Self-Assembled Carbon Nanotube Electrode Arrays: Effect of Length

Feb 2, 2009 - Paul K. Eggers , Paulo Da Silva , Nadim A. Darwish , Yi Zhang , Yujin Tong , Shen Ye , Michael N. Paddon-Row , and J. Justin Gooding...
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J. Phys. Chem. C 2009, 113, 3203–3211

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Self-Assembled Carbon Nanotube Electrode Arrays: Effect of Length of the Linker between Nanotubes and Electrode Alison Chou, Paul K. Eggers, Michael N. Paddon-Row, and J. Justin Gooding* School of Chemistry, The UniVersity of New South Wales, Sydney, New South Wales 2052, Australia ReceiVed: October 18, 2008; ReVised Manuscript ReceiVed: December 8, 2008

The influence of the length of the carbon chain of a self-assembled monolayer (SAM) on gold electrodes on the electrochemical performance of carbon nanotube arrays attached to the SAM was explored. Four electrode constructs were assessed, all of which were modified with four different lengths (C2-C11) of amine-terminated alkanethiols. The four constructs were gold electrodes modified (1) with SAMs alone, (2) with carbon nanotubes randomly dispersed onto the SAM-modified electrodes by drop coating, (3) with vertically aligned carbon nanotubes formed by self-assembly onto the SAMs, and (4) with vertically aligned nanotubes with ferrocene attached to the nanotubes. By use of ruthenium hexaammine as a redox probe, the attachment of the carbon nanotubes to the SAM, either randomly dispersed or aligned, enabled electrochemistry to be observed at SAMs that were passivating prior to attachment of the nanotubes. The electrochemistry decayed exponentially with methylene chain length as expected but with a surprisingly low attenuation factor (β value) for the nanotube-modified surfaces. For randomly dispersed nanotubes, the β value was 0.27 per -CH2- (s ) 0.04, n ) 4), and for the vertically aligned nanotubes, 0.66 per -CH2- (s ) 0.04, n ) 4). A similar β value of 0.62 per -CH2- for vertically aligned nanotubes with ferrocene attached provided good evidence that the results with ruthenium hexaammine were due to tunneling through the SAM rather than electrochemistry proceeding via defects in the SAM or the nanotubes penetrating the SAM to the underlying electrode. 1. Introduction Nanotube-modified electrodes are attracting enormous interest mainly because of their favorable electrochemistry and small size.1 The former advantage is exemplified by the ability to detect electroactive molecules of biological interest at significantly lower redox potentials than many other electrode materials such as hydrogen peroxide,2 nicotinamide adenine dinucleotide (NADH),3 and DNA.4 The latter advantage is demonstrated in research where carbon nanotubes have been plugged into individual enzymes,5-8 thus allowing electrochemistry to be performed in confined environments. Within this incredibly active research field there are many approaches to modifying electrodes. Modifying electrodes by randomly dispersing the nanotubes over an electrode surface is by far the most common.1 However, how the carbon-nanotube-modified electrode is configured is of great importance to the final electrochemical properties.1,9,10 We have concentrated much of our research efforts on carbon-nanotube-modified electrodes upon understanding what parameters influence the electrochemical response of the final electrode. This work has demonstrated that (1) with purified single-walled carbon nanotubes (SWNTs), the ends of the tubes are much more electroactive than the walls;9 (2) electron transfer through the carbon nanotubes decreases linearly with increased length; (3) electron transfer along the tubes was much more rapid than across tubes;10 and (4) dialysis to remove residual acids increases the rate of electron transport along nanotubes.11 These studies have pointed very strongly to fabricating carbon-nanotube-modified electrodes with the nanotubes aligned normal to the bulk electrode surface. The two main strategies for fabricating vertically * To whom correspondence [email protected].

should

be

addressed:

e-mail

aligned carbon-nanotube-modified electrodes is to grow them vertically12-15 or to self-assemble them vertically.1 We5,8-10,16,17 and others7,18-21 have explored the latter approach of selfassembling carbon nanotubes using variants of an original approach demonstrated by Liu and co-workers.22-25 Modification of electrodes with aligned carbon nanotubes using the self-assembly strategy basically involves acid shortening the carbon nanotubes such that the open tubes have carboxylic acid moieties at their open ends.9,23 The carboxylic acid moieties of the nanotubes dispersed in ethanol are activated with dicyclohexylcarbodiimide and then exposed to aminoalkanethiol self-assembled monolayer (SAM) -modified gold electrode. Amine groups on the distal end of the SAM react with carboxyl groups on the ends of the nanotubes to give nanotubes that stand normal to the electrode surface. Atomic force microscopy (AFM), scanning electron microscopy, and electrochemistry all confirm the nanotubes stand normal to the surface as schematically represented in Figure 1a. In our work the choice of 2-amino-1-ethanethiol (AET) to modify the gold electrode was to ensure electron tunneling through the SAM by minimizing the distance between the nanotubes and the bulk gold electrode. In contrast, Diao et al.18,19 used an almost identical procedure but with C11 11-amino-1-undecanethiol, which forms a passivating SAM on the electrode surface. Despite the blocking nature of the monolayer, upon attachment of the aligned carbon nanotubes, electrochemistry from a redox species in solution similar to a bare electrode was observed. Consistent with these observations has been other work by Shapter and co-workers,17,21 where aligned carbon nanotubes self-assembled onto highly doped silicon with a thin layer of passivating silicon dioxide also showed pronounced electrochemistry with redox-active species in solution only after assembly of the tubes. There are also similar examples with

10.1021/jp809235x CCC: $40.75  2009 American Chemical Society Published on Web 02/02/2009

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Chou et al. TABLE 1: Double-Layer Capacitance Data for NH2(CH2)nSH-SAM on a Gold Electrodea system

Cdl, µF cm-2

bare Au AET/Au AHT/Au AOT/Au AUT/Au

73.5 14.5 9.85 8.03 6.12

a

Capacitances of AET, AHT, AOT, and AUT monolayer films were determined from currents in the double-layer region of cyclic voltammograms: Cdl ) 1/2[(iE,anodic s iE,cathodic)/(dE/dt)], where iE,anodic and siE,cathodic are, respectively, anodic and cathodic charging currents measured at a potential (E) in the double-layer region. dE/ dt is the scan rate (-0.1 V/s).

Figure 1. Schematic illustrations of the constructions of the interfacial composition of chemically aligned oxidatively shortened nanotubes. Two possible electron-transport mechanisms are shown: (a) nanotubes vertically aligned on aminoalkanthiol SAM, with electron tunnels through the nanotube and through the aliphatic chain of the alkanethiol monolayer, or (b) tubes penetrating between the SAM forming a direct link with the gold, held in place by hydrophobic interactions with the SAM.

carbon nanotubes randomly dispersed onto an electrode modified with a passivating SAM where electrochemistry was observed after assembly of the tubes.20,26-28 The question arises as to why, after attachment of carbon nanotubes, an electrode modified with a passivating SAM can exhibit electrochemistry similar to when the SAM is absent. We can envisage two main possibilities as depicted in Figure 1. That is, either the nanotubes are sitting on the surface of the SAM as originally designed (Figure 1a) and electrons tunnel across the SAM more efficiently when the nanotubes are present, or the presence of the nanotubes is causing defects in the SAM, which allow the nanotubes to penetrate through the SAM and make direct contact with the electrode surface such that tunneling through the SAM is not required (Figure 1b). Diao et al.18 have discussed this issue in detail but did not consider the scenario in Figure 1b. It was suggested that, in such systems, electron transfer proceeds via a three-step process: (i) electron tunneling from the gold electrode through the SAM, (ii) electron transport within the SWNTs, and (iii) heterogeneous electron transfer from the ends or sidewalls of the SWNTs to the redoxactive species in solution. The reason electrons could transfer efficiently through the monolayer when the nanotubes were present was attributed to the nanotubes acting as “electron transfer stations” where the large π-conjugated system within the nanotubes can act as both an electron acceptor and an electron donor. Mao and co-workers28 suggested a similar threestep process for electron transfer at randomly dispersed nanotubes on SAM-modified electrodes.

The purpose of this paper is to provide data to support one of the two hypothesized electrode structures depicted in Figure 1 by assembling shortened SWNTs onto alkanethiol SAMmodified electrodes where the SAM is a variety of lengths. Electrodes were modified with SAMs of 2-amino-1-ethanethiol (AET, C2), 6-amino-1-hexanethiol (AHT, C6), 8-amino-1octanethiol (AOT, C8), and 11-amino-1-undecanethiol (AUT, C11) followed by assembly of aligned SWNTs or randomly dispersed SWNTs. These electrodes were characterized electrochemically either by use of ruthenium hexaammine as a redox species in solution or by attaching ferrocenemethylamine to the distal end of the nanotubes (see Figure 2). The experimental design described above aims to make all aspects of preparation of the different electrodes identical with the exception of the number of carbons in the alkanethiol molecule with which the gold surface is modified. In this way, variations in the electrochemistry can be attributed to the different length SAMs between the electrode and the nanotubes. This will enable the two possibilities in Figure 1 to be differentiated. According to studies of electron transfer in organic monolayers,29-32 the rate of electron transfer is expected to decrease exponentially as the distance between the redox center and the electrode surface increases. The through-bond electron transfer rate for a surface confined species is expressed as

kET ) k0 exp(-βd)

(1)

where k0 is a pre-exponential factor, β is the electron tunneling constant, and d is the distance between the redox center and the electrode surface. The tunneling constant can be obtained from the slope of ln k versus d, using the number of methylenes within the alkyl chain. If the nanotubes have direct access to the electrode surface, then the electron transfer rate constant of the attached ferrocene should be invariant with the length of thiol linker. When the nanotubes are specifically attached to the alkanethiol SAM only, the electron transfer kinetics is expected to vary exponentially with the length of the alkanethiol linker. 2. Experimental Section 2.1. Chemicals and Reagents. 2-Amino-1-ethanethiol (AET, C2), and 1,3 dicyclohexylcarbodiimide (DCC) were purchased from Sigma-Aldrich Chemical Co. (Sydney, Australia), and nitric acid, sulfuric acid, and perchloric acid were from Ajax Chemical Company (Sydney, Australia). 6-Amino-1-hexanethiol (AHT, C6), 8-amino-1-octanethiol (AOT, C8), and 11-amino1-undecanethiol (AUT, C11) were from Dojindo Molecular Technologies, Inc. (Japan). Single-walled carbon nanotubes (SWNTs) were HiPco tubes from Carbon Nanotechnologies Inc. Ferrocenemethylamine (Fc) was synthesized by Dr. Michael

Linker Lengths

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Figure 2. Schematic illustrations of the constructions of the 4-base self-assembled monolayers by which electrodes were modified. Electrochemical performances of electrodes modified with the SAM alone, with randomly dispersed SWNTs and aligned carbon nanotubes with ruthenium hexaammine in solution and with ferocene attached, were compared.

Jones following a procedure by Kraatz.33 All aqueous solutions were prepared with Milli-Q water. 2.2. Nanotube Shortening Procedure. The SWNTs were shortened according to the procedure developed by Liu et al.22 A portion (2 mg) of HiPco SWNTs was added to 10 mL of an acid mixture of 3:1 concentrated sulfuric acid and concentrated nitric acid. This mixture was sonicated in a 55 Hz sonicator bath for 4 h. The cut tubes were collected by vacuum filtration on a 0.22 µm nitrocellulose membrane (Millipore, Sydney, Australia) and washed with water until the pH of the filtrate tested neutral, before it was dispersed in 10 mL of ethanol. Typically, a 4-h sonication generates nanotube fragments with a mean length of about 500 nm.9,10 2.3. Preparation of Alkanethiol SAMs. Alkanethiols, NH2(CH2)nSH (n ) 2, 6, 8, or 11), in which the alkyl chains have different lengths were adsorbed from solution onto gold by immersing Au electrodes in 1 mM AET, AHT, AOT, or AUT in ethanol for 24 h. AET, AHT, AOT, and AUT form self-assembled monolayers (SAMs) on gold by chemisorption and these are denoted as AET/Au, AHT/Au, AOT/Au, and AUT/ Au. 2.4. Preparation of Nanotube-Modified Electrodes. SWNTs cut for 4 h were vertically assembled (VA) onto the aminoalkanethiol-modified Au electrodes via DCC coupling as outlined previously.11 Briefly, AET/Au, AHT/Au, AOT/Au, and AUT/Au were each immersed in a 200 µL ethanol solution containing 4-h-cut SWNTs (0.2 mg mL-1) and 2 mM DCC for 4 h. Extensive characterization via AFM, Raman spectroscopy, and electrochemistry has shown that this procedure produces vertically aligned nanotubes.9,16,21,34 These are henceforth referred to as SWNT-VA/AET/Au, SWNT-VA/AHT/Au, SWNTVA/AOT/Au, and SWNT-RD/AUT/Au. To give randomly dispersed nanotubes in electrodes, an aliquot of 5 µL (ca. 1 µg SWNTs) of 4-h-cut SWNTs in ethanol (0.2 mg ml-1) was drop-coated onto the alkanethiol-modified Au electrode, henceforth abbreviated as SWNT-RD/AET/Au, SWNT-RD/AHT/Au, SWNT-RD/AOT/Au, and SWNT-RD/ AUT/Au. To attach ferrocene to carbon-nanotube-modified electrodes, freshly prepared SWNT-VA/AET/Au, SWNT-VA/AHT/Au, SWNT-VA/AOT/Au/Au, and SWNT-VA/AUT/Au electrodes, where the carboxylic acid moieties on the ends of the carbon nanotubes are activated with DCC, were immersed in a 10 mM ferrocenemethylamine solution in ethanol for 6 h. These are henceforth abbreviated as Fc-SWNT-VA/AET/Au, Fc-SWNTVA/AHT/Au, Fc-SWNT-VA/AOT/Au, and Fc-SWNT-VA/ AUT/Au. 2.7. Electrochemical Measurements. Solution-phase Ru(NH3)63+/2+ and Fe(CN)64-/3- measurements were made in 0.05 M phosphate and 0.05 M KCl at pH 7.4. Measurements for Ru(NH3)63+/2+ were performed after the sample solution was

purged with nitrogen gas for 10 min. Surface-confined ferrocenemethylamine was measured in 0.1 M HClO4.35 All solutions were prepared with Milli-Q water. All electrochemistry was carried out at room temperature at ∼25 °C with a three-electrode configuration using a homemade polycrystalline gold disk with a diameter of about 1 mm as described previously,36 Pt counterelectrode, and Ag/AgCl reference. Cyclic voltammetry was performed with a BAS 100B potentiostat (BAS, Lafayette Inc.), while impedance measurements were collected with a Solartron 1287 electrochemical interface with a Solartron 1260 impedance/gain-phase analyzer.

3. Results 3.1. Characterization of Aminoalkanethiol Monolayer as a Tunneling Barrier on Gold Electrodes. If the construct in Figure 1b is possible, the SAM must contain a significant number of defects. As indicated by Su et al.28 and others,17-19,21 good-quality SAMs are essentially crystallinelike and therefore will not allow the nanotubes to penetrate the SAM. Previous studies on nanotubes assembled on SAM-modified electrodes gave little information regarding the quality of the underlying SAM with the exception that the SAM was shown to block a solution-bound redox-active species from reaching the electrode.16,18-20,26 As in many examples,17-19,21 the blocking of a redox species from the electrode surface could be due to either a physical barrier or an electrostatic barrier; such measurements are not a good indicator of the absence of any defects in the SAM. One of the important parameters in preparing SAMs with few defects is to use very smooth gold surfaces.37 As none of these previous samples were prepared on smooth gold surfaces, there is a distinct possibility that the SAMs prepared will have defects. To test whether the construct in Figure 1b is even possible, SAMs with many pinholes were prepared by assembling the SAMs on polycrystalline gold surfaces that were shown to produce SAMs with a significant number of defects.37 The capacitances of SAMs formed from the four different aminoalkanethiols were measured in a solution of 0.05 M phosphate and 0.05 M KCl at pH 7.4 from cyclic voltammetry (Table 1). A plot of the reciprocal of capacitance versus length of the alkyl chain was linear with a slope of 0.013 cm2/µF per CH2 group for mercaptohexyl to undecyl species (C6-C11). This slope is significantly smaller than the value (0.055 cm2/µF) for n-alkanethiol monolayers (CnSH, n > 9) reported by Porter et al.38 and others.39-41 The capacitive behavior of these assemblies resembles that predicted by the Helmholtz theory of the electrical double layer, which models the electrochemical interfaces as

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Figure 3. (a) Comparison of AET-, AHT-, AOT-, and AUT-modified Au, (---) before and (s) after 4-h-cut SWNT attachment by (a) random dispersion or (b) vertical alignment, in an aqueous solution of 0.05 M phosphate and 0.5 M KCl, pH 7.4, containing 10 mM Ru(NH3)63+ at scan rate 0.1 V s-1.

TABLE 2: Summary of Cyclic Voltammetric Data for Ru(NH3)63+/2+ at Alkanethiol/Au and SWNT/Alkanethiol/Aua E°′, mV

ipa/ipc

kapp,b cm s-1

60 ( 5 61 ( 2

-152 ( 2 -144 ( 1

1.03 ( 0.01 0.94 ( 0.02

64 ( 1 104 ( 4 134 ( 49 106 ( 13 62 ( 1 106 ( 9 173 ( 11 211 ( 6

-147 ( -157 ( -156 ( -160 ( -142 ( -150 ( -152 ( -198 (

0.85 ( 0.08 1.00 ( 0.1 0.77 ( 0.02 0.80 ( 0.01 0.94 ( 0.01 1.01 ( 0.04 0.79 ( 0.01 0.38 ( 0.09

(169 ( 31) × 10-3 (124 ( 3) × 10-3 (4.3 ( 0.3) × 10-4 (1.1 ( 0.1) × 10-4 (9.6 ( 0.9) × 10-6 (43 ( 26) × 10-3 (9.3 ( 8.4) × 10-3 (7.5 ( 6.0) × 10-3 (3.6 ( 1.7) × 10-3 (105 ( 39) × 10-3 (6.5 ( 4.4) × 10-3 (1.4 ( 0.3) × 10-3 (0.3 ( 0.1) × 10-3

electrode modification

∆Ep, mV

bare Au AET/Au AHT/Au AOT/Au AUT/Au SWNT-RD/AET/Au SWNT-RD/AHT/Au SWNT-RD/AOT/Au SWNT-RD/AUT/Au SWNT-VA/AET/Au SWNT-VA/AHT/Au SWNT-VA/AOT/Au SWNT-VA/AUT/Au

4 3 4 1 2 4 1 5

a Scan rate ) 0.1 V s-1, reference electrode Ag/AgCl; each data point is derived from three measurements. b Apparent electron-transfer rate constant.

an ideal capacitor.42 An ideal capacitor for which the reciprocal of the capacitance is

C-1 ) d/0

(2)

where d is the thickness of the dielectric medium,  is the dielectric constant of the separation medium, and 0 is the permittivity of free space. Thus, simplistically, an increase of C-1 corresponds to an increase of d (the film thickness) or the change of dielectric constant. The slope of the line of best fit through these data yields a dielectric constant of 11.3. This value is larger than that of 2.6 for n-alkanethiol monolayers,38 suggesting that the monolayer is “wet”. “Wet” indicates that the monolayer is loosely packed with pinholes that are filled up with water or a more porous structure that water generally penetrates. If a dielectric constant of 78 is assumed for water and a value of 2 for a pinhole-free monolayer consisting of all hydrocarbons, then a dielectric constant of 11.3 indicates the SAM contains about 12% water.

With this level of defects in the SAMs, it seems conceivable that nanotubes could penetrate to the electrode surface if such an interaction is favorable. The nanotubes used in this study are single-walled carbon nanotubes with diameters around 1 nm; hence, if any nanotubes can penetrate the defects in the SAMs it would be these tubes. 3.2. Electron Transfer to Ru(NH3)63+/2+ as an Electroactive Species in Solution. Electrochemical behavior of AET/ Au, AHT/Au, AOT/Au, and AUT/Au electrodes in the presence of 10 mM Ru(NH3)63+ is shown in Figure 3. The cyclic voltammogram (CV) of AET/Au electrode displayed welldefined redox waves corresponding to the Ru(NH3)63+/2+ redox transformation with a peak separation of ∼60 mV, indicative of an electrochemically reversible one-electron process.43 Compared to the reversible redox behavior on AET/Au-modified surface, the CV at the AHT/Au electrode showed a peak separation greater than 279 mV at a scan rate of 0.1 V s-1. This was due to the sluggish electron-transfer kinetics through

Linker Lengths

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Figure 4. Cyclic voltammograms at 0.1 V s-1 of Ru(NH3)63+ in aqueous 0.05 M phosphate buffer, pH 7.4. CVs were obtained after immersing AUT/Au in a 4-h-cut SWNT solution for the specified amount of time for nanotube attachment.

Figure 5. Plots of the logarithm of tunneling rate constant versus number of aminothiol chain carbon atoms recorded for (a) aminothiol monolayers, (b) SWNT-VA/SAM/Au, and (c) SWNT-RD/SAM/Au.

the AHT monolayer. With AOT/Au- and AUT/Au-modified electrodes, no distinct peaks associated with the Ru(NH3)63+/2+ redox process could be observed. This passivation of the Ru(NH3)63+ electrochemistry, despite the SAM being wet and hence bearing pinholes, is attributed to a combination of the SAM acting as a barrier for close approach of Ru(NH3)63+/2+ to the electrode and of electrostatic repulsion between positively charged Ru(NH3)63+ ions and the NH2-terminated SAM. The cyclic voltammograms (CVs) tended to that of a bare Au electrode with pronounced anodic and cathodic waves when AET/Au, AHT/Au, AOT/Au, and AUT/Au electrodes were further modified with either randomly dispersed SWNTs (Figure 3a) or vertically aligned SWNTs (Figure 3b). The corresponding CV for each of the modifications is shown in Figure 3 (solid line). ∆Ep for SWNT-RD/AET/Au and SWNT-RD/AHT/Au was 60 mV; ∆Ep for SWNT-RD/AOT/Au was 67 mV; and ∆Ep for SWNT-RD/AUT/Au was 135 mV. For the vertically aligned samples, the ∆Ep values were SWNT-VA/AET/Au, 60 mV; SWNT-VA/AHT/Au, 76 mV; SWNT-VA/AOT/Au, 83 mV; and SWNT-VA/AUT/Au, 204 mV. Thus the presence of nanotubes appears to switch on an efficient pathway for electron transfer that was absent without nanotubes. Previous AFM studies5 have shown that the density of nanotube self-assembled on a surface increases with immersion time. CVs of ruthenium hexaammine at electrodes prepared with different incubation times in SWNT solution are shown in Figure 4. Both the size of the redox peaks in Ru(NH3)63+ and the peak separation are influenced by the time the SAM-modified surface was incubated in SWNT solution, an observation consistent with that of Diao et al.18 ∆Ep for AUT/Au immersed in SWNT solution decreased with increasing incubation time: it was 393 mV after 30 min, 210 mV after 60 min, and 183 mV after 120

min. Diao et al.18 have shown, using Raman spectroscopy, that this increase in peak current and decrease in peak separation correlates with an increase in surface-bound nanotubes. Hence, Figure 4 shows that “switching on” of Ru(NH3)63+ electrochemistry upon adsorption of nanotubes onto the SAM is dependent on the time and hence coverage of the nanotubes. A summary of the electrochemical results for Ru(NH3)63+/2+ at AET/Au, AHT/Au, AOT/Au, and AUT/Au in the absence of SWNTs and in the presence of both randomly dispersed and vertically aligned nanotubes is presented in Table 2. In cases where reversible voltammograms were observed, the peak separation value was used to calculate the rate constant according to Nicholson theory.44 A literature value of 6.0 × 10-6 cm2 s-145 for the diffusion coefficient of Ru(NH3)63+/2+was used to calculate the electron-transfer rate constant. Ruthenium hexaammine is a highly reversible, single-electron, outer-sphere redox species.46 Rate constants for the Ru(NH3)63+/2+ couple have been reported as ∼0.4 cm s-1 at mercury47 and 0.24 cm s-1 at polished glassy carbon electrodes.46 Berchmans et al.48 reported 0.14 cm s-1 for a bare Au surface and 0.0041 cm s-1 for 3-mercaptopropionic acid at ν ) 50 mV s-1. The kET value obtained at the bare Au presented here agrees well with the value reported by Berchmans et al.48 For AHT/Au, AOT/Au, and AUT/Au, where redox peaks are not well-defined, an estimate of the apparent electron-transfer rate constant (kapp) was calculated according to ref 49:

kapp ) RT/(F2RctC)

(3)

where R is the gas constant, T is temperature, F is the Faraday constant, Rct is the charge-transfer resistance (ohms per square centimeter), and C is the concentration of the redox couple. The charge-transfer resistance values were obtained from impedance spectroscopy. There are a number of trends that are clear from Table 2. First, with the exception of the SWNT-VA/SWNT/AUT/Au electrode, in all cases the formal potential (E°′) is consistently around -151 mV (s ) 6, n ) 9) and the ratio of anodic to cathodic currents is close to 1. Both these parameters indicate a well-behaved electrochemical system. It is clear from the data in Table 2 that as the length of the SAM carbon chain increases, the separation between anodic and cathodic peaks also increases. This indicates a decrease in the rate of electron transfer, as is expected for through-bond electron transfer. This observation is the same regardless of whether the electrode is modified only with the SAM or with randomly dispersed or vertically aligned carbon nanotubes. The variation in kapp for the three-electrode systems is shown in Figure 5 as a plot of log kapp versus length of the linker, where the distance is taken as the number of methylene units. As expected, the variation in ln kET is linear

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Figure 6. Cyclic voltammograms of (a) Fc-SWNT-VA/AET/Au, (b) Fc-SWNT-VA/AHT/Au, (c) Fc-SWNT-VA /AOT/Au, and (d) Fc-SWNTVA/AUT/Au in 0.1 M HClO4 at 0.1 V s-1.

TABLE 3: Rate Constants Obtained from Cyclic Voltammetry for Ferrocenemethylamine Immobilized for Fc-SWNT-VA/ NH2(CH3)nSH/Aua AET AHT AOT AUT

Efwhm, mV

∆Ep, mV

E°′

181 ( 20 165 ( 22 186 ( 19 177 ( 13

57 ( 8 88 ( 3 189 ( 13 250 ( 101

375 ( 9 415 ( 20 349 ( 8 370 ( 59

Γa, mol cm-2 (8.0 (4.1 (5.6 (2.3

( ( ( (

a Errors correspond to the standard deviation of six independent measurements. peak area, and Γa is coverage from the anodic peak area.

with linker length, but what is unexpected is that for the different systems, the decay constant (β value) is quite different. For electron tunneling through saturated organic molecules, such as the alkanethiols studied here, such a plot is expected to be linear with a decay constant (β value) of approximately 1.1 per methylene.30 For the aminoalkanethiols (Figure 5a), the plot is linear with a slope of 1.05 per methylene (-CH2-) group (s ) 0.15, n ) 4), which is consistent with the expected value. However, with the SWNT-modified electrodes, although the plots are also linear, the β value is smaller: 0.66 per -CH2- (s ) 0.04, n ) 4) for the vertically aligned samples (Figure 5b) and 0.27 per -CH2- (s ) 0.04, n ) 4) for the randomly dispersed samples (Figure 5c). These values of β are still, however, within values quoted for other systems, although these are for saturated systems. For example, Ohta et al.50 reported a β value of 0.23 Å-1 (equivalent to 0.3 per -CH2-) for electron tunneling from cyanine dye to viologen through a fatty acid, and in DNA systems values between 1.4 and 0.1 Å-1 (1.8 and 0.13 per methylene group) have been reported for double strands,51-55 while Vanmaekelbergh and co-workers56,57 have quoted β-values of 0.5 Å-1 (0.6 per -CH2-) for CdSe quantum dots linked to gold surfaces by nonsaturated dithiols. However, very recently β values have been quoted of 0.69 Å-1 (0.88 per -CH2-) for 4-nm gold particles and 0.40 Å-1 (0.51 per -CH2-) for 2-nm gold particles.58 The linearity of the plots in Figure 5 suggests that length of the linker between nanotubes and electrode influences the electron-transfer process according to an exponential decay implied by the superexchange mechanism. As indicated by the small β value, the length dependence displayed by the randomly dispersed SWNT system is smaller than by the aminothiol SAMs alone. This may provide evidence either for electrostatic accumulation of Ru(NH3)63+ with the change in interface from positively charged to negatively charged upon adsorption of SWNTs or for adsorption of SWNTs onto the SAM, disrupting the SAM. Any significant disruption of the SAM could provide pinholes by which the Ru(NH3)63+ could access the electrode directly and hence cause the unexpected switching on of electrochemistry upon attachment of nanotubes to the SAM. The lower attenuation factor observed with randomly dispersed nanotubes relative to vertically aligned tubes is consistent with

4.2) 1.3) 5.6) 2.9) b

× × × ×

10-11 10-11 10-11 10-11

Γc/Γab

kapp, s-1

1.16 ( 0.41 1.14 ( 0.74 0.97 ( 0.87 1.6 ( 1.1

187 ( 98 10.6 ( 4.2 4.70 ( 3.78 0.63 ( 0.45

Γc corresponds to coverage calculated from the cathodic

nanotubes disrupting the SAM. This is because the randomly dispersed nanotube system has significantly more nanotubes on the SAM surface compared with the vertically aligned system. We are, however, mainly interested in the electrode constructs with vertically aligned nanotubes. To investigate whether disruption of the SAM by the nanotubes was responsible for the onset of electrochemistry upon attachment of nanotubes, vertically aligned nanotubes were assembled, followed by covalent attachment of ferrocenemethylamine to the nanotubes as described previously.10,11 With these constructs, pinholes cannot contribute to the electrochemistry via freely diffusing species reaching the electrode, as all redox centers are attached to the nanotubes. Such an experimental strategy is more problematical and ambiguous with randomly dispersed nanotubes because electron transfer between the bulk electrode and ferrocene is much slower with randomly dispersed tubes and the pathway of electron transfer is very ill-defined.10 3.3. Electron Transfer to Ferrocenemethylamine Covalently Linked to VA-SWNT/n-Aminoalkanethiol/Au. Figure 6 shows representative cyclic voltammograms for FcSWNT-VA/AET/Au, Fc-SWNT-VA/AHT/Au, Fc-SWNT-VA /AOT/Au, and Fc-SWNT-VA/AUT/Au in 0.1 M HClO4 at 0.1 V s-1. Fc-SWNT-VA/AET/Au (Figure 6a) shows redox peaks corresponding to Fc/Fc+ redox couple at E°′ ) ∼0.38 V vs Ag/AgCl, in accordance with previous measurements on ferrocene-terminated alkanethiol SAMs35,59-62 and at nanotubemodified electrodes.10,11,17 Integration of anodic peak area in the voltammograms yields a surface coverage well below the theoretical maximum value (4.5 × 10-10 mol cm-2), based on the assumption of hexagonal packing of the ferrocene moiety. The ratio of anodic to cathodic peak areas, Γc/Γa, was close to 1, again providing evidence for close to ideal electrochemistry (Table 3). The full widths at half-maximum (Efwhm) of the cyclic voltammograms for Fc-SWNT-VA/NH2(CH3)nSH/Au are similar with values for AET ) 181 ( 20 mV, AHT ) 165 ( 22 mV, AOT ) 186 ( 19 mV, and AUT ) 177 ( 13 mV. The broad Efwhm values suggest some deviation from ideal electrochemical behavior.59,63 However, they are consistent with previous observations where the electrode interface is fabricated by placing a SAM on the electrode and then attaching the

Linker Lengths

Figure 7. Linker length dependence of the ferrocene electron transfer rate constant for Fc-SWNT-VA /NH2(CH3)nSH/Au. Data points represent average values and standard deviations of six independent measurements for each alkanethiol.

ferrocene.64-66 This “asymmetric broadening” of the voltammograms in electroactive SAMs, as indicated by the greater than ideal Efwhm and nonzero ∆Ep at 0.1 V s-1, has been attributed to the ferrocene existing in a range of environments.67,68 Similar results are obtained for the other three linkers. For four different electrode constructs with ferrocene attached, the peak current exhibited a linear dependence on the scan rate, which is indicative of surface-confined direct electrochemistry.43 Separation between oxidation and reduction peaks increased with increasing scan rate. The value of ∆Ep for a given scan rate also increased with longer linkers between the nanotubes and the gold electrode. Separation between anodic and cathodic peaks, ∆Ep, increasing with the potential scan rate is indicative of an electrode reaction controlled by electron-transfer kinetics. The apparent rate constant for electron transfer, kapp, was calculated from the variation in peak separation with scan rate by the methodology of Laviron.69 As can be seen from Table 3, kapp varies by ∼300-fold with the chain length of the linker and is largest for the shortest linker (AET). The large uncertainties reflect the heterogeneous nature of the electrodes and the complexity in their assembly. These ferrocene electron-transfer rate constants obtained at the vertically aligned nanotube surface for the present study are smaller than the electron-transfer rate constants for ferrocene through alkanethiol SAMs reported by Liu et al.64 They reported electron-transfer rate constants of 2450 ( 495, 285 ( 106, and 5.2 ( 1.3 s-1 for ferrocenemethylamine immobilized covalently on mixed monolayers containing a methyl-terminated diluent for 6-, 8-, and 11-carbon alkanethiols. The slower electrontransfer rate is expected due to the extra bonds required to attach the ferrocene to the nanotube over the number used by Liu et al.64 The value of kapp demonstrates how rapidly electron transport occurs through the nanotubes, as the nanotubes in this study have a mean length of 507 nm.9 A plot of ln kapp as a function of number of carbons in the linker (spacer distance, d) between the gold surface and SWNTs is shown in Figure 7 with a β value of 0.62 (s ) 0.06, n ) 12). This β value is very similar to that observed for the vertically aligned system with ruthenium hexaammine in solution (Figure 4). This result provides good evidence that switching on of electrochemistry upon attachment of nanotubes is not due to disruption of the SAM causing pinholes and that electron tunneling through the linker between the gold electrode and nanotubes is the limiting process. 4. Discussion In all electrode constructs investigated that contained carbon nanotubes, the apparent rate constant for electron transfer was

J. Phys. Chem. C, Vol. 113, No. 8, 2009 3209 observed to decay exponentially with distance. However, the β value was not the 1.1 per -CH2- expected but a lower value of 0.27 for randomly dispersed SWNTs with Ru(NH3)63+ in solution, 0.67 with vertically SWNTs with Ru(NH3)63+ in solution, and 0.62 per -CH2- for vertically aligned SWNTS with covalently bound ferrocenemethylamine. The similarity in the β value when the redox-active species is attached to the SAM versus when it is freely diffusing in solution suggests the Vertically aligned SWNTs are sitting on the surface of the SAM, the obserVed electrochemistry proceeds Via the through-bond mechanism, and the electron traVels through both alkanethiol and nanotubes attached to the SAM. This also implies that any nanotubes which create defects within the SAM do not produce a noticeable effect in these measurements. Despite the experiments presented in this paper providing strong evidence that when the nanotubes are covalently attached to the SAM, electron transfer proceeds via tunneling through the linking SAM, two questions still arise. Why is the electrochemistry of redox-active species in solution or attached to the ends of nanotubes switched on when the nanotubes are attached to an otherwise passivating SAM? And if tunneling is proceeding through the linking SAM, why is the β value significantly lower than the expected 1.1 per -CH2-? The first question has been discussed by both Diao et al.18 and Mao and co-workers.28 In fact, this phenomenon of electrochemistry of a redox species in solution being switched on by the presence of a conducting material attaching to a passivating SAM on an electrode surface is not unique to nanotubes. Natan and coworkers70 were the first to observe this switching effect with gold nanoparticles on a SAM-modified platinum electrode. Prior to attachment of the nanoparticles, the electrochemistry was suppressed by the SAM. After gold nanoparticles were attached, electrochemistry of methyl viologen in solution was observed. Schriffin and co-workers71-73 reported a similar observation with gold and platinum nanoparticles in a multistep approach. First a gold electrode was passivated with a 1,9-nonanedithiol SAM, and then attachment of gold nanoparticles switched’ electrochemistry of ferricyanide on, followed by the modification of the surface-bound nanoparticles with another passivating SAM of 1,9-nonanedithiol that switched the electrochemistry off again. This process could be repeated several times, with no electrochemistry if the surface was terminated with a SAM and good electrochemistry if the surface was terminated with a nanoparticle. With these studies in mind, we have made modified electrodes with exactly the same four amine-terminated alkanethiols used in the current study and attached 12-nm gold nanoparticles.74 As with the carbon nanotubes, at otherwise passivating SAMs, attachment of the gold nanoparticles switched on the electrochemistry of ruthenium hexaammine. Recently, Fermin and co-workers75-78 in a series of elegant papers showed a similar phenomenon at carboxylic acid-terminated SAMs. In that work polycationic polyelectrolytes were adsorbed onto the SAM, followed by attachment of 20-nm gold nanoparticles. Fermin and co-workers75-78 showed that attachment of the nanoparticles caused a dramatic decrease in charge-transfer resistance and the onset of Faradaic peaks for potassium ferricyanide/ferrocyanide. They also showed that the chargetransfer resistance and peak current were sensitive to the amount of nanoparticles adsorbed,77 an observation consistent with our data depicted in Figure 4. In fact, the Fermin work shows that the electron-transfer rate constant for a single gold particle is the same as the one observed at polycrystalline gold.77 That is, the electrochemistry is the same as though the SAM and polyelectrolyte between the bulk gold electrode and the particle

3210 J. Phys. Chem. C, Vol. 113, No. 8, 2009 is absent. Hence, the observations we and others18,28 have made with carbon-nanotube-modified electrodes are not unique to nanotubes alone but may be a feature of conducting nanomaterials. Diao et al.18 proposed that the SWNTs act as Coulombic islands (electron relay stations) that can shuttle electrons between bulk gold electrode and redox species in solution. The π-conjugated system within the nanotubes would make it possible for nanotubes to accept or donate electrons depending on the potentialofthebulkelectrode.Similarly,Murrayandco-workers79,80 showed that gold nanoparticles can accept or donate several electrons (depending on their size) and hence can also act as a Coulombic island. Diao et al.18 have put forward an elegant argument as to how this is possible by the potential shifting the Fermi level of the electrode above that of the nanotubes, such that electrons transfer into the nanotubes, or below the nanotubes, such that electrons are accepted from the nanotubes. Implicit in this description is good electronic coupling between electrode and nanotube, which is provided by the covalent bond between vertically aligned SWNTs and the SAM. The importance of the covalent bond for good electronic coupling to allow efficient electron transfer is well demonstrated by comparison of SAMs with attached redox-active species compared with redox-active species in solution.81,82 Similarly, Whitesides and co-workers83 have demonstrated the importance of chemical bonds for electron transfer through electrode-organic moleculeelectrode molecule junctions. They have shown that the most efficient electron transfer through the junction, as a function of potential, is achieved with a covalent bond. In the case of randomly dispersed SWNTs, however, there is no covalent bond. The work by Whitesides and co-workers83 also shows that more rapid electron transfer proceeds through junctions that contain ionic and hydrogen bonds than through junctions that contain van der Waals bonds. In the absence of a chemical bond, through-space electron transfer will have to proceed, which, as observed with passivation of the electrode in the absence of nanotubes, is far less efficient. In the case of the randomly dispersed SWNTs, where in this study the carboxylic acidterminated ends will form hydrogen bonds and ionic bonds with the amine-terminated SAM, the electronic coupling is sufficient for redox chemistry of Ru(NH3)63+ to be observed at these SWNT-modified electrodes. This brings us to the lower than expected β value. We do not have a complete explanation at this time, but work with nanoparticle-modified electrodes, along with our data presented here, warrants further discussion. Our study is the first to explore the impact of linker length on the performance of nanotubemodified electrodes. Certainly, in the case of electron transfer to ferrocene attached to the ends of aligned SWNTs, the uncertainty in each rate measurement is high which mean there is high uncertainty in the β value. However, the β value calculated from attached ferrocene is very similar to that calculated for vertically aligned SWNTs with the species in solution. Hence it seems unlikely that the lower-than-expected β value is due to uncertainty in the line of best fit. The similarity between the β value when the redox-active species is attached to the nanotubes and when in solution strongly suggests that the solution-based electrochemistry is not occuring via pinholes. The low β values of 0.6-0.7 we record with vertically aligned nanotube-modified electrodes are consistent with the values we observe for similar electrodes where the nanotubes are replaced by gold nanoparticles (β value 0.4-0.5 Å-1).58 In the study by Wang et al.58 where conductivity of nanoparticle networks was measured, particles are separated by alkanedithiols of different lengths. The β values achieved were shown to be a function of

Chou et al. particle size, which provides strong evidence for quantum effects influencing electron transfer through SAMs. Although considerably more work remains to be performed, the Wang study may provide the first insight as to why such low β values were measured with nanotube-modified electrodes. Further clues may come from the work by Fermin and co-workers78 on assemblies of gold electrode-SAM-polyelectrolyte-nanoparticle. In that work, where distance dependence on the rate of charge transfer was explored, the charge-transfer resistance was shown to be independent of the length of the SAM above five methylene groups. The implication of this observation is that the ratelimiting step is the charging of nanoparticles by the redox-active species. The distance independence is rationalized by a “hot electron transfer” mechanism. Note that this system is a little different from our nanotube experiments and the other nanoparticle studies discussed here because the polyelectrolyte used to anchor the nanoparticles to the SAM also concentrates the redox-active species in the vicinity of the nanoparticles, and both the nanoparticle and the redox probe (ferricyanide) are negatively charged. Whether these differences could account for the difference in distance dependence behavior is unclear at this time, but it is apparent that nanoscale materials play an important part in this unusual electron-transfer behavior. Thus we speculate that electron transfer at these electrode constructs may be influenced by both tunneling through the SAM and charging the nanotubes. 5. Conclusions The initial premise of this work was to determine whether nanotube electrode arrays linked to electrodes via long-chain self-assembled monolayers was due to nanotube penetrating the insulating layer or whether tunneling through the passivating layer was facilitated by the nanotubes. To answer this question, SAMs with considerable defects of four different lengths were prepared and nanotubes were attached randomly or aligned. Electrochemistry was performed with redox species in solution or attached to the nanotubes. The results provide strong evidence for tunneling through the SAM rather than nanotubes disrupting the SAM. The electron-transfer rate at the nanotube-modified electrodes decayed exponentially with distance but with a lowerthan-expected attenuation factor. We speculate that attachment of nanotubes to the SAM increases electronic coupling between the underlying electrode and redox species in solution, with the nanotube serving as a redox relay station. It is suggested that the rate of electron transfer may be influenced both by SAM length and by charge transfer to the nanotubes, hence causing a lower-than-expected β value. Acknowledgment. This research was supported under the Australian Research Council’s Discovery Projects funding scheme (Project DP0556397). References and Notes (1) Gooding, J. J. Electrochim. Acta 2005, 50, 3049–3060. (2) Wang, J.; Musameh, M.; Lin, Y. H. J. Am. Chem. Soc. 2003, 125, 2408–2409. (3) Musameh, M.; Wang, J.; Merkoci, A.; Lin, Y. H. Electrochem. Commun. 2002, 4, 743–746. (4) Heng, L. Y.; Chou, A.; Yu, J.; Chen, Y.; Gooding, J. J. Electrochem. Commun. 2005, 7, 1457–1462. (5) Liu, J. Q.; Chou, A.; Rahmat, W.; Paddon-Row, M. N.; Gooding, J. J. Electroanal. 2005, 17, 38–46. (6) Yu, X.; Chattopadhyay, D.; Galeska, I.; Papadimitrakopoulos, F.; Rusling, J. F. Electrochem. Commun. 2003, 5, 408–411. (7) Patolsky, F.; Weizmann, Y.; Willner, I. Angew. Chem., Int. Ed. 2004, 43, 2113–2117.

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