Langmuir 2008, 24, 11239-11245
11239
Electroactive Dipyrromethene-Cu(II) Self-Assembled Monolayers: Complexation Reaction on the Surface of Gold Electrodes Iwona Szyman´ska,† Magdalena Stobiecka,† Czesława Orlewska,‡ Taoufik Rohand,§ Dimitri Janssen,§ Wim Dehaen,§ and Hanna Radecka*,† Institute of Animal Reproduction and Food Research of Polish Academy of Sciences, Tuwima 10, 10-747 Olsztyn, Poland, Department of Organic Chemistry, Medical UniVersity of Gdan´sk, Al. J. Hallera 107, 80-416 Gdan´sk, Poland, and Chemistry Department, UniVersity of LeuVen, Celestijnenlaan 200F, B-3001 LeuVen, Belgium ReceiVed April 15, 2008. ReVised Manuscript ReceiVed July 9, 2008 In the work presented, thiol- and COOH-terminated dipyrromethene derivatives have been applied for gold electrode modification. Dipyrromethene deposited onto a solid support, after binding Cu2+, can act as a redox active monolayer. The complexation of Cu(II) ions has been performed on the surface of gold electrodes modified with dipyrromethene. The characterization of dipyrromethene-Cu(II) self-assembled monolayers (SAMs) has been done by cyclic voltammetry (CV), wettability contact angle measurements, and atomic force microscopy (AFM). The new electroactive monolayer could be applied for the immobilization of proteins and ssDNA or for electrochemical anion sensing without redox markers in the solution.
Introduction The majority of the biological reactions that sustain life occur at aqueous/lipid interfaces, with outstanding selectivity and sensitivity being their main features. Therefore, they attract the attention of scientists who try to mimic these processes in order to develop new devices which might be applied in biosensors, microelectronics, catalysis, and medicine.1-6 Much progress has been made during the last 20 years in attaining electrodes with controllable surface properties. One of the most popular techniques to create well-defined functional surfaces is the formation of self-assembled monolayers (SAMs).7 A major class of SAMs is based on the strong chemical adsorption of thiols, disulfides, sulfides, and other related molecules to the surfaces of noble metals, particularly gold, as well as platinum and mercury. This phenomenon was discovered by Allara and Nuzzo8,9 in 1983, and since then a vast amount of research has been carried out in this field, for example, by Whitesides and co-workers.10,11 The advantages of SAMs include simplicity of preparation, versatility, stability, reproducibility, and the possibility of introducing different chemical functionalities with a high level of order at the molecular dimension. Several reports on the use of SAMs to improve selectivity and sensitivity of gold electrodes in a broad range of electroanalytical applications have been * To whom correspondence should be addressed. Telephone: (48) 89 5234636. Fax: (48) 89 5240124. E-mail:
[email protected]. † Polish Academy of Sciences. ‡ Medical University of Gdan´sk. § University of Leuven.
(1) Allara, D. L. Nature 2005, 437, 638–639. (2) Chandler, D. Nature 2005, 437, 640–647. (3) Atencia, J.; Beebe, D. J. Nature 2005, 437, 648–655. (4) Tanaka, M.; Sakmann, E. Nature 2005, 437, 656–663. (5) Yin, Y.; Alivisatos, P. Nature 2005, 437, 664–669. (6) Barth, J. V.; Costantini, G.; Kern, K. Nature 2005, 437, 671–679. (7) Love, J. Ch.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103–1169. (8) Allara, D. L.; Nuzzo, R. G. U.S. Patent Application 389, 775, June 18, 1982; U.S. Patent 4,690,715, September 1, 1987. (9) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481–4483. (10) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 3665-3666, 5897-5898, 6560-6560. (11) Bain, C. D.; Whitesides, G. M. Science 1988, 240, 62–63.
published.12-17 Despite the promising properties of SAMs, they also have some weak points. The most important is the presence of pinholes and defects, which have an effect on the faradaic response of blocking monolayers.18 Attaching a redox center to the SAMs is a possible solution of the above problem. In electroactive monolayers, all redox centers are located on the outer surface of the SAM. Because of this, they display many advantages in comparison to monolayers without redox centers: close packing prevents motion of the redox centers toward the electrode, toward a pinhole or a defect. Therefore, the faradaic current due to redox centers at (or near) pinholes and defects becomes a negligible component of the total faradaic current. Because the electroactive centers are combined with the electrode surface, the diffusion does not have any influence on cyclic voltammetric responses. Thus, the double-layer correction for the surface concentration versus the bulk concentration is not necessary. On the other hand, the new double-layer effect becomes important due to the changing oxidation state of the surface confined redox centers. Voltammetric waves frequently deviate from the ideal behavior, depending on such factors as the dielectric constants of film and solution, the concentration of electroactive adsorbate and supporting electrolyte, and the film thickness. The distortions caused by double-layer effects are less significant when the distances between redox active units are greater and when the electrolyte concentration is high. Thus, the formation of mixed SAMs with a relatively low concentration of redox sites and working in concentrated electrolyte is highly recommended.18,19 (12) Andolfi, L.; Bruce, D.; Cannistraro, S.; Canters, G. W.; Davis, J. J.; Hill, H. A. O.; Crozier, J.; Verbeet, M.Ph.; Wrathmell, C. L.; Astier, Y. J. Electroanal. Chem. 2004, 565, 21–28. (13) Radecka, H.; Szyman´ska, I.; Pietraszkiewicz, O.; Pietraszkiewicz, M.; Aoki, H.; Umezawa, Y. Anal. Chem. (Warsaw) 2005, 50, 85–101. (14) Freire, R. S.; Kubota, L. T. Electrochim. Acta 2004, 49, 3795–3800. (15) Xia, J.; Wei, W.; Hu, Y.; Tao, H.; Wu, L. Anal. Sci. 2004, 20, 1037–1041. (16) Ma, X.; Liu, X.; Xiao, H.; Li, G. Biosens. Bioelectron. 2005, 20, 1836– 1842. (17) Xu, X.; Liu, S.; Ju, H. Sensors 2003, 3, 350–360. (18) Finklea, H. O. In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker Inc.: New York, 1996; Vol. 19, p 109.
10.1021/la801164f CCC: $40.75 2008 American Chemical Society Published on Web 09/10/2008
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Experimental Section
Figure 1. Chemical structures of dipyrromethene-thiol (DPT 1) and dipyrromethene-COOH (DPC 2).
The most popular electroactive SAMs contain ferrocenes,20-24 ruthenium complexes,25,26 viologens,27,28 metalloporphyrins,29-31 cytochromes,32,33 and quinones.34,35 Here, we present a new method for the creation of electroactive monolayers based on the complexation reaction of Cu(II) occurring on the surface of a gold substrate, previously modified with dipyrromethene derivative SAMs (Figure 1). The redox active surface obtained in the above way was characterized by wettability contact angle measurements, cyclic voltammetry, and atomic force microscopy. The Cu(II)-dipyrromethene complex immobilized in SAMs could interact with another dipyrromethene which could contain functional groups in the meso position, such as NH2, COOH, and OH. Such SAMs can be applied for the detection of anions. Also, they might be suitable for the immobilization of proteins. This would be particularly beneficial in the case of proteins which do not possess an electroactive center. In the work presented, the complexation reaction on the surface of Cu(II)-dipyrromethene SAMs has been carried out with a dipyrromethene possessing a COOH group in the meso position. (19) Bard, A. J.; Faulkner, L. R. Electrochemical Methods - Fundamentals and Applications, 2nd ed.; John Wiley & Sons Inc.: New York, 2001. (20) Sek, S.; Maicka, E.; Bilewicz, R. Electrochim. Acta 2005, 50, 4857– 4860. (21) Grygołowicz, E.; Wygldacz, K.; Sek, S.; Bilewicz, R.; Brzoˇzka, Z.; Malinowska, E. Sens. Actuators, B 2005, 111-112, 310–316. (22) Chdsey, C. E. D. Science 1991, 251, 919–922. (23) Richardson, J. N.; Peck, S. R.; Curtin, L. S.; Tender, L. M.; Terrill, R. H.; Carter, M. T.; Murray, R. W.; Rowe, G. K.; Creager, S. E. J. Phys. Chem. 1995, 99, 766–772. (24) Tender, L.; Carter, M. T.; Murray, R. W. Anal. Chem. 1994, 66, 3173– 3181. (25) Finklea, H. O.; Hanshew, D. D. J. Electroanal. Chem. 1993, 347, 327– 340. (26) Finklea, H. O.; Ravenscroft, M. S.; Snider, D. A. Langmuir 1993, 9, 223–227. (27) Lee, K. A. B. Langmuir 1990, 6, 709–712. (28) Katz, E.; Itzhak, N.; Willner, I. Langmuir 1993, 9, 1392–1396. (29) Shankaran, D. R.; Narayanan, S. S. Bull. Chem. Soc. Jpn. 2002, 75, 501– 505. (30) Zak, J.; Yuan, H.; Ho, M.; Woo, K. L.; Porter, M. D. Langmuir 1993, 9, 2772–2774. (31) Hutchison, J. E.; Postlethwaite, T. A.; Murray, R. W. Langmuir 1993, 9, 3277–3283. (32) Tarlov, M. J.; Bowden, E. F. J. Am. Chem. Soc. 1991, 113, 1847–1849. (33) Song, S.; Clark, R. A.; Bowden, E. F.; Tarlov, M. J. J. Phys. Chem. 1993, 97, 6564–6572. (34) Hong, H. G.; Park, W.; Yu, E. Bull. Korean Chem. Soc. 2000, 21, 23–25. (35) Mebrahtu, T.; Berry, G. M.; Bravo, B. G.; Michelhaugh, S. L.; Soriaga, M. P. Langmuir 1988, 4, 1147–1151.
Materials. The dipyrromethene derivatives dipyrromethenethiol (DPT 1) and dipyrromethene-COOH (DPC 2) (Figure 1) were synthesized at the Chemical Department of Leuven University, by a procedure that has already been published.36 N-(3-Dimethylaminopropyl)-N′-ethylcarbodiimide hydrochloride (EDC), 2-aminoethanethiol hydrochloride (AET, 98%), copper(II) acetate, ethylene glycol, and CH2I2 were obtained from Fluka-SigmaAldrich (Poznan´, Poland). All aqueous solutions were prepared with deionized and charcoaltreated water (resistivity of 18.2 MΩ cm) purified with a Milli-Q reagent grade water system (Millipore, Bedford, MA). Preparation of Electrodes. Two types of electrodes were used for the experiments: gold disk electrodes, 2 mm2 area (Bioanalytical Systems (BAS), West Lafayette, IN) and gold substrates, 1 cm × 1 cm of mica covered by 10 nm Ti and 100 nm Au (IMEC, Leuven, Belgium). The gold disk electrodes were polished with wet 0.3 and 0.05 µm alumina slurry (Alpha and Gamma Micropolish; Buehler, Lake Bluff, IL) on a flat pad for at least 10 min and rinsed repeatedly with water and finally cleaned in a sonicator (1 min). The polished electrodes were then dipped in 0.5 M KOH solution, deoxygenated by purging with argon for 15 min, and the potential was cycled between -400 and -1200 mV (versus a Ag/AgCl reference electrode) with a scan rate of 100 mV s-1 until the cyclic voltammograms showed no further change. The gold substrates were washed with Milli-Q water and subsequently with acetone followed by cleaning in a UV/ozone chamber during 15 min. The cleanliness of the gold surface was checked by measuring the contact angle. Modification of Gold Electrodes Using a Thiol Dipyrromethene Derivative. The modification procedures using a thiol dipyrromethene derivative (DPT 1) have been illustrated in Scheme 1 in the Supporting Information. Both types of electrodes were soaked in a chloroform solution of 1.0 mM DPT 1 and 0.01 mM dodecanethiol (DDT) for 0.5 h at room temperature. Next, after washing with chloroform, the electrodes were dipped in ∼1 mM Cu(CH3COO)2 in water for 3 h, and for 12 h in a chloroform-methanol mixture (1:1). The modification solutions were put into the tubes (8 mm diameter, with no flat bottom) or special vessels. After dipping the electrodes, the tubes and vessels were sealed with Teflon tape in order to protect the solvent from evaporation. Modification of Gold Electrodes Using a COOH-Terminated Dipyrromethene Derivative. The modification procedures using a COOH-terminated dipyrromethene derivative (DPC 2) has been illustrated in Scheme 2 in the Supporting Information. After cleaning, the gold surface of the electrodes was immediately immersed into a 0.1 M solution of AET in ethanol for 3 h. The electrodes were then modified with a solution of 0.001 MDPC 2 and 0.26 M EDC in chloroform and methanol (volume ratio 1:1 v/v) for 18 h. Next, they were exposed to a solution of 0.001 M copper acetate in chloroform and methanol (ratio 1:1 v/v) for 12 h. Subsequently, the electrodes were dipped into a solution of 0.001 M DPC 2 in chloroform and methanol (ratio 1:1 v/v) for 18 h. The modification solutions were put into the tubes (8 mm diameter, with no flat bottom). After dipping the electrodes, the tubes were sealed with Teflon tape in order to protect the solvent from evaporation. After modification, the electrodes were rinsed with ethanol or chloroform from the modification solution. All modified gold electrodes were stored at 4 °C in a 0.1 M KCl solution until use. Electrochemical Measurements. All electrochemical measurements were performed with a potentiostat-galvanostat AutoLab (Eco Chemie, Utrecht, Netherlands) apparatus with a three electrode configuration. Potentials were measured versus the Ag/AgCl electrode, and a platinum wire was used as the auxiliary electrode. Cyclic voltammetry (CV) was performed, and the potential was (36) Orlewska, C.; Toppet, S.; Dehaen, W. Synth. Commun. 2005, 35, 1953– 1959.
Dipyrromethene-Cu(II) Self-Assembled Monolayers cycled from -200 to 700 mV with a scan rate from 10 to 9000 mV s-1 for gold electrodes modified with DPT 1/DDT/Cu and AET/ DPC 2/Cu. Wettability Contact Angle Measurements. Wetting contact angles were measured with an OCA 15 instrument from Dataphysics, Germany. A small droplet (1 µL) of Milli-Q water, ethylene glycol, and CH2I2 was placed on a horizontal gold substrate (1 cm × 1 cm), and the contact angle between the substrate and the edge of the drop was measured. Atomic Force Microscopy (AFM). Each step of the modification of the gold substrates (1 cm × 1 cm) according to procedures illustrated in Schemes 1 and 2 in the Supporting Information was characterized by an AFM system from Universal SPM Quesant. The AFM images were obtained using the intermittent contact mode with an NSC16 tip (W2C, Si3N4), which oscillates at ∼170 kHz resonant frequency above the sample surface. The radius of tip curvature was ∼10 nm.
Results and Discussion Dipyrromethenes are suitable molecules for creating complexes with transition metal ions.37 Also, they can interact with each other via hydrogen bonding. This is not desired in the present approach. In order to increase the distance between the dipyrromethene units and to improve the order of the SAMs, mixed monolayers were formed using a CHCl3 solution containing DDT and DPT 1 in a 100:1 ratio. This mixed monolayer has already been characterized38 and led to a quite hydrophobic and well-organized surface. It has been proved that the distances between dipyrromethene molecules immobilized on the surfaces of gold electrodes are sufficient to prevent the creation of hydrogen bonding between them. Therefore, such mixed SAMs formed with the chloroform solution of DDT and DPT 1 in a 100:1 ratio have been used in the present study for efficient immobilization of Cu(II). Wettability and Surface Free Energies of Dipyrromethene Derivative SAMs. Contact angle measurements have been reported to provide a relatively rapid and simple means of assessing surface hydrophilicity/hydrophobicity. The contact angle (Θeq) formed by a droplet of a probe liquid on a surface is ruled by the Young equation.39 It has already been reported that the values of static contact angles show that the SAMs created with DPT 1/DDT are quite hydrophobic.38 The value of the static contact angle measured for water was approximately 85°. Additional incorporation of Cu(II) into DPT 1/DDT SAMs decreased its value to 72°. After interaction of the DPT 1/DDT/Cu SAM with COOH-terminated dipyrromethene (DPC 2), the hydrophobicity of the surface slightly increased (Table 1). The gold substrates modified through the creation of amide bonds between the NH2 group of a 2-aminoethanethiol SAM and DPC 2 are quite hydrophilic. The static contact angle measured for water was approximately 35°. This monolayer is less ordered in comparison with a DPT 1/DDT SAM, and this might be the reason for its lower hydrophobicity. The incorporation of Cu(II) and subsequently DPC 2 into the AET/DPC 2 SAM gradually increased the contact angle measured for water (Table 1). (37) Bru¨ckner, C.; Karunaratne, V.; Rettig, S. J.; Dolphin, D. Can. J. Chem. 1996, 74, 2182–2193. (38) Szyman´ska, I.; Orlewska, C.; Janssen, D.; Dehaen, W.; Radecka, H. Electrochim. Acta 2008, 53, 7932–7940. (39) Young, T. Philos. Trans. R. Soc. London 1805, 95, 65. (40) Ulman, A. Thin Solid Films 1996, 273, 48–53. (41) Janssen, D.; De Palma, R.; Verlaak, S.; Heremans, P.; Dehaen, W. Thin Soid Films 2006, 515, 1433–1438. (42) Greiveldinger, M.; Shanahan, M. E. R. J. Colloid Interface Sci. 1999, 215, 170–178. (43) Norris, J.; Giese, R. F.; van Oss, C. J.; Costanzo, P. M. Clays Clay Miner. 1992, 40, 327–334. (44) van Oss, C. J.; Chaudhury, M. K.; Good, R. J. Chem. ReV. 1988, 88, 927–941.
Langmuir, Vol. 24, No. 19, 2008 11241 Table 1. Values of Static Contact Angles Measured for Water, Ethylene Glycol, and CH2I2 and the Total Solid/Water Free Energy43,44 Calculated for Each Step of Gold Substrate Modification type of modification
water (°)
ethylene glycol (°)
CH2I2 (°)
γSLsolid-water (mJm-2)
Au/AET/DPC 2 Au/AET/DPC 2/Cu Au/AET/DPC 2/Cu/DPC 2 Au/DPT 1/DDT Au/DPT 1/DDT/Cu Au/DPT 1/DDT/Cu/DPC 2
35.5 ( 1.0 44.6 ( 3.7 49.7 ( 4.0 85.0 ( 4.0 72.7 ( 5.1 76.5 ( 0.1
55.0 ( 0.5 45.9 ( 7.9 27.8 ( 3.7 59.3 ( 4.2 49.0 ( 3.7 45.7 ( 0.1
30.4 ( 0.7 26.9 ( 3.0 23.6 ( 2.9 27.3 ( 4.0 36.2 ( 2.6 41.8 ( 0.2
7.9 ( 0.7 3.8 ( 1.9 1.1 ( 1.5 40.7 ( 4.1 19.8 ( 3.8 24.5 ( 0.1
Wetting contact angles allow one to determine the surface free energy.40-42 This parameter could be a good probe of the chemical affinity of the surfaces studied. The surface free energies of each step of the modification using DPT 1 and DPC 2 were determined from the contact angles of water, ethylene glycol (polar liquids), and CH2I2 (apolar) according to Young’s equation.39 The liquids selected are chemically inert with respect to the SAMs studied. The contact angle values were determined within approximately 5 s. This short time should effectively limit any surface modification.42 For the determination of the interfacial free energy of the SAMs studied, the acid/base theory developed by van Oss et al. was applied.43,44 This approach is suitable for the measurement of surface tension for polar surfaces. According to this theory, the interaction between a liquid and a solid is interpreted as the interaction between an acid and a base. The surface free energy consists of apolar Lifshitz-van der Waals (LW) interactions such as dipole-dipole, dipole-induced dipole, and fluctuating dipole-induced dipole. The second component of the free energy (polar interactions) is based on the interaction of electron acceptor and electron donor sites (Lewis acids and bases, AB). Two parameters are specified for the AB component: electron donor character (γ-) and electron acceptor character (γ+). The Young equation for polar surfaces based on acid-base (van Oss) theory is as follows:43,44
0.5(1 + cos θ)γLV ) √γSLWγLLW + √γS+γL- + √γS-γL+
(1)
where θ is the value of contact angle, γ is the surface tension (surface free energy), the subscripts S and L refer to the solid and liquid and the superscripts LW, “+”, and “-” refer to Lifshitz-van der Waals, acid, and base components, respectively. The value of the surface tension components of the solid-vapor + γLW S , γS , and γS can be determined by solving a set of simultaneous equations, one for each kind of liquid used, with the restriction that two of them should be polar and one apolar. For the measurements reported here, polar water and ethylene glycol and apolar CH2I2 were used. The surface tension values (γLV, liquid-vapor) for these liquids and their acid and base components were taken from the literature.43,44 The total free energy of the solid/vapor is given by43,44
γSV (total) ) γSLW + γSAB ) γSLW + 2√γS+γS-
(2)
where AB denotes Lewis acids and bases. Calculations were done using contact angles obtained for water based on the Young equation:
γSL ) γSV - γLV cos Θ
(3)
where γSL, γSV, and γLV are the surface free energies of the solid-liquid (SL), solid-vapor (SV), and liquid-vapor (LV), respectively.
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The values of the total solid/water free energy calculated for each step of gold substrate modification using eqs 1 and 2 are summarized in Table 1. The solid/water surface free energy of the DPT 1 /DDT SAM is ∼5 times higher than that of the AET/DPC 2 SAM (Table 1). However, after incorporation of Cu(II) into both types of SAMs, a decrease of ∼50% of the solid/water surface free energy was observed. The next step of modification, complexation with DPC 2, slightly increased the surface energy of the DPT 1/DDT/Cu SAM (Table 1). In the literature, there are many critical reports on contact angle measurements and contact angle interpretation in terms of solid surface tension.45-48 The authors pointed out that the contact angles should be used only with caution for determining solid surface tensions. They concluded that the acid/base or electron acceptor/donor theory seem very reasonable, but their mathematical presentation is still inadequate.42,45-47 There is still no absolute criterion for choosing a given Lewis acid/base ratio for water.42 Despite these limitations, the surface free energy calculated according to the acid/base van Oss theory seems to be a very useful parameter for comparison of the relative chemical affinity of SAMs. Intermolecular recognition phenomena are the basis of a majority of chemical and biochemical sensing events occurring at the lipophilic/aqueous interfaces. Therefore, in view of future application of SAMs studied for chemical sensing, the total solid/ water free energy is a very important parameter. Electrochemical Properties of Dipyrromethene-Cu(II) SAMs. The main purpose of the study presented is the incorporation of redox centers into the monolayers by a complexation reaction carried out on the surface of the SAM. The dipyrromethene derivatives display a high affinity toward transition metal cations.37 As an example, Cu(II) has been selected. The gold electrodes modified with dipyrromethene derivatives according to procedures illustrated in Schemes 1 and 2 in the Supporting Information have been explored by cyclic voltammetry performed in 0.1 M KCl. The DPT 1/DDT and AET/DPC 2 SAMs were dipped into an ∼1 mM solution of Cu(CH3COO)2 in a chloroform-methanol (1:1) mixture. After this derivatization, the presence of Cu(II) was checked by cyclic voltammetry, performed in 0.1 M KCl. Both types of SAMs contained Cu(II). Quasi-reversible reduction and oxidation peaks of Cu(II) were observed. Representative CVs are presented in Figures 2A and 3A. It has already been reported that mixed DPT 1/DDT created from a chloroform solution of these compounds in a 1:100 molar ratio displayed the highest affinity toward protons.38 In this type of mixed SAMs, the distance between DPT molecules is sufficient to prevent the formation of hydrogen bonding. In such type of SAMs, the molecules of dipyrromethene displayed the highest affinity toward protons, meaning that they also are ready to bind transition metal cations. Therefore, this type of mixed DPT 1/DDT SAM was selected as the most suitable for the incorporation Cu(II). For this type modification, the reduction and oxidation peaks were observed at Epc) 0.017 ( 0.002 V and Epa) 0.208 ( 0.023 V, respectively (Figure 2A, Table 2). The peak separation ∆Ep ) 0.191 V indicates that the reversibility of the redox processes is quite low. It is consistent with quasi-reversible kinetics. (45) Kwok, D. Y.; Neumann, A. W. AdV. Colloid Interface Sci. 1999, 81, 167–249. (46) Kwok, D. Y.; Neumann, A. W. Colloids Surf., A 2000, 161, 31–48. (47) Kwok, D. Y. Colloids Surf., A 1999, 156, 191–200. (48) Li, W.; Amirfazli, A. J. Colloid Interface Sci. 2005, 292, 195–201.
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Figure 2. CV curves for gold electrodes modified by a DPT 1/DDT/Cu SAM (A) and DPT 1/DDT/Cu/DPC 2 SAM (B) measured in 0.1 M KCl. Scan rates 20-1000 mV s-1, reference electrode Ag/AgCl, counter electrode Pt.
A better reversibility was observed in the case of SAMs created with AET/DPC 2 according to the procedure illustrated in Scheme 2 in the Supporting Information (see Experimental Section). Representative CVs are illustrated in Figure 3A. The peak separation ∆Ep ) 0.074 V indicates the good reversibility of the redox processes proceeding on the electrode surface. In this type of SAM, the Cu(II) redox center is located closer to the electrode surface in comparison to the DPT 1/DDT SAM. This might be the main reason of the better reversibility. The distance of the Cu(II) redox center from the electrode surface for both types of SAMs (Scheme 3, Supporting Information) has been estimated based on molecule modeling, using CAChe WorkSystem v.7.5.0.85, Fujitsu Ltd., FQS, Krako´w, Poland. The geometry optimization of the analyzed SAMs was done using the AM1 geometry procedure in vacuum (MOPAC 2002 version 2.5.3, JJP Stweward, Fujitsu Limited, Tokyo, Japan). The next step of modification, complexation with DPC 2, eliminated the cathodic peak in the case of DPT 1/DDT/Cu/DPC 2 (Figure 2B). On the other hand, the anodic peak has been shifted to a more positive potential (Table 2). In the case of AET/DPC 2/Cu/DPC 2, the anodic peak was shifted slightly into a negative potential, whereas the cathodic peak remained almost at the same position (Figure 3B, Table 2). The system gained better reversibility while the peak separation was 0.053 V (Table 2). This might indicate that the Cu-redox centers, fully complexed with two dipyrromethene molecules, could undergo only anodic processes. This suggests that the copper complex with two molecules of dipyrromethene stabilizes the Cu(II) oxidation state.
Dipyrromethene-Cu(II) Self-Assembled Monolayers
Figure 3. CV curves for gold electrodes modified by a AET/DPC 2/Cu SAM (A) and AET/DPC 2/Cu/DPC 2 SAM (B) measured in 0.1 M KCl. Scan rates 20-1000 mV s-1, reference electrode Ag/AgCl, counter electrode Pt.
Therefore, its reduction was not observed within the potential window applied for the present study. The inserted plots of the anodic and cathodic peak currents versus scan rate show a linear relation up to 1000 mV s-1 (Figures 2 and 3), thus confirming the immobilized state of the DPT 1/Cu(II) and DPC 2/Cu(II) complexes at the I and II modification steps. The CV curves in Figure 2 represent nonideal voltammetric behavior. The generation or disappearance of net charges at the substrate electrode/solution interface during the course of the redox process would change the interfacial potential distribution. This double-layer effect would result in the distortion of the voltammogram. The theoretical treatments of such phenomenon, introduced by Laviron49,50 and recently by Smith and White,51 reviewed by Honeychurch and Rechnitz,52 was developed by Ohtani et al.53,54 They explained the changes in the potential distribution at the electrode substrate/solution interface in terms of the formation of ion pairs and/or triple ions. According to this, if the positive charge of redox-active groups immobilized on a SAM are neutralized with the electrolyte anions at the interface, the shape of the voltammetric wave becomes close to that of the ideal Nerstian with a full width at half-maximum (fwhm) of 90.6 mV (at 25 °C), but the peak potential is negatively shifted with decreasing interface charge value.54 (49) Laviron, E. J. Electroanal. Chem. 1979, 100, 263–270. (50) Laviron, E. J. Electroanal. Chem. 1979, 101, 19–28. (51) Smith, C. P.; White, H. S. Anal. Chem. 1992, 64, 2398–2405. (52) Honeychurch, M. J.; Rechnitz, G. A. Electroanalysis 1998, 10(5), 285– 293. (53) Ohtani, M. Electrochem. Commun. 1999, 1, 488–492. (54) Ohtani, M.; Kuwabata, S.; Yoneyama, H. Anal. Chem. 1997, 69, 1045– 1053.
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In order to check if the ion-paring has an influence on the potential distribution of the electrodes studied, the experimental results presented in Figures 2A and 3A were compared with results calculated with the JAVA applet.53 The simulated CV closest to the experimental CV, measured at ν ) 100 mV s-1, for an electrode modified with DPT 1/Cu(II), was obtained with using the formation constant of the ion-pair equal to 1. This indicated that the interface net ionic charge is +0.5. So, only half of the redox surface confined units are neutralized by anions from the electrolyte. On the other hand, the experimental CV obtained for DPC 2/Cu(II)/Au electrode at ν ) 100 mV s-1 was closest to the simulated one, when the formation constant of the ion-pair was equal 100. This means that the interface net ionic charge is +0.01. So, in this case, almost all surface redox confined units are neutralized with electrolyte anionic species. Both types of electrodes fulfilled the diagnostic criteria which indicated the double-layer effects: plots of Ia and Ic versus ν are linear and passing through the origin (inset in Figures 2A and 3A) and full widths at half-maximum are broader than 90.6 mV (Table 2). The model developed by Ohtani et al.54 considered the monolayer with redox centers located on the surface of the monolayer or at the some well-defined plane, called the plane of electron transfer (PET). In general, the double-layer effects are amplified if the redox centers are buried in the monolayer. This was also observed in the research presented. The simulated CVs could be obtained by using the JAVA applet53 for electrodes containing Cu(II) covalently bound only by one dipyrromethene molecule. In these cases, the redox units are located on the monolayer surface. The binding of the next dipyrromethene molecule makes Cu(II) buried in the monolayer. For these modifications, because of the relatively low reversibility of redox processes, the calculations of the kinetic parameters (R and k) were not possible. Therefore, the model developed by Ohtani et al.53 for estimation of double-layer effects was not applicable. Electron Transfer Rate Constants. Kinetic data of the electron transfer between the Cu(II) centers and the gold surface modified with DPT 1/DDT and AET/DPC 2 SAMs were obtained from the relationship between the log scan rate versus the anodic and cathodic peak potential.49,50 The scan rate was changed from 10 to 9000 mV s-1. The electrodes studied showed quasireversible behavior for DPT 1/DDT and reversible behavior for AET/DPC 2. The peak potential separation significantly increased above ∼1000 mV s-1 for DPT 1/DDT and AET/DPC 2 SAMs. The electron transfer coefficient R and the rate constant k have been calculated based on the generally applicable Laviron’s procedure49,50 using the following equations:
Epa)E0a - (RT ⁄ RnF) ln(RTka ⁄ RnFνa)
(4)
Epc)E0c - (RT ⁄ (1 - R)nF) ln(RTkc ⁄ (1 - R)nFνc)
(5)
where νa and νc indicates the critical scan rates which are obtained from extrapolating the linear portion of the Epa and Epc versus log scan rate plots, respectively. R can be determined from
R ⁄ (1 - R) ) νa ⁄ νc
(6)
The estimated electron transfer coefficient R values are collected in Table 3. For SAMs containing Cu(II) complexed by only one dipyrromethene molecule, the electron transfer coefficients are similar, in the ranges 0.67 ( 0.06 and 0.71 ( 0.09 (n ) 5) for DPT 1/DDT/Cu and AET/DPC 2/Cu, respectively. The binding of the next dipyrromethene molecule makes Cu(II) buried in the monolayer. These SAMs displayed relatively low reversibility
11244 Langmuir, Vol. 24, No. 19, 2008
Szyman´ska et al.
Table 2. Electrochemical Parameters of Different Types of SAMsa type of modification
Epa (V)
Epc (V)
∆Ep (V)
E0′ (V)
Efwhm (anodic) (V)
Efwhm (cathodic) (V)
Au/DPT 1/DDT/Cu Au/DPT 1/DDT/Cu/DPC 2 Au/AET/DPC 2/Cu Au/AET/DPC 2/Cu/DPC 2
0.208 ( 0.023 0.309 ( 0.023 0.299 ( 0.006 0.275 ( 0.007
0.017 ( 0.002 – 0.225 ( 0.006 0.222 ( 0.013
0.191 – 0.074 0.053
0.113 ( 0.013 – 0.262 ( 0.006 0.249 ( 0.009
0.261 ( 0.021 0.258 ( 0.005 0.213 ( 0.018 0.156 ( 0.006
0.135 ( 0.009 – 0.155 ( 0.009 0.162 ( 0.030
a Epa and Epc are the anodic and cathodic peak potentials of Cu, respectively, ∆Ep is the potential difference, E0′ was estimated as (Epa + Epc)/2, and Efwhm is the full width at half-maximum in the CVs obtained for electrodes measured in 0.1 M KCl at 100 mV s-1.
Table 3. Electron Transfer Coefficient, r, Rate Constant, k (s-1),49,50 and Surface Coverage, Γ (mol cm-2),19,40 for the DPT 1/DDT/Cu and AET/DPC 2/Cu SAMs type of modification Au/DPT 1/DDT/Cu Au/AET/DPC 2/Cu
R
k (s-1)
Γ (mol cm-2)
0.67 ( 0.06 0.71 ( 0.09
0.74 ( 0.21 0.44 ( 0.15
6.4 ( 1.8 × 10-11 2.7 ( 1.0 × 10-10
of redox processes (Figures 2B and 3B). So, the calculations of the kinetic parameters (R and k) was not possible. The transfer coefficient R shows how the total free energy of activation for electron transfer is divided between the anodic and cathodic steps. When the barrier for the oxidation is lower than that for the reduction, R is greater than half.19 This was observed also in our study. It has been also proved that redox systems having longer chains in the SAMs gave higher R values than those having shorter chains in the SAMs.55 The charge transfer coefficient R has been used for the calculation of the electron transfer rate constant k. For the calculations, the following equations have been applied:49,50
k ) RnFνc ⁄ RT ) (1 - R)nFνa ⁄ RT
(7)
The rate constants k ) 0.74 ( 0.21 and 0.44 ( 0.15 s-1 were very similar for DPT 1/DDT/Cu and AET/DPC 2/Cu, respectively. As the rate constant shows how the redox couple is kinetically facile, it means that systems with a small value of k reach equilibrium slower than systems possessing a larger value of k.19 In the literature, it has been studied how the chain length influences the value of the rate constant for different redox probes. Generally, by increasing the chain length in the SAMs, the rate constant can be decreased compared to the bare surface.18,55,56 However, when the heteroatoms, especially oxygen or a heteroatom connected with double bonds, are replacing -CH2 units in the chain, a decrease of the rate constant can be observed.57,58 Thus, a possible explanation of the results, which we obtained for both types of modifications (DPT 1/DDT/Cu and AET/DPC 2/Cu SAMs), can be as follows. Even though the distance of Cu from the surface of the electrode is greater in the case of DPT 1/DDT in comparison with the AET/DPC 2 SAM, the values of the rate constant are almost the same. Probably, the -CO-NH- group, which is present in case of the AET/DPC 2 SAM, decreases the electron tunneling in comparison with the DPT 1/DDT, in which only -CH2 units are present. The similar phenomenon was reported by Cheng et al.58 They studied the structural dependence of long-range electron transfer for ferricyanide and osmium(III)tris(bipyridyl) at Au electrodes coated with self-assembled monolayers of HO(CH2)n-X(CH2)mSH. When X was substituted by an ether group, the electron coupling across the monolayer decreased in comparison to the case when X was a -CH2 group. (55) Hanshew, D. D.; Finklea, H. O. J. Am. Chem. Soc. 1992, 114, 3173–3181. (56) Finklea, H. O.; Hanshew, D. D. J. Electroanal. Chem. 1993, 347, 327– 340. (57) Finklea, H. O.; Liu, L.; Ravenscroft, M. S.; Punturi, S. J. Phys. Chem. 1996, 100, 18852–18858. (58) Cheng, J.; Sa`ghi-Szabo´, G.; Tossell, J. A.; Miller, C. J. J. Am. Chem. Soc. 1996, 118, 680–684.
So, not only the distance of the redox units from the electrode surface but also the dielectric properties of the system affect the electrochemical behavior through the changing double-layer effect.18,19,57,58 Surface Coverage. The value of the surface coverage19,40 was calculated according the equation:
Ip)n2F2AΓν ⁄ 4RT ) nFQν ⁄ 4RT
(8)
where Γ is the surface coverage of the redox center (mol cm-2), A is the electrode area (0.02 cm2), and Q is the quantity of charge (C) which is determined based on the integration of the voltammetric peaks (Figures 2A and 3A). The symbols n, F, R, and T have their usual meaning. The results of determination yield Γ values of 6.4 ( 1.8 × 1011 mol cm-2 for DPT 1/DDT/Cu and 2.7 ( 1.0 × 10-10 mol cm-2 for AET/DPC 2/Cu. So, as expected, we observed a lower surface coverage in the case of the mixed monolayer DPT 1/DDT/ Cu in comparison with the one based on AET/DPC 2/Cu. AFM Characterization of Dipyrromethene Derivative SAMs Incorporated with Cu(II). AFM offers the possibility of imaging nonconductive samples. Recently, AFM images of monolayers of cyclodextrins,59 porphyrins,60 and metallofunctionalized dendrimers61 are just a few examples. In order to visualize the surfaces of the SAMs under study, flat gold electrodes (1 cm × 1 cm mica, covered with 10 nm Cr and 100 nm Au) were modified according to the procedures illustrated in Scheme 1 in the Supporting Information. Before the modification, the gold plates were annealed in hydrogen flame to obtain a smooth surface of the gold grains. For the evaluation of the surface topology, the intermittent contact mode of AFM in air was used. There are several advantages to image in intermittent contact mode. The tip makes contact with the sample only at the very extreme of its oscillation. Also, the contact time is very brief. Because of this, shear forces and compressive forces can be very small, and surface-tip adhesive forces can be overcome. Therefore, the intermittent contact mode was very suitable for screening the gold substrate modified with dipyrromethene derivatives. The images did not change after scans, indicating a strong modification on the surface. Selected representative images are illustrated in Figure 4. The gold modified with the DPT 1/DDT SAM created a uniform surface (Figure 4A). After the next modification steps, the surface morphology became more ordered. The rows with a width of ∼0.75 nm are clearly visible (Figure 4B). We conclude that this order occurred due to hydrogen bonding between COOH groups present on the DPC 2 units. A similar topography was observed for the surface modified with AET/DPC 2/Cu/DPC 2 (results not shown). (59) Schonherr, H.; Beulen, M. W. J.; Bugler, J.; Huskens, J.; van Veggel, F. C. J. M.; Reinhoudt, D. N.; Vansco, G. J. J. Am. Chem. Soc. 2000, 122, 4963–4967. (60) Duong, B.; Arechabaleta, R.; Tao, N. J. J. Electroanal. Chem. 1998, 447, 63–69. (61) Martinovic, J.; Chiorces-Paquim, A.-M.; Diculescu, V. C.; Wyk, J. V.; Iwuoha, E.; Baker, P.; Mapolie, S.; Oliveira-Brett, A.-M. Electrochim. Acta 2008, 53, 4907–4919.
Dipyrromethene-Cu(II) Self-Assembled Monolayers
Figure 4. AFM images of a DPT 1/DDT SAM (a) and after copper and dipyrromethene-COOH complexation of DPT 1/DDT/Cu/DPC 2 (b).
Hydrogen bonding between adenine and thymine molecules deposited on the graphite surface have been reported by Uchihasi et al.62 They have obtained high-resolution images by applying noncontact AFM operating in an ultrahigh vacuum. One of the main parameters limiting the resolution of AFM images is the geometry of the AFM tip.62 For the research presented, a tip with approximately 10 nm radius of curvature was used. Therefore, obtaining AFM images with molecular resolution was not possible.
Conclusions It has been proved that dipyrromethene derivatives, having high affinity toward transition metal ions, are suitable for the formation of SAMs with redox centers by performing the successive complexation reactions on the electrode surface. As the example of transition metal ions, Cu(II) was selected. The obtained electroactive SAMs were characterized by wet(62) Uchihashi, T.; Ishida, T.; Komiyama, M.; Ashino, M.; Sugawara, Y.; Mizutani, W.; Yokoyama, K.; Morita, S.; Tokumoto, H.; Ishikawa, M. Appl. Surf. Sci. 2000, 157, 244–250.
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tability contact angle measurements, cyclic voltammetry, and atomic force microscopy. The better reversibility of the redox processes proceeding on the electrode surface was observed for dipyrromentene SAMs with the Cu(II) redox center located closer to the electrode surface. On the other hand, the electron transfer rate constants were very similar for both systems studied, 0.74 ( 0.21 and 0.44 ( 0.15 s-1 for DPT 1/DDT/Cu and AET/DPC 2/Cu, respectively. This indicates that the electron transfer rate constant depends not only on the distance of the redox centers from the electrode surface but also on the presence of heteroatoms such as nitrogen or oxygen. Their presence changed the dielectric properties of the system. The proposed method of formation of electroactive SAMs might be useful for derivatization of gold surfaces with functional groups such as COOH, NH2, or OH. The possibility of the attachment of biomolecules to dipyrromethene-Cu(II) SAMs with controlled orientation offers an excellent framework for the study of interactions occurring in interfacial environments between biomolecules of the type protein-protein, protein-hormone, or ssDNA-drugs. Having surface confined Cu(II) centers, the system proposed might respond toward interfacial molecular recognition events without the redox markers in the solution. This is particularly beneficial, because some of them might cause the loss of biomolecule activity. By performing the transition metal coordination on the electrode surface by dipyrromethene derivatives, many electroactive monolayers could be designed and used to define paths of electron transfer into and out of the biomolecules. They could work also as the transducing layer of biosensors for exploring the interfacial molecular recognition processes. This research is currently in progress in our laboratory. Acknowledgment. This work was supported by EU Grant COST D31/0021/05, a grant from the Polish Ministry of Science and Higher Education 19/COS/2006/3, and Grant PBZ KBN 098/T09/2003, bilateral Flemish-Polish Grant No. BIL1/4, University of Leuven, Belgium and Institute of Animal Reproduction and Food Research of Polish Academy of Sciences, Olsztyn, Poland. The facilities offered by IMEC vzw, Leuven, Belgium for performing the measurements presented are gratefully acknowledged. W.D. further acknowledges the Ministerie voor Wetenschapsbeleid, the K.U. Leuven, and F.W.O. for financial support. Supporting Information Available: Schematic representations of each modification step of Au/DPT 1/DDT/Cu/DPC 2 (Scheme 1) and Au/AET/DPC 2/Cu/DPC 2 (Scheme 2), and schematic representation of complexes DPT 1/DDT/Cu/DPC 2 and AET/DPC 2/Cu/DPC 2 estimated based on molecule modeling (Scheme 3). This material is available free of charge via the Internet at http://pubs.acs.org. LA801164F
