Langmuir 1998, 14, 2343-2347
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Desorption of 4-Aminobenzenethiol Bound to a Gold Surface Nobuyuki Mohri,* Satoru Matsushita, and Morimasa Inoue Hyogo Prefectural Institute of Industrial Research, 3-1-1 Yukihira-cho, Suma-ku, Kobe 654, Japan
Kenichi Yoshikawa Graduate School of Human Informatics, Nagoya University, Nagoya 464-01, Japan Received July 9, 1997. In Final Form: February 11, 1998 Desorption of 4-aminobenzenethiol (ABT) bound to a gold surface in bulk ethanol solution was studied by the measurement of the UV-absorption of the solution, from the points of both thermodynamics and kinetics. By eliminating the kinetic effects, we evaluated the equilibrium ratio of the adsorbed species with respect to the initial concentration of ABT in the solution. We found that the gold surface is fully covered by ABT above the bulk concentration of 0.5 mmol/dm3. By analyzing the kinetics of the surface reaction from adsorbed ABT to 4,4′-diaminodiphenyl disulfide (DDD) and of the desorption of DDD, we deduced the rate of desorption of DDD from a fully covered gold surface. We found that the rate of desorption is extremely small, 1.6 × 10-5 min-1, due to rather strong binding of ABT to the native gold surface. These results suggest that there are two different processes for the desorption of DDD from a gold surface; one is from a native surface and the other is from a modified surface.
Introduction Modification of solid surfaces with functional moieties has attracted considerable interest with regard to the development of biosensing systems, bioreactors, and so on. Since the chemosorption of thiol and disulfide onto a gold surface can provide a relatively stable modified surface by a covalent bond, these compounds have been frequently applied as linker agents to modify a solid surface using various functional moieties.1-3 Ever since Nuzzo and Allara first reported the adsorption of bifunctional organic disulfide onto a gold surface in 1983,4 there have been many studies on the characterization of thiol compounds bound to a gold surface.5-18 The mechanism of this chemical modification, especially with regard to its kinetics, has also been studied with the (1) Katz, E. Y.; Solov’ev, A. A. J. Electroanal. Chem. 1990, 291, 171. (2) Takehara, K.; Takemura, H.; Ide, Y.; Okayama, S. J. Electroanal. Chem. 1991, 308, 345. (3) Collison, M.; Bowden, E. F.; Tarlov, M. J. Langmuir 1992, 8, 1247. (4) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481. (5) Nuzzo, R. G.; Zegarski, B. R.; Dubois, L. H. J. Am. Chem. Soc. 1987, 109, 733. (6) Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (7) Pale-Grosdemange, C.; Simon, E. S.; Prime, K. L.; Whitesides, G. M. J. Am. Chem. Soc. 1991, 113, 12. (8) Laibinis, P. E.; Whitesides, G. M.; Allara, D. L.; Tao, Y.-T.; Parikh, A. N.; Nuzzo, R. G. J. Am. Chem. Soc. 1991, 113, 7152. (9) Sabatani, E.; Rubinstein, I. J. Electroanal. Chem. 1987, 219, 365. (10) Bryant, M. A.; Joa, S. L.; Pemberton, J. E. Langmuir 1992, 8, 753. (11) Duevel, R. V.; Corn, R. M. Anal. Chem. 1992, 64, 337. (12) Kim, Y.-T.; Bard, A. J. Langmuir 1992, 8, 1096. (13) Taniguchi, I.; Iseki, M.; Yamaguchi, H.; Yasukouchi, K. J. Electroanal. Chem. 1985, 186, 299. (14) Ihs, A.; Liedberg, B. J. Colloid Interface Sci. 1991, 144, 282. (15) Ulman, A. An Introduction to Ultrathin Organic Films from Langmuir-Blodgett to Self-Assembly; Academic Press: San Diego, CA, 1991; pp 279-283. (16) Li, Y.; Huang, J.; McIver, R. T., Jr.; Hemminger, J.C. J. Am. Chem. Soc. 1992, 114, 2428. (17) Bertilsson, L.; Liedberg, B. Langmuir 1993, 9, 141. (18) Anderson, M. R.; Gatin, M. Langmuir 1994, 10, 1638.
aid of techniques such as ellipsometry,19 contact angle measurement,19 quartz crystal microbalance method20-23 and scintillation counting of radiolabeled thiols,24 and electrochemical measurement.25 However, unfortunately there seems to have been no report on the observation for the change in the bulk solution. The experimental study not only on the solid surface but also on the solution is considered to be important because the system is consisted with the adsorbate on the solid surface and the adsorbent in the bulk solution. Recently, we performed a kinetic study of the adsorption of 4-aminobenzenethiol (ABT) onto the surface of gold powder by UV measurement on the ethanol solution.26 It was found that ABT is adsorbed onto the gold surface and then spontaneously oxidized to 4,4′-diaminodiphenyl disulfide (DDD) and that all the ABT molecules in the solution are changed into DDD within a few hours. This indicates the existence of the process of desorption of DDD, the oxidized product of ABT, from the gold surface.26 Thus, it may be useful to identify suitable conditions for minimizing the rate of desorption from the gold surface, since stable binding between a linker reagent and the gold surface is an essentially important factor for the practical application of a modified surface. There have been some studies on the desorption of thiol bound to a gold surface using XPS,27 scintillation counting,24 and (19) Bain, C. D.; Troughton, E. B.; Tao, Y.-T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem. Soc. 1989, 111, 321. (20) Thomas, R. C.; Sun, L.; Crooks, R. M.; Ricco, A. J. Langmuir 1991, 7, 620. (21) Schelenof, J. B.; Dharia, J. R.; Xu, H.; Wen, L.; Li, M. Macromolecules 1995, 28, 4290. (22) Karpovich, D. S.; Bleanchard, G. J. Langmuir 1994, 10, 3315. (23) Schessler, H. M.; Karpovich, D. S.; Blanchard, G. J. J. Am. Chem. Soc. 1996, 118, 9645. (24) Schelenof, J. B.; Li, M.; Ly, H. J. Am. Chem. Soc. 1995, 117, 12528. (25) Lang, H.; Duschl, C.; Vogel, H. Langmuir 1994, 10, 197. (26) Mohri, N.; Inoue, M.; Arai, Y.; Yoshikawa, K. Langmuir 1995, 11, 1612. (27) Hutt, D. A.; Leggett, G. J. Langmuir 1997, 13, 3055.
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voltammetric measurement.28 However in these studies, only the change of the gold surface has been noticed without any quantitative observation on the solution composition. In the present study, we examined the desorption of ABT bound to a gold surface by the aid of measurements by UV absorption and XPS. The equilibrium of the desorption process was analyzed together with the transient kinetic effect through the observation of the change in the chemical composition of the bulk solution. We paid attention to the mechanism of adsorption and oxidation of ABT on the gold surface. Experimental Section Materials. ABT was purchased from Aldrich Chemical Co. Gold powder, obtained through 100 mesh sieves (Kojundokagaku, Saitama), was cleaned with ethanol before use. Unless otherwise stated, analytical grade chemicals were obtained from commercial sources and used without further purification. Equilibrium. The adsorption equilibrium of ABT onto gold was measured as follows. About 0.1 g of gold powder was mixed with 10 mL of an ethanol solution of ABT in a Teflon vessel. The vessel was immersed in a thermobath at 20 °C and shaken for 2 days. The UV spectrum of the supernatant solution was then obtained with a Shimazu spectrophotometer UV-365. After 24 h, the spectrum of the solution did not vary further with time. The surface area of the gold powder was measured by the BET method using a Perkin-Elmer sorptometer 212D. Desorption of Adsorbed ABT. About 0.5 g of gold powder was mixed with 50 mL of a 1 mmol/dm3 ethanol solution of ABT in a Teflon vessel. The vessel was immersed in a thermobath at 20 °C and shaken for 5 days. The solution was then poured out from the vessel, and the powder was washed with about 250 mL of fresh ethanol. After it was confirmed that the washing effluent exhibited no UV adsorption, the dried modified powder was put into 50 mL of fresh ethanol to observe desorption. The time course of the UV spectrum of the supernatant, which was immersed in a thermobath at 20 °C and shaken for 27 days, was then measured with the above-mentioned spectrophotometer.
Results Preparation of Gold Powder Fully Covered with ABT. To clarify the process of the desorption of the bound species, we must prepare gold powder that was fully covered with a monolayer of ABT. In the past studies, to obtain the fully covered gold surface,29,30 the thiol solution, around 1 mmol/dm3, was used in a tentative manner. As far as we know, in the past there has been no definite evaluation of the necessary concentration to accomplish the full coverage. We, thus, tried to measure the degree of coverage in a quantitative manner, based on eq 1. Figure 1 shows the relationship of the coverage ratio of ABT (P) for the surface of the gold powder to the initial concentration, C0, of ABT in the solution:
P ) {β - (β2 - 4Rγ)1/2}/2R ) f(C0)
(1)
where R ) Qs‚R, β ) (γ + R + 1/b), and γ ) C0. P and R represent the coverage ratio, Q/Qs, of the modified gold surface and the mass-to-volume ratio, M/V, respectively. Q, Qs, C0, M, V, and b are the quantity of DDD adsorbed on 1 g of gold powder at equilibrium, the maximum amount of DDD on 1 g of gold powder, the initial concentration of ABT in the solution, the mass of gold powder used, the (28) Walczak, M. M.; Popenoe, D. D.; Deinhammer, S.; Lamp, B. D.; Chung, C.; Porter, M. D. Langmuir 1991, 7, 2687. (29) Sabatani, E.; Rubinstein, I. J. Phys. Chem. 1987, 91, 6663. (30) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 91, 6663.
Figure 1. Relationship between P, the coverage ratio of the gold surface, and C0, the starting concentration of ABT in an ethanol solution, derived from the Langmuir equation, where Qs and b are 3.16 × 10-6 mol/g of Au and 3.66 × 105 dm3/mol, respectively.
volume of solution used, and an adsorption coefficient, respectively. Qs and b can be estimated experimentally by the Langmuir isotherm as
C/Q ) 1/(b‚Qs) + (1/Qs)C
(2)
where C is the concentration of DDD at the adsorption equilibrium. In the present experiment, Qs and b were 3.16 × 10-6 mol/g of Au and 3.66 × 105 dm3/mol, respectively. The relationship between C/Q and C is linear, with a correlation coefficient of 0.999. Equation 1 can be obtained by introducing P, R, and C0 ()C + Q‚R) into eq 2 and reformulating, where P e 1. Since R was about 10 (g/dm3) in this equilibrium experiment, in which about 0.1 g of gold powder and 10 cm3 of ABT solution were used, eq 1 indicates that the gold powder is fully covered in an ABT solution above 0.5 mmol/dm3. To confirm the suitability of using eq 1 to prepare a fully covered gold powder, we immersed a modified gold powder, prepared in 1 mmol/dm3 of ABT solution, in 0.1268 mmol/dm3 of ABT solution and observed the time course of UV absorption by the solution. This spectrum changed rapidly, as in our previous experiment.26 However, the time course had two clear isosbestic points at around 236 and 266 nm. The presence of isosbestic points is due to the oxidation of ABT to DDD and to the cessation of adsorption of ABT on the gold surface. If more adsorption or desorption is caused by oxidation, the isosbestic points should not be clear, as observed in the previous study.26 Thus, we prepared a fully covered gold powder in 1 mmol/ dm3 of ABT solution. Desorption of Bound ABT into Fresh Ethanol. About 0.5 g of fully covered gold powder was used to observe the desorption of bound ABT. Part A of Figure 2 shows the UV spectrum of ethanol in which fully covered gold powder was immersed for 2 days. The pattern of the spectrum is the same as that of an ethanol solution of DDD, indicating that DDD is desorbed from the gold surface. Part B of Figure 2 shows the expected spectrum of an ethanol solution, when ABT that is bound to the gold surface desorbs to achieve equilibrium. In comparing spectrum A with spectrum B, it is clear that the absorbance in spectrum A at 256 nm, where adsorption is maximum, is only about 10-15% of that in spectrum B. The spectrum in part B was obtained as follows. Since the coverage ratio, P, in a 1 mmol/dm3 solution of ABT can be calculated by using eq 1, the quantity of bound ABT
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Langmuir, Vol. 14, No. 9, 1998 2345
Figure 2. UV spectrum of an ethanol solution. Part A shows 2 days of desorption, and part B shows a solution which is expected to show the maximum desorption of bound ABT.
Figure 4. XPS spectrum of S2p electrons of a modified gold powder: (A) gold powder from part C immersed in ethanol for 27 days; (B) gold powder prepared in a 1 mmol/dm3 ethanol solution of ABT; (C) gold powder from part B sufficiently washed with ethanol.
Figure 3. Time course of U, the ratio of desorbed ABT to desorbable ABT.
can be estimated. In the ethanol solution formed by the desorption of bound ABT, the coverage ratio, P′, at an established equilibrium can also be calculated by using eq 1, where the concentration of a hypothetical DDD solution formed by all of the bound ABT is turned to C0. Next, multiplying C0 by (P - P′) gives the concentration of DDD, Ce, at the established equilibrium. The desired spectrum is given by multiplying the optical density of DDD by Ce. This spectrum is equivalent to that given by the maximum desorption of bound ABT. Figure 3 shows the time course of U for 27 days. U reflects the ratio of the quantity desorbed to the maximum desorption, C/Ce, obtained by the ratio of the absorbance of an ethanol solution to that in spectrum B at 256 nm. This figure indicates that the desorption of bound ABT barely exceeds 60% of the maximum over 27 days. Since C0 and Ce in this experiment were 3.18 × 10-5 mol/dm3 and 7.88 × 10-6 mol/dm3, only about 16% of the ABT covering the gold powder was desorbed. This means that ABT is tightly bound to the gold surface. XPS of Bound ABT. Part A of Figure 4 shows the XPS spectrum for S2p electrons of a modified gold powder that has been immersed in ethanol for 27 days in the above experiment. We can find two strong signals at about 162 and 163 eV.
To obtain further insight on this signal pattern, we measured the XPS spectrum of dried gold powder with two different manners of preparation: one was soaked in a 1 mmol/dm3 ethanol solution of ABT, and the other was extensively washed after the soaking in the solution. The spectrum of S2p electrons of the former gold powder is shown in part B of Figure 4. The spectrum has a strong peak at around 163 eV, accompanying the two shoulders. The spectrum differs from that of the ABT powder, which has a peak and a shoulder at around 163 and 164 eV, assigned as for S2p3/2 and S2p1/2 electrons, respectively. On the other hand, the modified gold powder, which has been extensively washed by ethanol, has a signal pattern similar to that of typical S2p electrons of the ABT powder, except for the shift of the strong peak and shoulder to a lower binding energy by about 1 eV, as shown in part C of in Figure 4. Thus, the ABT, shown in part B of Figure 4, suggests the existence of two different species of ABT attaching on the surface: one is directly attached on the surface, and the other is attributable to the pile over the directly attaching one. As the binding energy of the S2p electrons in part B is the same as that in the powder, the signals at 163 and 164 eV in part B are expected to be due to the electrons of the randomly piling ABT on the covered gold surface. On the other hand, the shifted signal in part C shows ABT tightly bound to the gold surface, because the signal survives even after extensive washing. Thus, the 162-eV signal in part B is due to the S2p3/2 electrons of the directly attached species, and another signal based on S2p1/2 electrons would overlap with the signal around 163 eV. Comparison of the signal pattern in part A to that in part C in Figure 4 suggests that part A shows an XPS spectrum of the S2p electrons of the species bound to the gold surface except for weakened strength. Thus, part A in Figure 4 indicates that the bound species remain on the gold surface after immersion in ethanol for 27 days.
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Figure 5. Relationship of dU/dt and U to the desorption of bound ABT. Part A is estimated experimentally, while parts B and C are simulated, using eqs 4 and 8, respectively.
Discussion Simulation of the Desorption of Bound ABT. In a previous report, we discussed the kinetics of the adsorption and desorption of ABT using Scheme 1,26
Scheme 1 adsorption
Figure 6. Relation between d[DDD]/dt and [DDD] at 20 °C, shown by eq 8.
where k3 is the rate constant for the adsorption of desorbed DDD. The rate equations can be expressed as follows:
dx/dt ) -k1xS(t)
(5)
dy/dt ) k1xS(t) - k2y + k3zS(t)
(6)
dz/dt ) k2y - k3zS(t)
(7)
oxidation
8 ABT-Au 98 ABT 9 k 1
desorption
THL-Au 9 8 DDD k 2
where k1 and k2 are the rate constants for adsorption and desorption, respectively. ABT-Au and THL-Au indicate the physically and chemically, or covalently, bonded species of ABT and THL (thiolate of ABT) on the gold surface, respectively. On the basis of Scheme 1, we derived eq 3, which shows the relationship between d[DDD]/dt and [DDD], and determined that the rate constant k2 is 3.2 × 10-2 min-1.
d[DDD]/dt ) k2(C0 - [ABT] - [DDD])
(3)
where [ABT] and [DDD] represent the concentrations of ABT and DDD, respectively. Equation 3 can be rewritten as eq 4 by using U.
dU/dt ) k2(C0/Ce - U)
(4)
Part B of Figure 5 shows a simulated relationship between (dU/dt) and U, based on eq 4, where C0 and Ce are 3.18 × 10-5 and 7.88 × 10-6 mol/dm3, respectively. Part A of the same figure shows the relationship obtained from our present experiment. In comparing the two lines, we find that the experimental velocity is much lower than the simulated value. Next, we tried another simulation according to Scheme 2, which considers the adsorption of desorbed DDD,
Scheme 2 adsorption k1
oxidation
ABT 98 ABT-Au 98 desorption k2
THL-Au {\ } DDD k 3
adsorption
where x and y indicate the number of molecules of ABT and the adsorbed species (ABT-Au and THL-Au), respectively, and z indicates twice the number of DDD molecules. If S0, θ, and N represent the total surface area of the gold powder used, the area covered by an ABT molecule, and Avogadro’s constant, respectively, eq 7 can be rewritten as
d[DDD]/dt ) k2C0 - (k2 + k3S0 - k3θNVC0)[DDD] k3θNV[DDD]2 (8) Figure 6 shows the relationship between (d[DDD]/dt) and [DDD]. A quadratics relationship is seen over 40 × 10-6 mol/dm3 on the [DDD] axis. By approximating the line to eq 8, k2, k3, and θ are (8.7 ( 1.3) × 10-3 min-1, (2.3 ( 0.6) × 10-4 min-1 dm-2, and 29 ( 5 Å-2/molecule, respectively. k2 is slightly smaller than that given in Scheme 1. We can now rewrite eq 8 as follows by introducing U.
(dU/dt) ) k2C0/Ce - (k2 + k3S0 - k3θNVC0)U k3θNVCeU2 (9) Part C of Figure 5 shows the simulated relationship by applying the above values of k2, k3, and θ to eq 9. The present simulated line, (part C), is not very different from that of part B, which also indicates that the experimental velocity (part A) is much less than the simulated value. Therefore, it is clear that the desorption of bound ABT from a sufficiently washed modified gold surface differs from that in the previous kinetics study. Estimation of Another Desorption Constant for Bound ABT. Since Scheme 3 can be applied to the
Desorption of 4-Aminobenzenethiol
Langmuir, Vol. 14, No. 9, 1998 2347 Scheme 4
Scheme 5
Figure 7. Linear approximation for the relation between dU/ dt and U, using eq 10.
desorption and adsorption of DDD from/onto a gold surface, eq 10 can also be derived.
Scheme 3 desorption kb
} DDD ABT-Au or THL-Au {\ k c
adsorption
dU/dt ) kbC0/Ce - (kb + kcS0 - kcθNVC0)U
(10)
where kb and kc correspond to k2 and k3 in Scheme 2, respectively. A linear relationship between dU/dt and U is expected at a U value of less than 0.3, as shown in Figure 7. Thus, we can estimate the rate constant of desorption, kb, to be 1.6 × 10-5 min-1. This value is about one-sixtieth of that obtained by Schelenof et al. for the desorption of octadecanethiol from gold surface in n-hexane.24 This means the binding of ABT-Au in ethanol is stronger than that between octadecanethiol and Au in n-hexane. On the other hand kb is much smaller than k2, indicating that kb is another rate constant for the desorption of bound ABT. Since kb is obtained for the desorption of bound ABT from a sufficiently washed surface, it can be considered to be attributable to the desorption of tightly attached species. If so, what causes the larger rate constant k2? As described above, we observed the larger value not only in the presence of fresh gold powder but also for powder that was fully covered by ABT. This observation indicates that a covered gold surface also plays an important role in the
desorption of bound ABT. Therefore, we can consider that the larger value, k2, results from the desorption of DDD from a modified gold surface. Kim31 and Freeman32 observed the formation of a multilayer of mercaptan on the metal surface. It may also be possible in the present study that the gold surface is covered with the multilayers of ABT. However we observed that the absorbance of the ABT solution remained essentially constant during some days except for the initial change during several hours. In addition, the comparison of the coverage surface area of a ABT molecule, about 39 × 10-20 m2, estimated from the Langmuir isotherm, to other studies on the coverage area of a single thiolate molecule33-38 indicates that ABT forms a monolayer on the surface. Consequently, we can conclude that the oxidation of ABT to DDD on a gold surface and the desorption of DDD occur through two different processes. One is oxidation and subsequent desorption from a native surface, as shown in Scheme 4. The other process is oxidation on a gold surface covered by ABT or THL and subsequent desorption from the covered surface, as shown in Scheme 5. We should emphasize again that kb is much smaller than k2. Acknowledgment. We are extremely grateful to H. Yoshioka and Dr. F. Monjushiro for conducting the XPS analysis. This work was supported financially by the New Energy and Industrial Technology Development Organization (NEDO) of Japan. LA9707639 (31) Kim, Y.-T.; McCarley, R. L.; Bard, A. J. Langmuir 1993, 9, 1941. (32) Freeman, T. L.; Evans, S. D.; Ulman, A. Langmuir 1995, 11, 4411. (33) Nuzzo, R. G.; Zegarski, B. R.; Dubois, R. M. J. Am. Chem. Soc. 1987, 109, 733. (34) Dubois, L. H.; Zegarski, B. R.; Nuzzo, R. G. Proc. Natl. Acad. Sci. U.S.A. 1987, 84, 4739. (35) Sun, L.; Thomas, R. C.; Crooks, R. M.; Ricco, A. J. Am. Chem. Soc. 1991, 113, 2085. (36) Thomas, R. C.; Sun, L.; Crooks, R. M. Langmuir 1991, 7, 620. (37) Shimazu, K.; Yagi, I.; Sato, Y.; Uosaki, K. Langmuir 1992, 8, 1385. (38) Pan, J.; Tao, N.; Lindsay, S. M. Langmuir 1993, 9, 1556.
