ARTICLE pubs.acs.org/JPCC
Voltammetric and Electrochemical Impedance Study of Ferrocenyl Containing β-Peptide Monolayers on Gold Paula A. Brooksby,*,† Kelly H. Anderson,† Alison J. Downard,‡ and Andrew D. Abell*,§ †
Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch, New Zealand MacDiarmid Institute for Advanced Materials and Nanotechnology, Department of Chemistry, University of Canterbury, Private Bag 4800, Christchurch, New Zealand § School of Chemistry & Physics, The University of Adelaide, Adelaide, SA 5005, Australia ‡
bS Supporting Information ABSTRACT: An investigation of the kinetics of electron transfer though a self-assembled monolayer containing long chain alkanethiol and β-peptide components and a terminating ferrocene group has been carried out at a polycrystalline gold electrode. Cyclic voltammetry and electrochemical impedance spectroscopy in aqueous and mixed aqueousmethanol solutions showed the rate of electron transfer varied from approximately 5 in water to 29 s1 in methanol. This difference is attributed to a change in the alkanethiol tilt angle in the film in response to hydrogen bonding disruption of the peptide to allow greater molecular motion during electron transfer.
’ INTRODUCTION Electron transfer (ET) through long-chain alkanethiol selfassembled monolayers (SAMs) is considered to occur via an electron tunneling mechanism.1,2 The rate of ET through C15 and C16 alkanethiols containing a terminal ferrocene (Fc) group is of the order of 120 s1, depending on whether CO2 or CONH groups are used to link Fc to the spacer and also the identity of the coadsorbed diluent in the SAM.36 Conversely, the mechanism of ET through long helical peptides is considered to be electron hopping.710 While this remains contentious, the rate is generally 4 or more orders of magnitude greater than for alkanethiols with a similar number of groups in the molecular chain.11 Indeed the ET rate through peptides very much depends on the sequence of amino acids and hence the number of intra- and intermolecular hydrogen bonds and the dipole moment, but also the type of linker attachment to the gold electrode and the molecular dynamics of the molecule.1215 ET processes within organized protein structures have been extensively studied due to anticipated applications in the field of molecular electronics12 and to assist in the elucidation of ET processes in natural bioelectrochemical systems such as photosynthesis.7 In general, R-helical peptides are good mediators of ET between donor and acceptor sites as evidenced by the smaller tunneling β decay constants (45% alcohol. The simultaneous behavior of the anodic peaks was complex and unpredictable, with the double peaks either replaced by one peak or one of the doublet peaks becoming more dominant. Voltammograms of SAMMe in aqueous and methanolic solutions in the presence of a ferri- and ferrocyanide were not significantly different from each other. Note that solutions of ferrocyanide in methanolic solutions were unstable and developed a distinctive Prussian blue color in approximately 30 min. Thus, electrochemistry in this solution was performed as quickly as possible. The addition of methanol did not significantly alter the capacity of SAMMe (Figure 4B) in the potential region where Fc would normally be observed. However, at more positive potentials, the
Figure 4. Cyclic voltammograms from: (A) One freshly prepared SAMFc surface recorded in (black) aqueous and (red) 50% methanol. (B) One freshly prepared SAMMe surface recorded in (black) aqueous and (red) 50% methanol. All solutions contain 1.0 M NaClO4 electrolyte. Scan rates = 50 mV s1. Potentials for methanolic solutions offset (0.15 V) to the SCE scale.
capacity in aqueous solutions is slightly greater. The capacity of the SAMFc layer at potentials greater than (200 mV is similar to that of SAMMe, but at E1/2 (Fc/Fcþ) it is difficult to compare the films with certainty using voltammetry. The ferri- and ferrocyanide redox probe response at SAMMe (Figure 3c) clearly indicates the film has pinhole type defects, and it appears that methanol does not significantly increase the number or size of these defects. However, the reduction of the fwhm of the Fc peaks (SAMFc in Figure 4A) in methanol indicates that the redox centers are more homogeneously located in the presence of methanol or that ET is faster in the methanolic solutions compared with aqueous solution. The Fc peak current density response with increasing scan rate is shown in Figure 5, and this confirms that the voltammetric signal arises from a surface-bound Fc species in both media. The linear regions of the plots in methanolic solution extend to 1 V s1, which is an order of magnitude greater than that in aqueous solvents. In addition, a plot showing the relationship between Ep (as an overpotential) against log(ν) for each solvent is shown in Figure 6. In this plot, Ep increases rapidly in aqueous solutions, especially on the oxidation branch, with increasing scan rate, but when alcohol is present, the Ep values are relatively constant with ΔEp at 3040 mV until ∼1 V s1. Thus, the addition of alcohol causes the quasi-reversible ET behavior in aqueous solutions to move toward that of a reversible system. Finally, the ET coefficient (R) in aqueous solutions was calculated as 0.5 based on the limiting slope to the data in Figure 6. 7520
dx.doi.org/10.1021/jp112278g |J. Phys. Chem. C 2011, 115, 7516–7526
The Journal of Physical Chemistry C
Figure 5. Graph showing peak current density against scan rate from cyclic voltammograms of SAMFc recorded in (A) aqueous and (B) 50% aqueousmethanol solutions containing 1.0 M NaClO4.
The voltammetric behavior of the SAMFc in aqueous solutions is predictable based on typical responses from similarly constructed R-peptide monolayers. Voltammetric studies in mixed solvent systems have not been performed previously. While the apparent similarity of the double layer capacitance for SAMMe in aqueous and methanolic suggests well-formed monolayers of β-peptide assemblies, the decreased fwhm peaks for Fc and apparent much faster ET kinetics in methanol are unexpected. Thus, the structure of SAMFc and SAMMe films and the associated ET were examined further using EIS. Electrochemical Impedance Spectroscopy (EIS). As noted above, cyclic voltammetry and potential step chronoamperometry are commonly employed to evaluate the ET mechanism and kinetics of Fc-terminated peptides in SAMs. It is well-known that the time to charge the electrode double layer during CA is critically dependent on the electrode area. However, determining the charging time is typically performed at a bare electrode surface, but with a film present the charging time increases by as much as 4 orders of magnitude depending on the nature of the SAM.34 Such an increase may further complicate the currentpotential transients. The advantage of EIS is that interfacial parameters describing capacities, resistances, and diffusion events can be observed explicitly, but the difficulty with this technique is in the choice of a suitable model of circuit elements that appropriately describes the physical interface. Elucidating the correct model is assisted by observing the EIS behavior under the influence of independent variables such as electrolyte concentration or applied potential. A schematic of the circuit model typically used to describe a metalSAM (without a redox moiety) electrode using an adjacent electrolyte solution with a redox species present is shown in Figure 7A. The component elements include (i) a solution resistance (Rs), (ii) the uncompensated solution resistance (Ru) that exists between the WE and the RE, (iii) the resistance through the SAM (RSAM) that arises from the diffusion of ionic species through the monolayer possibly attributable to collapsed sites and pinholes, (iv) the charge transfer resistance (Rct) associated with ET from the redox species, (v) the stray
ARTICLE
Figure 6. Graph showing peak overpotential against log (scan rate) from cyclic voltammograms of SAMFc recorded in (black circles) aqueous and (red triangles) 50% methanolic solutions containing 1.0 M NaClO4.
capacitance (Cstray) that exists in every cell but is only observed in dilute electrolyte solutions when the current between the WE and the CE finds other pathways, (vi) the capacitance of the double layer (CDL), and (vii) the Warburg impedance (Zw) related to a diffusion process of, typically, a solution redox species. The CDL is usually modeled as a constant phase element (CPE) because a polycrystalline metal electrode has surface irregularities and therefore behaves less than the ideal. A RSAM element is not present in the absence of a SAM, and Rct and Zw elements are not observed in the absence of a solution redox species. Thus, at the bare gold electrode the model is composed of Rs and CDL in series, but in increasingly dilute electrolyte solutions (
