3602
Langmuir 1998, 14, 3602-3606
Ruthenium Tetraammine Chemistry of Self-Assembled Monolayers on Gold Surfaces: Substitution and Reactivity at the Monolayer Interface Jian Luo and Stephan S. Isied* Department of Chemistry, Rutgers, The State University of New Jersey, Piscataway, New Jersey 08854 Received January 5, 1998
Different methods for preparing a variety of trans-tetraammine-ruthenium complexes on self-assembled monolayer (SAM) surfaces on gold electrodes are described. The neutral complex, trans-[RuII(NH3)4(SO3)(H2O)], was bound directly to terminal pyridine or imidazole groups on preformed SAM surfaces and oxidized to form trans-[RuIII(NH3)4(SO4)(PyMeNH)]-SAM surfaces. The kinetics of substitution of trans[RuII(NH3)4(SO3)(H2O)] onto pyridine and imidazole groups on preformed SAM surfaces and the ligand substitution of N-heterocycles (such as isonicotinamide, imidazole, and other biological molecules such as ATP (adenosine triphosphate)) on the resulting ruthenium(III) SAM surfaces were studied and found to be slightly slower than those for the corresponding reactions in aqueous solutions. The rate of substitution of the ruthenium(II) complexes onto terminal pyridine or imidazole groups on preformed SAM surfaces depends greatly on the charge of the ruthenium complex, with faster substitution occurring for neutral complexes.
The interfacial chemistry of ordered molecular assemblies, especially self-assembled monolayers (SAMs), is a subject of considerable current interest, with SAMs on gold surfaces being the most versatile and thoroughly studied.1 These well-defined and ordered surfaces have been used to study electron-transfer kinetics2-4 and interfacial phenomena.5 The ability to vary the terminal functional group on SAMs makes them attractive for numerous applications including patterning6 and sensing7 as well as for the modification of surface properties such as wetting,8 corrosion, and adhesion.9 SAMs substituted with transition metal complexes such as ferrocene derivatives and ruthenium pentaammines have also been used * To whom correspondence should be addressed. E-mail:
[email protected]. (1) (a)The first SAMs of disulfides on gold were reported by: Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481-4483. (b) For a review, see: Ulman, A. Chem. Rev. 1996, 96, 1533-1554 and references therein. (2) (a) Sabatani, E.; Rubinstein, I. J. Phys. Chem. 1987, 91, 66636669. (b) Cheng, J.; Sa`ghi-Szabo´, G.; Tossell, J. A.; Miller, C. J. J. Am. Chem. Soc. 1996, 118, 680-684. (c) Guo, L.-H.; Facci, J. S.; McLendon, G. J. Phys. Chem. 1995, 99, 8458-8461. (3) (a) Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301-4306. (b) Chidsey, C. E. D. Science 1991, 251, 919-922. (4) (a) Finklea, H. O.; Hanshew, D. D. J. Am. Chem. Soc. 1992, 114, 3173-3181. (b) Finklea, H. O.; Hanshew, D. D. J. Electroanal. Chem. 1993, 347, 327-340. (c) Finklea, H. O.; Ravenscroft, M. S.; Snider, D. A. Langmuir 1993, 9, 223-227. (d) Ravenscorft, M. S.; Finklea, H. O. J. Phys. Chem. 1994, 98, 3843-3850. (5) (a) Jones, T. A.; Perez, G. P.; Johnson, B. J.; Crook, R. M. Langmuir 1995, 11, 1318-1328. (b) Chailapakul, O.; Crook, R. M. Langmuir 1995, 11, 1329-1340. (c) Cheng, Q.; Brajter-Toth, A. Anal. Chem. 1995, 67, 2767-2775. (6) Wollman, E. W.; Kang, D.; Frisbie, C. D.; Lorkovic, I. M.; Wrighton, M. S. J. Am. Chem. Soc. 1994, 116, 4395-4404. (7) (a) Mandler, D.; Turyan, I. Electroanalysis 1996, 8, 207-213. (b) Creager, S. E.; Olsen, K. G. Anal. Chim. Acta 1995, 307, 277-289. (8) Engquist, I.; Lestelius, M.; Liedberd, B. Langmuir 1997, 13, 40034012. (9) For a review, see: Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932-950.
as models for understanding the distance dependence of interfacial electron-transfer reactions.3,4 While the studies with [Ru(NH3)5]-SAMs resulted in valuable information concerning the properties of these monolayers, the kinetic inertness of the Ru(NH3)5 group precluded any further derivatization of these ruthenium SAM surfaces. Ruthenium tetraammines such as trans-[RuII(NH3)4(H2O)] bound to SAMs provide opportunities for the synthesis of SAM surfaces capable of binding organic and biological molecules. These new SAM surfaces can be used for sensing and analyzing for specific molecules by electrochemical techniques. Herein we report the substitution properties of ruthenium tetraammine monolayers on gold surfaces by using the neutral complex trans-[RuII(NH3)4(SO3)(H2O)] as an intermediate for binding to pyridine and imidazole groups on preformed SAM surfaces. Understanding the reactivity of these ruthenium ammines toward SAM surfaces resulted in the development of a new synthetic route for obtaining aquo tetraammine ruthenium SAM surfaces, trans-[RuII(NH3)4(H2O)(PyMeNH)]-SAM surfaces (PyMeNH is 4-aminomethylpyridine appended on the SAM surface). The substitution of a variety of ligands, such as isonicotinamide, on these novel ruthenium surfaces was studied and compared to those of similar processes in aqueous solution. Experimental Section Materials. 4-Aminomethylpyridine (PyMeNH2), histamine dihydrochloride [(Hist-NH3)]Cl2, 1-(3-dimethylaminopropyl)-3ethylcarbodiimide hydrochloride (EDC), lipoic acid (LA), and histidine (His) (all purchased from Aldrich) and adenosine 5′triphosphate (ATP) (Sigma) were used without further purification. Isonicotinamide (isn) and imidazole (im) were used after recrystallization from water. 11-Mercaptoundecanoic acid (11MerA) was made from 11-bromoundecanoic acid (Aldrich) following the literature procedure.10 (10) Troughton, E. B.; Bain, C. D.; Whitesides, G. M. Langmuir 1988, 4, 365-385.
S0743-7463(98)00044-4 CCC: $15.00 © 1998 American Chemical Society Published on Web 06/06/1998
Ruthenium Tetraammine Chemistry of SAMs on Gold Surfaces The trans-[RuII(NH3)4(SO2)Cl]Cl and trans-[RuIII(NH3)4(SO4)(Hist-NH3)]Cl2 were prepared using literature procedures,11 and trans-[RuIII(NH3)4(SO4)(Hist-NH3)]Cl2 was then purified by two reprecipitations from HCl/ethanol. The ruthenium complex containing the ester of lipoic acid ([RuIII(NH3)4(SO4)(Hist-NH-LA)]Cl) was made by coupling lipoic acid with trans-[RuIII(NH3)4(SO4)(Hist-NH3)]Cl2 via hydroxysuccinimide active ester12 and then purifying the product on a C-18 column (YMC Co., Ltd.). This compound was characterized by HPLC and electrochemistry. The 4-aminomethylpyridine ester of 11-mercaptoundecanoic acid (PyMeNH-11-MerA) was synthesized in two steps: 11bromoundecanoic acid was first coupled to PyMeNH2 via the hydroxysuccinimide active ester method;12 then the bromide was converted to the mercapto group using the same method as in the synthesis of 11-MerA, except the compound was extracted by ethyl acetate and recrystallized twice from ethyl acetate/ hexane to yield ∼20% product. Mp: 53-55 °C. 200 MHz 1H NMR (in CD3Cl): 8.56 (d, 2H, PyH), 7.33 (d, 2H, PyH), 6.01 (s, 1H, NH), 4.51 (d, 2H, CH2Py), 2.52 (m, 2H, SCH2), 2.28 (t, 2H, CH2CO), 1.64 (m, 4H, CH2), 1.29 (m, 12H, CH2). UV: 256 nm. Modification of Gold Electrodes. Gold disk working electrodes were purchased from Bioanalytical Systems (West Lafayette, IN) (MF2014). Before modification, the gold electrodes were polished with a 0.03 mm alumina slurry (Union Carbide, Indianapolis, IN), rinsed thoroughly with water, and then etched with dilute aqua regia (6:3:1 water/concentrated HCl/concentrated HNO3) for 5 min13 followed by copious rinsing with water and ethanol. For the preparation of trans-[RuIII(NH3)4(SO4)Hist-NH-LA]SAMs, the electrode was incubated in an aqueous solution of trans-[RuIII(NH3)4(SO4)(Hist-NH-LA)]Cl (ca. 0.1 mM) overnight, rinsed with water, and then immersed in the electrochemical cell to measure the extent of substitution. For the preparation of trans-[RuIII(NH3)4(SO4)PyMeNH-11MerA]-SAMs, the electrode was incubated in an ethanol solution of PyMeNH-11-MerA (ca. 1 mM) overnight and, then suspended in an argon-saturated, aqueous solution of trans-[RuII(NH3)4(SO3)(H2O)] (10 mg of NaHCO3 + 5 mg of [RuII(NH3)4(SO2)Cl]Cl/1 mL H2O) (this light yellow solution is highly O2 sensitive and turns pink and even blue in the presence of O2). The ruthenium substitution on the SAM was completed within 10 min, after which the electrode was immersed in a 0.3% H2O2 dilute H2SO4 solution (ca. pH ) 2) for 10 s, rinsed thoroughly with water, and then used in the electrochemical cell. For the preparation of trans-[RuIII(NH3)4(SO4)Hist-NH-11MerA]-SAMs, the electrode was first incubated in an ethanol solution of 11-MerA (ca. 1 mM) overnight and then suspended in a phosphate solution (pH ) 7) of 5% (w/v) EDC and ca. 3 mM trans-[RuIII(NH3)4(SO4)(Hist-NH3)]Cl2 for 3 h. The modified electrode was rinsed with water and put into the electrochemical cell. Instruments. Electrochemical experiments were carried out in a conventional three-electrode glass cell containing the modified working gold electrode, a platinum wire auxiliary electrode, and a saturated sodium chloride calomel reference electrode (all the potentials reported in this paper are versus SCE). All electrochemical experiments were performed under argon at room temperature with a BAS 100A electrochemical analyzer (Bioanalytical Systems, West Lafayette, IN).
Results and Discussion The ruthenium tetraammine-modified SAM surfaces were prepared using three different methods, as shown in Scheme 1. In method 1 the appropriate ruthenium thiol complex was synthesized and used directly for SAM formation on (11) (a) Isied, S. S.; Taube, H. Inorg. Chem. 1976, 15, 3070-3075. (b) Brown, G. M.; Sutton, J. E.; Taube, H. J. Am. Chem. Soc. 1978, 100, 2767-2774. (12) Anderson, G. W.; Zimmerman, J. E.; Callahan, F. M. J. Am. Chem. Soc. 1964, 86, 1839-1842. (13) (a) Creager, S. E.; Hockett, L. A.; Rowe, G. K. Langmuir 1992, 8, 854-961. (b) Hockett, L. A.; Creager, S. E. Langmuir 1995, 11, 23182321.
Langmuir, Vol. 14, No. 13, 1998 3603 Scheme 1
gold electrode surfaces. The tetraammine ruthenium SAMs using trans-[RuIII(NH3)4(SO4)(Hist-NH-LA)]Cl were prepared following this method. Similar methods were used by Finklea et al. for the preparation of [Ru(NH3)5]SAMs.4 In method 3, a ruthenium complex with a free amino group such as trans-[Ru(NH3)4(SO4)(Hist-NH3)]Cl2 was bound to a SAM surface derivatized with a carboxylic acid using EDC as the coupling reagent. This approach has been used to covalently immobilize organic and biological molecules onto SAM surfaces.14 Method 2 is a new synthetic approach developed in this work, where the substitution of the ruthenium complex is carried out directly onto the already formed SAM surfaces having terminal N-heterocyclic functional groups. Direct substitution on the preformed monolayer can be simple and quantitative and does not require a separation and purification of the ruthenium complexes required in methods 1 and 3. Also the side reactions associated with the reactivity of the alkanethiol and the ruthenium complexes can be avoided. Using this technique, trans[RuII(NH3)4(SO3)(H2O)] was successfully immobilized onto a PyMeNH-11-MerA SAM electrode surface and the substitution reaction was monitored electrochemically. In all three methods shown in Scheme 1, the sulfato Ru(III) complexes are the resulting intermediates. When the kinetically inert sulfato Ru(III) complex is reduced electrochemically to the Ru(II) form, the SO42- ligand is quickly released (107 M-1).19 When a concentrated aqueous solution of [RuII(NH3)5(H2O)]2+ (0.02 M) was left in contact with a PyMeNH11-MerA-modified electrode for 24 h, no detectable amount of the corresponding [Ru(NH3)5]-SAM was observed. These results strongly argue for the importance of the charge and hydrophobicity of [RuII(NH3)5(H2O)]2+ in its substitution on organic membrane surfaces. Recently using aqueous THF (rather than water), [RuII(NH3)5(H2O)]2+ was successfully incorporated onto pyridine type SAM surfaces,17 presumably because of the more fluid structure of the SAM surface and/or the ion pairing of the ruthenium complex in aqueous THF solutions, making it more conducive to the substitution of [RuII(NH3)5(H2O)]2+on the pyridine SAM surfaces. The rate of substitution of the tetraammine complex trans-[RuII(NH3)4(SO3)(H2O)] onto SAM surfaces is reduced by only a factor of 30 (from its substitution rate in aqueous solution), while the substitution rate of the pentaammine complex [RuII(NH3)5(H2O)]2+ onto SAM surfaces is reduced by more than 4 orders of magnitude from its substitution rate in aqueous solution.20 This rate difference can be ascribed to the charge neutralization at the Ru(II) center by the sulfito ligand, resulting in a neutral species more compatible with the aqueous organic inter(16) Koval, C. A.; Anson, F. C. Anal. Chem. 1978, 50, 223-229. (17) Ryswyk, H. V.; Turtle, E. D.; Watson-Clark, R.; Tanzer, T. A.; Herman, T. K.; Chong, P. Y.; Waller, P. J.; Taurog, A. L.; Wagner, C. E. Langmuir 1996, 12, 6143-6150. (18) Nagano, K.; Tsukahara, H.; Kinoshita, H.; Tamura, Z. Chem. Pharm. Bull. 1963, 11, 797-805. (19) Shepherd, R. E.; Taube, H. Inorg. Chem. 1973, 12, 1392-1401. (20) Although no substitution was observed within 24 h at 0.02 M [RuII(NH3)4(H2O)]2+ aqueous solution, it was assumed that 5% of the reaction could not be detected, and the calculation was adjusted accordingly.
Ruthenium Tetraammine Chemistry of SAMs on Gold Surfaces
Langmuir, Vol. 14, No. 13, 1998 3605 Table 2. Substitution of Isonicotinamide (isn) on Ruthenium Monolayers [isn], M
pH
103kaquo,c s-1
103kisn,d s-1
102k,e M-1 s-1
[Ru(NH3)4(H2O)PyMeNH]-SAMsa
Figure 2. Cyclic voltammograms of [RuII(NH3)4(SO3)PyMeNH]-SAMs (dashed line, obtained before the electrode was dipped in H2O2) and [RuIII(NH3)4(SO4)PyMeNH]-SAMs (solid line, obtained just after the same electrode was dipped in H2O2 for 10 s) in a 1 M Na2SO4 solution (pH ∼ 5). The sweep rate was 100 mV/s.
face (the SAM surface). The charge on the ruthenium complex thus plays a critical role in determining their rate of substitution onto SAM surfaces (i.e., substitution of the neutral trans-[RuII(NH3)4(SO3)(H2O)] is more facile than that of the cationic [RuII(NH3)5(H2O)]2+ onto SAM surfaces). Substitution of Isonicotinamide (isn) onto trans[RuII(NH3)4(H2O)PyMeNH]-SAM Surfaces. The sulfate ruthenium(III) SAMs, trans-[RuIII(NH3)4(SO4)PyMeNH]-SAMs (eq 3), were obtained by oxidation of the corresponding sulfito SAM surfaces (prepared as in eq 2) by suspending the electrode in 0.3% H2O2 (pH ) 2-3) for about 10 s (Figure 2) (similar to the procedure used in solution for oxidation of the sulfito complexes to the corresponding sulfate complexes).11 The trans-[RuIII(NH3)4(SO4)PyMeNH]-SAMs were washed, and upon reduction to Ru(II) (by keeping the potential at -0.3 V (versus SCE)), the sulfate ligand is replaced by water.21 A variety of ligands can then be introduced onto these trans-[RuII(NH3)4(H2O)PyMeNH]-SAM surfaces. Isonicotinamide was chosen as the test ligand because of its well-known coordination to Ru(II) ammines in solution.11 The substitution of isn on the [RuII(NH3)4(H2O)PyMeNH]SAM surfaces occurred at a rate similar to that observed for the reaction of isn with trans-[RuII(NH3)4(pyridine)(H2O)]2+ in aqueous solution (Table 2). The rate of substitution of isn for H2O on the [RuII(NH3)4(H2O)PyMeNH]-SAM surfaces was studied by monitoring the current decrease of the aquo peak at E°′ ∼ -19 mV and the current increase due to the formation of the isn peak, trans-[RuII(NH3)4(isn)PyMeNH]-SAM (E°′ ∼ +232 mV), with time using cyclic voltammetry (CV) (Figure 3). The aquo ion current decreases in two stages: the first current decrease is rapid (
