pubs.acs.org/Langmuir © 2009 American Chemical Society
)
Ferrocenyl Alkanethiols-Thio β-Cyclodextrin Mixed Self-Assembled Monolayers: Evidence of Ferrocene Electron Shuttling Through the β-Cyclodextrin Cavity Marco Frasconi,‡ Andrea D’Annibale,† Gabriele Favero, ,† Franco Mazzei,‡ Roberto Santucci,§ and Tommaso Ferri*,† Dipartimento di Chimica, Sapienza Universit a di Roma, P.le Aldo Moro, 5 - 00185 Roma, Italy, ‡Dipartimento di Chimica e Tecnologie del Farmaco, Sapienza Universit a di Roma, P.le Aldo Moro, 5 - 00185 Roma, Italy, and § Dipartimento di Medicina Sperimentale e Scienze Biochimiche, Universit a di Roma “Tor Vergata”, Via Montpellier, 1 - 00133 Roma, Italy. Now at Dipartimento di Chimica e Tecnologie del Farmaco, Sapienza Universit a di Roma, P.le Aldo Moro, 5-00185, Roma, Italy. )
†
Received May 27, 2009. Revised Manuscript Received September 1, 2009 This paper reports the preparation and characterization of an Au electrode modified with self-assembled alkane ferrocenes, in the absence and in the presence of β-cyclodextrins ( βCD). Electrode modification with ferrocene derivatives was achieved through a self-assembled monolayer (SAM) approach, using ferrocenyl hexane thiol (FcC6) and ferrocenyl undecane thiol (FcC11); the same was also done using per-6-thio-β-cyclodextrin. The different SAMs prepared were characterized by both cyclic voltammetry and electrochemical surface plasmon resonance (EC-SPR). The behavior of both single and binary monolayers including their interfacial reorganization was investigated and critically discussed, according to the nature of the SAM used. Cyclic voltammetry combined with SPR measurements revealed the reorientation of the SAM concomitant with the oxidation of ferrocene moieties. In particular, the electron shuttling of FcC11 through the βCD cavity (mixed SAM) was also evidenced by both SPR and the electrocatalytic oxidation of ferro(II)cyanide.
Introduction Assembly of nanometer-scaled building blocks into device configurations is an intensely investigated research in nanotechnology.1-4 The development of self-assembly methods for the construction of monolayer films on surfaces provides a mean to control and manipulate the interfacial characteristics,5 thus attracting considerable interest nowadays, owing to their wide potential applications.6 In addition, structurally well-defined layers on solid surfaces has been exploited to investigate fundamental issues of electron transfer between electrode and redox couple, often difficult to study on bare (naked) surfaces.7 In this sense, the surfaces modified by alkanethiol SAM linked to electroactive molecules, like ferrocene and its derivatives, have been proven to be a versatile building block with reversible redox activity on the modified electrodes.8,9 In particular, due to the rapid heterogeneous electron transfer rate, the use of ferrocenes for modifying electrode surfaces is very attractive in electroche*Corresponding author.
[email protected]. (1) Whitesides, G. M.; Grzybowski, B. Science 2002, 295, 2418. (2) Alivisatos, A. P. ACS Nano 2008, 2, 1514. (3) Katsonis, N.; Lubonska, M.; Pollard, M. A.; Feringa, B. L.; Rudolf, P. Prog. Surf. Sci. 2007, 82, 407. (4) Haick, H.; Cahen, D. Prog. Surf. Sci. 2008, 83, 217. (5) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. Rev. 2005, 105, 1103. (6) Arya, S. K.; Solanki, P. R.; Datta, M.; Malhotra, B. D. Biosens. Bioelectron. 2009, 24, 2810. (7) Leger, C.; Bertrand, P. Chem. Rev. 2008, 108, 2379. (8) Li, M. X.; Cai, P.; Duan, C. Y.; Lu, F.; Xie, J.; Meng, Q. J. Inorg. Chem. 2004, 43, 5174. (9) You, C. C.; W€urthner, F. J. Am. Chem. Soc. 2003, 125, 9716. (10) Napper, A. M.; Liu, H.; Waldeck, D. H. J. Phys. Chem. B 2001, 105, 7699. (11) Liu, J.; Paddon-Row, M. N.; Gooding, J. J. J. Phys. Chem. B 2004, 108, 8460. (12) Yu, J.; Shapter, J. G.; Johnston, M. R.; Quinton, J. S.; Gooding, J. J. Electrochim. Acta 2007, 52, 6206.
Langmuir 2009, 25(22), 12937–12944
mical applications.10,11 Ferrocene compounds are excellent candidates for molecular memory devices,12 for new electrochemical pH sensors13 and in bioelectronics.14 In this case, ferrocenes work as redox mediators between an electrode and the redox-active center of the enzyme. In fact, an efficient electrical wiring of redox enzymes with electrodes is the fundamental prerequisite to provide reliable biosensing devices.15 Ferrocenes are very promising also for creating electronic readouts of biomolecular function,16 the assembly of nanocircuit elements, or the conversion of biocatalytic processes into electrical power.17,18 In molecular electronics, special interest is growing toward those redox-switchable molecules which trigger a difference in conductive states.19 In particular, the preparation of molecular devices by means of self-assembly of redox active molecules and supramolecular chemistry is a promising method to obtain selective structures, relatively simple, that are difficult to prepare through other techniques.20 For these reasons, it is important to investigate in detail how molecules behave at electrode interfaces and, in particular, to understand the electron transport phenomena of redox active molecules through these supramolecular structures immobilized at interfaces. The preparation and characterization of a polycrystalline Au electrode chemically modified with self-assembled monolayers of (13) Hickman, J. J.; Ofer, D.; Laibinis, P. E.; Whitesides, G. M.; Wrighton, M. S. Science 1991, 252, 688. (14) Willner, I.; Katz, E. Angew. Chem., Int. Ed. 2000, 39, 1180. (15) Wang, J. Chem. Rev. 2008, 108, 814. (16) Willner, I.; Heleg-Shabtai, V.; Katz, E.; Rau, H. K.; Haehnel, W. J. Am. Chem. Soc. 1999, 121, 6455. (17) Willner, I.; Yan, Y. -M.; Willner, B.; Tel-Vered, R. Fuel Cells 2009, 1, 7. (18) Heller, A. Phys. Chem. Chem. Phys. 2004, 6, 209. (19) Crespo-Biel, O.; Lim, C. W.; Jan Ravoo, B.; Reinhould, D. N.; Huskens, J. J. Am. Chem. Soc. 2006, 128, 17024. (20) Ludden, M. J. W.; Reinhould, D. N.; Huskens, J. Chem. Soc. Rev. 2006, 35, 1122.
Published on Web 10/06/2009
DOI: 10.1021/la9018597
12937
Article
ferrocenyl alkane thiols, pure or mixed with a suitably thiolated βCD, is reported herein. βCD is an important and widely studied example of host molecular receptor due to its high affinity for hydrophobic molecules in aqueous media.21 The organized selfassembled lipoyl-β-cyclodextrin derivative monolayer on a gold surface allow docking of molecules with ultimate control over binding thermodynamics and kinetics, and positioning with molecular accuracy. The characterization of prepared surfaces was carried out by both cyclic voltammetry (CV) and surface plasmon resonance (SPR) spectroscopy. The SPR technique allows detection of physicochemical changes occurring in thin films adsorbed on a Au surface;22,23 when coupled with electrochemistry (EC-SPR), it permits detection of the optical and electrochemical properties of the adsorbed layer as well as thickness changes of ultrathin films during redox reactions. In view of such a unique feature, EC-SPR has found wide application as dynamic tool for monitoring electrochemical polymerization,24-26 kinetics of nanofilms formation,27,28 redox-induced conformational changes of enzymes,29 and biosensing.30-32 In previous papers, we reported the preparation and characterization of chemically modified electrodes based on βCD monolayers self-assembled on a gold electrode surface.33,34 In particular, per-6-thio-β-cyclodextrin was used as modifying agent: the exhaustive substitution of primary hydroxyl groups of βCD with thiol groups ensures spontaneous adsorption of these molecules on a Au electrode.35 Furthermore, the chemisorption of the modified βCD on gold electrode allows one to recognize electroactive species able to form inclusion complex with βCD by means of electrochemical experiments,36 in the meantime excluding a great part of interfering species unable to permeate the βCD cavity.37,38 In this paper, EC-SPR is employed to shed more light on redoxinduced reorganization and thickness changes of ferrocenyl alkane thiol, in the presence of βCD SAM. By measuring ECSPR angular changes concomitant with potential steps, we determined a time scale for the rapid, redox-induced formation of the inclusion complex between ferrocene and βCD. The influence of hydrocarbon chain length of the monolayers on EC-SPR signal was also investigated. Data obtained are of relevance because the knowledge of the mechanism(s) governing the redox process at a modified electrode is crucial for the (21) Rekharsky, M. V.; Inoue, Y. Chem. Rev. 1998, 98, 1875. (22) Knoll, W. Annu. Rev. Phys. Chem. 1998, 49, 569. (23) Homola, J. Chem. Rev. 2008, 108, 462. (24) Kang, X.; Jin, Y.; Cheng, G.; Dong, S. Langmuir 2002, 18 1713. (25) Sriwichai, S.; Baba, A.; Deng, S.; Huang, C.; Phanichphant, S.; Advincula, R. C. Langmuir 2008, 24, 9017. (26) Jiang, X.; Cao, Z.; Tang, H.; Tan, L.; Xie, Q.; Yao, S. Electrochem. Commun. 2008, 10, 1235. (27) Baba, A.; Park, M. K.; Advincula, R. C.; Knoll, W. Langmuir 2002, 18, 4648. (28) Norman, L. L.; Badia, A. Langmuir 2007, 23, 10198. (29) Zhai, P.; Guo, J.; Xiang, J.; Zhou, F. J. Phys. Chem. C 2007, 111, 981. (30) Iwasaki, Y.; Horiuchi, T.; Niwa, O. Anal. Chem. 2001, 73, 1595. (31) Liu, J.; Tian, S.; Tiefenauer, L.; Nielsen, P. E.; Knoll, W. Anal. Chem. 2005, 77, 2756. (32) Wang, J.; Wang, F.; Chen, H.; Liu, X.; Dong, S. Talanta 2008, 75, 666. (33) D’Annibale, A.; Regoli, R.; Sangiorgio, P.; Ferri, T. Electroanalysis 1999, 11, 505. (34) Favero, G.; Campanella, L.; D’Annibale, A.; Ferri, T. Microchem. J. 2004, 76, 77. (35) Finklea, H. O. Electrochemistry of organized monolayers of thiols and related molecules on electrodes, In Electroanalytical Chemistry; Bard, A. J., Rubinstein, I., Eds.; Marcel Dekker: New York, 1996; Vol 19. (36) Hapiot, F.; Tilloy, S.; Monfier, E. Chem. Rev. 2006, 106, 767. (37) Bender, M. L.; Komiyama, M. Cyclodextrin Chemistry; Springer-Verlag: New York, 1978. (38) Maistrenko, V. N.; Gusakov, V. N.; Sangalov, E. Y. J. Anal. Chem. 1995, 50, 528.
12938 DOI: 10.1021/la9018597
Frasconi et al.
development and the proper utilization of molecular electronic devices.
Experimental Section Reagents. All the reagents used throughout along this work were products of analytical grade from either Carlo Erba or Fluka. The synthesis of per-6-thio-β-ciclodextrin (βCD) was performed according to the method previously reported.39,40 Both FcC6 and FcC11, being commercially unavailable, were prepared according to the following general scheme:
The synthesis of ω-ferrocenyl alkanethiols was carried out according to literature procedures.41 The obtained products show spectral properties (1H, 13C NMR, and IR spectra) in agreement with those previously reported for the same compounds.42,43 Apparatus. 1H and 13C NMR spectra were recorded on a Varian model XL 200 Gemini (Varian, Palo Alto, California, USA) operating at 200 MHz for 1H and at 50.7 Hz for 13C. A Shimadzu model IR-470 was used for recording IR spectra. All electrochemical measurements were carried out by a PAR model 273 potentiostat/galvanostat controlled by PAR model 270 electrochemical software (EG&G Instruments, Princeton, NY, USA). A conventional three-electrode setup was employed, where the potential of the working electrode (either self-assembled monolayer electrodes or bare Au one) was always referred to a saturated calomel electrode (SCE) and a platinum ring was used as auxiliary electrode. pH measurements were carried out at room temperature (21 ( 1 °C) by using a model 2002 Crison pH meter (Crison, Alella, Spain). Surface plasmon resonance experiments were carried out by an ESPRIT instrument (Echo Chemie B.V., Ultrech, The Netherlands) coupled with a potentiostat (μAUTOLAB) from Echo Chemie (Ultrecht, Netherlands). The ESPRIT instrument is based on the Kretschmann configuration44 with a scanning angle setup. In this system, the intensity of the reflected light is minimum in the resonance angle. This angle can be measured over a range of 4° in this equipment by using a diode detector. The incidence angle was varied by using a vibrating mirror (rotating over an angle of 5° at 77 Hz in 13 ms), which directs p-polarized laser light onto a 1 mm 2 mm spot of the sensor disk via a hemi cylindrical prism of BK7 glass. In each cycle, the reflectivity curves were scanned on both forward and backward movements of the mirror. In this vibrating mirror setup, the resolution was 1 m°. The light source of the system is composed of the laser diode with emission wavelength of 670 nm. In the experiments, a gold sensor disk was mounted into a precleaned SPR cuvette, made in Teflon. The solutions were injected into the cuvette by a syringe with a (39) Rojas, M. T.; K€oniger, R.; Stoddart, J. F.; Kaifer, A. E. J. Am. Chem. Soc. 1995, 117, 336. (40) Ashton, P. R.; K€oniger, R.; Stoddart, J. F. J. Org. Chem. 1996, 61, 903. (41) Creager, S. E.; Rowe, G. K. J. Electroanal. Chem. 1994, 370, 203. (42) Stiles, R. L.; Balasubramanian, R.; Feldberg, S. W.; Murray, R. W. J. Am. Chem. Soc. 2008, 130, 1856. (43) Hoertz, P. G.; Niskala, J. R.; Dai, P.; Black, H. T.; You, W. J. Am. Chem. Soc. 2008, 130, 9763. (44) Kretschmann, E. Z. Phys. 1971, 241, 313.
Langmuir 2009, 25(22), 12937–12944
Frasconi et al.
Article
stainless steel needle. The gold sensor disks used for SPR, from Xantec Bioanalytical (Munster, Germany), were constituted of a 50-nm-thick gold sensing layer deposited onto a glass microscopic slide already covered with a 1.5 nm Ti adhesion layer. Before use, Au surfaces were shortly dipped in a piranha solution (conc. H2SO4 and 33% H2O2 in a 3:1 ratio), and the resulting oxide layer was removed by leaving the substrates in absolute ethanol for 10 m. Electrode Modification. Prior to chemical modification, the electrode surface was first polished by diamond paste (1 μm) and then by alumina slurry (0.3 μm). After each polishing treatment, the electrode was abundantly rinsed with deionized water and sonicated in water for 30 s; successively, after further abundant rinsing, the electrode was dried under nitrogen stream. Before use, the cleanliness of the electrode surface was checked by recording cyclic voltammograms of reversible electrochemical markers such as ferricyanide or a water-soluble Fc derivative (as ferrocene monocarboxylic acid (FcA)). The chemical modification of the Au electrode was achieved by dipping the bare electrode overnight in 1 10-3 mol/L solutions of either per-6-thio-βCD in DMSO/H2O (60:40 v/v) or ferrocenyl alkane thiols in ethanol. Mixed monolayers were prepared by successive treatments of per-6-thio-βCD as first and then the chosen ferrocenyl alkane thiol to fill the βCD interstitial area. Prior to use, the modified electrode was abundantly rinsed with ethanol and water. The mixed alkanethiol-βCD SAMs were prepared, paying particular attention to allow the alkanethiol assembly only in the free interstitial area (among assembled βCD). To this end, the βCD-covered electrode was dipped in a 1 10-3 mol/L of FcA solution to fill βCD cavities, and after a few minutes, the same solution was made millimolar in alkanethiol (by adding a suitable volume of an ethanolic solution thereof). Procedures. A 0.1 mol/L NaClO4 solution was used for all measurements. Differently from other media that cause signal decrease during continuous potential cycling (due to loss of ions from the ferrocenyl sites), this electrolyte ensures long-term stability of the Fc electrochemical signal.45,46 Voltammetric measurements were carried out on oxygen-free solutions: prior to measurements, the solutions were purged by UPP nitrogen for at least 5 min and, the N2 atmosphere was maintained during the voltammetric measurements. For the SPR experiments, the water was left to flow over the gold sensor disk until a stable baseline was observed. Then, the water was removed from the cell and the gold surface modified by adding into the cell 1 10-3 mol/L solution of either per6thio-βCD in DMSO/H2O (60:40 v/v) or chosen ferrocenyl alkane thiol dissolved in ethanol. The volume of the solution in the cell was controlled. Again, the cell was washed with solution and the resonant angle was set up for about 10 min. All ESPR curves reported in the paper were suitably subtracted of the blank contribution, generally constituted by the SAM missing Fc electroactive head groups.
Results Pure FcC6 and FcC11 SAMs. Figure 1A shows the CV at a bare and Fc6 modified Au electrode. The voltammogram of the Fc-modified electrode shows two sharp peaks due to the oxidation and reduction of the anchored Fc. The voltammogram is not as symmetric as expected: the anodic peak is better-shaped than cathodic one. The formal potential (E°0 ) determined for the redox process is 334 ( 5 mV (vs SCE), a value 48 mV more positive than that determined for soluble FcA (diffusion-controlled process).33 On the other hand, the difference between anodic and cathodic (45) Popenoe, D.; Deinhammer, R. S.; Porter, M. D. Langmuir 1992, 8, 2521. (46) Zhang, L.; Godinez, L. A.; Lu, T.; Gokel, G. W.; Kaifer, A. F. Angew. Chem., Int. Ed. 1995, 34, 235.
Langmuir 2009, 25(22), 12937–12944
peak potentials (ΔEp) is lower than that determined for a diffusion-controlled process47 (as expected for a monoelectronic-confined system) and assumes a constant value of 32 mV for scan rates (v) e 100 mV/s, in agreement with literature data.48 In addition, the anodic to cathodic peak intensity ratio is higher than unity, analogous to what was observed for the charge ratio relative to the two processes. Analysis of the influence of the v (20 mV/s to 2 V/s range) on Ip and Ep reveals a typical behavior of immobilized systems: the peak intensity (Ip) changes linearly with the potential scan rate (not shown), while the logarithm plot of peak potential versus potential scan rate (not shown) provides evidence for a transition that, reversible at low scan rate, becomes irreversible at high scan rates. From the shift of the anodic and cathodic peak potentials as a function of the scan rate, the electron transfer rate constant was then estimated using the Laviron method .49 The data at first were fitted using the classical Butler-Volmer equation for a surfaceconfined redox reaction. The charge transfer coefficient (R) and the electron transfer rate constant (kET) are reported with formal potential (E°0 ) in Table 1. We found that the fitting on the cathodic branch was not too satisfactory, a result that discloses the occurrence of interactions between the immobilized redox probes.50 To take into account these slight interactions, voltammograms were simulated using the general expression developed by Laviron for a surface-confined reaction in which weak interaction among immobilized molecules take place.51 In this simulation, the same value of kET was assumed and the following values determined: β = -0.4, γ = 0.7, λ = 0, and μ = 0, for the four interaction parameters, in order to gain the best-fit curves. The asymmetry of the CV may be explained by considering the peculiarity of the system under study. During the anodic scan, the Fcs undergoing charge transfer are energetically equivalent hence, as expected, the relative peak is well-shaped. Conversely, the cathodic branch shows a main peak that is lower than the corresponding anodic one and is followed by two shoulders (ill-defined additional peaks). This suggests that the same energetic equivalence of electroactive species cannot be stated for cathodic scan. A possible explanation is provided here: the integration of the anodic peak area gives a value of 2.37((0.11) μC involved in Fc oxidation; since the electrode area is 4.91 mm2, we calculate that approximately 5.0 10-10 mol/cm2 of Fc derivatives are anchored onto the electrode. Although slightly higher than the theoretical value (4.5 10-10 mol/cm2 or 4.8 10-10 mol/cm2) as expected from previous works,52-54 this value appears reliable for a close-packed alkane Fc layer, particularly if we consider the real electrode area (a roughness factor up to 1.2 is usual for a polycrystalline Au electrode). On what is stated above, each Fc derivative molecule should cover ∼33 A˚2. By adopting the same calculation for the cathodic branch, it emerges that the charge value strictly depends on the criteria used for its evaluation. More specifically, if only the main cathodic peak is considered, the charge value is significantly smaller than that of the anodic peak; conversely, a charge equivalent to that of (47) Bard, A. J.; Faulkner, L. R. Electrochemical methods: fundamental and applications, 2nd ed.; John Wiley & Sons: New York, 2001. (48) Yao, X.; Yang, M. L.; Wang, Y.; Hu, Z. Sens. Actuators, B 2007, 122, 351. (49) Laviron, E. J. Electroanal. Chem. 1979, 101, 19. (50) Lee, L. Y. S.; Sutherland, T. C.; Rucareanu, S.; Lennox, R. B. Langmuir 2006, 22, 4438. (51) Laviron, E.; Roullier, L. J. Electroanal. Chem. 1980, 113, 65. (52) Chidsey, C. E.; Bertozzi, C. R.; Putvinsky, T. M.; Mujsce, A. M. J. Am. Chem. Soc. 1990, 112, 4301. (53) Viana, A. S.; Jones, A. H.; Abrantes, L. M.; Kalaji, M. J. Electroanal. Chem. 2001, 500, 220. (54) Ju, H.; Leech, D. Langmuir 1998, 14, 300.
DOI: 10.1021/la9018597
12939
Article
Frasconi et al.
Figure 1. (A) CVs recorded at an Au electrode before (dashed line) and after (solid line) modification with a FcC6 SAM; v = 50 mV/s. (B) EC-SPR dip shift of FcC6 SAM; v = 50 mV/s. Measurements were carried out in a 0.1 mol/L NaClO4 solution. Table 1. Formal Potential (E°0 ), Electron Transfer Rate Constant (kET), and Charge Transfer Coefficient (r) Determined for the Ferrocene-Based Electroactive SAMs Investigated SAM
E°0 (mV vs SCE)
kET (s-1)
R
FcC6 FcC11 FcC6-βCD FcC11-βCD
334 ( 5 349 ( 7 330 ( 6 359 ( 9
182 ( 9 43 ( 11 178 ( 19 15 ( 6
0.52 ( 0.03 0.53 ( 0.02 0.49 ( 0.04 0.46 ( 0.02
the anodic peak is determined if the two additional peaks are included. This is consistent with the view that, if initially all the Fc molecules in the SAM are energetically equivalent, once the anodic scan proceeds the formed ferricenium ions (Fcþ)-being charged species-repeal each other. Being anchored on the surface, to minimize the electrostatic repulsion effects, they are forced to change their spatial arrangement at the interface changing, for instance, the tilt angle55 or flipping the cyclopentyldiene ring around the Fc-C bond at the end of SAM.53 As evidenced by the signal, this entails the Fcþ formed during the anodic scan possibly being grouped into inequivalent reducible classes differing in height and consequently falling at different potential values according to their different overpotentials.56,57 In order to shed deeper light on the events occurring during potential scanning, EC-SPR experiments were carried out. To put in sharper evidence the role of the Fc head groups present on the SAM, an appropriate blank was subtracted from all the SPR curves of ferrocene-containing alkanethiol SAMs. The blank measurement is represented for each SAM by a SPR curve obtained for a SAM of an alkanethiol of same length without the terminal Fc group. The same approach was also followed for mixed SAMs (see below). In agreement with literature,48 EC-SPR performed on hexanethiol SAM produced only a very small change (∼0.001°) of dip shift vs potential. Since, the (dip shift vs potential) behavior reflects the interface changes in terms of thickness or dielectric constant value (charge density), or even both of them, this suggests that in this case the superficial electron density at the Au electrode surface does not change appreciably with potential. By contrast, for SAMs containing Fc head groups the dip shift vs potential changes significantly and a sigmoid curve is obtained (Figure 1B). The curve shape is not affected by the potential scan rate and shows no hysteresis when the scanning is (55) Ye, S.; Sato, Y.; Uosaki, K. Langmuir 1997, 13, 3157. (56) Uematsu, T.; Kuwabata, S. Anal. Sci. 2008, 24, 307. (57) Ye, S.; Yashiro, A.; Sato, Y.; Uosaki, K. J. Chem. Soc., Faraday Trans. 1996, 3813.
12940 DOI: 10.1021/la9018597
reversed. The vertical inflection point value obtained is very close to the formal potential of the CV curve, since it represents the interfacial arrangement at equimolar Fc and Fcþ concentration.48 The potential limiting values of the dip shift correspond to fully reduced and oxidized anchored Fc groups, while the intermediate points are related to their relative amounts. It can be pointed out that the curve shape resembles that of a stationary voltammogram. When FcC11 SAM is used in place of an FcC6 SAM, a CV curve displaying sharper peaks is obtained (Figure 2A). The determined formal potential (E°0 =349 ( 4 mV) is 15 mV higher than that of the hexane derivative, and a slightly smaller anodiccathodic peak separation is observed. The charge amount involved in the process, which is determined from the anodic peak area, is 2.72 ( 0.15 μC, a value slightly higher than that determined for the corresponding process of the hexane derivative (see above). This may be ascribed to the stronger Van der Walls interactions arising among longer side chains, which favor a tighter packing of the molecule.58 The charge value calculated allows determination of the superficial ferrocenyl concentration, 5.7 10-10 mol/cm2, from which a single ferrocenyl molecule covers 29 A˚2. Analysis of the dependence of the peak potentials on the scan rate reveals higher asymmetry with respect to FcC6, as demonstrated from the values of the interaction parameters, β=-0.5, γ=0.9, λ = 0.1, and μ=0, compatible with stronger interactions arising as the redox probe surface concentration is increased. The EC-SPR curve of such a SAM is similar in shape to that of the hexane derivative, but the dip shift change (Δθ0.0 V - Δθ0.7 V) is twice as great: 28.5 vs 12.4 (Figure 2B). Mixed FcC6-βCD and FcC11-βCD SAMs. To obtain mixed SAMs, the electrodes covered with a βCD SAM were treated with FcC11 (or FcC6) ethanolic solutions. Since the Fc forms a stable inclusion complex with βCD,59-61 the βCD cavities were filled with Fcs (added in solution as FcA) so that the ferrocenyl alkanethiol, added in solution, was forced to adsorb on the interstitial Au area (among βCD molecules) to form ordered mixed SAMs. Figure 3A shows the CVs of a βCD SAM-covered Au electrode before and after its treatment with FcC6. Whereas in the former (58) Uosaki, K.; Sato, Y.; Kita, H. Langmuir 1991, 7, 1511. (59) Bertrand, G. L.; Faulkner, J. R.; Han, J. S. M.; Armstrong, D. W. J. Phys. Chem. 1989, 93, 6863. (60) Wu, J.; Toda, K.; Tanaka, A.; Sanemasa, I. Bull. Chem. Soc. Jpn. 1998, 71, 1615. (61) Matsue, T.; Evans, D. H.; Osa, T.; Kobayashi, N. J. Am. Chem. Soc. 1985, 107, 3411.
Langmuir 2009, 25(22), 12937–12944
Frasconi et al.
Article
Figure 2. (A) CVs recorded at an Au electrode before (dashed line) and after (solid line) modification with a FcC11 SAM; v = 50 mV/s. (B) EC-SPR dip shift of FcC11 SAM as a function of potential; v = 50 mV/s. Measurements were carried out in a 0.1 mol/L NaClO4 solution.
Figure 3. (A) CVs recorded at a β-CDX SAM modified electrode in the absence (dashed line) and in the presence (solid line) of assembled FcC6 (FcC6-βCD). v = 50 mV/s. (B) EC-SPR dip shift of FcC6-βCD SAM as a function of potential. v = 50 mV/s. Measurements were carried out in a 0.1 mol/L NaClO4 solution.
case no faradic current is observed (black curve), two peaks appear after FcC6 self-assembly due to the electrochemical activity of Fc (red curve). As observed for pure FcC6 SAMs, also in this case asymmetric CVs are obtained. The anodic-cathodic peak separation was approx 30 mV and the formal potential 330 ( 4 mV. On the whole, these values do not differ significantly from those determined for a pure ferrocenyl SAM. By integrating the anodic peak area, a charge exchange of 464 ( 18 nC is obtained, corresponding to the 19.6% of that determined for a pure FcC6 SAM. Since the βCD-covered area is approx 80% of the total surface,33 it results in all Fc derivatives covering the remaining available surface (98 1012 A˚2). It follows that each Fc derivative molecule covers 34 A˚2, which is the same value determined for a pure FcC6 SAM; this suggests that SAMs essentially maintain the same structure independently from the presence of βCD on the electrode. This is confirmed by the change in the dip shift when potential is cyclically scanned. The dip shift change (Δθ0.0 V - Δθ0.7 V) for the FcC6-βCD SAM is 8.8 m° (Figure 3B), i.e., approximately 70% of the value recorded for a pure FcC6 SAM (12.4 m°). Figure 4A shows the CVs recorded by a βCD SAM (black curve) and by FcC11-βCD (red curve). Different from FcC6-βCD SAM, FcC11-βCD SAM generates well-shaped voltammograms, with current values comparable to those shown by shorter derivatives. This suggests that a similar coverage takes place. The SPR dip shift of FcC11-βCD mixed SAM is shown in Figure 4B. The dip shift may be expected to be similar to that of Langmuir 2009, 25(22), 12937–12944
FcC6-βCD, because passing from pure to mixed SAM, the superficial charge density change should be independent by the alkane length. This is not true for the film thickness that is dramatically affected: really, passing from a pure FcC11 to a FcC11-βCD SAM the average thickness decreases. Different from what was observed for FcC6-based SAMs, the EC-SPR curve of the FcC11-βCD SAM, shown in Figure 4B, is characterized by a dip shift change (Δθ0.0 V - Δθ0.7 V) larger than that determined for a pure FcC11 SAM (33.4 m° vs 28.5 m°). Analysis of the influence of the v on Ep for the mixed SAMs reveals nearly ideal behavior, as revealed from the good fitting of the CV data using the Butler-Volmer equation for a surfaceconfined redox reaction. The values of several parameters as the formal potential, the electron transfer rate constant, and the charge transfer coefficient obtained from CV data analysis for all SAMs investigated are reported in Table 1. The influence of potential scan rate (range 50-500 mV/s) on EC-SPR response is shown in Figure 5. In this study, the potential scanning started from 0.7 V. From the figure, it appears clear that, contrary to FcC6-βCD, the FcC11-βCD behavior strongly depends on the scan rate and its direction. Electrocatalytic Oxidation of Ferro(II)cyanide. The reversible redox process of potassium ferrocyanide observed at bare Au (E°0 =191 ((2) mV vs SCE) is no longer detected at the βCD SAM-covered Au electrode.33 As expected, the βCD SAM provides an electrode surface not accessible to relatively large DOI: 10.1021/la9018597
12941
Article
Frasconi et al.
Figure 4. (A) CVs recorded at a βCD SAM modified electrode in the absence (dashed line) and in the presence (solid line) of self-assembled
FcC11 (βCD-FcC11). v = 50 mV/s. (B) EC-SPR dip shift of βCD-FcC11 SAM as a function of potential. v = 50 mV/s. Measurements were carried out in a 0.1 mol/L NaClO4 solution.
Figure 5. Influence of potential scan rate on SPR response. (A) SPR dip shift vs potential for a FcC11-βCD SAM at scan rate: 50 mV s-1; 150 mV s-1; 400 mV s-1. Measurements were carried out in a 0.1 mol/L NaClO4 . (B) Potential scan rate-dependence of the SPR signal for a FcC11-βCDX SAM (b) and a FcC6-βCDX SAM (O).
charged molecules as ferrocyanide,62 whose direct electron transfer is prevented. Once the interstitial area of the βCD SAM is covered by ferrocenyl alkanethiol derivatives, the Fcþ-induced oxidation of ferrocyanide is expected, since the formal potentials of the anchored Fcþ/Fc and of soluble ferri/ferrocyanide significantly differ. Thus, a catalytic current should be generated in the presence of soluble ferrocyanide only if Fcþ is electrochemically regenerated at the electrode surface. The CVs recorded at the FcC11-βCD electrode in the presence of different ferrocyanide concentrations are reported in Figure 6. The behavior typical of a catalytic process, due to the shuttling action of the Fc anchored to the surface by the undecane residue, is observed. Contrary to ferrocyanide, Fc permeates the βCD cavity and forms a stable inclusion complex. Interestingly, no significant catalytic current was detected when CVs were run at a FcC6-βCD electrode in the presence of the same ferrocyanide concentrations.
Discussion In order to provide a rationale for the results described above (which can be summarized in the different EC-SPR behavior (62) Tamura, S.; Tagaki, W.; Nakahara, H.; Fukuda, K. Chem. Lett. 1986, 15, 1933.
12942 DOI: 10.1021/la9018597
shown by ferrocenyl alkanes of different lengths, and in Fc terminal groups inclusion into the βCD cavity), it is necessary to consider the factors that contribute to the SPR angle change. At a SAM-modified surface, three major parameters are supposed to affect the SPR angle, namely, (i) the electron density charge at the metal surface (4σ), (ii) the monolayer thickness change (4d), and (iii) the refractive index change (4n). In general, when an external electrode potential (4V) is applied the dependence of the SPR angular shift (4θR) from such parameters can be expressed as follows:63 ΔθR ðλÞ ΔnðλÞ Δd Δσ ¼ c1 þ c2 þ c3 ΔV ΔV ΔV ΔV where c1, c2, and c3 are three distinct constants and λ is the wavelength of the incident light. Here, we shall consider how the structure and the composition of a SAM affect each of the three factors. SPR depends on the electrons density at the metal surface; thus, any surface charging effect (4σ) induced by a potential change will provoke an angular change. On the basis of the zero charge potential of the metal/electrolyte interface,64 it may be asserted that, upon sweeping the gold electrode potential toward a positive (63) Wang, S.; Boussaad, S.; Tao, N. J. In Surface Science Series, Rusling, J. F., Ed.; Marcel Dekker: New York, 2003; Vol 111; pp 213-251. (64) Garcia, G.; Macagno, V. A.; Lacconi, G. I. Electrochim. Acta 2003, 48, 1273.
Langmuir 2009, 25(22), 12937–12944
Frasconi et al.
Figure 6. CVs recorded at a FcC11-βCD SAM modified electrode in a 0.1 mol/L NaClO4 solution presence of ferro(II)cyanide (mM): 0.00 (a); 0.05 (b); 0.10 (c); 0.20 (d); 0.40 (e); 1.00 (f); 2.00 (g). (v = 50 mV/s).
value, the electron deficient state of the surface results in a positive SPR angle shift (which reflects the positive increase of the interfacial charge due to the non faradic double layer charging).65 This phenomenon likely provides the highest contribution to the SPR angle changes in the absence of redox process (as observed for the alkanethiol and alkanethiol-βCD SAMs). However, since the same potential changes were applied to both FcC6 and FcC11, in the absence and presence of βCD, the third term of the equation reported above is expected to remain constant. A change of the SAM thickness (4d) is also expected to shift the SPR angle. Fc oxidation yields the Fcþ, which is repelled (by electrostatics) from the positively charged gold surface: this results in a smaller tilt angle and in a consequential thickness increase, which determines a negative dip shift.48 This is in agreement with previous work which reports an increase in thickness of FcC6 following a positive potential scan that was reported to shift the dip in a negative direction66 even though an increase in the thickness of alkanthiol SAMs is usually accompanied by a positive dip shift. The application of a potential to pure ferrocenyl alkanethiol SAMs was reported to result in rotation of the ferrocene moiety,53 although the EC-SPR measurements at a ferrocenyl alkanethiol monolayer seem to suggest that tilt angle changes have greater influence on the observed SAM thickness change.48 From such considerations, it may be assessed that the SAM average thickness changes during redox reactions and induces a shift of the SPR angle consistent with a reorganization of the Fc SAM. We found that such a shift is greater for FcC11 with respect to FcC6 (Figure 1B and Figure 2B). Such a difference may also be explained by taking into account the different lengths of alkane thiol derivative used to build the SAM: larger thickness and the closer packing thereof, in addition to greater influence on the dielectric constant value. By passing from pure to mixed SAMs, the negative dip shift change is different according to the length of anchoring group used; in particular, it slightly decreases for FcC6-based SAMs (see Figures 1B and 3B), while it increases for FcC11 ones (Figures 2B and 4B).
(65) Garland, J. E.; Assiongbon, K. A.; Pettit, C. M.; Roy, D. Anal. Chim. Acta 2003, 475, 47. (66) Xiang, J.; Gou, J.; Zhou, F. Anal. Chem. 2006, 78, 1418. (67) Porter, M. C.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559.
Langmuir 2009, 25(22), 12937–12944
Article
If we consider that a pure FcC6 SAM67 has a slightly greater thickness (of an extent equal to the ferrocene group size) than that of a closely packed βCD SAM,68 the diminished negative dip shift shown by mixed SAM is probably correlated with the lower superficial charge density now involved rather than the small thickness decrease.69 In mixed SAMs, FcC6 covers only limited surface portions among the βCDs (island-like structure); from a redox point of view, it behaves as in a pure SAM where electron transfer occurs by a tunneling process. For mixed FcC11-βCD SAM, the penetration of the Fc group into the βCD cavity (with the consequent formation of the inclusion complex) may represent an additional feature contributing to the SPR angle shift; in view of that, once oxidized the Fcþ moiety should be better solvated in the aqueous phase. This effect can be possible only in the case of FcC11-based mixed SAMs, where the C11 alkyl chain can bend to allow permeation of Fc groups into the βCD cavity with formation of an inclusion complex. In addition, the migration of bulk solvated ions onto the SAM surface increases the refractive index (4n) at the interface, thus determining a further SPR angle shift. By considering the (above-mentioned) surface coverage degree of the Fc, the presence of ions over the FcC6-βCD SAM is expected to be less extended than for FcC6 SAM, suggested by the different SPR angle shift observed. Conversely, in the case of FcC11-βCD and FcC11 SAMs the contribution to the SPR angle change due to ionic motion over the film is much less significant when compared to the SAM thickness change. Nevertheless, if we consider the percentage of the mixed SAMs vs pure Fc SAMs, an increase of the negative dip shift is observed for both FcC6 and FcC11 SAMs, likely ascribed to a decrease of the refractive index caused by water loading in mixed SAMs. Surface hydratation includes the water molecules inserted into the βCD molecules as well as those already present, with expected changes in the orientation of the ferrocene moieties. As stated, the Fc groups anchored to the electrode by a C6 chain transfer electrons by a tunneling process in the case of pure and mixed SAMs. Conversely, different electron transfer mechanisms can take place in the case of FcC11-based SAMs. In particular for FcC11-βCD SAM besides to a tunneling process (occurring at pure SAM), a direct electron transfer may also occur favored by the bending of peripheral FcC11 molecules of the islands allowing Fc penetration into the βCD cavity and formation of the inclusion complex (see Figure 7). This hypothesis seems to be supported by the data relative to the dip shift change as a function of potential scan rate. As shown in Figure 5A, the dip shift is lower at higher scan rate, consistent with a kinetic limitation for formation of the Fc/βCD inclusion complex. Further evidence is also provided by two curves shown in Figure 5B; although shifted along the Y-axis, they show a similar trend at scan rates of g200 mV/s. This means that under the conditions investigated both systems show similar behavior, which is determined by the difficulty of molecules to change their spatial arrangement (tilt angle change and/or cyclopentyldiene rings rotation) upon oxidation of Fc groups. Conversely, at scan rate
