pubs.acs.org/Langmuir © 2010 American Chemical Society
Optically Activated Uptake and Release of Cu2þ or Agþ Ions by or from a Photoisomerizable Monolayer-Modified Electrode† Junji Zhang,‡,§ Michael Riskin,‡ Ran Tel-Vered,‡ He Tian,*,§ and Itamar Willner*,‡ ‡
Institute of Chemistry, The Hebrew University of Jerusalem, Jerusalem, Israel, and §Laboratory for Advanced Materials, Institute of Fine Chemicals, East China University of Science and Technology, Shanghai 200237, PR China Received October 11, 2010. Revised Manuscript Received November 15, 2010 Di-(N-butanoic acid-1,8-naphthalimide)-piperazine dithienylethene was covalently linked to a cysteamine monolayer associated with a Au surface to yield a photoisomerizable monolayer composed of the open or closed dithienylcyclopentene isomers (3a or 3b), respectively. Electrochemical and XPS analyses reveal that the association of metal ions to the monolayer is controlled by its photoisomerization state. We find that Cu2þ ions reveal a high affinity for the open (3a) monolayer state, Ka = 4.6105 M-1, whereas the closed monolayer state (3b) exhibits a substantially lower binding affinity for Cu2þ, Ka=4.1104 M-1. Similarly, Agþ ions bind strongly to the 3a monolayer state but lack binding affinity for the 3b state. The reversible photoinduced binding and dissociation of the metal ions (Cu2þ or Agþ) with respect to the photoisomerizable monolayer are demonstrated, and the systems may be used for the photochemically controlled uptake and release of polluting ions. Furthermore, we demonstrate that the photoinduced reversible binding and dissociation of the metal ions to and from the photoisomerizable electrode control the wettability properties of the surface.
The functionalization of surfaces with photoisomerizable monolayers such as azobenzenes,1 nitrospiropyrans,2 or dithienylethenes3 has found growing interest as a means to control the physical or chemical properties of surfaces. For example, azobenzene layers on surfaces revealed photoswitchable hydrophilic/hydrophobic functions upon photoisomerization between the cis and trans photoisomer states.4 Similarly, the photostimulated uptake and release of substrates (porphyrins) to photoisomerizable monolayers was demonstrated.5 Nitrospiropyran-monolayer-modified electrodes were used to control electron-transfer processes at electrode surfaces6 and to photoswitch electrocatalytic processes in the presence of Pt NPs.7 Similarly, a photoisomerizable redox-active phenoxynaphthacene quinone monolayer was implemented for the activation of electron-transfer cascades.8 Also, dithienylethene-modified surfaces were suggested as functional systems for information storage,9 optical switches,10 and optoelectronic devices.11 Azobenzene-containing molecular “wires” were also used to “shuttle” molecular components with light and to photoactivate molecular machinery functions.12 † Part of the Supramolecular Chemistry at Interfaces special issue. *Corresponding authors. E-mail:
[email protected];
[email protected] (1) (a) Archut, A.; V€ogtle, F.; De Cola, L.; Azzellini, G. C.; Balzani, V.; Ramanujam, P. S.; Berg, R. H. Chem.;Eur. J. 1998, 4, 699. (b) Shipway, A. N.; Willner, I. Acc. Chem. Res. 2001, 34, 421. (2) Berkovic, G.; Krongaus, V.; Weiss, V. Chem. Rev. 2000, 100, 1741. (3) Irie, M. Chem. Rev. 2000, 100, 1685. (4) Hu, J.; Zhang, J.; Liu, F.; Kittredge, K.; Whitesell, J. K.; Fox, M. A. J. Am. Chem. Soc. 2001, 123, 1464. (5) Callari, F. L.; Sortino, S. J. Mater. Chem. 2007, 17, 4184. (6) (a) Willner, I.; Doron, A.; Katz, E.; Levi, S. Langmuir 1996, 12, 946. (b) Doron, A.; Katz, E.; Tao, G.; Willner, I. Langmuir 1997, 13, 1783. (7) Niazov, T.; Shlyahovsky, B.; Willner, I. J. Am. Chem. Soc. 2007, 129, 6374. (8) Doron, A.; Portnoy, M.; Lion-Dagan, M.; Katz, E.; Willner, I. J. Am. Chem. Soc. 1996, 118, 8937. (9) Baron, R.; Onopriyenko, A.; Katz, E.; Lioubashevski, O.; Willner, I.; Wang, S.; Tian, H. Chem. Commun. 2006, 2147. (10) Kudernac, T.; van der Molen, S. J.; van Wees, B. J.; Feringa, B. L. Chem. Commun. 2006, 3597. (11) Katsonis, N.; Kudernac, T.; Walko, M.; van der Molen, S. J.; van Wees, B. J.; Feringa, B. L. Adv. Mater. 2006, 18, 1397. (12) Willner, I.; Pardo-Yissar, V.; Katz, E.; Ranjit, K. T. J. Electroanal. Chem. 2001, 497, 172.
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The assembly of monolayers on surfaces consisting of ligands that specifically bind metal ions has been extensively used to control the hydrophilic/hydrophobic properties of the surfaces, to act as sensor devices, and to facilitate the binding of proteins to surfaces. For example, Agþ- or Hg2þ-functionalized monolayermodified electrodes revealed electroswitchable hydrophobic/hydrophilic properties upon the reduction/oxidation of the ions.13,14 Sensing of metal ions (e.g., Cu2þ) was demonstrated with a thiolated acetylacetonate monolayer associated with electrodes,15 and the association of proteins to electrodes via Ni2þ-Histag ligands was reported.16 Also, chiroselective electron transfer to redox proteins (cytochrome C) using a chiral metal complex monolayer acting as a mediator was reported.17 Supramolecular Cu2þ catenane or rotaxane complexes were assembled as monolayers on electrodes, and the electrochemically driven pirouetting within these molecular devices was reported.18 Similarly, monolayers consisting of metal-ligand complexes on surfaces were examined as transistor-like molecular electronic devices19 or as nanoscale molecular-wire charge-transporting units.20 Metal ion-ligand complexes were also conjugated to photoisomerizable components and linked to electrode surfaces in the form of monolayers. The physical or chemical properties of such surfaces were controlled by light and/or by electrical signals. For example, the ligation of a photoisomerizable diarylethene to a Cu2þ complex associated with a surface enabled the reversible control (13) Riskin, M.; Basnar, B.; Chegel, V. I.; Katz, E.; Willner, I.; Shi, F.; Zhang, X. J. Am. Chem. Soc. 2006, 128, 1253. (14) Riskin, M.; Basnar, B.; Katz, E.; Willner, I. Chem.;Eur. J. 2006, 12, 8549. (15) Rubinstein, I.; Steinberg, S.; Tor, Y.; Shanzer, A.; Sagiv, J. Nature 1988, 332, 426. (16) Balland, V.; Lecomte, S.; Limoges, B. Langmuir 2009, 25, 6532. (17) Takahashi, I.; Inomata, T.; Funahashi, Y.; Ozawa, T.; Masuda, H. Chem.;Eur. J. 2007, 13, 8007. (18) (a) Raehm, L.; Kern, J.; Sauvage, J.-P.; Hamann, C.; Palacin, S.; Bourgoin, J. Chem.;Eur. J. 2002, 8, 2153. (b) Weber, N.; Hamann, C.; Kern, J.; Sauvage, J.-P. Inorg. Chem. 2003, 42, 6780. (c) Collin, J.-P.; Mobian, P.; Sauvage, J.-P.; Sour, A.; Yan, Y.-M.; Willner, I. Aust. J. Chem. 2009, 62, 1231. (19) Albrecht, T.; Guckian, A.; Ulstrup, J.; Vos, J. G. Nano Lett. 2005, 5, 1451. (20) Tang, J.; Wang, Y.; Klare, J. E.; Tulevski, G. S.; Wind, S. J.; Nuckolls, C. Angew. Chem., Int. Ed. 2007, 46, 3892.
Published on Web 12/03/2010
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of the wettability of the surface.21 Also, the photoisomerization of a nitrospiropyran monolayer on electrodes to the nitromerocyanine photoisomer state enabled the photochemical patterning of the surface and the selective binding of ions to the nitromerocyanine domains. For example, the selective binding of Agþ or Co2þ ions to the ligand sites, followed by the electrochemical reduction of the ions, led to the formation of patterned domains of Ag0 nanoclusters22 or of magnetic Co0 nanoclusters.23 In the present study, we address the photochemical control of the binding and dissociation of metal ions to and from a photoisomerizable dithienylethenemonolayer-functionalized electrode, respectively. We discuss the electrochemical features and wettability of the resulting monolayer-modified surface.
Results and Discussion The synthesis and properties of dithienylcyclopentene, DTE (1a), and its derivatives were previously reviewed.24 The compound undergoes reversible photochemical isomerization (Scheme 1A, eq 1). Irradiation of 1a with UV light yields the closed isomer (1b), and the illumination of 1b with visible light regenerates the open isomer (1a). The DTE backbone was introduced into a compound of higher complexity, di(N-butanoic acid-1,8-naphthalimide)-piperazine dithienylethene (2a). Compound 2a was synthesized according to a previously described procedure25 and exhibits similar reversible photoisomerization features (Scheme 1A, eq 2). Compound 2a includes two piperazine units that act as ligands for the association of metal ions. Furthermore, its imide residues are functionalized with butanoic acid tethers that enable the covalent attachment of 2a to surfaces. Accordingly, 2a was covalently linked to a cysteaminemodified Au surface to yield a surface-bound monolayer of 3a (Scheme 1B). The resulting monolayer did not reveal any electrical response in either of its two isomer states (3a/3b). The monolayermodified electrodes in the two photoisomer states was then interacted with Cu2þ or Agþ ions, and the electrochemical features of the resulting Cu2þ-(3a/3b) or Agþ-(3a/3b) complexes as well as the photochemical interconversion of the metal ion photoisomerizable complexes were characterized (Scheme 2). Figure 1A shows the cyclic voltammograms, at different scan rates corresponding to the Cu2þ ion coordinated to the 3a open photoisomer monolayer (Cu2þ-3a). A quasi-reversible redox wave corresponding to the Cu2þ/Cu0 redox process, at around E °= 0.2 V versus SCE, is observed. Coulometric analysis of the reduction wave indicates a surface coverage of ca. 3.6 10-11 mol 3 cm-2. The peak currents of the redox wave reveal a linear dependence on the potential sweep rate (Figure 1A, inset), consistent with the formation of a surface-confined redox species. Additional chronoamperometric experiments (Figure S1, Supporting Information) revealed that the electron-transfer rate constant between the electrode and the Cu2þ-3a complex is ca. ket=34 s-1. An analysis of the chronoamperometric transient was performed according to eq 3,26 IðtÞ ¼ Q1 k et e - k et t
ð3Þ
where ket is the electron-transfer rate constant and Q1 is the charge associated with the reduction of the Cu2þ complex. The analysis yielded a surface coverage of Cu2þ-3a corresponding (21) Driscoll, P. F.; Purohit, N.; Wanichacheva, N.; Lambert, C. R.; McGimpsey, W. G. Langmuir 2007, 23, 13181. (22) Riskin, M.; Willner, I. Langmuir 2009, 25, 13900. (23) Riskin, M.; Gutkin, V.; Felner, I.; Willner, I. Angew. Chem., Int. Ed. 2008, 47, 4416. (24) Tian, H.; Yang, S. Chem. Soc. Rev. 2004, 33, 85. (25) Zhang, J.; Tan, W.; Meng, X.; Tian, H. J. Mater. Chem. 2009, 19, 5726. (26) Katz, E.; Willner, I. Langmuir 1997, 13, 3364.
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to 3.5 10-11 mol 3 cm-2, which is similar to the value derived from the coulometric analysis of the voltammograms. Figure 1B depicts the cyclic voltammograms corresponding to the Cu2þ ions coordinated to the closed photoisomer state (3b) at different scan rates. Differential pulse voltammetry experiments (Supporting Information, Figure S2) indicate that the oxidation potential of the Cu2þ-3b complex is negatively shifted by ca. ΔE = 130 mV with respect to the Cu2þ-3a complex. The anodic peak currents of the redox waves of Cu2þ-3b show a linear dependence on the scan rate (Figure 1B, inset), implying that the complex is a surface-confined redox species. The surface coverage of Cu2þ bound to the 3b photoisomer state corresponds to 1.6 10-11 mol 3 cm-2, indicating that ca. 55% of Cu2þ associated with the 3a monolayer state, was dissociated. Figure 2A, curve a, shows the voltammetric responses of the 3a-modified electrode that was pretreated with different concentrations of Cu2þ. The current responses are intensified as the concentration of Cu2þ during the pretreatment stage was increased, and they level off at a concentration of ca. 0.2 mM that leads to the saturation of the ligands associated with the electrode. From this calibration curve, the association constant of Cu2þ to the 3a ligand was estimated to be Ka (Cu2þ-3a)=4.6 105 M-1 (further details in Supporting Information). Similarly, Figure 2A, curve b, depicts the amperometric responses of the 3b-modified electrode that was pretreated with different concentrations of Cu2þ. The derived association constant, Ka (Cu2þ-3b)=4.1104 M-1, indicates that the binding of Cu2þ ions to the photoisomer state (3b) is substantially lower. The different affinities of the Cu2þ ions for the two photoisomer states of the monolayer enabled the cyclic photostimulated uptake and release of the ions to and from the monolayer, respectively. Irradiation of the monolayer at λ=302 nm resulted in the closed 3b isomer state and the low amperometric response of the electrode (Figure 2B, curve b). Photoisomerization of the electrode to the 3a state, using irradiation at λ>530 nm, leads to the effective association of Cu2þ ions to the monolayer, and to a high amperometric response (curve a). By the cyclic photoisomerization of the monolayer-modified electrode between the 3a and 3b states, the uptake and release of the Cu2þ ions are demonstrated. (Up to eight reversible cycles were performed with no noticeable effect on the uptake/release of the Cu2þ ions.) It should be noted that a monolayer of 1,2-bis[2-methyl-5-(4-(N-carboxymethyl)pyridyl)-3-thienyl] cyclopentene (4a), lacking the piperazine residues, was similarly immobilized onto the Au surface (Scheme 1C). The resulting photoisomerizable monolayer lacked any association affinity for Cu2þ to the open and closed states (5a and 5b). These results indicate that the piperazine units act as ligands for the association of Cu2þ ions. We find that the photoisomer state 3a exhibits a higher affinity for binding the Cu2þ ions than does the 3b state. Presumably, the steric flexibility of the ligands in the structure of 3a allows access for the ions to form the complex. The photoisomerizable monolayer-modified electrode also reveals photoswitchable binding and release functions toward Agþ ions. Figure 3A shows cyclic voltammograms at different scan rates corresponding to the association of Agþ ions to the 3a-functionalized monolayer electrode. A quasi-reversible wave corresponding to the Agþ/Ag0 redox is observed at a middle-point potential of ca. E = 0.2 V versus SCE. The anodic peak currents show a linear dependence on the scan rate, consistent with the formation of a surface-confined Agþ-3a complex (Figure 3A, inset). Figure 3B, curve b, depicts a cyclic voltammogram corresponding to the interaction of the 3b-monolayer electrode and Agþ ions. No electrical response of the functionalized electrode is observed, suggesting that no complex between 3b and Agþ ions is formed. Further support that Agþ lacks a binding affinity for 3b was obtained by DOI: 10.1021/la1040807
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Zhang et al. Scheme 1. Reversible Photochemical Isomerization of DTEa
a (A) Equation 1: Reversible photochemical isomerization of dithienylcyclopentene, DTE, between the open (1a) and closed (1b) states. Equation 2: Synthesis of di(N-butanoic acid-1,8-naphthalimide)-piperazine dithienylethene and its photoisomerization in the aqueous solution phase. (B) Assembly of the 3a monolayer-modified Au electrode and its photoisomerization to the 3b state. (C) Assembly of the 5a monolayer-modified Au electrode and its photoisomerization to the 5b state.
interacting the closed isomer (2b) with Agþ in solution. No spectral changes were observed for 2b, suggesting that no coordination complex was formed. Indeed, the reversible cyclic photoisomerization of 1382 DOI: 10.1021/la1040807
the monolayer-modified electrode between the 3a and 3b states, and in the presence of Agþ, leads to high amperometric responses corresponding to the Agþ-3a-monolayer state and only to background Langmuir 2011, 27(4), 1380–1386
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Figure 1. (A) Cyclic voltammograms corresponding to a Au electrode modified with 3a-Cu2þ. Scan rates: (a) 10, (b) 50, (c) 75, (d) 100, and (e) 200 mV s-1. (Inset) Plot of the anodic peak current as a function of the scan rate. (B) Cyclic voltammograms corresponding to a Au electrode modified with 3b-Cu2þ, using scanning conditions similar to those in plot A. (Inset) Plot of the anodic peak current as a function of the scan rate. Prior to the measurements, the electrodes were immersed in an aqueous solution of 20 mM Cu2þ for 4 h and then washed with water. All measurements were performed in an aqueous solution of 0.1 M NaNO3. Scheme 2. Binding and Release of Ions to and from the Photoisomer States of Bis-piperazine-Functionalized Dithienylethene
amperometric responses of the 3b-monolayer electrode that lacks Agþ ions (Figure 3B, inset). Thus, the photoisomerization of the monolayer leads to the cyclic uptake of Agþ ions by the 3afunctionalized electrode and to the release of Agþ ions upon the photoisomerization of the monolayer to the 3b state. Langmuir 2011, 27(4), 1380–1386
Coulometric analysis associated with the cyclic voltammogram of the Agþ-3a-monolayer electrode yielded a surface coverage of the complex that corresponded to 1.6 10-10 mol 3 cm-2. Chronoamperometric measurements revealed that the rate constant of electron transfer from the electrode to the Agþ-3a complex DOI: 10.1021/la1040807
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Figure 2. (A) Differential pulse voltammetry (DPV) responses obtained upon the interaction of (a) the 3a-monolayer-modified Au electrode and (b) the 3b-monolayer-modified Au electrode, with variable concentrations of Cu2þ. Differential current values were recorded at E = 0.35 and 0.22 V versus SCE for the 3a- and 3b-monolayer-modified electrodes, respectively. (B) Cyclic voltammetry scans corresponding to (a) the 3a-Cu2þ-monolayer-modified Au electrode and (b) the 3b-Cu2þ-monolayer-modified Au electrode. (Inset) Amperometric responses at E = 0.35 V versus SCE obtained upon the cyclic reversible photoisomerization of the 3a/ 3b-Cu2þ-modified electrode at a scan rate of 200 mV s-1. All measurements were performed in an aqueous solution of 0.1 M NaNO3.
corresponded to ket =105 s-1. An analysis of the chronoamperometric transient according to eq 3 led to a surface coverage of 1.8 10-10 mol 3 cm-2, in a good agreement with the value derived from the coulometric analysis of the voltammogram. Figure S3 depicts the amperometric responses of the Agþ-3a monolayermodified electrode generated in the presence of variable concentrations of Agþ ions. The derived association constant corresponds to Ka=3.0 105 M-1. In contrast to the previous system, where both the 3a and 3b photoisomer states revealed affinities for Cu2þ ions (the open 3a state revealed enhanced binding properties), we find that Agþ ions bind only to the open photoisomer state (3a) and lack a binding affinity to the 3b state. This difference may be attributed to the different dimensions of the Agþ and Cu2þ ions (ionic radii of 1.26 and 0.73 A˚, respectively). That is, the larger Agþ ion cannot bind to the sterically crowded piperazine ligand present in 3b. The photoinduced reversible binding of Cu2þ or Agþ ions to the 3a/3b photoisomer states of the monolayer was further confirmed by photoelectron X-ray spectroscopy measurements (Figure 4). The XPS spectra corresponding to the Cu2þ-3a and the Cu2þ-3b monolayer states are depicted in Figure 4A, panels I and II, respectively. The XPS doublet band of the Cu2þ ions 1384 DOI: 10.1021/la1040807
Figure 3. (A) Cyclic voltammograms corresponding to a Au electrode modified with 3a-Agþ. Scan rates: (a) 10, (b) 50, (c) 75, (d) 100, and (e) 200 mV s-1. (Inset) Plot of the anodic peak current as a function of the scan rate. (B) Cyclic voltammograms corresponding to (a) the 3a-Agþ-modified electrode and (b) the 3b-modified electrode treated with Agþ. Scan rate: 100 mV s-1. Prior to the measurements, the electrodes were immersed for 4 h in an aqueous solution of 20 mM Agþ and were then washed with water. All measurements were performed in an aqueous solution of 0.1 M NaNO3. (Inset) Amperometric responses, at E = 0.26 V vs SCE, obtained upon the cyclic photoisomerization of the (3a/3b)-Agþ-modified electrode.
associated with the 3a monolayer is substantially higher in intensity compared to that of the Cu2þ ions bound to the 3b monolayer. These results are consistent with the electrochemical studies that indicate the high association affinity of Cu2þ to the 3a ligand, as compared to that of the 3b ligand. Also, the XPS spectrum of the Agþ ions associated with the 3a monolayer state is depicted in Figure 4B. The 3b monolayer state treated with Agþ does not Langmuir 2011, 27(4), 1380–1386
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Figure 4. High-resolution XPS spectra at a normal photoelectron emission angle (0°) corresponding to (A) (panel I) the 3a-Cu2þ monolayermodified electrode and (panel II) the 3b-Cu2þ monolayer-modified electrode. (B) The 3a-Agþ monolayer-modified electrode. Blue lines indicate experimental spectra, and purple lines indicate deconvolution curves.
association of an ionic species to the electrode. The photoisomerization of the monolayer to the 3b state and the release of Agþ yield a surface with increased hydrophobicity. In conclusion, the present study has introduced a new photoisomerizable monolayer-functionalized Au surface that enabled the photostimulated uptake and release of Cu2þ or Agþ ions. Such functionalized surfaces may be used for the uptake of ions from contaminated solutions and the controlled release of the contaminants for secondary treatment while recycling the activeion-uptaking matrix.
Experimental Section Figure 5. Contact angle changes measured for a droplet of 1 mM Agþ solution placed on a 3b-monolayer-modified Au electrode upon cyclic photoisomerization of the monolayer between (a) the 3b and (b) the 3a states.
show any XPS signal for the Agþ ions. These results are consistent with the electrochemical measurements that demonstrate a high association affinity of Agþ ions to the 3a monolayer and a lack of binding affinity to the 3b monolayer state. Also, the XPS measurements allow us to quantify the relative contents of Cu2þ associated with the 3a or 3b monolayers by comparing the atomic concentrations of the ions relative to the constant carbon atom concentration of the monolayer constituents. We find that the Cu2þ concentrations correspond to 0.66 and 0.27 At% upon their association to the 3a and 3b monolayers, respectively. This implies that the coverage of Cu2þ on the 3a monolayer state is ca. 2.5-fold higher than on the 3b state, consistent with the coulometric analyses. Furthermore, we find that the wetting properties of the monolayer are photoswitchable. Whereas the contact angles of the 3a and 3b monolayers on the Au electrode are nearly indistinguishable, 75 ( 1°, the association of Agþ to the 3a monolayer surface leads to a surface with enhanced hydrophilicity. By the cyclic photoisomerization of the monolayer between the 3a and 3b states, the contact angle of an aqueous 1 mM AgNO3 droplet placed on the monolayermodified electrode was switched between low, 65 ( 1°, and high, 73 ( 1°, values, respectively (Figure 5). These results indicate that the association of Agþ to the 3a-monolayer-modified electrode leads to a surface with enhanced hydrophilicity, consistent with the Langmuir 2011, 27(4), 1380–1386
Materials. Di(N-butanoic acid-1,8-naphthalimide)-piperazine dithienylethene was synthesized according to a previously described procedure.25 Silver nitrate was purchased from Riedel-de Haen. All other materials were obtained from Sigma-Aldrich. Ultrapure water from NANOpure Diamond (Barnstead Int., Dubuque, IA) was used throughout the experiments. Modification of the Electrodes. Clean Au wire (0.5 mm diameter) was immersed overnight in an aqueous solution that contained 10 mM cysteamine. The cysteamine-modified electrode was washed thoroughly in water and immersed in a dimethyl sulfoxide/water (1:9 v/v) solution that contained 1 mM 2a. 1-(3Dimethylaminoproyl)-3-ethylcarbodiimide hydrochloride (EDC, 5 mM) and N-hydroxysulfo succinimide sodium salt (NHS, 5 mM) were then added, and the electrode was kept immersed in the dark for 3 h. The 3-modified electrode was washed thoroughly and then interacted for 4 h in a 20 mM Cu(NO3)2 or a 20 mM AgNO3 solution to obtain the respective metal-ion-bound 3-monolayer-modified electrodes. Prior to the electrochemical measurements, the electrodes were thoroughly washed with an aqueous solution of 0.1 M NaNO3, and the electrochemistry of the metal ions associated with the surface was recorded in a 0.1 M NaNO3 electrolyte solution. For the reversible uptake/release of the metal ions, the electrode modified with the respective metal ion/3a or 3b state was irradiated in a pure buffer solution. The electrodes were then allowed to interact with 20 mM Cu2þ or Agþ ions for 4 h, and the electrochemistry of the resulting monolayers was recorded in a pure 0.1 M NaNO3 electrolyte solution in the absence of the respective metal ions (Cu2þ or Agþ). Instrumentation. The metal-ion-bound 3-monolayer-modified electrodes were photoisomerized from the open, 3a-(Cu2þ/Agþ), to the closed, 3b-(Cu2þ/Agþ), states using a UV lamp (Upland, DOI: 10.1021/la1040807
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Article P = 8 W) at λ = 302 nm for 20 min. Reversible photoisomerization, from the closed to the open states, was performed using a Xe lamp (Oriel Instruments, model 6255OF, 150 W) in an Oriel Research Housing (model 66002 with an Oriel 68700 power supply) at λ > 530 nm for 40 min. These transitions were carried out in an aqueous solution of 0.1 M NaNO3. Electrochemical measurements were performed using a PC-controlled (Autolab GPES software) potentiostat/galvanostat (μAutolab, type III). A graphite rod (d = 5 mm) was used as the counter electrode, and the reference electrode was an SCE. Static contact angle measurements were performed on the modified Au by using a CAM 2000 optical-angle analyzer (KSV Instruments). XPS spectra were measured on a Kratos Axis Ultra X-ray photoelectron spectrometer equipped with a Mg KR source (hν = 1253.6 eV, 150-225 W). The spectra (with an experimental resolution of less than 0.5 eV) were obtained for normal emission by using a pass energy of 20 eV with 0.1 eV steps. The XPS binding (27) Wagner, C. D.; Riggs, W. M.; Davis, L. E.; Moulder, J. F.; Muilenberg, G. E. In Handbook of X-Ray Photoelectron Spectroscopy; Perkin-Elemer: Eden Prairie, MN, 1979.
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Zhang et al. energy was calibrated with respect to the peak position of C 1s at 285.0 eV. Measurements were performed under 1.5 10-9 Torr. The Cu 2p3/2 and the Ag 3d5/2 peaks were measured at 933.1 and 368.1 eV, respectively.27 Prior to the fitting of the peaks, the background noise was subtracted using the Shirley system. Data analysis was performed using Vision processing reduction software (Kratos Analytical Ltd.) and CasaXPS (Casa Software Ltd.).
Acknowledgment. This research was supported by the IsraelChina Cooperation Program, The Israel Ministry of Science, and the EU MOLOC project. J.Z. acknowledges the Chinese Scholarship Council. M.R. acknowledges a CAMBR fellowship. Supporting Information Available: Chronoamperometric transients and differential pulse voltammograms for the (3a/3b)Cu2þ monolayer-modified Au electrode, amperometric responses for the 3a-Agþ monolayer-modified Au electrode pretreated with different Agþ concentrations, and the derivation of the association constants. This material is available free of charge via the Internet at http://pubs.acs.org.
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