J. Phys. Chem. C 2007, 111, 1176-1179
Electron-Donor-Acceptor Fullerene Derivative Retained on Electrodes Using SC3 Hydrophobin Yohann Corvis,† Kinga Trzcinska,‡ Rick Rink,§ Petra Bilkova,| Ewa Gorecka,‡ Renata Bilewicz,† and Ewa Rogalska*,⊥ Department of Chemistry, Warsaw UniVersity, ul. Pasteura 1, 02-093 Warsaw, Poland, Department of Chemistry, Warsaw UniVersity, Al. Z˙ wirki i Wigury 101, 02-089 Warsaw, Poland, Biomade Technology Foundation, Nijenborgh 4, 9747 AG Groningen, The Netherlands, Institute of Physics, Academy of Sciences of the Czech Republic, Na SloVance 2, CZ-182 21 Praha 8, Czech Republic, and Groupe d’Etude des Vecteurs Supramole´ culaires du Me´ dicament, UMR 7565 CNRS/UniVersite´ Henri Poincare´ Nancy 1, Faculte´ des Sciences, BP 239, 54506 VandoeuVre-le` s-Nancy cedex, France ReceiVed: August 14, 2006; In Final Form: October 2, 2006
We report a novel method of preparing electrodes coated with an electron-donor-acceptor fullerene. The tailor-made fullerene derivative with possible use for future photovoltaic devices was blended into a selfassembled SC3 hydrophobin matrix. Hydrophobin, a small, naturally occurring protein, has the remarkable property of adhering to almost any surface. The blend, immobilized by physisorption on glassy carbon electrodes, was electrochemically functional. The ease and rapidity of preparation, as well as the chemical and physical robustness of the system obtained, show that the biomimetic approach used for elaborating nanostructured materials might be of interest for technological applications.
1. Introduction Organic photovoltaic devices spark great interest as possible inexpensive alternatives to inorganic solar cells.1 In the research on such devices, fullerenes are often used as electron acceptors.2,3 Absorption of a photon leads to the creation of an exciton on the electron donor. Upon relaxation of the donor, an electron is transferred to the acceptor fullerene sphere. However, the bottleneck in preparing efficient, fullerene-based photovoltaic devices is the immobilization of fullerenes on electrodes. The fullerenes can be deposited on electrodes by evaporation or spin coating. These techniques are frequently used with blends in which the fullerene or its derivative is mixed with a conjugated polymer.1,4,5 The performance of such blends is strongly limited by the quality of bulk heterojunctions between the donor and acceptor.6,7 Because charge transfer can occur only if the donor and acceptor are closely packed, typically at distances below 10 nm, these materials are elaborated into interwoven network structures. Photovoltaic devices can also be prepared by depositing onto the electrodes alternating fullerene and electrondonor monolayers by, for example, the Langmuir-Blodgett method.8 However, preparing such structures is delicate because of the poor amphiphilicity of fullerene derivatives. Recently, mesogenic fullerene derivatives forming columnar- and smecticphase liquid crystals were synthesized with the aim of improving the contact between the donor and acceptor sublayers. In these materials, self-microsegregation occurs between the donor and acceptor molecular units.9,10 It should be noted that developing tailor-made molecules with adequate liquid-crystal and electrondonor-acceptor properties that can be processed into electrodes is not always easy. * Corresponding author. E-mail: [email protected]
† Warsaw University, ul. Pasteura 1. ‡ Warsaw University, Al. Z ˙ wirki i Wigury. § Biomade Technology Foundation. | Academy of Sciences of the Czech Republic. ⊥ Universite ´ Henri Poincare´.
In the present work, a new method of immobilizing a fullerene derivative on electrodes has been developed. The agent used for fullerene immobilization was a protein of fungal origin, SC3 hydrophobin.11-21 SC3 has a high affinity for hydrophilic/ hydrophobic interfaces at which it self-assembles into stable amphipathic films.19-22 Our group showed recently that it was possible to use hydrophobin films as matrices for immobilizing synthetic molecules of different structures and properties,23 as well as biomolecules such as enzymes24 and co-enzymes,23 on various solid substrates. On the other hand, hydrophobin-lipid matrices allow metal cation immobilization.25 In this work, the immobilization was achieved by adsorbing protein/fullerene blends onto the electrodes. The voltammetric behavior of the fullerene derivative immobilized in the hydrophobin matrix was compared to that observed in solution. The results obtained led us to believe that the fullerene derivative described in this article can be used to develop new functional surfaces for photovoltaic applications. The immobilization of the fullerene via hydrophobin can be considered as an interesting alternative to the existing methods. 2. Experimental Section The fullerene derivative in solution was studied using cyclic voltammetry (CV). The experiments were done using an Autolab potentiostat (ECO Chemie) with a three-electrode arrangement using a silver/silver chloride (Ag|AgCl) electrode as the reference electrode, a platinum foil electrode as the counter electrode, and a glassy carbon electrode (GCE, BAS, 3-mm diameter) as the working electrode. The reference electrode was separated from the working solution by an electrolytic bridge filled with 0.1 M tetrabutylammonium tetrafluoroborate (TBABF4)/dichloromethane (CH2Cl2) or tetrahydrofuran (THF) solution. The reference electrode potential was calibrated using the ferrocene electrode process in the same electrolytes. The 0.1 M TBABF4, in CH2Cl2 or THF, was used as a supporting
10.1021/jp0652401 CCC: $37.00 © 2007 American Chemical Society Published on Web 12/16/2006
Electron-Donor-Acceptor Fullerene Derivative
J. Phys. Chem. C, Vol. 111, No. 3, 2007 1177 TABLE 1: Formal Potentials for the Fullerene Derivative Dissolved in CH2Cl2 or Immobilized in the Hydrophobin Matrix on the GCEa dissolved fullerene immobilized fullerene
+1.22 (0.73) +1.00b
-0.59 (-1.08) -0.60
-0.95 (-1.44) -0.94
-1.44 (-1.93) -
a Values of E1/2 approximated as (Epa + Epc)/2 are given versus Ag|AgCl electrode and, in parentheses, versus a ferrocene internal standard. b Irreversible signal of biphenyl oxidation.
Figure 1. (A) Structure of the fullerene derivative and (B) electron transfer from the biphenyl electron-donor moieties to the fullerene electron-acceptor sphere.
electrolyte. Argon was used to deaerate the solution, and an argon blanket was maintained over the solution during the experiments. SC3 hydrophobin was purified from the culture medium of Schizophyllum commune as described previously.26 Prior to use, freeze-dried SC3 protein was dissolved in 100% trifluoroacetic acid, which was then gently removed by evaporation in a stream of filtered air. The dried monomeric SC3 was dissolved in Millipore water without agitation. Glassy carbon electrodes (GCEs) are hydrophobic; their hydrophobicity is somewhat lower than that of Teflon. The contact angle values are 88.7° and 108° for air/water/GCE and air/water/Teflon, respectively.24,27 GCEs were coated with an SC3/fullerene blend by self-assembly from aqueous/THF solution over the course of ∼30 min. To this end, 20 µL of the SC3/fullerene blend was placed on each GCE. The SC3/fullerene blend was prepared extemporaneously by mixing the water SC3 solution (concentration 0.3 mg mL-1, 21.1 mM) with the THF/ fullerene solution (1 mg mL-1) at room temperature. The functionalized GCEs were stored dry at 4 °C. The electrolyte used in the experiments with immobilized fullerene was a 0.1 M TBABF4 solution, and the solvent used was either CH2Cl2 or THF. CH2Cl2 was used as the solvent for the anodic oxidation experiments because the range of positive potentials accessible in THF is too narrow to record the oxidation peaks of the biphenyl moiety. Multiple-scan cyclic voltammetry experiments lasted approximately 1 h. After being used in a series of experiments, the electrode was stored dry at 4 °C, and the next series of experiments was performed 24 h later. The stability of the functionalized GCEs was tested over 48 h. 3. Results and Discussion The fullerene derivative used in this study bears an aromatic electron-donor unit attached through a long alkyl spacer to a
C60 fullerene sphere (Figure 1). The synthesis of this derivative was described recently.28 This molecular structure was elaborated to prevent fullerene-donor interactions in the ground state. Separating the fullerene unit and mesogenic core with an alkyl chain allows better space separation between the negative and positive charges. Consequently, slower charge recombination and photocurrent amplification are expected. Here, the electrochemical properties of the fullerene derivative were investigated in solution and immobilized on electrodes using cyclic voltammetry. The values of the formal potentials, E1/2, obtained for each redox step in the voltammograms are reported in Table 1. E1/2 was approximated as (Epa + Epc)/2, where Epa and Epc are the oxidation and reduction peak potentials, respectively. Five reduction processes corresponding to the consecutive, one-electron reduction steps of the C60 fullerene sphere are easily resolved in the negative potential region (Figure 2A). The last peak is enhanced and has a small anodic counterpart pointing to a catalytic process possibly involving trace amounts of water present in the solution. It can be noted that the average distance between the peaks corresponding to successive electron transfers is around 0.5 V (Figure 2A). The first two peaks, corresponding to the C60/C60- and C60-/C602- reduction/reoxidation steps are reversible. The Epa - Epc value was found to be 61-65 mV for these peaks, which confirms that these are one-electron processes (Epa - Epc ) 58 mV for an ideal reversible one-electron process).29 In the positive potential region (Figure 2B), one reversible couple of anodic/cathodic peaks is observed. The value of Epa - Epc ) 44 mV indicates that these peaks correspond to a one-electron oxidation/reduction process at each biphenyl moiety of the fullerene derivative (Figure 1B).30 The dependence of the linear peak current versus the square root of the scan rate (xV; Figure 2A, inset) for the first reduction and oxidation step shows that the overall electrochemical process is diffusion-controlled, with a diffusion coefficient equal to 3.6 × 10-6 cm2 s-1. To immobilize the fullerene derivative on the electrodes, a highly tensioactive protein that self-assembles at the interfaces, SC3 hydrophobin,11-19 was used. It was shown recently that this protein is useful as an agent for retaining metal cations, different synthetic molecules, and enzymes on various types of solid supports. The systems obtained were functional in both aqueous solutions and organic solvents.23-25 Here, the fullerene derivative was incorporated into the hydrophobin matrix using a 1:1 v/v water/THF solution, and CV experiments with the immobilized fullerene were performed in CH2Cl2. The results obtained with the fullerene derivative incorporated in the hydrophobin matrix are shown in Figure 3. Two welldeveloped fullerene reduction/oxidation signals corresponding to the C60/C60- and C60-/C602- couples can be seen in the potential window available with CH2Cl2 (Figure 3A). It is important to note that a preliminary oxidation of the biphenyl triggers an increase of the signal corresponding to the first step
1178 J. Phys. Chem. C, Vol. 111, No. 3, 2007
Figure 2. Cyclic voltammograms for 0.5 mM fullerene solution. The fullerene derivative was dissolved in (A) 0.1 M TBABF4/THF, (B) 0.1 M TBABF4/CH2Cl2. Scan rate ) 10 mV s-1 (solid line) and 100 mV s-1 (dotted line). Inset: First reduction peak current versus the square root of the scan rate (xV).
of fullerene reduction (Figure 3B). The latter phenomenon shows charge transfer between the electron donor and acceptor. On the other hand, one additional signal, corresponding to the C602-/ C603- redox process, could be resolved when THF was used as the solvent (Figure 4). Notably, in neither of the two solvents was the fullerene adsorbed on the bare GCE, as indicated by the linearity of the peak current versus xV dependence (Figure 1A, inset). The E1/2 potentials for the dissolved and immobilized fullerene are collected in Table 1. Multiple voltammetric cycles performed using electrodes modified with the fullerene/hydrophobin blend showed that the latter system was stable. Indeed, the signals remained unchanged in several series of experiments performed over 48 h. Moreover, the shape of the curve indicates the good reversibility of the process, and the linear dependence of the peak current on the scan rate (Figure 3, inset) shows that the fullerene derivative is immobilized on the electrode surface. A small separation between the oxidation and reduction peaks might indicate that the electroactive centers of the fullerene derivative are separated from the electrode by a layer of hydrophobin. It is worth noting that hydrophobin allowed the retention of the fullerene derivative on the electrode in the presence of CH2Cl2, a solvent that dissolves this derivative. The hydrophobicity of the fullerene derivative indicates that it is retained in the hydrophobin matrix by nonpolar interactions.24
Corvis et al.
Figure 3. Cyclic voltammograms obtained with the fullerene immobilized in the hydrophobin matrix. Supporting electrolyte ) 0.1 M TBABF4/CH2Cl2. (A) Two peaks corresponding to C60/C60- and C60-/ C602- couples are resolved. Scan rate ) 10 mV s-1 (solid line) and 100 mV s-1 (dotted line). Inset: First reduction peak current versus scan rate. (B) Preliminary oxidation of the biphenyl amplifies the fullerene C60/C60- reduction signal for the scan rate of 100 mV s-1 compared to that in panel A.
Figure 4. Cyclic voltammetry experiments performed in 0.1 M TBABF4/THF with GCEs functionalized with the fullerene/hydrophobin blend (solid line). Reference curve: GCE after the adsorption of the fullerene in the absence of hydrophobin (dotted line). Scan rate ) 100 mV s-1.
4. Conclusions In summary, we have demonstrated that the fullerene-based electron-donor-acceptor system obtained is electrochemically functional and stable enough to withstand organic solvents,
Electron-Donor-Acceptor Fullerene Derivative namely, THF and CH2Cl2. Because the hydrophobin layer allows easy electron transfer from the electrode to the fullerene moiety, this method can be considered as promising for the development of fullerene-based electron-donor-acceptor devices. Photoelectrical experiments with fullerene-modified electrodes are underway in our laboratories. Acknowledgment. We thank Prof. Jo´zef Mieczkowski for synthesising the fullerene derivative. Insightful discussions with Dr. Karin Scholtmeijer, Dr. Wim Meijberg, and Dr. Hans J. Hector from BioMaDe (Groningen, The Netherlands) are greatly appreciated. Y.C. acknowledges a Ph.D. fellowship (Bourse Docteur Inge´nieur) from BioMaDe Technology Foundation and the Centre National de la Recherche Scientifique, a FrenchPolish bilateral project Polonium (no. 09184SC), and a Warsaw University postdoctoral fellowship. This work was supported by a Research Training Network grant (Grant HPRN-CT-2 00200171, to Y.C. and P.B.). The technical assistance of Jean-Louis Vaucher, Alexis Martin, and Bruno Hinck from Service Technique de l’UHP is acknowledged. We thank Brian DeWitt for proof-reading the manuscript. References and Notes (1) Winder, C.; Sariciftci, N. S. J. Mater. Chem. 2004, 14, 1077. (2) Reed, C. A.; Bolskar, R. D. Chem. ReV. 2000, 100, 1075. (3) Thomas, K. G.; George, M. V.; Kamat, P. V. HelV. Chim. Acta 2005, 88, 1291. (4) Wang, Y. Nature 1992, 356, 585. (5) Hoppe, H.; Egbe, D. A. M.; Muehlbacher, D.; Sariciftci, N. S. J. Mater. Chem. 2004, 14, 3462. (6) Yu, G.; Gao, J.; Hummelen, J. C.; Wudl, F.; Heeger, A. J. Science 1995, 270, 1789. (7) Yang, F.; Shtein, M.; Forrest, S. R. Nat. Mater. 2005, 4, 37. (8) Hirano, C.; Imae, T.; Fujima, S.; Yanagimoto, Y.; Takaguchi, Y. Langmuir 2005, 21, 272. (9) Campidelli, S.; Lenoble, J.; Barbera, J.; Paolucci, F.; Marcaccio, M.; Paolucci, D.; Deschenaux, R. Macromolecules 2005, 38, 7915.
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