated by potentiometric titration of micellar solutions of various surfactants.9-11 Since the SAM is a plane monolayer fixed on a solid substrate, it can be investigated by surface analysis techniques, such as contact angle titration,12 quartz crystal microbalance,13-16 capacitance measurement,17-19 inter* Corresponding author. E-mail: kuwabata@ chem.eng.osaka-u.ac.jp. Telephone & Fax: +81-6-6879-7372. (1) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 44814483. (2) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437-463. (3) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol. 19, pp 109335. (4) Ulman, A. Chem. Rev. 1996, 96, 1533-1554. (5) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321-335. (6) Bain, C. D.; Whitesides, G. M. Angew. Chem., Int. Ed. Engl. 1989, 28, 506-512. (7) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87-96. (8) Laibinis, P. E.; Fox, M. A.; Folkers, J. P.; Whitesides, G. M. Langmuir 1991, 7, 3167-3173. (9) Van Voorst Vader, F. Trans. Faraday Soc. 1961, 57, 2263-2271. (10) Van Voorst Vader, F. Trans. Faraday Soc. 1963, 59, 1225-1230. (11) Tokiwa, F.; Ohki, K. J. Phys. Chem. 1967, 71, 1824-1829. (12) Creager, S. E.; Clarke, J. Langmuir 1994, 10, 3675-3683. (13) Wang, J.; Frostman, L. M.; Ward, M. D. J. Phys. Chem. 1992, 96, 5224-5228.

facial force, and probe spectroscopy.20-23 In addition to such analyses using high-level instruments, conventional electrochemical measurements, in particular linear sweep voltammetry, are powerful for investigating the delicate properties of SAMs. Reductive desorption of alkanethiol SAMs is frequently utilized to know the amounts of the adsorbed molecules, the stability of the monolayer, the domain distribution in SAMs composed of different thiol molecules, and so on.24-29 In our previous paper, we reported shortly that electrochemical desorption reaction measured by voltammetry is also very useful to evaluate pKa values of carboxyl groups of 3-mercaptopropionic acid SAMs.30 This method allows direct determination of not only the pKa value of the arrayed carboxyl groups but also that of the molecules dissolved in solution simultaneously from the obtained titration curve. In this paper, to explicate (14) Shimazu, K.; Teranishi, T.; Sugihara, K.; Uosaki, K. Chem. Lett. 1998, 669-670. (15) Sugihara, K.; Teranishi, T.; Shimazu, K.; Uosaki, K. Electrochemistry 1999, 67, 1172-1174. (16) Sugihara, K.; Shimazu, K.; Uosaki, K. Langmuir 2000, 16, 71017105. (17) Bryant, M. A.; Crooks, R. M. Langmuir 1993, 9, 385-387. (18) Aoki, K.; Kakiuchi, T. J. Electroanal. Chem. 1999, 478, 101107. (19) Kakiuchi, T.; Iida, M.; Imabayashi, S.; Niki, K. Langmuir 2000, 16, 5397-5401. (20) Hu, K.; Bard, A. J. Langmuir 1997, 13, 5114-5119. (21) Van der Vegte, E. W.; Hadziioannou, G. J. Phys. Chem. B 1997, 101, 9563-9569. (22) Vezenov, D. V.; Noy, A.; Rozsnyai, L. F.; Lieber, C. M. J. Am. Chem. Soc. 1997, 119, 2006-2015. (23) Wallwork, M. L.; Smith, D. A.; Zhang, J.; Kirkham, J.; Robinson, C. Langmuir 2001, 17, 1126-1131. (24) Widrig, C. A.; Chung, C.; Porter, M. D. J. Electroanal. Chem. 1991, 310, 335-359. (25) Walczak, M. M.; Pnpenoe, D. D.; Deinhammer, R. S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687-2693. (26) Hobara, D.; Ota, M.; Imabayashi, S.; Niki, K.; Kakiuchi, T. J. Electroanal. Chem. 1998, 444, 113-119. (27) Hobara, D.; Sasaki, T.; Imabayashi, S.; Kakiuchi, T. Langmuir 1999, 15, 5073-5078. (28) Munakata, H.; Kuwabata, S.; Ohko, Y.; Yoneyama, H. J. Electroanal. Chem. 2001, 496, 29-36. (29) Imabayashi, S.; Iida, M.; Hobara, D.; Feng, Z. Q.; Niki, K.; Kakiuchi, T. J. Electroanal. Chem. 1997, 428, 33-38. (30) Munakata, H.; Kuwabata, S. Chem. Commun. 2001, 1338-1339.

10.1021/la048878h CCC: $27.50 © 2004 American Chemical Society Published on Web 10/08/2004

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its mechanism in detail, we investigate the effect of various ω-functional groups on the desorption behavior of SAMs. In particular, the desorption reactions of thiol molecules having carboxyl and amino terminals have been thoroughly examined as a method to evaluate the pKa values of these terminal groups. Experimental Section Materials. n-Alkanethiols terminated by methyl groups [HS(CH2)n-CH3 with n ) 1, 2, 7, 9 (Wako Pure Chemicals)], hydroxyl groups [HS-(CH2)n-OH with n ) 2, 4 (Wako Pure Chemicals) and n ) 6, 8 (Dojindo Laboratories)], carboxyl groups [HS(CH2)n-COOH with n ) 2, 5, 7, 10 (Dojindo Laboratories)], and amino groups [HS-(CH2)n-NH2 with n ) 2, 8, 11 (Dojindo Laboratories)] were reagent grade chemicals and were used without specific purification. Water used for preparation of electrolyte solutions was purified by a Milli-Q Gradient A10 (F > 18.2 MΩ cm). All other chemicals were of reagent grade. An electrode substrate used in this study was prepared by vacuum evaporation of Au (300 nm thickness) on a freshly cleaved natural mica sheet, which was heated at 300 °C for 2 h at least prior to the evaporation and during the Au deposition. The pressure chosen was below 5 × 10-6 Torr. It was confirmed by observation with a scanning tunneling microscope that the obtained Au electrode has atomically flat (111) terraces. A thiol SAM was formed by immersing the Au substrate for 1 h in an ethanol solution containing one of the above-mentioned thiols at 1 mmol dm-3. The prepared electrode was then rinsed with ethanol and water. Methods. Electrochemical experiments were carried out using a glass tube cell whose ends were open. The SAM-coated Au electrode was attached at the bottom hole of the cell with a Tefloncoated O-ring to give an active electrode area of 0.40 ( 0.02 cm2, which was estimated by measuring the charges involved in oxidation of chemically adsorbed iodine.31-33 After pouring the electrolyte solution, the cell was capped with a silicon rubber equipped with a reference electrode of Ag|AgCl in KCl-saturated aqueous solution and a Pt foil counter electrode. The electrochemical detection of reductive desorption of the thiol SAMs was conducted by linear sweep voltammetry using an ALS-701 electrochemical analyzer. The potential sweep rate chosen was 200 mV s-1 to obtain sufficiently high sensitivities for all SAMs except for the case of the 3-mercaptopropionic acid SAM where definite reduction waves due to the desorption reaction could be obtained by careful experiments with the sweep rates ranging from 10 to 1000 mV s-1. The electrolyte solution used was deaerated by bubbling nitrogen gas for 15 min at least prior to the desorption experiments. The software supplied with the ALS electrochemical analyzer possessed a function capable of integration of redox waves appearing in the obtained voltammograms, allowing electrochemical estimation of the amount of thiol molecules adsorbed on the Au electrode based on the fact that the reductive desorption of thiol is induced by one electron reaction.

Results and Discussion OH-Terminated Thiol SAM. Figure 1 shows linear sweep voltammograms of 2-mercaptoethanol (2-ME) SAMcoated Au(111) electrodes taken in 0.1 mol dm-3 phosphate buffer solutions having different pHs from 4.53 to 11.49. Cathodic waves representing reductive desorption of thiol appeared in all cases, and the peak potential was positively shifted with decreasing pH of the electrolyte solution. It is also recognized that the decrease in pH caused broadening of the wave and lowering of its height. Although such changes in the shape of the wave occurred, the charges calculated by integration of cathodic currents (31) Rodriguez, J. F.; Mebrahtu, T.; Soriaga, M. P. J. Electroanal. Chem. 1987, 233, 283-289. (32) Walczak, M. M.; Alves, C. A.; Lamp, B. D.; Porter, M. D. J. Electroanal. Chem. 1995, 396, 103-114. (33) Oyamatsu, D.; Nishizawa, M.; Kuwabata, S.; Yoneyama, H. Langmuir 1998, 14, 3298-3302.

Figure 1. Linear sweep voltammograms of 2-mercaptoethanol SAMs on Au(111) electrodes taken at 200 mV s-1 in 0.1 mol dm-3 phosphate buffer having the given pH values.

Figure 2. Plots of peak potentials (a) and fwhm (b) of cathodic waves due to reductive desorption of 2-ME SAMs as a function of the pH of the electrolyte solution.

gave almost the same value of (8.0 ( 0.4) × 10-5 C cm-2, which corresponded to (8.3 ( 0.4) × 10-10 mol cm-2 of 2-ME adsorbed on the electrode. Thiol molecules closely packed in a SAM are arrayed on Au(111) with the (x3 × x3)R30° structure, giving a molecular density of 7.6 × 10-10 mol cm-2.24 The fact that the experimentally obtained values were close to this value suggested that the 2-ME SAM kept the packed structure in the solutions of different pHs just before the electrochemical reduction. To clarify the dependency of the cathodic wave on solution pH, the peak potentials (Ep) and full widths at half-maximum (fwhm) of the waves are plotted as a function of pH, as shown in Figure 2. In many other studies, the reductive desorption of an alkanethiol SAM was usually made in strong alkaline solutions because it is expected that the thiolate anions are completely dissolved in solution after desorption from the electrodes.24-29 Then, the reaction can be formulated as follows:

Au-S-R + e- f Au + R-S-

(1)

where R denotes an alkyl group. The Nernst equation of this reaction can be expressed as follows:

E ) E0 +

RT aAu-SR ln nF aRS-

(2)

However, if the solution pH decreases, protonation of thiolate (eq 3) should occur just after the desorption reaction.

pH-Dependent Reductive Desorption of SAMs

R-S- + H+ / R-SH

Langmuir, Vol. 20, No. 23, 2004 10125

(3)

Then, the total reaction in the lower pH region can also be expressed by combination of eq 1 and eq 3 as follows:

Au-S-R + H+ + e- f Au + R-SH

(4)

The dissociation constant of thiol is expressed by

Ka )

aRS-aH+ aRSH

aRS- ) ared

Ka aH+ + Ka

(5)

(5′)

where ared is the sum of aRS- and aRSH, which corresponds to the activity of total reduced (desorbed) species. Equation 2 is transformed to eq 6 by substitution of eq 5′.

E ) E0 +

) E0 +

RT ln nF

aox Ka ared aH+ + Ka

aox RT aH+ + Ka RT ln ln + nF ared nF Ka

(6)

Then, the reversible half-wave potential (E1/2) at 25 °C can be written as eq 7.

E1/2 ) E0 - 0.591 log Ka + 0.0591 log(aH+ + Ka) (7) This equation predicts the following E1/2 shifts.

E1/2 ) E0

for pH > pKa

E1/2 ) E0 - 0.0591pH

for pH < pKa

(8) (9)

The Ep shifts shown in Figure 2a give a bending point at pH ) 9.5, below which Ep shifts with a slope of -59 mV/ pH are seen. This pH value is perfectly in accordance with the pKa value of 2-ME (9.5).34 Precise analysis of the electrochemical desorption of a thiol SAM cannot be made by only the Nernst equation because it is not a simple reversible redox reaction. However, the results obtained for the 2-ME SAM suggest that the use of this equation is allowed for the general analysis of Ep shifts of the SAM desorption. The plots of fwhm as a function of pH shown in Figure 2b show that the peak shape becomes broad with a decrease in pH, while the sharpest shape is obtained at pH > pKa of 2-ME. Similar changes in the shape of the reduction peak due to thiol desorption have been observed for the electrochemical desorption of thiols having different alkyl chain lengths; the shape becomes sharp as chain length increases. Aoki and Kakiuchi succeeded in simulating the cathode waves due to thiol desorption by introducing a honeycomb model for the arrangement of thiol molecules.35 They found that changes in the peak shape are depicted by consideration of the interaction between neighboring thiol molecules and that between a thiol molecule and species in solvent, the latter of which insert in vacant sites appearing in the initial stage of thiol (34) Jencks, W. P.; Regenstein, J. In Handbook of Biochemistry and Molecular Biology; Fasman, G. D., Ed.; CRC Press: Cleveland, 1976; Vol. 3, pp 305-351. (35) Aoki, K.; Kakiuchi, T. J. Electroanal. Chem. 1998, 452, 187192.

Figure 3. Plots of the peak potentials of cathodic waves due to reductive desorption of OH-terminated alkanethiol SAMs of n ) 4 (O), 6 (4), and 8 (0) as a function of the pH of the electrolyte solution. Voltammograms were measured at 200 mV s-1 in 0.1 mol dm-3 phosphate buffer.

desorption. When the former and the latter parameters are denoted by uM and uV, respectively, a parameter denoting their difference (u ) uM - uV) is related to the fwhm of the desorption peak; an increase in u reduces the fwhm value. In the case of thiol molecules having different chain lengths, it is considered that the uM determines the peak shape with the assumption that uV is constant regardless of the chain length. In the case of 2-ME as shown in Figure 1, however, it is unlikely that uM values are largely changed by pH changes. It would be better to assume that uM is constant, while an increase in uV makes the peak broad as pH decreases. According to a surface potential study of the SAM by Ulman et al., a sulfur of a thiol molecule adsorbed on Au has some negative charges,36 which should have electrostatic interaction with H3O+ in solution. Since the definition of u implies that an increase in uV and a decrease in uM are equivalent, it can be speculated that an increase in the interaction between sulfur and H3O+ caused by decreasing pH has the same effect as the case of a decrease in the chain length of a thiol molecule. Consequently, the reduction wave becomes broad as pH decreases. In the region where pH is larger than the pKa of thiol, since a proton is not bound to R-S- even in the dissolved condition, it is not expected that negative charges of the adsorbed thiol interact with H3O+, giving the sharpest reduction waves as shown in Figure 2b. Figure 3 shows plots of the Ep obtained for desorption of three different OH-terminated thiol SAMs having alkyl chains (HS-(CH2)n-OH with n ) 4, 6, 8) as a function of the pH of the electrolyte solution. As the alkyl chain of thiols became longer, Ep shifted to negative potentials. A similar relationship has already been reported for other kinds of thiol SAMs.29 However, it was found in this study that the relationship between Ep and the length of the alkyl chain was kept with the same ratio, regardless of solution pH. In the case of the SAM of HS-(CH2)n-OH, -51 mV/n was estimated as an average value. CH3-Terminated Thiol SAM. Figure 4 shows plots of Ep resulting from reductive desorption of normal CH3terminated alkanethiols having various lengths of the alkyl chain (HS-(CH2)n-CH3 with n ) 1, 2, 7, 9) as a function of pH. In the same manner as the case of the OH-terminated thiol SAM, two pH regions are recognized: a pH-independent region and a linear change region. The pH value of 9.6-10.7 where the bending point is observed agrees well with the pKa of the thiol group in (36) Evans, S. D.; Ulman, A. Chem. Phys. Lett. 1990, 170, 462-466.

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Figure 4. Plots of the peak potentials of cathodic waves due to reductive desorption of CH3-terminated alkanethiol SAMs of n ) 1 (O), 2 (4), 7 (0), and 9 (3) as a function of the pH of the electrolyte solution. Voltammograms were measured at 200 mV s-1 in 0.1 mol dm-3 phosphate buffer.

a CH3-terminated alkanethiol, which is slightly larger than that of the OH-terminated one (e.g., the pKa of an ethanethiol is 10.534). It is, therefore, suggested that desorption of CH3-terminated SAMs also proceeds according to the reactions of eq 1 and eq 4 in solutions of pH > pKa and pH < pKa, respectively. However, the slope of Ep at pH < pKa was estimated to be -33 mV/pH, which was apparently much smaller than the slope expected from the Nernst equation. The desorption of alkanethiol SAMs with eq 1 in strong alkaline solution requires charge compensation by electrolyte cations for generated thiolate anions. In an initial stage of the reaction, since thiolate anions generate at the interior of the monolayer, cations must penetrate through the packed alkyl layer. Therefore, the properties of electrolyte cations must influence the desorption behavior of alkanethiol SAMs, and the validity of this assumption has been clarified by voltammetry studies on desorption of SAMs with several kinds of electrolyte cations and their concentrations.24 Comparison of results shown in Figure 4 with those shown in Figure 2a allows one to recognize the importance of the surface properties of the SAM for desorption. If Ep values obtained in the pH-independent region for the OH-terminated thiol SAM and the CH3terminated one having the same length are compared, the former SAM is apparently desorbed at more positive potentials. This seems to be understood by consideration of the hydrophilicity of the terminal groups. It should be easier for electrolyte cations to approach and go into the monolayer having hydrophilic -OH groups on its surface than the completely hydrophobic -CH3 group. Then, a decrease in the blocking effect of the terminal group results in a positive shift of the reduction wave. Such blocking properties of SAMs observed in voltammetric responses, caused by the affinity between the terminal group and reactive species, have been reported by some research groups.37,38 In the case of the reductive desorption in the pH region where linear change in Ep is observed, protons as well as electrolyte cations are included in the reaction and the ratio of the contribution of both species to the reaction depends on H+ activity in solution. As mentioned above, in an initial stage of the desorption, species present in the electrolyte solution have to penetrate the monolayer to compensate charges of the generated thiolate anions. This must be true at pH < pKa not only for electrolyte cations (37) Giz, M. J.; Duong, B.; Tao, N. J. J. Electroanal. Chem. 1999, 465, 72-79. (38) Takehara, K.; Takemura, H.; Ide, Y. Electrochim. Acta 1994, 39, 817-822.

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Figure 5. Plots of the peak potentials of cathodic waves due to reductive desorption of COOH-terminated alkanethiol SAMs of n ) 2 (O), 5 (4), 7 (0), and 10 (3) as a function of the pH of the electrolyte solution. Voltammograms were measured at 200 mV s-1 in 0.1 mol dm-3 phosphate buffer.

but also for H3O+. The permeability of H3O+ into SAMs by the kind of terminal group has been discussed by Chidsey and Loiacono, measured by proton reduction on SAM-coated Au electrodes.39 It has been reported that proton reduction is easier at hydrophilic-terminated SAMs such as -OH and -COOH groups. In the case of -OHterminated thiol SAMs, protons can easily approach the surface of the SAM and penetrate to the bottom of the SAM (sulfur atom) through the vacant site. Then the H+ activity is similar to that in the electrolyte solution. Thus, the potential shift of Ep would roughly correspond to H+ activity in the electrolyte solution. On the other hand, since it would be difficult for the proton to approach the hydrophobic -CH3 surface of the SAM, H+ activity around the generated thiolate anions might be different from that in the electrolyte solution. This could give a slope of Ep versus pH plots much smaller than 59 mV/pH and shifts of the bending points to the alkaline direction by increasing the chain length. The finding that the degree of change in fwhm caused by pH change was much smaller than the case of OH-terminated thiol SAMs seems to be another fact suggesting high blocking effects of CH3-terminated thiol SAMs, although the results are not shown here. In addition, it is noteworthy that the slope of Ep at pH < pKa kept almost the same values regardless of length of alkyl chain (Figure 4). This behavior was also observed in the cases of OH-terminated SAMs (Figures 2a and 3). Since H+ (H3O+) is smaller than K+, H+ can easily move through the vacant site in the SAM, once it passes the terminal groups of the SAM. Thus, these results suggest that the kinetics of the desorption reaction initiated by the proton permeation is mainly determined not by the blocking effect of intermediate alkyl groups but by that of the terminal group of the SAM (-OH or -CH3). COOH-Terminated Thiol SAM. Figure 5 shows relationships between Ep and pH obtained for COOHterminated alkanethiol SAMs. All SAMs except the 11mercaptoundecanoic acid (n ) 10) SAM exhibit two distinct bending points. Since the pKa of the -SH group of COOHterminated thiol is shifted to larger than 12 due to the inductive effect of negatively charged -COO-,40 the bending point representing the pKa of -SH cannot be observed in the pH region chosen in this study. In our (39) Chidsey, C. E. D.; Loiacono, D. N. Langmuir 1990, 6, 682-691. (40) Perrin, D. D.; Dempsey, B.; Serjeant, E. P. pKa Prediction for Organic Acids and Bases; Chapman and Hall: London, 1981.

pH-Dependent Reductive Desorption of SAMs

Figure 6. Titration curves obtained from the peak potentials of cathodic waves due to reductive desorption of the COOHterminated alkanethiol SAM with n ) 2 taken at 10 (O), 50 (4), 200 (0), 500 (3), and 1000 ()) mV s-1.

previous paper, it has been reported for 3-mercaptopropionic acid (MPA, n ) 2) SAMs that the titration profile obtained is useful to determine the pKa value of the carboxyl group of the MPA SAM as well as that of dissolved MPA. In this case, the bending points appear at pH ) 4.3 and 7.5. Since the former value coincides with the original pKa of MPA, the linear Ep change with a slope of -50 mV/pH observed at pH < 4.3 indicates occurrence of the reaction represented by eq 4, where R is (CH2)2-COOH. The pKa of the carboxyl group of the adsorbed thiol SAM was also investigated by electrochemical quartz crystal microbalance (EQCM)13-16 and capacitance17-19 experiments, and values larger than the original value (4.3) have been reported. Then, if pH ) 7.5 where the other bending point appears is assumed to be the pKa of the -COOH group of adsorbed thiol molecules, the profile of Ep versus pH shown in Figure 5 can be explained. The reaction represented by eq 4, where R is (CH2)2-COO-, takes place at pH > 7.5. In the pH region between 4.3 and 7.5, the carboxyl group in the MPA SAM is -COOH but it must be deprotonated when MPA molecules are desorbed. Accordingly the desorption reaction in this pH region can be given by

Au-S-(CH2)2-COOH + e- f Au + HS-(CH2)2-COO- (10) Since the deprotonation of the carboxyl group and the protonation of the generated thiolate take place simultaneously, proton is excluded from the reaction, resulting in lower pH dependence. However, a slope around -10 mV/pH was observed and the slope was increased as the length of thiol was longer. This phenomenon was also investigated by varying the potential sweep rates, and the results are summarized in Figure 6. Apparently, a decrease in potential sweep rate makes the slope of Ep plots at 4.3 < pH < 7.5 smaller without any shift of the locations of both bending points. If relatively high sweep rates were chosen for the voltammetry measurements, the electrode potential would be changed at a rate higher than the rate of penetration of H+ through the MPA SAM. It is, then, supposed that the peaks of the cathodic waves do not appear at potentials that are expected from the pH of the electrolyte solution. These results suggest that the intramolecular transfer of H+ is slightly influenced by the

Langmuir, Vol. 20, No. 23, 2004 10127

Figure 7. Plots of the peak potentials of cathodic waves due to reductive desorption of NH2-terminated alkanethiol SAMs of n ) 2 (O), 8 (4), and 11 (0) as a function of the pH of the electrolyte solution. Voltammograms were measured at 200 mV s-1 in 0.01 mol dm-3 phosphate buffer containing 0.1 mol dm-3 K2SO4.

blocking effect of intermediate alkyl chains, which is not the dominant factor in the case of -OH- and -CH3terminated thiol SAMs, although the precise mechanism is still unclear. NH2-Terminated Thiol SAM. To extend the applicability of the SAM-desorption technique for evaluating the surface basicity, the titration curves for alkanethiols having amino groups were investigated, and the obtained results are shown in Figure 7. All titration curves indicate two distinct bending points at pH ) 6.4-6.8 and pH ) 8.6-8.9. The latter values agree well with the pKa values of primary alkylamines. Then, the linear change in Ep observed at pH > 8.9 is attributable to eq 4, where R is (CH2)n-NH2 . However, the observed slopes around -30 mV/pH were smaller than the ideal value although the hydrophilic amino groups were exposed on the surface of the monolayer. This would be due to the strong interaction between the neighboring amino groups. If such stronger interaction contributed to a closely packed monolayer, it would prevent penetration of species in the electrolyte solution. The interaction between amino groups was investigated by chemical force microscopy.21-23 In their reports, the adhesion force between NH2 groups was stronger than that between -COOH groups, which was not expected from their calculated relative hydrogen bond strengths. Appropriate factors causing this phenomenon have not yet been elucidated, but the fact that the interaction between arrayed amino groups is stronger than that for carboxyl groups seems to be one of the reasons. The linear Ep change with slopes around -30 mV/pH was also observed at pH < 6.4. The appropriate reaction in this pH region is eq 4, where R is (CH2)n-NH3+. It has already been reported that the pKa of the amino group in SAMs tends to be smaller than the original value; for example, 5.3 (capacitance titration)19 and 3-7 (chemical force microscopy)21-23 were reported. If pH values of 6.4-6.8 where the bending points appeared were assumed to be the pKa values of amino groups of the NH2-terminated SAMs, the titration profiles shown in Figure 7 could be understood. In the pH region between the pKa of the NH2terminated thiol and the pKa of its SAM, NH2 of the adsorbed thiol is protonated when it is desorbed from the electrode. Then, the reaction is formulated as

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Au-S-(CH2)n-NH2 + e- + 2H+ f Au + HS-(CH2)n-NH3+ (11) The equation includes two protons because of simultaneous protonation of thiolate and amino groups. This should give a dependence of Ep on pH whose magnitude is greater than that observed in other pH regions. In the case of the SAM of 2-aminoethanethiol, Ep changed at 6.4 < pH < 8.6 with a slope of -110 mV/pH, which is close to the value expected from the Nernst equation including one electron and two protons. However, such a characteristic profile became dull with an increase in length of the thiols due probably to kinetic factors mentioned above. Conclusions It has been demonstrated in this study that reductive desorption of alkanethiol SAMs measured by conventional voltammetry becomes a useful way to evaluate the properties of functional groups substituted on the terminals of alkanethiols. The pH dependency of the peak

Munakata et al.

potentials of the reductive desorption wave is a significant clue to elucidate the basicity of functional groups in arrayed molecules in SAMs and those dissolved in solution. That also gives some information concerning packing conditions of the monolayer and affinity of the layer surface. The latter information is obtained from the degree of deviation of the cathodic wave from the ideal one based on the Nernstian behavior. However, a concrete way to evaluate its degree was not developed in this study. To develop such a way, it would be necessary to elucidate kinetic factors controlling the shape and position of the cathodic wave, and successive research works are underway, aiming at development of a method to clarify the SAM structure by electrochemical means. Acknowledgment. This research work is partially supported by CREST of the Japan Science and Technology Corporation and by a Grant-in-Aid for Scientific Research on Priority Areas (417) from the Ministry of Education, Culture, Sports, Science and Technology, Japan. LA048878H