Langmuir 2008, 24, 11253-11260
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Reversible Electrochemical Switching of Polyelectrolyte Brush Surface Energy Using Electroactive Counterions Evan Spruijt,† Eun-Young Choi,† and Wilhelm T. S. Huck* MelVille Laboratory for Polymer Synthesis, Department of Chemistry, UniVersity of Cambridge, Lensfield Road, Cambridge, CB2 1EW United Kingdom ReceiVed June 25, 2008. ReVised Manuscript ReceiVed August 8, 2008 Polyelectrolyte brushes with electroactive counterions provide an effective platform for surfaces with electrochemically switchable wetting properties. Polycationic poly(2-(methacryloyloxy)-ethyl-trimethyl-ammonium chloride) (PMETAC) brushes with ferricyanide ions ([Fe(CN)6]3-) were used as the electrochemically addressable surface. After a negative potential of -0.5 V was applied to the [Fe(CN)6]3--coordinated PMETAC brushes, the [Fe(CN)6]3- species were reduced to [Fe(CN)6]4-, and the surface became more hydrophilic. By application of alternating negative and positive potentials, PMETAC brushes were switched reversibly between the reduced state ([Fe(CN)6]4-) and oxidized state ([Fe(CN)6]3-), resulting in reversible changes in water contact angles. The time required for a complete contact angle change can be tuned from 1 to 20 s, by changing the brush thickness and the concentration of supporting electrolyte. We present an electrochemical brush transport model that includes the electrochemical reaction at the charged electrode and describes ion transport through the brush phase covering the electrode. The model quantitatively describes the response of the contact angle (hydrophilicity) to the applied voltage as a function of background ionic strength and brush thickness, supporting the proposed mechanism of ion transport through the brush and electrochemical reaction at the electrode. A typical diffusion constant for ferricyanide in a PMETAC brush of any thickness in 5 mM KCl supporting electrolyte was found to be 2 × 10-15 m2 s-1, 5 to 6 orders of magnitude smaller than its bulk solution value.
Introduction The design of stimuli-responsive surfaces with tunable properties is an ongoing challenge in materials science.1 Among the tunable surface properties, control of surface energy, or wettability, has been the focus of considerable attention due to potential applications in sensors and displays.2-9 The use of an electrochemical potential provides a powerful tool to tune the wettability, as it does not require external reagents and allows an easy integration with electronic devices.10 In most literature studies, either electroactive self-assembled monolayers or thin films of polymers with * To whom correspondence should be addressed. E-mail: wtsh2@ cam.ac.uk. † These authors contributed equally to this work.
(1) Lahann, J.; Langer, R. MRS Bull. 2005, 30, 185–188. (2) Nakajima, A.; Hashimoto, K.; Watanabe, T. Monatsh. Chem. 2001, 132, 31–41. (3) Blossey, R. Nat. Mater. 2003, 2, 301–306. (4) Liu, Y.; Mu, L.; Liu, B. H.; Kong, J. L. Chem.sEur. J. 2005, 11, 2622– 2631. (5) Parkin, I. P.; Palgrave, R. G. J. Mater. Chem. 2005, 15, 1689–1695. (6) Sun, T. L.; Feng, L.; Gao, X. F.; Jiang, L. Acc. Chem. Res. 2005, 38, 644–652. (7) Wang, R.; Hashimoto, K.; Fujishima, A.; Chikuni, M.; Kojima, E.; Kitamura, A.; Shimohigoshi, M.; Watanabe, T. Nature 1997, 388, 431–432. (8) Liang, L.; Feng, X. D.; Liu, J.; Rieke, P. C.; Fryxell, G. E. Macromolecules 1998, 31, 7845–7850. (9) Feng, X. J.; Feng, L.; Jin, M. H.; Zhai, J.; Jiang, L.; Zhu, D. B. J. Am. Chem. Soc. 2004, 126, 62–63. (10) Choi, I. S.; Chi, Y. S. Angew. Chem., Int. Ed. 2006, 45, 4894–4897. (11) Abbott, N. L.; Gorman, C. B.; Whitesides, G. M. Langmuir 1995, 11, 16–18. (12) Albagli, D.; Wrighton, M. S. Langmuir 1993, 9, 1893–1897. (13) Lahann, J.; Mitragotri, S.; Tran, T. N.; Kaido, H.; Sundaram, J.; Choi, I. S.; Hoffer, S.; Somorjai, G. A.; Langer, R. Science 2003, 299, 371–374. (14) Liu, Y.; Mu, L.; Liu, B. H.; Zhang, S.; Yang, P. Y.; Kong, J. L. Chem. Commun. 2004, 1194–1195. (15) Riskin, M.; Basnar, B.; Chegel, V. I.; Katz, E.; Willner, I.; Shi, F.; Zhang, X. J. Am. Chem. Soc. 2006, 128, 1253–1260. (16) Sondag-Huethorst, J. A. M.; Fokkink, L. G. J. Langmuir 1994, 10, 4380– 4387. (17) Wang, X. M.; Gershman, Z.; Kharitonov, A. B.; Katz, E.; Willner, I. Langmuir 2003, 19, 5413–5420.
electroactive groups have been used to change the surface wetting under electrochemical control.10-21 In addition, the chemical nature of the counterions in self-assembled monolayers has been reported to affect wettability.22-24 The advantages of using polyelectrolyte brushes as tunable surfaces are the control over their thickness up to several hundreds of nanometers, making small defects irrelevant for the surface properties,25 and the presence of “free” counterions, homogeneously distributed inside the brush, which can easily be exchanged to alter the surface properties. We have previously demonstrated that the wetting properties of polyelectrolyte brushes can be tuned to a large extent by changing the chemical nature of these counterions.26 In that study, polyelectrolyte brushes bearing quaternary ammonium groups (QA+) were shown to change their wettability from very hydrophilic (θAW ≈ 15°) to relatively hydrophobic (θAW ≈ 90°) upon changing counterions from polyphosphate to bis(trifluoromethane)sulfonimide. In this study, we exploited the use of electrochemical potentials to change the redox state of counterions inside polyelectrolyte brushes and, as a consequence, the wettability of the brushes in the dry state. Ferricyanide ([Fe(CN)6]3-) is used as a counterion with reversible redox chemistry to allow reversible switching. (18) Bunker, B. C.; Huber, D. L.; Kushmerick, J. G.; Dunbar, T.; Kelly, M.; Matzke, C.; Cao, J. G.; Jeppesen, J. O.; Perkins, J.; Flood, A. H.; Stoddart, J. F. Langmuir 2007, 23, 31–34. (19) Feng, X. J.; Jiang, L. AdV. Mater. 2006, 18, 3063–3078. (20) Isaksson, J.; Tengstedt, C.; Fahlman, M.; Robinson, N.; Berggren, M. AdV. Mater. 2004, 16, 316–320. (21) Xu, L. B.; Chen, W.; Mulchandani, A.; Yan, Y. S. Angew. Chem., Int. Ed. 2005, 44, 6009–6012. (22) Chi, Y. S.; Lee, J. K.; Lee, S.; Choi, I. S. Langmuir 2004, 20, 3024–3027. (23) Lee, B. S.; Chi, Y. S.; Lee., J. K.; Choi, I. S.; Song, C. E.; Namgoong, S. K.; Lee, S. G. J. Am. Chem. Soc. 2004, 126, 480–481. (24) Shen, Y. F.; Zhang, Y. J.; Zhang, Q. X.; Niu, L.; You, T. Y.; Ivaska, A. Chem. Commun. 2005, 4193–4195. (25) Edmondson, S.; Osborne, V. L.; Huck, W. T. S. Chem. Soc. ReV. 2004, 33, 14–22. (26) Azzaroni, O.; Brown, A. A.; Huck, W. T. S. AdV. Mater. 2007, 19, 151– 154.
10.1021/la801994b CCC: $40.75 2008 American Chemical Society Published on Web 09/09/2008
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Figure 1. (a) Schematic representation of the brushes used in this study. (b) Reversible changes of the contact angle of electroactive PMETAC brush-modified Au (17 nm thick) in 5 mM KCl supporting electrolyte upon application of +0.5 and -0.5 V potentials for 30 s. (c) ATR-FTIR traces corresponding to the first 4 switching cycles as numbered in (b). (d) Cyclic voltammograms of a 16 nm thick [Fe(CN)6]3--coordinated PMETAC brush in 50 mM KCl supporting electrolyte, with a scan rate of 20 mV s-1. Curves correspond to total immersion times in the electrochemical cell: (following the large arrow) 0, 10, 20, 30, 50, 75, and 115 min.
Recently, we have reported the electrochemical characterization of poly(2-(methacryloyloxy)-ethyl-trimethyl-ammonium chloride) (PMETAC) brushes with ferricyanide counterions.27 Here, we will demonstrate how cycling between the reduced and oxidized state of the electroactive counterion leads to reversible switching of the contact angles in the dry state (as shown schematically in Figure 1a). Moreover, we will show how one can control the characteristic time of contact angle switching by changing brush thickness, applied potential, and supporting electrolyte concentration. Finally, we will show that dissipation of the multivalent electroactive species out of the brush is even more restricted than charge transport within the brush, confining specific counterions to the brush for considerable times, in which they can be reduced and oxidized multiple times. The time-resolved control over wettability is closely related to the strongly restricted transport and charge transfer kinetics of the counterions inside the polyelectrolyte brushes. We will show in the last section of this paper that progression of the redox reaction in the brush occurs via diffusion, as opposed to (27) Choi, E. Y.; Azzaroni, O.; Cheng, N.; Zhou, F.; Kelby, T.; Huck, W. T. S. Langmuir 2007, 23, 10389–10394. (28) Jones, D. M.; Brown, A. A.; Huck, W. T. S. Langmuir 2002, 18, 1265– 1269.
electron tunneling, and that diffusion coefficients for the tri- or tetravalent counterions are strongly reduced compared to their bulk solution value as a result of strong binding to the QA+ groups. In addition, charge transfer between the electrode and the electroactive counterions is believed to be retarded by the brush-electrode interface. Both factors contribute to the slow progression of the redox reaction in the brush, and together these effects adequately describe the effect of brush thickness, applied potential, and supporting electrolyte concentration on the contact angle switching time. A model in which both effects are included results in diffusion coefficients that are 5 to 6 orders of magnitude smaller than their bulk solution values. The typical time for complete switching of the wettability at the brush surface ranges from 1 to 20 s, even for very thin brushes (down to 15 nm).
Experimental Section Materials. Mercaptoundecane, CuCl, CuCl2, 2,2′-dipyridyl (BiPy), 2-(methacryloyloxy)-ethyl-trimethyl-ammonium chloride (METAC), KCl, and potassium ferricyanide (K3Fe(CN)6) were purchased from Aldrich. Inhibitor was removed from the METAC monomer by running it through a neutral alumina column. ω-Mercaptoundecylbromoisobutyrate was synthesized from mercaptoundecane according to a published procedure.28 All other chemicals were used as received.
ReVersible Switching of PMETAC Brushes Synthesis of Polyelectrolyte Brushes. Cationic brushes of poly(2(methacryloyloxy)-ethyl-trimethyl-ammonium chloride) (PMETAC) were grown on Au substrates modified with ω-mercaptoundecylbromoisobutyrate (thiol initiator) using aqueous atom transfer radical polymerization (ATRP) according to a previously described procedure.29-31 The polymerization solution for PMETAC brushes was prepared as follows: a solution of 13 g of METAC, 10 mL of water, 15 mL of methanol, 325 mg of 2,2′-dipyridyl (BiPy), and 7 mg of copper(II) chloride (CuCl2) was stirred and degassed by purging with N2 for 30 min at room temperature. To this solution, 100 mg of copper(I) chloride (CuCl) was added, and purging was continued for 15 min. The final composition of the polymerization solution was [METAC]/[BiPy]/[CuCl]/[CuCl2] ) 100:5:2:0.1 in MeOH/H2O (3:2). This solution was transferred to degassed Schlenk tubes containing the initiator-modified Au substrates, adding enough solution to immerse the substrates completely. After various reaction times, the samples were removed, washed with water and then methanol, and dried under a stream of N2. After drying, the brushes were immersed in 0.1 M K3Fe(CN)6 for 30 min to exchange the Clfor [Fe(CN)6]3- counterions. Characterization of Polyelectrolyte Brushes. Brush thicknesses were measured using an R-SE ellipsometer with a 632.8 nm laser at a 70° angle of incidence. Surface roughness was determined by analysis of 1 µm × 1 µm areas of atomic force microscopy (AFM) images, taken on a Agilent Series 4500 SPM instrument (Agilent Technologies) in MAC mode. The presence of [Fe(CN)6]3-/4counterions inside the PMETAC brushes was confirmed by attenuated total reflection Fourier transform infrared (ATR-FTIR) spectroscopy (Perkin-Elmer Instruments, Spectrum One). ATR-FTIR spectra were recorded with 12 scan averaging and a resolution of 4 cm-1. Electrochemical Measurements. All electrochemical measurements were carried out on an Autolab PGSTAT 30 instrument (Eco Chemie) at room temperature in a three-electrode cell, containing the [Fe(CN)6]3--coordinated PMETAC brush on approximately 1 cm2 Au as the working electrode (WE), a platinum plate (1 cm2) as the counter electrode (CE), and Ag/AgCl in 3 M KCl as the reference electrode (RE). KCl solutions of different ionic strength were used as supporting electrolyte. After filling the electrochemical cell with supporting electrolyte and connecting the electrodes, N2 was purged through the cell for 10 min. Two types of electrochemical measurements were carried out. Cyclic voltammograms were measured between the limits of -0.2 and +0.5 V with a scan rate of 20 mV s-1, taking a five scan average and data separation of 2.4 mV. All cyclic voltammetry (CV) data were corrected for charging currents. Chronoamperometry involves a discrete potential step from E0 to E1, whereby the resulting current decay is measured as a function of time. These measurements were carried out with a time interval of 0.01 s, a total decay time of 20 s, and an equilibration time of 30 s, unless stated otherwise. Positive potential steps (E1 - E0 < 0) were carried out from a starting potential (E0) of 0.0 V; negative potential steps were carried out from a starting potential of 0.4 V. Contact Angle Measurements. Samples for contact angle measurements were taken out of the electrochemical cell and rinsed thoroughly with Milli-Q water to wash off the supporting electrolyte solution. It was found to be very important to dry the brushes completely. Therefore, the rinsed samples were first dried thoroughly under a stream of N2 and subsequently dried to completeness under vacuum. Advancing water contact angles (θAW) were measured using a home-built stage with a computer-controlled microsyringe and digital camera. An infusion rate of 2 µL/min was used. Contact angle measurements were repeated at least six times for every sample and every potential step. The presented data are averages of these values. The concomitant measurements of contact angles and ATRFTIR were all carried out with the same sample. The time-resolved contact angle measurements in different supporting electrolyte (29) Jones, D. M.; Huck, W. T. S. AdV. Mater. 2001, 13, 1256–1259. (30) Azzaroni, O.; Moya, S.; Farhan, T.; Brown, A. A.; Huck, W. T. S. Macromolecules 2005, 38, 10192–10199. (31) Cheng, N.; Azzaroni, O.; Moya, S.; Huck, W. T. S. Macromol. Rapid Commun. 2006, 27, 1632–1636.
Langmuir, Vol. 24, No. 19, 2008 11255 concentrations were also carried out with the same sample, with the electroactive counterions exchanged with fresh [Fe(CN)6]3- before every decay. Only on ideal, smooth, and homogeneous surfaces, the Young equation, relating the equilibrium contact angle (θY) to the surface energies, is applicable.32 The surfaces under study here are not ideal, however, and many factors, such as heterogeneity, small variation in roughness, and swelling of the brush under a water droplet, will make quantification of the surface energies problematic. By measuring a dynamic, advancing contact angle of a pure liquid, the effects of swelling and other physical reactions are minimized. For relatively smooth, potentially heterogeneous surfaces, θAW can be expected to be a good approximation of θY.32 For the sake of completeness with respect to this experimental technique, the typical receding contact angle for a [Fe(CN)6]3--coordinated PMETAC brush is reported here to be 10 ( 3°. The hysteresis between advancing and receding contact angles is attributed to strong interactions between the brushes and water. Nevertheless, it is not the aim of this paper to quantify surface energies or wettability through contact angle data. The presented contact angle data serve to show a qualitative change in wettability of the polyelectrolyte brushes.
Results and Discussion Contact Angle Switching. A series of PMETAC brushes (Figure 1a), 13-33 nm thick, with [Fe(CN)6]3- counterions were prepared as described previously.27 Figure 1b shows the effect of an applied electrochemical potential on the wettability of a 17 nm thick polyelectrolyte brush in the dry state. During four switching cycles of alternating positive and negative potentials, contact angles exhibited a reversible change from relatively hydrophilic (26-27°, negative potential, [Fe(CN)6]4--coordinated brush) to relatively hydrophobic (41-44°, positive potential, [Fe(CN)6]3--coordinated brush). ATR-FTIR confirms the reversible oxidation and reduction of the [Fe(CN)6]3-/4- species during the switching cycles (Figure 1c). The presence of [Fe(CN)6]3was identified from a strong and characteristic peak at 2115 cm-1; [Fe(CN)6]4- was identified from a peak at 2040 cm-1.33 The ATR-FTIR data in Figure 1c show that the oxidation and reduction reactions go nearly to completion for every switching cycle, that they are reversible, and that no significant loss of electroactive counterions occurs during one switching cycle. The origin of the difference in contact angles is believed to lie in the difference in ion-pairing between the quaternary ammonium groups (QA+) in the polymer and the two redox states of the counterions. In our recent studies, we observed that scarcely hydrated, large, and highly polarizable species such as ClO4- and bis(trifluoromethane) sulfonimide (TFSI) interact very strongly with the QA+ groups leading to hydrophobic collapse by a loss of water accompanied by conformational changes.26 In the present case, the oxidized form of the electroactive counterion used here ([Fe(CN)6]3-) is better polarizable than the reduced form ([Fe(CN)6]4-) and will therefore lead to a stronger interaction with the QA+ groups and a more hydrophobic surface. The electrochemical characteristics of the brushes, as determined by cyclic voltammetry, indeed suggest that the interaction between [Fe(CN)6]3- and QA+ groups is stronger than that for the [Fe(CN)6]4- ions. Table 1 shows that the absolute values of the cathodic peak current densities (ipc), corresponding to the reduction of [Fe(CN)6]3-, are all higher than those of the anodic peak current densities (ipc), corresponding to the oxidation of [Fe(CN)6]4-. This asymmetry between ipc and ipa is believed to be related to the ion-pairing interaction between fixed ions and (32) Kwok, D.; Neumann, A. AdV. Colloid Interface Sci. 1999, 81, 167–249. (33) Pharr, C. M.; Griffiths, P. R. Anal. Chem. 1997, 69, 4665–4672. (34) Shen, G.; Tercero, N.; Gaspar, M. A.; Varughese, B.; Shepard, K.; Levicky, R. J. Am. Chem. Soc. 2006, 128, 8427–8433.
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Table 1. Anodic (ipa) and Cathodic (ipc) Peak Current Densities Obtained from CV in 5 mM KCl as Supporting Electrolyte, Corresponding to 17 and 33 nm Thick Electroactive PMETAC Brushes with Different Scan Rates 17 nm thick PMETAC scan rate (mV s) 5 10 20 40 70 100
33 nm thick PMETAC
ipa (µA cm-2)
ipc (µA cm-2)
ipc (µA cm-2)
ipc (µA cm-2)
3.48 6.71 11.61 19.73 30.19 40.31
-4.35 -7.83 -13.54 -22.09 -32.24 -41.51
6.93 13.68 25.13 44.46 68.33 89.81
-7.68 -15.66 -29.23 -52.24 -78.91 -101.41
redox species in the polyelectrolyte brush, with weaker interactions reducing the peak current.34 To illustrate the stability of this noncovalently bound electrochemically active surface, we followed the complete replacement of electroactive species by ions of the supporting electrolyte at a concentration of 50 mM. Figure 1d shows the changes in the cyclic voltammogram of a 16 nm thick PMETAC brush as a function of the time of immersion in a 50 mM KCl solution. In the first 10 min, the peak current decreased by 15%, corresponding to an approximately equal decrease in the amount of electroactive counterions. Furthermore, after 50 min, the shape of the voltammogram started to change significantly, with no clear anodic peak being visible. After 2 h, no further changes were recorded. These observations indicate that exchange of counterions is not significant on the time scale of contact angle and chronoamperometric experiments, which is 20 s at most. Finally, it is worth mentioning that exchange of the multivalent [Fe(CN)6]3-/4- counterions takes significantly longer (i.e., 2 h) than exchange of monovalent chloride or perchlorate counterions, which goes to completion within a few minutes in a 100 mM solution of another salt, as reported by Azzaroni et al.26 Effect of Brush Thickness on Time-Resolved Contact Angle Change. Changes in the wettability of PMETAC brushes were followed as a function of time. Figure 2a shows the time-resolved development of the advancing water contact angle of a series of polyelectrolyte brushes of various thicknesses. The electroactive ions in the brush were reduced by applying a potential of -0.5 V. Contact angles could be determined with a typical standard deviation of 3° for a certain sample. Additional differences in contact angles between the samples under otherwise identical conditions can be caused by a different surface roughness (30 nm: rms roughness 1.3 ( 0.2 nm; 16 nm: rms roughness 1.7 ( 0.1 nm) and relative humidity at the time of measurement. Initial contact angles (i.e., after 0 s of reduction) of all samples were found to lie between 35 and 45°. Final contact angles (i.e., after 15-30 s of reduction) were found to lie between 18 and 25°, with lower values generally corresponding to thicker brushes in both cases. To be able to compare the contact angle decreases for samples of different thickness, changes in contact angle with respect to their final state have been plotted in Figure 2a. It can be seen that the contact angle decrease is faster for the thinner brushes. This effect can be explained by the transport of charges in the polyelectrolyte brush. As is known, only the outermost 1-2 nm of the polyelectrolyte brush affects the value for sessile drop contact angles.35,36 The slower change in contact angle observed for the thicker brushes indicates that transport of charge from the electrode to the top of the brush, where the contact angle is measured, takes longer for thicker brushes. Progression of the reversible redox reaction inside the PMETAC brush was also followed by ATR-FTIR, as shown in (35) Bain, C. D.; Whitesides, G. M. J. Am. Chem. Soc. 1988, 110, 5897–5898. (36) Zeiri, L.; Efrima, S.; Deutsch, M. Langmuir 1996, 12, 5180–5187.
Figure 2b, for a 33 nm thick polyelectrolyte brush. Figure 2a already showed that wetting properties of this brush show no further change after roughly 10 s. Therefore, ATR-FTIR spectra have been measured up to 10 s. These data confirm that the surface of the PMETAC brush becomes more hydrophilic as more [Fe(CN)6]4- species are generated. Characteristic decay times (τd) for the decrease in contact angle were obtained from fits to the data in Figure 2a, which are described at the very end of the text. In the inset of Figure 2a, these decay times have been plotted as a function of the brush thickness, showing that contact angle decrease is indeed slower for increasing brush thickness. Maximum decay times that have been measured under these circumstances are about 8 s. Effect of Supporting Electrolyte Concentration on TimeResolved Contact Angle Change. Figure 2c shows the effect of the supporting electrolyte concentration on the change in contact angle for a 25 nm thick brush (rms roughness 1.6 ( 0.2 nm). The wettability clearly changes more rapidly in solutions with a higher supporting electrolyte concentration. The inset of Figure 2c shows that characteristic decay times indeed decrease with increasing supporting electrolyte concentration. At a concentration of 10 mM, the decrease in contact angle was almost complete within the first 2 s. This indicates that transport of charges in the brush occurs slower in a lower supporting electrolyte concentration. We will now discuss the model that provides an explanation for this effect. Transport of Counterions in PMETAC Brushes. Change of the wettability of the brush surface as a function of a redox reaction at the electrode at the bottom of the brush requires charge transport inside the brush. In the current electrochemical setup with stationary electrodes and no stirring, the charge transport mechanism involves either movement of the electroactive ions by diffusion or hopping/tunneling of electrons from one redox site to another.37 Examples of both transport mechanisms have been reported in electrochemistry. Electron hopping was found to be the transport mechanism in many polyelectrolyte multilayers, such as poly(viologen)-poly(styrene sulfonate) assemblies, where the redox sites are usually fixed.38 Ion diffusion has been reported for monovalent ions through thin films (up to 45 nm) of polyelectrolytes, with the lowest reported diffusion coefficient of 7 × 10-16 m2 s-1 for [Ru(bpy)2(py)Cl]+.39 There are several arguments that indicate ion diffusion rather than electron hopping is the charge transport mechanism in PMETAC brushes. The first argument is the observed dependence of charge transport on the supporting electrolyte concentration. In Figure 2c, it is seen that the contact angle decrease occurs more rapidly for higher concentrations of supporting electrolyte. An analogous effect is found in chronoamperometric experiments. Figure 3a shows the current decays for a 21 nm thick brush in supporting electrolyte of various concentrations. Again, a more rapid decay is observed in higher supporting electrolyte concentrations. These dependences of charge transport on the supporting electrolyte concentration can be explained in the context of ion diffusion. For diffusion-controlled processes to show a dependence of contact angle or current decay on the supporting electrolyte concentrations, the diffusion coefficient and hence the flux of ions should depend on electrolyte concentration. Such a dependence has been reported for ferrocyanide ions in polyelectrolyte multilayers and was related to (37) Bard, A. J.; Faulkner, L. R. Electrochemical Methods, Fundamentals and Applications, 2nd ed.; Wiley: New York, 2001. (38) Schlenoff, J. B.; Laurent, D.; Ly, H.; Stepp, J. AdV. Mater. 1998, 10, 347–349. (39) Ikeda, T.; Schmehl, R.; Denisevich, P.; Willman, K.; Murray, R. W. J. Am. Chem. Soc. 1982, 104, 2683–2691.
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Figure 2. (a) Changes in contact angle as a function of time for [Fe(CN)6]3--coordinated PMETAC brushes of various thickness (green circle, 13 nm; blue tilted square, 24 nm; and black square, 33 nm) at a potential of -0.5 V in 5 mM KCl supporting electrolyte. The solid lines are fits of the data to the diffusion equation in a finite layer (see end of text). (b) ATR-FTIR spectra of the 33 nm thick brush in (a) as a function of time. (c) Similar to (a) for a 25 nm thick brush in supporting electrolyte of various concentrations (black circle, 0.1 mM KCl; purple square, 1 mM KCl; blue ×, 5 mM KCl; and red tilted square, 10 mM KCl). The insets in (a) and (c) show the characteristic decay times (τd) from the fits in the main figures as a function of brush thickness and supporting electrolyte concentration, with solid lines representing a theoretical relation between (τd) and L in (a) and an empirical relation between (τd) and cKCl in (c).
Figure 3. Chronoamperometry (a) current decays for a 21 nm thick [Fe(CN)6]3--coordinated PMETAC brush after a potential step to +0.5 V in supporting electrolyte of various concentrations (black circle, 0.1 mM KCl; purple square, 1 mM KCl; blue tilted square, 5 mM KCl; red triangle, 10 mM KCl; and green ×, 50 mM KCl) and (b) current decays at various potential steps for the same sample in 10 mM KCl. The inset in (a) is an enlargement, showing the current decay in 0.1 mM KCl. Only data points for which the faradaic current was much larger than the capacitive current, as checked with a Cl--coordinated PMETAC brush, are shown.
the local displacement of a charge equivalent of the ferrocyanide by electrolyte ions, thus enhancing the mobility of the ferrocyanide species.40 Analogous in this case, the electrolyte can screen the local potential of the QA+, making the electroactive species more mobile. In contrast, this dependence cannot be explained by an electron tunneling mechanism, since this mechanism shows no dependence on the supporting electrolyte concentration.41 Finally, it has been argued that such an effect could be caused by an ohmic potential drop, which is related to the electrolyte (40) Farhat, T. R.; Schlenoff, J. B. Langmuir 2001, 17, 1184–1192. (41) Miller, J. R.; Beitz, J. V.; Huddleston, R. K. J. Am. Chem. Soc. 1984, 106, 5057–5068.
concentration and the distance between the working electrode (WE) and reference electrode (RE).37 However, this explanation was shown to be false in this case, as varying the distance between the RE and WE only had a small effect on the measured decay in the case of the 0.1 mM electrolyte, and no measurable effect for other electrolyte concentrations. The second argument favoring ion diffusion over electron tunneling as the charge transport mechanism in PMETAC brushes is the dependence of the peak current density on the scan rate (V). In Figure 4, a plot of ip versus V is shown, based on the data from Table 1. This figure shows, in contrast to our previous publication,27 a linear dependence of both ipa and ipc on V, a
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Figure 4. Cathodic (ipc) and anodic (ipa) peak current densities from cyclic voltammetry (Table 1) for a 17 nm thick [Fe(CN)6]3--coordinated PMETAC brush versus V in 5 mM KCl as supporting electrolyte.
distinctive feature of diffusion processes, whereas electron hopping would show a linear relation between ip and V. The deviation from linearity at low V in Figure 4 is attributed to the fact that charge transfer resistance at the electrode-brush interface limits the current in this range, as will be discussed at the end of the text. A third argument to exclude electron hopping as a plausible mechanism for charge transport is the relatively large distance between neighboring redox sites. The concentration of electroactive species can be estimated from the charge density inside PMETAC brushes, reported previously.27 Taking one species of [Fe(CN)6]3- for every three charged sites, the concentration of [Fe(CN)6]3- inside the brush is approximately 300 mM. This leads to an average intersite distance of 18 Å. The electron hopping rate constant decreases exponentially with distance (ketR exp(-βr)), and typical exponential decay constants (β) for proteins and nonconjugated organic materials are of the order of 1.4 Å-1.42 This implies that on the experimental time scale (10 s) electrons can hop from site to site over a typical distance of 25 Å, starting at the electrode surface.41 This distance is obviously much smaller than the typical thickness of the PMETAC brushes (15-30 nm). The above three arguments strongly suggest that ion diffusion is responsible for charge transport in the polyelectrolyte brush. In order to find the actual diffusion coefficients of the [Fe(CN)6]3-/4species in the polyelectrolyte brush, the chronoamperometric data in Figure 3 were analyzed. For pure diffusion from an infinite bulk solution to a surface, the diffusion coefficient can be obtained from the slope of a so-called Cottrell plot (i versus 1/V), as depicted by the dotted line in Figure 5a.37 The data points in Figure 5a are the Cottrell representation of the data points in Figure 3, which have no straight line part at all. This deviation has two origins, resulting in a characteristic S-shaped curve. At large times, deviation from linearity is due to the confinement of the electroactive species in a finite layer.43 At very short times, deviation from linearity can be caused by slow reaction kinetics at the electrode-brush interface.43 This means that the charge transfer from the electroactive species to electrode is not necessarily fast compared to the diffusion process, due to a significant charge transfer resistance at the electrode, as was suggested in the discussion of Figure 4. It is believed that this resistance results from the organic monolayer that was used to initiate the ATRP polymerization. Indeed, in our recent electrical impedance spectroscopy (EIS) studies, we have reported on these (42) Moser, C. C.; Keske, J. M.; Warncke, K.; Farid, R. S.; Dutton, P. L. Nature 1992, 355, 796–802. (43) Montella, C. J. Electroanal. Chem. 2002, 518, 61–83.
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charge transfer resistances.44 We found that charge transfer resistances were of the same order of magnitude as the total resistance within the PMETAC brush. In addition to the absence of a straight line part in a Cottrell plot (Figure 5a), two characteristic features in chronoamperometry indicate charge transfer limitations in a diffusion-controlled process, according to an analysis by Montella.43 The first is the presence of a maximum in a IV versus log(t) plot (Figure 5b), whereas unrestricted diffusion would show a horizontal line. The second feature is a typical slope of a log(i) versus log(t) between 0 and -0.5 (0 > ∂ log(i)/∂ log(t) > -0.5), measured for short times, which was also verified. All three conditions are met in this system. We therefore conclude that diffusion and slow charge transfer kinetics mutually determine the decay of current inside the polyelectrolyte brush and the decay of the contact angle at the surface of the dry brush. Montella derived the solution to the diffusion equation describing the concentration of a diffusing species as a function of position and time for a charge transfer limited diffusion process in one dimension with one zero-flux boundary condition (top of the brush, Figure 1d) and one boundary determined by a modified reaction kinetics equation. From this solution, the current decay in time can be derived.43
(
∞
∑
Λ c(x, t) ) c0 + ∆c 1 - 2 2 2 n)1 Λ + Λ + bn
((
cos bn 1 cos(bn)
D)
)) ×
( ))
exp b tan b ) Λ
x L
bn2t τd
(1) (2)
2
L τd
(3)
( )
∞ bn2t Λ2 ∆Q exp I(t) ) 2 τd n)1 Λ2 + Λ + b 2 τd
∑
n
(4)
In the above equations, L is the thickness of the finite layer, τd is related to the diffusion constant by eq 3, and ∆Q is the total amount of faradaic charge that passes the electrode after a potential step (i.e., the amount of charge being oxidized or reduced). Λ is related to the charge transfer limitations and to parameter b via eq 2 (refers to the nth positive root). For Λ f ∞, the above equations simplify to unrestricted diffusion in a thin layer; for Λ f 0, there will be no concentration gradients, because diffusion is much faster than charge transfer at the electrode. Finally, as Montella stressed, the above equations are only applicable under the condition that the diffusion coefficient is independent of the potential step size, when large potential steps are being used (Figure 3b, several hundreds of millivolts). This condition was found to be met. Figure 5a and b shows two-parameter fits (and Λ) of the current decays in Figure 3a to eq 4. The number of charged species in the brushes was calculated from the thickness, the electrode area, and the previously reported charge density.27 In addition to the fits shown in Figure 5a and b, current decays for 4 different brush thicknesses (13-21 nm), 5 different supporting electrolyte concentrations (0.1, 1, 5, 10, and 50 mM), and 10 different potential steps have been fitted. The potential steps were chosen 27 symmetrically around the value of E1/2 app of 0.2 V. Figure 5c and (44) Zhou, F.; Hu, H.; Yu, B.; Osborne, V. L.; Huck, W. T. S.; Liu, W. Anal. Chem. 2007, 79, 176–182.
ReVersible Switching of PMETAC Brushes
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Figure 5. (a) Cottrell representation and (b) Cottrell function plot of the chronoamperometric data from a 21 nm thick [Fe(CN)6]3--coordinated PMETAC brush at a potential of +0.5 V and different concentrations of supporting electrolyte: black circle, 0.1 mM KCl; purple square, 1 mM KCl; blue tilted square, 5 mM KCl; red triangle, 10 mM KCl; and green ×, 50 mM KCl. Solid lines represent fits to the diffusion model with charge transfer limitations derived by Montella.43 Dashed lines represent hypothetical curves for unrestricted diffusion. (c) Diffusion parameters derived from the fits in (a) and (b) as a function of supporting electrolyte concentration. Symbols indicate different samples of [Fe(CN)6]3--coordinated PMETAC brushes: black circle, 13 nm; blue square, 16 nm; red triangle, 18 nm; and green solid tilted square, 21 nm. The solid line is a power-law fit to all data. (d) Lambda parameters derived from the fits in (a) and (b) as a function of applied potential. Symbols indicate different concentrations of supporting electrolyte: purple solid square, 1 mM KCl; blue solid tilted square, 5 mM KCl; red solid triangle, 10 mM KCl; and green ×, 50 mM KCl. The solid line is a guide to the eye.
d shows the results in terms of diffusion coefficients (from eq 3) and Λ parameters. In general, the data can be described well by mixed diffusion and charge transfer limitations. As a result of different assumptions, the reported fitting parameters are however subject to some uncertainty and serve to show trends and differences in order of magnitude, rather than being absolute values. Figure 5c shows that the diffusion coefficient of the redox species inside the PMETAC brush is independent of the brush thickness, implying that the brush densities are identical for all brushes. The typical values of D (10-16-10-14 m2 s-1) are significantly smaller than the diffusion coefficient of free ferriand ferrocyanide in aqueous solution (7.6 × 10-10 and 6.3 × 10-10 m2 s-1, respectively).45 The presented diffusion coefficients are averages over all potential steps, with the error bars representing their standard deviation. These relatively small error bars show that the diffusion coefficients are apparently independent of the potential step, and hence, application of the diffusion model (eq 4) is allowed. The clear dependence of D on the supporting electrolyte concentration is explained by local screening of the QA+ potential. Figure 5d shows that the parameter Λ increases with increasing size of the potential step (from E1/2 app ) 0.2), with no significant dependence on supporting electrolyte concentration. This is a (45) Ramesham, R. Thin Solid Films 1999, 339, 82–87.
result of the effect of an “overpotential” on charge transfer kinetics. The overpotential can speed up charge transfer, by providing energy to overcome the charge transfer limitation barrier. At small overpotentials, Λ can be as small as 0.1, implying a significant contribution of charge transfer limitations to the slow current decay. At larger overpotentials, Λ increases to 0.5. At these values of Λ, the apparent diffusion coefficient (when ignoring the effect of charge transfer limitations) was estimated to be only 20% of the actual diffusion coefficient (Figure 5c) by Montella.43 Besides for fitting the current decays, the applied diffusion model can also be used to fit concentration profiles (eq 1). By setting x ) L and assuming that the value of the contact angle is (ideally) a linear combination of the contribution of ferri- and ferrocyanide ions at the surface (θAW ) RθFe3+ + βθFe2+), the contact angle data can be fitted using the applied diffusion model. This has indeed been done for all contact angle decreases, and the resulting fits are presented as solid lines in Figure 2a and c in this paper. The inset in Figure 2a shows the predicted quadratic dependence of the characteristic decay time on the brush thickness (eq 3). The inset in Figure 2c shows an empirical relation between the decay time and the supporting electrolyte concentration, since the supporting electrolyte was not explicitly taken into account in the presented diffusion model. The observation that the relatively simple eq 1 with only two parameters determining the
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decrease time is able to describe all contact angle data reasonably well confirms the applicability of the diffusion model.
Conclusions In this study, we have demonstrated how polyelectrolyte brushes with electroactive counterions can lead to switchable surface properties. Electroactive counterions can be easily introduced to cationic PMETAC brushes by immersion in an aqueous solution. By applying alternating positive and negative potentials, ferri/ferrocyanide ions ([Fe(CN)6]3-/4-) can be subsequently oxidized and reduced, leading to a change in contact angle for the dry brush from hydrophilic (θAW ≈ 22°) at negative potentials to more hydrophobic (θAW ≈ 40°) at positive potentials. The difference in contact angle is believed to be the result of a weaker ion-pairing between [Fe(CN)6]4- and the quaternary ammonium groups of the PMETAC brush compared to [Fe(CN)6]3-. Changes in contact angles have been measured as a function of time. This led to the conclusion that the decrease is faster for thinner brushes than for thicker brushes and faster for reduction in high than in low supporting electrolyte concentration. Using these variables, the time for a complete decrease of the contact angle upon reduction can be tuned from less than 1 s to more than 20 s. This relatively slow change of contact angle is a result of the restricted diffusion of counterions inside the polyelectrolyte brush and a charge transfer restriction at the electrode-brush interface. From fits of chronoamperometric data and contact angle decreases to a model, we determined that
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the ferri/ferrocyanide species has a typical diffusion coefficient of 10-16-10-14 m2 s-1, depending on supporting electrolyte concentration, compared to an ∼10-9 m2 s-1 value for free diffusion. The highly restricted transport of electroactive ions inside the polymer brushes shows that polyelectrolyte brushes can be used as selective surfaces for the transport of different ionic species with, for example, catalytic or pharmaceutical activity. In contrast to polyelectrolyte multilayers and spin-coated polymer films, polyelectrolyte brushes do have a homogeneous distribution of charges and can confine specific ionic species within the brush and selectively transport them to and from the surface by electrochemical control. This makes it, for example, possible to use polyelectrolyte brushes in (bio)chemical sensors with in situ measurement of the released quantity by probing the electrochemical response and controlled release of specific ionic species by application of a selective potential.46 Acknowledgment. We thank N. Cheng and T. S. Kelby for their assistance with PMETAC brush synthesis and AFM surface roughness measurements and P. M. Biesheuvel (Wageningen University) for his valuable remarks. E.S. acknowledges the Dr. Hendrik Muller’s Vaderlandsch Fonds and the Royal Netherlands Chemical Society (KNCV) for financial support. LA801994B (46) Tam, T. K.; Ornatska, M.; Pita, M.; Minko, S.; Katz, E. J. Phys. Chem. C 2008, 112, 8438–8445.
