DOI: 10.1021/la9018597 12937Langmuir 2009, 25(22), 12937–12944 Published on Web 10/06/2009

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 AldoMoro, 5 - 00185 Roma, Italy, ‡Dipartimentodi Chimica e Tecnologie del Farmaco, Sapienza Universit�a di Roma, P.le AldoMoro, 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 alkaneferrocenes, in the absence and in the presence of β-cyclodextrins (βCD). Electrode modification with ferrocenederivatives 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 SAMspreparedwere characterized by both cyclic voltammetry and electrochemical surface plasmon resonance (EC-SPR). Thebehavior of both single and binary monolayers including their interfacial reorganization was investigated and criticallydiscussed, according to the nature of the SAMused. Cyclic voltammetry combinedwith SPRmeasurements revealed thereorientation of the SAM concomitant with the oxidation of ferrocene moieties. In particular, the electron shuttlingof FcC11 through the βCD cavity (mixed SAM) was also evidenced by both SPR and the electrocatalytic oxidation offerro(II)cyanide.

Introduction

Assembly of nanometer-scaled building blocks into deviceconfigurations is an intensely investigated research in nano-technology.1-4 The development of self-assembly methods forthe construction of monolayer films on surfaces provides a meanto control and manipulate the interfacial characteristics,5 thusattracting considerable interest nowadays, owing to their widepotential applications.6 In addition, structurally well-definedlayers on solid surfaces has been exploited to investigate funda-mental issues of electron transfer between electrode and redoxcouple, often difficult to study on bare (naked) surfaces.7 In thissense, the surfaces modified by alkanethiol SAM linked toelectroactive molecules, like ferrocene and its derivatives, havebeen proven to be a versatile building block with reversible redoxactivity on the modified electrodes.8,9 In particular, due to therapid heterogeneous electron transfer rate, the use of ferrocenesfor modifying electrode surfaces is very attractive in electroche-

mical applications.10,11 Ferrocene compounds are excellent can-didates for molecular memory devices,12 for new electrochemicalpH sensors13 and in bioelectronics.14 In this case, ferrocenes workas redox mediators between an electrode and the redox-activecenter of the enzyme. In fact, an efficient electrical wiring of redoxenzymes with electrodes is the fundamental prerequisite toprovide reliable biosensing devices.15 Ferrocenes are very promis-ing also for creating electronic readouts of biomolecular func-tion,16 the assembly of nanocircuit elements, or the conversion ofbiocatalytic processes into electrical power.17,18

In molecular electronics, special interest is growing towardthose redox-switchable molecules which trigger a difference inconductive states.19 In particular, the preparation of moleculardevices by means of self-assembly of redox active molecules andsupramolecular chemistry is a promising method to obtainselective structures, relatively simple, that are difficult to preparethrough other techniques.20 For these reasons, it is important toinvestigate in detail how molecules behave at electrode interfacesand, in particular, to understand the electron transport phenom-ena of redox active molecules through these supramolecularstructures immobilized at interfaces.

The preparation and characterization of a polycrystalline Auelectrode chemically modified with self-assembled monolayers of

*Corresponding author. tommaso.ferri@uniroma1.it.(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) L�eger, 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.

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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.

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1122.

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ferrocenyl alkane thiols, pure or mixed with a suitably thiolatedβCD, is reported herein. βCD is an important and widely studiedexample of host molecular receptor due to its high affinity forhydrophobic molecules in aqueous media.21 The organized self-assembled lipoyl-β-cyclodextrin derivative monolayer on a goldsurface allow docking of molecules with ultimate control overbinding thermodynamics and kinetics, and positioning withmolecular accuracy.

The characterization of prepared surfaces was carried out byboth cyclic voltammetry (CV) and surface plasmon resonance(SPR) spectroscopy. The SPR technique allows detection ofphysicochemical changes occurring in thin films adsorbed on aAu surface;22,23 when coupled with electrochemistry (EC-SPR), itpermits detection of the optical and electrochemical properties ofthe adsorbed layer as well as thickness changes of ultrathin filmsduring redox reactions. In view of such a unique feature, EC-SPRhas found wide application as dynamic tool for monitoringelectrochemical polymerization,24-26 kinetics of nanofilms for-mation,27,28 redox-induced conformational changes of enzymes,29

and biosensing.30-32

In previous papers, we reported the preparation and character-ization of chemically modified electrodes based on βCD mono-layers self-assembled on a gold electrode surface.33,34 Inparticular, per-6-thio-β-cyclodextrin was used as modifyingagent: the exhaustive substitution of primary hydroxyl groupsof βCD with thiol groups ensures spontaneous adsorption ofthesemolecules on aAu electrode.35 Furthermore, the chemisorp-tion of the modified βCD on gold electrode allows one torecognize electroactive species able to form inclusion complexwith βCD by means of electrochemical experiments,36 in themeantime excluding a great part of interfering species unable topermeate the βCD cavity.37,38

In this paper, EC-SPR is employed to shedmore light on redox-induced reorganization and thickness changes of ferrocenylalkane thiol, in the presence of βCD SAM. By measuring EC-SPR angular changes concomitant with potential steps, wedetermined a time scale for the rapid, redox-induced formationof the inclusion complex between ferrocene and βCD. Theinfluence of hydrocarbon chain length of the monolayers onEC-SPR signal was also investigated. Data obtained are ofrelevance because the knowledge of the mechanism(s) governingthe redox process at a modified electrode is crucial for the

development and the proper utilization of molecular electronicdevices.

Experimental Section

Reagents. All the reagents used throughout along this workwere products of analytical grade from either Carlo Erba orFluka.

The synthesis of per-6-thio-β-ciclodextrin (βCD) was per-formed according to the method previously reported.39,40 BothFcC6 and FcC11, being commercially unavailable, were preparedaccording to the following general scheme:

The synthesis of ω-ferrocenyl alkanethiols was carried outaccording to literature procedures.41 The obtained products showspectral properties (1H, 13C NMR, and IR spectra) in agreementwith those previously reported for the same compounds.42,43

Apparatus. 1H and 13C NMR spectra were recorded on aVarian model XL 200 Gemini (Varian, Palo Alto, California,USA) operating at 200 MHz for 1H and at 50.7 Hz for 13C. AShimadzu model IR-470 was used for recording IR spectra.

All electrochemical measurements were carried out by a PARmodel 273 potentiostat/galvanostat controlled by PAR model270 electrochemical software (EG&G Instruments, Princeton,NY, USA). A conventional three-electrode setup was employed,where the potential of theworking electrode (either self-assembledmonolayer electrodes or bare Au one) was always referred to asaturated calomel electrode (SCE) and a platinum ring was usedas auxiliary electrode. pHmeasurementswere carried out at roomtemperature (21( 1 �C) by using a model 2002 Crison pHmeter(Crison, Alella, Spain).

Surface plasmon resonance experiments were carried out by anESPRIT instrument (EchoChemieB.V.,Ultrech,TheNetherlands)coupled with a potentiostat (μAUTOLAB) from Echo Chemie(Ultrecht, Netherlands). The ESPRIT instrument is based on theKretschmann configuration44 with a scanning angle setup. In thissystem, the intensity of the reflected light is minimum in theresonance angle. This angle can bemeasured over a range of 4� inthis equipment by using a diode detector. The incidence angle wasvaried by using a vibrating mirror (rotating over an angleof 5� at 77 Hz in 13ms), which directs p-polarized laser light ontoa 1mm� 2mmspotof the sensor disk via a hemi cylindrical prismofBK7 glass. In each cycle, the reflectivity curveswere scanned onboth forward and backward movements of the mirror. In thisvibrating mirror setup, the resolution was 1 m�. The light sourceof the system is composed of the laser diode with emissionwavelength of 670 nm. In the experiments, a gold sensor diskwas mounted into a precleaned SPR cuvette, made in Teflon. Thesolutions were injected into the cuvette by a syringe with a

(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.

(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.

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stainless steel needle. The gold sensor disks used for SPR, fromXantec Bioanalytical (Munster, Germany), were constituted of a50-nm-thick gold sensing layer deposited onto a glassmicroscopicslide 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 oxidelayer was removed by leaving the substrates in absolute ethanolfor 10 m.

Electrode Modification. Prior to chemical modification, theelectrode surface was first polished by diamond paste (1 μm) andthen by alumina slurry (0.3 μm). After each polishing treatment,the electrode was abundantly rinsed with deionized water andsonicated in water for 30 s; successively, after further abundantrinsing, the electrodewas dried under nitrogen stream. Before use,the cleanliness of the electrode surface was checked by recordingcyclic voltammograms of reversible electrochemicalmarkers suchas ferricyanide or a water-soluble Fc derivative (as ferrocenemonocarboxylic acid (FcA)).

The chemicalmodification of the Au electrode was achieved bydipping the bare electrode overnight in 1 � 10-3 mol/L solutionsof either per-6-thio-βCD inDMSO/H2O (60:40 v/v) or ferrocenylalkane thiols in ethanol. Mixed monolayers were prepared bysuccessive treatments of per-6-thio-βCD as first and then thechosen ferrocenyl alkane thiol to fill the βCD interstitial area.Prior to use, the modified electrode was abundantly rinsed withethanol and water. The mixed alkanethiol-βCD SAMs wereprepared, paying particular attention to allow the alkanethiolassembly only in the free interstitial area (among assembledβCD). To this end, the βCD-covered electrode was dipped in a1 � 10-3 mol/L of FcA solution to fill βCD cavities, and after afew minutes, the same solution was made millimolar in alka-nethiol (by adding a suitable volume of an ethanolic solutionthereof).

Procedures. A 0.1 mol/L NaClO4 solution was used for allmeasurements. Differently from other media that cause signaldecrease during continuous potential cycling (due to loss of ionsfrom the ferrocenyl sites), this electrolyte ensures long-termstability of the Fc electrochemical signal.45,46 Voltammetric mea-surements were carried out on oxygen-free solutions: prior tomeasurements, the solutions were purged by UPP nitrogen for atleast 5 min and, the N2 atmosphere was maintained during thevoltammetric measurements.

For the SPR experiments, the water was left to flow over thegold sensor disk until a stable baseline was observed. Then, thewater was removed from the cell and the gold surface modifiedby adding into the cell 1 � 10-3 mol/L solution of either per-6thio-βCD in DMSO/H2O (60:40 v/v) or chosen ferrocenylalkane thiol dissolved in ethanol. The volume of the solution inthe cell was controlled. Again, the cell was washed with solutionand the resonant angle was set up for about 10 min.

All ESPR curves reported in the paper were suitably subtractedof the blank contribution, generally constituted by the SAMmissing Fc electroactive head groups.

Results

Pure FcC6 and FcC11 SAMs. Figure 1A shows the CV at abare and Fc6 modified Au electrode. The voltammogram of theFc-modified electrode shows two sharp peaks due to the oxida-tion and reduction of the anchored Fc. The voltammogram is notas symmetric as expected: the anodic peak is better-shaped thancathodic one. The formal potential (E�0) determined for the redoxprocess is 334( 5mV (vs SCE), a value 48mVmore positive thanthat determined for soluble FcA (diffusion-controlled process).33

On the other hand, the difference between anodic and cathodic

peak potentials (ΔEp) is lower than that determined for adiffusion-controlled process47 (as expected for a monoelectro-nic-confined system) and assumes a constant value of 32 mV forscan rates (v) e 100 mV/s, in agreement with literature data.48 Inaddition, the anodic to cathodic peak intensity ratio is higher thanunity, analogous to what was observed for the charge ratiorelative to the two processes.

Analysis of the influence of the v (20 mV/s to 2 V/s range) on Ipand Ep reveals a typical behavior of immobilized systems: thepeak intensity (Ip) changes linearly with the potential scan rate(not shown), while the logarithm plot of peak potential versuspotential scan rate (not shown) provides evidence for a transitionthat, reversible at low scan rate, becomes irreversible at highscan rates.

From the shift of the anodic and cathodic peak potentials as afunction of the scan rate, the electron transfer rate constant wasthen estimated using the Lavironmethod .49 The data at first werefitted using the classical Butler-Volmer equation for a surface-confined redox reaction. The charge transfer coefficient (R) andthe electron transfer rate constant (kET) are reported with formalpotential (E�0) in Table 1. We found that the fitting on thecathodic branch was not too satisfactory, a result that disclosesthe occurrence of interactions between the immobilized redoxprobes.50 To take into account these slight interactions, voltam-mograms were simulated using the general expression developedby Laviron for a surface-confined reaction in which weak inter-action among immobilized molecules take place.51 In this simula-tion, the same value of kET was assumed and the following valuesdetermined: β = -0.4, γ = 0.7, λ = 0, and μ = 0, for the fourinteraction parameters, in order to gain the best-fit curves.

The asymmetry of the CVmay be explained by considering thepeculiarity of the system under study. During the anodic scan, theFcs undergoing charge transfer are energetically equivalent hence,as expected, the relative peak is well-shaped. Conversely, thecathodic branch shows a main peak that is lower than thecorresponding anodic one and is followed by two shoulders(ill-defined additional peaks). This suggests that the same ener-getic equivalence of electroactive species cannot be stated forcathodic scan. A possible explanation is provided here: theintegration 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 deri-vatives are anchored onto the electrode. Although slightly higherthan the theoretical value (4.5 � 10-10 mol/cm2 or 4.8 � 10-10

mol/cm2) as expected from previous works,52-54 this valueappears reliable for a close-packed alkane Fc layer, particularlyif we consider the real electrode area (a roughness factor up to 1.2is usual for a polycrystalline Au electrode). On what is statedabove, each Fc derivative molecule should cover ∼33 A2.

By adopting the same calculation for the cathodic branch, itemerges that the charge value strictly depends on the criteria usedfor its evaluation. More specifically, if only the main cathodicpeak is considered, the charge value is significantly smaller thanthat of the anodic peak; conversely, a charge equivalent to that of

(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.

(47) Bard, A. J.; Faulkner, L. R. Electrochemical methods: fundamental andapplications, 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.

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Article Frasconi et al.

the anodic peak is determined if the two additional peaks areincluded. This is consistent with the view that, if initially all the Fcmolecules in the SAM are energetically equivalent, once theanodic scan proceeds the formed ferricenium ions (Fcþ)-beingcharged species-repeal each other. Being anchored on the sur-face, to minimize the electrostatic repulsion effects, they areforced to change their spatial arrangement at the interfacechanging, for instance, the tilt angle55 or flipping the cyclopen-tyldiene ring around the Fc-C bond at the end of SAM.53 Asevidenced by the signal, this entails the Fcþ formed during theanodic scan possibly being grouped into inequivalent reducibleclasses differing in height and consequently falling at differentpotential values according to their different overpotentials.56,57

In order to shed deeper light on the events occurring duringpotential scanning, EC-SPR experiments were carried out. To putin sharper evidence the role of the Fc head groups present on theSAM, an appropriate blank was subtracted from all the SPRcurves of ferrocene-containing alkanethiol SAMs. The blankmeasurement is represented for each SAM by a SPR curveobtained for a SAM of an alkanethiol of same length withoutthe terminal Fc group. The same approach was also followed formixed SAMs (see below). In agreement with literature,48 EC-SPRperformed on hexanethiol SAM produced only a very smallchange (∼0.001�) of dip shift vs potential. Since, the (dip shiftvs potential) behavior reflects the interface changes in terms ofthickness or dielectric constant value (charge density), or evenboth of them, this suggests that in this case the superficial electrondensity at the Au electrode surface does not change appreciablywith potential. By contrast, for SAMs containing Fc head groupsthe dip shift vs potential changes significantly and a sigmoid curveis obtained (Figure 1B). The curve shape is not affected by thepotential scan rate and shows no hysteresis when the scanning is

reversed. The vertical inflection point value obtained is very closeto the formal potential of the CV curve, since it represents theinterfacial arrangement at equimolarFc andFcþ concentration.48

The potential limiting values of the dip shift correspond to fullyreduced and oxidized anchored Fc groups, while the intermediatepoints are related to their relative amounts. It can be pointed outthat the curve shape resembles that of a stationary voltammogram.

When FcC11 SAM is used in place of an FcC6 SAM, a CVcurve displaying sharper peaks is obtained (Figure 2A). Thedetermined formal potential (E�0 =349( 4 mV) is 15 mV higherthan that of the hexane derivative, and a slightly smaller anodic-cathodic peak separation is observed.

The charge amount involved in the process, which is deter-mined from the anodic peak area, is 2.72 ( 0.15 μC, a valueslightly higher than that determined for the corresponding processof the hexane derivative (see above). This may be ascribed to thestronger Van der Walls interactions arising among longer sidechains, which favor a tighter packing of the molecule.58 Thecharge value calculated allows determination of the superficialferrocenyl concentration, 5.7 � 10-10 mol/cm2, from which asingle ferrocenyl molecule covers 29 A2.

Analysis of the dependence of the peak potentials on the scanrate reveals higher asymmetry with respect to FcC6, as demon-strated from the values of the interaction parameters, β=-0.5,γ=0.9, λ= 0.1, and μ=0, compatible with stronger interactionsarising as the redox probe surface concentration is increased.

The EC-SPR curve of such a SAM is similar in shape to that ofthe 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 βCDSAMwere treatedwith FcC11 (or FcC6) ethanolic solutions. Since the Fc forms astable inclusion complexwithβCD,59-61 theβCDcavitieswere filledwith Fcs (added in solution as FcA) so that the ferrocenyl alka-nethiol, added in solution,was forced to adsorbon the interstitialAuarea (among βCD molecules) to form ordered mixed SAMs.

Figure 3A shows theCVs of a βCDSAM-coveredAu electrodebefore and after its treatment with FcC6. Whereas in the former

Figure 1. (A) CVs recorded at an Au electrode before (dashed line) and after (solid line) modification with a FcC6 SAM; v=50mV/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 334( 5 182( 9 0.52( 0.03FcC11 349( 7 43( 11 0.53( 0.02FcC6-βCD 330( 6 178( 19 0.49( 0.04FcC11-βCD 359( 9 15( 6 0.46( 0.02

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107, 3411.

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case no faradic current is observed (black curve), two peaksappear after FcC6 self-assembly due to the electrochemicalactivity of Fc (red curve). As observed for pure FcC6 SAMs, alsoin this case asymmetric CVs are obtained. The anodic-cathodicpeak separation was approx 30 mV and the formal potential330( 4 mV. On the whole, these values do not differ significantlyfrom 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 thatdetermined for a pure FcC6 SAM. Since the βCD-covered areais approx 80% of the total surface,33 it results in all Fc derivativescovering the remaining available surface (98� 1012 A2). It followsthat each Fc derivative molecule covers 34 A2, which is the samevalue determined for a pure FcC6 SAM; this suggests that SAMsessentially maintain the same structure independently from thepresence ofβCDon the electrode. This is confirmed by the changein the dip shift when potential is cyclically scanned.

The dip shift change (Δθ0.0 V - Δθ0.7 V) for the FcC6-βCDSAM is 8.8 m� (Figure 3B), i.e., approximately 70% of the valuerecorded for a pure FcC6 SAM (12.4 m�).

Figure 4A shows the CVs recorded by a βCD SAM (blackcurve) and byFcC11-βCD(red curve).Different fromFcC6-βCDSAM, FcC11-βCDSAMgenerates well-shaped voltammograms,with current values comparable to those shown by shorterderivatives. This suggests that a similar coverage takes place.

The SPR dip shift of FcC11-βCD mixed SAM is shown inFigure 4B. The dip shift may be expected to be similar to that of

FcC6-βCD, because passing from pure to mixed SAM, thesuperficial charge density change should be independent by thealkane length. This is not true for the film thickness that isdramatically affected: really, passing from a pure FcC11 to aFcC11-βCD SAM the average thickness decreases.

Different from what was observed for FcC6-based SAMs, theEC-SPR curve of the FcC11-βCD SAM, shown in Figure 4B, ischaracterized by a dip shift change (Δθ0.0 V-Δθ0.7 V) larger thanthat 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 SAMsreveals nearly ideal behavior, as revealed from the good fitting ofthe CV data using the Butler-Volmer equation for a surface-confined redox reaction.

The values of several parameters as the formal potential, theelectron transfer rate constant, and the charge transfer coefficientobtained from CV data analysis for all SAMs investigated arereported in Table 1.

The influence of potential scan rate (range 50-500 mV/s) onEC-SPR response is shown inFigure 5. In this study, the potentialscanning started from0.7V. From the figure, it appears clear that,contrary to FcC6-βCD, the FcC11-βCD behavior stronglydepends on the scan rate and its direction.Electrocatalytic Oxidation of Ferro(II)cyanide. The

reversible redox process of potassium ferrocyanide observed atbareAu (E�0=191 ((2)mVvs SCE) is no longer detected at theβ-CD SAM-covered Au electrode.33 As expected, the βCD SAMprovides an electrode surface not accessible to relatively large

Figure 2. (A) CVs recorded at anAu electrode before (dashed line) and after (solid line) modification with a FcC11 SAM; v=50mV/s. (B)EC-SPR dip shift of FcC11 SAM as a function of potential; v= 50mV/s. Measurements were carried out in a 0.1 mol/L NaClO4 solution.

Figure 3. (A) CVs recorded at a β-CDX SAMmodified electrode in the absence (dashed line) and in the presence (solid line) of assembledFcC6 (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 werecarried out in a 0.1 mol/L NaClO4 solution.

12942 DOI: 10.1021/la9018597 Langmuir 2009, 25(22), 12937–12944

Article Frasconi et al.

charged molecules as ferrocyanide,62 whose direct electron trans-fer is prevented.

Once the interstitial area of the βCD SAM is covered byferrocenyl alkanethiol derivatives, the Fcþ-induced oxidation offerrocyanide is expected, since the formal potentials of theanchored Fcþ/Fc and of soluble ferri/ferrocyanide significantlydiffer. Thus, a catalytic current should be generated in thepresence of soluble ferrocyanide only if Fcþ is electrochemicallyregenerated at the electrode surface.

The CVs recorded at the FcC11-βCD electrode in the presenceof different ferrocyanide concentrations are reported in Figure 6.The behavior typical of a catalytic process, due to the shuttlingaction of the Fc anchored to the surface by the undecane residue,is observed. Contrary to ferrocyanide, Fc permeates the βCDcavity and forms a stable inclusion complex. Interestingly, nosignificant catalytic current was detected when CVs were run at aFcC6-βCD electrode in the presence of the same ferrocyanideconcentrations.

Discussion

In order to provide a rationale for the results described above(which can be summarized in the different EC-SPR behavior

shown by ferrocenyl alkanes of different lengths, and in Fcterminal groups inclusion into the βCD cavity), it is necessaryto consider the factors that contribute to the SPR angle change.At a SAM-modified surface, three major parameters are sup-posed to affect the SPR angle, namely, (i) the electron densitycharge at the metal surface (4σ), (ii) the monolayer thick-ness change (4d), and (iii) the refractive index change (4n). Ingeneral, when an external electrode potential (4V) is applied thedependence of the SPR angular shift (4θR) from such parameterscan be expressed as follows:63

ΔθRðλÞΔV

¼ c1ΔnðλÞΔV

þ c2Δd

ΔVþ c3

Δσ

ΔV

where c1, c2, and c3 are threedistinct constants andλ is thewavelengthof the incident light. Here, we shall consider how the structure andthe composition of a SAM affect each of the three factors.

SPRdepends on the electrons density at themetal surface; thus,any surface charging effect (4σ) induced by a potential changewill provoke an angular change. On the basis of the zero chargepotential of the metal/electrolyte interface,64 it may be assertedthat, upon sweeping the gold electrode potential toward a positive

Figure 4. (A) CVs recorded at a βCD SAMmodified electrode in the absence (dashed line) and in the presence (solid line) of self-assembledFcC11 (βCD-FcC11). v=50mV/s. (B) EC-SPR dip shift of βCD-FcC11 SAMas a function of potential. v=50mV/s.Measurements werecarried 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/LNaClO4 . (B) Potential scan rate-dependence of the SPR signal for aFcC11-βCDX SAM (b) and a FcC6-βCDX SAM (O).

(62) Tamura, S.; Tagaki, W.; Nakahara, H.; Fukuda, K. Chem. Lett. 1986, 15,1933.

(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.

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Frasconi et al. Article

value, the electron deficient state of the surface results in a positiveSPR angle shift (which reflects the positive increase of theinterfacial charge due to the non faradic double layer charging).65

This phenomenon likely provides the highest contribution to theSPR angle changes in the absence of redox process (as observedfor the alkanethiol and alkanethiol-βCD SAMs). However, sincethe samepotential changeswere applied tobothFcC6 andFcC11,in the absence and presence ofβCD, the third termof the equationreported above is expected to remain constant.

A change of the SAMthickness (4d) is also expected to shift theSPR angle. Fc oxidation yields the Fcþ, which is repelled (byelectrostatics) from the positively charged gold surface: thisresults in a smaller tilt angle and in a consequential thicknessincrease, which determines a negative dip shift.48 This is inagreement with previous work which reports an increase inthickness of FcC6 following a positive potential scan that wasreported to shift the dip in a negative direction66 even though anincrease in the thickness of alkanthiol SAMs is usually accom-panied by a positive dip shift. The application of a potential topure ferrocenyl alkanethiol SAMs was reported to result inrotation of the ferrocene moiety,53 although the EC-SPR mea-surements at a ferrocenyl alkanethiol monolayer seem to suggestthat tilt angle changes have greater influence on the observedSAM thickness change.48 From such considerations, it may beassessed that the SAM average thickness changes during redoxreactions and induces a shift of the SPR angle consistent with areorganization of the Fc SAM. We found that such a shift isgreater for FcC11 with respect to FcC6 (Figure 1B andFigure 2B). Such a difference may also be explained by takinginto account the different lengths of alkane thiol derivative used tobuild the SAM: larger thickness and the closer packing thereof, inaddition to greater influence on the dielectric constant value.

Bypassing frompure tomixedSAMs, thenegative dip shift changeis different according to the length of anchoring group used; inparticular, it slightly decreases for FcC6-based SAMs (see Figures 1Band 3B), while it increases for FcC11 ones (Figures 2B and 4B).

If we consider that a pure FcC6 SAM67 has a slightly greaterthickness (of an extent equal to the ferrocene group size) than thatof a closely packed βCDSAM,68 the diminished negative dip shiftshown by mixed SAM is probably correlated with the lowersuperficial charge density now involved rather than the smallthickness decrease.69 In mixed SAMs, FcC6 covers only limitedsurface portions among the βCDs (island-like structure); from aredox point of view, it behaves as in a pure SAM where electrontransfer occurs by a tunneling process.

For mixed FcC11-βCD SAM, the penetration of the Fc groupinto the βCD cavity (with the consequent formation of theinclusion complex)may represent an additional feature contribut-ing to the SPR angle shift; in view of that, once oxidized the Fcþ

moiety should be better solvated in the aqueous phase. This effectcan be possible only in the case of FcC11-based mixed SAMs,where the C11 alkyl chain can bend to allow permeation of Fcgroups into the βCD cavity with formation of an inclusioncomplex. In addition, the migration of bulk solvated ions ontothe SAM surface increases the refractive index (4n) at the inter-face, thus determining a further SPR angle shift. By consideringthe (above-mentioned) surface coverage degree of the Fc, thepresence of ions over the FcC6-βCD SAM is expected to be lessextended than for FcC6 SAM, suggested by the different SPRangle shift observed. Conversely, in the case of FcC11-βCD andFcC11 SAMs the contribution to the SPR angle change due toionic motion over the film ismuch less significant when comparedto the SAM thickness change. Nevertheless, if we consider thepercentage of the mixed SAMs vs pure Fc SAMs, an increase ofthe negative dip shift is observed for both FcC6 and FcC11SAMs, likely ascribed to a decrease of the refractive index causedby water loading in mixed SAMs. Surface hydratation includesthe water molecules inserted into the βCD molecules as well asthose already present, with expected changes in the orientation ofthe ferrocene moieties.

As stated, the Fc groups anchored to the electrode by a C6chain transfer electrons by a tunneling process in the case of pureandmixed SAMs.Conversely, different electron transfermechan-isms can take place in the case of FcC11-based SAMs. Inparticular for FcC11-βCD SAM besides to a tunneling process(occurring at pure SAM), a direct electron transfermay also occurfavored by the bending of peripheral FcC11 molecules of theislands allowing Fc penetration into the βCD cavity and forma-tion of the inclusion complex (see Figure 7).

This hypothesis seems to be supported by the data relative tothe dip shift change as a function of potential scan rate. As shownin Figure 5A, the dip shift is lower at higher scan rate, consistentwith a kinetic limitation for formation of the Fc/βCD inclusioncomplex. Further evidence is also provided by two curves shownin Figure 5B; although shifted along the Y-axis, they show asimilar trend at scan rates of g200 mV/s. This means that underthe conditions investigated both systems show similar behavior,which is determined by the difficulty of molecules to change theirspatial arrangement (tilt angle change and/or cyclopentyldienerings rotation) upon oxidation of Fc groups. Conversely, at scanrate <200 mV/s whereas for FcC11-βCD the dip shift changeincreases as the scan rate decreases, in the case of FcC6-βCD itremains constant. This suggests that, whereas at high scan rateswhat is observed is correlated to the bulk Fc (the chain lengthseems to play a negligible role), at lower scan rates the role played

Figure 6. CVs recorded at a FcC11-βCDSAMmodified electrodein a 0.1mol/LNaClO4 solutionpresence 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).

(65) Garland, J. E.; Assiongbon, K. A.; Pettit, C. M.; Roy, D. Anal. Chim. Acta2003, 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.

(68) Damos, F. S.; Cruz, R. C. S.; Sabino, A. A.; Eberlin, M. N.; Pilli, R. A.;Kubota, L. T. J. Electroanal. Chem. 2007, 601, 181.

(69) Beulen, M. W. J.; B€ugler, J.; de Jong, M. R.; Lammerink, B.; Huskens, J.;Sch€onherr, H.; Vancso, G. J.; Boukamp, B. A.; Wieder, H.; Offenh€auser, A.;Knoll, W.; van Veggel, F. C. J.M.; Reinhoudt, D. N.Chem.;Eur. J. 2000, 6, 1176.

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Article Frasconi et al.

by peripheral FcC11 molecules forming inclusion complex withβCD becomes important.

The kinetics of the process appears to depend on both the chainbending and the inclusion process; formation of the inclusioncomplex is favored at lower scan rates. Acting as an electronshuttle, this interaction could really play an important role (asredox mediator) when working with large species unable, asferrocyanide, to permeate the βCD cavity (see below).

The hypothesis of involvement of inclusion complex formationin the case of FcC11-βCD seems to be further supported both bykinetic parameters reported in Table 1 and by the electrocatalyticoxidation of ferrocyanide. Indeed, from data of Table 1 it appearsthat the values of the formal potential and the charge transfercoefficient do not differ significantly, while the electron transferrate constants change as a function of the alkane chain length.70

Further, the comparisonof the kET values determined for theFc6/Fc6-βCD couple and the Fc11/FcC11-βCD couple (see Table 1)reveals an unexpected behavior. More specifically, the valuesdetermined for Fc6 either alone ormixed with βCD suggest that asimilar electrochemical process occurs. By contrast, in the case ofFc11, the rate constant determined for the mixed SAM is aboutone-third with respect to that determined for pure Fc11. This isstill unclear andmaybe explained by considering the formation ofan inclusion complex of Fc with βCD, which is characterized by aown kinetics of formation, as a consequence of the differentenvironments surrounding the redox center of FcC11 and FcC11-βCD SAMs. In fact, the nonideal electrochemical behavior of theFcC11SAM(evidenced by the value of the interaction parametersβ, γ, λ, and μ) is consistent with a structural disorder of the SAM,due to steric crowding of the Fc groups. Although the kinetic andthermodynamic ferrocene centers are solely assumed to be iden-tical, this is however a quite rare event due to the nonhomogeneityof the ferrocene moieties within the SAM.71 Conversely, a nearlyideal behavior for FcC11-βCD SAMs suggests the existence ofweak interactions between the ferrocene/ferriceniummolecules anda high homogeneous environment around the redox centers,reflecting a highdegree of self-organization affordedby the selectedimmobilization procedure.

As shown in Figure 7, the undecane bending allows both Fcpermeation into the cavity (which favors direct electron transfer)and Fcþ interaction with ferrocyanide outside the cavity (whichallows a mediated electron transfer). As a matter of fact, the Fc-to-Fcþ oxidation provokes the cation to jump out of the cavity,

since only nonpolar species can be hosted inside.72 In solution,Fcþ is promptly reduced by the ferrocyanide present in solution,and the resulting Fc can be oxidized again at the electrode onceincluded in the βCD cavity, according to the following scheme:

Of course, this gives rise to a continuous cycle.

Conclusions

In conclusion, from the preparation and characterizationofAuelectrodes modified with two ferrocene derivatives (FcC6 andFcC11) self-assembled on the surface with or without βCD, dataobtained provide the following information:

1 As expected, pure ferrocenyl alkane thiol SAMs be-have very similarly; the small differences which havebeen detected by SPR can be ascribed to the influenceof SAMs thickness.

2 In thecaseofmixedSAMswhich in this investigationwereconstituted by βCD with either FcC6 or FcC11 as inter-stitial filler, the behavior shown strictly depended on thetype of ferrocenyl derivatives is used. More specifically,a FcC6-βCD SAMs behave as FcC6 pure SAMs,

with electrons being transferred exclusively by atunneling process. The small differences ob-served by EC-SPR can be ascribed mainly tothe lower charge density of mixed SAM.

b When a mixed SAM with FcC11 in place ofFcC6 is used, a quite different EC-SPR behavioris observed. This is accountable to a differentstructural rearrangement and a more complexcharge transfer process involved: the bending ofa peripheral longer alkyl chain allows the Fcgroup to permeate the βCD cavity, thus givingplace to an inclusion complex. Hence, the elec-trons, besides the tunneling process, can also bedirectly transferred by Fc included in βCD cavity.

3 The electrocatalytic oxidation of ferrocyanide (a com-pound unable to permeate βCD cavity), in charge to Fcgroups, provides further evidence for the role played byperipheral FcC11 and by the βCD cavity. This is ofgreat relevance for developing biosensor devices.

Figure 7. Scheme of inclusion complex formation following electron transfer between peripheral FcC11 and βCD in FcC11-βCD SAM.

(70) Weber, K.; Hockett, L.; Creager, S. J. Phys. Chem. B 1997, 101, 8286.(71) Goujon, F.; Bonal, C.; Limoges, B.; Malfreyt, P. Langmuir 2009, 25, 9164.

(72) Nijhuis, C. A.; Boukamp, B. A.; Ravoo, B. J.; Huskens, J.; Reinhoudt,D. N. J. Phys. Chem. C 2007, 111, 9799.