Preparation, Characterization, and Electrocatalytic Activity of Surface

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Preparation, Characterization, and Electrocatalytic Activity of Surface Anchored, Prussian Blue Containing Starburst PAMAM Dendrimers on Gold Electrodes E. Bustos, J. Manrı´quez, G. Orozco, and Luis A. Godıńez* Electrochemistry Department, Centro de Investigacio´ n y Desarrollo Tecnolo´ gico en Electroquı´mica S.C., P.O. Box 064, C.P. 76700, Pedro Escobedo, Quere´ taro, Me´ xico Received October 12, 2004. In Final Form: January 4, 2005 Gold bead electrodes were modified with submonolayers of 3-mercaptopropionic acid or 2-aminoethanethiol and further reacted with poly(amidoamine) (PAMAM) dendrimers (generation 4.0 and 3.5, respectively) to obtain films on which Prussian Blue (PB) was later absorbed to afford mixed and stable electrocatalytic layers. Experiments carried out with these novel materials not only showed an improved surface coverage of PB on the dendrimer modified electrodes as compared to PB modified gold electrodes prepared under acidic conditions, but also showed an increased stability at neutral pH values for one of the dendrimer containing substrates where the PB film on a bare gold electrode is simply not formed. The dendrimer modified electrodes were also tested as electrocatalytic substrates for the electroxidation of L(+)-ascorbic acid (AA), and it was found that their sensitivity as well as the corresponding detection limits were improved as compared to the voltammetric response of a Au-PB modified electrode. On the basis of UV-visible (UV-vis) spectroscopy and electrochemical experiments, it is suggested that the PB molecules are located within the dendritic structure of the surface attached PAMAM dendrimers.

Introduction Prussian Blue (PB), or iron(III) ferrocyanide (Fe4[Fe(CN)6]3‚14H2O),1 is one of the longest known coordination compounds1a,2 which, by virtue of its redox and electrochromic properties, has been extensively studied in basic and technologically oriented research. Due to its low solubility in aqueous acidic solutions,3 PB has been successfully adsorbed on the surfaces of different electrode materials and it has been found to be a good electrocatalyst for several specific reactions, among which the electrooxidation of L(+)-ascorbic acid (AA)3b,4 and the electroreduction of H2O22b,4b,c,5e are two of the most representative heterogeneous reactions explored so far. The main problem of the chemically modified electrodes reported to date, however, relies on the fact that the electrocatalytic film of PB is stable only at low pH values (pH < 5),2b,4b-c,5e and therefore, its integrity and activity are seriously compromised by bulk and local changes in pH that often appear * Corresponding author. E-mail: [email protected]. Phone: 52-442-211-6026. Fax: 52-442-211-6001. (1) (a)Buser, H. J.; Ludi, A. Chem. Commun. 1972, 1299. (b) Buser, H. J.; Schwarzenbach, D.; Petter, W.; Ludi, A. Inorg. Chem. 1977, 16, 2704. (c) Dostal, A.; Meyer, B.; Scholz, F.; Schro¨der, U.; Bond, A. M.; Marken, F.; Shaw, Sh. J. J. Phys. Chem. 1995, 99, 2096. (d) Buschmann, W. E.; Miller, J. S. Inorg. Chem. 2000, 39, 2411. (e) Verdaguer, M.; Galvez, N.; Garde, R.; Desplanches, C. ECS Interface 2002, 28. (2) (a)Garcia-Jarenõ, J. J.; Navarro, J. J.; Roig, A. F.; Scholl, H.; Vicente, F. Electrochim. Acta 1995, 9, 1113. (b) Karyakin, A. A.; Karyakina, E. E. Russ. Chem. Bull. Int. Ed. 2001, 10, 1811. (3) (a) Ellis, D.; Eckhoff, M.; Neff, V. D. J. Phys. Chem. 1981, 85, 1225. (b) No´brega, J. A.; Lopes, G. S. Talanta 1996, 43, 971. (4) (a) Li, F.; Dong, Sh. Electrochim. Acta 1987, 10, 1511. (b) Scharf, U.; Grabner, E. W. Electrochim. Acta 1996, 2, 233. (c) Jaffari, S. A.; Turner, A. P. F. Biosens. Bioelectron. 1997, 1, 1. (d) Castro, S. S. L.; Balbo, V. R.; Barbeira, P. J. S.; Stradiotto, N. R. Talanta 2001, 55, 249. (e) Selvaraju, T.; Ramaraj, R. Electrochem. Commun. 2003, 5, 667. (5) (a) Karyakin, A. A.; Gitelmacher, O. V.; Karyakina, E. E. Anal. Chem. 1995, 67, 2419. (b) Karyakin, A. A.; Karyakina, E. E.; Gorton, L. Talanta 1996, 43, 1597. (c) Karyakin, A. A.; Karyakina, E. E.; Gorton, L. J. Electroanal. Chem. 1998, 456, 97. (d) Karyakin, A. A.; Karyakina, E. E.; Gorton, L. Anal. Chem. 2000, 72, 1720. (e) Moscone, D.; D’Ottavi, D.; Palleschi., G.; Amine, A. Anal. Chem. 2001, 73, 2529. (f) O’Halloran, M. P.; Pravda, M.; Guilbault, G. G. Talanta 2001, 55, 605.

as a consequence of electron-transfer events in the interphasial region. Dendrimers, on the other hand, also known as cascade polymers, arborols, or hyper-ramified polymers, are a relatively new class of materials that since their appearance in the scientific scene about 20 years ago have attracted the attention of several research groups around the world.6 Dendrimer molecules can be basically described as the result of the sequential modification of a polyfunctional core with multifunctional monomers, also called dendrons. These monomeric units are designed in such a way that, after each growing step, while the radius of the polymer is increased in a linear fashion, the number of terminal groups in the polymer grows in a geometric way as a consequence of the hyper-ramified nature of the dendrons. This feature confers dendrimer molecules a radial gradient of properties that make them a very attractive class of species.6e,7 As can be appreciated in the scheme presented in Figure 1, dendrimers resemble covalent micelles characterized by well defined cavities8 responsible for their particular endoreceptor properties9 and terminal groups that define the solubility,6b the reactivity,10 and the exoreceptor characteristics of the molecule.11 Due to their intrinsic and exciting properties, (6) (a) Freˆchet, J. M. Science 1994, 263, 1710. (b) Newkome, G. R.; Moorefield, C. N.; Vo¨gtle, F. Dendritic Molecules: Concepts, Syntheses and Perspectives; VCH: Weinheim, Germany, 1996. (c) Zeng, F.; Zimmerman, S. Chem. Rev. 1997, 97, 1681. (d) Bosman, A. W.; Janssenand, H. M.; Meijer, W. Chem. Rev. 1999, 99, 1665. (e) Crooks, R. M.; Lemon, B. I., III; Sun, L.; Yeung, L. K.; Zhao, M.; Dendrimers: Design, Dimension and Function; Springer-Verlag: Berlin, 2001. (f) Froehling, P. E. Dyes Pigm. 2001, 48, 187. (g) Tully, D. C.; Freˆchet, J. M. Chem. Commun. 2001, 1229. (7) (a) Zeng, F.; Zimmerman, S. C. Chem. Rev. 1997, 97, 1681. (b) Miura, N.; Dubin, P. L.; Moorefield, C. N.; Newkome, G. R. Langmuir 1999, 15, 4295. (8) Lee, S. C.; Parthasarathy, R.; Duffin, T. D. Y.; Botwin, K.; Zobel, J.; Beck, T.; Lange, G.; Kunneman, D.; Janssen, R.; Rowald, E.; Voliva, C. F. Biomed. Microdevices 2001, 1, 53. (9) (a) Maijer, E. W.; Jansen, J. F. G. A.; De Brabander-van den Berg, M. M. Polym. Mater. Sci. Eng. 1995, 73, 123. (b) Balzani, V.; Ceroni, P.; Gestermann, S.; Gorka, M.; Kauffmann, C.; Maestri, M.; Vo¨gtle, F. Chem. Phys. 2000, 4, 224.

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Figure 1. Scheme showing the systems studied. Whereas 1 and 2 represent Starburst (PAMAM) generation 4.0 and 3.5 dendrimers, respectively (64 -NH2 and -COOH terminal groups), 3 and 4 correspond to the HS(CH2)2COOH and HS(CH2)2NH3Cl species that were used to modify the gold substrates.

dendrimers have been studied as an important new class of potential drug delivery systems,12 protein models,13 molecular antennas,14 and catalytic materials.15 Although most of the work with dendrimers has been carried out in solution, these compounds have also been used to modify electrode surfaces and some recent reports indicate that these materials are capable of increasing the concentration of hydrophobic molecules at the electrode-solution interface, improving in this way the sensitivity as well as the selectivity of certain specific electrochemical reactions.16 In this work, we report the preparation and characterization of poly(amidoamine) (PAMAM)-PB nanocomposite films anchored on previously thiol modified gold electrodes and some preliminary results on the electrocatalytic activity of the resulting films in aqueous media. Experimental Section PAMAM generation 4.0 (1) and generation 3.5 (2) dendrimers (bearing 64 -NH2 and -COOH terminal functional groups, respectively), HS(CH2)2COOH (3), HS(CH2)2NH3Cl (4), and 1-(3(10) Jendruch-Borkowski, B.; Awad, J.; Wasgestian, F. J. Inclusion Phenom. Macrocyclic Chem. 1999, 35, 355. (11) (a) Balogh, L.; Tomalia, D. A. J. Am. Chem. Soc. 1998, 120, 7355. (b) Zhou, Y.; Bruening, M. L.; Bergbreiter, D. E.; Crooks, R. M.; Wells, M. J. Am. Chem. Soc. 1996, 118, 3773. (12) (a) Jansen, J. F. G. A.; De Brabander-van den Berg, E. M. M.; Meijer, E. W. Science 1994, 266, 1226. (b) Jansen, J. F. G. A.; De Brabander-van den Berg, E. M. M.; Meijer, E. W. J. Am. Chem. Soc. 1995, 117, 4417. (13) Zimmerman, S. C.; Wendland, M. S.; Rakow, N. A.; Zharov, I.; Suslick, K. S. Nature 2002, 418, 399. (14) Archut, A.; Azzellini, G. C.; Balzani, V.; De Cola, L.; Vo¨gtle, F. J. Am. Chem. Soc. 1998, 120, 12187. (15) (a) Zhao, M.; Crooks, R. M. Angew. Chem., Int. Ed. 1999, 38, 364. (b) Chechik, V.; Crooks, R. M. J. Am. Chem. Soc. 2000, 122, 1234. (c) Yeung, L. K.; Crooks, R. M. Nano Lett. 2001, 1, 14. (16) Ledesma-Garcıá, J.; Manrı´quez, J.; Gutie´rrez-Granados, S.; Godıńez, L. A. Electroanalysis 2003, 7, 657.

(dimethylamino)propyl)-3-ethylcarbodiimide hydrochloride (EDC) of the best commercially available quality were obtained from Aldrich and used without further purification. Analytical grade Ru(NH3)6Cl3 and L(+)-ascorbic acid (AA) were obtained from Strem Chemicals and from Mallinckrodt, respectively. The supporting electrolyte phosphate buffer solutions (pH 2, 7, and 12; ionic strength, I, of 0.1) were prepared from H3PO4, KH2PO4, Na2HPO4, NaOH, and KOH which were obtained from J. T. Baker. HPLC grade methanol, K4Fe(CN)6‚3H2O, and FeCl3 were also obtained from J. T. Baker. All aqueous solutions were prepared with deionized water (F g 18 MΩ‚cm). The electrochemical experiments reported in this work were performed using a PCI4/300 Potentiostat/Galvanostat/ZRA controlled with a Gamry Instruments software version 4.10 2002 that was installed in a Pentium based PC. The cyclic voltammetry (CV) experiments were performed at 298 K with a 10 mL volume cell equipped with a gold bead working electrode, a platinum counter electrode, and a saturated calomel reference electrode. Before the CV experiments were carried out, the electrolytic solutions were deoxygenated by bubbling ultrapure nitrogen (PRAXAIR, grade 5.0) for at least 10 min. Gold bead electrodes were prepared as previously described in the literature17 by melting the tip of a Au wire (99.999%, 1.0 mm diameter, Premion from Alfa) using a gas burner obtained from Flamineta. For characterization purposes, these Au bead electrodes were immersed in 0.5 mM Ru(NH3)6Cl3 solutions and the geometric area was computed from the CV response of the reversible couple at different scan rates using the RandlesSevick equation.16 It is important to point out that the geometric area thus computed was then considered to be equal to the real area, since electrodes prepared in this way have been reported to have roughness factors between 1.1 and 1.2.17 Electrodes which showed distorted signals for the Ru redox process during the area calculation experiments were further introduced in the flame and subsequently rinsed with plenty of water until a satisfactory CV response was obtained. Electrodes that did not show a clean capacitive region as well as a clear Ru(III)|Ru(II) reversible (17) Godıńez, L. A.; Castro, R.; Kaifer, A. Langmuir 1996, 12, 5087.

Starburst PAMAM Dendrimers on Gold Electrodes

Figure 2. UV-vis spectra of 50 µM PB (A) in the absence and in the presence of 1 µM dendrimers (B) 2 and (C) 1 in phosphate buffered aqueous solution (pH 2, I ) 0.1) at 298 K. electrochemical signal (∆Ep ≈ 60 mV, Ipc/Ipa ≈ 1) after a couple of cleaning cycles were discarded. As can be seen in Figure 1, Au bead electrodes were modified with thiols 3 and 4 followed by the covalent attachment of PAMAM dendrimers 1 and 2 (to prepare the functionalized surfaces Au-3-1 and Au-4-2) using typical self-assembling monolayer and peptidic bond formation protocols as described below. PB incorporation on the surface of the substrates Au, Au-3-1, and Au-4-2, on the other hand, was carried out by dipping the Au or the Au modified electrodes in aqueous phosphate buffered solutions (I ) 0.1) of pH values 2, 7, and 12 also containing equimolar quantities (1.0 mM) of FeCl3 and K4Fe(CN)6‚ 3H2O for 12 h at 298 K under a N2 atmosphere. The electrodes prepared in this way were then taken out from the modifying solution, rinsed with plenty of deionized water, and introduced in a 10 mL electrochemical cell on which the CV experiments discussed below were performed. The UV-visible (UV-vis) spectra were acquired at 298 K using an Agilent model 8453 UV-vis spectrophotometer with a spectral resolution of 2 nm. Raman spectra were obtained using a Renishaw spectrometer using a CCD detector. The laser (λ ) 785 nm) was focused on a 2 µm spot on top of the sample, using an optic Olympus microscope with an objective lens magnification of 50×, and the scattering signal was collected at 180°. Instrument control, data acquisition, and data manipulation were conducted using the GRAMS32 software. Each sample was analyzed by collecting spectra every 2 µm in a linear mapping with a length of 18 µm. The mapping analysis was made in three different zones randomly selected on the sample surface, and all the measurements were made at 298 K.

Results and Discussion PB can be formed by adding FeCl3 and K4Fe(CN)6‚3H2O in equimolar quantities to an aqueous acidic solution. In this way, a pH 2 phosphate buffer solution (I ) 0.1) turns blue upon the addition of FeCl3 and K4Fe(CN)6‚3H2O (1.0 mM each) and a precipitate of PB is formed on time.1a,e,f,5e,f,18 When the same experiment is made in the presence of either 50 µM PAMAM G4.0 (1) or 50 µM PAMAM G3.5 (2), however, the precipitate does not appear even after 2 days, suggesting an interaction between the dendrimer molecules and the PB species (and/or the Fe3+ and Fe(CN)64- ions) that prevents the precipitate from appearing. To find out if PB was actually being formed in the solutions surveyed, aliquots of these solutions were taken and diluted in the same buffer solution, and the UV-vis spectra were obtained as described in the Experimental Section. As can be observed in Figure 2, the spectroscopic response of the three solutions considered

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shows a peak at 697 nm, which has been reported to correspond to a characteristic charge-transfer band of PB (see Figure 2).3b,18c,19 The spectra of the individual species 1, 2, FeCl3, and K4Fe(CN)6‚3H2O or the phosphate buffer solution, on the other hand (data not shown), do not show this peak, revealing that PB is present in the three solutions and that, due to the relative intensity of the relevant peaks, the PB species is actually stabilized in solution by the dendrimer molecules by means of an interaction that is essentially independent of the terminal groups of the dendrimer molecule. The latter observation, under the assumption of similar absorption coefficients, suggests that the interaction of the PB species and the two dendrimer molecules must take place within the polymeric PAMAM host,9 an idea that is fully consistent not only with the characteristic complexation ability of the well defined void regions of these hyperbranched molecules6b,20 but also with a previously detected PBPAMAM dendrimer interaction.19d On the basis of the idea that PAMAM dendrimers are capable of hosting PB molecules, a concept that was reinforced by the experiments that will be later described, we turned our attention to a way of incorporating these hyperbranched molecules into a gold electrode surface, as was previously described by Crooks and co-workers.20 As can be seen in Figure 1, the strategy consisted of modifying clean gold electrodes with either 3 or 4 to construct modified substrates that could be further reacted with the relevant dendrimer molecules, 1 and 2, that bear complementary peripheral functional groups. In this way, clean gold electrodes were prepared as small beads from the tip of a pure gold wire, as described in the Experimental Section. Chemical modification of the clean Au substrates with either 3 or 4 was then carried out by exposing the substrate to 1.0 mM methanolic solutions of the relevant thiol for 12 h at room temperature. Surface modification was confirmed by electrochemical desorption experiments of the previously adsorbed thiol in 0.5 M KOH aqueous solutions, since under these pH conditions the evolution of hydrogen is seriously hindered and the thiol reductive signal is fully resolved.21 The resulting reductive desorption curve observed during the first cycle for at least three different substrates was then used to obtain the charge involved in the process and to compute, along with the electrode area previously obtained, surface coverage values of the thiol molecules chemically adsorbed on the Au surface.22 The average values thus obtained were 1.60 × 10-10 and 2.05 × 10-10 mol cm-2 for 3 and 4, which not (18) (a) Curtman, L. J. Ana´ lisis Quı´mico Cualitativo: Basado en las Leyes de Equilibrio y en la Teorıá de la Ionizacio´ n; Editora Nacional Me´xico: Me´xico City, 1963; pp 221-226. (b) Svehla, G. Vogel’s Qualitative Inorganic Analysis, 6th ed.; Longman Group: Essex, U.K., 1987; pp 169-170. (c) Christie, R. M. RSC Paperbacks: Colour Chemistry; The Royal Society of Chemistry: Cambridge, U.K., 2001; p 158. (d) Ellis, D.; Eckhoff, M.; Neff, V. D. J. Phys. Chem. 1981, 85, 1225. (19) (a) Guo, Y.; Guadalupe, A. R.; Resto, O.; Fonseca, L. F.; Weisz, S. Z. Chem. Mater. 1999, 11, 135. (b) Saliba, R.; Agricole, B.; Mingotaud, Ch.; Ravaine, S. J. Phys. Chem. B 1999, 103, 9712. (c) Pyrasch, M.; Tieke, B. Langmuir 2001, 17, 7706. (d) Zamponi, S.; Kijak, A. M.; Sommer, A. J.; Marassi, R.; Kulesza, P. J.; Cox, J. A. J. Solid State Electrochem. 2002, 6, 528. (e) Pyrasch, M.; Toutianousch, A.; Jin, W.; Schneptf, J.; Tieke, B. Chem. Mater. 2003, 15, 245. (f) Uemura, T.; Kitagawa, S. J. Am. Chem. Soc. 2003, 125, 7814. (20) (a) Crooks, R. M.; Zhao, M.; Sun, L.; Chechik, V.; Yeung, L. K. Acc. Chem. Res. 2001, 34, 181. (b) Niu, Y.; Sun, L.; Crooks, R. M. Macromolecules 2003, 36, 5725. (c) Tokuhisa, H.; Zhao, M.; Baker, L. A.; Phon, V. T.; Dermody, D. L.; Garcıá, M. E.; Peez, R. F.; Crooks, R. M.; Mayer, Th. M. J. Am. Chem. Soc. 1998, 120, 4492. (d) Zhou, Y.; Bruening, M. L.; Bergbreiter, D. E.; Crooks, R. M.; Wells, M. J. Am. Chem. Soc. 1996, 118, 3773. (21) Manrı´quez, J.; Juaristi, E.; Munõz-Mun˜iz, O.; Godıńez, L. A. Langmuir 2003, 19, 7315.

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only are consistent with previously reported values for the adsorption of similar compounds on Au monocrystalline electrodes22,23 but also reveal that the chemically adsorbed thiols actually form submonolayers that cover about 20 and 26% of the whole electrode surface. It is interesting to note that, although the fraction of the electrode surface chemically modified is far less than unity, previous studies suggest that the thiol molecules in this submonolayer are not forming islands or clusters but instead are individually distributed across the electrode surface.21 Therefore, the modified electrodes thus prepared can be considered as “homogeneous” surfaces with uniform distribution of solution facing carboxylic or ammine functional groups that are susceptible to chemical modification, as will be described in the following paragraph. Surface anchoring of dendrimers 1 and 2 on the previously thiol modified gold electrodes, Au-3 and Au-4, was carried out by means of peptidic bond formation using traditional peptide chemistry protocols. On the basis of previous studies carried out by us21 and others,24 modified gold electrodes were immersed in 21 µM methanolic solutions of the relevant dendrimer molecule (see Figure 1) in the presence of 5.0 mM EDC for a 12 h period at room temperature without stirring. Under these conditions, EDC, a well-known coupling activation agent,24 was assumed to promote the formation of peptide bonds, as schematically presented in Figure 1. Dendrimer attachment to the previously thiol modified gold surfaces was verified by following changes in the voltammetric response of the Ru(NH3)63+ ion, an electroactive probe molecule, using the clean and modified gold electrodes under study under neutral pH conditions. As can be seen in Figure 3, the CV responses of 2.0 mM solutions of Ru(NH3)63+ on Au and Au-3 or Au-4 are very different compared to those obtained using the dendrimer containing Au-3-1 and Au-4-2 modified electrodes. Focusing on the shape of the response curves,25 it is clear that, whereas the voltammetric waveforms for the Ru(III) cation show the typical reversible shape when either the clean Au or the short-thiol modified electrodes are employed, the corresponding electrochemical response on the dendrimer modified substrates is characterized by a distorted shape that suggests a complex behavior in which partial dendrimer induced surface blocking is probably combined with pinhole Ru(III) diffusion toward the electrode (22) (a) Swalen, J. D.; Allara, D. L.; Andrade, J. D.; Chandross, E. A.; Garoff, S.; Israelachvili, J.; McCarthy, T. J.; Murria, R.; Pease, R. F.; Rabolt, J. F.; Wynne, K. J.; Yu, H. Langmuir 1987, 3, 932. (b) Dubois, L. H.; Nuzzo, R. G. Annu. Rev. Phys. Chem. 1992, 43, 437. (c) Arias, F.; Godıńez, L. A.; Wilson, S. R.; Kaifer, A. E.; Echegoyen, L. J. Am. Chem. Soc. 1996, 118, 6086. (d) Ulman, A. Chem. Rev. 1996, 96, 1533. (23) Cheng, I. F.; Quintus, F.; Korte, N. Environ. Sci. Technol. 1997, 31, 1074. (24) (a) Collinson, M.; Bowden, E. F.; Tarlov, M. J. Langmuir 1992, 8, 1247. (b) Willner, I.; Lion-Dagan, M.; Marx-Tibbon, S.; Katz, E. J. Am. Chem. Soc. 1995, 117, 6581. (c) Wells, M.; Crooks, R. M. J. Am. Chem. Soc. 1996, 118, 3988. (25) Inspection of Figure 3 also shows changes in limiting currents that are consistent with the postulated interphasial structures. In this way, ionized thiol molecules, negatively charged for 3 and positively charged for 4 at pH 7,21 either increase or decrease the peak currents of the cationic electroactive probe in the voltammograms of parts A-ii or B-ii of Figure 3 as compared to the response obtained using the clean gold electrodes. This effect, that can be explained in terms of electrostatic interactions between the positively charged Ru probe and the modified electrode surface,16 also operates when the dendrimer compounds 1 or 2 are further incorporated on the corresponding electrode surface. As can be seen by comparing part A-ii with part A-iii of Figure 3 and part B-ii with part B-iii of Figure 3, the changes in the limiting current of the Ru probe molecule as compared to those obtained with the thiol modified gold electrodes also reflect the electrostatic interaction between the charged surface confined dendrimer molecule, positively charged for the Au-3-1 surface and negatively charges for the Au-2-4 substrate under neutral pH conditions,21 and the positively charged Ru species.

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Figure 3. CV responses of a 2.0 mM Ru(NH3)6Cl3 phosphate buffered solution (pH 7, I ) 0.1) at 298 K and 100 mV s-1 carried out using (A-i and B-i) clean gold, (A-ii) Au-3, (B-ii) Au-4, (A-iii) Au-3-1, and (B-iii) Au-4-2 electrodes.

surface.26 Although the precise nature of the observed changes in parts A-iii and B-iii of Figure 3 as compared to the typical reversible response of the Ru(III) species remains unclear due to the complex combination of effects that take place, it is obvious that these changes are induced by the presence of the dendrimer compounds at the electrode-solution interface. To further probe this point and to verify that peptidic bond formation between the dendrimer and thiol molecules actually takes place, control experiments were performed in which the two dendrimer modified substrates were prepared as previously described but without adding the coupling agent EDC. Under these conditions, electrostatic adsorption of the PAMAM species also takes place,21 giving rise to CV responses similar to those presented in parts A-iii and B-iii of Figure 3 (data not shown). When these electrodes are taken out from the electroactive solution, however, rinsed, and immersed in pure supporting electrolyte solution under stirring conditions, the dendrimer compounds are released from the electrode surface, as evidenced by the fully reversible response that is obtained when the electrodes are later introduced in the neutral 2.0 mM Ru(III) containing (26) Shin-Jung, Ch.; Su-Moon, P. Bull. Korean Chem. Soc. 2002, 23, 699.


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Figure 4. CV responses of PB modified Au, Au-4-2, and Au-3-1 surfaces in pH 2 (A, B, and C, respectively) and pH 7 (D, E, and F respectively) phosphate buffered solutions (I ) 0.1). The voltammograms were obtained at 100 mV s-1 and 298 K.

solution. Similar experiments using the Au-3-1 and Au4-2 substrates, on the other hand, showed that, after the rinsing process just described, the Ru(III) CV response observed in parts A-iii and B-iii of Figure 3 is fully retained probing in this way that for the dendrimer modified electrodes under study, peptidic bond formation takes

place between the PAMAM dendrimer molecules and the corresponding surface attached thiol species. Surface modification of Au and the dendrimer containing electrodes, Au-3-1 and Au-4-2, to incorporate PB species was carried out in the following stage by exposing the substrates to phosphate buffered solutions of pH 2, 7, and

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12 (I ) 0.1) also containing FeCl3 and K4Fe(CN)6 (1.0 mM each) as described in the Experimental Section. Electrodes treated in this way were then taken out from the modifying solution, rinsed with plenty of deionized water, and immersed in pure buffer solutions in which the CV response was recorded. As expected, no PB electrochemically related signals could be identified in any of the substrates prepared at pH 12, since under these conditions PB readily dissociates into Fe(OH)3 and Fe(CN)64-.18a,b Figure 4 shows the representative voltammograms under acidic and neutral conditions of the dendrimer containing electrodes along with those obtained using the same PB adsorption technique on a clean gold electrode as a reference. As can be readily seen by inspection of the relevant voltammograms, all the surfaces under study showed the PB characteristic responses2b,4c,27 at pH 2 values (two peaks around 0.2 and 0.9 V vs SCE, respectively), and only the Au-3-1 substrate showed the electrochemical signals of PB under neutral pH conditions. To verify that PB was adsorbed on the electrode surfaces that showed the characteristic electrochemical signals, cylic voltammograms at 20, 40, 60, 80, and 100 mV s-1 were obtained for each system and the current densities of the anodic peak located around 0.2 V versus SCE were plotted against the scan rate, giving rise to linear relationships (data not shown) that clearly demonstrated that the PB molecules are actually confined to the electrode surface.28 The formation of PB species at pH values smaller than 5 was expected, as it has been thoroughly reported in the literature,2b,4b,c,5e but PB films are known to be unstable at neutral pH conditions (see Figure 4D and E), and therefore, the response of Figure 4F was somewhat surprising. The existence of PB in the Au-3-1 modified electrode prepared in a pH 7 buffer solution was however rationalized as a consequence of the dendrimer protonated state that, by virtue of the pKa of the peripheral functional groups of 1 (-NH3+/-NH2, pKa 9.52),20c must result in a localized low pH value at the electrode-electrolyte interface even under neutral bulk solution conditions. To test this hypothesis, PB modified electrodes were prepared using the clean gold and the two dendrimer modified substrates using a pH 2 phosphate buffer medium as previously described. These electrodes were then sequentially rinsed and immersed in phosphate buffer solutions of pH 2, 4, 7, 9, and 12 (I ) 0.1), on which CV was performed in order to detect and quantify the presence of PB on the electrode surface. Quantification of the surface attached PB film on each buffered solution was carried out by the computation of surface coverage values, Γ (using the anodic peak located around 0.2 V vs SCE, Γ ) ∫(i dt/fnFA) ) Q/fnFA, where Q is the charge in coulombs, f is the roughness factor (1.1), n is the number of electrons involved in the process (1 e-), F is the Faraday constant (96 485 C mol-1), and A is the geometric area of the working electrode in square centimeters), from the CV data, allowing us to construct the curves of Γ versus pH that are presented in Figure 5. Inspection of the data presented in this figure reveals two fundamentally important points. First, it is interesting to note that, at pH 2, PB was

preferentially adsorbed on the two dendrimer containing electrodes, since the Γ values calculated are roughly equal to each other and about 7 times larger that those obtained in unmodified gold electrodes on which PB was confined. The fact that Γ is relatively large and almost the same for both dendrimer modified electrodes is fully consistent not only with the dendrimer-PB interaction previously observed but also with the idea of a PB-dendrimer intermolecular interaction that preferentially takes place within the PAMAM host. The second important point is derived from the inspection of the shape of the curves presented in Figure 5 from which it can be readily seen that, whereas the PB film on Au and on Au-4-2 quickly loses stability at pH conditions above 4, the PB electroactivity on the Au-3-1 modified electrode decreases drastically only after the pH has reached basic conditions. Considering that the pKa values of 1 and 2 in solution are 9.5220c and 3.79,21 respectively, the previously postulated explanation of an improved stability of the Au-3-1-PB substrate based on the protonated state of 1 at pH < 9.52, and the consequent existence of local acidic conditions at the electrode-solution interface, is clearly supported on the basis of the data presented in Figure 5. Further evidence of the presence of PB in the electrodes under study was obtained from surface Raman spectroscopy experiments, which also allowed us to explore the distribution of PB molecules on the electrode surfaces. As can be seen in the Raman spectra presented in Figure 6, for instance, the three samples Au-PB, Au-4-1-PB, and Au-3-1-PB prepared under pH 2 conditions (parts A, B, and C of Figure 6, respectively) show PB characteristic peaks at 2089, 2125, and 2147 cm-1, which have been assigned to the Raman vibrational frequencies of the bridging cyanide ligands in PB.29 The homogeneity of each PB film, on the other hand, was studied by performing a linear mapping in which, for each 2 µm point along a 18 µm line randomly selected on

(27) (a) Garcıá-Jarenõ, J. J.; Navarro, J. J.; Roig, A. F.; Scholl, H.; Vicente, F. Electrochim. Acta 1995, 9, 1113. (b) Garcıá-Jarenõ, J. J.; Navarro-Laboulais, J.; Vicente, F. Electrochim. Acta 1996, 17, 2675. (c) Jayarama, R. S.; Fritz, S. A. D. J. Electroanal. Chem. 1996, 403, 209. (d) Garcıá-Jarenõ, J. J.; Benito, D.; Navarro-Laboulais, J.; Vicente, F. J. Chem. Educ. 1998, 7, 881. (e) Guo, Y.; Guadalupe, A. R.; Resto, O.; Fonseca, L. F.; Weisz, S. Z. Chem. Mater. 1999, 11, 135. (f) Ba´rcena, S. M.; Scholz, F. J. Electroanal. Chem. 2002, 528, 27. (28) Bard, A. J.; Faulkner, L. R. Electrochemical Methods: Fundamentals and Applications; John Wiley & Sons: New York, 2001.

(29) (a) Lipkowski, J.; Ross, Ph. N. Adsorption of Molecules at Metal Electrodes; VCH: New York, 1992; pp 318-320. (b) Chalmers, J. M.; Griffiths, P. R. Handbook of Vibrational Spectroscopy: Sample Characterization and Spectral Data Processing; John Wiley & Sons: New York, 2002; Vol. 3, p 1845. (c) Chalmers, J. M.; Griffiths, P. R. Handbook of Vibrational Spectroscopy: Applications in Industry, Materials and the Physical Science; John Wiley & Sons: New York, 2002; Vol. 4, p 2915. (d) Zamponi, S.; Kijak, A. M.; Sommer, A. J.; Marassi, R.; Kulesza, P. J.; Cox, J. A. J. Solid State Electrochem. 2002, 6, 528. (d) Cao, X.; Wang, R.; Wang, G.; Zhang, Z. Dianhuaxue 2001, 1, 71.

Figure 5. Curves of PB surface coverage, Γ, vs pH of (A) Au, (B) Au-4-2, and (C) Au-3-1 modified electrodes in phosphate buffered aqueous solutions (I ) 0.1) at 298 K.


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Figure 7. Schematic distribution of PB integral fraction peak intensities, φ, of a linear Raman spectroscopy mapping (18 µm) randomly taken on Au-3-1-PB, Au-4-2-PB, and Au-PB substrates. The square markers and the rectangular boxes represent the mean φ value and the area in which 75% of the data falls, respectively, for each substrate. The extremes of the vertical lines, on the other hand, indicate the maximum and minimum integral fractional values obtained for each surface.

Figure 8. Cyclic voltammograms for the electro-oxidation of 0.1 mM AA on (A) Au and (B) Au-3-1-PB in phosphate buffered solutions (pH 2, I ) 0.1) at 100 mV s-1 and 298 K.

Figure 6. Raman spectra of (A) Au-PB, (B) Au-4-2-PB, and (C) Au-3-1-PB modified electrodes at 298 K.

each electrode substrate, the integration of the peaks in the 1950-2200 cm-1 range was performed. The resulting integration values were divided by the largest integration value obtained in each substrate and presented in Figure 7 as a fraction, φ, along with statistical information on the data obtained. Whereas the square markers and the boxes shown in Figure 7 represent the mean fractional integrated values and the zone in which 75% of the data falls, respectively, the extreme points of the vertical lines indicate the maximum and minimum fractional values that were obtained for each substrate. It is thus interesting to note from Figure 7 that the difference between the largest and smallest fractional integrated values (i.e., the maximum dispersion of the data samples) is larger for the

Au-4-2-PB when compared to that obtained for the Au3-1-PB modified electrode and also smaller than that measured for the Au-PB surface. Our data therefore suggest that, whereas PB is far more dispersed in the Au-PB substrate and relatively homogeneously distributed in the Au-3-1-PB substrate, an intermediate situation holds for the carboxylated dendrimer modified electrode Au-4-2-PB. Dendrimer molecules are known to be relatively permeable to small molecules in solution, and therefore, the possibility of using the PB-dendrimer modified electrodes as electrocatalytic substrates for the electroxidation of AA3b,4a,30 in aqueous media was explored. Preliminary CV experiments with clean gold electrodes as well as with all the PB modified substrates showed that, as reported in the literature,3b,4 PB films have a catalytic effect on the anodic electrochemical response of AA that is reflected by a substantial increase in the related oxidation current density (see Figure 8). (30) Lund, H.; Baizer, M. M. Organic Electrochemistry: an Introduction and a Guide, 3rd ed.; Marcel Dekker: New York, 1991; pp 645647.

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Table 1. Surface Coverage Values of PB Prepared under Acidic Conditions and Electroanalytical Data for AA Electro-oxidation Using the Different Substrates Considered in This Study, in Phosphate Buffered Solutions of pH 2 and 7 (I ) 0.1) at 298 K electrode

Γ/10-9 mol cm-2

Au Au-PB Au-4-2-PB Au-3-1-PB

0 1.02 7.28 7.58

Au Au-PB Au-4-2-PB Au-3-1-PB

0 0.63 2.73 2.85

r2

DL/10-6 mol L-1

QL/10-6 mol L-1

pH 2 (I ) 0.1) J ) 0.34[AA] - 1.64 J ) 1.24[AA] - 0.69 J ) 2.08[AA] + 2.28 J ) 2.08[AA] - 1.41

0.9966 0.9993 0.9982 0.9989

33.19 9.10 5.41 5.40

110.62 30.34 18.03 18.01

pH 7 (I ) 0.1) J ) 0.43[AA] + 1.80 J ) 1.58[AA] + 13.52 J ) 1.17[AA] + 7.80 J ) 20.60[AA] + 87.18

0.9944 0.9987 0.9976 0.9944

111.95 30.13 40.64 2.32

373.15 100.42 135.47 7.72

linear equation

Experiments carried out at pH 2 under constant stirring conditions (900 rpm) at a constant applied potential of 0.8 V versus SCE (a potential sufficiently anodic to oxidize the AA molecules on any of the substrates surveyed) and at different concentrations of AA in the 0-120 µM concentration range showed linear responses (r2 > 0.99) for the limiting current densities, J, versus AA concentration with different slopes for all the substrates considered in this study. As can be observed in Figure 9 and the corresponding data presented in Table 1, for instance, the J versus AA concentration response of the PB modified gold electrode shows a slope (related to sensitivity) about 3.5 times larger as compared to that observed for clean gold substrates and approximately 2 times smaller than those obtained using any of the PB-PAMAM dendrimer modified electrodes, Au-3-1-PB or Au-4-2-PB. The fact that the electrochemical behaviors observed for the two PB-PAMAM dendrimer modified electrodes are practically identical to each other, on the other hand, supports not only our previous finding on an increased PB surface coverage promoted by the anchored dendrimer molecules (see Table 1) but also the idea of a PB-dendrimer interaction that is essentially independent of the dendrimer peripheral functionalities and, therefore, must be taking place within the hyper-ramified polymer.6b,20 The linear behavior observed in all the curves presented in Figure 9 suggests, on the other hand, that passivation effects at the electrode surfaces studied are essentially absent in the AA concentration range explored and, in view of the improved detection and quantification limits (DL ) 3σ/m and QL ) 10σ/m, respectively, where σ is the

standard deviation of the data points and m is the slope of the linear relationship)31 that can be attained using surface anchored PB-PAMAM dendrimer nanocomposites, it is also possible to anticipate the use of these materials as a potentially important new family of materials for electroanalytical applications. On the basis of the results shown in Figures 4 and 5, however, the main potential application of these novel substrates in the electroanalytic field could derive from the dendrimer induced stability of PB that characterizes Au-3-1-PB films in neutral aqueous media. In this way, electrodes were prepared in pH 2 buffer solutions and similar electrochemical experiments to those previously discussed were carried out using AA dissolved in pH 7 buffer solutions. As can be observed in the resulting J versus AA concentration responses presented in Figure 10, the Au-3-1-PB substrate showed a substantially larger slope (about 20 times) than those observed for all the other surfaces under study, a fact that not only reflects the presence of the electrocatalytic PB film on the electrode surface but also results in an improved DL and QL (see Table 1) when compared to those obtained from the analysis of the data of either Au, Au-PB, or Au-4-2-PB. In this way, it is clear that, under neutral pH conditions, the electrocatalysis of AA seems to be possible only by using a Au-3-1-PB modified electrode that, among the systems surveyed, is the unique heterogeneous film capable of stabilizing PB on the electrode surface and maintaining at the same time the necessary permeability to keep the interface accessible to electroactive species in solution.30

Figure 9. Curves of J vs [AA] in the 0-120 µM concentration range obtained in phosphate buffered pH 2 aqueous solution (I ) 0.1), under constant stirring (900 rpm) and an applied potential of 0.8 V vs SCE using (A) Au, (B) Au-PB, (C) Au-42-PB, and (D) Au-3-1-PB modified electrodes.

Figure 10. Curves of J vs [AA] in the 0-120 µM concentration range obtained in phosphate buffered pH 7 aqueous solution (I ) 0.1), under constant stirring (900 rpm) and an applied potential of 0.8 V vs SCE using (A) Au, (B) Au-PB, (C) Au-42-PB, and (D) Au-3-1-PB modified electrodes.


Conclusions In this work, it has been shown that PB interacts with PAMAM dendrimer molecules in aqueous media and that this interaction, which seems to take place inside the dendrimer host, can be exploited to prepare stable PBdendrimer nanocomposite films that can be readily constructed on previously thiolated Au electrode surfaces. Due to the permeability and hosting properties of the surface attached dendrimers studied, the PB molecules were also shown to retain their electrocatalytic properties and work efficiently as catalysts for the electro-oxidation of L(+)-ascorbic acid in aqueous solution. Taking advan(31) (a) Wilson, A. L. Talanta 1973, 20, 725. (b) Long, G. L.; Winefordner, J. D. Anal. Chem. 1983, 55, 713A. (c) Bond, A. M.; Kratsis, S.; Mitchell, S.; Mocak, J. Analyst 1997, 122, 1147.

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tage of the acid/base properties of the peripheral groups of the ammine terminated PAMAM dendrimer molecules employed, it was also possible to maintain acidic conditions at the electrode-solution interface while working with neutral aqueous solutions of ascorbic acid, thus making it possible to prepare and use PB modified electrodes in a pH range in which the electrocatalytic film has been reported to be unstable. Acknowledgment. The authors thank the Mexican Council for Science and Technology (CONACyT, grant J-34905-E) for financial support of this work. E. B. and J. M. also acknowledge CONACyT for their graduate fellowships. LA047478R

Preparation, Characterization, and Electrocatalytic Activity of Surface

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