J. Phys. Chem. C 2008, 112, 6811-6815
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The Effect of Surface Preparation on Apparent Surface pKa’s of ω-Mercaptocarboxylic Acid Self-Assembled Monolayers on Polycrystalline Gold Stuart C. Burris,* Yi Zhou, Winston A. Maupin, Andrew J. Ebelhar, and Michael W. Daugherty Chemistry Department, Western Kentucky UniVersity, 1906 College Heights BouleVard, Bowling Green, Kentucky 42101-1079 ReceiVed: September 2, 2007; In Final Form: February 20, 2008
We have measured the apparent surface pKa values of self-assembled monolayers (SAMs) formed from 3-mercaptopropionic acid (3-MPA) and thioctic acid (TA) on evaporated gold film electrodes with a range of surface roughness (Ra from 1.3 to 6.3 nm as measured by AC mode atomic force microscopy). The surface roughness is induced and controlled with electrochemical etching via multiple potential cycles between 0.2 and 1.5 V (versus Ag/AgCl/1.0 M KCl) in 100 mM H2SO4 with KCl concentrations ranging from 0.10 to 10 mM. The pKa’s are measured by determining the capacitance of the surface using electrochemical impedance spectroscopy in a range of controlled ionic strength (µ ) 0.25 M) buffer solutions. The results for 3-MPA are striking, exhibiting a strong logarithmic correlation (R2 ) 0.988) between the KCl concentration used in the electrochemical etching solutions and the apparent surface pKa. Over the range of KCl concentrations, the surface pKa of 3-MPA shifts from 6.5 to 8.4. However, the relationship between the KCl concentrations in the electrochemical etching solutions and the apparent surface pKa of TA is flat at 7.3 ( 0.2 with an R2 value of less than 0.3 for both linear and logarithmic relationships.
1. Introduction The last two decades have produced nearly uncountable volumes of work on systems utilizing self-assembly1,2 for surface modification. This approach has allowed the creation of many different types of modified surfaces with highly desirable functions such as chemical sensors,3,4 binding promoters for electron-transfer kinetics studies,5,6 lithography platforms,7,8 etc. Self-assembled monolayers (SAMs) formed on gold using thiol compounds with acidic functionalities exposed to solution have found utility in nearly all of these areas. Carboxylic acid groups are especially useful in cases where further reaction with the surface is desired such as protein immobilization.9,10 An important aspect of controlling these interactions is knowledge of the difference between the solution property values of the surface modifiers, such as pKa, and their related properties once confined to the surface, such as pK1/2 or apparent surface pKa. Numerous recent studies have probed the acidity of ω-mercaptocarboxylic acid SAMs on gold using a variety of techniques, including contact angle measurements,11,12 chemical force titrations,13,14 quartz crystal microbalance (QCM),15 and electrochemical methods.12,16-18 Several of these12,14,15,17,18 explicitly report apparent surface pKa values for 3-mercaptopropioinic acid (3-MPA); however, they do not broadly agree with one another. The contact angle measurements of Zhao and co-workers12 on a relatively rough 0.05-µm alumina polished gold wire set the value at 5.6, while their measurements using ferricyanide as an electrochemical probe place the value at 5.2 on this substrate.19 Hu and Bard14 report the apparent surface pKa of 3-MPA on template-stripped gold surfaces20 at 7.7. Shimazu et al.15 determined the value to be 5.8 using evaporated gold on a quartz crystal in the QCM technique. Niki’s group * Corresponding author. Phone: (270) 745-2973. Fax: (270) 745-6293. E-mail:
[email protected].
used a double-layer capacitance titration17 to place the value at about 8 on a Au(111) substrate. Dai and Ju18 used an electrochemical technique and surfaces similar to those of Zhao and co-workers12 and ostensibly agreed with their measurement, reporting an apparent surface pKa of 5.3 for 3-MPA in the context of a chain-length dependence study. Many other reports corroborate this range of values for the apparent surface pKa of 3-MPA. A close inspection of the gold substrates used in these studies places them in three categories. The first is the extremely rough, those of Dia and Ju18 and Zhao and co-workers,12 consisting of mechanically polished gold wires encased in glass. These surfaces exhibit the most acidic apparent surface pKa values for 3-MPA, around 5.2. The second is the intermediate roughness of evaporated gold on quartz in the QCM study by Shimazu et al.15 This somewhat more moderate level of surface roughness produces 5.8 for the apparent surface pKa of 3-MPA. The third group contains the extremely smooth substrates, Niki’s Au(111)17 and the template-stripped gold substrate of Hu and Bard14 with apparent surface pKa reports of 8 and 7.7, respectively. A similar set of observations and a very insightful interpretation was made by Leopold and Bowden.21 However, their concern was primarily with much longer-chain ω-mercaptocarboxylic acids than what we focus on here. Specifically, they explored 11-mercaptoundecanoic acid and 14-mercaptotetradecanoic acid in the context of studying the effect of gold substrate topography on cytochrome c voltammetry.22 This Article reports the systematic investigation of the effect of the gold substrate surface preparation on the apparent surface pKa of 3-MPA on gold films evaporated on glass as measured by double-layer capacitance titrations in a controlled ionic strength buffer system. The surface morphology is monitored by atomic force microscopy to confirm that the surface preparation by electrochemical etching is providing different environments at different etchant concentrations.
10.1021/jp077052w CCC: $40.75 © 2008 American Chemical Society Published on Web 04/08/2008
6812 J. Phys. Chem. C, Vol. 112, No. 17, 2008 Parallel results are presented for thioctic acid to provide a comparison to 3-MPA with a molecule that is not likely to be as well packed. 2. Experimental Section Materials. Evaporated gold film substrates were purchased from EMF Corp. (Ithaca, NY) and consisted of a 25 × 25 × 1.6 mm glass substrate with a 100-nm Au layer over a 5-nm Ti layer (to promote adhesion). These were used as received, with no solvent cleaning prior to the electrochemical etching described below. Sulfuric acid, potassium chloride, denatured reagent ethanol (containing 5% 2-propanol and 5% methanol by volume), anhydrous citric acid, boric acid, anhydrous dibasic potassium phosphate, potassium hydroxide, and hydrochloric acid were all purchased in reagent grade or better from Fisher Scientific and used as received. The 3-mercaptopropionic acid (3-MPA) (99+%) and DL-thioctic acid (TA) (98+%) were purchased from Acros Organics. These were also used as received. Water was of Type 1 quality. Electrochemical Etching. Cyclic voltammetry was performed with a Princeton Applied Research (Oak Ridge, TN) PARSTAT 2263 Advanced Electrochemical Workstation, operating with Electrochemistry PowerSUITE. The reference electrode was Ag/AgCl with 1.0 M KCl, and a platinum wire was used as the counter electrode. The KCl concentration was kept at 1.0 M in the reference electrode to minimize the amount of additional chloride ion introduced to the etching solution by leakage through the frit. The gold surfaces were electrochemically etched in 100 mM H2SO4 containing varying concentrations of KCl (10, 5.0, 2.0, 1.0, 0.50, 0.20, and 0.10 mM) via potential cycling 10 times in each of three potential ranges: 0.2-0.9 V, 0.2-1.2 V, and 0.2-1.5 V.23,24 The electrochemical cell was constructed from polycarbonate material and used a double-seal viton O-ring with an inner diameter of 0.64 cm, exposing 0.32 cm2 of the electrode surface. All solutions used for electrochemical etching were purged with UHP argon for 10 min to significantly reduce the oxygen level. Atomic Force Microscopy. Surface topography images of representative electrodes were acquired by atomic force microscopy (AFM) with a Molecular Imaging PicoPlus AFM system (Molecular Imaging, Tempe, AZ) operating in Acoustic AC mode. Silicon probes (Budget Sensors Tap300Al, Sofia, Bulgaria) were used with the Molecular Imaging small multipurpose scanner (10 µm maximum scan range). Surface roughness data were obtained using the statistical quantities function in the AFM data analysis program Gwyddion.25 Gwyddion calculates average roughness (Ra) as an arithmetical mean deviation as specified in ASME standard B46.1. Surface Modification. Self-assembled monolayers of the 3-MPA and TA compounds were formed by exposing the surface of the electrode in the electrochemical cell to a 1.0 mM solution of the compound in denatured reagent ethanol. Minimum exposure times were 12 h for the 3-MPA and 24 h for the TA. Double-Layer Capacitance Titrations. A nitrogen-free buffer utilizing citrate, borate, and phosphate ions in the manner of Teorell and Stenhagen26,27 was used to control the pH over the range of 5.0-10.0 in increments as small as 0.1 pH unit during the double-layer capacitance titrations. The ionic strength of the buffers was controlled to within 10% at 0.25 M to minimize ion density effects from the solution as the pH was changed. This was accomplished by careful preparation of stock solutions of citrate, borate, and phosphate; standardization of the potassium hydroxide and hydrochloric acid solutions; close
Burris et al. tracking of the volumes of each component used to reach the target pH for each buffer; and dilution of all solutions to the same ionic strength. A minimum of 2 min was allowed for the surface to equilibrate after each buffer solution was introduced to the electrochemical cell. Electrochemical impedance spectroscopy (EIS) was also performed with the PARSTAT 2263. All EIS experiments were conducted at zero volts versus the reference electrode described above. The EIS experiments all utilized a 10 mV amplitude sine wave, and equivalent circuit analysis was accomplished with ZSimpWin (EChem Software, Ann Arbor, MI). The buffer solutions used in the EIS experiments were not purged, as oxygen does not play a role at the potential where these experiments were performed. 3. Results Electrochemical Etching. The effects of the varying concentrations of KCl in the electrochemical etching solution are illustrated in the cyclic voltammograms shown in Figure 1. Focusing on the plot for 10 mM KCl (in blue), the anodic peak for the formation of [AuCl4]- is observed at 1.16 V. The shoulder on the positive potential side of this peak is due to the formation of gold oxide on the surface. In this same voltammogram, the reduction of [AuCl4]- is observed in the cathodic peak at 0.64 V, while the reduction of the surface oxide occurs at 0.87 V. This is in accordance with the well-known behavior of gold in an acidified chloride ion solution.24 An analysis of the peak currents for the formation of the [AuCl4]- shows a good correlation with the KCl concentration down to the 0.50 mM level as shown in Figure 2. Below this level, the formation of [AuCl4]- is not observed in the cyclic voltammetry. This indicates that it is unlikely that KCl levels below about 0.50 mM will have a pronounced effect on the surface morphology. Atomic Force Microscopy. Figure 3 shows topographs for representative surfaces etched in the 10, 1.0, and 0.10 mM KCl as well as an example of an unetched surface. In viewing these data, take note of the z-axis ranges for the four topographs, which vary from just over 12 nm for the unetched surface to 65 nm for the most heavily etched surface. The unetched surface displays small spheroid features with diameters from 10 to 50 nm, while the most heavily etched surface has larger spheroid and ellipsoid features from 60 to 200 nm in diameter. While these observations provide only a qualitative to semiquantitative measure of the effects of the electrochemical etching process on the surface morphology, more quantitative, exemplary average roughness data (for typically 2 µm scans) are summarized in Figure 4. These data show that above a KCl concentration of 0.50 mM, there is a marked increase in the surface roughness, which linearly follows the KCl concentration. Below the 0.50 mM KCl level, the surface roughness values are grouped only slightly above the typical unetched surface. Double-Layer Capacitance Titrations. The key experimental measurement in this study was the capacitance of the electrical double layer (Cdl) at the SAM-solution interface. The EIS data collected on all of the electrodes display a very slightly depressed semicircle in the capacitance plane characteristic of rough surfaces.28 Because of this, Cdl could be estimated here by an equivalent circuit analysis of the EIS data using a simple RC-type equivalent circuit with a constant phase element (CPE)28 [RQ]. However, because the depression of the semicircular data in the capacitance plane is very slight (frequency power values typically greater than 0.95), the simple RC circuit was used here. Figure 5 shows a typical double-layer capacitance titration curve for 3-MPA. The general procedure for determining these
Surface pKa’s of ω-Mercaptocarboxylic Acid
J. Phys. Chem. C, Vol. 112, No. 17, 2008 6813
Figure 1. Cyclic voltammograms of the final cycle of the electrochemical etching process for representative electrodes. Etching solutions contain 100 mM H2SO4, and the concentration of KCl is indicated for each in the figure.
the KCl concentration in the electrochemical etching solution. A parallel process for the TA-modified surfaces yielded no such correlation with surface preparation, producing a constant apparent surface pKa value of 7.3 across all surface preparations. 4. Discussion
Figure 2. Peak current density for the formation of [AuCl4]- versus KCl concentration in the etching solutions.
curves was to initially collect duplicate run EIS data sets at pH increments of approximately 0.5 units and immediately determine the capacitance at each pH. This allowed the estimation of the apparent surface pKa value to within about one unit. EIS data were considered valid if the duplicate runs in the first pass agreed to within 5%. A second pass was made through the buffers, and the pH increment was dropped to as low as 0.1 unit in the range of the initial apparent surface pKa estimate while being widened to approximately 1.0 units above and below that range. In this pass, the EIS data were considered valid if the duplicate runs agreed to within 2%. The first derivative of the second pass was used to determine the apparent surface pKa value manifested for that electrode. Valid data sets for two to four separate electrodes were used for each level of electrochemical etching. A plot of the apparent surface pKa of 3-MPA versus the KCl concentration in the electrochemical etching solution is shown in Figure 6. As indicated on the figure, there is a high degree of correlation (R2 ) 0.988) between the apparent surface pKa of 3-MPA and
We interpret our results in the context of a shift in the hydrogen-bonding environment available to the surface-confined 3-MPA molecules under the varying conditions of surface roughness in relation to Benjamin and co-workers’29 recent theoretical study. On the smoother surfaces, those only slightly perturbed from the state of an unetched substrate, the most basic apparent surface pKa’s (8.3-8.4) are observed. These are approximately four units away from the solution pKa of 3-MPA, which is 4.3. These SAMs are likely dominated by chain-tochain hydrogen-bonding interactions because the acid groups certainly have easy access to one another. While the average roughness levels notable from Figures 3 and 4 do not necessarily imply a specific level of atomic scale roughness, we believe these values should correlate well with atomic scale roughness. The measured surface roughness values of representative electrodes etched with 0.10 and 0.20 mM KCl are less than 1.5 nm. The lower roughness values on the more lightly etched surfaces should provide sufficient proximity between molecules to allow for chain-to-chain hydrogen bonding because the length of the 3-MPA molecule and the surface roughness are only different by a factor of approximately 2.5. This is further plausible due to the limited amount of van der Waals interactions that can be expected between chains because there are only two methylene groups per chain in 3-MPA. On the rougher surfaces, those most heavily etched by the KCl solutions, there is only about a two unit shift from the solution pKa to 6.4. These SAMs are likely dominated by hydrogen bonding to water and possibly
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Figure 3. Topography of (a) an unetched surface and etched surfaces (b) 0.10 mM KCl, (c) 1.0 mM KCl, and (d) 10 mM KCl. The maximum scale heights are (a) 12.2 nm, (b) 18.1 nm, (c) 29.5 nm, and (d) 65 nm.
Figure 4. Average roughness (Ra) versus KCl concentration in the etching solutions. Trend line and equation are for the five most concentrated KCl levels. Dashed lines show a two standard deviation range for the roughness of an unetched substrate.
other components of the buffer solution rather than chain-tochain hydrogen bonding, although this will also ultimately depend upon the size of lateral surface features at the atomic scale. The measured surface roughness values of representative electrodes etched with 5.0 and 10 mM KCl are between 4 and 7 nm. These roughness levels are approaching a full order of
Figure 5. A typical double-layer capacitance titration curve for 3-MPA. Electrode was etched with 0.5 mM KCl solution.
magnitude larger than the approximate length of a 3-MPA molecule. Between these extremes there is a gradual, logarithmic change in the apparent surface pKa, representing a gradual shift from water-acid hydrogen bonding to acid-acid hydrogen bonding. In contrast, the apparent surface pKa values for TA are stable at 7.3 ( 0.2 for all of the surface preparations tested. We interpret this to indicate the general inability of TA to
Surface pKa’s of ω-Mercaptocarboxylic Acid
J. Phys. Chem. C, Vol. 112, No. 17, 2008 6815 tion due to the strong possibility of having two connection points to the gold surface (via the disulfide bond). Our results should prove interesting to those wishing to build upon acid-functionalized SAMs, especially those wishing to follow the more economical route of polycrystalline gold substrates. We have shown that the surface preparation has a pronounced effect on the apparent surface pKa for 3-MPA. Based on this, it is quite plausible that controlling the surface roughness will allow for the tailoring of a surface for greater or lesser reactivity within a given pH range for any particular acidic SAM.
Figure 6. Apparent surface pKa values for 3-MPA as a function of KCl concentration in the etching solutions.
undergo chain-to-chain hydrogen bonding. Our results also follow other reports for the apparent surface pKa of TA.16 To our knowledge, this is the only report where the apparent surface pKa of SAMs formed with an ω-mercaptocarboxylic acid has been studied under the controlled conditions of both bulk solution ionic strength and gold substrate surface roughness. While our measurements do corroborate other values in the literature for the extremes of very smooth and very rough substrates, we also present primary measurements on substrates of intermediate roughness. We agree with the basic notion of the two possible states presented by Gershevitz and Sukenik.30 However, our measurements are of an averaged property of the electrode surface, which will be driven by the average of the acidity of the molecules on the surface. Their measurements are spectroscopic and have the ability to probe both states simultaneously due to the different bands observed for monomeric, dimeric, and oligomeric species on the surface. 5. Conclusions The apparent surface pKa values of 3-MPA SAMs and TA SAMs on gold film electrodes with varying degrees of roughness have been determined by double-layer capacitance titrations in controlled ionic strength buffers. The apparent surface pKa of 3-MPA is observed to shift approximately two to four units from the solution pKa, correlating to the degree of surface roughness induced by electrochemical etching. The apparent surface pKa values fit a logarithmic function of the KCl concentration in the etching solution with a high degree of correlation (Figure 5) and correspond well with literature reports on other gold surfaces where smoother surfaces produce larger shifts in the apparent surface pKa. However, the relationship between the KCl concentration used for etching and the surface pKa’s of TA is flat, with pKa’s of 7.3 ( 0.2. We conclude this is due to the opportunity for much stronger chain-to-chain hydrogenbonding interaction occurring in the 3-MPA monolayers on smoother surfaces than in the TA monolayers. The TA monolayers are less likely than the 3-MPA monolayers to be packed in a fashion to allow chain-to-chain hydrogen-bonding interac-
Acknowledgment. We thank Kentucky EPSCoR for a Research Startup Fund award for S.C.B. and the Division of Materials Research of the National Science Foundation for DMR-0520789, which funded the purchase of the AFM. References and Notes (1) Ulman, A. Chem. ReV. 1996, 96, 1533. (2) Gooding, J. J.; Mearns, F.; Yang, W.; Liu, J. Electroanalysis 2003, 15, 81. (3) Schlereth, D. D. Compr. Anal. Chem. 2005, 44, 1. (4) Zhang, S.; Cardona, C. M.; Echegoyen, L. Chem. Commun. 2006, 43, 4461. (5) Kasmi, A. E.; Wallace, J. M.; Bowden, E. F.; Binet, S. M.; Linderman, R. J. J. Am. Chem. Soc. 1998, 120, 225. (6) Clark, R. A.; Bowden, E. F. Langmuir 1997, 13, 559. (7) Huck, W. T. S. Angew. Chem., Int. Ed. 2007, 46, 2754. (8) Fuierer, R. R.; Gorman, C. B. Encycl. Nanosci. Nanotechnol. 2004, 5, 851. (9) Petrovic, J.; Clark, R. A.; Yue, H.; Waldeck, D. H.; Bowden, E. F. Langmuir 2005, 21, 6308. (10) Collinson, M.; Bowden, E. F.; Tarlov, M. J. Langmuir 1992, 8, 1247. (11) Creager, S. E.; Clarke, J. Langmuir 1994, 10, 3675. (12) Zhao, J.; Wei, L.; Li, Q.; Yang, X. R.; Wang, E. K.; Dong, S. J. Electroanalysis 1999, 11, 1108. (13) Smith, D. A.; Wallwork, M. L.; Zhang, J.; Kirkham, J.; Robinson, C.; Marsh, A.; Wong, M. J. Phys. Chem. B 2000, 104, 8862. (14) Hu, K.; Bard, A. J. Langmuir 1997, 13, 5114. (15) Shimazu, K.; Teranishi, T.; Sugihara, K.; Uosaki, K. Chem. Lett. 1998, 7, 669. (16) Chang, Q.; Brajter-Toth, A. Anal. Chem. 1996, 68, 4180. (17) Kakiuichi, T.; Iida, M.; Imabayashi, S.-I.; Niki, K. Langmuir 2000, 16, 5397. (18) Dai Ju, H. Phys. Chem. Chem. Phys. 2001, 3, 3769. (19) Yu, H.-Z.; Wang, Y.-Q.; Cheng, J.-Z.; Zhao, J.-W.; Cai, S.-M.; Inokuchi, H.; Fujishima, A.; Liu, Z.-F. Langmuir 1996, 12, 2843. (20) Hegner, M.; Wagner, P.; Semenza, G. Surf. Sci. 1993, 291, 39. (21) Leopold, M. C.; Black, J. A.; Bowden, E. F. Langmuir 2002, 18, 978. (22) Leopold, M. C.; Bowden, E. F. Langmuir 2002, 18, 2239. (23) Nahir, T. M.; Bowden, E. F. J. Electroanal. Chem. 1996, 410, 9. (24) Oesch, U.; Janata, J. Electrochim. Acta 1983, 28, 1237. (25) Version 2.7 of Gwyddion was utilized. Gwyddion is free and open source software covered by the GNU General Public License. It can be downloaded at http://gwyddion.net. (26) Teorell, T.; Stenhagen, E. Biochem. Z. 1938, 299, 416. (27) Perrin, D.; Dempsey, B. Buffers for pH and Metal Ion Control; Chapman and Hall: London, 1974; p 54. (28) MacDonald, J. R. Impedance Spectroscopy: Emphasizing Solid Materials and Systems; John Wiley & Sons: New York, 1987; p 13. (29) Winter, N.; Vieceli, J.; Benjamin, I. J. Phys. Chem. B 2008, 112, 227. (30) Gershevitz, O.; Sukenik, C. N. J. Am. Chem. Soc. 2004, 126, 482.
