Electrochemical Transduction on Self-Assembled Monolayers: Are

Article pubs.acs.org/Langmuir

Electrochemical Transduction on Self-Assembled Monolayers: Are Covalent Links Essential?† Pierre-Yves Blanchard, Séverine Boisard, Marylène Dias, Tony Breton, Christelle Gautier,* and Eric Levillain* LUNAM Université, Université d’Angers, CNRS UMR 6200, Laboratoire MOLTECH-Anjou, 2 bd Lavoisier, 49045 Angers Cedex, France.

ABSTRACT: Electrochemical transduction without covalent links between redox and complexant units in a complexing selfassembled monolayer has been established. The results demonstrate that transduction depends on the crown ether/ferrocene ratio and appears to be tunable.

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INTRODUCTION

sophisticated targets including both complexant and redox moieties. On the basis of recent work focused on the strong influence of the spatial proximity between redox and complexant centers on the transduction efficiency in solution,8 we have investigated the electrochemical transduction of complexation on mixed SAMs elaborated from 1-aza-18-crown-6 (CE) and ferrocene (Fc) thiol derivatives (Scheme 1). Mixed SAMs were elaborated by the successive adsorption of CE and Fc. Because the selectivity of 1-aza-18-crown-6 toward barium had already been established (binding constant K > 104 in solution),8−10 experiments were first carried out with Ba2+.

During the past decade, increasing attention has been dedicated to the design and elaboration of redox-responsive selfassembled monolayers (SAMs) for various applications.1,2 Previous works have resulted in some elegant examples that incorporate sophisticated receptors on the electrode surfaces. To enhance the interfacial reactivity, several works have resorted to the elaboration of mixed SAMs in order to favor active molecule accessibility via an alkanethiol as a dilutant.3,4 Much more rarely, a few works have been dedicated to the elaboration of mixed SAMs in order to study the intermolecular reactivity between two immobilized units.5−7 Ten years ago, Reinhoudt and co-workers showed the existence of through-space communication in mixed acidferrocene SAMs.5 They observed a pH-dependent electrochemical response of ferrocene-immobilized moieties caused by the proximity of carboxylic acids in their acidic or basic forms. They raised a fundamental question about the influence of cation binding on the electrochemical response of an independent immobilized redox entity. In other words, is a covalent link between the redox probe and the host unit necessary to ensure electrochemical transduction? The present study is part of this set of problems. Our goal was to observe an electrochemical transduction via a noncovalent approach (i.e., without a redox/complexant link). This represents a significant challenge because it would avoid synthesis difficulties encountered in the preparation of © 2012 American Chemical Society

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EXPERIMENTAL SECTION

Synthesis. Ferrocenylalkanethiol11,12 and crown ether13 derivatives were synthesized according to the described protocols. Working Electrode Preparation. Au substrates were prepared by the deposition of ca. 5 nm of chromium followed by ca. 100 nm of gold onto a glass substrate through a shadow mask (MECACHIMIQUE/France) using a physical vapor deposition system (PVD ME300 PLASSYS/France) and were made immediately before use.14,15 This protocol provides reproducible Au(111) surfaces with high crystallographic quality and low roughness (Ra < 2 nm). The surfaces do not undergo post-treatment after completion. Received: January 5, 2012 Revised: July 10, 2012 Published: July 25, 2012 12067

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Scheme 1. Schematic Representation of a Mixed SAM Fc/CE before and after Complexation with Ba2+

Figure 1. (a) CVs recorded on a mixed Fc/CE SAM (Γ(Fc) = 1.2 × 10−10 mol cm−2) in 0.1 M n-Bu4NPF6/CH3CN before (−) and after (---) the addition of 1 × 10−6 mol Ba(ClO4)2. (b) CVs recorded on the same SAM after the addition of Ba(ClO4)2, before (---) and after a rinsing with CH3CN (···). The scan rate is 100 mV s−1. Ca(ClO4)2, Mg(ClO4)2, Na(ClO4), or Li(ClO4)) in CH3CN (10−4 M final concentration in the electrochemical cell).

Mixed Fc/CE SAMs were obtained by immersing the Au substrate for 30 min in a solution of CE (1 mM, CH3CN) and then in a solution of Fc (1 mM, CH3CN). To obtain the expected Fc surface coverages, the time of immersion in Fc solution was adjusted. It varied from 1 s to 6 h. Experimental Fc surface coverages (Γ(Fc)) were estimated by integrating the voltammetric signal of SAMs. A mixed Fc/dodecanethiol SAM was obtained by first dipping the substrate for 30 min in a solution of dodecanethiol (1 mM, CH2Cl2). After being rinsed with CH2Cl2 and CH3CN, the dodecanethiol SAM was immersed in a solution of Fc (1 mM, CH3CN) for 1 s. Electrochemical Measurements. Electrochemical experiments were carried out with a Biologic SP-300 potentiostat at 293 K. Cyclic voltammetry was performed in dry HPLC-grade CH3CN with tetrabutylammonium hexafluorophosphate (nBu4NPF6) as the supporting electrolyte in a three-electrode cell equipped with a platinumplate counter electrode. The reference electrode was Ag/AgNO3 (0.01 M CH3CN). Complexation experiments were performed in the electrochemical cell. A working electrode functionalized with mixed Fc/CE SAM was immersed in 10 mL of 0.1 M n-Bu 4 NPF6 /CH 3 CN. Cyclic voltammogramms were first recorded on mixed SAMs immediately after preparation and rinsing. Second, they were recorded after the addition of 100 μL of the 10−2 M perchlorate cation (Ba(ClO4)2,

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RESULTS AND DISCUSSION Cyclic voltammetry (CV) was used to probe the complexation between cations in solution and CE immobilized on the electrode surface because it makes the direct probing of the electroactive immobilized entities possible.4,16−18 Figure 1 shows CVs recorded on an Fc/CE SAM before and after the addition of Ba2+. After an injection of 1 μmol of Ba2+ on a mixed Fc/CE SAM (Γ(Fc) = 1.2 × 10−10 mol cm−2, estimated from the integration of the Fc oxidation signal), a +30 mV shift is noticeable on the CV (Figure 1a). This mixed SAM was then rinsed with acetonitrile, transferred in a free cation electrolytic solution, and studied again by CV (Figure 1b). The apparent redox potential was shifted toward negative potentials, returning to the value previously obtained before cation addition. A second injection of cations led to the same potential shifts (i.e., +30 then −30 mV). The positive shift upon addition of Ba2+ to the mixed SAM could be attributable to a complexation phenomenon. To 12068

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validate this hypothesis, a blank experiment was undertaken on a SAM in which Fc entities were diluted with inactive alkanethiol (Figure 2).

Figure 3. ΔE (E0(Fc) after 1 × 10−6 mol cation injection − E0(Fc) before cation injection) vs normalized Fc surface coverage (Γ(Fc)/ Γmax(Fc)), recorded on mixed Fc/CE SAMs in 0.1 M n-Bu4NPF6/ CH3CN by CV for Ba2+ (black circles), Ca2+ (blue squares), Mg2+ (green triangles), Na+ (yellow triangles), and Li+ (red diamonds). The scan rate is 100 mV s−1.

Figure 2. Blank experiment: CVs recorded on a mixed Fc/ dodecanethiol SAM ((Γ(Fc) = 2.1 × 10−11 mol cm−2) in 0.1 M nBu4NPF6/CH3CN before (−) and after (---) the addition of 1 × 10−6 mol of Ba2+. The scan rate is 100 mV s−1.

(corresponding to Γ(Fc) = 1 × 10−10 mol cm−2, CE/Fc = 2). Therefore, the electrochemical transduction is more efficient when Fc moieties are hyperdiluted in a crown ether matrix. This result is quite transposable with that already obtained in solution by Dieing et al.21 In this study, the positive shift recorded after the addition of sodium on a TTF-bis(crown) molecule was twice as pronounced as that observed on a TTFmono(crown) molecule. Therefore, a rich environment of complexed crown ethers enhances electrochemical transduction. Similar behaviors were observed after the addition of Ca2+ or Mg2+. As already observed in solution with 1-aza-18-crown-6, binding constant values of Ca2+8,10 and Mg2+10 are the same order of magnitude as that observed for Ba2+. On the contrary, the apparent redox potential remained almost unchanged upon addition of Na+ or Li+ on mixed Fc/CE SAMs with several CE surface coverages. The weak influence induced by the addition of Li+ in solution on the potential shift is in agreement with the low binding constant value already observed between the host and the guest entities. The results obtained with Na+ followed the same trend as ones obtained with Li+ (i.e., ΔE ≈ 0 mV). However, the binding affinity value between Na+ and CE in solution is between those obtained for Li+ and Ba2+. In spite of this weak difference between the results obtained for Na+ in solution and on the surface, the electrochemical transduction efficiency observed on mixed Fc/CE SAMs without covalent links between redox and complexant units agrees with the binding constant values obtained in solution.

After the addition of Ba2+ on a mixed Fc/dodecanethiol SAM (Γ(Fc) = 2.1 × 10−11 mol cm−2), the Fc signal appeared to be slightly lower but the apparent potential remained strictly unchanged. This result supports the electrochemical transduction of the Ba2+ complexation on Fc/CE SAMs and suggests that the positive shift of the potential transduced an unfavorable oxidation of the immobilized Fc units. Ferricinium cations are destabilized by repulsive Coulombic interactions with cations complexed by crown ether.19 It is noteworthy that a larger potential shift was observed on a mixed Fc/CE SAM at lower Fc coverage (Γ(Fc) = 2.2 × 10−11 mol cm−2): this time, the addition of Ba2+ involved a +50 mV shift of the Fc redox signal. After the SAM was rinsed with acetonitrile, the initial redox potential was also recovered in this case (−50 mV shift). This observation raises another question: how does the transduction efficiency evolve when the Fc/CE ratio adsorbed on the surface is tuned? To answer part of this question, SAMs having different Fc surface coverages were tested with respect to cation complexation. Figure 3 shows the electrochemical shifts obtained by CV after a 1 μmol addition of several cations (Ba2+, Ca2+, Mg2+, Na+, and Li+) on Fc/CE SAMs characterized by Fc surface concentrations varying from 1.3 × 10−11 to 5 × 10−10 mol cm−2. Normalized Fc surface coverages (Γ(Fc)/Γmax(Fc)) were calculated from the maximum Fc surface coverage (Γmax(Fc) = 5 × 10−10 mol cm−2, estimated from the integration of the redox signal obtained for a compact Fc monolayer), which agrees with a related value in the literature for similar monolayers.20 To estimate the Fc/CE ratio on the surface, the maximum crown ether surface coverage (Γmax(CE) = 2.5 × 10−10 mol cm−2, estimated by the quartz crystal microbalance) is also required. These experiments suggest the replacement of one CE with two Fc derivatives during the exchange stage. Figure 3 shows that the potential shift after Ba2+ injection is a function of the Fc surface concentration (•). Indeed, the shift is low from Γ(Fc)/Γmax(Fc) > 0.8 and becomes much more important for normalized Fc surface coverages lower than 0.2

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CONCLUSIONS The validity of the principle of electrochemical transduction without a covalent link between redox and complexant units in a complexing self-assembled monolayer has been confirmed. The results demonstrate that transduction depends on the crown ether/ferrocene ratio and appears to be tunable. These results raise open-ended questions concerning the role of the Fc/CE ratio and the influence of the nanoscale 12069

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(13) Douglass, E. F. J.; Driscoll, P. F.; Liu, D.; Burnham, N. A.; Lambert, C. R.; McGimpsey, W. G. Effect of Electrode Roughness on the Capacitive Behavior of Self-Assembled Monolayers. Anal. Chem. 2008, 80, 7670−7677. (14) Aleveque, O.; Blanchard, P. Y.; Gautier, C.; Dias, M.; Breton, T.; Levillain, E. Electroactive Self-Assembled Monolayers: Laviron’S Interaction Model Extended to Non-Random Distribution of Redox Centers. Electrochem. Commun. 2010, 12, 1462−1466. (15) Aleveque, O.; Gautier, C.; Dias, M.; Breton, T.; Levillain, E. Phase Segregation on Electroactive Self-Assembled Monolayers: A Numerical Approach for Describing Lateral Interactions between Redox Centers. Phys. Chem. Chem. Phys. 2010, 12, 12584−12590. (16) Trippé, G.; Oçafrain, M.; Besbes, M.; Monroche, V.; Lyskawa, J.; Le Derf, F.; Sallé, M.; Becher, J.; Colonnac, B.; Echegoyen, L. SelfAssembled Monolayers of a Tetrathiafulvalene-Based Redox-Switchable Ligand. New J. Chem. 2002, 26, 1320−1323. (17) Moore, A. J.; Goldenberg, L. M.; Bryce, M. R.; Petty, M. C.; Monkman, A. P.; Marenco, C.; Yarwood, J.; Joyce, M. J.; Port, S. N. Cation recognition by self-assembled layers of novel crown-annelated tetrathiafulvalenes. Adv. Mater. 1998, 10, 395−398. (18) Reynes, O.; Bucher, C.; Moutet, J.-C.; Royal, G.; Saint-Aman, E. Electrochemical Sensing of Dihydrogen Phosphate and Adenosine-5′triphosphate Anions by Self-Assembled Monolayers of (Ferrocenylmethyl)trialkylammonium Cations on Gold Electrodes. Inorg. Chim. Acta 2008, 361, 1784−1788. (19) Medina, J. C.; Goodnow, T. T.; Bott, S.; Atwood, J. L.; Kaifer, A. E.; Gokel, G. W. Ferrocenyldimethyl-[2.2]-cryptand: Solid State Structure of the External Hydrate and Alkali and Alkaline-EarthDependent Electrochemical Behaviour. J. Chem. Soc., Chem. Commun. 1991, 290−292. (20) Considering that ferrocene molecules are modeled as spheres with a diameter of 6.6 Å and are connected to the electrode through alkanethiol chains modeled as cylinders with a diameter of 4.6 Å, the maximum ΓFc for a monolayer has been estimated to be around 4.5 × 10−10 mol cm−2, assuming hexagonal closest-packing. Chidsey, C. E. D.; Bertozzi, C. R.; Putvinski, T. M.; Mujsce, A. M. Coadsorption of Ferrocene-Terminated and Unsubstituted Alkanethiols on Gold: Electroactive Self-Assembled Monolayers. J. Am. Chem. Soc. 1990, 112, 4301−4306. (21) Dieing, R.; Morisson, V.; Moore, A. J.; Goldenberg, L. M.; Bryce, M. R.; Raoul, J. M.; Petty, M. C.; Garín, J.; Savirón, M.; Lednev, I. K.; Hester, R. E.; Moore, J. N. Crown-Annelated Tetrathiafulvalenes: Synthesis of New Functionalised Derivatives and Spectroscopic and Electrochemical Studies of Metal Complexation. J. Chem. Soc., Perkin Trans. 2 1996, 8, 1587−1593.

distribution of redox and host sites on the transduction efficiency. Further work will be dedicated to the characterization of mixed electroactive and complexing SAMs in order to establish relationships between the nanoscale distribution of redox/host sites and their complexing ability.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected] and eric.levillain@ univ-angers.fr. Fax: +33241735405. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the Centre National de la Recherche Scientifique (CNRS France), the Agence Nationale de la Recherche (ANR France), and the Région des Pays de la Loire (France).

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DEDICATION Dedicated to the memory of Dr Nuria Gallego.

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