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Aug 3, 2010 - A gold electrode is sequentially modified by the SAM of n-octadecyl ... graphene/SAM modified Au electrode possesses electrochemical ...
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J. Phys. Chem. C 2010, 114, 14243–14250

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Study of Heterogeneous Electron Transfer on the Graphene/Self-Assembled Monolayer Modified Gold Electrode by Electrochemical Approaches Xiang Xie, Keke Zhao, Xiaodong Xu, Wenbo Zhao, Shujuan Liu, Zhiwei Zhu, Meixian Li, Zujin Shi,* and Yuanhua Shao* Beijing National Laboratory for Molecular Sciences, College of Chemistry and Molecular Engineering, Peking UniVersity, Beijing 100871, China ReceiVed: March 18, 2010; ReVised Manuscript ReceiVed: July 18, 2010

The electrochemical behaviors of graphene sheets attached to a self-assembled monolayer (SAM) on a gold electrode have been investigated. A gold electrode is sequentially modified by the SAM of n-octadecyl mercaptan (C18H37SH), followed by controllable adsorption of graphene sheets to obtain a graphene/SAM modified Au electrode. The graphene/SAM modified Au electrode is characterized electrochemically by using ruthenium hexaammine (Ru(NH3)63+) as a redox probe, and by atomic force microscopy (AFM) and scanning electron microscopy (SEM). The experimental results indicate that the heterogeneous electron transfer (ET) blocked by the SAM can be restored by graphene sheets and that the graphene/SAM modified Au electrode has a smaller interfacial capacitance, as compared with that of a bare Au electrode. The apparent ET rate constant of Ru(NH3)63+, kapp, on the graphene/SAM modified Au electrode has been also evaluated quantitatively by cyclic voltammetry (CV) and scanning electrochemical microscopy (SECM), and is equal to 4.2 × 10-2 and 6.8 × 10-2 cm s-1, respectively. In addition, the electrochemical responses of free bases of DNA (guanine (G), adenine (A), thymine (T), and cytosine (C)) on the graphene/SAM modified Au electrode show that the graphene/SAM modified Au electrode possesses electrochemical properties similar to those of a graphene modified electrode rather than an Au electrode. Introduction Graphene, the basic building block for graphitic materials, is a two-dimensional and one-atom-thick sheet of sp2 bonded carbon.1 Because of its unique optical,2 electronic,3 mechanical,4 thermal,5 and electrochemical properties,6 graphene has attracted much attention since it was first experimentally discovered in 2004 by micromechanical cleavage.3 Graphene has shown several advantages compared with those of other carbon materials, such as graphite or carbon nanotubes, in electrochemical applications.7 First, graphene has a huge surface area (2630 m2 g-1), which is much larger than that of graphite (∼10 m2 g-1) or carbon nanotubes (1315 m2 g-1).8 The high surface area is very attractive for energy storage applications, such as, in supercapacitors,6 fuel cells,9 and rechargeable Li ion batteries.10 Second, graphene exhibits high conductivity and mediates electron transfer (ET) at its edge planes.11 Those properties have potential applications in electrochemical sensing and biosensing.12 Third, graphene bears high π-conjunction and hydrophobic properties, which can provide a platform for immobilizing organic and inorganic molecules.13 Finally, graphene is usually obtained from graphite, and thus, it is much easier to have high purity graphene in contrast to the production of carbon nanotubes.14 In addition, graphene nanoplatelets formed by stacked graphene sheets with a typical thickness of 2-10 graphene layers are equally important. Until now, most of the electrochemical investigations of graphene and graphene sheets were based on the comparisons with carbon nanotubes and other graphite materials due to the similar electrochemical properties.15 The spontaneous formation of a self-assembled monolayer (SAM) of alkanethiols onto a gold electrode can establish useful * To whom the correspondence should be addressed. E-mail: zjshi@ pku.edu.cn (Z.S.); [email protected] (Y.S.).

structures on the electrode surface. The SAM of long carbon chains acts as an insulating barrier which can block ET between the bare electrode and redox species in solution.16 However, with nanomaterials attached to the SAM by either covalently17 or noncovalently18 bonding, ET can be restored, and the redox processes through the SAM can be investigated. On the basis of such a strategy, metal nanoparticles17a-e,18a-d and carbon nanotubes17f,g,18e,f have been employed to modify the SAM coated electrodes and used for the development of sensors and detection methods. The mechanism of ET restoration has been proposed.17,18 The observed enhancement of ET kinetics based on the adsorption of nanomaterials is subjected to further quantitative investigation of electronic communication between nanostructures and electrode surfaces. Murray et al.19 have estimated a heterogeneous ET rate constant in the order of 100 s-1 between Au electrodes and Au nanoparticles immobilized by carboxylate bridges using transient electrochemical techniques such as cyclic voltammetry and potential step approaches. Both techniques are experimentally straightforward, and the variation of the rate constant with a wide range of potentials can be observed. However, these methods are limited by the high resistive potential drop and double layer charging current, which may affect the reliability of the results obtained. Scanning electrochemical microscopy (SECM) is a powerful technique for studies of electrochemical processes in localized spaces including the sensing of electron, ion, and molecular transport in a wide range of materials.20 Compared with other electrochemical techniques, SECM has advantages of the steadystate approach, such as eliminating charging current. In addition, the small size of the SECM tip can minimize the iR drop and achieve very high mass transport, which can be used to measure fast charge (electron and ion) transfer processes. SECM was

10.1021/jp102446w  2010 American Chemical Society Published on Web 08/03/2010

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SCHEME 1: Graphical Illustration of the Protocol for the Fabrication of a Graphene/SAM Modified Gold Electrode and the Mechanism of Heterogeneous ET on the Graphene Modified Electrode

previously reported to study adsorption kinetics of n-alkanethiols on a gold electrode21 and the menadione permeability through molecular monolayers.22 Mirkin et al.23 have developed a new approach for measuring the rate constants of ET across SAM by SECM. By using a high concentration of redox mediator in solution, very fast heterogeneous (108 s-1) and bimolecular (1011 mol-1 cm3 s-1) ET rate constants have been obtained. The present work studies the fabrication and characterization of graphene/SAM modified gold electrode and the kinetics and mechanism of graphene sheet mediated ET on such chemically modified electrodes. As illustrated in Scheme 1, n-octadecyl mercaptan (C18H37SH) has been chosen as a bifunctional bridge to form a stable SAM on the surface of a bare gold electrode by the formation of Au-S bonds. This is because, as reported previously,17,18 carbon chains are long enough that the SAM can block ET on the gold electrode almost completely. Then, graphene sheets can be absorbed onto the hydrophobic monolayer with strong hydrophobic interaction and π-conjunction. Atomic force microscopy (AFM) and scanning electron microscopy (SEM) images show that graphene sheets can form random nano- or submicroarrays on the SAM. The electrochemical performances of the graphene/SAM modified gold electrode are investigated by cyclic voltammetry (CV), electrochemical impedance spectroscopy (EIS), differential pulse voltammetry (DPV) and SECM. The experimental results show that the blocked ET by the SAM with Ru(NH3)63+ as the indicator is restored almost completely after the adsorption of graphene sheets, which are consistent with observations reported by others.17,18 SECM results indicate that the rate constants of both electron tunneling through the SAM and fast ET on the graphene/SAM modified Au electrode. In addition, by using free bases of DNA as redox probes, we have observed that the graphene/SAM modified Au electrode has the similar electrochemical properties of a graphene modified electrode rather than an Au electrode. Experimental Section Chemicals. n-Octadecyl mercaptan (C18H37SH) and ruthenium hexaammine chloride (Ru(NH3)6Cl3) were purchased from Fluku and Aldrich, respectively. N,N-Dimethylformamide (DMF) and other reagents were purchased from Beijing Chemical Corporation. All chemicals were at least of analytical grade. All of the aqueous solutions were prepared with doubly distilled water. Graphene sheets were synthesized by an arc-discharge method.24 Fabrication of the Graphene/SAM Modified Au Electrodes. An Au electrode was polished with alumina powder (1.0 and 0.05 µm) and sonicated in ethanol and doubly distilled water for 3 min. Then, the electrode was immersed in a concentrated sulfuric acid and hydrogen peroxide mixture (3:1) (Caution: this solution is a very strong oxidizing agent and is very dangerous to handle in the laboratory; protective equipment including gloves, goggles, and face shields should be used at all times; and all operations should be completed in

the experimental hood) for 20 min to remove all adsorbed alkanethiols from previous experiments and from other possible contamination, then thoroughly rinsed with doubly distilled water. After that, the electrode was electrochemically cleaned by consecutive cycling in 0.50 M H2SO4 between -0.20 and 1.50 at 0.10 V/s until a characteristic cyclic voltammogram of a clean Au electrode was obtained. The clean electrode was immersed in an ethanol solution containing 4.0 mM C18H37SH for at least 24 h at room temperature (22 ( 2 °C) to form SAM on the surface. The SAM modified Au electrode (denoted as SAM/Au electrode) was then rinsed with ethanol and dried under pure N2 flow. Graphene sheets can be adsorbed onto the SAM/Au electrode by putting it into a graphene dispersion in DMF (1.0 mg/mL) for 30 min at room temperature, and finally, the graphene/SAM/Au electrode was thoroughly rinsed with DMF and doubly distilled water and then dried using pure N2 flow before used. Apparatus and Measurements. CV and DPV were performed with a CHI 660C electrochemical workstation (CH Instruments Inc.). EIS was performed with Autolab PGSTAT 302N featuring a Frequency Response Analyzer module. The bare Au and modified Au electrodes were used as working electrodes and a platinum wire and a saturated calomel electrode (SCE) as the counter and reference electrodes, respectively. All of the solutions were purged with pure nitrogen for at least 15 min to remove oxygen prior to experiments. The parameters applied for DPV were as follows: 50 mV amplitude, 50 ms pulse width, 4 mV step, and 200 ms pulse period. EIS was recorded at the equilibrium potential (Eeq ) -0.18 V), with an amplitude of 10 mV rms and in the frequency range between 10 mHz and 10 kHz. The SECM approach curves were obtained with a 25 µm diameter Pt SECM tip using a CHI 900 SECM setup (CH Instruments Inc.), employing a three-electrode cell. The bare Au and modified Au electrodes were used as working electrodes, and a platinum wire and a Ag/AgCl wire acted as the counter and reference electrodes, respectively. The RG value (RG ) rg/a, where rg is the radius of insulating layer + a, and a is the radius of the tip) of the tip used in this work was about 5. The tip was polished with 0.05 µm alumina before each experiment. The bare Au and modified Au electrodes were attached to the bottom of the Teflon cell used as substrates. Scanning electron microscopy (SEM) images were obtained with Hitachi S-4800. Atomic force microscopy (AFM) measurements were conducted using a Veeco DiInnova system in tapping mode. Results and Discussion Characterization of Graphene Sheets and the Graphene/ SAM Modified Gold Electrode. The synthesized graphene sheets are first dispersed in ethanol and then deposited onto the surface of the Si/SiO2 substrate. The morphology and height of graphene are characterized by AFM. A typical AFM image in Figure 1 shows that they are graphene sheets but not a single

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Figure 1. Tapping mode AFM image and height profiles of graphene sheets on the Si/SiO2 substrate.

Figure 2. Typical (a) AFM and (b) SEM images of graphene sheets deposited onto the surface of the SAM modified Au electrodes by immersing the SAM modified Au electrodes into a graphene dispersion in DMF (1.0 mg/mL) for 30 min.

Figure 4. Nyquist plots for Au electrodes with different surface conditions obtained in 0.10 M PBS containing 1.0 mM Ru(NH3)6Cl3 (pH 7.0): (a) bare Au, (b) SAM/Au, and (c) GN/SAM/Au electrodes.

Figure 3. Cyclic voltammograms of (a) bare Au, (b) SAM/Au, and (c) GN/SAM/Au electrodes in 0.10 M PBS containing 1.0 mM Ru(NH3)6Cl3(pH 7.0). The scan rate is 50 mV/s.

graphene and the thickness of graphene sheets to be generally around 0.7-1.5 nm corresponding to 2-4 layers.24 The area of the graphene samples is in the range of 1.0 × 104-4.0 × 104 nm2. The surface of the graphene sheets/SAM modified gold electrode (denoted as GN/SAM/Au electrode) is characterized by AFM and SEM, and which can provide information about how graphene sheets are distributed on the surface of SAM/Au electrode. Because of their hydrophobic property and high π-conjunction, graphene sheets adsorbed onto the SAM are as nano- and submicro-isolated islands, rather than a homogeneous monolayer, as shown in Figure 2a (AFM image). The SEM image shown in Figure 2b provides more detailed structures of graphene islands, from which the high density of edge-plane graphene sheets can be observed. However, it is hard to see whether the graphene sheets could penetrate the SAM on the basis of these images. Before the attachment of graphene sheets, the electrochemical properties of the SAM/Au electrode are characterized by CV and EIS. Figure 3 shows the voltammetric responses for the bare Au, SAM/Au, and GN/SAM/Au electrodes in the presence of 1.0 mM Ru(NH3)63+. As shown in Figure 3a, a pair of welldefined redox peaks is obtained on the bare Au electrode.25 Compared with the nice CV curve on the bare Au electrode,

no redox peaks are obtained on the SAM/Au electrode, indicating that the SAM greatly inhibits the heterogeneous ET between the electrode and the redox species in solution (Figure 3b) (for more details, see Figure S1 in Supporting Information). After the adsorption of graphene sheets on the surface of the SAM, the cyclic voltammogram (Figure 3c) shows a nearly reversible ET behavior with a peak separation of about 60 mV just as that observed for a bare gold electrode under the same conditions and suggesting that the heterogeneous ET blocked by the SAM is largely restored by graphene sheets. As shown in Figure 4, the charge transfer resistance (Rct) on the SAM/Au electrode is very large due to the insulating feature of the SAM (Figure 4b). After the adsorption of graphene sheets, the Rct is significantly decreased (Figure 4c), even to the same level as that of the bare Au electrode (Figure 4a), showing that ET is again much more effective and that the adsorption of graphene sheets does restore ET. The electrochemical behaviors of the GN/SAM/Au electrode are very similar to those of nanomaterials18 formed on the insulating monolayer deposited on gold electrodes. We have found that the adsorption time and concentration of graphene dispersions have great influence on the electrochemical behaviors of the GN/SAM/Au electrode. At lower concentration (1.0 × 10-3mg/mL), the peak separation decreases, while the current density increases as the time of adsorption is increased up to 30 min, as shown in Figure 5a, indicating the increasing of surface density of graphene sheets, which is consistent with previous works.26 But a higher concentration (1.0 mg/mL) leads essentially to nice redox curves within a very short time, as shown in Figure 5b (also see Figure S2 in the Supporting Information). It is clear that the current-potential relationship is dramatically affected even after 1 min of deposition. An adsorption time of up to 5 h shows no significant changes in the cyclic voltammograms. These results indicate that the interaction between graphene sheets and the

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Figure 5. Cyclic voltammograms as a function of adsorption time in 0.10 M PBS containing 1.0 mM Ru(NH3)6Cl3 (pH 7.0). The scan rate is 50 mV/s. The SAM/Au electrodes were immersed in (a) 1.0 × 10-3 mg/mL and (b) 1.0 mg/mL graphene dispersions in DMF with different time. (b) The adsorption time is from inner to outer: 1, 10, 60, 120, 180, 240, and 300 min.

SAM surface is strong and stable enough by the π-π stacking interaction and the hydrophobic interaction. To confirm the key role of graphene sheets in the restoration of ET, whether the graphene sheets can penetrate the SAM needs to be discussed. We suggest that graphene sheets might not penetrate through the SAM. This is because the formation of the SAM of alkanethiols usually undergoes a crystallization step and forms a compact and rigid structure due to the formation of Au-S bonds and the strong hydrophobic interaction among the long alkyl chains.27 It has been reported that the SAM is able to keep its integrity unless under extreme conditions, such as an extreme electric field or high temperature.27a In the present case, the CV result in Figure 3b (also Figure S1 in Supporting Information) shows that no significant Faradiac current could be observed on the SAM/Au electrode and that this is the characteristic of the heterogeneous ET through the SAM via a tunneling process, rather than a pinhole effect.28 However, Gooding et al.18f studied the effect of the length of linkers between carbon nanotubes and the gold electrode and reported that SWNTs, covalently or randomly dispersed onto insulating organic films, were sitting on the surface instead of intercalating into the SAM. Thus, the observed restoration of ET on the GN/ SAM/Au electrode might not be due to the penetration or intercalation of graphene sheets through the SAM. We have also run a blank experiment to rule out the possibility that C18H37SH molecules desorbed in DMF and formed pinholes during the attachment of graphene sheets. The newly prepared SAM/Au electrode is immersed in DMF, but without graphene sheets, for at least 24 h at room temperature. The cyclic voltammetric curve of the DMF treated SAM/Au

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Figure 6. (a) Cyclic voltammograms of the GN/SAM/Au, DMF treated SAM/Au and untreated SAM/Au electrodes in 0.10 M PBS containing 1.0 mM Ru(NH3)6Cl3 (pH 7.0). The scan rate is 50 mV/s. (b) The magnified cyclic voltammograms of DMF treated and untreated SAM/ Au electrodes.

electrode is almost identical to that of untreated SAM/Au electrode, as shown in Figure 6, indicating that no detectable desorption or local defects are generated in the SAM. Thus, the solvent (DMF) does not disturb the SAM. The restoration of heterogeneous ET on the GN/SAM/Au electrode is consistent with the observations of nanomaterial (covalently17 and noncovalently18) modified gold electrodes via insulating organic films. Schiffrin et al.17b observed the restoration of ET blocked by the SAM of 1,9-nonanedithiol between the gold electrode and redox species in solution with the attachment of gold nanoparticles onto the SAM. In the following investigations,17c,d they believed that at least two processes in series present in the charge transfer system: electron tunneling from the metal electrode to nanoparticles and ET from the nanoparticles to the redox species in solution. Mao et al.18e prepared the multiwalled carbon nanotube (MWNTs) film electrodes by controllable adsorption of MWNTs onto the hydrophobic monolayer of C18H37SH self-assembled onto a gold electrode through the hydrophobic interaction between MWNTs and the SAM. They suggested a similar two-step process for ET at randomly dispersed MWNTs on the SAM/Au electrode. Similar observations have also been reported by Gooding’s group.18f Fermin et al.18a-d studied the ET restoration processes on the basis of metal nanoparticles which could be attached to the self-assembled films by electrostatic interactions, and they proposed that the restoration of ET was a resonant hot electron transfer process. Possibly similar to the role of nanomaterials,17,18 graphene sheets adsorbed on the SAM/Au electrode can also relay ET between the gold electrode and the species in solution through the insulating SAM. The mechanism of whole charge transfer on the GN/SAM/Au electrode can be described in the following two steps: (1) heterogeneous ET between graphene sheets and redox species in solution and (2) electron tunneling between the gold electrode and graphene sheets, as schematically shown

Heterogeneous Electron Transfer in Scheme 1. The first step in the ET process is similar to that of a graphene electrode.11c In the second step, the large π-conjugated system would make graphene sheets to be both acceptor or donator of electrons, which allows not only facile ET between graphene sheets and redox species but also efficient electron tunneling between graphene sheets and the gold electrode.17,18 Thus, graphene sheets here can serve as electron relay stations that mediate electrons between the gold electrode and redox species. However, compared with the bare Au electrode in Figure 3, the peak currents of the GN/SAM/Au electrode are always slightly smaller, although the specific area of electrode is greatly increased after the adsorption of graphene. This phenomenon could be possibly elucidated by the similar diffusion characteristics of the GN/SAM/Au electrode to those of closely packed microelectrode arrays.29 For the microelectrode arrays controlled by mass transport, when the diffusion layer thickness is small compared to the size of the active spots and the distance between them, the measured area reflects the real surface area of the rough electrode; however, if the diffusion layer thickness is much larger than the size of the active spots and the distance between them, the separated diffusion fields merge into a single larger field, which has the same diffusion area as that of the entire array.30 For the GN/SAM/Au electrode which is covered by a blocking monolayer with an attachment of graphene sheets through which ET can be restored, the size of adsorbed graphene sheets and the distance between them are much smaller compared with the diffusion layer thickness (10-3 cm) built up under the experimental conditions, as shown as in Figure 2. Thus, the diffusion layers generated by graphene sheets will overlap and finally produce one uniform diffusion layer, which has the same geometric area as that of the underlying gold electrode. However, compared with a bare Au electrode, graphene is antifouling and easily modified due to high π-conjunction and hydrophobic properties. Therefore, the GN/ SAM/Au electrode is not only useful for fundamental studies but also for electrochemical applications. Electron Transfer Kinetics on the GN/SAM/Au Electrode. As a two-dimensional carbon material, graphene has the same basal and edge planes as those of HOPG.7 It is known that the rate of ET at the edge planes of carbon materials is several orders of magnitude higher than that at the basal planes. The reason is that the electronic density of states (DOS) near the Fermi level of the edge planes is much larger than that of the basal planes due to the existing defects on the edge planes.7 With a higher DOS, there are more electrons with suitable energy for ET to a redox system.31 After adsorbed onto the SAM, graphene sheets exposed abundance edge planes to the electrolyte and induced sufficient DOS for fast ET. However, as a zero-gap semiconductor, graphene bears remarkably high electron mobility and can enhance ET after absorbtion onto the SAM/Au electrode.1 Thus, the adsorption of graphene sheets onto the SAM/Au electrode can mediate heterogeneous ET effectively, which would be used in many fields of electrochemical implication and analysis. As shown in Figure 2b, the GN/SAM/Au electrode could possess the electrochemical behavior of edge planes. Herein, we have explored the ET kinetics of Ru(NH3)63+on the GN/ SAM/Au electrode using both CV and SECM in a more quantitatively way. On the basis of the cyclic voltammograms of Ru(NH3)63+on the GN/SAM/Au electrode (Figure S4 in Supporting Information), the apparent rate constants of ET, kapp, on the GN/SAM/Au electrode can be calculated according to

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Figure 7. SECM approach curves obtained on (a) the SAM/Au and (b) the GN/SAM/Au substrates in 0.10 M PBS containing 1.0 mM Ru(NH3)6Cl3 (pH 7.0). The tip scan rate is 0.4 µm/s. The solid line is the theoretical SECM curve.

the Nicholson theory,32 and the kapp value obtained is 4.2 × 10-2 cm s-1. Since ET kinetics obtained for a conventional electrode by cyclic voltammetry is always affected by the iR drop, we have also employed SECM to evaluate the rate constant of ET reactions by fitting theoretical and experimental approach curves obtained with the GN/SAM/Au electrode as a substrate. The approach curves are taken with a Pt tip approaching the SAM/ Au and GN/SAM/Au electrodes in 0.10 M PBS containing 1.0 mM Ru(NH3)63+ (pH 7.0). The potentials applied at the tip and substrate are chosen from cyclic voltammograms so as to reduce Ru(NH3)63+ at the tip and to oxidize it back at the substrate under diffusion controlled conditions. The expression of the tip current is given as a function of the normalized tip-substrate separation L (d/a) as20

ITk )

+ [ L(10.78377 + 1/Λ) ]

0.68 + 0.3315 exp(-1.0672/L) × (1 - ITins /ITc) + ITins (1) 1 + F(L, Λ) where Λ ) kd/D, with k the apparent limiting heterogeneous rate constant, D the aqueous diffusion coefficient, and F(L,Λ) ) (11 + 7.3Λ)/[Λ(110 - 40L)]. ITc and ITins represent the tip current for the conductive and insulating substrate, respectively. The expressions for these as a function of the normalized distance L ) d/a are as follows:

ITc ) 0.78377/L + 0.3315 exp(-1.0672/L) + 0.68

(2) ITins ) 1/(0.15 + 1.5358/L + 0.58 exp(-1.14/L) + 0.0908 exp[(L - 6.3)/1.017L)]) (3) ITk, ITc, and ITinsare normalized by the tip current at an infinite tip-substrate distance. Figure 7a shows the normalized current-distance curve for a Pt tip held at -0.5 V (vs Ag/AgCl) approaching the SAM/ Au electrode in the presence of 1.0 mM Ru(NH3)63+. The substrate potential, Esub, is kept at 0.1 V (vs Ag/AgCl). The apparent limiting heterogeneous rate constant is obtained by fitting the theoretical and experimental approach curves. The feedback current is low enough that the approach curve fits the theory for an insulating substrate, and the apparent heterogeneous rate constant is too low to be measured under these

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Figure 8. Cyclic voltammograms obtained at (a) bare Au, (b) SAM/ Au, and (c) GN/SAM/Au electrodes in 0.10 M PBS with saturated N2 (pH 7.0). The scan rate is 50 mV/s.

conditions. Thus, for the SAM/Au electrode, the current could only be attributed to direct electron tunneling. This result is consistent with earlier studies of the good blocking properties of long-chain alkylthiol monolayers,21-23 and shows that the SAM is densely packed and essentially pinhole free. After adsorbed onto the SAM/Au electrode, graphene sheets can restore ET between the gold electrode and the redox species in solution. The tip current changes to positive feedback while approaching the GN/SAM/Au electrode, as shown in Figure 7b. The apparent heterogeneous rate constant is evaluated to be equal to 6.8 × 10-2cm s-1 by fitting the theoretical and experimental approach curves. This rate constant is consistent with the reported value.33 However, the value obtained by CV is lower and is probably due to the iR drop contributions for the measured peak potential. This further confirms that the ET on the GN/SAM/Au electrode is restored and that the redox process on the modified electrode is effective. Interfacial Capacitance of the GN/SAM/Au Electrode. Figure 8 shows the interfacial capacitances of bare Au, SAM/ Au, and GN/SAM/Au electrodes in 0.10 M PBS with saturated N2. The interfacial capacitance of the SAM/Au electrode (Figure 8b) decreases significantly compared to that of the bare Au electrode (Figure 8a) due to an insulating SAM. However, after the adsorption of graphene sheets onto the SAM/Au electrode, ET is restored mostly without significantly increasing the interfacial capacitance (Figure 8c). As a result, the capacitance of the GN/SAM/Au electrode is slightly larger than that of the SAM/Au electrode but much smaller than that of the bare Au electrode. On the basis of the Helmhotlz theory,34 the double layer interface is considered as an ideal capacitor, and the capacitance can be described as follows:

1/C ) dm /ε0εmA

(4)

with dm being the thickness of the SAM; ε0, the standard dielectric constant; εm, the dielectric constant of the SAM; and A, the real surface area. The SAM formed by alkanethiols with long chains has a low dielectric constant compared with that of a gold electrode and results in a low capacitance. After the adsorption of graphene sheets, the small capacitance of the GN/ SAM/Au electrode could be possibly explained by the microelectrode array theory.35 The interface can be regarded as an array of capacitors in parallel, and the total capacitance (CT) of the electrode is as follows:

CT ) θCGN + (1 - θ)CSAM

(5)

Figure 9. Differential pulse voltammograms on the GN/SAM/Au electrodes with (solid line) or without (dotted line) a mixture of G, A, T, and C. Concentrations for different species: G, A, T, or C, 100 µM. Electrolyte: 0.10 M pH 5.0 acetate buffer.

where θ is the fraction of the surface coverage of graphene sheets onto the SAM/Au electrode. CGN and CSAM are the capacitance of the SAM/GN/solution and SAM/solution, respectively. Graphene sheets can restore ET on the SAM/Au electrode. Therefore, the capacitance of the GN/SAM/Au electrode is slightly larger than that of the SAM/Au electrode. Meanwhile, graphene sheets could not cover the surface of the SAM totally, so that the capacitance is much smaller than that of the bare Au electrode. The reduced capacitance of the GN/ SAM/Au electrode here is similar to that with carbon nanotubes assembled onto the SAM/Au electrode as reported previously.18e The small capacitance is very useful for electrochemical analysis, such as the determination of trace species. Surface Electrochemical Property of the GN/SAM/Au Electrode. Although ET can be restored with nanomaterials attached to the SAM, whether the nanomaterial modified SAM/ Au electrode possesses electrochemical properties of a nanomaterial modified electrode or an Au electrode is still a matter of debate.17,18 Because of the rapid, simple, and low-cost detection, electrochemical techniques have been widely used for DNA analysis,36 and the direct electrochemical oxidation of DNA is one of the simplest methods.35,37 To achieve such a purpose, it is necessary for an electrode to have a wide potential window and sufficiently high electrochemical or catalytic activity. Until now, there have been relatively few reports for the simultaneous detection of all of the four DNA bases (guanine (G), adenine (A), thymine (T), and cytosine (C)) by direct oxidation without a prehydrolysis step.12b,e,36 Niwa et al.12b and Dong et al.12e have reported that graphene or graphene oxide modified electrodes could be used to detect directly the four DNA bases. In the present case, the four DNA bases are chosen as the redox probes and to explore whether they can be detected directly on the GN/SAM/Au electrode. Because of the relatively narrow potential window and low ET kinetics, as well as the fouling at the surface, the bare gold electrode could not simultaneously detect all of the four DNA bases (see Figure S6 in Supporting Information). However, the current signals of G, A, T, and C on the GN/SAM/Au electrode are all well separated, as shown in Figure 9. This is possibly attributed to the high density of the edge planes of graphene sheets, on the GN/SAM/Au electrode, which could provide many active sites and would be beneficial for accelerating ET between the graphene modified electrode and species in solution.7,30 However, graphene sheets with unique electronic structure and remarkably high electron mobility would be the another reason for the detection of the four free bases of DNA. However, the electrochemical properties of the Au electrode, as mentioned

Heterogeneous Electron Transfer above, cannot be restored. Thus, the GN/SAM/Au electrode possesses electrochemical properties similar to those of a graphene modified electrode rather than an Au electrode. We believe that these results may also provide some insights into the understanding of surface electrochemical properties of other nanomaterials assemblies. Conclusions We have successfully fabricated a GN/SAM/Au electrode. AFM and SEM studies show that random nano- or submicroarrays of graphene sheets can be formed onto the surface of the SAM/Au electrode. The kinetics of the ET of Ru(NH3)63+ is effectively blocked by the SAM of alkanehiols with long carbon chains. However, the adsorption of graphene sheets can restore heterogeneous ET between the gold electrode and redox species in solution and reduce the interfacial capacitance. We believe that graphene sheets, as electron relay stations, can accept or donate electrons to mediate electron transfer between the electrode and redox species in solution. An advantage of SECM measurements is that it is carried out under steady-state conditions so that iR drop and charging current can be overcome. The apparent rate constant of ET on the GN/SAM/Au electrode measured by both cyclic voltammetry and scanning electrochemical microscopy is 4.2 × 10-2 and 6.8 × 10-2 cm s-1, respectively. Furthermore, by using free bases of DNA as redox probes, we have observed that the GN/ SAM/Au electrode has electrochemical properties similar to those of a graphene sheets modified electrode rather than an Au electrode. Acknowledgment. The financial support for this work from the National Natural Science Foundation of China (20735001, 20771010, and 20628506) and the Innovation Team Programs of the Ministry of Education of China is acknowledged. Supporting Information Available: Characterization of the SAM/Au electrode; the effect of the concentration of graphene sheets in DMF dispersions on the fabrication of the GN/SAM/ Au electrode; the effect of sonication in DMF for the performances of the GN/SAM/Au electrode; measurement of the rate constant by CV; studies of the surface morphology of the different Au electrodes by SECM; and DPVs of the bare Au electrode with and without the four bases in DNA. This material is available free of charge via the Internet at http://pubs.acs.org. References and Notes (1) Geim, A. K.; Novoselov, K. S. Nat. Mater. 2007, 6, 183. (2) (a) Nair, R. R.; Blake, P.; Grigorenko, A. N.; Novoselov, K. S.; Booth, T. J.; Stauber, T.; Peres, N. M. R.; Geim, A. K. Science 2008, 320, 1308. (b) Shi, Y.; Fang, W.; Zhang, K.; Zhang, W.; Li, L. J. Small 2009, 5, 2005. (3) Novoselov, K. S.; Geim, A. K.; Morozov, S. V.; Jiang, D.; Zhang, Y.; Dubonos, S. V.; Firsov, A. A.; Grigorieva, I. V. Science 2004, 306, 666. (4) Bunch, J. S.; van der Zande, A. M.; Verbridge, S. S.; Frank, I. W.; Tanenbaum, D. M.; Parpia, J. M.; Craighead, H. G.; McEuen, P. L. Science 2007, 315, 490. (5) Yu, A.; Ramesh, P.; Itkis, M. E.; Bekyarova, E.; Haddon, R. C. J. Phys. Chem. C 2007, 111, 7565. (6) Stoller, M. D.; Park, S.; Zhu, Y.; An, J.; Ruoff, R. S. Nano Lett. 2008, 8, 3498. (7) Pumera, M. Chem. Rec. 2009, 9, 211. (8) Pumera, M.; Smid, B.; Veltruska, K. J. Nanosci. Nanotechnol. 2009, 9, 2671. (9) Wang, X.; Zhi, L.; Mullen, K. Nano Lett. 2008, 8, 323. (10) (a) Yoo, E. J.; Kim, J.; Hosono, E.; Zhou, H.-S.; Kudo, T.; Honma, I. Nano Lett. 2008, 8, 2277. (b) Takamura, T.; Endo, K.; Fu, L.; Wu, Y.; Lee, K. J.; Matsumoto, T. Electrochim. Acta 2007, 53, 1055.

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