Photochemically Controlled Electrochemical Deposition and

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Photochemically Controlled Electrochemical Deposition and Dissolution of Ag0 Nanoclusters on Au Electrode Surfaces† Michael Riskin, Eugenii Katz, Vitaly Gutkin, and Itamar Willner* Institute of Chemistry and Center of Nanoscience and Nanotechnology, The Hebrew UniVersity of Jerusalem, Jerusalem 91904, Israel ReceiVed April 24, 2006. In Final Form: July 10, 2006 A photoisomerizable thiolated nitrospiropyran SP, (1a), monolayer is assembled on a Au electrode by the primary deposition of thiolated nitromerocyanine isomer 1b as a monolayer on the electrode, followed by the irradiation of the surface with visible light, λ > 475 nm. The surface coverage of nitrospiropyran units (1a) on the electrode is 2 × 10-10 mole cm-2. Irradiation of the electrode with UV light, 320 nm < λ < 360 nm, results in the nitromerocyanine, MR, monolayer on the electrode that binds Ag+ ions to the phenolate units. The Ag+ ions associated with the MR monolayer undergo cyclic reduction to surface-confined Ag0 nanoclusters, and reoxidation and dissolution of the Ag0 nanoclusters to Ag+ ions associated with the monolayer are demonstrated. The electron-transfer rate constants for the reduction of Ag+ to Ag0 and for the dissolution of Ag0 were determined by chronoamperometry and correspond to ketred ) 12.7 s-1 and ketox ) 10.5 s-1, respectively. The nanoclustering rate was characterized by surface plasmon resonance measurements, and it proceeds on a time scale of 10 min. The size of the Ag0 nanoclusters is in the range of 2 to 20 nm. The electrochemically induced reduction of the MR-Ag+ monolayer to the MR-Ag0 surface and the reoxidation of the MR-Ag0 surface control the hydrophilic-hydrophobic properties of the surface. The advancing contact angle of the MR-Ag0-functionalized surface is 59°, and the contact angle of the MR-Ag+-monolayerfunctionalized surface is 74°. Photoisomerization of the Ag0-MR surface to the Ag0-SP state, followed by the oxidation of the Ag0 nanoclusters, results in the dissolution of the Ag+ ions into the electrolyte solution.

Introduction Control of the hydrophilic and hydrophobic properties of surfaces by means of external signals has recently attracted attention.1 Photochemical, 2 electrical, 3 magnetic,4 and chemical5 stimuli were used to switch the hydrophilic and hydrophobic properties of surfaces. Recently, the electrochemical reduction of a Ag+-thiolated monolayer to a Ag0-nanocluster-functionalized monolayer and the reversible oxidation of the Ag0 nanoclusters to the Ag+-thiolated monolayer were reported as a means to switch the interface between hydrophobic and hydrophilic states, respectively.6 Different photoisomerizable compounds were reported to reversibly bind and dissociate metal ions by light.7,8 Also, photoisomerizable monolayers associated with electrode †

Part of the Electrochemistry special issue. * To whom all correspondence should be addressed. E-mail: [email protected]. Tel: 972-2-6585272. Fax: 972-2-6527715. (1) Liu, Y.; Mu, L.; Liu, B.; Kong, J. Chem.sEur. J. 2005, 11, 2622-2631. (2) (a) Wang, X.; Zeevi, S.; Kharitonov, A. B.; Katz, E.; Willner, I. Phys. Chem. Chem. Phys. 2003, 5, 4236-4241. (b) Feng, X. J.; Feng, L.; Jin, M. H.; Zhai, J.; Jiang, L.; Zhu, D. B. J. Am. Chem. Soc. 2004, 126, 62-63. (c) Rosario, R.; Gust, D.; Garcia, A. A.; Hayes, M.; Taraci, J. L.; Clement, T.; Dailey, J. W.; Picraux, S. T. J. Phys. Chem. B 2004, 108, 12640-12642. (d) Jiang, W. H.; Wang, G. J.; He, Y. H.; Wang, X. G.; An, Y. L.; Song, Y. L.; Jiang, L. Chem. Commun. 2005, 3550-3552. (3) (a) Lahann, J.; Mitragotri, S.; Tran, T. N.; Kaido, J. H.; Sundaram, I. S.; Choi, S. H.; Somorjai, G. A.; Langer, R. Science 2003, 299, 371-374. (b) Wang, X.; Kharitonov, A. B.; Katz, E.; Willner, I. Chem. Commun. 2003, 1542-1543. (c) Katz, E.; Lioubashevsky, O.; Willner, I. J. Am. Chem. Soc. 2004, 126, 1552015532. (4) Katz, E.; Sheeney-Haj-Ichia, L.; Basnar, B.; Felner, I.; Willner, I. Langmuir 2004, 20, 9714-9719. (5) (a) Jiang, Y. G.; Wang, Z. Q.; Yu, X.; Shi, F.; Xu, H. P.; Zhang, X. Langmuir 2005, 21, 1986-1990. (b) Yu, X.; Wang, Z. Q.; Jiang, Y. G.; Shi, F.; Zhang, X. AdV. Mater. 2005, 17, 1289-1293. Whitesides, G. M.; Laibinis, P. E. Langmuir 1990, 6, 87. (6) Riskin, M.; Basnar, B.; Chegel, V. I.; Katz, E.; Willner, I.; Shi, F.; Zhang, X. J. Am. Chem. Soc. 2006, 128, 1253-1260. (7) (a) Blank, M.; Soo, L. M.; Wassermann, N. H.; Erlanger, B. F. Science 1981, 214, 70-72. (b) Shinkai, S.; Shigematsu, K.; Ogawa, T.; Minami, T.; Manabe, O. Tetrahedron Lett.1980, 21, 4463-4466. (c) Shinkai, S.; Minami, T.; Kusano, Y.; Manabe, O. J. Am. Chem. Soc. 1982 , 104, 1967-1972.

surfaces were widely employed to control electrocatalytic and bioelectrocatalytic processes, to reversibly bind biomolecules, and to act as logic gates and information processing systems.9,10 In these contexts, nitrospiropyran derivatives associated with electrodes were used to regulate electron-transfer processes at electrodes and to reversibly associate/dissociate biomolecules to and from the electrode surface.11 Also, the photoisomerizable functions of nitrospiropyran compounds were used to develop logic gates in solutions12 and to bind metal ions selectively to the merocyanine photoisomer state.13 Different studies have addressed the formation of complexes between metal ions, such as Na+, Li+, Hg2+, Zn2+, Cd2+, and Cu2+, with merocyanine (8) (a) Shinkai, S.; Shigematsu, K.; Kusano, Y.; Manabe, O. J. Chem. Soc., Perkin Trans. 1981, 1, 3279-3283. (b) Shinkai, S.; Nakaji, T.; Ogawa, T.; Shigematsu, K.; Manabe, O. J. Am. Chem. Soc. 1981, 103, 111-115. (c) Shinkai, S.; Ogawa, T.; Kusano, Y.; Manabe, O.; Kikukawa, K.; Goto, T.; Matsuda, T. J. Am. Chem. Soc. 1982, 104, 1960-1967. (9) (a) Shipway, A. N.; Willner, I. Acc. Chem. Res. 2001, 34, 421-432. (b) Willner, I. Acc. Chem. Res. 1997, 30, 347-356. (c) Lion-Dagan, M.; Katz, E.; Willner, I. J. Am. Chem. Soc. 1994, 116, 7913-7914. (d) Willner, I.; Lion-Dagan, M.; Marx-Tibbon, S.; Katz, E. J. Am. Chem. Soc. 1995, 117, 6581-6592. (10) (a) Doron, A.; Portnoy, M.; Lion-Dagan, M.; Katz, E.; Willner, I. J. Am. Chem. Soc. 1996, 118, 8937-8944. (b) Willner, I.; Pardo-Yissar, V.; Katz, E.; Ranjit, K. T. J. Electroanal. Chem. 2001, 497, 172-177. (c) Liu, Z.; Hashimoto, K.; Fujishima, A. Nature 1990, 347, 658-660. (d) Wang, R.; Jiang, L.; Iyoda, T.; Fujishima, A. Langmuir 1996, 12, 2, 2052-2057. (11) (a) Willner, I.; Doron, A.; Katz, E.; Levi, S.; Frank, A. J. Langmuir 1996, 12, 946-954. (b) Willner, I.; Doron, A.; Katz, E. J. Phys. Org. Chem. 1998, 11, 546-560. (c) Blonder, R.; Levi, S.; Tao, G. L.; Ben-Dov, I.; Willner, I. J. Am. Chem. Soc. 1997, 119, 10467-10478. (d) Doron, A.; Katz, E.; Tao, G.; Willner, I. Langmuir 1997, 13, 1783-1790. (12) (a) Andreasson, J.; Terazono, J.; Albinsson, B.; Moore, T. A.; Moore, A. L.; Gust, D. Angew. Chem., Int. Ed. 2005, 44, 7591-7594. (b) Gust, D.; Moore, T. A.; Moore, A. L. Chem. Commun. 2006, 11, 1169-1178. (c) Straight, S. D.; Andreasson, J.; Kodis, G.; Bandyopadhyay, S.; Mitchell, R. H.; Moore, T. A.; Moore, A. L.; Gust, D. J. Am. Chem. Soc. 2005, 127, 9403-9409. (13) (a) Voloshin, N. A.; Chernyshev, A. V.; Metelitsa, A. V.; Besugliy, S. O.; Voloshina, E. N.; Sadimenko, L. P.; Minkin, V. I. ARKIVOC 2004, xi, 16-24. (b) Wojtyk, J. T. C.; Kazmaier, P. M.; Buncel, E. Chem. Commun. 1998, 17031704. (c) Wojtyk, J. T. C.; Kazmaier, P. M.; Buncel, E. Chem. Mater. 2001, 13, 2547-2551. (d) Tahara, R.; Morozumi, T.; Nakamura, H. J. Phys. Chem. B 1997, 101, 7736-7743.

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Scheme 1. Assembly of the Photoisomerizable Nitrospiropyran/Nitromerocyanine Monolayer on a Gold Electrode and the Reversible Electrochemical Generation and Dissolution of Ag0 Nanoclusters on the Monolayer

derivatives.14 Also, the formation of metal ion-modified thiolated monolayers and the reductive metallization of the ions to surfaceconfined clusters were examined.15 In the present study, we describe the photochemically controlled electrochemical deposition and dissolution of Ag0 nanoclusters on an electrode by the photoisomerization of the thiolated nitrospiropyran monolayer associated with a Au electrode. We demonstrate that the functionalized surface allows the reversible formation of Ag0 nanoclusters and the subsequent photochemical/electrochemical dissolution of them. Besides the demonstration that the surface properties (e.g., hydrophilicity/hydrophobicity) can be controlled by the reversible electrochemical formation of Ag0 nanoclusters and their dissolution, the results have a broader impact because they reveal a method to concentrate metal ions on surfaces and (14) (a) Winkler, J. D.; Deshayes, K.; Shao, B. In Biological Applications of Photochemical Switches; Morrison, H., Ed.; Wiley-Interscience: New York, 1993; pp 169-196. (b) Shimidzu, T.; Yoshikawa, M. J. Membr. Sci. 1983, 13, 1-13. (c) Sasaki, H.; Ueno, A.; Anzai, J.; Osa, T. Bull. Chem. Soc. Jpn. 1986, 59, 1953-1956. (d) Inouye, M.; Ueno, M.; Kitao, T.; Tsuchiya, K. J. Am. Chem. Soc. 1990, 112, 8977-8979. (e) Winkler, J. D.; Deshayes, K. J. Am. Chem. Soc. 1987, 109, 2190-2191. (15) (a) Kind, H.; Bittner, A. M.; Cavalleri, O.; Kern, K. J. Phys. Chem. B 1998, 102, 7582-7589. (b) Cavalleri, O.; Kind, H.; Bittner, A. M.; Kern, K. Langmuir 1998, 14, 7292-7297.

release them in a controlled process. Thus, the system might be employed for the removal of toxic ions from the environment. Experimental Section Au-coated (50 nm gold layer) glass plates (22 × 22 mm2) (Analytical µ-Systems, Germany) were used as working electrodes. Prior to the modification, the Au surfaces were flame-annealed for 5 min in an n-butane flame and cooled for 10 min under a weak stream of Ar. The thiol derivative 1-(4-mercaptobutyl)-3,3′-dimethyl-6′,8′dinitrospiro(2′H-1-benzopyran-2′,2-indoline) (1a) was synthesized according to the published procedure11a and assembled on Au electrodes as shown in Scheme 1, using an ethanolic solution of 1b with a modification time of 2 h at 70 °C. The obtained monolayer was then washed with ethanol and water, dried, and irradiated for 15 min under visible light (λ > 475 nm) to produce the spiropyran (SP) state. The electrode was then immersed into a 0.05 M K2SO4 solution at pH 7.4 and irradiated with long-wavelength UV light, 320 nm < λ < 360 nm, to generate the ca. 10% deprotonated merocyanine monolayer (i.e., the MR state; MR pKa is ca. 8.6).16 The modification with silver ions took place by the subsequent (16) Kato, A.; Aizawa, M.; Suzuki, S. J. Membr. Sci. 1976, 1, 289.

Deposition and Dissolution of Ag0 Nanoclusters immersion of the MR-Au electrode in the dark in a 0.05 M K2SO4 aqueous solution at pH 7.4 that included 25 mM AgNO3 (after deaeration for 15 min with Ar). The resulting electrode was thoroughly washed with water, dried with nitrogen, and cycled 5-10 times in 0.05 M K2SO4 at pH 7.4 using a scan rate 20 mV s-1 to exclude the presence of nonspecifically adsorbed silver. 6-Nitro-1′,3′,3′-trimethylspiro(2H-1-benzopyran-2,2′-indolin) 1′,3′dihydro-1′3′3′ trimethyl-6-nitrospiro (2a) was purchased from Aldrich. Ultrapure water from a Nanopure (Barnstead) source was used throughout this work. Electrochemical, Contact Angle, Surface Plasmon Resonance (SPR), Energy Dispersive Spectroscopy (EDS), Scanning Electron Microscopy (SEM), and Spectrophotometric Measurements. The cyclic and linear sweep voltammetry as well as in situ measurements under a constant applied potential on the modified electrode were performed using an electrochemical PC-controlled analyzerpotentiostat IVIUM (PalmSensPC software). The chronoamperometric measurements were performed on an electrochemical analyzer potentiostat/galvanostat (EG&G model 283) connected to a computer (EG&G software no. 270/250). In situ electrochemical measurements coupled with static contact angle measurements were performed on the modified Au-coated glass plates using a CAM2000 optical angle analyzer (KSV Instruments, Finland) and the PalmSense potentiostat. An aqueous droplet of ca. 20 µL of 0.05 M K2SO4 at pH 7.4 with the controlled diameter of the footprint of 0.3 cm was deposited from a syringe on the modified surface (wired as a working electrode). Then the counter electrode wire (Pt wire with a diameter of 0.1 mm) and the quasi-reference electrode (Ag wire with a diameter of 0.1 mm) were introduced into the droplet.17 All of the potentials are reported versus the silver wire quasi-reference electrode, calibrated according to the relation EAg/AgCl(KCl,sat) + 0.11 V ) EAg wire. Prior to contact angle measurements, the droplet was kept under the applied potential for 5 min in order to achieve its equilibrium shape. The images and the contact angles of the droplets were recorded, and each contact angle value was measured at least 10 times. The derived average contact angle values have a precision of (0.1°. The Ag+MR/Au system was electrochemically reduced to generate Ag0 nanoclusters associated with the modified MR surface and then back oxidized to the Ag+ ionic state. These electrochemical processes were performed using a three-electrode configuration in the droplet while measuring the contact angles in situ. The SEM measurements were performed on the respective modified Au-coated glass plates using a UHR FEG SEM, model SIRION (FEI), with a resolution of 1.5 nm. The EDS analysis was performed using a Phoenix spectrometer (EDAX) with a resolution of 128 eV. Quantification of the atomic percentages in the EDS spectra was done by a ZAF matrix correction system. The surface plasmon resonance (SPR) Kretschmann-type spectrometer NanoSPR (light-emitting diode light source λ ) 670 nm) and Au-covered glass slides (1.5 cm2 area exposed to the solution) from Analytical µ-Systems (Germany) were used in this work. An auxiliary Pt and a quasi-reference Ag electrode made from wires of 0.5 mm diameter were part of the cell, thus allowing in situ electrochemical SPR measurements upon performing the electrochemical processes on the modified surface. Absorption spectra were recorded on a Shimadzu UV-2401 PC UV-vis spectrophotometer. The association constant for the Ag+merocyanine complex was determined spectroscopically at 25° ( 1° using the Benesi-Hildebrand relation.18

Results and Discussion The assembly of the photoisomerizable monolayer on the Au electrode is depicted in Scheme 1. In the first step, the thiolated nitromerocyanine MR, 1b, was assembled on a Au electrode. (The MR state exhibits improved solubility in an ethanol solution.) The nitrospiropyran (SP) monolayer photoisomerizes by UV (17) Wang, X.; Gershman, Z.; Kharitonov, A. B.; Katz, E.; Willner, I. Langmuir 2003, 19, 5413-5420. (18) Benesi, H.; Hildebrand, J. H. J. Am. Chem. Soc. 1949, 71, 2703-2707.

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Figure 1. Absorption spectra of (a) nitromerocyanine (2b) generated by the irradiation of nitrospiropyran (2a), 320 nm < λ < 360 nm (0.3 mM in acetone), and the subsequent spectral changes upon the addition of different concentrations of AgNO3: (b) 0.03, (c) 0.3, and (d) 3 mM.

light (320 < λ < 360) to the nitromerocyanine (MR) state, whereas the MR monolayer is back isomerized to the SP monolayer by visible light (λ > 475 nm). The surface coverage of the nitrospiropyran monolayer was determined by coulometric analysis of the 4e-/4H+ irreversible electrochemical reduction of the nitro substituents to the NHOH group to be 0.21 nmole cm-2. Taking into account the footprint area of 1a, the surface coverage corresponds to ca. 80% of a random, densely packed monolayer. On the basis of the photochemical features of photoisomerizable compound 2 in solution, we assume a 9598% photochemical interconversion between states 1a and 1b on the surface. The electrodes used for the determination of the surface coverage were discarded after the electrochemical analysis of the surface content because the monolayer is irreversibly transformed to a nonphotoisomerizable state. Treatment of the MR monolayer with Ag+ ions yields the phenolate-Ag+ complex, whereas no binding of Ag+ to the SP-monolayer-electrode was detected. The binding properties of Ag+ to the merocyanine state were examined in an acetone solution by following the spectral changes of 2 in the presence of variable concentrations of Ag+ (Figure 1).13 Using the Benesi-Hildebrand relation,18 the derived association constant corresponds to Ka ) 1.2 × 104 M-1. Curve a of Figure 2A shows the cyclic voltammogram corresponding to the merocyanine-functionalized monolayer treated with Ag+ ion solution and effectively rinsed afterward. A quasi-reversible wave corresponding to the reduction of Ag+ to Ag0 is observed at 0.06 V, accompanied by an oxidation wave at 0.2 V. Coulometric analysis of the Ag+ reduction wave indicates a surface coverage of 0.19 nmole cm-2, consistent with the surface coverage of the SP/MR monolayer, provided the ratio of the bound silver ions and MR molecules of the monolayer is ca. 1:1. The Ag+-MR monolayer reveals reasonable stability, and the same cyclic voltammogram could be recorded upon applying 50 scans of the potential. Curve b of Figure 2A shows the cyclic voltammogram (10th cycle) after exposing the Ag+-MR surface to irradiation by visible light, which resulted in the isomerization of the MR monolayer back to the nitrospiropyran form. No redox waves were observed in this case (after several cycles), suggesting that all of the redox species dissolved into the bulk. Figure 2B shows the cyclic voltammograms of the Ag+-MR-monolayer electrode recorded at different scan rates. The anodic or cathodic

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Figure 3. Chronoamperometric transients corresponding to (a) the reduction of the Ag+-MR to the Ag0-MR state by stepping the potential from 0.3 to -0.25 V and (b) the oxidation of the Ag0-MR to the Ag+-MR state by stepping the potential from -0.25 to 0.3 V. (Insets) Analysis of the cathodic transient (a) and of the anodic transient (b), according to eq 1. The experiments were performed in 0.05 M K2SO4, pH 7.4, under Ar.

Figure 2. (A) Cyclic voltammograms of (a) the Ag+-MR functionalized electrode and (b) after photoisomerization of the Ag0MR, λ > 475 nm, and cycling between the potentials of 0.3 and -0.25 V for three cycles. Data were recorded in 0.05 M K2SO4, pH 7.4, scan rate 20 mV s-1, under Ar. (Inset) Cyclic anodic responses observed upon the oxidation of Ag0 nanoclusters associated with the MR monolayer (a points) and the electrical responses of the SP monolayer (b points) after electrochemical dissolution of the Ag0 nanoclusters. (B) Cyclic voltammograms of the Ag+-MR/Au electrode recorded at different potential scan rates: (a) 10, (b) 20, (c) 30, (d) 50, (e) 75 and (f) 100 mV s-1. (Inset) Dependence of the anodic (a) and cathodic (b) peak currents of the cyclic voltammograms on the potential scan rate. The experiments were performed in 0.05 M K2SO4, pH 7.4, under Ar.

peak currents reveal a linear relation with the scan rate, Figure 2B inset, implying that the Ag+/Ag0 electroactive species are indeed surface-confined. Treatment of the SP-monolayerfunctionalized electrode with the Ag+ solution followed by rinsing of the surface did not lead to any electrochemical response, indicating that no Ag+ ions were associated with the SPmonolayer electrode. Irradiation of the Ag0-MR-monolayer electrode with visible light (λ > 475 nm) resulted in the SP monolayer with physically adsorbed Ag0 nanoclusters that were completely removed from the surface upon three potential cycles in the region of 0.3 to -0.25 V and back, Figure 2A, curve b. That is, the transformation of the Ag0 nanoclusters to Ag+ in the presence of the SP monolayer resulted in the dissolution of the ions into the electrolyte solution. Re-photoisomerization of the monolayer to the MR state and its treatment with Ag+ ions regenerated the stable Ag+-MR-monolayer electrode that revealed the stable quasi-reversible Ag+/Ag0 transformation. The inset of Figure 2A shows the cyclic, photochemically controlled electrochemical generation of the Ag0 nanoclusters on the MR monolayer (points “a”), followed by the photoisomerization of the monolayer to the SP state and the electrochemical dissolution of the Ag0 nanoclusters (points “b”). The photochemical regeneration of the MR-monolayer state and its treatment with

Ag+ ions restored the electrochemical activity of the Ag+/Ag0 redox process. The electron-transfer rate constant for the reduction of the Ag+-MR-monolayer electrode was characterized by chronoamperometric experiments. Curve a of Figure 3 shows the current transient observed upon the reduction of the Ag+-MR monolayer to theAg0 state, whereas curve b depicts the current transient observed upon the oxidation of Ag0 to the Ag+-MR monolayer. These current transients follow equation 1, where ket corresponds to the respective electron-transfer rate constant and Q corresponds to the charge associated with the respective electron-transfer process (reduction or oxidation).19

I(t) ) ketQ exp(-kett)

(1)

The insets of Figure 3 show the analysis of the respective current transients according to eq 1. The electron-transfer rate constant for the reduction of Ag+ to Ag0 corresponds to 12.7 s-1, and the surface concentration of silver, calculated from the charge associated with the process, is 0.2 nmole cm-2. The electrontransfer rate constant for the oxidation of the Ag0 monolayer to the Ag+-MR state is ket ) 10.5 s-1, and the surface concentration of silver, calculated from the charge associated with the process, is 0.21 nmole cm-1. The values are in good agreement with the coulometric analysis of the cyclic voltammograms. The results also indicate that the Ag+ and Ag0 species stay intact with the monolayer upon the redox reactions. To further understand the electrochemical transformation of the Ag+-MR monolayer to the Ag0-MR state, we performed SEM measurements on the Au substrate. Figure 4A shows the SEM image of the Ag+-MR-monolayer-modified Au surface whereas Figure 4B shows the SEM image of the surface after the reduction of the Ag+-MR monolayer. Clearly, upon the reduction of the Ag+-MR monolayer, clusters of variable dimensions in the region of 2 to 20 nm are observed. The EDS analysis of the clusters, Figure 4B, inset b, indicates that the clusters consist of silver with a characteristic band at 3 keV. The EDS spectrum of a domain that lacks clusters reveals the characteristic spectrum of the Au background surface. The EDS spectrum of the Ag+-MR monolayer, Figure 4A, inset a, shows a minute band of silver species. The two-dimensional coverage of the Ag+ ions leads to a low-intensity band. The SEM images (19) Katz, E.; Willner, I. Langmuir 1997, 13, 3364-3373.

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Figure 4. (A) SEM image of the Ag+-MR-functionalized Au surface generated by the application of a potential corresponding to 0.25 V for 5 min and the corresponding EDS spectrum (a) of the Ag+-MR-functionalized Au surface. (B) SEM image of the Ag0-MR-functionalized Au surface, generated by the application of a constant potential of -0.25 V for 10 min and the corresponding EDS spectra of (a) a domain lacking the Ag0 nanoclusters and (b) a domain containing Ag0 nanoclusters.

shown in Figure 4 are fully reversible. The oxidation of the surface that includes the Ag0 nanoclusters on the MR monolayer restores the surface shown in Figure 4A, and the re-reduction of the Ag+-MR monolayer regenerates the Ag0 nanoclusters. Also, the photoisomerization of the Ag0-MR monolayer to the Ag0 nanoclusters associated with the SP monolayer, followed by the oxidation of the nanoclusters, results in a surface that does not yield any further formation of the Ag0 nanoclusters. This latter result is consistent with the electrochemical experiments that demonstrated that the oxidation of the Ag0-SP monolayer configuration causes the dissolution of the resulting Ag+ ions into the bulk. Control experiments reveal that the unmodified gold substrate exhibits a surface morphology identical to that of the thiol-modified surface (Figure 4A), implying that the structural features correspond to grain boundaries of the gold support. Furthermore, the image shown in Figure 4B suggests that the electrochemically generated Ag0 nanoclusters preferably localize at these grain boundaries. The interaction of the localized plasmon absorbance of metallic nanoparticles with the surface plasmon of the Au surface leads

to a substantial shift in the surface plasmon resonance energy.20 Indeed, Au nanoparticles were used as labels to follow biorecognition events between antigens and antibodies21 or nucleic acids and DNA22 and also to monitor biocatalytic transformations23 by following the surface plasmon resonance (SPR) shifts as a result of the interaction of the localized plasmon of Ag0 nanoparticles with the surface plasmon. The electrochemically induced formation of Ag0 nanoclusters on the surface suggests that such interactions between the Ag0 nanoclusters and the surface plasmon should exist, and thus the surface plasmon resonance features of the system could reflect the Ag+/Ag0 ion/nanocluster transitions. Figure 5A shows SPR spectra observed upon the (20) (a) Lyon, L. A.; Pena, D. J.; Natan, M. J. J. Phys. Chem. B 1999, 103, 5826-5831. (b) Chah, S.; Hutter, E.; Roy, D.; Fendler, J. H.; Yi, J. Chem. Phys. 2001, 272, 127-128. (21) Lyon, L. A.; Musick, M. D.; Natan, M. J. J. Anal. Chem. 1998, 70, 5177-5183. (22) He, L.; Musick, M. D.; Nicewarner, S. R.; Sallinas, F. G.; Benkovic, S. J.; Natan, M. J.; Keating, C. D. J. Am. Chem. Soc. 2000, 122, 9071-9077. (23) Zayats, M.; Pogorelova, S. P.; Kharitonov, A. B.; Lioubashevski, O.; Katz, E.; Willner, I. Chem.sEur. J. 2003, 9, 6108-6114.

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Figure 6. Cyclic in situ contact angles of a 0.05 M K2SO4 aqueous droplet, pH 7.4, corresponding to (a) the Ag+-MR monolayer state and (b) the Ag0-MR monolayer configuration. Points c and d correspond to the contact angles of SP and MR monolayers in the absence of Ag+, respectively.

Figure 5. (A) Time-dependent SPR spectra upon reduction of the Ag+-MR monolayer to the Ag0-MR state by applying a potential step from 0.3 to -0.25 V. (Inset) Time-dependent reflectivity changes at a constant angle of 62° upon reduction of the Ag+-MR monolayer to the Ag0-MR state. (B) Time-dependent SPR spectra upon oxidation of the Ag0-MR monolayer to the Ag+-MR state by applying a potential step from -0.25 to 0.3 V. (Inset) Time-dependent reflectivity changes at a constant angle of 62° upon oxidation of the Ag0-MR monolayer to the Ag+-MR state. Point a shows the timing of oxidative potential step application on the Ag0-MR-modified electrode. (C) Reversible changes in the reflectivity values of the Ag+/0/MR/Au system at a constant angle of 62° upon cyclic application of potentials corresponding to 0.3 and -0.25 V on the modified electrode. The experiments were performed in 0.05 M K2SO4, pH 7.4, under Ar.

reduction of the Ag+-MR monolayer to the Ag0 nanoclusters. With respect to the time-dependent shift to lower SPR angles, the inset of Figure 5A shows the time-dependent shifts at a constant reflectivity angle of 62°. The time-dependent reflectivity changes are slow and proceed for ca. 10 min until they reach a constant equilibrium value. We realize that the kinetics of the SPR reflectivity changes are significantly different as compared to the electron-transfer rates for the reduction of the Ag+-MR monolayer to the Ag0-MR state. This apparent discrepancy is explained by the fact that the chronoamperometric measurements follow the electron-transfer rate for the reduction of Ag+ to the respective Ag0 atoms, a process that is relatively fast, whereas the changes in the SPR spectra monitor the dynamics of

nanoclustering of Ag0 on the surface, a process that is substantially slower. That is, the SPR spectra monitor the lateral movement of the Ag0 atoms and their clustering process on the surface. It should be noted that the clusters are not washed off of the nitromerocyanine monolayer. Furthermore, the generated Ag0 nanoclusters reveal a relatively stable configuration (on a time scale of at least 24 h), and no further agglomeration of the nanoclusters was detected. These results indicate that the Ag0 nanoclusters exhibit surface affinity to the nitromerocyanine monolayer. The stability of the nanoclusters against further agglomeration presumably originates from their concentration in stabilizing surface grain boundaries as indicated by the SEM image (Figure 4B). Figure 5B depicts the change in the SPR spectra of the system upon oxidation of the Ag0 nanoclusters associated with the MR monolayer. The minimum reflectivity (SPR) angle is shifted to a higher value, characteristic of the Ag+-MR monolayer. Figure 5C shows the cyclic changes in the minimum reflectivity angle upon the electrochemically induced Ag0 nanocluster formation and dissolution. The inset of Figure 5B shows the kinetics of the changes in the reflectivity angle (at θ ) 62°) as a result of the oxidation of the Ag0 nanoclusters to the Ag+-MR monolayer. These changes proceed relatively fast and are completed within ca. 0.6 min. The time scale for the dissolution of the Ag0 nanoclusters reflected by the SPR spectrum is similar to the electron-transfer rate for the oxidation of the nanoclusters. Another aspect related to the Ag+/Ag0 electrochemical transitions on the MR monolayer involves the control of the hydrophobic/hydrophilic properties of the interface. In our previous study,6 we demonstrated that upon electrochemical transformation of a Ag+-thiolated monolayer to a Ag0-nanocluster-modified monolayer the surface properties turn from hydrophobic to hydrophilic, respectively. Figure 6 shows that the contact angle value of the Ag+-MR monolayer is 74 ( 2°, whereas the contact angle of the surface consisting of Ag0 nanoclusters on the MR monolayer is 59 ( 2°. That is, the Ag+MR monolayer reveals enhanced hydrophobicity, as compared to that of the Ag0-MR state. This surface property is fully reversible, and upon cycling the monolayer between the Ag+MR and Ag0-MR states the surface is transformed from a hydrophilic to hydrophobic interface, points a and b, respectively. Photoisomerization of the monolayer followed by the oxidation of the Ag0 nanoclusters yields a surface with a contact angle of 81°, point c, reflecting hydrophobic properties of the monolayer with respect to the nitrospiropyran monolayer. Photoisomerization of the nitrospiropyran monolayer to the MR monolayer state (lacking Ag+) yields a surface of enhanced hydrophilicity,

Deposition and Dissolution of Ag0 Nanoclusters

exhibiting a contact angle of 75°, point d. The changes in the contact angle values corresponding to the Ag+- and Ag0nitromerocyanine surfaces are moderate. The use of other metals (e.g. Hg2+) might, however, enhance the difference in the hydrophobic/hydrophilic properties of the surfaces. Our studies enabled the kinetic analysis of electron transfer and metal nanoclustering on the surface. Besides the interesting basic phenomena associated with the electrochemical Ag+/Ag0 transformations on the photoisomerizable nitrospiropyran/MR monolayer, we revealed that macroscopic hydrophobic/hydrophilic properties of the surfaces are regulated by these coupled electrochemical/photochemical transformations. To conclude, the present study has introduced a nitrospiropyran monolayer associated with a Au surface as a photocontrolled matrix for the selective uptake of Ag+ by the merocyanine isomer and the reversible reduction to Ag0 clusters. Electrochemical oxidation of the Ag0 nanoclusters resulted in the redistribution of the Ag+ ions on the MR monolayer, without leakage of the

Langmuir, Vol. 22, No. 25, 2006 10489

ions to the electrolyte solution. Photoisomerization of the Ag0MR-monolayer-modified electrode to the Ag0-SP state, followed by electrochemical oxidation of the Ag0 clusters to Ag+, resulted in the dissolution and elimination of the ions from the surface. These results suggest that functional surfaces for the removal of the metallic ions (e.g., pollutants) and their subsequent removal from the surface by coupled electrochemical/photochemical means may be realized. Acknowledgment. This study is supported by the GermanIsraeli Program (DIP) and by the Israel Science Foundation. We also thank the unit for Nanoscopic Characterization of The Hebrew University of Jerusalem. Supporting Information Available: SPR spectra of the MR monolayers at different potentials and in the absence of Ag+. This material is available free of charge via the Internet at http://pubs.acs.org. LA061101Z