Pt Nanoparticle

Nov 24, 2008 - The 4-ATP SAM layer was not stable to repeated electrochemical ... KozuchStefan FrielingsdorfOliver LenzMaria-Andrea MroginskiPeter ...
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Langmuir 2008, 25, 534-541

An Electrochemical Study of 4-Aminothiophenol/Pt Nanoparticle Multilayers on Gold Electrodes Cuijie Jiang, Joanne M. Elliott,* David J. Cardin, and Shik Chi Tsang Department of Chemistry, The UniVersity of Reading, Whiteknights Campus, Reading, RG6 6AD, United Kingdom ReceiVed August 7, 2008 Quartz crystal microbalance (QCM) measurements of the formation of a 4-aminothiophenol (4-ATP) self-assembled monolayer (SAM) at a gold electrode showed that a surface coverage of 118 ng cm-2 was obtained after a 3 h exposure period, indicating that good surface coverage was achieved. Cyclic voltammetry of the ferricyanide redox couple across this SAM modified surface produced similar results to those of a bare electrode; however, the electroreduction of oxygen was found to be impaired. The 4-ATP SAM layer was not stable to repeated electrochemical oxidation and reduction; it is believed that the 4-ATP SAM layer was first converted to a 4′-mercapto-N-phenylquinone diimine (NPQD) layer followed by subsequent formation of a 4′-mercapto-N-phenylquinone monoimine (NPQM) layer. We also report a quartz crystal microbalance study of the attachment of platinum nanoparticles to such SAM modified electrodes. We show that five times the amount of platinum nanoparticles can be attached to a 4-ATP modified electrode surface (observed frequency change -187 Hz) compared with an NPQD modified electrode surface (observed frequency change -35 Hz). The presence of the platinum particles was confirmed electrochemically by their surface electrochemical properties, which were different from those of the underlying gold electrode. It is believed that this is the first time that such direct evidence of electrochemical communication between platinum nanoparticles and a SAM modified electrode surface has been obtained. It was also shown to be possible to build up multilayer SAM/ nanoparticle modified surfaces while maintaining efficient electrochemical communication. Up to three SAM/nanoparticle sandwich layers were constructed.

1. Introduction Self-assembled monolayers (SAMs) were first reported in the 1980s1,2 and have been used to decorate a wide variety of surfaces. Simple alkanethiol and alkanedithiol monolayers adsorbed on a Au(111) surface to form a well-defined and densely packed SAM have been well-studied.3-5 Detailed studies concerning the surface organization of variable chain length, n-alkyl thiol, SAM layers have also been undertaken, and it was shown that long alkyl chain thiols induce better surface organization than short alkyl chain thiols, due to greater van der Waals interactions between the long tail groups.6 Difunctionalized SAM layers, in particular aromatic ones,7-9 have attracted much attention in the past decade. Aromatic molecules have a highly conjugated system, which is expected to induce a strong interaction with the metal surface; however, the steric effect of the ring structure may also lead to increased molecular disorder within the SAM. Generally, therefore aromatic thiol SAMs have been found to be much less reproducible. For example, it was reported by Whelan et al. that an almost upright orientation of phenylthiol occurred, leading to * To whom correspondence should be addressed. E-mail: j.m.elliott@ rdg.ac.uk. (1) Nuzzo, R. G.; Allara, D. L. J. Am. Chem. Soc. 1983, 105, 4481–4483. (2) Love, J. C.; Estroff, L. A.; Kriebel, J. K.; Nuzzo, R. G.; Whitesides, G. M. Chem. ReV. 2005, 105, 1103–1169. (3) Widrig, C. A.; Alves, C. A.; Porter, M. D. J. Am. Chem. Soc. 1991, 113, 2805–2810. (4) Ulman, A. Chem. ReV. 1996, 96, 1533–1554. (5) Bain, C. D.; Troughton, E. B.; Tao, Y. T.; Evall, J.; Whitesides, G. M.; Nuzzo, R. G. J. Am. Chem, Soc. 1989, 111, 321–335. (6) Porter, M. D.; Bright, T. B.; Allara, D. L.; Chidsey, C. E. D. J. Am. Chem. Soc. 1987, 109, 3559–3568. (7) Mendes, R. K.; Freire, R. S.; Fonseca, C. P.; Neves, S.; Kubota, L. T. J. Braz. Chem. Soc. 2004, 15, 849–855. (8) Boer, B.; Meng, H.; Perepichka, D. F.; Zheng, J.; Frank, M. M.; Chabal, Y. J.; Bao, Z. Langmuir 2003, 19, 4272–4284. (9) Jiang, W.; Zhitenev, N.; Bao, Z.; Meng, H.; Abusch-Magder, D.; Tennant, D.; Garfunkel, E. Langmuir 2005, 21, 8751–8757.

the formation of a high coverage SAM.10 In contrast, it is interesting to note that, according to the reports of Tao et al.11 and Dhirani et al.,12 aromatic thiols form poorly defined monolayers. Scanning tunneling microscopy (STM) images of their systems indicated that a disordered coverage pattern was obtained. However, the degree of order was generally found to increase when the number of aromatic rings was increased or when a methylene group was introduced between the phenyl ring and the thiol headgroup.11,12 In the light of the various findings, the fundamentals of the adsorption of phenylthiol on a Au(111) surface were thoroughly studied more recently by Hush and co-workers,13 and detailed information on the structure, energetics, and nature of the chemical bonding were provided. Recent developments in nanotechnology have seen the introduction of SAM layers as protective groups for the stabilization of nanoparticles14,15 and also as a means of attachment of nanoparticles to electrode surfaces.16,17 The fabrication of electrodes modified with nanoparticles has been the focus of recent attention owing to the unique electronic, optical, catalytic, and biological properties of the particles.18-22 (10) Whelan, C. M.; Smyth, M. R.; Barnes, C. J. Langmuir 1999, 15, 116–126. (11) Tao, Y.-T.; Wu, C.-C.; Eu, J.-Y.; Lin, W.-L.; Wu, K.-C.; Chen, C. Langmuir 1997, 13, 4018–4023. (12) Dhirani, A.-A.; Zehner, R. W.; Hsung, R. P.; Guyot-Sionnest, P.; Sita, L. R. J. Am. Chem. Soc. 1996, 118, 3319–3320. (13) Bilic´, A.; Reimers, J. R.; Hush, N. S. J. Chem. Phys. 2005, 122, 094708. (14) Brust, M.; Walker, M.; Bethell, D.; Schiffrin, D. J.; Whyman, R. J. Chem. Soc., Chem. Commun. 1994, 801, 802. (15) Schaaff, T. G.; Shafigullin, M. N.; Khoury, J. T.; Vezmar, I.; Whetten, R. L. J. Phys. Chem. B 2001, 105, 8785–8796. (16) Abdelrahman, A. I.; Mohammad, A. M.; Okajima, T.; Ohsaka, T. J. Phys. Chem. B 2006, 110, 2798–2803. (17) Wang, L.; Bai, J.; Huang, P.; Wang, H.; Zhang, L.; Zhao, Y. Electrochem. Commun. 2006, 8, 1825–1829. (18) Sagara, T.; Kato, N.; Nakashima, N. J. Phys. Chem. B 2002, 106, 1205– 1212. (19) Chirea, M.; Garcı´a-Morales, V.; Manzanares, J. A.; Pereira, C.; Gulaboski, R.; Silva, F. J. Phys. Chem. B 2005, 109, 21808–21817.

10.1021/la802567a CCC: $40.75  2008 American Chemical Society Published on Web 11/24/2008

Electrochemical Study of 4-ATP Multilayers on Au

A wide variety of stabilized metal nanoparticles may be prepared. First, consider SAM-stabilized gold nanoparticles, in this case, the chemical bond (Au-S) between the nanoparticles and the SAM layer is very strong, thus preventing chemical displacement reactions from being a useful surface attachment method. However, it is possible to anchor difunctionalized thiol (those modified with carboxylic acid or amine end groups) modified nanoparticles to electrodes to form mono- or multinanoparticle layers on electrode surfaces by electrostatic interaction or by formation of covalent bonds.23,24 Secondly, consider citrate-stabilized nanoparticles, in this case, a carboxylic acidderivatized nanoparticle surface is presented and this also opens up the possibility of exploiting electrostatic forces for surface attachment.25,26 The development of methods ensuring the successful attachment of such particles to an electrode surface is crucial to the exploitation of the novel properties of metal nanoparticles. This paper deals with the fabrication of electrodes modified with SAM layers with different surface properties for the improved attachment of nanoparticles. The mechanical stability, the chemical nature, and the reactivity of a SAM all play a role in determining its usefulness for the attachment of nanoparticles. Many different SAMs have been studied in the hope of attaching nanoparticles to an electrode surface. For example, Ohsaka’s group16 has studied a multilayer film of Au nanoparticles on a gold electrode using 1,4-benzenedimethanethiol (1,4-BDMT) as the SAM, based on the layer-by-layer (LBL) technique. This nanoparticle layer has shown good catalytic activity for the reduction of oxygen and good electrical communication with the gold electrode surface. Another example of a molecule used to form SAMs is that of 4-aminothiophenol (4-ATP). It is wellknown that a 4-ATP SAM undergoes electrochemical oxidation in acidic27 or neutral28 solution at an electrode surface to form a dimer [4′-mercapto-N-phenylquinone diimine (NPQD)] modified surface. Head-to-tail coupling between two adjacent aminothiophenol molecules occurs to form the dimer, as shown in Scheme 1. In addition, Lukkari et al.29 have shown that the oxidized dimer surface (NPQD) can be easily hydrolyzed to a 4′-mercapto-N-phenylquinone monoimine (NPQM) surface with extensive voltammetric cycling, also summarized in Scheme 1. To the best of our knowledge, there have been no reports of a pristine 4-ATP SAM being used to attach Pt nanoparticles to an electrode surface. However, Wang and co-workers17 have presented a self-assembled monolayer of Au nanoparticles on an NPQD surface (formed by the electrochemical oxidation of the original 4-ATP surface on a gold electrode). In their study, a Au nanoparticle layer on a surface modified by a mixture of NPQD and 1,4-BDMT supported on a gold electrode was also investigated. The results showed that Au nanoparticles on an NPQD/1,4-BDMT electrode gave a bigger oxidation peak current for naphthol and better catalytic activity compared with the Au nanoparticles on an NPQD modified electrode. However, (20) Stolarczyk, K.; Palys, B.; Bilewicz, R. J. Electroanal. Chem. 2004, 564, 93–98. (21) Wang, M.; Wang, L.; Wang, G.; Ji, X.; Bai, Y.; Li, T.; Gong, S.; Li, J. Biosens. Bioelectron. 2004, 19, 575–582. (22) Nakamura, F.; Ito, E.; Hayashi, T.; Hara, M. Colloids Surf., A 2006, 284, 495–498. (23) Chan, E. W. L.; Yu, L. Langmuir 2002, 18, 311–313. (24) Okamura, M.; Kondo, T.; Uosaki, K. J. Phys. Chem. B 2005, 109, 9897– 9904. (25) Yang, M.; Zhang, Z. Electrochim. Acta 2004, 49, 5089–5095. (26) Raj, C. R.; Abdelrahman, A. I.; Ohsaka, T. Electrochem. Commun. 2005, 7, 888–893. (27) Hayes, W. A.; Shannon, C. Langmuir 1996, 12, 3688–3694. (28) Raj, C. R.; Kitamura, F.; Ohsaka, T. Langmuir 2001, 17, 7378–7386. (29) Lukkari, J.; Kleemola, K.; Meretoja, M.; Ollonqvist, T.; Kankare, J. Langmuir 1998, 14, 1705–1715.

Langmuir, Vol. 25, No. 1, 2009 535 Scheme 1. Schematic Representation of the Electrochemical Oxidation of 4-ATP supported on a Gold Electrodea

a

Peaks A, B, and C refer to Figure 7a.

neither of the modified electrodes produced an enhanced oxidation current compared with the bare gold electrode surface. Our research interest has focused on the development of a simple, yet effective, route to the surface attachment of platinum nanoparticles to underlying gold electrodes via the use of SAM layers. In particular, gold electrode surfaces were modified with Pt nanoparticles, tethered by either 4-ATP or NPQD molecules (hereafter referred to as an “ATP-Pt” or an “NPQD-Pt” modified surface). As mentioned before, a 4-ATP monolayer film on a gold electrode can easily be electrochemically oxidized to form an NPQD film. Therefore, it is of great relevance to identify the properties of these two different films and find out which performs best (in terms of simplicity, overall performance, and stability) for the attachment of nanoparticles. The results shown here indicate that use of a 4-ATP SAM layer outperforms the use of an NPQD SAM layer in all of these areas. In particular, we report that a 5-fold increase in the amount of citrate-stabilized Pt nanoparticles attached to an ATP modified gold electrode was observed compared to an NPQD modified gold electrode. In addition, because the system studied involved the attachment of Pt nanoparticles to a gold electrode (via the SAM layer), the presence of the Pt nanoparticles could be identified by means of the unique surface electrochemistry of Pt. These results suggest that 4-ATP SAM layers provide a far superior route to the successful attachment of nanoparticles to electrode surfaces. This work is of particular relevance to the fields of sensor development and catalysis.

2. Experimental Section 2.1. Reagents. The following reagents were purchased from the suppliers shown and used as received: hydrogen hexachloroplatinate(IV) solution (Pt 30%, Alfa Aesar), trisodium citrate dihydrate (99+%, Aldrich), sodium borohydride (g96%, Fluka), 4-aminothiophenol (4-ATP, 96%, Acros), potassium ferricyanide (99+%, Aldrich), strontium nitrate (99+%, Adrich), potassium phosphate

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Jiang et al. Scheme 2. Schematic Representation for the Formation of a Multilayer Film of Pt Nanoparticles Linked by 4-ATP on a Gold Electrode

Figure 1. (a) TEM image of the citrate-stabilized Pt nanoparticles and (b) size distribution graph of the citrate-stabilized Pt nanoparticles, where n ) 364 and average particle diameter ) 4.8 nm.

dibasic trihydrate (99+%, Aldrich), and potassium dihydrogen phosphate (99+%, Aldrich). 2.2. Instruments. A Philips CM20 analytical transmission electron microscope was used to take the TEM images of the Pt nanoparticles. A QCM200 quartz crystal microbalance with an ATcut, 5 MHz piezoelectric quartz crystal (Au/Cr crystal, 1 in. in diameter) was used for all QCM measurements. The exposed electrode area in contact with the liquid was about 1.37 cm2. 2.3. Synthesis of the Citrate-Stabilized Pt Nanoparticles.30 A total of 50 mL of 2.8 mM aqueous trisodium citrate dihydrate solution was added to 100 mL of 0.4 mM aqueous hydrogen hexachloroplatinate(IV) solution at room temperature. An amount of 10 mL of 12 mM sodium borohydride was introduced dropwise with vigorous stirring, and the pale yellow solution turned dark brown in 5 min. The dark brown colloidal solution was stirred for 4 h and stored in a refrigerator at 4 °C until ready for further use. 2.4. QCM Measurements. A temperature controlled water bath and jacketed cell were used during the QCM measurements to control the temperature at 25 °C. (30) Yang, J.; Lee, J. Y.; Deivaraj, T. C.; Too, H. P. J. Colloid Interface Sci. 2004, 277, 95–99.

2.4.1. Precleaning of the Gold Electrode. Some of the crystals were precleaned31 in a piranha solution (30% H2O2/98% H2SO4, 1:3 by volume) at room temperature for 10 s and then rinsed with distilled water and dried in a gentle stream of nitrogen gas. The treatment was repeated twice with a fresh piranha solution. Alternatively, in some cases, the crystals were used as received. 2.4.2. SAM Layer Formation. In general, the gold crystal installed in the crystal holder was dipped into the pure ethanol solution for ∼2 h, and any change in frequency was recorded. Concentrated 4-ATP in ethanol was then injected into the system, providing a diluted 30 mM solution of 4-ATP, and again any change in frequency was recorded for another 3 h. This was followed by rinsing with first ethanol and second pure water. 2.4.3. Attachment of Nanoparticles. The 4-ATP modified gold electrode was dipped into distilled water for ∼2 h, and the change in frequency was recorded. The crystal was then dipped into the Pt nanoparticle solution overnight (∼12 h). The crystal was then rinsed and dipped into distilled water for ∼2 h, and the change in frequency was recorded again. 2.4.4. Formation of Multilayers. The aforementioned procedures (sections 2.4.2 and 2.4.3) were repeated as necessary to produce a multilayer modified electrode (as shown in Scheme 2). 2.5. Cyclic Voltammetric Measurements. A three-electrode cell with a gold quartz crystal as the working electrode, a platinum gauze as the counter electrode, and a Hg/Hg2SO4 reference electrode was used at room temperature. A scan rate of 100 mV s-1 was used for the majority of measurements. Experiments were performed both in 10 mM K3[Fe(CN)6] in 0.1 M Sr(NO3)2 solution and in 0.1 M buffer solution (pH 7, 0.1 M KH2PO4 + 0.1 M K2HPO4 · 3H2O).

3. Results and Discussion 3.1. Formation of ATP and ATP-Pt Modified Gold Electrodes. Figure 1 presents a TEM image of the well-dispersed citrate-stabilized Pt nanoparticles, and the size distribution graph shows that the majority of nanoparticles were between 3 and 5 nm in diameter. Attempts to anchor these nanoparticles to an electrode surface using a variety of SAMs were made. Quartz crystal microbalance measurements were used to monitor the step by step electrode modification process, and cyclic voltammetry was used to monitor changes in the electroactivity of the modified electrode surface. (31) Kim, D. H.; Noh, J.; Hara, M.; Lee, H. Bull. Korean Chem. Soc. 2001, 22, 276–280.

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Figure 2. (a) Cyclic voltammograms of 10 mM K3[Fe(CN)6] in 0.1 M Sr(NO3)2 at (i) a bare QCM gold electrode, (ii) an ATP(1) modified QCM gold electrode, and (iii) an ATP-Pt(1) modified QCM gold electrode. Scan rate: 100 mV s-1. (b) QCM “frequency shift versus time” data for the formation of ATP(1) modified QCM gold electrode in 30 mM ethanol solution of 4-ATP for 3 h.

Prior to surface modification, cyclic voltammetry (CV) of the redox couple [Fe(CN)6]3-/4- was used to study the electrochemical behavior of the bare electrode. Results were as expected and are shown in Figure 2a (i). Subsequently, stage one of the modification process involved exposing the gold quartz crystal surface to a solution containing 4-ATP. During exposure, 4-ATP was expected to self-assemble at the electrode surface, leading to the formation of a SAM. Figure 2b shows the frequency change observed during the adsorption of 4-ATP. As can be seen, a stable response was seen initially, allowing the effect of the 4-ATP injection to be clearly observed. Upon injection, a sudden and marked decrease in frequency occurred, as expected. After ∼300 s, the frequency stabilized; exposure was continued for ∼3 h, and the overall change in frequency observed was -6.7 Hz. All frequency changes are summarized in Table 1. The Sauerbrey equation32 (see eq 1) relates the mass change per unit area on the QCM electrode surface to the observed change in oscillation frequency of the crystal:

∆f ) -Cf ∆m

(1)

where Cf is the sensitivity factor for the crystal, and Cf ) 56.6 Hz µg-1 cm2 for a 5 MHz AT-cut quartz crystal at room temperature. According to the Sauerbrey equation, when ∆f ) -6.7 Hz, then the surface coverage per unit area of 4-ATP on the Au (32) Sauerbrey, G. Z. Phys. 1959, 155, 206–222.

Figure 3. QCM “frequency shift versus time” data for the formation of (a) ATP(2) and (b) ATP(3) modified QCM gold electrodes in 30 mM ethanol solution of 4-ATP for 3 h. Table 1. QCM “Frequency Shift” Data for the Formation of ATP(1), ATP(2), ATP(3), ATP-Pt(1), ATP-Pt(2), ATP-Pt(3), and NPQD-Pt Modified QCM Gold Electrodesa electrodes

frequency shift, ∆f (Hz)

ATP(1) ATP(2) ATP(3) ATP-Pt(1) ATP-Pt(2) ATP-Pt(3) NPQD-Pt

-6.7 -19.5 -17.0 -187 -151 -133 -35

a The ∆f value presented in the table is the final QCM reading at end of the 3 hr, or overnight, dipping.

electrode surface corresponds to 118 ng cm-2 (5.73 × 1014 molecules cm-2). This value agrees well with the coverage values previously reported (110 and 117 ng cm-2) for a fairly close packed 4-ATP monolayer on Au.33-35 Note that “per unit area” refers to the geometric area of the QCM crystal. CV of the redox couple [Fe(CN)6]3-/4- was also used to study the electrochemical behavior of the 4-ATP modified electrode (hereafter referred to as an “ATP(1)” surface). A voltammogram representative of the ATP(1) modified crystal is shown in Figure 2a (ii). Comparing Figure 2a (i) with Figure 2a (ii), it can be seen that in both cases a well-defined redox process was observed with peak to peak separations of 192 and 194 mV, respectively, (33) Gole, A.; Sainkar, S. R.; Sastry, M. Chem. Mater. 2000, 12, 1234–1239. (34) Kim, Y. T.; McCarley, R. L.; Bard, A. J. J. Phys. Chem. 1992, 96, 7416– 7421. (35) Xie, Q.; Zhang, Y.; Xiang, C.; Tang, J.; Li, Y.; Zhao, Q.; Yao, S. Anal. Sci. 2001, 17, 613–620.

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and in both cases the magnitude of the cathodic process matched that of the anodic process. These results show that 4-ATP does not block the electron transfer process and allows good communication with the underlying gold electrode. It is important to note here that cycling the ATP(1) modified electrode in this fashion is well-known to change the nature of the surface.29 The ATP(1) surface dimerizes to form an NPQD modified surface. Thus, if the ability of an ATP modified surface to attach nanoparticles is to be examined, then cycling in this fashion is not recommended. The results presented here are shown purely to confirm that electron transfer across the ATP(1) layer can indeed occur, and in all other experiments the ATP(1) layer was not cycled in this way. In stage two of the modification process, the ATP(1) modified (uncycled) electrode was exposed to a Pt nanoparticle solution overnight. The QCM was again used to measure the frequency shift (∆f) of the modified gold crystal when exposed to a solution containing Pt nanoparticles. However, in this case, it was not possible to measure the frequency change continuously, instead the frequency of the crystal was recorded prior to exposure, and this measurement was repeated after exposure. The change in frequency observed was -187 Hz, and according to the Sauerbrey equation this corresponds to a mass change per unit area (∆m) of 3304 ng cm-2. Estimating the relative molecular mass of a typical citrate-stabilized Pt nanoparticle (4.8 nm in diameter) to be 83 × 104 g mol-1, then this mass change corresponds to 2.4 × 1012 particles cm-2. It should be noted that two possible modes of surface attachment exist. In the first case, citrate-stabilized Pt nanoparticles may be attached to the ATP(1) modified electrode by electrostatic interaction of “-COO-” and “-NH3+” groups; in the second case, it may be that an amide bond forms the link between the SAM layer and the nanoparticles. It is difficult, at this stage, to identify the nature of the surface attachment, given the available information, but it is clear that the modified electrode appears to be quite stable over time. This modified surface will subsequently be referred to as “ATP-Pt(1)”. A theoretical coverage may be estimated by assuming first an average Pt nanoparticle diameter of 4.8 nm (see Figure 1b), second spherical nanoparticles, and third creation of a closepacked Pt nanoparticle monolayer. This ideal coverage is 5.6 × 1012 particles cm-2. Therefore, an ideal complete coverage of citrate-stabilized Pt nanoparticles would produce a mass change per unit area of ∼8000 ng cm-2, which agrees well with the reference result.36 The QCM result of 3304 ng cm-2 for ATPPt(1) thus provides a surface coverage of 43%. However, it has been reported that, based on geometrical principles, only 50-60% of a total available surface area is accessible for a random attachment of particles to a surface,36,37 and based on this estimate a surface coverage of 43% represents a relatively high coverage. CV of the redox couple [Fe(CN)6]3-/4- was again used to study the electrochemical behavior of the ATP-Pt(1) electrode. The results are shown in Figure 2a (iii); a well-defined redox process was observed with a peak to peak separation of 202 mV, and the magnitude of the cathodic process matched that of the anodic process. These results may be interpreted in two ways. In the first case, these results may indicate that the ferricyanide redox couple reacts at the outer surface of the modified electrode, in which case the results show good electrochemical communication (36) Brust, M.; Etchenique, R.; Calvo, E. J.; Gordillo, G. J. Chem. Commun. 1996, 1949–1950. (37) Finegold, L.; Donnell, J. T. Nature 1979, 278, 443–445. (38) Finklea, H. O.; Snider, D. A.; Fedyk, J.; Sabatani, E.; Gafni, Y.; Rubinstein, I. Langmuir 1993, 9, 3660–3667. (39) El-Deab, M. S.; Okajima, T.; Ohsaka, T. J. Electrochem. Soc. 2003, 150, A851-857.

Jiang et al.

Figure 4. Cyclic voltammograms of 10 mM K3[Fe(CN)6] in 0.1 M Sr(NO3)2 at (i) an ATP-Pt(2) modified QCM gold electrode and (ii) an ATP-Pt(3) modified QCM gold electrode. Scan rate: 100 mV s-1.

across the nanomodified surface. In the second case, these results may indicate that the ferricyanide redox couple is able to penetrate the nanomodified surface and subsequently react at the ATP modified electrode interface [as in Figure 2a (ii)], in which case little information regarding electrochemical communication across the nanomodified surface may be obtained. However, as there is no change in the magnitudes of the current passed in Figure 2a (i), (ii), and (iii), this suggests that the area of the electrode across which diffusion can occur remains the same, and thus, it is believed that the data represent electrochemistry occurring at the external surface of the ATP-Pt(1) modified electrode. In subsequent stages of modification, the process of exposing the ATP-Pt(1) modified electrode to 4-ATP once again followed by exposure to the nanoparticle solution was repeated twice, leading to the formation of modified surfaces referred to as ATP(2), ATP-Pt(2), ATP(3), and ATP-Pt(3) (see Scheme 2). QCM was again used to measure the frequency shift of the modified gold crystal during these further modification stages. Figure 3 shows the frequency change during the adsorption process of 4-ATP (a) at the ATP-Pt(1) modified crystal to form an ATP(2) modified surface and (b) at the ATP-Pt(2) modified crystal to form an ATP(3) modified surface. As can be seen, an initial stable response was seen for the modified ATP-Pt(1) and ATP-Pt(2) electrodes, allowing the effect of 4-ATP injection to be clearly observed. Upon injection, a sudden and marked decrease in frequency occurred, indicating the successful formation of the ATP(2) and ATP(3) surfaces. After ∼300 s, the frequency stabilized; the overall change in frequency recorded after ∼3 h was -19.5 and -17.0 Hz, respectively. These changes in frequency were much larger than those observed when the quartz crystal was initially exposed to 4-ATP. This larger decrease may be explained by consideration of the larger surface area provided by the ATP-Pt(1) and ATP-Pt(2) nanoparticle modified surfaces compared with the underlying gold electrode surface, thus allowing greater attachment of 4-ATP in subsequent layers. CV of the redox couple [Fe(CN)6]3-/4- was not used to study the electrochemical behavior of the ATP(2) and ATP(3) modified electrodes, for the reasons stated earlier. Next, consider the formation of ATP-Pt(2) and ATP-Pt(3) modified surfaces: the ATP(2) and ATP(3) modified electrodes were exposed to a Pt nanoparticle solution overnight to create ATP-Pt(2) and ATP-Pt(3) modified surfaces, respectively. The change in frequency observed was -151 and -133 Hz, respectively. According to the Sauerbrey equation, this corre-

Electrochemical Study of 4-ATP Multilayers on Au

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Figure 5. Cyclic voltammograms of 10 mM K3[Fe(CN)6] in 0.1 M Sr(NO3)2 at (a) a bare QCM gold electrode and (b) an ATP-Pt(3) modified QCM gold electrode. Scan rate variable: 10, 20, 50, 100, 200, and 500 mV s-1. The direction of the arrows indicates increasing scan rate.

sponds to a mass change per unit area (∆m) of 2668 and 2350 ng cm-2, respectively. These values are slightly less than that observed for the formation of the ATP-Pt(1) modified electrode. CV of the redox couple [Fe(CN)6]3-/4- was again used to study the electrochemical behavior of the ATP-Pt(2) and ATP-Pt(3) modified electrodes. Results are shown in Figure 4. As Figure 4 (i) shows, the results collected for the ATP-Pt(2) modified surface were very similar to those observed for the ATP-Pt(1) modified surface and indicate that electron transfer across the modified electrode is possible. However, Figure 4 (ii) shows that while electron transfer across the ATP-Pt(3) modified electrode is still possible, the magnitude of the current passed is slightly lower, and the peak to peak separation is slightly larger (238 mV). It should also be noted that while voltammetry across the ATP-Pt(3) modified electrode was relatively stable, it was noted that some increase in the peak to peak separation was observed as the electrode aged. CV at a range of scan rates was performed for such an aged ATP-Pt(3) modified electrode, and comparisons were made to a bare gold quartz crystal electrode. The data are shown in Figure 5. As parts (a) and (b) of Figure 5 show, voltammetry at both electrodes becomes more reversible as the scan rate is lowered. In both cases, a linear variation between peak height and scan rate1/2 was found, and the corresponding peak to peak separations are shown in Table 2. As can be seen, the peak to peak separations are quite large in both cases. Unfortunately, these data cannot be used to determine the standard rate constant for electron transfer, as both the uncompensated resistance (determined to be

Figure 6. (a) Cyclic voltammograms of (i) a bare QCM gold electrode and (ii) an ATP(1) modified QCM gold electrode in 0.1 M phosphate buffer solution (pH 7.0). Scan rate: 100 mV s-1. (b) Cyclic voltammograms of an ATP-Pt(3) modified QCM gold electrode in 0.1 M phosphate buffer solution (pH 7.0). Scan rate variable: 10, 20, 50, 100, 200, and 500 mV s-1. The direction of the arrows indicates increasing scan rate. (c) Cyclic voltammograms of an ATP-Pt(3) modified QCM gold electrode in (i) 0.1 M phosphate buffer solution (pH 7.0) and (ii) 0.1 M degassed phosphate buffer solution. Scan rate: 100 mV s-1.

in the range 22-65 Ω) for this large area QCM crystal and the current flowing (1-5 mA) are quite large. These factors coupled together introduce significant distortion (iRdrop) to the data and mean that peak to peak separations cannot be used to calculate kinetic data. The electrochemical behavior of the nanoparticle modified electrode was compared with that of the ATP modified surface and the bare electrode in a buffer solution (pH 7.0). Cyclic voltammograms in buffer solution of a bare gold electrode and ATP(1) are shown in Figure 6a. First, consider the bare electrode

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Figure 7. Cyclic voltammograms obtained during the electrochemical transformation of a 4-ATP modified QCM electrode in (a) 0.1 M phosphate buffer solution (pH 7.0) and (b) 10 mM K3[Fe(CN)6] in 0.1 M Sr(NO3)2solution. Scan numbers 1, 11, 21, 31, 41, 51, 61, 71, 81, 91, 101, and 111 are shown. Scan rate: 100 mV s-1. Table 2. Peak to Peak Separations Measured from the Cyclic Voltammetric Data in Figure 5 for (a) the Bare Gold QCM Electrode and (b) the ATP-Pt(3) Modified QCM Electrode peak to peak separation, ∆Ep (mV) scan rate (mV s-1)

bare gold electrode

ATP-Pt(3)

500 200 100 50 20 10

302 226 186 154 120 106

558 420 340 262 210 168

data, the current due to the reduction of oxygen plateaus at a voltage of -0.88 V and the maximum current observed was ∼ -163 µA. Second, consider the ATP(1) modified electrode, which did not show any voltammetric response for the reduction reaction. This is to be expected, as it is well-known that a SAM on the electrode surface can hinder the electron transfer process between an electroactive species in the solution and the underlying electrode depending upon the reaction mechanism.38,39 Interestingly, the ATP(1) electrode only blocks electron transfer associated with the reduction of oxygen but not electron transfer associated with the [Fe(CN)6]3-/4- redox couple. Ohsaka et al.28 reported a similar phenomenon and explained this unusual behavior as being due to the different reaction mechanisms. In the case of oxygen reduction, the mechanism may be described as an inner sphere electron transfer, while the [Fe(CN)6]3-/4- redox couple forms the classic example of an outer sphere electron

transfer reaction. Data for the nanoparticle modified electrodes showed a different behavior. Data for the ATP-Pt(3) modified gold electrodes are presented in Figure 6b. This shows the cyclic voltammograms collected under different conditions of scan rate. Classic voltammetric data for Pt surface chemistry were observed, indicating the successful attachment of Pt nanoparticles to the modified electrode and confirming good electrochemical communication between the platinum nanoparticles and the underlying gold electrode. The classic behavior of Pt may be identified first by the nature of the oxidation signal observed at voltages greater than -0.2 V (corresponding to the formation of platinum oxide); indeed the reduction peak observed at ∼ -0.3 V corresponds to the subsequent removal of this platinum oxide layer. Second, the enhanced reduction and oxidation currents observed as the voltage applied to the electrode is lowered (corresponding to the formation and subsequent removal of a layer of platinum hydride) confirm the presence of an electroactive Pt surface. When the buffer solution was degassed, as shown in Figure 6c, the effect was to shift the voltammogram upward, by approximately 0.1 mA. This effect indicates that a background signal due to oxygen electroreduction was present. However, it was not possible to deconvolute the surface chemistry signal from the oxygen reduction signal using these basic electrochemical techniques. It is believed that this is the first time that such direct evidence of electrochemical communication between Pt nanoparticles and a modified electrode surface has been collected. 3.2. Formation of NPQD and NPQD-Pt Modified Gold Electrodes. As stated earlier, stage one of the modification process involved exposing the gold quartz crystal surface to a solution containing 4-ATP. The adsorption process was again monitored using the QCM, and Figure 2b shows typical results collected. Again, the frequency changes observed are summarized in Table 1. The NPQD modified gold electrode was formed by the electrochemical oxidation of the ATP(1) modified gold electrode, and this was achieved either by cycling in buffer solution or by cycling in the [Fe(CN)6]3-/4- redox couple solution as shown in Figure 7. Figure 7a shows the cyclic voltammograms obtained by cycling the ATP(1) electrode in 0.1 M phosphate buffer solution (pH 7.0) between -0.8 and 0.3 V for six cycles. In the first cycle, two oxidation peaks were visible at ∼ -0.20 and +0.20 V [labeled as peaks “A” and “B” in Figure 7a]. On the reduction scan, two peaks were observed at ∼ -0.25 and -0.55 V [labeled as peaks “C” and “D” in Figure 7a]. The intensity of all the peaks decreased with continuous cycling, and after six cycles only the two small peaks “A” and “C” were observed. According to the literature,27-29 the peak “B” corresponds to the oxidation of the ATP(1) modified surface and hence the formation of an NPQD modified surface (see Scheme 1). Peaks “A” and “C” correspond to the reversible oxidation and subsequent reduction of the NPQD layer as shown in Scheme 1. Clearly, cycling an ATP(1) modified surface only once or twice is sufficient to change its nature. In the literature, it has been reported that further oxidation of an NPQD surface may result in the formation of an NPQM or another more complicated surface. In order to investigate this further, extensive voltammetry in the presence of the [Fe(CN)6]3-/4- redox couple was carried out; this type of approach has the advantage that it is possible to monitor the electroactivity of the electrode surface simultaneously. Figure 7b thus presents typical cyclic voltammograms obtained by extensive cycling of the ATP(1) electrode in the [Fe(CN)6]3-/4- redox couple solution [every 10th scan is shown, and the total number of scans performed was ∼111]. Indeed, the ATP(1) modified surface is initially converted into an NPQD

Electrochemical Study of 4-ATP Multilayers on Au

surface, and upon extensive cycling it is believed that an NPQM type film results. The electron transfer properties of an ATP(1) and an NPQD modified surface are different from those of an NPQM type surface, and this is apparent in Figure 7b where a slow and steady loss in the ability of the film to transfer electrons is observed. As mentioned earlier, the ATP(1) electrode was transformed into an NPQD electrode after just a few scans, and a well-defined redox peak for [Fe(CN)6]3-/4- was still observed. However, the well-defined redox peak for [Fe(CN)6]3-/4- gradually decreased with extensive cycling. Obviously, the new type of electrode blocks the electron transfer process completely and has significantly different electron transfer properties from the ATP(1) or NPQD modified electrodes. Indeed, there is some controversy in the literature concerning the nature of the products resulting from the electrochemical oxidation of 4-ATP. For example, Lukkari et al. have reported that, in solution where the pH is g6, oxidation leads to the formation of electroinactive products, while Ohsaka et al. have shown that in solution where the pH is ∼7.2 a stable surface confined redox species is produced.28,29 The results shown here support both reports, as it is shown that the initial transformation of the ATP(1) modified surface to an electroactive NPQD one is quite rapid and that only extensive cycling will result in the formation of a surface modified by NPQM and perhaps other electroinactive products. It is interesting to note that the mass of the electrode (measured prior to and after the above experiments) was not observed to change significantly. In the second stage of modification, the NPQD modified surface (obtained after 5-6 cycles in buffer solution) was exposed to the citrate-stabilized Pt nanoparticle solution overnight and the frequency change observed was -35 Hz (summarized in Table 1). This modified surface is subsequently referred to as an NPQDPt surface. ∆f data for the formation of the ATP-Pt(1) electrode was -187 Hz, and this shows that 5 times less Pt nanoparticles were attached to the NPQD electrode than to the ATP(1) electrode. According to the theory of Lukkari et al.29 and Ohsaka et al.,28 a head-to-tail coupling between two adjacent 4-ATP molecules occurs to form the dimeric system NPQD (see Scheme 1), resulting in a halving of the N-coordination sites available on the electrode surface. It is believed that the fewer available “-NH3+” groups on the NPQD surface allow fewer Pt nanoparticles to become attached to it. In an effort to increase the number of nanoparticles attached to the NPQD surface, the electrode was exposed to the nanoparticles containing solution for a second night. The oscillation frequencies of the quartz crystal were measured before and after exposure to the nanoparticle solution, and no further changes in frequency were apparent, indicating that maximum uptake of nanoparticles had already been achieved. CV in the [Fe(CN)6]3-/4- redox couple solution was performed at the NPQDPt surfaces (after both 1 and 2 days exposure). The voltammograms were found to remain the same, that is, they were identical to the bare electrode data. Once again, cyclic voltammetry of the different surfaces in buffer solution was studied, and these results are shown in Figure 8. Data for a bare electrode are shown for comparison. First, consider an NPQD modified surface. The results for this surface were very similar to those observed for the ATP(1) modified

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Figure 8. Cyclic voltammograms of (i) a bare QCM gold electrode, (ii) an NPQD modified QCM gold electrode, (iii) an NPQD-Pt modified QCM gold electrode prepared by 1 day exposure, and (iv) an NPQD-Pt modified gold QCM electrode prepared by 2 days exposure in 0.1 M phosphate buffer solution (pH 7.0). Scan rate: 100 mV s-1.

surface: no electrochemical signal was apparent. However, although the NPQD-Pt surface did show some enhanced reduction current, the results were still poor in comparison to those of the bare electrode. No evidence for the surface chemistry of the attached Pt nanoparticles was found. These results are also very poor in comparison to the ATP-Pt(3) surface (see Figure 6b), and this could be due to the fact that many fewer Pt nanoparticles have been attached to the NPQD electrode surface compared with the ATP(1) electrode surface, as indicated by the QCM results. These data strongly suggest that a 4-ATP modified electrode interacts much more strongly with Pt nanoparticles than an NPQD electrode and produces an active Pt electrode surface.

4. Conclusions Two different modified gold electrode surfaces, ATP-Pt and NPQD-Pt, have been investigated in this paper using quartz crystal microbalance measurements. A 5-fold increase in the amount of Pt nanoparticles that can be attached to an ATP modified gold electrode compared to an NPQD modified gold electrode was found. In addition, the presence of the Pt nanoparticles, and direct electrochemical communication to them, was established by studying their cyclic voltammetric behavior in buffer solution. Classic cyclic voltammograms of Pt were observed, confirming the presence of Pt nanoparticles on the modified surface. It was also shown to be possible to build up multilayer SAM/nanoparticle modified surfaces while, at the same time, maintaining good electrochemical communication. A 4-ATP SAM can thus be said to provide a very successful route for attachment of nanoparticles in this way. Acknowledgment. The authors acknowledge Dr. Peter Harris and Dr. Chris Stain for technical assistance with the TEM images and the University of Reading for funding. LA802567A